Epimers with distinct mechanical behaviours

Abstract

This study highlights the impact of relative stereochemistry in epimer compounds on their mechanical properties; the crystals of one series of esters are ductile and deform plastically upon bending, whereas the other series are all brittle. Nanoindentation studies show that the hardness, H, and elastic moduli, E, of the brittle crystals are substantially larger than those of the ductile ones. For the brittle crystals, the H values range from 153(10) to 293(37) MPa and E from 2.85(0.33) to 9.10(0.51) GPa, whereas for the ductile crystals, the H values range from 76(2) to 125(11) MPa and E from 1.40(0.36) and 2.75(0.06) GPa. These are rationalized by recourse to the distinct crystal structural features, especially in terms of interdigitation in the molecular planes in the brittle crystals and slip planes in the ductile crystals. The indentation fracture toughness, K_c, values of the (2′S) crystals are higher than those typically reported for molecular crystals, due to the corrugated nature of their crystal packing which enhances the crack tortuosity. The K_c values are in the range 0.215 (0.08) to 0.278 (0.06) MPa m^½ and the brittleness index values are in the range 711(19) to 1053(50) m^−½.