Revisiting the thermosensitivity of poly(acrylamide-co-diacetone acrylamide)

The lower critical solution temperature (LCST) behavior of poly(acrylamide-co-diacetone acrylamide) (poly(AM-co-DAM)) copolymer in aqueous solutions was studied. The results demonstrate the LCST linearly decreases as the molar fraction of DAM (f _DAM) increases. In the range of f _DAM < 0.36, the transmittance increases as f _DAM decreases because the more hydrophilic copolymer chains can form looser aggregates with a lower refractive index. The transmittance exhibits a minimum when f _DAM is less than 0.28 as the chains form micelle-like structure with a size smaller than the wavelength. The LCST decreases with the initial polymer concentration, but it levels off when the polymer concentration is high enough. Moreover, no hysteresis can be observed in the change of transmittance during the heating-cooling process because no additional hydrogen bonds are formed in the collapsed state due to the steric hindrance of the large side groups in DAM units.