Molecular orientation evolution during low‐density polyethylene blown film extrusion using real‐time Raman spectroscopy

Abstract

Real-time polarized Raman spectroscopy was used in this study to measure the molecular orientation evolution during blown film extrusion of low-density polyethylene (LDPE). Spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near the nip rolls. The trans C-C symmetrical stretching vibration of polyethylene (PE) at 1132 cm −1 was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c-axis of the orthorhombic crystal, and was used to solve a set of intensity ratio equations to obtain second ( P 2 (cos θ) ) and fourth ( P 4 (cos θ ) ) moments of the orientation distribution function. The orientation parameters (P 2 , P 4 ) were found to increase along the axial distance in the film line even past the frost-line height (FLH). The P 2 values also showed an increasing trend with crystalline evolution during extrusion, consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. It was also found that the integral ratio (I 1132 /I 1064 ) obtained from a single, ZZ-back-scattered mode can provide a reasonable estimate of molecular orientation. These results indicate the potential of real-time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion.

Real‐time crystalline orientation measurements during low‐density polyethylene blown film extrusion using wide‐angle X‐ray diffraction

“…It is also important to note that significant orientation occurs past the FLH, even after the bubble has solidified. This result is consistent with our earlier study [6] using real‐time Raman spectroscopy, which confirmed that uniaxial orientation parameters ( P 2 , P 4 ) increase along the axial distance in the film line even past the FLH. Thus, refinement of chain orientation continues even after the blown film diameter is locked into place.…”

“…The orientation factors corroborate the effect of stress on the orientation within the bubble for two different take‐up speeds. The orientation profiles are consistent with earlier polarized Raman spectroscopy results [6] and small‐angle light scattering [26], and confirm that orientation takes place even past the FLH during the film blowing process. These results provide a fundamental understanding of the microstructural transformation occurring during the process.…”

“…The technique requires little sample preparation and is not influenced by humidity in the environment. The potential of real‐time Raman spectroscopy as a rapid microstructure monitoring tool during blown film extrusion has been demonstrated [6–8]. However, Raman spectroscopy is not a primary measurement technique to obtain crystallinity and orientation in fibers or films [9–11].…”

Spectroscopic elucidation of structure-property relations in filaments melt-spun from amorphous polymers

“…In the as-spun (undrawn) state, the filaments are in a highly un-oriented condition and this expected high degree of isotropy is confirmed in Figure 6 by the fact that the : ratios are close to 1 for all vibration bands under consideration, where a value close to 1 implies that the intensities of the polarized Raman bands in the two directions are indistinguishable [44]. Further, Figure 6 shows that as the draw ratio increases, the polarization ratio for spectroscopic bands located along the polymer backbone such as for C-C stretching (ν C-C ) as well as CH 2 rocking (ρ CH2 ) and twisting (τ CH2 ) decreases for both COP and CoPA, a trend clearly observed by other authors for highly oriented fibers and films [43][44][45][46]. In contrast, Figure 6b shows how the polarization ratio obtained from the secondary amide C=O stretching vibration band located at 1638 cm -1 is directly proportional to the degree of draw ratio imparted to the CoPA fiber.…”

Polymer crystallization studies under processing-relevant conditions at the SAXS/WAXS DUBBLE beamline at the ESRF

“…Recently, efforts to monitor the structure evolution during film blowing in real time have been made, using techniques such as small-angle light scattering (Bullwinkel et al, 2001), Raman spectroscopy (Gururajan & Ogale, 2009) or X-ray diffraction using sealed tubes (Gururajan et al, 2008). While the advantages of using commercial X-ray sources for structural analysis are evident, they still lack the collimation which is so beneficial when using synchrotron radiation.…”