|Santiago Cintron, Michael|
|French, Alfred - Al|
Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2011
Publication Date: 2/16/2011
Citation: Santiago Cintron, M., Johnson, G.P., French, A.D. 2011. Young’s modulus calculations for cellulose Iß by MM3 and quantum mechanics. Cellulose. 18(3):505-516.DOI:10.1007/s10570-011-9507-1.
Interpretive Summary: Young's modulus is a measure of the resistance to deformation of a flexible material. In the case of cellulose, it quantifies the ability of the material to undergo changes in length as elongation or compression forces are applied. The modulus can be calculated by performing stretching tests on cotton fibers and studying the resulting structural changes with spectroscopic methods or, as in this study, by stretching molecular models in a computer program. However, reported experimental and calculated values show large ranges. With this study we attempt to establish the roles of intermolecular hydrogen bonding and other molecular details in computer-based modulus determinations. To achieve this, modulus calculations with molecular mechanics and quantum mechanics were performed with short-form cellulose models capable of intramolecular hydrogen bonds, as well as, some related models in which hydrogen bonding was eliminated. These models, however, did not correspond well with recent modulus determinations. Model with longer chains of cellulose (up to 40 glucose units) provided values comparable to reported values. Results suggest that intramolecular hydrogen bonding contributes considerably to the resistance to deformation observed in cellulose.
Technical Abstract: Quantum mechanics (QM) and molecular mechanics (MM) calculations were performed to elucidate Young’s moduli for a series of cellulose Iß models. Computations using the second generation empirical force field MM3 with a disaccharide cellulose model, 1,4'-O-dimethyl-ß-cellobioside (DMCB), and an analogue, 2,3,6,2',3',6'-hexadeoxy-1,4'-O-dimethyl-ß-cellobioside (DODMCB), that cannot make hydrogen bonds reveal a considerable contribution of intramolecular hydrogen bonding to the molecular stiffness of cellulose Iß; the moduli for DMCB and DODMCB being 84.9 GPa and 38.1 GPa, respectively. QM calculations confirm this contribution with modulus values of 99.7 GPa for DMCB and 33.0 GPa for DODMCB. However, modulus values for DMCB were considerably lower than values previously reported for cellulose Iß. MM calculations with extended cellulose chains (10-40 glucose units) resulted in modulus values, 126.0-146.7 GPa, more akin to the values reported for cellulose Iß. Comparison of the cellodecaose model, 1,4'-O-dimethyl-ß-cellodecaoside (DMCD), modulus with the that of its hydrogen bonding-deficient analogue, 2,3,6,2',3',6'-hexadeoxy-1,4'-O-dimethyl-ß-cellodecaoside (DODMCD), corroborates the observed stiffness conferred by intramolecular hydrogen bonds; the moduli for DMCD and DODMCD being 126.0 GPa and 63.1 GPa, respectively. Additional MM3 determinations revealed that modulus values were not strongly affected by intermolecular hydrogen bonding, with multiple strand models providing values similar to the single strand models; 90.8 GPa for a 7-strand DMCB model and 129.0 GPa for a 7 strand DMCD model.