The unit cell expansion of branched polyethylene as detected by Raman spectroscopy: an experimental and simulation approach

Raman spectroscopy has become a commonly used technique for the study of phase structure in semicrystalline polymers [1–8]. Among them, polyethylene (PE) is one of the more deeply explored materials. However, there are still some remaining questions concerning the effect molecular and structural features in the intensity and position of characteristic Raman signals. The Raman crystallinity band appearing in linear PE at 1415 cm is thought to arise from factor group splitting, where the two PE chains packed in the orthorhombic unit cell cause the CH2 bending vibrational mode to split with a second component appearing at 1440 cm [4, 9, 10]. The other characteristic band appearing at 1460 cm has been shown to arise from superposition of rocking combination modes, which interact with the crystal mode with symmetry B3g [11]. Changes in the crystalline phase, i.e. in the unit cell parameters for example as a result of temperature changes or the inclusion of short branches, could affect this phenomenon resulting on changes in the interchain energy interactions [8, 9]. The shift in the position of the 1415 cm band towards a higher wavenumber observed as crystallinity decreases has been associated with a lower effectiveness of the interchain interaction within the lattice, and hence to lateral disorder in the crystals. Lagaron has recently found a correlation between the shift of this crystalline band and the macroscopic density for a set of PE samples with different molecular architectures [8]. However, the variation did not result in a linear correlation, probably due to the molecular heterogeneity of the selected samples. In this work we carried out a careful analysis of the orthorhombic crystalline band observed by Raman spectroscopy, i.e., the CH2 bending 1415 cm –1 mode, as a function of crystal features obtained from X-Ray scattering (WAXS) and differential scanning calorimetry (DSC), for a series of model ethylene/1-hexene copolymers, with varying comonomer content, obtained in our laboratory from single-site catalyst polymerization (see Table 1). The materials are characterized by having a narrow molecular weight distribution (Mw/Mn=2) and a homogeneous comonomer distribution. The weight average molecular weight (Mw) of the samples ranges between 120 and 300 kg mol. We will interpret the experimental shift observed in terms of the variations in the crystalline orthorhombic structure found in the materials. Moreover, we will compare the experiments with computed results calculated at the local Density Functional Theory (DFT) level. For that purpose, normal modes of vibration were obtained for the orthorhombic cells with different parameters derived from the X-ray data. The splitting of the single-chain vibrational active modes into the bands at 1415 and 1440 cm, can be observed in Fig. 1a for the linear PE sample. The separation between these two bands has been obtained for all polymers studied and listed in Table 1. The exact J. Otegui J. F. Vega (&) S. Martin V. Cruz A. Flores J. Martinez-Salazar Departamento de Fisica Macromolecular, Instituto de Estructura de la Materia, CSIC Serrano 113 bis, Madrid 28006, Spain e-mail: imtv477@iem.cfmac.csic.es