9 Diffusion in glassy and semicrystalline polymers

Abstract

Wide attention has been paid in recent years to diffusion through polymeric materials for its interesting applications in various technological fields [91Vie]. Examples are permselective membranes for separation of gases, vapors and liquids, food packing, protective coatings and controlled drug delivery systems. Moreover, diffusion studies have provided valuable information on the structure of polymers and timedependent changes in the glassy state. Particularly gas molecules were often used as structure-sensitive probes of variable size.Since this volume is concerned with diffusion in solids, the present chapter covers diffusion in glassy polymers and semicrystalline polymers with a glassy amorphous phase. The diffusion behavior of semicrystalline polymers is essentially governed by the amorphous phase. For crystalline polymers the reader is referred to Chap. 8 on molecular solids.Diffusion characteristics of polymers can be markedly affected by the presence of additives, such as plasticizers and stabilizers, and by residual solvent. In glassy polymers the polymer molecular weight generally has no influence unless it is very low [82Toi2]. The non-equilibrium nature of the glassy state gives rise to a history dependence of transport properties and the appearance of local inhomogenities [91Vie, 94Pet, 89Kor]. The latter are produced by the residual mobility of different frozen chains, which are unable to relax to thermal equilibrium because of inhibited segmental motions. The influence of history on current and future performance of glassy polymers is well documented for thermal and mechanical treatment as well as for processing conditions. Furthermore, many studies have reported penetrant-induced history-dependent effects, often termed conditioning in connection with transport in glassy polymers. History also affects the orientation of molecules and the crystallinity of semicrystalline polymers. History effects manifest themselfs in time-dependent and hysteretic behavior. For instance, structural relaxation of glassy polymers, which is accompanied by annealing of excess volume, may lead to a marked decrease of penetrant diffusivities [90Ehl, 94Zha]. On experimental time scales diffusivities finally approach a time-independent value being characteristic of the well relaxed glassy state. An example of penetrant-induced conditioning and hysteresis is the exposure of glassy polymers to relatively high CO 2 pressures, which induces a volumetric expansion effect. A substantial increase in excess volume persists long after the penetrant has been removed. No hysteretic effects are usually observed, even at high pressure in the MPa range, for gases such as He, N 2 and even CH 4 [89Kor]. The lower condensability, and hence lower solubility of these supercritical gases, is presumable responsible for their lack of conditioning capability.In view of the abovementioned complications it is obvious that diffusion data in glassy polymers are often not very meaningful without detailed information on sample preparation and history. ...

Free Volume and Transport Properties in Highly Selective Polymer Membranes

“…Using these values and the value of l = 0.5 nm, one obtains activation energies E between 0.1 and 0.3 eV depending on the diameter of the gas molecule. These values are in the range of experimental values ( E = 0.1−0.5 eV) reported for glassy polymers . Details of the fits are given in ref .…”

“…The logarithmic deviation between two diffusion coefficients can be defined as For the diffusion coefficients in Figure the difference in the activation energy between the data taken from ref and the present one is Δ E ≈ 50 meV. This is of the order of 10% of measured activation energies . This difference can be explained by differences in the polymer structures.…”

“…The parameters resulting from a fit to the log D vs 1/ V h data in Figure by means of the classical free volume relation, given in eq 2, are shown in Table . The D 0 values are of the order of those from Arrhenius plots of log D against 1/ T . Substituting eq 8 into eq 6 yields In this equation the only free parameters are and l .…”

“…In membrane applications, however, polymers with high permeability and high selectivity are needed. This has stimulated many investigations during the past decades. − Free volume has proven to have a strong influence in diffusion and permeation properties. − …”

“…This has stimulated many investigations during the past decades. [2][3][4][5][6][7][8][9][10] Free volume has proven to have a strong influence in diffusion and permeation properties. [11][12][13][14][15][16] The present paper is concerned with the correlation between free volume and transport properties.…”