Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B17: Dynamics of Glassy Polymers Under Nanoscale Confinement IIFocus Recordings Available
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Sponsoring Units: DPOLY DSOFT DCP Chair: Kathleen McEnnis, New Jersey Institute of Technology Room: McCormick Place W-184BC |
Monday, March 14, 2022 11:30AM - 12:06PM |
B17.00001: Insights into the physics of glass formation from simulations of thin film dynamics Invited Speaker: David S Simmons A major motivation for 35 years of research on nanoconfined glass formation has been the hope that confinement and broken symmetry near interfaces might reveal the details of dynamical correlations in glass forming liquids and thus resolve the nature of glass formation itself. A central challenge has been lack of a settled, cohesive picture of the dependences of altered dynamics on position, domain size, and temperature. Here, I discuss results establishing a cohesive phenomenology of interface and confinement effects on glass formation at the longest timescales accessible to simulation. Key findings include a temperature-invariant fractional reduction in activation barriers, an exponential gradient of activation barriers and glass transition temperatures in the first 5-10 nm near the free surface, saturation of the range of this gradient upon cooling, and the emergence of non-interface finite-size effects in thin films. I report on new results establishing the long-ranged fate of this gradient beyond the near-surface exponential. We find that the phenomenology of altered thin films dynamics is profoundly at odds with predictions of a number of theories of glass formation. On the other hand, the phenomenology is predicted near-quantitatively at a zero-parameter level by the Elastically Collective Nonlinear Langevin Equation (ECNLE) Theory of Schweizer and coworkers. This theory predicts altered dynamics in films as a result of alteration and truncation of local caging constraints and long-ranged elastic activation barriers. Together with complementary evidence for the success of the ECNLE theory in predicting bulk glass formation, these findings may have major implications for our understanding of the underlying mechanism of glass formation. |
Monday, March 14, 2022 12:06PM - 12:18PM |
B17.00002: An Experimental and Modeling Comparison of the Dynamics of Capped and Freestanding Films Ronald P White, Jane E Lipson, Jijian Song, Simone Napolitano We compare the influence of two different types of interfaces on the segmental dynamics of poly(2-chlorostyrene) (P2ClS): the simple free surface of freestanding films vs. the solid adsorbing surface of Aluminum (Al) capped films. Data on both have been measured via dielectric spectroscopy, and to understand and quantify the change in free space at these two types of interfaces we use the Cooperative Free Volume (CFV) rate model. Though adsorbing interfaces are often considered to be a source of slower dynamics, meta-stable packing imperfections at the interface can lead to more free space (lower density) near the interface and enhanced dynamics. CFV calculations show that the Al-capped interface does provide an excess of free space relative to bulk, and this excess is almost half as large as that at the P2ClS free surface. These calculations also show that "capping" an 18 nm thick freestanding film has an equivalent effect on the dynamics as applying a 19 MPa pressure increase. We also discuss the importance of accounting for the volume sensitivity of the bulk material, and, whether this can be sufficient to predict the magnitude of the confinement effect. |
Monday, March 14, 2022 12:18PM - 12:30PM |
B17.00003: Understanding Fracture Behavior of Brittle Polymeric Glasses Chaitanya Ramanand Gupta, Shi-Qing Wang This presentation attempts to understand the crack growth and propagation in brittle glassy polymers, which, in theory, is amenable to study using Linear Elastic Fracture Mechanics (LEFM). We attempt to explore the basic concepts of fracture mechanics using polymethyl methacrylate, polystyrene, physically aged polylactic acid and polycarbonate. Stress-optical measurements around a deliberately introduced single edged notch during uniaxial deformation allows us to examine the stress intensification around this defect to better understand the role of flaws. While models explaining the mechanics and strength of glassy polymers at a chain-network level have been around for some time [1,2] these pictures are more idealistic, i.e., assumes the absence of flaws. Thus, understanding the role of flaws in mechanical responses will extend our knowledge about mechanical characteristics of these glassy polymers. |
Monday, March 14, 2022 12:30PM - 12:42PM |
B17.00004: Role of local structure in the enhanced dynamics of deformed polymer glasses Entao Yang, Robert Riggleman External stress can accelerate the molecular mobility of polymer glasses by several orders of magnitude. The changes in polymer dynamics are commonly interpreted through the Eyring model, which invokes an empirical activation volume whose origin remains poorly understood. In this study, we performed molecular dynamics simulations for polymer glasses under a series of constant stresses and propose an extension of the Eyring model with a machine-learned structural field, softness. Our model connects the activation volume, which is believed to be an empirical parameter, to a structural property (softness) for the first time. We show that stress has a heterogeneous effect on the dynamical enhancement, which depends on local structure. Our model explains the narrower distribution of relaxation time and decrease of dynamical heterogeneity in glasses under creep deformation, which has been observed in previous experiments and simulations. Finally, our results also suggest there is a threshold stress after which dynamical enhancement grows exponentially with stress. |
Monday, March 14, 2022 12:42PM - 12:54PM |
B17.00005: Using the TNM model to reproduce reported anomalous structural recovery data in light of temperature variability in vacuum oven-based experiments Shuang Jin, Gregory B McKenna We report a study on the possible reasons for the apparent two mechanism structural (enthalpy) recovery reported by Cangialosi et al. [Cangialosi, D., Boucher, V. M., Alegría, A., & Colmenero, J. (2013). Physical Review Letters, 111(9), 095701], which challenges the general view of the isothermal structural recovery behavior of glasses being smooth functions of aging time as observed in extensive dilatometric experiments by Kovacs [Kovacs, A. J. (1964). In Fortschritte der hochpolymeren-forschung, pp. 394-507. Springer, Berlin, Heidelberg.], and a second plateau was not reproduced in the work from Koh and Simon [Koh, Y. P., & Simon, S. L. (2013). Macromolecules, 46(14), 5815-5821.] in one year aging experiments. We examine the possibility that the poor temperature control of a typical vacuum oven could explain the anomalous results. We used the Tool-Narayanaswamy-Moynihan (TNM) model of structural recovery to calculate the effect of typical vacuum oven temperature variation on the structural recovery of polystyrene and find that we can reproduce the reported experimental results. The issue of temperature control using a vacuum oven is discussed, and data from thermocouple measurements of temperature at various locations inside a vacuum oven are shown. |
Monday, March 14, 2022 12:54PM - 1:06PM |
B17.00006: Glass transition of random heteropolymer melts Tianyi Jin, Shayna Hilburg, Alfredo Alexander-Katz Random heteropolymers (RHPs) have been widely used as toy models for protein folding, and in many industrial and biological scenarios. Here, we use atomistic MD simulations to explore the factors affecting glass transition temperature (Tg) of a complex class of RHPs. Our random copolymers consist of four monomers with a methacrylate backbone. We explore the dynamical features in polymer melts of random sequence mixtures and show that instead of a sharp glass transition as happen in PMMA, RHPs show a soft transition. However, we show that in both systems there exists an “entropy crisis”, i.e. the backbone configurational entropy becomes extinct. Interestingly, the chain mobility depends on the contents of ionic monomers and types of counterions when they are present. The spatial heterogeneity during the glass transition is also found to be related to both chemical structure and sequence. The comparison with the single-chain shows the effects of breathing dynamic in aqueous systems or vacuum environments. Our work thus indicates that RHP melts are excellent system to investigate the effects of chemistry on Tg. This work is supported by the Defense Threat Reduction Agency contract HDTRA11910011. |
Monday, March 14, 2022 1:06PM - 1:18PM |
B17.00007: Model for mechanical behavior of glassy polymers Grigori A Medvedev, James M Caruthers Existing constitutive models of glassy polymers are incapable of even qualitatively describing the stress-strain response seen in the course of a four-step deformation experiment (constant strain rate loading-partial unloading-creep-reloading) reported by Dreistadt et al. [1] Specifically, the second stress overshoot observed during reloading is not predicted by the models. There are other features in the thermomechanical behavior of glassy polymers observed experimentally that also cannot be described by existing constitutive models, e.g. deep glass aging. We propose a new model, where an additional physical mechanism accounting for changes in the packing efficiency is included. The model describes material response during basic deformation experiments, the second stress overshoot in the four-step experiment and other features observed experimentally. Further validation experiments for testing the new model will be proposed. |
Monday, March 14, 2022 1:18PM - 1:30PM |
B17.00008: The molecular weight dependence of Tg in polymers Peter D Olmsted, Johan Mattsson, Matthew Reynolds, Robin Masurel, Daniel Baker The polymer glass transition temperature Tg(M) decreases with decreasing molecular weight M. Early theories related this to the increasing free volume (Fox Flory), to configurational entropy for short chains and/or to chain flexibility (Gibbs-DiMarzio, Generalized Entropy Theory). A more recent theory combined ideas from Mode Coupling Theory with activated behaviour, accounting for polymer flexibility and packing to provide precise predictions (Schweizer et al.). Here we review and evaluate these theories in the context of experiments on Tg(M) on many polymer chemistries, from monomer to the "infinite" chain limit; these data show a general M-dependence that can be simply related to measures of chain bulkiness and flexibility, and are a challenge to existing theories. |
Monday, March 14, 2022 1:30PM - 1:42PM |
B17.00009: Overaging under stress in polymer glasses? Larger yield stress despite faster segmental dynamics! Masoud Razavi, Enran Xing, Mark D Ediger Understanding the relationship between physical aging, structural relaxation time and yield stress in polymer glasses is crucial for modeling and long-term prediction of mechanical properties of these materials. If two identical polymer glasses with the same thermal history are aged under the same conditions, except in one case the polymer glass is held under stress and in the other is aged without stress, the stress-aged sample can develop a larger yield stress than the quiescently-aged sample. The increased yield stress of the stress-aged sample has been interpreted as accelerated aging or “overaging”. A similar feature was initially reported for Nylon 6,10 by Kramer. However, it is important to test whether the yield stress properly indicates the age of a polymer glass. If so, then the segmental relaxation time of an overaged glass should be longer than the corresponding quiescently-aged sample. Direct measurements of structural relaxation time using a probe reorientation technique enable us to address this issue. Here using PMMA glasses we examine a case where the stress-aged sample, despite having the larger yield stress value, has a shorter structural relaxation time in comparison to the quiescently-aged sample. |
Monday, March 14, 2022 1:42PM - 1:54PM |
B17.00010: Effect of large deformation on the linear viscoelastic response of a glassy polymer Hosup Song, James M Caruthers The state of a glassy material changes as a result of a loading-unloading cycle, provided the loading is sufficiently large to take the material past yield. This is evidenced by the well-known observation that the DSC trace upon heating following a load-unload cyclel exhibits a broad exothermic sub-Tg peak not present in a reference material not subjected to deformation. We now report that the loading-unloading cycle also significantly changes the viscoelastic spectrum of a PMMA-PBMA co-polymer deformed at Tg-35°C. The large deformation has been performed in uniaxial extension and the frequency dependent linear viscoelastic behavior has been obtained in shear. The implications of the observed effect of deformation on the viscoelastic spectrum for fundamental understanding of the glassy state as well as the constitutive modeling of glassy polymers will be discussed. |
Monday, March 14, 2022 1:54PM - 2:06PM |
B17.00011: Study of Sub-Tg Relaxation in Plasticized Polycarbonate with the Quartz Crystal Microbalance Qifeng Wang, Christopher L Soles, Jack F Douglas, Kenneth R Shull Quartz crystal resonators are well-known for their sensitivity to the film mass and are commonly referred to as quartz crystal microbalances (QCM). A unique aspect of quartz crystal resonators is that they are also able to accurately probe the viscoelastic properties in the megahertz frequency regime. Here we discuss the use of the rheometric quartz crystal microbalance (RheoQCM) as a fixed frequency rheometer operating at 15 MHz to study the rheological properties of bisphenol-A polycarbonate (PC) from -30 °C up to 180 °C. The effects of dioctyl terephthalate (DOTP) and the solvent, 1,4-dioxane, on the viscoelastic response were investigated. The plasticizer increases the modulus in the glassy regime and decreases the magnitude of a sub- Tg relaxation peak centered near 50 ° C. The data were analyzed with an open-source Python-based interface developed for use with a low-cost vector network analyzer, using methods that can also be applied to data obtained from commercial QCM-D instrumentation. |
Monday, March 14, 2022 2:06PM - 2:18PM |
B17.00012: Long-wavelength fluctuations and dimensionality crossover in confined liquids Jing Yang The phase behavior of liquids confined in a slit geometry does not reveal a crossover from a three- to a two-dimensional behavior as the gap size decreases. The hexatic phase in two dimensions only occurs in liquids in a monolayer. In this work, the dimensionality crossover becomes apparent in the lateral size dependence of the relaxation dynamics of confined liquids. We develop a Debye model for the density of vibrational states of confined liquid systems and perform extensive numerical simulations. In these systems, the amplitude of vibrational motion is enhanced by Mermin-Wagner fluctuations or the Debye-Waller factor- by a quantity that scales as the inverse gap width and is proportional to the logarithm of the aspect ratio, a clear signature of two-dimensional behavior. As the temperature or lateral system size increases, the crossover to a size-independent relaxation dynamics occurs when structural relaxation takes place before the vibrational modes with the longest wavelength develop. Our results show that confined systems exhibit a gradual dimensionality crossover controlled by the gap width and the temperature, which is appreciable when investigating the lateral size dependence of the dynamics. The physics of confined liquids are thus richer than previously realized. |
Monday, March 14, 2022 2:18PM - 2:30PM |
B17.00013: Acrylic Paints: An Atomistic View of Polymer Structure and Effects of Environmental Pollutants Aysenur Iscen, Nancy C Forero-Martinez, Omar Valsson, Kurt Kremer Acrylic polymers are a major component of acrylic paints used in many contemporary artwork and cultural heritage objects. Although acrylics are praised for many of their properties, one disadvantage of acrylics is their glass transition temperature close to room temperature. At low temperatures the paints face the danger of cracking while a temperature that is too high makes the surface of the paints sticky and more prone to collecting impurities. Most of these objects are stored in museums under conditions that are difficult to monitor. Damage in cultural heritage conservation emerges from the interplay of various complex processes occurring at the microscopic scale. Through a multi-scale computational methodology, we aim at a fundamental understanding of these processes in acrylic materials present in contemporary paintings. Here we develop fully atomistic models to understand the structure of two types of acrylic copolymers and their interactions with environmental pollutants. In addition, our model forms the foundation for developing mesoscopic and continuum models that will allow us to access longer time and length scales to further our understanding of the degradation of artwork. |
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