Elastic Properties, Thermal Expansion, and Polymorphism of Acetylsalicylic Acid

High quality single crystals of acetylsalicylic acid (aspirin) with dimensions up to about 30 cm3 were grown from saturated solutions. The complete elastic stiffness tensor was determined by two different ultrasonic techniques. The thermal expansion coefficients were measured by dilatometry. The thermal expansion is anisotropic, with a minimum of the longitudinal effect parallel to the direction [010]. The expansion perpendicular to this direction is larger by a factor of 2. The anisotropy of the longitudinal elastic stiffness is less pronounced; the difference between the minimum and maximum values is approximately 30%. The maximum elastic stiffness is observed along the directions of the hydrogen bonds linking the centrosymmetric acetylsalicylic acid dimers. The experimentally determined elastic stiffness coefficients and the observed morphology have been compared to results from atomistic model calculations. This comparison allows us to assess the reliability of predictions made with respect to a potential polymorph.