Pursuit of the correlation between yield strength and crystallinity in sintering-molded UHMWPE

Abstract

Crystallinity (Xc) is a crucial crystallographic factor that related closely to mechanical properties in linear polyethylene or high density polyethylene (HDPE), whose molecular weight (MW) is at a 105 level. However, as a typically intractable-processing polymer, the structure-property relationship concerned Xc is still ambiguous for ultrahigh molecular weight polyethylene (UHMWPE) with MW﹥1.5 × 106. The reason is that high entanglement density hinders chain mobility in the course of UHMWPE crystallization, thus crystalline degree in UHMWPE molding parts is difficult to be artificially modulated within a wide scope. In this study, low chain-entanglement status of UHMWPE melt was achieved by (1) utilizing lowly entangled nascent UHMWPE synthesized via the single-site Ziegler-Natta catalyst, and (2) decreasing the sintered temperature. It is impressive that partial disentanglement reserved in the melt is favorable for obtaining higher crystallinity. Meanwhile, an isothermal crystallization treatment was additionally adopted to further improve the capacity of regulating crystallization after the traditional two-step sintering procedure. By a combination of varying the initial entanglement in nascent powders, sintering temperature, isothermal crystallization duration, and molecular weight of UHMWPE, the Xc of the compression-sintered products has been successfully adjusted in the relatively broad range of 29%≦ Xc ≦ 65%, which offered an opportunity to validly survey the correlation between mechanical property and crystallinity. Particular attention was paid to the yield strength. A linear master straight-line can be plotted as yield strength vs. Xc for all of the samples inspected, indicating crystallinity also plays a dominant role on mechanical performances of UHMWPE sintering parts, as same in the case of HDPE. However, the mechanical reinforcement induced by increment of Xc in UHMWPE is more efficient than in HDPE. The yield strength of UHMWPE is much higher than that of HDPE at the same Xc, indicating the amorphous phase or entanglements in UHMWPE is crucial to determine the mechanical property. These findings are of significance for guiding the manufacturing of high-performance UHMWPE materials via crystallization manipulation.