J. Phys. II France
Volume 6, Number 5, May 1996
Page(s) 617 - 637
DOI: 10.1051/jp2:1996202
J. Phys. II France 6 (1996) 617-637

Isotropic and Anisotropic Composition Fluctuations Close to the Order-to-Disorder Transition in an Asymmetric Diblock Copolymer Melt Subjected to Reciprocating Shear Fields

Kristoffer Almdal1, Kell Mortensen1, Kurt A. Koppi2, Matthew Tirrell2 and Frank S. Bates2

1  Department of Solid State Physics, Risø National Laboratory, P.O Box 49, 4000 Roskilde, Denmark
2  Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55405-0132, USA

(Received 11 September 1995, received in final form 1 February 1996, accepted 8 February 1996)

An asymmetric poly(ethylene-propylene)-poly(ethylethylene) (PEP-PEE) diblock copolymer was examined near the order-to-disorder transition temperature, $T_{\rm ODT}$, by small-angle neutron scattering as a function of shear rate, $\dot{\gamma}$. Heating in the absence of shear disorders the material from a hexagonal structure at $T_{\rm ODT}=155\pm 1 ^\circ{\rm C}$. Application of a reciprocating simple shear field markedly influences the phase behavior near the ODT. Application of shear while heating increases $T_{\rm ODT}$ consistent with the theoretical prediction, $T_{\rm ODT}\sim\dot{\gamma}^{2} $. During fast cooling studies we observe an additional characteristic temperature, $T_{\rm S}$, at which the material spontaneously orders, which we interpret as a stability (i.e., spinodal) limit. this result also agrees with the theoretical prediction, $\tau_{\rm S}\sim\dot{\gamma}^{-1/3}$. Upon cessation of shear at temperatures close to but above the quiescent $T_{\rm ODT}$ a transient phase is observed before the system macroscopically disorders. The transient structure has a symmetry compatible with spheres arranged on a body centered cubic (BBC) space lattice, or undulating cylinders of the minority component (PEE). Anisotropic (i.e. lattice pinned) fluctuations can also be observed close to, but below, $T_{\rm ODT}$ upon heating the hexagonal structure under shear free conditions.

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