Growth, microstructure, and failure of crazes in glassy polymers

We report on an extensive study of craze formation in glassy polymers. Molecular dynamics simulations of a coarse-grained bead-spring model were employed to investigate the molecular level processes during craze nucleation, widening, and breakdown for a wide range of temperature, polymer chain length $N$, entanglement length $N_e$ and strength of adhesive interactions between polymer chains. Craze widening proceeds via a fibril-drawing process at constant drawing stress. The extension ratio is determined by the entanglement length, and the characteristic length of stretched chain segments in the polymer craze is $N_e/3$. In the craze, tension is mostly carried by the covalent backbone bonds, and the force distribution develops an exponential tail at large tensile forces. The failure mode of crazes changes from disentanglement to scission for $N/N_e\sim 10$, and breakdown through scission is governed by large stress fluctuations. The simulations also reveal inconsistencies with previous theoretical models of craze widening that were based on continuum level hydrodynamics

Tensile Forces and Shape Entropy Explain Observed Crista Structure in Mitochondria

A model is presented from which the observed morphology of the inner mitochondrial membrane can be inferred as minimizing the system's free energy. Besides the usual energetic terms for bending, surface area, and pressure difference, our free energy includes terms for tension that we believe to be exerted by proteins and for an entropic contribution due to many dimensions worth of shapes available at a given energy. In order to test the model, we measured the structural features of mitochondria in HeLa cells and mouse embryonic fibroblasts using 3D electron tomography. Such tomograms reveal that the inner membrane self-assembles into a complex structure that contains both tubular and flat lamellar crista components. This structure, which contains one matrix compartment, is believed to be essential to the proper functioning of mitochondria as the powerhouse of the cell. We find that tensile forces of the order of 10 pN are required to stabilize a stress-induced coexistence of tubular and flat lamellar cristae phases. The model also predicts \Deltap = -0.036 \pm 0.004 atm and \sigma=0.09 \pm 0.04 pN/nm

Shear yielding of amorphous glassy solids: Effect of temperature and strain rate

We study shear yielding and steady state flow of glassy materials with molecular dynamics simulations of two standard models: amorphous polymers and bidisperse Lennard-Jones glasses. For a fixed strain rate, the maximum shear yield stress and the steady state flow stress in simple shear both drop linearly with increasing temperature. The dependence on strain rate can be described by a either a logarithm or a power-law added to a constant. In marked contrast to predictions of traditional thermal activation models, the rate dependence is nearly independent of temperature. The relation to more recent models of plastic deformation and glassy rheology is discussed, and the dynamics of particles and stress in small regions is examined in light of these findings

Simulations of the Static Friction Due to Adsorbed Molecules

The static friction between crystalline surfaces separated by a molecularly thin layer of adsorbed molecules is calculated using molecular dynamics simulations. These molecules naturally lead to a finite static friction that is consistent with macroscopic friction laws. Crystalline alignment, sliding direction, and the number of adsorbed molecules are not controlled in most experiments and are shown to have little effect on the friction. Temperature, molecular geometry and interaction potentials can have larger effects on friction. The observed trends in friction can be understood in terms of a simple hard sphere model.Comment: 13 pages, 13 figure

Computational studies of contact time dependence of adhesive energy due to redistribution of the locations of strong specific interfacial interactions.

The work required to pull a polymeric material from a solid surface with which it connects through hydrogen bonding has been studied by means of molecular dynamics simulations of a coarse-grained bead-spring model. In our simulations, the work, and hence the adhesive energy, increases with the time for which the polymeric material and the surface have been in contact. The work of adhesion G contains a reversible component due to interfacial molecular interactions, as well as an irreversible one, due to dissipative processes. Our data indicate that an increase in the irreversible and not in the reversible work causes G to increase with prolonged contact time. Hence, the phenomenon cannot be attributed to the formation of more or stronger interfacial bonds. Instead, we attribute this increase to a slow redistribution of the beads on the polymer chains that form hydrogen bonds with the surface. Two ways in which this takes effect could be specified. One is that the formation of long loops-chain sections between adjacent bonds-is suppressed after prolonged contact. Another is that over time bonds get distributed more evenly over the polymer backbone and among the polymer chains

Effects of strong confinement on the glass-transition temperature in simulated atactic polystyrene films

We have performed molecular dynamics simulations to explore the influence of confinement on the glass-transition temperature Tg for supported atactic-polystyrene (aPS) thin films of different thickness (1-10 nm) and different strengths of attraction to the substrate (0.1-3.0 kcal/mol). The aPS films have been equilibrated in a melt at 540 K and further cooled down with a constant cooling velocity of 0.01 K/ps below Tg to room temperature, 300 K. On the basis of the density measurements, we have defined three different (substrate, middle, and surface) layers for each film. We found that the monomers close to the surface and in the substrate layer are partially oriented, which leads to more effective monomer packing. For the whole film the average density-based Tg value remains almost constant for films down to 2 nm thickness, where the middle layer vanishes. For the middle layer itself Tg does not depend on the total film thickness, while an increase up to 70 K is measured for the substrate layer depending on the strength of attraction to the actual substrate. The surface layer remains liquidlike in the whole temperature range (300-540 K). We claim that the redistribution of mass in the three film layers may explain the change with film thickness of the average Tg, if the latter is determined from linear fits of the average glass and melt densities

Simulated glass transistion in free-standing thin polystyrene films

In this article, we investigate the glass transition in polystyrene melts and free-standing ultra-thin films by means of large-scale computer simulations. The transition temperatures are obtained from static (density) and dynamic (diffusion and orientational relaxation) measurements. As it turns out, the glass transition temperature of a 3 nm thin film is 60 °K lower than that of the bulk. Local orientational mobility of the phenyl bonds is studied with the help of Legendre polynomials of the second-order P2(t). The and relaxation times are obtained from the spectral density of P2(t). Our simulations reveal that interfaces affect and -relaxation processes differently. The relaxation rate is faster in the center of the film than near a free surface; for the relaxation rate, an opposite trend is observed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1160-1167, 201

Interfacial and topological effects on the glass transition in free-standing polystyrene films

United-atom molecular-dynamics computer simulations of atactic polystyrene (PS) were performed for the bulk and free-standing films of 2 nm – 20 nm thickness, for both linear and cyclic polymers comprised of 80 monomers. Simulated volumetric glass-transition temperatures (Tg) show a strong dependence on the film thickness below 10 nm. The glass-transition temperature of linear PS is 13% lower than that of the bulk for 2.5 nm-thick films, as compared to less than 1% lower for 20 nm films. Our studies reveal that the fraction of the chain-end groups is larger in the interfacial layer with its outermost region approx. 1 nm below the surface than it is in the bulk. The enhanced population of the end groups is expected to result in a more mobile interfacial layer and the consequent dependence of Tg on the film thickness. In addition, the simulations show an enrichment of backbone aliphatic carbons and concomitant deficit of phenyl aromatic carbons in the interfacial film layer. This deficit would weaken the strong phenyl-phenyl aromatic (-) interactions and, hence, lead to a lower film-averaged Tg in thin films, as compared to the bulk sample. To investigate the relative importance of the two possible mechanisms (increased chain ends at the surface or weakened - interactions in the interfacial region), the data for linear PS are compared with those for cyclic PS. For the cyclic PS the reduction of the glass-transition temperature is also significant in thin films, albeit not as much as for linear PS. Moreover, the deficit of phenyl carbons in the film interface is comparable to that observed for linear PS. Therefore, chain-end effects alone cannot explain the observed pronounced Tg dependence on the thickness of thin PS films; the weakened phenyl-phenyl interactions in the interfacial region seems to be an important cause as well