AN NMR-STUDY OF PLASTICIZATION AND ANTIPLASTICIZATION OF A POLYMERIC GLASS

Films of glassy polycarbonate (BPA-PC) containing relatively low concentration of the diluent di-n-butyl phthalate were studied in two series of NMR experiments. In the first series proton T1ρ measurements were made on polycarbonate with deuterated methyl groups containing perdeuterated diluent up to a concentration of 25 wt %. Under these circumstances the proton relaxation times monitor the π flips of the phenylene groups that is related to the principle sub-glass transition mechanical loss. As the phthalate diluent is first added, the modulus increases slightly and the loss peak is suppressed. The T1ρ at 44 kHz data show a minimum at about -40 °C which corresponds to the mechanical loss at 1 Hz and a temperature of -100 °C. The T1ρ minimum is changed by the addition of diluent in a selective manner. At a diluent concentration of 5 wt % the low-temperature part of the minimum corresponding to the most mobile phenylene groups is suppressed. The remaining minimum is narrower and is shifted slightly to higher temperatures by the removal of the component of the distribution of relaxation times which leads to the low-temperature side. As the concentration of diluent is raised to 10 and 25 wt %, a second lower temperature minimum appears. This minimum is attributed to enhanced motion of repeat units in contact with microscopic clusters of diluent. A lattice model is presented which accounts for suppression of motion at low concentrations and the appearance of the second minimum at higher concentrations. Suppression is associated with improved packing caused by the presence of an isolated diluent molecule in contact with a polymeric repeat unit while enhanced mobility is caused by the presence of a microscopic diluent cluster in contact with a repeat unit. This same lattice model is used to reinterpret a carbon-13 spin diffusion study of the same diluent-polymer system where one ester group on the diluent is labeled. The lattice interpretation of the spin diffusion data reinforces the earlier interpretation in terms of a preferred spatial location of the diluent relative to the polymeric repeat unit. The new interpretation also indicates an increased rate of spin diffusion per diluent molecule at low concentrations, which indicates stronger interactions with the polymer in this regime, in agreement with the suppression of motion observed in the proton T1ρ data. © 1990, American Chemical Society. All rights reserved.