A hydrogel is a crosslinked network of polymer chains swollen by water. When immersed in an aqueous medium, a hydrogel will swell by taking up water until the osmotic pressure set by mixing between water and polymer is balanced by the free energy required to stretch polymer chains. When considered at a length scale greater than the sub-micrometer scale inhomogeneity of the gel network structure, the unconstrained gel swells isotropically and reaches a macroscopically stress free state. However, when a sheet of gel is attached to either a non-swelling rigid substrate or a gel that swells by a different amount, the resulting mechanical constraints generate stress within the gel, leading to the out-of-plane deformations of the gel. In this thesis, we study and harness the instabilities of these mechanically constrained hydrogels, especially thin hydrogel sheets with thicknesses of 10–100 micrometers that swell and deswell rapidly (less than 10 seconds for sufficiently hydrophilic gels). We create hydrogel based micro-systems, where we locally apply mechanical constraints on the swelling of hydrogel sheets, and therefore, the gels deform out-of-plane into 3D shapes. Next, we experimentally characterize and analyze the deformed hydrogels, elucidate the mechanisms underlying the observed deformation using finite element analysis, and finally utilize these methods to fabricate stimuli-responsive surfaces and structures. As the first example, we attach a thin film of hydrogel on a rigid substrate, inducing an elastic creasing instability in which the surface of the hydrogel locally folds against itself. Through the chemical modification of the hydrogel surfaces that undergo the creasing instability, we fabricate dynamic surfaces that hide and display biomolecular patterns in response to an external stimulus and show how these materials hold promise for applications in studying cell mechanics and creating lab-on-a-chip devices. Next, we use a grayscale gel lithography to two-dimensionally patterned discretely varying swelling ratios within a hydrogel sheet of ∼ 10 micrometer thickness. This finite-thickness, differentially growing hydrogel sheet undergoes out-of-plane deformation as it swells and adopts a configuration that is determined by the initially prescribed local swelling ratios and minimizes the overall elastic deformation energy, i.e. the sum of stretching and bending energies. Additionally, we introduce a halftone-style two-level grayscale gel lithography, which prescribes effectively continuous metrics on the hydrogel sheets by patterning hexagonal arrays of dots that locally vary in their sizes and swell less than the background. This platform, grayscale gel lithography, provides opportunities both for asking fundamental questions about the mechanics of non-Euclidean plates, as well as for designing stimuli-responsive micro-devices.

The deformation behavior of several different polyurethane thermoplastic elastomers was investigated. Four chemically distinct systems were synthesized, each with hard segment contents of 30, 40, and 50 percent by weight for a total of twelve different elastomers. Chemical components for the elastomer synthesis were as follows: diisocyanates, 4,4$\prime$-Diphenyl Methanediisocyanate (MDI) and 4,4$\prime$-Dicyclohexyl Methanediisocyanate (H$\sb{12}$MDI), diol chain extenders, Butanediol (BD) and 4,4$\prime$-Bis(6-hydroxyhexoxy) biphenyl (BHHBP), and a soft segment oligomer of 2000 molecular weight Poly(tetramethylene oxide) ether (PTMO). Elastomers synthesized with H$\sb{12}$MDI were found to contain hard segment which were essentially glassy in nature while those elastomers based on MDI contained crystalline hard segments. Infrared (IR) analysis was used to investigate the extent of hydrogen bonding in the hard segment as well as to assess segmental orientation with deformation. Crystalline hard segment materials were found to exhibit higher levels of hydrogen bonding. A significant difference in the orientation behavior of the hard segments as a function of elongation was observed while little difference in the soft segment orientation behavior was noted. The development of crystallinity in the soft segments of the materials was followed with both wide angle x-ray scattering (WAXS) and deformation calorimetry. Soft segments were found to undergo strain induced crystallization at extensions greater than 100 percent. No change was observed in the hard segment structure after deformation with WAXS, although an internal energy increase in all of the elastomers was measured after a complete deformation cycle of extension and retraction. The elastomers based upon H$\sb{12}$MDI were found to exhibit superior mechanical properties, especially reduced permanent set and hysteresis. This improvement in properties was primarily associated with decreased hard segment chemistry and amount. Other mechanical properties investigated included stress relaxation, dynamic mechanical thermal analysis, and stress development with film casting.

The non-Newtonian rheology of dilute polymer solutions can be traced to the flow-induced rearrangements in the conformation of flexible, isolated macromolecules. Such rearrangements are most pronounced in strong elongational flows; when such a flow is imposed on a random coil polymer for a sufficient time, the chain can become highly distorted and the fluid properties highly non-Newtonian. A direct determination of the magnitude of flow-induced polymer stretching will lead to a better understanding of the physical mechanisms responsible for the unique rheology of polymer solutions. In this study, light scattering methods are employed for the first time to probe the deformation of flexible polymer coils in dilute solutions undergoing uniaxial elongational flows. Such flows are created at the stagnation point region of an opposed jets flow device. This device is positioned so that its stagnation point can be superimposed on the scattering volume of a specially constructed variable angle light scattering instrument. Flow birefringence is also monitored. After suitable analysis, the two optical measurements provide the radius of gyration parallel to the stretching direction and the degree of segmental orientation; both quantities are averages over a 100 $\mu$m size region. Data are collected from the good solvent pair polystyrene/toluene as a function of elongation rate and polymer molecular weight. The scattering results indicate limited stretching, even at elongation rates greatly exceeding the reciprocal of the Zimm relaxation time. No evidence is found for complete chain unravelling. The birefringence does saturate at large elongation rates, indicating that segmental orientation and global deformation are not necessarily achieved simultaneously. It is suggested that measurements performed at the stagnation point actually reflect a larger spatial region of limited average residence time and that this time is sufficient for segmental orientation but not for chain unravelling. In addition to flexible polymer solutions, birefringence is also used to monitor the behavior of stiff polymers in the same flow. Distinctions between flexible and stiff chains are readily discerned in the spatial variation of the birefringence intensity.

<p> One of the major barriers to expanding the capacity of large-scale electrochemical energy storage within batteries is the threat of a catastrophic failure. Catastrophic battery pack failure can be initiated by a defect within a single battery cell. If the failure of a defective battery cell is not contained, the damage can spread and subsequently compromise the integrity of the entire battery back, as well as the safety of those in its surroundings. Replacing the volatile, flammable liquid electrolyte components found in most current lithium ion batteries with a solid polymer electrolyte (SPE) would significantly improve the cell-level safety of batteries; however, poor ionic conductivity and restricted operating temperatures compared to liquid electrolytes have plagued the practical application of SPEs. Rather than competing with the performance of liquid electrolytes directly, our approach to developing SPEs relies on increasing electrolyte functionality through the use of block copolymer architectures. </p><p> Block copolymers, wherein two or more chemically dissimilar polymer chains are covalently bound, have a propensity to microphase separate into nanoscale domains that have physical properties similar to those of each of the different polymer chains. For instance, the block copolymer, polystyrene-<i>b</i>-poly(ethylene oxide) (SEO), has often been employed as a solid polymer electrolyte because the nanoscale domains of polystyrene (PS) can provide mechanical reinforcement, while the poly(ethylene oxide) microphases can solvate and conduct lithium ions. Block copolymer electrolytes (BCEs) formed from SEO/salt mixtures result in a material with the bulk mechanical properties of a solid, but with the ion conducting properties of a viscoelastic fluid. The efficacy SEO-based BCEs has been demonstrated; the enhanced mechanical functionality provided by the PS domains resist the propagation of dendritic lithium structures during battery operation, thus enabling the use of a lithium metal anode. The increase in the specific energy of a battery upon replacing a graphite anode with lithium metal can offset the losses in performance due to the poor ion conduction of SPEs. However, BCEs that enable the use of a lithium anode and have improved performance would represent a major breakthrough for the development of high capacity batteries. </p><p> The electrochemical performance of BCEs has a complex relationship with the nature of the microphase separated domains, which is not well-understood. The objective of this dissertation is to provide fundamental insight into the nature of microphase separation and self-assembly of block copolymer electrolytes. Specifically, I will focus on how the ion-polymer interactions within a diverse set of BCEs dictate nanostructure. Combining such insight with knowledge of how nanostructure influences ion motion will enable the rational design of new BCEs with enhanced performance and functionality. </p><p> In order to facilitate the study of BCE nanostructure, synchrotron-based X-ray scattering techniques were used to study samples over a wide range of length-scales under conditions relevant to the battery environment. The development of the experimental aspects of the X-ray scattering techniques, as well as an improved treatment of scattering data, played a pivotal role in the success of this work. The dissemination of those developments will be the focus of the first section. </p><p> The thermodynamic impact of adding salt to a neutral diblock copolymer was studied in a model BCE composed of a low molecular weight SEO diblock copolymer mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), a common salt used in lithium batteries. In neutral block copolymers (BCPs), self-assembly is a thermodynamically driven process governed by a balance between unfavorable monomer contacts and the entropy of mixing. When the enthalpic and entropic contributions to free energy are similar in magnitude, a block copolymer can undergo a thermally reversible phase transition from an ordered to a disordered nanostructure. We used temperature-dependent small angle X-ray scattering (SAXS) to observe this transition in the model SEO/LiTFSI system. Unlike neutral BCPs, which to a first approximation are single component systems, the SEO/LiTFSI system demonstrated the thermodynamically stable coexistence phases of ordered lamellae and disordered polymer over a finite temperature window. Analysis of the lamellar domains revealed an increase in salt concentration during the ODT, indicating local salt partitioning due to the presence of nanostructure.</p><p> The performance of BCEs can also be improved by chemically functionalizing one of the polymer blocks by covalently attaching the salt anion. Since the cation is the only mobile species, these materials are coined single-ion conducting block copolymers. Single ion conduction can improve the efficiency of battery operation. In order for cation motion to occur in single-ion conducting block copolymers, it must dissociate from the backbone of the anion-containing polymer block. This direct coupling of ion dissociation (and hence conduction) and nanostructure has interesting implications for BCE performance. (Abstract shortened by ProQuest.) </p>

<p> Polymer blending facilitates the combination of the attractive attributes of two or more polymers while compensating for the unfavorable ones. Most polymers are thermodynamically incompatible with one another, and their blending yields a two-phase microstructure. This morphology generally determines the mechanical and rheological properties of the blend system which then determine its applications. Morphology development typically involves deformation of the dispersed phase followed by drop breakup. However, drop coalescence competes with this process, and ultimately a balance must be reached between these two competing processes. Extensional flow fields are known to promote drop breakup and are especially important for blends with high viscosity ratios, that is for blends where the viscosity of the dispersed phase is at least about 3.8 times greater than that of the matrix phase. Coalescence may be attenuated by compatibilizers that modify the interface between the polymer phases. Nanoparticles with tuned surface chemistry may also be used as compatibilizers. A combination of extensional flow and nanoparticle stabilization should, therefore, result in a fine, stable morphology. </p><p> To begin the investigation toward the effects of extensional flow blending with and without the incorporation of nanoparticles, preliminary results were obtained using two different polymer blend systems: polycarbonate (PC)/styrene acrylonitrile (SAN) and polystyrene (PS)/linear low-density polyethylene (LLDPE). However, the majority of the presented results involve blends of high-density polyethylene (HDPE) dispersed in PS. With this blend system, with the material grades selected, the viscosity ratio exceeded 3.8 over the entire domain of deformation rates anticipated in the processing used. Coarse blends of various compositions were formulated using shear flow in an internal mixer or in a twin-screw extruder. These blends were subjected to extensional flow in converging dies of different geometries and where more than one stretching episode was possible; the temperature, total strain, and flow rate were varied, among other factors, in a systematic manner. Experiments were repeated in the presence of various grades of fumed nanosilica of different sizes and surface treatments, which imparted different surface tension and relative surface polarity (hydrophilic versus hydrophobic) for the nanoparticles. The mixing sequence was varied including premixing the nanosilica into the thermodynamically non-preferred polymer phase. </p><p> Scanning electron microscopy (SEM) was used to determine the size and size distribution of the dispersed polymer phase. The material was typically sectioned in the flow direction, but sectioning in the direction perpendicular to flow and etching, or selectively dissolving, one phase or the other was also investigated. The primary effect of extensional flow blending was to reduce the volume-average diameter of the dispersed polymer phase, especially with increasing strains and flow rates, or strain rates, which is directly dependent on both. Finding suitable conditions for the nanoparticles to selectively localize at the HDPE/PS interface was challenging, but relatively small amounts of nanoparticles dispersed in the PS matrix decreased the volume-average diameter of HDPE drops. When the nanosilica was preloaded into the HDPE dispersed phase, very coarse initial blends were produced which then exhibited dramatic decreases in phase size with extensional flow. These and other results are properly organized and presented.</p>

The primary objective of this research is to investigate the morphological and mechanical properties of composite materials and porous materials prepared by thermally induced phase separation. High melting crystallizable diluents were mixed with polymers so that the phase separation would be induced by the solidification of the diluents upon cooling. Theoretical phase diagrams were calculated using Flory-Huggins solution thermodynamics which show good agreement with the experimental results. Porous materials were prepared by the extraction of the crystallized diluents after cooling the mixtures (hexamethylbenzene/polyethylene and pyrene/polyethylene). Anisotropic structures show strong dependence on the identity of the diluents and the composition of the mixtures. Anisotropic crystal growth of the diluents was studied in terms of thermodynamics and kinetics using DSC, optical microscopy and SEM. Microstructures of the porous materials were explained in terms of supercooling and dendritic solidification. Dual functionality of the crystallizable diluents for composite materials was evaluated using isotactic polypropylene (iPP) and compatible diluents that crystallize upon cooling. The selected diluents form homogeneous mixtures with iPP at high temperature and lower the viscosity (improved processability), which undergo phase separation upon cooling to form solid particles that function as a toughening agent at room temperature. Tensile properties and morphology of the composites showed that organic crystalline particles have the similar effect as rigid particles to increase toughness; de-wetting between the particle and iPP matrix occurs at the early stage of deformation, followed by unhindered plastic flow that consumes significant amount of fracture energy. The effect of the diluents, however, strongly depends on the identity of the diluents that interact with the iPP during solidification step, which was demonstrated by comparing tetrabromobisphenol-A and phthalic anhydride. A simple method to prepare composite surfaces that can change the wettability in response to the temperature change was proposed and evaluated. Composite surfaces prepared by nanoporous alumina templates filled with polymers showed surface morphology and wettability that depend on temperature. This effect is attributed to the significant difference in thermal conductivity and the thermal expansion coefficient between the alumina and the polymers. The reversibility in thermal response depends on the properties of the polymers.

In this work the effect of elasticity on turbulent drag reduction due to polymers is investigated using a hybrid Direct Numerical Simulation (D.N.S) and Langevin dynamics approach. Simulations are run at a friction Reynolds number of Reτ = 560 for 960.000 dumbbells with Deborah numbers of De = 0, De = 1, and De = 10. The conclusions are that it is possible to simulate a drag reduced flow using hybrid D.N.S. with Langevin dynamics, that polymers, like other occurrences of drag reduction, reduce drag through streak stabilization, and that the essential property of polymers and fibers in having a drag reducing effect is their ability to exert a torque on the solvent when they orientate in the boundary layer of the turbulent flow.

The ability of block copolymers (BCPs) to self-assemble into well-defined arrays of nanoscopic structures has enabled them to be used as templates and scaffolds for the fabrication of nanostructured materials. Such materials find applications in several fields including microelectronics, photonics and sensors. In this dissertation nanostructured silicate films were fabricated by performing phase selective silica chemistry within self-assembled BCP templates using a discrete two step replication process: (i) template formation and (ii) supercritical fluid assisted silica deposition. The use of supercritical CO2 as a reaction medium enabled facile transfer and diffusion of silicate precursor within the BCP film without disturbing its order. The sensitivity of the chosen precursor to acid, helped to control the silica condensation at nanoscopic and at microscopic length scales. Removal of templates yielded mesoporous silicate films in which the porous geometry can be completely controlled over multiple length scales. The use of BCP films with cylindrical domains oriented normal to the substrate as templates for phase selective silica deposition yielded arrays of perpendicular nanochannels in silica films. Obtaining such morphology in mesoporous materials has proven to be challenging, although they are promising candidates for applications ranging from catalysis to sensors and the separations. To fabricate directly patterned mesoporous silicate films, a photo acid generator was added in the BCP templates. Before performing phase selective precursor condensation, the templates were exposed to UV radiation through a photomask that has microscopic features. Photo-lithographic exposure triggered area selective generation of acid, which in turn led to patterned formation of silicate network. Because the acid generated in UV exposed field segregate further into hydrophilic domains of the BCP, precursor condensation is controlled in micro and nano-scopic length scales. De-templating via calcination yielded patterned mesoporous silicate films. These mesoporous silicate films inherited two levels of porosity. Microscopic features were inherited from the photomask and the nanoscopic features were inherited from the phase separated block copolymer template. Direct definition of the former followed by replication of pattern obviated the need for additional etching-cleaning steps and offers a cost-effective and compressed process routine for device level structures as small as 90 nm.

Studies of the electrophoretic transport of charged macromolecules have been performed to elucidate the molecular mechanisms involved in gel electrophoresis and to discriminate among various theories which attempt to describe chain motion in gels. Experiments with high molecular weight, synthetic polyelectrolytes establish gel electrophoresis as a viable technique for characterizing polymers according to chain length distribution. Mobility has been measured as a function of degree of polymerization, electric field strength, gel concentration, ionic strength, chain topology, and charge density. Trends for synthetic polyelectrolytes, principally poly(styrenesulfonate), qualitatively match those observed for DNA. By comparison of the electrophoretic mobility of polyions in the absence and presence of a gel, the importance of interactions between polymer and gel fibers is demonstrated. High resolution separations which occur in the presence of a gel are a result of interactions of between the polyion and gel fibers. The frequency of such interactions is influenced by the size of the polymer chain relative to the mesh spacing in the gel matrix. Entanglement of the polymer in the matrix plays a fundamental role in the separation process. Variations in mobility among chains with different topologies (linear, star-branched, and circular molecules) provide strong evidence for the recently proposed "entropic barriers" transport theory for weakly entangled chains. For highly entangled linear polymers, theories based on reptation appear to predict the proper scaling dependence of mobility on chain length at low electric field strengths.

Electrophoretic transport of highly charged polyelectrolytes in the presence or absence of a gel matrix has been studied, with the objective of an improved understanding at a molecular level of the transport mechanisms responsible for molecular weight discrimination. In the presence of a gel and low applied fields, based on the ratio of the probe size to gel mesh spacing, three transport mechanisms are identified as confinement level increases: sieving, entropic barriers, and reptation. For sieving models the widely accepted notion that nonspherical and flexible molecules can be represented as effective spheres during motion through highly porous gels is shown to be a poor approximation. At intermediate confinement, probe mobility trends are explained by the entropic barriers theory, where spatial variations in chain entropy control the motion. In the most confined state, transport is governed by reptation. Capillary electrophoresis is established as the method of choice for measurements of the free-solution mobility as a function of ionic strength and chain length. A traditionally assumed chain-length independent free-solution mobility is monitored by the method, but a transition to molecular weight dependence is noted for flexible short chains of degree of polymerization less than 50. Increasing ionic strength decreases mobility because the cylindrically symmetric counter-ion cloud is shifted towards smaller radial distances from the chain backbone; this shift increases the hydrodynamic forces propagated to the chain by these ions. The trends with ionic strength, however, do not follow previous theories.