Molecular dynamics simulations of polymers reinforced with nanoscopic filler particles reveal the mechanisms by which nanofillers improve the toughness of the material. We find that the mobility of the nanofiller particle, rather than its surface area, controls its ability to dissipate energy. Our results show similarities between the toughening mechanisms observed in polymer nanocomposites and those postulated for biological structural materials such as spider silk and abalone adhesive.
We use self-consistent mean field methods and analytical theory to determine the behavior of AB copolymers at the interface between two incompatible homopolymers, A and B. We calculate the reduction in interfacial tension, γ, resulting from the copolymers localizing at the A/B interface. We examine the effects of chain length, composition, and molecular architecture on the efficiency of the copolymers. In particular, we compare the interfacial behavior of different linear copolymers (random, alternating, and diblock) and various branched copolymers (stars and combs). At fixed molecular weight, the diblock copolymers are the most efficient at reducing γ. However, when we compare random and comb copolymers with diblocks at different molecular weights, we observe that the longer random or comb copolymers are more efficient than short diblocks. These studies allow us to predict the reduction in interfacial tension produced by a wide variety of copolymers and, thereby, permit a rational design of cost-effective and efficient compatibilizers.
Strong dependence of the crystal orientation, morphology, and melting temperature (Tm) on the substrate is observed in the semicrystalline polyethylene thin films. The Tm decreases with the film thickness decrease when the film is thinner than a certain critical thickness, and the magnitude of the depression increases with increasing surface interaction. We attribute the large Tm depression to the decrease in the overall free energy on melting, which is caused by the substrate attraction force to the chains that competes against the interchain force which drives the chains to crystallization.
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