Effects of Polymer Molecular Weight and Temperature on Case II Transport MASUD M. HASSAN, CHRISTOPHER J. DURNING Department of Chemical Engineering and Applied Chemistry, Columbia University, 812 S.W. Mudd Bldg., 520 W. 120th St., New York, New York 10027 Received 15 March 1999; revised 21 June 1999; accepted 28 June 1999 ABSTRACT: We investigated the effects of polymer molecular weight and temperature on Case II transport in the poly(methyl methacrylate)/methanol (PMMA/MeOH) sys- tem by a laser interferometric technique, using monodisperse polymer samples. Both the induction process and the steady-state front propagation were investigated. The data gave the volume fraction of MeOH in the swollen layer behind the moving front, , the steady state front speed, , and the characteristic induction time, t ind . Values of  separated into two groups, independent of molecular weight within each group. Significantly lower values of  were found for polymers with molecular weight above the critical threshold for entanglement which can be explained by unrelaxed entangle- ments in the swollen layer. The Case II front velocity was independent of molecular weight for molecular weights at, or above, the critical weight for entanglement, sug- gesting that anelastic deformation processes other than simple viscous flow control the front propagation. Analysis of induction time data shows that the film surface proper- ties differ from those of the bulk. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3159 –3171, 1999 Keywords: Case II transport; non-Fickian diffusion; glassy polymer; swelling; inter- ferometry INTRODUCTION Case II transport is an unusual form of gradient diffusion, readily observed in glassy polymer/sim- ple fluid systems in sorption experiments. During Case II, a sharp fluid concentration front develops after first contact between the dry polymer and the fluid which then propagates at nearly con- stant speed into the polymeric material, reminis- cent of a wave propagation phenomenon. This striking non-Fickian transport process arises from the anelastic, non-linear nature of the poly- mer glass. A phenomenological understanding of the process has been developed through careful experiments. For example, Kramer et al. 1–4 mea- sured the fluid concentration profiles during the initial stages of Case II in a variety of systems. Their measurements showed that the fluid con- centration at the polymer surface relaxes auto- catalytically after first contact, causing an initial delay or “induction time” in the fluid mass uptake kinetics by the polymer. After the initial surface relaxation, a sharp fluid concentration profile de- velops and propagates into the polymer sample at constant speed causing the fluid uptake to in- crease linearly with time thereafter. Thomas and Windle 5 proposed a model for Case II accounting for glassy polymer’s retarded response to local changes in fluid chemical poten- tial. They assumed the polymer’s response to be slow, viscous flow. Although oversimplified, the model captures all the essential qualitative fea- tures of Case II transport. Using this simple non- Correspondence to: C. J. Durning (E-mail: cjd2@columbia. edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, 3159 –3171 (1999) © 1999 John Wiley & Sons, Inc. CCC 0887-6266/99/223159-13 3159