3D Directional Temperature Responsive (N-(DL)-(1-Hydroxymethyl) Propylmethacrylamide-co-n-Butyl Acrylate) Colloids and Their Coalescence
Originally published in Macromolecules 2008, 41, 352-360
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A new family of colloidal dispersions containing N-(DL)-(1-hydroxymethyl) propylmethacrylamide (DL-HMPMA) and n-butyl acrylate (nBA) monomers was developed which exhibits temperature-responsive behavior. Particle size analysis, along with UV-vis and 1H NMR spectroscopic measurements, showed that aqueous p(DL-HMPMA/nBA) particle dispersions display second-order lower critical solution temperature (IILCST) in the 27-37 °C range at which the particle size decreases from 81 to 47 nm. When p(DL-HMPMA/nBA) colloidal particles were coalesced to form films, which is facilitated by the lower Tg nBA component that maintains an adequate free volume for temperature-responsive DL-HMPMA segments, coalesced films exhibited 3D directional responsiveness to temperature. While shrinkage occurs in the x-y plane directions (length and width, respectively), expansion in the z direction (thickness) above II-LCST is observed. Computer thermodynamics molecular simulations combined with the experimental data showed that conformational changes of the side chains and the collapse of the p(DL-HMPMA/nBA) copolymer backbone are responsible for directional temperature responsiveness. To the best of our knowledge, this is the first study that shows selective 3D directional temperature-responsiveness of polymer films. These studies also show that the total free energy (G) of the system increases by 150 kcal/mol which results from the diminishing entropic term above II-LCST which, in addition to polar and hydrogen bonding interactions, contributes to conformational changes near II-LCST.