Cellulose molecular weight averages

VI. Molecular Weight Averages and Molecular Weight Distribution of Cellulose Nitrate... 

Table I. Molecular Weight Averages of Cellulose by Viscometric and GPC Weight Average Methods Using Extended Chain Length Model (Q Factor)...

The M.S. of a hydroxypropyl cellulose sample is 3.5. The D.P. of the cellulose used to prepare the sample was 120. Calculate the cellulose molecular weight and the grams of propylene oxide used in the synthesis. Assume the average number of hydroxypropyl units per side chain is 5. Calculate the D.S. 

Cellulose forms the major structural unit of plants. Wood is about 50% cellulose cotton fibers are almost entirely cellulose. Cellulose consists of an unbranched chain of glucose units, witii molecular weights averaging more than 500,000 amu. The structure of cellulose is shown in Figure 2530 . At first glance... 

Polysaccharide synthesis is under enzymatic control, but does not occur from a template as in protein synthesis. For this reason, each molecule of a particular polysaccharide will have its own unique molecular weight. The molecular weight of a carbohydrate polymer is usually expressed as an average. Starch or cellulose chains, for example, may vary by several hundred thousand in their molecular weights between individual molecules. For an excellent review of carbohydrate chemistry, see Binkley (1988). 

Polysaccharide solubility in aqueous solutions usually is dependent on polymer size and its allied three-dimensional structure. Even water-insoluble carbohydrates may be solubilized by controlled hydrolysis of o-glycosidic linkages to create smaller polysaccharide molecules. Thus, cellulose may be solubilized by heating in an alkaline solution until the polymers are broken up sufficiently to reduce their average molecular weight. Many such soluble forms of common polysaccharides are available commercially. 

Narrow Molecular Weight Triacetate Fractions. Narrow molecular weight cellulose triacetate fractions were obtained by both fractional precipitation and preparative GPC as described above. The number average molecular weight (1 ) of the various fractions and cuts was determined by high speed membrane osmometry. A linear dependence of GPC elution volume on log molecular weight for all cellulose triacetate fractions was found in both methylpyrroli-done and dichloromethane. 

Narrow Molecular Weight Triacetate Calibration. A linear relationship was found when log against the elution volumes of various cellulose triacetate fractions was plotted. For narrow molecular weight distribution triacetate fractions, the GPC experimental average molecular weight, termed can be expected to conform... 

The changes that occur when cellulose is subjected to hydrolysis all follow from the destruction of the 1,4-glucoside bond. Thus the cellulose chains are shortened and a decrease in the average molecular weight takes place and the cellulose fibres... 

The use of gel permeation chromatography (GPC) in studies of cellulose and cellulose derivatives has expanded greatly within the past few years. In common with similar studies of other polymers the matter of calibration of the GPC instrument and of conversion of GPC data into average molecular weight or average degree of polymerization (DP) of the polymer is of considerable importance. 

A relationship has been developed by means of which more valid values are obtained by GPC for the average degrees of polymerization for cellulose. This can be extended to other polymers. With automation of data acquisition and computer processing of data, a differential molecular weight distribution and complete information on DP of the sample are available in very short order with a low degree of error. The ready availability of narrow, well characterized polystyrene fractions for calibration makes this procedure highly attractive in view of the lack of similar standards of cellulose. 

A comparison of the weight-average molecular weights of cellulose computed from both Equations 3 and 4 and from viscometric data are shown in Table II. It is apparent that the agreement between viscometric... 

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