Calorimetric Study of Order-Disorder Transition of Block Copolymers

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Block copolymers such as stirene-isoprene diblock copolymers (SI) exhibit the microphase separation with a regular crystalline lattice structure. With increasing temperature, they transform from the ordered phase structure to a disordered state. Around the order-disorder transition temperature (ODT), the dynamics of the chains and the physical properties change dramatically. This study aims to clarify the relationship between the thermodynamic properties and the chain dynamics. Firstly we constructed an adiabatic calorimeter for the accurate measurements of heat capacity of block copolymers. For thermometer, a resistance thermometer (Rd-Fe) calibrated at Oxford instrument Co.was used and the resistance was measured with a bridge made by ASL Co. Thus we measured temperatures before and after the supply of electric energy E at a precision of 0.0001K.The sample cell was doubly surrounded by adiabatic walls and the temperature of the wall was regulated to be the same as the sample cell within the error of 0.01K.The cell was gold-plated to prevent the radiation. The readings of the meters and the temperature control were performed with two computers. The heat capacity of toluene solution of SI (20wt%) was measured. The SI with total molecular weight of 142000 and the stirene content of 35.9% was prepared by anionic polymerization. Small lambda type anomaly is found around 228K and can be ascribed to the ODT.The extra heat capacity due to the ODT is compared with the Leibler theory. Although the temperature dependence of the heat capacity agrees with the theory, the intensity was ca.ten times higher than the theory. Beside the calorimetric study, we investigated the dielectric relaxation and small angle X-ray scattering for semi dilute solutions of SI.It was found that the dielectric relaxation time for the normal mode process of the isoprene block chain is bimodal in the ordered state but it changes into unimodal above the ODT.