Simultaneous multiple sample light scattering detection of LCST during copolymer synthesis
C. A. McFaul, A. M. Alb, M. F. Drenski, W. F. Reed*, "Simultaneous multiple sample light scattering detection of LCST during copolymer synthesis", Polymer 52 (2011) 4825-4833
Abstract
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.
