Continuous and discontinuous volume-phase transitions in surface-tethered, photo-crosslinked poly(N-isopropylacrylamide) networks

The water-polymer demixing behavior of surface-tethered poly(N-isopropylacrylamide) networks copolymerized with x mol% of photo-crosslinkable methacryloyloxybenzophenone (MaBP) (x = 1, 3, 5, 10%) was characterized with neutron reflection and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Neutron reflection revealed that water is expelled discontinuously at low crosslink densities and continuously at high crosslink densities. The demarcation between the two behaviors occurred roughly at the critical point as measured by cloud point experiments. The neutron reflection experiments further revealed that the discontinuous concentration jump at low crosslink densities takes place in the presence of significant amounts of water and that water is not completely expelled in the process, with 2-3 water molecules remaining per polymer segment after the collapse of the network, independent of crosslink density. Parallel measurements with ATR-FTIR confirm that the transition is driven by dehydration of the isopropyl groups, with water remaining confined between the amide groups even at temperatures well above the demixing temperature. The internal water, however, is readily exchanged with deuterium oxide at temperatures up to 100 degrees C. This exchange points to the absence of a hydrophobic skin or physical barrier that would prevent water from completely leaving the film above the demixing temperature.