Asymmetric block copolymers in neutral good solvents: self-diffusion through the ordering transition

✍ Scribed by Timothy P. Lodge; Matthew A. Blazey; Z. Liu; Ian W. Hamley

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 689 KB
• Volume: 198
• Category: Article
• ISSN: 1022-1352
• DOI: 10.1002/macp.1997.021980406

✦ Synopsis

Abstract

The self‐diffusion coefficients of a styrene‐isoprene diblock and a styrene‐isoprene‐styrene triblock copolymer have been measured as functions of concentration in the neutral good solvent toluene. The sample block molecular weights are 1.0 × 10^4^ and 5.0 × 10^4^ g/mol for the diblock, and 1.0 × 10^4^, 1.0 × 10^5^ and 1.0 × 10^4^ g/mol for the triblock, and the samples are designated SI‐60 and SIS‐120, respectively. Synchrotron small‐angle X‐ray scattering was utilized to locate the ordering transitions, and to identify the morphologies. SI‐60 exhibited an order‐order transition from a hexagonal to a cubic structure (i.e., from cylinders to spheres), prior to disordering, whereas SIS‐120 only exhibited the hexagonal phase. These results were compared to the melt, where both materials exhibit the cylinder‐to‐sphere transition. The concentration range for diffusion measurements extended into the ordered state: weight fractions up to 0.78 for SI‐60 and 0.62 for SIS‐120, where the corresponding ordering concentrations are 0.67 and 0.56. Diffusion measurements were performed by forced Rayleigh scattering and by pulsed‐field‐gradient NMR. The results indicate that substantial concentration fluctuations arise in solution prior to the ordering transition, and that these fluctuations act to retard chain diffusion. However, from comparison of the diblock and triblock mobilities, it appears that the retardation of mobility in the fluctuating and ordered states is due more to the increased monomeric friction in the (cylindrical) styrene‐rich microdomains, than to the thermodynamic barrier for escape of the styrene blocks from these microdomains.