Diffusion in Model Networks as Studied by NMR and Fluorescence Correlation Spectroscopy

We have studied the diffusion of small solvent molecules (octane) and larger hydrophobic dye probes in octane-swollen poly(dimethyl siloxane) linear-chain solutions and end-linked model networks, using pulsed-gradient nuclear magnetic resonance (NMR) and fluorescence correlation spectroscopy (FCS), respectively, focusing on diffusion in the bulk polymer up to the equilibrium degree of swelling of the networks, that is, 4.8 at most. The combination of these results allows for new conclusions on the feasibility of different theories describing probe diffusion in concentrated polymer systems. While octane diffusion shows no cross-link dependence, the larger dyes are increasingly restricted by fixed chemical meshes. The simple Fujita free-volume theory proved most feasible to describe probe diffusion in linear long-chain solutions with realistic parameters, while better fits were obtained assuming a stretched exponential dependence on concentration. Importantly, we have analyzed the cross-link specific effect on probe diffusion independently of any specific model by comparing the best-fit interpolation of the solution data with the diffusion in the networks. The most reasonable description is obtained by assuming that the cross-link effect is additive in the effective friction coefficient of the probes. The concentration dependences as well as the data compared at the equilibrium degrees of swelling indicate that swelling heterogeneities and diffusant shape have a substantial influence on small-molecule diffusion in networks.

Phenomenology of Self-Diffusion in Star-Branched Polyisoprenes in Solution

We have measured polymer and solvent self-diffusion at 50°C in equiarmed star-branched polyisoprenes of low molecular weights in solution with C6F5Cl and CCl4, using pulsed-gradient spin-echo methods supplemented by NMR T2 relaxation. In solutions and melts of stars with M(arm) ≃ 104 and f ≤ 18, entanglements are less pronounced than in linear polymers of equal molecular weight. Polymer diffusion depends differently on M(arm) and f, the f dependence becoming weak at high f at all solvent concentrations. In dilute solution, diffusion behavior shows that these stars are relatively highly expanded; at high polymer concentrations, segmental diffusion mechanisms begin to contribute. Free volume theories explain solvent but not polymer diffusion. © 1982, American Chemical Society. All rights reserved

Phenomenology of Self-Diffusion in Star-Branched Polyisoprenes in Solution

We have measured polymer and solvent self-diffusion at 50°C in equiarmed star-branched polyisoprenes of low molecular weights in solution with C6F5Cl and CCl4, using pulsed-gradient spin-echo methods supplemented by NMR T2 relaxation. In solutions and melts of stars with M(arm) ≃ 104 and f ≤ 18, entanglements are less pronounced than in linear polymers of equal molecular weight. Polymer diffusion depends differently on M(arm) and f, the f dependence becoming weak at high f at all solvent concentrations. In dilute solution, diffusion behavior shows that these stars are relatively highly expanded; at high polymer concentrations, segmental diffusion mechanisms begin to contribute. Free volume theories explain solvent but not polymer diffusion. © 1982, American Chemical Society. All rights reserved

Self-Diffusion of Three-Armed Star and Linear Polybutadienes and Polystyrenes in Tetrachloromethane Solution

We have used the pulsed-gradient spin-echo method to measure the self-diffusion of linear and three-armed star-branched polystyrenes and polybutadienes (3 × 103 M 106) dissolved in CC14 at 50 °C. We detect no evidence of cooperative diffusion in the semidilute regime. The polymer diffusivities at infinite dilution obey Flory's theory, while their concentration dependences in the dilute regime follow a Pyun-Fixman model. No differences between linear and three-armed molecules are observed in this experiment. © 1983, American Chemical Society. All rights reserved