To understand, at a molecular scale, the effect of water on the structure of the amorphous region of polyamide 6,6 (PA6,6), atomistic molecular dynamics simulations have been carried out. Our results concerning the very local water organization relative to PA moieties agree qualitatively with a two-step sorption model. The first sorption mode seems to be saturated well below the lowest water concentration studied (5% relative to the amorphous phase). Moreover, above this saturation, the overall water organization displays at 300 K larger clusters than the 2−3 molecules usually assumed in the literature. The temperature dependence of free volume, hole size, and hydrogen bonding has also been investigated. It shows a transition between plasticized and antiplasticized behavior.

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 χ, 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 χ parameters, in good agreement with results reported by others. The χ 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 χ 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 a specific 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.

We consider the microscopic mechanisms of damaging in plasticized cellulose acetate under tensile stress. We show how they appear and develop during the course of deformation until failure. By using scanning transmission electron microscopy, we observe the presence of cavities and the coexistence of homogeneous and fibrillar crazes. Ultrasmall-angle X-ray scattering experiments allow for describing the onset of damaging and the growth of crazes and for measuring the volume fractions of damages as well as their shapes and sizes at different stages during deformation. We propose that damages are initiated by the nucleation of cavities in the vicinity of pre-existing impurities. We then show that their initial growth after nucleation is blocked at a size of order 100 nm by strain hardening in their immediate vicinity where deformation and stress are amplified. Increasing the stress further leads to a new growth regime for a small fraction of crazes. Ultimate failure is due to the propagation of this small number of crazes and not to the accumulation of crazes and a coalescence process. We propose that this growth process is the consequence of homogeneous nucleation of new cavities just in front of existing ones. The volume fractions of damage remain very low, of order of 10–4 at failure. The strain hardening behavior appears to be the key for preventing an early failure of the material and conferring high ductility to the material.

The interaction between cellulosic material and benzophenone was studied by molecular modeling. A model of the crystalline part of a native microfibril was built from previously published coordinates of the I allomorph. This model presents three faces, namely (200), (110), and (11 h0), of about the same surface area. The energetical and geometrical characteristics of the benzophenone adsorption onto this microfibril were studied with a Monte Carlo protocol. It was shown that the interaction does occur on the three faces and was stabilized by both van der Waals and electrostatic forces. On the hydrophobic (200) face, a large number of interacting sites without specific geometry were sampled by the adsorbing molecule. The hydrophilic surfaces, (110) and (11 h0), also have many interaction sites, but in contrast, the orientation of the adsorbed molecules is more strict. These two hydrophilic surfaces display equivalent behavior. Three surfaces (crystalline (11 h0) and (200) and amorphous) subjected to periodic boundary conditions were also generated to study the process of the benzophenone monolayer formation. The calculated data showed that locally the amorphous surface displayed very favorable topology for benzophenone adsorption in which both van der Waals and electrostatic interactions were maximized. After fulfillment of these optimal sites, the amorphous surface behaves like the crystalline surfaces for which the adsorption sites are nonspecific. Finally, the interface between cellulose/benzophenone monolayer and water was studied by molecular dynamics. The density profiles showed that the benzophenone molecules penetrated the amorphous phase while they remained at the surface in the crystalline models.