Block or ordered copolymers

The long sequences of A and B units of a block copolymer can be arranged in several different ways. A diblock copolymer, schematically represented as 

It is also characterized by the molecular weight of each block. A multiblock, or repeating copolymer can be represented in general as 

Before examining the crystallization behavior of block copolymers it is necessary to hrst understand the nature of the melt. This is an important concern since it is from this state that crystals form and into which they melt. For reasons that will become apparent this is a particularly important consideration in understanding the crystallization and melting of block copolymers. The melt of a block copolymer is 

Consider now a block copolymer composed of two chemically dissimilar blocks each of which is noncrystalline. The same factors that are involved in homopolymer mixing will still be operative so that phase separation would be a priori expected. However, since the sequences in the block copolymer are covalently linked, macrophase separation characteristic of binary blends is prevented. Instead, microphase separation and the formation of separate domains will occur. The linkages at the A-B junction points further reduce the mixing entropy. There has to be a boundary between the two species and the junction point has to be placed in this interphase. The interphase itself will not be sharp and will be composed of both A and B units. Mixing of the sequences, and homogeneity of the melt, will be favored as the temperature is increased. There is then a transition temperature between the heterogeneous and homogeneous melt, known as the order-disorder transition. 

A schematic illustration of the major domain structures that are found in pure amorphous block copolymers is illustrated in Fig. 5.25.(183) Here the diblock copolymer poly(styrene)-poly(butadiene) is taken as an example. In (a) poly-(styrene) spheres are clearly seen in a poly(butadiene) matrix the spheres change to cylinders with an increase in the poly(styrene) content, as in example (b). With a further increase in the poly(styrene) concentration, alternating lamellae of the two species are observed (c). At the higher poly(styrene) contents, (d) and (e), the situation is reversed. Poly(butadiene) cylinders, and then spheres, now form in a poly(styrene) matrix. More quantitative descriptions of the domain structures have been given.(184,186,187) Crystallization and melting often occur to or from heterogenous melts with specific microphase structures. 

Since block copolymers can be synthesized in a variety of chain architectures, we consider first diblock copolymers in which only one block can crystallize. An example of the overall crystallization kinetics of a diblock copolymer, atactic poly(styrene)-poly(ethylene oxide), is illustrated in Fig. 10.25.(48) Here the relative fraction transformed is plotted against log time, at different crystallization temperatures. Only the poly(ethylene oxide) block, M = 9900, crystallizes 

When the noncrystallizable block in a diblock copolymer is rubber-like the isotherm shapes are very similar to those of the parent homopolymers.(55,56) This situation exists even when the crystallization occurs from a well-defined melt struc-ture.(55,57,58) However, at a fixed undercooling, there is a reduction in the overall crystallization and spherulite growth rates.(55) When the growth rates of ethylene oxide-butadiene block copolymers, and the corresponding homopolymer, are plotted against l/AT it is found, with the exception of the lowest content ethylene oxide polymer, that a set of parallel straight lines results irrespective of the iiutial melt domain structure.(55) This result implies that the products of interfacial free energies for nucleation are similar to one another. 

The situation is quite different when the noncrystallizing component is a glass. When Tg of the noncrystallizable block is greater than of the crystallizable block 

Another example of confined crystalUzation is illustrated by the block copolymer hydrogenated poly(butadiene) x)ly(vinyl cyclohexane).(59) The glass temperature of poly(vinyl cyclohexane) is about 145 °C, well above the melting temperature of 

The differences in time scale, relative to the initial domain structures, are also reflected in the detailed kinetics, the isotherm shapes and the Avrami exponents.(59a) When there is connectivity between the domain structures, the usual sigmoidal shaped isotherm results. However, when the crystalline block is confined to a specific domain, first-order crystallization kinetics result. This corresponds to a derived Avrami equation with n = 1. The significance of these results will be discussed shortly in terms of other findings. 

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Block or ordered copolymers this postulate, it is found that... 

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