Browsing by Subject "Block Copolymers"

Now showing 1 - 8 of 8
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    Block Copolymer and Silica Modified Epoxy Structural Adhesives
    (2021-06) Pang, Vincent
    Block copolymers have been studied extensively as toughening agents for epoxies over the past two decades, where most work has been focused on improving the bulk properties of epoxies. However, epoxies are commonly utilized in applications where bulk properties are only part of the story. We begin to address this by studying block copolymer modified epoxies as structural adhesives and exploring the impacts of these additives on adhesion strength. Poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) block copolymers were synthesized and dispersed in epoxy formulations, forming spherical micelle and bilayer vesicle structures. Both morphologies led to significant increases in critical strain energy release rate, GIc, over the neat epoxy with no reduction in the elastic modulus. Spherical micelle modified epoxy adhesives showed up to a 46% enhancement to single-lap-joint shear adhesion strength. Electron micrographs of the fracture surfaces indicated that micelle cavitation plays a role in both the toughening and adhesion strength enhancements of epoxies. In contrast, a 28% reduction in adhesion strength was observed in the bilayer vesicle modified epoxies.Recently, there has also been a growing interest in studying epoxy composites with both rubbery and rigid particle modifiers. We explore this by extending our understanding of block copolymer toughening to composites containing both block copolymer and silica nanoparticles. Nanosilica and spherical micelle-forming PEP-PEO modifiers were dispersed both individually and together in epoxy formulations. The nanosilica and PEP-PEO modifiers formed uniform dispersions when added individually in the matrix but led to limited aggregation of nanosilica particles when added together. Incorporating nanosilica in neat and block copolymer modified epoxies led to increases to both fracture toughness and elastic modulus. Combining both additives led to additive toughening beyond that obtained with the individually modified epoxies. The extent of these improvements also increased with nanosilica loading up to 25 wt%. Electron micrographs of the failure surfaces revealed both micelle cavitation and nanosilica debonding occurring in concert. These findings were taken further in a final study on how silica and block copolymer modified epoxy adhesives impact toughness and adhesion strength. Micelle-forming PEP-PEO and various types of silica particle modifiers were again dispersed in epoxy formulations. All modifiers were well dispersed when added individually to the matrix. When added together, microsilica particles remained well-dispersed, in contrast to the partial aggregation observed with nanosilica. Increases to both fracture toughness and elastic modulus were observed when all types of silica were incorporated, without significant effects from silica particle size or surface functionalization. Adhesion strength was increased by approximately 50% when block copolymer was added and was not affected by the silica. Overall, the incorporation of silica improved both elastic modulus and fracture toughness without compromising the adhesion strength enhancement from block copolymer micelles.
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    Block copolymers in ionic liquids.
    (2009-06) Simone, Peter Mark
    In this thesis the self-assembly behavior of block copolymers diluted with ionic liquids has been investigated. Initial experiments involved characterizing the selfassembly of poly(styrene-b-methyl methacrylate) (PS–PMMA) and poly(butadiene-bethylene oxide) (PB–PEO) copolymers at dilute concentrations (~1 wt%) in the ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF6]) and 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). Dynamic light scattering and cryogenic transmission electron microscopy results showed that the ionic liquids behave as selective solvents for the PMMA and PEO blocks of the copolymers, and that the micelle morphology and self-assembly behavior of the block copolymers in the ionic liquids was analogous to that observed in conventional solvents. At increased solution concentrations (≥ 20 wt%) the lyotropic mesophase behavior for PB–PEO diluted with [BMI][PF6] and [EMI][TFSI], and poly(styrene-bethylene oxide) (PS–PEO) diluted with [EMI][TFSI] was investigated via small angle X-ray scattering. These experiments showed a microstructure phase progression with addition of ionic liquid that was analogous to that expected for an increase in the PEO volume fraction of the bulk copolymers. Additionally, an increase in the lamellar microstructure domain spacing with ionic liquid content indicated that both ionic liquids behave as strongly selective solvents for the PEO blocks of the copolymers. The ionic conductivity of the concentrated PS–PEO/[EMI][TFSI] solutions was measured via impedance spectroscopy, and found to be in the range of 10−3 S/cm at elevated temperatures (~100 °C). Additionally, the ionic conductivity of the solutions was observed to increase with both ionic liquid content and molecular weight of the PEO blocks of the copolymer. Finally, preliminary investigations of the microstructure orientation in thin films of a concentrated PS–PEO/[EMI][TFSI] solution were conducted. The copolymer microstructure was observed to align perpendicular to the film surface with short term (≤ 2 hours) thermal annealing. Longer term thermal annealing resulted in a transition to parallel alignment of the copolymer microstructure relative to the film surface.
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    Degradable Polymersomes for Targeted Drug Delivery
    (2013-08) Petersen, Matthew
    Chemotherapy today is often accompanied by major side effects due to delivery of toxic drugs to healthy tissue in addition to diseased cells. Targeted drug delivery offers the possibility of minimizing these side effects by specific delivery to cancer cells using targeted nanocarriers that enhance drug accumulation in tumors and facilitate target-specific cellular uptake. Polymersomes, vesicles self-assembled from polymeric amphiphiles, are an attractive targeted vehicle, as they are capable of encapsulating both hydrophobic and hydrophilic drugs, have lengthy circulation times in vivo, and can employ degradable functionality for triggered release of payload and clearance from the body. This thesis reports on efforts to enhance the capabilities of degradable polymersomes for targeted delivery. First, targeting functionality is incorporated into polymersomes of the block copolymer poly(ethylene oxide)-b-poly(γ-methyl-ε-caprolactone) by incorporating the reactive vinyl sulfone group into the amphiphile's hydrophilic terminus, allowing site-selective reaction with cysteine-functionalized targeting peptides following self-assembly. The performance of targeted delivery using this polymersome is then evaluated in vitro. Binding and delivery to model cell lines for targeted and bystander cells is tracked using nontargeted polymersomes and compared to that for polymersomes using a high- or low-affinity ligand. Polymer degradation is also tracked both in simple media and during cellular delivery. Finally, a new monomer is developed incorporating acid-labile acetal functionality into a cyclic polyester. The polymerization of this monomer to two distinct polymers is also characterized and the degradation behavior of both polymers evaluated.
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    Fabricating robust nanoporous materials and polymer electrolyte membranes from reactive block copolymers by metathesis reactions.
    (2009-06) Chen, Liang
    Polymer membranes have been studied for several decades, and conventional utility of these membranes has been realized over a wide range of applications such as water purification, gas separation, and fuel cells. Separation performance of polymer membranes for these applications can be evaluated from different aspects including selectivity, material cost, and thermal and mechanical stabilities. This research focused on the design of crosslinked polymer membranes from reactive block copolymers, which could possess some attractive features such as controlled functionality and morphology. Novel block copolymers containing a chemically crosslinkable block and a chemically modifiable block were of particular interest. Metathesis reactions were employed to crosslink these copolymers with additional tough polymers such as polycyclooctene (PCOE) and polydicyclopentadiene (polyDCPD), to realize enhanced toughness in the resulting materials. First, a norbornene-functionalized polystyrene-polylactide (PNS-PLA) block copolymer was synthesized. A self-assembled blend containing this copolymer and DCPD was cured by metathesis reactions, and nanoporous monoliths with a cylindrical morphology were successfully produced after removing the PLA component. These monoliths exhibited pronounced mechanical and thermal stabilities superior to nanoporous polystyrene (PS). Second, polymerization induced phase separation (PIPS) during the ring-opening metathesis polymerization (ROMP) of DCPD in the presence of the PNS-PLA copolymer rendered continuous PLA nanodomains in a crosslinked PNS/polyDCPD matrix. Upon etching the PLA component, the resultant nanoporous membranes exhibited well-defined percolated nanopores and good thermal and mechanical stabilities. Preliminary diffusion measurements demonstrated potential utility of such membranes in ultrafiltration. Third, crosslinked polymer electrolyte membranes (PEMs) were fabricated from a PNS-poly(n-propyl-p-styrenesulfonate) (PSSP) block polymer and COE/DCPD via the PIPS scheme followed by deprotection of the PSSP block. These PEMs possessed a bicontinuous morphology and mechanical and thermal robustness. Select PEMs exhibited high proton conductivity similar to Nafion at high humidity and reduced methanol crossover. Tunable domain size and mechanical strength of the resulting PEMs are advantageous attributes of this preparation protocol. Additionally, the use of such PEMs for NH3 separation from gas mixtures was demonstrated. The appendix chapter represented our efforts to produce amine-functionalized polymer membranes from PNS-poly(dimethylaminoethylmethacrylate) (PNS-PDMAEMA) and COE by metathesis reactions, potentially useful for CO2 separation from gas mixtures.
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    Interfaces and Fluctuations in Diblock Copolymer Melts and Ternary Mixtures with Homopolymers
    (2020-10) Yadav, Mridul
    Self-consistent field theory (SCFT) is a powerful tool for study of equilibrium phase behavior of block copolymer melts and mixtures. However, it fails to take into account the effect of fluctuations. This leads to some incorrect predictions for the phase behavior including the nature and location of order-disorder tran- sitions, associated phenomemon and phases. SCFT solutions are also limited by finite amount of computer resources available for numerical calculations. This is especially true of highly swollen self-assembled structures with large characteris- tic length scales. Accurate thermodynamic description of the interfaces in these swollen structures is sufficient to describe the phase behavior in this limit. Here we present our studies of the phase behavior of and interfaces in block copolymer melts and mixtures where we incorporate fluctuation effects and study highly swollen phases. The different approaches we take are, in a large part, guided by the limitations of SCFT.We present the first theoretical study into the role of conformational asym- metry in ternary mixtures of AB diblock copolymers, A homopolymers and B homopolymers which are high molecular counterparts of oil-water-surfactant mixtures. We show that the sign of the spontaneous curvature of an asym- metric diblock copolymer monolayer in these ternary mixtures is controlled by competition between swelling and stretching of copolymer brushes. We explore the phase behavior in highly swollen limit using the analytical Helfrich theory of bending elasticity for diblock copolymer monolayers. Further, we present a generalized version of the Helfrich theory that eliminates arbritrary choices in controlling variables needed to stabilize interfaces and presents a simplified description of these monolayers as a pseudo-one component system. We demon- strate the utility of our generalized theory by presenting an accurate thermody- namic description of a metastable phase in the highly swollen limit. Fluctuation effects are considered in the latter part of this thesis. In neat melts of volumetrically symmetric diblock copolymers, we view the strongly correlated disordered phase, near the order-disorder transition to the ordered lamellar phase, as an ensemble of multiple network topologies. We claim that the entropy associated with this ensemble is a constant per junction of the network. We present a free energy model of the disordered phase as the free energy per junction of a surrogate ordered network phase stabilized by this constant junction entropy. We test this claim and the predictions from this model by comparing to results from molecular simulations. We then extend this model to the ternary mixtures. We incorporate the effect of interfacial fluctuations in these mixtures through their effect on the renormalization of rigidities in the disordered phase and an undulation pressure in the lamellar phase. Using these models we predict phase behavior that is consistent with reported behavior in experiments and simulations.
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    Polythiophene-containing block copolymers for organic photovoltaic applications.
    (2009-08) Boudouris, Bryan W.
    Poly(3-alkylthiophene)s (P3ATs) have become the most common electron-donating material in organic photovoltaics (OPVs), and recent advances in the fabrication of polythiophene-fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies of up to ~6% to be realized. This efficiency has only been possible through enhancements in the active layer microstructure. This key factor allowed for better separation of the bound electron-hole pair (exciton), generated by absorption of light. Understanding how exciton dissociation and the active layer morphology affect device performance will facilitate cell optimization, ultimately leading to higher device efficiencies. Consequently, we developed two new classes of polythiophene-based block copolymers to better understand these phenomena. First, we synthesized well-defined diblock and triblock copolymers with the structures: poly(3-alkylthiophene)-b-polylactide (P3AT-PLA) and polylactide-b-poly(3-alkylthiophene)-b-polylactide (PLA-P3AT-PLA). We have observed that kinetic factors dominate phase separation for a semicrystalline polythiophene block. However, if the polythiophene moiety was amorphous the polymers self-assembled into thermodynamically stable, ordered microstructures with domain spacings on the scale of interest for charge separation in OPV cells (ca 30 nm). Polylactide was chosen as the second moiety in the block copolymers because it could be selectively etched from the polythiophene matrix with a gentle alkaline bath. This procedure led to the formation of nanoporous templates that could generate ordered bulk heterojunctions. In the second approach, P3AT chain ends were terminated with fullerene to create an internal electron acceptor-donor-acceptor, methylfulleropyrrolidine-poly(3-alkylthiophene)-methylfulleropyrrolidine (C60-P3AT-C60). Microphase separation occurred between the polymer chain and fullerene end groups, which suggested the creation of two distinct semicrystalline regimes. A compositionally similar blend of P3HT and C60 showed a similar microstructure. This comparable domain formation, coupled with the possibility of enhanced charge transfer, makes C60-P3AT-C60 a promising candidate as a material in bulk heterojunction organic photovoltaic devices.
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    Quantum Dot Dispersion in Block Copolymer Matrices
    (2018-08) Wenger, Whitney
    Quantum dots (QDs) have demonstrated viability for a wide set of applications ranging from bioimaging to electronics. Their unique size-tunable band gaps and accessibility for roll-to-roll processing via solution synthesis makes them promising candidates in many of these areas. Several of these applications benefit from carefully manipulated spacing in QD films, often achieved through integration of QDs in a polymer matrix. Although many studies have achieved varying degrees of success in dispersing QDs in polymer matrices, there remains much to be understood about the path dependency of QD integration and how QDs may be integrated into sphere-forming polymer matrices. In this work, CdSe QDs were synthesized via a hot injection technique in an air-free environment. These QDs were fabricated in a range of sizes based on the reaction time and were evaluated for their crystal structure, absorbance, fluorescence, ligand coverage, and dispersion in various solvents. The resulting QDs feature a wurtzite crystal structure and exhibit narrow absorbance and emission peaks. The QDs are well stabilized in nonpolar solvents like hexane and toluene via trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) ligands. In the first approach towards QD integration in polymer matrices, the native TOPO ligands were exchanged for a poly(ethylene glycol) ligand functionalized with a thiol end group. The resulting QDs were qualitatively and quantitatively analyzed to determine the ligand density on the QD surface and the QD dispersion in different solvents. After ligand exchange, the QDs were no longer dispersible in nonpolar solvents like hexane but formed stable dispersions in solvents like tetrahydrofuran, chloroform, and water. Upon ligand exchange, 50-85% of the original ligands were removed and an average of 2-4 poly(ethylene glycol) ligands were installed on each QD surface. The extent of QD dispersion in various homopolymers was evaluated using transmission electron microscopy (TEM). CdSe QDs were mixed with poly(lactic acid) in chloroform and dropcast into thin films for TEM. The resulting films indicate phase separation of the polymer and the QDs where the spacing between QDs does not change upon addition of the polymer. In a separate study, CdSe QDs were mixed with poly(butadiene) in n-hexane and dropcast into thin films for TEM. These films demonstrated an increase in spacing between the QDs of roughly 2 nm. However, the majority of the polymer does not end up in the space between QDs and is phase separated from the QD crystal phase. Finally, CdSe QDs after ligand exchange with PEG were mixed with poly(lactic acid) in chloroform and dropcast into thin films for TEM. The resulting films indicate some mixing between the QDs and the polymer. Finally, the extent of QD dispersion in various diblock copolymers was evaluated using small angle x-ray scattering (SAXS), TEM, and fluorescence measurements. CdSe QDs were mixed with a lamellae-forming poly(ethylene-b-cyclohexylethylene) in benzene and dried and heated to T ≈ 200°C and then cooled to 140°C to induce polymer ordering. The resulting solid composites exhibited aggregates of QDs via SAXS and TEM measurements. In a separate study, CdSe QDs were mixed with a body-centered cubic forming poly(styrene-b-butadiene) in benzene. Two types of samples were prepared: one formed from drying the polymer and QDs from the solvent and subsequently heating to 140°C, and one formed from dropcasting the dispersion of polymer and QD from the solvent. The resulting solid composite prepared with temperature processing was microtomed and exhibited QD aggregation in TEM. The dropcast sample exhibited phase separation of the QDs and the polymer. Finally, CdSe QDs were mixed with a body-centered cubic poly(lactide-b-butadiene) in a mixture of n-hexane and chloroform. Micelles with the minority poly(butadiene) block on the outside were formed in similar mixtures of n-hexane and chloroform and observed via dynamic light scattering. QDs were added to the polymer in a mixture of n-hexane and chloroform after micellization (at higher hexane concentrations) and before micellization (at lower hexane concentrations); in the latter case, hexane was added to induce polymer micellization after the addition of QDs. These dispersions were dropcast into films for TEM, which revealed similar film structures for both samples where QDs appeared at the interstices between roughly spherical shapes; these shapes were attributed to the formation of polymer microemulsions during the drying process. The effect of chloroform and n-hexane content of the solvent mixture on the composite formation was tested for various mixtures of n-hexane and chloroform. TEM of QD and polymer samples prepared from these solvent mixtures showed phase separation of QDs and polymer at low n-hexane concentrations and the polymer microemulsion structure for higher n-hexane concentrations. Although this work focused on the integration of CdSe QDs, the insights demonstrated here should prove relevant for other nanoparticle-composite systems.
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    Uniaxial Extensional Behavior of A–B–A Thermoplastic Elastomers: Structure-Properties Relationship and Modeling
    (2015-05) Martinetti, Luca
    At service temperatures, A–B–A thermoplastic elastomers (TPEs) behave similarly to filled (and often entangled) B-rich rubbers since B block ends are anchored on rigid A domains. Therefore, their viscoelastic behavior is largely dictated by chain mobility of the B block rather than by microstructural order. Relating the small- and large-strain response of undiluted A–B–A triblocks to molecular parameters is a prerequisite for designing associated TPE-based systems that can meet the desired linear and nonlinear rheological criteria. This dissertation was aimed at connecting the chemical and topological structure of A–B–A TPEs with their viscoelastic properties, both in the linear and in the nonlinear regime. Since extensional deformations are relevant for the processing and often the end-use applications of thermoplastic elastomers, the behavior was investigated predominantly in uniaxial extension. The conceptual basis of the theories underlying each topical area was explained while the emphasis was kept on fundamental principles and the molecular viewpoint. The analysis herein is independent from the specific choice of the constituent blocks and thus applies to any microphase-segregated thermoplastic elastomer of the A–B–A type. The unperturbed size of polymer coils is one of the most fundamental properties in polymer physics, affecting both the thermodynamics of macromolecules and their viscoelastic properties. Literature results on poly(D,L-lactide) (PLA) unperturbed chain dimensions, plateau modulus, and critical molar mass for entanglement effect in viscosity were reviewed and discussed in the framework of the coil packing model. Self-consistency between experimental estimates of melt chain dimensions and viscoelastic properties was discussed, and the scaling behaviors predicted by the coil packing model were identified. Contrary to the widespread belief that amorphous polylactide must be intrinsically stiff, the coil packing model and accurate experimental measurements undoubtedly support the flexible nature of PLA. The apparent brittleness of PLA in mechanical testing was attributed to a potentially severe physical aging occurring at room temperature and to the limited extensibility of the PLA tube statistical segment. The linear viscoelastic response of A–B–A TPEs was first examined at temperatures where the A domains are glassy. Characteristic length scales and tube model parameters were determined, and the role of the glassy A domains on the entangled rubbery B network was assessed. Thermo-rheological complexity, observed near and below Tg,A, was attributed to augmented motional freedom of the B block ends at the corresponding A/B interfaces, in harmony with the theoretical treatment of thermo-rheological complexity for two-phase materials developed by Fesko and Tschoegl. When the magnitude of the steepness index was taken into account, the shift behavior was analogous to the response measured for pure B melts. Building upon the procedure proposed by Ferry and co-workers for entangled and unfilled polymer melts, a new method was developed to extract the matrix monomeric friction coefficient ζ0 from the linear response behavior of a filled system in the rubber-glass transition region, and to estimate the size of Gaussian submolecules. Stress relaxation beyond the path equilibration time was found qualitatively and quantitatively compatible with dynamically undiluted arm retraction dynamics of entangled dangling structures (originating either from a fraction of triblock chains having one end residing outside A domains or from diblock impurities). By employing tube models and rubber elasticity theories, suitably modified to account for microphase-segregation, the linear elastic behavior across the rubbery plateau and up to the entanglement time was modeled, and a simple analytical expression relating the Langley trapping factor with the fraction of entangled and unentangled dangling structures of the material was obtained. The critical-gel-like behavior typical of A–B–A TPEs at service temperatures approaching Tg,A was analyzed in terms of a power-law distribution of relaxation times derived from the wedge distribution, shown to be equivalent to Chambon–Winter's critical gel model and to the mechanical behavior of a fractional element. A relation between the observed power-law exponent and molecular structure was established. The measured low-frequency response, originating from the incipient glass transition of the A domains, was exploited and extrapolated to lower frequencies via a sequential application of the fractional Maxwell model and the fractional Zener model. With only a few, physically meaningful material parameters a realistic description of the A–B–A self-similar relaxation was obtained over a frequency range much broader than the experimental window and not accessible via time-temperature superposition. The relationship between large-strain response and network structure of A–B–A triblocks was investigated, by examining (1) the effect of linear relaxation mechanisms on the tensile behavior, (2) the sources of elastic and viscoelastic nonlinearities, and (3) the strain rate dependence of the ultimate properties. Because of the numerous typos that appear in the original papers as well as in a recent Macromolecules review, a detailed analysis of the Edwards–Vilgis slip-link model was performed and the main steps leading to the determination of the chemical and topological contributions to the reduced stress were outlined. After establishing an operational definition of initial modulus for critical-gel-like materials subjected to start-up extensional tests, it was possible to determine the relationship between the dimensionless stress in tensile tests at constant strain rate and the step-strain extensional damping function. Based on the molecular picture of the strain-induced structural changes gained from exposing time and strain effects, the governing mechanism of rupture was identified with ductile/fragile rupture of A domains. To the best of our knowledge, this is the first experimental evidence linking the strain rate dependence of ultimate properties of triblock TPEs to the strain-induced glass-rubber transition of the domains. In addition, experimental results on the ultimate properties of A–B–A/B–A blends were consistent with this mechanism of rupture. For the first time in the literature, the complex high-dimensional rheological signature of chewing gum was analyzed, especially in response to nonlinear and unsteady deformations in both shear and extension. A unique rheological fingerprint was obtained that is sufficient to provide a new robust definition of chewing gum that is independent of specific molecular composition.