Rheological Behavior of Blends of Metallocene Catalyzed Long-Chain Branched Polyethylenes. Part I: Shear Rheological and Thermorheological Behavior

Long-chain branched metallocene-catalyzed high-density polyethylenes (LCB-mHDPE) were solution blended to obtain blends with varying degrees of branching. A high molecular LCB-mHDPE was mixed with low molecular LCB-mHDPE at varying concentrations. The rheological behavior of those low molecular LCB-mHDPE is similar but their molar mass and molar mass distribution are significantly different. Those blends were characterized rheologically to study the effects of concentration, molar mass distribution, and long-chain branching level of the low molecular LCB-mHDPE. Owing to the ultra-long relaxation times of the high molecular LCB-mHDPE, the blends exhibited a clearly more long-chain branched behavior than the base materials. The thermorheological complexity analysis showed an apparent increase in the activation energies E_a determined from G′, G″, and especially δ. E_a(δ), which for LCB-mHDPE is a peak function, turned out to produce even more pronounced peaks than observed for LCB-mPE with narrow molar mass distribution and also LCB-mPE with broader molar mass distribution. Thus, it is possible to estimate the molar mass distribution from the details of the thermorheological complexity.

Rheological properties and crystallization behaviors of long chain branched polyethylene prepared by melt branching reaction

Long chain branched polyethylene (LCBPE) without gel was prepared by melt branching reaction in a Haake torque rheometer in the presence of peroxide and different multi-functional acrylate monomers, and the optimal reaction time was determined according to the transient torque curves. The Fourier transform infrared (FTIR) results indicated that multi-functional monomers had been grafted onto HDPE backbone. Rheometer, 13C NMR, and high-temperature gel permeation chromatography (HT-GPC) coupled with triple detectors were used to characterize the microstructure of the LCBPE. The results showed the LCB content and the degree of branching increased with the increasing of functionality of the multi-functional monomers. Moreover, the LCBPE samples exhibited higher apparent zero shear rate activation energy and clear strain-hardening behavior compared with pure HDPE. Various rheological plots including viscosity, storage modulus, loss angle, and Cole-Cole plots were used to distinguish LCBPE from linear HDPE. A possible mechanism for melt branching reaction was also discussed in this paper. Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were used to study the influences of LCB on the crystallization behavior and crystal morphology of all samples. It was found that the melt temperature and crystal morphologies of LCBPE were evidently different from that of pure HDPE due to the introduction of LCB.