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Crystallisation behaviour and crystal rearrangement of poly(ethylene oxybenzoate)
Materials Science and Engineering A 413–414 (2005) 435–441
Crystallisation behaviour and crystal rearrangement of poly(ethylene oxybenzoate) E. N´un˜ ez, P. Garc´ıa, U.W. Gedde ∗ School of Chemical Science and Engineering, Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received in revised form 18 July 2005
Abstract One complex fact of polymer crystallisation is that polymer crystals have a tendency to rearrange with time. In this paper, poly(ethylene oxybenzoate)s (PEOB) with different degrees of polymerisation ranging from 5 to 30 have been studied by differential scanning calorimetry and polarised microscopy. The samples showed a great tendency for crystal rearrangement during heating to the melting point, even at high heating rates. The relationship between melting point and crystallisation temperature was analyzed and the Hoffman–Weeks method was found to be unsuitable for determining the equilibrium melting point of these polymers. It is proposed that fast crystal rearrangement, which is a characteristic feature of poly(ethylene oxybenzoate), is the reason for the inadequacy of the Hoffman–Weeks method to obtain reliable estimates of the equilibrium melting point. Polarised microscopy showed, remarkably in view of the low molar mass of the polymers, the formation of perfect banded spherulites. Linear growth rate data suggested that the branched polymers crystallised more slowly than their linear analogues, presumably due to differences in the equilibrium melting point. © 2005 Elsevier B.V. All rights reserved. Keywords: Polymer crystallisation; Melting; Crystal rearrangement; Equilibrium melting point
1. Introduction One of the recently raised questions is concerned with the possible existence of metastable phases during the early stages in polymer crystallisation [1]. It is believed that the initially formed organized structures, of either crystalline or mesomorphic nature, rearrange rapidly at the crystallisation temperature. This together with the ‘slow’ experimental traditionally used techniques, such as electron microscopy, has made it very difficult to study early stage polymer crystallisation. Simulation techniques or the use of molecularly constrained polymers, in which crystal rearrangement is inhibited, could be suitable alternatives to approach the problem. In previous studies [2–4], star polyesters with crystallisable poly(ε-caprolactone) arms were investigated and the results indicated that the constrain imposed by the cores on the crystallisable arms retarded crystal rearrangement. ‘Fast’ experimental techniques such as synchrotron X-ray diffraction could help to get more information about the early crystallisation stages for these polymers.
∗
Corresponding author. Tel.: +46 8 790 7640; fax: +46 8 208 856. E-mail address: [email protected] (U.W. Gedde).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.168
This paper presents data on poly(ethylene oxybenzoate) (PEOB), which is an aromatic poly(ester ether) with a chemical structure similar to that of poly(ethylene terephthalate). Korematsu and Kuriyama [5] reported two crystal modifications of PEOB (denoted α and β), which differ in the chain conformation. Lotti et al. [6] and Finelli et al. [7] reported on the thermal properties and crystallisation behaviour of a ¯ n = 7000 g mol−1 . Differsample of PEOB with molar mass M ential scanning calorimetry of samples isothermally crystallised indicated multimodal melting, which the authors attributed to melting of less perfect crystallites followed by crystallisation into thicker crystals and final melting. The equilibrium melting point of the polymer was determined to be 232 ◦ C using the Hoffman–Weeks method. The crystallinity obtained from X-ray diffraction data was between 21 and 55%. The heat of fusion at the equilibrium melting point was determined to be 96 kJ kg−1 [7]. In this paper, the crystallisation and melting behaviour of a series of PEOBs with degree of polymerisation ranging from 5 to 30 has been studied by polarised microscopy and differential scanning calorimetry. The materials showed a great tendency for crystal rearrangement before melting even at high heating rates without any visible change of the morphology as revealed
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by polarised microscopy. The equilibrium melting points of samples isothermally crystallised to different degrees of crystallinity have been determined according to the Hoffman–Weeks method. The results showed that changes in the crystallisation times (i.e. degree of crystallinity) used had a tremendous effect on the equilibrium melting points obtained by extrapolation. It may thus be concluded that this methodology is not suitable for polymers showing fast crystal rearrangement during heating. 2. Experimental 2.1. Materials The materials studied were kindly provided by the Polymer IRC group in the University of Leeds (UK). They consist of a series of p-poly(ethylene oxybenzoate)s prepared by co-polymerisation of methyl 4-(2-hydroxyetoxy)benzoate (AB) and dimethyl 5-(2-hydroxyetoxy)isophthalate (AB2 ) monomers in two different proportions: (i) homopolymer from AB monomer (i.e. linear PEOB) and (ii) copolymer based on 99 mol% of AB monomer and 1 mol% of AB2 monomer. The chemical structures of the monomers used are shown in Fig. 1. Details regarding the synthesis of the polymers have been presented by Anderson [8]. NMR spectroscopy was performed on a Bruker Avance 400 MHz NMR instrument. Since the polymers were not soluble in common organic solvents, a few drops of trifluoroacetic acid (TFA) were added to the prepared sample of CDCl3 and polymer to enhance the solubility of the polymer. Proton NMR spectra were acquired with a spectral window of 20 ppm, an acquisition time of 4 s and a relaxation delay of 1 s. 13 C NMR spectra were acquired with a spectral window of 240 ppm, an acquisition time of 0.7 s and a relaxation delay of 2 s. The determination of the degree of polymerisation (n) of the samples was performed according to [8]: n=
[CH2 ] 3 [CH3 ] 2
(1)
where CH2 is the intensity of peak b (in Fig. 2) from the two hydrogen in the aromatic ring closest to the carbonyl and CH3
Fig. 2. Proton NMR spectrum used to calculate the degree of polymerisation (n) of the samples studied.
is the intensity of peak f (in Fig. 2) from the three hydrogen in the methyl ester end group. Eq. (1) can also be used to determine n for the copolymers, because they only contained 1 mol% of AB2 units. The peak from TFA is appeared as peak a in Fig. 2. The values obtained for n were in the range of 5–30. The homopolymers were denoted as Ln (n specifying the degree of polymerisation) and the copolymers with 1% of branching unit were denoted as Bn . 2.2. Polarised microscopy Spherulitic structure and crystallisation kinetics were studied in a Leitz Ortholux POL BK II optical microscope equipped with crossed polarisers and a Mettler Hot Stage FP 82HT controlled by a Mettler FP90 Central Processor. The samples were prepared by depositing a small amount of sample onto microscopy slides preheated to 250 ◦ C in the hot stage. Cover sheets were pressed onto the molten sample to obtain a thin polymer film between the glass slides. The samples were then heated to 230 ◦ C, kept at this temperature for 1 min, and cooled at 20 ◦ C min−1 to the selected crystallisation temperature. The microscopic images of growing spherulites were recorded under isothermal conditions by a Leica DC 300 CCD camera for further analysis with Leica IM50 software. The linear growth rate measurements were performed after calibration with a scale. Finally, the samples were quenched from the melt in air and their melting was recorded at a heating rate of 5 ◦ C min−1 . 2.3. Differential scanning calorimetry (DSC)
Fig. 1. Chemical structure of the monomers used in the polymerisation: methyl 4-(2-hydroxyetoxy)benzoate (AB) and dimethyl 5-(2-hydroxyetoxy) isophthalate (AB2 ).
Samples of 5 ± 1 mg were encapsulated in 40 l aluminium pans and analyzed in a temperature- and energy-calibrated Mettler Toledo DSC 820 using nitrogen as purge gas. The samples were held in the melt (230 ◦ C) during 5 min in order to erase the effects of previous thermal and mechanical history, cooled at 10 ◦ C min−1 to 0 ◦ C, held at this temperature for 5 min and then heated again to 230 ◦ C at 10 ◦ C min−1 . The heats of crystallisation and fusion, Hc and Hf , were calculated, respectively, from the area under the crystallisation exotherm recorded during the cooling and the area under the melting endotherm. The
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mass crystallinity was calculated by the total enthalpy method [9] using as the heat of fusion of 100% crystalline PEOB the values obtained by Finelli et al. [7]. The crystallisation and melting temperatures, Tc and Tm , were calculated as the minimum of the crystallisation peak and the maximum of the melting peak, respectively. In the cases where the peaks appeared as ‘shoulders’ due to their proximity to another peak, their maximum or minimum values were estimated by eye. Fig. 3 shows a typical thermogram of the samples studied and the nomenclature used for the different data obtained. In order to study the crystal rearrangement of the PEOB crystals different experiments were conducted. First, the melting curves of samples of each material were recorded at different heating rates, after a specific crystallisation procedure: cooling from 230 to 25 ◦ C at 30 ◦ C min−1 . In another series of experiments, the heating rate was maintained constant at 10 ◦ C min−1 and the cooling rate varied. Samples quenched from the melt in cold water were also analyzed. Finally, the samples were crystallised at isothermal conditions in the range of 168–208 ◦ C after 5 min storage at 230 ◦ C and 10 ◦ C min−1 cooling to the selected crystallisation temperature. Calorimetric data were recorded during both the cooling phase and during the isothermal stage to make certain that all
437
Fig. 3. Example thermogram of the samples studied, showing the nomenclature used to describe the crystallisation and melting temperatures and enthalpies.
crystallisation occurred during the isothermal stage. The samples were crystallised under isothermal conditions to different crystallinities by varying the crystallisation time in order to study its effect on the equilibrium melting point obtained by extrapolation according to the Hoffman–Weeks method. The melting peak temperature data used for extrapolation was recorded at 10 ◦ C min−1 .
Fig. 4. Polarised photomicrographs of samples of: (a) L16.7 crystallising at 176 ◦ C; (b) L16.7 crystallising at 184 ◦ C; (c) B5.8 crystallising at 164 ◦ C; and (d) B15.8 crystallising at 180 ◦ C. The scale bar corresponds to 50 m.
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Table 1 Thermodynamic data of the samples studied Sample
Crystallisation Tc1
(◦ C)
Tc2
Melting (◦ C)
Tc3
(◦ C)
Hc1
(J g−1 )
Tm1
(◦ C)
Crystallinity Tm2
(◦ C)
Tm3
(◦ C)
Hf
(J g−1 )
Hf − Hc2 (J g−1 )
χc a (%)
L6.4 L9.1 L16.7 L30
151.97 165.27 166.08 173.00
– 176.9 – –
175.62 180.26 190.53 195.61
59.98 65.87 63.01 64.35
188.90 191.49 202.67 212.03
199.97 – 211.47 –
207.99 209.29 216.96 220.19
61.47 62.03 65.63 62.23
53.42 57.99 54.41 53.15
64.03 64.61 68.36 64.82
B5.3 B7.7 B12 B15.8 B30
148.27 154.36 167.54 174.40 159.91
– 171.54 175.21 – –
154.42 169.28 188.10 201.13 194.15
50.13 56.63 64.74 60.52 57.71
171.50 183.01 199.69 211.60 207.94
– – – – –
192.62 202.76 212.81 217.28 216.14
50.38 55.36 58.96 56.60 57.90
49.43 53.48 55.19 52.27 50.87
52.48 57.67 61.42 58.96 60.31
a
Crystallinity calculated using 96 J g−1 as Hf0 [7].
3. Results and discussion
account for some of the deviation of the data point associated with B30 .
3.1. Spherulitic morphology 3.3. Crystal rearrangement The polymers crystallised forming spherulites with circular extinction rings or bands and with a strictly circular periphery (Fig. 4a–d). The spherulites exhibited negative birefringence, and their impingement was tight and clean in all cases, suggesting that no macro-segregation occurred. The band spacing increased when increasing Tc (Fig. 4a and b), which is in accordance with earlier reported data for other polymers [10]. For the samples with a lower degree of polymerisation, the banding and the spherulite periphery was slightly coarser (Fig. 4c and d). No change of shape, band spacing or sign was observed in the spherulites before final melting. Instead, the material lost its birefringence gradually and the observed superstructures faded away progressively until they melted completely. The crystal rearrangement, which was evident from the analyses of the calorimetric data, had no visible effect on the morphology as revealed by polarised microscopy. A remarkable finding was the fact that the samples studied, being very low molar mass, showed exclusively a spherulitic texture. It is common that low molar mass polymers show axialites. 3.2. Thermodynamic data Thermodynamic data of some of the polymers studied obtained from differential scanning calorimetry (DSC) are presented in Table 1. Data for the crystallisation and melting temperatures (Tc1 and Tm3 ) as a function of the degree of polymerisation are also presented in Fig. 5. Both the crystallisation and melting peak temperatures increased with increasing degree of polymerisation for the homopolymers. The copolymers showed the same basic behaviour as the homopolymers except for the sample with n = 30 (B30 ), which had a lower Tc than B15.8 and L30 . The depression in the crystallisation temperature for this particular sample is most probably due to the presence of non-crystallisable branches. In addition, the presence of branches in the B samples causes an error in the calculated n-value, which may
The polymers studied exhibited a profound tendency for crystal rearrangement during heating, as observed in the multimodal melting traces obtained. Multiple endothermal peaks can appear for several reasons: (i) the initial presence of two or more populations of crystals differing in thickness or crystal unit cell; (ii) melting and reorganization of the first formed crystals into thicker crystals during the DSC scan. The first alternative seems unlikely in this case because only one crystal phase of PEOB has been reported to be stable at the experimental conditions used in this study [5], and because there are no reasons to expect the systematic presence of two crystal populations of different size in the samples with the polymerisation and crystallisation methods used. Figs. 6a shows the melting curves of L9.1 , previously crystallised from the melt at 30 ◦ C min−1 and melted at different heating rates. Increasing the heating rate had no significant effect in the reorganization ability of the crystals. The low-and hightemperature peaks appeared even at the highest heating rates. Fig. 6b shows the melting curves of the same polymer, previously crystallised from the melt at different cooling rates and melted at 10 ◦ C min−1 . The melting traces were very similar for the samples crystallised during 10–20 ◦ C min−1 . However,
Fig. 5. Crystallisation temperature Tc1 for Ln (䊉) and Bn () and melting temperature Tm3 for Ln (), Bn () plotted as a function of the degree of polymerisation (n).
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on a linear extrapolation of Tm − Tc data according to: 1 Tc 0 Tm = + Tm 1 − β β
where β is the crystal thickening factor (β = Lc /L∗c ), where L∗c is the thickness of the virgin crystal with the melting point equal to the crystallisation temperature. According to Eq. (2), the equilibrium melting point is obtained by extrapolation of Tm − Tc data to Tm = Tc , provided that β is the constant for different samples used for extrapolation. The validity of Eq. (2) requires also that the stability of the first formed crystal is just slightly greater than the minimum requirement, i.e. L∗c = Lc,min + δLc ≈ Lc,min , where Lc,min is the minimum crystal thickness corresponding to a melting point equal to the crystallisation temperature. Serious doubts have been raised about the validity of the Hoffman–Weeks method by showing that the extrapolated equality Tm = Tc occurred at a finite crystal thickness in several semicrystalline polymers [16]. Furthermore, the thickness of the virgin crystals exceeds often the minimum thickness by a significant δLc value [17,18], which gives the Tm − Tc relationship a curvature, which is often seen in practice [4]. In this study, the melting point was taken as the melting peak temperature on samples isothermally crystallised at different temperatures. Different series of experiments were performed varying the crystallinity by changing the crystallisation time at each crystallisation temperature. The results obtained from the Hoffman–Weeks extrapolations for some of the samples studied are presented in Table 2, where a, b and r2 correspond, respectively, to the slope coefficient, the ordinate at the origin and the coefficient of determination of the optimum straight line. The obtained Tm0 values were extremely sensitive to even minor variations in the crystallinity. Small changes in the melting point data at a given crystallisation temperature data may have a profound effect on the extrapolation. The alteration was in some cases so strong that the slope coefficient was higher than 1, and the extrapolated line showed no intersection with the equilibrium line (Table 2). Unstable crystals that rearrange rapidly show melting temperatures that depend strongly on the crystallisation time. However, the Tm0 values obtained for series of samples with different constant degrees of crystallinity were consistent for some of the samples (B12 had the best consistency). However, the consistency of the extrapolations can be considered to take place at random. Figs. 7a and b present examples of Hoffman–Weeks extrapolations for samples of B12 and B10.3 , representing, respectively, the case of consistent and inconsistent
Fig. 6. Melting traces of L9.1 (a) crystallised at 30 ◦ C min−1 and melted at different heating rates; (b) crystallised at different cooling rates and melted at 10 ◦ C min−1 .
a significant difference was observed for the sample quenched in ice water from the melt; the low-temperature melting peak was absent and a new cold-crystallisation peak appeared during heating. This suggests that the polymer remained essentially amorphous during quenching, and that it showed cold crystallisation during the subsequent heating. 3.4. Equilibrium melting point The equilibrium melting point (Tm0 ) of a polymer can be obtained in different ways [11–15], most of them based on extrapolations of experimental data. One of the most used methods is that proposed by Hoffman and Weeks [12], which is based Table 2 Results from the equilibrium melting point calculations Sample
L16.7 B7.7 B10.3 B12 B15.8 a
100% of attainable crystallinity
30% of attainable crystallinity
20% of attainable crystallinity
Tm0 (◦ C)
a
b
r2
Tm0 (◦ C)
a
b
r2
Tm0 (◦ C)
a
b
r2
244.2
0.777 1.114 0.917 0.723 0.545
54.46 12.31 25.27 63.76 100.5
0.994 0.988 0.997 0.995 0.999
229.6 273.0
0.731 0.903 1.268 0.714 0.551
61.66 26.84 −36.88 65.59 99.42
0.999 0.999 0.996 0.995 0.997
237.24 239.26 222.43 227.04 222.43
0.741 0.840 0.911 0.697 0.552
61.73 38.28 34.46 68.88 99.58
0.999 0.998 0.995 0.999 0.997
a
305.2 230.1 220.8
a
229.32 370.29
The extrapolation line gave no intersection with the equilibrium line.
(2)
440
E. N´un˜ ez et al. / Materials Science and Engineering A 413–414 (2005) 435–441
Fig. 8. Linear growth rate plotted as function of crystallisation temperature for L7.3 (䊉), L9.1 (), L16.7 (), L30 (), B5.8 (), B7.7 (), B10.3 (♦), B12 ().
Fig. 9. Linear growth rate plotted as function of crystallisation temperature for L6.4 (䊉), L7.3 ()), L9.1 (), L16.7 (), B5.3 (), B5.8 (), B7.7 (), B10.3 (♦), B12 (). The lines are exponential fits to the experimental data. The insert figure shows the crystallisation temperature plotted as a function of the degree of polymerisation at a growth rate G = 0.45 m s−1 for Ln (䊉) and Bn (). Fig. 7. Melting point as a function of crystallisation temperature of samples crystallised to different degrees of crystallinity: 100% (), 30% (䊉) and 20% () of attainable crystallinity; (a) B12 and (b) B10.3 .
results. These results suggest that the Hoffman–Weeks method is unsuitable for the determination of the equilibrium melting point for these particular polymers. It is proposed that fast crystal rearrangement, which is a characteristic feature of PEOB, is the reason for the inadequacy of the Hoffman–Weeks method to obtain reliable estimates of the equilibrium melting point. 3.5. Crystallisation kinetics The linear growth rate was determined by measuring the radii of freely growing spherulites at a constant crystallisation temperature. The values presented are averages of 1–5 growing units; a typical value was 3. In general, the variation in the growth rate within a given sample was small (less than 3%). The spherulite growth was strictly linear; the coefficient of determination (r2 ) of the optimum straight line was greater than 0.99 in all cases. Fig. 8 shows the linear growth rate plotted as function of crystallisation temperature for some of the polymers studied. As expected, the crystallisation rate decreased with increasing crystallisation temperature and increased with n for the same crystallisation temperature. The linear growth rate was analyzed by comparing the crystallisation temperatures of the different samples at a specified linear growth rate, 0.45 m s−1 (Fig. 9). The growth rate versus
crystallisation temperature data were fitted to an exponential function. The crystallisation temperature values corresponding to G = 0.45 m s−1 were obtained and plotted against the degree of polymerisation (Fig. 9). This crystallisation temperature increased with increasing n in a similar fashion as is typical for the equilibrium melting point. Surprisingly, the branched polymers with the lowest degree of polymerisation (n ≤ 6) showed a higher crystallisation temperature than their linear analogues. For the majority of the samples (n > 8), the crystallisation temperature was higher for the linear polymers than for the branched polymers. These data suggest that the equilibrium melting point is lower for the copolymers than for the homopolymers at a given degree of polymerisation. 4. Conclusions Poly(ethylene oxybenzoate)s with degree of polymerisation between 5 and 30 have been studied by polarised microscopy and differential scanning calorimetry. The polymers showed a profound tendency for crystal rearrangement during melting even at high heating rates, as observed in the multimodal melting curves. However, the spherulite morphology and band spacing remained unchanged prior to final melting. The relation between crystallisation temperature and melting point was analyzed for samples crystallised to different crystallinities. The Hoffman–Weeks extrapolation method was unsuitable to calculate the equilibrium melting point of the samples studied because the Tc − Tm data were sensitive to the
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variations in crystallisation time, which led to significant variations in the equilibrium melting points obtained. The linear growth rate data suggest that the equilibrium melting point is lower for the copolymers than for the homopolymers at a given degree of polymerisation. Acknowledgements The financial support from the Swedish Research Council (grant #621-2004-2699) is gratefully acknowledged. Thanks are extended to Dr. M. Buzza (Polymer IRC group, University of Leeds, UK) for supplying the polymers studied and P. Antoni and Prof. E. Malmstr¨om, at our Department, for determining the molar masses of the polymers. References [1] G. Strobl, Eur. Phys. J. E 3 (2000) 165. [2] E. N´un˜ ez, U.W. Gedde, Polymer 46 (2005) 5992. [3] E. N´un˜ ez, C. Ferrando, E. Malmstr¨om, H. Claesson, U.W. Gedde, J. Macromol. Sci. Phys. 43 (2004) 1213.
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[4] E. N´un˜ ez, C. Ferrando, E. Malmstr¨om, H. Claesson, P.E. Werner, U.W. Gedde, Polymer 45 (2004) 5251. [5] M. Korematsu, S. Kuriyama, Nippon Kagaku Zasshi. 81 (1960) 852. [6] N. Lotti, V. Siracusa, L. Finelli, A. Munari, P. Manaresi, J. Therm. Anal. Calorim. 66 (2001) 827. [7] L. Finelli, V. Siracusa, N. Lotti, A. Munari, M. Gazzano, J. Polym. Sci. B: Polym. Phys. 40 (2002) 1354. [8] I.C. Anderson, Synthesis and properties of branched poly(ethylene oxybenzoate)s, Doctoral Thesis, University of Leeds, UK, 2000. [9] U.W. Gedde, Polymer Physics, Kluwer Academic Publishers, Dordrecht, Netherlands, 1995. [10] A. Keller, J. Polym. Sci. 39 (1959) 151. [11] B. Wunderlich, Macromolecular Physics, in: Crystal Melting, vol. 3, Academic Press, New York, 1980. [12] J.D. Hoffman, J.J. Weeks, J. Res. Natl. Bur. Stand. 66A (1962) 13. [13] J.N. Hay, J. Polym. Sci. Polym. Chem. 14 (1976) 2845. [14] P.J. Flory, A. Vrij, J. Am. Chem. Soc. 85 (1963) 3548. [15] M.G. Broadhurst, J. Res. Natl. Bur. Stand. 70 (1966) 481. [16] B. Heck, T. Hugel, M. Iijima, E. Sadiku, G. Strobl, New J. Phys. [Electronic Publication]. 1 (1999), Article 17. [17] P.J. Barham, R.A. Chivers, A. Keller, J. Martinez-Salazar, S.J. Organ, J. Mater. Sci. 20 (1985) 1625. [18] S.Z.D. Cheng, J. Chen, J.S. Barley, A. Zhang, A. Habenschuss, P.R. Zschack, Macromolecules 25 (1992) 1453.
Crystallisation behaviour and crystal rearrangement of poly(ethylene oxybenzoate) E. N´un˜ ez, P. Garc´ıa, U.W. Gedde ∗ School of Chemical Science and Engineering, Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received in revised form 18 July 2005
Abstract One complex fact of polymer crystallisation is that polymer crystals have a tendency to rearrange with time. In this paper, poly(ethylene oxybenzoate)s (PEOB) with different degrees of polymerisation ranging from 5 to 30 have been studied by differential scanning calorimetry and polarised microscopy. The samples showed a great tendency for crystal rearrangement during heating to the melting point, even at high heating rates. The relationship between melting point and crystallisation temperature was analyzed and the Hoffman–Weeks method was found to be unsuitable for determining the equilibrium melting point of these polymers. It is proposed that fast crystal rearrangement, which is a characteristic feature of poly(ethylene oxybenzoate), is the reason for the inadequacy of the Hoffman–Weeks method to obtain reliable estimates of the equilibrium melting point. Polarised microscopy showed, remarkably in view of the low molar mass of the polymers, the formation of perfect banded spherulites. Linear growth rate data suggested that the branched polymers crystallised more slowly than their linear analogues, presumably due to differences in the equilibrium melting point. © 2005 Elsevier B.V. All rights reserved. Keywords: Polymer crystallisation; Melting; Crystal rearrangement; Equilibrium melting point
1. Introduction One of the recently raised questions is concerned with the possible existence of metastable phases during the early stages in polymer crystallisation [1]. It is believed that the initially formed organized structures, of either crystalline or mesomorphic nature, rearrange rapidly at the crystallisation temperature. This together with the ‘slow’ experimental traditionally used techniques, such as electron microscopy, has made it very difficult to study early stage polymer crystallisation. Simulation techniques or the use of molecularly constrained polymers, in which crystal rearrangement is inhibited, could be suitable alternatives to approach the problem. In previous studies [2–4], star polyesters with crystallisable poly(ε-caprolactone) arms were investigated and the results indicated that the constrain imposed by the cores on the crystallisable arms retarded crystal rearrangement. ‘Fast’ experimental techniques such as synchrotron X-ray diffraction could help to get more information about the early crystallisation stages for these polymers.
∗
Corresponding author. Tel.: +46 8 790 7640; fax: +46 8 208 856. E-mail address: [email protected] (U.W. Gedde).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.168
This paper presents data on poly(ethylene oxybenzoate) (PEOB), which is an aromatic poly(ester ether) with a chemical structure similar to that of poly(ethylene terephthalate). Korematsu and Kuriyama [5] reported two crystal modifications of PEOB (denoted α and β), which differ in the chain conformation. Lotti et al. [6] and Finelli et al. [7] reported on the thermal properties and crystallisation behaviour of a ¯ n = 7000 g mol−1 . Differsample of PEOB with molar mass M ential scanning calorimetry of samples isothermally crystallised indicated multimodal melting, which the authors attributed to melting of less perfect crystallites followed by crystallisation into thicker crystals and final melting. The equilibrium melting point of the polymer was determined to be 232 ◦ C using the Hoffman–Weeks method. The crystallinity obtained from X-ray diffraction data was between 21 and 55%. The heat of fusion at the equilibrium melting point was determined to be 96 kJ kg−1 [7]. In this paper, the crystallisation and melting behaviour of a series of PEOBs with degree of polymerisation ranging from 5 to 30 has been studied by polarised microscopy and differential scanning calorimetry. The materials showed a great tendency for crystal rearrangement before melting even at high heating rates without any visible change of the morphology as revealed
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by polarised microscopy. The equilibrium melting points of samples isothermally crystallised to different degrees of crystallinity have been determined according to the Hoffman–Weeks method. The results showed that changes in the crystallisation times (i.e. degree of crystallinity) used had a tremendous effect on the equilibrium melting points obtained by extrapolation. It may thus be concluded that this methodology is not suitable for polymers showing fast crystal rearrangement during heating. 2. Experimental 2.1. Materials The materials studied were kindly provided by the Polymer IRC group in the University of Leeds (UK). They consist of a series of p-poly(ethylene oxybenzoate)s prepared by co-polymerisation of methyl 4-(2-hydroxyetoxy)benzoate (AB) and dimethyl 5-(2-hydroxyetoxy)isophthalate (AB2 ) monomers in two different proportions: (i) homopolymer from AB monomer (i.e. linear PEOB) and (ii) copolymer based on 99 mol% of AB monomer and 1 mol% of AB2 monomer. The chemical structures of the monomers used are shown in Fig. 1. Details regarding the synthesis of the polymers have been presented by Anderson [8]. NMR spectroscopy was performed on a Bruker Avance 400 MHz NMR instrument. Since the polymers were not soluble in common organic solvents, a few drops of trifluoroacetic acid (TFA) were added to the prepared sample of CDCl3 and polymer to enhance the solubility of the polymer. Proton NMR spectra were acquired with a spectral window of 20 ppm, an acquisition time of 4 s and a relaxation delay of 1 s. 13 C NMR spectra were acquired with a spectral window of 240 ppm, an acquisition time of 0.7 s and a relaxation delay of 2 s. The determination of the degree of polymerisation (n) of the samples was performed according to [8]: n=
[CH2 ] 3 [CH3 ] 2
(1)
where CH2 is the intensity of peak b (in Fig. 2) from the two hydrogen in the aromatic ring closest to the carbonyl and CH3
Fig. 2. Proton NMR spectrum used to calculate the degree of polymerisation (n) of the samples studied.
is the intensity of peak f (in Fig. 2) from the three hydrogen in the methyl ester end group. Eq. (1) can also be used to determine n for the copolymers, because they only contained 1 mol% of AB2 units. The peak from TFA is appeared as peak a in Fig. 2. The values obtained for n were in the range of 5–30. The homopolymers were denoted as Ln (n specifying the degree of polymerisation) and the copolymers with 1% of branching unit were denoted as Bn . 2.2. Polarised microscopy Spherulitic structure and crystallisation kinetics were studied in a Leitz Ortholux POL BK II optical microscope equipped with crossed polarisers and a Mettler Hot Stage FP 82HT controlled by a Mettler FP90 Central Processor. The samples were prepared by depositing a small amount of sample onto microscopy slides preheated to 250 ◦ C in the hot stage. Cover sheets were pressed onto the molten sample to obtain a thin polymer film between the glass slides. The samples were then heated to 230 ◦ C, kept at this temperature for 1 min, and cooled at 20 ◦ C min−1 to the selected crystallisation temperature. The microscopic images of growing spherulites were recorded under isothermal conditions by a Leica DC 300 CCD camera for further analysis with Leica IM50 software. The linear growth rate measurements were performed after calibration with a scale. Finally, the samples were quenched from the melt in air and their melting was recorded at a heating rate of 5 ◦ C min−1 . 2.3. Differential scanning calorimetry (DSC)
Fig. 1. Chemical structure of the monomers used in the polymerisation: methyl 4-(2-hydroxyetoxy)benzoate (AB) and dimethyl 5-(2-hydroxyetoxy) isophthalate (AB2 ).
Samples of 5 ± 1 mg were encapsulated in 40 l aluminium pans and analyzed in a temperature- and energy-calibrated Mettler Toledo DSC 820 using nitrogen as purge gas. The samples were held in the melt (230 ◦ C) during 5 min in order to erase the effects of previous thermal and mechanical history, cooled at 10 ◦ C min−1 to 0 ◦ C, held at this temperature for 5 min and then heated again to 230 ◦ C at 10 ◦ C min−1 . The heats of crystallisation and fusion, Hc and Hf , were calculated, respectively, from the area under the crystallisation exotherm recorded during the cooling and the area under the melting endotherm. The
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mass crystallinity was calculated by the total enthalpy method [9] using as the heat of fusion of 100% crystalline PEOB the values obtained by Finelli et al. [7]. The crystallisation and melting temperatures, Tc and Tm , were calculated as the minimum of the crystallisation peak and the maximum of the melting peak, respectively. In the cases where the peaks appeared as ‘shoulders’ due to their proximity to another peak, their maximum or minimum values were estimated by eye. Fig. 3 shows a typical thermogram of the samples studied and the nomenclature used for the different data obtained. In order to study the crystal rearrangement of the PEOB crystals different experiments were conducted. First, the melting curves of samples of each material were recorded at different heating rates, after a specific crystallisation procedure: cooling from 230 to 25 ◦ C at 30 ◦ C min−1 . In another series of experiments, the heating rate was maintained constant at 10 ◦ C min−1 and the cooling rate varied. Samples quenched from the melt in cold water were also analyzed. Finally, the samples were crystallised at isothermal conditions in the range of 168–208 ◦ C after 5 min storage at 230 ◦ C and 10 ◦ C min−1 cooling to the selected crystallisation temperature. Calorimetric data were recorded during both the cooling phase and during the isothermal stage to make certain that all
437
Fig. 3. Example thermogram of the samples studied, showing the nomenclature used to describe the crystallisation and melting temperatures and enthalpies.
crystallisation occurred during the isothermal stage. The samples were crystallised under isothermal conditions to different crystallinities by varying the crystallisation time in order to study its effect on the equilibrium melting point obtained by extrapolation according to the Hoffman–Weeks method. The melting peak temperature data used for extrapolation was recorded at 10 ◦ C min−1 .
Fig. 4. Polarised photomicrographs of samples of: (a) L16.7 crystallising at 176 ◦ C; (b) L16.7 crystallising at 184 ◦ C; (c) B5.8 crystallising at 164 ◦ C; and (d) B15.8 crystallising at 180 ◦ C. The scale bar corresponds to 50 m.
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Table 1 Thermodynamic data of the samples studied Sample
Crystallisation Tc1
(◦ C)
Tc2
Melting (◦ C)
Tc3
(◦ C)
Hc1
(J g−1 )
Tm1
(◦ C)
Crystallinity Tm2
(◦ C)
Tm3
(◦ C)
Hf
(J g−1 )
Hf − Hc2 (J g−1 )
χc a (%)
L6.4 L9.1 L16.7 L30
151.97 165.27 166.08 173.00
– 176.9 – –
175.62 180.26 190.53 195.61
59.98 65.87 63.01 64.35
188.90 191.49 202.67 212.03
199.97 – 211.47 –
207.99 209.29 216.96 220.19
61.47 62.03 65.63 62.23
53.42 57.99 54.41 53.15
64.03 64.61 68.36 64.82
B5.3 B7.7 B12 B15.8 B30
148.27 154.36 167.54 174.40 159.91
– 171.54 175.21 – –
154.42 169.28 188.10 201.13 194.15
50.13 56.63 64.74 60.52 57.71
171.50 183.01 199.69 211.60 207.94
– – – – –
192.62 202.76 212.81 217.28 216.14
50.38 55.36 58.96 56.60 57.90
49.43 53.48 55.19 52.27 50.87
52.48 57.67 61.42 58.96 60.31
a
Crystallinity calculated using 96 J g−1 as Hf0 [7].
3. Results and discussion
account for some of the deviation of the data point associated with B30 .
3.1. Spherulitic morphology 3.3. Crystal rearrangement The polymers crystallised forming spherulites with circular extinction rings or bands and with a strictly circular periphery (Fig. 4a–d). The spherulites exhibited negative birefringence, and their impingement was tight and clean in all cases, suggesting that no macro-segregation occurred. The band spacing increased when increasing Tc (Fig. 4a and b), which is in accordance with earlier reported data for other polymers [10]. For the samples with a lower degree of polymerisation, the banding and the spherulite periphery was slightly coarser (Fig. 4c and d). No change of shape, band spacing or sign was observed in the spherulites before final melting. Instead, the material lost its birefringence gradually and the observed superstructures faded away progressively until they melted completely. The crystal rearrangement, which was evident from the analyses of the calorimetric data, had no visible effect on the morphology as revealed by polarised microscopy. A remarkable finding was the fact that the samples studied, being very low molar mass, showed exclusively a spherulitic texture. It is common that low molar mass polymers show axialites. 3.2. Thermodynamic data Thermodynamic data of some of the polymers studied obtained from differential scanning calorimetry (DSC) are presented in Table 1. Data for the crystallisation and melting temperatures (Tc1 and Tm3 ) as a function of the degree of polymerisation are also presented in Fig. 5. Both the crystallisation and melting peak temperatures increased with increasing degree of polymerisation for the homopolymers. The copolymers showed the same basic behaviour as the homopolymers except for the sample with n = 30 (B30 ), which had a lower Tc than B15.8 and L30 . The depression in the crystallisation temperature for this particular sample is most probably due to the presence of non-crystallisable branches. In addition, the presence of branches in the B samples causes an error in the calculated n-value, which may
The polymers studied exhibited a profound tendency for crystal rearrangement during heating, as observed in the multimodal melting traces obtained. Multiple endothermal peaks can appear for several reasons: (i) the initial presence of two or more populations of crystals differing in thickness or crystal unit cell; (ii) melting and reorganization of the first formed crystals into thicker crystals during the DSC scan. The first alternative seems unlikely in this case because only one crystal phase of PEOB has been reported to be stable at the experimental conditions used in this study [5], and because there are no reasons to expect the systematic presence of two crystal populations of different size in the samples with the polymerisation and crystallisation methods used. Figs. 6a shows the melting curves of L9.1 , previously crystallised from the melt at 30 ◦ C min−1 and melted at different heating rates. Increasing the heating rate had no significant effect in the reorganization ability of the crystals. The low-and hightemperature peaks appeared even at the highest heating rates. Fig. 6b shows the melting curves of the same polymer, previously crystallised from the melt at different cooling rates and melted at 10 ◦ C min−1 . The melting traces were very similar for the samples crystallised during 10–20 ◦ C min−1 . However,
Fig. 5. Crystallisation temperature Tc1 for Ln (䊉) and Bn () and melting temperature Tm3 for Ln (), Bn () plotted as a function of the degree of polymerisation (n).
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on a linear extrapolation of Tm − Tc data according to: 1 Tc 0 Tm = + Tm 1 − β β
where β is the crystal thickening factor (β = Lc /L∗c ), where L∗c is the thickness of the virgin crystal with the melting point equal to the crystallisation temperature. According to Eq. (2), the equilibrium melting point is obtained by extrapolation of Tm − Tc data to Tm = Tc , provided that β is the constant for different samples used for extrapolation. The validity of Eq. (2) requires also that the stability of the first formed crystal is just slightly greater than the minimum requirement, i.e. L∗c = Lc,min + δLc ≈ Lc,min , where Lc,min is the minimum crystal thickness corresponding to a melting point equal to the crystallisation temperature. Serious doubts have been raised about the validity of the Hoffman–Weeks method by showing that the extrapolated equality Tm = Tc occurred at a finite crystal thickness in several semicrystalline polymers [16]. Furthermore, the thickness of the virgin crystals exceeds often the minimum thickness by a significant δLc value [17,18], which gives the Tm − Tc relationship a curvature, which is often seen in practice [4]. In this study, the melting point was taken as the melting peak temperature on samples isothermally crystallised at different temperatures. Different series of experiments were performed varying the crystallinity by changing the crystallisation time at each crystallisation temperature. The results obtained from the Hoffman–Weeks extrapolations for some of the samples studied are presented in Table 2, where a, b and r2 correspond, respectively, to the slope coefficient, the ordinate at the origin and the coefficient of determination of the optimum straight line. The obtained Tm0 values were extremely sensitive to even minor variations in the crystallinity. Small changes in the melting point data at a given crystallisation temperature data may have a profound effect on the extrapolation. The alteration was in some cases so strong that the slope coefficient was higher than 1, and the extrapolated line showed no intersection with the equilibrium line (Table 2). Unstable crystals that rearrange rapidly show melting temperatures that depend strongly on the crystallisation time. However, the Tm0 values obtained for series of samples with different constant degrees of crystallinity were consistent for some of the samples (B12 had the best consistency). However, the consistency of the extrapolations can be considered to take place at random. Figs. 7a and b present examples of Hoffman–Weeks extrapolations for samples of B12 and B10.3 , representing, respectively, the case of consistent and inconsistent
Fig. 6. Melting traces of L9.1 (a) crystallised at 30 ◦ C min−1 and melted at different heating rates; (b) crystallised at different cooling rates and melted at 10 ◦ C min−1 .
a significant difference was observed for the sample quenched in ice water from the melt; the low-temperature melting peak was absent and a new cold-crystallisation peak appeared during heating. This suggests that the polymer remained essentially amorphous during quenching, and that it showed cold crystallisation during the subsequent heating. 3.4. Equilibrium melting point The equilibrium melting point (Tm0 ) of a polymer can be obtained in different ways [11–15], most of them based on extrapolations of experimental data. One of the most used methods is that proposed by Hoffman and Weeks [12], which is based Table 2 Results from the equilibrium melting point calculations Sample
L16.7 B7.7 B10.3 B12 B15.8 a
100% of attainable crystallinity
30% of attainable crystallinity
20% of attainable crystallinity
Tm0 (◦ C)
a
b
r2
Tm0 (◦ C)
a
b
r2
Tm0 (◦ C)
a
b
r2
244.2
0.777 1.114 0.917 0.723 0.545
54.46 12.31 25.27 63.76 100.5
0.994 0.988 0.997 0.995 0.999
229.6 273.0
0.731 0.903 1.268 0.714 0.551
61.66 26.84 −36.88 65.59 99.42
0.999 0.999 0.996 0.995 0.997
237.24 239.26 222.43 227.04 222.43
0.741 0.840 0.911 0.697 0.552
61.73 38.28 34.46 68.88 99.58
0.999 0.998 0.995 0.999 0.997
a
305.2 230.1 220.8
a
229.32 370.29
The extrapolation line gave no intersection with the equilibrium line.
(2)
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Fig. 8. Linear growth rate plotted as function of crystallisation temperature for L7.3 (䊉), L9.1 (), L16.7 (), L30 (), B5.8 (), B7.7 (), B10.3 (♦), B12 ().
Fig. 9. Linear growth rate plotted as function of crystallisation temperature for L6.4 (䊉), L7.3 ()), L9.1 (), L16.7 (), B5.3 (), B5.8 (), B7.7 (), B10.3 (♦), B12 (). The lines are exponential fits to the experimental data. The insert figure shows the crystallisation temperature plotted as a function of the degree of polymerisation at a growth rate G = 0.45 m s−1 for Ln (䊉) and Bn (). Fig. 7. Melting point as a function of crystallisation temperature of samples crystallised to different degrees of crystallinity: 100% (), 30% (䊉) and 20% () of attainable crystallinity; (a) B12 and (b) B10.3 .
results. These results suggest that the Hoffman–Weeks method is unsuitable for the determination of the equilibrium melting point for these particular polymers. It is proposed that fast crystal rearrangement, which is a characteristic feature of PEOB, is the reason for the inadequacy of the Hoffman–Weeks method to obtain reliable estimates of the equilibrium melting point. 3.5. Crystallisation kinetics The linear growth rate was determined by measuring the radii of freely growing spherulites at a constant crystallisation temperature. The values presented are averages of 1–5 growing units; a typical value was 3. In general, the variation in the growth rate within a given sample was small (less than 3%). The spherulite growth was strictly linear; the coefficient of determination (r2 ) of the optimum straight line was greater than 0.99 in all cases. Fig. 8 shows the linear growth rate plotted as function of crystallisation temperature for some of the polymers studied. As expected, the crystallisation rate decreased with increasing crystallisation temperature and increased with n for the same crystallisation temperature. The linear growth rate was analyzed by comparing the crystallisation temperatures of the different samples at a specified linear growth rate, 0.45 m s−1 (Fig. 9). The growth rate versus
crystallisation temperature data were fitted to an exponential function. The crystallisation temperature values corresponding to G = 0.45 m s−1 were obtained and plotted against the degree of polymerisation (Fig. 9). This crystallisation temperature increased with increasing n in a similar fashion as is typical for the equilibrium melting point. Surprisingly, the branched polymers with the lowest degree of polymerisation (n ≤ 6) showed a higher crystallisation temperature than their linear analogues. For the majority of the samples (n > 8), the crystallisation temperature was higher for the linear polymers than for the branched polymers. These data suggest that the equilibrium melting point is lower for the copolymers than for the homopolymers at a given degree of polymerisation. 4. Conclusions Poly(ethylene oxybenzoate)s with degree of polymerisation between 5 and 30 have been studied by polarised microscopy and differential scanning calorimetry. The polymers showed a profound tendency for crystal rearrangement during melting even at high heating rates, as observed in the multimodal melting curves. However, the spherulite morphology and band spacing remained unchanged prior to final melting. The relation between crystallisation temperature and melting point was analyzed for samples crystallised to different crystallinities. The Hoffman–Weeks extrapolation method was unsuitable to calculate the equilibrium melting point of the samples studied because the Tc − Tm data were sensitive to the
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variations in crystallisation time, which led to significant variations in the equilibrium melting points obtained. The linear growth rate data suggest that the equilibrium melting point is lower for the copolymers than for the homopolymers at a given degree of polymerisation. Acknowledgements The financial support from the Swedish Research Council (grant #621-2004-2699) is gratefully acknowledged. Thanks are extended to Dr. M. Buzza (Polymer IRC group, University of Leeds, UK) for supplying the polymers studied and P. Antoni and Prof. E. Malmstr¨om, at our Department, for determining the molar masses of the polymers. References [1] G. Strobl, Eur. Phys. J. E 3 (2000) 165. [2] E. N´un˜ ez, U.W. Gedde, Polymer 46 (2005) 5992. [3] E. N´un˜ ez, C. Ferrando, E. Malmstr¨om, H. Claesson, U.W. Gedde, J. Macromol. Sci. Phys. 43 (2004) 1213.
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