Synthesis and Characterization of Co-Modified Polyurethane Nanocomposite Latexes by Terminal and Pendant Fluoroalkyl Segments

Abstract

To further improve the hydrophobic and thermal properties of fluorinated polyurethane (FPU), we initially prepared the fluorine- and silicon-containing pendant groups’ diols (DEFA and DESiA) by Michael addition reaction. Next, we synthesized a series of co-modified polyurethane nanocomposite (TPFPU-SiO₂) latexes by terminal and pendant fluoroalkyl segments via varying the additive amount of silica sols. Structure and performance properties of the target materials were characterized by IR, TEM, TGA, DSC, XRD, XPS, SEM, AFM, and contact angle measurements. Results showed that with the increase in silica sols dosage, the mean particle size of the TPFPU-SiO₂ latexes was increased and their emulsion stability gradually decreased. The thermal stability of the TPFPU-SiO₂ films were gradually increased and hydrophobicity of the TPFPU-SiO₂ films increased first and then slightly decreased. The maximum water contact angle on the TPFPU-SiO₂ films was 119.8° and the lowest water absorptivity was 2.49%. Thus, these novel fluorinated polyurethane nanocomposite latexes can find valuable applications in heat-resistant and anti-fouling coatings.

1. Introduction

Waterborne polyurethane has advantages such as an eco-friendly environment, good mechanical properties, and easy modification. In recent years, it has been widely used in many industrial fields such as leather coating, fabric coating, adhesives, and building insulation materials [1,2,3,4]. However, the shortcomings of poor water resistance limits its further development. Therefore, many researchers devote themselves to improving the poor water resistance of waterborne polyurethane [5,6,7].

Fluorocarbon chain segments have low surface energy, strong chemical and thermal stability, etc. Thus, they are often chosen to improve the water tolerance of waterborne polyurethane. So far, fluorinated polyurethanes are involved in several methods [8,9,10,11,12,13,14,15,16,17,18,19,20], amongst which fluorinated hyperbranched polyurethane or the fluoroalkyl group at the end of the polymer chain are proved more effective in hydrophobic modification. In order to pursue the balance between the hydrophobic modification effectiveness and the appropriate cost of waterborne polyurethane, we prepared the co-modified polyurethane by the terminal and pendant fluoroalkyl segments, and then cross-linked linear polyurethane molecules by the trimethylolpropane from our previous work [21], starting from the perspective of the molecular structure design. At last, the water resistance of the obtained waterborne fluorinated polyurethane was well improved.

Studies have also shown that nanosilica can form crosslinking points (Si-O-Si and Si-O-C) with polyurethane molecules through physical adsorption or chemical reaction, thus forming a three-dimensional network structure, which endows the material with good heat stability and mechanical properties [22]. In addition, the embedded nanoparticles can increase the microscopic roughness of hydrophobic polyurethane film, which can further improve the hydrophobicity of polyurethane.

In this context, a reactive silicon-containing chain extender was first prepared by Michael addition reaction, and it was combined with the modified polyurethane molecules through the end and the suspended fluoroalkyl segments. Those modified polyurethane molecules were cross-linked by pentaerythritol (PETP) and then emulsified. During the emulsification process, a different quantity of silica sols were added to conduct the condensation reactions between siloxy groups and silanol groups. Finally, a series of the terminal and pendant fluoroalkyl segments co-modified polyurethane nanocomposite latexes were prepared and the effect of the silica sols dosage on properties of polyurethane latex and its film was emphatically investigated.

2. Materials and Methods

2.1. Materials

Aminopropyltriethoxy silane (APTES), of analytical purity, was purchased from Hubei Wuda Silicone New Material Co., Ltd., China. 4-methoxyphenol of analytical purity was bought from China Tianjin Komeo Chemical Reagent Co., Ltd. Hydroxyethyl acrylate (HEA), triethylamine (TEA), acetone, 1,4-Butanediol (BDO), pentaerythritol (PETP), N-methyl-2-pyrrolidone (NMP), and Dibutyltin dilaurate (DBTDL) were reagents with analytical purity and were provided by China Tianjin Fuchen Chemical Reagent Co., Ltd. 2,2-dimethylol propionic acid (DMPA) and isophorone diisocyanate (IPDI) were analytically pure reagents and were purchased from China Jining Baiyi Chemical Co., Ltd. The industrial products dodecafluoroheptyl methacrylate (G04) and perfluorohexyl ethanol (S104) were supplied by Xuejia Fluorosilicone Chemical Co., Ltd. in Harbin, China. Polybutylene glycol adipate (CMA-1044) was produced by Huada Chemical Group Co., Ltd. in Yantai, China with an average molecular weight of 1000 g/mol. Diethanol amine (DEOA) was purchased from Aladdin Biochemical Technology Co., Ltd. in Shanghai, China with analytical purity. Silicon wafers were purchased from Songjiang Silicon Material Co., Ltd. in Shanghai, China, and were washed and treated on the basis of our previous works [15,16,17,21].

2.2. Fabrication of the Silicon-Containing and the Fluorine-Containing Chain Extenders (DESiA and DEFA)

The silicon-containing chain extender (DESiA) was prepared in detail as follows. In total, 4.42 g of APTES and 0.0465 g of 4-methoxyphenol were added into a three-neck flask equipped with a thermometer, a reflux condenser, and a nitrogen catheter; then, 4.88 g of HEA was slowly dropped into it with gentle stirring under ice cooling and continued to react at 40 °C for 6 h under N₂ atmosphere. After the reaction was completed and the ambient temperature in the flask had reached room temperature, raw product DESiA was repeatedly extracted to eliminate the unreacted reagents using deionized water, and the water was removed using reduced pressure distillation. The whole reaction process of the silicon-containing chain extender (DESiA) is illustrated as procedure (1) in Scheme 1. Additionally, the fluorine-containing chain extender (DEFA) was synthesized according to our previous work [21].

2.3. Preparation of the Terminal and Pendant Fluoroalkyl Segments Co-Modified Polyurethane Nanocomposite (TPFPU-SiO₂) Latexes

Firstly, the given amounts of CMA-1044 and IPDI were added in order into a 250 mL three-necked flask where the N₂ gases were then filled for half an hour. Next, the reaction mixture was vigorously stirred and heated to 80 °C. Three drops of DBTDL as catalyst were added into the system and the reaction time was 1 h. Then, predetermined amounts of DEFA, DESiA, BDO, and DMPA were added to the mixture and this reaction was kept for another 2.5 hrs. Then, NCO-capped fluorinated PU prepolymer was obtained. Afterwards, the N-methyl-2-pyrrolidone solutions of given S104 and PETP were dripped into the system successively and reacted for 2 h, respectively. After this, the stoichiometric portion of TEA was added into the system as the neutralizing agent and the neutralization reaction was proceeded at 40 °C for half an hour. Soon after, a white and viscous co-modified polyurethane (TPFPU) by the terminal and pendant fluoroalkyl segments was obtained. At last, the slightly blue TPFPU-SiO₂ nanocomposite latexes with about 30% solid content were prepared by stirring and dispersing TPFPU with deionized water and different portions of silica sols. The whole reaction process of TPFPU-SiO₂ is illustrated by Scheme 1 and the preparation formulas of the TPFPU-SiO₂ latexes are also listed in

Table 1. The synthetic formulas for the TPFPU-SiO₂ nanocomposite latexes.

Chemicals	TPFPU-SiO2-0	TPFPU-SiO2-1	TPFPU-SiO2-2	TPFPU-SiO2-3	TPFPU-SiO2-4
PETP	0.39	0.39	0.39	0.39	0.39
IPDI	11.11	11.11	11.11	11.11	11.11
CMA-1044	18.50	18.50	18.50	18.50	18.50
DMPA	1.87	1.87	1.87	1.87	1.87
BDO	0.18	0.18	0.18	0.18	0.18
DEFA	2.53	2.53	2.53	2.53	2.53
DESiA	2.27	2.27	2.27	2.27	2.27
S104	4.21	4.21	4.21	4.21	4.21
Silica sols	0	2.05	4.10	6.15	8.20

. The theoretical mass ratios of silica nanoparticles in series of the TPFPU-SiO₂ are increased by degrees as follows: 0, 5%, 10%, 15%, and 20%, respectively.

2.4. Preparation of the Latex Films

At first, about 30 g of TPFPU-SiO₂ latex was carefully weighed and poured onto the PTFE plate, then it was dried for 72 h at room temperature to form the TPFPU-SiO₂ film. Next, this TPFPU-SiO₂ film was dried for 24 h at 65 °C in an oven. After the latex film had cooled down, it was gently stripped from the PTFE plate and washed several times with absolute ethyl alcohol and deionized water, respectively. Finally, this latex film was dried in a vacuum oven for future use.

2.5. Test Methods

IR spectra of the samples were characterized by an IR spectrum tester (VECTOR-70, Bruker Ltd., Bremen, Germany) within a scope of 400–4000 cm⁻¹. The scanning frequency was 32 times and the resolution was 2 cm^–1, respectively. KBr coating method was utilized to determine the reactant, HEA, and the purified substance, DESiA, while KBr pellets method was to test the selected TPFPU-SiO₂-2 polymer.

The laser particle size instrument (Nano-ZS, Malvern Co., Ltd., Whitnash, UK) was applied for testing the particle’s size and its distribution of the TPFPU-SiO₂ latexes. The appearance of emulsion was checked by visual inspection. The emulsion stability was tested by centrifugal method. The TPFPU-SiO₂-2 latex was diluted to a suitable concentration and then colored using 2 wt% phosphotungstic acid solution. Next, it was observed and some photographs were taken by transmission electron microscope (TEM, Tecnai G2 F20 S, FEI. Co., Ltd., Hillsboro, OR, USA).

The heat endurance test of the samples was examined by a thermogravimeter (Q500, TA Co., Ltd., New Castle, DE, USA) in an inert atmosphere with a heating rate of 10 °C/min and a temperature range of 600 °C. The DSC test was conducted with a thermal analyzer (Q2000, TA Ltd., New Castle, DE, USA) at a heating rate of 10 °C/min and the temperature ranged from −60 °C to 100 °C.

The crystal structure of reagent CMA-1044 and the TPFPU-SiO₂ polymers was tested with an X-ray diffractometer (D8 Advanced, Bruker Ltd., Bremen, GER). Hereinto, CuK_α radiation (α = 0.154 nm) was used and generator parameters were set as 45 mA and 45 kV, respectively. The diffraction angle ranged from 5° to 60°; the diffraction step size and the diffraction step in continuous mode were adopted as 0.02° and 0.1 s, respectively.

The element composition of the samples was determined by an X-ray photoelectron spectrometer (Axis Supra, Kratos Analytical Co., Ltd., Stretford, UK). Hereinto, the X-ray source was the monochromatic Al K_α rays, the angle of the incidence was 90°, and the vacuum value was 1.2 × 10⁻⁸ Pa in the analysis room. The C_1s binding energy value (284.8 eV) of carbon contamination on the sample surface was set as a reference for the line deviation.

The surface morphologies of the TPFPU-SiO₂ films were observed using scanning electron microscope (VEGA3, TESCAN Ltd., Brno, CZ, USA). All the samples were fixed on the carrier table with conductive adhesive and then coated with gold under vacuum. The magnification and acceleration voltage were 200 times and 10.0 kV, respectively.

The fine morphologies of several TPFPU-SiO₂ were examined by a Nanoscope IIIA AFM (Digital Instruments Ltd., Boston, MA, USA) on silicon wafer. The ambient temperature and relative humidity were controlled as 20 ± 0.5 °C and 49.5 ± 2%, respectively. The test mode was selected as tapping mode.

The hydrophobicity of the TPFPU-SiO₂ films was indicated as water contact angle (WCA) and water absorption (Q). Thereinto, contact angle goniometer (OCA20, Dataphysics Ltd., Filderstadt, GER) was used to determine the WCA, and the injecting volume of distilled water was 5 μL. To reduce experimental errors, each sample was tested nine times, and the average of the test results was used as the final value. For testing water absorption, a piece of the TPFPU-SiO₂ films with a suitable size was placed for 48 h in room temperature distilled water, and then fetched from the water. Next, excess free waters were absorbed with filter paper. At last, Q could be calculated using the following formula:

$$Q=\frac{W2-W1}{W1}\times 100\%$$

where W₂ and W₁ are represented as the weight of the wet film with water and the dry film.

The TPFPU-SiO₂ films were firstly cut into dumbbell-shaped pieces (width: 3 mm, length: 10 mm) and their mechanical properties were detected using a universal testing machine (UTM2102, Sansizongheng Technology Co., Ltd., Shenzhen, China). Hereinto, the extension rate was set as 5 mm/min at room temperature.

3. Results and Discussion

3.1. IR Analysis

Figure 1 provides the IR spectra of the reactant, HEA, and the silicon-containing chain extender (DESiA). It shows that the signals at 3040 cm⁻¹ and 1641 cm⁻¹ resulted from the classical absorbed signals of the alkene in HEA. The wide absorption peak at 3400 cm⁻¹ and the weak pinnacles at 990 cm⁻¹ were due to stretching and bending vibrations of –OH, respectively. The signals at 2944 cm⁻¹, 2875 cm⁻¹, and 1302 cm⁻¹ were ascribed to the stretching and bending vibrations absorption peaks of –CH₂–. The absorption peaks at 1276 cm⁻¹ and 1195 cm⁻¹ originated from the asymmetric and symmetric stretching vibrations of ester group C-O. The strong absorption peak at 1731 cm⁻¹ resulted from the carbonyl group. However, the absorption peak of alkene vanished at 1641 cm⁻¹, while new absorption peaks appeared at 1248 cm⁻¹ and 800 cm⁻¹ in Figure 1b, which should be due to the bending vibration peaks of –CH₂– and Si-C from silane coupling agent. In short, the above fact has confirmed that HEA and APTES completed the Michael addition reaction to obtain the due silicon-containing chain extender.

FTIR spectrum of the representative co-modified polyurethane nanocomposite TPFPU-SiO₂-2 is shown in Figure 2. The strong signals at 3360 cm⁻¹ and 1540 cm⁻¹ were due to the stretching vibration and bending vibration of N-H in urethane groups and the absorption peaks at 1731 cm⁻¹ and 1302 cm⁻¹ resulted from the signals of C=O and C-N in urethane groups [23]. The signals at 2946 cm⁻¹, 2870 cm⁻¹, and 1302 cm⁻¹ were ascribed to the stretching and bending vibrations absorption peaks of –CH₂–. The absorption peaks at 1236 cm⁻¹ and 1092 cm⁻¹ were derived from the stretching vibrations of C-O and C-O-C and the absorption peaks at 1150 cm⁻¹, 732 cm⁻¹, and 700 cm⁻¹ were obtained from the stretching vibration and bending vibration of –CF [17]. The absorption peaks at 1020 cm⁻¹ 783 cm⁻¹, and 468 cm⁻¹ should be due to the asymmetric and symmetric stretching vibration peaks [22] and the bending vibration peaks of Si-O-Si bonds. The above results indicate that DESiA and DEFA were successfully embedded into the PU backbones and the terminal and pendant fluoroalkyl segments co-modified polyurethane nanocomposite were acquired via the present methods.

3.2. The Physicochemical Properties of the TPFPU-SiO₂

The physicochemical properties of emulsions such as emulsion stability, average particle size and its dispersity, and particle morphology play an important role in their application. Thus, we used several experimental methods to test some physicochemical properties of the TPFPU-SiO₂ latexes; the results can be seen in

Table 2. The physicochemical properties of the TPFPU-SiO₂ nanocomposite latexes.

Product No.	Appearance	Stability	Average Particle Size (nm)	Polydispersity Index
TPFPU-SiO2-0	Transparent, pale blue	Precipitation- and stratification-free	60.25	0.124
TPFPU-SiO2-1	Transparent, pale blue	Precipitation- and stratification-free	76.35	0.141
TPFPU-SiO2-2	Translucent, pale blue	Little precipitation	90.11	0.185
TPFPU-SiO2-3	Translucent, cloudy	Sectional stratification	110.65	0.229
TPFPU-SiO2-4	Non-transparent	Gels	159.08	0.296

.

It shows that emulsion appearance gradually changed from transparent with blue light to translucent or even cloudy to opaque and emulsion stability changed from no precipitation and stratification to partial delamination or gelling, which demonstrates that emulsion stability becomes worse with an increasing amount of the nanosilica addition. Meanwhile, average particle size increased from 60.25 nm to 159.08 nm and the corresponding polydispersity index from 0.124 to 0.296, with the increasing mass amount of the nanosilica addition increasing from zero to 20%. Those consequences possibly resulted from the following fact. At first, there is only a certain amount of reactive alkoxy group on both sides of the polyurethane main chain, which can interact with Si-OH on the surface of silica sol. Secondly, besides the above condensation reaction, the nanosilica particles in the water system can also exert self-condensation reaction. Therefore, agglomeration could occur, the emulsion balance would be broken, and finally the emulsion stratification and even the generation of gel could be engendered when the adding volume of silica sol was too high.

Taking TPFPU-SiO₂-2 as the representative, the TEM result is shown in Figure 3. It can be seen from the TEM image scale that the particle morphology was approximately regular spherical, with a particle diameter of about 70 nm. Considering the influence of hydration layer on DLS measurement, DLS measurement results are in good agreement with TEM measurement results.

3.3. Thermal Behaviors of the TPFPU-SiO₂

The thermal behavior of several TPFPU-SiO₂ polymers was determined using TGA and DSC and the results are shown in Figure 4 and Figure 5. As shown in Figure 4, the initial decomposition temperature of a series of TPFPU-SiO₂ polymers was about 190 °C, and the weight loss at this stage may be caused by impurities or small molecular products. Their corresponding T_d50% temperatures were 332 °C, 340 °C, 346 °C, 354 °C, and 361 °C, respectively, which demonstrates that the thermal stability of the TPFPU-SiO₂ polymers is strengthened in order. Those consequences should be due to the crosslinking effect of nanosilica particles amongst the polyurethane molecules chains, which makes the crosslinking density of the latex film enlarged and thermal stability enhanced. The residual amount at 600 °C was 1.52%, 6.31%, 11.74%, 15.90%, and 19.89%, respectively, which should the resulted of mainly inorganic SiO₂ and undecomposed ingredients.

As shown in Figure 5, only one T_g in all the TPFPU-SiO₂ curves appeared and were severally −49.2 °C, −43.2 °C, −40.3 °C, −35.0 °C, and −29.1 °C, which should be from the soft polyester segments. As the introduction of nanoparticles increased gradually, the crosslinking degree of the polyurethane molecules segments increased, which restricted the free movement of the soft molecular chains, and Tg was increased gradually.

3.4. XRD

To obtain the fine structure of co-modified polyurethane nanocomposites, the raw material, CMA-1044, and the TPFPU-SiO₂-1, TPFPU-SiO₂-2, and TPFPU-SiO₂-3 polymers were tested by XRD instrument; Figure 6 presents the results. Two distinct pinnacles appeared at 2θ = 21° and 24° in the CMA-1044 curve, respectively, which indicates that CMA-1044 has crystallinity. However, those above pinnacles at 2θ = 21° and 24° were gradually weakened, and a broad dispersion peak about at 2θ = 18° emerged with the increasing additional amount of nanoparticles SiO₂. Those consequences indicate that three TPFPU-SiO₂ polymers are mainly amorphous and the crystallinity of the soft segment decreases gradually with the increasing amount of nanoparticles introduced because of their crosslinking effect and restriction of the molecular chains’ movement.

3.5. Surface Analysis

Taking TPFPU-SiO₂-1, TPFPU-SiO₂-2, and TPFPU-SiO₂-3 as the representatives, their surface chemical element compositions were detected using the XPS technique and the wide spectra are shown in Figure 7. The results show that six peaks appeared at the binding energies of 101, 150, 284, 398, 531, and 686 eV, belonging to Si_2p, Si_2s, C_1s, N_1s, O_1s, and F_1s, respectively [24]; furthermore, the intensity of the silicon element gradually increased from TPFPU-SiO₂-1 to TPFPU-SiO₂-3, while the other intensities remained almost constant. This result is apparently consistent with the actual situation, where the dosages of all reagents are the same, except the silica sols, in different synthetic formulas of the TPFPU-SiO₂, aiming to explore the effect of the silica sols’ dosage on properties of polyurethane latex and its film.

3.6. Film Morphologies

The micromorphology of TPFPU-SiO₂-1, TPFPU-SiO₂-2, and TPFPU-SiO₂-3 latex films was examined by scanning electron microscope and the results are displayed in Figure 8. Three TPFPU-SiO₂ latex films all presented as smooth appearance with some granules on their surfaces. Furthermore, with the increase in the introduction of nanoparticles, the number of these granules increased gradually and the surface roughness increased. This is probably due to the deposition of silica nanoparticles in the form of chemical bonding or physical blending on the film surface. However, the exact particle size could not be obtained due to small magnification.

A more precise method is needed to acquire the detailed information concerning the TPFPU-SiO₂ polymer. Thus, fine morphologies of TPFPU-SiO₂-1, TPFPU-SiO₂-2, and TPFPU-SiO₂-3 polymers on the silicon wafer were observed with an AFM instrument and the results are presented in Figure 9. In addition, some roughness factors, such as root mean square roughness (Rq) and average roughness (Ra), are also given in

Table 3. Roughness parameters of the TPFPU-SiO₂ latex films.

Samples	Rq (nm)	Ra (nm)
TPFPU-SiO2-1	2.310	1.389
TPFPU-SiO2-2	2.710	2.050
TPFPU-SiO2-3	4.190	2.964

. From the AFM topographies, three polymers all presented heterogeneous rough structure and microphase separation model. There were some dark sections and numerous particles with diverse brightness and sizes. The dark part was derived from the soft segment of PU, while the bright particles resulted from silica nanoparticles (big bright particles) or the interaction between the hard segment of PU and the fluoroalkyl group (small bright spots). Furthermore, the quantity of big bright particles increased with the increase in the introduction of nanoparticles, which confirms that the AFM results are in accordance with the SEM results. For the roughness parameters of three TPFPU-SiO₂ polymers, both R_q and R_a were in the ascending order of TPFPU-SiO₂-1, TPFPU-SiO₂-2, and TPFPU-SiO₂-3. In addition, sizes of the big bright particles, i.e., silica nanoparticles, were estimated about 70 nm from the data scale on AFM topography.

3.7. Hydrophobicity of the TPFPU-SiO₂ Latex Films

The water contact angle (WCAs) and water absorption of TPFPU-SiO₂ films were measured, and the results are shown in

Table 4. The hydrophobicity of the TPFPU-SiO₂ latex films.

Product No.	Water Contact Angle (°)	Water Absorptivity (%)
TPFPU-SiO2-0	102.4 ± 1.8	5.43 ± 0.06
TPFPU-SiO2-1	116.7 ± 4.1	3.86 ± 0.11
TPFPU-SiO2-2	119.8 ± 2.4	2.49 ± 0.04
TPFPU-SiO2-3	118.2 ± 3.6	3.50 ± 0.13
TPFPU-SiO2-4	117.6 ± 2.8	3.67 ± 0.25

. The results show that, compared with the TPFPU-SiO₂-0 films where the silica sols dose was zero, the hydrophobicity of the other TPFPU-SiO₂ films was greatly increased. Moreover, it increased first and then slightly decreased with the increase in the introduction of nanoparticles. The best hydrophobicity was obtained when the theoretical mass ratio of silica sols was 10 wt% and the WCA and water absorption of TPFPU-SiO₂ films were 119.8° and 2.49%, respectively. As is known, the hydrophobicity of one material surface is related to its composition and surface microstructure. According to the preparation formula of the TPFPU-SiO₂ and the results of XPS test, the fluorine contents in organic moieties of the TPFPU-SiO₂ polymer were almost the same, while the dose of silica nanoparticles added was continuously increased. Herein, nanosilica particles could play a crosslinked role in the polyurethane molecule chains and would make the latex film dense; as a result, hydrophobicity of the latex film was improved. For another, the embedded nanoparticles can increase the micro-roughness of the nanocomposite latex film; based on the Wenzel model [25], it is known that the rougher the hydrophobic surface is, the larger the static contact angle on its surface is. However, since the nanosilica is hydrophilic, it will lead to a slight decrease in the hydrophobicity of the TPFPU-SiO₂ films when the imported amount of nanosilica is excessive. Therefore, in this sense, the micro- and nano-scaled rough structure may play an important role in the hydrophobicity of the hydrophobic surface in the present work.

3.8. The Mechanical Properties of the TPFPU-SiO₂ Latex Films

The tensile strength and elongation at a break were tested for the mechanical properties of the TPFPU-SiO₂ films, and results are shown in Figure 10. With the increase in the embedded nanosilica amount, tensile strength firstly increased and then decreased; elongation at a break decreased continuously. The possible reason is due to the increase in hydrogen bonds or interactions amongst the PU molecular chains, which will intensify the rigidity and reduce the flexibility of the film. The reason may also be due to the cross-linking effect of nanosilica, which increases the interaction between the molecular chains of polyurethane and enhances the rigidity of the film (the tensile strength is increased), but decreases the flexibility. However, when the amount of embedded nanosilica is too large, it may produce over-crosslinking, resulting in the molecular chain having difficulty moving freely and a declining tensile strength. These results are also consistent with the related literatures [26,27].

4. Conclusions

The fluorine- and silicon-containing chain extenders (DEFA and DESiA) were firstly synthesized by a Michael addition reaction. Then, a series of the terminal and pendant fluoroalkyl co-modified polyurethane (TPFPU-SiO₂) latexes were also prepared via varying the additive amount of silica sols. Results showed that with the increase in silica sols dosage, the mean particle size of the TPFPU-SiO₂ latexes was increased and their emulsion stability gradually decreased. Thermal stability of the TPFPU-SiO₂ films were gradually increased and hydrophobicity of the TPFPU-SiO₂ films increased first and then slightly decreased. The maximum water contact angle on the TPFPU-SiO₂ films was 119.8° and the lowest water absorptivity was 2.49%. Thus, these novel fluorinated polyurethane nanocomposite latexes can find valuable applications in heat-resistant and anti-fouling coatings.