@article{aberg_ozcam_majikes_seyam_spontak_2008, title={Extended chemical crosslinking of a thermoplastic polyimide: Macroscopic and microscopic property development}, volume={29}, ISSN={["1022-1336"]}, DOI={10.1002/marc.200800230}, abstractNote={AbstractPolyimides are well established as gas separation membranes due to their intrinsically low free volume and correspondingly high H2 selectivity relative to other gases such as CO2. Prior studies have established that H2/CO2 selectivity can be improved by crosslinking polyimides with diamines differing in spacer length. In this work, we follow the evolution of macroscopic and microscopic properties of a commercial polyimide over long crosslinking times (tx) with 1,3‐diaminopropane. According to spectroscopic analysis, the crosslinking reaction saturates after ≈24 h, whereas tensile, nanoindentation and stress relaxation tests reveal that the material stiffens, and possesses a long relaxation time that increases with increasing tx. Although differential scanning calorimetry shows that the glass transition temperature decreases systematically with increasing tx, permeation studies indicate that the permeabilities of H2 and CO2 decrease, while the H2/CO2 selectivity increases markedly, with increasing tx. At long tx, the polyimide becomes impermeable to CO2, suggesting that it could be used as a barrier material.magnified image }, number={17}, journal={MACROMOLECULAR RAPID COMMUNICATIONS}, author={Aberg, Christopher M. and Ozcam, Ali E. and Majikes, Jacob M. and Seyam, Mohamed A. and Spontak, Richard J.}, year={2008}, month={Sep}, pages={1461–1466} } @article{aberg_seyam_lassell_bronstein_spontak_2008, title={In situ Growth of Pd Nanoparticles in Crosslinked Polymer Matrices}, volume={29}, ISSN={["1521-3927"]}, DOI={10.1002/marc.200800505}, abstractNote={AbstractPolymer nanocomposites continue to receive considerable attention as multifunctional hybrid materials, with most nanocomposites fabricated by physical dispersion of surface‐functionalized nanoscale objects. In this study, we explore the viability of growing Pd‐containing nanoparticles from Na2PdCl4 in two different polymers: hypercrosslinked polystyrene (HPS) and an aromatic polyimide (PIm). In HPS, single Pd‐containing nanoparticles possessing a relatively narrow size distribution (ca. 1–4 nm) form upon reduction of the divalent PdCl ions. Single nanoparticles with a broad size distribution ranging from ≈2 to 16 nm develop in PIm, which simultaneously undergoes chemical crosslinking during ion reduction. Such hybrid materials hold promise in molecular catalysis and gas separation.magnified image }, number={24}, journal={MACROMOLECULAR RAPID COMMUNICATIONS}, author={Aberg, Christopher M. and Seyam, Mohamed A. and Lassell, Scott A. and Bronstein, Lyudmila M. and Spontak, Richard J.}, year={2008}, month={Dec}, pages={1926–1931} } @article{patel_aberg_sanchez_capracotta_martin_spontak_2004, title={Morphological, mechanical and gas-transport characteristics of crosslinked poly(propylene glycol): homopolymers, nanocomposites and blends}, volume={45}, ISSN={["1873-2291"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-3342879397&partnerID=MN8TOARS}, DOI={10.1016/j.polymer.2004.06.024}, abstractNote={Linear polyethers possess unusually high CO2 solubility and, hence, selectivity due to the presence of accessible ether linkages that can interact with the quadrupolar moment of CO2 molecules. In this work, membranes derived from crosslinked poly(propylene glycol) diacrylate (PPGda) oligomers differing in molecular weight (M), as well as PPGda nanocomposites containing either an organically-modified montmorillonite clay or a methacrylate-terminated fumed silica are investigated and compared with highly CO2-selective poly(ethylene glycol) diacrylate (PEGda) homopolymer and nanocomposite membranes previously reported. The rheological and permeation properties of PPGda depend sensitively on M, with the elastic modulus decreasing, but CO2 permeability and CO2/H2 selectivity increasing, with increasing M. Incorporation of either nanofiller into PPGda enhances the elastic modulus and reduces the gas permeability in the resultant nanocomposites without strongly affecting CO2/H2 selectivity. Blending PPGda and PEGda prior to chemical crosslinking yields binary membranes that exhibit intermediate gas-transport properties accurately described by a linear rule of mixtures.}, number={17}, journal={POLYMER}, author={Patel, NP and Aberg, CM and Sanchez, AM and Capracotta, MD and Martin, JD and Spontak, RJ}, year={2004}, month={Aug}, pages={5941–5950} }