Browsing by Subject "Block Copolymer"

Now showing 1 - 8 of 8
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    Block copolymer modified epoxy: role of epoxy crosslink density.
    (2010-03) Thompson, Zachary John
    Epoxies of systematically varying crosslink density containing 5% by weight of a poly(ethylene oxide)-b-poly(ethylene-alt-propylene) (OP) block copolymer were prepared and characterized. The block copolymer self-assembled to form particles with diameters ranging from 15 to 100 nm. Transmission electron microscopy of the modified epoxies revealed that the block copolymer nanostructure can be altered by changing the epoxy crosslink density. The block copolymer structures displayed a decrease in surface curvature as the crosslink density was reduced. The strain energy release rate, Gc, of the block copolymer-modified epoxies, which can be related to fracture resistance, increased dramatically with a decrease in the epoxy network crosslink density and plateau at a value 13 times greater than the unmodified material. This trend was observed with both high and low molecular weight OP additives. The toughening behavior is dependent on the block copolymer nanostructure in highly crosslinked system while lightly crosslinked block copolymer-modified epoxies display similar fracture resistances for each block copolymer additive. Scanning electron microscopy of fracture surfaces revealed extensive voiding and plastic deformation near the crack tip of the modified epoxies. Addition of the block copolymer did not appreciably decrease the Young's modulus or glass transition temperature compared to the unmodified material. Epoxies with varying concentrations of block copolymer additive were prepared and further demonstrated the role of crosslink density on improving fracture resistance. Lightly crosslinked epoxies were able to maintain extraordinary toughness at block copolymer concentrations as low as 1 wt%. Increasing crosslink density decreases the toughening ability of the block copolymer and a higher concentration is required to provide adequate fracture resistance.
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    Block Copolymers for Sustainable Thermoplastic Elastomers and Nanoparticle Fabrication
    (2017-12) Nasiri, Mohammadreza
    The first part of this thesis focuses on the preparation and characterization of sugar-derived monomers and polymers. Chapter 2 describes direct modification of glucose to produce new sustainable and functional polymers. Glucose acrylate tetraacetate (GATA) was synthesized and shown to provide a useful glassy component for developing an innovative family of elastomeric and adhesive materials. A series of diblock and triblock copolymers of GATA and n-butyl acrylate (nBA) were created via RAFT polymerization. These block copolymers were investigated as thermoplastic elastomers (TPEs) and while the peel adhesion results were desirable, moderate mechanical properties were observed. As described in Chapter 3, further structural and chemical modifications were employed to improve the performance of these block copolymers. Isosorbide was also modified to prepare acetylated acrylic isosorbide (AAI), as another sugar-based glassy component. RAFT polymerization was employed to prepare ABA triblock copolymers of GATA and AAI with nBA. Comprehensive adhesion testings were conducted and adhesion properties comparable to many commercial pressure sensitive adhesives were observed. Additionally, GATA-derived triblock copolymers were chemically modified to promote self-complementary hydrogen bonding in their glassy domains, resulting in significant enhancement in their mechanical strength. The improvements observed in the properties of these materials as a result of such non-covalent interactions allows for improved design of sustainable, sugar-derived polymers as high performance TPEs. The second part focuses on controlled fabrication of cylindrical nanoparticles, described in Chapter 4. Block copolymers containing immiscible segments can self-assemble to generate ordered nanostructures, such as cylinders of one block in a matrix of the other in the bulk, which can then be sectioned on the nanoscale using a microtome (nanoskiving). Dispersing these sections in a selective solvent for the matrix block results in nanocylinders. In one example, we utilized a poly(N,N-dimethylacrylamide)-block-poly(styrene) (PDMA-PS) copolymer containing 36% by volume of PS. This composition was selected as it self-assembles into cylinders of PS in a matrix of PDMA. The cylinders were aligned using a channel die and the aligned samples were subsequently sectioned using a microtome. The resulting sections were dispersed in water, a selective solvent for the PDMA matrix, affording PS nanocylinders with a PDMA corona.
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    Cryo-SEM Study of Nanostructure Development of latex dispersions and block copolymer solutions
    (2008-11) Gong, Xiaobo
    High resolution cryogenic scanning electron microscopy (cryo-SEM) was used to study the physics of latex film formation. Fast freezing, controlled freeze-drying and annealing under vacuum, followed by room-temperature and cryogenic SEM demonstrated that van der Waals force alone can compact a latex coating under conditions devoid of surface tension and capillary forces. Rewetting tests of the annealed coatings shed light on distinguishing elastic and viscoelastic deformation. Key factors affecting the freeze-thaw (F/T) stability of polymer latexes were studied. The nanostructural changes during freeze-thaw cycles were visualized by cryo-SEM. Reducing Tg and modulus of the polymer, latex particle size, amount of protective functional groups, molecular weight and addition of coalescent all lead to reduced F/T stability. Both the freezing and thawing rates have strong impact on F/T stability. Both functional acid monomer type and degree of neutralization in pre-emulsion greatly influence the ability of the latex and titanium dioxide (TiO2) particles to interact with each other which prevents TiO2 particle aggregation. Latexes incorporated with vinylphosphonic or itaconic acid show better TiO2 efficiency than latexes with acrylic acid or methacrylic acid. For acid monomers with high water solubility, higher degree of neutralization in pre-emulsion yields in general lower TiO2 efficiency. Cryo-SEM was employed to further understand the nature of nanostructure deduced by small angle x-ray scattering (SAXS) for poly(butadiene-b-ethylene oxide) diblock copolymers solutions, as a function of copolymer concentration and block copolymer composition. The SAXS measurements and cryo-SEM images reveal a new type of network morphology, comprised of a random arrangement of interconnected cylinders, in addition to the other classical structures.
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    Data for Single Gyroid in H-shaped Block Copolymers
    (2023-10-05) Park, Sojung; Bates, Frank S; Dorfman, Kevin D; dorfman@umn.edu; Dorfman, Kevin D; Dorfman Research Group - University of Minnesota Department of Chemical Engineering and Materials Science
    This data set contains the input and output files from the PSCF C++ program used for the self-consistent field theory (SCFT) simulation in "Single Gyroid in H-shaped Block Copolymers." Self-consistent field theory was used to investigate the equilibrium phase behavior of H-shaped block copolymers. With this dataset, users should be able to regenerate all the calculations that appeared in the paper, using the open-source C++ SCFT program available on GitHub (https://github.com/dmorse/pscfpp).
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    Exfoliated zeolite sheets and block copolymers as building blocks for composite membranes.
    (2009-08) Maheshwari, Sudeep
    Mixed matrix materials, comprising of zeolites incorporated in suitable matrix (polymeric or inorganic), are promising as future membrane materials with high permselectivity. However, they suffer from the drawback of low productivity due to increase in the membrane thickness by incorporation of micron-sized zeolites crystals as well as the low-permeability matrices employed currently. Nanocomposite membranes, consisting of thin zeolite sheets (~2 nm) embedded in an appropriate matrix, can provide a solution to this problem. This thesis addresses some of the material challenges to make such nanocomposite membranes. A high permeability polymer was synthesized by combining the glassy polystyrene (PS) with the rubbery polydimethylsiloxane (PDMS) in a block copolymer architecture. The mechanical toughness of the material was optimized to facilitate the fabrication of thin free standing films and its gas transport properties were evaluated. The PS-PDMS-PS triblock copolymers were successfully hydrogenated for the first time to obtain the PCHE-PDMS-PCHE triblock copolymers (PCHE stands for polycyclohexylethylene). The hydrogenation reaction proceeded without any polymer chain breaking and the resultant polymer showed some interesting, rather unexpected thermodynamic properties. These polymeric materials are potentially useful as the matrix of nanocomposite membranes. Highly crystalline zeolite sheets were obtained by exfoliation of zeolite lamellae. Preservation of crystal morphology and pore structure, which presents a major challenge during the exfoliation process, was successfully addressed in this work by judicious choice of operating conditions. Lamellae were exfoliated by surfactant intercalation and subsequently melt processing with polymers, resulting in polymer nanocomposites containing thin zeolite sheets (~2.5 nm) with well preserved pore structure. A method to obtain polymer-free exfoliated sheets was also developed to facilitate the fabrication of inorganic composite membranes. These zeolite sheets can be used as the selectivity-enhancement additive in composite membranes.
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    Network morphologies in monodisperse and polydisperse multiblock terpolymers.
    (2009-02) Meuler, Adam James
    Multiply continuous network morphologies were previously identified in “monodisperse” (polydispersity index (PDI) < ~1.1 in all blocks) poly(isoprene-bstyrene- b-ethylene oxide) (ISO) triblock terpolymers. This work extends the investigation of multiply continuous network structures to two other classes of multiblock terpolymers: i) “monodisperse” OSISO pentablocks and ii) polydisperse ISO triblocks. The OSISO pentablocks are synthesized using a protected initiation strategy that required the development of the functional organolithium 3-triisopropylsilyloxy-1- propyllithium (TIPSOPrLi). TIPSOPrLi may be used to prepare α-hydroxypolystyrene with narrower molecular weight distributions (PDI ~ 1.1) than are attainable using the commercially available 3-tert-butyldimethylsilyloxy-1-propyllithium. A telechelic triblock terpolymer (HO-SIS-OH) with narrow molecular weight distributions in all blocks is prepared using TIPSOPrLi. A series of OSISO pentablocks is synthesized from this parent triblock, and a stable region of O70 (the orthorhombic Fddd network) is identified between two-domain lamellae (LAM2) and three-domain lamellae (LAM3) in OSISO materials. This sequence of morphologies was previously reported in ISO triblocks with comparable compositions. Mechanical tensile testing reveals that an OSISO sample with a lamellar mesostructure fractures in a brittle fashion at a strain of 0.06. An OSISO containing the O70 network, in contrast, has a strain at failure of 1.3, even though the crystallinity of the terminal blocks is above the brittle threshold established in other multiblock materials. This improved toughness is attributed to the combined effects of a triply continuous morphology and an intrinsically tough SIS core. The ISO triblock studies probe the stability of network morphologies with respect to polydispersity in the polystyrene and poly(ethylene oxide) chains. Three series of ISO triblocks with polydisperse (PS PDI = 1.16, 1.31, 1.44) polystyrene blocks are prepared by anionic polymerization. While the network “window” in the PS PDI = 1.16 series is comparable in width and location to the window reported in the “monodisperse” ISO materials, it apparently shrinks for the higher PS PDI values. Only lamellar mesostructures are reported in the PS PDI = 1.31 materials, and network morphologies are identified over only a narrow range of compositions in the PS PDI = 1.44 samples. Polydispersity does not always destabilize network morphologies, however, as broadening the molecular weight distribution of the terminal poly(ethylene oxide) block drives a morphological transition from lamellae to the coreshell gyroid network. This result demonstrates that polydispersity can be used to tune block terpolymer phase behavior and stabilize technologically useful network mesostructures. Self-consistent field theory calculations augment the experimental analysis and offer insight into the physics underlying the polydispersity-driven morphological changes.
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    Renewable Aliphatic Polyester Block Polymer Thermoplastic Elastomers
    (2014-04) Martello, Mark
    The performance of thermoplastic elastomers is predicated on their ability to form mechanically tough physically crosslinked elastomeric networks at low temperatures and be able to flow at elevated temperatures. This dissertation focuses on renewable aliphatic polyester block polymers with amorphous polylactide (PLA) and their performance as TPEs. The goal of this work was to enhance the mechanical toughness of PLA containing TPEs; fundamental properties ranging from chemical composition and phase behavior, molecular architecture and melt processability, to melt polymerization strategies were investigated. ABA triblock polymers with PLA end-blocks and rubbery mid-blocks from substituted lactones comprised of poly(6-methyl-ε-caprolactone)(PMCL), poly(δ-decalactone), and poly(ε-decalactone)(PDL) were produced by sequential ring-opening polymerizations in the bulk. The bulk microstructure of symmetric PLA-PMCL-PLA and PLA-PDL-PLA triblock polymers formed long-range ordered morphologies and the interaction parameter of the repeat units was determined. High molar mass triblocks exhibited elastomeric behavior with good tensile strengths and high elongations. Small triblocks were coupled to produced (PLA-PDL-PLA)n multiblock polymers with high molar mass and accessible order-disorder transitions allowing for melt processing via injection molding. The mechanical toughness of the multiblocks was comparable to the high molar mass triblocks. The controlled polymerization of renewable δ-decalactone was accomplished with an organocatalyst at low temperatures in the bulk to maximize the equilibrium conversion of the monomer.
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    Thermoresponsive hydrogels from ABC triblock terpolymers
    (2014-03) Zhou, Can
    Two-compartment hydrogels, which are three-dimensional networks with two distinguishable hydrophobic domains, have been prepared from aqueous self-assembly of poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEP-b-PEO-b-PNIPAm, PON) triblock terpolymers. The PON terpolymers were synthesized using a combination of anionic and reversible addition-fragmentation chain transfer (RAFT) polymerization. They self-assembled into well-defined micelles with hydrophobic PEP cores surrounded by hydrophilic PEO-PNIPAm coronae at low temperatures and these micelles associated to form larger aggregated structures upon heating above the lower critical solution temperature (LCST) of PNIPAm in dilute aqueous solutions (0.5 and 0.05 wt%). At higher polymer concentrations (1-5 wt%), micellar aggregation manifests itself as gelation on heating due to the non-covalent association of PNIPAm blocks. The separation of micellization and gelation leads to the formation of a two-compartment network with a very high fraction of bridging conformations for the PEO midblocks. Therefore, gelation can be achieved at a much lower concentration, with a much higher modulus at a given polymer concentration and a much sharper sol-gel transition, as compared to poly(N-isopropylacrylamide)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (NON) copolymer hydrogels, in which both looping and bridging conformations are possible. The formation of a micellar network with two discrete PEP and PNIPAm hydrophobic domains in PON hydrogels is verified by cryogenic scanning electron microscopy (cryo-SEM) and cryogenic transmission electron microscopy (cryo-TEM) experiments and is further confirmed by small-angle neutron scattering (SANS) measurements of two PON triblocks with a normal PNIPAm and a deuterated PNIPAm block. This study confirms the assumption that the formation of two-compartment networks in PON terpolymer hydrogels results in better gelation properties compared with NON copolymer hydrogels. In addition to temperature, it is desirable to have other stimuli such as pH to control the polymer self-assembly. Therefore, poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide-co-acrylic acid) (PO(N/A)) triblock terpolymers in which the PNIPAm block contains a small fraction of AA monomers were prepared to achieve the dual pH- and temperature-sensitive micellar aggregation and gelation in aqueous solutions.Finally, the self-assembly of PON triblock terpolymers in the ionic liquid 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]) shows well-defined sol-gel transitions upon cooling with a lower gelation concentration and a higher modulus when compared with NON copolymers, which further confirms that ABC triblock terpolymers can be beneficial for gel formation in comparison to ABA triblock copolymers. Overall, we demonstrated that the rational design of two immiscible, hydrophobic endblocks in ABC triblocks is crucial for the preparation of compartmentized hydrogels with improved gelation properties. These studies will help guide the design and development of new systems with enhanced performance.