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Modifications to the aqueous solution self-assembly and thermoresponsive properties of poly(N-isopropylacrylamide) (PNIPAm) can be achieved by hydrophobic end-group functionalization and incorporation of hydrophilic N,N-dimethylacrylamide (DMA) repeat units. Although these variations have been studied separately in the past, the simultaneous effects of both modifications have not been investigated systematically. Herein, we report the synthesis of six NIPAM and DMA based statistical, ABA triblock, and ABABA pentablock copolymers using reversible addition-fragmentation chain transfer (RAFT) polymerization, each containing one or two dodecyl hydrocarbon end-groups. Assembly into nanoscale particles and thermoresponsive properties in phosphate buffered saline were studied using light scattering and diffusion ordered NMR spectroscopy. Cloud points (T cp) remained between 30-45 °C, notwithstanding hydrophobic modification. Copolymers with two alkyl tails assembled into flower-like micelles below the T cp. The monofunctional statistical copolymer formed a core-shell assembly and the monofunctional ABA and ABABA polymers were molecularly dissolved. Above the T cp, reversible precipitation was observed in all systems except for the monofunctional ABABA pentablock, which unexpectedly formed uniform large (>100 nm diameter) aggregates interpreted as mesoglobules. These results demonstrate surprising and delicately balanced tradeoffs between short non-polar end groups and tailored hydrophobicity in the nanoscale self-assembly of PNIPAm based copolymers in water near the lower critical solution temperature.
Bibliographical noteFunding Information:
We acknowledge the financial support of The Dow Chemical Company in this study. We gratefully thank Prof. Marc A. Hillmyer at the University of Minnesota, as well as Dr. Jodi Mecca, Dr. Tim Young, and Dr. William W. Porter III at The Dow Chemical Company, for helpful discussions. We also thank Ziang Li for assistance with the dynamic light scattering analysis and Dr. Christopher J. Ellison for the use of a DMF SEC-MALS. J. M. T. acknowledges funding support by the National Science Foundation Graduate Research Fellowship under Grant No. 00006595 and the University of Minnesota Doctoral Dissertation Fellowship. M. L. O. acknowledges funding support by the National Science Foundation Graduate Research Fellowship under Grant No. 00039202. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Award Number DMR-1420013.
© 2019 The Royal Society of Chemistry.
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- Period 6