Controllable Reversible Addition–Fragmentation Termination

Jan 31, 2014 - The CRAFT monomers employed have highly efficient stress relaxation ... a remarkably enhanced efficacy with the inclusion of relatively...
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Controllable Reversible Addition−Fragmentation Termination Monomers for Advances in Photochemically Controlled Covalent Adaptable Networks Christopher R. Fenoli,† James W. Wydra,‡ and Christopher N. Bowman*,†,‡ †

Department of Chemistry and Biochemistry, University of Colorado Boulder, 215 UCB, Boulder, Colorado 80309-0215, United States ‡ Department of Chemical and Biological Engineering, University of Colorado Boulder, 3415 Colorado Avenue, UCB 596 JSC Biotechnology 530, Boulder, Colorado 80309, United States

ABSTRACT: With the advent of systematically designed controllable reversible addition−fragmentation termination (CRAFT) compounds, we have identified structure−property relationships related to the RAFT compositional structure as it impacts photoplasticity in covalent adaptable networks (CANs). In this study, we have expanded the range and functional capabilities of addition−fragmentation capable network forming monomers by synthesizing and evaluating systematically varying CRAFT monomers with the general formula ABCBA. Subsequent assessment of the impact of these monomers on photoplasticity and stress relaxation was performed. Structural variation of the A and B segments, in particular, imparts increased efficiency and efficacy in stress relaxation and photoplasticity. The CRAFT monomers employed have highly efficient stress relaxation properties demonstrating stress reduction of up to 54% and 75%, respectively, in postpolymerization network photoplasticity experiments. Furthermore, polymerization stress reduction in purely acrylate and acrylate−thiol networks with CRAFT monomers shows a remarkably enhanced efficacy with the inclusion of relatively small amounts of the monomers. With a loading of only 1.5 wt % of the alkene trithiocarbonate monomer in each system more than 75% stress reduction was achieved.

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INTRODUCTION

covalently cross-linked, but contain specific functionalities, either in the network backbone or at cross-link points, which allow the bonded structure of the network to rearrange in response to the application of a specific stimulus13,14 without suffering from the drawbacks of permanent cross-link degradation. Several different chemical architectures have been used to produce CANs including transesterification reactions,15 thermoreversible Diels−Alder structures,16 and radical-mediated addition−fragmentation chain transfer (AFT) reactions.13,17,18 Networks that incorporate AFT-capable moieties have the distinct advantage of undergoing radical-catalyzed bond exchange, where the radicals are generated by one of several means including by exposure to light.17,18 Here, UV light and a traditional radical photoinitiator (2,2-dimethoxy-2-phenylacetophenone) are used to generate radicals at ambient temperature. Under these circumstances, photochemical activation of these CANs enables spatiotemporal control of the network rearrangement that causes the transition from a conventional

In recent years, the ability of adaptable thermoset polymer networks to be smart materials and respond to an external stimulus,1−4 thereby altering the polymer network, has generated considerable interest by affording the ability to induce shape change, self-healing,5−9 photoplasticity, and mechano-patterning10,11 in response to external stimuli within covalently cross-linked polymer networks. Within traditional thermosetting polymers, e.g. elastomers, the cross-linked network of covalent bonds forms a polymer network that is permanent and static in nature. Alternatively, thermoplastics contain no permanent cross-links within the polymer structure and have the ability to flow under appropriately applied stresses. Recently, there has been an increasing interest in dynamic elastomeric materials that have the material properties of cross-linked networks with the capability of undergoing plastic deformation to promote rearrangement of their permanent shape and structure upon application of an appropriate stimulus such as light. Dynamic materials, known as covalent adaptable networks (CANs), bridge this gap by being comprised of cross-linked networks capable of dynamic, internal bond exchange when exposed to the appropriate stimuli.12 Broadly, CANs are polymer networks that are © 2014 American Chemical Society

Received: December 13, 2013 Revised: January 23, 2014 Published: January 31, 2014 907

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Figure 1. Radical-mediated AFT bond-exchange mechanism with a trithiocarbonate moiety internal to the polymer network. Going from left to right, a radical generated initially by light exposure adds across the trithiocarbonate within the cross-linked network structure to form a ternary radical. The ternary radical, I1, then rearranges and fragments, either re-forming the original structures or replacing the original cross-link with a new cross-link, I2.

thioether moieties,21 thiol−ene,31 and methacrylate−silica composites32 have demonstrated the ability to reduce stress levels that are particularly important in alleviating polymerization-induced shrinkage stress in coatings, dental materials, and other thermosetting polymer applications. Radicals that are generated during or after polymerization reduce the shrinkage stress that arises during polymerization33−35 or when external stresses and light are concurrently applied postpolymerization, leading to spatiotemporally controlled creep behavior.13 These prior approaches represent promising developments in the field of stimuli responsive polymers, and yet, they often necessitate long light exposure times, high loading of the AFT-capable moiety, or marginal stress relaxation. The ability to synthesize and implement AFT-capable monomers with systematically varying structures capable of being incorporated into a polymer network utilizing shorter light exposure times, lower AFTcapable moiety loading, and greater stress relaxation is appealing for numerous photochemical network modification strategies. The development of CANs has enabled a thermoset material to flow postpolymerization without negatively impacting material properties and represents a significant advancement in materials science. The ability to improve these materials by increasing process efficiencies, understanding structure−property relationships, and increasing the overall rate of these processes would have a significant impact in the continued development and application of responsive materials. The two primary AFT-capable moieties that have been used to date in CANs are allyl sulfides13 and trithiocarbonates (TTC).29 One interesting class of these species is monomer structures that are broadly represented by the structure C−B−A−B−C. A represents the core of the molecule that is AFT-capable allowing for network rearrangement. B represents a linking group that dictates many of the mechanical and chemical attributes of the polymer, and furthermore; B directly affects the efficiency of the initial AFT process through inductive or resonance stabilizing effects on the fragmenting radical. C represents a functional group that is capable of participating in one of many network forming polymerization reactions. Despite these potential benefits, there have been only a few instances where monomers that are capable of AFT have been (meth)acrylate or bis-norbornene capped, which allows direct incorporation of the AFT-capable moiety into the cross-linked polymer structure.9,11,13,21,23,30,29 Here, (meth)acrylate-functionalized trithiocarbonates and allyl sulfides with systematically varying structures are synthesized and referred to as controlled reversible addition−fragmentation termination (CRAFT)36 monomers; i.e., AFT-capable compounds that have been specifically designed to control the chain transfer reaction process by their specific synthetic structure and leaving radical stability are described, and their efficiency in photoinduced

thermoset-like material into a thermoplastic-like material that flows in response to an applied stress during the irradiation, whether that stress is externally applied or whether it arises due to polymerization shrinkage. The AFT process (Figure 1) involves a radical adding across the double bond of an AFT-capable moiety to form a ternary radical intermediate (I1). This intermediate, which is generally much less reactive to further direct addition to double bonds and radical termination, then cleaves (i.e., fragments) in one of three possible manners. Upon cleavage, the fragmentation reforms a double bond (I2) and another radical species (I3), which subsequently continues the AFT process. In two of the possible fragmentation reactions, the original network bond is cleaved and replaced by a new network bond, promoting relaxation and/or adaptation of the network. This process has been coined “photoplasticity” as a predominantly elastic network is converted into one capable of flowing and exhibiting plastic deformation when and where it is exposed to light.13,14 (Meth)acrylates, allylics, epoxies, thiols, enes, ynes, and numerous other functional groups are all capable of reacting to form networks, and all could be considered as possibilities here.19−23 Epoxy,21 (meth)acrylate,19 norbornene,24 and vinyl ether10,13 functional monomers of this general structure have all been previously made. Monomers of these types have been used in numerous network-forming polymerization reactions including simple radical-mediated reaction of vinyls such as (meth)acrylates,19 thiol−ene reactions with one of many vinyl groups such as norbornenes and allyl ethers,10,13,24 thiol− acrylate Michael addition reactions,25 and traditional epoxy− amine thermosetting reactions.26,27 In particular, for the acrylate monomers containing AFT synthons, a polymer network is formed either by a photocatalyzed radical polymerization or by a base (or nucleophile)-catalyzed thiol−Michael addition reaction. In all cases, the monomers were demonstrated to form adaptable networks mediated either during or after polymerization by radical generation. The incorporation of AFT-capable moieties throughout a polymer network enables numerous photochemical network modification and stress relaxation strategies, all based on the photoplasticity effect. As previously demonstrated,13 when these networks undergo AFT, they flow in response to stresses, whether those stresses are internal and arise due to polymerization induced shrinkage or whether the stresses are externally applied after polymerization. Building on the work of Lai and co-workers,28 Matyjaszewski and co-workers functionalized a carboxy-terminated TTC, thereby enabling radical polymerization with methacrylate functionalities and demonstrated the ability to form CANs that have self-healing properties.9,29 Photoresponsive elastomers have also been used in contact-free mechanopatterning11 and telechelic photopolymers through photocontrolled radical polymerization.30 Epoxy allylic di908

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Scheme 1. Trithiocarbonate (left) and Allyl Sulfide (right) Based CRAFT Monomers Which Are Polymerized into Polymer Networks via Thiol−Michael Addition or Radical Photopolymerization in This Studya

a

Each of the monomers follows the general C−B−A−B−C motif with systematically varying linker and AFT moiety. In order to slow down the reaction to workable rates, this formulation is catalyzed with 1% imidazole in 5 drops of DMSO. For postpolymerization stress relaxation studies, specimens (10 mm × 3 mm × 0.5 mm) for DMA were prepared and strained to 10% strain. The samples were held for 2 min to equilibrate at the desired stress and strain level. Following the 2 min equilibration, the samples were irradiated for 30 min using 365 nm light at 20 mW/cm2. The stress relaxation plots were constructed using a normalized percent stress vs time. This was achieved by measuring the stress relaxation from the time the resins were exposed to the UV light until no more stress relaxation was seen. Determining an exponential fit decay constant for the stress relaxation, which was constructed using OriginPro software, assessed the characteristic time for complete stress relaxation to occur. General Procedure for Thiol−Acrylate Polymerization. Samples were formulated with a 3:1 acrylate:thiol ratio based on the moles of functional groups comprised of bisphenol A ethoxylated diacrylate/PETMP diluted by varying the weight percent of CRAFT monomer from 10% to 1.5% depending on the efficiency of the monomer. All resins contained 0.25% DMPA as photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min. General Procedure for Acrylate Homopolymerization. Samples were formulated with 70 wt % bisphenol A ethoxylated diacrylate, 28.5−20 wt % tetraethylene glycol diacrylate, diluted by varying the weight percent of this overall monomer formulation with the CRAFT diacrylate monomers depending on the efficiency of the monomer. All resins contained 0.25% DMPA as a photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min.

stress relaxation during and postpolymerization is demonstrated (Scheme 1).

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EXPERIMENTAL SECTION

Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) was obtained from Evans Chemetics. Irgacure 651 (2,2-dimethoxy-2-phenylacetophenone, DMPA) was obtained from BASF (formerly Ciba). Tetraethylene glycol diacrylate (TEGDA), containing 150−200 ppm MEHQ as an inhibitor, bisphenol A ethoxylated diacrylate (BisDA), Mn = 668, 250 ppm monomethyl ether hydroquinone as inhibitor, triethylamine, and imidazole were all purchased from Sigma-Aldrich. CRAFT monomers used in this study are depicted in Scheme 1. All chemicals were used without further purification unless otherwise noted. A setup, consisting of a cantilever beam-based tensometer (American Dental Association Health Foundation) coupled with a Fourier transform infrared (FTIR) spectrometer (Nexus 6700, ThermoFisher Scientific), was used to simultaneously monitor shrinkage stress and functional group conversion. The sample (6 mm diameter and 1 mm thickness) is placed between two quartz rods in the tensometer, each of which is treated with a methacrylate functional silane, and irradiated with UV light (Acticure 4000, EXFO) filtered with a 365 nm filter, at 5 mW/cm2 for 10 min. The lower rod is fixed while the upper one is free to move. A LVDT (linear variable differential transformer) measures the deflection of the cantilever beam as the sample polymerizes and shrinks and converts it to a stress measurement. Near-infrared transmitting fiber-optic cables from the FTIR spectrometer monitored the real-time conversion by tracking the acrylate and methacrylate peaks centered at 6165 cm−1, signifying the CC−H stretching overtone. In the postpolymerization stress relaxation studies, the elastic moduli (E′) and glass transition temperatures (Tg’s) of the polymerized samples were measured by dynamic mechanical analysis (DMA, TA Instruments Q800). Stress relaxation properties were evaluated for CRAFT monomers by incorporating them into an elastomeric network containing 1% radical photoinitiator (DMPA). Prepolymerized networks were constructed using a base-catalyzed thiol−Michael “click” reaction by reacting stoichiometric mixtures (1:1 acrylate:thiol functional group ratio) of multifunctional thiol and acrylate-based CRAFT monomers in the presence of a base catalyst. The thiol−Michael gel was catalyzed with 1 wt % triethylamine for all systems, except the phenyl allyl sulfide formulation. Because of the increased stability of the acrylate intermediate during the thiol− Michael reaction, triethylamine catalyzed this reaction spontaneously.

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RESULTS AND DISCUSSION Multifunctional methacrylates and acrylates (i.e., (meth)acrylates) are among the most reactive monomers polymerizable by free radicals and often exhibit a large buildup of internal stress when cross-linked. The inclusion of CRAFT monomers into the polymerization mixture enables the network to rearrange, i.e. adapt, during, and or after, the polymerization to accommodate the shrinkage without developing stress. When monomers containing AFT-capable moieties have been used previously to alleviate stress, they have been used in high overall monomer concentrations or as the entire (meth)acrylic portion of the formulation. In this study, we explore the effects of adding small amounts of the CRAFT monomers to the polymer formulation. The highest levels of the CRAFT monomers added to the systems are 10% relative 909

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Figure 2. Stress reduction in thiol−acrylate photopolymerizations including a trithiocarbonate acrylate containing CRAFT monomers. (A) Stress evolution and (B) acrylate conversion as a function of time. Samples were formulated with a 3:1 acrylate:thiol ratio based on the moles of functional groups from bisphenol A ethoxylated diacrylate/PETMP diluted by varying the weight percent to the overall monomer formulation with the trithiocarbonate diacrylates as follows: 10% alkyl (■), 5% S,S′-bis(isobutyric acid)-trithiocarbonate (●), 1.5% alkene (▲), 2.0% benzyl (×), and the control with no trithiocarbonate (◆). All resins contained 0.25% DMPA as the photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min.

bonate, and 1.5 wt % alkene trithiocarbonate as the CRAFT monomers exhibited only modest reductions in conversion under these conditions. Figure 3 presents the data for the

to the total resin formulations. Furthermore, in this study, we focus on stress relaxation studies in mixed-mode photopolymerizations, where multifunctional thiols are copolymerized with the acrylic monomer systems, chain-growth acrylate homopolymerizations, and postpolymerization stress relaxation of networks formed by thiol−Michael reactions with these monomers. In a mixed-mode thiol−acrylate photopolymerization, in which multifunctional thiols are copolymerized with the acrylic monomer systems, the presence of the thiol leads to extensive chain transfer, which leads to the formation of a more homogeneous cross-linked network. More importantly, the presence of the thiol implies that both carbon-centered radicals and thiyl radicals will be present during the polymerization of thiol−acrylate resins. The thiyl radicals are capable of reacting with allyl sulfides to induce reversible addition−fragmentation, while the carbon-centered radicals are capable of reversible addition−fragmentation with the trithiocarbonate moieties. Thiol−ene photopolymers react via a step-growth reaction mechanism that leads to a uniform polymer network and reduced shrinkage and stress; however, these systems often exhibit reduced mechanical behavior and lower glass transition temperatures relative to conventional (meth)acrylate photopolymers. Hence, a network consisting of both acrylate and thiol functionalities containing the CRAFT monomers results in a polymer network containing benefits of the acrylate and thiol systems along with the potential for a greater reduction of shrinkage stress through incorporation of the CRAFT monomers. Ultimately, studies here focused on polymerization stress for a combination of a core diacrylate monomer (BisDA) in a 3:1 acrylate:thiol ratio based on the moles of the functional groups with the multithiol, PETMP, and with up to 10 wt % of the CRAFT (meth)acrylate monomers (Figures 2A,B and 4A,B). The trithiocarbonate or allyl sulfide concentrations were selected depending upon the performance of the CRAFT agent, designed to achieve significant stress relaxation while minimizing the reduction in (meth)acrylate conversion. As seen in Figure 2, the control BisDA/PETMP system with no trithiocarbonate monomer exhibited shrinkage stress slightly above 1 MPa, whereas each of the systems with trithiocarbonate monomers, even at these low levels, exhibited stresses less than 0.4 MPa. The systems with 10 wt % alkyl tirhtiocarbonate, 5 wt % S,S′-bis(isobutyric acid)-trithiocar-

Figure 3. Stress evolution evaluating acrylate conversion in thiol− acrylate photopolymerizations including a trithiocarbonate acrylate containing CRAFT monomers. Samples were formulated with a 3:1 acrylate:thiol ratio based on the moles of functional groups from bisphenol A ethoxylated diacrylate/PETMP diluted by varying the weight percent to the overall monomer formulation with the trithiocarbonate diacrylates as follows: 10% alkyl (■), 5% S,S′bis(isobutyric acid)-trithiocarbonate (●), 1.5% alkene (▲), 2.0% benzyl (×), and the control with no trithiocarbonate (◆). All resins contained 0.25% DMPA as the photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min.

shrinkage stress as a function of conversion for these trithiocarbonate systems. Here, the pronounced effect of small amounts of the CRAFT monomer on the stress development is best observed, and the relative differences between the various CRAFT monomers are noted. In acrylate homopolymerizations as noted by the data in Figure 3, at higher double bond conversions, dramatic increases in shrinkage stress occur rapidly, associated with the vitrification process in these networks. By allowing for network rearrangement before and after vitrification, the CRAFT monomers exhibit a remarkable ability to alleviate shrinkage stress by promoting network rearrangement well past the gel point, without significantly impacting the acrylate conversion (Figure 3) or mechanical behavior of the resulting polymer. The trithiocarbonates all have enhanced radical stability as compared to the propagating radical, owing to having three 910

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Figure 4. Stress reduction in thiol−acrylate photopolymerizations including allyl sulfide−acrylate containing CRAFT monomers. Samples were formulated with a 3:1 acrylate:thiol ratio based on the moles of functional groups comprised of bisphenol A ethoxylated diacrylate/PETMP with 10 wt % to the overall monomer formulation of the allyl sulfides as follows: 10% alkyl allyl sulfide diacrylate (■), 10% phenyl allyl sulfide diacrylate (▲), and the control with no CRAFT monomer (◆). All resins contained 0.25% DMPA as photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min.

sulfur atoms surrounding the radical intermediate. This increased radical stability promotes the addition−fragmentation reaction and stress evolution by enhancing the reaction rate with the AFT-capable moiety. At as little as 1.5 wt %, the alkene trithiocarbonate monomer shows a remarkable capacity to lower the polymerization stress by 78% with 96% overall acrylate conversion. As shown schematically in Figure 1, we attribute this phenomenon to the intermediate radical stability (I1) and a low activation energy barrier between the leaving allylic (R) radical and the trithiocarbonate radical intermediate (I1). When the activation barrier between the two aforementioned species is increased, the trithiocarbonate loading must be increased to maintain the same polymerization stress relaxation. This behavior is best illustrated by the alkane trithiocarbonate, where 10% loading is required to achieve comparable stress relaxation in comparison to the alkene trithiocarbonate. Moreover, in all the trithiocarbonate monomers, there is a remarkable decrease in the exponential stress growth in relation to conversion when evaluating the last 10% of acrylate conversion in comparison to the control. As the polymerization rate slows toward the conclusion of the polymerization, the relative RAFT chain transfer effectiveness increases, thereby reducing the dramatic stress growth typically seen in the final stages of the polymerization. In the thiol− acrylate polymerization, the aforementioned trend of stress reduction due to the intermediate radical stability and a low activation energy barrier between the leaving radical (R) and the radical intermediate is present in the allyl sulfides as well. In the thiol−acrylate polymerization containing PETMP, the liberation of thiyl radicals allows the allyl sulfides to perform as completely reversible chain transfer agents and exhibit similar behavior as the trithiocarbonate monomers. The phenyl allyl sulfide exhibits increased radical intermediate stability, which is attributed to the presence of the adjacent phenyl groups, and the leaving phenyl thiyl radical stability enhances cleavage of the intermediate. These factors allow for enhanced efficacy of the propagating radical species and control of the stress evolution; hence, the phenyl allyl sulfide allows for 81% stress reduction with 97% acrylate conversion in the thiol−acrylate polymerization. With the loss of the neighboring phenyl groups, as in the case with the alkyl allyl sulfide, where the phenyl groups are replaced by alkyl chains, the activation barrier between the intermediate and the leaving radical (R) increases promoting greater stress evolution at a 10% loading, in comparison to the phenyl counterpart. Since the trithiocar-

Figure 5. Stress evolution evaluating acrylate conversion in thiol− acrylate photopolymerizations including allyl sulfide−acrylate containing CRAFT monomers. (A) Shrinkage stress and (B) acrylate conversion as a function of time. Samples were formulated with a 3:1 acrylate:thiol ratio based on the moles of functional groups comprised of bisphenol A ethoxylated diacrylate/PETMP with 10 wt % to the overall monomer formulation of the allyl sulfides as follows: 10% alkyl allyl sulfide diacrylate (■), 10% phenyl allyl sulfide diacrylate (▲), and the control with no CRAFT monomer (◆). All resins contained 0.25% DMPA as photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min.

bonates and allyl sulfides are completely reversible in the thiol− acrylate system, we have postulated that the phenyl allyl sulfide and alkene trithiocarbonate exhibit the greatest stress reduction, within their respective RAFT core moieties (A), due to the intermediate radical stability and a low activation energy barrier between the leaving (R) radical and the radical intermediate. However, the trithiocarbonates still have an enhanced intermediate radical stability (I1), in comparison to the allyl sulfide radical intermediate, allowing for overall enhanced stress relaxation performance in the alkene trithiocarbonate with respect to the phenyl allyl sulfide. In addition to understanding CRAFT monomer behavior in a thiol−acrylate polymerization in which both thiyl and acrylic radicals are present, the CRAFT monomers were incorporated into (meth)acrylate homopolymerization reactions to understand their capacity for reducing stress in those systems where only the (meth)acrylic radicals exist. In each system containing a CRAFT monomer, a percentage of the reactive diluent, TEGDA, was removed and replaced by the CRAFT monomer. In all in situ polymerizations the relative amount of the CRAFT monomer was adjusted based on the effect of the CRAFT monomer on the polymerization rate and acrylate conversion. 911

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Figure 6. (A) Stress evolution over time. (B) Conversion of monomer over time. Samples were formulated with 70 wt % bisphenol A ethoxylated diacrylate, 28.5−20 wt % tetraethylene glycol diacrylate, diluted by varying the weight percent to the overall monomer formulation with the trithiocarbonate diacrylates as follows: 10% alkyl (■), 5% S,S′-bis(isobutyric acid)-trithiocarbonate (●), 1.5% alkene (▲), 2.0% benzyl (×), and control (◆). All resins contained 0.25% DMPA as photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min.

By balancing the amount of CRAFT monomer in the system, the systems are able to rearrange and relax stress without significantly affecting polymerization rates and overall acrylate conversions. Each of these polymerizations was monitored for simultaneous measurement of the (meth)acrylate conversion and stress relaxation (Figures 6A,B, 7, 8A,B, and 9).

Figure 9. Stress evolution evaluating acrylate conversion in acrylate photopolymerizations including allyl sulfide−acrylate containing CRAFT monomers Samples were formulated with 70% bisphenol A ethoxylated diacrylate, 20% tetraethylene glycol diacrylate, and 10 wt % of the corresponding alkyl allyl sulfide (■), phenyl allyl sulfide (▲), diphenyl allyl sulfide (●), and control (◆). All resins contained 0.25% DMPA as photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min. Figure 7. Stress evolution evaluating acrylate conversion in acrylate photopolymerizations including trithiocarbonate-acrylate containing CRAFT monomers. Samples were formulated with 70 wt % bisphenol A ethoxylated diacrylate, 28.5−20 wt % tetraethylene glycol diacrylate, diluted by varying the weight percent to the overall monomer formulation with the trithiocarbonate diacrylates as follows: 10% alkyl (■), 5% S,S′-bis(isobutyric acid)-trithiocarbonate (●), 1.5% alkene (▲), 2.0% benzyl (×), and control (◆). All resins contained 0.25% DMPA as photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min.

Immediately, it is notable that the magnitude of the shrinkage stress in these systems, and in particular in the control sample comprised of a 70/30 mixture of bisphenol A ethoxylated diacrylate and tetraethylene glycol diacrylate, is much higher than in the thiol−acrylate systems. Here, the control sample achieves a maximum stress of nearly 3 MPa as compared with the slightly above 1 MPa for the thiol−acrylate control in Figure 2. The homopolymerization reaction leads to the formation of a more highly cross-linked system and a higher glass transition temperature network as compared to the thiol−

Figure 8. (A) Stress evolution over time. (B) Conversion of monomer over time. Samples were formulated with 70% bisphenol A ethoxylated diacrylate, 20% tetraethylene glycol diacrylate, and 10 wt % of the corresponding alkyl allyl sulfide (■), phenyl allyl sulfide (▲), diphenyl allyl sulfide (●), and control (◆). All resins contained 0.25% DMPA as photoinitiator and were irradiated at 5 mW/cm2 at 365 nm for 10 min. 912

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Figure 10. Photoresponsive formulation and stress relaxation properties for trithiocarbonate-based networks. The resins were formulated with a 1:1 stoichiometric ratio based on functional groups of PETMP and TEGDA and the RAFT trithiocarbonate at 33 mol % based on functional groups comprising 50% of the acrylate functionalities as follows: alkyl trithiocarbonate (dashed), S,S′-bis(isobutyric acid)-trithiocarbonate (solid), alkene trithiocarbonate (dotted), and benzyl trithiocarbonate (dot-dashed). The RAFT component incorporates trithiocarbonate functional groups into the network strands. Photoinduced stress relaxation (normalized) was performed at 20 mW/cm2 irradiation using 1 wt % DMPA at 365 nm for 30 min.

Figure 11. Photoresponsive formulation and stress relaxation properties for allyl sulfide-based networks. The resins were formulated with a 1:1 stoichiometric ratio based on functional groups of PETMP and TEGDA and the RAFT allyl sulfide at 33 mol % of the functional groups comprising 50% of the acrylate functionalities as follows: alkyl allyl sulfide (solid), phenyl allyl sulfide (dotted), diphenyl allyl sulfide (dashed). Photoinduced stress relaxation (normalized) was performed at 20 mW/cm2 irradiation using 1 wt % DMPA at 365 nm for 30 min.

systems. With loadings of 10 wt % or less, all of the trithiocarbonate monomers decrease polymerization-induced shrinkage stress by 51%−75% and still exhibit a decrease in the exponential stress growth in the final stages of the homopolymerization. The alkene trithiocarbonate exemplifies the superlative structural composition, which allows for increased chain transfer by having an ideal activation energy barrier between the leaving (R) radical and the radical intermediate. In contrast, the allyl sulfides without the ability to be completely reversible lose the ability to efficiently reduce the stress evolution in acrylate homopolymerizations. Most of the allyl sulfides showed only marginal stress reduction in the acrylate homopolymerization and no impact on the exponential stress evolution at the final stages of the polymerization, and yet, by systematic structural variation and lowering the activation energy barrier between the leaving radical (R) and the radical intermediate, the phenyl allyl sulfide was able to reduce 45% of the stress evolution in the homopolymerization. In addition to the benefits of these monomers in regards to reducing polymerization-induced shrinkage stress, polymer networks formed even from a relatively small amount of these CRAFT monomers also exhibit excellent postpolymerization photoplasticity behavior. In particular, we explored the systematic variation of attributes that might control the relaxation behavior in these networks. We focused on the inclusion of relatively small fractions of the CRAFT monomers into networks. Here, we specifically assess the effects of the type

acrylate polymerization that contains some step growth nature. The trithiocarbonates, in contrast to the allyl sulfides, are completely reversible in meth(acrylate) homopolymerization chain transfer processes, allowing these compounds to have a greater impact on stress evolution, but also, this reversibility allows for repeated rearrangement of the forming tangled polymer network to alleviate stress.29,30 As predicted, the trithiocarbonates with their ability to have fully reversible AFT in a meth(acrylate) system, where only carbon-centered radicals are generated, show the greatest reduction in shrinkage stress during (meth)acrylate homopolymerizations. The same structure−activity relationships and trends between the trithiocarbonate monomers, in relationship to the overall stress evolution in the thiol−acrylate polymerizations, are observed in the acrylate homopolymerizations. The distinct difference observed in the acrylate homopolymerizations is the direct impact of the type of CRAFT moiety, as observed in comparing the allyl sulfide and trithiocarbonate monomer additions. In an acrylate homopolymerization, where no thiyl radicals are generated, the allyl sulfide CRAFT monomers are no longer fully reversible and lose their effectiveness at repeated chain transfer and network rearrangement. In addition, the ability of the allyl sulfides to control the exponential stress development at the final stages of the polymerization is lost. However, during the in situ polymerization relaxation studies, the trithiocarbonates, upon irradiation, continue to show a remarkable improvement in the extent of stress relaxation in acrylate 913

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of AFT moiety, the linker, and the loading level of the CRAFT monomer on the relaxation behavior. Stress relaxation properties were evaluated for CRAFT monomers by incorporating them into an elastomeric network containing 1% radical photoinitiator (DMPA). The networks were constructed using a base-catalyzed thiol−Michael “click” reaction by reacting stoichiometric mixtures (1:1 acrylate: thiol functional group ratio) of multifunctional thiols and acrylatebased CRAFT monomers in the presence 1% triethylamine or 0.8% imidazole as a base catalyst. Utilizing this nonphoto, nonradical mediated polymerization mechanism, the photoinitiator is dormant and unconsumed during the thiol−Michaelbased polymerization, remaining in the network for subsequent induction of the desired postpolymerization photoinduced stress relaxation at any point in the future. The CRAFT monomer-containing networks were evaluated on a dynamic mechanical analyzer (DMA) by inducing a strain of 10%, allowing for a 2 min equilibration hold, and then irradiating uniformly at 20 mW/cm2 for 30 min and evaluating the stress relaxation (Figures 10 and 11). The initial time points in Figures 10 and 11 denote the time the light was turned on after a strain of 10% was achieved and the 2 min equilibrium period was implemented. The phenyl allyl sulfide and benzyl trithiocarbonate resins where half of the overall acrylates are derived from the CRAFT monomer at 20 mW/cm2 relieved 75% and 54% of the stress, respectively. It should be noted that in these experiments the residual stress is dictated by a combination of the rate of addition−fragmentation, the effectiveness of each bond shuffling event to promote relaxation within the network, and the continual generation of radicals to initiate the process. In fact, at some point, as indicated by these results, the initiator will be depleted, and little addition−fragmentation will occur due to a lack of radical generation. Here, despite having only a fraction of the network cross-links containing a RAFT-capable moiety, this outcome demonstrates a significant improvement based on increasing the chain transfer efficiency through synthetic modification of the initial CRAFT monomers. In our postpolymerization stress relaxation studies, we hypothesize that the stability of the leaving carbon radical (I3) in the CRAFT systems dominates their ability to relax stress due to the stability of the radical intermediate. Furthermore, as a measure of the rate of the process, a characteristic time for complete stress relaxation to occur was assessed by determining an exponential decay constant for the stress relaxation in each system, as presented in Table 1. The stress relaxation data suggest that as the leaving radical (I3) becomes more stable, the amount of stress relaxation increases (Figure 12). These effects are clearly seen when comparing the less stable alkyl leaving radical to the more stable benzyl radical in both the allylic sulfide and trithiocarbonate systems. Second, the allyl sulfides have an increased amount of stress relaxation with an increased relaxation rate in comparison to their trithiocarbonate counterparts. This phenomenon is explained by two factors. In the allyl sulfide systems, where the radical intermediate is not as stable as its trithiocarbonate counterpart, and the leaving radical is a more stable thiyl radical, not a carbon radical, the system generates more I3 radicals that react to propagate the chain transfer process at an accelerated rate. For postpolymerization stress relaxation, we believe these factors are what allow the allyl sulfides to alleviate a greater amount of stress at an increased rate in comparison to its trithiocarbonate counterpart

Table 1. Relaxation Times Needed To Achieve Complete Network Relaxation Assessed by a Simple Exponential Fit for the Various CRAFT Monomers in Postmodification Thiol−Michael Resinsa RAFT monomer alkyl trithiocarbonate S,S′-bis(isobutyric acid)-trithiocarbonate (Mat TTC) alkene trithiocarbonate benzyl trithiocarbonate alkyl allyl sulfide phenyl allyl sulfide biphenyl allyl sulfide

relaxation time (min)

relaxation (%)

3.1 13

12 30

1.2 2.2 2.3 1.2 10

45 54 52 75 19

a

The resins were formulated with a 1:1 stoichiometric ratio based on functional groups of PETMP, TEGDA, and the RAFT monomer comprising 50% of the acrylates.

Figure 12. Radical stability chart going left to right from the least stable methyl radical to the most stable benzyl radical.

(Table 1). However, in the allyl sulfide diphenyl system, it is hypothesized that further enhanced radical stability of the leaving group (I3) generates a radical that attacks neighboring centers (Cm), so slowly that little relaxation in the network is seen. The stress relaxation rate depends on the overall rate of addition−fragmentation. Unlike in controlled radical polymerization, where the goal is to shift the equilibrium away from the propagating radical, here, the goal is one of kinetics, i.e., have the overall AFT rate proceed as rapidly as possible. To achieve this end, both the addition and fragmentation steps must proceed efficiently without large activation barriers between any of the steps and without the formation of an overly stable intermediate. Thus, it is desirable for the leaving radical to be as reactive as possible but also to have the tricentered radical intermediate possessing an intermediate stability. If the radical intermediate is too unstable and disfavored thermodynamically, then the activation barrier for addition will be too large while if the radical is too stable, fragmentation will proceed too slowly to promote stress relaxation. Interestingly, the phenomenon of stress relaxation past the gel point is seen in all of these systems both during polymerization and in postpolymerization relaxation studies. While a covalent network would exist in nearly every one of these cases even if all the AFT-capable moieties were cleaved, these networks are all capable of significant stress relaxation. This behavior indicates that while a network would exist even in the absence of the cross-links containing AFT-capable moieties, the cleavage and re-formation of these bonds does enable relaxation, of at least some of the remainder of the network. Ultimately, despite the presence of a relatively small fraction of cleavable bonds, the bond shuffling process is imparting mobility to the remainder of the network that allows even those portions of the network that remain chemically coupled to relax and adapt to their new conditions. 914

dx.doi.org/10.1021/ma402548e | Macromolecules 2014, 47, 907−915

Macromolecules

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CONCLUSIONS The CRAFT monomers developed here exhibit excellent poststress and in-situ stress relaxation properties with the remarkable ability to exhibit controllable properties that can be applied to a variety of polymer networks. With the systematic variation of monomer structure, the CRAFT monomers have varying radical stability of the leaving radical (I3), which enables control of AFT/photoplasticity in networks of multiple types and compositions. Furthermore, the concept of control of the leaving radical (I3) enables understanding of the linker variations and CRAFT monomer loading that control stress relaxation during polymerization and in networks. This allows for inclusions of small fractions of CRAFT monomers into otherwise consistent formulations for enablement of significant adaptability and stress relaxation during and after polymerization. This understanding of stress relaxation represents an enablement for enhanced efficiency of these new CRAFT monomers to promote polymer development within a variety of material applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.N.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Industry University Cooperative Research Center for Fundamentals and Applications of Photopolymerizations and NSF CBET 1264298.

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dx.doi.org/10.1021/ma402548e | Macromolecules 2014, 47, 907−915