VISCOELASTIC PROPERTIES OF THERMOREVERSIBLE GELS FROM CHEMICALLY-MODIFIED PVCS

Two homologous series of polymers based on poly(vinyl chloride) (PVC) have been prepared by chemical reduction of PVC with lithium aluminum tetrahydride and by chemical substitution of PVC with sodium benzeneth iolate. The general formula for both series of copolymers is (CH2CHCl) (m)-(CH2-CHX)(n), with X = H or SC6H5 for the first or second reaction , respectively, and n varying from 0 to 50% or more. The microstructur e of both series of copolymers has been analyzed in terms of stereoreg ularity and chemical composition distribution by C-13- and H-1-NMR spe ctroscopy. It is found that, for a given copolymer composition in both series, the chain length is the same and Cl atoms from isotactic and atactic placements are preferentially eliminated. Gels of these-two se ries of copolymers of different molar composition were prepared at sev eral concentrations in a Series of diester like solvents, varying the acid and alcohol size of the diester. The viscoelastic properties of t hese gels using parallel-plate shear and tensile compression modes wer e evaluated asa function of temperature. The storage moduli were analy zed using different new theories proposed for the elasticity of gels: those of Clark and Ross-Murphy and of Jones and Marques. These results provide evidence for an enthalpic origin of the elasticity of PVC gel s. This interpretation is in agreement with the results obtained from small-angle neutron scattering for both series of gels in deuterated d iethyl oxalate, in which a fiber-like structure for PVC gels is assume d.