Chinese Journal of Polymer Science Vol. 28, No. 5, (2010), 745−751
Chinese Journal of Polymer Science © Chinese Chemical Society Institute of Chemistry, CAS Springer-Verlag Berlin Heidelberg 2010
SYNCHROTRON INVESTIGATION ON MESOMORPHIC STRUCTURE OF sPP AND POLY(ETHYLENE-CO-OCTENE) IN THEIR INITIAL CRYSTALLIZATION STAGE* Rong Cheng, Hui-ying Wen, Hong-fei Li, Shi-chun Jiang** and Li-jia An State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
Zhong-hua Wu Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China Abstract Using synchrotron SAXS and WAXS, we investigated crystalline structures including crystallite size, lamellar thickness, long spacing of syndiotactic polypropylene (sPP) and poly(ethylene-co-octene) (PEcO) that crystallized from the sheared melt. The independence of the shear effects on the crystalline structures and the shear related lamellar orientation indicate the stable mesophase. Experiments on the different shear effects on polymer various crystalline structures confirm the multi-step polymer crystallization process. Keywords: Polymer crystalline structures; Shear effects; Mesophase.
INTRODUCTION The geometrical nature and mechanism of polymer crystallization of flexible chain molecules were in a state of intense disaccord for long time[1]. Crystallization in polymer systems which transfers the entangled macromolecules in the melt into a semi-crystalline state is a process of primary importance and has been studied since long time. Different from low molar mass compounds polymer systems never turn into perfect crystals but end up in a metastable state which is crystalline in part. A semi-crystalline polymer is usually composed of lamellar crystallites which are separated by amorphous layers. Structure parameters like the volume fraction of the crystallites or their thickness are kinetically controlled and changed with the crystallization conditions. Usually crystallites get thicker when the temperature of crystallization is increased. This is accompanied by a decrease in the rate of crystallization described by an exponential law. Studies of polymer crystallization ask for an understanding of these dependencies based on the knowledge of the molecular processes controlling the transformation[2]. Investigations of the kinetics of polymer crystallization are usually carried out with the aid of DSC, SAXS, WAXS, dilatometry, time dependent vibrational spectroscopy or light scattering experiments. Although different properties and probing different physical values are used to derive information about a sample’s crystallinity, all these methods end up having a similar sensitivity: reliable data can only be obtained for crystallinities already in *
This work was financially supported by he National Natural Science Foundation of China (Nos. 20574069, 50773082, 20974077, 20620120105), the Fund for Creative Research Groups (No. 50621302). Synchrotron experiments at Beam line A2 were supported by HASYLAB project (II20070004). ** Corresponding author: Shi-chun Jiang (蒋世春), E-mail:
[email protected] Received September 2, 2009; Revised December 2, 2009; Accepted December 16, 2009 doi: 10.1007/s10118-010-9130-x
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the range of some per cent of the final value. Before reaching the final size of spherulites in the order of several microns, the earliest growing spherulites which can be detected by these measurements already have sizes of several hundred nanometers. Therefore, this is far away from the initial stages of spherulite nucleation and growth. These early stages have recently gained particular importance in the discussion of the basic mechanism of polymer crystallization[3−5]. There are several experimental observations that lead to an assumption of crystal nucleation and growth which includes an intermediate phase, although a commonly accepted view is not yet evolved. Some authors propose a preceding coverage of the whole volume by a mesomorphic phase which develops by a mechanism resembling a spinodal process while others point at indication of nucleation for mesomorphic phase formation. In this paper, we show the results providing evidence of preordered or mesomorphic phase before polymer crystallization via shearing the polymer melt. Syndiotactic polypropylene (sPP) is a typical flexible chain crystallizable polymer. Much is already known about the structure of semicrystalline sPP[6−8]. Morphological studies have shown that sPP forms both hedrites and spherulites, depending on the crystallization temperature Tc. Since the early work of Nata and Corradini it is known that the chain conformation corresponds to a sequence for each syndiotactic repeating unit, setting up a helix with two syndiotactic units per turn with a period of 0.74 nm[9]. There are three types of crystalline forms: forms I, forms II and forms III[10−15]. In the three crystalline forms, form I is considered to be the most stable which has a T2G2 chain conformation setting up a helix with two syndiotactic units per turn and always obtained by melt-crystallization[14, 16]; form II[17−20] is metastable presenting (T4) chain in trans-planar conformation and form III is most unstable showing (T6G2T2G2) helical conformations[21, 22]. The short branches in poly(ethyleneco-octene) copolymers can give rise to a distribution of crystallizable ethylene sequence in the chains which has great influence on physical parameters according to the thermodynamic model[23]. Sequences with comparable length are favored to proceed during the process of crystallization for thermodynamic reasons and other incomparable sequences are retarded from crystallization. Furthermore, bulky aliphatic branches with six carbon atoms are expelled from the crystal lattice and reversely the short-chain branches will hinder the crystallization of ethylene units[24]. It is idealized that all sequence of the same length merge into extended-sequence crystallites of a related thickness with infinite lateral dimension. Whereas, the thorough selection and segregation of ethylene sequences is never reached for a number of restrictions. The evidence was found for a stable mesomorphic structure of syndiotactic polypropylene and serial octene contents of poly(ethylene-co-octene) with shear before crystallization. The results indicated that the lamellae orientation increased as well as the long period and the lamellar crystal thickness did not change with shear rate increasing. sPP and PEcO crystalline structure and the crystallite orientation have not been affected by shear. We are convinced that the result is an evidence for the mesomorphic phase before the crystallization. EXPERIMENTAL Materials and Measurements The sample of sPP is a commercial brand obtained from FINA Oil, Brussels. It has 83% syndiotactic pentades and a molar mass Mw = 6 × 104 (Mw/Mn = 4). Poly(ethylene-co-octene) copolymers of the metallocene catalyst type with 2%, 7%, 12%, 14% and 25% octene per weight of counits, were produced by Dow Chemicals Europe. A Linkam CSS450 shear cell provided sPP with temperature and shear control. The samples were heated to melt temperatures (about 50 K higher than melting point) with a rate of 30 K/min and maintained for 5 min to eliminate residual effects, then cooled to crystallization temperatures with a rate of 30 K/min. A step shear for 5 s was applied to the sample whenever it reached the isothermal crystallization temperatures. X-ray scattering measurements were carried out with synchrotron beam line at 1W2A (SAXS station) of BSRF, Beijing, China and the photon factory (PF) of the High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. RESULTS AND DISCUSSTION The transition time of sPP crystallized at 100°C are 1002 s, 1335 s and 1475 s investigated by light attenuation,
Synchrotron Investigation on Mesomorphic Structure of sPP and PEcO
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Fig. 1 2D SAXS patterns of sPP crystallization at 112°C with different shear rate a) γ& = 0 s−1; b) γ& = 1 s−1; c) γ& = 5 s−1; d) γ& = 20 s−1
simultaneous synchrotron SAXS and WAXS measurements respectively[25]. The different experimental methods are corresponding to various scales of structure transitions. The reported results indicated that the structure of sPP can be observed from big to small in turn during the crystallization. The accepted depiction of the development of a spherulitic is the array of lamellar crystals by the nucleation of one lamella and the subsequent spawning of new lamellae at the surfaces of existing crystals. How can a later formed structure be observed first? The only possibility is to create the mesomorphic phase before a lamellae and spherulite growing. Figure 1 shows two-dimension SAXS results of sPP crystallized at 112°C with different shear rate before crystallization. From Fig. 1 one can obviously find the lamella orientation induced by shearing, the similar results can be found in Fig. 2 as an example for PEcO with 12% octene crystallization with shear. In two directions i.e., shearing direction and perpendicular to shearing direction, the X-ray scattering intensity as function of the scatter vector with different shear rates obtained from Fig. 1 is shown in Fig. 3. The intensity of the scattering increases with the shear rate increasing in shearing direction, which implies that the orientation in crystallized sPP is enhanced with shearing. According to the Bragg’s law, L = 2π/qmax, the peak position qmax does not change either in the shear direction or in the direction perpendicular to the shear direction, which indicates that the shear does not have an obvious influence on the long period. The precise information about structure parameters such as the average lamellar thickness and the long spacing can be gained by the one-dimensional electron intensity correlation function K(z) defined by Strobl which can be derived from Fourier transformation of scattering curves.
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K ( z) =
1 2π
∫
∞
0
I ( s ) s 2 cos( sz )ds
(1)
where s = 2sinθ/λ = q/2π, the modulus of the momentum transfer vector q, 2θ being the scattering angle. The calculated long period and lamellar thickness in shear direction and perpendicular to the shear direction are shown in Fig. 4, Fig. 5 and Fig 6. One can find that there is almost no difference of long period and lamellar thickness between the shearing direction and perpendicular to the shearing direction, which indicates that shearing could not affect the lamellae structure even it could induce different orientation in different directions. 0 s−1
2 s−1
5 s−1
10 s−1
20 s−1
Fig. 2 SAXS patterns of PEcO with 12% octene subsequent to isothermal crystallization at 97°C with indicated shear rate
Fig. 3 Iq2 versus scattering vector q of sPP along the meridian after isothermal crystallization at 112°C with different shear rates: (a) shearing direction, (b) perpendicular to shearing direction
Fig. 4 Lamellae parameters: long period (L), crystal lamellar thickness (Lc) obtained from SAXS data of sPP after isothermal crystallization at 112°C with different shear rates
Synchrotron Investigation on Mesomorphic Structure of sPP and PEcO
Fig. 5 Shear rate dependent long period (L) obtained from SAXS data of PEcO subsequent to isothermal crystallization with different octene contents
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Fig. 6 Shear rate dependent llamella thickness (Lc) obtained from SAXS data of PEcO subsequent to isothermal crystallization with different octene contents
WAXD results prove that shearing does not affect sPP crystallization form as shown in Fig. 7. In order to observe shearing effect on sPP crystallite orientation, two-dimension WAXS was performed, and the results are shown in Fig. 8. Figure 8 shows 2D WAXD patterns of the crystallized sPP obtained after cooling of the sheared melt to the crystallization temperature. Figure 8(a) shows the patterns obtained without shearing. Figures 8(b), 8(c) and 8(d) show the 2D WAXD patterns of sPP after shearing with shear rate 1, 5, and 20 s−1, respectively. Figure 8 indicates there is no obvious orientation of sPP crystals.
Fig. 7 WAXD of sPP after isothermal crystallization at 112°C with shear (shear rate is 20 s−1) and without shear
From SAXS and WAXS results one can find that the effects of shearing on the melting polymer before crystallization are only valid for the orientation of lamellae. However, the long period, lamellae thickness, crystallite size and even the orientation of crystallites almost have not been affected by shear. This is the evidence that the polymer crystallization process starts with the attachment of chain sequences from the melt onto the lateral growth face of a layer with a mesomorphic inner structure. The mesomorphic structure has formed before shearing and polymer crystallization.
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Fig. 8 2D WAXS patterns of sPP after crystallization at 112°C with different shear rates a) γ& = 0 s−1; b) γ& =1 s−1; c) γ& =5 s−1; d) γ& = 20 s−1
As mentioned above, the sheared sPP melt at 112°C primarily consists of high-temperature stable mesomorphic structure before crystallization. Upon shearing, the mesomorphic structure is orientated beyond the mesomorphic phase size; the later formed crystals within the primarily formed stable mesomorphic structure before shearing could not be affected or oriented by shearing. The SAXS and WAXD results confirm the observation in the sPP crystallization initial stage that a stable mesomorphic structure formed before crystallization. Our work provides a possible explanation of this effect. CONCLUSIONS In conclusion, on the basis of the observations that shearing before crystallization only induced lamellar orientation and could not affect sPP and PEcO crystalline structure and orientation of the crystallite, we are convinced that a stable mesomorphic ordering structure formed during the initial stage crystallization of the flexible chain polymers sPP and PEcO before shearing far from the nucleation and crystal growth.
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