A comparative experimental and theoretical study between heteroarm star and diblock copolymers in the microphase separated state

A pronounced difference in the behavior of heteroarm star copolymers from the conventional diblock copolymers is analyzed theoretically and experimentally on the example of symmetric polystyrene-poly(2-vinylpyridine) (PS(6)P2VP(6)) star block copolymer melts. Microphase separation of PS(6)P2VP(6) stars with volume fraction of PS varying from 0.30 to 0.75 is studied using transmission electron microscopy (TEM). The observed (lamellar, cylindrical and undefined "bicontinuous-like") structures often turned out to be different from that expected for the corresponding diblock copolymers. The dependence of the long period of lamellar structures of heteroarm star block copolymers on block length is found to deviate in a systematic way from N-2/3 law (known for diblock copolymers), especially for the stars of high functionality. The theoretical model proposed to describe an extra-stretching of star block copolymers near the lamellar interface is found to be successful in prediction of block length and star functionality dependence of the lamellar period as it agrees remarkably well with the present and other available experimental data for both symmetric and asymmetric heteroarm star block copolymers.