1
Thermal degradation of PZT/PVDF in function of ceramic grain size and the concentration.
Sérgio Firmino Mendes1, Carlos Miguel Costa1, Vitor Sencadas1, Mário Pereira1, Aiying Wu2, Paula Maria Vilarinho2, Rinaldo Gregorio Jr.3, Senentxu Lanceros-
Méndez1,4
(1) Center/Department of Physics, Universidade do Minho Campus de Gualtar, 4710- 057 Braga, Portugal
(2) Department of Ceramic and Glass Engineering, CICECO, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
(3) Department of Materials Engineering, Universidade Federal de São Carlos, C.P.
676, São Carlos, SP, CEP 13565-905, Brazil
(4) INL-International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal
*corresponding author: [email protected]
Abstract:
Poly(vinylidene fluoride)/Pb(Zr0.53Ti0.47)O3,([PVDF]1−x/[PZT]x) composites of volume fractions x and (0–3) type connectivity were prepared in the form of thin films. PZT powders with average grain sizes of 0.2, 0.84, and 2.35 μm in different volume fraction of PZT up to 40 % were mixed with the polymeric matrix. The influence of the inorganic particle size and its content on the thermal degradation properties of the composites was then investigated by means of thermo-gravimetric analysis. It is observed that filler size affects more than filler concentration the degradation temperature and activation energy of the polymer. In the same way and due to their larger specific area, smaller particles leave larger solid residuals after the polymer degradation. The polymer degradation mechanism is not significantly modified by the presence of the inorganic fillers. On the other hand, an inhibition effect occurs due to the presence of the fillers, affecting particularly the activation energy of the process
2 Keywords: Composite material, TGA, Thermal degradation kinetics, activation energy
1. Introduction:
Polymer composites are widely used in everyday applications such as construction, electronics, consumer products and even in transportation. The final properties of the polymer composite are strong influenced by the reinforced particles dimensions, specific surface area and microstructure of the dispersed phase. A growing interest in the developement of polymer-clay composites has been oberved recently [1, 2]. Due to the strong tht can possibly occur between polymer and ceramic materials, a widely variety of hibrids have been developed with enhanced thermal, mechanical, and excellent barrier properties when compared to the pure materials. However, most applications require prolongated service at high temperatures [3, 4].
Composites are materials that results from the combination of two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure. By combining materials with different properties is expected to obtain a composite material with enhanced properties when compared to any one of its constituents and for the purpose intended [5].
Newnham et. al. [6] were the first to designate the connectivity term, which indicates how the different phases of the composite materials are interconnected. The micro and nanocomposites with a dielectric ceramic phase embedded in a polymeric matrix have been studied over the years due to the interest in increase the dielectric permittivity of the composite by incorporate an high dielectric ceramic particle and thereby improving its electroactive properties [7-10].
Poly(vinylidene fluoride) (PVDF) is a widely used material in polymer industry for applications from pipes, water treatment, gas separation membrane, sensors and actuators, due to its outstanding thermal, mechanical and electrical properties, high aging and corrosion resistance [11, 12]. PVDF is also being widely investigated due to
3 its exceptional pyro- and piezoelectric properties among the polymeric systems. These properties are in the origin of various applications, especially in the field of sensor and actuator devices and technologies [11, 12]. PVDF have at least four polymorphs known as , , and -phase, but the crystalline phase with best ferroelectric and piezoelectric properties is the -PVDF. Commonly, -phase films are obtained by stretching -phase films at temperatures between 70 ºC than 100 ºC and for stretch ratios from 2 until 5 [13-15]. PVDF electroactive phase content and crystallininty degree are influenced by stretching ratio and temperature [13, 15], which in turn will affect the electroactive properties of the polymer.
Lead Zirconate Titanate (PZT) is a ceramic with material with chemica structure Pb(ZrxTi1-x)O3 and crystallizes in a perovskite structure [16]. The phase diagram is complex, but one of the most interesting issues is the existence of the morphotropic phase boundary (MPB) dividing the ferroelectric region in two parts: rhombohedral crystalline phase region, rich in Zr atoms and a tetragonal crystalline phase region rich in Ti atoms. At room temperature the MPB is placed in the region Zr/Ti = 52/48 [17, 18]. At the MPB the dielectric and piezoelectric response of the ceramic filler ir the largest.
In order to increase the electrical properties of the polymer matrix and not lo loose the mechanical ones, it is common to produce micro and nanocomposite materials which the matrix is a polymer, filled with ceramnic particles. The combination of an electroactive polymer like PVDF with PZT ceramic powder, leads to composites materials with enhanced electrical properties without loose the benefits of the mechanical properties of the poymeric matrix [19].
It is known that degradation of polymers and composites includes all changes in chemical structures and physical properties due to external pysical or chemical stresses caused by chemical reactions, involving bond scissions in the backbone of the molecules, wich leads to materials with different characteristics from those of the starting materials [20].
Liau et. al. studied the thermal degradation of polymer based composite materials with fillers of ceramic materials. There shows that the inclusion of ceramics materials (Al2O3
and AlN) accelerate the thermal degradation of the composite material compared to the polymeric matrix (PVB) for the values of activation energy determined [21]. Park et.
al., two years later, noted that the thermal stability of carbon-carbon composites with increase of oxidation resistant filler (MoSi2), during the graphitization process. It has
4 been found that 12-20 wt% filler on the basis of the resin matrix leads to an improvement of degradation temperature and to an effectively increase of activation energy of the composites [22].
In 2004, Liu et al evaluated the thermal stability of epoxy-silica nanocomposites by thermogravimetric analysis. Verified that the introduction of nanoscale silica into epoxy resins certainly improved their thermal stability and reduced their weight loss rates [23].
Campos et. al. study the effect of the inclusion of CaCO3 particles in PVDF matrix and they found that the inclusion of the ceramic particles increased the thermal stability of the polymeric matrix, and no changes were observed for the melting temperature of the composite film [24].
PVDF composites and copolymers with ceramic fillers (PZT, BaTiO3) have great technological interest because of their versatility in the attainment of thin and high flexible films for electronics applications.
The purpose of this work is to understand how the processing conditions of the PVDF/PZT films affect the thermal degradation of the composites in comparison to the pure polymer. The effect of the PZT filler particles in the thermal stability of the films will be also discussed. The thermal degradation of PVDF/PZT micro and nanocomposites under a nitrogen atmosphere will be analyzed and discussed by the Ozawa-Flynn-Wall theory in order to understand the influence of the size and amount of the ceramic particles filler in the thermal stability of the composites.
2. Theory:
Thermal properties can be expressed by the value of the activation energy, which can give an idea about the reactions conditions in the chemical process, and provide information related to the thermal stability and expected lifetime of a compound at a given temperature.
In general, the kinetics of the mass loss process can be investigated from the thermogravimetric data. The general model for the thermal decomposition of a homogeneous system has the following form [20, 25, 26]:
5
T f
tk k
Eq. 1
where represents the degree of conversion of the sample under degradation, defined by:
w w
w w t
0
0 Eq. 2
where 𝑤0, 𝑤(𝑡) and 𝑤∞ are the weights of the sample before degradation at time t and after complete degradation, respectively. The rate constant 𝑘(𝑡) suffers changes with absolute temperature according to the Arrhenius equation. 𝑓(𝛼) represents the net results of elementary steps, as the polymer degradation are often chain reaction. For solid state reactions 𝑓(𝛼) = (1 − 𝛼)𝑛, where n is the reaction order, assumed to remain constant during the degradation process.
a) Different Heating Rates: Ozawa-Flynn-Wall method
The isoconversional method of Ozawa-Flynn-Wall (OFW)[27, 28] is a method which assumes that the conversion function 𝑓(𝛼) does not change with the variation of the heating rate for all values of the degree of conversion 𝛼. It involves a measuring of the temperatures corresponding to fixed values of 𝛼 from the experiments at different heating rates 𝛽. In this formalism:
RT f E
R
AEact act
ln ln
ln Eq. 3
where A is a pré-exponential factor (min-1).
If the calculated values are the same for various values of 𝛼, the existence of a single step reaction can be concluded. On contrary, a change of 𝐸𝑎𝑐𝑡 with increasing degree of conversion is an indication of a complex reaction mechanism that invalidates the separation of the variables involved in the OFW analysis [29]. This method is a powerful tool to study the degradation kinetics of the thermogravimetric data obtained when studying complex processes like thermal degradation of polymers. This method
6 allow determining the activation energy without a previous knowledge of the reaction order [29].
Experimental:
Composites of PVDF with PZT [Pb(Zr0.53Ti0.47)O3] were prepared by dispersing the ceramic powder in a solution of PVDF in dimethylacetamide (DMA). The initial concentration of the solution was 0.2 g PVDF (Foraflon F4000-Atochem) per milliliter of DMA. The average size of the ceramic particles used in this work was 0.2, 0.84, 1.68 and 2.35 μm. The system was kept inside an oven at 120 ºC during 60 min. This time was enough to insure the removal of all solvent by evaporation.
After evaporation of the solvent, the sample was melted at 220 ºC for 10 min, removed from the oven, and cooled down at room temperature. After this procedure, the crystalline phase present in the polymer is -PVDF [13].
The volume percentage of the ceramic varied from 5-40%. Percentages higher than 40%
were not used in order to guarantee 0-3 connectivity and to maintain the high flexibility of the films [30].
Thermogravimetric analyses (TGA) were carried out using Pyris 1 TGA – Perkin-Elmer under nitrogen atmosphere supplied at a constant 50 mL min−1 flow rate. The sample holders used were crucibles of ceramic with capacity of 60 μL. The samples were subjected to different heating rates of 5 ± 0.1 up 30 ± 0.4 K min−1, between 300 and 1100 K, in order to evaluate the degradation kinetics of the process.
Results and Discussion
To evaluate the thermal stability of polymers and composites used in the thermogravimetric analysis (TGA) [31, 32].
For all applications, comprehend how the degradation is crucial to understand the advantages and limitations of the material.
The initial temperature of degradation, Tinicial, is defined as the temperature at which the material begins to lose mass, ie the temperature to which the material maintains its
7 properties. The study of decomposition kinetics, Tonset and Tmax can help identify the mechanisms of degradation.
Normally, TGA results are not easy to understand because they have a strong influence of the geometry and mass of sample, heating rate and gas environment. In this work an effort was made to keep the sample weight and the environment gas pressure constant for all experiments.
The figure 1 shows the TGA data obtained for various heating rates for the sample with 5% PZT and ceramic grain size of 200 nm.
150 300 450 600 750
40 50 60 70 80 90 100
675 700 725 750 775 800
-9 -6 -3 0
5 ºC/min 10 ºC/min 15 ºC/min 20 ºC/min
[dm/dT]/m0 / K-1
Temperature / K
Weight Loss / %
Temperature / ºC
5 ºC/min 10 ºC/min 15 ºC/min 20 ºC/min
Figure 1 – TGA results obtained for the 5% PZT, = 200 nm at different heating rates.
It is observed a major weight loss process between 350 and 500 ºC. This is due to degradation of polymeric matrix [20, 26].
For all the thermal degradation curves are identified various temperatures: the initial temperature, Tinicial, and the onset temperature, Tonset, that is calculated by extending the pre-degradation portion of the curve to the point of the interception with a line draw as a tangent to the steepest portion of the mass curve occurring during degradation [25, 33, 34]. The two temperatures are indicated for all heating rate in the tab. 1.
Table 1 – TGA results for all heating rate for 5% PZT, = 200 nm sample.
8 Heating Rate Tonset (ºC) Tmáx (ºC) Residue at 700ºC (%)
5ºC/min 422 434 48
10ºC/min 443 454 49
15ºC/min 452 468 51
20ºC/min 449 466 52
For a more detailed analysis of the TGA curves, we use the DTG, which is the differential degradation curve. These curves are shown in Figure 2. The maximum of DTG is defined to the temperature of maximum loss weight rate and these values are including in table 1.
To determine the activation energy of the sample is considered the method of Ozawa- Flynn-Wall.
In figure 3 a), are shown of ln vs 1000/T, which the straight line correspond of activation energy.
Figure 3 – a) Ozawa-Flynn-Wall plots of the 5% PZT, = 200 nm at different conversions. b) Evolution of the activation energy for the OFW method.
The activation energy medium for -PVDF is 76.5 kJ/mol [20]. Comparing this value of activation energy for the polymeric matrix without ceramic material with the values of figure 3 b shows that the ceramic material influences the degradation of the polymeric matrix. The reason for this may be due to the percentage and grain size of the ceramic are small and that there is chemical interaction between the ceramic and polymer.
To understand the percentage and grain size of the ceramic really influence the kinetics degradation of the polymer was done the same study but for the sample with 10% PZT and =0.84m.
9 The figure 4a and 4b shows the TGA and DTG data respectively, obtained for various heating rates for the sample with 10% PZT and ceramic grain size of 0.84m.
Figure 4 – a) TGA results and b) DTG results obtained for the 5% PZT, = 0.84m at different heating rates.
In the table 2 shows the measured temperature (Tinitial, Tonset and Tmax) and weigth for this sample.
Table 2 – TGA results for all heating rate for 10% PZT, = 0.84m sample Heating Rate Tinicial (ºC) Tonset (ºC) Tmáx (ºC) Residue at 700ºC (%)
5ºC/min 339 433 461 39
10ºC/min 319 445 475 43
20ºC/min 345 466 490 41
30ºC/min 326 464 502 43
Observing the values in figure 4 a) and b) and table 2 verified the shift to higher temperatures as the heating rate increases.
The activation energy was calculated using the Ozawa-Flynn-Wall method.
In figure 5 a), are shown of ln vs 1000/T, which the straight line correspond of activation energy.
Figure 5 – a) Ozawa-Flynn-Wall plots of the 10% PZT, =0.84m at different conversions. b) Evolution of the activation energy for the OFW method.
With the increase in percentage and grain size of the ceramic there is an increase in activation energy compared with the polymeric matrix. This may be related to the increase of the interface between the particles of ceramic material and polymeric matrix.
10 The percentage and grain size of the ceramic material influence the degradation kinetics of polymeric material and have a very important role in the interface of the respective materials.
For determine the parameters of ceramic material, percentage an grain size, influence over the kinetics degradation of the polymeric matrix, do a study of thermal degradation while maintaining the same heating rate.
The figure 6 a) and b) respectively, shows the TGA and DTG dates obtained for various percentages for the sample with same ceramic grain size of 0.2m and heating rate.
Figure 6 – a) TGA results and b) DTG results obtained for the =0.2m at different percentages with same heating rate.
In the table 3 shows the measured temperature (Tinitial, Tonset and Tmax) and weigth for these samples.
Table 3 – TGA results for same heating rate for =0.2m sample at different amounts percentages.
Samples Tinicial (ºC) Tonset (ºC) Tmáx (ºC) Residue at 700ºC (%)
=200nm; 5% PZT 342 428 462 (460) 50
=200nm; 10% PZT 370 443 463 55
=200nm; 20% PZT 350 440 480 66
Observing at figures 6 a), b) and table 3 verified that the composite material with a higher percentage of ceramic material increases the initial temperature and maximum temperature.
For determine the activation energy of these samples will be used the Horowitz-Metzger method.
11 Figure 7 – a) Plots of ln
ln
1
1
vs for the =0.2m at different percentages with same heating rate. b) Evolution of the activation energy as a function of percentage of PZT.Analyzing the figure 7 b), is observed that the activation energy increases with the increase in the percentage of ceramic material. This may be related to two aspects:
- Reduced amount of polymeric material in the composite material;
- Brittle to ductile behaviour which can improve the degree of adhesion at interfaces between fillers and matrix.
The figure 8 a) and b) respectively, shows the TGA and DTG dates obtained for various ceramic grain size for the sample with same percentage ceramic of 10% and heating rate.
Figure 8 – a) TGA results and b) DTG results obtained for the PZT at different ceramic grain size with same heating rate.
In the table 4 shows the measured temperature (Tinitial, Tonset and Tmax) and weigth for these samples.
Table 4 – TGA results for same heating rate for PZT sample at different amounts ceramic grain size.
Observing at figures 8 a), b) and table 4 verified that the composite material with a higher grain size of ceramic material increases the initial temperature and maximum temperature. For determine the activation energy of these samples will be used the Horowitz-Metzger method.
12 Figure 9 – a) Plots of ln
ln
1
1
vs for the 10%PZT at different ceramic grain size with same heating rate. b) Evolution of the activation energy as a function of ceramic grain size.Analyzing the figure 9 b), is observed that the activation energy increases with the increase in the grain size of ceramic material. For the grain size of 0.84mm decreases the activation energy due to the heterogeneity of the sample [30].
Due to the increased grain size of ceramic material, it appears that there is not chemical bond between the filler and matrix, by increasing the activation energy.
Conclusion
Thermogravimetry has been used to study the thermal degradation of the PZT/PVDF under a nitrogen atmosphere at different heating rates, percentage and grain size of ceramic material.
Thermal stability of the sample was analysed and the obtained results showed that the Tonset and Tmax shifts to higher temperatures with increasing heating rate.
It is observed that the degradation thermal kinetics depend on the percentage and grain size of ceramic material.
Considering the values of activation energy determined by the Horowitz-Metzger method, is concluded that the degradation kinetics depends more on the grain size than the percentage of the ceramic material.
Acknowledgements
This work is funded by FEDER funds through the “Programa Operacional Factores de Competitividade–COMPETE” and by national funds by FCT- Fundação para a Ciência e a Tecnologia, project references PTDC/CTM/69316/2006, PTDC/CTM-NAN/112574/2009, and NANO/NMed- SD/0156/2007. S. Firmino Mendes, C.M. Costa and V. Sencadas and thank to FCT grants SFRH/BD/22506/2005, SFRH/BD/68499/2010 and
13 SFRH/BPD/63148/2009, respectively. The authors also thank support from the COST Action MP1003, the “European Scientific Network for Artificial Muscles”
(ESNAM).
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