Session H48: Polymer Gels and Solutions

Phase Segregation and Dynamics in Strongly Interacting Small Molecule Additive - Block Copolymer Surfactant Complexes

Rheology and Small Angle X-Ray Scattering (SAXS) were used to investigate order to disorder transitions (ODTs) and disorder to order transitions (DOTs) of poly(ethyleneoxide-b-propyleneoxide-b-ethyleneoxide) block copolymer surfactants mixed with hydrogen-bond-donating small molecule additives. A series of additives having a core benzene ring and systematic variation in the number of carboxylic or hydroxyl groups attached to the ring were of particular interest. Ordered cylindrical morphologies, confirmed using SAXS, were obtained only in a certain additive concentration region. ODTs were characterized by sudden changes in the linear viscoelastic properties in low frequency region upon increasing temperature. The locations of ODTs varied widely with hydrogen-bond-donating ability of the functional group and were found to be strongly dependent on the number of functional groups attached to the ring. For a given additive, the temperature at which ODT occur was strong function of the additive loading, whereas the linear viscoelastic properties of the ordered state were little changed upon varying additive concentration in ordered region. The location and dynamics of DOTs upon cooling were comparable to the ODTs upon heating. Studies using these model systems provide insight into the design of well-ordered hybrid materials.   

Using Mesoscopic Models to Design Strong and Tough Biomimetic Polymer

Using computational modeling, we investigate the mechanical properties of polymeric materials composed of coiled chains, or globules, which encompass a folded secondary structure and are crosslinked by labile bonds to form a network. In the presence of an applied force, the globules can unfold into linear. Our goal is to determine how to tailor the labile intra- and inter-molecular bonds within the network to produce material exhibiting both toughness and strength. We use the lattice spring model (LSM) to simulate the globules and the crosslinked network. We utilize our modified Hierarchical Bell model (MHBM) to simulate the rupture and reforming of $N$ parallel bonds. We demonstrate that the mechanical properties of the system are sensitive to the values of $N_{in}$ and $N_{out}$, the respective values of $N$ for the intra- and inter-molecular bonds. We find that the strength of the material is mainly controlled by the value of $N_{out}$, with the higher value of $N_{out}$ providing a stronger material. We also find that if $N_{in}$ is smaller than $N_{out}$, the globules can unfold under the tensile load before the sample fractures and thus, can increase the ductility of the sample.   

Synthesis and Characterization of Poly(hydroxyethyl methacrylate) Hydrogels Bearing Reversibly Associating Side Groups

Poly(hydroxyethyl methacrylate) (poly(HEMA)) is a technologically important hydrogel that can be processed into different shapes and is best known for its role in contact lenses. However, applications of water swollen polyHEMA are limited by its poor mechanical properties. We are studying the influence of reversibly associating side groups on the behavior of poly(HEMA) hydrogels. In non-polar media, it is well known that ureidopyrimidinone (UPy) groups self-associate to form hydrogen bonded dimers (DDAA); however their behavior in water-swollen hydrogels is unclear. A series of poly(HEMA) linear polymers of controlled molecular weight with varying UPy content have been prepared using a reversible addition-fragmentation chain transfer (RAFT) polymerization technique. UPy content significantly reduces water swelling and improves mechanical properties. The degree of hydrogen bonding within water swollen hydrogels is studied, and properties of functional hydrogel polymers and networks are compared to an unswollen hydrophobic analog.   

Thermoreversible Supramolecular Ion Gels via Hydrogen Bonding

Ion gels are a novel class of functional materials of broad interest for advanced applications. We have developed a thermoreversible supramolecular ion gel system consisting of a poly(2-vinylpyridine-$b$-ethylene oxide-$b$-2-vinylpyridine) (P2VP-PEO-P2VP) triblock copolymer, a poly(4-vinylphenol) (PVPh) linear homopolymer, and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]), where a highly inter-connected transient polymer network is formed by hydrogen bonding between the P2VP endblocks and PVPh cross-linkers. This system exhibits novel physical properties, such as interesting dynamics and homopolymer clustering in the cross-links. The applicability of time-temperature superposition to this system is striking, resulting in a master curve that extends over 20 orders of magnitude in reduced frequency. The hydrogen-bonded phase can arrange into a hexagonally packed cylindrical morphology with long-range ordering, which reveals very slow kinetics and is thermodynamically stable only within a narrow temperature window. The highly tunable relaxation dynamics as well as shear modulus might enable materials design for specific applications.   

Hydrogen Bonding Based Layer-by-Layer Assembly of Poly(vinyl alcohol) with Weak Polyacids

Multilayer thin films that consist of poly(vinyl alcohol) (PVA) and weak polyacids such as poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) were prepared by hydrogen bonding interactions. Both the degree of hydrolysis and molecular weight of PVA were investigated in terms of their influence on growth behavior and pH stability. Multilayer films containing PVA and PAA could be assembled successfully only by using partially hydrolyzed PVA and low pH solutions. By comparing films containing PAA with those containing a more strongly interacting partner, PMAA, it was shown that the extent of PVA hydrolysis becomes significant only when weak hydrogen bonding pairs such as PVA and PAA were used. pH-triggered dissolution experiments demonstrated that the degree of hydrolysis can be used as an additional parameter by which to tune the pH stability of the film. Also, the presence of an abundance of free hydroxyl and carboxylic acid groups in the multilayer allowed enhanced pH stability to be obtained by thermal and chemical methods as well as numerous opportunities for post-assembly functionalization.   

Gelation in a model 1-component system with adhesive hard-sphere interactions

Colloidal dispersions can undergo a dynamical arrest of the disperse phase leading to a system with solid-like properties when either the volume fraction or the interparticle potential is varied. Systems that contain low to moderate particulate concentrations form gels whereas higher concentrations lead to glassy states in which caging by nearest neighbors can be a significant contributor to the arrested long-time dynamics. Colloid polymer mixtures have been the prevalent model system for studying the effect of attraction, where attractions are entropically driven by depletion effects, in which gelation has been shown to be a result of phase separation [1]. Using the model 1-component octadecyl coated silica nanoparticle system, Eberle et al. [2] found the gel-line to intersect the spinodal to the left of the critical point, and at higher concentrations extended toward the mode coupling theory attractive driven glass line. . We continue this study by varying the particle diameter and find quantitative differences which we explain by gravity. \\ \\ 1. Lu, P.J., et al., Nature, 2008. 453(7194): p. 499-504.\\ 2. Eberle, A.P.R., N.J. Wagner, and R. Castaneda-Priego, Physical Review Letters, 2011. 106(10).   

Concentration Fluctuations in Polymer Solutions under Extensional Flow

Polymer solutions under flow are known to exhibit stress-concentration coupling that can anisotropically amplify concentration fluctuations. This phenomenon has been extensively studied for shear flow, but is less well understood for extensional flows. Using a two-fluid model, we study concentration fluctuation amplification in polymer solutions under a variety of extensional flows, including mixed shear and extension.   

Crossover length scale for viscosity experienced by Gold nanoparticles in semi dilute polymer solution

Gold nanoparticles (Au NPs) were used as a probe to characterize the crossover length scale for viscosity in semi dilute poly (ethylene glycol) (PEG)-water solutions. Fluctuation correlation spectroscopy (FCS) was used to measure the diffusion of these NPs as a function of their size (5-20 nm), PEG concentration (0 to 40\%w/w) and PEG molecular weight (5 kDa to 35 kDa). Our results indicate that for particles with radius R $>$ R$_{g}$, usual hydrodynamic conditions can be applied, but for particles with radius R $\leq$ R$_{g}$, the diffusion is approximately an order of magnitude faster than that predicted by Stokes Einstein (SE) relation. The results imply that radius of gyration R$_{g}$ of the polymer gives the crossover length scale from nanoviscosity to macroviscosity. The relative viscosity experienced by the particles was scaled as $\eta$/ $\eta$$_{0}$ = exp (b (R$_{g}$/$\xi$) $^{a}$), where $\eta$$_{0}$ is the water viscosity, $\xi$ is the correlation length, a = 0.70 $\pm$ 0.03 and b = 1.59 $\pm$ 0.07.   

Kinetics of Narrowly dispersed Latex Formation in a Surfactant-free Emulsion Polymerization of Styrene in Acetone-Water Mixture

The kinetics of narrowly dispersed latex formation in a surfactant-free emulsion polymerization of styrene in acetone-water was studied by a combination of transmission electron microscopy and light scattering. The critical nuclei were experimentally observed and the formation of narrowly dispersed PS latex is proved to be originated from competitive growth kinetics. Spherical nuclei were regenerated via a microphase inversion of PS oligomer in 50{\%} volume fraction acetone-water mixture at 70$^{\circ}$C. They follow a polydispersed log-normal distribution and the smallest nucleus with Rs 1.1nm is similar to critical nuclei. Note the spherical nuclei are not necessarily narrowly dispersed. Competitive growth kinetics makes smaller nuclei grow much faster than large nuclei in the subsequent polymerization process, resulting in narrowly dispersed PS latex. Two kinds of PS seed particles were added, separately, into two parallel surfactant-free emulsion polymerization batches of styrene in acetone-water mixture at 70$^{\circ}$C. It was found that the size of seed particles almost does not change, but the small size PS latex grows rapidly. Our fitting results proves competitive growth kinetics proposed by Vanderhoff and coworkers.   

Cluster Growth in Aqueous Sugars Observed by Dynamic Light Scattering

Dynamic light scattering of aqueous sugar solutions as a function of sugar concentration and temperature reveal the development of sugar clusters occurring in two stages. At low volume fractions of sugar, a so-called cluster phase consisting of nearly monodisperse clusters forms with a mean cluster mass that increases in proportion to the volume fraction. At a critical volume fraction, near where the clusters begin to overlap, a second stage ensues wherein cluster-cluster aggregation forces a more rapid, power law growth in advance of a percolation threshold observed near 83 wt{\%} sugar.   

Boson peak in L-cysteine: a Raman scattering study

The Boson peak is a distinctive feature of many glassy and disordered crystalline solids. Recently it has been suggested that similar feature may be correlated to anharmonic transitions observed in macromolecules like DNA and proteins. In the present work we studied the low frequency ($15-600$ cm$^{-1}$) Raman scattering response of L-cysteine and L-Cysteine hydrochloride with different hydration levels in the $15-270$ K temperature range. Our analyzes will be concerned to understand the water rule in the Boson peak inelastic light signal, its correlation to the dynamic transitions at T$^{*}\sim 80$ K and T$_{D}\sim 280$ K, and its microscopic origin as well.   

Low temperature dynamic transitions of L-cysteine and L-proline amino acids: a specific heat study

Studies have shown that several macromolecules present two dynamic transitions at T* $\sim $ 80 K and TD $\sim $ 280 K which have intrinsic correlation to their biological activity. The present work concerns the detailed analysis of the low temperature transition at T* by specific heat. This transition is usually described as originated on the CH2SH dynamics. We compared the experimental results with simulations based on rigid rotor specific heat model by Caride and Tsallis [J. Stat. Phys. \textbf{35}, 187 (1984)] and we found an excellent agreement.