L. E. St. Pierre

The effect of polymer—filler interaction energy on the T′g of filled polymers

Abstract The glass transition temperatures of composites of four polymers containing moderate to high concentrations of silicas of different surface properties were measured by means of the torsional pendulum. The shift in T g of the composites to higher temperatures as compared to that for the unfilled polymer was correlated in terms of the polymer—filler interaction energies which were measured by the heats of adsorption of the model compounds of the polymers on the filler surfaces. The shift in T g is directly related to the extent of polymer—filler interaction energy. It is small in the absence of a strong polar interaction but increases for the strongly adsorbing polymers. At an average polymer film thickness of 20–31 A, loss peaks due to the adsorbed as well as unadsorbed polymer were obtained. Thus the effect of polymer-filler interaction energy on the short range molecular motion in composites is restricted to short distances (less than 20 A), comparable with the range of the surface force field.

Adsorption of poly(dimethyl siloxanes) from solution on silica: 1

The adsorption of a broad range of poly(dimethyl siloxanes) on silica, treated and untreated, has been studied. Three solvents, n-hexane, CCl4, and benzene were employed. Adsorption was molecular weight dependent and more polymer adsorbed from n-C6H14 than from CCl4. Only negative adsorption from C6H6 solution was observed. The results are rationalized on the basis of the polymer—solvent interaction values (χ), the solvent—adsorbent and the solute—adsorbent adsorption energies. It is concluded that the loss in configurational entropy on adsorption of the macromolecules is a major factor in determining the adsorption isotherms.

Radiolytic Scissioning in Polydimethylsiloxane Networks

Abstract γ-radiation induced backbone cleavage in stressed polydimethylsiloxane networks displays the stress relaxation profile expected for random scission of a network possessing a Gaussian distribution of chain lengths. G (scission) as measured by stress relaxation, is found to be dose rate dependent below 0.4 Mr/hr, but does not reveal any atmospheric or temperature effects. The value of G (scission) = 0.7, obtained at a dose rate of 0.4 Mr/hr, is in good agreement with that determined from sol-dose measurements. The postulated mechanism of scission is based upon a three step reaction scheme involving a siliconium ion produced initially by γ-ray cleavage of an SiO bond. The ion is capable of reacting with a neighbouring chain before it is terminated by an electron or some other neutralizing species. Mathematical analysis of this mechanism leads to the observed dose rate dependence.

Radiation Induced Stress Relaxation of Silica-Polydimethylsiloxane Composites

Abstract A study of G (scission) for silica filled dimethylsiloxane composites has been carried out by measurement of stress relaxation in a y-ray flux. The results show the same trend as those derived from a combination of sol-dose and swelling experiments. G (scission) is found to be strongly dependent on the weight fraction of filler in the composite, as well as on the particle size. A model involving reaction of physisorbed polymer chains with electrons and/or positive holes, generated within the silica by γ-radiation, is proposed to explain the increase in the scission yield. The model also explains the observation of a large negative activation energy on the basis of an increase in the rate of annihilation of the holes and electrons within the silica particles.