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 t he conventional diblock copolymers is analyzed theoretically and experiment ally on the example of symmetric polystyrene-poly(2-vinylpyridine) (PS(6)P2 VP(6)) star block copolymer melts. Microphase separation of PS(6)P2VP(6) st ars with volume fraction of PS varying from 0.30 to 0.75 is studied using t ransmission electron microscopy (TEM). The observed (lamellar, cylindrical and undefined "bicontinuous-like") structures often turned out to be differ ent from that expected for the corresponding diblock copolymers. The depend ence of the long period of lamellar structures of heteroarm star block copo lymers on block length is found to deviate in a systematic way from N-2/3 l aw (known for diblock copolymers), especially for the stars of high functio nality. The theoretical model proposed to describe an extra-stretching of s tar block copolymers near the lamellar interface is found to be successful in prediction of block length and star functionality dependence of the lame llar period as it agrees remarkably well with the present and other availab le experimental data for both symmetric and asymmetric heteroarm star block copolymers.