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ABREVIERI
1. OBTINEREA NANOCOMPOZITELOR POLIMERICE
1.1. Introducere
1.2. Clasificarea nanocompozitelor polimerice
1.3. Tipuri de polimeri utilizati pentru obtinerea nanocompozitelor polimerice
1.4. Nanocompozite pe baza de polimeri biodegradabili utilizabili in aplicatii biomedicale
1.5. Metode de obtinere a nanocompozitelor polimerice pe baza de nanoargile
1.5.1. Metode Intercalative
A-Inserarea lanturilor macromoleculare sau a unui prepolimer prin intercalarea din solutie in silicati stratificati
B- Intrcalarea din topitura
1.5.2. Metode "In Situ" (cresterea lanturilor macromoleculare prin polimerizare in situ in silicati stratificati)
2. CORELAREA STRUCTURA-PROPRIETATI
2.1. Caracterizarea fizico-chimica a materialelor
Metodele (fizice si sau chimice) de analiza
2.2. Studiu comparativ polimeri-nanocompozite
2.2.1. Influenta nanoumplurii asupra proprietatilor fizico-chimice
2.2.2. Biodegradabilitatea si nanocompozite biodegradabile utilizabile in aplicatii biomedicale
ABREVIERI
ABS - acrilonitrilbutadienstiren
HDPE - polietilena de densitate mare (inalta)
Li+ - ionul de litiu
MMA - monomer metilmetaacrilat
MMT - montmorillonit
OMLS - silicati stratificati modificati organic
PA - poli acrilat
PC - policarbonat
PE - polietilena
PES - polietersulfona
PETF - polietilentereftalat
PHB - polihidroxibutirat
PHV - polihidroxivalerat
PLA - polilactida
PLS - polimer/silicat stratificat
PP - polipropilena
PMMA - polimetilmetaacrilat
PS - polistiren
PVC - policlorura de vinil
SAN - copolimer stiren-acrilonitril
REZUMAT
Referatul prezinta in prima parte stadiul actual in domeniul obtinerii si caracterizarii materialelor nanocompozite polimerice. Deasemenea sunt aratate principalele aplicatii ale acestor tipuri de materiale.
In partea a doua a referatului sunt prezentate corelatiile dintre structura si proprietatile materialelor nanocompozite polimerice. Deasemenea in aceasta parte sunt prezentate influenta nanoumpluturii (incluziunilor nanometrice) asupra proprietatilor fizico-chimice si biodegradabilitatea nanocompozite utilizabile in aplicatii biomedicale.
Cuvinte cheie: polimer, nanocompozit, incluziune, biodegradabil, aplicatii biomedicale, transport medicamente, implanturi.
1. OBTINEREA NANOCOMPOZITELOR POLIMERICE
1.1. Introducere
In ultimii ani au atras un interes deosebit din partea comunitatilor stiintifica si industriala obtinerea si caracterizarea structurilor nanocompozite polimerice, precum si descoperirea unor noi aplicatii ale acestora. Acest interes deosebit se datoreaza proprietatilor remarcabile ale materialelor nanocompozite polimerice daca le comparam cu polimerii actuali si cu macro- sau micro-compozitele conventionale [1]. Proprietatile nanocompozitelor polimerice (mult imbunatatite fata de materialele conventionale) se refera la elasticitatea , rezistenta mecanica , rezistenta termica la incalzire , permeabilitate scazuta pentru gaze [9-13], usurinta de a arde cu flacara (flamabilitate) [14-18] si la cresterea biodegradabilitatii
Pe de alta parte a fost manifestat un interes deosebit pentru fundamentarea teoretica si aplicatiile practice privind metodele de preparare si proprietatile acestor materiale [20-35], ele reprezentand sisteme-model unice pentru studierea structurii si dinamicii polimerilor in medii restranse sau limitate [36-42].
O metoda utilizata deseori pentru imbunatatirea proprietatilor mecanice ale materialelor polimerice nanocompozite este aceea a intari structura acestora prin includerea unor fibre, plachete (nanoplaci, nanopelete) sau particule [43]. O practica obisnuita pentru obtinerea unor proprietati deosebite este aceea de a introduce, adauga, spori numarul de fibre, plachete sau particule in matricea-suport, proces prin care se fabrica materialele compozite (cu caracteristici superioare fazelor luate individual). Prin utilizarea acestei metode se imbunatatesc proprietatile polimerilor, fara a se altera greutatea specifica (densitatea) sau caracteristica de a fi ductile . Proprietatile polimerilor se vor imbunatati chiar si in cazul unui continut scazut de filer (material de umplutura) [47,50,51].
In ultimii ani au aparut tehnici de procesare care permit obtinerea incluziunilor de dimensiuni nanometrice [43]. Incluziunile nanometrice se definesc ca fiind acele incluziuni care prezinta cel putin o dimensiune in domeniul 1-100 nm
In ultimii ani cercetatorii in domeniu au incercat diverse tehnici de obtinere a matricei polimerice nanocompozite. Printre aceste tehnici amintim amestecarea in topitura si polimerizarea "in situ". Este dificila realizarea unei tehnici universale pentru obtinerea nanocompozitelor polimerice datorita diferentelor fizice si chimice intre sisteme, precum si datorita diverselor tipuri de echipamente disponibile cercetatorilor. Astfel, aceste tehnici diferite vor determina obtinerea unor rezultate diferite
In ultima decada a secolului XX, majoritatea aplicatiilor polimerilor s-au limitat la obtinerea de ambalaje de tip plastic. Aceste matariale plastice sunt, in general, poliolefine (PP, PE, PS sau PVC). obtinute din prelucrarea chimica a combustibililor fosili. Cand materialele plastice ajung in mediul inconjurator ele reprezinta reziduri nedegradabile, constituindu-se intr-o problema de mediu la nivel global. Una din metodele des utilizate in ultimul timp pentru a reduce cantitatea de reziduri de mase plastice a fost incinerarea acestor polimeri dar produsul final este dioxidul de carbon (responsabil de efectul de sera) sau diverse alte gaze cu potential poluant. Alta metoda ar fi reciclarea acestor materiale, dar aceasta abordare este consumatoare de timp si energie (indepartarea altor reziduri, separarea pe categorii de mase plastice, spalarea, uscarea, reprocesarea etc.) iar produsul final are calitatea inferioara produsului initial. Tinand cont de aceste considerente, este necesara obtinerea unor polimeri ecologici (polimeri verzi) care sa nu contina ingredienti toxici si care sa poata fi degradati in mediul inconjurator. Din aceste motive, comunitatea stiitifica si cea industriala acorda o atentie deosebita dezvoltarii materialelor biodegradabile cu propritati controlate.
Polimerii biodegradabili se definesc ca fiind polimerii care sufera o scindare a lantului polimeric, scindare indusa microbiologic, avand ca rezultat final mineralizarea materialului respectiv [61]. Astfel, acesti polimeri isi gasesc aplicatii in dezvoltarea de materiale ecologice, cu potential poluant redus. Procesul de biodegradare este influentat de anumiti factori, cum ar fi: pH, temperatura, umiditate, grad de oxigenare sau prezenta anumitor metale.
Domeniul transportului substantelor farmacologic-active (medicamentelor) in organism se dezvolta rapid, captand tot mai mult atentia oamenilor de stiinta, a responsabililor cu promovarea farmaceutica si a patronatelor industriale [62]. Visul actual al cercetatorilor farmacisti este de a realiza sisteme de transport eficiente si precise al medicamentelor catre locurile de actiune ale acestora in organism [62]. Una din metodele de interes pentru transportul medicamentelor in organism este metoda incapsularii substantei farmacologic-actice in nanoparticule cu dimensiuni sub 100 nm [62]. Chiar daca aceasta nanotehnologie este la inceput, deja s-au proiectat sisteme nanocompozite de transport bazate pe nanocapsule, sisteme micelare sau nanoparticule [63,64]. Unul dintre marile avantaje ale acestor sisteme sub-micrometrice este acela ca determina o penetrabilitate intracelulara superioara particulelor micrometrice
1.2. Clasificarea nanocompozitelor polimerice
Nanotehnologia poate fi definita ca: "realizarea, procesarea, caracterizarea si utilizarea materialelor, dispozitivelor si sistemelor cu dimensiuni cuprinse in domeniul 0,1-100 nm care prezinta proprietati fizice, chimice si biologice noi, superioare datorita dimensiunilor nanometrice" [65,66]. Interesul actual in nanotehnologie este reprezentat de nano-biotehnologie, nano-sisteme, nano-electronica si materiale nano-structurate (in care se includ, ca parte semnificativa, nano-compozitele)
Materialele utilizate ca adaos, aditiv, intaritor sau umplere (reinforcement) la producerea nanocompozitelor pot fi particule (metale, combinatii ale siliciului si alti compusi anorganici sau organici), materiale stratificate (grafit, silicati stratificati sau alte minerale stratificate) sau materiale fibroase (nanofibre sau nanotuburi) (fig. 1).
Fig. 1 Cle trei tipuri principale de nano-materiale folosite la obtinerea nanocompozitelor
In functie de materialele utilizate ca adaos (aditiv de umplutura) pentru intarire (termen original in engleza - reinforcement), nanocompozitele se impart in:
Nanocompozite bazate pe nanoparticule
Nanocompozite bazate pe nanoplachete (nanostratificate)
Nanocompozite bazate pe nanofibre.
1.2.1. Nanocompozite bazate pe nanoparticule
Compozitele bazate pe particule sunt probabil cele mai folosite in materiale din zilele noastre. De obicei, particulele sunt adaugate pentru a spori elasticitatea matricei si pentru a cresterea rezistentei. Prin reducerea dimensiunii particulelo0r catre domeniul nanometric, se pot obtine materiale noi, cu proprietati superioare celor originale.
1.2.2. Nanocompozite bazate pe nanoplachete (nanostratificate)
Cele mai utilizate plachete sunt grafitul si argila. Ca materiale brute, argila si grafitul au o structura stratificata. Pentru ca folosirea acestor materiale sa fie eficienta, straturile trebuie separate si dispersate in faza matricei (fig. 2).
Fig. 2 Morfologia nanocompozitelor polimer/argila: (a) miscibil conventional, (b) intercalat si exfoliat partial, (c) intercalat si dispersat complet (d) exfoliat si dispersat complet
1.2.3. Nanocompozite bazate pe nanofibre
Nanofibrele de carbon (crescute din faza de vapori) au fost utilizate la intarirea diferitelor tipuri de polimeri (PP, PC, nylon, PES, ABS, PETF etc.). Nan 343b17d ofibrele carbonice prezinta morfologii diferite (fig. 3), de la structuri dezordonate tip lemn de bambus la structuri ordonate tip grafit cu straturi in forma de cescuta (sau cupa)
In functie de taria interactiilor interfaciale dintre matricea polimerica si silicatul stratificat (modificat sau nu) nanocompozitele PLS se clasifica in trei categorii (fig 4), din punct de vedere termodinamic:
Nanoplacheta
(nanoplacuta, foita) de argila Intercalat Intercalat si floculat Exfoliat
Fig. 4 Tipurile de nanocompozite PLS
1.3. Tipuri de polimeri utilizati pentru obtinerea nanocompozitelor polimerice
Vollenberg si Heikens [52] au reusit sa produca nanocompozite de calitate buna prin amestecarea particulelor de umplutura cu matricea polimerica. Astfel, au utilizat PS, SAN, PC sau PP pentru includerea particulelor sferice de alumina de 35 sau 400 nm si includerea particulelor sferice de sticla de 4, 30 sau 100 nm. Fractia volumica a particulelor a fost cuprinsa in domeniul 0-25 %. Prepararea nanocompozitelor polimerice s-a realizat prin dizolvarea polimerului intr-un solvent polar si amestecarea nanosferelor (nanoperlelor) in acest produs timp de cateva ore.
Chan si colab. [54] au produs nanocompozite cu matrice polimerica de PP si nanoparticule de carbonat de calciu prin amestecarea in topitura a componentelor. Mai intai componentele au fost uscate intr-un cuptor la 120 °C, apoi au fost racite la temperatura camerei. Polimerul PP a fost amestecat intai cu un antioxidant. Nanoparticulele de carbonat de calciu, cu un diametru de 44 nm au fost adaugate cu grija, lent, sub agitare continua un timp stabilit. Dispersia a fost buna pentru fractii volumice ale umpluturii de 4,8 % si 9,2 %. La o fractie volumica de 13,2 % a avut loc fenomenul de agregare.
Petrovic si colab. [53] au dezvoltat un nanocompozit pe baza de poliuretan si silice (dioxid de siliciu). Silicea a fost mai intai amestecata cu poliol. Amestecul a fost apoi tratat cu diisocianat la 100 °C, timp de 16 ore in prezenta unui catalizator de Cocurre 55 aflat intr-o concentratie de 0,1 %.
Yang si colab. [56] au utilizat tehnica polimerizarii "in situ" pentru a obtine nanocompozite. Matricea este formata de poliamid-6 iar ca incluziuni s-au folosit particule de dioxid de siliciu. Dimensiunea particulelor a fost de 50 nm, obtinandu-se o buna dispersie dar prin utilizarea unor particule de 12 nm in diametru s-a produs fenomenul de agregare.
Ash si colab. si Siegel si colab. [70,71] au obtinut nanocompozite prin utilizarea polimerului PMMA ca matrice iar ca incluziuni u fost folosita pulberea de alumina. Pulberea de alumina (particulele) a fost adaugata la monomerul MMA dispersandu-se prin sonicare in solutia de vascozitate scazuta. Apoi au fost adaugate un initiator si un agent de transfer in lant. Polimerizarea a fost desfasurata sub atmosfera de azot, iar produsul final a fost fin divizat si uscat in vid.
Li si colab. [59] au utilizat o metoda originala pentru obtinerea nanocompozitelor ce sunt bazate pe HDPE si PP. S-a amestecat cele doua componente topite HDPE (75 %) si PP (25 %) apoi au fost obtinute benzi prin extrudere. Aceste benzi au fost apoi taiate in bucati mai mici si prelucrate in topitura prin extrudere sau turnare. Astfel a fost obtinut un material nanocompozit cu matrice de HDPE si nanofibre de PP cu dimensiuni de 30-150 nm in diametru.
1.4. Nanocompozite pe baza de polimeri biodegradabili utilizabili in aplicatii biomedicale
Nanotehnologia reprezinta "obtinerea, caracterizarea si utilizarea materialelor, dispozitivelor si sistemelor cu dimensiuni cuprinse in domeniul 0,1-100 nm" [65,66]. Nanotehnologia se ocupa si de obtinerea materialelor nano-structurate (in care se includ, ca parte semnificativa, nano-compozitele)
Nanocompozitele bazate pe polimeri biodegradabili si-au gasit foarte multe aplicatii datorita proprietatilor lor remarcabile.
Polimerii biodegradabilli pot fi obtinuti din surse biologice (fig. 5,6) cum ar fi: porumbul, celuloza, chitosanul, gelatina, amidonul, acidul lactic (dimerul ciclic - lactida din care se obtine PLA sau pot fi sintetizati de catre bacterii din molecule de mici dimensiuni de acid butiric sau acid valeric cand rezulta PHB sau PHV [72].
Celuloza
Chitosan
Gelatina
Fig. 5 Structurile moleculare ale celulozei si gelatinei
Alta modalitate de a obtine polimeri biodegradabili este folosirea poliesterilor alifatici si a copoliesterilor aromatici-alifatici din produsele petroliere.
Fig. 6 Structura moleculara a unui polimer tip PLA
Generatia urmatoare de materiale (materialele viitorului) este reprezentata de nanocompozitele bazate pe polimeri biodegradabili, asa-numitele nanocompozite verzi sau ecologice
Aplicatiile biomedicale ale nanocompozitelor polimerice biodegradabile sunt ingineria tesuturilor [74], realizarea materialelor biocompatibile pentru implanturi [62], transportul medicamentelor in organism prin metoda incapsularii substantei farmacologic-actice in nanostructuri cu dimensiuni sub 100 nm [62
In ultimii ani au fost dezvoltate diferite tipuri de materiale intaritoare pentru nanocompozitele polimerice biodegradabile, cum ar fi
Un interes particular, deosebit, il reprezinta nanocompozitele polimerice bazate pe silicati stratificati modificati organic (OMLS) datorita proprietatilor superioare rasinilor polimerice nemodificate [61]. Aceste proprietati superioare sunt valabile in general pentru un continut redus de silicat (≤ 5 % procente masa). Pe baza tariei interactiilor dintre polimer si OMLS, sunt posibile doua structuri de nanocompozite, din punct de vedere termodinamic (fig. 7): nanocompozite intercalate si nanocompozite exfoliate. In cazul nanocompozitelor intercalate lanturile polimerice aflate in structura silicata sunt dispuse regulat din punct de vedere cristalografic indiferent de raportul dintre polimer si OMLS. In cazul nanocompozitelor exfoliate, straturile individuale de silicat sunt separate in matricea polimerica si se afla la anumite distante unele de altele in functie de continutul in OMLS al nanocompozitului.
INTERCALAT EXFOLIAT
Fig. 7 Doua tipuri de aranjamente posibile din punct de vedere termodinamic pentru un nanocompozit polimer/silicat
1.5. Metode de obtinere a nanocompozitelor polimerice pe baza de nanoargile.
O metoda foarte buna pentru obtinerea nanocompozitelor pe baza de nanoargile este intercalarea polimerilor in golurile din structura silicatului, folosind doua abordari
- inserarea unor monomeri potriviti in golurile dintre straturile de silicati urmata de polimerizare
- inserarea directa a lanturilor polimerilor in golurile silicatilor din solutie sau din topitura.
In ultimii ani intercalarea din topitura a devenit o metoda foarte atractiva pentru obtinerea nanocompozitelor polimer/OMLS deoarece aceasta metoda este compatibila cu tehnicile industriale moderne. Aceasta metoda presupune calirea materialului (incalzirea si racirea) static sau in cadrul operatiei de debitare (taiere, forfecare).
1.5.1. Metode Intercalative.
A-Inserarea lanturilor macromoleculare sau a unui prepolimer prin intercalarea din solutie in silicati stratificati.
B- Intercalarea din topitura.
1.5.2. Metode "In Situ" (cresterea lanturilor macromoleculare prin polimerizare in situ in silicati stratificati)
2. CORELAREA STRUCTURA-PROPRIETATI
Micromecanica teoretica actuala se bazeaza pe faptul ca proprietatile materialelor compozite, (ex. modulul de elasticitate Young) depind de proprietatile constituientilor, fractia volumica a componentelor, forma si aranjamentul incluziunilor, precum si de interfata matrice-incluziuni.
Aceasta teorie prevede faptul ca proprietatile materialelor compozite sunt independente de dimensiunile incluziunilor; acest lucru fiind in general valabil pentru sistemele in care au fost incluse particule/fibre cu dimensiuni micrometrice. Pentru sistemele nanocompozite nu este valabila aceasta teorie
Chan si colab. [54] au afirmat faptul ca anumite proprietati, cum ar fi modulul de elasticitate si fortele de tensiune scad in cazul nanocompozitelor cu matrice polipropilenica datorita modificarilor procesului de nucleatie, modificari determinate de nanoparticule (fig. 8).
Fig. 8 (a) PP pur; (b) PP continand 9,2 % filer (umplutura), procent volumic
Singh et al. [79] au studiat modificarile rezistentei la rupere a unei rasini poliesterice, modificari datorate adaugarii de particule de alumina cu dimensiunea de 20, 3,5 si 100 nm in diametru. Fig. 9 prezinta o crestere initiala a rezistentei la rupere, urmata de o scadere a acesteia la fractii volumice mari pentru particulele utilizate. Acest fenomen este atribuit aglomerarii nanoparticulelor in cazul maririi volumului de particule.
Fig. 9 Rezistenta la rupere (valorile normalizate) functie de fractia volumica procentuala a particulelor de aluminiu [79]
2.1. Caracterizarea fizico-chimica a materialelor
Morfologia la scala nanometrica deschide posibilitatea dezvoltarii unor modele de studiu interfaciale care permit evaluarea structurii si dinamicii catenelor inlantuite prin utilizarea tehnicilor conventionale macroscopice cum ar fi calorimetria cu scanare diferentiala, curentul stimulat termic, reometria, rezonanta magnetica nucleara sau diferitele metode spectroscopice [36,37,80,81].
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2. CORELAREA STRUCTURA-PROPRIETATI
2.1. CARACTERIZAREA FIZICO-CHIMICA A MATERIALELOR
Metodele (fizice si sau chimice) de analiza
Nanocompozitele se compun dintr-un polimer si un silicat stratificat (modificat sau nu) frecvent prezentand proprietati mecanice si de material imbunatatite remarcabil cand le comparam cu cele ale polimerilor clasici continand o mica cantitate (£ 5%) de silicat stratificat. Imbunatatirile includ un coeficient mai mare, crescand durabilitatea si rezistenta termica, scazand permeabilitatea gazului si inflamabilitatea , si crescand biodegradabilitatea a polimerilor biodegradabili.
Fig. 66. Imagini in camp stralucitor TEM ale (a) PLACN12 (C8, 1,7%) si (b) PLACN2 (C16, 3%)
Motivul principal al acestor proprietati imbunatatite din nanocompozite este interactiunea interfaciala mai puternica dintre matrice si silicatul stratificat, comparat cu sistemele conventionale ranforsate cu umplutura.
2.1.1.Proprietati mecanice.
2.1.1.1.Analize mecanice dinamice.
Analizele mecanice dinamice (DMA) masoara raspunsul dat de un material la o deformatie oscilatorie (aici in modul tensiune-torsiune) ca o functie de temperatura. Rezultatele DMA sunt compuse din trei parametrii: (a) coeficientul de stocare (GI); (b) coeficientul de pierdere (GII), si (c) tang d, raportul (GII/ GI), folositor pentru determinarea pozitiei tranzitiilor mobilitatii moleculare, cum ar fi temperatura tranzitiei sticloase (Tg).
DMA au fost folosite pentru a studia dependenta temperaturii de coeficientul de stocare al PMMA pe pentru matia de nanocompozit sub conditii experimentale diferite Fig. 71 aratand dependenta temperaturii de GI si tang d, pentru variate nanocompozite bazate pe copolimeri, si corespunzator amestecurilor fara argila de copolimer/QA+. Masuratorile au fost realizate folosind un instrument RDAII cu frecventa oscilatorie de 6,28 rad/s, cu o amplitudine de 0,05%, si o rata de incalzire de 20C/min.
Fig. 67. Exemple WAXD de smectite, MMT, si mica nanocompozite cu C16 organo-argila si acelasi continut de argila
Fig. 68. Prezentarea schematica a straturilor de silicat in OMLS si in diferite nanocompozite
Fig. 69. C18-MMT, diferiti PBSCNs Fig. 70. Imagini TEM de PBSCN cu 2,8 % MMT
si PBS pur. Linia punctata indica locul
silicatului (001) reflectat de C18-MMT
dispersat in matricea PBS
Pentru amestecurile PMMA/SPN10 si PMMA/QA+, nu este nici o diferenta intre dependenta de temperatura de GI sau tang d (vezi Fig. 71a). Pana la incorporarea de AEA, PAA, si AA pana la PMMA, o crestere puternica in GI se dezvolta la toate temperaturile in timp ce matricea copolimera apare. Acest comportament indica ca adaugarea de copolimer are un efect puternic asupra proprietatilor elastice ale matricelor corespunzatoare. Pe de alta parte, peak-urile tang d ale nanocompozitelor sunt indreptate catre temperaturi mai mici decat pentru amestecurile fara argila corespunzatoare. Comparate cu alte sisteme, comportamentul ui PMMA-AA(1)/SPN10 este putin diferit; peste Tg, GI este aproape acelasi pentru nanocompozite si matricele fara argila corespunzatoare, desi o extindere si o mica schimbare de temperatura la tang d este observata. Comportamentul poate fi facut la intercalarea lanturilor copolimerice in galeriile straturilor de argila, care conduc la impiedicarea mobilitatii ale segmentelor copolimrului aproape de interfata. Aceasta presupunere a fost sprijinita de modelele WAXD ale acestor nanocompozite (vezi Fig. 21d), care prezinta slab, dar peak-uri semnificative din planurile expandate (002) si (003).
Fig. 71. Dependenta temperaturii de GI si tang d pentru nanocompozite si amestecurilor copolymer/QA+ corespunzatoare fara argila
Fig. 72. Dependenta temperaturii de GI; GII si tang d pentru matricea PP-MA
si diferite PPCNs
Fig. 72 arata dependenta temperaturii de GI, GII si tang d de diferite PPCNs si matricea corespunzatoare PP-MA. Pentru toate PPCNs, este o puternica intensificare a modulilor peste nivelul temperaturii investigate, care indica raspunsurile plastice si elastice ale PP fata de deformatie sunt puternic influentate in prezenta de OMLS. Sub Tg, intensificarea lui GI este clar in PPCNs intercalat.
In contrast, curbele tang d pentru PPCNs arata doua peak-uri: unul la -50C si altul peak clar intre 50 si 900C. McCrum si colaboratorii au demonstrat ca curba tang d a PP trei exponati relaxati localizati in vecinatatea - 800C (g), 100C (Tg) si 1000C (a). Relaxarea dominanta la circa 100C este relaxarea sticla-cauciuc a portiunii amorfe de PP.
Fig. 73. Spectre ale dinamicii mecanice: (a) modulul de stocare, EI; (b) modulul de pierdere, EII; si (c) factorul de pierdere tang d ca o functie de temperature pentru PP pur si PPCN
Proprietatile mecanice dinamice ale PP si PPCNs pure preparate cu EM-MMT sunt aratate in Fig. 73 [304]. Aceste rezultate arata clar ca incorporarea de EM-MMT in matricea PP reprezinta o remarcabila crestere in rigiditate si o descrestere in tang d. Curbele GI arata un palier elastic largit, aratand ca adaugarea de EMMMT induce un efect de intarire; la temperaturi foarte mari acest efect de intarire a rigiditatii. Acest comportament in plus indica sporirea stabilitatii termo-mecanice a acestor materiale la temperaturi inalte.
Similar cu sistemele anterioare, acesti PPCNs arata doua peak-uri in tang d (vezi Fig. 73c). Procesul dominant de relaxare la circa 100C este relaxarea sticlos-elastica a portiunii amorfe de PP. Aparitia peak-ului slab sub pentru ma de umar la circa 1000C este asociat cu regiuni (a-faze) cristaline ale PP. Un alt fenomen interesant este acela ca valorile Tg ale PPCNs nu descresc mai mult peste un continut de EM-MMT de 3%. Oricum, mecanismul pentru acest comportament ne ste inca inteles.
Imbunatatirea remarcabila in GI; in plus puternica interactie dintre matricea de OMLS, este observata clar in nanocompozitele N6/OMLS
Fig. 74 reprezinta dependenta temperaturii de GI, GII si tang d a matricei N6 si diferite nanocompozite (N6CNs). Detaliile ale dinamicii temperaturii din testul de rampa pentru N6 pur si diferite N6CNs sunt sumarizate in Tabelul 12. Toate N6CNs arata o mare crestere in modul la toate temperaturile.
Cresterile in GI observate la dimensiunea particulelor de argila dispersate este in plus demonstrata in PLACNs . In ordine pentru a intelege efectul compatibilizarilor peste proprietatile morfologice si mecanice, autorii deasemenea au preparat PLACNs cu foarte mici cantitati de oligo(ε -caprolactona) (a-PCL).
Detaliile si denumirile compozitiei pentru diferite tipuri de nanocompozite sunt prezentate in Tabelul 8
Fig. 74. Dependenta temperaturii de GI; GII si tang d pentru matricea N6 si diferiti N6CNs.
Tabelul 12
Testele sumare de DMA pentru N6 si diferiti N6CNs sub diferite niveluri de temperatura
Partile a si b ale Fig. 75 arata dependenta temperaturii de GI; GII si tang d a matricii de PLA si respectiv a diferitelor PLACNs. Pentru toate PLACNs, sporirea de GI poate fi vazuta peste nivelul de temperatura investigat cand comparat cu matricea, indica ca C18-MMT are un efect puternic asupra proprietatilor elastice ale PLA pur. Sub Tg, sporirea de GI este clara pentru diferite intercalate PLACNs. Pe de alta parte, toate PLACNs arata o mai mare crestere in la temperaturi mari comparat fata de cele ale matricelor de PLA.
Fig. 75. Dependenta de temperatura a GI; GII si nivelul lor tang d pentru PLACNs si matricele corespunzatoare: (a) fara o-PCL si (b) cu o-PCL
Acest lucru este facut celor doua intariri mecanice de catre particulele de argila si intercalare extinsa la temperaturi mari . Peste Tg, cand materialele devin moi, efectul de intarire ale particulelor de argila devin proeminente fata de miscarea restrictiva a lanturilor de polimer. Acesta este acompaniat de observarea cresterii de GI.
La cealalta extrema, probele PLACN4 si PLACN5 arata cresteri mari ale GI comparate cu cele ale probei de PLACN2 cu comparabila argila adaugata, si fata de matricele PLA/o-PCL continand pana la 0,5% o-PCL (vezi Tabelul 9 Fig. 75b). prezenta unor mici cantitati de o-PCL nu conduc la o mare schimbare sau largire a curbelor tang d Oricum, o mare crestere in GI peste Tg devine evidenta, indicand ca marea anisotropie ale particulelor floculate dispersate sporeste componentul tinta. Valorile lui GI la diferite niveluri de temperatura ale diferitelor PLACNs si matricele corespunzatoare fara argila sunt reprezentate in Tabelul 13. PLACNs cu o foarte mica cantitate de o-PCL (PLACN4 si PLACN5) expun o foarte mare sporire a proprietatilor mecanica comparate cu cele ale PLACN cu comparabila argila adaugata (PLACN2). Factorul esential care guverneaza sporirea proprietatilor mecanice in nanocompozite este nivelul aspectului al particulelor de argila disperate
Tabelul 13
Valoarea GI a diferite PLACNs si matricele corespunzatoare fara argila la diferite niveluri de temperatura
Din figurile TEM (vezi Fig. 62), este clar vazut ca, in prezenta unei foarte mici cantitati de o-PCL, floculatia particulelor de argila dispersate au loc, iarasi datorita puternicelor interactiuni margine-margine ale particulelor de argila. Nivelele aspectului 2D ale particulelor de argila dispersate Lclay/dclay estimate din observarea TEM sunt 22 pentru PLACN4 si 12 pentru PLACN2 (vezi Tabelul 9). Acest mare nivel de aspect conduce la sporirea observata a proprietatilor mecanice.
Tabelul 8
Compozitia si parametrii caracteristici ai diferitelor PLACNs bazate pe PLA, o-PCL si C18-MMT
Tabelul 9
Comparatie intre factorii de pentru mare a PLACN2 si PLACN4 obtinuta din structurileWAXD si obervatiile TEM
Fig. 62. Imagini campuri stralucitoare TEM: (a) PLACN2 ( 10,000), (b) PLACN4 ( 10,000), (c) PLACN2 ( 400,000), si (d) PLACN4 ( 40,000). Entitatile intunecate sunt sectiunile de trecere a OMLS intercalat, si zonele luminoase sunt matricele
Ipoteza ca o crestere in GI depinde direct de nivelul aspectului al particulelor de argila dispersate este de asemenea clar observate in PBSCNs. Dependenta temperaturii de GI pentru PBS si diferiti PBSCNs sunt prezentate in Fig. 76a. Natura cresterii lui GI in PBSCNs cu temperatura este cumva diferita fata de teoriile bina stabilite, care explica comportamentul similar observat in sistemele care sunt fie intercalate (PP-MA/MMT) sau exfoliate (N6/MMT) . In sistemul precedent, GI creste tipic cu circa 40-50°%, cand comporat cu cele ale matricii mult sub Tg, in timp ce peste Tg, este o mare crestere (>200%) in GI. Acest comportament este comun pentru nanocompozitele prezntate mai sus, si motivul a fost aratat a fi efectul puternic de intarire al particulelor de argila peste Tg (cand materialele devin moi). Oricum, in cazul PBSCNs, ordinea cresterii in GI este aproape aceiasi sub si peste Tg, si acest comportament poate avea loc la un extreme de mic Tg (-290C) al matricii de PBS.
Peste nivelul de temperatura de - 50 pana la -100C, cresterea in GI este de 18% pentru PBSCN1, 31% pentru PBSCN2, 67% pentru PBSCN3 si 167% pentru PBSCN4 comparate cu cele ale PBS pur. In plus, la temperatura camerei PBSCN3 si PBSCN4 arata o mare crestere in GI, 82 si 248%, respectiv, comparate cu cele de PBS pur, in timp ce cele de PBSCN1 si PBSCN2 sunt mai mari cu 18,5 si 44% La 900C. Doar PBSCN4 prezinta o foarte mare crestere a GI comparat cu celelalte trei PBSCNs.
In contrast, PBSCNs preparat cu qC16-sap prezinta o relativa mica crestere a GI comparativ cu cele de PBSCNs preparate cu C18-MMT (vezi Fig. 76b). Pentru PBSCN6, crestereea in GI este de 102,5% la -50 0C, 128,6% la 25 0C si 100% la 90 0C, comparat cu PBS. Aceste valori sunt mult mai mici comparate cu cele de PBSCN4, desi ambele specii contin un comparabil continut in argila (parte anorganica). Doi factori pot fi sugerati pentru o foarte mare crestere a modulului in cazul PBSCN4, comparat cu cel de PBSCN6, unul din factori este foarte marele format al aspectului al particulelor de argila dispersate si alt factor este bine ordonata structura intercalata in PBSCN4.
Fig. 76. Dependenta temperaturii GI; GII si formatul lor tang d pentru (a) PBSCNs (preparat cu C18-MMT) si PBS pur, (b) PBSCNs
Fig. 77. Grafice ale GInanocomposite/GImatrix vs. vol% de argila pentru nanocompozite diferite. Coeficientul Einstein kE este aratat cu numar in chenar, liniile arata rezultatele calculate de teoria lui Halpin si Tai's cu variate kE.
Dependenta continutului de argila de GI pentru diferite tipuri de nanocompozite obtinute mult sub Tg sunt prezentate in Fig. 77, aratand coeficientul Einstein, kE; care este derivat folosind expresia teoretica a lui Halpin si Tai's modificata de Nielsen si repezinta formatul aspectului (L/D) al particulelor dispersate de argila fara intercalare. expresia Haplin-Tai's-Nielsen al modulului nanocompozitelor, GInanocomposite este dat de
Aici, GImatrix si GIclay sunt module de stocare ale matricei (aici PLA, PBS, PP-MA si N6) si respectiv argila. X este o constanta care depinde de tipul structurii nanocompozitului, este legata de formatul aspectului, si ψclay si ψm sunt fractiuni de volum al argilei intarite si respectiv maximul fractiuni de volum de umplere (in general egal cu 0,63). Presupunerea ca valoarea lui GIclay este egala cu 170 GPa , permitand estimarea dependentei compozitiei de GInanocomposite/GImatrix folosind ecuatia de mai sus. In plus, valorile lui kE au fost estimate prin selectarea unei valori apropiate din cele mai bune potrivite pentru experiment obtinand graficele GInanocomposite/GImatrix vs. ψclay (vezi Fig. 77
Din Fig. 77, este clar observat ca PBSCNs prezinta o mare crestere in GI comparat cu al altor nanocompozite avand acelasi continut de argila in matrice. PPCNs sunt bine cunoscute ca sisteme intercalate, N6CNs sunt bine stabilite ca nanocompozite exfoliate, PLACNs sunt considerate nanocompozite intercalate-si-floculate, in timp ce PBSCNs sunt nanocompozite intercalate-si-extins floculate . Datorita interactiunii puternice dintre gruparilor hidroxilate capat-capat, particulele de argila sunt uneori floculate in matricea de polimer. Ca rezultat al acestei floculatii, lungimea particulelor de argila creste enorm, rezultand intr-o crestere corespunzatoare peste toate formatului aspectului. Pentru prepararea unei mase moleculare mari PBS, grupari finale diisocianat sunt in general folosite ca o prelungire de lant . Aceste grupari finale de isocianat fac legaturi uretanice cu cu cele de hidroxi terminale LMW PBS. Fiecare masa moleculara mare a lantului PBS doua din acest tip de legaturi, si de aici (vezi ilustrarea schematica in Fig. 78
Fig. 78. Formarea legaturilor uretanice in mase moleculare mari PBS
Fig. 79. Formarea legaturilor de hidrogen intre PBS si argila, care conduce la floculatia silicatilor stratificati dispersati
Aceste legaturi tip uretanive conduc la o puternica interactie cu suprafata silicata prin formarea legaturilor de hidrogen, si de aici floculatie puternica (vezi Fig. 79). Pentru acest motiv, formatul aspectului al particulelor de argila dispersate este mult mai mare in cazul lui PBSCNs comparat cu alte nanocompozite, si de aici marea intensificare a modulului.
Fig. 80. Ilustrarea schematica pentru formarea legaturilor de hidrogen in nanocompozite
N6/MMT.
2.1.2. Proprietatile de intindere
Modulul de intindere al unui material polimeric a fost prezentat a fi remarcabil imbunatatit cand nanocompozitele sunt formate cu silicati stratificati. Nanocompozitele N6 preparate prin inele intercalate in situ deschizand polimerizarea ε - caprolactama, conducand la formarea unor nanocompozite exfoliate, prezinta o crestere drastica in prorietatile de intindere la mai degraba mici continuturi de umplutura. Motivul principal pentru drastica imbunatatire in modulul de intindere in nanocompozitele N6 este puternica interactiune dintre matrice si silicati stratificati prin formarea legaturilor de hidrogen, cum este aratat in Fig. 80
In cazul nanocompozitelor, intinderea imbunatatita a modulului depinde direct pe media lungimii particulelor de argila dispersate, si de aici formatul aspectului. . Fig. 81 reprezinta dependenta modulului de intindere E masurat la 1200C pentru nanocompozite exfoliate N6 cu diferite continuturi de argila, obtinute prin polimerizarea intercalata in situ a ε-caprolactama in prezenta de acid aminododecanoic protonat -MMT modificat si saponit. Mai mult, diferenta in exfolierea intinsa, cum se observa pentru nanocompozite bazate pe N6 sintetizate prin polimerizarea intercalata in situ a ε-caprolactama folosind Na+ - MMT si diferiti acizi, influentat puternic modulul nanocompozitelor.
Tabelul 14
Proprietatile mecanice ale IpotNCH sintetizat in prezenta acidului fosoric si prin metoda intercaltiva in situ.
Tabelul 14 prezinta modulul de intindere al 1potNCH impreuna cu N6 pur si NCH preparate prin polimerizare in situ intercalata cu deschidere de inel a ε-caprolactama . Modulul excelent in cazul 1potNCH este atribuita dispersarii uniforme a silicatilor stratificat. In plus, 1potNCH si-a imbunatatit proprietatile mecanice cand comparam cu NCH.
Tabelul 14 summarizes the tensile modulus of
1potNCH together with neat N6 si NCH prepared
via in situ intercalative ring-opening polymerization
of1 -caprolactam . The excellent modulus in the
case of 1potNCH is attributed to the uniformly
dispersed silicate layers. Furthermore, 1potNCH has
improved mechanical properties when compared with
NCH. The polymer matrix in the nanocomposites
prepared by a one pot synthesis is the homopolymer of
N6, whereas in the case of NCH prepared via
intercalative ring-opening polymerization, the matrix
is a copolymer of N6 si a small amount of N12. The
presence of N12 may give rise to the lower modulus.
One can observe variations of the modulus of the
nanocomposites based on the various kinds of
acids used to catalyze the polymerization
The WAXD peak intensity (Im; inversely related to
the exfoliation of clay particles) also depends on the
nature of the acid used to catalyze the polymerization
process. pentru an increase in the Im values, a parallel
decrease in the modulus is observed, indicating that
exfoliated layers are the main factor responsible pentru
the stiffness improvement. Intercalated particles,
having a less important aspect ratio, play a minor
role. These observations are further confirmed in
Fig. 82, which presents the evolution of the tensile
modulus at room temperature of N6 nanocomposites
obtained by melt extrusion as a function of the filler
content
The effect of MMT content si N6 molecular
weight on the tensile modulus of nanocomposites
prepared using MMT modified with (HE)2M1R1 is
shown in Fig. 83. The addition of organoclay leads to
a substantial improvement in stiffness pentru the
composites based on each of the three N6, i.e.
LMW, MMW si HMW. Interestingly, the stiffness
increases with increasing matrix molecular weight at
any given concentration even though the moduli of the
neat N6 are all quite similar. Tabelul 15 summarizes the
moduli si other mechanical properties of the virgin
Fig. 81. Effect of clay content on tensile modulus in case of
N6/OMLS nanocomposites prepared via melt extrusion
Reproduced from Kojima, Usuki, Kawasumi, Okada, Kurauchi and
Kamigaito by permission of
Materials Research Society,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1598
materials si selected (HE)2M1R1 organoclay/N6
nanocomposites. The slightly larger modulus of
2.82 GPa pentru LMW may be the result of a higher
degree of crystallinity, creating faster crystallization
kinetics during the cooling of the specimen during
injection molding.
Similar trends with respect to the level of
organoclay content si molecular weight are evident
in the yield strength results. The dependence of yield
strength on MMT content si molecular weight is
shown in Fig. 84. Yield strength increases with the
content of MMT, however, there are notabelul differences
in the level of strength improvement pentru pure
polyamides. The HMW- si MMW-based nanocomposites
show a steady increase in strength with content
of clay, while the LMW-based nanocomposites show
a less pronounced effect. The differences in strength
improvement with respect to molecular weight are
very prominent at the highest clay content. The
increase in strength relative to the virgin matrix pentru the
HMW composite is nearly double to that of the LMW
composite.
The relationship between MMT content and
elongation at break pentru the different matrices is
shown in Fig. 85 pentru two different rates of extension.
Fig. 85a shows that the virgin polyamides are very
ductile at a test rate of 0.51 cm/min. With increasing
clay content the ductility gradually decreases, however,
the HMW si MMW based composites attain
reasonable levels of ductility at MMT concentrations
as high as 3.5 wt%. The elongation at break pentru the
LMW-based nanocomposites decreases rapidly at low
MMT content (around 1 wt%). The larger reduction in
the LMW-based systems may be due to the presence
of stacked silicate layers, as seen in TEM photographs
(see Fig. 46). In contrast, the higher testing rate of
5.1 cm/min yields similar trends, as shown in Fig. 85b
but the absolute level of the elongation at break is
significantly lower. Interestingly, the strain at break
for LMW composites is relatively independent of the
rate of extension, similar to what has been observed in
glass fiber reinforced composites. Even at the highest
clay content, the HMW composite exhibits ductile
fracture, whereas the LMW- si MMW-based
nanocomposites fracture in a brittle manner at the
highest clay content.
In the case of PPCNs, most studies report the
tensile properties as a function of clay content. The
results of an Instron study of a neat-PP/f-MMT
composite compared to a PP/2C18-MMT 'conventional'
composite are shown in Fig. 86. In PP/layered
silicate nanocomposites, there is a sharp increase in
tensile modulus pentru very small clay loading
(#3 wt%), followed by a much slower increase
beyond a clay loading of 4 wt%. This is behavior
characteristic of PLS nanocomposites. With an
increase in clay content, strength does not change
markedly compared to the neat-PP value, si there is
only a small decrease in the maximum strain at break.
Conventional composites of PP with the same fillers
do not exhibit as much of an improvement in their
tensile modulus. On the other hand, as the PP/layered
silicate interaction is improved, pentru example when
Fig. 82. Dependence of tensile modulus ðEÞ on clay content
measured at 120 8C . Reproduced from Alexander si Doubis
by permission of Elsevier Science
Ltd,
Fig. 83. Effect of MMT content on tensile modulus pentru LMW,
MMW, si HMW based nanocomposites . Reproduced from
Fornes, Yoon, Keskkula si Paul by permission of Elsevier Science
Ltd,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1599
MA functional groups are incorporated into the
polymer, the stress is much more efficiently transferred
from the polymer matrix to the inorganic filler,
resulting in a higher increase in tensile properties.
Parts a si b of Fig. 87 represent the dependence of
the tensile modulus si strength on MMT content of
various PPCNs prepared by melt extrusion of PP-MA
and C18-MMT, respectively. The modulus of the
PPCNs systematically increases with increasing clay
content, as does the tensile strength up to 4 wt%,
where it levels off.
If the interaction between nanocomposite components
is not thermodynamically favorable, these
properties will change during processing because the
nanocomposite structure will change. Recent work by
Reichert et al. systematically found the
dependencies on compatibilizer functionality and
organic modification, si revealed that considerable
tensile property enhancement could be achieved only
when appropriate PP-MA compatibilizers were used
to pretreat the OMLS in conjugation with specific
organic modifications of the MMT. Similar materials
under different processing conditions showed much
smaller improvements in the practical material
properties
The tensile properties of various PPCNs prepared
with EM-MMT, a new type of co-intercalated MMT,
are summarized in Fig. 88 [304]. The PPCN containing
1 wt% EM-MMT is abbreviated as PPCN1, while
the PPCNs with 3, 5 si 7 wt% of EM-MMT are
abbreviated as PPCN3, PPCN5, si PPCN7, respectively.
The tensile strength of the PPCNs increase
rapidly with increasing EM-MMT content from 0 to
5 wt%, but the trend is less pronounced when the clay
content increases beyond 5 wt%. A similar trend is
observed pentru the tensile modulus. In contrast, the
notched Izod impact strength of the PPCNs is
Tabelul 15
Mechanical properties of some N6/(HE)2M1R1 nanocomposites
N6/(HE)2M1R1 nanocomposites Modulus
(GPa)
Yield strength
(MPa)
Straina
Elongation at break
(%) crosshead speed
Izod impact strength
(J/m)
0.51 cm/min 5.1 cm/min
LMW
0.0 wt% MMT 2.82 69.2 4.0 232 28 36.0
3.2 wt% MMT 3.65 78.9 3.5 12 11 32.3
6.4 wt% MMT 4.92 83.6 2.2 2.4 4.8 32.0
MMW
0.0 wt% MMT 2.71 70.2 4.0 269 101 39.3
3.1 wt% MMT 3.66 86.6 3.5 81 18 38.3
7.1 wt% MMT 5.61 95.2 2.4 2.5 5 39.3
HMW
0.0 wt% MMT 2.75 69.7 4.0 3.4 129 43.9
3.2 wt% MMT 3.92 84.9 3.3 119 27 44.7
7.2 wt% MMT 5.70 97.6 2.6 4.1 6.1 46.2
Reproduced from pentru nes, Yoon,
Keskkula si Paul by permission of Elsevier Science Ltd,
a Strain at yield point measured during modulus si yield strength testing using a crosshead speed of 0.51 cm/min.
Fig. 84. Effect of MMT content on yield strength pentru LMW,
MMW, si HMW based nanocomposites . Reproduced
from pentru nes, Yoon, Keskkula si Paul by permission of Elsevier
Science Ltd,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1600
constant, within experimental error, in the EM-MMT
content range between 0 si 7 wt%.
The evolution of the tensile modulus pentru the epoxy
matrix with three different types of layered silicates is
presented in Fig. 89 [228]. A C18-MMT, a C18Amagadiite,
and a magadiite modified with methyloctadecylammonium
cation (C18A1M-magadiite)
were used pentru nanocomposite preparation. This figure
shows a significant increase in the modulus pentru the
MMT-based nanocomposites with filler content of
4 wt%. According to the present authors, this behavior
is due to the difference in layer charge between
magadiite si MMT. Organomagadiites have a higher
layer charge density, si subsequently higher alkylammonium
content than organo-MMT. As the
alkylammonium ions interact with the epoxy resin
while polymerizing, dangling chains are pentru med, and
more of these chains are pentru med in the presence of
organomagadiites. These dangling chains are known
to weaken the polymer matrix by reducing the degree
of network cross-linking, then compromising the
reinforcement effect of the silicate layer exfoliation.
For thermoset matrices, a significant enhancement
in the tensile modulus is observed pentru an exfoliated
structure when alkylammonium cations with different
chain length modified MMTs were used pentru nanocomposite
preparations, with the exception of the
MMT modified with butylammonium, which only
gives an intercalated structure with a low tensile
modulus.
In a recent work, Mulhaupt et al. reported the
correlations between polymer morphology, silicate
structure, stiffness, si toughness of thermoset
Fig. 85. Effect of MMT content on elongation at break pentru LMW, MMW, si HMW based nanocomposites at a crosshead speed of
(a) 0.51 cm/min si (b) 5.1
cm/min Reproduced from pentru nes, Yoon,
Keskkula si Paul by permission of Elsevier Science Ltd,
Fig. 86. Tensile characterization of the PP/f-MMT nanocomposites
(B) by Instron. pentru comparison, conventionally filled PP/2C18-
MMT 'macro' composites are also shown (W) . Reproduced
from Manias, Touny, Strawhecker, Lu si Chang by permission of
American Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1601
nanocomposites as a function of layered silicate type
and content. They suggest that the main factor pentru the
matrix stiffness improvement resides in the pentru mation
of supramolecular assemblies obtained by the presence
of dispersed anisotropic laminated nanoparticles.
They also describe a stiffening effect when the
MMT is modified by a functionalized organic cation
(carboxylic acid or hydroxyl groups) that can strongly
interact with the matrix during curing.
The tensile properties of APES/Cloisite 30B and
APES/Cloisite 10A nanocomposites at various clay
contents are presented in Tabelul 16 [398]. In
comparison to the APES, the tensile strength and
modulus have been improved with a slight decrease in
elongation at break. APES/Cloisite 30B nanocomposites
exhibit a much higher tensile strength and
modulus compared to the APES/Cloisite 10A nanocomposites.
This is also attributed to the strong
interaction between the APES matrix si Cloisite
30B. These results further confirm the importance of
strong interaction between matrix si clay, which
ultimately leads to better overall dispersion, as
already observed by TEM analysis.
4.1.3. Flexural properties
Nanocomposite researchers are generally interested
in the tensile properties of final materials, but
there are very few reports concerning the flexural
properties of neat polymer si its nanocomposites
with OMLS. Very recently, Sinha Ray et al.
reported the detailed measurement of flexural properties
of neat PLA si various PLACNs. They
conducted flexural property measurements with
injection-molded samples according to the ASTM
D-790 method. Tabelul 17 summarizes the flexural
modulus, flexural strength, si distortion at break of
neat PLA si various PLACNs measured at 25 8C.
There was a significant increase in flexural modulus
for PLACN4 when compared to that of neat PLA,
followed by a much slower increase with increasing
OMLS content, si a maximum at 21% pentru PLACN7.
Fig. 88. Effect of clay loading on: (a) tensile modulus, si (b) tensile strength of PPCNs . Reproduced from Liu si Wu by permission of
Elsevier Science Ltd,
Fig. 87. Relative moduli of various PP-based nanocomposites, each
normalized by modulus of the respective neat PP. (a) PP-based
nanocomposites with: f-MMT (B), C18-MMT (K), si 2C18-MMT
(W). (b) PP-g-MA/2C18-MMT nanocomposite (B) si PP hybrids
with various PP-g-MA pretreated organically modified MMT: C18-
MMT right triangle open), C18-MMT (W, K), si C8-MMT (K, K)
. Reproduced from Manias, Touny, Strawhecker, Lu and
Chang by permission of American
Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1602
On the other hand, the flexural strength si distortion
at break shows a remarkable increase with PLACN4,
then gradually decreases with OMLS loading.
According to the author, this behavior may be due
to the high OMLS content, which leads to brittleness
in the material.
4.2. Heat distortion temperature
Heat distortion temperature (HDT) of a polymeric
material is an index of heat resistance towards applied
load. Most of the nanocomposite studies report HDT
as a function of clay content, characterized by the
procedure given in ASTM D-648. Kojima et al.
first showed that the HDT of pure N6 increases up to
90 8C after nanocomposite preparation with OMLS.
In their further work they reported the clay
content dependence of HDT of N6/MMT nanocomposites.
In N6/MMT nanocomposites, there is a
marked increase in HDT from 65 8C pentru the neat N6
to 152 8C pentru 4.7 wt% nanocomposite. Beyond that
wt% of MMT, the HDT of the nanocomposites level
off. They also conducted HDT tests on various N6
nanocomposites prepared with different lengths of
clay si found that the HDT also depends on the
aspect ratio of dispersed clay particles . Like all
other mechanical properties, the HDT of 1potNCH is
higher than that of NCH prepared by in situ
intercalative polymerization
Since the degree of crystallinity of N6 nanocomposites
is independent of the amount si nature of
clay, the HDT of N6 nanocomposites is due to the
presence of strong hydrogen bonds between the
matrix si silicate surface (see Fig. 80). Although
N6 in nanocomposites results in a different crystal
phase (g-phase) than that found in pure N6, this
different crystal phase is not responsible pentru the higher
mechanical properties of N6 nanocomposites because
the g-phase is a very soft crystal phase. The increased
mechanical properties of N6 nanocomposites with
increasing clay content is due to the mechanical
reinforcement effect.
The nano-dispersion of MMT in the PP matrix
also promotes a higher HDT . The HDT of PP
and its nanocomposites based on f-MMT and
Tabelul 16
Tensile properties of APES/Closite 30B nanocomposites
Closite 30B content
(wt%)
Modulus
(kgf/cm2)
Strength
(kgf/cm2)
Elongation at break
Reproduced from Lee, Park, Lim, Kang, Li, Cho si Ha by
permission of Elsevier Science Ltd,
Fig. 89. A comparison of (A) the tensile strengths si (B) tensile moduli pentru epoxy nanocomposites prepared from C18A-MMT, C18Amagnitide,
and C18A1M-magnitide. The silicate loading was determined by calcining the composites in air at 650 8C pentru 4 h at a heating rate of
2 8C/min . Reproduced from Wang, Lan si Pinnavaia by permission
of American Chemical Society,
Tabelul 17
Comparison of materials properties between neat PLA si various
PLACNs prepared with octadecyltrimethylammonium modified
MMT
Materials properties PLA PLACN4 PLACN5 PLACN7
Bending modulus (GPa) 4.8 5.5 5.6 5.8
Bending strength (MPa) 86 134 122 105
Distortion at break (%) 1.9 3.1 2.6 2
Reproduced from Sinha Ray, Yamada, Okamoto si Ueda by
permission of Elsevier Science Ltd,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1603
alkylammonium modified MMT are summarized in
Tabelul 18. Like previous systems, there is a
significant increase in HDT, from 109 8C pentru the
neat PP to 152 8C pentru a 6 wt% of clay, after which
the HDT of the nanocomposites levels off. This
improvement in HDT pentru neat PP after nanocomposite
preparation originates from the greater mechanical
stability of the nanocomposite as compared to
neat PP, since there is no increase in melting point
of neat PP after nanocomposite preparation.
The nanodispersion of octdecyltrimethylammonium
modified MMT (qC18-MMT) in neat PLA
also promotes a higher HDT. Sinha Ray et al.
examined the HDT of neat PLA si various PLACNs
with different load conditions. As seen in Fig. 90a
there is a marked increase of HDT with an
intermediate load of 0.98 MPa, from 76 8C pentru the
neat PLA to 93 8C pentru PLACN4. This value gradually
increases with increasing clay content, si in the
case of PLACN7 with 7 wt% of OMLS, the value
increases to 111 8C.
On the other hand, an imposed load dependence on
HDT is clearly observed in the case of PLACNs.
Fig. 90b shows the typical load dependence in
PLACN7. In the case of high load (1.81 MPa), it is
very difficult to achieve high HDT enhancement
without a strong interaction between the polymer
matrix si OMLS, as observed N6 based nanocomposites.
For PLACNs, the values of Tm do not change
significantly as compared to that of neat PLA.
Furthermore, in WAXD analyses up to 2 u¼ 708; no
large shifting or pentru mation of new peaks in the
crystallized PLACNs was observed. This suggests
that the improvement of HDT with intermediate load
originates from the better mechanical stability,
reinforcement by the dispersed clay particles, and
higher degree of crystallinity si intercalation.
The increase of HDT due to clay dispersion is a
very important property improvement pentru any polymeric
material, not only from application or industrial
point of view, but also because it is very difficult to
achieve similar HDT enhancements by chemical
modification or reinforcement by conventional filler.
4.3. Thermal stability
The thermal stability of polymeric materials is
usually studied by thermogravimetric analysis (TGA).
The weight loss due to the pentru mation of volatile
products after degradation at high temperature is
monitored as a function of temperature. When the
heating occurs under an inert gas flow, a non-oxidative
degradation occurs, while the use of air or oxygen
allows oxidative degradation of the samples. Generally,
the incorporation of clay into the polymer matrix
was found to enhance thermal stability by acting as a
Tabelul 18
HDT of PP/MMT nanocomposites si the respective unfilled PP
Organically modified mmt (wt%) HDT (8C)
PP/f-MMT PP/alkyl-MMT
3 144 ^ 5 130 ^ 7a
6 152 ^ 5 141 ^ 7b
Reproduced from Manias, Touny, Strawhecker, Lu si Chang
by permission of American Chemical
a C18-mmt filler, extruder processed.
b 2C18-MMT filler, twin-head mixer.
Fig. 90. (a) Organoclay (wt%) dependence of HDT of neat PLA si various PLACNs. (b) Load dependence of HDT of neat PLA si PLACN7
. Reproduced from Sinha Ray, Yamada, Okamoto si Ueda
by permission of Elsevier Science Ltd,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1604
superior insulator si mass transport barrier to the
volatile products generated during decomposition.
Blumstein first reported the improved thermal
stability of a PLS nanocomposite that combined
PMMA si MMT clay. These PMMA nanocomposites
were prepared by free radical polymerization of
MMA intercalated in the clay. He showed that the
PMMA that was intercalated (d spacing increase of
0.76 nm) between the galleries of MMT clay resisted
the thermal degradation under conditions that would
otherwise completely degrade pure PMMA. TGA data
revealed that both linear si cross-linked PMMA
intercalated into MMT layers have a 40-50 8C higher
decomposition temperature. Blumstein argues that the
stability of the PMMA nanocomposite is due not only
to its different structure, but also to the restricted
thermal motion of the PMMA in the gallery.
Recently, there have been many reports concerned
with the improved thermal stability of nanocomposites
prepared with various types of OMLS and
polymer matrices . Very recently, Zanetti
et al. conducted detailed TG analyses of
nanocomposites based on EVA. The inorganic phase
was fluorohectorite (FH) or MMT, both exchanged
with octadecylammonium cation. They found that the
deacylation of EVA in nanocomposites is accelerated,
and may occur at temperatures lower than those pentru
the pure polymer or corresponding microcomposite
due to catalysis by the strongly acidic sites created by
thermal decomposition of the silicate modifier. These
sites are active when there is intimate contact between
the polymer si the silicate. Slowing down the
volatilization of the deacylated polymer in nitrogen
may occur because of the labyrinth effect of the
silicate layers in the polymer matrix
In air, the nanocomposite exhibits a significant
delay in weight loss that may derive from the barrier
effect caused by diffusion of both the volatile thermooxidation
products to the gas si oxygen from the gas
phase to the polymer. According to Gilman et al.
this barrier effect increases during volatilization
owing to the reassembly of the reticular of the silicate
on the surface.
Fig. 91 represents the TGA analysis of a phosphonium-
PS nanocomposite compared with virgin
PS. It shows that the thermal stability of the
nanocomposite is enhanced relative to that of virgin
PS , si the typical the onset temperature of
the degradation is about 50 8C higher pentru the
nanocomposites. From Fig. 91 it is clearly observed
that the degradation mechanism of phosphonium
nanocomposites is somehow different from the others;
there is a second step in the degradation. This second
step accounts pentru about 30% of the degradation of the
phosphonium-PS nanocomposite, si must be attributed
to some interaction between the clay si the PS
that serves to stabilize the nanocomposite. The most
likely explanation is that the higher decomposition
temperature of the phosphonium clay provides pentru the
formation of char at a more opportune time to retain
the PS. In the case of ammonium clays, char
formation occurs earlier si can be broken up by
the time the polymer degrades.
The variation of the temperature at which 10%
degradation occurs pentru all three nanocomposites is
shown as a function of the amount of clay in Fig. 92
. Even with as little as 0.1 wt% of clay present in
the nanocomposite, the onset temperature was significantly
increased.
Fig. 93 [275] shows the TGA results pentru pure PSF
and pentru nanocomposites containing 1 si 5 wt% of the
OMLS. The approximate decomposition temperatures
of these three materials were 494, 498 si 513 8C,
respectively. There were significant increases in
thermal stability resulting from the exfoliated clay
platelets, which may be due to kinetic effects, with the
platelets retarding diffusion of oxygen into the
polymer matrix.
The thermal stability of the PCL-based composites
has also been studied by TGA. Generally, the
degradation of PCL fits a two-step mechanism
; first random chain scission through
Fig. 91. TGA curves pentru polystyrene, PS si the nanocomposites
. Reproduced from Zhu, Morgan, Lamelas si Wilkei by
permission of American Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1605
pyrolysis of the ester groups, with the release of CO2,
H2O si hexanoic acid, then in the second step, 1 -
caprolactone (cyclic monomer) is pentru med as a result of
an unzipping depolymerization process. The thermograms
of nanocomposites prepared with Mont-Alk and
pure PCL recovered after clay extraction are presented
in Fig. 94 [171]. Both intercalated si exfoliated
nanocomposites show higher thermal stability when
compared to the pure PCL or to the corresponding
microcomposites. The nanocomposites exhibit a 25 8C
high in decomposition temperature at 50% weight loss.
The shift of the degradation temperature may be
ascribed to a decrease in oxygen si volatile
degradation products permeability/diffusivity due to
the homogeneous incorporation of clay sheets, to a
barrier of these high-aspect ratio fillers, si char
formation. The thermal stability of the nanocomposites
systematically increases with increasing clay, up to a
loading of 5 wt%.
Different behavior is observed in synthetic biodegradable
aliphatic polyester BAP/OMLS nanocomposite
systems, in which the thermal degradation
temperature si thermal degradation rate systematically
increases with increasing amounts of OMLS
up to 15 wt% . The TGA results pentru neat BAP
and various nanocomposites are presented in Fig. 95
Fig. 94. Temperature dependence of the weight loss under an air
flow pentru neat PCL si PCL nanocomposites containing 1, 3, 5,
and 10 wt% (relative to inorganics) of MMT-Alk
Reproduced from Lepoittevin, Devalckenaere, Pantoustier, Alexandre,
Kubies, Calberg, Jerome si Dubois by permission of
Elsevier Science Ltd,
Fig. 93. TGA curves (relative weight loss as a function of
temperature) pentru (a) pure polysulfone, (b) nanocomposite with
1 wt% clay, si (c) nanocomposite with 5 wt% clay
Reproduced from Sur, Sun, Lyu si Mark by permission of
Elsevier Science Ltd,
Fig. 92. Temperature of 10% mass loss pentru nanocomposites as a
function of the fraction of clay . Reproduced from Zhu, Morgan,
Lamelas si Wilkei by permission of American Chemical Society,
Fig. 95. TGA of BAP/organically modified MMT with different
organoclay . Reproduced from Lim, Hyun, Choi si Jhon by
permission of American Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1606
Like PS-based nanocomposites, a small amount of
clay also increased the residual weight of BAP/
OMMT because of the restricted thermal motion of
the polymer in the silicate layers. The residual weight
of various materials at 450 8C increased in the order
BAP , BAP03 , BAP06 , BAP09 , BAP15 (here
number indicates wt% of clay). These improved
thermal properties are also observed in other systems
like SAN , the intercalated nanocomposite
prepared by emulsion polymerization.
The role of clay in the nanocomposite structure may
be the main reason pentru the difference in TGA results of
these systems compared to the previously reported
systems. The clay acts as a heat barrier, which
enhances the overall thermal stability of the system,
as well as assist in the pentru mation of char after thermal
decomposition. In the early stages of thermal
decomposition, the clay would shift the decomposition
to higher temperature. After that, this heat barrier effect
would result in a reverse thermal stability. In other
words, the stacked silicate layers could hold accumulated
heat that could be used as a heat source to
accelerate the decomposition process, in conjunction
with the heat flow supplied by the outside heat source.
4.4. Fire retardant properties
The Cone calorimeter is one of the most effective
bench-scale methods pentru studying the fire retardant
properties of polymeric materials. Fire-relevant properties
such as the heat release rate (HRR), heat peak
HRR, smoke production, si CO2 yield, are vital to
the evaluation of the fire safety of materials.
In 1976 Unitika Ltd,
potential flame retardant properties of N6/layered
silicate nanocomposites . Then in 1997 Gilman
et al. reported detailed investigations on flame
retardant properties of N6/layered silicate nanocomposite
. Subsequently, they chose various types of
nanocomposite materials si found similar reductions
in flammability . Recently, Gilman reviewed
the flame retardant properties of nanocomposites in
detail . Since the decreased flammability of
nanocomposites is one of the most important properties,
the results of some of the most recent studies on
flame retardant properties of nanocomposites are
reported in the following.
Tabelul 19 represents the cone calorimeter data of
three different kinds of polymer si their nanocomposites
with MMT. As shown in Tabelul 19, all of the
MMT-based nanocomposites reported here exhibit
reduced flammability. The peak HRR is reduced by
50-75% pentru N6, PS, si PP-g-MA nanocomposites
. According to the authors, the MMT must be
nanodispersed pentru it to affect the flammability
of the nanocomposites. However, the clay need
not be completely delaminated. In general, the
nanocomposites' flame retardant mechanism involves
Tabelul 19
Cone calorimeter data of various polymers si their nanocomposites with OMLS
Sample
(structure)
% residue yield
Peak HRR
(kW/m2)
(D%)
Mean HRR
(kW/m2)
(D%)
Mean Hc
(MJ/kg)
(m2/kg)
Mean CO yield
(kg/kg)
N6 1 1010 603 27 197 0.01
N6 nanocomposite 2% (delaminated) 3 686 (32) 390 (35) 27 271 0.01
N6 nanocomposite 5% (delaminated) 6 378 (63) 304 (50) 27 296 0.02
PS 0 1120 703 29 1460 0.09
PS-silicate mix 3% (immiscible) 3 1080 715 29 1840 0.09
PS-nanocomposite 3% (intercalated/delaminated) 4 567 (48) 444 (38) 27 1730 0.08
PSw/DBDPO/Sb2O3) 30% 3 491 (56) 318 (54) 11 2580 0.14
PpgMA 5 1525 536 39 704 0.02
PpgMA-nanocomposite 2% (intercalated/delaminated) 6 450 (70) 322 (40) 44 1028 0.02
PpgMA-nanocomposite 4% (intercalated/delaminated) 12 381 (75) 275 (49) 44 968 0.02
Heat flux, 35 kW/m2. Hc; specific heat of combustion; SEA, specific extinction area. Peak HRR, mass loss rate, si SEA data,
measured at 35 kW/m2, are reproducible to within ^10%. The carbon monoxide si heat of combustion data are reproducible to within
^15%. Reproduced from Gilman,
American Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1607
a high-performance carbonaceous-silicate char, which
builds up on the surface during burning. This insulates
the underlying material si slows the mass loss rate of
decomposition products.
In a recent study, Zhu et al. reported the fire
retardant properties of PS/MMT nanocomposites,
which were prepared using three different types of
new organically modified MMT (see Fig. 25). They
initially used phosphonium salt pentru the modification of
clay, si then examined the differences between
organo ammonium si phosphonium salt treatments
of clay fillers in nanocomposites towards thermal
stability. The peak HRR pentru PS si the three
nanocomposites is also shown graphically in Fig. 96
As mentioned above, the suggested mechanism by
which clay nanocomposites function involves the
formation of a char that serves as a barrier to both
mass si energy transport . As the fraction of clay
increases, the amount of char that can be pentru med
increases, si the rate at which heat is released
decreases. One of these nanocomposites, OH-16, is
mostly intercalated. This yields a slight reduction in
the rate of heat release compared with the other two
systems, which contain a significant exfoliated
fraction. This observation again supports the suggestion
that an intercalated material is more effective than
an exfoliated material in fire retardance
In contrast, the decrease in the rate of heat release
corresponds to (1) a decrease in mass loss rate si the
amount of energy released by the time PS has ceased
burning, si (2) a modest increase in the time at
which the peak heat release is reached. The production
of a char barrier must serve to retain some of
the PS, si thus both the energy released si the mass
loss rate decrease. The amount of smoke evolved and
specific extinction area also decrease with the
formation of the nanocomposites. There is some
variability in the smoke production. Although it is
observed that the pentru mation of the nanocomposites
reduces smoke production, the presence of additional
clay does not continue this smoke reduction.
4.5. Gas barrier properties
Clays are believed to increase the barrier properties
by creating a maze or 'tortuous path' (see Fig. 97) that
retards the progress of the gas molecules through the
matrix resin. The direct benefit of the pentru mation of
such a path is clearly observed in polyimide/clay
nanocomposites by dramatically improved barrier
properties, with a simultaneous decrease in the
thermal expansion coefficient . The
polyimide/layered silicate nanocomposites with a
small fraction of OMLS exhibited reduction in the
permeability of small gases, e.g. O2, H2O, He, CO2,
and ethylacetate vapors . pentru example, at 2 wt%
clay loading, the permeability coefficient of water
vapor was decreased ten-fold with synthetic mica
relative to pristine polyimide. By comparing nanocomposites
made with layered silicates of various
aspect ratios, the permeability was seen to decrease
with increasing aspect ratio.
Oxygen gas permeability has been measured pentru
near to exfoliated PLA/synthetic mica nanocomposites
prepared by Sinha Ray et al. . The relative
permeability coefficient value, i.e. PPLACN=PPLA
where PPLACN si PPLA are the nanocomposite and
pure PLA permeability coefficient, respectively, is
plotted as a function of the wt% of OMLS in
Fig. 98. The data are analyzed with the Nielsen
theoretical expression , allowing prediction of
Fig. 96. Peak HRRs pentru PS si the three nanocomposites
Reproduced from Zhu, Morgan, Lamelas si Wilkei by permission
of American Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1608
gas permeability as a function of the length si width
of the filler particles, as well as their volume fraction
in the PLA-matrix.
The H2O-vapor permeability pentru the PUU/OMLS
nanocomposites is presented in Fig. 99 in terms of
Pc=
nanocomposite ðPcÞ to that of the neat PUU ðPoÞ
The nanocomposite pentru mation results in a dramatic
decrease in H2O-vapor transmission through the PUU
sheet. The solid lines in Fig. 99 are based on
the tortuosity model pentru aspect ratios of 300 and
1000. The comparison between the experimental
values si the theoretical model suggests a gradual
change in the effective aspect ratio of the filler.
Although the enhancement in barrier properties in
nanocomposites is well known, the dependence on
factors such as the relative orientation si dispersion
(intercalated, exfoliated or some intermediate) is not
well understood. Very recently, Bharadwaj
addressed the modeling of barrier properties in
PLS nanocomposites based completely upon
the tortuosity arguments described by Nielsen
The correlation between the sheet length, concentration,
relative orientation, si state of aggregation
is expected to provide guidance in the design of
better barrier materials using the nanocomposite
approach.
The presence of filler, spherical, plate, cylindrical,
etc. introduces a tortuous path pentru a diffusing
penetrant. The reduction of permeability arises from
the longer diffusive path that the penetrants must
travel in the presence of filler, as shown in the inset in
Fig. 100. A sheet-like morphology is particularly
efficient at maximizing the path length due to the large
length-to-width ratio, when compared to other filler
shapes such as spheres or cubes. The tortuosity factor
tis defined as the ratio of the actual distance d0 that a
penetrant must travel to the shortest distance d that it
would travel in the absence of barriers. It is expressed
in terms of the length L; width W; si volume fraction
Fig. 98. Oxygen gas permeability of neat PLA si various PLACNs
as a function of OMLS content measured at 20 8C si 90% relative
humidity. The filled circles represent the experimental data.
Theoretical fits based on Nelson tortuousity model
Reproduced from Sinha Ray, Yamada, Okamoto, Ogami and
Ueda by permission of American
Chemical
Fig. 99. Relative H2O vapor permeability pentru the PUU nanocomposites. The nanocomposite pentru mation results in a dramatic decrease in H2O
vapor transmission through the PUU membrane. The solid lines represent the theoretical value pentru aspect ratios ¼ 300 si 1000 . Reproduced
from Xu, Manias, Snyder, Runt by
permission of American Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1609
of the sheets fs as
t¼
d0
d ¼ 1 þ
L
2W
fs ð5Þ
The effect of tortuosity on the permeability is
expressed as
Ps
Pp ¼
1 2 fs
t ð6Þ
where Ps si Pp represent the permeabilities of the
nanocomposite si pure polymer, respectively. In the
present case, the polygonal aluminosilicate sheets are
approximated as disks with a mean diameter L ranging
from 30 to 200 nm si a width of 1 nm.
Although the above equations were developed to
model the diffusion of small molecules in conventional
composites, they do extremely well in reproducing
experimental results pentru the relative permeability in
PLS nanocomposites. The key assumption of this
model is that the sheets are placed in an arrangement
such that the direction of diffusion is normal to the
direction of the sheets. Clearly, this arrangement
results in the highest tortuosity, si any deviation
from the arrangement where the sheet normal lies
perpendicular to the film plane would in fact lead to
deterioration of the barrier properties. A range of
relative sheet orientations with respect to each other
and to the plane of the film is shown in Fig. 101
4.6. Ionic conductivity
Solvent-free electrolytes are of much interest
because of their charge-transport mechanism and
their possible applications in electrochemical devices.
Fig. 100. Effect of sheet orientation on the relative permeability in exfoliated PLS nanocomposites at fs ¼ 0:05 si W ¼ 1 nm. The
illustrations show the definition of the direction of preferred orientation ðnÞ of the silicate sheet normals ð pÞ with respect to the film plane.
Illustration pentru three values of the order parameter ðSÞ-1/2, 0, si 1 are also shown . Reproduced from Bharadwaj by permission of
American Chemical
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1610
Vaia et al. studied the preparation of PEO/
layered silicate nanocomposites to fine tune the ionic
conductivity of PEO. An intercalated nanocomposite
prepared by melt intercalation of PEO (40 wt%) into
Liþ-MMT (60 wt%) was shown to enhance the
stability of the ionic conductance at lower temperatures
when compared to a more conventional PEO/
LiBF4 mixture. This improvement in conductivity is
explained by the fact that PEO is not able to
crystallize when intercalated, eliminating the presence
of crystallites, which are non-conductive. The
higher conductivity at room temperature, compared to
conventional PEO/LiBF4 electrolytes with a single
ionic conductor, makes these nanocomposites promising
new electrolyte materials. The same type ionic
conductivity behavior was observed in a poly[bis
(methoxy-ethoxy) ethoxy phosphazene/Naþ-MMT
nanocomposites prepared by Hutchison et al.
4.7. Optical transparency
Although layered silicates are microns in lateral
size, they are just 1 nm thick. Thus, when single layers
are dispersed in a polymer matrix, the resulting
nanocomposite is optically clear in visible light.
Fig. 102 presents the UV/visible transmission spectra
of pure PVA si PVA/Naþ-MMT nanocomposites
with 4 si 10 wt% MMT. The spectra show that the
visible region is not affected by the presence of the
silicate layers, si retains the high transparency of
PVA. pentru UV wavelengths, there is strong scattering
and/or absorption, resulting in very low transmission
of UV light. This behavior is not surprising, as the
typical MMT lateral sizes are 50-1000 nm.
Like PVA, various other polymers also show
optical transparency after nanocomposite preparation
with OMLS
4.8. Biodegradability of biodegradable
polymers-based nanocomposites
Another interesting si exciting aspect of nanocomposite
technology is the significant improvement
of biodegradability after nanocomposite preparation
with OMLS. Tetto et al. first reported results
on the biodegradability of nanocomposites based on
Fig. 101. Effect of incomplete exfoliation on the relative permeability. The illustrations show the effect of having one, two, si four sheet
aggregates dispersed throughout the matrix. The plot shows the relative permeability as a function of the aggregate width at several different
lengths of the sheets at fs
¼ 0:05 . Reproduced from Bharadwaj by
permission of American Chemical Society,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1611
PCL, reporting that the PCL/OMLS nanocomposites
showed improved biodegradability compared to pure
PCL. The improved biodegradability of PCL after
nanocomposites pentru mation may be due to a catalytic
role of the OMLS in the biodegradation mechanism,
but it this is not clear.
Recently, Lee et al. reported the biodegradation
of aliphatic polyester-based nanocomposites
under compost. Parts a si b of Fig. 103 show the
clay content dependence of the biodegradation of
APES-based nanocomposites prepared with two
different types of clays. They assumed that the
retardation of biodegradation was due to the improvement
of the barrier properties of the aliphatic APSE
after nanocomposite preparation with clay. However,
they do not provide permeability data.
Very recently, Sinha Ray et al. reported
the biodegradability of neat PLA si the corresponding
nanocomposites prepared with octadecyltrimethylammonium-
modified MMT (C18C3-MMT), along
with a detailed mechanism of the degradation. The
compost used was prepared from food waste, si tests
were carried out at a temperature of (58 ^ 2)8C.
Fig. 104a shows the recovered samples of neat PLA
and PLACN4 (C18C3-MMT ¼ 4 wt%) from compost.
The decrease in Mw si residual weight
percentage of the initial test samples are also reported
in Fig. 104b. Obviously, the biodegradability of neat
PLA is significantly enhanced after nanocomposite
preparation with C18C3-MMT. Within one month,
both the extent of Mw si the extent of weight loss are
at the same level pentru both neat PLA si PLACN4.
However, after one month, a sharp change occurs in
the weight loss of PLACN4, si within 2 months it is
completely degraded by compost.
The presence of terminal hydroxylated edge
groups in the silicate layers may be one of the factors
responsible pentru this behavior. In the case of PLACN4,
the stacked (,4 layers) si intercalated silicate layers
are homogeneously dispersed in the PLA matrix (from
TEM image), si these hydroxy groups start heterogeneous
hydrolysis of the PLA matrix after absorbing
water from the compost. This process takes some time
to start. pentru this reason, the weight loss si degree of
hydrolysis pentru PLA si PLACN4 are almost the same
up to 1 month (see Fig. 104b). However, after 1 month
there is a sharp weight loss in the case of PLACN4
compared to that of PLA. That means that 1 month is
the critical timescale to start heterogeneous hydrolysis,
and due to this type of hydrolysis, the matrix
decomposes into very small fragments si eventually
disappears with the compost. This assumption was
confirmed by conducting the same experimental
procedure with PLACN prepared with dimethyldioctdecylammonium
salt modified synthetic mica, which
Fig. 102. UV-vis transmittance spectra of PVA si PVA/MMT
nanocomposites containing 4 si 10 wt% MMT . Reproduced
from Strawhecker si Manias by permission of American Chemical
Fig. 103. Biodegradability of APES nanocomposites with: (a) Closite 30B si (b) Closite 10A . Reproduced from Lee, Park, Lim, Kang,
Li, Cho si Ha by permission of
Elsevier Science Ltd,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1612
has no terminal hydroxylated edge group. The same
degradation tendency was found with PLA
A respirometric test has also been used to study the
degradation of the PLA matrix in a compost environment
at (58 ^ 2)8C . pentru this test compost
was made from bean-curd refuse, food waste, and
cattle feces. Unlike the weight loss, which reflects the
structural changes in the test sample, CO2 evolution
provides an indicator of the ultimate biodegradability
of PLA in PLACN4 (prepared with (N(coco alkyl)N,N-
[bis(2-hydroxyethyl)]-N-methylammonium modified
synthetic mica), via mineralization, of the samples.
Fig. 105 shows the time dependence of the degree of
biodegradation of neat PLA si PLACN4, indicating
that the biodegradability of PLA in PLACN4 is
enhanced significantly. The presence of MEE clay
may cause a different mode of attack on the PLA
component due to the presence of hydroxy groups.
Details regarding the mechanism of biodegradability
can be found in relevant literature
4.9. Other properties
PLS nanocomposites also show improvement in
most general polymeric properties. pentru example, in
addition to the decreased permeability of liquids and
gases, nanocomposites also show significant improvement
in solvent uptake. Scratch resistance is another
property that is strongly enhanced by the incorporation
of layered silicates
The potential to use PANI-based nanocomposites
as electrorheologically sensitive fluids , or to use
Fig. 104. (a) Real picture of biodegradability of neat PLA si PLACN4 recovered from compost with time. Initial shape of the crystallized
samples was 3 £ 10 £ 0.1 cm3. (b) Time dependence of residual weight, Rw si of matrix, Mw of PLA si PLACN4 under compost at
(58 ^ 2) 8C . Reproduced from Sinha Ray, Yamada, Okamoto si Ueda
by permission of Elsevier Science Ltd,
S. Sinha Ray, M. Okamoto / Prog. Polym. Sci. 28 (2003) 1539-1641 1613
the combination of dispersed layered silicates in a
liquid crystal medium is also an attractive application.
This could result in a stabelul electro-optical device that
is capable of exhibiting a bistabelul si reversible
electro-optical effect between an opaque si transparent
state
Finally, nanocomposites have been used in highly
technical areas such as in the improvement of ablative
properties in aeronautics
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