The acetylation of cellulose in ionic liquids (ILs) and the subsequent dry/wet spinning of these processing solutions were investigated. The homogeneous acetylation in ILs was carried out using different molar ratios of acetic anhydride to the anhydroglucose unit (AGU). The obtained solutions of cellulose acetates were characterised analytically by means of rheological methods. DS and of the prepared cellulose acetates were determined. Furthermore the processing solutions of the acetylation were shaped by means of a dry/wet spinning process to fibres with varied properties. The resulting fibre properties were discussed in consideration of the DS and in comparison with unsubstituted cellulose fibres.
Porous cellulose acetate butyrate foams with a bimodal cell size distribution were produced using supercritical carbon dioxide as a blowing agent. It is demonstrated that the cell size distribution is tunable, due to the semi‐crystalline nature of the polymer. The resulting morphology will either be homogeneous or bimodal, depending on the depressurization rate. Mercury intrusion porosimetry shows that the produced cellulose acetate butyrate foams possess an open cellular structure.
Cellulose‐based fibers were prepared by electrospinning from cellulose dissolved in NaOH/urea in the presence of a small amount of polyol binders. The as‐spun products were examined with SEM. Pure cellulose solution did not produce fibrous materials, because it often formed spherical nanoparticles with diameters ranging from 100 to 300 nm. However, bicomponent fibrous materials were obtained successfully from mixtures of cellulose and HMPEG or PVA by electrospinning. The cellulose/HMPEG electrospun fibers had average diameters of 400 nm. The content of NaOH and urea as well as the stiffness of cellulose chains were found to have significant effect on the electrospinning process.
PEG is used to reinforce chitosan‐based hydrogels through the formation of ester and amide linkages. The reinforced PEG/chitosan (RPC) hydrogels exhibit significant enhancements in tensile modulus and elongation compared with neat chitosan. Other properties are thoroughly investigated and indicate that the physicochemical and in vitro degradation properties of the RPC hydrogels depend on the amount and molecular weight of the PEG. The RPC hydrogels can control evaporative water loss at a suitable rate to maintain a moist environment. In terms of in vitro biological properties, 3T3 fibroblasts show good viability with the RPC hydrogels, which indicates that the RPC hydrogels may be used as wound dressing materials.
A comparative study of the preparation and properties of composites of PCL with cellulose microfibres (CFs) containing butanoic‐acid‐modified cellulose (CB) or PCL grafted with maleic anhydride/glycidyl methacrylate as compatibilizers, is reported. The composites are obtained by melt mixing and analyzed using SEM, DSC, TGA, XRD, FT‐IR, NMR and tensile tests. An improved interfacial adhesion is observed in all compatibilized composites, as compared to PCL/CF. The crystallization behavior and crystallinity of PCL is largely affected by CF and CB content. Composites with PCL‐g‐MAGMA display higher values of tensile modulus, tensile strength and elongation at break.
Cellulose microfibers were modified with two different bi‐functional monomers. Composites of EVA copolymer with modified and unmodified cellulose were prepared by melt mixing. The samples were analyzed by SEM, XRD, FT‐IR, DSC, TGA, DMTA and tensile mechanical tests. SEM showed that the presence of reactive groups on cellulose surface enhanced the compatibility, improving the fiber/matrix interfacial adhesion. FT‐IR disclosed the occurrence of chemical reactions between the functionalized cellulose and polymer chains. The incorporation of fibers affected the crystallization behaviour and crystallinity of the polymer matrix. Composites with GMA modified cellulose displayed better compatibility, higher thermal and mechanical properties.
The thermal behavior of cellulose dissolved in ionic liquids was studied in comparison to NMMO solutions. The cellulose solutions were characterized by reaction calorimetry and UV‐vis spectroscopy. Generation of chromophoric substances in cellulose/IL solutions is minimized by exposing to temperatures of above 100 °C for longer time periods. Dynamic calorimetric investigations revealed first thermal activities above 180 °C applying EMIMac and above 200 °C for BMIMCl and five other ILs tested. Moreover, even in the case of modified cellulose/IL solutions, e.g., activated charcoal, only a slight decline of onset temperatures was registered compared to modified cellulose/NMMO solutions.
Porous membranes were fabricated using chitosan and poly(DL ‐lactide) or poly(L ‐lactide) blends through a combinatorial technique. Well‐controlled porous structures could be achieved by optimizing processing conditions. The ductility and toughness of dry porous membranes were improved by incorporating an increased amount of chitosan, and the physical strength and dimensional stability of hydrated porous membranes were preserved if the components were used in a suitable ratio. Although there were measurable differences in the pore‐size distributions of membranes with the same composition prepared under identical conditions, this showed no effect in their dry states.
Temperature‐responsive PVCL homopolymers and functional PVCL polymers containing carboxylic acids are prepared in organic and aqueous solutions. PVCL bulk polymers are characterized using 1H NMR, photometry, ATR‐FTIR, and thermal analysis. A finite phase transition at 37–40 °C occurs in aqueous solutions of PVCL and PVCL‐COOH. PVCL and PVCL‐COOH polymers are electrospun into fibers ranging from 100 to 2300 nm in diameter. PVCL/cellulose bi‐component films are obtained by electrospinning of CA and PVCL followed by alkaline hydrolysis. These tunable thermo‐responsive PVCL/cellulose nanofibers have potential applications in developing affinity membranes.
Strong honeycomb like nanocomposite sponges were fabricated from starch and PVA by using repeated cycles of freezing and thawing and reinforcing with cellulose whiskers. Their structure and properties were investigated with WAXD, FT‐IR, SEM, DMTA, rheological measurements, and LSCM. The results revealed that the repeated freezing/thawing cycles induced a physically crosslinked chain packing between starch and PVA, as well as a phase separation caused by the crystalline ice and syneresis. Thus, larger pores and tougher walls emerged in the sponges, leading to a high swelling degree. The sponges reinforced with cellulose whiskers exhibited improved dimensional stability and enhanced strength. These nanocomposite sponges are promising for wound dressing and tissue engineering applications.
The report shows that simple LbL deposition of positively charged chitosan and negatively charged heparin can be used to efficiently modify the native surface of both NiTi and Ti without any previous treatments. Moreover, mineralization of the polymer multilayers with calcium phosphate leads to surfaces with low contact angles around 70 and 20° for NiTi and Ti, respectively. This suggests that a polymer multilayer/calcium phosphate hybrid coating could be useful for making NiTi or Ti implants that are at the same time antibacterial (via the chitosan), suppress blood clot formation (via the heparin), and favor fast endothelialization (via the improved surface hydrophilicity compared to the respective neat material).
The degradation of cellulose to lmw samples with $\overline {DP} _{{\rm w}} $ varying from 15 to 130 is investigated. Cellulose samples prepared from the hydrolysis of regenerated cellulose fibers in dilute HCl possess $\overline {DP} _{{\rm w}} $ = 50. Applying homogenous degradation of microcrystalline cellulose in H3PO4 at RT for 3 weeks, samples with $\overline {DP} _{{\rm w}} $ = 35 and a PDI of 1.58 are obtained. Decreasing the hydrolysis temperature to 8 °C results in lmw cellulose with $\overline {DP} _{{\rm w}} $ > 70. Fractionation in DMA/LiCl provides samples with $\overline {DP} _{{\rm w}} $ = 12 to 130, together with a narrow molecular weight distribution. Detailed structural analysis by 2D NMR spectroscopy reveals that the prepared lmw celluloses are suitable as mimics for cellulose.
This is the first report on a thermoformable bionanocomposite based on a natural nanocrystal and formed by grafting long polymer chains onto the surface of microcrystalline cellulose. For the cellulose nanocrystal‐graft‐poly(ε‐caprolactone), the “graft from” strategy contributed to long and dense “plasticizing” PCL tails onto the CN surface as the key of thermoforming. The grafted PCL chains shielded the hydrophilic surface of CN and, hence, showed high water‐resistance. Moreover, a strategy for developing new bionanocomposite materials based on natural nanocrystals has been presented.
The electrospinnability of hordein and gliadin from acetic acid solutions is demonstrated and compared to that of zein. The relation between protein conformations in solution and the properties of post‐spun fibers are studied by means of CD, FTIR, TEM, DLS, viscosity measurements, SEM, and mechanical testing. The results show that both electrospinnability of the proteins and mechanical properties of post‐spun fibers are significantly influenced by solution protein conformations. Ultrafine fibers are fabricated at optimized concentrations, while large compact aggregates caused by hydrophobic interactions have a negative effect on the formation of continuous protein fibers. Moreover, hordein fibers show a lower cytotoxicity than gliadin and zein fibers.
Low density polyethylene (LDPE) was prepared into micro‐ or submicro‐spheres or nanofibers via melt blending or extrusion of cellulose acetate butyrate (CAB)/LDPE immiscible blends and subsequent removal of the CAB matrix. The sizes of the PE spheres or fibers can be successfully controlled by varying the composition ratio and modifying the interfacial properties of the blends. The surface structures of LDPE micro‐ or submicro‐spheres and nanofibers were analyzed using SEM and FTIR‐ATR spectroscopy. In addition, the crystalline structures of the LDPE nanofibers were characterized.
Natural fiber reinforced polymer composites are lightweight, economical and available in a variety of forms. They have low densities, comparable material properties, high molding flexibility and are environmentally friendly, making them a conceivable alternative to traditional fillers like mica, calcium carbonate and glass. By modifying either the resin system or the natural fiber, biocomposites can be designed for different applications ranging from products of commodity to aerospace, examples including electroactive papers, fuel cell membranes, controlled drug release mechanisms and biosensors. This review aims to analyze the advancement in the application of cellulose based materials in different sectors with a discussion of fundamental research in these areas.
The effects of process engineering in the fabrication of PHBV, PLA and their blends prepared by melt blending are studied. The elongation of an optimized blend can be improved by 148 and 250% over the virgin PHBV and PLA polymers, respectively. DSC shows that the two polymers are immiscible in blends of any composition. The crystallinity of PHBV is hindered by the presence of PLA. UV‐Vis demonstrates the opacity of the blend with incorporation of PHBV to the PLA phase. The observed tensile modulus of the optimized sample is compared with theoretical values from the rule of mixtures. Gordon‐Taylor's equation is applied on the glass transition temperatures for theoretical modeling to explain the miscibility of the polymers.
In this study, the sol‐gel transition temperature of a thermosensitive chitosan system was measured using SAOS, in‐real time FTR and multi‐frequency SAOS excitation. From FT analysis, we found that the intensity of the harmonics stayed constant while the chitosan system remained in the solution state, while it increased passed the gelation point. Multi‐frequency SAOS excitation was also carried out using a summation function of sine waves that allowed performing the measurements in the LVR. This last technique could determine the unique (frequency independent) critical sol‐gel transition temperature, and was found to be less tedious than the application of the traditional Chambon and Winter's method.
A novel mesoporous material (“nanopaper”) prepared from template‐synthesized, polyelectrolyte‐stabilized polymer nanotubes is reported. The stacked network of completely collapsed, flat nanotubes forms the porous structure, which has a water‐vapor permeability that can be tuned by the stabilizer. The transport mechanism is elucidated based on microscopy, thermal analysis, spectroscopy characterization, and mass‐transfer theory. The results suggest that the nanotube surface plays a key role in the through‐film transport process. This effect vanishes in the more open films formed from micro‐fibrillated cellulose having similar fibril diameters. Nanopaper mechanical properties are also reported. With a pore structure and functionality that can be varied, nanopaper is a promising functional membrane.
A new, nickel‐coated graphite resistance‐change‐based method for gel‐point determination for epoxy‐based thermoset resins is presented and compared with DSC and rheological methods. Gelation times determined by this new method are in very good agreement with conventional techniques; this new method is potentially simpler and less time consuming than existing ones.
