Effect of structure and plasticizer on the glass transition of adsorbed polymer
"The effect of molecular mass on adsorbed poly(methyl methacrylate) (PMMA) on silica was determined using temperature-modulated differential scanning calorimetry (TMDSC). A two-component model, based on loosely-bound polymer (glass transition temperature, Tg, similar to that of bulk) and tightly-bound polymer (Tg higher than that of bulk) was used to interpret the thermograms. Similar amounts of tightly-bound polymer were found for 450 and 85 kDa PMMA (0.78 mg/m²), while the 32 kDa sample had a smaller amount (0.48mg/m²). A comparison of adsorbed PMMA, poly(vinyl acetate) (PVAc), and poly(methyl acrylate) (PMA) on silica was made using the same technique. The amounts of tightly-bound PMMA (0.78 mg/m²), PVAc (0.78 mg/m²), and PMA (0.72 mg/m²), adsorbed on silica surfaces did not differ much. The ratios of heat capacity increments of the loosely- and tightly-bound components (ΔCpA/ΔCpB) at Tg, indicating the relative mobility of the two components, were also estimated to be about 1.8. The dynamics of the phenyl-deuterated plasticizer di(propylene glycol) dibenzoate in bulk and adsorbed PVAc on silica were probed using solid-state ²H NMR. The dynamics of this plasticizer in the plasticized-bulk polymer system were found to be heterogeneous. For adsorbed PVAc on silica, the dynamics of the plasticizer was found to be even more motionally heterogeneous than that observed in bulk samples. The NMR results provided solid evidence that, when there is only a small amount of adsorbed polymer (i.e., 0.8 mg/m²), the plasticizer has very little or almost no effect on the dynamics of that polymer. The plasticizer was observed to be effective at higher adsorbed amounts"--Abstract, page iv.
Missouri University of Science and Technology
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