Homopolymer and small-molecule tracer diffusion in a gyroid matrix

Forced Rayleigh scattering was used to measure the tracer diffusion coeffic ients of the photochromic dye tetrathioindigo (TTI) and a 1,4-polyisoprene (PI) homopolymer (8000 g/mol) in a poly(styrene-b-isoprene) (SI) diblock co polymer matrix that formed a bicontinuous gyroid microstructure. The dibloc k copolymer contained 63% polystyrene (PS) by Volume and had a total molecu lar weight of 21,300 g/mol. Rheology and small-angle X-ray scattering confi rmed that the diblock copolymer microphase-separated into the bicontinuous gyroid over the temperature range 60-230 degreesC, where the sample disorde red. For both the TTI and PI tracers, two distinct modes of transport were observed. The faster mode displayed a temperature dependence consistent wit h diffusion within a PI matrix, whereas the slower mode had a temperature d ependence more similar to diffusion within PS. The fast diffusivities were both over an order of magnitude lower than in a corresponding PI homopolyme r matrix. For TTI, this was attributed to the preferential selectivity of t he dye for PS and, therefore, an averaging of the mobility between the PS a nd PI domains. The slow mode was consistent with a small fraction of the TT I dye molecules becoming trapped within the much slower PS domains. Far the PI tracer, the reduction in the diffusion coefficient for the fast mode wa s attributed to a combination of the tortuosity of the struts, the suppress ion of constraint release within the diblock matrix, and additional frictio n due to the presence of some styrene segments within the PI domains, The i nevitable presence of grain boundaries or defects within the matrix interru pted the percolation of the PI struts, thereby forcing some of the PI trace rs to diffuse through PS. Consequently, the slow mode was attributed to the diffusion through these defects, where the PI diffusion was retarded by bo th the increased segmental friction and the thermodynamic barrier to enteri ng the PS domains. (C) 2001 John Wiley & Sons, Inc.