Gas Transport Properties of Poly(trimethylsilylpropyne) and Ethylcellulose Filled with Different Molecular Weight Trimethylsilylsaccharides: Impact on Fractional Free Volume and Chain Mobility


This work aims to systematically investigate the gas transport behavior in two glassy polymers. One is the rigid, high fractional free volume (FFV) poly(1-trimethylsilyl-1-propyne) [PTMSP], and the other is the relatively flexible, considerably lower FFV ethylcellulose (EC). Both polymer systems are filled with a series of various molecular weight (Mw) trimethylsilylsaccharides [TMSS] (trimethylsilylglucose (Mw = 180) [TMSG], trimethylsilyldextran1 (Mw = 900-1200) [TMSD1], and trimethylsilyldextran500 (Mw = 350K-550K) [TMSD500]). The consistent trend of decreasing gas permeability, diffusivity, and solubility with increasing load of the TMSS fillers was observed in the PTMSP/TMSS system. In addition, the extent of reduction of gas permeability, diffusivity, and solubility in these composites is closely related to the Mw of TMSS fillers at an equivalent load of various TMSS in the PTMSP matrix. For example, the PTMSP permeability to nitrogen reduced 227-fold, 43-fold, and 4-fold when filled with constant 27.2% TMSG, TMSD1, and TMSD500, respectively. The diffusivity decreased 45-fold, 21-fold, and 3-fold, and the solubility decreased 5.0-fold, 2.0-fold, and 1.3-fold, respectively. The decreases in permeability, diffusivity, and solubility are directly related to the decrease of FFV in PTMSP caused by the incorporation of the various Mw fillers. In contrast to the decrease of permeability observed in the PTMSP/TMSS system, a systematic increase of gas permeability and diffusivity was obtained for the EC/TMSS system with increasing load of TMSS fillers. However, no consistent change of solubility was observed in EC/TMSS. Moreover, the gas diffusivity increase for the EC/TMSS system correlated well with the Mw of the TMSS fillers, in contrast to the permeability increase. For example, when TMSG, TMSD1, and TMSD500 were used as fillers, the permeability to nitrogen of EC composites with 32.1% fillers increased 1.75-fold, 1.81-fold, and 1.64-fold, respectively, compared to that in unfilled EC. The diffusivity increased 3.32-fold, 1.84-fold, and 1.31-fold, and the solubility increased -1.87-fold, 0-fold, and 1.25-fold, respectively. All applied TMSS fillers led to an increase of gas diffusivity, which can be attributed to an increased chain mobility. The chain mobility changes in EC/TMSS resulted in changes of the excess FFV of EC and therefore altered the gas solubility. The increase extent of chain mobility was the highest with the lowest Mw TMSS.
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