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Study of the thermal properties of miscible blends between poly(ether ketone ketone) (PEKK) and polyimide
Accepted Manuscript Study Of The Thermal Properties Of Miscible Blends Between Poly(Ether Ketone Ketone) (Pekk) And Polyimide S. Dominguez, C. Derail, F. Leonardi, J. Pascal, B. Brule PII: DOI: Reference:
S0014-3057(14)00381-4 http://dx.doi.org/10.1016/j.eurpolymj.2014.10.024 EPJ 6613
To appear in:
European Polymer Journal
Received Date: Accepted Date:
16 August 2014 31 October 2014
Please cite this article as: Dominguez, S., Derail, C., Leonardi, F., Pascal, J., Brule, B., Study Of The Thermal Properties Of Miscible Blends Between Poly(Ether Ketone Ketone) (Pekk) And Polyimide, European Polymer Journal (2014), doi: http://dx.doi.org/10.1016/j.eurpolymj.2014.10.024
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STUDY OF THE THERMAL PROPERTIES OF MISCIBLE BLENDS BETWEEN POLY(ETHER KETONE KETONE) (PEKK) AND POLYIMIDE S. Dominguez1, C. Derail1*, F. Leonardi1, J. Pascal2, B. Brule2 1
Université de Pau et des Pays de l’Adour, IPREM UMR CNRS 5254, Equipe de Physique et Chimie des Polymères, 2, Avenue du Président Angot, 64053 Pau, France 2
CERDATO, ARKEMA, 27470 Serquigny, France
Corresponding Author : [email protected], +33 (0)559 407 707 Abstract - This study deals with the enhancement of the thermal behavior of thermostable thermoplastic polymers dedicated to high performance composites. Semi-crystalline Poly(Ether Ketone Ketone) and amorphous poly(Imide) have been blended at different ratios by using a high temperature co-rotative twin screw extruder and injected using an injection-molding machine with a “cold” mold. Injected samples have been characterized by different techniques and we especially discuss how crystallinity affects the composition of the amorphous phase and consequently the variation of the glass transition temperature value according to the composition that we propose to model by a modified Gordon-Taylor law. Keywords: thermostable polymers, miscible blends, thermomechanical analysis
1
Introduction The thermal transitions of semi-crystalline polymers generally can give two main informations: (i) the using temperature can be defined from the glass transition temperature, Tg, and (ii) the processing temperature is defined knowing the melt temperature. To improve the end-use temperature domain of a given polymer, a way consists in choosing a high Tg. Nevertheless, for semi-crystalline polymers, a high Tg generally implies a high melt temperature, Tm, as these two values are linked.1 Van Krevelen indeed has found that Tg=2/3 Tm. Miscible blends of polymers have been widely studied in literature whatever the nature (i) blends of two amorphous polymers,2,3 (ii) blends of one semi-crystalline and one amorphous polymers,4,5 and (iii) blends of two semi-crystalline polymers.6,7 If we focus on the glass-transition temperature of blends containing two miscible amorphous polymers, various laws have been proposed to understand and to model its evolution. The simplest and well-known one is the Fox law which models the behavior of an ideal blend and neglects the influence of interactions and free volume variations of the constituents.8 For most of the blends, the use of other models is necessary to describe correctly the experimental data. The widely used Gordon - Taylor model adds a parameter, noted K, which takes into account the free volume variation of the polymers in the blends (Eq. 1, where Tg,b is the Tg of the blend, Tg,1 and Tg,2 the Tg of each polymer composing the blend and ϕ1 , ϕ 2 the mass fractions of each neat polymer).9 ϕ T + Kϕ 2Tg 2 Eq. 1 Tg ,b = 1 g1 ϕ1 + Kϕ 2 The models of Kwei and Braun – Kovacs equations take into account the interactions between polymers.10 For more complex behaviors such as sigmoidal behavior, Schneider gave other equation.11 The literature on thermoplastic thermostable blends shows that mixing poly(ether aryl ketone)s (PEAKs) and a thermoplastic polyimide can lead to miscible blends.6 The variation of the Tg of poly(ether ether ketone) (PEEK) mixed with poly(ether imide) (PEI) has already been studied and it was found that these blends are miscible in the amorphous state. Concerning the PEEK/PEI blends, a phase separation is observed in all the range of studied compositions in the semi-crystalline state.12,13 Blends of poly(ether ketone ether ketone ketone) (PEKEKK) with PEI have also been studied and are miscible.14 Poly (ether ketone ketone) (PEKK) exhibits a good miscibility with a semicrystalline thermoplastic polyimide, the N-TPI (New Thermoplastic Polyimide).6 Binary blends containing at least one semi-crystalline compound and based on poly(ether ketone ketone) (PEKK) or PEEK blended with PEI have been studied.15,16 The crystallization rate of the semi-crystalline polymer with a lower Tg decreases when its content decreases in the blend. Indeed the increase of Tg implies the decrease of the mobility of its polymer chains in the miscible blends. On the contrary, the crystallization rate of the higher Tg semi-crystalline polymer increases when its amount in the blend decreases. This behavior can be interpreted by an increase of the mobility of the polymer chains. Then, the crystallization of these miscible polymer blends can cause a phase segregation leading to a broad glass transition temperature, or to the apparition of two different Tg corresponding respectively to the two single phases.12,13 PEKKs (poly(ether ketone ketone)s) are semi-crystalline thermoplastic thermostable statistical copolymers.17,18 The PEKK thermal properties vary with the amount of isophtalic or terephtalic moieties in its polymer chain. 6,19,20 Indeed, a decrease of the melt temperature (Tm) of PEKK (from 393 °C to 310 °C) and its crystallization speed has been observed when the ratio of isophtalic 2
moieties in its chain increases. One can note that the glass transition temperature is not affected by this parameter in the amorphous state. In the present study, four different blends, composed of a semi-crystalline polymer blended with a higher Tg amorphous polymer will be analyzed by focusing on the dependence of the glass transition temperature and alpha mechanical relaxation temperature (Tg and Tα respectively) with composition.
Experimental Semi-crystalline polymers PEKK1 and PEKK2 are semi-crystalline polymers provided by ARKEMA with two different ratios of isophtalic moieties in the polymer chain equal to 0.2 and 0.4, respectively. This composition doesn’t influence the maximum weight crystallinity level which is equal to around 35% but influences strongly the crystallization rate. The glass transition temperature (Tg = 159°C) is not affected by the composition contrary to the melt temperature (Tm) respectively equal to 360°C and 310°C. PEKK polymers crystallize under different forms. In this paper, we have decided to analyze results considering a constant enthalpy of crystallization to calculate the weight crystallinity level which will be the base of our analysis of the variation of the Tg as proposed hereafter.
Amorphous Polymers PEI and PI exhibit a glass transition temperature equal to 220°C and 247°C respectively. These amorphous polymers were purchased from Sabic (PEI : Ultem 1000, PI : Extem VH 1003). Processing Each blend is melted and mixed in the same conditions, using a co-rotative Benchtop 16mm Twin Screw Extruder type LTE 16-40 from LABTECH Engineering. The ratio screw length out of diameter (L/D) is equal to 40. The screw diameter (D) is 16mm and the screw is composed of kneading and conveying elements. The distribution of those elements along the screw and the barrel, composed of 10 zones are described in Fig. 1. The temperature profile is displayed in Tables 1 and 2. The extruded blends are then injected using a 65 tons injection–molding machine from DK Leguen Hemidi, with a screw diameter of 28mm. The injection-molding parameters are reported in Table 3. The specimens obtained by using these conditions, and particularly the mold temperature (90°C), are amorphous. To make them crystallize, an annealing at 250°C during 2 hours is performed. A comparison between the properties of amorphous and semi-crystalline blends is then possible. Characterizations Glass-transition temperatures and crystallinity ratios have been measured with a Differential Scanning Calorimetry apparatus DSC Q100 from TA instruments. The crystallinity level is derived from the measurement of the enthalpy of crystallization from melt with a cooling rate of 20°C.min-1. The enthalpy of crystallization of a 100% crystalline PEKK is equal to 130J.g-1. 21 The Tg are measured during the second heating at 20°C.min1 , after the first cooling at 20°C.min-1. Tmax applied during DSC analysis was equal to 400°C for PEKK 2 and 450°C for PEKK 1. The α relaxation temperatures are measured performing a thermomechanical analysis with a shear dynamic rheometer Anton Paar MCR 301. The rectangular torsion geometry is performed on a sample cut in injection molded bars. The analysis is carried out at a 3
heating rate of 2°C.min-1 and with an angular frequency of 1rad.s-1 in the linear domain early defined. The Tα is measured at the maximum of tan (δ). Results and Discussion Glass-transition region The αrelaxation temperatures of our blends have first been studied in the amorphous state, using the amorphous injection molded samples. The thermomechanical analysis, presented in Figure 2 for blends and pure polymers, exhibit only one peak in the transition region. For blends, the width of tan(δ) peak being not larger than the peaks of the pure polymers, one can assume that the polymers are miscible. In the same time, one can notice that DSC measurements also show a single Tg (Fig.1SD and Fig.2SD, supporting data). In our measurement conditions, one can notice that Tg=Tα. In the amorphous state, the variation of Tα as a function of the composition of blends follows a Gordon-Taylor law (Eq.1). The value of the K parameter for PEKK1/PEI and PEKK2/PEI blends equals 0.73. For PEKK1/PI blends, K=0.5 and for PEKK2/PI blends K=0.47. The crystallization effect on the glass-transition region has also been observed on these blends. First, for the high crystallization rate polymer, PEKK1, the effect of crystallization during cooling (20°C.min-1, Tmax = 450°C) has been investigated by DSC. The raw data are reported in Fig.1SD for the PEKK1/PEI blends. The Tg of the blends and the pure PEKK1 becomes broad, due to the increase of the crystallinity ratio, but there is no evidence of a phase segregation, as only one Tg has been measured. Nevertheless, the blends do not crystallize at their maximum capacity when the PEI amount becomes too important. The Tg composition dependence and the corresponding crystallinity ratio of PEKK1/PEI blends are reported in fig.3. The results put into relief the deviation of the glass-transition temperature of semicrystalline blends from the Gordon-Taylor law predicted for amorphous blends, particularly between 20 and 50% PEI content. This can be explained by the modification of composition of the amorphous phase when the PEKK crystallizes in the blend. Indeed, as we assume that only PEKK crystallizes, we postulate, as an evidence, that it is consumed from the amorphous phase to form the crystalline phase. Consequently, the PEI content in the amorphous phase of semi-crystalline blends is higher than in an amorphous blend with the same global composition. Nevertheless, some blends containing PEKK1 have not totally crystallized during the cooling step, and the PEKK2 blends do not crystallize in these conditions. Then, annealed and crystallized specimens have been produced. These specimens have been characterized by thermomechanical analyses. The peaks of tan(δ) of the crystallized specimens are broader than the amorphous ones (Fig.4). This result is in agreement with the DSC signatures obtained for the PEKK1/PEI blends. Moreover, some PEKK1/PEI blends exhibit a very broad tan(δ) peak certainly due to a phase segregation, and two convoluted peaks are observable (Fig.5). The annealed blends have a Tα much higher than the Tg measured by DSC (Fig.6). Indeed, the Tα of the pure PEKK is higher than that of the PEKK that had crystallized during cooling. This increase also affects the Tα of the blends. It is important to note that the Tα improvement due to the crystallization depends on the way the PEKK has crystallized, and also on the isophtalic content in the PEKK chain.10 Indeed, semi-crystalline blends represented in Fig.3 have crystallized during cooling corresponding to an anisothermal crystallization from melt. On the contrary, the semicrystalline blends in Fig.6 have been quenched, and then have crystallized during 4
annealing. It was an isothermal cold crystallization from a quenched amorphous state. This difference of processing can explain the differences observed between results reported in Fig.6 and Fig.3, although the crystallinity level remains similar.16 Blends containing PEKK2 do not exhibit any phase segregation and remain miscible in the whole range of composition, showing only one Tα (Fig.7). This result is in agreement with other studies showing the better miscibility of this kind of PEKK with polyimides.6 But, all these blends do not completely crystallize. Indeed, the PEKK2 crystallizes very slowly, and its crystallization rate decreases with the amount of polyimide in the blend. Thus, a longer annealing could be required to make completely crystallized these blends. For some compositions, blends containing PEKK1 show phase segregation when they crystallize. For PEKK1/PEI blends, the segregation occurs at 50% and 60% of PEI contents, when it is completely crystallized. Indeed, no phase segregation has been observed when this blend crystallizes during the cooling at 20°C.min-1. PEKK1/PI blends segregate from 30% to 70% of PI, when they have completely crystallized (Fig.6). To explain the phase separation of some of the annealed blends, one can consider two hypotheses (i) the blends have an upper critical solution temperature above 250°C, but below 360°C (melt state). Then, the quenched samples have no time to reorganize themselves to let the phase separation occur. On the contrary, during the annealing, the macromolecules are enough mobile to reorganize themselves into two different phases, (ii) during isothermal crystallization, the polyimide, which is rejected out of the PEKK crystallites, has not enough mobility to disperse itself into the blend, creating a gradient of polyimide concentration near the crystallites. Then, depending on which hypothesis has to be considered, the differences observed in the segregation domain of PEKK1/PEI and PEKK1/PI can be explained in two different ways. (i) PEI is more compatible with PEKK than PI. Thus, phase segregation of PEKK/PEI blends occurs in a thinner range of composition than PEKK/PI blends. This would also mean that PEKK2 has a better compatibility with polyimides than PEKK1. (ii) The mobility of the PI chains is lower than the mobility of PEI chains. Then, when the polyimide is rejected from the crystalline phase, PEI can diffuse easier than PI in the amorphous phase of the blend. Thus, there is not a too much PEI concentration around the PEKK crystallites that could induce phase segregation. This can also explain why there is no phase segregation observe for PEKK2/PEI or PI blends. Indeed, as the PEKK2 crystallizes very slowly, PEI or PI has much time to diffuse in the amorphous phase of the blend. As the differences of compatibility of PEKK1 and PEKK2 can be hardly explained (no differences in the amorphous state, even in the Gordon Taylor constant K, have been observed), the second hypothesis seems to be more realistic.
Modelling The evolution of the Tg of amorphous blends follows a Gordon-Taylor law. Then, the crystallization of the blends changes the value of the Tg and the Tα. Two phenomena have to be taken into account for the Tg/Tα composition dependence modelling. To model the Tg composition dependence of the blends that have crystallized in the DSC apparatus oven, only the amorphous phase composition change during crystallization has been taken into account. Then, the minimum amorphous PEKK content in the amorphous phase of the blends, noted θPEKK, has been calculated from Eq.2, considering that the maximal crystallinity level of a blend is proportional to φPEKK (as only PEKK crystallizes). Indeed, we aim at calculating the maximal Tg obtainable, so we replace the total PEKK content φPEKK by θPEKK in the Gordon-Taylor law. This leads to Eq.3, that permits to calculate the maximal Tg of a blend knowing the characteristics of the pure polymers only. 5
Eq. 2
wc,PEKK is the maximum crystallinity level of PEKK. T g ,b =
θ PEKK Tg , PEKK + K (1 − θ PEKK )Tg , PEI θ PEKK + K (1 − θ PEKK )
Eq. 3
Concerning the annealed blends, the results show that they have crystallized. To model the increasing of the Tα, one can consider a linearly growth of the Tα with the crystallinity.15 Then, a new parameter is set, ΔTα = Tα, semi-crystalline – Tα, amorphous. As Tα evolves linearly with the crystallinity level, so does ΔTα. Moreover, when the crystallinity level evolves linearly with φPEKK, ΔTα is proportional to φPEKK. As a consequence, one adds a new parameter in Eq.3 to describe more precisely the Tα evolution of miscible annealed PEKK/PEI samples (Eq.4, Fig.6) Tα , m =
θ PEKK Tα , PEKK + K (1 − θ PEKK )Tα , PEI + ϕPEKKΔΤα , PEKK θ PEKK + K (1 − θ PEKK )
Eq. 4
Concerning the PEKK2 based blends, even with the annealing, the whole crystallization for all blends is not reached. Then, the Eq.4 can be modified to take into account the experimental crystallinity of each blend. Nevertheless, the crystallinity level of each blend has to be measured. θPEKK gets another expression (Eq.5) and the modified Gordon-Taylor law is transformed into Eq.6. θ PEKK ,exp = Tα , m =
ϕ PEKK − wc ,exp
Eq. 5
1 − wc ,exp
θ PEKK ,exp Tα , PEKK + K (1 − θ PEKK ,exp )Tα , PEI θ PEKK ,exp + K (1 − θ PEKK ,exp )
+
wc ,exp wc
ΔΤα , PEKK
Eq.6
The models are in good agreement with the experimental results. The hypothesis of linear evolution of Tα with crystallinity level is then reasonable to foresee the Tα of crystallized blends. It is important to note that for our blends, a non-negligible influence of the thermal treatments of PEKK/polyimide blends is put into relief. To correctly foresee the glass transitions of these blends, the effects of crystallinity have to be taken into account, and those ones depend on the way the polymers have crystallized. In this case, the polymer blend processing is then something important to control that will lead the thermal properties of the blends, particularly the ones based on PEKK1.
Rubbery plateau domain The thermomechanical analyses (Fig 3-SD - supporting data) show that the plateau modulus of the same semi-crystalline blends discussed previously decreases with the polyimide content. We have reported the value of G’ at the minimum of tan(δ) as a function of PEI content in Fig. 8 and 9 according to the state of the blend. In the amorphous state, there is no significant influence of the composition on the plateau modulus (Fig. 8). For semi-crystalline one, we have reported the variation of the plateau modulus as a function of the crystallinity ratio in Fig. 9. 6
These results can be described by the law proposed by Van Krevelen and reported in Eq. 7 where Gsc is the rubbery modulus of the semi-crystalline polymer, Gc is the rubbery modulus of the 100% crystalline polymer and Gam is the rubbery modulus of the amorphous polymer.1 Gc value has been found at 8.106 Pa, applying the model to experimental data and Gam has been measured using data reported in Fig. 8. Gsc = wc (Gc − Gam ) + Gam 2
Eq. 7
The evolution of the rubbery plateau demonstrates the importance of the crystallinity level on the mechanical properties of the blends. It is necessary to make a compromise between the improvement of the glass transition and the decrease of the rubbery moduli, to obtain good properties after Tg. Conclusions We have studied experimental variations of Tα and Tg PEKK/Polyimide blends. In the amorphous state, they are completely miscible. We have shown that a good control on crystallinity improves the value of the Tg, for a given composition. We have proposed a model that could describe the Tg and the Tα variation of the semi-crystalline blends. Some of the blends let appear phase segregation when crystallization occurs during annealing, due to the expulsion of the non-crystalline part out of the forming crystallites. The Tg improvement using the method of blends induce a decrease of the rubbery moduli, that has to be taken into account to adjust the mechanical properties as a function of the enduse applications.
Acknowledgements This work benefits from the support of BPI France within the ASTech and Aerospace Valley clusters and links to the COMPTINN project which deals with composites dedicated to structural applications during long lasting and intermediate temperatures for civil aeronautics. SD, CD and FD thank ARKEMA to provide PEKK samples.
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[18] M. G. Zolotukhin, D. R. Rueda, M. Bruix, M. E. Cagiao, F. J. Balta Calleja, A. Bulai, N. G. Gileva, and L. Van der Elst, “Aromatic polymers obtained by precipitation polycondensation: 5. 1H and 13C n.m.r. study of poly(ether ketone ketone)s,” Polymer, vol. 38, no. 14, pp. 3441–3453, 1997. [19] K. H. Gardner, B. S. Hsiao, R. R. Matheson, and B. A. Wood, “Structure, crystallization and morphology of poly (aryl ether ketone ketone),” Polymer, vol. 33, no. 12, pp. 2483–2495, 1992. [20] R. K. Krishnaswamy and D. S. Kalika, “Glass transition characteristics of poly(aryl ether ketone ketone) and its copolymers,” Polymer, vol. 37, no. 10, pp. 1915–1923, mai 1996. [21] M. Hojjati, J. Chen, and A. Yousefpour, “Crystallization kinetics of Poly (Ether Ketone Ketone) PEKK,” in International SAMPE Symposium and Exhibition (Proceedings), 2007, vol. 52.
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TABLES Table 1 - Temperature profile for blends composed of PEKK1 Zone
Die (10)
T (°C)
360
9
8
7
6
5
4
3
2
1
380 380 390 390 380 380 370 360 340
Table 2 - Temperature profile for blends composed of PEKK2 Zone Die 9 8 7 6 5 4 3 2 1 T (°C) 350 350 350 350 340 340 330 320 310 305
Table 3 – Injection parameters T3, T2, and T1 are the 3 barrel temperatures from die to hopper. T die (°C)
T3 (°C)
T2 (°C)
T1 (°C)
380
380 360 350
Injection speed (mm/s) 80
P t t T holding holding cooling mold (bar) (s) (s) (°C) 1000
10
5
7
90
FIGURES AND CAPTIONS Figure 1 – Screw profile “screw” : conveying elements, “discs” : kneading elements – the first three zones contain conveying screw elements Figure 2 – tan(δ) versus temperature of PEKK2/PEI amorphous blends : PEKK2: PEKK2 /PEI 80/20: 70/30: 60/40: 50/50: + 40/60: - 30/70: 20/80: + PEI:
;
Figure 3 – Glass-transition temperatures of amorphous ( ) and semi-crystalline ( blends of PEKK1/PEI with their crystallinity ratio ( ) versus PEI content.
)
Figure 4 – tan(δ) versus temperature of PEKK2/PEI semi-crystalline blends : PEKK2: PEKK2 /PEI 80/20: 70/30: 60/40: 50/50: + 40/60: 30/70: 20/80: + PEI:
;
Figure 5 – tan(δ) versus temperature of PEKK1/PEI semi-crystalline blends : PEKK1 /PEI 70/30: 60/40: 50/50: + 40/60: 30/70: Figure 6 – Tα versus polyimide content of PEKK1/PEI amorphous ( blends and PEKK1/PI amorphous ( ) and annealed ( ) blends Gordon-Taylor law K=0,73 ( ), K=0,5 ( ) Equation 4 K=0,73 ( ), K=0,5 ( )
)and annealed (
)
Figure 7 – Tα versus polyimide content of PEKK2/PEI amorphous ( blends and PEKK2/PI amorphous ( )and annealed ( ) blends Gordon-Taylor law K=0,73( ), K=0,47( ) Equation 6 K=0,73 ( ), K=0,47 ( )
)and annealed (
)
Figure 8 – Rubbery plateau modulus versus polyimide content of each amorphous blend : PEKK 1/PEI ( ), PEKK1/PI ( ), PEKK2/PEI ( + ), PEKK2/PI ( ) Figure 9 –Plateau modulus versus crystallinity level of semi-crystalline blend : PEKK1/PEI ( ), PEKK1/PI ( ), PEKK2/PEI ( ), PEKK2/PI ( )
11
Figure 1
12
Figure 2
4 3.5 3
tan (δ)
2.5 2 1.5 1 0.5 0
150
200
Temperature (°C)
13
250
230
0.4
220
0.35
210
0.3
200
0.25
190
0.2
180
0.15
170
0.1
160
0.05
150
0
0.2
0.4
0.6
PEI contents
14
0.8
1
0
weight fraction crystallinity
Tg (°C)
Figure 3
Figure 4
3 2.5
tan (δ)
2 1.5 1 0.5 0
150
200
Temperature (°C)
15
250
Figure 5
1 0.9 0.8
tan (δ)
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
150
250
Temperature (°C)
16
Figure 6
17
Figure 7
250
Tα (°C)
230
210
190
170
150
0
0.2
0.4
0.6
polyimide content
18
0.8
1
Figure 8
Rubbery moduli (Pa)
1.E+07
1.E+06 0
0.2
0.4
0.6
polyimide content
19
0.8
1
Figure 9
Rubbery moduli (Pa)
1.E+08
1.E+07
1.E+06 0
0.05
0.1
0.15
0.2
0.25
crystallinity level
20
0.3
0.35
0.4
Graphical abstract
21
S0014-3057(14)00381-4 http://dx.doi.org/10.1016/j.eurpolymj.2014.10.024 EPJ 6613
To appear in:
European Polymer Journal
Received Date: Accepted Date:
16 August 2014 31 October 2014
Please cite this article as: Dominguez, S., Derail, C., Leonardi, F., Pascal, J., Brule, B., Study Of The Thermal Properties Of Miscible Blends Between Poly(Ether Ketone Ketone) (Pekk) And Polyimide, European Polymer Journal (2014), doi: http://dx.doi.org/10.1016/j.eurpolymj.2014.10.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
STUDY OF THE THERMAL PROPERTIES OF MISCIBLE BLENDS BETWEEN POLY(ETHER KETONE KETONE) (PEKK) AND POLYIMIDE S. Dominguez1, C. Derail1*, F. Leonardi1, J. Pascal2, B. Brule2 1
Université de Pau et des Pays de l’Adour, IPREM UMR CNRS 5254, Equipe de Physique et Chimie des Polymères, 2, Avenue du Président Angot, 64053 Pau, France 2
CERDATO, ARKEMA, 27470 Serquigny, France
Corresponding Author : [email protected], +33 (0)559 407 707 Abstract - This study deals with the enhancement of the thermal behavior of thermostable thermoplastic polymers dedicated to high performance composites. Semi-crystalline Poly(Ether Ketone Ketone) and amorphous poly(Imide) have been blended at different ratios by using a high temperature co-rotative twin screw extruder and injected using an injection-molding machine with a “cold” mold. Injected samples have been characterized by different techniques and we especially discuss how crystallinity affects the composition of the amorphous phase and consequently the variation of the glass transition temperature value according to the composition that we propose to model by a modified Gordon-Taylor law. Keywords: thermostable polymers, miscible blends, thermomechanical analysis
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Introduction The thermal transitions of semi-crystalline polymers generally can give two main informations: (i) the using temperature can be defined from the glass transition temperature, Tg, and (ii) the processing temperature is defined knowing the melt temperature. To improve the end-use temperature domain of a given polymer, a way consists in choosing a high Tg. Nevertheless, for semi-crystalline polymers, a high Tg generally implies a high melt temperature, Tm, as these two values are linked.1 Van Krevelen indeed has found that Tg=2/3 Tm. Miscible blends of polymers have been widely studied in literature whatever the nature (i) blends of two amorphous polymers,2,3 (ii) blends of one semi-crystalline and one amorphous polymers,4,5 and (iii) blends of two semi-crystalline polymers.6,7 If we focus on the glass-transition temperature of blends containing two miscible amorphous polymers, various laws have been proposed to understand and to model its evolution. The simplest and well-known one is the Fox law which models the behavior of an ideal blend and neglects the influence of interactions and free volume variations of the constituents.8 For most of the blends, the use of other models is necessary to describe correctly the experimental data. The widely used Gordon - Taylor model adds a parameter, noted K, which takes into account the free volume variation of the polymers in the blends (Eq. 1, where Tg,b is the Tg of the blend, Tg,1 and Tg,2 the Tg of each polymer composing the blend and ϕ1 , ϕ 2 the mass fractions of each neat polymer).9 ϕ T + Kϕ 2Tg 2 Eq. 1 Tg ,b = 1 g1 ϕ1 + Kϕ 2 The models of Kwei and Braun – Kovacs equations take into account the interactions between polymers.10 For more complex behaviors such as sigmoidal behavior, Schneider gave other equation.11 The literature on thermoplastic thermostable blends shows that mixing poly(ether aryl ketone)s (PEAKs) and a thermoplastic polyimide can lead to miscible blends.6 The variation of the Tg of poly(ether ether ketone) (PEEK) mixed with poly(ether imide) (PEI) has already been studied and it was found that these blends are miscible in the amorphous state. Concerning the PEEK/PEI blends, a phase separation is observed in all the range of studied compositions in the semi-crystalline state.12,13 Blends of poly(ether ketone ether ketone ketone) (PEKEKK) with PEI have also been studied and are miscible.14 Poly (ether ketone ketone) (PEKK) exhibits a good miscibility with a semicrystalline thermoplastic polyimide, the N-TPI (New Thermoplastic Polyimide).6 Binary blends containing at least one semi-crystalline compound and based on poly(ether ketone ketone) (PEKK) or PEEK blended with PEI have been studied.15,16 The crystallization rate of the semi-crystalline polymer with a lower Tg decreases when its content decreases in the blend. Indeed the increase of Tg implies the decrease of the mobility of its polymer chains in the miscible blends. On the contrary, the crystallization rate of the higher Tg semi-crystalline polymer increases when its amount in the blend decreases. This behavior can be interpreted by an increase of the mobility of the polymer chains. Then, the crystallization of these miscible polymer blends can cause a phase segregation leading to a broad glass transition temperature, or to the apparition of two different Tg corresponding respectively to the two single phases.12,13 PEKKs (poly(ether ketone ketone)s) are semi-crystalline thermoplastic thermostable statistical copolymers.17,18 The PEKK thermal properties vary with the amount of isophtalic or terephtalic moieties in its polymer chain. 6,19,20 Indeed, a decrease of the melt temperature (Tm) of PEKK (from 393 °C to 310 °C) and its crystallization speed has been observed when the ratio of isophtalic 2
moieties in its chain increases. One can note that the glass transition temperature is not affected by this parameter in the amorphous state. In the present study, four different blends, composed of a semi-crystalline polymer blended with a higher Tg amorphous polymer will be analyzed by focusing on the dependence of the glass transition temperature and alpha mechanical relaxation temperature (Tg and Tα respectively) with composition.
Experimental Semi-crystalline polymers PEKK1 and PEKK2 are semi-crystalline polymers provided by ARKEMA with two different ratios of isophtalic moieties in the polymer chain equal to 0.2 and 0.4, respectively. This composition doesn’t influence the maximum weight crystallinity level which is equal to around 35% but influences strongly the crystallization rate. The glass transition temperature (Tg = 159°C) is not affected by the composition contrary to the melt temperature (Tm) respectively equal to 360°C and 310°C. PEKK polymers crystallize under different forms. In this paper, we have decided to analyze results considering a constant enthalpy of crystallization to calculate the weight crystallinity level which will be the base of our analysis of the variation of the Tg as proposed hereafter.
Amorphous Polymers PEI and PI exhibit a glass transition temperature equal to 220°C and 247°C respectively. These amorphous polymers were purchased from Sabic (PEI : Ultem 1000, PI : Extem VH 1003). Processing Each blend is melted and mixed in the same conditions, using a co-rotative Benchtop 16mm Twin Screw Extruder type LTE 16-40 from LABTECH Engineering. The ratio screw length out of diameter (L/D) is equal to 40. The screw diameter (D) is 16mm and the screw is composed of kneading and conveying elements. The distribution of those elements along the screw and the barrel, composed of 10 zones are described in Fig. 1. The temperature profile is displayed in Tables 1 and 2. The extruded blends are then injected using a 65 tons injection–molding machine from DK Leguen Hemidi, with a screw diameter of 28mm. The injection-molding parameters are reported in Table 3. The specimens obtained by using these conditions, and particularly the mold temperature (90°C), are amorphous. To make them crystallize, an annealing at 250°C during 2 hours is performed. A comparison between the properties of amorphous and semi-crystalline blends is then possible. Characterizations Glass-transition temperatures and crystallinity ratios have been measured with a Differential Scanning Calorimetry apparatus DSC Q100 from TA instruments. The crystallinity level is derived from the measurement of the enthalpy of crystallization from melt with a cooling rate of 20°C.min-1. The enthalpy of crystallization of a 100% crystalline PEKK is equal to 130J.g-1. 21 The Tg are measured during the second heating at 20°C.min1 , after the first cooling at 20°C.min-1. Tmax applied during DSC analysis was equal to 400°C for PEKK 2 and 450°C for PEKK 1. The α relaxation temperatures are measured performing a thermomechanical analysis with a shear dynamic rheometer Anton Paar MCR 301. The rectangular torsion geometry is performed on a sample cut in injection molded bars. The analysis is carried out at a 3
heating rate of 2°C.min-1 and with an angular frequency of 1rad.s-1 in the linear domain early defined. The Tα is measured at the maximum of tan (δ). Results and Discussion Glass-transition region The αrelaxation temperatures of our blends have first been studied in the amorphous state, using the amorphous injection molded samples. The thermomechanical analysis, presented in Figure 2 for blends and pure polymers, exhibit only one peak in the transition region. For blends, the width of tan(δ) peak being not larger than the peaks of the pure polymers, one can assume that the polymers are miscible. In the same time, one can notice that DSC measurements also show a single Tg (Fig.1SD and Fig.2SD, supporting data). In our measurement conditions, one can notice that Tg=Tα. In the amorphous state, the variation of Tα as a function of the composition of blends follows a Gordon-Taylor law (Eq.1). The value of the K parameter for PEKK1/PEI and PEKK2/PEI blends equals 0.73. For PEKK1/PI blends, K=0.5 and for PEKK2/PI blends K=0.47. The crystallization effect on the glass-transition region has also been observed on these blends. First, for the high crystallization rate polymer, PEKK1, the effect of crystallization during cooling (20°C.min-1, Tmax = 450°C) has been investigated by DSC. The raw data are reported in Fig.1SD for the PEKK1/PEI blends. The Tg of the blends and the pure PEKK1 becomes broad, due to the increase of the crystallinity ratio, but there is no evidence of a phase segregation, as only one Tg has been measured. Nevertheless, the blends do not crystallize at their maximum capacity when the PEI amount becomes too important. The Tg composition dependence and the corresponding crystallinity ratio of PEKK1/PEI blends are reported in fig.3. The results put into relief the deviation of the glass-transition temperature of semicrystalline blends from the Gordon-Taylor law predicted for amorphous blends, particularly between 20 and 50% PEI content. This can be explained by the modification of composition of the amorphous phase when the PEKK crystallizes in the blend. Indeed, as we assume that only PEKK crystallizes, we postulate, as an evidence, that it is consumed from the amorphous phase to form the crystalline phase. Consequently, the PEI content in the amorphous phase of semi-crystalline blends is higher than in an amorphous blend with the same global composition. Nevertheless, some blends containing PEKK1 have not totally crystallized during the cooling step, and the PEKK2 blends do not crystallize in these conditions. Then, annealed and crystallized specimens have been produced. These specimens have been characterized by thermomechanical analyses. The peaks of tan(δ) of the crystallized specimens are broader than the amorphous ones (Fig.4). This result is in agreement with the DSC signatures obtained for the PEKK1/PEI blends. Moreover, some PEKK1/PEI blends exhibit a very broad tan(δ) peak certainly due to a phase segregation, and two convoluted peaks are observable (Fig.5). The annealed blends have a Tα much higher than the Tg measured by DSC (Fig.6). Indeed, the Tα of the pure PEKK is higher than that of the PEKK that had crystallized during cooling. This increase also affects the Tα of the blends. It is important to note that the Tα improvement due to the crystallization depends on the way the PEKK has crystallized, and also on the isophtalic content in the PEKK chain.10 Indeed, semi-crystalline blends represented in Fig.3 have crystallized during cooling corresponding to an anisothermal crystallization from melt. On the contrary, the semicrystalline blends in Fig.6 have been quenched, and then have crystallized during 4
annealing. It was an isothermal cold crystallization from a quenched amorphous state. This difference of processing can explain the differences observed between results reported in Fig.6 and Fig.3, although the crystallinity level remains similar.16 Blends containing PEKK2 do not exhibit any phase segregation and remain miscible in the whole range of composition, showing only one Tα (Fig.7). This result is in agreement with other studies showing the better miscibility of this kind of PEKK with polyimides.6 But, all these blends do not completely crystallize. Indeed, the PEKK2 crystallizes very slowly, and its crystallization rate decreases with the amount of polyimide in the blend. Thus, a longer annealing could be required to make completely crystallized these blends. For some compositions, blends containing PEKK1 show phase segregation when they crystallize. For PEKK1/PEI blends, the segregation occurs at 50% and 60% of PEI contents, when it is completely crystallized. Indeed, no phase segregation has been observed when this blend crystallizes during the cooling at 20°C.min-1. PEKK1/PI blends segregate from 30% to 70% of PI, when they have completely crystallized (Fig.6). To explain the phase separation of some of the annealed blends, one can consider two hypotheses (i) the blends have an upper critical solution temperature above 250°C, but below 360°C (melt state). Then, the quenched samples have no time to reorganize themselves to let the phase separation occur. On the contrary, during the annealing, the macromolecules are enough mobile to reorganize themselves into two different phases, (ii) during isothermal crystallization, the polyimide, which is rejected out of the PEKK crystallites, has not enough mobility to disperse itself into the blend, creating a gradient of polyimide concentration near the crystallites. Then, depending on which hypothesis has to be considered, the differences observed in the segregation domain of PEKK1/PEI and PEKK1/PI can be explained in two different ways. (i) PEI is more compatible with PEKK than PI. Thus, phase segregation of PEKK/PEI blends occurs in a thinner range of composition than PEKK/PI blends. This would also mean that PEKK2 has a better compatibility with polyimides than PEKK1. (ii) The mobility of the PI chains is lower than the mobility of PEI chains. Then, when the polyimide is rejected from the crystalline phase, PEI can diffuse easier than PI in the amorphous phase of the blend. Thus, there is not a too much PEI concentration around the PEKK crystallites that could induce phase segregation. This can also explain why there is no phase segregation observe for PEKK2/PEI or PI blends. Indeed, as the PEKK2 crystallizes very slowly, PEI or PI has much time to diffuse in the amorphous phase of the blend. As the differences of compatibility of PEKK1 and PEKK2 can be hardly explained (no differences in the amorphous state, even in the Gordon Taylor constant K, have been observed), the second hypothesis seems to be more realistic.
Modelling The evolution of the Tg of amorphous blends follows a Gordon-Taylor law. Then, the crystallization of the blends changes the value of the Tg and the Tα. Two phenomena have to be taken into account for the Tg/Tα composition dependence modelling. To model the Tg composition dependence of the blends that have crystallized in the DSC apparatus oven, only the amorphous phase composition change during crystallization has been taken into account. Then, the minimum amorphous PEKK content in the amorphous phase of the blends, noted θPEKK, has been calculated from Eq.2, considering that the maximal crystallinity level of a blend is proportional to φPEKK (as only PEKK crystallizes). Indeed, we aim at calculating the maximal Tg obtainable, so we replace the total PEKK content φPEKK by θPEKK in the Gordon-Taylor law. This leads to Eq.3, that permits to calculate the maximal Tg of a blend knowing the characteristics of the pure polymers only. 5
Eq. 2
wc,PEKK is the maximum crystallinity level of PEKK. T g ,b =
θ PEKK Tg , PEKK + K (1 − θ PEKK )Tg , PEI θ PEKK + K (1 − θ PEKK )
Eq. 3
Concerning the annealed blends, the results show that they have crystallized. To model the increasing of the Tα, one can consider a linearly growth of the Tα with the crystallinity.15 Then, a new parameter is set, ΔTα = Tα, semi-crystalline – Tα, amorphous. As Tα evolves linearly with the crystallinity level, so does ΔTα. Moreover, when the crystallinity level evolves linearly with φPEKK, ΔTα is proportional to φPEKK. As a consequence, one adds a new parameter in Eq.3 to describe more precisely the Tα evolution of miscible annealed PEKK/PEI samples (Eq.4, Fig.6) Tα , m =
θ PEKK Tα , PEKK + K (1 − θ PEKK )Tα , PEI + ϕPEKKΔΤα , PEKK θ PEKK + K (1 − θ PEKK )
Eq. 4
Concerning the PEKK2 based blends, even with the annealing, the whole crystallization for all blends is not reached. Then, the Eq.4 can be modified to take into account the experimental crystallinity of each blend. Nevertheless, the crystallinity level of each blend has to be measured. θPEKK gets another expression (Eq.5) and the modified Gordon-Taylor law is transformed into Eq.6. θ PEKK ,exp = Tα , m =
ϕ PEKK − wc ,exp
Eq. 5
1 − wc ,exp
θ PEKK ,exp Tα , PEKK + K (1 − θ PEKK ,exp )Tα , PEI θ PEKK ,exp + K (1 − θ PEKK ,exp )
+
wc ,exp wc
ΔΤα , PEKK
Eq.6
The models are in good agreement with the experimental results. The hypothesis of linear evolution of Tα with crystallinity level is then reasonable to foresee the Tα of crystallized blends. It is important to note that for our blends, a non-negligible influence of the thermal treatments of PEKK/polyimide blends is put into relief. To correctly foresee the glass transitions of these blends, the effects of crystallinity have to be taken into account, and those ones depend on the way the polymers have crystallized. In this case, the polymer blend processing is then something important to control that will lead the thermal properties of the blends, particularly the ones based on PEKK1.
Rubbery plateau domain The thermomechanical analyses (Fig 3-SD - supporting data) show that the plateau modulus of the same semi-crystalline blends discussed previously decreases with the polyimide content. We have reported the value of G’ at the minimum of tan(δ) as a function of PEI content in Fig. 8 and 9 according to the state of the blend. In the amorphous state, there is no significant influence of the composition on the plateau modulus (Fig. 8). For semi-crystalline one, we have reported the variation of the plateau modulus as a function of the crystallinity ratio in Fig. 9. 6
These results can be described by the law proposed by Van Krevelen and reported in Eq. 7 where Gsc is the rubbery modulus of the semi-crystalline polymer, Gc is the rubbery modulus of the 100% crystalline polymer and Gam is the rubbery modulus of the amorphous polymer.1 Gc value has been found at 8.106 Pa, applying the model to experimental data and Gam has been measured using data reported in Fig. 8. Gsc = wc (Gc − Gam ) + Gam 2
Eq. 7
The evolution of the rubbery plateau demonstrates the importance of the crystallinity level on the mechanical properties of the blends. It is necessary to make a compromise between the improvement of the glass transition and the decrease of the rubbery moduli, to obtain good properties after Tg. Conclusions We have studied experimental variations of Tα and Tg PEKK/Polyimide blends. In the amorphous state, they are completely miscible. We have shown that a good control on crystallinity improves the value of the Tg, for a given composition. We have proposed a model that could describe the Tg and the Tα variation of the semi-crystalline blends. Some of the blends let appear phase segregation when crystallization occurs during annealing, due to the expulsion of the non-crystalline part out of the forming crystallites. The Tg improvement using the method of blends induce a decrease of the rubbery moduli, that has to be taken into account to adjust the mechanical properties as a function of the enduse applications.
Acknowledgements This work benefits from the support of BPI France within the ASTech and Aerospace Valley clusters and links to the COMPTINN project which deals with composites dedicated to structural applications during long lasting and intermediate temperatures for civil aeronautics. SD, CD and FD thank ARKEMA to provide PEKK samples.
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TABLES Table 1 - Temperature profile for blends composed of PEKK1 Zone
Die (10)
T (°C)
360
9
8
7
6
5
4
3
2
1
380 380 390 390 380 380 370 360 340
Table 2 - Temperature profile for blends composed of PEKK2 Zone Die 9 8 7 6 5 4 3 2 1 T (°C) 350 350 350 350 340 340 330 320 310 305
Table 3 – Injection parameters T3, T2, and T1 are the 3 barrel temperatures from die to hopper. T die (°C)
T3 (°C)
T2 (°C)
T1 (°C)
380
380 360 350
Injection speed (mm/s) 80
P t t T holding holding cooling mold (bar) (s) (s) (°C) 1000
10
5
7
90
FIGURES AND CAPTIONS Figure 1 – Screw profile “screw” : conveying elements, “discs” : kneading elements – the first three zones contain conveying screw elements Figure 2 – tan(δ) versus temperature of PEKK2/PEI amorphous blends : PEKK2: PEKK2 /PEI 80/20: 70/30: 60/40: 50/50: + 40/60: - 30/70: 20/80: + PEI:
;
Figure 3 – Glass-transition temperatures of amorphous ( ) and semi-crystalline ( blends of PEKK1/PEI with their crystallinity ratio ( ) versus PEI content.
)
Figure 4 – tan(δ) versus temperature of PEKK2/PEI semi-crystalline blends : PEKK2: PEKK2 /PEI 80/20: 70/30: 60/40: 50/50: + 40/60: 30/70: 20/80: + PEI:
;
Figure 5 – tan(δ) versus temperature of PEKK1/PEI semi-crystalline blends : PEKK1 /PEI 70/30: 60/40: 50/50: + 40/60: 30/70: Figure 6 – Tα versus polyimide content of PEKK1/PEI amorphous ( blends and PEKK1/PI amorphous ( ) and annealed ( ) blends Gordon-Taylor law K=0,73 ( ), K=0,5 ( ) Equation 4 K=0,73 ( ), K=0,5 ( )
)and annealed (
)
Figure 7 – Tα versus polyimide content of PEKK2/PEI amorphous ( blends and PEKK2/PI amorphous ( )and annealed ( ) blends Gordon-Taylor law K=0,73( ), K=0,47( ) Equation 6 K=0,73 ( ), K=0,47 ( )
)and annealed (
)
Figure 8 – Rubbery plateau modulus versus polyimide content of each amorphous blend : PEKK 1/PEI ( ), PEKK1/PI ( ), PEKK2/PEI ( + ), PEKK2/PI ( ) Figure 9 –Plateau modulus versus crystallinity level of semi-crystalline blend : PEKK1/PEI ( ), PEKK1/PI ( ), PEKK2/PEI ( ), PEKK2/PI ( )
11
Figure 1
12
Figure 2
4 3.5 3
tan (δ)
2.5 2 1.5 1 0.5 0
150
200
Temperature (°C)
13
250
230
0.4
220
0.35
210
0.3
200
0.25
190
0.2
180
0.15
170
0.1
160
0.05
150
0
0.2
0.4
0.6
PEI contents
14
0.8
1
0
weight fraction crystallinity
Tg (°C)
Figure 3
Figure 4
3 2.5
tan (δ)
2 1.5 1 0.5 0
150
200
Temperature (°C)
15
250
Figure 5
1 0.9 0.8
tan (δ)
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
150
250
Temperature (°C)
16
Figure 6
17
Figure 7
250
Tα (°C)
230
210
190
170
150
0
0.2
0.4
0.6
polyimide content
18
0.8
1
Figure 8
Rubbery moduli (Pa)
1.E+07
1.E+06 0
0.2
0.4
0.6
polyimide content
19
0.8
1
Figure 9
Rubbery moduli (Pa)
1.E+08
1.E+07
1.E+06 0
0.05
0.1
0.15
0.2
0.25
crystallinity level
20
0.3
0.35
0.4
Graphical abstract
21