The role of polymer phase on rigidity percolation of ABS−spherical nickel microparticles composites

Abstract

Percolation and rigidity transitions are very important phenomena in random heterogeneous microstructures, as various physical properties change abruptly at some critical volume fractions of one phase. The concepts of percolation and rigidity transitions are critical in various fields of sciences, which deal with the formation of networks in a structure. Significant efforts have been made to address the theory of percolation and rigidity transitions due to the universality of these transitions. Percolation and rigidity transitions are characterized by two parameters: exponents and the thresholds. Exponents are the measures of the sharpness of the transitions and thresholds are the critical volume fractions, where these transitions take place. Exponents are often regarded universal, as theoretically exponents are independent of geometrical details of a random microstructure. However, thresholds strongly depend on geometrical parameters. Metal-polymer particulate composites are good systems to study percolation and rigidity thresholds, as metals and polymers have contrasting electrical and mechanical properties. The electrical resistivity of these composites is expected to decrease drastically at the percolation threshold, as a result of point contacts between the metal particles in a continuous network of metal particles. A rigidity threshold leads to a significant increment in mechanical strength and modulus, as a result of the networks of metal particles supporting load. The rigidity threshold is greater than percolation threshold, as connectivity is a necessary but not a sufficient criterion for rigidity. Rigidity transition in a particulate metal-polymer composite is the focus in the present study. In the present study composites of ABS polymer filled with nickel particles of diameter 10 μm was prepared by hot compression moulding. Composites were prepared for various volume fractions, in 0.1 volume fraction increments till a maximum 0.6 volume fraction of nickel. Electrical resistivity and compression tests were carried out at 25 C for all composites, and the pure ABS. Percolation threshold was found to be ~ 0.4, as indicated by a drastic drop in electrical resistivity. Rigidity threshold was found to be ~ 0.6 as indicated by significant increment in Young's moduli and compressive strengths. Rigidity transitions have been found in various systems in the past. The numerical value of the exponent is used to explain various load transferring mechanisms, although a clear picture is far from being achieved. The rigidity transition in a particulate polymer composite can happen by formation of load bearing network of rigid particles. Soft polymer phase plays important role in the rigidity transition. The polymer present adjacent to particles in the composites behaves differently compared to the bulk, depending on the chemical and physical interactions with the particles surface. Polymer in this region is called interphase. It was reported in past that presence of an immobilized interphase region at the polymer-particle interface can enhance the reinforcement effect. However, the role of interphase is expected to be insignificant for microparticles composites compared to the nanocomposites. The present study primarily emphasizes on the composite of 0.6 volume fraction of nickel (ABS-0.6 nickel), which is beyond the rigidity threshold. It was demonstrated that under compressive loading, the soft polymer phase can play an important role, which does not come necessarily from an interphase region. The importance of the polymer phase is indicated by a significant difference in the compressive strength of ABS-0.6 nickel between room temperature and a temperature (120 C) above glass transition of the ABS. The glass transition temperature of ABS is 110 C. Differential scanning calorimetry (DSC) indicated that the glass transition temperature (Tg) of ABS-0.6 nickel is same as the pure ABS, suggesting an insignificant effect of immobilized polymer. The difference in the deformation mechanism is investigated with the help of digital image correlation (DIC). In situ compression tests were carried out to study the state of stress in the nickel phase. In situ compression test of the ABS-0.6 nickel composite at 25 C by synchrotron x-ray diffraction shows that nickel particles are in a compressive state of stress. The strain distribution on the surface of the composite, as indicated in the DIC results, and the high compressive stress in the nickel phase shown in situ x-ray results suggest the presence of force chains, which carries compressive load through a network of nickel particles. The formation of force chain was supported by the results of confined compression test, which shows pressure sensitivity of the ABS-0.6 nickel composite. Formation of force chains is indicated by the ratio of radial to axial stress < 1 in confined compression test. Poisson's ratio measurements by DIC show that the ABS-0.6 nickel composite is under volume contraction in a compression test at the linear and strain hardening stage of the true stress-strain curve. The polymer phase present in the composite experiences volume contraction, as the volume of the composite reduces under uniaxial compression. The ABS behaves like a perfectly plastic material, as demonstrated by confined compression tests. Being a perfectly plastic material, the polymer phase exerts reaction stress in a response to the volume reduction of the polymer phase. It is suggested in this study that the reaction force exerted by the polymer phase prevents the force chains from buckling at 25 C. Compression tests were carried out at 120 C to study the effect of polymer properties on the strength of the ABS-0.6 nickel composites. DIC results of the compression tests show marked difference in strain distribution on a surface of ABS-0.6 nickel at 25 C and 120 C. The DIC results show significant strain heterogeneity at 25 C. The strain heterogeneity is an indication of force chain formation. However, the strain distribution at 120 C shows homogeneous deformation till the peak stress is reached. Shear banding was observed beyond the peak stress, a result of possible buckling of the particle networks. ABS does not behave as a perfectly plastic material at 120 C, as it shows pressure sensitivity indicated by confined compression test. Thus, the reaction stress to the volume contraction is significantly lower. Therefore, the polymer does not prevent the particle network from buckling at 120 C, indicated by a substantial drop in overall compressive strength.