Probe mobility

Figure B8.2.1 shows the fluorescence spectra of DIPHANT in a polybutadiene matrix. The h/lu ratios turned out to be significantly lower than in solution, which means that the internal rotation of the probe is restricted in such a relatively rigid polymer matrix. The fluorescence intensity of the monomer is approximately constant at temperatures ranging from —100 to —20 °C, which indicates that the probe motions are hindered, and then decreases with a concomitant increase in the excimer fluorescence. The onset of probe mobility, detected by the start of the decrease in the monomer intensity and lifetime occurs at about —20 °C, i.e. well above the low-frequency static reference temperature Tg (glass transition temperature) of the polybutadiene sample, which is —91 °C (measured at 1 Hz). This temperature shift shows the strong dependence of the apparent polymer flexibility on the characteristic frequency of the experimental technique. This frequency is the reciprocal of the monomer excited-state...

L. E. Morrison and G. Weber, Biological membrane modeling with a liquid/liquid interface. Probing mobility and environment with total internal reflection excited fluorescence, Biophys. J. 52, 367-379 (1987). 

The reduction of the long-range diffusivity, Di by a factor of four with respect to bulk water can be attributed to the random morphology of the nanoporous network (i.e., effects of connectivity and tortuosity of nanopores). For comparison, the water self-diffusion coefficient in Nafion measured by PFG-NMR is = 0.58 x 10 cm s at T = 15. Notice that PFG-NMR probes mobilities over length scales > 0.1 /rm. Comparison of QENS and PFG-NMR studies thus reveals that the local mobility of water in Nafion is almost bulk-like within the confined domains at the nanometer scale and that the effective water diffusivity decreases due to the channeling of water molecules through the network of randomly interconnected and tortuous water-filled domains. ... 

Eq. 1. CT = area of the CT-band, LE = area of the locally excited state C = constant that contains all photophysical parameters for both the charge transfer and the locally excited state k, t = rate constant for the formation of the CT -state from the LE-state, this value is strongly dependent on the probe mobility. 

Typical spectra obtained for the probed attached at the network junction are shown in Figs. 2 and 3 The probe in THF possesses the highest ratio for CT/LE. An increased intensity for the LE was measured for the swollen sample. This effect can be explained by different polarity /mobility of the probe One can assume that covalent bonded probes possess another probe mobility than free dissolved probe molecules. Furthermore, the covalent bonded probe molecule that shows a higher polarity in comparison to the siloxane chains is located at the network junction. The attached probe molecule is surrounded mainly by siloxane chains of the network. Addition of polar swelling solvents leads to an increase of the CT-emission and the ratio CT/LE is mainly influenced by the composition of polymer and swelling agent (compare spectra for dried and swollen N1 samples in Fig. 2). Therefore, the covalently bonded probe shows another fluorescence behavior in comparison to the free dissolved probes that can be surrounded also by solvent molecules. 

In the case of probe mobility different cases can be described. 

This equation, as a double reciprocal plot, is similar to Eq. (5) but is applied to probe mobility, not quencher dynamics. The same values for the dissociation rate constants (Table 17) were obtained when employing these two different methods with quenchers in different phases, suggesting that the underlying assumptions for the derivation of Eqs. (27) and (28) were reasonable. The entry rate constants were diffusion controlled, and the exit rate constants varied by a factor of 3. In analogous fashion to the polycyclic aromatic hydrocarbon probes, the exit rate constants were faster for the more polar ketones p-methoxyacetophenone and acetophenone compared to isobutyro-phenone or propiophenone [193,194],... 

Data on the structure density increase resulting from microbiological impact are confirmed by the data obtained by EPR spectroscopy by radical probe correlation time [1], The radical probe mobility decreases with the increase of microbiological impact duration. This may testify to packing of the leather tissue structure as a result of microflora impact on the material. 

Raccis R, Roskamp R, Hopp I, Menges B, Koynov K, Jonas U, Knoll W, Butt HJ, Fytas G (2011) Probing mobility and structural inhomogeneities in grafted hydrogel films by fluorescence correlation spectroscopy. Soft Matter 7(15) 7042-7053... 

At fixed polymer concentration, p/fio decreases with increasing matrix molecular weight. Rodbard and Chrambach found that p is not independent of M, consistent with many other results on probe electrophoresis and sedimentation. Nonlinear (in E) mobility behavior was observed, namely the probe mobility increased at larger applied fields. The nonlinearity was more pronounced at larger polymer concentrations. The dependence of p upon E could be said to be shear thinning, but if so the relevant shear rate (for example, involving a thin layer around each probe) must be quite large, because direct measurement at lower shear rates found no dependence of the macroscopic on /c. 

This reveals the important effect of the chemical structure of the matrix. The variation of the correlation time of the rotational motion of the probe is an indirect reflection of the difference in free volume offered by the polymer and accessible to the fluorescence probe to achieve its conformational transition. The results would suggest that the polymer chain flexibility (or free volume) around the polymer backbone rs different in each family of matrices. But in each family of polymers, the probe mobility is modulated by the same free volume fluctuations as the polymer segments. 

In Figure 11.6, the dependences of effective correlation time for initial and cold-rolled mats on ozone oxidation time are represented. The figure shows that for the first hour there is a sharp drop of the i values and then for next 3 hours the decrease of i is very small, that is, the dynamic characteristic is stabilized. After 4 hours of oxidation, the final stage shows the initiation of correlation time decrease. Note that the initial PHB mats and the cold-rolled mats demonstrate the symbatic probe rotation when in accordance with temperature- and water-influence data the mobility of TEMPO in the cold-rolled oxidized mat is decreased relative to the initial mat after ozonolysis. Taking into account the previous characteristics of crystalline structure and ESR data in amorphous area of the PHB and PHBV films after ozone exposition [23], it can be assumed that at the first stage of oxidation the partial destruction of macromolecules occurs that leads to the increase of probe mobility. On this stage only more accessible and defect molecules take part in reaction with ozone which are situated in less dense fields of PHB. After their concentration depletion the PHB-ozone interaction is stabilized for the next 3 hours that could be treated as induction period (the plateau in the Figure 11.6) and than the oxidation... 

The further exposure of the polymer to ozone leads to the probe mobility increase related with oxidative destruction of PHB. It is worth to note that the drop in rotation mobility for the mats after eold-rolling is less pronounced than for the initial mats. Taking into account the previous ESR data this effect enables one to suggest that the cold-rolling leads to denser field formation in the intercrystalline area of PHB furthering its stabilization against attack of temperature, water and oxidative agent. 

---