Molecular Dynamics of Poly(ethylene glycol) and Poly(propylene glycol) Copolymer Networks by Broadband Dielectric Spectroscopy

Molecular Characterization of Polymer Networks

Polymer networks are complex systems consisting of molecular components. Whereas the properties of the individual components are typically well understood by most chemists, translating that chemical insight into polymer networks themselves is limited by the statistical and poorly defined nature of network structures. As a result, it is challenging, if not currently impossible, to extrapolate from the molecular behavior of components to the full range of performance and properties of the entire polymer network. Polymer networks therefore present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. A barrier to sophisticated molecular approaches to polymer networks is that the techniques for characterizing the molecular structure of networks are often unfamiliar to many scientists. Here, we present a critical overview of the current characterization techniques available to understand the relation between the molecular properties and the resulting performance and behavior of polymer networks, in the absence of added fillers. We highlight the methods available to characterize the chemistry and molecular-level properties of individual polymer strands and junctions, the gelation process by which strands form networks, the structure of the resulting network, and the dynamics and mechanics of the final material. The purpose is not to serve as a detailed manual for conducting these measurements but rather to unify the underlying principles, point out remaining challenges, and provide a concise overview by which chemists can plan characterization strategies that suit their research objectives. Because polymer networks cannot often be sufficiently characterized with a single method, strategic combinations of multiple techniques are typically required for their molecular characterization.

Fast secondary dynamics for enhanced charge transport in polymerized ionic liquids

Segmental dynamics is considered as a major factor governing ionic conductivity of polymerized ionic liquids (PILs), envisioned as potential electrolytes in fuel cells and batteries. Our dielectric studies performed in T-P thermodynamic space on ionene, composed of the positively charged polymer backbone and freely moving anions, indicate that other relaxation modes, completely ignored so far, can affect the charge transport in PILs as well. We found that fast mobility manifested by a secondary β process promotes segmental dynamics and thereby increases ionic conductivity making the studied material a first coupled PIL of superionic properties. The molecular mechanism underlying such a β process has been identified as Johari-Goldstein relaxation giving experimental proof that fast secondary relaxations of intermolecular origin exist also in PILs and thereby reveal a universal character.

The effect of cross-linking on the molecular dynamics of the segmental and β Johari-Goldstein processes in polyvinylpyrrolidone-based copolymers

The effect of the cross-link density on the molecular dynamics of copolymers composed of vinylpyrrolidone (VP) and butyl acrylate (BA) was studied using differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). A single glass transition was detected by DSC measurements. The dielectric spectra exhibit conductive processes and three dipolar relaxations labeled as α, β and γ in the decreasing order of temperatures. The cross-linker content affects both α and β processes, but the fastest γ process is relatively unaffected. An increase of cross-linking produces a typical effect on the α process dynamics: (i) the glass transition temperature is increased, (ii) the dispersion is broadened, (iii) its strength is decreased and (iv) the relaxation times are increased. However, the β process, which possesses typical features of a pure Johari-Goldstein relaxation, unexpectedly loses the intermolecular character for the highest cross-linker content.

Subambient temperature CO(2) and light gas permeation through segmented block copolymers with tailored soft phase

The permeation properties of a series of block copolymers based on poly(ethylene oxide)-ran-poly(propylene oxide) (PEO-ran-PPO) soft segments and monodisperse tetra-amide (T6T6T) hard segments have been studied. The polyether soft segment used in the current study differs from the commonly used pure poly(ethylene oxide) (PEO) soft segment by the fact that it contains 25 wt % randomly distributed poly(propylene oxide) (PPO). The presence of the methyl group of PPO suppresses crystallization of the soft segment and strongly improves the permeability of these materials, especially at subambient temperatures. In addition, the unique monodisperse character of the hard segment ensures a very well phase-separated morphology, resulting in a very pure soft phase. The soft segment length of these block copolymers was varied between 1000 and 10000 g/mol (62-89 wt %). High soft segment concentrations and flexibility were obtained resulting in high CO(2) permeabilities (up to 570 Barrer at 50 degrees C). Due to the random distribution of PPO in the predominantly PEO based soft segment crystallization of PEO was not observed at temperatures as low as -10 degrees C. CO(2) permeabilities exceeding 200 Barrer could be obtained at this low temperature. The CO(2)/light gas selectivity in these materials is governed by the solubility selectivity and consequently only slightly lowered because of the introduction of PPO in the soft segment. Comparison with literature revealed that this block copolymer system has exceptionally high CO(2) permeabilities combined with reasonable CO(2)/light gas selectivities. It is very interesting in CO(2) separation processes where subambient conditions are present (e.g., separation of CO(2) from natural gas), as at these low temperatures, one can take maximum advantage of the increased separation ability of the polymer materials while maintaining excellent transport characteristics.