Compositions that photocure with

Hybrid systems are understood to be photocurable compositions that cure by both ionic and free radical mechanisms. As can be seen in Chapter 2, section 2.3.4. photoinitiators were actually designed for use with systems that contain both type of monomers, or prepolymers, and both to cure by both mechanisms. One such example are combinations of phosphonium and sulfonium salts described in that chapter ... 

A photocurable ink composition that contains a dendrimer has been described. The dendritic polymer has a molecule structure in which a functional group is dense on the surface with high density in comparison to a linear polymer. 

Diacrylate monomers have been prepared that are photocurable in visible light and that have small polymerization shrinkage and high X-ray contrast properties. When polymerized with 0.01 to 0.04 pm glass powder, these dental composites were easily machined into artificial teeth. 

Typical physical properties obtainable with UV cured silicones are provided In Table I. Incorporation of reactive unsaturation into the silicone polymer backbone In combination with a photosensitization system provided the photocure capability. Properties of a standard heat-cured encapsulant developed for use on semiconductor devices, Dow Corning HIPEC R-6103, are provided for comparative purposes. Clearly, introduction of a photocrosslinking mechanism into a siloxane type composition has afforded the desired result. The one-part, solventless, UV curable silicone composition cured rapidly upon exposure to UV radiation, providing a cured composition which has retained the typical properties that make silicones so attractive for protection of semiconductor devices. 

While the photocurable polymers used for S-FlL avoid the viscosity and thermal concerns associated with the thermoplastics used in NIL, they are not without then-problems. During polymerization, the photocurable polymers undergo a volumetric shrinkage or densification due to chemical bond formation. Consequently, the feature size, shape, and position could be affected. Studies have shown, however, that by controlling the resist composition, the shrinkage is limited to the z-direction, resulting in a reduced aspect ratio [131,137],... 

Several publications suggest that an optimum concentration of a pbotoinitiator should be such that the optical density or absorbance of the reacting composition should be 0.434 to yield an optical density at which maximum radiation is absorbed. This number has meaning, however, only for ideal systems, cured by monochromatic radiation. With commercial sources of radiation that consist of 20 to 40 emission lines this does not apply. Also, it is important to know that variations in the concentrations of a photoinitiator within various locations of the film that is being photocured, dramatically affect flic locations where the radiation is absorbed, such as at the surface, flnoughout the material, or at some other location. 

Few new observations in the photocurable hyperbranched polyester/eutectic nematic liquid crystals (E7) system was reported by Kim et al. [163] Higher order mesophase is induced at some specific compositions, which is found absent in their individual components and possible escape of some ingredients observed during prolonged annealing. It is observed that in the eutectic mixture some components is expelled due to its weak affinity with the hyperbranched polyester. The induced smectic phase at 63-93 wt % E7 in the blend is influenced by the shift of eutectic nematic point. The presence of isotropic (I), isotropic + nematic (I + N), pure nematic (N), nematic +smectic A (N + SmA), and induced smectic A (SmA) phase is dependent on the composition and temperature. 

Glass fiber reinforced composites based on epoxy-acrylate modified UPRs were studied [228]. The authors showed that UPRs, endcapped with acrylate groups and diluted with reactive multifunctional acrylic and allylic monomers in the presence of a photoinitiator, can be photocrosslinked with UV radiation as glass fiber laminates in a rapid process. It was found that the physical properties of the photo-crosslinked laminates are well correlated with the molecular weight of the polyester, the amount of multifunctional monomer added, and the glass fiber content. A greater improvement of the tensile and flexural properties of the photocured products was observed for multifunctional acrylate or acrylether monomers added to the UPR (Table 31) than for allylic monomers. 

Other synthetic fibers, as well as natural fibers, were employed in epoxy-based multicomponent systems. Thus, a comparative study between polymeric fibers reinforced epoxy resins and natural fibers reinforced ones, all obtained by UV curing, was performed [186]. Isotactic PP (iPP) fibers modified with 20 wt% EVA and hemp fibers were employed, while the matrices used were epoxy acrylate and epoxy methacrylate. Data indicated that relatively regular distribution of fibers was achieved the addition of fibers caused an increase in Shore hardness of the epoxy methacrylate based composites the epoxy acrylated composites showed a decrease in hardness when EVA-modified iPP fibers were used, whereas hemp fibers caused an opposite effect. Even more, the iPP fibers reinforced photocurable composites displayed a brittle to ductile fracture transition. 

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