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2007, 111, 11642-11645 Published on Web 09/18/2007
Conformation Orientation of Syndiotactic Polypropylene Induced by Deformation at Different Temperatures Xiuqin Zhang, Lei Kong, Joerg Rottstegge, Duanfu Xu, and Dujin Wang* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics/Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: July 29, 2007; In Final Form: September 8, 2007
Two syndiotactic polypropylene (sPP) sample films (S0 and S90) with different initial structures were prepared in this paper by isothermal crystallization from the melt at 0 and 90 °C, respectively. The polymorphic transitions of the two samples induced by deformation at different temperatures (20, 40, and 60 °C) were investigated by rheo-optical FTIR spectroscopy. The results indicated that stretching leads to the transition from the helical to trans-planar conformation and improves the orientation of both helical and trans-planar conformations for the sPP films. With increasing stretching temperature, the conformational transitions for the two sPP samples are all suppressed, and the orientation behavior of the two samples appears completely different. The orientation degree of S0 decreases with increasing stretching temperature, while that of S90 increases. These results suggest that the stretching temperature and initial structure have great influence on the conformation transition and orientation behavior of the sPP sample. On the basis of the experimental results, the schematic illustration of the conformation transition and orientation behavior of sPP during stretching has been proposed.
Introduction Syndiotactic polypropylene (sPP) has complex polymorphs. Four crystalline forms and a mesophase of sPP have been described so far.1-8 The crystalline forms I and II adopt the (t2g2)n helical conformation, whereas forms III and IV present chains in trans-planar and (t6g2t2g2)n conformations, respectively. The mesomorphic form of sPP is characterized by disorder in the lateral packing of chains in the trans-planar conformation. It has been recognized that stress can induce the transition of the conformation and crystalline form of sPP.9-15 Furthermore, the initial morphology of sPP, the stretching conditions, and the stereoregularity influence the structure transition.9-11,13,14,16,17 It was found that form I with a helical conformation can transform into form III or a mesophase with the trans-planar conformation during stretching. With the increase of stretching temperature, the structural transition of sPP samples is depressed or disappears. For the low stereoregular sPP, it is difficult to obtain the crystalline form III even at high strain.18 These experimental results help to understand the complicated structure transition of sPP samples, while the orientation behavior of different structures has been only marginally discussed in the literature. Infrared spectroscopy is a powerful technique to investigate the orientation of macromolecular chains as well as the condensed structure of polymeric materials. More specifically, the rheo-FTIR spectroscopy can simultaneously detect the structure transition and orientation behavior of polymer materials. However, up to now, only a few articles dealt with the structure transition and orientation behavior of sPP using rheo* To whom correspondence should be addressed. Phone: 86-1082618533. Fax: 86-10-82612857. E-mail:
[email protected].
10.1021/jp075995n CCC: $37.00
optical FTIR spectroscopy.13,15,19 Kannan reported the structure transition and orientation behavior by uniaxial stretching of sPP films (up to 200%) as a function of temperature (25-70 °C).13 The results showed that the helical conformation does not orient appreciably at higher temperature, though it is present beyond the yield point, while the trans-planar chains show a significant increase in dichroism. The orientation of different structures is related to the content of structure during stretching, which is controversial compared with other results.19,20 Therefore, this research is not enough to provide comprehensive understanding of the orientation behavior of different structures in sPP and their dependence on the initial morphology and the stretching temperature. In the present work, the conformational variation of sPP samples was systematically investigated at different stretching temperatures by a rheo-optical FTIR spectrometer, which was fabricated in our lab. In the process of stretching, the gradual orientation of the chain conformation was manifestly observed. These results may provide a more comprehensive understanding of the conformation transition and orientation behavior of sPP at different stretching temperatures and their dependence on the initial structure. Experimental Section Materials. Syndiotactic polypropylene was obtained from Atofina Company, with the following basic parameters: weightaveraged molecular weight Mw ) 8.6 × 104 g/mol (GPC), polydispersity Mw/Mn ) 4.1, and melting temperature 130 °C (DSC). The sPP sample was, in addition, analyzed by 13C NMR spectroscopy at 120 °C on a Bruker AM-300 FT-NMR spectrometer. The polymer showed 80% syndiotactic pentads. © 2007 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 111, No. 40, 2007 11643 but no line drawing phenomenon. As strain reaches 100%, sPP films have already undergone yielding and come into the plateau zone. Therefore, the present research focused on the structure changes of sPP after yielding. For oriented samples, the light absorption of bands (A0) can be acquired by eq 1
A0 ) (Ax + Ay + Az)/3
(1)
Under the uniaxial stretching, an approximation is utilized by assuming transverse isotropy (Ax ) Az) A⊥). In such a case Figure 1. In situ absorbance ratio of 977-963 cm-1 bands for sPP as a function of tensile strain at different stretching temperatures.
Sample Preparation. The sPP pellets were molded in a hot press at 200 °C, forming 0.1-0.15 mm thick films, which were rapidly quenched to 0 °C or room temperature and subsequently isothermally crystallized at 0 (sample S0) or 90 °C (sample S90) for 65 h, respectively. Rheo-FTIR Spectra Measurement at Different Temperatures. The IR spectra were measured by using a Bruker EQUINOX 55 spectrometer equipped with a homemade variable temperature cell,21 inside which polymer films could be drawn to a certain ratio at different temperatures. A resolution of 4 cm-1 was adopted, and 32 scans were accumulated. Samples S0 and S90 were first drawn up to ) 100, 150, 200, 250, 300, and 400% at different temperature on the abovedescribed homemade tensile machine, which was installed inside of the IR detection chamber. Because the melting or transition temperature of the mesophase in sPP is in the range of 20-60 °C, the stretching temperatures in the present work were set at 20, 40, and 60 °C. The sPP films were mounted on two mobile clamps, which can be moved back and forth, resulting in stretching or relaxing of the materials at a controlled rate. The initial length of the sample was 10 mm, and the deformation velocity was set to 10 mm/min. The drawn samples were analyzed by the FTIR spectrometer under tension. During stretching, S0 and S90 showed usual plastic deformation via necking at different stretching temperatures (20, 40, and 60 °C)
A0) (A|| +2A⊥)/3
(2)
The absorption polarized parallel to the stretching direction (A||) and the absorption polarized perpendicular to the stretching direction (A⊥) were measured. According to eq 2, the A0 can be calculated and used to characterize the conformational content. In order to characterize the orientation of the stretched films of S0 and S90, an important parameter, the dichroic ratio (D), was used to describe the orientation of the macromolecular chains
D) A||/A⊥
(3)
Results and Discussion The initial structures of S0 and S90 films have been described in detail in our previous paper.20 In brief, the initial structure of sample S0 consists of form I crystals with a helical conformation and trans-planar mesophase, while sample S90 allows the prevalent crystallization of form I. The crystallinity and crystallite size of S90 are all higher than those of S0. Infrared spectroscopic analysis showed that the bands of the helical conformation of S90 are very evident and well developed, indicating that the helical conformation is the major form. S0, however, presents a more trans-planar conformation compared with that of S90. In order to quantitatively describe the conformation changes of sPP samples during stretching at different temperatures, the
Figure 2. The dichroic ratios (D) of sPP samples tensilely deformed to different strains at different temperatures.
11644 J. Phys. Chem. B, Vol. 111, No. 40, 2007
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Figure 3. The schematic conformation transition and orientation model for sPP with a different initial structure at different temperatures.
absorptions of 977 (helical) and 963 cm-1 (trans-planar) are computed according to eq 2. The ratio of 977 (helical) to 963 cm-1 (trans-planar) (A977/A963) is compared, which is commonly used to characterize the variation of the relative content of helical and trans-planar conformations. This ratio is expressed in Figure 1 as a function of the tensile strain at different temperature for S0 and S90 samples. It is observed that for S0 and S90, at the beginning of stretching at 20 °C (
