High-temperature BCC lattice formation/disappearance in triblock copolymer gels: morphological transition or order frustration?

R. Kleppinger, N. Mischenko, L. Theunissen, H. Reynaers, K. Mortensen*, K. Almdal*, M.H.J. Koch#
Dept. of Chemistry KULeuven, Celestijnenlaan 200F, B-3001 BELGIUM; * Risoe National laboratory, Denmark; #EMBL-Outstation, Germany
Triblock ABA copolymer gels, based on a three-dimensional physical network of polystyrene-rubber-polystyrene copolymers in a relatively low molar mass (Mw ~ 400-500) solvent selective for the midblock (B), are soft solid materials[1-2]. They have high elasticity and deformability (100 - 1000%, depending on molar mass and block ratio of the copolymer[3-4]. At ambient temperature a short-range ordered arrangement of the network nodes (spherical domains of the endblocks) results in a morphology intermediate between liquid-like and highly distorted crystalline one [2,5]. For some gels a real polycrystalline (BCC) structure has been observed developing within certain temperature range [6,7]. It however tends to disappear beyond these temperature limits. While the existence of the high- temperature limit sounds reasonable, the reason of the low-temperature one is disputable. It is worth to mention that BCC structure is also reported for micellar solutions of the same triblock copolymer in a different solvent (for instance, in hexane at room temperatures) although they do not have elastic rubber properties.

At ambient temperature structure of the network in our gels can be described in terms of a highly distorted crystalline lattice of close-packed spheres (CPS, either HCP or FCC) or in terms of liquid with effective hard-sphere interaction. Therefore two possible mechanisms exist for the low-temperature transformation: frustration of the BCC morphology or a change to a different one. We are going to discuss this matter on the basis of real-time SAXS scans during cooling- heating or isothermal annealing and equilibrium SANS measurements.

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