Molecular simulation study of the effect of cross-linker on the properties of poly(N-isopropyl acrylamide) hydrogel

Molecular simulation techniques have been utilised to investigate the effect of cross-linker type on the structural and dynamical properties of a temperature-sensitive hydrogel, poly (N-isopropyl acrylamide) (PNIPAM) across its lower critical solution temperature (LCST). PNIPAM exhibits an LCST at similar to 305 K, above which it collapses and below which it is swollen. Molecular dynamics simulations of PNIPAM hydrogel cross-linked with N,N'-methylene bisacrylamide (BIS) and ethylene glycol dimethacrylate (EGD) cross-linkers were carried out below, at and above its LCST (namely 300, 305 and 310 K, respectively). Structural analysis indicates that the cross-linkers did not affect the temperature of the onset of the LCST, but did affect the degree of swelling and pore size distribution, where the EGD-cross-linked hydrogel exhibited a greater degree of structural change than that of the BIS-cross-linked hydrogel. We believe that this could be attributed to the longer chain length and more flexible nature of the EGD cross-linker compared to the BIS cross-linkers.