On the loops-to-bridges ratio in ordered triblock copolymers: An investigation by dielectric relaxation spectroscopy and computer simulations

Dielectric relaxation spectroscopy and computer simulations have been used to elaborate on the loops/bridges population ratio in ordered poly(styrene-block-cis-isoprene-block-styrene), PS-PI-PS, triblock copolymer lamellae. The dynamics of the PT blocks are examined dielectrically for a PS-PI-PS copolymer having symmetrically inverted dipoles along the midblock backbone together with its diblock PS-PI precursor; the present work is an extension of a previous attempt (Watanabe, H. Macromolecules 1995, 28, 5006) addressing the loop/bridge ratio on the basis of a difference in the dynamics of these chains. The experiments indicate that the relaxation of the PS-PI-PS triblock lamella at T < T-g(PS) is significantly affected by preannealing at T > T-g(PS), whereas the relaxation of the PS-PI lamella is insensitive to this annealing. These results as well as changes in the data for the triblock lamella with the sample preparation method should be related to structural changes occurring only for the triblock lamella, i.e., changes in the loop/bridge population distribution as well as formation of mutually knotted loops/bridges on annealing/sample preparation. In the computer simulations, equal mobilities are assumed for the PS and PT blocks, and thus, the PS-PI junctions are allowed to move rather freely under the segregation potential. The equilibrium bridge fraction is estimated and, in agreement with theoretical predictions, is found in the range of 0.50-0.37 as the molecular weight increases. The computer simulations suggest that the relaxation intensity of loops is almost twice that of bridges with both, however, having similar relaxation rates. The dynamics of loops and bridges are found not very different from that of the diblocks (except for the lowest molecular weight); however, the total intensity for the triblocks at equilibrium is much larger than that for the respective diblocks, which may be related to the observed increase of the intensity of the triblock lamella on annealing. However, the dielectric behavior of the diblock and triblock lamellae observed for T < T-g(PS) was not explained from the simulation. These results, indicating an important influence of the junction mobility on the block dynamics, in turn suggest that the (unknotted) loops and bridges can be dielectrically distinguished only when the junction motion is essentially frozen in order to enhance the thermodynamic effect on the block motion due to the density-preserving requirement.