MEASURING THE STABILITY OF POLYMER-GRAFTED NANOPARTICLES IN POLYMER MATRICES
Daniel Sunday1 and David Green2
1 Polymers Division, Material Measurement Laboratory, NIST, Gaithersburg MD
2 Chemical Engineering Department, University of Virginia, Charlottesville, VA
Integrating nanoparticles into a polymer matrix can generate materials which combine the best properties from both components. Many researchers have observed that the property improvement derived from the nanoparticles scales with decreasing particle size. Therefore it is desirable to integrate increasingly smaller nanoparticles into composite materials, but as the size decreases the driving force for aggregation increases. As a result, one of the challenges in the design of next generation materials is to optimize the distribution of nanoscale fillers in a polymeric matrix. A promising approach to tailoring composite morphologies is to graft polymer chains to the particle surface. When the grafted chains are chemically identical to the matrix chains the particle distribution is then determined by the ratio of the grafted chain length (N) to matrix chain length (P) and the graft density (σ).
In this study, polystyrene (PS) grafted silica nanoparticles were synthesized using controlled living polymerization techniques and integrated into PS matrices. The distribution of the particles in the PS matrices was evaluated with four separate techniques; small angle x-ray scattering (SAXS), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and rheological measurements. SAXS and TEM provided the most direct evaluation of the particle distribution, DSC providing information on the interactions between the grafted and matrix chains and rheology providing an indirect measure of the particle distribution over the largest sample volume of any of the techniques. Results from the different techniques were compared and used to produce a phase diagram. Additionally DSC measurements show the gradual expulsion of the matrix chains from the grafted chains through the second order autophobic transition, compared to the sharp, first order allophobic transition occurring at lower graft densities. Particle size was also varied in order to evaluate the impact of particle curvature on the phase diagram.