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In molten polymers, three types of structural heterogeneities have to be considered: (1) heterogeneities caused by short range order; (2) fluctuating bundles of more or less parallel chains which are much larger than the domains with short range order; (3) small crystals which are thermodynamically not stable caused by heterogeneous fluctuations as discussed by Frenkel. The definition of the three types of heterogeneities, their mutual relation and the theories by which they are predicted are discussed. Special attention is given to the problem of characterization of bundles and to the question of the existence of bundles. Experimental methods, by which heterogeneities can he detected, are critically discussed and the results are reviewed. In the glassy state, the heterogeneities mentioned above also occur. Here, however, they do not fluctuate but are frozen in. In some substances, in addition, ordered regions of another kind occur which disappear after heating above the glass transition temperature. The possible nature of these regions is discussed.

Structural heterogeneities in glassy and molten polymers