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

Abstract

Poly( N-isopropylacrylamide), PNIPAM, undergoes phase separation, upon heating under semi-dilute conditions, at 32°C, the lower critical solution temperature (LCST). Upon subsequent cooling, the dispersion rapidly clears again, at the LCST, to form a single, homogeneous phase. In this paper we report that fluorescence techniques, including quenching and time-resolved anisotropy measurements (TRAMS) on ultra-dilute (10 −3 wt%) aqueous solutions of an acenaphthylene labelled sample, provide conclusive evidence that a coil collapse mechanism is implicated in the thermoreversible behaviour: the polymer undergoes a conformational switch from an open coil below 32°C to a compact, globular structure above the LCST. TRAMS 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 accessibility of the label to aqueous-borne quenchers is also apparent at temperatures in excess of 32°C. PNIPAM is capable of solubilising low molar mass species in its compact form: changes in the vibrational fine structure of the emission spectrum of pyrene have been used to monitor uptake and release of the probe. Excited state lifetime measurements have also proven to be sensitive monitors of the conformational switch of PNIPAM: at temperatures greater than 32°C, τ increases to ca. 160 ns which is indicative of pyrene sequestered in a hydrophobic, protective environment. Release of the probe into the aqueous phase results in a dramatic reduction of τ to ca. 130 ns which is characteristic 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 organic guests) of PNIPAM as it is raised through its LCST. Information of this type is not readily obtained through other techniques and fluorescence approaches should prove invaluable in future investigations of the effects of chemical modification in attempts to manipulate the LCST of NIPAM-based systems.