Aging and Energy Landscapes: Application to Liquids and Glasses

The equation of state for a liquid in equilibrium, written in the potential energy landscape formalism, is generalized to describe out-of-equilibrium conditions. The hypothesis that during aging the system explores basins associated to equilibrium configurations is the key ingredient in the derivation. Theoretical predictions are successfully compared with data from molecular dynamics simulations of different aging processes, such as temperature and pressure jumps.Comment: RevTeX4, 4 pages, 5 eps figure

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 aging and plastic deformation in polymer glasses

We study the effect of physical aging on the mechanical properties of a model polymer glass using molecular dynamics simulations. The creep compliance is determined simultaneously with the structural relaxation under a constant uniaxial load below yield at constant temperature. The model successfully captures universal features found experimentally in polymer glasses, including signatures of mechanical rejuvenation. We analyze microscopic relaxation timescales and show that they exhibit the same aging characteristics as the macroscopic creep compliance. In addition, our model indicates that the entire distribution of relaxation times scales identically with age. Despite large changes in mobility, we observe comparatively little structural change except for a weak logarithmic increase in the degree of short-range order that may be correlated to an observed decrease in aging with increasing load.Comment: 9 pages, 12 figure

Local dynamics and deformation of glass-forming polymers : modelling and atomistic simulations

The research described in the present thesis is about glassy phenomena and mechanical properties in vitrifiable polymer materials. Glasses are solid materials, but, in contrast to crystals, the structure is disordered. Polymers are macromolecular chains formed by covalently linking a very large number of repeating molecular building blocks or monomers. Polymeric materials are easy to reshape and reuse. Also they are lightweight and often transparent. These characteristics make them ideal materials for commodity products such as compact discs, safety helmets, or vandal-proof glazing. Some glassy polymers are also biocompatible, so that they can be used in medical applications. For a successful usage of polymer glasses it is necessary to understand and predict their behaviour under various circumstances. Although many new insights have been acquired over the last decades still a lot of questions remain open. Upon vitrifying a polymer melt the relaxation times and the viscosity increase dramatically. Accompanied with this increase various glassy phenomena are observed — in particular dynamical heterogeneities and non-Gaussian displacements of particles. The comprehension of the striking viscosity increase and the two phenomena mentioned above is still far from complete. During the straining of a polymer glass typical mechanical characteristics are observed, of which the magnitude can vary enormously between different types of polymers. A wellknown example of a polymer glass is atactic polystyrene. In its glassy state polystyrene is usually found to be very brittle. Within a few percent of elongation the material breaks. This behaviour is caused by a relatively high yield tooth in combination with a relatively low strain-hardening modulus. Other polymers, such as bisphenol-A polycarbonate, show a tough response; a test bar can easily be extended to twice its original length. In spite of much study, the physical (molecular) origin of this difference in mechanical behaviour is still not clear. Below the glass transition rubber-elasticity theory appears to be invalid, as it fails to explain the observation that the strain-hardening modulus of polystyrene in its glassy phase is about two orders of magnitude higher than its modulus in the rubbery state. Our main goals are to acquire a better understanding at the molecular scale of heterogeneous and non-Gaussian dynamics and mechanical deformation of glassy polymers and to differentiate chemistry-specific from more universal physical properties. These goals are achieved by carrying out molecular-dynamics simulations on glass-forming systems. In addition, the results are elucidated by the usage of simple physical models. The simulations consist of solving the equations of Newton, a coupled set of differential equations with a given force field and initial conditions. The force field describes the interactions between the various particles. As the main interest is in glassy polymers, most simulations are done for a united-atom model of polystyrene. In the simulation run several characteristics of the glass transition are identified. As is typical for other vitreous systems as well, anomalous, non-Gaussian displacements play an important role near the glass transition for polystyrene; the same observation has been made for a dendritic melt and a colloid-like system. For all these systems of different architecture we have described some essential features of this non-Gaussian behaviour with a simple one-particle model in an effective field. The non-Gaussian behaviour is mainly caused by the cage-to-cage motion of the constituent particles, whereby the cage is formed by interactions with neighbouring particles. By means of the model the height of the so-called non-Gaussian parameter can be interpreted as a measure for the ratio of the root-mean-square displacement within the cage and the effective jump length between cages, without the assumption of any heterogeneity of glassy dynamics in the sense of sitespecific relaxation times. The maximum of the non-Gaussian parameter occurs in each case at the crossover from the cage regime to the (sub)diffusive regime and is connected to the cage-escape time. For the colloid-like system also the shape of the time-dependent non-Gaussian parameter is described well by the model (chapter 3). Dynamical heterogeneity, a phenomenon observed in many experiments on glasses, is also found in the simulation result of the polystyrene phenyl-ring-flip movement (to which the mechanical gamma relaxation is ascribed). This means that some phenyl rings behave very differently than others within a typical simulation run. Different relaxation times and activation enthalpies associated with the flip are determined using various methods. A particular result of the study of the phenyl flip is that an enthalpy barrier determined solely from structural properties is in accordance with an activation enthalpy acquired by analyzing the dynamics of the phenyl rings, even in the presence of dynamical heterogeneity. The heterogeneity arises because of the following mechanism. The conformation of the backbone determines to a large extent the barrier of the phenyl-ring flip. Eventually the relaxation of the backbone is becoming so slow upon cooling down that the phenyl ring is unable to access the conformation-dependent state with the lowest flip barrier within the corresponding barrier-jump time. The phenyl rings are trapped instead in various other states with accompanying different energy barriers. These states are available because of the disordered nature of the material. The mechanism just described for the heterogeneous dynamics in the phenyl-ring flip movement becomes stronger upon cooling down towards the glass transition; eventually the relaxation becomes more Arrhenius-like below the glass transition temperature because only the fastest phenyl flips occur within the accessible observation time (chapter 4). By doing uniaxial-stress extension and compression simulations the stress-strain relation of polystyrene has been measured under various conditions. Although the cooling and deformation velocities in the simulations differ many orders of magnitude from their values in usual experiments, the characteristic features of the experimental stress-strain relation are well reproduced, which allows one to study the origin of the yield tooth and strain hardening. It is observed that the strain-hardening modulus increases with increasing pressure, an effect not described by rubber-elasticity theory. Also it is observed that the thermal history is not completely erased by the mechanical deformation. The picture arising from this study is that the yield peak in polystyrene is mostly mediated by interchain energetic interactions. A net debonding of these interactions is likely causing this yield peak and the subsequent strain softening. The positiveness of the strain-hardening modulus in polystyrene is mainly due to intrachain interactions (chapter 5). From our comparative study of polystyrene and polycarbonate it can be concluded that strain hardening in polymer glasses such as these two polymers is likely caused mainly by the following mechanism. During uniaxial extension a glassy chain adopts a more stretched and hence more inflexible state, also at a local scale. Due to interactions with other particles non-affine displacements take place. The non-affine response is stronger at shorter length scales, but as the deformation proceeds and the effective flexibility decreases also longer length scales are affected. This is accompanied with more bond-altering processes and implies an increase in the rate of energy dissipation, causing in turn an increase in stress upon straining the polymer material further (chapter 6). All these results show that simple physical models supported and tested by results of molecular-dynamics simulations (in which typical physical phenomena observed in real experiments can be reproduced) provide a fruitful approach in understanding glassy material

Effect of Chain Rigidity on Network Architecture and Deformation Behavior of Glassy Polymer Networks

Processing carbon fiber composite laminates creates molecular-level strains in the thermoset matrix upon curing and cooling which can lead to failures such as geometry deformations, micro-cracking, and other issues. It is known strain creation is attributed to the significant volume and physical state changes undergone by the polymer matrix throughout the curing process, though storage and relaxation of cure-induced strains remain poorly understood. This dissertation establishes two approaches to address the issue. The first establishes testing methods to simultaneously measure key volumetric properties of a carbon fiber composite laminate and its polymer matrix. The second approach considers the rigidity of the polymer matrix in regards to strain storage and relaxation mechanisms which ultimately control composite performance throughout manufacturing and use. Through the use of a non-contact, full-field strain measurement technique known as digital image correlation (DIC), we describe and implement useful experiments which quantify matrix and composite parameters necessary for simulation efforts and failure models. The methods are compared to more traditional techniques and show excellent correlation. Further, we established relationships which represent matrix-fiber compatibility in regards to critical processing constraints. The second approach involves a systematic study of epoxy-amine networks which are chemically-similar but differ in chain segment rigidity. Prior research has investigated the isomer effect of glassy polymers, showing sizeable differences in thermal, volumetric, physical, and mechanical properties. This work builds on these themes, and shows the apparent isomer effect is rather an effect of chain rigidity. Indeed, it was found that structurally-dissimilar polymer networks exhibit very similar properties as a consequence of their shared average network rigidity. Differences in chain packing, as a consequence of chain rigidity, were shown to alter the physical, volumetric, and mechanical properties of the glassy networks. Chain rigidity was found to directly control deformation mechanisms, which were related to the yielding behavior of the epoxy network series. The unique benefit to our approach is the ability to separate the role of rigidity – an intramolecular parameter – from intermolecular phenomena which otherwise influence network properties

A New Phenomenon: Sub-Tg, Solid-State, Plasticity-Induced Bonding in Polymers

Polymer self-adhesion due to the interdiffusion of macromolecules has been an active area of research for several decades [70, 43, 62, 42, 72, 73, 41]. Here, we report a new phenomenon of sub-Tg, solid-state, plasticity-induced bonding; where amorphous polymeric films were bonded together in a period of time on the order of a second in the solid-state at ambient temperatures nearly 60 K below their glass transition temperature (Tg) by subjecting them to active plastic deformation. Despite the glassy regime, the bulk plastic deformation triggered the requisite molecular mobility of the polymer chains, causing interpenetration across the interfaces held in contact. Quantitative levels of adhesion and the morphologies of the fractured interfaces validated the sub-Tg, plasticity-induced, molecular mobilization causing bonding. No-bonding outcomes (i) during the compression of films in a near hydrostatic setting (which inhibited plastic flow) and (ii) between an 'elastic' and a 'plastic' film further established the explicit role of plastic deformation in this newly reported sub-Tg solid-state bonding