Tightly Bound PMMA on Silica Has Reduced Heat Capacities

The heat capacities of very small adsorbed amounts of poly(methyl methacrylate) on high-surface-area silica (Cab-O-Sil) were measured using temperature-modulated differential scanning calorimetry (TMDSC) using a quasi-isothermal method and interpreted via different models. The composition-dependent heat capacities of the adsorbed samples were measurably less than those predicted with a simple mixture model. A two-state model, composed of tightly and loosely bound polymer, fits the data better with heat capacities of the tightly bound polymer found to be 70-80% (glassy region) and 70-94% (rubbery region) of that of the bulk polymer at the same temperatures. The amount of tightly bound polymer was estimated to be about 1.2 mg/m² (about 1 nm thickness) in both the glassy and rubbery regions, consistent with heat flow measurements. The data sets were also extensive enough to model them with a more detailed layered gradient model, including a nonzero heat capacity for the polymer at zero adsorbed amount, which increased based on an exponential growth function to bulk polymer value of the heat capacity away from the surface. More importantly, this gradient model mimicked the experimental dependence on adsorbed amounts in the tightly bound adsorbed amount region (approximately 1 mg/m²). This model provided, for the first time, an experimental estimate of the heat capacity of the polymer adsorbed closest to the surface. The fractional heat capacity of the adsorbed polymer closest to the silica surface, relative to bulk polymer, increased with temperature from 0.3 (well below) to 0.8 (well above the bulk T_g). It was also possible to estimate the exponential growth parameter of the development from the initial heat capacities to the bulk heat capacity as 0.4 to 0.6 mg/m², identifying a distance scale (0.3 to 0.5 nm) consistent with the notion of a transition from tightly bound to loosely bound polymer.

Rational design of nanoparticle/monomer interfaces: a combined computational and experimental study of in situ polymerization of silica based nanocomposites

Methylmethacrylate monomers/silica nanoparticles interfaces are investigated using simulations and experiments. This allowed to understand and to control interfaces structures. On this basis, an improved in situ polymerization process is proposed.

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.

Effect of particle structure and surface chemistry on PMMA adsorption to silica nanoparticles

The interphase layer of polymers adsorbed to silica surfaces can be affected by the surface silanol density as well as the relative size of the polymer compared with the size of the adsorbing substrate. Here, the nonequilibrium adsorption of PMMA onto individual colloidal Stober silica (SiO(2)) particles, where R(particle) (100 nm) > R(PMMA) (approximately 6.5 nm) was compared with the adsorption onto fumed silica, where R(particle) (7 nm) approximately R(PMMA) (6.5 nm) < R(aggregate) (approximately 1000 nm), as a function of both silanol density [SiOH] and hydrophobility. In the former case, TEM images showed that the PMMA adsorbed onto individual nanoparticles, so that the number of PMMA chains/bead could be calculated, whereas in the latter case bridging of PMMA between aggregates occurred. The anchoring point densities were comparable to the silanol densities, suggesting that PMMA adsorbed as trains rather than loops. For hydrophilic SiO(2), T(g) increased with [SiOH], as more carbonyl groups hydrogen bonded to the silanols, and was independent of particle morphology. For methylated silica, (CH(3))(3)SiO(2), the adsorption isotherms were identical for colloidal and fumed silica, but T(g) was depressed for the former, and comparable to the bulk value for the latter. The increased T(g) of PMMA adsorbed onto fumed (CH(3))(3)SiO(2) was attributed to the larger loops formed by the bridging PMMA chains between the silica aggregates.