Kinetics and Mechanisms of the Sphere-to-Rod and Rod-to-Sphere Transitions in the Ternary System PS₃₁₀-b-PAA₅₂/Dioxane/Water

Abstract

The relaxation kinetics and transition mechanisms of the sphere-to-rod and rod-to-sphere transitions occurring in polystyrene-b-poly(acrylic acid) (PS-b-PAA) aggregates have been examined using solution turbidity measurements and transmission electron microscopy. The copolymer PS310-b-PAA52 has been found to self-assemble into aggregates with spherical, rodlike, and vesicular shapes in dioxane-water mixtures. Recently, the morphological phase diagram for this ternary system was constructed in order to determine the concentration boundaries associated with each aggregate architecture and the regions of morphological coexistence. 1 It had previously been found that a sudden alteration of the solvent composition near one of these boundaries could induce a morphological transition. 2 This approach has been used to investigate the kinetics and mechanisms of the sphere-to-rod and rod-to-sphere transitions. The transformation of spherical micelles to rodlike aggregates occurs through a two-step mechanism; it begins with the fast, adhesive collisions of spheres resulting in the formation of irregular "pearl necklace" intermediate structures. This is followed by the reorganization of the necklace intermediates to form smooth rods. The rod-to-sphere transition also involves two steps. A bulb develops quickly on one or both ends of the rod, and then the bulbs are slowly pinched off to release free spheres in solution. The sphereto-rod transition occurs at comparable rates to the reverse process. The effect of the jump magnitude, the initial solvent content, and the copolymer concentration on the relaxation rates was also investigated.

Undulated cylinders of charged diblock copolymers

“…This procedure yields a spherical cap radius R s very close to the ideal spherical micelle size. This characteristic "bulbous" end shape has been widely observed in a number of polymeric micelle systems [9,14] , and is similar to the ends conjectured to appear in the context of bicontinuous phases of emulsions [25]. However, despite the preference for a spherical micelle configuration indicated by these bulbous end shapes, the free energy always has a minimum at finite neck size.…”

“…We conclude that higher order curvature terms to equation (14) are required to understand the local stability of undulated cylinders in terms of universal geometrical measures of the block interface [25,39]. This is, perhaps, not surprising since higher order corrections introduce length scales on the order of R c for axisymmetric shapes [39].…”

“…In the context of diblock copolymers [35,36] or fluid membrane shapes [37], the additional curvature terms modify the shape and stability of various structures. Higher order curvature terms are typically neglected as being subleading, but we will show equation (14) fails to describe our results, even qualitatively, without them. We restrict ourselves to interfaces belonging to the unduloid family of surfaces capped at both ends.…”

“…While similar experiments have address the morphological transition between wormlike micelles and spheres for neutral block copolymers [9,14], the added control provided by modulating the charge and salt conditions of the polyelectrolyte domains allow for a systematic investigation of, in particular, the cylinder to sphere (CS) transition. Bendejacq et al [13] have performed such experiments at controlled salinity and ionic strength and find that when the polyelectrolyte domains are highly charged, the CS transition proceeds by the undulation of the cylindrical micelles followed by the eventual breakup into spheres.…”

Synthesis and Self-assembly Behavior of Photo-responsive Diblock Copolymers with Different Hydrophilic/Hydrophobic Ratios

“…同时, 当亲疏水比例为 2.3 时, 形成了花生型、棒 状结构. 相关研究曾表明 [14] , 引起自组装体形貌改变的 因素包括共聚物亲疏水链段长度比、溶剂组成、温度以 及添加剂等. 当其中的一个参数改变时, 比如亲疏水比 例发生变化, 形成聚集体结构的平衡会被打破, 导致聚 集体形貌转变 [15] .…”