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A differential scanning calorimetry method to study polymer photoperoxidation
Polymer Testing 20 (2001) 765–768 www.elsevier.com/locate/polytest
A differential scanning calorimetry method to study polymer photoperoxidation Y. Ouldmetidji, L. Gonon *, S. Commereuc, V. Verney Laboratoire de Photochimie Mole´culaire et Macromole´culaire, UMR CNRS 6505, Universite´ Blaise Pascal (Clermont-Ferrand), F-63177 Aubie`re Cedex, France Received 27 December 2000; accepted 15 February 2001
Abstract A main challenge in the field of polymer aging is the detection of extremely small amounts of peroxide formed in the primary steps of oxidation. This is a necessary condition to be able to predict physical changes and to remedy the resulting loss of properties. All experiments carried out to assess the properties of peroxide species are generally based on direct or indirect chemical titration. However, all of these techniques are limited by the molecular accessibility of chemical reagents in the macromolecular medium. As the decomposition of these species is known to be strongly exothermic, we used differential scanning calorimetry (DSC) measurements to quantify the level of peroxides formed from the photo-oxidation carried out under accelerated conditions. This method appears to be more sensitive than chemical titration in the detection of small amounts of peroxide structures. Moreover, it gives additional information such as their thermal range and kinetics of decomposition. 2001 Elsevier Science Ltd. All rights reserved. Keywords: DSC; Peroxide; Photo-oxidation
1. Introduction A main challenge in the field of polymer aging is the detection of extremely small amounts of molecular modification occurring in the primary steps of oxidation. This is a necessary condition to be able to predict physical changes and to remedy the resulting loss of properties. The first step is the formation of peroxide species on the polymer backbone. These groups are the key products for understanding the mechanism as well as for gaining better insight into the function of stabilizers [1–3]. All experiments carried out to assess the properties of peroxide species are generally based on direct or indirect chemical titration. The most common technique is the well-known iodide method based on the reduction of peroxide functions by sodium iodide. However, all of these
techniques are limited by the molecular accessibility of chemical reagents in the macromolecular medium. The aim of this study is to use a physical method in the condensed state to determine the peroxide concentration resulting from photo-oxidation of the polymer. As the decomposition of these species is known to be strongly exothermic, we used differential scanning calorimetry (DSC) measurements to quantify the level of peroxides formed in the photo-oxidation carried out under accelerated conditions (SEPAP unit). The first step of this study was to calibrate the energy level of the heat of decomposition of molecular peroxide model molecules dipped or undipped in the polymer matrix.
2. Experimental * Corresponding author. Fax: +33-4-73-40-77-00. E-mail address: [email protected] Gonon).
(L.
All experiments were performed with a differential scanning calorimeter (DSC TA Instruments 2920) at a heating rate of 20°C min⫺1. The apparatus calibration
0142-9418/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 0 2 4 - 1
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Y. Ouldmetidji et al. / Polymer Testing 20 (2001) 765–768
was performed with indium and lead as standard materials. As the heat flow recorded is significantly affected by the quantity of sample tested, the sample weights were kept constant (8 mg). Lauroyl and dicumyl peroxides from Aldrich were used to monitor the heat of decomposition of the peroxidic bonds under a thermal treatment. In a first step, DSC experiments were performed on pure molecules to assess their thermal properties. In a second step, the peroxides were introduced in a polymer matrix in order to define the exothermic flow generated by the radical reactivity of the macromolecular species. Homogeneous materials were obtained by dissolving the peroxide molecule and the polymer material in chloroform. Solvent evaporation was performed under vacuum at room temperature, and complete elimination of the solvent was checked by the disappearance of the infrared absorption at 760 cm⫺1 characteristic of the C–Cl stretching vibration. The peroxide concentration was kept at a constant value of 600 mmol kg⫺1 in each analysis. The polymers used in this study were: polystyrene, cis-polybutadiene, and cis- and trans-polyisoprene from Aldrich; styrene–isoprene–styrene (SIS) copolymer from National Starch and Chemical; and polyoctenamer (Vestenamer) from CREANOVA (Table 1). To demonstrate the ability of this method to measure a peroxide concentration, DSC experiments were performed with photo-oxidized SIS and polyoctenamer polymers under accelerated conditions [4]. In parallel, results were compared with classical chemical titration of peroxide species by the iodine method [5].
3. Principle of the measurements As the bond energy for the O–O peroxidic bond is low (about 143 kJ mol⫺1 [6–8]), its progressive decomposition is a rapid process under an external influence (heat, light radiation, etc.). The radical species induced by this
decomposition are extremely reactive and can lead to radical reactions within their environment. For example, the peroxide decomposition can lead to the formation of a carboxylated product. The covalent bond energy (about 740 kJ mol⫺1) released from this reaction is higher than the O–O bond energy [6–8]. As a consequence, this reaction is an exothermic process. Because of the intrinsic chemical reactivity of macromolecular structures, the exothermic heat flow must be monitored with regard to the polymeric matrix. In order to achieve a general scheme of the exothermic response, different compositions of polymeric material and peroxide species were studied. The objective of the present study was to demonstrate that the energy released can be monitored under a thermal treatment with a calorimetric apparatus and correlated with peroxide concentration. To achieve this analysis, it is necessary to perform a preliminary calibration of the heat flow with regard to the peroxide structure and the polymeric material. Once this is done, the concentration of peroxide is then directly proportional to the measured heat flow.
4. Results and discussion 4.1. Molecular peroxide Fig. 1 shows the thermogram recorded during the thermal decomposition of lauroyl peroxide. The first endothermic transition detected at about 60°C corresponds to melting of the peroxide. At higher temperature one can see a strong exothermic heat flow associated with radical reactions induced by decomposition of the covalent peroxide bond. One can see that this decomposition begins at about 80°C and that the maximum of the exothermic transition is reached at 114°C (under the experimental conditions defined previously) [9]. As we can observe,
Table 1 Polymer matrix materials
Fig. 1.
DSC thermogram of lauroyl peroxide.
Y. Ouldmetidji et al. / Polymer Testing 20 (2001) 765–768
767
Table 2 Calculated heat flow associated with the radical reactions induced Broken bonds O–O 2C–C
Formed bonds ⌬H=⫺143 kJ mol−1 ⌬H=2×(⫺344) kJ mol−1
Total ⌬H=⫺143⫺688=⫺831 kJ mol−1
⌬H=⫺344 kJ mol−1 ⌬H=2×[⌬H(C=O)⫺⌬H(C–O)]=2×[⫺724⫺ (⫺350)]=⫺748 kJ mol−1 Total ⌬H=⫺748⫺344=⫺1092 kJ mol−1 C–C 2C=O
no reaction is detected at a temperature higher than 140°C. The exothermic heat flow associated with the thermal decomposition is 684 J g⫺1. As the molar mass of this molecule is 398 g mol⫺1, the heat flow generated per peroxide molecule is then equal to E=⫺272 kJ mol⫺1. A scheme of the radical reactions induced by thermal decomposition of the peroxide molecule is as follows:
The energy of the broken and formed bonds is reported in Table 2. The energy balance is then equal to the difference of the energy associated with the formed bonds and the energy associated with the broken bonds, and equivalent to ⫺260 kJ mol⫺1 [6–8]. This value is in good agreement with the experimental result. In the case of dicumyl peroxide, determination of the energy of decomposition was not possible because a significant amount of energy was consumed by partial vaporization of the by-products.
Fig. 2. DSC thermogram of lauroyl peroxide associated with a polymer matrix.
4.2. Association of a molecular peroxide with a polymer matrix The association of this molecular peroxide with a polymeric matrix allows propagation of the radical reactions induced by the peroxil radical, R–O–O쐌. As can be observed in Fig. 2, two exothermic peaks are detected. The first relates to the thermal decomposition of the peroxide and the second to propagation of the radical reactions up to a temperature of 240°C. In the case of dicumyl peroxide, one can observe that its decomposition and the induced reactions occur in the same range of temperature and as a consequence only one exothermic signal is recorded (Fig. 3). Moreover, the exothermic flow per mole of peroxide was found to be constant whatever the molecular peroxide used. The results obtained with all of the polymers studied are reported in Table 3. One can observe that the exothermic flow depends on the chemical nature of the poly-
Fig. 3. DSC thermogram of dicumyl peroxide associated with a polymer matrix.
mer matrix. In the case of saturated hydrocarbon macromolecular chains, the energy recorded is a constant value (about 300 kJ mol⫺1) regarding the experimental uncertainties. This energy corresponds to the heat flow generated by recombination of the radicals formed from cleavage of the peroxide bond. These reactions cannot propagate to the neighboring molecules. On the other hand, if the molecular backbone contains reactive groups like unsaturated bonds, propagation of the radical reac-
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Y. Ouldmetidji et al. / Polymer Testing 20 (2001) 765–768
Table 3 Measured heat flow associated with a polymer matrix Polymer matrix
Heat flow for one mole of peroxide (kJ mol⫺1)
Polystyrene Polyoctenamer trans-Polyisoprene cis-Polyisoprene Styrene–isoprene–styrene Polybutadiene
260 290 450 670 530 1350
Fig. 5. Peroxide titration (䊊, iodine; 䊏, DSC methods) versus irradiation time for photo-oxidized polyoctenamer.
Fig. 4. Peroxide titration (䊊, iodine; 䊏, DSC methods) versus irradiation time for photo-oxidized SIS copolymer.
media. The high heat flow induced by the thermal decomposition of peroxide groups allows a precise determination of macromolecular peroxides, especially when the amount of peroxide is very low. Compared with chemical titration, this methodology is more interesting because it does not imply dissolution of the material in an organic solvent which can sometimes be difficult, for example when aging leads to crosslinking. Moreover, this method brings additional information such as the thermal stability of the macromolecular peroxide species.
References tions can occur leading to an increase in the energy released. The heat flow generated depends on the intrinsic reactivity of the chemical structures associated with the stereochemistry (cis or trans double bonds) and also on their relative distance from the initiation center. As a consequence, a preliminary calibration must be performed to relate the exothermic heat flow to the peroxide concentration. Fig. 4 and Fig. 5 present examples of the thermal analysis of oxidized materials obtained after accelerated photoaging [10]. A very good superposition is obtained with the classical titration of peroxide by the iodine method.
5. Conclusion The DSC method appears a powerful technique to assess the peroxide concentration in macromolecular
[1] J. Jayaseharam, A.K. Nanda, K. Kishore, Polymer 41 (2000) 5721–5727. [2] V. Verney, A. Michel, Polymer Process Engineering 4 (24) (1986) 321–335. [3] J. Carlsson, M. Wiles, Macromolecules 2 (6) (1969) 587. [4] J. Lemaire, Journal de Chimie Physique 95 (1998) 1386. [5] J. Scheirs, D.J. Carlson, S.W. Bigger, Polymer Plastic Technology and Engineering 34 (1) (1995) 97–116. [6] P.W. Atkins, Physical Chemistry, 5th ed., Oxford University Press, UK, 1994. [7] G. Franz, R.A. Sheldon, Ullmann’s Encyclopedia of Industrial Chemistry, vol. A18, VCH, Germany, 1985, pp. 261–311. [8] Q. Zhu, X.M. Zhang, A.J. Fry, Polymer Degradation and Stability 57 (1997) 43–50. [9] B. Catoire, V. Verney, A. Michel, Makromolecular Chemie, Macromolecular Symposium 25 (1989) 199–208. [10] L. Gonon, S. Commereuc, V. Verney, Journal of International Polymer Analysis and Characterization 6 (1–2) (2000) 75–87.
A differential scanning calorimetry method to study polymer photoperoxidation Y. Ouldmetidji, L. Gonon *, S. Commereuc, V. Verney Laboratoire de Photochimie Mole´culaire et Macromole´culaire, UMR CNRS 6505, Universite´ Blaise Pascal (Clermont-Ferrand), F-63177 Aubie`re Cedex, France Received 27 December 2000; accepted 15 February 2001
Abstract A main challenge in the field of polymer aging is the detection of extremely small amounts of peroxide formed in the primary steps of oxidation. This is a necessary condition to be able to predict physical changes and to remedy the resulting loss of properties. All experiments carried out to assess the properties of peroxide species are generally based on direct or indirect chemical titration. However, all of these techniques are limited by the molecular accessibility of chemical reagents in the macromolecular medium. As the decomposition of these species is known to be strongly exothermic, we used differential scanning calorimetry (DSC) measurements to quantify the level of peroxides formed from the photo-oxidation carried out under accelerated conditions. This method appears to be more sensitive than chemical titration in the detection of small amounts of peroxide structures. Moreover, it gives additional information such as their thermal range and kinetics of decomposition. 2001 Elsevier Science Ltd. All rights reserved. Keywords: DSC; Peroxide; Photo-oxidation
1. Introduction A main challenge in the field of polymer aging is the detection of extremely small amounts of molecular modification occurring in the primary steps of oxidation. This is a necessary condition to be able to predict physical changes and to remedy the resulting loss of properties. The first step is the formation of peroxide species on the polymer backbone. These groups are the key products for understanding the mechanism as well as for gaining better insight into the function of stabilizers [1–3]. All experiments carried out to assess the properties of peroxide species are generally based on direct or indirect chemical titration. The most common technique is the well-known iodide method based on the reduction of peroxide functions by sodium iodide. However, all of these
techniques are limited by the molecular accessibility of chemical reagents in the macromolecular medium. The aim of this study is to use a physical method in the condensed state to determine the peroxide concentration resulting from photo-oxidation of the polymer. As the decomposition of these species is known to be strongly exothermic, we used differential scanning calorimetry (DSC) measurements to quantify the level of peroxides formed in the photo-oxidation carried out under accelerated conditions (SEPAP unit). The first step of this study was to calibrate the energy level of the heat of decomposition of molecular peroxide model molecules dipped or undipped in the polymer matrix.
2. Experimental * Corresponding author. Fax: +33-4-73-40-77-00. E-mail address: [email protected] Gonon).
(L.
All experiments were performed with a differential scanning calorimeter (DSC TA Instruments 2920) at a heating rate of 20°C min⫺1. The apparatus calibration
0142-9418/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 1 ) 0 0 0 2 4 - 1
766
Y. Ouldmetidji et al. / Polymer Testing 20 (2001) 765–768
was performed with indium and lead as standard materials. As the heat flow recorded is significantly affected by the quantity of sample tested, the sample weights were kept constant (8 mg). Lauroyl and dicumyl peroxides from Aldrich were used to monitor the heat of decomposition of the peroxidic bonds under a thermal treatment. In a first step, DSC experiments were performed on pure molecules to assess their thermal properties. In a second step, the peroxides were introduced in a polymer matrix in order to define the exothermic flow generated by the radical reactivity of the macromolecular species. Homogeneous materials were obtained by dissolving the peroxide molecule and the polymer material in chloroform. Solvent evaporation was performed under vacuum at room temperature, and complete elimination of the solvent was checked by the disappearance of the infrared absorption at 760 cm⫺1 characteristic of the C–Cl stretching vibration. The peroxide concentration was kept at a constant value of 600 mmol kg⫺1 in each analysis. The polymers used in this study were: polystyrene, cis-polybutadiene, and cis- and trans-polyisoprene from Aldrich; styrene–isoprene–styrene (SIS) copolymer from National Starch and Chemical; and polyoctenamer (Vestenamer) from CREANOVA (Table 1). To demonstrate the ability of this method to measure a peroxide concentration, DSC experiments were performed with photo-oxidized SIS and polyoctenamer polymers under accelerated conditions [4]. In parallel, results were compared with classical chemical titration of peroxide species by the iodine method [5].
3. Principle of the measurements As the bond energy for the O–O peroxidic bond is low (about 143 kJ mol⫺1 [6–8]), its progressive decomposition is a rapid process under an external influence (heat, light radiation, etc.). The radical species induced by this
decomposition are extremely reactive and can lead to radical reactions within their environment. For example, the peroxide decomposition can lead to the formation of a carboxylated product. The covalent bond energy (about 740 kJ mol⫺1) released from this reaction is higher than the O–O bond energy [6–8]. As a consequence, this reaction is an exothermic process. Because of the intrinsic chemical reactivity of macromolecular structures, the exothermic heat flow must be monitored with regard to the polymeric matrix. In order to achieve a general scheme of the exothermic response, different compositions of polymeric material and peroxide species were studied. The objective of the present study was to demonstrate that the energy released can be monitored under a thermal treatment with a calorimetric apparatus and correlated with peroxide concentration. To achieve this analysis, it is necessary to perform a preliminary calibration of the heat flow with regard to the peroxide structure and the polymeric material. Once this is done, the concentration of peroxide is then directly proportional to the measured heat flow.
4. Results and discussion 4.1. Molecular peroxide Fig. 1 shows the thermogram recorded during the thermal decomposition of lauroyl peroxide. The first endothermic transition detected at about 60°C corresponds to melting of the peroxide. At higher temperature one can see a strong exothermic heat flow associated with radical reactions induced by decomposition of the covalent peroxide bond. One can see that this decomposition begins at about 80°C and that the maximum of the exothermic transition is reached at 114°C (under the experimental conditions defined previously) [9]. As we can observe,
Table 1 Polymer matrix materials
Fig. 1.
DSC thermogram of lauroyl peroxide.
Y. Ouldmetidji et al. / Polymer Testing 20 (2001) 765–768
767
Table 2 Calculated heat flow associated with the radical reactions induced Broken bonds O–O 2C–C
Formed bonds ⌬H=⫺143 kJ mol−1 ⌬H=2×(⫺344) kJ mol−1
Total ⌬H=⫺143⫺688=⫺831 kJ mol−1
⌬H=⫺344 kJ mol−1 ⌬H=2×[⌬H(C=O)⫺⌬H(C–O)]=2×[⫺724⫺ (⫺350)]=⫺748 kJ mol−1 Total ⌬H=⫺748⫺344=⫺1092 kJ mol−1 C–C 2C=O
no reaction is detected at a temperature higher than 140°C. The exothermic heat flow associated with the thermal decomposition is 684 J g⫺1. As the molar mass of this molecule is 398 g mol⫺1, the heat flow generated per peroxide molecule is then equal to E=⫺272 kJ mol⫺1. A scheme of the radical reactions induced by thermal decomposition of the peroxide molecule is as follows:
The energy of the broken and formed bonds is reported in Table 2. The energy balance is then equal to the difference of the energy associated with the formed bonds and the energy associated with the broken bonds, and equivalent to ⫺260 kJ mol⫺1 [6–8]. This value is in good agreement with the experimental result. In the case of dicumyl peroxide, determination of the energy of decomposition was not possible because a significant amount of energy was consumed by partial vaporization of the by-products.
Fig. 2. DSC thermogram of lauroyl peroxide associated with a polymer matrix.
4.2. Association of a molecular peroxide with a polymer matrix The association of this molecular peroxide with a polymeric matrix allows propagation of the radical reactions induced by the peroxil radical, R–O–O쐌. As can be observed in Fig. 2, two exothermic peaks are detected. The first relates to the thermal decomposition of the peroxide and the second to propagation of the radical reactions up to a temperature of 240°C. In the case of dicumyl peroxide, one can observe that its decomposition and the induced reactions occur in the same range of temperature and as a consequence only one exothermic signal is recorded (Fig. 3). Moreover, the exothermic flow per mole of peroxide was found to be constant whatever the molecular peroxide used. The results obtained with all of the polymers studied are reported in Table 3. One can observe that the exothermic flow depends on the chemical nature of the poly-
Fig. 3. DSC thermogram of dicumyl peroxide associated with a polymer matrix.
mer matrix. In the case of saturated hydrocarbon macromolecular chains, the energy recorded is a constant value (about 300 kJ mol⫺1) regarding the experimental uncertainties. This energy corresponds to the heat flow generated by recombination of the radicals formed from cleavage of the peroxide bond. These reactions cannot propagate to the neighboring molecules. On the other hand, if the molecular backbone contains reactive groups like unsaturated bonds, propagation of the radical reac-
768
Y. Ouldmetidji et al. / Polymer Testing 20 (2001) 765–768
Table 3 Measured heat flow associated with a polymer matrix Polymer matrix
Heat flow for one mole of peroxide (kJ mol⫺1)
Polystyrene Polyoctenamer trans-Polyisoprene cis-Polyisoprene Styrene–isoprene–styrene Polybutadiene
260 290 450 670 530 1350
Fig. 5. Peroxide titration (䊊, iodine; 䊏, DSC methods) versus irradiation time for photo-oxidized polyoctenamer.
Fig. 4. Peroxide titration (䊊, iodine; 䊏, DSC methods) versus irradiation time for photo-oxidized SIS copolymer.
media. The high heat flow induced by the thermal decomposition of peroxide groups allows a precise determination of macromolecular peroxides, especially when the amount of peroxide is very low. Compared with chemical titration, this methodology is more interesting because it does not imply dissolution of the material in an organic solvent which can sometimes be difficult, for example when aging leads to crosslinking. Moreover, this method brings additional information such as the thermal stability of the macromolecular peroxide species.
References tions can occur leading to an increase in the energy released. The heat flow generated depends on the intrinsic reactivity of the chemical structures associated with the stereochemistry (cis or trans double bonds) and also on their relative distance from the initiation center. As a consequence, a preliminary calibration must be performed to relate the exothermic heat flow to the peroxide concentration. Fig. 4 and Fig. 5 present examples of the thermal analysis of oxidized materials obtained after accelerated photoaging [10]. A very good superposition is obtained with the classical titration of peroxide by the iodine method.
5. Conclusion The DSC method appears a powerful technique to assess the peroxide concentration in macromolecular
[1] J. Jayaseharam, A.K. Nanda, K. Kishore, Polymer 41 (2000) 5721–5727. [2] V. Verney, A. Michel, Polymer Process Engineering 4 (24) (1986) 321–335. [3] J. Carlsson, M. Wiles, Macromolecules 2 (6) (1969) 587. [4] J. Lemaire, Journal de Chimie Physique 95 (1998) 1386. [5] J. Scheirs, D.J. Carlson, S.W. Bigger, Polymer Plastic Technology and Engineering 34 (1) (1995) 97–116. [6] P.W. Atkins, Physical Chemistry, 5th ed., Oxford University Press, UK, 1994. [7] G. Franz, R.A. Sheldon, Ullmann’s Encyclopedia of Industrial Chemistry, vol. A18, VCH, Germany, 1985, pp. 261–311. [8] Q. Zhu, X.M. Zhang, A.J. Fry, Polymer Degradation and Stability 57 (1997) 43–50. [9] B. Catoire, V. Verney, A. Michel, Makromolecular Chemie, Macromolecular Symposium 25 (1989) 199–208. [10] L. Gonon, S. Commereuc, V. Verney, Journal of International Polymer Analysis and Characterization 6 (1–2) (2000) 75–87.