Segmental dynamics in poly(methyl acrylate) on silica: effect of surface treatment.

The effect of surface treatment on the dynamics of adsorbed poly(methyl acrylate) (PMA) was studied using deuterium NMR and temperature-modulated differential scanning calorimetry (TMDSC). The solid-state deuterium NMR experiments were performed using PMA-d(3), deuterated on the methyl group. The line shape changes for PMA-d(3) were followed as a function of temperature and compared for the polymer on untreated silica, organically modified (treated) silica (reacted with hexamethyltrisilazane), and in bulk. The dynamics of PMA-d(3) on treated silica was found to be intermediate between that of the polymer adsorbed on untreated silica and that of the bulk polymer, i.e., the treated silica caused a restriction on the dynamics of the polymer as compared to bulk, but not as dramatically as that on untreated silica. Similar to the dynamics on untreated silica, the dynamics on treated silica showed a broad heterogeneity with a superposition of more-mobile and less-mobile components. Two molecular mass samples were also studied (38 and 77 kDa) with the molecular mass dependence on the treated or untreated silica being weaker than that in bulk. The TMDSC thermograms of the samples were consistent with the NMR results, with the glass transition region for the PMA-d(3) on the treated silica being in between that of the bulk and that on the untreated silica.

Segmental dynamics of poly(methyl acrylate)-d3 adsorbed on anopore: a deuterium NMR study.

The segmental dynamics of poly(methyl acrylate-d3) (PMA-d3) adsorbed in the pores of anopore membranes has been investigated using deuterium NMR over the temperature range 25-80 degrees C. The onset of the NMR glass-transition temperature (Tg) for the adsorbed samples was approximately 15 degrees C higher than that for the bulk sample. The adsorbed polymer contained segments with restricted mobility (glassy), even at the highest temperatures studied, at which the bulk polymer showed only mobile segments. The spectra from samples with different adsorbed amounts of PMA-d3, between 1.1 and 4.2 mg/m2, were similar in their temperature-dependent mobilities. Neither was there much difference in the spectra of PMA-d3 on anopore samples with pore sizes of 0.2 and 0.02 microm. However, for a solvent-washed sample with an adsorbed amount of 0.7 mg/m2, additional restriction in PMA-d3 mobility was observed.

Segmental dynamics in poly(methyl acrylate) on silica: molecular-mass effects.

The effect of molecular mass on the segmental dynamics of poly(methyl acrylate) (PMA) adsorbed on silica was studied using deuterium quadrupole-echo nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry. Samples adsorbed on silica (all about 1.5 mg PMA/m2 silica) were shown to have more restricted segmental mobility, and higher Tg's, than the corresponding bulk PMA samples. Around the glass-transition region, adsorbed samples exhibited segmental mobility, which could be classified as heterogeneous due to a superposition of more-mobile and less-mobile components present in the deuterium NMR spectra. This heterogeneity was consistent with a motional gradient with more-mobile segments near the polymer-air interface and the less-mobile species near the polymer-silica interface. The mobility of the adsorbed 77 kDa PMA sample was the lowest among the four different molecular-mass samples studied. Samples studied with masses both larger and smaller than 77 kDa had larger mobile-component fractions in the adsorbed polymer. The additional mobility was attributed to the presence of either longer tail and loop conformations in the higher molecular-mass samples or the inherent mobility of the tails in the lower molecular-mass samples on the surface.

Molecular mass and dynamics of poly(methyl acrylate) in the glass-transition region.

The segmental dynamics of bulk poly(methyl acrylate) (PMA) were studied as a function of molecular mass in the glass-transition region using 2H NMR and modulated differential scanning calorimetry (MDSC). Quadrupole-echo 2H NMR spectra were obtained for four samples of methyl-deuterated PMA-d3 with different molecular masses. The resulting spectra were fit using superpositions of simulated spectra generated from the MXQET simulation program, based on a model incorporating nearest-neighbor jumps from positions on the vertices of a truncated icosahedron (soccer-ball shape). The lower molecular-mass samples, influenced by the presence of more chain ends, showed more heterogeneity (broader distribution) and lower glass transitions than the higher molecular-mass samples. The MDSC experiments on both protonated and deuterated samples showed behavior consistent with the NMR results, but temperature shifted due to the different frequency range of the measurements in terms of both the position and breadth of the glass transition as a function of molecular mass.