Solvent effects on chain fluctuations in highly concentrated polyisobutylene solutions. A new description derived from high resolution NMR spectroscopy

Abstract

The purpose of the present paper is twofold. First it is shown that a standard analysis of high resolution NMR spectra can be defined to derive a physical parameter Δ, reflecting chain dynamics in polymer systems. This property was illustrated by observing temperature and solvent concentration effects on high resolution NMR spectra of protons linked to high molecular weight polyisobutylene chains. Three types of solvents were used, giving analogous results: CDCl3, CS2, and C7D8. Observations were made on broad concentration and temperature ranges. Attention was particularly focused on highly concentrated systems which are not easily observed from viscoelastic experiments. Then the description of solvent concentration and temperature effects on NMR data is investigated. It is shown with great accuracy that the parameter Δ−1 varies according to a linear function of the ratio c=ns/nm, where ns and nm are the solvent molecule and monomeric unit number, respectively: Δ−1=Δ−10 +δ−1c (δ−1 is a constant characterizing the polymer–solvent system). This linear representation is shown to apply also to the description of concentration effects on diffusion parameters previously measured on other polyisobutylene–solvent systems. This linear representation is contrasted to the conventional Williams–Landel–Ferry (WLF) representation used to describe free volume effects usually considered as well-reflected by diffusion parameters. Also, temperature effects on NMR data were not found to be described according to the WLF form but according to the following representation: the product TΔ−1 is a linear function of the absolute temperature T. It is concluded that chain fluctuations observed from NMR lines cannot be characterized by parameters such as the steady-state viscosities derived from viscoelastic measurements.