The Dynamic Properties, Temperature Transitions, and Thermal Stability of Poly(etherether ketone)- Thermotropic Liquid Crystalline Polymer Blends

ATUL MEHTA and A. I. ISAYEV"

Institute of PoZymer Engineering University of Akron

Akron, Ohio 44325-0301

Differential scanning calorimetry, dynamic mechanical measurements, and thermal gravimetric analysis techniques have been used to thermally character- ize blends of poly(etherether ketone)-thermotropic liquid crystalline polymer (LCP) based on hydroxybenzoic and hydroxynaphthoic acid (HBA/HNA). Based upon differential scanning calorimetry and dynamic mechanical measurements, these blends have been shown to be incompatible in the entire range of concen- tration. For these blends the glass transition temperature of both components does not change much with composition. Dynamic mechanical measurements performed under torsional and flexural modes of testing and different samples geometries indicate that the dynamic properties depend a lot on the above two factors. Anisotropy in these blends was studied by performing dynamic mea- surements in flow and transverse directions. The effect of orientation is found to be predominant. Dynamic mechanical properties tend to improve in the flow direction, whereas in the transverse direction they are found to decrease with increasing LCP concentration.

INTRODUCTION low its melting point. Thus, melt processible ther-

he increased knowledge of process-structure- T property relationships in recent years have ac- celerated the development of high performance polymers such as high modulus fibers and super tough polymer blends. Fiber-reinforced polymer composites made from graphite, boron and glass fibers have found enhanced applications in aircraft, automotive, and marine industries. Highly oriented materials have been produced by the modification of conventional polymers ( 1 , 2) and by the design of rod-like, liquid crystalline polymers.

Liquid crystalline polymers ( 1 , 3-7) have found application as high modulus fibers and films with unique properties due to the formulation of ordered lyotropic solutions or thermotropic melts. These polymers transform easily into highly oriented, ex- tended chain structures in the solid state. The in- vention of Kevlar, a high modulus lyotropic aro- matic polyamide as commercialized by DuPont (8). have opened new areas of application leading to the replacement of inorganic fibers. However, Kevlar is not melt processable; it decomposes sufficiently be-

*To whom correspondence should be addressed

Y *

motropic liquid crystalline polymers (LCP), have found tremendous application in the polymer mar- ket (9- l l).

Many researchers have studied polymer blends in which one component is liquid crystalline in nature (12- 17). In general, these researchers have eluci- dated the rheological, mechanical, and morphologi- cal characteristics of such blends. In depth studies on thermal and dynamic mechanical properties have not been detailed.

Polymer blends are of significant interest to poly- mer producers and processors. Under static loading, the performance of thermoplastics can be predicted using the tensile modulus. However, when the ma- terial is subjected to periodic force, the amplitude of the resultant vibration depends on both the elastic and the damping properties of the material. Since polymers are viscoelastic in nature, the use of dy- namic mechanical testing provides a useful charac- terization technique for such materials. These tests provide substantial understanding of the materials response to frequency and temperature dependent loads. In general, properties such as modulus, glass transition, and secondary relaxation transitions can be obtained.

POLYMER ENGINEERING AND SCIENCE, MidJuly 7991, Vol. 31, No. 13 963

Atul Mehta and A. I . Isayev

In this communication, we thus describe the ther- mal and the dynamic mechanical behavior of PEEK- LCP blends. The effect of sample geometry and the mode of testing in dynamic properties have been studied by subjecting the samples to different meth- ods of testing such as single cantilever, torsion, and three point bending. Anisotropy of dynamic proper- ties are studied by testing the samples in the flow and transverse directions. In addition, the thermal stability of these blends is studied by using the thermal gravimetric analysis technique.

EXPERIMENTAL

Materials and Method of Investigations

The material used in this study was Polyetherether ketone (PEEK) Victrex 380G supplied by Imperial Chemical Industries, Ltd., and a thermotropic liquid crystalline polymer based on wholly aromatic copolyester (HBA/HNA) Vectra 950A obtained from the Celanese Research Company. Both of these ma- terials were obtained in pellet form.

Blends of PEEK with varying amounts of LCP were prepared by extrusion, through a Koch six elements static mixer attachment connected to the exit of one single screw extruder (Killion, Inc.), at 350°C and pelletized using a Killion pelletizer. The pellets thus obtained were then dried in an air oven.

Injection molding experiments have been carried out using a Boy 15s reciprocating screw injection molding machine with a clamping force of 24 tons and a maximum shot size of 5 x m3. A volu- metric flow rate of 4.64 x lop5 m3/sec, correspond- ing to the maximum injection speed, was used to mold ASTM Standard Tensile Bars (STB's) of dimen- sions 0.155m x 0.0125m x 0.0033m and mini ten- sile bars (MTB's) of dimensions 0.0635m x 0.0031m x 0.0015m for dynamic property measurements. Melt and mold temperatures were 350°C and 150°C. respectively. An injection pressure of 13.8 MPa was used. End-gated disks with a diameter of 0.0635 m and a thickness of 0.001 m were also molded at the maximum injection speed.

A Differential Scanning Calorimeter (DSC) (Model 9100, DuPont) has been used in order to determine transition temperatures of the pure samples and the blends in nitrogen environment. A heating rate of 2O"C/min was used. A Thermal Gravimetric Ana- lyzer (TGA) (Model 951, DuPont) was used to study the thermal stability of the blends in air. Isothermal experiments at 350°C in 1 hour and nonisothermal experiments at a heating rate 20°C/min were car- ried out in order to determine the weight loss and degradation temperature. These studies of pure LCP and PEEK were carried out on pellets as obtained from the manufacturers.

A Rheometrics Mechanical Spectrometer (model RMS-605, Rheometrics, Inc.) has been used to deter- mine the transition temperature and dynamic me- chanical properties under torsion and three point bending mode for the blends and pure components. Samples with dimensions of 0.05111 x 0.0125m X

0.0033m, used to carry out these experiments, were cut from injection molded STBs. The dynamic mea- surements have been preformed at a frequency of l rad/s and a strain amplitude of 0.2%. All the sam- ples were subjected to a temperature sweep from 25°C to 350°C under a heating rate of l"C/min. Results have been recorded at 5°C interval with a thermal soak time of 2 mins between each measure- ment. The dynamic properties employing a temper- ature sweep of 3O"C-35O0C with a heating rate of l"C/min were also obtained using Dynamic Me- chanical Thermal Analyzers (Polymer Laboratories, Inc.) in a single cantilever bending mode. MTBs with dimensions of 0.02m x 0.0031m x 0.0015m were cut using a diamond saw and mounted in a single cantilever geometry with a span length of 0.008m. All the tests have been carried out at a frequency of 1 Hz and a peak-to-peak displacement of 6.4 x m. A medium size clamp, type B, was used to hold the samples. Dynamic properties in the flow and transverse directions, under the above given condi- tions, were also measured for rectangular samples with dimensions of 0.02m x 0.006m x 0.001m cut from injection molded disks.

RESULTS AND DISCUSSIONS

The typical DSC traces for PEEK, LCP, and their blend are shown in Fig. 1. Features of particular interest as seen are the glass transition temperature, the cold crystallization exotherm, and the melting endotherms. Tg of pure PEEK is not evident from this figure, but has been found to be around 145°C as reported in the literature (18).

Pure PEEK shows a melting endotherm, T,, at about 338°C whereas for LCP it occurs at about 278°C. The melting endotherm of blends show val- ues close to the T, of PEEK. This can be clearly seen from Fig. 2, where the melting point of PEEK in the blends has been plotted against increasing LCP concentration. Even at high LCP loading, there is hardly a variation of 10°C in the T,'s of blends when compared to that of PEEK. Such type of melt-

--- 0.05

I 75 125 175 225 275 325 375

TEMPERATURE ('C )

Fig. 1 . DSC thermograms of PEEKILCP composites at different LCP concentration obtained at heating rate of 20"Clmin. 1-100/0; 2-97.512.5: 3-9515: 4-90110; 5- 75/25; 6-50-50; 7-25/75; 8-10190; 9-0/100.

964 POLYMER ENGINEERING AND SCIENCE, Mid-July 1991, YO/. 31, NO. 13

Stability of PEEK-Thermotropic LCP Blends

0

ing behavior can be attributed to the difference in crystal size of the pure polymers and to the large difference (3 1 J/g) in the heat of fusion of PEEK and LCP. Evidently, the melting point of blends is that of PEEK itself. Two melting peaks are observed only at 75% and 90% LCP loadings. According to Flory (19), for two phase systems, the decrease in melting point is due to the decrease in the chemical poten- tial of the crystalline phase. Figure 3 shows the heat of fusion for PEEK in each blend plotted vs. blend composition. The value shown for pure PEEK, after subtracting the heat of cold crystallization yields a percent crystallinity under these conditions of 22% when compared to the estimated value of 130 J /g for 100% crystalline PEEK (18). This is nearly half of what is found to be the maximum crystallinity for this polymer and reflects partly to the fact that heating does not allow enough time to develop full crystallinity. The heat of fusion shows a pronounced maximum at an LCP content of 2.5% by weight and then is found to be constant and equal to that of pure PEEK with increasing LCP content and de- creased only at 90% LCP content. This initial in- crease is an indirect indication of variation in per- cent crystallinity of the blends. Further, it suggests that addition of small amounts of LCP enhances the formation of crystallites or the crystallization pro- cess. A similar trend is seen for the area under the cold crystallization exotherm in the heating scan.

270 L

0 0 0 0

260 L--d 0 10 20 30 40 50 60 70 80 90 100

WT. 9. LCP

Fig. 2. Melting temperature as a function of LCP concen- tration for PEEK in PEEKILCP composites as obtained from DSC measurements.

NORMALlLfO DSC R t S U L f

50

0

2o t 0

1 0 1 ' ' ' ' ' ' ' . 0 10 20 30 40 50 60 70 80 90 1

WT. % LCP

Fig. 3. Heat of fusion as a function of LCP concentration for PEEK in PEEK ILCP composites as obtained from DSC measurements.

Figure 4 shows similar maximum for 2.5% Lcp blend. However, heat of the cold crystallization of blends is substantially higher than that of pure PEEK. Figure 5 shows the variation in the cold crystallization temperature during the heating scan. Pure LCP does not show any cold crystallization exotherm whereas a maxima and minima is found to occur at 2.5 and 50 weight percent of LCP con- centration, respectively.

Figure 6 shows the DCS traces of PEEK/LCP blends when cooled from the melt state. From this figure it is noticed that PEEK and LCP crystallize at about 285°C and 236°C respectively. In the blends, the crystallization temperature of PEEK is the domi- nant factor. This is shown in Fig. 7. Additional crystallization peak corresponding to the crystalliza- tion temperature of LCP appears only at high con- centration of LCP (above 50%). Figure 8 shows the plot of heat of crystallization of PEEK during cooling blends vs. the blend composition. It is observed that the area under the crystallization exotherm initially increases at an LCP content of 2.5% LCP after which it is found to be equal to that of pure PEEK with increasing LCP content. This again indicates that LCP acts as a nucleating agent for PEEK at LCP concentration of 2.5%, indicating that small addi- tion of the LCP can enhance the crystallization of PEEK.

w [L: 180 3 l- 0 a DSC

0 I75 0

9 I- 170 1 0 0 O 0

f3 150

0 10 20 30 40 50 60 7 0 80 90 100

WT. Z LCP

Fig. 4. Heat of cold crystallization as a function of LCP concentration for PEEK in PEEKILCP composites as ob- tained from DSC measurements.

7 NORMALIZED DSC RLSULI

L - J o 10 20 30 40 50 60 70 80 90 100

WT z LCP Fig. 5. Cold crystallization temperature as a function of LCP concentration for PEEK in PEEKILCP composites as obtained from DSC measurements.

POLYMER ENGiNEERlNG AND SCIENCE, Mid-July 1991, Yo/. 31, NO. 13 965

Atul Mehta and A. I . Isayev

\

1 z S B

urn X \

X w

0.5

290

280

270

260

250

240

230

2 2 0 L

- * O 0 0 0 0

-

-

0 -

' ' ' ' ' ' ' ' '

320 8 1

300 310 I DSC

Fig. 7. Crystallization temperature during cooling as a function of LCP concentration for PEEK in PEEKILCP composites as obtained from DSC measurements.

NORMAULED DSC RLSULI L

50

1 0 1 . ' ' ' " J 0 10 20 30 40 50 60 70 80 YO 100

WT 9. I C P

Fig. 8. Heat of crystallization during cooling as a func- tion of LCP composition for PEEK in PEEK ILCP compos- ites as obtained from DSC measurements.

Most polymer mixtures are not compatible and, thus, form phases that can be quantitatively identi- fied by their transition temperature. Researchers (20-22) have indicated that the glass transition tem- perature of two component materials which are compatible can be described by Fox (23) relation- ship which is given as:

where Tg, is the glass transition temperature of the blend, W, and W2 represent the weight fractions of the individual components which possess glass transition temperature of Tg, and Tg,, respectively.

Figure 9 shows the compositional dependence of Tg of PEEK in PEEK/LCP system obtained from DSC, DMTA, and torsional modes of testing and the predicted Tg from the Fox equation. One can hardly find any variation in the Tg values with testing method. It is noticed that there is a slight shift in experimentally measured Tg with an increase of LCP content. However, large differences occur in the experimental and predicted Tg values. This indi- cates that the PEEK-LCP system is incompatible at all concentrations of LCP. Similar observations have also been made by Isayev and Modic (12) and Isayev and Swaminathan (13) during their studies on poly- carbonate (PC)-LCP and polyetherimide (PE1)-LCP blends, respectively.

Figures 10 through 1 2 show the dynamic storage modulus E', tan 6 and loss modulus E", respec- tively, as a function of temperature for PEEK, LCP, and their blends. From Fig. 10a it can be noticed that the storage modulus at 30°C is found to in- crease with increasing LCP concentration. This indi- cates that LCP reinforces PEEK. However, from Fig. 10b it can be seen that the value of E' remains constant above 75% LCP concentration and is equal to that of pure LCP.

The effect of addition of LCP to PEEK on the highest temperature dispersion peak can be seen from Figs. 11a and I l b , respectively. From Fig. 1 l a it can be observed that for pure PEEK tan 6 shows a maximum at around 151°C. This is be- lieved to be the QI relaxation for PEEK as referred in the literature (24). At this temperature the storage modulus is found to drop precipitously (refer to Fig. 1 Oa) and whereas the loss modulus shows a maxi- mum (Fig. 12a). The temperature at which tan 6 shows a maximum has been considered to be the glass transition of the material. The height of the tan6 peak is found to decrease with increase in LCP content as seen from Figs. 1 1 a and 1 1 b, respec- tively. However, there is hardly a shift of 10°C in the

0 10 20 30 40 50 60 70 80 YO 100

WEIGHT Z OF I C P

Fig. 9. Comparison of experimental glass transition tem- perature as a function of LCP concentration for PEEK in PEEK ILCP composites to that obtained using the Fox equation.

966 POLYMER ENGINEERING AND SCIENCE, Mid-July 1991, Vol. 31, No. 13

Stabili ty of PEEK-Therrnotropic LCP Blends

50 100 150 200 250 300 350 TEMPERATURE ('C )

(a)

Fig. lO(a). Storage modulus (E') as a function of tempera- ture at different LCP concentrations for PEEK ILCP com- posites as obtained f rom single cantilever mode testing. (A) 0% LCP; (B) 2.5 % ; (C) 5 % LCP; (D) 10% LCP; (E) 25 % LCP; IF) 50% LCP.

2 50 100 150 200 250

TEMPERATURE ( ' C )

(b)

Fig. 101b). (A) 75% LCP; (B) 90% LCP; (C) 100% LCP.

temperature at which tan6 shows a maximum. For pure LCP this transition temperature is found to occur at around 100°C. This has been reported to be the Tg of this material (7). Two Tg's corresponding to the Tg of pure materials is not evident from Fig. 1 1 a. This may be attributed to the interference from the amorphous region of PEEK and its effect on the glass transition of LCP. However, the 75% and 90% LCP blends as shown in Fig. 11 b show two distinc- tive Tg's, one at around 100°C and the other at about 140°C. The evidence of two Tg's suggest that the blends are incompatible at all concentrations. The loss modulus E" as shown in Figs. 1 2 a and 1 2 b as a function of temperature is found to increase with increase in LCP concentration at 30°C. How- ever, the height of loss modulus peak at a tempera- ture where tan6 showed a maximum is found to decrease with increase in LCP content. The relative decrease in the height of tan6 peak and E" at Tg is

.42 . . . . ,' . . . . , 1 ' ' . ' ' I ' ' ' '

%

,336 - I.". . ..-- A

I 50 100 150 200 250 300 3

TEMPERATURE ('C )

(a)

0

Fig. 1 1 (a). T a n 6 as a function of temperature at different LCP concentrations for PEEKILCP composites as ob- tained f rom single cantilever mode of testing (A) 0% LCP; (B) 2.5% LCP; (C) 5% LCP; (D) 10% LCP; (E) 25% LCP; (FI 50 % LCP.

.42

,336

,252

ul

5

,168

,004 C

I - ci" .-- 50 100 150 200 250 1

IEFlPERATURE ('C 1

(b)

Fig. 11 (b). (A) 75% LCP; (B) 90% LCP; (C) 100% LCP.

related to increase in the extent of crystallinity in the polymers since the transition behavior is associ- ated with the local mobility of the polymer chains in the amorphous region of the polymer. From Figs. 10, 11, and 12, a transition to melting can also be seen in the range from 270°C for LCP to 340°C for PEEK, with the blends exhibiting this transition in- termediate to the extreme values of pure polymers but closer to the melting temperature of PEEK. This transition corresponds to the melting of the crys- talline domains.

Figure 13 shows the variation in E' as a function of LCP concentration at 30°C and 200°C. These data are taken from E' versus temperature func- tions shown in Fig. 10. At both temperatures, E' is found to increase with increase in LCP concentra- tion. The increase in E' with an increase in LCP concentration is an indication of reinforcement and can be attributed to the change in the morphology.

POLYMER ENGINEERING AND SCIENCE, Mid-July 1991, Vol. 31, No. 13 967

Atul M e h t a a n d A. I . Zsayev

0 TEMPERATURE ('C 1

(a) Fig. 12(a). Loss modulus (E") as a function of tempera- ture at dqferent LCP concentrations f o r PEEKILCP com- posites as obtained from single Cantilever mode of test- ing. (A) 0% LCP: (B) 2.5% LCP; (C) 5% LCP (D) 10% LCP (E) 25 % LCP (F) 50 % LCP.

gr--- " ' ' ' ' ' "---1 C - .j ;-

I

5 --L

50 100 150 200 250 300

TEMPERATURE ('C 1

(b)

Fig. 12(b). (A) 75% LCP; (B) 90% LCP; (C) 100% LCP.

71 1 0 30" C A 200" C

3

8- *- I ::

0 10 20 30 40 50 60 70 80 90 100 % LCP

Fig. 13. Storage modulus (El) as a function of LCP con- centration for PEEK ILCP composites, injection molded as MTB and tested in single cantilever mode of testing em- ploying a temperature sweep.

Three point bending and torsional experiments were performed on STBs of PEEK, LCP, and their blends in order to study the effect of sample geome- try and the mode of testing on the dynamic mechan- ical properties. Figure 14 shows the flexural and torsional dynamic moduli as a function of LCP con- centration as obtained from the three point bending and torsional experiments in comparison with sin- gle cantilever mode of testing of MTB's. An increas- ing trend with increase in LCP concentration can be noticed in cases of the three point bending of STB's and single cantilever experiments in MTB's. In addi- tion, the storage modulus determined in three point bending mode of testing are higher than that deter- mined in single cantilever mode of testing. This is despite the fact that MTB's used in the latter testing are measured to have higher tensile mechanical properties than STB's. Torsion (shear) storage mod- ulus is found to be unaffected due to the addition of LCP. The storage torsion modulus values are much lower than the flexural modulus values. This may be due to the slippage between the layers of the reinforcing species when the polymer is deformed in shear. The obtained results suggest that the dy- namic modulus depends on the sample geometry as well as the mode of testing.

The effect of orientation on the dynamic storage modulus of PEEK, LCP, and their blends is shown in Fig. 15. It can be observed that in the flow direction the modulus increases with increasing LCP content whereas it decreases in the transverse direction. The difference in the two values is found to increase suggesting an increase in the level of anisotropy with increasing LCP content. This indicates that orientation plays a predominantly role in governing the dynamic mechanical properties of these polymer blends.

Figure 16 shows the relative thermal stability in air of PEEK, LCP, and their blends. Wrights (25) approach has been used to define the degradation temperature in order to avoid any ambiguity. The degradation temperature has been defined as the temperature at which the polymer lost 1% of its weight during the non-isothermal heating scan.

10 r I

A TORSIONAL MODE (STB)

0 3-POINT BENDING MODE (STB) ,., I I

ul 0 SINGLE CANTILEVER MODE (MTB) 3 _I

3- 6

L1 - 4 u

0 0 10 20 30 40 50 60 7 0 80 90 100

% LCP

Fig. 14. Torsional modulus as a function of LCP concen- tration for PEEKILCP blends injection molded as STB and its comparison with the flexural modulus as ob- tained from DMTA and 3-point bending experiments.

968 POLYMER ENGINEERING AND SCIENCE, Mid-July 1991, Vol. 31, No. 13

Stability of PEEK-Thermotropic LCP Blends

- -w 4 v

From the figure it can be noticed that PEEK de- grades at around 570°C. whereas LCP has a degra- dation temperature of about 490°C. The degrada- tion temperature decreases with increase in LCP concentration. Such behavior can be attributed to the degradation of LCP in the blend.

Figure 17 shows the percentage weight loss as a function of LCP concentration when the polymer is tested under isothermal conditions in air at 350°C for 60 minutes. It was noticed that pure PEEK shows negligible weight loss whereas LCP showed a loss of 1.1 %. Some scatter is observed at low LCP content.

1 0 FLOW DlRCCTlON

- 0 1RhNSVLRSE DIRECTION

*--- I

i'

-I

0 10 20 30 40 50 60 70 80 90 100 W T % LCP

Fig. 15. Storage modulus (E') i n the f low and transverse directions as a funct ion of LCP concentration obtained f r o m sample cut f r o m injection molded disks and employ- ing single cantilever mode of testing.

4 6 0 4 : : : : : : ; : : I 0 10 20 30 40 50 60 70 80 90 100

WEIGHT X OF LCP

Fig. 16. Degradation temperature as a funct ion of LCP concentration for PEEK ILCP composites as obtained f r o m non-isothermal gravimetric analysis.

1.4

1.2

- 1.0

$ 0.8

* 0.6 z E 0.4

0.2

0.0

h

K

c

w

0 A

IrMPERATURL = 350" C

0 0 n

0 0

I

0

10 20 30 40 50 60 70 80 90

WEIGHT Z OF LCP

Fig. 17. Loss i n weight percent of PEEK ILCP composites as a funct ion of LCP concentration as obtained f r o m isothermal thermal gravimetric analysis.

Such behavior may be due to the compositional difference in the tested sample due to its small size. The increase in percent weight loss is due to the vaporization of LCP domains. The weight loss re- ported here is based on total weight of blend. Thus, the weight loss of LCP in blend is higher than that in the pure LCP.

CONCLUSIONS

Thermal analysis of PEEK, LCP, and their blends were studied using DSC and DMTA under different testing modes. These studies indicated that these blends were incompatible in the entire range of LCP concentration. As found from the DSC studies, melt- ing of PEEK is not affected much due to the addition of LCP. The cold crystallization temperature and the crystallization temperature during cooling also show similar type of behavior. Heat of fusion, heat of cold crystallization during heating scan, and heat of crystallization during cooling were found to pass through a maximum at the 2.5% LCP content. Heat of fusion and heat of crystallization of the other blends was equal to that of pure PEEK and de- creased only at the LCP content of 90%. However, heat of cold crystallization during heating of blends of other LCP concentrations was higher than that of PEEK. The dynamic properties were found to de- pend on the mode of testing as well as the sample geometry. Shear modulus, measured in the tor- sional mode of testing, does not change with the inclusion of LCP. Flexural modulus was found to increase with LCP concentration indicating the rein- forcement of PEEK with LCP. Degradation tempera- ture, as measured by thermal gravimetric analysis technique, was found to decrease with increase in LCP content. The weight loss was found to increase with increasing LCP concentration.

ACKNOWLEDGMENTS

The authors wish to thank Mr. George Carter and Mr. Don Askea of B. F. Goodrich Company for their help in carrying out the dynamic mechanical mea- surements.

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