@article{appaw_gilbert_khan_2007, title={Viscoelastic behavior of cellulose acetate in a mixed solvent system}, volume={8}, ISSN={["1526-4602"]}, DOI={10.1021/bm0611681}, abstractNote={The effect of increasing water composition on the rheological and microstructural behavior of a ternary cellulose acetate (CA)/N,N-dimethylacetamide (DMA)/water system is examined. Addition of water to the CA/DMA system results in enhanced steady shear viscosity and dynamic viscoelastic properties and ultimately to phase-separated gel formation. The changes in dynamic rheological behavior of the system during gelation correlate well with the combined solubility parameter (delta) and, in particular, the Hansen hydrogen-bonding solubility parameter index (delta(h)) of the solvent system, suggesting hydrogen-bonding interactions may be the major route initiating the sol-gel process. For all gels studied, the elastic modulus and the critical stress to yield shifts to higher values with increasing CA concentration and/or water content. In addition, the elastic modulus exhibits a power-law behavior with water content, with the same power-law exponent observed for gels containing different CA concentrations. Addition of water leads to formation of a denser gel network, as evidenced from direct visualization of the gel microstructure through confocal microscopy.}, number={5}, journal={BIOMACROMOLECULES}, author={Appaw, Collins and Gilbert, Richard D. and Khan, Saad A.}, year={2007}, month={May}, pages={1541–1547} }
 @article{bastidas_venditti_pawlak_gilbert_zauscher_kadla_2005, title={Chemical force microscopy of cellulosic fibers}, volume={62}, ISSN={0144-8617}, url={http://dx.doi.org/10.1016/j.carbpol.2005.08.058}, DOI={10.1016/j.carbpol.2005.08.058}, abstractNote={Atomic force microscopy with chemically modified cantilever tips (chemical force microscopy) was used to study the pull-off forces (adhesion forces) on cellulose model surfaces and bleached softwood kraft pulp fibers in aqueous media. It was found that for the –COOH terminated tips, the adhesion forces are dependent on pH, whereas for the –CH3 and –OH terminated tips adhesion is not strongly affected by pH. Comparison between the cellulose model surfaces and cellulosic fibers under our experimental conditions reveal that surface roughness does not affect adhesion strongly. X-ray photoelectron spectroscopy (XPS) and Fourier Transformed Infrared (FTIR) spectroscopy reveal that both substrate surfaces have homogeneous chemical composition. The results show that chemical force microscopy can be used for the chemical characterization of cellulose surfaces at a nano-level.}, number={4}, journal={Carbohydrate Polymers}, publisher={Elsevier BV}, author={Bastidas, J and Venditti, R and Pawlak, J and Gilbert, R and Zauscher, S and Kadla, J}, year={2005}, month={Dec}, pages={369–378} }
 @book{kadla_gilbert_venditti_kubo_2004, place={Washington, DC}, title={Porous fibers from natural/synthetic polymer blends}, volume={6,765,028}, number={2004 July 20US6765028B2}, institution={U.S. Patent and Trademark Office}, author={Kadla, J. F. and Gilbert, R. D. and Venditti, R. A. and Kubo, S.}, year={2004}, month={Jul} }
 @article{dai_gilbert_kadla_2004, title={Synthesis, characterization, and mesophase formation of phenylacetoxy cellulose and its halogenated derivatives}, volume={5}, ISSN={["1526-4602"]}, DOI={10.1021/bm034245q}, abstractNote={A series of phenylacetoxy cellulosics with degrees of substitution (DS) between 1.4 and 3.0 and different halogenation (2-chloro, 3-chloro, 4-chloro, 2,4-dichloro, 3,4-dichloro, and 4-bromo) were synthesized. All the prepared phenylacetoxy cellulosics were soluble in dimethylformamide (DMF) and DMAc. The solubility increased with increasing DS. Mesophases were observed for all of the phenylacetoxy cellulosics with low to medium DS (DS < 2.5) in DMF and DMAc. Non- or mono-halogeneated phenylacetoxy cellulosics with high DS (DS > 1.9) were soluble in methylene chloride (CH2Cl2), whereas those with very low DS or di-halogenation on the phenyl ring were only slightly swollen or partially soluble in CH2Cl2. Non- and mono-halogenated phenylacetoxy cellulosics were soluble in DMSO and formed liquid crystals regardless of the DS, in contrast to CH2Cl2 solutions which display liquid crystalline behavior at medium to high DS (DS > 1.9) only. The solubility of the di-halogenated phenylacetoxy cellulosics in DMSO was limited to approximately 40 wt %.}, number={1}, journal={BIOMACROMOLECULES}, author={Dai, QZ and Gilbert, RD and Kadla, JF}, year={2004}, pages={74–80} }
 @article{kadla_kubo_venditti_gilbert_compere_griffith_2002, title={Lignin-based carbon fibers for composite fiber applications}, volume={40}, ISSN={0008-6223}, url={http://dx.doi.org/10.1016/S0008-6223(02)00248-8}, DOI={10.1016/S0008-6223(02)00248-8}, abstractNote={Carbon fibers have been produced for the first time from a commercially available kraft lignin, without any chemical modification, by thermal spinning followed by carbonization. A fusible lignin with excellent spinnability to form a fine filament was produced with a thermal pretreatment under vacuum. Blending the lignin with poly(ethylene oxide) (PEO) further facilitated fiber spinning, but at PEO levels greater than 5%, the blends could not be stabilized without the individual fibers fusing together. Carbon fibers produced had an over-all yield of 45%. The tensile strength and modulus increased with decreasing fiber diameter, and are comparable to those of much smaller diameter carbon fibers produced from phenolated exploded lignins. In view of the mechanical properties, tensile 400–550 MPa and modulus 30–60 GPa, kraft lignin should be further investigated as a precursor for general grade carbon fibers.}, number={15}, journal={Carbon}, publisher={Elsevier BV}, author={Kadla, J.F and Kubo, S and Venditti, R.A and Gilbert, R.D and Compere, A.L and Griffith, W}, year={2002}, pages={2913–2920} }
 @article{kadla_kubo_venditti_gilbert_2002, title={Novel hollow core fibers prepared from lignin polypropylene blends}, volume={85}, ISSN={["0021-8995"]}, DOI={10.1002/app.10640}, abstractNote={Polymer blending is a convenient method to develop products with desirable properties. The chemical and physical properties of the polymer blends are dependent on monomer type(s), molecular weight, and distribution of the respective polymers. 1 Most polymers are immiscible due to a low entropy of mixing.1-3 Only through specific intermolecular interactions can favorable polymer blending occur and composite materials with desirable properties be produced. Nonetheless, a large number of technologically interesting polymers are multiphase inhomogeneous materials.4 Properties such as prevention and control of gas and liquid permeation, gas and liquid adsorption/desorption and transmission, and reflection of light are dependent on phase behavior, and more specifically on the size of the domains within the material. Lignin, second only to cellulose in natural abundance, is an amorphous natural polymer existing in the cell wall of plants. 5 Its utilization in solid material systems is constrained by the extensive crosslinking, strong intramolecular interactions, and high molecular weight of most lignins, which upon heating decompose rather than soften and flow. However, through polymer blending6-13 or lignin derivatization13-19 these interactions are disrupted, thus altering the lignin's viscoelastic properties and allowing for flow. Recently, we have developed lignin fibers for carbon fiber applications through thermal spinning of lignin-synthetic polymer blends. 20 Through careful control of processing conditions, miscible continuous fibers are produced.21 However, fibers with core-shell morphology can be formed through manipulation of processing conditions and/or blend composition. The premise behind this method for the production of porous hollow-core lignin fibers is based on the differences in thermal stabilities of polymers (Scheme 1).22 This method uses selected polymers with different thermal stabilities, i.e., one tends to crosslink at high temperatures while the other one melts and flows from within the fiber. If the affinities of the two polymers are weak enough, they will form an immiscible two phase material. Thermal treatment will produce pores because of the melting or pyrolysis of the low melting polymer. At high levels of the low melting polymer, hollow fibers may be formed. Controlling the structure of the polymer blends will provide a way to obtain fibers with tailored porous hollow core morphology. Production of porous hollow-core fibers from the thermal treatment of immiscible polymer blended fibers containing a meltable polymer and a thermally stable polymer. In this study we have selected a commercial technical lignin and polypropylene as the thermally stable polymer and thermally unstable one, respectively. Here we report a preliminary study on the porous hollow-core fibers produced from this method. Hardwood Kraft lignin, HWKL (Indulin® AT—Westvaco Corp.) was desalted and thermally processed prior to fiber blending using a previously published method, 20 and mechanically blended with polypropylene (PP, syndiotactic, M̄w ∼127,000, Aldrich). The blend ratios were set at 87.5/12.5, 75/25, 50/50 (HWKL/PP) by weight. Control fibers were made with no PP and no lignin. The samples were then extruded in an Atlas mixer/extruder with a 1/32-inch spinneret to produce fine fibers. The optimal temperatures for fiber production were obtained by slowly increasing the temperature until fiber formation occurred. Fiber spinning was then performed isothermally at that temperature. The spinning temperature is closely related to the molten viscosity of the sample; a low spinning temperature is indicative of a low molten viscosity. The spinning temperature was 220°C for the HWKL and did not change with increasing PP content. Phase immiscibility in the lignin-PP blend is shown in Figure 1, in which a melting point (Tm) and glass transition point (Tg) of the PP phase and a Tg of the lignin phase are distinctly observed at all ratios of lignin/PP. Further, the Tg of the lignin and PP phases appear independent of the lignin/PP ratio, further indicating negligible mixing of the two polymers. Differential scanning calorimetry profiles of lignin-PP fibers of varying lignin/PP ratios. The spun fibers were heated to 250°C to remove the polypropylene and induce crosslinking of the lignin phase. Upon heating, the PP flowed from the fiber and was visually observed to pool outside of the fiber. Fiber stability was dependent on heating rate and PP blend content. Increasing the heating rate above 1.5°C min−1 resulted in the fibers in contact with one another fusing together. As the temperature increased at a heating rate below 1.5°C min−1 the Tg of the lignin increases faster than the temperature, maintaining the material in the glassy state (Tg > T): nontacky. At heating rates above 1.5°C min−1, the crosslinking reactions are not able to maintain Tg > T, and the material devitrifies entering the liquid rubbery state, tacky, and thus fuse together. Gillham and coworkers have described such phenomena in continuous heating transformation (CHT) diagrams. 23 Thus, the thermoplastic character of the lignin is changed to thermosetting, enabling the lignin fibers to maintain fiber form while the PP is removed. Figure 2 shows the HWKL/PP (75/25) fibers before (green fibers) and after (crosslinked fibers) thermal treatment. Stretching of the green fibers (magnified inset) reveals the polypropylene phase as distinct “strings” dispersed throughout the lignin fiber. In the corresponding thermally treated fibers the removal of the polypropylene phase(s) produces the observed porous microstructure. Scanning electron micrographs of HWKL/PP (75/25) fibers. The fiber to the left is prior to thermal treatment. It has been stretched to produce fractures along the fiber. The inset shows the “stringy” polypropylene phase dispersed throughout the lignin phase at a point of fracture. The fiber to the right is after thermal treatment. It has been cut at a 30° angle with respect to the fiber axis to show the extent of the porous structure and the hollow-core nature of the fiber. In this article, we described the production of porous hollow fibers made from a wood-based biopolymer and a recyclable petrochemical polymer. It is expected that these fibers relative to solid lignin fibers will have high specific surface area and a high flexural strength/mass ratio due to the lignin shell/hollow core morphology. We are now studying the effect of the mixing ratio, molecular weight and temperature on the dispersion of PP in the lignin matrix, and the relation of the initial dispersion and the pore size distribution in the final product fibers. Production of activated porous carbon fibers is also a current subject and research is underway. The authors would like to thank Oak Ridge National Laboratories and NC State University for the financial support. SEM photographs were courtesy of Oak Ridge National Laboratories.}, number={6}, journal={JOURNAL OF APPLIED POLYMER SCIENCE}, author={Kadla, JF and Kubo, S and Venditti, RA and Gilbert, RD}, year={2002}, month={Aug}, pages={1353–1355} }
 @article{stauffer_venditti_gilbert_kadla_2002, title={Removing paraffin-based wax coatings from old corrugated containers using supercritical carbon dioxide}, volume={83}, ISSN={0021-8995 1097-4628}, url={http://dx.doi.org/10.1002/app.10248}, DOI={10.1002/app.10248}, abstractNote={Supercritical carbon dioxide (SC-CO 2 ) extractions of paraffin-based wax coatings from saturated and curtain-coated old corrugated containers (OCC) are reported. Extractions were performed in a 500-mL reactor (300 bar, 100°C, 50 g CO 2 /min and 1 h). Wax removal efficiencies of 98 and 70% for saturated and curtain-coated OCC, respectively, were obtained. Under similar conditions, extractions in the presence of water resulted in an extraction efficiency of 99% for saturated OCC. Decreasing the operating pressure to 200 bar decreased the extraction efficiency to approximately 50%. Gas chromatography (GC) of the wax coatings on OCC, before and after extraction with SC-CO 2 , showed a slight shift in the molecular weight distribution of the paraffin wax (after SC-CO 2 extraction) toward higher molecular weights for both saturating wax and curtain-coating wax. There was no evidence of chemical degradation or modification of the paraffin wax coatings by SC-CO 2 The packing density, packing arrangement, and dimensions of the curtain-coated OCC in the extraction apparatus affected the extraction efficiency. Loose packing compared to tight packing, 1 X 1 cm squares versus 1 X 20 cm strips, had higher extraction efficiencies; a random packing arrangement was better than packing with the fluting material in the direction of SC-CO 2 flow.}, number={12}, journal={Journal of Applied Polymer Science}, publisher={Wiley}, author={Stauffer, Thad C. and Venditti, Richard A. and Gilbert, Richard D. and Kadla, John F.}, year={2002}, month={Jan}, pages={2699–2704} }
 @article{stauffer_venditti_gilbert_kadla_chernyak_montero_2001, title={Supercritical carbon dioxide dewaxing of old corrugated containers}, volume={81}, ISSN={["0021-8995"]}, DOI={10.1002/app.1533}, abstractNote={Wax-coated old corrugated containers (OCC) are not part of the paper recycling stream because a process to remove the wax coating is not presently available. Residual waxes influence fiber–fiber bonding, reducing the paper properties of recycled OCC as well as the paper machine operating efficiency. A procedure to dewax OCC is a major objective of the paper industry. Here we describe a novel process to quantitatively dewax OCC by using supercritical carbon dioxide to remove the wax. The results obtained for the extraction of both saturated and curtain-coated waxed containers are reported and compared with Soxhlet extraction with hexane. Quantitative removal of the waxes was obtained under a variety of operating conditions. Gas chromatographic analysis of the extracted paraffin wax shows that supercritical fluid extraction does not chemically alter the paraffin wax, indicating the recovered wax may be recycled. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1107–1114, 2001}, number={5}, journal={JOURNAL OF APPLIED POLYMER SCIENCE}, author={Stauffer, TC and Venditti, RA and Gilbert, RD and Kadla, JF and Chernyak, Y and Montero, GA}, year={2001}, month={Aug}, pages={1107–1114} }
 @article{kadla_gilbert_2000, title={Cellulose structure: A review}, volume={34}, number={3-4}, journal={Cellulose Chemistry and Technology}, author={Kadla, J. F. and Gilbert, R. D.}, year={2000}, pages={197–216} }
 @article{gilbert_venditti_zhang_koelling_2000, title={Melt spinning of thermotropic cellulose derivatives}, volume={77}, DOI={10.1002/(SICI)1097-4628(20000711)77:2<418::AID-APP19>3.3.CO;2-9}, abstractNote={The melt spinning of two thermotropic cellulose derivatives—trimethyl silyl cellulose and phenyl acetoxy cellulose—is described in this article. Removal of the substituents was facile, rapid, and essentially complete. Both the melt-spun and regenerated fibers had banded textures typical of fibers spun from a liquid crystalline phase. The regenerated cellulose fibers had high strengths and moduli compared to viscose rayon and Lyocel cellulose fibers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 418–423, 2000}, number={2}, journal={Journal of Applied Polymer Science}, author={Gilbert, R. D. and Venditti, Richard and Zhang, C. S. and Koelling, K. W.}, year={2000}, pages={418–423} }
 @article{ryu_gilbert_khan_1999, title={Influence of cationic additives on the rheological, optical, and printing properties of ink-jet coatings}, volume={82}, number={11}, journal={TAPPI Journal}, author={Ryu, R. Y. and Gilbert, R. D. and Khan, S. A.}, year={1999}, month={Nov}, pages={128–134} }
 @inbook{gilbert_kadla_1999, title={Polysaccharides-cellulose in biopolymers from renewable resources}, booktitle={Biopolymers from renewable resources}, publisher={New York: Springer-Verlag}, author={Gilbert, R. D. and Kadla, J. F.}, year={1999}, pages={47–90} }
 @inbook{venditti_chang_gilbert_1999, place={Appleton, WI}, title={Stickies measurement based on deposition used at North Carolina State University}, volume={IV}, booktitle={Paper Recycling Challenge: Process Control & Mensuration}, publisher={Progress in Paper Recycling}, author={Venditti, R.A. and Chang, H. M. and Gilbert, R. D.}, editor={Doshi, E.M. and Dyer, J.Editors}, year={1999}, pages={103–104} }
 @article{joyce_gilbert_khan_1997, title={The influence of carboxymethylated chitosan on the structure, rheology, and dewatering properties of clay}, volume={80}, number={5}, journal={TAPPI Journal}, author={Joyce, M. K. and Gilbert, R. D. and Khan, S. A.}, year={1997}, pages={185–190} }
 @misc{morgan_bartula_gilbert_1987, title={Aquatic event timing device}, volume={4,657,403}, number={1987 Apr. 14}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Morgan, J. A. and Bartula, W. and Gilbert, R. D.}, year={1987} }
 @misc{gilbert_stannett_kim_1976, title={Polyanhydroglucose biodegradable polymers and process of preparation}, volume={3,950,282}, number={1976 Apr. 13}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Gilbert, R. D. and Stannett, V. T. and Kim, S. L.}, year={1976} }
 @article{appaw_gilbert_khan_kadla, title={Phase separation and heat-induced gelation characteristics of cellulose acetate in a mixed solvent system}, volume={17}, number={3}, journal={Cellulose}, author={Appaw, C. and Gilbert, R. D. and Khan, S. A. and Kadla, J. F.}, pages={533–538} }