Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session K23: Polymer Glasses and Glass Formation |
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Sponsoring Units: DPOLY Chair: Robert Hoy, University of South Florida Room: Room 215 |
Tuesday, March 7, 2023 3:00PM - 3:36PM |
K23.00001: Break
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Tuesday, March 7, 2023 3:36PM - 3:48PM |
K23.00002: Crossover expressions for the craze extension ratio of semiflexible polymer glasses Kai Nan, Robert S Hoy, Joseph Fox D Dietz Kramer’s formulas for the craze extension ratio λcraze and fracture stretch λfrac accurately predict these quantities for flexible polymer glasses with entanglement length Ne much larger than their Flory characteristic ratio C∞, but break down badly for maximally entangled glasses (MEGs) with Ne = C∞ because they do not account for chain stretching at the Kuhn segment scale. Accounting for this stretching yields alternative expressions that accurately predict λcraze and λfrac in MEGs [J. D. Dietz, K. Nan and R. S. Hoy, Phys. Rev. Lett. 129, 127801 (2022)], but these expressions break down as Ne/C∞ increases. In this talk, we will present crossover expressions that accurately predict λcraze and λfrac in flexible and semiflexible polymer glasses spanning the entire range of chain stiffnesses and Ne/C∞ values for which the glasses’ parent melts remain isotropic. We will explain the physics underlying these expressions in terms of how Kuhn-segment scale stretching in craze fibrils gets progressively stronger as polymers get stiffer. |
Tuesday, March 7, 2023 3:48PM - 4:00PM |
K23.00003: Theory for the strain hardening of glassy polymers Didier R LONG, Thomas Merlette, Jerome Hem, Caroline Crauste, Sergio Ciliberto, Florence Clement We propose a model for the strain-hardening of glassy polymers. We attribute the latter to the increase of free energy barriers for α-relaxation as a consequence |
Tuesday, March 7, 2023 4:00PM - 4:12PM |
K23.00004: Structural Relaxation during the Strain-rate Switching of Polymeric Glasses Pradip Kumar Bera, Grigori A Medvedev, Mark D Ediger In the flow state of a polymeric glass under deformation, when the strain rate is abruptly lowered, stress undershoot occurs. This cannot be explained by a simple transition of the system between two minima in the potential energy landscape (PEL). We stretch poly(methyl methacrylate) glass films at a strain rate of 10-5 s-1 at a temperature Tg-20K and measure in situ the segmental relaxation time using a photobleaching technique. The segmental relaxation time provides access to the position of the system on the PEL (or more generally, a state variable). After the yield, in the steady flow region, we switch the applied strain rate to 10-6 s-1 and observe the stress undershoot. The segmental relaxation time shows a delay in the response during the strain rate switching. We have extracted the characteristic timescale associated with this memory/delay, which is ~ ta, the segmental relaxation time in the undeformed state. After the initial delay, the segmental relaxation time reaches a steady state value that depends upon the applied strain rate and is independent of the previous mechanical history. These results support the approach, taken in several models, that postulates an internal variable that does not respond instantaneously to a change in the strain rate. |
Tuesday, March 7, 2023 4:12PM - 4:24PM |
K23.00005: Mechanistic Origins of Glass Transition Dependence on Molecular Weight in Linear Homopolymers William F Drayer, David S Simmons The dependence of the glass transition temperature (Tg) on molecular weight in linear homopolymers is canonically understood by the Flory-Fox equation. |
Tuesday, March 7, 2023 4:24PM - 4:36PM |
K23.00006: Molecular signatures of the glass transition in polymers Tianyi Jin, Connor W Coley, Alfredo Alexander-Katz The glass transition temperature (Tg) is one of the most fundamental properties of polymers. Tg is predicted by some theories as a sudden change in a "macroscopic" quantity (e.g. compressibility). However, for systems with "soft" glass transitions where the change is gradual it becomes hard to pinpoint precisely the transition temperature as well as the set of molecular changes occurring during this transition. Here, we introduce two new molecular signatures for the glass transition of polymers that exhibit clear changes as one approaches Tg: i) differential change of the probability distribution of dihedral angles as a function of temperature, and ii) the distribution of fractional of time spent in the different torsional states. These new signatures provide insights into the glass transition in polymers by directly exhibiting the concept of spatial heterogeneity and dynamical ergodicity breaking in such systems, as well as provide a key step to quantitatively obtaining the transition temperature from molecular characteristics of the polymeric systems. To end this talk we will discuss the generalization of these signatures to colloidal systems. |
Tuesday, March 7, 2023 4:36PM - 4:48PM |
K23.00007: Effect of Comonomer Sequence Distribution on the Glass Transition Temperature of Poly(D,L-lactic-co-glycolic acid) Samruddhi M Patil, You-Yeon Won We developed a facile method (named “feed rate-controlled polymerization (FRCP)”) for producing statistically monomer sequence-controlled aliphatic copolyesters, such as uniform poly(lactic-co-glycolic acid) (PLGA). Using this FRCP method, monodisperse PLGA homopolymers having an identical molecular weight with varying degrees of monomer (lactide (LA) and glycolide (GL)) sequence uniformity (uniform vs. gradient PLGA) at various different monomer compositions (LA/GL ratios) were prepared and used to investigate the effect of LA/GL sequence distribution on the glass transition properties of PLGA polymers. The Tg vs. copolymer composition plot showed a significant negative deviation (< ~ 8 K) from the Flory-Fox prediction, indicating a repulsive interaction between the LA and GL monomers. We found that experimental trends between Tg and measures of monomer sequence lengths (monomer triad distribution and composition) quantitatively agree with the predictions of both the Johnston theory (based on the free volume concept) and the Barton theory (based on the configurational entropy concept) with no adjustable parameters. We will also discuss how loaded drugs having asymmetric affinities for LA and GL impact the Tg properties of uniform and gradient PLGA materials. |
Tuesday, March 7, 2023 4:48PM - 5:00PM |
K23.00008: Modeling Glass-Forming Systems to Illuminate Connections Between Experimental Properties Ronald P White, Jane E Lipson In this talk we will discuss our recent work in the modeling of dielectric spectroscopy data on glass-forming polymers, nanocomposites, and interfaces, as well as our current research on rheological properties, ellipsometry of polymer films to obtain free volume, and expansivities of melts and glasses under mechanical constraint. The thread that ties together all of these projects is our goal to relate thermodynamic concepts like expansivity, random close-packing, and free volume to dynamics measurements such as segmental relaxation times (τ) and glass transition temperatures (Tg). Our modeling begins with analysis of thermodynamic data (e.g. PVT data) to reveal the system's characteristic volume at random close-packing (Vcp), from which free volume, Vfree(T,P) = V(T,P) − Vcp, can be defined. This definition is fundamental and can be unambiguously applied; it does not a priori assume a connection to dynamics and does not require any dynamics data, yet, it correlates strongly with Tg and τ. For example, knowledge of Vfree(T,P) allows us to predict the volume-based contribution to dynamics, because of the general form lnτ(T,P) ∼ f(T)×(1/Vfree(T,P)) resulting from our model for segmental relaxation. |
Tuesday, March 7, 2023 5:00PM - 5:12PM |
K23.00009: Enhancing the Dielectric Response of the ‘Hidden’ Secondary β-Relaxation in Polystyrene Caitlin M Probert The relaxation dynamics of polymers generally involve a series of molecular relaxations (α, β, γ…), and understanding these relaxations is important both for the fundamental understanding of polymer glass formation and the design of polymer glasses for specific applications. In particular, direct links have been identified between the secondary β-relaxation and the structural α-relaxation, where the latter defines the glass transition, suggesting that the β-relaxation is a precursor to structural dynamics in polymers. It is thus important to determine how the β-relaxation in polymers varies with parameters such as chain-length or `local’ chain characteristics. To achieve this, broadband dielectric spectroscopy (BDS) is a widely used technique that probes the reorientation of permanent dipoles over a wide frequency range. We here focus on the behaviour of one of the most investigated polymers - polystyrene (PS). PS is characterized by a relatively low segmental dipole moment, which makes the study of secondary relaxations using BDS difficult; BDS data for PS typically demonstrate a so-called excess wing on the high-frequency side of the dielectric α-relaxation response [1], and its interpretation is unclear. To address this, we add polar probe molecules to PS [2], and investigate the resulting effects on the BDS response. We find that addition of suitable polar probes leads to the observation of a molecular relaxation (here termed β*-relaxation). Comparisons of the properties of this β*-relaxation with results from small amplitude oscillatory shear rheology, dynamic mechanical analysis, as well as literature data, provide strong evidence that the dielectric β*-relaxation is directly related to a secondary β-relaxation that is present, but not fully observable, in BDS experiments on pure PS. Using the technique of probe addition, we determine the β-relaxation of PS for varying molecular weights, and we investigate the effects of probe characteristics on the observations. |
Tuesday, March 7, 2023 5:12PM - 5:24PM |
K23.00010: A model for translational and orientational relaxation in glass forming materials Grigori A Medvedev Spectroscopic techniques and the MD simulations allow for studying molecular motions in glass forming materials, which are believed to hold the key to understanding the glass transition phenomena. The mean squared displacement and the orientational autocorrelators exhibit several generic features, including: the sub-diffusive regime, dynamic heterogeneity, distinct α- and β-processes, and the fact that in some cases the cos2θ orientational autocorrelator decays slower than the cosθ autocorrelator. A mechanism responsible for these features is poorly understood. The current models are based on the picture of effective energy barriers, which carries a number of well-known conceptual difficulties. In this communication, a random walk model is proposed that does not involve energy barriers. The model relies on the tortuosity of the path a molecule travels to account for the apparent slowing down of mobility. The model captures characteristic features of the translational and orientational relaxation in glass forming materials. |
Tuesday, March 7, 2023 5:24PM - 5:36PM |
K23.00011: Glycerol glass under pressure Cade M Vallero, Muhtar Ahart, Seiji Kojima, Russell J Hemley Glycerol is a medium fragility glass-forming fluid widely used in research applications and as a cryoprotectant. However, numerous questions remain concerning its behavior on compression through its high-pressure glassy state. We report high-pressure measurements of the elastic properties, relaxation times, photoelastic constants, and Cauchy-like relations of glycerol to 14 GPa by Brillouin scattering. In addition, the boson peak was measured to 10 GPa by Raman scattering. Using direct volumetric measurements, we also determine the pressure dependence of the Cp/Cv ratio. The combined results provide a deeper analysis into the thermodynamics of this glass forming material to be inspected. |
Tuesday, March 7, 2023 5:36PM - 5:48PM |
K23.00012: Film thickness dependence of the kinetic stability and glass transition temperature of ultrastable and rejuvenated PS films prepared by vapor deposition James A Forrest, Junjie Yin We have used ellipsometry to study the properties of ultrastable and rejuvenated films of near-oligomeric PS with thickness ranging from 10-100 nm. The measured Tg of the rejuvenated films show a reduced Tg value with decreasing thickness as seen for many other PS samples. While a Tg measured from cooling is not possible for the as-deposited stable glass system, the onset temperatures (Tons) for rejuvenation show the same film thickness dependence such that the difference between Tg and Tons is a constant temperature. The thin films also show evidence for a portion of the sample already being in the normal state before the first heating. The results are discussed in terms of studies of thin films of stable molecular glass as well as thin films of high Mw PS produced by spin casting. |
Tuesday, March 7, 2023 5:48PM - 6:00PM |
K23.00013: Space that cannot be accessed is not free: Why volume and dynamics decouple in nanocomposite matrices Jane E Lipson, Ronald P White We report on our first modeling studies involving nanocomposite systems: polyvinyl acetate (PVAc) and polystyrene (PS), each with silica nanoparticles. In each case, experimental data showed that the segmental relaxation times and glass transition temperatures of the polymer matrix material in the nanocomposite were essentially unchanged relative to those of the neat polymer, however, the specific volume of the former was higher than that of the latter (with temperature fixed). We have demonstrated for many systems that segmental relaxation times at fixed temperature track with density changes, as described using our Cooperative Free Volume (CFV) model. So it was provocative to encounter examples where this connection seemed to break down – but does it, really? This talk will provide the answer, and yield insight as to how nanoparticle presence can affect thermophysical properties, and therefore dynamic response. |
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