Browsing by Subject "polymer"

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Application of Fluorous Polymer Matrixes in Ion-Selective Elcetrodes
(2016-08) Lugert-Thom, Elizabeth
Polyperfluoro(4-vinyloxy-1-butene), which is also known as Cytop, and poly[4,5-difluoro-2,2,-bis (trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) copolymers with dioxole monomer contents of 65% or 87% (known as Teflon AF1600 and Teflon AF2400, respectively) were plasticized with four fluorous compounds. While plasticization of all polymers with perfluoroperhydrophenanthrene, perfluoro(1-methyldecalin), a perfluorotetraether with three trifluoromethyl side groups and one hydrogen atom, and a linear perfluorooligoether with an average of 14.3 ether groups per molecule was successful, these four plasticizers affected the twelve blends very differently. A threshold of plasticization beyond which further increases in the plasticizer volume fraction did not further affect the glass transition temperature, Tg, was observed for some blends. Also, the limit of miscibility ranged from as low as 20% plasticizer content to complete miscibility at all volume fractions. The blends of Teflon AF2400 or Teflon AF1600 with high contents of the linear perfluorooligoether provided Tg values as low as –114 ºC, lower than for any other fully miscible blend. The occurrence of two glass transitions in an intermediate range of plasticizer volume ratios for these two types of blends can be explained by distinct local environments rather than macroscopic phase separation, as anticipated by the Lodge-McLeish model. In spite of the widespread use of perfluorinated solvents with amino and ether groups in a variety of application fields, the coordinative properties of these compounds are poorly known. It is generally assumed that the electron withdrawing perfluorinated moieties render these functional groups rather inert, but little is known quantitatively about the extent of their inertness. This chapter reports on the interactions between inorganic monocations and perfluorotripentylamine and 2H-perfluoro-5,8,11-trimethyl-3,6,9,12-tetraoxapentadecane, as determined with fluorous liquid-membrane cation-selective electrodes doped with tetrakis[3,5-bis(perfluorohexyl)phenyl]borate salts. The amine does not undergo measurable association with any ion tested, and its formal pKa is shown to be smaller than –0.5. This is consistent with the nearly planar structure of the amine at its nitrogen center, as obtained with density functional theory calculations. The 2HPFTE interacts very weakly with Na+ and Li+. Assuming 1:1 stoichiometry, formal association constants were determined to be 2.3 and 1.5 M-1, respectively. This disproves an earlier proposition that the Lewis base character in such compounds may be non-existent. Due to the extremely low polarity of fluorous solvents and the resulting high extent of ion pair formation, a fluorophilic electrolyte salt with perfluoroalkyl substituents on both the cation and the anion had to be developed for these experiments. In its pure form, this first fluorophilic electrolyte salt is an ionic liquid with a glass transition temperature, Tg, of -18.5 ºC. Interestingly, the molar conductivity of solutions of this salt increases very steeply in the high concentration range, making it a particularly effective electrolyte salt. Fluorous media are the least polar and polarizable condensed phases known. Their use as membrane materials considerably increases the selectivity and robustness of ion-selective electrodes (ISEs). In this research, a fluorous amorphous perfluoropolymer was used for the first time as a matrix for an ISE membrane. Electrodes for pH measurements with membranes composed of poly[4,5-difluoro-2,2,-bis(trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) (known as Teflon AF) as polymer matrix, a linear perfluorooligoether as plasticizer, sodium tetrakis(3,5-bis(perfluorohexyl)phenyl)borate providing for ionic sites, and bis[(perfluorooctyl)propyl]-2,2,2-trifluoroethylamine as H+-ionophore were investigated. All electrodes had excellent potentiometric selectivities, showed Nernstian responses to H+ over a wide pH range, exhibited enhanced mechanical stability and maintained their selectivity over at least four weeks. For membranes of low ionophore concentration, the polymer affected the sensor selectivity noticeably at polymer concentrations exceeding 15%. Also, the membrane resistance increased quite strongly at high polymer concentrations, which cannot be explained by the Mackie-Meares obstruction model. The selectivities and resistances depend on the polymer concentration because of a functional group associated with Teflon AF2400, with a concentration of one functional group per 854 monomer units of the polymer. In the fluorous environment of these membranes, this functional group binds to Na+, K+, Ca2+, and the unprotonated ionophore with binding constants of 103.5, 101.8, 106.8 and 104.4 M–1, respectively. Potentiometric and spectroscopic evidence indicates that these functional groups are COOH groups formed by the hydrolysis of carboxylic acid fluoride C(꞊O)F groups originally present in Teflon AF2400. The use of higher ionophore concentrations removes the undesirable effect of these COOH groups almost completely. Alternatively, the C(꞊O)F groups can be eliminated chemically. In this work we demonstrate the remarkable stability of fluorous-based ion-selective electrode (ISE) membranes by exposing them to a cleaning-in-place treatment, CIP, as it is used in many industrial processes. The sensing membranes were made up of a linear perfluoropolyether as membrane matrix, 0.5 mmol/kg ionic sites (tetrakis[3,5-bis(perfluorohexyl)phenyl]borate), 2 mmol/kg ionophore (tris[(perfluorooctyl)propyl]amine or tris[(perfluorooctyl)pentyl]amine), and Teflon AF2400. To mimic a typical CIP treatment, the electrodes were repeatedly exposed for 30 min to 3.0% NaOH solution at 90 ºC (pH ≈12.7). After ten exposures and a total of 5 h at 90 ºC, the fluorous sensing membranes doped with the more selective ionophore still showed the ability to respond with a theoretical (Nernstian) slope without loss in selectivity. Addition of a fluorophilic electrolyte salt reduced the membrane resistance by an order of magnitude.
Characterizing mVenus adsorption to photodegraded polyethylene using circular dichroism and fluorescence spectroscopy
(2022-08) Amaris, Althea
Due to their versatility and relative cost-effectiveness, plastics as a material have gained increasing popularity and are heavily utilized by almost every major industry in the modern day. Their exponential rate of production coupled with a lack of proper disposal methods, however, have resulted in the global environmental issue of plastic pollution. Upon entering the ecosystem, plastic surfaces can act as a foundation for the formation of microbial communities known as biofilms. An initial key step to biofilm growth is the attachment of bacterial surface proteins onto the polymer. In this study, we examine structural changes of a “hard” model protein in the presence of environmentally relevant plastics. Using the intrinsic probes of the mVenus protein, a model yellow fluorescent protein (YFP), we study its structural response to variably photo-aged polyethylene (PE) through circular dichroism (CD) and tryptophan (W)/YFP-fluorescence spectroscopy. Upon binding to aged PE, mVenus undergoes mild secondary structure rearrangement. Interestingly, a forbidden transition in W-fluorescence is observed, evolving from the interaction between the sole tryptophan in mVenus and the increasingly hydrophilic surface of PE as the polymer is progressively photo-oxidized. The beta barrel and beta sheet structure of mVenus retains the overall stability of the protein, whereas the local structure and turn regions accommodate the protein-polymer interactions based on polymer surface chemistry. We can therefore start to predict that proteins bind variably during the initial docking of cells as the secondary structure behaves distinctly based on the age of the film to which it attaches. The dependence of protein docking on the extent of PE-irradiation reveals that film age, polymer type, and structural stability can either accelerate or inhibit biofilm growth.
Connecting macroscopic properties to microstructure of block copolymer materials through simulation
(2024-05) Collanton, Ryan
Due to their molecular topology, block copolymers exhibit rich and unique physical properties that make them of interest for use in a wide variety of applications. In this thesis, we first discuss the methodology and development of software for performing self-consistent field theory calculations. We then investigate the properties of block copolymers and their microscopic origin in two distinct contexts. First, we consider the equilibrium states of materials consisting solely of block copolymers or of majority block copolymer. Block copolymers are well-known to self-assemble into ordered microstructures. These microstructures constitute a balance of entropic and enthalpic contributions to the free energy. The particular microstructure that forms depends on the block copolymer chemistry, the temperature, and if it is a multi-component system, the blending fraction. In addition to the long-established classical phases such as lamellae or hexagonal cylinders, near the turn of the century, theoretical efforts led to predictions of the stability of more complicated phases in block copolymer melts such as the Frank-Kasper A15 phase might be stable in block copolymer melts. Furthermore, Frank-Kasper phases, originally discovered in metals in the 1950s, had been reported to form in other soft matter systems such as lipid-containing micelles. They nonetheless remained elusive in block copolymer materials until 2010 when the Frank-Kasper σ phase was serendipitously discovered. This discovery was followed by the discovery of many more complex ordered microstructures in polymer melts and blends, leading to questions about what drives these phases to be stable. In this work, we calculate structures of diblock copolymer melts using self-consistent theory and perform a novel analysis that connects free energy to crystal structure. We find that the transition from the body-centered cubic (bcc) phase to the σ phase is driven by a decrease in enthalpy, and that this decrease is a result not of a decrease in contact area but instead of a sharpening of the interface. This contradicts previous explanations employing strong-stretching theory. We then transition to investigate the properties of immiscible polymer blends compatibilized with block copolymers as small weight-fraction additives. Due to vanishing entropy of mixing, all commercially relevant polymers are thermodynamically immiscible. In blends, this manifests as large phase-separated domains with weak separating interfaces across which there is a very low degree of entanglement between dissimilar polymers. Thus, blends of immiscible polymers exhibit poor mechanical properties. However, these blends are of interest because of their potential relevance in multi-functional materials or in recycling of highly heterogeneous waste streams. Early efforts attempted to improve blend properties without any additives by refining the structure of the interface to promote entanglement. In the past two decades, however, attention has turned to block copolymers as potential additives to achieve the two goals of compatibilization: reduction of domain size and improvement of mechanical properties. Block copolymers are known to act in a surfactant-like fashion by reducing interfacial tension at homopolymer-homopolymer interfaces. Furthermore, they have been found to improve mechanical strength through anchoring mechanisms such as entanglement and co-crystallization. Diblock copolymers were first investigated for their performance as “compatibilizers”, and recent work has shown other architectures such as linear multiblock copolymers to significantly outperform diblock copolymers. In this work, we investigate the thermodynamic and mechanical properties of polymer blends compatibilized by linear multiblock copolymers using coarse-grained molecular dynamics. We find that the density and uniformity of interfacial crossings determined the reduction of interfacial tension. Furthermore, we identify the existence of an optimal loading of copolymer that maximizes toughness and strain-at-break, and examine the mechanism of failure of a glassy compatibilized blend under uniaxial elongation. We show that failure occurs via a two-step mechanism that involves cavitation at the interface followed by simultaneous re-densification and chain pullout. This mechanism is qualitatively different from failure of a single-component polymer glass, but leads to a nearly identical stress-strain response.
Fabrication of Zeolite MFI Membranes on Low Cost Polymer Supports
(2017-07) Zhang, Han
About 10~15% of total energy consumption in US is attributed to energy intensive chemical separation processes, such as distillation. The alternative membrane-based separation could save up to 90% energy consumption with outstanding separation performance. Zeolite MFI membranes have been demonstrated for xylene and butane isomer separations with high separation factors and permeances. However, high cost and scale-up difficulty prevent the commercialization of MFI membranes in industries. This dissertation attempts to explore the methods for MFI membranes supported on low cost polymer supports. The major challenge is the stability of polymer support during the detemplation treatment of the MFI membrane after secondary growth. Two mild detemplation methods, thermal treatment at 280 °C and UV/ozone treatment, were identified with sub-100 nm MFI membranes supported on quartz supports. These two methods were then applied to MFI membranes supported on mesh-polyethersulfone (PES) supports and MFI membranes supported on mesh-polybenzimidazole supports. However, cracks formed after the treatments due to the damage of polymer layer by UV light and the mismatch of linear thermal expansion co-efficient, respectively. Another approach, which utilizes the open-pore MFI nanosheets, have been demonstrated. The organic structure directing agents (OSDA) occluded inside the micropores of nanosheets were removed by successive piranha solution treatment, while the crystallinity and morphology were still preserved that confirmed by X-ray diffraction(XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and gas adsorption. The simple deposition of such open-pore MFI nanosheets on porous PBI support, without the need of secondary growth and detemplation, exhibited n-/iso-butane ideal selectivity of 5.4 with n-butane permeance of 3.5×10-7 mol/m2-s-Pa. In addition, the nanosheet exfoliation yield was significantly improved by an oligomeric polystyrene resin. Ultrafiltration polymer hollow fibers were also prepared as suitable supports for nanosheet coating.
Investigating the Interactions of Polymeric Excipients with Poorly Water-Soluble Drugs as Means for Pharmaceuticals Bioavailability Enhancement
(2019-02) Purchel, Anatolii
Oral administration is the most preferable route of drug delivery, especially during prolonged therapy of chronic diseases. Unfortunately, many effective pharmaceuticals are poorly water-soluble, which leads to decreased bioavailability and shelf life. One of the ways to improve drug solubility and efficacy is to prepare an amorphous solid dispersion (ASD) with a polymer excipient. It is important that the polymer matrix of an ASD will stabilize the drug in the amorphous state and maintain its supersaturated concentration long enough in the dissolution media. Some of the commercial polymeric systems have shown a positive impact on drug dissolution, but most of them are difficult to characterize due to high polydispersity and system complexity. Most of the available excipients that improve dissolution of poorly water-soluble drugs tend to form nano-aggregates in the solution. Thus, in order to understand structure-property relationships better, various polymers were explored, which self-assemble into micelle-like structures or exist as free polymer chains in the solution, as excipients for dissolution of a model drugs such as probucol and phenytoin. Reversible addition-fragmentation chain transfer (RAFT) polymerization was used as a controlled polymerization technique to obtain well-defined polymers of polystyrene, poly(acrylic acid), N-isopropylamide, 4-vinylpyridine, N,N-dimethylacrylmide, and trehalose-derived monomers. The polymers were characterized by nuclear magnetic resonance (NMR) spectroscopy and size exclusion chromatography (SEC). The effects of nano-aggregation in ASDs, polymer charge, H-bonding and hydrophobic interactions on drug dissolution were determined. Caco-2 cell permeability assay was applied to determine cell permeability of drugs in some of the obtained formulations.
Poly(isoprenecarboxylates) from Glucose via Anhydromevalonolactone
(2018-01-04) Ball-Jones, Nicolas; Hoye, Thomas R.; Fahnhorst, Grant W.; hoyex001@umn.edu; Hoye, Thomas R.
These are raw data files obtained during development of the following manuscript: Ball-Jones, N. R.; Fahnhorst, G. W.; Hoye, T.R. "Poly(isoprenecarboxylates) from Glucose via Anhydromevalonolactone" ACS Macro Lett. 2016, 1128–1131. The abstract of this document is the following, "A short and efficient synthesis of a series of isoprenecarboxylic acid esters and their corresponding polymers is presented. The base-catalyzed eliminative ring opening of anhydromevalonolactone (3) provides isoprenecarboxylic acid (6-H), which was further transformed to the isoprenecarboxylic acid esters. Reversible addition–fragmentation chain-transfer (RAFT) polymerization was used to synthesize high molecular weight (>100 kg mol–1) poly(isoprenecarboxylates) with dispersities (Đ) of ca. 1.5. The glass transition temperatures (Tg) and entanglement molecular weights (Me) of the poly(isoprenecarboxylates) were determined and showed similar trends to the Tg and Me values for analogous poly(acrylate esters). These new glucose-derived materials could provide a sustainable alternative to poly(acrylates).
A Polymer Driveshaft for use in Orbital and Rotational Atherectomy
(2016-01) Grothe, Preston
Driveshafts used in atherectomy medical devices are often comprised of coiled or braided metal wires. These constructions are designed to tolerate delivery through tortuous vessels and can endure high speed rotation used during activation of the atherectomy treatment. This research investigated polymer driveshaft designs, which were comprised of polymer inner and outer layers, and coiled or braided stainless steel wires. The polymer driveshaft materials included polyimide, nylon 12, and polytetrafluoroethylene (PTFE). Mechanical testing of polymer driveshafts was conducted to determine material response in bending, tension, compression, and torsion. The polymer driveshaft test results were then compared with current coiled metal wire driveshaft constructions. The investigation identified polymer driveshaft options that could feasibly work in an atherectomy application.
Polymerization Kinetics of Cyclic Esters by Metal Alkoxide Complexes and Catalytic Decarbonylation of Bio-Derived Carboxylic Acids to Commodity Alkenes
(2014-05) Miranda, Maria
Plastic materials are an integral part of modern life; however, nearly every plastic, or polymer, is derived from petroleum resources, which are non-sustainable, non-degradable, and can be toxic to humans and the environment. Developing methodologies to synthesize and characterize alternative materials that are degradable, safe, and sustainable has therefore been a vibrant research area. This thesis describes two approaches towards the development of sustainable polymers and monomers (the building blocks from which polymers are made). The first aims to understand the fundamental mechanistic details of metal-catalyzed ring-opening polymerization of renewable cyclic ester monomers to degradable polyesters. The second targets the catalytic synthesis of common petroleum-based monomers from sustainable and biomass-derived carboxylic acids.
Quantifying Polymer Surface Degradation Using Fluorescence Spectroscopy
(2023) Tigner, Jonathan
One solution to minimizing plastic pollution is to improve reuse and recycling strategies. Recycling, however, is limited by the overall degradation of plastics being used. Photochemical or thermal driving forces facilitate the incorporation of oxygen into the backbone and chain cleavage; yet, current techniques for monitoring this plastic degradation fail to observe early stages of degradation, which is key for optimizing reusability. This research seeks to develop a cheap, reproducible, and nondestructive technique for monitoring degradation of polyethylene and polypropylene materials using Nile red as a fluorescent probe. Changes in Nile red’s fluorescence spectra were observed upon exposure to stained, aged polyethylene and polypropylene samples. As the surface hydrophobicity of the plastic decreases, Nile red’s fluorescence signal undergoes corresponding signal shift to longer wavelengths (lower energy). The trends seen in the fluorescent profile were related to more commonly used measurements of plastic degradation, namely carbonyl index from infrared spectroscopy and bulk crystallinity from calorimetry. Results demonstrate clear trends in fluorescence spectra shifts as related to the chemical and physical changes to the plastics, with trends dependent on polymer type but independent of polymer film thickness. The strength of this technique is divided into two defined fits of the fluorescence signal; one fit characterizes the degradation throughout the whole range of degradative oxidation and the other is tailored to provide insight into the early stages of degradation. Overall, this work establishes a characterization tool that assesses the extent of plastics’ degradation, which may ultimately impact our ability to recover plastics and minimize plastic waste.
Ring-Opening Polymerization as a Platform for Tailored Polymers from Isosorbide and Other Renewable Feedstocks
(2020-10) Saxon, Derek
To withstand the critical need for plastics, we must innovate how polymers are constructed and deconstructed. Isosorbide and other renewable feedstocks have shown exceptional promise as replacements for commodity plastics. The work in thesis describes ring-opening polymerization as a previously unexplored strategy to synthesize polymers primarily from isosorbide, as well as several other renewable feedstocks. We describe traditional and contemporary approaches to synthesizing polymers from isosorbide along with the current challenges faced (Chapter 1). Initial efforts were aimed at developing polyethers with isosorbide in the backbone through ring-opening polymerization of an annulated isosorbide derivative, ultimately providing control over both the polymer microstructure and macromolecular architecture, enabling cyclic or linear polymers to be targeted (Chapter 2). This work is a stepping-stone for polymerization of complex heterocycles from renewable feedstocks. We then turned our focus to polycarbonate analogs to the poly(meth)acrylates previously developed in our lab (Chapter 3). Specifically, we established a method for the rapid synthesis of chemically recyclable, functional (co)polycarbonates with tailored thermal properties from isosorbide and other renewably derived alcohols. The polycarbonates were then redesigned to exploit industrial waste streams—specifically glycerol and carbon dioxide—to construct the value-added polymer backbone (Chapter 4). Tandem functionalization and ring-opening polymerization is being pursued to afford polycarbonates with 100% renewable content. These efforts may facilitate the development of commercially relevant sustainable polycarbonates with tailored properties that work toward eliminating plastic waste streams.
Shear-Banding In Entangled Polymer Solutions Under Large Amplitude Oscillatory Shear: A Confocal Rheometry Study
(2020-04) Shin, Seunghwan
We use a large aspect-ratio, planar-Couette shear cell to explore the flow properties of entangled polymer solutions, with a special focus on a long-standing problem of shear-banding in polymer solutions/melts. We first analyze the velocity profiles of entangled DNA solutions under large amplitude oscillatory shear (LAOS) inside the shear cell. We vary a gap between the shearing plates and Weissenberg number ($\mathrm{Wi}$) to construct phase diagrams quantifying the degree of wall-slip and shear-banding at different conditions. We observe transitions from normal linear shear profiles to wall-slip dominant and finally to shear-banding profiles with increasing $\mathrm{Wi}$. We further explore the dynamics of micron-sized tracer particles embedded in the solutions to study the microscopic origin of the shear-banding. Tracer particles in the shear frame exhibit transient super-diffusivity and strong dynamic heterogeneity localized in the high-shear-rate band. The probability distribution functions of particle displacements follow a power-law scaling at large displacements, indicating a L\'{e}vy-walk-type motion, reminiscent of tracer dynamics in entangled wormlike micelle solutions and sheared colloidal glasses. We further characterize the length and time scales of the abnormal dynamics of tracer particles. Based on them, we hypothesize that the unusual particle dynamics arise from localized shear-induced chain disentanglement. Next, we experimentally investigate a penetration of edge-induced disturbances and its influence on shear-banding flows. Edge instabilities have been pointed out as one of the possible experimental artifacts leading to apparently heterogeneous shear profiles. Simulations suggested even a mild edge disturbance can penetrate deeply along a vorticity direction to cause apparent gradient-banding of a velocity profile, potentially misleading experimentalists. We measure velocity profiles at different locations to reveal penetrating behavior of edge disturbances and test authenticity of the observed shear-banding flows. Under a weak oscillatory shear ($\mathrm{Wi} < 1$) where DNA solutions display a linear shear profile with wall slip, the penetration depth of the edge disturbance was on the order of the gap thickness , similar to a behavior in Newtonian fluids. Under a strong shear ($\mathrm{Wi} > 1$) where shear-banding flows are developed, the penetration depth was estimated as 20 $H$ along the flow direction while it was still on the order of the gap thickness along the vorticity direction. Furthermore, we find that the shear-banding profiles persist deep inside the sheared fluid, where the influence of edge disturbances diminishes. Our findings suggest a long penetration of the edge disturbance and also demonstrates the authentic nature of the observed shear-banding polymers. Shear-induced microscopic conformational change of individual polymer chains that trigger shear banding still remains an open question. To attain information about chain-end distributions and its dynamics, we synthesize dumbbells consisting of two spherical colloidal tracer particles connected by $\lambda$-DNA linkers and track their 2D-projected configurations and motions in the two shear-bands. We observe preferable alignment along the flow direction, enhanced translation/rotation in the high-shear-rate band. Coupling between translational/rotational dynamics and stronger correlation between chain extension and translation are also found in the high-shear-rate band. We hypothesize a formation of the localized low viscosity zones which allow the enhanced dynamics and chain extension in the high-shear-rate band.
Silicon quantum dot luminescent solar concentrators
(2019-08) Hill, Samantha
Silicon quantum dots (Si QDs) have previously been established as a unique class of quantum-confined materials with potential for a wide variety of optoelectronic applications. In this work, we examine their application to luminescent solar concentrators, or LSCs, for the first time by developing high-quality Si QD / polymer nanocomposites. By encasing Si QDs with in a polymer slab, most of their photoluminescence becomes trapped via total internal reflection and escapes only at the slab edges where solar cells can be placed to harvest the concentrated light. We find that Si QDs are suitable for such LSC devices due to their unique combination of indirect band gap absorption with efficient photoluminescence. The resulting low overlap between the absorption and photoluminescence spectra yields low reabsorption losses in large-area LSCs without the use of rare or toxic elements in the luminophore. We demonstrate effective Si QD LSC prototypes consisting of flexible and rigid bulk nanocomposites as well as films on glass using methacrylate-based polymers. We find the Si QDs maintain their optical properties throughout radically-initiated polymerization processes but are prone to forming light scattering agglomerates in the solid phase. These agglomerates drastically reduce the LSC waveguiding efficiency due to their light scattering properties. We find that light scattering from these nanocomposites increases with Si QD concentration. One approach for improving the dispersion of the Si QDs within solid polymers is to choose surface ligands which mimic the structure of the encasing polymer. We demonstrate this with ester-capped Si QDs compared to alkane-capped Si QDs in poly(methyl methacrylate), or PMMA. Furthermore, we find that fast polymer solidification rates also reduce the formation of light scattering agglomerates. We show ester-Si QD / PMMA films cast from prepolymer solutions have an order of magnitude higher concentration limit before the onset of light scattering compared to their bulk-polymerized counterparts. Overall, this work establishes Si QDs as a promising luminophore for visibly transparent LSCs which may be used in the future for solar harvesting windows and architectural elements or in concert with other LSCs to form more efficient tandem structures.
Solid-State Block Polyelectrolytes
(2020-07) Goldfeld, David
While polyelectrolytes are, in general, hydrophilic and soluble in water, there are many applications that benefit from immobilized solid-state charged materials, including membrane separations and batteries. One convenient method to immobilize polyelectrolytes in a solid-state configuration is using block polymer materials self-assembled to contain charged polyelectrolyte domains immobilized by neutral supporting domains. We used this strategy to work towards charge mosaic materials, a proposed design for a piezodialysis-based water desalination system. In a charge mosaic membrane, there are both positively and negatively charged polymer domains that are spatially separated and independently cross the thickness of the material. In Chapter 2, we first developed a new technique to integrate this design into thin films. Through the synthesis of neutral ABC triblock polymers, we casted thin films with three microphase separated domains. We then demonstrated the functionalization of these materials in a mild, 2-in-1 postpolymerization modification that converted the A domain (poly(n-propyl styrene sulfonic ester)) to a negatively charged polyanion and the C domain (poly(vinylbenzyl chloride)) to a positively charged polycation in a mild, single step vapor exposure. While these materials demonstrated successful microphase separation with a simple functionalization that maintained morphology, they had poor long-range order and suffered from brittle mechanical properties that prevented their effective use as active layers in membrane separations. During the synthesis of an ABC triblock polymer for charge mosaic applications, we found a previously unreported miscibility between polystyrene and poly(vinylbenzyl chloride). Although both polymers had been used together in a number of previous applications, their solid-state structure had never been adequately explored. In Chapter 4, we attempted to characterize the Flory-Huggins interaction parameter between these two polymers using small-angle X-ray scattering of homogeneous polymer blends. We then synthesized a vinylbenzyl chloride derivative, vinylbenzyl nitrate, that demonstrated both microphase separation from polystyrene as well as facile postpolymerization modification and explosive properties. Chapter 3 attempted to solve the mechanical problems associated with the triblock polymers by integrating poly(styrene sulfonic ester) and poly(vinylbenzyl chloride) into a system that undergoes polymerization-induced microphase separation (PIMS). PIMS monoliths were made through a simple radical polymerization initiated in a homogeneous mixture of a macroinitiator dissolved in mono- and di-functional monomers. The PIMS technique results in strong materials that contain a bicontinuous structure comprising a percolating macroinitiator domain crossing the thickness of the crosslinked matrix. We used PIMS to produce solid monoliths using the neutral polyelectrolyte precursors previously used in the ABC triblock polymers. The macroinitiator domain was then functionalized to yield either a positively charged polycation material or a negatively charged polyanion material, confirmed using oppositely charged dyes and infrared characterization. The integration of both positive and negative charges into a PIMS system is approached in Appendix A. Simply mixing macroinitiators, a procedure based on previous literature, showed unexpected macrophase separation in the monolith. The use of block polymer macroinitiators to overcome solubility differences between the segments is presented as a potential solution and the synthesis of the first block polymer PIMS is demonstrated. Finally, we introduced the potential of using a polyelectrolyte as the matrix domain. Appendix B presents a proposed model for controlling swelling in the PIMS polyelectrolyte domain. By adding a poly(lactide) block to the poly(styrene sulfonic ester) macroinitiator, a degradable domain is introduced that can be selectively removed to free swelling space for the polyelectrolyte. We hypothesize that control over the swelling will provide a model system for the systematic variation of ion conductivity and provide insights into the fundamental effects of ion density, water content, and morphology. A slightly different project is explored in Chapter 5, where we develop a poly(lactide) based foam for use in floral foam applications. We formulate a mix of surfactants that make the hydrophobic, low melt-strength poly(lactide) into a rigid, low density (<0.02 g·cm–3), hydrophilic foam that readily absorbs water and supplies it to inserted flowers. The foam is compostable and made from renewable materials, making it a significant improvement over the petroleum based, non-degradable materials that are currently commercially available.
Supporting Data for "Effects of Electrolytes on Thermodynamics and Structure of Oligo(ethylene oxide)/Salt Solutions and Liquid–Liquid Equilibria of a Squalane/Tetraethylene Glycol Dimethyl Ether Blend"
(2021-01-22) Shen, Zhengyuan; Chen, Qile P; Lodge, Timothy P; Siepmann, J Ilja; siepmann@umn.edu; Siepmann, J Ilja
Data including input/output and restart files for all the systems, analysis codes (python, fortran, cpp), and figures in the paper "Effects of Electrolytes on Thermodynamics and Structure of Oligo(ethylene oxide)/Salt Solutions and Liquid–Liquid Equilibria of a Squalane/Tetraethylene Glycol Dimethyl Ether Blend". Sample movie files of the production trajectory are provided.
Synergistic Effects of Multiple Aging Stressors on HDPE and HDPP
(2020-12) Finke, Adam
The service life for power cables in our nation’s nuclear power plants have been extended beyond 40 years, however no reliable test has been found for determining the remaining lifetime of a cable in service. With experimental data and simulated models, a test and material lifetime prediction are being developed based on changes in electrical, mechanical, and chemical properties as a function of service conditions, temperature, aqueous exposure, material, and time. While it may be next to impossible to test every condition or environment for a material lifetime study, analytical models based on accelerated aging can attempt to predict both tested and untested conditions. By using models obtained from experimental data produced during accelerated aging studies, changes in mechanical properties, chemical properties, and even visual properties can be combined to better understand and more accurately predict aging.The first step to understanding insulation degradation is to understand how environmental conditions like water, temperature, and mineral or an aqueous environment affects polymer aging and how that might accelerate or decelerate aging. The aim of this thesis is to cover what effects combining these conditions might have on polyethylene and polypropylene tensile samples at a moderately elevated temperature. Additionally, how might cycling of conditions age or degrade these samples differently from samples continuously submerged. Tensile properties, hardness measurements, and surface chemical characterization of carbonyl formation through the attenuated total reflection were measured and calculated to determine the synergistic effects of aging polyethylene and polypropylene in distilled water, a copper sulfate solution, and Harrison’s solution at 90ºC through common mechanical properties and aging indicators. Additional studies were started aging both materials at elevated temperatures and oxygen partial pressures, and submersion in water. Results suggest that at 90ºC these mixed conditions did not accelerate aging over dry aged samples within a 16-week period and more time or a greater temperature would be required to create a greater difference between conditions.

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