Thermodynamics of sorption in and free volume of poly(5,6-bis(trimethylsilyl)norbornene)

Thermodynamics of sorption of n-alkanes C4–C11 in poly(5,6-bis(trimethylsilyl)norbornene) was studied using the Inverse Gas Chromatography (IGC). Temperature dependences of the specific retention volume Vg and solubility coefficient S of various alkanes were obtained in the range 60–140 °C. For solubility coefficients, the following equation holds: ln S = 3.44 ( T c / T ) 2 − 3.90 , where Tc is the critical temperature of solutes, T is the experimental temperature and S is expressed in cm3(STP)/cm3 atm. The partial molar enthalpies ΔHm and entropies ΔSm of mixing of different n-alkanes in this glassy polymer vary in much wider range than in rubbers and display positive correlations: for more exothermic mixing process the larger negative ΔSm values are observed. It was shown that the ΔHm in poly(5,6-bis(trimethylsilyl)norbornene) pass through a minimum, when the size of solutes increases. The coordinates of ΔHm at minimum versus solute size provide an estimate of the size of free volume elements in this polymer. This conclusion was supported by variation of permeability in different glassy polymers studied using IGC and by the results of other methods for probing free volume in glassy polymers.