Photopolymerization kinetics of multifunctional monomers
Introduction
Photoinduced polymerization is a rapidly expanding technology resulting from its main advantages: the process is solvent-free, energy efficient and generally economical [1]. It has found extensive application for producing photoactive polymer-based systems used in the coating industry, paints or printing inks, adhesives, composite materials and dental restorative formulations. Its unique feature — occurrence of polymerization only in illuminated areas — enables the generation of high resolution images for the production of printing plates, optical discs and microcircuits. More recently, applications for this technology have included three-dimensional stereolithography and holographic recordings [1], [2], [3], [4], [5], [6], [7]. Acrylate-based resins (formula I) are the most widely used light-curable systems because of the high reactivity of the acrylate monomers.
Tetrafunctional (meth)acrylate monomer with R=H or CH3 and R′=short chain alkylene, arylene, polyester, polyurethane, polysiloxane
UV-irradiation in the presence of a suitable photoinitiator is one of the most efficient methods for the generation of highly cross-linked polymers from multifunctional monomers. Polymerizations can be carried out under a wide range of conditions, including varations in monomer structures, the number and type of reactive functional groups, temperature, atmosphere, irradiation rate and photoinitiator type. Because of the importance of free-radical photopolymerizations of multifunctional monomers and the effect of the polymer structure on material properties, it is necessary to study the influence of reaction conditions and monomer structure on the polymerization kinetics.
The photopolymerization of multifunctional monomers has been the subject of many comprehensive review articles and books [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], which have appeared recently. These reviews were devoted to various aspects of the process, like kinetics, UV equipment, photoinitiators, polymerization systems, applications. In this article, special emphasis will be given to the determination of rate coefficients, the kinetics of photocrosslinking process and its modeling and to the influence of various factors, both chemical as well as physical, on the chemistry of network formation. To emphasize the special features of the kinetics of photocrosslinking processes, a comparison with the kinetics of linear polymerizations will be made.
Although interest in cationic photopolymerizations has increased recently, this review deals mainly with (meth)acrylic compounds and radical photopolymerizations, since these monomers and this type of photocuring are still the most popular and most widely applied.
This article does not pretend to be exhaustive and the references cited cover mainly the last 10 years of work in this area.
Section snippets
Initiating systems
Classification of initiators is based on the type of polymerization system they initiate, i.e. free radical, cationic or anionic [1], [3], [4], [5], [6], [7], [8], [9], [10], [13], [14], [17], [18]. There are also a few cases of initiators (e.g. iodonium and sulphonium salts, arene complexes), which are able to initiate polymerizations via both cationic and radical processes.
Many efficient photoinitiators have been developed, and efforts to obtain more efficient photoinitiator systems that
Linear vs. crosslinking systems
From the point of view of elementary reaction steps, free-radical crosslinking polymerization does not differ from the linear polymerization of monovinyl compounds and involves initiation, propagation, chain transfer and termination, where transfer and termination can occur by several mechanisms [49]. However, the kinetics of network formation show special features arising from the participation of a multi-unsaturated monomer and are complicated by other factors not observed in linear
Determination of rate coefficients
The determination of actual polymerization rate coefficients for crosslinking systems is very difficult due to the complicated behavior of the reaction with respect to reaction kinetics, especially the very early onset of autoacceleration. Although a number of studies have appeared in which rate coefficient were estimated [91], [95], [96], [97], [98], [99], [100], [101], [102], [103], [104], [105], [106], [107], [108], [109], [110], [111], [112], [113], [114], [115], the calculated values were
Effect of various parameters on the polymerization kinetics
The photopolymerization behavior, i.e. the polymerization rate, extent of conversion, etc. highly depend on the reaction conditions and monomer structure. In this review, this subject will be only briefly presented, focusing mainly on more recent studies in this area.
Conclusions
The photopolymerization of multifunctional monomers is a complicated process, especially with respect to reaction kinetics. Besides difficulties associated with studying heterogeneous, insoluble systems, the kinetics of network formation involve phenomena not observed in linear polymerizations, especially the immediate onset of autoacceleration and radical trapping at early reaction stages as well as a dominance of reaction diffusion as the means of migration of radical centers. Most of the
Acknowledgements
This work was supported by the Committee of Scientific Research, Poland, grant DS-32/10/2000.
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