Thermoplastic High Strain Multishape Memory Polymer: Side-Chain Polynorbornene with Columnar Liquid Crystalline Phase

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1 Thermoplastic High Strain Multishape Memory Polymer: Side-Chain Polynorbornene with Columnar Liquid Crystalline Phase Ruiying Zhao, Tipeng Zhao, Xuqiang Jiang, Xin Liu, Dong Shi, Chenyang Liu, Shuang Yang,* and Er-Qiang Chen* As a class of stimuli-responsive materials, shape memory polymers (SMPs) have received great attention due to their scientific interest and promising applications in advanced technologies in different areas. [1,2] Dual-SMPs can memorize a programmed temporary shape defined by the applied force and fixed by switching the system from one state to the other. Various molecular relaxations and phase transitions, e.g., vitrification, crystallization, or less commonly used liquid crystal (LC) transition, can be employed to fix the temporary shape. It is also demonstrated that multi-smps can be fabricated based on multiple transitions or a broad transition. [3] When the reversible transition is triggered by external stimuli, [1,4] the SMP will recover to its permanent shape defined by a network embedded in the system. The network shall be robust enough to resist the plastic deformation when the temporary shape is programmed. The network can be made of chemical or physical crosslink points, leading to SMPs that are thermoset or thermoplastic. [1] While the former gives more stable shape memory performance, the latter is attractive due to the flexibility of processing. However, physical crosslinks based on noncovalent bond interactions, e.g., chain entanglement, hydrogen bond, and ionic interaction, are often less stable. [5] Chain sliding or reorganization occurred during deformation will result in poor shape memory properties. This is a fatal weakness for many thermoplastic SMPs, particularly when a large shape change is demanded, [6] such as in some biomedical devices [2a] and package materials. [6d] To obtain better SMPs combining R. Y. Zhao, Dr. T. P. Zhao, X. Q. Jiang, Dr. X. Liu, D. Shi, Prof. S. Yang, Prof. E.-Q. Chen Beijing National Laboratory for Molecular Sciences Key Laboratory of Polymer Chemistry and Physics of Ministry of Education Center for Soft Matter Science and Engineering College of Chemistry Peking University Beijing , China shuangyang@pku.edu.cn; eqchen@pku.edu.cn Prof. C. Y. Liu Beijing National Laboratory for Molecular Sciences CAS Key Laboratory of Engineering Plastics Institute of Chemistry The Chinese Academy of Sciences Beijing , China DOI: /adma thermoplastic and stable network, one elegant approach is to follow the strategy of vitrimer. [7a] One can make the network using dynamic covalent bonds, which can allow the SMP to be reshaped at high temperatures with the aid of a catalyst. [7] On the other hand, new thermoplastic SMPs with pure physical crosslink network are still desirable. For example, an excellent multi-smp of a compositional gradient copolymer is recently reported, [3d] showing a microphase-separated structure similar to the thermoplastic elastomer of styrene-butadiene-styrene (SBS) triblock copolymer. Nevertheless, to make the thermoplastic SMPs with both ideal shape fixity (R f ) and shape recovery (R r ) for high strain (e.g., strain >400%) remains a great challenge. [5,6b] While the commonly applied physical crosslinks show their limitation, we attempt to find a new type of physical crosslink by utilizing a columnar LC structure. Here we report a novel thermoplastic high strain SMP of hemiphasmid sidechain polynorbornene (P1; Figure 1a). P1 exhibits a hexagonal columnar LC (Φ H ) phase and a broad Φ H -isotropic transition. It renders both the R f and R r approaching 100% for dual-shape memory effect (SME), even when a high strain of 600% is applied. It is also a multi-smp. For triple and quadruple-sme, with the total strain higher than 400%, it can still give R f quite high and R r > 95% at each step. P1 is the first SMP based on columnar phase. Without any chemical crosslinks, the excellent SME is due to the fact that P1 can form the supramolecular multichain columns composed of several chains laterally associating together. Such multichain columns, i.e., the building blocks of Φ H phase, can provide a strong physical crosslink network that an outstanding SMP requires (Figure 1a). P1 with a molecular weight (MW) of g mol 1 was synthesized by ring-opening metathesis polymerization. [8] The synthesis and molecular characterization are shown in the Supporting Information. The hemiphasmid side chain of P1 contains a biphenyl group and a mini-dendron bearing three dodecyl tails in series, which is wedge-shaped (Figure 1a), favoring the formation of columnar phase. 1D X-ray diffraction (XRD) evidences the Φ H phase (Figure 1b), showing three low angle diffractions with the scattering vector (q) ratio of 1: 3: 4. Upon heating from 30 to 70 C the diffractions become sharper and stronger. At 80 C the first peak drops drastically and the others disappear, indicating that P1 enters the isotropic state, which can be confirmed by polarized optical microscopy observation (Figure S1, Supporting Information). This transition is enantiotropic. Cooling can restore the Φ H phase (Figures S2 and S3, Supporting Information) (1 of 6) wileyonlinelibrary.com 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Figure 1. a) Chemical structure of P1 and the schematic illustration of multichain columns acting as physical crosslinks. b) 1D XRD patterns recorded upon heating. c) 2D XRD pattern of a stretched P1 at RT. The X-ray beam is perpendicular to the stretch direction (the meridian). d) Reconstructed relative electron density map of P1. The color from blue to red represents the electron density from low to high. The molecular packing of P1 was further investigated by 2D XRD using a uniaxially stretched sample. In Figure 1c, the (hk) diffractions of Φ H appear on the equator, indicating the column axis of P1 parallel to the stretch direction (the meridian). The high angle scattering concentrates more or less on the meridian at 0.44 nm, indicating that the side chains tend to perpendicular to the column axis. The 2D electron density map (EDM) calculated based on the (hk) diffractions reveals that the P1 column adopts a microphase-separated structure along the radial direction. The alkyl tails with the lowest density occupy the column periphery and the main chains locate at the center (Figure 1d). The a parameter of 6.04 nm determined for the Φ H of P1 is notably large. We estimated the number of repeating units (Z rep ) packed in a unit column stratum with the thickness of 0.44 nm (see the Supporting Information). Taking the measured density of g cm 3, the Z rep value calculated is of 8. It is physically unreasonable to compress a P1 segment of 8 repeating units into a 0.44 nm thick column stratum. To fulfill the space, the repeating units shall come from several P1 chains. The column is thus a multichain column rather than a single-chain column encountered in other columnar sidechain polymers. [9] For different hemiphasmid side-chain polymers we studied recently, the multichain column is identified; [10] for others reported earlier, [11] Ungar also suggests that the columnar phase should compose of the multichain columns with the polymer backbones threading through the column center. [12] The multichain columns endow P1 with unique mechanical property and relaxation behavior. Figure 2a depicts a rheology temperature sweep curve measured using a sample preannealed 24 h at 70 C. P1 is soft at room temperature (RT), with the shear storage modulus (G ) of Pa. When heated from 40 to 80 C to cross over the Φ H -to-isotropic transition, the G decreases more than two orders of magnitude. Thus P1 changes from soft (G 10 6 Pa) at RT to ultrasoft (G 10 4 Pa) in isotropic state. This rheology measurement properly reflects the phase transition of P1, agreeing with the XRD results. At above 80 C, P1 exhibits the second G plateau before entering the terminal flow region at 105 C, evidencing the physical crosslink network for P1 even after the Φ H phase was melted. Figure 2a also reveals the broad and complex relaxation of P1. From 40 to 80 C, two tanδ peaks are observed. They might relate to the reorientation of LC domains under shear mode, [13] and the activation of main and sidechain motion within the columns. Given the phase structure unchanged below 80 C, the continuous decline of G suggests a wide distribution of Φ H domains with different sizes and columnar imperfections. While the stable ones can sustain until 80 C, the small or less perfect Φ H domains melt earlier, causing the G decreasing. Such a broad transition is of particular interest, [3c] making P1 not only the dual but also the multi-smp. We first verify that P1 is a high strain dual-smp with excellent R f and R r (calculation of R f and R r ; see the Supporting Information). Dynamic mechanical analysis (DMA) was utilized to quantitatively characterize the dual-sme. As shown in Figure 2b, with the deformation and recovery temperature (T d and T r ) at 75 C (slight below the isotropic temperature) and the fixing temperature (T f ) at 25 C, P1 displays both R f and R r close to 100% for the strain under load (ε S0,load ) of 250%. In the middle of the transition, at T d = T r = 58 C, R f is of 93% with ε S0,load of 375% and R r is of 99% (Figure 2c). Although P1 just possesses the physical crosslinks, the time taken for recovery is 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (2 of 6)

3 Figure 2. a) Rheology temperature sweep curve of P1 (frequency: 1 rad s 1, amplitude: 1%). b,c) Dual-shape memory cycle at T d = T r = 75 C (R f : 98.2%, R r : 99.5%) and at T d =T r = 58 C (R f : 93.2%, R r : 99.3%), respectively, with T f = 25 C. Solid line: strain; dotted line: temperature; and dashed line: stress. d) Visual demonstration for dual-shape memory property of P1 when deformed at 80 C. The strain fixed is up to 600%. short (Table S1 and Video S1, Supporting Information). P1 can even memorize the ultrahigh strain up to 600% (beyond the extensional limit of DMA) efficiently but without reducing the shape fixity and shape recovery (Figure 2d and Figure S4 and Table S2 of the Supporting Information). In consecutive dualshape memory cycles, P1 can exhibit outstanding R f and R r in every cycle (Figure S5, Supporting Information). Figure 3 describes the multi-sme of P1. For the triple- SME (Figure 3a and Figure S6 of the Supporting Information) measured by DMA, the permanent shape S0 was deformed at 76 C (T d1 ) and fixed at 57 C (T f1 ) to yield the first temporary shape S1; it was further deformed at 57 C (T d2 ) and fixed at 25 C (T f2 ) to yield the second temporary shape S2 with a high strain of ε S2,load > 400%. The shape fixing is more efficient at the lower temperature [R f (S1 S2) of 92.8% vs R f (S0 S1) of 85.8%] (Table S3, Supporting Information). Upon reheating to 57 C (T r1 ) and then 76 C (T r2 ), the sample recovered to S1 and S0 in sequence. For both of the shape recoveries, R r s > 96%. The quadruple-sme of P1 is shown in Figure 3b,c. In one shape memory cycle (Figure 3b), three temporary shapes (S1, S2, and S3) with the final strain of 425% can be effectively programmed; reheating to respective temperatures leads to the correspondingly recovered shapes (S2 rec, S1 rec, and S0 rec ). All the three R r s are above 95%. For S3 fixed at 25 C, the R f is 91%; for S1 and S2 fixed at 63 and 53 C in the middle of the transition, the R f s are still quite high (>60%). The pictures in Figure 3c display that P1 can memorize complex temporary shape (e.g., spiral shape) imposed by the external field. The aforementioned results confirm the versatile SME of P1. Chemically crosslinked LC polymers with smectic or nematic transition can be SMPs. [14] Here, the thermoplastic SMP of P1 relies on the Φ H, showing high R f and R r even for a very large strain ( 600%) and also excellent multi-sme. Consequently, P1 distinguishes from other thermoplastic SMPs. [1,5] We consider that the outstanding SME of P1 stems from the multichain columns of its Φ H phase. Under a large strain, SMPs having only the conventional chain entanglements hardly avoid plastic flow, resulting in moderate or low R r. [1] For the high MW P1, each long chain can pass through several LC domains, joining with different chains to form the multichain columns that can act as the netpoints (Figure 1a). Coupled with the chain entanglements in amorphous region, they form a robust network. The cyclic tensile test at 70 C indicates that in every step the stretched P1 sample can recover to its original length after releasing the stress (Figure 4a). The elastic recovery approaching 100% at 70 C (Figure S7, Supporting Information) evidences the characteristic thermoplastic elastomer behavior of P1. In the stress relaxation experiment (Figure 4b), a P1 sample after preannealing 24 h at 70 C was stretched and held at 200% strain at RT. The stress relaxed rapidly from 4 to 2.5 MPa within 1 min; further relaxation was rather slow, reaching a stress of 1.3 MPa at 90 min. Figure 4a,b confirms a strong physical crosslink network embedded in P1. We found that the better developed Φ H phase would give the better mechanical properties and shape memory performance. We compared the samples thermally annealed at 25, 50, and 70 C for 24 h prior to the measurements at RT. 1D XRD results reveal that the treatment at higher annealing temperatures (T a s) can give the stronger and sharper (10) diffraction (Figure 4c), indicating the better LC structure. Figure 4d presents the stress strain curves of the three annealed samples (3 of 6) wileyonlinelibrary.com 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Figure 3. DMA data of a) triple and b) quadruple-shape memory cycle of P1. Solid line: strain; dotted line: temperature; and dashed line: stress. The details of R f s and R r s are summarized in Table S3 in the Supporting Information. c) Visual demonstration for quadruple-shape memory of P1. at RT. The 70 C-annealed P1 shows the stress up to 8.5 MPa at a high strain of 700%, more than double the value of the 25 C-annealed sample. Clearly, elevating T a enhances the yielding stress and rate of strain hardening, suggesting an increase in the number of physical crosslinks that shall be resulted from the improved Φ H phase. With a more robust network and enhanced mechanical properties, the 70 C-annealed P1 sample exhibits a much better SME than the 25 C-annealed one (Figure S8, Supporting Information). Essentially belonging to physical crosslinks, the multichain columns of P1 can prevent the plastic deformation to a large extent, at least on the time scale of our experiments (minutes to hours), suggesting the very slow multichain column relaxation. We presume that it is associated with the unique chain packing within the column. The 2D EDM suggests the P1 main chains at the column center surrounded by the side chains (Figure 1d). To maximize the entropy, the side chains from different chains will select radial directions arbitrarily and therefore interlock with each other. Such a side-chain arrangement can be achieved by rotating the single bonds on the P1 backbone that can also lead the main chains to intertwine partially. High-T a annealing can promote this process. Once the interlocking is formed, pulling out chains from the multichain column becomes difficult. The chain relaxation toward the unlocked state requires the cooperative motion of different main and side chains and thus is slow. We note that residuals of the multichain columns can exist even after entering the isotropic state. Figure S9 (Supporting Information) shows that the 70 C-annealed sample presents significantly higher G of the second plateau at above 80 C compared to that annealed at 25 C, which indicates more physical crosslinks and thus a stronger physical crosslink network. This can be due to the fact that more individual wellorganized multichain columns resulted from annealing P1 at 70 C can survive above the isotropic temperature, albeit the Φ H phase is vanished. Employing the physical crosslink network to define the permanent shape near the isotropic temperature, we fixed the temporary shape of P1 based on the Φ H formation upon cooling. As aforementioned, the broad Φ H -isotropic transition of P1 corresponds to the wide distribution of LC domains with different melting temperatures. Therefore, this broad transition can be considered as a continuous distribution of many individual transitions, each corresponding to the LC domain with a particular melting temperature. According to different deformation temperatures, a series of individual LC transitions are activated in a shape memory cycle, leading to the multi-sme. [3c] In summary, we synthesized a novel hemiphasmid sidechain LC polynorbornene P1, which demonstrates an outstanding shape memory performance. Particularly, P1 is a highstrain multi-smp with high shape fixity and shape recovery. We 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (4 of 6)

5 Figure 4. a) Cyclic tensile test for P1 at 70 C. b) Stress relaxation experiment performed on a 70 C-annealed P1 sample at RT. c) 1D XRD patterns and d) stress strain curves (strain rate: 15 mm min 1 ) measured at RT for the P1 samples preannealed at 25, 50, and 70 C. Inset of (c): Peak full-width at half-maximum (FWHM, dot) and peak area (square) for (10) diffraction of the samples annealed at different T a s. consider that its versatile SME roots in the physical crosslink network composed of multichain columns and the broad LC transition. When P1 chains pass through multichain columns placed at different locations, a robust physical crosslink network is generated, which can resist the plastic flow during the programmed deformation. We presume that the robustness of physical crosslink netpoints, i.e., the multichain columns, can arise from the interlocking of chains within the confined space of the column. However, the detailed chain packing scheme and the relaxation behaviors demand in-depth studies. For example, the strain was observed to reduce with increasing the shape memory cycles (Figure S5, Supporting Information), of which the mechanism is unclear now. We anticipate that further investigation will uncover the unique properties of the multichain columns and, moreover, promote the more sophisticated applications of columnar side-chain polymers. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was supported by the Major State Basic Research Development Program (Grant No. 2011CB606004) and the National Natural Science Foundation of China (Grant Nos , , and ). Received: November 2, 2016 Revised: December 9, 2016 Published online: January 24, 2017 [1] a) A. Lendlein, S. Kelch, Angew. Chem. Int. Ed. 2002, 41, 2034; b) C. Liu, H. Qin, P. T. Mather, J. Mater. Chem. 2007, 17, 1543; c) Q. Zhao, H. J. Qi, T. Xie, Prog. Polym. Sci. 2015, 49, 79; d) C. L. Lewis, E. M. Dell, J. Polym. Sci., Part B: Polym. Phys. 2016, 54, [2] a) A. Lendlein, R. Langer, Science 2002, 296, 1673; b) J. L. Hu, S. J. Chen, J. Mater. Chem. 2010, 20, 3346; c) M. Behl, M. Y. Razzaq, A. Lendlein, Adv. Mater. 2010, 22, [3] a) I. Bellin, S. Kelch, R. Langer, A. Lendlein, Proc. Natl. Acad. Sci. USA 2006, 103, 18043; b) X. F. Luo, P. T. Mather, Adv. Funct. Mater. 2010, 20, 2649; c) T. Xie, Nature 2010, 464, 267; d) Y. W. Luo, Y. L. Guo, X. Gao, B. G. Li, T. Xie, Adv. Mater. 2013, 25, 743. [4] a) A. Lendlein, H. Jiang, O. Junger, R. Langer, Nature 2005, 434, 879; b) H. Koerner, G. Price, N. A. Pearce, M. Alexander, R. A. Vaia, Nat. Mater. 2004, 3, 115; c) R. Mohr, K. Kratz, T. Weigel, M. Lucka-Gabor, M. Moneke, A. Lendlein, Proc. Natl. Acad. Sci. USA 2006, 103, 3540; d) W. M. Huang, B. Yang, L. An, C. Li, Y. S. Chan, Appl. Phys. Lett. 2005, 86, [5] a) K. Nakyama, Int. Polym. Sci. Technol. 1991, 18, 43; b) X. Z. Gu, P. T. Mather, Polymer 2012, 53, 5924; c) Y. Shao, C. Lavigueur, X. X. Zhu, Macromolecules 2012, 45, 1924; d) Y. Zhu, J. L. Hu, (5 of 6) wileyonlinelibrary.com 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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