- Email: [email protected]
Thermal and photooxidation of high styrene–butadiene copolymer (SBC)
Polymer Degradation and Stability 86 (2004) 11e23 www.elsevier.com/locate/polydegstab
Thermal and photooxidation of high styreneebutadiene copolymer (SBC) Norman S. Allena,), Adriana Barcelonaa, Michele Edgea, Arthur Wilkinsona, Carmen Galan Merchanb, V. Ruiz Santa Quiteriab a
Chemistry and Materials Department, Faculty of Science and Engineering, The Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK b Repsol-YPF, Centro Tecnolo´gico Repsol-YPF, Carretera Extremadura Km 18, 28931 Mo´stoles, Madrid, Spain Received 5 August 2003; received in revised form 24 September 2003; accepted 1 October 2003
Abstract The thermal and photooxidation of high styreneebutadiene copolymer (SBC) with high styrene content (K-resin) have been studied using a variety of analytical and spectroscopic methods including yellowness, luminescence and FTIR spectroscopy coupled with crosslinking and hydroperoxide analysis in order to understand the nature of the processes involved. FTIR and luminescence analysis show complex oxidation processes with some distinct features associated with each phase. Rates of thermal oxidation on oven ageing show carbonyl growth increases with increasing temperature and is autocatalytic at 110 (C while at 90 (C an initial induction period is evident. Typical autocatalytic growth and decay of hydroperoxides are also observed at both temperatures with higher concentrations being observed at 90 (C. High degrees of crosslinking as well as yellowness (discolouration) show a similar pattern. Fluorescence analysis confirms a rapid initial disruption of the polystyrene excimers coupled with the formation of long wavelength emitting polyconjugated chromophores, possibly, stilbene type in nature giving rise to the colour formation. Oxidation is due primarily to the olefinic vinyl groups and acetophenone end groups with FTIR absorptions at 966 and 1695 cmÿ1 respectively. Thermal oxidation gives rise to a predominant absorption associated with ester groups at 1730e1740 cmÿ1. Anhydrides, aromatic ketones, aldehydes, lactones/peracids and a,b-unsaturated carbonyl species are also formed in this matrix coupled with rapid hydroperoxidation as shown by hydroxyl group formation. Analysis of the gel matrix showed similar functionalities with both styrenic and olefinic/butadiene phases being involved as well as additional conjugated vinyl group formation. Irradiation of the SBC was undertaken with polychromatic light (Microscal), 365 nm and 254 nm light. Under all three conditions long induction periods were observed prior to carbonyl group formation with 254 nm exposure showing only weak formation due to their high photolytic instability under this condition. Hydroperoxide concentration increased slightly initially and then achieved a steady state under 254 nm irradiation. The Microscal and 365 nm light gave similar rates of oxidation as determined by carbonyl index coupled with a gradual increase in stable hydroperoxide concentrations. Weak crosslinking was evident only under 254 nm light while excimer aggregates were destroyed rapidly with slower rates for Microscal and 365 nm irradiations. Strong discolouration was also evident under 254 nm light compared with the longer wavelength sources. Irradiation gave predominantly carboxylic acid groups at 1716 cmÿ1 with lesser evident formation of more active anhydrides, aromatic ketones, aldehydes, lactones/peracids and a,b-unsaturated carbonyl species due to their photolytic instability. Phosphorescence analysis shows the presence of initial active acetophenone groups which during irradiation grow rapidly initially followed by their rapid decomposition (instability and reactivity). On thermal oxidation they grow to a steady state. The olefinic vinyl groups at 966 cmÿ1 are one of the major causes of the instability associated with hydroperoxidation to form vinyl hydroperoxides. These groups in-turn break down to give unsaturated carbonyl groups and crosslinked products. They will also give rise to end-chain aliphatic macroradicals capable of further oxidation. Thus, end-chain oxidation is a predominant process at the interphase boundary of the soft aliphatic and hard aromatic segments with the immediate autocatalytic formation of high concentrations of primary hydroperoxides during the early stages of oxidation. Phosphorescence analysis also indicated the presence of initial acetophenone chromophores, which are associated with polystyrene end groups formed by chain breakage at the aliphatic links. These species can act as initial active sensitive sites for further breakdown, possibly via a thermally induced hydrogen atom abstraction process to give benzaldehyde and benzoic acid. These active end groups show a typical autocatalytic growth and decay process on irradiation. The end-chain
) Corresponding author. Tel.: C44-161-247-1432; fax: C44-161-247-6357. E-mail address: [email protected] (N.S. Allen). 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.10.010
12
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
aliphatic radicals then become the sites for further initial rapid hydroperoxidation and crosslinking reactions, which can give rise to complex gel formation. Reaction mechanisms and coloured reaction products are proposed. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: SBC-resins; Styreneebutadiene copolymer; Elastomers; Thermal oxidation; Photooxidation; Luminescence; Infra-red analysis
1. Introduction High styrene resins, styreneebutadiene copolymers (SBC) are a family of clear resins commercialised first in the early 1970s by Phillips Petroleum as K-resins and have since grown steadily in the market place as more applications find a niche for this material where clarity and impact strength are required. Coupled with these requirements is their low density compared to other clear plastics and high versatility in processing. Research studies on their degradation, is so far limited although the degradation processes in styrene block copolymers (SBCs) are known, to occur in both the polystyrene and elastomer phases [1]. However, the elastomer phase is considered more liable to degradation because its low Tg promotes permeability toward oxygen diffusion. The unsaturated SBCs are considered similar to styreneebutadiene-rubber and natural rubber, respectively, as regards resistance to oxidation, ozone attack and UV irradiation [2]. However, hydrogenated SBCs are claimed to have improved resistance toward degradation processes [3], a typical example being styreneeethylenee butyleneestyrene block copolymer (SBC). Although there are claims that SBC materials undergo degradation and chain scission, there is no experimental evidence to support this statement in terms of detailed mechanistic studies. The stabilisation of SBCs and composites involve the use of synergistic mixtures of hindered phenolic and phosphite antioxidants [1] while in recent work hydrogen atom donating hydrocarbon solvents has been claimed to inhibit the degradation SBCs [4,5]. In an earlier study on the thermal oxidation of SBC oxidation and chain scission were found to dominate at the boundary of the polystyreneeolefin phases [6]. This was found to give rise to the formation of acetophenone end groups on the styrene units and carboxylic acids on the olefin chain ends. Concurrent and further reactions gave rise to the formation of anhydrides and peresters/ acids in the longer term together with vinyl and a,b-unsaturated carbonyl products, predominantly carboxylic acids. The olefin phase was found to exhibit severe oxidation and crosslinking associated with the initial formation of unstable primary hydroperoxide species. The presence of a hindered phenolic antioxidant and phosphite were also highly synergistic in inhibiting oxidation and phase separations at the boundaries by destroying the acetophenone end groups and preventing
excimer disaggregation. Thus, in this work we have extended our previous study to include a thermal and photooxidation study on the behaviour of a high styrene content styreneebutadiene copolymer (SBC). These have been studied using a variety of analytical and spectroscopic methods including luminescence and FTIR spectroscopy coupled with yellowing measurements, crosslinking and hydroperoxide analysis in order to understand the nature of the processes involved. As for thermal oxidation the degradation reactions appear to proceed in two distinct phases with the predominant chemistry occurring in the elastomer region but with no evidence for crosslinking. Rate processes and product distributions were also notably different.
2. Experimental 2.1. Materials The SBC sample used in this study was supplied by Repsol-YPF S.A., in Madrid and was an unstabilised experimental grade product. The polymer was synthesized by anionic polymerisation in cyclohexane with n-butyllithium as the initiator, epoxidized soybean oil as a coupling agent and an alcohol as a deactivating agent. The polymerisation was carried out in stainless steel pressurised vessel. The block copolymer was made by sequential addition of monomers. The initial temperature was in the range 60e70 (C and was allowed to rise without cooling. After complete conversion, an amount of epoxidized soybean oil was added in order to obtain the desired molecular weight distribution. Finally, an amount of alcohol was added to deactivate the living ends of the polymer. The product was isolated by quenching the polymer solution into hot water and vacuum dried at 80 (C. The materials were all compression moulded at 150 (C. All the solvents used in this work were of ‘Analar’ purity and obtained from Aldrich Chemical Company, Gillingham, UK. 2.2. Crosslinking Sample of aged SBC were soxhlet extracted with chloroform for 12 h, the thimble weighed after 2 h drying at 60 (C.
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
13
Samples were also extracted with chloroform in glass tubes maintained in a water bath at 60 (C for 24 h followed by separation of the gel, drying at 60 (C for 2 h and then weighing the residue. 2.3. FTIR analysis The samples of SBC films were placed in a transmission cell fitted to a Nicolet 510 FTIR (Fourier Transform Infra-Red) spectrophotometer (DTGS detector) with air purge. Spectra were made up of 50 scans with a resolution of 2 cmÿ1. Samples were also measured by total internal reflection using an ATR accessory.
Fig. 1. Carbonyl index growth versus thermal ageing time for SBC at (B) 90 and (C) 110 (C.
2.4. Luminescence analysis Luminescence analysis was undertaken using a PerkineElmer Model LS-50B research spectrometer. Fluorescence spectra were obtained at ambient conditions on thin films using a front face accessory, while phosphorescence spectra were obtained at 77 K on thin strips of film placed in quart tube cells.
2.5. Yellowness index Colour formation was obtained using yellowness index (ASTM 313) via a Gretag Spectral Eye Colour measurement instrument (Colour data Systems Ltd., Wirral, UK). 2.6. Photooxidation
2.4.1. Microstructure The styrene and the styrene block contents were determined by 1H Nuclear Magnetic Resonance (NMR) spectroscopy. 1H NMR spectra were acquired at 300 MHz with a Bruker AC-300 Spectrometer in solution (1%) of Cl4C/CDCl3, 90/10. 2.4.2. Molecular weight distribution The molecular weights were determined by gel permeation chromatography (GPC) based on the calibration curve of polystyrene standards using a set of three ultrastyragel linear columns. The elution solvent was terahydrofuran (THF) at 35 (C. The flow rate was maintained at 1.5 ml minÿ1 and the sample concentration was 0.2%.
Irradiation studies were undertaken in a Microscal Unit (High pressure Hg/W 500 watt source) and Black Body Temperature of 50 (C (available from Microscal Ltd., London, UK). Samples were also irradiated using a 400 watt 254 low pressure mercury lamp (Applied Photophysics, London) as well as a 365 nm 8 watt fluorescent tube (10 nm band-width half peak height). Samples were rotated periodically for consistency in exposure. Samples for light exposure were 200 mm thick. 2.7. Hydroperoxide concentration Hydroperoxide concentrations were determined via the standard iodometric analysis method [6e8].
Table 1 Properties of SBC resin used Properties
SBC resin
1
H NMR Styrene, % (styrene block) 1,2 Butane, % 1,4 Butane, % GPC Mw Mn I.H. Antioxidants, HPLC Structure after coupling S: polystyrene block B: polybutadiene block
71.5 (100) 3.3 25.2 152,139 87,187 1.74 None S1-S2-B1-S3-B2-Li1 C S2-B1-S3-B2-Li2 C S3-B2-Li3
Fig. 2. % Crosslinking versus oven ageing time for SBC at (C) 90 and (B) 110 (C.
14
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Fig. 3. Yellowness index versus thermal ageing time for SBC at (C) 90 and (B) 110 (C.
Fig. 6. Phosphorescence emission spectra (EX = 250 nm) of unstabilised SBC before and after different periods of irradiation at 365 nm.
3. Results and discussion
Fig. 4. Hydroperoxide concentration in ppm versus thermal oxidation time for SBC at (C) 90 and (B) 110 (C.
Fig. 5. Relative excimer emission intensity versus thermal ageing time for SBC at (C) 90 and (B) 110 (C.
Styreneebutadiene copolymer is a complex system being dependent on the nature of the starting materials, their composition and the manufacturing process [1]. The properties of the SBC sample utilised in this study is shown in Table 1 with the respective molecular weight and structure. No stabilisers were utilised in this work in order to underpin the nature of the mechanistic processes involved.
Fig. 7. Carbonyl index growth versus photoageing time for SBC with different light sources.
15
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Fig. 8. % Crosslinking versus 254 nm irradiation time for SBC.
Fig. 11. Relative excimer emission intensity versus photoageing time for SBC with different light sources.
3.1. Thermal oxidation The rate of carbonyl growth [7] in the SBC polymer used here is shown in Fig. 1 at 90 and 110 (C. Here the rate is seen to be autocatalytic at 110 (C whereas at 90 (C the rate is autoretarding with a long induction period. In our earlier work on SBC crosslinking was Table 2 FTIR bands assigned to thermal and photooxidised SBC-resins Wavenumber (cmÿ1) Group Thermal 3600e3200
Fig. 9. Yellowness index growth versus photoageing time for SBC with different light sources.
1775 1739 1735
1730 1715 1695 1685 1639 Photooxidation 3600e3200
1775a 1738b
1716 1700 1697c 1685d a b
Fig. 10. Hydroperoxide concentration versus photooxidation time (h) for SBC in the Microscal (C), 365 nm light (B) and 254 nm light (-).
c d
Hydroxyls (alcohols/peroxides/ hydroperoxides) Anhydride/g-lactone/peracids Aliphatic ester Aliphatic esters/carboxylic acids/benzoic acid (monomeric form)/d-lactone Aliphatic ester Carboxylic acids/aliphatic esters Aromatic ketones/ acetophenone groups a,b-Unsaturated carbonyls a,b-Unsaturated carbonyls Hydroxyls (alcohols/peroxides/ hydroperoxides) Anhydride/g-lactone/peracids Aliphatic esters/carboxylic acids/benzoic acid (monomeric form)/d-lactone Carboxylic acids/aliphatic esters Carboxylic acids/benzoic acid (monomeric form) Aromatic ketones/ acetophenone groups a,b-Unsaturated carbonyls ÿ1
For 254 nm: 1772 cm . For 254 nm: 1740 cmÿ1. For 254 nm: 1696 cmÿ1. For 254 nm: 1684 cmÿ1.
Remark OeH stretch (associated) C]O stretch C]O stretch C]O stretch
C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch OeH stretch (associated) C]O stretch C]O stretch
C]O stretch C]O stretch C]O stretch C]O stretch
16
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
found to be predominant during thermal ageing in the aliphatic phase. The data in Fig. 2 show the rates of crosslinking for SBC at 90 and 110 (C. Both exhibit a short induction period followed by a very rapid autocatalytic growth in crosslinking. Over a period of 100 h up to 50% by weight of gel was formed. As will be discussed later, unlike SBS, SBC exhibits crosslinking within the styrene as well as the butadiene phase. Discolouration is also a notable feature of the thermal ageing of the SBC and is consistent with the carbonyl and crosslinking growths (Fig. 3). At 90 (C there is a long induction period prior to the observed yellowing while at 110 (C the discolouration is strong following a short induction period. These trends are consistent with the rates of hydroperoxide formation (Fig. 4). At both temperatures there is a rapid autocatalytic growth in concentration followed by a rapid decline with 90 (C showing an initial induction period, higher concentration of species and longer time maximum. The interesting feature of the results is that at both temperatures there is a decrease in hydroperoxides with prolonged oxidation below the initial value confirming the high instability of the species formed. Luminescence ( fluorescence and phosphorescence) analysis provides more specific information on photoactive chromophores in the polymer either present initially or formed during the oxidation/degradation processes. Such species may also be consumed by the ensuing degradation reactions/products. These species are normally carbonylic in nature or vinyl groups or aromatic species. In the case of aliphatic species these can be ketonic or aldehydic in nature either alone or
2.0
coupled with vinyl groups. Carboxylic acids and ester groups, hydroperoxides and peresters/acids are nonluminescent following excitation by light energy. Such species have higher energy excitation bands (below 250 nm) and tend to dissipate their absorbed energy through chain scission reactions rather than emit light. Fluorescence analysis of the SBC material as film using the reflective mode shows a strong emission band centred at 335e340 nm, which is associated primarily with the presence of excimer sites in the polystyrene phase. This is due to the association of a ground-state styrene unit with an excited unit and emits to the red of the styrene monomer emission [8]. The distinct observation of excimer emission is supportive of the presence of discrete polystyrene phases and is unaffected by the ‘‘olefin’’ phase. Upon degradation, however, there is a rapid loss and reduction in the excimer sites due to chain scission and disaggregation of the styrene units (Fig. 5). This process occurs rapidly during the early stages of the degradation process at 110 (C. As found previously for SBC [6] magnification of the fluorescence spectra with degradation also showed the initial formation of a longer wavelength emitting species i.e. a broad spectral shift to 350e380 nm. Under light exposure (later) the instability of such chromophores will not give rise to such an apparent spectrum as was observed thermally [6]. It is known that in the degradation of polystyrene, phenylevinyl units are formed similar to that of stilbene and that these can contribute to yellowing [8e10]. The absence of other stronger emitting chromophores, such as a,b-unsaturated carbonyls, suggests that if indeed they are formed then
CR-38A@365nm-84h
1.5
1601.08
Abs 1.0
1372.03
1181.36 1155.17
1372.03 1311.83
1181.42 1154.74
1068.59
968.94
540.54
0.5 2.0
CR-38A@110C-50h
1.5 1739.70 1696.57
Abs 1.0
1583.22
1069.38 972.44 908.71
767.43705.10
539.49
0.5 2.0
CR-38A@microscal-0h
1.5
Abs 1.0 0.5 2000
1500
cm-1
1000
500
Fig. 12. FTIR spectra 2000e500 cmÿ1 of SBC-resin (SBC) before and after ageing for 84 h at 365 nm and 50 h at 110 (C.
17
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
autocatalytic growth in carbonyl formation at similar rates. With the 365 nm source being very weak in energy (8 watts) this is surprising but nevertheless a clear indication of the development and importance of reactive hydroperoxides in the oxidation process. The destructive effect of the 254 nm light source is very apparent with a strong autoretarding effect on carbonyl growth. This is due to not only the photolysis and direct screening by the styrenic units due to the thickness of the films utilised in this work, but also the photolytic instability of any generated carbonyl groups. This effect was accompanied by some gel formation as indicated in Fig. 8. However, consistent with our work on SBC photooxidation [12] no evident crosslinking was found. The SBC was also found to discolour under irradiation with the 254 nm giving rise to the greatest effect (Fig. 9). Again, as for carbonyl index the 365 nm and Microscal irradiations were similar. For the films used in this work the irradiation wavelengths would be transmissive. However, the magnitude of discolouration was significantly less than that produced during thermal ageing. In part, this is due to many of the products being transient in nature under light exposure. Hydroperoxide formation under the three irradiation conditions is shown in Fig. 10. Here there is a gradual increase in concentration with the 365 nm light and Microscal conditions
they must be non-emissive (as in carboxylic acids). Low temperature phosphorescence analysis is also valuable for probing the formation or presence of carbonylic species either of an aromatic or aliphatic nature. The interesting feature of this analysis was the observation of the initial presence of acetophenone chromophores in the polymer (Fig. 6). It is to be noted that such chromophores were observed in SBC by FTIR (later). Thus, acetophenone groups are rapidly formed during oxidation in the polystyrene phase due to end-chain oxidation and scission as was reported earlier for SBC [6]. 3.2. Photooxidation Irradiation studies in this work were undertaken using polychromatic and monochromatic light sources. In the former case this would relate to sunlight exposure i.e. for wavelengths greater than 300 nm while 365 nm light would induce photolysis of hydroperoxide groups but not carbonyl species (unless heavily conjugated). Irradiation under 254 nm light is destructive to the styrene units as well as other potential light absorbing species such as carbonyl groups. Carbonyl index curves in Fig. 7 show an interesting comparison. All three light exposures display an initial induction period with both the Microscal and 365 nm sources then exhibiting an
2.0
1.8 1716.46 1.6
1601.08
1.4
1.2
1716
Abs
1738
1.0
1700 1697
0.8
1685
0.6 1775
1740 17351716
0.4
1700 1696 1684
1772 0.2
2000
1800
1600
cm-1 Fig. 13. FTIR spectra 2000e1500 cmÿ1 of SBC-resin (SBC) before and after photoageing.
18
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23 2.0 1.8
CR-38A@110C-50h
1.6
1372.03 1311.83
1.4 1739.70 1696.57
1.2
Abs 1.0
1181.42 1154.74
1028.93 1069.38 972.44908.71
767.43705.10
539.49
1583.22
0.8 0.6 0.4 0.2 2.0 CR-38A@110C-0 1.8 1.6 1.4 1.2
Abs 1.0 0.8 0.6 0.4 0.2 2000
1500
1000
cm-1 Fig. 14. FTIR spectra 2000e500 cmÿ1 of SBC-resin (SBC) before and after ageing for 50 h at 110 (C.
exhibiting similar rates with the latter at a somewhat higher level. As for the carbonyl data the 254 nm irradiation gave a small initial increase but thereafter autoretarded with a long plateau. Clearly under this exposure condition the hydroperoxides are photolysed rapidly as soon as they are formed. Disruption of the polystyrene aggregates also takes place under irradiation with the greatest effect being observed under 254 nm light (Fig. 11). Here the styrene absorbs the light directly. However, the interesting feature of this data is the destruction of the styrenic aggregates under 365 nm light. Here traces of any initial acetophenone groups could trigger reactions and disrupt the styrene units as discussed later. Also, on photooxidation acetophenone chromophores were formed rapidly after an initial 24 h period (Fig. 6). Phosphorescence emission spectra versus irradiation for the SBC are given in Fig. 6. Consistent with our earlier work on SBC their concentration then declines rapidly on further irradiation due to their transient nature and subsequent photoreactivity in the polymer. This effect was also seen under the Microscal irradiation. The species appeared to be destroyed under 254 nm light exposure. 3.3. FTIR analysis To determine the chemical changes induced in the SBC FTIR analysis was undertaken on oxidised films of the material by transmission. Table 2 summarises the main functional group frequencies observed in the SBC
for both thermal and photooxidised materials. Fig. 12 shows typical FTIR changes observed in the thermal and photoaged SBC materials in the region 2000e500 cmÿ1. These are typical spectra as shown for 365 nm irradiation and 110 (C thermal for 50 h. Strong Table 3 FTIR bands assigned to thermal and photogenerated soluble/gel fractions Wavenumber (cmÿ1) Group Thermal gel fraction 3600e3200 Hydroxyls (alcohols/ peroxides/hydroperoxides) 1775 Anhydride/g-lactone/peracids 1739 Aliphatic ester/benzoic acid (monomeric form)/d-lactone 1725 Aliphatic ketones 1715 Carboxylic acids 1695 Aromatic ketones/ acetophenone groups 1639 a,b-Unsaturated carbonyls 1620/1600 Conjugated vinyls Photo gel fraction (254 nm) 3600e3200 Hydroxyls (alcohols/ peroxides/hydroperoxides) 1775 Anhydride/g-lactone/peracids 1739/1730 Aliphatic esters/benzoic acid (monomeric form)/d-lactone 1716 Carboxylic acids/aliphatic esters 1695 Aromatic ketones/ acetophenone groups 1639 a,b-Unsaturated carbonyls
Remark OeH stretch (associated) C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch OeH stretch (associated) C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch
19
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23 0.10
CR-38A@90C-120h(gel)
0.08
Abs
0.06
1725 1739 1715 1695 1639 1775
0.04 0.02 0.08
CR-38A@254nm-120h(gel)
0.06
Abs 0.04
1451.88 1492.57
0.02 0.00 0.08
965.80
1600.90
1028.09
756.46
668.46
CR-38A@110C-50h(gel)
0.06
Abs 0.04 0.02 0.00 2000
1500
1000
cm-1 Fig. 15. FTIR spectra 2000e500 cmÿ1 of SBC-resin (SBC) gel after ageing for 120 h at 254 nm, 50 h at 110 (C and 120 h at 90 (C.
carbonyl absorptions are observed in both cases with a maximum at 1716 cmÿ1 due to carboxylic acids plus weak shoulders at 1775 cmÿ1 due to anhydrides/ lactones, 1738 cmÿ1 due to esters and possibly benzoic acid, 1700 cmÿ1 due to carboxylic acids and aromatic ketones such as acetophenone at 1697 cmÿ1 [8,11,12]. Derivatisation with sodium hydroxide treatment (2 M) removed the band at 1716/1700 cmÿ1 confirming the presence of carboxylic acids. Absorption due to a,b-unsaturated carbonyl species is seen at 1685 cmÿ1. Aromatic carbonyls may also be absorbing at 1700 and 1685 cmÿ1. Above 1000 cmÿ1 other absorption changes were observed due to ether formation (eCeO) indicating the possible instability and transient nature of any formed vinyl groups except those associated with carbonyl groups albeit weakly. Below 1000 cmÿ1 was the notable disappearance of the main chain olefinic 1,4-trans-vinyl groups at 966 cmÿ1. These appear to be readily removed during both oxidation processes and have been reported on in earlier work on SB rubber [13,14]. Similar species were also formed during thermal oxidation but unlike photooxidation where the maximum peak was at 1716 cmÿ1 due to carboxylic acids thermal oxidation generated primarily ester groups. Expanded carbonyl spectra are shown in Fig. 13 for irradiated samples displaying weak shoulders for the other products whereas thermally oxidised SBC displays quite strong shoulders due to other products i.e.
aromatic carbonyls at 1695 cmÿ1. This is shown more clearly in the expanded spectrum in Fig. 14. Other potential absorbing species are also given in Table 2 and their mode of formation is discussed below. Here, strong hydroxyl absorption was observed in the SBC at 3430 cmÿ1 due to the formation of associated groups. The 254 nm irradiation gave weak absorptions as discussed above. Thus, oxidation appears to be consistent with rapid hydroperoxidation of the SBC, with the olefinic vinyl groups being one of the main points of attack Fig. 14. The FTIR absorption bands observed in the separated thermal and photoaged gel products are compared in Table 3 by transmission. Spectral examples are displayed in Fig. 15. As can be seen the 254 nm spectrum is very weak but upon expansion some products were identifiable and given in Table 3. For the thermally aged gel material the main absorption peak was found to be centred at 1725 cmÿ1 due to ketonic groups. Other product groups were similar to those found for the liquid fractions.
3.4. Mechanistic processes Schemes 1e4 depict the various functional group changes consistent with those seen in the FTIR spectra and phosphorescence analysis and are listed in Tables 2
20
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Scheme 1. Oxidation and removal of vinyl groups via hydroperoxides.
and 3. Firstly, the dominant formation of primary carboxylic acids throughout both the oxidation processes suggests the predominance of an end-chain oxidation step and this is consistent with the phosphorescence analysis data below. The only evident vinyl group formation in FTIR analysis was associated with carbonyl groups and stilbenic chromophores. The above analysis indicates that chain degradation, scission and oxidation are occurring primarily at two sites namely, the styreneebutadiene phase boundary and in-chain oxidation of the vinyl groups. This is confirmed by the rapid formation of end-chain carboxylic acids (FTIR at 1716 cmÿ1) and concurrent formation of acetophenone end groups as measured by phosphorescence analysis. The latter are observed in the FTIR during thermal oxidation and are transient during photooxidation due to their further photolysis. Scheme 1 depicts oxidation of the in-chian olefinic vinyl groups to give unsaturated hydroperoxides. These will break down further to yield a,b-unsaturated carbonyl groups and end-chain macroradicals [13,14]. The latter can oxidise further to form carboxylic acids via aldehydes and then peracids (1720 and 1775 cmÿ1) (Scheme 2). Lactones are known to be formed in polyolefin oxidation by an intra-molecular backbiting reaction (H-atom abstraction) by a carboxyl radical.
The esters, peresters and anhydrides are formed through appropriate radical recombination reactions and alcohols through hydrogen atom abstraction. Unsaturated carbonyl products (e.g. carboxylic acids) can be formed by further hydrogen atom abstraction reactions down the chain. Hydroperoxide build-up in the olefin phase is predominant as shown by the FTIR analysis and will be representative of the total amount of hydroperoxidation in the SBC and not just end-group oxidation. During thermal oxidation in-chain oxidation and crosslinking may also predominate via hydroperoxides and the formation of peroxy radicals to give crosslinked material (Scheme 3). Breakage of the olefinestyrene chain will result in the formation of benzyl radicals which can then be oxidised to form hydroperoxides (Scheme 4). These species, breakdown easily to give acetophenone end groups, which are shown by the phosphorescence emission. Phenyl-propene or stilbene groups can also be formed by further hydrogen atom transfer reactions resulting in the release of acetophenone as a product (Scheme 4). Subsequent photoreactions of the acetophenone end groups can give rise to the formation of benzaldehyde and benzoic acid products. One interesting observation in this work is the significance in the consumption of the excimer styrene units and the direct consumption of the acetophenone
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
21
Scheme 2. End-chain olefin oxidation via hydroperoxides.
units initially present and/or formed in the SBC. The latter species can be reactive as has already been shown in earlier work for aromatic ketones. These chromophores can react through a photochemically generated hydrogen atom abstraction process to give ketyl radicals (see Scheme 4 at end) and alkyl radicals. The rapid consumption of the excimer sites certainly indicates disruption of the styrenic conjugation along the chains into smaller fragments. The ring absorption peak at 756 cmÿ1 appeared to be unaffected by all oxidation processes in line with an earlier report [13]. In future analysis light stabiliser interactions will be of interest.
4. Conclusions This work shows a number of interesting features associated with SBC thermal and photooxidation/degradation processes. Thermal and photooxidation give rise to oxidation and chain scission at the boundary of the polystyreneeolefin phases in terms of the analysis of data presented here. This gives rise to the formation of acetophenone end groups on the styrene units and carboxylic acids on the olefin chain ends. Concurrent and further reactions give rise to the formation of ketones, anhydrides and peresters/acids in the longer term together with a,b-unsaturated carbonyl products,
22
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Scheme 3. In-chain oxidation of olefin via hydroperoxides and crosslinking.
predominantly esters during thermal oxidation and carboxylic acids during photooxidation. It is understood in the context of the structure of SBC that tertiary groups in the butadiene units and olefinic vinyl groups would also be labile centres for hydroperoxidation.
There is no evidence for crosslinking in the SBC during long wavelength irradiation unlike that for thermal oxidation. Only high energy radiation in the far UV causes crosslinking and decomposes the hydroperoxide and carbonylic groups. The olefin phase exhibits severe
Scheme 4. Polystyrene oxidation via hydroperoxides and coloured products.
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
oxidation associated with the initial formation of unstable primary hydroperoxide species while the polystyrene phase exhibits oxidation to form acetophenone end groups. The latter are transient and photounstable in the longer term causing further photoreactions.
Acknowledgements The authors thank Repsol-YPF, for material costs in support of one of them (CL).
References [1] Mistrali F, Proni A. Styrenic block copolymers. In: Whelan A, Lee KS, editors. Developments in rubber technology, vol. 3. London: Applied Science Publishers Ltd.; 1982. [2] Haws JR, Wright RF. Block copolymers. In: Walker BM, editor. Handbook of thermoplastic elastomers. New York: Van Nostrand Reinhold Co.; 1979.
23
[3] Gergen WP, Lutz RG, Davidson S. Hydrogenated block copolymers in IPNS. In: Golden G, editor. Thermoplastic elastomers. 2nd ed. Munich: Hanser Verlag; 1996. [4] Holden G. Applications of thermoplastic elastomers. In: Holden G, editor. Thermoplastic elastomers. 2nd ed. Munich: Hanser Verlag; 1996. [5] Kubo J, Onzuka H, Akiba M. Polym Deg Stabil 1994;45:27. [6] Allen NS, Edge M, Wilkinson A, Liauw CM, Mourelatou D, Barrio J, et al. Polym Deg Stabil 2001;7:113e22. [7] Allen NS, Edge M. Fundamentals of polymer degradation and stabilisation. Oxford: Elsevier Applied Science Publishers; 1992. [8] McKellar JF, Allen NS. Photochemistry of man-made polymers. London: Elsevier Applied Science Publishers; 1979. [9] Fox RB, Price TR, Cozzens RF. ACS Symp. Sers. No: 25. In: Labana SS, editor. UV light induced reactions in polymers. 1976, No. 17. 242 pp. [10] Basile LJ. J Chem Phys 1962;36:2204. [11] Allen NS, Parkinson A, Loffelman FF, Susi PV. Angew Makromol Chemie 1983;116:203e19. [12] Allen NS, Edge M, Wilkinson A, Liauw CM, Luengo C, Barrio J, et al. J Photochem Photobiol, Chem Ed 2004;162:41. [13] Adam C, Lacoste J, Lemaire J. Polym Deg Stabil 1989;26:269. [14] Ghaffar A, Scott A, Scott G. Eur Polym J 1977;13:83.
Thermal and photooxidation of high styreneebutadiene copolymer (SBC) Norman S. Allena,), Adriana Barcelonaa, Michele Edgea, Arthur Wilkinsona, Carmen Galan Merchanb, V. Ruiz Santa Quiteriab a
Chemistry and Materials Department, Faculty of Science and Engineering, The Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK b Repsol-YPF, Centro Tecnolo´gico Repsol-YPF, Carretera Extremadura Km 18, 28931 Mo´stoles, Madrid, Spain Received 5 August 2003; received in revised form 24 September 2003; accepted 1 October 2003
Abstract The thermal and photooxidation of high styreneebutadiene copolymer (SBC) with high styrene content (K-resin) have been studied using a variety of analytical and spectroscopic methods including yellowness, luminescence and FTIR spectroscopy coupled with crosslinking and hydroperoxide analysis in order to understand the nature of the processes involved. FTIR and luminescence analysis show complex oxidation processes with some distinct features associated with each phase. Rates of thermal oxidation on oven ageing show carbonyl growth increases with increasing temperature and is autocatalytic at 110 (C while at 90 (C an initial induction period is evident. Typical autocatalytic growth and decay of hydroperoxides are also observed at both temperatures with higher concentrations being observed at 90 (C. High degrees of crosslinking as well as yellowness (discolouration) show a similar pattern. Fluorescence analysis confirms a rapid initial disruption of the polystyrene excimers coupled with the formation of long wavelength emitting polyconjugated chromophores, possibly, stilbene type in nature giving rise to the colour formation. Oxidation is due primarily to the olefinic vinyl groups and acetophenone end groups with FTIR absorptions at 966 and 1695 cmÿ1 respectively. Thermal oxidation gives rise to a predominant absorption associated with ester groups at 1730e1740 cmÿ1. Anhydrides, aromatic ketones, aldehydes, lactones/peracids and a,b-unsaturated carbonyl species are also formed in this matrix coupled with rapid hydroperoxidation as shown by hydroxyl group formation. Analysis of the gel matrix showed similar functionalities with both styrenic and olefinic/butadiene phases being involved as well as additional conjugated vinyl group formation. Irradiation of the SBC was undertaken with polychromatic light (Microscal), 365 nm and 254 nm light. Under all three conditions long induction periods were observed prior to carbonyl group formation with 254 nm exposure showing only weak formation due to their high photolytic instability under this condition. Hydroperoxide concentration increased slightly initially and then achieved a steady state under 254 nm irradiation. The Microscal and 365 nm light gave similar rates of oxidation as determined by carbonyl index coupled with a gradual increase in stable hydroperoxide concentrations. Weak crosslinking was evident only under 254 nm light while excimer aggregates were destroyed rapidly with slower rates for Microscal and 365 nm irradiations. Strong discolouration was also evident under 254 nm light compared with the longer wavelength sources. Irradiation gave predominantly carboxylic acid groups at 1716 cmÿ1 with lesser evident formation of more active anhydrides, aromatic ketones, aldehydes, lactones/peracids and a,b-unsaturated carbonyl species due to their photolytic instability. Phosphorescence analysis shows the presence of initial active acetophenone groups which during irradiation grow rapidly initially followed by their rapid decomposition (instability and reactivity). On thermal oxidation they grow to a steady state. The olefinic vinyl groups at 966 cmÿ1 are one of the major causes of the instability associated with hydroperoxidation to form vinyl hydroperoxides. These groups in-turn break down to give unsaturated carbonyl groups and crosslinked products. They will also give rise to end-chain aliphatic macroradicals capable of further oxidation. Thus, end-chain oxidation is a predominant process at the interphase boundary of the soft aliphatic and hard aromatic segments with the immediate autocatalytic formation of high concentrations of primary hydroperoxides during the early stages of oxidation. Phosphorescence analysis also indicated the presence of initial acetophenone chromophores, which are associated with polystyrene end groups formed by chain breakage at the aliphatic links. These species can act as initial active sensitive sites for further breakdown, possibly via a thermally induced hydrogen atom abstraction process to give benzaldehyde and benzoic acid. These active end groups show a typical autocatalytic growth and decay process on irradiation. The end-chain
) Corresponding author. Tel.: C44-161-247-1432; fax: C44-161-247-6357. E-mail address: [email protected] (N.S. Allen). 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.10.010
12
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
aliphatic radicals then become the sites for further initial rapid hydroperoxidation and crosslinking reactions, which can give rise to complex gel formation. Reaction mechanisms and coloured reaction products are proposed. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: SBC-resins; Styreneebutadiene copolymer; Elastomers; Thermal oxidation; Photooxidation; Luminescence; Infra-red analysis
1. Introduction High styrene resins, styreneebutadiene copolymers (SBC) are a family of clear resins commercialised first in the early 1970s by Phillips Petroleum as K-resins and have since grown steadily in the market place as more applications find a niche for this material where clarity and impact strength are required. Coupled with these requirements is their low density compared to other clear plastics and high versatility in processing. Research studies on their degradation, is so far limited although the degradation processes in styrene block copolymers (SBCs) are known, to occur in both the polystyrene and elastomer phases [1]. However, the elastomer phase is considered more liable to degradation because its low Tg promotes permeability toward oxygen diffusion. The unsaturated SBCs are considered similar to styreneebutadiene-rubber and natural rubber, respectively, as regards resistance to oxidation, ozone attack and UV irradiation [2]. However, hydrogenated SBCs are claimed to have improved resistance toward degradation processes [3], a typical example being styreneeethylenee butyleneestyrene block copolymer (SBC). Although there are claims that SBC materials undergo degradation and chain scission, there is no experimental evidence to support this statement in terms of detailed mechanistic studies. The stabilisation of SBCs and composites involve the use of synergistic mixtures of hindered phenolic and phosphite antioxidants [1] while in recent work hydrogen atom donating hydrocarbon solvents has been claimed to inhibit the degradation SBCs [4,5]. In an earlier study on the thermal oxidation of SBC oxidation and chain scission were found to dominate at the boundary of the polystyreneeolefin phases [6]. This was found to give rise to the formation of acetophenone end groups on the styrene units and carboxylic acids on the olefin chain ends. Concurrent and further reactions gave rise to the formation of anhydrides and peresters/ acids in the longer term together with vinyl and a,b-unsaturated carbonyl products, predominantly carboxylic acids. The olefin phase was found to exhibit severe oxidation and crosslinking associated with the initial formation of unstable primary hydroperoxide species. The presence of a hindered phenolic antioxidant and phosphite were also highly synergistic in inhibiting oxidation and phase separations at the boundaries by destroying the acetophenone end groups and preventing
excimer disaggregation. Thus, in this work we have extended our previous study to include a thermal and photooxidation study on the behaviour of a high styrene content styreneebutadiene copolymer (SBC). These have been studied using a variety of analytical and spectroscopic methods including luminescence and FTIR spectroscopy coupled with yellowing measurements, crosslinking and hydroperoxide analysis in order to understand the nature of the processes involved. As for thermal oxidation the degradation reactions appear to proceed in two distinct phases with the predominant chemistry occurring in the elastomer region but with no evidence for crosslinking. Rate processes and product distributions were also notably different.
2. Experimental 2.1. Materials The SBC sample used in this study was supplied by Repsol-YPF S.A., in Madrid and was an unstabilised experimental grade product. The polymer was synthesized by anionic polymerisation in cyclohexane with n-butyllithium as the initiator, epoxidized soybean oil as a coupling agent and an alcohol as a deactivating agent. The polymerisation was carried out in stainless steel pressurised vessel. The block copolymer was made by sequential addition of monomers. The initial temperature was in the range 60e70 (C and was allowed to rise without cooling. After complete conversion, an amount of epoxidized soybean oil was added in order to obtain the desired molecular weight distribution. Finally, an amount of alcohol was added to deactivate the living ends of the polymer. The product was isolated by quenching the polymer solution into hot water and vacuum dried at 80 (C. The materials were all compression moulded at 150 (C. All the solvents used in this work were of ‘Analar’ purity and obtained from Aldrich Chemical Company, Gillingham, UK. 2.2. Crosslinking Sample of aged SBC were soxhlet extracted with chloroform for 12 h, the thimble weighed after 2 h drying at 60 (C.
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
13
Samples were also extracted with chloroform in glass tubes maintained in a water bath at 60 (C for 24 h followed by separation of the gel, drying at 60 (C for 2 h and then weighing the residue. 2.3. FTIR analysis The samples of SBC films were placed in a transmission cell fitted to a Nicolet 510 FTIR (Fourier Transform Infra-Red) spectrophotometer (DTGS detector) with air purge. Spectra were made up of 50 scans with a resolution of 2 cmÿ1. Samples were also measured by total internal reflection using an ATR accessory.
Fig. 1. Carbonyl index growth versus thermal ageing time for SBC at (B) 90 and (C) 110 (C.
2.4. Luminescence analysis Luminescence analysis was undertaken using a PerkineElmer Model LS-50B research spectrometer. Fluorescence spectra were obtained at ambient conditions on thin films using a front face accessory, while phosphorescence spectra were obtained at 77 K on thin strips of film placed in quart tube cells.
2.5. Yellowness index Colour formation was obtained using yellowness index (ASTM 313) via a Gretag Spectral Eye Colour measurement instrument (Colour data Systems Ltd., Wirral, UK). 2.6. Photooxidation
2.4.1. Microstructure The styrene and the styrene block contents were determined by 1H Nuclear Magnetic Resonance (NMR) spectroscopy. 1H NMR spectra were acquired at 300 MHz with a Bruker AC-300 Spectrometer in solution (1%) of Cl4C/CDCl3, 90/10. 2.4.2. Molecular weight distribution The molecular weights were determined by gel permeation chromatography (GPC) based on the calibration curve of polystyrene standards using a set of three ultrastyragel linear columns. The elution solvent was terahydrofuran (THF) at 35 (C. The flow rate was maintained at 1.5 ml minÿ1 and the sample concentration was 0.2%.
Irradiation studies were undertaken in a Microscal Unit (High pressure Hg/W 500 watt source) and Black Body Temperature of 50 (C (available from Microscal Ltd., London, UK). Samples were also irradiated using a 400 watt 254 low pressure mercury lamp (Applied Photophysics, London) as well as a 365 nm 8 watt fluorescent tube (10 nm band-width half peak height). Samples were rotated periodically for consistency in exposure. Samples for light exposure were 200 mm thick. 2.7. Hydroperoxide concentration Hydroperoxide concentrations were determined via the standard iodometric analysis method [6e8].
Table 1 Properties of SBC resin used Properties
SBC resin
1
H NMR Styrene, % (styrene block) 1,2 Butane, % 1,4 Butane, % GPC Mw Mn I.H. Antioxidants, HPLC Structure after coupling S: polystyrene block B: polybutadiene block
71.5 (100) 3.3 25.2 152,139 87,187 1.74 None S1-S2-B1-S3-B2-Li1 C S2-B1-S3-B2-Li2 C S3-B2-Li3
Fig. 2. % Crosslinking versus oven ageing time for SBC at (C) 90 and (B) 110 (C.
14
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Fig. 3. Yellowness index versus thermal ageing time for SBC at (C) 90 and (B) 110 (C.
Fig. 6. Phosphorescence emission spectra (EX = 250 nm) of unstabilised SBC before and after different periods of irradiation at 365 nm.
3. Results and discussion
Fig. 4. Hydroperoxide concentration in ppm versus thermal oxidation time for SBC at (C) 90 and (B) 110 (C.
Fig. 5. Relative excimer emission intensity versus thermal ageing time for SBC at (C) 90 and (B) 110 (C.
Styreneebutadiene copolymer is a complex system being dependent on the nature of the starting materials, their composition and the manufacturing process [1]. The properties of the SBC sample utilised in this study is shown in Table 1 with the respective molecular weight and structure. No stabilisers were utilised in this work in order to underpin the nature of the mechanistic processes involved.
Fig. 7. Carbonyl index growth versus photoageing time for SBC with different light sources.
15
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Fig. 8. % Crosslinking versus 254 nm irradiation time for SBC.
Fig. 11. Relative excimer emission intensity versus photoageing time for SBC with different light sources.
3.1. Thermal oxidation The rate of carbonyl growth [7] in the SBC polymer used here is shown in Fig. 1 at 90 and 110 (C. Here the rate is seen to be autocatalytic at 110 (C whereas at 90 (C the rate is autoretarding with a long induction period. In our earlier work on SBC crosslinking was Table 2 FTIR bands assigned to thermal and photooxidised SBC-resins Wavenumber (cmÿ1) Group Thermal 3600e3200
Fig. 9. Yellowness index growth versus photoageing time for SBC with different light sources.
1775 1739 1735
1730 1715 1695 1685 1639 Photooxidation 3600e3200
1775a 1738b
1716 1700 1697c 1685d a b
Fig. 10. Hydroperoxide concentration versus photooxidation time (h) for SBC in the Microscal (C), 365 nm light (B) and 254 nm light (-).
c d
Hydroxyls (alcohols/peroxides/ hydroperoxides) Anhydride/g-lactone/peracids Aliphatic ester Aliphatic esters/carboxylic acids/benzoic acid (monomeric form)/d-lactone Aliphatic ester Carboxylic acids/aliphatic esters Aromatic ketones/ acetophenone groups a,b-Unsaturated carbonyls a,b-Unsaturated carbonyls Hydroxyls (alcohols/peroxides/ hydroperoxides) Anhydride/g-lactone/peracids Aliphatic esters/carboxylic acids/benzoic acid (monomeric form)/d-lactone Carboxylic acids/aliphatic esters Carboxylic acids/benzoic acid (monomeric form) Aromatic ketones/ acetophenone groups a,b-Unsaturated carbonyls ÿ1
For 254 nm: 1772 cm . For 254 nm: 1740 cmÿ1. For 254 nm: 1696 cmÿ1. For 254 nm: 1684 cmÿ1.
Remark OeH stretch (associated) C]O stretch C]O stretch C]O stretch
C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch OeH stretch (associated) C]O stretch C]O stretch
C]O stretch C]O stretch C]O stretch C]O stretch
16
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
found to be predominant during thermal ageing in the aliphatic phase. The data in Fig. 2 show the rates of crosslinking for SBC at 90 and 110 (C. Both exhibit a short induction period followed by a very rapid autocatalytic growth in crosslinking. Over a period of 100 h up to 50% by weight of gel was formed. As will be discussed later, unlike SBS, SBC exhibits crosslinking within the styrene as well as the butadiene phase. Discolouration is also a notable feature of the thermal ageing of the SBC and is consistent with the carbonyl and crosslinking growths (Fig. 3). At 90 (C there is a long induction period prior to the observed yellowing while at 110 (C the discolouration is strong following a short induction period. These trends are consistent with the rates of hydroperoxide formation (Fig. 4). At both temperatures there is a rapid autocatalytic growth in concentration followed by a rapid decline with 90 (C showing an initial induction period, higher concentration of species and longer time maximum. The interesting feature of the results is that at both temperatures there is a decrease in hydroperoxides with prolonged oxidation below the initial value confirming the high instability of the species formed. Luminescence ( fluorescence and phosphorescence) analysis provides more specific information on photoactive chromophores in the polymer either present initially or formed during the oxidation/degradation processes. Such species may also be consumed by the ensuing degradation reactions/products. These species are normally carbonylic in nature or vinyl groups or aromatic species. In the case of aliphatic species these can be ketonic or aldehydic in nature either alone or
2.0
coupled with vinyl groups. Carboxylic acids and ester groups, hydroperoxides and peresters/acids are nonluminescent following excitation by light energy. Such species have higher energy excitation bands (below 250 nm) and tend to dissipate their absorbed energy through chain scission reactions rather than emit light. Fluorescence analysis of the SBC material as film using the reflective mode shows a strong emission band centred at 335e340 nm, which is associated primarily with the presence of excimer sites in the polystyrene phase. This is due to the association of a ground-state styrene unit with an excited unit and emits to the red of the styrene monomer emission [8]. The distinct observation of excimer emission is supportive of the presence of discrete polystyrene phases and is unaffected by the ‘‘olefin’’ phase. Upon degradation, however, there is a rapid loss and reduction in the excimer sites due to chain scission and disaggregation of the styrene units (Fig. 5). This process occurs rapidly during the early stages of the degradation process at 110 (C. As found previously for SBC [6] magnification of the fluorescence spectra with degradation also showed the initial formation of a longer wavelength emitting species i.e. a broad spectral shift to 350e380 nm. Under light exposure (later) the instability of such chromophores will not give rise to such an apparent spectrum as was observed thermally [6]. It is known that in the degradation of polystyrene, phenylevinyl units are formed similar to that of stilbene and that these can contribute to yellowing [8e10]. The absence of other stronger emitting chromophores, such as a,b-unsaturated carbonyls, suggests that if indeed they are formed then
CR-38A@365nm-84h
1.5
1601.08
Abs 1.0
1372.03
1181.36 1155.17
1372.03 1311.83
1181.42 1154.74
1068.59
968.94
540.54
0.5 2.0
CR-38A@110C-50h
1.5 1739.70 1696.57
Abs 1.0
1583.22
1069.38 972.44 908.71
767.43705.10
539.49
0.5 2.0
CR-38A@microscal-0h
1.5
Abs 1.0 0.5 2000
1500
cm-1
1000
500
Fig. 12. FTIR spectra 2000e500 cmÿ1 of SBC-resin (SBC) before and after ageing for 84 h at 365 nm and 50 h at 110 (C.
17
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
autocatalytic growth in carbonyl formation at similar rates. With the 365 nm source being very weak in energy (8 watts) this is surprising but nevertheless a clear indication of the development and importance of reactive hydroperoxides in the oxidation process. The destructive effect of the 254 nm light source is very apparent with a strong autoretarding effect on carbonyl growth. This is due to not only the photolysis and direct screening by the styrenic units due to the thickness of the films utilised in this work, but also the photolytic instability of any generated carbonyl groups. This effect was accompanied by some gel formation as indicated in Fig. 8. However, consistent with our work on SBC photooxidation [12] no evident crosslinking was found. The SBC was also found to discolour under irradiation with the 254 nm giving rise to the greatest effect (Fig. 9). Again, as for carbonyl index the 365 nm and Microscal irradiations were similar. For the films used in this work the irradiation wavelengths would be transmissive. However, the magnitude of discolouration was significantly less than that produced during thermal ageing. In part, this is due to many of the products being transient in nature under light exposure. Hydroperoxide formation under the three irradiation conditions is shown in Fig. 10. Here there is a gradual increase in concentration with the 365 nm light and Microscal conditions
they must be non-emissive (as in carboxylic acids). Low temperature phosphorescence analysis is also valuable for probing the formation or presence of carbonylic species either of an aromatic or aliphatic nature. The interesting feature of this analysis was the observation of the initial presence of acetophenone chromophores in the polymer (Fig. 6). It is to be noted that such chromophores were observed in SBC by FTIR (later). Thus, acetophenone groups are rapidly formed during oxidation in the polystyrene phase due to end-chain oxidation and scission as was reported earlier for SBC [6]. 3.2. Photooxidation Irradiation studies in this work were undertaken using polychromatic and monochromatic light sources. In the former case this would relate to sunlight exposure i.e. for wavelengths greater than 300 nm while 365 nm light would induce photolysis of hydroperoxide groups but not carbonyl species (unless heavily conjugated). Irradiation under 254 nm light is destructive to the styrene units as well as other potential light absorbing species such as carbonyl groups. Carbonyl index curves in Fig. 7 show an interesting comparison. All three light exposures display an initial induction period with both the Microscal and 365 nm sources then exhibiting an
2.0
1.8 1716.46 1.6
1601.08
1.4
1.2
1716
Abs
1738
1.0
1700 1697
0.8
1685
0.6 1775
1740 17351716
0.4
1700 1696 1684
1772 0.2
2000
1800
1600
cm-1 Fig. 13. FTIR spectra 2000e1500 cmÿ1 of SBC-resin (SBC) before and after photoageing.
18
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23 2.0 1.8
CR-38A@110C-50h
1.6
1372.03 1311.83
1.4 1739.70 1696.57
1.2
Abs 1.0
1181.42 1154.74
1028.93 1069.38 972.44908.71
767.43705.10
539.49
1583.22
0.8 0.6 0.4 0.2 2.0 CR-38A@110C-0 1.8 1.6 1.4 1.2
Abs 1.0 0.8 0.6 0.4 0.2 2000
1500
1000
cm-1 Fig. 14. FTIR spectra 2000e500 cmÿ1 of SBC-resin (SBC) before and after ageing for 50 h at 110 (C.
exhibiting similar rates with the latter at a somewhat higher level. As for the carbonyl data the 254 nm irradiation gave a small initial increase but thereafter autoretarded with a long plateau. Clearly under this exposure condition the hydroperoxides are photolysed rapidly as soon as they are formed. Disruption of the polystyrene aggregates also takes place under irradiation with the greatest effect being observed under 254 nm light (Fig. 11). Here the styrene absorbs the light directly. However, the interesting feature of this data is the destruction of the styrenic aggregates under 365 nm light. Here traces of any initial acetophenone groups could trigger reactions and disrupt the styrene units as discussed later. Also, on photooxidation acetophenone chromophores were formed rapidly after an initial 24 h period (Fig. 6). Phosphorescence emission spectra versus irradiation for the SBC are given in Fig. 6. Consistent with our earlier work on SBC their concentration then declines rapidly on further irradiation due to their transient nature and subsequent photoreactivity in the polymer. This effect was also seen under the Microscal irradiation. The species appeared to be destroyed under 254 nm light exposure. 3.3. FTIR analysis To determine the chemical changes induced in the SBC FTIR analysis was undertaken on oxidised films of the material by transmission. Table 2 summarises the main functional group frequencies observed in the SBC
for both thermal and photooxidised materials. Fig. 12 shows typical FTIR changes observed in the thermal and photoaged SBC materials in the region 2000e500 cmÿ1. These are typical spectra as shown for 365 nm irradiation and 110 (C thermal for 50 h. Strong Table 3 FTIR bands assigned to thermal and photogenerated soluble/gel fractions Wavenumber (cmÿ1) Group Thermal gel fraction 3600e3200 Hydroxyls (alcohols/ peroxides/hydroperoxides) 1775 Anhydride/g-lactone/peracids 1739 Aliphatic ester/benzoic acid (monomeric form)/d-lactone 1725 Aliphatic ketones 1715 Carboxylic acids 1695 Aromatic ketones/ acetophenone groups 1639 a,b-Unsaturated carbonyls 1620/1600 Conjugated vinyls Photo gel fraction (254 nm) 3600e3200 Hydroxyls (alcohols/ peroxides/hydroperoxides) 1775 Anhydride/g-lactone/peracids 1739/1730 Aliphatic esters/benzoic acid (monomeric form)/d-lactone 1716 Carboxylic acids/aliphatic esters 1695 Aromatic ketones/ acetophenone groups 1639 a,b-Unsaturated carbonyls
Remark OeH stretch (associated) C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch OeH stretch (associated) C]O stretch C]O stretch C]O stretch C]O stretch C]O stretch
19
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23 0.10
CR-38A@90C-120h(gel)
0.08
Abs
0.06
1725 1739 1715 1695 1639 1775
0.04 0.02 0.08
CR-38A@254nm-120h(gel)
0.06
Abs 0.04
1451.88 1492.57
0.02 0.00 0.08
965.80
1600.90
1028.09
756.46
668.46
CR-38A@110C-50h(gel)
0.06
Abs 0.04 0.02 0.00 2000
1500
1000
cm-1 Fig. 15. FTIR spectra 2000e500 cmÿ1 of SBC-resin (SBC) gel after ageing for 120 h at 254 nm, 50 h at 110 (C and 120 h at 90 (C.
carbonyl absorptions are observed in both cases with a maximum at 1716 cmÿ1 due to carboxylic acids plus weak shoulders at 1775 cmÿ1 due to anhydrides/ lactones, 1738 cmÿ1 due to esters and possibly benzoic acid, 1700 cmÿ1 due to carboxylic acids and aromatic ketones such as acetophenone at 1697 cmÿ1 [8,11,12]. Derivatisation with sodium hydroxide treatment (2 M) removed the band at 1716/1700 cmÿ1 confirming the presence of carboxylic acids. Absorption due to a,b-unsaturated carbonyl species is seen at 1685 cmÿ1. Aromatic carbonyls may also be absorbing at 1700 and 1685 cmÿ1. Above 1000 cmÿ1 other absorption changes were observed due to ether formation (eCeO) indicating the possible instability and transient nature of any formed vinyl groups except those associated with carbonyl groups albeit weakly. Below 1000 cmÿ1 was the notable disappearance of the main chain olefinic 1,4-trans-vinyl groups at 966 cmÿ1. These appear to be readily removed during both oxidation processes and have been reported on in earlier work on SB rubber [13,14]. Similar species were also formed during thermal oxidation but unlike photooxidation where the maximum peak was at 1716 cmÿ1 due to carboxylic acids thermal oxidation generated primarily ester groups. Expanded carbonyl spectra are shown in Fig. 13 for irradiated samples displaying weak shoulders for the other products whereas thermally oxidised SBC displays quite strong shoulders due to other products i.e.
aromatic carbonyls at 1695 cmÿ1. This is shown more clearly in the expanded spectrum in Fig. 14. Other potential absorbing species are also given in Table 2 and their mode of formation is discussed below. Here, strong hydroxyl absorption was observed in the SBC at 3430 cmÿ1 due to the formation of associated groups. The 254 nm irradiation gave weak absorptions as discussed above. Thus, oxidation appears to be consistent with rapid hydroperoxidation of the SBC, with the olefinic vinyl groups being one of the main points of attack Fig. 14. The FTIR absorption bands observed in the separated thermal and photoaged gel products are compared in Table 3 by transmission. Spectral examples are displayed in Fig. 15. As can be seen the 254 nm spectrum is very weak but upon expansion some products were identifiable and given in Table 3. For the thermally aged gel material the main absorption peak was found to be centred at 1725 cmÿ1 due to ketonic groups. Other product groups were similar to those found for the liquid fractions.
3.4. Mechanistic processes Schemes 1e4 depict the various functional group changes consistent with those seen in the FTIR spectra and phosphorescence analysis and are listed in Tables 2
20
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Scheme 1. Oxidation and removal of vinyl groups via hydroperoxides.
and 3. Firstly, the dominant formation of primary carboxylic acids throughout both the oxidation processes suggests the predominance of an end-chain oxidation step and this is consistent with the phosphorescence analysis data below. The only evident vinyl group formation in FTIR analysis was associated with carbonyl groups and stilbenic chromophores. The above analysis indicates that chain degradation, scission and oxidation are occurring primarily at two sites namely, the styreneebutadiene phase boundary and in-chain oxidation of the vinyl groups. This is confirmed by the rapid formation of end-chain carboxylic acids (FTIR at 1716 cmÿ1) and concurrent formation of acetophenone end groups as measured by phosphorescence analysis. The latter are observed in the FTIR during thermal oxidation and are transient during photooxidation due to their further photolysis. Scheme 1 depicts oxidation of the in-chian olefinic vinyl groups to give unsaturated hydroperoxides. These will break down further to yield a,b-unsaturated carbonyl groups and end-chain macroradicals [13,14]. The latter can oxidise further to form carboxylic acids via aldehydes and then peracids (1720 and 1775 cmÿ1) (Scheme 2). Lactones are known to be formed in polyolefin oxidation by an intra-molecular backbiting reaction (H-atom abstraction) by a carboxyl radical.
The esters, peresters and anhydrides are formed through appropriate radical recombination reactions and alcohols through hydrogen atom abstraction. Unsaturated carbonyl products (e.g. carboxylic acids) can be formed by further hydrogen atom abstraction reactions down the chain. Hydroperoxide build-up in the olefin phase is predominant as shown by the FTIR analysis and will be representative of the total amount of hydroperoxidation in the SBC and not just end-group oxidation. During thermal oxidation in-chain oxidation and crosslinking may also predominate via hydroperoxides and the formation of peroxy radicals to give crosslinked material (Scheme 3). Breakage of the olefinestyrene chain will result in the formation of benzyl radicals which can then be oxidised to form hydroperoxides (Scheme 4). These species, breakdown easily to give acetophenone end groups, which are shown by the phosphorescence emission. Phenyl-propene or stilbene groups can also be formed by further hydrogen atom transfer reactions resulting in the release of acetophenone as a product (Scheme 4). Subsequent photoreactions of the acetophenone end groups can give rise to the formation of benzaldehyde and benzoic acid products. One interesting observation in this work is the significance in the consumption of the excimer styrene units and the direct consumption of the acetophenone
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
21
Scheme 2. End-chain olefin oxidation via hydroperoxides.
units initially present and/or formed in the SBC. The latter species can be reactive as has already been shown in earlier work for aromatic ketones. These chromophores can react through a photochemically generated hydrogen atom abstraction process to give ketyl radicals (see Scheme 4 at end) and alkyl radicals. The rapid consumption of the excimer sites certainly indicates disruption of the styrenic conjugation along the chains into smaller fragments. The ring absorption peak at 756 cmÿ1 appeared to be unaffected by all oxidation processes in line with an earlier report [13]. In future analysis light stabiliser interactions will be of interest.
4. Conclusions This work shows a number of interesting features associated with SBC thermal and photooxidation/degradation processes. Thermal and photooxidation give rise to oxidation and chain scission at the boundary of the polystyreneeolefin phases in terms of the analysis of data presented here. This gives rise to the formation of acetophenone end groups on the styrene units and carboxylic acids on the olefin chain ends. Concurrent and further reactions give rise to the formation of ketones, anhydrides and peresters/acids in the longer term together with a,b-unsaturated carbonyl products,
22
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
Scheme 3. In-chain oxidation of olefin via hydroperoxides and crosslinking.
predominantly esters during thermal oxidation and carboxylic acids during photooxidation. It is understood in the context of the structure of SBC that tertiary groups in the butadiene units and olefinic vinyl groups would also be labile centres for hydroperoxidation.
There is no evidence for crosslinking in the SBC during long wavelength irradiation unlike that for thermal oxidation. Only high energy radiation in the far UV causes crosslinking and decomposes the hydroperoxide and carbonylic groups. The olefin phase exhibits severe
Scheme 4. Polystyrene oxidation via hydroperoxides and coloured products.
N.S. Allen et al. / Polymer Degradation and Stability 86 (2004) 11e23
oxidation associated with the initial formation of unstable primary hydroperoxide species while the polystyrene phase exhibits oxidation to form acetophenone end groups. The latter are transient and photounstable in the longer term causing further photoreactions.
Acknowledgements The authors thank Repsol-YPF, for material costs in support of one of them (CL).
References [1] Mistrali F, Proni A. Styrenic block copolymers. In: Whelan A, Lee KS, editors. Developments in rubber technology, vol. 3. London: Applied Science Publishers Ltd.; 1982. [2] Haws JR, Wright RF. Block copolymers. In: Walker BM, editor. Handbook of thermoplastic elastomers. New York: Van Nostrand Reinhold Co.; 1979.
23
[3] Gergen WP, Lutz RG, Davidson S. Hydrogenated block copolymers in IPNS. In: Golden G, editor. Thermoplastic elastomers. 2nd ed. Munich: Hanser Verlag; 1996. [4] Holden G. Applications of thermoplastic elastomers. In: Holden G, editor. Thermoplastic elastomers. 2nd ed. Munich: Hanser Verlag; 1996. [5] Kubo J, Onzuka H, Akiba M. Polym Deg Stabil 1994;45:27. [6] Allen NS, Edge M, Wilkinson A, Liauw CM, Mourelatou D, Barrio J, et al. Polym Deg Stabil 2001;7:113e22. [7] Allen NS, Edge M. Fundamentals of polymer degradation and stabilisation. Oxford: Elsevier Applied Science Publishers; 1992. [8] McKellar JF, Allen NS. Photochemistry of man-made polymers. London: Elsevier Applied Science Publishers; 1979. [9] Fox RB, Price TR, Cozzens RF. ACS Symp. Sers. No: 25. In: Labana SS, editor. UV light induced reactions in polymers. 1976, No. 17. 242 pp. [10] Basile LJ. J Chem Phys 1962;36:2204. [11] Allen NS, Parkinson A, Loffelman FF, Susi PV. Angew Makromol Chemie 1983;116:203e19. [12] Allen NS, Edge M, Wilkinson A, Liauw CM, Luengo C, Barrio J, et al. J Photochem Photobiol, Chem Ed 2004;162:41. [13] Adam C, Lacoste J, Lemaire J. Polym Deg Stabil 1989;26:269. [14] Ghaffar A, Scott A, Scott G. Eur Polym J 1977;13:83.