The synthesis and characterization of films and nanofibers from novel poly(N,N'-dimethyl acrylamide)-graft-poly(dimethyl siloxane) amphiphilic hydrogels

Wagenaar, Stefan (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: A series of graft copolymers comprised of poly(N,N’-dimethyl acrylamide) (PDMAA) and poly(dimethyl siloxane) (PDMS) were synthesized for the first time by way of free radical polymerization. The copolymer compositions were found to closely match the feed ratios of each reaction. HPLC separation showed that the samples were composed of a hompolymer and copolymer fraction, with the homopolymer fraction rapidly diminishing as the amount of PDMS monomer in the feed was increased. Nanofibres were successfully produced from the material by single needle electrospinning to produce fibres with an average diameter of around 700 nm. Films and nanofibres of the material were then analysed by way of DSC, FTIR, HPLC, SEM, Solid state NMR, TGA and WAXD, as well as conventional swelling studies. The presence of the hydrophobic PDMS grafts was effective in producing a physically crosslinked hydrogel network, as the materials were able to absorb up to 1.6 times their own weight in water and were held together by the hydrophobic clusters. The nanofibres were able to absorb as much as 10 times their weight in water, with water retention undergoing a sharp reduction with increasing PDMS content. Swelling occurred rapidly, with the films reaching around 90% of their equilibrium swelling in less than 5 minutes when immersed in water. SEM images of the nanofibres before and after water exposure revealed that they lacked dimensional stability and lose their fibrous structure when wet. The stability improved with increased PDMS content. The effective crosslinking density of the samples was calculated in order to investigate the effect of the PDMS content on crosslinking density, and it was shown that a non-linear relationship existed between the PDMS content and the resultant swelling of the material. DSC analysis was used to elucidate the water structure inside of the films and fibres, and the relative fractions of freezing and non-freezing water was determined. DSC analysis of dry films showed that the material’s glass transition occurred around 90°C, which is near the expected value for poly(N,N’-dimethyl acrylamide). However, the physical properties more closely resembled that of a rubber-like compound due to the large weight percentage of included PDMS. Determination of the material’s morphology showed that it underwent a morphological transition above a threshold PDMS content, changing from a highly phase segregated system to one with a higher degree of homogeneity. T1ρ relaxation data from solid state NMR showed that the respective values for the PDMAA and PDMS segments differed significantly in the low PDMS content samples, but converged with increasing PDMS content. Additionally, in the nanofibre state it was also found that the samples showed a lower degree of phase separation and that the PDMS grafts were more isolated, which would result in a lower effective crosslinking density. This correlated well with other observed data. SEM images of fracture surfaces from swollen samples revealed island-like structures which are believed to be primarily associated with the PDMS domains. This correlated well with other evidence from solid state NMR and swelling studies. Finally, tensile tests illustrated that the new material had improved physical properties over the homopolymer material.

AFRIKAANSE OPSOMMING: ‘n Reeks van entkopolimere, poli(N,N’-dimetiel akrilamied)-ent-poli(dimetiel siloksaan), is vir die eerste keer gesintetiseer deur middel van vrye radikaal polimersasie. HPLK skeiding het aangedui dat die monsters homo- en kopolimeer fraksies bevat, en dat die fraksie homopolimeer vining afneem met toenames in in PDMS inhoud. Nano-vesels is gevorm met electrospinning en vesels met ‘n gemiddelde deursnit van 700 nm is verkry. Films en vesels van die materiaal is geanaliseer met DSK, FTIR, HPLK, SEM, vaste fase KMR, TGA en X-straal analise, sowel as water absorbsie studies. Dit is bevind dat die teenwoordigheid van die hidrofobiese poli(dimetiel siloksaan) (PDMS) doeltreffend was in die skepping van ‘n hidrogel, aangesien die materiaal tot 1.6 keer sy eie gewig in water kon absorbeer, as gevolg van die samespanning van die hidrofobiese PDMS. Die nano-vesels kon tot 10 keer hul gewig in water absorbeer, maar dit was hoogs afhanklik van die PDMS inhoud. Die films het baie vinning water geabsorbeer en omtrent 90% versadiging bereik na slegs 5 minute. Die vesels was egter onstabiel in water en het geneig om hul strukteer te verloor na ‘n kort tydperk. Hierdie stabiliteit het verbeter met verhogings in die PDMS inhoud. Die kruis bindings digtheid is uitgewerk om die effek van die PDMS inhoud daarop te bepaal, and dit is bevind dat daar ‘n nie-lineêre verhouding was tussen die twee veranderlikes bestaan as gevolg van die teenwoordigheid van komplekse morfologiese rangskikkings. DSK analise is gebruik om die water struktuur met betrekking tot die vriesbare en onvriesbare water fraksies te bepaal. DSK analise van droëe monsters het aangedui dat die polimeer se glas oorgangs temperatuur om en by 90°C was, naby aan die verwagte temperatuur vir poli(N,N’-dimetiel akrilamied). Die fisiese eienskappe was egter meer kenmerkend van ‘n rubberagtige materiaal as gevolg van groot gewig persentasie PDMS. ‘n Ondersoek om die materiaal se morfologie te bepaal het aangedui dat dit ‘n morfologiese oorgang ondergaan bo ‘n kritieke PDMS inhoud, en dat dit dan verander vanaf ‘n hoogs geskeide stelsel na ‘n meer homogene een. T1ρ data vanaf vaste fase KMR het verskil in die monsters met ‘n lae PDMS inhoud vir die PDMAA en PDMS gedeeltes, maar was eenders in die monsters met n hoër hoeveelheid PDMS. In die nano-vesels was die skeiding tussen die gedeeltes amper onopmerkbaar aangesien die PDMS gedeeltes hoogs geisoleerd was, wat sou veroorsaak dat die vesels ‘n lae kruis bindings digtheid sou hê. Mikro beelde vanaf electron mikroskopie op die oppervlaktes van monsters wat volledig geswel is en toe oopgekraak is onder lae temperatuur het ‘n rangskikking van “eiland” strukture aangetoon wat waarskynlik afkomstig is vanaf PDMS reike areas. Die bevindings het sterk ooreengstem met alle vorige gevolgtrekkings.Laastens, meganiese toetse het aangetoon dat die nuwe materiaal se fisiese eienskappe ‘n verbeter het teenoor die homopolimeer.

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