Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F11: Physics of Natural Polymers, Polymer Hybrids, and AssembliesFocus
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Sponsoring Units: DPOLY Chair: Bradley Olsen, MIT Room: 270 |
Tuesday, March 14, 2017 11:15AM - 11:27AM |
F11.00001: Simulation Studies of LCST-like Phase Transitions in Elastin-like Polypeptides (ELPs) and Conjugates of ELP with Rigid Macromolecules Joshua Condon, Tyler Martin, Arthi Jayaraman We use atomistic (AA) and coarse-grained (CG) molecular dynamics simulations to elucidate the thermodynamic driving forces governing lower critical solution temperature (LCST)-like phase transition exhibited by elastin-like peptides (ELPs) and conjugates of ELP with other macromolecules. In the AA simulations, we study ELP oligomers in explicit water, and mark the transition as the temperature at which they undergo a change in ``hydration'' state. While AA simulations are restricted to small systems of short ELPs and do not capture the chain aggregation observed in experiments of ELPs, they guide the phenomenological CG model development by highlighting the solvent induced polymer-polymer effective interactions with changing temperature. In the CG simulations, we capture the LCST polymer aggregation by increasing polymer-polymer effective attractive interactions in an implicit solvent. We examine the impact of conjugating a block of LCST polymer to another rigid unresponsive macromolecular block on the LCST-like transition. We find that when multiple LCST polymers are conjugated to a rigid polymer block, increased crowding of the LCST polymers shifts the onset of chain aggregation to smaller effective polymer-polymer attraction compared to the free LCST polymers. These simulation results provide guidance on the design of conjugated bio-mimetic thermoresponsive materials, and shape the fundamental understanding of the impact of polymer crowding on phase behavior in thermoresponsive LCST polymer systems. [Preview Abstract] |
Tuesday, March 14, 2017 11:27AM - 11:39AM |
F11.00002: Role of Hydrogen Bonding on Nonlinear Mechano-Optical Behavior of $L$-Phenylalanine-based Poly(ester urea)s. Keke Chen, Jiayi Yu, Gustavo Guzman, S. Shams Es-haghi, Matthew L. Becker, Miko Cakmak The uniaxial mechano-optical behavior of a series of amorphous $L$-phenylalanine-based poly(ester urea) (PEU) films was studied in the rubbery state using a custom real-time measurement system. When the materials were subjected to deformation at temperatures near the glass transition temperature ($T_{\mathrm{g}})$, the photoelastic behavior was manifested by a small increase in birefringence with a significant increase in true stress. At temperatures above $T_{\mathrm{g}}$, PEUs with a shorter diol chain length exhibited a liquid-liquid ($T_{\mathrm{ll}})$ transition at about 1.06 $T_{\mathrm{g}}$ (K), above which the material transforms from a heterogeneous ``liquid of fixed-structure'' to a ``true liquid'' state. The initial photoelastic behavior disappears with increasing temperature, as the initial slope of the stress optical curves becomes temperature independent. Fourier transform infrared spectra of PEUs revealed that the average strength of hydrogen bonding diminishes with increasing temperature. For PEUs with the longest diol chain length, the area associated with N-H stretching region exhibits a linear temperature dependence. The presence of hydrogen bonding enhances the ``stiff'' segmental correlations between adjacent chains in the PEU structure. As a result, the photoelastic constant decreases with increasing hydrogen bonding strength. [Preview Abstract] |
Tuesday, March 14, 2017 11:39AM - 11:51AM |
F11.00003: High Deformability, Particle Size Distribution and Hydration of Phytoglycogen Nanoparticles Benjamin Baylis, John Dutcher We have used atomic force microscopy to resolve a large discrepancy between the size of monodisperse phytoglycogen nanoparticles measured using small angle neutron scattering (SANS) and dynamic light scattering (DLS), and to calculate the effect of hydration on the nanoparticle size. The AFM measurements are challenging because of the ``stickiness'' and deformability of the soft nanoparticles. By significantly reducing the interaction between the AFM tip and the ``sticky'' nanoparticles, we were able to obtain high quality images in both air and water. We found that the adsorbed particles are highly deformed, forming pancake-like objects on hydrophilic mica surfaces. By measuring the distribution of isolated particle volumes in air, we calculated the average effective spherical diameter of the particles. Comparing nanoparticle aggregates in both air and water allowed the determination of the hydration of an individual nanoparticle. Our results are in excellent agreement with the diameter determined using SANS, providing insight into the unusual diffusion dynamics that is measured in DLS. These measurements illustrate the distinct advantages of AFM over other imaging techniques for visualizing nanoscopic soft objects in a liquid environment. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:27PM |
F11.00004: Self-Assembly and Responsiveness of Polypeptide-Based Star and Triblock Copolymers Invited Speaker: Daniel Savin This study involves the bottom-up design and tunability of responsive, peptide-based block polymers. The self-assembly of amphiphilic block polymers is dictated primarily by the balance between the hydrophobic core volume and the hydrophilic corona.~ In these studies, amphiphilic triblock and star copolymers containing poly(lysine) (PK), poly(leucine) (PL) and poly(glutamic acid) (PE) were synthesized and their solution properties studied using dynamic light scattering, circular dichroism spectroscopy and transmission electron microscopy.~The peptide block in these structures can serve to introduce pH responsiveness (in the case of PK and PE), or can facilitate the formation of elongated or kinetically-trapped structures (in the case of PL.) This talk will present some recent studies in solution morphology transitions that occur in these materials under varying solution conditions. As the topological complexity of the polymers increases from diblock to linear triblock or star polymers, the solution morphology and response becomes much more complex. We present a systematic series of structures, with increasing complexity, that have applications as passive and active delivery vehicles, hydrogels, and responsive viscosity modifiers. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F11.00005: Structure of Hydrophobically Modified Phytoglycogen Nanoparticles John Atkinson, Jonathan Nickels, John Dutcher, John Katsaras Phytoglycogen is a highly branched, polysaccharide nanoparticle produced by some varieties of plants including sweet corn. These particles are attractive candidates for cosmetic, industrial and biomedical applications. Many of these applications result from phytoglycogen's unique interaction with water: (1) high solubility; (2) low viscosity and high stability in aqueous dispersions; and (3) a remarkable capacity to sequester and retain water. Neutron scattering measurements of native phytoglycogen revealed that the particles have uniform size, uniform radial particle density, and a high level of hydration. Hydrophobically modifying the outer surface of the hydrophilic nanoparticles opens up new applications in food and biomedicine, such as solubilizing and stabilizing bioactive compounds. One such modification is octenyl succinate anhydride (OSA), where the hydrophobicity can be tuned by adjusting the degree of substitution. I will present the results of small angle neutron scattering (SANS) measurements of aqueous dispersions of OSA-modified phytoglycogen with two different degrees of modification. Contrast series SANS measurements have yielded information about the radial density profile, providing insight into the nature of the chemical modification of the particles. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F11.00006: Rheology of Aqueous Dispersions of Phytoglycogen Nanoparticles Hurmiz Shamana, John Dutcher Phytoglycogen is a natural, highly branched polysaccharide nanoparticle extracted and purified from sweet corn. The nanoparticles possess many unusual properties that suggest a broad range of applications in cosmetics, food and nutrition, and biomedicine. These applications stem from a strong interaction between the nanoparticles and water, which has motivated our studies of aqueous phytoglycogen dispersions. We have measured the rheology of the dispersions as a function of phytoglycogen concentration C. Unlike other polysaccharides such as starch, we find that the zero-shear viscosity of phytoglycogen dispersions remains very low over an extended range of C, increasing significantly only for C \textgreater 20{\%} w/w. These results imply that the particles do not interact significantly until they are forced into contact at very high concentrations. This is consistent with our small angle neutron scattering measurements that show that the particle spacing becomes equal to the particle diameter for C \textasciitilde 25 {\%} w/w [1]. [1] J.D. Nickels, J. Atkinson, E. Papp-Szabo, C. Stanley, S.O. Diallo, S. Perticaroli, B. Baylis, P. Mahon, G. Ehlers, J. Katsaras and J.R. Dutcher. Biomacromolecules 17, 735-743 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F11.00007: Equilibrium swelling, interstitial forces and water structuring in phytoglycogen nanoparticle films Michael Grossutti, Eric Bergmann, John Dutcher Phytoglycogen is a natural, highly branched polymer of glucose that forms dendrimeric nanoparticles. This special structure leads to a strong interaction with water that produces exceptional properties such high water retention, low viscosity and high stability of aqueous dispersions. We have used ellipsometry at controlled relative humidity (RH) to measure the equilibrium swelling of ultrathin films of phytoglycogen, which directly probes the interstitial forces acting within the films. Comparison of the swelling behaviour of films of highly branched phytoglycogen to that of linear and slightly branched polysaccharides in both the high and low disjoining pressure regimes shows that chain architecture plays an important role in determining the short-range repulsion of the chains at low RH and the hydration forces at high RH. By combining ellipsometry with infrared spectroscopy, we find a correlation between the structural rearrangement of the hydrogen-bonding network of the tightly bound hydration water and the inter-chain separation in the highly branched phytoglycogen nanoparticles. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F11.00008: Tunable mechanical properties of green solid films based on deoxyribonucleic acids Hisao Matsuno, Yuma Morimitsu, Noboru Ohta, Hiroshi Sekiguchi, Atsushi Takahara, Keiji Tanaka Promoting green innovation to establish a worldwide low-carbon society is an urgent priority. We here show that solid films made from deoxyribonucleic acid (DNA) can be used as a structural material. The great advantage of DNA films over the ones made from synthetic polymers is that the mechanical properties are controllable, from glassy to rubbery, via semicrystalline by simply regulating the water content in the film. Why such unique mechanical properties can be manifested by the DNA films is determined from structural analyses using Fourier-transform infrared spectroscopy and wide-angle X-ray diffraction measurements. With increasing water content, the conformation of DNA was changed from A-form in an amorphous state to B-form in a partially packed one. DNA in the B-form became densely packed as the film was stretched. Also, DNAs were intermolecularly cross-linked using 2,5-hexanedione based on reductive amination induced by 2-picoline borane in aqueous phase. Cross-linking points were directly observed by atomic force microscopy. The tensile properties of cross-linked films were much better than those of non-cross-linked DNA films. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F11.00009: Versatile Assemblies of Zwitterionic Giant Surfactants Toward Two-dimensional ``Nano-disks''. Stephen Cheng, Zhiwei Lin, Jian Sun, Wei Zhang Two-dimensional (2D) structures and materials have sparked considerable interests because of their unique dimension-dependent optical, mechanical and electric properties. Despite the recent significant advances in bottom-up nanoscale fabrication approaches, the preparation of 2D circular shape nanostructures (``nano-disks'') has remained a grand challenge. In this work, a specific non-crystallization approach is developed to prepare 2D ``nano-disks'' by the self-assembly of zwitterionic giant surfactants composed of positively and negatively charged fullerene-based heads tethered by a hydrophobic polystyrene (PS) tail. Deriving from the separation of assembled 3D cylindrical colloids with internal disk stacked structures, the 2D ``nano-disks'' with uniform diameter and controllable thicknesses are achieved. The 2D ``nano-disks'' and their 3D cylindrical stacking colloids can be controlled by varying pH value in the solution. These hierarchically assembled structures resemble 2D ``nano-disk'' thylakoids and their 3D stacking structures of grana within the chloroplasts of plant cells in nature. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F11.00010: Bioinspired Nanocellulose Based Hybrid Materials With Novel Interfacial Properties Sinan Keten This talk will overview a simulation-based approach to enhancing the mechanical properties of nanocomposites by utilizing cellulose $-$ the most abundant and renewable structural biopolymer found on our planet. Cellulose nanocrystals (CNCs) exhibit outstanding mechanical properties exceeding that of Kevlar, serving as reinforcing domains in nature's toughest hierarchical nanocomposites such as wood. Yet, weak interfaces at the surfaces of CNCs have so far made it impossible to scale these inherent properties to macroscopic systems. In this work, I will discuss how surface functionalization of CNCs influences their properties in their self-assembled films and nanocomposites with engineered polymer matrices (Xin et al. Nano Letters, 2015, Fox et al. Applied Materials and Interfaces, 2016). Specifically, the role of ion exchange based surface modifications and polymer conjugation will be discussed, where atomistic and coarse-grained simulations will reveal new insights into how superior mechanical properties can potentially be attained by hybrid constructs. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F11.00011: Rapid Configurational Fluctuations in a Model of Methylcellulose Xiaolan Li, Kevin Dorfman Methylcellulose is a thermoresponsive polymer that undergoes a phase transition at elevated temperature, forming fibrils of a uniform diameter. However, the gelation mechanism is still unclear, in particular at higher polymer concentrations. We have investigated a coarse-grained model for methylcellulose, proposed by Larson and coworkers, that produces collapsed toroids in dilute solution with a radius close to that in experiments. Using Brownian Dynamics simulations, we demonstrate that this model’s dihedral potential generates ``flipping events", which helps the chain to avoid kinetic traps by undergoing a sudden transition between a coiled and a collapsed state. If the dihedral potential is removed, the chains cannot escape from their collapsed configuration, whereas at high dihedral potentials, the chains cannot stabilize the collapsed state. We will present quantitative results on the effect of the dihedral potential on both chain statistics and dynamic behavior, and discuss the implication of our results on the spontaneous formation of high-aspect ratio fibrils in experiments. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F11.00012: Effect of cellulose nanocrystals on crystallization kinetics of polycaprolactone Kalman Migler, Debjani Roy, Anthony Kotula, Bharath Natarajan, Jeffrey Gilman, Douglas Fox The development of biocompatible polymer composites that enhance mechanical properties while maintaining thermoplastic processability is a longstanding goal in sustainable materials. Here we compatibilize a crystallizable polymer and a nano-fiber via surface modification and study the properties and crystallization kinetics of the resulting composite. First we demonstrate that polycaprolactone (PCL) and cellulose nanocrystals (CNCs) can be well-compatibilized by replacing the Na+ of sulfated cellulose nanocrystals (Na-CNCs) with tertiary butyl ammonium cations and then melt mixing via twin-screw extrusion. Transmission electron microscope and high temperature melt rheology show that the modified CNCs were dispersed in the polymer matrix. We find the crystallization kinetics are substantially affected by the CNC as indicated by the simultaneous measures of modulus and conformational states; higher loadings of CNCs accelerated the kinetics. We further correlate the crystallization kinetics, mechanical properties and stability. [Preview Abstract] |
Tuesday, March 14, 2017 2:03PM - 2:15PM |
F11.00013: Self-Renewing Microns-Thick Biopolymer Brush Made of Hyaluronan under Active Synthesis W. Wei, J. Washburn, P. Weigel, J.E. Curtis Hyaluronan (HA) is a large anionic polysaccharide distributed throughout many vertebrate tissues. We introduce a technology to produce dynamic HA polymer brush interfaces. The strategy relies on the enzyme hyaluronan synthase (HA synthase), which synthesizes and extrudes HA polymers up to 20 microns in length. We show that interfaces decorated by HA synthase-rich membrane fragments robustly produce polymer brushes of predictable heights and concentration profiles. The brush thickness can be tuned by the duration of growth or the enzyme density in the membranes. The system is self-renewing in that old polymers desorb and new polymers are produced. The brush can also be replenished after enzymatic removal multiple times. The large extent of the polymer interface allows for characterization of the brush architecture and for studying dynamic processes inside the brush using optical microscopy. At low ionic strengths (1 mM), we measure one of the largest polymer brushes yet reported, an average of 7.8 microns thick. For applications that require a stable brush interface, we have covalently reinforced the HA to the surfaces, and demonstrated that the brush is stable for at least two months. This self-renewing, dynamic biopolymer brush has great potential as a new biomaterial for implants, antifouling, tissue engineering and drug delivery. [Preview Abstract] |
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