Polymer-Solvent Molecular Compounds
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Molecular weights were employed for calculation of the molar concentration of all of the polymer solutions, in units of mol L Photoluminescence studies of solutions of the MEH-PPV with three molecular weights in toluene and chloroform were prepared in the concentration range from 10 -8 mol L -1 to 10 -4 mol L Films were prepared by casting of the solutions in both solvents on a Petri dish, with slow evaporation under a saturated solvent atmosphere, at room temperature, for 30 hours.
Later, the films were dried in an oven at a temperature of ca. Film thicknesses were ca. This temperature is roughly at the glass transition, previously determined by DSC, but below the onset of the decomposition temperature estimated by TGA under a non-oxidative atmosphere. We determined the crystallinity of the neat polymer samples and of the films by X-ray diffraction using a model XD-3A X-ray diffractrometer, with CuKa radiation, in the range ca.
Samples were supported on aluminum plates. The degree of crystallinity was determined by deconvolution of the diffraction peaks measured relative to the scattering band, which defines the c RX value. We also determined the glass transition by differential scanning calorimetry. The glass transition temperatures were determined using the data from the second heating cycle.
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Fluorescence spectra of polymer solutions and polymer films were obtained. The polymer for these solutions was dissolved under stirring, during several hours and then, the solution was maintained in dark in a sealed flask. The spectral range was from to nm for the excitation spectra and from to nm for emission spectra. The method of group contribution 35 was used, according to:. The solvents with the closest delta values to these were toluene 8.
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The value for a M pristine sample was ca. Compared to other reports, the crystallinity is lower, probably due to the lower molecular weight employed here. In addition, films of MEH-PPV produced from chloroform solutions seem to exhibit lower crystallinity than films from toluene solutions. Decomposition temperatures were determined by thermogravimetric analysis TGA Table 1.
Since O 2 enhances efficiency of the decomposition, 37 to prevent or at least minimize the thermal degradation, we performed the annealing processes at mild conditions near the glass transition and under a dynamic vacuum. Although X-ray diffraction showed some crystallinity, no relevant transitions could be assigned to melting in the DSC curves.
Absorbances of solutions with several concentrations show a deviation from Lambert-Beer's law for concentrations greater than 10 -6 mol L Samples with higher molecular weights are less soluble undergoing deviations of the Beer-Lambert law at lower concentrations. On the other hand, for dilute samples, the excitation band peaked at nm, which differs from the maximum of the absorption spectra.
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Figure 4 shows the excitation and fluorescence emission spectra of MEH-PPV, at the lowest concentration, 10 -8 mol L -1 , in both solvents. As noted, the emission band is virtually independent of the excitation wavelengths characteristic of the emission from an isolated excited state singlet intrachain exciton. The blue shift of emission observed for the solvent with larger polarity could, in principle, be interpreted by the larger stabilization of the electronic ground state of the MEH-PPV in comparison with the stabilization of the electronic excited state.
Figure 4 also shows that the emission band is broader for the chloroform solution, which can be explained by greater conformational disorder. It can be deconvoluted in three vibronic bands Figure 5 using the software Origin version 6.
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For comparison, the full width at the half maximum FWHM of the vibronic band at nm for MEH-PPV M51 was determined by deconvolution with values of cm -1 and cm -1 for films spun from toluene and chloroform, respectively. Although the deconvolution of the spectra is always an arbitrary procedure because we can choose the type of the functions gaussian or lorentizian or combined functions , we define the initial values of FWHM and the maximum of the peaks, there is no doubt that the relative intensities of the and bands are different for chloroform and toluene solutions.
Taking these values, the Huang-Rhys parameter, S, equation 2 and geometrical relaxation energy equation 3 can be determined. The geometrical relaxation energies are, meV and meV for chloroform and toluene solutions, respectively. Thus the higher intensity ratio for chloroform is attributed to the preferential solvation of the lateral groups while toluene preferentially solvates the backbone. Moreover, the solvation layer formed by chloroform molecules surrounding the lateral groups tightens the polymer chain to maximize the solvent-moiety interactions.
Since the lateral groups can display several possible special orientations, several types of partially folded conformations can take place and, as a consequence, the fluorescence spectra are broadened. Differently, toluene and other aromatic solvents undergo preferential interaction with the conjugated polymer backbone vinylene-phenylene groups , which adopt more planar conformations.
More limited distributions of conformers always lead to narrower fluorescence bands because the number of emissive Franck-Condon FC states is more limited. The solubility parameters for toluene and chloroform are relatively similar, 8. The appropriateness of a solvent can not be easily represented by a single solubility parameter.
Additional complexity is introduced when solvents induce conformational changes of the conjugated polymer chain that modify the effective lengths and the size distribution of the conjugated emissive segments. Consequently, the solvatochromism and the spectral profile of the fluorescence and of the absorption band depend on several factors: on the overlap of the vibronic progression, on the configurational imperfections extrinsically imposed by the solvent and intrinsically imposed by chemical defects, on the configurational disorder, on the dynamics of polymer relaxation and on the dynamics of photophysical processes.
Fluorescence spectra of MEH-PPV in more concentrated solutions, 10 -7 mol L -1 Figure 6 , are slightly red-shifted nm compared with the spectra of a 10 -8 mol L -1 solution Figure 5. This red-shift is independent of the excitation wavelengths and is attributed to the inner-filter effect, as often observed for concentrated solutions. The aggregates of MEH-PPV emit at nm in coincidence with the vibronic band of the intra-chain isolated chromophore, which explains the relative increase of the intensity at nm. Similar results were obtained for samples with other molecular weights and are omitted.
Aggregates increase the relative intensity of the band because their emission is red-shift compared with the isolated lumiphore are dimmers. If we assume that these aggregates are dimes and that this red-shift originates from a exciton splitting, 45 the expected value of the Davidov coupling is 2B cm -1 , which is in the range of the exciton splitting observed for dimerization of small molecules 46,47 and other conjugated polymers. As noted in Figure 7 , the fluorescence emissions for solutions of 10 -7 mol L -1 are also red-shifted compared with the spectra of samples with lower concentration.
Larger red-shifts occur for chloroform solutions of M from nm to nm compared to toluene from nm to nm. This effect is much more pronounced for toluene solutions of M As previously commented, the red-shift of the fluorescence spectra resulted from: i inner-filter effect produced by the higher optical density of the more concentrated solutions; ii the conformational changes of the polymer chain that modify the effective size of the intra-chain conjugation; iii the formation of aggregates that increases the relative intensity of the red-edge vibronic band nm and, iv interaction with the solvent, producing solvatochromic effects.
In principle, the inner-filter effect depends only on the number of chromophoric units present in solution, being independent of the solvent if differences of solubility are ignored. The more plausible reason for the larger changes for M compare Figure 7a , c , and e or 7b , d and f , is the decrease of the solubility of longer chains. Nevertheless, there are several reasons for our belief that aggregation predominates over the conformational disorder: i there is a relative increase of the band at nm where the emission of aggregates predominates; ii this relative intensity is more pronounced for higher molecular weight samples, which one expects to be less soluble; iv there is a remarkable decrease of the entire intensity signal, which is compatible with the decrease of the quantum yield of aggregates compared to the isolated lumophores.
On the other hand, increase of the conformational disorder should produce different types of changes of the spectral profile, such as: i broadening of the emission band occurs when the conformational disorder increases; ii more flexible conformations produce shorter conjugation lengths leading to the blue-shift of the emission spectrum. Thus, considering that none of these two behaviors were observed and considering that the solubility decreases with the increase of the molecular weight, we conclude that the major reason for the relative increase of the intensity at nm is the aggregation of the polymer in concentrated solutions.
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Interaction with solvents with larger dipole moments should shift the spectra to the blue or to the red, depending on the stabilization of the electronic excited state. Wszystkie nowe produkty.
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Guenet, Jean-Michel ;. Date of Publication: 7 November The discount is only available for 'Alert of Favourite Topics' newsletter recipients. Click here to subscribe. Add to basket. Add to wishlist. What is wishlist? This book covers both biopolymers and synthetic polymers. It uses temperature-concentration phase diagrams abundantly for describing the systems.