Ordering kinetics and dynamics of diblock copolymer solutions

Date of Completion

January 2005

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Block copolymer solutions have been widely used in gels, coatings, adhesives, surfactants and drug delivery systems. Understanding the self-assembling behavior, especially the ordering process of micelles in the concentrated regime, is of great importance to the processing and application of these complex systems. An abnormal thermally induced gelation was observed in squalane solutions of a high-molecular-weight polystyrene-b-ethylene-alt-propylene (SEP) block copolymer, and was attributed to the formation of the BCC phase from a nonequilibrium disordered state. Both rheology and SAXS measurements were used for the investigations. The ordering mechanism of the BCC phase in SEP/squalane solutions was proved to be “nucleation and growth” by SAXS experiments in both deep and shallow quenches. The ordering half time (t0.5) showed a minimum at an intermediate temperature. This is similar to the crystallization process of polymer melts. In the case of a shallow quench, the ordering process of block copolymer solutions was controlled by the nucleation effects, and t0.5 was found to vary with quenching depth as: ln t0.5 ∝ (TODT − T)−n with n varying around 2. For deep quenches t0.5 followed an Arrhenius relationship with temperature, indicating the dominance of the diffusion effects. The activation energy of the ordering process following a deep quench showed no significant variation with concentration, but it changed from 70 to 40 kJ/mol as the molecular weight of the copolymer increased. ^ The disordered micelles in SEP/squalane solutions were found to be thermalrheologically simple. At 30°C, from intermediate to low frequencies, the master curves of G′ and G″ of the disordered block copolymer solutions displayed two separate relaxation processes. Rheological tests showed that the fast relaxation mode corresponds to reptation of individual corona chains tethered on the cores of the micelles, similar to a star polymer. The slow relaxation mode accounts for a terminal flow behavior of the disordered block copolymer solutions and was attributed to the diffusion of individual micelles, which in return determines the ordering kinetics of block copolymer solutions in deep quenches. The rheological and phase behavior of diblock copolymer solutions can be effectively tuned by adding homopolymer of the corona block of the micelles. In SEP/squalane solutions, adding PEP homopolymer with high enough molecular weight inhibited the formation of the ordered BCC structure and decreased the zero-shear viscosity of the disordered solutions. SAXS measurement clearly showed that this behavior was caused by the depletion effects of the free homopolymer chains, which shrink the corona chain conformation and thus decrease the micellar repulsion interaction range and intensity. ^

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