Xueming Zhang

Studies on the properties and formation mechanism of flexible nanocomposite hydrogels from cellulose nanocrystals and poly(acrylic acid)

A novel series of nanocomposite hydrogels based on cellulose nanocrystals (CNCs) and poly(acrylic acid) (PAA) have been synthesized by in situ free radical polymerization within an aqueous medium. Rheological measurements were applied to monitor the gelation process and results indicated that the gelation took place as monomers (acrylic acid, AA) grafted from the CNC surface and PAA chains entangled to produce flexible CNC–PAA gels. By tailoring the concentration of CNC (CCNC) over a wide range of 0.02–1 wt%, two critical CCNC, C* and C**, were found which corresponded to polymer chains that occurred in overlapping entanglements and promoted conformational rearrangements on the basis of earlier gel precursors, respectively. The formation mechanism of CNC based nanocomposite hydrogels, in which the nanoparticles transformed from the isolated state below C* to the spatially continuous percolation structure above C**, was proposed. The CNC–PAA gels exhibited excellent, composition-dependent mechanical properties, such as a large elongation ratio (>1100%) and high tensile strength (>350 kPa). Transmission electron microscopy (TEM) revealed that the CNCs were surrounded by grafted chains and formed inter-connected network structures, where the CNCs acted as multifunctional cross-links with an average effective functionality of 75. The mechanical measurements indicated that the increase of CCNC led to an increase in the hydrogels viscous characteristics and contributed to the energy dissipating mechanism, which was responsible for CNC–PAA gels excellent flexibility. The swelling and partial dissolution behaviors of the hydrogels were examined, focusing on the effect of CCNC on the gels characteristic partial deswelling and gel-to-sol transition. Some new chain entanglements were formed under concentrated conditions after drying treatment above the glass transition temperature (Tg) which was verified by observation of the greater tensile strength and modulus. All the results corresponded to the self-consistent network structure model for CNC–PAA gels.

Synthesis and characterization of mechanically flexible and tough cellulose nanocrystals–polyacrylamide nanocomposite hydrogels

The unique combinations of hard and soft components with core/shell structures were proposed to synthesize high strength nanocomposite hydrogels. The elastomeric hydrogels containing rod-like cellulose nanocrystals (CNCs) core and polyacrylamide shell were made from aqueous solutions via free radical polymerization in the absence of chemical cross-links. The obtained hydrogels possessed greater tensile strength and elongation ratio when compared with chemically cross-linked counterparts. Oscillatory shear experiments indicated that CNCs interacted with polymer matrix via both chemical and physical interactions and contributed to the rubbery elasticity of the hydrogels. The nanocomposite hydrogels were more viscous than the chemical hydrogels, suggesting the addition of CNC led to the increase of energy dissipating and viscoelastic properties. The network structure model was proposed and it suggested that the high extensibilities and fracture stresses were related to the well-defined network structures with low cross-linking density and lack of noncovalent interactions among polymer chains, which may promote the rearrangements of network structure at high deformations.

High strength of hemicelluloses based hydrogels by freeze/thaw technique

Novel hydrogels were prepared from hemicelluloses, polyvinyl alcohol (PVA), and chitin nanowhiskers through 0, 1, 3, 5, 7, and 9 times of freeze/thaw cycle. These hydrogels were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), swelling property, and compressive strength. The repeated freeze/thaw cycles induced physically crosslinked chains packing among these polymers, and a phase separation caused by the hydrogen bonds. Larger pores led to a high swelling degree, whereas the formation of compact structure after multiple freeze/thaw cycles resulted in high mechanical strength and thermal stability. The highest compressive strength of these hydrogels was achieved by the 9 times of freeze/thaw cycles with compressive stress of 10.5 MPa. This work provides a remarkable way for the preparation of hydrogels with good mechanical properties by physical method.

Synthetic and viscoelastic behaviors of silica nanoparticle reinforced poly(acrylamide) core–shell nanocomposite hydrogels

The poor mechanical strength of traditional synthesized hydrogels remains the greatest challenge to their performance as tissue engineering materials. Unique combinations of hard and soft components were used to design and synthesize elastomeric nanocomposite hydrogels with a core–shell microstructure. The nanocomposite hydrogels consisting of multifunctional cross-links of silica nanoparticles (SNPs) as the nucleus and poly(acrylamide) (PAM) as the shell were prepared by mixing both components in solution, and photo-crosslinking by UV radiation. Transmission electron microscopy (TEM) observations confirmed the existence of a core–shell morphology, and dynamic light scatting (DLS) measurements were applied to monitor the particle conformation changes near the critical gelation condition. The results indicated that the concentration of the core significantly affected the mechanical properties of the hydrogel and the unique three-stage arrangement process exhibited an average effective functionality around 28. The molecular weight of grafted polymer chains was monitored and it was found that the grafted PAM chain had a high value of 2.46 × 105 g mol−1, and it remained almost constant when the SNP concentration (CSNP) exceeded the critical value. The mechanical behavior of the nanocomposite hydrogels was studied with elongation under large strains and it was found that the hydrogels exhibited excellent tensile properties, including a modulus 9–38 kPa, a fracture strain 700–1000%, and a fracture stress 80–250 kPa, depending on the SNP concentrations. Stress relaxation experiments indicated that the nanocomposite hydrogels were more viscous than comparable chemical gels and were more efficient at the energy dissipation process, which is related to the orientation of SNPs and relaxation of the PAM chains. Further development of the synthetic platform would potentially lead to valuable knowledge for the design of high performance hydrogels in biomedical and pharmaceutical applications.

Green films from renewable resources: properties of epoxidized soybean oil plasticized ethyl cellulose films.

Epoxidized soybean oil (ESO), which is a biomass-derived resource, was first used as a novel plasticizer for ethyl cellulose (EC) film preparation. Surface morphologies, mechanical performances, thermal properties, oxygen and water vapor permeabilities of plasticized EC films were detected in detail to evaluate the plasticizing effect of ESO and explore the plastication mechanisms. Results showed that ESO was an effective plasticizer that outstripped conventional plasticizers, i.e. dibutyl phthalate (DBP) and triethyl citrate (TEC) in producing high-quality films. Especially, at plasticizer concentrations of 15-25%, ESO-EC films had preferable mechanical properties and better thermal stability, as well as non-flammability. In addition, the water vapor permeability of ESO-EC films was lower than that of traditional plasticized films. Their oxygen permeability was also remained in a low level. These outstanding performances were related to the relatively high molecular weight, hydrophobicity, chemical structure of ESO, and the intermolecular interactions between ESO and EC chains.

Assessment of integrated process based on hydrothermal and alkaline treatments for enzymatic saccharification of sweet sorghum stems.

In this study, sweet sorghum stem was subjected to hydrothermal pretreatment (HTP) and alkaline post-treatment to enhance its saccharification ratio by reducing its recalcitrance. The results showed that the HTP (110-210°C, 0.5-2.0h) significantly degraded hemicelluloses, and the pretreatment at the temperature higher than 190°C led to the partial degradation of the cellulose. As compared to the sole HTP, the integrated process removed most of lignin and hemicelluloses, which incurred a higher cellulose saccharification ratio. Under an optimum condition evaluated (HTP at 170°C for 0.5h and subsequent 2% NaOH treatment), 77.5% saccharification ratio was achieved, which was 1.8, 2.0 and 5.5 times as compared to the only HTP pretreated substrates, alkaline treated substrates alone and the raw material without pretreatment, respectively. Clearly, the integrated process can be considered as a promising approach to achieve an efficient conversion of lignocellulose to fermentable glucose.

Simple Approach to Synthesize Amino-Functionalized Carbon Dots by Carbonization of Chitosan

Carbon dots (CDs) as a new series of fluorescent nanomaterials have drawn great attention in recent years owning to their unique properties. In this paper, a simple carbonization approach to synthesize amino-functionalized CDs was developed by using chitosan as the carbon precursor. The as-prepared CDs possessed desirable amino function group on their surface and exhibited bright luminescence with absolute quantum yield (QY) of 4.34%, excitation-, pH-dependent and up-conversion fluorescence behaviors. Furthermore, we have investigated the cytotoxicity and biocompatibility of the as-prepared CDs, which demonstrated that the as-prepared CDs have the potential applications in biosensing, cellular imaging and drug delivery.

Fabrication and Characterization of Regenerated Cellulose Films Using Different Ionic Liquids

The demand for substitution of fossil-based materials by renewable bio-based materials is increasing with the fossil resources reduction and its negative impacts on the environment. In this study, environmentally friendly regenerated cellulose films were successfully prepared using 1-allyl-3-methylimidazolium chloride (AmimCl), 1-butyl-3-methylimidazolium chloride (BmimCl), 1-ethyl-3-methylimidazolium chloride (EmimCl), and 1-ethyl-3-methylimidazolium acetate (EmimAc) as solvents, respectively. The results of morphology from scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that all the cellulose films possessed smooth, highly uniform, and dense surface. The solid-state cross-polarization/magic angle spinning (CP/MAS) 13C NMR spectra and X-ray diffraction (XRD) corroborated that the transition from cellulose I to II had occurred after preparation. Moreover, it was shown that the ionic liquid EmimAc possessed much stronger dissolubility for cellulose as compared with other ionic liquids and the cellulose film regenerated from EmimCl exhibited the most excellent tensile strength (119 Mpa). The notable properties of regenerated cellulose films are promising for applications in transparent biodegradable packaging and agricultural purpose as a substitute for PP and PE.

Characteristics and enzymatic hydrolysis of cellulose-rich fractions from steam exploded and sequentially alkali delignified bamboo (Phyllostachys pubescens)

In this study, cellulose-rich fractions from bamboo were prepared with steam explosion pretreatment (SEP) followed by a successive alkaline delignification to improve the enzymatic digestibility for an efficient bioethanol production. The cellulose-rich fractions obtained were characterized by FT-IR, XRD, CP/MAS (13)C NMR, SEM, and BET surface area. It was found that the SEP alone significantly removed partial hemicelluloses, while the synergistic treatment by SEP and alkaline delignification removed most hemicelluloses and lignin. Results from enzymatic hydrolysis showed that SEP alone improved the enzymatic hydrolysis rate by 7.9-33.1%, while the synergistic treatment by SEP and alkaline delignification enhanced the rate by 45.7-63.9%. The synergistic treatment by SEP at 2.0 MPa for 5 min with water impregnation followed by a successive alkaline delignification with 0.5% NaOH and 70% ethanol containing 1.5% NaOH resulted in a maximum enzymatic hydrolysis rate of 70.6%.

Enhancement of Lignin Biopolymer Isolation from Hybrid Poplar by Organosolv Pretreatments

Lignocellulosic biomass is an abundant renewable resource that has the potential to displace petroleum in the production of biomaterials and biofuels. In the present study, the fractionation of different lignin biopolymers from hybrid poplar based on organosolv pretreatments using 80% aqueous methanol, ethanol, 1-propanol, and 1-butanol at 220°C for 30 min was investigated. The isolated lignin fractions were characterized by Fourier transform infrared spectroscopy (FT-IR), high-performance anion exchange chromatography (HPAEC), 2D nuclear magnetic resonance (2D NMR), and thermogravimetric analysis (TGA). The results showed that the lignin fraction obtained with aqueous ethanol (EOL) possessed the highest yield and the strongest thermal stability compared with other lignin fractions. In addition, other lignin fractions were almost absent of neutral sugars (1.16–1.46%) though lignin preparation extracted with 1-butanol (BOL) was incongruent (7.53%). 2D HSQC spectra analysis revealed that the four lignin fractions mainly consisted of -O-4′ linkages combined with small amounts of - and -5′ linkages. Furthermore, substitution of in -O-4′ substructures had occurred due to the effects of dissolvent during the autocatalyzed alcohol organosolv pretreatments. Therefore, aqueous ethanol was found to be the most promising alcoholic organic solvent compared with other alcohols to be used in noncatalyzed processes for the pretreatment of lignocellulosic biomass in biorefinery.