|A.H.Hofman at rug.nl|
Hierarchical Self-Assembly of Supramolecular Comb-Shaped Block Copolymers
Chemically different polymers, e.g. polystyrene (PS) and poly(methyl methacrylate) (PMMA), are often immiscible due to their chain-like structure. When both homopolymers are covalently linked to each other, the interface between the two compounds in such diblock copolymers is again minimized in order to reduce the unfavorable interaction. However, due the connection between the blocks the phase separation is now confined to the molecular level. This phenomenon is known as microphase separation. In the simplest case, i.e. a linear PA-b-PB diblock copolymer, depending on the composition, molecular weight and the interaction parameter several fascinating mesostructures are formed upon phase separation, including lamellae, cylinders, spheres and even bicontinuous network morphologies.
Figure 1: transmission electron micrographs of double-comb diblock copolymers with high (a) and low (b) comb density. The large TEM images are stained with iodine (P4VP appears dark), while the small ones are unstained (both small length scales become visible).
Besides being interesting for application in lithography, optics, compatibilizers, membranes and electronic devices, the phase behavior of block copolymers is also interesting from a fundamental point of view. More complicated polymer architectures, like triblock terpolymers, star polymers and multiblock copolymers, often result in more complex phase behavior, but the synthesis of such materials requires very inert reaction conditions and special equipment. A more subtle approach for the preparation of complex polymer architectures is supramolecular chemistry. In our research we introduce side chains to a polymer backbone via this route: hydrogen bonding between poly(4-vinylpyridine) (P4VP) and 3-pentadecylphenol (3-PDP) amphiphiles, and the repulsion of its aliphatic tail gives rise to lamellar structure formation, similar to the process observed in block copolymers. When combined with PS-b-P4VP diblock copolymers, the self-assembly of such PS-b-P4VP(3-PDP) comb-coil diblock copolymers results in hierarchical (i.e. structure-in-structure) morphologies. Very recently the system was extended to a symmetric double hydrogen bond accepting diblock copolymer based on poly(N,N-dimethylacrylamide) (PDMA) and P4VP. Addition of stoichiometric quantities of 3-PDP resulted in a double perpendicular lamellar-in-lamellar morphology.  Further optimization of this system by moving to a slightly different diblock copolymer (P4VP-b-PAPI, N-acryloylpiperidine, API) and amphiphiles with a longer tail (3-nonadecylphenol, 3-NDP) gave rise to even more exotic self-assembly: double perpendicular lamellae-in-lamellae were found in stoichiometric complexes, while double parallel lamellae-in-lamellae were characterized in complexes with lower comb densities (figure 1). These observations are in excellent agreement with our previously performed theoretical analysis on (A-comb-C)-b-(B-comb-C) double-comb diblock copolymers.
As our supramolecular approach turned out to be very successful, we are currently further exploring the phase diagram of this new system (molecular weight, block composition and comb density) and expanding the architecture of the backbone to ABC triblock terpolymers. Potential applications of the double-comb diblock copolymers include their use in photonic crystals and template fabrication.
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|Laatst gewijzigd:||01 juli 2015 10:53|