Publications
1986
Surface interactions, as revealed by surface force studies between mica sheets in 0.1-0.2 M KNO3 aqueous electrolyte, are critically examined for three different types of macromolecules adsorbed on the surfaces: flexible uncharged polymer, flexible charged polyelectrolyte, and rigid rod protein. Aspects of specific and nonspecific effects, such as surface conformation, the origin of repulsive interactions and the possibility of attractive bridging, as well as the question of equilibrium and reversibility for such adsorbed macromolecular layers, are compared and contrasted.
Self-diffusion in polymer melts has been extensively studied and is reasonably well understood in terms of the reptation model1-4; the related phenomenon of mutual diffusion in miscible blends of chemically different polymers has received little attention, despite its practical relevance and implications for physical processes, such as phase separation kinetics. In such blends, attractive interactions between the monomers, when summed over a polymer chain, may lead to large enthalpic driving forces favouring the mixing; this in turn results in a mutual diffusion rate which is rapid compared with the entropically driven self-diffusion5, and which is strongly composition-dependent6. We have measured mutual diffusion as a function of composition in one such binary blend, polyvinyl chloride (PVC)/polycaprolactone (PCL), and report here that the mutual diffusion coefficient is strongly enhanced in the middle of the composition range. This result is qualitatively, though not quantitatively, in accord with the results of some recent theoretical treatments5-8.
The variation of the effective adsorbed layer thickness δeffwith molecular weight M for a polymer adsorbed at a mica-solvent interface under good solvent conditions, as revealed by force measurements, is analyzed. The observed variation, δeff∝M0-43, is shown to be consistent with a scaling form for the extension of the polymer from the surface. The corresponding effective thickness of adsorbed layers in poor solvents is briefly considered.
We evaluate the translational diffusion coefficients and longest relaxation times for entangled linear n-mers, for f-arm star branched polymers (nb-mers/arm), and for cyclic (closed ring) polymers (nR-mers) in a fixed entanglement network and also in a melt of entangled linear p-mers. \u201cTube-renewal\u201d effects for the latter case are reexamined, taking into account both hydrodynamic interactions and especially the interdependence of p-mer constraints about a diffusing n-mer; the characteristic tube-renewal time in this case becomes τtube~ n2p5/2, in contrast with earlier proposals. At very high n values one expects an unscreened form for the tube-renewal time, τtube~ n3/2p3, equivalent to the \u201chydrodynamic sphere\u201d diffusion of the n-mer in the p-mer melt. The diffusion coefficients and longest relaxation times for stars in a fixed entanglement lattice are calculated with a diffusion equation approach as Ds ~ nb-1 exp(-αnb/nc) and τ8 ~ nbexp(αnb/nc), with a a constant and nc the entanglement degree of polymerization, a form similar (in the dominant exponential term) to earlier calculations. For rings the corresponding expressions are DR~ exp(-βnR/nc) and rR~ exp(βnR/nc), where β ≳ α, though these are limiting forms valid for high nR(≳20nc); at lower nRvalues ring polymers may exhibit a quasi-linear behavior. When incorporated in melts of linear p-mers, the diffusion and relaxation of both stars and rings is dominatedfor nband nR greater than some crossover value of order 10ncby tube-renewal effects mediated by reptation of the linear melt matrix.
Analytical expressions, based on a mean-field model and derived in a previous paper (part 1: Klein, J.; Pincus, P. A. Macromolecules 1982,15,1129), for the interaction between two parallel plates bearing adsorbed polymer in poor-solvent medium, are solved numerically to yield (a) segmental density profiles of the adsorbed polymer and (b) interaction-energy vs. plate-separation profiles. All parameters appearing in these calculations are obtainable from bulk data such as the polymer-solvent phase diagram and adsorbance measurements. The calculated results are critically compared with model experiments on forces between smooth mica surfaces bearing polystyrene in cyclohexane in poor-solvent conditions: the agreement is qualitatively very good and quantitatively fair. Some improvements to the model are suggested.