Chapter 25: Molecular Solids

Simple molecular crystals such as ice, solid carbon dioxide, and benzene demonstrate characteristically low melting points and substantial thermal expansion due to their limited cohesive energies. Water exhibits a remarkable diversity of high-pressure phases and amorphous solid variants. The chapter explores amphiphilic molecules, which contain both hydrophobic and hydrophilic domains and spontaneously assemble in aqueous environments to form organized structures such as spherical micelles, cylindrical arrangements, and bilayer configurations. The treatment of polymers comprises a substantial portion of the chapter, covering the synthesis and properties of these macromolecules constructed from repeating monomer units. Molecular weight characterization distinguishes between number-average and weight-average measures, while polymer chain architecture encompasses linear, branched, and cross-linked network topologies. Temperature-dependent behavior differentiates thermoplastic materials from thermosetting polymers. Polymerization mechanisms include chain-growth addition processes and step-growth condensation routes. Stereochemical configurations—isotactic, syndiotactic, and atactic arrangements—fundamentally influence mechanical behavior. Random walk statistical models describe polymer chain conformation and end-to-end distance distributions. Block copolymers undergo phase separation to generate ordered microstructures with lamellar or hexagonal symmetries, enabling sophisticated nanocomposite applications. Conducting and superconducting polymers leverage electron delocalization through conjugated backbones, exemplified by polyacetylene and charge-transfer complexes like TTT-TCNQ. The chapter addresses biological macromolecules, particularly the complementary base-pairing mechanisms in deoxyribonucleic acid and ribonucleic acid double helices. Viral architectures, composed of nucleic acid cores surrounded by protein shells, display symmetries ranging from icosahedral to helical geometries. Fullerene-derived solids, encompassing pristine fullerites and alkali metal-intercalated fullerides, exhibit high-temperature superconducting properties. Carbon nanotubes, formed by rolling graphene sheets into cylindrical structures, are characterized by chirality descriptors and possess exceptional mechanical strength.