No abstract

By the use of small angle neutron scattering, the chain conformation in a polymer nanocomposite was studied as a function of the nanoparticle fraction for two different molecular weights. A repulsive system was realized in mixing poly(ethylene-propylene) and hydrophobically modified silica. All nanocomposite SANS data exhibit a pronounced scattering peak at intermediate momentum transfers, which is closely connected with the particle structure. Even under contrast matching conditions, the hydrophobic surface layer of the nanoparticles was found to contribute significantly to the scattering signal. In particular, in the short chain matrix the peak origin is exclusively related to direct particle scattering. In the long chain matrix, an additional peak contribution is present. Possible origins include void correlation scattering or polymer correlation scattering. We show unambiguously that the conformation of short chains with a molecular weight of 3000 g/mol is not visibly disturbed by the presence of the nanoparticles. In contrast to that a polymer matrix with 50 000 g/mol chains is affected by the particle presence. The chain radius of gyration R g decreases.

The rheological behavior of poly(ethylene-alt-propylene) (PEP), polydimethylsiloxane (PDMS) and polyisoprene (PI, two molecular weights: 70k and 200k) melts containing polyhedral oligomeric silsesquioxane (POSS) molecules was investigated by means of small angle scattering (SAXS and SANS) and oscillatory shear rheology. The dependence of the nanocomposite viscosity on the polymer–particle solubility and polymer molecular weight was studied. At high filler fractions all polymers exhibited hydrodynamic reinforcement of the plateau modulus quantified by a Guth Gold relation. Additionally, the PEP and PI70k systems showed a transition from liquid-like to solid-like rheological behavior. SAXS results enabled us to relate this behavior to the formation of a POSS network or POSS particle jamming, respectively. At low filler degrees, the zero shear viscosity of the nanocomposites was strongly dependent on the polymer solubility and entanglement number. We observed a viscosity decrease in the filled PDMS and PI200k samples, a constant viscosity in the PEP samples and regular reinforcement in the PI70k samples. These results are compared to the predictions of regular plasticization, and then quantitatively discussed in terms of the recently proposed model by Ganesan and Pyramitsyn, as well as the model by Wang and Hill. In particular the latter is shown to constitute a suitable means to quantify the results in terms of a layer of reduced polymer viscosity surrounding each nanoparticle.

We studied unentangled poly(ethylene-alt-propylene) (PEP) in a composite with hydrophobic silica particles as a function of the filler concentration. Our neutron spin echo (NSE) experiments cover both the internal dynamics as well as the center of mass diffusion beyond the Rouse time. The key experimental results are (i) all of the chains are equally mobile, (ii) the basic segmental (Rouse) relaxation rate is unaffected even at highest filler concentrations, and (iii) apparently the obstacles reduce significantly the translational center of mass motion. This happens, even in the case when the particles do not significantly confine the polymer. (iv) A transition from regular to anomalous diffusion in the Rouse regime at the highest particle fractions is clearly evidenced. In order to understand the microscopic mechanisms underlying the experimental observations, we performed coarse grained simulations. We demonstrate that the geometrical confinement only affects the dynamics at a long time scale outside the experimental window and therefore it is not able to explain the results found in the NSE experiments. The consideration of inter-chain interactions, however, results in a significant influence even at shorter times and a quantitative agreement between the experiments and simulations was found. The simulations clearly demonstrate that the interfaces cause a deceleration of the chains in their close vicinity. Then the interchain interactions carry this slowing down to the other chains at a time-scale of the Rouse relaxation time. Hence, in the experimental datasets an overall slowing down is observed.

By means of rheology and neutron spin echo spectroscopy, both a macroscopic and a microscopic view on the relaxation dynamics of polymers in the presence of non-attractive nanoparticles could be realized. The results enabled us to show a direct correlation between the microscopically observed confinement length and the macroscopically measured dissipation maximum, which corresponds to the terminal relaxation time. Moreover, the impact of nanoparticle presence on the effectiveness of polymer contour length fluctuations and constraint release is discussed.