Chapter 13
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Molecular Weights and Molecular-Weight Distributions of Cellulose and Cellulose Nitrates During Ultrasonic and Mechanical Degradation Marianne Marx-Figini Polymer Division, INIFTA, National University, La Plata, Argentina
Cellulose and cellulose nitrate were subjected to depolymerization by ultrasonication and high-speed mechanical agitation (stirring). Both treatments resulted in significant, but different, depolymerization with the resulting degradation products having considerably narrower molecular weight distributions than the parent substrates. In all cases a level-off degree of polymerization was reached which was different for unsubstituted cellulose and the derivatives. The results obtained with unsubstituted cellulose are explained with the existence of "weak links" in cellulose backbones.
The treatment of polymers with ultrasound has received considerable attention in the past (1-6). Cellulose depolymerization by ultrasonification, by contrast, has been largely overlooked (7-8); additionally, this work predates the advent of modern and reliable cellulose molecular weight determination methodology. Since one of the unusual features of polymer degradation with ultrasound is the narrowing of the molecular weight distribution in addition to the apparent existence of a limiting level-off DP, it was of interest to reexamine cellulose (derivative) degradation by ultrasound in view of the potential of generating cellulose preparations with narrow molecular weight distributions. Alternatively, this task requires time consuming fractional precipitation for preparative size exclusion chromatography (SEC) experiments. Likewise, cellulose degradation studies using high speed mechanical agitation (9-12) were performed at a time when reliable molecular weight methodology did not exist. Since an understanding of the molecular weight behavior of cellulose under mechanical stress can provide insight into the effect of industrial processing 184
©1998 American Chemical Society In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
185
on macromolecular properties of cellulose, cellulose depolymerization during mechanical high speed stirring was investigated as well.
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Experimental Section L Determination of Molecular Weights and Molecular Weight Distributions: Molecular weights were determined viscosimetrically as well as by size exclusion chromatography (SEC). The resulting molecular weight data were interpreted in terms of degrees of polymerization (DPs) so as to reach a universally applicable polymeric size parameter that is dependent only on the number of repeat units and not on the average weight or substitution pattern of repeat units. DP is independent of the nature and degree of substitution of the anhydroglucose repeat unit. Viscosimetric measurements were carried out in ethyl acetate and cupriethylenediamine (cuen) solution for cellulose nitrate and unsubstituted cellulose, respectively, according to Marx-Figini and Schulz (13). All measurements were performed with solutions whose concentration was adjusted so that η was between 0.3 and 0.6. All intrinsic viscosity measurements were expressed as D P using Equations 1 to 4. 8 ρ
ME.
=5.70XDP
fo] a
=1.06xDP
E
076
(1)
and [ η ]
(3)
and h ]
0ϋβη
c u e n
=2.29xDP
=0.42xDP
0 7 6
(2) forDP>950
(4)forDP10,000 and U 3,000. Degradation is independent of applied shear forces. The level-off DP-value is reached rapidly. Once reached, the limiting DP, which is independent from DPQ, no longer declines. Both observations, the existence of a level-off DP and the narrowing of U during degradation, suggest that mechanical shear forces degrade cellulose by cleaving "weak links" present in native cellulose. The results observed in this study are consistent with this hypothesis (21). The results therefore suggest that mechanical treatments of cellulose during industrial processing, including storage, are responsible for significant molecular degradation. 0
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In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
