Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S20: Polymer Glasses |
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Sponsoring Units: DPOLY Chair: Rodney Priestly Room: 405 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S20.00001: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S20.00002: Segmental mobility measured during constant strain rate deformation of poly(methyl methacrylate) glasses Kelly Christison, Benjamin Bending, Josh Ricci, Mark Ediger We have measured segmental mobility in poly(methyl methacrylate) glasses during constant strain rate deformation using a dye reorientation method. At 19 K below the glass transition temperature and for strain rates between 5.5x10$^{\mathrm{-6}}$ and 1.5x10$^{\mathrm{-4}}$ s$^{\mathrm{-1}}$, mobility increases as yield is approached, after which, it remains constant. In the post-yield regime, higher strain rates are found to be correlated with higher values of mobility. These results are consistent with the simulations of Riggleman et al. and the theory of Chen and Schweizer. On a log-log plot of mobility versus strain rate, our data falls on two parallel lines with slopes of -1. Data associated with high strain rates falls on a line consistent with the theory of Chen and Schweizer. Low strain rate data falls on a separate line shifted toward lower mobility. To our knowledge, this behavior is not predicted by existing simulations or modeling approaches. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S20.00003: Multi-step loading/unloading experiments that challenge constitutive models of glassy polymers James Caruthers, Grigori Medvedev The mechanical response of glassy polymers depends on the thermal and deformational history, where the resulting relaxation phenomenon remains a significant challenge for constitutive modeling. For strain controlled experiments the stress response is measured during loading/unloading ramps and a constant strain. By judiciously combining the basic steps, a set of multi-step experiments have been designed to challenge existing constitutive models for glassy polymers. A particular example is the ``stress memory'' experiment, i.e. loading through yield, unloading to zero stress, and holding at final strain, where the subsequent evolution of the stress exhibits an overshoot. The observed dependence of the overshoot on the loading strain rate cannot be explained by the models where the relaxation time is a function of stress or strain. Another discriminating multi-step history experiment involves strain accumulation to test the common assumption that the phenomenon of strain hardening is caused by a purely elastic contribution to stress. Experimental results will be presented for a low Tg epoxy system, and the data will be used to critically analyze the predictions of both traditional viscoelastic/viscoplastic constitutive models and a recently developed Stochastic Constitutive Model. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S20.00004: Challenges in predicting non-linear creep and recovery in glassy polymers Grigori Medvedev, James Caruthers The phenomenon of non-linear creep of amorphous polymeric glasses is difficult to predict using the traditional viscoelastic and viscoplastic constitutive frameworks, where two features present a particular challenge: (i) the tertiary stage of the creep and (ii) the recovery from large creep upon removal of the load. Representative examples of these two nonlinear responses will be shown for lightly cross-linked PMMA and an epoxy material, where the creep and recovery behavior has been studied as a function of temperature and aging time. The acceleration of creep during the tertiary stage is not caused by damage since the original dimensions of a cross-linked sample are fully recoverable by annealing above Tg. The assumption that the relaxation time is a function of strain runs into qualitative problems when predicting multi-step constant strain rate loading experiments. Recovery from creep as predicted by the constitutive models where the relaxation time depends on the deformation history is too abrupt compared to the experiment - this known as the ``accelerated aging'' problem. A recently developed Stochastic Constitutive Model that acknowledges dynamic heterogeneity in the glass state naturally predicts both the tertiary creep and the smooth recovery from creep. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S20.00005: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S20.00006: Stress relaxation behavior of polymer glasses in uniaxial extension Panpan Lin, Shiwang Cheng, Jianning Liu, Shi-Qing Wang Ductile polymers can undergo large tensile extension upon mechanical precondition (i.e. milling and melt-stretching). In the post-yield regime the tensile stress can still grow with the extension partially because of the elastic energy buildup as the chain tension grows from the stretching of the chain network [1]. To learn more about the nature of the mechanical stress, we carried out a series of stress relaxation experiments of both milled and melt-stretched PC. We found rescaling behavior, i.e., the stress relaxation is faster from a faster tensile extension by exactly the same amount. In other words, for an extension made at a cross-head speed of V$_{1}$, the stress relaxation occurs on a time scale of t$_{1}$. Then the stress relaxation from an extension produced at V$_{2}$ \textgreater V$_{1}$ occurs on a time scale of t$_{1}$(V$_{1}$/V$_{2})$. This is true for a range of nearly five orders of magnitude in V. We have studied this surprising scaling law as a function of the precondition. \\[4pt] [1] ``Strain hardening in homogeneous deformation of polymer glasses,'' P. P. Lin \textit{ et al.}, Phys. Rev. Lett., under review. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S20.00007: The effect of normal stress on the rheology of sub-micron thick polymer melt Janet Wong, Aleks Ponjavic The rheology of sub-micron thick polymer melt confined and sheared between a sphere and a flat surface (resulting in a circular point contact) is examined by obtaining the local through-thickness flow profiles of the melt. The effect of normal stress exerted to the melt is investigated. The rheology of the melt and the mechanical response of the fluid system are then correlated. The possibility of rheological heterogeneity within the confined melt is also explored. It is observed that behaviour of the confined melt is insensitive to the range of shear rate tested. Normal stress exerted, on the other hand, influences the rheology of the confined melt significantly. A critical normal stress exists below which Couette-like flow profiles are observed. Above the critical normal stress, the flow profiles signify plug-flow. This can be due to pressure-induced polymer glass transition. The existence of a critical stress is confirmed by the variation of local flow profiles within the point contact that closely resembles the normal pressure distribution in the contact. While a switch in flow behaviour occurs at a critical normal stress, the corresponding change in mechanical response in terms of measured friction forces is only marginally. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S20.00008: Scalar softness field correlates to molecular rearrangements for a thermal polymer glass Anton Smessaert, J\"{o}rg Rottler A fundamental challenge in the field of amorphous materials is to understand the structural causes for the spatial distribution of plastic events. Recent studies suggest that the low frequency vibrational modes encode information about structurally weak regions. Such ``soft spots'' were shown to strongly correlate to molecular rearrangements for an athermal amorphous solid in 2D~[1]. Building on these ideas, we construct a scalar ``softness field'' from a weighted superposition of low frequency modes and we show that this field identifies regions in which particles undergo rearrangements. We test the predictive strength of the field computationally for a 3D polymer glass model in a quiescent state at several temperatures. Rearrangements are identified as particle hops using a previously introduced detection algorithm~[2]. We find that hops are clustered in regions of large softness, and present a quantitative analysis of the correlation. The autocorrelation of the field shows that the soft regions are long lived compared to the timescales of the rearrangements. Furthermore, we find that particles hop preferentially along soft directions that are predicted by the softness field.\\[4pt] [1] M.L. Manning \& A.J. Liu,PRL 107,108302(2011)\\[0pt] [2] A. Smessaert \& J. Rottler,PRE 88, 022314(2013) [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S20.00009: Pressure-induced rheological transition of polymer melt Luca di Mare, Janet Wong Experiments have recently shown that a critical normal stress exists where the flow of a polymer melt under shear transits from Couette flow to plug flow as the normal stress exerted onto the melt increases. It has been conjectured that the observation is related to pressure-induced glass transition of the polymer melt. Experimentally this is challenging to verify. Hence MD simulations are carried out to elucidate the origin of such transition. The simulation consists of model polymer chains being sheared between two hard walls under isothermal conditions. The conformation, the density distribution, the dynamics of the chains, the viscosity of the melts, and the through-thickness velocity profiles are simulated by varying the shear velocity, the molecular weight of the chains and its distribution, and the normal stress of the system. The simulated results are then compared with experimental observations. Preliminary results show that the through-thickness viscosity of the melt is heterogeneous under high normal stress conditions. This can result in non-linear velocity profiles resemble experimental findings. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S20.00010: Developing a molecular picture for polymer glasses under large deformation Shi-Qing Wang, Shiwang Cheng, Panpan Wang Polymer glasses differ from most other types of glassy materials because they can be ductile under tensile extension. Remarkably, a ductile polymer can turn brittle and vice versa. For example, upon cooling, the glass changes from ductile to brittle at a temperature known as the brittle-ductile transition temperature (BDT). Aging causes the ductile glass to be brittle. Mechanical ``rejuvenation'' or pressurization brings a brittle glass into a ductile state. Finally, one glass can be ductile 100 degrees below T$_{\mathrm{g}}$ while another polymer is already brittle even just 10 degree below T$_{\mathrm{g}}$. Polystyrene and bisphenol A polycarbonate are at the two extremes in the family of polymer glasses. How to rationale such a wide range of behavior in terms of a molecular picture has been a challenging task. What is the role of ``chain entanglement''? Since many of the procedures including the temperature change do not alter the ``chain entanglement'', it is clearly insufficient to explain the nature of the BDT in terms of the entanglement density. Our work attempts to answer the question of what then is the role of chain networking. We have formulated a molecular picture that presents a unifying and coherent explanation for all the known phenomenology concerning the BDT and condition for crazing. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S20.00011: Enhancing polymer T$_{\mathrm{g}}$ and tuning mechanical properties with stiff molecular additives Jayachandra Hari Mangalara, David Simmons Small-molecule additives are commonly employed to alter glass formation, mechanical, and transport properties of polymers. For example, plasticizers are used to suppress polymer T$_{\mathrm{g}}$ and soften the glassy state, while antiplasticizers, which stiffen the glassy state of a polymer while suppressing its T$_{\mathrm{g}}$, are employed to enhance protein and tissue preservation. Recent advances in the understanding of additives' effects on glass formation suggest that additional combinations of temperature-dependent alterations to properties including T$_{\mathrm{g}}$, viscosity, and glassy moduli can be obtained via rational selection of additive properties. Here we employ coarse-grained molecular dynamics simulations to study the effect of introducing a stiff molecular additive to an unentangled polymer melt. Results indicate that, in contrast to plasticizer and classical antiplasticizer additives, these stiff molecular additives enhance the T$_{\mathrm{g}}$ of the matrix polymer. We further examine the impact of these additives on glassy moduli and yield stress of the polymer. These results highlight the importance of additive stiffness as a design parameter enabling more rational control of glass formation behavior. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S20.00012: Disorder-driven glass transition of polymers Alessio Zaccone, Eugene Terentjev The mechanical response of solids depends on temperature because the way atoms and molecules respond collectively to deformation is affected at various levels by thermal motion. This is a fundamental problem of solid state science and plays a crucial role in materials science. In glasses the vanishing of shear rigidity upon increasing temperature is the reverse process of the glass transition. It remains poorly understood due to the disorder leading to nontrivial (nonaffine) components in the atomic displacements. Our theory explains the basic mechanism of the melting transition of amorphous (disordered) solids in terms of the lattice energy lost to this nonaffine motion, compared to which thermal vibrations turn out to play only a negligible role. The theory is in good agreement with classic data on melting of amorphous polymers (for which no alternative theory can be found in the literature) and offers new opportunities in materials science. Ref: A. Zaccone \& E.M. Terentjev, Phys. Rev. Lett. 110, 178002 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S20.00013: Exploring chain tension in cold drawing of polymer glasses Shiwang Cheng, Panpan Lin, Mesfin Tsige, Shi-Qing Wang Ductile polymer glasses can undergo large tensile extension (cold draw) to double its original length either homogeneously or through necking. The corresponding tensile stress is typically much higher than the rubbery elastic modulus. Apart from the plastic component, there is also an energetic contribution to the mechanical stress. The origin of this elastic stress appears to arise from the existence of a chain network. The elastic yielding phenomenon [1] indicates that significant chain tension builds up during the cold drawing. Atomistic molecular dynamics simulation is carried out to delineate the nature of the chain tension and explore the suggestion of bond distortion in deformation of polymeric glasses. In a simple model to mimic a polymer glass with sufficient chain networking, we found evidence for the bond distortion that grows with the degree of extension. \\[4pt] [1] ``Elastic yielding in cold drawn polymer glasses well below the glass transition temperature,'' S. W. Cheng and S. Q. Wang, \textit{Phys. Rev. Lett. }\textbf{110}, 065506 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S20.00014: Nonlinear mechanics of thermoreversibly associating dendrimer glasses Arvind Srikanth, Robert S. Hoy, Berend C. Rinderspacher, Jan W. Andzelm The integration of thermoreversibly associating groups into polymers produces a wide variety of complex behavior arising from the finite lifetime of the ``sticky,'' thermoreversible bonds. Using hybrid molecular dynamics / Monte Carlo simulations, we characterize the nonlinear mechanical properties of associating trivalent dendrimer network glasses with a focus on their energy dissipation properties. Various combinations of sticky bond (SB) strength and kinetics are employed. The toughness (work to fracture) of these systems displays a surprising deformation-protocol dependence; different association parameters optimize different properties. In particular, ``strong, slow'' SBs optimize strength, while ``weak, fast'' SBs optimize ductility via self-healing during deformation. We relate these observations to breaking, reformation, and partner switching of SBs during deformation. These studies point the way to creating associating-polymer glasses with tailorable mechanical properties. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S20.00015: Measuring the nanoscale properties of laser-deposited glassy polymer nanodroplets Kimberly Shepard, Craig Arnold, Rodney Priestley Glassy polymer nanodroplets are fabricated via the Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique using short deposition times. At longer deposition times, the nanodroplets act as nanoscale building blocks, forming nanostructured bulk films with thickness on the order of microns. These nanostructured glassy films exhibit unique properties, including 40{\%} reduced density along with a 40K increase in the glass transition temperature compared with glasses prepared by cooling from the liquid state. Indirect experimental study of the thermal properties of the nanoscale features has indicated that the stability of the bulk film may be a result of the nanostructure. Here, we directly measure the properties of the nanoscale building blocks and connect the results to observations about the global film properties. Heated atomic force microscopy is used to measure the volume of individual nanodroplets as they are heated in situ. MAPLE-deposited droplets exhibit large excess volumes and enhanced thermal stability compared with similarly-sized droplets prepared from polymer nanoparticles. We discuss this behavior in the context of the MAPLE process of nanodroplet formation. [Preview Abstract] |
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