Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A25: Deformation, Flow and Relaxation of Melt and Glassy Polymers |
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Sponsoring Units: DPOLY Chair: Robert Riggleman, University of Pennsylvania Room: BCEC 160A |
Monday, March 4, 2019 8:00AM - 8:12AM |
A25.00001: Dynamic Mechanical Spectrum of an Epoxy Polymer James M Caruthers, Yelin Ni, Grigori Medvedev Dynamic mechanical experiments were performed on an epoxy thermoset with a Tg=101°C over a wide temperature range from -150°C to 260°C. Time-temperature superposition (TTS) is only observed at temperatures above Tg+100°C where the network relaxation is the dominant relaxation process. At lower temperatures, both above and below Tg, TTS is clearly violated. The traditional representation of the relaxation spectrum as a sum of several broad empirical functions each with its own independent temperature dependence that are often designated as α-, β-, etc. processes cannot describe the set of isotherms. Using a procedure that includes Tikhonov regularization with adaptive meshing, a relaxation spectrum consisting of discrete processes emerges. It is shown that determination of the spectrum is robust, resulting in a continuous spectrum at high temperatures and discrete peaks at lower temperatures. Each relaxation process has its own temperature dependence, which becomes stronger as Tg is approached. Below Tg the relaxation time of each process is affected differently by aging. The implications of a discrete spectrum of relaxation times vs. the more commonly assumed continuous distribution of relaxation processes for a theory of the amorphous state will be discussed. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A25.00002: Different temperature dependence of nonlinear rheological responses in melt stretching Jianning Liu, Shiqing Wang Decades of research in nonlinear polymer rheology [1] have not fully cleared all the confusion in the literature including one concerning the alleged difference in the rheological responses of entangled solutions and melts to fast extensional deformation. The idea of nematic interaction modification of chain friction has caused Huang et al. [2] to find support from the examination of three polystyrene solutions where the solvent is polystyrene of respective molecular weight equal 1 kg/mol (that has Tg = 20 oC), 2 kg/mol and 4 kg/mol with Tg ~ 75 oC. We refute this idea by investigating similar PS solutions to indicate a different interpretation by showing how a breakdown of the time-temperature superposition occurs in nonlinear responses of these solutions, causing the 1K solution to respond more strongly than a 5K solution where the 1K solution is examined at a lower temperature. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A25.00003: Brittle-Ductile Transition of a polymer glass in the presence of a crack Yexin Zheng, Mesfin Tsige, Shiqing Wang Fracture behavior of polymer glasses under deformation has been widely studied in the experiment. However, molecular-level understanding of crack propagation in polymer glasses is still lacking. In this work, the standard bead-spring model based molecular dynamic simulation was modified to gain microscopic insights into the crack propagation in glassy polymers. In the presence of a pre-existing crack, the transition from brittle failure to shear yielding in extension was investigated by varying the temperature, the length of the pre-crack length and deformation rate. The results from the simulation are consistent with experiment. Our simulation results also show that the brittle-ductile transition temperature is strongly dependent on the strength of the pair potential. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A25.00004: The relationship between unstable localized vibrational modes and dynamical heterogeneity in glass formers Wengang Zhang, Jack Douglas, Francis Starr The heterogenous dynamics of glass-forming materials is commonly characterized by the spatially correlated particles (clusters) with high and low mobilities, and are helpful for explaining the temperature dependence of the non-Arrhenius behavior of relaxation time. Due to the inherent difficulty to experimentally observe heterogeneous dynamical clusters, we consider the possibility that the localized unstable modes of the vibrational spectrum may be related to dynamical clusters. Specifically, we analyze the instantaneous normal modes (INMs) associated with the Hessian matrix of glass-forming liquids. We find that the particles that contribute most to the unstable, localized modes form spatially correlated clusters. We quantify the degree to which these clusters correlate to string-like cooperative motions and mobile clusters, and how polymer topology affects these clusters. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A25.00005: Making glassy semicrystalline polylactic acid ductile Masoud Razavi, Shiqing Wang We borrow the insights that explain the molecular mechanism for the brittle-to-ductile transition in polymeric glasses [1] to study the mechanics of semicrystalline polymers. Polyesters such as polylactic acid (PLA) and poly(ethylene terephthalate) (PET) are known to be brittle in their fully crystalline form. PLA is brittle in its amorphous state because of the rapid physical aging that cannot be avoided. Such poor mechanical properties are the bottleneck preventing the wider application of PLA to replace the petroleum-based polymers including PET. We demonstrate a strategy to make PLA tough and heat resistant with zero shrinkage at 100 oC along with a molecular picture detailing the structure-property relationship. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A25.00006: Comparing Segmental Dynamics in Polymer Glasses during Deformation: PLA and PMMA Trevor Bennin, Joshua V Ricci, Mark Ediger Polymer glasses are ubiquitous engineering materials, but their deformation is not well understood at a fundamental level. Here, the segmental dynamics of glassy poly(lactic acid) (PLA) were measured with a probe reorientation technique during constant strain rate tensile deformation at temperatures between Tg – 25 K and Tg – 15 K and local strain rates between 10-5 s-1 and 10-4 s-1. The relaxation time of the undeformed PLA is about 104 s and decreases by up to a factor of 30 during deformation. The relaxation time in the plastic flow regime shows a temperature dependence of about 30 K/decade and is related to the local strain rate through a power-law relationship with an exponent near -1 for all investigated temperatures. We also find that the KWW β parameter for PLA increases during deformation from 0.42 to 0.60, indicating a narrowing of the distribution of relaxation times. These results are very similar to those previously reported on lightly crosslinked poly(methyl methacrylate) (PMMA) glasses, suggesting that these are universal effects of constant strain rate deformation on the segmental dynamics of polymer glasses. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A25.00007: A Simple Mean-Field Description of the Viscosity of Glass-Forming Polymers Valeriy Ginzburg The low-temperature behavior of glass-forming liquids is still poorly understood. Here, we propose a simple model with no divergence of relaxation time at any finite temperature. We hypothesize a “key degree of freedom” (KDF) associated with the Arrhenius-Andrade-Eyring transition state mechanism of the liquid flow. We then assume that the KDF energy levels are quantized in increments of ε (where ε can be an energy of a gauche’-trans-gauche kink1 or another low-energy excitation). In that case, the “fictive temperature”, Tf, is proportional to the KDF internal energy, determined by the Planck-Bose-Einstein statistics, Tf = <u >/kB = (ε/kB)(exp(βε) – 1)-1, where β = ( kBT)-1. The temperature-dependent viscosity (or the shift factor) is then estimated in the standard fashion, ln(η) = const + (Ea/ kBTf) = const + (Ea/ε)(exp(βε) – 1) (where Ea is the Arrhenius activation energy). This expression is similar to the Mauro2 equation and is shown to agree very well with multiple experimental data sets for both polymers and organic liquids. The theory can be further extended to describe the behavior of random copolymers and miscible blends. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A25.00008: Correspondence Between the Configurational Enthalpy Model and the Relaxation Dynamics of Simulated Amorphous Polymers above Tg Brett Savoie, Grigori Medvedev, James M Caruthers Glass forming materials exhibit dramatic mobility decreases of up to ten orders of magnitude as their temperature approaches Tg. This slow-down exhibits a super-Arrhenian temperature dependence that is diagnostic for testing competing theories of glass formation. Using experimental data, it was recently shown that for 21 molecular glass formers a simple one-parameter model based on the excess enthalpy accurately predicts the super-Arrhenian behavior, whereas the traditional configurational entropy model of Adam-Gibbs shows significant deviations. Extending this analysis to polymeric materials is challenging since the requisite experimental data is limited. Here we present simulations on several common small molecules and polymers to explore the temperature dependence of the polymer dynamics in the supercooled regime, making comparisons with the new configurational enthalpy model and experimental data where available. We show that the configurational enthalpy model accurately describes the temperature and pressure dependence of the mobility slowdown in the studied polymers. Moving forward this provides justification for using simulations to investigate the molecular mechanisms responsible for the accurate predictions of the configurational enthalpy model. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A25.00009: Chain-length dependent relaxation dynamics in glass-forming polymers Daniel Baker, Robin Masurel, Matthew Reynolds, Peter Olmsted, Johan Mattsson We present experimental and computational results on the effects of molecular size, monomer chemistry, and flexibility on the relaxation behaviour of glass-forming polymers. We show experimental data from broadband dielectric spectroscopy, calorimetry, and oscillatory rheology together with RIS computer simulation calculations. We discuss the length-dependence of molecular relaxation dynamics, chain conformation and shape; and relate these features to the glass transition temperature. We discuss the links between glass-formation in small molecules and oligomeric/polymeric glass-formers and the dependences on polymer chemistry and flexibility. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A25.00010: Dynamics of Viscoelastic Filaments based on Onsager Principle Jiajia Zhou, Masao Doi When a polymer solution is uniaxially stretched and held fixed at both ends, the solution quickly separates into droplets connected by strings and takes the beads-on-string structure. The string then becomes thinner by capillary forces. Here we develop a theoretical framework on viscoelastic fluids based on Onsager principle, and apply it to the dynamics of viscoelastic filaments. We show that the beads-on-string structure is a thermodynamic quasi-equilibrium state, and derive an equation for the coexistence condition in the pseudo-equilibrium state. Using the condition, we solve the evolution equation analytically and show that the string radius and the tensile stress vary exponentially as predicted by the classical theory of Entov and Hinch [J. Non-Newtonian Fluid Mech. 72, 31 (1997)], but the prefactor for the tensile stress is different from their theory and agrees with the numerical solutions of Clasen et al. [J. Fluid Mech. 556, 283 (2006)]. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A25.00011: Visco-Elastic Property Reconstitution in Chain-Like Polymers Manon Heili, John Kieffer We investigate the elastic deformation and structural reorganization process in a chain-like polymer, e.g., polyvinylidene fluoride (PVDF) during straining and stress relaxation. Using a unique experimental setup in which a miniature tensile tester is placed into the optical path of a Brillouin light scattering (BLS) system, we simultaneously measure the adiabatic and isothermal elastic moduli of the polymer as a function of the strain history. Upon straining, elastic moduli of the polymer drop instantaneously, while during stress relaxation at constant strain both the static and adiabatic moduli reconstitute, tending towards the unstrained values. While such behavior is expected for the former, it is not for the latter. Although a detailed analysis allows us to identify a spectrum of relaxation rates, general visco-elastic theory (e.g., Maxwell-Wiechert model) is not adequate to describe the observed behavior, not even that of the static moduli. An improved description of material’s visco-elastic response requires taking the structural rearrangements during straining and relaxation into account, and that the relaxation mechanisms, as well as the inherent elastic properties of the strained and unstrained polymer are indeed different. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A25.00012: Tube models are not compatible with the slip-link model for entangled star polymers Jay Schieber, Konstantin Taletskiy The dynamics of entangled linear and star polymers have been described reasonably successfully by both the slip-link model and various tube models. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A25.00013: Thermal and viscoelastic properties of selectively hydrogenated poly(1,1-diphenylethylene-alt-butadiene) Sungmin Park, Gagan Kangovi, Sangwoo Lee The synthetic utility of 1,1-diphenylethylene (DPE) to produce nearly perfectly alternating copolymers of DPE and vinyl monomers is well-established, but the physical properties of the alternating DPE copolymers have not been well documented. We characterized the thermal and viscoelastic properties of nearly perfectly alternating copolymer of 1,1-diphenylethylene (DPE) and butadiene and its selectively hydrogenated polymers. The entanglement molecular weights of the DPE-butadiene alternating copolymer and hydrogenated copolymers are measured 20 – 22 kg/mol. Interestingly, the glass transition temperatures of the DPE-butadiene copolymers decrease from 120 °C to 100 °C as the copolymer is selectively hydrogenated, and this behavior is opposite to the earlier report on the increasing glass transition temperature of hydrogenated DPE and styrene copolymers. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A25.00014: The Density Fluctuations of Polycarbonate under Deformation by Time-resolved small angle X-ray Scattering Shotaro Nishitsuji, Hiroshi Ito, Masaru Ishikawa, Takashi Inoue, Mikihito Takenaka Polycarbonate(PC) in an excellent thermoplastic with high impact strength, good transparency and so on. However, PC becomes brittle by annealing below the glass transition temperature. The mechanism of this phenomenon, which is so-called physical aging, is not clarified. To understand this phenomenon, we focus on density fluctuations of PC. By using a strong X-ray source of synchrotron, this density fluctuations can be detected under deformation. In this study, the change with density fluctuations of PC under deformation is investigated by using time-resolved small angle X-ray scattering. PC sample used in this study is a commercial polymer (viscosity average molecular weight Mv=19,500). Time-resolved small angle X-ray scattering is carried out under a constant load at BL05XU, SPring-8, Japan. At steady state, the isotropic scattering pattern is detected. After applying a load, the scattering pattern become anisotropic and the scattering intensity is enhanced in the parallel direction of load. This means the density fluctuations are enhanced by deformation. After necking, the density fluctuations are enhanced at an accelerating rate. We will discuss the detailed change of density fluctuations in this presentation. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A25.00015: Accelerated and depressed aging of PS blocks under 3D nanoconfinement in diblock copolymers Mingchao Ma, Yunlong Guo In this work, physical aging of polystyrene (PS) blocks in two representative diblock copolymers were investigated by a differential scanning calorimeter. Aging of PS blocks was remarkably accelerated in polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), compared to the homo-PS. In addition, the enthalpy results demonstrate that the aging rate of PS blocks in PS-b-PMMA increases with larger PMMA/PS mole ratio. On the other hand, the aging rate of PS blocks in polystyrene-block-poly(n-butyl methacrylate) (PS-b-PnBMA) is lower than the relaxation rate of corresponding neat PS. Together with morphology in theses copolymers, the aging results imply that the hard three-dimensional confinement induced by the PMMA blocks accelerates the aging rate of relatively soft PS blocks in copolymer, while the sluggish aging of PS blocks in PS-b-PnBMA is attributed to the extremely soft confinement formed by PnBMA blocks, which was liquid in the aging process due to its low glass transition temperature. |
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