Disentangling and Lamellar Thickening of Linear Polymers during Crystallization: Simulation of Bimodal and Unimodal Molecular Weight Distribution Systems.

We have performed coarse-grained molecular dynamics simulations to study the isothermal crystallization of bimodal and unimodal molecular weight distribution (MWD) polymers with equivalent average molecular weight (Mw). By using primitive path analysis, we can monitor the entanglement evolution during the process of crystallization. We have discovered a quantitative correlation between the degree of disentanglement and crystallinity, indicating that chain disentanglement permits the process of crystallization. In addition, the crystalline stem length also displays a linear relation with the degree of disentanglement at different temperatures. Based on the observation in our simulations, we can build a scenario of the whole process of chain disentangling and lamellar thickening on the basis of chain sliding diffusion. Furthermore, we have enough evidence to infer that the temperature dependence of crystalline stem length is basically a result of temperature dependence of chain sliding diffusion. Our observations are also in agreement with Hikosaka's sliding diffusion theory. Compared to the unimodal system, the disentanglement degree of the bimodal system is more delayed than its crystallinity due to the slower chain sliding of the long-chain component; the bimodal system reaches a larger crystalline stem length at all temperatures due to the promotion of higher chain sliding mobility of the short-chain component.

Role of the Intercrystalline Tie Chains Network in the Mechanical Response of Semicrystalline Polymers.

We examine the microscopic origin of the tensile response in semicrystalline polymers by performing large-scale molecular dynamics simulations of various chain lengths. We investigate the microscopic rearrangements of the polymers during tensile deformation and show that the intercrystalline chain connections known as tie chains contribute significantly to the elastic and plastic response. These results suggest that the mechanical behavior of semicrystalline polymers is controlled by two interpenetrated networks of entanglements and tie chains.

Crystallization of finite-extensible nonlinear elastic Lennard-Jones coarse-grained polymers.

The ability of a simple coarse-grained finite-extensible nonlinear elastic (FENE) Lennard-Jones (LJ) polymer model to be crystallized is investigated by molecular dynamics simulations. The optimal FENE Lennard-Jones parameter combinations (ratio between FENE and LJ equilibrium distances) and the optimal lattice parameters are calculated for five different perfect crystallite structures: simple tetragonal, body-centered tetragonal, body-centered orthorhombic, hexagonal primitive, and hexagonal close packed. It was found that the most energetically favorable structure is the body-centered orthorhombic. Starting with an equilibrated polymer liquid and with the optimal parameters found for the body-centered orthorhombic, an isothermal treatment led to the formation of large lamellar crystallites with a typical chain topology: folded, loop, and tie chains, and with a crystallinity of about 60%-70%, similar to real semicrystalline polymers. This simple coarse-grained Lennard-Jones model provides a qualitative tool to study semicrystalline microstructures for polymers.

Correlation of structure and mechanical response in solid-like polymers.

Employing large scale molecular dynamics simulations, we measure the uniaxial tensile response of amorphous and semicrystalline states of a coarse-grained PVA bead-spring model. The response beyond the elastic limit encompasses strain-softening and strain-hardening regimes. To understand the underlying mechanisms of plastic deformation, we analyse conformational and structural changes of polymers. In particular, we characterise the volume distribution of crystalline domains along the stress-strain curve. The strain-softening regime in semicrystalline samples is dominated by deformation of crystalline parts, while strain-hardening involves unfolding and alignment of chains in both amorphous and crystalline parts. Comparing the tensile response of semicrystalline and amorphous polymers, we find similar conformations of polymers for both systems in the strain-hardening regime.

Plastic Deformation Mechanisms of Semicrystalline and Amorphous Polymers.

We use large-scale molecular dynamics simulations to investigate plastic deformation of semicrystalline polymers with randomly nucleated crystallites. The strain-softening regime is dominated by deformation of crystallites via reorientation of chain-folded lamellae toward the tensile axis, fragmentation of largest crystalline domains, and a partial loss of crystallinity. The strain-hardening regime coincides with unfolding of chains and recrystallization as a result of strain-induced chain alignment. These observed deformation mechanisms are consistent with experimental findings. We compare the tensile behavior of semicrystalline polymers with their amorphous counterparts at temperatures above and below the glass transition temperature.

Nanoscale buckling deformation in layered copolymer materials.

In layered materials, a common mode of deformation involves buckling of the layers under tensile deformation in the direction perpendicular to the layers. The instability mechanism, which operates in elastic materials from geological to nanometer scales, involves the elastic contrast between different layers. In a regular stacking of "hard" and "soft" layers, the tensile stress is first accommodated by a large deformation of the soft layers. The inhibited Poisson contraction results in a compressive stress in the direction transverse to the tensile deformation axis. The hard layers sustain this transverse compression until buckling takes place and results in an undulated structure. Using molecular simulations, we demonstrate this scenario for a material made of triblock copolymers. The buckling deformation is observed to take place at the nanoscale, at a wavelength that depends on strain rate. In contrast to what is commonly assumed, the wavelength of the undulation is not determined by defects in the microstructure. Rather, it results from kinetic effects, with a competition between the rate of strain and the growth rate of the instability.

Mechanical testing of glassy and rubbery polymers in numerical simulations: role of boundary conditions in tensile stress experiments.

We use coarse-grained molecular dynamics simulations to perform tensile test deformation on glassy and rubbery polymer samples using two types of driving for the deformation. We compare the outcome from a standard homogeneous deformation procedure with that of a boundary driven procedure in which the sample is driven by the nanometric equivalent of grips. No significant difference is observed in both uniaxial and triaxial tensile experiments. Implications for testing the behavior of nonhomogeneous polymer materials are briefly discussed.

Polymer chain generation for coarse-grained models using radical-like polymerization.

A versatile method is proposed to generate configurations of coarse-grained models for polymer melts. This method, largely inspired by chemical "radical polymerization," is divided in three stages: (i) nucleation of radicals (reacting molecules caching monomers), (ii) growth of chains within a solvent of monomers and (iii) termination: annihilation of radicals and removal of residual monomers. The main interest of this method is that relaxation is performed while chains are generated. Pure mono and polydisperse polymer melts are generated and compared to the configurations generated by the push off method from Auhl et al. [J. Chem. Phys. 119, 12718 (2003)]. A detailed study of the static properties (radius of gyration, mean square internal distance, entanglement length) confirms that the radical-like polymerization technique is suitable to generate equilibrated melts. Moreover, the method is flexible and can be adapted to generate nanostructured polymers, namely, diblock and triblock copolymers.