The Effect of Molecular Architecture on the Phase Diagram and Mobility of a Polymer Blend Exhibiting a Lower Critical So
- PDF / 344,944 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 72 Downloads / 169 Views
THE EFFECT OF MOLECULAR ARCHITECTURE ON THE PHASE DIAGRAM AND MOBILITY OF A POLYMER BLEND EXHIBITING A LOWER CRITICAL SOLUTION TEMPERATURE: CYCLES MIXED WITH LINEAR CHAINS. Maria M. Santore, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015. Gregory McKenna and Charles Han, Polymer Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 ABSTRACT We examine the role of molecular architecture on the phase diagram of the PS/PVME (poly[styrenel /poly[vinyl methyl ether]) blend, a mixture which in previous studies with linear chains exhibited a lower critical solution temperature, (LCST) i.e. it phase separated on heating. In this investigation, two blends with components exceeding the critical molecular weight for entanglement were compared: one consisting of linear PS and PVME and a second with cyclic PS and linear PVME. Cloud point experiments over a broad composition range reveal that the blend containing cyclic PS undergoes phase separation at temperatures 7-8 oC higher than the analogous linear blend. In other words, the mixture of cycles and linear chains is more thermodynamically stable than the mixture of two linear chains. The LCST nature of the system facilitates examining chain mobility by considering the phase separation kinetics. Time-resolved light scattering studies of blends near their critical compositions tracked the spinodal decomposition following a rapid temperature jump from the one-phase to the two-phase region. An analysis of the scattering intensity growth ultimately led to mutual diffusion coefficients whose temperature dependence confirmed the observed cloud points. An approximation of the second derivative of the free energy function based on SANS studies of the linear PS/PVME blend allowed us to estimate mutual mobilities. The values determined for the cycle-containing blend were considerably lower than those for the blend of linear chains at these molecular weights. INTRODUCTION Macrocyclic polymers are of special interest to the polymer physicist, because the ring topology presents a unique opportunity to test established theories such as reptation. Given the perception that, unlike their linear analogs, rings simply cannot reptate, basic questions arise. For instance, what is the relationship between viscosity and molecular weight for rings, and how does the viscosity of a cyclic sample compare with that of linear chains? Considerable work on these viscosity issues with cyclic polystyrene has been undertaken by Roovers 1 and McKenna et al.,2,3 with the main conclusions that like linear chains, cycles exhibit a power law dependence on molecular weight; that low molecular weight cycles are less viscous than their linear analogs presumably due to reduced entanglements; and that small amounts of linear contaminants drastically increase the viscosity of a cyclic sample. The equally important dynamic issue of tracer diffusion of cycles through linear matrices has been studied by forward recoil spectrometry with a focus on the influence of molec
Data Loading...
