poly (ether imide) blends

0584-8539193$6.00+0.00 0 1993 Pergamon Press Ltd

Specmchimica Acta, Vol. 49A. No. 516. pp. 753-758. 1993 Printed in Great Britain

A Fourier transform Raman spectroscopy study of the crystallization behaviour of poly (ether ether ketone)/poly (ether huide) blends B. J. BRISCOEand B. H. STUART Department of Chemical Engineering, Imperial College, London SW7 2BY, U.K.

and S. ROSTAMI ICI Wilton Materials Research Centre, P.O. Box 90, Wilton, Middlesbrough, Cleveland TS6 fBE, U.K. (Received 17 March 1992; accepted 6 May 1992) Abstract-This paper describes a study of the crystallization behaviour of blends consisting of the polymers poly (ether ether ketone) (PEEK) and poly (ether imide) (PEI) using Fourier transform Raman spectroscopy. The annealing process of PEEK was followed for the virgin polymer and also when it was blended with PEI. The data presented show that the crystallization process of PEEK is inhibited by the presence of the PEI, but the extent of the crystallinity is increased. There is also evidence that the presence of the PEI induces premelting of the PEEK.

POLY (ETHER ETHER KETONE) (PEEK) is a tough semi-crystalline thermoplastic polymer with attractive mechanical properties [l] and hence the polymer is currently finding use as a matrix material for high performance composites and in engineering applications. Recently, several papers have reported miscibility studies of PEEK blended with the amorphous polymer poly (ether imide) (PEI) [2,3]. These polymers are apparently completely miscible in the amorphous state and PEEK/PEI blends are of particular interest because PEI can be used as a joining agent for PEEK and PEEK composite parts

PI.

CREVECOEUR and GROENINCKX[3] carried out a study of the miscibility and crystallization behaviour in blends of PEEK and PEI using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and small-angle X-ray scattering. The components of these blends were deduced to be completely miscible in the amorphous state over a range of composition and, in particular, exhibited a single glass transition temperature (Tg). Further, the rate of crystallization of PEEK is lowered substantially by the addition of PEI to the blend. As PEEK in the pure state crystallizes relatively rapidly, and there is a desire to optimize crystallinity, there is practical interest in arresting the crystallization kinetics of PEEK or its blends; normally the maximum crystallinity obtained with PEEK is cu 45%. Phase segregation occurs as PEEK crystallizes in the blends and the blends containing up to 50 wt% PEEK show some crystallinity associated with the PEEK phase. The amorphous region will thus be richer in PEI. The crystallization rate of the PEEK phase in its blends with PEI is significantly affected, although no significant effects on the resulting crystal melting temperatures were noticed for the blends composed with PEI by CREVECOEUR and GROENINCKX [3]. The total degree of PEEK crystallinity in these blends was reduced compared with the virgin PEEK [4]. Fourier transform infrared spectroscopy (FT-IR) is now a commonly used technique for the examination of polymer blends [5-91. The technique has been particularly successful for providing information about polymer miscibility. However, Fourier transform (FI) Raman spectroscopy, regarded as a complementary technique to IR spectroscopy, has been comparatively under-utilized for studying blends. Most Raman studies reported have been concerned with homopolymers. FT-Raman spectroscopy has the distinct advantage of requiring minimal sample preparation, but still provides a 753

754 significant

B. J. BRISCOE et amount

of structural

information.

al.

In this paper,

FT-Raman

spectroscopy

is

used to examine the effect of PEI on the crystallization behaviour of PEEK via annealing and to estimate quantitatively the crystallinity of the various samples produced.

EXPERIMENTAL Polymer samples were supplied by ICI Materials, Wilton, U.K. Each sample was heated to 400-420°C then quenched in liquid nitrogen before use. DSC was performed on a Perkin-Elmer 7 Series Thermal Analysis system at a heating rate of 20°C min-‘. All the FT-Raman spectra were recorded using a Bomem Ramspec 152 equipped with an indium-gallium-arsenide (InGaAs) photodiode detector and using 1.05 W of 1064 nm radiation from a Quantronix Series 100 Nd-YAG laser focused to a spot size of approximately 1 mm. EVERALL et al. [lo] noted in their FT-Raman study of PEEK that laser powers of about 1 W focused to a 1 mm spot produce crystallinities of the order 30-40%. However, the current study found that crystallization was not induced under similar conditions. On crystallization, amorphous PEEK transforms from clear to opaque material: under the above conditions our samples remained clear at room temperature. In addition, comparison of the spectra of amorphous and crystalline (ca 30%) PEEK recorded under the same conditions shows notable differences, also indicating that the laser conditions described here do not induce crystallization. For each spectrum 100 scans were co-added, apodized with a cosine function and Fourier transformed to give a resolution of 4 cm-‘. For the temperature studies, the samples (dimensions ca 10 x 10 mm; thickness 0.5 mm) were mounted into the hot stage of a Linkam THMS 600 temperature cell. The sample was allowed to thermally equilibrate at the appropriate temperature for 5 min before instituting a spectral scanning of 10min at each temperature. Thus, the thermal history of the sample is not readily prescribed. However, each sample underwent a comparable thermal treatment. At high temperatures some distortion of the baselines of the spectra was observed. As this distortion occurred mainly in the higher frequency range (> 3000 cm-‘) the intensities and frequencies of the major Raman modes of PEEK were regarded as reliable at relatively high temperatures.

RESULTS AND DISCUSSION

The chemical structure of PEEK is shown in Fig. 1. PEEK has a TBof 145°C and a melting point (T,,,) of around 340°C [ll]. A typical DSC trace of amorphous PEEK, which has been quenched from 42O”C, is shown in Fig. 2. The amorphous PEEK crystallizes rapidly at around 175°C as shown in this figure. By comparison, PEI has a high Tgof 215”C, and its structure is also shown in Fig. 1. CREVECOEURand GROENINCKX [3] used DSC to determine the Tg of a range of PEEK/PEI blends. The blend under current study here is 50 wt% PEEK/SO wt% PEI, which was shown to have a Tg of around 180°C.

Fig. 1. The structural repeat units of PEEK and PEI.

Crystallization behaviour of PEEKlPEI

Ii.0

5;.0

10;. 0

15:. 0

200.0 -r- ~~~ 250.0 I

blend6

300.0 I

755

350.0 I

400.0 r-

temperature / “C Fig. 2. A DSC trace of PEEK.

The Raman bands of PEEK have been assigned in several recent studies and are discussed in detail elsewhere [lo, 12,131. Current FI-Raman spectra of both crystalline and amorphous PEEK are shown in Fig. 3. The spectra of pure PEI and of a 50% PEEK/SO% PEI blend are also shown in Fig. 3. It was found that the spectra of the homopolymers overlapped in certain spectral ranges. However, the carbonyl stretching modes of PEEK and PEI occur at frequencies free from mutual interference. The carbonyl band of PEEK is a useful measure of crystallinity [12,14]. This band is broad and consists of a series of overlapping component bands, each at slightly different frequencies, depending upon the environments of the C=O bonds. Figure 4 shows that the maximum of the C=O stretching mode is observed at 1651 cm-’ in amorphous PEEK, while the maximum of the same mode occurs at 1644 cm-’ in crystalline PEEK. The spectra of 50% PEEK/SO%PEI and pure PEEK were recorded at a range of temperatures from ambient to the melting point. The frequencies of the carbonyl mode of the pure PEEK polymer and its blend were measured and plotted as a function of temperature (Fig. 5). The standard deviation of the frequency values is about 0.2 cm-‘. The location of the corresponding carbonyl mode of PEI was found to be insensitive to changes in temperature. In the case of pure PEEK, as the temperature increases, the C=O frequency gradually shifts to a lower wavenumber until it reaches 250°C where it is observed at 1644 cm-‘. There is a sharp shift after a temperature of 140°C is attained, which is in the region of the Tr After 250°C the carbonyl frequency rapidly begins to increase. These observations indicate that, up to 250°C there is a gradual crystallization of the originally amorphous polymer, after which the polymer begins to melt and then the spectral properties appear like those of the original amorphous material. There are notable changes to the thermal behaviour of the carbonyl mode of PEEK when it is blended with PEI. Figure 5 shows that the carbonyl frequency of the blended PEEK remains relatively constant in the range 1651-1652 cm-’ until a temperature of 150°C. In the temperature range 150-180°C the frequency shifts sharply to a lower value around 1644 cm-‘. After a temperature of about 275°C has been reached, the frequency increased sharply to a value associated with the amorphous state of the polymer. It is interesting, but probably fortuitous, that there is a detectable discontinuity in this spectra at the Tg identified by CREVECOEUR and GROENINCKX[3]. In summary, the presence of the PEI suppresses the crystallization process of PEEK prior to Tg after which there is a SA(A) 46:5/6-K

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B. J. BRISCOEet al.

amorphous

PEEK

crystalline PEEK

PEI

50% PEEK /50%

I.~.,...r...,,,.,.,.

1900

1640

PEI

,,

1380

1120

860

600

wavenumber I cm-l

Fig. 3. IT-Raman

spectra of PEEK, PEI and a 50% PEEK/SO% PEI blend at 25°C.

I...,...,...,...I..,

1680

,,

1652

1624

15%

1568

1540

wavenumber I cm-’

Fig. 4. The carbonyl region of the Raman spectra of crystalline and amorphous PEEK at 25°C

Crystallization behaviour of PEEWPEI blends

757

1654-

smorphous

0

100

200

300

region

400

-lqJ Fig. 5. The carbonyl frequencies of PEEK and a 50% PEEK/SO% PEI blend as a function of temperature.

marked increase in crystallinity; while the virgin material crystallization appears to occur prior to Tg within the time-scale of the current measurements. As the PEEK T,,, is approached there is an indication that the PEI induces premelting. The results shown here generally support the findings of CREVECOEUR and GROENINCKX[3]. The rate of crystallization of PEEK was found by FT-Raman spectroscopy to be lowered by the addition of PEI. In the blend the crystallization process does not begin until a temperature of 180°C is reached, compared with about 140°C for pure PEEK. The manner in which PEEK crystallizes is similar. However, it appears that the initial rate of crystallization after 180°C is higher, but the total amounts of crystallinity in the blend are greater than that of PEEK. This is shown by a difference of approximately 2cm-’ between the spectra in the region near 25O“C, where the polymer has been thermally crystallized. The frequency is lower in the case of the blend, indicating a more crystalline polymer at this stage. If it is assumed that there is a linear relationship between the Raman frequency of the PEEK carbonyl mode and the percentage crystallinity in PEEK, then we can estimate that the 2 cm-’ difference in the spectra corresponds to an increase of about 5% in the crystallinity of PEEK compared with the virgin system. The interpretation of this observation differs from the conclusions made by CREVECOEURand GROENINCKX[3], who argued that the theory that PEEK crystallizes independently from the amorphous PET phase and also proposed that the crystallinity of PEEK does not change significantly with PEI concentration. It may also be surmised from the results presented here that the melting temperature of PEEK is lowered by the presence of PEI. While pure PEEK melts at about 34O”C, in the blend the T,,, is reduced slightly. Again, this differs from the observations of CREVECOEURand GROENINCKX[3], who were not able to determine a difference in the T,,, of PEEK using DSC heating traces for PEEK and its blend with PEI. There is an inference that the presence of PEI in some way disrupts the crystal lattice of the PEEK, although this should be regarded as only a tentative suggestion.

CONCLUSIONS We have shown that FI-Raman spectroscopy can be used to examine the crystallization processes of both pure PEEK and PEEK blended with PEI. This study has shown

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that the rate of crystallization of PEEK is inhibited by the presence of PEI up to the Tg of PEEK, after which crystallization proceeds rapidly. It was also noted that PEI induces premelting in the blend as the T,,, of PEEK is approached. The amount of crystallinity induced in PEEK is increased by the blending with PEI and in the case of 50% PEEK/SO% PEI this increase is tentatively computed to be in the order of about 5%. Acknowledgement-The publish this work.

authors wish to thank ICI Materials, Wilton, for financial support and permission to

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