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Spectroscopic, photocalorimetric and holographic measurements are conducted to investigate effects of stoichiometric thiol-to-ene ratio on the polymerization dynamics, refractive index modulation, recording sensitivity and polymerization shrinkage of volume gratings recorded in silica nanoparticle-polymer composite films based on step-growth radical addition polymerization. It is found that the polymerization rate of the composite system is maximized at the stoichiometric thiol-ene composition. It is also found that while the refractive index modulation and the recording sensitivity are maximized at the stoichiometric thiol-ene composition, polymerization shrinkage decreases with increasing the thiol monomer fraction. A negative correlation between gel point conversion and shrinkage is observed.
©2011 Optical Society of America
1. Introduction
Since 2002 we have studied a new class of holographic dry photopolymers, the so-called photopolymerizable nanoparticle-polymer composites (NPCs) [1–6], where inorganic or organic nanoparticles are uniformly dispersed in (meth)acrylate monomers capable of the chain-growth polymerization. The inclusion of nanoparticles increases the saturated refractive index modulation (Δnsat) and the material recording sensitivity (S) that exceed the required minimum values of 5 × 10−3 and 500 cm/J, respectively, for holographic data storage (HDS) media [7]. At the same time the mechanical and thermal stability (polymerization shrinkage and thermal expansion/refractive index changes) of recorded volume holograms can be improved with inorganic nanoparticles dispersed in NPCs [8]. High spatial frequency cut off of recorded volume holograms was also observed in NPCs, which is typical characteristic of multi-component photopolymer systems including NPCs and polymer-dispersed liquid crystals [9]. By taking advantage of holographic assembly of nanoparticles in NPCs [10], NPCs have also been used as holographic diffractive elements for neutron quantum beams [11,12]. When they are considered as HDS media, however, their polymerization shrinkage is still larger than 0.5% (the required shrinkage criterion for HDS media [7]) although shrinkage can be reduced as low as 1% by nanoparticle dispersion.
To reduce shrinkage with NPCs further, we recently proposed the use of thiol-ene photopolymerizations that proceeded via a step-growth radical addition mechanism [13–15]. The main idea of using the step-growth radical addition polymerization for volume holographic recording is explained as follows: while high-molecular-weight polymer is formed immediately in chain-growth radical polymerization of crosslinking (meth)acrylate monomers, it is not obtained until the later stage of the polymerization in the step-growth polymerization process [16]. As a result, gelation takes place late in conversion in the step-growth polymerization, leading to reduced volume (bulk) shrinkage and stress [17]. Indeed, we demonstrated that shrinkage reduction as low as 0.3% was possible with Δnsat and S as high as 8 × 10−3 and 1014 cm/J, respectively, in the green by using NPCs with organic nanoparticles and the stoichiometric mixture of primary mercaptopropionate trithiols and allyl ether trienes [14,15]. We also showed that another NPC system with silica nanoparticles and the stoichiometric mixture of secondary dithiol and allyl triazine triene exhibited high transparency, Δnsat as large as 1 × 10−2, S as high as 1615 cm/J and shrinkage as low as 0.4% [18]. Because of the dispersion of inorganic silica nanoparticles and the use of the allyl triazine triene monomer the thermal stability was much improved as compared with the former thiol-ene based NPC system. It is not clear yet, however, how the stoichiometric thiol-to-ene ratio has the impact on shrinkage and holographic recording performance such as Δnsat and S. Indeed, it is known that the polymer conversion dynamics is strongly influenced by the stoichiometry of thiol-ene monomers as well as by their functionalities [19,20]. This paper describes an experimental investigation of effects of the stoichiometric thiol-to-ene ratio on the properties of photopolymerization and volume holographic recording in the recently introduced thiol-ene based NPC system [18] at a wavelength of 532 nm.
2. Experimental
2.1 Sample preparation
A secondary dithiol monomer, 1,4-bis(3-mercaptobutyryloxy) butane (dithiol, Showa Denko K.K.) and a triene monomer, triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO, Aldrich), were employed and mixed at different stoichiometric ratios of thiol-to-ene functional groups (r). The reason for the use of this thiol-ene formulation is that while the dithiol exhibits high self-life stability and low odor, TATATO possesses the rigid structure of the triazine group, the high electron density of the double bond and homopolymerization characteristics giving fast thiol-ene polymerization rates, moderately late gel point conversion and increased cross-linking network density [21]. These characteristics improve the holographic recording and thermal properties [18]. The chemical structures of the dithiol and TATATO are shown in Figs. 1(a) and 1(b), respectively. This thiol-ene mixture was dispersed with inorganic silica nanoparticles (the average size of 13 nm) at the optimum concentration of 25 vol.% giving large Δnsat and high S at the stoichiometric thiol-ene composition [18]. Arefractive index difference between the cured thiol-ene mixture and silica nanoparticles is close to 0.1. Such a relatively large difference in refractive index would lead to large Δnsat. In addition, the silica nanoparticle dispersion provides better thermal stability of a recorded hologram as compared with the organic nanoparticle dispersion since the coefficient of linear thermal expansion for inorganics is much smaller than that for organics and the sign of the thermo-optic coefficient (dn/dT) is opposite between inorganics and organics [8]. To photo-sensitize the thiol-ene monomer mixture in the green, we employed a titanocene organo-metallic complex (Irgacure 784, Ciba) in combination with benzoyl peroxide (BzO2, Aldrich), known as a visible initiator/sensitizer system for thiol-ene polymerization [22]. To obtain efficient step-growth thiol-ene polymerization, we employed Irgacure 784 of 2 wt.% and BzO2 of 2.5 wt.% with respect to various thiol-ene monomer formulations at different rs. These were mixed, together with silica nanoparticles dissolved in methyl isobutyl ketone [2], to the thiol-ene blend. The mixed syrup was cast on a 10-μm spacer-loaded glass plate. It was dried in an oven and was finally covered with another glass plate to make film samples for holographic measurements. Linear absorption coefficients of these film samples before and after incoherent light curing were measured to be 10 and 1 cm−1, respectively, at a recording wavelength of 532 nm [18], indicating that their optically available thicknesses were thicker than the required minimum HDS media thickness of 500 μm [7]. Detailed sample preparation for spectroscopic and calorimetric measurements was described in [18].
2.2 Experimental methods
A real-time Fourier transform infrared spectrometer (Nicolet 6700, ThermoFisher Scientific) with a liquid N2 cooled MCT detector was used to measure the photo-induced conversion dynamics of thiol and ene functional groups under 532-nm incoherent light and ambient conditions. The curing light intensity was set to be 5mW/cm2 corresponding to the optimum intensity for holographic recording in our thiol-ene based NPC system [18]. The photo-differential scanning calorimetry was also conducted by using a photocalorimeter (Q200, TA instrument) to evaluate time-dependent relative conversion (α) and polymerization rate (Rp) of various thiol-ene monomer formulations having different values for r under the isotherm condition at 25 °C. Here α(t) is defined as [23]
where Q(t) and QT are the cumulative exothermic heat flow evolved at a given time t and the total heat flow of evolution, respectively. Rp was given by taking time derivative of α(t).In holographic measurements we employed a conventional two-beam interference setup to write an unslanted transmission grating of 1-μm spacing with two mutually coherent s-polarized beams of equal intensities from a cw Nd:YVO4 laser operating at a wavelength of 532 nm. The recording intensity (I0) was 5 mW/cm2. We monitored the time evolution of the diffraction efficiency (η), defined as the ratio of the 1st-order diffracted signal to the sum of the 0th- and 1st-order signals, at a photo-insensitive wavelength of 633 nm. The effective thickness (l) of each sample was estimated from a least-squares curve fit to the Bragg-angle detuning curve of the saturated diffraction efficiency (ηsat) with Kogelnik’s formula for an unslanted transmission grating [24]. Then, Δnsat was extracted from Bragg-matched ηsat with a help of Kogelnik’s formula and l. Note that Δnsat measured at 633 nm was converted to that at 532 nm by multiplying the former by a factor being the ratio of Δnsat at 532 nm to that at 633 nm. Applying the factor to the buildup dynamics of the refractive index modulation (Δn) measured at 633 nm, we obtained the buildup dynamics of Δn at 532 nm [14,18]. S was defined as [(∂η1/2/∂t|t = τ)/(I0l)], where τ is the induction time period, and was calculated from the buildup dynamics of η at 532 nm. The out-of-plane fractional thickness change (σ) due to polymerization shrinkage was evaluated by means of a Dhar et al.’s holographic method [8,25]. It is reported recently that polymerization shrinkage is lower for recording in an acrylamide-based photopolymer film at higher recording intensities and at larger grating spacing [26]. In our present study σ was evaluated for 10-μm-thick film samples at fixed recording intensity (5 mW/cm2) and grating spacing (1 μm) in order to find a dependence of σ on r. It would be a future subject of investigation on a dependence of shrinkage on recording intensities and grating spacing for NPC systems.
3. Results and discussion
3.1 Photopolymerization kinetics
Figure 2 shows parametric plots of thiol and ene functional group conversions for various thiol-ene formulations at different values of r without [Fig. 2(a)] and with [Fig. 2(b)] silica nanoparticle dispersion. Solid lines plotted in Figs. 2(a) and 2(b) correspond to ideal thiol-ene reaction obeying the step-growth polymerization mechanism by which the radical addition of a thiol to an ene funcitonal group takes place with the final product resulting from copolymerization of the ene with the thiol. Note that data at r = 0.50 and 2.00 are not shown in Fig. 2(b) because thiol-ene mixtures at these stoichiometric ratios with silica nanoparticle dispersion did not produce stable holograms with measurable ηsat. It can be seen from Fig. 2(a) that each parametric plot more or less follows the corresponding solid line. This trendindicates that photopolymerization reactions for samples without nanoparticle dispersion obey the step-growth polymerization mechanism. Final thiol and ene functional group conversions for the stoichiometrically balanced sample (r = 1) are 100%, so that no unreacted monomer is left. A stoichiometrically imbalanced sample gives the final conversion of an excess functional group which is lower than 100%, implying that either completely or partially unreacted monomers are left in a sample after photopolymerization. Ene functional group conversion slightly dominates over thiol functional group conversion for samples at r≤1 due to partial homopolymerization of TATATO having the high electron density. On the other hand, when silica nanoparticles are incorporated in thiol-ene formulations, the photopolymerization dynamics become quite different from those shown in Fig. 2(a), as shown in Fig. 2(b). First, final thiol and ene functional group conversions for the stoichiometrically balanced sample cannot reach 100%. Second, thiol and ene functional group conversions stop in the middle of polymerization for the stoichiometrically imbalanced sample at r = 0.75 (i.e., ene rich in the mixture). Third, the reaction of a thiol tends to dominate over that of an ene for the stoichiometrically imbalanced samples at r = 1.25 and 1.50 (i.e., thiol rich in the mixture), which is pronounced with increasing r. The reasons for such observations are unclear. We speculate that silica nanoparticles interfere with ene monomer’s thiol-ene reaction and homopolymerization events [18] and they facilitate the reaction of excessive thiol monomers with carbon radicals and/or initiating benzoyl oxy radicals that arise after decomposition of the complex of BzO2 with the photo-excited isomer of Irgacure 784 [22]. For samples with silica nanoparticle dispersion it can be estimated that while the fraction of completely unreacted thiol (ene) monomers is (1−0.90)2 = 1.0% [(1−0.85)3 = 0.3%] at r = 1, they are 18.1% (13.6%), 6.2% (~0%) and 13.0% (~0%) at r = 0.75, 1.25 and 1.50, respectively. This means that when the photopolymerization process reaches the steady state, completely unreacted thiol monomers are present in all the samples with silica nanoparticle dispersion and they are more than completely unreacted ene monomers.
Fig. 2 Parametric dependences of thiol and ene functional group conversions for various thiol-ene formulations at different values for r (a) without and (b) with silica nanoparticle dispersion.
Figure 3 shows parametric plots of α and Rp for thiol-ene formulations at different values of r without [Fig. 3(a)] and with [Fig. 3(b)] silica nanoparticle dispersion. It can be seen that Rp is the largest at r = 1 for samples with and without silica nanoparticle dispersion. This trend is quite different from the case of thiol-ene based polymer dispersed liquid crystals that exhibit higher Rp of ene monomer with excess thiol monomer (i.e., the thiol-ene formulation at r>1) [19]. A reduction in Rp is seen for samples with silica nanoparticle dispersion due, probably, to the inhibition of the thiol-ene polymerization by silica nanoparticles. Despite this reduction in Rp it is expected that the largest values for Δnsat and S are obtained with thestoichiometrically balanced sample. We could also evaluate gel point conversion (αc) [16] from Fig. 3. It is defined as the conversion point when gelation of a polymerizing material starts. It was reported that when α began to saturate after its rapid increase during the initial stage of the thiol-ene reaction, the elastic modulus started increasing [27]. The similar trend was reported on the relation between α and the shrinkage stress in the thiol-ene polymerization [26]. Based on these observations, we could define αc to be α at which Rp is peaked. Note that the so-defined αc tends to slightly lower than those determined from the temporal evolution of the elastic modulus and the shrinkage stress. It can be seen from Figs. 3(a) and 3(b) that while αc monotonically increases with increasing r for thiol-ene formulations without silica nanoparticle dispersion, its change is not large for thiol-ene formulations with silica nanoparticle dispersion. According to Flory-Stockmayer theory for gelation [16,28,29], αc is given by
where fthiol (fene) is the functionality of thiol (ene) monomer and ρ is the stoichiometric imbalance defined as NA/NB in which NA and NB are the numbers of A and B functional groups with NB≥NA. Equation (2) shows that αc is higher with non-stoichiometric compositions and/or with low thiol and ene functionalities. Table 1 summarizes αcs obtained from Figs. 3(a) and 3(b) for thiol-ene forumulations at different values of r, together with those calculated from Eq. (2). When r is smaller than unity (i.e., excess ene monomers in the mixture), αc decreases, which is an opposite trend to the theoretical prediction. We speculate that this discrepancy is caused by partial homopolymerization of TATATO since the stoichiometrically imbalanced samples at r<1 exhibit the homopolymerization leading to lower αc, as seen in Fig. 2. This result suggest that non-stoichiometric thiol-ene formulations at r>1 provide low shrinkage.Fig. 3 Parametric plots of α and Rp for various thiol-ene formulations at different values of r (a) without and (b) with silica nanoparticle dispersion.
Table 1. Gel Point Determination for Samples without and with Silica Nanoparticle Dispersion
3.2 Holographic recording characteristics
In what follows we describe the results of holographic measurements for samples dispersed with 25 vol.% silica nanoparticles. Figure 4 shows the buildup dynamics of Δn as a function of recording time for samples at different values for r. Once again, we note that results for four types of samples at r = 0.75, 1.00, 1.25 and 1.50 are shown since other non-stoichiometric samples r = 0.50 and 2.00 did not produce stable holograms with measurable ηsat. It can be seen that the stoichiometric sample provides the largest Δnsat. It can also be seen that all the samples exhibit similar induction time periods. This is so because the same relative volume ofthe initiator/sensitizer to the thiol-ene monomers was used in all the samples. Dependences of Δnsat and S on r are plotted in Fig. 5(a) . It can be seen that both Δnsat and S decrease when r departs from unity. Such a trend can be explained by the fact that the highest value for Rp is obtained with the stoichiometric sample as seen in Fig. 2(b). Also, unreacted excess monomers left in non-stoichiometric samples would lower the refractive index contrast under holographic exposure. Figure 5(b) shows a dependence of σ on r. It can be seen that, as expected from Table 1, σ monotonically decreases with increasing r. The observed trend can be explained as follows: homopolymerization of TATATO likely taken place in samples at r<1 results in an increase in σ. When thiol monomers are excessive (samples at r>1), homopolymerization of TATATO is negligible and unreacted thiol monomers with low viscosity act as plasticizers during holographic exposure. Therefore, volume shrinkage is relaxed and σ decreases with increasing r. To meet the required criteria of Δnsat (≥5 × 10−3), S (≥500 cm/J) and σ (≤0.5%) for HDS media, we find that r needs to be between 1.0 and ~1.3.
Figure 6 shows a parametric dependence of σ on αc. In this case we used αcs of the samples without silica nanoparticle dispersion as shown in Table 1 in order to make the dependence clearer. It can be seen that σ decreases with an increase in αc, showing a negative correlation between αc and σ.
3. Conclusion
We have investigated the impact of stoichiometric thiol-ene ratio on the properties of photopolymerization and volume holographic recording in our recently reported thiol-ene NPC system. This thiol-ene NPC system at stoichiometric composition meets the required criteria of Δnsat, S and σ for HDS media in the green. Spectroscopic, photocalorimetric and holographic measurements have been conducted to measure the polymerization dynamics and holographic grating characteristics including shrinkage. We have found that the thiol-ene stoichiometry strongly influences on the dynamics of thiol and ene functional group conversions and therefore on Rp for samples with silica nanoparticle dispersion. We have also found that αc increases with increasing r. We have shown that while Δnsat and S are maximized at r = 1, σ can be reduced with increasing r. We have also shown that there exists a negative correlation of αc with σ. Such a correlation must also be seen in other holographic photopolymer systems and can be used for the material design strategy for holographic applications.
Acknowledgments
This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan under grant 23656045 and by JST Innovation Satellite Ibaraki under grant 04-123.
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