Photochemical calorimetry

C) Changing the composition of the system Titration calorimetry , Flow calorimetry , photochemical calorimetry , sample insertion calorimetry ... 

Photocalorimetry is a technique for determining the ordinary enthalpy (AH) of a reaction but, unlike conventional calorimetry, the reaction is light induced,191 Essentially, the procedure involves measuring the rates of heat production in two irradiated solutions, one containing an absorbing but unreactive substance and the other containing the photosensitive compound. The difference between these rates, per mole of reaction, gives the AH for the photochemical process. 

The chemical structure of the polyimide polymers (named PI-1 and PI-2) studied by Sekkat et al. is shown in Figure 12.12. They prepared the polymer samples by spin-casting onto glass substrates. PTl was cast from a cyclohexanone solution and PI-2 from 1,1,2,2- tetrachloroethane. The Tg values of PI-1 and PI-2 were determined to be 350°C and 252 C, respectively, by scanning calorimetry method. The thicknesses of the PI-1 and PI-2 films were, respectively, approximately 0.72 im and 0.14 im, and their respective optical densities were approximately 0.79 and 0.3 at 543.5 nm. Details of the preparation and characterization of the samples can be found in References 3 and 20. In their EFISH experiment, a typical corona poling technique was used to pole the samples, with a dc electric field about 2-3 MV/cm across a 1-2 lm thick polymer film. They used the SHG output from the EFISH experiment to in situ monitor the photochemical change in the third-order susceptibility of the PI-1 and PI-2 polymers. 

The present preliminary study of rapid photopolymerization reactions by differential calorimetry shows this method to have great potential utility. Future studies involving simpler reaction systems and better controlled reaction conditions will display the power and sophistication of the differential calorimetric technique in photochemical research. 

Differential calorimetry has been applied to the study of rapid photopolymerizations. This new technique holds great promise for basic and applied research on photopolymerization and other photochemical reactions. The method requires only a few milligrams of sample, can be used on network-forming systems, and can approximate actual conditions of thin film and coating technologies. 

Figure 11 Potential energy surafes of cis-trans isomerization of 8b. (Reprinted from ref. 114 Potential energy surfaces of a one-way photoisomerizing olefin studied by photoacoustic calorimetry and X-ray crystallography, T, Aral, H. Okamoto, K. Tokumaru, T. Ni, R. A. Caldwell, and K. Ueno, J. Photochem. Photobiol. A Chem. IS, 85-90 (1993) copyright (1998), with permission fiom Elsevier Science.)...

Lewis JE, Moore TA, Benin D, Gust D, Nicodem D and NoneU S (1994) The triplet energy of a carotenoid pigment determined by photoacoustic calorimetry. Photochem Photobiol 59S 35S... 

Pulsed, time-resolved photoacoustic calorimetry employs a pulsed uv laser source to induce a photochemical reaction in the molecule of... 

Most studies have dealt either with the free radical polymerization of hydrophobic monomers—e.g., styrene [56-89], methyl methacrylate (MMA) [68,73,74,84,86,90-93] or derivatives [2,94,97], and butyl acrylate (BA) [98-100]—within the oily core of O/W microemulsions or with the polymerization of water-soluble monomers such as acrylamide (AM) within the aqueous core of W/O microemulsions [101-123]. In the latter case, the monomer is a powder that has to first be dissolved in water (1 1 mass ratio) so that the resulting polymer particles are swollen by water, in contrast with O/W latex particles, where the polymer is in the bulk state. The polymerization can be initiated thermally, photochemically, or under )>-radiolysis. The possibility of using a coulometric initiation for acrylamide polymerization in AOT systems was also reported [120]. Besides the conventional dilatometric and gravimetric techniques, the polymerization kinetics was monitored by Raman spectroscopy [73,74], pulsed UV laser source [72,78], the rotating sector technique [105,106], calorimetry, and internal reflectance spectroscopy [95]. 

In order to determine the energies of dissociation of the metal-ligand bond, the following methods are commonly utilized calorimetric combustion, solution calorimetry, ion cyclotron resonance, mass spectrometry, ion beam technique, equilibrium constant measurements, kinetic methods, and photochemical determination of the threshold wavelength of the photolytic dissociation of the metal-ligand bond. ... 

There are some less widely studied types of inorganic photochemical systems. There is the phenomenon of photochromism, in which a system is driven one way by light and returns either thermally or photochemically. An example for us was the family of transition metal dithizonate complexes [51]. Photochemistry and calorimetry may be combined in photocalorimetry, to obtain enthalpies of reactions which are not clean or are not obtainable at all thermally [52], Similar information can be obtained from the photoacoustic effect [53]. Another interesting phenomenon is that of chemiluminescence, that is, the chemical production of an excited state reaction product, and the related process of electrogenerated chemiluminescence. This type of emission can be detected even when... 

SCAIANO, J.C., R.W. Redmond, B. Mehta, and J.T. Arnason Efficiency of the Photoprocesses Leading to Singlet Oxygen ( Ag) Generation by a-Terthienyl Optical Absorption, Optoacoustic Calorimetry and Infrared Luminescence Studies. Photochem. Photobiol. 54, 655 (1990). 

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