Investigations On The Effect Of Entanglement Molecular Weight On Crystallization Behavior Of Polyethylene

Polyethylene（PE）is a typical semi-crystalline polymer with extremely high critical cooling rate(>10⁶ K·s^-1),which normally formed with semi-crystalline structure in general production and application conditions.This semi-crystalline structure consists of crystalline and amorphous regions interconnected by molecular chains.The entanglements between molecular chains have significant effect on the formation of crystalline structure and the properties of polymer materials.Entanglement molecular weight（_e）is not only an important parameter of polymer fluid,but also a crucial factor affecting the crystallization structure and behavior of polymer.In this thesis,the effects of_e on the crystallization structure and behavior of polyethylene were investigated mainly via differential scanning calorimetry（DSC）,wide angle X-ray scattering（WAXS）,small angle X-ray scattering（SAXS）.The basic crystallization properties of PE were studied via DSC experiments of high-density polyethylene（HDPE）and ultra-high molecular polyethylene（UHMPE）.The effect of the added chain entanglements on the crystallization behavior of PE was discussed;The experimental results showed that metallocene polyethylene（m PE）samples have linear structure and only a small amount of long-chain branch（LCB）,which length is greater than 2_e.It was found that under the condition of similar molecular weight distribution,several times of weight average molecular difference did not cause significant impact on the crystallization structure and behavior of m PE samples.However,significant long relaxation behaviors and order of magnitude differences in viscosity were observed via dynamic rheological experiments,indicated that the m PE formed long chains fractal structure.Successive self-nucleation and annealing（SSA）was used to characterize the length of crystallizable methylene sequence（MSL）.It was found that the lamellar thickness calculated via Thomson-Gibbs equation exhibits two scaling relationships with MSL related to the_e of the PE,reflected the limiting effect of chain entanglements on the crystallization of PE.The experimental results of irradiation cross-linked polyethylene（XLPE）showed that once the number-average molecular weight between cross-linked points（_c）of XLPE was larger than_e,the stable crystal（Orthorhombic）could be formed,otherwise the metastable crystal（Hexagonal）appeared.In the meantime,it was found that the crystallization kinetic tend to the formation of metastable crystal compared with stable one under the same supercooling.The inverse lamellar thickness((9(9_c^-1)and lateral coherence lengths(₁₀₀^-1)of XLPE samples obtained via WAXS and SAXS exhibited linear relationship with crystallization temperature（_c）.These phenomena could be explained by the multi-step growth crystallization model proposed by Strobl,that is,hexagonal crystals formed at the initial stage of crystallization could not complete the transformation to orthorhombic crystals under the topological constraints of the cross-linked network,exist stably in a small size at room temperature and pressure.