1. Introduction to
photopolymerization.
As an environmentally friendly technology, photopolymerization in which
liquid monomers or oligomers are transformed into solid materials under
irradiation of light (ultraviolet, visible or infrared light) holds
these advantages of low volatile organic compounds (VOC), fast curing,
energy saving, environmental protection, and low temperature compared
with traditional thermal polymerization.[1-6] In
addition, photopolymerization is widely used in many fields due to its
unique ā5Eā properties (efficiency, enabling, economical, energy
saving and environmental friendly).[3, 7-12] The
history of photopolymerization can be traced back to the 19th century
when people observed that styrene polymerized into glassy resin under
light. Ostromislenski, the first researcher of photopolymerization,
discovered that the number of constitutional units of product far
exceeded the number of photons absorbed in photopolymerization reaction
of vinyl bromide, and concluded that the reaction was chain
reaction.[13] The 1960s was the heyday of basic
research on photochemistry, its theory gradually improved, and began to
be widely applied to actual industrial production.
A complete photopolymerization system generally consists of monomer,
oligomer and photoinitiator (PI), all components are
indispensable.[14] Monomers, also known as active
diluents, can reduce the viscosity of polymerization system and improve
solubility, more importantly, they contain one or more functional groups
that can participate in the polymerization reaction such as double
bonds, epoxy, oxetane, thiol, etc. Oligomers, whose molecular weights
range from several hundred to tens of thousands, have high viscosity and
also contain some polymerizable functional groups to participate
polymerization. The proportion of monomers and oligomers is large in
whole polymerization system, and the properties of polymerization
materials are largely determined by them. PI, as an essential component
accounting for 1-10 wt % of photopolymerization system, can produce
active species under irradiation of light to initiate polymerization
reaction, which has great influence on the rate of photopolymerization
and the performance of polymerization
material.[15-16]
The procedure of the photopolymerization reaction is exhibited in Figure
1, firstly, PI of ground state undergoes electron transition to excited
state under irradiation of light, then generate active species, which
are free radicals or cations generally. Active species can interact with
functional groups of monomers or oligomers to initiate chain reactions,
resulting in the formation of polymer.