Morphological and mechanical properties of semicrystalline polymers are strongly influenced by flow-induced crystallization during processing. We perform extensive molecular dynamics simulations with more than 1 million atoms to describe orientation, drawability, and crystallization of entangled polyethylene melts under uniaxial tensions at three different strain rates and after a subsequent cooling. During tensile deformation at the lowest strain rate of 10 7 s −1 , the polyethylene melt experiences entanglement loss and crystal nucleation. At higher strain rates of 10 8 and 10 9 s −1 , we observe a higher degree of chain alignment and void formation in addition to disentanglement and crystal nucleation. Chain segments make sharp turns relative to the neighboring chain orientations at the entanglement points, which manifests as a bimodal distribution of the local order parameter. Upon cooling below the melting temperature, semicrystalline polyethylene with a crystallinity close to 50% is formed. The entanglements are located in the amorphous regions of the semicrystalline polyethylene, with some located in the crystal/amorphous interface region. The chain ends of the semicrystalline polyethylene are preferentially localized at the crystal/amorphous interface, which is in agreement with recent experimental results.
Cellulose nanofibrils (CNFs) are a class of cellulosic nanomaterials with high aspect ratios that can be extracted from various natural sources. Their highly crystalline structures provide the nanofibrils with excellent mechanical and thermal properties. The main challenges of CNFs in nanocomposite applications are associated with their high hydrophilicity, which makes CNFs incompatible with hydrophobic polymers. In this study, highly transparent and toughened poly(methyl methacrylate) (PMMA) nanocomposite films were prepared using various percentages of CNFs covered with surface carboxylic acid groups (CNF-COOH). The surface groups make the CNFs interfacial interaction with PMMA favorable, which facilitate the homogeneous dispersion of the hydrophilic nanofibrils in the hydrophobic polymer and the formation of a percolated network of nanofibrils. The controlled dispersion results in high transparency of the nanocomposites. Mechanical analysis of the resulting films demonstrated that a low percentage loading of CNF-COOH worked as effective reinforcing agents, yielding more ductile and therefore tougher films than the neat PMMA film. Toughening mechanisms were investigated through coarse-grained simulations, where the results demonstrated that a favorable polymer-nanofibril interface together with percolation of the nanofibrils, both facilitated through hydrogen bonding interactions, contributed to the toughness improvement in these nanocomposites.
The morphological and mechanical properties of entangled ABA triblock copolymer gels, where solvent were selective to the midblock, were studied as a function of polymer concentration using a novel dissipative particle dynamics model which includes a modified segmental repulsive potential that restricts chain crossing. Morphological properties, such as micelle size, distance between micelles, and the bridge fraction, were calculated as a function of concentration. Although the micelle size was shown to have a strong dependence on concentration, the bridge fraction and distance between micelles were shown to plateau at moderate concentrations. Deformation under uni-axial tension was also performed to extract the cross-link and entanglement contribution to the elastic modulus. Scaling results qualitatively agree with other theoretical predications.
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