Cellulose, chain scission

Scheme 7 Cellulose-chain scission as a consequence of homolytic reactions involving NMMO-derived radicals...

Subsequent reactions are strongly dependent on the chemical nature of the polymer. Recombination of radicals to form a new chemical bond is often observed and is the key process in radiation-induced cross-linking. Examples of polymers in which cross-linking is favored include polyolefins such as polyethylene (PE), natural rubber, or poly-dimethylsiloxane (PDMS). In other polymers, including most fluori-nated polymers, poly(methyl methacrylate) (PMMA), and natural polymers such as DNA and cellulose, chain scission is favored, leading to degradation of the polymer (for a more comprehensive list, see Drobny [2, p. 21]). 

Scheme 4 The hydrolytic scission of a cellulose chain providing the sites for free radical oxidation.

Depolymerization of some natural polymers is another typical example. Milling of chitin or chitosan, at ambient temperature, leads to cleavage of the cellulose polymeric chain. Scission of 1,4-glucosidic bonds takes place, and the radicals formed recombine. Based on electron spin resonance, Sasai et al. (2004) monitored both the homolysis and the radical recombination. The recombination led to the formation of midsize polymeric chains only. Some balance was established between the homolytic depolymerization and the size-limited recombination of the radicals primarily formed. 

Resins may influence phototendering of rayons. Wood (26) has reported that viscose rayon fabrics treated with urea formaldehyde or thiourea formaldehyde resin are protected from the degradative effects of mercury vapor lamp radiation. The mechanism of the protective effect is not fully understood as yet. Possibly resins can quench free radicals formed during irradiation. Work with resin-treated cotton indicates that simultaneous scission of cellulose chain molecules and resin-cellulose bonds occurs on exposure to light (63). 

Thermal Degradation. Thermal degradation follows different paths depending upon whether moisture and oxygen are present. Above 140 °C, however, moisture does not influence degradation. Continuous exposure to heat below the pyrolysis temperature can produce chain scission and autoxidation within the cellulose chains (23,24). To produce thermally degraded samples for evaluation, cotton fabrics were exposed in a forced convection oven at 168° 4°C. Fabric was removed at intervals ranging from 24r-211 hr for examination. 

Natural or artificially accelerated aging of papermaking pulps is characterized by two important reactions, scission of the polymeric cellulose chains and some cross-linking reaction (7,8,9), the exact nature of which remains unknown. Since the mechanical properties of aged paper are modified by these two simultaneous reactions, it was of interest to determine whether these chemical effects influence the thermograms of artificially aged papers. 

Acid-catalyzed hydrolytic degradation of cellulose proceeds according to the principles of chemical kinetics. Nonetheless, concepts of kinetics have not been widely applied in the literature concerning the conservation of cellulosic materials. Thirty years ago, McBurney (I) provided an excellent exposition of this subject. We will review the subject in the light of developments since that time (2) and will present examples from the literature and from our own work to illustrate ways in which an analysis of the kinetics of chain scission can help conservators better understand the deterioration of cellulose-based materials. 

To express the results in terms of a few distinctly linear stages is a simplification. We have reviewed a number of factors that can influence the ease of chain scissioning. A point that we have not discussed, however, is the fact that several researchers (31, 32) have suggested that high-molecular-weight native celluloses may have a significant crystallite or chain-dislocation unit of the order of DP = 500. The overall rate must certainly represent transitions between the predominating influence of one or another of the factors noted. 

Simultaneously with Charlesby s findings, work along similar lines was carried out in G. E. s Research laboratories in Schenectady (22) and also in Research Institutes in the Soviet Union, although the latter only became known several years later (23). The results of this research demonstrated that in addition to polyethylene, many other polymers could be cross-linked by radiation. These include silicones, rubber, poly (vinyl chloride), polyacrylates and, to a lesser extent, polystyrene. In contrast, polymers such as polymethacrylates, polyisobutylene, polytetrafluoroethylene and cellulose underwent "degradation" by main-chain scission. These early findings were confirmed and extended to other compounds by numerous studies. 

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