Benzocyclobutene resin with m-carborane cages and a siloxane backbone: a novel thermosetting material with high thermal stability and shape-memory property

Abstract

In this paper, we report the design, synthesis and properties of novel cross-linked benzocyclobutene resin with m-carborane cages and a siloxane backbone. The synthesis process of the m-carborane derivative containing the benzocyclobutene functional group was simple and efficient. ¹H, ¹³C, and ²⁹Si NMR confirmed the structure of the dibenzocyclobutene-functional m-carborane (DBMC). The novel polymer film was readily fabricated through ring-opening polymerization of the benzocyclobutene unit. Differential scanning calorimetry (DSC) and FT-IR were used to study the curing behavior. Thermogravimetric analysis (TGA) showed that the cured film had outstanding thermal stability. The cross-linked film also exhibited good shape-memory property, and had the capacity to address challenges in advanced aerospace. The boron in the cured DBMC was abundant and showed homogeneous distribution at the microscale. These data imply that the cured DBMC is suitable for use as a neutron shielding material, especially in aerospace applications such as neutron shielding garments.

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Introduction

Neutrons are subatomic particles with no net electric charge, and are commonly used in nuclear reactors for producing nuclear energy and radiation therapy for cancer.^1–3 Because of the uncharged nature of neutrons, they readily pass through most materials and interact with the nuclei of the target atoms. Exposure to neutron radiation can be particularly hazardous to bodily tissues because of the interaction of neutrons with the nuclei of the atoms in the body. Nuclear plant workers, radiation therapist and aircraft crew are most vulnerable to exposure of neutrons. Hence, there is a significant demand for effective shielding gears with superior physical properties.⁴ Due to lightweight shielding materials are preferred in mobile nuclear devices and manned spacecrafts, polymeric composites are particularly excellent candidates as shielding materials.^5–8

Neutron undergoes nuclear reaction with ¹⁰B atom to produce α particle, ⁷Li atom and γ ray, which opens door for the preparation of materials with neutron-trapping ability.^9,10 Neutron shielding polymeric composites are currently prepared by blending boron-rich particles such as boron carbide and boron nitride with hydrogen-rich polymers such as epoxy resin, polyethylene and polypropylene.^11–14 The aggregation of particles is a big challenge in the preparation of polymer-based materials, and boron-rich particles blended in the polymer matrix can easily be removed by washing.¹⁵ Besides, the thermal properties and flexibilities of these polymer matrices cannot meet the demands of aerospace shielding materials. On the other hand, polymers with shape-memory property are widely used to save space and minimize weight during rocket launching.^16,17 The neutron shielding material with shape-memory property is highly expected in space vehicles. Hence, the research and development of neutron shielding polymeric materials with homogeneous distribution of boron element and superior properties such as flexibility and shape-memory property is essential to the application of special fields such as aerospace.

Icosahedral carboranes (C₂B₁₀H₁₂) are polyhedral boron-cluster compounds including: o-carborane, m-carborane and p-carborane. These clusters are thermal stable and rich in boron content.^18–22 By chemical incorporation of icosahedral caboranes, carborane-containing polymers with neutron-trapping ability can be prepared. Carborane-containing polymers such as polysiloxanes and polyimides have been prepared as advanced materials with unique thermostabilities. These polymers with high boron contents (more than 10 wt%) show outstanding thermal stabilities.^15,23–30 However, the synthetic routes of carborane-containing polymers are relatively complicated, hence novel carborane-containing polymers through simple synthetic process with high yield are highly desirable. On the other hand, benzocyclobutene-based polymers have been recognized as high performance materials due to the good thermal stability, chemical resistance and low dielectric constants. In addition, the reaction of benzocyclobutene (BCB) is free of catalyst and initiator.^31–39 Thus, the combination of icosahedral carboranes and benzocyclobutene could obtain a new neutron shielding material with superior properties such as high thermal stability and shape-memory property. The soft segments highly contribute to the elastic mechanical properties of the shape memory polymers. Siloxane chain is an excellent candidate for the soft segments because of its high flexibility and excellent thermostability.⁴⁰

In this study, bis(hydroxydimethylsilyl)-m-carborane was prepared by chemical modification of m-carborane, and then reacted with chlorodimethylsilane in the presence of excessive triethylamine to obtain bis(hydriodimethylsilyl)-m-carborane (BHMC). 4-(1′,1′-Dimethyl-1′-vinyl)-silylbenzocyclobutene (4-DMVSBCB) was synthesized through the cross coupling reaction of benzocyclobutene functional Grignard reagent and vinyldimethylchlorosilane. Dibenzocyclobutene-functional m-carborane (DBMC) was obtained with high yield by the hydrosilylation reaction of 4-DMVSBCB and BHMC. The novel benzocyclobutene resin was prepared by thermal treatment of the DBMC. The mechanical and thermal properties of the cured DBMC are excellent, which is able to meet the requirements of neutron shielding materials. The cross-linked film also exhibited good shape-memory property, which can play an important role in saving space in the restricted space and has a high innovation potential in space vehicles such as space-efficient neutron shielding garments.

Experimental section

Materials

m-Carborane was purchased from KATCHEM. Acetone (99%), dichlorodimethylsilane (99%), chlorodimethylsilane (98%) and Karstedt catalyst were purchased from Sigma-Aldrich. N-Butyllithium (2.5 M in n-hexane) and mesitylene were purchased from Acros Organics. 4-Bromobenzocyclobutene (97%) was purchased from Chem-target Technologies Co. Ltd. Magnesium ribbon, iodine, diethyl ether, tetrahydrofuran and n-hexane were obtained from Nanjing Reagent Corporation, China. m-Carborane was dried under vacuum. Magnesium ribbon was washed with dilute hydrochloride acid and dried under nitrogen. Diethyl ether, acetone, mesitylene and tetrahydrofuran were dried over sodium and distilled under nitrogen.

Characterization

¹H, ¹³C and ²⁹Si NMR spectra were obtained at ambient temperature on Bruker DRX-400 spectrometer (¹H NMR, 400 MHz; ¹³C NMR, 100 MHz; ²⁹Si NMR 80 MHz) with tetramethylsilane (TMS) as the standard. Chemical shifts were reported relative to TMS (δ ¹H = 0.00 ppm, δ ¹³C = 0.00 ppm, δ ²⁹Si = 0.00 ppm). Differential scanning calorimetry (DSC) was measured on METTLER TOPEM TM DSC under nitrogen flow (50 ml min⁻¹). Thermogravimetric analysis (TGA) was performed on NETZSCH STA 449 F3 TG/DSC with a heating rate of 10 °C min⁻¹ under argon or air flow (50 ml min⁻¹). Dynamic mechanical analysis (DMA) was carried out on a Dynamic Mechanical Analyzer (METRAVIB DMA + 450, France). The viscoelastic properties were measured at a heating rate of 3 °C min⁻¹ from −50 °C to 125 °C, and at a frequency of 1 Hz. Ultraviolet-visible (UV-vis) spectra were recorded with a UV1800PC UV-vis spectrophotometer operating at 1 nm resolution. Scanning Electron Microscopy (SEM) was performed using a Hitachi S-4800. Accelerating the voltage to 10 kV with Au coating of the sample was used to image the fractured surface morphology.

Synthesis of compound 1

Compound 1 (see Scheme 1) was prepared according to the literature.⁴¹ A solution of 2.5 M n-BuLi in n-hexane (44.0 ml, 110 mmol) was dropwise added into a solution of m-carborane (7.20 g, 50 mmol) in 200 ml of dry diethyl ether at −20 °C under argon by using standard Schlenk techniques. The mixture was stirred for 3 h at room temperature. Then, 25 ml of anhydrous dichlorodimethylsilane (33.32 g, 258 mmol) was added all at once at −50 °C under argon, and the mixture was stirred for 3 h at 0 °C and 24 h at room temperature, respectively. Then, diethyl ether, n-hexane and excessive amounts of dichlorodimethylsilane were removed by vacuum evaporation. The remaining liquid with high boiling points was dissolved into 250 ml of dry acetone, and 25 ml of deionized water (25.00 g, 1388 mmol) was quickly added into the solution at 0 °C. The mixture was stirred for 3 h at 0 °C and 24 h at room temperature. The crude product was obtained after the removal of acetone and deionized water under vacuum conditions. The purified product was obtained through washing with n-hexane and vacuum drying at 45 °C (yield: 88%). ¹H NMR (DMSO; δ, ppm): 0.12 (Si–CH₃, 12H, s), 1.50–3.20 (B–H, 10H, br m), 6.47 (Si–OH, 2H, s). ¹³C NMR (DMSO; δ, ppm): −0.97 (Si–CH₃), 69.19 (C_Cb). ²⁹Si NMR (DMSO; δ, ppm): 3.25.

Scheme 1 Synthetic route of compound 1.

Synthesis of compound 2 (BHMC)

Compound 2 (see Scheme 2) was prepared by condensation reaction of compound 1 and chlorodimethylsilane with triethylamine as acid binding agent. Compound 1 (29.25 g, 100 mmol), chlorodimethylsilane (20.28 g, 210 mmol) and 250 ml of anhydrous diethyl ether were added into a flask under argon by using standard Schlenk techniques. The mixture was cooled to −10 °C with stirring, trimethylamine (21.25 g, 210 mmol) in 50 ml of anhydrous diethyl ether was added through a dropping funnel within 1 h, and the reaction mixture was stirred at room temperature for 12 h. Then, the mixture was filtrated through silica gel to remove the inorganic impurities and the solvent was removed by rotary evaporation. Finally, the residue was purified with reduced pressure distillation to obtain BHMC with a yield of 93% as a colorless viscous liquid. ¹H NMR (CDCl₃; δ, ppm): 0.18–0.20 (Si–CH₃, 12H, m), 1.50–3.20 (B–H, 10H, br m), 4.69 (Si–H, 2H, m). ¹³C NMR (CDCl₃; δ, ppm): 0.24 (Si–CH₃), 0.60 (Si–CH₃), 68.50 (C_Cb). ²⁹Si NMR (CDCl₃; δ, ppm): −4.35, 0.85.

Scheme 2 Synthetic route of BHMC.

Synthesis of compound 3 (4-DMVSBCB)

Compound 3 (see Scheme 3) was prepared according to the previous work.³⁸ Pieces of magnesium ribbon (0.96 g, 40 mmol), small amounts of iodine and a few drops of 4-bromobenzocyclobutene were added to a flask under argon by using standard Schlenk techniques. The color of liquid turned from brown to colorless after instant heating to 100 °C, which indicated the success of initiation. A solution of 4-bromobenzocyclobutene (6.40 g, 35 mmol) in 25 ml of tetrahydrofuran was dropwise added into the flask at room temperature, and the mixture was stirred for 10 h at 40 °C. The mixture was naturally cooled to room temperature, 5 ml of vinyldimethylchlorosilane (5.42 g, 38 mmol) was added into the front reaction mixture at 0 °C, and the mixture was stirred for 3 h at 0 °C and 24 h at 40 °C, respectively. Then, the mixture was filtrated through silica gel to remove the inorganic impurities. After removal of the solvents by rotary evaporation, the liquid residue was finally passed through silica gel column chromatography using n-hexane as an eluent to give 3 as a colorless liquid (yield: 80%). ¹H NMR (CDCl₃; δ, ppm): 0.33 (Si–CH₃, 12H, s), 3.16 (CH₂–CH₂, 4H, s), 5.71–6.06 (CH=CH₂, 4H, m), 6.24–6.32 (CH=CH₂, 2H, m), 7.05–7.38 (Ar-H, 3H, m). ¹³C NMR (CDCl₃; δ, ppm): 2.70 (Si–CH₃), 29.77 (Ar-CH₂), 29.89 (Ar-CH₂), 121.94 (Ar), 127.58 (Ar), 132.13 (CH=CH₂), 132.53 (CH=CH₂), 136.41 (Ar), 138.40 (Ar), 145.59 (Ar), 147.12 (Ar). ²⁹Si NMR (CDCl₃; δ, ppm): −10.52.

Scheme 3 Synthetic route of 4-DMVSBCB.

Synthesis of compound 4 (DBMC)

Compound 4 (see Scheme 4) was prepared by hydrosilylation of compound 2 and 3. Compound 3 (3.84 g, 20.4 mmol), compound 2 (4.08 g, 10 mmol) and 50 ml of anhydrous toluene and were charged to a flask under argon by using standard Schlenk techniques. 100 μl of Karstedt catalyst was added using a microsyringe at ambient temperature. After stirring at room temperature for 1 h, the system was heated to 90 °C over a period of 24 h to insure the hydrosilylation complete. After removal of the solvents by rotary evaporation, the liquid residue was finally passed through silica gel column chromatography using n-hexane as an eluent to give 5 as a colorless viscous liquid (yield: 90%). ¹H NMR (CDCl₃; δ, ppm): 0.18–0.27 (Si–CH₃, 36H, m), 0.43–0.49 (O–Si–CH₂–CH₂, 4H, m), 0.63–0.68 (O–Si–CH₂–CH₂, 4H, m), 1.50–3.15 (B–H, 10H, br m), 3.22 (Ar-CH₂, 4H, s), 7.08–7.39 (Ar-H, 3H, m). ¹³C NMR (CDCl₃; δ, ppm): 0.39 (Si–CH₃), 0.53 (Si–CH₃), 0.62 (Si–CH₃), 7.29 (O–Si–CH₂–CH₂), 10.18 (O–Si–CH₂–CH₂), 29.77 (Ar-CH₂), 29.89 (Ar-CH₂), 68.64 (C_Cb), 121.89 (Ar), 127.36 (Ar), 136.86 (Ar), 137.29 (Ar), 145.54 (Ar), 146.90 (Ar). ²⁹Si NMR (CDCl₃; δ, ppm): −1.20, −0.89, 11.10.

Scheme 4 Synthetic route of DBMC.

Synthesis of pre-crosslinked polymer

A mesitylene solution of compound 4 at a mass–volume concentration of 15% wt was added in a dry and clean reactor, and the mixture was heated for 10 h under the nitrogen atmosphere at 175 °C. A transparent liquid with relatively high viscosity was obtained after the solvent was evaporated under vacuum at 120 °C.

Formation of polymer film

After degassing in a vacuum oven at 120 °C for 0.5 h, the mold with 2.0 g of pre-crosslinked polymer was heated stepwise at 180 °C for 2 h, 240 °C for 2 h and 280 °C for 2 h, respectively. After cooling down to room temperature, the transparent film was obtained.

Results and discussion

Preparation and characterization of monomers

The condensation reaction between Si–Cl and Si–OH represents an efficient method for the formation of Si–O–Si linkage. Si–OH functional m-carborane was generated by hydrolysis of m-carborane with difunctional Si–Cl groups, which could be stored for a long time without spoiling. The condensation reaction was taken at low temperature, which was beneficial to the stability of Si–C_carborane bond in m-carborane. In this work, difunctional m-carborane with Si–OH groups (1) was synthesized from m-carborane followed by carbolithiation of m-carborane, nucleophilic substitution of chlorosilane, and hydrolysis of m-carborane with Si–Cl groups (Scheme 1). The structure of compound 1 was examined by ¹H, ¹³C and ²⁹Si NMR, and the detailed data are shown in the ESI (Fig. S1–S3†). The protons of B–H in m-carborane cage were presented at 1.5–3.2 ppm, and those of Si–OH in compound 1 were presented at 6.47 ppm. Moreover, as could be seen from ¹³C NMR spectrum, the peaks of the carbon atoms in compound 1 appeared at −0.97 and 69.19 ppm, respectively. The peak appearing at 69.19 ppm was ascribed to the carbon atoms in the m-carborane moiety of compound 1, and the peak at −0.97 ppm was attributed to the carbon atoms of Si–CH₃. The single peak located at 3.25 ppm appeared in the ²⁹Si-NMR spectrum of compound 1. Thus, all data confirmed the chemical structure of compound 1.

Compound 2 (BHMC) was synthesized through the reaction of chlorodimethylsilane and bis(hydroxydimethylsilyl)-m-carborane (Scheme 2). The structure of BHMC was confirmed by ¹H NMR, ¹³C NMR and ²⁹Si NMR spectroscopy (see the ESI†). In the ¹H NMR spectrum, the peaks at around 0.18–0.20 ppm were derived from Si–CH₃ groups. The signals of protons in m-carborane cage were located at 1.5–3.2 ppm. The singlet at 4.69 ppm was assigned to Si–H groups. In the ¹³C NMR spectrum, the peak at 68.50 ppm was attributed to the carbon atoms of the m-carborane, the peaks at 0.24 and 0.60 ppm were attributed to the carbon atoms attached to Si atoms. In the ²⁹Si NMR spectrum, the peaks around 0.85 ppm were attributed to elemental Si of Si–(CH₃)₂, and the signals at −4.35 ppm were derived from elemental Si of Si–H. All these characteristic NMR data indeed matched the expected BHMC structure.

Compound 3 (4-DMVSBCB) was synthesized through the cross-coupling reaction between benzocyclobutene-functional Grignard reagent and vinyldimethylchlorosilane (Scheme 3). The structure of 4-DMVSBCB was confirmed by ¹H NMR, ¹³C NMR and ²⁹Si NMR spectroscopy (see the ESI†). In the ¹H NMR spectrum, the signals located in 0.33 and 3.16 ppm were assigned protons in Si–CH₃ groups and cyclobutene groups, and the signals of vinyl groups and benzene rings were observed. In the ¹³C NMR spectrum, the peaks at 121.94, 127.58, 136.41, 138.40, 145.59 and 147.12 ppm were attributed to the carbon atoms of the benzene. The resonances of olefinic carbon atoms appeared at 132.13 and 132.53 ppm. The peaks at 29.77 and 29.89 ppm were assigned to the methylene carbons of the BCB ring. The singlet at 2.70 ppm was attributed to the –CH₃ attached to Si. In the ²⁹Si NMR spectrum, the singlet appeared at −10.52 ppm. All the NMR data was in good agreement with the chemical structure of 4-DMVSBCB.

Compound 4 (DBMC) was synthesized by the hydrosilylation reaction between 4-DMVSBCB and BHMC (Scheme 4). The structure of DBMC was confirmed ¹H NMR, ¹³C NMR and ²⁹Si NMR spectra. ¹H NMR spectrum of DBMC is shown in Fig. 1. In the ¹H NMR spectrum, DBMC showed the signals located between 0.18 and 0.27 ppm, which were attributed to the Si–CH₃ groups. A resonance assigned to the protons of cyclobutene groups was observed at 3.22 ppm. The signals of hydrogens in carborane cage were highly coupled, so a broad shoulder around 1.50–3.15 ppm was monitored. The shifts between 7.08 and 7.39 ppm were stemmed from the protons in the benzene ring. It should be pointed out that the resonances between 0.43 and 0.68 ppm were assignable to the protons of methylene groups resulting from β addition between 4-DMVSBCB and BHMC, which meant that the hydrosilylation was mainly in the fashion of β addition. It was consistent with the DEPT-135 NMR spectrum of DBMC (see the ESI†).

Fig. 1 ¹H NMR spectrum of DBMC.

The ¹³C NMR spectrum of DBMC is showed in Fig. 2. The signals at 0.39, 0.53 and 0.62 ppm were assigned to Si–CH₃ groups, and the cyclobutene groups were identified in 29.77 and 29.89 ppm. The signals at 121.89, 127.36, 136.86, 137.29, 145.54 and 146.90 ppm were ascribed to the benzene ring. The existence of methylene groups between Si atoms was proved by the signals at 7.29 and 10.18 ppm. The signal of resonance at 68.64 ppm was characteristic of the carbon nucleus in m-carborane cages. The existence of elemental Si was shown by signals at −1.20, −0.89 and 11.10 ppm in the ²⁹Si NMR spectrum (Fig. 3). All the NMR spectra proved that the DBMC obtained was consistent with the designed structure.

Fig. 2 ¹³C NMR spectrum of DBMC.

Fig. 3 ²⁹Si NMR spectrum of DBMC.

Curing behavior

Upon heating, the four-membered-ring of benzocyclobutene opens to produce a highly reactive o-quinodimethane intermediate. This active intermediate has a tendency to form a product with eight-membered-ring via self-coupling or produces a copolymer through Diels–Alder reaction. The curing behavior of DBMC was characterized by DSC, and the results are shown in Fig. 4. The curing onset temperature of DBMC was at about 180 °C, and the curing peak temperature located at 260 °C. The crosslinking was attributed to the ring opening of benzocyclobutene groups in DBMC, similar to the curing behaviors of the compounds containing benzocyclobutene (Scheme 5).³¹ According to the DSC result, the pre-crosslinked polymer was treated at 180 °C for 2 h, 240 °C for 2 h and 280 °C for 2 h to form film, respectively.

Fig. 4 DSC traces of DBMC.

Scheme 5 Process of curing reaction.

Fig. 5 shows the FT-IR spectra for DBMC and cured DBMC. The characteristic absorption for the deformation vibration of C–H in the strained four-membered ring of benzocyclobutene appeared at 1472 cm⁻¹. The strong peak at 1256 cm⁻¹ was attributable to the stretching vibration of Si–CH₃. The characteristic peak of BCB monomers at 1472 cm⁻¹ moved to 1495 cm⁻¹ after the polymerization, while the characteristic peak of Si–CH₃ at 1256 cm⁻¹ remained unchanged. The absorption peaks at 881 cm⁻¹ belonging to the in-plane ring stretching vibration of C–H in the four-membered ring of benzocyclobutene group disappeared after the curing reaction, suggesting that DBMC was fully crosslinked. The film of cured DBMC was transparent, implying that no macroscopic phase separation occurred. The fractured surface morphology of film was investigated by SEM, as shown in Fig. 6. The fractured surface of the film was smooth and had no pores, which meant that there was no production of gaseous by-products or volatilization of lower molecular species. The film of cured DBMC was homogeneous at the micro-scale.

Fig. 5 FT-IR spectra of DBMC and cured DBMC.

Fig. 6 SEM image of the fractured surface of cured DBMC.

Physical properties of cured DBMC

The thermal stability of the cured polymers was evaluated by TGA under argon and air atmosphere, which are shown in Fig. 7. The weight loss temperatures of 5% (T_d5%), the weight loss temperatures of 10% (T_d10%) and the weight retention at 1000 °C for the cured DBMC are summarized in Table 1. The cured polymers were highly thermally stable with T_d5% exceeding 430 °C in argon and air, which were significantly higher than those of epoxy resin, polyethylene and polypropylene. The results implied that the cured DBMC had high thermostability under argon and air atmosphere. Furthermore, the cured DBMC showed a residual weight of 59% at 1000 °C in air. This high thermostability was attributed to the high boron content and good thermal stability of m-carborane cage. The boron and silicon element turned into the corresponding oxides in the presence of oxygen, so the residue weight in air was higher than that in argon. The thermal stability of the cured DBMC is inferior than those of the carborane-containing polymers such as poly(carborane-siloxane-acetylene) and carborane-containing polyimides, but it is still significantly higher than those of traditional neutron shielding polymeric composites.^{13,15,25–30}

Fig. 7 TGA curves of cured DBMC under argon and air.

Table 1 Thermal stability of the cured DBMC

Sample	Atmosphere	Td5% (°C)	Td10% (°C)	Char yield^a
a At 1000 °C.
Cured DBMC	Ar	433	446	26%
Cured DBMC	Air	444	457	59%

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Fig. 8 shows the UV-visible transmittance spectra of cured DBMC. The thickness of the film used for UV-vis measurement was about 30 μm. It showed transmittances of higher than 90% in a range of wavelengths from 800 nm to 400 nm. It was also shown that the film had strong absorption in extreme ultraviolet region. The sample film with thickness of 0.5 mm in the insert of Fig. 8 looked colorless and transparent.

Fig. 8 UV-vis spectrum of the cured DBMC. The insert is an image of the cured DBMC film on a page, indicating good transparence of the cured DBMC.

The thermomechanical properties of the cured DBMC were analyzed with DMA, as shown in Fig. 9. The glass transition temperature (T_g) can be evaluated from the peak of the loss tangent (tan δ). The loss tangent of the cured DBMC showed a single peak at 49.3 °C. It was observed that the tensile storage modulus (E′) decreased slowly with the increase of temperatures in glassy state, but there was an essential decrease around T_g. The storage modulus of the cured DBMC at 0 °C, 25 °C (T_g − 25 °C), 50 °C (T_g) and 75 °C (T_g + 25 °C) were 36.4 MPa, 71.0 MPa, 1.0 MPa and 0.7 MPa, respectively. From the DMA results, we could speculate that the cured DBMC transformed from the glass like state to the highly elastic state when the temperature was above 50 °C. According to the analysis from SEM and DMA results, m-carborane cages were homogenously distributed in the polymer matrix. The boron element of the cured DBMC was homogeneously distributed at micro-scale, which was beneficial to improving the absorption efficiency of neutrons.

Fig. 9 DMA curve of the cured DBMC.

To explore whether the cured DBMC has thermally induced shape memory property, we conducted the following experiment. Firstly, the film was heated to 75 °C, a temperature higher than T_g, at which the sample was in elastic state, and then it was shaped into a temporary twisted “O” shape under external stress. Then the sample was cooled to 0 °C (a temperature lower than T_g) and maintained for 10 min under external stress. After the stress was released, twisted “O” shape was fixed with internal stress stored in the sample. The shape recovery process was realized by heating (Fig. 10). When the sample was heated to 55 °C, the frozen chains start to move, and the temporary shape “O” was recovered to the original shape due to the release of internal stress.

Fig. 10 Shape memory process of the cured DBMC.

The shape memory behavior of the cured DBMC was further examined by a bending test using rectangular strip specimen (30 mm × 5 mm × 0.5 mm) as the permanent shape according to the previous literatures.^42–45 The specimen of the cured DBMC was heated up to 75 °C (T_g + 25 °C) in an oven and held for 1 min for full heating. Then the specimen was slowly pushed into a “U” shape aluminum mold with another aluminum bar by applying a constant force. It was subsequently cooled to 25 °C (T_g − 25 °C) with the mold constrained for 20 s to fix the temporary shape. Then the specimen was released from the “U” shape mold. The deformed specimen was again heated up to 75 °C (T_g + 25 °C). The shape recovery angle was determined by measuring the θ angle between the straight ends of the bent specimen. The shape fixity ratio (R_f) and the shape recovery ratio (R_r) were calculated as R_f = (180° − θ_s)/180° and R_r = θ_r/(180 − θ_s), where θ_s represents the slack or released angle after cooling and mould removal, while θ_r is the recovered angle. In addition, the shape recovery speed was evaluated by determining the time to full recovery, T_r.

The shape memory behavior of the first five shape memory cycles for the cured DBMC is described in Table 2. It could observed that the recovery time was no longer than 20 s, indicating that the cured DBMC sample achieved shape recovery in a short period of time. R_f and R_r for the first five shape memory cycles for the cured DBMC were all greater than 98%. The results showed that the cured DBMC exhibited excellent shape memory behavior.

Table 2 Shape memory behavior of the first five cycles for the cured DBMC

Sample	Shape memory results	1st	2nd	3rd	4th	5th
Cured DBMC	Rf (%)	99	98	99	99	99
	Rr (%)	99	99	100	99	100
	Tr (s)	14	16	15	17	14

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Conclusions

In summary, dibenzocyclobutene-functional m-carborane (DBMC) was synthesized in the first place by one-step hydrosilylation reaction. Upon treatment of DBMC via prepolymerization and postpolymerization procedure, the cured DBMC with cross-linked network structure was obtained. The obtained resin is of high thermal stability. Due to a combination of high thermal stability and boron-riched structure, the as-prepared resin has potential application in future neutron shielding fields such as aerospace and nuclear power station. The cured DBMC with shape memory property could be further fabricated into space-efficient neutron shielding garments which are lightweight and have a small packing volume.

Acknowledgements

The authors thank the Program for Changjiang Scholars and Innovative Research Team in University and the Fundamental Research Funds for the Central Universities (1104020505) for financially supporting this research.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27647k

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