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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Linear Viscoelastic Properties of Putative Cyclic Polymers Synthesized by Reversible Radical Recombination Polymerization (R3P)

Linear viscoelastic properties in both melt and solution states are reported for a series of poly(3,6-dioxa-1,8-octanedithiol) (polyDODT) made by reversible radical recombination polymerization (R 3 P) under conditions designed to produce linear (LDODT), cyclic (RDODT) and linear-cyclic mixtures (LRDODT). PolyDODT is amorphous (T g < -50 °C) and highly flexible (entanglement molecular weight M e,lin ≈ 1850 g/mol for LDODT). PolyDODT’s low T g and low M e,lin enable characterization over a wide dynamic range and a wide range of dimensionless weight-average molecular weight Z w = M w /M e,lin . Measurements at temperatures from -57 °C to 100 °C provide up to 18 deades of reduced frequency, which is necessary to characterize RDODT melts with Z w from 23 to 300. The two highest molecular weight polymers in the present RDODT series have such high M w (406k and 556k g/mol) that mass spectrometry, NMR spectroscopy, and even chemical assays for chain ends are unable to rule out up to 2 mol% linear contaminant. By studying the samples in solution (using dilution to reduce Z w ) we could compare their dynamics with those of previously established high-purity polystyrene (PS) rings (limited to Z w ≤ 13.6). RDODT solutions with Z w < 15 (concentrations <5wt% for RDODT-406k and 556k) have dynamic moduli G^* that accord with LCCC-purified PS rings in terms of the frequency dependence (including the absence of a plateau), the progression of shapes of G* as a function of Z w , and the linear scaling of their zero-shear viscosity η0 with M w . The shape of G* as a function of Z w for solutions of RDODT-406k and -556k also accord with lower M w RDODT melts (which have ≤ 1.3mol% linear contaminant). Thus, measurement of the linear viscoelastic properties of appropriate concentrations of high M w (>200k g/mol) putative cyclic polymers, in which linear chains evade spectroscopic detection, may provide an alternative means (though not fully proven) of validation of sample purity. When Z w > 15 (including all seven RDODT melts and eight of their solutions), G* has a rubbery plateau. This suggests that the onset of entanglement-like behavior in rings requires 4-5-fold greater Z w than is required for linear chains. Further, the plateau moduli of RDODT samples are indistinguishable from G N o of the corresponding LDODT (melt or matched-concentration solutions). In entangled linear polymers, the observation that G N o is independent of Z w follows from limitations on lateral fluctuations due to neighboring chains becoming independent of position along a given chain. The present results for RDODT suggest that this holds for sufficiently long endless chains, too. While the RDODT have the same G N o as entangled LDODT, when Z w > 60 the terminal relaxation, if reached at all, of RDODT extends to orders of magnitude lower frequency than an entangled linear polymer of the same Z w . Consequently, the viscosity of RDODT with Z w > 60 increases with Z w much more strongly than the 3.4-power observed for entangled linear polymers. Lastly, these novel polymers, with disulfide-linked backbone and broad relaxation time distribution may prove important in relation to biodegradable elastomers and materials with exceptional low-frequency dissipation, extending at least 12 decades below the onset of the rubbery plateau.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

Incorporation of catechyl monomers into lignins: lignification from the non-phenolic end via Diels–Alder cycloaddition?

Canonical lignification occurs via the coupling of phenolic radicals, in which chain extension can occur only from phenolic ends of growing polymer chains. Radical coupling of catechyl monomers, including caffeyl and 5-hydroxyconiferyl alcohols, gives rise to benzodioxane units in the polymer. Anticipating that a catechol could oxidize to its o-benzoquinone analog under the dehydrogenative (oxidative) conditions of lignification, we examined the possibility that an o-benzoquinone, as the diene component, could also incorporate into lignin via another mechanism, the Diels–Alder cycloaddition reaction. The o-benzoquinone derived from methyl 5-hydroxyvanillate and 4-O-methylconiferyl alcohol served as models for the diene and dienophile, respectively, and produced Diels–Alder products in vitro. Two types of Diels–Alder products were found: (i) when the 1,2-diketone of the quinone acts as the diene in a hetero-Diels–Alder reaction, a benzodioxane structure was produced with a different regiochemistry than the benzodioxane isomer produced via radical coupling; (ii) when the quinone's diene participated in the Diels–Alder reaction, a distinctive oxatricyclo structure was produced. Both features may be used as markers for the occurrence of Diels–Alder reactions in lignification. Examination of natural lignins derived from catechyl monomers, however, did not reveal evidence for such products. The conclusion is that the only significant reactions in lignification are combinatorial radical coupling reactions of the single-electron-oxidized phenolics and that polymer chain extension therefore occurs only from the phenolic end-units even in the special case of plants that utilize catechyl monomers for lignification.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1985

Stable polymeric carbon radicals. Part 2: Attempts at the preparation of polyradicals of the triphenylmethyl type linked by P-phenylene units

As starting materials for the preparation of polyradicals of triphenylmethyl type linked by p-phenylene units bis(4-iodophenylmethane) and bis(4-iodo-2,5-dimethyl-phenylmethane) were synthesized by a Sandmeyer reaction from the corresponding diamino compounds and subsequently transformed into the corresponding polymeric hydrocarbons 6a and 6b by an Ullmann condensation. In the following step 6a and 6b were brominated at the tert. carbon atom by means of N-bromosuccinimide. The reaction of the resulting poly (4,4'-biphenylylen-alpha-bromobenzylidene)s (7a and 7b) with mercury afforded the corresponding radicals, the ESR spectra of which were recorded. From the methyl substituted polymer 7b poly (2,2'5,5-tetramethyl-4,4'-bi-phenylylen)phenylmethylidyne was formed, whereas the unsubstituted product 7a was transformed into a para-quinoide polymer with radical properties.

Braun, D.↗
DOE OSTI2020

Probing Membrane Degradation with Varying Catalyst Layer Thickness and Carbon Support

Understanding membrane degradation continues to be of significant importance when studying fuel cell durability. Fuel cell performance and durability depends heavily on the membrane's chemical and mechanical integrity. In Nafion® membranes, hydroxyl radicals are the primary cause of chemical membrane degradation as they attack the membrane's polymer structure. Radical formation can ensue via peroxide decomposition at the cathode or gases reacting on Pt particles within the membrane. Here, a previous research effort on membrane degradation as a function of ultra-low Pt loadings (≤ 0.1 mgPt cm -2 ) showed the degradation rate increases with Pt loading. This is most likely due to more heterogenous sites for radical formation. Manufacturing MEAs with ultra-low Pt loadings can yield thin, non-uniform catalyst layers. In this research effort we studied membrane degradation with ultra-low Pt loadings while varying catalyst layer thickness, I/C ratio, and types of carbon support.

25 ENERGY STORAGE↗
NASA NTRS1989

ESR Measurement Of Crystallinity In Semicrystalline Polymers

Photogenerated free radicals decay at different rates in crystalline and amorphous phases. Degree of crystallinity in polymer having both crystalline and amorphous phases measured indirectly by technique based in part on electron-spin-resonance (ESR) spectroscopy. Accuracy of crystallinity determined by new technique equals or exceeds similar determinations by differential scanning calorimetry, wide-angle x-ray scattering, or measurement of density.

Kim, Soon Sam↗
DOE OSTI2020

Computational and Experimental Evaluation of Peroxide Oxidants for Amine-Peroxide Redox Polymerization

Amine–peroxide redox polymerization (APRP) is the prevalent method for producing radical-based polymers in the many industrial and medical applications where light or heat activation is impractical. We recently developed a detailed description of the APRP initiation process through a combined computational and experimental effort to show that APRP proceeds through SN2 attack by the amine on the peroxide, followed by the rate-determining homolysis of the resulting intermediate. Using this new mechanistic understanding, a variety of peroxides were computationally predicted to initiate APRP with fast kinetics. In particular, the rate of APRP initiation can be improved by radical and anion stabilization through increased p-electron conjugation or by increasing the electrophilicity of the peroxy bond through the addition of electron-withdrawing groups. On the other hand, the addition of electron-donating groups lowered the initiation rate. These design principles enabled the computational prediction of several new peroxides that exhibited improved initiation rates over the commonly used benzoyl peroxide. For example, the addition of nitro groups (NO2) to the para positions of benzoyl peroxide resulted in a theoretical radical generation rate of 1.9 × 10–9 s–1, which is ~150 times faster than the 1.3 × 10–11 s–1 radical generation rate observed with unsubstituted benzoyl peroxide. These accelerated kinetics enabled the development of a redox-based direct-writing process that exploited the extremely rapid reactivity of an optimized redox pair with a custom inkjet printer, capable of printing custom shapes from polymerizing resins without heat or light. Furthermore, the application of more rapid APRP kinetics could enable the acceleration of existing industrial processes, make new industrial manufacturing methods possible, and improve APRP compatibility with biomedical applications through reduced initiator concentrations that still produce rapid polymerization rates.

functional polymers↗
DOE OSTI2025

Bottom-Up Simulation, Reconstruction, and Quantification of Macromolecule Sequences from Experimental Polymerizations

Motivated by the canonical sequence–structure–function paradigm, tools to characterize chemical patterning in natural biomacromolecules, from proteins to nucleic acids, have grown exponentially in recent years. However, analogous strategies for synthetic macromolecules remain in nascent stages, complicated by sequence polydispersity and analytical limitations. To address this, we have developed a comprehensive and open-source Python package, PRISM (polymer rate insights and sequence modeling), an end-to-end workflow that provides a path from experimental kinetics measurements to quantitative and qualitative metrics for describing chemical patterning in stochastic polymers. First, a numerical integration strategy was constructed to simulate and fit experimental data from reversible addition–fragmentation chain transfer (RAFT) polymerization kinetics, enabling the facile estimation of relevant reactivity ratios. These ratios were then used in a mechanism-specific stochastic kinetic simulation strategy to simulate sequence ensembles corresponding to model systems spanning experimental copolymers, classes of statistical polymers (e.g., alternating, block, and gradient), and multiblock copolymers. Lastly, inspired by sequence homology metrics from bioinformatics, we introduce visualization strategies and quantitative metrics to facilitate comparisons of different sequence ensembles. As the sequence–structure–function paradigm becomes increasingly central in de novo design of synthetic macromolecules, this toolkit provides a first step toward accurate and representative sequence description and featurization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Hairy nanoparticles by atom transfer radical polymerization in miniemulsion

Polymer nanoparticles with various architectures and functionalities are promising materials in numerous fields. Miniemulsion polymerization is one of the suitable pathways to prepare polymer nanoparticles since each droplet could act as a “nanoreactor”. A “smart” atom transfer radical polymerization (ATRP) catalytic system comprising Cu-TPMA/DS – (TPMA = tris(2-pyridylmethyl)amine, DS – = dodecyl sulfate anion) ion pair catalyst was efficiently applied in miniemulsion ATRP at low catalyst concentrations. Herein, hairy nanoparticles consisting of hydrophobic poly(n-butyl methacrylate) (P(BMA/EGDMA), EGDMA = ethylene glycol dimethacrylate) network “core” and hydrophilic oligo(ethylene oxide) methyl ether methacrylate (OEOMA) chains as “hair” were prepared by miniemulsion ATRP and a successive chain extension by aqueous ATRP from the particle surface. The addition of an inimer, 2-(α-bromoisobutyryloxy)ethyl methacrylate (HEMA-iBBr) to the P(BMA/EGDMA) network introduced more ATRP initiation sites for ATRP of OEOMA, enabling adjustment of the particle size from >400 nm to <150 nm. The miniemulsion system remained stable after the one pot synthesis of P(BMA/EGDMA)-g-POEOMA and P(BMA/EGDMA/HEMA-iBBr)-g-POEOMA, and the resulting polymer nanoparticles were re-dispersed in water for further modification. A zwitterionic monomer, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA), was grafted from P(BMA/EGDMA/HEMA-iBBr)-g-POEOMA. The resulting hairy nanoparticles provide an avenue for the design and preparation of novel nanostructured materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Tuning dispersity of linear polymers and polymeric brushes grown from nanoparticles by atom transfer radical polymerization

Molecular weight distribution imposes considerable influence on the properties of polymers, making it an important parameter, impacting morphology and structural behavior of polymeric materials. Atom transfer radical polymerization (ATRP) has established itself as a powerful tool to prepare polymers with predetermined molecular weight, preserved chain-end functionality, and low dispersity. More recently, ATRP has also been shown to provide a means to deliberately broaden molecular weight distributions, and, via retaining living chain-ends, to enable the formation of block copolymers with designed block dispersity, featuring new microstructures and potentially attractive properties. Here, similar methodologies have been developed to facilitate tuning of the dispersity of polymeric brushes grown from nanoparticles thus resulting in hybrid materials with enhanced fracture toughness and high inorganic content. Recent advances have given access to brush architectures comprised of uni- and bimodal block copolymers with unique morphologies along with interesting mechanical, thermal, and optical properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1987

Mechanisms of polymer degradation using an oxygen plasma generator

An RF oxygen plasma generator was used to produce polymer degradation which appears to be similar to that which has been observed in low Earth orbit. Mechanisms of this type of degradation were studied by collecting the reaction products in a cryogenic trap and identifying the molecular species using infrared, mass spectral, and X-ray diffraction techniques. No structurally dependent species were found from Kapton, Teflon, or Saran polymers. However, very reactive free radical entities are produced during the polymer degradation, as well as carbon dioxide and water. Reactions of the free radicals with the glass reaction vessel, with copper metal in the cold trap, and with a triphenyl phosphate scavenger in the cold trap, demonstrated the reactivity of the primary products.

Colony, Joe A.↗
DOE OSTI2021

Investigation of $\gamma$-irradiated polyvinylidene fluoride and its acute toxicity

Polyvinylidene fluoride (PVDF) is an industrial polymer with many applications. The EPR spectrum of polyvinylidene fluoride (PVDF) irradiated by γ-rays at low temperature (-196°C) provides evidence for the presence of radicals formed by loss of a fluorine atom from PVDF, and polyenyl radicals. Polyenyl radicals dominate in the polymer irradiated at room temperature. Peroxide radicals formed as a result of oxidation of primary free radicals by atmospheric oxygen are observed in the EPR spectrum of a polymer irradiated in air, or when air is introduced to a polymer irradiated in a vacuum. Carbonization and oxidation of γ-irradiated PVDF were observed in the XPS spectrum. A model for polymer degradation based on correlated G3(MP2) molecular orbital theory electronic structure calculations is described in this report. The presence of HF in the products and a low yield of products with the composition CxFyHz indicate the absence of noticeable thermal destruction of the main polymer chain upon heating to 220°C of both the initial and γ-irradiated PVDF. Although any HF released during the PVDF dehydrofluorination would be toxic, a study of the acute toxicity of γ-irradiated PVDF showed that the polymer does not exhibit toxicity after a single intragastric route of administration to mice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Modeling Synergistic Fuel Cell Membrane Degradation with Mitigating Effects of Cerium

During operation, polymer-electrolyte-membrane (PEM) fuel cells undergo mechanical and chemical degradation mechanisms, which behave synergistically and lead to accelerated membrane degradation over time. This study builds upon previous modeling work on mechanical degradation as described by a pinhole in the membrane and the effects of cerium on chemical degradation.1 By combining these two models, analysis can be carried out on the coupled degradation methods and how the mitigation effects of cerium disrupt the degradation cycle. The mechanical model represents a pinhole in the membrane using an effective void fraction, which allows for increased gas crossover through the membrane and impacts the membrane transport and mechanical properties. A microkinetic model for the chemical degradation is included in the model, including attack of the membrane polymer by hydroxyl radicals as well as the mitigation reaction for quenching of hydroxyl radicals with cerium. A concentrated solution theory approach is used to model the transport of cerium ions throughout the cell.2 The model results show how the location of cerium in the cell can be used to prevent chemical degradation. In addition, the model shows how cerium slows down the rate of pinhole growth by reducing the gas crossover and membrane thinning rate. Finally, the model can be used to optimize the distribution of cerium in the membrane and catalyst layers by balancing trade-offs between lowering degradation rates and decreasing fuel cell performance.

25 ENERGY STORAGE↗