Application of differential scanning calorimetry to the identification of crosslinks in cellulose

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Abstract
Thermograms of reaction products of cellulose with dimethylolurea (DMU) or dimethylolethyleneurea (DMEU) homopolymers show the peak associated with cellulose decomposition is shifted up by as much as 20°C. Solubility measurements in cupriethylenediamine (Cuene) indicate the homopolymers are present as side groups on the cellulose. Cellulose reacted with DMU or its homopolymer is only insoluble in Cuene when oxymethylene crosslinks are simultaneously formed by CH2O. These crosslinks do not affect the thermal stability of cellulose. Linkages to DMEU or its homopolymer are relatively stable to alkaline hydrolysis and may be distinguished from oxymethylene linkages by comparison of thermograms and solubilities before and after acid hydrolysis. Comparison of thermograms of cellulose with those of the n‐methylol compounds, their homopolymers, physical mixtures of cellulose and the homopolymers, and reaction products of cellulose with DMU, DMEU, and CH2O, demonstrated that, depending upon the reaction conditions, CH2O evolution, in situ homopolymenzation, cellulose crosslinking by CH2O, by the n‐methylol compounds, and reaction with their oligomers may occur. Reaction variables, viz., pH, temperature, nature and concentration of catalyst, the time‐temperature interval between application of the n‐methylol compound and the final reaction, and the type of n‐methylol compound determine which reaction predominates. The modification of cellulose by DMU shows a greater dependence on catalyst type than does DMEU which readily polymerizes to low molecular‐weight products and evolves CH2O which crosslinks the cellulose.