Reorientation of crystalline and noncrystalline regions in regenerated cellulose fibers and films tested in uniaxial tension

The orientation of molecular chains in regenerated cellulose films and fibers was characterized using in situ wide‐angle X‐ray diffraction and birefringence measurements coupled with tensile tests. Generally, an increase in the degree of preferred orientation in the direction of applied strain was observed during testing. For both types of specimen this relationship was clearly linear, irrespective of whether the volume‐averaged preferred orientation or the orientation in the crystalline and noncrystalline regions was considered. Interestingly, the rate of change in orientation induced by external strain was significantly higher for noncrystalline regions when compared with that of crystalline regions. This difference was more pronounced for cellulose fibers when compared with films. Upon the reversal of straining in cellulose films until zero stress, the degree of orientation diminished in a linear fashion. However, a large part of the orientation, both crystalline and noncrystalline, induced by tensile straining remained permanent and increased further when straining was resumed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 297–304, 2008     

Surface properties of cellulose modified by imidazolidinone

The influence of the modification of cellulose fibres by the imidazolidinone derivative 1,3-dimethyl-4,5-dihydroxyethylene urea (DMeDHEU) on fibre surface free energy and electrochemical potential was studied. The presence of DMeDHEU in the cellulose structure was confirmed by infrared spectral analysis. The surface free energy of untreated and treated cellulose fibres was determined from the results of thin-layer wicking, where the rate of liquid penetration into the cellulose fabric was measured. Using the van Oss-Chaudhury-Good theoretical approach, apolar, γ _S ^LW , polar electron-acceptor, γ _S ⁺, and electron-donor, γ _S ⁻, components of the surface free energy were calculated. The electrokinetic potential was determined from the results of steaming potential measurements. The results revealed that the incorporation of DMeDHEU into the cellulose structure lead to a decrease in the value of γ _S ⁻, whereas the values of γ _S ^LW and γ _S ⁺ remained almost unchanged. Despite their decreased γ _S ⁻ value, the treated cellulose fibres still represent a monopolar solid with a strongly expressed electron-donor component. The values of ΔG _iw and ΔG _iwi suggested that both untreated and treated cellulose samples could be considered hydrophilic substrates. The results of the electrokinetic potential measurements showed that the consumption of cellulose hydroxyl groups in the crosslinking reaction with DMeDHEU did not decrease the electrokinetic properties of the cellulose surface.     

On the determination of crystallinity and cellulose content in plant fibres

A comparative study of cellulose crystallinity based on the sample crystallinity and the cellulose content in plant fibres was performed for samples of different origin. Strong acid hydrolysis was found superior to agricultural fibre analysis and comprehensive plant fibre analysis for a consistent determination of the cellulose content. Crystallinity determinations were based on X-ray powder diffraction methods using side-loaded samples in reflection (Bragg-Brentano) mode. Rietveld refinements based on the recently published crystal structure of cellulose Iβ followed by integration of the crystalline and amorphous (background) parts were performed. This was shown to be straightforward to use and in many ways advantageous to traditional crystallinity determinations using the Segal or the Ruland–Vonk methods. The determined cellulose crystallinities were 90–100 g/100 g cellulose in plant-based fibres and 60–70 g/100 g cellulose in wood based fibres. These findings are significant in relation to strong fibre composites and bio-ethanol production.     

Solvent and enzyme induced recrystallization of mechanically degraded hemp cellulose

The structural degradation of cellulose fibre from hemp (Cannabis Sativa L.) by a ball-milling process and the recrystallization behavior of the product were studied. A linear increase in the Brunauer–Emmett–Teller specific surface area was observed; indicating the fibre bundles were being crushed and disrupted to single fibres, which was confirmed by SEM. An increase in the milling duration gradually destroyed the crystalline structure of the cellulose fibres, observed by a reduction of the 002 plane intensity in wide angle X-ray scattering measurements. The crystalline order index calculated from the area ratio of the 002 to the 021, 10 and 002 planes was decreased from 65 to 36 after milling for 330 min. Subsequently the lower thermal stability of ball-milled fibre was observed from a decrease in the temperature at the maximum mass loss rate using thermogravimetry. An increase in solvent polarity, solvent-fibre ratio, agitation speed and drying rate resulted in the rearrangement of ball-milled cellulose crystalline structure to a greater order. Moreover, an increase in the BET specific surface area and the amorphous fraction improved the scouring efficiency of the ball-milled cellulose using the pectate lyase enzyme (EC. 4.2.2.2).     

The effect of barley husk arabinoxylan adsorption on the properties of cellulose fibres

This study investigates the adsorption of (glucurono)arabinoxylan (GAX) on cellulose fibres and the properties thereof. A water-soluble GAX, from barley husks (Hordeum vulgare), was isolated using chlorite delignification and alkaline extraction followed by enzymatic purification. The isolated GAX fraction showed an arabinose to xylose ratio of 0.22 and a weight average molar mass of 20,200 g/mol, as determined by size exclusion chromatography (SEC) in DMSO:H₂O. The GAX was adsorbed on cellulose fibres under well controlled conditions, where temperature and initial concentration of GAX proved to be important parameters in controlling the level of adsorption. The adsorption process was also dependent on xylan molecular structure. Carbohydrate analysis on the modified fibres showed a preferential adsorption of low substituted xylans (arabinose to xylose ratio of ∼0.10). During the adsorption process the GAX solution was analyzed using SEC-RI-MALLS in aqueous solvent, which demonstrated a molecular xylan adsorption on cellulose fibres. Additionally, a decrease in light scattering responses, which corresponds to an adsorption of aggregated xylan and/or xylan with a great tendency towards self-association, could be observed during the adsorption process. This was demonstrated by adsorption of GAX on regenerated cellulose fibres (Lyocell), which compared to native fibres possesses a relatively smooth fibre surface. Atomic force microscopy analysis visualised a heterogeneous decoration of the Lyocell fibres with xylan agglomerates. The effect of GAX adsorption on paper strength was also investigated. A GAX modified kraft pulp showed an evident increase in tensile strength, which might be due to a retained fibre–fibre bonding ability for xylan coated fibrils after drying and rewetting.     

Polymerization of pyrrole on cellulose fibres using a FeCl3 impregnation- pyrrole polymerization sequence

Polypyrrole was polymerized on the surface of cellulose fibres using a sequence of fibre impregnation in FeCl₃ solutions, thickening and re-dispersion in a pyrrole solution. ζ-Potential and adsorption isotherms of the FeCl₃-cellulose systems showed that the adsorption of iron III was associated with the formation of free Fe³⁺ cations in the impregnation liquor. Moreover, under the test conditions applied, the amount of adsorbed iron III was not sufficient to promote the polymerization of a adequate amount of pyrrole on the fibre surface. Optimization of the polymerization reaction required that the FeCl₃ concentration in the impregnation liquor be increased to approximately 1 mol/l with a subsequent decrease of pH to approximately1.8. Based on scanning electron (SEM) micrographs and the low cellulose polymerization degree measured after pyrrole polymerization, we concluded that the decrease in the electric resistance of bulky polypyrrole/cellulose compounds was associated with a not negligible degradation of the cellulose fibres due to acid hydrolysis and the subsequent impossibility to prepare hand sheets with modified fibres due to the insufficient strength of the wet fibre network. The results of this investigation bring into question the use of FeCl₃-pyrrole-cellulose systems for the elaboration of conducting paper sheets with good and stable mechanical properties.     

Swelling of carboxymethylated cellulose fibres

Swelling of cotton cellulose fibres having different proportions of carboxyl groups in the H-form was studied. The carboxyl groups were introduced by carboxymethylation under different reaction conditions. By studying the swelling of modified cellulose samples (water retention value of non-dried fibre) it was shown that the concentration of sodium hydroxide was the dominant factor among the investigated reaction parameters. The number of acidic groups was found to play a significant but not determinative role in the level of improvement in swelling caused by carboxymethylation. A linear correlation was observed between swelling and iodine sorption capacity. The degree of collapse of the highly accessible structure of cellulose during drying (hornification) was larger in the case of more accessible carboxymethylated fibres than for the alkali treated sample. The degree of hornification increased with growing swellability and with growing number of carboxyl groups in the investigated interval (40–120 mmol carboxyl/mol cellulose). This type of modified cellulosic fibre could be used for enhanced entrapping and release of chemicals. This revised version was published online in August 2006 with corrections to the Cover Date.     

Swelling and dissolution of cellulose, Part V: cellulose derivatives fibres in aqueous systems and ionic liquids

The swelling and dissolution mechanisms of several cellulose derivatives (nitrocellulose, cyanoethylcellulose and xanthate fibres) are studied in aqueous systems (N-methylmorpholine-N-oxide—water with various contents of water, hydroxide sodium—water) and in ionic liquids. The results are compared with the five modes describing the swelling and dissolution mechanisms of cotton and wood cellulose fibres. The mechanisms observed for the cellulose derivatives are similar to the ones of cotton and wood fibres. Swelling by ballooning is also seen with cellulose derivatives, showing that this phenomenon is linked to the fibre morphology, which can be kept after undergoing a heterogeneous derivatisation. Patrick Navard and Thomas Heinze—Members of the European Polysaccharide Network of Excellence (EPNOE),     

New Method for Determining the Degree of Cellulose I Crystallinity by Means of FT Raman Spectroscopy

A Raman crystallinity index – Xc_Raman – characterizing the degree of crystallinity of partially crystalline cellulose I samples was created, utilizing the crystallinity dependence of CH₂ bending modes. For calibration, physical mixtures containing different mass fractions of crystalline cellulose I and its amorphous form were prepared. Crystallinities from 0 to 60% were generated. Relative intensity ratios of the Raman lines I and I characterizing crystalline and amorphous parts of cellulose I correlated linearly with the mass fraction of crystalline cellulose I of the mixtures. Xc_Raman values of microcrystalline celluloses of different origins and varying degree of crystallinity correlated reasonably with results obtained from NMR spectroscopy (Xc_NMR values).     

Preparation and characterization of cellulose nanocomposite films with two different organo-micas

The mechanical properties, morphologies, and gas barriers of hybrid films of cellulose with two different organoclays are compared. Dodecyltriphenyl-phosphonium-mica (C₁₂PPh-mica) and hexadecyl-mica (C₁₆-mica) were used as reinforcing fillers in the fabrication of the cellulose hybrid films. The cellulose hybrid films were synthesized from N-methyl-morpholine-N-oxide (NMMO) solutions with the two organo-micas, and solvent-cast at room temperature under vacuum, yielding 15–20 μm thick films of cellulose hybrids with various clay contents. We found that the addition of only a small amount of organoclay is sufficient to improve the mechanical properties and gas barriers of the cellulose hybrid films. Even polymers with low organoclay contents (1–7 wt %) were found to exhibit much higher strength and modulus values than pure cellulose. The addition of C₁₂PPh-mica was more effective than that of C₁₆-mica with regards to the initial tensile modulus, whereas the addition of C₁₆-mica was more effective than that of C₁₂PPh-mica with regards to the gas barrier of the cellulose matrix. The intercalations of the polymer chains in the clays were examined with wide-angle X-ray diffraction (XRD) and electron microscopy (SEM and TEM).     

Hybrid electrospun nonwovens from chitosan/cellulose acetate

Co-mixtures of chitosan (CS) and cellulose acetate (CA) were electrospun into fibrous webs from a binary co-solvent containing 70:30 trifluoroacetic acid (TFA): methylene chloride (DCM). Fibrous webs were produced from CS/CA in ratios (wt%) of 20:80, 40:60, 50:50 and 60:40. As determined by SEM analysis, 12% polymer solutions of CS/CA 60:40 produced structures with uniform bead free fibre morphologies with an average fibre diameter of 458 nm. FTIR-spectroscopy confirmed the presence of CS in the as-spun fibres in the form of chitosan-amine trifluoroacetate salts (NH₃ ⁺CF₃COO⁻). Uniform mixing of the CS and CA components was confirmed by DSC analyses. Alkaline neutralisation of the chitosan amine salts was explored as a means of increasing wet stability. The as-spun fibres were found to be relatively unstable in aqueous medium due to the solubility of the chitosan amine salts. Alkaline post-neutralisation was evaluated as means of minimising weight loss and maximising retention of fibrous structure.     

All-cellulose composites from unbleached hardwood kraft pulp reinforced with nanofibrillated cellulose

In the present work, nanofibrillated cellulose (NFC) from bleached eucalyptus pulp was prepared, characterized and used as reinforcement in an unbleached eucalyptus fiber matrix. First, the NFC was fabricated through TEMPO-mediated oxidation and characterized for the degree of polymerization, water retention value, cationic demand and carboxyl content. Intrinsic mechanical properties were also calculated by applying the rule of mixtures, which determines the coupling (f _c) and efficiency factor (η _e) of cellulose nanofibrils within the matrix. The results showed that the average intrinsic tensile strength and Young’s modulus of NFC are estimated to be 6,919 MPa and 161 GPa, respectively. After characterization, the NFC was used as reinforcement in the preparation of biocomposites in the form of paper handsheets, which were physically and mechanically analyzed. The presence of NFC induced an increase in the density of biocomposites and significant enhancement of the mechanical properties as well as an important reduction in porosity. Finally, f _c and η _e were determined from the mean intrinsic properties.     

Ligament-like tough double-network hydrogel based on bacterial cellulose

Using the double-network (DN) method, bacterial cellulose/polyacrylamide (BC/PAAm) DN gels able to sustain not only high elongation but also high compression have been synthesized by combining BC gel as the first network with PAAm as the second network in the presence of N,N′-methylene bisacrylamide (MBAA) as a cross-linker. This DN gel was obtained by modifying the monomer concentration of the second network, acrylamide monomer (AAm) and MBAA, and by controlling the water content of the first network, BC gel. The mechanical properties are discussed in term of the swelling degree (q), which is independent of the concentration of AAm and MBAA. It was found that, for BC/PAAm DN gels with the first network formed from BC gel with high q (BC _q=120), the tensile and compressive modulus (E) scales with q as E μ q ^{- 2} E \propto q^{ - 2} . The tensile fracture stress, σ _F, of this DN gel was almost independent of q, that is s_\textF μ q⁰, \sigma_{\text{F}} \propto q^{0}, but the compressive fracture stress, σ _F, scaled with q as E μ q ^{- 2} E \propto q^{ - 2} . Meanwhile, the tensile and compressive fracture strain (ε _F) of the gel is almost independent of q, which is caused by AAm concentration change, but linearly increased with q, which is caused by MBAA concentration change. Furthermore, by decreasing the water content of the BC gel prior to polymerization of the second (PAAm) network, a ligament-like tough BC/PAAm DN gel could be obtained with tensile strength of 40 MPa.     

Phase transformations in microcrystalline cellulose due to partial dissolution

All-cellulose composites were prepared by partly dissolving microcrystalline cellulose (MCC) in an 8.0 wt% LiCl/DMAc solution, then regenerating the dissolved portion. Wide-angle X-ray scattering (WAXS) and solid-state ¹³C NMR spectra were used to characterize molecular packing. The MCC was transformed to relatively slender crystallites of cellulose I in a matrix of paracrystalline and amorphous cellulose. Paracrystalline cellulose was distinguished from amorphous cellulose by a displaced and relatively narrow WAXS peak, by a 4 ppm displacement of the C-4 ¹³C NMR peak, and by values of T₂(H) closer to those for crystalline cellulose than disordered polysaccharides. Cellulose II was not formed in any of the composites studied. The ratio of cellulose to solvent was varied, with greatest consequent transformation observed for c < 15%, where c is the weight of cellulose expressed as % of the total weight of cellulose, LiCl and DMAc. The dissolution time was varied between 1 h and 48 h, with only small additional changes achieved by extension beyond 4 h.     

Synthesis of water-soluble cationic cellulose derivatives with tertiary amino groups

Water-soluble cellulose derivatives with tertiary amino groups up to substitution degree 0.8 (4.2% of coupled nitrogen) were prepared in a controlled manner by the interaction of cellulose acetate with N,N-diethylepoxypropylamine. It was shown that two reactions take place simultaneously, i.e., hydrolysis of acetyl groups and aminoalkylation of free hydroxyl groups of cellulose. The amino groups coupled to the cellulose are of middle basicity with pK _α ~9.5. Solubility of the products was found to be determined by the chemical composition of the cellulose derivative.     

The thermal expansion of cellulose II and IIIII crystals

Highly crystalline samples of cellulose II and III_II have been prepared from repeated mercerization of ramie fibers and supercritical ammonia treatment of the mercerized ramie fibers, respectively. The thermal expansion behavior of cellulose II and III_II was investigated using X-ray diffraction at temperatures ranging from room temperature to 250 °C. With increasing temperature, the unit cell of cellulose II expanded in the lateral directions and contracted in the longitudinal direction, with the a and b axes increasing by 0.54 and 3.4%, respectively, and the c axis decreasing by 0.09%. The anisotropic thermal expansion in these three directions was closely related to the crystal structure and the hydrogen bonding in cellulose II. A similar anisotropic thermal expansion was also observed in cellulose III_II. Cellulose III_II expanded in the lateral direction but contracted in the longitudinal direction.     

Effects of temperature and molecular weight on dissolution of cellulose in NaOH/urea aqueous solution

Dissolution of cellulose having different viscosity-average molecular weight (M _η ) in 7 wt%NaOH/12 wt%urea aqueous solution at temperature from 60 to −12.6°C was investigated with optical microscope, viscosity measurements and wide X-ray diffraction (WXRD). The solubility (S_a) of cellulose in NaOH/urea aqueous solution strongly depended on the temperature, and molecular weight. Their S_a values increased with a decrease in temperature, and cellulose having M _η below 10.0 × 10⁴ could be dissolved completely in NaOH/urea aqueous solution pre-cooled to −12.6°C. The activation energy of dissolution (E_a,s) of the cellulose dissolution was a negative value, suggesting that the cellulose solution state had lower enthalpy than the solid cellulose. The cellulose concentration in this system increased with a decrease of M _η to achieve about 8 wt% for M _η of 3.1 × 10⁴. Moreover, cellulose having 12.7 × 10⁴ could be dissolved completely in the solvent pre-cooled to −12.6°C as its crystallinity (χ _c) decreased from 0.62 to 0.53. We could improve the solubility of cellulose in NaOH/urea aqueous system by changing M _η , χ _c and temperature. In addition, the zero-shear viscosity (η ₀ ) at 0°C for the 4 wt% cellulose solution increased rapidly with an increase of M _η , as a result of the enhancement of the aggregation and entanglement for the relatively long chains.     

Effect of sodium hydroxide pre-treatment on the optical and structural properties of lyocell

Lyocell is a type of regenerated cellulose. Fibres spun from cellulose solution in N-methylmorpholine-N-oxide hydrate consist of crystalline cellulose II and amorphous cellulose. Lyocell fabrics were treated with aqueous sodium hydroxide solution (NaOH) to study the influence of alkali on optical and structural properties. It was observed that sodium hydroxide treatment causes the density, orientation and crystallinity of lyocell fibre to decrease with increasing sodium hydroxide concentration, a corresponding decrease in tensile strength is also observed. The greatest change in fibre properties occurs between 3.0 and 5.0 mol dm⁻³ NaOH. This is attributed to the onset of formation of Na-cellulose II at 3.0 mol dm⁻³ NaOH; a fully formed Na-cellulose II structure is expected above 6.8 mol dm⁻³ NaOH. Formation of Na-cellulose II causes plasticization of the lyocell fibres as both inter- and intra-molecular hydrogen bonds are broken by these higher sodium hydroxide concentrations.     

Stoichiometry and stability of cellulose-hydrazine complexes

Highly crystalline cellulose samples from green algae (cellulose I) and mercerized ramie (cellulose II) were treated with anhydrous hydrazine and the resulting complexes were analyzed by synchrotron X-ray diffraction and thermogravimetry. Cellulose I-hydrazine complex could be fully described by a two-chain monoclinic unit cell, a = 0.879 nm, b = 1.076 nm, c = 1.038 nm, and γ = 122.0°, with space group P2₁. Cellulose II-hydrazine complex prepared from mercerized ramie gave a different two-chain monoclinic unit cell, a = 1.042 nm, b = 1.046 nm, c = 1.038 nm, γ = 129.7°, also with space group P2₁. Though having different crystal structures, the number of hydrazine molecules per glucopyranoside residue was 0.82 for cellulose I-complex and 0.93 for cellulose II-complex, probable stoichiometric value of 1.0. Hydrazine could be extracted from the complexes by organic solvents retaining the crystalline orders, resulting in the allomorphic conversion to cellulose III_I and cellulose III_II, both having non-staggered chain arrangements. These features are similar to those of cellulose-ethylenediamine complexes.     

Approximate solution of the autocatalytic hydrolysis of cellulose

Depolymerization of cellulose in homogeneous acidic medium is analyzed on the basis of autocatalytic model of hydrolysis with a positive feedback of acid production from the degraded biopolymer. The normalized number of scissions per cellulose chain, S(t)/n° = 1 − C(t)/C₀, follows a sigmoid behavior with reaction time t, and the cellulose concentration C(t) decreases exponentially with a linear and cubic time dependence, C(t) = C₀exp[−at − bt ³], where a and b are model parameters easier determined from data analysis.