Counits Content and Stretching Temperature-Dependent Critical Stress for Destruction of γ Crystals in Propylene–Ethylene Random Copolymers

Abstract

Tensile deformation behavior of three random propylene–ethylene copolymers with the same molecular weight and different contents of counit was investigated at different temperatures from room temperature to close to melting point via tensile tests, step-cycle tests, and in situ wide-angle X-ray diffraction techniques. Upon stretching, the original crystalline lamellae must be destroyed, generating new highly oriented ones. A critical stress has been suggested, under which the original crystallites can be destructed. The propylene–ethylene copolymer samples in pure γ-form transformed gradually into α-form during tensile stretching. This crystalline transition proceeded via a destruction (melting) of the original γ-form crystals followed by recrystallization of the freed polymeric chain segments into α-form along stretching direction. This result provides a marker for investigating the critical stress mentioned above. Such critical stresses triggering the destruction of γ-form crystals for the propylene–ethylene copolymers of different ethylene counit contents were successfully calculated. It turned out that this critical stress depended on the ethylene counit content and stretching temperature. Samples with less ethylene counits show higher critical stress because of a lower degree of insertion of the ethylene counits into the crystalline unit cell than samples with higher ethylene counit content. The critical stresses remained constant when the samples were stretched at low-temperature region, whereas they decreased significantly at high stretching temperatures close to the melting points due to strong thermal distortion of the crystalline lattices, making the crystallites less stable.

Mechanism of Significant Improvement of Large Strain Elasticity in Soft Propylene–Ethylene Random Copolymer via Blending with Hard Propylene–Ethylene Coplymer

“…The cross-hatch structure was rather stable as the azimuthal angle of the branching at 172° remained the same even after recovery. Three-peak profiles were observed in Figures c and d, which was similar to the azimuthal intensity distribution of (008) plane of γ-form , with a second preferred orientation at an angle to the fiber axis but not the orientation of (040) plane of α-form even though the diffractions of the two crystal planes almost located at the same position. Thus, it is possible to conclude that the few original crystals in iPPcoE23 before stretching are “γ-form-like” crystals.…”

Interplay between Macroscopic Stretching and Microscopic Phase Transition Revealed in Butene-1/1,5-Hexadiene Random Copolymers

“…It has been pointed that crystallinity may affect strain hardening for the semicrystalline polymers . Since the increase of MCP co-unit concentration decreases the degree of crystallinity, the Einstein equation G /(1 + 2.5 x c ) can be used to eliminate the effect of crystallinity, x c . − In this equation, crystallinities were determined from the in situ WAXD results of strain hardening after the complete phase transition (i.e., with the pure transformed form I) and the x c values kept nearly constant during stretching in this stage. The results of Figure b demonstrate that the corrected strain hardening modulus is around 7 MPa for all MCP copolymers, independent of the crystallinity and co-unit concentration.…”

Influence of ethylene/propylene sequences on the thermal and mechanical properties of propylene‐based elastomers

“…In addition, PBEs can be a perfect candidate for polyolefin compounding modification benefit from the excellent compatibility. For example, PBEs blended with polypropylene can improve the transparency and impact resistance of the product and improve its processing performance 6–9 . Exxon Mobil has blended PBE (16 wt%) with iPP (84 wt%) to prepare an SMMS nonwoven that can be used in a three‐dimensional leak‐proof nappy partition design.…”

“…For example, PBEs blended with polypropylene can improve the transparency and impact resistance of the product and improve its processing performance. [6][7][8][9] Exxon Mobil has blended PBE (16 wt%) with iPP (84 wt%) to prepare an SMMS nonwoven that can be used in a threedimensional leak-proof nappy partition design. Heidari et al 10 significantly improved the impact strength with the increase of PBEs concentration.…”