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Previously, the orientation structure of ultrahigh-molecular-weight polyethylene (UHMWPE) for artificial joints was considered to be unchanged after irradiation crosslinking. Therefore, much of the research related to the long-term failure of artificial joints has focused on material improvements. In this study, ultrasmall-angle X-ray scattering (USAXS) and the small/wide-angle X-ray scattering (SAXS–WAXS) combined technique reveal that the orientation structures of UHMWPE materials at all scales (nanoscale to microscale) are responsible for the long-term failure of artificial joints. To further illustrate the formation of these hierarchical oriented structures, a simple model is presented. In this model, first, the migration of free radicals plays a vital role, and the different steric hindrances in different directions directly lead to uneven migration behavior of free radicals. Second, the uneven migration of free radicals contributes to an inhomogeneous concentration of free radicals, thus resulting in observable crosslinking nonuniformities. Finally, all the hierarchical structural nonuniformities promote long-term failure of artificial joints after long-term wear.

Polybutene-1 (PB-1) always crystallizes into its metastable form II when cooled from a molten state because of its lower nucleation barrier, while the stable form I exhibits superior properties compared to form II, in particular, its resistance to creep and fracture. We investigated the polymorph selection of PB-1 during crystallization under high gas/supercritical fluid (SCF) pressure using a high-pressure chamber and differential scanning calorimeter. By applying pressurized gases/SCFs in PB-1 during crystallization, the obtained crystal form was changed from the kinetically favored metastable form II to the most stable form I′, which is desirable because of its outstanding mechanical properties. The formation of form I′ can also be promoted by increasing the crystallization temperature or by applying gases/SCFs with high solubility in PB-1. The enhancement of chain mobility under high-pressure gases/SCFs and at high crystallization temperatures plays a critical role in the polymorph selection. The increase in chain mobility results in an increase in entropy reduction and an increased surface free energy during nucleation. Therefore, the critical nucleus size required to achieve thermodynamic stability increases, and more entanglement would be needed to form the larger crystal nucleus, which has a greater effect on form II (which has a lower surface free energy under routine conditions) than on form I′. Furthermore, with increased CO2pressure, we observed a greater reduction in the melting point in form II than in form I/I′. The increase in chain mobility would intensify the local melting in the crystal growth front, where the relaxation of the chains in the interface between the crystal and amorphous phase occurs stronger in the αc-mobile form II. Under high gas/SCF pressure, the formation of form I′ tends to dominate during the crystallization of PB-1, and the crystallization of the kinetically favored form II was suppressed because of the increased chain mobility.

As for the solid–solid transformation from form II to form I in polybutene-1, the intercrystalline links in amorphous phase and the temperature difference between crystallization and phase transition were thought to be essential in previous studies for generating the internal stress to promote nucleation and growth of the phase transition. In this work, we report experimental evidence showing that such a transition can be accomplished without supercooling at the temperature of crystallization directly in a low molar mass sample where there are almost no intercrystalline links. This result reveals two significant aspects of the form II to I transition in polybutene-1 against conventional wisdom: (1) the nucleation barrier of the transition is very low and can be overcome by thermal fluctuation; (2) entanglements and intercrystalline links in high molar mass systems play a role of stabilizing the metastable form II via hindering the translational movements of chains in crystalline phase and slowing the relaxation of amorphous chains during phase transition. Direct formation of form I′ from melt begins in the early stage of the phase transition and proceeds during the phase transition. The melting temperatures of these form I′ crystals increase with the aging temperature. Moreover, the structure and melting point of transformed form I highly depend on the annealing temperature.

To decrease the dipole polarization rate and reduce the dielectric constant of poly(aryl ether ketone) (PAEK) resin, 1,4-di(4-fluorobenzoyl) cyclohexane (DFBCH), a weakly polarizing cyclohexane-based monomer, was designed and synthesized as the primary reactant. The bulky fluorene group was incorporated to increase the free volume of the resin, further reducing the dielectric constant. Additionally, hydroquinone with a symmetric and regular structure was utilized to enhance the molecular chain’s regularity and reduce dipole relaxation, further lowering the resin’s dielectric constant and dielectric loss. The PFQEKs series resins exhibited excellent thermal stability with glass transition temperature (Tg) ranging from 222 to 239 °C and 5% weight loss (Td5%) ranging from 458 to 463 °C, with different monomer ratios. As the hydroquinone content increased, the dielectric constant (Dk) and dielectric loss (Df) of the resin decreased significantly, with Dkranging from 2.92 to 2.77 and Dfranging from 0.011 to 0.008 at 10 GHz.