Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X42: Polymeric Glasses |
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Sponsoring Units: DPOLY Chair: Mark Ediger, University of Wisconsin--Madison Room: A302/303 |
Thursday, March 24, 2011 2:30PM - 2:42PM |
X42.00001: Molecular Mobility on the Surface of Glassy Tris-naphthylbenzene (TNB) Zahra Fakhraai, Chad Daley, Stephen F. Swallen, Daniel Scifo, James A. Forrest, Mark D. Ediger Mechanical relaxation measurements on the surface of polymeric glasses show that as the bulk material falls out of equilibrium at Tg a thin layer at the surface behaves like a liquid with relaxation times that are orders of magnitude faster and more weakly temperature dependent compared to those of the bulk glass. However the origin of this phenomenon remains elusive. Recently exceptionally stable glasses of small organic molecules have been produced by physical vapor deposition at temperatures below Tg, suggesting that these glasses also exhibit enhanced surface mobility. In this study gold nanoparticles were used to probe micron size meniscus formation on the surface of organic glass former TNB below Tg, a direct evidence of surface mobility in this material. Neutron scattering measurements of inter-diffusion between stacks of d-TNB and regular TNB layers during the deposition suggest that the temperature dependence of the diffusion on the surface is very similar to what is observed on polymeric films. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X42.00002: Tuning the Dynamics of Penetrant Transport in Glassy Polymers through Network Structure Modification Adam Ekenseair, Nicholas Peppas The relative rates of the diffusional and relaxational processes during the absorption of penetrant molecules in glassy polymers determine the nature of the transport process and lead to Fickian, Case II, and anomalous absorption behavior. While previous models account for anomalous behavior, there is still a disconnect between theory and experiment, as data must be fit to the model with previously determined independent parameters. With trends leading to smaller device scales and increasingly complex polymer structures, there is a need for a quantitative understanding of the manner in which a polymer's network structure alters both the rate and the mode of penetrant transport. To this end, the effects of the basic network parameters of PMMA, including the degree of crosslinking, polymer mesh size, and the crosslink interchain bridge length, on the integral sorption of methanol were studied utilizing gravimetric integral sorption studies. The effects of sub-T$_{g}$ annealing/aging, temperature, and the presence of un-reacted monomer were also investigated. Controlling the relative timescale of the relaxational process by altering the polymer network structure was shown to directly influence the Case II front propagation velocity and control the overall nature of the observed transport behavior. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X42.00003: A Surrogate for Debye-Waller Factors from Stokes Shifts Marcus Cicerone, Qin Zhong, Madhusudan Tyagi We show that short-time relaxation behavior characteristic of the intermediate scattering function at $q$ near the peak in the static structure factor can be obtained from time-resolved Stokes shifts (TRSS) in glassforming materials. We extract Debye- Waller factor ($\langle u^{2}\rangle$) analogs from the TRSS data from four glassforming liquids and apply these to a proposed relationship between $\alpha$ relaxation and the Debye-Waller factor; $\tau_{\alpha}=\tau _{\infty }\; \mbox{E}xp\left[ \frac{a^{2}}{2\langle u^{2}\rangle}+\frac{\sigma _{a^{2}}^{2}}{8\langle u^{2}\rangle^{2}} \right]$. This putative relationship has previously been evaluated using experimental Debye-Waller factors obtained in the time range (40 to 2000) ps. We show that the relation yields physically meaningful fit values only when relaxation on a 1 ps timescale is considered. We also observe an unexpected dependence of short-time Debye-Waller factors on fragility. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X42.00004: Comparison of the KWW and BSW Model Descriptions of the Dynamic Responses of Polymeric and Colloidal Glass Formers Ben Xu, Gregory B. McKenna In this work, we present the results of the KWW and BSW\footnote{M. Baumg\"{a}rtel, A. Schausberger, H.H. Winter, Rheol. Acta. 29:400--408 (1990). } descriptions of the dynamic data for a colloid and a polymer (PVAc) in their respective glass transition regions. It was found that the KWW function is not able to describe the dynamic data for the colloidal system, while BSW function, provides an acceptable description to the dynamic response of the polymer. The fitting parameters n$_{e}$ and n$_{g}$ in the BSW function, which indicate the slopes of the relaxation spectrum, remain constant at different temperatures consistent with the validity of the time-temperature superposition principle. We also used the G$_{g}$ obtained from the KWW and BSW functions, where appropriate, to evaluate the Dyre shoving model.\footnote{J. C. Dyre, N. B. Olsen, T. Christensen, Physical Review B.53, 5 (1996).} Here, as is the case for small molecule glass formers, we found the temperature dependences of the G$_{g}$ highly sensitive to the model chosen to describe the experimental data. This suggests that evaluation of the shoving model requires very broad frequency and temperature experiments beyond those normally performed in dynamic rheometry. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X42.00005: Aging and structural recovery behaviors in epoxy films subjected to carbon dioxide plasticization jumps: Evidence for a new glassy state Gregory McKenna, Shankar Subramanian, Jing Zhao, Mataz Alcoutlabi, Lameck Banda Structural recovery and physical aging of glassy polymers after temperature jumps have been very well studied in the literature. On the contrary, there is only limited work available on the aging and recovery behaviors of glassy polymers subjected to plasticizer jumps. We have shown in our previous works, using strong and weakly polar plasticizers that qualitatively they mimic the behaviors of temperature jumps but quantitatively they are different [1, 2]. In this work, we further investigate this anomalous behavior by studying the structural recovery and physical aging of an epoxy film subjected to carbon dioxide pressure jumps and compare the results with temperature jump experiments such that the final conditions are identical. The results are surprising and we observe evidence for existence of a new glassy state. \\[4pt] [1] Zheng. Y., and McKenna, G.B., \textit{Macromolecules,} \textbf{36}, 2387-2396, 2003\\[0pt] [2] Alcoutlabi, M., Briatico-Vengosa, F., and McKenna, G.B., JPSB., \textbf{40}, 2050-2064, 2002 [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X42.00006: Effect of Quench Conditions on the Subsequent Physical Aging Rate of Polymer Glasses Laura Golick, Paul Yoon, Andy Pahner, Connie Roth We investigate the stability of polymer glasses when thermally quenched under different conditions. Ellipsometry is used to measure the physical aging rate of polystyrene (PS) films supported or transferred onto silicon wafers. The aging rate quantifies the time-dependent decrease in film thickness that results from the increase in average film density during aging. Although all films are subsequently aged in a supported state, we observe significant differences between films quenched in a free-standing compared to supported state. Films quenched in a free-standing state exhibit a strong thickness dependence to their physical aging rate at micron length scales, an order of magnitude or two larger than thicknesses where nanoconfinement effects on the glass transition and modulus are typically observed. In contrast, supported films do not display any film thickness dependence to their aging rate at this large length scale. This indicates that the physical aging of the material is strongly dependent on conditions during the formation of the glassy state. In an effort to determine the key factors underlying the aging dynamics, we have measured the physical aging rate of supported PS films quenched at various controlled rates. In addition, we have explored the effects of quenching free-standing films held on different frames such that either biaxial or uniaxial stress is applied. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X42.00007: Low Temperature Flow of PVC Chains Wei Chen, Gi Xue PVC is usually processed at temperature above 180 $^{\circ}$C, however, it starts to degrade at 130 $^{\circ}$C. If PVC can flow at temperatures below glass transition temperature (Tg), the manufacturing procedure will be energy-conserving and environment-friendly. We find that PVC powders with controlled inter-segment van der Waals attraction can be compressed into a transparent pellet with high modulus at low temperatures. The molecular mechanism underlying this phenomenon involves shear-induced unjamming transition. PVC chains are unjammed by cold-pressing freeze-dried powder with decreased packing density. Because the Tg of freeze-dried PVC is dramatically reduced to the test temperatures under compression, PVC chains are able to flow by applying pressure solely. These results help us better understanding glass transition and can possible to develop a theory for cold processes. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X42.00008: Evolution of Entanglements During Crazing of Glassy Polymers Ting Ge, Mark O. Robbins, Robert Hoy, Stefanos Anogiannakis, Christos Tzoumanekas, Doros Theodorou Craze formation increases the fracture energy of glassy polymers by orders of magnitude. The polymer volume expands by an extension ratio which is assumed to be determined by the entanglement network. We test this assumption with molecular simulations that use the Contour Reduction Topological Analysis (CReTA) algorithm to follow topological constraints (TCs) associated with the entanglement network. The TCs are identified with contacts between chains after applying CReTA. Within systematic errors, crazing does not change the number of TCs or the distribution of chemical distances between them. Moreover, about 75{\%} of the contacts remain between the same chains at nearly the same location. The 25{\%} of contacts that change do not reflect a comparable loss of entanglements. Instead, small displacements within the tube change which chains contact after CReTA. This interpretation is tested by adding fixed crosslinks to a sparse entanglement network and crazing preoriented samples. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X42.00009: Surface Softening in Polymers and Their Nanocomposites Determined by Surface Mechanical Properties through Spontaneous Particle Embedment Taskin Karim, Gregory McKenna In the present work, we have used the particle embedment technique with sub-micron particles to estimate the surface modulus of epoxy/POSS composites at a temperature far below the glass transition temperature. The embedment of the particle is determined from atomic force microscope measurements and the modulus was determined using the elastic analysis of Johnson, Kendall and Roberts (JKR) with surface energy estimates of the work of adhesion as the driving force for embedment. The surface modulus values were found much smaller than the macroscopic modulus values. The maximum embedment depth obtained for all surfaces was low enough so that it did not cause plastic deformation on the surface. The maximum stress values on all surfaces induced by the particle embedment were estimated to verify the expected response in close to the linear regime. \\[4pt] [1] K. L. Johnson, K. Kendall and A. D. Roberts, \textit{P. Royal Society of Lonodon A, }\textbf{324}, 301-313 (1971). \\[0pt] [2] J. H. Teichroeb and J. A. Forrest, \textit{Physical Review Letter, }\textbf{91}, 016104 (2003). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X42.00010: Effect of molecular weight on gold nanoparticle embedding into polystyrene films near and below the bulk glass transition temperature Chad Daley, Dongping Qi, James Forrest We use gold nanoparticle embedding to probe the surface properties of glassy polystyrene films at temperatures ranging from a few degrees above to 10's of degrees below the bulk glass transition temperature (T$_{g})$. These studies employed monodisperse polymer samples with molecular weights (M$_{w})$ ranging from 3000-80000 kg/mol. A qualitative change in the surface response is observed between the high M$_{w}$ and low M$_{w}$ regimes. At low M$_{w}$ a buildup of polymer material forms around the base of the nanoparticles similar to the observed behavior in molecular glasses. For the higher molecular weights this buildup is not observed and the system instead relaxes through nanoparticle embedding. We also observe changes in the complete embedding process observed only near the bulk T$_{g}$. These changes suggest that nanoparticle embedding can be used as a probe of polymer entanglements. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X42.00011: Glass Transition and Free Volume Behavior in Epoxy-amine Network Glasses: Effect of Diiamine Isomers Sergei Nazarenko, Mukul Kaushik, Matthew Jackson, Jeffrey Wiggins A systematic investigation of the effect of meta and para isomers of diamino diphenyl sulfone (DDS) crosslinker on glass transition temperature (T$_{g})$ and free volume properties of DGEBF based epoxy-amine network was carried out. The pressure volume temperature (PVT) properties were measured experimentally from 0 to 120 MPa and 30\r{ }C to 240\r{ }C in a high pressure dilatometer-type PVT apparatus. It was observed that the glass transition temperature of epoxy system with para isomer is higher than the one consisting of meta isomer by 30 \r{ }C. PVT data were also fitted using Simha-Somcynsky, equation of state to calculate occupied and free volume. Positron annihilation lifetime spectroscopy (PALS) was used to calculate average hole free volume below and above glass transition temperature. The average free volume size in para isomer cured systems is larger than in meta isomer cured systems below their glass transition temperature, while in the melt state they are the same. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X42.00012: Difference in the heat capacity and the coefficient of thermal expansion responses during thermal cycling Grigori Medvedev, Eun-Woong Lee, James Caruthers An observation that different experimental methods give different values of Tg is part of the lore of the field of the glassy polymers. We report on a careful study of a series of polymeric systems both thermoplastic and thermoset, including PMMA, PC, PS, and 3,3' DDS Epon 825, conducted using DSC and TMA techniques. We found that for the same thermal history the heat capacity and the coefficient of thermal expansion (both measured upon heating) as functions of temperature transition from the glassy asymptote to the equilibrium asymptote at significantly different temperatures; this difference was in the range from 8 to 17 degrees, depending on the system. We argue that such a large difference in the enthalpy and volume responses during the same thermal history is inconsistent with the commonly used material clock models, but is consistent with the view of the glassy materials as containing dynamically heterogeneous regions. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X42.00013: Formation of Glassy Polymer Films by Matrix Assisted Pulsed Laser Evaporation Rodney Priestley, Yunlong Guo, Craig Arnold The properties of glasses strongly depend on the path to glass formation. The most common method of making polymer glasses is by cooling from the liquid state. Recently, it has been shown that alternative routes to the vitreous state can lead to dramatically improved glassy-state stability. In this talk, we present our initial work on the thermal and kinetic stability of glassy polymer films prepared by Matrix Assisted Pulsed Laser Evaporation (MAPLE). In comparison to glassy films prepared by spin coating, MAPLE-deposited glassy films can exhibit higher glass transition temperatures and greater kinetic stability. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X42.00014: Characterization of pre-mature nanocomposite crazes Gregory N. Toepperwein, Juan J. de Pablo Crazing is a unique mode of failure by which polymer strands are stretched into a periodic array of columns. It has been shown that these crazes follow cavitation under deformation. Inclusion of nanoparticles drastically alters the glass transition temperature and the globally measurable mechanical properties of these polymer glasses. However, limited literature exists to explain the behavior on the lengths scales of the heterogeneous domains within the glass in the context of nanocomposites. In this work, we investigate the nucleation and growth of voids that precede craze formation to elucidate the role these inclusions play in failure and further characterize the pre-mature craze itself. Extensive Molecular Dynamics and Monte Carlo simulations of highly entangled polymer nanocomposites allow for calculation of local densities, local elastic moduli, and local orientation of additives. We find that the site of void formation is inexorably linked to the local mechanical properties of polymer. This relationship is more evident upon the inclusion of reinforcing additives which induce a broader distribution of local moduli leading to the nucleation of more, smaller voids. Within the developing craze, larger additives resist incorporation, but those that do are subject to ordering. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X42.00015: Structural Characterization of a Polymer of Intrinsic Microporosity: X-ray Scattering With Insight From Molecular Dynamics Simulations Amanda G. McDermott, Gregory S. Larsen, Peter M. Budd, Coray M. Colina, James Runt Polymers of intrinsic microporosity (PIMs) are high-T$_{g}$, amorphous materials exhibiting high gas permeability and a large concentration of pores smaller than 2 nm, arising from a combination of rigid segments and sites of contortion. Structures generated by molecular dynamics simulations accurately reproduce characteristic scattering features from PIM-1 at high $q$, allowing us to investigate their origin by examining partial structure factors. Unlike scattering patterns typical of nonporous amorphous polymers, broad $q$ range PIM scattering patterns include a shoulder at the size scale corresponding to pore sizes measured by other techniques. We discuss the development of a model for extracting pore sizes from scattering patterns. [Preview Abstract] |
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