Mechanism of the temperature-dependent

Polymer Degradation and Stability 92 (2007) 1977e1985 www.elsevier.com/locate/polydegstab

Mechanism of the temperature-dependent degradation of polyamide 66 films exposed to water Elsa Silva Gonçalves, Lars Poulsen, Peter R. Ogilby* ˚ rhus, Denmark Department of Chemistry, University of Aarhus, DK-8000 A Received 2 April 2007; received in revised form 8 August 2007; accepted 15 August 2007 Available online 22 August 2007

Abstract Experiments were performed to elucidate the degradation mechanism of hot-pressed polyamide 66 upon exposure to water. For films exposed to water over the temperature range 25 Ce90 C, degradation was monitored using FTIR and solid-state 13C NMR spectroscopies. The data are consistent with a mechanism in which (1) a radical is formed on the methylene carbon adjacent to the amide nitrogen, (2) this radical reacts with oxygen to form a hydroperoxide, and (3) the hydroperoxide decomposes to form an imide or a hydroxylated amide, both of which may cleave leading to chain scission. Water appears to facilitate degradation by increasing the flexibility of the polymer matrix through swelling rather than acting as a reactive species, at least at the early stages of the process. An apparent activation energy of 15 2 kJ/mol is observed for the early stages of degradation, suggesting that segmental motions in the polymer associated with water and oxygen sorption or inter-chain radical reactions are indeed key components of the degradation process. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: FTIR; Oxygen; Polyamide 66; Water sorption; Radicals

1. Introduction Engineering plastics are widely used in applications where the polymer is exposed directly to water (e.g., purification membranes, pipes, and pump parts). In many cases, water initiates degradation reactions in the polymer which, in turn, can limit its use in a given application. Polyamide 66 (PA66, Fig. 1) is one commonly used material that will degrade upon exposure to water. For the present study, we set out to elucidate the chemical changes in PA66 films upon exposure to water under a variety of different conditions. The degradation of polyamides when exposed to water has been examined by Mikolajewski et al. [1], Vachon et al. [2], and lately by Bernstein et al. [3]. A common aspect of these studies is the focus on degradation-dependent changes in mechanical properties, sample weight and oxygen consumption.

* Corresponding author. Tel.: þ45 8942 3863; fax: þ45 8619 6199. E-mail address: [email protected] (P.R. Ogilby). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.08.007

Mikolajewski et al. studied the mechanical properties of PA66 exposed to water at temperatures between 50 C and 90 C. Moreover, using low molecular weight amides as model compounds, they concluded that the primary degradation path involved oxidation of the methylene carbon adjacent to the nitrogen of the amide. This yields an imide, which, in turn, facilitates hydrolytic cleavage of the carbonenitrogen bond (Scheme 1). Vachon et al. [2] found that the rate at which mechanical properties of PA66 change in water was both pH and temperature-dependent over the range 60 Ce80 C with activation energies ranging from 60 to 95 kJ/mol. Furthermore, they found the degradation to be inhibited by antioxidants and, by analyzing dissolved samples of the polymer, they established that hydroperoxides and carboxylic acids were formed. These results led to the suggestion that the degradation reactions involved not just oxygen but also free-radicals. In a more recent study, Bernstein et al. [3] examined the tensile strength of PA66 under humid conditions at temperatures from 37 to 138 C and confirmed the importance of oxygen in the water-dependent degradation of PA66; the extent to

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E.S. Gonçalves et al. / Polymer Degradation and Stability 92 (2007) 1977e1985

the brown edges) was cut away and experiments were performed on the remaining clear film. 2.2. Water baths

which the tensile strength changed was significantly enhanced in the presence of oxygen. Although mechanical testing certainly yields information about the accumulated effects of degradation, it does not give information about events at a molecular level. To elucidate initial stages of the water-dependent chemical change, we set out to investigate the degradation of PA66 films using FTIR microscopy. This spectroscopic technique allows us to assess the presence (or absence) of specific functional groups which, in turn, is most useful in identifying degradation intermediates and products. We have used this approach in a number of previous studies of polymer degradation [4e6]. For the present study, we specifically set out to ascertain if, in the early stages of PA66 degradation, water acts as (1) a reactive species, or (2) a facilitator of degradation by increasing the flexibility and mobility of the polymer matrix through swelling. Furthermore, we set out to investigate the role of molecular oxygen and radicals, and to quantify the effect of temperature at the early stages of PA66 degradation.

A given hot-pressed PA66 film was immersed in distilled water contained in an opaque glass jar under air, unless otherwise specified. Solutions of antioxidants consisted of distilled water saturated with 8-hydroxyquinoline (Merck, product # 107089) (pH ¼ 7). Solutions of iron(III) were made using distilled water and FeCl3(H2O)6 (Riedel-de Hae¨n, product # 31232). Solutions with iron concentrations ranging from 2 106 M (pH ¼ 5.5) to 1 104 M (pH ¼ 3.7) were prepared. The pH of the iron solutions was defined solely by the added ferric chloride; no other substances (e.g., buffer, acid or bases) were added to these solutions. The films and solutions used for the experiments performed under a nitrogen atmosphere were kept under a flow of nitrogen for 1 h prior to immersion of the sample in the water. After being immersed in the respective water baths for the desired time period, polymer samples were dried in a desiccator containing silica gel. The samples immersed in solutions containing 8-hydroxyquinoline or iron(III) were first rinsed with distilled water before drying. Prior to recording FTIR spectra, the samples were taken out of the desiccator and exposed to the ambient atmosphere for w12 h. Samples exposed to water at temperatures greater than 70 C were difficult to handle due to increasing brittleness with immersion time.

2. Experimental

2.3. Spectroscopic measurements

2.1. Polymer samples

As in our previous studies [4e6], FTIR spectra were recorded using a Bruker IFS-66v/S spectrometer operated in the continuous scan mode. The polymer films were mounted on the stage of a microscope (Bruker IRscopeII) attached to the spectrometer, and spectra were recorded from small spatial domains (w70 mm in diameter). The size of the domain was determined by the iris aperture at the image plane of the microscope [7]. When quantifying the effects of exposure to water, only IR bands with an absorbance less than 1.0 were used. To avoid artifacts due to differences in absorbance caused by material loss and thickness variations across the samples, the IR spectra were normalized using an internal spectral reference band. The band used as an internal reference was that at 930 cm1 which is assigned to the CeCO stretch in the crystalline phase of PA66 [8,9]. It is accepted that diffusion of molecules (e.g., oxygen, water) does not occur in the crystalline domains of semicrystalline polymers which, in turn, precludes degradation in these domains [10,11]. We confirmed this latter point using solid-state 13C NMR measurements performed on degraded samples (see Section 3.1). Using NMR measurements, we also confirmed that the amount of crystallinity in the film does not change as a function of degradation (i.e., resonances assigned to crystalline domains neither increase nor decrease in intensity during degradation). As such, use of the 930 cm1 band as a reference standard for the IR experiments is indeed reasonable.

Fig. 1. Molecular structure of PA66. The methylene carbon adjacent to the amide nitrogen, denoted by aN, is indicated with an arrow.

Additive-free PA66 (Aldrich, product # 42,917-1) was received in the form of pellets. PA66 films were prepared by hot-pressing the pellets at 275 C in a procedure that has been previously described [5]. For the present experiments, 15 mm thick, optically transparent films were prepared. The films showed some degradation at the edges of the sample, manifested as a slight brown colouring. The brown colouring is likely a result of oxygen diffusion from the edges of the hot-pressing platens into the polymer melt where it could then react with PA66 to form oxidation products. The part of the sample that had suffered such thermo-oxidation (i.e.,

Scheme 1.


PA66 degradation was quantified as a function of the integrated absorbance of a given IR band normalized against the internal reference band, and expressed as the ratio of the integrated absorbance after degradation (A) to the absorbance prior to water exposure (A0). The solid-state 13C NMR spectra were recorded on a Varian Inova spectrometer by means of cross-polarization from 1H to the observed 13C (300 and 75 MHz for 1H and 13C, respectively), using a 5 mm cross-polarization/ magic-angle spinning (CP/MAS) probe [12]. The spectra were recorded at a spinning speed of 5 kHz. 3. Results and discussion 3.1. General background and mechanistic framework The degradation of hot-pressed PA66 films exposed to water was monitored under a variety of conditions. In the present context, degradation means any change at the molecular level. Such degradation was principally monitored using FTIR microscopy. However, some solid-state 13C NMR experiments were also performed. In addition to the spectroscopic changes described below, several macroscopic changes became apparent when the samples were exposed to water at temperatures greater than 70 C. In these cases, the samples became brittle, and the brittleness increased with immersion time and temperature. At the limit of long immersion times (>48 h), the samples completely deteriorated (i.e., crumble when touched). Furthermore, yellowing of the samples was also observed. Yellowing of PA66 has been reported in studies dealing with photo- or thermal-degradation. In these cases, the colour change has been attributed either to the formation of pyrrole derivates [13], conjugated enal or enone groups [14], or a-ketoamide groups [15]. Upon exposure of PA66 to water, several distinct changes in the FTIR spectra of the samples are observed. Before discussing these explicit changes, it is pertinent to note that the data presented here were obtained from samples that had been exposed to distilled water (pH ¼ 6). However, data were also recorded from PA66 samples exposed to deionized (pH ¼ 6) and, independently, tap (pH ¼ 7) water. These differences in the water to which the samples had been exposed did not appear to influence the results obtained. Changes observed for selected domains of the IR spectra are shown in Fig. 2. The bands used to quantify degradation are marked with an arrow pointing in the direction of the changes with increasing water temperature. The band assignments are given in Table 1. Three of these bands (i.e., the 3080, 1740, and 1370 cm1 bands) likely reflect vibrational modes of functional groups in both amorphous and crystalline domains of the polymer. However, as already discussed, we expect any water-dependent changes in the band intensity to only reflect changes that occur in the amorphous domain. In Fig. 2a, a significant decrease in the absorbance at 3500 cm1 can be observed as the water temperature to which the sample is exposed is increased. This part of the spectrum contains an overlap of the bands associated with non-

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hydrogen-bonded NeH and OeH stretching modes. An OeH contribution may originate from absorbed water molecules [22] or from hydroperoxides generated during the processing of the polymer [17]. Consequently, the decrease of absorbance at 3500 cm1 could reflect the temperature-dependent decomposition of hydroperoxides and/or changes in the molecular structure of the polymer leading to a decrease in water content. With this in mind, it is important to note that this 3500 cm1 band is the most intense in a sample that had yet to be immersed in water. The band at 3080 cm1 is observed to shift to lower frequencies, and this shift has been assigned [23] to a decrease in water content in the polymer film. Recall that, for the films that had been immersed in water, the given sample was exposed to a drying cycle prior to FTIR measurements. Thus, our spectra reflect the equilibrium water content of the dried PA66 films, which, in turn, reflects water uptake during immersion and subsequent water loss during drying. In the midrange of the spectrum, the intensity of most bands is observed to decrease slightly with an increase in the temperature of the immersion bath (Fig. 2b). However, the bands between 1700 and 1500 cm1, which correspond to the carbonyl stretching and the CeH deformations, are unsuitable for quantitative analysis due to their high absorbance (A > 1). The bands at 1370, 1180 and 1140 cm1, marked with an arrow, meet the criteria for quantitative analysis (A < 1) and were used to monitor the progress of the degradation. As outlined in Table 1, these latter bands have been assigned to different vibrational modes of the amide and to the methylene group adjacent to the nitrogen (i.e., aN). The decrease in the absorbance of these bands thus indicates that these functional groups are consumed during the degradation process. To the high frequency side of the 1700e1500 cm1 bands, a small band with a maximum centered around 1740 cm1 is found to increase in intensity with an increase in the temperature of the water immersion bath (Fig. 2b and c). In studies of PA66 photo- [19] and thermal-oxidation [24], this band has been assigned to the formation of imide groups during the degradation process. The appearance of this imide band thus suggests that the degradation of PA66 in water occurs through a path similar to that of photo- and thermal-oxidation. This conclusion is in agreement with the results of Mikolajewski et al. [1] who, using low molecular weight amides, showed that the primary degradation path in water between 50 C and 90 C is oxidation of the carbon adjacent to the nitrogen of the amide to yield an imide. Solid-state 13C NMR studies performed on non-degraded PA66 samples and samples that had been immersed in water for 6 days at 45 C are consistent with the IR data and the suggested formation of an imide. The only NMR peak observed to change in intensity upon exposure of the PA66 films to water was that at 40.50 ppm. This peak has previously been assigned to the aliphatic carbon adjacent to the nitrogen (i.e., aN) in amorphous PA66 [25]. These results strongly suggest that the aN carbon in amorphous PA66 is indeed altered upon exposure to water, whereas the complementary aN carbon in crystalline PA66 is not involved in the degradation process (the latter has a chemical shift of 42.91 ppm).

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E.S. Gonçalves et al. / Polymer Degradation and Stability 92 (2007) 1977e1985 Table 1 Assignment of IR bands used to quantify degradation IR Band (cm1)a

Assignment

Reference

3080 1740 1370 1180 1140 930

NeH deformation þ CeN stretch Imide group CeNeH deformation CH2eNH (amorphous) CeO deformation (amorphous) CeCO stretch (crystalline)

[16] [17e19] [20] [17,21] [8,9] [8,9]

a

We denote the respective bands by the approximate wavenumber at which the band maximum occurs. In some cases (e.g., the 3080 cm1 band), this maximum shifts slightly during the degradation process (see text).

Unfortunately, the degradation-dependent appearance and disappearance of carbonyl functional groups could not be monitored using solid-state 13C NMR spectroscopy. The inherent peak broadening of the carbonyl groups of PA66 at w170 ppm prevents us from discriminating between amide groups and other carbonyl groups with similar chemical shifts. Accordingly, using solid-state 13C NMR, we cannot detect the appearance of an imide carbonyl whose chemical shift is around 170 ppm [26]. 3.2. The temperature dependence of the degradation The data in Fig. 2 clearly show that the extent of PA66 degradation depends on the temperature of the water to which the polymer sample is exposed. Thus, the data imply that temperature activated reactions are involved in the degradation. With this in mind, the degradation of PA66 was analysed as a function of the immersion time in water as well as a function of the exposure temperature. In Fig. 3, data for samples immersed in water at temperatures between 25 C and 90 C are shown. These particular data were obtained by monitoring changes in the integrated intensity of the 3080 cm1 IR band. At each temperature, we observe an exponential decrease in the band absorbance when plotted against the elapsed exposure time to water. Moreover, the extent of this absorbance change increases with an increase in the water temperature. The integrated absorbance of the 3080 cm1 band varies only slightly with exposure time for temperatures lower than or equal to 40 C. At 50 C, the changes become more pronounced and, above

Fig. 2. Normalized IR spectra for PA66 films that had been exposed to water for 2 days at 40 C, 60 C, 70 C and 80 C. A spectrum from a sample that had not been exposed to water is also shown (dotted line). The bands used for monitoring the degradation are marked with an arrow. (a) With an increase in the water temperature, one sees a decrease in the intensity of bands assigned to non-hydrogen-bonded NeH and OeH stretching modes (w3500 cm1), and to the combination band of the NeH deformation and the CeN stretching modes (3080 cm1). (b) With an increase in the water temperature, one sees a decrease in the intensity of bands assigned to the CeNeH in-plane bending mode (1370 cm1), the methylene group adjacent to the nitrogen in the amorphous phase (1180 cm1), and the CO twisting mode in the amorphous phase (1140 cm1). (c) Expanded view of the spectrum over the range 1780e 1680 cm1. With an increase in the water temperature, one sees the increase of the band assigned to an imide group (w1740 cm1).


Fig. 3. Changes in normalized integrated absorbance as a function of the elapsed exposure time, in hours, of PA66 films exposed to water at 25 C (e,e), 30 C (eCe), 40 C (e6e), 50 C (e;e), 60 C (e>e), 70 C (e=e), 80 C (e8e) and 90 C (e+e). Data were obtained using the IR band at 3080 cm1. With an increase in exposure time, one sees a decrease in the intensity of the band at 3080 cm1 (Note: the observed spectral shift of this band, see text, is smaller than the wavenumber range over which we integrate to obtain the band intensity). The decrease of the 3080 cm1 band intensity with time also gets more pronounced with an increase in the temperature of the water immersion bath. Each point in this plot corresponds to data recorded from an independent PA66 film immersed in an independent bath. The error bars shown derive from multiple recordings of spectra at different positions on the given film. Moreover, in some cases, a given point in the plot may reflect an average of data recorded from more than one film.

60 C, the changes are significant. Similar trends were observed for the 1370, 1180 and 1140 cm1 bands. The increasing rate of degradation with increasing temperature suggests that the degradation is indeed due to one, or several, temperature activated reactions. In a first-order kinetic analysis of the data in Fig. 3 (i.e., in a plot of ln(A/A0) against elapsed exposure time), rate constants for the evolution of the 3080 cm1 band were obtained. The data for samples immersed in water at temperatures over the range 25 Ce80 C exhibit linear Arrhenius behaviour (Fig. 4), and yield an apparent activation energy of 15 2 kJ/mol. We discuss possible implications of this apparent activation barrier in Section 3.4 below. (Note: the data point at 90 C was ignored in this Arrhenius analysis because it appears to deviate somewhat from the linear trend observed for the data over the range 25 Ce80 C. This could indicate that, above 80 C, a different degradation path becomes dominant. On the other hand, a linear fit to all the data over the range 25 Ce90 C is not unreasonable, and yields a similar activation energy; 13 1 kJ/mol.) An analogous Arrhenius treatment of data recorded using the 1140 cm1 band yielded a correspondingly small apparent activation energy. The appearance of the 1740 cm1 band assigned to a carbonyl group in an imide was also monitored. The intensity of this band increased with the immersion time in water and with the water temperature in the early stages of the experiment (Fig. 2c). The time-dependent changes in the intensity of the 1740 cm1 band recorded at water temperatures of

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Fig. 4. Arrhenius plot for the degradation of PA66 films immersed in water between 25 C and 90 C. The first-order rate constants, k, were obtained using changes in the intensity of the 3080 cm1 IR band (see Fig. 3). The solid line is a linear fit to the data obtained over the range 25 Ce80 C.

25 C, 45 C, and 50 C are shown in Fig. 5. Because this 1740 cm1 band appears only as a weak shoulder on more dominant bands (Fig. 2b and c), we want to be somewhat circumspect in our analysis of these data. In the least, it was difficult to accurately monitor changes in this band, especially after prolonged periods of immersion at higher temperatures. 3.3. Role of oxygen and radical intermediates Experiments were performed to investigate the possibility that both molecular oxygen as well as radical intermediates

Fig. 5. Changes in normalized integrated absorbances as a function of the elapsed exposure time, in hours, for PA66 films exposed to water at 25 C (e-e), 40 C (e 1.5). There was also little evidence of the characteristic OeH stretching mode of a carboxylic acid, which indicates that the accumulated concentration in the film of this cleavage product is small. As mentioned previously, by monitoring changes in the intensity of the 3080 cm1 IR absorption band, we found an apparent activation energy of 15 2 kJ/mol for the degradation of PA66 upon exposure to water. Moreover, we obtained a correspondingly small apparent activation energy upon treatment of data recorded using the 1140 cm1 IR band. Although the data recorded from both bands document changes in the PA66 amide functional group (see Table 1), such an activation energy barrier is clearly too small to correspond to bond cleavage reactions. Rather, it is likely that this value of w15 kJ/mol characterizes processes associated with conformation changes and segmental motion in the polymer that, in turn, facilitate either water and/or oxygen sorption

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or inter-chain interactions (e.g., hydrogen atom abstraction in Step III of our mechanism in Scheme 2). In making this assignment, it is important to recognize that although we spectroscopically monitor chemical changes in the system, this does not require the apparent activation energy we obtain to correspond specifically to these particular reactions. In light of our Arrhenius data, we note that activation energies ranging from 4 to 95 kJ/mol have been reported for a number of different processes in polyamides. Vachon et al. [2] found pH-dependent activation energies ranging from 60 to 95 kJ/mol for the degradation of PA66 in water between 60 C and 80 C based on the loss of mechanical properties. Activation energies varying between 36 and 59 kJ/mol have been reported for the diffusion of water in PA66 [29,41,42]. The disruption of hydrogen bonds in PA66 has been reported to have an activation energy that can range from 4 to 40 kJ/ mol [43,44]. Studies of segmental motion in dry polyamide samples yield activation energies over the range 33e84 kJ/ mol depending on the type of motion [45e47]. Finally, and perhaps most pertinent to our own work, it has been shown that the energy barrier for a given motion in the polymer decreases with an increase in the amount of water sorbed by the polymer [47]. 4. Conclusions Using FTIR and solid-state 13C NMR spectroscopies, we have investigated the early degradation stages of hot-pressed PA66 upon exposure to water over the temperature range of 25 Ce90 C. The degradation was found to involve both molecular oxygen and radical intermediates, and proceeds via the formation of an imide. Simple thermally-activated amide hydrolysis was found to be insignificant. In the presence of water, the mechanism of PA66 degradation is thus essentially thermooxidative, but this process is facilitated by water sorption as a consequence of what is most likely a water-dependent increase in the flexibility of the polymer matrix. This latter phenomenon is consistent with an apparent activation energy for the degradation of only 15 2 kJ/mol. Water appears to be important as a reactive species only in the later stages of the degradation process. The water-dependent degradation of hot-pressed PA66 appears to derive from and reflect the prior thermal history of the sample. This is an important point to be considered when using samples that have been injection-moulded. Acknowledgments This work was supported by grants from The Danish Graduate School of Polymer Science, Grundfos Management A/S, and The Danish National Research Foundation under a block grant for the Center for Oxygen Microscopy and Imaging. We also thank Morten V. Ørsnæs and Carsten Bloch of Grundfos Management A/S for informative discussions. Moreover, we are grateful for the help of Professor Hans Jørgen Jakobsen in recording the solid-state 13C NMR spectra.

References [1] Mikolajewski E, Swallow JE, Webb MW. Wet oxidation of undrawn nylon 66 and model amides. J Appl Polym Sci 1964;8(5):2067e93. [2] Vachon RN, Rebenfel L, Taylor HS. Oxidative degradation of nylon 66 filaments. Text Res J 1968;38(7):716e28. [3] Bernstein R, Derzon DK, Gillen KT. Nylon 6.6 accelerated aging studies: thermal-oxidative degradation and its interaction with hydrolysis. Polym Degrad Stab 2005;88(3):480e8. [4] Dam N, Ogilby PR. On the mechanism of polyamide degradation in chlorinated water. Helv Chim Acta 2001;84(9):2540e9. [5] Zebger I, Goikoetxea AB, Jensen S, Ogilby PR. Degradation of vinyl polymer films upon exposure to chlorinated water: the pronounced effect of a sample’s thermal history. Polym Degrad Stab 2003;80(2): 293e304. [6] Zebger I, Elorza AL, Salado J, Alcala AG, Goncalves ES, Ogilby PR. Degradation of poly(1,4-phenylene sulfide) on exposure to chlorinated water. Polym Degrad Stab 2005;90(1):67e77. [7] Snyder JW, Zebger I, Gao Z, Poulsen L, Frederiksen PK, Skovsen E, et al. Singlet oxygen microscope: from phase-separated polymers to single biological cells. Acc Chem Res 2004;37(11):894e901. [8] Starkweather HW, Moynihan RE. Density, infrared absorption, and crystallinity in 66-nylon and 610-nylon. J Polym Sci 1956;22(102):363e8. [9] Jakes J, Krimm S. Valence force field for amide group. Spectrochim Acta A 1971;27(1):19e34. [10] Satoto R, Subowo WS, Yusiasih R, Takane Y, Watanabe Y, Hatakeyama T. Weathering of high-density polyethylene in different latitudes. Polym Degrad Stab 1997;56(3):275e9. [11] Vergelati C, Imberty A, Perez S. Water-induced crystalline transition of polyamide 6,6 e a combined X-ray and molecular modeling approach. Macromolecules 1993;26(17):4420e5. [12] Jakobsen HJ, Daugaard P, Langer V. CP/MAS NMR at high speeds and high fields. J Mag Reson 1988;76(1):162e8. [13] Marek B, Lerch E. J Soc Dyers Color 1965;81(11):481e7. [14] Fromageot D, Roger A, Lemaire J. Thermooxidation yellowing of aliphatic polyamides. Angew Makromol Chem 1989;170:71e85. [15] Li R, Hu X. Study on discoloration mechanism of polyamide 6 during thermo-oxidative degradation. Polym Degrad Stab 1998;62(3):523e8. [16] Cannon CG. The infra-red spectra and molecular configurations of polyamides. Spectrochim Acta 1960;16(3):302e19. [17] Thanki PN, Singh RP. Photo-oxidative degradation of nylon 66 under accelerated weathering. Polymer 1998;39(25):6363e7. [18] Tang L, Sallet D, Lemaire J. Photochemistry of polyundecanamides .1. Mechanisms of photo-oxidation at short and long wavelengths. Macromolecules 1982;15(5):1432e7. [19] Roger A, Sallet D, Lemaire J. Photochemistry of aliphatic polyamides .4. Mechanisms of photooxidation of polyamide-6, polyamide11, and polyamide-12 at long wavelengths. Macromolecules 1986; 19(3):579e84. [20] Matsui H, Arrivo SM, Valentini JJ, Weber JN. Resonance Raman studies of photoinduced decomposition of nylon-6,6: product identification and mechanistic determination. Macromolecules 2000;33(15):5655e64. [21] Vasanthan N, Murthy NS, Bray RG. Investigation of brill transition in nylon 6 and nylon 6,6 by infrared spectroscopy. Macromolecules 1998; 31(23):8433e5. [22] Wu YJ, Xu YZ, Wang DJ, Zhao Y, Weng SF, Xu DF, et al. FT-IR spectroscopic investigation on the interaction between nylon 66 and lithium salts. J Appl Polym Sci 2004;91(5):2869e75. [23] Lim LT, Britt IJ, Tung MA. Sorption and transport of water vapor in nylon 6,6 film. J Appl Polym.Sci 1999;71(2):197e206. [24] Do CH, Pearce EM, Bulkin BJ, Reimschuessel HK. FT-IR spectroscopic study on the thermal and thermal oxidative-degradation of nylons. J Polym Sci A 1987;25(9):2409e24. [25] Kubo K, Ando I, Shiibashi T, Yamanobe T, Komoto T. Conformations and C-13 NMR chemical-shifts of some polyamides in the solid-state as studied by high-resolution C-13 NMR-spectroscopy. J Polym Sci B 1991;29(1):57e66.

E.S. Gonçalves et al. / Polymer Degradation and Stability 92 (2007) 1977e1985 [26] Szelejewska-Wozniakowska A, Chilmonczyk Z, Les A, Cybulski J, Wawer I. 13C cross-polarization magic angle spinning NMR and gauge-independent atomic orbital, coupled Hartree-Fock calculations of buspirone analogues: part 2. Hydrochlorides and perchlorates of 1-arylpiperazine-4-alkylimides. Solid State Nucl Magn Reson 1999; 14(1):59e65. [27] Gijsman P, Tummers D, Janssen K. Differences and similarities in the thermooxidative degradation of polyamide-46 and polyamide-66. Polym Degrad Stab 1995;49(1):121e5. [28] Kettle GJ. Variation of glass-transition temperature of nylon-6 with changing water-content. Polymer 1977;18(7):742e3. [29] Camacho W, Hedenqvist M, Karlsson S. Near infrared (NIR) spectroscopy compared with thermogravimetric analysis as a tool for on-line prediction of water diffusion in polyamide 6,6. Polym Int 2002;51(12):1366e70. [30] Koros WJ. Barrier polymers and structures. In: ACS symposium series, vol. 423. Washington, D.C.: American Chemical Society; 1990. [31] Schnabel W. Polymer degradation. New York: Macmillan/Hanser; 1981. [32] Grassie N, Scott G. Polymer degradation and stabilisation. Cambridge University Press; 1985. [33] Bolland JL, Gee G. Kinetic studies in the chemistry of rubber and related materials. 2. The kinetics of oxidation of unconjugated olefins. Trans Faraday Soc 1946;42(3e4):236e43. [34] Lock MV, Sagar BF. Autoxidation of N-alkylamides. Part I. N-acylamides as oxidation products. J Chem Soc B 1966;7:690e6. [35] Sagar BF. Autoxidation of N-alkyl amides. Part III. Mechanism of thermal oxidation. J Chem Soc B 1967;10:1047e61. [36] Sebenda J, Lanska B. Effect of polymerization conditions on the thermooxidation of nylon-6. J Macromol Sci Pure Appl Chem 1993;A30(9e10):669e78.

1985

[37] Barb WG, Baxendale JH, George P, Hargrave KR. Reactions of ferrous and ferric ions with hydrogen peroxide. Part I. The ferrous ion reaction. Trans Faraday Soc 1951;47(5):462e500. [38] Allen NS, Harrison MJ, Ledward M, Follows GW. Thermal and photo-chemical degradation of nylon 6,6 polymer: part III e influence of iron and metal deactivators. Polym Degrad Stab 1989;23(2): 165e74. [39] Gray NF. Drinking water quality problems and solutions. Chichester: Wiley; 1996. [40] Guidelines for drinking-water quality. 3rd ed, vol. 1. Geneva: World Health Organization; 2004. [41] Tokoro T, Hackam R. Loss and recovery of hydrophobicity, surface energies, diffusion coefficients and activation energy of nylon. IEEE Trans Dielect Elec Insul 1999;6(5):754e62. [42] Ishak ZAM, Berry JP. Hygrothermal aging studies of short carbon-fiberreinforced nylon-6.6. J Appl Polym Sci 1994;51(13):2145e55. [43] Rosen SL. Fundamental principles of polymeric materials. New York: Barnes and Noble; 1971. [44] Ericksen RH. Creep of aromatic polyamide fibres. Polymer 1985;26(5):733e46. [45] Goudeau S, Charlot M, Mu¨ller-Plathe F. Mobility enhancement in amorphous polyamide 6,6 induced by water sorption: a molecular dynamics simulation study. J Phys Chem B 2004;108(48): 18779e88. [46] Miura H, Hirschinger J, English AD. Segmental dynamics in the amorphous phase of nylon-66: solid-state H-2 NMR. Macromolecules 1990;23(8):2169e82. [47] Steeman PAM, Maurer FHJ. Dielectric-properties of polyamide-4,6. Polymer 1992;33(20):4236e41.