Diffusion of Liquid Polystyrene into a Glassy Poly(phenylene oxide) Matrix. Diffusion Mechanisms and Experimental Verification

Abstract

The diffusion of a liquid polymer into a glassy polymer matrix has been studied in a range of temperatures below the glassy matrix glass transition temperature (T g) and for different diffusion times. The liquid polymer used is low-molecular-weight polystyrene (PS) with a narrow molecular weight distribution, and the glassy matrix is poly(phenylene oxide); the two are miscible at any concentration. A simple physical diffusion model is proposed to correlate and predict diffusion rates, assuming a relatively rapid dissolution of the high-Tg polymer at the liquid-solid interface and a relatively slow diffusion process that produces a thick interphase. The local chemical compositions, local glass transition temperatures, and local PS monomeric friction coefficients change markedly along the diffusion path across the interphase; these changes are well predicted by the diffusion model and have also been experimentally verified. The large changes in local T g values cause huge changes in the PS monomeric friction factor across the interphase, and this fact explains the asymmetrical local chemical composition profiles experimentally measured for the PS-rich interphase. The results obtained by other authors for the diffusion of liquid polymers and bulky plasticizers into glassy matrixes are analyzed and discussed on the basis of the diffusion model predictions, and it is found that all of them behave following the same pattern as was observed in our experiments. It is concluded that the Case II diffusion mechanism must not be expected for the diffusion of liquid polymers into glassy matrixes, because of the negligible osmotic pressure. Furthermore, all of the analyzed data for diffusion of liquid polymers and bulky plasticizers into glassy matrixes show evidence for relatively rapid dissolution of the glassy matrix at the interface, together with a relatively slow diffusion process across the interphase.

Liquid‐Glassy Polymer Diffusion: Effects of Liquid Molecular Weight and Temperature

“…[11][12][13][14][15][16] One group of authors has extended the concept of case II to explain the characteristics of the liquid/glassy polymer diffusion. [14,17,18] The more solid argument in favor of a mechanism of diffusion controlled by the mechanical relaxation of the solid matrix has been the characteristic shape of the concentration profiles of the liquid polymer, that closely resemble those observed in case II.…”

“…These ideas have been questioned by other group of authors, who have suggested that the growth mechanism for these interphases may be diffusion controlled, similar to that observed in liquidliquid polymer diffusion between species with different physical properties. [13,15] The main argument against case II in these cases has been pointed toward the extremely low osmotic suction values associated with the large polymer molecules, insufficient to trigger the mechanism of mechanically controlled liquid penetration described above. [16] The polystyrene (PS)-poly(phenylene oxide) (PPO) polymer pair is an ideal system to test these ideas.…”

“…Instantaneous velocities were calculated from the derivative of the front advances versus time plot as detailed elsewhere. [11,13] To focus only on the effect of temperature, each set of data correspond to similar stages in the diffusion process as they were calculated at the same value of liquid-polymer concentration at the plateau region (F pl ) and from experiments involving similar initial thicknesses for the low-T g layer (60-70 mm). The solid symbols correspond to data of the l-PS0.7/PPO system, with V df values calculated at two stages of the diffusion process, F pl ¼ 0.6 (triangles) and F pl ¼ 0.…”

“…The marked asymmetry in the predicted concentration profiles originates in the dramatic changes of the monomeric friction coefficient values along the diffusion coordinate, in turn controlled by the local T g through the WLF scaling, as extensively discussed in our previous work. [13] The predicted kinetics of the diffusion fronts are compared with experimental data through Figure 3(a)-3(b). To calculate front advances from the calculated profiles, we used the PS concentration at the plateau region and the area under the profile, the same methodology employed to obtain the front advances from the experimental data.…”

Fast Transient Fluorescence Technique for Studying Homopolymer Mobility in a Swelling Gel

“…[25] More recently, the diffusion of liquid polystyrene into a glassy poly(phenylene oxide) was reported. [26] A simple physical model is proposed to correlate and predict diffusion rates assuming a relatively rapid dissolution of the high T g polymer at the liquid/solid interphase. For a long time, the common belief has been that the mobile chains do not penetrate into gel, even if they are compatible with the crosslinked chains.…”