Size-Dependent Behavior of Macromolecular Solids I:
Molecular Origin of the Size Effect
W. Wei; David C.C. Lam

Source CMES: Computer Modeling in Engineering & Sciences, Vol. 64, No. 2, pp. 213-226, 2010
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Abstract Molecular rotation is the elastic deformation mechanism underpinning macroscopic deformation in macromolecular solid. In this investigation, molecular mechanic simulations are used to investigate the effect of size on the higher order material properties macromolecular solid. The rotational behavior of molecular coils embedded in beams was examined as a function of the beam size in tension, and in bending where the strain gradients in the bent direction are size-dependent. Analysis showed that the effective elastic modulus is size dependent when strain gradients are significant in bending, but not in tension. Analysis of the molecular rotation behavior indicated that the increase in the effective elastic modulus is generated by increase in molecular rotations with strain gradients. This implies that the higher order strain gradient stiffening has the same underlying deformation mechanism as conventional elastic deformation. Further analysis confirmed that the individual higher order material length scale parameter l2is uniquely related to the elastic modulus of individual solid studied. These results suggested that size-stiffening in polymers is not a new deformation mechanism, but is a new association of strain gradients with molecular rotations in the solid.
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