Segmental Dynamics of Bulk Poly(vinyl acetate)-d3 by Solid-State 2H NMR: Effect of Small Molecule Plasticizer

The effect of dipropyleneglycol dibenzoate, a plasticizer, on the glass-transition temperature (T-g) of poly(vinyl acetate) was studied using deuterium solid-state NMR and modulated differential scanning calorimetry (MDSC) from 0 to 20% plasticizer content. Quadrupole-echo H-2 NMR spectra were obtained for methyl deuterated PVAc-d(3) samples with different plasticized amounts. The T-g's of different plasticized samples were determined from NMR as the temperatures at which the deuterium powder patterns collapsed. It was found that the T-g's decreased by approximately 6 degrees C for every 5% increment in the plasticizer content and that the trends in the NMR-determined T-g's, that is, T-g(NMR), were consistent with those determined by modulated differential scanning calorimetry (MDSC). The T-g(NMR) values were about 36 degrees C above those of the T-g(DSC) values. This difference in the T,'s was due to the different time scales of the two experiments which could be accounted for on the basis of time-temperature superposition principles. The experimental NMR line shapes were fitted using a set of simulated spectra generated from the MXQET Simulation program. The spectra were based on a model of nearest-neighbor jumps on a truncated icosahedron (soccer ball). The resulting average correlation times were also found to fit a time-temperature superposition with the same parameter. While the T-g was decreased by the amount of plasticizer, it was found that the breadth of the transitions from either the NMR line shapes or the MDSC thermograms did not seem to change much with the amount of added plasticizer.