The b-Spherulite Morphology of Isotactic Polypropylene Investigated by Raman Spectroscopy J. MARTIN, P. BOURSON,* A. DAHOUN, and J. M. HIVER Laboratoire Mate ´riaux Optiques, Photonique et Syste `mes, Universite ´ Paul Verlaine de Metz et Supe ´lec, UMR CNRS 7132, 2 rue Edouard Belin, 57070 Metz, France (J.M., P.B.); Institut Jean Lamour, De ´partement SI2M, UMR CNRS 7198, Nancy Universite ´ et Ecole Nationale Supe ´rieure des Mines de Nancy, Parc de Saurupt, 54042 Nancy, France (A.D., J.M.H.) Raman spectroscopy is used to investigate the a- and b-crystalline polymorphs of isotactic polypropylene. Raman spectra of the polymorphs show some minor differences, in particular the wavenumber shifting of certain scattering bands, which reflect the environment of an isolated macromolecular chain within the packing unit cell of the different crystal forms. For example, the shifting of the 842 cm 1 scattering band, which is the most susceptible band to cause intermolecular interactions, is used to generate a structural map of one b-spherulite among a-spherulites with a well-defined resolution. Anisotropy of the crystalline phase orientation within a b-spherulite is also investigated by performing polarized Raman spectroscopy measurements. Differences in the polarized spectra are then finely exploited to map the radial lamellae orientation distribution within one b-spherulite. Index Headings: Raman spectroscopy; Isotactic polypropylene; Crystalline polymorphs; Lamellae orientation. INTRODUCTION Crystalline morphology of semi-crystalline polymers con- trols the ultimate properties of the material. 1 Polymorphism is one of the most important aspects of it. Isotactic polypropylene (IPP) has three different crystalline polymorphs referred to as the a-, b-, c- and one liquid crystal form named the smectic phase. 2–4 All have been identified by X-ray scattering. Crystallization of each depends strongly on the thermo- mechanical conditions of cooling. 5 The predominant a-form presents a monoclinic unit cell structure and is principally obtained under typical industrial conditions. 6,7 The b-form shows a hexagonal unit cell structure and its appearance is favored under conditions of shear stress and the use of nucleating agents. 3,8,9 The c- and smectic forms are identified as a triclinic and a pseudo-hexagonal unit cell structure, respectively, and have a minor occurrence. 10–12 The individual chains in all the crystalline forms have a 3 1 helical conformation with three monomer units per helix. Vibrational analysis of semi-crystalline polymer, such as infrared (IR) or Raman spectroscopy, are particularly well- adapted for the characterization of the overall chain structure regularity because IR absorption and Raman scattering spectra are sensitive to intra-molecular interactions. 13 For example, Snyder et al. 14 shows that it is possible to distinguish unambiguously the three isomeric configurations of polypro- pylene with Raman spectroscopy. Minogianni et al. 15 and Gatos et al., 16 who exploited the Raman sensibility to IPP 3 1 helical conformation, propose a spectral criterion of the crystallinity degree. On the contrary, it is often assumed that inter-molecular interactions between neighboring polymer chains in the crystal lattice can be ignored since IR absorption and Raman scattering spectra are insensitive to the lateral order of the crystalline phase. 17 Thus, a handful of works have compared the different crystalline polymorphs of IPP. Results collected by Beckett et al. 18 and Goldstein et al. 19 on far-IR experiments show absorption band splitting, which is discussed in terms of inter-molecular vibrational coupling within the crystal unit cell. Chalmers et al. 20 tend to review Raman band shifting noted for the a-, b-, c-, and smectic polymorphs of IPP in a large wavenumber range of about 40–3100 cm 1 . In recent papers Ellis et al. 21,22 have used the minor differences noted between the MIR spectra of the a- and b-IPP forms to generate maps of b-spherulites. Images obtained have a poor resolution related to the low spatial resolution of IR micro-spectroscopy (10–15 lm). 23 The same authors have performed high- resolution IR micro-spectroscopy experiments using synchro- tron IR sources and show an enhancement of the image resolution. 24 By contrast, it is easier to achieve a beam waist of better than 1 lm in Raman micro-spectroscopy by the use of visible laser sources and microscope objective lenses. In this paper we identify differences between the Raman spectra of the a- and b- polymorphs of IPP and use them to construct structural maps of the crystalline morphology with high resolution. In the second part, we focus our attention on crystalline anisotropy in such structures. EXPERIMENTAL Materials. The used IPP was manufactured and processed by Atochem (ref.: 3050 MN1). The isotactic grade has a broad molecular weight distribution, as assessed by gel permeation chromatography, with M ¯ w ¼ 75 940 g/mol and M ¯ n ¼ 26 2200 g/ mol. Pellets of the polymer were subsequently processed by intrusion in a thick rectangular mold (300 3 200 3 15 mm). This technique consists of slowly extruding the melt at a temperature of 230 8C in the mold maintained at 30 8C feeding under an extruding pressure of 6 MPa during the cooling sequence (240 s). Thermo-mechanical conditions of the intrusion process lead to a predominant a-spherulitic morphol- ogy but also favor b-spherulite crystallization in the inner region of the plate, as is seen schematically in Fig. 1. To verify the presence of the b-spherulites, external and internal regions are independently characterized by X-ray scattering experi- ments using the K a radiation of copper (k ¼ 0.1542 nm). Patterns are recorded at room temperature with a Philips Geiger counter X-ray diffractometer at 1 degree per minute over a 2h Bragg’s angle range of 10–40 degrees. X-ray patterns given in Fig. 2 show that external regions are only composed of a- spherulites since scattering bands located at 2h ¼ 14.12, 16.9, 18.52, 21.28, 21.80, and 21.86 degrees are attributed to the a- (110), a-(040), a-(130), a-(111), a-(131), and a-(041) Received 23 March 2009; accepted 22 September 2009. * Author to whom correspondence should be sent. E-mail: abdesselam. dahoun@mines.inpl-nancy.fr. Volume 63, Number 12, 2009 APPLIED SPECTROSCOPY 1377 0003-7028/09/6312-1377$2.00/0 Ó 2009 Society for Applied Spectroscopy