Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S08: Polymer GlassesLive
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Sponsoring Units: DPOLY Chair: David Simmons |
Thursday, March 18, 2021 11:30AM - 11:42AM Live |
S08.00001: A “universal" dependence of the glass transition temperatures in polymers on molecular weight? Peter Olmsted, Matthew Reynolds, Daniel L. Baker, Robin Masurel, Johan Mattsson The glass transition temperature Tg in polymers increases with molecular weight M, but the detailed Tg(M) dependence in polymers is not well understood. Evidence has accumulated that many polymers have a characteristic Tg(M) that has three regimes: (I) short oligomers which are fairly stiff near Tg; (II) oligomers with masses between roughly one and some 10s of a Rouse mass; and (III) much longer polymers, typically in the entangled and/or fully Gaussian regime. Here we present new data from a wide range of different polymers, and together with literature data demonstrate a remarkable scaling form. We show that these data are not consistent with existing theories of Tg(M). We show from primary (alpha) and secondary (beta and gamma) relaxations that this behaviour can be qualitatively understood in terms of a dynamic facilitation picture in which all of the dynamics are scaled by the molecular weight of the smallest excitation. We compare and contrast this with theories based on free volume, configurational entropy, molecular dynamics simulations, polymer-based mode coupling theories, and activated liquid-state dynamics. |
Thursday, March 18, 2021 11:42AM - 11:54AM Live |
S08.00002: Effect of entanglements on the glass transition behavior of polymer chains in a melt Manjesh Singh, Hsiao-Ping Hsu, Kurt Kremer In the present work, we study the effect of entanglements on the glass transition behavior of high molecular weight polymers, by comparing disentangled and entangeled melts of high molecular weight polystyrene of identical molecular weight. Single-chain-nanoparticles (SCNPs) were used to prepare the disentangled melt. The SCNPs were prepared by electrospraying technique and characterized using scanning electron microscope and atomic force microscope techniques. Glass transition temeprature (Tg) of disentangled and entangled polymer melts were measured using three different techniques: differential scanning calorimetry, Brillouin light spectroscopy and rheological experiments. We also performed bead-spring polymer model based molecular dynamics simulations. A recently developed approach was used to compare entangled and disentangled polymer melts under cooling. While our experiments suggest a small decrease in the glass transition temperature of films of nanoparticles in comparison to entangled melts, simulations do not observe any significant difference, despite rather different chain conformations. |
Thursday, March 18, 2021 11:54AM - 12:06PM Live |
S08.00003: Block Copolymer Micelle Core Tg Can Be Determined by Corona NMR T2 Relaxation Measurements Hyun Chang Kim, Daniel Fesenmeier, Hansol Wee, You-Yeon Won The conformational and dynamic properties of PEG corona chains in PS-PEG, PtBMA-PEG and PLGA-PEG block copolymer micelles were investigated by 1H NMR spectroscopy. The PEG corona chains of PtBMA-PEG and PLGA-PEG micelles are fully hydrated. The PEG corona chains of PS-PEG micelles, on the other hand, are only partially hydrated; significant portions of the PEG segments are collapsed onto the surface of the PS core domain in order to minimize unfavorable contact between the PS and water. 1H NMR spin-spin transverse (T2) relaxation measurements suggest that the mobility of the collapsed PEG corona segments of PS-PEG micelles is about 5 – 8 times slower than that of the fully hydrated PEG segments, and is significantly influenced by the glass transition of the micelle core domain. The micelle core Tg values determined by PEG corona T2 relaxation measurements for PS-PEG and PtBMA-PEG micelles agreed well with the Tg values obtained by measurements of fluorescence from FCVJ-loaded PS-PEG and PtBMA-PEG micelles. Therefore, this study establishes that corona T2 measurements can be used to determine the micelle core Tg. |
Thursday, March 18, 2021 12:06PM - 12:18PM Live |
S08.00004: Surface glass transition of polymer films supported by a weakly interacting substrate Jinsong YAN, Jianquan Xu, Ophelia K.C Tsui, Lu-Tao Weng Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and contact angle measurements were used to measure the surface glass transition temperature, Tgsurf, of polystyrene (PS) films with molecular weights from 12k to 1119 kg/mol. For the films supported by silica, as the film thickness, h, was decreased, Tgsurf decreased in a manner similar to » Tgfilm – 20 K (where Tgfilm is the Tg of the films), supporting correlation between Tgsurf and Tgfilm. At the same time, the width of the surface glass transition, W, broadened noticeably when h fell below » Rg + 10 nm, where Rg is the radius of gyration the polymer. We have also studied Tgsurf of PS films supported by polydimethylsiloxane (PDMS) - a substrate with weaker surface interactions with PS than silica does. A similar but bigger reduction in Tgsurf and a similar broadening in W were observed with decreasing h. We tentatively interpret these results based on de Genne’s sliding chain motion. |
Thursday, March 18, 2021 12:18PM - 12:30PM Live |
S08.00005: Sequence Effects on the Glass Transition - Suppression from Block to Alternating Copolymers William Drayer, David Simmons There is evidence that the glass transition temperature (Tg) of copolymers is sensitive to their sequence, potentially providing a new strategy for polymer property control at fixed composition. However, the nature of this sequence-dependence remains poorly understood. Here, we report on results of bead-spring molecular dynamic simulations probing the effect of copolymer sequence on Tg suppression. Results indicate that the Fox equation for mixing fails to predict the qualitative behavior of Tg; however, two mechanisms for Tg suppression emerge. For large positive chi, in the diblock limit, sequence dictates the domain size, modulating interfacial alterations in the glass transition temperature. Notably, as sequences become more like the alternating chain, much larger suppression is observed along with the expected loss of well-defined domains. Results suggest that, in this regime, direct sequence-modulated modification of segmental packing and localization length-scale mediates sequence control of Tg. The findings provide guidance towards design of copolymers with targeted transport and dynamical properties. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S08.00006: Structural relaxation in a polymeric glass well below Tg Hosup Song, Grigori Medvedev, James M Caruthers Glass formers below Tg exhibit slow relaxation toward equilibrium state in a process known as physical aging. The time to reach equilibrium increases drastically as the temperature decreases so that it is impractical to achieve equilibration for temperatures below Tg-15°C. Consequently, it is typically assumed that at temperatures well below Tg physical aging ceases in glasses. We report on linear viscoelastic experiments on an epoxy material, where the linear viscoelastic shear storage and loss isotherms were obtained for a wide temperature range both above Tg and deep in the glassy state. At each temperature a frequency scan from 10-2 Hz to 101.5 Hz was performed first after 2 hr of annealing and then repeated after 6 hr of annealing. Somewhat unexpectedly, even at temperature as low as Tg-280°C there is a noticeable systematic difference between 2 hr and 6 hr isotherms. At room temperature (Tg-160°C) the measurements were performed at longer times of up to 2 months, where systematic shift in the isotherms continued. These results indicate that, although the rate of equilibration slows down progressively with decrease in temperature, not all relaxation processes are frozen even well below Tg. |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S08.00007: Temperature dependence of the relaxation time for a family of glass formers with systematically varying architecture Grigori Medvedev, Yelin Ni, James M Caruthers Using the mechanical and dielectric relaxation experiments, the linear relaxation behavior for series of diglycidyl ether of bisphenol-A (DGEBA) based polymers was investigated. The set of materials included: (i) the DGEBA monomer, (ii) a DGEBA based linear polymer (i.e. phenoxy) and (iii) a series of DGEBA based epoxy networks where various diamine cross-linking agents were used. For two of the diamine crosslinking agents the tightness of the network was systematically varied from loose to extremely tight. With the exception of loose networks and phenoxy, all systems exhibited just the α-relaxation process above Tg. In contrast, the loose networks and phenoxy exhibit the α-relaxation and a lower frequency α+-relaxation process that has a different temperature dependence than the main α-process. There are only two distinct logaT vs T dependencies, which are both super-Arrhenian – branch A and branch B, where branch B has a stronger temperature dependence. The monomer and the linear polymer follow the A branch and all the tighter networks, despite their chemical variety, exhibit the same B branch temperature dependence when adjusted for Tg. Intriguingly, in case of the loose networks the α+-process follows the A branch and the α-process follows the B branch. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S08.00008: Using TS2 Model to Describe the Dynamics of Amorphous Random Copolymers or Miscible Polymer Blends near the Glass Transition Valeriy Ginzburg The glass transition in miscible polymer blends or random copolymers has been an important area of research since at least the 1950-s. While Fox, Gordon-Taylor, Kwei, and other equations are able to successfully describe the dependence of Tg on the blend or copolymer composition, they are usually less definitive about the prediction of the fragility and the dependence of the relaxation time on temperature. Here, we extend our recent lattice model of glass-forming systems (two-state, two-(time)scale or TS21) to the cases of miscible blends and random copolymers. In particular, we apply the model to the case of PS-PMMA random copolymers, and successfully describe the dielectric spectroscopy results for the α- and β-relaxation times. These results establish mixing rules to enable the generalization of the TS2 approach to heterogeneous systems (phase-separated polymer blends, thin films, and nanocomposites). |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S08.00009: Temperature Dependence Free Volume and Dynamic Heterogeneity in Polymer Melt He Cheng, Zehua Han, Guisheng Jiao, Charles C Han Free volume and dynamic heterogeneity may be the most frequently used concepts in polymer physics, but they have never been observed directly in experiment. Here, by a combination of neutron scattering and molecular dynamic simulation, the temperature dependence of free volume and dynamic heterogeneity can be observed in a Polystyrene melt. Neutron scattering profiles shows that only the segment-segment peak moves to higher scattering vector with the decrease of temperature. It indicates that the volume of the polymer melt decreases with the decease amplitude of thermal fluctuation. When it is close to the glass transition temperature, polymer chains are confined in the cage from its neighboring chains. The fractional free volume can be defined by the volume outside the confinement in the cage over the cage volume. Glass formation happens when the cage size is small enough, either temperature or pressure changes, to freeze the out-of-cage motion. During solidification, dynamic heterogeneity develops over a long range of time and length scales, and string-like cooperative rearrangement regions are observed. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S08.00010: Mobility in Pressure-Densified and Pressure-Expanded Polystyrene Glass: Tests of the KAHR Model xiao zhao, Luigi Grassia, Sindee L Simon The mobility of pressure-densified and pressure-expanded polystyrene glass is explored using a custom-built pressurizable dilatometer. Various pressure-volume-temperature (PVT) histories are performed, including pressure up- and down-jumps in the glassy state followed by temperature ramps with (or without) the succeeding isothermal aging, as well as conventional isobaric temperature scans and isothermal pressure scans. The isobaric cooling and heating scans and the isothermal pressure scans, as well as the pressure-dependent glass transition temperature (Tg), are quantitively described by the Kovacs-Aklonis-Hutchinson-Ramos (KAHR) model of structural recovery. On the other hand, the behavior of pressure-densified and pressure-expanded glass is not captured by the conventional KAHR model. The model predictions improve for changes to the equation for the relaxation time and its dependence on the fictive temperature and pressure. The physics "missing" from the conventional KAHR model is further probed by examining the isothermal aging of the pressure-densified and pressure-expanded glasses. In these aging experiments, a memory-like effect is observed that allows rigorous testing of the modified model. |
Thursday, March 18, 2021 1:30PM - 1:42PM Live |
S08.00011: Effects of Coarse-Graining on Molecular Dynamics Simulations of Craze Formation in Polystyrene Glass Jiuling Wang, Ting Ge, Mark Owen Robbins Crazing is a unique failure mechanism of glassy polymers. Craze forms through a fibril-drawing process at constant stress S, and its density ρcraze is lower than the density ρglass of the glass. We perform molecular dynamics simulations of craze formation in glassy polystyrene using a United-Atom (UA) model and a Coarse-Grained (CG) model. The extension ratio Λ=ρglass/ρcraze is almost the same for the two models and agrees with the experimental value. This is consistent with the argument that Λ is determined by the entanglement molecular weight Me, which is identical in the simulations and experiments. The structure of a craze as characterized by the distribution of fibril diameters and the structure factor is almost identical in the UA and CG simulations, showing that the CG model preserves the structural correlations. The drawing stress SCG in the CG simulation is smaller than SUA at the same drawing velocity in the UA simulation. This reduction reflects that the coarse-graining leads to a smoother potential with lower energy barriers for craze formation. The same large fraction (70%-80%) of SCG and SUA is dissipative stress, showing that the coarse-graining preserves the relative contributions from the energy dissipation and internal energy change during craze formation. |
Thursday, March 18, 2021 1:42PM - 1:54PM Live |
S08.00012: Fracture Mechanical Behaviors of Various Polymers from Brittle Glasses to Elastomers Shi-Qing Wang, Travis Smith, Chaitanya Ramanand Gupta We expand our research on mechanical performance of glassy and semicrystalline polymers to include evaluation of their toughness in the language of fracture mechanics, i.e., in terms of critical energy release rate at which crack growth become inevitable. By showing how various thermoplastics can be made to avoid brittle fracture in plane-strain deformation, we dive into the molecular origins of structural failures as observed in the form of crack propagation and search for alternative explanation of fracture mechanical properties of thermoplastics and elastomers. |
Thursday, March 18, 2021 1:54PM - 2:06PM Live |
S08.00013: Mechanical spectral hole burning in glassy polymers Satish Chandra Hari Mangalara, Shreejaya Paudel, Gregory B McKenna Polymeric glasses in deep glassy state exhibit excellent mechanical properties and are commonly used in engineering applications. Linear viscoelastic properties of polymers are well characterized by mechanical and di-electric spectroscopies, whereas methods such as large amplitude oscillatory shear (LAOS) provide material response under large non-linear deformation for polymer solutions and melts. Non-linear dynamics of glassy polymers has seldom been reported and an understanding is important from both fundamental and practical perspectives. In the current work, mechanical spectral hole burning (MSHB), an extension of LAOS methodology, has been applied on two amorphous glassy polymers, poly(methyl methacrylate) (PMMA) and polycarbonate to investigate the non-linear dynamics close to the β transition [1]. Results from polycarbonate show a weak hole intensity compared to PMMA thus relating the hole burning event with the dynamic heterogeneity as related to the occurrence of strong β transition. |
Thursday, March 18, 2021 2:06PM - 2:18PM Live |
S08.00014: Understanding the chain-length-dependent relaxation dynamics in polymeric glass-formers Johan Mattsson, Daniel L. Baker, Matthew Reynolds, Robin Masurel, Peter D Olmsted The glass transition temperature Tg in polymers increases with increasing molecular weight M, but the detailed Tg(M) dependence is not well understood. Here, we present experimental results of the M-dependence of both the structural (α) relaxation process, which controls the glass transition, and faster secondary relaxation processes for polymers of varying chain flexibility. Based on our results, we propose that these relaxations are linked through dynamic facilitation. This leads to the conclusion that the chain-length-dependent α relaxation, and thus Tg(M), is controlled by a relatively ‘local’ fundamental relaxation, for which the relevant metric is linked to local chain flexibility. Thus, following earlier work of many others, we argue that local dihedral barriers play an important role in controlling the dynamics. We identify regimes in M where intra- and inter-molecular relaxation dynamics play different roles in defining the dynamics. We argue that this naturally gives rise to clear differences to the behaviour observed in non-polymeric glass-formers with simpler ‘rigid’ structures, or in barrier-free models of polymers. |
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