Origin of Extended-chain Lamellae in Polyethylene

Synergistic Effect of Pressurization Rate and β-Form Nucleating Agent on the Multi-Phase Crystallization of iPP

Using a homemade pressure device, we explored the synergistic effect of pressurization rate and β-form nucleating agent (β-NA) on the crystallization of an isotactic polypropylene (iPP) melt. The obtained samples were characterized by combining small angle X-ray scattering and synchrotron wide angle X-ray diffraction. It was found that the synergistic application of pressurization and β-NA enables the preparation of a unique multi-phase crystallization of iPP, including β-, γ- and/or mesomorphic phases. Pressurization rate plays a crucial role on the formation of different crystal phases. As the pressurization rate increases in a narrow range between 0.6–1.9 MPa/s, a significant competitive formation between β- and γ-iPP was detected, and their relative crystallinity are likely to be determined by the growth of the crystal. When the pressurization rate increases further, both β- and γ-iPP contents gradually decrease, and the mesophase begins to emerge once it exceeds 15.0 MPa/s, then mesomorphic, β- and γ- iPP coexist with each other. Moreover, with different β-NA contents, the best pressurization rate for β-iPP growth is the same as 1.9 MPa/s, while more β-NA just promotes the content of β-iPP under the rates lower than 1.9 MPa/s. In addition to inducing the formation of β-iPP, it shows that β-NA can also significantly promote the formation of γ-iPP in a wide pressurization rate range between 3.8 to 75 MPa/s. These results were elucidated by combining classical nucleation theory and the growth theory of different crystalline phases, and a theoretical model of the pressurization-induced crystallization is established, providing insight into understanding the multi-phase structure development of iPP.

CRISTAPRESS: an optical cell for structure development in high-pressure crystallization

An original optical high-pressure cell, named CRISTAPRESS, has been especially designed to investigate phase transitions of complex liquids, i.e., polymers, polymer blends, nano-composites, etc. The design of the cell is based on the optical properties of morphological entities through in situ light depolarizing microscopic observations. Pressure up to 200 MPa with a fine temperature control up to 300 °C can be applied. A striking advantage of this cell is the possibility to select the pressure transmitting medium that can be water, silicone oil, a fluid in the supercritical state, etc. The potential of the novel technique was demonstrated by carrying out time-resolved measurements during polymer crystallization induced by water pressure. These preliminary experimental investigations permit to discriminate the role of the barometric and thermal histories on the kinetics of polymer growth, as well as on the subsequent morphologies. It should lead to new reliable crystallization kinetics models.

A study of the nanostructure and tensile properties of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) has gained worldwide acceptance as a bearing material used in orthopaedic implants. Despite its widespread use, inherent properties of the polymer continue to limit the wear resistance and the clinical lifespan of implanted knee and hip prosthetics containing UHMWPE components. The degree of crystallinity of UHMWPE is known to strongly influence several of its tensile mechanical properties such as Young's modulus, yield stress, strain-hardening rates, work of fracture and ultimate tensile properties. In this study, medical grade UHMWPE was subjected to four different crystallization conditions resulting in UHMWPE with a range of crystalline morphologies. Thereafter, the crystalline nanostructure was quantitatively characterized using a combination of ultra-small angle X-ray scattering and differential scanning calorimetry. Low-voltage scanning electron microscopy was employed as a supplementary technique to compare the crystalline morphology resulting from each crystallization condition. In addition, uniaxial tensile tests were performed to assess the effects of crystallization conditions on the mechanical properties of UHMWPE. This study showed that while crystallization conditions strongly influenced the morphology of UHMWPE, in most cases the mechanical properties of the material were not significantly affected.