Influence of methanol conditioning and physical aging on carbon spin-lattice relaxation times of poly(1-trimethylsilyl-1-propyne)

Molecular-level changes in poly(1-trimethylsilyl-1-propyne) [PTMSP] were observed using solid-state C-13 NMR. These changes are attributed to the effect of polymerization catalyst on polymer configuration and the influence of methanol conditioning and physical aging on chain mobility. Chain mobility is deduced from carbon spin-lattice relaxation times (T-1). In the solid-state C-13 NMR spectrum of TaCl5-synthesized PTMSP. the alpha-methyl carbon resonance is split into two peaks (25.5 and 29.5 ppm), indicating the existence of both cis and trans isomers. In the spectrum of the NbCl5-catalyzed polymer, however, the peak assigned to the trans isomer is absent, suggesting that PTMSP synthesized with NbCl5 catalyst has a more regular configuration than PTMSP prepared with TaCl5 Catalyst. For all PTMSP films, T-1 values decrease during methanol conditioning and then increase upon aging, indicating that molecular motion is enhanced by methanol treatment and reduced by aging. Methanol conditioning enhances carbon molecular motion by a similar amount in both NbCl5- and TaCl5-synthesized PTMSP, except for the mobility of carbons assigned to the trans isomer (which notably exhibits the largest percentage decrease in relaxation time). In the initial state, the presence of the trans isomer, which has less rotational freedom than the cis isomer, resulted in TaCl5-synthesized PTMSP having higher T, values than the NbCl5-catalyzed counterpart. Although the trans isomer present in the TaCl5-catalyzed membrane imparts rigidity, it also induces disorder in chain packing. As the TaCl5-catalyzed PTMSP membrane was previously shown to be more permeable than the NbCl5-catalyzed PTMSP, disorder in chain packing seems to be more important than polymer mobility when high permeability is desired. (C) 2004 Elsevier B.V. All rights reserved.