Photocalorimetry reaction

Differential photocalorimetry (DPC) is included here since the instrument used is essentially an adaptation of DSC instrumentation. The photocalorimeter comprises a DSC instrument with a UV/visible source mounted on top, such that light of appropriate wavelength or wavelength region from the source is focused onto the measuring head (both reference and sample pans). The most frequent use of DPC is in the study of polymer cure reactions, but it may also be used to follow such as UV degradation. 

Some of these problems can be overcome with a different calorimetric design (see later discussion). Other problems, which are more dependent on the chemistry and physics of the process under study than on the instrumentation, require careful attention. Unnoticed side reactions or secondary photolysis are examples, but one of the most serious error sources in photocalorimetry is caused by the quantum yield values, particularly, as explained, when they are small. Unfortunately, many literature quantum yields are unreliable, and it is a good practice to determine n for each photocalorimetric run. Errors in

inner filter effects, that is, photon absorption by reaction products. 

What can we conclude from all these data Although the two photocalorimetric values are in excellent agreement with each other, these values are problably less accurate than the reaction-solution calorimetric result. In any case, the 4 kJ mol-1 discrepancy is not a cause of concern regarding the general usefulness and reliability of carefully made photocalorimetry experiments. 

Y. Harel, A. W. Adamson. Photocalorimetry. 2. Enthalpies of Ligand Substitution Reactions of Some Group 6 Metal Carbonyl Complexes in Solution. J. Phys. Chem. 1982, 86, 2905-2909. 

All the acylated polyimides retained solubility in organic solvents. This gave us reason to believe that photopolymerisation via the double bonds of polyimides could be conducted by the method of differential scanning photocalorimetry. This method, which is widely covered in the literature [73-76], is based on on the principle that heat released during any reaction can be measured. 

Photopolymerization of the system we have studied appears to proceed by the common mechanism in which termination occurs by reaction between two macroradicals. Analysis of the photocalorimetry traces at different light intensities for our initiator-monomer system shows no evidence for a... 

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. 

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

Experimental Analysis Despite the large number of works devoted to the investigation of the radical R" reactivity, the determination of the experimental values of the radical addition rate constants to a monomer double bond remains difficult. Only a small part of these values is based on a direct detection of the produced radical (flash photolysis, ESR). In fact, radicals are very often difficult to observe because of their absorption band below or close to 300 nm. Moreover, the classical overlap of the absorption spectra in the UV (due to the presence of different transient and ground-state species inherent to the production route when using laser flash photolysis) often prevents the direct observation. Indirect methods were tentatively used but they involve a complex set of reactions and are rather difficult to carry out. The same holds true for laser-induced photocalorimetry [265]. 

Phosphorus tribromide stereochemistry, 36 Phosphorus trichloride metal complexes solvolysis, 418 stereochemistry, 36 Phosphorus trifluoride stereochemistry, 36 Phosphorus trihalides stereochemistry, 36 Phosphorus triiodidc stereochemistry, 36 Photoaquation solid state, 471 Photocalorimetry, 410 Photochemical reactions, 397 applications, 408 mechanisms, 385 solid state, 470 Photochromism, 409 Photolysis... 

In classical photocalorimetry the processes are also light-induced but as this technique relies on temperature or heat flux measurements, only reactions involving stable, long-lived species, can be examined. See, e.g. Harel, Y. Adamson, A. W. J Phvs. Chem. 1986, 90, 6693 and references cited therein. 

THERMAL ANALYSIS STUDIES The results of our model study indicates that [2.2.1]bicyclic olefins do exhibit a high relative reactivity to the addition of thiols. We have also initiated a study of the polymerization reaction with differential photocalorimetry (photo DSC). The results of this study indicate that the enthalpy of the photopolymerization increases to a maximum of ca. 224 J/g at 320°K and decreases thereafter as the temperature continues to increase (illustrated in Figure 9). The enthalpy per equivalent of ene and thiol is roughly 100,000 J/eq (23.9 Kcal/eq) at 320°K. 

Differential Photocalorimetry (DPC) (19.201. The polymerization being an exothermal process, the reaction can be monitored in real time by differential scanning calorimetry (DSC). From the recorded thermogram which shows the variation of the heat flow with the irradiation time, the rate of polymerization can be directly calculated, provided the standard heat of polymerization (AHq) is known. For acrylic monomers, AHg values are usually in the range of 78 to 86 kJ mol depending on the monomer considered. 

The experimental procedure involves initiation of the polymerization by irradiation followed by cutting off the light after a certain time at a degree of conversion chosen, and monitoring the reaction in the dark. As experimental methods, both isothermal differential scanning calorimetry (photocalorimetry, photo-DSC) [2,6, 7, 18-32] and real-time infrared... 

It should be emphasized that the k thus calculated, describing the fraction of radicals immobilized and deactivated in the network per unit of time, corresponds to radicals that are registered as inactive on the time-scale measured by the experimental method used, for instance, photocalorimetry. It is possible that for longer reaction times they will react further but with very slow, undetectable rates due to very limited monomer diffusion or due to various relaxation processes [38 0],... 

DPA 7 aluminum pans and covers are intended to be used with a photo-calorimetric accessory. The covers are actually quartz windows that allow ultraviolet (UV) energy from the accessory light source to transfer to the sample specimen. Photocalorimetry is used to study UV-curing reactions. A crimper is not required with these pans. 

Since polymerisation processes are mainly exothermic reactions in which each additional chain formation step generates a defined amount of heat, the reaction process can be monitored directly and continuously. Combining DSC with an irradiation unit enables photoinitiated reactions to be studied. Cure data and extent of polymerisation are determined by chemical parameters such as photoinitiation, resin components, and additives and by technical parameters such as radiation source, temperature, and environmental conditions. With photocalorimetry all these parameters can be investigated individually and coordinated for optimum results. 

As a result of these reactions the materials eventually crosslink and become set] that is, they lose the ability to flow or to be dissolved. Cure most often is thermally activated hence the term thermoset, but network-forming materials whose cure is light- or radiation-activated are also considered to be thermosets (see Section 2.11, on differential photocalorimetry). Some thermosetting materials, such as certain adhesives, crosslink by a dual-cure mechanism, that is, by either heat or light activation. In contrast to the values for crosslinked elastomers or rubbers, the glass transition temperature of thermosets is generally above room temperature. 

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... 

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