Anomalous Swelling Behavior\nof Poly(<i>N</i>‑vinylimidazole)‑<i>l</i>‑Poly(tetrahydrofuran)\nAmphiphilic Conetwork in Water Studied by Solid-State NMR and Positron\nAnnihilation Lifetime Spectroscopy

Abstract

Poly­(<i>N</i>-vinylimidazole) homopolymer (PVIm)\nand\npoly­(<i>N</i>-vinylimidazole)-<i>l</i>-poly­(tetrahydrofuran)\n(PVIm-<i>l</i>-PTHF), a novel amphiphilic polymer conetwork\n(APCN), were synthesized to compare their solid state structure and\ninvestigate the swelling behavior of this unique conetwork in water.\nA short-range ordered structure stabilized by second-order interactions\nbetween the imidazole pendant groups was found in the PVIm homopolymer\nand in the PVIm phase of the dry conetwork as revealed by solid-state\nNMR ¹³C cross-polarization magic angle spinning (CP MAS)\nand two-dimensional ¹H–¹³C frequency-switched\nLee–Goldburg (FSLG) HETCOR measurements. With increasing swelling\nratio, structural and conformational changes were recognized in the\nhydrophilic PVIm phase of the APCN. In the kinetic swelling study,\nunexpectedly, four different swelling ranges were identified by gravimetric\nmeasurements, solid-state NMR methods, and positron annihilation lifetime\n(PAL) spectroscopy before the APCN reached its equilibrium swelling\nstate. In the first period, which takes place in several minutes,\nthe ordered structure disintegrates in the PVIm phase and the water\nuptake is relatively slow. This structural realignment is followed\nby the main course of water uptake governed by Fickian diffusion in\nthe second stage. Close to the equilibrium swelling ratio, the swelling\ncurve becomes nonmonotonic caused by a realignment of main chains\nof the hydrophilic phase in the third stage of swelling. Thus, by\nthe unique combination of conventional swelling kinetics and solid-state\nNMR as well as PAL spectroscopies for investigating the aqueous swelling\nof the PVIm-<i>l</i>-PTHF amphiphilic polymeric conetwork,\nit was revealed that unexpected noncontinuous swelling occurs which\nis due to structural changes of the PVIm component in the conetwork\nin the course of this process.