Probing the Scope of Crystallization-Driven Living Self-Assembly: Studies of Diblock Copolymer Micelles with a Polyisoprene Corona and a Crystalline Poly(ferrocenyldiethylsilane) Core-Forming Metalloblock

Abstract

Diblock copolymers with a crystalline poly(ferrocenyldimethylsilane) (PFDMS) core-forming block have been previously shown to self-assemble in selective solvents for a coblock such as polyisoprene (PI) to form cylindrical micelles (where the material is asymmetric, with a PI:PFDMS block ratio of ca. >5:1) or platelets (where the block ratio is ca. 1:1). Moreover, upon addition of further cylinder-forming block copolymer to the ends of the crystalline cores of the cylinders, the faces of the platelets and also the surfaces of thin films of PFDMS homopolymer has been shown to initiate the further growth of cylinders by a living self-assembly process. This is believed to involve an epitaxial growth mechanism. To obtain more detailed insight and to examine the generality of this behavior, we report here detailed comparative studies of the analogous poly(ferrocenyldiethylsilane) (PFDES) homopolymer and the corresponding PI−PFDES diblock copolymers. Significantly, although PI-b-PFDES diblock copolymers with a semicrystalline PFDES core-forming block show similarities to their known PFDMS counterparts in terms of their self-assembly behavior in selective solvents for PI, important differences were also observed. For example, a pronounced tendency of PFDES diblock copolymers to form tape-like structures in solution was noted for PI:PFDES block ratios of ca. 6:1. Uniform cylindrical structures were obtained as the exclusive morphology by increasing the length of the PI block to block ratios of 19:1. Nevertheless, the successful crystallization-driven living self-assembly of PFDES block copolymers involving homoepitaxial growth was demonstrated by the addition of block copolymer unimers to preformed stub-like crystallites formed by sonication. This allowed controlled growth of monodisperse cylinders with the length controlled by the unimer to seed ratio. However, heteroepitaxial growth of PFDES block copolymer from seed micelles of the PFDMS analogue (and vice versa) could not be accomplished. This may be a consequence of the lattice mismatch between the materials and/or kinetic effects. The results demonstrate that crystallization-driven living self-assembly is not limited to PFDMS diblock copolymers and suggest that, although significant differences in self-assembly behavior are likely, this process may be expected to be applicable to other diblock copolymers with a semicrystalline core-forming block.

Crystallization-Driven Self-Assembly of Block Copolymers with a Short Crystallizable Core-Forming Segment: Controlling Micelle Morphology through the Influence of Molar Mass and Solvent Selectivity

“…The change in morphological preference from cylinders to platelets under conditions of improved solvation of the core-forming block has been previously noted for other crystalline-coil block copolymers such as poly(ethylene-b-butadiene) (PEO-b-PB) and poly-(isoprene-b-ferrocenyldiethylsilane) (PI-b-PFDES) systems. 65,87 Detailed structural information on the crystalline PFS core within cylindrical PFS x -b-P2VP 6x micelles was obtained by comparing the spot-type SAED pattern of ordered PFS x -b-P2VP 6x micelles (Figure 10b) to that for a rectangular platelet of PFS-b-PI obtained previously. The spot-type [001] zone ED patterns collected from the cylindrical micelles with similar orientations and the PFS-b-PI platelet 37 were essentially identical.…”

Synthesis and solution self-assembly of block copolymers with a gradient, crystallizable polyferrocenylsilane core-forming metalloblock

“…The progress of the PFS polymerization reaction can be conveniently monitored visually since completion of the polymerization is indicated by a colour change of the solution from dark red to amber. In contrast to the anionic polymerization of FDMS in THF at room temperature, where complete monomer consumption takes place within less than 30 min, the formation of PFDES under similar conditions is considerably slower, and an amber colour indicating full conversion was obtained aer 4 h. 56 In order to gain insight into the structure of the statistical copolymers PFDMS-co-PFDES, a kinetic study was performed by mixing equal amounts (within the experimental error) of FDMS and FDES, according to Scheme 2, and the results are presented in Table 2. The composition of the copolymers at various time intervals was determined with 1 NMR spectroscopy by integrating the FDMS repeating unit signals (Si-CH 3 at 0.55 ppm) against the FDES protons (Si-CH 2 CH 3 at 1.22 ppm).…”

“…All reactions and manipulations were performed under an atmosphere of prepuried nitrogen using either standard Schlenk techniques or in an argon atmosphere MBraun MB150B-G glovebox. Dimethylsila [1]ferrocenophane and diethylsila [1]ferrocenophane were prepared as described previously 56,66 and purication prior to polymerization reactions was achieved by 3 consecutive recrystallizations/sublimations. 63 n-Butyllithium (1.6 M) in hexane, sec-butyllithium (1.4 M) in cyclohexane and isoprene were all purchased from Aldrich.…”

“…For example, in the case of PI-b-PFS systems (PI ¼ polyisoprene) the metalloblock was either poly(ferrocenyldimethylsilane) 55 (PFDMS) or poly(ferrocenyldiethylsilane) (PFDES). 56 Signicantly, unlike in the case of PFDMS and the Ge analog, 57 attempts to achieve heteroepitaxial growth between PFDMS micelles and PFDES unimers were unsuccessful. 56 In order to better understand the underlying mechanism and to further expand the scope of crystallization-driven selfassembly processes, detailed studies of crystalline-coil block copolymers with controlled variations in the PFS block structure are desirable.…”

“…56 Signicantly, unlike in the case of PFDMS and the Ge analog, 57 attempts to achieve heteroepitaxial growth between PFDMS micelles and PFDES unimers were unsuccessful. 56 In order to better understand the underlying mechanism and to further expand the scope of crystallization-driven selfassembly processes, detailed studies of crystalline-coil block copolymers with controlled variations in the PFS block structure are desirable. In this paper we report studies of asymmetric PIb-PFS diblock copolymers, where the crystallizable core-forming PFS block contains a mixture of ferrocenyldimethylsilane (FDMS) and ferrocenyldiethylsilane (FDES) repeating units.…”