Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W18: Focus Session: Polymer Network Mechanics I |
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Sponsoring Units: DPOLY Chair: Catalin Picu, Rensselaer Polytechnic Institute Room: B117 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W18.00001: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W18.00002: Possible explanation of anomalous ductility in thermoset/thermoplastic polymer alloys Debashish Mukherji, Cameron Abrams Mechanical properties of highly cross-linked polymer (HCP) networks, e.g., thermosets, can be significantly modified by adding linear polymer chains, e.g., thermoplastics. In this work, we study thermoset/thermoplastic polymer alloys by means of large scale molecular dynamics simulations (MD) of a coarse-grained model. We focus here on the effect of linear chain mass fraction $\Gamma_l$, for different chain lengths $N_l$, and strain rates $\dot{\varepsilon}$. Our results show that tensile strain (i.e; strain to break) decreases with increasing mass fraction $\Gamma_l$, up to a threshold value $\Gamma_{l}^*$, beyond which it increases with $\Gamma_l$. This non-monotonic behavior, which we call ``anomalous ductility", is qualitatively independent of $\dot{\varepsilon}$ and $N_l$, so long as fracture occurs in bulk. $\Gamma_{l}^*$ decreases with increasing chain length and we observe microscopic evidence that this threshold value signifies the onset of interchain interactions. A simple scaling argument suggests that $\Gamma_{l}^*$ is related to the overlap concentration of the thermoplastic homopolymer in the cured thermoset matrix. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W18.00003: Toughness and Fracture Energy of PDMS Bimodal and Trimodal Elastomers Claude Cohen, Geoffrey Genesky PDMS bimodal and trimodal end-linked elastomers display remarkable ultimate properties in uniaxial extension when the molar masses of the precursor chains are widely separated and are present in an appropriate range of concentrations. The mass of the long chain component in these networks remains dominant. These elastomers can be stretched to large elongations before fracture while displaying an upturn in stress at high strain when the short chains are near their overlap concentration and will support the applied load. The fracture energy of pre-cut bimodal and trimodal networks on the other hand does not exhibit a pronounced improvement over that of unimodal networks. It appears to be governed by the average molar mass of the ``effective'' elastic strand length in each network that is based on its elastic modulus. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W18.00004: Novel Telechelic Polymer Hydrogels using Homo and Hetero-combinations of End-blocks Rajiv R. Taribagil, Marc A. Hillmyer, Timothy P. Lodge The last two decades have seen telechelic triblock polymers support increasing number of applications as stabilizers and flow modifiers in fields as varied as pharmaceutics, paints and oil recovery. Mainly consisting of a solvophilic mid-block end-capped with solvophobic blocks, their use as flow altering additives stems from their ability to form gels comprising hydrophobic junctions, with hydrophilic blocks bridging these junctions. Previous studies have shown that the chemical nature of the end-block has a direct bearing on the morphology of the gel, which in turn affects its macroscopic properties. We have conducted an examination of this relationship using a variety of techniques including cryogenic scanning electron microscopy (cryo-SEM), small angle neutron scattering (SANS) and rheology. Using homo and hetero-combinations of poly(1,2-butadiene) and poly(perfluoropropylene oxide) as hydrophobic end-blocks with poly(ethylene oxide) as the hydrophilic mid-block, we have demonstrated that the hydrogels obtained are distinct in morphology and physical properties. The study seeks to highlight the importance of controlling end-blocks in triblock polymers as a potential tool towards manipulating the physical properties of gels. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W18.00005: Tunable Strain-Stiffening of Physically Associating Networks Kendra A. Erk, Kenneth R. Shull Many biological networks become stiffer when mechanically deformed. This strain-stiffening behavior is difficult to reproduce in synthetic networks due in part to the propensity of synthetic networks to strain soften prior to the onset of stiffening. However, physically associating polymer networks frequently display stiffening behavior without softening. Here, strain-stiffening of a model elastic network of physically associating polymers is characterized by shear rheometry. The network is composed of a triblock copolymer dissolved in a midblock-selective solvent. Experiments demonstrate a correlation between network structure -- specifically, variation in midblock length -- and the onset of strain-stiffening, quantified using an exponential strain energy function. Fracture-like breakdown of the network is observed at large strain. The work performed here benefits from studying networks with a well-defined molecular structure and tailorable chain architecture. Control over the structure of these networks allows for the creation of synthetic networks with tunable strain stiffening. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W18.00006: Buckling instabilities in patterned, poly(N-isopropylacrylamide) microgels Ryan Toomey, Samuel DuPont, Ryan Cates Stimuli-sensitive hydrogels facilitate reconfigurable microstructures with response integrated at the material level. Response is engendered by a competing mechanism: the elasticity of the network counterbalances expansion by the solvent. If the strength of expansion can be controlled by an environmental cue, the hydrogel can be adjusted in situ. The equilibrium state occurs when the osmotic stress exerted by the solvent in the gel equals the osmotic pressure of the solvent outside the gel. For a free structure, the equilibrium state corresponds to homogenous swelling. If a free surface of the gel is mechanically constrained, however, the dimensions available for the relief of the osmotic stress are reduced, resulting in non-uniform or inhomogeneous swelling. In this study, we demonstrate how mechanical constraints impose differential gel swelling, leading to complex three-dimensional structures that arise from two-dimensional poly(N-isopropylacrylamide) microstructures. Depending on the initial geometry of the constrained gel, three general modes of swelling-induced deformation can be observed: lateral differential swelling, bulk sinusoidal buckling, and surface wrinkling. Through confocal microscopy and 3D image rendering, the mechanics of swelling has been evaluated and compared to linear elastic theory. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 1:03PM |
W18.00007: Strain hardening, fracture and toughening mechanisms in self-assembling gels Invited Speaker: Polymer gels based on triblock copolymers in a midblock-selective solvent are excellent model systems for studying fundamental features of network mechanics. We use a series of acrylic triblock gels that exhibit both a concentration dependent ``structural'' gelation phenomenon and a temperature dependent ``dynamic'' gelation phenomenon. Dynamic gelation is controlled by the exchange kinetics of endblock between different aggregates, and is strongly temperature dependent because of an underlying glass transition of the endblock aggregates. Structural gelation is controlled by midblock chains which form a percolated network of endblock aggregates above a critical concentration. The focus of this talk is on the non-linear mechanical response of these materials, including strain hardening and fracture in both extension and in shear. We also discuss the design of high toughness ionically crosslinked gels with a type of double network structure. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W18.00008: Wrinkling and strain softening in polymer supported networks of pristine single-wall carbon nanotubes Erik K. Hobbie, Daneesh Simien, Jeffrey Fagan, Ji Yeon Huh, Jun Young Chung, Steven Hudson, Jan Obrzut, Christopher Stafford Single-wall carbon nanotubes (SWCNTs) represent mesoscale analogs of semiflexible polymers. We study the deformation mechanics of quasi two-dimensional SWCNT networks by assembling them as thin isotropic films on stretched polymer substrates. After the strain is released, the morphology and topography of the deformed membranes are characterized through a combination of optical microscopy, light scattering, atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and impedance spectroscopy. Above a critical surface density, films assembled from SWCNTs of well-defined length and/or electronic type (metallic vs. semiconducting) exhibit a strongly nonlinear response under prestrains of 1{\%} to 20{\%}. The strain dependence of the wrinkling wavelength suggests that the films soften dramatically under small deformation, and we develop a model to extract the network modulus and yield strain from the nonlinear mechanical response. The softening results from local anisotropy that develops under the Poisson expansion of the soft polymer support, which induces bond slip and the rupture of network junctions. This loss of connectivity leads to a drop in electrical conductivity normal to the direction of strain. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W18.00009: Creasing Instability of Hydrogel Surfaces: Nucleation, Growth Dynamics {\&} Hysteresis Ryan Hayward, Jinhwan Yoon, Jungwook Kim The free surface of a hydrogel film supported on a rigid substrate will become unstable to formation of sharp folds when placed under sufficiently large compressive swelling stresses. We seek to exploit this mechanical creasing instability to design smart polymer films with reversible stimuli-responsive properties, however many fundamental questions remain about the mechanism and dynamics of crease formation. We have studied the process of crease nucleation and growth, as well as presence of hysteresis in the level of compressive stress at which folds form and disappear using temperature-responsive surface attached hydrogels. To further understand these processes, we have also studied the influence of skin layers coated on the gel surface and defects in the gel. These studies provide important insights into the mechanism of crease formation and how to control the onset and disappearance of creases. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W18.00010: Entanglement swelling in polymer glasses Joshua McGraw, Kari Dalnoki-Veress A polymer system in which the chains are much longer than the entanglement molecular weight is well entangled. When a glassy polymer film composed of such chains is uniaxially strained, deformations called crazes may be formed. It is well established that the study of crazes can reveal much about the nature of entanglements. Here, we present results of crazing experiments in which well entangled polystyrene networks have been diluted with various weight fractions of polystyrene with molecular weights in the vicinity of the entanglement molecular weight. Upon dilution, the systems assume an effective reduction in the entanglement density which is a function of both the weight fraction and molecular weight of the small chains. A model which combines simple ``chain packing'' with ``binary contact'' ideas is proposed. The model is found to quantitatively describe measurements in systems with two and three molecular weight components, and can easily be extended to polydisperse systems. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W18.00011: Long-range correlations and solution of boundary value problems for semi-flexible polymer networks Catalin Picu, Hamed Hatami-Marbini Semi-flexible random fiber networks are present in biological and non-biological systems such as the cytoskeleton, tissue scaffolds and cellulose structures. In this work we show that long-range correlations exist in structural (e.g. density) and mechanical (e.g. elastic moduli) parameters of the network. All these quantities have large fluctuations with position and the magnitude of the fluctuations exhibits power law scaling. This observation makes solving boundary value problems defined over domains containing such networks difficult. The use of standard homogenization arguments becomes problematic. We present an inexpensive technique based on the stochastic finite element method that can be used for solving boundary value problems over such networks. [Preview Abstract] |
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