Introduction

Chapter 4 is principally concerned with the crystal structure of polymers, that is to say the shape and size of the smallest unit of a crystal-the unit cell-and the arrangement of the atoms within it. The present chapter considers the structure of polymers on a scale much larger than the unit cell. Not all polymers crystallise and, even in those that do, there is always some remaining non-crystalline material, as shown for instance by the presence of broad halos in WAXS patterns from unoriented polymers in addition to any sharp rings due to crystalline material. The widths of the rings due to the crystallites indicate that, in some polymers, the crystallite dimensions are only of the order of tens of nanometres, which is very small compared with the lengths of polymer chains, which may be of order 3000nm measured along the chain, i.e. about 100 times the dimensions of crystallites. The following questions therefore arise.

1. (i) How can long molecules give rise to small crystallites?

2. (ii) What are the sizes and shapes of polymer crystallites?

3. (iii) How are the crystallites disposed with respect to each other and to the non-crystalline material?

4. (iv) What is the nature of the non-crystalline material?

These questions are the basis of polymer morphology, which may be defined as the study of the structure and relationships of polymer chains on a scale large compared with that of the individual repeat unit or the unit cell, i.e. on the scale at which the polymer chains are often represented simply by lines to indicate the path of the backbone through various structures.