@article{park_shin_yoo_zoppe_park_2015, title={Delignification of Lignocellulosic Biomass and Its Effect on Subsequent Enzymatic Hydrolysis}, volume={10}, ISSN={["1930-2126"]}, DOI={10.15376/biores.10.2.2732-2743}, abstractNote={The effect of delignification on the enzymatic hydrolysis of biomass was investigated to determine how different delignification processes affect enzymatic hydrolysis conversion yields. Oxygen, hydrogen peroxide, and sodium chlorite treatments were performed, and the structural and chemical changes in the biomass were evaluated. Sodium chlorite delignification proved the most effective process to remove lignin in hardwood samples, followed by oxygen delignification. Hydrogen peroxide delignification was not as effective as the other two methods. As for the enzymatic conversion of carbohydrates after delignification processes on hardwood, oxygen and sodium chlorite treatments substantially improved conversion yields as the number of successive treatments was increased, compared to untreated hardwood samples. Changes in α-cellulose after delignification were less substantial than those of hardwood samples, and corresponding conversion yields were also lower. Delignification-induced structural changes in treated substrates might be responsible for the changes in carbohydrate conversion yield observed following subsequent enzymatic hydrolysis.}, number={2}, journal={BIORESOURCES}, author={Park, Junyeong and Shin, Heenae and Yoo, Seunghyun and Zoppe, Justin O. and Park, Sunkyu}, year={2015}, pages={2732–2743} }
 @article{kafle_lee_shin_zoppe_johnson_kim_park_2015, title={Effects of Delignification on Crystalline Cellulose in Lignocellulose Biomass Characterized by Vibrational Sum Frequency Generation Spectroscopy and X-ray Diffraction}, volume={8}, ISSN={["1939-1242"]}, DOI={10.1007/s12155-015-9627-9}, number={4}, journal={BIOENERGY RESEARCH}, author={Kafle, Kabindra and Lee, Christopher M. and Shin, Heenae and Zoppe, Justin and Johnson, David K. and Kim, Seong H. and Park, Sunkyu}, year={2015}, month={Dec}, pages={1750–1758} }
 @article{zoppe_venditti_rojas_2012, title={Pickering emulsions stabilized by cellulose nanocrystals grafted with thermo-responsive polymer brushes}, volume={369}, ISSN={0021-9797}, url={http://dx.doi.org/10.1016/j.jcis.2011.12.011}, DOI={10.1016/j.jcis.2011.12.011}, abstractNote={Cellulose nanocrystals (CNCs) from ramie fibers are studied as stabilizers of oil-in-water emulsions. The phase behavior of heptane and water systems is studied, and emulsions stabilized by CNCs are analyzed by using drop sizing (light scattering) and optical, scanning, and freeze-fracture electron microscopies. Water-continuous Pickering emulsions are produced with cellulose nanocrystals (0.05-0.5 wt%) grafted with thermo-responsive poly(NIPAM) brushes (poly(NIPAM)-g-CNCs). They are observed to be stable during the time of observation of 4 months. In contrast, unmodified CNCs are unable to stabilize heptane-in-water emulsions. After emulsification, poly(NIPAM)-g-CNCs are observed to form aligned, layered structures at the oil-water interface. The emulsions stabilized by poly(NIPAM)-g-CNCs break after heating at a temperature above the LCST of poly(NIPAM), which is taken as indication of the temperature responsiveness of the brushes installed on the particles and thus the responsiveness of the Pickering emulsions. This phenomenon is further elucidated via rheological measurements, in which viscosities of the Pickering emulsions increase on approach of the low critical solution temperature of poly(NIPAM). The effect of temperature can be counterbalanced with the addition of salt which is explained by the reduction of electrostatic and steric interactions of poly(NIPAM)-g-CNCs at the oil-water interface.}, number={1}, journal={Journal of Colloid and Interface Science}, publisher={Elsevier BV}, author={Zoppe, Justin O. and Venditti, Richard A. and Rojas, Orlando J.}, year={2012}, month={Mar}, pages={202–209} }
 @article{zoppe_österberg_venditti_laine_rojas_2011, title={Surface Interaction Forces of Cellulose Nanocrystals Grafted with Thermoresponsive Polymer Brushes}, volume={12}, ISSN={1525-7797 1526-4602}, url={http://dx.doi.org/10.1021/bm200551p}, DOI={10.1021/bm200551p}, abstractNote={The colloidal stability and thermoresponsive behavior of poly(N-isopropylacrylamide) brushes grafted from cellulose nanocrystals (CNCs) of varying graft densities and molecular weights was investigated. Indication of the grafted polymer brushes was obtained after AFM imaging of CNCs adsorbed on silica. Also, aggregation of the nanoparticles carrying grafts of high degree of polymerization was observed. The responsiveness of grafted CNCs in aqueous dispersions and as an ultrathin film was evaluated by using light scattering, viscosimetry, and colloidal probe microscopy (CPM). Light transmittance measurements showed temperature-dependent aggregation originating from the different graft densities and molecular weights. The lower critical solution temperature (LCST) of grafted poly(NiPAAm) brushes was found to decrease with the ionic strength, as is the case for free poly(NiPAAm) in aqueous solution. Thermal responsive behavior of grafted CNCs in aqueous dispersions was observed by a sharp increase in dispersion viscosity as the temperature approached the LCST. CPM in liquid media for asymmetric systems consisting of ultrathin films of CNCs and a colloidal silica probe showed the distinctive effects of the grafted polymer brushes on interaction and adhesive forces. The origin of such forces was found to be mainly electrostatic and steric in the case of bare and grafted CNCs, respectively. A decrease in the onset of attractive and adhesion forces of grafted CNCs films were observed with the ionic strength of the aqueous solution. The decreased mobility of polymer brushes upon partial collapse and decreased availability of hydrogen bonding sites with higher electrolyte concentration were hypothesized as the main reasons for the less prominent polymer bridging between interacting surfaces.}, number={7}, journal={Biomacromolecules}, publisher={American Chemical Society (ACS)}, author={Zoppe, Justin O. and Österberg, Monika and Venditti, Richard A. and Laine, Janne and Rojas, Orlando J.}, year={2011}, month={Jun}, pages={2788–2796} }
 @article{peresin_habibi_zoppe_pawlak_rojas_2010, title={Nanofiber Composites of Polyvinyl Alcohol and Cellulose Nanocrystals: Manufacture and Characterization}, volume={11}, ISSN={["1526-4602"]}, DOI={10.1021/bm901254n}, abstractNote={Cellulose nanocrystals (CN) were used to reinforce nanofibers in composite mats produced via electrospinning of poly(vinyl alcohol) (PVA) with two different concentrations of acetyl groups. Ultrathin cross-sections of the obtained nanocomposites consisted of fibers with maximum diameters of about 290 nm for all the CN loads investigated (from 0 to 15% CN loading). The electrospinning process did not affect the structure of the PVA polymer matrix, but its degree of crystallinity increased significantly together with a slight increase in the corresponding melting temperature. These effects were explained as being the result of alignment and enhanced crystallization of PVA chains within the individual nanofibers that were subjected to high shear stresses during electrospinning. The strong interaction of the PVA matrix with the dispersed CN phase, mainly via hydrogen bonding or bond network, was reduced with the presence of acetyl groups in PVA. Most importantly, the elastic modulus of the nanocomposite mats increased significantly as a consequence of the reinforcing effect of CNs via the percolation network held by hydrogen bonds. However, this organization-driven crystallization was limited as observed by the reduction in the degree of crystallinity of the CN-loaded composite fibers. Finally, efficient stress transfer and strong interactions were demonstrated to occur between the reinforcing CN and the fully hydrolyzed PVA electrospun fibers.}, number={3}, journal={BIOMACROMOLECULES}, author={Peresin, Maria S. and Habibi, Youssef and Zoppe, Justin O. and Pawlak, Joel J. and Rojas, Orlando J.}, year={2010}, month={Mar}, pages={674–681} }
 @article{zoppe_habibi_rojas_venditti_johansson_efimenko_österberg_laine_2010, title={Poly(<i>N</i>-isopropylacrylamide) Brushes Grafted from Cellulose Nanocrystals via Surface-Initiated Single-Electron Transfer Living Radical Polymerization}, volume={11}, ISSN={1525-7797 1526-4602}, url={http://dx.doi.org/10.1021/bm100719d}, DOI={10.1021/bm100719d}, abstractNote={Cellulose nanocrystals (CNCs) or nanowhiskers produced from sulfuric acid hydrolysis of ramie fibers were used as substrates for surface chemical functionalization with thermoresponsive macromolecules. The CNCs were grafted with poly(N-isopropylacrylamide) brushes via surface-initiated single-electron transfer living radical polymerization (SI-SET-LRP) under various conditions at room temperature. The grafting process was confirmed via Fourier transform IR spectroscopy and X-ray photoelectron spectroscopy and the different molecular masses of the grafts were quantified and found to depend on the initiator and monomer concentrations used. No observable damage occurred to the CNCs after grafting, as determined by X-ray diffraction. Size exclusion chromatography analyses of polymer chains cleaved from the cellulose nanocrystals indicated that a higher degree of polymerization was achieved by increasing initiator or monomer loading, most likely caused by local heterogeneities yielding higher rates of polymerization. It is expected that suspension stability, interfacial interactions, friction, and other properties of grafted CNCs can be controlled by changes in temperature and provide a unique platform for further development of stimuli-responsive nanomaterials.}, number={10}, journal={Biomacromolecules}, publisher={American Chemical Society (ACS)}, author={Zoppe, Justin O. and Habibi, Youssef and Rojas, Orlando J. and Venditti, Richard A. and Johansson, Leena-Sisko and Efimenko, Kirill and Österberg, Monika and Laine, Janne}, year={2010}, month={Sep}, pages={2683–2691} }
 @article{zoppe_peresin_habibi_venditti_rojas_2009, title={Reinforcing Poly(ε-caprolactone) Nanofibers with Cellulose Nanocrystals}, volume={1}, ISSN={1944-8244 1944-8252}, url={http://dx.doi.org/10.1021/am9003705}, DOI={10.1021/am9003705}, abstractNote={We studied the use of cellulose nanocrystals (CNXs) obtained after acid hydrolysis of ramie cellulose fibers to reinforce poly(epsilon-caprolactone) (PCL) nanofibers. Chemical grafting with low-molecular-weight PCL diol onto the CNXs was carried out in an attempt to improve the interfacial adhesion with the fiber matrix. Grafting was confirmed via infrared spectroscopy and thermogravimetric analyses. The polymer matrix consisted of electrospun nanofibers that were collected as nonwoven webs. The morphology as well as thermal and mechanical properties of filled and unfilled nanofibers were elucidated by scanning electron microscopy, differential scanning calorimetry, and dynamic mechanical analysis, respectively. The addition of CNXs into PCL produced minimal changes in the thermal behavior of the electrospun fibers. However, a significant improvement in the mechanical properties of the nanofibers after reinforcement with unmodified CNXs was confirmed. Fiber webs from PCL reinforced with 2.5% unmodified CNXs showed ca. 1.5-fold increase in Young's modulus and the ultimate strength compared to PCL webs. Compared to the case of grafted nanocrystals, the unmodified ones imparted better morphological homogeneity to the nanofibrillar structure. The grafted nanocrystals had a negative effect on the morphology of nonwoven webs in which individual nanofibers became annealed during the electrospinning process and, therefore, could not be compared to neat PCL nonwoven webs. A rationalization for the different effects of grafted and unmodified CNXs in reinforcing PCL nanofibers is provided.}, number={9}, journal={ACS Applied Materials & Interfaces}, publisher={American Chemical Society (ACS)}, author={Zoppe, Justin O. and Peresin, Maria S. and Habibi, Youssef and Venditti, Richard A. and Rojas, Orlando J.}, year={2009}, month={Aug}, pages={1996–2004} }