Polymer properties, such as their mechanical strength, barrier properties, and dielectric response, can be dramatically improved by the addition of nanoparticles. This improvement is thought to be because the surface area per unit mass of particles increases with decreasing particle size, R, as 1/R. This favorable effect has to be reconciled with the expectation that at small enough R the nanoparticles must behave akin to a solvent and cause a deterioration of properties. How does this transition in behavior from large solutes to the solvent limit occur? We conjecture that for small enough particles the layer of polymer affected by the particles (“bound” polymer layer) must be much smaller than that for large particles: the favorable effect of increasing particle surface area can thus be overcome and lead to the small solvent limit with unfavorable mechanical properties, for example. To substantiate this picture requires that we measure and compare the “bound polymer layer” formed on nanoparticles with those near large particles with equivalent chemistry. We have implemented a novel strategy to obtain uniform nanoparticle dispersion in polymers, a problem for many previous works. Then, by combining theory and a suite of experimental techniques, including differential scanning calorimetry and positron annihilation lifetime spectroscopy, we show that the immobilized poly(2-vinylpyridine) layer near 15 nm diameter silica particles (∼1 nm) is considerably thinner than that at flat silica surfaces (∼4 to 5 nm), which is the limit of an infinitely large particle. We have also determined that the changes in the polymer’s glass-transition temperature due to the presence of this strongly interacting surface are very small in both well-dispersed nanocomposites and thin films (<100 nm). Similarly, the polymer’s fragility, as determined by dielectric spectroscopy, is also found to be little affected in the nanocomposites relative to the pure polymer. While a systematic study of the dependence of the bound polymer layer thickness on particle size remains an outstanding challenge, this first study provides conclusive evidence for the hypothesis that the bound polymer layer can be significantly smaller around nanoparticles than at chemically similar flat surfaces.

The frequency-dependent shear moduli were measured over a range of temperature for polyisoprene (PI, M = 22 000 and 78 000), poly(vinylethylene) (PVE, M = 10 000 and 120 000), and their blends. The longest relaxation times of each component in the blends were extracted by a modified Tsenoglou mixing rule, and converted to monomeric friction factors via the reptation model. Dielectric relaxation and pulsed field gradient NMR were used to follow the terminal relaxation and diffusion, respectively, of a low molecular weight PI (M = 1300) in blends with perdeuterated PVE (M = 2300), for a range of temperature and composition. In this case monomeric friction factors were extracted via the Rouse model. The friction factors for PI via all three techniques agreed quantitatively, and the friction factors for both components were independent of molecular weight. Segmental dynamics of each component in PI/PVE blends by NMR methods, reported in the literature, could be compared directly with the terminal relaxation data. In all cases, the segmental and terminal dynamics exhibited equivalent dependences on temperature and composition. The predictions of the model of Lodge and McLeish were compared to the combined data, by calculating component self-concentrations and effective glass transition temperatures using a single temperature-independent length scale. As anticipated in the original model, this length scale is close to the Kuhn length of the particular component. In the case of PI, the model provides a quantitative description over the entire composition and temperature range studied. For PVE the agreement is not as quantitative, but it is still quite satisfactory.

Molecular dynamics and morphology in the blends of a network-forming reactive polymer and an amphiphilic block copolymer were examined as a function of the advancement of chemical reactions. In the blends containing a triblock copolymer, both microscopic (domains of the order of micrometers) and nanoscopic (domains of the order of nanometers) phase separations were observed during network formation. Interestingly, only nanoscopic phase separation was found in the blends containing a diblock copolymer. The shape and the origin of these nanoscopic features were investigated by atomic force microscopy and were found to be a function of blend composition. A concept was advanced of the three-phase nanostructured morphology that begins to form with self-assembly of one block and continues to develop during network formation in the postassembly stage. The changes in relaxation dynamics that accompany network formation were monitored by broad-band dielectric relaxation spectroscopy (DRS) and were shown to represent a signature of the morphological state of the blend. The ability of DRS to identify and deconvolute various relaxation processes during network formation and phase separation is noteworthy and should be exploited as means of monitoring and controlling the development of nanostructured morphology in these complex systems.

The reorientational dynamics of dipoles in poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends were investigated by dielectric spectroscopy. Measurements were performed over a wide range of temperature and frequency, and previously unavailable results are reported. Various relaxation processes were identified and their locations assigned to the different morphological regions in PVDF, PMMA, and PVDF/PMMA blends. The development of a relaxation process associated with the imperfections in the crystalline phase was recorded both in pure PVDF and in crystalline blends. An amorphous interphase and a liquid-like amorphous phase are present in pure PVDF, but their relaxation dynamics are dielectrically indistinguishable, giving rise to a single non-Arrhenius relaxation mechanism. In the crystalline blends, however, the interphase is devoid of PMMA, and its relaxation dynamics are readily distinguishable from those of the PVDF/PMMA miscible phase. Interestingly, the relaxation dynamics in the interphase were found to vary as a function of blend composition. Since the relaxation processes are governed by cooperativity, an explanation of our findings is offered in terms of the interactive nature of the relaxation process.