Self-healing of covalently cross-linked polymers under an ambient visible light in the bulk state, in air, at room temperature using radical exchange of thiuram disulfide units is reported. The successful attachment of surfaces of cut pieces proceeded under ambient conditions under exposure to visible light from a commercial tabletop lamp, as confirmed by bending and tensile tests.
Well‐healed: Polymers cross‐linked with trithiocarbonate (TTC) units were prepared by a RAFT polymerization. The repeatable self‐healing systems or macroscopic fusions were accomplished by UV irradiation of the cross‐linked polymer in solution and in the bulk state. The macroscopic fusion of completely separated parts was successful (see pictures, BA=n‐butyl acrylate).
The effects of side chain length on the molecular aggregation states and surface properties of poly(fluoroalkyl acrylate) [PFA-Cy, where y is the fluoromethylene number of the Rf groups] thin films were systematically investigated. Spin-coated PFA-Cy thin films were characterized by static and dynamic contact angle measurements, X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry, and wide-angle X-ray diffraction. The receding contact angles showed small values for PFA-Cy with short side chain (y e 6) and increased for ones with y g 8. It has been revealed that PFA-Cy with y g 8 was crystallized and formed ordered structures. These results suggest that the water-repellent mechanism of PFA-Cy can be attributed to the presence of highly ordered fluoroalkyl chains at the outermost surfaces. The results of XPS in the dried and hydrated states and the contact angle measurement in water indicate that the contact angle for water is lowered by exposure of the carbonyl groups to the water interface through reorientation of short fluoroalkyl chains.
The film thickness dependence of surface structure for immiscible
polystyrene/poly(methyl
methacrylate) (PS/PMMA) films was investigated on the basis of atomic
force microscopic observation
and X-ray photoelectron spectroscopic measurement. In the case of
the PS/PMMA film of 25 μm thickness,
the air−polymer interfacial region was covered with a PS rich
overlayer due to its lower surface free
energy compared with that of PMMA and a well-defined macroscopic
phase-separated structure was formed
in the bulk phase. Also, in the case of the PS/PMMA thin film of
100 nm thickness, the phase-separated
structure, in which the PMMA rich domains separated out of the PS rich
matrix, formed at the film
surface. The formation of the surface structure for the PS/PMMA
thin film can be attributed to either
the chain conformation or chain aggregation structure being frozen at
the air−polymer interfacial region
before the formation of a PS rich overlayer due to the fairly fast
evaporation of solvent molecules. On
the other hand, the two-dimensional PS/PMMA ultrathin film of 10.2 nm
thickness did not show distinct
phase-separated structure. When the film thickness became thinner
than 10.2 nm, the two-dimensional
PS/PMMA ultrathin film of 6.7 nm thickness showed fine and distinct
phase-separated structure with
the domain size of a few hundred nanometers. This structure can be
designated as “mesoscopic phase-separated structure”. The surface phase state for the
two-dimensional PS/PMMA ultrathin films can be
explained by the film thickness dependence of both the interaction
parameter and the degree of
entanglement among polymer chains.
The surface wettabilities of polymer brushes with hydrophobic and hydrophilic functional groups were discussed on the basis of conventional static and dynamic contact angle measurements of water and hexadecane in air and captive bubble measurements in water. Various types of high-density polymer brushes with nonionic and ionic functional groups were prepared on a silicon wafer by surface-initiated atom-transfer radical polymerization. The surface free energies of the brushes were estimated by Owens-Wendt equation using the contact angles of various probe liquids with different polarities. The decrease in the water contact angle corresponded to the polarity of fluoroalkyl, hydroxy, ethylene oxide, amino, carboxylic acid, ammonium salt, sulfonate, carboxybetaine, sulfobetaine, and phosphobetaine functional groups. The poly(2-perfluorooctylethyl acrylate) brush had a low surface free energy of approximately 8.7 mN/m, but the polyelectrolyte brushes revealed much higher surface free energies of 70-74 mN/m, close to the value for water. Polyelectrolyte brushes repelled both air bubbles and hexadecane in water. Even when the silicone oil was spread on the polyelectrolyte brush surfaces in air, once they were immersed in water, the oil quickly rolled up and detached from the brush surface. The oil detachment behavior observed on the superhydrophilic polyelectrolyte brush in water was explained by the low adhesion force between the brush and the oil, which could contribute to its excellent antifouling and self-cleaning properties.
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