Surface-Localized Chemically Modified Reduced Graphene Oxide Nanocomposites as Flexible Conductive Surfaces for Space Applications.

Thermoplastic polymers are a compelling class of materials for emerging space exploration applications due to their wide range of mechanical properties and compatibility with a variety of processing methods, including additive manufacturing. However, despite these benefits, the use of thermoplastic polymers in a set of critical space applications is limited by their low electrical conductivity, which makes them susceptible to static charging and limits their ability to be used as active and passive components in electronic devices, including materials for static charge dissipation, resistive heaters, and electrodynamic dust shielding devices. Herein, we explore the microstructural evolution of electrically conductive, surface-localized nanocomposites (SLNCs) of chemically modified reduced graphene oxide and a set of thermoplastic polymers as a function of critical thermal properties of the substrate (melting temperature for semi-crystalline materials or glass transition temperature for amorphous materials). Selected offsets from critical substrate temperatures were used to produce SLNCs with conductivities between 0.6-3 S/cm and surface structures, which ranged from particle-rich, porous surfaces to polymer-rich, non-porous surfaces. We then demonstrate the physical durability of these electrically conductive SLNCs to expected stress conditions for flexible conductive materials in lunar applications including tension, flexion, and abrasion with lunar simulant. Small changes in resistance (R/R0 < 2) were measured under uniaxial tension up to 20% strain in high density polyethylene and up to 500 abrasion cycles in polysulfone, demonstrating the applicability of these materials as active and passive flexible conductors in exterior lunar applications. The tough, electrically conductive SLNCs developed here could greatly expand the use of polymeric materials in space applications, including lunar exploration, micro- and nano-satellites, and other orbital structures.

Synergistic Reinforcement of Composite Hydrogels with Nanofiber Mixtures of Cellulose Nanocrystals and Chitin Nanofibers.

Simultaneous incorporation of cellulose nanocrystals (CNCs) and chitin nanofibers (ChNFs) into a polyvinyl alcohol (PVA) matrix opens possibilities for customization of more environmentally friendly composite materials. When used in tricomponent composite hydrogels, the opposite surface charges on CNCs and ChNFs lead to the construction of beneficial nanofiber structures. In this work, composite hydrogels containing CNCs, ChNFs, or their mixtures are produced using cyclic freeze-thaw (FT) treatments. When considering different compositions and FT cycling, tricomponent composite hydrogels containing a specific ratio of CNCs/ChNFs are shown to have promising mechanical performance in comparison to other samples. These results together with results from water absorption, rheological, and light scattering studies suggest that the CNC/ChNF structures produced property improvement by concurrently accessing the stronger interfacial interactions between CNCs and PVA and the longer lengths of the ChNFs for load transfer. Overall, these results provide insight into using electrostatically driven nanofiber structures in nanocomposites.

Acrylic Functionalization of Cellulose Nanocrystals with 2-Isocyanatoethyl Methacrylate and Formation of Composites with Poly(methyl methacrylate).

Cellulose nanocrystals (CNCs) derived from renewable plant-based materials exhibit strong potential for improving properties of polymers by their dispersal in the polymer matrix as a composite phase. However, the hydrophilicity and low thermal stability of CNCs lead to compromised particle dispersibility in common polymers and limit the processing conditions of polymer-CNC composites, respectively. One route that has been explored is the modification of CNCs to alter surface chemistry. Acrylic materials are used in a broad class of polymers and copolymers with wide commercial applications. Yet, the available methods for adding groups that react with acrylics to enhance dispersion are quite limited. In this work, a versatile chemical modification route is described that introduces acryloyl functional groups on CNCs that can in turn be polymerized in subsequent steps to create acrylic-CNC composites. The hydroxyl group on CNC surfaces was reacted with the isocyanate moiety on 2-isocyanatoethyl methacrylate (IEM), a bifunctional molecule possessing both the isocyanate group and acryloyl group. The resulting modified CNCs (mCNCs) showed enhanced hydrophobicity and dispersibility in organic solvent relative to unmodified CNCs. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis verified the surface modification and allowed an estimation of the degree of modification as high as 0.4 (26.7% surface hydroxyl substitution CNC). The modified CNCs were copolymerized with methyl methacrylate, and the composites had improved dispersion relative to composites with unmodified CNCs and enhanced (104%) tensile strength at 2 wt % CNC when compared to the neat poly(methyl methacrylate) (PMMA), indicating a benefit of the reactive acryloyl groups added to the CNC surface. Overall, the modification strategy was successful in functionalizing CNCs, opening possibilities for their use in organic media and matrices.

Controlling Barrier and Mechanical Properties of Cellulose Nanocrystals by Blending with Chitin Nanofibers.

Chitin nanofibers (ChNFs) and cellulose nanocrystals (CNCs) have been proposed as materials for renewable packaging with low O2 transmission that protect food, medicine, and electronics. A challenge in biomass-derived functional materials is tuning both barrier and mechanical properties, while minimizing process steps. A concept that merits additional study in this field is tuning of the barrier and mechanical properties by use of oppositely charged biomass-derived fibers, through interactions that support dense film formation. We report free-standing films formed by solution casting of blends of aqueous suspensions of CNCs and ChNFs with either low degree of acetylation (LChNFs, higher charge) or high degree of acetylation (HChNFs, lower charge). While neat CNC films had the highest O2 permeability (OP), the OP was lowered by 91% by addition of at least 25 wt % LChNFs to CNCs to an OP value near 1.7 cm3 µm/m2/d/kPa. Interestingly, blends of CNCs with less highly charged, larger HChNFs had equivalently lower OP as with LChNFs. The tensile strength and strain at break of blended ChNF/CNC films was optimal compared to neat cellulose or chitin when at least 50 wt % LChNFs or HChNFs were blended with CNCs. We show that the ability to tune properties of ChNF/CNC blends was coincident with the formation of aggregates of chitin and cellulose nanomaterials, which appear to support formation of dense layers of tortuous fiber networks.

Site-Selective Modification of Cellulose Nanocrystals with Isophorone Diisocyanate and Formation of Polyurethane-CNC Composites.

The unequal reactivity of the two isocyanate groups in an isophorone diisocyante (IPDI) monomer was exploited to yield modified cellulose nanocrystals (CNCs) with both urethane and isocyanate functionality. The chemical functionality of the modified CNCs was verified with ATR-FTIR analysis and elemental analysis. The selectivity for the secondary isocyanate group using dibutyl tin dilaurate (DBTDL) as the reaction catalyst was confirmed with (13)C NMR. The modified CNCs showed improvements in the onset of thermal degradation by 35 °C compared to the unmodified CNCs. Polyurethane composites based on IPDI and a trifunctional polyether alcohol were synthesized using unmodified (um-CNC) and modified CNCs (m-CNC). The degree of nanoparticle dispersion was qualitatively assessed with polarized optical microscopy. It was found that the modification step facilitated superior nanoparticle dispersion compared to the um-CNCs, which resulted in increases in the tensile strength and work of fracture of over 200% compared to the neat matrix without degradation of elongation at break.

Copolymer-mediated synthesis of hydroxyapatite nanoparticles in an organic solvent.

The objective of this research is to develop a nanoparticle synthesis scheme that controls nanoparticle shape and surface chemistry concomitantly. Specifically, a method to synthesize hydroxyapatite nanoparticles using a dispersed block copolymer template is explored, which produces spherical and needle-shaped nanoparticles, and at the end of the synthesis, the block copolymer is retained as a surface coating on the nanoparticles. This strategy has been used previously with double-hydrophilic block copolymers (DHBCs) as the dispersed template; however, in this work, an alternative block copolymer chemistry is explored in an effort to extend this method to synthesis in organic solvents, producing nanoparticles that are organophilic instead of hydrophilic. The hydroxyapatite nanoparticles were synthesized using poly(methyl methacrylate)-b-poly(methacrylic acid) (PMMA-b-PMAA) as the dispersed template and tetrahydrofuran as the solvent. The synthesis proceeds following the ionization of the PMAA block of the copolymer and association between this ionized group and the calcium precursor ions. To investigate the degree of shape control available, the concentration of block copolymer solution and the amount of precursor were systematically varied, and the synthesized HAp nanoparticles were characterized. SEM images showed that needle and spherical HAp nanoparticles could be synthesized by changing the block copolymer concentration. TGA, FT-IR, and XRD results indicated that the block copolymer used for synthesis remained on the HAp particle surface. Overall, these results indicate that the shape of the nanoparticles produced by this method was related to the Ca(2+)/COO(-) mole ratio used during synthesis, similar to results obtained with DHBC template synthesis. The qualitative agreement between the shape control mechanisms in the two synthesis schemes suggests that this relationship could be general to the overall synthesis scheme and provide a mechanism for controlling nanoparticle shape with many block copolymer chemistries.

Enabling nanoparticle networking in semicrystalline polymer matrices.

Among the physical and chemical attributes of the nanocomposite components and their interactions that contribute to the ultimate material properties, nanoparticle arrangement in the matrix is a key contributing factor that has been targeted through materials choices and processing strategies in numerous previous studies. Often, the desired nanocomposite morphology contains individually dispersed and distributed nanoparticles. In this research, a phase-segregated morphology containing nanoparticle networks was studied. A model nanocomposite system composed of calcium phosphate nanoparticles and a poly(3-hydroxybutyrate) matrix was produced to understand how polymer crystallization and crystal structure can facilitate the formation of a phase-segregated morphology containing nanoparticle networks. Two chemically similar calcium phosphate nanoparticle systems with different shapes, near-spherical and nanofiber, were synthesized for use in the nanocomposites. The different shapes were used independently in nanocomposites in an attempt to understand the effect of the nanoparticle shapes on crystallization-mediated nanoparticle network formation. The resulting nanocomposites were characterized to establish the effects of component interactions on the polymer structure. Additionally from the viscoelastic properties, structure-property relationships in these materials can be defined as a function of nanoparticle shape and concentration. The results of this research suggest that when the nanocomposite components are not strongly interacting, polymer crystallization may be used as a forced assembly method for nanoparticle networks. Such a methodology has applications to the design of functional polymer nanocomposites such as biomedical implant materials and organic photovoltaic materials where judicious choice of nanoparticle-polymer pairs and control of polymer crystal nucleation and growth processes could be used to control the length scale of phase segregation.