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liquid crystalline polymer ternary blends—I. Calorimetric studies and morphology
Eur. Polym. J. Vol. 32, No. 3, pp. 349-363, 1996 Copyright 0 1996 Elsevier Science Ltd
Pergamon
Printed in Great Britain. All rights reserved 0014-3057/96 $15.00 + 0.00
POLYETHERIMIDE/POLY(ETHER ETHER KETONE)/LIQUID CRYSTALLINE POLYMER TERNARY BLENDS-I. CALORIMETRIC STUDIES AND MORPHOLOGY A. R. MORALES Department
of Materials
Engineering,
(Received
IO October
and R. E. S. BRETAS*
Universidade SSo Paula,
Federal Brazil
de ‘So
1994; accepted in final form
Carlos,
3 February
13565-905
SBo Carlos,
1995)
Abstract-The crystallization kinetics of polyetherimide (PEI)/poly(ether ether ketone) (PEEK)/liquid crystalline polymer ternary blends were studied by differential scanning calorimetry. Non-isothermal and isothermal crystallizations at different temperatures were analyzed by using the Avrami approach. The morphology obtained after isothermal crystallization at 315°C was observed by polarized light optical microscopy (PLOM). The miscibility region changed with the annealing temperature. The Avrami parameters, n and k, changed with the crystallization temperature and composition. A decrease in crystallization rates as the amount of PEI increases was observed; also the overall crystallization rate was
deendent on the crystallization temperature of the blend. From PLOM observations, it was concluded that heterogeneous nucleation is the main nucleation mechanism for the PEEK crystallization even in the blends.
INTRODUCTION
impact strength was maximum changed from 60 to 80% PEEK content, probably because crystallization enriched the amorphous phase in PEI. In recent work [7], researchers developed a novel ternary blend of PEEK, PEI and an LCP. The miscibility of these ternary blends, in the solid state, studied by dynamic mechanical analysis (DMA), using the r, criteria, was measured before and after annealing. Miscible and partially miscible compositions were observed and correlated with the morphology and mechanical properties of the blends. It was observed that, after annealing, compositions with the highest elastic moduli were partially miscible and with a fibrillar morphology, while compositions with the highest ultimate tensile strength were also partially miscible but with a droplet-like morphology. The cold crystallization temperature of the PEEK decreased with the increase in the amount of PEEK and the decrease of the amount of PEI, indicating that the presence of the PEI and LCP modified the PEEK crystallization rate. Due to the limitations of the DMA technique, calorimetric and morphological studies of these blends were proposed in order to observe this influence. These studies constitute the object of this work.
Crystallization rates in binary blends of a crystallizable polymer with another miscible, non-crystallizable polymer will be different from the pure polymer because of a dilution effect [l]. The result is usually a decrease in crystallization rate proportional to the increase in the amount of the non-crystallizable component. The radial growth rate G will depend on the molecular weight of the non-crystallizable polymer and on the crystallization temperature of the blend. The crystallization kinetics of pure poly(ether ether ketone) (PEEK) have been extensively studied [2, 31; it has been shown that the isothermal crystallization follows an Avrami behavior, with the exponent n being about three, indicating heterogeneous nucleation and three-dimensional spherulitic growth. A large part of the non-isothermal crystallization has been attributed to secondary processes. The behavior of PEEK in binary blends is somehow different and depends on the other component; for example, in blends with an amorphous phenolphthalein PEEK, PEK-C [4], the exponent n was found to decrease from 3.8 to 2.6 and the increase in PEK-C was found to inhibit the PEEK crystallization rate in the blend. Studies of blends with a liquid crystalline polymer (LCP) [S], indicated that a small addition of the LCP could enhance the crystallization of the PEEK without affecting its melting behavior. In blends with amorphous polyetherimide (PEI) [6], it was found that, upon annealing, the concentration at which the
Materials
*To whom all correspondence
The polymers used were PEI (Ultem 1000, Genera1 Electric), PEEK (Victrex 450G, ICI Co.) and an LCP (HX4000, DuPont). The LCP is a polyester based on terephthalic acid, phenylhydroquinone and hydroquinone.
should
be addressed. 349
EXPERIMENTAL
350
A. R. Morales
and R. E. S. Bretas
Blending The blends were prepared tested ternary compositions Comoosition
number
as described elsewhere were as follows: PEIIPEEKILCP
[7]. The
(wt%)
80/10/10 60110130 60/30;10 40140120 4OjlOj50 30/6O/lO 30/30/40 20/20/60 10/80/10 10160130 10/40;50 10110180
2 3 4 5 6 8 9 10 11 12
The binary compositions (wt%) were: PEEK/PEI: lo/SO. 30/70, SO/SO, 70/30 and 90/10; PEEK/HX4000: 30/70, SO/SO, 70/30 and 80/20; and PEI/HX4000: 30/70 and SO/SO. Annealing conditions The samples were annealed at two different conditions: one at 200°C (below the PEI T,), for 5 days and the other at 260°C (above the PEI r,), also for 5 days, in order to obtain samples as close as possible to the thermodynamic equilibrium. Deferential scanning calorimetry Non-isothermal
and isothermal
crystallizations
were per-
formed with a differential scanning calorimeter DSC 2910, from DuPont, under an N, atmosphere. For non-isothermal crystallization the scanning was performed on heating and on cooling. On heating, the heating rate was lO”C/min, and on cooling, the samples were first melted at 380°C for 2 min, and then cooled at - lO”C/min. For isothermal crystallization the samples were first melted at 380°C for 2&n, and then cooled to the isothermal crystallization temperature (315, 313, 309 and 304°C). The calorimetric data reported in this paper are the average of two, and in some cases three, scans. Polarized light optical microscopy In order to observe the blend morphology, samples were cut with an ultramicrotome (20 pm), pressed between two glass microscope slides, and thermally-treated in a vacuum oven (Five-2. EDG Eauinamentos e Controles Ltda). They were melted at 400 and’38O”C for 30 min, slowly .cooleh (3-S”C/min) to 315”C, maintained at this temperature for 3 hr and again slowly cooled to room temperature. Polarized
light optical micrographs (Carl Zeiss) microscope.
were obtained using a Jenaval
RESULTS AND DISCUSSION
Non-isothermal crystallization A typical DSC scan of the ternary blends, on heating, is shown in Fig. 1. When Tg was difficult to detect it was assumed to be the peak of the first derivative of the curve of heat flow versus temperature. Figure 2 shows the miscibility diagrams calculated by measuring the glass transition temperatures of the blends, on heating, before and after annealing. Figure 2(a) shows the miscibility diagram before annealing. Regarding the binary compositions, it can be observed that the PEEKjPEI and the PEI/HX4000 blends are miscible and the PEEK/HX4000 compositions are partially miscible, before annealing. It can
also be observed that the miscibility diagram of the ternary compositions has two separated regions: one with two partially miscible phases and the other with only one phase. In the partially miscible or two phase region the LCP rich phase remains almost unaltered (the LCP 7’p remains almost the same in all the partially miscible compositions). However, the second phase, PEEK or PEI rich phase, has a T, intermediate between the original 7, of these two polymers. After annealing at 2OO”C, the diagram modified slightly; the T8 values were higher than before annealing, as observed in Fig. 2(b). Composition 6 changed from miscible to partially miscible, probably due to the PEEK phase separation and subsequent crystallization; due to the PEEK crystallization. the LCP is rejected and two phases are formed: one rich in PEEK and the other rich in LCP. Composition 3 changed from partially miscible to miscible; probably all the PEEK crystallized and the remaining amorphous phase was only an LCP. Annealing at 260°C also modified the diagram, decreasing the miscibility window, as shown in Fig. 2(c). Some compositions showed three values of 7’g, corresponding to three phases. r, of the pure LCP decreased from 206 to 162 C, indicating that annealing at 260 C promoted plasticization of the LCP domains. However, it is also possible that this r, corresponds to a different glass-solid transition of this LCP. The main difference between these last two diagrams is the miscibility of the PEEKjHX4000 blends; at 200 C they seem to be partially miscible. while at 260’C they seem to be miscible. Figure 3 shows the melting behavior, on heating of the blends, before and after annealing. The annealing at 200-C did not modify the melting behavior of the blends as shown in Fig. 3(a); however, after annealing at 260-C, both PEEK and LCP each showed two melting peaks, as observed in Fig. 3(b). The presence of two fusion peaks in a slightly similar PEEK has already been observed and has been interpreted as either being a melting/recrystallization process occurring at around 260, C or due to the presence of two lamellar populations, each one having its own lamellar thickness and melting temperature [8,9]. Another work [lo] has also found two melting endotherms for pure PEEK, both varying with the heating rate. As the heating rate is increased, the heat of fusion of the low temperature endotherm increases and that of the high temperature endotherm decreases. Thus, it seems that these endotherms are highly dependent on the heating rate. In an LCP, the double melting has many interpretations: it can be due to a solid-nematic transition [l 1, 121, to interconvertible forms of polymers, which differ only in crystal size and properties, to fundamental differences in crystal morphology, or to true polymorphism. In the blends, 24 melting peaks can be observed, depending on the compositions; however, due to the closeness of these melting temperatures it is difficult to correlate each one with the corresponding polymer. It is only clear that annealing at 26O’C affects the pure polymers by melting/recrystallization or reorganization and that the blending between them and another component, amorphous PEI, alters this behavior in a very complex manner.
351
Calorimetric and morphological studies of ternary blends
150
200
250
300 TEMPERATURE
350
400
1%)
Fig. I. Typical DSC scan of the ternary blends.
Table 1 shows the cold crystallization temperatures on heating (7’,) and on cooling (T,,) and the glass transition temperatures on cooling (T,.) of the polymers and blends as received. It can be seen that, on heating, no cold crystallization was observed for the LCP and that up to 40 wt% PEEK, this temperature remains almost unaltered. Compositions of 40/40/20 and 10/40/50 have the same amount of PEEK; however, their crystallization temperatures are far apart by 35”C, indicating that both PEI and LCP are influencing the PEEK crystallization. In the first blend there is a dilution effect; that is the reason the crystallization temperature is higher. On cooling, both PEEK and LCP showed cold crystallization; the blends had crystallization tem-
peratures whose values were lower than the values of the pure polymers, indicating that either both polymers were miscible in the melt state and on cooling, phase separation occurred, lowering both crystallization temperatures, or co-crystallization occurred (each crystallite was composed of PEEK, LCP and amorphous PEI). It seems also that either the LCP did not act as a nucleating agent for the PEEK or the PEI dilution effect was strong, because in the blends where the amount of PEEK was high (composition 9), T, was lower than for the pure PEEK indicating that crystallization was delayed. All the other compositions showed two crystallization peaks, indicating separate crystallization of the LCP-rich phase and the PEEK-rich phase.
A. R. Morales
352 Table I. Crystallization
temperatures
and
R. E. S. Bretas
and glass transition
temperatures
of polymers
and blends
Composition
PEl/PEEK/LCP (wt%)
Tc (“C) WC)
O/I 00/o o/o/ 100
169
Tc’ ( C)
Tg’ (“C)
304 277
274 2731282 2741294 2731283 2691276 275/280 275/281
141 206 215 195 194/208 191/206 1731206 1711206 188/206 I761206 1741206
loo/o/o
I 2 5 I2 8 3 7 4
Isothermal
80/10/10 60/10/30 40/10/50 I o/ I o/so 20/20/60 60/30/10 30/30/40 40/40/20
231 237 234 252 239 232
II
10/40/50
197
2771295
154/208
6
30/60/10
210
2671295
163
IO
10/60/30
I87
2721293
I50
9
IO/SO/IO
I83
299
149
crystallization
dQ/dt = 0, where Q(t) is the heat flow rate. The data were plotted using the Avrami equation:
Figure 4 shows data of isothermal crystallization at 315°C. The times used for the calculation were: t, = stabilization time, t, = induction time, t, = time at which crystal growth begins and t,,, = time at
HX
1 - X, = exp( -kt”) where X, = volume fraction crystalhnity at time t, k = constant containing the nucleation and growth
4000 206
w
205
PEEK
214
PEI 0
ONE PHASE q TWO PHASES Fig.
2(a)
See caption on page 354
Calorimetric
and morphological
parameters, and n = constant that depends on the mechanism of nucleation and the form of crystal growth. These values were determined from curves of log[ - ln(1 - X,/X,)] (where X, is the volume fraction crystallinity at infinite time) versus log t, as shown in Fig. 5. Each curve showed two regions, one linear followed by a roll-off at longer times. Thus, two values for n can be calculated; however, Avrami analysis of the “roll-over” portion of the curves is not appropriate since secondary crystallization is not predicted by his approach. These data are presented in Table 2. In a study of pure PEEK [13], the authors found a dual mechanism for crystallization, one corresponding to n, = 2.5 and the other to n2 = 1.5, both independent of crystallization temperature. Our values for pure PEEK, however, changed with the crystallization temperature. Another study [ 141found 1.76 < n, < 2.60 and 1.49 < n2 < 2.03, depending on the holding time at the melting temperature. In our case this time was constant. Thus, on average, our values are similar to those of the literature.
studies
of ternary
blends
353
In the blends, the values of n varied also with the crystallization temperature and composition. If we analyse these values at 315°C for example, it can be seen that in compositions 3 and 7, crystallization was not observed; in both blends the dilution effect was predominant, inhibiting crystallization. Compositions 4 and 11 have the same amount of PEEK; however the n and k values are different. The k values reflect the nucleation rate and the growth rate or the overall crystallization rate. Thus, in composition 4, the overall crystallization rate is higher than in composition 11; this is an indication that the higher amount of LCP in composition 11 reduces the overall crystallization rate of the PEEK, probably due to its partial miscibility with this polymer. However, the induction time for composition 4 is higher than for composition 11, probably due to the dilution effect promoted by the PEI. In compositions 6 and 10, the values of n are similar, indicating spherulitic growth from sporadic or heterogeneous nuclei while the overall crystallization rate is slightly higher in composition 10 than in 6, as expected. In composition 9, these values are the same
HX 4000 204
PEEK
PEI
0 ONE PHASE &I TWO PHASES Fig. 2(b) See caption on page 354
354
A. R.
Morales and R. E. S. Bretas
as before. It can also be observed that the highest n values are associated with the highest crystallization temperatures. High values of n represent greater dimensionality in the growth process [15]. However, all these observations need to be confirmed by polarized light optical microscopy. As stated before, the k values reflect the overall crystallization rate. In pure PEEK, k increases with a decrease in crystallization temperature, as expected. This increase occurs up to approximately the temperature where the maximum crystallization rate should occur. In compositions 6, 9, 10 and 11 the same trend can be observed; however, in compositions 7 and 4 an inverse behavior is observed. In composition 7, k increases and later decreases with crystallization temperature; in composition 4, there is a decrease of the crystallization rate with temperature. It should be noticed that both compositions have equal amounts of PEI and PEEK, which forms a miscible binary blend. Probably as the crystallization temperature decreases and approaches the T, of PEI it gets harder for the PEEK to diffuse away from
the PEI, due to the crescent immobilization of these macromolecules. It can also be observed that the induction time decreases with the crystallization temperature for all the blends, and at a given crystallization temperature, compositions with high concentrations of PEI need higher times for crystal growth to begin and higher times for maximum crystallization rate to occur. It can also be seen that, at a given crystallization temperature, the higher the amount of PEEK in the blend, the smaller ti and t,,,. Finally, as expected, t,,, decreases with crystallization temperatures. The fold surface free energy of PEEK, cr,, in the blends was tentatively calculated by using the following expression [4] for miscible blends: c( = (In k/n) + {U*/R[C,
-[l
+ T - T,(q)]}
- 2aT;(cp)/b,fAHm”
AT(q)]
x In C#J* = In G, - rboaa, Tmc(~)/ x Kf AHm”TAT(4)
HX 4000 162
7 l3l 151 167 102
0
ONE PHASE
@I TWO PHASES Fig. 2(c) Fig. 2. Glass transition temperatures of the blends obtained by DSC, on heating: 0 (one T,), 0 (two T,). (a) As received; (b) after annealing at 200°C; (c) after annealing at 260°C.
(1)
Calorimetric and morphological studies of ternary blends where U* = activation energy for reptation of PEEK macromolecules in the melt = 8380 J/mol, C, = WLF constant = 5 1.6”C, Tg = glass transition temperature of the blend (composition dependent), 0 = lateral free energy surface of PEEK = 19 erg/cm2, rm’(cp) = equilibrium melting temperature of the crystallizable component (composition dependent), b, = thickness of the PEEK surface nucleus = 0.2929 nm, f= 2T/(Tm” + T), AHm” = equilibrium heat of fusion of PEEK = 130 J/g, cp = volume fraction of the crystallizable polymer, G, = preexponential factor (independent of temperature), r = parameter characteristic of the crystallization growth regime (4 for regime I and III and 2 for regime II); it was assumed regime III of crystallization for the PEEK, and K = Boltzman constant. It needs to be pointed out that equation (1) was derived for a binary miscible blend where one of the components is a crystallizable polymer and the other, is a non-crystallizable one, that will act as a diluent. In our ternary system, two polymers are capable of crystallization; however, it is known that at low concentrations, the HX4000 acts as a flow promoter (or a “plasticizant agent”) of both the PEEK and PEI-rich phases lowering the viscosities of the blends [16]. Therefore, we will assume, as a crude appoxima-
355
tion, that the only crystallizable polymer in this ternary system is the PEEK, and that both HX4000 and PEI act as diluents. The calculated value of o, for pure PEEK was found to be 47.6 erg/cm2, close to values found in the literature [4, 171; however, the values of 0, for the ternary blends varied from 47.6 to -40.27 erg/cm2. These unrealistic negative values suggest that some of the assumptions used to apply equation (1) need to be reviewed. This is the subject of another work [17]. Polarized
right optical microscopy
Figure 6 shows the polymers and some blends melted at 380°C and isothermally crystallized at 315°C. PEEK had a fine grained texture; the LCP showed a “schlieren” texture, probably nematic. In blends with the same amount of PEEK (compositions 6 and 10, 4 and 11, and 3 and 7) the increase in the amount of LCP produced a finer texture, while the increase in PEI produced an interconnected structure made of PEI domains. These morphologies can be observed in Figs 6(c)(f). These last blends had a higher elongation at maximum load [7] than the others. At high concentrations of PEEK (composition 9), Fig. 6(g), a fine grained morphology is again observed. The partial miscibility between the three
PEEK
PEI
340 Fig. 3(a) See caption ooerleaJ
A. R. Morales
356
and R. E. S. Bretas
HX4000 299-
306
PEI
PEEK
287-
-
339 Fig. 3(b)
Fig. 3. Melting temperatures of the blends, obtained by DCS, on heating. before and after annealing; 0 (one T,,,), 0 (two or more T,,,). (a) As received and after annealing at 200°C; (b) after annealing at 26O’C.
components can be observed in Fig. 6(h) (composition 12) where PEIjPEEK domains are dispersed in an LCP rich matrix. Figure 7 shows the micrographs of the polymers and some blends melted at 400°C and isothermally crystallized at 3 15°C. PEEK showed a coarser texture at 38O”C, while the LCP had a different morphology, with circular domains instead of rod-like domains. This last texture can be a result of degradation or the formation of an isotropic melt. In blends with the same amount of PEEK, the increase in LCP again produced a finer texture, while the increase in PEI induced the formation of a spherulitic morphology in PEEK as shown in Figs 7(c)+f). The amount of spherulites decreased as the amount of PEEK decreased and the amount of PEI increased. Due to the small amount of nuclei, large and well defined spherulites grew, with approximately the same size. Compositions 10/80/10 and 80/10/10 also showed large PEEK spherulites, while composition lO/lO/SO did not, as observed in Figs 7(g) and (h), respectively. Probably, at 380°C the PEEK did not melt completely, and a high amount of the previous nuclei remained; so, on cooling, a high amount of very small
size spherulites (not detected by PLOM) grew. However, at 400°C PEEK melted completely and heterogeneous (or athermal) nucleation occurred (because the spherulites have all almost the same size). The PEI and LCP also have influence; at OjlOOjO no spherulites were observed at both melting temperatures; however, at lO/SO/lO, after melting at 4OO”C, large spherulites appeared, indicating that the LCP domains could have acted as nucleating agents for the PEEK. CONCLUSIONS
Even though this work is a preliminary study of the calorimetric behavior and morphology of PEI/PEEK/LCP ternary blends, some conclusions can be drawn. The miscibility region changed with annealing temperature, due to phase separation, PEEK crystallization and reorganisation of LCP domains. Miscibility is a complex issue when both polymers are crystallizable. PEEK and HX4000 seem (at least visually, by PLOM) to be miscible at high temperatures [17]. At room temperature, before annealing, they seem to be
Calorimetric and morphological studies of ternary blends Table
2. Avrami
357
analysis
Avrami Comnosition
Darameters
PEEK
315 c
313°C
309 c
2.4
2. I
2.0 0.8
1.7 2.1 x IO -6
0.9 1.3 x 10-S
I.5 x 10-4
0.5
0
0
6.0
3.3
0.9
3 (60/30/10)
304 c
2.6
2.5
5.6 x IO-'
2.9
4.8
16.9
14.3
2.7
2.7
2.1 2.0x10
4.2 ;I,-"
4 (40/40/20)
“I 3 k
6 (30/60/10)
5.5
1.5
14.3
12.4
2.0
2.4
2.8
3.1
9.9 x IO x 21.5
9.6
53.4
31.5
20.0
2.9
2.5
2.4
3.2
2.8
1.7
8 x IO-"
19.1
6.4
0.5
14.1
4.0
3.2
3.0
2.3
“i
I.3 I"
2.6 x IO '
11.3
3.3
2.0
16.2
8.2
3.0
2.5 -
2.3
I.8 x 10-7
1.5 4.1 x IO '
5.6
4.0
0
15.7
9.7
5.4
2.9
2.9
2.3
n2
k
7.3 x IO
21.7
3.5 x 10-v
9 (10/80/10)
7.0 x 10-7
30.8
2.4
IO (10/60/30)
7 x 10-x
30.2
3.3 ,O~lO
6.2 x IO '
1.7
I.7
8.5 x lO-9
2.6 x IO '
3.0
0.5
0
12.2
7.6
3.1
partially miscible by DMTA [7, 171. However, by scanning electron microscopy (SEM) [7], no two phases are observed. After annealing, they again seem to be miscible (by DMTA, DSC and SEM). Before annealing, we cannot assume that the PEEK/HX4000 blends are at equilibrium because the PEEK has not fully crystallized. Thus, we assume that the annealed blends are more thermodynamically stable than the non-annealed ones, and therefore PEEK and HX4000 are miscible at room temperature. The Avrami parameters, n and k, changed with the crystallization temperature and composition. Compositions with high concentrations of PEI or LCP needed higher times for crystal growth to begin and higher times for maximum crystallization to occur. As observed also in binary blends, in some of the ternary blends there is a decrease in crystallization rates as the amount of non-crystallizable component (PEI) increases; and as also observed in binary blends, the overall crystallization rate is dependent on the crystallization temperature of the ternary blend.
8.2 x IO-'
14.5
I.8
3.4 x 10-9
2.0 8
39.4
3.1 I.0 x 10-h
4.5 x IO 8
10.7
The spherulitic morphology of PEEK in these blends is dependent on the melting temperature and composition. Our results of PLOM cannot be compared with the isothermal crystallization results because we did not use a hot stage to measure crystallization rates; however, it can be concluded from both techniques that heterogeneous nucleation is the main nucleation mechanism for the PEEK crystallization even in the blends. Evidently, the influence of the LCP on the nonisothermal crystallization is different on heating than on cooling. On heating, the LCP has organized domains that will not need further rearrangement (or crystallization); thus, no cold crystallization for the LCP is observed and only amorphous PEEK will crystallize. Therefore the influence of the amorphous diluent (PEI) will be strong. On cooling, however, the LCP organized domains will solidify (or crystallize) from the melt, and a cold crystallization temperature is observed. In this case, both PEI and LCP will strongly influence the PEEK crystallization by diluting it. Data of the crystallization kinetics of PEEK/ HX4000 blends confirm this last observation [17].
358
A. R. Morales
and R. E. S. Bretas
0.8-
0.6 -
g
0.4-
PEEK
0 10/80/10
Ei s
0.230/60/10
!G O-
30/
30/40
-0.2-
- 0.4b
I 10
I 20
I 30
I 40 TIME
Fig. 4. Isothermal
crystallization
Fig. 5. Avrami
at 315’ C, obtained
plots at 315’C
I 50
I 60
I 70
I 80
(min.)
by DSC. for PEEK
for PEEK
and some of the blends
and some of the blends.
I
Calorimetric
and morphological
studies
of ternary
blends
359
t\. R. Morales
and R. E. S. Bretas
Calorimetric
and morphological
studies
of ternary
blends
361
362
A. R. Morales
and R. E. S. Bretas
Calorimetric
and morphological
Acknowledgemenrs-The authors would like to thank Prof. Donald G. Baird, in whose laboratories the blends were prepared, FAPESP (92-0990-2) and Volkswagon Foundation (I-69693).
studies
8. 9.
REFERENCES 1. J. P. Runt
2. 3. 4. 5. 6.
and L. M. Martynowicz. In Multicomponent Polvmer Materials (edited by D. R. Paul and L. H. Spelling). ACS, WashingtonDC (1986). P. Cebe and S. D. Hone. Poivmer 27. I183 (19861. A. Jonas and R. Legrasl Polimer 32,‘15, 2641 (1691). G. C. Alfonso, V. Chiappia, J. Liu and E. Sakiku. Eur. Polym. J. 27, 795 (1991). A. Mehta and A. I. Isayev. Polym. Eng. Sci. 31, 963 (1991). J. E. Harris and L. M. Robeson. In Confemporary Topics in Polymer Science, Vol. 6, Multicomponent Macromolecular Systems (edited by Bill M. Culbertson) p. 520 (1989).
10. 11. 12. 13. 14. 15. 16. 17.
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blends
363
R. E. S. Bretas and D. G. Baird. Polymer 33, 5233 (1992). D. J. Blundell, J. J. Liggat and A. Flory. Polymer 33, 2475 (1992). A. Jonas, R. Legras and J. P. Issi. Polymer 32, 3364 (1991). Y. Lee and R. S. Porter. Macromolecules 21. 2770 (1988). Polymeric Liquid Crystals (edited by Alexandre Blumstein). Plenum Press. New York (1985). S. Z: D. Cheng. M&omolecules il, 2475 (1988). C. N. Velisaris and J. C. Seferis. Poiym. Eng. Sci. 26, 1574 (1986). Y. Deslandes, M. Day, N. F. Sabir and T. Suprunchuk. Polym. Comp. 10, 360 (1989). L. H. Sperling. Introduction to Physical Polyner Science. Wiley Interscience, New York (1986). R. E. S. Bretas, D. Collias and D. G. Baird. Polym. Eng. Sci. 34, 1492 (1994). B. Carvalho and R. E. S. Bretas. J. Appl. Polym. Sci. 55, 233 (1995).
Pergamon
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POLYETHERIMIDE/POLY(ETHER ETHER KETONE)/LIQUID CRYSTALLINE POLYMER TERNARY BLENDS-I. CALORIMETRIC STUDIES AND MORPHOLOGY A. R. MORALES Department
of Materials
Engineering,
(Received
IO October
and R. E. S. BRETAS*
Universidade SSo Paula,
Federal Brazil
de ‘So
1994; accepted in final form
Carlos,
3 February
13565-905
SBo Carlos,
1995)
Abstract-The crystallization kinetics of polyetherimide (PEI)/poly(ether ether ketone) (PEEK)/liquid crystalline polymer ternary blends were studied by differential scanning calorimetry. Non-isothermal and isothermal crystallizations at different temperatures were analyzed by using the Avrami approach. The morphology obtained after isothermal crystallization at 315°C was observed by polarized light optical microscopy (PLOM). The miscibility region changed with the annealing temperature. The Avrami parameters, n and k, changed with the crystallization temperature and composition. A decrease in crystallization rates as the amount of PEI increases was observed; also the overall crystallization rate was
deendent on the crystallization temperature of the blend. From PLOM observations, it was concluded that heterogeneous nucleation is the main nucleation mechanism for the PEEK crystallization even in the blends.
INTRODUCTION
impact strength was maximum changed from 60 to 80% PEEK content, probably because crystallization enriched the amorphous phase in PEI. In recent work [7], researchers developed a novel ternary blend of PEEK, PEI and an LCP. The miscibility of these ternary blends, in the solid state, studied by dynamic mechanical analysis (DMA), using the r, criteria, was measured before and after annealing. Miscible and partially miscible compositions were observed and correlated with the morphology and mechanical properties of the blends. It was observed that, after annealing, compositions with the highest elastic moduli were partially miscible and with a fibrillar morphology, while compositions with the highest ultimate tensile strength were also partially miscible but with a droplet-like morphology. The cold crystallization temperature of the PEEK decreased with the increase in the amount of PEEK and the decrease of the amount of PEI, indicating that the presence of the PEI and LCP modified the PEEK crystallization rate. Due to the limitations of the DMA technique, calorimetric and morphological studies of these blends were proposed in order to observe this influence. These studies constitute the object of this work.
Crystallization rates in binary blends of a crystallizable polymer with another miscible, non-crystallizable polymer will be different from the pure polymer because of a dilution effect [l]. The result is usually a decrease in crystallization rate proportional to the increase in the amount of the non-crystallizable component. The radial growth rate G will depend on the molecular weight of the non-crystallizable polymer and on the crystallization temperature of the blend. The crystallization kinetics of pure poly(ether ether ketone) (PEEK) have been extensively studied [2, 31; it has been shown that the isothermal crystallization follows an Avrami behavior, with the exponent n being about three, indicating heterogeneous nucleation and three-dimensional spherulitic growth. A large part of the non-isothermal crystallization has been attributed to secondary processes. The behavior of PEEK in binary blends is somehow different and depends on the other component; for example, in blends with an amorphous phenolphthalein PEEK, PEK-C [4], the exponent n was found to decrease from 3.8 to 2.6 and the increase in PEK-C was found to inhibit the PEEK crystallization rate in the blend. Studies of blends with a liquid crystalline polymer (LCP) [S], indicated that a small addition of the LCP could enhance the crystallization of the PEEK without affecting its melting behavior. In blends with amorphous polyetherimide (PEI) [6], it was found that, upon annealing, the concentration at which the
Materials
*To whom all correspondence
The polymers used were PEI (Ultem 1000, Genera1 Electric), PEEK (Victrex 450G, ICI Co.) and an LCP (HX4000, DuPont). The LCP is a polyester based on terephthalic acid, phenylhydroquinone and hydroquinone.
should
be addressed. 349
EXPERIMENTAL
350
A. R. Morales
and R. E. S. Bretas
Blending The blends were prepared tested ternary compositions Comoosition
number
as described elsewhere were as follows: PEIIPEEKILCP
[7]. The
(wt%)
80/10/10 60110130 60/30;10 40140120 4OjlOj50 30/6O/lO 30/30/40 20/20/60 10/80/10 10160130 10/40;50 10110180
2 3 4 5 6 8 9 10 11 12
The binary compositions (wt%) were: PEEK/PEI: lo/SO. 30/70, SO/SO, 70/30 and 90/10; PEEK/HX4000: 30/70, SO/SO, 70/30 and 80/20; and PEI/HX4000: 30/70 and SO/SO. Annealing conditions The samples were annealed at two different conditions: one at 200°C (below the PEI T,), for 5 days and the other at 260°C (above the PEI r,), also for 5 days, in order to obtain samples as close as possible to the thermodynamic equilibrium. Deferential scanning calorimetry Non-isothermal
and isothermal
crystallizations
were per-
formed with a differential scanning calorimeter DSC 2910, from DuPont, under an N, atmosphere. For non-isothermal crystallization the scanning was performed on heating and on cooling. On heating, the heating rate was lO”C/min, and on cooling, the samples were first melted at 380°C for 2 min, and then cooled at - lO”C/min. For isothermal crystallization the samples were first melted at 380°C for 2&n, and then cooled to the isothermal crystallization temperature (315, 313, 309 and 304°C). The calorimetric data reported in this paper are the average of two, and in some cases three, scans. Polarized light optical microscopy In order to observe the blend morphology, samples were cut with an ultramicrotome (20 pm), pressed between two glass microscope slides, and thermally-treated in a vacuum oven (Five-2. EDG Eauinamentos e Controles Ltda). They were melted at 400 and’38O”C for 30 min, slowly .cooleh (3-S”C/min) to 315”C, maintained at this temperature for 3 hr and again slowly cooled to room temperature. Polarized
light optical micrographs (Carl Zeiss) microscope.
were obtained using a Jenaval
RESULTS AND DISCUSSION
Non-isothermal crystallization A typical DSC scan of the ternary blends, on heating, is shown in Fig. 1. When Tg was difficult to detect it was assumed to be the peak of the first derivative of the curve of heat flow versus temperature. Figure 2 shows the miscibility diagrams calculated by measuring the glass transition temperatures of the blends, on heating, before and after annealing. Figure 2(a) shows the miscibility diagram before annealing. Regarding the binary compositions, it can be observed that the PEEKjPEI and the PEI/HX4000 blends are miscible and the PEEK/HX4000 compositions are partially miscible, before annealing. It can
also be observed that the miscibility diagram of the ternary compositions has two separated regions: one with two partially miscible phases and the other with only one phase. In the partially miscible or two phase region the LCP rich phase remains almost unaltered (the LCP 7’p remains almost the same in all the partially miscible compositions). However, the second phase, PEEK or PEI rich phase, has a T, intermediate between the original 7, of these two polymers. After annealing at 2OO”C, the diagram modified slightly; the T8 values were higher than before annealing, as observed in Fig. 2(b). Composition 6 changed from miscible to partially miscible, probably due to the PEEK phase separation and subsequent crystallization; due to the PEEK crystallization. the LCP is rejected and two phases are formed: one rich in PEEK and the other rich in LCP. Composition 3 changed from partially miscible to miscible; probably all the PEEK crystallized and the remaining amorphous phase was only an LCP. Annealing at 260°C also modified the diagram, decreasing the miscibility window, as shown in Fig. 2(c). Some compositions showed three values of 7’g, corresponding to three phases. r, of the pure LCP decreased from 206 to 162 C, indicating that annealing at 260 C promoted plasticization of the LCP domains. However, it is also possible that this r, corresponds to a different glass-solid transition of this LCP. The main difference between these last two diagrams is the miscibility of the PEEKjHX4000 blends; at 200 C they seem to be partially miscible. while at 260’C they seem to be miscible. Figure 3 shows the melting behavior, on heating of the blends, before and after annealing. The annealing at 200-C did not modify the melting behavior of the blends as shown in Fig. 3(a); however, after annealing at 260-C, both PEEK and LCP each showed two melting peaks, as observed in Fig. 3(b). The presence of two fusion peaks in a slightly similar PEEK has already been observed and has been interpreted as either being a melting/recrystallization process occurring at around 260, C or due to the presence of two lamellar populations, each one having its own lamellar thickness and melting temperature [8,9]. Another work [lo] has also found two melting endotherms for pure PEEK, both varying with the heating rate. As the heating rate is increased, the heat of fusion of the low temperature endotherm increases and that of the high temperature endotherm decreases. Thus, it seems that these endotherms are highly dependent on the heating rate. In an LCP, the double melting has many interpretations: it can be due to a solid-nematic transition [l 1, 121, to interconvertible forms of polymers, which differ only in crystal size and properties, to fundamental differences in crystal morphology, or to true polymorphism. In the blends, 24 melting peaks can be observed, depending on the compositions; however, due to the closeness of these melting temperatures it is difficult to correlate each one with the corresponding polymer. It is only clear that annealing at 26O’C affects the pure polymers by melting/recrystallization or reorganization and that the blending between them and another component, amorphous PEI, alters this behavior in a very complex manner.
351
Calorimetric and morphological studies of ternary blends
150
200
250
300 TEMPERATURE
350
400
1%)
Fig. I. Typical DSC scan of the ternary blends.
Table 1 shows the cold crystallization temperatures on heating (7’,) and on cooling (T,,) and the glass transition temperatures on cooling (T,.) of the polymers and blends as received. It can be seen that, on heating, no cold crystallization was observed for the LCP and that up to 40 wt% PEEK, this temperature remains almost unaltered. Compositions of 40/40/20 and 10/40/50 have the same amount of PEEK; however, their crystallization temperatures are far apart by 35”C, indicating that both PEI and LCP are influencing the PEEK crystallization. In the first blend there is a dilution effect; that is the reason the crystallization temperature is higher. On cooling, both PEEK and LCP showed cold crystallization; the blends had crystallization tem-
peratures whose values were lower than the values of the pure polymers, indicating that either both polymers were miscible in the melt state and on cooling, phase separation occurred, lowering both crystallization temperatures, or co-crystallization occurred (each crystallite was composed of PEEK, LCP and amorphous PEI). It seems also that either the LCP did not act as a nucleating agent for the PEEK or the PEI dilution effect was strong, because in the blends where the amount of PEEK was high (composition 9), T, was lower than for the pure PEEK indicating that crystallization was delayed. All the other compositions showed two crystallization peaks, indicating separate crystallization of the LCP-rich phase and the PEEK-rich phase.
A. R. Morales
352 Table I. Crystallization
temperatures
and
R. E. S. Bretas
and glass transition
temperatures
of polymers
and blends
Composition
PEl/PEEK/LCP (wt%)
Tc (“C) WC)
O/I 00/o o/o/ 100
169
Tc’ ( C)
Tg’ (“C)
304 277
274 2731282 2741294 2731283 2691276 275/280 275/281
141 206 215 195 194/208 191/206 1731206 1711206 188/206 I761206 1741206
loo/o/o
I 2 5 I2 8 3 7 4
Isothermal
80/10/10 60/10/30 40/10/50 I o/ I o/so 20/20/60 60/30/10 30/30/40 40/40/20
231 237 234 252 239 232
II
10/40/50
197
2771295
154/208
6
30/60/10
210
2671295
163
IO
10/60/30
I87
2721293
I50
9
IO/SO/IO
I83
299
149
crystallization
dQ/dt = 0, where Q(t) is the heat flow rate. The data were plotted using the Avrami equation:
Figure 4 shows data of isothermal crystallization at 315°C. The times used for the calculation were: t, = stabilization time, t, = induction time, t, = time at which crystal growth begins and t,,, = time at
HX
1 - X, = exp( -kt”) where X, = volume fraction crystalhnity at time t, k = constant containing the nucleation and growth
4000 206
w
205
PEEK
214
PEI 0
ONE PHASE q TWO PHASES Fig.
2(a)
See caption on page 354
Calorimetric
and morphological
parameters, and n = constant that depends on the mechanism of nucleation and the form of crystal growth. These values were determined from curves of log[ - ln(1 - X,/X,)] (where X, is the volume fraction crystallinity at infinite time) versus log t, as shown in Fig. 5. Each curve showed two regions, one linear followed by a roll-off at longer times. Thus, two values for n can be calculated; however, Avrami analysis of the “roll-over” portion of the curves is not appropriate since secondary crystallization is not predicted by his approach. These data are presented in Table 2. In a study of pure PEEK [13], the authors found a dual mechanism for crystallization, one corresponding to n, = 2.5 and the other to n2 = 1.5, both independent of crystallization temperature. Our values for pure PEEK, however, changed with the crystallization temperature. Another study [ 141found 1.76 < n, < 2.60 and 1.49 < n2 < 2.03, depending on the holding time at the melting temperature. In our case this time was constant. Thus, on average, our values are similar to those of the literature.
studies
of ternary
blends
353
In the blends, the values of n varied also with the crystallization temperature and composition. If we analyse these values at 315°C for example, it can be seen that in compositions 3 and 7, crystallization was not observed; in both blends the dilution effect was predominant, inhibiting crystallization. Compositions 4 and 11 have the same amount of PEEK; however the n and k values are different. The k values reflect the nucleation rate and the growth rate or the overall crystallization rate. Thus, in composition 4, the overall crystallization rate is higher than in composition 11; this is an indication that the higher amount of LCP in composition 11 reduces the overall crystallization rate of the PEEK, probably due to its partial miscibility with this polymer. However, the induction time for composition 4 is higher than for composition 11, probably due to the dilution effect promoted by the PEI. In compositions 6 and 10, the values of n are similar, indicating spherulitic growth from sporadic or heterogeneous nuclei while the overall crystallization rate is slightly higher in composition 10 than in 6, as expected. In composition 9, these values are the same
HX 4000 204
PEEK
PEI
0 ONE PHASE &I TWO PHASES Fig. 2(b) See caption on page 354
354
A. R.
Morales and R. E. S. Bretas
as before. It can also be observed that the highest n values are associated with the highest crystallization temperatures. High values of n represent greater dimensionality in the growth process [15]. However, all these observations need to be confirmed by polarized light optical microscopy. As stated before, the k values reflect the overall crystallization rate. In pure PEEK, k increases with a decrease in crystallization temperature, as expected. This increase occurs up to approximately the temperature where the maximum crystallization rate should occur. In compositions 6, 9, 10 and 11 the same trend can be observed; however, in compositions 7 and 4 an inverse behavior is observed. In composition 7, k increases and later decreases with crystallization temperature; in composition 4, there is a decrease of the crystallization rate with temperature. It should be noticed that both compositions have equal amounts of PEI and PEEK, which forms a miscible binary blend. Probably as the crystallization temperature decreases and approaches the T, of PEI it gets harder for the PEEK to diffuse away from
the PEI, due to the crescent immobilization of these macromolecules. It can also be observed that the induction time decreases with the crystallization temperature for all the blends, and at a given crystallization temperature, compositions with high concentrations of PEI need higher times for crystal growth to begin and higher times for maximum crystallization rate to occur. It can also be seen that, at a given crystallization temperature, the higher the amount of PEEK in the blend, the smaller ti and t,,,. Finally, as expected, t,,, decreases with crystallization temperatures. The fold surface free energy of PEEK, cr,, in the blends was tentatively calculated by using the following expression [4] for miscible blends: c( = (In k/n) + {U*/R[C,
-[l
+ T - T,(q)]}
- 2aT;(cp)/b,fAHm”
AT(q)]
x In C#J* = In G, - rboaa, Tmc(~)/ x Kf AHm”TAT(4)
HX 4000 162
7 l3l 151 167 102
0
ONE PHASE
@I TWO PHASES Fig. 2(c) Fig. 2. Glass transition temperatures of the blends obtained by DSC, on heating: 0 (one T,), 0 (two T,). (a) As received; (b) after annealing at 200°C; (c) after annealing at 260°C.
(1)
Calorimetric and morphological studies of ternary blends where U* = activation energy for reptation of PEEK macromolecules in the melt = 8380 J/mol, C, = WLF constant = 5 1.6”C, Tg = glass transition temperature of the blend (composition dependent), 0 = lateral free energy surface of PEEK = 19 erg/cm2, rm’(cp) = equilibrium melting temperature of the crystallizable component (composition dependent), b, = thickness of the PEEK surface nucleus = 0.2929 nm, f= 2T/(Tm” + T), AHm” = equilibrium heat of fusion of PEEK = 130 J/g, cp = volume fraction of the crystallizable polymer, G, = preexponential factor (independent of temperature), r = parameter characteristic of the crystallization growth regime (4 for regime I and III and 2 for regime II); it was assumed regime III of crystallization for the PEEK, and K = Boltzman constant. It needs to be pointed out that equation (1) was derived for a binary miscible blend where one of the components is a crystallizable polymer and the other, is a non-crystallizable one, that will act as a diluent. In our ternary system, two polymers are capable of crystallization; however, it is known that at low concentrations, the HX4000 acts as a flow promoter (or a “plasticizant agent”) of both the PEEK and PEI-rich phases lowering the viscosities of the blends [16]. Therefore, we will assume, as a crude appoxima-
355
tion, that the only crystallizable polymer in this ternary system is the PEEK, and that both HX4000 and PEI act as diluents. The calculated value of o, for pure PEEK was found to be 47.6 erg/cm2, close to values found in the literature [4, 171; however, the values of 0, for the ternary blends varied from 47.6 to -40.27 erg/cm2. These unrealistic negative values suggest that some of the assumptions used to apply equation (1) need to be reviewed. This is the subject of another work [17]. Polarized
right optical microscopy
Figure 6 shows the polymers and some blends melted at 380°C and isothermally crystallized at 315°C. PEEK had a fine grained texture; the LCP showed a “schlieren” texture, probably nematic. In blends with the same amount of PEEK (compositions 6 and 10, 4 and 11, and 3 and 7) the increase in the amount of LCP produced a finer texture, while the increase in PEI produced an interconnected structure made of PEI domains. These morphologies can be observed in Figs 6(c)(f). These last blends had a higher elongation at maximum load [7] than the others. At high concentrations of PEEK (composition 9), Fig. 6(g), a fine grained morphology is again observed. The partial miscibility between the three
PEEK
PEI
340 Fig. 3(a) See caption ooerleaJ
A. R. Morales
356
and R. E. S. Bretas
HX4000 299-
306
PEI
PEEK
287-
-
339 Fig. 3(b)
Fig. 3. Melting temperatures of the blends, obtained by DCS, on heating. before and after annealing; 0 (one T,,,), 0 (two or more T,,,). (a) As received and after annealing at 200°C; (b) after annealing at 26O’C.
components can be observed in Fig. 6(h) (composition 12) where PEIjPEEK domains are dispersed in an LCP rich matrix. Figure 7 shows the micrographs of the polymers and some blends melted at 400°C and isothermally crystallized at 3 15°C. PEEK showed a coarser texture at 38O”C, while the LCP had a different morphology, with circular domains instead of rod-like domains. This last texture can be a result of degradation or the formation of an isotropic melt. In blends with the same amount of PEEK, the increase in LCP again produced a finer texture, while the increase in PEI induced the formation of a spherulitic morphology in PEEK as shown in Figs 7(c)+f). The amount of spherulites decreased as the amount of PEEK decreased and the amount of PEI increased. Due to the small amount of nuclei, large and well defined spherulites grew, with approximately the same size. Compositions 10/80/10 and 80/10/10 also showed large PEEK spherulites, while composition lO/lO/SO did not, as observed in Figs 7(g) and (h), respectively. Probably, at 380°C the PEEK did not melt completely, and a high amount of the previous nuclei remained; so, on cooling, a high amount of very small
size spherulites (not detected by PLOM) grew. However, at 400°C PEEK melted completely and heterogeneous (or athermal) nucleation occurred (because the spherulites have all almost the same size). The PEI and LCP also have influence; at OjlOOjO no spherulites were observed at both melting temperatures; however, at lO/SO/lO, after melting at 4OO”C, large spherulites appeared, indicating that the LCP domains could have acted as nucleating agents for the PEEK. CONCLUSIONS
Even though this work is a preliminary study of the calorimetric behavior and morphology of PEI/PEEK/LCP ternary blends, some conclusions can be drawn. The miscibility region changed with annealing temperature, due to phase separation, PEEK crystallization and reorganisation of LCP domains. Miscibility is a complex issue when both polymers are crystallizable. PEEK and HX4000 seem (at least visually, by PLOM) to be miscible at high temperatures [17]. At room temperature, before annealing, they seem to be
Calorimetric and morphological studies of ternary blends Table
2. Avrami
357
analysis
Avrami Comnosition
Darameters
PEEK
315 c
313°C
309 c
2.4
2. I
2.0 0.8
1.7 2.1 x IO -6
0.9 1.3 x 10-S
I.5 x 10-4
0.5
0
0
6.0
3.3
0.9
3 (60/30/10)
304 c
2.6
2.5
5.6 x IO-'
2.9
4.8
16.9
14.3
2.7
2.7
2.1 2.0x10
4.2 ;I,-"
4 (40/40/20)
“I 3 k
6 (30/60/10)
5.5
1.5
14.3
12.4
2.0
2.4
2.8
3.1
9.9 x IO x 21.5
9.6
53.4
31.5
20.0
2.9
2.5
2.4
3.2
2.8
1.7
8 x IO-"
19.1
6.4
0.5
14.1
4.0
3.2
3.0
2.3
“i
I.3 I"
2.6 x IO '
11.3
3.3
2.0
16.2
8.2
3.0
2.5 -
2.3
I.8 x 10-7
1.5 4.1 x IO '
5.6
4.0
0
15.7
9.7
5.4
2.9
2.9
2.3
n2
k
7.3 x IO
21.7
3.5 x 10-v
9 (10/80/10)
7.0 x 10-7
30.8
2.4
IO (10/60/30)
7 x 10-x
30.2
3.3 ,O~lO
6.2 x IO '
1.7
I.7
8.5 x lO-9
2.6 x IO '
3.0
0.5
0
12.2
7.6
3.1
partially miscible by DMTA [7, 171. However, by scanning electron microscopy (SEM) [7], no two phases are observed. After annealing, they again seem to be miscible (by DMTA, DSC and SEM). Before annealing, we cannot assume that the PEEK/HX4000 blends are at equilibrium because the PEEK has not fully crystallized. Thus, we assume that the annealed blends are more thermodynamically stable than the non-annealed ones, and therefore PEEK and HX4000 are miscible at room temperature. The Avrami parameters, n and k, changed with the crystallization temperature and composition. Compositions with high concentrations of PEI or LCP needed higher times for crystal growth to begin and higher times for maximum crystallization to occur. As observed also in binary blends, in some of the ternary blends there is a decrease in crystallization rates as the amount of non-crystallizable component (PEI) increases; and as also observed in binary blends, the overall crystallization rate is dependent on the crystallization temperature of the ternary blend.
8.2 x IO-'
14.5
I.8
3.4 x 10-9
2.0 8
39.4
3.1 I.0 x 10-h
4.5 x IO 8
10.7
The spherulitic morphology of PEEK in these blends is dependent on the melting temperature and composition. Our results of PLOM cannot be compared with the isothermal crystallization results because we did not use a hot stage to measure crystallization rates; however, it can be concluded from both techniques that heterogeneous nucleation is the main nucleation mechanism for the PEEK crystallization even in the blends. Evidently, the influence of the LCP on the nonisothermal crystallization is different on heating than on cooling. On heating, the LCP has organized domains that will not need further rearrangement (or crystallization); thus, no cold crystallization for the LCP is observed and only amorphous PEEK will crystallize. Therefore the influence of the amorphous diluent (PEI) will be strong. On cooling, however, the LCP organized domains will solidify (or crystallize) from the melt, and a cold crystallization temperature is observed. In this case, both PEI and LCP will strongly influence the PEEK crystallization by diluting it. Data of the crystallization kinetics of PEEK/ HX4000 blends confirm this last observation [17].
358
A. R. Morales
and R. E. S. Bretas
0.8-
0.6 -
g
0.4-
PEEK
0 10/80/10
Ei s
0.230/60/10
!G O-
30/
30/40
-0.2-
- 0.4b
I 10
I 20
I 30
I 40 TIME
Fig. 4. Isothermal
crystallization
Fig. 5. Avrami
at 315’ C, obtained
plots at 315’C
I 50
I 60
I 70
I 80
(min.)
by DSC. for PEEK
for PEEK
and some of the blends
and some of the blends.
I
Calorimetric
and morphological
studies
of ternary
blends
359
t\. R. Morales
and R. E. S. Bretas
Calorimetric
and morphological
studies
of ternary
blends
361
362
A. R. Morales
and R. E. S. Bretas
Calorimetric
and morphological
Acknowledgemenrs-The authors would like to thank Prof. Donald G. Baird, in whose laboratories the blends were prepared, FAPESP (92-0990-2) and Volkswagon Foundation (I-69693).
studies
8. 9.
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363
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