A self-healing PDMS elastomer based on acylhydrazone groups and the role of hydrogen bonds

Accepted Manuscript A self-healing PDMS elastomer based on acylhydrazone groups and the role of hydrogen bonds Dong-Dong Zhang, Ying-Bo Ruan, Bao-Qing Zhang, Xin Qiao, Guohua Deng, Yongming Chen, Chen-Yang Liu PII:

S0032-3861(17)30537-2

DOI:

10.1016/j.polymer.2017.05.060

Reference:

JPOL 19720

To appear in:

Polymer

Received Date: 24 March 2017 Revised Date:

9 May 2017

Accepted Date: 25 May 2017

Please cite this article as: Zhang D-D, Ruan Y-B, Zhang B-Q, Qiao X, Deng G, Chen Y, Liu C-Y, A selfhealing PDMS elastomer based on acylhydrazone groups and the role of hydrogen bonds, Polymer (2017), doi: 10.1016/j.polymer.2017.05.060. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

A Self-Healing PDMS Elastomer Based on Acylhydrazone Groups and the Role of Hydrogen Bonds Dong-Dong Zhang1,2, Ying-Bo Ruan1,2, Bao-Qing Zhang1,2, Xin Qiao1, Guohua

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Deng3,*, Yongming Chen4,*, and Chen-Yang Liu1,2,* 1

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CAS Key Laboratory of Engineering Plastics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100190, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China 4 Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Materials Science and Engineering, Sun Yat-Sen University, NO. 135, Xingang Xi Road, Guangzhou 510275, China

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*Corresponding author: E-mail: [email protected] (G. H. Deng); [email protected] (Y. M. Chen) ; [email protected] (C. Y. Liu)

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ACCEPTED MANUSCRIPT ABSTRACT

A PDMS elastomer based on acylhydrazone groups with both acid- and self-healing

properties

was

successfully

prepared

from

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heat-assisted

tetra-acylhydrazine-terminated PDMS and terephthalaldehyde through solution casting. The good healing performance was obtained with catalytic acetic acid for 24

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h at 25 °C or by annealing at 120 °C for 2 h. The elastomer exhibited a reversible

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transition near 80 °C observed by rheological measurements and variable-temperature FTIR, which corresponded to the dissociation and reconstruction of hydrogen bonds between acylhydrazone groups. Since the non-equimolar sample presented similar behaviors

with

the

equimolar

sample,

it

verifies

that

the

reversible

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dissociation/reformation of hydrogen bonds dominates the heat-assisted self-healing process. This finding will enable better understanding of the contribution of hydrogen bonding interactions in acylhydrazone self-healing systems, thus promoting the

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development of self-healing bulk materials based on acylhydrazone groups.

Keywords: Self-healing; PDMS elastomer; acylhydrazone groups; hydrogen bonds

2

ACCEPTED MANUSCRIPT 1. Introduction Self-healing refers to the ability to autonomously heal damage, which also means without any external intervention [1]. So far, the major findings in this field can be

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divided into two categories: extrinsic self-healing and intrinsic self-healing [2]. The former requires a pre-embedded healing agent, represented by the epoxy system [3], which has the problem that the healing agent may be exhausted. In contrast, the latter

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heals crack using the structural characteristics of polymers, providing a material with

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the advantage of healing repeatedly and reversibly. For the intrinsic self-healing polymers, the reported healing modes include supramolecular interactions (such as hydrogen bonding [4,5], π-π stacking [6,7], and metal coordination [8,9]) and dynamic covalent bonds (such as acylhydrazone bonds [10-13], disulfide bonds

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[14-16], Diels-Alder reactions [17-19], and imine bonds [20]). The acylhydrazone group, arising from the reaction between acylhydrazine and aldehyde, has dynamic characteristics, provided by both the reversible acylhydrazone bond and the hydrogen

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bonding sites [21]. The acylhydrazone bond can be activated under mild conditions,

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showing both temperature and pH responsiveness. Lehn’s group [22-25] performed a comprehensive and in-depth study of linear polyacylhydrazones. They investigated the change of mechanical and optical properties caused by the exchange reaction of acylhydrazone bonds [22-24]. Later, double dynamic self-healing polymers were also prepared [25]. In our previous work, the acylhydrazone bond was used to construct dynamic polymer gels based on poly(ethylene glycol) (PEG), having self-healing properties 3

ACCEPTED MANUSCRIPT [10-12], exhibiting sol–gel transitions in response to pH changes, and demonstrating rapid adhesion between hydrogel and organogel [13]. White and coworkers achieved restoration of large-scale damage by developing a vascular-like repair system

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involving the two-stage chemistry of rapid acylhydrazone formation and slow polymerization [26]. Self-healing bulk materials or elastomers [14,15,17,18] are more difficult to fabricate due to the nature of the very slow diffusion of segments in

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polymer melts or solids. Recently, Schubert and coworkers [27] prepared an

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acylhydrazone bond-crosslinked self-healing bulk material for the first time by using 2-hydroxyethyl methacrylate as the backbone. The material can heal a deep scratch as long as 1 cm in length under 125 °C. The healing mechanism was attributed to the exchange reaction between acylhydrazone bonds, but the role of hydrogen bonds

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existing in plenty in this system was omitted. It is worth noting that hydrogen bonds can easily form between C=O and N-H in acylhydrazone groups, which is similar to other multiple hydrogen bonds systems formed between C=O and N-H [28-32].

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In this study, a self-healing bulk material crosslinked by acylhydrazone groups

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was prepared. Here, polydimethylsiloxane (PDMS), one of the common commercial elastomers, was employed as the backbone to replace PEG because the latter is in the semi-crystalline state (hindering the self-healing behavior) at room temperature. The self-healing properties of the PDMS elastomers were investigated in detail. The healing mechanism and the role of H-bonds were explored with the help of rheology, variable-temperature FTIR, and other characterization methods. The present study will provide some guidance for designing self-healing materials based on 4

ACCEPTED MANUSCRIPT acylhydrazone groups.

2. Experimental section

Bis(3-aminopropyl)-terminated Mn=2500

g/mol)

was

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2.1 Materials. Polydimethylsiloxane

(H2N-PDMS-NH2,

from

Methyl

purchased

Sigma-Aldrich.

acrylate,

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terephthalaldehyde, and dichloroacetic acid were obtained from J&K Scientific Ltd.

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Hydrazine hydrate (80% in water, w/w), methanol, toluene, chloroform, acetic acid (HAc), triethylamine (Beijing Chemical Reagent Co.). All the chemicals were used as received. 2.2 Sample preparation. of

tetra-acylhydrazine-terminated

PDMS

(A4).

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Synthesis

Tetra-acylhydrazine-terminated PDMS (A4) was synthesized by modification of H2N-PDMS-NH2 through two steps as shown in Fig. 1a. First, methyl

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propionate-terminated PDMS was synthesized through Michael addition with methyl 13

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acrylate. lH NMR and the corresponding

C NMR (Figs. S1 & S2) showed that the

methyl propionate functionality at the PDMS ends was close to 100%. Then, A4 with 89% functionality (Figs. S3 & S4) was obtained by the nucleophilic substitution of methyl propionate-terminated PDMS with hydrazine hydrate. Experimental details are summarized in the Supporting Material.

5

Fig.

1.

Gelators’

structure

used

in

this

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study:

(a)

Synthesis

of

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tetra-acylhydrazine-terminated PDMS (A4), and (b) terephthalaldehyde (B2). Gel preparation. Predetermined amount of A4 and B2 were dissolved in toluene to observe the gelation. The gelator mass concentration was fixed at 20 wt%. The mixed solution of A4 and B2 with equimolar functional groups was cast into a round

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Teflon mold with 20 mm in diameter. The mold containing samples was kept in a desiccator saturated with toluene vapor for 24 h to obtain the stable gel samples for

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rheological tests and self-healing study. Gel–sol transition was conducted using dichloroacetic acid and triethylamine as pH regulators. Organogels with

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non-equimolar functional groups were also prepared with the same condition. Elastomer preparation. The PDMS elastomer was prepared through solvent

evaporation of organogels at room temperature over 5 days and further dried for 48 h under vacuum at 60 °C. The resulting elastomer samples were ca. 0.7 mm in thickness. The elastomer was annealed at 120 °C to reach the equilibrium state. Rheological measurement was conducted using a TA ARES-G2 strain-controlled rheometer with 8 mm parallel-plate geometry under N2 atmosphere to monitor the 6

ACCEPTED MANUSCRIPT change of modulus. Recycling study. The recycling experiment was conducted in a FM450 vacuum mould pressing machine. Small pieces of A1B1 elastomer samples were reprocessed

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under a pressure of 10 MPa for 15 min at 120 °C and then a round film was obtained. Similar process was repeated twice, triangular and pentangular films were got respectively. All the recycled films had a thickness of 1 mm.

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2.3 Characterization.

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Acid-assisted and Heat-assisted healing. The samples were cut into dumbbell-shaped films (length 25 mm, width 8 mm; the length and the width of the middle (neck) part of the sample are 4 and 3 mm, respectively). For good contact between the fracture surfaces, all the elastomer samples for tensile tests were cut

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partially in the thickness direction with 0.01 mm left. All the samples were coated with the same amount of HAc. The healing automatically occurred at room temperature for 24 h or 48 h before the healed samples were subjected to stress–strain

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tests at room temperature. Heat-assisted healing was conducted at 100 °C or 120 °C

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for different time to evaluate the effect of temperature and time on healing efficiency. Tensile measurements were conducted at 25 °C with Instron 3365. The

deformation rate was set at 5 mm/min. Healing efficiency was calculated from the ratio of tensile strength of the healed samples to that of the original samples. Each measurement was repeated at least three times. Rheological

measurements.

For

H2N-PDMS-NH2,

methyl

propionate-terminated PDMS, and A4, viscosities at 25 °C were measured on a 7

ACCEPTED MANUSCRIPT stress-controlled rheometer of TA AR-2000ex with the strain amplitude of 10% and the angular frequency ranging from 0.1 to 100 rad/s. For organogels, dynamic frequency sweep was carried with AR-2000ex at 25 °C (the strain amplitude of 0.5%

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and the angular frequency ranging from 0.01 to 100 rad/s). For elastomer samples, dynamic temperature sweep was conducted using a TA ARES-G2 strain-controlled rheometer with 8 mm parallel-plate geometry under N2

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atmosphere. The temperature range was -140 to 200 °C, and the heating rate was

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5°C/min. Dynamic frequency sweep was conducted at several temperatures (T=30, 50, 75, 100, 120, 150 and 200 °C) to construct the time-temperature superposition master curve. When cooling-heating cycles were performed, the temperature range was 30-120 °C, and the cooling/heating rate was 5°C/min.

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FT-IR experiment. Thermo Fisher Nicolet 6700 spectrometer with the resolution of 4 cm-1 was performed. For each spectrum, 32 scans were accumulated. Sample was prepared by mixed gelator solution (toluene, 20 wt %) casting on the

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surface of a KBr plate. The temperature of sample was controlled by a Linkman FTIR

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600 hot stage. The temperature range was 30 to 120 °C, and the heating/cooling rate was 3 °C/min.

3. Results and Discussions 3.1 Properties of the organogel. The gelation of A4 and terephthalaldehyde (B2) in toluene was investigated. When the gelator mass concentration was fixed at 20 wt%, a gel with equimolar 8

ACCEPTED MANUSCRIPT functional groups was formed in 6 h. A room temperature dynamic frequency sweep also indicated the formation of a gel (Fig. 2a). The gel exhibited reversible gel-sol phase transition in dichloroacetic acid and triethylamine (Fig. 2b) and could be

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repaired with acetic acid (HAc) (Fig. 2c). In summary, a self-healing PDMS organogel based on acylhydrazone groups was prepared, and its self-healing and gel-sol transition behavior was similar to that of the PEO hydrogels reported in our

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previous work [10, 11].

Fig. 2. Dynamic frequency sweep curves of the PDMS gel at 25 °C (Insert: picture of the formation of polymer gel); (b) gel–sol phase transition of the PDMS gel in toluene triggered by pH; (c) HAc-assisted healing properties of the PDMS gel. 3.2 Self-healing properties of PDMS elastomer. The PDMS elastomer was prepared through solvent evaporation at room temperature over 5 days and dried under vacuum at 60 °C for 48 h. Since the modulus of the PDMS elastomer was found to increase with time and temperature, the 9

ACCEPTED MANUSCRIPT elastomer was annealed at 120 °C to reach the equilibrium state. The modulus change during heat treatment was monitored using rheological measurements. The modulus increased gradually with time during the first hour of annealing (Fig. S5a, red points),

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but it remained basically constant during the second hour of annealing (blue points). The modulus measured at 30 °C before and after the second hour of annealing at 120 °C also gave a similar result (Fig. S5b). This indicated that the approximate

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equilibrium state could be reached after annealing at 120 °C for 1 h, because high

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temperatures may be favorable for both the exchange reaction of acylhydrazone bonds and the reversible formation of hydrogen bonds in this system. Therefore, all the elastomer samples used in the present work were treated at 120 °C for 1 h in order to ensure the same initial state.

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Lehn and coworkers have found that both acid catalysts and heating can promote the exchange reaction in linear polyacylhydrazones [33]. Thus, the elastomers based on acylhydrazone bonds should possess both acid- and heat-assisted self-healing

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capabilities. We first evaluated the effect of HAc on the healing behavior at room

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temperature. Trace amounts of HAc were coated on the cut section, and the samples were then repaired for a certain period. Fig. 3a shows the tensile stress–strain curves of the original and acid-repaired samples. The initial region of the stress–strain curve of the healed sample basically overlapped with that of the original sample, and the healed samples restored 60% of their fracture strength. When the healing time was extended to 48 h, the healing efficiency increased to 85% (Fig. 3a). This indicated that HAc was suitable for use as the acid catalyst in this system. 10

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Stress (MPa)

(a) 2.0 OR HAc@24h HAc@48h

1.5

1.0

0.0 0

50

100

150

Strain (%) (b)2.5 OR

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100°C@2h 120°C@2h

1.5 1.0 0.5 0.0 0

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Stress (MPa)

2.0

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0.5

50

100

150

Strain (%)

(c)2.5

1.5 1.0

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Stress (MPa)

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2.0

OR 120°C@1h 120°C@2h 120°C@4h

0.5

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0.0

0

50

100

150

Strain (%)

Fig. 3. Acid- and heat-assisted self-healing study of the PDMS elastomer. Typical tensile stress–strain curves of samples (a) catalyzed by HAc at 25 °C for different times, 24 h and 48 h; (b) healed for 2 h at different temperatures, 100 °C and 120 °C; (c) healed at 120 °C for different periods, 1 h, 2 h and 4 h. For comparison, tensile curves of the original sample are also shown. Note that each measurement was repeated at least three times, as shown in Fig. S6. The thermal repair behavior of the elastomer was also studied. Fig. 3b shows the effect of healing temperature with a healing time of 2 h. The repair efficiency was 11

ACCEPTED MANUSCRIPT approximately 60% at 100 °C and 120% (slightly larger than 100% due to the uncertainty of the strain at break) at 120 °C, mainly because the exchange of the dynamic acylhydrazone groups at 120 °C made the network more stable, since the

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sample will continuously approach the final equilibrium state with increasing healing time, as shown in Fig. S5c. However, the sample cannot be healed at lower temperatures, e.g. 80 °C, which is similar to results in literature [27], because the

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diffusion was relatively slow at this low temperature. More importantly, the

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dissociation of acylhydrazone groups was just activated around 80 °C, which will be discussed in section 3.3. Fig. 3c shows the effect of the healing time with the healing temperature fixed at 120 °C. After 1 h, the repair efficiency was 99%, increasing to 120% after 2 h. However, when the healing time was further extended to 4 h, the

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elongation at break decreased probably due to degradation as a result of the longer thermal history at 120 °C, although this sample had a higher modulus, considering that the exchange of the dynamic acylhydrazone groups is “live” in nature. Therefore,

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the optimal heat healing performance was obtained at 120 °C for 2 h.

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Fig. 4. Recycling study: reprocessing process of A1B1 elastomer by repeatable hot compression molding under a pressure of 10 MPa for 15 min at 120 °C. All the

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recycled films had a thickness of 1 mm.

In addition, the elastomer can be recycled by utilizing the heat exchange properties of the acylhydrazone groups. For example, the elastomer can be hot-pressed repeatedly to different shapes at 120 °C (Fig. 4). Compared with the

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original sample, the three recycled samples have similar mechanical properties, and can restore about 75% of their fracture strength (Fig. S7). In summary, a PDMS

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elastomer based on acylhydrazone groups with both acid- and heat-assisted

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self-healing properties has been successfully prepared. 3.3 Mechanism of Self-healing PDMS Elastomer and the Role of Hydrogen Bonds.

A dynamic temperature scan was performed on the PDMS elastomer based on

acylhydrazone groups to explore the healing mechanism (Fig. 5a). The glass transition temperature of the elastomer was near -120 °C, which is consistent with that of PDMS reported in the literature [34]. The small peak of tan δ at -90 °C may correspond to the 13

ACCEPTED MANUSCRIPT relaxation process of the methylene segments connected to the acylhydrazone bond moiety (Fig. 1a). At the high temperature region, the elastomer exhibited two plateaus (moduli were 0.98 and 0.18 MPa, respectively). The transition temperature of the two

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platforms was at approximately 80 °C (the peak of tan δ). Finally, the elastomer could flow above 200 °C, which should correspond to the dissociation of acylhydrazone bonds. Fig. 5b shows the time-temperature superposition master curve of the

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elastomer at the reference temperature of 100 °C. Clearly, there also existed two

(a)

9

10

G' G" tanδ

8 7

10

6

10

5

10

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G',G" (Pa)

10

1

10

0

10

tanδ

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plateaus, a transition region between them and a low-frequency flow region.

-1

10

4

10

10 -100 -50 0 50 100 150 200 Temperature (°C)

-2

7

10

1

10

0

10

-1

10

-2

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(b)10

G' G" tanδ

6

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G',G"(Pa)

10

5

10

4

tanδ

3

10

10

3

10

-6

10

-4

10

-2

10

0

10

2

10

ω (rad/s)

14

4

10

6

10

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10

8

10

7

10

6

10

5

10

4

10

3

G' G" tanδ

0

50

10

0

10

-1

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10

A4

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1

0.1

-2

100 150 200

Temperature(°C)

η (Pa·s)

1

10

-100 -50

(d)

10

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10

9

tanδ

G',G" (Pa)

(c)

(H3COOC)2-PDMS-(COOCH3)2 H2N-PDMS-NH2

0.01 0.1

1

10

ω (rad/s)

100

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Fig. 5. (a) Dynamic temperature sweep curves of the PDMS (A1B1) elastomer; (b) master curves of the A1B1 elastomer, the reference temperature was 100 °C; (c) dynamic temperature sweep curves of the A1B0.5 elastomer; (d) viscosities of H2N-PDMS-NH2, methyl propionate-terminated PDMS, and A4 at 25 °C, (Insert:

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A4).

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structural formula of interchain hydrogen bonds between acylhydrazine end groups in

There were a few conjectures for the transition at approximately 80 °C: the

crystallization transition, the dissociation of acylhydrazone bonds at low temperatures, and the dissociation of H-bonds. First, DSC and WAXD results indicated that there was no crystallization–melting transition in this system (Fig. S8). In addition, the lack of hysteresis in the cooling–heating cycles (Fig. S9) indicated that this was a reversible process. Second, the four acylhydrazine groups in gelator A4 were

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80 °C, since the dissociation temperatures of hydrogen bonds in similar systems have generally ranged from 80 to 100 °C as reported in the literature [31,32]. For example, Long et al. studied Upy-containing PS, PI and PS-b-PI by NMR and rheology and

rheological

techniques

to

study

PIB

systems

modified

with

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utilized

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found that the multiple hydrogen bonds were destroyed at 80 °C [31]. Binder et al.

thymine/2,6-diaminotriazine end groups and found that the hydrogen bond failed at 80 °C in both systems [32]. It can be seen that the dissociation temperatures of hydrogen bonds in the literature are consistent with the transition temperature (80 °C)

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observed in this study for the PDMS elastomer.

Gelation by the condensation polymerization of multiple functional groups

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normally requires an equimolar condition. If a non-equimolar sample could be gelled and present the same transition at 80 °C and also the self-healing property, it will

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prove that other interactions instead of dynamic acylhydrazone bonds were the dominated effect to understand the mechanism of self-healing. Therefore, two non-equimolar A4/B2 experiments were designed. The molar ratio of the aldehyde group/acylhydrazine group was changed to 0.5 (A1B0.5) or 2 (A1B2), with the gelator total concentration fixed at 20 wt%. Theoretically, systems with large non-equivalent gelators have difficulty achieving gelation. However, the A1B0.5 sample could form a gel within 12 h (Fig. S10a). In fact, the A1B0.5 and A1B1 16

ACCEPTED MANUSCRIPT organogels exhibited similar rheological behaviors (Fig. S10b). This implied that there were other interactions besides acylhydrazone bonds in the system. In contrast, no gel was formed for the A1B2 sample even after 7 days (Fig. S10a). Moreover,

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when the solvent was removed, the A1B0.5 elastomer could be prepared from the organogel. Its moduli-temperature curve (Fig. 5c) is very similar to A1B1, including the transition between the two plateaus at approximately 80 °C. The A1B0.5

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elastomer also presented acid- and heat-assisted healing properties (Fig. S11).

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Since the dissociation temperature of multiple hydrogen bonds is close to 80 °C [31,32], it is reasonable to consider the role of hydrogen bonds in acylhydrazone groups. First, the C=O and N-H groups in acylhydrazone bonds can form a double hydrogen-bonding interaction (cyan-colored bonds in Fig. 6a) [35]. Therefore, a 20%

between

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A1B0.5/toluene solution can be completely gelled by inter-chain hydrogen bonds acylhydrazone

groups

(Fig.

6b).

However,

in

this

case,

the

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acylhydrazone-forming reaction only promoted the chain propagation instead of the crosslink due to the non-equimolar nature of A1B0.5. Moreover, C=O and N-H

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groups in acylhydrazine end groups can also form double hydrogen bonds (yellow-colored bonds in Figs. 6a and 6b), which can be confirmed by the viscosity measurements in Fig. 5d. The viscosity of the acylhydrazine-terminated PDMS, A4, is nearly two orders of magnitude higher than those of bis(3-aminopropyl)-terminated PDMS and methyl propionate-terminated PDMS, because interchain hydrogen bonds between acylhydrazine end groups can extend the polymer chain and then enhance the viscosity (structural formula in Fig. 5d). Finally, it is worth noting that double 17

ACCEPTED MANUSCRIPT hydrogen bonds can also be formed between the acylhydrazone groups and the acylhydrazine groups (green-colored bonds in Fig. 6a & Fig. 6b). Three kinds of H-bonds which can play the role of crosslinks in this system were summarized in Fig.

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6a and 6b.

Fig. 6. Schematic illustrations of network structure (a) Three kinds of hydrogen

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bonding interactions in the system; (b) interchain hydrogen bonds between acylhydrazine groups in the non-equimolar sample A1B0.5; (c) interchain hydrogen

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bonds in the equimolar sample A1B1. Therefore, the gelation of the A1B1 solution and the self-healing properties of its

elastomer can be explained by the synergic effect of the acylhydrazone bond and the hydrogen bonds. First, the acylhydrazone-forming reaction of two acylhydrazine end groups in A4 with B2 promoted the chain extension. Since the other two unreacted acylhydrazine end groups in A4 were located near each other, another B2 molecule was more likely to react with them to form an intra-chain ring structure (Fig. 6c) 18

ACCEPTED MANUSCRIPT rather than an inter-chain crosslinked structure. On the other hand, inter-chain hydrogen bonds formed between acylhydrazone bonds served as cross-linking points (Fig. 6c), which is similar to inter-chain hydrogen bonds in the non-equimolar sample

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A1B0.5 (Fig. 6b). In contrast, excess B2 in the A1B2 sample acted as capping agents and the chain extension was then hindered, and thus, the system could not be gelled by inter-chain hydrogen bonds. Since these hydrogen bonds dissociated near 80 °C,

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the A1B1 and A1B0.5 elastomers presented a reversible transition here (Figs. 5a to

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5c), and the thermal repair behavior could be observed above this transition (Figs. 3b & 3c). In other words, the dissociation/reformation of H-bonds (Figs. 6b and 6c) occurred around 80 °C dominates the heat assisted self-healing capabilities. Furthermore, the magnitude of two plateau moduli before and after this transition

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at 80 °C can be estimated. There are two networks in these elastomers: the inter-chain hydrogen bond network (Mx) and the PDMS entanglement network (Me), which is

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trapped by the H-bond network [36,37]. At ambient temperature, the plateau modulus was contributed by these two parts, with the sum as follows:

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 1 1  G ≅ Gx + Ge ≈ ρ RT  +   M x Me 

(1)

The Mx value (~3000 g/mol) can be calculated from eq 1 using the plateau

modulus (0.98 MPa) obtained from the G’ value where tan δ has a minimum [38]. Therefore, the calculated value of Mx is very close to the molecular weight of A4 (3040 g/mol), indicating that there was a relatively perfect inter-chain crosslinked network in the low temperature region and that most of the building blocks were 19

ACCEPTED MANUSCRIPT involved in the formation of the network. Above 80 °C, the H-bond network was destroyed, and the plateau modulus thus mainly came from the entanglement network (Ge) of the PDMS long chains. Indeed, both Figs. 5a and 5b show that the platform

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modulus obtained by tan δ minimum method was approximately 0.18 MPa, which is consistent with the entangled plateau modulus of PDMS (0.2 MPa) reported in the literature [39]. Note that the Me of PDMS was about 12000 g/mol, which was much

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larger than the MW of A4, thus the plateau above 80 °C was ascribed to the chain

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entanglements of the chain-extension PDMS by acylhydrazone-forming reaction, which is similar to results reported by Gent and coworkers [40]. Conclusively, H-bonds mechanism can quantitatively explain the transition at 80 °C and the self-healing property of PDMS elastomers.

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To further verify the hydrogen bonding mechanism, variable-temperature FTIR was carried out with the A1B1 elastomer. It has been reported that the C=O stretching

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and N-H in-plane bending would move toward higher and lower wavenumbers, respectively, as the hydrogen bonding weakened [41,42]. As shown in Fig. 7a, the

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stretching vibration peak of C=O at 1670 cm-1 gradually shifted to 1674 cm-1 with the temperature increasing from 30 °C to 120 °C, whereas the in-plane bending vibration peak of N-H shifted from 1558 cm-1 to 1549 cm-1. The changes of these infrared peaks were reversible (Fig. 7b). These results indicated that the reversible transition at 80 °C corresponded to the dissociation and reformation of hydrogen bonds.

20

1670

1549 120°°C 80°°C

1558

1750

(b)

heating

1700

1650

1600

1550

1674

cooling

1670

1549 120°°C 80°°C

1558

30°°C

1500

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1700

30°°C

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absorbance(a.u.)

(a) absorbance(a.u.)

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1650

1600

1550

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1500

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Fig. 7. FTIR spectra of the A1B1 elastomer upon (a) heating and (b) cooling.

Schubert and coworkers reported the self-healing behavior of methacrylate

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copolymers crosslinked by acylhydrazone groups [27]. The repair of scratches was attributed to the exchange reaction between acylhydrazone bonds. However, in their paper, the pure triethyleneglycol methylether methacrylate system displayed no

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healing abilities even though its acylhydrazone content was 5 mol%, and only polymers or copolymers containing 2-hydroxyethyl methacrylate (HEMA) displayed healing abilities at high temperatures (from 100 °C to 150 °C, 24 h or longer). The

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hydroxyl groups (-OH) existing in every HEMA monomer unit can easily form

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hydrogen bonds with C=O groups in ester bonds or acylhydrazone bonds, but the role of hydrogen bonds on the self-healing properties was ignored. Moreover, their system not only presented an extremely wide glass transition

region of 0-90 °C (Fig. 1 of ref. 27) but also exhibited a strongly annealing-dependent behavior. The Tg of the annealed sample increased to 70 °C from 48 °C (the original sample) after annealing for 7 days at 100 °C. The authors presumed that the rearrangement of the polymer network resulted from the (re)formation of hydrogen 21

ACCEPTED MANUSCRIPT bonds. Therefore, the reversible dissociation/reformation of hydrogen bonds may dominate the thermal repair process of their system, which is similar to the findings in

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our study.

4. Summary

successfully

prepared

from

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In summary, a self-healing PDMS gel based on acylhydrazone groups was tetra-acylhydrazine-terminated

PDMS

and

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terephthalaldehyde. The PDMS elastomer was further fabricated through solution casting. The elastomer had excellent acid- and heat-assisted self-healing properties. The rheological study showed that the system presented a reversible transition near 80 °C. Through comparative study of the gelation process of the non-equimolar

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system and the temperature response of the elastomer, we tend to believe that this reversible transition corresponds to the dissociation and reconstruction of hydrogen

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bonds between acylhydrazone groups. Combined with variable-temperature FTIR results, the contribution of hydrogen bonding interactions in an acylhydrazone

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self-healing system was elucidated for the first time. This work laid a solid foundation for introducing acylhydrazone bonds into other elastomers, such as polybutadiene and polyisoprene.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://

22

ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the NSFC (Grant Nos. 21374127 and 21674122), the

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National Basic Research Program of China (973 Program, No. 2014CB643601).

References

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Applications, in: S.K. Ghosh (Ed), Self-Healing Materials: Fundamentals, Design

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Strategies, and Applications, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim,

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[3] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, S. Viswanathan, Autonomic Healing of Polymer Composites, Nature 409 (2001) 794-797.

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[4] P. Cordier, F. Tournilhac, C. Soulié-Ziakovic, L. Leibler, Self-Healing and

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[10] G. Deng, C. Tang, F. Li, H. Jiang, Y. Chen, Covalent Cross-Linked Polymer Gels with Reversible Sol-Gel Transition and Self-Healing Properties, Macromolecules 43 (2010) 1191-1194.

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Dual-Responsive Sol-Gel Transitions, ACS Macro Lett. 1 (2012) 275-279. [12] F. Liu, F. Li, G. Deng, Y. Chen, B. Zhang, J. Zhang, C.Y. Liu, Rheological Images of Dynamic Covalent Polymer Networks and Mechanisms behind Mechanical and Self-Healing Properties, Macromolecules 45 (2012) 1636-1645. [13] G. Deng, Q. Ma, H. Yu,; Y. Zhang, Z. Yan,; F. Liu, C.Y. Liu, H. Jiang, Y. Chen, Macroscopic Organohydrogel Hybrid from Rapid Adhesion between Dynamic 24

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Exchange of Disulfide Bonds, Chem. Mater. 26 (2014) 2038-2046. [15] A. Rekondo, R. Martin, A. Ruiz de Luzuriaga, G. Cabañero, H.J. Grande, I. Odriozola, Catalyst-free Room-Temperature Self-Healing Elastomers Based on

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Aromatic Disulfide Metathesis, Mater. Horiz. 1 (2014) 237-240.

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[16] S.Y. An, S.M. Noh, J.H. Nam, J.K. Oh, Dual Sulfide-Disulfide Crosslinked Networks with Rapid and Room Temperature Self-Healability, Macromol. Rapid Commun. 36 (2015) 1255-1260.

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ACCEPTED MANUSCRIPT Polym. Sci. 30 (2005) 814-831. [22] T. Ono, T. Nobori, J.M. Lehn, Dynamic Polymer Blends-Component Recombination between Neat Dynamic Covalent Polymers at Room Temperature,

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[26] S.R. White, J.S. Moore, N.R. Sottos, B.P. Krull, W.A. Santa Cruz, R.C.R.

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Gergely, Restoration of Large Damage Volumes in Polymers, Science 344 (2014) 620-623.

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ACCEPTED MANUSCRIPT Self-Assemble into Strong and Tough Self-Healing Materials, Chem. Commun. 50 (2014) 10868-10870. [29] N. Roy, E. Buhler, J.M. Lehn, The Tris-Urea Motif and Its Incorporation into

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Polydimethylsiloxane-Based Supramolecular Materials Presenting Self-Healing Features, Chem. Eur. J. 19 (2013) 8814-8820.

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Supramolecular Polymer Materials: Chain Extension of Telechelic Polymers Using a

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Reactive Hydrogen-bonding Synthon, Adv. Mater. 12 (2000) 874-878.

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[32] F. Herbst, K. Schröter, I. Gunkel, S. Gröger, T. Thurn-Albrecht, J. Balbach, W.H. Binder, Aggregation and Chain Dynamics in Supramolecular Polymers by Dynamic

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ACCEPTED MANUSCRIPT Crystals of N-Acylhydrazone Derivatives: Single-Crystal X-ray, Solid-State NMR, and Computational Studies, Cryst. Growth Des. 14(2014) 2263-2281. [36] S.K. Patel, S. Malone, C. Cohen, J.R. Gillmor, R.H. Colby, Elastic Modulus and

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Equilibrium Swelling of Poly(dimethylsiloxane) Networks, Macromolecules 25 (1992) 5241-5251.

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Networks Formed by Associating Telechelic Chains, Macromolecules 49 (2016)

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7510-7524.

[38] C.Y. Liu, J. He, E. van Ruymbeke, R. Keunings, C. Bailly, Evaluation of Different Methods for the Determination of the Plateau Modulus and the Entanglement Molecular Weight, Polymer 47 (2006) 4461-4479.

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[39] L.J. Fetters, D.J. Lohse, R.H. Colby, Chain Dimensions and Entanglement Spacings, in: J.E. Mark (Ed.), Physical Properties of Polymers Handbook, 2nd ed., Springer, New York, 2007, pp 447-454.

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[40] A.N. Gent, G.L. Liu, M. Mazurek, Experimental Study of Molecular

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Entanglement in Polymer Networks, J. Polym. Sci. B Polym. Phys. 32 (1994) 271-279.

[41] A. Zhang, L. Yang, Y. Lin, L. Yan, H. Lu, L. Wang, Self-Healing Supramolecular Elastomers

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Polydimethylsiloxanes: Synthesis and Characterization, J. Appl. Polym. Sci. 129 (2013) 2435-2442. [42] M. Aiba, T. Higashihara, M. Ashizawa, H. Otsuka, H. Matsumoto, Triggered 28

ACCEPTED MANUSCRIPT Structural Control of Dynamic Covalent Aromatic Polyamides: Effects of Thermal Reorganization Behavior in Solution and Solid States, Macromolecules 49 (2016)

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2153-2161.

29

ACCEPTED MANUSCRIPT Highlights Self-healing PDMS elastomers based on acylhydrazone groups were prepared.

●

A reversible transition was observed around 80 °C.

●

Role of H-bonds on self-healing properties was elucidated

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ACCEPTED MANUSCRIPT Supporting Information for

A Self-Healing PDMS Elastomer Based on Acylhydrazone Groups and the Role of Hydrogen Bonds

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Dong-Dong Zhang1,2, Ying-Bo Ruan1,2, Bao-Qing Zhang1,2, Xin Qiao1, Guohua Deng3,*, Yongming Chen4,*, and Chen-Yang Liu1,2,*

1

Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education,

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4

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Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100190, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

Department of Polymer and Materials Sciences, School of Chemistry and Chemical Engineering,

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Sun Yat-Sen University, NO. 135, Xingang Xi Road, Guangzhou 510275, China

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ACCEPTED MANUSCRIPT Table of Contents Experimental Section ...................................................................................................................... 3

1. Synthesis of methyl propionate-terminated PDMS ...................................................... 3 1.1 Degree of functionality of methyl propionate-terminated PDMS ......................... 3 2. Synthesis of tetra-acylhydrazine-terminated PDMS (A4) .............................................. 3

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2.1 Degree of functionality of tetra-acylhydrazine-terminated PDMS (A4) ................. 4 3. Characterization ............................................................................................................ 4 3.1 Nuclear magnetic resonance (NMR) ..................................................................... 4 3.2 Differential scanning calorimetry (DSC) ................................................................ 4

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3.3 Wide-angle X-ray diffraction (WAXD) .................................................................... 4 Figures.............................................................................................................................................. 5

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Supporting References..................................................................................................................13

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Experimental Section 1. Synthesis of methyl propionate-terminated PDMS A modified procedure published in the literature [1] was adopted. A typical

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procedure was as follows: H2N-PDMS-NH2 (Mn=2500 g/mol) (12.3 g, 4.92 mmol) and 75 mL methanol were added in a 150 mL flask. The mixture was deoxygenated by bubbling a steady stream of N2 under stirring for no less than 30 min. Then methyl acrylate (5.7 mL, 63.3 mmol) was added to the reaction flask. After stirring at room

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temperature in the dark for 48 h, all the volatile chemicals were removed under reduced pressure to obtain the colorless oily liquid product ( 13.3 g, yield: 93%). lH 13

respectively.

C NMR were shown as Fig. S1 and Fig. S2,

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NMR and the corresponding

1.1 Degree of functionality of methyl propionate-terminated PDMS By comparing the peak area of h with that of b or c in Fig. S1, the degree of

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terminal methyl propionate functionality was nearly 100%. In 13C NMR (Fig. S2), no other peaks were observed. These results proved that methyl propionate functionality at PDMS ends was close to 100%.

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2. Synthesis of tetra-acylhydrazine-terminated PDMS (A4) Tetra-acylhydrazine-terminated PDMS was synthesized by modifying our

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previously reported procedure [2]. A general procedure: Methyl propionate-terminated PDMS (12.7 g, 4.47 mmol) and 35 mL hydrazine hydrate (80% in water, w/w) were added in a 100 mL flask. The mixture was deoxygenated by bubbling a steady stream of N2 under stirring for no less than 30 min. After stirring at 80 °C for 24 h, the polymer solution was concentrated and diluted with chloroform, washed with 1:1 water: brine and brine. The organic layer was then dried over anhydrous Na2SO4 before the solvent was removed under reduced pressure to give the viscous product A4 (7.4 g, yield: 74%). lH NMR and the corresponding 13C NMR were shown as Fig. S3 and Fig. S4, respectively. After reaction, the methyl peak (h) of methyl s3

ACCEPTED MANUSCRIPT propionate-terminated PDMS was completely disappeared. 2.1 Degree of functionality of tetra-acylhydrazine-terminated PDMS (A4) By comparing the peak area of i with that of b or c in Fig. S3, the degree of terminal acylhydrazine functionality in A4 was nearly 89%. In

3. Characterization

H NMR and

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3.1 Nuclear magnetic resonance (NMR) 1

C NMR of A4 (Fig.

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S4), no other peaks were observed.

13

C NMR spectra were recorded on a Bruker Avance III 400 HD

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instrument at room temperature with CDCl3 as a solvent and an internal standard. Chemical shifts were reported in parts per million (ppm, δ scale) relative to the residual signal of the solvent.

3.2 Differential scanning calorimetry (DSC)

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DSC (TA Q2000 Instruments) was used to study the crystallization behavior of the A1B1 elastomer. The sample was first heated at 150 °C for 2 min to eliminate thermal history under nitrogen atmosphere, followed by cooling to -60 °C at a rate of

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50 °C/min and held for 2 min at this temperature. After that, the sample was heated to 150 °C at 10 °C/min.

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3.3 Wide-angle X-ray diffraction (WAXD) WAXD measurement was carried out at room temperature on a round polymer

film (diameter: 8 mm, thickness: 0.7 mm) using a Rigaku D/MAX-RB diffractometer (40 kV, 100 mA) with CuKa radiation. The scanning range of the Bragg 2q angle varied from 1° to 50° at a scanning rate of 3°/ min.

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Figures

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Fig. S1. 1H NMR spectrum of methyl propionate-terminated PDMS in CDCl3.

Fig. S2. 13C NMR spectrum of methyl propionate-terminated PDMS in CDCl3.

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Fig. S3. 1H NMR spectrum of gelator A4 in CDCl3.

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Fig. S4. 13C NMR spectrum of gelator A4 in CDCl3.

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(a) 10

G',G"(Pa)

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Fig. S5. (a) Dynamic time sweep curves of A1B1 elastomer isothermal at 120 °C; (b) dynamic frequency sweep curves of the sample at 30 °C before and after isothermal at 120 °C for 1 h and another 1 h; (c) enlarged plot of Fig. S5a, which indicated that G’ continuously increase and approach the final equilibrium value with increasing annealing (or healing) time.

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(b) 2.0

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(OR in Fig. 3), (b) for HAc-assisted healing samples (HAc@48h in Fig. 3a), and (c) for heat-assisted healing samples (120 °C@2h in Fig. 3b and 3c). These curves demonstrated a good reproducibility.

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Intensity(a.u.)

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Fig. S8. (a) DSC (b) WAXD curves of A1B1 elastomer. Both showed no

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cooling heating G' G"

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Fig. S9. Cooling–heating cycles of A1B1 elastomer.

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A1B0.5

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Fig. S10. (a) Pictures of the gelation of PDMS polymer organogel with different number of functional groups: A1B1, A1B0.5 and A1B2; (b) storage modulus (G′) and loss modulus (G″) vs angular frequency (ω) of the A1B1 and A1B0.5 organogel

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Fig. S11. Acid- and heated-assisted self-healing study of A1B0.5 elastomer.

Supporting References

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[1] A.D. Meltzer, D.A. Tirrell, A.A. Jones, P.T. Inglefield, D.M. Hedstrand, D.A. Tomalia, Chain Dynamics in Poly(amido amine) Dendrimers. a Study of

13

C NMR

Relaxation Parameters, Macromolecules 25 (1992) 4541–4548. [2] G. Deng, C. Tang, F. Li, H. Jiang, Y. Chen, Covalent Cross-Linked Polymer Gels with Reversible Sol-Gel Transition and Self-Healing Properties, Macromolecules 43 (2010) 1191-1194.

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