Gas permeability and free volume of highly branched substituted acetylene polymers

Gas permeation, sorption, and structural properties of two highly branched polyacetylenes, poly [1-phenyl-2-[p-(triphenylsilyl)phenyl] acetylene] (PPh SiDPA) and poly [1-phenyl-2-[p-(triisopropylsilyl)phenyl] acetylene] (PPrSi DPA), are reported. Structurally, both polymers have much in common; howeve r, their transport properties are quite different. PPhSiDPA has dramaticall y lower permeability coefficients than PPrSiDPA. For example, the Oz permea bility coefficients of PPhSiDPA and PPrSiDPA are 12 x 10(-10) and 230 x 10( -10) cm(3) (STP) cm/(cm(2) s cmHg), respectively at 22 degreesC. Gas solubi lity is very high in PPrSiDPA, similar to that observed in poly(1-trimethys ilyl-1-propyne) (PTMSP), the most permeable polymer known. Gas solubility c oefficients of PPhSiDPA are 2-3 times lower than those of PPrSiDPA. Free vo lume size and size distribution were characterized using positron annihilat ion lifetime (PAL) spectroscopy. Results from these studies (i.e., a bimoda l size distribution of free volume elements and large free volume element s ize) are consistent with the observed transport and sorption properties of PPrSiDPA. In contrast, the PAL spectrum of PPhSiDPA is similar to that of c onventional low free volume glassy polymers.