Self-Healable and Reprocessable Polysulfide Sealants Prepared from

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Self-Healable and Reprocessable Polysulfide Sealants Prepared from Liquid Polysulfide Oligomer and Epoxy Resin Wentong Gao, Mengyao Bie, Fu Liu, Pengshan Chang, and Yiwu Quan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05285 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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ACS Applied Materials & Interfaces

Self-Healable and Reprocessable Polysulfide Sealants Prepared from Liquid Polysulfide Oligomer and Epoxy Resin Wentong Gao, Mengyao Bie, Fu Liu, Pengshan Chang, and Yiwu Quan* Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China KEYWORDS: Sealants; Self-healing; Reprocessing; Thiol-terminated polysulfide oligomer; Epoxy resin

ABSTRACT: Polysulfide sealants have been commercially applied in many industrial fields. This paper studied the self-healing property of the epoxy resin cured polysulfide sealants for the first time. The obtained sealants showed a flexible range of the ultimate elongation from 157 to 478% and tensile strength from 1.02 to 0.75 MPa corresponding to different polysulfide oligomers. By taking advantage of the dynamic reversible exchange of disulfide bonds, the polysulfide sealants exhibited good self-healing ability under a moderate thermal stimulus. Higher molecular weight and lower cross-linking degree polysulfide oligomer was helpful to improve the ultimate elongation and healing efficiency of the polysulfide sealants. After subjected to a temperature of 75 °C for 60 min, the tensile strength and ultimate elongation of a fully cut sample LP55-F were both restored to 91% of the original, without sacrificing the sealing property. Moreover, the sample also exhibited excellent reshaping and reprocessing

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abilities. These outcomes offer a paradigm towards the sustainable industrial applications of the polysulfide-based sealants. INTRODUCTION Thanks to the lower cost, excellent adhesion, good low-temperature property, low water-vapor permeability as well as high resistance to UV radiation and environmental degradation, polysulfide sealants have a wide range of applications in industry.1 Thiol-terminated polysulfide oligomers are most commonly used in the preparation of the polysulfide sealants due to their highly reactive terminal mercaptans, which can be easily converted into polymeric networks by oxidation or using other active groups, such as epoxy, isocyanate, and vinyl.2−4 To date, the curing kinetic analysis,5 mechanical properties,6 compression behavior,7 thermal stability,8 and photo-degradation process9 of the polysulfide sealants have been studied extensively. However, to the best of our knowledge, the self-healing and reprocessing abilities of the polysulfide sealants are not yet investigated. Recently, intrinsic self-healing materials have drawn widespread attention because of their built-in capacity of repairing physical damages caused by environmental and mechanical fatigue.10−12 There are two well-established strategies to develop intrinsic self-healing materials,13 including reversible dynamic covalent chemistry and noncovalent chemistry. In contrast to noncovalent interactions such as π−π stacking,14,15 hydrogen bonding,16−18 and metalion binding,19,20 the stronger nature of dynamic covalent chemistry provides the possibility to endow the systems with superior mechanical strength and stability, which has been widely adopted in the design of various smart materials.21−26 Diels−Alder/retro-Diels−Alder reactions,27−29 transesterification,30,31 acylhydrazone bonds,32,33 and disulfide bonds34−36 are typical examples of dynamic covalent chemistry for the self-healing materials. Among them,


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disulfide bonds have attracted much attention because they can undergo exchange reaction under a moderate temperature. Some pioneering researchers have utilized various disulfide-containing segments for preparing self-healing materials, such as bis(4-aminophenyl) disulfide in poly(ureaurethane)21,37

and

fiber-reinforced

composites,38

disulfide

bonds

in

thiol-terminated

poly(ethylene glycol)-based hydrogels,39 bis(2-methacryloyloxyethyl) disulfide in poly(n-butyl acrylate)-based polymers,40,41 disulfide and polysulfide bonds in vulcanized rubber,42−44 dual sulfide-disulfide in cross-linked p(2-ethylhexyl methacrylate-co-2-ydroxyethyl methacrylate),45 and poly(ethyl formal disulfide) in epoxidized polysulfide-based materials.46−48 Inspired by the above research, we expected that the polysulfide sealants should have selfhealing ability owing to abundant disulfide bonds in their networks. Herein, we prepared the sealants from the thiol-terminated polysulfide oligomers and epoxy resins, and discussed the effects of the molecular weight and cross-linking degree of polysulfide oligomers on the mechanical, thermal properties, and self-healing responses of the sealants in detail. Additionally, the reshaping and reprocessing abilities of the polysulfide sealants were also examined. After healing at 75 °C for 60 min, the obtained sample LP55-F achieved a healing efficiency of 91%. Even after three repeated reprocessing, the reprocessed sample still exhibited good mechanical properties similar to those of the original. Finally, we designed an aromatic terminated polysulfide oligomer as an additive (5 wt%) for LP55-F to improve its room-temperature selfhealing ability. The self-healing process of the polysulfide sealants via disulfide metathesis is speculated in Scheme 1.


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Scheme 1. Schematic Representation of Disulfide Metathesis in the Polysulfide Sealants.

EXPERIMENTAL SECTION Materials. Liquid thiol-terminated polysulfide oligomers with different molecular weights and cross-linking degrees were supplied by Thiokol Co. Japan (The main properties of the polysulfide oligomers are listed in Table 1). Bisphenol A epoxy resin (NPEL-128, with an epoxy equivalent weight 190 g/mol) and bisphenol F epoxy resin (NPEL-170, with an epoxy equivalent weight 170 g/mol) were supplied by Shenzhen Jiadida Chemical Co. China. 2((benzyloxy)methyl)oxirane was supplied by Aladdin Chemical. 2,4,6-Tris(dimethy-laminomethyl)phenol was commercially available material. All materials were used as received without further purification. Table 1. The Main Properties of Thiol-Terminated Polysulfide Oligomers. polysulfide oligomer

LP3

LP23

LP2

LP32

LP55

average molecular weight

1000

2500

4000

4000

4000

2%

2%

2%

0.5%

0.05%

5.9−7.7

2.5−3.5

1.5−2.0

1.5−2.0

1.5−2.0

cross-linking agenta mercaptan fraction (−SH)% a

In the synthesis of liquid polysulfide oligomers, bis-2-chloroethyl formal is the usually used monomer and 0.05−2% 1,2,3-trichloropropane is added as a cross-linking agent.2

Synthesis of the Polysulfide Sealants. The polysulfide sealants were prepared by using an equivalent molar ratio of the polysulfide oligomer and epoxy resin. This reaction was effectively catalyzed by 1 wt% 2,4,6-Tris(dimethyl-lamino-methyl)phenol. All precursors (the polysulfide


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oligomer, epoxy resin, and catalyst) were completely mixed by a mechanical stirrer at room temperature and degassed by a DAC 150FV high-speed mixer (FlackTec Co., USA) at 3000 r/min for 2 min. The bubble-free mixture was poured onto the PTFE mold and cured at room temperature for 12 h. Then the mold was put in an oven and the temperature was kept at 75 °C for 24 h. The compositions of the polysulfide sealants are listed in Table 2. Instrumentation and Characterization. Fourier transform infrared spectra were monitored using a Nexus 870 (Thermo Nico-let Co., USA) FTIR spectrophotometer over the range of 4000−400 cm-1. KBr discs were used as supports. Swelling experiments were performed by immersing the samples (20 mm × 20 mm, thickness 2 mm) in toluene at 25 °C until reaching the equilibrium of solvent absorption. Thereafter, excessive surface toluene was removed by blotting with a piece of filter paper and the swollen samples were weighted using an electronic digital balance with an accuracy of 0.1 mg. Then the samples were dried at 100 °C in the vacuum oven for 24 h. Each result was obtained by repeating the test with three samples. The swelling ratio is defined as Swelling ratio = Mt/M0 × 100%, where M0 and Mt are the mass of the test samples before swelling and after immersion, respectively. The gel fraction is defined as Gel fraction = Mw/M0 × 100%, where M0 and Mw are the mass of the test samples before swelling and after drying, respectively. Tensile properties were measured on an Instron 3366 Universal Materials Testing Machine (Instron Co., USA) with a speed of 50 mm/min at 23 °C. The cured film was cut into dumb-bellshaped samples (total length 50 mm, thickness 2 mm, width of parallel part 4 mm). The tensile strength and ultimate elongation were obtained from the stress−strain curves. Each result was obtained by repeating the test with at least three samples. The hardness of the samples was


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measured using a Frank hardness tester with a Shore A durometer (Shanghai Microcre LightMachine Tech Co., China). Differential scanning calorimetry (DSC) tests were performed on a DSC apparatus of MettlerToledo (Mettler-Toledo GmbH, Germany) at a heating rate of 20 °C/min from -75 to 150 °C under a nitrogen atmosphere. The glass transition temperature (Tg) values were deduced from the inflection point of the DSC curves on the second heating run. Thermogravimetric analysis (TGA) was performed on PerkinElmer TA 2100-SDT 2960 (Perkin Elmer Instruments, USA). The samples were heated at a rate of 10 °C/min from 50 to 500 °C under an air atmosphere. Compression stress relaxation tests were performed on an Instron 3366 Universal Materials Testing Machine (Instron Co., USA). The compression percentage of the cylindrical shape samples (diameter 28 mm, thickness 12 mm) was fixed to 25% of the original thickness with a displaced control at the crosshead rate of 10 mm/min at 75 °C. The compression stress data were recorded until the stress retention ratio was less than 5%. The stress retention ratio is defined as Stress retention ratio = Ft/F0 × 100%, where F0 is the stress when the strain just reaches 25% and Ft is the stress at time t. Steady-state rheological relaxation tests were performed on a HAAKE Rheo-Stress 600 (Thermo Electron Co., Germany) rheometer instrument with 35 mm parallel-plate geometry. The cured film was cut into disk-shaped samples (diameter 30 mm, thickness 2 mm). The experiments were performed by applying a deformation step of 1% strain and recording the shear stress evolution. All of the rheological data were analyzed by IRIS software. For self-healing tests, the sample was cut into two pieces using a clean blade, and then the two pieces were put back together to allow healing by heating or UV irradiating. The UV light was


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generated by a 300 W CEL-HXF300 Xenon lamp (Beijing Aulight Co., China) at an intensity of 10 mW/cm2 and a wavelength of 350 nm. The optical microscope images were recorded using an MT 1100T metallurgical optical microscope (Meiji Techno Co., Japan). The sealing experiments were performed according to ASTM E96. The open cups containing anhydrous calcium chloride were sealed with the samples (thickness 2 mm) and placed into a controlled environment at 90 ± 2% relative humidity under 25 °C. The sealed cups were weighted using an electronic digital balance with an accuracy of 0.1 mg and each result was obtained by three repeated tests. The water absorption is defined as Water absorption = Mn/M0 × 100%, where M0 is the mass of the anhydrous calcium chloride before test and Mn is the mass change of the anhydrous calcium chloride for different times. Reshaping and reprocessing experiments were carried out in a hot pressing at 75 °C and 5 MPa for 20 min. The samples in the form of a cylinder or chopped pieces were placed in a 2 mm thick mold and compressed in the hot pressing. RESULTS AND DISCUSSION Synthesis. The preparation of the polysulfide sealants is presented in Figure 1. Equal molar polysulfide oligomer and epoxy resin were completely mixed and the reaction was effectively catalyzed by the tertiary amine. Although this step-growth polymerization usually gives a linear polymer, the cross-linked sealants were finally obtained due to the presence of some branched chains in polysulfide oligomers, which were produced by adding 0.05−2% 1,2,3-trichloropropane as a cross-linking agent during the synthesis of polysulfide oligomers.2


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Figure 1. Schematic representation of the reaction between the polysulfide oligomer and epoxy resin.

The completion of the curing process was verified by infrared spectroscopy. After the reaction, the peak of S−H stretching at 2563 cm−1 completely disappeared as shown in Figure 2a. The swelling ratio and gel fraction were determined to characterize the relative cross-linking degree of the polysulfide sealants. After immersing in toluene at 25 °C for 120 h, the weight of test samples did not increase any more to reach the equilibrium of solvent absorption. The swelling ratio and gel fraction values are outlined in Table 2. With the same cross-linking degree of polysulfide oligomers, the swelling ratios of LP3-F, LP23-F, and LP2-F increase significantly from 38.1, 59.6 to 75.9%, but the gel fractions are almost unaffected. Moreover, as for the samples based on the same molecular weight of polysulfide oligomers, the swelling ratios further increase with the decrease of cross-linking degree. All of the swollen samples after drying still remain over 80% of the initial weight, indicating a nearly complete reaction of epoxy groups with thiols.


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Table 2. The Compositions and Swelling, Mechanical Properties of the Polysulfide Sealants. compositions

mechanical properties

sample

polysulfide (g)

epoxy (g)

swelling ratio (%)

LP3-F

100

34b

38.1

93.6

1.02

157

48

LP23-F

100

13.6b

59.6

91.4

0.90

214

45

LP2-F

100

8.5b

75.9

92.1

0.87

225

43

LP32-F

100

8.5b

93.8

87.7

0.84

299

42

LP55-F

100

8.5b

106.3

84.4

0.75

478

38

LP55-A

100

9.5c

103.5

85.1

0.77

423

39

b

gel fraction (%)

tensile strength (MPa)

ultimate elongation (%)

hardness (Shore A)

Bisphenol F epoxy resin. cBisphenol A epoxy resin.

Figure 2. (a) FTIR spectra of the mixture of the polysulfide oligomer and epoxy resin before and after curing. (b) Tensile stress−strain curves of the polysulfide sealants.

Mechanical and Thermal Properties. The tensile stress−strain curves of the polysulfide sealants are shown in Figure 2b and the results are summarized in Table 2. The sealants exhibit a flexible range of ultimate elongation from 157 to 478% meanwhile maintaining a reasonable tensile strength of over 0.7 MPa, which is comparable to that of metal oxide cured polysulfide


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polymers. As presented in Figure 2b, the ultimate elongation increases from 157, 214 to 225% and the tensile strength decreases from 1.02, 0.90 to 0.87 MPa in the order of LP3-F, LP23-F, and LP2-F. This phenomenon can be explained by the fact that higher molecular weight polysulfide oligomer means a longer flexible chain and fewer rigid bisphenol-coupling units in the sealants. In addition, LP2-F, LP32-F, and LP55-F, based on the different cross-linking degree of polysulfide oligomers, show an increase in the ultimate elongation from 225, 299 to 478%, but a slight change in tensile strength. Compared with LP55-F, LP55-A exhibits a decrease in elongation at break resulting from the use of more rigid bisphenol A epoxy resin. As for the self-healing materials, the healing time is closely related to the Tg of the materials, and a lower Tg is favorable to ensure the macroscopic mobility of the chains for achieving a selfhealing response. The Tg of the polysulfide sealants were measured according to DSC tests and there are no signs of residual exothermal effect in the first heating run from room temperature to 150 °C, demonstrating complete reaction in the produced sealants. The DSC curves obtained from the second heating run are shown in Figure 3a. By comparing LP3-F, LP23-F, and LP2-F, it can be observed that the Tg exhibits a gradual decrease from -34.9, -42.5 to -45.4 °C. The reason lies in that there are fewer bisphenol epoxy units dispersing in the soft-segment matrixes of the higher molecular weight polysulfide oligomer based sample. It is interesting to note that the cross-linking degree of the polysulfide oligomer only has a weak effect on the Tg of the corresponding sample, with a slight decrease of Tg in the order of LP2-F, LP32-F, and LP55-F. Besides, introducing the more rigid bisphenol A units (LP55-A) increases the Tg for 5.3 °C in comparison with that of LP55-F. Since the healing progress is promoted by a thermal stimulus, we checked the thermal stability of the sealants by TGA experiments in an air atmosphere. As observed from the curves in Figure


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3b, all samples exhibit a similar thermal decomposition behavior and the decomposition temperatures corresponding to 5% weight loss are around 280 °C despite different dosages of the epoxy resins employed. The results verify that the thermal stability of bisphenol structure is superior to that of polysulfide unit. This can be attributed to the presence of the S−S and C−S bonds in polysulfide unit, which are obviously less stable than C−O or C−C bonds at high temperatures as indicated by their dissociation enthalpies.49

Figure 3. (a) DSC curves of the polysulfide sealants. (b) TGA curves of the polysulfide sealants.

Due to the exchangeable dynamic bonds, the self-healing materials have an evident stress relaxation behavior, which can be used to evaluate the self-healing ability in a certain degree.42,43,50 Therefore, we are interested in exploring the compression stress relaxation behaviors of the polysulfide sealants. The tests are terminated once the compression retention ratio drops to below 5%. After the compression tests, all of the samples produced a permanent compression deformation. The terminated time of LP3-F, LP23-F, and LP2-F are 442, 320, and 247 s at 75 °C, respectively, as shown in Figure 4a. By comparing LP2-F, LP32-F, and LP55-F, it can be seen that LP55-F takes the shortest time to relax stress. This is indicative of a faster


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internal creep of chains and exchange of disulfide bonds in the samples based on a higher molecular weight and lower cross-linking degree polysulfide oligomer.

Figure 4. (a) Compression stress relaxation curves of the polysulfide sealants at 75 °C. (b) Steady-state rheological relaxation curves of the polysulfide sealants at 75 °C.

The rheological behaviors of the samples were measured by steady-state rheological relaxation tests. As depicted in Figure 4b, the shear modulus of all samples exhibits a prominent decrease over the relaxation time range at 75 °C, indicating that under these experimental conditions, these sealants behave more like a viscoelastic liquid than a viscoelastic solid.51 The time for shear modulus to drop to 1000 Pa displays a gradual decrease from above 1000 s of LP3-F, 520 s of LP2-F, 356 s of LP55-A to 157 s of LP55-F. As expected, LP55-F shows a more quickly descent process, revealing that the sample is more likely to become viscous and thus potentially facilitates the self-healing.52 On the basis of the above investigations, it can be inferred that LP55-F, with the lowest Tg and fastest stress relaxation, is expected to have the best self-healing ability. Self-Healing Properties. The comparison of mechanical properties before and after healing can be used to quantitatively evaluate the self-healing efficiency.46,53 The healing efficiency for


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each sample was calculated according to the following equation: Healing efficiency = Et/E0 × 100%, where E0 is the ultimate elongation or tensile strength of original samples and Et is the ultimate elongation or tensile strength of healed samples at time t. To begin with, we studied the effect of the stimulating temperature on the healing process. After healing at 75 °C for 60 min or at 50 °C for 240 min, the healing efficiencies of a fully cut sample LP55-F all reached 90%, which is generally considered a good value for an intrinsic self-healing material.48 However, even after healing at 25 °C for 24 h, the ultimate elongation and tensile strength of LP55-F were only restored to 49 and 73% of the original, respectively. Then, with the temperature fixed at 75 °C, we further investigated the effects of molecular weight and cross-linking degree of polysulfide oligomers on the healing efficiency of the polysulfide sealants, and the results are shown in Figure 5. As can be seen from Figure 5a, the time for healing efficiencies exceeding 90% (recovery time) are 240 and 180 min for LP23-F and LP2-F, respectively. However, the healing efficiency of LP3-F only reaches 80% even after healing for 480 min. Figure 5b shows that the recovery time decreases from 240 min of LP55-A, 180 min of LP2-F, 120 min of LP32F to 60 min of LP55-F. The similar trends of recovery time deduced from the tensile strength tests are observed from Figure 5c,d. On the basis of the above results, it can be concluded that higher molecular weight and lower cross-linking degree polysulfide oligomer, as well as flexible epoxy resin, is favored to improve the self-healing ability of the polysulfide sealants. These results are in good agreement with the compression stress relaxation and rheological behaviors of the samples.


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Figure 5. Healing efficiencies of the polysulfide sealants deduced from the ultimate elongation (a and b) and tensile strength (c and d) at 75 °C for different times.

To further observe the self-healing behavior of the polysulfide sealants, we recorded the optical images of LP55-F during the healing process. First, LP55-F was cut into two pieces using a clean blade. Then the two pieces were put back together to allow healing at 75 °C. The optical images after healing for 0, 30, 60, and 120 min are shown in Figure 6a. The initial cut around 75 µm on the surface of the sample was clearly observed, whereas the cut remained only a slight trace after healing at 75 °C for 60 min. With continued thermal stimulus for an extra 60 min, the cut healed completely, producing a homogeneous sample without any scars. In order to show a more intuitive recovery result, one of the two pieces was stained with black dye to make the cut


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region more distinguishable. After healing at 75 °C for 60 min, the healed sample can sustain a large strain once again and the stretching behavior is shown in Figure 6c.

Figure 6. (a) Optical microscope images of LP55-F after healing at 75 °C for different times. (b) Optical images of LP-55-F before and after healing at 75 °C. (c) The healed film before and after stretching.

Disulfide exchange reaction can also be triggered by UV irradiation, which has been applied in the reversible adhesion and surface functionalization.54,55 Herein, we further examined the effect of UV irradiating stimulus on the self-healing behavior of the polysulfide sealant. Figure 7 presents the optical images and mechanical properties of LP55-F after healing under UV light for different times. As shown in Figure 7, UV irradiation can indeed effectively promote the selfhealing process of LP55-F. After exposure to UV light for 30 min, the cut area significantly decreased and the sample maintained a tensile strength of 0.55 MPa, about 73% of the original strength. Under UV irradiation for 120 min, the cut on the surface almost disappeared, despite a negligible trace left. Meanwhile, the tensile strength was restored to 0.59 MPa, which is relatively lower than that of the sample after thermal healing at 75 °C for 120 min (see Figure 7b). This may be explained that the thermal stimulus results in a more uniform healing effect for damages inside the sample than the UV irradiation.


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Figure 7. (a) Optical microscope images of LP55-F after healing under UV light for different times. (b) Tensile stress−strain curves of LP55-F after healing at 75 °C or under UV light for different times.

Sealing Property. The sealing performance is an essential evaluation index for sealants and the sealing property of the obtained polysulfide sealants was evaluated according to ASTM E96. First, the open cups containing anhydrous calcium chloride were sealed with the glass, original, cut, or healed samples of LP55-F, respectively (Figure 8a). Then the sealed cups were placed into the controlled environment (90 ± 2% relative humidity and 25 °C) to examine the water absorptions (Figure 8b), and the results are summarized in Table 3. It can be found that the cup sealed with the original sample maintained a low water absorption of 0.87% after 120 h, which is comparable to 0.62% of the glass sealed cup (control). By contrast, the water absorptions of the cups sealed with cut samples (cut-1 and cut-2) increased dramatically, reaching 13.4 and 15.4% after 120 h, respectively. However, the water absorptions of healed-1 and healed-2 both dropped to around 1.0% after 120 h, which is similar to that of the original. Clearly, the healed samples showed a recovery of their sealing integrity.


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Figure 8. (a) The test cups sealed with the glass, original, cut, or healed samples of LP55-F. (b) Water absorptions of the test cups.

Table 3. Water Absorptions of the Test Cups. water absorptions (%) test time (h)

original

cut-1

cut-2

healed-1

healed-2

control

24

0.17

0.91

2.74

0.18

0.19

0.13

48

0.34

2.88

5.37

0.37

0.40

0.23

72

0.51

5.83

8.48

0.61

0.61

0.38

96

0.66

9.58

12.0

0.82

0.80

0.50

120

0.87

13.4

15.4

1.01

0.98

0.62

Reshaping and Reprocessing Abilities. The reprocessing ability of materials is vital to the sustainable industrial applications. Herein, we selected LP55-F as the example to investigate the reshaping and reprocessing abilities of the polysulfide sealants. A cylindrical sample was placed in a 2 mm thick mold. After compressed in a hot pressing at 75 °C and 5 MPa for 20 min, the sample was finally reshaped into a homogeneous film as shown in Figure 9a. The reshaped film has an ultimate elongation of 458% and tensile strength of 0.75 MPa, maintaining the similar mechanical properties to those of the film produced from the starting liquid components.


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Figure 9. (a) Reshaping of LP55-F from a cylinder to a homogeneous film at 75 °C and 5 MPa for 20 min. (b) Reprocessing of the small pieces through compression molding at 75 °C and 5 MPa for 20 min for three repeated events. (c) Tensile stress−strain curves of the reprocessed films.

In order to verify the reprocessing ability of the polysulfide sealants, the original film was cut into small pieces and intentionally exposed to air at ambient temperature for 4 h. Then the small pieces were subjected to a hot pressing at 75 °C and 5 MPa for 20 min, resulting in a homogeneous film. Subsequently, a second and third repeat were performed and similar films were obtained (see Figure 9b). The ultimate elongation/tensile strength for the first, second, and third reprocessing films are 430%/0.75 MPa, 443%/0.72 MPa, and 436%/0.68 MPa, which are all comparable to 478%/0.75 MPa of the original film. This indicates that the reprocessing of


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these sealants is not sensitive to air exposure and the polysulfide sealants exhibit the potential for recyclability. Room-Temperature Self-Healing Ability. Generally, the room-temperature self-healable materials are highly desired for practical applications.21,45 The healing efficiencies of LP55-F deduced from the ultimate elongation were measured to be 42, 49, and 55% after healing at 25 °C for 12, 24, and 48 h, respectively. In order to improve the room-temperature healing ability of LP55-F, we synthesized an aromatic terminated polysulfide oligomer (see Supporting Information for details) as an additive (5 wt%) for LP55-F in an attempt to enhance the molecular mobility, thereby increasing the disulfide exchange probability in the networks.47 Figure 10 shows the schematic representation of the disulfide exchange reaction in LP55-F and the modified LP55-F networks. In this case, the disulfide exchange reaction in the modified LP55-F can occur not only among the main chains, but also between the main chain and the aromatic terminated polysulfide oligomer.

Figure 10. Schematic representation of the disulfide exchange reaction in LP55-F and the modified LP55-F networks.


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The viscoelastic properties of LP55-F before and after modification were compared based on steady-state rheological relaxation tests at 25 °C, as shown in Figure 11. It was found that LP55-F needed about 6850 s to relax until 20% of the initial shear modulus value. However, the modified LP55-F took only about 640 s for the same relaxation process. This result indicated a more viscous network in the modified LP55-F, thus implying an enhancement of the mobility of chains and a better self-healing ability at room temperature. Thereafter, we measured the mechanical properties and healing efficiencies of the samples at 25 °C for different times (Table 4). As expected, the addition of the aromatic terminated polysulfide oligomer effectively improved the healing ability of the sealant. The healing efficiencies of the modified LP55-F deduced from the ultimate elongation and tensile strength reached 86 and 96% after 24 h, respectively, which are about 1.8 and 1.3 times those values of LP55-F (49 and 73%).

Figure 11. Steady-state rheological relaxation curves of LP55-F and the modified LP55-F at 25 °C.

Table 4. Mechanical Properties and Healing Efficiencies of LP55-F and the Modified LP55F at 25 °C for Different Times. original mechanical properties

healing efficiencies (%) from the ultimate elongation

from the tensile strength

sample

tensile strength (MPa)

ultimate elongation (%)

12 h

24 h

48 h

12 h

24 h

48 h

LP55-F

0.75

478

42

49

55

67

73

80

modified LP55-F

0.70

498

70

86

95

89

96

99


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CONCLUSIONS In summary, we have studied the self-healing and reprocessing abilities of the polysulfide sealants. By taking advantage of the reversible exchange of disulfide bonds, the polysulfide sealants exhibited good self-healing ability under a moderate thermal stimulus. The higher molecular weight and lower cross-linking polysulfide oligomer, as well as flexible epoxy resin, is favorable to improve the self-healing ability of the polysulfide sealants. After healing at 75 °C for 60 min, the mechanical properties of LP55-F were restored to over 90%, without sacrificing the original sealing property. Moreover, LP55-F also exhibited excellent reprocessing ability, keeping almost unchanged mechanical properties even after three repeated reprocessing events. Finally, the room-temperature self-healing ability of LP55-F was significantly improved by the addition of an aromatic terminated polysulfide oligomer. Easy preparation from commercially available raw materials and rapid self-healing and recycling abilities make this kind of polysulfide sealants suitable for a wide range of sustainable industrial applications. ASSOCIATED CONTENT Supporting Information. Synthesis and FTIR spectra of the aromatic terminated polysulfide oligomer. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +86 25 89683289. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the Nanjing University Innovation and Creative Program for PhD candidate. REFERENCES (1) Ghatge, N. D.; Vernekar, S. P.; Lonikar, S. V. Polysulfide Sealants. Rubber Chem. Technol. 1981, 54, 197-210. (2) Usmani, A. M. Chemistry and Technology of Polysulfide Sealants. Polym-Plast. Technol. 1982, 19, 165-199. (3) Lowe, G. B. The Cure Chemistry of Polysulfides. Int. J. Adhes. Adhes. 1997, 17, 345-348. (4) Zhang, G. Z.; Fan, Z. K.; Quan, Y. W.; Chen, Q. M. The Preparation and Physical Properties of Polysulfide-Based Elastomers through One-Pot Thiol-Ene Click Reaction. Express Polym. Lett. 2013, 7, 577-584. (5) Zhou, B. J.; He, D.; Quan, Y. W.; Chen, Q. M. The Investigation on the Curing Process of Polysulfide Sealant by In Situ Dielectric Analysis. J. Appl. Polym. Sci. 2012, 126, 1725-1732. (6) Matsui, T.; Miwa, Y. Detection of a New Crosslinking and Properties of Liquid Polysulfide Polymer. J. Appl. Polym. Sci. 1999, 71, 59-66. (7) Lu, Y.; Shen, M. X.; Ding, X. D.; Quan, Y. W.; Chen, Q. M. Compression Set Property and Stress-Strain Behavior During Compression of Polysulfide Sealants. J. Appl. Polym. Sci. 2010, 115, 1718-1723. (8) Radhakrishnan, T. S.; Rao, M. R. Characterization of Cured Polysulfide Polymers by Thermal Degradation: Pyrolysis-GC and Thermogravimetric Studies. J. Appl. Polym. Sci. 1987, 34, 1985-1996. (9) Mahon, A.; Kemp, T. J.; Coates, R. J. Thermal and Photodegradation of Polysulfide PrePolymers: Products and Pathways. Polym. Degrad. Stabil. 1998, 62, 15-24.


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Table of Contents


28

Self-Healable and Reprocessable Polysulfide Sealants Prepared from

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