Self-Assembled Structures in Diblock Copolymers with Hydrogen-Bonded Amphiphilic Plasticizing Compounds

Abstract

Hydrogen-bonding amphiphilic low molecular weight plasticizing compounds to one block of diblock copolymers to form supramolecular comblike blocks leads to hierarchical self-assembly at the block copolymer (long) and amphiphile (short) length scales, in which lamellar-in-lamellar order and the related phase transitions have previously been shown to allow thermal switching of electrical and optical properties [Science 1998, 280, 557; Nat. Mater. 2004, 3, 872]. In this work other hierarchies and phase transitions are systematically searched, a particular interest being hierarchies containing gyroid structures and the related order−order transitions. Polymeric supramolecular comb−coil diblock copolymers consisting of a polystyrene (PS) coillike block and a supramolecular comblike block based on poly(4-vinylpyridine) (P4VP) are used, where the pyridines are either directly hydrogen bonded with 3-pentadecylphenol (PDP), i.e., PS-block-P4VP(PDP)1.0, or first protonated with methanesulfonic acid (MSA) and then hydrogen bonded to PDP, i.e., PS-block-P4VP(MSA)1.0(PDP)1.0. In this way the comblike block can be noncharged or charged. The morphologies were determined using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) at different temperatures. In the case of PS-block-P4VP(PDP)1.0, all classical diblock copolymer morphologies were observed at room temperature, where the P4VP(PDP)1.0 domains contain an additional lamellar structure due to the supramolecular comblike blocks. Here we report novel gyroid and hexagonal perforated layer morphologies, i.e., where the PS and P4VP(PDP)1.0 blocks form gyroid or hexagonal perforated layer order and the P4VP(PDP)1.0 domains have an internal lamellar order. Heating past ca. T = 60 °C causes an order−disorder transition within the P4VP(PDP)1.0 domains. Further heating leads to gradually reduced hydrogen bonding strength, and importantly PDP becomes soluble in PS at T > ca. 120 °C. At such temperatures PDP is found in both the P4VP and PS domains, thus leading to changes in the relative volume fractions of the domains, which in turn leads to order−order transitions. In PS-block-P4VP(MSA)1.0(PDP)1.0, typically lamellar and cylindrical block copolymeric structures were observed, where there was an additional internal lamellar order within the P4VP(MSA)1.0(PDP)1.0 domains. Coincidentally, an order−disorder transition within the P4VP(MSA)1.0(PDP)1.0 domains takes place at T = ca. 125 °C. Above that temperature, PDP is in both PS and P4VP(MSA)1.0 domains, but most interestingly at ca. T > 175 °C PDP becomes a nonsolvent for P4VP(MSA)1.0 and it is therefore expelled to predominantly to the PS domains. This manifests as an order−order transition. All samples exhibit at least two thermoreversible order−order transitions, and some of them show even five consecutive self-assembled phases as a function of temperature. Besides being amphiphilic, PDP can also be regarded as a plasticizer, i.e., relatively nonvolatile solvent, for the P4VP, PS, and P4VP(MSA)1.0 with characteristic phase behaviors. This, in combination with the comb−coil diblock copolymer composition and the reversibility of the hydrogen bonding, enables to achieve thermoreversible transition sequences that are not easily accessible only by changing the Flory−Huggins interaction parameter χ by temperature, for example transitions from a lamellar to spherical structure. The combination of phase behaviors of self-assembly and polymer/plasticizer mixtures allows new structural hierarchies and phase transitions that may lead to new types of responsive materials.