Study of the solid-state morphology of linear low-density polyethylene blown films by Fourier transform infrared spectroscopy

In this work, orientations of crystallites and chain segments in the amorphous phase as well as the relative amounts of tie molecules (TMs) of linear low density polyethylene (LLDPE) were studied. Correlations between the morphological structures and mechanical properties were also investigated. Orientational features were determined using a tilted angle Fourier transform infrared (FTIR) technique. A new method based upon FTIR was developed to measure the relative amounts of TMs. The measured orientation factors show that the crystalline a-axis orientation is along the machine direction (MD) of the LLDPE blown films studied in this work and increases with increasing drawdown ratio (DDR). The crystalline b-axis orientation is perpendicular to the MD, with relatively balanced distribution in both the transverse direction (TD) and the normal direction (ND). The crystalline c-axis does preferred to lie perpendicular to the MD. The a- and b-axes orientations indicate that LLDPE crystallites are not in an isotropic state; instead, regional orientation exits. The chain segments in the amorphous phase are relatively randomly oriented. Films with lower degrees of crystalline orientation show higher relative amounts of TMs. Both MD tear resistance and TD tear resistance increase when the crystalline a-axis exhibits a higher degree of orientation along the MD while the crystalline b-axis is oriented more toward the TD and the ND respectively. Orientation of the crystallites has little influence on dart impact strengths of the films. However, dart impact strength is closely related to the amount of TMs in such blown films. Films with greater amounts of TMs show higher dart impact strengths.