Polarised fluorescence

The principle of the method is similar to that of the infrared method except that the polymer is illuminated by polarised radiation of a suitable wavelength to induce fluorescence and the emitted radiation is examined using an analyser. Information is thus obtained about molecular orientation with respect to two directions, the transmission directions of the polarizer and analyser. 

The emission and absorption axes usually either coincide or are separated by a small angle. If they coincide and both the exciting and the analysed fluorescent light are polarised parallel to the draw direction in a uniaxially oriented polymer, then a = = 9, where 6p is the angle between the absorption axis and the draw direction. The intensity observed will thus be proportional to (cos Op. It is easy to show that, if the incident 

Because the angle 0 refers to the absorption/emission axis of the fluorescent molecule, the method does not characterise the orientation of the polymer molecules directly when probe molecules are used rather than fluorescence of the polymer itself. Some relationship between the orientations of the probes and the polymer chains must then be assumed if information about the orientation distribution of the chains is to be deduced. 

Biaxially oriented samples can be characterised by making tilted-film measurements somewhat similar to those made for the infrared characterisation of biaxial samples. 

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Polarised fluorescence has been used to study the orientation of non-crystalline polymer chains and a fluorescence technique has been developed to quantify the amount of microgel in polyacryl-amide. Time resolved photoluminescence analysis of poly( -phenyl-... 

In Table 3 the orientation information which can be obtained from these various structural techniques is summarised. This table also shows the part of the molecular structure which is being characterised, and some of the theoretical and experimental limitations of each method. A further technique, that of polarised fluorescence has been added. This technique is exactly analogous in its orientation aspects to Raman spectroscopy. The distinction between the two techniques lies in the fact that in the Raman effect, the lifetime of the process is of the order of the vibrational period ( 10 s) whereas fluorescence occurs after much longer occupancy of the transition state ( 10 s). 

Fig. I. Apparatus used to make polarised fluorescence intensity measurements from the top (a), the cell compartment (bl, and from the side (c). Xenon arc (Xe), monochromator (M), lens (L). filter (Fi), polariser (Pi), outside shell (OS), inside shell (IS), ciaet (Cu), slide (S), mask (Ma), filter (F2K polariser (Pi), photo-cathode (PC). photomultiplier (PM), power supply (PS), microammeter (A), locking collar (C), scale (Sc), slide holder (H), hole in mask (Ho), movable support (Su).

This could help to account for the alternation in peripheral bond-lengths which is observed. There is good evidence from absorption and polarised fluorescence spectra and from magnetic circular dichroism to support this viewpoint [120,125]. 

PPP films can also be prepared by electropolymerisation imder either reductive or oxidative conditions, but the EL properties have been found to be highly dependent on the polymerisation conditions [51]. A study of the PL efficiency of PPP thin films of varying chain length concluded that for highly ordered PPP films a chain length of 25-30 units was optimal [52]. Oriented films of PPP have been prepared by a friction deposition method and found to show highly polarised fluorescence [53]. 

Figure 6, Polarised fluorescence excitation spectra of the PS2 RC complex at 0.20% (w/v) TX-100. Emission detected at 672nm [-J, and 684nm [o—o—oj.

Surface Lateral resolution, mapping/imaging Ear-field microscopy (reflection, transmission, polarised, fluorescence, phase-contrast, interference) near-field microscopy (AFM/SPM morphology, micro-roughness) elemental imaging... 

London, Chapman Hall, 1997, p.181-233. 9 INFRARED DICHROISM, POLARISED FLUORESCENCE AND RAMAN SPECTROSCOPY... 

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