-Advanced Polymer Monitoring Technologies, Inc.
-Polymer Standards Service
-Brookhaven Instruments Corp.
-International Specialty Products
This work has two objectives in the investigation of polymer solution lower critical solution temperature (LCST): First, to develop a new instrument to monitor LCST onset during copolymer synthesis by coupling several thermostatted light scattering ﬂow cells to the output stream of an ACOMP system . Second, to use this to investigate effects on LCST when N-isopropylacrylamide (NIPAM) is copolymerized with a charged comonomer, styrene sulfonate (SS). This comonomer pair has widely separated reactivity ratios. SS is rapidly consumed, yielding composition drift toward NIPAM rich polymer. High content SS chains inhibited LCST, and SS was dramatically effective at suppressing LCST down to copolymers of 5% molar SS in 10 mM NaCl aqueous solvent. LCST for higher content SS chains was elevated and required signiﬁcant additional ionic strength.
Detection of LCST behavior during polymer synthesis has been demonstrated with a new embodiment of ACOMP instrumentation. This opens many possibilities for understanding and optimizing the development of stimuli-responsive polymers. While the present work has used only free radical polymerization, the ACOMP technique is independent of the reaction chemistry, and has been used in a very wide array of reactions. One potentially fruitful area will be the application of this SGA approach to controlled polymerization techniques, allowing one both to understand the fundamental physical processes controlling the LCST and to tailor the LCST behavior and the speciﬁc chemical architecture of the resulting polymer.
Charged comonomers suppress LCST-induced chain collapse and aggregation in a way that neutral comonomers do not. This suppression is an intra-chain effect, and is strongly dependent on ionic strength. That is, both the critical value of Finst,SS at which suppression of LCST begins and the associated temperatures may be a function of the Debye screening length of the solution, persistence length, and possibly other length scales and details of linear charge distribution in the copolymer chains.
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