Since their naissance in the 2000s, various micro or nanomotors with powerful functions have been proposed. Among them, polymer-based micro or nanomotors stand out for the easy processing and facile functionalization, holding immense potential for bioapplications. In this review, fabrication of polymer-based micro or nanomotors and their applications in biomedical areas are covered. Classic manufacturing approaches as well as cutting-edge techniques are discussed with representative works highlighted. Current challenges and future prospects are presented in the hope of pointing new research directions to facilitate practical translations of micro/nanomotors.
The symmetry of polymer crystals greatly affects the optical, thermal conductivity and mechanical properties of the materials. Past studies have shown that the two-dimensional (2D) confined crystallization of polymer nanorods could produce anisotropic structures. However, few researchers have focused on understanding confined nanostructures from the perspective of crystal symmetry. In this research, we demonstrate the molecular chain self-assembly of tetragonal crystals under cylindrical confinement. We specifically selected poly(4-methyl-1-pentene) (P4MP1) with a 41 or 72 helical conformation (usually crystallizing with a tetragonal lattice) as the model polymer. We found a coherent crystal branching of the tetragonal crystal in the P4MP1 nanorods. The unusual 45°- and 135°-{200} diffractions and the meridional 220 diffraction (from 45°-tilted crystals) have shown a uniform crystal branching between the a
1-axis crystals and the 45°-tilted crystals in the rod long axis, which originates from a structural defect associated with tetragonal symmetry. Surprisingly, this chain packing defect in the tetragonal cell can be controlled to develop along the rod long axis in 2D confinement.
Helical chain assembly has a great influence on the physical properties of polymeric materials. It has been known that the newly developed, anisotropic nanostructure of polymers in nanoconfinement is significantly different from that in the bulk. In this research, we demonstrate 3 1 helical chain assembly in isotactic polypropylene (i-PP) under 2D confinement in two main crystalline phases: αand γ-phase crystals. Samples synthesized with a metallocene catalyst yield the γ-phases. We found the integrated assembly of the threefold helical chains in both αand γ-phase crystals under confinement. The helical chain packing at the "four-methyl growth face" prefers to be aligned with the rod long axis. Furthermore, we demonstrate two unit-cell orientations of the γphase crystal in which the c γ -axis is consistent with the radial or axial direction under cylindrical confinement. The formation mechanism for the radial γ-phase crystals lies in that the rapid growth direction of the α-phase lamellar from which the γ-phase crystals are epitaxially grown is kinetically favored to be consistent with the rod long axis. More interestingly, the new axial γ-phase crystal reflects a complex, multi-fold structural ensemble of threefold helices in two phases via epitaxial relationships.
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