(578b) The Effects of Copolymerized Blocks On the Self-Assembly of Acrlyic Terpolymers in Solution

Obtaining the optimal polymer microstructure with the optimal chemical domains for specific applications is difficult.  Because of this, there has been much research into how to manipulate the microstructures of self-assembling polymers such that desired chemical functionalities and microstructures can be simultaneously achieved.  The specific microstructure formed in a phase-separated system is determined by minimizing the free energy of the polymer chain confirmations and the interfacial energy between the two microdomains.  To date, the thermodynamics that define the interfacial curvature, and therefore, the microstructure have largely been tuned by changing polymer composition.  Other techniques for tuning the thermodynamics include additives (solvents, surfactants or counter ions), and manipulation of architectures (miktoarm star, janus particles and graft copolymers).  Terpolymers offer another variable for changing the microstructure at a specific composition, namely the sequencing of the blocks.  For example, it has been shown that two triblock terpolymers, ABC and ACB, both with the same block sizes, do not self-assemble into the same microstructures.  Over the past twenty years the interest in polymers with three chemically unique repeat units has increased.  Compared to diblock copolymers, triblock terpolymers have been shown to self-assemble into a greater number of microstructures.  One of the exciting promises of terpolymers is the wider availability of multi-continuous phases. 

Here, two of the three monomers in an acrylic terpolymer consisting of hydroxyethyl acrylate (HEA), methyl acrylate (MA), and octyl acrylate (OA) are copolymerized, and the effects of the architecture on the self-assembled microstructures in water and tetrahydrofuran are examined.  The copolymerization of the HEA and MA or OA and MA is expected to dampen the interfacial energy between domains, and the entropic penalties of self-assembling, allowing for greater segregation.  All of the polymers were synthesized using reversible addition chain transfer (RAFT) polymerization techniques.  The entirety of this work has studied four different terpolymer compositions with a constant molecular volume, and four different architectures.  Here, we will limit the discussion to two of the compositions.  The first composition had 69 repeat units of HEA, 20 repeat units of MA and 9 repeat units of OA; the second composition had 54 repeat units of HEA, 20 repeat units of MA and 17 repeat units of OA.  The first two architectures were triblocks, HEA – b – OA – b – MA and HEA – b – MA – b – OA.  The third and fourth architectures consisted of one homoblock and one copolymerized block, HEA – b – (OA – co – MA) and (HEA – co – MA) – b – OA.  The monomer conversion was verified gravimetrically, and molecular weight was determined by gel permeation chromatography after each monomer addition.  All of the polymers were equilibrated at four concentrations in water and four concentrations in tetrahydrofuran.  Small angle x-ray scattering was used to characterize morphologies of the neat polymers and equilibrated samples.

Lamellar and hexagonal microstructures have been identified, and differences in self-assembled microstructures based on architectures were observed.  The results indicate a strong segregation for the terpolymers with copolymerized blocks.  Further characterization of these systems using cryo-TEM is being conducted.