NMR observation of the swelling process of polydimethylsiloxane networks. Average orientational order of monomeric units

Abstract

The swelling process of several polydimethylsiloxane gels was observed from the relaxation process of the magnetization transverse component of protons bound to elementary chains. Polymeric gels were studied in equilibrium with solvent vapor, using chloroform. A model of progressive swelling process from the dry gel to the equilibrium with pure solvent liquid is proposed from observed elementary chain properties. At small swelling degrees, gels behave like strongly entangled chains in concentrated solutions; then, elementary chains are desinterspersed and a packing condition may be applied from a characteristic swelling degree to equilibrium swelling. In this concentration range, gels swollen by different solvents behave the same way at a given swelling degree. Trapped entanglements are perceived. All these properties are derived from residual dipolar interactions associated with average orientational order of monomeric units induced by chain elongation. This residual energy is shown to be proportional to (r/N)2, with r the end-to-end vector and N the number of links of a chain. The C theorem proposed by De Gennes is extended to NMR properties. It is clearly shown that NMR observed on a chain segment may be sensitive to both its overall dimensions (30 Å) and its internal isomerization properties.