Crystalline Morphologies of Poly(butadiene)-b-Poly(ethylene oxide) Block Copolymers in n-Heptane
This thesis reports the development of micellar crystalline morphologies in a selective solvent. The phase diagram of solution morphologies as a function of the molecular composition of the semicrystalline poly(butadiene)-b-poly(ethylene oxide)(PB-b-PEO) block copolymers was investigated. The crystalline morphologies discussed here have been generated from selective solvent condition (70°C in n-heptane) via two thermal pathways: (A) by direct immersion into liquid nitrogen, where n-heptane becomes a poor solvent for both blocks at very low temperatures, and (B): by quenching to the crystallization temperature of the PEO, i.e., determined by the length of PEO block. In pathway B, n-heptane is a poor solvent only for the PEO block. At 70°C, the block copolymers self-assembled into micellar structures consisting of a PEO molten core and a soluble PB corona. As crystallization takes place in the PEO core, a fast quenching into liquid nitrogen results in the formation of crystalline micelles retaining the shape present in the molten state at 70°C (pathway A). In the case of pathway B, the competition between the PEO core crystallization and the self-assembly of the micellar units, is the driving force that dictates the morphological development, therefore crystallization breaks out the melt morphology. These studies, demonstrated that the PB-b-PEO block copolymers are a promising system models for developing a general route towards tunable crystalline morphologies. In a symmetric PB-b-PEO block copolymer, crystalline morphologies like spheres and meanders formed upon quenching into liquid nitrogen and at 30°C, respectively. The meander morphology consisting of branched lamellae with a crystalline PEO ribbon-like core and ellipsoidal endings was observed for the first time in solution. Investigations of the crystal development revealed that this structure formed via crystallization-induced aggregation of spherical micelles upon cooling. A systematic study of the effect of crystallization kinetics on the formed morphology upon crystallization-induced aggregation of spherical micelles of a symmetric PB-b-PEO block copolymer was discussed. We demonstrated that the resulting morphology is controlled by two competitive effects, namely, by the nucleation and growth of the PEO micellar core: at lower crystallization temperatures (Tc ≤ 30°C), a high nucleation rate leads to a meander-like morphology formation, whereas at higher crystallization temperatures (Tc > 30°C), a low nucleation rate favors the formation of twisted lamellae. For a highly asymmetric PB-b-PEO block copolymer, crystallization at -30°C induced the formation of crystalline micelles retaining the spherical shape present in the molten state at 70°C. However, a quenching into liquid nitrogen facilitated a transition to rod-like micelles caused by changes of solvent quality for the PB coronar chains. This triggers the onset of an interfacial instability, therefore the spherical micelles preferred to reorganize into a morphology with a smaller interfacial curvature. The low crystallinity of the PEO core imposed a stronger tendency of the rods to aggregate and to thicken into more stable morphologies as needle-like structures, with a preferred growth direction along the long axis. Finally, the micellar morphology diagram of the PB-b-PEO block copolymers has been studied as a function of the crystallization temperature and molecular composition of the blocks via two thermal pathways. Pathway A allowed morphological transitions from spheres to rods, worms or twisted cylinders with the increase of the crystalline content of the PEO core. In Pathway B, the sequence of spheres, cylinders, lamellae, platelets and dendrites structures is observed with the increases of the PEO block length. The aggregation number of the spherical micelles is affected by the weight fraction and crystallinity of the PEO block. Moreover, an increased chain folding was observed at a high PEO composition which reduced the lamellar thickness of the crystals. The competition between the PEO core crystallization and the aggregation of the micellar units leads to coexistence regions of lamellae with platelets and cylinders with platelets. The novelty of this thesis relies on the development of novel crystalline morphologies in a selective solvent, as well as, in the detailed analysis of the major parameters that govern morphological formation in a controlled manner.
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