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Chapter 17
Some Thermodynamic Considerations of the Lower Disorder-to-Order Transition of Diblock Copolymers 1
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M. Pollard , Ο. K. C. Tsui , T. P. Russell , Α. V. Ruzette , A. M. Mayes , and Y. Gallot 3
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Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003 Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA 02139 Institut Charles Sadron, Strasbourg, France
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The lower critical ordering of diblock copolymers is an entropically driven phase transition that is accompanied by a negative volume change on mixing. Small angle neutron scattering (SANS) studies of the phase transition under hydrostatic pressure has shown a very large pressure coefficient δΤ/δΡ = 147°C/kbar. Differential scanning calorimetry studies of the phase transition show that the transition from the disordered to the ordered state is endothermic with an enthalpy, ΔΗ ~0.2 J/g. X-ray reflectivity studies of thin copolymer films as a function of temperature exhibits the characteristic thermal expansion of the copolymer film with a discrete change in the film thickness at the transition that corresponds to a 0.35 % volume change. This agrees, within the same order of magnitude, with what would be predicted from the Clapeyron equation.
The order-disorder transition (ODT) in diblock copolymers is a fluctuation induced first-order phase transition that separates a high temperature phase mixed state from a low-temperature microphase separated state. At temperatures below the ODT, also called the upper critical ordering transition (UCOT), repulsive interactions between the monomeric segments of the two blocks drive a segmental demixing that forms a microphase separated structure comprised of nanometer-sized domains. At temperatures above the ODT, thermal energy is sufficient to overcome the unfavorable interactions and segmental mixing occurs. In general, the ODT is determined physically by a balance between energetic and entropie factors unique to block copolymers (1). The coupling between the dynamical behavior of macromolecules and these molecular-level drivers for self-assembly at the ODT has produced a sustained academic interest in the subject of diblock copolymer ordering. Industrial interest in the understanding of the ODT stems from the desire to control © 2000 American Chemical Society
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262 the processability of block copolymers that are used as thermoplastic elastomers, viscosity stabilizers, and pressure sensitive adhesives (2). Very few studies, however, have been devoted to the more fundamental aspects of the phase transition, in particular to the measurement of first-order thermodynamic parameters, such as volume and enthalpy, which change discontinuously at the ordering transition. The importance of these parameters to our experimental and theoretical knowledge of block copolymer phase behavior was recently demonstrated by Hajduk et al (3), who examined the ODT phase behavior of a poly(styrene-&/
