Shape Memory Composites Based on Electrospun Poly(vinyl alcohol

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Shape Memory Composites Based on Electrospun Poly(Vinyl alcohol) Fibers and a Thermoplastic Polyether Block Amide Elastomer Anuja Shirole, Janak Sapkota, E. Johan Foster, and Christoph Weder ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00834 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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ACS Applied Materials & Interfaces

Shape Memory Composites Based on Electrospun Poly(Vinyl alcohol) Fibers and a Thermoplastic Polyether Block Amide Elastomer Anuja Shirole, Janak Sapkota, E. Johan Foster# and Christoph Weder* Adolphe Mekle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, Switzerland *

#

Corresponding author. Email: [email protected]

Current address: Virginia Tech, Macromolecules and Interfaces Institute, Department of Materials Science & Engineering, 445 Old Turner Street, 213 Holden Hall, Blacksburg VA 24061, USA

Abstract The present study aimed at developing new thermally responsive shape-memory composites, that were fabricated by compacting mats of electrospun poly(vinyl alcohol) (PVA) fibers and sheets of an thermoplastic polyether block amide elastomer (PEBA). This design was based on the expectation that the combination of the rubber elasticity of the PEBA matrix and the mechanical switching exploitable through the reversible glass transition (Tg) of the PVA filler could be combined to create materials that display shape memory characteristics as an emergent effect. Dynamic mechanical analyses (DMA) show that at upon introduction of 10-20% w/w PVA fibers, the room-temperature storage modulus (E′) increased by a factor of 4-5 in comparison to the neat PEBA, and they reveal a stepwise reduction of E′ around the Tg of PVA (85 °C). This transition could indeed be utilized to fix a temporary and recover the permanent shape. At low strain, the fixity was 66 ± 14% and the recovery of 98 ± 2%. Overall, the data validate a simple and practical strategy for the fabrication of shape memory composites that involves a melt compaction process and employs two commercially available polymers.

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Keywords Composites, shape memory polymer, electrospinning, poly(vinyl alcohol), thermoplastic polyether block amide elastomer

Introduction Stimuli-responsive polymers, which change their mechanical characteristics in response to a predefined stimulus in a highly selective manner, are attracting considerable current interest and are considered useful for a broad range of technologically relevant applications.1-5 Shape memory polymers, which can be programmed to adopt a ‘fixed’ temporary shape and be converted to their ‘memorized’ original shape when an external stimulus (e.g. thermal, mechanical, chemical, light, electrical) is applied, represent a subset of this class of materials.6-10 Polymers can be bestowed with shape-memory characteristics by combining the effect of rubber elasticity with a switching element that can be addressed to enable (i.e., during loading and unloading) or prevent (i.e., while a temporary shape is fixed) elastic deformation. Rubber elasticity is typically imparted via a network structure that involves covalent or physical cross-links, whereas discrete reversible phase transition in the polymer can be used as the switch. On the basis of these simple design guidelines, a plethora of different shape-memory polymers has been developed. The most widely studied design involves thermally switchable shape-memory polymer networks, in which the glass transition of an amorphous phase or the melting of crystalline domains are employed as switching elements.8, 11-13 An alternative approach involves the fabrication of (nano) composites that consist of an elastic matrix and a micro- or nano-sized filler that is responsible for fixing the temporary shape.14 This can be achieved by either a thermally or a chemically induced phase transition of the filler or the formation / break-up of a mechanically reinforcing network on account of changing the 2 ACS Paragon Plus Environment

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interactions between the filler particles.15-16 One intriguing feature of shape-memory composites is that they display emergent properties, i.e., a characteristic behavior that is not found in either of the components alone. From an application point of view, the possibility to create novel materials with new properties from known components that are readily available without new chemistry is particularly attractive. In this context, the fabrication of shape memory composites in which a responsive filler is fabricated by electrospinning has recently gained significant traction.17-18 This process is a well-established and convenient method to fabricate (mats of) a wide range of polymeric fibers that have a high surface to volume ratio and is applicable to many polymers.19,20-21 Shape memory composites research field is now increasingly focusing on incorporation of electrospun micro or nano fibers in composites to impart shape memory effect activated by many different stimuli in these materials.22 For example, Leng and co-workers recently demonstrated a heat- and water-triggered shape memory effect for a bio-compatible hybrid membrane based on poly(caprolactone) and poly(ethylene oxide) that was fabricated via electrospinning technology.23 The same group also investigated the shape-memory behavior of electrospun chitosan/poly(ethylene oxide) composite membranes.24 Mather et al. recently reported polymer composites fabricated by dual electrospinning and compression molding, composed of poly(vinyl acetate) (PVAc) and poly(ε-caprolactone) (PCL) which demonstrated shape memory-assisted self-healing property.25 Similar approaches were used to obtain shape memory composites containing poly(ε-caprolactone) (PCL) and a thermoplastic polyurethane.17 Leng and coworkers reported an electrically activated shape memory nanocomposite consisting of a Nafion® matrix and electrospun polyacrylonitrile-based carbonized nanofibers, by solution impregnation and a compaction process.26 In the present study, we report the fabrication of shape memory composites by a simple melt compaction process and employing two widely used commercially available polymers, i.e., 3 ACS Paragon Plus Environment


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electrospun fibers of semicrystalline poly(vinyl alcohol) (PVA) as the switching element, and a thermoplastic polyether block amide elastomer (PEBA) as the rubber-elasticity providing matrix (Figure 1). While the architecture is somewhat reminiscent of the composites recently reported by Korley and coworkers,27-28 who incorporated electrospun PVA mats as filler in poly(vinyl acetate) and an ethyleneoxide-epichlorohydrin copolymer, the latter materials were studied in the context of waterinduced mechanical adaptation and do not exhibit shape memory behavior. Also, in most of these systems, the composites were fabricated using solvent impregnation approach or dual electrospinning followed by compression molding, whereas our approach does not involve any solvent and uses most widely used technologically relevant electrospinning process to fabricate shape memory composites using commercially available polymeric materials.

Experimental Section Materials. Poly(vinyl alcohol) (PVA, 99% hydrolyzed, weight-average molecular weight, Mw, = 124,000 g·mo1-1, Tg ~85 °C, Tm ~220 °C), rhodamine B isothiocyanate, fluorescein 5(6)isothiocyanate, anhydrous dimethyl sulfoxide (DMSO), and solvent grade methanol (MeOH) were purchased from Sigma-Aldrich and used without purification. The polyether block amide (PEBA) thermoplastic elastomer employed (Pebax® 6333, Tg ~0 °C, Tm ~170 °C) was supplied by Arkema. Electrospinning of the PVA Fibers. PVA (10% w/w) was dissolved in deionized water by stirring at 90 °C for 3 h. The resulting solution was subjected to electrospinning, using a setup that consisted of a syringe pump (NE-4000 model from KF Technology), a 20 mL polymer syringe fitted with a stainless steel needle with an outer diameter of 0.8 mm, and an aluminum foil as a stationary collector fixed 15 cm away from the needle tip (Figure 1). The feed rate was set to 0.020 mL·min-1 and the applied 4 ACS Paragon Plus Environment

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voltage, generated by a SL30 model, from Spellman HV Electronics Ltd, was 25 kV. The spinning time was varied to make PVA fiber mats of desired weight with different average thickness (~50-100 µm) for fabrication of various compositions. Fabrication of PEBA/PVA Composites by Melt-Impregnation of Electrospun PVA Mats. PEBA films with a thickness of 100 µm were produced by compression molding the as-received polymer in a Carver® press under a pressure of 3 bar at 180 °C for 5min between Teflon® sheets using spacers to control the thickness. The electrospun PVA mats were subsequently sandwiched between two PEBA films and this assembly was compacted in a Carver® press under a pressure of 3 bar at 180 °C for 5 min (Figure 1). The resulting composite films had a thickness of ~220-280 µm. Three different compositions containing 10, 15, or 20% w/w of the electrospun PVA mat in PEBA were prepared. For instance, to prepare the 10% w/w composite, a PVA mat weighing 0.25 g was sandwiched between two PEBA films weighing 1.125 g each and the assembly was compacted as detailed above.

Figure 1. Schematic representation of the fabrication process of PVA fibers and PEBA/PVA composites. The electrospun PVA mats were sandwiched between two PEBA films and the composite film was produced by meltimpregnation/compaction.

PEBA/PVA/PEBA Reference Films. For reference purposes, PEBA/PVA/PEBA multilayered films containing 10% w/w PVA were prepared under similar conditions as the PEBA/PVA composites, but employing PVA films instead of the PVA mats. The PVA films used had a thickness of 150 µm and 5 ACS Paragon Plus Environment


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were obtained by solution casting 50 mL of a 1% w/w aqueous PVA solution into a Petri dish (diameter 7.5 cm) and drying at 70 °C for 48 h. Scanning Electron Microscopy. To investigate the morphology of the electrospun PVA mats, the composites, as well as the multilayer reference films, cross sections of the samples cut by normal surgical blade at room temperature were analyzed by scanning electron microscopy (SEM), using a TESCAN MIRA3 octane pro operated at an accelerating voltage of 5-8 kV. All samples were coated with gold (thickness 3 nm) prior to analysis. The diameter of the PVA fibers was determined by analyzing at least 5 independent SEM images with image analysis software ‘ImageJ’, measuring the width of more than 15-20 individual nanofibers per image. An average diameter of 757 ±264 nm was found. Confocal Microscopy of Composites Made from Components Labelled with Fluorescent Dyes. PVA was labelled with rhodamine isothiocyanate by stirring a solution of the polymer (7.00 g) and the dye (0.35 mg) in dry DMSO (70 mL) at 90 °C for 3 h. The polymer was isolated by precipitation in methanol (500 mL), washing with methanol (300 mL), filtration, and drying at 60 °C for 24 h. An aqueous solution of the labelled PVA (10% w/w) was subjected to electrospinning under the same conditions as for the non-labelled PVA. PEBA (4.00 g) was labelled with fluorescein isothiocyanate (2 mg) by melt mixing the two components in twin screw extruder (DACA micro-compounder) at 180 °C for 10 min. Mats of the rhodamine isothiocyanate labelled PVA and composites comprised of 10% w/w of these mats and the fluorescein isothiocyanate labelled PEBA were produced using the same protocols as for the non-labelled components. Cryo-sliced (using a surgical blade) sections of a film with a thickness of about ~150 µm were imaged by confocal optical microscopy on a laser scanning microscope (Zeiss LSM 710, Carl Zeiss, AG, Germany) using an excitation wavelength of 488 nm for fluorescein and 561 nm for rhodamine. 6 ACS Paragon Plus Environment

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Dynamic Mechanical Analysis (DMA). The mechanical properties of the composites and the PEBA/PVA/PEBA reference films were analyzed using a TA Instrument DMA Q800. The experiments were carried out in tensile mode with a strain amplitude of 15 µm and at a frequency of 1 Hz in the temperature range of -50 to 250 °C applying a heating rate of 5 °C·m-1. The samples were prepared by cutting strips with a width of ~6 mm and length of ~10 mm. The mechanical data shown in Table 1 and values quoted for E′ in the text represent averages of 3–5 independent measurements ± standard deviations. Shape Memory Analysis. The shape memory behavior of the PEBA/PVA composites and the PEBA/PVA/PEBA reference films was studied by DMA in controlled force mode, using the same equipment and sample geometry as detailed above and conducting experiments under either low strain (3-5%) or high strain (25-40%) conditions. A strain abort step was utilized to achieve the targeted strains. Samples were heated to 90 °C, deformed by applying forces of up to 10 N (low strain) and 18 N (high strain) at rates of and 0.05 or 0.08 N·min-1, respectively. The samples were maintained elongated at this temperature for 10 min, before they were cooled to 10 °C at a rate of 5 °C·min-1 to fix the temporary shape. Keeping the strain measurement on, the applied force was removed and the samples were heated at a rate of 5 °C·min-1 to 120 °C (low strain) or 90 °C (high strain) and kept at the final temperature for 10 min to recover the original shape. Three cycles were conducted for each sample and the fixity (%) and recovery (%) for each cycle was calculated according to the Eq. (1) and (2) respectively:17

% Fixity =

ε − ε × 100 … … … … … … … … … … … … … … … … … … … … … … … (1) ε − ε

% Recovery =

ε − ε × 100 … … … … … … … … … … … … … … … … … … … … … … . . (2) ε − ε



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Where, ε is the strain after stretching, ε is the strain after unloading, ε is the recovered strain after heating, and ε is the initial strain. Differential Scanning Calorimetry (DSC). DSC measurements were performed under N2 using a Mettler-Toledo STAR system. The typical procedure included heating and cooling cycles of ~10 mg of the sample in a DSC pan from 0 to 250 to 0 °C using a heating and cooling rates of 10 °C·min-1. The glass transition temperature (Tg) was determined from the midpoint of the DSC curve in the vicinity of the glass−rubber transition, while the melting temperature (Tm) was established as the highest temperature point of the melting endotherm. Thermogravimetric Analyses (TGA). The thermal stability of the materials studied was probed by thermogravimetric analysis using a Mettler-Toledo STAR thermogravimetric analyzer under nitrogen atmosphere in the range of 25 to 600 °C with a heating rate of 10 °C·min-1 using ~6 mg of the sample.

Results and Discussion The shape memory composites investigated in this study were composed of electrospun mats of poly(vinyl alcohol) (PVA) and a commercial thermoplastic polyether block amide elastomer (PEBA) composed of flexible polyether and rigid polyamide segments. While neither of the two already widely commercially used components displays appreciable shape memory behavior, their combination was designed to promote this effect as an emergent characteristic. The shape memory effect in the new composites is enabled by the elastic nature of the PEBA (whose physical network is responsible for the shape recovery based on entropic grounds) and the ability to thermally switch the mechanical properties of the semicrystalline PVA filler between a rigid (below the glass transition) and a soft (above the glass transition) state, which permits thermal fixation/release of a temporary shape. The 8 ACS Paragon Plus Environment

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PEBA/PVA composites were prepared by electrospinning ~50-100 µm thick PVA mats, sandwiching these between two PEBA films of a thickness of ~100 µm, and compacting this assembly at 180 °C in a hot press (see Experimental Section for details). Three different compositions containing 10, 15, or 20% w/w of the electrospun PVA mat in PEBA were prepared. Multilayered PEBA/PVA/PEBA reference films containing 10% w/w PVA were prepared under similar conditions, but employing PVA films instead of the PVA mats. Photographs of an electrospun PVA mat, a control PEBA/PVA/PEBA multilayer film and a PEBA/PVA composite film containing 10% w/w PVA are shown in Figure 2a-c. The multilayer control film is largely transparent and colorless (Figure 2b) and appears more homogenous than the PEBA/PVA composite film (Figure 2c), mainly due to thickness variation of the PVA mat. The apparent homogeneity of the composite films appears to decrease with increasing PVA content (Figure S1) and the samples show an increasing yellow hue, which is assigned to (minor) thermal degradation of the PVA. Since the yellowing is virtually absent in the control PEBA/PVA/PEBA multilayer films, we speculate that this effect is related to the large surface area of the electrospun PVA mat.

Figure 2. Pictures of (a) an electrospun PVA mat; (b) a control PEBA/PVA/PEBA multilayer film containing 10% w/w PVA; and (c) a PEBA/PVA composite film containing 10% w/w PVA. Shown images are ca. 2 cm x 2 cm.



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The morphology of the samples prepared was studied by scanning electron microscopy (SEM) and confocal optical microscopy. The SEM image of the electrospun PVA mat (Figure 3a) shows the typical nonwoven structure of PVA fibers with an average diameter of 757 ± 264 nm. The crosssection of the control PEBA/PVA/PEBA multilayer film (Figure 3b) shows an alternating layered structure of the neat PEBA and PVA films. The SEM images of the PEBA/PVA composite films (Figure 3c-e) containing 10, 15, or 20% w/w of the PVA mat shows nicely that the PVA fibers remain intact and that the fibrous mats are well impregnated with PEBA. However, a PVA-rich layer that is sandwiched between two PEBA-rich layers can be clearly discerned.

Figure 3. Scanning electron microscopy (SEM) images of (a) an electrospun PVA mat (b) a reference PEBA/PVA/PEBA multilayer film (white arrows indicating the PEBA layers, the red arrow the PVA layer), and (c-e) the PEBA/PVA composite films containing 10, 15, and 20% w/w PVA respectively. All scale bars represent 50 µm.


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To render the composites suitable for characterization by confocal optical microscopy, PVA and PEBA were labeled with rhodamine and fluorescein, respectively. Images of the rhodamine labelled PVA fibrous mat (Figure 4a) show randomly oriented fibers and mirror the observation made by SEM. Images of the labeled PEBA/PVA 10% w/w composite (Figure 4b-d) reveal the layered nature of the composite better than the SEM images. Images acquired upon simultaneous excitation of both components (Figure 4b), or the PEBA (Figure 4c) and PVA fiber mat (Figure 4d) only, show a core that is rich in PVA (but not void of PEBA), a relatively narrow, transient border between the layers, and a shell that is exclusively formed by PEBA.

Figure 4. Confocal optical microscopy images of (a) rhodamine labeled PVA fibers and (b-d) PEBA/PVA composites containing 10% w/w PVA made from fluorescein labelled PEBA and a rhodamine labelled PVA mat. Images were acquired upon excitation of both components (b) and the (c) fluorescein labelled PEBA layer or (d) rhodamine labelled PVA layer only. All scale bars represent 100 µm.



The thermal stability of the PEBA/PVA composites and the constituent polymers was determined by thermogravimetric analysis (TGA) under inert nitrogen atmosphere (Figure 5). The thermal stability of the neat PEBA is clearly the highest, with an onset of decomposition around 400 °C, whereas decomposition of the electrospun PVA mat starts already at around 220 °C. The composites show TGA traces that reflect a combination of the degradation profiles of their constituents. Importantly, none of the samples displays an appreciable weight loss at the processing temperature (180 °C). 120 100

Weight (%)

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Figure 5. Thermogravimetric analysis (TGA) curves of the neat PEBA, the neat PVA mat, the control PEBA/PVA/PEBA multilayer film containing 10% w/w PVA, and the PEBA/PVA composites.

Differential scanning calorimetry (DSC) experiments were performed with the neat PEBA, the PVA mat, the control PEBA/PVA/PEBA multilayer film containing 10% w/w PVA, and the PEBA/PVA composites (Figure 6) to explore the thermal transitions in these materials. The first heating curves (Figure 6a) show the expected endothermic peaks associated with the melting of PEBA (except in the PVA mat) and PVA (except in the neat PEBA) at ~170 and ~225 °C, respectively. The DSC trace of the PVA mat also shows a pronounced transition with maximum at ~85 °C, which is associated with the glass transition of this polymer; unlike in the dynamic mechanical analyses discussed below, this transition is not visible in the DSC traces of the composites, on account of the small heat flow


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associated with this transition and the low PVA content in the composites. In the first cooling traces, the expected crystallization peaks of both PVA and PEBA at ~190 °C and ~120 °C, respectively, can be seen; interestingly, the incorporation of the fibrous PVA mat into the PEBA increases the crystallization temperature of the PEBA from ~120 °C (neat PEBA) to ~135 °C (composites), most probably due to nucleation effects (Figure 6b).

(a) 60

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Figure 6. DSC curves showing (a) the first heating and (b) the first cooling cycle of neat PEBA, the neat PVA mat, the control PEBA/PVA/PEBA multilayer film containing 10% w/w PVA, and the PEBA/PVA composites with 10, 15, or 20% PVA. The traces are vertically shifted for clarity.

The mechanical characteristics of the PEBA/PVA composites and the reference materials were investigated as a function of temperature by dynamic mechanical analysis (DMA). The DMA trace of the neat PEBA seems to suggest a broad glass transition around 0 °C, shows a rubbery plateau above this temperature, and reveals mechanical failure due to melting around 170 °C (Figure 7). The storage modulus (E′) of the matrix polymer is already rather high; 365 MPa at room temperature (25 °C) and 109 MPa at 100 °C (Table 1). The introduction of the PVA mat leads to a significant reinforcement, on account of the semicrystalline nature of the PVA, both above and below the PVA’s glass transition temperature (Tg ~85 °C). For instance at 100 °C, E′ increased from 109 MPa (neat PEBA) to 310, 359, 13 ACS Paragon Plus Environment


and 410 MPa upon introduction of 10, 15, or 20% w/w PVA, respectively (Table 1). A similar trend was observed at 25 °C (i.e. below the PVA’s Tg), where E′ increased from 365 MPa (neat PEBA) up to 1.8 GPa (composite with 20% w/w PVA). The DMA traces further demonstrate that the mechanical coherence of all PVA-containing samples above the melting temperature of the PEBA (around 170 °C) is improved. The rubbery plateau between the melting temperatures of PEBA and PVA becomes more pronounced as the PVA content is increased, as a result of the load-bearing nature of the PVA mat or film, respectively. Importantly, all PVA-containing composites show a step-wise decrease of E′ upon being heated above the PVA’s glass transition temperature (Figure 7). This feature is essential for the shape-memory behavior of these materials, which relies on the possibility to “switch” the mechanical

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Figure 7. Dynamic mechanical analysis (DMA) traces of films of the neat PEBA, the control PEBA/PVA/PEBA multilayer film containing 10% w/w PVA, and the PEBA/PVA composites.

characteristics of the PVA filler by heating/cooling through this transition and using the resulting mechanical change to fix/release a temporary shape. The composites containing 15 or 20% w/w PVA and the 10% w/w PVA containing control multilayer film show an additional plateau between 170 and 14 ACS Paragon Plus Environment

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220 °C, i.e., above the failure temperature of the PEBA matrix. In this regime the PVA mat (or film in the case of the reference film) becomes the sole load-bearing element. A comparison of the DMA traces of the PEBA/PVA/PEBA multilayer control and the PEBA/PVA composite with the same PVA content (10% w/w) shows that the two materials display a similar behavior thermomechanical, although the stiffness of the composite was significantly higher. This is likely due to the large surface area of the PVA fibers which enables strong interactions between the filler and matrix polymer. Table 1. Storage modulus (E′) of films of the neat PEBA, the control PEBA/PVA/PEBA multilayer film containing 10% w/w PVA, and the PEBA/PVA composites.

PEBA PEBA / PVA Control (10% w/w) PEBA / PVA (10 % w/w) PEBA / PVA (15 % w/w) PEBA / PVA (20 % w/w) a

E′ (MPa) at 25 °Ca 365 ± 44 828 ± 80 1343 ± 163 1347 ± 264 1826 ± 89

E′ (MPa) at 100 °Ca 109 ± 13 232 ± 19 310 ± 38 359 ± 28 411 ± 21

Determined by DMA. Results are averages of 3-5 independent measurements ± standard deviation.

The shape memory behavior of the PEBA/PVA composites was first qualitatively evaluated by subjecting the composites to simple heating-deformation / cooling / heating cycles. Figure 8 shows images of such a cycle, in which a film of the PEBA/PVA composite with 20% w/w PVA was heated to 90 °C (i.e., above the Tg of the PVA, Figure 8a) and the sample was formed into a spiral shape at this temperature. The sample was held in the spiral shape and was cooled to room temperature. As seen from the picture shown in Figure 8b, the spiral shape could readily be fixated by this process, while subsequent heating to 90 °C led to full recovery of the original shape (Figure 8c,d) We note that hot silicon oil was used for the recovery in the experiment documented in Figure 8, but an identical



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result was observed when water was used (Figure S2). Indeed, despite the hydrophilic nature of the PVA filler, the equilibrium water uptake of PEBA/PVA composites was very low (4%, Figure S3).

Figure 8. Pictures showing the shape memory behavior of a PEBA/PVA 20% w/w composite film. (a)The sample was

heated to 90 °C, formed into a spiral shape, and the temporary shape was fixed by cooling to ambient temperature (b). The original shape was recovered (c, d) upon heating the sample again to 90 °C in silicon oil.

The shape memory behavior was quantitatively evaluated under well-controlled conditions in a DMA that permitted conducting experiments in controlled force mode. Experiments were conducted under either low strain (3-5%) or high strain (25-40%) conditions. Samples were heated to 90 °C, deformed by applying forces of up to 10 N (low strain) and 18 N (high strain) at rates of 0.05 or 0.08 N·min-1, respectively. This temporary shape was then fixed by cooling to 10 °C, while keeping the stress constant. The applied stress was subsequently removed, and the fixity was calculated using Eq. 1. The original shape was recovered by heating the sample to achieve recovery, the extent of which was calculated using Eq. 2. These cycles were repeated multiple times to explore possible hysteresis effects. We note that under high strain conditions, maximum recovery was achieved at 90 °C, i.e., at 16 ACS Paragon Plus Environment

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the same temperature that was used for programming, while under low-strain conditions a slightly higher temperature (120 °C) had to be used. The shape memory curves of the PEBA/PVA composite containing 20% w/w PVA are shown in Figure 9a,b and values for fixity and recovery for all composites are compiled in Figure 9c,d. Figure 9a reveals that under low-strain conditions, the shape memory cycle is reversible and virtually no hysteresis is observed, this can also be seen in bar graphs that show average values for the first (Figure 9c) and consecutive shape memory cycles (Figure 9d).

6

(b) 50

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40 Strain (%)

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) C (°

) C (°

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re tu ra pe m Te

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MP a)

00

Str ess (

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Str ess (M Pa ))

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tu ra pe m Te

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

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% Fixity % Recovery 0%LS

0

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% Fixity % Recovery 0%LS

% w/w of PVA

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% w/w of PVA

Figure 9. Shape memory cycles of a PEBA/PVA 20% w/w composite under (a) low strain (LS) and (b) high strain (HS)

conditions. Samples were heated to 90 °C, before a force of up to 10 N (low strain, rate = 0.5 N/min) or up to 18 N (high strain, rate = 0.8 N/min) was applied. The temporary shapes were fixed by cooling to 10 °C at a rate 5 °C/min, while keeping the stress constant. The applied stress was removed, and the original shape was recovered by heating the sample back to 120 °C (low strain) or 90°C (high strain) at a rate of 5°C/min. The bar graph shown in (c,d) compile fixity and recovery values for (c) the first shape memory cycle and (d) successive shape memory cycles. Data shown are average values from 3 individual samples and error bars are standard deviations.



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Analysis of the data from 2nd and 3rd cycles, using Eqs. 1-2 results an appreciable fixity of 66 ± 14 % and a very good recovery of 98 ± 2%. The shape memory graph reveals a clear step-wise transformation, from the temporary to the original shape, as the composite is heated from below to above the Tg of the PVA and shows that full recovery, necessitates heating to a temperature that is slightly above the programming temperature (at least at the heating rate employed). When the shape-memory experiment was conducted under high stress/strain conditions, a significant hysteresis was observed in the first cycle (Figure 9b,c) which was absent in subsequent cycles. A comparison with the data acquired for the neat PEBA (Figures S4 and S5) suggests that this effect is largely related to irreversible morphological changes of the thermoplastic elastomer matrix, such as chain slippage and re-orientation of the hard segments, which are well known to be caused in such materials under high strain.29-30,31-32 Following the protocol used in a previous study,16 in which an initial ‘conditioning’ cycle was employed to “pre-stretch” the material with the objective to eliminate the otherwise appreciable hysteresis from the analysis, the shape memory characteristics were extracted from the average of second and third cycle. This resulted a high fixity of 75 ± 10% and a recovery of 96 ± 3% (Figure 9b,d). The fixity of the composites is higher in the first cycle than in the consecutive cycles (from which the above values were extracted), which might at least to some extent be associated with the hysteresis observed in the first cycle.

Similar trends were observed for

composites with 10 and 15% w/w PVA (Figures S4, S5), although the values for fixity and recovery decreased with decreasing PVA content. Also the reference composites containing 10% w/w PVA film shows similar shape memory behavior comparable to the composite with 10% w/w PVA fibers (Figure S4-S6), but delamination is observed in this multilayered structure (Figure S7) restricting the potential application as a shape memory material. Comparative experiments with the neat PEBA (Figure S4, S5) reveal that the neat matrix polymer does not show any appreciable shape memory


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behavior. Further, one might speculate that the semicrystalline PVA on its own might have a shape programming capability between Tg and Tm. While shape memory cycles on neat PVA mats under low and high strain conditions (Figure S8) show that the neat PVA mat has good shape fixity when programmed at 90ºC, the mat is unable to recover to the original shape. The latter aspect is related to the fibrous nature of the object and largely irrecoverable deformation of the mesh-like structure upon programming. Thus the data confirm that the shape memory effect is indeed the result of the combination of the two components. Conclusion In summary, we have shown that PVA fibers prepared by electrospinning can serve as a thermally responsive reinforcing filler that is useful to bestow an elastomeric PEBA matrix with shape memory characteristics. A sequential process that involves electrospinning of a fibrous PVA mat and melt impregnation/compaction of this filler above the melting temperature of the PEBA, but below the melting temperature of the PVA can be used to create composites in which the PVA fibers retain their fibrous morphology. The mechanical properties of the PEBA/PVA composites thus made reveal a significant reinforcement below and above the Tg of PVA fibers vis a vis the neat PEBA matrix and the materials display a step-wise modulus decrease around the Tg of PVA. Qualitative as well as quantitative experiments confirm that the combination of the rubber elasticity of the PEBA matrix and the mechanical switching exploitable through the reversible glass transition of the PVA filler leads to materials that display shape memory characteristics as an emergent effect. Possible applications likely require a reduction of the hysteresis that is largely inherent to the matrix polymer used. A larger ratio of the stiffness at usage and programming temperatures (which generally leads to better recovery but reduced recovery stress) should be achievable by utilizing fibers of a PVA copolymer with reduced



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crystallinity. Overall, it appears that the approach used here is general and may be exploitable in different types of PEBA/PVA composites, e.g. using conventionally spun PVA fibers.

Supporting Information Pictures of PEBA/PVA composites containing 15% and 20% w/w PVA; graphs, bar graphs, and images characterizing the shape memory behavior of additional films of the neat PEBA, the PVA mat, the PEBA/PVA/PEBA reference composite, and PEBA/PVA composites, additional DSC data, and results of swelling experiments in water.

Acknowledgements The authors gratefully acknowledge financial support received from the Swiss National Science Foundation (NRP 62, NRP 66: Resource Wood, Nr. 406640_136911/1), the Adolphe Merkle Foundation, and Sonova AG. The authors thank Marc Karman for assistance with Figure 1.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

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Shape Memory Composites Based on Electrospun Poly(vinyl alcohol

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