- Email: [email protected]
PC block copolymers
European Polymer Journal 39 (2003) 1081–1089 www.elsevier.com/locate/europolj
Effects of annealing on crystallinity and phase behaviour of PET/PC block copolymers Paola Marchese a
a,*
, Annamaria Celli a, Maurizio Fiorini a, Marina Gabaldi
b
Dipartimento di Chimica Applicata e Scienza dei Materiali, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy b Centro Ricerche ‘‘Giulio Natta’’, Basell, Piazzale Donegani 12, 44100 Ferrara, Italy Received 24 October 2002; received in revised form 29 November 2002; accepted 29 November 2002
Abstract The effects of the annealing on the properties of PET/PC block copolymers, obtained by reactive blending in the presence of different catalysts and for different mixing times, have been studied. The annealing, performed in conditions that promote the crystallization, has been used to better understand the role of block length in determining the phase behaviour. The copolymers characterized by blocks with molecular weight larger than 8000 are able to reorganize towards more ordered domains. This rearrangement maintains the phase separation, as two crystalline phases are present before and after annealing, due to the immiscibility of long blocks. In copolymers characterized by blocks with molecular weight equal to about 2500, that is the higher limit for the miscibility in the amorphous state in PET/PC block copolymers studied in this work, the rearrangement of the chains during annealing causes a phase separation leading to two crystalline phases. Only in the copolymers with molecular weight of blocks lower than 1500, the very short block length hinders the crystallization: therefore, only in this case a phase separation does not take place after annealing. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: BPA-polycarbonate; Poly(ethylene terephthalate); Block copolymer; Annealing; Phase behaviour
1. Introduction Annealing of a polymer involves thermal treatments at temperatures below the melting point for prolonged times. Different phenomena can take place at microscopical level, such as reorganization of amorphous segment through chain transportation, crystallization of amorphous chains, reorganization of crystal regions, rejection of defects, changes of state stress, modification of structure and/or chemical composition [1]. At macroscopical level the effects of annealing can be important to improve the mechanical properties (as impact strength). For these reasons, the consequences of an-
*
Corresponding author. Fax: +39-051-209-3218. E-mail address: [email protected] Marchese).
(P.
nealing on the final properties of macromolecules can be very significant not only for an academic interest but also from a practical point of view. In previous works we described that it is possible to obtain PET/PC copolymers by reactive blending [2–4]: the chemical structure and the thermal properties of these materials were analysed as a function of the nature of catalyst and the mixing time in Brabender. The 1 HNMR and differential scanning calorimetry (DSC) analyses indicated that the PET/PC final materials were either block or random copolymers, depending on the block length. By considering only the block copolymers, it resulted that their phase behaviour was strictly connected to the chemical architecture. In particular, an homogeneous amorphous phase, without the presence of crystalline phases, was observed in samples characterized by a sequence of monomeric units lower than 15. In copolymers with longer blocks, the occurrence of a
0014-3057/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0014-3057(02)00377-4
1082
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
reactions, was purchased from Aldrich and used as supplied. PET/PC blends were prepared by mixing PET and PC samples in a 1:1 weight ratio in a Brabender (PlastiCorder PL 2000) at 270 °C and 30 rpm. Samples were taken from the mixer at different reaction times (5, 10, 20, 30 and 60 min) and cooled in air. Table 1 describes the samples analysed in this work, including some properties previously discussed [4] and useful to better understand the thermal behaviour of the samples. The sample code used is XX–YY, where XX is the catalyst code and YY is the mixing time in Brabender. Mn PET blocks , representing the molecular weight of the PET blocks, was determined from intrinsic viscosity data of the separated PET blocks obtained by selective degradation. The DSC data reported in Table 1 refer either to the Tg values of PET and PC phases, when the copolymers present two different glass transitions, or to the Tg of the single phase ðTg sp Þ, when the copolymer is characterized by only one homogeneous amorphous phase. Moreover the melting temperature and the corresponding melting enthalpy are reported. A blend between 50 wt.% of PET and 50 wt.% of PC has been obtained by mixing in Brabender for 2 min PET, prepared by using Ti(OBu)4 , and PC in the presence of DNOP: this sample, identified by Ti-2/DNOP, is a mechanical mixture between the two homopolymers and does not show any evidences of transesterification reactions [4].
phase separation, due to a crystallization process, was evidenced. For this reason, it appeared very interesting to study how a thermal treatment, performed at the conditions where the crystallization can occur, influences the phase behaviour as a function of the chemical architecture. The analysed materials, in form of cast films, were subjected to annealing for different times at 160 °C, that is a temperature located between the Tg and Tm of the two homopolymers, i.e. in the temperature range where their crystallization is possible. This thermal treatment can cause a reorganization of both amorphous and crystalline phases, inducing a modification in the final microstructure [5]. It is important to underline that during the annealing any possible change in the chemical structure of the materials, due to transesterification reactions, already observed during annealing of blends and copolymers with polyesters [6–10], have been avoided. In fact, an inhibitor of the transesterification reactions has been added. Therefore, the effects of the thermal treatments on the characteristics and modifications of the crystalline phase have been analysed, by using DSC and wide angle X-ray diffraction (WAXS).
2. Experimental 2.1. Materials The preparation of the PET/PC samples and their characteristics are described in the previous works [2–4]. Briefly, the polycarbonate of bisphenol A (PC) (intrinsic viscosity equal to 0.65 dL g1 in chloroform at 30 °C) was kindly supplied by Enichem. Various PET specimens were prepared in our laboratory by reacting dimethyl terephthalate and ethylene glycol in the presence of different catalysts, based on lanthanides (cerium, erbium, europium, samarium and terbium) (Table 1 in Ref. [4]). The catalysts are described with the same codes used in Ref. [4]: Ce, Er, Eu, Sm, Tb. Di-n-octadecyl phosphite (DNOP), an inhibitor of transesterification
2.2. Annealing experiments The annealing experiments were performed on cast films, prepared by dissolving the samples in a mixture of dichlorometane/1,1,1,3,3,3-hexafluoro-2-propanol (90/10 vol%). For PC only dichloromethane was used. The concentration of the solutions was 0.04 g/ml. In all the solutions DNOP (0.5 wt.% with respect to the polymers) was added to avoid the transesterification reactions during thermal treatments at high temperatures. After a slow evaporation, the cast films were dried
Table 1 Analysed samples and their characteristics Sample
Mn
Sm-10 Eu-10 Ce-10 Er-10 Tb-10 Eu-30 Sm-20 Sm-30
8000 10,100 13,000 9700 8600 2300 2700 1500
a b
PET blocks
a
Tg
PET phase
(°C)
78 77 76 74 74
Calculated according to Eq. (3) in Ref. [4]. Calculated respect to the PET present in the blend.
Tg
PC phase
(°C)
Tg
138 (2nd scan) 140 (2nd scan) 137 142 (2nd scan) 139 90 92 92
sp
(°C)
Tm (°C)
DHm b (J/g)
248 252 258 253 256 238 – –
21.6 23.2 31.2 30.0 31.4 2.0 – –
under vacuum at 90 °C for 12 h and, then, annealed in an oven at the annealing temperature Ta ¼ 160 °C in nitrogen atmosphere for annealing times ðta Þ equal to 16, 24, 72 and 120 h. As the samples maintained their colour at the surface after annealing, apparently degradation processes do not occur. In fact, it is reported that for PET and PC degradation takes place at higher Ta [11,12]. 2.3. Thermal analysis The thermal analysis was carried out in a DSC-7 Perkin–Elmer calorimeter, calibrated with high purity standards. The annealed samples were analysed from 50 to 280 °C at 20 °C/min (1st scan). The two or three melting peaks, when present, are indicated as Tm0 and Tm00 or Tm0 , Tm00 and Tm000 , corresponding to the temperature positions of the first, second and third peak, respectively. The melting enthalpies are calculated from the area between the curve and the baseline, including all the endothermic phenomena.
Heat Flow (mW)
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
1083
a)
b)
c)
100
150
200
250
Temperature ( °C) Fig. 1. DSC thermograms at 20 °C/min for (a) PET before annealing; (b) PET after annealing at Ta ¼ 160 °C for 72 h; (c) PET after annealing at Ta ¼ 160 °C for 72 h, treated with an isotherm at 205 °C for 10 min during the scan.
2.4. X-ray diffraction analysis A Philips PW1710 diffractometer, employing CuKa radiation, was used. Diffractograms were recorded by continuous scanning at the rate of 0.05°/min over the range of 2h from 5° to 40°.
3. Results and discussion 3.1. Annealing on PET and PC homopolymers In order to better understand the effects of the annealing on the thermal behaviour of PET/PC copolymers, the first analysis has been carried out on pure homopolymers, in form of cast films. Fig. 1 evidences that PET is crystalline before annealing (curve a) and shows a more complex melting process after annealing at Ta ¼ 160 °C for ta ¼ 72 h (curve b): a new peak at Tm0 ¼ 194 °C and a shoulder in the original peak in the range of 210–230 °C appear. The peak at low temperature is generally attributed to the melting of crystals formed during secondary crystallization [13–15]. According to Alfonso et al. [11], at Ta < 170 °C the amorphous chain segments, originally trapped in the ordered blocks, can gain enough mobility to reorganize themselves and to form new ordered zones, inducing the new melting process at 194 °C. As the melting temperature reflects the crystal perfection, the low Tm0 value suggests that the new crystalline zones are characterized by low perfection and are not in the state of the maximum stability.
In order to interpret the broad shoulder in curve b, the annealed sample has been heated in DSC up to 205 °C, held at this temperature for 10 min and then heated to 280 °C [16]. In Fig. 1, curve c, it is evident that, after the isotherm at 205 °C, a new melting peak appears at 216 °C and the high endotherm is slightly shifted to higher temperature. This indicates that PET is able to reorganize the imperfect crystals during the heating [11,14,17, 18], leading to an increase of Tm00 . As a consequence, the evaluation of the melting enthalpies from the melting curves is doubtful. Fig. 2 reports the X-ray diffraction pattern (WAXS) obtained from the PET film before annealing (curve a): the breadth of the reflections indicate that PET crystals are characterized by low perfection [17]. It is noteworthy that an identical spectrum is obtained after annealing. Then, the effects of the annealing are not revealed by WAXS, in spite of the fact that they have high influence on the melting behaviour. Similar results are reported in literature [11,17] and suggest that the crystalline phase does not improve its perfection during annealing. This result confirms that a reorganization of the amorphous regions is the most probable phenomenon occurring in PET during annealing. With regard to the PC films, it is well known that PC is an amorphous polymer, that thermally crystallizes slowly and to a limited extent. However, cast films can crystallize due to the effect of solvents that promote chain motions and improves an internal order [19]. Moreover, it is reported in literature that also additives can induce PC crystallization [20]. As DNOP has been
1084
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Intensity
a)
b)
c) 0
10
20
30
40
2θ Fig. 2. WAXS patterns of (a) PET before annealing; (b) PC before annealing; (c) Ti-2/DNOP blend before annealing.
a)
Heat Flow (mW)
b) c) d)
e)
100
150
200
250
Temperature (°C) Fig. 3. DSC thermograms at 20 °C/min for (a) original PC sample; (b) PC cast film prepared without DNOP; (c) PC cast film prepared with DNOP; (d) PC cast film prepared with DNOP and annealed at Ta ¼ 160 °C for 72 h; (e) PC cast film after annealing at Ta ¼ 160 °C for 72 h, treated with an isotherm at 200 °C for 10 min during the scan.
added to all the blends, it is necessary to preliminary understand the effect of DNOP on the thermal behaviour of PC. In Fig. 3 the DSC curves (1st scan) of the original PC (curve a), of the PC film obtained from solution without DNOP (curve b), of the PC film obtained from solution with DNOP (curve c) are reported. It is interesting to
observe that only the original PC is amorphous, showing the glass transition at 153 °C. All the other samples present a melting process with a broad peak at about 230–237 °C. Then, as PC can crystallize from solution both with and without DNOP, the ability of PC to form a crystalline phase can be attributed to the solvent effect. After the annealing (curve d), the behaviour of PC is very similar to that observed for PET: the melting process becomes more complex, showing a peak at low temperature (Tm0 ¼ 190 °C) and a more pronounced shoulder in the main peak. The presence of a low temperature melting peak in PC has been observed by Sohn et al. [21] after isothermal treatments at temperatures between 165 and 185 °C. This endotherm was associated to a secondary crystallization that led to fringed micellae secondary crystals. In fact, even if for PC the induction time for crystallization is very high, the rate of formation of new crystals is enhanced when other crystals are already present. An isothermal treatment in DSC, similar to that described for PET, has been applied also on the PC sample annealed at 160 °C for 72 h. In this case, an isotherm at 200 °C for 10 min has been performed (curve e): after the isotherm the PC sample shows a more intense shoulder at 218 °C respect to the reference thermogram of the annealed sample (curve d). Then, also in PC the metastable secondary crystals can rearrange through melting–recrystallization–remelting processes. In spite of its rigid backbone, PC, if crystalline, is able to reorganize during heating like other polyesters with more flexible chains [22]. In the X-ray diffraction pattern of the PC, analysed before annealing (Fig. 2, curve b), a sharp diffraction peak (2h ¼ 17:3°), attributed to the monoclinic PC [23], is evident. Like for PET, the annealing does not induce a modification in the pre-existing crystalline phase of PC. Jonza and Porter [23] report the same result for crystalline PC annealed at 197 °C for 94 h, while only for higher Ta , for example 230 °C, the annealing causes the diffraction peaks to sharpen and the weaker ones to stand out, due to the lamellar thickening, as confirmed by small-angle X-ray diffraction. Therefore, during the annealing at 160 °C of both PC and PET a reorganization of the amorphous phase towards new crystals at low perfection takes place and does not seem to significantly modify the original crystalline phase. In this contest, it seems interesting to analyse the effects of the annealing on a mechanical mixture between PET and PC homopolymers (Ti-2/DNOP), that is characterized by a complete lack of miscibility [4]. The cast film, analysed in DSC before annealing, shows a single melting peak at 250 °C (Fig. 4, curve a). By considering the low tendency of PC to crystallize and the temperature of the melting peak, it is expected that the crystalline phase is due only to the PET crystals. However, the
Heat Flow (mW)
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
dicating the formation of crystals at low perfection. The WAXS analysis shows a pattern very similar to that reported in curve c of Fig. 2, suggesting that modifications, in terms of unit cell, degree of crystallinity and crystal size do not take place during the thermal ageing. Therefore, during annealing, the PET and PC chains in the blend have the same behaviour shown by the isolated homopolymers. This is a consequence of the immiscibility of the two homopolymers in the absence of transesterification reactions.
a)
3.2. Annealing on PET/PC copolymers
b)
100
1085
150
200
250
Temperature (°C) Fig. 4. DSC thermograms at 20 °C/min for (a) Ti-2/DNOP blend before annealing; (b) Ti-2/DNOP blend after annealing at Ta ¼ 160 °C for 72 h.
DHm equal to 42 J/g results to be a too high value for the crystalline phase of PET: it corresponds to a improbable degree of crystallinity equal to 60% (heat of fusion for 100% crystalline PET equal to 135.8 J/g [24]). The X-ray diffraction analysis on the sample before annealing is reported in Fig. 2, curve c. By comparing the WAXS spectra of PET and PC, it is noteworthy that the reflection of PC crystals coincides with one of the reflections of PET: for this reason, it is not possible to identify the PC crystalline phase by the presence of its sharp peak at 17.3°. However, in spite of the low degree of resolution of the reflections, it is possible to compare the intensities of the diffraction peaks of the pure PET with those obtained from the blend (curves a and c): it is evident that in the blend the reflection centred at 17° is more intense respect the other signals in the pure PET. This result indicates that this reflection is the sum of two peaks, one due to PET crystals and the other one attributed to PC crystals. Then, both the crystalline phases of PET and PC are present. This result, that justifies the high value of DHm , suggests that also PC is able to crystallize in the blend: the most probable reason is the use of the solvent during the sample preparation. It is significant that the DSC trace before annealing (curve a in Fig. 4) does not evidence the melting peak characteristic of the PC crystals at about 230 °C: probably the melting process of the PC crystals is hidden in the shoulder of the main peak. The effects of the annealing at 160 °C for 72 h are shown in the DSC trace reported in curve b of Fig. 4: it is evident a new peak at low temperature (199 °C), in-
The study of the effects of the annealing on PET/PC copolymers has been carried out by differentiating the copolymers according to their block length, that resulted the key parameter to determine their thermal properties [4]. The Mn PET blocks value, available for all the samples here analysed, is used to distinguish three different systems: (i) block copolymers with Mn PET blocks P 8000; (ii) block copolymers with Mn PET blocks 2500; (iii) block copolymers with Mn PET blocks 1500. From the results reported in Ref [4], it is expected that in a copolymer the value of Mn PC blocks is not too different from the corresponding Mn PET blocks . The DSC characterization of these samples, reported in Table 1, was carried out in the previous work [4] on cast films heated to 280 °C for 1 min and, then, fast cooled to room temperature in order to cancel the previous thermal history. It is evident that the copolymers with Mn PET blocks P 8000 are characterized by a phase separation: two amorphous and one crystalline phases are present, as shown by two glass transition processes and one melting peak. On the other hand, the copolymers with Mn PET blocks equal to about 2500, present a single homogeneous amorphous phase and a little tendency to crystallize, as evident from the DHm ¼ 2:0 J/g for Eu-30. This capacity completely disappears for Mn PET blocks lower than 1500. In this work the DSC analysis has been performed directly on the cast films, before and after annealing: only the 1st scan is considered in order to evidence the differences in chain reorganization ability. The discrepancies in Tm and DHm between the results reported in Table 1 and those here obtained for Sm-10 and Eu-10 samples at ta ¼ 0 (Table 2) are due to the different thermal history. In particular, if PC is able to crystallize due to the solvent effect, only in the 1st scan it should be possible to evidence its crystalline phase. 3.2.1. Block copolymers with Mn PET blocks P 8000 As already described, the copolymers with Mn PET blocks P 8000 are characterized by phase separation, due to the immiscibility of the long PET and PC blocks. In curve a of Fig. 5 the DSC trace for Sm-10
1086
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Table 2 Melting temperatures and enthalpies measured for copolymers with Mn PET blocks P 8000 after annealing at Ta ¼ 16 °C for the times ta Sample ta (h)
Tm0 (°C)
Tm00 (°C)
Tm000 (°C)
DHm a (°C)
Sm-10
0 24 72 120
– 184 184 189
– 216 216 216
232 240 242 241
29.8 38.9 39.9 42.6
Eu-10
0 24 72 120
– 183 186 189
– 216 218 217
237 242 244 246
19.8 38.8 41.5 43.8
Ce-10
24 72 120
186 187 189
215 217 215
247 247 245
34.3 42.3 44.0
Er-10
24 72 120
183 185 189
216 215 214
243 243 243
40.8 41.8 45.3
Tb-10
24 72 120
184 186 190
215 214 214
246 245 243
39.0 44.0 43.8
Calculated respect to the weight of the sample.
Heat Flow (mW)
a)
b)
c)
d)
100
150
200
250
Temperature (°C)
Heat Flow (mW)
a
analysed copolymers belonging to this class: Table 2 reports the melting temperatures and total DHm obtained after annealing. It can be seen that the 1st melting peak is always located in the region of 183–190 °C and is shifted to 3–6 °C higher temperatures by passing from ta ¼ 24 to 120 h. In analogy with what found for the homopolymers, the presence of a new melting process at low temperature suggests the formation of secondary crystals during annealing. The increment of Tm0 with ta suggests that these new crystals, characterized by a low perfection degree, improve their crystalline state by increasing ta [11,14]. For the high endotherm, it is evident that, respect to the not annealed sample, Tm000 increases of 8 °C with the annealing. Then, both Tm00 and Tm000 do not vary during the thermal treatments as a function of ta . As the peak at Tm000 is reasonably due to the melting of crystals originated by primary crystallization, the treatment at 160 °C has an initial effect of improving the perfection of the original crystals. In fact, it is expected that in block copolymers the crystals have a high level of imperfections. The peak at Tm00 changes its shape with ta : even if Tm00 corresponds to the melting temperature of PC, from the previous results it was found that the endotherm of PC crystals is not evident in DSC traces when PET is also present. Then, in order to try to understand the origin of the peak at Tm00 , isothermal treatments have been performed in DSC. In Fig. 6 some results are reported: the Eu-10 sample, annealed at 160 °C for 120 h and characterized by the DSC thermogram shown in curve a, has
a)
b)
Fig. 5. DCS thermograms at 20 °C/min for Sm-10 annealed at Ta ¼ 160 °C for (a) ta ¼ 0; (b) ta ¼ 24 h; (c) ta ¼ 72 h; (d) ta ¼ 120 h.
c)
100
before annealing is reported: a broad melting peak is evident, confirming the presence of crystallinity. After the annealing at 160 °C for times varying from 24 to 120 h, three melting peaks appear in the thermograms, in the temperature range from 185 to 240 °C (curves b–d in Fig. 5). The results are similar for all the
150
200
250
Temperature (°C) Fig. 6. DSC thermograms at 20 °C/min for Eu-10: (a) annealed at Ta ¼ 160 °C for 120 h; (b) annealed at Ta ¼ 160 °C for 120 h, treated with an isotherm at 200 °C for 10 min during the scan; (c) annealed at Ta ¼ 160 °C for 120 h, treated with an isotherm at 220 °C for 10 min during the scan.
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Intensity
been analysed at 20 °C/min from 50 to 280 °C with 10 min isotherm at 200 °C (curve b) or 220 °C (curve c): these temperatures correspond to the end of the first and second melting peaks, respectively. After these two isotherms, the melting process shows an increment in the intensity of the second peak (curve b) and the appearance of a new peak at about 230 °C (curve c). These results suggest the capability of the copolymers to recrystallize during the treatment at high temperatures. In front of this results, the peak at Tm00 can be attributed to the melting of the secondary crystals melted at Tm0 and then recrystallized during the heating scan. As the recrystallization process takes place always in the same temperature range above Tm0 , Tm00 is not affected by the time of annealing. Because of the melting–recrystallization processes, the total DHm is not completely meaningful. However, it is interesting to observe in Table 2 that the DHm is characterized by a tendency to increase by 4–10 J/g as a function of the annealing times. The WAXS analysis can be useful to shed light on some characteristics of the crystals present in the samples. Fig. 7 compare the diffraction spectra of Sm-10 before and after annealing: it is evident that both PET and PC crystalline phases are present. It can be surprising that PC is able to crystallize also in copolymers: this behaviour can be justified by a solvent effect on the primary crystallization. Moreover, during primary and secondary crystallizations, the presence of blocks of PET can lead to better mobility of the neighbouring PCsegments, improving the crystallization process [25]. In the cast film before annealing the broad and not resolved WAXS peaks indicate the presence of imperfect crystals: these induce the wide melting peak observed in
b) a) 0
10
20
30
40
2θ (°) Fig. 7. WAXS patterns of (a) Sm-10 before annealing; (b) Sm10 annealed at Ta ¼ 160 °C for 72 h.
1087
the DSC first scan (Fig. 5, curve a). After 72 h of annealing at 160 °C, the peak slightly sharpen and increase in intensity, suggesting an improvement in the crystal perfection and in crystallinity degree. This behaviour justifies the higher values of Tm000 and should be in agreement with the apparent increment of DHm observed in DSC analysis. In analogy with that found for the homopolymers, the most important effect of the annealing is a secondary crystallization that induces mainly rearrangements of the amorphous chains towards more ordered regions and causes an increment of the total crystallinity. As these copolymers are characterized by immiscibility of the two amorphous phases, the annealing should not increase the phase separation that is originally present in the samples and attributed to the long block sequences.
3.2.2. Block copolymers with Mn PET blocks 2500 Eu-30 and Sm-20 are examples of block copolymers characterized by Mn PET blocks equal to about 2500: this value has been recognized as the maximum block length able to form a miscible amorphous phase after the fast cooling from the melt [4]. Only Eu-30 is characterized by a low level of crystallinity: probably during the fast cooling from the melt a small fraction of PET is able to crystallize. The DSC thermograms of both cast films before annealing show a melting peak with low intensity at 224 and 220 °C for Eu-30 and Sm-20 respectively: this behaviour confirms that during the film preparation some crystals are formed, even if their perfection is very low, as shown by the low melting temperature. The effects of the annealing at 160 °C for 72 h on the thermal behaviour of these copolymers are described in Fig. 8, curves a and b respectively. It can be seen that the thermograms show two melting processes at Tm0 ¼ 186 and 172 °C and Tm00 ¼ 227 and 230 °C for Eu-30 and Sm20 respectively. The total DHm is about 27 J/g for both samples, even if this value is apparent because of the possible presence of melting–recrystallization processes. In any case, these results indicate that the annealing increases the initial crystallinity: beside an improvement of the pre-existing crystals perfection, it is expected mainly the formation of new crystalline regions from the amorphous phase. The WAXS analysis confirms the very low crystallinity level of the samples, as shown in curve a of Fig. 9 for Sm-20. After annealing, the reflections of both PET and PC crystalline phases are evident (curve b of Fig. 9), even if the crystal are strongly imperfect. Then, respect to the block copolymers characterized by Mn PET blocks P 8000, in this case the PET and PC blocks show a greater difficulty in organizing from a disordered homogeneous state towards an ordered phase. This process, possible only if the samples spent
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Heat Flow (mW)
1088
b)
a)
100
150
200
250
Temperature (°C) Fig. 8. DSC thermograms at 20 °C/min for (a) Eu-30 annealed at Ta ¼ 160 °C for 72 h; (b) Sm-20 annealed at Ta ¼ 160 °C for 72 h.
Intensity
d) c)
3.2.3. Block copolymers with Mn PET blocks < 1500 Sm-30 is an example of copolymer characterized by a very short block length. This chemical architecture induces the formation of a single amorphous phase if the sample is quenched from the melt [4]. The cast film, analysed before and after annealing, does not show melting endotherms in DSC thermograms, suggesting that the crystallization of the sample is not improved by the presence of the solvent and by the thermal treatments. The WAXS patterns, obtained before and after annealing, are reported in curve c and d of Fig. 9 respectively: the amorphous halo of the sample is present in both cases. Then, the sample retains its amorphous state even after annealing. By comparing the behaviour of Sm-20 and Sm-30, it is noteworthy that both samples appear homogeneous, without phase separation, after quenching from the melt (Table 1). If the samples are subjected to the conditions favourable to crystallization, Sm-20 is able to crystallize during annealing, through a new organization of the amorphous phase: this phenomenon increases the phase separation between PET and PC by crystallization. Sm30, on the contrary, does not modify the phase behaviour with annealing at 160 °C. This different behaviour can be explained by considering that Sm-30 is characterized by very short block length (corresponding to sequences of 10 monomeric units), that hinders any crystallization process even if the sample is maintained in temperature and time conditions favourable to crystallization. Then, in Sm-30 the phase separation cannot take place. For this reason, the occurrence of a single stable phase, that can be maintained also during thermal treatments, is observed in PET/PC copolymers only when the sequences of PET (and probably also of PC) monomeric units are lower than 10.
b) a) 0
10
20
30
40
2θ (°) Fig. 9. WAXS patterns of (a) Sm-20 before annealing; (b) Sm20 after annealing at Ta ¼ 160 °C for 72 h; (c) Sm-30 before annealing; (d) Sm-30 after annealing at Ta ¼ 160 °C for 72 h.
enough time at temperatures favourable to the chain reorganization, is attributed to the short block length that does not promote a crystallization process and the formation of crystals at high perfection. As a consequence, the annealing at 160 °C for long times promotes phase separation, induced by crystallization, in partially miscible copolymers. Similar results are reported for PC blended with PBT or amide modified PBT [26].
4. Conclusions The effect of the annealing on block PET/PC copolymers has been discussed in terms of ability to crystallize, that is strictly dependent on the chemical architecture. For copolymers with Mn PET blocks P 8000 it was observed that the annealing at 160 °C causes an increment of crystallinity, as confirmed by WAXS analysis. This effect has been interpreted as due to the secondary crystallization and, in particular, to the reorganization of the chains in the amorphous phase towards PET and PC crystalline phases. This process induces multiple melting peaks in DSC, due to a melting–recrystallization phenomena, involving the new crystals at low perfection. As a consequence, the annealing of block copolymers, characterized by a phase separation before ageing, practically does not modify the phase behaviour of the system.
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Instead, for the copolymers with Mn PET blocks 2500, the blocks are short enough to form an homogeneous amorphous phase, but long enough to crystallize during a suitable thermal treatment. It resulted that the annealing at 160 °C promotes the crystallization of both PET and PC blocks, causing a phase separation from the homogeneous amorphous state. This is the case of Eu-30 and Sm-20 samples. On the other hand, when the copolymers are characterized by a very short block length ðMn PET blocks < 1500Þ, the annealing has not effect on the phase behaviour: the samples remain in the amorphous state also during the thermal treatments that should improve the crystallization. This behaviour is observed for Sm-30. As a conclusion, the thermal treatments at high temperatures, in the range where the crystallization of PET and PC is possible, evidence the rearrangements that can occur in block copolymers and the role of the block length on the phase behaviour.
References [1] Wunderlich B. Crystal nucleation, growth, annealing. In: Macromolecular physics, vol. 2. New York: Academic; 1976. [2] Ignatov VN, Carraro C, Tartari V, Pippa R, Scapin M, Pilati F, et al. Polymer 1997;38:201–5. [3] Pila F, Fiorini M, Berti C. Effects of catalysts in the reactive blending of bisphenol A polycarbonate with poly(alkyleneterephthalate)s. In: Fakirov S, editor. Transreactions in condensation polymers. Weinheim: WileyVCH; 1999 [Chapter 2]. [4] Marchese P, Celli A, Fiorini M. Macromol Chem Phys 2002;203:695–704. [5] Debier D, Jonas AM, Legras R. J Polym Sci, Part B: Polym Phys 1998;36:2197–210.
1089
[6] Porter RS, Wang LH. Polymer 1992;33:2019. [7] Fakirov S, Sarkissova M, Denchev Z. Macromol Chem Phys 1996;197:2837–67. [8] Denchev Z, Sarkissova M, Fakirov S, Yilmaz F. Macromol Chem Phys 1996;197:2869–87. [9] Fakirov S, Sarkissova M, Denchev Z. Macromol Chem Phys 1996;197:2889–907. [10] Denchev Z, Sarkissova M, Radusch H-J, Luepke T, Fakirov S. Macromol Chem Phys 1998;199:215–21. [11] Alfonso GC, Pedemonte E, Ponzetti L. Polymer 1979;20: 104–12. [12] Mark HF, Bikales NM, Overberger CG, Menges G. In: Encyclopedia of polymer science and engineering. 2nd ed. New York: Wiley Interscience; 1988. p. 655. [13] Wang Z-G, Hsiao BS, Sauer BB, Kampert WG. Polymer 1999;40:4615–27. [14] Lin S-B, Koenig JL. J Polym Sci, Polym Symp 1984;71: 121–35. [15] Lu XF, Hay JN. Polymer 2001;42:9423–31. [16] Roberts RC. J Polym Sci, Polym Lett 1970;8:381–4. [17] Holdsworth PJ, Turner-Jones A. Polymer 1971;12:195– 208. [18] Fakirov S, Fisher EW, Hoffmann R, Schmidt GF. Polymer 1977;18:1121. [19] Garbauskas MF. Polycarbonate crystallinity. In: Legrand DJ, Bendler JT, editors. Handbook of polycarbonate science and technology. New York: Marcel Dekker; 1996. [20] van Bennekon ACM, Pluimers DT, Bussino J, Gaymans RJ. Polymer 1997;38:3017. [21] Sohn S, Alizadeh A, Marand H. Polymer 2000;41:8879– 86. [22] Mendez G, Muller AJ. J Therm Anal 1997;50:593–602. [23] Jonza JM, Porter RS. J Polym Sci, Part B: Polym Phys 1986;24:2459–72. [24] Jog JP. JMS-Rev Macromol Chem Phys 1995;C35:531–53. [25] Wings N, Trafara G. Angew Makromol Chem 1994;217: 91–105. [26] van Bennekom ACM, van den Berg D, Bussink J, Gaymans RJ. Polymer 1997;38:5041–9.
Effects of annealing on crystallinity and phase behaviour of PET/PC block copolymers Paola Marchese a
a,*
, Annamaria Celli a, Maurizio Fiorini a, Marina Gabaldi
b
Dipartimento di Chimica Applicata e Scienza dei Materiali, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy b Centro Ricerche ‘‘Giulio Natta’’, Basell, Piazzale Donegani 12, 44100 Ferrara, Italy Received 24 October 2002; received in revised form 29 November 2002; accepted 29 November 2002
Abstract The effects of the annealing on the properties of PET/PC block copolymers, obtained by reactive blending in the presence of different catalysts and for different mixing times, have been studied. The annealing, performed in conditions that promote the crystallization, has been used to better understand the role of block length in determining the phase behaviour. The copolymers characterized by blocks with molecular weight larger than 8000 are able to reorganize towards more ordered domains. This rearrangement maintains the phase separation, as two crystalline phases are present before and after annealing, due to the immiscibility of long blocks. In copolymers characterized by blocks with molecular weight equal to about 2500, that is the higher limit for the miscibility in the amorphous state in PET/PC block copolymers studied in this work, the rearrangement of the chains during annealing causes a phase separation leading to two crystalline phases. Only in the copolymers with molecular weight of blocks lower than 1500, the very short block length hinders the crystallization: therefore, only in this case a phase separation does not take place after annealing. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: BPA-polycarbonate; Poly(ethylene terephthalate); Block copolymer; Annealing; Phase behaviour
1. Introduction Annealing of a polymer involves thermal treatments at temperatures below the melting point for prolonged times. Different phenomena can take place at microscopical level, such as reorganization of amorphous segment through chain transportation, crystallization of amorphous chains, reorganization of crystal regions, rejection of defects, changes of state stress, modification of structure and/or chemical composition [1]. At macroscopical level the effects of annealing can be important to improve the mechanical properties (as impact strength). For these reasons, the consequences of an-
*
Corresponding author. Fax: +39-051-209-3218. E-mail address: [email protected] Marchese).
(P.
nealing on the final properties of macromolecules can be very significant not only for an academic interest but also from a practical point of view. In previous works we described that it is possible to obtain PET/PC copolymers by reactive blending [2–4]: the chemical structure and the thermal properties of these materials were analysed as a function of the nature of catalyst and the mixing time in Brabender. The 1 HNMR and differential scanning calorimetry (DSC) analyses indicated that the PET/PC final materials were either block or random copolymers, depending on the block length. By considering only the block copolymers, it resulted that their phase behaviour was strictly connected to the chemical architecture. In particular, an homogeneous amorphous phase, without the presence of crystalline phases, was observed in samples characterized by a sequence of monomeric units lower than 15. In copolymers with longer blocks, the occurrence of a
0014-3057/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0014-3057(02)00377-4
1082
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
reactions, was purchased from Aldrich and used as supplied. PET/PC blends were prepared by mixing PET and PC samples in a 1:1 weight ratio in a Brabender (PlastiCorder PL 2000) at 270 °C and 30 rpm. Samples were taken from the mixer at different reaction times (5, 10, 20, 30 and 60 min) and cooled in air. Table 1 describes the samples analysed in this work, including some properties previously discussed [4] and useful to better understand the thermal behaviour of the samples. The sample code used is XX–YY, where XX is the catalyst code and YY is the mixing time in Brabender. Mn PET blocks , representing the molecular weight of the PET blocks, was determined from intrinsic viscosity data of the separated PET blocks obtained by selective degradation. The DSC data reported in Table 1 refer either to the Tg values of PET and PC phases, when the copolymers present two different glass transitions, or to the Tg of the single phase ðTg sp Þ, when the copolymer is characterized by only one homogeneous amorphous phase. Moreover the melting temperature and the corresponding melting enthalpy are reported. A blend between 50 wt.% of PET and 50 wt.% of PC has been obtained by mixing in Brabender for 2 min PET, prepared by using Ti(OBu)4 , and PC in the presence of DNOP: this sample, identified by Ti-2/DNOP, is a mechanical mixture between the two homopolymers and does not show any evidences of transesterification reactions [4].
phase separation, due to a crystallization process, was evidenced. For this reason, it appeared very interesting to study how a thermal treatment, performed at the conditions where the crystallization can occur, influences the phase behaviour as a function of the chemical architecture. The analysed materials, in form of cast films, were subjected to annealing for different times at 160 °C, that is a temperature located between the Tg and Tm of the two homopolymers, i.e. in the temperature range where their crystallization is possible. This thermal treatment can cause a reorganization of both amorphous and crystalline phases, inducing a modification in the final microstructure [5]. It is important to underline that during the annealing any possible change in the chemical structure of the materials, due to transesterification reactions, already observed during annealing of blends and copolymers with polyesters [6–10], have been avoided. In fact, an inhibitor of the transesterification reactions has been added. Therefore, the effects of the thermal treatments on the characteristics and modifications of the crystalline phase have been analysed, by using DSC and wide angle X-ray diffraction (WAXS).
2. Experimental 2.1. Materials The preparation of the PET/PC samples and their characteristics are described in the previous works [2–4]. Briefly, the polycarbonate of bisphenol A (PC) (intrinsic viscosity equal to 0.65 dL g1 in chloroform at 30 °C) was kindly supplied by Enichem. Various PET specimens were prepared in our laboratory by reacting dimethyl terephthalate and ethylene glycol in the presence of different catalysts, based on lanthanides (cerium, erbium, europium, samarium and terbium) (Table 1 in Ref. [4]). The catalysts are described with the same codes used in Ref. [4]: Ce, Er, Eu, Sm, Tb. Di-n-octadecyl phosphite (DNOP), an inhibitor of transesterification
2.2. Annealing experiments The annealing experiments were performed on cast films, prepared by dissolving the samples in a mixture of dichlorometane/1,1,1,3,3,3-hexafluoro-2-propanol (90/10 vol%). For PC only dichloromethane was used. The concentration of the solutions was 0.04 g/ml. In all the solutions DNOP (0.5 wt.% with respect to the polymers) was added to avoid the transesterification reactions during thermal treatments at high temperatures. After a slow evaporation, the cast films were dried
Table 1 Analysed samples and their characteristics Sample
Mn
Sm-10 Eu-10 Ce-10 Er-10 Tb-10 Eu-30 Sm-20 Sm-30
8000 10,100 13,000 9700 8600 2300 2700 1500
a b
PET blocks
a
Tg
PET phase
(°C)
78 77 76 74 74
Calculated according to Eq. (3) in Ref. [4]. Calculated respect to the PET present in the blend.
Tg
PC phase
(°C)
Tg
138 (2nd scan) 140 (2nd scan) 137 142 (2nd scan) 139 90 92 92
sp
(°C)
Tm (°C)
DHm b (J/g)
248 252 258 253 256 238 – –
21.6 23.2 31.2 30.0 31.4 2.0 – –
under vacuum at 90 °C for 12 h and, then, annealed in an oven at the annealing temperature Ta ¼ 160 °C in nitrogen atmosphere for annealing times ðta Þ equal to 16, 24, 72 and 120 h. As the samples maintained their colour at the surface after annealing, apparently degradation processes do not occur. In fact, it is reported that for PET and PC degradation takes place at higher Ta [11,12]. 2.3. Thermal analysis The thermal analysis was carried out in a DSC-7 Perkin–Elmer calorimeter, calibrated with high purity standards. The annealed samples were analysed from 50 to 280 °C at 20 °C/min (1st scan). The two or three melting peaks, when present, are indicated as Tm0 and Tm00 or Tm0 , Tm00 and Tm000 , corresponding to the temperature positions of the first, second and third peak, respectively. The melting enthalpies are calculated from the area between the curve and the baseline, including all the endothermic phenomena.
Heat Flow (mW)
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
1083
a)
b)
c)
100
150
200
250
Temperature ( °C) Fig. 1. DSC thermograms at 20 °C/min for (a) PET before annealing; (b) PET after annealing at Ta ¼ 160 °C for 72 h; (c) PET after annealing at Ta ¼ 160 °C for 72 h, treated with an isotherm at 205 °C for 10 min during the scan.
2.4. X-ray diffraction analysis A Philips PW1710 diffractometer, employing CuKa radiation, was used. Diffractograms were recorded by continuous scanning at the rate of 0.05°/min over the range of 2h from 5° to 40°.
3. Results and discussion 3.1. Annealing on PET and PC homopolymers In order to better understand the effects of the annealing on the thermal behaviour of PET/PC copolymers, the first analysis has been carried out on pure homopolymers, in form of cast films. Fig. 1 evidences that PET is crystalline before annealing (curve a) and shows a more complex melting process after annealing at Ta ¼ 160 °C for ta ¼ 72 h (curve b): a new peak at Tm0 ¼ 194 °C and a shoulder in the original peak in the range of 210–230 °C appear. The peak at low temperature is generally attributed to the melting of crystals formed during secondary crystallization [13–15]. According to Alfonso et al. [11], at Ta < 170 °C the amorphous chain segments, originally trapped in the ordered blocks, can gain enough mobility to reorganize themselves and to form new ordered zones, inducing the new melting process at 194 °C. As the melting temperature reflects the crystal perfection, the low Tm0 value suggests that the new crystalline zones are characterized by low perfection and are not in the state of the maximum stability.
In order to interpret the broad shoulder in curve b, the annealed sample has been heated in DSC up to 205 °C, held at this temperature for 10 min and then heated to 280 °C [16]. In Fig. 1, curve c, it is evident that, after the isotherm at 205 °C, a new melting peak appears at 216 °C and the high endotherm is slightly shifted to higher temperature. This indicates that PET is able to reorganize the imperfect crystals during the heating [11,14,17, 18], leading to an increase of Tm00 . As a consequence, the evaluation of the melting enthalpies from the melting curves is doubtful. Fig. 2 reports the X-ray diffraction pattern (WAXS) obtained from the PET film before annealing (curve a): the breadth of the reflections indicate that PET crystals are characterized by low perfection [17]. It is noteworthy that an identical spectrum is obtained after annealing. Then, the effects of the annealing are not revealed by WAXS, in spite of the fact that they have high influence on the melting behaviour. Similar results are reported in literature [11,17] and suggest that the crystalline phase does not improve its perfection during annealing. This result confirms that a reorganization of the amorphous regions is the most probable phenomenon occurring in PET during annealing. With regard to the PC films, it is well known that PC is an amorphous polymer, that thermally crystallizes slowly and to a limited extent. However, cast films can crystallize due to the effect of solvents that promote chain motions and improves an internal order [19]. Moreover, it is reported in literature that also additives can induce PC crystallization [20]. As DNOP has been
1084
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Intensity
a)
b)
c) 0
10
20
30
40
2θ Fig. 2. WAXS patterns of (a) PET before annealing; (b) PC before annealing; (c) Ti-2/DNOP blend before annealing.
a)
Heat Flow (mW)
b) c) d)
e)
100
150
200
250
Temperature (°C) Fig. 3. DSC thermograms at 20 °C/min for (a) original PC sample; (b) PC cast film prepared without DNOP; (c) PC cast film prepared with DNOP; (d) PC cast film prepared with DNOP and annealed at Ta ¼ 160 °C for 72 h; (e) PC cast film after annealing at Ta ¼ 160 °C for 72 h, treated with an isotherm at 200 °C for 10 min during the scan.
added to all the blends, it is necessary to preliminary understand the effect of DNOP on the thermal behaviour of PC. In Fig. 3 the DSC curves (1st scan) of the original PC (curve a), of the PC film obtained from solution without DNOP (curve b), of the PC film obtained from solution with DNOP (curve c) are reported. It is interesting to
observe that only the original PC is amorphous, showing the glass transition at 153 °C. All the other samples present a melting process with a broad peak at about 230–237 °C. Then, as PC can crystallize from solution both with and without DNOP, the ability of PC to form a crystalline phase can be attributed to the solvent effect. After the annealing (curve d), the behaviour of PC is very similar to that observed for PET: the melting process becomes more complex, showing a peak at low temperature (Tm0 ¼ 190 °C) and a more pronounced shoulder in the main peak. The presence of a low temperature melting peak in PC has been observed by Sohn et al. [21] after isothermal treatments at temperatures between 165 and 185 °C. This endotherm was associated to a secondary crystallization that led to fringed micellae secondary crystals. In fact, even if for PC the induction time for crystallization is very high, the rate of formation of new crystals is enhanced when other crystals are already present. An isothermal treatment in DSC, similar to that described for PET, has been applied also on the PC sample annealed at 160 °C for 72 h. In this case, an isotherm at 200 °C for 10 min has been performed (curve e): after the isotherm the PC sample shows a more intense shoulder at 218 °C respect to the reference thermogram of the annealed sample (curve d). Then, also in PC the metastable secondary crystals can rearrange through melting–recrystallization–remelting processes. In spite of its rigid backbone, PC, if crystalline, is able to reorganize during heating like other polyesters with more flexible chains [22]. In the X-ray diffraction pattern of the PC, analysed before annealing (Fig. 2, curve b), a sharp diffraction peak (2h ¼ 17:3°), attributed to the monoclinic PC [23], is evident. Like for PET, the annealing does not induce a modification in the pre-existing crystalline phase of PC. Jonza and Porter [23] report the same result for crystalline PC annealed at 197 °C for 94 h, while only for higher Ta , for example 230 °C, the annealing causes the diffraction peaks to sharpen and the weaker ones to stand out, due to the lamellar thickening, as confirmed by small-angle X-ray diffraction. Therefore, during the annealing at 160 °C of both PC and PET a reorganization of the amorphous phase towards new crystals at low perfection takes place and does not seem to significantly modify the original crystalline phase. In this contest, it seems interesting to analyse the effects of the annealing on a mechanical mixture between PET and PC homopolymers (Ti-2/DNOP), that is characterized by a complete lack of miscibility [4]. The cast film, analysed in DSC before annealing, shows a single melting peak at 250 °C (Fig. 4, curve a). By considering the low tendency of PC to crystallize and the temperature of the melting peak, it is expected that the crystalline phase is due only to the PET crystals. However, the
Heat Flow (mW)
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
dicating the formation of crystals at low perfection. The WAXS analysis shows a pattern very similar to that reported in curve c of Fig. 2, suggesting that modifications, in terms of unit cell, degree of crystallinity and crystal size do not take place during the thermal ageing. Therefore, during annealing, the PET and PC chains in the blend have the same behaviour shown by the isolated homopolymers. This is a consequence of the immiscibility of the two homopolymers in the absence of transesterification reactions.
a)
3.2. Annealing on PET/PC copolymers
b)
100
1085
150
200
250
Temperature (°C) Fig. 4. DSC thermograms at 20 °C/min for (a) Ti-2/DNOP blend before annealing; (b) Ti-2/DNOP blend after annealing at Ta ¼ 160 °C for 72 h.
DHm equal to 42 J/g results to be a too high value for the crystalline phase of PET: it corresponds to a improbable degree of crystallinity equal to 60% (heat of fusion for 100% crystalline PET equal to 135.8 J/g [24]). The X-ray diffraction analysis on the sample before annealing is reported in Fig. 2, curve c. By comparing the WAXS spectra of PET and PC, it is noteworthy that the reflection of PC crystals coincides with one of the reflections of PET: for this reason, it is not possible to identify the PC crystalline phase by the presence of its sharp peak at 17.3°. However, in spite of the low degree of resolution of the reflections, it is possible to compare the intensities of the diffraction peaks of the pure PET with those obtained from the blend (curves a and c): it is evident that in the blend the reflection centred at 17° is more intense respect the other signals in the pure PET. This result indicates that this reflection is the sum of two peaks, one due to PET crystals and the other one attributed to PC crystals. Then, both the crystalline phases of PET and PC are present. This result, that justifies the high value of DHm , suggests that also PC is able to crystallize in the blend: the most probable reason is the use of the solvent during the sample preparation. It is significant that the DSC trace before annealing (curve a in Fig. 4) does not evidence the melting peak characteristic of the PC crystals at about 230 °C: probably the melting process of the PC crystals is hidden in the shoulder of the main peak. The effects of the annealing at 160 °C for 72 h are shown in the DSC trace reported in curve b of Fig. 4: it is evident a new peak at low temperature (199 °C), in-
The study of the effects of the annealing on PET/PC copolymers has been carried out by differentiating the copolymers according to their block length, that resulted the key parameter to determine their thermal properties [4]. The Mn PET blocks value, available for all the samples here analysed, is used to distinguish three different systems: (i) block copolymers with Mn PET blocks P 8000; (ii) block copolymers with Mn PET blocks 2500; (iii) block copolymers with Mn PET blocks 1500. From the results reported in Ref [4], it is expected that in a copolymer the value of Mn PC blocks is not too different from the corresponding Mn PET blocks . The DSC characterization of these samples, reported in Table 1, was carried out in the previous work [4] on cast films heated to 280 °C for 1 min and, then, fast cooled to room temperature in order to cancel the previous thermal history. It is evident that the copolymers with Mn PET blocks P 8000 are characterized by a phase separation: two amorphous and one crystalline phases are present, as shown by two glass transition processes and one melting peak. On the other hand, the copolymers with Mn PET blocks equal to about 2500, present a single homogeneous amorphous phase and a little tendency to crystallize, as evident from the DHm ¼ 2:0 J/g for Eu-30. This capacity completely disappears for Mn PET blocks lower than 1500. In this work the DSC analysis has been performed directly on the cast films, before and after annealing: only the 1st scan is considered in order to evidence the differences in chain reorganization ability. The discrepancies in Tm and DHm between the results reported in Table 1 and those here obtained for Sm-10 and Eu-10 samples at ta ¼ 0 (Table 2) are due to the different thermal history. In particular, if PC is able to crystallize due to the solvent effect, only in the 1st scan it should be possible to evidence its crystalline phase. 3.2.1. Block copolymers with Mn PET blocks P 8000 As already described, the copolymers with Mn PET blocks P 8000 are characterized by phase separation, due to the immiscibility of the long PET and PC blocks. In curve a of Fig. 5 the DSC trace for Sm-10
1086
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Table 2 Melting temperatures and enthalpies measured for copolymers with Mn PET blocks P 8000 after annealing at Ta ¼ 16 °C for the times ta Sample ta (h)
Tm0 (°C)
Tm00 (°C)
Tm000 (°C)
DHm a (°C)
Sm-10
0 24 72 120
– 184 184 189
– 216 216 216
232 240 242 241
29.8 38.9 39.9 42.6
Eu-10
0 24 72 120
– 183 186 189
– 216 218 217
237 242 244 246
19.8 38.8 41.5 43.8
Ce-10
24 72 120
186 187 189
215 217 215
247 247 245
34.3 42.3 44.0
Er-10
24 72 120
183 185 189
216 215 214
243 243 243
40.8 41.8 45.3
Tb-10
24 72 120
184 186 190
215 214 214
246 245 243
39.0 44.0 43.8
Calculated respect to the weight of the sample.
Heat Flow (mW)
a)
b)
c)
d)
100
150
200
250
Temperature (°C)
Heat Flow (mW)
a
analysed copolymers belonging to this class: Table 2 reports the melting temperatures and total DHm obtained after annealing. It can be seen that the 1st melting peak is always located in the region of 183–190 °C and is shifted to 3–6 °C higher temperatures by passing from ta ¼ 24 to 120 h. In analogy with what found for the homopolymers, the presence of a new melting process at low temperature suggests the formation of secondary crystals during annealing. The increment of Tm0 with ta suggests that these new crystals, characterized by a low perfection degree, improve their crystalline state by increasing ta [11,14]. For the high endotherm, it is evident that, respect to the not annealed sample, Tm000 increases of 8 °C with the annealing. Then, both Tm00 and Tm000 do not vary during the thermal treatments as a function of ta . As the peak at Tm000 is reasonably due to the melting of crystals originated by primary crystallization, the treatment at 160 °C has an initial effect of improving the perfection of the original crystals. In fact, it is expected that in block copolymers the crystals have a high level of imperfections. The peak at Tm00 changes its shape with ta : even if Tm00 corresponds to the melting temperature of PC, from the previous results it was found that the endotherm of PC crystals is not evident in DSC traces when PET is also present. Then, in order to try to understand the origin of the peak at Tm00 , isothermal treatments have been performed in DSC. In Fig. 6 some results are reported: the Eu-10 sample, annealed at 160 °C for 120 h and characterized by the DSC thermogram shown in curve a, has
a)
b)
Fig. 5. DCS thermograms at 20 °C/min for Sm-10 annealed at Ta ¼ 160 °C for (a) ta ¼ 0; (b) ta ¼ 24 h; (c) ta ¼ 72 h; (d) ta ¼ 120 h.
c)
100
before annealing is reported: a broad melting peak is evident, confirming the presence of crystallinity. After the annealing at 160 °C for times varying from 24 to 120 h, three melting peaks appear in the thermograms, in the temperature range from 185 to 240 °C (curves b–d in Fig. 5). The results are similar for all the
150
200
250
Temperature (°C) Fig. 6. DSC thermograms at 20 °C/min for Eu-10: (a) annealed at Ta ¼ 160 °C for 120 h; (b) annealed at Ta ¼ 160 °C for 120 h, treated with an isotherm at 200 °C for 10 min during the scan; (c) annealed at Ta ¼ 160 °C for 120 h, treated with an isotherm at 220 °C for 10 min during the scan.
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Intensity
been analysed at 20 °C/min from 50 to 280 °C with 10 min isotherm at 200 °C (curve b) or 220 °C (curve c): these temperatures correspond to the end of the first and second melting peaks, respectively. After these two isotherms, the melting process shows an increment in the intensity of the second peak (curve b) and the appearance of a new peak at about 230 °C (curve c). These results suggest the capability of the copolymers to recrystallize during the treatment at high temperatures. In front of this results, the peak at Tm00 can be attributed to the melting of the secondary crystals melted at Tm0 and then recrystallized during the heating scan. As the recrystallization process takes place always in the same temperature range above Tm0 , Tm00 is not affected by the time of annealing. Because of the melting–recrystallization processes, the total DHm is not completely meaningful. However, it is interesting to observe in Table 2 that the DHm is characterized by a tendency to increase by 4–10 J/g as a function of the annealing times. The WAXS analysis can be useful to shed light on some characteristics of the crystals present in the samples. Fig. 7 compare the diffraction spectra of Sm-10 before and after annealing: it is evident that both PET and PC crystalline phases are present. It can be surprising that PC is able to crystallize also in copolymers: this behaviour can be justified by a solvent effect on the primary crystallization. Moreover, during primary and secondary crystallizations, the presence of blocks of PET can lead to better mobility of the neighbouring PCsegments, improving the crystallization process [25]. In the cast film before annealing the broad and not resolved WAXS peaks indicate the presence of imperfect crystals: these induce the wide melting peak observed in
b) a) 0
10
20
30
40
2θ (°) Fig. 7. WAXS patterns of (a) Sm-10 before annealing; (b) Sm10 annealed at Ta ¼ 160 °C for 72 h.
1087
the DSC first scan (Fig. 5, curve a). After 72 h of annealing at 160 °C, the peak slightly sharpen and increase in intensity, suggesting an improvement in the crystal perfection and in crystallinity degree. This behaviour justifies the higher values of Tm000 and should be in agreement with the apparent increment of DHm observed in DSC analysis. In analogy with that found for the homopolymers, the most important effect of the annealing is a secondary crystallization that induces mainly rearrangements of the amorphous chains towards more ordered regions and causes an increment of the total crystallinity. As these copolymers are characterized by immiscibility of the two amorphous phases, the annealing should not increase the phase separation that is originally present in the samples and attributed to the long block sequences.
3.2.2. Block copolymers with Mn PET blocks 2500 Eu-30 and Sm-20 are examples of block copolymers characterized by Mn PET blocks equal to about 2500: this value has been recognized as the maximum block length able to form a miscible amorphous phase after the fast cooling from the melt [4]. Only Eu-30 is characterized by a low level of crystallinity: probably during the fast cooling from the melt a small fraction of PET is able to crystallize. The DSC thermograms of both cast films before annealing show a melting peak with low intensity at 224 and 220 °C for Eu-30 and Sm-20 respectively: this behaviour confirms that during the film preparation some crystals are formed, even if their perfection is very low, as shown by the low melting temperature. The effects of the annealing at 160 °C for 72 h on the thermal behaviour of these copolymers are described in Fig. 8, curves a and b respectively. It can be seen that the thermograms show two melting processes at Tm0 ¼ 186 and 172 °C and Tm00 ¼ 227 and 230 °C for Eu-30 and Sm20 respectively. The total DHm is about 27 J/g for both samples, even if this value is apparent because of the possible presence of melting–recrystallization processes. In any case, these results indicate that the annealing increases the initial crystallinity: beside an improvement of the pre-existing crystals perfection, it is expected mainly the formation of new crystalline regions from the amorphous phase. The WAXS analysis confirms the very low crystallinity level of the samples, as shown in curve a of Fig. 9 for Sm-20. After annealing, the reflections of both PET and PC crystalline phases are evident (curve b of Fig. 9), even if the crystal are strongly imperfect. Then, respect to the block copolymers characterized by Mn PET blocks P 8000, in this case the PET and PC blocks show a greater difficulty in organizing from a disordered homogeneous state towards an ordered phase. This process, possible only if the samples spent
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Heat Flow (mW)
1088
b)
a)
100
150
200
250
Temperature (°C) Fig. 8. DSC thermograms at 20 °C/min for (a) Eu-30 annealed at Ta ¼ 160 °C for 72 h; (b) Sm-20 annealed at Ta ¼ 160 °C for 72 h.
Intensity
d) c)
3.2.3. Block copolymers with Mn PET blocks < 1500 Sm-30 is an example of copolymer characterized by a very short block length. This chemical architecture induces the formation of a single amorphous phase if the sample is quenched from the melt [4]. The cast film, analysed before and after annealing, does not show melting endotherms in DSC thermograms, suggesting that the crystallization of the sample is not improved by the presence of the solvent and by the thermal treatments. The WAXS patterns, obtained before and after annealing, are reported in curve c and d of Fig. 9 respectively: the amorphous halo of the sample is present in both cases. Then, the sample retains its amorphous state even after annealing. By comparing the behaviour of Sm-20 and Sm-30, it is noteworthy that both samples appear homogeneous, without phase separation, after quenching from the melt (Table 1). If the samples are subjected to the conditions favourable to crystallization, Sm-20 is able to crystallize during annealing, through a new organization of the amorphous phase: this phenomenon increases the phase separation between PET and PC by crystallization. Sm30, on the contrary, does not modify the phase behaviour with annealing at 160 °C. This different behaviour can be explained by considering that Sm-30 is characterized by very short block length (corresponding to sequences of 10 monomeric units), that hinders any crystallization process even if the sample is maintained in temperature and time conditions favourable to crystallization. Then, in Sm-30 the phase separation cannot take place. For this reason, the occurrence of a single stable phase, that can be maintained also during thermal treatments, is observed in PET/PC copolymers only when the sequences of PET (and probably also of PC) monomeric units are lower than 10.
b) a) 0
10
20
30
40
2θ (°) Fig. 9. WAXS patterns of (a) Sm-20 before annealing; (b) Sm20 after annealing at Ta ¼ 160 °C for 72 h; (c) Sm-30 before annealing; (d) Sm-30 after annealing at Ta ¼ 160 °C for 72 h.
enough time at temperatures favourable to the chain reorganization, is attributed to the short block length that does not promote a crystallization process and the formation of crystals at high perfection. As a consequence, the annealing at 160 °C for long times promotes phase separation, induced by crystallization, in partially miscible copolymers. Similar results are reported for PC blended with PBT or amide modified PBT [26].
4. Conclusions The effect of the annealing on block PET/PC copolymers has been discussed in terms of ability to crystallize, that is strictly dependent on the chemical architecture. For copolymers with Mn PET blocks P 8000 it was observed that the annealing at 160 °C causes an increment of crystallinity, as confirmed by WAXS analysis. This effect has been interpreted as due to the secondary crystallization and, in particular, to the reorganization of the chains in the amorphous phase towards PET and PC crystalline phases. This process induces multiple melting peaks in DSC, due to a melting–recrystallization phenomena, involving the new crystals at low perfection. As a consequence, the annealing of block copolymers, characterized by a phase separation before ageing, practically does not modify the phase behaviour of the system.
P. Marchese et al. / European Polymer Journal 39 (2003) 1081–1089
Instead, for the copolymers with Mn PET blocks 2500, the blocks are short enough to form an homogeneous amorphous phase, but long enough to crystallize during a suitable thermal treatment. It resulted that the annealing at 160 °C promotes the crystallization of both PET and PC blocks, causing a phase separation from the homogeneous amorphous state. This is the case of Eu-30 and Sm-20 samples. On the other hand, when the copolymers are characterized by a very short block length ðMn PET blocks < 1500Þ, the annealing has not effect on the phase behaviour: the samples remain in the amorphous state also during the thermal treatments that should improve the crystallization. This behaviour is observed for Sm-30. As a conclusion, the thermal treatments at high temperatures, in the range where the crystallization of PET and PC is possible, evidence the rearrangements that can occur in block copolymers and the role of the block length on the phase behaviour.
References [1] Wunderlich B. Crystal nucleation, growth, annealing. In: Macromolecular physics, vol. 2. New York: Academic; 1976. [2] Ignatov VN, Carraro C, Tartari V, Pippa R, Scapin M, Pilati F, et al. Polymer 1997;38:201–5. [3] Pila F, Fiorini M, Berti C. Effects of catalysts in the reactive blending of bisphenol A polycarbonate with poly(alkyleneterephthalate)s. In: Fakirov S, editor. Transreactions in condensation polymers. Weinheim: WileyVCH; 1999 [Chapter 2]. [4] Marchese P, Celli A, Fiorini M. Macromol Chem Phys 2002;203:695–704. [5] Debier D, Jonas AM, Legras R. J Polym Sci, Part B: Polym Phys 1998;36:2197–210.
1089
[6] Porter RS, Wang LH. Polymer 1992;33:2019. [7] Fakirov S, Sarkissova M, Denchev Z. Macromol Chem Phys 1996;197:2837–67. [8] Denchev Z, Sarkissova M, Fakirov S, Yilmaz F. Macromol Chem Phys 1996;197:2869–87. [9] Fakirov S, Sarkissova M, Denchev Z. Macromol Chem Phys 1996;197:2889–907. [10] Denchev Z, Sarkissova M, Radusch H-J, Luepke T, Fakirov S. Macromol Chem Phys 1998;199:215–21. [11] Alfonso GC, Pedemonte E, Ponzetti L. Polymer 1979;20: 104–12. [12] Mark HF, Bikales NM, Overberger CG, Menges G. In: Encyclopedia of polymer science and engineering. 2nd ed. New York: Wiley Interscience; 1988. p. 655. [13] Wang Z-G, Hsiao BS, Sauer BB, Kampert WG. Polymer 1999;40:4615–27. [14] Lin S-B, Koenig JL. J Polym Sci, Polym Symp 1984;71: 121–35. [15] Lu XF, Hay JN. Polymer 2001;42:9423–31. [16] Roberts RC. J Polym Sci, Polym Lett 1970;8:381–4. [17] Holdsworth PJ, Turner-Jones A. Polymer 1971;12:195– 208. [18] Fakirov S, Fisher EW, Hoffmann R, Schmidt GF. Polymer 1977;18:1121. [19] Garbauskas MF. Polycarbonate crystallinity. In: Legrand DJ, Bendler JT, editors. Handbook of polycarbonate science and technology. New York: Marcel Dekker; 1996. [20] van Bennekon ACM, Pluimers DT, Bussino J, Gaymans RJ. Polymer 1997;38:3017. [21] Sohn S, Alizadeh A, Marand H. Polymer 2000;41:8879– 86. [22] Mendez G, Muller AJ. J Therm Anal 1997;50:593–602. [23] Jonza JM, Porter RS. J Polym Sci, Part B: Polym Phys 1986;24:2459–72. [24] Jog JP. JMS-Rev Macromol Chem Phys 1995;C35:531–53. [25] Wings N, Trafara G. Angew Makromol Chem 1994;217: 91–105. [26] van Bennekom ACM, van den Berg D, Bussink J, Gaymans RJ. Polymer 1997;38:5041–9.