As cotton cellulose deteriorates, it begins to lose both the flexural strength and the cohesive strength of the material.  In other words, the paper can no longer withstand folding without breaking and its ability to hold together when pulled upon is greatly diminished.  However, very old paper as far back as 15th-16th century and even earlier maintains some integrity and does not crumble completely.  We are studying the late-stage deterioration of cellulose  to better understand the effects that chemical degradation has on the physical strength of the paper.

Current research focuses on:

• Examining hydrolytic deterioration of cellulose in aging cotton paper in order to gain insight into the course of hydrolysis in the amorphous phase.
• Exploring the chemical mechanism for strength loss in paper at late stages to better understand the ability of paper to maintain physical integrity.
• Correlating MW changes with physical strength properties to be able to assess the current condition of old papers.
• Assessing the kinetic model for degraded cellulose throughout the lifetime of an aging cotton paper. 

Changes in the amorphous cellulose of cotton paper as it degraded was studied through an accelerated oven aging experiment.  The molecular weight changes (measured by GPC), paper strength, and oligomer production in the soluble fraction (measured by ESI-MS) were tracked as the cotton paper degraded and analyzed to develop a model describing the degradation of the amorphous regions of the cotton paper.  The results indicate that scission reactions passed through three stages as shown in the schematic below.  In Stage I, a single amorphous tie chain is broken during the hydrolytic reaction.  In stage II, an increasing amount of chain breaking will occur on already broken tie chains creating fragments no longer attached to the crystalline cellulose.  These scissions that occur on already broken tie chains will have no effect on strength loss and thus the strength loss progressively slows as the reaction continues.  In Stage III, nearly all the tie chains have been broken and strength derived from the connectivity of the cellulose network is nearly gone.

Monte Carlo modeling was used to generate a reaction kinetics curve and to model the oligomer production and strength loss for degradation of a semi-crystalline polymer assuming the kinetic rate was constant. This simulation indicates that there are three stages of degradation corroborating the structural model shown above.  It is clear from the model that the distortion of the underlying constant reaction rate is a result of the increasingly nonrandom reaction localized nearer the amorphous tie chain ends.

A correlation was inferred between the strength loss and the chain breaks calculated from the molecular weight changes and the Monte Carlo modeling.  The studies indicate that for chain-breaking occurring at a constant rate, the strength loss was roughly exponential with time.  However, there still remained a residual strength in the paper at very late stages of degradation where there are very few surviving tie chains.  Thus the fiber, which once was a series of crystals held together by amorphous tie chains and attractive forces is now in the very degraded paper held together only by attractive forces which impart the paper's strength.

For more information:
        Stephens, C. H. et al. Biomacromolecules (2008) (Abstract) (pdf)