Ian Soutar

Time-resolved fluorescence anisotropy studies of the temperature-induced intramolecular conformational transition of poly(N-isopropylacrylamide) in dilute aqueous solution

Abstract Time-resolved fluorescence anisotropy measurements have been performed upon an acenaphthylene-labelled (0.5 mol%) sample of poly( N -isopropylacrylamide), PNIPAM, in dilute solutions in both methanol and water as solvents. In methanol, segmental relaxation of PNIPAM follows an Arrhenius dependence upon temperature over the range investigated (279–323K) characterized by an ‘activation energy’ of 13.4(±0.5) kJ mol −1 . This is only slightly greater (by ca 2.4 kJ mol −1 ) than that of solvent flow and it is likely that specific dipolar interactions between the PNIPAM and the methanol determine the macromolecular dynamics in this solvent. In aqueous solution, the segmental mobility of PNIPAM exhibits a dramatic thermoreversible discontinuity at ca 32°C. This change in conformational behaviour occurs at the polymers lower critical solution temperature in aqueous media. This observation, supports the proposition (Winnik, F. M. Polymer 1990, 31, 2125) that the thermally-induced separation in this system occurs by a ‘dual mode’ mechanism wherein intermolecular aggregation is preceded by intramolecular conformational shrinkage of the polymer coils.

Photophysics of carbazole‐containing systems. 1. Dilute solution behavior of poly(N‐vinyl carbazole) and N‐vinyl carbazole/methyl acrylate copolymers

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical behaviors of poly(N-vinyl carbazole), PNVCz and a series of N-vinyl carbazole-methyl acrylate (NVCz-co-MA) copolymers in dilute solution as a function of both NVCz composition and temperature. A kinetic scheme, intended to describe intramolecular excimer formation across the entire NVCz composition range, is proposed. In low aromatic content copolymers, two monomer species (unquenched and quenched monomer) and two excimer species (the sandwich-like excimer and a higher energy excimer) exist. The contribution from monomer emission to the overall fluorescence decreases with increasing NVCz content through increased excimer formation: this is likely to be consequent upon (1) an increase in the number of excimer forming sites, and (2) increasing efficiency of energy transfer from the excited monomers to the excimer forming sites. In the homopolymer, PNVCz, the only emission that can be observed on a nanosecond timescale is excimeric. This fluorescence appears to originate from three excimer species (the sandwich-like excimer, and two higher energy forms). For the homopolymer, the current observations are consistent with the model proposed by Vandendriessche and De Schryver [Polym. Photochem. 7, 153 (1986)].

Time-resolved fluorescence anisotropy studies of the cononsolvency of poly(N-isopropyl acrylamide) in mixtures of methanol and water

Time-resolved anisotropy measurements (TRAMS) have provided invaluable information concerning the molecular interactions responsible for cononsolvency in the poly(N-isopropylacrylamide), PNIPAM, H2O, and methanol ternary system. TRAMS have successfully monitored the intramolecular segmental dynamics of an acenaphthylene labelled sample, ACE–PNIPAM, revealing details about the conformation adopted by the polymer constituent as a function of both alcohol composition and temperature of the system. In pure aqueous solution, ACE–PNIPAM undergoes a conformational transition from an expanded solvent swollen structure to a compact globule at the lower critical solution temperature (LCST). With increasing alcohol content up to 55% v/v of methanol there is both a reduction in the magnitude and an onset temperature of the LCST of ACE–PNIPAM. From 57.5–65% v/v methanol, ACE–PNIPAM forms an extended solvent swollen structure: observation of an LCST at higher polymer concentrations (e.g., 0.1 wt%) is a consequence of intermolecular aggregation between expanded chains.

Manipulating the thermoresponsive behaviour of poly(N-isopropylacrylamide) 3. On the conformational behaviour of N-isopropylacrylamide graft copolymers

The thermally triggered conformational change of poly(N-isopropyl acrylamide), PNIPAM, in aqueous media occurs at the lower critical solution temperature, LCST. Manipulation of the switch can be achieved via simple free radical copolymerisation, for example. However, the magnitude of the transition is reduced which has a detrimental effect on the solubilisation and controlled release properties of the polymer. In an attempt to over come these limitations the effect of architecture on the thermal response has been examined through syntheses of a range of fluorescently labelled graft copolymers. To examine the effect of topography, samples containing a NIPAM-based backbone and NIPAM branches have been prepared. Simultaneous variation of the entropic and enthalpic contribution to the thermal response has been achieved through syntheses of macromolecules containing a dimethylacrylamide-based backbone and NIPAM grafts. Time-resolved fluorescence anisotropy (TRFA) measurements have been successful in determining the onset, magnitude and dispersion of the LCST of these samples and, by selective labelling of sites, have provided information concerning the role of backbone and graft on the resultant thermorespsonive behaviour. TRFA measurements confirm that the temperature of the conformational switch can be varied through simultaneous manipulation of the entropic and enthalpic contribution to the thermal response of the graft copolymers, whilst maintaining the magnitude of the transition.

Time-resolved fluorescence anisotropy studies of the interaction of N-isopropyl acrylamide based polymers with sodium dodecyl sulphate

Time-resolved anisotropy measurements have been used to probe the interaction between poly(N-isopropylacrylamide), PNIPAM, and sodium dodecyl sulphate, SDS, in aqueous solution. Investigation of the intramolecular segmental dynamics of an acenaphthylene labelled sample, ACE-PNIPAM, below the lower critical solution temperature, LCST, has revealed that at the critical aggregation concentration, CAC, the polymer changes its conformation from an expanded water swollen structure to a surfactant swollen coil of reduced mobility. At temperatures in excess of the LCST, addition of SDS to ACE-PNIPAM induces a change in macromolecular shape from a collapsed globule to an expanded surfactant swollen coil. The presence of SDS at concentrations in excess of the CAC elevates the temperature of the conformational transition and corresponding LCST.The CAC was observed to decrease as a function of hydrophobic modification of PNIPAM. This was considered to reflect hydrophobic cavity formation between styrene units in the copolymer samples which enhance aggregation with SDS.

Luminescence studies of polymer matrices: 1. Phosphorescence of benzophenone dispersed in poly(methyl methacrylate)

Abstract The photophysical behaviour of benzophenone (BP) in poly(methyl methacrylate) (PMMA) has been studied as a function of temperature. Time-resolved phosphorescence data reveal that at no temperature between 77 and 420K can the decay behaviour of the triplet state of BP in PMMA be described adequately by a single exponential function. This contrasts with the phosphorescence behaviour of BP in low temperature, low molecular mass, glassy matrices and that reported previously for BP dissolved in PMMA at temperatures below the onset of the β relaxation of the polymer. The complexity of the phosphorescence decay kinetics of the BP/PMMA system increases at temperatures greater than that of the β transition of the polymer host. This has been rationalized in terms of the onset of enhanced energy transfer interactions between the BP triplet excited state and the PMMA solvent, as proposed earlier. However, the function derived previously on the basis of diffusion-controlled quenching of the triplet state of BP by the PMMA host, did not model, adequately, the time-resolved phosphorescence data obtained in the current study. In this respect, the form of the quenching influence exerted by the polymer matrix upon the photophysical behaviour of the BP solute remains to be explained. Notwithstanding the absence of a complete rationalization of the influence of the PMMA host upon the photophysical behaviour of the BP guest, it has been shown that both the intensity of phosphorescence emitted by the BP and its mean decay time provide means whereby transitions within the polymer matrix can be detected. In this respect the data are not equivalent: the lifetime data ‘sense’ the presence of the α′ transition (at ∼345 K) of the PMMA. This transition is not apparent in the temperature dependence of intensity data which, in common with the lifetime data, are sensitive to the onset of both the γ and β transitions of the polymer. The relevance of the data in respect of establishing phosphor/polymer combinations for application as phosphorescent coding media is discussed.

Luminescence studies of polyelectrolyte behaviour in solution—I. Accessibility of naphthalene-based labels of poly(methacrylic acid) to mobile low molar mass species in aqueous media

Abstract A novel approach to the study of the conformational behaviour of water-soluble polymers is reported. The method is based upon the generation of stabilized, room temperature phosphorescence using mobile “heavy-atom” promoters (Tl+). The conjunct roles of phosphorescence intensity and lifetime data in providing information upon polyelectrolyte behaviour is illustrated, using samples of poly(methacrylic acid) labelled with 1-vinylnaphthalene or acenaphthylene as luminescent labels. The resultant data are compared with those obtained using more conventional fluorescence methodologies.

Luminescence studies of polyelectrolyte behaviour in solution: 2. The application of steady-state fluorescence anisotropy measurements to the study of the conformational behaviour of poly(methacrylic acid) in dilute aqueous solution

Abstract Steady-state anisotropy and fluorescence-quenching measurements have been used to study the segmental relaxation of poly(methacrylic acid) (PMAA) in dilute aqueous solution. None of the quenchers (CH 3 NO 2 , Tl + and I − ) employed in this work exhibited truly dynamic quenching of the excited states of the fluorescent species (0.5 mol% copolymerized acenaphthylene and I-vinylnaphthalene) used to label the PMAA. As a consequence, the dependence of the measured anisotropy r upon the relative, quenched fluorescence intensity could not be used as a means of determination of the intrinsic anisotropy r 0 . Furthermore, the influence of a static component in the quenching process rendered plots of r −1 as a function of fluorescence lifetime less reliable for estimation of r 0 than would be the case with a purely dynamic quencher. (This limitation is a direct consequence of the difficulties encountered in the measurement of r itself in the presence of significant static quenching.) Consequently, under these conditions, steady-state fluorescence anisotropy measurements have been shown to be of limited value in studying the conformational behaviour of PMAA. In contrast, time-resolved measurements have allowed the determination of rotational correlation times which are internally consistent and independent of the choice of fluorescent label at the pH values studied.

Communication. Water-soluble copolymers: a new class of media for fluorescence and phosphorescence analyses in aqueous systems?

The first observations of room temperature stabilized phosphorescence (RTSP) from analytes solubilized in aqueous media by a water-soluble copolymer are reported. Pyrene and benzophenone were dispersed in the hydrophobic domains created within the coils of a copolymer of 1-vinylnaphthalene and methacrylic acid, containing 36 mol-% of 1-vinylnaphthalene. The hydrophobic cavities allow sufficient interactions with heavy ions, such as Tl+, dissolved in the aqueous phase, to produce effective population of the triplet states of the sequestered analytes. In addition, the cavities are sufficiently protective to the excited states of the occluded guest molecules that room temperature phosphorescence can be generated readily with simple nitrogen purging. In terms of convenience of sample preparation, water-soluble copolymers offer distinct advantages over alternative media, such as doped micelles, for the creation of RTSP. These preliminary studies point the way towards the ‘custom design’ of copolymers for specific analytical applications involving aromatic hydrocarbon analytes in aqueous media.

Photophysics of Carbazole-Containing Systems. 2. Fluorescence Behaviour of Poly(N-Vinyl Carbazole) and N -Vinyl Carbazole/Methyl Acrylate Copolymer Films

The fluorescence behaviour of thin films of poly(N-vinyl carbazole), PNVCz, and a series of N-vinyl carbazole/methyl acrylate, NVCz/MA, copolymers has been investigated using both steady-state and time-resolved emission techniques. The fluorescence of PNVCz at 298 K is dominated by emission from two excimeric traps. Trap I has the conventional ‘sandwich’ structure formed from two fully overlapped aromatic rings. The other (trap II) is a species involving two partially overlapped carbazole substituents. The fluorescences of the NVCz/MA copolymers contain contributions from unassociated or monomeric carbazoyl excited states which increase in prominence as the aromatic content of the system is reduced. The influence of intermolecular interactions in creating excimer traps is apparent: even at a carbazole content of 8.3 mol%, excimer emission is evident. In PNVCz and copolymers of higher aromatic contents, the microtacticity of the macromolecule exerts a dominant influence upon the photophysical behaviour of the bulk polymer despite the moderating effects of intermolecular interactions between chromophores. As the temperature of the system is reduced, the contribution from trap II to the overall fluorescence from the polymer films increases relative to that made by emission from trap I. At 77 K, the fluorescence of PNVCz contains a significant contribution from trap II emission. At 298 K, fluorescence from the conventional excimer, trap I, dominates the steady-state emission spectrum. On the basis of observations upon time-resolved fluorescence data, it is proposed that restrictions to the reorientation of carbazole substituents which occur at low temperature serve to inhibit the conversion of a proportion of the trap II sites into fully overlapped excimers, thereby reducing depletion of the trap II population. Implicit to this explanation of the photophysical characteristics of PNVCz and the higher content NVCz/MA copolymers, is the belief that the high concentration of excimer-forming trap sites mitigates against significant energy migration between carbazole substituents which might otherwise populate the excimer traps. These observations are of importance to considerations of the photoconductivity displayed by polymers derived from NVCz.