Project: Order-order and order-disorder transitions of block copolymer solutions using small angle X-ray and neutron scattering

Part I: Epitaxial Transitions among FCC, HCP, BCC, and Cylinder Phases in a Block Copolymer Solution.

Block copolymers belong to a broad class of compounds that can self-assemble into ordered phases with periodic microstructures. In particular, when marginally good solvent was used, intriguing morphological change has been observed by varying temperature in a narrow range of polymer concentration presumably due to the reduced repulsive interaction between the microphase-separated domains. When a sphere-forming block copolymer is used, subtle order-order transitions from fcc/hcp to bcc/hcp to hex occur by raising temperature. A bcc phase was grown epitaxially from the fcc and this transformation is related to the Bain distortion, which is well-established in simple metals. Interestingly, the parallelism between the close-packed planes, i.e., (111)_fcc and (110)_bcc, was preserved during these transformations. The orientational relationship between the fcc and the bcc crystals are identical to the Kurdjumov-Sachs and Nishiyama-Wasserman ORs. The epitaxial relationship between hcp and hexagonal cylinder is also described for the first time in block copolymers, whereas the bcc/cylinder epitaxy is as previously established. Based on our experimental results, we were able to locate a low-energy pathway among fcc, hcp, bcc, and cylinders as a function of temperature.

Part II: Kinetics Study between Cylinders and Spheres by Small Angle X-ray Scattering (SAXS).

The phase behavior of a symmetric styrene-isoprene (SI) diblock copolymer in a styrene-selective solvent, diethyl phthalate, was investigated by in-situ small angle X-ray scattering (SAXS) on shear-oriented solutions. At room temperature, solutions in the concentration range f = 0.25 ~ 0.45 adopt a close packed lattice of micelles, a mixture of fcc and hcp structures. Upon heating, for solutions with polymer volume fractions f = 0.25 ~ 0.32, the fcc/hcp structure transformed to the bcc structure, following the epitaxial pathways established previously. In contrast, for f = 0.33 ~ 0.45, a transition from an fcc/hcp to a bcc/hcp mixture occurred, and then the bcc and hcp structures transformed to the cylinder phase simultaneously. At this point, one important question can be raised. While the cylinder phase is formed by both the bcc and the hcp phases with the increase in temperature, what is the ordering pathway from the cylinder upon cooling? We observed hysterisis between heating scan and cooling scan, namely, super-cooled cylinders were formed by cooling scan. The stability of these super-cooled cylinders is quite dependent on concentration and for f > 0.40, the super-cooled cylinders do not go back to spheres even the annealing time for one month. This transition kinetics between spheres and cylinders are confirmed by a combination of rheology and static birefringence.

Part III: Study on the Order-disorder Transition (ODT) of Block Copolymer Solutions Using Small Angle Neutron Scattering (SANS).

The order-disorder transition (ODT) of PS-b-PI diblock copolymer in solvents of varying selectivity for a specific block is investigated with small angle neutron scattering (SANS). In order to understand the subtle morphological changes more clearly, one of the diblock copolymer is deuterated (i.e., PS-b-dPI and dPS-b-PI) and both diblock copolymers were synthesized by anionic polymerization. By using the contrast matching method, SANS experiments were performed for PS-dPI in DEP/DBP, which make us possible to see the “dPI core”, and dPS-PI in dC14/C14 to see the behavior of “dPS core”. Two polymer concentrations were selected to place two kinds of ODT, i.e., bcc-to-disordered and cylinder-to-disordered. The decrease in the core size with increase in temperature is clearly monitored. Furthermore clear “cmt” was observed for both concentrations, which is about 20 ^oC higher than ODT. Based on these results, the detailed explanation for the ODT and the solution behavior above ODT temperature were suggested.