The isothermal degradation of some polyetherketones: a comparative kinetic study between long-term and short-term experiments
Introduction
In recent years the interest of the aerospace industry has been devoted to the development of low density materials to be used in structural applications and, in particular, of high performance composites with polymer matrix. Themosetting matrices are widely used for this purpose, but their brittle nature to impact-initiated failure and the high production costs of their composites limit their use and drive the industries to examination of thermoplastics. Since these polymers can be subjected to high temperature not only in manufacturing, compounding and processing stages, but also in service or during repair, an important aspect of the development of modern engineering plastics is directed toward producing thermally stable thermoplastics keeping good physical and mechanical properties as well as good resistance to oxidative and chemical breakdown at high temperature. Since the thermal stability of a polymer is connected with the initial degradation temperature and the rate of degradation, the study of the kinetics of degradation and the determination of the associated kinetic parameters, particularly the apparent activation energy values, appear to be an interesting topic of research in this field.
We have in progress a comparative kinetic study on the degradation, in various environments and experimental conditions, of aromatic thermoplastic polyethers containing ketone and sulfone groups [1], [2], [3], [4], [5], [6], which are economically accessible polymers showing favourable properties for use as matrices for high performance composite materials, because of the presence of fully oxidized and flexibilizing ether and carbonyl groups [7], [8], [9], [10], [11].
Short-term isothermal experiments are usually performed to determine kinetic parameters associated with the thermal degradation of polymers, but the apparent activation energy values and the corresponding degradation curves thus determined might not be suitable to represent the behaviour of polymers in service because they are usually obtained at temperatures near (or, in some cases, above) the melting temperatures.
In this work we made a long-term isothermal degradation study on the following thermoplastic polyetherketones having very similar structures(where: Ar=1,4-substituted phenylene).
The experiments were carried out in static air atmosphere for more than 3 years, at a temperature (270 °C) about 150 °C lower than the melting temperatures of the polymers. The results obtained were compared with those from a set of short-term isothermal measurements in a range (350–530 °C) more extended than that (470–530 °C) we used in the past for analogous measurements on the same polymers in the molten state [2]. In such a way we could consider also temperatures lower than the melting temperatures.
The purposes of this work were the following:
- 1.
to check if the apparent activation energy of degradation (Ea) is the same near the melting temperature and at largely lower temperatures;
- 2.
to find the V=f(t) equations at the various temperatures used for isothermal experiments;
- 3.
to verify if it is possible to predict similar equations at lower temperatures by experiments carried out at higher temperatures;
- 4.
to have information about the time needed to reach appreciable weight loss, to explore the possibility of performing degradation experiments below the glass transition temperature in reasonable times.
Section snippets
Materials
Polymers were synthesized according to the procedure previously reported [12] and were used in powder form. Polymers were dried under vacuum at room temperature and kept in a desiccator under vacuum until use.
Viscosity measurements
An Ostwald viscometer was used to determine the inherent viscosity (ηinh=lnηr/C) of polymers. Measurements were performed at 30 °C (±0.1 °C), in sulfuric acid solution at a concentration of 0.5 g/dl.
Calorimetric measurements
A Mettler DSC 20 calorimeter coupled with a Mettler TC 10A processor as control and
Results and discussion
The melting temperatures of the polymers were first determined by DSC. Multiple unresolvable melting endotherms were observed in all cases, thus suggesting the presence of more than one crystal forms in the “as made” polymers. In particular, two DSC peaks were found for polymers 1 and 3, and three DSC peaks for polymer 2. The peak temperatures were taken as melting temperatures due to the impossibility of determining the onset values. The percentages of crystallinity were calculated by
Acknowledgements
The authors are grateful to the Italian M.U.R.S.T. for financial support.
References (23)
- et al.
Polymer
(1999) - et al.
Polymer
(2000) - et al.
Polymer
(1992) - et al.
Polym. Degrad. Stab.
(1993) - et al.
Polymer
(1994) - et al.
Polym. Eng. Sci.
(1996) - et al.
Macromol. Chem. Phys.
(1997) - et al.
Polym. Eng. Sci.
(2000) - Abate L, Blanco I, Pappalardo A, Pollicino A. J Therm Anal Cal...
- Critchley JP, Knight GJ, Wright WW. Heat-resistant polymers. New York and London: Plenum;...
Phil. Trans. Roy. Soc. Lond.
Cited by (37)
Thermal behaviour of a series of novel aliphatic bridged polyhedral oligomeric silsesquioxanes (POSSs)/polystyrene (PS) nanocomposites: The influence of the bridge length on the resistance to thermal degradation
2014, Polymer Degradation and StabilityCitation Excerpt :Due to this reason the Ti values so obtained largely depend on the slope of the descending piece of TG curves. So we preferred to determine temperature at 5% mass loss which is correlated with initial decomposition temperature [43,44], but is, in our opinion, more reliable to compare the thermal stability of various compounds because it is not depending on the kinetics of the most advanced degradation stages. Also, this choice is in agreement with those of some our previous works, then allowing to us to compare the results and, then, to make a classification of the resistance to the thermal degradation.
The regression of isothermal thermogravimetric data to evaluate degradation E<inf>a</inf> values of polymers: A comparison with literature methods and an evaluation of lifetime prediction reliability
2011, Polymer Degradation and StabilityCitation Excerpt :The answer, in our opinion, is the following: because for these polymers only a single degradation stage occurs in the considered large temperature range, so the determined Ea is the same independently on the degree of degradation of polymer. In previous papers [17–19] we set up a simple isothermal method to obtain reliable degradation Ea values of polymers directly from experimental TG curves, without complicated calculations and without any assumption or restriction. Since in those studies we used polymers having very similar structures, in this work we have applied our method to determine the apparent degradation activation energy of some polymers (PE, PS, PC and PMMA) having various structures different from those of polyaromatics we used in the past, aiming to verify its general validity, independently on the nature of degraded polymer.
The influence of sulfonation degree on the thermal behaviour of sulfonated poly(arylene ethersulfone)s
2010, Polymer Degradation and Stability