Reduced Plastic Dilatancy in Polymer Glasses

Polymer Science Series A, 2010

Molecular dynamic simulation of low temperature plastic deformation (T def = 50 K, T def /T g ≤ 0.3) is studied for glassy polymethylene under the regime of active uniaxial compression and tension for a cell composed of 64 chains containing 100 -CH 2 groups in each (as united atoms) and with periodic boundary conditions. Thirty two such cells are created, and, in each cell, polymethylene chains in the statis tical coil conformation are independently constructed. The cells are subjected to isothermal uniaxial com pression at T def = 50 K by ε = 30% and by ε = 70% under uniaxial tension. In the course of loading, a σ-ε diagram is recorded, while the mechanical work spent on deformation, the changes in the overall potential energy of the system, and the contributions from various potential interactions (noncovalent van der Waals bonds, chemical links, valence and torsional angles) are estimated. The results are averaged over all 32 cells. The relaxation of stored potential energy and residual strain after complete unloading of the deformed sample is studied. The relaxation of stored energy and residual strain is shown to be incomplete. Most of this energy and strain is stored in the sample at the deformation temperature for long period. The conformational com position of chains and the average density of polymer glass during loading are analyzed. Simulation results show that inelastic deformations commence not with the conformational unfolding of coils but with the nucleation of strain bearing defects of a nonconformational nature. The main contribution to the energy of these defects is provided by van der Waals interactions. Strain bearing defects are nucleated in a polymer glass during tension and compression primarily as short scale positive volume fluctuations in the sample. During tension, the average density of the glass decreases; during compression, this parameter slightly increases to ε ≈ 8% and then decreases. An initial increase in the density indicates that, during compression and at ε < 8%, coils undergo compactization via an increase in chain packing. During compression, the concentration of trans conformers remains unchanged below ε ≈ 8% and then decreases. During compression, it means that in a glass, coils do not increase their sizes at strains below ε ≈ 8%. During tensile drawing, coils remain unfolded below ε ≈ 35%; at higher strains, coils become enriched with trans conformers or unfold. At this stage, the concentration of trans conformers linearly increases. The development of a strain induced excess volume (strain bearing defects) entails an increase in the potential energy of the sample. Under the given conditions of deformation, nucleation of strain bearing defects and an increase in their concentration are found to be the only processes occurring at the initial stage of loading of glassy polymethylene. The results of computer aided simulation are compared with the experimental data reported in the literature.

2014

Deformation of a solid usually leads to an increase in the internal potential energy U in of the solid. In inelastic and plastic deformation, the increase in U in is due to the formation of excited structural defects in the solid under the action of the external force (e.g., in crystals, dislocations form). The storage of U in has been studied experimentally in detail for crystalline metals 2], rubbers , and glassy organic polymers . However, experimental measurements on polymer systems cannot determine the contributions of inter actions of various types to the increase in U in . Today, computer simulation can identify such contributions and can suggest what types of interactions con trol the energy storage and influence the mechanical behavior of material . In this work, we performed molecular dynamic simulation of uniaxial compres sive and tensile deformation of a full atom model of amorphous glassy polymethylene and analyzed the contributions of interactions of various types to the increase in its potential energy.

Bulletin of the American Physical Society, 2017

Impact of melt-deformation on molecular structure and mechanical behavior of glassy polymers 1 JIANNING LIU, XIAOXIAO LI, ZHICHEN ZHAO, SHI-QING WANG, Department of Polymer Science, University of Akron -This work studies effects of melt deformation such as extension and compression on mechanical behavior of glassy polymers. Depending on how the entanglement network is altered during melt deformation, mechanical properties of polystyrene and poly(methyl methacrylate) are changed at temperatures below Tg. Conversely, the observed mechanical behavior below Tg reveals how molecular structures at segmental levels have undergone distortion due to melt stretching or shear. This research expands well beyond our previous investigations that have demonstrated how and why melt-stretched PS and PMMA turns ductile at room temperature.... 1 and why a cold-drawn ductile polymer glass produces significant retractive stress upon annealing above the cold-drawing temperature.. 2 .1.Wang, S.-Q.; Cheng, S.; Lin, P.; Li, X. A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses.

Journal of Polymer Science Part B: Polymer Physics, 2010

An experimental study was made of the effects of prior molecular orientation on large tensile deformations of polystyrene in the glassy state. A new hybrid glass-melt constitutive model is proposed for describing and understanding the results, achieved by parallel coupling of the ROLIEPOLY molecularly-based melt model with a model previously proposed for polymer glasses. Monodisperse and polydisperse grades of polystyrene are considered. Comparisons between experimental results and simulations illustrate that the model captures characteristic features of both the melt and glassy states. Polystyrene was stretched in the melt state and quenched to below T g , and then tensile tested parallel to the orientation direction near the glass transition. The degree of strain-hardening was observed to increase with increasing prior stretch of molecules within their entanglement tubes, as predicted by the constitutive model. This was explored for varying temperature of stretching, degree of stretching, and dwell time before quenching. The model in its current form, however, lacks awareness of processes of subentanglement chain orientation. Therefore, it underpredicts the orientation-direction strain hardening and yield stress increase, when stretching occurs at the lowest temperatures and shortest times, where it is dominated by subentanglement orientation.

Polymer, 2003

The influence of network density on the strain hardening behaviour of amorphous polymers is studied. The network density of polystyrene is altered by blending with poly(2,6-dimethyl-1,4-phenylene-oxide) and by cross-linking during polymerisation. The network density is derived from the rubber-plateau modulus determined by dynamic mechanical thermal analysis. Subsequently uniaxial compression tests are performed to obtain the intrinsic deformation behaviour and, in particular, the strain hardening modulus. At room temperature, the strain hardening modulus proves to be proportional to the network density, irrespective of the nature of the network, i.e. physical entanglements or chemical cross-links. With increasing temperature, the strain hardening modulus is observed to decrease. This decrease appears to be related to the influence of thermal mobility of the chains, determined by the distance to the glass-transition temperature ðT 2 T g Þ:

Physical Review Letters, 2004

Molecular simulations of a model, deeply quenched polymeric glass show that the elastic moduli become strongly inhomogeneous at length scales comprising several tens of monomers; these calculations reveal a broad distribution of local moduli, with regions of negative moduli coexisting within a matrix of positive moduli. It is shown that local moduli have the same physical meaning as that traditionally ascribed to moduli obtained from direct measurements of local constitutive behaviors of macroscopic samples.

Europhysics Letters (EPL), 2005

Molecular-dynamics (MD) simulations have been performed for two amorphous polymers with extremely different mechanical properties, atactic polystyrene (PS) and bisphenol A polycarbonate (PC), in the isotropic state and under load. The glass transition temperatures, Young moduli, yield stresses and strain-hardening moduli are calculated and compared to the experimental data. Both chemistry-specific and mode-coupling aspects of the segmental mobility in the isotropic case and under the uniaxial deformation have been identified. The mobility of the PS segments in the deformation direction is increased drastically beyond the yield point. A weaker increase is observed for PC.

Bulletin of the American Physical Society, 2015

Submitted for the MAR15 Meeting of The American Physical Society What deformation does and does not do in ductile polymer glasses JIANNING LIU, SHI-QING WANG, Department of Polymer Science, University of Akron-Entangled polymeric liquids have so far only shown strain softening, signified by stress overshoot upon startup shear. We show for the first time that entangled solutions of polymers with high glass transition temperature undergoes non-Gaussian chain stretching, losing its ability to yield through chain disentanglement. The stronger than linear increase of the shear stress with the strain ends with a sharp decline, forming a cusp. In situ visualization by particle-tracking velocimetry confirms that the solution undergoes abrupt rupture at a shearing plate in the sample interior. The rapid sample recoils elastically, producing the observed stress drop.

The Journal of Chemical Physics, 2013

Glassy polymers show "strain hardening": at constant extensional load, their flow first accelerates, then arrests. Recent experiments under such loading have found this to be accompanied by a striking dip in the segmental relaxation time. This can be explained by a minimal nonfactorable model combining flow-induced melting of a glass with the buildup of stress carried by strained polymers. Within this model, liquefaction of segmental motion permits strong flow that creates polymer-borne stress, slowing the deformation enough for the segmental (or solvent) modes to then re-vitrify. Here we present new results for the corresponding behavior under step-stress shear loading, to which very similar physics applies. To explain the unloading behavior in the extensional case requires introduction of a 'crinkle factor' describing a rapid loss of segmental ordering. We discuss in more detail here the physics of this, which we argue involves non-entropic contributions to the polymer stress, and which might lead to some important differences between shear and elongation. We also discuss some fundamental and possibly testable issues concerning the physical meaning of entropic elasticity in vitrified polymers. Finally we present new results for the startup of steady shear flow, addressing the possible role of transient shear banding.

The Journal of Chemical Physics

Fast deformation of entangled melts is known to cause chain stretching due to affine-like straining of the entanglement network. Since the chain deformation may also result in perturbations of covalent bond angles and bond length, there are always possible enthalpic effects. In this study we first subject polystyrene and PMMA of different molecular weights to either uniaxial melt extension or planar extension and subsequently impose rapid thermal quenching to preserve the chain deformation. Then such pre-melt-deformed samples are annealed at various temperatures below the glass transition temperature Tg. During annealing, these samples can undergo appreciable contraction on a time scale much shorter than the alpha relaxation time. Significant retractive stress is observed when such contracting samples are held fixed during the annealing. The stress level can be nearly as high as the Cauchy stress produced during melt stretching. These observations not only allowed us to investigate glassy chain dynamics as well as the molecular nature of mechanical stress but may also suggest that pre-melt-stretched polymers can cause segmental mobilization in the glassy state. The available evidence indicates that the retractive stress is enthalpic in origin, associated with the conformational distortion at the bond level produced by melt stretching.