Transport phenomena within block copolymers: The effect of morphology and grain structure
In the present study, a fundamental understanding of the structure-property correlation with emphasis on the mass transport properties of lamellar block copolymers has been accomplished. A gravimetric sorption apparatus has been developed and constructed to measure the permeation and diffusion coefficients for a variety of polystyrene-polybutadiene and polystyrene-polyisoprene neat diblock and graft copolymers, using small molecule gases. The effect of structural ordering in the nanometer scale, morphology, and the micron-scale, grain structure, has been investigated.
The effect of morphology on mass transport properties in polystyrene-polybutadiene (SB) diblock copolymers is studied by selective solvent casting. This is the first study to employ selective solvent casting to circumvent the thermodynamics and generate both lamellar and cylindrical morphologies at the same volume fraction, and thus decouple the two important variables, the volume fraction and morphology. It is found that the excluded effect of morphology is much less significant than anticipated.
The effect of grain structure on mass transport properties in polystyrene-polyisoprene (SI) diblock copolymers is studied by solution casting at different rates. In the slow-cast materials, the lamellae are oriented almost parallel to the film surface with larger grain sizes. On the other hand, smaller grains are formed with a random orientation in the fast-cast materials. The effects of lamellae orientation and grain size are decoupled by employing a ‘random-column model’ by Permnath to predict the permeation coefficients in the fast- and slow-cast materials. This is the first study to investigate the effect of grain structure in block copolymers without the macroscopic orientation artificially induced. It is found that the lamellar orientation is the dominant parameter for mass transport behavior.
The unusual phase behaviors are shown in the new blend materials of A 2B-type single graft copolymers and their homopolymers. The homopolymer blending has induced a substantial shift in the order-order transitions (OOTs) and results in complex morphologies not possible for neat diblock copolymers. This has enhanced the number of design variables for new materials development.