Multi- and triblock copolymers based on 1,5-dioxepan-2-one/epsilon-caprolactone (DXO/CL) were investigated by MALDI-TOF MS to determine the influence of copolymer composition and architecture on the molecular structures at the individual chain level. The copolymer compositions, average block lengths, and molecular weights were determined by H-1 and C-13 NMR and by SEC, respectively. The structures of polyether-ester oligomers (linear, cyclic) as well as the chemical structures of their end groups were established on the basis of their MALDI-TOF mass spectra. The mass spectrum of PDXO homopolymer was relatively simple, however, complex mass spectra were obtained in the case of multi- and triblock copolymers and the mass spectra clearly discerned the molecular level effect of copolymer composition and copolymer type.
Aliphatic polyesters prepared by ring-opening polymerization of lactones are now used worldwide as bioresorbabale devices in surgery (orthopaedic devices, sutures, stents, tissue engineering, and adhesion barriers) and in pharmacology (control drug delivery). This review presents the various methods of the synthesis of polyesters and tailoring the properties by proper control of molecular weight, composition, and architecture so as to meet the stringent requirements of devices in the medical field. The effect of structure on properties and degradation has been discussed. The applications of these polymers in the biomedical field are described in detail.
A new route for the design of renewable hydrogels is presented. The soluble waste from masonite production was isolated, fractionized, and upgraded. The resulting hemicellulose rich fraction was alkenyl-functionalized and used in the preparation of covalently cross-linked hydrogels capable of sustained release of incorporated agents. Said hydrogels showed a Fickian diffusion-based release of incorporated bovine serum albumin. Also, a method for the coating of seeds with hydrogel was developed. The sustained release of incorporated growth retardant agents from the hydrogel coating on rape seeds was shown to enable the temporary inhibition of germination.
Interfacial properties of two types of mucins have been investigated at the aqueous solution/poly(methyl methacrylate) (PMMA) interface. One is commercially available bovine submaxillary mucin, BSM, which consists of alternating glycosylated and nonglycosylated regions. The other one is a recombinant mucin-type fusion protein, PSGL-1/mIgG(2b), consisting of a glycosylated mucin part fused to the Fc part of an immunoglobulin. PSGL-1/mIgG(2b) is mainly expressed as a (timer upon production. A quartz crystal microbalance with dissipation was used to study the adsorption of the mucins to PMMA surfaces. The mass of the adsorbed mucin layers, including the adsorbed mucin and water trapped in the layer, was found to be significantly higher for PSGL-1/mIgG(2b) than for BSM. Atomic force microscopy with colloidal probe was employed to study interactions and frictional forces between mucin-coated PMMA surfaces. Purely repulsive forces of steric origin were Observed between PSGL-1/mIgG(2b) mucin layers, whereas a small adhesion was detected between BSM layers and attributed to bridging. Both mucin layers reduced the friction force between PMMA surfaces in aqueous solution. The reduction was, however, significantly more pronounced for PSGL-1/mIgG(2b). The effective friction coefficient between PSGL-1/mIgG(2b)-coated PMMA surfaces is as low as 0.02 at low loads, increasing to 0.24 at the highest load explored, 50 nN. In contrast, a friction coefficient of around 0.7 was obtained between BSM-coated PMMA surfaces. The large differences in interfacial properties for the two mucins are discussed in relation to their structural differences.
Ethylhydroxyethyl cellulose (EHEC) of three different viscosity classes (EHEC I, II, and III) was analyzed by programmed cross-flow asymmetrical flow field-flow fractionation coupled to multiangle light scattering and refractive index detectors to determine their size and molar mass distribution. Two size populations were detected in the two lower viscosity classes, EHEC I and II, one high molar mass and one ultrahigh molar mass (UHM). The two covered molar masses from 10(4) up to 10(9) g.mol(-1). The highest viscosity class EHEC III was less size-dispersed covering molar masses from 5x10(5) to 5x10(7) g.mol(-1). Filtering of the EHEC II solution removed small amounts of compact UHM material. Enzyme treatments were performed on EHEC II to further characterize it. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and anion ion-exchange chromatography coupled to pulsed amperometric detection showed that the UHM component contained EHEC.
The new combinations of monomers presented in this work were evaluated in order to create an elastic material for potential application in soft tissue engineering. Thermoplastic elastomers (TPE) of trimethylene carbonate (TMC) with L-lactide (LLA) and 1,5-dioxepan-2-one (DXO) have been synthesized using a cyclic five-membered tin alkoxide initiator. The block copolymers were designed in such a way that poly(trimethylene carbonate-co1,5-dioxepan-2-one) formed an amorphous middle block and the poly(L-lactide) (PLLA) formed semicrystalline terminal blocks. The amorphous middle block consisted of relatively randomly distributed TMC and DXO monomer units, and the defined block structure of the PLLA terminal segments was confirmed by C-13 NMR. The properties of the TMC-DXO-LLA copolymers were compared with those of triblock copolymers based either on LLA-TMC or on LLA-DXO. Differential scanning calorimetry and dynamic mechanical analysis data confirmed the micro-phase separation in the copolymers. The mechanical properties of the copolymers were evaluated using tensile testing and cycling loading. All of the copolymers synthesized showed a highly elastic behavior. The properties of copolymers could be tailored by altering the proportions of the different monomers.
Cellulose nanocomposites can be considered for semistructural load-bearing applications where modulus and strength requirements exceed 10 GPa and 100 MPa, respectively. Such properties are higher than for most neat polymers but typical for molded short glass fiber composites. The research challenge for polymer matrix biocomposites is to develop processing concepts that allow high cellulose nanofibril (CNF) content, nanostructural control in the form of well-dispersed CNF, the use of suitable polymer matrices, as well as molecular scale interface tailoring to address moisture effects. From a practical point of view, the processing concept needs to be scalable so that large-scale industrial processing is feasible. The vast majority of cellulose nanocomposite studies elaborate on materials with low nanocellulose content. An important reason is the challenge to prevent CNF agglomeration at high CNF content. Research activities are therefore needed on concepts with the potential for rapid processing with controlled nanostructure, including well-dispersed fibrils at high CNF content so that favorable properties are obtained. This perspective discusses processing strategies, agglomeration problems, opportunities, and effects from interface tailoring. Specifically, preformed CNF mats can be used to design nanostructured biocomposites with high CNF content. Because very few composite materials combine functional and structural properties, CNF materials are an exception in this sense. The suggested processing concept could include functional components (inorganic clays, carbon nanotubes, magnetic nanoparticles, among others). In functional three-phase systems, CNF networks are combined with functional components (nanoparticles or fibril coatings) together with a ductile polymer matrix. Such materials can have functional properties (optical, magnetic, electric, etc.) in combination with mechanical performance, and the comparably low cost of nanocellulose may facilitate the use of large nanocomposite structures in industrial applications.
In this work, the problem to disperse cellulose nanocrystals (CNC) in hydrophobic polymer matrices has been addressed through application of an environmentally friendly chemical modification approach inspired by clay chemistry. The objective is to compare the effects of unmodified CNC and modified CNC (modCNC) reinforcement, where degree of CNC dispersion is of interest. Hydrophobic functionalization made it possible to disperse wood-based modCNC in organic solvent and cast well-dispersed nanocomposite films of poly(vinyl acetate) (PVAc) with 1-20 wt % CNC. Composite films were studied by infrared spectroscopy (FT-IR), UV-vis spectroscopy, dynamic mechanical thermal analysis (DMTA), tensile testing, and field-emission scanning electron microscopy (FE-SEM). Strongly increased mechanical properties were observed for modCNC nanocomposites. The reinforcement efficiency was much lower in unmodified CNC composites, and specific mechanisms causing the differences are discussed.
Lignosulfonates are by-products from the sulfite pulping process. During this process, lignin is liberated from pulp fibers through sulfonation and washed away. As a consequence, the lignosulfonate molecules contain both hydrophobic and hydrophilic moieties. Lignosulfonates are low-value products with limited performance and are used as such as binders, surfactants, and plasticizers in concrete. Lignosulfonates face strong competition from synthetic petroleum-based plasticizers with superior quality. Therefore, increasing the performance of lignosulfonates is desirable not only from a sustainability point of view but also to expand their usage. One important aspect that describes how well lignosulfonates can act as plasticizers is the molecular weight. In this paper, the molecular weight of four commercial lignosulfonates is increased through oxidation by two laccases without utilization of mediators. Different parameters to obtain maximal molecular weight increase were identified and the technical significance of the experiments is discussed.
The effective use of biodegradable polymers relies on the ability to control the onset of and time needed for degradation. Preferably, the material properties should be retained throughout the intended time frame, and the material should degrade in a rapid and controlled manner afterward. The degradation profiles of polyester materials were controlled through their miscibility. Systems composed of PLLA blended with poly[(R,S)-3-hydroxybutyrate] (a-PHB) and polypropylene adipate (PPA) with various molar masses were prepared through extrusion. Three different systems were used: miscible (PLLA/a-PHB5 and PLLA/a-PHB20), partially miscible (PLLA/PPA5/comp and PLLA/PPA20/comp), and immiscible (PLLA/PPA5 and PLLA/PPA20) blends. These blends and their respective homopolymers were hydrolytically degraded in water at 37 degrees C for up to I year. The blends exhibited entirely different degradation profiles but showed no diversity between the total degradation times of the materials. PLLA presented a two-stage degradation profile with a rapid decrease in molar mass during the early stages of degradation, similar to the profile of PLLA/a-PHB5. PLLA/a-PHB20 presented a single, constant linear degradation profile. PLLA/PPA5 and PLLA/PPA20 showed completely opposing degradation profiles relative to PLLA, exhibiting a slow initial phase and a rapid decrease after a prolonged degradation time. PLLA/PPA5/comp and PLLA/PPA20/comp had degradation profiles between those of the miscible and the immiscible blends. The molar masses of the materials were approximately the same after 1 year of degradation despite their different profiles. The blend composition and topographical images captured at the last degradation time point demonstrate that the blending component was not leached out during the period of study. The hydrolytic stability of degradable polyester materials can be tailored to obtain different and predetermined degradation profiles for future applications.
The fabrication of reusable, sustainable adsorbents from low-cost, renewable resources via energy efficient methods is challenging. This paper presents wet-stable, carboxymethylated cellulose nanofibril (CNF) and amyloid nanofibril (ANF) based aerogel-like adsorbents prepared through efficient and green processes for the removal of metal ions and dyes from water. The aerogels exhibit tunable densities (18-28 kg m-3), wet resilience, and an interconnected porous structure (99% porosity), with a pH controllable surface charge for adsorption of both cationic (methylene blue and Pb(II)) and anionic (brilliant blue, congo red, and Cr(VI)) model contaminants. The Langmuir saturation adsorption capacity of the aerogel was calculated to be 68, 79, and 42 mg g-1for brilliant blue, Pb(II), and Cr(VI), respectively. Adsorption kinetic studies for the adsorption of brilliant blue as a model contaminant demonstrated that a pseudo-second-order model best fitted the experimental data and that an intraparticle diffusion model suggests that there are three adsorption stages in the adsorption of brilliant blue on the aerogel. Following three cycles of adsorption and regeneration, the aerogels maintained nearly 97 and 96% of their adsorption capacity for methylene blue and Pb(II) as cationic contaminants and 89 and 80% for brilliant blue and Cr(VI) as anionic contaminants. Moreover, the aerogels showed remarkable selectivity for Pb(II) in the presence of calcium and magnesium as background ions, with a selectivity coefficient more than 2 orders of magnitude higher than calcium and magnesium. Overall, the energy-efficient and sustainable fabrication procedure, along with good structural stability, reusability, and selectivity, makes these aerogels very promising for water purification applications.
The possibility of forming self-organized films using only charge-stabilized dispersions of cellulose I nanofibrils with opposite charges is presented, that is, the multilayers were composed solely of anionically and cationically modified microfibrillated cellulose (MFC) with a low degree of substitution. The build-up behavior and the properties of the layer-by-layer (LbL)-constructed films were studied using a quartz crystal microbalance with dissipation (QCM-D) and stagnation point adsorption reflectometry (SPAR). The adsorption behavior of cationic/anionic MFC was compared with that of polyethyleneimine (PEI)/anionic MFC. The water contents of five bilayers of cationic/anionic MFC and PEI/anionic MFC were approximately 70 and 50%, respectively. The MFC surface coverage was studied by atomic force microscopy (AFM) measurements, which clearly showed a more dense fibrillar structure in the five bilayer PEI/anionic MFC than in the five bilayer cationic/anionic MFC. The forces between the cellulose-based multilayers were examined using the AFM colloidal probe technique. The forces on approach were characterized by a combination of electrostatic and steric repulsion. The wet adhesive forces were very long-range and were characterized by multiple adhesive events. Surfaces covered by PEU/anionic MFC multilayers required more energy to be separated than surfaces covered by cationic/anionic MFC multilayers.
There are numerous examples where animals or plants synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by fibrous biopolymers. Cellulose, the world's most abundant natural, renewable, biodegradable polymer, is a classical example of these reinforcing elements, which occur as whiskerlike microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, with the consequence of axial physical properties approaching those of perfect crystals. This quite "primitive" polymer can be used to create high performance nanocomposites presenting outstanding properties. This reinforcing capability results from the intrinsic chemical nature of cellulose and from its hierarchical structure. Aqueous suspensions of cellulose crystallites can be prepared by acid hydrolysis of cellulose. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspension by subsequent vigorous mechanical shearing action. During the past decade, many works have been devoted to mimic biocomposites by blending cellulose whiskers from different sources with polymer matrixes.
We report the mechanical strength of native chitin nanofibrils. Highly crystalline alpha-chitiri nanofibrils Were purified from filaments produced by a microalgae Phaeocystis globosa, and two types of beta-chitin nariofibrils were purified from pens of a squid Loligo bleekeri and tubes of a tubeworm Lamellibrachia satsuma, with relatively low and high crystallinity, respectively. These chitin nanofibrils were fully dispersed in water. The strength of individualized nanofibrils was estimated using cavitation induced tensile fracture of nanoscale filaments in a liquid medium. Both types of beta-chitin nanofibrils exhibited similar strength values of approximately 3 GP; in contrast, the alpha-chitin nanofibrils exhibited a much lower strength value of 1.6 GPa. These strength estimates suggest that the tensile strength of chitin nanofibrils is governed by the molecular packing modes of chitin rather than their crystallinity.
The temperature-dependence of xyloglucan (XG) adsorption onto smooth cellulose model films regenerated from N-methylmorpholine N-oxide (NMMO) was investigated using surface plasmon resonance spectroscopy, and it was found that the adsorbed amount increased with increasing temperature. This implies that the adsorption of XG to NMMO-regenerated cellulose is endothermic and supports the hypothesis that the adsorption of XG onto cellulose is an entropy-driven process. We suggest that XG adsorption is mainly driven by the release of water molecules from the highly hydrated cellulose surfaces and from the XG molecules, rather than through hydrogen bonding and van der Waals forces as previously suggested. To test this hypothesis, the adsorption of XG onto cellulose was studied using cellulose films with different morphologies prepared from cellulose nanocrystals (CNC), semicrystalline NMMO-regenerated cellulose, and amorphous cellulose regenerated from lithium chloride/dimethylacetamide. The total amount of high molecular weight xyloglucan (XGHMW) adsorbed was studied by quartz crystal microbalance and reflectometry measurements, and it was found that the adsorption was greatest on the amorphous cellulose followed by the CNC and NMMO-regenerated cellulose films. There was a significant correlation between the cellulose dry film thickness and the adsorbed XG amount, indicating that XG penetrated into the films. There was also a correlation between the swelling of the films and the adsorbed amounts and conformation of XG, which further strengthened the conclusion that the water content and the subsequent release of the water upon adsorption are important components of the adsorption process.
A process has been developed to create self-supporting hydrogels with low solids content (down to 0.5 wt %) and anisotropic aerogels with a low density (down to 5 kg/m(3)) from cellulose nanofibrils (CNFs). The CNF networks were formed by vacuum filtration of dilute dispersions (0.2 wt %) of 90% CNFs and 10% alginate. We call this process "the dynamic CNF network approach" since the solids content of these hydrogels can be tuned in the range of 0.5-3 wt % by reswelling the filter cakes in a medium with a controlled osmotic pressure. These hydrogels are significantly stronger than the 1-2 wt % CNF gels typically used to prepare hydrogels and aerogels because the dynamic CNF networks are formed below their arrested state threshold (ca. 0.5 wt %) and are thus homogeneous. The vacuum filtration leads to a directional reswelling vertical to the plane of the filter cake, and this is crucial in order to turn a two-dimensional (2D) shape, cut from the filter cake, into a 3D hydrogel without distorting the 2D shape. The anisotropic swelling was used to create intricate 3D-shaped hydrogels and solved some of the issues involved in the degassing and molding of high-viscosity CNF gels. Multivalent ions were used to lock the CNF and alginate networks at the desired solids content and 3D shape, and resulted in an increase by an order of magnitude in storage modulus. Moreover, the self-supporting nature of the hydrogels allowed us to freeze-cast them into anisotropic aerogels with the same 3D shape without using any container. The 5 kg/m(3) aerogel had a specific modulus of 43 kN m/kg and an anisotropy index of 12, which are impressive properties in relation to earlier experiences. The process can be used for applications where a precise control of density and shape is critical.
Bioactive and reinforced poly(ϵ-caprolactone) (PCL) films were constructed by incorporation of cellulose derived reduced nanographene oxide (r-nGO) carbon nanodots. Two different microwave-assisted reduction routes in superheated water were utilized to obtain r-nGO and r-nGO-CA. For the latter, a green reducing agent caffeic acid (CA), was incorporated in the reduction process. The materials were extruded and compression molded to obtain proper dispersion of the carbon nanodots in the polymer matrix. FTIR results revealed favorable interactions between r-nGO-CA and PCL that improved the dispersion of r-nGO-CA. r-nGO, and r-nGO-CA endorsed PCL with several advantageous functionalities including improved storage modulus and creep resistance. The considerable increase in storage modulus demonstrated that the carbon nanodots had a significant reinforcing effect on PCL. The PCL films with r-nGO-CA were also evaluated for their osteobioactivity and cytocompatibility. Bioactivity was demonstrated by formation of hydroxyapatite (HA) minerals on the surface of r-nGO-CA loaded nanocomposites. At the same time, the good cytocompatibility of PCL was retained as illustrated by the good cell viability to MG63 osteoblast-like cells giving promise for bone tissue engineering applications.
Biodegradable polymers complement recyclable materials in battling plastic waste because some products are difficult to recycle and some will end up in the environment either because of their application or due to wear of the products. Natural biopolymers, such as cellulose, are inherently biodegradable, but chemical modification typically required for the obtainment of thermoplastic properties, solubility, or other desired material properties can hinder or even prevent the biodegradation process. This Review summarizes current knowledge on the degradation of common cellulose derivatives in different laboratory, natural, and man-made environments. Depending on the environment, the degradation can be solely biodegradation or a combination of several processes, such as chemical and enzymatic hydrolysis, photodegradation, and oxidation. It is clear that the type of modification and especially the degree of substitution are important factors controlling the degradation process of cellulose derivatives in combination with the degradation environment. The big variation of conditions in different environments is also briefly considered as well as the importance of the proper testing environment, characterization of the degradation process, and confirmation of biodegradability. To ensure full sustainability of the new cellulose derivatives under development, the expected end-of-life scenario, whether material recycling or "biological"recycling, should be included as an important design parameter.
Multifunctional three-dimensional (3D) scaffolds were targeted by surface grafting cellulose-derived nanographene oxide (nGO) on the surface of porous poly(epsilon-caprolactone) (PCL) scaffolds. nGO was derived from cellulose by microwave-assisted carbonization process and covalently grafted onto aminolyzed PCL scaffolds through an aqueous solution process. Fourier transform infrared spectroscopy and thermogravimetric analysis both verified the successful attachment of nGO and scanning electron microscopy depicted a homogeneous dispersion of nGO over the scaffold surface. Mechanical tests were performed and demonstrated a significant increase in compressive strength for the nGO grafted scaffolds. Grafting of nGO was also shown to induce mineralization with the formation of calcium phosphate precipitates on the surface of the scaffolds with the size increasing with higher nGO content. The potential of surface-grafted nGO as a nanocarrier of an antibiotic drug was also explored. The secondary interactions between nGO and ciprofloxacin, a broad-spectrum antibiotic used in the treatment of osteomyelitis, were optimized by controlling the solution pH. Ciprofloxacin was found to be adsorbed most strongly in its cationic form at pH 5, in which pi-pi electron donor-acceptor interactions predominate and the adsorbed drug content increased with increasing nGO amount. Further, the release kinetics of the drug were investigated during 8 days. In conclusion, the proposed simple fabrication process led to a scaffold with multifunctionality in the form of improved mechanical strength, ability to induce mineralization, as well as drug loading and delivery capability.
Glycerol-plasticized wheat gluten was explored for producing soft high-density biofoams using dry upscalable extrusion (avoiding purposely added water). The largest pore size was obtained when using the food grade ammonium bicarbonate (ABC) as blowing agent, also resulting in the highest saline liquid uptake. Foams were, however, also obtained without adding a blowing agent, possibly due to a rapid moisture uptake by the dried protein powder when fed to the extruder. ABC's low decomposition temperature enabled extrusion of the material at a temperature as low as 70 °C, well below the protein aggregation temperature. Sodium bicarbonate (SBC), the most common food-grade blowing agent, did not yield the same high foam qualities. SBC's alkalinity, and the need to use a higher processing temperature (120 °C), resulted in high protein cross-linking and aggregation. The results show the potential of an energy-efficient and industrially upscalable low-temperature foam extrusion process for competitive production of sustainable biofoams using inexpensive and readily available protein obtained from industrial biomass (wheat gluten).
In 1628, the Swedish warship Vasa capsized on her maiden voyage and sank in the Stockholm harbor. The ship was recovered in 1961 and, after polyethylene glycol (PEG) impregnation, it was displayed in the Vasa museum. Chemical investigations of the Vasa were undertaken in 2000, and extensive holocellulose degradation was reported at numerous locations in the hull. We have now studied the longitudinal tensile strength of Vasa oak as a function of distance from the surface. The PEG-content, wood density, and cellulose microfibril angle were determined. The molar mass distribution of holocellulose was determined as well as the acid and iron content. A good correlation was found between the tensile strength of the Vasa oak and the average molecular weight of the holocellulose, where the load-bearing cellulose microfibril is the critical constituent. The mean tensile strength is reduced by approximately 40%, and the most affected areas show a reduction of up to 80%. A methodology is developed where variations in density, cellulose microfibril angle, and PEG content are taken into account, so that cell wall effects can be evaluated in wood samples with different rate of impregnation and morphologies.
Several key enzymes in lignin biosynthesis of Populus have been down-regulated by transgenie approaches to investigate their role in wood lignification and to explore their potential for lignin modification. Cinnamate 4-hydroxylase is an enzyme in the early phenylpropanoid pathway that has not yet been functionally analyzed in Populus. This study shows that down-regulation of cinnamate 4-hydroxylase reduced Klason lignin content by 30% with no significant change in syringyl to guaiacyl ratio. The lignin reduction resulted in ultrastructural differences of the wood and a 10% decrease in wood density. Mechanical properties investigated by tensile tests and dynamic mechanical analysis showed a decrease in stiffness, which could be explained by the lower density. The study demonstrates that a large modification in lignin content only has minor influences on tensile properties of wood in its axial direction and highlights the usefulness of wood modified beyond its natural variation by transgene technology in exploring the impact of wood biopolymer composition and ultrastructure on its material properties.
This Article reports the influence of the protein network structure on the mechanical properties of foams produced from commercial wheat gluten using freeze-drying. Foams were produced from alkaline aqueous solutions at various gluten concentrations with or without glycerol, modified with bacterial cellulose nanosized fibers, or both. The results showed that 20 wt % glycerol was sufficient for plasticization, yielding foams with low modulus and high strain recovery. It was found that when fibers were mixed into the foams, a small but insignificant increase in elastic modulus was achieved, and the foam structure became more homogeneous. SEM indicated that the compatibility between the fibers and the matrix was good, with fibers acting as bridges in the cell walls. IR spectroscopy and SE-HPLC revealed a relatively low degree of aggregation, which was highest in the presence of glycerol. Confocal laser scanning microscopy revealed distinct differences in HMW-glutenin subunits and gliadin distributions for all of the different samples.
This paper describes a novel method for introducing the RGD cell adhesion peptide to enhance cell adhesion onto bacterial cellulose (BC). BC and cotton linters as reference were modified with xyloglucan (XG) and xyloglugan bearing a GRGDS pentapeptide. The adsorptions followed Langmuir adsorption behavior, where both XGs probably decorate the cellulose surfaces as a monolayer. The adsorption maximum of the XGs reached around 180 mg/g on BC and only about three times as much on cotton linters. The adsorption was verified with colorimetric methods. The specific surface area of BC measured with XG and XG-GRGDS was about 200 m(2)/g and was almost three times less for cotton linters, 60 m2/g. The difference in the amounts of XGs adsorbed might be explained by the swollen network of bacterial cellulose and a more exposed and accessible bulk as compared to cotton linters. The nanocellulose material was modified homogeneously throughout the material, as seen by the z-scan in confocal microscopy. Moreover, the modification in the water phase, in comparison with organic solvents, was clearly advantageous for preserving the morphology, as observed with SEM. The modification slightly increased the wettability, which might explain the decrease in or undetectable adsorption of adhesive protein shown by QCM-D. Initial cell studies showed that adhesion of human endothelial cells is enhanced when the BC hydrogel is modified with XG-GRGDS. QCM-D studies further revealed that the cell enhancement is due to the presence of the RGD epitope on XG and not to a nonspecific adsorption of fibronectin from cell culture medium. Optimization and proliferation studies of human endothelial cells onto bacterial cellulose modified with XG-GRGDS are currently being carried out at the Vascular Engineering Center, Sahlgrenska University Hospital, Gothenburg.
This study focused on the assembly characteristics of debranched xylan onto cellulose surfaces. A rye arabinoxylan polymer with an initial arabinose/xylose ratio of 0.53 was debranched with an oxalic acid treatment as a function of time. The resulting samples contained reduced arabinose/xylose ratios significantly affecting the molecular architecture and solution behavior of the biopolymer. With this treatment, an almost linear xylan with arabinose DS of only 0.04 was obtained. The removal of arabinose units resulted in the self-assembly of the debranched polymer in water into stable nanoparticle aggregates with a size around 300 nm with a gradual increase in crystallinity of the isolated xylan. Using quartz crystal microbalance with dissipation monitoring, the adsorption of xylan onto model cellulose surfaces was quantified. Compared to the nonmodified xylan, the adsorption of debranched xylan increased from 0.6 to 5.5 mg m(-2). Additionally, adsorption kinetics suggest that the nanoparticles rapidly adsorbed to the cellulose surfaces compared to the arabinoxylan. In summary, a control of the molecular structure of xylan influences its ability to form a new class of polysaccharide nanoparticles in aqueous suspensions and its interaction with nanocellulose surfaces.
A critical aspect in materials design of polymer nanocomposites is the nature of the nanoparticle/polymer interface. The present study investigates the effect of manipulation of the interface between cellulose nanofibrils (CNF) and poly(methyl methacrylate) (PMMA) on the optical, thermal, and mechanical properties of the corresponding nanocomposites. The CNF/PMMA interface is altered with a minimum of changes in material composition so that interface effects can be analyzed. The hydroxyl-rich surface of CNF fibrils is exploited to modify the CNF surface via an epoxide-hydroxyl reaction. CNF/PMMA nanocomposites are then prepared with high CNF content (similar to 38 wt %) using an approach where a porous CNF mat is impregnated with monomer or polymer. The nanocomposite interface is controlled by either providing PMMA grafts from the modified CNF surface or by solvent-assisted diffusion of PMMA into a CNF network (native and modified). The high content of CNF fibrils of similar to 6 nm diameter leads to a strong interface and polymer matrix distribution effects. Moisture uptake and mechanical properties are measured at different relative humidity conditions. The nanocomposites with PMMA molecules grafted to cellulose exhibited much higher optical transparency, thermal stability, and hygro-mechanical properties than the control samples. The present modification and preparation strategies are versatile and may be used for cellulose nanocomposites of other compositions, architectures, properties, and functionalities.
Microbubbles (MBs) are commonly used as injectable ultrasound contrast agent (UCA) in modern ultrasonography. Polymer-shelled UCAs present additional potentialities with respect to marketed lipid-shelled UCAs. They are more robust; that is, they have longer shelf and circulation life, and surface modifications are quite easily accomplished to obtain enhanced targeting and local drug delivery. The next generation of UCAs will be required to support not only ultrasound-based imaging methods but also other complementary diagnostic approaches such as magnetic resonance imaging or computer tomography. This work addresses the features of MBs that could function as contrast agents for both ultrasound and magnetic resonance imaging. The results indicate that the introduction of iron oxide nanoparticles (SPIONs) in the poly(vinyl alcohol) shell or on the external surface of the MBs does not greatly decrease the echogenicity of the host MBs compared with the unmodified one. The presence of SPIONs provides enough magnetic susceptibility to the MBs to accomplish good detectability both in vitro and in vivo. The distribution of SPIONs on the shell and their aggregation state seem to be key factors for the optimization of the transverse relaxation rate.
In the context of three-dimensional (3D) cell culture and tissue engineering, 3D printing is a powerful tool for customizing in vitro 3D cell culture models that are critical for understanding the cell-matrix and cell-cell interactions. Cellulose nanofibril (CNF) hydrogels are emerging in constructing scaffolds able to imitate tissue in a microenvironment. A direct modification of the methacryloyl (MA) group onto CNF is an appealing approach to synthesize photocross-linkable building blocks in formulating CNF-based bioinks for light-assisted 3D printing; however, it faces the challenge of the low efficiency of heterogenous surface modification. Here, a multistep approach yields CNF methacrylate (CNF-MA) with a decent degree of substitution while maintaining a highly dispersible CNF hydrogel, and CNF-MA is further formulated and copolymerized with monomeric acrylamide (AA) to form a super transparent hydrogel with tuneable mechanical strength (compression modulus, approximately 5-15 kPa). The resulting photocurable hydrogel shows good printability in direct ink writing and good cytocompatibility with HeLa and human dermal fibroblast cell lines. Moreover, the hydrogel reswells in water and expands to all directions to restore its original dimension after being air-dried, with further enhanced mechanical properties, for example, Young’s modulus of a 1.1% CNF-MA/1% PAA hydrogel after reswelling in water increases to 10.3 kPa from 5.5 kPa.
This work demonstrates a versatile and environmentally friendly route for the development of new orthogonal monomers that can be used for postfunctionalizable polymer networks. A monomer containing both vinyl ether (VE) and cyclic disulfide moieties was synthesized via enzyme catalysis under benign reaction conditions. The bifunctional monomer could be polymerized to form macromolecues with differing architectures by the use of either cationic or radical photo polymerization. When cationic polymerization was performed, a linear polymer was obtained with pendant disulfide units in the side chain, whereas in the presence of radical initiator, the VE reacted with the disulfide to yield a branched structure. The monomer was thereafter used to design networks that could be postfunctionalized; the monomer was cross-linked with cationic initiation together with a difunctional VE oligomer and after cross-linking the unreacted disulfides were coupled to RhodamineVE by radical UV-initiation.
Functionalized wheat gluten (WG) protein particles with the ability to absorb fluids within the superabsorbent range are presented. Ethyleneditetraacetic dianhydride (EDTAD), a nontoxic acylation agent, was used for the functionalization of the WG protein at higher protein content than previously reported and no additional chemical cross-linking. The 150-550 μm protein particles had 50-150 nm nanopores induced by drying. The EDTAD treated WG were able to absorb 22, 5, and 3 times of, respectively, water, saline and blood, per gram of dry material (g/g), corresponding to 1000, 150 and 100% higher values than for the as-received WG powder. The liquid retention capacity after centrifugation revealed that almost 50% of the saline liquid was retained within the protein network, which is similar to that for petroleum-based superabsorbent polymers (SAPs). An advantageous feature of these biobased particulate materials is that the maximum swelling is obtained within the first 10 min of exposure, that is, in contrast to many commercial SAP alternatives. The large swelling in a denaturation agent (6 M urea) solution (about 32 g/g) suggests that the secondary entangled/folded structure of the protein restricts protein network expansion and when disrupted allows the absorption of even higher amounts of liquid. The increased liquid uptake, utilization of inexpensive protein coproducts, easy scalable protocols, and absence of any toxic chemicals make these new WG-based SAP particles an interesting alternative to petroleum-based SAP in, for example, absorbent disposable hygiene products.
Cellulose fibers, in the form of a conventional filter paper, have been modified by reacting the hydroxyl groups on the fiber surface with 2-bromoisobutyryl bromide, followed by grafting using ATRP conditions. The papers were first grafted with methyl acrylate (MA), rendering the paper very hydrophobic as reported in an earlier work. The papers were analyzed by gravimetry, FT-IR, ESCA, and AFM. To verify that the polymerization from the surface was living, a second layer of another, hydrophilic, polymer, 2-hydroxyethyl methacrylate (HEMA), was grafted upon the PMA layer, creating a block-copolymer graft from the fibers. After the layer of PHEMA had been attached, contact angle measurements were no longer possible, because of the absorbing nature of PHEMA-grafted layer. This indicates that a copolymer had indeed been formed on the surface. FT-IR showed a large increase in carbonyl content after the PHEMA-grafting, which further proves that a layer of PHEMA was attached to the PMA layer. This goes to show that the hydrophilic/ hydrophobic behavior of a cellulose surface can be tailored by the use of living/controlled radical polymerization methods such as ATRP.
The principal purpose of the investigation was to clarify the mechanisms behind the stabilizing action of cellulose nanofibrils (CNFs) in wet-stable cellulose foams. Following the basic theories for particle-stabilized foams, the investigation was focused on how the surface energy of the stabilizing CNF particles, their aspect ratio and charge density, and the concentration of CNF particles at the air-water interface affect the foam stability and the mechanical properties of a particle-stabilized air-liquid interface. The foam stability was evaluated from how the foam height changed over time, and the mechanical properties of the interface were evaluated as the complex viscoelastic modulus of the interface using the pendant drop method. The most important results and conclusions are that CNFs can be used as stabilizing particles for aqueous foams already at a concentration as low as 5 g/L. The major reasons for this were the small dimensions of the CNF and their high aspect ratio, which is important for gel-formation and the complex viscoelastic modulus of the particle-filled air-water interface. The influence of the aspect ratio was also demonstrated by a much higher foam stability of foams stabilized with CNFs than of foams stabilized by cellulose nanocrystals (CNC) with the same chemical composition. The charge density of the CNFs affects the level of liberation within larger aggregates and hence also the number of contact points at the interface and the gel formation and complex viscoelastic modulus of the air-water interface. The charges also result in a disjoining pressure related to the long-range repulsive electrostatic pressure between particle-stabilized bubbles and hence contribute to foam stability. (Figure Presented).
A contact-active antibacterial approach based on the physical adsorption of a cationic polyelectrolyte onto the surface of a cellulose material is today regarded as an environment-friendly way of creating antibacterial surfaces and materials. In this approach, the electrostatic charge of the treated surfaces is considered to be an important factor for the level of bacteria adsorption and deactivation/killing of the bacteria. In order to clarify the influence of surface charge density of the cellulose on bacteria adsorption as well as on their viability, bacteria were adsorbed onto cellulose model surfaces, which were modified by physically adsorbed cationic polyelectrolytes to create surfaces with different positive charge densities. The surface charge was altered by the layer-by-layer (LbL) assembly of cationic polyvinylamine (PVAm)/anionic cellulose nanofibril/PVAm onto the initially differently charged cellulose model surfaces. After exposing the LbL-treated surfaces to Escherichia coli in aqueous media, a positive correlation was found between the adsorption of bacteria as well as the ratio of nonviable/viable bacteria and the surface charge of the LbL-modified cellulose. By careful colloidal probe atomic force microscopy measurements, it was estimated, due to the difference in surface charges, that interaction forces at least 50 nN between the treated surfaces and a bacterium could be achieved for the surfaces with the highest surface charge, and it is suggested that these considerable interaction forces are sufficient to disrupt the bacterial cell wall and hence kill the bacteria.
Macromolecular dynamics in biological systems, which play a crucial role for biomolecular function and activity at ambient temperature, depend strongly on moisture content. Yet, a generally accepted quantitative model of hydration-dependent phenomena based on local relaxation and diffusive dynamics of both polymer and its adsorbed water is still missing. In this work, atomistic-scale spatial distributions of motional modes are calculated using molecular dynamics simulations of hydrated xyloglucan (XG). These are shown to reproduce experimental hydration-dependent C-13 NMR longitudinal relaxation times (T-1) at room temperature, and relevant features of their broad distributions, which are indicative of locally heterogeneous polymer reorientational dynamics. At low hydration, the self-diffusion behavior of water shows that water molecules are confined to particular locations in the randomly aggregated XG network while the average polymer segmental mobility remains low. Upon increasing water content, the hydration network becomes mobile and fully accessible for individual water molecules, and the motion of hydrated XG segments becomes faster. Yet, the polymer network retains a heterogeneous gel-like structure even at the highest level of hydration. We show that the observed distribution of relaxations times arises from the spatial heterogeneity of chain mobility that in turn is a result of heterogeneous distribution of water-chain and chain chain interactions. Our findings contribute to the picture of hydration-dependent dynamics in other macromolecules such as proteins, DNA, and synthetic polymers, and hold important implications for the mechanical properties of polysaccharide matrixes in plants and plant-based materials.
The elastic tensors of chitin and chitosan allomorphs were calculated using density functional theory (DFT) with and without the dispersion correction and compared with experimental values. The longitudinal Young's moduli were 114.9 or 126.9 GPa for alpha-chitin depending on the hydrogen bond pattern: 129.0 GPa for beta-chitin and 191.5 GPa for chitosan. Furthermore, the moduli were found to vary between 17.0 and 52.8 GPa in the transverse directions and between 2.2 and 15.2 GPa in shear. Switching off the dispersion correction led to a decrease in modulus by up to 63%, depending on the direction. The transverse Young's moduli of a-chitin strongly depended on the hydroxylmethyl group conformation coupled with the dispersion correction, suggesting a synergy between hydrogen bonding and dispersion interactions. The calculated longitudinal Young's moduli were, in general, higher than experimental values obtained in static conditions, and the Poisson's ratios were lower than experimental values obtained in static conditions.
The Young's modulus of multilayer films containing nanofibrillated cellulose (NFC) and polyethyleneimine (PEI) was determined Using the strain-induced elastic buckling instability for mechanical measurements (SLEBIMM) technique.(1) Multilayer films were built up on polydimethylsiloxane substrates using electrostatic layer-by-layer assembly. At 50% relative humidity, SIEBIMM gave a constant Young's modulus of 1.5 +/- 0.2 GPa for 35-75 run thick films. Conversely, in vacuum, the Young's modulus was 10 times larger, at 17.2 +/- 1.2 GPa. A slight decrease in buckling wavelength with increasing strain was observed by scanning electron microscopy with in situ compression, and above 10% strain, extensive cracking parallel to the compressive direction occurred. We conclude that whereas PEI acts as a "glue" to hold multiple layers of NFC together, it prevents full development of hydrogen bonding and specific fibril-fibril interactions, and at high humidity, its hygroscopic nature decreases the elastic modulus when compared with pure NFC films.
Ternary poly(N-isopropylacrylamide)/poly(2-alkyl-2-oxazoline)/poly(2-hydroxyethy l methacrylate) (PNIPAAm/PROZO/PHEMA) hydrogels were prepared by the free-radical copolymerization of N-isopropylacrylamide (NIPAAm), 2-hydroxyethyl methacrylate (HEMA), and poly(2-alkyl-2-oxazoline) (PROZO) multifunctional macromonomers. The resulting polymeric materials were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), as well as by equilibrium swelling experiments. All synthesized hydrogels display temperature sensitivity in the 28-38 degrees C range. A high rate of response was registered as compared to that of materials based only on PNIPAAm. The swelling-deswelling peculiar behavior was related to the chemical composition (hydrophile/hydrophobe balance), the length of the inserted PROZO sequence, and inner morphology, an aspect which points on its possible control by synthesis. It was evidenced that the architecture of the resulting porous materials has a high order degree, emerging from the self-assembling of the microgel particles, which provided numerous, nearly uniform, large water release channels.
A hydrogel that can deliver both proteins and cells enables the local microenvironment of transplanted cells to be manipulated with a single injection. Toward this goal, we designed a hydrogel suitable for the co-delivery of neural stem cells and chondroitinase ABC (ChABC), a potent enzyme that degrades the glial scar that forms after central nervous system (CNS) injury. We leveraged the inverse electron-demand Diels-Alder reaction between norbomene and methylphenyltetrazine to form rapidly gelling (<15 min) crosslinked methylcellulose (MC) hydrogels at physiological temperature and pH, with Young's modulus similar to that of brain tissue (1-3 kpa), and degradable, disulfide-containing crosslinkers. To achieve tunable, affinity-controlled release of a ChABC-Src homology 3 (SH3) fusion protein, we immobilized norbornene-functionalized SH3-binding peptides onto MC-methylphenyltetrazine and observed release of bioactive ChABC-SH3 over 4 days. We confirmed cytocompatibility by evaluating neural progenitor cell survival and proliferation. The combined encapsulation of neural stem cells and chondroitinase ABC from one hydrogel lays the framework for future in vivo studies to treat CNS injuries.
The effects and mechanism of pulp delignificabon by laccases in the presence of redox mediators have been investigated on unbleached eucalyptus kraft pulp treated with laccases from Pycnoporus cinnabarinus (PcL) and Myceliophthora thermophila (MtL) and 1-hydroxybenzotriazole (HBT) and methyl syringate (MeS) as mediators, respectively. Determination of the corrected kappa number in eucalyptus pulps after the enzymatic treatments revealed that the PcL-HBT system exhibited a more remarkable delignification effect than the MtL-MeS system. To obtain further insight, lignin-carbohydrate complexes were fractionated and subsequently characterized by nuclear magnetic resonance, thioacidolysis (followed by gas chromatography and size exclusion chromatography), and pyrolysis-gas chromatography-mass spectrometry (pyrolysis-GC-MS) analyses before and after the enzymatic treatments and their controls. We can conclude that the laccase mediator treatments altered the lignin structures in such a way that more lignin was recovered in the xylan-lignin fractions, as shown by Klason lignin estimation, with smaller amounts of both syringyl (5) and guaiacyl (G) uncondensed units, as shown by thioacidolysis and gas chromatography, especially after the PcL-HBT treatment. The laccase mediator treatment produced oxidation at C alpha and cleavage of C alpha and C beta bonds in pulp lignin, as shown by pyrolysis-GC-MS. The general mechanism of residual lignin degradation in the pulp by laccase-mediator treatments is discussed in light of the results obtained.
A pH- and light-responsive polymer has been synthesized from softwood kraft lignin by a two-step strategy that aimed to incorporate diazobenzene groups. Initially, styrene oxide was reacted with the phenolic hydroxyl groups in lignin, to offer the attachment of benzene rings, thus creating unhindered reactive sites for further modifications. The use of advanced spectroscopic techniques H-1 and P-31 NMR, UV and FTIR) demonstrated that the reaction was quantitative and selective toward the phenolic hydroxyl groups. In a second step, the newly incorporated benzene rings were reacted with a diazonium cation to form the target diazobenzene motif, whose formation was again thoroughly verified. As anticipated, the diazobenzene-containing kraft lignin derivatives showed a pH-dependent color change in solution and light-responsive properties resulting from the cis-trans photoisomerization of the diazobenzene group.
The chemical strategy presented herein is the nondestructive preparation of resorbable polymer scaffolds with heparin covalently bonded to the surface and an osteoinductive growth factor, recombinant human bone morphogenetic protein-2, immobilized in the heparin layer. The coupling scheme involves functionalization of surfaces by grafting in the vapor phase with poly(L-lactide) and poly (e-caprolactone) films chosen as representative substrates. The biocompatibility of functionalized surfaces was verified by a much improved attachment and proliferation of mesenchymal stem cells (MSC).
A water-based one-pot synthesis strategy for converting cellulose nanofibrils (CNF) into a hydrophobic and processable biopolymer grade is devised. CNF was chemically modified through admicellar polymerization, producing fibrils coated with fatty acrylate polymers. The proposed modification targets a change in the interfibrillar interactions and improved CNF compatibility with a degradable plastic composite matrix, poly(butylene adipate-co-terephthalate), PBAT in composites prepared by melt extrusion. CNF had a clear reinforcing effect on PBAT, increasing Young's modulus by at least 35% and 169% at 5 and 20% (w/w) CNF content, respectively. However, unmodified CNF showed aggregation, poor adhesion in the matrix, and severely impaired the ductility of PBAT. CNF modified by admicellar polymerization was homogeneously dispersed in the PBT matrix and showed significantly better preservation of the elongation properties compared to unmodified CNF, especially at 5% (w/w) addition level.
Biobased free-standing films and coatings with low oxygen permeability were designed from a wood hydrolysate according to a recovery and formulation procedure that provides added value to wood converting industrial processes. Wood components released to the wastewater in the hydrothermal treatment of spruce wood were recovered and converted to an oligo- and polysaccharide-rich, noncellulosic fraction that was utilized in film formulations in a range of concentrations and compositions. Free-standing smooth and transparent films as well as coatings on thin PET were prepared and characterized with respect to oxygen permeability, tensile properties, structure, and water vapor transmission. With oxygen permeabilities as low as below I cm(3) mu m m(-2) day(-1) kPa(-1) and with adequate mechanical properties, the films and coatings show promising property profiles for renewable packaging applications.
The development of wood-based thermoplastic polymersthat can replacesynthetic plastics is of high environmental importance, and previousstudies have indicated that cellulose-rich fiber containing dialcoholcellulose (ring-opened cellulose) is a very promising candidate material.In this study, molecular dynamics simulations, complemented with experiments,were used to investigate how and why the degree of ring opening influencesthe properties of dialcohol cellulose, and how temperature and presenceof water affect the material properties. Mechanical tensile properties,diffusion/mobility-related properties, densities, glass-transitiontemperatures, potential energies, hydrogen bonds, and free volumeswere simulated for amorphous cellulosic materials with 0-100%ring opening, at ambient and high (150 degrees C) temperatures, withand without water. The simulations showed that the impact of ringopenings, with respect to providing molecular mobility, was higherat high temperatures. This was also observed experimentally. Hence,the ring opening had the strongest beneficial effect on "processability"(reduced stiffness and strength) above the glass-transition temperatureand in wet conditions. It also had the effect of lowering the glass-transitiontemperature. The results here showed that molecular dynamics is avaluable tool in the development of wood-based materials with optimalthermoplastic properties.