Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate)

Polymer Degradation and Stability 91 (2006) 60e68 www.elsevier.com/locate/polydegstab

Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate) K. Chrissafis a, K.M. Paraskevopoulos a, D.N. Bikiaris b,* a

Solid State Physics Section, Physics Department, Aristotle University of Thessaloniki, GR-541 24 Thessaloniki, Macedonia, Greece b Laboratory of Organic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-541 24 Thessaloniki, Macedonia, Greece Received 8 February 2005; received in revised form 21 April 2005; accepted 28 April 2005 Available online 27 June 2005

Abstract The preparation of the biodegradable aliphatic polyester poly(propylene succinate) (PPSu) using 1,3-propanediol and succinic acid is presented. Its synthesis was performed by two-stage melt polycondensation in a glass batch reactor. The polyester was characterized by gel permeation chromatography, 1H NMR spectroscopy and differential scanning calorimetry (DSC). It has a number average molecular weight 6880 g/mol, peak temperature of melting at 44
C for heating rate 20
C/min and glass transition temperature at 36
C. After melt quenching it can be made completely amorphous due to its low crystallization rate. According to thermogravimetric measurements, PPSu shows a very high thermal stability as its major decomposition rate is at 404
C (heating rate 10
C/min). This is very high compared with aliphatic polyesters and can be compared to the decomposition temperature of aromatic polyesters. TG and Differential TG (DTG) thermograms revealed that PPSu degradation takes place in two stages, the first being at low temperatures that corresponds to a very small mass loss of about 7%, the second at elevated temperatures being the main degradation stage. Both stages are attributed to different decomposition mechanisms as is verified from activation energy determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures is auto-catalysis with activation energy E Z 157 kJ/mol while the second mechanism is a first-order reaction with E Z 221 kJ/mol, as calculated by the fitting of experimental measurements. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Poly(propylene succinate); Aliphatic polyester; Biodegradable polymer; Thermal degradation; Thermogravimetry

1. Introduction Nowadays the development and application of biodegradable polymers instead of those traditionally used, has received a great attention from the viewpoint of environmental protection and resource recycle. There are three broad categories of biodegradable polymers: naturally occurring, like cellulose, starch, chitin etc., modified natural polymers, like cellulose acetate, chito-

* Corresponding author. Tel.: C30 2310 997812; fax: C30 2310 997769. E-mail address: [email protected] (D.N. Bikiaris). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.04.028

san, esterified starch etc., and those synthesized chemically or by micro-organisms. These polymers are degraded in the natural environment into small metabolite compounds by the enzymes that micro-organisms (bacteria and fungi) contain, as direct or indirect biological activity. In the final stage the water soluble oligomers are metabolised by intracellular enzymes into water and carbon dioxide under aerobic conditions, while energy is gained for micro-organisms. Among synthetic polymers, aliphatic polyesters have attracted considerable attention as they combine the features of biodegradability, biocompatibility and physical or chemical properties, comparable with some of the extensively used polymers like LDPE, PP, etc.

K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68

Aliphatic polyesters due to their favourable features of biodegradability and biocompatibility are becoming commercially available and increased progressively in number. Some of them are: polycaprolactone (PCL), poly(hydroxybutyrate) (PHB) and its copolymer with hydroxyvaleric acid poly(3-hydroxybutyrate-co-3hydroxyvalerate) (BiopolÒ) which can also be produced in vivo at very high molecular weights by microorganisms, poly(L-lactide), poly(butylene succinate) (PBSu), poly(butylene succinate-co-adipate) (BionolleÒ), EASTAR BIOÒ a copolyester based on 1,4-butanediol, adipic acid and terephthalic acid, EcoflexÒ, an other aliphatic aromatic copolyester from BASF, and poly(ester amide)s with trade name (BAK 1095Ò) [1e3]. These polyesters show a wide variety in their physical and mechanical properties being directly comparable with that of many traditional and nonbiodegradable polymers and especially the commodities like LDPE and PP. Biodegradable products that can be produced with the use of traditional techniques of polymer processing and technology are blown bottles, foam sheets, flat and split yarns, paper laminate, and injection-moulded products. Among them, films for packaging materials and agricultural uses have received much attention. Additionally, several polyesters derived from different lactides and lactones can be used as drug carriers for controlled release devices and for biomedical applications, their advantage being their biocompatibility and higher hydrolysability in the human body [4]. In the present study a biodegradable polyester poly(propylene succinate) (PPSu) was synthesized and characterized. The difficulties in its preparation until now were attributed to the fact that 1,3-propanediol was not available in the market in sufficient quantity and purity, a few years ago. However, in the last years more attractive processes have been developed for its production such as selective hydration of acrolein, followed by catalytic hydrogenation of the intermediate 3-hydroxypropionaldehyde [5,6] or hydroformylation of ethylene oxide [7]. Recently, Du Pont and Shell Chemical companies have developed 1,3-propanediol commercial products with low cost and high quality by using different methods. 1,3-PD is a valuable chemical intermediate that has found extended applications as a monomer for the production of polyesters, like poly(propylene terephthalate) (PPT) [8]. PPT can be used for the preparation of fibres that have high performances, desired mechanical and hydrolytic properties and high thermal stability. 1,3-PD is considered to be one of the bulk chemicals that will be produced in large scale in the next few years. PPSu that was prepared and studied in the present work beyond its biodegradability presents the additional advantage that it can be produced from oligomers derived from renewable sources. The production of chemicals and fuels by using alternative sources instead

61

of hydrocarbons (oil), has received great attention as a green feed stock manufacture. 1,3-Propanediol and succinic acid that were used in the present study as monomers for the preparation of biodegradable polyester PPSu are two of the many chemicals produced by using green techniques. Until now 1,3-PD has been manufactured mainly by chemical synthesis, which requires high temperature, high pressure and expensive catalysts. Last decade much effort has been paid to its production by bioprocesses on large scales. Many micro-organisms like Klebsiella, Citrobacters, Enterobacter, Lactobacillus and Clostridia are able to convert glycerol or glucose to 1,3-PD via fermentation processes [9e12]. Experimental investigations showed that the fermentation is a complex bioprocess taking place mostly in two stages while microbial growth is subjected to multiple inhibitions of substrate and by-products. On the other side, succinic acid is a dicarboxylic acid used as monomer for the preparation of aliphatic and fully biodegradable polyesters. It can be produced petrochemically from butane and also by anaerobic fermentation, requiring glucose and CO2, decreasing the pollution caused from petrochemical process. Glycerol and wood hydrolysates can also be used as carbon sources for succinic acid production, during fermentation by Mannheimia succiniciproducens bacterium [13,14]. From our previous study it was verified that PPSu has higher biodegradation rate compared with the other two familiar polyesters poly(ethylene succinate) and poly(butylene succinate) [15]. This is attributed to the lower degree of crystallinity that PPSu has compared with the other two polyesters. However, there is no information in the literature about its thermal stability at elevated temperatures. The aim of the present work is to follow the thermal degradation of PPSu and to study its kinetics during heating in an inert atmosphere, which is important in order to establish its thermal stability during processing. 1.1. Kinetic methods Kinetic information can be extracted from dynamic experiments by means of various methods. All kinetic studies assume that the isothermal rate of conversion da/dt, is a linear function of the temperature-dependent rate constant, k(T ), and a temperature-independent function of the conversion, f(a), that is: da ZkðTÞfðaÞ dt

ð1Þ

a being the fractional extent of reaction. In Eq. (1) f(a) depends on the particular decomposition mechanism. According to Arrhenius equation: kðTÞZA eE=RT

ð2Þ

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K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68

where A is the pre-exponential factor, that is assumed to be independent of temperature, E is the activation energy, T the absolute temperature, and R is the gas constant. Combining Eqs. (1) and (2) we have
 da E ZA exp  fðaÞ ð3Þ dt RT For non-isothermal measurements at constant heating rate b Z dT/dt, Eq. (3) transforms to
 da E b ZA exp  fðaÞ ð4Þ dT RT Activation energy E can be calculated by various methods. The first one is based on Kissinger’s method [16,17] that has been used in the literature in order to determine activation energy from plots of the logarithm of the heating rate versus the inverse of the temperature at the maximum reaction rate in constant heating rate experiments. The activation energy can be determined by Kissinger method without a precise knowledge of the reaction mechanism, using the following equation: ! b E ln Cconst ð5Þ Z Tp2 RTp where b is the heating rate, Tp is the temperature corresponding to the inflection point of the thermal degradation curves which correspond to the maximum reaction rate, and R is the gas constant. The second method, the isoconversional method of Ozawa, Flynn and Wall (OFW) [18,19] is, in fact, a ‘‘model-free’’ method which assumes that the conversion function f(a) does not change with the alteration of the heating rate for all values of a. It involves the measuring of the temperatures corresponding to fixed values of a from experiments at different heating rates b. Therefore, plotting ln(b) against 1/T according to Eq. (4) in the form of   AfðaÞ E lnðbÞZln ð6Þ  da=dt RT should give straight lines the slope of which is directly proportional to the activation energy (E/R). If the determined activation energy is the same for the various values of a, the existence of a single-step reaction can be concluded with certainty. On the contrary, a change in E with increasing degree of conversion is an indication of a complex reaction mechanism that invalidates the separation of variables involved in the OFW analysis [20]. These complications are serious, especially in the case where the total reaction involves competitive reaction mechanisms. The third method is also an isoconversional one. Based on Eq. (1) and the Arrhenius Eq. (2), Friedman [21,22] proposed to apply the logarithm of the con-

version rate da/dt as a function of the reciprocal temperature, from Eq. (4) in the form of

 da A E ð7Þ ln Zln Clnð fðaÞÞ  dT b RT It is obvious from Eq. (7) that if the function f(a) is constant for a particular value of a, then the sum ln( f(a)) C ln A/b is also constant. By plotting ln(da/ dT )i against 1/Ti, the value of the E/R for a given value of a can be directly obtained. Using this equation, it is possible to obtain values for E over a wide range of conversions.

2. Experimental 2.1. Materials Succinic acid (purum 99C%) was purchased from Aldrich Chemical Co. 1,3-Propanediol (CAS Number: 504-63-2, purity: O99.7%) was kindly supplied by Du Pont de Nemours Co. Tetrabutyl titanate (TBT), used as catalyst, was of analytical grade and it was purchased from Aldrich Chemical Co. Polyphosphoric acid (PPA) used as heat stabilizer was supplied from Fluka. All other materials and solvents used for the analytical methods were of analytical grade. 2.2. Synthesis of polyesters Synthesis of aliphatic polyester PPSu was performed following the two-stage melt polycondensation method (esterification and polycondensation) in a glass batch reactor [8,23]. In brief, the proper amount of succinic acid and 1,3-PD in a molar ratio 1/1.1 and the catalyst TBT (3 ! 104 mol TBT/mol SA) were charged into the reaction tube of the polycondensation apparatus. The apparatus with the reagents was evacuated several times and filled with argon in order to remove oxygen. The reaction mixture was heated at 190
C under argon atmosphere and stirring at a constant speed (500 rpm) was applied. This first step (esterification) is considered to be complete after the collection of theoretical amount of H2O, which was removed from the reaction mixture by distillation and collected in a graduate cylinder. In the second step of polycondensation, PPA was added (5 ! 104 mol PPA/mol SA), in order to prevent side reactions such as etherification and thermal decomposition. A vacuum (5.0 Pa) was applied slowly over a period time of about 30 min, to avoid excessive foaming and to minimise oligomer sublimation, which is a potential problem during the melt polycondensation. The temperature was slowly increased to 230
C, while stirring speed was also increased to 720 rpm. The polycondensation continued for about 60 min. After the end

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K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68

of the polycondensation reaction, the polyester was easily removed, milled and washed with methanol. 2.3. Polymer characterization 2.3.1. Gel permeation chromatography (GPC) GPC analysis was performed using a Waters 150
C GPC equipped with differential refractometer as detector and three ultrastyragel (103, 104, 105 A˚) columns in series. CHCl3 was used as the eluent (1 ml/min) and the measurements were performed at 35
C. Calibration was performed using polystyrene standards with a narrow molecular weight distribution. 2.3.2. Nuclear magnetic resonance (NMR) 1 H NMR spectra of polyesters were obtained with a Bruker spectrometer operating at a frequency of 400 MHz for protons. Deuterated chloroform (CDCl3) was used as solvent in order to prepare solutions of 5% w/v. The number of scans was 10 and the sweep width was 6 kHz. 2.3.3. Thermal analysis A PerkineElmer, Pyris 1 differential scanning calorimeter (DSC), calibrated with indium and zinc standards, was used. A sample of about 10 mg was used for each test, placed in an aluminium pan and heated to 25
C above the melting point of PPSu at a heating rate 20
C/min. The sample remained at that temperature for 5 min in order to erase any thermal history. After that it was quenched into liquid nitrogen and scanned again using the same heating rate as before. The glass transition temperature (Tg), the melting temperature (Tm) and the heat of fusion (DHm) were measured. Thermogravimetric analysis was carried out with a SETARAM SETSYS TG-DTA 1750
C. Samples (11 G 0.5 mg) were placed in alumina crucibles. An empty alumina crucible was used as reference. Samples were heated from ambient temperature to 500
C in a 50 ml/min flow of N2. Heating rates of 5, 10, 16 and 22
C/min were used and continuous records of sample temperature, sample weight, its first derivative and heat flow were taken.

3. Results and discussion 3.1. Synthesis and characterization of poly(propylene succinate) The polymerisation process for poly(propylene succinate) preparation involves two steps according to the well-known process used for polyester synthesis [23]. In the first stage, called esterification, succinic acid reacts with 1,3-propanediol and water is eliminated as byproduct. The reaction takes place at elevated tempera-

ture (190
C), so the formed water can be easily removed from the reactor and oligomers are prepared. In order to increase the molecular weight in the second, polycondensation stage, the prepared oligomers are condensed at high temperature (230
C) with the application of high vacuum. The reactions that take place during these stages and the procedure that was used for the synthesis of poly(propylene succinate) are presented in Scheme 1. The prepared polyester has a yellowish-brown colour maybe due to the use of tetrabutyl titanate as catalyst and is very soft because of its low melting point. It has an average number molecular weight 6880 g/mol as found by GPC and average weight molecular weight 17 900 g/mol (Fig. 1). The 1H NMR spectrum of PPSu has the characteristic single peak at 2.63 ppm attributed to methylene proton a of succinic acid, a triple peak 4.09e4.21 ppm attributed to c protons and a multiple peak between 1.9e2.02 ppm corresponding to d protons (Fig. 2). Furthermore, some other peaks but in lower intensity can also be found, which is an indication of oligomers. This can be attributed to the low molecular weight of the prepared polyester. c

d

c

a

a

O-CH2-CH2-CH2-O-C-CH2-CH2-C O

O

n

DSC thermogram for PPSu as received from the glass reactor has shown a peak temperature of melting close to 44
C for heating rate 20
C/min. Compared with the other two familiar polyesters poly(ethylene succinate) and poly(butylene succinate) it can be characterized that it is very small since they have melting temperatures at 104
C and 112
C, respectively. After quenching from its melt PPSu can be made completely amorphous. Besides, the significant portion of amorphous material was also proved by the Cp increase observed in the glass transition which is at 36
C. Furthermore, as shown in Fig. 3 even during a second heating PPSu cannot be crystallized and due to its slow crystallization rate remains amorphous. No melting peak can be recorded during this second run. However, by heating the quenched sample at 5
C/min the recorded thermogram is completely different. Beyond the Tg that is recorded at the same temperature as before, a cold crystallization (Tcc) is observed with a maximum at 21.4
C and a melting peak at 43
C. 3.2. Thermogravimetric analysis Thermal degradation of PPSu was studied by determining its mass loss during heating. In Fig. 4 are

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K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68

1st Step: Esterification n HOOC - CH2 - CH2- COOH + 190 °C Ti(O4H9)4

Oligomers

a

n HO -CH2-CH2-CH2 - OH

- H2O c

d

O-CH2CH2CH2-O-C-CH2CH2-C-O O

O

n

2nd Step: Polycondensation 230 °C Ti(O4H9)4 low vacuum

- H2O - HO-CH2CH2CH2- OH 4

O-CH2CH2CH2-O-C-CH2CH2-C-O O

O

2

0

Chemical shift (ppm) Fig. 2. 1H NMR spectra of PPSu.

n

poly (propylene succinate) Scheme 1. Procedures and chemical reactions during preparation of succinate polyesters.

presented the mass loss (TG%) and the derivative mass loss (DTG) curves of PPSu at heating rate 10
C/min. From the thermogravimetric curve it can be seen that PPSu shows a relatively good thermal stability since no significant weight loss (only1.2%) occurred until 300
C. As can be seen in the curve of DTG, in the early stages of the decomposition, there is a small shoulder peak probably due to a slight difference in the slope of decomposition curve of TG. Until 366
C, where the maximum rate of this decomposition step appears, the volatile matter corresponds to about 7 wt% of initial

weight. Such a pre-major weight-loss stage was also mentioned in poly(propylene terephthalate) (PPT) with low number average molecular weights ranged between 13 000 and 23 000 g/mol, where the first decomposition step that corresponded to small weight loss 2e4% of PPT was sensitive to molecular weight and was attributed to the volatilisation of small molecules, residual catalysts, 1,3-propanediol and carbon dioxide that evolved from chain ends. [24,25]. Thus, the temperature at the maximum weight-loss rate of this stage increases significantly with molecular weight while the weight loss decreases steadily. Taking this finding into consideration this decomposition step is attributed to degradation and volatilisation of the oligomers

Tm=44°C

First run

Endothermic

Mw

W

Mn

a Tg=-36°C

Tm=43°C b

Heating rate 20°C/min

Tg=-36°C c Mz

Heating rate 5°C/min Tc=21.4°C

-50 0

20000

40000

60000

80000

100000

120000

Molecular Weight (g/mol) Fig. 1. Molecular weight distribution of PPSu measured by GPC analysis.

0

50

Temperature (°C) Fig. 3. DSC thermograms of (a) PPSu as received from glass reactor, (b) after quenching in liquid nitrogen and heated at 20
C/min, and (c) after quenching in liquid nitrogen and heated at 5
C/min.

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K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68

DTG

60 40

TG%

80

20 0 250

300

350

400

450

500

T (°C) Fig. 4. Thermal degradation of PPSu, mass loss (TG%) and derivative mass loss (DTG) versus temperature with heating rate b Z 10
C/min.

detected with 1H NMR spectroscopy. From preliminary studies of the monomers it was found that succinic acid degrades at temperatures close to 200
C and 1,3propanediol at slight higher temperatures. However, both are fully decomposed at 300
C. These data strengthen our hypothesis that the first decomposition step is due to oligomer degradation. Furthermore, from kinetic studies of poly(3-caprolactone) (PCL) which also degrades in two steps, it was verified with TGA/FTIR and mass spectroscopy that water, carbon dioxide and hexanoic acid are the volatile products of the first decomposition step taking place at lower temperatures [26]. After that temperature the polyester decomposes quickly and loses almost its whole weight (about 99 wt%) at 460
C. No char residue was left. As can be seen from the peak of the first derivative, the temperature at which PPSu decomposition gains the highest rate is at 404
C for heating rate 10
C/min. This temperature is very high taking into account that it is an aliphatic polyester and the temperature is comparable to decomposition temperatures reported for aromatic polyesters of terephthalic (PET, PBT, PPT) and naphthalic acid like PEN [27,28]. Furthermore, this temperature is much higher compared to other aliphatic polyesters like poly(L-lactide) in which major degradation step appears at temperatures lower than 370
C, as well as from poly[(R)-3-hydroxybutyrate] and poly(3-caprolactone) [29e31] From the above it can be concluded that PPSu although it has low melting point presents a very high thermal stability. In order to be analysed more deeply the degradation mechanism of PPSu, it is important that kinetic parameters (activation energy E and pre-exponential factor A) and conversion function f(a) are to be evaluated. The relationship between kinetic parameters and conversion (a) can be found by using the mass loss curves recorded in TG dynamic thermograms. The thermogravimetric curves of PPSu heated in N2 at different heating rates are shown in Figs. 5 and 6 from

temperature above the melting point till 500
C. It is clear from the DTG plots that the peak temperature, Tp, shifts to higher values with increasing heating rate. An increase of 26
C in the initial thermal decomposition temperature is measured. At the same time the mass loss remains stable (98.9% G 0.1%) for all the different heating rates. The activation energy of degradation of the studied polyester was estimated using Ozawa, Flynn and Wall (OFW), Friedman and Kissinger’s methods. Firstly, the isoconversional Ozawa method was used to calculate the activation energy for different conversion values by fitting the plots of log b versus 1/T. Some of the Ozawa plots are shown in Fig. 7 and all data are summarized in Table 1. Fig. 7 were presented the fitting the data straight lines, with correlation coefficient greater than 0.999. These lines are nearly parallel (0.1 ! a ! 0.95), thus indicating the applicability of Ozawa method to our system in the conversion range presented [18]. Secondly, Friedman method was used by plotting ln(da/dT ) against 1/T for a constant a value and calculated the activation energy. The results are shown also in Table 1. Comparing the results of the application of the two methods, we observe that the values calculated by Friedman method are slightly higher than those of Ozawa method. However, both methods reveal the same trend of activation energies for the whole conversion range studied. It is followed from Table 1 that the dependence of E on the a value can be separated in two main distinct regions, the first for values of a up to 0.1, in which E presents a monotonous increase and the second (0.1 ! a ! 0.95) in which E can be considered as having a constant average value. E does not significantly change for conversions between 0.1 and 0.95 (Table 1) indicating that the degradation of PPSu takes place through the cleavage of linkages with similar bond energies. The dependence of E on a is an indication of a complex reaction with the participation

100 80

TG%

100

4

60

1 2 3

40 20 0 250

300

350

400

450

500

T (°C) Fig. 5. TG curves at different heating rates b. (1) 5
C/min, (2) 10
C/ min, (3) 16
C/min, (4) 22
C/min.

66

K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68 Table 1 Activation energies of PPSu using Ozawa and Friedman methods 1

Conversion a

Activation energy Ozawa method (kJ/mol)

Activation energy Friedman method (kJ/mol)

0.02 0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95 0.98

139 G 12.9 178 G 4.5 199 G 2.5 209 G 6.6 213 G 7.4 216 G 6.3 218 G 4.4 221 G 2.5 221 G 1.2 119 G 1.1 219 G 1.4 222 G 2.0 234 G 6.7

157 G 8.9 190 G 5.1 209 G 9.5 215 G 9.4 219 G 6.4 227 G 14.3 221 G 8.4 230 G 2.5 219 G 2.5 228 G 0.6 239 G 3.6 262 G 5.6 247 G 17.7

DTG

2

3

4

300

250

350

450

400

500

T (°C) Fig. 6. DTG curves at different heating rates b. (1) 5
C/min, (2) 10
C/min, (3) 16
C/min, (4) 22
C/min.

of at least two different mechanisms, from which one has quite small effect in mass loss. This conclusion is associated with the already mentioned beginning observation of overlapped peak in DTG diagram with a very small contribution in mass loss. The first mechanism corresponds to the part where small loss appears while the second part, where the substantial mass loss takes place, is attributed to the main decomposition mechanism, each one mechanism presenting different activation energy. This in accordance with other studies in biodegradable polyesters such as PCL, in which two mechanisms were recorded with different activation energies for each other [29,32,33]. For comparative purpose, we calculated the activation energy by Kissinger method. The value of E (204 G 1.9 kJ/mol) was obtained from the slope of ln(b/Tp2) versus 1/Tp plot with correlation coefficient greater than 0.9999, and is given in Fig. 8. This value is lower than the one calculated with the other two methods, Ozawa and Friedman. However, it is almost

identical with the value calculated for other aliphatic polyester PCL (203 kJ/mol) [34]. In order to determine the nature of these two mechanisms through the comparison of the experimental and theoretical data, initially it is considered that the degradation of PPSu can be described only by a single mechanism that corresponds to the main mass loss, without presuming the exact mechanism. Then, knowing this mechanism ( f(a)) and the values of E and A, are determined the parameters of the other mechanism corresponding to the small mass loss, aiming to achieve the better possible agreement between experimental and theoretical data. To determine the conversion function f(a) we used a method referred to as the ‘‘model fitting method’’ [35,36]. This method that does not assume the knowledge of E and f(a) in advance, was applied simultaneously on the experimental data taken at the heating rates b Z 5, 10, 16 and 22
C/min. For the fitting were used 16 different kinetic models. In Fig. 9 can be seen the results of this fitting. The form of the conversion function, obtained by fitting is

1.4 -14 1.2

2

3

4

5

-14.4

1

2

ln( /T )

log

1

-14.8

0.8 -15.2 0.6 1.35

1.4

1.45

1.5

1.55

1.6

-1

1000/T (K )

-15.6 1.44

1.46

1.48

1.5 -1

Fig. 7. Ozawa plots at fractional extent of reaction (1) a Z 0.95, (2) a Z 0.8, (3) a Z 0.5, (4) a Z 0.3, (5) a Z 0.1.

1000/T (K ) Fig. 8. Kissinger plot of PPSu.

1.52

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K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68

4. Conclusions

TG%

80 60

3 1 2

40

4

20 0 250

300

350

400

450

500

T (°C) Fig. 9. Fitting and experimental data TG curves for all the different heating rates b for one reaction mechanism. (1) 5
C/min, (2) 10
C/ min, (3) 16
C/min, (4) 22
C/min.

f(a) Z (1  a)n, with exponent value n Z 0.993, activation energy E Z 221 kJ/mol and pre-exponential factor log A (s1) Z 15. The correlation coefficient was 0.99972. Comparing these result the nature of mechanism can be evaluated and it can be defined as first class mechanism, which coincides with the mechanism that is generally used for the description of mass loss in polyesters [36,37]. The value of the activation energy is between the limits of the calculated values from the Ozawa and Friedman methods and it is higher than the one calculated with Kissinger’s method. As it can be seen the fitting to the experimental data is very good for all the range of 0.07 ! a ! 0.95. A slight deviation appeared mainly in the starting temperature area, where we have noticed the small overlapped peak in the DTG curve. When the value of the activation energy that is used for fitting is the one calculated from Kissinger’s method, the results are not better compared with the above ones. For the determination of the first stage mechanism we assume the followings: (a) the two mechanisms follow each other, (b) this mechanism which we try to identify corresponds to mass loss of order 7%, according to the experimental results, (c) this mechanism according to the literature is mechanism of nth-order auto-catalysis that is described by equation f(a) Z f(a) Z (1  a)nam. Taking into account the increase of E in the region of values 0 ! a ! 0.1, it is considered that E has values between 120e190 kJ/mol. The agreement of experimental and theoretical results (Fig. 10), leads to a small further improvement of fitting (correlation factor 0.99982) that is remarkable mainly in the first stages of mass loss. From the new fitting, taking into account and the additional mechanism, we have for this mechanism activation energy E Z 157 kJ/mol, preexponential factor log A (s1) Z 10.7, and factors n Z 0.73 and m Z 1.4 ! 104.

PPSu prepared by melt polycondensation has number average molecular weight 6880 g/mol and shows melting at 44
C, glass transition at 36
C and cold crystallization at 21.4
C. The mass loss of PPSu is accomplished in two stages, from which the small one corresponding to a mass loss of 7%, is distinguishable only in DTG diagrams, with overlapping contributions. The two stages are connected with different degradation mechanisms that take place during PPSu thermal decomposition at different temperatures. The activation energies for all values of a, were determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. Comparing the dependence of activation energy on the a value, two regions of E values were identified. The first for a ! 0.1, where the value of E is increased monotonically, and a second one for a O 0.1 with the value of E almost constant. This constitutes a clear indication that the kinetic description of mass loss can be accomplished through two different mechanisms. This conclusion is consistent with the appearance of a small overlapped peak (shoulder) in DTG diagrams. From kinetic study it was concluded that the main mechanism of mass loss can be better described by the equation f(a) Z (1  a)n, with activation energy E Z 221 kJ/mol, pre-exponential factor log A (s1) Z 15 and exponent value n Z 0.99, being a firstorder reaction. The additional mechanism that takes place at lower temperatures and is related to the much smaller mass loss, is nth-order auto-catalysis, described by the equation f(a) Z (1  a)nam with the values, activation energy E Z 157 kJ/mol, pre-exponential factor log A (s1) Z 10.7, and factors n Z 0. 73 and m Z 1.4 ! 104.

100 80

TG%

100

60 4

40

1

2 3

20 0 250

300

350

400

450

500

o

T ( C) Fig. 10. Fitting and experimental data TG curves for all the different heating rates b for two reaction mechanisms. (1) 5
C/min, (2) 10
C/ min, (3) 16
C/min, (4) 22
C/min.

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K. Chrissafis et al. / Polymer Degradation and Stability 91 (2006) 60e68

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