Meisha L. Shofner

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.

Polymer nanocomposite processing, characterization, and applications 2012

1 Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA 2 Department of Polymer Engineering, The University of Akron, 250 South Forge Street, Akron, OH 44325, USA 3 DST/CSIR National Centre for Nano-Structured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa 4 School of Mechanical and Aerospace Engineering, Queen’s University, Belfast, UK

Synthesis of polymer-decorated hydroxyapatite nanoparticles with a dispersed copolymer template

In this research, a nanoparticle synthesis method was explored in order to produce nanoparticles with different shapes and polymer-coated surfaces. Specifically, a poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA) block copolymer was employed as a dispersed template for the controlled synthesis of hydroxyapatite (HAp) nanoparticles in water. Six different synthesis conditions were used to produce different nanoparticle shapes, and characterization of these nanoparticles was performed to understand the level of morphological control available with this synthesis method. SEM images showed that needle and spherical nanoparticles could be produced by changing the block copolymer concentration. The shape of the nanoparticles produced by this synthesis method was shown to be related to the relative concentrations of the calcium precursor and the block copolymer used. Significant quantities of the block copolymer used for synthesis remained on the particle surface, constituting a functionalized moiety and providing an opportunity to control component interactions in composites.

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.

Membranes for Kraft black liquor concentration and chemical recovery: Current progress, challenges, and opportunities

ABSTRACT Membranes can significantly reduce energy consumption during concentration of black liquor (BL) in the Kraft papermaking process, but the harsh conditions (pH ~12, 80°C–95°C, ~15 wt% solids) make this challenging. We elucidate challenges and opportunities for membranes in BL applications. We critically review membrane materials, processes, and operational modes investigated in the literature. Future advances will involve fabrication of higher-rejecting (≥95% lignin and inorganics), BL-resistant, NF, and RO membranes. Opportunities exist for molecular sieving and electrically driven membranes to recover other valuable chemicals such as carboxylic acids. We also discuss the economics of BL concentration with a single-stage membrane process.

Fluorinated Single Wall Nanotube/Polyethylene Composites for Multifunctional Radiation Protection

Abstract : Fluorinated Single Wall Nanotubes (f-SWNTs) have been processed in polyethylene by an incipient wetting technique to achieve a well dispersed nanocomposite for radiation protection. In some cases, samples were further processed using the rapid prototyping method of extrusion freeform fabrication. Composites were exposed to 40 MeV proton radiation with a flux of about 1.7x10(exp 7) protons/sq cm/sec to a total fluence of 3x10(exp 10) protons/sq cm. This exposure is consistent with a long-term space mission in low earth orbit. The samples were evaluated by means of Raman spectroscopy and thermogravimetric analysis (TGA). These results were compared to the unexposed composite and unfilled polymer samples. This study has focused on the stability of the nanotube composites when exposed to radiation and prior to hydrogen exposure. It was shown that the stability of the functional group is not constant with SWNTs produced by different processes and that radiation exposure is capable of defluorinating SWNTs in polyethylene.

Effect of Matrix Crystallinity on Nanocomposite Properties

The properties of two nanocomposite systems with semi-crystalline matrices were reported. Differences in achievable dispersion and thermomechanical properties were related to the matrix crystallinity. A lesser amount of matrix crystallinity aided in dispersion and led to greater mechanical reinforcement at temperatures above the glass transition.Copyright

Hierarchical Composites Containing Carbon Nanotubes

Hierarchical composites containing carbon nanotubes (CNTs) have the potential to possess improved multifunctional properties as well as unique sensing/active capabilities due to the inherent properties of CNTs (i.e., mechanical, electrical, and thermal). The purpose of this chapter is to review the current state of the art in this research area and highlight opportunities for future research. Specifically, three construction schemes used to produce CNT hierarchical composites are reviewed: dispersed systems, fiber coatings, and CNT structures. In these construction schemes, CNTs are used as a performance additive to reduce matrix mobility, as a sizing to improve adhesion between the matrix and the microscale fiber, and as the building blocks of structures such as fibers to more fully exploit the mechanical properties of CNTs, respectively. To date, research results have indicated that these strategies produce composites with improved properties, and most frequently those improved properties are mechanical properties such as strength, modulus, and fracture toughness. Based on these results, further activities aimed at understanding these property increases in terms of modeling as well as more research activity aimed at producing CNT fibers and exploiting other CNT properties will lead to improved approaches to composite design which merit the routine use of CNTs in structural composites.