Fabrication of Liquid Crystalline Polyurethane Networks with a

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Fabrication of liquid-crystalline polyurethane networks with pendant azobenzene group to access thermal/photo-responsive shape-memory effects Zhi-Bin Wen, Dan Liu, Xiao-Yang Li, Chenhui Zhu, Ren-Fan Shao, Rayshan Visvanathan, Noel A. Clark, Ke-Ke Yang, and Yu-Zhong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05280 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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ACS Applied Materials & Interfaces

Fabrication

of

Liquid-Crystalline

Polyurethane

Networks with Pendant Azobenzene Group to Access Thermal/Photo-Responsive Shape-Memory Effects Zhi-Bin Wen†‡, Dan Liu†, Xiao-Yang Li†, Chen-Hui Zhu§, Ren-Fan Shao‡, Rayshan Visvanathan‡, Noel A. Clark*‡, Ke-Ke Yang*† and Yu-Zhong Wang† † Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China. ‡ Department of Physics and Soft Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA. §

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720,

USA

KEYWORDS: polyurethane network, side-chain liquid crystalline, thermal responsive, tripleshape memory effect, thermal responsive, photo-responsive

ABSTRACT: Herein, we report a novel thermal/photo-responsive shape-memory polyurethane network with pendant azobenzene group by utilizing its anisotropic-isotropic phase transitions and photo-responsive feature concurrently. To achieve this goal, the side-chain liquid-crystalline

ACS Paragon Plus Environment

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polyurethane networks based on pendant azobenzene group (SCLCPU(AZO)-Ns) were developed in a well-defined architecture. The smectic C nature of liquid crystalline phase in the polyurethane networks was confirmed by differential scanning calorimetry (DSC), polarized optical microscopy (POM), 1D and 2D wide angle X-ray (WAXD). The well-defined architecture made SCLCPU(AZO)-N display two distinct transition temperatures (Ttrans) (Tg and Tcl) with a difference about 40 °C. Consequently, the excellent triple-shape memory effect in this network was demonstrated by cyclic thermomechanical analyses. Making full use of the transcis photoisomerization of azobenzene, the reversible bending and unbending behaviors were realized under the light irradiation with 450 and 550 nm wavelength, respectively.

INTRODUCTION Shape-memory polymers (SMPs) are stimuli-responsive materials which can be recovered from a temporary shape to a permanent shape by exposing to an external stimuli,1-4 such as heating,5-9 cooling,10 pH,11 magnetic field,12 light,13-16 solvent,17 or moisture.18 SMPs show great potential applications in varied fields due to this unique characteristic.19-22 Indeed, most of SMPs are intrinsically heat and chemo-responsive.23 The thermally-activated phase transitions of polymers have been widely employed as molecular switch in SMPs, and the transition temperatures (Ttrans) for shape fixing and recovery can be associated with glass transition (Tg),24-29 melting temperature (Tm)30-36 as well as liquid-crystalline clearing temperature (Tcl).37-41 Although the SMPs based on Tg and Tm have been intensively investigated, Tcl-based SMPs have a prompt response with a small temperature interval (1~ 5 °C) mostly. Recently, the SME on thermo-responsive LCEs or LCNs has established a successful development through the order-disorder of thermotropic phase transitions.37, 39-40, 42-45 Mather and


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co-workers40 reported the exceptional SME on the smectic C main-chain LCEs. Rajeswari M. Kasi et al.37 successfully synthesized a series of chiral side-chain polynorbornene LCNs, showing well-separated Tg and Tcl within microphase separated morphologies and leading to the triple-shape memory effect (TSME). In our previous work,46 we also developed a side-chain polyurethane LCN, which exhibited TSME depending on Tg and Tcl and a multi-shape memory effect based on a broad Ttrans. Actually, some liquid-crystalline mesogens show multi-responsive character, which inspires us to develop multi-responsive SMPs incorporating just single liquid-crystalline group. As we know, azobenzene is one of the most widely selected chromophores employed for producing photo-responsive materials owing to its unique nature of reversible trans-cis photoisomerization upon exposition to UV-Vis irradiation, and it can be utilized to realize reversible contraction and expansion, bending and unbending, and three dimensional movements.47 Ikeda and Yu prepared the first film of azo-LCNs with the ability of bending along chosen direction irradiating with linearly polarized light.48 The new three dimensional movement of azo-LCNs composite materials also was reported by the same group.49 More recently, White et al. reported the design of glassy azo-LCNs exhibiting dual-responsive (thermal and light) shape memory effect (SME). However, the Tg but not the Tcl of the azo-LCNs was used to trigger the thermally-induced SME.14 Here, we report a thermal/photo-responsive shape-memory polyurethane network (SCLCPU(AZO)-N) with pendant azobenzene group by utilizing its thermotropic phase transitions and photo-responsive feature concurrently. For thermal-actuation, two distinct thermal transitions (Tg and Tcl) are also expected considering the rigidity of rod-like molecular shape of azobenzene group,50 which encourages us to expect excellent TSME. As a result, the


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thermal behaviors and the thermal/photo-responsive SME are investigated systematically in this work.

EXPERIMENTS AND METHODS Materials: 2-amino-6-methoxybenzothiazole, N, N-dimethylformamide (DMF) and 1,10dibromodecane were supplied by Alfa Aesar. 1,6-hexamethylene diisocyanate (HDI) were provided by Sigma-Aldrich. Pentaerythritol (PTOL) and Dibutyltindilaurate (DBTDL) were purchased from Sinopharm Chemical Reagent Co.. PTOL was sublimated in vacuo before use. All other chemicals were purchased from Kelong Reagent Corp. (China) and used as received. Synthesis of 2-(4'-Hydroxyphenylazo)-6-methoxybenzothiazole (1): In a 500 mL roundbottomed flask, 2-amino-6-methoxybenzothiazole (5.0 g, 27.8 mmol) was dissolved in 125 ml glacial acetic acid. After adding 100 mL dilute sulphuric acid (60%), the mixture was kept at ~5 °C in the ice bath. On the other hand, sodium nitrite (1.9 g, 27.8 mmol) dissolved in 62.5 mL water was cooled and then dropped into the mixture. With 1 h constant stirring, the diazonium salt was obtained and then added dropwise into an ice-cold solution of phenol (2.6 g, 27.8 mmol) in 75 mL ethanol below 5 °C. The reaction mixture was kept stirring for 1 h after complete addition, and 1 N NaOH (aq.) was used to increase the pH to 6–7. Finally, the red-brown solid was obtained after subsequently stirring for 1 h, filtrating, washing with water as well as methanol and recrystallizing from ethanol. Yield: 6.1 g (77%). 1H NMR (400MHz, DMSO-d6): δ (ppm) =10.91 (s, 1H, -OH), 7.97-8.04 (d, 2H, Ar-H), 7.87-7.95 (d, 2H, benzo-H), 7.66-7.70 (d, 1H, Ar-H), 7.15-7.20 (q, 1H, Ar-H), 6.99-7.05 (m, 2H, benzo-H), 3.88 (s, 3H, -OCH3). Synthesis of 2-[4'-(10-Bromohexyloxy)phenylazo]-6-methoxybenzothiazole (2): 1 (4.0 g; 14 mmol) was dissolved in 100 ml acetone. Then potassium iodide (trace) and potassium


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carbonate (10.0 g; 70 mmol) were added as well as 1,10-Dibromohexane (32 mL; 140 mmol). The reactant was heated to reflux for 24 h under N2. The filtrate was collected and concentrated. Then the obtained solid was dissolved in chloroform and slowly poured into excess hexane. The mixture was filtered and the precipitate was purified by column chromatography (eluent: dichloromethane/hexane (2:1)). Yield: 5.0 g (70%). 1H NMR (400 MHz, CDCl3): δ (ppm): 8.018.09 (m, 3H, Ar-H, benzo-H), 7.30-7.34(d, 1H, Ar-H), 7.10-7.15 (q, 1H, Ar-H), 7.01-7.07 (m, 2H, benzo-H), 3.93 (s, 1H, -OCH3), 4.05-4.13 (t, 2H, -O-CH2-), 3.40-3.47 (t, 2H, -CH2-Br), 1.801.92 (m, 4H, -CH2-CH2-Br, - CH2-CH2-O), 1.25-1.55 (m, 12H, -CH2-). Synthesis of 2-[4'-(10-bis(2-Hydroxyethyl)amino)-phenylazo]-6-methoxybenzothiazole (3): A solution of 2-propanol (50 mL), 2 (2.52 g; 5 mmol), diethanolamine (2.67 g; 25 mmol) and potassium iodide (trace) was kept in a 100 ml round bottom flask and refluxed. After 18 h, the reaction mixture was evaporated and dissolved in dichloromethane. The organic phase was washed with water three times. Finally, the combined organic was concentrated under reduced pressure and dried overnight in vacuum to obtain product 3 (93%). 1H NMR (400 MHz, DMSOD6): δ (ppm): 8.01-8.09 (m, 3H, Ar-H, benzo-H), 7.30-7.34(d, 1H, Ar-H), 7.10-7.15 (q, 1H, ArH), 7.01-7.07 (m, 2H, benzo-H), 3.93 (s, 1H, -OCH3)，4.05-4.13 (t, 2H, -O-CH2-), 3.63-3.69 (t, 4H, -CH2-O), 2.68-2.74 (t, 4H, -CH2-N), 2.54-2.60 (t, 2H, -CH2-N), 2.47 (brs, 2H, OH), 1.801.92 (m, 4H, -CH2-CH2-N, -CH2-CH2-O), 1.25-1.55 (m, 12H, -CH2-). Synthesis of SCLCPU(AZO) precursor and SCLCPU(AZO)-N: The diol monomer 3 (0.5287 g; 1 mmol) was dissolved in 5 ml dry DMF at 80 °C with N2. Then stoichiometric HDI and DBTDL (0.2% with respect to the diol)51 were added. The mixture was stirred for 3 h to obtain precursor 4. On the route V, the dry methanol (10 mmol) was added to obtain the linear SCLCPU(AZO), which precipitated with anhydrous ether and dried at 40 °C for two days. Yield:


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62%-85%. On the route VI, an appropriate amount of PTOL was used to crosslink the linear precursor and the solution was poured into a horizontal Teflon dish before the viscosity increased dramatically. The crosslinking reaction proceeded in a glass autoclave at 80 °C under N2. Finally, the network was obtained after one day in oven and two days under vacuum at 50 °C. Characterization and Measurements. Nuclear magnetic resonance (NMR): A Bruker AV400 (Bruker, Switzerland) was used to record 1H NMR spectra at room temperature. Solvent: deuterated chloroform (CDCl3) or dimethyl sulfoxide (DMSO); internal reference: tetramethyl silane (TMS). Gel permeation chromatography (GPC): The molecular weight (Mw, Mn and polydispersity index Mw/Mn) of SSCLCPU(AZO) was determined by GPC on a Waters instrument, which equipped with a Waters model 717 autosampler, a 2414 refractive index detector and a model 1515 pump. Tetrahydrofuran (THF) acted as the eluent at a flow rate of 0.6 mL min-1. The column temperature was kept at 40 °C. Polystyrene standard was used for constructing calibration curves. Dynamic mechanical analysis(DMA): Thermomechanical properties of the samples were carried out on DMA Q800 (TA Instruments, USA), with a heating rate of 3 °C min-1 from 0 °C to 120 °C and a frequency of 1 Hz. Differential scanning calorimetry (DSC): DSC was recorded on DSC-Q200 (TA Instrument, USA), over the temperature ranges 0 °C-120 °C with a heating or cooling rate of 10 °C min-1. Wide-angle x-ray scattering (WAXS): WAXS were performed on Forvis Technologies x-ray instrument. The source is a 30 W Genix 3D x-ray generator with Cu anode (wavelength, λ = 1.5405 Å and energy= 8.05092 keV). The detector is Dectris Eiger R 1M with 0.075 x 0.075


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mm2 pixel size. The beam size at the sample position is 0.8 x 0.8 mm2, and the flux is 4 x 107 xray photons/ s. The LC films were placed inside Instec hot stage and measured in transmission mode. The calibrant used is AgBe, and the sample-to-detector distance is about 200 mm. The xray scattering patterns were analyzed and plotted in Igor Pro software. The d-spacing for each film is calculated using the Bragg’s equation below: λ = 2 sin

(1)

where λ is the wavelength of x-ray (1.5405 Å), d is the layer spacing, and is the scattering angle. All samples were measured at 70 °C. Polarized optical microscopy (POM): LC textures of the SCLCPU(AZO)-Ns were obtained by a Nikon Fi1 polarizing optical microscope (POM) equipped with a hot stage. In order to determine the liquid crystal phase in shape memory effect. DMA Q800 with controlled force mode was used to prepare the sample as follows: the film was kept at 120 °C for 5 min to isotropy phase. Then it was stretched to 50% strain, kept at 70 °C (cooling rate: 5 °C min-1) and for 30 min before releasing force. The swelling ratio and gel content of the SCLCPU(AZO)-N: The SCLCPU(AZO)-Ns were cut into small pieces, then swelled in chloroform and extracted by chloroform at room temperature for 24 h. The degree of swelling (S(%)) and the gel content (G(%)) were calculated according to our previous work. 46 Crosslinking densities of the SCLCPU(AZO)-N: Cross-linking density (v) was computed from the volume fraction of the swollen polymer v252 using Flory–Rhener equation:53-54 =

[ ]

(2)

/ /

Here, Vs is the molar volume of the solvent and χ is the polymer–solvent interaction parameter, which is related to the solubility parameters of the polymer (δpolymer) and the solvent


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(δsolvent). The calculation of cross-linking density is detailed in Supporting Information. Triple-shape Memory Properties: The triple-shape memory experiments refer to our previous work,46 which were measured on a DMA Q800 (TA Instruments, USA) under controlled force mode. The testing program was available in Supporting Information. Photoisomerization properties: UV−Vis absorption spectra of solutions were taken on an Agilent 8453 spectrometer in 1 cm path length quartz cells. The photoinduced bending−unbending behaviors of the film were studied as follow: the film was first heated to 105 °C (above Tcl) for 3 min, then stretched to 20% strain, cooled to 64 °C (below Tcl) immediately in order to freeze the temporary shape and finally removed the external stress. Put the thin films of LC sample on a glass substrate, which laid on a hot stage (kept at 64 °C) and exposed to unpolarized visible light. Light irradiation was performed using Xe lamp (CEL-HXF300 power system, China) through a band-pass filter (450 or 550 nm). The intensity was ~20 mW cm−2. RESULTS AND DISCUSSION Synthesis of SCLCPU(AZO)-Ns: The synthetic route of SCLCPU(AZO)-Ns is outlined in Scheme 1. Firstly, the monomer (the functional azobenzene diol 3) was synthesized from 2amino-6-methoxybenzothiazole with three steps via intermediate products (1 and 2) according to the literature.55 Then, the linear SCLCPU(AZO) precursor 4 was polymerized from monomer 3 and HDI. Finally, SCLCPU(AZO)-N was obtained by crosslinking precursor 4 with Pentaerythritol (PTOL) according to our previous work.46 Scheme 1. Synthesis routes of (3) the diol monomer, linear polymer: SCLCPU(AZO), and polyurethane network: SCLCPU(AZO)-N. Conditions: i) a) NaNO2, CH3COOH-H2SO4; b) phenol, NaOH, 0-5 °C, ii) Br(CH2)10Br, K2CO3, acetone, KI (trace), refluxed, 24 h; iii)


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HN(CH2CH2OH)2, 2-propanol, refluxed, 18 h; iv) OCN(CH2)6NCO, DMF, DBTL, 80 °C, 3 h; v) CH3OH, 80 °C, 3 h; vi) C(CH2OH)4, 80 °C.

The structure of SCLCPU(AZO) was elucidated by 1H NMR (400 MHz, CDCl3) (Figure 1), the peaks of the protons in side-chain were found in 7.01−8.10 ppm (δHo, δHp, m, 4H, Ar-H), 7.01−8.10 ppm (δHi, δHm, δHn, m, 3H, benzo-H), 4.10 ppm (δHa, t, 2H, Ar-O–CH2), 3.93 ppm (δHk, s, 3H, Ar-OCH3), 1.80-1.92 (δHc, m, 4H, -CH2-CH2-N, -CH2-CH2-O), 1.25-1.55 (δHd, m, 12H, -CH2-); and the characteristic peaks of the protons in main-chain were found in 4.22 ppm (δHe, t, 2H, COOCH2CH2N), 2.72 ppm (δHf, t, 4H, COOCH2CH2N), 3.15 ppm (δHh, m, 4H, – CH2CH2NH), 5.13 ppm (δHg, brs, 1H, –NH), 3.53 ppm (δHj, s, 6H, -OCH3), 1.25-1.55 (δHi, m, 8H, -CH2-). According to 1H NMR, the SCLCPU(AZO) was successfully prepared.


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Figure 1. 1H NMR spectrum of SCLCPU(AZO)6.2k in CDCl3.

The thermal properties and SME of the network usually are influenced by the crosslinking density. Here, by adjusting the feed ratio of diol/diisocyanate to obtain precursors with different segment chain lengths, four networks were developed. The detailed information of molecular weight obtained by GPC and 1H NMR of SCLCPU(AZO)s are reported in Table 1. The numberaverage molecular weight (Mn) recorded by GPC ranged from 6.2 to 2.7×103 with the polydispersity ranging from 1.39 to1.79. Mn was also calculated from 1H NMR result which was found to coincide with what from GPC. Table 1. Molecular parameters of SCLCPU(AZO)s Diol:HDI

Conversion

Samples

Mn,NMRa -1

Mnb

Mwb -1

-1

PDIb

(molar ratio)

(%)

(g mol ) (g mol ) (g mol )

SCLCPU(AZO)6.2k

1:1.3

85

7900

6200

9700

1.56

SCLCPU(AZO)4.2k

1:1.5

80

6100

4200

7500

1.79

SCLCPU(AZO)3.4k

1:1.7

73

3600

3400

5400

1.57

SCLCPU(AZO)2.7k

1:2.0

62

2300

2700

3800

1.39


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a

Calculated by 1H NMR. b Measured by GPC.

Swelling test and crosslinking density were carried out and the results are listed in Table 2. In detail, the gel content (G) of networks was around 75%. The values decreased slightly when the molecular weight of SCLCPU(AZO) precursor increased. From this results, we can conclude that the crosslinking efficiency of all networks are similar, most of precursor was enter into the network, therefore, the crosslink density was mainly effected by the polymer chain length between two crosslink points which depended on molecular weight of precursor. Also, it makes sense that the crosslink density of networks should decrease while the molecular weight of SCLCPU(AZO) precursor increased from 2.7k to 6.2k, it has been identified by the results that swelling ratio (S) increased from 779% to 1145%. Crosslinking densities (v) of SCLCPU(AZO)Ns were estimated by Flory–Rhener equation and the results are correlated to G and S, which strictly dependent on molecular weight of relevant SCLCPU(AZO) precursor: longer polymer chain length within a network resulting in lower crosslinking density. Table 2. Gel content (G), swelling ratio (S) and crosslinking densities (v) of SCLCPU(AZO)-Ns. Sample

G (%)

S (%)

v (mol cm-3)

SCLCPU(AZO)6.2k-N 73.5±1.2 1145±42 7.45×10-5 SCLCPU(AZO)4.2k-N 75.2±1.6 1022±57 9.30×10-5 SCLCPU(AZO)3.4k-N 78.5±0.8 847±19

1.35×10-4

SCLCPU(AZO)2.7k-N 78.2±0.8 779±24

1.60×10-4

Thermal behaviors of SCLCPU(AZO)-Ns: The thermal properties of a SMP play an important role to achieve a thermally-induced SME. In this work, Tg and Tcl are expectantly used


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as Ttrans to drive TSME. Therefore, DSC and DMA analyses were utilized to investigate the thermal properties of SCLCPU(AZO)-Ns. Figure 2 shows the DSC curves of SCLCPU(AZO)-Ns and the relevant values are listed in Table 3. All samples present two thermal transitions assigned to Tg and Tcl, influenced by the architectures of networks. For Tg of SCLCPU(AZO)-Ns, a slight increase was detected while the backbone chain lengths decrease from 6.2k to 2.7k, which may be caused by the restriction of the polymer chains due to the increase of the crosslinking density. Meanwhile, a downtrend for Tcl varying from 93.7 °C to 89.3 °C is observed as well as the relevant enthalpy. This tendency maybe attributed to the reduction of chain flexibility in PU backbone and the decrease of the pendant LC mesogens with shorter backbone chain length. Comparing with the previous work, it is worth noting that the architectures of materials were adjusted by enhancing the rigidity of LC mesogen to obtain a larger temperature deference between Tg and Tcl ( ⊿ T ＞ 40 °C). In accordance, it is suggested that these new networks with two completely independent Ttrans have enormous potential in triple-shape memory materials.56


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Figure 2. DSC curves of SCLCPU(AZO)-Ns: (A) cooling scan (B) heating scan. Cooling or heating rate: 10 °C min-1. Table 3. The relevant values of SCLCPU(AZO)-Ns determined by DSC Tg

Tcl

∆Hcl

Tcl

∆Hcl

(°C)

(°C)a)

(J*g-1) a)

(°C) b)

(J*g-1)b)

SCLCPU(AZO)6.2k-N

44.3

93.7

5.719

90.3

5.555

SCLCPU(AZO)4.2k-N

44.5

92.1

4.299

88.4

4.282

SCLCPU(AZO)3.4k-N

45.0

90.4

3.323

87.2

3.280

SCLCPU(AZO)2.7k-N

46.3

89.8

2.942

86.5

2.672

Samples

a)

Tcl and b)Tcl: the phase transition from LC phase to isotropic phase during heating and cooling, respectively)


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The storage modulus-temperature curves of SCLCPU(AZO)-Ns are presented in the Figure 3. Two obvious drops are recorded in the storage modulus (E’) curve, assigned to Tg and Tcl. When the temperature is below Tg, all samples exhibit quite high E', with a slight higher value for samples with a shorter chain length backbone. At a temperature closed to Tg, the E' declines dramatically. This Ttrans is also clearly observed on tan δ curve as showed by the broad characteristic peak. Tcl is detected at about 100 °C and decreases with shortening the backbone chain length in networks, which is observed in DSC. Unfortunately, the rubber plateau cannot be detected on DMA.

Figure 3. Storage modulus-temperature curves of SCLCPU(AZO)-Ns.

In order to determine the liquid phase in shape memory effect. DMA Q800 with controlled force mode was used to prepare the sample. Figure 4(A) exhibits LC textures recorded by POM of the stretched SCLCPU(AZO)-Ns from isotropic melts to liquid crystal phase during the first cooling process captured at 70 °C. After being stretched, the main chain of polymer has the same direction as the elongation orientation. When the stretched direction is parallel to analyzer, it was bright for SCLCPU(AZO)6.2k-N, while the view changes to dark with a small tilted angle. As a result, the director of liquid crystal molecule does not agree well with the polymer main chain,


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which is a typical feature of smectic C phase (SmC). Unfortunately, the result cannot be exactly determined on other films. Microstructural Analysis: To investigate detailed microstructure of the LC phase, WAXD analysis is employed. Figure 4(B) exhibits 2D WAXD results of the stretched films at 70 °C. Obviously, all samples show strong anisotropy in the scattering pattern. It seems that the LC molecular long axis (the director) parallels the stretching direction in the stretched samples. In small angle, the four spot patterns show that the layer normal direction is tilted away from the stretching direction and the molecular long axis is close to the stretching direction (SmC). The d spacing values of specimens are summarized on Figure 4(C). In the 1D WAXD pattern, two sharp diffraction peaks (d001 and d002, respectively) were obvious at small angle confirming the smectic phase. The result is mainly in keeping with POM and DSC observations.


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Figure 4. (A) Liquid crystalline textures of the stretched SCLCPU(AZO)-Ns recorded by POM at 70°C; thickness: ~20 µm; (B) 2D WAXD images of the stretched SCLCPU(AZO)-Ns at 70°C; (C) 1D WAXD curves of the stretched SCLCPU(AZO)-Ns at 70°C: (a) SCLCPU(AZO)6.2k-N; (b) SCLCPU(AZO)4.2k-N; (c) SCLCPU(AZO)3.4k-N; (d) SCLCPU(AZO)2.7k-N;

Triple-shape Memory Effect: Since the SCLCPU(AZO)-Ns display two independent Ttrans (Tg and Tcl) with a temperature difference of more than 40 °C, allowing to observe a TSME with


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two temporary shapes without obvious mutual interference. The cyclic thermomechanical test conducted by DMA in a controlled-force mode was used to determine the TSME of networks. Considering the Tg and Tcl of the networks, the three characteristic programming temperatures were selected as follows: Tlow (Tg -20 °C), Tmid (Tg + 20 °C) and Thigh (Tcl +10 °C), respectively. The testing procedures and calculated equations are described in the Experiments section. The Rf and Rr were computed from the 2-4 cycles. Figure 5 illustrates the typical TSME of SCLCPU(AZO)6.2k-N determined by DMA and photos. Triple shape memory properties for all samples are summarized in Table 4. Obviously, the shape memory performance was affected by the architectures of networks evidently. In the programming step, temporary shape S1 was fixed by forming LC phase. When shortening the PU backbone, the orientation of the LC mesogen became weakened, in accordance with the decrease of relevant enthalpy in DSC analyses. It is clear that Rf(S1) reduced from 94.4% to 85.4% as the chain length of backbone ranges from 6.2 K to 2.7 K. In addition, Rf(S2) (temporary shape S2), dominated by Tg, were all above 99% without obvious change. In the recovery programming, according to the larger temperature interval (⊿T＞40 °C), the recovery behaviors associated with Tg and Tcl are completely independent. It is clear that the total recovery properties are delighted because of Rr(S2→S0) of all SCLCPU(AZO)-Ns are beyond 95%. Precisely, when the crosslinking densities of networks increased, Rr(S2→S1) ranged from 95.0% to 96.0%, and Rr(S1→S0) from 95.3% to 99.8%. As expected, TSME was successfully realized in the materials.


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Figure 5. TSME of SCLCPU(AZO)6.2k-N. (A) TSME of SCLCPU(AZO)6.2k-N recorded from cyclic thermal mechanical test. (B) Digital photos and the mechanism of a TSME cycle of SCLCPU(AZO)6.2k-N. Table 4. Triple shape memory properties of SCLCPU(AZO)-Ns recorded from cyclic thermal mechanical tests Rf (%)a

Rr (%)b

Samples S0 → S1

S1 → S2

S2 → S1

S1 → S0

S2 → S0

SCLCPU(AZO)6.2k-N

94.4±0.2

99.8±0.0

95.0±0.1

95.3±1.5

95.1±0.6

SCLCPU(AZO)4.2k-N

93.3±0.4

99.7±0.0

92.4±0.4

97.7±2.0

95.8±1.2

SCLCPU(AZO)3.4k-N

87.4±0.1

99.8±0.0

96.5±1.0

99.2±0.5

98.0±0.7


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SCLCPU(AZO)2.7k-N a

85.4±0.0

99.8±0.1

96.0±0.9

99.8±0.7

98.2±0.7

Rf: average shape fixity for 2-4 cycle; bRr: average shape recovery for 2-4 cycle.

Photo-responsive feature: Azobenzene owns the excellent capacity of reversible trans-cis isomerization upon exposition to UV-Vis.47 In this work, the lights with wavelength of 450 nm and 550 nm were employed to record UV–Vis absorption spectra of SCLCPU(AZO)6.2k in dilute DMSO solution (~2.5×10−6 M) (Figure 6). The UV–Vis absorption maxima (λmax) peak of studied polymer in solution was about 435 nm, which assigned to π–π* transition of the trans isomer. The absorption peaks due to the π–π* transition and n–π* transition of the cis isomer were observed at around 345 nm and 550 nm respectively.57 When the solution was exposed to visible light (λ=450 nm), the sample underwent trans to cis isomerization as showed by the dramatically decrease of the characteristic absorption peak of trans isomer while the cis isomer gradually enhanced to stabilization within about 30s (Figure 6(A)). As expected, UV–Vis absorption spectra returned to initial state upon irradiation wavelength of λ=550 nm (Figure 6(B)). It is mentioned that the reversible trans-cis isomerization can be introduced by visible light. It has been reported that photo-induced bending−unbending motions in azobenzene containing LC polymer films were caused by photo-induced motions.48 The shrinking behavior only occurred in the surface of the film while the remaining parts of the film did not change irradiating with UV-light. After heating or exposing to visible light, the cis-azobenzene state got back to trans-azobenzene state and as a consequence the bent film revert back to its original state. When the temperature was kept at 64 °C (above the Tg and below the Tcl of SCLCPU(AZO)6.2k-N), bending−unbending behaviors of the network were investigated (Figure 6(C)). The concave shape (b) was obtained from a flat shape (a) within 7 s upon irradiation with


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visible light at λ=450 nm visible. Subsequent irradiation at λ=550 nm visible induced a reverse motion and the film gradually got back to the initial flat (c) within only 12 s. It can be assumed that this sample exhibits an excellent photo-responsive characteristic to trigger the SME.14

Figure 6. The UV-Vis absorption spectra of SCLCPU(AZO)6.2k in DMSO solution (2.0 x10-6 mol/L) upon irradiation with visible light: (A) 450 nm; (B) 550 nm; the arrows show the increment of irradiation time; (C) Photos show the bending and unbending behaviors of SCLCPU(AZO)6.2k-N exposing to visible light. Size: 15 mm x 3 mm x 80 um.

CONCLUSION


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In present work, the side-chain liquid-crystalline polyurethane networks containing azobenzene (SCLCPU(AZO)-Ns) were synthesized to carry out excellent thermal/photo-responsive SME. The architectures of networks could be adjusted by the molecular weight of the precursors detected by 1H NMR. DSC, WAXD and POM analysis proved the SmC phase of the SCLCPU(AZO)-Ns. As confirmed by DSC, the networks displayed two distinct Ttrans (Tg and Tcl, ⊿T＞40 °C), which could be utilized as Ttrans to trigger the TSME. The excellent TSME of samples was recorded by DMA on the cyclic thermomechanical test. It is clear that the Rf(S1) based on Tcl ranged from 94.4% to 85.4% as the chain length of backbone ranged from 6.2 K to 2.7 K. Meanwhile, the Rf(S2) determined by Tg kept on a high level (~99%). The total recovery properties were delighted since Rr(S2→S0) of all SCLCPU(AZO)-Ns were beyond 95%. As the crosslinking density increased, Rr(S2→S1) ranged from 95.0% to 96.0%, and Rr(S1→S0) varied from 95.3% to 99.8%. Based on the trans-cis photoisomerization of azobenzen, bending and unbending of materials were observed upon irradiation with visible light at different wavelength of visible 450 nm and 550 nm.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The calculation of crosslinking density, the testing program of TSME and supplementary figures.

AUTHOR INFORMATION

Corresponding Author


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* Ke-Ke Yang. E-mail: [email protected]. *Noel A. Clark. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge use of facilities and instrumentation supported by NSF MRSEC Grant DMR-1420736 (University of Colorado, Boulder). This work was supported financially by the National Science Foundation of China (51473096, 51421061), the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1026), and the State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2016-2-06).

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TOC


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Fabrication of Liquid Crystalline Polyurethane Networks with a

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