Abstract
A series of waterborne polyurethane/inorganic (WPU/TiO2) hybrid composites were synthesised by a sol–gel process on the basis of isophorone diisocyanate, polyether polyol (GE-210), dimethylolpropionic acid, tetrabutyl titanate (TBT) and 3-glycidyloxypropyl trimethoxysilane as a coupling agent. The physical properties of the WPU and WPU/TiO2 dispersions and hybrids were measured. Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy and X-ray diffraction were used to assess the fracture surface morphology and the dispersions of the WPU/TiO2 hybrids. The scanning electron microscopy, transmission electron microscopy and atomic force microscopy results showed that the TiO2 particles were dispersed homogeneously in the WPU matrix in nanoscale. The prepared hybrids showed good thermal stability and mechanical properties in comparison with pure WPU and showed tunable transparence with the TBT fraction in the film. Through suitable adjustment of TBT content, some thin hybrids have potential applications, such as coatings, leather finishing, adhesives, sealants, plastic coatings and wood finishes.
Introduction
Organic–inorganic composites have attracted much interest in recent years as they usually not only occupy the properties of organic polymers (for example, flexibility, ductility and dielectric) but also show particular properties of inorganic materials such as photocatalysis.1–3 Therefore, these could be widely used in many fields. Polyurethane (PU) is an important elastomer that has various useful properties, including high impact strength, high elasticity and good adhesion. Although materials with urethane structures exhibit good properties, their use as surface coatings in outdoor applications is limited by the sensibility to the photooxidative degradation, which can lead to discolouration and even total destruction.4–6
In recent years, titania has gained increasing interest as an inorganic unit for hybrid production 7 , 8 owing to its particular properties (i.e. X-rays, high refractive index and photocatalytic properties) that can lead to new organic–inorganic materials. The most common way is the sol–gel procedure, which provides an easy, cost effective and excellent way to incorporate inorganic compounds into an organic binder. Therefore, the sol–gel method has been widely used to produce homogeneous microstructures and coatings with organic–inorganic hybrid solids at a rather low temperature.9–11 To improve the compatibility between the organic and inorganic phases, it is essential to prove chemical linkages between the soft organic and hard inorganic phases. This can be achieved using coupling agents, such as amine terminated alkoxysilanes, 3-aminopropyl triethoxysilane, epoxy capped alkoxysilanes 3-glycidyloxypropyl trimethoxysilane (GLYMO), isocyanato capped alkoxysilane 3-triethoxysilylpropyl isocyanate, etc. 12 , 13
Ou et al.14 reported the preparation of polypropylene/polyamide/functionalised TiO2 nanocomposites. The result replayed to incorporate toluence diisocyanate (TDI) functionalised TiO2, which could improve the stiffness and ductility, as expected, for its high specific surface. Mazzocchetti et al.15 reported the synthesis and characterisation of radiopaque organic–inorganic hybrids composed of poly(1,4-butyleneglutarate) and titanium oxide (titania) using the sol–gel method. Jeon et al.16 reported the preparation of PU/hybrid composites with nanosize hydrophobic silica (Aerosil R812) and 3-aminopropyltriethoxysilane as the coupling agent by the sol–gel process.
In this work, a series of waterborne PU/inorganic (WPU/TiO2) hybrids were prepared by the sol–gel technique. The synthesised hybrids were characterised with Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) methods. The other properties of the WPU/TiO2 hybrids, such as tensile strength, hardness and water resistance values, were also determined.
Experimental
Materials
Polyether polyol (GE-210, 56 mg KOH/g) was produced by Yutian Petrochemical Co. (Qingdao, China). Isophorone diisocyanate was supplied by Hüls Chemical Co. (Krefeld, Germany). Dimethylolpropionic acid was produced by Perstop Co. (Helsingborg, Sweden). N-Methyl-2-pyrrolidone, acetone, dibutylbis (lauroyloxy)tin, triethylamine, tetrabutyl titanate (TBT) and ethylenediamine were obtained from Guoyao Chemical, Ltd (Shanghai, China). 3-Glycidyloxypropyl trimethoxysilane was purchased from Nanjing Shuguang Chemical Plant (Nanjing, China).
Preparation of WPU dispersion
Polymerisation was performed in a 250 mL, round bottom, four-necked separable flask with a mechanical stirrer, thermometer and condenser with a drying tube. Certain amounts of GE-210, isophorone diisocyanate and dibutylbis (lauroyloxy)tin as a catalyst were charged into the dried flask. With stirring, the mixture was heated to 80°C for ∼2 h to obtain the NCO terminated prepolymer. A certain amount of dimethylolpropionic acid dissolved in N-methyl-2-pyrrolidone was added the prepolymer at 80–85°C for another 2 h so that the NCO terminated prepolymer containing carboxyl group was obtained. Then, the reactants were cooled to 60°C. The obtained NCO terminated prepolymer was neutralised by adding triethylamine at 40°C for ∼45 min. Finally, the neutralised prepolymer was cooled to room temperature and poured into cold deionised water after stirring at high speed, and ethylenediamine was added as chain extender. The WPU dispersion was obtained. The synthetic route of the WPU dispersion is shown in Fig. 1.
Synthetic route of WPU
Preparation of organic/inorganic hybrid composite dispersion
Organic/inorganic (WPU/TiO2) hybrid material dispersions containing different inorganic contents were synthesised via the sol–gel process. In this study, a homogeneous organic solution was obtained from the coupling agents GLYMO and WPU at 40°C for 1 h. The homogeneous inorganic solution was a hydrolysed TBT mixture, which was prepared with deionised water, hydrochloric acid, ethanol and TBT in a conical flask and was stirred for 1 h. Then, it was added carefully to the organic solution and reacted for another 1 h. The temperature was cooled to room temperature and kept stirring for 12 h. Varying the proportion of TBT, a series of WPU/TiO2 hybrids were obtained. In this article, the hybrids with different TBT contents of 0·3, 0·5, 1·0, 1·5 and 3·0% are expressed as WPU/TiO2-1, WPU/TiO2-2, WPU/TiO2-3, WPU/TiO2-4 and WPU/TiO2-5 respectively, as shown in Table 1. The synthetic route of the WPU/TiO2 hybrid is shown in Fig. 2.
Synthetic route of WPU/TiO2 hybrid composite
Some physical properties of WPU and WPU/TiO2 dispersions
In this experiment, when the TBT content was >3·0%, the hybrid dispersion was unstable and appeared as a gel. Therefore, experiments with greater percentages of TBT were not performed.
Membrane preparation of WPU and WPU/TiO2
The membranes were prepared by casting the newly synthesised WPU or hybrid dispersions onto a poly(tetrafluoroethylene) at room temperature for 2 days; this was followed by drying at 60°C for 3 h. This trend of drying was just for slow drying. It was also possible to evaporate the solvent at a fixed temperature, either at room temperature or at elevated temperature. After demoulding, the films were stored in a desiccator at room temperature for further studies.
Particle size and particle size distribution (PSD) of WPU and WPU/TiO2 dispersions
The WPU and WPU/TiO2 samples were added to 100 mL test tubes and diluted with deionised water. The particle diameter and the PSD of the WPU and WPU/TiO2 dispersions were measured by a laser particle size analyser (BIC-9010; Brookhaven Instrument Co., Holtsville, NY, USA).
Surface tension of WPU and WPU/TiO2 dispersions
Surface tension σ is an important parameter of the physical properties for the application performances of materials. In this study, the measurement of σ was conducted on a single tube manner set-up by the maximum air bubble method, as shown in Fig. 3. When the liquid end of the capillary was tangent with the sample's liquid end, the liquid went up along the capillary. A piston of the tap funnel was opened, and water dropped slowly to reduce the system pressure. In this way, the liquid surface of the capillary had a bigger pressure than that of the cuvette, and a pressure difference was formed. When the pressure, which was the result of the pressure difference acting on the capillary surface, was a little larger than σ of the capillary nozzle liquid, the air bubble transgressed from the capillary nozzle. The maximum pressure maximum was obtained from a manometer, and the value is expressed by equation (1)
Measurement equipment of surface tension
When the air bubble transgressed from the capillary nozzle, the pressure was πr2Pmax, where r is the radius of the capillary. The pressure, which was brought by σ of the air bubble in the capillary, was 2πrσ. The two pressure values were equal and are described by equation (2)
Tensile strength and elongation at break of WPU and WPU/TiO2 membranes
Tensile strength testing and elongation at break testing for all the specimens were carried out on a tensile tester (KY-8000A; Jiangdu Kaiyuan Test Machine Co., Ltd, Jiangdu, China) at room temperature at a speed of 50 mm min−1. All the measurements had an average of three runs. The dumbbell type specimen was 30 mm length at two ends, 0·2 mm thick and 4 mm wide at the neck.
Water absorption of WPU and WPU/TiO2 membranes
The water absorption values of the WPU and WPU/TiO2 membranes were obtained as follows. Preweighed dry membranes (20×20 mm) were immersed in distilled water at 25°C. After 24 h, the membranes were then blotted with filter paper and weighed. The water absorption w was calculated as follows
Structure characterisation and optical transparency
The FTIR spectra of the WPU and WPU/TiO2 membranes were obtained on a KBr powder with an FTIR spectrometer (Avatar 360; Nicolet, Madison, WI, USA). A minimum of 32 scans was signal averaged with a resolution of 2 cm−1 in the 4000–400 cm−1 range. The UV–Vis spectra of the WPU and WPU/TiO2 membranes were recorded with a UV–Vis spectrometer (UV-2450; Shimadzu, Kyoto, Japan) in the 350–800 nm range at 25°C.
Thermal properties
The DSC and TGA of samples were performed on a Netzsch instrument (STA 449C; Netzsch, Seligenstadt, Germany). The programmed heating rang was from room temperature to 500°C at a heating rate of 10°C min−1 under a nitrogen atmosphere. The measurement was taken using 6–10 mg samples. The DSC and TGA curves were recorded.
Scanning electron microscopy
To investigate the morphology of the WPU and WPU/TiO2 membranes, fracture surfaces were investigated with a 20 kV accelerating voltage with a field emission SEM (S-4800; Hitachi Corp., Tokyo, Japan).
Transmission electron microscopy
The morphology of the composite particles was observed by TEM (Tecnai 12; Philips Company, Eindhoven, The Netherlands) with an acceleration voltage of 120 kV. The composite particles were dispersed in deionised water in an ultrasonic bath for 10–30 min and then prepared by the deposition of the emulsion onto a copper net after it was stained by phosphor-wolframic acid.
Atomic force microscopy
The AFM topographies were obtained with a Digital Instruments Multimode IIIa AFM (Veeco Instruments Inc., Plainview, NY, USA) equipped with an E-scanner. Tapping mode silicon nitride cantilevers (TESP – tapping etched silicon probe) with nominal spring constants of 20–100 N m−1 and nominal resonance frequencies of 200–400 kHz were used. A piece of freshly cleaned mica (about 4×4 μm) was used as a substrate of film preparation. To minimise possible contamination of the surface by ambient air, each sample was freshly prepared just before the AFM experiments.
X-ray diffraction
The XRD patterns were recorded by reflection scan with nickel filtered Cu Kα radiation (D8; Bruker-AXS, Karlsruhe, Germany). The X-ray generator was run at 50 kV and 70 mA. All of the XRD measurements were performed at 2θ values between 3 and 60°.
Results and discussion
Properties of WPU and WPU/TiO2 hybrid material dispersion
The test results of the aqueous dispersion, such as storage stability σ, particle size and PSD, are shown in Table 1. All of the dispersions were stored at room temperature for 12 months and exhibited satisfactory stability. Compared with pure WPU dispersion, the particle sizes of all the WPU/TiO2 hybrid dispersion increased, indicating that a large number of titania particles dispersed in the PU latex. The particle size depends on the hydrolysis (rate of nucleation) and condensation rates (rate of growth).17 The hybrid dispersion had the maximum diameter of 121·3 nm when the TBT content was 0·5% (WPU/TiO2-2). With increasing ratio of TBT, the particle size increased. This was due to the increase in the –Ti(OCH3)3 concentration and an enhancement in the rates of hydrolysis and the condensation reaction, which induced consequently the growth of the particles. However, with the increase in TBT content, the particle size decreased. It was maybe due to the condensation reaction steps of TBT in the WPU solution following the sequence: nucleation, nucleus propagation formed colloid and particle growth through colloid collision. However, the WPU chain in the solution inhibited particle growing, and the growing particles could not form the bigger particle. The PSDs of the WPU and WPU/TiO2 series dispersions are shown in Table 1. Obviously, the WPU/TiO2 hybrids had narrower PSD values than the pure WPU. The result indicated that there were excellent compatibilities between the organic and inorganic phases.
σ is a phenomenon caused by the cohesive forces between liquid molecules. It is an effect within the surface film of a liquid that causes the film to behave like an elastic sheet. The knowledge of σ is useful for many applications and processes as σ governs the chemical and physical behaviours of liquids. It can be used to determine the quality of numerous industrial products, such as paints, inkjet products, detergents, cosmetics, pharmaceuticals, lubricants, pesticides and food products. In addition, it has a profound effect on some steps in industrial processes, such as adsorption, distillation and extraction. Many methods have been established to measure the σ of liquids, such as the capillary rise method, the drop weight method, the du Nouy ring method, the Wilhelmy plate method, the spinning drop method, the pendant drop method and the sessile drop method. The choice of the method depends on the nature and stability of the liquid being measured, measurement conditions, precision, reliability and instrumentation cost. Among these methods, drop weight can be considered as one of the oldest, and it is still widely used. This method is popular because it is inexpensive and the set-up is simple. A typical drop weight apparatus consists of a single dripping tip of known diameter, a liquid delivery system and a weighing balance. In this study, the measurement of σ was conducted on a single tube manner set-up by the maximum air bubble method. The smaller σ is, the better the performance of infiltration is. The σ values of the WPU and WPU/TiO2 hybrid dispersions are listed in Table 1. The σ of the WPU/TiO2 hybrids with 0·3 and 0·5%TBT increased, and σ decreased for 1·0%TBT and then increased for 3·0%. The results also showed that all the dispersions had smaller σ values than water (72 mN m−1) and had little apparent change in σ. When the TBT content is 1·0% (WPU/TiO2-3), the material reached the minimum, showing an excellent infiltrative performance in the substrate.
Mechanical properties of WPU and WPU/TiO2 composites
The tensile strengths of the WPU and WPU acrylate (WPUA)/TiO2 hybrid composites are shown in Table 2. The results indicated that the WPU/TiO2 hybrid composites had higher tensile strength than the pure WPU. The tensile strength of the WPU/TiO2 hybrid composites was different with the TBT content. From Table 2, the tensile strength of the WPU/TiO2 hybrid composites increased a little with increasing initial TBT content up to ∼0·5% and then dropped. When the TBT content was >1·5%, the tensile strength increased. As a result, the mechanical properties of WPU could be improved by modification of the coupling agent and a certain amount of TBT. The elongation at break values of the pure WPU and WPUA/TiO2 hybrid composites are shown in Table 2. It was particularly interesting that the elongation at break also increases initially with TBT content. This was ascribed to the deformation mechanism of WPU. In the case of WPU, which is composed of hard and soft segment domains, the hard segment lamellae sensitive to applied stress could be tilted towards the stretching direction at low strain, and in the high strain, they aligned parallel to the stretching direction. Thus, WPU maintained its stress capacity at a relatively high strain without any breakdown of the amorphous soft chains. In this study, the WPU/TiO2 hybrid composites endured higher stress because of the incorporation of titania, although an excess of TBT negatively affected the elongation of the samples. That is, as the TBT content increased initially, the formed titania made the composites more reinforced gradually without hindering elastic deformation of the soft segments; this resulted in higher breaking strengths and elongation at break. However, the incorporation of excess titania hindered the deformation of soft segments in WPU and, thus, led to the lower elongation at break.
Some mechanical properties of WPU and WPU/TiO2 membranes
The water absorption values of the pure WPU and WPUA/TiO2 hybrid composites are also shown in Table 2. The water absorption values of the hybrids were lower than that of the pure WPU when the TBT content was below 0·5%; this enhanced the water resistance. The water absorption reached a minimum of 31·16% when the TBT content was 0·3%. The obtained results indicated that hybrid composites had the best water resistance.
Spectra (UV–Vis) of WPU and WPU/TiO2 composites
Transparency is an important parameter for many applications in coatings and films, especially for water based polymer systems. Figure 4 shows the UV–Vis spectra of the WPU and WPU/TiO2 hybrid composites in the wavelength range of 350–800 nm. As shown in the inset (Fig. 4), the transmittance of pure WPU at 633 nm decreased from 99 to 88·8%. The transmittance gradually increased from 88·8 to 95·9% for the hybrid with 1·5% TBT content. This was because of the smaller refractive index of pure titania compared to WPU. Thus, the refractive index decreased with increasing titania content. At low titania content (<1·0%), the titania particle was isolated as a dispersive heterogeneous phase in the hybrid matrix, which resulted in a serious light scattering. This led to a hazy thin film. At high titania content, the titania particles were blended into WPU to form an aggregation phase in the hybrid matrix. This led to an increase in the transparency of the hybrid film. These results suggested that the transparency of the prepared hybrid thin film could be tunable through the adjustment of the titania content.
Spectra (UV–Vis) of WPU and WPU/TiO2 hybrid composites
Spectra (FTIR) of WPU and WPU/TiO2 composites
The FTIR spectra of the WPU and WPU/TiO2 hybrid composites are shown in Fig. 5. As shown in Fig. 5, there was a strong absorption peak at about 1678–1740 cm−1, which corresponded to the carbamate carbonyl of –C = O groups. The bands of C–H and N–H were observed at 2970 and ∼3300 cm−1 respectively. Clearly, there were wide OH stretching regions centred at 3000 cm−1 and spanning over 1000 cm−1, where the vibrations of Ti–OH and absorbed water appear. The broad absorption region ranging from 400 to 1000 cm−1 was typical of Ti–O–Ti vibrations of amorphous titanium oxide. 18 , 19 The absorption peak was broader with the increase in TiO2 content. The band at 654 cm−1 was due to the presence of Ti–O vibration in compounds where the oxygen atom was linked to an organic residue. The absorption at 654 cm−1 was attributed to the Ti–O stretching vibration of the Ti–O–C groups, formed by the WPU chain with the TiO2 network. The band changes at 1180 and 1030 cm−1 are attributed to the bond of Ti–OH on three –OH groups. The sharp absorption at ∼925 cm−1 was typical of the Ti–O–Si network, which was due to the polycondensation between the siloxane side the of coupling agent GLYMO and the hydrolysis of TBT, forming the Ti–O–Si network and generating the interpenetrating polymer network between the organic and inorganic phases.
Spectra (FTIR) of WPU and WPU/TiO2 hybrid composites
X-ray diffraction of WPU and WPU/TiO2 composites
Further evidence of the homogeneity can be gleaned from wide angle X-ray analysis of the films. Figure 6 shows the XRD curves for the WPU and some WPU/TiO2 hybrid composites. For pure WPU, a nearly amorphous diffraction peak was seen near 2θ = 19·5°; this indicated the crystallinity of polyether segments.20 With the inclusion of 0·5, 1·0 and 1·5% respectively, the broad reflection was still present with an additional sharp peak at 2θ = 9·3, 21·1, 28·5, 29·2 and 30·6°. These sharp reflections corresponding to the crystalline monomeric TBT were not detected in any of the films prepared from samples ranging from as depicted WPU/TiO2-2 to WPU/TiO2-4. The obtained results indicated that the membranes existed as an amorphous form. The increase in titania contents results in the increase in the intensity of the sharp peak mentioned above, confirming that the soft segments tend to crystallise, generating better defined peaks. This was because of the formation of a uniform three-dimensional network structure in the hybrid composites, which interspersed with the WPU segment. Then, the organic and inorganic uniform cross-networks were formed between TiO2 and WPU, limiting the TiO2 nucleation and crystal growing.
Curves (XRD) of WPU and some WPU/TiO2 hybrid composites
Scanning electron microscopy of WPU and WPU/TiO2 composites
The cross-sections of the WPU and WPU/TiO2-2 hybrid composite observed by SEM are shown in Fig. 7. Evidently, the surface of WPU/TiO2 was relatively smooth and with almost no cracks; it showed some relatively rule cracks in the pure WPU. This was because of the chemical bond formed between PU and TBT in the presence of GLYMO and then engender the cross-network, improving the compatibility between organic and inorganic phases. It was apparent that the incorporation of TBT and GLYMO induced a more miscible structure between the hard and soft segments of WPU.
Images (SEM) of WPU and WPU/TiO2-2 hybrid composite
Transmission electron microscopy of WPU and WPU/TiO2 composites
To obtain further characterisation of the membranes, such as the microphase structure and compatibility, TEMs of the WPU and WPU/TiO2 hybrid composites were observed and shown in Fig. 8. As shown in Fig. 8, the morphology of the WPU/TiO2 hybrids did not show any segregation or aggregation of the TiO2 particles but exhibited a finer structure compared to that of pure WPU. This experimental evidence indicated that the nano-TiO2 was completely chemically reacted with the hard segments. The result indicated that the incorporation of TBT in the molecular chains of WPU enhanced the miscibility between these incompatible hard and soft segments to produce a more homogenous structure. The formation of titania particles in the as prepared WPU and coupling agent GLYMO matrix indicated that the –Ti(OCH3)3 groups may have functioned as internal bridging groups between organic and inorganic segments for sol–gel reactions of TBT molecules. On the other hand, the large surface area of the TiO2 nanoparticles that created a large interaction zone with the WPU segments was thought to be another major reason for the high miscibility of the WPU hard and soft segments. It was well established in Fig. 8 that the high surface area of the nanoparticles led to a large volume fraction of the polymer matrix, which in turn led to the different bulk properties of the polymer nanocomposite that was enhanced by the interaction zone. Depending on the degree of interaction between the polymer and the nanoparticles, this interaction region had a higher or lower mobility than that of the bulk material. 21 , 22 However, the WPU chain in the solution inhibited the particle growth, causing the growing particle to not form a bigger particle. Therefore, the particle size was about 10–40 nm, as shown in Fig. 8, and the size slightly increased as the TBT content increased. The increase in the TiO2 particle size clearly resulted from the increase in the aggregation tendency as the TBT content and the TiO2 particle number were increased. These micrographs showed the fine interconnected or continuous phases morphologies. The authors estimate that these materials will be pretty good for practical applications.
Images (TEM) of WPU, WPU/TiO2-3 and WPU/TiO2-4 hybrid composites
Atomic force microscopy of WPU and WPU/TiO2 composites
The AFM study demonstrated an accurate photopattern of the hybrid film interface. Figure 9 shows the three-dimensional AFM images of the WPU and WPU/TiO2-2 hybrid composite. As shown in Fig. 9, the hybrid showed a rough surface morphology, whereas the pure WPU film had a much smoother appearance. This was because the lower surface energy of the TBT molecular with hydrophobic groups created surface roughness, which resulted in an increase in the surface area. Such a surface morphology enhanced the water and friction resistances.23
Images (AFM) of WPU and WPU/TiO2-2 hybrid composite
Thermal properties of WPU and WPU/TiO2 composites
Figure 10 shows the DSC scan curves for the WPU and WPU/TiO2 hybrid composites. The WPU/TiO2 hybrid composites had a higher glass transition temperature Tg of hard segment than the pure WPU. There was only a glass transition temperature in all the membrane samples. The result indicated that the polymers had a strong and uniform interaction between WPU and titania nanoparticles, which was due to the bridge effect by GLYMO. The decomposition temperature of the WPU/TiO2 membrane samples increased with the increase in TBT content.
Curves (DSC) of WPU and WPU/TiO2 hybrid composites
Figure 11 shows the TGA scan curves for the WPU/TiO2 hybrid composites. From the TGA curves of the hybrids, the small weight loss at lower temperature (<180°C) was probably due to the evaporation of residual water and physically absorbed water. There were three steps (at 180–280, 280–360 and 360–500°C respectively) in the TGA curves of the hybrids. The first step was ascribed to the introduction of the Ti–O network in the WPU/TiO2 hybrid based on the IR spectrum analysis. The other two steps at 280–360 and 360–500°C were caused by the decomposition of residual organic group. The weight of the residue in the hybrid film at 500°C was proportional to the titanium and silicon content, and the residual components were mostly titania and silica. The pure WPU exhibited a weight loss starting at ∼45°C and a rapid weight loss at 170–360°C. When the temperature was >360°C, the WPU decompounded completely. It can be seen that the WPU/TiO2 hybrids exhibited better thermal stability.
Curves (TGA) of WPU/TiO2 hybrid composites
Conclusions
A series of WPU/TiO2 hybrid composites were synthesised by the sol–gel process. The WPU/TiO2 hybrid composites showed a homogeneous morphology, which supported the compatibility improvements by SEM, TEM and AFM measurements. Compared with the pure WPU aqueous dispersion, the particle sizes of the hybrid aqueous dispersion increased from 70·7 to 121·3 nm. The WPU/TiO2 hybrid composites had higher tensile strength than the pure WPU; this was due to chemical network formations between the organic and inorganic phases. It illuminated the Ti–O bond formed in the hybrids through IR spectra, and TiO2 was amorphous in the hybrid film toward XRD. The SEM and AFM analyses also demonstrated that the hybrids had a smooth surface and superior film forming properties. The TGA demonstrates that the WPU/TiO2 hybrids exhibited better thermal stability.
Footnotes
Acknowledgements
This project was supported by the Natural Science of Jiangsu Province (grant no. BK2008247), China, Post-doctoral Science Foundation (grant no. 20070420973) and the Jiangsu Planned Projects for Post-doctoral Research Funds (grant no. 0602037B).