4. Characteristics of cellulose acetates 4.1 Characterization and physical properties of cellulose acetates

An overview is provided of the basic features of cellulose acetate of various degree of substitution in the solid and liquid crystalline state as well as in solution. These features represent a necessity for an understanding of the properties of these cellulose derivatives and further for mixed esters, which are not presented in this paper. Specifically, the crystal structure of cellulose triacetate will be addressed as well as structures in dilute and semi-dilute solutions. Thermal, viscoelastic and further properties in the solid state are discussed as well as flow behavior of solutions and their application in molecular weight determination, including false viscosity of commercial 2.5 cellulose acetates.     

Arabinoxylan structure affects the reinforcement of films by microfibrillated cellulose

The chemical structure of rye arabinoxylan (rAX) was systematically modified, exploiting selective enzymes to mimic different naturally occurring xylans, i.e., its degree of substitution (DS) was decreased using α-l-arabinofuranosidase, and a controlled decrease in the degree of polymerization (DP) was performed using endo-1,4-β-d-xylanase. The arabinose to xylose ratio was decreased from 0.45 to 0.27 and the weight-average molar mass was decreased from 184,000 to 49,000 g/mol. The resulting samples were used to prepare films, as such, and with 15% (wt. − %) softwood-derived microfibrillated cellulose (MFC) to obtain novel plant-derived biocomposite materials. The enzymatic tailoring of rAX increased the crystallinity of films, evidenced by X-ray diffraction studies, and the addition of MFC to the debranched, low DS rAX induced the formation of ordered structures visible with polarizing optical microscopy. MFC decreased the moisture uptake of films and increased the relative humidity of softening of the films, detected with moisture scanning dynamic mechanical analysis. For the first time, the chemical structure of xylan was proven to significantly affect the reinforcement potential of nano-sized cellulose, as the tensile strength of films from high DP rAXs, but not that of low DP rAXs, clearly increased with the addition of MFC. At the same time, MFC only increased the Young’s modulus of films from rAX with high arabinose content, regardless of DP.     

Optical-activity anisotropy of ordered phases in cellulose acetates

The optical activity of anisotropic solutions of cellulose diacetate in nitromethane and dimethyl sulfoxide and of diand triacetate films with vitrified ordered structure has been studied. The systems under study are characterized by high specific optical rotation [α], suggesting formation of a cholesteric mesophase. The value of [α] is found to depend on the angle of rotation of the samples relative to the direction of the polarization vector of an incident light beam in the plane perpendicular to this beam (the anisotropy of optical activity). This dependence (indicatrix) shows an irregular pattern and, when plotted in Cartesian coordinates, can be described with a distorted sinusoid. The data on the resolution of indicatrices into harmonic constituents and isolation of contributions due to isotropic components and anisotropic components, each of which is determined by the structural element with the corresponding asymmetry, are analyzed.     

Applications of Baylis-Hillman acetates: one-pot, facile and convenient synthesis of substituted γ-lactams

A simple, convenient and one-pot transformation of the acetates of Baylis-Hillman adducts into substituted γ-lactams, that is, (E)-5-alkyl-3-arylidenepyrrolidin-2-ones via treatment with nitroalkanes in the presence of a base, followed by reductive cyclization, using Fe/AcOH, is described.     

4.3 Degradation and modification of cellulose acetates by biological systems

A survey is given on recent findings in the enzymology of cellulose acetate degradation. Acetyl esterases have been identified as the principal enzymes, initiating cellulose acetate degradation as a prerequisite for endoglucanase-catalyzed cellulose acetate depolymerisation. Acetyl esterases are provided by nature to deacetylate naturally occurring partly acetylated polysaccharides, i.e. xylan and chitin. Accordingly they are not designed to attack high DS cellulose acetate. Under these circumstances acetyl esterases require a pretreatment of cellulose acetate, leading to some reduction in DS, in case highly substituted material should be degraded. One of these treatments is composting under the conditions of which a partial deacetylation may occur under the action of heat and high pH, facilitating the accessibility for acetyl esterases. However from the present knowledge it cannot be excluded that certain microbial specialists exist, being capable to degrade high DS cellulose acetate.     

Cross-coupling of benzylic acetates with arylboronic acids: one-pot transformation of benzylic alcohols to diarylmethanes

Benzylic acetates reacted with arylboronic acids in the presence of a DPEphos-[Pd(eta3-C3H5)Cl]2 catalyst when tert-amyl alcohol was used as a solvent, and the catalytic cross-couplings produced diarylmethanes in high yields (up to 94% isolated yield).     

Determination of the substitution pattern in the polymer chain of cellulose acetates

Cellulose acetates have been investigated on different structural levels. Based on the determination of the acetate pattern of the monomers, a method has been developed to describe the distribution of the acetyl groups in the polymer chain. This approach comprises permethylation with methyl triflate, deacetylative deuteromethylation under alkaline conditions, random cleavage, remethylation with methyl-d₃, FAB-MS analysis and comparison of the experimental data with those calculated for a random acetate pattern. Results are discussed with respect to the conditions of the acetylation reactions. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3011–3016, 1999     

Probing the dependence of the properties of cellulose acetates and their films on the degree of biopolymer substitution: use of solvatochromic indicators and thermal analysis

Although cellulose acetates, CAs, are extensively employed there is scant information about the systematic dependence of their properties on their degree of substitution, DS; this is the subject of the present work. Nine CAs samples, DS from 0.83 to 3.0 were synthesized; their films were prepared. The following solvatochromic probes have been employed in order to determine the empirical polarity, E _T(33); “acidity, α”; “basicity, β”, and “dipolarity/polarizability, π*” of the casted films: 2,6-dichloro-4-(2,4,6-triphenyl-pyridinium-1-yl) phenolate, WB; 4-nitroaniline; 4-nitroanisole; 4-nitro-N,N-dimethylaniline; 2,6-diphenyl-4-(2,4,6-triphenyl-pyridinium-1-yl)phenolate, RB. Additionally, two systems, ethanol plus ethyl acetate (EtOH–EtAc), and cellulose plus cellulose triacetate, CTA, were employed as models for CAs of different DS. Regarding the model systems, the following was observed: (i) For EtOH–EtAc, the dependence of all solvatochromic parameters on the “equivalent-DS” of the binary mixture was non-linear because of preferential solvation; (ii) The dependence of E _T(33) on equivalent DS of the cellulose–CTA films is linear, but the slope is smaller than that of the corresponding plot for CAs. This is attributed to the more efficient hydrogen bonding in the model system, a conclusion corroborated by IR measurements. The dependence of solvatochromic parameters of CAs on their DS is described by the simple equations; a consequence of the substitution of the OH by the ester group. The thermal properties of bulk CAs samples were investigated by DSC and TGA; their dependence on DS is described by simple equations. The relevance of these data to the processing and applications of CAs is briefly discussed.     

Structural change and redispersion characteristic of dried lignin-containing cellulose nanofibril and its reinforcement in PVA nanocomposite film

Cellulose - Having the advantages of a higher yield, lower cost and less environmental impact, lignin-containing cellulose nanofibrils (LCNFs) obtained by mechanically fibrillating unbleached pulps...     

CPMAS 13C NMR and X-ray studies of cellooligosaccharide acetates as a model for cellulose triacetate

A series of crystalline oligomers from α-D -cellobiose octaacetate through α-D -cellohexaose eicosaacetate were prepared by homogeneous acetylation of the corresponding cellooligosaccharides and characterized by cross-polarization and magic angle sample spinning (CPMAS) carbon-13 nuclear magnetic resonance (¹³C NMR) spectroscopy and X-ray analysis to obtain the structural models of cellulose triacetate (CTA) in the solid state. Progressing toward the hexamer, the NMR spectral features of the oligomers, in comparison with two allomorphs of CTA I and CTA II, gradually approached those of CTA I. Specifically, chemical shifts of both the hexamer and pentamer were in agreement with those of CTA I. In addition, X-ray diffraction patterns of the oligomers established that the crystalline pentamer and hexamer had a CTA I lattice despite recrystallization from ethylacetate-n-hexane. Therefore, we conclude that the pentamer and hexamer are useful models for the CTA I structure. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4100–4107, 1999     

Simple, facile and one-pot conversion of the Baylis-Hillman acetates into 3,5,6-trisubstituted-2-pyridones

A facile route for the synthesis of novel 3,5,6-trisubstituted-2-pyridones from the acetylated Baylis-Hillman esters with β-enamino esters or β-enamino nitriles in one pot with good yields is described.     

Langmuir-Blodgett films of cellulose nanocrystals: preparation and characterization

The goal of this work is the preparation of monolayers of cellulose I nanocrystals providing flat crystalline cellulose surfaces. Suspensions of cellulose nanocrystals were prepared by hydrolyzing ramie and tunicin fibers with sulfuric acid. Due to surface grafted sulfate groups, the negatively charged, rod-like cellulose nanocrystals were found to form stable layers at the air-water interface in the presence of a cationic amphiphilic molecule such as dioctadecyldimethylammonium (DODA) used in this work. These layers were formed at different cellulose-DODA weight ratios, compressed and analyzed by tensiometry, ellipsometry and Brewster angle microscopy. At low cellulose concentrations the layers are discontinuous, becoming dense and homogeneous upon reaching a critical weight ratio, which depends on the aspect ratio of the cellulose nanocrystals. After transfer onto silicon wafers, the surface composition and morphology as well as the thickness of the films were examined by X-ray photoelectron spectroscopy, ellipsometry and atomic force microscopy. The results indicate that they are monolayer films, well structured, relatively smooth and pure. These films offer a crystalline and easily reproducible model cellulose surface.     

Charging and swelling of cellulose films

Charging and swelling of cellulose in aqueous environments are of highest interest with respect to the performance of cellulose based products and applications. To unravel the interplay between ionization and structural features of the biopolymer hydrogel we compared non-crosslinked and crosslinked cellulose thin films based on a determination of the Donnan potential [S.S. Dukhin, R. Zimmermann, C. Werner, J. Colloid Interface Sci. 274 (2004) 309] from microslit electrokinetic (streaming potential/streaming current) experiments and layer thicknesses from ellipsometry in aqueous electrolyte solutions. The pH dependence of the Donnan potential, reflecting the ionization of carboxylic acid groups within the cellulose films, was found to be significantly different from the related trend of the streaming current which reflects the characteristics of the topmost surface of the layers: While carboxylic acid groups on the surface of the films dissociate as isolated functionalities, the electrostatic interactions of ionized groups within the cellulose layers cause an incomplete dissociation (pK shift) of the carboxylic acid and a layer expansion (swelling) in the alkaline pH range. The system was found to restrict its volume charge density even after structural restrictions (crosslinking) of the layer and at lower ionic strength of the solutions through a further decrease of the degree of dissociation of the carboxylic acid functions. These findings were attributed to the local accumulation of the carboxylic acid groups caused by preferential oxidation of the amorphous regions of the cellulose and to the ordered water structure within the layer.     

The interaction of cellulose and montmorillonite in a hydrothermal process

Journal of Sol-Gel Science and Technology - In this paper, the interaction of cellulose and montmorillonite in a hydrothermal process was discussed. The preparation of the composite was performed...     

Oriented cellulose films and fibers from a mesophase system

Cellulose mesophases were obtained by preparing concentrated solutions of cellulose (20–55%) in a mixture of N-methyl-morpholine N-oxide (MMNO) and water. The anisotropy depends on four interconnected parameters: the temperature of the solution which, in general, must be lower than 90°C; the concentration of cellulose which must exceed 20%; a water content such that the mole ratio water/anhydrous MMNO is smaller than unity; and the degree of polymerization of the dissolved cellulose. The anisotropic cellulose solutions can readily be oriented during extrusion or casting thus giving fibers or films which upon regeneration exhibit high orientation.     

Features of complexing of molecular NMR relaxation probes with cellulose acetates in solutions

The method of active and inert NMR relaxation probes can be used for determining the degree of substitution of cellulose acetates. The formation of hydrogen bonds between the hydroxyl and acetate groups of cellulose acetate is more probable than between the hydroxyl groups, which results in relative large shielding of the free OH groups in the polymer in samples with high degrees of substitution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1761–1765, August, 1989.We would like to thank N. I. Naimark for providing the samples of the cellulose acetates.     

Preparation and study of polyacryamide-stabilized silver nanoparticles through a one-pot process

A one-pot route was illustrated to synthesize stable well-dispersed silver colloids stabilized by polyacrylamide on a large scale. Reduction of silver ions and polymerization of acrylamide occurred almost simultaneously in the absence of a commonly used reducing agent and initiator. A possible mechanism for the formation of silver nanoparticles with bimodal size distribution was proposed. The structure and composition of the obtained nanoparticles were characterized carefully. Furthermore, light scattering simulation and UV-vis absorption studies confirmed that the obtained colloids were the mixture of Ag and Ag2O nanoparticles. The presence of silver oxide layers on the nanoparticle surface should be responsible for the broadening of the surface plasmon band of silver nanoparticles. Ag2O layers could be added or removed from Ag nanoparticle surfaces by the addition of HNO3, HAc, or NaCl solution to the as-obtained silver colloids.     

Investigation of titanium-catalysed dehydrogenative coupling and hydrosilylation of phenylhydrogenosilanes in a one-pot process

Titanium-catalysed dehydrocondensation and hydrosilylation of primary, secondary and tertiary phenylsilanes have been investigated in a one-pot process with Cp₂Ti(OPh)₂ as catalyst by NMR studies. Only primary and secondary silanes were found to undergo simultaneous dehydrogenative coupling and hydrosilylation reactions to produce the functional polysilanes.