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Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization
European Polymer Journal 39 (2003) 2315–2322 www.elsevier.com/locate/europolj
Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization Fajun Zhang *, Yumei Gong, Tianbai He
*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jilin 130022, PR China Received 28 March 2003; received in revised form 5 August 2003; accepted 6 August 2003
Abstract The multiple melting behavior of several commercial resins of isotactic polypropylene (iPP) and random copolymer, poly(propylene-co-ethylene) (PPE), after stepwise isothermal crystallization (SIC) were studied by differential scanning calorimeter and wide-angle X-ray diffraction (WAXD). For iPP samples, three typical melting endotherms appeared after SIC process when heating rate was lower than 10 °C/min. The WAXD experiments proved that only a-form crystal was formed during SIC process and no transition from a1- to a2-form occurred during heating process. Heating rate dependence for each endotherm was discussed and it was concluded that there were only two major crystals with different thermal stability. For the PPE sample, more melting endotherms appeared after stepwise isothermal crystallization. The introduction of ethylene comonomer in isotactic propylene backbone further decreased the regularity of molecular chain, and the short isotactic propylene sequences could crystallize into c-form crystal having a low melting temperature whereas the long sequences crystallized into a-form crystal having high melting temperature. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Polypropylene; DSC; WAXD; Melt; Stepwise crystallization
1. Introduction Crystallization and melting behavior of a-form iPP has been widely studied [1–15] and the feature of melting curves of iPP as functions of crystallization temperature Tc is characterized as follows [1–3]: (1) Melting curves of samples crystallized at very low Tc (<120 °C) show double a-peaks due to the recrystallization; (2) Melting curves after crystallization at medium Tc (120–136 °C) *
Corresponding authors. Present address: Unite de Physique et de Chimie des Hauts Polymeres, Universite Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium. Tel.: +32-10-47-30-88; fax: +32-10-45-15-93 (F. Zhang), Tel.: +86-431-5262123; fax: + 86-431-5262126 (T. He). E-mail addresses: [email protected] (F. Zhang), tbhe@ ciac.jl.cn (T. He).
comprise a single relatively sharp peak; (3) Melting curves of a-iPP crystallized at high Tc (>136 °C) show peak duplication. Interpretation of peak duplication of isothermally crystallized a-iPP is much more complicated and controversial and many works have been done [4–16]. Generally speaking, there are four candidates to explain the cause of double melting endotherms: (1) molecular weight, chain regularity and the segregation process during crystallization [4–7]; (2) the co-existence of multiple crystal phase, i.e. a1, a2 in a-form iPP [8–11]; (3) lamellae thickening process [12,13] and (4) the melting of different type of spherules in iPP [14,15]. Recently, Naiki et al. [16] investigated the crystal ordering from the a1 to the a2 phase of iPP. Their results indicated that although the a1 to a2 transition really occurred during heating scan, the DSC melting curves was only one broad endotherm, i.e. the recrystallization
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.08.001
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process from a1 to a2 was not the necessary condition of the double melting peaks. The origin of double melting behavior has been investigated. Using the permanganic etching technique, Norton and Keller [17] studied the crosshatched feature of iPP and found radial lamellae thicker than tangential lamellae. Janimak et al. [18] proposed that the crosshatched lamellae grew later than the radial lamellae at higher temperatures, thus showing a thinner lamellar thickness. Mezghani and Phillips [19] found that the birefringence of the a-spherulites increased as melting proceeds until reaching a high temperature, which was the direct result of the melting of tangential lamellae; this also demonstrated that radial lamellae were thicker than tangential lamellae. The work of Al-Raheil et al. [14,15] indicated that iPP sample crystallized at temperatures below 132 °C, both a- and b-spherulites was formed. The melting point of a-spherulite was 12 °C higher than that of b-spherulite, so the DSC heating curves showed double melting endotherms. When crystallization temperature higher than 132 °C, only aspherulite was formed, the first peak represented the melting of crosshatched lamellae in a-spherulite, while the second peak was correlated to melting of the radial and the reorganized tangential lamellae. Zhu et al. [20] also found that when the crystallization temperature was higher than 136 °C, two kinds of lamellae with different thickness were developed during isothermal crystallization. For chemical-compositional copolymer such as ethylene/a-olefin copolymer, the molecular chains are interrupted by comonomer units. However, from the point of view of the crystallization behavior, any two (or more) chemically dissimilar monomers or co-units, incorporated into the chain will constitute a copolymer [21]. Depending on the nature of the repeating unit, structural isomers, geometric or stereo can also be present in the chain. Thus, the isotactic polypropylene with certain isotacticity can be viewed as a stereo-copolymer with comonomer units having methyl groups positioned in different direction [2]. In propylene stereo-copolymers, the stereo co-units can enter the lattice on an equilibrium basis [21]. Thus, the crystals incorporated by stereo co-units will have lower stability than pure crystals. It is expected that introduction of chemical co-unit such as ethylene comonomer in propylene backbone, will further decrease the regularity of molecular chain. This stimulates us to study the melting behavior of iPP by using stepwise isothermal crystallization (SIC) process. The SIC process has been used to investigate the heterogeneity of ethylene/a-olefin copolymers [22,23]. In this work, it was found that several melting endotherms (at least three for iPP and more for PPE) were shown after stepwise isothermal crystallization process and the origin of the multiple endotherms were studied by DSC and WAXD.
2. Experimental section Samples used in this work were commercial isotactic polypropylene (iPP) resins (PP1, PP2) and random copolymer with ethylene as comonomer (PPE). They were provided by Shanghai Jinshan Petrochemical Corporation (China). The molecular weight and its distribution of the samples were measured by gel permeation chromatography (GPC). The isotacticity and comonomer content were measured by high temperature solution 13 C-NMR. Detailed molecular characteristics were listed in Table 1. DSC measurements were conducted on a computerized Perkin–Elmer differential scanning calorimeter, Model DSC-7. The samples were weighted in the range 4–6 mg. The temperature reading and calorific measurement were calibrated by using standard indium. The iPP and PPE samples crimped in aluminum pans were first heated to 200 °C under a dry nitrogen atmosphere and held for 5 min to remove residual crystals, and then the cooling and subsequent heating curves (50–200 °C) were recorded at a rate of 10 °C/min. Stepwise isothermal crystallization process: The iPP samples were first heated to 200 °C under a dry nitrogen atmosphere and held for 5 min, and then stepwisely crystallized at preset temperatures. The crystallization temperatures were 152, 148, 144, 140, 136 and 132 °C and each temperature was held for 4 h. For PPE, the samples were melted at 180 °C for 5 min, crystallization temperatures were 140, 137, 134, 131, 128, 125, 120 and 115 °C and each temperature was held for 4 h. The molecular weight and its distribution of the samples treated by SIC process were measured by GPC. Although the samples would be degraded during long time treatment, the results indicated that for most of cases no obvious thermal degradation occurred. After complete crystallization, the samples were quenched to 50 °C, and then the heating curves with 5 °C/min were recorded. In the case of heating rate dependence experiments, four different scanning rates (2.5, 5, 10, 20 °C/min) were used. In order to diminish the effect of instrument lag, the melting peak temperatures were calibrated as follow: standard indium was heating up from 50 to 200 °C with different rates (2.5, 5, 10, 20 °C/min) and the melting points were 156.42, 156.75, 157.27, 158.28 °C increased with 0.33, 0.85 and 1.86 °C respectively. Thus, the
Table 1 Molecular characterization of the samples used in this work Sample
Mw /1000
Mw =Mn
Isotacticity (%)
Comonomer (%)
PP1 PP2 PPE
406 370 275
6.39 8.60 5.07
93.9 95.2 93.3
– – E: 4.68
F. Zhang et al. / European Polymer Journal 39 (2003) 2315–2322
temperature for each endotherm of iPP sample obtained at the scan rate of 5 °C/min would minus 0.4 °C, temperatures obtained at the rate of 10 °C/min would minus 0.9 °C and temperatures obtained at the rate of 20 °C/ min would minus 1.9 °C. The crystal structure of polymer film with 0.1 mm thick was detected by wide-angle X-ray diffraction (WAXD) after SIC process. The WAXD experiments were carried out using a Rigaku 8 kW rotating anode generator and diffractometer operated in the reflection mode. Medium and low temperature attachment was used in the temperature region studied and the heating speed was 5 °C/min. The monochromatized X-ray beam was CuKa radiation with a wavelength of 0.15438 nm. The continuous mode was used to scan 2h angles between 5° and 40° with scanning rate of 4°/min, the sampling width was 0.02°.
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molecules will crystallize first if the system can reach equilibrium during cooling process. However, the fast cooling rate and large supercooling eliminate the difference on the crystallization ability, and both the long and short molecules crystallize at the same temperature range (Fig. 1). Here, SIC process was used to try to investigate the heterogeneity of iPP molecules with different molecular weight and regularity. Fig. 2a shows the heating curves of PP1 sample after SIC process with a heating rate 2.5 °C/min. It is obvious that the melting range is broadened and three melting peaks (171.5, 163.7 and 157.55 °C) are observed. We name the high, middle and low temperature peaks as Thigh , Tmid and Tlow respectively. SIC process has been carried out for many iPP samples, and at least three endotherms similar to PP1 are observed. Nevertheless,
3. Results and discussion 3.1. Isotactic polypropylene with stereo co-unit Fig. 1 gives the DSC cooling and subsequent melting curves of PP1 sample between 50 and 200 °C. The crystallization and melting temperature are 113.1 and 157.5 °C. The crystallization enthalpy (DH ) is 91.8 J/g corresponding to the degree of crystallinity of 0.443 [2]. The cooling curves show a single exotherm with a large supercooling (about 50 °C) corresponding to the melting endotherm at the subsequent heating curve. According to the Flory’s theory [24], the long molecules with high regularity have the more driving force for crystallization than short molecules with low regularity at the same crystallization temperature. It means that the long
Fig. 1. DSC cooling and subsequent heating curves of PP1 sample, scanning rate 10 °C/min.
Fig. 2. DSC heating curves of iPP samples (a) PP1 and (b) PP2 after stepwise isothermal crystallization, heating rate 2.5 °C/min.
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some samples show more than three endotherms. For example, the heating curve of PP2 (Fig. 2b) after SIC process shows four endotherms, another small endotherm appears at the highest temperature. The crystal structure of iPP sample after thermal treatment by stepwise crystallization is still a-form, which is supported by room temperature WAXD experiment as shown in Fig. 3a. Fig. 3b shows the X-ray diffraction profiles of PP1 sample in the range of 2h ¼ 30–40° after stepwise isothermal crystallization. The peaks are shifting to smaller angles due to thermal expansion of the crystal lattice. The most significant is that the 2 3 1 and 1 6 1 reflections in the vicinity of 2h ¼ 31:2° (at high temperature) characteristic of the a2 phase [16] are observed at even room temperature (2h ¼ 32:1°) and its intensity changes little with in-
Fig. 3. (a) Room temperature WAXD profile of PP1 sample after stepwise crystallization. (b) Temperature dependence of X-ray diffraction profiles of PP1 sample in the range of 2h ¼ 30–40° after treatment as in Fig. 2. The characteristic 2 3 1 and 1 6 1 reflections of a2-form crystal appear even at 25 °C.
creasing temperature. This result indicates that the recrystallization process from a1 to a2 transition does not occur during heating scan. It has been pointed out that the shape of the DSC curve is determined in general by three competing processes [25]: melting of primary crystallites, recrystallization and reorganization of the sample as it is heated during the scan. On heating in the DSC, small molten lamellae can recrystallize into larger and thicker lamellae with higher melting points. Reorganization is refolding process in the solid state (without melting), i.e. lamellae thickening through sliding diffusion. Both these phenomena depend on their rates in relation to the DSC scan rate. In Fig. 4a, we present the results of original DSC heating curves in different rates for PP1 samples after thermal treatment by stepwise crystallization. Fig. 4b shows the relationship of three endotherms as function of heating rate, the melting points used in this figure have been calibrated (see Section 2). The high melting endotherm Thigh changes its size slightly and shifts to high temperature with increasing heating rate; while the middle-melting endotherm Tmid decreases its size, shifting to low temperature with increasing heating rate and disappear when heating rate is higher than 10 °C/min. The point indicated by arrow in Fig. 4b does not belong to Tmid , which will be discussed latter. The low temperature melting endotherm Tlow increases its size dramatically and shifts its position to a low temperature first, and then a constant temperature is reached. It is obvious that Thigh and Tlow correspond to the melting of primary crystals while Tmid corresponds to the melting of recrystallized crystals of Tlow . The crystals melt at Thigh have high stability corresponding to the thick and perfect crystals formed by long molecules with high tacticity, the crystals melt at Tlow have relative low stability corresponding to the thin and imperfect crystals formed by short molecules with low tacticity. The crystals in Tlow firstly melt and part of them can recrystallize into crystals with high stability and remelt at Tmid when heating rate is slow. When fast heating rate is used, the crystals have no sufficient time to recrystallize and melt at their entropyproduction melting temperature [26], i.e. the metastable lamellae melt directly into a melt of equal metastability. It is surprised to see that another endotherm (167.1 °C as indicated by arrow in Fig. 4a, we name it as Tnew ) appears between Thigh and Tlow in the curve of 20 °C/min. It is unique for PP1 and no such endotherm is observed for other samples. It cannot be the middle-melting endotherm because it has higher melting temperature than other middle-melting endotherms and the Tmid is caused by recrystallization process, it should shift to low temperature with increasing heating rate. Thus, the new endotherm indicates that there is another type of crystal having the stability between crystals in Thigh and Tlow , and the reason of absence in low heating rate DSC curves is because of the low content and hidden by the overlap-
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Fig. 4. (a) Heating rate dependence of DSC heating curves of PP1 samples after stepwise isothermal crystallization from isotropic melt. (b) The relationship of melting endotherms as function of heating rate. The melting points have been calibrated (see Section 2). (c) Heating rate dependence of PP2 after stepwise isothermal crystallization. Note that the endotherm (in Fig. 4a) and the point (in Fig. 4b) indicated by arrow presents a new endotherm.
ping between Tmid and Thigh . When heating rate increased, the Tmid shifts to the low temperature and decreases its size, while Tnew changes little and then it appears. It is worth noting that the melting behavior for PP1 sample is general, Fig. 4c shows a set of DSC heating traces of another iPP sample (PP2) after treated by SIC process. The three typical endotherms show similar heating rate dependence to PP1. The highest melting endotherm for PP2 is also heating rate dependence. When heating rate higher than 10 °C/min, it disappears. This result indicates that even the crystals melt at higher temperature also unstable at slow heating rate, and part of crystals recrystallize into more stable crystals and melt at the highest temperature. Early works [14,15,17– 20] on the origin of the duplicate melting behavior of iPP
indicate that it was due to melting of the two major lamellae come from the two types of lamellae in aspherulites. In our case, although three or more melting endotherms have been observed, there are still two major crystals in the samples during SIC process, this conclusion can be deduced by heating rate dependence of each endotherm (Fig. 4). However, because of the molecular heterogeneity, the crystals have low stability, when slow heating rate is carried out, recrystallization process leads to multiple melting behavior. 3.2. Poly(propylene-co-ethylene) with chemical co-units Introduction of ethylene comonomer into the polypropylene chains further increase the molecular
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heterogeneity i.e. decrease the chain regularity and the average crystalline sequence length. Fig. 5 shows the cooling and subsequent melting curves of PPE sample. The crystallization and melting points of PPE are 98.5 and 141.9 °C respectively. The crystallization enthalpy (DH ) is 67.0 J/g corresponding to the degree of crystallinity of 0.324, much lower than that of iPP samples. The shoulder appears at higher temperature of heating curve indicates there are still some molecules with low comonomer content and high isotacticity, which can crystallize at high temperature. After stepwise isothermal crystallization, the melting curve (Fig. 6) show about 7 recognizable peaks or shoulders, and this result
is quite similar to that of ethylene/a-olefin copolymer [22,23]. It is obvious that the multiple melting behavior is due to the segregation process. Early studies have been associated the formation of the c-form with chemical heterogeneity in the polypropylene chain caused by stereoirregularity or by copolymerization [27–32]. Random copolymers of propylene with 2.5–20 wt.% of other 1-olefins may crystallize preferably in the c-form and Thomann et al. [28,29] proved that iPP with stereoirregularities also tended to crystallize in the c-form. All these works [27–32] indicate that the c-form crystals develop from the short isotactic propylene sequences. Thus, the low temperature endotherms in Fig. 6 maybe correspond to the melting of cform crystals. Fig. 7 shows the WAXD curves of PPE sample before and after stepwise crystallization. It indicates that there is little c-form crystals formed during cooling process. But after stepwise crystallization, the intensity of typical c-form (1 1 7) reflection increases dramatically. It is necessary to know which endotherm in Fig. 6 corresponds to the melting of c-form crystals. The cform crystals formed by short crystalline sequence at ambient condition usually have lower melting temperature than that of a-form crystals formed by long crystalline sequence. The c-form (1 1 7) reflection peak in WAXD curves is used to detect the c-form, the heating rate is also 5 °C/min same to the DSC heating curve (Fig. 6). As shown in Fig. 8, that the c-form (1 1 7) reflection peak is completely disappear at 145 °C for PPE after stepwise isothermal crystallization. Thus, we can cursory determine that the melting endotherms lower than 145 °C belong to c-form crystal. Compared with Fig. 6, we can calculate the content of c-form from the
Fig. 6. DSC heating curves of PPE sample after stepwise isothermal crystallization from the melt (180 °C), heating rate 5 °C/min and the heating curve has been decomposed by PeakFit software.
Fig. 7. Room temperature WAXD profile of PPE samples cooling from the melt (10 °C/min) and after stepwise crystallization.
Fig. 5. DSC cooling and subsequent heating curves of PPE sample, scanning rate 10 °C/min.
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units as indicated by three or four low temperature melting shoulders. While the long isotactic propylene sequences or the molecules with low comonomer content and high isotacticity will still crystallize in a-form crystals.
4. Conclusions
Fig. 8. Temperature dependence of X-ray diffraction profiles of PPE sample after stepwise crystallization.
Table 2 The peak area and content of each endotherm of PPE sample after stepwise isothermal crystallization Peak
Tm (°C)
Area (a.u.)
Content (%)
1 2 3 4 5 6 7
117.5 123.0 128.3 133.0 138.0 145.5 156.0
0.244 0.468 0.510 0.480 1.371 5.840 5.376
1.71 3.28 3.57 3.36 9.59 40.87 37.62
area of each endotherm. Table 2 lists the content of each endotherm after decomposed the melting curve in Fig. 6. If the sixth endotherm (145 °C) is included, the content of c-form crystal is 62.4%. Certainly, in this method, calculation of the proportion of c-form from the DSC melting endotherms requires the assumption that the melting enthalpy for both c- and a-forms are the same. This assumption is reasonable because there is only a small energy difference between the two polymorphs (a and c) [28]. According to the method proposed by Turner Jones [27], c-form percentage of the total crystallinity is 42%, quite different to the results calculated by peak area. From the decomposition result, we can see that the fifth and the sixth endotherms are overlapped each other, so the sixth endotherm (145 °C) originates from the melting of both c- and a-form crystals. The multiple melting behavior of propylene-co-ethylene copolymer is quite complicated than that of iPP, the random distribution of ethylene co-unit in propylene molecular chain increases the molecular heterogeneity, short propylene sequences cannot participate in the formation of a-form crystals, but crystallize into c-form crystals. The c-form crystal size strongly depends on the length of propylene sequence between two ethylene co-
We reported here the results of thermal analysis and WAXD of stereo and chemical propylene copolymers after stepwise isothermal crystallization. Multiple melting endotherms (three or more) were observed and their heating rate dependence was also discussed. It was concluded that although multiple melting behavior of aiPP after SIC process were observed, there were only two major crystals formed during SIC process and the molecular heterogeneity led the crystals had different stability, thus recrystallization process occurred during slow heating scan. When chemical co-unit was introduced, the molecular heterogeneity increased and short propylene sequences crystallized into c-form crystals at low temperature, while long sequences crystallized into a-form crystals. The difference in thermal stability of different type of crystal was the origin of multiple melting behavior of PPE sample.
Acknowledgements This work is subsidized by the Special Funds for Major State Basic Research Projects of China and supported by the National Science Foundation of China. The authors thank Prof. Decai Yang (State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, PR China) for useful discussions.
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Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization Fajun Zhang *, Yumei Gong, Tianbai He
*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun, Jilin 130022, PR China Received 28 March 2003; received in revised form 5 August 2003; accepted 6 August 2003
Abstract The multiple melting behavior of several commercial resins of isotactic polypropylene (iPP) and random copolymer, poly(propylene-co-ethylene) (PPE), after stepwise isothermal crystallization (SIC) were studied by differential scanning calorimeter and wide-angle X-ray diffraction (WAXD). For iPP samples, three typical melting endotherms appeared after SIC process when heating rate was lower than 10 °C/min. The WAXD experiments proved that only a-form crystal was formed during SIC process and no transition from a1- to a2-form occurred during heating process. Heating rate dependence for each endotherm was discussed and it was concluded that there were only two major crystals with different thermal stability. For the PPE sample, more melting endotherms appeared after stepwise isothermal crystallization. The introduction of ethylene comonomer in isotactic propylene backbone further decreased the regularity of molecular chain, and the short isotactic propylene sequences could crystallize into c-form crystal having a low melting temperature whereas the long sequences crystallized into a-form crystal having high melting temperature. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Polypropylene; DSC; WAXD; Melt; Stepwise crystallization
1. Introduction Crystallization and melting behavior of a-form iPP has been widely studied [1–15] and the feature of melting curves of iPP as functions of crystallization temperature Tc is characterized as follows [1–3]: (1) Melting curves of samples crystallized at very low Tc (<120 °C) show double a-peaks due to the recrystallization; (2) Melting curves after crystallization at medium Tc (120–136 °C) *
Corresponding authors. Present address: Unite de Physique et de Chimie des Hauts Polymeres, Universite Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium. Tel.: +32-10-47-30-88; fax: +32-10-45-15-93 (F. Zhang), Tel.: +86-431-5262123; fax: + 86-431-5262126 (T. He). E-mail addresses: [email protected] (F. Zhang), tbhe@ ciac.jl.cn (T. He).
comprise a single relatively sharp peak; (3) Melting curves of a-iPP crystallized at high Tc (>136 °C) show peak duplication. Interpretation of peak duplication of isothermally crystallized a-iPP is much more complicated and controversial and many works have been done [4–16]. Generally speaking, there are four candidates to explain the cause of double melting endotherms: (1) molecular weight, chain regularity and the segregation process during crystallization [4–7]; (2) the co-existence of multiple crystal phase, i.e. a1, a2 in a-form iPP [8–11]; (3) lamellae thickening process [12,13] and (4) the melting of different type of spherules in iPP [14,15]. Recently, Naiki et al. [16] investigated the crystal ordering from the a1 to the a2 phase of iPP. Their results indicated that although the a1 to a2 transition really occurred during heating scan, the DSC melting curves was only one broad endotherm, i.e. the recrystallization
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.08.001
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process from a1 to a2 was not the necessary condition of the double melting peaks. The origin of double melting behavior has been investigated. Using the permanganic etching technique, Norton and Keller [17] studied the crosshatched feature of iPP and found radial lamellae thicker than tangential lamellae. Janimak et al. [18] proposed that the crosshatched lamellae grew later than the radial lamellae at higher temperatures, thus showing a thinner lamellar thickness. Mezghani and Phillips [19] found that the birefringence of the a-spherulites increased as melting proceeds until reaching a high temperature, which was the direct result of the melting of tangential lamellae; this also demonstrated that radial lamellae were thicker than tangential lamellae. The work of Al-Raheil et al. [14,15] indicated that iPP sample crystallized at temperatures below 132 °C, both a- and b-spherulites was formed. The melting point of a-spherulite was 12 °C higher than that of b-spherulite, so the DSC heating curves showed double melting endotherms. When crystallization temperature higher than 132 °C, only aspherulite was formed, the first peak represented the melting of crosshatched lamellae in a-spherulite, while the second peak was correlated to melting of the radial and the reorganized tangential lamellae. Zhu et al. [20] also found that when the crystallization temperature was higher than 136 °C, two kinds of lamellae with different thickness were developed during isothermal crystallization. For chemical-compositional copolymer such as ethylene/a-olefin copolymer, the molecular chains are interrupted by comonomer units. However, from the point of view of the crystallization behavior, any two (or more) chemically dissimilar monomers or co-units, incorporated into the chain will constitute a copolymer [21]. Depending on the nature of the repeating unit, structural isomers, geometric or stereo can also be present in the chain. Thus, the isotactic polypropylene with certain isotacticity can be viewed as a stereo-copolymer with comonomer units having methyl groups positioned in different direction [2]. In propylene stereo-copolymers, the stereo co-units can enter the lattice on an equilibrium basis [21]. Thus, the crystals incorporated by stereo co-units will have lower stability than pure crystals. It is expected that introduction of chemical co-unit such as ethylene comonomer in propylene backbone, will further decrease the regularity of molecular chain. This stimulates us to study the melting behavior of iPP by using stepwise isothermal crystallization (SIC) process. The SIC process has been used to investigate the heterogeneity of ethylene/a-olefin copolymers [22,23]. In this work, it was found that several melting endotherms (at least three for iPP and more for PPE) were shown after stepwise isothermal crystallization process and the origin of the multiple endotherms were studied by DSC and WAXD.
2. Experimental section Samples used in this work were commercial isotactic polypropylene (iPP) resins (PP1, PP2) and random copolymer with ethylene as comonomer (PPE). They were provided by Shanghai Jinshan Petrochemical Corporation (China). The molecular weight and its distribution of the samples were measured by gel permeation chromatography (GPC). The isotacticity and comonomer content were measured by high temperature solution 13 C-NMR. Detailed molecular characteristics were listed in Table 1. DSC measurements were conducted on a computerized Perkin–Elmer differential scanning calorimeter, Model DSC-7. The samples were weighted in the range 4–6 mg. The temperature reading and calorific measurement were calibrated by using standard indium. The iPP and PPE samples crimped in aluminum pans were first heated to 200 °C under a dry nitrogen atmosphere and held for 5 min to remove residual crystals, and then the cooling and subsequent heating curves (50–200 °C) were recorded at a rate of 10 °C/min. Stepwise isothermal crystallization process: The iPP samples were first heated to 200 °C under a dry nitrogen atmosphere and held for 5 min, and then stepwisely crystallized at preset temperatures. The crystallization temperatures were 152, 148, 144, 140, 136 and 132 °C and each temperature was held for 4 h. For PPE, the samples were melted at 180 °C for 5 min, crystallization temperatures were 140, 137, 134, 131, 128, 125, 120 and 115 °C and each temperature was held for 4 h. The molecular weight and its distribution of the samples treated by SIC process were measured by GPC. Although the samples would be degraded during long time treatment, the results indicated that for most of cases no obvious thermal degradation occurred. After complete crystallization, the samples were quenched to 50 °C, and then the heating curves with 5 °C/min were recorded. In the case of heating rate dependence experiments, four different scanning rates (2.5, 5, 10, 20 °C/min) were used. In order to diminish the effect of instrument lag, the melting peak temperatures were calibrated as follow: standard indium was heating up from 50 to 200 °C with different rates (2.5, 5, 10, 20 °C/min) and the melting points were 156.42, 156.75, 157.27, 158.28 °C increased with 0.33, 0.85 and 1.86 °C respectively. Thus, the
Table 1 Molecular characterization of the samples used in this work Sample
Mw /1000
Mw =Mn
Isotacticity (%)
Comonomer (%)
PP1 PP2 PPE
406 370 275
6.39 8.60 5.07
93.9 95.2 93.3
– – E: 4.68
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temperature for each endotherm of iPP sample obtained at the scan rate of 5 °C/min would minus 0.4 °C, temperatures obtained at the rate of 10 °C/min would minus 0.9 °C and temperatures obtained at the rate of 20 °C/ min would minus 1.9 °C. The crystal structure of polymer film with 0.1 mm thick was detected by wide-angle X-ray diffraction (WAXD) after SIC process. The WAXD experiments were carried out using a Rigaku 8 kW rotating anode generator and diffractometer operated in the reflection mode. Medium and low temperature attachment was used in the temperature region studied and the heating speed was 5 °C/min. The monochromatized X-ray beam was CuKa radiation with a wavelength of 0.15438 nm. The continuous mode was used to scan 2h angles between 5° and 40° with scanning rate of 4°/min, the sampling width was 0.02°.
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molecules will crystallize first if the system can reach equilibrium during cooling process. However, the fast cooling rate and large supercooling eliminate the difference on the crystallization ability, and both the long and short molecules crystallize at the same temperature range (Fig. 1). Here, SIC process was used to try to investigate the heterogeneity of iPP molecules with different molecular weight and regularity. Fig. 2a shows the heating curves of PP1 sample after SIC process with a heating rate 2.5 °C/min. It is obvious that the melting range is broadened and three melting peaks (171.5, 163.7 and 157.55 °C) are observed. We name the high, middle and low temperature peaks as Thigh , Tmid and Tlow respectively. SIC process has been carried out for many iPP samples, and at least three endotherms similar to PP1 are observed. Nevertheless,
3. Results and discussion 3.1. Isotactic polypropylene with stereo co-unit Fig. 1 gives the DSC cooling and subsequent melting curves of PP1 sample between 50 and 200 °C. The crystallization and melting temperature are 113.1 and 157.5 °C. The crystallization enthalpy (DH ) is 91.8 J/g corresponding to the degree of crystallinity of 0.443 [2]. The cooling curves show a single exotherm with a large supercooling (about 50 °C) corresponding to the melting endotherm at the subsequent heating curve. According to the Flory’s theory [24], the long molecules with high regularity have the more driving force for crystallization than short molecules with low regularity at the same crystallization temperature. It means that the long
Fig. 1. DSC cooling and subsequent heating curves of PP1 sample, scanning rate 10 °C/min.
Fig. 2. DSC heating curves of iPP samples (a) PP1 and (b) PP2 after stepwise isothermal crystallization, heating rate 2.5 °C/min.
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some samples show more than three endotherms. For example, the heating curve of PP2 (Fig. 2b) after SIC process shows four endotherms, another small endotherm appears at the highest temperature. The crystal structure of iPP sample after thermal treatment by stepwise crystallization is still a-form, which is supported by room temperature WAXD experiment as shown in Fig. 3a. Fig. 3b shows the X-ray diffraction profiles of PP1 sample in the range of 2h ¼ 30–40° after stepwise isothermal crystallization. The peaks are shifting to smaller angles due to thermal expansion of the crystal lattice. The most significant is that the 2 3 1 and 1 6 1 reflections in the vicinity of 2h ¼ 31:2° (at high temperature) characteristic of the a2 phase [16] are observed at even room temperature (2h ¼ 32:1°) and its intensity changes little with in-
Fig. 3. (a) Room temperature WAXD profile of PP1 sample after stepwise crystallization. (b) Temperature dependence of X-ray diffraction profiles of PP1 sample in the range of 2h ¼ 30–40° after treatment as in Fig. 2. The characteristic 2 3 1 and 1 6 1 reflections of a2-form crystal appear even at 25 °C.
creasing temperature. This result indicates that the recrystallization process from a1 to a2 transition does not occur during heating scan. It has been pointed out that the shape of the DSC curve is determined in general by three competing processes [25]: melting of primary crystallites, recrystallization and reorganization of the sample as it is heated during the scan. On heating in the DSC, small molten lamellae can recrystallize into larger and thicker lamellae with higher melting points. Reorganization is refolding process in the solid state (without melting), i.e. lamellae thickening through sliding diffusion. Both these phenomena depend on their rates in relation to the DSC scan rate. In Fig. 4a, we present the results of original DSC heating curves in different rates for PP1 samples after thermal treatment by stepwise crystallization. Fig. 4b shows the relationship of three endotherms as function of heating rate, the melting points used in this figure have been calibrated (see Section 2). The high melting endotherm Thigh changes its size slightly and shifts to high temperature with increasing heating rate; while the middle-melting endotherm Tmid decreases its size, shifting to low temperature with increasing heating rate and disappear when heating rate is higher than 10 °C/min. The point indicated by arrow in Fig. 4b does not belong to Tmid , which will be discussed latter. The low temperature melting endotherm Tlow increases its size dramatically and shifts its position to a low temperature first, and then a constant temperature is reached. It is obvious that Thigh and Tlow correspond to the melting of primary crystals while Tmid corresponds to the melting of recrystallized crystals of Tlow . The crystals melt at Thigh have high stability corresponding to the thick and perfect crystals formed by long molecules with high tacticity, the crystals melt at Tlow have relative low stability corresponding to the thin and imperfect crystals formed by short molecules with low tacticity. The crystals in Tlow firstly melt and part of them can recrystallize into crystals with high stability and remelt at Tmid when heating rate is slow. When fast heating rate is used, the crystals have no sufficient time to recrystallize and melt at their entropyproduction melting temperature [26], i.e. the metastable lamellae melt directly into a melt of equal metastability. It is surprised to see that another endotherm (167.1 °C as indicated by arrow in Fig. 4a, we name it as Tnew ) appears between Thigh and Tlow in the curve of 20 °C/min. It is unique for PP1 and no such endotherm is observed for other samples. It cannot be the middle-melting endotherm because it has higher melting temperature than other middle-melting endotherms and the Tmid is caused by recrystallization process, it should shift to low temperature with increasing heating rate. Thus, the new endotherm indicates that there is another type of crystal having the stability between crystals in Thigh and Tlow , and the reason of absence in low heating rate DSC curves is because of the low content and hidden by the overlap-
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Fig. 4. (a) Heating rate dependence of DSC heating curves of PP1 samples after stepwise isothermal crystallization from isotropic melt. (b) The relationship of melting endotherms as function of heating rate. The melting points have been calibrated (see Section 2). (c) Heating rate dependence of PP2 after stepwise isothermal crystallization. Note that the endotherm (in Fig. 4a) and the point (in Fig. 4b) indicated by arrow presents a new endotherm.
ping between Tmid and Thigh . When heating rate increased, the Tmid shifts to the low temperature and decreases its size, while Tnew changes little and then it appears. It is worth noting that the melting behavior for PP1 sample is general, Fig. 4c shows a set of DSC heating traces of another iPP sample (PP2) after treated by SIC process. The three typical endotherms show similar heating rate dependence to PP1. The highest melting endotherm for PP2 is also heating rate dependence. When heating rate higher than 10 °C/min, it disappears. This result indicates that even the crystals melt at higher temperature also unstable at slow heating rate, and part of crystals recrystallize into more stable crystals and melt at the highest temperature. Early works [14,15,17– 20] on the origin of the duplicate melting behavior of iPP
indicate that it was due to melting of the two major lamellae come from the two types of lamellae in aspherulites. In our case, although three or more melting endotherms have been observed, there are still two major crystals in the samples during SIC process, this conclusion can be deduced by heating rate dependence of each endotherm (Fig. 4). However, because of the molecular heterogeneity, the crystals have low stability, when slow heating rate is carried out, recrystallization process leads to multiple melting behavior. 3.2. Poly(propylene-co-ethylene) with chemical co-units Introduction of ethylene comonomer into the polypropylene chains further increase the molecular
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heterogeneity i.e. decrease the chain regularity and the average crystalline sequence length. Fig. 5 shows the cooling and subsequent melting curves of PPE sample. The crystallization and melting points of PPE are 98.5 and 141.9 °C respectively. The crystallization enthalpy (DH ) is 67.0 J/g corresponding to the degree of crystallinity of 0.324, much lower than that of iPP samples. The shoulder appears at higher temperature of heating curve indicates there are still some molecules with low comonomer content and high isotacticity, which can crystallize at high temperature. After stepwise isothermal crystallization, the melting curve (Fig. 6) show about 7 recognizable peaks or shoulders, and this result
is quite similar to that of ethylene/a-olefin copolymer [22,23]. It is obvious that the multiple melting behavior is due to the segregation process. Early studies have been associated the formation of the c-form with chemical heterogeneity in the polypropylene chain caused by stereoirregularity or by copolymerization [27–32]. Random copolymers of propylene with 2.5–20 wt.% of other 1-olefins may crystallize preferably in the c-form and Thomann et al. [28,29] proved that iPP with stereoirregularities also tended to crystallize in the c-form. All these works [27–32] indicate that the c-form crystals develop from the short isotactic propylene sequences. Thus, the low temperature endotherms in Fig. 6 maybe correspond to the melting of cform crystals. Fig. 7 shows the WAXD curves of PPE sample before and after stepwise crystallization. It indicates that there is little c-form crystals formed during cooling process. But after stepwise crystallization, the intensity of typical c-form (1 1 7) reflection increases dramatically. It is necessary to know which endotherm in Fig. 6 corresponds to the melting of c-form crystals. The cform crystals formed by short crystalline sequence at ambient condition usually have lower melting temperature than that of a-form crystals formed by long crystalline sequence. The c-form (1 1 7) reflection peak in WAXD curves is used to detect the c-form, the heating rate is also 5 °C/min same to the DSC heating curve (Fig. 6). As shown in Fig. 8, that the c-form (1 1 7) reflection peak is completely disappear at 145 °C for PPE after stepwise isothermal crystallization. Thus, we can cursory determine that the melting endotherms lower than 145 °C belong to c-form crystal. Compared with Fig. 6, we can calculate the content of c-form from the
Fig. 6. DSC heating curves of PPE sample after stepwise isothermal crystallization from the melt (180 °C), heating rate 5 °C/min and the heating curve has been decomposed by PeakFit software.
Fig. 7. Room temperature WAXD profile of PPE samples cooling from the melt (10 °C/min) and after stepwise crystallization.
Fig. 5. DSC cooling and subsequent heating curves of PPE sample, scanning rate 10 °C/min.
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units as indicated by three or four low temperature melting shoulders. While the long isotactic propylene sequences or the molecules with low comonomer content and high isotacticity will still crystallize in a-form crystals.
4. Conclusions
Fig. 8. Temperature dependence of X-ray diffraction profiles of PPE sample after stepwise crystallization.
Table 2 The peak area and content of each endotherm of PPE sample after stepwise isothermal crystallization Peak
Tm (°C)
Area (a.u.)
Content (%)
1 2 3 4 5 6 7
117.5 123.0 128.3 133.0 138.0 145.5 156.0
0.244 0.468 0.510 0.480 1.371 5.840 5.376
1.71 3.28 3.57 3.36 9.59 40.87 37.62
area of each endotherm. Table 2 lists the content of each endotherm after decomposed the melting curve in Fig. 6. If the sixth endotherm (145 °C) is included, the content of c-form crystal is 62.4%. Certainly, in this method, calculation of the proportion of c-form from the DSC melting endotherms requires the assumption that the melting enthalpy for both c- and a-forms are the same. This assumption is reasonable because there is only a small energy difference between the two polymorphs (a and c) [28]. According to the method proposed by Turner Jones [27], c-form percentage of the total crystallinity is 42%, quite different to the results calculated by peak area. From the decomposition result, we can see that the fifth and the sixth endotherms are overlapped each other, so the sixth endotherm (145 °C) originates from the melting of both c- and a-form crystals. The multiple melting behavior of propylene-co-ethylene copolymer is quite complicated than that of iPP, the random distribution of ethylene co-unit in propylene molecular chain increases the molecular heterogeneity, short propylene sequences cannot participate in the formation of a-form crystals, but crystallize into c-form crystals. The c-form crystal size strongly depends on the length of propylene sequence between two ethylene co-
We reported here the results of thermal analysis and WAXD of stereo and chemical propylene copolymers after stepwise isothermal crystallization. Multiple melting endotherms (three or more) were observed and their heating rate dependence was also discussed. It was concluded that although multiple melting behavior of aiPP after SIC process were observed, there were only two major crystals formed during SIC process and the molecular heterogeneity led the crystals had different stability, thus recrystallization process occurred during slow heating scan. When chemical co-unit was introduced, the molecular heterogeneity increased and short propylene sequences crystallized into c-form crystals at low temperature, while long sequences crystallized into a-form crystals. The difference in thermal stability of different type of crystal was the origin of multiple melting behavior of PPE sample.
Acknowledgements This work is subsidized by the Special Funds for Major State Basic Research Projects of China and supported by the National Science Foundation of China. The authors thank Prof. Decai Yang (State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, PR China) for useful discussions.
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