Polymeric gels. NMR study of elementary chain swelling. Effect of trapped topological constraints
Résumé
This work aims at studying the progressive swelling process of three polydimethylsiloxane covalent gels, synthetized at several volume fractions, νc ; two good solvents (chloroform and heptane) and a theta-solvent (bromocyclohexane) were used as swelling agents. i) The macroscopic swelling was found to obey a simple addition law of volumes of solvent and polymer. ii) The transverse magnetic relaxation process of protons linked to PDMS chains is shown to reflect a progressive swelling process of elementary chains occurring according to two well defined steps. The first one (1 < q < qd ~ 3), is a progressive desinterspersion of chains; monomeric units are replaced by solvent molecules; the relaxation rate δ was shown to obey a superposition property : δ/ν c ∝ (qν5/8c)-0.6±0.05 ; in the second step (qd < q ≲ 12), δ was found to obey another superposition property : δ ∝ q2/3 ν 5/3c. In the swelling ratio range 1 < q ≤ qd and in the presence of good solvents, elementary chains have Gaussian properties; these are conveniently identified using a theta-solvent; these properties progressively disappear (q > qd) and an excluded volume effect arises ; this is fully observed on gels at equilibrium with a liquid solvent. Superposition properties are interpreted from a basic formula describing the transfer of localization of NMR properties from a local space-scale to a semi-local one.
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