Thermal degradation of poly(ethylene oxide–propylene oxide–ethylene oxide) triblock copolymer: comparative study by SEC/NMR, SEC/MALDI-TOF-MS and SPME/GC-MS
Introduction
Poloxamer materials are synthetic ABA type triblock copolymers of ethylene oxide and propylene oxide. A is made of hydrophilic poly(ethylene oxide) (PEO) chains and B of more hydrophobic poly(propylene oxide) (PPO) segments. Poloxamers were first synthesised in 1954 by Lundsted and Ile [1] when trying to develop new surface-active agents with new properties. There are several differences comparing poloxamers to classic surfactants. Firstly they exhibit a molar mass range 1000–15,000 whereas most other surfactant series have much lower mass. Then they have two hydrophilic groups, whereas most non-ionic surfactants have only one. The particular physical properties of poloxamers are used in pharmaceuticals, in drug delivery systems, and in other drug and medical studies as the following functions: emulsifying, thickening, coating, solubilising, dispersing and foaming.
Non-ionic surfactants composed of polyoxyalkene chains are very sensitive to autoxidation. The reaction occurs with the degradation of the hydrophilic chain resulting in the loss of tensile properties. Hydroperoxides and radicals formed in the autoxidation reaction are responsible for degradation and ageing in several kinds of commercial products in which the surfactants are added as minor components, for example in pharmaceutical or cosmetic products.
The mechanism of degradation of polyethylene oxide chain is quite similar to that of hydrocarbon chains, but the presence of oxygen in the molecules strongly activates the process by increasing the labile nature of protons on α-carbon atoms. Several autoxidative mechanisms of degradation of PEO (inert atmosphere) giving low molecular weight products have been presented. By pyrolysis Madorsky and Straus [2] observed the formation of oligomers of PEO as well as formaldehyde, ethanol, carbon dioxide and water. Grassie and Perdoma Mendoza [3] added methane, ethylene oxide and derivatives of acetaldehyde to this list. More recently attention was focused on the chain-ends of the polymer after pyrolysis [4], [5], [6]. It was concluded that at the lowest temperature (150°C) the dominant products result from the preferred cleavage of C–O bonds. At the highest temperature (550°C) C–C cleavage and dehydration become more favourable.
In the presence of oxygen degradation of the polymer occurs in a very different manner as hydroperoxides are more readily formed on α-carbon atoms of the PEO chains. Progress of reaction is accompanied by decomposition of the hydroperoxides and by random chain scission at C–O and C–C bonds in a ratio depending on the temperature of oxidation. At 50°C Morlat and Gardette [7] found no evidence of homolysis of C–O bonds. C–C cleavage is the main mechanism of degradation. The same thing was previously observed by Yang et al. [8] at 150°C. It is only for higher temperatures that C–O cleavages do occur significantly.
PPO is less thermally stable than PEO because formation of a radical on a tertiary carbon of PPO is more probable than on a secondary carbon of PEO. However when studying the thermoxidation of PPO, authors have been arguing whether the secondary alkoxy radicals play a major role or not on the degradation of PPO. By NMR spectroscopy, Griffith et al. [9] monitored the end groups of PPO degraded at 125°C. Starting from dihydroxyl terminated PPO, they found after oxidation, acetate and formate end groups in a ratio of 2:3, as well as ketone chain-ends. These chain-ends were explained by taking into account the breakdown of both tertiary and less probable secondary hydroperoxide in some extent. Kemp and co-authors [10] used MALDI-TOF MS to monitor the thermoxidation of PPO at 155°C. They found that the degradation pathways implied a major role of the secondary alkoxy radical. These results support previous studies by Lemaire and co-authors [11]. By NMR spectroscopy Yang et al. [8] followed the thermal degradation of both PEO and PPO at 150°C. For oxidised PEO, formate end groups appear through intramolecular decomposition and esterification of hydroxyl chain-ends. For PPO they did show the importance of esterification of hydroxyl chain-ends during the oxidation, and suggested the predominance of the tertiary alkoxy radical at the beginning of the polymer breakdown.
In order to solve the problem of the preferential degradation of PPO initiated on the secondary or the tertiary carbon of the polymer backbone we proposed to follow the thermoxidation of a poloxamer at a low temperature (80°C) slightly above the melting point of the material. This should also enable us to see what happens at the beginning of the degradation and confirm the preferential breakdown of the PPO block compare to the PEO block. To reach these goals we will have in a first step to study carefully the composition and structure of the commercial poloxamer utilised. One weakness of the previous studies was the high temperature usually chosen to oxidise the polymers (over 120°C) making difficult to observe the start of the degradation. Another weakness was the use of a single analytical technique at a time (usually NMR spectroscopy or MALDI-TOF mass spectrometry (MS)) to monitor the degradation. For this reason we compare in this paper results obtained by size exclusion chromatography (SEC), MALDI-TOF MS and NMR for the polymer matrix analysis, and solid phase microextraction–gas chromatography-mass spectrometry (SPME–GC-MS) for the volatile products of degradation.
Section snippets
Material
The material used in this study is commercially available from BASF, known also under the name poloxamer 407 and trademark Pluronic® F127. According to the manufacturer this polymer has a molecular weight ranging from 9840 to 14,600 g mol−1 and the following structure: H(O–CH2–CH2)a–(O–CH2–CH(CH3)–)b–(O–CH2–CH2)a–OH. At the maximum of the molecular weight distribution, a=101 and b=56.
Poloxamer 407 contains 100 ppm of the antioxidant butylated hydroxytoluene (BHT).
The melting point of the block
Characterisation of virgin copolymer
Poloxamer 407 is made by the addition of propylene oxide to a propylene glycol initiator to form a polyoxypropylene glycol having a molar mass of approximately 4000. This is performed at an elevated temperature and pressure in an anhydrous and inert atmosphere in the presence of an alkaline catalyst (in this case KOH). Once all propylene oxide has reacted, ethylene oxide is added in a controlled way to form two polyoxyethylene blocks. The product is then neutralised with an acid, typically
Conclusions
The combination of MALDI-TOF-MS for the analysis of oligomers, SPME/GC-MS for the analysis of low molecular weight compounds and 1H NMR for chain-end determination is very successful. The thermal oxidation of poloxamer 407 at 80°C in air proceeds in three steps: After an induction period depending on the quantity of antioxidant present in the polymer (21 days for 100 ppm BHT), the degradation starts through a six-ring intramolecular decomposition reaction of the PPO block of the copolymer. This
Acknowledgements
Financial support from ‘KTH prioriterad satsning’ for PhD position for G. Gallet is gratefully acknowledged. Professor G. Montaudo is acknowledged for discussions and suggestions. L. Schitter and G. Impallomeni are thanked for technical assistance and suggestions.
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