Studies on waterborne polyurethanes based on new medium length fluorinated diols

Journal of Fluorine Chemistry 175 (2015) 12–17

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Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor

Studies on waterborne polyurethanes based on new medium length fluorinated diols Junpei Li, Xingyuan Zhang *, Zheng Liu, Weihu Li, Jiabing Dai CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 23 November 2014 Received in revised form 25 February 2015 Accepted 26 February 2015 Available online 6 March 2015

A novel fluorinated diol 3-(bis-(N,N-dihydroxyethyl)) dodecafluoroheptyl acrylate (DEFA) was for the first time synthesized via Michael addition reaction between diethanolamine and dodecafluoroheptyl acrylate, followed by a series of waterborne fluorinated polyurethane emulsions IPDI-DEFA-PU synthesis using isophorone diisocyanate (IPDI), poly(tetramethylene oxide glycol) (PTMG), dimethylolpropionic acid (DMPA) and 1,4-butanediol (BDO) as main starting materials, and DEFA as a chain extender. The structure of DEFA and IPDI-DEFA-PU was confirmed by Fourier transform infrared (FTIR) spectroscopy and proton nuclear magnetic resonance (1H NMR). It was found that the fluorine content increased from 0 to 15.1%, and the particle size of emulsion increased from 102 nm to 260 nm, whereas the surface tension of latex film decreased from 42.9 mN/m to 16.5 mN/m; while the water contact angle increased from 718 to 1048, the swelling ratio of latex film in water and cyclohexane decreased from 15.8% to 1.1% and from 28.8% to 3.4%, respectively. Meanwhile, water/organic solvent resistance and thermal stability, mechanical properties of IPDI-DEFA-PU were all improved. As tensile strength increased from 9 MPa to 15 MPa, extensibility decreased from 520% to 280%. ß 2015 Elsevier B.V. All rights reserved.

Keywords: Medium length fluorinated diols Polyurethane emulsion Water contact angle Surface tension Swelling ratio

1. Introduction In recent years, waterborne polyurethane (WPU) has been widely used in leather, adhesive, plastic, textile, water-based ink, water-based wood coatings, external and industrial coating industry due to its good cohesiveness, filming property, abrasion resistance, low temperature resistance and other physical mechanical properties, as well as its good performance against the properties of incombustibility, non-toxicity and less pollution to the environment. The research and development work is focused mainly on WPU containing anionic hydrophilic groups, especially those into which dimethylolpropionic acid (DMPA) or dimethylol butanic acid (DMBA) is introduced. The anionic polyurethane aqueous dispersions are usually obtained by self-emulsification in water after being neutralized with tertiary amine. However, the hydrophilic nature of ionic groups or water-soluble segments of waterborne type polyurethanes inevitably poses some problems associated with poor water resistance and solvent resistance, and bad mechanical properties. In order to improve the shortcoming,

* Corresponding author. Tel.: +86 551 63607484; fax: +86 551 63607484. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.jfluchem.2015.02.015 0022-1139/ß 2015 Elsevier B.V. All rights reserved.

some strong hydrophobic groups especially fluorocarbon segment, are commonly introduced into the molecular chain. The modified waterborne fluorinated polyurethane combines some of the virtues of WPU and fluorinated polymer, such as high thermal stability, good chemical resistance, low water absorptivity (water resistance), attractive surface properties, excellent flexibility, good wearability, and high weatherability [1–4]. So far, the general method of introducing fluorocarbon segment mainly focused on using small molecule containing fluorine and fluorinated macromolecular glycol as a chain extender, blocking fluorine chemically into polyurethane (PU) chain [5–12], or forming hybrid emulsion using polymerized fluorine-containing acrylate with polyurethane [1320]. These methods generally embedded fluorine atoms into molecular main chain. During the formation of latex film, the migration quantity of fluorine to the film surface does not work well because the movement of fluorinecontaining segments with low surface energy is restricted by the movement of full molecular chains. In WPU systems, too longer hydrophobic segments will interfere with the mobility and the liquid character of the micro-phases, whereas too shorter segments will interfere with the hydrophobic phase separation from the aqueous phase [21]. Also, the main disadvantage of a fluorinated monomer is its relatively high cost. Therefore, the

J. Li et al. / Journal of Fluorine Chemistry 175 (2015) 12–17

CF3

O

HOCH2CH2

N H

HOCH2CH2

CH2 CH C O

CH2

DEA

HOCH2CH2 HOCH2CH2

CF3

CF CFHCF CF3 DFHA

N CH2CH2

O

CF3

C O

CFCFHCF CF3

CH2

CF3

DEFA Scheme 1. Synthesis of DEFA.

content of fluorinated monomers should be minimized while the low value of surface tension was obtained (water/oil repellency). In view of these problems, the synthesis of a novel fluorinated diol 3-(bis-(N,N-dihydroxyethyl)) dodecafluoroheptyl acrylate (DEFA) by using diethanolamine and dodecafluoroheptyl acrylate was carried out. This was followed by the preparation of a polyurethane aqueous emulsion IPDI-DEFA-PU with fluorinated medium branched chain using DEFA as a chain extender. Based on the measurement and analysis of the surface tension, contact angle with water, the swelling ratio in water/cyclohexane and stress– strain curve for the latex film, different contents of fluorinated moieties in IPDI-DEFA-PU with relevant performances was systematically investigated. By choosing suitable techniques and appropriate fluorine content, the stable IPDI-DEFA-PU aqueous emulsion and the latex film with low surface tension can be successfully prepared. 2. Results and discussion 2.1. Structure analysis using FTIR and 1H NMR As shown in Scheme 1, the double bond in DFHA will gradually disappear as the reaction progresses and DEFA is formed. In order to understand the generation of DEFA during its synthesis, infrared spectroscopy was used to monitor the reaction process. FTIR spectra monitor the range between 1550 and 1850 cm1 for the mixtures of DEA, DFHA and DEFA at different reaction times (Fig. 1). The absorption peak located at 1734 cm1 is attributed to stretching vibration of the ester carbonyl bond (C5 5O), while that located at 1640 cm1 can be attributed to the stretching vibration of the alkene bond (C5 5C). Fig. 1 shows that the peak at 1640 cm1 gradually decreased with the increase of reaction time. This suggested that an addition reaction between the NH group of DEA and the C5 5C of DFHA occurred, hence suggesting that DEFA was generated. For the absorption peak of 1734 cm1, the peak symmetry gradually changed and peak location moved to a higher wavenumber. A blue shift of 24 cm1 from 1734 cm1 to 1758 cm1 was observed at the end of the reaction. The phenomena of a blue shift can be attributed

to the reduction of the C5 5C group and the break of the conjugate system in DFHA with the addition reaction, which also proves that the amount of DEFA increases gradually with the increase of reaction time. As can be seen from the infrared spectra (Fig. 1), the reaction goes to completion within 8 h. The molecular structure of DEFA can be further confirmed by 1H NMR spectroscopy (Fig. 2). Peak a located at 2.28 ppm is contributed by the proton of methylene connected to carbonyl group, peak b located at 2.62 ppm is produced by the proton of methylene connected with nitrogen, and peak c located at 3.63 ppm is attributed to the proton of methylene connected to hydroxyl group. Due to the coupling effect of the adjacent methylene proton, it appeared as a triplet peak. Peak d located at 3.88 ppm only appears as unimodal form, contributed by the proton of hydroxyl group as there is no adjacent proton coupling effect. Peak e was located between 4.65 and 4.80 ppm and was assigned to the protons of –CFCH2O– group, it appears as multiple peaks because of common coupling effect between a and b fluorine atoms. A slight difference with the other peak is the appearance of f, which is contributed by the proton of –CFH. Due to the coupling action of adjacent fluorine atoms connected to –CFCF3 and –CF(CF3)2, all bimodal appeared as multiple peak between 4.89 and 5.09 ppm. The 1H NMR spectral analyses demonstrated that the ratio of integrated values of peaks was 2:6:4:1.5:0.7, approximately 2:6:4:2:1, which revealed the presence of 2, 6, 4, 2 and 1 protons at chemical shifts 2.28, 2.62, 3.63, 4.654.80 and 4.89–5.09 respectively. 2.2. Chain structure analysis of IPDI-DEFA-PU by FTIR The chain structure of IPDI-DEFA-PU was confirmed by FTIR spectroscopy. FTIR spectra of DEFA-PU0 and DEFA-PU5 are shown in Fig. 3. Whether samples contained fluorine or not, the typical absorption peaks of polyurethane are observed at 3329 cm1 [n(NH)], 2863–2943 cm1 [n(CH2) and n(CH3)], 1710 cm1 [n(C5 5O)], 1532 cm1 [d(N–H)], 1108 cm1 [n(C–O–C)] and 1236 cm1 [n(CN)] in both spectra. The peak at 1236 cm1 of DEFA-PU5 is overlapped by the strong peak located at 1193 cm1 [n(CF)]. In addition, absorptions at 1282 cm1 and 1193 cm1 can be assigned to the stretching vibration of C–F bonds in polyurethane chains of DEFA-PU5. Moreover, the peak located at 836 cm1 was assigned to the characteristic absorption peak of the CF3 group. The appearance of three peaks indicated that the structure of DEFA as a chain extender has been incorporated into the molecular chain of IPDI-DEFA-PU. The absorbance peaks disappear gradually at around 2260 cm1 (NCO), which indicates that all the NCO groups of the isocyanate were reacted. These changes in peak observation indicate that IPDI-DEFA-PU with fluorinated medium branched chain is synthesized successfully.

8h 6h 4h 2h 0h

1758 1734

1640

1800

1700

1600

Wavenumber ( cm-1)

Fig. 1. FTIR spectra of DEFA at different reaction times.

13

Fig. 2. 1H NMR spectrum of DEFA.

J. Li et al. / Journal of Fluorine Chemistry 175 (2015) 12–17

14

following geometric-mean equation [23].

DEFA-PU0

1=2

g L1 ð1 þ cos u1 Þ ¼ 2ðg dL1 g dS Þ þ 2ðg pL1 g pS Þ1=2 1=2 g L2 ð1 þ cos u2 Þ ¼ 2ðg dL2 g dS Þ þ 2ðg pL2 g pS Þ1=2

1236 DEFA-PU5

3329 836

1532

1710

1282 1193

3500

3000

1 500

1000

Wavenumber (cm-1) Fig. 3. FTIR spectra of DEFA-PU0 and DEFA-PU5.

2.3. Particle size of IPDI-DEFA-PU emulsion It is generally observed that the particle size of polyurethane emulsion is 50–400 nm. By increasing the fluorine content of polyurethane, surface energy could be reduced as well as the water and organic solvent resistance could be improved. The relationship between particle size and fluorine content is shown in Fig. 4. It can be observed that with increasing fluorine content of IPDI-DEFA-PU, the particle size of the emulsion increases gradually (from 102 nm for DEFA-PU0 to 260 nm for DEFA-PU5). The increase of fluorine content indicates the increase of DEFA embedded into the molecular chain, resulting in the quantitative increase of medium branched chain linked to IPDI-DEFA-PU chain. Due to high hydrophobicity of the medium branched chain, the particle size of emulsion is enlarged during the formation of emulsion particles. The interaction between the fluorine content, the particle size and the emulsion stability should be taken into account as enlarged particle size has a certain influence on the emulsion stability. To meet the specific requirement in practice, DEFA content should be appropriately controlled. 2.4. Surface tension of IPDI-DEFA-PU latex film The properties of water and organic solvent resistance for waterborne polyurethane are relatively poor due to higher surface tension of the hydrophilic groups contained in the molecular chain. Fluorine component incorporated into the molecular chain in general can reduce the surface tension and improve water and organic solvent resistance. The surface tension of IPDI-DEFA-PU latex film with different DEFA content can be calculated using the

(1)

where gS represents the surface tension of latex film, and can be divided into two items of dispersion force g dS and polarity force g pS . gL1 and gL2 are the surface tensions of two testing liquids, and also can be divided into dispersion force (g dL1 and g dL2 ) and polarity force (g pL1 and g pL2 ) respectively. Here g S ¼ g dS þ g pS , g L1 ¼ g dL1 þ g pL1 and g L2 ¼ g dL2 þ g pL2 . Therefore, if g dL1 , g pL1 , g dL2 and g pL2 of two testing liquids are known, g dS and g pS can be calculated according to Eq. (1) by measuring the contact angles u1 and u2 of two testing liquids on the surface of latex film, and can further calculate the value of the surface tension gS. The testing liquids are deionized water (L1) and methylene iodide (L2), and the corresponding g dL1 , g pL1 , g dL2 , g pL2 are 21.8, 51.0, 49.5 and 1.3 mN/m, respectively. It could be seen from Fig. 5(a) that the surface tension of latex film decreased gradually with increasing fluorine content. When fluorine content was lower than 8%, the surface tension decreased linearly with increasing fluorine content. However, the decreasing trend of surface tension with a slower rate when fluorine content was higher than 8%. While the fluorine content increased from 0 to 15.1%, the surface tension decreased from 42.9 mN/m to 16.5 mN/ m, a reduction of 62%. Surface tension less than 20 mN/m is reached with a fluorine content higher than 9.13%. It is observed that fluorinated medium branched chains linked to polyurethane main chains enhanced its migration to the film surface, resulting in a significant decrease of surface tension with the increase of fluorine content. 2.5. Water contact angle of IPDI-DEFA-PU latex film High water contact angles/low surface energy (tension) was used as criteria of water repellent/antifouling properties in this study. The water contact angle of IPDI-DEFA-PU latex film with different fluorine content is shown in Fig. 5(b). It could be seen that the contact angle increased linearly with the increase of fluorine content when fluorine content was 8% or less. But when fluorine content was higher than 8%, the contact angle increased at a slower rate. This change in contact angle trend was opposite to that of surface tension. During the formation of latex film, the migration quantity of fluorinated moieties to the film surface works well for DEFA-PU1-5 because the movement of fluorine-containing segments (multiple micro-phase separation) with medium length chains is very easy, and even fluorine-containing segments with low surface energy are restricted by the movement of full molecular chains. But when fluorine content was higher than 1 10

Contact angle (o)

250

Partcle size (nm)

45

1 05

300

200 150

1 00

40

b

35

95 90

30

85

25

80 75

20

a

70

100 0

2

4

6

8

10

12

14

Fluorine content (wt %) Fig. 4. Particle size of IPDI-DEFA-PU emulsion.

16

65

Surface tension (mN/m)

1108

15 0

2

4

6

8

10

12

Fluorine content (wt %)

14

16

Fig. 5. Surface tension and water contact angle of IPDI-DEFA-PU latex film.

J. Li et al. / Journal of Fluorine Chemistry 175 (2015) 12–17

100

30

DEFA-PU5 DEFA-PU4 DEFA-PU3 DEFA-PU2 DEFA-PU1 DEFA-PU0

80

25

b

20

Mass (%)

Swelling ratio (%)

15

15

a

10

60 40 20

5 0

0

2

4

6

8

10

12

14

100

16

Fluorine content (wt %)

200

300

400

o

Temperature ( C)

500

600

Fig. 6. Swelling ratio of IPDI-DEFA-PU latex film in water (a) and cyclohexane (b).

Fig. 7. TGA curves of IPDI-DEFA-PU latex films under nitrogen atmosphere.

8%, the migration quantity of fluorinated moieties to the film surface does not work effectively because the movement of fluorine-containing segments is restricted by the capacity of the film surface, and thus contact angle increased at a slower rate. Overall, fluorocarbon fragments were found to migrate to the film surface more effectively with an increase in fluorine content, resulting in a decreased surface tension. From this it was found that the contact angle increased from 718 (DEFA-PU0) to 1048 (DEFA-PU5). The increase of contact angle indicated that water resistance of the film was improved.

obvious. With the increase of fluorine content, T1 and T2 gradually increased. Therefore, it can be concluded that the thermal stability after introducing fluorinated monomer for DEFA-PU1-5 is better than the pure PU of DEFA-PU0.

The water resistance of the latex film can also be appraised from Fig. 6(a). The curve indicates a relationship between swelling ratio of the film in water and fluorine content. With increasing of fluorine content, the swelling ratio decreased gradually from 15.8% for the sample DEFA-PU0 to 1.1% for the sample DEFA-PU5. This clearly showed that the water resistance of the film had been improved greatly due to the introduction of fluorinated medium branched chain. The relationship between swelling ratio of the film in cyclohexane and fluorine content (Fig. 6(b)), revealed the organic solvent resistance for the latex film. Although the swelling ratio in cyclohexane is higher than that in water for all samples, the swelling ratio decreases similarly with the increase of fluorine content. The reduction magnitude is significantly higher than that in water, from 28.8% for the sample DEFA-PU0 to 3.4% for the sample DEFA-PU5. This is likely attributed to the migration rate of the fluorocarbon component as well as urethane component to the film surface than urethane in different solvents [14]. Thus it can be seen that the introduction of fluorinated medium branched chain in polyurethane molecules can improve the performance of organic solvent resistance. 2.7. Thermal stability of IPDI-DEFA-PU latex film In general, the molecular chain in waterborne polyurethane is a linear structure and its thermal stability is relatively poor. It was found that introduction of a fluorine component in the molecular chain could increase its thermal stability [24]. It can be seen from the thermogravimetric analysis (TGA) curves (Fig. 7) that the initial decomposition temperature increases from 230 8C for the sample DEFA-PU0 to 300 8C for the sample DEFA-PU5. Generally speaking, the degradation of the urethane component begins first from its hard segment (T1) and the soft segment starts to decompose at a temperature (T2) of 350 8C or above. In this case, the relationship between the degradation temperature and fluorine content is

The stress–strain curves of IPDI-DEFA-PU latex film are shown in Fig. 8. The curves present a typical mechanics characteristic for amorphous polymer with a good elasticity. The tensile strength of the film increased, and the elongation breaking point was reduced with an increase in fluorine content. For the latex film without containing fluorine (DEFA-PU0), the breaking elongation can reach up to 520%; the elongation at breaking point was reduced to 280% as the fluorine content increases to 15.1%. Meanwhile, the tensile strength also increases from 9 MPa for the sample DEFA-PU0 to about 15 MPa for the sample DEFA-PU5 as the amount of fluorinated medium branched chain in polyurethane plays an important role in the mechanical properties of the film. Although the increase for the component of fluorinated medium branched chain will increase the formed emulsion particle size, the medium branched chain in the molecular chain may increase the degree of chain entanglement in the packing process of molecular chain during film formation from the emulsion. This results in the improvement of film toughness and the increase of tensile strength. In this way, fluorinated polyurethane materials with good water and organic solvent resistances, as well as certain mechanical strength can be prepared practically through the content control of fluorinated medium branched chain. 20

DEFA-PU5 DEFA-PU4 DEFA-PU3 DEFA-PU2 DEFA-PU1 DEFA-PU0

15

Stress (MPa)

2.6. Water and organic solvent resistance of IPDI-DEFA-PU latex film

2.8. Mechanical properties of IPDI-DEFA-PU latex film

10

5

0

0

100

200

300

400

500

Strain (%) Fig. 8. Stress–strain curves of IPDI-DEFA-PU latex films.

600

J. Li et al. / Journal of Fluorine Chemistry 175 (2015) 12–17

16

NCO

CH3

H O CH2CH2CH2CH2 O nH

CH2NCO

CH3

CH3

PTMG

IPDI

O

O

NHC R1 CNH R2 n NCO 1

OCN R2

Prepolymer I

CH3 HO CH2 4 OH , DEFA

HOCH2-C-CH2OH COOH

BDO

DMPA

O

CH3

O

O

O

OCN R3 NHCOCH2 -C-CH2 O C NH n2 R3 NHCO R 4 OCNH n3R3 NCO COOH

Prepolymer II

TEA

O

CH3

O

OCN R3 NHCOCH2 -C-CH2 O C NH C ON CH2CH3 H O

O n2

R3

3

O

NHCO R 4 OCNH n R3 NCO 3 H2O

IPDI-DEFA-PU aqueous emulsion

R1=

O CH2CH2CH2CH2 O n O

O

R2 =

CH3

CH3

CH2 CH3

NHC R 1 CNH R2 n1

R3=

R2

R4=

O CH2 4O

or

OCH2CH2 OCH2CH2

CF3

O N CH2CH2

C O

CH2

CF3

CFCFHCF CF3

Scheme 2. Synthetic process of IPDI-DEFA-PU aqueous emulsion.

3. Conclusions

4. Experimental

A novel fluorinated diol 3-(bis-(N,N-dihydroxyethyl)) dodecafluoroheptyl acrylate (DEFA) was synthesized via Michael addition reaction between diethanolamine and dodecafluoroheptyl acrylate. IPDI-DEFA-PU polyurethane emulsions with fluorinated medium branched chain were synthesized by self-emulsification method using IPDI, PTMG, DMPA and BDO as main raw materials, and DEFA as a chain extender. Effect of physical properties on IPDI-DEFA-PU with fluorine content is well studied. It was found that the fluorine content increased from 0 to 15.1%, the emulsion particle size increased from 102 nm to 260 nm, and surface tension of latex film decreased from 42.9 mN/m to 16.5 mN/m while the water contact angle increased from 718 to 1048. The swelling ratio of latex film in water and cyclohexane decreased from 15.8% to 1.1% and from 28.8% to 3.4%, respectively. Meanwhile, water and organic solvent resistance and thermal stability were all improved. Also tensile strengths increased from 9 MPa to 15 MPa, and extensibility decreased from 520% to 280%.

4.1. Materials Dodecafluoroheptyl acrylate (DFHA) was the product of Xeogia Fluorine-Silicon Chemical Co., Ltd. Isophorone diisocyanate (IPDI) was purchased from Junsei Chemical Co., Ltd. DFHA and IPDI were vacuum distilled before use. Polytetramethylene ether glycol (PTMG, Mn = 2000) was supplied by Daicel Chemical Industries, Ltd. and desiccated at 110 8C before use. Dimethylolpropionic acid (DMPA) was provided by Aldrich Chemical Company and used as received. Diethanolamine (DEA), 1,4-butanediol (BDO), triethylamine (TEA) used as neutralization agent, acetonitrile or acetone used as solvent, and di-n-butyltin dilaurate (DBTDL) used as catalyst were all purchased from Shanghai Chemical Reagent Co., Ltd. 4.2. Synthesis of DEFA The synthetic route of DEFA is shown in Scheme 1. A mixture of DEA and DFHA (molar ratio is 1.1:1) was added into a four-neck

J. Li et al. / Journal of Fluorine Chemistry 175 (2015) 12–17 Table 1 Compositions of IPDI-DEFA-PU. Sample

IPDI:PTMG:DMPA (mol:mol:mol)

BDO (mol)

DEFA (mol)

WF (wt%)

DEFA-PU0 DEFA-PU1 DEFA-PU2 DEFA-PU3 DEFA-PU4 DEFA-PU5

7:1:1.37 7:1:1.37 7:1:1.37 7:1:1.37 7:1:1.37 7:1:1.37

3.83 3.06 2.55 2.01 1.28 0

0 0.77 1.28 1.82 2.55 3.83

0 3.2 6.1 8.4 11.1 15.1

flask equipped with a mechanical stirrer, a thermometer, a reflux condenser, a nitrogen catheter, and using acetonitrile as solvent. The reaction was carried out at 80 8C for 8 h under N2 atmosphere with agitation. Crude product of DEFA was prepared after cooling the reaction system to ambient temperature. DEFA was obtained by repeated extraction using deionized water to remove unreacted DEA and acetonitrile, and the excess DFHA is removed by reduced pressure distillation to yield DEFA as a light yellow oil. 4.3. Synthesis of IPDI-DEFA-PU The synthetic process of IPDI-DEFA-PU aqueous emulsion is shown in Scheme 2. IPDI and PTMG were firstly added into a fourneck flask equipped with a mechanical stirrer, a thermometer, a reflux condenser, and a nitrogen catheter. The prepolymerization of polyurethane was carried out at 90 8C under N2 atmosphere for 2 h. Prepolymer I with active NCO group was obtained. Then DMPA and BDO were added into the reactor and reacted at 70 8C for 1.5 h. During the prepolymerization, a proper level of acetone was needed to reduce viscosity of the system. Subsequently, DEFA was added into the system and reacted at 70 8C for 2 h until prepolymer II was obtained using DBTDL (0.05–0.1 wt%) as a catalyst. NCO content during the polymerization was monitored via a standard di-n-butylamine titration test method [22]. As a neutralization agent, TEA was added to neutralize the carboxylic group in the side chain of polyurethane. The molar ratio of TEA to the carboxylic group was fixed at 1.05:1 to guarantee complete neutralization. Finally, a high speed shearing rate (2500 r/min) was used to emulsify the solution for 5 min after suitable deionized water was added into the mixture. The WPU emulsion with fluorinated medium branched chain was obtained after removing acetone using a rotary evaporator under reduced pressure. The solid content of the obtained emulsion was 30% (w/w). On the basis of the method, a series of IPDI-DEFA-PU aqueous emulsions were prepared and their basic formulations are given in Table 1 (WF is the fluorine content of IPDI-DEFA-PU latex film). 4.4. Sample preparation The latex film was prepared by casting IPDI-DEFA-PU emulsion on a leveled PTFE template at room temperature for 7 days, and then vacuum drying to a constant weight at 60 8C. The prepared latex film with thickness of about 100 mm were chosen for further investigation. 4.5. Characterization FTIR spectra were recorded using a thin latex film (thickness less than 2 mm) on a Bruker Vector-22 spectrophotometer. 1H NMR spectra were collected on a Bruker AC-300 NMR spectrometer

17

with tetramethylsilane (TMS) as an internal standard and deuterated chloroform (CDCl3) as a solvent. Particle size of IPDIDEFA-PU emulsions was determined using a NicompTM 380 particle size analyzer (PSS, USA). The contact angle and the surface tension were measured at 25 8C by the sessile-drop method using a contact angle goniometer (JC 2000C1, Shanghai Zhongchen Digital Technical Equipment Ltd.) with a temperature control system. The whole measuring process was monitored by an imaging apparatus. The result reported was the mean value of three times for each sample. Surface tension of the IPDI-DEFA-PU latex film was calculated by the geometric-mean method [23], and deionized water and methylene iodide were used as test liquids. Water and organic solvent resistance of IPDI-DEFA-PU latex films were characterized by the swelling ratio of film soaked in water or cyclohexane, respectively. The film was cut into 3 cm  3 cm piece and dried in a vacuum oven for 24 h to determine dry weight M0. The sample weight absorbed water or cyclohexane M was determined by wiping off the surface water or cyclohexane with a piece of filter paper. The swelling ratio was calculated using the formula: swelling ratio (%) = [(M  M0)/M0]  100%. Each sample was repeated five times and a mean value was calculated. TGA was carried out on a thermal analyzer (Shimadzu TGA-50). Stress– strain curves were measured using a TY8000 tensile tester (Jiangdu Tianyuan Test Machinery Co., Ltd.) with a cross head speed of 100 mm/min at 25 8C. The dumbbell-shaped specimen was 20 mm in length and 4 mm wide at the extension part. Acknowledgements Financial supports from the National High Technology Research and Development Program of China (No. 2014AAQ00294) and the National Natural Science Foundation of China (No. 51073144) are acknowledged. We also thank Dr. Robert Harper in Institute of Pharmaceutical Sciences (King’s College London) for proof-reading and discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

[23] [24]

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