Orientation and Epitaxy in the Injection-Molded Bars of Linear Low-Density Polyethylene/Isotactic Polypropylene Blends: An Infrared Dichroism Measurement

Abstract

In this article, molecular orientation of crystalline and amorphous phases of both linear low-density polyethylene (LLDPE) and isotactic polypropylene (iPP) and epitaxy in the LLDPE/iPP blends prepared via dynamic packing injection molding have been investigated with the aid of polarized Fourier transform infrared (FTIR) spectroscopy and two-dimensional X-ray scattering (2D-WAXS). In LLDPE-rich blends, LLDPE was oriented along the shear flow direction, and iPP kept very low orientation. No epitaxial growth between LLDPE and iPP was observed. However, for the blends with LLDPE content less than 50 wt %, where iPP was the matrix and LLDPE formed the droplets, iPP was highly oriented along shear flow direction and LLDPE epitaxially grew onto iPP. The contact planes were (100)LLDPE and (010)iPP, and LLDPE molecules were about 50 degrees apart from the shear direction. The epitaxy fraction in LLDPE/iPP blends could be tentatively calculated from the values of orientation function of the LLDPE crystalline a-axis (fa) and that of crystalline b-axis (fb). Both tensile strength and tensile modulus decreased with the increase of LLDPE composition, indicating they were composition dependent, though shear-induced orientation and epitaxial crystallization might play some role. The impact strength of the blend samples reached a plateau in the LLDPE composition range of 20-50%, due to the contribution of orientation and epitaxy.

Insight into understanding the evolution of the epitaxy crystallization in isotactic polypropylene and polyethylene blends

“…The crystallinity Xc of component i in the blends was calculated by the equation where ∆H i is the enthalpy of fusion of component i , directly obtained from DSC and φ i is the mass fraction of component i in the blends. The enthalpy of fusion of 100% crystalline polymer is 293 and 207 J/g for HDPE and iPP, respectively …”

“…Inspired by this mechanism, a few novel processing methods have been promoted to successfully manufacture the special samples with epitaxy crystallization nowadays. Fu's research group has made great efforts on the epitaxy crystallization formed during dynamic packing injection molding . Under the extensive shear field, iPP melt can be oriented, and thus, its crystallization rate can be significantly accelerated, on which PE can subsequently epitaxial grow and form the epitaxy crystallization.…”

Mechanical Performance of Injection‐Molded Poly(propylene): Characterization and Modeling

“…Therefore, it is attempted to link the degree of orientation of both anisotropic systems to the anisotropic constants. The c ‐axis orientation in the solid state drawn tapes is probed using WAXD since this particular orientation is proven to be equivalent with the degree of c ‐axis orientation obtained from FTIR using the 998 cm −1 vibration 29. Figure 9 shows the anisotropic constants as a function of orientation factor of the c ‐axis.…”

Simultaneously enhancing strength and toughness for impact polypropylene copolymers by regulating the dispersed phase with high density polyethylene

“…Some of the nanoscale particles even play the role of nucleation agent and inuence toughness through reducing spherulite size. 10,[22][23][24][25] It was reported that the introduction of polyethylene into PP brought dramatic improvement in toughness 23,24 and J. C. Feng et al 23 believed that it should be mainly ascribed to the reduced spherulite size of PP by the introduction of HDPE. 20,21 In addition, incorporation of another crystalline polymer in the modied polymer can also improve toughness without serious sacrice of stiffness and various toughening mechanisms have been presented.…”