Unprecedented scissor effect of macromolecular cross-linkers on the glass transition temperature of poly(N-vinylimidazole), crystallinity suppression of poly(tetrahydrofuran) and molecular mobility by solid state NMR in poly(N-vinylimidazole)-l-poly(tetrahydrofuran) conetworks

Abstract

Unexpected correlations have been found between structural parameters and glass transition temperatures (T_g) of poly(N-vinylimidazole) (PVIm) and crystallinity of poly(tetrahydrofuran) (PTHF) in a series of novel, unique PVIm-l-PTHF amphiphilic conetworks synthesized in broad composition ranges via free radical copolymerisation of VIm and semicrystalline, methacrylate-telechelic PTHF macromolecular cross-linkers with varying M_n from 2170 to 10 000 g mol⁻¹. Differential scanning calorimetry (DSC) investigations revealed microphase separation between the covalently bonded PVIm and PTHF components, that is two distinct T_gs corresponding to the respective polymers (PVIm and PTHF) were obtained in these optically clear, transparent materials. Complete microphase separation, i.e. absence of mixed phases, was also confirmed by solid state NMR measurements. The T_g of the PVIm phase significantly decreases with increasing PTHF content, and Fox–Flory type correlation was surprisingly found between the T_g of PVIm and its M_c (average molecular weight between cross-links). This striking finding indicates a unique, unpredicted scissor effect of the macromolecular PTHF cross-linker in these materials, i.e. with respect to glass transition, PVIm behaves as individual chains between cross-links. The molecular mobility in the PVIm chain segments obtained by solid state NMR investigations shows a similar trend as a function of M_c. In the DSC thermograms, the semicrystalline PTHF has a sharp endothermic melting peak (T_m) indicating partial crystallisation of this polymer. It was found that the T_m and the crystalline fraction (X_c) of the PTHF phase are suppressed by even a minimal content of PVIm phase in the conetworks. Even complete diminishing of X_c occurs in conetworks with lower than 40 wt% PTHF of the lowest M_n (2170 g mol⁻¹). Unexpectedly, T_m linearly decreases with M_c in conetworks with constant M_n of PTHF. These data indicate that the decrease of both T_m and X_c of PTHF is not only composition dependent, but the MW of the macromolecular PTHF cross-linker and the M_c of the PVIm component also have effects on these parameters. These results also indicate that chemical bonding of polymer chains in conetworks yields novel materials with unprecedented property variation. This provides unique opportunities for fine tuning of the investigated fundamental material properties, i.e. T_g, T_m and X_c, within certain ranges in the novel PVIm-l-PTHF amphiphilic conetworks by selecting the proper synthesis parameters, that is, composition and MW of the telechelic PTHF macromonomer cross-linker.