Simultaneous Microscopic Structure Characteristics of Shape-Memory

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Simultaneous Microscopic Structure Characteristic of Shape-Memory Effect of Thermo-Responsive Poly (vinylidene fluoride-co-hexafluoropropylene) Inverse Opals Maohua Quan, Bowen Yang, Jingxia Wang, Haifeng Yu, and Xinyu Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17230 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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ACS Applied Materials & Interfaces

Simultaneous Microscopic Structure Characteristic of Shape-Memory Effect of Thermo-Responsive Poly (vinylidene fluoride-co-hexafluoropropylene) Inverse Opals Maohua Quan1, Bowen Yang 4, Jingxia Wang2,3*, Haifeng Yu4* and Xinyu Cao5 1

Institute for Advanced Materials and Technology, University of Science and Technology

Beijing, Beijing, 100083, China 2

CAS Key Laboratory of Bio-Inspired Materials and Interfacial Sciences, Technical Institute of

Physics and Chemistry, Chinese Academy of Science, Beijing, 100190, China 3

School of Future Technologies, University of Chinese Academy of Sciences, Yanqihu Campus,

Huaibei Town, Huaibei Zhuang Huairou District, Beijing 101407, China 4

Department of Material Science and Engineering, College of Engineering, Peking University,

Beijing, 100871, China 5

Key Laboratory of Green Printing, Institute of Chemistry Chinese Academy of Sciences,

Beijing, 100190, China

ACS Paragon Plus Environment

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KEYWORDS inverse opals, dimensional effect, nanoscale deformed, thermo-responsive shape memory, PVDF-HFP

ABSTRACT The paper presents a simultaneous microscopic structure characteristic of shape memory Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) inverse opals together with a bulk PVDF-HFP by SEM. The materials show thermo-sensitive micro-shape-memory property, accompanying with a reversible and modulated optical property. The introduction of the inverse opal structure into the SMP material render a recognition ability of micro-structure change aroused from complex environmental signals by optical signal, the micro-structure change can be simultaneously detected by SEM. Furthermore, this feature was applied as a reversible write/erase of fingerprint pattern through press-stimulus and solvent-induced effect, together with changes of morphology/optical signal. This micro-shape-memory property can be attributed to the shrink/swell effect of polymer chain from external stimuli combined with microscopic structure of inverse opals. It will trigger a promising way towards designing reversible micro-deformed actuators.


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1. Introduction Shape-memory polymers (SMPs) 1-24 have attracted wide research attention because they enable to be fixed into the temporary shape and recover to original shape at temperature field1-3 and their extensive applications in sensor,4 artificial muscles,5-7 implants for invasive surgery4 and etc. Recently, these studies have been focused on the development of “smarter” composite materials14,15 by combining with large stress/strain recovery of SMP.10-13 For example, Zhao16 presented structure color reporting wetting/dewetting process via SMP’s stress/strain behavior. Jiang23 reported a novel SMP that can be “cold” programmed based on chromogenic inverse opals, the programmed process is recorded by structure color/optical signal. SMP materials have been explored for an intriguing potential based on inverse opal structure. However, little attention is paid on microscopic, nano-scale23,24 deformed behavior of SMP, it is especially important for the optimization of physics properties of device19-22,25-27 and the development of smart materials based on SMP.28 Herein, we report a simultaneous microscopic structure characteristic of SMP from single material embedded self-inverse opal structure of Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). There describes a morphological deformed behavior of thermal-sensitive SMP inverse opals (SMPIO) at phase change temperature ranges. SMPIO shows a reversible deformed behavior at Tm1＜T＜Tm2 (in this case, Tm1 ， Tm2 ， and Tm3 indicates the melting point of the three crystal phase of PVDF-HFP copolymer respectively), and an irreversible collapse of polymer network at Tm2＜T＜Tm3. The surface interpolated inverse opal can repair shape changes in micro-nanoscale at T＜Tm1, and the deformed degree is monitored by structure color/reflection spectra. Distinguished from Traditional thermo-responsive SMP, the film interpolated inverse opal introduce more functionals. Especially, it can record the deformation by spectra signal or structure color. The


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deformed microstructure can be simultaneously detected by SEM images, and the measured micro-deformation data is corresponding to the calculated value as well. Furthermore, the sample demonstrates reversible information storage functionality based on similar “cold” programmed procedure, i.e. solvent-induced shape recovery by press-deformed character of SMPIO. The “smart” material from single material PVDF-HFP can be attributed to the combined effect of microscopic structure of inverse opal and the shrinking/swelling property of interpolated polymer chain. This will open a new door for the creation of new functional materials or design of the microstructural acting on the nano-scale. 2. Results and Discussion Figure 1A demonstrates the fabrication process of multi-functional PVDF-HFP inverse opals. First, the solution of PVDF-HFP (in N,N-Dimethylformamide, 10 wt%) was infiltrated into the interstice of as-prepared silica opals in Figure 1A1, which were obtained from assembling the silica particles on the glass substrate by vertical deposition method.12 Notably, the infiltrated sample was a two-layer structure owing to excess infiltration of the polymer into the interstice of silica opals (Figure 1A2). The upper part is the pure polymer film, the bottom one is a composite film of the polymer infiltrated silica opals. After drying for 2 h (115 ℃) the silica opals was removed by being immersed in hydrofluoric acid for 5 h (Figure 1A3). Finally the inverse opal of PVDF-HFP was obtained (Figure 1A4). As expected, the resultant sample is the combination of two parts owing to the excess infiltration of the copolymer as shown in Figure 1B: the upper layer contains hexagonal array of porous microstructures (Figure 1C), possessing ordered inverse opals with thickness of 2 µm. The bottom one is of bulk PVDF-HFP copolymer film with thickness of 16 µm (Figure 1B and 1D). The thickness of the resulting sample can be modified from 18 to 35 µm by adjusting the dosage of PVDF-HFP solution in the embedding process


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(Figure S1, Supporting Information). The water contact angles of the top and bottom surface is 65.5⁰ and 114⁰ respectively, as shown in the inserted Figure1C and 1D. XPS analysis in Figure S2 indicates that a spot of SiO2 was remained on porous PVDF-HFP surface that lead to the top surface of smart film presents hydrophilic feature.

Figure 1. (A) Schematic illustration of the fabrication process of multi-functional PVDF-HFP inverse opal together with PVDF-HFP copolymer, including (A1) the infiltrating of the polymer solution into the air gaps of silica opal, (A2) forming a complex of opal and copolymer, (A3) dissolving the opal temperate by wet etching, and (A4) yielding the inverse opal after template removal. (B~D) SEM images of the sample. (B) Crosssection SEM images of the sample, with top layer of inverse opal 2 µm, bottom layer of pure polymer film of 16 µm. (C) Top-view and (D) Down-view SEM images of inverse opal; the inset is the optic image of the water droplet on the corresponding film.

The as-prepared PVDF-HFP inverse opals present a typical thermal-memorized behavior accompanied with a similar morphological recovery in Figure 2A. At first, the sample adopts a flat-sheet state (the inserted Figure 2A1), revealing an ordered hexagonal pores array with uniform pore dimension of 280 nm in SEM image (Figure 2A1), corresponding to a reflection


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peak λmax of 533 nm in Figure 2B (black arrow line). Then it turns to an expected tube-like shape by enforced at 40 ℃, exhibiting a microscopic quadrilateral array with extended pore dimension of ~ 642 nm (Figure 2A2). And the deformed state is fixed after being cooled to room temperate and removed the force (the inserted Figure 2A2). In this case, the spectra were red-shifted to 540 nm (magenta arrow) or blue-shifted to 520 nm (blue arrow) for different deformed positions since the sample may expand or shrink (Figure 2B) at different region. Subsequently, the sample recovers to the original flat-sheet state when being exposed to temperatures over Tm1 (i.e. 49.4 ℃) (the inserted Figure 2A3), accompanied with a shrinkage of pore dimension to ~330 nm with separated quadrilateral array by thick wall (thickness of 194 nm) above 60℃ (Figure 2A3). Where, λmax of the sample recovered to ~543 nm owing to a larger shrinkage than that of elastic strain of polymer chains over Tm2. As matter of fact, the sample reveals a macroscopic recovery but leaving a microscopic morphologic change from a well-ordered hexagonal pores array to extended quadrilateral pores during released stress/strain process in Figure 2A. For the macroscopic sample, the thermal-memorized behavior is similar to that of bulk PVDF-HFP (Figure S3 Supporting Information) except a lower deformed temperature of 49.4 ℃, compared with that of 50 ℃ for bulk PVDF-HFP. The polymer chains of inverse opals provide a tensile force for deformed process. But the deformed degree of a pore depends on the bending direction of inverse opal as well. The mode 1 , 2 indicates different deformed direction (Figure 2C and 2D). The pores extend along X axis for large strain in mode 1 (Figure 2C), and extend along Y axis in mode 2. The arrows on Figure 2C indicate the bending direction of the sample, the pores are drew along X direction with pore size extending from 553 to 953 nm. In model 2, the maximum deformed pore is 1100 nm along Y direction noted by inserted Figure 2D and S4 (Supporting Information). The sample exhibited fixing ratios (Rf) of 40 %/33 % for mode 1/2,


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respectively, presenting a reversible macroscopic deformation but an inconsistent microscopic deformation (Figure S5 supporting Information), and 10 cycles macroscopic bending behavior in Figure S5 (A). Here, the maximum or minimum wavelength was reduced by 2–3 % during a single cycle. It could be deduced that the deformation was not perfectly restored during the cycle due to permanent deformation of the structure.

Figure 2. (A) in-situ photos and SEM images (scale bar: 500nm) of the sample during shape-memory process and deformed mode of the sample. (A1) a flat sample, after the sample was programmed into tube (A2) at 25 ℃ and fixed by heating up 40 ℃ at 5 min, (A3) the sample recovered toward a flat initial state as heating above 60 ℃. (A4) The sample has two bending models. The sample shows bending behavior when inverse opal is at the bottom layer of the sample (model 1), compared with that inverse opal is at the top layer (model 2). The deformed direction of porous is defined by using a projected coordinate system. (B) The reflection spectra of inverse opal structures of the sample for the various deformed state. (C) SEM images of the deformed model 1 and (D) deformed model 2. In the model 1, the strain of deformed porous stems from extended porous along


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direction of X axis, compared with that the model 2 stems from extended porous of Y direction. The position of the maximum deformed degree of the extended porous is the lowest curvature radius of the bending sample.

To understand the heat effect of inverse opals toward the micro-deformed behavior, we investigated the relationship between the phase transition temperature of porous polymer and the corresponding morphological evolution. The phase change point of copolymer is 85 ℃ and 136 ℃, respectively (Figure S7, Supporting Information). Clearly, little morphological evolution is observed for the sample when being heated from 25 to 85 ℃, according with an unchanged reflection signal in position (Figure 3A). In comparison, an obvious morphological evolution is observed over 85 ℃, corresponding to a large spectra shift in Figure 3B. Concretely, the initial inverse opal reveals an ordered hexagonal pores array (Figure 3C1) with roughly pore dimension of 294 nm, corresponding to spectra of 565 nm. Then the pores were drew along horizontal direction, accompanied with a shrinkage of its vertical direction (Figure 3C2) toward a quadrilateral shape with long/short axis of 444/218 nm above 100 ℃, responding to an obvious blue-shift to 545 nm in the spectra. Subsequently, the pores were extended along the horizontal direction (Figure 3C3) resulting in a shrinkage of other direction and blue-shift of the spectra signal to 527 nm, with the average long/short axis of 478/209 nm at 125 ℃ respectively. Finally, the copolymer chains fleetly shrink along vertical direction, leading to a dramatic decrease of the pore size over 130 ℃ (Figure 3C4), with average long/short axis of 811/139 nm respectively. In this case, there observed a large blue-shift and the fall of the spectra intensity, or even a disappearance of the spectra. Furthermore, the continuous rising temperature causes a mixing and disturbance of the periodic structure, contributing to a blue-shift of spectrum (from 567 to 470 nm) in 97 nm and a fall of the reflection intensity in Figure 3B. The connecting bridges


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between the unit of air-cavity networks are collapsed to lower the surface energy at Tm2＜T＜Tm3, the polymer chains get closer each other until air-cavity networks completely close above Tm3. The copolymer chains become mobile and have no effective dimensional effect over Tm2. Such an irreversible deformed phenomenon indicates that this dynamic behavior of thermo-SMP chains in a melting stage. Additional experiments revealed that the heating rate can’t limit the deformation of the elastic ordered macropores or maintain the size of the pores in Figure S8 (Supporting Information). This deformed behavior of the inverse opal is irreversible during heating process (above Tm2). Based on above-mentioned extended pore dimension and blue-shift spectra, it can be speculated that the depth of the pore decrease greatly, which determines the spectra position as indicated in Figure S9 (Supporting Information). When assuming a constant volume for the polymer chain during the deformed process, the effective diameter C33 (Figure S9 Supporting Information) of spherical air-cavity can be speculated based on the original pore size and the deformed morphology from corresponding SEM images. It is calculated that the pore size is 262 (Figure 3C2), 253 (Figure 3C3), and 225 nm (Figure 3C4), respectively for the deformed process. There is shrinkage of ~ 10.88 %, 13.95 %, and 23.46 %, compared with the original macropores with size of 294 nm (Figure 3C1). Accordingly, λmax of the deformed inverse opals can be estimated by combining Bragg’ with Snell’ s law,28 λmax = (8/3)1/2D(d/d0)(neff2-sin2θ)1/2, where D is the diameter of the silica particle, (d/d0) is the deformed degree of pore, neff is the effective refractive index, and θ is the incidence angle, respectively. In this case, neff = (ɸpn2p+(1-ɸp) n2m)1/2 from Maxwell-Garnett approximation. As a result, λmax blue-shifted to 543, 525 and 470 nm at 105 ℃, 125℃, 130 ℃ respectively. The values are also in according with λmax of 545, 527, 469 nm calculated from Snell’s law under similar conditions.34,35 The triggered SM effect is arose


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from temperature-induced dimensional effect of SMP chains, rather than inhomogeneous swelling induced by disordered arrays.23 This phenomenon demonstrates the potential application of SMPs for temperature alarm by optical signal.36

Figure 3. (A, B) reflection spectra and (C) SEM images of the sample were taken at the different phase change temperature. (C1) is original porous with ordered array. (C2) is deformed porous with ordered array above 100℃. (C3) is disordered porous above 125 ℃. (C4) is the porous that tends to collapse over 130 ℃.

To further understanding the SM functionality of the as-fabricated sample, it was developed for the press-solvent induced information-storage property at T＜Tm1. The fingerprint pattern was obtained by pressing the finger at the top surface of sample for 5 s in Figure 4A and Figure S10. An alternated pattern of dark and light is presented in magnified optic image (Figure 4A2 and 4B). Particularly interesting, the pattern can be removed by immersing the sample in ethanol, it is similar to a reversible write-erase procedure, indicating an allowance of re-arrangement of fingerprint pattern by ethanol. Furthermore, the sample experienced changing color/spectra signal from bright red (655 nm, green line, region 1) to pale red (629 nm, yellow line, region 2)


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during the procedure in Figure 4B. The blue-shift of spectra is due to the reduced pores size caused by stress.37 Afterwards the sample is immersed into ethanol and solvent evaporation, λmax red-shifted to 638 nm. This result suggested that SM network could compensate net compressive strain. Figure 4C, 4D and Figure S11 (Supporting Information) showed SEM images of the sample after experiencing press/recovery by solvent removal of the fingerprint, respectively. Clearly, an obvious dent-shape print was left on the sample after pressing process (Figure 4C). There appear light lines (optic image Figure 4A2) with a width of about 99.65 µm (i.e. distance between two lines of fingerprint) in the press (Figure S11, Supporting Information). In comparison, the print is removed and the microstructure is recovered after solvent treatment (Figure 4D), layers of collapsed pores appear in the fingerprint region (i.e. the dark lines in optic image Figure 4A2), indicating stress-induced disordered structure.23 When being immersed into ethanol, the copolymer chains are in tensile stress configurations. The stress disappears and triggers an instantaneous shrinkage of polymer chain upon removing the solvent, resulting in chemo-responsive behavior. The micro-deformed shape can recover original state, indicates that SMP builds up entropic elasticity matched the gap pressure after solvent removal. This phenomenon is similar to a reported literature 24 that fixed SM shape by suitable stress and shape recovery is triggered by solvent treatment, which provides larger capillary pressure to compensate compressive strain of inverse opal network.


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Figure 4. (A) In-situ photos of sample taken before and after fingerprint process. (A1) The press fingerprint on the top surface of the SMP film leads to an additional deformation into fingerprint pattern, and (A2) their sequential recovery to initial state after drying out of EtOH. (B) The optical image of fingerprint pattern on the top surface after the small strains induced deformation of flexible network, which results in a color change. The scale bar is 200 µm. The inset are the reflection spectra of corresponding region, which is obtained from the top surface after press fingerprint and drying out of EtOH, respectively. (C) and (D) SEM images of the top surface of the sample after press fingerprint and drying out of EtOH, respectively. (E) The recyclability of the write-erase transition for 3D nano-structure pattern change.

In principle, polymer is an intrinsically viscoelastic material,8 which tends to recover to a permanent state due to entropy elasticity of the polymer chains.38 Polymer network shows a heatsensitive SM effect and solvent-sensitive recovery property.39 After ethanol evaporation, the shrinking strain arises by the capillary condensation, which can rapidly trigger the recovery of initial shape from the deformed network structure. The strain energy from shrinking stress equals to the entropy energy.23,24,40 By comparison, the deformed force being imposed onto the top


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surface (the value is more than entropy elasticity of the polymer chains) would lead to a compromised SM behaviors, that is reflected as “permanent” deformation and color. Figure 4E shows a reversible write-erase transition, it takes 10 cycles with little change. In fact, only small strain of inverse opal was involved in the input/output of fingerprint pattern process. The sample with good cycling stability is promising for the material’s application in information storage device and strain sensor. They provide a large-scale fabrication for sensitive and stable resistance response on deformations. In a word, as-prepared PVDF-HFP inverse opals showed responsive SM property at different thermal-responsive temperature in Figure 5. In this case, the flexibility of polymer chain plays a key role on SM property41,42 at three different phase transition temperature.43 The sample shows press-inducted microscopic SM property below Tm1: the macropores of inverse opal deformed by small press along direction of axial load, this deformation can be released by ethanol immersion.17 Such phenomenon indicates that inverse opal structure leads to an increase of threedimensional space of polymer chains and a reduced volume of physical entanglement of polymer chains. The sample shows temperature-induced recovered character of memory shape above Tm1. There is shrinkage of macropores of inverse opal above Tm2, because of the melt state for crystal phase. When T＞Tm3, the inverse opal structure disappears owing to melting polymer.


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Figure 5. Schematic illustration for microscopic SM effects of PVDF-HFP copolymer film. (A) The sample has inverse opal structure from network chains of copolymer with SM effect. (B) In small strains field, the porous were in shrinkage condition along direction of axial load under low temperature. In the solvent field, the porous were in elongate condition. When the solvent evaporated, the porous have transformed from elongate to shrinkage, combining released the strain of inverse opal. (C) When the temperature is between Tm1~85 ℃, the porous show recovery of deformed behavior with an outer force. (D) The porous show gradually shrink above 85℃, fleetly shrink until disappear as above 130℃.

3. Conclusion The paper presents a microscopic thermo-deformed behavior of a thermo-responsive inverse opal SMP. It enables to recognize micro-pressure signals at room temperature, and favored with a solvent-triggered shape-recovery. The paper discusses the impact of the elastic artificial periodic network structure on SMP film that a microscopic thermo-deformed and press-induced behavior can be recorded by SEM. This material is interesting for elastic artificial periodic network structure, thermo property of self SMP and press-induced property. Polymer chains of SMP are


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capable of responding to different environmental signals and recovery microscopic deformations by reversible bending and twisting through the plasticizing effect of periodic network. The top layer (i.e., elastic artificial inverse opal structure) provides an optical signal for the intriguing SM behavior at micro-nanoscale. The SMPIO with microscopic SM effect have potential applications in biological field, which will provides a fabrication approach for “smart” material through microscopic deformed SMP. Experimental Section Fabrication of smart film: PVDF-HFP, N,N-Dimethylformamide, ethyl alcohol and toluene were purchased from Aladdin. PVDF-HFP, Mark-Houwink parameters: a=0.901, K =0.0000112721, Mw 230800, Mn 140600. The random copolymer contains 15 wt% of HFP. All chemicals were used without further purification. The opals were carefully deposited onto glass substrate from the ethanol dispersion of monodisperse silica spheres using the vertical assembly method at 40℃ for 24 h. Silica opal coated on PVDF-HFP/DMF sol is embedded by thermal annealing, which are then removed by HF etching, thereby yielding inverse opal on the top surface of PVDF-HFP film. The overall procedure for creating multifunction film is illustrated in Figure 1A. Data analysis: There must be located that the temperature is main factor that affect the evaporation time (i.e. main factor affecting the solvent-responsive property of the sample). In order to explore the correlation between the room temperature and the evaporation time observed the CA of 2 µL water on the sample under similar conditions, as shown in Figure S12 (Supporting Information). The water CA of the bottom surface was dropped to 90⁰ in 7 min. Maxwell’s equation was utilized to calculate the effective refractive index (neff). The size of


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the pore of the inverse opal was derived from SEM images. The refractive index of the PVDFHFP copolymer was calculated to be 1.56. The shrinking ratio, which is defined as (d-d0)/d0, where d is the effective diameter of the deformed pores after heating, can then be calculated. d0 is the diameter of the original pores. Characterization: SEM imaging was performed on a Hitachi S-4800 Field emission microscope and Zeiss Supra 55 Field emission microscope. The bend sample was fixed in the specimen platform for SEM. SM testing was carried out on a Nikon Digital-Single Lens Reflex and DataPhysics (Germany) OCA20 contact-angle system at ambient temperature. The photo of the film was obtained a Digital microscope Leica DVM6. Vis Reflectance Spectroscopy was taken by system microscopy (Olympus BX51, Japan). The X-ray diffraction measurements were conducted on a PANalytical EMPYREAN X-ray diffractometer. Thermal property was characterized by Diamond differential scanning calorimeter (DSC). The layer of bulk PVDFHFD was fixed in the heater for research process of morphological change. Thermal deforming analysis was carried out on Q5000IR from 25 to 140 °C at the heating rate of 2, 5 and 10 °C/min, respectively. XPS was used for the analysis chemical element of the bulk PVDF-HFD surface by using Al Kα radiation (185 W). ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Figures S1, S2, and Table 1, estimate thickness and chemical element of bulk PVDF-HFP film.


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Figures S3-S12, the calculating formulas for analysis the effective diameter of spherical aircavity, and the calculating formulas of the fixing (Rf) and recovery ratios (Rr) of SMPIO. AUTHOR INFORMATION Corresponding Author * Jingxia Wang, E-mail: [email protected] *Haifeng Yu, E-mail: [email protected] Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the financial support by MOST of China (2017YFA0204504, 2016YFA0200803), and the National Natural Science Foundation of China (Grant Nos. 51403017, 51673207, 51373183, 51373179, 51322301 and 51573005). REFERENCES


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Table of Contents Graphic

The paper presents a shape memory Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDFHFP) inverse opals together with a bulk PVDF-HFP, which shows thermo-sensitive microshape-memory property, reversible write/erase of fingerprint pattern through press-stimulus and solvent-induced effect.


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Simultaneous Microscopic Structure Characteristics of Shape-Memory

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