Fluorescence investigations of the thermally induced conformational transition of poly(N-isopropylacrylamide)

critical solution temperature (LCST). Upon subsequent cooling, the dispersi on rapidly clears again, at the LCST, to form a single, homogeneous phase. In this paper we report that fluorescence techniques, including quenching a nd time-resolved anisotropy measurements (TRAMS) on ultra-dilute (10(-3) wt %) aqueous solutions of an acenaphthylene labelled sample, provide conclusi ve evidence that a coil collapse mechanism is implicated in the thermorever sible behaviour: the polymer undergoes a conformational switch from an open coil below 32 degreesC to a compact, globular structure above the LCST. TR AMS reveal that a marked reduction in the segmental mobility of the polymer occurs at the onset of the LCST. In addition, a dramatic change in the acc essibility of the label to aqueous-borne quenchers is also apparent at temp eratures in excess of 32 degreesC. PNIPAM is capable of solubilising low molar mass species in its compact for m: changes in the vibrational fine structure of the emission spectrum of py rene have been used to monitor uptake and release of the probe. Excited sta te lifetime measurements have also proven to be sensitive monitors of the c onformational switch of PNIPAM: at temperatures greater than 32 degreesC, t au increases to ca. 160 ns which is indicative of pyrene sequestered in a h ydrophobic, protective environment. Release of the probe into the aqueous p hase results in a dramatic reduction of tau to ca. 130 ns which is characte ristic of pyrene dispersed in water. These data highlight the potential of NIPAM based polymers to act as carriers in controlled release applications. Fluorescence techniques have proved capable of monitoring changes in chain mobility, the degree of coil compaction and solubilization capacity (for or ganic guests) of PNIPAM as it is raised through its LCST. Information of th is type is not readily obtained through other techniques and fluorescence a pproaches should prove invaluable in future investigations of the effects o f chemical modification in attempts to manipulate the LCST of NIPAM-based s ystems. (C) 2001 Published by Elsevier Science Ltd.