Estimation of free volume in poly(trimethylsilyl propyne) by positron annihilation and electrochromism methods

Abstract

SYNOPSISThe free volume of poly( trimethylsilyl propyne) (PTMSP), a glassy polymer distinguished by extremely high gas permeability and diffusion coefficients, was determined by means of two probe techniques-the positron annihilation method and the electrochromism method. Lifetime analysis of positron annihilation spectra revealed an additional long-lived component that is not typical for most of previously studied polymers. A model that has been developed earlier predicts that this lifetime corresponds to the larger size of free-volume elements having an intrinsic radius of about 6 A. Therefore, PTMSP should be similar to porous inorganic sorbents. A comparison with the literary data for zeolites and silica gel confirms this. According to the electrochromism method, the temperature dependence of the size of free-volume elements for PTMSP is much weaker than that for regular glassy polymers like polystyrene. It was concluded that the great free volume in glassy PTMSP is associated with very loose packing of the chains. 0 1993 John Wiley & Sons, Inc. INTRODUCTIONPoly( 1-trimethylsilyl-1-propyne) (PTMSP) synthesis, which has been described by Masuda et al., has drawn great attentionThe gas permeability coefficients ( P ) of PTMSP are at least 1-2 orders of magnitude higher than those of the most permeable synthetic polymers, rubbery or glassy. Additionally, it has been shown that this polymer is characterized by high diffusion coefficients ( D ) , solubility coefficients (S) , and great Langmuir hole saturation constant Ch. This parameter of sorption isotherms is usually considered as a rough measure of the excess free volume in glassy polymers. Interestingly, for this polymer, anomalously low densities have been r e p~r t e d ,~ which for some samples are as low as 0.7 g/cc.It was only natural to explain all these peculiarities as the manifestation of the greater free volume * To whom correspondence should be addressed. of PTMSP. Indeed, it has been assumed that the larger free volume associated with such structural features of glassy PTMSP as the rigid main chain and the bulky Si ( CH3)3 side-chain group is responsible for the observed unusual properties of this p01ymer.~-~ Recently, so-called probe methods have been extensively used to estimate the free volume or characteristic size of its elements in glassy polyrner~.~-~ This work presents the results of the study of PTMSP by means of the positron annihilation method and the electrochromism method. For comparison, two other polymers were studied. The first, poly( vinyl trimethylsilane) (PVTMS) is a vinylictype polymer having the same side-chain group as has PTMSP attached directly to main chain. Transport and thermodynamic properties of PTMSP and PVTMS have been compared earlier.3~'0 The other polymer, polystyrene, was considered as a "typical" glassy polymer whose transport and other physicochemical properties have been extensively studied, including the investigations by means of positron annihilation and electrochromism methods.

Positron annihilation lifetime (PAL) in poly[1-(trimethyl-silyl)propine] (PTMSP): Free volume determination and time dependence of permeability

“…In a previous study of PAL spectroscopy performed by Yampol'skii et al 39 on PTMSP in air, the intensities and lifetimes of the two longest annihilation spectra components reported (I, = 5; 73 = 2468 ps; I4 = 30; 74 = 6668 ps) are practically the same as our data in Table I11 for the sample in air. The difference in the experimental data under vacuum and in air induced us to investigate the role of the external atmosphere on PAL spectra.…”

Ending Aging in Super Glassy Polymer Membranes

“…The pendant trimethylsilyl groups, key to propping main chains open and delivering free volume, dominate interchain interaction and chain packing, a key driver of free volume and hence permeability. [28] The overall mechanism revealed in these data is that the PTMSP is tethered by the inclusion of pendant methyl groups within PAF-1 pores, affecting the freedom of main-chain segments, and leaving trimethylsilyl groups unchanged (see Figure 1 B which shows schematically how PTMSP chains are tethered within the pores of PAF-1). Compared to T 1 values of PTMSP/MOF nanocomposites, it is evident that the tethering mechanism is not valid for all porous particles.…”

Detailed-atomistic molecular modeling of small molecule diffusion and solution processes in polymeric membrane materials

“…It clearly reveals regions of high segmental packing density adjacent to rather large voids with a tendency to a partially continuous hole phase with lateral void-widths between about 5 and 20 Å . In this sense the structure of PTMSP membranes can be described as microporous 86) and micro-heterogeneous 87) . These results can be compared with available experimental data from positron annihilation studies.…”

“…Consolati et al 88) , for instance, reported a bimodal distribution of smaller holes with a diameter of 9 Å and larger ones of 15 Å . Yampol'skii et al 86) , on the other hand determined a spectrum of void diameters between 6 and 12 Å . Both results are not too far away from the findings from simulated PTMSP structures reported here.…”