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Article

Millisecond Response of Shape Memory Polymer NanocompositeAerogel Powered by Stretchable Graphene Framework

Fan Guo, Xiaowen Zheng, Chunyuan Liang, Yanqiu Jiang, Zhen Xu, Zhongdong Jiao, Yingjun Liu,Hong Tao Wang, Haiyan Sun, Lie Ma, Weiwei Gao, Andreas Greiner, Seema Agarwal, and Chao Gao

ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00428 • Publication Date (Web): 23 Apr 2019

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1

Millisecond Response of Shape Memory Polymer Nanocomposite

Aerogel Powered by Stretchable Graphene Framework

Fan Guo1, Xiaowen Zheng1, Chunyuan Liang2, Yanqiu Jiang1, Zhen Xu1,5*, Zhongdong Jiao3,

Yingjun Liu1, Hong Tao Wang2, Haiyan Sun4, Lie Ma1*, Weiwei Gao1, Andreas Greiner6,

Seema Agarwal6, Chao Gao1*

1MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of

Polymer Science and Engineering, 2Institute of Applied Mechanics, 3State Key Laboratory of

Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, China.

4Hangzhou Gaoxi Technology Co., Ltd., Hangzhou 310027, P. R. China.

5National Key Laboratory of Science and Technology on Advanced Composites in Special

Environments, Harbin Institute of Technology, Harbin 150080, P. R. China.

6University of Bayreuth, Faculty of Biology, Chemistry and Earth Sciences, Macromolecular

Chemistry II and Bayreuth Center for Colloids and Interfaces, Universitätsstraße 30, 95440,

Germany.

*Corresponding author: zhenxu@zju.edu.cn (Z.X.); liema@zju.edu.cn (L.M.),

chaogao@zju.edu.cn (C.G.)

ABSTRACT

Shape memory polymers (SMPs) change shapes as-designed through altering the chain

segment movement by external stimuli, promising wide uses in actuator, sensor, drug delivery,

deployable devices. However, the recovery speed of SMPs is still far slower than the

benchmark shape memory alloys (SMAs), originating from their intrinsic poor heat transport

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and retarded viscoelasticity of polymer chains. In this work, monolithic nanocomposite

aerogels composed of bi-continuous graphene and SMP networks are designed to promote the

recovery time of SMP composites to a record value of 50 milliseconds, comparable to SMA

case. The integration of stretchable graphene framework as fast energy transformation grid

with ultrathin polycaprolactone (PCL) nanofilms (tunable in 2.5-60 nm) enables the rapid

phase transition of SMP under electrical stimulation. The graphene-SMP nanocomposite

aerogels, with density of ~10 mg cm-3, exhibit fast response (175±40 mm s-1), large

deformation (~100%) and wide response bandwidth (0.1~20 Hz). The ultrafast response of

SMP nanocomposite aerogels confers extensive uses in sensitive fuses, micro-oscillators,

artificial muscles, actuators and soft robotics. The design of bi-continuous ultralight aerogel

can be extended to fabricate multifunctional and multi-responsive hybrid materials and

devices.

KEYWORDS: graphene aerogels, shape memory polymers, fast responsive shape memory,

3D printing, polycaprolactone

Shape memory materials (SMMs) feature the ability of shape recovery under external

stimulus and exhibit a wide range of applications from aerospace, robotics to biomaterials.1,2

SMM mainly encompasses two categories: shape memory alloys (SMAs) and shape memory

polymers (SMPs). Recently, SMPs have attracted increasing attention for their many merits,

such as low cost, high energy efficiency, large deformation (up to 800%), easy processability,

good bio-compatibility and degradability, as compared with SMAs.3-8 However, SMPs exhibit

long response time (τ, the time recovering back to original shape under external stimulus),

usually at minute scale, which is far inferior to those of SMAs (< 1 s).9-13 The long response

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time of SMPs originates from the intrinsic low heat transmission efficiency (for example, 0.3

W mK-1 for the prevailing polycaprolactone, PCL) and the retarded viscoelasticity of bulk

polymers, in contrast to the fast solid-to-solid phase transition between martensitic and

austenitic phase of SMAs.10,14,15 The slow response of SMPs becomes the Achilles' heel that

greatly limits their extensive uses in high frequency actuations, sensors, etc.16

To circumvent the intrinsic slow response issue of SMPs, many actuation methods have

been proposed, including Joule heating,17-27 photothermal conversion,28-31 solvent

interaction32-37 and so on, to replace tardily direct heating.38-40 For Joule heating, percolation

networks of conductive fillers, such as carbon nanotubes, silver nanofiber, graphene, etc.,17-27

have been constructed to realize electric triggering in thermally induced SMPs (Figure 1a).

For example, Cho et. al. prepared carbon nanotube/polyurethane composites and achieved 10

s response under ~40 V.17 Li and coworkers fabricated graphene/trans-1,4-polyisoprene (TPI)

hybrid foams which could recover to the original shape in ~7 s under 6 V.22 However, the

response time of SMP composites with percolation structure is still limited by the low heating

rate due to low electrical conductivity of conductive networks as well as long heat

transmission distance in SMP bulk matrix (micro scale to millimeter scale as usual). In

addition, the limited elongation strain of conductive path decreases the fracture strain,

sacrificing the favorable large deformation of SMP (Figure S1). To date, how to improve the

response time of SMP composites to the standard of benchmark SMAs still remains a great

challenge.

Here, we achieve nanocomposite aerogels composed of bi-continuous graphene and SMP

nano-networks which show ultrafast response up to a record millisecond scale comparable

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with current SMAs. The rapid electro-thermal conversion of graphene framework and the

extremely short heat transmission distance in PCL nanofilms collectively enable 50 ms

response at a low electrical field of 0.1 V mm-1 (~0.5 W mg-1). Because of the high

stretchability of graphene framework, the nanocomposite aerogels exhibit large elongation

(~100%), high fixing (97%) and recovering ratio (98%) at 100% tensile strain.

Different from the conventional bulk SMP composites (Figure 1a), Figure 1b depicts our

design of graphene-SMP (G-SMP) nanocomposite aerogels to achieve fast response. The

monolithic graphene framework serves as an integral energy transformation grid for fast

thermal generating and injecting. The PCL nano-coating possesses uniform thickness ranging

from 2.5 nm to 60 nm and therefore extremely short energy transmission distance (h),

accelerating the phase transition of PCL. Consequently, the response speed of nanocomposite

aerogels can be improved by about orders of magnitude, outperforming bulk SMP composites

with percolating conductive networks and much larger h.

Figure 1. a) Conventional electric response SMP composite composed of percolating

+

-

a b

SMP bulk

matrix

(above micro

scale)

Percolating

conductive

network

+-

Stretchable graphene

framework

Thermal Transmission

Distance (2.5-60 nm)

SMP composite Graphene/SMP nanocomposite aerogel

h

Post infiltration

Monolithic Energy

Transformation Grid

PCL

nanofilm

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conductive network and SMP bulk matrix. The heat transmission distance (h) is micro to

millimeter scale. b) Schematic illustration of fabrication process of graphene/SMP

nanocomposite aerogel that encompasses monolithic graphene framework and PCL nanofilm.

RESULTS AND DISCUSSIONS

Typically, the G-SMP nanocomposite aerogel (Figure 2a,b) was prepared by a post

infiltration of PCL macromonomer/toluene solution into graphene aerogel framework. The

schematic diagram of shape memory effect of PCL and characteristic spectra of PCL

macromonomer in our work are shown in Figure S2. The hybrid aerogel encompasses two

parts: a continuously connected graphene power grid and active PCL nanofilms uniformly

attached on basal surface of graphene sheets. We picked stretchable graphene frameworks

with programmed truss structure as the efficient power network, which were fabricated by

ion-aided gel 3D printing, followed by freeze drying and heat annealing. The detailed

procedure was described in the experimental section and our recent works.41,42

The graphene frameworks have low density, only of 8 mg cm-3, yet possess large tensile

strain (~100%), high electrical conductivity (~1100 S m-1) and thermal conductivity (~ 3.6 W

m-1 K-1) (Figure S3), which are both more than one order of magnitude higher than those of in

situ self-assembled graphene aerogels (87 S m-1, 0. 2W m-1 K-1).43-46 The coordination of low

density and high conductivity of G-SMP nanocomposite aerogel originates from the uniform

alignment of graphene sheets along the printed filament axis within graphene framework,

guided by the uniaxial extrusion through nozzle during ink-printing. The constituent filaments

have a core-shell structure: a dense shell (Figure 2c) with graphene sheets stacked densely on

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surface and a porous core with axially aligned empty cells (Figure 2d,e). The regular

alignment of graphene sheets was identified not only on the shell but also in the core, thereby

conferring the graphene framework with high electrical and thermal conductivity,

outperforming random graphene aerogels with the similar density.22,44,47,48 The combination of

low specific heat capacity (~10 J K-1 mol-1 at 273 K)49 and high electrical conductivity of G-

aerogel implies the high efficient energy conversion and transmission in the framework, thus

in favor of fast response.

During the shape recovery process of SMP composites, the rate of diffusion has a square

root time dependence with the diffusion distance,50 which means the transmission rate of

external energy decreases steeply with the increasing thickness of SMP. In this context, we

take another vital strategy to accelerate the response by shortening the diffusion distance.

Graphene framework was used as template to absorb PCL macromonomer/ toluene solution

with low concentrations (~0.1 mg mL-1) and then uniform PCL films of nanoscale thickness

were formed after thermo-crosslinking (Figure 2f,g). The aligned porous structure kept

invariable and the absorbed PCL nanofilms uniformly sandwiched graphene cell wall after

infiltration and drying (Figure 2d,e).

As the inner surface of graphene aerogel framework is completely exposed to infiltration,

the thicknesses of absorbed PCL layers (d) have a linear correlation with the weight ratio (α)

of PCL to graphene. We disassembled G-SMP nanocomposite aerogel by short ultrasonication

to extract individual hybrid flakes dispersed in ethanol (poor solvent for PCL), and assessed

their thickness by atomic force microscopy (AFM). Typically, the hybrid sheets were

measured as thick as ~10 nm at α of 0.43 (Figure 2h), 100% higher than that of bare graphene

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wall (~5 nm), which implies that the double-side coated PCL films have an average thickness

of 2.5 nm. From the statistical data of AFM tests (Figure S4), we concluded a linear

relationship between d and α as: 𝑑 = 11𝛼 (𝑛𝑚), indicating the facile control of thickness (i.e.

the thermal diffusion distance, h) by tuning the weight ratio of PCL.

Solution infiltration obtained a uniform coating of PCL nanofilms on graphene surface.

The arithmetical mean deviation of the profile (Ra) of hybrid sheets is ~1.0 nm, equal to that

(~ 0.96 nm) of neat graphene flakes (Figure S5c). The uniformity of PCL nanofilms results

from the good wettability of PCL macromonomer /toluene solution on graphene sheets. We

examined the affinity between graphene and PCL macromonomer/toluene solution by contact

angle meter. The contact angle between solution and graphene film (20 μm thickness) was

measured as less than 10o, ensuring the formation of spreading liquid film (Figure S5e). A 5.3

μm thick PCL film, after drying, was coated evenly onto graphene film (measured by a step

profiler) and dramatically reduced the surface roughness (Ra) from 0.956 μm to 0.303 μm

(Figure S5f), in according with uniform coating of PCL onto individual graphene flakes in

frameworks as shown in Figure 2h.

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Figure 2. a) A digital photograph and b) SEM image of G-SMP nanocomposite aerogel with

density of ~10 mg cm-3. SEM images of the c) surface and d, e) cross section of G-SMP

nanocomposite aerogel. The arrows indicate the arrangement direction of sandwiched sheets.

Inset in (d) is the schematic core-shell model of graphene framework. f, g) Energy dispersive

X-ray (EDX) mapping images (carbon and oxygen) of sandwiched sheets composed of

graphene and PCL nanofilm. Inset in (f) is the corresponding SEM image. h) AFM image of

hybrid sheets with 0.43 PCL (α, weight ratio of PCL to graphene) deposited on silicon

substrate.

The high conductivity of graphene power network and ultrashort thermal diffusion

distance of PCL nanofilms greatly promote the response time of G-SMP composites. Taking

10 μm

c

200 μm

d

60 μm

e

25 μm

10 nm

5 μm

h

1 mm

a b

5 mm

f

10 μmO

g

Graphene 5 nm

10nm

PCL 2.5 nm

PCL 2.5 nmC

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the G-SMP with ~0.35 (α) PCL as an example, we investigated the response time activated by

electric current under the tracking by a high-speed camera (100 FPS). The recovery frequency

was defined as the reciprocal of recovery time. Real-time snapshots taken within 50

milliseconds (Figure 3a) revealed the fast and homogeneous deformation of our G-SMP

triggered by electric stimulus (Movie S1, Supporting Information). The shape recovery of the

G-SMP from the temporary (Lt) to the original length (Lo) can be triggered by applied voltage

down to 2 V (~0.07 V mm-1) and reach about 100% tensile strain (δ, δ=(𝐿𝑡 − 𝐿𝑜) 𝐿𝑜⁄ × 100)

in 0.74 s. With increasing voltage, the response time of G-SMP sharply reduced and

eventually shortened to 50 ms with a low bias of 4 V，reaching the response time scale of

SMAs (Figure 3b; Figure S6). The actuation time of our G-SMP is reduced by two orders of

magnitude compared with previously reported results (> 1 s),9-13 which breaks the limit of

actuation time of SMP composites and shows comparable response time to SMA (Figure 3c).

Notably, our aerogel demonstrated large deformation simultaneously that outperforms the

ordinary SMMs.1-13

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Figure 3. a) The images of G-SMP nanocomposite aerogel with 0.35 PCL (α) under 4 V (0.5

W mg-1) during shape recovery process. See also Movie S1 (Supporting Information). b) The

tendency of length changes of G-SMP nanocomposite aerogel versus time under 4 V during

one typical shape recovery process. c) Comparison of maximum actuation frequency (1/τ) and

maximum deformation strain (tensile/compression) of our G-SMP nanocomposite aerogel

with various SMPs and SMAs.

The electric-triggered responsive behavior of single hybrid flake was tracked by in-situ

transmission electron microscope (Figure 4a). Following the same procedure in G-SMP

monoliths, the hybrid sheet transformed as programed from an extended shape (permanent

shape in Figure 4a) into a scroll under the together action of compression force and applied

voltage by a gold probe, and eventually maintained this temporary shape after releasing the

probe. Once contacting with the biased probe, the fast generated Joule heat turned PCL into

Lt

Lo

a30 ms

48 %

0 ms

100 %

50 ms

0 %

SMP

SMA

10-4

10-3

10-2

10-1

100

101

102

10-1

100

101

102

103

Electricity

Light

Solvent

Heat

Ma

x s

tra

in (

%)

Max frequency (Hz)

cG-SMP30

38

3139

4029

18

20

22

32

33 3435

36

3-8

1 10 100

0

25

50

75

100

125

Tensile

str

ain

(δ，

%)

Time (ms)

4 V

b

δ=(𝐿𝑡 − 𝐿𝑜) 𝐿𝑜⁄ × 100

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viscous state and drove the scrolled sheet recover to the permanent extended shape (Movie

S2, Supporting Information). The quick shape response to the electrical stimulus was repeated

for several recovery cycles and no sign of agglomeration was observed, exhibiting the high

mechanical stability (Figure S7).

To establish a quantified understanding on the shape recovery behavior of G-SMP

nanocomposites aerogel, we proposed a one-dimensional steady heat conduction model on the

energy conversion and transmission process for a hybrid flake unit according to Fourier’s law

of heat transport

𝑞 = −κ𝑇𝑐−𝑇ℎ

𝑑 (for 1D transmit through thickness, d) (1)

where q is the heat flux (amount of thermal energy flowing through a unit area per unit time),

κ is the coefficient of thermal conductivity of PCL, 𝑇ℎ (𝑇𝑐 ) is temperature of hot (cold)

interface of PCL membrane, and d is thickness, which is equal to heat transmission distance in

1D model (Figure S8). q comes from the Joule heat generated by graphene network when the

current passes through. Due to low specific heat capacity (~10 J K-1 mol-1 at 273 K), high

electrical conductivity (~1000 S m-1) and low density (10 mg cm-3), graphene frameworks

conversed electrical energy to thermal energy rapidly and reached saturated temperature

within tens of milliseconds (Figure S9) under different electrical power density (ranging from

0.04~1.1 W mg-1). Under invariant applied power, 𝑇𝑐 can be calculated as a function of film

thickness 𝑇𝑐 = 𝑇ℎ −𝑞

𝜅𝑑 . 𝜅 of PCL is relatively small due to the absence of electron

contribution to heat conduction in polymers, in order to keep 𝑇𝑐 above the melting

temperature of PCL (Tm-PCL), d should be thin enough to compensate the small 𝜅. Based on the

above equation, reducing PCL film thickness (d) has an impact on shortening response time

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by shortening the thermal diffuse distance through PCL nanofilms.

Differential scanning calorimeter (DSC) can be used to determine thermal transmission of

polymers by measuring heat flux differentials between sample and reference. Generally, the

crystallization temperature (Tc) and melting temperature (Tm) in DSC tests would separate due

to the thermal hysteresis caused by thermal diffusivity in thermal insulating polymers. The

larger the temperature difference (ΔT), the slower the thermal diffusivity. In the DSC heating

and cooling cycle (Figure 4b), the transient temperature difference (ΔT) decreased from 26 oC

of pure PCL to 10 oC of G-SMP nanocomposites with d below 50 nm as concluded in Figure

4c. This distinctive difference of ΔT indicates that increasing film thickness inhibits the

thermal diffusivity velocity through SMP and ultimately results in long response time of

monolithic composites.20,34 It is observed that Tc and Tm increased dramatically with the

decreasing thickness of PCL nanofilms. This is possibly caused by the confined

nanocrystallization and homogeneous nucleation of PCL on graphene surface, that serves as

heterogeneous nucleation agents. This trend has been observed by theoretical analyses and

laboratory tests.51-54 Apart from the thermal diffusivity difference, the underneath graphene

surface possibly homogenizes the polymer crystallinity regions and this homogenization

effect weakens as the coating thickness of PCL increases,55,56 resulting the narrower half-peak

height (WHPH) of the crystallization transition in thinner PCL coatings. As PCL content and

the thickness decrease, the detected narrower temperature difference and crystallization

transition indicate that the thermal response of phase transition in PCL is accelerated by

graphene networks and the response time of composites was greatly improved as shown in

Figure 4d.

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Theoretical deduction and experimental analysis validate that the fast response of G-SMP

nanocomposite aerogels originates from the nano thickness of PCL coating. In theory, the

relationship in recovery time (𝜏), thickness (d) and applied power density (P) can be summed

up as two empirical equations based on Equation 1 (details as shown in Supporting

Information):

For the constant d, 1

𝜏 ~ P. (2)

For the invariable P, 𝜏 =𝑎

𝐴𝑑2 −

𝑓0

𝜅𝐴𝑑 (3)

where the constant A =κ∙(𝑇ℎ−𝑇𝑐)

𝑆ρ𝐶𝑝∙(𝑇𝑚−𝑇𝑐), f0 represents the reliance force from graphene

framework. In this case, the saturated time of graphene frameworks (τ0) is constant and can be

omitted in order to simplify recovery time calculating. From Equation 3, τ increases as the

PCL nanofilm thickens if 𝑑 >𝑓0

𝑘𝑎. While 𝑑 ≤

𝑓0

𝑘𝑎, the temporary shape cannot be fixed because

the fixing force of glassy PCL cannot resist the resilience force of graphene frameworks. In

experiments, recovery time shortens hyperbolically with increasing power density in G-SMP

nanocomposites with different thicknesses of PCL nanofilms, fitting the predicted function in

Equation 2 (Figure 4d, black lines). The relationship between τ and d under the same P is

consistent with quadratic parabola, as calculated in formulas (3) (Figure S10).

Taking G-SMP nanocomposite aerogel as a whole, the improvement of thermal

diffusivity in basic hybrid flake makes a direct contribution to lessen its recovery time. With

PCL thickness reduced from 11 nm to ~3.7 nm (α, from 1 to 0.35), τ of G-SMP

nanocomposites steeply decreased by two orders of magnitude, from 2.2 s to 50 ms, under

adequate power density (>0.5 W mg-1), which is corresponding to the tendency in DSC tests

(Figure 4c). In addition, τ of G-SMP with 0.35 PCL (α) decreased from 1 s to 50 ms with P

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increasing from 0.1 to 0.5 W mg-1 (equivalent to 2 V - 4 V), (Figure 4d, square). When P

reached a certain intensity (saturation power, ~0.5 W mg-1), τ would keep almost unchanged.

Figure 4. a) In-situ TEM observation of a single hybrid flake in one deformation-retraction

cycle. b) DSC thermograms of G-SMP nanocomposite aerogels with increasing weight ratios

of PCL to graphene (α). Both heating and cooling rates were 5 oC min-1 for all tests. c) The

influence of PCL thickness (weight ratio) on temperature difference. d) The tested recovery

time (𝜏) as a function of power density (P) on G-SMP nanocomposite aerogels. All the open

symbols are experimental data and curves are fittings by Equation 2. Dash line, saturated

power density. Inset shows the recovery time-power density curves with linear axes.

We further measured thermal and electrical conductivity of monolithic nanocomposite

aerogel as a function of PCL content. With the increase of PCL content from 0 to 95 wt. %,

thermal and electrical conductivity of macroscopic G-SMP remained nearly unchanged at

about 3.5 W mK-1 and 1000 S m-1 respectively (Figure 5a). The stability of equivalent thermal

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.810

-3

10-2

10-1

100

101

3.7 nm

7.3 nm

11 nm

Re

co

ve

ry t

ime

(τ，

s)

Power density (P, W/mg)

Permanent shape Permanent shape

Pure PCL

Programming Temporary shape Recovery

10 20 30 40 50 60 70

-6

-3

0

3

6

Toc

Tm

Tom

exo

Melting

Pure PCL

8

2

1

0.3

0.1

He

at flo

w (

mW

)

Temperature (oC)

CrystalizationTc

increasing PCL dose

𝑇 − 𝑇𝑐 = 𝑇

b d

Free end T<Tm

Fixed end

T<Tm

Energ

ized

T>Tm

Energ

ized

T>Tm T<Tm120 nm

c

a

0.2 0.4 0.6

0

3

6

9

0 25 50 75 1005

10

15

20

25

30

PCL thickness （nm）

ΔT

(oC

)

0

2

4

6

8

We

igh

t ra

tio

(α

)

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conductivity (𝜆𝑒) of composite resulted from the constant porosity of graphene frameworks

with same density, based on equation 𝜆𝑒 = (1 − 𝜀)𝜆𝑔 + 𝜀𝜆𝑝 , where 𝜀 is the porosity of

graphene framework, 𝜆𝑔 and 𝜆𝑝 are thermal conductivity of graphene and PCL respectively

(details see Supporting Information).57,58 The unchanged electrical conductivity demonstrated

the conductive graphene network was well preserved during infiltration-air-dry-crosslinking

process. The Young’s modulus of the monolithic nanocomposite aerogel increased with

increasing PCL content and thickness of the nanofilm (Figure S10d,e).

As to the mechanical properties of the monolithic G-SMP nanocomposite aerogel,

negligible attenuation in elongation and shape memory capability was observed compared

with pristine PCL. Shape fixing ratio (Rf) and recovery ratio (Rr) are two important

parameters reflecting the shape memory capability (detailed definitions are given in

Supporting Information). In the first five cycles, Rf and Rr were all beyond 97% (Figure 5b;

Figure S11a), demonstrating that soft inner conductive networks and nano sized SMP had no

negative effect on shape memory capability but accelerated response time of shape memory

composites.40,59 The cyclic shape-memory curves under fixed strains showed the recovery

stress for G-SMP nanocomposite aerogels (Figure S11c). Plateau stress level (σp) reached

during fixed-strain recovery of G-SMP nanocomposite aerogels with 50% PCL is 11.9 kPa,

while σp of aerogels with ~90% PCL reached 21.7 kPa, deviating from a linear increasing of

PCL thickness from 11 nm to 99 nm. The normalized stress generation (NSG) of G-SMP

nanocomposite aerogels with 50% PCL (~0.95) is slightly larger than that of aerogels with

90% PCL (~0.92). We reasoned that this deviation on stress can be attributed to strong

influence of graphene surface on the homogeneity of PCL layers and their crosslinking

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level,60 which is reflected in the thermal phase transition shift in Figure 4 b and c. Besides the

molecular mechanism, the increasing thickness also affects the deformation mode from

buckling for thin layers to bending for thick ones, resulting the nonlinear stress increasing

with thickness of the monolithic aerogels composite. Tracking the mico morphology of G-

SMP nanocomposite aerogel at tensile and recovery state by SEM (Figure S12), we observed

that cell walls gradually crumple from extended conformations while the aerogel recovers to

the original shape.

Figure 5. a) Thermal conductivity and electrical conductivity versus PCL content. Insets

show the infrared images of composites. b) Cyclic shape-memory behavior of G-SMP

nanocomposite aerogel (α, ~1) at a 100% actuation strain. Detailed explanation of a typical

cycle is shown in Figure S11b.

Our G-SMP nanocomposite aerogels with millisecond response can be applied as a high-

speed fuse to protect circuit elements from current overloading. We designed a simple circuit

with a LED bulb, a G-SMP fuse and a DC power supply (Figure 6a). Once the current

exceeded the critical value, the G-SMP component retracted and disconnected the circuit

immediately for protection. For instance, the LED bulb continuously lightened (State 1) when

0 20 40 60 80 100

3

6

9

12

Th

erm

al co

md

uctivity (

W/m

K)

PCL content (%)

102

103

104

105

Ele

ctr

ic c

om

du

ctivity (

S/m

)

0%

95%

25 30 35 40 45 50 55

L, W, T: 4.7, 1, 1 mm

0 50 100 150 200 250

0

25

50

75

100

125

Str

ain

(%

)

Time (min)

-30

0

30

60

90

Te

mp

era

ture

(oC

)

0

2

4

6

8

10

Fo

rce

(m

N)

a b

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applied 100 mA current. While the current rose to 400 mA, the bulb blinked (State 2) and

went off immediately within 134 ms (State 3) due to protective function of G-SMP fuse. The

higher current generated more Joule heat and heated the fuse to ~80 oC，thus triggered the

shape recovery process of G-SMP.

The fast responsiveness merit allows G-SMP nanocomposite aerogels to couple with a

high frequency electromagnetic field and work as a micro SMP oscillator. We constructed the

oscillator by assembling G-SMP with an electromagnet as shown in Figure 6b. By applied

different impulse voltages with matched frequency and phase on electromagnet (Ve) and G-

SMP (Vf), the G-SMP responded and oscillated through four typical stages with negligible

hysteresis (80 ms), compared with the frequency of impulse voltages (Figure 6c,d). The SMP

oscillator reciprocated quickly and steadily as the Ve and Vf changed within 20 cycles by

virtue of the fast response and mechanical robustness (Figure 6e). The high oscillator

frequency of G-SMP composites together with high stretchability and good designability

promises them as technically feasible micro driver in smart device or robotics.

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Figure 6. a) A high-speed fuse of G-SMP nanocomposite. When the current exceeded the

critical value, the crystalline fraction melted leading the structure to recover to its original

shrinked state. b) Illustration of the high speed G-SMP oscillator. c) The original state (Ⅰ)

and three critical states (Ⅱ, Ⅲ, Ⅳ) during one oscillation cycle. d) Impulse voltage on G-

SMP (Vf), electromagnet (Ve) and the consequent distance change in a cycle. e) Stability of G-

SMP oscillator in 20 cycles.

CONCLUSION

In summary, we fabricated millisecond response shape memory composites with bi-

continuous networks by introducing highly conductive intact graphene framework and PCL

nanofilms. This structural design extends the limit of SMP recovery time to 50 ms and

provides an effective approach for achieving SMP composites with fast response and large

deformability, outperforming the current common SMMs. The efficient energy transformation

0 5 10 15 20 25 30 35 40

Time (s)

0.0 0.5 1.0 1.5 2.0

Vf

Dis

tance

Ve

Time (s)

80 ms

Ⅰ Ⅱ Ⅲ Ⅳ

Ⅰ

Ⅱ Ⅲ

Ⅳ

off off offonoff offon on

90 oC

21

55

73

28

a b

c

d

100 mA 400 mA

0 ms 134 ms

broken

LED

Fu

se

Vib

ra

tio

n

+-

+-

+-

Electromagnet

G-SMP

ElectromagnetG-SMP

oscillatorIronflake

e

1 2 3

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of integral graphene grid and nano-scale energy transmission distance in PCL nanofilm

collectively confer our G-SMP nanocomposite aerogel with satisfying recovery time

comparable with SMAs. In addition, owing to the stretchable 3D printed graphene

framework, G-SMPs possess large deformation strain and controlled deformation ability

simultaneously. The combination of fast response, stretchability and designability provides

our shape memory composites with tremendous opportunities for high frequency SMP device

in smart devices and micro robotics.

EXPERIMENT SECTION

Fabrication of stretchable graphene frameworks. Aqueous GO solution (20 mg ml-1,

lateral width of 10-20 μm) was purchased from Hangzhou Gaoxi Technology Co. Ltd.

(www.gaoxitech.com). The typical fabrication process of GO aerogel was described as

follows: 0.5 g of 2.22 wt.% CaCl2 aqueous solution (11 mg CaCl2) was gradually added to

GO solution (GO~20 mg ml-1) with stirring, then a conditioning mixer (AR-100, THINKY)

was used to further mix the ink at 3000 rpm for 15 min. The prepared inks were used to

fabricate 3D structure using a robotic deposition device (TH-206H, TIANHAO TECHNIC,

China). The inner diameter of the printing nozzles (r) is 250 μm. The 3D GO structures were

printed onto glass substrate in air, with a constant extruding pressure of 0.2 bar. The initial

nozzle height from the substrate was set at around 0.8r to help the adhesion onto the substrate,

and the moving speed of the nozzle ranges from 4 to 6 mm s-1. The as-printed 3D GO aerogels

were then frozen in liquid nitrogen and subsequently freeze-dried for 24 h. The as-prepared

binary aerogels were compressed to a certain ratio and then reduced by reducing gas (HI or

N2H4) or heat annealing at 3000 K for 30 min with a slow heating rate of 3 K min-1 under

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inert gas.

Synthesis of PCL macromonomer. Glycerol (29.6 L, 1 mmol) was placed in a schlenk

bottle and dehydrated under reduced pressure for 12 h prior to polymerization. Distilled ε-

caprolactone (55.2 g, 480 mmol) and a catalytic amount of tin hexanoate were added under

flowing dry nitrogen. The mixture was stirred for 24 h at 120 C under a nitrogen atmosphere.

After dilution with a little amount of tetrahydrofuran, the solution was dropwise added into

cold methanol. The precipitated polymer was repeatedly washed with methanol to remove the

unreacted monomer and catalyst, then dried under vacuum. Then, 20 g obtained polymer, 3.01

mL acryloyl chloride and 5.79 mL triethylamine were added into dehydrated tetrahydrofuran

(THF) at room temperature for 24 h. The mixture was precipitated and washed with cold

methanol. After the purification, the macromonomer was dried under reduced pressure for 2

days. The structure of 3-branched PCL was confirmed by FTIR (Brucker TENSOR II,

Germany) and 1H NMR (500MHz, CDCl3). The grafting degree of acrylate end-groups were

48.3%. Its number-average molecular weight was evaluated as 26901 g mol-1 by gel

permeation chromatography (GPC, Wyatt)

Fabrication of G-SMP nanocomposite aerogels. G-SMP nanocomposite aerogels were

prepared by a post infiltration of SMP toluene solution into graphene aerogel framework.

First, the PCL macromonomer/toluene solution with a concentration of ~0.1 mg mL-1 was

prepared by dissolving PCL macromonomer and crosslinking agent (BPO, ~0.01 mg mL-1) in

toluene. Then the as-prepared stretchable graphene frameworks were immersed in the SMP

solution for 45 min in a beaker, which was placed under a tightly sealed bell jar. After

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infiltration, the samples were heated to 90 oC in the vacuum oven to dry the toluene and

induce crosslinking reaction, resulting G-SMP nanocomposite aerogels.

Characterizations. SEM inspections were taken on Hitachi S4800 field emission system.

Raman spectra were taken on a Renishaw in Via-Reflex Raman microscopy at an excitation

wavelength of 532 nm. The stress-strain tests were performed on a Computer Control

Electronic Universal Testing Machine (RGWT-4000-20, REGER). TEM images of the hybrid

flake were taken on a JEM-2100 HR-TEM with a X-Nano TEM holder with both functions of

nanomanipulation and electrical biasing developed at the Center for X-Mechanics of Zhejiang

University (Figure S13). XRD data were collected on a X’ Pert Pro (PANalytical)

diffractometer using monochromatic Cu 17 Kα1 radiation (λ = 1.5406 Å) at 40 kV. DSC

curves were taken on TA Q200.

ASSOCIATED CONTENT

The authors declare no competing financial interest.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Nos. 51703194,

51533008 and 51603183), National Key R&D Program of China (No. 2016YFA0200200),

Fujian Provincial Science and Technology Major Projects (NO. 2018HZ0001-2), Hundred

Talents Program of Zhejiang University (188020*194231701/113), Key Research and

Development Plan of Zhejiang Province (2018C01049), and Foundation of National Key

Laboratory on Environment Effects (No. 614220504030717)

Supporting Information.

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The Supporting Information is available free of charge on the ACS Publications website at

http://pubs.acs.org.

Detailed fitting process of empirical equation, definitions of shape fixing ratio and recovery

ratio, experimental details and additional experimental data on the SEM ,TEM and AFM

micrographs of hybrid sheets, electro-thermal conversion curves of G-SMP nanocomposite

aerogel, TGA curves of G-SMP nanocomposite aerogels.

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ACS Paragon Plus Environment

ACS Nano

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ACS Paragon Plus Environment

ACS Nano

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