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

The overall goal of this two-stage reaction is to help overcome many of the technical barriers of liquid-crystalline elastomers, such as how to utilize a facile, tailorable, and scalable reaction or how to mechanically program a sample for thorough mechanical actuation. This method gives us a versatile platform to investigate key questions in the field of LCEs as well as make these materials more accessible to researchers without extensive backgrounds in chemistry and synthesis. One advantage of using this technique is that by using thiol and acrylate chemistries, it gives researchers a very easy way to investigate structure, property, performance relationships by simply adjusting the amounts and types of monomers and mesogens used.

The creation of a monodomain in liquid-crystalline elastomer has always been a challenge, especially a relatively thick sample. Generally individuals known to look at crystalline-elastomer will struggle because of the difficulty of the synthesis. Demonstrating this procedure will be undergraduate students Victoria Dorr and Michael Bollinger.

To begin, add 4 grams of RM257 into a 30-milliliter vial. RM257 is a diacrylate mesogen and is received as a powder. Dissolve the RM257 by first adding 40 weight percent of toluene.

Then heat to 80 degrees Celsius on a hot plate. It typically takes less than 5 minutes to dissolve the RM257 into a solution. After cooling the solution to room temperature, add 0.217 grams of the tetrathiol cross-linking monomer PETMP.

The next step is to add the dithiol monomer. In this study, EDDET was chosen over shorter, lower molecular weight dithiols that have an extremely strong odor in the liquid form. Add 0.9157 grams of EDDET to result in a molar ratio of thiol functional groups between PETMP and EDDET of 15 to 85 or 15 mole percent PETMP.

Then dissolve 0.0257 grams of the photoinitiator HHMP into the solution. HHMP is used to enable the second-stage photopolymerization reaction and can be omitted if the second-stage reaction will not be utilized. Prepare a separate solution of a catalyst by diluting dipropylamine or DPA, with toluene at a ratio of 1:50.

Add 0.568 grams of diluted catalyst solution to the monomer solution, which corresponds to 1 mole percent of catalyst with respect to the thiol functional groups. Mix vigorously on a vortex mixer. Avoid adding undiluted catalysts to the solution, as this will likely result in extremely rapid localized polymerization and will prevent manipulation of the polymer solution into the desired molds.

Immediately after mixing, place the monomer solution in a vacuum chamber for 1 minute at 508 millimeters of mercury to remove any air bubbles caused by mixing. 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. Allow the reaction to proceed for at least 12 hours at room temperature. The solution will begin to gel within the first 30 minutes.

Then place the samples in a vacuum chamber at 80 degrees Celsius and 508 millimeters of mercury for 24 hours to evaporate the toluene. Once completed, the samples should have a glossy white and opaque appearance at room temperature. Repeat the procedure to tailor the ratio of tetrafunctional to difunctional thiol monomers with ratios of 25:75, 50:50, and 100:0, respectively.

Prepare an HDPE custom dog-bone mold with gauge length of 25 millimeters and cross-sectional area of 1 millimeter by 5 millimeters. Using a glass pipette, fill each mold cavity with the 15 mole percent PETMT monomer solution until it is flush with the top of the mold. Allow the samples to cure and dry as before.

Next set two pieces of reflective laser tape 5 to 7 millimeters apart within the gauge length of the specimen. Load the specimen into a mechanical tester equipped with a laser extensometer, thermal chamber, and a 500 Newton load cell. Use wedge or self-tightening grips to secure the specimen.

Self-tightening grips can help prevent the sample from dislodging at high strain values. Align the laser extensometer properly to track the accurate change in length as a function of applied strain. Using a permanent marker, mark a dot on the other side of each piece of reflective tape, recording the length between the dots.

Strain the specimens at room temperature with a displacement rate of 0.2 millimeters per second to 100, 200, 300, or 400 percent strain. While maintaining the desired strain level, expose the sample to a 365 nanometer UV light source at an intensity of approximately 10 milliwatts per square centimeter for 10 minutes while holding a UV lamp approximately 150 millimeters from the sample. Unload the sample and then heat it above the isotropic transition temperature, or TI, to induce actuation.

Allow the sample to cool back to room temperature and record the length between the dots. Then calculate fixity using the equation listed in the text protocol. Next cut a 30-millimeter length sample from the center portion of the programmed specimen.

Load the sample properly into a dynamic mechanical analysis or DMA, tester. Test the sample in tensile mode with active length measuring 13 to 15 millimeters, taking care not to overtighten the grips on the test coupon. Equilibrate the sample at 120 degrees Celsius under a preload of 0 Newton, followed by cooling of the sample from 120 to 25 degrees Celsius at a rate of 3 degrees Celsius per minute.

Maintain the preforce at 0 Newtons for the entire test. By tailoring the molar ratio of thiol functional groups between the dithiol and tetrathiol monomers, a wide range of mechanical properties is demonstrated in the materials. The modulus, failure strain, and soft elasticity plateau are all influenced by the amount of tetrafunctional cross-linker present during the first-stage Michael addition reaction.

The glass transition behavior can also be tailored via the molar ratio of tetrathiol cross-linker used during synthesis. The glass transition temperature, pneumatic modulus, and isotropic modulus increase with increasing cross-linker concentration. These materials display unique thermomechanical behavior, in that their tan delta functions remain elevated after the glass transition and throughout the pneumatic region.

The failure strain of these materials as a function of temperature follows the same shape as the tan delta function, reaching a maximum near the glass transition temperature. By utilizing this platform, the fixity, or efficiency of locking the monodomain of the second-stage reaction, can be measured as a function of applied strain. Furthermore, the magnitude of thermal actuation is seen to increase linearly with applied programming strain.

Using the multi-stage thiol acrylate reaction makes it much easier to process and synthesize the LCEs. Visual demonstration of this method is critical to show that anyone can explore these fascinating materials with commercially available mesogens and monomers. After watching this video, you should have a good understanding of how to create your own tailorable liquid-crystalline elastomers and program them for reversible thermal actuation.

Due to the scalable nature of the thiol acrylate reaction, we're able to create small-scale features in large samples that have the potential to be used in biomedical devices. Once learned, a large batch of liquid-crystalline elastomer samples takes about an hour to mix. We let our samples cure overnight, dry the next day, and then they're ready for testing, which is faster than many techniques.

While attempting this procedure, it's important to remember to have excess acrylate groups to enable the second-stage photopolymerization reaction. Of course if you keep the stoichiometric balance of acrylate to thiol groups, you can still synthesize polydomain samples. Following this procedure, other method like photopatterning can be performed in order to specially tailor the mechanical property of the elastomer and control the transition temperature between polydomain, monodomain, and isotropic state.

The technique now opens doors for material science engineers all across the globe and provides the means to explore the mechanical and optical properties of LCEs in a pretty facile manner. Don't forget that working with organic solvents and monomers is hazardous and precautions such as wearing personal protective equipment and working in a hood fan should always be taken when performing this procedure.