Poly(vinyl alcohol) particle-reinforced elastomer composites with water-active shape-memory effects

European Polymer Journal 53 (2014) 230–237

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Poly(vinyl alcohol) particle-reinforced elastomer composites with water-active shape-memory effects Tongfei Wu a,b, Kevin O’Kelly b, Biqiong Chen a,⇑ a b

Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK Department of Mechanical and Manufacturing Engineering, Trinity College Dublin, College Green, Dublin 2, Ireland

a r t i c l e

i n f o

Article history: Received 11 July 2013 Received in revised form 20 January 2014 Accepted 28 January 2014 Available online 3 February 2014 Keywords: Polymer–matrix composites Mechanical properties Shape-memory effects Smart materials

a b s t r a c t Polymer composites with excellent water-active shape-memory effects (SMEs) were successfully prepared using polyvinyl alcohol (PVA) submicron particles as the SME-activating phase and thermoplastic polyurethane (TPU) as the resilient source and matrix. The incorporation of high-modulus, hydrophilic PVA particles (with a size of 80–200 nm) improved the Young’s modulus and water uptake of elastomeric and hydrophobic TPU significantly, which leads to large changes of the modulus upon exposure to water. Water acted as a plasticizer of PVA phase, decreasing its modulus. Such modulus changes were reversible and dependent on the content of PVA particles, which can be up to 16 times. As a result, the shape-memory performance of the composite was also dependent on the content of PVA particles. The PVA particle-reinforced TPU composite containing 15 vol.% PVA particles exhibited the best SMEs among the composites investigated, with the shape fixing and shape recovery ratios being 97% and 97%, respectively. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Polymers with shape-memory effects (SMEs) are a type of important stimuli-responsive materials with the ability to recover their original shape upon exposure to external stimuli [1–3]. Numerous structures have been developed and widely studied for the potential in many areas such as biomedical devices, textiles, energy, bionics engineering, and civil engineering [4]. The ingredient for SMEs is a resilient physically/chemically cross-linked network with a reversible switching transition of its resilience. The switching transition involves a marked change in the modulus of the materials with SMEs upon exposure to stimuli for the shape recovery. The majority of current reversible switching is physically and thermally activated through temperature change. The transition temperature can be ⇑ Corresponding author. Tel.: +44 (0) 114 222 5958; fax: +44 (0) 114 222 5943. E-mail address: biqiong.chen@sheffield.ac.uk (B. Chen). http://dx.doi.org/10.1016/j.eurpolymj.2014.01.031 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.

the glass transition temperature (Tg, in amorphous segments) [5–7], melting temperature (Tm, in semicrystalline segments) [8], or liquid crystalline clearing temperature (Tcl, in liquid crystalline segments) [9]. The temperature change can be achieved by using an external heat source, electric field [10] or even electromagnetic field [11,12] depending on the nature of the constituents. The reversible switching can also be activated by athermal effects, such as crystallization-dissolving transition induced by small molecule absorption/desorption (e.g., water) [13]. The reversible switching can even be reversible cross-linking reactions induced by light irritation [14,15], temperature change [16], or redox treatments using small molecules [17], and supramolecular association/disassociation induced by light irritation [18] or pH change [19]. Recently, research interest into water-active SMEs has been rapidly growing, due to the potential for biomedical applications [2], such as minimally invasive medical devices [20]. Since the first report based on crystallization-dissolving transition in 2005 [13], water-active SMEs

T. Wu et al. / European Polymer Journal 53 (2014) 230–237

have been observed in many polymer materials [20–24]. They are mainly achieved by two methods. One is based on the glass transition as the source of switching [21,23]. Water penetrates into the hydrophilic segments of the polymer network and increase the flexibility of the macromolecules through its plasticizing effect [25]. Tg of the hydrophilic segments drops to a temperature below the desired temperature of the shape recovery and/or deformation (e.g., room temperature and body temperature) [26,27]. Another method to obtain outstanding wateractive SMEs is the incorporation of hydrophilic fillers in an elastomer matrix [20,22,24]. For instance, water-active SMEs are achieved in cellulose nanocrystal (CNC)-elastomer composites [20,22]. Polymer composites are two- or multi-phase materials that consist of a polymeric matrix and filler(s) and exhibit superior and possibly distinct properties compared to the properties of their constituents [28,29]. In a polymer composite, the interfacial bonding between individual filler particles is a crucial parameter in governing the mechanical behavior of the composite, especially when a percolated network of the filler is present [30,31]. CNCs are able to form a high-modulus and interconnected CNC phase in the matrix via hydrogen bonding that can be altered by water [32]. The recovery force is the resilient force of the distorted matrix, but is stored in the temporary shape by this strong interconnected CNC phase. The modulus of dry CNC is one thousand times higher than that of the matrix [33]. This huge difference in modulus guarantees the excellent shape fixing ability, the ability of switching segments to fix the mechanical deformation [34], of CNC phase. Upon wetted by water, water penetrated into CNC phase, leading to the softening of CNC phase and resulting in the shape recovery under the entropy resilience of the matrix [33,35,36]. In comparison with the SMEs based on water-active glass transition, those based on hydrophilic filler–elastomer composites offer a higher recovery stress, a larger maximum strain and more flexibility [4], and their working temperature is the rubbery range of the elastomeric matrix. Based on the latter method, we have developed clay–elastomer composites by using poly(methacrylic acid)-grafted clay (PMAA-g-clay) as the water-active filler and thermoplastic polyurethane (TPU) as the elastomer matrix, which exhibits a good water-active SME (the shape fixing and recovery ratios being 82% and 91%, respectively, with 10.4 vol.% of PMAA-g-clay) [37]. In this work, we report another type of TPU composites with water-active SMEs based on the hydrophilic filler– elastomer system and using pre-made polyvinyl alcohol (PVA) particles as the filler in the elastomer-matrix composites. PVA is a semicrystalline and hydrophilic synthetic polymer, derived from polyvinyl acetate through hydroxylation. PVA has been of great interest and widely used in pharmaceutical and biomedical applications for decades, because of its desirable characteristics, such as low protein adsorption characteristics, biocompatibility, high water solubility, and chemical resistance [38,39]. PVA particles were prepared from PVA crystals by ultrasonication with the presence of borates. TPU was selected as the resilient matrix due to its elastomeric nature and good biomedical properties [40,41]. The term polymer composites instead

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of polymer blends is used in this paper because the highmodulus PVA particles were prepared before they were mixed with TPU and used as the reinforcing filler in the composite, and there was no blending between TPU and PVA molecules. PVA particle-reinforced TPU composites with different contents of PVA particles were prepared by solution mixing/casting, and subjected to structural characterization, swelling tests in water, mechanical analysis in wet/dry conditions and SME assessments. 2. Experimental 2.1. Materials Polyvinyl alcohol (PVA) with weight-average molecular weight of 89,000–98,000 and hydrolysis degree of 99+%, boric acid, sodium hydroxide (NaOH), acetone and tetrahydrofuran (THF) were purchased from Sigma–Aldrich. Commercial thermoplastic polyurethane (TPU) (IROGRAN PS 455-203) with a Shore A hardness of about 78 and a density of 1.19 g cm3 was obtained from Huntsman. All materials were used as received unless otherwise specified. 2.2. Preparation of PVA particles 200 mL of PVA aqueous solution (1 wt.%) was prepared by stirring at 95 °C for 8 h. The PVA solution was cooled to room temperature (22 °C), poured into a metal mold, and then frozen at 20 °C for 24 h to allow for the crystallization of PVA chains [42]. Subsequently, it was thawed at room temperature and stirred for 12 h. PVA crystals were collected and purified by centrifugation with water for three times. The PVA crystals were put into 100 mL of a solution containing 0.05 g mL1 of boric acid and 0.03 g mL1 of NaOH. After stirring for 2 h, the mixture was ultrasonicated at 200 W and 24 kHz (UP200S, Hielscher) for 10 min. 100 mL of acetone was added into the mixture to aggregate PVA particles which were then collected and purified by centrifugation with THF for three times to remove water. The PVA particles were finally dispersed in 50 mL of THF and kept for further use after ultrasonication for 5 min. 5 g of PVA particle THF dispersion was weighed and dried to determine the content of PVA particles. The concentration of PVA particles in THF dispersion was 0.5 wt.%. 2.3. Preparation of PVA particle-reinforced TPU composites TPU was dissolved in THF at a concentration of 10 wt.% by stirring for 12 h. Desired amounts of TPU and PVA particles were weighed, mixed and stirred for 6 h before they were cast into Teflon dishes. The solvent was evaporated in a fume cupboard for 24 h and subsequently dried under vacuum at 40 °C for 24 h to remove any residual solvent. The thicknesses of as-prepared films were between 0.10 and 0.13 mm. The composites containing 2, 4, 8 and 16 wt.% of PVA particles were prepared and designated as TPU2, TPU4, TPU8 and TPU16, respectively. Pristine TPU films and PVA particle films were prepared under the same conditions.

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2.4. Characterization and measurements Fourier transform infrared spectroscopy (FTIR) was carried out on a Spectrum 100 spectrophotometer (Perkin Elmer) with attenuated total reflectance (ATR) in the wavenumber region of 4000–400 cm1 with a resolution of 1 cm1. The density of PVA particles was measured by an analytical balance with density kits (Mettler Toledo) using ethanol. PVA particles and the fracture surface of PVA particle-reinforced TPU composite (TPU16) fractured in liquid nitrogen were characterized by scanning electron microscopy (SEM), executed at an acceleration voltage of 10 kV (Inspect F, FEI). Prior to SEM observations, all the samples were sputter-coated with gold using an SPI sputter coater to enhance the conductivity. The swelling degrees of PVA particle-reinforced TPU composites were defined as (Wwet  Wdry)/Wdry  100%. Wdry was the weight before immersion in distilled water and Wwet the weight after immersion. Five specimens were tested for each sample. Tensile tests were carried out at room temperature using a H25K-Suniversal testing machine (Hounsfield) at a constant strain rate (50 mm min1). Samples for the tensile tests were cut from cast films into a rectangular shape (5 mm  60 mm). A 10N load cell was used and the distance between the sample grips was 40 mm. The average Young’s modulus for each sample was calculated from five specimens. The shape-memory effect was evaluated according to a method of wetting–stretching–drying cycles described by Zhu et al. [20]. Initially a straight strip of the composite film was immersed in distilled water for 1 h. It was then removed from water and stretched to a strain em (100%) at ambient temperature (22 °C). After 100% strain being kept for 4 h to dry the sample, the residual strain eu(N) was measured in the stress-free state; N is the cycle number. The shape fixing ratio (Rf) was defined as eu(N)/em [43]. Finally, the strip was immersed in the solution again for 1 h to recover its original length and its residual strain ep(N) was measured in the stress-free state. The shape recovery ratio (Rr) was defined as [em  ep(N)]/ [em  ep(N  1)] (ep(0) = 0) [43]. To start the next cycle, the strip was removed from the solution and stretched to 100% strain (based on the original length).

3. Results and discussion 3.1. Characterization of PVA particle-reinforced TPU composites FTIR spectra of as-received commercial PVA powder and the PVA particlesproduced in this work are shown in Fig. 1. The absorption bands in the range from 3600 to 3000 cm1 are attributed to the stretching vibration of O–H in PVA and the peak at 1070 cm1 to C–O in PVA [44]. The peaks at 1375 cm1, 1100 cm1 and 925 cm1 belong to the characteristic stretching vibrations of B–O [45], confirming the presence of borate ions in PVA particles. The borate ions react with the adjacent alcohol groups [46], cross-linking PVA chains on the surface of PVA particles, and stabilize the PVA particles through electrostatic repulsion. SEM images of PVA particles are shown in

Fig. 1. FTIR spectra of as-received PVA powder and as-prepared PVA particles.

Fig. 2a. The size of PVA particles is relatively uniform and in the range of 80–200 nm (Inset). They adhere to each other after dried from the THF suspension, suggesting strong filler–filler interactions between PVA particles. The PVA particles disperse well in TPU matrix, even at a high particle concentration. No large particles can be seen in the SEM image for TPU 16 (Fig. 2b). The nominal volume fractions of PVA particles (UPVA) in PVA particle-reinforced TPU composites are calculated according to UPVA = WPVA/[WPVA + (1  WPVA)qPVA/qTPU]  100% and summarized in Table 1. The density (q) is 1.28 g cm3 for PVA particles as experimentally measured and 1.19 g cm3 for TPU.

3.2. Water-induced modulus changes of PVA particlereinforced TPU composites Fig. 3a shows the swelling behavior of PVA particlereinforced TPU composites. It can be seen that all PVA particle-reinforced TPU composites absorb water rapidly within the first 20 min, and then gradually reach equilibrium after 40–60 min. The absorption rate increases with the PVA particle content. Fig. 3b shows the equilibrium degree determined after 2 h of immersion in water. It can be seen that the equilibrium degree depends on the content of PVA particles, again increasing with the content of PVA particles. The equilibrium degree shows a more dramatic increase when the content of PVA particles is above 1.9 vol.% (indicated by the arrow). This transition indicates the formation of a percolated network of PVA particles in the composites. The percolation threshold should be between 1.9 and 3.7 vol.%, which is close to the values found for polymer composites containing well-dispersed submicron particles (carbon black), reported in the literature, being 3 vol.% [47]. Young’s moduli for PVA particle-reinforced TPU composites determined from tensile testing are shown in Table 1. The modulus of as-prepared samples increases with increasing content of PVA particles due to the reinforcement effect of PVA particles. Young’s modulus of PVA crystals is in the range of 200–220 GPa [48], much

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Fig. 3. Swelling behavior of PVA particle-reinforced TPU composites in distilled water (a) and the equilibrium swelling degrees of PVA particlereinforced TPU composites as a function of PVA particle content (b).

Fig. 2. SEM images of PVA particles (a) and fracture surface of TPU16 (b).

higher than that for TPU (3.9 MPa). For samples wetted by water, the modulus decreases slightly with increasing content of PVA particles and is much lower than the values for the as-prepared dry samples. The modulus reductions increase with the content of PVA particles from 19.1% for TPU2 to 99.7% for TPU16. Since there are almost no effects of wetting on the modulus of TPU matrix (Table 1, Row 1), the reductions of modulus in the composites (Rows 2–5)

are mainly due to the plasticizing effect of water molecules on PVA particle phase. By combining the moduli with the equilibrium swelling degrees, it can be deduced that the modulus reduction increases with increasing equilibrium swelling degree. It is notable that all the wet composites are able to restore their high modulus after re-drying (Table 1, Column 6), demonstrating reversible response to water. The recovered modulus is slightly higher than the original value for the as-prepared composites, especially for the composites containing more PVA particles. This may be because water absorption increases the mobility of PVA chains on the particle surface which could allow for stronger adhesion between the neighboring particles and form stronger percolated network of the PVA particle phase.

Table 1 Composition and Young’s modulus of PVA particle-reinforced TPU composites. Sample

TPU TPU2 TPU4 TPU8 TPU16

WPVA (wt.%)

0 2 4 8 16

UPVA (vol.%)

0 1.9 3.7 7.5 15

Young’s modulus (MPa) As-prepared

Wet

Re-dried

3.9 ± 0.9 4.2 ± 1.9 4.5 ± 1.6 12.3 ± 2.7 34.9 ± 3.2

3.8 ± 0.7 3.4 ± 0.6 3.2 ± 0.9 2.7 ± 0.7 2.1 ± 0.8

4.0 ± 0.8 4.4 ± 1.3 4.7 ± 1.8 15.1 ± 2.8 36.7 ± 3.9

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In order to study the reinforcement of PVA particles in composites and the water-induced reduction of modulus, the Halpin-Tsai model (Eqs. (1) and (2)) [49,50] and percolation model (Eqs. (3)–(5)) [33,51] were employed.

E ¼ ð1 þ gnUf ÞEm =ð1  gUf Þ

ð1Þ

g ¼ ðEf =Em  1Þ=ðEf =Em þ nÞ

ð2Þ

E ¼ ½ð1  2w þ wUf ÞEf Em þ ð1  Uf ÞwE2f =½ð1  Uf ÞEf þ ðUf  wÞEm 

ð3Þ

with w ¼ 0 Uf < Uc w ¼ Uf ½ðUf  Uc Þ=ð1  Uc Þ

ð4Þ 0:4

Uf  Uc

ð5Þ

Here E, Ef and Em are the Young’s moduli of the composite, filler and matrix, respectively. Uf is the volume fraction of filler in the composite, while Uc is the percolation threshold. n is the reinforcement factor; n = 2 in the Halpin-Tsai model for conventional polymer composites filled with spherical particles [52]. Ef in the Halpin-Tsai model refers to the modulus of PVA particles, which was considered as 200 GPa here [48], while Ef in the percolation model refers to the modulus of PVA particle film (Fig. 4), which was measured as 505 MPa. Uc was assumed as 3.5 vol.% based on the results of swelling test and the trend of the modulus data. Fig. 5 shows data plots of the experimentally determined E of PVA particle-reinforced TPU composites, along with theoretical values predicted by the Halpin-Tsai model and the percolation model. It is evident from the figure that the experimental moduli of PVA particle-reinforced TPU composites match well the values predicted by the percolation model at the content of PVA particles > 3.5 vol.%. The significant mechanical reinforcement observed above 3.5 vol.% is attributable to the formation of a strong percolating filler network, similar to those found in CNC–TPU composites [20,22,37]. The

Fig. 4. SEM image of top surface of PVA particle film cast from THF dispersion. Inset: a photograph of a free-standing PVA particle film.

Fig. 5. Young’s moduli of PVA particle-reinforced TPU composites in combination with their modeled values.

values predicted by the Halpin-Tsai model fit the experimental moduli well when the PVA particle content 6 3.7 vol.%, but they are much lower than the experimental moduli when the PVA particle content > 3.7 vol.%. This is due to the fact that the pronounced filler–filler interactions are not considered in the model [53]. This reversible and large modulus change of PVA particle-reinforced TPU composites sets the stage for the water-active SME in hydrophilic filler–elastomer composites, permitting the original shape to recover under the stored resilient force [22]. 3.3. Shape-memory properties of PVA particle-reinforced TPU composites Fig. 6 shows the values of Rf and Rr of each composite for each cycle. Rf increases with the cycle number in the initial cycles, especially for the composites with less PVA particles, which is presumably due to gradual elimination of the sample processing history [43] and rearrangement of PVA particles during stretching. The average Rf (from five cycles) increases significantly with the content of PVA particles from 67% in TPU2 to 97% in TPU16. The PVA particle networks serve as physical barriers throughout the TPU matrix preventing the stretched elastomer from entropic recovery back to the original length. A higher content of PVA particles provides stronger networks, thus facilitating the fixity of shape. In contrast, Rr is more sensitive to the cycle number in the initial cycles for the composites with more PVA particles than those with less PVA particles. A higher content of PVA particles forms more physical barriers during entropy recovery of TPU, thus creating a higher irreversible strain in the initial cycle. These effects are eliminated by the rearrangement of PVA particles during repeated stretching. In general, the values of Rf and Rr for each sample are relatively stable after the initial cycles, showing good repeatability. The average Rr from cycle 2 to cycle 6 for all the samples is in the range of 97– 98%. The composite TPU16 shows the best combination of Rf and Rr among all the composites studied, being 97% and 97%, respectively. Fig. 7 demonstrates the SMEs of TPU16.

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Fig. 6. Shape memory properties of PVA particle-reinforced TPU composites: shape fixing ratio (a) and shape recovery ratio (b).

A pre-wetted TPU16 strip is stretched and fully dried at ambient temperature (22 °C) for 4 h. The deformed strip is kept in stress-free state for 16 h, and there is no change in length, indicating the stable temporary shape. The deformed TPU16 strip changes back to its original shape after immersed in water for 1 h, showing excellent water-active shape-memory behavior. In summary, the water-active modulus change arising from the plasticization effect of water on the PVA particle phase and the resilient nature of TPU form the ingredients for water-active SMEs. The filler phase creates hydrophilic channels in the hydrophobic matrix that aid the diffusing of water into the composite, leading to a recoverable modulus drop. On the other hand, the use of a hydrophobic matrix restricts the water uptake and avoids the fragmentation of the percolated PVA particle phase, ensuring the mechanical strength of the material. PVA particle-reinforced TPU composites with water-active SMEs have promise in various applications, such as sensors [2], smart packaging [20], body liquid-responsive medical devices [37], and morphing structures with the ability to alter their shape when a particular stimulus is applied (e.g. expandable stents [54], self-closing foams [55] and self-folding hinges [56]). Similar to CNC-elastomer nanocomposites [20,33,36,57] and PMAA-g-clay-elastomer composites [37], PVA particle-reinforced TPU composites studied in this work also successfully mimic the mechanically

Fig. 7. Water-active shape-memory effect of TPU16: the original length of wet TPU16 as indicated by the two marks on the strip (a) pre-treated by immersion in distilled water for 1 h, stretched wet TPU16 (b), deformed TPU16 after drying at ambient temperature (22 °C) for 4 h (c and d), deformed TPU16 at the beginning of immersion in distilled water (e), and shape-recovered TPU16 after immersed in distilled water for 1 h (f and g).

adaptive properties of the sea cucumber dermis [33]. Their modulus changes from the wet to the dry state are reversible, and the reduction factor is 16 from 34.9 to 2.1 MPa for TPU16 upon exposure to water. The mechanically adaptive property of PVA particle-reinforced TPU composites, together with fast water absorption, offers potential for the composites to be used as water-sensitive switches or other force-control devices triggered by wetting or drying. 4. Conclusions We successfully developed composites with wateractive SMEs by using polyvinyl alcohol particles and thermoplastic polyurethane. PVA particles were in the

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submicrometer size (80–200 nm), prepared from PVA crystals by ultrasonication with the presence of borates. Strong percolated PVA particle phase in PVA particle-reinforced TPU composites was achieved at over 3.5 vol.% PVA particles as determined by swelling, and theoretical modeling of the mechanical results. The swelling degree of TPU increased with increasing content of PVA particles, by 48.5 wt.% for TPU16. The incorporation of PVA particles improved the modulus of TPU significantly (by up to 795% for TPU16) because of the much higher modulus of the filler compared to that of the matrix and the formation of strong percolation networks. Water molecules reduced the modulus of PVA particle phase due to their plasticizing effect, thus decreasing the modulus of the composite as a whole. The modulus reduction was reversible and increased with increasing content of PVA particles (by up to 99.7% for TPU16). The moduli of dry PVA particle-reinforced TPU composites could be predicted by the percolation model, confirming the existence of percolation in the composites. The water-induced modulus change in the PVA particle phase and the resilient nature of TPU led to water-active SMEs. The incorporation of PVA particles into TPU significantly enhanced the shape fixity, with Rf increasing from 67% in TPU2 to 97% in TPU16, while the shape recovery ratio maintaining relatively high values, being 97–98%. This work provides a facile approach to the fabrication of novel composites with water-active SMEs and mechanically adaptive functions.

[12]

[13]

[14] [15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

Acknowledgements

[23]

We thank the Irish Research Council for Science, Engineering & Technology and the University of Sheffield for funding.

[24]

References [1] Huang WM, Yang B, Zhao Y, Ding Z. Thermo-moisture responsive polyurethane shape-memory polymer and composites: a review. J Mater Chem 2010;20(17):3367–81. [2] Yakacki C, Gall K. Shape-memory polymers for biomedical applications. In: Lendlein A, editor. Shape-memory polymers, vol. 226. New York: Springer, Berlin Heidelberg; 2010. p. 147–75. [3] Behl M, Zotzmann J, Lendlein A. Shape-memory polymers and shapechanging polymers. In: Lendlein A, editor. Shape-memory polymers, vol. 226. Springer Berlin Heidelberg; 2010. p. 1–40. [4] Meng H, Li G. A review of stimuli-responsive shape memory polymer composites. Polymer 2013;54(9):2199–221. [5] Voit W, Ware T, Dasari RR, Smith P, Danz L, Simon D, et al. Highstrain shape-memory polymers. Adv Funct Mater 2010;20(1): 162–71. [6] Rousseau IA, Xie T. Shape memory epoxy: composition, structure, properties and shape memory performances. J Mater Chem 2010;20(17):3431–41. [7] Kalogeras IM, Lobland HEH. The nature of the glassy state: structure and transitions. J Mater Ed 2012;34(3–4):69–94. [8] Ji FL, Hu JL, Li TC, Wong YW. Morphology and shape memory effect of segmented polyurethanes. Part I: With crystalline reversible phase. Polymer 2007;48(17):5133–45. [9] Ahn SK, Deshmukh P, Kasi RM. Shape memory behavior of side-chain liquid crystalline polymer networks triggered by dual transition temperatures. Macromolecules 2010;43(17):7330–40. [10] Cho JW, Kim JW, Jung YC, Goo NS. Electroactive shape-memory polyurethane composites incorporating carbon nanotubes. Macromol Rapid Commun 2005;26(5):412–6. [11] Cuevas JM, Alonso J, German L, Iturrondobeitia M, Laza JM, Vilas JL, et al. Magneto-active shape memory composites by incorporating

[25]

[26]

[27] [28]

[29] [30]

[31]

[32]

[33]

[34] [35]

[36]

ferromagnetic microparticles in a thermo-responsive polyalkenamer. Smart Mater Struct 2009;18(7). Koerner H, Price G, Pearce NA, Alexander M, Vaia RA. Remotely actuated polymer nanocomposites[mdash]stress-recovery of carbon-nanotube-filled thermoplastic elastomers. Nat Mater 2004;3(2):115–20. Huang WM, Yang B, An L, Li C, Chan YS. Water-driven programmable polyurethane shape memory polymer: demonstration and mechanism. Appl Phys Lett 2005;86(11):1–3. Lendlein A, Jiang H, Junger O, Langer R. Light-induced shapememory polymers. Nature 2005;434(7035):879–82. Wu L, Jin C, Sun X. Synthesis, properties, and light-induced shape memory effect of multiblock polyesterurethanes containing biodegradable segments and pendant cinnamamide groups. Biomacromolecules 2010;12(1):235–41. Defize T, Riva R, Raquez J-M, Dubois P, Jérôme C, Alexandre M. Thermoreversibly crosslinked poly(e-caprolactone) as recyclable shape-memory polymer network. Macromol Rapid Commun 2011;32(16):1264–9. Aoki D, Teramoto Y, Nishio Y. SH-containing cellulose acetate derivatives: preparation and characterization as a shape memoryrecovery material. Biomacromolecules 2007;8(12):3749–57. Kumpfer JR, Rowan SJ. Thermo-, photo-, and chemo-responsive shape-memory properties from photo-cross-linked metallosupramolecular polymers. J Am Chem Soc 2011;133(32):12866–74. Han XJ, Dong ZQ, Fan MM, Liu Y, Li JH, Wang YF, et al. PH-induced shape-memory polymers. Macromol Rapid Commun 2012;33(12):1055–60. Zhu Y, Hu J, Luo H, Young RJ, Deng L, Zhang S, et al. Rapidly switchable water-sensitive shape-memory cellulose/elastomer nano-composites. Soft Matter 2012;8(8):2509–17. Chen M-C, Tsai H-W, Chang Y, Lai W-Y, Mi F-L, Liu C-T, et al. Rapidly self-expandable polymeric stents with a shape-memory property. Biomacromolecules 2007;8(9):2774–80. Mendez J, Annamalai PK, Eichhorn SJ, Rusli R, Rowan SJ, Foster EJ, et al. Bioinspired mechanically adaptive polymer nanocomposites with water-activated shape-memory effect. Macromolecules 2011;44(17):6827–35. Jung YC, So HH, Cho JW. Water-responsive shape memory polyurethane block copolymer modified with polyhedral oligometric silsesquioxane. J Macromol Sci, Phys 2006;45(6):1189. Luo H, Hu J, Zhu Y, Zhang S, Fan Y, Ye G. Achieving shape memory: reversible behaviors of cellulose–PU blends in wet–dry cycles. J Appl Polym Sci 2012;125(1):657–65. Tsavalas JG, Sundberg DC. Hydroplasticization of polymers: model predictions and application to emulsion polymers. Langmuir 2010;26(10):6960–6. Yang B, Huang WM, Li C, Li L, Chor JH. Qualitative separation of the effects of carbon nano-powder and moisture on the glass transition temperature of polyurethane shape memory polymer. Scr Mater 2005;53(1):105–7. Lv H, Leng J, Liu Y, Du S. Shape-memory polymer in response to solution. Adv Eng Mater 2008;10(6):592–5. Work WJ, Horie K, Hess M, Stepto RFT. Definition of terms related to polymer blends, composites, and multiphase polymeric materials. Pure Appl Chem 2004;76(11):1985–2007. Hull D. An introduction to composite materials. Cambridge: Cambridge University Press; 1981. p. 3. Bréchet Y, Cavaillé JY, Chabert E, Chazeau L, Dendievel R, Flandin L, et al. Polymer based nanocomposites: effect of filler–filler and filler– matrix interactions. Adv Eng Mater 2001;3(8):571–7. Kopczynska A, Ehrenstein GW. Polymeric surfaces and their true surface tension in solids and melts. J Mater Ed 2007;29(3– 4):325–440. Favier V, Cavaille JY, Canova GR, Shrivastava SC. Mechanical percolation in cellulose whisker nanocomposites. Polym Eng Sci 1997;37(10):1732–9. Capadona JR, Shanmuganathan K, Tyler DJ, Rowan SJ, Weder C. Stimuli-responsive polymer nanocomposites inspired by the sea cucumber dermis. Science 2008;319(5868):1370–4. Ratna D, Karger-Kocsis J. Recent advances in shape memory polymers and composites: a review. J Mater Sci 2008;43(1):254–69. Hsu L, Weder C, Rowan SJ. Stimuli-responsive, mechanicallyadaptive polymer nanocomposites. J Mater Chem 2011;21(9):2812–22. Dagnon KL, Shanmuganathan K, Weder C, Rowan SJ. Water-triggered modulus changes of cellulose nanofiber nanocomposites with hydrophobic polymer matrices. Macromolecules 2012;45(11): 4707–15.

T. Wu et al. / European Polymer Journal 53 (2014) 230–237 [37] Wu T, O’kelly K, Chen B. Poly(methacrylic acid)-grafted clay– thermoplastic elastomer composites with water-induced shapememory effects. J Polym Sci Part B: Polym Phys 2013;51(20): 1513–22. [38] DeMerlis CC, Schoneker DR. Review of the oral toxicity of polyvinyl alcohol (PVA). Food Chem Toxicol 2003;41(3):319–26. [39] Baker MI, Walsh SP, Schwartz Z, Boyan BD. A review of polyvinyl alcohol and its uses in cartilage and orthopedic applications. J Biomed Mater Res, B 2012;100B(5):1451–7. [40] Pedicini A, Farris RJ. Mechanical behavior of electrospun polyurethane. Polymer 2003;44(22):6857–62. [41] Foks J, Janik H. Microscopic studies of segmented urethanes with different hard segment content. Polym Eng Sci 1989;29(2):113–9. [42] Hassan CM, Ward JH, Peppas NA. Modeling of crystal dissolution of poly(vinyl alcohol) gels produced by freezing/thawing processes. Polymer 2000;41(18):6729–39. [43] Lendlein A, Kelch S. Shape-memory polymers. Angew Chem Int Ed 2002;41(12):2034–57. [44] Daniliuc L, De Kesel C, David C. Intermolecular interactions in blends of poly(vinyl alcohol) with poly(acrylic acid)—1. FTIR and DSC studies. Eur Polym J 1992;28(11):1365–71. [45] Jia YZ, Gao SY, Jing Y, Zhou YA, Xia SP. FTIR spectroscopy of magnesium tetraborate solution. Chem Pap 2001;55(3):162–6. [46] Harada A, Takagi T, Kataoka S, Yamamoto T, Endo A. Boron adsorption mechanism on polyvinyl alcohol. Adsorption 2011;17(1):171–8. [47] Segal E, Tchoudakov R, Narkis M, Siegmann A. Thermoplastic polyurethane–carbon black compounds: structure, electrical conductivity and sensing of liquids. Polym Eng Sci 2002;42(12): 2430–9. [48] Matsuo M, Harashina Y, Ogita T. Effects of molecular orientation and crystallinity on measurement by X-ray diffraction of the crystal

[49]

[50] [51]

[52] [53]

[54]

[55]

[56]

[57]

237

lattice modulus of poly(vinyl alcohol) prepared by gelation/ crystallization from solution. Polym J (Tokyo, Japan) 1993;25(4):319–28. Zhao X, Zhang Q, Chen D, Lu P. Enhanced mechanical properties of graphene-based poly(vinyl alcohol) composites. Macromolecules 2010;43(5):2357–63. Schaefer DW, Justice RS. How nano are nanocomposites? Macromolecules 2007;40(24):8501–17. Ouali N, Cavaillé J, Perez J. Elastic, viscoelastic and plastic behavior of multiphase polymer blends. Plast Rub Compos Pro 1991;16(1):55–60. Affdl JCH, Kardos JL. The Halpin-Tsai equations: a review. Polym Eng Sci 1976;16(5):344–52. Hajji P, Cavaillé JY, Favier V, Gauthier C, Vigier G. Tensile behavior of nanocomposites from latex and cellulose whiskers. Polym Compos 1996;17(4):612–9. Yakacki CM, Shandas R, Lanning C, Rech B, Eckstein A, Gall K. Unconstrained recovery characterization of shape-memory polymer networks for cardiovascular applications. Biomaterials 2007;28(14):2255–63. Li G, Nettles D. Thermomechanical characterization of a shape memory polymer based self-repairing syntactic foam. Polymer 2010;51(3):755–62. Felton SM, Tolley MT, Shin B, Onal CD, Demaine ED, Rus D, et al. Selffolding with shape memory composites. Soft Matter 2013;9(32):7688–94. Shanmuganathan K, Capadona JR, Rowan SJ, Weder C. Stimuliresponsive mechanically adaptive polymer nanocomposites. ACS Appl Mater Interfaces 2010;2(1):165–74.