Order in side-chain liquid crystalline diblock copolymers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2001.Includes bibliographical references.The architecture of side-chain liquid crystalline diblock copolymers (SCLCBC's) involves order on two different but interdependent length scales. Through molecular-scale interactions, liquid crystalline moieties self-assemble into a variety of mesophases, and on a larger length scale, chemical differences between the two blocks can result in phase-segregated block copolymer microstructures. This combination of organizational tendencies offers experimentally and theoretically challenging problems that precede a wide array of applications. Using sequential anionic polymerization we synthesized a large number of well-defined SCLCBC's. These materials consist of an amorphous, polystyrene (PS) block and a methacrylate-based, LC block with side-chain mesogens that exhibit the Sc* mesophase. The samples' block copolymer morphology was evaluated using a combination of small angle X-ray scattering (SAXS) and electron microscopy, and thermal transitions were identified using polarized microscopy, calorimetry, and elevated temperature-SAXS. We found the formation of micron-sized domains and focal-conic superstructures to depend on the length of the PS block and the overall molecular weight. A morphological phase diagram was constructed based on LC volume fraction for molecular weights up to 40,000 Daltons. A broad lamellar regime was identified that extends to unusually low LC compositions. At intermediate LC fractions, just over 50 %-wt. LC, where one would expect lamellar morphologies for analogous coil-b-coil diblocks, morphologies are predominately lamellar, but exhibit perforated defects that connect the lamellar LC microdomains.(cont.) A morphology consisting of hexagonally-packed PS cylinders was observed at 79 %-wt. LC, and at very high LC volume fractions (85 %-wt.) an unusual layered morphology was observed in both bulk and thin film studies. For this sample, the microstructure has a periodicity of -70 Angstroms and forms highly ordered micron-size monodomains. SAXS and TEM data suggest that the LC domains consist of smectic bilayers, and the amorphous polystyrene domains are highly oblate spheres that arrange hexagonally between smectic bilayers. We investigated the interdependence of block copolymer morphology and LC thermotropic phase behavior using elevated temperature SAXS. Order-disorder and order-order block copolymer transitions were located and compared to the LC isotropization temperature. For materials with low LC volume fractions, and low overall molecular weight, LC isotropization is shown to trigger morphological order-disorder transitions (ODT's). In other samples the LC clearing point precedes the ODT temperature, and this temperature difference largely depends on the length of the LC block. One sample, with 58 wt-% LC, undergoes a thermal-reversible LC triggered order-order transition between a predominately lamellar morphology with cylindrical defects and a completely lamellar morphology. These and other observations are explained on the basis of conformational asymmetry. Our analysis indicates that the length of the LC block and the related block copolymer superstructure are key parameters to controlling both LC mesophase and morphology. A simple free energy model was developed to capture the interplay between liquid crystalline and block copolymer morphology ....by Mitchell Lewis Anthamatten.Ph.D

Porous polymer by vapor deposition polymerization

Thesis (Ph. D.)--University of Rochester. Dept. of Chemical Engineering, 2015.Macro-porous polymers are most commonly prepared by solution- or melt-phase methods including polymerization induced phase separation (PIPS), thermal induced phase separation (TIPS) and other phase inversion techniques. While these techniques have achieved exquisite control of pore size and porosity and are advancing technologies related to membranes for separations, drug delivery, and cellular scaffolding of tissues or implants, it remains challenging to form porous polymer as highly conformal layers or to deposit precise amounts of porous polymer onto targeted areas. This thesis develops multi-component vapor deposition polymerization (VDP) techniques that force phase separation of as-deposited species, while, at the same time, reactive polymerization is occurring, leading to kinetically trapped macro-scale structure and morphology. It shows that rapid film growth rates can be achieved by initiated chemical vapor deposition (iCVD) of poly(glycidyl methacrylate) from supersaturated monomer vapor. Further, template-free methods were applied to fabricate continuousphase, porous polymer films by simultaneous phase separation during vapor deposition polymerization. To further understand the process, the degree of interaction between condensed species was systematically varied and experiments were conducted using three different porogens with different cohesive energy densities. Experiments show that the morphology and porosity of the as-deposited polymer thin films depend on deposition rate, crosslinker density, the mass transfer mobilities of phase-separating species, and the interaction energies between species. Chemical crosslinking around condensed porogen during vapor deposition polymerization offers morphological control of porous polymer within thin, conformal layers. In principle, this strategy could be translated to line-of-sight vapor deposition methods, enabling porous polymer to be grown through a pattern mask, or even directly onto part’s surface. The ability to control solid/ porous membrane growth and feature size is relevant to the future work including laser fusion targets fabrication, stimuli-responsive porous hydrogel thin films, and multi-stimuli responsive polymers

Diffusion through Reversibly Associating Polymer Networks

Many natural macromolecules, like proteins and DNA, are equipped with site-specific, non-covalent molecular interactions. These interactions lead to intricate secondary structures such as the double helix, self-assembled phospholipid membrane bilayers, and precisely folded protein structures that are vital to life and rely on non-covalent interactions to "guide" molecular organization. Mankind has begun to borrow such concepts from nature to engineer responsive materials. In recent years, for example, the use of strong, highly directional hydrogen bonds has enabled polymers with temperature-tunable architectures to be engineered. Site-specific hydrogen bonding and ionic interactions in solution can lead to aggregation, gelatin, or sudden viscosity changes that are triggered by slight changes in polymer concentration, pH, or temperature.3·5 In the melt, rigid and elastic polymer networks can be reversibly transformed into a low viscosity polymer melt simply by heating. This new materials concept is playing an important role in the development of recyclable (thermoplastic) elastomers. The quest to fully understand structure-property relationships of polymers decorated with hydrogen bonding groups has opened a new field at the interface of polymers and supramolecular chemistry. Our laboratory has synthesized novel polymer networks which contain both covalent crosslinks and non-covalent crosslinks (Figure 1). Non-covalent crosslinks arise from the presence of ureidopyrimidone (UPy) sidegroups, which are well known to undergo strong, yet reversible, H-bond association. At low temperatures, the rate of H-bond dissociation is slow, and the material behaves as if it is highly crosslinked-like a rigid solid. At higher temperatures, H-bond dissociation is fast, and the material behaves as if it only contains covalent rosslinks-like a soft elastomer. Consequently, mechanical properties show unusual temperature dependence. Shape-memory responses of these and similar networks have been carefully studied. While a great deal of research has focused on controlling polymer rheological properties, to our knowledge no studies have examined how reversible association affects molecular transport. The primary goal of our research is to determine how the rate of molecular transport across dynamic hydrogen bonded networks depends on temperature. We hypothesize that the presence of long-lived hydrogen-bonds will decrease the rate of molecular transport at lower temperatures. A secondary question we raise is how the molecular size of the penetrants influences molecular diffusion . To address this issue, molecular transport of ethanol across polymer networks is compared to that of a much larger organic dye

Fabrication of structured polymer films using vapor deposition techniques

Thesis (Ph. D.)--University of Rochester. Dept. of Mechanical Engineering, 2009.New techniques to fabricate structured polymer films using chemical vapor deposition were developed and studied. Two different vapor deposition approaches, one using step-growth polymerization and another using chain-growth polymerization, were employed. The primary objective of this thesis was to determine the feasibility of controlling morphology of vapor-deposited polymer films by introducing non-reactive, immiscible components into the vapor deposition process. Poly(amic acid)s, condensation polymers and precursors to rigid-rod polybenzoxazoles (PBO), formed upon co-deposition of 3, 3’-dihydroxybenzidine (DHB) and pyromellitic dianhydride (PMDA). Deposited coatings were cured under inert gas conditions and resulted in the conversion to semi-aromatic polybenzoxazoles at around 550 °C. Physical and chemical changes occurring during the curing process were studied with FT-IR, TGA and nanoindentation experiments. Successful fabrication of PBO films provided a platform to study simultaneous film growth and phase separation in vapor-deposited condensation polymers. Control of morphology of vapor-deposited condensation polymer films was achieved by the fabrication of polyimide/CuPc composite films, which were made from the co-evaporation of 4,4’-oxydianiline (ODA) and 3,3’,4,4’-biphenyl tetracarboxylic dianhydride (BPDA) in the presence of non-reactive, third-component CuPc. Spectroscopy experiments confirm the formation of polyimide segments and suggest that embedded CuPc molecules have less mobility compared to pure CuPc films. Electron microscopy and XRD studies show evidence of embedded CuPc particles at the surface and in the bulk of fine lateral structure with a length scale of about 100 nm. Fabrication of poly (methyl methacrylate) (PMMA) films using initiated chemical vapor deposition (iCVD) provided a platform to investigate film growth and phase separation for a classical chain-growth polymer. An axisymmetric, multi-component iCVD apparatus was designed to study the vapor-phase growth of glassy PMMA films. Key reactor operating parameters, including the hot-zone temperature, reactor base-pressure, substrate temperature, and the monomer/initiator molar feed ratio were systematically varied to understand film growth kinetics. The non-reactive solvent-vapor, t-butanol, was then introduced into the deposition process to promote polymer film dewetting. When solvent-vapor is used, non-equilibrium dewetted structures comprising of randomly distributed polymer droplets were observed. The length-scale of observed topographies, determined using power spectral density (PSD) analysis, ranges from 5 to 100 microns and can be influenced by deposition conditions, especially the carrier gas and solvent-vapor flowrates. Control over lateral length-scale is demonstrated by preparation of hierarchal “bump-on-bump” topographies. Autophobic dewetting of PMMA from SiOx/Si substrate during iCVD process is attributed to a thin film instability driven by both long range van der Waals forces and short range polar interactions

Polymer networks containing reversibly associating side-groups

Thesis (Ph. D.)--University of Rochester. Dept. of Chemical Engineering, 2011.Supramolecular polymers consist of low molar mass subunits that non-covalently bind together through hydrogen bonding or other non-covalent interactions, forming macromolecular assemblies. Site-specific and reversible hydrogen bonds and other non-covalent interactions are increasingly employed to modify bulk polymer properties, enabling thermoplastic elastomers and self-healing polymers. In this thesis, I investigate how hydrogen bonding groups directly bonded onto an elastic polymer network affect material properties. A lightly crosslinked covalent network containing hydrogen bonding side-groups (ureidopyrimidinone, UPy) was synthesized. This architecture results in a novel shape-memory effect, and the molecular events resulting in this behavior were deduced. Further, to systematically evaluate how thermomechanical properties are related to network architecture, a new photo-crosslinking route was developed to prepare shape-memory elastomers. This method enables melt-processing of shape-memory elastomers into complex permanent shapes, and samples can be prepared with much higher UPy-content. Furthermore, the covalent and non-covalent crosslink density can be accurately controlled. Dynamic mechanical analysis on photo-crosslinked shape-memory elastomers revealed that dynamic crosslinks behave nearly as effectively as permanent crosslinks below the UPy hydrogen bond transition. Compared to linear polymers bearing identical hydrogen bonding groups, the synthesized dynamic networks exhibit an enhanced temperature dependence of mechanical properties. This indicates that the covalent network supports cooperative hydrogen bonding. This finding will guide researchers to more effectively employ non-covalent interactions within bulk polymer materials. Mass transport through dynamic networks was also studied using multi-photon fluorescence recovery after photobleaching (MP-FRAP). In contrast to viscous relaxation, small molecule mass transport through the dynamic networks is limited by the density of hydrogen bonds instead of their exchange rate

Design and synthesis of novel proton conducting electrolytes: protogenic liquid crystals and electrospun quaternary ammonium ionomers

Thesis (Ph. D.)--University of Rochester. Dept. of Chemical Engineering, 2012Highly conductive and durable proton exchange membranes (PEMs) are critical to realizing alternative energy applications including fuel cells, batteries, and photochemical water splitting. A major challenge is to fabricate materials with high ionic conductivity at high operating temperature that can maintain mechanical stability. This thesis investigates (i) the use of liquid crystalline (LC) order to improve anhydrous proton transport, and (ii) the use of electrospinning to form mechanically stable membranes with high ion content. Two novel smectic liquid crystalline (LC) mesogens (Imi-COOH and Imi-DAH) were designed and synthesized with a diacylhydrazine core and a terminal proton-conducting imidazole group. In principle smectic LC ordering may offer proton-conductive pathways between smectic layers while retaining a high level of liquid-like mobility. Imi-COOH forms a smectic LC mesophase and exhibits mesoscopic order up to 230 °C, whereas Imi-DAH exhibits a less ordered nematic phase. Impedance spectroscopy studies indicate that LC ordering combined with acid-base exchange results in significantly higher proton conductivity in Imi-COOH’s LC state (δ ~10-5 S/cm) compared to Imi-DAH (δ ~10-7 S/cm). However, for Imi-DAH and DAH, the imidazole groups are relatively dilute, and LC ordering alone within microscopic polydomains does not result in significantly higher conductivity. The activation energy for proton transport is shown to be higher in the LC state compared to the isotropic liquid. Thermal activation of targeted LCs above their clearing temperature was comparable to that of liquid imidazole (~0.2-0.4 eV) and this energy scale agrees with Grothuss-like proton transport. The second part of this thesis investigates a novel approach to improve mechanical properties of PEMs through electrospinning. A series of quaternary ammonium polysulfone (QAPS) solutions with ion contents, ranging from 0.5-1.7 mmol/g, were electrospun and solvent-cast. With increasing solution concentrations, the electrospun morphology evolved from stretched spheres to uniform bead-free fibers. The as-spun QAPS mats were treated to increase membrane density and to promote 3D fibrous interconnection. Considering their high ion exchange capacities (ICEs), the electrospun membranes exhibited favorable mechanical strength compared to solution-cast films. Future composite membranes, derived from those reported here, may offer low cost, high IEC, and mechanical durability

Synthesis and phase behavior of end-functionalized associating polymers

Thesis (Ph. D.)--University of Rochester. Dept. of Mechanical Engineering, Materials Science Program, 2010.We have explored polymer blend phase behavior in the presence of multiple hydrogen bonding end-groups. This work details the synthesis of functionalized polymers and their subsequent use in miscibility studies. The synthesis of end-functionalized hydrogen bonding polymers and the investigation of their physical properties and miscibility is presented. Mono-functional and telechelic ureidopyrimidinone (UPy) functionalized polymers were prepared by two main routes: post-polymerization functionalization (of commercially available or synthesized polymers); and polymerization of monomers using a functionalized initiator. UPy-functionalized polymers were prepared with a variety of polymer backbones including poly(ethylene oxide)s; poly(butadiene)s, poly(dimethyl siloxanxe)s; poly(styrene)s and poly(methyl methacrylate)s. The most successful route to polymers with UPy end-groups was atom transfer radical polymerization (ATRP) using a UPy-functionalized initiator, followed by atom transfer radical coupling (ATRC). The incorporation of ureidopyrimidinone end-groups was shown to affect the physical properties of the polymer backbone. Parent polymers that were liquids became viscous liquids or waxy solids upon UPy-functionalization of chain end. UPy-functionalization of a hydroxyl-terminated polybutadiene (HO-PB-OH) resulted in a waxy solid while the HO-PB-OH precursor was a viscous liquid. The thermal properties of functionalized polymers also differed from those of the unfunctionalized parent polymers. Hot-stage optical microscopy revealed that UPy-functionalized PEO displayed a depressed melting point relative to the analogous unfunctionalized precursor. Differential scanning calorimetry was also used to investigate the synthesized UPy-polymers. UPy-functionalized polystyrenes and poly(methyl methacrylate)s showed an increased Tg compared to the equivalent homopolymer standards. This increased Tg was determined to be dependent upon the fraction of UPy groups present and chemical cleavage of the end-groups resulted in Tgs near those observed for polymer standards of comparable molecular weight. Aggregation of UPy end-groups in solution was observed using gel permeation chromatography. Aggregation was only observed for telechelic samples of low molecular weight, indicating that the aggregation of end-groups is dependent upon the concentration of the end-groups. The effect of UPy end-groups on blend miscibility was studied in solution using laser light scattering and in the melt state was using laser light scattering, optical microscopy and differential scanning calorimetry. The incorporation of associating groups onto one end of either blend component decreases miscibility relative to the unfunctionalized parent blends. Lower miscibility was also observed for blends in which both components were mono-functionalized with associating end-groups. The largest decrease in miscibility was observed for blends containing telechelic UPy-functionalized polymers, which were immiscible across the entire composition range

A Hexanuclear Copper Arylselenolate: Synthesis, Structure and Proposal for Its Rearrangement

Copper(I) oxide reacts with selenol 2,4,6-Pri3C6H2-SeH in ethanol to give the hexanuclear copper(I) selenolate {Cu[Se(2,4,6-Pri3C6H2)]}6 of 6, the solid state structure of which consists of a disordered Cu6 octahedron embedded in an antiprism formed by six selenium centres of the selenolate residues; this leads us to propose an exchange mechanism for the equilibration of all copper positions