The discussion in the previous chapter on structure at the molecular level is now extended to include an examination of the more macroscopic features of polymers. This is important because most useful commercial polymers are heterogeneous with properties that depend sensitively upon the dimensional scale of the different component structures in the material. Such is the case for partially crystalline polymers, blends and composites, segregated block copolymers, filled and plasticised systems. Various processing steps can radically alter the scale of heterogeneity, notably thermal treatment. Blending component polymers to achieve desirable properties also introduces many factors which influence the degree of miscibility in the system; method of mixing, solvents, molecular weight and polydispersity, tacticity, the weight fractions of polymer components and the presence or absence of specific chemical entities which act as compatibility promoters.

Clearly, dimensional scale is all important in defining structural heterogeneity. Before discussing the specific contribution of NMR as such, a brief digression is in order to clarify the way in which different experimental and theoretical approaches relate to one another.

Experimental probes of heterogeneity: an overview

The sensitivity of different experimental probes to dimensional scale spans many orders of magnitude. Criteria such as experimental procedure and inherent detection limits of the measuring equipment can lead to differences even within a given technique.