Chemiluminometry of Cellulosic Materials - ACS Symposium Series

Chapter 33

Chemiluminometry of Cellulosic Materials 1

Matija S t r l i č 1, Drago Kočar , and Jana K o l a r

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Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch033

1

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Slovenia National and University Library, Ljubljana, Slovenia 2

The emission of light as a result of chemical reaction during degradation of polymers has been studied since the early 1960s. Many polymers have been studied in depth so far, and chemiluminometry has entered industrial labs as a routine investigation technique. It is particularly attractive due to the absence of sample preparation, its non-invasiveness and simplicity of instrumentation. The data can be rapidly obtained, often in the early stages of oxidation, and the technique is complementary with other approaches. However, due to the often-encountered multitude of simultaneous chemiluminescent reactions, the interpretation of data is rarely straightforward. 1,2

Introduction Chemiluminescence is light emission as a consequence of relaxation of a species excited in an elementary process of a chemical reaction. This elementary process proceeds with a rate, which is related to the kinetics of the overall reaction. In the commercially available chemiluminometers, measurements proceed using photomultipliers and photon counters. The signal /, in s" , is thus proportional to 1

ι = ο. Μ Φ

Ψ

dt

© 2007 American Chemical Society

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

531

532 where [L] is the concentration of the chemical species giving rise to chemiluminescence, t is time, G is the geometric parameter (including the size and specific surface of the sample, absorption of light in the sample, etc.) and φ is the quantum efficiency with a typical value of ~10" . Since photomultipliers usually in use have maximum sensitivity in the interval -300-650 nm, the reaction should also comply with the condition: AH > 185 - 400 kJ mol" . 9


1

As a rule, although not exclusively, light emission is a consequence of an exothermal process and is frequently observed during atmospheric oxidation of materials. A comprehensive review of chemiluminescence measurements and data evaluation during polymer oxidation was edited by Zlatkevich. Only recently, research on natural polymers, including cellulose, also intensified. The commercially available luminometers for polymers and non-volatiles are produced by Tohoku Electronic Industrial Co., Japan, and by the Polymer Institute of the Slovak Academy of Sciences, Slovakia. Lately, a separate sampling unit has been constructed enabling us to measure chemiluminescence in atmosphere containing a pre-set fraction of water vapor. Samples are heated in a compartment flushed with nitrogen, oxygen, or mixtures thereof. The temperature interval usually in use is 30-250 °C. Sample holders can accommodate samples in solid form (films, foils, powders etc.) or non-volatile liquids. The amount needed is a few milligrams. Thick samples or granules can lead to an unwanted temperature gradient across the sample. Sample preparation is usually not needed, however, sample size, specific surface and other morphological properties also influence the measurements. Typically, isothermal, dynamic, and perturbation experiments are performed. During the latter, an experimental parameter is changed abruptly, e.g. atmosphere composition, sample irradiation, etc. In the following subchapters, we will briefly review the state-of-the-art. 3

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Chemiluminescence of Cellulose in Inert Atmosphere At elevated temperatures, chemiluminescence of cellulose can readily be observed even in inert atmosphere. This indicates that even reactions other than oxidation can lead to light emission. In cellulose, this was shown to be a consequence of the transglycosidation reaction. It was also shown that the decrease of degree of polymerisation of cellulose during isothermal heating in a nitrogen atmosphere can be correlated with an integrated chemiluminescence signal at 180, 190 and 200 °C. The rate of transglycosidation is affected by the presence of alkali-earth metal carbonates: the higher the electronegativity of the alkali-earth metal, the higher the rate constant of chain scission and chemiluminescence intensity also increases correspondingly. The hypothesis that a complex between a metal ion and the glycosidic oxygen is formed has thus 9

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533 been put forward. Additionally, this indicates that even trace quantities of extraneous material can have a strong effect on the chemiluminescence signal. This should be taken into account when studying real paper samples. Also, i f a sample is not flushed well with inert atmosphere prior to an experiment, oxygen remaining in the fibrous structure has a pronounced effect. Dynamic experiments in inert atmosphere, during which the temperature is increased at some constant rate, are of considerable importance. Several different processes lead to light emission during these experiments (Figure l ) . Without a pre-treatment, a monotonously increasing signal is indicative of thermolysis (Figure 1C). If the sample is pre-oxidised, a peak appears with the maximum situated at approximately 130-150 °C (Figure 1A). The peak area was shown to correlate with peroxide content in pulp, determined titrimetrically. The area can be calculated by deconvolution of the signal, supposing two independent phenomena: decomposition of peroxides and thermolysis. 10


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Figure 1. Dynamic chemiluminometric experiments in nitrogen atmosphere with sulphite bleached pulp: (A) pre-oxidised in oxygen atmosphere, 80 °C, 30 min; (B) irradiated under a 60-W incandescent light source at a 25-cm distance from sample; (C) without pre-treatment. Rate of temperature increase: 2.5 °C min . (Reproduced with permission from reference 9. Copyright 2000 Elsevier.) 1

To explain the phenomenon leading to the peak with maximum at approximately 85 °C (curve Β in Figure 1), it was proposed that recombination


534 of charge-transfer complexes takes place after irradiation, supported by the low apparent activation energy of the process, 20.5 kJ mol" . Other chemiluminescent phenomena at low temperatures were observed after plasma or laser treatments of paper. ' 1

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Chemiluminescence of Cellulose in Oxidative Atmosphere Cellulose, being a heterochain polymer with a number of variously reactive hydroxyl groups, decomposes via the acid-catalysed hydrolytic mechanism. The relative importance of this well-researched degradation pathway decreases with increasing p H of the macromolecular environment, while the importance of oxidative degradation increases. 13

Figure 2. Isothermal chemiluminescence of cellulose samples (Whatman filter paper) impregnated with phosphate buffers of different pH as indicated. Conditions: 180°C, 0 atmosphere. (Reproduced with permission from reference 10. Copyright 2001 Elsevier.) 2

In samples of acidic character, oxidation and acid-catalysed hydrolysis can take place simultaneously and light emission is also observed, albeit less intensively than in alkaline ones (Figure 2). The relatively complex autoxidation


535 degradation pathway is predominant in cellulose in a mildly alkaline environment The reaction scheme in Figure 3 is thought to adequately describe the process of oxidation of organic polymers, including cellulose. The autoxidation scheme was originally used to explain autoxidation of simple hydrocarbons in solution, and its application to heterogeneous systems, such as atmospheric oxygen/cellulose, is not without risks. E.g., the addition of 0 to P* is a diffusion-controlled reaction, with activation energy 0 kJ m o l ' , and at ambient conditions, other reactions of P* are negligible. However, i f diffusion of oxygen to reaction sites is impaired, e.g. due to slow diffusion in crystalline regions, the relative importance of other reactions of P* may increase. Additionally, mobility of polymeric chains is lower in comparison to the mobility of low-molecular-weight compounds in solutions. Differences in mobility may easily lead to differences in rates of reactions; and instead of one rate of reaction we may well have to speak of a distribution of rates. The shape of curves in Figure 3 is typical for polymers with a short length of autoxidation chains. This conclusion is further supported by the generally low steady-state content of peroxides in cellulose samples during oxidative degradation. 14,15

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PO'+HO'

p*

POJ

Figure 3. The Bolland-Gee autoxidation reaction scheme. Native cellulose polymer is denoted as PH.

Chemiluminometry was successfully used to study certain reactions taking place during cellulose autoxidation. Pre-oxidation of samples prior to a dynamic experiment in inert atmosphere gives rise to a chemiluminescence peak, the area


536 of which corresponds to the content of peroxides in the sample formed during the pre-oxidation treatment. This approach can be used for determination of the content of peroxides in samples of similar origin. Using pullulan samples of different molecular weights and thus different content of aldehyde end groups, the correlation between peroxide and aldehyde group content was obtained as shown in Figure 4. Chromatographic analyses of samples that were subjected to accelerated degradation at 80 °C, 65% R H , indicated a correlation between peroxide content and rate of chain scission. Thus, the mechanisms of autoxidation initiation as discussed for simple carbohydrates ' may be applied. The build-up and the fate of peroxides during degradation is of primary importance in studies of oxidation. Using a chromatographic method, formation of hydroperoxides during accelerated ageing at 80 °C, 65% R H was followed and shown to be fairly rapid, while the content was found to be extremely low. Using chemiluminometry, the formation or decomposition of peroxides can also be easily followed. At room temperature, peroxides formed during oxidation for 60 min in an oxygen atmosphere at 80 °C degrade quickly, with a steady state attained in 100 min, apparently following first-order kinetics with a calculated activation energy E = 75 kJ mol" . 20


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Chemiluminometry of Cellulosic Materials - ACS Symposium Series

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