Triazole-forming waterborne polyurethane composites fabricated with silane coupling agent functionalized nano-silica

Journal of Colloid and Interface Science 361 (2011) 483–490

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Triazole-forming waterborne polyurethane composites fabricated with silane coupling agent functionalized nano-silica Daoxing Sun a,c,⇑, Xiao Miao a, Kejie Zhang b, Hern Kim c, Yongan Yuan c a

College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao 266042, China Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Ministry of Education, Nanjing 210094, China c Energy and Environment Fusion Technology Center, Department of Environmental Engineering and Biotechnology, Myongji University, Yongin, Kyonggi-do 449-728, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 15 March 2011 Accepted 18 May 2011 Available online 26 May 2011 Keywords: Triazole Click chemistry Waterborne polyurethane Nano-silica

a b s t r a c t In the research work, ‘‘click’’ chemistry was used to modify waterborne polyurethane (WPU) with silane coupling agent (SiCA) functionalized nano-silica. The modified WPU (CWPU) was characterized by FTIR, scanning electron microscope (SEM), thermogravimetric analysis (TGA) and contact angle measurement. The experiment results show that the modification can improve the thermal stability, hardness, and water or weather resistance of CWPU. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction WPU has received increasingly attention because of its environmentally friendly nature, good process ability and versatile structure–property relationships. It has already been widely used in adhesives, coatings, textile sizing, and so forth [1–3]. However, some inferior properties, such as low mechanical strength, low water and solvent resistance, restrict its high performance utility to some extent. It is important to modify WPU via various methods to enhance its properties. Recently, nano structured hybrid organic–inorganic composites based on organic polymer and inorganic nano minerals have played a significant role in the WPU modification, because this method exhibits a remarkable improvement in the mechanical strength and heat resistance compared to that of conventional WPU materials. On the other hand, nano-sized silica has been widely used to enhance the performance of WPU in hardness, temperature and weather resistance [4–6]. Unfortunately, due to the high surface free energy, nano-silica aggregates easily, and this limits its applications in a large extend. Besides, nano-silica treated with SiCA can increase its dispersion, but this treatment is not easy to control.

⇑ Corresponding author at: College of Environment and Safety Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. Fax: +86 532 84022961. E-mail address: [email protected] (D. Sun). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.05.062

Click chemistry, as termed by Sharpless and coworkers in 2001, won a lot of attention because of its high selectivity, quantitative yield, mild reaction conditions and absence of by-products [7]. Besides, click chemistry reactions can link particular groups or compounds together through triazole ring and fabricate functional materials. Recently it becomes a very powerful tool in the modification of nano-materials and carbohydrate compounds [8–11], also in the construction of star polymers [12–19] and polyurethane [20–22]. In this study, we introduced ‘‘click’’ chemistry to the modification of WPU by the reaction between alkyne-functionalized WPU and functionalized nano-silica with silane coupling agent azide (azide-SiCA). This modification was more efficient than conventional methods which by reactions between hydroxyls existing in SiO2 and isocyanate groups [22–26]. 2. Experimental 2.1. Materials Nano-silica (15 nm, 360 m2/g) was purchased from Nanjing, China and dried at 120 °C for 12 h before use. Polycarbonatediol (L5651, 2000 g/mol), polyether polyol (PTMG, 2000 g/mol), 1,6hexanediol, polyether diol (TDIOL, 1000 g/mol), dimethylol propionic acid (DMPA), and isophorone diisocyanate (IPDI) were dried and degassed at 75 °C for 5 h in vacuum oven. 1-Methyl-2-pyrrolidone (NMP), triethylamine (TEA) and acetone were water-free over 4 Å molecular sieve for days before use. Diethyl malonate,

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the copper catalyst based on either CuBr/PMDETA (0.1 equivalent each according to the alkyne content) or CuSO4
5H2O/Naasc (0.05 and 0.1 equiv, respectively). The reaction was performed under nitrogen atmosphere at 50 °C, and the reaction was tracked by FTIR (see Scheme 4).

LiAlH4 (Aladdin), NaN3, propargyl bromide and sodium ascorbate were purchased from Kermel. 3-Chloropropyl-triethoxysilane was used as a coupling agent and purchased from Shandong, China. 2.2. Synthesis of 2,2-di (prop-2-ynyl) propane-1,3-diol (DPPD) Base on the typical preparation procedure in the previous report [27], as the first step, diethyl malonate and sodium ethoxide were used to produce diethyldipropargyl malonate. 1H NMR(CDCl3): 4.16 (4H, CH2), 2.91 (4H, CH2), 1.98 (2H, „CH), 1.19 (6H, CH3). FTIR (KBr, cm 1): m(„CH) = 3300 cm 1, m(C„C) = 2119 cm 1. Then diethyldipropargyl malonate was reduced with LiAlH4 in THF in the second step. 1H NMR (CDCl3): 3.74 (4H, CH2), 2.60 (2H, OH), 2.37 (4H, CH2), 2.06 (2H, CH). FTIR (KBr, cm 1): m(„CH) = 3300 cm 1, m(C„C) = 2119 cm 1. The procedure for the synthesis of DPPD was shown in Scheme 1.

2.6. Characterization The sample films were prepared by casting the WPU onto Teflon surface and dried at room temperature for three days, and then dried at 70 °C for 24 h. After stripping off, the films were stored in a desiccator to avoid moisture. The FTIR spectra were determined by FTIR spectrophotometer (FTIR-8400, Japan). The NMR spectra were recorded on NMR spectrometer (Bruker AV-500, Germany). CDCl3 was used as solvent and tetramethylsilane as an internal standard. The elemental analysis was carried out by Vario EL III instrument (Germany). The graft degree of 3-azidopropyl-triethoxysilane onto the surface of silica was determined by the nitrogen content analysis of the samples. Thermogravimetric analysis (TGA) of the functionalized nano-silica was carried out using a Netzsch STA449C instrument (Germany) with a heating rate 10 °C/ min under nitrogen flow. Transmission electronic microscopy (TEM) images were recorded on JEM-2000EX (Japan). The morphology of nano-silica particles dispersed in WPU was observed by SEM (JSM-6700F, Japan). Static contact angle for water (WCA) was measured with JC 2000C1 contact angle goniometer (Zhongchen Co., China) at RT. Water absorption was measured by immersing WPU films in water for 24 h, and it was calculated by the samples’ weight before and after in water. UV–visible transmission spectrum was obtained using TU-1901 UV–visible spectrometer. Pencil hardness of CWPU films was measured using a QHQ hardness tester according to GB/T 6739-1996.

2.3. Synthesis of alkyne-containing waterborne polyurethane L5651, PTMG, TDIOL and IPDI were introduced to a round-bottom flask, and the mixture was heated to 90 °C under agitating in N2 atmosphere for 2 h to obtain NCO-terminated prepolymer. After cooling to 45 °C, DMPA, 1,6-hexanediol, DPPD, NMP and dibutyltin dilaureate were added, then the mixture was heated up to 80 °C under stirring until the reaction of DPPD with IPDI finished (indicated by TLC). After cooling to room temperature (RT), TEA was fed into the flask and mixed for 15 min, and then distilled water was added dropwise under vigorous stirring at last. The synthesis procedure of alkyne-functionalized WPU was shown in Scheme 2. 2.4. Synthesis of azide-SiCA functionalized nano-silica particles (ASiFS) 3-Chloropropyltriethoxysilane (50 mmol), NaN3 (60 mmol), KI (6 mmol) and DMF were added into a 250 ml flask, the reaction was performed at 100 °C for 24 h. After completion of the reaction and purification, a colorless liquid (3-azidopropyltriethoxysilane) was obtained. IR (KBr plates): 2097 cm 1 (AN@N+@N A); 1H NMR (500 MHz, CDCl3): d 0.65 (2H, CH2Si), 1.20 (6H, CH3), 1.69 (2H, CCH2C), 3.24 (2H, CH2N3), 3.80 (4H, OCH2); 13C NMR (500 MHz, CDCl3) d7.64 (SiCH2), 18.19 (CCH2C), 22.67 (CH2N3), 53.80 (NCH2), 58.40 (OCH2). Next, nano-silica suspension liquid was prepared by dispersing nano-silica in NMP with sonication, then it was mixed with 3-azidopropyltriethoxysilane and stirred at 110 °C for 12 h. After washing with dichloromethane, methanol, water and acetone respectively, the production was centrifuged. This procedure repeated four times and the residual volatile was removed in vacuo. The modified nano-silica was obtained finally. IR (KBr plates): 2097 cm 1 (AN@N+@N A); 1100 cm 1 (SiAOASi) (see Scheme 3).

3. Results and discussion 3.1. Nano-silica functionalized with azide-SiCA ASiFS was an important intermediate for the CWPU, and the chlorine atoms in 3-chloropropyltriethoxysilane were easily converted into azide groups after the nucleophilic substitution reaction between 3-chloropropyltriethoxysilane and sodium azide. The FTIR spectrum shows (see Fig. 1) the characteristic absorption at 1100 cm 1 due to SiAOASi. The spectrum of azido-silica displayed new peak at 2100 cm 1 (azide group) compared with pristine silica, proving the successful graft of azide-SiCA onto the surface of nano-silica particles. The grafting ratio was determined by the nitrogen content of elemental analysis and TGA. The loading of active sites were calculated based on the elemental analysis. The surface concentration of bonded species (azide-SiCA) on the surface of nano-silica particles was calculated according to the equation proposed by Unger group [28]. The amount of azide-SiCA grafted onto the nano-silica was determined by using the percentage of nitrogen, and the nitrogen content was 1.65% according to the elemental analysis. The result revealed that the grafting density of azide-SiCA onto silica was 1.2 lmol/m2.

2.5. Huisgen 1,3-dipolar cycloaddition between azide-SiCA functionalized of nano-silica and alkyne-containing waterborne polyurethane In a round-bottom flask, the alkyne-containing WPU (1.2 eq of alkyne functions) was mixed with ASiFS (1 eq of azide functions),

O

O

O

C

C

NaOCH2CH3

CH3CH2OH

O

O

O

O

C

C

OH O

LiAlH 4,THF

Br

Scheme 1. The synthesis procedure of DPPD.

4h, r. t

OH

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HO-

-OH

+ HO

OH

+ O

C N CH2

TDIOL

PTMG

L5651

-OH

+ HO

CH3

H3C

IPDI

N C O CH3

90

O=C=N

N=C=O

CH3 HO CH2

C CH2 OH

HO

+

OH OH H2C CH2 C H2C CH2

OH

COOH

1,6-hexanediol

DMPA

C

CH2

+ HO CH2

80

HOOC

CH

C

C

CH2 OH

CH3

trimethylol propane

DPPD

CH

C CH2

C

CH CH

dibutyltin dilaureate

O=C=N

OH

N=CO COOH

C

C

CH

CH COOH

HC

C

C

CH

O=C=N

water

triethylamine

alkyne-functionalized WPU Scheme 2. Synthesis of alkyne-functionalized WPU.

(OEt)3Si

Cl

NaN3

N3

(OEt)3Si

a -1

SiO2 OEt -O Si -O

2100cm

N3

b

N3

Scheme 3. Synthesis of azide-SiCA functionalization of nano-silica particles.

Si-O-Si

0

500

1000 1500 2000 2500 3000 3500 4000 4500

Fig. 1. FTIR spectrum of (a) azido-silica and (b) pristine silica nanoparticles.

Scheme 4. Scheme of the Huisgen 1,3-dipolar cycloadditon.

TGA was also used to confirm the successful graft of azide-SiCA onto the surface of silica. Fig. 2 shows the TGA curves traced the pristine and the azide-SiCA grafted nano-silica (ASiFS). For the pristine nano-silica, the weight loss was 3.46% from RT to 230 °C due to the absorbed water [4,29]. The weight loss was 4.06% from 230 °C to 900 °C, resulting from dehydration condensation reaction of

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101 100 99 98

Residual weight (%)

97 96 95 94 93

a

92 91 90 89 88

b 0

100

200

300

400

500

600

700

800

900

Temperature (ºC) Fig. 2. TGA curves of (a) pristine and (b) azide-grafted nano-silica.

silanol groups (SiAOH) of nano-silica. For ASiFS, the 4.07% weight loss from RT to 230 °C resulted from vaporization of small-molecules. But the weight loss from 230 °C to 900 °C was 7.26%, there was an additional 3.20% weight loss compared to the 4.06% weight loss of pristine silica. This was ascribed to the decomposition of the azide-SiCA grafted onto the surface of nano-silica. The dispersion of nano-silica was studied by TEM (see Fig. 3). Fig. 3a shows that pristine nano-silica particles overlapped each other and single particle could hardly be observed distinctly, which indicates that they were aggregated in some degree. Compared to pristine silica, although ASiFS (Fig. 3b) particles were still aggregated in some degree, the transparency of ASiFS was better. This can be ascribed to the fact that the surface modification layer which kept nano-silica particles apart and the grafting density (1.2 lmol/m2) of azide-SiCA onto silica was so small that barrier effect of silica particles aggregation was reduced.

followed by TLC using mixture of ethyl acetate and petroleum ether (1:1) as developer. The completion of reaction between DPPD and IPDI was indicated by the disappearance of DPPD spots on TLC plate. In order to further prove the linkage of alkyne diol and the WPU, the alkyne-functionalized WPU was characterized by FTIR (Fig. 4). The absorption peak at 2100 cm 1 (C„C) was attributed to the terminal alkyne groups of DPPD in the alkyne-functionalized WPU (Fig. 4b). Additionally, Fig. 4 shows the absorption peak at 3310 cm 1 belonging to NAH stretch in WPU. The absorption peaks at 2980–2850 cm 1 were ascribed to CAH stretch of CH3, CH2 and CH. The absorption peak at 1690 cm 1 was related to the C@O carbonyl stretch of urethane. The absorption peak at 2270 cm 1 belonging to NCO groups, indicating the existence of residual NCO in the formulation design. The thermal stability of the alkyne-functionalized WPU was also studied by TGA curves from Figs. 5 and 6. The incorporation of DPPD can improve the flame retardancy of WPU. The char yield increased with the increase of DPPD. This could be attributed to the fact that alkyne groups as a cross-linker can form reticulated alkenes and participate in cyclo-trimerization reaction. The char layer covering the material as protective barrier prevented the formation of volatile compounds and the heat transfer into inner part of the material [21,30], which can make the material flame retarding. 3.3. CWPU modification with nano-silica The WPU/nano-silica composite attracts great interest of researchers [4,5,31] because it often exhibits remarkable improvement in materials properties, such as heat resistance and mechanical properties. Click chemistry as a promising tool to fabricate functional materials, has been appreciated in recent years [7,9,11,14,15,27]. The IR spectroscopy of CWPU was shown in Fig. 7. Fig. 7 shows the IR peak at around 2100 cm 1 disappeared, and the stronger SiAO peak located at 1100 cm 1 and C@O peak located at 1730 cm 1 are observed after the reaction, indicating the success of ‘‘click’’ modification of CWPU with nano-silica.

3.2. Characterization of alkyne-functionalized WPU

3.4. The morphology of nano-silica in CWPU

Alkyne groups were successfully incorporated into WPU by the reaction between DPPD and IPDI. The reaction process was

Nano-silica grafted onto the WPU backbone by the ‘‘click’’ reaction between the alkyne-functionalized WPU and ASiFS. The

Fig. 3. TEM images of pristine (a) and azido-SiCA functionalized nano-silica (b).

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100

b

90

2100cm -C= C-

a

80

Transmittance (%)

Transmittance (%)

a

70 60

b

50 40 30 20 10

2000

0

500

2050

2100

2150

2200

1000 1500 2000 2500 3000 3500 4000 4500

0 500

1000

1500

2000

2500

3000

3500

4000

4500

Wavenumber (cm-1)

wavenumber (cm-1) Fig. 4. FTIR spectra of WPU (a) and alkyne-functionalized WPU (b).

Fig. 7. FTIR spectroscopy of alkyne-functionalized WPU (a) and CWPU (b).

the bonds between nano-silica and WPU enhanced the miscibility. However, relatively high silica concentration may result in aggregation of silica particles (Fig. 8, WPUi10).

100 80

Weight (%)

3.5. TGA of CWPU 60 40

WPUs WPU-DPPD(5%) WPU-DPPD(10%)

20

WPU-DPPD(15%)

0

0

100

200

300

400

500

600

700

800

Temperature (ºC) Fig. 5. TGA curves of WPU and DPPD/WPU.

2.0 1.8

Final char yield (%)

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0

2

4

6

8

10

12

14

16

Incorporated DPPD (mol%)

TGA experiment was carried out to examine the effect of ASiFS on the thermal stability of CWPU. Fig. 9 shows the TGA curves of WPU and CWPU. The TGA curves starting at 30 °C and ending at about 550 °C exhibited three distinct weight loss stages. The weight loss temperature range of the first stage was from 25 °C to 300 °C, resulting from the vaporization of the residual water, loss of the oligomer and the by-products existing in the WPUi film. The second stage from 300 °C to 350 °C originated from the decomposition of urethane group of CWPU. The third stage of the temperature from 350 °C to 450 °C might be attributed to the decomposition of the soft segment of polyurethane. Fig. 10 shows the DTG curves of the samples. The effect of ASiFS on the thermal stability of CWPU was studied on the basis of the temperature at weight loss of 10%, 30%, 50% and maximum decomposition of the samples (Table 1). It can be seen from Fig. 9 and Table 1 that T0.1 of pristine WPU was higher than that of WPUi1, WPUi3, WPUi5 and WPUi10. It can be ascribed to no oligomer, by-products or unreacted DPPD were existent in WPU. It can be seen from Fig. 9 that the maximum thermal degradation temperatures of DPPD ranged from about 180 to 250 °C. However, compared to conventional WPU, T0.3, T0.5 and Tmax of CWPU shifted to high temperature, indicating that ASiFS can enhance the thermal stability of CWPU. This can be explained by following: CWPU modified with ASiFS developed an interpenetrating or crosslinking network, which enhanced the strength and limited the movement of CWPU molecules. Additionally, the SiAO bond energy existing in CWPU was greater than that of CAO bond which improved thermal resistance of WPU [24,25]. In addition, Tmax, T0.3 and T0.5 of WPUi5 and WPUi10 were slightly lower than that of WPUi1 and WPUi3, may be due to the aggregation of excessive nano-silica particles.

Fig. 6. The char yield of the WPU-DPPD vs. DPPD content incorporated in WPU.

3.6. Contact angle measurement microstructure of nano-silica grafting CWPU was studied by SEM as shown in Fig. 8. The nano-silica content in WPUi1, WPUi3, WPUi5 and WPUi10 was 1%, 3%, 5% and 10% respectively. Generally, nano-silica can be uniformly dispersed into CWPU by ‘‘click’’ reaction. This is mainly because functionalized nano-silica with azide-SiCA formed network structure in CWPU. Moreover,

The increased WCA of CWPU showed the successful grafting of ASiFS to the WPU backbone (see Fig. 11). For example, the pristine WPU showed a low WCA droplet (i.e., 61°). Upon addition of ASiFS to the WPU, the WCA increased to 63°, 91°, 83° and 72° for WPUi1, WPUi3, WPUi5 and WPUi10, respectively. This indicated that the hydrophobicity of CWPU can be improved by ASiFS.

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Fig. 8. SEM pictures of pristine WPU and CWPU.

Table 1 Effects of ASFS on the heat resistance of CWPU.

100 100

DPPD

80

Weight (%)

Weight (%)

80

60

60 40 20

40

a:WPU b:WPU-SiO2(1%) c:WPU-SiO2(3%) d:WPU-SiO2(5%) e:WPU-SiO2(10%)

20

0

0

0

100

0

T0.3 (°C)

T0.5 (°C)

Tmax (°C)

293 281 282 276 279

317 323 323 313 322

336 346 342 330 340

328 336 340 331 342

Temperature (?)

400

e

d

c

300

T0.1 (°C)

WPU WPUi1 WPUi3 WPUi5 WPUi10

T0.1, T0.3, T0.5, and Tmax refer to the temperature at weight loss of 10%, 30%, 50% and maximum decomposition.

100 200 300 400 500 600 700

a

200

Designation

500

600

b 700

Temperature (ºC) Fig. 9. Thermal degradation behaviors of DPPD, WPU and CWPU.

It is well known that surface roughness and surface energy affect the WCA. Therefore, the linkage of ASiFS and the WPU matrix changed the surface characteristics of the hybrid materials and led to a lower surface energy [32,33]. However, it also can be seen from Fig. 11 that the WCA of CWPU grew with the content of nano-silica and then decreased when the content was more than 3%. This was mainly ascribed to the fact that excessive silica resulted in partial agglomeration of nano-silica particles.

4 2

d

0

DTG %/min

3.7. Influence of nano-silica content on water absorption

a c

-2 -4

b

a:WPU b:WPUSi1 c:WPUSi3 d:WPUSi5 e:WPUSi10

-6 -8 -10 -12

e

-14 100

200

300

400

500

600

Temperature/ ºC Fig. 10. DTG curves of WPU and CWPU.

700

800

The influence of the content of nano-silica on the water absorption of CWPU was shown in Fig. 12. The water absorption decreased with the increase of nano-silica when the content was less than 3%. This indicated that the incorporation of nano-silica into CWPU by click reaction enhanced water resistance by the formation of ASiAOASiA hydrophobic network and higher crosslinking density between corsslink points. The ASiAOASiA bonds formed by the polycondensation reaction between silanol groups of nano-silica and the hydroxides from the hydrolysis of the ethoxy groups of SiCA in CWPU. Additionally, SiCA is a trifunctional material in terms of reactive ethoxy groups and therefore were able to form 3D chemical bonds between WPU molecules [24]. However, the water absorption no further varied with the increase of nanosilica content above 3%; this may be ascribed to the agglomeration

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Fig. 11. WCA pictures of WPU and CWPU.

Table 2 Effect of silica on the pencil hardness of CWPU films.

Fig. 12. Influence of the content of nano-silica on water absorption of CWPU.

of excessive nano-silica which decreased the above mentioned effect of nano-silica.

3.8. UV–vis spectra Fig. 13 shows the transmittance of the CWPU hybrid composites. Fig. 13 illustrates that the transmittance of the samples decreased with the increase of nano-silica. This indicated that the incorporation of ASiFS into WPU could improve the weather resistance due to the effect of UV shield of nano-silica. It was also

100

a b

Transmittance (%)

80

d

c e

60

a WPU

40

b WPUSi1 c WPUSi3

20

d WPUSi5 e WPUSi10

0 300

400

500

600

Wavelenght (nm) Fig. 13. Transmittance variation of the hybrid films with different silica content.

Designation

Pencil hardness

WPU WPUi1 WPUi3 WPUi5 WPUi10

HB H 2H 2H 2H

attributed to the fact that the silica particles were isolated as ‘‘dispersive’’ heterogeneous phase in the hybrid matrix resulting in a serious light scattering [34]. 3.9. Influence of nano-silica content on the hardness of CWPU Table 2 reveals that the pencil hardness of the CWPU films containing different nano-silica content was improved. The hardness increased from HB of WPU to 2H of WPUi3 with the increase of nano-silica content, also because of polycondensation reaction between hydroxyl on the SiO2 surface and hydroxyl formed by the SiCA. Dimers were first synthesized, and then the linear and network structures formed gradually; finally, the organic and inorganic phases interpenetrating polymer network (IPN) was generated, which improved the strength of composites. However, the hardness of CWPU films was constant from WPUi3 to WPUi5. This may be due to the excessive nano-silica in CWPU that led to nonuniform dispersion, and did no help to the formation of IPN [26].

4. Conclusions We showed an effective strategy in construction modified WPU with silane coupling agent and azide functionalized nano-silica using ‘‘click’’ chemistry. WPU bearing alkyne as pendant groups was synthesized by incorporating an alkyne diol (DPPD), and TGA and IR spectra proved the linkage of alkyne diol and WPU. This linkage improved the flame retardancy of the WPU. The copper catalyzed Huisgen 1,3-dipolar cycloaddition was undertaken between alkyne-functionalized WPU and ASiFS particles. These results illustrated that the thermal stability, hardness, and water and weather resistance of the CWPU were all improved when nano-silica was incorporated into WPU by ‘‘click’’ reaction. Therefore, it is a better conjugation technique to fabricate functional materials.

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Acknowledgments This work was supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2010BL022). The author is thankful for support by grant from the Qingdao Science Foundation, Shandong Province, China (Grant No. 8-1-3-17-jch). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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