<p>The microphase segregation behavior, which exhibits periodic patterns on a mesoscale, has been found in many systems where it has demonstrated its extreme industrial usefulness in the diblock copolymer system. When studying more general isotropic colloidal systems, periodic microphases should ubiquitously emerge in systems for which short-range inter-particle attraction is frustrated by long-range repulsion~(SALR). The morphological richness of these phases makes them desirable material targets, but our relatively coarse understanding of even simple models hinders controlling their assembly both from thermodynamic and dynamic points of view. The thermodynamic question is what should be the appropriate potential to stabilize microphases, such as cluster crystal, cylindrical, double gyroid and lamellar, while the dynamic question is whether the current experiments are long enough for these phases to appear. This dissertation will focus on solving these two parts of problems and hopefully guide the experiments to discover a simple material that can have microphase segregation behavior. In order to answer the thermodynamic problem of the stability of the microphases, we use a novel thermodynamic integration method as well as density functional methods in comparing the free energy of the microphases with uniform liquids. With the thermodynamic integration, we locate FCC-cluster, cylindrical, double gyroid and lamellar phases as well as nontrivial interplay between cluster, gel and microphase formation for a model microphase former. We also extended the methods to the model with a shorter and longer repulsion region where we found that the shorter region of the system may be in the Wigner glass of clusters of different sizes rather than the microphases. We also compare our simulation results with that from the density functional theory where we demonstrate that the classical density functional theory is qualitatively right but the simple improvement of the radial distribution functional by assuming the system is the same as Percus-Yevick hard sphere does not make a quantitatively difference. Our finding confirms that if the colloidal system has proper SALR potential as well as the right regime of area fraction and temperature, the microphase will be found in these systems. We then answer the second question which is whether the slow dynamics hinders the formation of microphases. We study the modeled microphase former and track the change of the first peak in the structure factor as well as the structural correlation time. We found that the system has a very complex dynamical regimes, including homogeneous fluid, void micelle, liquid gel and solid gel. The system becomes extreme slow in the solid gel regime but if in the regime that density and temperature are near the order-disorder transition, the lamellar self-assembly is much faster than the relaxation time of the solid gel which may explain why in the experimental system, the colloids seem stuck forever. We have collaborated with an experimental group to realize the SALR self-assemble behavior in a well controlled system. We have calibrated the system using a high precision thermodynamic integration by determining and matching the critical point and triple point of the experiments when the system is set up in the purely attractive regime. The system, however, becomes unpredictable when it goes into the SALR system where both higher body and other interactions become dominant. Finally, we try to extend our system to a spherocylinder model, which is an anisotropic particles with SALR. We have developed a novel cell list method here to accelerate the simulation. By determining the percolation transition and the order parameter, we find that the simple anisotropic interaction will introduce a much complex phase behavior of the system even in the disordered regime. We have identified several disordered phases, including homogeneous liquid, micellar liquid, free rotator gel, nematic gel and smectic gel.</p> / Dissertation

This dissertation is aimed at understanding the microstructure evolution in styrene — diene block copolymer — based pressure-sensitive adhesive compositions in melt. The work also focuses on determining the microphase separation mechanism in adhesive melts containing various amounts of low molecular weight resin (tackifiers) blended with styrene — diene block copolymers. To understand the correlation between adhesive morphology and their dynamic mechanical behavior, small angle X-ray scattering (SAXS) and rheological analysis were performed on blends with different compositions. A modified Percus — Yevick model combined with Gaussian functions was used fit the liquid like disordered and bcc — ordered peaks of the SAXS intensity profiles. The morphological parameters derived from SAXS analysis corresponded to features such as the size and extent of ordering of the microphase separated polystyrene domains. The variation in these parameters with respect to temperature and adhesive composition correlated reasonably well with the trends observed in the shear modulus measured using rheological analysis. It was found that the ordering of polystyrene domains was influenced by the tackifier content in the adhesive blends. Polymer chain mobility was determined to be the dominant factor governing ordering kinetics, which depended on both the quench temperature and tackifier content in the blends. The addition of increasing amounts of tackifier eventually leads to a shift from a nucleation and growth type mechanism to a spinodal decomposition mechanism for phase separation and ordering. The compatibility of the tackifier with the polystyrene chains had a significant impact on the morphological transitions and microphase separation in adhesive blends. The blends containing a styrene — incompatible tackifier showed ordering over a broader range of temperatures compared to the blends containing a polystyrene — compatible tackifier. / Ph. D.

This dissertation presents the synthesis and studies of polymer brush-supported organocatalysts and asymmetric mixed homopolymer brushes grafted on particles. The brushes were synthesized from initiator-functionalized particles by surface-initiated “living” radical polymerizations. Polymer brush-supported organocatalysts were designed to combine the advantages of both soluble polymer- (high activity) and crosslinked insoluble polymer-supported catalysts (recyclability). Chapter 1 describes the synthesis of a polymer brush-supported 4-N,N-dialkylaminopyridine (DAAP) catalyst from initiator-functionalized latex particles by surface-initiated nitroxide-mediated radical polymerization (NMRP). The hairy particles efficiently catalyzed the acylation of secondary alcohols and Baylis-Hillman reaction and were recycled  six times with no or negligible decrease in the reaction yield. Chapter 2 presents the synthesis of a thermosensitive polymer brush-supported DAAP by surface-initiated atom transfer radical polymerization (ATRP) from silica particles with addition of a free initiator. Both hairy particles and the free copolymer formed from the free initiator were used as catalysts for hydrolysis of nitrophenyl acetate at various temperatures. Below the lower critical solution temperature (LCST), the activity of hairy particles was very close to that of the free copolymer. LCST transitions exerted different effects on the reactions catalyzed by hairy particles and the free copolymer. Chapters 3 and 4 present the studies of the effects of chain length disparity and grafting density on phase morphology of mixed brushes grafted on silica particles. A series of mixed poly(tert-butyl acrylate) (PtBA)/polystyrene (PS) brushes with a fixed PtBA molecular weight and various PS molecular weights were grown from silica particles functionalized with a monochlorosilane-terminated asymmetric difunctional initiator (Y-initiator) by sequential ATRP and NMRP. The total grafting densities of these brushes were 0.6 – 0.7 chains/nm2. The morphology of mixed brushes evolved from isolated PS nanodomains, to short PS cylinders, to a nearly bicontinuous nanostructure, and a two-layered nanostructure with the change of chain length disparity of two homopolymers. To study the grafting density effect, a set of high density asymmetric mixed brushes with total grafting densities of 0.9 – 1.2 chains/nm2 was prepared from triethoxysilane- terminated Y-initiator-functionalized silica particles. The feature sizes of the patterns formed from high density mixed brushes were much smaller than those of lower density mixed brushes.

A series of immiscible crystalline-crystalline diblock copolymers, poly(ethylene oxide)-b-(£`-caprolactone) (PEO-b-PCL), were blended with (3-phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-yl) methanol (Pa-OH). FT-IR analyses provide that the ether group of PEO is a stronger hydrogen bond acceptor than the carbonyl group of PCL with the hydroxyl group of Pa-OH. Pa-OH after curing results in the excluded and confined PCL phase based on differential scanning calorimeter (DSC) analyses. In addition, the mesoporous structure was proved with the increasing the ratio of PCL to PEO in block copolymers by small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) analyses and N2 adsorption-desorption isotherms (BET) The poly(styrene-b-4-vinyl pyridine) diblock copolymer was blended with Pa-OH monomer. FT-IR analyses demonstrate the intermolecular hydrogen bonding interaction between the pyridine group of P4VP and the hydroxyl group of Pa-OH. After curing, the block copolymers were incorporated into polybenzoxazine resin to access the nanostructure through the reaction induced microphase separation mechanism by TEM and SAXS analyses.

The fabrication of one-dimensional (1-D) polymeric photonic crystals from the self-assembly of ultra-high-Mw polystyrene-b-polyisoprene (PS-PI) block copolymers (BCPs) were conducted in this study. Well-ordered microphase-separated lamellar structures can be observed in the ultra-high-Mw PS-PI BCPs in the bulk by transmission electron microscopy (TEM) and ultra-small angle X-ray scattering (USAXS). To fabricate large-area and well-oriented lamellar microstructures with parallel orientation to the substrate, substrate-induced microstructural orientation with the accompanying solvent annealing method (i.e., solvent-induced orientation) was carried out in the PS-PI film. By grazing-incidence ultra-small angle X-ray scattering (GIUSAXS), scanning probe microscope (SPM) and cross-sectional TEM morphological observation, identification of the microstructural orientation in the PS-PI film can be achieved. A disordered wormlike morphology is observed in the as-spun PS-PI thin film from toluene on the PS-grafting substrate and on neat glass or wafer. This is attributed to the fast solidification of the disordered microstructure due to fast evaporation rate of the toluene solvent. After solvent annealing by the PS-selective or PI-selective solvents such as divinylbenzene (DVB) (neutral but highly PS-selective), benzene (PS-selective) and cyclohexane (PI-selective), parallel lamellar microstructures can be obtained in the PS-PI films on the PS-grafting substrate. By contrast, the coexistence of parallel and perpendicular lamellar microstructures is obtained in the PS-PI film from toluene after solvent annealing by neutral toluene on the PS-grafting substrate or by PS-selective benzene on the neat glass or wafer. This indicates that the formation of the parallel lamellar microstructures is mainly determined by both solvent-induced and substrate-induced orientation. In contrast to the as-spun disordered morphology from toluene, well-ordered parallel lamellar microstructures with few defects was found in the as-spun PS-PI film from DVB on the PS-grafting substrate, whereas parallel lamellar microstructures with many defects was observed in the as-spun PS-PI film from DVB on the neat glass or wafer. This further demonstrates that the PS-grafting substrate indeed plays an important role on the fabrication of well-ordered parallel lamellar microstructures. Interestingly, once the initial morphology of the PS-PI BCP reaches a relative stable state (i.e., parallel lamellar microstructures versus disordered wormlike morphology), it is hardly to trigger the microstructural reorientation by the subsequent solvent annealing. We suggest that the stable initial morphology in the PS-PI film may create high energy barrier for microstructural reorientation. With the controllable microstructural orientation, a PS-PI thick film having large-area and well-oriented parallel lamellar microstructures can be successfully carried out. Therefore, 1-D polymeric photonic crystals from the self-assembly of the lamella-forming PS-PI BCPs can be achieved. The in-situ UV reflectance spectra show that the reflective band shifts from ultraviolet wavelength to visible wavelength was observed in the lamella-forming PS-PI thick film with elapse of time by solvent annealing. Notably, the band gap can be recovered to the initial state once the solvent is removed, indicating the reversible process. As the results, the solvatochromic BCP photonic crystals can be successfully carried out by the manipulation of the solvent swelling in the large-area and well-oriented lamella-forming PS-PI BCP film.

Double crystalline block copolymers (BCPs), syndiotactic poly(4-methyl-1-pentene)-b-poly(L-lactide) (sPMP-PLLA) and syndiotactic poly(4-methylstyrene)-b-poly(L-lactide) (sPMS-PLLA), were synthesized to examine crystallization effect on the self-assembled morphologies in the double crystalline BCPs. Because of the stainable chemical structures, morphological observation can be carried out in these double crystalline BCPs. Also, different microphase-separated structures including lamellae and hexagonally packed cylinders were explored to study the shape effect for double crystallization. Based on differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) results, both sPMP and PLLA blocks are able to crystallize in the sPMP-PLLA BCP (fsPMPv=0.52) at the crystallization temperature (Tc) from 80¢XC to 120¢XC. Notably, temperature-dependent phase transitions between the PLLA polymorphisms are obtained by WAXD. By using small-angle X-ray scattering (SAXS) and transmission electron microscope (TEM), the microphase-separated lamellar structures can be observed in the sPMP-PLLA BCP (fsPMPv=0.52). Also, the preservation of the lamellar morphology at all Tcs (80¢XC~120¢XC) indicates that the sPMP and PLLA crystallization can be strongly confined within the lamellar microstructures due to the strong segregation strength of the sPMP-PLLA (fsPMPv=0.52) BCP. This can be further demonstrated by the ambiguous birefringence under polarized light microscope (PLM). According to the time-resolved SAXS and WAXD profiles at 90oC and 110oC, the sPMP block crystallizes first and induces the enlargement of the BCP long period. Also, the leading sPMP crystallization gives rise to the robust lamellar microstructural template and result in strong confinement for the subsequent PLLA crystallization. In the sPMS-PLLA BCP (fsPMSv=0.58), the microphase-separated lamellar nanostructures can be found by SAXS and TEM. DSC analysis shows that PLLA block is able to crystallize as Tc=90¢XC~100¢XC; the sPMS block is able to crystallize as Tc ≥120oC. By self-nucleation processes, both sPMS and PLLA blocks are able to crystallize. Therefore, by the manipulation of the respective crystallization, two-stage crystallization and coincident crystallization, systematic studies in the semi-crystallization, double crystallization and coincident double crystallization with the accompanying environmental Tg effect and BCP segregation strength can be carried out in the lamella-forming sPMS-PLLA (fsPMSv=0.58) BCP. By SAXS and TEM, the microphase-separated lamellar microstructures can be preserved in the self-assembly of the sPMS-PLLA (fsPMSv=0.58) BCP whatever the PLLA crystallization occurs under hard confinement (Tc,PLLA<Tg,sPMS) or soft confinement(Tc,PLLA˃Tg,sPMS). For the sPMS crystallization under soft confinement, the lamellar microstructures can be preserved as Tc,sPMS ≤140oC, whereas the breakout morphology by the sPMS crystallization is found as Tc,sPMS ≥150oC. As a result, the final morphologies is strongly dependent on the BCP segregation strength in the lamella-forming sPMS-PLLA (fsPMSv=0.58) BCP. In sPMS-PLLA BCP (fsPMSv=0.7), hexagonally-packed PLLA cylinders in the sPMS matrix are obtained by SAXS and TEM. DSC analysis shows that the sPMS block is able to crystallize as Tc=130¢XC~180¢XC, whereas no PLLA crystallization can be found in the cylinder-forming sPMS-PLLA BCP (fsPMSv=0.7). This indicates that the 2-D cylindrical shape might give rise to the strong confined effect and result in non-crystallizable PLLA. According SAXS and TEM results, the intrinsic hexagonally-packed cylinders can be preserved after the sPMS crystallization at 130oC due to the strong BCP segregation strength. By contrast, the crystallization driving force may overwhelm the microphase separation so as to form breakout morphology in the sPMS-PLLA (fsPMSv=0.7) BCP as Tc≥150¢XC.

No description available.

This study examined several features of flexible polyurethane foams from a structure-property perspective. A major part of this dissertation addresses the issue of connectivity of the urea phase and its influence on mechanical and viscoelastic properties of flexible polyurethane foams and their plaque counterparts. Lithium salts (LiCl and LiBr) were used as additives to systematically alter the phase separation behavior, and hence the connectivity of the urea phase at different scale lengths. Macro connectivity, or the association of the large scale urea rich aggregates typically observed in flexible polyurethane foams was assessed using SAXS, TEM, and AFM. These techniques showed that including a lithium salt in the foam formulation suppressed the formation of the urea aggregates and thus led to a loss in the macro level connectivity of the urea phase. WAXS and FTIR were used to demonstrate that addition of LiCl or LiBr systematically disrupted the local ordering of the hard segments within the microdomains, i.e., it led to a reduction of micro level connectivity or the regularity in segmental packing of the urea phase. Based on these observations, the interaction of the lithium salt was thought to predominantly occur with the urea hard segments, and this hypothesis was confirmed using quantum mechanical calculations. Another feature of this research investigated model trisegmented polyurethanes based on monofunctional polyols, or "monols", with water-extended toluene diisocyanate (TDI) based hard segments. The formulations of the monol materials were maintained similar to those of flexible polyurethane foams with the exceptions that the conventional polyol was substituted by an oligomeric monofunctional polyether of ca. 1000 g/mol molecular weight. Plaques formed from these model systems were shown to be solid materials even at their relatively low molecular weights of 3000 g/mol and less. AFM phase images, for the first time, revealed the ability of the hard segments to self-assemble and form lath-like percolated structures, resulting in solid plaques, even though the overall volume of the system was known to be dominated by the two terminal liquid-like polyether segments. In another aspect of this research, foams were investigated in which the ratios of the 2,4 and 2,6 TDI isomers were varied. The three commercially available TDI mixtures, i.e., 65:35 2,4/2,6 TDI, 80:20 2,4/2,6 TDI, and 100:0 2,4/2,6 TDI were used. These foams were shown to display marked differences in their cellular structure (SEM), urea aggregation behavior (TEM), and in the hydrogen bonding characteristics of the hard segments (FTIR). Finally, the nanoscale morphology of a series of 'model' segmented polyurethane elastomers, based on 1,4-butanediol extended piperazine based hard segments and poly(tetramethylene oxide) soft segments, was also investigated using AFM. The monodisperse hard segments of these 'model' polyurethanes contained precisely either one, two, three, or four repeating units. Not only did AFM image the microphase separated morphology of these polyurethanes, but it also revealed that the hard domains preferentially oriented with their long axis along the radial direction of the spherulites which they formed. / Ph. D.

An examination of the ordering process of block copolymer microstructure with respect to concentration was performed. Specifically, the process of solution casting block copolymer films was studied using small-angle X-ray scattering (SAXS). A method for determining the volume fraction of ordered phase in solution as the system dried was developed and used to analyze the solution casting process in several different block copolymer films in the neutral solvent toluene; these polymers include poly(styrene-b-butadiene), poly(styrene-b-isoprene-b-styrene), poly(styrene-b-butadiene-b-styrene), and several poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) polymers with different block fractions. A method was also developed for studying different drying rates of these films at a constant temperature. Temperature quenches of poly(styrene-b-isoprene-b-styrene) were performed to evaluate the effect of concentration on ordering rate. In all cases studied, an ordering layer was observed where self-assembly was thermodynamically favorable. This layer steadily grew until it reached the bottom substrate, resulting in a two-step ordering process. In the case of the styrene/diene copolymers, a constant polymer concentration was observed in the ordering layer as it grew to encompass the entire film. Kinetic entrapment was observed in the case of the diblock copolymer, as the system with a medium drying rate with respect to the other two experienced faster kinetics than the other two systems. For the two triblock copolymers, it was found that similar kinetics were observed with respect to the ordering layer concentration, largely due to skinning on the surface allowing time for lower sections of the film to order more completely. In the acrylate copolymers studied, the kinetics were not able to be evaluated with respect to drying rate. This was due to domain compression that cause a disordering of ordered microstructure as solvent was removed. This disordering was attributed to interfacial disruption caused by the compression in the film. In addition, a significant decrease in domain spacing was observed to occur in the vertical direction as a result of compression in that direction and pinning of the film to the substrate in the horizontal direction. Finally, the Avrami kinetic model was fit to several concentration of styrene/isoprene triblock copolymers as they ordered after a temperature quench. A U-shaped curve was observed in the system, as a result of competition between chain mobility effects and thermodynamic effects that occur as polymer concentration increases away from the CODT. It was found that the Avrami exponent remained constant over all concentrations, and an empirical model was fit to find the various rate constants at each polymer concentration. / Ph. D.

No description available.