Impact of molecular structure on water vapour sorption properties in nanostructured polymeric films

Cloete, Valeska (2011-12)

Thesis (PhD)--Stellenbosch University, 2011.

Thesis

ENGLISH ABSTRACT: In this study, the use of surfactants, plate-like clays, organophilic molecules and side-chain crystallinity was investigated for their impact on the transport mechanisms of water vapour through polymer films. A model polymer latex, poly(styrene-co-butyl acrylate), was prepared using miniemulsion polymerization. Three different types of surfactants, sodium dodecyl benzene sulfonate (SDBS, an anionic surfactant), octyl phenol ethoxylate (OPE, a nonionic surfactant) and dodecyl ammonium- 3-butenoate (DA3B, a reactive surfactant) were used to stabilize the latex. Films were prepared from the resultant latices and their water vapour sorption behaviour determined across a water vapour partial pressure range of 0.1 to 0.9. Sigmoidal kinetic behaviour was seen for all three films, with the DA3B stabilized film exhibiting high diffusion coefficients compared to films stabilized with SDBS and OPE. The thermodynamic behaviour of the films differed and was dependent on the reactivity of the surfactant. SDBS and OPE stabilized films exhibited Flory- Huggins behaviour, while the DA3B stabilized film followed Henry’s Law. Despite significant differences in terms of these properties, the permeability coefficients were similar for the three films across the water vapour partial pressure range evaluated. The impact of sodium montmorillonite (Na-MMT) clay and an organophilic modifier, 2- acrylamido-2-methylpropanesulfonic acid (AMPS), on the water vapour sorption properties of poly(styrene-co-butyl acrylate) was evaluated. These polymer clay nanocomposites (PCNs) were synthesized using miniemulsion polymerization. The resultant latex films were characterized and used for water vapour sorption analyses. It was shown that complete exfoliation of the Na-MMT was necessary to minimize the equilibrium water vapour uptake. Even when Na-MMT was completely exfoliated, the amount of water vapour sorbed by the PCN was high and this was attributed to the hydrophilic nature of the clay. Using a least squares regression fit, good correlation was obtained between the experimental isotherms and the sorption behaviour predicted by the Dual Mode Sorption model which was originally developed for polymers in their glassy state. The impact of side chain crystallinity on the water vapour sorption properties of poly(methyl methacrylate-co-octadecyl acrylate) was evaluated. These random copolymers containing increasing amounts of octadecyl acrylate, and therefore increasing degrees of crystallinity, were synthesized using solvent polymerization. Although it could be expected that side chain crystallinity would be the main contributing factor resulting in a reduction in the diffusion coefficient, it was shown that the methyl group on the á- carbon of the vinyl group in the methacrylate reduced the diffusion to a greater extent through the increased stiffness of the polymer backbone. This was also reflected in poly(methyl methacrylate-co-octadecyl acrylate) having a greater activation energy for diffusion compared to polyoctadecyl acrylate.

AFRIKAANSE OPSOMMING: In hierdie studie is die gebruik van sepe, plaatagtige kleie, organofiliese molekules en sykettingkristalliniteit ondersoek ten opsigte van die impak op transportmeganismes van waterdamp deur polimeerfilms. ‘n Model polimeerlateks, polistireen-ko-butielakrilaat, is voorberei deur miniemulsiepolimerisasie. Drie verskillende tipes sepe, natriumdodekielbenseensulfonaat (NDBS, ‘n anioniese seep), oktielfenoletoksilaat (OFE, ‘n nie-ioniese seep) en dodekielammonium-3-butenoaat (DA3B, ‘n reaktiewe seep) is gebruik om die lateks te stabiliseer. Films is van die resultante lateks voorberei en hul waterdampsorpsie –eienskappe oor die parsiële waterdampdrukreeks van 0.1 tot 0.9 bepaal. Sigmodale kinetiese gedrag is vir al drie films waargeneem, met die DA3B gestabiliseerde film wat hoër diffusiekoëffisiënte toon in vergelyking met die films wat met NDBS en OFE gestabiliseer is. Die termodinamiese gedrag van die films het verskil en was afhanklik van die reaktiwiteit van die seep. NDBS en OFE gestabiliseerde films het Flory-Huggins gedrag getoon, terwyl die DA3B gestabiliseerde film Henry se Wet gevolg het. Ten spyte van die beduidende verskille ten opsigte van hierdie eienskappe was die permitiewe koëffisiënte soortgelyk vir die drie films regoor die parsiële waterdampdrukreeks wat vir die evaluasie gebruik is. Die impak van natriummontmorilloniet (Na-MMT) klei en ‘n organofiliese modifiseerder, 2- akrielamido-2-metielpropaansulfoonsuur (AMPS), op die waterdampsorpsie-eienskappe van polistireen-ko-butielakrilaat is geevalueer. Hierdie polimeer-klei-nanosaamgesteldemateriale (PKNe) is gesintetiseer deur van miniemulsiepolimerisasie gebruik te maak. Die resultante lateksfilms is gekarakteriseer en gebruik vir waterdampsorpsie analises. Daar is getoon dat algehele afskilfering van die Na-MMT nodig was om die ewewigswaterdampopname te minimaliseer. Selfs wanneer Na-MMT algeheel afgeskilfer was, was die hoeveelheid waterdamp gesorbeer deur die PKN hoog en kan dit toegeskryf word aan die hidrofiliese karakter van die klei. Deur ‘n kleinste-kwadrate-regressie passing te doen, is ‘n goeie korrelasie verkry tussen die eksperimentele isoterme en die sorpsie gedrag voorspel deur die Dubbelmodussorpsiemodel wat oorspronklik ontwikkel is vir polimere in hul glasagtige toestand. Die impak van sykettingkristalliniteit op die waterdampsorpsie-eienskappe van poli(metielmetakrilaat-ko-oktadekielakrilaat) is ondersoek. Hierdie ewekansige kopolimere wat toenemende hoeveelhede oktadekielakrilaat, en dus toenemende grade van kristalliniteit bevat, is gesintetiseer deur van oplossingspolimerisasie gebruik te maak. Alhoewel dit te wagte was dat sykettingkristalliniteit die hoofbydraende faktor is in die redusering van die diffusiekoeffisiente, is daar getoon dat die metielgroep aan die α-koolstof van die vinielgroep in die metakrilaat die diffusie tot 'n groter mate gereduseer het deur toenemende styfheid van die polimeerrugraat. Dit is ook gereflekteer deur poli(metielmetakrilaat-ko-oktadekielakrilaat) wat 'n groter aktiveringsenergie vir diffusie het in vergelyking met polioktadekielakrilaat.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/17890
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