Poly(N-vinylimidazole)-l-poly(propylene glycol) amphiphilic conetworks and gels: molecularly forced blends of incompatible polymers with single glass transition temperatures of unusual dependence on the composition

Abstract

A series of macroscopically homogeneous poly(N-vinylimidazole)-l-poly(propylene glycol) (PVIm-l-PPG) (“l” stands for “linked by”) amphiphilic conetworks (APCNs) composed of otherwise incompatible polymers were successfully synthesized in a broad composition range (34–88 wt% PPG) by free radical copolymerization of hydrophilic N-vinylimidazole (VIm) and hydrophobic poly(propylene glycol) dimethacrylate (PPGDMA) macromolecular cross-linkers. Strikingly, while PVIm and PPGDMA homopolymers are immiscible and their blends have two distinct glass transition temperatures (T_g), the PVIm-l-PPG conetworks possess only one T_g indicating the absence of considerable phase separation in the conetworks, which was also confirmed by AFM measurements. This is in sharp contrast to the two T_gs of APCNs reported so far in the literature, on the one hand. On the other hand, the T_g values do not follow known correlations between T_g and composition, like the Fox equation or additive rule, widely applied for compatible polymers. These results indicate a strong interpolymer interaction on the molecular level between the PVIm and PPG chains in these new APCNs resulting in single T_g. Thermogravimetric analysis (TGA) shows that degradation of the conetworks occurs at high temperatures in two stages without sharp changes, but with a transition period in between. The DTG curves indicate that the components retain their chemical integrity to certain extent in these APCNs. The amphiphilic nature of the PVIm-l-PPG conetworks was confirmed by their composition dependent swelling in both polar (water, ethanol) and nonpolar (THF) solvents, that is in spite of the lack of phase separation, these new materials behave as either hydrogels or hydrophobic gels (organogels) depending on the swelling medium in a broad composition range.