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Diol-functionalized trisaminocyclopropenium (TACP) carbocations were used as chain extenders in a two-step synthesis of a segmented polyurethane. Differential scanning calorimetry demonstrated significant differences in the crystallization behavior of the poly(tetramethylene oxide) soft segment when minor changes were made to the TACP structure and when compared to a control that was chain extended with butane diol. Fourier transform infrared spectroscopy was used to characterize the different level of hydrogen bonding in the polymers and showed that the bulky, charged TACP chain extender limited hydrogen bonding interactions when compared to the control. Dynamic mechanical analysis was used to probe the thermomechanical behavior of polymers that showed that the TACP-containing polymers were much more resistant to flow at high temperatures when compared to the control. Small-angle X-ray scattering showed a phase separated morphology for all the polymers tested. Tensile testing of the TACP polyurethanes demonstrated an elastic response over a wide range of strain, followed by a significant strain hardening. These results suggest a morphology of ionic aggregates rather than hard segment physical cross-links.

The effects of incorporating metal‐binding ligands as chain extenders in polyurethane elastomers were investigated. Segmented polyurethanes based on 2 kDa poly(tetramethylene oxide) (PTMO) and 4,4‐methylenebis(cyclohexyl isocyanate) were polymerized using a two‐step process in which 2,6‐bis(1‐ethyl‐5‐(methoxymethyl)‐1H‐benzo[d]imidazol‐2‐yl)pyridine was added as a chain extender. The resulting polyurethanes were then metallated using stoichiometric amounts of Zn(II) metal salts with different counterions. The resulting metallopolymers have substantially improved Young's moduli, increased failure stress, and improved thermomechanical behavior. The materials were microphase‐separated into anisotropic hard domains within a PTMO matrix. Simultaneous small‐angle X‐ray scattering and tensile testing revealed the minority hard segment domains remain relatively intact during elongation, likely due to the strength of the metal–ligand complex. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1744–1757

Covalent organic frameworks (COFs) are highly modular porous crystalline polymers that are of interest for applications such as charge-storage devices, nanofiltration membranes, and optoelectronic devices. COFs are typically synthesized as microcrystalline powders, which limits their performance in these applications, and their limited solubility precludes large-scale processing into more useful morphologies and devices. We report a general, scalable method to exfoliate two-dimensional imine-linked COF powders by temporarily protonating their linkages. The resulting suspensions were cast into continuous crystalline COF films up to 10 cm in diameter, with thicknesses ranging from 50 nm to 20 μm depending on the suspension composition, concentration, and casting protocol. Furthermore, we demonstrate that the film fabrication process proceeds through a partial depolymerization/repolymerization mechanism, providing mechanically robust films that can be easily separated from their substrates.

Reclaimed materials such as waste plastics can be utilized in additive manufacturing to improve the self-reliance of warfighters on forward operating bases by cutting costs and decreasing the demand for the frequent resupplying of parts by the supply chain. In addition, the use of waste materials in additive manufacturing in the private sector would reduce cost and increase sustainability, providing a high-value output for used plastics. Experimentation is conducted to process polyethylene terephthalate bottles and packaging into filament that can then be used for additive manufacturing methods like fused filament fabrication, without the use of additives or modification to the polymer. The chemistry of different polyethylene terephthalate recycled feedstocks was evaluated and found to be identical, and thus mixed feedstock processing is a suitable approach. Rheological data showed drying of the recycled polyethylene terephthalate led to an increase in the polymer’s viscosity. Thermal and mechanical properties were evaluated for filament with different processing conditions, as well as printed and molded specimens. Crystallinity ranged from 12.2 for the water cooled filament, compared to 24.9% for the filament without any active cooling. Tensile results show that the elongation to failure was similar to an injection molded part (3.5%) and tensile strength of 35.1 ± 8 MPa was comparable to commercial polycarbonate-ABS filament, demonstrating the robustness of the material. In addition, three point bending tests showed a similar load at failure for a select long-lead military part printed from the recycled filament compared to parts printed from commercial filament. Thus filament from recycled polyethylene terephthalate has the capability for replacing commercial filament in printing a diverse range of plastic parts.

Understanding multicomponent diffusion in polymers on the molecular-scale could lead to optimization of many practical processes. One important example is the removal of a toxic chemical (penetrant) from polyurethanes, which serve as the binder in many coatings technologies. This work is an equilibrium molecular dynamics (MD) study to characterize the molecular-scale hydrogen bonding (H-bonding) interactions in ternary penetrant, solvent, and polyurethane systems, and how these H-bonds influence the corresponding diffusivities. Homomorphic series of penetrant and solvent species in which molecular size and shape are kept constant while varying polarity or number of H-bonding sites are used to study the influence of hydrogen bond probability and strength on diffusivity. It is found that H-bonding between all species in the ternary mixture as well as penetrant-solvent collisions play a role in determining penetrant diffusivity. The findings provide insight into solvent selection criteria to increase the diffusivity of H-bonding penetrants that are absorbed in polyurethanes for extraction and decontamination applications.

A fundamental understanding of chemical interactions and transport mechanisms that result from introducing multiple chemical species into a polymer plays a key role in the development and optimization of membranes, coatings, and decontamination formulations. In this study, we explore the solvent-assisted desorption of a penetrant (2,5-lutidine) in polyurethane with aprotic (acetonitrile) and protic (methanol) solvents. Chemical interactions between solvent, penetrant, and polymer functional groups are characterized via time-resolved Fourier transform infrared spectroscopy (FTIR) during single and multicomponent exposures. For both solvents, an increase in the extraction rate of the penetrant is observed when the solvent is applied during desorption. Inspection of the FTIR spectra reveals two potential mechanisms that facilitate the enhanced desorption rate: (1) penetrant/solvent competition for hydrogen donor groups on the polymer backbone and (2) disruption of the self-interaction (cohesive forces) between neighboring polymer chains. Finally, the aprotic solvent is found to generate an order of magnitude greater desorption rate of the penetrant, which is attributed to a greater disruption of the self-interaction during penetrant desorption compared to the protic solvent and the inability of an aprotic solvent to form larger and potentially slower penetrant-solvent complexes.

Poly(urethane-urea) (PUU) has been infused into ultrahigh volume fraction carbon nanotube (CNT) forests using a heat-curable polymer formula. Polymer nanocomposites with carbon nanotube volume-fractions of 1%, 5%, 10%, 20%, and 30% were fabricated by overcoming densification and infusion obstacles. These polymer nanocomposites were nanoindented quasi-statically and dynamically to discern process-structure-(mechanical) property relations of polymerizing PUU in such densely-packed CNT forests. A 100× increase in indentation modulus has been observed, which is attributed not only to CNT reinforcement of the matrix, but also to molecular interactions in the matrix itself. Quasi-static elastic moduli ranging from 10 MPa–4.5 GPa have been recorded. Storage modulus for all materials is found to track well at loadings of 200 Hz, with little effect observed from increasing CNT volume fraction. Keywords: carbon nanotubes; polymer nanocomposites; polyurethane urea; self-assembly, United States. Army Research Office (Contract W911NF-13-D-0001)

Polymers prepared by ring opening methathesis polymerization contain double bonds along the backbone and are susceptible to oxidation during high temperature processing or device operation. To overcome this, we report a simple method to improve the oxidative stability of ROMP based polymers by inclusion of a small percentage of monomer functionalized with a radical scavenger in the feedstock. The oxidation induction temperature was improved from 111°C up to 262°C when 8 mol% of the functionalized comonomer was included in the feedstock. Modest improvements in the thermal decomposition temperature were also observed.

This work incorporates ML complexes as unbound entities that interact with ML complexes bound to the backbone of the polymer. The π–π interactions and Coulombic forces between bound and unbound ML complexes hold the ML-rich phase together and result in improved mechanical properties over polymers containing only the bound ML complexes. The ML-rich phase formed ordered, cylindrical domains. The storage modulus, surface elastic modulus, and high temperature stability of these metallopolymers increased with increasing concentration of ML complex in the polymer while an optimal concentration and morphology are necessary to improve the strength and creep resistance of the polymer. Ultimately, the successful addition and patterning of unbound ML complexes as a hard phase in a polymer matrix provides an important template for the design of a new type of supramolecular nanocomposite.

Epoxies and polyurethanes are both important classes of materials with wide-ranging applications. The merging of both chemistries presents new opportunities to investigate materials with potentially unique or improved performance. In this work, a series of formulations with varying levels of epoxy and cyclic carbonate monomers were reacted with a multifunctional amine to produce network polymers with hybrid functionality. The spectroscopic, swelling, thermal-mechanical, tensile, and adhesive behaviors were evaluated. The materials performed as expected based on the proposed network structure as determined through small-molecule model studies. In particular, the hybrid network polymers performed admirably as adhesives with significantly improved performance over an epoxy–amine control.

Evaluation and prediction of multicomponent diffusion in polymers is an important area of research since it occurs in several fields associated with mass transport including membrane separations, preparation of polymer films, and material decontamination. The functional dependence of the diffusivity on the composition in a multicomponent system is critical for accurately predicting transport in these processes. Here, the composition dependent diffusivity of 2,5-lutidine in polyurethane with acetonitrile as a co-penetrant was experimentally determined using Fourier transform infrared spectroscopy (FTIR) to acquire differential sorption curves for lutidine across the composition space and fitting a multicomponent diffusion model to the data. The free volume theory of diffusion was used to develop a mathematical expression for the composition-dependent diffusivity, and shows good agreement with the experimental data for both single and multicomponent cases. The lutidine diffusivity was found to vary up to two orders of magnitude with ACN concentration, and the importance of capturing this dependence quantitatively is highlighted by applying the multicomponent diffusion model with the best-fit values to predict previously published data for lutidine desorption from polyurethane.

The organocatalyzed aminolysis of cyclic carbonates to form (poly)hydroxyurethanes is an important reaction as an alternative to isocyanate based chemistries. In an effort to increase reaction rates between cyclic carbonates and amines to lower cure times and increase molecular weights, various organocatalysts were surveyed. Reaction rates between monofunctional model cyclic carbonates and amines were determined in the presence of a variety of organocatalysts, which operate under different mechanisms to promote the reaction. Of the catalysts investigated, TBD was the most potent, presumably due to its bifunctional activity. TBD was also used to promote step-growth polymerization between difunctional cyclic carbonates and amines. The polymers produced in the presence of TBD at room temperature were much higher in molecular weight than polymers produced in the absence of catalyst at both room temperature and at 80 °C.

This work studies the properties of poly(n-butyl acrylate) functionalized with 2,6-bis(1′-methylbenzimidazolyl)pyridine ligand and cross-linked with either copper(II), zinc(II), or cobalt(II) metal ions. Because of phase separation between the metal–ligand complex and the polymer matrix, these polymers have a rubbery plateau modulus that is 10 times higher than expected based on the theory of rubber elasticity. Differences in the metal–ligand bond strength influence the mechanical behavior at high temperature and strains. Because of the particularly weak bond strength associated with the copper–ligand bond, the metallopolymer containing copper degrades at a lower temperature and has lower yield strength, ultimate tensile strength, and creep resistance than polymers containing cobalt and zinc. To tune the properties of the polymer further, a polymer is made with both copper and cobalt ions. The hybrid polymer combines the properties of the stiffer cobalt-containing polymer with the more compliant copper-co...

We present calculations on metal–ligand complexes for the evaluation of mechanical properties as they pertain to the inclusion in polymer-linked supramolecular complexes. To this end, we investigate the energy profiles of stretching various complexes according to external forces exerted on each complex via the attached polymer strands. Zn 2+ and Fe 2+ complexated by 2,6-bisbenzimidazolyl-pyridine (BP) were considered in the presence of tetrafluoro borate. We find that the yield characteristics are subject to a complex interplay of steric and electronic effects of the ligands and metal center.

Methods for removing the residual Grubbs' third generation catalyst from polymers prepared by ring opening metathesis polymerization are reported. Two strategies were investigated for reduction of the residual catalyst in the final polymer product. The first strategy involved the use of heterogeneous functionalized particles to scavenge the catalyst from the solution. Filtration of the particles followed by precipitation produced polymers with 10–60 ppm residual catalyst, depending on the type of particle used, surface functional groups, and number of equivalents. The second strategy used small organic molecules that could coordinate to the metal species and modify the solubility of the catalyst, facilitating partitioning of the catalyst into the precipitation solvent. Several types of molecules with varied functionality reduced the residual catalyst level to 30–120 ppm, depending on the loading. Hydrogenation of the polymer backbone followed by precipitation lowered the residual Ru content from 195 ppm to 10 ppm, suggesting that the difficulty of completely removing the catalyst could be a result of coordination of the metal species to the double bonds in the polymer backbone. Reducing the amount of trace catalyst significantly improved the oxidative stability of the polymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

A method for producing well-defined azobenzene functionalized polymers via ring-opening metathesis polymerization (ROMP) is described. ROMP of an isomerically pure exo-norbornene ester tethered to an azobenzene unit in the presence of the third generation Grubbs' catalyst was investigated in detail. The polymerization was determined to be living on the basis of a linear relationship between Mn and [M]/[I] and the ability to quantitatively produce chain extended homopolymers. The polymer's physical response to plane-polarized light was studied in thin films and latex particles. A relief grating was inscribed on the surface of a thin film by irradiation with an interference pattern of linearly polarized light. The same material was used to prepare isotropic colloidal particles of the polymer. These latex particles were transformed to ellipsoids in response to irradiation by linearly polarized light perpendicular to the sample surface.

To mimic the three-dimensional (3-D) globular architecture resulting from the precise positioning of hydrophobic/hydrophilic domains (blocks) of naturally occurring proteins, water-soluble linear and star homopolymers of N,N'-dimethylacrylamide (DMA) were synthesized with prescribed molecular weights via reversible addition-fragmentation chain transfer (RAFT) polymerization and subsequently used as macro chain transfer agents for block copolymerization with N-isopropylacrylamide (NIPAM). For the star block copolymers, the interior block consisted of NIPAM while the exterior block was DMA. Since polyNIPAM thermally switches from hydrophilic to hydrophobic, the 3-D solution conformations of the polymers were studied as a function of temperature using differential scanning calorimetry (DSC), static light scattering (SLS), and dynamic light scattering (DLS). The polymers were observed to form monodisperse aggregates in an aqueous pH 4 buffer solution when heated above the lower critical solution temperature (LCST) of polyNIPAM. The temperature at which the polymers aggregated and the size of the aggregates were dependent on the NIPAM block length and the core architecture. A simple model based on an optimal area per headgroup was used to analyze our experimental findings and was useful for predicting the final size and molecular weight of the aggregates formed.

This report describes the construction of an optical test system to measure the electro-optic coefficients of poled polymers directly. The poling process in polymers is the spontaneous orientation of the dissolved dye molecule, the chromophore, in the presence of an applied electric field. This spontaneous orientation confers upon the poled polymer a variety of useful optical effects, the linear electro-optic effect being chief among them. The optical test system measures the electro-optic coefficient directly by measuring the relative retardance of one polarization, the p-polarization, relative to the other polarization present, the s-polarization. This method has advantages over previous methods that have relied on measuring the very weak second-harmonic signal emitted from poled polymers. This report describes the measurement of the electro-optic coefficient of a number of electro-optic chromophores synthesized in the Weapons and Materials Research Directorate (WMRD) and intended for use in electro-optic modulation devices.

This report summarizes efforts to date toward preparing electro-optical polymer by ring opening metathesis polymerization. Initial work involved preparing model electro-optical polymers with tunable glass transitions. Random copolymerization of a norbornene ester tethered to E-O chromophore Disperse Red 1 (DR1) with various levels exo-N-Phenylnorbornene-5,6-dicarboxamide (NDI) and 5-carbomethoxy-2-norbornene produced materials with controlled Tgs. The Tg could be varied from 89 to 189 deg C by modifying the feed ratio of the 3 monomers enabling materials to be produced according to desired processing conditions. The polymers were tested for electro-optical activity but displayed very little or no alignment along an electric field. The lack of activity is potentially the result of residual polymerization catalyst remaining in the final polymer product. To probe this further, a comprehensive investigation was performed to discover purification methods capable of reducing residual catalyst to acceptable levels. Initially, baseline levels were established for polymer purified via traditional methods involving multiple precipitations. The residual catalyst levels ranged from 240 to 140 ppm depending on the number precipitations. Several different strategies were employed to further reduce the trace metal remaining in the final polymer product. The first strategy involved the use of catalyst modifiers and various adsorbents. None of the methods explored reduced the catalyst level below 100 ppm while significantly reducing the yield of polymer due to adsorption of the polymer. The second strategy involved the use of heterogeneous functionalized particles to scavenge the catalyst from the solution. Filtration of the particles followed by precipitation produced polymers with 10 60 ppm residual catalyst, depending on the type of particle used, surface functional groups, and number of equivalents.

The development of materials with combined mechanical properties remains a significant challenge in polymer synthesis. The preparation of such materials could potentially impact a wide range of Army applications. In this work, bio-inspired metallo- supramolecular polymers were designed, synthesized, and characterized. We anticipate these polymers will display enhanced mechanical properties due to the presence of intermediate strength metal-ligand bonds present in the polymer backbone, which forms a compacted, modular molecular topology. The 2,6-bisbenzimidazolylpyridine ligand was prepared and incorporated into a model polymer backbone via thiol-ene click chemistry. The resulting polymer was thoroughly characterized by 1H Nuclear Magnetic Resonance (NMR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Binding of the ligand with Zn2+ and Co2+ was studied using ultraviolet/visible (UV/Vis) spectroscopy, which demonstrated a 2:1 ligand-to-metal ratio.

We investigate the ability of a cationic comb polymer composed of a poly(trimethylammonium iodide ethyl methacrylate) (PTMAM) backbone and uncharged poly(ethylene glycol) (PEG) teeth to stabilize aqueous silica suspensions of varying ionic strength and pH. Both PTMAM-g-PEG and its homopolymer backbone, PTMAM, are synthesized via reversible addition-fragmentation chain transfer followed by quaternization of the pendant amine groups with methyl iodide. Through a combination of polymer adsorption, zeta potential, and sedimentation measurements as well as confocal imaging of sediment structures, we find that PTMAM-g-PEG imparts stability over a broad range of solution conditions, where pure PTMAM fails.