A molecular modeling study of binary blend compatibility of polyamide 6 and poly(vinyl acetate) with different degrees of hydrolysis: An atomistic and mesoscopic approach

The binary blend compatibility of polyamide 6 (PA6) with poly(vinyl alcohol) (PVOH), poly(vinyl acetate) (PVAC), and partially hydrolyzed PVAC was studied for a wide range of compositions, by atomistic and mesoscopic modeling. The Flory-Huggins interaction parameter chi, calculated for these mixtures by atomistic modeling, showed that favorable interactions develop for PVAC with a low hydrolysis degree for a specific composition and also for compositions rich in either component. The effect of the PVAC hydrolysis degree on the mixture compatibility was explained in terms of the reduced ability of the acetylated chains to form hydrogen bonds, which in turn, may result in weaker intramolecular interactions. Such an effect may also be due to the more extended conformations assumed by these chains because of the bulky side groups. Calculations at high temperature gave small negative chi parameters, in good agreement with results reported by others. The chi and other structure-dependent parameters, derived from the atomistic level, were supplied to coarse-grained (mesoscopic) simulations. The length and time scales spanned by these simulations were relevant to real application scales. The kinetics of phase separation were examined via the structure factor and the growth rate of the domain size. Larger domain sizes were observed for the less hydrolyzed mixtures due to smaller chi parameters which implied higher temperature simulations for these mixtures. The exponent of the domain growth for the symmetric 1/1 and asymmetric 2/1 mixtures was found in the range 0.2-0.28 irrespective of time step. At equilibrium, the degree of order of the phases formed was extracted. This was found to decrease as the hydrolysis degree of PVAC decreased for aspecific composition, indicating improved compatibility, in line with internal experimental findings and with results reported in the literature. The incompatible mixtures showed macrophase separation with a wide density spectrum. Compatibility was obtained for all PVAC/PA6 mixtures and also for the mixture of PVAC with 75% degree of hydrolysis for the composition 1/3 v/v, rich in PA6.