Session Y18: Elastomers and Gels

Dynamics of Poly(ethylene glycol) Based Gels Created in situ with Polyisocyanates

Low-Tg amorphous polyesters condensed from poly(ethylene glycol) and isophthalate were prepared having alcohol end groups and purified. These diol polymers were then end-linked using polyisocyanates as the crosslinking agent because of their high reactivities with alcohol end groups and lack of a condensation byproduct. Gels with varying concentrations of crosslinkers were created to prepare samples above and near the gel point. The diol polymers and polyisocyanates were mixed and loaded into the rheometer at ambient temperature, and then cured at 60C to create gels. Linear viscoelastic measurements determined the modulus of these gels, which was correlated with the gel fraction determined by solvent extraction of the soluble portion.   

Dynamics of Main Chain Liquid Crystal Elastomers by X-ray Scattering

Main chain liquid crystal elastomers (MCLCEs) are crosslinked polymer networks that incorporate liquid crystal mesogens into the backbone and combine anisotropic order of liquid crystals and elastic properties of polymers. We investigated the structural response and relaxation dynamics of several siloxane-based smectic-C MCLCEs upon the application of strain, after removal of strain, and with temperature. These elastomers relaxed under constant strain as evident from the changes in the smectic-C tilt angle. At low strains, the MCLCEs exhibited reversible elastic behavior but had anelastic response beyond a threshold strain of $\sim$50\%. The MCLCE films' structure gradually changed with increasing strain at room temperature and revealed a strain dependent enhancement of the smectic order. At strains beyond 50\%, a monodomain structure from an initially polydomain state was obtained. Quantitative analysis of the structural evolution with strain, time, and temperature provide important insight into their molecular level relaxation.   

Segmental order of entangled polymer networks is controlled by monomer fluctuations along the confining tube

The tube model of entangled chains is applied to compute segment fluctuations and segmental orientational order parameter in polymer networks. The latter is essential for interpreting NMR measurements of entangled polymer networks. The sliding motion of monomers along the tube axis leads to a non-homogeneous reduction of segmental order along the chain. For network strands of length $N$ much larger than entanglement length $N_{e}$, the average segmental order decreases $\sim(N_{e}N)^{-1/2}$ in marked contrast to the $1/N_{e}$ contribution of entanglements to network elasticity. As consequence, network modulus is not proportional to segmental order in entangled polymer networks. Monte Carlo simulation results of polymer networks over a wide range of molecular weights are in quantitative agreement with the theoretical predictions. The impact of entanglements on these properties is directly tested by comparing with simulations where entanglement constraints are switched off.   

Effects of charge inhomogeneities on the phase behavior of Polyelectrolyte gels

Networks of Polyelectrolyte chains (PE gels) express a wide range of functionality and are intelligent models to study biological responsive materials such as chromosomes. Electrostatic interactions originating from the charge inhomogeneities compete with the short range interactions that result in a rich phase behavior not well explained by the existing models. We study the electrostatic effects in PE gels within two different schemes - a Poisson-Boltzmann and a Green's function formalism. While a numerical free energy minimization is used in the former scheme, a novel Monte Carlo approach is utilized in the latter. We discuss the influence of density and dielectric inhomogeneities together with dynamic fluctuations on the swelling, the collapse, and the phase segregation behavior of PE gels.   

Yielding Mechanisms in Associating Telechelic Protein Hydrogels

Understanding the flow of associating polymer hydrogels is important for the development of injectable biomaterials. We have investigated gels formed by telechelic proteins composed of associating coiled-coil endblocks linked by a polyelectrolyte midblock domain under both large amplitude oscillatory shear (LAOS) and capillary flow. These materials show dramatic yielding behavior that allows them to flow under low shear stress. Upon cessation of shear they almost instantaneously heal to full elastic strength, making them promising as injectable hydrogels. LAOS experiments and flow visualization suggest that these properties occur due to the formation of shear bands at high strain rates. A theoretical treatment of molecular configuration during flow has been developed to understand the molecular origins of the rheological response in telechelic polymer gels. Polymer chains are modeled as dumbbells with a FENE chain potential, and force-activated kinetics of endgroup attachment and detachment that satisfy detailed balance are used to model the transition of chains between looped, bridged, and detached chain configurations.   

Cell Migration on crosslinked gelatin based scaffolds

The goal of this work was to develop a novel biomaterial to be used as a scaffold for tissue engineering, particularly in the field of dermal healing. The biodegradable hydrogels were prepared through cross linking of gelatin with microbial transglutaminase(mTG) in an aqueous solution and were found to be stable at 37 degrees celcius. Hydrogels were synthesized by cross-linking gelatin with mTG in a ratio of 25:1, 5:1 (gelatin:mTG). Cell proliferation was studied for a period of 8 days showing that cell proliferates better on 25:1 gelatin-mTG hydrogel. Single cell migration for 16 hrs revealed than cells migrated faster and in same direction on 25:1 compared to 5:1 gelatin/mTG hydrogel substrate. En-mass cell migration for 24hrs of also indicated that the cells migrated faster and in a well defined direction following each other on 25:1 compared to the control and 5:1. Cellular traction forces, hence, played a major role in cell migration on these substrates.   

Incompressible Biaxial Nematic Liquid Crystal Elastomers for Artificial Muscles

Assuming only incompressibility, in the simplest complete theory for monodomain liquid crystalline elastomers as rectangular parallelepipeds, we find two biaxial nematic phases, $N_{2+}$ and $N_{2-}$ with a first order $N_{2+}-N_{2-}$ transition exhibiting spontaneous shape change. We identify $N_{2+}$ as icosahedral ($Y_{h}$) and $N_{2-}$, as simple orthorhombic ($D_{2h}$) nematics. Using standard orientational mechanics, we derive the stress-strain behavior before swelling to pin-point the recently discovered elastic strain limits for swelling with $5CB$.   

Crystallization Induced Structuring and Properties of CEC-P-CEC Elastomeric Multiblock Terpolymers

We report the synthesis and characterization of a new class of block copolymer based thermoplastic elastomers that microphase separate in response to block crystallization. Five relatively monodisperse CEC-P-CEC block terpolymers containing glassy poly(cyclohexyl-ethylene) (C), semi-crystalline poly(ethylene) (E) and elastomeric poly(ethylene-alt-propylene) (P) were synthesized in a heptablock architecture by anionic polymerization and catalytic hydrogenation. Microphase segregation is induced from a homogeneous state by E crystallization, resulting in a bicontinuous morphology with CEC and P sub-structures leading to a useful combination of tensile modulus, tensile strength and elastic recovery. This molecular design decouples the material processing temperature from copolymer composition and molecular weight due to a favorable combination of block-block interactions in the melt state.   

Interpenetrating Polymer Network Hydrogels Based on VP/MMA Gel and PLGA Diacrylate Macromers

Hydrogels have been widely used in biomedical applications due to their biocompatibility, similar physical properties to human tissue and appropriate mechanical properties. A thorough understanding of their swelling behavior is necessary to be able to choose the most suitable hydrogel and to applying it optimally. The long term goal of our research is to develop hydrogel systems with controllable swelling behavior for medical/surgical use. For this purpose, interpenetrating polymer network (IPN) hydrogels have been prepared based on the N-vinyl-2-pyrrolidone (VP)/methyl methacrylate (MMA) copolymeric gel and poly(DL-lactic-co-glycolic acid) (PLGA) diacrylate macromers as well as semi-IPN VP/MMA and PLGA hydrogels. The thermal, morphological, mechanical and physical properties of the hydrogels have been characterized and the potential for surgical use verified. This presentation will concentrate on the studies of the swelling kinetics and equilibrium swelling ratios of the hydrogels. In addition, very recent results will be presented on how additions of PLGA can be used to manipulate the swelling behavior of the hydrogel system.   

Photo-Crosslinkable Shape-Memory Elastomers Containing Hydrogen-Bonding Side-Groups

Lightly crosslinked poly(butyl acrylate) networks containing self-complementary hydrogen bonding side-groups (e.g. ureidopyrimidinones) can exhibit unique shape-memory effects. Conventional free-radical solution polymerization of monomer mixtures offers a simple approach to achieving these networks, but presents the following limitations: (i) the H-bonding side- group content is limited by its solubility; (ii) prepared networks can only be studied in their crosslinked form; and (iii) upon solvent removal, intrinsic stress is generated making shape-memory responses difficult to interpret. Here, photo-polymerization of linear prepolymers is reported as an approach to overcome these limitations. A series of linear poly (butyl acrylate)s containing light-sensitive benzophenone side- groups and H-bonding side-groups were prepared using free radical polymerization. These pre-polymers can be fully characterized in solution (NMR, GPC). Moreover, following solvent removal, they can be molded into any desired shape and subsequently photo-crosslinked to form shape-memory elastomers. The impact of H-bonding side-group content and the benzophenone side-group content on the mechanical properties of the photo-crosslinked polymer will be discussed. A constitutive model is also developed to interpret the mechanical response of these shape-memory elastomers.   

Structural changes of semicrystalline polyolefin block copolymer elastomers during step cycle mechanical processing

Development of stereo- and regioselective catalysts has led to the capability to produce multiblock copolymers with crystalline isotactic or syndiotactic polypropylene blocks and ethylene-r-propylene rubbery blocks which have excellent elastomeric properties. During step cycle mechanical processing the crystals can plastically deform and transform from lamellae into rod-like fibrils. The stress-strain tensile curves provide evidence of dramatic changes in the mechanical properties and small angle and wide angle X-ray scattering experiments as well as real space imaging bring a better understanding of the structural changes of the crystals during such processing.   

Origin of cononsolvency, based on the structure of tetrahydrofuran--water mixture

Poly ($N$-isopropylacrylamide) (PNIPAM) is soluble in (cold) water and THF, but insoluble in mixtures of the two in certain conditions. The origin of this cononsolvency phenomenon was studied from the point of view of mixed solvent structures. Dynamic light scattering, small angle neutron scattering, and viscometry were used to study this phenomenon. From this study, the dynamic equilibrium of THF-water compositional fluctuations in the mixed solvent system was found to be the main cause for this cononsolvency effect.