(251e) Synthesis of Amphiphilic Copolymers and Assessment of Their Critical Aggregation Concentrations Via Fluorescence Techniques: Styrene/Acrylic Acid Diblock, Random and Gradient Copolymers

Amphiphilic copolymers of styrene (S) and acrylic acid (AA) differing in the sequence distribution are synthesized by hydrolysis of S/tert-butyl acrylate (tBA) copolymer precursors. While the random copolymers of S/tBA are made via conventional free radical polymerization using AIBN as the initiator, the diblock and gradient copolymers are prepared via nitroxide-mediated controlled radical polymerization (NM-CRP). In particular, the diblock copolymers are made by two subsequent NM-CRP batches of S and tBA, and the gradient copolymers are synthesized via a semi-batch method where the tBA monomer is continuously fed into styrene NM-CRP, thereby a gradual increase in the tBA concentration along the copolymers is attained. Two copolymer compositions (50 mol% styrene and 70 mol% styrene) per each sequence distribution (random, diblock and gradient) are produced.

After hydrolysis of the S/tBA precursor copolymers into S/AA, the amphiphilic copolymers are characterized for the critical aggregation concentration (CAC) in a water-rich solvent condition (20:80 v/v THF:H2O) containing trace amount of pyrene. We monitor the evolution of vibronic band intensity (I1 and I3) from the pyrene emission fluorescence spectra as a function of the copolymer concentration. The fluorescence probe (pyrene) can effectively reflect its micro-polarity by favorable penetration into the hydrophobic, styrene-rich aggregate cores above a critical copolymer concentration. The intensity ratio, I3/I1, from the pyrene emission spectra exhibits a substantial increase in the value above a certain S/AA copolymer concentration where the pyrene molecules begin to partition into the styrene-rich cores of the aggregates. Below the critical copolymer concentration where the S/AA copolymers are expected to exist as free chains, the pyrene emission spectra and I3/I1 show the pattern typical to polar environment. We show that every S/AA copolymer, regardless of the sequence distribution, forms aggregates above a critical concentration, but with noticeable differences in the micro-environment of the aggregate cores depending on the S/AA sequence distribution as observed by the pyrene fluorescence measurements. In addition, we employ UV-transmittance and dynamic light scattering measurements in order to further investigate the differences in the CAC and the aggregate sizes influenced by the sequence distribution of hydrophobic (styrene) and hydrophilic (acrylic acid) repeat units along the copolymers.