Morphology and properties of shape memory thermoplasticpolyurethane composites incorporating graphene-montmorillonitehybrids

Xing Zhou,1 Bin Hu,1 Wen Qiang Xiao,1 Lei Yan,1 Zheng Jun Wang,1 Jian Jun Zhang,1 Hai Lan Lin,1

Jun Bian ,1 Yun Lu2

1College of Materials Science and Engineering, Xi-hua University, Chengdu Sichuan 610039, China2Department of Mechanical Engineering, Graduate School of Science and Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku262-8522, JapanCorrespondence to: J. Bian (E - mail: bianj@mail.xhu.edu.cn)

ABSTRACT: A novel hybrid containing graphene oxide (GO) and montmorillonite (MMT) was first synthesized by solution reaction.

Then shape memory thermoplastic polyurethane (TPU) composites incorporating MMT–GO hybrid was fabricated via melt blending.

Infrared spectra indicated that GO and MMT have been combined together through chemical hydrogen bonding. Tensile tests showed

that MMT-GO hybrids provided substantially greater mechanical property enhancement than using MMT or GO as filler alone. With

only 0.25 wt % loading of MMT–GO hybrid (the mass ratio of MMT: GO is 1:1), there was a relatively high improvement in tensile

properties of TPU composites, compared with those of TPU/GO and TPU/MMT composites at the same filler content. Thermal anal-

ysis indicated that MMT-GO hybrids enhanced the thermal decomposition temperatures of TPU composites. Shape memory property

tests showed that the shape fixing rate of TPU composites was effectively enhanced by incorporating MMT–GO hybrid. VC 2017 Wiley

Periodicals, Inc. J. Appl. Polym. Sci. 2017, 135, 46149.

KEYWORDS: composites; graphene oxide; shape memory property; thermoplastic polyurethane (TPU)

Received 29 August 2017; accepted 3 December 2017DOI: 10.1002/app.46149

INTRODUCTION

Thermoplastic polyurethane (TPU), possessing rubbery elastic-

ity, and easy processability, availability of raw materials, molecu-

lar–structural controllable, good compatibility with the human

body, and readily biodegradable, is becoming one of the most

important candidate polymers in artificial tissues and organs,1,2

bio-medicine,3 flame-retarded, and package materials.4 Espe-

cially, TPU detains high elastic deformations and can operate

over a broad range of temperatures, endowing it becomes an

important shape memory material5–7 with a great potential

applied in intelligent materials area. However, TPU is suffered

from relatively low mechanical and thermal properties inquired

in engineering filed and insufficient shape memory property in

intelligent materials area. Therefore, there is great interest in

using various fillers to fabricate TPU composites with aim to

achieve high performance and high functionality of TPU due to

significant multifunctional property enhancements can be

observed in these systems. Since Finnigan et al.8 reported first

in the literature of the preparation of TPU nanocomposites

based on organosilicates by a melt compounding technique,

significant number of works has been published regarding the

processing and characterization of the fabricated TPU nanocom-

posites for high performance and multifunctions.9 Most of these

nanocomposites used nano-clays,10–13 nano-SiO2,14 carbon

nanotubes (CNTs),15–18 as well as carbon nano-fibers19 as fillers.

In contrast, using graphene or graphene derivatives as fillers is

superior to those traditional fillers because a significant perfor-

mance enhancement can be achieved only a small amount of fil-

ler is needed. Therefore, in recent years, a large number of

studies on TPU/graphene composites have been reported and a

remarkable progress has been achieved. Quan et al.20 prepared

graphene nanoparticles (GNPs) filled TPU nanocomposites via

a solution method. It was concluded that the mechanical, ther-

mal, flaming, and electrical properties of TPU were significantly

enhanced by GNPs. Nguyen et al.21 prepared TPU/FGS nano-

composites via in situ intercalative polymerization followed by a

casting film process. They concluded that FGS has a high affin-

ity for TPU and it was an effective and convenient new material

for the modification of TPU. Kim et al.22 compared the effects

of different processes and dispersion methods of exfoliated gra-

phene on the gas barrier and electrical conductivity of TPU/

VC 2017 Wiley Periodicals, Inc.

J. APPL. POLYM. SCI. 2017, DOI: 10.1002/APP.4614946149 (1 of 9)

graphene nanocomposites. In summary, previous reports indi-

cate that graphene could be effectively used in place of other

nano-sized fillers, such as CNTs and nanoclays. In our previous

work,23 microwave exfoliated graphene oxide (GO) reinforced

TPU nanocomposites have been prepared. The mechanical

properties and electrical conductivity of nanocomposites have

improved substantially by the incorporation of graphene.

Graphene has been widely used as functional fillers for fabricat-

ing polymer composites due to its excellent performances.24–27

However, complete exfoliation of graphene layers to sheets is

difficult because of ready aggregation of graphene layers and

poor compatibilities between polymers and graphene. Up to

date, chemical functionalization by using the oxygen containing

functional groups of GO is one of the most effective methods

for modifying the surface of graphene. Typical functionalization

method, such as “grafting” has been reported in our previous

works.28–30 These methods showed some advantageous in

improving the compatibilities and interfacial strength as well.

However, it is also suffered from obvious shortages, such as los-

ing of some properties of the composites because of the choice

of graft monomer, and the most important issue is that grafting

methods still cannot hinder effectively the aggregation of GO

sheets due to the tangles come from grafted chains. Obviously,

these shortages can be solved if mutual “barrier effects” between

graphene sheets can be designed and introduced.

Interestingly, Jiang et al.,31 Hsiao et al.,32 Kou et al.,33 and Bian

et al.34 have reported that the aggregation trend of GO sheets

can be tackled by combine different dimensional fillers to make

hybrids by various methods, such as silica nanoparticles-coated

GO/SiO2 nanohybrids31–33 and CNTs–GO hybrids.34 These

nanohybrids have been used in polymer composites and signifi-

cant improvements in mechanical, thermal, and conductive

properties have been achieved. Due to the space-layer barrier

effects of SiO2 nanoparticles to GO sheets, GO/SiO2 hybrid

plates could be individually dispersed in many kinds of solvents

and facilitated to better processibility. These discoveries inspired

us to explore an effective method to promote the dispersibility

of graphene with the help of the mutual barrier effects.

Montmorillonite (MMT), a so-called “universal material”, has

been widely applied in polymer nanocomposites after effectively

chemical treatment and dispersing. However, due to existence

of a large amount of inorganic ions in its interlayers, it is hard

for polymers to intercalate in it. Pluta et al.35 prepared MMT/

PLA composites by using Na1 modified MMT, the effects of

MMT on the thermal properties of PLA were studied. Han

et al.36 prepared MMT–GO hybrids by ultrasonicating the aque-

ous solution containing GO and MMT, followed by reducing

with hydrated hydrazine to obtain MMT–rGO, and then MMT–

rGO/PVA nanocomposites were fabricated by mixing, stirring,

and drying the solution of MMT–rGO and PVA. The hybrid fil-

ler synergistically enhanced the mechanical properties of PVA.

GO are made by treating graphene through chemical method,

the carboxyl groups on it forms hydrogen bonds with the

hydroxyl groups on the surface of MMT. So MMT–GO hybrids

can be readily prepared through chemical bonding of MMT and

GO. The interlayer interpenetration between MMT and GO is

expected to contribute to the stripping of both materials. It is

benefit for hampering MMT and GO agglomeration when the

hybrids are added into the polymer matrix. In this study,

MMT–GO hybrids (MMT–GO) with different mass ratio of

MMT:GO through solution reaction grafting method, and then

TPU/MMT–GO composites with different GO contents were

prepared by melt blending, and its microstructure, mechanical,

thermal and shape memory properties were investigated

systematically.

EXPERIMENTAL

Materials

TPU (Bayer-385S) used in this study was provided by Germany

Bayer Ltd. (Leverkusen, Germany). Natural graphite powder

[NGP, SP-2 (C> 99%, D 5 5 mm)] was purchased from Qing-

dao Tianhe Graphite Ltd. (QingDao, China). The alkyl quater-

nary ammonium salt-modified MMT was purchased from

Zhejiang Fenghong New Material Co. Ltd. (Zhejiang, China).

Other reagents, including KMnO4 (A.R.), NaNO3 (A.R.), con-

centrated H2SO4 (>96%), H2O2, and anhydrous ethanol

(C2H5OH, A.R.) were kindly provided by KeLong Reagent Ltd.

(Chengdu, China) and used as received.

Preparation of GO–MMT Hybrids (MMT–GO)

Graphite oxide used in this research was synthesized from the

NGP by graphite oxidation with KMnO4 in concentrated H2SO4

Table I. formula Used for Fabricating TPU/MMT–GO Composites

Sample No. TPU (wt %) MMT–GO (wt %) MMT:GO (mass ratio) MMT (wt %) GO (wt %)

1 100 — — — —

2 99.75 — — 0.250 —

3 99.75 — — — 0.250

4 99.75 0.25 1:1a 0.125 0.125

1:2b 0.083 0.167

2:1c 0.167 0.083

5 99.50 0.50 1:1 0.250 0.250

6 99.00 1.00 1:1 0.500 0.500

7 98.00 2.00 1:1 1.000 1.000

Note: a MG1; b MG2; c MG3.

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according to the procedures depicted in Ref. 28. The obtained

graphite oxide was then sonicated to produce GO. In order to

investigate the influence of different mass ratio of MMT: GO on

the structure and property of final composites, three kinds of

MMT–GO hybrids with different mass ratio of MMT:GO, that

is 1:1, 1:2, and 2:1, labeled as MG1, MG2 and MG3, respec-

tively, have been prepared. In a typical experiment, 0.125 g

MMT was added to 45 mL deionized water in a three-necked

flask and stirred for 6 h so that MMT can dissolve and disperse

homogenously. 0.125 g GO was dissolved in 85 mL deionized

water and sonicated for 1 h. Then the GO suspension was

mixed with the MMT suspension, followed by stirring for 1 h,

then vacuum filtered and washed thoroughly by using anhy-

drous ethanol. The filtered film was dried at 60 8C for 24 h to

obtain MG1. Other hybrids, such as MG2 and MG3 were pre-

pared by the same procedure.

Preparation of TPU/MMT–GO Composites

TPU/MMT–GO composites containing various MMT–GO con-

tents were prepared by melt compounding method. Before

Figure 1. FTIR spectra of GO, MMT, and MG1. [Color figure can be

viewed at wileyonlinelibrary.com]

Scheme 1. Synthesis procedure of MMT-GO.

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processing, TPU was dried under vacuum at 80 8C for 6 h. In

order to improve the dispersion of MMT–GO in TPU/MMT-g-

GO composites, a coating method reported in our previous

work37 was applied. In particular, MMT–GO was first dispersed

in anhydrous ethanol by sonication for 1 h, TPU particles were

then added to MMT–GO solution and the sonication was con-

tinued for 1 h. Finally, the solvent was evaporated at 80 8C

resulting in a complete coverage of TPU particles with MMT–

GO hybrids. The obtained solid mixtures were then melt-

blended at 180 8C for 15 min under a blending speed of 50 r/

min in a compounder (HL-200 type, Science Education Instru-

ment Factory of Jilin University, China). Finally, the compounds

were hot-pressed at 190 8C on a plate vulcanization machine

(XLB type, QingDao, China) to make plates, followed by cutting

into dumbbell specimens (62.5 3 3.25 3 1 mm3) for tensile

and shape memory property tests. For comparison purpose,

TPU/GO and TPU/MMT composites were prepared under the

identical processing conditions as those of TPU/MMT–GO

composites. The MMT or GO content in TPU/MMT or TPU/

GO composites is consistent with the optimal MMT–GO con-

tent in TPU/MMT–GO composites. The MMT–GO contents in

the composites were set at 0, 0.25, 0.5, 1, and 2 wt %. The for-

mula used for fabricating TPU/MMT–GO composites is identi-

fied in Table I.

Characterizations

Fourier transform infrared spectroscopy (FTIR) spectra of the

samples were carried out on a Nicolet 380 type FTIR instru-

ment (Thermo Fisher Scientific). The powders of GO, MMT,

and MMT–GO were uniformly mixed with KBr by grinding in

an agate mortar, followed by pressing to a flake before FTIR

measurements.

X-ray diffraction (XRD) was performed on a DX-2500 type

XRD diffraction instrument (Hao Yuan Instrument Co., Ltd.)

with Cu–Ka radiation (k 5 0.154 nm) source. Scans were taken

from 0.58 to 408 with a step of 28 at 25 kV and 15 mA. The

GO, MMT, and MMT–GO samples were in fine powder form,

whereas TPU and TPU/MMT–GO composites were from hot-

pressed specimens.

Thermogravimetric analysis (TGA) was carried out on a

STA449 F3 type thermal analysis instrument (Netzsch Company,

Germany) in a temperature ranging from 30 to 800 8C at a heat-

ing rate of 10 8C/min in a argon environment.

The morphologies of TPU/MMT–GO composites fracture were

observed using a field emission scanning electron microscope

(FESEM, Quanta FEG 250, FEI Company). The fracture surfaces

of composites were coated with gold before SEM examination.

Tensile testing was performed on an electronic universal (ten-

sile) testing machine (CMT6104, Shenzhen Suns Technology

Co., Ltd.). The specimens were elongated at a rate of 50 mm/

min at room temperature.

The shape memory tests were performed on an electronic uni-

versal (tensile) testing machine. A gauge length of L0

(L0 5 25 mm was first marked in the center of the sample; put

the sample into a 50 8C oven for 1 h. Then kept the environ-

mental temperature at 50 8C, stretched the samples to a fixed

elongation L1 (L1 5 150 mm) under a constant stress. With the

stress on, the samples were cooled for 5 min, and then trans-

ferred rapidly to a 25 8C oven. 30 min later, the gauge length

in the center of the sample was measured and marked as L2.

Finally, heated the samples to 50 8C again and remained for 30

min, the gauge length in the center of the sample was measured

and marked as L3. The shape recovery rate (Rr) and the shape

fixation rate (Rf) were calculated according to the following

formula:

Rr5L22L3

L22L0

Rf 5L22L0

L12L0

RESULTS AND DISCUSSION

Micro-Structure of Fillers and TPU/MMT-GO

The possibility of chemical interactions in MMT–GO hybrid

can be identified by FTIR test. Figure 1 shows the FTIR spectra

of GO, MMT, and MG1. As shown in the inset in Figure 1, GO

Figure 2. XRD patterns of: (a) GO, MMT, and MMT–GO hybrids and

(b) TPU/MG1 composites. [Color figure can be viewed at wileyonlineli-

brary.com]

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shows a sharp characteristic peak at 1720 cm21, which corre-

sponds to stretching vibrating absorption peaks from C@O

(carboxyl groups). For MMT, the peaks detected at 2800–

3000 cm21 are the CAH stretching and bending vibrating

absorption peaks of the alkyl quaternary ammonium salt. Other

characteristic peaks, such as the strong absorption peak of

SiAO stretching vibration peak (1039 cm21), bending vibration

of SiAOASi (468 cm21), and SiAOAAl (520 cm21) can be

observed. Due to the presence of large amounts of hydroxyl

groups in the structure of MMT, the vibrating absorption peaks

of AOH are obviously detected at 3400 and 1630 cm21. For

MG1, the characteristic absorption peak of GO at 1720 cm21

disappears and the intensity of AOH vibrating absorption peaks

at 3400 cm21 decreases due to the formation of hydrogen bonds

between GO and MMT. Scheme 1 represents the most probable

interaction model between GO and MMT. The interactions

between GO and MMT provide positive contribution to the

homogeneous dispersion of MMT–GO hybrid, and conse-

quently, to the reinforcement capacity of hybrid to the final

properties of TPU/MMT–GO composites.

Figure 2 shows the XRD patterns of GO, MMT, and MMT–GO

hybrids [Figure 2(a)] and TPU/MG1 composites [Figure 2(b)].

According to the Bragg formula, the d-spacing of GO, MMT,

and MMT–GO hybrids were calculated and the results are listed

in Table II. It can be seen that the characteristic diffraction

peaks of GO and MMT appear at 2u 5 108 and 2u 5 38, respec-

tively. Compared to original GO, the diffraction peaks observed

at 108 in the MMT–GO hybrids moved to lower angles, the

width and the intension of the peak become wide and weak,

indicating that addition of MMT does not change the crystal

form of GO, but promoting the exfoliation of GO, and as a

result, the d-spacing of GO in MMT–GO hybrids increased. In

contrast, the diffraction peak of original MMT observed at 38

moves to higher angles in the MMT–GO hybrids, leading to the

decreasing of d-spacing from 1.261 of MMT to 0.911 nm of

MG2 hybrids. The decreasing in d-spacing indicated that MMT

exists in several MMT sheets in MMT–GO hybrids.

In Figure 2(b), pure TPU matrix shows a very strong broad

peak at about 2u 5 208 of (110) reflection plane with inter-

chain spacing of 0.442 nm. Compared to pure TPU, the charac-

teristic diffraction peak at 208 for TPU/MMT–GO composites

shifted slightly to the left, while the peak broadens and the

intensity increases with the addition of MG1, which implies

that the MG1 significantly affects the micro-structural phases of

the TPU matrix. The diffraction peaks corresponding to the

MMT (2u 5 38) and to the GO (2u 5 108) are nearly absent in

XRD patterns of the TPU/MG1 composites when MG1 content

is lower than 2.0 wt %. This may be due to effective dispersion

the MG1 within the TPU matrix given the low MG1 content.

Thermal Property of TPU/MMT–GO Composites

Figure 3 shows the thermal decomposition curves of TPU and

TPU/MG1 composites. Figure 3(a) is the TGA curves and Fig-

ure 3(b) is the DTG curves. From Figure 4(b), it is found that

the Tmax of TPU/MG1 composites are lower than that of pure

TPU, but the decreasing amplitude levels off because MMT–GO

destroys the interaction between TPU molecules and makes the

composites to overcome the interaction by absorbing the heat

Table II. XRD Results of Fillers

2uGO (8) 2uMMT (8) dGO (nm) dMMT (nm)

GO 11.09 — 0.404 —

MMT — 3.77 — 1.261

MG1 10.10 4.70 0.439 0.940

MG2 10.79 4.85 0.411 0.911

MG3 9.23 4.13 0.418 1.069

Note: d 5 nk/2sinu.

Figure 3. Thermal decomposition curves of TPU/MG1 composites. (a) TGA curves and (b) DTG curves. [Color figure can be viewed at wileyonlinelibrary.com]

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when were heated. However, in Figure 3(a), the thermal stability

of TPU/MG1 composites at high temperatures was enhanced,

when the content of MG1 is 1 wt %, the thermal stability of

TPU/MG1 composite has the best thermal stability. This can be

explained, on one hand, by the uniform dispersion of MMT–

GO in TPU forming a network structure, on the other hand, by

part of the TPU molecular chains inserted into the MMT–GO

layers preventing the heat conduction. But with the increasing

of MMT–GO, MMT–GO was agglomerated in the TPU matrix,

which caused the reduction of the thermal stability of TPU/

MMT–GO composites.

Figure 4 is the melting curve obtained by DSC tests. According

to the melting curve, the melting temperature (Tm) of the TPU/

MMT–GO composites is higher than that of the pure TPU.

This is can be attributed to the presence of MMT–GO restrict-

ing the activities of TPU molecular chains.

Tensile Property of TPU/MMT–GO Composites

Figure 5 shows the tensile property of TPU/MMT–GO compo-

sites. From Figure 5(a), it can be seen that the mass ratio of

MMT:GO has an apparent enhancement effect on the tensile

property of composites. The tensile stress at definite elongation

of TPU/MMT–GO composites is higher than that of pure TPU,

which may be related to the interfacial bonds between TPU and

MMT–GO. The hydrogen bonds between AOH group of

Figure 4. DSC heating thermograms of TPU/MG1 composite with differ-

ent MG1 contents. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5. Tensile properties of TPU/MMT–GO composites with various filler contents and MMT/GO mass ratio: (a) stress–strain curves of TPU/MG1

composite with different MG1 contents; (b) stress at definite elongation of TPU/MG1 composite with different MG1 contents; (c) stress–strain curves of

TPU/MMT-GO composite under various mass ratio of MMT/GO; (d) stress at definite elongation of TPU/MMT–GO composite under various mass

ratio of MMT/GO. [Color figure can be viewed at wileyonlinelibrary.com]

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MMT–GO and ANHCOOA group of TPU molecular chains

create a good interfacial stress transferring. The tensile proper-

ties of TPU/MG1, TPU/MG2, and TPU/MG3 composites are

better than TPU/MMT or TPU/GO composites, indicating that

MMT and GO are able to synergistically strengthen TPU. MMT

and GO in MMT–GO hybrid materials promoted each other’s

stripping, which is better for GO to play its role of high

strength enhancement effect on TPU. Moreover, TPU/MG1

showed the highest stress at a definite elongation compared

with those of TPU/MG2 and TPU/MG3 composites. Therefore,

MG1 hybrid was selected to further investigate the effects of

MG1 contents on the tensile property of TPU composites. The

results can be seen in Figure 5(c,d). With the increasing of

MG1 contents, the tensile stress at a definite elongation

increased. TPU/MG1 containing 0.25 wt % MG1showed the rel-

atively superior tensile properties, in the full range of MG1 con-

tent used.

Morphology of TPU/MMT–GO Composites

Figure 6 shows a typical overview on the fracture surfaces of

TPU/MMT–GO composites with different MMT–GO contents.

The MMT–GO hybrid has been dispersed uniformly in the

TPU matrix [Figure 6(a)]. But in Figure 6(b), there was a slight

agglomeration happening in TPU matrix, which is caused by

the reunion at high MG1 content (up to 2 wt %). In Figure

6(c,d), both of the composites contain 0.25 wt % of MMT–GO,

respectively. A “silk-like” phenomenon appeared in the TPU/

MG2 and TPU/MG3 composites compared to Figure 6(a), indi-

cating that the TPU/MG2 and TPU/MG3 composites have good

toughness, MG2 and MG3 have both enhancing and toughening

effects on TPU, and the effects are better than that of MG1.

This is consistent with the tensile testing results.

Shape Memory Property of TPU/MMT–GO Composites

The shape memory performances of TPU/MMT–GO compo-

sites are shown in Figure 7. MMT-GO formed physical cross-

linking points in TPU through hydrogen bonding between

MMT–GO and TPU, which limited the movements of TPU

molecular chains, resulting in a significant increasing in shape

fixing ratio (Rf) compared to that of pure TPU. It was found

from XRD testing results that the crystallinity of TPU/MMT–

GO composites decreased with the increasing of MMT–GO,

which also led to the increasing of Rf. Because of the intercala-

tion structure of TPU/MMT–GO composites, the existence of

MMT–GO destroyed the linkages between TPU molecular

chains. As a result, the ability of deformative reversibility of soft

segments was weakened and the ability of hard segments mem-

orizing the initial shape of the composites was reduced during

the shape memory tests along with the increasing of MMT–GO,

leading to decreasing of shape recovery ratio (Rr).

All the fillers used in this research can form a physical hydrogen

bonding with the TPU matrix, restricting the activities of the

TPU molecular chains. In Figure 7(b), MMT–GO hybrids have

better improving effect on Rf of TPU than by using GO or MMT

alone. That is because TPU molecular chains were inserted into

the layers of MMT–GO and were connected with MMT–GO by

hydrogen bonding, makes it easier to fix the deformation of

TPU at low temperature. The effect of MMT–GO contents on

Figure 6. Fracture surfaces SEM images of TPU/MMT–GO composites. (a) TPU/MG1 (0.25 wt %); (b) PU/MG1 (2 wt %) ; (c) TPU/MMT–GO (0.25

wt %, MMT:GO 5 1:2); (d) TPU/MMT–GO (0.25 wt %, MMT:GO 5 2:1).

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the tensile strength shows the same trends as that on the Rr for

TPU/MMT–GO composites.

The shape memory mechanism of the TPU used in this research

is shown in Figure 8. Ts and Th are the melting temperature of the

soft segment and hard segment of TPU, respectively. During the

shape recovery process, when heating to T (Ts<T<Th), the

molecular chains of soft segment of TPU turned from crystalline

into amorphous state, then stretching TPU to a certain tensile

deformation under a constant stress. Quickly cooling TPU to

freeze the TPU molecular chains aims to fix the deformation.

Finally, heating up again to T, TPU recover the initial shape. In

this process, the hard segment acts as the fixed phase to provide

the recovery stress and remember the initial shape of TPU, while

the soft segment implements the shape fixing and recovery

behaviors.

CONCLUSIONS

MMT–GO hybrids reinforced TPU composites have been fabri-

cated by melt blending. FTIR and XRD tests show that the

MMT promotes the stripping of GO layers, and the GO and

MMT are linked together by hydrogen bonding. Compared to

pure TPU, MMT–GO effectively improved the Rf of TPU, and

the tensile properties are improved too. Furthermore, MMT and

GO synergistically have better enhancement on TPU than by

using GO or MMT alone. The TPU/MMT–GO composites have

an intercalation structure, the existence of MMT–GO destroyed

the linkages between itself and TPU molecular chains are pro-

posed to be responsible for the above results. When the mass

ratio of MMT:GO is 1:1, TPU/MMT–GO composites showed

the best performance.

ACKNOWLEDGMENTS

The authors thank the Open Research Subject of Key Laboratory of

Special Materials and preparation Technology (grant no. szjj2017-

066, szjj2015-084, szjj2015-086), National Undergraduate Training

Programs for Innovation and Entrepreneurship (grant no. 2017-

XX), Xihua University Teaching Support Program (grant no.

02020597) and Youth Fund Project of Sichuan Provincial Educa-

tion Department (grant no. 2017ZB0422) for the financial sup-

ports of this work.

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Figure 7. Shape memory testing results of TPU/MMT–GO composites. (a)

Shape fixing ratio (Rf) and shape recovery ratio (Rr) of TPU/MG1 under

various MG1 contents; (b) shape fixing ratio (Rf); and shape recovery ratio

(Rr) of TPU/MMT-GO composite under various mass ratio of MMT/GO.

Figure 8. Shape memory mechanism of the TPU/MMT–GO composites.

[Color figure can be viewed at wileyonlinelibrary.com]

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