S1

Supporting Information

Programmable and Sophisticated Shape-Memory Behavior via

Tailoring Spatial Distribution of Polymer Crosslinks

Yaxin Qiu1 Qianru Wanyan1 Wenting Zhang1 Suna Yin1 Defeng Wu1,2*

(1 School of Chemistry & Chemical Engineering, Yangzhou University, Jiangsu, 225002, P. R. China)

(2 Provincial Key Laboratories of Environmental Engineering & Materials, Jiangsu, 225002, P. R. China)

* Corresponding author, dfwu@yzu.edu.cn

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2020

S2

Table S1. The mechanical parameters of Cx samples.....................................................................S4

Table S2. The bulk and network parameters of Cx samples...........................................................S5

Table S3. The Simha-Boyer (SB) parameters of Cx samples .........................................................S6

Table S4. The WLF parameters of Cx samples ..............................................................................S7

Table S5. The relative parameters of isomerization energy of samples .........................................S8

Table S6. The shape memory parameters of Cx samples ...............................................................S9

Table S7. Shear strengths of the bonded sheet pairs using pre-Cx as adhesives after curing.......S10

Figure S1. The photos of (a) pre-cross-linked PMA (pre-Cx) and (b) the cross-linked PMA (Cx)

samples. .........................................................................................................................................S11

Figure S2. The stress-strain traces of Cx samples. .......................................................................S12

Figure S3. 1H NMR spectra of (a) pre-C6, (b) pre-C8, (c) pre-C10 and (d) pre-C12. .................S13

Figure S4. The carbon atom contents of Cx samples....................................................................S14

Figure S5. Master curves of dynamic storage moduli of Cx samples...........................................S15

Figure S6. Calculated values of (a) free volume fractions and (b) isomerization energies of Cx

samples against carbon numbers of used diol cross-linkers. .........................................................S16

Figure S7. Apparent activation energy (Ea) against Mc of Cx samples ........................................S17

Figure S8. (a) Optical pictures of the loaded substrate pairs bonded with Cx and the fracture

surfaces of different substrates after lap shear tests, and SEM images of fracture surfaces of (b)

iron based sheets and (c) PET sheets. ............................................................................................S18

Figure S9. (a) The stress-strain curves of combined Cx ribbons (30 mm×2 mm×1.5 mm) (Cx/Cx

compound joints) and the pictures of compound joints with different parts (b) before and (c) after

tensile tests.....................................................................................................................................S19

Figure S10. (a) Schematics of the points on the combined C6/C12 sample collected for the EDX

tests and (b) collected carbon atom contents of C6/C12 sample at different points......................S20

S3

Figure S11. (a) Responsive sensitivity and recovery rates of the Cx samples; (b) shape recovery

process of a Cx twisted ribbon and (c) infrared thermal images of twisted part of ribbon. ..........S21

Video S1. Bulk appearance and properties of pre-cross-linked PMA (pre-Cx)

Video S2. Shape-memory behavior of a ribbon sample with single component (C8) (95 oC)

Video S3. Shape-memory behavior of a ribbon sample composed of four parts with various spatial

distributions of networks (C6+C8+C10+C12) (95 oC)

Video S4. Fabricating a flower-like sample using four kinds of Cx samples (C6+C8+C10+C12)

Video S5. Fabricating robotic arms using pre-Cx as the joint connecting wires

Video S6. Programable (step-by-step) motions of a robotic arm driven by SME of Cx samples

Video S7. Shape recovery processes of Cx twisted ribbons

S4

Table S1. The mechanical parameters of Cx samples

samplesYoung's modulus

(MPa)

yield strength

(MPa)

elongation at break

(%)

C6 2079.95±233.12 73.44±6.37 7.16±0.95

C8 2027.04±165.78 63.81±5.61 7.69±0.86

C10 1636.95±98.18 49.71±3.79 8.73±0.75

C12 1416.98±62.37 43.46±2.64 11.15±0.12

S5

Table S2. The bulk and network parameters of Cx samples

samples gel content (%) ρ (kg m-3) Tg (K) E (MPa) a) ν (103mol m-3) Mc (g mol-1)

C6 97.6 1.09×103 359.55 12.27 1.231 885.29

C8 97.3 1.15×103 349.75 11.89 1.223 939.41

C10 97.4 1.22×103 341.65 11.58 1.216 1000.41

C12 97.4 1.29×103 336.15 11.51 1.227 1053.94

a) E is dynamic storage modulus at rubbery plateau.

Notes: the bulk densities of cross-linked samples increase with increased carbon numbers of diol

crosslinkers. All samples have almost the same gel contents. Therefore, the bulk density alteration

is nearly independent on the crosslinks in this work. Two possible reasons are proposed here. On

the one hand, the densities of diol molecules increase with increased carbon numbers (for instance,

1,6-hexanediol: 0.96×103 kg m-3, 1,8-octanediol: 1.05×103 kg m-3, and 1,10-decamethylenediol:

1.09×103 kg m-3). This leads to increase of densities of cross-linked system when using long-chain

diol as crosslinker. On the other hand, the reactive ability of diol reduces with increased aliphatic

chain length. This leads to the formation of ineffective crosslinks, indicating an increased amounts

of grafting chains. Those long side chains (grafted chains), to some extent, promote entanglements

of poly(L-malic acid) chains (currently it is hard to evaluate the critical entanglement molecular

weight of poly(L-malic acid), but the molecular weight of side chain of C12 samples is almost half

of that of poly(L-malic acid) oligomer), which has contribution to the increase of bulk densities,

also.

S6

Table S3. The Simha-Boyer (SB) parameters of Cx samples

samples Tg (K) G L free,exs,SB g/V V

C6 332.32 6.954×10-5 4.144×10-4 3.449×10-4 0.124

C8 326.76 7.898×10-5 5.041×10-4 4.251×10-4 0.139

C10 321.42 8.628×10-5 5.371×10-4 4.508×10-4 0.145

C12 308.77 9.247×10-5 5.967×10-4 5.042×10-4 0.156

S7

Table S4. The WLF parameters of Cx samples

samples b C1 fg (WLF)

C6 1 9.79144 0.04435

C8 1 7.97366 0.05446

C10 1 7.06198 0.06149

C12 1 6.52238 0.06657

S8

Table S5. The relative parameters of isomerization energy of samples

samples gT fg u ε u-ε

C6 0.124 0.04435 9.17798 5.40172 3.77626

C8 0.139 0.05446 8.29976 4.95420 3.34556

C10 0.145 0.06149 7.74149 4.74162 2.99987

C12 0.156 0.06657 7.37985 4.33405 3.04580

S9

Table S6. The shape memory parameters of Cx samples

samples fixing temp.

(Tfix)

recovery

temp.

(Trec)

shape fixity ratio

(Rf)

shape recovery

ratio

(Rr)

C6 35 oC 100 oC 95.8 % 97.2 %

C8 25 oC 90 oC 95.6 % 98.9 %

C10 15 oC 80 oC 95.7 % 99.8 %

C12 5 oC 70 oC 98.6 % 95.2 %

Notes: the samples show the shape fixity ratio (Rf) and the shape recovery ratio (Rr), which are

calculated according to:

(1)S1 S0f

S0, load S0

R

(2)S0, recover S0r

S1 S0

R

where the primary strain, the strain under load, temporary shape in the strain S0 S0, load S1

without load, and the strain after recovery. All samples present the Rf and Rr values S0, recover

higher than 95%.

S10

Table S7. Shear strengths of the bonded sheet pairs using pre-Cx as adhesives after curing a)

adhesives glass (MPa) iron (MPa) wood (MPa) polyester (MPa)

pre-C6 67.15±5.88 69.94±6.58 55.46±8.34 6.11±1.95

pre-C8 56.23±4.37 60.23±3.92 32.69±7.51 6.84±0.85

pre-C10 43.95±5.65 44.71±4.33 22.43±3.28 7.73±1.22

pre-C12 35.37±2.26 38.69±2.75 13.83±3.15 10.04±1.89

a) All sheet pairs were cured at 130 oC for 24 h under N2.

Note: The adhesion capacity of pre-Cx to the substrates reduces with weakened surface polarities

of those substrates, following the order of metal>glass>wood>plastics, and also depends on the

chain lengths of cross-linker diols in the pre-Cx system. For the substrates with stronger polarities

(glass, iron), shear strengths increase with decreased chain lengths of cross-linker diols because of

increased polarities of pre-Cx adhesives. Whereas for the plastics, shear strengths show opposite

trend, increasing with lengthened aliphatic chains of cross-linker diols, which is due to improved

interfacial compatibility between plastics and pre-Cx adhesive arising from decreased polarity of

pre-Cx. Details are discussed around Figure S7.

S11

Figure S1. The photos of (a) pre-cross-linked PMA (pre-Cx) and (b) the cross-linked PMA (Cx)

samples.

Note: pre-Cx sample behaves like transparent plasticine, shapeable and sticky, and hence can be

used as the glue/adhesive or the shaped semi-finished devices. After further crosslinking (also

called heat treatment or curing), transparent Cx sample is obtained. It has very good mechanical

strengths (43-73 MPa) and moduli (1.4-2.0 GPa), which depend on the chain lengths of diols

strongly (see Figure S2 & Table S2).

(a) (b)

S12

0.00 0.02 0.04 0.06 0.08 0.10 0.12

0

10

20

30

40

50

60

70

80

C6 C8 C10 C12

stre

ss(M

Pa)

strain

Figure S2. The stress-strain traces of Cx samples.

S13

6 5 4 3 2 1 0

dca'

ab e

3,42

51

ppm

in

tens

ity

6

O CH CH2 C O CH C

CO CH2

C

n m

O

O O

O

OH

O

OH

O CH CH2 C

O

CO

OH

O CH C

CH2

O

CO

OH

12

34

56

a' a

d

c b

e

pre-C6

6 5 4 3 2 1 0

pre-C8

inte

nsity

ppm

dca'ab e

3~6

27

18

OHC

H2C C O

HC C

CO CH2

C

n m

O

O O

O

O

OHC

H2C C

O

CO

OH

OHC C

CH2

O

CO

OH

OH

OH

12

34

56

78

e

a' a

d

c b

6 5 4 3 2 1 0

inte

nsity

ppm

pre-C10

dca'ab e

3~8

2

91

10

OHC

H2C C O

HC C

CO CH2

C

n m

O

O O

O

O

OHC

H2C C

O

CO

OH

OHC C

CH2

O

CO

OH

OH

OH

12

34

56

78

109

a' a

d

c b

e

6 5 4 3 2 1 0

inte

nsity

pre-C12

ppm

dca'ab e

3~102

11112

O CH CH2 C O CH C

CO CH2

C

n m

O

O O

O

O

O CH CH2 C

O

CO

OH

O CH C

CH2

O

CO

OH

OH

OH

12

34

56

78

109

1112

e

a' a

d

c b

Figure S3. 1H NMR spectra of (a) pre-C6, (b) pre-C8, (c) pre-C10 and (d) pre-C12.

Note: 1H NMR spectra of pre-Cx samples reveal new chemical shift (a' 4.44 ppm ppm) ascribed

to the methylene protons close to newly formed ester groups. The new shift at 4.03 ppm is also a

solid evidence of successful esterification between PMA backbone chain and diols.

(a) (b)

(c) (d)

S14

0.0

0.5

1.0

1.5

2.0

carb

on a

tom

con

tent

(%)

C12C10

C8

samples

C6

Figure S4. The carbon atom contents of Cx samples.

Note: The results were obtained by energy dispersive X-ray (EDX) spectrometric microanalysis

with a scanning electron microscopy (SEM, Zeiss-Supra55, Germany). It is clear that the carbon

atom contents of Cx samples increase with increase of chain lengths of diol crosslinkers.

S15

106107108109

1010

106107108109

1010

106107108109

1010

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103106107108109

1010

frequency (Hz)

stor

age

mod

ulus

(Pa)

C6

C8

C10

C12

Figure S5. Master curves of dynamic storage moduli of Cx samples.

S16

6 8 10 120.00

0.04

0.08

0.12

0.16

0.20

carbon numbers of diols

f g (fr

ee v

olum

e fra

ctio

n)

WLF eq. SB eq.

(a)

6 8 10 12

2

4

6

8

10

carbon numbers of diols

ener

gy (k

J m

ol-1)

u u-

(b)

Figure S6. Calculated values of (a) free volume fractions and (b) isomerization energies of Cx

samples against carbon numbers of used diol cross-linkers.

S17

850 900 950 1000 1050 1100

180

210

240

270

300

E a (k

J m

ol-1)

Mc

Figure S7. Apparent activation energy (Ea) against Mc of Cx samples

Note: Arrhenius equation is used here to calculate apparent activation energy (Ea) of cross-linked

PMAs:

(3)aT

r

1 1- ln ( )EK T T

is shift factor, K is Boltzmann’s constant and Tr is reference temperature. is obtained by T T

TTS of dynamic modulus using 30 oC as Tr. Ea can then be calculated. It is clear that Ea reduces

with increased Mc. The difference of Ea implies a difference of temperature in a given relaxation

time, or a difference of relaxation time under a given temperature. 1, 2 Thus, C12 has rapider/easier

relaxations than C6 because the former possesses looser crosslinking networks and higher free

volume fraction relative to the latter.

(1) Zheng, N.; Hou, J. J.; Xu, Y.; Fang, Z. Z.; Zou, W. K.; Zhao, Q.; Xie, T. Catalyst-Free Thermoset

Polyurethane with Permanent Shape Reconfigurability and Highly Tunable Triple-Shape Memory

Performance. ACS Macro Lett. 2017, 6, 326-330.

(2) Ge, Q.; Luo, X. F.; Iversen, C. B.; Mather, P. T.; Dunna, M. L.; Qi, H. J. Mechanisms of triple-shape

polymeric composites due to dual thermal transitions. Soft Matter 2013, 9, 2212-2223.

S18

Figure S8. (a) Optical pictures of the loaded substrate pairs bonded with Cx and the fracture

surfaces of different substrates after lap shear tests, and SEM images of fracture surfaces of (b)

iron based sheets and (c) PET sheets.

Note: Pre-Cx can be used as adhesive to various substrates (metal, glass, wood, and plastics,

etc.). After curing, those substrate pairs bonded with Cx show good shear strengths, which is

strongly dependent on the polarities of substrates (Table S1 of SI). Cx has the best adhesion with

iron and glass, and as a result, the fracture surface is rather rough. The smoothest surface indicates

that the adhesion between Cx and plastics is the lowest, which is in consistent with shear strength

testing results. The adhesion capacity of pre-Cx to the substrates reduces with weakened surface

polarities of those substrates, following the order of metal>glass>wood>plastics, and also depends

on the chain lengths of cross-linker diols in the pre-Cx system. Therefore, one can determine

appropriate pre-Cx and substrate to prepare smart device according to the requirements of

applications.

(a)

(b) (c)

S19

0.00 0.02 0.04 0.06 0.08 0.10 0.120

10

20

30

40

50

60

70

80

strain

stre

ss (M

Pa)

C6/C8 C8/C10 C10/C12 C6/C12

(a)

Figure S9. (a) The stress-strain curves of combined Cx ribbons (30 mm×2 mm×1.5 mm) (Cx/Cx

compound joints) and the pictures of compound joints with different parts (b) before and (c) after

tensile tests.

Note: The yield strengths of a compound joint are ranged in between those of two bulks, close to

that of weaker one. For instance, the strength of C6/C8 joint is about 65 MPa (between 73 MPa of

C6 and 64 MPa of C8), and the strength of C6/C12 joint is about (between 73 MPa of C6 and 45

MPa of C12, Figure S2). This means that the two parts have a good connection at joint. As a

result, all joint samples show bulk fracture (see the arrow in Figure S9c), instead of interfacial one

after tensile tests.

interface interface(b) (c)

S20

0 10 20 30 40 50

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

car

bon

atom

con

tent

(%)

scanning position (mm)

transition region

C12

C6

(b)

Figure S10. (a) Schematics of the points on the combined C6/C12 sample collected for the EDX

tests and (b) collected carbon atom contents of C6/C12 sample at different points.

Note: EDX was used to detect interfacial thickness of a compound joint. Taking the combined

C6/C12 ribbon (~5.4 cm length) as an example, the carbon atom contents increased rapidly in the

joint region from C6 side to C12 one. However, the transition region is very narrow (~0.2-0.3 cm).

In the following shaping stage when the C6/C12 ribbon was used as a compound joint, the C6 part

experienced twisting and the C12 one experienced bending, which were independent with each

other. In this process, the jointed region might experience slight deformation, but its size was too

small to affect subsequent shape recovery of C6 or C12 part. Besides, C6 and C12 were used for

completing different actions, and those actions were asynchronous and mainly performed by the

main parts of C6 ribbon and C12 one, instead of the interfacial region (Figure 10a). Therefore, the

existence of interfacial layer (jointed region between C6 and C12) with good interfacial adhesion

(Figure S9) might not have evident influence on the C6/C12 compound joint to complete a whole

action step by step.

(a)

S21

0

5

10

15

20

25

resp

onsi

ve ti

me

(s)

recovery rate

C12C10C8C6samples

responsive time(a)

0.0

0.5

1.0

1.5

2.0

2.5

re

cove

ry ra

te (r

ad/s

)

Figure S11. (a) Responsive sensitivity and recovery rates of the Cx samples; (b) shape recovery

process of a Cx twisted ribbon and (c) infrared thermal images of twisted part of ribbon.

Note: The responsive sensitivity and recovery rate were evaluated roughly through the following

ways: the twisted ribbon connected with a U-shaped bar was place into a thermal environment,

starting timing, till the shape recovery completed. The time to the moment when the bar began its

rotation is defined as the responsive time. The average rate was defined as the time spent rotating

half a turn. All samples were pre-twisted with the same level, and rest for 1 day. The tests were

performed in a closed environment, ensuring stable heating. Details could be found in Video S7 of

SI.

(b)

(c)