Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S52: Polymeric Elastomers and Gels |
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Sponsoring Units: DPOLY Chair: Sangwoo Lee, Rensselaer Polytech Inst Room: LACC 512 |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S52.00001: Physical gelation and spinodal decomposition of poly(N-isopropylacrylamide) aqueous solutions measured by rheometry Chi Wang, Ya-Chen Chuang Aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) undergoes sophisticated phase transitions at 20–33 °C. At this temperature range, the solution exhibits a phase behavior of lower critical solution temperature and physical gel formation. The transition temperatures of PNIPAM solutions of various concentrations were determined using two different methods, namely, turbidity and rheological measurement. A quasi-equilibrium phase diagram was then constructed. Analyses of loss modulus G″ and storage modulus G′ give rise to the spinodal temperature (Ts), macroscopic gel temperature (Tgel) and the temperature at which G′ starts to rise (Tonset,G’) due to the thermal concentration fluctuations when the spinodal is approached. Tgel was obtained from the crossover temperature of the G′ and G″ curves, and Ts was derived based on the Fredrickson–Larson–Ajji–Choplin mean field theory. Depending upon solution concentration, the measured Tonset,G’ is approximately 3–10 °C lower than the derived Ts. Upon heating, the one-phase solution undergoes pronounced concentration fluctuations at a temperature above Tonset,G’. The concentration fluctuations subsequently triggered the physical gelation process to develop a macroscopic-scale gel network at Tgel before spinodal decompsotion at Ts. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S52.00002: Topological Structure of Networks Formed from Symmetric Four-arm Precursors Tzyy-Shyang Lin, Rui Wang, Jeremiah Johnson, Bradley Olsen Compared to gels formed by asymmetrically coupling bifunctional polymer chains and tetrafunctional junctions, gels formed via A-B type end-linking of symmetric tetra-arm star polymer precursors were found to exhibit higher strength and lower spatial heterogeneity. While loops constituting of odd number of junctions are forbidden by precursor chemistry, these materials still contain topological defects which affect their properties. Here, we demonstrate the superposition of the dilution effect and chain-length effect on loop formation, and show that the entire network topology can be characterized by the measurement of just secondary loops. The loop fractions predicted by the topological simulations are in excellent agreement with the experimental measurements of model tetra-PEG hydrogel. In addition, the delay of gel point as a function of secondary loop fraction is quantitatively predicted without any fitting parameter. Furthermore, careful comparison reveals similarity between the behavior of loops consisting of 2n junctions in the symmetric system and that of loops consisting of n junctions in the asymmetrical system, suggesting analogies between the chemically distinct networks. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S52.00003: Failure of a Polystyrene-Polyisoprene-Polystyrene Gel in Mineral Oil Satish Mishra, Rosa Maria Badani Prado, Thomas Lacy, Santanu Kundu Fracture of gels subjected to load during their applications is an unwanted phenomenon. Here, we present the fracture behavior of a gel consisting of polystyrene-polyisoprene-polystyrene in mineral oil, a midblock selective solvent. The collapsed polystyrene blocks form aggregates acting as the crosslinking points, whereas, the solvated polyisoprene blocks bridge those aggregates. Fracture in these gels mostly occurs by the endblock pullout from the aggregates. Fracture energy for these gels depends on the chain pullout energy and on the viscous dissipation process. Tensile test results capture an increase in fracture stress and strain with the increasing strain rates. Quasistatic fracture tests capture the crack-tip velocity dependence of fracture energy for these gels. The creep tests performed over a range of applied stress values indicate a decrease in failure time with the increase in applied stress. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S52.00004: Patterning with loops to dynamically reconfigure polymer gels Santidan Biswas, Victor Yashin, Anna Balazs The structural and mechanical properties of gels can be controlled by promoting the unfolding (and refolding) of loops embedded within the networks. As a loop unfolds, the released chain length can increase the extensibility and reconfigurability of the gel. Here, we develop a theoretical model that couples the elasticity of the gel to the dynamic transitions occurring in loops that lie between the crosslinks. Using this model, we show that a thermally-induced swelling of the gel generates an internal strain, which unfolds the loops and thereby further increases the degree of gel swelling. We exploit this cooperative behavior to reconfigure the gel by patterning the location of the loops within the sample. Through this approach, we convert flat, two-dimensional layers into three-dimensional forms and introduce architectural features into initially uniform 3D slabs. At a fixed temperature, an applied force produces analogous structural transformations. The shape-changes are reversible, i.e., the systems return to their original structure when the temperature is reset or the force is removed. The reported findings provide guidelines for creating materials that interconvert thermal, chemical and mechanical energy to perform useful work. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S52.00005: Tough self-healing elastomers by molecular enforced integration of covalent and reversible bonds Liheng Cai, Jinrong Wu, David Weitz Self-healing polymers crosslinked by solely reversible bonds are intrinsically weaker than common covalently crosslinked networks. Introducing covalent crosslinks into a reversible network would improve mechanical strength. It is challenging, however, to apply this concept to "dry" elastomers, largely because reversible crosslinks are often polar motifs, whereas covalent crosslinks are nonpolar motifs. These two types of bonds are intrinsically immiscible without cosolvents. Here we design and fabricate a hybrid polymer network by crosslinking randomly branched polymers carrying motifs that can form both reversible hydrogen bonds and permanent covalent crosslinks. The randomly branched polymer links such two types of bonds and forces them to mix on the molecular level without cosolvents. This enables a hybrid "dry" elastomer that is very tough with fracture energy 13500J/m2 comparable to that of natural rubber. Moreover, the elastomer self-heals at room temperature with a recovered tensile strength 4 MPa, comparable to the pristine strength of existing self-healing polymers. The concept of forcing covalent and reversible bonds to mix at molecular scale to create a homogenous network is quite general and should enable development of tough, self-healing polymers of practical usage. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S52.00006: Dynamic Mechanical Behavior of Second-Generation Dendronized Wedge-Type Polymer Zhiyuan Qian, Yung Pyo Koh, Alice Chang, Tzu-Pin Lin, Pablo E. Guzman, Robert Grubbs, Sindee Simon, Gregory McKenna The dynamic mechanical behavior of second-generation dendronized wedge-type polymer synthesized by ring-opening metathesis polymerization have been measured. By applying time-temperature superposition (TTS) principle, the master curve was constructed over the frequencies ranging from terminal regime to glassy regime. An extremely low glassy modulus of 100 MPa is observed which may be due to the bulky densely placed side groups in the structure. Upon further investigation, the apparent extremely low rubbery plateau of approximately 16 kPa in the dynamic response and Van Gurp-Palmen plot is found to be related to the steady state recoverable compliance. Absolute heat capacity measurements showed large heat capacity (CP) in the glassy regime and a small ΔCP of 0.076 J/(gK) at the glass transition, which indicates a high degree of freedom in the glassy state. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S52.00007: Self-Assembled Structural Colloids of Nematic Liquid Crystal Polymer and Elastomer Wei-Shao Wei, Yu Xia, Sophie Ettinger, Shu Yang, A. G. Yodh Self-organized nematic liquid crystal polymer (NLCP) microstructures and corresponding cross-linked liquid crystal elastomer (LCE) microstructures were fabricated and investigated. The NLCP structures were generated by temperature quenching of LCP droplets. The drops then evolved morphologically into rough spheres, flower-shaped drops, and filaments with sub-micron diameters. The control parameters dictating the final morphologies include temperature, pre-polymer chain length, and surfactant concentration in the aqueous background. Interestingly, the morphologies varied reversibly with temperature cycling. Finally, useful target morphologies were photo-crosslinked to make LCEs with well-maintained director fields and smooth surfaces. The schemes thus introduce a novel procedure for synthesizing LCE fibers. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S52.00008: Simulation for stiffening of reconstituted silk fibroin gels during deformation Peiran Jin, Peter Olmsted Silk from silkworms has been used in the textile industry for thousands of years. It consists of β-sheet structures due to hydrophobic domains. Large hydrophobic domains interspaced with smaller hydrophilic domains generate the strength and impressive mechanical property of silk. Recently, a physical electrogel(e-gel) was made by reconstituting Bombyx mori silk into stable aqueous solutions and then applying small DC electric field [Tabatabai et al, Soft Matter 11 (2015) 756]. The e-gels exhibit distinctive strain hardening and are partially recoverable from strain. To study the structure change and nonlinear behavior of the gel, we build a coarse-grained model of fibroin protein polymers, which comprise crystallizable domains and amorphous domains. We find that the kinetics of unfolding and refolding of crystallizable domains changes the number and functionality of crosslinks in the physical network, and thus contributes to the strain hardening of the gel and the non-recoverable strain. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S52.00009: Tailoring Poisson's Ratio via the Directed Self-Assembly of Liquid Crystal Elastomers Anesia Auguste, Eric Harper, Timothy White Auxetic materials are of great interest due to their superior toughness, resilience and shear resistance compare to positive Poisson’s ratio materials. Auxetic materials are negative Poisson’s ratio materials that expand in the transverse direction when a tensile force is applied. Currently, most auxetic materials are porous or foam based materials. However, creating a non-porous auxetic material will lead to high density and effective stiffness materials with improved impact resistance and vibration damping. Here, we utilized ink-jet printing to prepare designer elastomers with distinct local elastic properties by controlling the alignment of the liquid crystal mesogens. Programming the local orientation and elastic properties of liquid crystal elastomers leads to the ability to tune the Poisson’s ratio in a non-porous elastomeric film. Both computational and experimental results will be shown. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S52.00010: Correlation of Monomer Source and Structure to the Morphology and Properties of Lignin Based Polyurethanes Umesh Shrestha, Jason Lang, Yun-yan Wang, Priya Sengupta, Arthur Ragauskas, Charles Wyman, Charles Cai, Mark Dadmun Lignin based polyurethanes (PU) have gained interest due to the possibility of using naturally available lignin as polyols to synthesize environmentally friendly PUs. The structural properties of the resultant PUs are dependent on the structure and type of polyol and isocyanate used during synthesis. Here, lignin based PUs were synthesized using lignin extracted from poplar wood with the novel CELF (Co-solvent Enhanced Lignocellulosic Fractionation) technology, as well as from soft wood, hard wood and wheat straw which react with the diisocyanates toluene diisocyanate-encapped PPG or hexamethylene diisocyanate. The morphology, mechanical and thermal properties of these PUs with various lignin fraction ranging from 30 to 60 wt% were studied. Independent of the type of the lignin used, increasing the fraction of lignin results in two-phase PUs. PUs synthesized from CELF lignin show that mechanical properties are highly reliant on the OH/NCO ratio used during synthesis. Dispersion of the lignin within PPG matrix depend on the fraction and origin of the lignin used, which dramatically impacted the measured mechanical properties. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S52.00011: The Role of Fiber Reinforced Gels in Maintaining Metastable Water in Plants Alexandre Ponomarenko, William Steinhardt, Fulton Rockwell, Shmuel Rubinstein, Noel Holbrook Plants resistance to droughts rely on the ability to stop the spread of gas embolism in the sap network. Dedicated porous membranes segment the sap network to prevent this gas embolism spread. With experiments on plants and on artificial composite materials we study how the composition of those membranes, a natural fiber reinforced gel, sets their fracture toughness and how it impacts the plants resistance to droughts. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S52.00012: Fracturing Biopolymer Gels with Bubbles Bradley Frieberg, Ray-Shimry Garatsa, John Bachert, Benjamin Crawshaw, Michael Liu, Ronald Jones, Edwin Chan The fracture behavior of thermoreversible biopolymer gels, such as gelatin gels, is important in applications such as drug encapsulation, food texture and cosmetics. In this contribution, we use a bubble inflation method called cavitation rheology to study the viscoplastic fracture behavior of gelatin gels as a function of the gel volume fraction. We find that gelatin gels fracture via a viscoplastic mechanism, which scales with the gel mesh size as a power law until a critical gel volume fraction is reached, and then transitions to fracture via plastic deformation. With the aid of neutron scattering measurements, we show that this critical gel volume fraction is determined by the thermal blob size of the gel that defines the transition between semi-dilute gel concentration versus concentrated gel condition. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S52.00013: Mullins effect in filled elastomers characterized by stretching measurements in various geometries Thanh-Tam Mai, Yoshihiro Morishita, Kenji Urayama It is well known that the filled elastomers exhibit the considerable degrees of stress-softening in the cyclic stretching tests (L. Mullins, 1947). The Mullins effect is closely related to the energy loss of filled elastomers subjected to cyclic deformation, and its full understanding is very important for the design of rubber products. Interestingly, the emergence of such stress softening is not limited to filled elastomers, and the similar phenomena including cyclic hardening are observed for both soft materials, such as polymer composites, biopolymer hydrogels or tissues, and hard materials, such as shape-memory alloys (K. M. Schmoller et al., 2013). |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S52.00014: Quenched Polyampholyte Hydrogels for Low-Temperature Energy Storage and Energy-Saving Smart Window Applications Hyun-Joong Chung, Xinda Li, Thanh-Giang La Quenched polyampholyte hydrogel is a tough hydrogel with self-healing ability, strong adhesion, and mechanical flexibility. Structure studies of polymer chains and water molecules in hydrogels made of a charge-balanced polyampholyte, a random copolymer of NaSS-co-MPTC, by SAXS and WAXS, respectively, suggested a networked globule structure in the charge-balanced polyampholyte hydrogels prevented freezing of water in the hydrogel, resulting in 'slush-like' ice formation. The amorphous water was confirmed at –45 °C was by solid-state 2H NMR. We also observed high ionic conductivity at low temperatures using EIS. Utilizing the scientific investigations, a flexible and self-healing supercapacitor with high energy density in low temperature operation was fabricated using a polyampholyte hydrogel electrolyte. At -30°C, the supercapacitor exhibited an energy density of 10.5 Wh/kg at a power density of 500 W/kg. We also modulated the overall hydrophilicity/phobicity of polyampholyte chains when synthesizing the random copolymer and adjusted the upper critical solution temperature (UCST) at high precision. Finally, we developed a stretchable, high-contrast, optically tunable stretchable smart window for energy saving applications. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S52.00015: Springy or Crumbly: Depends on crosslinking protocol when we make porous colloid/polymer hybrids Karthika S, Shivprasad Patil, Rajamohanan P.R, Guruswamy Kumaraswamy Recently, we demonstrated1 that macroporous scaffolds can be produced by ice-templating a dispersion of colloids, polymer and crosslinker and, by crosslinking this hybrid in the frozen state. Such scaffolds show elastic recovery even after 90% compression, despite being comprised predominantly of inorganic (> 90% w/w). We have demonstrated the formation of elastic sponges using different particles, polymers and crosslinking chemistries. In my talk, I will contrast these elastic sponges with those crosslinked after lyophilization to remove the ice. Both these scaffolds appear chemically and structurally identical. However, monoliths prepared in absence of ice are brittle and fail catastrophically at low compressive strains. We use rheology, SAXS, NMR and AFM to understand how the difference between elastic sponges and plastic monoliths (made by crosslinking after removal of ice crystals ). We show that crosslinking in the presence of ice results in spatially homogeneous crosslinking, while crosslinking after lyophilization renders the spatial distribution of crosslinks highly heterogeneous. Thus, remarkably, the presence (or absence) of macroscopic ice crystals influence the microscopic spatial localization of chemistry in these scaffolds2. |
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