Linear viscoelasticity and thermal degradation of hybrid nanocomposites: isotactic polypropylene reinforced with carbon nanofibres

Observations are reported in isothermal torsional oscillation tests on melts of isotactic polypropylene reinforced with carbon nanofibres. Prior to rheological tests, specimens were annealed at various temperatures ranging from Ta = 190 to 310 Cfor various amounts of time (from 15 to 120 min). Degradation of the melts driven by thermal treatment was observed as a pronounced reduction in their molecular weights. To describe the kinetics of thermal degradation, a nanocomposite melt is treated as an equivalent network of chains with attached side-groups. The presence of nanofiller is accounted for in terms of coefficients in the evolution equations only. Degradation of the network is thought of as a combination of two processes: (i) binary scissionof chains, and (ii) detachment of side-groups from the backbone and their subsequent annihilation. An integro-differential equation is developed for the concentration of chains with various lengths of backbone and various amounts of sidegroups per segment, and its explicit solution is derived. With reference to the concept of transient networks, constitutive equations are developed for the viscoelastic behavior of nanocomposite melts. A melt is treated as an equivalent network of strands bridged by junctions. The time-dependent response of the network is modelled as separation of active strands from and merging of dangling strands with temporary nodes. The stress-strain relations involve three adjustable parameters that are determined by matching the dependencies of storage and loss moduli on frequency of oscillations. The study focuses on the effect of molecular weight of the host matrix on the material constants in the constitutive equations.