Flow-induced mesophases in crystallizable polymers
In this chapter we review flow-induced mesophases in some crystalline polymers with rather different characteristics: polyethylene terephthalate, isotactic polypropylene and polydiethylsiloxane. Because these three polymers are representatives of flexible and semi-rigid-chain polymers, the occurrence of a mesophase suggests a similar behaviour for many other crystalline polymers. The question may be not so much whether crystalline polymers show mesophase behaviour, but rather under which conditions it can be obtained. A flow field turns out to be an effective tool to unravel structural information, even if hidden in quiescent conditions. The emergence of a mesophase affects strongly any subsequent crystallization. As a result of templating and nucleating effects, a mesophase may accelerate the crystallization rate, lead to different crystal modifications, change the morphology and guide the orientation of the crystals. On the basis of mesophase formation new insights into flow-induced polymer crystallization emerge, even though the nature of the mesophase ordering is still not fully understood. A re-evaluation of models for the polymer melt (random coil and folded-chain fringed-micellar grains) is proposed to understand the combined experimental observations on mesophase ordering and crystallization.