A Wavelength-Shifting Fluorescent Probe for Investigating Physical Aging Otto van den Berg, Wolter F. Jager,* Daniele Cangialosi, ‡ Jan van Turnhout, Peter J. T. Verheijen, ² Michael Wu 1 bbenhorst, § and Stephen J. Picken* Faculty of Applied Sciences, Department of Polymer Materials and Engineering, Delft UniVersity of Technology, Julianalaan 136, 2628BL Delft, The Netherlands, and Dutch Polymer Institute, PO Box 902, 5600 AX EindhoVen, The Netherlands ReceiVed August 13, 2004; ReVised Manuscript ReceiVed October 3, 2005 ABSTRACT: 7-(Dimethylamino)-1-methylquinolinium tetrafluoroborate (2), a stable color shifting mobility sensitive fluorescent probe, was employed for investigating physical aging in amorphous polymers. A linear correlation between the emission wavelength of 2 and the specific volume of the polymer was found in polycarbonate (PC) and poly(methyl methacrylate) (PMMA). The shift in emission wavelength measured during physical aging was used to calculate the temperature and time dependence of the effective relaxation time τ eff , employing an age-dependent Kohlrausch-Williams-Watts (KWW) equation. From these data and by comparison with data obtained by positron annihilation spectroscopy (PALS) in PC and volume relaxation in PC and PMMA, it was concluded that the fluorescence of 2 yields reliable information on physical aging of these polymers. The fluorescence method, employing a color-shifting probe, offers opportunities for investigating physical aging in powders and semicrystalline polymers, of specific components in blends and mixtures, and in confined systems. Introduction When amorphous polymers are cooled below the glass transition temperature, they fall out of equilibrium. The subse- quent time evolution of properties like volume, 1,2 enthalpy, 3,4 and creep toward equilibrium is a self-retarding process that is known as physical aging. Many studies have been made on the impact of physical aging on the mechanical properties of amorphous polymers. 5 Excellent reviews on physical aging of polymers and glass-forming liquids have been published. 6-8 Since physical aging has a profound influence, in particular on the long-term properties of polymers, a better understanding of this phenomenon is of great practical importance. In addition, physical aging provides information about the dynamics of supercooled liquids. Several reports on the behavior of both polymeric and nonpolymeric glass formers just below the glass transition temperature T g have been published recently. 9-11 These studies showed that the non-Arrhenius temperature dependence of the relaxation time of the R-process, usually well described by the phenomenological Vogel- Fulcher-Tammann (VFT) equation, 12-14 is replaced by a weaker temperature dependence. In a narrow temperature range below T g this dependence can be described by an Arrhenius law. 15 Fluorescent probes that detect medium mobility by changes in the emission intensity are known for a considerable time. Malononitrile-based fluorescent dyes, so-called rotor probes, 16 dominate this field of research. The singlet excited state of the dye has two major pathways to return to the ground state. The first and most common pathway is radiationless decay, a process that involves rotation within the excited molecule and requires free volume. The second pathway involves the emission of a photon from the planar molecule, which is only probable when rotation within the excited state is sterically hindered. Physical aging lowers the amount of free volume available for probe rotation and thus leads to an increase in emission intensity. The response of “rotor probe” doped in various polymers 17 showed asymmetry behavior and memory effects, 18 similar to those published for specific volume recovery experiments. 19 Recently, physical aging in thin films was investigated using the same type of probe molecule. 20 Although mobility is probed on a molecular scale, a direct connection between the bulk volume response and the response of the ensemble of fluorescent probes has been established. 21 Replacing intensity-changing fluorescent probes by wavelength- shifting ones would be a major improvement. Besides the intrinsic advantages of using fluorescent probes, such as the nondestructive nature of this technique that allows for remote sensing, fast acquisition that allows for on-line monitoring, and a relative freedom of sample size and geometry, wavelength- shifting probes offer additional advantages. Since the emission wavelength is a state variable, wavelength-shifting probes are self-referencing. This opens up a larger application window, since most requirements concerning sample composition and geometry, which limit the use of intensity changing probes, do not apply. Therefore, in addition to homogeneous, optically clear samples of well-controlled thickness, opaque and even light- scattering samples are accessible using wavelength-shifting probes. 22 Wavelength-shifting mobility-sensitive fluorescent probes generally belong to two classes of materials: the charge transfer (CT) and the charge resonance (CR) probes. CT probes have either a D-π-A 23 or D-σ-A 24 architecture, and CR probes are organic salts of the D-π-A + X - type. 25 Both classes of probes have been used for monitoring polymerization processes 23-25 and for polymer characterization, 26-29 but not for measuring physical aging. ² Faculty of Applied Sciences, Department of Process Systems Engineer- ing, Delft University of Technology. ‡ Present address: Fundacion Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastia ´n, Spain. § Present address: Laboratory for Acoustics and Thermal Physics, Department of Physics and Astronomy, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium. * Corresponding authors: W. F. Jager (W.F.Jager@tnw.tudelft.nl) or S. J. Picken (S.J.Picken@tnw.tudelft.nl). 224 Macromolecules 2006, 39, 224-231 10.1021/ma048329i CCC: $33.50 © 2006 American Chemical Society Published on Web 11/30/2005