Thermal Behavior of Transparent Poly(etheretherket0ne) (PEEK) Film

HEMANT GUPTA* and R. SALOVEY

University of Southern California Department of Chemical Engineering Los Angeles, California 90089-1 21 1

The thermal behavior of poly(etheretherket0ne) (PEEK) film heated in a n open differential scanning calorimetry (DSC) pan at 2O”C/min is distorted by relaxation of the strained film. PEEK film in a closed pan or quenched PEEK in open or closed pans shows a glass-transition temperature ( Tg) around 144”C, cold crys- tallization (-22 J/g) at 177”C, melt-temperature ( Tm) peaking at 335-34OoC, with an enthalpy of fusion of 32-34 J/g, and recrystallization on cooling a t 285°C. with a crystallization exotherm of about 40 J/g. The enthalpy of fusion decreases with increasing heating rate from 2-10OoC/min and approaches the enthalpy of cold crystallization. With increasing heating rate, further crystallization of PEEK during the DSC scan is suppressed. With increasing cooling rate, PEEK melt crystallizes at larger supercoolings to a lesser extent. Crystallization on cooling the melt was more complete than cold crystallization and annealing on heating.

INTRODUCTION

oly(etheretherket0ne) (PEEK) has received con- P siderable recent attention because of unusual physical properties (1, 2). High melting point, solvent resistance, toughness, low flammability, and a reten- tion of mechanical properties a t high temperatures make it a suitable thermoplastic matrix for composite materials in aerospace applications.

The condensation polymerization of alkali bis- phenates with activated aromatic dihalides was re- ported a s early as 1967 by Johnson, et al. (3). How- ever, precipitation during polymerization produced a low-molecular-weight polymer (4). Not until Atwood, et al. (5) used diphenyl sulphone was high-molecular- weight PEEK made. Recently, Kircheldrof and Bier (6) carried out bulk condensations of various silylated bisphenols and 4.4’ difluorobenzophenone at ele- vated temperatures between 220 and 350°C. using cesium fluoride as the catalyst, to synthesize PEEK. This reaction scheme did not require purification of the molten polymer from solvents or metal salts. The molecular structure of PEEK is

L 0-1 The solubility of the polymer is limited to a few

high boiling solvents, such as diphenyl sulphone, benzophenone, a-chloronaphthalene, and a mixture

*Current address: FerroCorporation. 5915 Rodeo Road. Los Angeles. CA 90016.

POLYMER ENGINEERING AND SCIENCE, APRIL 1990, Vol. 30, No. 8

of 1,2,4 trichlorobenzene (TCB) and phenol in low concentration (7). Del Rios (8) determined the molec- ular weight distribution of PEEK using gel permea- tion chromatography (GPC) at elevated temperature. Few studies have been published on the solution properties of PEEK because of insolubility at low temperatures and in conventional solvents.

Studies have been reported on the melting, crystal- lization, and morphology of PEEK (9-13). X-ray dif- fraction data indicate that the unit cell of PEEK crystals is orthorhombic with dimensions a = 0.755 to 0.788 nm, b = 0.586 to 0.594 nm, and c = 0.988 to 1.007 nm (14, 15). Diffraction patterns revealed that the radial growth direction in spherulites was along the b axis (16, 17). In a unit cell, phenyl rings form alternating angles of +/-40 degrees with re- spect to the plane of the zigzag backbone (1 7). From the view point of packing, the ketone and the ether linkage can be interchanged with minimum distor- tion.

Blundell and Osborn (9) first pointed out the close similarity between thermal behavior in PEEK and in poly(ethy1ene terephthalate) (PET), except that tran- sitions occur about 75°C higher in PEEK. Like PET, commercial PEEK is available in the amorphous state as transparent film, which can be obtained by very rapid cooling from temperatures above the melting point. Amorphous PEEK undergoes crystallization on heating to just above the glass-transition tempera- ture, referred to as “cold crystallization,” thus show- ing a strong tendency to crystallize from the super- cooled “glass.” The melting behavior of crystalline PEEK has been studied by differential scanning cal-

453

H . Gupta and R. Salovey

orimetry (DSC) (9-12) and by Fourier transform in- frared spectroscopy (FTIR) (1 3). Studies have been reported on the annealing behavior of pure PEEK (9, 12). as well as of composites, in which fibers are reported to act as nucleating surfaces within a PEEK matrix (1 8-2 1). Similar to observations on PET, mul- tiple melting peaks in DSC of PEEK have been ob- served by many workers as a result of annealing at various temperatures (1 2, 22-25).

Major efforts have been made to assess the crys- tallinity of thermally treated PEEK, since the degree of crystallinity influences mechanical properties (26, 27). Crystallinity measurements have involved wide- angle X-ray diffraction (WAXD) (9, 14, 26, 27), DSC (9, 28-30), and FTIR (13, 29). Crystallization kinetics have been studied by optical microscopy (1 6) and DSC (9, 28). Various forms of the Avrami equation have been used to model cold crystallization, as well as crystallization from the melt (1 1, 31, 32). An Avrami exponent of one for cold crystallization indicates that fibrillar crystal growth or that secondary crystalli- zation may be dominant (32). The Avrami exponent for melt crystallization has been found to be 3.0, consistent with spherulitic growth and heteroge- neous nucleation (10, 1 1). Nonisothermal crystalli- zation has been studied by Cebe and Hong (1 1) and the Avrami exponent has been found to vary from 4.7 to 7.8, indicating a strong time-dependent crystal growth. It is clear that the melt crystallization of PEEK produces a spherulitic morphology (17). Olley, et al. (33), using electron microscopy and etching techniques, found that the periodicity of the lamellae approximates 15 nm. Crystallization of quenched PEEK on heating above the glass-transition temper- ature (T4) , produced spherulites, a n order of magni- tude smaller, and smaller and thinner lamellae than for the melt crystallized material (33). A higher nu- cleation density was cited a s the reason for the re- duction in spherulite and lamellar size.

The study presented in this paper concerns the thermal behavior of PEEK film, a s measured by dif- ferential scanning calorimetry and thermomechani- cal analysis.

EXPERIMENTAL DETAILS

Materials

Transparent amorphous PEEK films were obtained from two sources: ICI Americas Inc., Films Division, provided transparent film samples (commercial name, Stabar) of 0.003-inch thickness; and Westlake Plastics Company provided thick transparent PEEK film (450 G) of 0.01-inch thickness extruded from PEEK resin obtained from ICI.

Thermal Analysis

A Dupont differential scanning calorimeter (DSC) with a pressure cell (Model 9 10) and a thermal analy- sis workstation (TA 9900) was used throughout this study. Amorphous polymer samples were prepared for DSC by quenching molten PEEK into liquid nitro-

gen, after heating at 38OOC for 5 min. Nonisothermal scans were usually carried out at a heating or cooling rate of 20"C/min in DSC. The effect of varying the heating and cooling rate were also examined. Here, liquid nitrogen or ice water were used as quenching media. All studies were carried out in a n open DSC pan, unless otherwise specified. Some samples were placed in the DSC pan, covered with a lid and crimped shut. The latter are referred to as closed pan meas- urements. DSC measurements were repeated five times using indium standards. The precision in tem- perature was f 0.5"C and in energy was f 1 J/g.

The degree of crystallinity (x) of PEEK was calcu- lated from

where AH is the heat of fusion per gram of PEEK obtained from a nonisothermal DSC scan and AEIf is taken as 130 J/g, corresponding to the heat of fusion of 100% crystalline PEEK (9).

Thermomechanical analysis (TMA) measurements were done (Dupont Thermomechanical Analyzer, Model 943) using a n expansion probe with a contact weight of 2 g and a heating rate of 5"C/min, with a helium flow of 30 cc/min. TMA measures changes in the linear dimension of the sample as a function of temperature.

Shrinkage studies were performed by placing a rectangular piece of film in a n oven at 385°C for 5 min and then quenching in ice water. The areas of films before and after shrinkage were recorded. Three samples of each film were analyzed. Thin Sta- bar films warped and rolled up at melt temperatures. Guide foils were used to prevent warping and allow shrinkage measurements.

RESULTS

Transparent PEEK film undergoes so-called "cold crystallization from the solid state on heating above the glass-transition temperature (1 45"C), changing to white and opaque. If the opaque film is heated above the melting point (340°C). the material be- comes transparent again and remains transparent if quenched into liquid nitrogen or into ice water from temperatures higher than the melting point.

A typical DSC thermogram of as-received PEEK film (0.003-inch thick) is shown in Fig. 1 . This ca- lorimetric experiment is done in a n open pan at a heating rate of 20"C/min at atmospheric pressure. The thermogram shows a n exotherm beginning a t 172°C due to crystallization from the solid state (cold crystallization) after the glass-transition inflection and melting beginning at about 327°C. The rate of cold crystallization was maximum (exothermic peak) at 177°C with a heat of crystallization of 19 J/g. The crystals melt with a n endothermic peak maximum temperature of 337°C and a n enthalpy of fusion of only 14.8 J/g. When the sample was cooled from the melt, Fig. 1 , recrystallization occurred at 284°C

454 POLYMER ENGINEERING AND SCIENCE, APRIL 7990, Yo/. 30, No. 8

Thermal Behavior of Transparent Poly(etheretherket0ne)

(peak) and the heat of crystallization approximated 40 J/g.

The same PEEK film when quenched in liquid ni- trogen from 400°C and then heated in the DSC at 20"C/min (Fig. 2) showed a n exothermic crystalliza- tion peak at 178°C with a cold crystallization en- thalpy of 22.3 J/g. Also, DSC displayed a substan- tially increased enthalpy of fusion (32 J/g) and a peak melting temperature of 336°C. Repeated quenching of the same sample from 400°C into liquid nitrogen showed no significant changes in crystalli- zation enthalpy or enthalpy of fusion. However, the cold crystallization temperature increased from 178.4 to 18 1.8"C in the second quench cycle. Ftgure 3 illustrates the thermogram obtained by

heating as-received PEEK film at 20"C/min in a closed pan. We observed a cold crystallization peak at 177°C with a n enthalpy of 23 J/g, a 20% increase from the open pan experiment. The enthalpy of fu- sion was about 34 J/g, and the fusion peak occurred at 340°C. Recrystallization occurred a t 292°C on cool- ing the melt, with a crystallization exotherm of about 42 J/g.

Closed pan measurements on quenched PEEK (Fig. 4) show very similar thermal behavior to open pan calorimetry on the quenched material. A cold crys- tallization peak occurs at 177°C after a Tg inflection a t 144°C. The cold crystallization exotherm exhibits a heat of crystallization approximating 22 J/g. A melting endotherm peaks at 335°C with a heat of fusion of 32 J/g. In general, actual crystallinities range from 1 1-33%.

Calorimetric results similar to those observed with PEEK film of 0.003-inch thickness were also exhib- ited by as-received and repeatedly quenched PEEK film of 0.01 -inch thickness. Following quenching, PEEK heated in a n open pan shows a much larger heat of fusion and a 20% larger heat of "cold crys- tallization than for commercial PEEK film.

Shrinkage studies on as-received films indicated shrinkage in one direction of 16.4% for the 0.01-inch film and 39.6% for the 0.003-inch film. The average shrinkage was calculated from changes in areas for three samples of each film. Results indicate uniaxial orientation of the PEEK molecules in each film.

Dimensional changes on heating 0.0 1-inch thick film and quenched PEEK were also studied by ther- momechanical analysis (TMA). The quenched sample

1 2.5

t I

- 0 . 5 L " ' ' " " " " ' " ~ " " ' ~ ' ~ ' " ~ ~ ' 1 ~ ~ ~ ~ 100 150 200 250 300 350 400 450

Ternperoture ("C)

Fig. 1. Heat ing a n d cooling of PEEKf i lm . PO"C/rnin, o p e n pan . At the e n d of the hea t ing cyc le , t h e base l ine i s d i sp laced for clarity.

Temperature ("C)

Fig. 3. Heat ing a n d cooling of PEEK f i l m . ZO"C/min, closed p a n . At t h e e n d of the hea t ing cyc l e , t h e base l ine i s d i sp laced f o r clarity.

d 0'0' ' '146 ' kdd ' ' '2kO ' ' 360 ' ' '3;; 400 Temperoture ("C)

Fig. 2. Heat ing of quenched PEEK, 20"C,/rnin. o p e n p a n . Fig. 4. Heat ing of quenched PEEK, 20"Clrnin. closed pan.

POLYMER ENGINEERING AND SCIENCE, APRIL 1990, Vol. 30, No. 8 455

H . Gupta and R. Salovey

began to soften on heating, even below the glass- transition temperature, after which the rate of pen- etration increased markedly. After 175°C. because of cold crystallization, further penetration of the probe was prevented, and the sample expanded linearly on heating until melting. The as-received film displayed a more complex pattern of probe penetration on heat- ing.

Further studies concern amorphous PEEK samples quenched into ice water from molten PEEK film (0.01-inch thick) after heating at 380°C for 5 min. To elucidate the nature of crystallization during DSC scanning, we performed calorimetric experiments in an open pan at different heating and cooling rates between 100 and 400°C. The effect of heating rate OR the crystallinity developed in amorphous quenched PEEK during the DSC scan was determined and compared with the results of similar variations in cooling rate from the melt. Results are summarized in Table 1 for heating and cooling rates between 2 and 10O0C/min. Temperatures recorded refer to cor- responding peaks.

DISCUSSION

The enthalpies of fusion of films measured in open or closed pan configurations are compared with those of quenched PEEK. While for quenched PEEK, open and closed pan DSC traces are very similar, marked calorimetric differences are observed between com- mercial films studied in open and closed pans. For example, in an open pan experiment, the enthalpy of fusion of PEEK films approximates 1 5 J/g. In a closed pan, the enthalpy of fusion of PEEK films is more than double, about 34 J/g. Similarly, the cold crys- tallization exotherm for PEEK film is 18.6 J/g in a n open pan, but 23.2 J/g in a closed pan. Also, melt crystallization on cooling molten as-received PEEK films occurred 6 4 ° C sooner (higher T J , with a slightly higher heat of crystallization, for crystalli- zation in a closed pan. In general, PEEK film heated or cooled in a closed pan approximates the melting and crystallization behavior of quenched PEEK [in either open or closed pan). We do not believe that the shape of the melting endotherm in Fig. 1 simply results from the loss of contact between the film and pan surface on shrinking of the film on heating in an open pan. Similar results are shown on heating thicker films, which do not shrink as much. More-

Table l. Calorimetric Behavior of PEEK at Various DSC Scan Rates.

From Solid From Melt Heating/Cooling

Rate Tg 1, AHc Tm AH, 1, AHc ("C/min) ("C) ("C) (J/g) ("C) (J/g) ("C) (J/g)

2 141 168 23 342 38.2 297 44.7 5 144 172 22 339 34.5 289 39

10 146 177 22 338 33.5 285 37.6 20 147 182 23 336 33.6 284 35.6 50 150 192 23 336 29 266.5 33

100 158 203 23 338 25 257 26

over, the calorimetric behavior of oriented films is reproducible, regardless of the sample direction se- lected. However, we cannot exclude the possibility that with wrinkling in the film in a n open pan, some contact may be lost with the pan surface.

We suggest that using a closed pan in DSC sup- presses the relaxation of strained polymer films and associated enthalpy changes before melting occurs. Molecular orientation of commercial films was intro- duced on shearing the melt during film formation. Orientation in as-received films was confirmed by thermomechanical analysis and shrinkage measure- ments. Due to the mechanical alignment of polymer molecules, needle-shaped crystallites form in the shear direction during cold crystallization, with fur- ther crystallization proceeding laterally on these crystallites as nucleation sites (34). The relationship between shrinkage and draw ratio is reviewed in a recently proposed phase transition model, which pre- dicts that the shrinkage is approximately equal to the draw ratio (35). Differences in the thermal profiles of oriented polymers, when scanned in either open or closed pans, have been observed by many workers (36). For PEEK film in a closed pan and quenched PEEK in open or closed pans, the experimental calor- imetric trace, before and after the melting endo- therm, is linear with respect to temperature. There- fore, the baseline definition is much better than for PEEK film in an open pan. Accordingly, the DSC enthalpy of fusion may not be accurately determined on heating a n unrestrained PEEK film.

In a n open pan DSC experiment, the heat capacity of the oriented film after the glass-transition temper- ature [between T4 and the onset of the cold crystalli- zation) extrapolated to beyond the melting tempera- ture was always found to be lower (absolutely) than that of the melt at the same temperature. Similar results were observed by Peterlin and Meinel (37) for drawn low-density polyethylene and later discussed by Wunderlich (38, 39). The deviation of the expected enthalpy of the melt is higher in the 0.003-inch film compared to 0.01-inch film. Quenched samples show no such deviation because quenched PEEK is not oriented.

Complex patterns of probe penetration shown by as-received films in TMA result from the superposi- tion of unidirectional shrinkage on the usual glass transition and cold crystallization. Again, the film contains oriented molecules that relax at Tg and crys- tallize above Tg.

From repeated calorimetric measurements on quenched PEEK, the heat of fusion was found to be about 9 to 12 J/g higher than the "cold crystalliza- tion exotherm (Fig. 2). Similar differences have been previously reported for PET (23). Perhaps a continu- ous increase in crystallinity occurs during heating in the intermediate temperature range, which is not detectable in DSC. The increase in crystallinity might involve a n increase in the total number of crystallites or the average perfection of existing crystallites. De- spite many studies on the subject (9, 1 l ) , the events

456 POLYMER ENGINEERING AND SCIENCE, APRIL 1990, Vol. 30, No. 8

Thermal Behavior of Transparent Poly(etheretherket0neJ

that take place during this intermediate range of temperature are not very well understood. Increased crystallinity is attributed to the melting of less perfect crystals and recrystallization into more perfect crys- tals, which melt at higher temperatures (9, 22). This constant reordering may continue during the DSC scan until the final melting of the most perfect crys- tals.

Information on the nature and speed of transitions occurring on heating quenched PEEK and cooling melts may be inferred from calorimetric studies at various DSC scan rates (Table 1 ). Tg is a fairly rapid transition and changes slightly, from 141 to 147"C, on raising the heating rate from 2 to 2O"C/min. How- ever, as is well known (40), Tg is a kinetic phenome- non and increases with increasing heating rate. On the other hand, it is possible that crystallization from the solid state is a relatively slower process, and the onset of such crystallization on heating quenched PEEK occurs at higher temperatures, as we raise the heating rate. However, the extent of such crystalli- zation is virtually constant, and the crystallization enthalpy was "independent" of heating rate.

Additional crystallization, occurring at various heating rates during the DSC scan, between cold crystallization and melting may be estimated from the difference between the enthalpy of fusion (melt- ing endotherm) and the enthalpy of cold crystalliza- tion (exothermic). This difference decreases from 16 to 2 J/g as the heating rate is increased from 2 to 10O0C/min. As the heating rate is reduced, the poly- mer undergoes increased reordering, leading to fur- ther crystallization on heating. However, the melting temperatures of the samples heated between 2 and 100"C/min are about the same, indicating that the perfection of the crystals has not changed. Actually, the melting point (T,n) decreases slightly on raising the heating rate as the annealing time during heating is reduced. However, at heating rates above 20°C/ min, T,,, may be increased slightly because of super- heating. At the very highest heating rates, both cold crystallization and melting peaks were broadened, perhaps because of increasing difficulties in temper- ature equilibration. Moreover, crystallization from the melt is somewhat slower, and the supercooling (T , - Tc) increases with cooling rate, especially at the higher cooling rates. Moreover, the crystallization enthalpy from the melt decreases with increasing cooling rate. A s the cooling rate is increased, the crystallinity of PEEK decreases as an increasing frac- tion of the molten polymer is quenched. At all cooling or heating rates, the crystallization exotherm (from the melt) exceeds the heat of fusion. We infer that crystallization from the melt is a more efficient proc- ess than cold crystallization, which involves crystal- lization in constrained regions, plus crystallization on annealing, before final melting.

CONCLUSIONS

PEEK film (0.003-inch thick) placed in a closed pan and quenched PEEK in either open or closed pans

show similar thermal behavior, at 2O"C/min, in DSC. After a glass-transition inflection near 144°C. a cold crystallization exotherm of about 22 J/g peaks at 177°C. An endothermic peak occurs at 335-340°C. with an enthalpy of fusion of 32-34 J/g. Recrystal- lization from the cooling melt occurs at 292°C with a crystallization exotherm of about 42 J/g. On the other hand, the thermal behavior of PEEK film heated in an open pan is distorted because of en- thalpy changes associated with the relaxation of the strained film. It is also possible that changes in di- mensions on heating the unrestrained film may alter thermal contact with the DSC pan and effect appar- ent enthalpy changes. Frozen-in molecular orienta- tion in PEEK film was evident in thermomechanical and shrinkage measurements.

On heating at 20"C/min, the heat of fusion was found to be about 10 J/g higher than the cold crys- tallization exotherm. The difference between the en- thalpies of fusion and cold crystallization decreases from 16 to 2 J/g, as the heating rate is increased from 2 to 100"C/min. Here, the enthalpy of cold crystallization was independent of heating rate, while the heat of fusion decreased with increasing heating rate. Apparently, annealing during heating in the calorimeter accounts for the increase in crys- tallinity between cold crystallization and fusion. With increasing heating rate, the semicrystalline PEEK is subjected to less annealing and recrystalli- zation is increasingly suppressed.

For crystallization from the melt, the supercooling increases and the crystallization exotherm decreases with increasing cooling rate, as an increasing frac- tion of the polymer melt is quenched. The crystalli- zation exotherm approached, but always exceeded, the melting endotherm. Crystallization from the melt was more complete than cold crystallization and an- nealing during the DSC scan.

ACKNOWLEDGMENTS

Financial support of the National Science Foun- dation from Grant No. DMR-8607466 and the Los Angeles Rubber Group Foundation (TLARGI) is grate- fully acknowledged. The authors also thank Ferro Corporation, Composities Division, for providing ex- perimental facilities and other resources that made this research possible. We acknowledge ICI Americas Inc. and Westlake Plastics Co. for kindly providing PEEK samples. We thank the reviewer for helpful comments.

1. 2. 3.

4.

5.

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