Concept of Structure–Property Relationships in Molecular Solids and Polymers

1.1 Introduction

Low molar mass organic molecules and polymeric materials are often found as solids and their physical properties are a consequence of the way in which the molecules are organized: their morphology. The morphology is a result of specific molecular interactions which control the processes involved in the individual molecules packing together to form a solid phase. Depending on the extent of the molecular organization, a crystalline solid, liquid crystals or amorphous solid may be formed. As we shall see later, the organization that is created at a molecular level sometimes also tells us about the macroscopic form of the material, but in other cases it does not, hence the subtitle of the book: 'from nano to macro organization'.

Synthetic polymers, often referred to as plastics, are familiar in the home as furniture, the frames for double glazed windows, shopping bags, furnishings (carpets, curtains and covering for chairs), cabinets for televisions and paper and paint on the walls. Outside the house plastics are used for rainwater pipes, septic, water and fuel storage tanks, garden furniture, water hoses, traffic cones and sundry other items which we see around us. Removal of all articles containing polymers from a room would leave it bare. Synthetic plastics form the basis for many forms of food packaging, containers for cosmetics, soft drink containers and the trays used in microwave cooking of food. Natural polymers such as wood, cotton and wool all exhibit a high degree of order and many biopolymers play a critical role in the human body.

Whilst a focus of this monograph is structure in polymeric materials, many of the factors that control the organization of these big molecules are best studied with lower molecular weight analogues. It is therefore appropriate to spend some time understanding small molecular systems before the consideration of the complexity of polymers is undertaken.

The physical properties of a material are dictated by its ability to self-assemble into a crystalline form. Polymer chemists have for many years sought to establish structure-property relationships that predict various physical properties from of knowledge the chemical structure of the polymer. Staudinger recognized that polymers or macromolecules are constructed by the covalent linking of simple molecular repeat units. This structure is implied in the phrase poly meaning many and mer designating the nature of the repeat unit. Thus poly(ethylene) is the linkage of many ethylene units:

H2C=C2H -> -(CH2-CH2)n-

Recognition of the nature of this process of polymerizationmade it possible to produce materials with interesting and useful properties, and brought about the discipline of polymer science. The value of 'n' indicates the number of monomers in the polymer chain.

In the last forty years, a very significant effort has been directed towards understanding the relation between the chemical structure of the polymer repeat unit and its physical properties. In the ideal situation, knowing the nature of the repeat unit it should be possible to be able to determine all the physical properties of the bulk solid. Whilst such correlations exist, they also require an understanding of the way in which the chemical structure will influence the chain-chain packing in forming the solid. Similar correlations can be created for the understanding of other forms of order in lower molecular weight materials.

1.2 Construction of a Physical Basis for Structure–Property Relationships

Why do structure-property relations apparently work?

1.2.1 Ionic Solids

In order to understand the basis for structure-property correlations, it is appropriate to consider the structure of simple ionic solids. A solid sodium chloride single crystal (Figure 1.1) can be constructed starting from the atomic species of sodium [Na+] and chlorine [Cl-]. Since each ion carries a single charge, pairing the ions to form an NaCl pair would create a lower energy state. This pair will have a minimum separation and would be charge neutral. The line formed between the two atoms can be considered to lie on the x-axis. If another pair of atoms is brought close to the first pair, then once more a minimum energy situation would be created if the two pairs of atoms are aligned but their orientations are in the opposite sense. This arrangement will have a lower energy than the isolated pair and the distance between the atoms will be slightly reduced compared with the isolated pair. A further reduction in energy will accompany bringing to the cluster a further pair of atoms. This latter pair will align in an opposite sense to the pair to which it attaches itself. Once more there will be a small change of separations to reflect the formation of a lower energy state. It is relatively easy to see that this process can be repeated and a sheet of atoms would be formed. As we will see later this same principle is used in considering attachment and growth of molecular and polymeric crystals.

The sheet of atoms formed by the process described above is not the lowest energy structure that can be formed. If this original sheet is sandwiched between similar states such that each of the Na and Cl atoms becomes surrounded by atoms of the opposite sign then a true minimum will be observed. If this order structure cannot be formed, because the entropy (disorder) is high, then the ensemble of atoms will be in the melt or gaseous state.

In the case of the NaCl crystal, this lowest energy structure is a cubic close-packed structure and results from each atom having six neighbouring atoms of opposite sign. Changes in the size of the ions and their charges lead to different types of packing being favoured. However, it is relatively easy to see that an average energy can be ascribed to a basic unit of the structure and this will reflect the physical properties of the bulk. Whilst the energy of the first pair can be calculated explicitly, adding additional elements means that the force field has to be averaged and will give rise to the problem of how one calculates the interaction of many bodies all interacting. The energy is the result of electrostatic (Coulombic) interactions between unit charges and in principle can be calculated by averaging all the interactions that will act on an atom chosen as the reference. The above example illustrates not only the lowering of the energy by surrounding an atom by other atoms but...