Polyacrylate Elastomer

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Interpenetrating Liquid Crystal Polyurethane/Polyacrylate Elastomer with Ultrastrong Mechanical Property Hai-Feng Lu, Meng Wang, Xu-Man Chen, Bao-Ping Lin, and Hong Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06757 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Journal of the American Chemical Society

Interpenetrating Liquid Crystal Polyurethane/Polyacrylate Elastomer with Ultrastrong Mechanical Property Hai-Feng Lu, Meng Wang, Xu-Man Chen, Bao-Ping Lin, Hong Yang* School of Chemistry and Chemical Engineering, Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, State Key Laboratory of Bioelectronics, Institute of Advanced Materials, Southeast University, Nanjing 211189, China KEYWORDS. Liquid crystal elastomer; interpenetrating polymer network; actuation; two-way shape memory material

ABSTRACT: Liquid crystal elastomer (LCE) materials, which have been developed and investigated for four decades, still lack real industrial applications. The fundamental obstacle is the modest force of LCEs generated in the LC-to-isotropic phase transition process, which is the most important actuation moment. Here we report an interpenetrating liquid crystal polyurethane/polyacrylate elastomer material, consisting of one main-chain polyurethane LCE and another liquid crystal polyacrylate thermoset networks which are simultaneously polymerized. This two-way shape memory material can reversibly shrink/expand under thermal stimulus, and shows ultrastrong actuation-mechanic properties. With a maximum shrinkage ratio of 86% at 140 oC which is beyond the LC-to-isotropic phase transition, its actuation blocking stress, actuation work capacity, breaking strength and elastic modulus reach 2.53 MPa, 1267.7 KJ/m3, 7.9 MPa and 10.4 MPa respectively. Such a LCE material can lift up a load 30000 times heavier than its own weight. We hope the outstanding mechanical properties of this IPN-LCE material would pave the way for the real industrial utilizations of LCEbased soft actuators.

INTRODUCTION Liquid crystal elastomers (LCEs),1-25 as a typical class of two-way shape memory materials, exhibit large deformation strain and outstanding shape-morphing reversibility which outweigh the performances of other soft actuator materials, such as hydrogels,26 conducting polymers27 and dielectric elastomers,28 and thus have received tremendous scientific attention. However, since the first invention by Finklemann in 1981,1 hundreds of research papers have been published in the past four decades, but LCEs still lack real industrial applications. The fundamental obstacle is the modest force of LCEs generated in the LC-to-isotropic phase transition process, which is indeed the essential actuation moment. For example, inspired by de Gennes’ LCE artificial muscle proposal in 1997,29 many research groups have developed diverse LCE contraction/expansion-actuating systems up till now.2-6 Compared with the real skeletal muscles,30,31 whose actuation stress, strain and elastic modulus are ca. 0.35 MPa, 40% and 10~60 MPa respectively as shown in Figure 1A, these LCE materials though could have large deformation strain,32-34 generated far-below-satisfactory forces, in particular poor elastic modulus showing 0.1~1 MPa beyond the LC-to-isotropic phase transition.35

Figure 1. (A) The actuation stress, strain and elastic modulus data of human skeletal muscles. (B) The chemical compositions of LCPA and LCPU systems of the designed IPN-LCE material. (C) Schematic illustration of the preparation protocol of IPN-LCE film.

To enhance the mechanical property of LCE materials has become one of the most important research objectives of LCE community. From the molecular structure perspective, main-chain LCE system is usually advantageous over side-chain LCE network in providing much higher stress and strain, because compared with the

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lateral attachment manner, direct incorporation of mesogens into the polymeric backbones would provide stronger chain anisotropy. Above the LC-to-isotropic phase transition, the elastic modulus and maximum stress of side-chain LCE materials were recorded in the range of 0.01~0.6 MPa, while those values of main-chain LCEs could reach to 0.5~1.4 MPa.2-5 For example, our group36 reported a two-step acyclic diene metathesis polymerization approach to synthesize a main-chain LCE material with ca. 1.3 MPa elastic modulus above its clearing point. Zhao37,38 and colleagues described a polyester-type main-chain LCE material with ca 1.3 MPa elastic modulus and ca. 0.45 MPa maximum (blocking) stress above the LC-to-isotropic phase transition. Recently, Ware39 applied the crystallization treatment on a main-chain thiol-acrylate LCE material and raised its maximum (blocking) stress and actuation work capacity up to 1.3 MPa and 730.5 kJ/m3.

polyurethane/polyacrylate elastomer material, consisting of one main-chain polyurethane LCE (named as LCPU) and another LC polyacrylate thermoset (named as LCPA) networks which are simultaneously polymerized. This material can reversibly shrink/expand under thermal stimulus, and shows ultrastrong actuation-mechanic properties.

RESULTS AND DISCUSSION

The newly emerged interpenetrating polymer network (IPN) consisting of two separate but interwoven macromolecular matrices, has been regarded as an efficient way to enhance the functional properties of polymeric materials.40,41 In LCE field, Zhao42,43 pioneered in combining IPN concept and liquid crystallinity. Ikeda,44-46 Broer and Schenning,47-50 Firestone,51 Park52 have also developed some interpenetrating liquid crystal elastomers (IPN-LCEs) or hydrogels as smart materials and biosensors. Although the mechanical properties of these IPN-LCE materials could be improved to some extent, in particular at ambient temperature, no breakthrough enhancement has ever been reported, possibly because of two technique issues: (1) all the above IPN-LCE materials were obtained by the sequential polymerization method which first prepared the LCE network, then introduced the second monomer through swelling into the first matrix, and performed photo-polymerization to form the second network. During this process, the swelling treatment might cause a non-uniform dispersion of the second monomer in LCE network and meanwhile decrease the chain anisotropy. (2) All the above IPN-LCE materials were limited to polyacrylate system, most of which were formed by one side-chain polyacrylate LCE and another non-LC polyacrylate interpenetrating networks. Since those non-LC polyacrylates with low glass transition could not contribute on enhancing the chain anisotropy, a scenario often observed was that these IPNLCE materials exhibited excellent mechanical properties at ambient temperature, but moderate performances beyond the LC-to-isotropic phase transition.

The IPN-LCE was composed of two crosslinked liquid crystal polymer networks, LCPA system and LCPU system as shown in Figure 1B. Specifically, the formulation of LCPA system included LC monomer 2-methyl-1,4phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate) (RM82, 21.60 mol%), hexane-1,6-diyl diacrylate (HADA, 2.16 mol%) and the photoinitiator 2,2-dimethoxy-1,2diphenylethanone (DMPA, 0.22 mol%), while LCPU system was formed by LC monomer 4-((6hydroxyhexyl)oxy)phenyl 4-((6-hydroxyhexyl)oxy) benzoate (Y1901, 21.60 mol%), bis(4isocyanatophenyl)methane (MDI, 38.00 mol%), the chain extender polyethylene glycol (PEG400, 14.46 mol%), the crosslinker 2-ethyl-2-(hydroxymethyl)propane-1,3-diol (TMP, 1.29 mol%) and the catalyst dibutyltin dilaurate (DBTL, 0.65 mol%). The preparation procedure of this IPN-LCE material is schematically illustrated in Figure 1C. All the starting reagents of LCPA and LCPU were dissolved in dry N,N-dimethylformamide which was stirred at 78 oC for 4.5 h. Most of the organic solvent (ca. 90 vol %) was then removed by vacuum to provide a viscous solution, which was immediately extruded through a syringe (needle hole: 1.51 mm diameter) into a polytetrafluoroethylene (PTFE) rectangular mold (3 cm long × 3 cm wide × 3 cm deep) and meanwhile illuminated under UV light (365 nm, 6.9 mW.cm-2), for a short time period of ca. 20 s. The mold was heated in a vacuum oven at 70 °C for 5.5 h. After cooling to room temperature, the resulting pre-crosslinked IPN-LCE film was carefully removed from the PTFE mold, and sliced into a strip (25.48 mm long × 6.91 mm wide × 0.68 mm thick) which was further longitudinally stretched to ca. 200% of its original length. The stretched strip was fixed on a hollow PTFE frame and kept at 100 °C for 48 h, meanwhile irradiated under UV light (365 nm, 7.2 mW.cm-2) from both sides of the film, to give the desired IPN-LCE sample. We also prepared pure LCPU and LCPA samples respectively for comparison purpose, the synthetic protocols are described in Supporting Information.

Confronting the above limitations, we propose an innovative strategy of building two interwoven macromolecular matrices both possessing liquid crystallinity, of which one LCE network shall be mainchain type and perform the essential actuation function, another crosslinked LC polymeric network as a supplementary frame shall maintain its chain anisotropy and preserve the mechanical property to some extent beyond the clearing point temperature (Tiso). Based on this design, we here report an interpenetrating LC

The mesomorphic properties of the above three samples were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) experiments. As shown in Figure 2 and Figure S10, pure LCPA as a highly crosslinked thermoset, had no LC-to-isotropic phase transition but only a glass transition temperature (Tg) at 110 °C. As to pure LCPU sample, its Tg and Tiso temperatures appeared at 9 °C and 120 °C, while those two values of the IPN-LCE material rose to 26 °C and 128 °C respectively. One-dimensional


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Journal of the American Chemical Society (1D) and two-dimensional (2D) WAXS patterns (Figure S13-15) demonstrated that all the three polymeric materials formed cybotactic nematic phase.53,54

Figure 2. DSC curves of LCPU (yellow), LCPA (blue) and IPN-LCE (red) recorded on (A) the second heating process and (B) the first cooling process.

Scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments were further performed to examine the interpenetrating polymer network structures formed between LCPU and LCPA matrices. As illustrated in Figure S16, the IPN-LCE strips were immersed into liquid nitrogen and broken along the stretching direction. SEM and AFM images showed that the fractured surfaces of pure LCPU (Figure S17A-C) and LCPA (Figure S17D-F) were nearly homogeneous, while the well-dispersed phase-separated domains with sizes of ca. 20~100 nm which are characteristic of IPN structures,45,58 were found in IPN-LCE sample (Figure S17G-I). In addition, Fourier-transform infrared spectroscopy (FT-IR) demonstrated the full conversion of the acrylate and isocyanate functional groups (Figure S8), a swelling experiment55 indicated that both LCPA and LCPU networks of the IPN-LCE sample were well formed (Figure S9). Combining all the above data with the DSC result showing a single Tg,51 we can conclude that a wellmixed interpenetrating polymer network has been successfully achieved in this IPN-LCE material. The thermal-induced shape morphing property of IPNLCE film was first examined by directly heating or cooling the sample on the top of a temperature-controlled hot stage, as shown in Figure 3A and Video S1. During the heating-cooling cycle, the IPN-LCE film showed a reversible shrinkage with a maximum strain ratio of -46%, which was defined as (L - Lo)/Lo (L is the longitudinal length of IPN-LCE film measured at any temperature and Lo is the longest length achieved at ambient temperature). The maximum shrinkage ratio of this material, defined as (Lo – Liso)/Liso (Liso is the shortest length achieved in the isotropic phase), was then determined as 86%. Under a 0.001 N preload static force, the varied strain of IPN-LCE film measured on a dynamic mechanical analyzer (DMA Q800, TA Instrument) using the isoforce mode, was plotted against temperature as presented in Figure 3B. The strain value showed an abrupt change from 0% to 46% at the temperature range of 86~140 °C during the heating process. On cooling, the strain of IPN-LCE sample had a sharp jump from -46% to 0% at the temperature range of 132~70 °C. The strain variation exhibited good reversibility and recyclability in 6 cycles. The actuating stress of IPN-LCE material was further investigated by

using the isostrain mode where the IPN-LCE ﬁlm was elongated by a constant 0.01% strain in tension. The generated contractile force by the IPN-LCE sample was measured along with the temperature variation as illustrated in Figure 3C. In a heating-cooling cycle, the blocking (maximum) stress value of this IPN-LCE material reached 2.53 MPa, which was almost twice the known highest record (1.3 MPa, 0.1% isostrain) of previous LCE actuators.39 For comparison, the actuation performances of pure LCPU and pure LCPA samples were also characterized on DMA by using the isoforce and isostrain modes respectively. As shown in Figure S22, the blocking stress values of LCPU and LCPA were 0.82 MPa and 0.17 MPa in the heating-cooling cycle. The maximum actuation strain ratio of LCPU film was -67%, whereas the actuation strain of pure LCPA film was nearly 0% since LCPA was a highly crosslinked thermoset. Moreover, pure LCPU film showed an irreversible one-way shape memory effect (Figure S22B), which has been previously observed in some chemically crosslinked polyurethane LCE materials.56,57 Apparently, introducing LCPA network into LCPU matrice would endow the corresponding IPN-LCE sample with enhanced actuation stress and two-way shape morphing capability, although its maximum actuation strain ratio was shortened to some extent. The quasi-static stress-versus-strain data of pure LCPA, pure LCPU and IPN-LCE samples at different temperatures (30~140 °C) were obtained by stretching the films along the mesogenic orientations, as shown in Figure S18-20. Since LCPA was a highly crosslinked thermoset, its elastic modulus and breaking strength were both very high in the whole experimental temperature range, and could maintain at 298 MPa and 27.3 MPa respectively even at 140 oC; whereas LCPU behaved as a typical LCE material, its elastic modulus and breaking strength dramatically decreased to below 1.0 MPa beyond its LC-to-isotropic phase transition (Figure 3D,E). The mechanical performance of IPN-LCE appeared like a compromised result of those of LCPA and LCPU. At 140 oC which was not only above the clearing temperature but also the critical point of achieving the maximum shrinkage of the elastomeric film, the elastic modulus and breaking strength of IPN-LCE sample reached respectively 10.4 MPa (Figure 3D) and 7.9 MPa (Figure 3E), which were both much higher than previously reported data (no more than 1.4 MPa) of LCE materials,33,35,36,38,59-61 which were listed in Table S1. In particular, as shown in Figure 3F, the elastic modulus of IPN-LCE at isotropic phase was nearly one order of magnitude higher than all the reported values of traditional LCEs,62-67 and for the first time matched the real application requirement (10~60 MPa modulus) of artificial skeletal muscles (Figure 1A).



Figure 3. (A) The shape morphing behavior of IPN-LCE film placed on the top of a glass slide whose surface temperature was controlled by a hot stage. (B) The representative strain (in isoforce mode) and the corresponding temperature diagram of IPNLCE film plotted against time. (C) Representative isostrain (0.01%) measurment of IPN-LCE film. (D) Representative elastic moduli and (E) breaking strengths of pure LCPU, pure LCPA and IPN-LCE samples measured at varied temperatures. (F) A comparative summary of the breaking strengths and elastic moduli (above Tiso) reported in previous representative works and this work.

The outstanding mechanical properties of this IPN-LCE material might derive from two factors: (1) since the mesogens of LCPU and LCPA systems were fully miscible, the main-chain backbones of LCPU could penetrate through the highly crosslinked LCPA frame which in a sense provided an additional physical crosslinking for the loosely chemical-crosslinked LCPU elastomer. (2) LCPA system as a liquid crystal thermoset had no isotropic phase which would preserve the chain anisotropy and the mechanical strength of the whole polymeric network insome extent when the LCPU matrix lost its long-range orientational order at high temperatures. This hypothesis could be verified by the POM result as illustrated in Figure S12, which still showed some birefringence at 140 oC while pure LCPU sample was completely dark beyond its LC-to-isotropic phase transition. Furthermore based on the XRD experiments (Figure S13,14), the order parameter of IPN-LCE material was calculated as ca. 0.18 while the value of the pure LCPU sample was nearly 0, at 140 °C, which also demonstrated that some anisotropic order still existed in the IPN-LCE network even after the material shrunk completely. In another hand, we could also invoke the shape of the LCPA micro/nanodomains as being anisotropic, elongated particles, which might play a reinforcing filler role similar to carbon nanotubes in charged classical polymers.68 Taking advantage of its ultrastrong mechanical property, this novel IPN-LCE material exhibited an outstanding heavy-lifting capability. Heated by a thermal coil equipment as shown in Figure S23, one single IPNLCE ribbon sample (20.1 mg) could lift up a counterpoise and a clamp (ca. 605.02 g in total), which were 30100 times heavier than the ribbon’s own weight, as

demonstrated in Figure 4A and Video S2. The detailed strain of IPN-LCE sample bearing the heavy load was plotted against temperature as shown in Figure 4B. Furthermore, the work capacity of this IPN-LCE sample was calculated as ca. 1267.7 KJ/m3 under a load of 5.95 N, which was ca. 31 times higher than the average value of skeletal muscles (40 KJ/m3).31,39 The detailed calculation is included in Supporting Information.

Figure 4. (A) Photo images of IPN-LCE film (20.1 mg) lifting up a load (ca. 605.02 g) in a heating/cooling cycle. (B) The strains vs. temperature diagram of this IPN-LCE film bearing a load 30100 times higher than its own weight.

CONCLUSION For nearly four decades, the modest mechanical property of LCE material has seriously impeded its industrial applications, which is the key scientific issue of this research field. Here we report an interpenetrating LCE material by introducing LC polyacrylate thermoset into main-chain polyurethane LCE network. This two-way shape memory material can reversibly shrink/expand under thermal stimulus. With a maximum shrinkage ratio of 86% at 140 oC which is above the LC-to-isotropic phase


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Journal of the American Chemical Society transition, its actuation blocking stress, actuation work capacity, breaking strength and elastic modulus reach 2.53 MPa, 1267.7 KJ/m3, 7.9 MPa and 10.4 MPa respectively, which are all the highest records of LCE materials till now. Most importantly, this IPN-LCE material is the first LCE actuator fulfilling all the actuation stress (> 0.35 MPa), strain (> 40%) and elastic modulus (> 10 MPa) requirements of artificial skeletal muscles indeed. We hope that this work would pave the way for the real industrial utilizations of LCE-based soft actuator materials.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details and data (PDF) Video S1: Thermal-induced reversible shrinkage of the IPN-LCE film (mp4) Video S2: A thermal actuation experiment of an IPN-LCE film (20.1 mg) lifting up a load of ca. 605 g (mp4)

AUTHOR INFORMATION Corresponding Author *(H.Y.) Telephone: 86 25 52090620. Fax: 86 25 52090616. E-mail: [email protected]

ORCID: Hong Yang: 0000-0003-4647-1388

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by Jiangsu Provincial Natural Science Foundation of China (BK20170024) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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SYNOPSIS TOC.


Polyacrylate Elastomer

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