Preparation of Epoxy Resins with Excellent Comprehensive Performance by Thiol-Epoxy Click Reaction

Preparation of Epoxy Resins with Excellent Comprehensive Performance by Thiol-Epoxy Click Reaction

Progress in Organic Coatings xxx (xxxx) xxxx Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.c...

3MB Sizes 0 Downloads 6 Views

Progress in Organic Coatings xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Preparation of Epoxy Resins with Excellent Comprehensive Performance by Thiol-Epoxy Click Reaction Yanfen Lua,1, Yimei Wanga,1, Sufang Chenb, Junheng Zhanga, Juan Chenga, Menghe Miaoc, Daohong Zhanga,* a

Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan 430074, China b Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan, Hubei 430073, China c CSIRO Manufacturing, 75 Pigdons Road, Waurn Ponds, Victoria 3216, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Hyperbranched polymers Epoxy resins Click reaction Free volume Strength

Simultaneous improvement on mechanical performance, thermal stability and crosslinking efficiency of epoxy resins is a large challenge. Here, we have synthesized a series of thiol-ended hyperbranched polymers (THBP-n) as curing agents to crosslink bisphenol-A epoxy resins by a thiol-epoxy click reaction. Compared with the common amine curing agent, the THBP-n could improve the crosslinking efficiency of DGEBA about 5 times. Increasing the thiol group number n from 3 to 6, 9 in THBP-n, the tensile strength, flexual strength, impact toughness of cured THBP-n/DGEBA materials increased simultaneously and sharply several times, besides their thermal degradation and glass transition temperatures raised to some extent. This improvement could be attributable to the synergetic effect about the minimization of internal molecular free volume, hyperbranched structure, and high-crosslinking density, substantiated by integrating techniques of dynamic light scattering, dynamic mechanical analysis, positron annihilation lifetime spectroscopy and SEM, and the novel mechanism was proposed. The thiol-epoxy click reaction will supply an energy-saving route for high-efficient crosslinking of thermosetting epoxy resins with excellent comprehensive performance.

1. Introduction Bisphenol-A epoxy resin (DGEBA) as an important thermoset is widely used in advanced electronic materials fields. However, the low curing speed and long curing time bring a large challenge for energysaving and cost. The invention of “Click” chemical reaction may supply an available approach to the challenge, due to the advantages, including fast reaction rate[1], high yield, solvent-free and mild reaction conditions [2]. The click reaction has been applied in the fields of DielsAlder cycloaddition [3], thiol-based reactions[4], and copper-catalyzed azide-alkyne cycloaddition [5]. Thiol-based reactions can be further categorized into thiol-ene[6,7], thiol-yne [8], thiol-isocyanate[9], thiolepoxy[10] and thiol-Michael reaction[11]. Thiol-epoxy click reaction has been widely used as a powerful tool to obtain high performance coatings[12], polymer synthesis[13,14], shape memory networks[15] and hydrogels[16,17]. Thermosets could be prepared rapidly using a base catalyst to initiate the thiol-epoxy click reaction between thiols and epoxides

[18,19]. Some transparent materials via the thiol-epoxy click reaction showed a lot of advantages for application in the field of electronic devices, including homogeneous crosslinked network, low curing shrinkage, narrow glass transition, etal. Guzmán et al[18] prepared thermosets by a click reaction between tri- or tetra-functional thiols and DGEBA basing an amine precursor as catalyst, and increasing functionality of thiols increased the glass transition temperature. They also used base catalyst 4-(N,N-dimethylaminopyridine) to carry out the thiol-epoxy reaction between thiols and a cycloaliphatic resin [20]. They observed that modulus and glass transition temperature (Tg) of the cured samples in the rubbery state increased with an increase in thiol functionality. Lee, et al[21] obtained various thiol-epoxy polymer networks from different multifunctional thiols and DGEBA, and they discovered that the curing efficiency, Tg, mechanical performance, and crosslinking density of cured networks were increased as the functionality of the thiols increased. But overall the performance of cured thiol-epoxy thermosets still falls behind expectations due to the limits of the functionality of the thiols.



Corresponding author E-mail address: [email protected] (D. Zhang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.porgcoat.2019.105436 Received 5 September 2019; Received in revised form 15 October 2019; Accepted 7 November 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Yanfen Lu, et al., Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.105436

Progress in Organic Coatings xxx (xxxx) xxxx

Y. Lu, et al.

stored for about 12 h. Their mechanical performance were measured according to the relative standard methods.

Because the large terminal groups of hyperbranched polymers (HBPs) can participate in network formation, HBPs can be used to improve the mechanical and thermal performance of cured materials [22,23]. Compared with linear polymers with the similar molecular weight, HBPs show better processability due to their branched structures and low entanglement, which reduces viscosity of reactive mixture[24]. Liu [25] added hyperbranched polysiloxane (HBPSi) into an epoxy-aromatic amine network, and found that the mechanical properties of the HBPSi-epoxy resin has a maximum. Mechanical performance of epoxy resin could be improved by increasing hyperbranched poly (urethane-phosphine oxide) (HPUPO) loading from 0 to 4 wt %[26]. Hyperbranched poly(amide-ester)s was also used as modifier for DGEBA to increase impact toughness without decreasing its thermomechanical properties[27]. Hyperbranched poly(aminomethylphosphine oxide-amine)[28] was used to enhance Tg and toughness of epoxy resin. The effect of hyperbranched polyester polythiol (H20-SH) on the properties of cured thiol-epoxy acrylate was studied by Guan and coworkers[29]. The tensile strength of the cured epoxy resins changed from 21 to 25 MPa with an increase in H20-SH content from 0 to 11 wt %, although their Tg and the tensile storage modulus (Tg +50 K) decreased from 134.3 to 106.9 °C and from 122.5 to 79.4 MPa, respectively. We have prepared thiol-ended hyperbranched polymers (THBP-n, n = 3, 6, 9, 12) [30] by an esterification reaction between 3-mercaptopropionic acid and hyperbranched polyesters, and further synthesized (EHBP-n) through thiol-ene click reaction between THBP-n and allylglycidyl ether. EHBP-n could improve simultaneously the tensile, flexural, impact and adhesive strengths of DGEBA about 38.9%, 35.3%, 158.6% and 96.2%, respectively. However, the low crosslinking efficiency and short polt life of curing agent is a large challenge to wide application. Here, THBP-n was used directly to cure DGEBA by thiolepoxy click reaction with high efficiency, and then effects of the thiol group number of THBP-n on performance of cured THBP-n/DGEBA materials were investigated in detail.

2.3. Characterizations FT-IR and 1H NMR data of THBP-n were obtained from a Bruker Vertex 70 FT-IR and Bruker Avance III-400 NMR spectrometers, respectively. The molecular weights of THBP-n were measued by a Bruker Matrix-assisted laser desorption ionization time-of-flight (Maldi-TOF) using α-cyano-4-hydroxycinnamic acid as matrix. The particle size of THBP-n and THBP-n/DGEBA blends with a condensation of 1 mg/mL in tetrahydrofuran were obtained from a Malvern laser particle size analyzer (Nano S90) at 25 °C. The impact toughness of cured samples was measured on a CEAST 9050 impact tester. Their tensile and flexural strength were tested on an Instron 5966 testing machine. Non-isothermal curing of THBP-n/ DGEBA blends and their Tg were determined by a NETZSCH DSC200 F3 differential scanning calorimetry from 30 to 200 °C and from 0 to 150 °C, respectively, at a constant heating rate of 10 °C/min. Thermal degradation temperature of cured materials was performed on a NETZSCH TG209 F3 thermogravimetric analyzer from 40 to 750 °C in 20 mL/min nitrogen and at a 10 °C/min of heating rate. The modulus of the cured samples (10 mm × 5 mm × 0.5 mm) was tested by a DMA (TA Q800) instrument from -120 to 200 °C by using the frequency of 1 Hz and at the rate of 10 °C/min. A RGM-1/APBS-2 (First Point Inc.) was used to investigate the positron annihilation lifetime spectroscopy (PALS) of cured samples. The fractured surface micrographs of the cured materials were observed by a SU8010 scanning electron microscope (SEM) at a 5 kV accelerating voltage. 3. Results and discussions 3.1. Curing process of THBP-n/DGEBA blends

Four types of thiol-ended hyperbranched polymers (THBP-n) was synthesized by the reaction between hydroxyl-ended polymers and 3mercaptopropionic acid according to our previous method [30]. Amine curing agent (DETA-AN) was produced by an addition reaction of equal molar acrylonitrile (AN) and diethylene triamine (DETA)[31]. Structures of THBP-n were shown in Scheme 1. Bisphenol-A epoxy resin (DGEBA) with an epoxy equivalent weight of 196 g/eq was obtained from Yueyang Chemical Corp., China. DMP-30 (2,4,6-Tris(dimethylaminomethyl)phenol) as an accelerator was supplied by Shanghai Meryer Chemical Technology Co., Ltd. Main FT-IR data of THBP-n: 1035 (-C-O-C-), 1734 (s, -C = O), 2571 (s, -SH), and 3440 (s, -OH). Main 1H-NMR data of THBP-n: δ ppm 0.960.89 (3 H), 1.32-1.21 (29 H), 1.70-1.61 (3-12 H), 2.72-2.64 (6-24 H), 2.80-2.72 (6-24 H), 3.55-3.39 (6-18 H), 4.11-4.05 (6 H), 4.35-4.19 (4260 H). Molecular weights from Maldi-TOF Mass spectra: THBP-3 (1495 g/mol), THBP-6 (1790 g/mol), THBP-9 (1975 g/mol) and THBP12 (2260 g/mol), respectively, being close to their theoretical molecular weights (1444, 1708, 1972 and 2237 g/mol).

All blends were mixed by quickly stirring for 1 min in a glass cup basing a stoichiometric amount of curing agents (DETA-AN or THBP-n) and DGEBA. And then dynamic measurements were used to investigate the curing process of the blends through a non-isothermal curing analysis, compared with DETA-AN/DGEBA sample, Fig. 1 shows the curing exotherms and integral curves of the THBP-n/DGEBA and DETA-AN/ DGEBA blends. The corresponding recorded calorimetric data of the curing process were summarized in Table 2. The THBP-n/DGEBA blends begun to cure at relatively high temperature of about 120130 °C, compared with the initial curing temperature (∼80 °C) of DETA-AN/DGEBA in Fig. 1a-b, indicating the good pot time of THBP-n/ DGEBA blends. It is much important that all THBP-n/DGEBA blends showed a narrow curing time range of 1.8-2.4 min in Fig.1c-d and Table 2, being much lower than that (∼ 9.9 min) of DETA-AN/DGEBA blend, and the crosslinking efficiency was improved about 5 times, indicating high reactivity and high curing efficiency of the thiol-epoxy click reaction. The thiol group number of THBP-n within thiol-epoxy matrix exhibits a clear and consistent influence on the Tp (Table 2). As the thiol group number of THBP-n increased, and the crosslinkable thiol group increased, resulting in high reactivity and rapid clich reaction speed, and finally resulting in decrease in the Tp of THBP-n/DGEBA blends, and the enthalpy increased due to the increase of the relative amount of epoxy groups in the blends in Table 2.

2.2. Preparation of THBP-n/DGEBA composites

3.2. Performances of cured THBP-n/DGEBA composites

According to the formulation in Table 1, both THBP-n and DGEBA were mixed and stirred in a glass cup at room temperature by using a glass stick for 5 min, and then DMP-30 was added into the glass cup and stirred for 3 min to obtain THBP-n/DGEBA blends. The blends were crosslinked at 40 °C/24 h and 80 °C/2 h in a silicone rubber mold. After that, the samples were cooled gradually to room temperature and

Fig. 2a-c shows that the mechanical performanceof the cured materials were strongly dependent on the thiol group number of THBP-n, including tensile, flexural and impact strength. Their mechanical properties followed an upward parabolic curve with maximum at n = 9 with an increase in thiol group number of THBP-n. The maximum flexural, tensile and impact strength of the cured THBP-9/DGEBA

2. Experimental 2.1. Materials

2

Progress in Organic Coatings xxx (xxxx) xxxx

Y. Lu, et al.

Scheme 1. Chemical structures of THBP-n.

DGEBA, THBP-6/DGEBA, THBP-9/DGEBA and THBP-12/DGEBA are 941.28, 2063.63, 3117.65 and 2654.83 N·mm, respectively, being well agreement with their impact strength in Fig. 2c. Fig. 3a and Table 3 also show that the Tg of the cured samples gradually increased with the thiol group number in THBP-n, being explained by the fact that the higher thiol group number of THBP-n in the formulation leads to higher crosslinking density. The differential scanning calorimetry (DSC) and thermogravimetric (TGA) curves of the cured THBP-n/DGEBA samples are shown in Fig. 3, and their characteristic data are summarized in Table 3. The changes of the temperatures corresponding to 5 % weight loss (Td5%), 10 % weight loss (Td10%) and the maximum weight loss (Tmax) were attributable to the variation in chemical structure[4,20] of THBP-n and crosslinking density[35]. All Td5%, Td10% and Tmax of cured THBP-n/DGEBA materials increased with an increase in thiol group number n (n = 3, 6, 9). The cured THBP-3/DGEBA materials displays the lowest Td5% because much more easily dehydration of a higher content of hydroxyl groups in THBP-3 than other THBP-n (n = 6, 9, 12). The cured THBP-9/DGEBA and THBP-12/DGEBA materials display the high thermal stability due to their high crosslinking density.

Table 1 Formulation of the blends. blends

DGEBA

THBP-3

THBP-6

THBP-9

THBP-12

DMP-30

THBP-3/DGEBA THBP-6/DGEBA THBP-9/DGEBA THBP-12/DGEBA

50.00 50.00 50.00 50.00

121.43 -

72.86 -

55.43 -

47.22

1.71 1.23 1.05 0.97

materials were 105.78 MPa, 61.56 MPa and 61.64 kJ/m2, respectively in Fig. 2a-c, being also higher than those of cured DETA-AN/ DGEBA[32]. The improvements on the cured THBP-n/DGEBA materials can be attributable to the synergetic effect of hydroxyl groups content, intramolecular cavities[31], crosslinking density[21,33], dispersion of THBP-n in DGEBA matrix and topological structure of crosslinked network. The stress-strain curves of cured THBP-n/DGEBA mateials are shown in Fig. 2d. The THBP-9/DGEBA shows the lowest strain and the highest stress at break. The elongation at break decreased from 98.47 % at n = 3 to 4.37 % at n = 9. The decrease of elongation at break of cured THBP-n/DGEBA materials was attributed to the reduced mobility of the molecular chains[34]. The relationship between impact force and displacement of the cured THBP-n/DGEBA materials are shown in Fig. 2e. With an increase in thiol group number, the impact force and displacement of cured THBP-n/DGEBA composites increased from 500.04 N to 592.57 N and from 3.85 mm to 7.38 mm, respectively, and then decreased. The cured THBP-9/DGEBA composites showed the highest impact force and displacement, indicating its high toughness. Toughness (area under the force-displacement plot) of the THBP-3/

3.3. Simultaneous enhancement mechanism of strength and ductility Fig. 4 shows the average particle size of the THBP-n and THBP-n/ DGEBA blends. The hydrodynamic diameter (DH)[36,37] of the particles was obtained by a translational diffusion coefficient (D) as follows: D=kT/3πη DH

(1)

where T is the absolute temperature; k is the Boltzmann constant; and η 3

Progress in Organic Coatings xxx (xxxx) xxxx

Y. Lu, et al.

Fig. 1. DSC and integral curves of THBP-n/DGEBA and DETA-AN/DGEBA blends.

is the viscosity of the solvent. The average hydrodynamic diameters of THBP-3, THBP-6, THBP-9 and THBP-12 are 288.85 nm, 172.70 nm, 44.37 nm and 22.01 nm respectively, indicating the smallest DH of the THBP-12, being attributable to the compact and hyperbranched structure[38] of the THBP-12 in tetrahydrofuran. With increasing molecular weight of THBP-n, the DH of THBP-n/DGEBA blends decreased from 648.48 nm to 584.24 nm, 329.17 nm, 211.76 nm, and 44.39 nm in Fig. 4b, respectively. Because the ellipsoidal THBP-n can enter into the interchains of entangled DGEBA and reduce entanglement and aggregation[39], resulting in lower average DH of THBP-n/DGEBA blends than pure DGEBA. The thermomechanical characteristics of the as cured THBP-n/

Table 2 Calorimetric data of THBP-n/DGEBA blends and DETA-AN/DGEBA blends. Blends

△Ha (J/g)

Tpb (℃)

Curing time (min)

THBP-3/DGEBA THBP-6/DGEBA THBP-9/DGEBA THBP-12/DGEBA DETA-AN/DGEBA

186.3 213.6 261.9 284.0 381.4

136.9 132.2 128.9 127.4 105.9

1.8 2.4 1.8 2.1 9.9

a b

Enthalpy evolved by dynamic curing. Temperature about the curing exotherm peak.

Fig. 2. The mechanical properties of cured THBP-n/DGEBA (a. tensile, b. flexural, c. impact, d. tensile curves, e. impact curves). 4

Progress in Organic Coatings xxx (xxxx) xxxx

Y. Lu, et al.

Fig. 3. The DSC (a) and TGA (b) curves of cured THBP-n/DGEBA materials.

constant (8.314 J/K/mol); and T is the temperature (K). The calculated ve of the cured THBP-n/DGEBA materials are shown in Fig. 5c. As n increased from 3 to 12, the crosslink density increased from 0.18 × 10-3 to 1.58 × 10-3 mol/cm3, being attributable to the larger number of crosslink junctions in THBP-n [18,42]. PALS is often used as an available method for analyzing free-volume of thermosets[23,43]. Fig. 6 shows PALS spectra of cured THBP-n/ DGEBA materials, and PALS data are supplied in Table 4. PALS data may be used to calculate the free volume fraction (fv). According to the Tao-Eldrup model, τ3, R, V, I3 and fv satisfy the following relationships:

Table 3 Thermal degradation data of cured THBP-n/DGEBA materials. Typical data

Td5% (°C)

Td10% (°C)

Tmax (°C)

Char yields at 750 °C (%)

Tg (°C)

THBP-3/DGEBA THBP-6/DGEBA THBP-9/DGEBA THBP-12/ DGEBA

243.0 296.1 307.4 307.1

297.2 318.5 323.2 322.5

335.5 350.8 351.6 349.6

11.98 10.57 11.64 10.73

37.1 44.7 59.8 62.8

1-R 1 2πR ⎞ ⎤ + sin ⎛ (τ3)-1=2 ⎡ ⎢ R + 0.166 2π R + 0.166 ⎠ ⎥ ⎝ ⎦ ⎣

DGEBA materials were studied using DMA. The effects of thiol group number on loss tangent (tan δ) and storage modulus (E′) are displayed in Fig. 5. With an increase in thiol group number (n) from 3 to 6, 9, and 12, the α-relaxation peak of the corsslinked THBP-n/DGEBA materials changes toward higher temperature, and their Tg increased from 54.0 to 68.6, 76.8 and 86.1 °C, respectively. The temperature at tan δ peak showed a similar variation but higher values than the Tg measured using DSC due to the effect of testing frequency. A narrow unimodal shape was observed in the tan δ curves in Fig. 5a, suggesting the cured materials without phase-separation. The curve corresponding to the THBP-12 had the highest tan δ peak temperature, in agreement with a dense thiol network structure with internal branching points[20]. The storage modulus (E′) curves of cured THBP-n/DGEBA (n = 3, 6, 9 and 12) materials are presented in Fig. 5b. Their E′ values in glassy state were reasonably stable at low temperatures, but a steep drop in modulus was observed by increasing temperature, indicating that the cured THBP-n/DGEBA materials entered into a rubbery state. The crosslink density (ve, mol/cm3) [40] of a cured material is often represented and can be determined by the following rubber elasticity theory [10,41]: Er=3νeRT

(3)

4πR3 3

(4)

f v = cVI3

(5)

V=

where τ3 is orthopositronium (o-Ps) life time (ns); V is the void volume (nm3); R is the vacancy radius (nm); I3 is the positron intensity (%); fv is the free volume fraction (%); c is a constant of 0.018 nm-3. fv is calculated from equations (3–5) and shown in Table 4. The o-Ps life time τ3 decreases with the increase of thiol number in THBP-n, indicating the decrease of average free volume void size in the cured materials. The o-Ps intensity I3 (void concentration)[44] in cured THBP9/DGEBA composites is greater than the other cured materials, indicating a higher number density of voids in cured material. The cured THBP-12/DGEBA has a smaller free volume voids V than the other cured materials due to its higher crosslink density. With the increase of thiol group number of THBP-n, the fv of the cured materials decreases significantly, being attributed to the smaller hydrodynamic diameter in Fig. 4, compact topological structure and packing of molecule chains [45]. A possiple mechanism of simultaneously strengthening and toughening DGEBA by THBP-n is proposed here and shown in Scheme 2,

(2)

where Er is the rubbery storage modulus at Tg+50 K; R is the gas

Fig. 4. The particle size of THBP-n and THBP-n/DGEBA blends. 5

Progress in Organic Coatings xxx (xxxx) xxxx

Y. Lu, et al.

Fig. 5. DMA curves of cured THBP-n/DGEBA materials (a. tan δ, b. storage modulus, c. crosslink density).

in Table 4. And the large number of terminal thiol groups in the THBP-9 increases the crosslinking density and thus improves the tensile and fluxual strength of the cured THBP-9/DGEBA material. So high crosslinking density, the low density of free volume voids (V), the low free volume fraction (fv) and the minimization of internal molecular free volume [47] will increase the tensile and flexural strength and decrease toughness of the cured materials. In addition, the pendent hydroxyl functional groups content, crosslinked topological structure and dispersion also have the effect on the mechanical properties. An appropriate amount of hydroxyl functional groups can deform and absorbs energy to improve impact toughness because it could not participate the curing reaction. Low DH can promote dispersion of THBP-n in DGEBA and form homogenous structure during curing [46], and dissipate impact energy, resulting in high toughness. Hyperbranched ellipsoidal structure can deform, dissipate and absorb much more impact energy than linear structure [11]. With an increase in the number of n in THBP-n, the functionality of thiol group increases from 3 to 6, 9, and 12, and the topological structure of THBP-n changes from linear to ellipsoidal shape, resulting in improvement on toughness. Therefore, both cured THBP-9/DGEBA and THBP-12/DGEBA composites show high mechanical strength. This reinforcement mechanism can be further explained by the fractured surface morphologies of cured THBP-n/DGEBA materials, taking cured THBP-3/DGEBA and THBP-9/DGEBA materials as examples. As shown in Fig. 7, the plastic zone of the fractured surfaces became larger with increasing thiol group number in the THBP-n. Fig. 7c-d show the “river-branch” patterns and the filamentous debonding on the fractured surface of the cured THBP-9/DGEBA materials, which could not be observed in THBP-3/DGEBA materials (Fig. 7a-b). This mechanism could be explained by an “in situ” reinforcing and toughening[22–24] mechanism.

Fig. 6. Positron annihilation lifetime spectroscopy of cured THBP-n/DGEBA materials. Table 4 PALS data of cured THBP-n/DGEBA materials. Typical data

τ3 (ns)

I3 (%)

R (nm)

V (nm3)

fv (%)

THBP-3/DGEBA THBP-6/DGEBA THBP-9/DGEBA THBP-12/DGEBA

1.753 1.728 1.605 1.578

27.00 26.74 28.02 27.98

0.2611 0.2585 0.2456 0.2426

0.0745 0.0724 0.0620 0.0598

3.62 3.48 3.13 3.01

using THBP-9 as an example. The mechanical performance of cured epoxy resins [11,46] is mainly decided by crosslinking density, intramolecular cavity, pendent group content, crosslinked topological structure and dispersion of hyperbranched polymers in matrix. Increasing crosslinking density and decreasing intramolecular cavities improve the flexural and tensile strength and impair toughness. As shown in Scheme 2, after mixing THBP-9 with the entangled DGEBA, the THBP-9 enters the cavity of DGEBA. The branching and ellipsoidal structure of THBP-9 causes the DGEBA to disentangle to a certain degree, resulting in a decrease of the DH and free volume (V) of the blends

4. Conclusions Thiol-ended hyperbranched polymers (THBP-n) were used as curing agent to crosslink DGEBA. We discovered that THBP-n could improve distinctly the curing efficiency and shorten curing time by thiol-epoxy click reaction. With the increase of thiol group number n of the THBP-n from 3 to 9, the glass transition and thermal decomposition

Scheme 2. The simultaneously strenghening and toughening mechanism. 6

Progress in Organic Coatings xxx (xxxx) xxxx

Y. Lu, et al.

Fig. 7. SEM micrographs of impact fractured surface of cured materials (a-b. THBP-3/DGEBA, c-d. THBP-9/DGEBA).

temperatures of the cured THBP-n/DGEBA materials increased, and their mechanical properties (tensile, flexural, and impact) also increased sharply. The maximum impact toughness, tensile and flexural strength achieved by cured THBP-9/DGEBA material were 61.54 kJ/ m2, 61.56 MPa and 105.78 MPa, respectively. A simultaneously reinforcing and toughening mechanism of epoxy resin by THBP-n was analyzed and proposed by integrating DMA, DLS, PALS and SEM techniques in detail.

chemistry, Macromolecules 49 (2016) 1296–1304. [9] J. Tan, C. Li, S. Dan, H. Li, J. Gu, B. Zhang, H. Zhang, Q. Zhang, One-step thiolisocyanate dispersion polymerization: Preparation of uniform, cross-linked and functional particles, Chemical Engineering Journal 304 (2016) 461–468. [10] A. Belmonte, X. Fernández-Francos, S. De la Flor, New understanding of the shapememory response in thiol-epoxy click systems: towards controlling the recovery process, Journal of Materials Science 52 (2017) 1625–1638. [11] S. Chen, Z. Xu, D. Zhang, Synthesis and application of epoxy-ended hyperbranched polymers, Chemical Engineering Journal 343 (2018) 283–302. [12] Y.H. Zhao, D. Vuluga, L. Lecamp, F. Burel, Photoinitiated thiol-epoxy addition for the preparation of photoinduced self-healing fatty coatings, RSC Advances 6 (2016) 32098–32105. [13] S. Binder, I. Gadwal, A. Bielmann, A. Khan, Thiol-epoxy polymerization via an AB monomer: Synthetic access to high molecular weight poly(β-hydroxythio-ether)s, Journal of Polymer Science Part A: Polymer Chemistry 52 (2014) 2040–2046. [14] X.J. Song, J. Hu, C.C. Wang, Synthesis of highly surface functionalized monodispersed poly(St/DVB/GMA) nanospheres with soap-free emulsion polymerization followed by facile “click chemistry” with functionalized alkylthiols, Colloids and Surfaces A: Physicochemical and Engineering Aspects 380 (2011) 250–256. [15] Y. Feng, Y. Hu, L. Man, T. Yuan, C. Zhang, Z. Yang, Biobased thiol-epoxy shape memory networks from gallic acid and vegetable oils, European Polymer Journal 112 (2019) 619–628. [16] N. Cengiz, J. Rao, A. Sanyal, A. Khan, Designing functionalizable hydrogels through thiol-epoxy coupling chemistry, Chemical communications 49 (2013) 11191–11193. [17] C.T. Huynh, F. Liu, Y. Cheng, K.A. Coughlin, E. Alsberg, Thiol-epoxy “click” chemistry to engineer cytocompatible PEG-based hydrogel for siRNA-mediated osteogenesis of hMSCs, ACS Applied Materials & Interfaces 10 (2018) 25936–25942. [18] D. Guzmán, X. Ramis, X. Fernández-Francos, A. Serra, Enhancement in the glass transition temperature in latent thiol-epoxy click cured thermosets, Polymers 7 (2015) 680–694. [19] J.A. Carioscia, J.W. Stansbury, C.N. Bowman, Evaluation and control of thiol–ene/ thiol–epoxy hybrid networks, Polymer 48 (2007) 1526–1532. [20] D. Guzmán, B. Mateu, X. Fernández-Francos, X. Ramis, A. Serra, Novel thermal curing of cycloaliphatic resins by thiol-epoxy click process with several multifunctional thiols, Polymer International 66 (2017) 1697–1707. [21] T.H. Lee, Y.I. Park, S.M. Noh, J.C. Kim, In-situ visualization of the kinetics of low temperature thiol-epoxy crosslinking reactions by using a pH-responsive epoxy resin, Progress in Organic Coatings 104 (2017) 20–27. [22] J. Zhang, S. Chen, Q. He, P. Guo, Z. Xu, D. Zhang, Toughening benzoxazines with hyperbranched polymeric ionic liquids: Effect of cations and anions, Reactive and Functional Polymers 133 (2018) 37–44. [23] J. Zhang, S. Chen, B. Qin, D. Zhang, P. Guo, Q. He, Preparation of hyperbranched polymeric ionic liquids for epoxy resin with simultaneous improvement of strength and toughness, Polymer 164 (2019) 154–162. [24] Y. Wang, S. Chen, W. Guo, M. Miao, D. Zhang, The precise effect of degree of branching of epoxy-ended hyperbranched polymers on intrinsic property and performance, Progress in Organic Coatings 127 (2019) 157–167. [25] H. Liu, X. Gao, B. Deng, G. Huang, Simultaneously reinforcing and toughening epoxy network with a novel hyperbranched polysiloxane modifier, Journal of

Declaration of Competing Interest All authors have declare without any conflict of interest. Acknowledgments We gratefully acknowledge the financial support of the National Natural Science Foundation of China (51573210, 51873233) and the Hubei Provincial Natural Science Foundation of China (2018CFA023). References [1] S. Chen, J. Zhang, J. Zhou, D. Zhang, A. Zhang, Dramatic toughness enhancement of benzoxazine/epoxy thermosets with a novel hyperbranched polymeric ionic liquid, Chemical Engineering Journal 334 (2018) 1371–1382. [2] C. Liu, T. Li, J. Zhang, S. Chen, Z. Xu, A. Zhang, D. Zhang, Preparation and properties of phosphorous-nitrogen containing UV-curable polymeric coatings based on thiol-ene click reaction, Progress in Organic Coatings 90 (2016) 21–27. [3] P. Buono, A. Duval, L. Averous, Y. Habibi, Thermally healable and remendable lignin-based materials through Diels-Alder click polymerization, Polymer 133 (2017) 78–88. [4] A. Lungu, J. Ghitman, A.I. Cernencu, A. Serafim, N.M. Florea, E. Vasile, H. Iovu, POSS-containing hybrid nanomaterials based on thiol-epoxy click reaction, Polymer 145 (2018) 324–333. [5] J.A. Shin, Y.G. Lim, K.H. Lee, Copper-catalyzed azide-alkyne cycloaddition reaction in water using cyclodextrin as a phase transfer catalyst, The Journal of Organic Chemistry 77 (2012) 4117–4122. [6] D. Zhang, C. Liu, S. Chen, J. Zhang, J. Cheng, M. Miao, Highly efficient preparation of hyperbranched epoxy resins by UV-initiated thiol-ene click reaction, Progress in Organic Coatings 101 (2016) 178–185. [7] K.L. Killops, L.M. Campos, C.J. Hawker, Robust, Efficient, and Orthogonal Synthesis of Dendrimers via Thiol-ene “Click” Chemistry, Journal of the American Chemical Society 130 (2008) 5062–5064. [8] A.B. Cook, R. Barbey, J.A. Burns, S. Perrier, Hyperbranched polymers with high degrees of branching and low dispersity values: Pushing the limits of thiol-yne

7

Progress in Organic Coatings xxx (xxxx) xxxx

Y. Lu, et al.

[37] W. Burchard, Dynamic Light Scattering Method of Polymer Solutions 101 (1996) 103–113. [38] H. Mori, D. Chan-Seng, H. Lechner, M. Zhang, A. Mueller, Synthesis and Characterization of Branched Polyelectrolytes, 1. Preparation of Hyperbranched Poly(acrylic acid) via Self-Condensing Atom Transfer Radical Copolymerization 35 (2002) 9270–9281. [39] M.G. Carneiro-da-Cunha, M.A. Cerqueira, B.W.S. Souza, J.A. Teixeira, A.A. Vicente, Influence of concentration, ionic strength and pH on zeta potential and mean hydrodynamic diameter of edible polysaccharide solutions envisaged for multinanolayered films production, Carbohydrate polymers 85 (2011) 522–528. [40] A.K. Guin, B.P. Mallik, S. Shreepathi, Electrochemical and mechanical studies on influence of curing agents on performance of epoxy tank linings, Progress in Organic Coatings 78 (2015) 340–347. [41] A. Belmonte, X. Fernández-Francos, S. De la Flor, À. Serra, Network structure dependence on unconstrained isothermal-recovery processes for shape-memory thiolepoxy “click” systems, Mechanics of Time-Dependent Materials 21 (2017) 133–149. [42] A. Belmonte, D. Guzmán, X. Fernández-Francos, S. De la Flor, Effect of the network structure and programming temperature on the shape-memory response of thiolepoxy “click” systems, Polymers 7 (2015) 2146–2164. [43] M. Gosecki, B. Zgardzinska, M. Gosecka, Temperature-Induced Changes in the Nanostructure of Hydrogels Based on Reversibly Cross-Linked Hyperbranched Polyglycidol with B(OH)4⊖ Ions, The Journal of Physical Chemistry C 120 (2016) 18323–18332. [44] N. Qi, W. Gong, Z. Wang, B. Wang, Y. Yang, C. Hu, Gas permeability and free volume hole properties of interpenetrating polymer network studied by positrons, Wuhan University Journal of Natural Sciences 12 (2007) 271–274. [45] L. Rey, J. Galy, H. Sautereau, G.P. Simon, W.D. Cook, PALS free volume and mechanical properties in dimethacrylate-based thermosets, Polymer International 53 (2004) 557–568. [46] N. Domun, H. Hadavinia, T. Zhang, T. Sainsbury, G.H. Liaghat, S. Vahid, Improving the fracture toughness and the strength of epoxy using nanomaterials - a review of the current status, Nanoscale 7 (2015) 10294–10329. [47] N.T. Tsui, A.J. Paraskos, L. Torun, T.M. Swager, E.L. Thomas, Minimization of Internal Molecular Free Volume: A Mechanism for the Simultaneous Enhancement of Polymer Stiffness, Strength, and Ductility, Macromolecules 39 (2006) 3350–3358.

Applied Polymer Science 135 (2018) 46340/1-46340/10. [26] C. Ma, S. Qiu, J. Wang, H. Sheng, Y. Zhang, W. Hu, Y. Hu, Facile synthesis of a novel hyperbranched poly(urethane-phosphine oxide) as an effective modifier for epoxy resin, Polymer Degradation and Stability 154 (2018) 157–169. [27] G.G. Buonocore, L. Schiavo, I. Attianese, A. Borriello, Hyperbranched polymers as modifiers of epoxy adhesives, Composites Part B: Engineering 53 (2013) 187–192. [28] C. Ma, S. Qiu, B. Yu, J. Wang, C. Wang, W. Zeng, Y. Hu, Economical and environment-friendly synthesis of a novel hyperbranched poly(aminomethylphosphine oxide-amine) as co-curing agent for simultaneous improvement of fire safety, glass transition temperature and toughness of epoxy resins, Chemical Engineering Journal 322 (2017) 618–631. [29] X. Guan, J. Gan, G. Chen, X. Huang, M. Lu, K. Wu, L. Liang, J. Shi, UV-cured hyperbranched polyester polythiol(H20-SH)-epoxy acrylate networks: Preparation, thermal and mechanical properties, Journal of Macromolecular Science, Part A 54 (2017) 662–668. [30] Y. Wang, S. Chen, X. Chen, Y. Lu, M. Miao, D. Zhang, Controllability of epoxy equivalent weight and performance of hyperbranched epoxy resins, Composites Part B: Engineering 160 (2019) 615–625. [31] D. Zhang, Y. Chen, D. Jia, Toughness and reinforcement of diglycidyl ether of bisphenol-A by hyperbranched poly(trimellitic anhydride-butanediol glycol) ester epoxy resin, Polymer Composites 30 (2009) 918–925. [32] Y.M. Wang, S.F. Chen, W.Q. Guo, M.H. Miao, D.H. Zhang, The precise effect of degree of branching of epoxy-ended hyperbranched polymers on intrinsic property and performance, Progress in Organic Coatings 127 (2019) 157–167. [33] D. Guzmán, X. Ramis, X. Fernández-Francos, S. De la Flor, A. Serra, Preparation of new biobased coatings from a triglycidyl eugenol derivative through thiol-epoxy click reaction, Progress in Organic Coatings 114 (2018) 259–267. [34] V. Mishra, J. Desai, K.I. Patel, High-performance waterborne UV-curable polyurethane dispersion based on thiol-acrylate/thiol-epoxy hybrid networks, Journal of Coatings Technology and Research 14 (2017) 1069–1081. [35] X. Wang, L. Zong, J. Han, J. Wang, C. Liu, X. Jian, Toughening and reinforcing of benzoxazine resins using a new hyperbranched polyether epoxy as a non-phaseseparation modifier, Polymer 121 (2017) 217–227. [36] N. Yevlampieva, A. Dobrodumov, O. Nazarova, O. Okatova, H. Cottet, Hydrodynamic Behavior of Dendrigraft Polylysines in Water and Dimethylformamide, Polymers 4 (2012) 20–31.

8