Infrared spectra of UHMW, HD, and HSG polyethylene. Stars and circles indicate the amorphous and the monoclinic bands of the material, respectively. The other peaks are assigned to the orthorhombic Pnam phase.
Upper panel: P–T path followed in the annealing procedure of polyethylene. Lower panel: comparison between the IR spectra measured before (A) and after (B) the annealing procedure and those calculated for the monoclinic A2/m (dashed line) and orthorhombic Pnam (full line) phases.
Selected IR spectra of crystalline polyethylene at 475 K, measured upon pressure increase (from top to bottom) from 0.7 to 53.8 GPa. At the bottom side it is reported the spectrum of the sample measured at the lowest pressure after the complete compression/decompression cycle.
Decomposition of the IR spectra of crystalline polyethylene measured at 425 K in the scissoring region through the orthorhombic Pnam to monoclinic P21 m transition. The formation of the monoclinic phase is revealed by the appearance of the new peak on the low frequency side of the orthorhombic doublet. The fit was performed with pseudo-Voigt functions.
Comparison between experimental (575 K) and simulated (0 K) infrared spectra at four selected pressures. The calculated spectra are shown with green (orthorhombic Pnam), red (monoclinic P21 m), and blue (monoclinic A2/m) lines, while the experimental spectra are reported as black lines. Correspondence of the peaks is also indicated. The theoretical bandwidths were fixed to 7 cm−1. The vertical scales of the computed spectra were multiplied by a frequency independent arbitrary factor.
Selected Raman spectra of crystalline polyethylene measured at 475 K upon pressure increase from 1.4 to 40.6 GPa. The strong band at ∼1340 cm−1 is due to the diamond.
Comparison between experimental (575 K) and simulated DFT (0 K) Raman spectra at three selected pressures. Green, red, and blue spectra are the calculated spectra relative to the orthorhombic (Pnam), monoclinic (P21 m), and monoclinic (A2 /m), respectively.
Pressure shift of infrared (empty circles) and Raman (red dots) frequencies, upon compression. The vibrational modes are labeled following the assignment for the polyethylene chain from Ref. 29. γ t , γ r , and γ w indicate twisting, rocking, and wagging modes, respectively, δ bending and ν stretching modes.
Pressure evolution of the IR spectra in a decompression measurement along the 610 K isotherm showing the orthorhombic-hexagonal-liquid phase transitions.
Phase diagram of crystalline polyethylene after the present work. The empty dots indicate the Pnam-P21/m phase transition as deduced by the appearance of the low frequency band in the IR spectra of the scissoring mode (see Fig. 4) measured upon compression. Full dots indicate the P-T values where the orthorhombic IR spectra were recovered during decompression experiments. The experimental points for the Pnam-P21/m and P21/m-A2/m phase transitions are from Ref. 4, whereas the dotted line is a schematic representation of the P21/m-A2/m phase boundary.
Vibrational coupling constant G/A (see Eq. (2)) vs R (see Eq. (3)) for the rocking and scissoring IR active modes at three different temperatures, and for the C–C stretching Raman mode at 475 K. The R values range between 3.9 Å (∼6 GPa) and 4.4 Å, being 4.5 Å; the ambient pressure value. Solid lines are drawn by using the power law reported in Eq. (5) with the n value fixed to –6. In the upper left panel the curves obtained from room temperature literature experimental15 (dotted) and theoretical17(full) frequencies are also reported.
Pressure evolution of the integrated absorbance of the wagging IR peak (see inset) at 475 K. The data have been obtained by fitting the band to a pseudo-Voigt profile.
Intensity of the IR active wagging mode of crystalline polyethylene as a function of density at different temperatures. Full (open) circles indicate the data upon compression (decompression) while n represents the exponent of the power law employed to fit the data (full lines, see Eq. (6)). The calculated (DFT) intensity (empty squares), scaled by an arbitrary factor, is also reported in the same density range. The full line reproducing the evolution with density is a power law with n = 2.64.
Article metrics loading...
Full text loading...