Critical phenomenon during photoinitiated gelation at different temperatures: A Photo-DSC study
Highlights
▸ The effect of temperature on photopolymerization was determined by Photo-DSC method. ▸ An increase in temperature helped to increase the propagation rate and resulted in higher conversion values. ▸ Gel fraction critical exponent (β) during the gel formation of acrylate with photoinitiator for various curing temperatures can be investigated by DSC. ▸ β displayed no variation during photopolymerization with various temperatures.
Introduction
Photoinitiated polymerization of multifunctional acrylates provides an easy and instant method for producing highly crosslinked networks. The excellent physical properties and low curing time of these crosslinked materials have led to a growing demand and widespread applications for these materials. Applications, such as curing of coatings on various materials, adhesives, printing inks and photoresists are well known in the field of interest. The photoinitiated polymerization of acrylates and methacrylates is one of the most efficient processes for the rapid production of polymeric materials with well-defined properties. These materials have found widespread use as coatings, imaging materials, photoresists and polymeric materials for many other applications. The photoinitiator plays a key role in UV-curable systems by generating the reactive species, free radicals or ions, which will initiate the polymerization of the multifunctional monomers and oligomers [1], [2], [3], [4], [5].
Photoinitiated radical polymerization may be initiated by bond cleavage (Type I) and H-abstraction (Type II) initiators [1]. Type II photoinitiators are based on compounds whose triplet-excited states readily react with hydrogen donors, thereby producing initiating radicals (Scheme 1) [2], [3], [4]. Because of the bimolecular radical generation process, they are generally slower than Type I photoinitiators, which form radicals unimolecularly.
Thiol and carboxylic acid derivatives of thioxanthone have recently been reported to initiate photopolymerization without co-initiators as they contain functional groups with a H-donating nature [6], [7], [8]. In addition, involvement of oxygen inhibition of photopolymerization suggests that thioxanthone–anthracene (TX–A) may find use in a variety of practical applications of photocuring in air [9]. A major advantage of thioxanthone-based initiators is related to their one component nature. They can serve as both a triplet photosensitizer [10], [11] and a hydrogen donor. Thus, this photoinitiator does not require an additional co-initiator, i.e., a separate molecular hydrogen donor. We recently reported [7] the use of a thiol derivative of thioxanthone (TX-SH) as a photoinitiator for free radical polymerization. The mechanism of the photoinitiation is based on the intermolecular reaction of the triplet 3TX-SH* with the thiol moiety of ground state TX-SH. The resulting thyl radical initiates the polymerization (Scheme 2).
Photoinitiated polymerization of multifunctional monomers results in crosslinked polymers that induce particular behaviours as regards kinetic reactions. Various parameters such as photoinitiator concentration, type of photoinitiator, light intensity and temperature affect the kinetic reaction processes during gelation [12]. Therefore, it is important to study the effect of temperature on the photopolymerization kinetics. Several researchers have been interested in this matter. Cook [13], Young and Bowman [14], and Andrzejewska [15] have thoroughly studied the temperature effect on the photopolymerization of multifunctional acrylates. Significantly, Cook [16] investigated the reactivity of a homologous series of bisphenol-A-based dimethacrylate resins from −40 °C to 160 °C and also, low-temperature photopolymerization of different acrylates was studied by Gao and Nie [17]. They found generally that the maximum rate of cure for photopolymerized acrylates increased with an increase of temperature due to an increase in propagation rate. Maffezzoli et al. reported that photopolymerization of an epoxy resin for stereolithography by Photo-DSC and increasing the irradiation intensity or the cure temperature led to an increase of both the rate of reaction and the degree of reaction [18].
When the glass transition temperature (Tg) of the polymer network is lower than the isothermal cure temperature, the polymerization reaction is kinetically controlled. When Tg of the network equals the isothermal cure temperature, vitrification occurs and diffusion of the reactive species becomes the limiting stage in the crosslinking reaction.
It is well known that free radical cross-linking polymerization (FCP) produces a network called a gel. The whole course of the bulk FCP is divided into three stages: low conversion stage, gel effect stage and glass effect stage [19], [20], [21]. It was observed that monomer conversion first increases very slowly, but then accelerates because of the gel effect [22]. When the reaction temperature is lower than the glass transition temperature of the polymer, the glass effect stage occurs as the last stage of polymerization. The glass transition temperature of polymers is customarily defined as the temperature at which the relaxation time on the monomer scale reaches about 100 s [23]. Radical chain polymerizations are often characterized by the presence of autoacceleration in the polymerization rate as the reaction proceeds [24].
Since the monomer and initiator concentrations decrease with time, usually one would expect the reaction rate to drop with the extent of conversion. However, exactly the opposite behaviour has been observed in many polymerization processes (the reaction rate increases with the conversion during the first stage of polymerization). This behaviour during the polymerization is referred to as the gel effect, also named the Trommsdorf effect or Norrish–Smith effect [25]. The gel effect corresponds to a dramatic increase in the rate of free radical polymerization and of the viscosity of the reaction medium. It is caused by the diffusion limitations in the reaction medium, which slows down the termination but not the propagation reaction. Norrish and Smith [25] postulated that the increased viscosity caused by monomers being converted to polymers resulted in a decrease in the mobility of the growing chains, making it more difficult for them to diffuse together and terminate. The term ‘gel effect’ was used due to the characteristic rise in viscosity accompanying the dramatic increase in polymer conversion [26]. Burnett and Melville [26], Schultz and Harbort [27] each independently performed polymerizations in the presence of solvent and both reached the conclusion that the gel effect was caused by increasing the bulk viscosity, as the solvent reduces the viscosity and delays the onset of the gel effect.
Quite recently, photopolymerization of 75 wt% EA and 25 wt% TPGDA mixture was monitored by employing the Photo-DSC technique in the presence of various thioxanthone based initiators [28], [29] and at different UV light intensities with TX-SH as the photoinitiator [29], [30].
In this work, the behaviour of photopolymerization kinetics of 80 wt% EA and 20 wt% TPGDA acrylates mixture with TX-SH photoinitiator was investigated at increasing temperatures from −15 to 125 °C by using the Photo-DSC technique. It was observed that all conversion curves during gelation present sigmoidal behaviour as predicted by the percolation model from which the critical exponents, β were produced. The averaged value for the critical exponents β was found to obey the percolation model, predicting that the universal behaviour holds near the glass transition point. The produced glass transition point tg, maximum rate of polymerization Rpmax and final conversion Cs values were found to be strongly correlated with the temperature in which the photoinitiated polymerization took place.
It is known that the gelation phase is not a transition one in the thermodynamic sense, being a geometrical one. As the subject of the critical phenomenon, it behaves like a second order phase transition constituting a universal class by itself. The exact solution of the gelation was given first by Flory and Stockmayer [31], [32] on a special lattice called the Bethe lattice on which the closed loops were ignored. An alternative to the chemical-kinetic theory is the lattice percolation model [33] where monomers are thought to occupy the sites of a periodic lattice and the chemical bonds corresponding to the edges randomly joining these sites with some probability p where p is the ratio of the actual number of bonds that have been formed between the monomers to the total possible number of such bonds. The gel point can be identified with the percolation threshold pc where, in the thermodynamic limit, the incipient infinite cluster starts to form; and the system behaves viscoelastically rigid [34], [35].
The predictions of these two theories about the critical exponents for the gelation are different from the point of universality. Consider, for example, the exponents β for the gel fraction G (the strength of the infinite network in percolation language) near the gel point, which is defined in Eq. (1):where the Flory–Stockmayer theory (the so-called classical or mean-field theory) gives β = 1 which is independent of the dimensionality, while the percolation studies based on computer simulations give β around 0.43 in three-dimensions [32], [36]. These two universality classes for the gelation problem are separated by a Ginzburg criterion [37] that depends upon the chain length between the branch points as well as the concentration of the non-reacting solvent. Critical percolation describes the polymerization of small multifunctional monomers [33], [34], [35].
Section snippets
Materials
2-Mercaptothioxanthone (TX-SH) was synthesized according to the previously described procedure [7]. Dimethylformamide (DMF, 99+%, Aldrich) was distilled over CaH2 under reduced pressure. Epoxy diacrylate (EA) and tripropyleneglycoldiacrylate (TPGDA) were obtained from Cognis France.
Photo differential scanning calorimeter (Photo-DSC)
The heat of the photoinitiated polymerization reaction was measured by means of a photo differential scanning calorimeter, as a good control of the reaction temperature [38]. The photoinitiated polymerization of
Results and discussion
Photo-DSC experiments are capable of providing reaction data in which the measured heat flow can be converted directly to the ultimate percentage conversion and polymerization rate for a given amount of formulation as in Eq. (3). It is important to study the effect of the temperature on the photopolymerization kinetics although photopolymerization reactions proceed at room temperature. However, the temperature at which the cure proceeds can affect the kinetics and the final cure values.
Conclusions
This work has presented a study in which the Photo-DSC technique was used to measure the critical exponents, β during the gel formation of EA and TPGDA mixtures with TX-SH photoinitiator for various curing temperatures at a constant light intensity (40 mW/cm2). It has to be emphasized that ( values do not vary at all during gelation for all samples conducted at various temperatures. However, it was observed that the other gelation parameters such as tg, Rpmax and final conversion (Cs) presented
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