Silica Ionic Hybrids

Mar 27, 2017 - Commercial polylactide (PLA) was converted and endowed with shape-memory properties by synthesizing ionic hybrids based on blends of ...
0 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Shape-Memory Behavior of Polylactide/Silica Ionic Hybrids Jérémy Odent,† Jean-Marie Raquez,‡ Cédric Samuel,§ Sophie Barrau,∥ Apostolos Enotiadis,† Philippe Dubois,*,‡ and Emmanuel P. Giannelis*,† †

Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States Laboratory of Polymeric and Composite Materials (LPCM), Center of Innovation and Research in Materials and Polymers (CIRMAP), University of Mons (UMONS), Place du Parc 23, 7000 Mons, Belgium § Department of Polymers and Composites Technology and Mechanical Engineering (TPCIM), Mines Douai, Rue Charles Bourseul 941, CS 10838, 59508 Douai, France ∥ Unité Matériaux et Transformations (UMET)−CNRS UMR 8207, Université Lille 1, 59655 Villeneuve d’Ascq, France ‡

S Supporting Information *

ABSTRACT: Commercial polylactide (PLA) was converted and endowed with shape-memory properties by synthesizing ionic hybrids based on blends of PLA with imidazolium-terminated PLA and poly[ε-caprolactone-co-D,L-lactide] (P[CL-co-LA]) and surfacemodified silica nanoparticles. The electrostatic interactions assist with the silica nanoparticle dispersion in the polymer matrix. Since nanoparticle dispersion in polymers is a perennial challenge and has prevented nanocomposites from reaching their full potential in terms of performance we expect this new design will be exploited in other polymers systems to synthesize well-dispersed nanocomposites. Rheological measurements of the ionic hybrids are consistent with the formation of a network. The ionic hybrids are also much more deformable compared to the neat PLA. More importantly, they exhibit shape-memory behavior with fixity ratio Rf ≈ 100% and recovery ratio Rr = 79%, for the blend containing 25 wt % im-PLA and 25 wt % im-P[CL-co-LA] and 5 wt % of SiO2−SO3Na. Dielectric spectroscopy and dynamic mechanical analysis show a second, low-frequency relaxation attributed to strongly immobilized polymer chains on silica due to electrostatic interactions. Creep compliance tests further suggest that the ionic interactions prevent permanent slippage in the hybrids which is most likely responsible for the significant shape-memory behavior observed.

■

INTRODUCTION

deployable and morphing structures, medical implants, actuators, and self-healing systems. Over the past decade, the design of shape-memory materials has focused on dynamic and supramolecular polymers, where a specific bond or interaction can be broken and reformed upon external stimulus.15,16 Besides their shape-shifting ability, such dynamic systems have the ability to adapt to their environment and possess a wide range of intriguing physical properties.17−22 Recent studies have successfully demonstrated various tunable materials based on a double-network design.23−27 For instance, Sun et al.26,27 designed hydrogels consisting of two interpenetrating ionically and covalently cross-linked networks with partial and even full self-recovery after rupture. They reported that these supramolecular hydrogels can be designed using a wide range of motifs exploiting strong and weak bonds; the permanent network of covalent cross-links preserves the memory of the initial state, whereas the sacrificial ionic network reversibly breaks and reforms to recover after experiencing a deformation.

Shape-memory polymers (SMPs) are a class of stimuliresponsive materials that have attracted widespread attention because of both scientific interest as well as their potential in various applications, especially in textiles,1 biomedicine2 (e.g., self-tightening sutures), and advanced engineering3−5 including the electronics and aerospace industries. When subjected to external stimuli such as a change in temperature, light, moisture, or magnetic field, SMPs can recover from a metastable state to their original shape.5−10 Among them, temperature responsive SMPs are the most promising and by far the most used in industry. They usually consist of a twocomponent system: permanent domains made of either chemical or physical cross-links and switching domains, which are associated with a suitable transition temperature (Ttrans).11−13 A sample cooled below Ttrans after deformation recovers its original shape when heated above Ttrans. The shape recovery is thought to be due to the entropic relaxation of the polymer chains.5,14 This feature is naturally very attractive and has made SMPs the focus of several scientific and engineering studies. The objective is to develop smart materials and integrate them in high performance applications such as © XXXX American Chemical Society

Received: January 26, 2017 Revised: March 9, 2017

A

DOI: 10.1021/acs.macromol.7b00195 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Synthesis of Rigid PLA and Soft P[CL-co-LA]. The synthesis of PLA and P[CL-co-LA] was carried out by bulk ring-opening polymerization (ROP) of L,L-lactide or ε-caprolactone/D,L-lactide (30 mol %) and promoted by 1,8-octanediol and tin(II) octanoate. The initial molar ratio of [alcohol]/[tin(II) octanoate] and [monomer]/[alcohol] was 100 and 200, respectively. The synthesis of PLA and P[CL-co-LA] was carried out for 1 and 3 h, respectively in a reactor (200 cm3) at 160 °C. The reaction was quenched using an ice bath. The crude product was dissolved in a minimum volume of CHCl3, followed by precipitation into a 10-fold excess of heptane. The polymers were recovered after filtration and drying under vacuum: PLA, yield ≈ 99%, Mn ≈ 30 500 g/mol, D ≈ 1.7, Tg ≈ 58 °C, Tm ≈ 169 °C ; P[CL-co-LA], yield ≈99%, Mn ≈ 30 000 g/mol, D ≈ 1.7, CL/ LA ≈ 72/28 mol %, Tg ≈ −37 °C, no Tm. Imidazolium-Terminated PLA and P[CL-co-LA]. First 1 equiv of either rigid PLA or soft P[CL-co-LA] synthesized as described above was dissolved in CH2Cl2 in a glass reactor, followed by addition of 10 equiv of 1-methyl-3-propionic acid−imidazolium bromide, 10 equiv of N,N′-dicyclohexylcarbodiimide, and 1 equiv of 4-(dimethylamino)pyridine under stirring at room temperature for 24 h. The endfunctionalized polymer was filtered and washed by CH2Cl2 in order to remove the dicyclohexylurea byproduct, and finally recovered by precipitation into a 10-fold excess of methanol, followed by filtration and drying under vacuum (im-PLA, yield ≈99%; im-P[CL-co-LA], yield ≈ 90%). The 1H NMR spectra (CDCl3, δ, ppm) at 8.15 (1H, s, −N−CH−N−) and 6.8 ppm (1H, s, −N−CHCH−N−) confirmed the incorporation of imidazolium moieties into the polymers (see Figure S2). Sulfonate Functionalization of Silica Nanoparticles. Sulfonate-modified silica nanoparticles were prepared following a previously reported method.28 In a flask, Ludox HS 30 colloidal silica (3 g) was diluted with deionized water (22 mL). In a separate flask, 3(trihydroxysilyl)-1-propanesulfonic acid (4 g) was also diluted with deionized water (20 mL). The colloidal silica suspension was slowly added to the SIT suspension, while stirring vigorously. To the mixture, a solution of sodium hydroxide solution (1 M) was added dropwise until a pH of about 5 was reached. The entire solution was then heated to 70 °C and stirred vigorously for 24 h. After that, the suspension was cooled to room temperature, placed into dialysis tubing and dialyzed against deionized water for 3 days while changing the water twice a day (d ≈ 17 ± 5 nm, MSO3H ≈ 1 mmol, Rorganic ≈ 24%, C ≈ 10 wt % in water). Preparation of Ionic Polylactide Hybrids. Imidazoliumterminated polymers (i.e., im-PLA and/or im-P[CL-co-LA]-IMID) and PLA pellets were dissolved in a minimal amount of DMF under stirring at 70 °C for 1 h. Separately, the 10 wt % solution of sulfonated silica nanoparticles was diluted with DMF and the resulting sulfonated silica was added to the polymer solution. After being stirred at 70 °C for an extra 3 h, the resulting solution was solvent-cast at 80 °C and then compression-molded at 180 °C for 5 min. Characterization Techniques. Proton nuclear magnetic resonance (1H NMR) spectra were recorded in CDCl3 using a Bruker AMX-500 spectrometer at a frequency of 500 MHz. Size-exclusion chromatography (SEC) was performed in THF (containing 2 wt % NEt3) at 35 °C using an Agilent liquid chromatograph equipped with a degasser, an isocratic HPLC pump (flow rate, 1 mL/min), autosampler (loop volume, 100 μL; solution concentration, 1 mg/mL), refractive index detector, and three columns: a guard column PLgel 10 μm and two columns PLgel mixed-B 5 μm. Molecular weight and molecular weight distribution were calculated by reference to a relative calibration curve made of polystyrene standards. Differential scanning calorimetry (DSC) was performed using a DSC Q2000 (TA Instruments) at heating and cooling rates of 10 °C/min under nitrogen flow (to avoid any thermal history effects, the second scan was used). Thermal gravimetric analysis (TGA) was performed using a TGA Q500 from TA Instruments at a heating rate of 20 °C/min under flowing nitrogen. Dynamic Light Scattering (DLS) was carried out in water (concentration less than 1 wt %) at 25 °C using a Malvern Zetasizer. Rheological measurements were carried out on an Anton-

Our group has developed organic−inorganic hybrids consisting of a soft polymeric canopy bound to a well-defined nanoparticle core by ionic interactions.28−30 Referred to as nanoscale ionic materials (NIMs), they are typically composed of a nanometer-sized inorganic cores, surface functionalized with a charged corona (e.g., sulfonate modified silica), in which an oppositely charged canopy consisting of a low molecular weight polymer (e.g., amine terminated poly(ethylene oxide)) is introduced to counterbalance the charge. Their hybrid nature allows for their properties to be easily engineered by selectively choosing their components. In addition, the rapid exchange between the ionically modified nanoparticles and the polymeric canopy provides opportunities for adaptive and/or multiresponsive properties.31 In this paper, we leverage the reversible ionic bonds in NIMs with the reinforcement effect of nanoparticles to demonstrate new hybrid systems with unique responses. Our design is based on hybrids of blends of imidazolium-terminated oligomers of glassy polylactide (PLA) and rubbery poly[ε-caprolactone-coD,L-lactide] (P[CL-co-LA]) (referred to as im-PLA and im-P[CLco-LA]) dispersed in commercial PLA with sulfonate-modified silica (SiO2−SO3Na) (the synthesis is schematically shown in Figure S1). The reversible/dynamic nature of the ionic interactions endows the hybrids with unique behavior and transformed commercial PLA from a nonresponsive to a shapememory polymer. This is extremely significant, since PLA demonstrating shape-memory is very attractive especially for medical implants, but only a limited shape-memory effect (i.e., relatively low recovery strain) for unmodified PLA has been demonstrated up to now.32−34 We believe that the design demonstrated in this study (i.e., systems based on ionic hybrids) is not limited to PLA and can be exploited more generally to endow other unresponsive polymers with high degree of shape-memory behavior.

■

EXPERIMENTAL SECTION

Materials. ε-Caprolactone (99%, Acros) was dried for 48 h over calcium hydride and distilled under reduced pressure. L,L-lactide or D,L-lactide (>99.5%, Purac) was stored in a glovebox. 1,8-octanediol (98%, Aldrich) was dried over molecular sieves (4 Å). Tin(II) octanoate (Sn(Oct)2) (95%, Aldrich) was used as a solution in dry toluene (0.01 M). A commercially available extrusion-grade PLA (NatureWorks 4032D) especially designed for production of biaxially oriented films was used as received ( M n = 133500 ± 5000 g/mol, Đ = 1.94 ± 0.06 as determined by size-exclusion chromatography, 1.4 ± 0.2% D-isomer content as determined by the supplier). Ludox HS 30 colloidal silica (mean diameter 18 nm, Aldrich), 3-(trihydroxysilyl)-1propanesulfonic acid (SIT, 40 wt %, Gelest), sodium hydroxide solution (1 M, Aldrich), 1-methylimidazole (99%, Aldrich), 3bromopropionic acid (97%, Aldrich), N,N′-dicyclohexylcarbodiimide (99%, Aldrich), 4-(dimethylamino)pyridine (99%, Aldrich), chloroform (CHCl3, > 99.9%, Aldrich), dichloromethane (CH2Cl2, > 99.8%, Aldrich), heptane (>99%, Aldrich), diethyl ether (>99%, Aldrich), methanol (>99.8%, Aldrich), dimethylformamide (DMF, > 99%, Aldrich) and dialysis tubing (Spectra/Por RC Biotech Membrane, 15K MWCO, 16 mm flat width) were obtained commercially and used without further purification. Synthesis of 1-Methyl-3-propionic Acid−Imidazolium Bromide. First, 1.1 equiv of 3-bromopropionic acid and 1 equiv of 1methylimidazolium were dissolved in dry CHCl3 into a glass reactor. The mixture was refluxed at around 60 °C for 24h with stirring. The crude product was recovered after precipitation into a 10-fold excess of diethyl-ether, filtration and drying under vacuum. Yield ≈ 87%. 1H NMR (CDCl3, δ, ppm): 2.1 (2H, t, CH2COOH), 3.1 (3H, s, CH3), 3.65 (2H, t, −N−CH2−), 6.7 and 6.8 (1H and 1H, s, −N−CHCH− N−), 8.15 (1H, s, −N−CH−N−). B

DOI: 10.1021/acs.macromol.7b00195 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Schematic of Ionic Hybrids Based on PLA and Imidazolium-Terminated PLA and P[CL-co-LA] Oligomers with Sulfonated Silica Nanoparticles

Paar rheometer using frequency sweeps and parallel plate geometry with a diameter of 25 mm. All measurements were performed at 180 °C with a strain of 1% to ensure that the deformations are in the linear viscoelastic regime. Transmission electron microscopy (TEM) was performed on a FEI Tecnai T12 Spirit Twin TEM/STEM microscope operated at 120 kV. The samples were cryomicrotomed at −100 °C by a Leica UCT microtome. Dielectric relaxation measurements were performed on a Broadband Dielectric Spectrometer from 10−1 to 106 Hz using an ALPHA analyzer supplied by Novocontrol. Samples were placed between two gold-plated electrodes (diameter 20 mm). Conductivity was measured at ambient temperature. The dc conductivity, σdc, of the samples was determined from the real part of the complex conductivity in the region of the low-frequency plateau, (∼10−1 Hz). Dielectric permittivity spectra were further collected isothermally from −70 to +100 °C. The derivative formalism was applied to the real part of permittivity ε′ as a function of the angular frequency ω for eliminating the contribution of dc conductivity as successfully exploited by Choi et al.35 on poly(ethylene) ionomers containing imidazolium pendant groups (see eq 1). εder = −

∂ε′(ω) π × 2 ∂(ln ω)

Rf =

Rr =

εun εext

(2)

εun − εf εext

(3)

Here εext is the maximum strain before the stress was released, εun is the strain after cooling and unloading, and εf is the remaining strain after heating during the recovery step. A good reproducibility was ensured by repeating the actuation program three times.

■

RESULTS AND DISCUSSION The design of polylactide- (PLA-) based materials with shapememory properties relies on the electrostatic (ionic) interactions between imidazolium-ended polymer segments of rigid polylactide (im-PLA) and soft poly[ε-caprolactone-co-D,Llactide] (im-P[CL-co-LA]), respectively, with sulfonate-modified silica nanoparticles (nanosilica−SO3Na) (Scheme 1) and the close location of the anionic sulfonate to the cationic imidazolium groups optimizing Coulomb interactions.36 The system was tuned by the introduction of both rigid im-PLA and soft im-P[CL-co-LA] within the (conventional) PLA matrix. Rheological measurements were used to confirm the importance of electrostatic interactions between the charged ionic groups in the hybrids. The storage modulus of PLA-based hybrids containing 5 wt % of SiO2−SO3Na is affected significantly by the addition of im-PLA (Figure 1). As the imPLA content increases, the storage modulus in the lower frequency regime increases by orders of magnitude and a gellike plateau can be seen. This well-known gel-like behavior (absent from the hybrids of neat PLA containing the same loading of silica nanoparticles) is attributed to the development of the ionic network of the imidazolium-terminated PLA with the charged silica.37 Using the Winter−Chambon criterion of tan δ being frequency independent (see Figure S3), we conclude that the ionic network is formed even at 10 wt % of im-PLA in the PLA matrix.38,39 Importantly, the electrostatic interactions assist with the silica nanoparticle dispersion in the polymer. As the amount of endfunctionalized im-PLA in the hybrids increases from 0 to 50 wt %, the nanoparticle dispersion improves (Figure 1 and Figure

(1)

Dynamic mechanical analysis was carried out at 55 °C in film shearing mode (specimens of 14 × 2 × 0.45 mm3) using Acoem, Metravib, DMA 1150. Frequency sweeps from 100 to 0.01 Hz were performed at a constant strain of 5 μm. Shape-Memory Characterization. Samples for shape-memory testing were cut from compression-molded thin films into rectangular specimens of approximately 30 × 5 x 0.15 mm3. Dynamic mechanical thermal analyses (DMTA) at ambient atmosphere used a stresscontrolled DMTA Q800 apparatus from TA Instruments in a film tension mode. The shape-memory behavior was characterized using a four-step program as follows: (1) deformation, the sample was elongated by applying a given load (stress ramp of 0.1 MPa until ca. 0.5 MPa, and strain around 100%) at a temperature higher than the glass transition temperature (Tg) of the neat PLA (T = 65 °C); (2) fixing, the sample was cooled at a rate of 2 °C/min to room temperature (25 °C) under a constant load; (3) unloading, the load was removed within 2 min (at intervals of 0.1 MPa until 0 MPa); and (4) recovery, the sample was heated to 65 °C at a rate of 2 °C/min. The shape-memory behavior was characterized by the shape fixity Rf (see eq 2) and the shape recovery Rr (see eq 3) ratios calculated using the following equations: C

DOI: 10.1021/acs.macromol.7b00195 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Storage modulus G′ as a function of angular frequency at 180 °C of PLA-based hybrids with 5 wt % of SiO2−SO3Na (black diamonds) and the corresponding hybrids containing 10 wt % (green triangles), 30 wt % (blue squares), and 50 wt % (red circles) of im-PLA. Cross-sectional TEM micrographs (with scales of 1 μm (left) and 200 nm (right)) for the hybrids based on neat PLA (bottom) and 50 wt % of im-PLA (top) are also included.

LA] stabilizes the final phase-morphology and minimizes their incompatibility with the PLA matrix. Consistent with our hypothesis, the introduction of the dynamic ionic interactions converts the brittle PLA matrix into a tough polymer able to withstand very high levels of deformation (see Figure S4). Owing to our interest in developing shape-memory polymers we focus next on characterizing the shape-memory effect (Figure 3). For testing, the samples were heated to 65 °C (T > Tg of the PLA matrix) and stretched to a given elongation (εext ≈ 100%) by applying a constant deformation stress (σapp = 0.1 MPa). Subsequently, the samples were cooled to room temperature (i.e., 25 °C) under constant stress. The shape fixity ratio, Rf, describes the ability of the material to maintain the deformation after cooling and release of the load while the shape recovery ratio, Rr, refers to the ability of the material to revert to (memorize) its original state after heating. Neat PLA displayed poor shape-memory properties, even, when a low stress is applied (Figure 3a). Addition of 5 wt % ionic silica nanoparticles (SiO2-SO3Na) to PLA leads to brittle materials with an even worse shape-memory behavior (Rr

2a). Note that nanoparticle dispersion in polymers is a perennial challenge has prevented nanocomposites from reaching their full potential in terms of performance. We posit that the new design can be exploited in other systems to readily synthesize well-dispersed nanocomposites with enhanced properties. The ionic interactions and formation of the ionic network can be used to explain the overall morphology of the hybrids (Figure 2). Note that im-PLA appears miscible with PLA leading to a uniform morphology of the polymer matrix. The only features seen in the TEM images are those of the silica nanoparticles (Figure 2a). In contrast, addition of im-P[CL-coLA] forms a second phase (Figure 2b,c). When im-P[CL-coLA] is added alone a double network is seen while the addition of both im-PLA and im-P[CL-co-LA] in PLA leads to dispersed spherical inclusions (Figure 2c). The morphologies of all samples are consistent with optimizing the location of silica particles to maximize electrostatic interactions between the sulfonate and imidazolium groups. Furthermore, the location of nanoparticles at the interface of both im-PLA and im-P[CL-coD

DOI: 10.1021/acs.macromol.7b00195 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 2. Cross sectional TEM micrographs (with scales of 5 μm (left) and 500 nm (center) for PLA/SiO2−SO3Na (5 wt %) hybrids containing (a) 50 wt % of im-PLA, (b) 50 wt % of im-P[CL-co-LA], and (c) 25 wt % of im-PLA and 25 wt % of im-P[CL-co-LA]. The schematics on the right are ideal representations of the observed microstructure for each sample.

Figure 3. Shape-memory properties of neat PLA (images a, a′, and a″) and PLA-based hybrids containing 25 wt % of im-PLA and 25 wt % of imP[CL-co-LA] with 5 wt % of SiO2−SO3Na (images b, b′, and b″). The initial length for each sample is 4 cm (images a and b). The samples are stretched at 65 °C to 8 cm, (100% of deformation) and fixed at this size by cooling to 25 °C (images a′ and b′). Then, the samples are reheated to 65 °C and their recovery is measured (images a″ and b″).

E

DOI: 10.1021/acs.macromol.7b00195 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Table 1. Shape-Memory Properties of Neat PLA and Corresponding PLA-Based Hybrids of im-PLA and im-P[CL-co-LA] with SiO2−SO3Naa entry

PLA (wt %)

im-PLA (wt %)

im-P[CL-co-LA] (wt %)

nanosilica−SO3Na (wt %)

Rf (%)

Rr (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

100 100 75 50 75 50 50 50 50 50 50 50 90 70

0 0 25 50 0 0 25 25 25 25 20 30 5 15

0 0 0 0 25 50 25 25 25 25 30 20 5 15

0 5 5 5 5 5 0 1 3 5 5 5 5 5

≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100 ≈100

10 2 5 26 21 48 17 55 65 79 12 23 59 73

As determined by DMTA tests in tension film mode (First equilibrate at 65 °C, ramp stress 0.1 MPa/min to 0.5 MPa, i.e. strain around 80%, equilibrate at 25 °C, ramp stress 0.1 MPa/min to 0 MPa, ramp temperature 2 °C/min to 65 °C). a

Figure 4. Storage modulus G′ (a) and complex viscosity η* (b) as a function of the angular frequency at 180 °C for PLA/im-PLA/im-P[CL-co-LA] (50/25/25 wt %) (black diamonds) and corresponding hybrids containing 1 wt % (green triangles), 3 wt % (blue squares), and 5 wt % (red circles) of SiO2−SO3Na.

decreases from 10 to 2%, respectively). In contrast, virtually all PLA-ionic hybrids show an improved shape memory effect. The best improvement (Rr ∼ 80%) is for the ionic hybrid consisting of 25 wt % im-PLA and 25 wt % im-P[CL-co-LA] (Figure 3b). Table 1 summarizes the shape-memory performance of PLAbased systems quantified by DMTA. Note that adding the imidazolium-terminated polymers to PLA but in the absence of silica nanoparticles (i.e., hybrids void of the ionic network) leads to a minor improvement (Rr = 20%). In contrast, the addition of the charged silica nanoparticles to form ionic hybrids improves dramatically Rr and the improvements scale with the amount of silica added. The Rr values for the hybrids containing 1, 3, and 5 wt % silica are 55, 65 and 79%, respectively. Note that the theoretical charge balance (i.e., 1:1 ratio sulfonate:imidazolium ratio) at this specific composition (i.e., 25 wt % im-PLA and 25 wt % im-P[CL-co-LA]) is achieved at about 5 wt % of silica. Note also that the ionic hybrid with the best shape-memory behavior is based on both im-PLA and im-P[CL-co-LA] surrounded by silica within the PLA phase (see Figure 2c). The shape-memory effect is highly

dependent on the silica content and the material composition. For instance, changing the ratio of im-PLA/im-P[CL-co-LA] from 1:1 led to significant decreases of shape-memory properties. The formation of the ionic network and unique morphologies seem to play a significant role in determing the shape memory behavior. In this respect, the rheological measurements discussed earlier are consistent with the formation of such unique ionic structures resulting in significant increases in the storage modulus and related complex viscosity in the low frequency regime (Figure 4). The shape memory properties are clearly governed by a complex synergy between the different components in the hybrids with the recovery performance linked to the polymer chain mobility near the glass transition.40−43 To probe this effect and in an effort to understand the structure-shape recovery relationship, the hybrids were characterized further by dielectric and mechanical spectroscopy. First, the α-relaxation process was investigated by dielectric spectroscopy (Figure 5). For neat PLA, the segmental α-relaxation could be readily observed between 60 and 80 °C with ωmax between 3 × 101-1 × F

DOI: 10.1021/acs.macromol.7b00195 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 5. Dielectric spectra of neat PLA and PLA based hybrids at different temperatures: 60 °C (blue triangles), 70 °C (green circles) and 80 °C (red squares) for (a) neat PLA, (b) PLA/im-PLA (50/50 wt %), (c) PLA/im-PLA/SiO2−SO3Na (50/50/5 wt %), (d) PLA/im-PLA/im-P[CL-coLA] (50/25/25 wt %), and (e) PLA/im-PLA/im-P[CL-co-LA]/SiO2−SO3Na (50/25/25/5 wt %). The asterisks and arrows indicate the second lowfrequency relaxation and the ωmax, respectively.

105 rad s−1 (Figure 5a) in good agreement with previously reported values by Mierzwa et al.44 The incorporation of 50 wt % of im-PLA in PLA shifts the segmental α-relaxation to higher frequencies by ca. 1 decade, which is probably due to the plasticization of the PLA matrix by the low-molecular weight im-PLA (Figure 5b). While the addition of silica nanoparticles to produce the ionic hybrids has a small effect on the segmental α-relaxation process, it also leads to a second relaxation at low frequencies (marked by an asterisk in the spectra) with a shift

by ca. 2−3 decades depending on temperature (Figure 5c). We believe that this second, low-frequency relaxation with moderate amplitude is due to the “immobilized” im-PLA interacting strongly with the silica nanoparticles via electrostatic interactions as discussed previously. The shift of the αrelaxation of PLA to higher frequencies upon addition of imPLA probably due to the plasticization of the PLA matrix by the low-molecular weight im-PLA is consistent with dynamic mechanical analysis (see Figure S5). Note that the DMA was G

DOI: 10.1021/acs.macromol.7b00195 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules performed at a different temperature compared to the dielectric relaxation spectroscopy measurements. A consistent albeit more complicated picture was observed for the neat im-PLA/im-P[CL-co-LA] and the corresponding hybrids. The spectra are dominated by a high-frequency relaxation in the range 103−104 Hz, which is specifically attributed to the terminal relaxation (i.e., rubbery-liquid transition) and/or normal relaxation modes of im-P[CL-coLA] while the α-relaxation of PLA seems to be depressed. Upon addition of the silica nanoparticles, the main relaxation becomes slower by approximately an order of magnitude and a second (weaker) relaxation at lower frequencies (as in the case of im-PLA) can be seen. As discussed above we attribute this second relaxation to “immobilized” im-PLA interacting strongly with the silica nanoparticles via electrostatic interactions. The intensity is lower though consistent with the lower amount of im-PLA in the sample. A common method to trigger shape memory in polymers is to first heat above the glass or melting transition for amorphous and crystalline polymers, respectively. After heating and deformation, the material is fixed by cooling. Subsequent heating allows the sample to recover its original shape. Shape recovery relies on the presence of reversible, physical or chemical cross-links that prevent permanent chain slippage.45 Physical cross-links in particular include ionic interactions, hydrogen bonding, π−π stacking, and metal−ligand interactions and because of their nature they are reversible.19 While the specific mechanism responsible for shape memory in our system is unclear at present we suspect that it is largely due to the presence of ionic interactions between the cationically charged imidazolium chains and the negatively charged silica nanoparticles. Note that the neat PLA or the silica hybrid based on PLA only show poor shape memory behavior with Rr of 10% and 2%, respectively (entries 1 and 2, Table 1). Ionic hybrids based on blends of 50 wt % im-PLA or 50 wt % im-P[CL-co-LA] in PLA (entries 4 and 6, Table 1) exhibit better Rr than the neat polymer or hybrids void of ionic interactions. In addition, shape memory behavior improves as the amount of silica nanoparticles (and the extent of ionic interactions) increases in the hybrids (entries 7 to 10, Table 1). The best performances are seen for samples containing equimolar amounts of both im-PLA and im-P[CL-co-LA] (entries 10, 13 and 14, Table 1). In addition to the ionic interactions, the microstructure and morphology of the sample appear to be important. For example, recall the composition based on 25 wt % im-PLA and 25 wt % im-P[CL-co-LA] is made up of dispersed spherical inclusions (Figure 2c). On the basis of the above, and similarly to the extensive literature on ionomer based shape-memory polymers,46 we propose that the ionic interactions facilitate shape memory by serving as a permanent network (i.e., the anionic silica nanoparticles act as netpoints between the cationic polymer chains). In an effort to better understand the mechanism, creep experiments were carried out (Figure 6). The creep compliance increases with stretching time for neat PLA and the PLA/imPLA/SiO2−SO3Na hybrid (50/50/5 wt %, respectively), showing an inflection point at intermediate stretching times. The first part is basically associated with the glass−rubbery transition, while the final part is related to the rubbery-liquid transition. The rubbery plateau appears very short (almost absent for the neat PLA) for these two systems with a liquid state obtained at short stretching times (