Influence of branch content, comonomer type, and crosshead speed on the mechanical properties of metallocene linear low-density polyethylenes

The effects of branch content (BC) and comonomer type on the mechanical properties of metallocene linear low-density polyethylene (m-LLDPEs) were studied by means of a stress-strain experiment at room temperature. A total of 16 samples with different BCs and comonomer types were used. In addition, the effect of cross-head speed on the mechanical properties of m-LLDPEs with different BCs was examined. The degree of crystallinity (X_t) of these copolymers was determined by differential scanning calorimetry. In addition, Ziegler-Natta linear low-density polyethylenes (ZN-LLDPEs) were also studied for comparison purposes. The increase in BC of m-LLDPEs decreased X_t and the modulus. However, the ZN-LLDPEs showed higher small-strain properties but lower ultimate properties than the m-LLDPEs with similar weight-average molecular weights and BCs. In comparison with low-BC resins, m-LLDPEs with high BCs exhibited a stronger strain hardening during the stress-strain experiments. Strain hardening was modeled by a modified Avrami equation, and the order of the mechanically induced crystal growth was in the range of 1-2, which suggested athermal nucleation. The crosshead speed was varied in the range 10-500 mm/min. For low-BC m-LLDPEs, there existed a narrow crosshead speed window within which the maxima in modulus and ultimate properties were observed. The location of the maxima were independent of BC. The effect of the crosshead speed on the mechanical properties of the m-LLDPEs was a strong function of BC. However, highly branched m-LLDPE in this experiment showed a weak dependence on the cross-head speed.