Linear melt rheology and small-angle X-ray scattering of AB diblocks vs A(2)B(2) four arm star block copolymers

The frequency dependent viscoelastic properties and lamellar spacing of thr ee symmetric styrene-isoprene (PS-PI) diblock copolymers are compared to th ose of their hetero-four-arm star counterparts. The PS and PI arm molecular weights of the three linear and three star samples are 10, 20, and 60 kg/m ol, respectively. All six samples were unoriented and had lamellar morpholo gy for temperatures less than T-ODT, the order-disorder temperature for eac h molecular-weight. The lamellar spacing D at the same temperature was foun d to scale with overall molecular weight N according to D similar to N-delt a, with delta approximate to 0.7 for both linear and stars. However, the st ar chains were consistently 5-10% more strongly stretched compared to their Linear counterparts. For the 10K arm materials, the critical frequency for the onset of mesophase relaxations to,) for the stars was found to be abou t 20 times smaller compared to the linears. This difference correlated very well with quantitative estimates of the inverse layer hopping time of the chains, suggesting that mesophase relaxations for the 10K arm materials may be controlled by layer hopping of chains. For the 10K and 20K arm material s, relaxation of the PS chain deformations are dominant for omega much grea ter than omega (PS)(term), whereas nonclassical terminal scaling of G', G" similar to omega (1/2) was observed for omega much less than omega (PS)(ter m) and T < T-ODT due to mesophase relaxations (<omega>(PS)(term) is the PS block terminal relaxation frequency). In addition, the linear rheology of t he linear and star analogues coincide for omega much greater than omega (PS )(term), but an additional shoulder emerges in the star materials for omega approximate to omega (PS)(term). By fitting to a simple model incorporatin g free chain Rouse dynamics and mesophase relaxations, we were able to obta in excellent quantitative fits to the 20K materials across the whole freque ncy range and conclude that the observed shoulder in the star materials was due to differences in the linear and star mesophase relaxations. The fitte d omega (e), and G(M0) (the mesophase modulus) values are in good agreement with Kawasaki-Onuki theory indicating that the mesophase relaxations of th e 20K arm materials may be controlled by collective hydrodynamic layer fluc tuations rather than layer hopping of chains. For the 60K arm materials qua litatively different behavior compared to the lower molecular weight sample s was observed: PI rate controlled relaxation with G', G" similar to omega (1/2) was observed for omega much greater than omega (PS)(term). We identif y this relaxation as a PI controlled mesophase relaxation. Theoretical esti mates of w, for this mechanism using Kawasaki-Onuki theory yield omega (e) much greater than omega (PS)(term) in support of our suggestion.