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

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 °C), while the acrylate groups attained between 70% to 78% conversion under the same conditions. The thiol-acrylate Michael addition 'click' 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 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 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 T_i. 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 δ function measure by DMA. The failure strain reached a maximum of 650% strain at 10 °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 δ curves. The storage modulus of all LCE networks displayed a distinct minimum that was roughly associated with T_i. The tan δ 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 T_g 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 T_i. This process can be repeated indefinitely. The shape-memory effect was exhibited when the bias stress was removed from the specimen when cooled below T_i to 22 °C, which is still 18 °C above T_g. 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., the gripped portion remained glossy white). Heating the sample above T_i 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 (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 T_i 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., 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 °C, and 90 °C, while the behavior shown in Figure 4B was monitored between -25 and 120 °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.

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 — RM 257), dithiol flexible spacer (2,20-(ethylenedioxy) diethanethiol — EDDET), and tetra-functional thiol crosslinker (pentaerythritol tetrakis(3-mercaptopropionate) — 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. Please click here to view a larger version of this figure.

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. Please click here to view a larger version of this figure.

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 ∂ 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. Please click here to view a larger version of this figure.

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 T_i 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 < T_i and T > T_i images were taken at 22 and 90 °C, respectively. Please click here to view a larger version of this figure.

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 °C on a hot plate. Error bars represent standard deviation. (B) The magnitude of actuation measured on DMA from -25 to 120 °C, the actuation increase with increasing of applied programming strain. Please click here to view a larger version of this figure.

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 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 T_i and E'_r were measured at the lowest point of the storage modulus vs. temperature curve.