Cellulose model compounds

C-NMR spectra of water soluble cellulose model compounds indicate that formaldehyde is capable of reacting with wood cellulose functions under hot press conditions as well as at room temperature yielding hemiacetals. The formation of hemiacetals is reversible, and thus constitutes a reservoir for formaldehyde storage. Due to its affinity for water, formaldehyde released during the manufacture of UF-resin bonded products will be retained in the aqueous phase of wood. Wood contains about 9 wt% of moisture. Most of this is in the S-2 secondary cell walls that consist mainly of wood cel IuIose. 

Based on the number and location of the maxima and minima in the relationship between the band intensities and the polarization of the incident light relative to the chain axis, the bands in the Raman spectrum of cellulose could be divided into four groups. The about the direction of the vibrational motions in cellulose. The directions of the vibrations are such that the major change in polarizability associated with the motions is either parallel or perpendicular to the chain axis. Raman spectra recorded from deu-terated celluloses allowed the vibrational modes involving C-H and 0-H motions to be identified. These spectra demonstrated that most of the modes are complex coupled vibrations. Normal coordinate analyses of cellulose model compounds were done to determine the types of motion most likely to occur in each region of the spectrum. The calculations also suggested that the vibrational motions are very complex. The information from the normal coordinate calculations. 

C. Schwarzinger, I. Tanczos and H. Schmidt, Levoglucosan, cellobiose and their acetates as model compounds for the thermally assisted hydrolysis and methylation of cellulose and cellulose acetate, J. Anal. Appl. Pyrol., 62, 179 196 (2002). 

The kinetics of homogeneous reaction of several reactive dyes of the vinylsulphone type with methyl-a-D-glucoside (7.9), selected as a soluble model for cellulose, were studied in aqueous dioxan solution. The relative reactivities of the various hydroxy groups in the model compound were compared by n.m.r. spectroscopy and the reaction products were separated by a t.l.c. double-scanning method [38]. The only sites of reaction with the vinylsulphone system were the hydroxy groups located at the C4 and C6 positions [39,40]. 

Fig. 6.2 Structural formulas of cellulose (a), example of hemicelluloses (b), and an example of lignin (c), showing a repeating unit as possible model compound.

The model compounds were exposed for 16 h at 80° to 80% relative humidity. Proportion (1.8 mmol per mmol of cellulose.)... 

It now appears that cellulose I is not exclusively the native polymorph present in all organisms. The results reported originally by Sisson (61), which provided evidence that cellulose II was the native polymorph present in Halicystis (Ulvophyceae) cell walls, were recently reinvestigated and confirmed (62). Additionally, cellulose II producing mutants of Aceiobacier have been isolated and analyzed with x-ray and low-dose electron diffraction (63). When cellotetraose is induced to crystallize in solution it forms a structure which has been used as a model compound approximating the crystallographic nature of cellulose II based on x-ray diffraction, electron diffraction and CP-MAS 13C NMR evidence (64). Significantly, in all cases where Aceiobacier cellulose synthase in vitro activity has been reported,... 

Initially, 100% microcrystalline cellulose (MCC) (Avicel PH 200, FMC Corp. Newark, Delaware, U.S.A.) powder was used as the model compound... 

Comparative investigations with related monomeric compounds, glucose pentaacetate, and cellobiose octaacetate indicate that, on a qualitative basis, these materials undergo photolytic changes in a manner similar to cellulose triacetate. These materials may serve as model compounds for future quantitative investigations of cellulose triacetate. 

Membrane Rejection. Both cellulose acetate and FT-30 composite membranes were evaluated for rejection of solutes. Sodium chloride rejections were confirmed and listed in Table III. Typical organic rejections of model compounds are listed in Tables IV and V for cellulose acetate and FT-30 composite membranes, respectively. Rejections were measured during screening and concentration tests solute levels were in the parts-per-billion range. Measurement of feed and permeate stream solute concentrations provided the necessary information to calculate solute rejection. Eq 1 was used to calculate rejection values. 

Table IV. Model Compound Rejection for the Cellulose Acetate Membrane from Screening and Concentration Test...

Chemical Class Model Compound Cellulose Acetate FT-30 ... 

The results of irradiating the pora-quinoid structures (compounds 1-4) absorbed onto cellulose sheets are summarized in Figure 2. Examination of the photolysis data indicates that with the notable exception of 2-methoxy-l,4-benzoquinone the remainingpara-quinone model compounds did not cause any further darkening of the handsheets during irradiation. Indeed, several of the quinones exhibit a minor brightening response upon photolysis which could be attributed to a photo-bleaching effect, but the lack of a complete mass balance makes this conclusion tenuous. 

Scheme 24 Proving the oxidizing effect of DMSO/phenyl isocyanate mixtures under conditions used for carbanilation of cellulose by means of sterically hindered alcohol model compounds...

The oxidative effect of the DMSO/PhNCO system on cellulose was confirmed by means of alcoholic model compounds (60, 62, 64) that were neatly oxidized into the corresponding ketones. The presence of the active species, the oxidatively acting sulfonium ylide 66, in the cellulose carbanilation mixture was proven by trapping with two reagents, a tocopherol-based compound (48) and a naphthoquinone (68) that was also used in a facile color test to estimate the degrading effect of certain carbanilation mixtures and conditions on cellulose. 

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