细菌纤维素(BC)增强PVA/PVP复合水凝胶的制备及性能表征

通过冷冻-熔融法制备了细菌纤维素/聚乙烯醇/聚乙烯吡咯烷酮(BC/PVA/PVP)双网络复合水凝胶,并采用X射线衍射,红外光谱,扫描电镜,力学性能测试等手段对凝胶的结构和性能进行表征.研究发现PVA、PVP通过氢键作用均匀地吸附于纤维微丝周围,将BC纤维有效地分开,因而干燥后的复合凝胶在热水中浸泡后仍可恢复原状;X射线...     

Effect of different additives on bacterial cellulose production by Acetobacter xylinum and analysis of material property

Bacterial cellulose (BC) demonstrates unique properties including high mechanical strength, high crystallinity, and high water retention ability, which make it an useful material in many industries, such as food, paper manufacturing, and pharmaceutical application. In this study, different additives including agar, carboxymethylcellulose (CMC), microcrystalline cellulose, and sodium alginate were added into fermentation medium in agitated culture to enhance BC production by Acetobacter xylinum. The optimal additive was chosen based on the amount of BC produced. The produced BC was analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). Among the evaluated additives, CMC yielded highest BC production (8.2 g/L) compared to the control (1.3 g/L). The results also indicated that CMC-altered BC production increased with CMC addition and reached saturation around 1%. The variation between replicates for all analysis was <5%. From XRD analysis, however, the crystallinity and crystal size decreased as CMC addition increased. FESEM results showed CMC-altered BC produced from agitated culture retained its interweaving property. TGA results demonstrated that CMC-altered BC had about 98% water retention ability, which is higher than BC pellicle produced with static culture. CMC-altered BC also exhibited higher T _max compared to control. Finally, DMA results showed that BC from agitated culture loses its mechanical strength in both stress at break and Young’s modulus when compared to BC pellicle. This study clearly demonstrated that addition of CMC enhanced BC production and slightly changed its structure.     

In situ modification of bacterial cellulose nanostructure by adding CMC during the growth of Gluconacetobacter xylinus

Many studies focus on bacterial cellulose (BC) functioning as multi-function bio-resource polymers, due to its fine fiber network, biocompatibility, high water holding capacity, and high mechanical strength. However, BC exhibits poor rehydration after drying due to its high crystallinity. This study added carboxymethylcellulose (CMC) to a BC producing culture medium, which interfered with the formation of BC structure in situ. This process created a modified BC called CBC, whose mechanical strength was found weaker than BC. Scanning electron microscope (SEM) images showed that the cellulose network in CBC became denser. X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) analysis demonstrated that the addition of CMC reduced crystallinity. CBC also exhibited the highest rehydration ratio because of the lowest crystallinity at the 1.0% CMC addition level.     

Homogeneous and porous modified bacterial cellulose achieved by in situ modification with low amounts of carboxymethyl cellulose

To improve the rehydration ability of bacterial cellulose (BC), many macromolecules have been used as modifiers in previous reports. However, the aggregation of additives in the BC matrix appears to be inevitable. We investigated different parts of a BC pellicle, which was achieved by in situ modification with carboxymethyl cellulose (CMC) in culture with Gluconacetobacter xylinus ATCC53582 or Enterobacter sp. FY-07. We observed a concentration gradient of CMC in the BC pellicle from G. xylinus ATCC53582, but not with Enterobacter sp. FY-07. Low concentrations of CMC (0.01 %, m/v) are sufficient to modify BC in situ in culture with Enterobacter sp. FY-07, in which CMC could sufficiently contact with the newly formed BC. The crystallinity of the modified BC decreased by more than 39.8 %, and its rehydration ability and water holding capacity increased by 43.3 and 31.0 %, respectively. Unlike the pellicle of modified BC achieved from obligate aerobes, such as G. xylinus ATCC53582, that produced by Enterobacter sp. FY-07 exhibited better homogeneity and porosity.     

纳米细菌纤维素膜的表征与生物相容性研究

利用木醋杆菌静态培养法制备的由纳米纤维组成的细菌纤维素膜具有超细的三维网络结构和适当的孔隙率. 利用光镜、扫描电镜和原子力显微镜对其进行结构表征发现, 细菌纤维素膜具有极为精细的纳米网络结构, 冻干膜的孔径约为0.6~2.8 μm; 纤维素带宽度约为50~80 nm. 采用湿重与浮重结合法测定烘干膜和冻干膜的孔隙率分别约为70%和90%. 由于细菌纤维素含有大量的羟基, 故烘干膜表现出极好的透湿性. 将细菌纤维素膜分别与成纤维细胞和软骨细胞进行复合培养, 并将成纤维细胞和细菌纤维素膜的复合物进行裸鼠皮下移植实验. 结果显示, 移植的复合物很好地融入了裸鼠正常皮肤, 成纤维细胞和软骨细胞在细菌纤维素表面形成连续的细胞层, 绿色荧光蛋白表达正常. 以上结果表明, 细菌纤维素膜非常适合细胞贴附和增殖, 表现出较好的生物相容性, 有望成为新型组织工程支架材料.     

Susceptibility of never-dried and freeze-dried bacterial cellulose towards esterification with organic acid

The susceptibility of (1) never-dried and (2) freeze-dried bacterial cellulose (BC) towards organic acid esterification is reported in this work. When never-dried BC (BC which was solvent exchanged from water through methanol into pyridine) was modified with hexanoic acid, it was found that the degree of substitution (DS) was significantly lower than that of hexanoic acid modified freeze-dried BC. The crystallinity of freeze-dried BC hexanoate was found to be significantly lower compared to neat BC and never-dried BC hexanoate. This result, along with the high DS indicates that significant bulk modification occurred during the esterification of freeze-dried BC. Such results were not observed for never-dried BC hexanoate. All these evidence point towards to fact that freeze-dried BC was more susceptible to organic acid esterification compared to never-dried BC. A few hypotheses were explored to explain the observed behaviour and further investigated to elucidate our observation; the effect of residual water in cellulose, the accessibility of hydroxyl groups and the crystal structure of never-dried and freeze-dried BC on the susceptibility of cellulose fibrils to esterification, respectively. However, the investigation of these hypotheses raised more questions and we are still left with the main question; why do BC nanofibres behave differently when modifying freeze-dried BC or never-dried BC?     

Development of Nanocomposites from Bacterial Cellulose and Poly(vinyl Alcohol) using Casting-drying Method

Nanocomposites of bacterial cellulose (BC) and poly(vinyl alcohol) (PVA) were prepared by cast-drying method as an easy way in producing nanocomposite films and to expand the use of BC. The contribution of PVA in nanocomposites was evaluated by measurement of cross-sectional surface, moisture uptake and mechanical properties. Morphological analysis shows that PVA covered a number of cellulosic fibres and formed denser material as a function of PVA addition. Based on the tensile test, the addition of PVA causes a very slight reduction compared with bacterial cellulose itself. The BC/PVA nanocomposites still have similar stiffness to BC with elongation at break less than 5%, while PVA film shows ductile properties with elongation at break more than 80%. On the other hand, the presence of BC fibres in the PVA matrix enhanced the tensile strength and the elastic modulus of pure PVA about two to three times, but it decreased the toughness of pure PVA. The highest tensile strength and elastic modulus of the nanocomposites are 164 MPa and 7.4 GPa, respectively at BC concentration of 64%. Increasing BC concentration is proportional to reducing moisture uptake of BC/PVA nanocomposites indicating that the existence of BC fibres inhibits moisture absorption.     

Preparation of biocompatible system based on electrospun CMC/PVA nanofibers as controlled release carrier of diclofenac sodium

Nanofibers of naturally modified polymer such as carboxymethyl cellulose (CMC) blended with poly(vinyl alcohol) (PVA) at different ratios was obtained by electrospinning technique. The blended solutions of CMC and PVA loaded with and without diclofenac sodium (DS) were electrospun using environmentally benign electrospinning technique in the absence of organic solvents. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR), thermogravimetric analysis (TGA) were used to investigate the surface morphology functional groups, as well as the thermal stability of DS loaded CMC/PVA nanofibers mat. The mechanical properties of the as prepared electrospun nanofibers was also evaluated. The entrapment efficiency and the in vitro release of DS loaded CMC/PVA nanofibers were characterized using UV-Vis spectroscopy. The obtained results displayed that the blended nanofibers have shown a smooth morphology, no beads formation when the concentration of CMC was equal or below 5% and beads formation above 5%. FTIR data demonstrated that there were good interactions between CMC and PVA possibly via the formation of hydrogen bonds. The electrospun blended CMC/PVA nanofibers exhibit good mechanical properties. From the in vitro release data, it was found that with the presence of CMC, the release of DS from the nanofibers mats became sustained controlled. Due to the biocompatibility and low cost of the two blended polymers (CMC and PVA), the blended nanofibers system can be considered as one of the promising materials for the preparation of excellent drug carrier.     

Water redispersible cellulose nanofibrils adsorbed with carboxymethyl cellulose

Cellulose nanofibrils (CNFs) are difficult to redisperse in water after they have been completely dried due to the irreversible agglomeration of cellulose during drying. Here, we have developed a simple process to prepare water-redispersible dried CNFs by the adsorption of small amounts of carboxymethyl cellulose (CMC) and oven drying. The adsorption of CMC onto CNFs in water suspensions at 22 and 121 °C was studied, and the adsorbed amount of CMC was measured via conductimetric titration. The water-redispersibility of dried CNFs adsorbed with different amounts of CMC was characterized by sedimentation test. Above a critical threshold of CMC adsorption, i.e. 2.3 wt%, the oven dried CNF–CMC sample was fully redispersible in water. The morphology, rheological, and mechanical properties of water-redispersed CNF–CMC samples were investigated by field emission scanning electron microscopy, viscosity measurement, and tensile test, respectively. The water-redispersed CNFs preserved the original properties of never dried CNFs. This new method will facilitate the production, transportation and storage, and large-scale industrial applications of CNFs.     

Effects of Electron Beam Irradiation on the Structural Properties of PVA/PAM/CMC Ternary Polymer Blends

Ternary miscible blends based on various ratios of poly(vinyl alcohol) (PVA), poly(acrylamide) (PAM) and carboxymethyl cellulose (CMC) were prepared by solution casting in the form of thin films. The structure‐property behavior of the ternary PVA/PAM/CMC blends, before and after they had been exposed to various doses of electron beam irradiation, was investigated by FT‐IR spectroscopy, SEM, XRD and stress‐strain curves. The visual observation showed that the cast films of the individual polymers PVA, PAM, and CMC and their blends over a wide range of composition are clear and transparent indicating the miscibility of PVA/PAM/CMC ternary blends. The FT‐IR analysis of pure polymers or their ternary blends before or after electron beam irradiation proved the formation of hydrogen bonding. In addition, it was found that the intensity of the different absorption bands depends on the ratio of PAM and CMC in the ternary blend. The XRD patterns showed that the peak position for the ternary blends decreases with increasing the ratio of CMC in the blend. However, the peak position for the ternary blend based on equal ratios of pure polymers was not affected by blending and was found in the same position as in the XRD pattern of pure PVA. The SEM micrographs give support to the visual observation indicating the complete miscibility of PVA/PAM/CMC ternary blends. The improvement in morphology leads to improvement in the tensile mechanical properties of the ternary polymer blends.     

In situ production of nanocomposites of poly(vinyl alcohol) and cellulose nanofibrils from Gluconacetobacter bacteria: effect of chemical crosslinking

Nanocomposites of poly(vinyl alcohol) (PVA) reinforced with bacterial cellulose (BC) were bioproduced by Gluconacetobacter genus bacteria. BC was grown from a culture medium modified with water-soluble PVA to allow in situ assembly and production of a novel nanocomposite that displayed synergistic property contributions from the individual components. Chemical crosslinking with glyoxal was performed to avoid the loss of PVA matrix during purification steps and to improve the functional properties of composite films. Reinforcement with BC at 0.6, 6 and 14 wt% content yielded nanocomposites with excellent mechanical, thermal and dimensional properties as well as moisture stability. Young’s modulus and strength at break increased markedly with the reinforcing BC: relative to the control sample (in absence of BC), increases of 15, 165 and 680 % were determined for nanocomposites with 0.6, 6 and 14 % BC loading, respectively. The corresponding increase in tensile strengths at yield were 1, 12 and 40 %, respectively. The results indicate an exceptional reinforcing effect by the three-dimensional network structure formed by the BC upon biosynthesis embedded in the PVA matrix and also suggest a large percolation within the matrix. Bonding (mainly hydrogen bonding) and chemical crosslinking between the reinforcing phase and matrix were the main contributions to the properties of the nanocomposite.     

Synthesis of cellulose derivative based superabsorbent hydrogels by radiation induced crosslinking

Hydrogels with high water uptake were prepared by ionizing radiation induced crosslinking in aqueous solutions of four cellulose derivatives (carboxymethylcellulose sodium salt—CMC-Na, methylcellulose—MC, hydroxyethylcellulose—HEC and hydroxypropylcellulose—HPC). The gel fraction increased with absorbed dose, while water uptake decreased. At high polymer concentrations lower gel fractions were found due to the lower polymer chain mobility and inhomogeneity at low water content. The swelling rate gradually slowed down after 4–5 h. CMC and HEC gels reached equilibrium after 24 h, while HPC and MC gels required longer immersion times. Gels showed second-order swelling kinetics in water. The mechanism of the water diffusion proved to be anomalous. In pure water, CMC gels showed the highest, while HPC and MC gels the lowest water uptake. The derivatives had different sensitivities to ionic strength in the swelling solution. The salt type also proved to be a significant factor at uniform ionic strength. Thus different cellulose derivative based gels may be preferred at various applications depending on the environment.     

Carboxymethylation of Bacterial Cellulose

The carboxymethylation of bacterial cellulose (BC) was studied under typical heterogeneous reaction conditions. It was found that the BC possesses a significantly lower reactivity compared to wood cellulose converted under comparable conditions. Moreover, water-solubility of carboxymethyl cellulose (CMC) obtained from BC appears at rather high degree of substitution of about 1.5 although a nearly statistical functionalization pattern was analyzed by HPLC. Obviously, the nano-structure of BC is important for the reactivity and the properties of the synthesized CMC like water-solubility.     

Cellulose loading and water sorption value as important parameters for the enzymatic hydrolysis of cellulose

The enzymatic hydrolysis of cotton raw cellulose (RC) samples, sieved RC samples through meshes <100 (CS1), 100–200 (C12), 200–400 (C24), mercerized RC samples (M-C), freeze-dried RC (RC-FD) samples, microcrystalline cellulose Avicel, bacterial cellulose (BC), raw sisal pulp and mercerized sisal pulp (S-M) was performed at cellulose-to-cellulase mass ratios of 1,000:1, 699:1, 400:1, 100:1 and 10:1. The index of crystallinity and water sorption values were quantified for all samples. The morphological features were analyzed by means of scanning electron microscopy (SEM). For cellulose-to-cellulase mass ratio of 100:1 and 10:1, the maximum hydrolysis extents of cellulose samples after 24 h reaction could not be correlated with their physical characteristics. However, hydrolyses of samples with large water sorption values were faster than those with lower water sorption values. The hydrolysis efficiency decreased when the cellulose-to-cellulase mass ratio was greater than 400:1; under this condition a remarkable dependence of the hydrolysis yield on the type of cellulosic sample was observed. The water sorption ability could be directly correlated with the hydrolysis extent, except for RC-FD and BC samples, which presented the lowest values. In the former, freeze-drying has led to pore collapse, with concomitant reduction of the amount of adsorbed water. For the latter sample, the densely packed structure made the water sorption slower than in all other samples. Despite of this fact, the presence of nanofibrils on the surface of BC (as detected by SEM) improved the enzyme adsorption, indicating that analysis by complementary techniques should be performed in order to predict the enzymatic hydrolysis efficiency.     

羧甲基纤维素钠和甲基纤维素在ZrO_2微粉上的吸附作用及其对悬浮体稳定性的影响

研究了ZrO2微粉自水中对羧甲基纤维素钠（CMC）和甲基纤维素（MC）的吸附作用及对ZrO2水悬浮作稳定性的影响．结果表明：（1）CMC的吸附等温线为Langmuir型,CMC可能以较为舒展的方式单层吸附于ZrO2。表面,由于有电性吸引作用在pH3．4时的极限吸附量较pH7．0时的略大;（2）MC的极限吸附量远大于CMC的,MC分子可能以线团状吸附,吸附层厚度可达14～56nm;（3）难较大浓度时MC比CMC更易于使ZrO2悬浮作稳定．     

Synthesis and characteristics of cellulose peroxides of the peracid type having a temperature-responsive function

The synthesis of cellulose peroxides of the peracid type having a temperature-responsive function was studied by using carboxymethyl cellulose (CMC) and acrylic acid (AA)-grafted cellulose, into which the temperature-responsive component, poly(N-isopropylacrylamide) [poly(NIPAAm)], was introduced by a photografting method (λ > 300 nm). Dissolving pulp from softwoods was used as cellulose sample. NIPAAm-grafted CMC samples prepared by photografting with CMC peroxide exhibited a slightly larger temperature-responsive character than the samples prepared by photografting with xanthone photoinitiator, where the grafted CMC samples swelled and shrank in water at 5 °C and 60 °C, respectively. Ungrafted and NIPAAm-grafted CMC samples were subjected to peroxidation with hydrogen peroxide in the presence of methanesulfonic acid. About 90% of the initial amount of peroxide on the ungrafted CMC sample disappeared after thermal decomposition at 50 °C for 60 min. On the other hand, about 50% of the peroxide on the NIPAAm-grafted CMC sample remained stable under the same conditions. Peroxides on AA/NIPAAm-grafted samples, which were prepared by photografting of AA/NIPAAm binary monomers followed by peroxidation with hydrogen peroxide, were more stable towards thermal decomposition than those on NIPAAm-grafted samples.     

Characterization of water-soluble exopolysaccharides from Gluconacetobacter xylinus and their impacts on bacterial cellulose crystallization and ribbon assembly

Gluconacetobacter xylinus has the ability to produce different types of water soluble exopolysaccharides (EPS). Those EPS have different levels of association to bacterial cellulose (BC). At least a portion of the EPS can be released from the BC by 0.1 to 4 M NaOH solution treatments. Hard to extract EPS (HE-EPS) released by 4 M NaOH solutions have been characterized and contain approximately 75 % mannose and 25 % glucose. To study the effect of the EPS on BC synthesis, purified EPS were added to the medium at the start of cultivation and the BC produced was characterized. Results showed that the presence of HE-EPS in the culture medium interfered with the alignment of the BC crystals, but did not reduce crystal size. This is in contrast to similar studies performed using xyloglucan, xylan and glucomannan. The width of the average ribbon increased by 60 % when HE-EPS levels increased in the medium, which indicated that the HE-EPS could also modulate the bundling of cellulose ribbons. Based on the data we propose a mechanism for how HE-EPS alters cellulose formation and assembly. The addition of HE-EPS disturbs the preferential crystal orientation and increases the spacing of cellulose microfibrils without affecting crystallization by associating with ordered cellulose prior to physical aggregation or bundling.     

Production of Microbial Cellulose by a Bacterium Isolated from Fruit

This study presents the production of bacterial cellulose (BC) by a bacterium isolated from a rotten fruit and its process optimization. Here, isolation and screening of potent cellulose producers were carried out from different natural sources, viz., soil, rotten fruits, and vegetables and vinegar. A total of 200 bacterial isolates were obtained, which were screened for cellulose production using Hestrin?CSchramm medium. A novel and potent cellulose-producing bacterium was newly isolated from a rotten fruit and identified as Gluconacetobacter sp. F6 through 16S ribosomal DNA sequencing and morphological, cultural, and biochemical characteristics. After optimization of culture conditions, including pH, temperature, agitation, carbon/nitrogen sources, and inducers, the BC production was greatly increased from 0.52 to 4.5?g/l (8.65-fold increase). The optimal culture medium contained 1% (w/v) glucose, 1.5% (w/v) yeast extract, 0.5% (w/v) peptone, 0.27% (w/v) disodium hydrogen phosphate, 0.115% (w/v) citric acid, and 0.4% (w/v) ethanol. BC produced was analyzed for the presence of cellulose fibrils by epiflourescent microscopy using Calcofluor white stain and scanning electron microscopy and confirmed by NMR. There are very scanty reports about the optimization of BC production by bacteria isolated from rotten fruits.     

改性羧甲基羟丙基纤维素在聚醋酸乙烯乳液中的增稠效应

用旋转粘度计研究了改性羧甲基羟丙基纤维素在聚醋酸乙烯乳液中的增稠效应。通过与非离子型羟乙基纤维素及离子型羧甲基纤维素的比较试验表明,改性羧甲基羟丙基纤维素用作聚醋酸乙烯乳液增稠剂有良好的增稠性、稳定性和配伍性。     

Tailored preparation of dual phase concomitant methylcellulose/poly(vinyl alcohol) physical hydrogel with tunable thermosensivity

A novel dual phase concomitant, methylcellulose sol@poly(vinyl alcohol) (MC/PVA) hydrogel, was prepared via physical mixing and subsequent freezing/thawing. MC/PVA hydrogel was stable within a wide temperature range, and exhibited reversible thermoresponsivity. The initial sol-gel transition temperatures of MC/PVA hydrogels containing 40, 45 and 50 wt% MC were 45.9, 42.0 and 45.5 °C, respectively. It was found that the crystallinity of these samples was 41.1%, 38.3% and 40.3%, respectively; all of them were lower than that of MC and PVA. The thermal responding rates of MC/PVA hydrogel composed of 30, 40, 45 and 50 wt% MC were about 2.85, 3.17, 5.74 and 8.58%/min, respectively. The fluorescence micrograph and scanning electron microscopy of MC/PVA hydrogel revealed that the micro MC sol phases were dispersed in whole PVA network. Moreover, the thermal transition behavior and interior morphology of MC/PVA hydrogel could be tailored with its composition.