Photopolymer-clay Nanocomposite Performance Paper Technical Utilizing Different Polymerizable Organoclays

By Soon Ki Kim and he effects of polymerizable While adding 3 wt% natural clay C. Allan Guymon organoclays on thermo- reduces WVTR 16% compared to that mechanical characteristics, of a neat system, polymerizable and Tgas barrier properties and thermal nonreactive organoclays induce stability have been studied about a 30% decrease, indicating utilizing a thiol- mixture. similar efficiency in reducing gas Photopolymerization kinetic behavior permeability through addition of shows that incorporation of thiolated organoclays when clays are modified organoclay into the thiol-acrylate with organic surfactants. composition induces a higher degree of step thiol-ene reaction than other Introduction organoclays. The addition of acrylated The incorporation of clays into a organoclay greatly increases tensile matrix with well exfoliated modulus with no significant change in morphologies can significantly enhance either elongation or glass transition many performance characteristics temperature of cured nanocomposites. of polymer composites (such as On the other hand, the incorporation mechanical and barrier properties, of thiolated organoclay decreases the thermal stability and chemical glass transition temperature due to resistance) with relatively small greater formation of flexible thio- amounts of loading in comparison linkages with higher thiol conversion. to systems based on microscale This enhanced flexibility results fillers.1-3 In-situ preparation of in significant increases in maximum polymer-clay nanocomposites based elongation of the nanocomposite, but on photopolymerization, on the other interestingly no reduction in Young’s hand, provides a simple way to produce modulus is observed. Significant nanoscale dispersion of clay particles, increases of 60% to 80% in toughness, avoiding several disadvantages of as determined by the area of stress- conventional processes involving strain curves in tensile experiments thermal degradation or releasing have been observed by adding only volatile organic compounds (VOC) 3 wt% acrylated or thiolated during the process.3-6 organoclays, while nonreactive To obtain the clay exfoliation organoclay actually decreases the necessary for achieving advanced toughness. Water vapor transmission performance in clay nanocomposites, rate (WVTR) of the photopolymer-clay modification of clay surfaces by suitable nanocomposites has also been examined. surfactants is required to improve

ISSUE 1 2013 RADTECH REPORT 33 compatibility in polymer matrices.3,4,7,8 thiol-acrylate-based photopolymers the reaction mechanism and enhanced However, most widely used have been studied.20-22 The interaction between the polymerizable nonreactive quaternary incorporation of polymerizable organoclays and systems surfactants9-11 may act as impurities organoclays increases both clay facilitate this reduction in shrinkage

Technical Paper Technical affecting the final performance because exfoliation and photopolymerization stress. they inherently do not participate rates significantly as also observed Likewise, differences in in the reaction.12 In in acrylate systems. Chemical polymerization behavior for this research, unique polymerizable compatibility between and organoclay-containing materials surfactants have been developed organoclay is of primary importance may affect final performance of the to overcome this disadvantage for achieving better clay exfoliation. nanocomposites based on the close of nonreactive surfactants. The Interestingly, the type of functional relation of polymer properties with incorporation of reactive groups on the group on the clay surface significantly both polymerization mechanism clay surfaces using photopolymerizable influences the polymerization behavior and resulting network structure. surfactants may also maximize the by affecting the stoichiometric balance Particularly in thiol-acrylate systems, interaction between clay and organic between thiol and double bond in the the reaction competition between the phase during and after polymerization. clay gallery. Thiolated organoclays thiol-ene step-growth mechanism and Previous results have demonstrated enhance thiol-ene reaction, while faster acrylate homopolymerization that the addition of polymerizable acrylated organoclays encourage chain often results in lower thiol conversion organoclays into various acrylate acrylate homopolymerization.21,22 than that of acrylate double bonds.25,26 photopolymer systems induce Changes in polymerization behavior This low thiol conversion further significant improvement in both with different organoclays also induce decreases the mechanical strength nanocomposite properties with significant differences in other aspects and barrier properties of thiol-acrylate enhanced clay exfoliation and of the photopolymer system. For photopolymers that already exhibit photopolymerization rate compared example, it was demonstrated that decreased glass transition temperatures to the system with conventional the addition of suitable polymerizable from the more flexible thio-ether nonreactive organoclays.13-14 Increases organoclays significantly reduces linkage.27,28 This shortcoming of thiol- of photopolymerization rate are inhibition and polymerization acrylate systems may be significantly also observed with incorporation of shrinkage of acrylate and thiol-acrylate improved by incorporating well- polymerizable organoclays, primarily systems.23,24 Addition of only 5 wt% dispersed organoclays that may not only due to greater immobilization of thiolated organoclays into thiol-acrylate increase the toughness of neat thiol-ene propagating radicals on the clay copolymerization systems effectively photopolymers, but also enhance gas surface, subsequently resulting in a eliminated oxygen inhibition during barrier properties by generating longer decrease of bimolecular termination of photopolymerization by reaching an path length for gas molecules.19,28-31 propagating radicals. Enhanced clay almost identical conversion in air and The inherent control of reaction exfoliation facilitates this mechanism . This improvement is observed mechanism by incorporating by increasing the effective surface area regardless of the basic acrylate polymerizable organoclays in the system. composition in the thiol-acrylate provides an intriguing means to Recent research has also focused formulations, whereas the addition control polymer properties. This on utilizing the potential of reactive of either nonreactive or acrylated research investigates the effects of clays in thiol-ene formulations. Due organoclay typically decreases the polymerizable organoclays on the to its unique step polymerization conversion and polymerization rate ultimate performance of photopolymer mechanism and properties, thiol-ene in air. Incorporation of only 5 wt% clay nanocomposites, including photopolymerization has been widely polymerizable organoclays modified gas barrier properties and various studied in the past decade to help with acrylate or thiol functional groups thermo-mechanical properties such overcome major drawbacks of free- induces up to a 90% decrease in the as glass transition temperature, polymerization such as oxygen shrinkage stress while the reduction storage modulus, ultimate elongation, inhibition and polymerization shrinkage by the same amount of nonreactive Young’s modulus and tensile strength during photopolymerization.15-19 The clays is less than 20%. Real-time in both acrylate and thiol-acrylate effects of polymerizable organoclays on infrared spectroscopy (RTIR) and photopolymers. Additionally, reaction polymerization behavior as well as simultaneous near-infrared conversion kinetics of systems with and without basic nanocomposite properties of the analysis suggest that both changes in organoclays is investigated to provide

34 RADTECH REPORT ISSUE 1 2013 context for the nanocomposite trimethylolpropane triacrylate dimethylammonium bromide (C16A behavior. Toughness of the material, (TMPTA), and -based surfactant) synthesized following determined by the area of stress- urethane-acrylate oligomer (CN9009, methodologies described previously.14,15 strain curves, was also compared MW ~2,000) were supplied by PSH2 thiol-functionalized organoclays as a parameter for discussing Sartomer Inc. (Exton, Penn.). were synthesized via Michael addition Paper Technical quantitative impact of organoclays on Trifunctional trimethylolpropane reaction between thiol groups of overall mechanical performance of trimercaptopropionate (TMPTMP) trifunctional TMPTMP and the acrylate nanocomposite materials. To study the was obtained from Aldrich. Cloisite groups from C16A acrylated organoclay effect of organoclays on improving gas Na (Southern Clay Products— based on prior procedures.16 The barrier properties of photopolymer-clay Gonzalez, Texas) was used for natural chemical structures of monomers and systems, water vapor permeability was montmorillonite clay. Cloisite 93A surfactants used in this research are examined based on an industrial ASTM (CL93A, Southern Clay Products), illustrated in Figure 1. Unless otherwise standard with different monomers as montmorillonite clay modified with noted, 0.5 wt% 2,2-dimethoxyphenyl well as organoclay type. The results dihydrogenated tallow, was used as a acetophenone (DMPA, Ciba Specialty from this study provide an effective typical nonreactive organoclay. Chemicals) was used as a free-radical platform for design of advanced To produce polymerizable photoinitiator in all experiments. All photopolymer-clay nanocomposites to organoclays, sodium cations between chemicals, including monomers and overcome drawbacks in conventional silicate platelets of Cloisite Na were clays, were used as received. acrylate photopolymer materials. exchanged using acrylate or thiol- functionalized quaternary ammonium Methods Experimental surfactants as described elsewhere.14 Thiol-ene photopolymerization Materials C16A acrylated organoclay-bearing, kinetics was studied using RTIR Tripropyleneglycol diacrylate acrylate-functional groups on the (Thermo Nicolet Nexus 670). RTIR (TrPGDA), polyethyleneglycol (600) clay surfaces were produced utilizing samples were prepared by sandwiching diacrylate (PEGDA, MW=742), hexadecyl-2-acryloyloxy(ethyl) monomer mixtures between two sodium chloride plates. Measurements were performed at ambient temperature Figure 1 after purging the RTIR chamber for Chemical structures of monomers used in this study six minutes with dry nitrogen gas. UV was provided by an optical fiber from a medium-pressure mercury lamp (EXFO Acticure). conversion was evaluated by monitoring the decrease in the height of the absorbance peak at 810 cm-1 for acrylate and at 2575 cm-1 for thiol.32,33 Photopolymerization reactions were initiated with a 365 nm light at 3.0 mW/cm². Dynamic mechanic analysis (DMA- Q800 DMA TA Instruments) was conducted to investigate the effect of organoclays on ultimate thermo- mechanical properties. To fabricate samples (2 mm x 13 mm x 25 mm) (A) tripropylene glycol diacrylate (TrPGDA); (B) polyethyleneglycol diacrylate (PEGDA600, MW=742); (C) trimethylolpropane tris(3-mercaptopropionate) (TMPTMP); and (D) for testing, liquid monomer mixtures diacrylate oligomer (CN9009). Additionally, structures of clay modifiers, including (E) methyl were injected between two microscope dihydrogenated tallow sulfonate, (CL93A); (F) hexadecyl-2-acryloyloxy(ethyl) dimethylammonium bromide (C16A); and (G) tetradecyl 2-(bis(3-mercaptopropionate) mercaptopropionyl slides endcapped with 2 mm spacers. trimethylolpropyl) acetocy(ethyl) dimethylammonium bromide (PSH2; Dithiol) are shown. The sample was then irradiated for 10 minutes on each side using 365

ISSUE 1 2013 RADTECH REPORT 35 nm UV at 3.6 mW/cm2 intensity under glass WVTR vessel (12 cm diameter organoclays than with conventional nitrogen atmosphere. To measure opening) with the prepared thin films, nonreactive organoclays. the modulus and glass transition 100g of distilled water was charged To verify this hypothesis, the temperature, samples were heated into the vessel. The sealed vessels effect of adding organoclays on Technical Paper Technical from -100°C to 100°C at 3°C/min. were placed in a convection hood thermo-mechanical property and Measurements were made using the two- controlled at 23°C and mass loss was polymerization behavior of thiol- point bending mode at 1Hz frequency. measured. WVTR per unit area was ene photopolymer-clay systems was Tensile experiments were calculated considering the thickness investigated with different reactive conducted based on DMA-controlled of composite films. and nonreactive organoclays. In force tensile mode with 0.5N/min ramp previous studies, low-molecular weight rate at 30°C using rectangular samples Results and Discussion monomers were used for thiol-acrylate (1mm x 5 mm x 20 mm), prepared Many results for organic-inorganic compositions as the high functional using the same process described composites have shown that the use of group concentration in these systems previously with 1 mm spacing glass well-dispersed inorganic fillers improve allowed good contrast in polymerization properties of polymer systems.1,4 If molds. The most important factor for kinetics.21,22 For examination of the achieving reproducibility of the tensile inorganic fillers can chemically react ultimate composite performance (such experiments was the force ramp rate. with the organic phase, interactions as mechanical and barrier properties), Whereas 2N/min ramp rate generated between the filler surface and however, polymerized materials need high experimental errors as high as polymer matrix may increase to a sufficient toughness to make and 20%, the error was reduced to about greater degree than in systems with handle thin films. 5% for all nanocomposite compositions nonreactive fillers.26 Interestingly, For instance, to conduct tensile by utilizing a relatively low ramp rate the incorporation of polymerizable tests using acrylate or thiol-acrylate of 0.5N/min. Young’s modulus was organoclays into photopolymerization formulations, use of suitable calculated utilizing the slope of the systems typically enhances the amounts of oligomers that increase stress-strain curve. polymerization rate, whereas the rate the toughness of cured is Water vapor transmission rate usually decreases when nonreactive needed because simple monomeric (WVTR) of photopolymer-clay clays are added due to reduction in compositions based on low-molecular nanocomposites including 3 wt% incident-light energy.13,14 In addition, weight multifunctional acrylate or (organo)clay was examined based on the type of functional group (e.g., thiol-acrylate mixtures are typically too an ASTM standard (ASTM E9634) (meth)acrylate or thiol) on the brittle or weak for such experiments utilizing thin polymer films in clay surface significantly affects the utilizing thin films. For this reason, comparison with neat systems. To polymerization kinetics and mechanism mixtures of polyethyleneglycol fabricate the 100 µm composite films, of thiol-acrylate systems.20-22 controlled amounts of liquid monomer The type of functional group diacrylate (PEGDA) and polyester- mixtures were placed between thick on the organoclay surfaces directs based urethane acrylate (CN9009) glass plates (150 mm x 150 mm x polymerization behavior by affecting were used for the oligomer system. 20 mm). Glass plates were covered the stoichiometric balance between Appropriate ratios were determined with 100 µm thick polyvinylidene thiol- and ene-functional groups in considering the flexibility and difloride film to allow easy release of the clay galleries and by affecting the strength of the cured films. the cured polymer films. On one of the the primary type of radicals on the Tripropyleneglycol diacrylate glass plates, thin tape spacers were clay surfaces, resulting in a different (TrPGDA) was also added to the attached to control the cured film degree of thiol-ene reaction with oligomer mixture to adjust the thickness. After placing the monomer different polymerizable organoclays. viscosity of liquid reactant mixture. mixtures on the bottom glass plate, Such results indicate a great degree For the thiol-acrylate mixture, trithiol bubbles were removed by applying a of interaction between the clay and TMPTMP was added to the basic vacuum to prevent pinhole formation. polymers when reactive functional acrylate mixture with CN9009/PEGDA/ Afterward, the upper plate was covered groups are incorporated on the clay TrPGDA using a 3:3:4 mass ratio. The and the plates were clamped. The mold surfaces. Therefore, it is reasonable to amount of TMPTMP was controlled was then exposed to 250~450 nm UV expect that greater improvement in based on the overall mole ratio light at 18 mW/cm2 for five minutes the ultimate composite performance between acrylate and thiol functional on each side. Before covering the could be achieved with polymerizable groups in the system.

36 RADTECH REPORT ISSUE 1 2013 Figure 2 Acrylate and thiol conversion profiles Technical Paper Technical

RTIR conversion profiles of CN9009/PEGDA/TRPGDA (3:3:4 by mass) mixtures including 20mol% thiol from TMPTMP with and without addition of 3 wt% organoclays. Shown are profiles of (A) acrylate conversion for neat CN9009/PEGDA/TRPGDA (), CN9009/PEGDA/TRPGDA/TMPTMP with 3 wt% CL93A (∇), 3 wt% C16A (), and 3 wt% PSH2 (◊) organoclay and (B) thiol conversion for neat CN9009/PEGDA/TRPGDA/TMPTMP (), CN9009/PEGDA/TRPGDA/TMPTMP with 3 wt% CL93A (), 3 wt% C16A (), and 3 wt% PSH2 (◊) organoclay. Photopolymerizations were initiated with 0.2 wt% DMPA using 365nm light at 3.0 mW/cm2.

Based on the potential impact of a nonreactive organoclay was added to termination that is more dominant in polymerization behavior on network low-molecular weight monomeric thiol- low-viscosity monomer systems. formation, understanding the acrylate polymerization systems. This Thiol conversion profiles in relationship between polymerization difference indicates that the increased Figure 2B, however, show much kinetics and final properties may system viscosity from incorporating different behavior based on the type be crucial for improving basic thiol- oligomers reduces bimolecular of organoclay. Incorporation of a acrylate materials. Therefore, the photopolymerization behavior Figure 3 was examined using RTIR kinetic experiments. The acrylate conversion Storage modulus profiles as a function of and thiol conversion as a function of temperature with addition of 5 wt% organoclays time for CN9009/PEGDA/TrPGDA systems, including 20 mol% thiol from TMPTMP, are shown in Figures 2 and 3 with and without 5 wt% organoclays. As shown in the reaction conversion profiles (Figure 2A), the incorporation of the different organoclays does not significantly change the acrylate conversion profiles. Ultimate acrylate conversion varies only from 0.90 for the system with nonreactive organoclay to 0.96 by adding acrylated polymerizable organoclay with the neat Storage modulus profiles of CN9009/PEGDA/TRPGDA (3:3:4 by mass) mixtures including system showing a conversion of 0.93. 20mol% thiol from TMPTMP with and without addition of 5 wt% organoclays. Shown are profiles for neat CN9009/PEGDA/TRPGDA/TMPTMP (), CN9009/PEGDA/TRPGDA/TMPTMP with 5 wt% The addition of nonreactive organoclay CL93A (), with 5 wt% C16A-acrylated organoclay (q), and 5 wt% PSH2 thiolated organoclay slightly decreases the polymerization (∆). Samples were photopolymerized with 0.5 wt% DMPA using 365 nm light at 3.6 mW/cm2. rate but not nearly to the degree observed in previous research21,22 when

ISSUE 1 2013 RADTECH REPORT 37 Figure 4 Acrylated organoclays Technical Paper Technical

Tan d profiles as a function of temperature of CN9009/PEGDA/TRPGDA (3:3:4 by mass) mixtures including 20mol% thiol from TMPTMP with increase of organoclay amount. Shown are profiles of the systems (A) with 1 wt%, 3 wt%, and 5 wt% C16A acrylated organoclays and (B) with 1 wt%, 3 wt%, and 5 wt% PSH2 thiolated organoclays. The profile for the neat CN9009/PEGDA/TRPGDA/TMPTMP system is included in each figure for comparison. Samples were photopolymerized with 0.5 wt% DMPA using 365 nm light at 3.6 mW/cm2.

thiolated organoclay induces significant increases the rubbery storage modulus, CN9009/PEGDA/TrPGDA/TMPTMP increases in photopolymerization whereas the modulus of the thiolated formulation. As shown in Figure 4A, rate and final thiol conversion. The organoclay system is slightly less no significant change in the glass incorporation of either nonreactive than that of the neat system. Based transition temperature, as indicated CL93A or acrylated C16A organoclay on similar or enhanced degrees of by the tan d peak of the system, is decreases ultimate thiol conversion acrylate homopolymerization in CL93A observed when increasing the amount to about 70% from 79% in the neat and acrylated organoclay system, the of C16A-acrylated organoclay up to 5 system, while the same amount of inorganic filler generates a higher wt%. The addition of PSH2 thiolated PSH2-thiolated organoclays increases modulus than in the neat system. organoclays, however, decreases the thiol conversion to 87%, corresponding On the other hand, for the thiolated glass-transition temperature (as shown to a 10% enhancement. Even in this organoclay systems where the thiol in Figure 4A) due to the enhancement oligomer-monomer mixture that is conversion is much higher than that of of thiol-ene reaction and greater similar to many practical formulations the neat system, the storage modulus flexibility in the network. The glass- in industrial applications, it appears decreases due to formation of more transition temperature decreases that ultimate thiol conversion can be flexible thio-ether linkages in the about 3°C from that of the neat system varied over a 20% range simply with polymer network, which mitigates by the addition of only 1 wt% thiolated changing the organoclay type. the strengthening effect from the organoclay and continues to decrease To determine whether these kinetic inorganic filler. This modulus behavior by increasing the clay amount up effects influence thermo-mechanical confirms that increased acrylate to 5 wt% with a substantial 6°C of properties of thiol-ene photopolymer- homopolymerization through addition substantial decrease. clay systems, dynamic mechanical of acrylated organoclay forms harder In many applications, tensile analysis (DMA) experiments were domains in the nanocomposites, while properties such as toughness, overall conducted for the same CN9009/ thiolated organoclays induce a more elongation and Young’s modulus PEGDA/TrPGDA/TMPTMP system, flexible network due to enhanced thiol- are practical indicators regarding including 20 mol% thiol. The storage ene step-growth reaction. material performance. To investigate modulus profiles as a function of Further evidence of this effect the organoclay effects on these temperature with addition of 5 wt% is provided by the glass transition mechanical properties of photopolymer organoclays are shown in Figure 3. The temperature behavior of these thiol- clay composites, tensile elongation incorporation of either nonreactive acrylate systems with addition of experiments were performed CL93A- or C16A-acrylated organoclay polymerizable organoclays into the utilizing DMA for the 20 mol% thiol

38 RADTECH REPORT ISSUE 1 2013 Figure 5 Tensile stress profile of thiolated organoclays Technical Paper Technical

DMA tensile profiles of CN9009/PEGDA/TRPGDA (3:3:4 by mass) mixtures including 20mol% thiol from TMPTMP with increase of organoclay amount. Shown are profiles of the systems (A) with 1 wt%, 3 wt%, and 5 wt% PSH2 thiolated organoclays and (B) with 1 wt%, 3 wt%, and 5 wt% C16A acrylated organoclays. The profile for the neat CN9009/PEGDA/TRPGDA/TMPTMP system is included in each figure for comparison. Samples were photopolymerized with 0.5 wt% DMPA using 365 nm light at 3.6 mW/cm2.

CN9009/PEGDA/TrPGDA/TMPTMP as compared to the neat system. These of nonreactive, acrylated and thiolated composition. Figure 5 shows the different results in tensile behavior organoclays into the thiol-acrylate tensile stress profiles as a function of are primarily due to the difference mixture. This comparison clearly strain until break of the cured films in polymerization mechanisms demonstrates how the organoclays produced with increased amounts of based on the type of reactive groups structure induces different thiolated or acrylated organoclays. in polymerizable organoclays. tensile properties in thiol-acrylate The stress-strain profile for the neat Again, based on facilitated acrylate photopolymer clay composites. While system is also included in each figure homopolymerization by incorporation adding nonreactive organoclay (CL93A) for comparison. By adding thiolated of acrylated organoclays, crosslinking slightly increases the stiffness with organoclays (as shown in Figure 5A), density is increased which induces some decrease in ultimate elongation, ultimate elongation to break for the greater stiffness in the composites. the addition of polymerizable photopolymer clay composites are Thiolated organoclays, on the other organoclays improves either ultimate enhanced while the slope of the hand, produce more flexible polymer elongation (thiolated organoclay) profiles, corresponding to the stiffness networks that are able to withstand or Young’s modulus (acrylated (or Young’s modulus) of the materials, greater deformation by enhancing the organoclay). This behavior also does not significantly change. The thiol-ene reaction that allows more indicates that the interaction between addition of only 3 wt% thiolated homogeneous polymerization with clay surfaces and polymer matrices are organoclay induces an enhancement of reduced crosslinking density based significantly improved by incorporating more than 70% in ultimate elongation. on the step-growth reaction. While reactive groups on the clay surfaces,14,21 Interestingly, the addition of acrylated one acrylate double bond acts as a not only by inducing enhanced clay organoclays significantly increases difunctional group in chain-acrylate exfoliation but also by reacting with the stiffness of the materials, while homopolymerization (which induce the monomers and oligomers during the the overall elongation to break does polymer network), the same double photopolymerization, forming strong not change significantly as shown in bond in a thiol-ene step mechanism covalent bonds between the filler Figure 5B. reacts with thiol only once and, surface and organic polymer networks. By adding 1, 3 and 5 wt% acrylated thus, becoming monofunctional and In addition to tensile strength and organoclay, the Young’s moduli of resulting in lower crosslinking density. elongation, the overall toughness of photopolymer-clay composites increase Figure 6 shows the overall a material is an important parameter 20% with 1 wt% organoclay to as comparison of organoclay effects in indicating energy absorptive capacity much as 80% with 5 wt% organoclay elongation properties by adding 3 wt% and durability of materials in many

ISSUE 1 2013 RADTECH REPORT 39 polymerizable organoclays induces 60 Figure 6 and 80% increases in toughness of the thiol-acrylate mixture compared to Overall comparison of organoclay effects in that of neat system, respectively, while Technical Paper Technical elongation properties nonreactive Cloisite 93A decreases the toughness about 10%. To determine the impact of polymerizable organoclay in the acrylate photopolymerization system without thiol, tensile experiments were performed to evaluate the formulation utilizing the basic acrylate mixture as summarized in Figure 7B. Incorporation of nonreactive CL93A organoclay slightly reduces the toughness again. The addition of the two polymerizable organoclays induces

Comparison of DMA tensile profiles by adding 3 wt% different type of organoclays into CN9009/ a 30 to 40% increase in toughness of PEGDA/TRPGDA (3:3:4 by mass) mixtures including 20mol% thiol from TMPTMP. The profile for the neat CN9009/PEGDA/TRPGDA/TMPTMP system is included for comparison. Samples were the acrylate mixture with thiolated 2 photopolymerized with 0.5 wt% DMPA using 365 nm light at 3.6 mW/cm . organoclay producing the greatest toughness increase. This trend is similar to that observed in the thiol- engineering applications. The normalized to the toughness of the neat acrylate mixtures but the degree of toughness of a material is typically system. Toughness of composites with enhancement is significantly smaller in defined as the overall energy needed 3 wt% of organoclays into thiol-acrylate each organoclay system. to deform the materials to failure mixture, including 20 mol% thiol, are It is also important to note that and is commonly found from the area compared. The average toughness the use of thiolated organoclay also under stress-strain curves in tensile for each composition was obtained increases the elongation of composite experiments.35 Figure 7A shows the based on three independent tensile films significantly, whereas adding toughness as calculated by the area experiments. The addition of either acrylated organoclay basically under the stress-strain curve and C16A-acrylated or PSH2-thiolated induces the highest Young’s modulus.

Figure 7 Tensile stress profile of thiolated organoclays

Comparison of relative toughness of systems without or with addition of 3 wt% different type of organoclays into (A) CN9009/PEGDA/TRPGDA (3:3:4 by mass) acrylate mixtures and (B) CN9009/PEGDA/TRPGDA (3:3:4 by mass) thiol-acrylate mixtures including 20mol% thiol from TMPTMP. Toughness is calculated from the area of stress-strain curves in Figures 5 and 6.

40 RADTECH REPORT ISSUE 1 2013 These results provide opportunity to control the mechanical properties of Figure 8 nanocomposites simply by choosing the organoclay type to control. For instance, Water loss through the composite films as a with significant enhancement in overall function of time Paper Technical toughness of both polymerizable organoclay systems, the use of acrylated organoclay can make materials stiffer while the addition of thiolated organoclay produces more flexible nanocomposites than the neat system. Gas barrier properties for many gases (such as oxygen and water vapor) are often important characteristics in many photopolymer applications (i.e., as coating and film packaging). It is well-known that the addition of plate shape inorganic fillers Water weight loss profiles as a function of time from water vapor permeation tests based on such as silicates and micas can often the ASTM E96 standard with and without addition of 3 wt% organoclay into CN9009/PEGDA/ TRPGDA (3:3:4 by mass) mixtures, including 8mol% thiol from TMPTMP.. Shown are profiles improve the gas barrier properties by normalized by film thickness with 100mm thick films for neat CN9009/PEGDA/TRPGDA/TMPTMP (), CN9009/PEGDA/TRPGDA/TMPTMP with 3 wt% CL Na natural clay (), with 3 wt% CL93A generating a longer diffusion path for organoclay (q), with 3 wt% C16A-acrylated organoclay (∆, ), and with 3 wt% PSH2 thiolated gaseous materials.19 To demonstrate organoclays (, ♦). Samples were photopolymerized with 0.5 wt% DMPA using 250~450 nm light at 18 mW/cm2. The result using Mylar PET (100mm) film ◊( ) is included for evaluation of the impact of organoclay on barrier experimental accuracy. properties, the WVTR through photopolymer clay nanocomposite films, including organoclays, was To investigate the organoclay compared to those of systems with compared. In studying the gas transfer effect on WVTR of photopolymer clay polymerizable organoclays. rate, homogeneity in the thickness systems, 3 wt% of various organoclays WVTR for each system is calculated of the thin film and the absence of were incorporated and compared from the overall water loss after defects are critical to achieve good with neat and natural clay systems. 28 days. This data is summarized in experimental results. The ultimate Figure 8 shows the water loss through Figure 9 after normalization for the cured-film thickness was thus the composite films as a function of film thickness. Each WVTR value is controlled to 100 microns with at time for the CN9009/PEGDA/TrPGDA thus based on a 100 micron thickness. most a 5% error. In addition, to make mixture, including 8 mol% thiol from As shown in Figure 9, WVTR of a thin films that are not easily damaged TMPTMP. The neat system is also 100 micron Mylar film is 4.3 g/m2/day during removal from the mold, the toughness of the cured film needed evaluated for comparison with the with literature value using similar 2 36 to be increased from that observed result for Mylar PET film included to experimental condition at 3.5 g/m /day. in the mechanical testing. To this confirm experimental reproducibility. The experimental value is slightly end, the thiol concentration of the Measurements were performed for higher than the reference values, basic thiol-acrylate formulation was 28 days by recording the weight change but experimental accuracy appears decreased to 8 mol%. The ratio of of each experimental set. As shown in reasonable. Addition of 3 wt% natural acrylic monomer composition was not Figure 8, adding 3 wt% organoclays clay (CL Na) reduces WVTR about changed. To compare the experimental more significantly decreases the rate 16% from the value of the neat reproducibility with an ASTM of water loss through the composite system. Further decrease in WVTR standard, 100 mm PET film (Mylar) film than adding CL Na natural clay, is observed when organoclays are was used as the control system. The but there appears to be no significant incorporated into the thiol-acrylate measured WVTR value of the PET difference between organoclay mixture. Approximately 30% reduction film was compared with the literature systems. Even the system with in WVTR is achieved by adding 3 wt% value obtained based on a similar nonreactive CL93A organoclay exhibits CL93A or polymerizable organoclays, methodology.36 a similar trend in weight change as suggesting that the clay effect on water

ISSUE 1 2013 RADTECH REPORT 41 gas barrier properties of clay Figure 9 nanocomposites are also influenced by the addition of organoclays. Water vapor transmission rate as a function of Both polymerizable and nonreactive

Technical Paper Technical (organo)clay type in the thiol-acrylate systems organoclays improve WVTR of the nanocomposites. While adding 3 wt% natural clay reduces WVTR 16% compared to that of neat system, polymerizable and nonreactive organoclays decrease it about 30%. w

Acknowledgements The authors acknowledge financial support from the Industry & University Cooperative Research Program (I/UCRC) for Photopolymerization Fundamentals and Applications, and the National Science Foundation Water Vapor Transmission Rate (WVTR) as a function of (organo)clay type in the thiol-acrylate systems based on CN9009/PEGDA/TRPGDA (3:3:4 by mass) mixtures including 8mol% thiol (CBET-0933450). from TMPTMP without or with addition of 3wt% different type of organoclay. WVTR is calculated utilizing the results from Figure 8. References 1. Rodriguez F, Cohen C, Ober CK and Archer LA, Principles of Polymer permeability is within the experimental organoclay induces higher thiol-ene Systems; Taylor & Francis, New York, error when clay is organically modified reaction than other organoclays— Ch.9:pp 405-408 (2003). 2. Vaia RA and Giannelis, EP,. Macromol and thereby dispersed with at least which has a significant effect on 30:7990-7999 (1997). an intercalated morphology. The nanocomposite characteristics. 3. Alexandre M and Dubois P, Mater Sci incorporation of thiolated organoclay The addition of only 5 wt% acrylate Eng 28:1-63 (2000). induces decreases in glass transition organoclay (which enhances acrylate 4. Ray SS and Okamoto M, Prog Polym temperature of the composites due homopolymerization) increases Sci 28:1539-1641 (2003). to increased thiol-ene reaction as the tensile modulus 80% with no 5. Uhl FM, Davuluri SP, Wong SC and Webster DC, Chem Mater 16:1135- discussed previously, which may significant changes in elongation and 1142 (2004). increase gas molecule passage through glass transition temperature. Thiolated 6. Beck E, Lokai E and Nissler H, RadTech the composite layer. This WVTR organoclay, on the other hand, with Int North America, Nashville, 160-161 behavior clearly shows that barrier greater thiol-ene polymerization (1996). 7. LeBaron PC, Wang Z and Pinnavaia TJ, properties are not detrimentally induces greater flexibility in the Appl Clay Sci 15, 11-29 (1999) . affected by incorporating thiolated nanocomposites with decreased glass 8. Ogawa M and Kuroda K, Bull. Chem organoclays into thiol-acrylate transition temperature. Adding 3 wt% Soc Jap 70:2593-2618 (1997). systems. The results are particularly thiolated organoclays induces a 9. Wang Z and Pinnavaia T J, Chem interesting because a 30% decrease 70% increase in elongation without Mater 10:1820-1826 (1998). in WVTR is possible by adding only significantly changing Young’s modulus. 10. Decker C, Zahouily K, Keller L, Benfarhi S, Bendaikha T and Baron J, J Mater 3 wt% organoclay. The toughness of nanocomposites, Sci 23:4831-4838 (2002). as measured by the area under 11. Uhl FM, Hinderliter BR and Davuluri Conclusion stress-strain curves, is substantially SP, Mater Res Soc 16:203-208 (2004). Incorporation of organoclay improved by adding polymerizable 12. Tan H, Nie J, J Appl Polym Sci affects polymerization behavior organoclays. The addition of 3 wt% 106:2656-2660 (2007). 13. Owusu-Adom K and Guymon CA, and ultimate nanocomposite acrylated or thiolated polymerizable Polymer 49:2636-2643 (2008). performance, including thermo- organoclays induces 60% and 80% 14. Owusu-Adom K and Guymon CA, mechanical and gas barrier properties increases in toughness, respectively, Macromol 42:180-187 (2009). in photocurable thiol-acrylate while nonreactive organoclay actually 15. Decker C, Polym Int 45:133-141 systems. Incorporation of thiolated decreases toughness. Additionally, (1998). Polymer International

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