Rheological Study of Order-to-Disorder Transitions and Phase Behavior of Block Copolymer–Surfactant Complexes Containing Hydrogen-Bonded Small Molecule Additives

Dynamic mechanical measurements were used to investigate the effect of small molecule additives on the order-todisorder transitions (ODTs) of Pluronic, poly(ethylene oxide) (PEO)−poly(propylene oxide) (PPO)−PEO triblock copolymer surfactant melts. The small molecule additives contain multiple functional groups (carboxyl or hydroxyl), which selectively interact with the PEO component of Pluronic via hydrogen bonding, thereby effectively increasing χ of the system and leading to microphase separation in otherwise disordered melts. The ODTs of these Pluronic/small-molecule-additive complexes can be detected by rheology since, upon increasing temperature, crossing the order-to-disorder transition temperature (TODT) results in a sharp decrease in the low frequency storage and loss moduli (G′ and G′′, respectively). The crystallization of the PEO component is suppressed with increasing additive loading due to strong hydrogen bond interactions. The TODT is strongly composition dependent and increases up to 145 °C for 20 wt % loading of a particular additive. TODT is also found to vary widely but systematically with the number, position and hydrogenbond-donating ability of the functional groups of the additive. Upon increasing temperature for high additive loadings, macrophase separation and crystallization of the additives can occur before the ODT is detected. ■ INTRODUCTION The phase behavior of block copolymers (BCPs) with two chemically distinct molecular segments is governed by the relative volume fraction of the blocks and the product χN, where χ and N are the Flory−Huggins interaction parameter of the blocks and the degree of polymerization, respectively. Microphase separation into ordered morphologies (lamellar, bicontinuous, cylindrical and spherical) occurs below the TODT for sufficiently large χN. Self-assembly of BCPs into nanostructured morphologies facilitates their use as materials for the fabrication of high density data storage media, high resolution etch masks, nanopores, nanowires, and nanopillers, and as templates for mesoporous inorganic materials. The increasing demand for higher density microelectronic structures and the associated well-ordered morphologies with small features (3−15 nm) requires increasing χ, to compensate for small N, in order to maintain a sufficiently large χN. Hydrogen bonding is one of the key mechanisms behind the formation of various supramolecular structures from small moieties. Taking advantage of versatility, directionality, and stability of the interactions, hydrogen bonding between small molecule building blocks has enabled researchers to create supramolecular assemblies exhibiting liquid crystalline behavior. Hydrogen bond assisted liquid crystalline supramolecular assemblies have also been achieved in blends containing polymers and mesogenic or nonmesogenic small molecules. Hierarchical assemblies at two different length scales have been demonstrated by the means of selective hydrogen bonding between mesogenic or amphiphilic additive molecules and a specific block of the block copolymers. Disorder-to-order and order-to-order transitions in the block copolymer can be introduced through the loading of the additive molecules. Owing to the mesogenic or amphiphilic nature, these additive molecules further self-organize within the host domain of the microphase separated block copolymers to result in composition dependent hierarchical morphologies. Recently, it has been shown that multiple-hydrogen-bonddonating small molecules can selectively associate with a particular block and induce microphase separation in otherwise disordered BCPs.The molecules used are nonmesogenic and nonamphiphilic, therefore no substructures are formed within the resulting microphase separated domains and the morphologies native to the block copolymers are preserved. The resulting well-ordered morphologies exhibit feature sizes of less than 15 nm, which is important for applications requiring small domains. Since the microphase separation is driven by the addition of the small molecule component, segregation strength, and consequently the order-to-disorder transition temperature (TODT) will naturally depend upon the strength Received: September 4, 2014 Revised: October 24, 2014 Published: November 12, 2014 Article pubs.acs.org/Macromolecules © 2014 American Chemical Society 8048 dx.doi.org/10.1021/ma501816d | Macromolecules 2014, 47, 8048−8055 and the number of hydrogen bond interactions presented by the additive and the loadings of the small molecule additive into the BCP. Studying ODTs in these BCP/small-moleculeadditive complexes is of particular interest. Block copolymers of PEO and PPO are commercially available under the trade name Pluronic. The hydrophobic PPO block and the hydrophilic PEO block give the molecule an amphiphilic nature typical of a nonionic surfactant. Pluronic block copolymers are usually disordered at room temperature and above in the melt due to their low molecular weight and weak repulsive interaction between PEO and PPO blocks. By taking advantage of the hydrogen bonding interaction, microphase separation can be induced by blending Pluronic block copolymers with homopolymers, small molecule additives, and functionalized nanoparticles. All of these additives bear hydrogen bond donating sites which selectively interact with the hydrophilic PEO blocks, increasing the effective χ parameter and thus leading to microphase