Figure 47 Fabrication process of lignin−gelatin hybrid hydrogel.
3.5. Applications of in photopolymerization other
fields
In addition to the above fields, the application of photopolymerization
is gradually expanding to other fields, such as frontal polymerization,
photochromic materials, self-healing materials and so on.
Frontal polymerization is a
self-propagating reaction where monomers are crosslinked into polymers
by the propagation from localized reaction zone to entire
system.[161-162] Generally speaking, no extra
energy is required except in initial stage.[162]According to the initiation sources, frontal polymerization can be
divided into the thermal-induced frontal polymerization (TFP) and
photo-induced frontal polymerization (PFP).[163]PFP is initiated by the irradiation usually UV
light,[164-166] however, as mentioned above, UV
light has many disadvantages and limitations in practical application,
it is a new strategy to use more secure NIR light as irradiation source
of frontal polymerization. Based on this, our group employed a
two-component system including cyanine (NIR sensitizer) and iodonium
salt to initiate the frontal polymerization of cationic polymerizable
monomers upon 808 nm NIR laser exposure.[57] The
heat released by NIR sensitizer and the polymerization reaction can
dissociate AIBN or TPED to achieve frontal polymerization of vinyl
ethers and epoxides. The frontal velocity and starting time can be
modulate by changing the different kinds of monomers, thermal initiators
and their concentration.
Solid-state photochromic
materials especially photochromic polymers have potential application
prospect in the fields of photoswitching devices, chemosensors,
solid-state optical devices, imaging and 3D optical-data storage
devices, and so on owing to their advantages of lightweight and good
processability.[167-171] At present, the main
preparation method of photochromic polymers is thermopolymerization,
which is complicated and energy consuming. Therefore, it is extremely
meaningful to develop a simple, efficient and environmentally friendly
preparation method of photochromic polymers.
Du et al.[172] designed and synthesized two novel
UV-LED-induced fluorophenyl oxime ester PIs (E-FBOXEs) with the ability
to prepare photochromic polymers, and then photochromic polymers based
on E-FBOXEs were prepared by fast photopolymerization. Proposed
mechanism for the photochromism of the film initiated by E-FBOXEs is
exhibited in Figure 48, Residual E-FBOXEs in photopolymerized films will
generate colored iminyl radicals in the planar conformation under
irradiation at 395 nm, and the recombination of the radicals to form
colorless E-FBOXEs in the nonplanar conformation under heating, which
results in the photochromism of the polymeric films.