Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V43: Polymer Glasses |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Connie Roth, Emory Univ Room: LACC 503 |
Thursday, March 8, 2018 2:30PM - 2:42PM |
V43.00001: How brittle-ductile transition (BDT) in uniaxial compression depends on the network structure affected by predeformation Jianning Liu, Xiaoxiao Li, Weiyu Wang, Zhichen Zhao, Jimmy Mays, Shiqing Wang Polystyrene of sufficiently high molecular weight is nevertheless brittle at room temperature upon tensile extension. However, the same brittle PS is actually ductile in uniaxial compression. Our recently proposed phenomenological molecular model1 can not only explain this intriguing different from a molecular viewpoint but also anticipate how predeformation such as melt stretching shifts the BDT. Experiments are carried out to verify our theoretical predictions. Specifically, we apply melt stretching to alter the chain network structure and examine the effects of melt stretching in comparison to the predicted responses. |
Thursday, March 8, 2018 2:42PM - 2:54PM |
V43.00002: Relationship between the mobility in glass forming liquids and excess thermodynamic quantities James Caruthers, Grigori Medvedev The key characteristic of the glass is the dramatic decrease in mobility as the temperature is decreased towards the glass transition temperature, where this decrease in mobility can be ten or more orders-of-magnitude more than expected from the Arrhenian response exhibited at temperatures well above Tg. One model that has been proposed to describe this super-Arrhenian behavior is due to Adams and Gibbs (J. Chem. Phys., 1965), where the logarithm of the mobility is postulated to be proportional to 1/TSc where Sc is the configurational entropy. Alternatively, in order to describe the nonlinear viscoelastic behavior of glassy polymers constitutive models have been developed (Caruthers, et al., Polymer, 2004; Medvedev and Caruthers, J. Rheology, 2013) where the logarithm of the mobility is postulated to be proportional to 1/Uc where Uc is the configurational internal energy. In this talk we will critically compare the ability of 1/TSc, 1/Uc and other configurational and excess thermodynamic quantities to quantitatively describe the mobility of equilibrium liquids above Tg as a function of temperature and pressure. |
Thursday, March 8, 2018 2:54PM - 3:06PM |
V43.00003: Does the “excess wing” in the relaxation spectrum of glass formers have a fine structure? Grigori Medvedev, Yelin Ni, James Caruthers The relaxation spectrum of glassy materials as determined by various spectroscopic techniques, including dielectric, light scattering, NMR as well as mechanical contains several universal features. In particular at higher frequencies than the α-relaxation peak one finds a broad wedge-like feature commonly referred to as the excess wing. Depending on the material, the excess wing may or may not be followed (when going to still higher frequencies) by another peak known as the β-relaxation peak. It is still a subject of debate whether the excess wing and the β-peak are manifestations of the same underlying mechanism. This is because the materials that only possess the excess wing under standard thermal history begin to exhibit a peak or at least a shoulder when aged for a sufficiently long time. We report on dynamic mechanical experiments on several epoxy systems quenched and aged. In the analysis we show that the relaxation spectrum needed to describe the sub-Tg data consists of multiple relatively narrow peaks (of which the β-peak is but one representative) rather than the continuous “wing”. The relaxation processes giving rise to these peaks have Arrhenian temperature dependence with the activation energies systematically ordered. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V43.00004: A molecular dynamics study of the effect of pre-deformation on compression of glassy polymers Yexin Zheng, Xiaoxiao Li, Shiqing Wang, Mesfin Tsige Previous and ongoing experimental studies have revealed intriguing effects of melt stretching on uniaxial extension and compression. For example, brittle PS turns ductile during extension after melt stretching.1 The effect is different for the same melt-stretched PS and PMMA on compression.2 We carry out molecular dynamic (MD) simulation using coarse-grained models for PS and PMMA to explore the molecular mechanism leading to activation of the glassy state during either extension or compression. In addition, the MD simulation allows us to reveal that the origin of stress in response to extension is different from that in compression. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V43.00005: Counter-Intuitive Stress Relaxation Behavior of Glass Forming Materials and the Ability of Rheologically Simple “Material Clock” Nonlinear Viscoelastic Modeling to Represent it Jamie Kropka, Kevin Long The observance of an increase in glassy polymer relaxation rates under a mechanical deformation is often referred to as deformation induced mobility (DIM). It has been argued that stress relaxation experiments can provide indirect evidence of this phenomenon. Recently, stress relaxation experiments have been interpreted as demonstrating a mobility decrease with increased deformation when very slow strain rates, 1.2 x 10-5 s-1, are used to apply the deformation. This would suggest against generality of DIM and would have significant implications to constitutive models founded on this principle. Here, we test the proposed interpretation by assessing the situation using an extensively validated, thermorheologically simple, “material clock” model, the Simplified Potential Energy Clock (SPEC) model. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V43.00006: Effect of chain stiffness on polymer glasses’ shear deformation micromechanisms Hong Nguyen, Robert Hoy Using molecular dynamics simulations, we study how chain stiffness affects how glassy polymers deform under applied shear. Loosely entangled systems composed of flexible chains exhibit strong shear banding and subsequent strain softening whereas tightly entangled systems composed of semiflexible chains exhibit neither of these. For all systems, inflection points in their stress-strain curves correspond to the onset of chain scission, but nonlinear strain hardening continues well beyond this point. Tightly entangled systems build up considerable elastic energy before fracturing via chain scission. Loosely entanged systems’ deformation is far more dissipative and their fracture occurs via chain pullout. Their much larger fracture strain produces far greater prefracture chain alignment, which in turn reduces interchain friction and further favors pullout over scission. We further explain these differences in terms of microscale plasticity metrics. Tightly entangled systems’ much higher rate of chain scission causes their plastic flow to be far more heterogeneous, and they ultimately fail along significantly sharper fracture planes than their loosely entangled counterparts. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V43.00007: Predicting glass transition temperatures from short-time simulations of PMMA and PS Zijun Lu, Philip Taylor, Solomon Duki, Mesfin Tsige
|
Thursday, March 8, 2018 3:54PM - 4:06PM |
V43.00008: The role of connectivity in polymer glasses Anna Lappala, Luke Sefton, Paul Fenimore, Eugene Terentjev
|
Thursday, March 8, 2018 4:06PM - 4:18PM |
V43.00009: Quantification of evaporatively purified polystyrene oligomers Adam Raegen, James Forrest We present a detailed study of the dependence of glass transition temperature on actual molecular weight for extremely monodisperse polymeric/oligomeric samples. Any synthetic polymerisation process leads to a final sample with an unavoidable degree of polydispersity. For example even with living polymerisation, this intrinsic polydispersity gives rise to a polymer/oligomer with nominally 8 repeat units (N = 8) that contains a non negligible fraction of chains with lengths N = 3 to 13. By employing vacuum distillation on commercially available “standard” polymer, we are able to obtain samples with a much sharper molecular weight profile. These samples (before and after distillation) are compared with MALDI-TOF spectroscopy as well as differential scanning calorimetry. Purified polymers are shown to have a polydispersity index lower than 1.005. We compare calorimetric glass transitions of the polymer standards with the behavior of purified samples. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V43.00010: Heat Capacity and Tg of Polycyanurates as a Function of Crosslink Density and Implications for the Kauzmann Paradox Evelyn Lopez, Yung Pyo Koh, Sindee Simon The absolute heat capacity and glass transition temperature (Tg) of a cyanurate trimer, cyanurate oligomers with different crosslink densities, and a fully crosslinked polycyanurate are measured by the step-scan method using conventional differential scanning calorimetry (DSC). The crosslink density (XD) is varied by changing the monofunctional to difunctional cyanate ester ratio from XD = 0 to 1. Preliminary results show that as the crosslink density increases, the step change in heat capacity at Tg (ΔCp) decreases and Tg increases, as expected. Interestingly, the glass and liquid heat capacities of all cyanurates are found to be on the same lines within experimental error, indicating that ΔCp depends only on Tg. Equations in the literature for the influence of molecular weight and crosslink density on Tg are tested for these series of materials. Furthermore, an extrapolation of the liquid heat capacity of the fully crosslinked polymer to lower temperatures can be performed and a test of the Kauzmann paradox can be made. The results indicate that, similar to our prior results of poly(α-methyl styrene), entropy decreases smoothly and slowly as temperature changes with no discernable transition in the temperature range where the Kauzmann temperature is expected to lie. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V43.00011: Explaining the T,V-Dependent Dynamics of Glass Forming Liquids: The Cooperative Free Volume Model Tested Against New Simulation Results Ronald White, Jane Lipson We present a rate model, the "cooperative free volume model" (CFV), to describe segmental relaxation times as a function of a system's free volume, Vfree, and its temperature, T. Our most important result is an analytic expression that provides a full pressure dependent description in the Arrhenius to non-Arrhenius crossover region and at all higher T. We demonstrate agreement with extensive simulation results for both simple Lennard-Jones fluids and 20-mer polymers. In the CFV model, free volume dictates the number of cooperating particles and thus the overall free energy of activation. This underlies the volume contribution to dynamics, and has been successfully applied to all systems tested. The CFV model predicts the non-Arrhenius behavior on isobars, with an energetic contribution from changing T and, simultaneously, a contribution from changing Vfree; the implication is a change in the number of cooperating particles. Furthermore, as the Arrhenius regime is approached, we show that the entropy of activation, and a significant contribution from the gas kinetic T-dependence, cannot be ignored. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V43.00012: Dynamic Behavior of Ultrastable Amorphous Teflon Films in the Deep Glassy Regime Heedong Yoon, Gregory McKenna Recently, we have succeeded in producing high molecular weight ultra-stable amorphous Teflon films by vapor pyrolysis deposition method [Macromolecules 2017, 50, 4562−4574.]. The films produced here exhibited 57 K fictive temperature (Tf) reduction when the substrate temperature was between 0.75 to 0.97 times the glass transition temperature (Tg). This large Tf reduction gives an opportunity to investigate the dynamic behavior in the deep glassy regime. In the current work, we use this ultra-stable glass to study the relaxation behavior of the polymer in the temperature range between Tf and Tg where the glass has lower enthalpy and specific volume than the equilibrium system. The relaxation behavior of stable Teflon films was measured using the TTU nano bubble inflation technique following Struik’s protocol. The results are compared with Vogel-Fulcher-Tammann (VFT) dynamics determined above Tg. We found that there is a strong deviation from VFT dynamics in the upper bound regime. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V43.00013: The highly ordered state of glassy polymers from quenching Youngjong Kang Kang, Jae Hyun Sim Controlling the degree of structural order in polymers have been issue of interest in polymer science and industry in that it widely influences optical, electrical, and mechanical properties. Highly-ordered structures of polymers, in general, has been obtained by controlling processing temperature but it costs significant time and energy. In this study, glassy polymers such as poly(methyl methacrylate) (PMMA) were successfully textured by quenching process. The mixture of Polymer/volatile cryatallizable additive (VCA) was molten and quenched. Large birefringence was observed by polarized optical microscopy and ellipsometry. First order transition and superheating effect were observed by differential scanning calorimetry. Melting temperature was 502 K for syndiotactic-PMMA. Strong in-plane diffraction patterns from grazing incident X-ray diffraction study also supports textured feature. Preparation of well-ordered and closely packed, stretched PMMA chains from quenching have not yet been reported. Interestingly molecular dynamics simulation showed stretched polymer chains in melt with VCA. The stretched chains were quickly trapped in ordered state. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V43.00014: Wrinkles and Cracks -- Quantifying the Mechanical Properties of Polymer Thin Films Shawn Chen, Caleb Fishell, Jae-Hwang Lee, Christopher Soles, Edwin Chan, Kenneth Shull To further engineer materials that are mechanically strong and fracture-resistant, direct knowledge of the key mechanical properties such as stiffness, strength, and ductility is a necessity. Previous works have demonstrated the capabilities of a combined wrinkling-cracking test to measure the elastic modulus of ultrathin glassy polymer films. It is shown that the modulus observed often deviate significantly from the bulk, and may undergo subcritical cracking from continuous loading conditions. In this study, the elastic and fracture properties of polycarbonate thin films is measured as a function of film thickness and strain rate. Additionally, using the crack density and channel crack growth, in combination with our numerical model, the strain energy release rate 𝒢 can be calculated to benchmark the failure criterion of the material in terms of a crack driving force that is independent of sample history and geometry. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V43.00015: Imaging strain field near the tip of a propoagting crack in drying colloidal films Han Gao, Ye Xu Drying-induced cracking is commonly seen in colloidal coatings like paints and is also a major limiting factor in developing solution-processed functional nanoparticle films. Due to its non-equilibrium nature, it is also challenging to directly study the mechanics, i.e. stress and strain, of cracks formed in those films. In this work, we use video microscopy to track a propogating crack in a colloidal nanoparticle film over the whole drying process, while quantifying microscopic strain field near the crack tip using fluorescent tracer particles. Our work will shed light on the underlying fracture mechanics that governs the cracking behaviors in drying colloidal films and can also provide a general tool in studying mechanics of other soft matter systems. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700