The influence of biaxial stretching process on LLDPE/COC blend: Structure–property relationships in mechanical and barrier performance

Linear low-density polyethylene (LLDPE) and cyclic olefin copolymer (COC, up to 15 wt%) were melt blended, followed by sequential biaxial stretching (3×5 ratio). COC with a bulk amorphous chain structure was confirmed to have restricted compatibility within LLDPE using SEM. Rheological and high-temperature tensile analyses demonstrated that dispersed COC domains act as interfacial constraints to partially restrict chain relaxation, but they became flexible at elevated stretching temperatures to facilitate deformation. In particular, the blend composition with 5 wt% COC (LC5) exhibited an optimal balance between chain mobility and interfacial confinement, and the sequentially biaxially stretched LC5 film (LC5-35) had an excellent transverse (TD) tensile strength of 101.2 MPa. Wide angle X-ray scattering (WAXS) analysis shows that stress was concentrated in certain crystal planes, leading to unique structural fragmentation, resulting in denser rearrangement of LC5-35 polymer chains. As a result, the water vapor permeability (WVP) was reduced by approximately 15% compared to biaxially stretched LLDPE (LC0-35). The effect of the biaxial stretching process on the structure-property relationships was observed from the overall performances of the LLDPE/COC blend, and these investigations confirmed the potential applications of stretched films as recyclable flexible packaging materials.