Probing Mechanical Properties Of Molecular Crystals with Nanoindentation : Applications to Crystal Engineering

Abstract

Crystal engineering is widely applied in the design of new solids with desired physical and chemical properties based on an understanding of intermolecular interactions in terms of crystal packing. The understanding of such structure-property correlations increased my interest in the modulation of macroscopic properties of solid compounds. Establishing connections between structure and macroscopic properties is a classical aspect of materials science and engineering. With the advent of the nanoindentation technique, it is now possible to make such a link between micro-level structures with mechanical properties of molecular solids - in other words, between chemistry and engineering. Nanoindentation is a quantitative probe for the assessment of mechanical behavior of small volume materials. In this technique, applied load and indenter depth penetration are measured simultaneously for a molecular crystal specimen, with high precision and resolution. From this data, one can obtain the elastic modulus and hardness of molecular crystals. Being able to accordingly assess the relative strengths of intermolecular interactions, such a technique has become relevant to the subject of crystal engineering. We have used nanoindentation to study the packing anisotropy of molecular crystals and to establish structure-property relationships. This thesis demonstrates that nanoindentation is a state-of-the-art technique to probe the mechanical properties of molecular crystals and assists the development of the subject of crystal engineering towards property design. Chapter 1 gives an overview of the development of crystal engineering from solid state organic chemistry and a brief introduction of the nanoindentation technique which has become relevant to the subject of crystal engineering to establish structure-property relationships. The study of the mechanical properties of molecular solids as a function of their crystal structures is a very active branch of crystal engineering. Chapter 2 explores the insights of well-known odd-even alternative mechanical, physical and thermal properties of α,ω-alkanedicarboxylic acids such as elastic modulus, hardness and melting temperature through nanoindentation technique. These properties are well correlated with their crystal structure packing. The odd acids were found to be softer and lower melting temperature as compared to the even ones, possibly due to the strained molecular conformations in the odd acids in easier plastic deformation. Shear sliding of molecular layers past each other during indentation is a key to the mechanism for plastic deformation in the molecular crystals. Relationships between structural features such as interplanar spacing, interlayer separation distance, molecular chain length and signatures of the nanoindentation responses, discrete displacement bursts have also been discussed in this chapter. Chapter 3 explores the use of the nanoindentation measurement as a signature response to study the microstructure that exists in a single crystal of organic solids. The analysis of microstructure through X-ray crystallography can be misleading. This is because crystal structures as determined from the single-crystal diffractometer data represent only space- and time-averaged structures. Thus, due to higher spatial resolution of the nanoindentation technique compared to X-ray diffraction (XRD) it become a local probe, which allows for discrimination between different microstructure or domains in the single crystal. Chapter 4 attempts to explore an understanding of the underlying relationship between crystal structure and the mechanical properties of molecular crystals which are relevant for the systematic design of organic solids with a desired combination of mechanical properties such as elasticity and hardness through crystal engineering. Elastic properties in molecular solids are largely determined by the isotropy of crystal packing. By using the techniques of crystal engineering, seven halogenated N-benzylideneanilines (Schiff bases) crystals have been systematically designed and observed common underlying structural features which lead to high flexibility and elasticity. Elasticity in those crystals arises from a criss-cross packing of molecular tapes in isotropic structures with energetically comparable halogen bonds (Cl···Cl or Cl···Br). The chapter also demonstrates that the solid solution strengthening can be effectively employed to engineer hardness of organic solids. High hardness can be attained by increasing lattice resistance to shear sliding of molecular layers during plastic deformation. Chapter 5 demonstrates the broad applications of mechanical properties of molecular solids in the context of the pharmaceutical industry, which can be understood through nanoindentation. Crystal engineering is applied in designing active pharmaceutical ingredients (APIs) so as to obtain materials that exhibit optimum combinations of important physicochemical properties such as solubility, dissolution rate, and bioavailability. In the context of industrial-scale pharmaceutical manufacturing, it can also be used to tune mechanical properties such as grindability and tabletability, which often determine the processing steps that are adopted. Hence, there is always interest in the crystal structure−mechanical property correlations of APIs. The study of the mechanical properties of polymorphic drugs is an important for developing an understanding of their stability in the solid state. Overall, the main aim of this thesis is to explore an understanding for establishing structure-mechanical properties correlations of molecular crystals with recent advances in the nanoindentation technique and to gain knowledge for the design and synthesis of new materials using the crystal engineering approach. Nanoindentation of molecular crystals provides insights related to crystal packing, interaction characteristics, polymorphism and topochemistry.