Restricted Dynamics in Polymer-Filler Systems
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Restricted Dynamics in Polymer-Filler Systems V. Arrighi, S. Gagliardi and Julia S. Higgins1 Department of Chemistry, Heriot Watt University, Edinburgh, UK 1 Imperial College, Dept of Chemical Engineering, UK
ABSTRACT Composites in which fibres or fillers are incorporated into a polymeric component exhibit improved mechanical strength compared to the polymer matrix. This reinforcement effect strongly depends on the properties of the interphase and the specific interactions between the polymer and the reinforcing additive. A wide range of experimental methods have been used to assess the effect of active fillers on the mobility of the polymer chains. The experimental results from NMR, dielectric spectroscopy and dynamical thermal analysis reveal that the mobility of chain units adjacent to the adsorbed surface differs considerably from the bulk. We have used quasielastic neutron scattering to investigate the dynamic properties of poly(dimethyl siloxane) (PDMS) filled with silica particles. This technique which probes the motion of the hydrogen atoms has been extensively used to study the local dynamics of polymeric materials. In this paper we show that QENS provides detailed information on the reduced mobility of chain segments in polymer-filler systems. QENS measurements were carried out on PDMS filled with hydrophilic Aerosil with different specific surface area (average diameter 7 and 20 nm). Detailed data analysis indicates that the QENS spectra of the polymer-filler composites can be described by the sum of two contributions: (a) a quasielastic component due to chains not affected by the presence of the fillers and (b) an elastic term from those chain segments strongly affected by the presence of fillers. The latter depends on the specific surface area of the particles and their weight fraction in the composite.
INTRODUCTION In the last few decades neutron scattering has emerged as an indispensable tool to study the dynamics of glass forming materials [1-3]. In order to explain the macroscopic properties of polymers an understanding of motions from the single bond length to the whole chain and beyond is necessary. These different length scales, from Å to hundreds of nanometers, are associated with very different dynamics which take place in the scale ranging from picoseconds to macroscopic times. Neutron energies (0.1-10 meV) are suitable to investigate energy transfer from µeV up to 200 meV and this corresponds to molecular motions ranging from molecular rotation, through vibration, to centre-of-mass translation. Various techniques such as Inelastic Neutron Scattering, Quasi Elastic Neutron Scattering and Neutron Spin-Echo make it possible to cover a frequency range and length scale, not otherwise accessible by conventional techniques, such as light scattering or NMR. KK4.4.1
In this study we demonstrate that neutron scattering is suitable to study the dynamics in composites. The reinforcement effect provided by the addition of fibres or fillers to a polymeric component depends both on the properties of the
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