Influence of Cyclodextrins on Thermosensitive and Fluorescent Properties of Pyrenyl-Containing PDMAA

A series of pyrenyl-containing PDMAA copolymers were prepared by free radical copolymerization of dimethylacrylamide (DMAA) with pyrenebutanoyloxy ethyl methacrylate (PyBEMA). The structure of as-prepared copolymers was characterized by UV, FT-IR and (1)H NMR spectroscopy. The effect of cyclodextrins (α-CD, β-CD and γ-CD) on the thermosensitivity and fluorescence of the copolymers in aqueous solutions were investigated. It was found that the as-prepared copolymers exhibit lower critical solution temperature (LCST)-type thermosensitivity. Cloud point (T(cp)) decreases with the increasing molar content of PyBEMA unit in the copolymers. T(cp) of the copolymers increases after the CD is added from half molar to equivalent amount relative to pyrenyl moiety, and that further adding twice equivalent CD results in a slight decrease in T(cp). The copolymers exhibit a pyrene emission located at 377 nm and a broad excimer emission centered at 470 nm. The copolymers in water present a stronger excimer emission (Intensity I(E)) relative to monomer emission (Intensity I(M)) than that in ethanol. The I(E)/I(M) values decrease after the addition of equivalent α-CD, β-CD and γ-CD into the copolymers in aqueous solution, respectively. The I(E)/I(M) values abruptly increase as the copolymers’ concentration is over 0.2 mg/L whether in ethanol solution or aqueous solution with or without CD, from which can probably be inferred that intra-polymeric pyrene aggregates dominate for solution concentration below 0.2 mg/L and inter-polymeric pyrene aggregates dominate over 0.2 mg/L. Furthermore, the formation of the CD pseudopolyrotaxanes makes it possible to form pyrene aggregates. For high concentration of 5 g/L, the copolymers and their inclusion complexes completely exhibit an excimer emission. The I(E) values abruptly increased as the temperature went up to T(cp), which indicates that the I(E) values can be used to research phase separation of polymers.