microscale

Accepted Manuscript Shape-memory polymer composites selectively triggered by near-infrared light of two certain wavelengths and their applications at macro-/microscale Liang Fang, Shunping Chen, Tianyu Fang, Jiaojiao Fang, Chunhua Lu, Zhongzi Xu PII:

S0266-3538(16)31363-X

DOI:

10.1016/j.compscitech.2016.11.018

Reference:

CSTE 6581

To appear in:

Composites Science and Technology

Received Date: 29 September 2016 Revised Date:

16 November 2016

Accepted Date: 19 November 2016

Please cite this article as: Fang L, Chen S, Fang T, Fang J, Lu C, Xu Z, Shape-memory polymer composites selectively triggered by near-infrared light of two certain wavelengths and their applications at macro-/microscale, Composites Science and Technology (2016), doi: 10.1016/ j.compscitech.2016.11.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Shape-Memory Polymer Composites Selectively Triggered by Near-Infrared Light of two Certain Wavelengths and their

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Applications at Macro-/Microscale

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Liang Fang*, Shunping Chen, Tianyu Fang, Jiaojiao Fang, Chunhua Lu*, Zhongzi Xu

State Key Laboratory of Materials-Oriented Chemical Engineering, College of

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Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China.

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 210009, P. R. China.

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Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),

*

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Nanjing Tech University, Nanjing 210009, PR China.

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Corresponding Authors: [email protected], [email protected]

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Abstract A cost-efficient filler system based on rare earth organic complexes of Yb(TTA)3Phen and Nd(TTA)3Phen is reported, showing selective photothermal effect to

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near infrared (NIR) light of 980 and 808 nm, respectively. The integration of the two fillers into a commercially available SMP of poly[ethylene-ran-(vinyl acetate)] (EVA)

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respectively provide selective NIR light responsive SMP composites, while their full shape recoveries triggered by NIR light of 980 or 808 nm are correspondingly realized.

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In addition, the NIR light irradiation periodicity is varied and the results suggests that reducing deformation temperature (Tdeform) of EVA caused the shape recovery at a relatively short exposure time, while long irradiation time must be allowed to trigger the shape deformation at a high Tdeform. A macroscaled actuator with multi-shaped variation

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is demonstrated via sequential irradiations of NIR lights of 980 and 808 nm. More interestingly, the shape recovery of a certain pattern on the micropatterned surface

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consisted of the two composites is triggered first upon one NIR irradiation before the remaining patterned area is stimulated using another NIR wavelength. This work

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integrates more capabilities into light-induced SMPCs, including multi-shape deformation both at macro and microscale, as well as unique responsive signal, in addition to the known remote and noncontact control.

Keywords: Smart materials, Functional composites, Polymer-matrix composites, Shape-memory Polymers, Near-Infrared Light

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ACCEPTED MANUSCRIPT 1. Introduction Shape-memory polymers (SMPs) have found applications in many fields, including biomedical devices, self-deployable instruments, actuators, and smart textiles, to name a

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few.[1-4] The trends towards investigating SMP structures at micro/nanoscale and miniaturization of SMP devices are also of increasing interest because of the usage in

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switching adhesion or optical property as well as directing cell growth.[5-9] Upon stimulus, usually heat, SMPs recall their original shape from programmed temporary

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one when the temperature goes beyond their switching temperature (Tsw).[4] In comparison with heat, light enables the “on-demand” triggering of SMPs in a remote, localized and non-contact manner with neglected intervention on surrounding circumstance.[10, 11] To date, introduction of photothermal fillers into thermally-induced

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SMPs have been widely reported as one convenient and commercially available approach to prepare such light-induced SMP composites (SMPCs).[12-23] Under

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irradiation, the fillers absorb the light energy and transfer it into heat, indirectly increasing the temperature of the composites. Shape recovery occurs when the

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temperature reaches the Tsw.

Functional photothermal fillers responsive to different light wavelengths, including

UV light, visible light, and infrared light, have been explored to fabricate light-induced SMPCs. A metallosupramolecular unit formed via coordinating Mebip ligands to Zn(NTf2)2 was introduced into an epoxy resin to turn UV light into heat.[14] In addition, a visible light beam of 532 nm was irradiated onto crosslinked polyethylene oxide with

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ACCEPTED MANUSCRIPT a very small amount of Au nanopowders.[15] The surface plasmon resonance of Au nanopowders resulted in the temperature increase and correspondingly the shape deformation. Light responsive SMPC micropillars in a hexagonal array were prepared

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based on epoxy/Au nanorods, while the visible light laser of 532 nm was also utilized as the light source.[17] One main advantage of visible light is the easy location of beam on

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the samples to achieve complex shape deformation.[16] In comparison with UV and visible light, near-infrared (NIR) light is not absorbed greatly by biological systems and

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thus considered as a safe actuation source to trigger the shape deformation of SMPCs.[24] Via selective deposition of black ink onto hinge area, irradiation of NIR light of 988 nm as another widely reported NIR light source resulted in the self-folding of polymer sheets.[19] And the NIR light of 808 nm was utilized to trigger the shape recovery of a

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polycarpolactone (PCL) based SMPCs mixed with multi-walled carbon nanotubes (CNTs) and reduced graphene oxides.[23] Besides, gold nanorods were introduced into

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crosslinked PCL and used the NIR light of 805 nm to stimulate the shape change of surface nanopatterns to direct cell growth.[9]

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SMPs with multiple shape variation have attracted significant attention.[25, 26] The

development of photothermal filler selectively responsive to a certain light wavelength has attracted interest to facilitate the complex or successive multi-shape change of polymers, exploring the applications in microfluidic devices or smart actuators. To date, however, only a few selective filler systems were successfully explored.[18, 27] Gold colloid and nanoshells which responded to the light of 532 and 832 nm, respectively,

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ACCEPTED MANUSCRIPT were developed.[27] The independent optical control of microfluidic valves prepared by the hydrogel nanocomposites was achieved via separate irradiation of the two beams.[27] Also, ferriferous oxide (Fe3O4) and CNTs generated heat at two very different

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radiofrequency ranges (296 kHz and 13.56 MHz), which were used to prepare smart SMPCs with multi-shape recovery upon separated radiofrequency actuations.[18] The

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combination of two materials responsive to different wavelengths enables the “on-demand” shape recovery, overcoming the drawback of conventional triple-shape

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memory polymer which has to recall its permanent shape following the changing order determined by the programming procedure.[28] To our knowledge, few reports concerned the selective photothermal system within the NIR window. More importantly, as far as micropatterned SMP surfaces are concerned, precise actuation of certain region within

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the whole pattern before the other area is activated can provide more flexibility of controlling the surface patterned morphologies and correspondingly offer more

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functions, which has been rarely discussed either. Furthermore, previous research mainly focused on continuous light irradiation,

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providing persistent energy input for heat accumulation and temperature increase. Actually, light is not only a form of energy, but can be presented in a signal manner as well. The irradiation of light onto SMPCs can also be achieved in a periodic way with different frequencies, which highly influences the heat accumulation, temperature, and correspondingly the shape recovery behavior of SMPCs. Recently, temperature-memory polymer (TMP) has been reported, the Tsw of which can be controlled by the

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ACCEPTED MANUSCRIPT deformation temperature (Tdeform).[29-32] One question that arises in this context is whether critical irradiation periodicity and frequency are required to recall the original shape from temporary shape which is deformed at a certain Tdeform? The positive answer

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may suggest that incorporation of selective photothermal filler into a TMP, producing TMP composites, can realize its response to not merely unique wavelength but to

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certain irradiation periodicity and frequency, enhancing the “communication” between actuators and operators.

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In the present work, a photothermal filler system based on organic rare earth complex is reported to trigger the shape recovery of SMPCs selectively to the NIR light of 808 or 980 nm. More specifically, Yb(TTA)3Phen and Nd(TTA)3Phen powders were prepared, while their structures and selective photothermal effects were studied. The

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two powders were subsequently incorporated into one commercial SMP/TMP of poly[ethylene-ran-(vinyl acetate)] (EVA) respectively to prepare the smart SMPCs,

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enabling the NIR responsive shape recovery to 808 or 980 nm. The effects of irradiation periodicity and frequency on the shape recovery of SMPCs were also examined. Finally,

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the applications of such SMPCs on the macroscale actuators and micropatterned surface, which can achieve multishaped deformation upon successive irradiations of NIR light of 808 and 980 nm, were demonstrated to prove their versatility.

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ACCEPTED MANUSCRIPT 2. Materials and experimental section 2.1 Materials EVA (H2031, VA content = 19%) was obtained from Sumitomo Chemical Company

isocyanurate

(TAI,

98%)

were

obtained

from

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(Japan). Benzophenone (BP, 98%) was purchased from Sigma Aldrich. Triallyl Aladdin,

China.

The

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α-thenoyltrifluroacetone (TTA) and 1,10-phenanthroline (Phen) were purchased from Sinopharm Chemical Reagent Company (China). And YbCl3·6H2O and NdCl3·6H2O

used without further purification.

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were provided by Funing Rare Earth Industrial Company (China). All chemicals were

2.2 Preparation of Yb(TTA)3Phen and Nd(TTA)3Phen

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The synthesized procedures of Yb(TTA)3Phen and Nd(TTA)3Phen complexes by co-precipitation method were as follow. The 1 mmol of YbCl3·6H2O or NdCl3·6H2O, 3

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mmol TTA and 1 mmol phen were dissolved in ethanol, respectively. Then the ethanol solution of TTA was first added to the three neck flask and it was stirred and refluxed at

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60 oC in a water bath. In addition, the YbCl3·6H2O or NdCl3·6H2O, ethanol solution and the ethanol solution of phen were added into the three neck flask, successively. The pH value of the reaction mixture was adjusted to 6-7 by dropping 1 mol·L-1 sodium hydroxide ethanol solution. Finally, the mixture was reacted in a 60 oC water bath for 6 h. After the reaction finished, the mixture was centrifuged at the speed of 10000 rpm to obtain the precipitation. The precipitation was centrifuged and washed with water and

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ACCEPTED MANUSCRIPT ethanol, and this process was repeated for three times to obtain the sample, and then the complexes were dried for 12 h in a vacuum oven of 50 o C.

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2.3 Mixing and UV Curing

The mixing and UV curing procedures has been described schematically in a

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previous report.[21] Solution casting methods were used to fabricate composite films. Yb(TTA)3Phen or Nd(TTA)3Phen was introduced into THF respectively and sonicated

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for 30 min. The EVA pellets, TAI (4 phr) and BP (4 phr) were added into the solution subsequently. The whole solution was stirred vigorously at 60 °C for 1 h. Next, the solution was poured into a Teflon mold, allowing the solvent evaporation naturally for at least 72 h. The obtained film was then placed into a vaccum oven at room

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temperature for another 24 h. Different powder loadings (2, 5, and 10 phr) were used in this case. The batches were named as Yb(Nd)-2, Yb(Nd)-5, and Yb(Nd)-10, respectively.

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The obtained film was placed between two glass plates, while the top plate was treated by mold-release agent (Trichloro(1H, 1H, 2H, 2H-perfluorooctyl)silane, J&K Chemical,

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China) and a Teflon film was placed between the material and the bottom plate to avoid sticking. The whole system was pressed by office clamps and heated at 140 °C for 10 min to achieve a thin film (thickness ~ 0.6 mm). UV light of 125 W was irradiated through the transparent top plate for 1 h to allow curing.

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ACCEPTED MANUSCRIPT 2.4 Irradiation Manner The 808 nm NIR light was generated by a laser driver (FC-808-10W, Xinchanye Corp., China), while the 980 nm NIR light was given by a laser diode driver

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(KS3-11312-912, BWT Corp., China). The light outgoing point was placed 1 cm above the sample. The power density was determined using an optical power/energy meter

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(Model 1918-R, Newport, USA) equipped with a thermopile detector (Model 818P-020-12, Newport, USA).

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The irradiation through a varied periodicity was achieved via self-made light-blocking boards equipped with a motor (See Supporting Information Fig. S1). A quarter, half, and three quarters of the round boards were cut out to realize the different irradiation time. The ratio of the irradiation time to the non-irradiation time within one

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cycle was increased gradually from 1/3, to 1/1 and 3/1, which is named as Periodicity I, Periodicity II, and Periodicity III, respectively. An arbitrary function generator

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(AFG-2005, Gwinstek Co., Taiwan) was connected with the motor to switching the

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rotation speed, controlling the exposure frequency.

2.5 Preparation, Programming, and Recovery of Micropatterned Surface The preparation of micropatterned EVA/composites surface was based on a soft

lithography technique (See Supporting Information Fig. S2). In step I, a polydimethylsiloxane (PDMS, Sylgard® 184, Dow Corning Corporation) template was created using the widely reported method, replicating the glass template with designed

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ACCEPTED MANUSCRIPT micropatterns. First, the glass template, which was rinsed using ethanol, was placed in a Teflon mold. PDMS prepolymer (prepolymer : crosslinker = 10:1) in xylene solution (75 wt%) was poured carefully into the mold, covering the glass template. The mold

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was placed in a vacuum over to allow the curing at 40 oC for 24 h before the PDMS template I was peeled off carefully. The same procedure was repeated via pouring

oven and also allowed the curing at 40 oC for 24 h.

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PDMS prepolymer solution onto the template I. The system was placed into the vacuum

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The PDMS template II was placed onto a glass slide was heated to 150 oC. A slice of EVA composite film was laid carefully onto the template and covered by a Teflon sheet. A 2 kg weight was located to achieve the hot-embossing of EVA into the template. After 5 min, the weight, glass slide and Teflon sheet were removed carefully. A pre-heated

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glass slide was used to scrape the remaining EVA composite film on the template surface slowly at 150 oC, leaving the EVA particles embedding in the holes of the

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PDMS template. After cooling, the remaining cylindrical particles were extracted from the holes using a tweeze and placed onto a glass slide carefully.

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In the last step, as the movable component, the prepared EVA particles containing

10 phr Yb(TTA)3Phen were inserted into the certain holes in PDMS template II one by one, contributing to the required pattern. Subsequently, EVA/Nd(TTA)3Phen particles were also inserted into certain holes, forming another micropatterns. Pure EVA film was hot-embossed into the remaining empty holes following the similar procedure as described above. Without the scraping, the whole film were cured under UV light

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ACCEPTED MANUSCRIPT irradiation for 30 min and peeled off from PDMS template II. The prepared micropatterned surface was heated to 60 oC and pressed downwards using a weight, before it was cooled down to room temperature. The NIR laser of 980

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nm was irradiated onto the EVA/Yb(TTA)3Phen patterned areas, while NIR light of 808

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nm was used to trigger the shape recovery of EVA/Nd(TTA)3Phen regions.

2.6 Characterization

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Fourier Transform Infrared Spectroscopy. The chemical structure of the prepared Yb(TTA)3Phen and Nd(TTA)3Phen, as well as TTA and Phen was examined using the Fourier transform-infrared spectroscopy (FT-IR, Vector 22, Bruker, USA). The samples were grinded with pottassium bromide (KBr) together and pressed into a testing tablet.

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Reflectance Measurements. A UV-vis-NIR spectrophotometer (UV-3101PC, Shimadzu Corp., Japan) was used to measure the reflectance spectra of Yb(TTA)3Phen

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and Nd(TTA)3Phen, as well as the raw materials. Thermogravimetric analysis. Thermogravimetric analysis (TGA, Netzsch SAT

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449C, Germany) was used to investigate the degradation behaviors of the Yb(TTA)3Phen and Nd(TTA)3Phen via scanning from room temperature to 800 ºC at the heating rate of 10 ºC·min-1 in an air flow. Scanning

electron

microscope.

The

morphology of

Yb(TTA)3Phen

and

Nd(TTA)3Phen powders were examined using a scanning electron microscopy (SEM, SU8010, Hitachi, Japan).

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ACCEPTED MANUSCRIPT Photothermal effect. The temperature on the samples upon the irradiation of NIR light was measured using a hand-held infrared camera (Xintest Company, China)

was used to characterize the powder dispersion in EVA matrix.

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Optical Microscope. An optical microscope (BA210, Motic China Group Co., Ltd.)

Swelling Experiments. Swelling experiments were carried out to analyze the

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crosslinked networks. One piece of the sample with the weight of m1 was stored in xylene for 72 hours. After removing from solvent, the sample weight (m2) was

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immediately measured in swollen state after cleaning extra solvent with tissue. The swollen sample was heated under 60 oC until dried completely and then measured the weight (m3). The gel content was (G) given by m3/m1×100%. The swelling degree (Q)

crosslinking density.

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was calculated using the equation as described in a previous report to determine the

Thermal Property Analysis. Differential scanning calorimetry (DSC, 204 Phoenix,

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Netzsch, Selb, Germany) was used to determine the thermal property using bulk samples with the scanning temperature range from room temperature to 150 ºC, before

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it was cooled to -75 ºC and then increased to 150 ºC. The heating and cooling rates were 10 ºC·min-1.

Mechanical Property. A tensile test instrument (MZ-2000c, Mingzhu Testing

Machinery Co., China) was used to perform the tensile test experiment via elongation of the dumbbell-shaped specimens until break at 100 mm·mm-1. Shape-Memory Effect. The shape-memory effect was determined on the basis of

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ACCEPTED MANUSCRIPT angle variation. The test specimen was placed onto a hot plate at 120 °C. After 5 min, the specimens were completely folded to 90° against a perpendicular glass plate tightly and kept for another 5 min before natural cooling to room temperature. Under the

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irradiation of NIR light, the instantaneous angles, θ, between the real-time location and the primary location of the bended leg were characterized with the error of ±2o. The

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shape recovery ratio, Rr, is determined as Rr = θ/90° × 100%.

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3. Results and Discussion

3.1 Structure and Property of Selective Photothermal Fillers

Yb(TTA)3Phen and Nd(TTA)3Phen powders were prepared using co-precipitation method according to a previous report.[33] The commercially available raw materials and

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the easy preparation approach make the organic complex cost efficient and attractive as the new photothermal filler. FTIR spectra of TTA, Phen, and the two rare earth organic

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complexes are presented in Fig. 1a to examine their chemical structures first. Two anti-symmetric stretch vibration peaks are observed in the FTIR curve of TTA. The ones

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at 1661 and 1642 cm-1 are attributed to the peaks of vC=O close to thienyl and strong electronegativity trifluoro-base, respectively.[34] After coordination of TTA with Yb(III) or Nd(III), the vC=O peak shifted to 1625 and 1601 cm-1, respectively.[34] Three characteristic ring stretching vibration bands of Phen are located at 1563, 851, and 736 cm-1, which varied to 1536, 842, and 715 cm-1 after coordination.[34] In addition, the peaks at 1309 and 1357 cm-1 can be attributed to vas(CF3) and vs(CF3).

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ACCEPTED MANUSCRIPT Fig. 1b shows the TGA curves of Yb(TTA)3Phen and Nd(TTA)3Phen in an air flow. Good thermal stability was achieved until 270 oC, while the first degradation stepping from 270 to around 350 oC was attributed to decomposition of ligand TTA. The second

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degradation stepping from 440 to 500 oC was considered as contributing by the decomposition of ligand Phen. In the two degradation steps, the weight loss was 60%

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and 20%, respectively, while 20% residual remains, which matched with the mass fraction of the different components in Yb(TTA)3Phen and Nd(TTA)3Phen. The TGA

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results in combination with FTIR suggest the success preparation of Yb(TTA)3Phen and Nd(TTA)3Phen, and the proposed chemical structures are depicted as shown in Fig. 1c. SEM was used to characterize the structure and size of the organic complex powders (Fig. 1d-e). The Yb(TTA)3Phen powder possessed sheet-shaped structures lower than 3

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µm, while small cluster-shaped powders at nanoscale were also found. The Nd(TTA)3Phen powders presented sheet structures with the main size distribution

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between around 1 and 5 µm, larger than its counterpart. The 4f orbit of rare earth ions possesses abundant energy levels, responsive to

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different light wavelengths. Yb3+ ions possess the highest absorption cross-section at 976 nm and complete the level transition 2I7/2 → 2F5/2, while Nd3+ is reported for its 4I9/2 → 4F5/2 transition and the strong absorption at ca. 800 nm.[35,

36]

To examine the

absorption selectivity of the prepared powders, the reflectance spectra of YbCl3, NdCl3, TTA, Phen, Yb(TTA)3Phen and Nd(TTA)3Phen are shown in Fig. 2a and 2b. The Yb3+ had a characteristic absorption range between 870 and 1060 nm with the peak located at

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ACCEPTED MANUSCRIPT 975 nm. The Nd3+ had more abundant absorption peaks, while an evident one was located at 805 nm. Whereas, the Yb3+ did not have absorption peaks close to 808 nm and no peak at 980 nm was found for Nd3+. The coordination procedure did not vary the

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absorption peaks of Yb(TTA)3Phen and Nd(TTA)3Phen. Therefore, it can be expected that Yb(TTA)3Phen and Nd(TTA)3Phen could present the selective photothermal effect

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upon the irradiation of 980 and 808 nm NIR light, respectively.

The NIR light of 808 and 980 nm were irradiated onto Yb(TTA)3Phen and

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Nd(TTA)3Phen powders, respectively, to observe the temperature increase with time, for the purpose of examining the selective photothermal effects of Yb(TTA)3Phen and Nd(TTA)3Phen. As shown in Fig. 2c, the 980 nm NIR light with a power density (PD) of 0.2 W·cm-2 heat the Yb(TTA)3Phen up to 41 oC in 30 s. Increasing PD to 0.3 and 0.5

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W·cm-2 further increased the balanced temperature to 67 and 96 oC in 30 s. The energy at the high energy level is unstable. Some energy can be released in the form of heat

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through non-radiative channels, which is also attributed to the quantum confinement of phonons and enhanced electron-phonon interaction. In comparison, carbon black, CNT,

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and Fe3O4 nanoparticles increased their temperatures to 320, 220, and 250 oC upon irradiation of the same NIR light with a high PD of 0.5 W·cm-1. The Nd(TTA)3Phen powder, however, varied negligibly with time, suggesting no photothermal effect for 980 nm NIR light. The photothermal effect of those examined powders or particles under NIR light of 808 nm was studied next (Fig. 2d). The Yb(TTA)3Phen powder did not enable temperature increase for 808 nm, while Nd(TTA)3Phen increased its

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ACCEPTED MANUSCRIPT temperature evidently. As shown in Fig. 2d, under the irradiation of 808 nm with PD of 0.2, 0.3, and 0.5 W·cm-1, the temperature increased to 85, 103, and 185 oC, indicating better photothermal capability than Yb(TTA)3Phen. Besides, the conventional black

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photothermal fillers also lead in the temperature increase upon the irradiation of 808 nm, indicating no selectively photothermal effect yet. These results demonstrate that

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Yb(TTA)3Phen and Nd(TTA)3Phen powders possessed selective photothermal effect for

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980 and 808 nm, respectively.

3.2 Structure and Property of Selective NIR Light Responsive Composites As composites, the effect of filler on material structure and property is of high priority. Because the photothermal effect of Yb(TTA)3Phen and Nd(TTA)3Phen cannot

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be comparable with conventional black fillers as mentioned above, the loading was set as 2, 5, and 10 phr, higher than the reported systems. The SMPC films were prepared

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using a solution casting method, while the dispersion of Yb(TTA)3Phen (10 phr) and Nd(TTA)3Phen (10 phr) in EVA matrix is examined using optical microscope as the

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examples (Fig. 3a and 3b). Both powders dispersed uniformly in polymer matrix, while the morphologies of Yb(TTA)3Phen and Nd(TTA)3Phen powders did not vary evidently. Swelling degree (Q) and gel content (G) are used as two parameters to evaluate how

Yb(TTA)3Phen and Nd(TTA)3Phen influenced the crosslinking reaction of the composites. As shown in Fig. 3c and 3d, the pure EVA specimen after UV curing had a Q and G of 245% and 90%, respectively, indicating the formation of sufficient

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ACCEPTED MANUSCRIPT crosslinking points as the netpoints. The introduction of Yb(TTA)3Phen or Nd(TTA)3Phen powders changed the value negligibly. Even after 10 phr powder was introduced, the Q and G for Yb(TTA)3Phen and Nd(TTA)3Phen composites were 252%

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and 89% as well as 237% and 86%, respectively. The concern that the organic ligand absorbing UV light would affect the UV curing did not occur. A wide melting transition

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peak of the neat EVA network ending at 92 °C with the peak located at Tm = 80 °C was found, as shown in Fig. 3e and 3f, while the weight crystallinity was calculated as 22%.

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Interestingly, the introduction of 2 phr Yb(TTA)3Phen or Nd(TTA)3Phen powders decreased the Tm from 80 to 76 °C, while the powders of 5 and 10 phr increased Tm to 83 °C. The slight variation in Tm might be attributed to the mild change in crystal size. The corresponding weight crystallinities were all around 22%, indicating the limited

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influence of the powders on the crystallization behavior of EVA because of the physical mixing manner and their microscaled size. Mechanical property of the prepared SMPCs

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was also examined. Integration of the powders to some extent decreased both the tensile strength and elongation at break of the composites. Moreover, the mechanical property

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of the SMPC, especially the elongation at break, was deteriorated more evidently by the Yb(TTA)3Phen.

Since negligible influence of Yb(TTA)3Phen and Nd(TTA)3Phen exerted on EVA

structures, their main function is to embed the selective photothermal effect. While NIR light of 980 nm was irradiated onto EVA/Yb(TTA)3Phen composites, the balanced temperature in 3 min increased greatly with light PD and powder content (Fig. 4a). For

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ACCEPTED MANUSCRIPT the loading of 2, 5, and 10 phr, the composite temperature increased linearly from room temperature to 61, 112, and 167 oC at the PD of 3.1 W·cm-1. The pure EVA did not increase its temperature obviously with the balanced temperature of 38 oC, while 10 phr

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Nd(TTA)3Phen also increased the temperature slightly to only 59 oC at the same PD. Because the powder of Nd(TTA)3Phen had better photothermal effect than its

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counterpart, lower PD of 808 nm NIR light is required to increase the temperature of EVA/Nd(TTA)3Phen composites. As shown in Fig. 4b, similarly, the temperature

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increased with light PD linearly and with powder content. The loading of 2, 5, and 10 phr powders enabled the temperature rise to 50, 93, and 120 oC at the PD of 1.0 W·cm-1. The 10 phr Yb (TTA)3Phen showed neglected effect on temperature increase, i.e., the balanced temperature was only 33 oC, close to the pure EVA (30 oC). Obviously, the

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introduction of Yb(TTA)3Phen and Nd(TTA)3Phen offered the composite selective photothermal capability for 980 and 808 nm, bringing in the selectively responsive

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shape-memory effect which is examined next. At Tdeform = 120

o

C the EVA composite films with different contents of

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Yb(TTA)3Phen and Nd(TTA)3Phen powders were bended to 90

o

and cooled down to

room temperature to fix the temporary shape. NIR light of 980 and 808 nm were irradiated onto those samples, while the shape recovery rate (Rr) was calculated and shown in Fig. 4c and 4d. Because Nd(TTA)3Phen did not own the photothermal capability for 980 nm, the shape recovery was not observed in EVA composite with 10 phr Nd(TTA)3Phen. The EVA/Yb(TTA)3Phen composites, on the other hand, performed

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ACCEPTED MANUSCRIPT the shape recovery under irradiation of 980 nm, particularly at the loading of 5 and 10 phr. Since the temperature increase was enhanced at a higher content, EVA composites with 10 phr Yb(TTA)3Phen reached full recovery at a smaller PD than the one with 5

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phr Yb(TTA)3Phen, while the Tsw was calculated as 73 and 86 oC, respectively. The NIR light of 808 nm instead did not trigger the shape change of EVA/Yb(TTA)3Phen with 10

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phr loading (Fig. 4d). Only the powder of Nd(TTA)3Phen can realize the shape recovery under the irradiation of 808 nm NIR light, while higher content (10 phr) also trigger the

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recovery under a smaller PD. The Tsw in this case was 103 oC. It seems that the apparent Tsw of EVA composites varied greatly with filler type and content for the reason discussed in a previous report which is related to the indirect heating manner of the matrix. Because of the selective photothermal effect of Yb(TTA)3Phen and

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Nd(TTA)3Phen powders, their composites with EVA presented the shape recovery to the

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certain wavelength of 980 and 808 nm, respectively.

3.3 Effect of Irradiation Periodicity and Frequency on Shape Recovery

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During the light irradiation onto SMPCs, photothermal fillers absorb the light and

transfer the energy into heat, increasing sample temperature. In addition to continuous irradiation, the influence of periodic irradiation with different frequency on the temperature and shape recovery was studied. The irradiation through a periodic manner was achieved via self-made light-blocking boards equipped with a motor. A quarter, half, and three quarters of the round boards were cut out to realize the different irradiation

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ACCEPTED MANUSCRIPT periodicity. As shown in Fig. 5a, in the named Periodicity I, II, and III, the ratio of the irradiation time to the non-irradiation time within one cycle was increased gradually from 1/3, to 1/1 and 3/1. An arbitrary function generator was connected with the motor

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to switching the rotation speed, for the purpose of controlling the variation frequency.

The temperature increase can be assumed to be in a linear manner with a constant

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rate of k1 in the case of no heat loss. While heat convection and heat radiation are considered as the main heat loss manner, how the temperature varied with time can be

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described using Equation 1, where k2(T-T0) and k3(T4-T04) are attributed to the heat loss due to heat convection and radiation, respectively. The heat conductivity can be neglected here because of the low thermal conductivity of air. 

 =  −   −  −    −  

(1)

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NIR light of 980 and 808 nm with a certain PD (1.4 and 0.65 W·cm-1) were first irradiated continuously onto EVA composites having 10 phr Yb(TTA)3Phen and

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Nd(TTA)3Phen powders, respectively, to reach a balanced temperature of T ≈ 100 oC, as shown in Fig. 5b-d (black curves, measured). The k1 was set as the slope value of the

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curve during the temperature increase step, while the k2 and k3 were obtained by fitting the curve within the balanced temperature range using Equation 1. Subsequently, instead of a continuous exposure, we simulated the temperature variation when the SMPCs were supposed to be exposed onto the NIR light having different irradiation periodicity (Periodicity I-III) and the variation frequencies were set as 0.1, 1, and 10 Hz as shown in Fig. 5b-d (red, blue, and pink curves, simulated). Since more time was

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ACCEPTED MANUSCRIPT allowed for light irradiation onto SMPCs with 10 phr Yb(TTA)3Phen (Fig. 5b-5d), the balanced temperature increased from 46 (Periodicity I), to 66 (Periodicity II), to 85 oC (Periodicity III). On the other hand, at a low irradiation frequency of 0.1 Hz, a larger

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temperature fluctuation of ±15 oC during the periodic PD change was achieved. Increasing the irradiation frequency reduced the temperature fluctuation range to ±2 (1

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Hz) and ±0.2 oC (10 Hz). The similar results can also be observed in the SMPCs with 10 phr Nd(TTA)3Phen (Fig. 5e-5g), while the balanced temperatures were 46 (Periodicity

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I), 66 (Periodicity II), and 82 oC (Periodicity III), respectively.

The irradiation periodicity can be used to control the balanced temperature, while the variation frequency only switched the temperature fluctuation range. Such capability can be connected with the temperature-memory effect, which is also presented by EVA.

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Instead of Tdeform = 120 oC, another two Tdeforms of 55 and 80 oC were used to deform EVA composites, respectively. NIR light of 980 and 808 nm with the PD of 1.4 and 0.65

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W·cm-1 were irradiated onto the composites with 10 phr Yb(TTA)3Phen and Nd(TTA)3Phen in the form of three different periodicities, while the irradiation

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frequency was set as 3.4 Hz at which the temperature fluctuation was neglected. The measured balanced temperature increased from 40.4 (Periodicity I), to 59.7 (Periodicity II) and 82.5 oC (Periodicity III), respectively, which matched with the simulated values very well. As shown in Fig. 5h, at Tdeform of 55 oC for the case of Yb(TTA)3Phen, the Rr increased from 6%, to 51%, and 86%, respectively, due to the increase in balanced temperature with more time allowing for exposure. The shape recovery capability,

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periodic irradiation. A similar result was also observed for composites with 10 phr Nd(TTA)3Phen (Fig. 5i).

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Here, we state that for light responsive SMPCs, four factors should be concerned with regard to the light source. The PD is a primary factor, i.e., critical PD must be

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offered to achieve sufficient heat accumulation, causing the temperature higher than the Tsw. The critical PD is determined by the type and content of the photothermal filler, and there is a need to reduce the PD for safety concern. Therefore, increasing the photothermal effect as well as their dispersion in polymer matrix is one research focus.

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Light wavelength is another important factor. As mentioned above, different photothermal fillers which are suitable for various wavelengths have been developed.

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The reported Yb(TTA)3Phen and Nd(TTA)3Phen here can be used widely as the selective photothermal filler for the safer NIR window. In addition to PD and

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wavelength as internal parameters of one light beam, how it is irradiated onto the composites can also be considered. The variation in irradiation periodicity and frequency has been shown in the present work to switch the balanced temperature and its fluctuation range. With the combination of temperature-memory effect, critical exposure periodicity is required to realize the shape recovery at a certain Tdeform. More specifically, reducing Tdeform can lead to the shape recovery at a relatively shorter

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ACCEPTED MANUSCRIPT exposure time, while long exposure time must be allowed to trigger the shape deformation at a high Tdeform. With the combination of certain NIR light wavelength and irradiation manner, the operator can embed unique response signal into the TMPs, i.e.,

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periodicity can trigger the shape deformation.

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via setting a suitable Tdeform, only the right NIR light wavelength and suitable irradiation

3.4 Applications in Macroscaled Actuator and Micropatterned Surface

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The combination of SMPCs responsive to different wavelengths has been used to present multiple shape change in an actuator upon the sequential irradiation of radiofrequency. The feasibility of using the prepared composites in a multishaped actuator upon the sequential irradiation of two NIR lights was demonstrated, while the

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working principle was visualized in Supporting Information Video S1 and Video S2 as well as the photo series displayed in Fig. 6. EVA/Yb(TTA)3Phen (10 phr) and

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EVA/Nd(TTA)3Phen (10 phr) which were bended to around 90 o at Tdeform at 60 oC, were connected by a pure EVA specimen. In the beginning, the 980 nm NIR light with the PD

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of 0.8 W·cm-2 was irradiated on the bended corner of EVA/Nd(TTA)3Phen, while no recovery (Fig. 6a). The laser was moved to trigger the EVA/Yb(TTA)3Phen on the other side, which present an evident shape recovery very quickly. Finally, the EVA/Nd(TTA)3Phen was exposed under 808 nm NIR light with the PD of 1.5 W·cm-2 to achieve selective recovery. The process was repeated again when 808 nm with the same PD was irradiated onto the EVA/Yb(TTA)3Phen first, before it was moved to

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ACCEPTED MANUSCRIPT trigger the shape recovery of EVA/Nd(TTA)3Phen (Fig. 6b). Similarly, the selective photothermal effects of Yb(TTA)3Phen and Nd(TTA)3Phen realized the multiple shape change with sequential irradiation as request.

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More importantly, we expanded such combination of two SMPCs responded to different wavelengths towards the micropatterned surfaces for the first time to our

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knowledge. Via imitating an ancient technique of movable type printing, the preparation of cured EVA surface consisting of arbitrary micropatterns of the two EVA/composites

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was achieved (see Experiment Section). As shown in Fig. 7a-d, the cured EVA/Yb(TTA)3Phen and EVA/Nd(TTA)3Phen spherical microparticles (diameter and height were 300 µm) as the movable components were transferred and attached onto a cured pure EVA film, forming the patterns of letter “N” and “J” respectively. The

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microparticles were programmed by compressing downwards using a weight at Tdeform of 60 oC and subsequent cooling. As shown in Fig. 7e and 7f, the diameters of the

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compressed microparticles became larger, indicating the decrease in the particle height. More specifically, as shown in Fig. 7g and 7h, after programming, the diameters of the

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micropartcles with a temporary shape increased from 300 to 400 µm. The NIR light of 980 nm was used first to precisely trigger the shape recovery of EVA/Yb(TTA)3Phen microparticles, forming the pattern of letter “N” (Fig.s 7i-7j), while the diameter reduced to 300 µm (Fig. 7k), indicating a complete shape recovery. Afterwards, the compressed EVA/Nd(TTA)3Phen microparticles (letter “J”) recovered to their original shape under the localized irradiation of 808 nm NIR light (Fig. 7m-7n). And as shown

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direct the cell behavior in a more complex manner.

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which can find potent applications in counterfeiting areas or act as smart substrate to

4. Conclusion

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Rare earth organic complexes of Yb(TTA)3Phen and Nd(TTA)3Phen were prepared and used as selective photothermal fillers for NIR light of 980 and 808 nm. The two fillers were introduced into EVA using a solution mixing method. Upon the irradiation of 980 nm, the EVA/Yb(TTA)3Phen composites presented the good shape recovery

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behavior, while 808 nm NIR light was used to selectively trigger the shape deformation of EVA/Nd (TTA)3Phen composites. The irradiation periodicity was also varied and

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the results indicated that a reduction in Tdeform caused the shape recovery at a relatively shorter exposure time, while long exposure time must be allowed to trigger the shape

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deformation at a high Tdeform. Finally, upon sequential irradiation of 980 and 808 nm NIR light, a macroscaled actuator presented multi-shaped deformation. More interestingly, the shape recovery of a certain area on the micropatterned surface was triggered first upon one irradiation before the other region was stimulated using another wavelength. One can expect that further work is focusing on the choice of suitable organic ligands with reactive groups which enable the in-situ polymerization of SMPCs

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Supporting Information

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Supporting Information is available from www.elsevier.com or from the authors.

Acknowledgements

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This work was sponsored by the Scientific Research Foundation for Returned Scholars (ZX15504320001), National Natural Science Foundation of China (51503098), and Preferred Program Foundation for Returned Scholars (ZX15512320008). The Priority Academic Program Development of the Jiangsu Higher Education Institutions

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(PAPD) is appreciated. Qing Lan Project, Six Talent Peaks Project in Jiangsu Province

References:

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(No. XCL-029) is gratefully acknowledged.

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Soc. 135 (2013) 12608-12611.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. (a) FTIR curves of TTA, Phen, as well as prepared Yb(TTA)3Phen and Nd(TTA)3Phen

powders.

(b)

TGA curves

of

prepared

Yb(TTA)3Phen

and

images of (d) Yb(TTA)3Phen and (e) Nd(TTA)3Phen powders.

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Nd(TTA)3Phen. (c) The proposed chemical structures of the two powders. (d, e) SEM

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Fig. 2. Reflectance spectra of (a) Yb(TTA)3Phen and (b) Nd(TTA)3Phen as well as their raw materials with BaSO4 as the reference. (c,d) Temperature increase of different

different power densities.

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photothermal fillers under the irradiation of NIR light of (c) 980 and (d) 808 nm with

Fig. 3. OM images of EVA composites with 10 phr (a) Yb(TTA)3Phen and (b) Nd(TTA)3Phen powders. Gel content (G) and swelling ratio (Q) of the EVA composites

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with different loadings of (c) Yb(TTA)3Phen and (d) Nd(TTA)3Phen powders. DSC heating curves of (e) EVA/Yb(TTA)3Phen and (f) EVA/Nd(TTA)3Phen composites. The

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powder loading were 0 (solid line), 2 (dash line), 5 (dotted line), and 10 phr (dash-dot line). (g, h) Variation of (g) tensile strength and (h) elongation at break of EVA

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composites with different power loadings. Fig. 4. (a-b) Photothermal effect and (c-d) shape recovery ratio of EVA composites with different loading of Yb(TTA)3Phen and Nd(TTA)3Phen under the irradiation of NIR light with (a, c) 980 and (b, d) 808 nm. Fig. 5. (a) The irradiation periodicity involved in the present work, while the ratio of light on/off was set as 1/3 (Periodicity I), 1/1 (Periodicity II), and 3/1 (Periodicity III).

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ACCEPTED MANUSCRIPT (b-g)

Simulated

temperatures

of

(b-d)

EVA/Yb(TTA)3Phen

and

(e-g)

EVA/Nd(TTA)3Phen under the irradiation of three periodicities. The frequency was 0.1 (red), 1 (blue), and 10 Hz (pink), while the black curve indicated the continuous

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irradiation to achieve a balanced temperature of 100 oC. (h, i) The shape recovery rate of (h) EVA/Yb(TTA)3Phen and (i) EVA/Nd(TTA)3Phen programmed at Tdeform of 55 and

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80 oC under the irradiation of the three periodicities.

Fig. 6. Photograph series of selective shape recovery demonstration experiment. (a) The

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NIR light of 980 nm was irradiated onto EVA/Nd(TTA)3Phen first before it was moved to trigger the shape recovery of EVA/Yb(TTA)3Phen. The EVA/Nd(TTA)3Phen was finally exposed to the NIR light of 808 nm. (b) The EVA/Yb(TTA)3Phen was irradiated under 808 nm first without shape recovery. The NIR light of 808 and 980 nm was used

sequentially.

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to stimulate the shape change of EVA/Nd(TTA)3Phen and EVA/Yb(TTA)3Phen

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Fig. 7. (a-d) EVA/Yb(TTA)3Phen and EVA/Nd(TTA)3Phen microparticles transferred to a cured EVA films, and compressed to fix a temporary shape (e-h). NIR lights of (i, j, k)

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980 nm and (m, n, l) were irradiated the shape recovery of EVA/Yb(TTA)3Phen and EVA/Nd(TTA)3Phen particles, forming the letter “N” and “L”, respectively. The elliptical and circle marks in figure b indicate the magnified areas for (c, g, k) EVA/Yb(TTA)3Phen and (d, h, l) EVA/Yb(TTA)3Phen particles.

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