Design of Multistimuli-Responsive Shape-Memory ... - ACS Publications

Sep 5, 2014 - Richard Todd,. †. Jean-Marie Raquez,*. ,‡ and Philippe Dubois. †. †. Laboratory of Polymeric and Composite Materials (LPCM), Cen...
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Design of Multistimuli-Responsive Shape-Memory Polymer Materials by Reactive Extrusion Florence Pilate,† Rosica Mincheva,† Julien De Winter,‡ Pascal Gerbaux,‡ Linbo Wu,§ Richard Todd,† Jean-Marie Raquez,*,‡ and Philippe Dubois† †

Laboratory of Polymeric and Composite Materials (LPCM), Center of Innovation and Research in Materials and Polymers (CIRMAP) and ‡Organic Synthesis and Mass Spectrometry Laboratory, Center of Innovation and Research in Materials and Polymers (CIRMAP)University of Mons−UMONS, Place du Parc 20, B-7000 Mons, Belgium § Department of Chemical & Biological Engineering, State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: Shape-memory polymers (SMPs) are a class of stimuli-responsive materials that have attracted tremendous attention in various applications, especially in the medical field. While most SMPs are thermally actuated, relating to a change of thermal transition (e.g., melting temperature), SMPs that can be actuated upon exposure to light are emerging. Recently, there has been new interest into multiple stimuli-responsive SMPs in order to cover the range of applications for these smart materials. In this work, poly(ester-urethane)s (PURs) made of heating-responsive poly(ε-caprolactone) (PCL) segments of various degrees of crystallinity and photoresponsive N,N-bis(2-hydroxyethyl) cinnamide (BHECA) monomer were successfully prepared using reactive extrusion technology to design dual-stimuli-responsive SMPs (DSRSMP). In order to tune the SMP properties (temperature or light), the crystallinity of the PCL segment was finely adjusted by the copolymerization of ε-caprolactone with para-dioxanone in bulk at 160 °C using tin(II) octoate. The resulting polyester segments were then coupled with BHECA using n-octyl diisocyanate at 130 °C. The SMP properties of resulting PURs were correlated with DSC and DMTA measurements. Further addition of di- and tetracinnamate PCL segments into these SMPs was also studied in order to enhance the photoactuated SMP properties.

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INTRODUCTION Since the 1980s, the class of smart materials known as shapememory polymers (SMPs) have been studied intensively due their capacity to memorize a temporary shape1−5 meaning that they can be employed as biomedical materials,6−10 as smart textiles,11 or as engineering thermoplastics. SMPs can deform themselves from a “dormant” temporary shape to the permanent shape upon the application of an external stimulus like a change in temperature, exposure to light, moisture, solvent, and so on.12−19 While chemo(including water) and thermoresponsive shape memory effects (SMEs) are directly related to the intrinsic features of a couple of polymers (vide supra),20 this behavior is however observed for some particular polymer morphology and programming (technically, the procedure for fixing the temporary shape (distorted shape)) in most SMPs (e.g., photobased SMEs).21 In this respect, heating-responsive SMPs, being the most representative SMPs,22,23 are built on network architecture (elastic component, called matrix and a transition component, named inclusion).20 They combine chemical or physical crosslinks (net-points), which define the permanent domain, and the switching domains, which are associated with a transition © XXXX American Chemical Society

temperature (Ttrans). In order to observe any memory effect, the polymer sample is heated above Ttrans and undergoes a constant deformation (e.g., elongation). The temporary form is then obtained by cooling the sample under deformation below Ttrans. After unloading, the sample is reheated above Ttrans to allow the entropic relaxation of the polymer chains and to recover its initial form. The permanent domain prevents the deformation of polymer chains upon the application of a stress at T > Ttrans, and the switching domain ensures the fixation of the temporary shape due the formation of strong reversible interactions.12 The domains can be either glassy or crystalline domains if Ttrans is glass temperature (T g ) or melting temperature (T m ), respectively. The abilities of SMPs to fix temporary shapes and recover permanent shapes are respectively described as shape fixity (Rf) and shape recovery (Rr). Very recently, a new concept has appeared from Huang et al.,20 revealing many approaches to optimizing the design and fabrication of shapememory materials. Received: June 6, 2014 Revised: August 27, 2014

A

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n-butyl acrylate (BA), hydroxyethyl methacrylate (HEMA), and ethylene glycol-1-acrylate-2-CA (HEA-CA) with poly(propylene glycol) dimethacrylate as the cross-linker. At 10% elongation for the grafted polymer containing 10−30 mol % of HEA-CA, they obtained maximum values of 38−52% for Rf and 97% for Rr. Like heating-responsive SMPs, photoresponsive SMPs have to combine permanent and switching domains, but the programming procedure to observe SMEs is slightly different. When the coiled segments of amorphous polymer chains are elongated by stretching the polymer film, UV light at λ > 260 nm is applied to fix them by forming new cross-links within the materials. After unloading, recovery is obtained by irradiation at λ < 260 nm, but it is rarely complete due to the difficulty to achieve the full photocleavage of dimers.35 In order to design biodegradable photoresponsive SMPs, Wu and co-workers have recently reported on the synthesis of multiblock PUR containing biodegradable segments and pendant cinnamide moieties.36 This synthesis was centered on the use of N,N-bis(2-hydroxyethyl)cinnamide (BHECA) as a photoresponsive comonomer, which can be prepared under mild conditions at high yield. In addition of the photoreactive cinnamide moiety, BHECA presents two reactive hydroxyl groups that can further react during polyaddition or polycondensation.37 In this regard, they synthesized PUR via a two-step condensation reaction using PCL of low molecular weight, poly(L-lactide) (PLLA) diols as biodegradable segments and hexamethylene diisocyanate (HMDI) as coupling agent. The soft domain within the resulting PURs was made from BHECA moieties and PCL, and the hard-phase was consisted of PLLA segments as permanent domain (physical cross-links). An Rf of >50% and Rr of >95% were reached by adjusting the composition between the different components. A minimum of 20 wt % of BHECA was recommended to reach correct fixation. A new tendency is arising to design more complicated SMPs with multishapes.38 For instance, Bellin et al.39 reported the first example of triple shape-memory polymers, starting from polymer networks, combining two segregated domains with two distinct T trans and by application of a two-step programming process. Triple and multishape SMPs can be also designed on the basis of a single but broad thermal transition.20,38 On the other hand, there has been a growing interest for the development of SMPs combining two (or even three) different external stimuli. The concept is to associate different actuation mechanisms for fixation.40 Although some multiresponsive SMEs have already been developed, most of them present an unwanted overlapping effect between the stimuli, i.e., direct and indirect heating effects such as the induction effect of magnetic nanoparticles under a magnetic field.41 From the information available to date, there are no studies describing multiresponsive SMPs where such overlapping effects have been overcome. As such, the aim of this study was to synthesize PURs made from a heating-responsive PCL-based segment with various degrees of crystallinity, photoresponsive BHECA monomer, and HMDI as coupling agent, yielding a dual heating- and photoresponsive SMP with no overlapping effects. These PURs were obtained using a DSM vertical twin-screw mini-extruder, thus, in the absence of any solvent. As both segments are related to two distinct stimuli a double actuation can be achieved on the basis of their inherent characteristics, namely on the fact that if a segment is used for the fixation of the temporary form, the other can act as the switching segment and

From all polymers, poly(ε-caprolactone) (PCL) has been intensively investigated as a switching segment for heatingresponsive SMPs.24 In addition to its inherent biocompatibility, Tm of PCL can be easily tuned by the variation of its chain length or by copolymerization with other cyclic esters such as glycolide.25 Furthermore, the Tm of PCL can be close to the body-temperature when the molecular weight is low enough.26,27 These features make SMPs interesting for biomedical applications. For instance, some PCL-based systems were designed by Lendlein et al.6 for their potential biomedical applications, such as implant materials for minimally invasive surgical procedures and smart degradable sutures. To design these PCL-based SMPs coupling reactions are carried out in solution between PCL segments, other (co)polyesters, e.g., poly(p-dioxanone) as hard segments, and diisocyanates as coupling agents, yielding poly(ester-urethane) (PUR)-based materials. More recently, to enlarge their processability, e.g. extrusion technology, photo- and thermoreversible reactions have been considered in the synthesis of these polymeric networks with shape-memory properties. For instance, thermoreversible cross-linked PCL-based PUR networks with shapememory properties were designed through Diels−Alder reactions by reactive extrusion technology.28 First, PCL-diol was end-functionalized into furfuryl moieties using furfuryl alcohol and methylene diphenyl 4,4′-diisocyanate. The networks were then obtained by a successive reaction between these PUR precursor (diene) and N,N-m-phenylene-bismaleimide (dienophile). The shape-memory properties could be modulated by partially replacing PCL-tetraol with PCL-diol. To avoid any processing stages at high temperatures, photosensitive groups can also be used to fix the permanent shape of heating-responsive SMPs. M. Nagata et al. prepared some PCL copolyesters with pendant coumarin moieties that are able to participate in photoreversible [2 + 2] cycloaddition reactions.29 At strains of 100−500%, they obtained excellent shape-memory properties: both Rf and Rr were of 88−100%. They observed that the degree of crystallinity of PCL segments and the crosslinking density had a strong influence on the shape-memory performances. Garle et al. investigated the influence of cinnamoyl moieties in semicrystalline PCL copolyester networks.30 Indeed, these moieties undergo photoreversible [2 + 2] cycloaddition reactions upon exposure to UV-light: at a wavelength of λ > 260 nm, cyclobutane rings are formed by dimerization of the double bonds from adjacent CA groups and at wavelengths of λ < 260 nm, the formed cyclobutane rings are photocleaved.31 Garle et al. also copolymerized a cinnamate modified lactone comonomer (CCL) with ε-caprolactone (CL) into various polymeric architectures and formed the covalent network after UV-curing.30 For the P(CL:CCL) (50:50) triblock copolyester where CL was the central block and CCL the end block, both Rf and Rr were above 99%, but the actuation strain remained very small (∼25%). In addition to heating-responsive SMPs, photoresponsive SMPs have attracted a recent attention because they give a quick, spatial and remote control.32,33Although light-irradiation was previously employed to promote the actuation of heatingresponsive SMPs through photothermal effect, Lendlein and co-workers showed the possibility to memorize a secondary shape involving reversible photochemical reactions at room temperature.34 They developed photoresponsive SMPs by incorporating cinnamic acid (CA) and cinnamylidene acetic acid (CAA) as “photo-responsive switches”. The first system described relied on the synthesis of grafted copolymers made of B

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vice versa. To modulate the shape-memory properties (thermal or light), the crystallinity degree of PCL segment was investigated and confirmed using DSC and DMTA measurements. The degree of crystallinity of the PCL segment was controlled by the bulk ring-opening copolymerization of εcaprolactone with para-dioxanone (PDX). This yielded blocklike copolymers with a tunable Tm in function of CL-molar content within the resulting copolymers.42 In order to enhance the light-responsive shape-memory properties, di- and tetracinnamate PCL were synthesized and incorporated into the PURs via swelling methods.

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Table 1. Molecular and Thermal Parameters of Copolymers Obtained by 1,8-Octanediol Initiated Ring-Opening (Co)polymerization of CL and PDX Catalyzed by Sn(Oct)2 CL (mol %)

M̅ n. (kg/mol)

no.

th

expa

th

expa

Đb

Y (%)c

Tm (°C)d

ΔHm (J/g)d

1 2 3 4

100 90 80 70

100 93 80 78

4.2 4.1 3.3 3.9

4.4 5.3 3.7 5.1

1.6 1.7 1.7 1.7

91 80 69 94

53 44 30 26

74 70 53 45

a

As determined by the proton 1H NMR technique recorded in CDCl3. As determined by SEC (THF + 2% NEt3) upon PS calibration. cAs determined by gravimetry. dAs determined by DSC analyses (from −80 to 100 °C at a heating rate of 10 °C min−1; second scan).

EXPERIMENTAL SECTION

b

Materials. α,ω-Dihydroxyl poly(ε-caprolactone) (PCL-diol) (CAPA2402, Mn = 4000 g mol−1; MWD = 1.48) (used for synthesis of PUR(1) and PCL dicinnamate) and α,α′,ω,ω′-tetrahydroxyl poly(εcaprolactone)tetraol (PCL-tetraol) (CAPA4801, Mn = 8000 g mol−1; MWD = 1.48) were kindly supplied by Perstorp and used as received. N,N-Bis(2-hydroxyethyl) cinnamamide (BHECA) was prepared from methyl cinnamate (VWR, for synthesis) and diethanolamine (VWR, for synthesis) in the presence of sodium methoxide (Acros, (30 wt %) solution in methanol) via amminolysis reaction, as reported by L. Wu et al.37 Hydroquinone (HQ ≥ 99%, Aldrich) and 1,8-diisocyanatooctane (ODI, Aldrich) were used as received. ε-Caprolactone (CL, for synthesis, Merck) was dried for 48 h over calcium hydride and distilled under reduced pressure. Crude 1,4dioxan-2-one (PDX, Jlight Chemicals Co, China) was first introduced in a previously flame-dried two-necked round-bottom flask under a nitrogen flow and added with dried toluene (10 wt/vol %). This solution was heated up to 100 °C until its complete dissolution, then cooled down to 50 °C and after which calcium hydride was added. After 24 h, toluene and PDX were distilled off under reduced pressure (ca. 10−1 mbar). In a final step purified PDX was recovered as a solid after toluene volatilization under reduced pressure and stored under nitrogen. Tin(II) octoate (Sn(Oct)2) (95%, Aldrich) was used as received and diluted in dry toluene (0.3 M). 1,8-Octanediol (98%, Aldrich) was dried by azeotropic distillation in toluene before use. trans-Cinnamoyl chloride (98%, Aldrich) was used as received and triethylamine (Aldrich) was dried using barium oxide. Synthesis of Poly(ε-Caprolactone-co-para-Dioxanone) Copolymers of Various Degrees of Crystallinity. The reactions were carried out in bulk in a high-pressure reactor from Autoclave, which was conditioned beforehand under nitrogen flow for 2 h at 200 °C. The copolymers were synthesized by the ring-opening polymerization (ROP) of ε-caprolactone and para-dioxanone initiated by 1,8octanediol. Tin(II) octoate was used as a catalyst at an initial [alcohol]/[tin(II) octoate] molar ratio equal to 100. The syntheses were conducted at 160 °C and 50 rpm under nitrogen atmosphere for 3 h. The crude products were dissolved in a minimum volume of CH2Cl2, followed by precipitation into a 7-fold excess of heptane. The copolyesters were then recovered by filtration and drying under a vacuum. Table 1 shows the molecular and thermal parameters of resulting copolyesters. Synthesis of PUR. Typically, PCL or P(CL-co-PDX) segments were introduced into a 15 cm3 twin-screw DSM microcompounder at 40 °C and 30 rpm. After melting, BHECA, hydroquinone (0.3 mol %) and ODI (1.1 equiv with respect to the overall hydroxyl content) were successively introduced at 80 °C and 30 rpm. Temperature and speed were then raised to 130 °C and 75 rpm, respectively. It is worth noting that ODI was selected here as its boiling temperature is higher than the processing temperature. The reaction was considered complete when the torque was constant. The resulting PUR networks were then recovered and hot-pressed (130 °C) as films. Synthesis of Cinnamate End-Functionalized poly(ε-Caprolactone) Oligomers. Di- and tetracinnamate-terminated PCL were synthesized by respective reactions of PCL diol or tetraol in the presence of cinnamoyl chloride and promoted with triethylamine as elsewhere reported.43 Here, PCL (diol or tetraol) precursor was

dissolved in dried CHCl3, followed by the addition of triethylamine. Cinnamoyl chloride (quadruple amount with respect to OH moles from PCL precursors) was previously solubilized in CHCl3 and the resulting solution was added dropwise to the reaction mixture put in an ice-bath. Then, the reaction medium was set under reflux for 24 h under stirring. After reaction, carbon black was added to absorb excess triethylamine for a few hours. Carbon black and NEt3·HCl salts were removed by filtration under vacuum. The functionalized polymer was then obtained by precipitation in heptane and drying under vacuum. Incorporation of Cinnamate End-Functionalized Poly(εCaprolactone) Oligomers in PUR. A film of PUR (thickness of 0.3 mm) was swollen in the presence of a solution containing 10 wt % cinnamoyl group end-functionalized PCL oligomers solubilized in CH2Cl2. After swelling, the film was taken out from the solution, dried under vacuum at room temperature and pressed again by compression molding. Characterizations. DSC measurements were performed using a DSC Q200 (or Q2000) from TA Instruments under nitrogen flow. Melting temperature and melting enthalpy of the PUR and polyesters were evaluated through the following “heat/cool/heat “ procedure: heating at 10 °C min−1 to 100 °C, cooling at 10 °C min−1 to −80 °C, heating at 10 °C min−1 to 100 °C. The first scan was to erase any prior thermal history of the samples. Tzero pans and lids were utilized and sample weights were generally in the range 5−10 mg. 1H NMR spectra were recorded on a Bruker AMX-500 spectrometer at a frequency of 500 MHz from solutions in CDCl3. ATR Fourier-transform infrared (FTIR) spectra were recorded using Bruker Tensor 17 spectrometer. Size-exclusion chromatography (SEC) was performed in THF (containing 2 wt % NEt3) (sample concentration: 1 wt %) at 35 °C using a polymer laboratories (PL) liquid chromatography instrument (Agilent Technology 1200 series) equipped with a PL-DG802 degazer, an isocratic HPLC pump LC1120 (flow rate: 1 mL min−1), a BasicMarathon Autosampler, a PL-RI refractive index detector and three columns: a guard column PLgel 10 μm (50 mm × 7.5 mm) and two columns PLgel mixed-B 10 μm (300 mm × 7.5 mm). Molar mass and molar mass distribution was calculated by reference to a relative calibration curve from polystyrene standards. Matrix-assisted laser desorption ionization time-of-flight (MALDI-ToF) mass spectrum was recorded using a QToF Premier mass spectrometer equipped with a nitrogen laser, operating at 337 nm with a maximum output of 500 J m−2 delivered to the sample in 4 ns pulses at a 20 Hz repeating rate. Time-of-flight mass analyses were performed in reflection mode at a resolution of about 10 000. The sample was analyzed using (DCTB) trans-2-[3-(4-tertbutylphenyl)-2-methylprop-2-enylidene] malononitrile. This matrix was prepared as a 40 mg mL−1 solution in CHCl3. The matrix solution (1 mg mL−1) was applied to a stainless steel target and air dried. The polymer sample was dissolved in THF to obtain 1 mg mL−1 solution in which 20 μL of NaI solution (2 mg mL−1 water:acetonitrile, 1:1, v/v) was added as cationization agent. Here, 1 μL aliquots of this solution were applied onto the target area already bearing the matrix crystals and air dried. For the recording of the single-stage MS spectra, the quadrupole (rf-only mode) was set to pass C

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ions from 500 to 20 000 Th, and all ions were transmitted into the pusher region of the time-of-flight analyzer where they were mass analyzed with 1 s integration time. Heating-Responsive Shape-Memory Characterization. Samples for heating-responsive shape-memory testing were cut from compression-molded thin films into rectangular shape-memory specimens of approximately 30 mm × 5 mm × 0.25 mm. Prior to the test, the films were irradiated at long wavelengths (λ = 365 nm) with a UV lamp from Spectroline (model ENF-240 cm3/FE, One 4-W BL, BLE-270W) that can also irradiate at short wavelengths (λ = 254 nm). Dynamic mechanical thermal analyses (DMTA) of the PURsystems were performed using a stress-controlled Q800 DMA apparatus in film tension. The shape-memory behavior was characterized using a four-step program as follows: (1) Deformation. Elongation of the sample by applying a given load at a temperature higher than the melting temperature of the PUR system (T = 65 °C). (2) Fixing. The sample was cooled at a rate of 2 °C min−1 to 0 °C under a constant load. (3) Unloading. The load was removed within 2 min. (4) Recovery. The sample was heated to 65 °C at a rate of 2 °C min−1. The shape-memory behavior was characterized by the shape fixity ratio, Rf, and by the shape recovery ratio, Rr. They were calculated using the followings equations:

⎛ε ⎞ R f = ⎜ un ⎟ ⎝ εext ⎠

(1)

⎛ ε − εf ⎞ R r = ⎜ un ⎟ ⎝ εun ⎠

(2)

Scheme 1. Synthesis of the Poly(Ester-Urethane) (PUR) Networks

moieties at high temperatures. A molar content in BHECA of 80 mol %, representing 15.5 wt %, was selected to achieve a good fixation in order to obtain heating-responsive SMPs, based on previous experiments (see Supporting Information Table 1S). It is worth noting that these PCL-based oligomers were obtained by Sn(Oct)2-catalyzed (co)polymerization of CL with PDX as initiated from 1,8-octanediol at 200 °C for 3 h (see Table 1). Table 2 shows the molecular and thermal Table 2. Molecular and Thermal Features of Resulting PURs Obtained by Chain-Extension from the PCL-Based Oligomers of Various Degrees of Crystallinitya

Where εun is the strain directly after cooling and unloading, εext is the strain obtained before the constant loading was released, and εf is the strain obtained directly after heating in the recovery step. A good reproducibility was achieved by repeating times the actuation program three times. Photoresponsive Shape-Memory Characterization. For these experiments, all necessary precautions were taken into consideration and strictly adhered to (proper source shielding to reduce UV exposure, adequate training in the hazards and the safe methods of using the equipment, welder’s masks, wearing of goggles, and face shields). Samples for light shape-memory tests were also cut from compression-molded thin films to form rectangular shape-memory specimens of approximately 60 mm × 10 mm × 0.25 mm. The films were placed between two clamps of a Zwick tensile bench. The distance between the two clamps was fixed at 3 cm (= Li). The films were elongated to 200% (Lext = 9 cm) at 50 mm min−1 and irradiated at 365 nm for 2 h. The lamp was positioned at a distance of 5 cm from the film. After unloading, the fixing ratio was determined by measuring the distance between the traces left by the clamps on the films (= Lun). The samples were then irradiated at 254 nm for 4 h and the recovery ratio was calculated by measuring the distance between the traces (= L4h). Rf and Rr were determined by simple measurements as follows:

⎛ L − Li ⎞ R f = ⎜ un ⎟ ⎝ Lext − L i ⎠

(3)

⎛ L − L4h ⎞ R r = ⎜ un ⎟ ⎝ Lun − L i ⎠

(4)

entry PUR PUR PUR PUR

(1) (2) (3) (4)

CL (mol %)b

Treact (min)

conv (%)c

Tm (°C)d

ΔHm (J/g)d

100 93 83 78

50 50 30 30

100 100 100 100

53 45 52 36

53.5 45.0 1.3 0.7

a

See Table 1. bCorresponding to the experimental values determined for the starting copolyesters. cAs determined by FT-IR measurements. d As determined by DSC measurements (from −80 to 100 °C at a heating rate of 10 °C min−1; second heating cycle).

features of the resulting PURs obtained by chain-extension from these PCL-based oligomers of various degrees of crystallinity. Note that PUR (1) was synthesized from a commercial α,ω-diol PCL-based oligomer of 4000 g mol−1. As far as the PURs are concerned, FTIR analysis confirmed the occurrence of the chain-extension reactions as shown by the absence of the band at 2200−2300 cm−1 assigned to the −N CO functionality. Also noted were the presence of stretching and bending vibrations at 1550−1650 cm−1 relative to amide I and amide II from urethane linkage and a stretching vibrations band at 1700 cm−1 relative to carbonyl from ester linkage (Figure 1). It has been reported that hydrogen bonding can establish between PUR chains. Whereas the −NH− moiety acts as a donor, the acceptor can be either the carbonyl group of urethane moieties and/or ester moieties along the polyester backbone. As a result, the PURs are poorly soluble in any good solvent for PCL, e.g. toluene or chloroform, therefore making molecular weight characterizations by GPC impossible. Even with the lack of solubility for the PURs, attempts were made to highlight the extent of the chain-extension reactions using 1H NMR techniques. For so doing, a 1H NMR spectrum could only be recorded for a PUR prepared from homo-PCL oligomers at a low molar content in BHECA, i.e. 20 mol %

Note that, based on kinetic studies, 4 h is the required time for reaching the maximum shape variation.

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RESULTS AND DISCUSSION A series of PURs were synthesized via a chain-extension reaction between α,ω-diol PCL-based oligomers of various degrees of crystallinity, BHECA monomer as photoreversible monomer and 1,8-octanediisocyanate as coupling agent (Scheme 1). A small quantity of hydroquinone was added to prevent any eventual cross-linking reaction between cinnamate D

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changing the molar composition in PCL-based segments used for the synthesis of PURs. The thermal shape-memory effect (SME) of the recovered PURs (according to Figure 2) was quantified using DMA in tensile mode testing (Figure 3). Before DMA characterizations,

Figure 1. FTIR spectrum of PUR (1) as derived from PCL-based oligomers (see Table 2, entry 1).

(Figure 1S; see the Supporting Information). In addition to the signals assigned to BHECA, 1,8-octanediisocyanate and PCLbased derivatives, the absence of α-hydroxymethylene endgroups from BHECA at 3.87 ppm and from PCL at 3.65 ppm confirms the full conversion of hydroxyl functionalities from both PCL and BHECA into the expected urethane functionalities. However, at 80 wt % BHECA, only FTIR characterizations could be studied and they highlighted the synthesis of PURs by chain-extension reactions. From Table 2, it is shown that all PURs are semicrystalline, as supported by the presence of a melting temperature (Tm) between 36 and 53 °C. In addition, the melting enthalpy (ΔHm) of PURs decreases with the molar PDX-content in PCL-based oligomers used for the chain-extension reactions, decreasing from 53.5 for homoPCL-based PUR (1) to 0.7 J g−1 for the PUR (4) based on copolyester segments containing 22 mol % in PDX (entries 1 and 4, Table 2). This demonstrates the possibility of tuning the thermal properties of PURs by

Figure 3. Cyclic thermomechanical process (DMTA) of PUR (1) containing a PCL/BHECA molar ratio of 20/80 (entry 1, Table 2).

the samples were first set under UV-irradiation (λ > 254 nm) for 2 h to fix the permanent domain related to the cycloaddition of cinnamate moieties from BHECA units.29 During testing, the samples were heated to 65 °C (T > Tm) and stretched to a given elongation (εext) by applying a constant deformation stress (σapp = 0.1 or 0.4 MPa). Subsequently, the sample was cooled to 0 °C under constant stress. The temporary shape was then achieved by releasing the stress and was characterized by an elongation noted εun. Upon heating to 65 °C, the permanent shape was recovered at elongation εf. In the case of the PUR (1) based on homo-PCL

Figure 2. Schematic pathways of the heating-based SME (left part) and the photobased SME (right part) of the dual-stimuli-responsive SMP (DSRSMP) represented below. Each segment is represented as shown in the legend. E

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oligomers, both Rf and Rr values proved to be close to 100% for εext of 80%, attesting for good thermally actuated SMEs. These results are in agreement with those reported on PCL-based copolyesters obtained by copolymerization of ε-caprolactone with a functionalized lactone bearing a cinnamate moiety but at εext below 25%.30 As far as PCL-based cooligoesters are concerned, increasing PDX molar content up to 17 mol % within PCL-based cooligoesters again gives excellent heatingresponsive SMEs for the corresponding PURs. For instance, the respective Rf and Rr values are of 92 and 97% for the PUR made of PCL-based cooligoesters containing 17 mol % PDX. However, at 22 mol % PDX, the resulting PUR does not have any heating-responsive SMEs and more likely acts as an elastic material due to the low ΔHm for the corresponding PUR. This observation highlights that a critical degree of crystallinity is required to reach acceptable temperaturedependent SMEs for the investigated PURs. Photoresponsive SME behavior was tested for these PURs using a tensile bench and a dual wavelength UV lamp (Figure 4). The proposed

Table 4. Photoresponsive Shape-Memory Properties of Resulting PURsa entry PUR PUR PUR PUR a

mechanism is based on a light induced reversible [2 + 2] cycloaddition reaction of the BHECA double bond, as it is clearly schematized in Figure 2. Film samples (60 × 10 × 0.3 mm) were prepared (see Experimental Section) and placed between the bench clamps, set 30 mm apart, and extended to 200% strain. The films were molded to be thin enough, with a maximum thickness of 0.3 mm, in order to enable an efficient irradiation of the UV light. The films were elongated at λ > 260 nm for 2 h in order to promote the [2 + 2] cycloaddition between cinnamate moieties and, therefore, to fix the temporary shape via these resulting cycloadducts from BHECA.36 It is worth noting that the crystallinity of PURs ensures the role of permanent domain in the case of lightactuated SMEs. After unloading, the films were irradiated under UV light at λ < 260 nm for 4 h to disassemble the cycloadducts. From Table 4, it is shown that the studied PURs exhibit photoresponsive SMEs. However, the photobased SMEs depend on the ΔHm for the corresponding PUR, and therefore on the molar PDX-content within those PURs. Increasing the molar PDX-content within the resulting PURs thereby increases Rr, while lowering Rf

a

(1) (2) (3) (4)

σapp (MPa)

Rf (%)

Rr (%)

0.1 0.1 0.4 0.4

100 100 92 ND

100 100 97 ND

Rr (%)

98 78 45 43

18 15 55 27

See Table 2.

Scheme 2. Synthesis of Cinnamate-Modified PCL Diol or Tetraol

Complete end-group functionalization was confirmed by 1H NMR analyses (see Figures 2S and 3S of the Supporting Information). For instance, the resonance signals relative to methylene (6.4 and 7.6 ppm) and phenyl (7.0 to 7.4 ppm) protons of cinnamate moieties were observed, together with the absence of protons relative to the initial α-hydroxymethylene end-groups of the di- and tetrahydroxyl-terminated PCL precursors. In addition, to get more information on the quantitative end-group modification, the PCL tetraol was also characterized by MALDI-MS after esterification by cinnamoyl chloride. As attested by the mass spectrum reported in Figure 4, one main distribution was observed which corresponds to Na+-

Table 3. Heating-Based SME of PURs as Obtained by ChainExtension from Different PCL-Based Copolyestersa entry

Rf (%)

values except for the last entry (entry 4, Table 4). The latter highlights poor photoresponsive SMEs as a result of the almost amorphous morphology of the PUR, which could not correctly fix the permanent shape during photoactuation. Interestingly enough, at 17 mol % PDX relative content (entry 3, Table 4), while Rf was merely of 45%, Rr reached a value as high as 55%, indicating that the photoresponsive SMEs can be enhanced when the ΔHm, and therefore the crystallinity degree of resulting PURs, decreased (up to a critical value). It is worth noting that the Rr values may appear relatively low with respect to the literature related to light-actuated SMEs. However, the extension level used in this work (above 200%) was much higher than those reported in the literature. In fact, the reported Rr and Rf values were calculated by considering the length after unloading instead the length at maximal strain (see the Experimental Section for the definition). In other terms, this work clearly attests for the synthesis of PURs exhibiting significantly better light-actuated SMEs. To further enhance the heating and photoresponsive SMEs, incorporation of cinnamate-terminated PCL into PURs was attempted using a solvent-cast procedure as reported by Lendlein et al.34 Di- and tetracinnamate-terminated PCL (linear and four-arm star-shaped) samples were obtained by esterification reactions of di- and tetrahydroxyl-terminated PCL, respectively, in the presence of cinnamoyl chloride and promoted by triethylamine at 90 °C overnight (Scheme 2).43

Figure 4. Extension and light-exposure (A) at λ > 260 nm and lightexposure (B) at λ < 260 nm.

PUR PUR PUR PUR

(1) (2) (3) (4)

See Table 2. F

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striking features were that the heating and photoresponsive SMEs are both enhanced on the addition of the cinnamateterminated PCL into PURs. Rf value of 66% and a Rr value of 76% were obtained for the photobased SME of the blend containing 4 wt % dicinnamate-terminated PCL, while values close to 100% were recorded for the temperature-SMEs. In addition, the shape-recovery of these PUR systems proceeded within 2 s in the case of heating-responsive SMEs and within 4 h in the photoresponsive SMEs (see the video and pictures in the Supporting Information), confirming the efficiency of both the temperature- and light-actuated SMEs. These results clearly highlight the beneficial effect of adding di- or tetracinnamateterminated PCL to PURs in order to design materials with dually actuated SMEs.

cationized four-arm star shaped PCL end-capped by the expected 4-cinnamoyl groups. This is readily confirmed by comparing the experimental data with the corresponding theoretical isotopic model (Figure 5). To be complete, two

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CONCLUSIONS

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ASSOCIATED CONTENT

PURs with double-SMEs were successfully obtained by chainextension between α,ω-diol PCL-based oligomers of various degrees of crystallinity, BHECA monomer as the photoreversible monomer and 1,8-octanediisocyanate as the coupling agent by reactive extrusion processing. In order to tune the heating and photoresponsive SMEs, the crystallinity of PCL segments was finely adjusted by the copolymerization of εcaprolactone with para-dioxanone in bulk at 160 °C using tin(II) octoate as a catalyst, and the resulting polyester segments were then coupled with BHECA in the presence of n-octyl diisocyanate at 130 °C. The SMP properties of resulting PURs were correlated to DSC and DMTA measurements. Further addition of di- and tetracinnamate PCL segments in these SMPs was also studied in order to enhance the photobased SMEs of the PURs.

Figure 5. MALDI mass spectrum for cinnamate-modified PCL tetraol, main distribution corresponds to Na+ cationized PCL. ▲ and ● are K+ and N(Et)3H+ cationized PCL, respectively.

weaker ion distributions were also observed and were attributed to the expected cinnamate-modified PCL sample here cationized by K+ and protonated triethylamine (N(Et)3H+). To demonstrate the enhancement of the heating and photoresponsive SMEs, the resulting di- and tetracinnamateterminated (linear and four-arm star-shaped) PCLs were incorporated into the PURs (3) (Table 3, entry 2). In practice, 10 wt % di- or tetracinnamate-terminated PCLs, together with PURs (3), were swollen in CHCl3 overnight. After swelling, the film was taken out of the PCL cinnamate solution in CHCl3 and the solvent was smoothly evaporated under reduced pressure. Then the resulting mixture was hot-pressed. The weight take-in was determined by the following equation: PM (%) = (Wt − W0)/W0 × 100 with Wt and W0 being the weight after and before swelling, respectively (Table 5). It was revealed that the weight take-in reached 4 and 6 wt % for the resulting blends containing the di- and tetracinnamate-terminated PCL, respectively. DSC analyses also revealed that ΔHm of resulting blends increased moderately after incorporation of di- and tetracinnamate-terminated PCLs into PURs (3). For instance, ΔHm passed from 0.5 to 2.66 J g−1 when 10 wt % dicinnamate PCL was added into PUR (3) (entry 3, Table 5). The most

S Supporting Information *

Table of temperature−SMP properties of PURs with varying molar contents in PCL and BHECA. 1H NMR spectra in CHCl3 of the PUR prepared with the PCL/BHECA molar content of 80/20% (entry 1, Table 1S), dicinnamate-terminated PCL and tetracinnamate-terminated PCL. Videos of temperature- and light-SMEs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Table 5. Photo- and Heating-Responsive Shape-Memory Properties of PUR (3) Containing Di- and TetracinnamateTerminated PCLa light

thermal

type

PM

Tm (°C)b

ΔHm (J g−1)b

strain (%)

Rf (%)

Rr (%)

σapp (MPa)

Rf (%)

Rr (%)

PUR (3) PUR (3) with tetracinnamate PCL PUR (3) with dicinnamate PCL

6 4

51.6 33.6 41.2

0.8 5 2.66

200 200 200

45 75 66

55 60 76

0.4 0.15 0.2

92 97 99

97 100 100

After swelling at 10 wt %; strain rate: 50 mm min−1. bAs determined by DSC measurements (from −80 to 100 °C at a heating rate of 10 °C min−1; second heating cycle).

a

G

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(33) Yu, Y.; Ikeda, T. Macromol. Chem. Phys. 2005, 206, 1705−1708. (34) Lendlein, A.; Jiang, H.; Junger, O.; Langer, R. Nature 2005, 434, 879−882. (35) Sodhi, J. S.; Rao, I. J. Int. J. Eng. Sci. 2010, 48, 1576−1589. (36) Wu, L.; Jin, C.; Sin, X. Biomacromolecules 2011, 12, 235−241. (37) Jin, C.; Sun, X.; Wu, L. Des. Monomers Polym. 2011, 14, 47−55. (38) Zhao, Q.; Behl, M.; Lendlein, A. Soft Matter 2013, 9, 1744− 1755. (39) Bellin, I.; Kelch, S.; Langer, R.; Lendlein, A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18043−18047. (40) Julich-Gruner, K. K.; Löwenberg, C.; Neffe, A. T.; Behl, M.; Lendlein, A. Macromol. Chem. Phys. 2013, 214, 527−536. (41) Mohr, R.; Kratz, K.; Weigel, T.; Lucka-Gabor, M.; Moneke, M.; Lendlein, A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3540−3545. (42) Raquez, J-.M.; Degee, P.; Dubois, P. Polym. Eng. Sci. 2005, 45, 622−629. (43) Sun, S-.J.; Yun, J. H.; Lee, S.; Park, J-.K.; Kim, D-.H.; Cho, K. Y. React. Funct. Polym. 2010, 70, 622−629.

ACKNOWLEDGMENTS LPCM would like to thank the Belgian Federal Government Office of Science Policy (SSTC- PAI 6/27) for general support and would also like to thank both Wallonia and the European Commission “FSE and FEDER” for financial support in the frame of Phasing-out Hainaut. F.P. is a FRS-FNRS Ph.D. student, and J.-M.R. is a FRS-FNRS research associate.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on October 2, 2014, with an error to Figure 2. The corrected version was reposted on October 10, 2014.

H

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