A Fully Biobased Encapsulant Constructed of Soy Protein and

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A Fully Bio-Based Encapsulant Constructed by Soy Protein and Cellulose Nanocrystal for Flexible Electromechanical Sensing Dan-yang Xie, Dan Qian, Fei Song, Xiu-Li Wang, and Yu-Zhong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01266 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017

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ACS Sustainable Chemistry & Engineering

1

A Fully Bio-Based Encapsulant Constructed by Soy

2

Protein and Cellulose Nanocrystal for Flexible

3

Electromechanical Sensing

4

Dan-Yang Xie, Dan Qian, Fei Song,* Xiu-Li Wang and Yu-Zhong Wang*

5

Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), College of

6

Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key

7

Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China.

8

Corresponding authors: E-mail: [email protected]; [email protected]

9

Tel: 86-28-85410755; Fax: 86-28-85410755

10 11

Abstract: Very recently, flexible electromechanical sensors are of particular interest

12

to next generation mobile applications. To enrich the flexible encapsulants, herein, a

13

fully bio-based film is developed by crosslinking soy protein isolate (SPI) with a

14

nanocrosslinker, aldehyde-bearing cellulose nanocrystal (CNC). Thanks to the

15

enhanced interfacial interaction between SPI and CNC resulting from the Millard

16

reaction, the protein-rich phase becomes more homogeneous with smaller domain size.

17

Compared with neat SPI film, the resultant composite films exhibit obviously

18

improved mechanical property and water resistance. In particular, the heat-sealing

19

property of such films is well maintained, which guarantee their application as

20

encapsulation layers to construct flexible electromechanical sensors. These results

21

indicate the green composite films hold promising applications as a universal

22

encapsulation material for integrate flexible movement-monitoring electronics.

23 24

Keywords:

25

Soy protein isolate; cellulose nanocrystal; interfacial interaction; encapsulant

26 27 28 29 30

ACS Paragon Plus Environment


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1

INTRODUCTION

2

With the fast development of flexible electronics, great and ever-increasing interest

3

has been aroused in wearable devices, roll-up displays, and bendable equipments.1-6

4

Although significant progress has been made for flexible current collectors, electrodes,

5

solid-state electrolytes and encapsulation materials, some shortages still exist because

6

the flexible substrates are generally non-degradable or stem from petroleum

7

resources.7-10 Development of flexible renewable bio-based encapsulants, therefore, is

8

more welcome for global green concerns. Among the big family of biomass, soy

9

protein isolate (SPI) has attracted considerable attention because of its low cost, good

10

processability, film-forming characteristic and gas barrier property, which exhibits

11

promising applications in biomedical and food fields.11-16 Nevertheless, in spite of

12

some physical and chemical modifications already performed on SPI, how to fabricate

13

SPI-based films with high mechanical property and water resistance is still a big

14

challenge.17-24

15

Cellulose nanocrystal (CNC), prepared by the acid-catalyzed hydrolysis of

16

cellulose, has been regarded as an attractive reinforcement filler over cellulose

17

whisker and microcrystalline cellulose for polymeric materials, because of its higher

18

specific surface area, degree of crystallinity, and superior mechanical properties.25-30

19

Up to now, the enhancement of mechanical property has been realized by

20

incorporating CNC in different matrix, including poly(lactic acid), bacterial polyester,

21

poly(vinyl alcohol), polyurethane, natural rubber, etc.31-35 Unfortunately, the

22

utilization of CNC to modify SPI is still limited. Till now, there have been only two

23

reports using CNC as a filler of SPI. Zhang and co-workers36 prepared SPI/CNC

24

composite films with improved tensile strength. The films, however, were brittle

25

because of lacking interfacial interaction between the filler and the protein matrix.

26

Furthermore, as described by Li, Shi and their co-workers,37 ethyleneglycol diglycidyl

27

ether was used as a crosslinker to connect SPI and silane-modified CNC together.

28

Although their interfacial interaction was improved, the crosslinking reaction has no

29

selectivity, which may result in the self-crosslinking of CNC as well as formation of

30

stress concentration. In addition, considering the requirement of application as


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1

artificial skins,38 the mechanical property of SPI-based films still need to be improved

2

further.

3

As well known, periodate oxidation of polysaccharides occurs specifically at the

4

secondary hydroxyl groups of C2 and C3 atoms, giving rise to highly reactive

5

aldehyde groups.39-41 The resulting oxidized cellulose can be used as a

6

macromolecular crosslinker for many biopolymers.42-46 Inspired by this fact, we

7

considered reforming CNC to a novel nano-crosslinker for SPI. In this work, the

8

nano-crosslinker was prepared by hydrolysis of ramie fiber and subsequent oxidation

9

with sodium periodate. Solution casting method was exploited to prepare SPI/CNC

10

nanocomposite films with different amounts of the nano-crosslinker. The

11

microstructure, water resistance and mechanical property of the films were

12

investigated. In addition, the bio-based films were explored as an encapsulant to

13

develop flexible electromechanical sensors.

14

MATERIALS AND METHODS

15

Materials

16

Ramie fiber (RF) with 65~67% cellulose was purchased from Sichuan JB & Ramie

17

Co. Ltd., China. Soy protein isolate (SPI, protein content 90%) was supplied by

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Chengdu Protein Food Company (China). Sodium periodate, thymol blue,

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hydroxylamine hydrochloride, hydrochloric acid, and methyl alcohol were purchased

20

from Kelong Chemicals Reagent Co. Ltd., China. Ethylene glycol, acetic acid, and

21

isopropyl alcohol were purchased from Zhiyuan Chemicals Reagent Co. Ltd., China.

22

Sodium hydroxide was purchased from Ruijinte Chemicals Reagent Co., Ltd., China.

23

Single-walled carbon nanotube (SWNT) was purchased from Timesnano (Chengdu,

24

China). All other reagents were of analytical grade and used without further

25

purification.

26

Preparation of CNC-CHO

27

At first, CNC was prepared according to a previous report.47 In brief, smashed RF

28

was treated with 5 wt% NaOH at 80°C for 2 h to remove hemicellulose, pectin, and



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1

lignin, followed by washing with deionized water and hydrolysis with 4 M HCl at

2

80°C for 8 h. After that, precipitate was obtained by removing the supernatant. CNC

3

suspension was obtained after the treatment with 0.5 wt% NaOH at 60°C for 1 h and

4

repeated centrifugation. CNC-CHO was then prepared by oxidizing CNC with 0.2 M

5

sodium periodate (2.2 mol for 1 mol of glucopyranose) at pH 3. The reaction was

6

performed in the dark at 30°C for 4 h and quenched by ethylene glycol. After

7

centrifugation, the CNC-CHO product was obtained and dried in vacuum before use.

8

Preparation of SPI/CNC-CHO composite films

9

SPI/CNC-CHO composite films were prepared by a solution-cast method.

10

CNC-CHO was mixed with SPI in water at weight ratios of 2.5/97.5, 5.0/95.0,

11

7.5/92.5 and 10.0/90.0, respectively. Total solid content of the film-forming solution

12

was fixed at 4.0%. Glycerol with the final concentration at 1.71% was added as a

13

plasticizer. The mixture was immediately treated using an ultrasonic cell crusher (JY

14

99-IIDN, SCIENTZ, China) for 30 min. The pH value was adjusted to 10 with 2 M

15

NaOH, at which the SPI molecules were unfolded sufficiently in favor of forming

16

films. After heating at 60 °C for 2 h and degassing, the SPI/CNC-CHO blend

17

solutions were cast on Teflon plates (15 cm × 15 cm) and dried at 45 °C for 24 h to

18

prepare composite films. The as-prepared films were hot-pressed at 150°C for 4 min.

19

Accordingly, the resulted films were named as SPI/CNC-5.0, SPI/CNC-CHO-2.5,

20

SPI/CNC-CHO-5.0, SPI/CNC-CHO-7.5, and SPI/CNC-CHO-10.0. In addition, neat

21

SPI and SPI/CNC films were prepared by the same method as controls.

22

Fabrication of SPI/CNC-CHO film-encapsulated electromechanical sensors

23

Typically, 25 mg SWNT was dispersed by ultra-sonication for 6 h in the presence

24

of 25 mg surfactant. After filtration, the SWNT membrane was obtained and washed

25

several times by acetone. The resulting SWNT membrane was encapsulated by a

26

SPI/CNC-CHO composite film with the assistance of heat-seal tester (DZ260B,

27

Zhejiang, China) at the heating temperature of 150 oC.

28


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1

Characterization

2

FTIR analysis was performed on a FTIR spectroscopy (NICOLET 6700) within

3

the wavenumber from 4000 to 400 cm-1. X-ray diffractometer analysis was conducted

4

by an XRD instrument (Philips X’Pert), and the scans were taken over the 2θ range

5

from 5° to 40° with the scanning rate of 2°/min. Sample morphology was investigated

6

by a scanning electron microscope (SEM, JSM-5900LV, JEOL, Japan). Samples were

7

fixed on aluminum stubs and coated with gold. A Transmission eletron microscope

8

(Tecnai G2 F20 S-TWIN, FEI, America) was used to investigate the morphology of

9

CNC before and after modification at the voltage of 200 kV. Dynamic mechanical

10

analysis was carried out using a dynamic mechanical analyzer (Model Q800, TA

11

Instruments, USA) in tensile mode. Samples (15 mm × 40 mm × 0.3 mm) were first

12

equilibrated at -70°C for 3 min and then scanned from -70°C to 180°C at the heating

13

rate of 5°C/mina and the frequency of 1 Hz.

14

Mechanical property

15

The tensile strength (TS) and elongation at break (ε) of the films were investigated

16

using a Universal Testing Machine (SANS CMT4104, SANS Group, China) with a

17

tensile rate of 20 mm/min according to the standard method of GB/T 1040.3-2006.

18

Samples were kept under the RH of 60% at room temperature before measurement.

19

Each sample was tested five times, and the testing temperature and RH conditions

20

were 25°C and 60%, respectively.

21

Water resistance

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Water resistance property was evaluated in terms of swelling test, water uptake, and

23

solubility test. All samples well cut into 2.0 × 2.0 cm2 and dried to constant weight.

24

The swelling characteristic of the film was determined by immersing dried sample to

25

swell in deionized water at 25°C for 6 h. The initial and final morphologies of the

26

sample were photographed. The expansion rate was calculated by the following

27

equation

28 29

Expansion rate = (Sf –Si)/Si × 100% where Sf and Si are the final and initial areas of testing samples.


(1),


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1

The kinetics of water uptake was determined by immersing dried samples in

2

deionized water at 25°C. At specific time intervals, the samples were taken out and

3

weighted. The water absorption was calculated by equation (2). The total soluble

4

matter (TSM) of the sample was calculated by equation (3) after reaching the water

5

absorption equilibrium

6

Water uptake = (mt – mi) mi × 100%

(2),

7

TSM = (mi –me)/mi × 100%

(3),

8

where mt is the weight of samples at time of t; mi is the initial weight of dried samples;

9

me is the final dry matter of samples. All experiments were carried out in triplicate.

10

Light transmittance performance

11

Transparency was determined by a UV-vis spectrometer (Varian Cary50, USA)

12

within the wavelength of 200 to 800 nm. Thickness of samples was 0.30 mm. The

13

measurements were performed five times for each sample.

14

Thermal stability

15

TGA measurement was carried out by a TG 209F1 (NETZSCH, Germany)

16

thermogravimetric analyzer at a heating rate of 10 K/min under the nitrogen flow of

17

50 mL/min with the temperature range from 40°C to 700°C.

18

Electromechanical property

19

The real-time I-t curve of the sealed SPI/CNC-CHO encapsulated SWNTs

20

membrane during its bending process was recorded by a Keithley 6487 picoammeter

21

digital meter at a constant voltage of 0.3 V. A Keithley 2400 picoammeter was used

22

to determine the electrical resistance of the composite film. Resistivity was calculated

23

using the following formula:

24

ρ = Rab/l

(4),

25

where R is the resistance of the sample, and a, b, and l represent width, thickness, and

26

length of the sample, respectively.

27


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1

Statistical analysis

2

Data was expressed as mean ± SD. Statistical analysis of differences was

3

performed by a two tailed unpaired Student’s t-test. P < 0.05 was considered

4

significant.

5

RESULTS AND DISCUSSION

6

At first, acid-catalyzed hydrolysis of cellulose fibers was performed to prepare CNC.

7

As shown in Figure 1a, rod-like CNC is detected with length-diameter ratio of 7.6.

8

After periodate oxidation, its morphology remains nearly unchanged and the

9

corresponding length-diameter ratio is slightly decreased to 7.1. In comparison to

10

CNC, the oxidized CNC exhibits a new peak at 1723 cm-1 attributed to newly formed

11

aldehyde group (Figure 1b). Moreover, the intensity of another peak at 893 cm-1

12

corresponding to hemiacetal vibration increases. The results confirm the successful

13

occurrence of oxidation of CNC to CNC-CHO.48 To guarantee the enhancement effect

14

of CNC, XRD measurement was conducted to investigate the structural change of

15

CNC after oxidation. As shown in Figure 1c, CNC exhibits a typical XRD pattern

16

assigning to cellulose I, which is the characteristic crystalline structure of native

17

cellulose.49 Four well-defined peaks at 2θ of 14.7°, 16.5°, 22.7° and 34° are attributed

18

to the planes of 110, 110, 200 and 004, respectively. In addition to that, two weak

19

peak locating at 2θ of 12.6° (101) and 20.7° (021), which belong to cellulose Ⅱ,50, 51

20

are determined. This transformation from cellulose I to cellulose II is caused by the

21

alkaline treatment during the preparation process of CNC.50 Furthermore, as

22

determined from the XRD patterns, the crystallinity of CNC is decreased from 88.4%

23

to 64.8% after oxidation. The decrease of crystallinity results from the opening of

24

glucopyranose rings and destruction of the ordered structure of CNC.51

_



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1 2

Figure 1. Structural characterization of CNC and CNC-CHO. a) Schematic illustration of the synthesis

3

of CNC-CHO as well as TEM images of CNC before and after oxidation. b) FTIR spectra and c) XRD

4

patterns of CNC and CNC-CHO.

5

6 7

Figure 2. a) FTIR spectra of CNC-CHO, SPI, and composite films. b) Schematic illustration of the

8

network formation in SPI/CNC-CHO composite film. c) Photos of neat SPI film, SPI/CNC and

9

SPI/CNC-CHO composite films before and after immersion in water: (1) SPI; (2) SPI/CNC-5.0; (3)

10

SPI/CNC-CHO-2.5; (4) SPI/CNC-CHO-5.0; (5) SPI/CNC-CHO-7.5; (6) SPI/CNC-CHO-10.0.


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1

Figure 2a illustrates the FTIR spectra of SPI, CNC-CHO, and SPI/CNC-CHO

2

composite films, from which we can see the stretching absorption band (1723 cm-1) of

3

free aldehyde of CNC-CHO disappears after the reaction with SPI. This suggests the

4

complete consumption of aldehyde groups, which is required for applications in food

5

and biomedical science. The network formation in the composite films is proposed in

6

Figure 2b, which illustrates the Maillard reaction between SPI and CNC-CHO. In

7

addition, another proof for the Maillard reaction is the color change of the resulting

8

films as shown in Figure 2c, that is, from pale yellow for neat SPI film or

9

SPI/CNC-5.0 (control group) to dark brown for SPI/CNC-CHO composite films. To

10

present the superiority of forming network between SPI and CNC, the SPI/CNC-CHO

11

composite films were immersed in water for a sufficient period (6 h) that was required

12

to reach equilibrium absorption state, as compared with control films (neat SPI and

13

SPI/CNC). In contrast to their obvious shape changes, the expansion behavior of

14

SPI/CNC-CHO composite films is remarkably prohibited (Table S1). In particular, the

15

expansion rate of SPI/CNC-CHO-10.0 film is decreased by 81.1% compared with

16

neat SPI film. Moreover, the expansion rate of SPI/CNC-CHO-5.0 film is lowered

17

than that of SPI/CNC-5.0 film, suggesting the crosslinking is favorable to the

18

enhancement of water resistance.

19 20

Figure 3. TEM images of SPI, SPI/CNC-5.0, and SPI/CNC-CHO-5.0 films. Dark and bright areas

21

represent the protein-rich and glycerol-rich domains, respectively. Marked with red circles are

22

observed independent CNC.

23

As previously reported by Zhang's group,19, 52 glycerol-plasticized soy protein film

24

has two domains, glycerol-rich domain and protein-rich domain, resulting from the

25

relatively high or low compatibility between SPI and glycerol. Therefore, two glass



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1

transition temperatures, Tg1 and Tg2 corresponding to the glycerol-rich and

2

protein-rich domains, can be detected by DMA for neat SPI film (Figure S1 and Table

3

S2). With the introduction of CNC, the both temperatures become higher. Compared

4

with neat SPI film, Tg1 and Tg2 of SPI/CNC-CHO composite films became closer,

5

indicating the more homogeneous distribution of glycerol in SPI matrix. To

6

understand the reason, TEM measurement was conducted. As referenced by previous

7

reports on other protein-based materials,53 the dark and bright areas in the TEM

8

images shown in Figure 3 represent the protein-rich and glycerol-rich phases,

9

respectively. As for neat SPI film, the protein-rich phase with two distinct

10

morphologies (microscale stripe-like and nanoscale sphere-like domains) is detected

11

dispersing in the glycerol-rich phase. For the SPI/CNC film, these characteristics are

12

still determined. As marked with red ellipses, furthermore, rod-like CNC with obvious

13

boundary can be seen in the composite. More interestingly, CNC prefers to locate at

14

the relatively hydrophilic glycerol-rich domains rather than the relatively hydrophobic

15

protein-rich domains. These results indicate the poor interfacial interaction between

16

SPI matrix and CNC. On the other hand, the presence of the hydrophilic CNC, which

17

is able to interact with glycerol, may cause the reduced location of the plasticizer at

18

protein-rich domains, thus weakening the plasticizing effect for SPI. Consequently,

19

the Tg1 and Tg2 detected for SPI/CNC-5.0 are reasonably higher than those of the neat

20

SPI modified with glycerol alone. In comparison, the microphase separation behavior

21

is significantly affected when incorporating CNC-CHO into SPI, that is, microscale

22

stripe-like domains disappear and a large number of nanoscale sphere-like domains

23

are detected homogeneously in the glycerol-rich phase. Besides that, rod-like CNC

24

cannot be easily seen any more. Therefore, it can be concluded that the interfacial

25

interaction is obviously increased and the glycerol molecules disperse more

26

homogeneously in the SPI matrix when using reactive CNC as the filler.


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1 2

Figure 4. a) Representative stress-strain curves and b) fracture toughness of SPI, SPI/CNC-5.0, and

3

SPI/CNC-CHO films. SEM images of the tensile fracture surfaces of (c) SPI/CNC-5.0 and (d)

4

SPI/CNC-CHO-5.0 films.

5

For practical applications, mechanical property is definitely an important issue that

6

should be considered. The typical stress-strain curves of film samples are shown in

7

Figure 4a, from which the TS and ε of neat SPI film are determined as 6.3 MPa and

8

200% (listed in Table S3). It should be noted that the TS and ε values are higher than

9

those reported previously14 because of the hot-pressing treatment, which can improve

10

the inter-diffusion of denatured SPI chains and result in more molecular

11

entanglements.23 Generally, the utilization of CNC can increase the TS as well as

12

decrease the ε of polymeric materials.30, 36, 54-58 In our work, similar results have been

13

obtained for SPI/CNC-5.0 composite film, that is, the TS and ε are increased and

14

decreased to 8.8 MPa and 143%, respectively. In comparison, introducing only 2.5%

15

of CNC-CHO into SPI makes TS and ε increased 49.2% and 13.0% to 9.4 MPa and

16

226%; this strengthen effect is even more obvious than CNC at higher dosage.

17

Notably, when the CNC-CHO content is increased to 10.0%, the TS of resulting

18

composite film achieves at 14.0 MPa and its ε maintains at 145%. Compared with

19

previous reports on cellulose fiber-reinforced SPI films,24,


37, 59

the interfacial


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1

crosslinking shows a superior effect on enhancement of mechanical property.

2

Furthermore, fracture toughness defined as the area under the stress-strain curves60 is

3

remarkably improved for SPI upon the introduction of CNC-CHO rather than CNC

4

(Figure 4b). To understand the reinforcement effects of CNC-CHO, the

5

microstructures of tensile fracture surfaces of SPI/CNC-5.0 and SPI/CNC-CHO-5.0

6

were investigated by SEM. From the images shown in Figure 4c, a large number of

7

nano-pores (marked with white arrows) are observed for SPI/CNC-5.0 film, resulting

8

from the pulling out of CNC. Compared with that, the SPI/CNC-CHO-5.0 film shows

9

smoother tensile fracture surface and white circle dots attributing to the broken CNC.

10

Moreover, the dots disperse homogeneously in the SPI matrix. These results,

11

consequently, well explain why the relatively high mechanical property is realized for

12

the SPI-based film materials. A corresponding illustration regarding the failure

13

mechanisms of SPI/CNC and SPI/CNC-CHO composite films is proposed in Figure 5.

14

As for the SPI/CNC, the filler, CNC, causes stress concentration under the stretching,

15

and the mechanical breakage prefers to occur at the interfaces between CNC and SPI

16

matrix. Meanwhile, the CNC bearing aldehyde groups acts as not only a

17

nano-reinforcement to increase mechanical strength but also a nano-crosslinker to

18

maintain the ductility. Owing to the strong interfacial interaction between CNC and

19

SPI, the alignment of the filler will orient along with the stretching direction, thereby

20

the cross-section of CNC (dot-like morphology) rather the whole CNC is observed in

21

SEM image and the resulting SPI/CNC-CHO films exhibit promoted mechanical

22

properties.

23 24

Figure 5. Schematic illustration of the network structures of a) SPI/CNC and b) SPI/CNC-CHO

25

composite films during stretching process.


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1

As well known, the poor water resistance of SPI film is one of the main obstacles to

2

real applications. Generally, the water resistance is obviously dependent on the filler

3

content as well as the crosslinking structure. Figure 6a shows the swelling behavior of

4

such films. Because of the high hydrophilicity, neat SPI film can absorb a large

5

amount of water with the equilibrium water uptake (EWU) of 650%. However, the

6

EWU is reduced by 33.4% upon the addition of CNC. The water resistance property is

7

further improved when using CNC-CHO as the filler, that is, the EWU of the

8

crosslinked composite film (SPI/CNC-CHO-10.0) is decreased to 155%, which is

9

76.1% lower than the neat SPI film. To quantitatively understand the contribution of

10

network formation to the water resistance, diffusion coefficient (D) was calculated

11

according to the followed equation19

12

௠೟ ି௠బ ௠ಮ

= 1−෍

௡ୀ଴ ஶ

଼

(ଶ௡ାଵ)మ ஠మ

exp ቂ

ି஽(ଶ௡ାଵ)మ ஠మ ௧ ସ୐మ

ቃ

(5),

13

where m0 and mt are the weights of the samples before and after swelling at time of t,

14

m∞ is the weight of sample at equilibrium swelling state, 2L is the thickness of the

15

film of the film sample. Within a short time, this equation can be written as:

16

௠೟ ି௠బ ௠ಮ

ଶ

஽

ଵ/ଶ

= ൬ ൰ ୐ π

‫ݐ‬ଵ/ଶ

(6),

17

When (mt-m0)/m∞ ≤ 0.5, the plots of ((mt-m0)/m∞)2 as a function of (4t/(πL2)) were

18

drawn for all samples, and the D value was calculated from the slope of the plots. As

19

shown in table S4, the D values of neat SPI and SPI/CNC-5.0 films are over 2.2,

20

while those of SPI/CNC-CHO films are below 1.7, indicating the positive effect of

21

crosslinking on the suppressed water absorption. In addition, the D values are

22

dependent on the amount of CNC-CHO used in the composite films. As a result, the

23

crosslinking extent is regarded essential to limit the expansion and swelling of the

24

SPI-based films. To further understand the crosslinking degree, TSM was determined

25

for the films. The TSM of neat SPI film is as high as 46.6%, which is decreased to

26

43.1% after the introduction of CNC, attributing to the lower net content of SPI in the

27

composite film. Notably, as for SPI/CNC-CHO films, their TSM is obviously

28

decreased. Nevertheless, the TSM value remains over 30% even if the amount of

29

CNC-CHO is 10.0%. This is due to the fact that the main soluble matter in the case is



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1

glycerol. Although the crosslinking is created in the films, the plasticizer is still free to

2

move and diffuse. As a result, it can be concluded that 5.0% of CNC-CHO is enough

3

to form stable network structures with SPI, protecting the film from corrosion by

4

water. However, for the mechanical property, the higher CNC-CHO amount, the better

5

tensile performance.

6 7

Figure 6. a) Swelling behavior, b) optical transmittance and c) thermal stability of SPI, SPI/CNC-5.0,

8

and SPI/CNC-CHO films. d) Real-time I-t curve of SWNT membrane sealed with SPI/CNC-CHO-5.0

9

during the bending/stretching cycles (“S” represents “stretching” and “B” represents “bending”).

10

Transparency, an important property for packaging films, was following evaluated.

11

As shown in Figure 6b, neat SPI film presents high transparency with the

12

transmittance of about 80% within the wavelength from 600 to 800 nm. Although the

13

incorporation of CNC-CHO results in a decrease in transparency, the transmittance is

14

still maintained over 60%. Compared with that, the optical transmittance is decreased

15

obviously in the case of adding CNC as the filler, because of its poor interfacial

16

interaction with matrix.61 Figure 6c illustrates the TGA curves of these films, from

17

which no obvious difference is detected. This indicates the thermal stability of


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Page 15 of 20

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1

SPI-based film is not deteriorated after the incorporation of CNC or CNC-CHO.

2

Thereafter, a typical sample composite film, SPI/CNC-CHO-5.0 with the electrical

3

resistivity of 1.7 × 103 Ω·m, was selected as an encapsulant to prepare an

4

electromechanical sensor, of which the appearance is shown in Figure 6d. From the

5

highly sensitive current response of the sensor to the bending treatment as well as the

6

comparable flexibility, optical transparency, non-toxic nature and easy encapsulation

7

of such SPI-based film to conventional polymers (polydimethylsiloxane for

8

instance),62, 63 we can conclude that using the fully bio-based film as the encapsulant

9

to construct movement-monitoring electronics is of great potential value.

10

CONCLUSIONS

11

CNC containing aldehyde groups is synthesized and employed as the reinforcement

12

material for SPI. Owing to the Millard reaction, the CNC-CHO filler also acts as a

13

nano-crosslinker in the composite films, resulting in obvious enhancement of the

14

interfacial interaction between SPI and CNC. The as-prepared SPI/CNC-CHO films

15

exhibit significantly improved mechanical property and water resistance as compared

16

with neat SPI film as well as the SPI/CNC film without the interfacial reaction.

17

Notably, the SPI/CNC-CHO films have heat-sealing property, which can be used to

18

encapsulate conductive SWNT membrane. The resultant electronic skin exhibit

19

current response to mechanical treatments, indicating the bio-based composite films

20

have potential applications in next-generation electromechanical sensing.

21

ASSOCIATED CONTENT

22

Supporting Information

23

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

24

website at DOI: XXXXX/acssuschemeng.XXXXX, including calculation of aldehyde

25

content, and results of DMA, expansion rate, tensile strength, elongation at break, and

26

water resistance.

27

ACKNOWLEDGMENT

28

This work was supported by the National Natural Science Foundation of China



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

(Grants 51403136 and 51421061), the Program for Changjiang Scholars and

2

Innovative Research Teams in University of China (IRT 1026), and the Fundamental

3

Research Funds for the Central Universities of China (2015SCU04A22).

4

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29 30 31 32 33 34 35 36



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Graphic Abstract

2

A Fully Bio-Based Encapsulant Constructed by Soy

3

Protein and Cellulose Nanocrystal for Flexible

4

Electromechanical Sensing

5

Dan-Yang Xie, Dan Qian, Fei Song,* Xiu-Li Wang and Yu-Zhong Wang*

6

Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), College of

7

Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key

8

Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China.

9

Corresponding authors: E-mail: [email protected]; [email protected]

10

Tel: 86-28-85410755; Fax: 86-28-85410755

11

12 13

Synopsis:

14

A fully bio-based encapsulant constituted of soy protein isolate and cellulose

15

nanocrystal is developed for construction of flexible electromechanical sensors.


Page 20 of 20

A Fully Biobased Encapsulant Constructed of Soy Protein and

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