The poly [(2-dimethylamino)ethyl methacrylate]-l-polyisobutylene (PDMAEMA-l-PIB) amphiphilic network (APN), a new class of cross-linked systems, was synthesized by the use of methacrylatetelechelic PIB obtained via quasiliving carbocationic polymerization. The swelling dynamics and the free volume changes of this APN was followed by simultaneous swelling and positron annihilation measurements. It was found that the lifetime of ortho positrons (o-Ps) increases with increasing swelling ratio (R), reaches a maximum at relatively low R, and decreases to a constant value at equilibrium swelling. These findings indicate the collapse of the hydrophobic PIB domains and expansion of the hydrophilic PDMAEMA phase in the network upon contacting with water. After reaching the maximum, the decrease of the lifetime parameter is caused by filling the free volume with water in the network. A striking observation was obtained for the o-Ps formation intensity as a function of time (or swelling ratio): the intensity rapidly decreases, and it reaches a minimum at very low R, at ∼10% of the equilibrium swelling ratio, and then increases to a constant value. These phenomena reveal important aspects of the structure of the free volume in the APN and provide fundamental information about the swelling dynamics. The minimum of the o-Ps intensity is reached at around 1:1 water/monomer units molar ratio in the network. Surprisingly, a similar phenomenon was observed when the monomer itself was mixed with water. Molecular modeling by ab initio calculations indicates that a 1:1 ringlike cluster may be formed between water and DMAEMA. On the basis of these results it is concluded that the surface of the free volume units in the APN become covered with water molecules quickly by interacting with the PDMAEMA chains at the beginning of swelling. This indicates that the free volume in the PDMAEMA-l-PIB APN is not composed of independent pores but of interconnected channels which allow rapid diffusion of water molecules to cover the surface of the free volume units. This fast process results in a quick surface structure reorganization, i.e., enrichment of the surface of the channels by PDMAEMA segments leading to the corresponding rapid decrease of o-Ps formation intensity, and to the simultaneous collapse of PIB domains yielding increase of free volume in the network and positron lifetime at the beginning of swelling. Results of this study also allow us to predict that the combined method presented here, i.e., simultaneous swelling kinetics and positron annihilation investigations, can be widely applicable for gaining new information on free volume structure, swelling dynamics, and interactions between material components and swelling agents for a large variety of networks,
Poly(N-vinylimidazole) homopolymer (PVIm) and poly(N-vinylimidazole)-l-poly(tetrahydrofuran) (PVIm-l-PTHF), a novel amphiphilic polymer conetwork (APCN), were synthesized to compare their solid state structure and investigate the swelling behavior of this unique conetwork in water. A short-range ordered structure stabilized by second-order interactions between the imidazole pendant groups was found in the PVIm homopolymer and in the PVIm phase of the dry conetwork as revealed by solid-state NMR 13 C cross-polarization magic angle spinning (CP MAS) and two-dimensional 1 H− 13 C frequencyswitched Lee−Goldburg (FSLG) HETCOR measurements. With increasing swelling ratio, structural and conformational changes were recognized in the hydrophilic PVIm phase of the APCN. In the kinetic swelling study, unexpectedly, four different swelling ranges were identified by gravimetric measurements, solid-state NMR methods, and positron annihilation lifetime (PAL) spectroscopy before the APCN reached its equilibrium swelling state. In the first period, which takes place in several minutes, the ordered structure disintegrates in the PVIm phase and the water uptake is relatively slow. This structural realignment is followed by the main course of water uptake governed by Fickian diffusion in the second stage. Close to the equilibrium swelling ratio, the swelling curve becomes nonmonotonic caused by a realignment of main chains of the hydrophilic phase in the third stage of swelling. Thus, by the unique combination of conventional swelling kinetics and solid-state NMR as well as PAL spectroscopies for investigating the aqueous swelling of the PVIm-l-PTHF amphiphilic polymeric conetwork, it was revealed that unexpected noncontinuous swelling occurs which is due to structural changes of the PVIm component in the conetwork in the course of this process.
Several poly(methyl methacrylate) samples of different molecular weight and dispersity were prepared and studied by positron lifetime spectroscopy. According to the lifetime measurements, the free volume holes were smaller and their distribution was narrower in monodisperse PMMA samples than in polydisperse samples. On the other hand, density measurements revealed that the overall free volume fractions are identical in both kinds of samples. To check these results, we constructed a model for the polymer structure and calculated free volume distributions in it. Theoretical results have not shown dispersity-dependent free volume distributions which suggested that the structure of compressed PMMA pellets differs from the ideal interaction-free structure considered in calculations. This latter result emphasizes the role of entanglements in the formation of the polymer structure.
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