This work was supported by NSF CAREER Award CMMI-1350436 as well as the University of Colorado Denver Center for Faculty Development. The authors would like to acknowledge Jac Corless, Eric Losty, and Richard Wojcik for their help in developing fixtures and molds for the synthesis and characterization of these materials. The authors would also like to thank Brandon Mang and Ellana Taylor for their preliminary characterization of the materials.

1. Preparation of Liquid Crystalline Elastomers LCEs
<ol>
	<li>Add 4 g of 4-bis-[4-(3-acryloyloxypropypropyloxy) benzoyloxy]-2-methylbenzene (RM257) into a 30 ml vial. RM257 is a di-acrylate mesogen and is received as a powder. Dissolve RM257 by adding 40 wt% (i.e.,&#160;1.6 g) of toluene and heat to 80 &#176;C on a hot plate. This process typically takes less than 5 min to dissolve the RM257 into a solution. 
	Note: Other solvents can be used to dissolve the RM257, such as dichloromethane (DCM), chloroform, and dimethylformamide; however, toluene was chosen because it allows the monomers to cure at RT without having the solvent evaporate quickly during reaction, while DCM and chloroform could evaporate quickly at RT before the Michael-addition reaction is completed. Dimethylformamide can dissolve RM257 immediately without heating, but requires very high temperatures to remove the solvent (~ 150 &#176;C). Kamal and Park used a combination of DCM and a liquid crystal, CB5, to dissolve RM257.31</li>
	<li>Cool the solution to RT. Add 0.217 g of pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), a tetra-functional thiol crosslinking monomer, and 0.9157 g of 2,2-(ethylenedioxy) diethanethiol (EDDET), a di-thiol monomer. The molar ratio of thiol functional groups between PETMP and EDDET is 15:85. This ratio will be referred to as 15 mol% PETMP throughout the study. 
	Note: If the RM257 recrystallizes during this process, temporarily place the vial back onto the 80 &#176;C hot plate until the monomer returns to solution. Cool the solution to RT before proceeding to the next steps.</li>
	<li>Dissolve 0.0257 g of (2-hydroxyethoxy)-2-methylpropiophenone (HHMP) into the solution. HHMP is a photoinitiator used to enable the second-stage photopolymerization reaction. This step can be skipped if the second-stage reaction will not be utilized.</li>
	<li>Prepare a separate solution of a catalyst by diluting dipropylamine (DPA) with toluene at a ratio of 1:50. Add 0.568 g of diluted catalyst solution to the monomer solution and mix vigorously on a Vortex mixer. This corresponds to 1 mol% of catalyst with respect to the thiol functional groups. 
	Note: Adding undiluted catalyst, such as DPA, to the solution will likely result in extremely rapid localized polymerization and will prevent manipulation of the polymer solution into the desired mold detailed in the next steps.</li>
	<li>Place the monomer solution in a vacuum chamber for 1 min at 508 mmHg to remove any air bubbles caused by mixing. Perform this step immediately after mixing.</li>
	<li>Immediately transfer the solution into the desired mold or inject the solution between two glass slides. Molds should be manufactured from HDPE. The molds do not need to be covered, as the Michael-addition reaction is relatively insensitive to oxygen inhibition.</li>
	<li>Allow the reaction to proceed for at least 12 hr at RT. The solution will begin to gel within the first 30 min.</li>
	<li>Place samples in a vacuum chamber at 80 &#176;C and 508 mmHg for 24 hr to evaporate the toluene. Once completed, the samples should have a glossy white and opaque appearance at RT.</li>
	<li>Repeat the procedure to tailor the ratio of tetra-functional to di-functional thiol monomers in step 1.2 with ratios of 25:75 50:50, and 100:0, respectively. A detailed table of the chemical formulations used for this study is shown in Table 1.</li>
</ol>
2. Kinetics Study of Two-stage Reaction with Real-time Fourier Transform Infrared
<ol>
	<li>Equip a spectrometer with a MCT/B detector and XT-KBr beam splitter.</li>
	<li>Prepare a mixture using the protocol outlined above in the Preparation of LCE section using 0.5 mol% of catalyst with respect to thiol functional groups and 0.5 wt% of photoinitiator. Two initiators were tested separately, 2-2-dimethoxy-2-phenylacetophenone (DMPA) and HHMP. DMPA is a more commonly used initiator, while HHMP is more stable at elevated temperatures.</li>
	<li>Place one drop of LCE mixture between NaCl crystals immediately after mixing using a glass pipette.</li>
	<li>Record spectra at a 2.92 sec sampling interval rate.</li>
	<li>Monitor the conversion of the thiol groups using a peak height profile with the S-H absorption peak at 2,571 cm-1 with a baseline of 2,614 - 2,527 cm-1.</li>
	<li>Monitor the conversion of the acrylate groups using a peak height profile with the C=C absorption peak at 810 cm-1 with a baseline of 829 - 781 cm-1.</li>
	<li>Allow the reaction to proceed under FTIR at RT until the thiol peak height plateaus, showing 100% conversion of thiol groups.</li>
	<li>Upon complete conversion of thiol group, turn on a 365-nm light source equipped with a light guide for 10 min to complete the polymerization of excess acrylates at 350 mW/cm2 intensity, which can be measured by a radiometer photometer.</li>
	<li>Monitor the conversion of the acrylate groups as described in 2.6.</li>
</ol>
3. Dynamic Mechanical Analysis (DMA)
<ol>
	<li>Prepare two glass slides by spraying the surfaces of the slides with a hydrophobic surface agent and rubbing the surfaces with a paper towel until dry.</li>
	<li>Stack slides together such that they are separated with a 1 mm spacer. Spacers can be cut by scoring and breaking a separate glass slide to measure approximately 25.4 mm x 5 mm x 1 mm. Clamp slides together using a binder clip at each end.</li>
	<li>Inject monomer solution between the slides using a glass pipette. This requires approximately 1.5 g of the prepared monomer solution.</li>
	<li>Allow the sample to cure for at least 12 hr according to step 1.7. Separate the glass slides and dry the sample according to Step 1.8.</li>
	<li>Using a razor blade or scissors, cut a rectangular test specimens with dimensions of 30 x 10 x 1 mm3.</li>
	<li>Load the sample properly into a DMA machine. Test the sample in tensile mode, with active length measuring 10 to 15 mm. Take care not to over-tighten the grips on the test sample, as 0.1 N-m is often too much torque when tightening the grips.</li>
	<li>Cycle the sample at 0.2% strain at 1 Hz from -50 to 120 &#176;C at a heating rate of 3 &#176;C/min. Set the force track to 125%.</li>
	<li>Measure the glass transition temperature (Tg) at the peak of the tan &#8706; curve.</li>
	<li>Measure the isotropic transition temperature (Ti) and the lowest point of the storage modulus curve.</li>
	<li>Measure the rubbery modulus, E&#39;r, at Ti+30 &#176;C.</li>
</ol>
4. Strain-to-failure Tests
<ol>
	<li>Prepare an HDPE mold by milling ASTM Type V dog-bone cavities at a depth of 1 mm.</li>
	<li>Using a glass pipette, fill each dog bone cavity until the monomer solution is flush with the top of the mold. Allow the samples to cure and dry according to Steps 1.7 and 1.8.</li>
	<li>Prepare 5 tensile specimens from LCE samples formulated with varying PETMP crosslinker concentrations of 15, 25, 50, and 100 mol%.</li>
	<li>Set two pieces of reflective laser tape 5 to 7 mm apart within the gage length of the specimen.</li>
	<li>Load the specimen into a mechanical tester equipped with a laser extensometer, thermal chamber, and 500 N load cell. Use self-tightening grips to secure the specimens, as specimens will dislodge from wedge grips at high strain values. Align the laser extensometer properly to track the accurate change in length as a function of applied strain.</li>
	<li>Strain the specimens at RT with a displacement rate of 0.2 mm/sec until failure. Define failure by the fracture of the specimen.</li>
	<li>Test additional specimens of 15 mol% PETMP crosslinking agent for strain-to-failure testing as a function of temperature. Test specimens at -40, -30, -20, -10, 0, 10, 22, 40, 60, and 80 &#176;C. Hold all specimens isothermally at the desired test temperature for 10 min prior to testing.</li>
</ol>
5. Shape Fixity and Actuation Tests
<ol>
	<li>Prepare an HDPE custom dog-bone mold with gage length of 25 mm and cross-sectional area of 1 mm x 5 mm.</li>
	<li>Prepare a 15 mol% PETMP monomer solution according to Steps 1.1 to 1.5.</li>
	<li>Using a glass pipette, fill each mold cavity until the monomer solution is flush with the top of the mold.</li>
	<li>Allow the samples to cure and dry according to Steps 1.7 and 1.8.</li>
	<li>Set two pieces of reflective laser tape 5-7 mm apart within the gage length of the specimen. Load the sample according step 4.5. Using a permanent marker, mark a dot in the other side of each piece of reflective tape. Record the length between the dots.</li>
	<li>Strain the specimens at RT with a displacement rate of 0.2 mm/sec to 100%, 200%, 300%, or 400% strain.</li>
	<li>While maintaining the desired strain level, expose the sample to a 365 nm UV light source at an intensity of ~10 mW/cm2 for 10 min by holding a UV Lamp approximately 150 mm from the sample.</li>
	<li>Unload the sample and then heat it above Ti to induce actuation. Allow the sample to cool back to RT and record the length between the dots.</li>
	<li>Calculate fixity using the following equation: 
	<img alt="Equation 1" src="/files/ftp_upload/53546/53546eq1.jpg" width="500" /> 
	where &#949;applied is programming strain before photo-crosslinking (measured by the laser extensometer) and &#949;fixed is the amount of permanent strain after photo-crosslinking (measured by the change in dot displacement).</li>
	<li>Cut a 30 mm length sample from the center portion of the programmed specimen.</li>
	<li>Load the sample properly into a DMA tester. Test the sample in tensile mode, with active length measuring 13 to 15 mm. Make sure not to over-tighten the grips on the test coupon.</li>
	<li>Equilibrate the sample at 120 &#176;C under a preload of 0 N. Cool the sample from 120 to -25 &#176;C at a rate of 3 &#176;C/min. Maintain the pre-force at 0 N for the entire test.</li>
</ol>

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R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanical+properties+of+monodomain+side+chain+nematic+elastomers.">Mechanical properties of monodomain side chain nematic elastomers.</a> Eur Phys J E. 14 (4), 311-321 (2004).</li><li>Behl, M., Lendlein, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Actively+moving+polymers.">Actively moving polymers.</a> Soft Matter. 3 (1), 58-67 (2007).</li><li>Rousseau, I. A., Mather, P. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape+memory+effect+exhibited+by+smectic-C+liquid+crystalline+elastomers.">Shape memory effect exhibited by smectic-C liquid crystalline elastomers.</a> J Am Chem Soc. 125 (50), 15300-15301 (2003).</li><li>Sauter, T., Heuchel, M., Kratz, K., Lendlein, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantifying+the+Shape-Memory+Effect+of+Polymers+by+Cyclic+Thermomechanical+Tests.">Quantifying the Shape-Memory Effect of Polymers by Cyclic Thermomechanical Tests.</a> Polym Rev. 53 (1), 6-40 (2013).</li><li>Clarke, S. M., Hotta, A., Tajbakhsh, A. R., Terentjev, E. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+cross-linker+geometry+on+equilibrium+thermal+and+mechanical+properties+of+nematic+elastomers.">Effect of cross-linker geometry on equilibrium thermal and mechanical properties of nematic elastomers.</a> Phys Rev E. 64 (6), (2001).</li><li>Ahir, S. V., Tajbakhsh, A. R., Terentjev, E. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-Assembled+Shape-Memory+Fibers+of+Triblock+Liquid-Crystal+Polymers.">Self-Assembled Shape-Memory Fibers of Triblock Liquid-Crystal Polymers.</a> Adv Func Mater. 16 (4), 556-560 (2006).</li></ol>

The authors have nothing to disclose.

Main-chain LCEs have been investigated for numerous potential applications ranging from actuators and sensors to artificial muscles. Unfortunately, synthesis and monodomain alignment remain significant challenges that prevent many of these applications from being fully realized.11 Recent work has explored new methods to help overcome these challenges, such as using exchangeable crosslinks to be able to re-program an aligned monodomain multiple times.33 The purpose of this study was to present a relatively unexplored approach to LCE synthesis and monodomain programming using a two-stage TAMAP reaction. The first-stage reaction is a &#34;click&#34; reaction based on a thiol-acrylate Michael addition using an amine catalyst. Due to the nature of this reaction, full conversion of thiol-acrylate Michael addition reaction was accomplished within 5 hr at RT using DPA as a catalyst (Figure 2). It is important to note that this was achieved with commercially available materials without purification and using a relatively simple &#34;mix-and-pour&#34; method. 0.5 mol% of DPA with respect to thiol functional groups was chosen in this study for the control it gave over polymerization rate, allowing transferring the monomers solution into the mold. It is very important to note that the polymerization rate of Michael addition is merely dedicated by the catalyst concentration. High catalyst concentration results an immediate gelation with high monomers conversions where is too low catalyst concentration allows slow conversions and often times high conversions cannot be achieved even as a function of time. Ultimately, the polymerization rate can be tuned by the catalyst concentration.34 One of the advantages this methodology offers is that the resulting intermediate polydomain LCE network is uniform and stable, such that the second-stage reaction can be delayed indefinitely. This can enable the synthesis and programming steps to be performed in separate laboratories. Furthermore, the second-stage reaction can be coupled with standard photolithography techniques to provide spatio-temporal control over photo-crosslinking.25 For preparation of our experimental samples, HHMP was used as a photoinitiator because of its stability in present of visible light and at elevated temperatures, allowing for samples to be thermally cycled for stress-driven actuation or the shape-memory effect without triggering the initiator. A separate photo-initiator was used for the FTIR portion of this study, helping illustrate that this methodology has the potential to be used with a variety of free-radical initiators to drive the second-stage reaction.
The presented TAMAP methodology offers facile control over the structure of the initial polydomain LCE network. The four LCE networks synthesized demonstrate a wide range of achievable thermomechanical properties by varying the ratio between the PETMP crosslinker and EDDET spacer. Failure strain decreased with increasing in the concentration of PETMP, while Tg and rubbery modulus (Er) increased with increasing in PETMP concentration. This behavior is explained as an increase in PETMP concentration increases the crosslinking density of the networks and restricts chain mobility within the network. The strain-to-failure behavior follows the inherent inverse relationship between rubbery modulus and failure strain as shown in other amorphous shape-memory polymer (SMP) networks.35 LCE systems with high failure strains are generally more desirable, as they allow for increased alignment of the monodomain with larger programming strains. The failure strain of our 15 mol% PETMP system was maximized when strained near Tg, as measured by the peak of tan &#8706;. This is also in good agreement with previous studies that demonstrated the maximum strain in amorphous SMP networks occurred between the onset of the glass transition and Tg;35,36 however, the LCE samples did not experience a rapid decrease in failure strain when heated above Tg, as shown in most elastomers.37 This can be attributed to the elevated tan &#8706; region that exists between Tg and Ti (i.e., the nematic phase). Previous groups have investigated and verified the unique tan &#8706; loss behavior in nematic LCE networks.38,39 This loss behavior is attributed to the soft-elasticity in the nematic phase, such that the anisotropic shape of the mesogens can accommodate strains by rotation without experiencing an increase in stress.
LCEs have generated a lot of scientific interest due to their stimuli-responsive shape-changing capabilities.40 Polydomain LCE samples can be programmed to demonstrate reversible stress-driven thermal actuation if stretched under a constant stress to program a temporary monodomain (Figure 4b-c); however, the shape-memory effect can also be realized in LCE networks.19,41 In this study, the TAMAP synthesized LCE samples could be programmed for shape memory at RT, in which a significant amount of strain remained stored in the sample even though the sample was above Tg. To enable stress free or &#34;hands-free&#34; actuation, the second stage photopolymerization reaction can be used to program a permanently aligned monodomain in stretched LCE samples. The efficiency of the second stage reaction can be examined by measuring fixity as a function of increasing stretch. It should be noted that fixity is a common metric used to evaluate the programming of SMP networks.42 In this study, samples were programed at different levels of strain (i.e.,&#160;100%, 200%, 300%, and 400%) and showed excellence fixity over 90%. Our results demonstrated that the magnitude of thermal actuation scaled with programming strain are in good agreement with previous results that link an increase in order parameter to increased mechanical actuation.43 For example, LCE samples that were photo-cured at 400% strain demonstrated on average 115% actuation when heated and cooled between 22 and 90 &#176;C and 207% actuation when heated and cooled between -25 and 120 &#176;C. Compared to other LCE studies, Ahir et al.44 reported 400% actuation in LC polymer fibers and Yang et al.22 reported 300% to 400% actuation for micro LCE pillars. It is important to note that the present study measures actuation differently from much of the LCE literature, which often calculates strain based on the length of the sample in the isotropic state. In this study, actuation strain is always based on the original length of the synthesized polydomain sample. This is more appropriate for the TAMAP methodology as it provides a more effective measure of the efficiency of the programming strain and photo-crosslinking on both strain fixity and recovery. Regardless, our reported actuation strains are still lower than 400% as reported in other studies. However, this TAMAP reaction is still relatively unexplored and the influence of photo-crosslinking has yet to fully be uncovered. While photo-crosslinking is necessary to fix a permanent monodomain, too much photo-crosslinking will prevent actuation from occurring. Theoretically, there should exist an optimal amount of photo-crosslinking to both stabilize the monodomain and allow for maximum actuation. Overall, the TAMAP methodology provides a powerful tool to synthesize LCE systems, tailor their structure, program permanent monodomain alignment, and ultimately explore this fascinating class of materials.

LCEs are a class of stimuli-responsive polymers that are capable of exhibiting mechanical and optical functionalities due the combination of liquid-crystalline (LC) order and rubber elasticity. These materials can demonstrate extraordinary changes in shape, soft-elasticity behavior, and tunable optical properties in response to a stimulus such as heat or light,1-3 which makes them suitable for many for potential technological applications such as artificial muscles,4,5 sensors,6 and actuators.6,7 LCEs have already been demonstrated in many applications such as micro-grippers for robotics,8 micro-electromechanical systems (MEMS),6,9 optical grating devices,10 tunable apertures,6,11 and microfluidic systems.12
The structural components that give rise to the ordered LC phases are called mesogens. Mesogens are the basis of the LC domains and are typically composed of two or three linearly connected aromatic rings with flexible ends. These moieties can be directly placed within the polymer backbone to create main-chain LCEs or as a side group (i.e.,&#160;side-on or end-on LCEs).1,13 Main-chain LCEs have generated a lot of interest due to their direct coupling between mesogenic order and polymer backbone conformations.4,14-17 This direct coupling allows main-chain LCEs to exhibit higher degrees of mesogen orientation, mechanical anisotropy, and strain actuation.17
Thermal actuation of LCEs relies on a reversible anisotropic-isotropic transition associated with LC order.2 To program an LCE for actuation, the mesogens must first be oriented along a director to form a monodomain (i.e.,&#160;anisotropic mesophase) and is often referred to as a liquid single-crystal elastomer. Actuation occurs as an aligned LCE is heated above an isotropic clearing temperature (Ti), which disrupts the order of the mesogens into an isotropic state and drives shape change. A monodomain can be formed temporarily by applying an external stress (i.e., hanging a weight) to a sample, which will align the polymer chains and orient the mesogens in the direction of the stress. Permanent programming of the monodomain can be achieved via a multi-step process, which involves producing a lightly cross-linked gel followed by immediate application of mechanical stress to induce orientation of the mesogens. Once aligned, the reaction is continued to established covalent crosslinks and stabilize the monodomain.18 Other &#34;one pot&#34; alignment techniques can be performed in the presence of electric fields or by surface alignment (i.e., rubbing polyimide on a glass slide) during polymerization; however, these methods are generally limited to thin film samples.1,16
Finkelmann and Bergmann introduced the first synthetic route for the preparation main-chain LCEs using one-step platinum-catalyzed hydrosilylation reaction of a divinyl mesogen and a tetra-functional siloxane crosslinker.15 This method has been widely adapted by many research groups to synthesize main-chain LCEs.17,19,20 Polyesterification and epoxy-based reactions have also been used to make main-chain LCEs.21 All of these methods require high purity starting materials and careful experimental conditions to prevent side reactions.1 Furthermore, these methods rely on random cross-linking of the monomers, resulting in poorly defined network structure. Therefore, it is more difficult to correlate the structure to the properties of LCEs. Recent studies have used click chemistry as a tool to prepare more uniform LCE networks; however, these reactions require custom-synthesized starting mesogenic and thiol monomers, which can be challenging to produce, and have been limited to prepare micron-sized actuators rather than bulk samples.22-24
Current challenges in the LCEs focus on how to develop synthetic methods that are facile, reproducible, and scalable to design tailored LCE networks with programmable monodomains. Recently, our group introduced a two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) methodology for the first time in mesomorphic systems to prepare nematic main-chain LCEs.25 Two-stage TAMAP reactions form dual-cure polymer networks, where the staging of the polymerization process allows modification of the polymer structure at two distinct time points. This strategy has been adapted in the past few years to design and fabricate other advanced materials, other than mesomorphic systems, such as micro-actuators,26 shape-memory polymers,27,28 and surface wrinkles.29,30 The TAMAP methodology utilizes a non-stoichiometric composition with an excess of acrylate functional groups. The first stage reaction is used to create a polydomain LCEs via the thiol Michael-addition reaction, which is self-limited by the thiol groups. This is an intermediate LCE network that would be capable of mesogenic domain orientation by applying mechanical stress. The polydomain resulting from the first-stage Michael-addition reaction is indefinitely stable and the alignment of the monodomain does not need to occur immediately after the reaction has completed. The second-stage photopolymerization reaction between excess acrylate groups is used to permanently fix an aligned monodomain and program the LCE for reversible and stress-free (i.e., &#34;hands free&#34;) actuation. The purpose of this study is to explore and demonstrate the robust nature of the TAMAP reaction to prepare main-chain LCEs by investigating the influence of crosslinking density and programed strain on the thermomechanics of the LCE systems. We demonstrate a wide range of thermomechanical properties and actuation performance that are achievable using this reaction.

<table><tbody><tr><td>1,4-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene;&amp;nbsp; &lt;strong&gt;RM257&lt;/strong&gt;</td><td>Wilshire Technologies</td><td>174063-87-7</td><td>Di-Acrylate Mesogen&amp;nbsp;</td></tr><tr><td>2,2&amp;rsquo;-(Ethylenedioxy) diethanethiol;&amp;nbsp; &lt;strong&gt;EDDET&lt;/strong&gt;</td><td>Sigma Aldrich</td><td>465178</td><td>Di-Thiol Spacer&amp;nbsp;</td></tr><tr><td>Pentaerythritol tetrakis (3-mercaptopropionate); &lt;strong&gt;PETMP&lt;/strong&gt;</td><td>Sigma Aldrich</td><td>381462</td><td>Tetra-Thiol Crosslinker&amp;nbsp;</td></tr><tr><td>Dipropylamine; &lt;strong&gt;DPA&lt;/strong&gt;</td><td>Sigma Aldrich</td><td>D214752</td><td>Catalyst</td></tr><tr><td>2-Hydroxy-4&amp;prime;-(2-hydroxyethoxy)-2-methylpropiophenone; &lt;strong&gt;HHMP&lt;/strong&gt;</td><td>Sigma Aldrich</td><td>410896</td><td>Photoinitiator&amp;nbsp;</td></tr><tr><td>2-2-dimethoxy-2-phenylacetophenone; &lt;strong&gt;DMPA&lt;/strong&gt;</td><td>Sigma Aldrich</td><td>196118</td><td>Photoinitiator</td></tr><tr><td>Toluene</td><td>Sigma Aldrich</td><td>244511</td><td>Solvent&amp;nbsp;&amp;nbsp;</td></tr></tbody></table>

synthesis of programmable main-chain liquid-crystalline elastomers using a two-stage thiol-acrylate reaction

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

In this study, the two-stage TAMAP reaction cure kinetics were investigated using real-time FTIR. An FTIR series study on the conversion of the thiol and acrylate groups as a function of time to capture both the stages of the reaction was implemented and the normalized results are shown in Figure 2A. The first-stage thiol-acrylate Michael-addition reaction was initiated via base catalysis using DPA as the catalyst and results in the formation of a crosslinked polymer network. At the end of this initial reaction, the thiol functional groups achieve close to 100% conversion within 5 hr of under ambient conditions (~22 &#176;C), while the acrylate groups attained between 70% to 78% conversion under the same conditions. The thiol-acrylate Michael addition &#39;click&#39; reaction is self-limiting in nature and can generate a step-growth, crosslinked, stable network in a facile manner based on the relative ratios of functional groups present. Subsequently, the second-stage photopolymerization reaction was initiated via exposure to UV irradiation and the remaining unreacted acrylate groups present within the network were further crosslinked to achieve a final acrylate functional group conversion near 100%. Two photoinitiators, HHMP and DMPA and their reaction kinetics were studied within the polymer networks and both were seen to efficiently create crosslinked acrylate networks at the end of second stage polymerization. The conversion of the acrylate groups as a function of the intensity of exposure was also studied and seen to correlate. Overall, it was observed that though a number of variables such as photoinitiators and exposure times could be varied, it was possible to efficiently attain high final conversion of the acrylates at the end of the second stage within 10 min even with relatively low levels of UV intensity (~10-25 mW/cm2 compared to 350 mW/cm2). Figure 2B shows the FTIR absorbance spectra of the two-stage reaction at 3 different time points, 0, 300, and 320 min. At time 0, the initial spectra captures the presence of both thiol and acrylate functional groups in their unreacted state. At the 300 min&#160;time point, by the end of the first-stage thiol-Michael addition reaction, the thiol and acrylate peak heights are seen to reduce considerably, thereby implying the reaction between the thiol and acrylate functional groups has progressed to completion. The thiol peak is measured to be close to 100% conversion at this point, whereas the acrylates are seen to be consumed up to 78%. The complete disappearance of the thiol peak is not observed, most likely as the presence of the thiol-Michael adduct from the first-stage reaction is seen to appear and overlap with the thiol peak at 2,571 cm-1. At the end of the second-stage photopolymerization reaction initiated via UV exposure, at the 320 min&#160;point, the acrylate conversion is seen to proceed to completion, implying 100% conversion of remaining acrylic double bonds within the network.32
The two-stage TAMAP methodology provides facile control to explore structure-property relationships in LCEs. The influence of crosslinking density on stress-strain behavior is shown in Figure 3A. Modulus and fracture stress were shown to increase with increasing PETMP content, while failure strain increased with decreasing PETMP content (Figure 3B). LCE samples with 50 and 100 mol% PETMP demonstrated initial elastic loading followed by a stress plateau and sharp increase in stress due to chain alignment. In comparison, samples with 15 and 25 mol% PETMP appeared to demonstrate more traditional elastomeric loading followed by an increase of stress due to chain alignment. All specimens tested showed a transition from white opacity to clear transparency when stretched (Figure 3E). It should be noted that all specimens maintained a large degree of permanent strain after fracture and did not recover to their original shape at RT; however, all specimens visually recovered to their original shape upon heating above Ti. The influence of temperature on failure strain was then investigated for the 15 mol% PETMP composition (Figure 3C). In the glassy state, LCE specimens exhibited brittle failure with no appreciable deformation. At the onset of the glass transition, the failure strain increased significantly and followed the general shape of the tan &#948; function measure by DMA. The failure strain reached a maximum of 650% strain at 10 &#176;C. Representative glass transition behavior for the four LCE network systems is shown in Figure 3D. All of the LCE networks displayed non-traditional behavior in both the storage modulus and tan &#948; curves. The storage modulus of all LCE networks displayed a distinct minimum that was roughly associated with Ti. The tan &#948; functions were represented by an initial peak followed by an elevated region that diminished as the sample was heated into the isotropic state (a representative curve can be seen in Figure 3C). For the four LCE systems tested, both Tg and rubbery modulus increased with increasing crosslinking density. A summary of thermo-mechanical properties of the four LCE systems can be seen in Table 2.
LCEs offer the ability to demonstrate both the shape-memory effect and reversible actuation (Figure 4). An unaligned polydomain specimen of 15 mol% PETMP was used to illustrate the different shape-switching pathways that can be programmed into the material (Figure 4A). Reversible stress-driven actuation is demonstrated by the pathway in Figure 4A-B-C. The polydomain specimen is stretched by hanging a 60.6 mN weight to apply a constant stress. This bias stress mechanically orients the mesogens into a transparent monodomain. The specimen contracts when heated to the isotropic state and elongates when cooled below Ti. This process can be repeated indefinitely. The shape-memory effect was exhibited when the bias stress was removed from the specimen when cooled below Ti to 22 &#176;C, which is still 18 &#176;C above Tg. While some elastic recoil was observed, a majority of the strain remained programmed into the material. It should be noted that the mesogens remained in a stable monodomain orientation, and there is a noticeable difference in optical properties within the free end of the sample where the clamp was attached (i.e.,&#160;the gripped portion remained glossy white). Heating the sample above Ti activated full shape recovery, indicating the shape-memory cycle follows the pathway of Figure 4A-B-D-E. The second-stage photopolymerization reaction can be used to achieve stress-free actuation without the need for a constant bias-stress or programming step between cycles. The temporarily aligned specimen was photo-cured using 365 nm light at ~10 mW/cm2 for 10 min&#160;(Figure 4F). The sample experienced minimal elastic recoil when unloaded due to the establishment of covalent crosslinks between the excess of unreacted acrylate groups (Figure 4G). Stress-free actuation was then activated by controlling the temperature about Ti using the reversible pathway in Figure 4G-H; however, it should be noted that the sample does not experience full recovery back to the initial shape of the specimen.
The influence of applied programming strain (i.e.,&#160;strain during photopolymerization) as function of fixity and actuation for the 15 mol% PETMP system is shown in Figure 5A. All specimens demonstrated fixity values higher than 90%. The amount of programming strain did not noticeably influence the fixity values for the strain range tested in this study. Conversely, actuation strain increased linearly with the amount of programming strain. On average, the actuation strain corresponded to approximately 30% of the programming strain value. Representative curves showing actuation as a function of temperature can be seen in Figure 5B. It should be noted that the actuation strain values in Figure 5A correspond to measurements between RT, 22 &#176;C, and 90 &#176;C, while the behavior shown in Figure 4B was monitored between -25 and 120 &#176;C. This expanded temperature range caused additional actuation strain to be realized: 80%, 102%, 125%, and 207% actuation strain for samples programmed at 100%, 200%, 300%, and 400% strain, respectively.
<img alt="Figure 1" src="/files/ftp_upload/53546/53546fig1highres.jpg" width="700" /> 
Figure 1. Schematic of Monodomain Programing via a Two-Stage Thiol-Acrylate Reaction. (A) A diacrylate mesogen (1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene &#8212; RM 257), dithiol flexible spacer (2,20-(ethylenedioxy) diethanethiol &#8212; EDDET), and tetra-functional thiol crosslinker (pentaerythritol tetrakis(3-mercaptopropionate) &#8212; PETMP) were selected as commercially available monomers. Non-equimolar monomer solutions were prepared with an excess of 15 mol% acrylate functional groups and allowed to react via a Michael addition reaction. Dipropyl amine (DPA) and (2-hydroxyethoxy)-2-methylpropiophenone (HHMP) were added as the respective catalyst and photo-initiator to the solutions. (B) Representative polydomain structure forms via Michael addition (first stage) with a uniform cross-link density and latent excess acrylate functional groups. (C) A mechanical stress is applied to the polydomain samples to orient the mesogens into a temporary monodomain. (D) A photopolymerization reaction (second stage) is used to establish crosslinks between the excess acrylate groups, stabilizing the monodomain of the sample. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53546/53546fig1highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53546/53546fig2highres.jpg" width="700" /> 
Figure 2. Kinetics Study of Michael Addition Reaction with Real-Time FTIR. (A) Representative two-stage thiol-acrylate reaction kinetics showing conversion as a function of time using DMPA photoinitiator. At the end of first stage, the thiol groups reached near 100% conversion while 22% of acrylate groups were unreacted. At the end of the second stage, unreacted acrylates reached 100% conversion. (B) FTIR absorbance spectra showing the thiol and acrylate conversion before curing at time 0, upon completion of the first stage at 300 min, and upon completion of the second stage at 320 min. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53546/53546fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53546/53546fig3highres.jpg" width="700" /> 
Figure 3. Thermomechanics of TAMAP LCE Systems. (A) Representative strain-to-failure curves of four LCE systems with 15 mol% excess acrylate and varying amount of PETMP crosslinker. (B) Failure strain as a function of PETMP crosslinker. (C) The influence of temperature on failure strain for an LCE system with 15 mol% PETMP. The failure strain is compared alongside the tan &#8706; function of the material measured by DMA. (D) Representative glass transition behavior of four LCE systems tested. (E) Image of a stretched LCE specimen with 15 mol% PETMP compared to an untested specimen. Error bars in (B) and (C) represent standard deviation. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53546/53546fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53546/53546fig4highres.jpg" width="700" /> 
Figure 4. Shape-Switching Pathways in an LCE. This schematic represents several different pathways available to achieve shape switching in LCEs. A custom dog-bone sample of 15 mol% PETMP is used in this demonstration with an initial shape of (A). Reversible stress-driven actuation is realized between (B-C) by adjusting the temperature about Ti while under a constant bias force (60.6 mN); the shape-memory effect is achieved by following the programming and recovery cycle of (A-B-D-E); and stress-free actuation can be activated thermally between (G-H) after a permanent monodomain has been programmed into the sample in step (F). The legend illustrates mesogen orientation in polydomain, monodomain, and isotropic states. T &#60; Ti and T &#62; Ti images were taken at 22 and 90 &#176;C, respectively. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53546/53546fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53546/53546fig5highres.jpg" width="700" /> 
Figure 5. Thermomechanical Response in Programmed-Monodomain LCE Systems: (A) Shape fixity represents the efficiency of permanently aligning monodomain and all of samples show fixity above 90%. The magnitude of actuation measured between 22 and 90 &#176;C on a hot plate. Error bars represent standard deviation. (B) The magnitude of actuation measured on DMA from -25 to 120 &#176;C, the actuation increase with increasing of applied programming strain. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/53546/53546fig5highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Name</td>
			<td>RM 257 (g)</td>
			<td>Toluene (g)</td>
			<td>PETMP (g)</td>
			<td>EDDET (g)</td>
			<td>HHMP (g)</td>
			<td>DPA (g)*</td>
		</tr>
		<tr>
			<td>15 mol% PETMP</td>
			<td>4.0</td>
			<td>1.6</td>
			<td>0.2166</td>
			<td>0.9157</td>
			<td>0.0272</td>
			<td>0.5681</td>
		</tr>
		<tr>
			<td>25 mol% PETMP</td>
			<td>4.0</td>
			<td>1.6</td>
			<td>0.3610</td>
			<td>0.8080</td>
			<td>0.0272</td>
			<td>0.5681</td>
		</tr>
		<tr>
			<td>50 mol% PETMP</td>
			<td>4.0</td>
			<td>1.6</td>
			<td>0.7219</td>
			<td>0.5386</td>
			<td>0.0272</td>
			<td>0.5681</td>
		</tr>
		<tr>
			<td>100 mol% PETMP</td>
			<td>4.0</td>
			<td>1.6</td>
			<td>1.4438</td>
			<td>0.0000</td>
			<td>0.0272</td>
			<td>0.5681</td>
		</tr>
		<tr>
			<td colspan="7">*DPA is diluted in toluene at a 1:50 ratio.</td>
		</tr>
	</tbody>
</table>
Table 1. Chemical Formulations for LCE Systems. Four different LCE systems used in this study. The naming convention is based on the molar ratio of thiol functional groups between PETMP and EDDET. All systems have an excess of 15 mol% acrylate functional groups. It should be noted, FTIR studies tested HHMP as well as DMPA as photoinitiators and reduced the amount DPA catalyst by half to help with the kinetic characterization. *DPA is diluted in toluene at a ratio of 1:50.
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Name</td>
			<td>Tonset (&#176;C)</td>
			<td>Tg (&#176;C)</td>
			<td>Ti (&#176;C)</td>
			<td>E&#39;r (MPa)</td>
		</tr>
		<tr>
			<td>15 mol% PETMP</td>
			<td>-6 &#177; 2</td>
			<td>3 &#177; 1</td>
			<td>62 &#177; 3</td>
			<td>0.18 &#177; 0.01</td>
		</tr>
		<tr>
			<td>25 mol% PETMP</td>
			<td>0 &#177; 2</td>
			<td>7 &#177; 1</td>
			<td>76 &#177; 2</td>
			<td>0.47 &#177; 0.05</td>
		</tr>
		<tr>
			<td>50 mol% PETMP</td>
			<td>8 &#177; 2</td>
			<td>16 &#177; 2</td>
			<td>78 &#177; 1</td>
			<td>0.78 &#177; 0.13</td>
		</tr>
		<tr>
			<td>100 mol% PETMP</td>
			<td>15 &#177; 1</td>
			<td>27 &#177; 1</td>
			<td>64 &#177; 3</td>
			<td>1.90 &#177; 0.13</td>
		</tr>
	</tbody>
</table>
Table 2. Summary of Thermomechanical Properties of LCE Systems. Dynamic Mechanical Analysis (DMA) test shows the thermomechanical properties of the initial polydomain LCE networks formed via the first-stage Michael-addition reaction. Both Ti and E&#39;r were measured at the lowest point of the storage modulus vs. temperature curve.

Watch this Scientific Journal Video about Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction at JoVE.com

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

A novel methodology is presented to synthesize and program main-chain liquid-crystalline elastomers using commercially available starting monomers. A wide range of thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation performance was dependent on the amount of applied strain during programming.

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline ...

synthesis-programmable-main-chain-liquid-crystalline-elastomers-using

Universidad de Cartagena UDEC

Research

JoVE Journal

Chemistry

21.4K Views.  University of Colorado Denver. A novel methodology is presented to synthesize and program main-chain liquid-crystalline elastomers using commercially available starting monomers. A wide range of thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation performance was dependent on the amount of applied strain during programming.

Video: Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

Preparation and Use of Samarium Diiodide (SmI2) in Organic Synthesis: The Mechanistic Role of HMPA and Ni(II) Salts in the Samarium Barbier Reaction

Although initially considered an esoteric reagent, SmI2 has become a common tool for synthetic organic chemists. SmI2 is generated through the addition of molecular iodine to samarium metal in THF.1,2-3 It is a mild and selective single electron reductant and its versatility is a result of its ability to initiate a wide range of reductions including C-C bond-forming and cascade or sequential reactions. SmI2 can reduce a variety of functional groups including sulfoxides and sulfones, phosphine oxides, epoxides, alkyl and aryl halides, carbonyls, and conjugated double bonds.2-12 One of the fascinating features of SmI-2-mediated reactions is the ability to manipulate the outcome of reactions through the selective use of cosolvents or additives. In most instances, additives are essential in controlling the rate of reduction and the chemo- or stereoselectivity of reactions.13-14 Additives commonly utilized to fine tune the reactivity of SmI2 can be classified into three major groups: (1) Lewis bases (HMPA, other electron-donor ligands, chelating ethers, etc.), (2) proton sources (alcohols, water etc.), and (3) inorganic additives (Ni(acac)2, FeCl3, etc).3
Understanding the mechanism of SmI2 reactions and the role of the additives enables utilization of the full potential of the reagent in organic synthesis. The Sm-Barbier reaction is chosen to illustrate the synthetic importance and mechanistic role of two common additives: HMPA and Ni(II) in this reaction. The Sm-Barbier reaction is similar to the traditional Grignard reaction with the only difference being that the alkyl halide, carbonyl, and Sm reductant are mixed simultaneously in one pot.1,15 Examples of Sm-mediated Barbier reactions with a range of coupling partners have been reported,1,3,7,10,12 and have been utilized in key steps of the synthesis of large natural products.16,17 Previous studies on the effect of additives on SmI2 reactions have shown that HMPA enhances the reduction potential of SmI2 by coordinating to the samarium metal center, producing a more powerful,13-14,18 sterically encumbered reductant19-21 and in some cases playing an integral role in post electron-transfer steps facilitating subsequent bond-forming events.22 In the Sm-Barbier reaction, HMPA has been shown to additionally activate the alkyl halide by forming a complex in a pre-equilibrium step.23
Ni(II) salts are a catalytic additive used frequently in Sm-mediated transformations.24-27 Though critical for success, the mechanistic role of Ni(II) was not known in these reactions. Recently it has been shown that SmI2 reduces Ni(II) to Ni(0), and the reaction is then carried out through organometallic Ni(0) chemistry.28
These mechanistic studies highlight that although the same Barbier product is obtained, the use of different additives in the SmI2 reaction drastically alters the mechanistic pathway of the reaction. The protocol for running these SmI2-initiated reactions is described.

Preparation and Use of Samarium Diiodide SmI2 in Organic Synthesis: The Mechanistic Role of HMPA and NiII Salts in the Samarium Barbier Reaction

A straightforward procedure for the preparation of samarium diiodide (SmI2) in THF is described. The role of two main additives namely hexamethylphosphoramide (HMPA) and Ni(acac)2 in Sm mediated reactions is demonstrated in the Sm-Barbier reaction.

Although initially considered an esoteric reagent, SmI2 has become a common tool for synthetic organic chemists. SmI2 is generated through the addition of ...

Amide Coupling Reaction for the Synthesis of Bispyridine-based Ligands and Their Complexation to Platinum as Dinuclear Anticancer Agents

Amide coupling reactions can be used to synthesize bispyridine-based ligands for use as bridging linkers in multinuclear platinum anticancer drugs. Isonicotinic acid, or its derivatives, are coupled to variable length diaminoalkane chains under an inert atmosphere in anhydrous DMF or DMSO with the use of a weak base, triethylamine, and a coupling agent, 1-propylphosphonic anhydride. The products precipitate from solution upon formation or can be precipitated by the addition of water. If desired, the ligands can be further purified by recrystallization from hot water. Dinuclear platinum complex synthesis using the bispyridine ligands is done in hot water using transplatin. The most informative of the chemical characterization techniques to determine the structure and gross purity of both the bispyridine ligands and the final platinum complexes is 1H NMR with particular analysis of the aromatic region of the spectra (7-9 ppm). The platinum complexes have potential application as anticancer agents and the synthesis method can be modified to produce trinuclear and other multinuclear complexes with different hydrogen bonding functionality in the bridging ligand.

This protocol describes the use of amide coupling reactions of isonicotinic acid and diaminoalkanes to form bridging ligands suitable for use in the synthesis of multinuclear platinum complexes, which combine aspects of the anticancer drugs BBR3464 and picoplatin.

Amide coupling reactions can be used to synthesize bispyridine-based ligands for use as bridging linkers in multinuclear platinum anticancer drugs. ...

The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ethylene-propylene rubber, grafted with maleic anhydride, is thermo-reversibly cross-linked in two steps. The pending anhydride moieties are first modified with furfurylamine to graft furan groups to the rubber backbone. These pendant furan groups are then cross-linked with a bis-maleimide via a Diels-Alder coupling reaction. Both reactions can be performed under a broad range of experimental conditions and can easily be applied on a large scale. The material properties of the resulting Diels-Alder cross-linked rubbers are similar to a peroxide-cured ethylene/propylene/diene rubber (EPDM) reference. The cross-links break at elevated temperatures (&#62; 150 &#176;C) via the retro-Diels-Alder reaction and can be reformed by thermal annealing at lower temperatures (50-70 &#176;C). Reversibility of the system was proven with infrared spectroscopy, solubility tests and mechanical properties. Recyclability of the material was also shown in a practical way, i.e., by cutting a cross-linked sample into small parts and compression molding them into new samples displaying comparable mechanical properties, which is not possible for conventionally cross-linked rubbers.

A simple two-step approach involving rubber modification and cross-linking yields fully reworkable, elastic rubber products.

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ...

Constructing Thioether/Vinyl Sulfide-tethered Helical Peptides Via Photo-induced Thiol-ene/yne Hydrothiolation

Here, we describe a detailed protocol for the preparation of thioether-tethered peptides using on-resin intramolecular/intermolecular thiol-ene hydrothiolation. In addition, this protocol describes the preparation of vinyl-sulfide-tethered peptides using in-solution intramolecular thiol-yne hydrothiolation between amino acids that possess alkene/alkyne side chains and cysteine residues at i, i+4 positions. Linear peptides were synthesized using a standard Fmoc-based solid-phase peptide synthesis (SPPS). Thiol-ene hydrothiolation is carried out using either an intramolecular thio-ene reaction or an intermolecular thio-ene reaction, depending on the peptide length. In this research, an intramolecular thio-ene reaction is carried out in the case of shorter peptides using on-resin deprotection of the trityl groups of cysteine residues following the complete synthesis of the linear peptide. The resin is then set to UV irradiation using photoinitiator 4-methoxyacetophenone (MAP) and 2-hydroxy-1-[4-(2-hydroxyethoxy)-phenyl]-2-methyl-1-propanone (MMP). The intermolecular thiol-ene reaction is carried out by dissolving Fmoc-Cys-OH in an N,N-dimethylformamide (DMF) solvent. This is then reacted with the peptide using the alkene-bearing residue on resin. After that, the macrolactamization is carried out using benzotriazole-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate (PyBop), 1-hydroxybenzotriazole (HoBt), and 4-Methylmorpholine (NMM) as activation reagents on the resin. Following the macrolactamization, the peptide synthesis is continued using standard SPPS. In the case of the thio-yne hydrothiolation, the linear peptide is cleaved from the resin, dried, and subsequently dissolved in degassed DMF. This is then irradiated using UV light with photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA). Following the reaction, DMF is evaporated and the crude residue is precipitated and purified using high-performance liquid chromatography (HPLC). These methods could function to simplify the generation of thioether-tethered cyclic peptides due to the use of the thio-ene/yne click chemistry that possesses superior functional group tolerance and good yield. The introduction of thioether bonds into peptides takes advantage of the nucleophilic nature of cysteine residues and is redox-inert relative to disulfide bonds.

We present a protocol for the construction of thioether/vinyl sulfide-tethered helical peptides using photo-induced thiol-ene/thiol-yne hydrothiolation.

Here, we describe a detailed protocol for the preparation of thioether-tethered peptides using on-resin intramolecular/intermolecular thiol-ene ...

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.

This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation ...

Synthesis of Soft Polysiloxane-urea Elastomers for Intraocular Lens Application

This study discusses a synthesis route for soft polysiloxane-based urea (PSU) elastomers for their applications as accommodating intraocular lenses (a-IOLs). Aminopropyl-terminated polydimethylsiloxanes (PDMS) were previously prepared via the ring-chain equilibration of the cyclic siloxane octamethylcyclotetrasiloxane (D4) and 1,3-bis(3-aminopropyl)-tetramethyldisiloxane (APTMDS). Phenyl groups were introduced into the siloxane backbone via the copolymerization of D4 and 2,4,6,8-tetramethyl-2,4,6,8-tetraphenyl-cyclotetrasiloxane (D4Me,Ph). These polydimethyl-methyl-phenyl-siloxane-block copolymers were synthesized for increasing the refractive indices of polysiloxanes. For applications as an a-IOL, the refractive index of the polysiloxanes must be equivalent to that of a young human eye lens. The polysiloxane molecular weight is controlled by the ratio of the cyclic siloxane to the endblocker APTMDS. The transparency of the PSU elastomers is examined by the transmittance measurement of films between 200 and 750 nm, using a UV-Vis spectrophotometer. Transmittance values at 750 nm (upper end of the visible spectrum) are plotted against the PDMS molecular weight, and &#62; 90% of the transmittance is observed until a molecular weight of 18,000 g&#183;mol&#8722;1. Mechanical properties of the PSU elastomers are investigated using stress-strain tests on die-cut dog-bone-shaped specimens. For evaluating mechanical stability, mechanical hysteresis is measured by repeatedly stretching (10x) the specimens to 5% and 100% elongation. Hysteresis considerably decreases with the increase in the PDMS molecular weight. In vitro cytotoxicity of some selected PSU elastomers is evaluated using an MTS cell viability assay. The methods described herein permit the synthesis of a soft, transparent, and noncytotoxic PSU elastomer with a refractive index approximately equal to that of a young human eye lens.

This study describes synthetic routes for aminopropyl-terminated polydimethylsiloxanes and polydimethyl-methyl-phenyl-siloxane-block copolymers and for soft polysiloxane-based urea (PSU) elastomers. It presents the application of PSUs as accommodating an intraocular lens. An evaluation method for in vitro cytotoxicity is also described.

This study discusses a synthesis route for soft polysiloxane-based urea (PSU) elastomers for their applications as accommodating intraocular lenses ...

Synthesis of Monodisperse Cylindrical Nanoparticles via Crystallization-driven Self-assembly of Biodegradable Block Copolymers

The production of monodisperse cylindrical micelles is a significant challenge in polymer chemistry. Most cylindrical constructs formed from diblock copolymers are produced by one of three techniques: thin film rehydration, solvent switching or polymerization-induced self-assembly, and produce only flexible, polydisperse cylinders. Crystallization-driven self-assembly (CDSA) is a method which can produce cylinders with these properties, by stabilizing structures of a lower curvature due to the formation of a crystalline core. However, the living polymerization techniques by which most core-forming blocks are formed are not trivial processes and the CDSA process may yield unsatisfactory results if carried out incorrectly. Here, the synthesis of cylindrical nanoparticles from simple reagents is shown. The drying and purification of reagents prior to a ring-opening polymerization of &#949;-caprolactone catalyzed by diphenyl phosphate is described. This polymer is then chain extended by methyl methacrylate (MMA) followed by N,N-dimethyl acrylamide (DMA) using reversible addition&#8722;fragmentation chain-transfer (RAFT) polymerization, affording a triblock copolymer that can undergo CDSA in ethanol. The living CDSA process is outlined, the results of which yield cylindrical nanoparticles up to 500 nm in length and a length dispersity as low as 1.05. It is anticipated that these protocols will allow others to produce cylindrical nanostructures and elevate the field of CDSA in the future.

Crystallization-driven self-assembly (CDSA) displays the unique ability to fabricate cylindrical nanostructures of narrow length distributions. The organocatalyzed ring-opening polymerization of &#949;-caprolactone and subsequent chain extensions of methyl methacrylate and N,N-dimethyl acrylamide are demonstrated. A living CDSA protocol that produces monodisperse cylinders up to 500 nm in length is outlined.

The production of monodisperse cylindrical micelles is a significant challenge in polymer chemistry. Most cylindrical constructs formed from diblock ...

Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and as soft robots. Here, we demonstrate the preparation of shape-responsive liquid crystal elastomers (LCEs) and LCE nanocomposites along with characterization of their shape-responsiveness, mechanical properties, and microstructure. Two types of LCEs &#8212; polysiloxane-based and epoxy-based &#8212; are synthesized, aligned, and characterized. Polysiloxane-based LCEs are prepared through two crosslinking steps, the second under an applied load, resulting in monodomain LCEs. Polysiloxane LCE nanocomposites are prepared through the addition of conductive carbon black nanoparticles, both throughout the bulk of the LCE and to the LCE surface. Epoxy-based LCEs are prepared through a reversible esterification reaction. Epoxy-based LCEs are aligned through the application of a uniaxial load at elevated (160 &#176;C) temperatures. Aligned LCEs and LCE nanocomposites are characterized with respect to reversible strain, mechanical stiffness, and liquid crystal ordering using a combination of imaging, two-dimensional X-ray diffraction measurements, differential scanning calorimetry, and dynamic mechanical analysis. LCEs and LCE nanocomposites can be stimulated with heat and/or electrical potential to controllably generate strains in cell culture media, and we demonstrate the application of LCEs as shape-responsive substrates for cell culture using a custom-made apparatus.

We demonstrate the preparation of siloxane-based and epoxy-based liquid crystal elastomers (LCEs) and LCE nanocomposites. The LCEs are characterized with respect to reversible strain, liquid crystal ordering, and stiffness. As a potential application, we demonstrate their use as shape-responsive substrates in a custom device for active cell culture.

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and ...

Bioengineering

Fabricating Degradable Thermoresponsive Hydrogels on Multiple Length Scales via Reactive Extrusion, Microfluidics, Self-assembly, and Electrospinning

While various smart materials have been explored for a variety of biomedical applications (e.g., drug delivery, tissue engineering, bioimaging, etc.), their ultimate clinical use has been hampered by the lack of biologically-relevant degradation observed for most smart materials. This is particularly true for temperature-responsive hydrogels, which are almost uniformly based on polymers that are functionally non-degradable (e.g., poly(N-isopropylacrylamide) (PNIPAM) or poly(oligoethylene glycol methacrylate) (POEGMA)). As such, to effectively translate the potential of thermoresponsive hydrogels to the challenges of remote-controlled or metabolism-regulated drug delivery, cell scaffolds with tunable cell-material interactions, theranostic materials with the potential for both imaging and drug delivery, and other such applications, a method is required to render the hydrogels (if not fully degradable) at least capable of renal clearance following the required lifetime of the material. To that end, this protocol describes the preparation of hydrolytically-degradable hydrazone-crosslinked hydrogels on multiple length scales based on the reaction between hydrazide and aldehyde-functionalized PNIPAM or POEGMA oligomers with molecular weights below the renal filtration limit. Specifically, methods to fabricate degradable thermoresponsive bulk hydrogels (using a double barrel syringe technique), hydrogel particles (on both the microscale through the use of a microfluidics platform facilitating simultaneous mixing and emulsification of the precursor polymers and the nanoscale through the use of a thermally-driven self-assembly and cross-linking method), and hydrogel nanofibers (using a reactive electrospinning strategy) are described. In each case, hydrogels with temperature-responsive properties similar to those achieved via conventional free radical cross-linking processes can be achieved, but the hydrazone cross-linked network can be degraded over time to re-form the oligomeric precursor polymers and enable clearance. As such, we anticipate these methods (which may be generically applied to any synthetic water-soluble polymer, not just smart materials) will enable easier translation of synthetic smart materials to clinical applications.

Protocols are described for the fabrication of degradable thermoresponsive hydrogels based on hydrazone cross-linking of polymeric oligomers on the bulk scale, microscale, and nanoscale, the latter for preparation of both gel nanoparticles and nanofibers.

While various smart materials have been explored for a variety of biomedical applications (e.g., drug delivery, tissue engineering, bioimaging, etc.), ...

Synthesis of Biocompatible Liquid Crystal Elastomer Foams as Cell Scaffolds for 3D Spatial Cell Cultures

Here, we present a step-by-step preparation of a 3D, biodegradable, foam-like cell scaffold. These scaffolds were prepared by cross-linking star block co-polymers featuring cholesterol units as side-chain pendant groups, resulting in smectic-A (SmA) liquid crystal elastomers (LCEs). Foam-like scaffolds, prepared using metal templates, feature interconnected microchannels, making them suitable as 3D cell culture scaffolds. The combined properties of the regular structure of the metal foam and of the elastomer result in a 3D cell scaffold that promotes not only higher cell proliferation compared to conventional porous templated films, but also better management of mass transport (i.e., nutrients, gases, waste, etc.). The nature of the metal template allows for the easy manipulation of foam shapes (i.e., rolls or films) and for the preparation of scaffolds of different pore sizes for different cell studies while preserving the interconnected porous nature of the template. The etching process does not affect the chemistry of the elastomers, preserving their biocompatible and biodegradable nature. We show that these smectic LCEs, when grown for extensive time periods, enable the study of clinically relevant and complex tissue constructs while promoting the growth and proliferation of cells.

This study presents a methodology to prepare 3D, biodegradable, foam-like cell scaffolds based on biocompatible side-chain liquid crystal elastomers (LCEs). Confocal microscopy experiments show that foam-like LCEs allow for cell attachment, proliferation, and the spontaneous alignment of C2C12s myoblasts.

Here, we present a step-by-step preparation of a 3D, biodegradable, foam-like cell scaffold. These scaffolds were prepared by cross-linking star block ...