[SY-J3] Multiscale molecular-dynamics simulations of structure and mechanics of polymer nanocomposites
Using multiscale modelling approach we performed molecular-dynamics simulations of both coarse-grained and detailed atomistic (polyimide R-BAPB) amorphous polymer melts consisting of non-entangled, non-crosslinked polymer chains. The inorganic filler surfaces were mimicked either by solid walls, or by explicit insertion of the filler particles. The rise in the glass-transition temperature with increase of the filler fraction was accompanied by a monotonic slowing-down of the relaxation of the incoherent scattering function on all simulated length scales. The filler surface roughness could also lead to slower segmental relaxation. Higher dynamic fragility was observed for smaller film thicknesses.
The cyclic shear deformation is performed to characterize macroscopic properties of the systems before and after filler insertion. The reported coarse-grained simulations show a strong decrease of the nanocomposites storage modulus with increasing strain amplitude, which is accompanied by a maximum in the loss modulus (the so-called Payne effect); the onset of the softening is observed in the linear regime of deformation at strain amplitude of about 0.01. Moreover, the dependence of the storage modulus on the instantaneous strain exhibits both softening and hardening regimes, in agreement with Large Amplitude Oscillatory Shear (LAOS) experiments. The observed hardening is caused by the shear-induced decrease of the non-affine diffusion of the polymer segments due to filler particles acting as effective crosslinks between polymeric chains and, hence, hindering diffusion. Moreover, the formation of glassy immobile layers at the nanoparticle interface strongly increases the storage modulus at low strain amplitudes. The strain softening with increasing strain amplitude is connected to the mobilization of these glassy layers and an increase in the dynamic heterogeneity of the polymer matrix.
The cyclic shear deformation is performed to characterize macroscopic properties of the systems before and after filler insertion. The reported coarse-grained simulations show a strong decrease of the nanocomposites storage modulus with increasing strain amplitude, which is accompanied by a maximum in the loss modulus (the so-called Payne effect); the onset of the softening is observed in the linear regime of deformation at strain amplitude of about 0.01. Moreover, the dependence of the storage modulus on the instantaneous strain exhibits both softening and hardening regimes, in agreement with Large Amplitude Oscillatory Shear (LAOS) experiments. The observed hardening is caused by the shear-induced decrease of the non-affine diffusion of the polymer segments due to filler particles acting as effective crosslinks between polymeric chains and, hence, hindering diffusion. Moreover, the formation of glassy immobile layers at the nanoparticle interface strongly increases the storage modulus at low strain amplitudes. The strain softening with increasing strain amplitude is connected to the mobilization of these glassy layers and an increase in the dynamic heterogeneity of the polymer matrix.