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Article

Insight into the Morphology, Hydrophobicity and Swelling Behavior of TiO2-Reinforced Polyurethane

by
Ivan S. Stefanović
1,*,
Jasna V. Džunuzović
1,2,
Enis S. Džunuzović
3,
Danijela V. Randjelović
1,
Vladimir B. Pavlović
4,
Andrea Basagni
5 and
Carla Marega
5
1
Institute of Chemistry, Technology and Metallurgy, National Institute of the Republic of Serbia, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
2
Institute of Chemistry, Technology and Metallurgy, Center of Excellence in Environmental Chemistry and Engineering, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia
3
Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
4
Faculty of Agriculture, University of Belgrade, Nemanjina 6, 11000 Belgrade, Serbia
5
Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italy
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(2), 231; https://doi.org/10.3390/coatings15020231
Submission received: 27 December 2024 / Revised: 29 January 2025 / Accepted: 13 February 2025 / Published: 14 February 2025
(This article belongs to the Special Issue Recent Innovations in Polyurethane Coatings and Films)
Browse Figures
Versions Notes

Abstract

:
In this research, the structure, morphology, hydrophobicity and swelling behavior of a polyurethane (PU) network and its composites (PUCs) were examined. PUCs were synthesized by the incorporation of different percentages (0.5, 1 and 2 wt.%) of unmodified or surface-modified TiO2 nanoparticles into a PU network based on polycaprolactone, aliphatic hyperbranched polyester and isophorone diisocyanate. In order to improve interfacial interactions, the surface of the TiO2 nanoparticles was chemically modified with lauryl gallate. The impact of the presence and content of unmodified or surface-modified TiO2 nanoparticles on the cross-sectional and surface morphology, swelling behavior and hydrophobicity of the PU network was assessed by different experiments. The obtained findings revealed that the incorporation of TiO2 nanoparticles brought a more pronounced irregular cross-sectional and rougher surface morphology, better microphase separation, higher values of the equilibrium swelling degree in tetrahydrofuran and toluene, and altered water contact angles compared to the neat PU. Based on the collected results, the practical applicability of the prepared PUCs may be in the area of protective coatings.

1. Introduction

Polyurethanes (PUs) are one of the most diverse polymer materials and therefore one of the most widely used worldwide. Today, their application is so widespread that they are used in the design of medical equipment and drug delivery vehicles, adhesives, elastomers, coatings, footwear and clothing, rigid and flexible foams, etc. [1,2,3,4]. Their wide practical applicability stems from the fact that they have good chemical, electrical and abrasion resistance, high bearing capacity, durability, flexibility, etc. Traditionally, PUs are synthesized by the reaction between polyols and isocyanates, but, lately, researchers have increasingly been resorting to new synthetic pathways that involve catalyzed non-isocyanate reactions in order to improve PU biocompatibility [5,6]. Their unique and tunable features, which can be easily adjusted by changing the structure of the reactants, depend also on the final molecular weight of the PUs, appearance of crystallization and additional interactions, density and surface morphology of the PUs, presence and extent of microphase separation, etc. [7,8].
Although PUs have good performances, there is a constant need for their improvement and promotion among the scientific and industrial ranks. Therefore, the creation of PUs with good water and solvent resistance and hydrolytic and enzymatic stability is always in demand. The incorporation of nanofillers into a PU structure is one of the most commonly used approaches to enhance the surface and weathering performances of PUs and extend their durability and thermal stability [9,10,11,12,13,14,15,16,17]. Due to the high surface/volume ratio, even with an extremely small amount of nanofiller, it is possible to achieve superior performances of PU composites compared to pure PUs. Still, to achieve a positive effect of added nanofillers on the targeted features of PUs, their suitable dispersion without agglomeration is a basic requirement. However, because of the large particle surface energy and incompatibility with the organic polymer matrix, nanoparticles tend to stick together and form agglomerates. Nano-sized TiO2 is intensively used as nanofiller for PUs due to its outstanding performances. Many attempts have been made to achieve the homogenous distribution of TiO2 nanoparticles in PUs and increase the contact surface and interfacial interactions between the TiO2 nanofiller and polymer matrix [18,19,20]. One way is to use a sonification process during preparation in which mechanical shear force is applied to separate the nanoparticles and prevent their agglomeration. However, this method does not prove to be sufficient to completely prevent the agglomeration of nanoparticles in PU since interfacial forces between individual nanoparticles are often stronger than applied mechanical forces [21]. Reid et al. [22] improved the dispersion of TiO2 nanoparticles in PU by their in situ synthesis in a prepolymer solution. Furthermore, Polizos et al. [23] synthesized in situ TiO2 nanoparticles in polyethylene glycol and then used the prepared composite as a filler for commercial thermoplastic PU in order to improve its electrical insulation properties. Accordingly, the researchers also tried to improve interactions between the PU matrix and nanoparticles by adequate surface modification of the nanoparticles [24,25,26,27]. Sabzi et al. [24] showed that the surface modification of TiO2 nanoparticles with amino propyl trimethoxy silane enables their good dispersion and enhances the UV resistance and mechanical properties of the prepared PU clear coatings. Similarly, Wu et al. [28] achieved the uniform dispersion of TiO2 nanoparticles in PU elastomers by their surface modification with a silane coupling agent. Furthermore, Behniafar et al. [29] showed that covalent linkage between TiO2 nanoparticles and PU can be formed using NH2-functionalized TiO2 nanoparticles. Zhang et al. [30] reported that the grafting of hyperbranched polymers onto TiO2 nanoparticle surfaces via a thiol-yne click chemistry reaction could decrease their tendency to aggregate within the PU matrix, increase surface roughness and, consequently, lead to better hydrophobicity. The application of TiO2 nanoparticles can slow down the photo-aging of the PUs, which includes the prevention of double bond oxidation, crosslinking breaks, the scission of different segments that can lead to discoloration and the appearance of cracks and coating delamination, all because of the great index of refraction of UV radiation [31,32]. Furthermore, TiO2 nanoparticles can also interact with soft or hard segments of PU, creating additional chemical or physical interactions, leading also to the improvement of PU features.
This study is a continuation of our research considering polyurethane composites (PUCs) based on polycaprolactone (PCL), isophorone diisocyanate (IPDI), aliphatic hyperbranched polyesters and unmodified and modified TiO2 nanoparticles. In our previous works, we examined the structural, thermal and mechanical properties of a series of PUs [33,34] and their composites [35]. In this paper, we investigate the surface properties of PU composites by examining their surface and cross-sectional morphology, swelling behavior, and hydrophobicity.

2. Materials and Methods

2.1. Materials

Poly(ε-caprolactone) diol (PCL, Mn ≈ 2000 g/mol, from Sigma-Aldrich, Darmstadt, Germany) was dried at 80 °C for 12 h before use. Boltorn® hydroxy-functional aliphatic hyperbranched polyester of the second pseudo generation (BH-20, Mn = 1340 g/mol) [36] from Perstorp Specialty Chemicals AB (Malmö, Sweden) was dried under a vacuum for 48 h at 50 °C. Isophorone diisocyanate (IPDI, from Sigma-Aldrich, Darmstadt, Germany) was used as received. The stannous octoate (Sn(Oct)2, from Sigma-Aldrich, Darmstadt, Germany, purity 95%) was used as a catalyst. Solvents, N,N-dimethylacetamide (DMAc, from Sigma-Aldrich, Darmstadt, Germany, purity 99%) and tetrahydrofuran (THF, from J. T. Baker, Gliwice, Poland, purity 99.99%) were previously purified and kept over molecular sieves (0.4 nm) before synthesis, while toluene (from Sigma-Aldrich, Darmstadt, Germany, purity 99.9%) was used as received. Dodecyl (lauryl) gallate (LG) was obtained from Sigma-Aldrich (Darmstadt, Germany). A commercially available TiO2 was purchased from Sigma-Aldrich (Darmstadt, Germany). Acetonitrile was supplied by Sigma-Aldrich (Darmstadt, Germany) and used as received.

2.2. Preparation of Polyurethane Composites

The complete procedures for the preparation of the neat PU based on PCL, IPDI and BH-20; the surface modification of TiO2 nanoparticles with lauryl gallate (TiO2-LG); and the synthesis of PUCs based on unmodified TiO2 and TiO2-LG as nanofillers are described elsewhere [33,35]. Briefly, 2 g of TiO2 was introduced into a 100 cm3 solution of lauryl gallate in acetonitrile (0.01 mol/dm3), followed by 10 min mixing in an ultrasonic bath. After the solution was left overnight, TiO2-LG nanoparticles were precipitated using a centrifuge, washed twice with acetonitrile and, finally, dried in a vacuum oven at 40 °C. Unmodified and modified TiO2 nanoparticles were added in amounts of 0.5, 1 and 2 wt.% relative to the total weight of reactants. In order to provide a homogenous dispersion of nanoparticles within the PU network structure, pre-determined amounts of nanofiller and THF were mixed in an ultrasonic bath for 20 min at room temperature. For the neat PU and all PUCs, the stoichiometric ratio between –NCO and –OH groups was kept constant at 1.1. At the end of the reaction, the mixtures were poured into Teflon dishes and placed in a force-draft oven to enable the curing of samples, first at 60 °C for 24 h and then at 80 °C for 3 h. The drying of samples was performed in a vacuum oven at 50 °C for 24 h. All samples were stored in desiccators at room temperature for two weeks before characterization.
Prepared PU composites are designated as PUC/Ti-xx or PUC/LG-xx, where “Ti” and “LG” indicate that they were prepared with TiO2 or TiO2-LG nanoparticles, respectively, while “xx” denotes the weight percentages of the added nanoparticles. The neat polyurethane network (without nanoparticles) is denoted simply as PU. Figure 1a shows the reactants and nanoparticles, while Figure 1b shows the preparation pathway of the PUCs.

2.3. Characterization

FTIR analysis was performed in attenuated total reflection (Everest ATR) mode using an FT-IR Nicolet SUMMIT (Thermo Scientific, Waltham, MA, USA) spectrometer. All spectra were recorded at the same conditions: resolution of 2 cm−1, 64 scans and wave number from 400 to 4000 cm−1.
The cross-sectional surface of the neat PU and its composites was investigated by scanning electron microscopy (SEM) using the XL30 scanning electron microscope (Philips, Eindhoven, The Netherlands), which contains a Gemini column (Zeiss Supra 35 VP, Oberkochen, Germany). All samples were Au-coated (15 nm thickness) in a high-vacuum evaporator before taking measurements.
Surface topography and roughness of the neat PU and its composites were examined using NTEGRA Prima atomic force microscope (NT-MDT, Moscow, Russia). Measurements were taken using NT-MDT NSG01/Pt silicon cantilevers. Atomic force microscopy (AFM) investigations collected 3D height and 2D phase AFM images of the neat PU and its composite films, measured under ambient conditions, using intermittent contact mode at a scan size of 20 × 20 μm2.
TEM micrographs of PUC films (with 0.5 and 2 wt.% of unmodified and modified TiO2 nanoparticles) were recorded on a JEOL JEM-1400 Plus Electron microscope (Jeol Ltd., Mitaka, Japan), operating at a voltage of 120 kV with a LaB6 filament, at 60k× magnification. Samples were first cut into 2 mm strips and then embedded in Araldite (Fluka, Buchs, Switzerland) at an elevated temperature. The PUC ultra-thin films were cut with a diamond knife (Diatome, Nidau, Switzerland) on Leica UC6 ultramicrotome (Leica Microsystems, Wetzlar, Germany) and afterwards mounted on holey-carbon coated grids.
Equilibrium swelling degrees, qe, of the neat PU and its composites in toluene and THF were determined by utilizing the conventional gravimetric method and the following equation:
q e = w − w 0 w 0
where w0 and w are the sample weights before and after the swelling. Square test specimens (1.0 × 1.0 × 0.1 cm3) were immersed in a selected solvent at room temperature and the weight of the swollen squares was periodically measured until a constant value was reached. The average qe of each sample was calculated using data from three separate swelling measurements.
The water absorption ability of the neat PU and its composites was examined by immersion of the investigated samples (1.0 × 1.0 × 0.1 cm3) in distilled water for 48 h at room temperature. The water absorption (in weight percent), WA, was calculated by
WA = w W − w W 0 w W 0 × 100
where wW0 is the weight before water immersion, while wW is the weight after the removal of the excess water from the sample. The average WA of samples was calculated by conducting three measurements for each sample.
Water contact angle (WCA) measurements were carried out on the Drop shape analyzer (DSA25 instrument, Hamburg, Germany) using the sessile drop method and Krüss Advance version 1.11.2.25901 software. For all measurements, the conditions during recordings were the same (room humidity of 55%, room temperature of 23 °C and needle diameter of 0.513 mm). The presented WCA values are the mean of five replicates.

3. Results and Discussion

3.1. FTIR Spectroscopy

For examination of the structure of the TiO2 nanoparticles, neat PU and its composites, the ATR-FTIR spectroscopic method was used. The obtained FTIR spectra are shown in Figure 2.
Figure 2a shows the FTIR spectrum of unmodified TiO2 nanoparticles, where intense and broad bands at ~3300 and ~700 cm−1, which originated from the stretching vibrations of –OH and Ti–O groups, were noticeable, respectively. Apart from that, in the FTIR spectrum of the TiO2-LG nanoparticles (Figure 2b), the bands from LG were visible, indicating the successful functionalization of the TiO2 nanoparticles [37]. These bands were visible at ~2900 cm−1, 1700 cm−1, 1610 cm−1 and 1220 cm−1 and arose from the stretching vibrations of the –CH2, C=O, aromatic ring and C–O groups, respectively.
FTIR spectra of the neat PU and its composites had similar bands at 3320–3365 cm−1 (stretching vibrations of –NH groups), 2950 and 2865 cm−1 (asymmetric and symmetric stretching vibrations of –CH2 groups, respectively), 1725 cm−1 (stretching vibrations of C=O groups), 1550 and 1240 cm−1 (stretching vibrations of amide II and amide III, respectively), 1460 cm−1 (deformation vibrations of –CH2 groups), and 1040–1200 cm−1 (asymmetric and symmetric stretching vibrations of C–O–C groups). The presence of quoted bands in the FTIR spectra of the prepared samples unequivocally confirmed the formation of a polyurethane structure [38,39].

3.2. SEM Analysis

SEM analysis was performed to examine and compare the micromorphology of the neat PU and its composites. The recorded micrographs of all samples (Figure 3) were obtained on the fractured cross-sectional films, previously frozen in liquid N2 and coated with Au.
As can be seen from the obtained micrographs, the neat PU had an irregular cross-sectional surface morphology. On the other hand, the micrographs showing the cross-sectional surface of the PUCs showed the impact of the TiO2 nanoparticles on the morphology of the PU. PUCs with the lowest content of TiO2 nanoparticles (samples with 0.5 wt.%) had less irregular and smoother cross-sectional surfaces than neat PU, while on the surface of PUCs with higher contents of TiO2 nanoparticles (1 and 2 wt.%), the creation of certain micro-structures (clusters or agglomerates) [35] of various sizes could be observed (predominantly visible and noticeable on the cross-sectional surface of PUC/Ti-2.0). It seems that the homogeneous dispersion of unmodified TiO2 nanoparticles inside the PU was quite challenging to achieve due to their high tendency to stick together [40,41]. Agglomerates, which certainly formed in the PUC if a higher content of unmodified TiO2 nanoparticles was added, can be regarded as weak points that can aggravate the mechanical properties of PUCs [42]. Contrarily, SEM micrographs of PUC/LG cross-sectional surfaces indicated that better dispersion of the TiO2 nanoparticles was accomplished after their surface modification with LG.

3.3. AFM Analysis

AFM analysis was performed to investigate the surface topography and roughness of the neat PU and PUCs. The obtained 3D height and 2D phase images at scan size 20 × 20 μm2 and in intermittent contact mode are presented in Figure 4 and Figure 5, respectively. Also, the surface roughness values, determined using Gwyddion 2.61 (free and open-source software, Czech Metrology Institute) software, are listed in Table 1.
From Figure 4, it can be observed that the surface of the PUCs became rougher after the incorporation of TiO2 nanoparticles (Table 1), whereby surface roughness values were the highest for composites with 2 wt.% of TiO2 nanoparticles (modified or unmodified). It can also be observed that values of surface roughness were higher for composites prepared with unmodified TiO2 nanoparticles. The reason for this lies in the fact that long alkyl chains from LG increased the repulsive force between modified TiO2 nanoparticles, which simultaneously led to their better dispersion in the polymer matrix and, consequently, a smoother surface [43].
The AFM results presented in Figure 5 confirmed the presence of microphase-separated morphology on the surface of neat PU and PUCs [34,44]. It can be observed that microphase separation was more pronounced in composites with unmodified TiO2 nanoparticles.

3.4. TEM Analysis

In order to examine the distribution of modified and unmodified TiO2 nanoparticles within the polymer matrix, TEM analysis was carried out. Figure 6 shows TEM micrographs of the selected PUCs with 0.5 wt.% and 2 wt.% of unmodified and modified TiO2 nanoparticles.
From the obtained TEM images, it can be noticed that the dispersion and size of the unmodified TiO2 nanoparticles inside the PU matrix were quite different from the dispersion and size of the modified TiO2 nanoparticles. Namely, the unmodified TiO2 nanoparticles showed a tendency to create bigger agglomerates, the size of which increased with increasing TiO2 nanoparticle content (between 500 and 650 nm in length). The reason for this behavior can be found in the fact that the unmodified TiO2 nanoparticles possessed high surface free energy, high surface areas and the possibility of strong hydrogen bonding between –OH groups on their surface between particles, which made it possible to group them together [9]. This agglomeration was not unexpected since other researchers also reported similar behavior [45,46,47]. Contrary to this, the modified TiO2 nanoparticles showed much better and relatively uniform dispersion inside the polymer matrix (Figure 6). Their individual sizes were much smaller than what was observed for unmodified ones (below 60 nm). This could be explained by the increased repulsive force between the modified TiO2 particles and the improved compatibility between the TiO2 nanoparticles and PU matrix [9,48]. The increase in the content of modified TiO2 nanoparticles (2 wt.%) did not significantly disrupt their dispersion within the PU.

3.5. Swelling Behavior

Values of the equilibrium swelling degrees of the neat PU and prepared composites in toluene and THF, as well as the percentage of water absorption, are listed in Table 2. According to these results, values of all three parameters were lower for the neat PU than for the composites due to the decrease in the crosslinking density by the incorporation of unmodified and modified TiO2 nanoparticles [35]. The increase of the filler content from 0.5 to 1.0 wt.% caused a further increase in the qe values, while composites prepared with 2 wt.% of unmodified or modified TiO2 nanoparticles showed a slight decrease in the qe values for toluene and THF. On the other hand, the increase in filler content induced a continuous decrease in WA values. In order to explore the kinetics of the swelling of the neat PU and prepared composites in all three solvents, curve-fitting by a pseudo-second-order kinetic model was applied [34], and the obtained results are presented in Figure 7 and Table 3. High values of the correlation coefficients (R2 ≥ 0.9974) indicated that the applied pseudo-second-order kinetic model was a good choice. It can be observed that experimentally obtained qe values were similar to the theoretical ones and had the same trend (Table 3). Calculated swelling rate constants showed no specific trend.
To investigate the diffusion mechanism of toluene, THF and water through the neat PU and prepared composites, the following power law expression was applied [49]:
w t w e = k t n
where k is the power law constant, n represents the exponent used to describe the diffusion mechanism and we and wt are the quantities of solvent penetrating the samples at equilibrium and time t. Since the equation was valid only for the case when wt/we ≤ 0.6, values of the exponent n were possible to determine only for specimens swollen in water (Table 4). According to Alfrey et al., if n < 0.5, it can be supposed that pseudo-Fickian diffusion occurs (neat PU and PUC/Ti-0.5) [50]. In this case, it was assumed that the polymer segments had low mobility and decelerated solvent penetration, likely due to the relatively high crosslinking density. The diffusion of water through all other investigated samples could be classified as so-called non-Fickian diffusion, according to the n values.
The results obtained by swelling measurements were also applied to evaluate the diffusion coefficients (D) of different solvents in the prepared samples, and the calculated values are listed in Table 4 [51,52]. The increase in D values for PUCs could be attributed to the lower crosslinking density of the composites compared to that of the neat PU [35]. As expected, values of D were the highest for THF since the swelling of the prepared samples was the greatest in THF, leading to the formation of larger gaps in samples through which the solvent could be transported.

3.6. Water Contact Angle Measurements

Contact angle measurements were performed to investigate the hydrophobicity/wettability features of the prepared samples. Figure 8 presents the obtained water contact angle (WCA) values and images of the water droplets on the surface of the neat PU and its composites.
Determined WCA values were in the range of 84.4 to 97.9° for the PU/Ti samples and 86.6 to 91.1° for the PU/LG samples. It can be observed that all composite samples (except PU/Ti-2.0) had WCA values lower than those of the neat PU (96.7°). WCA was the measure of the wettability of the solids with water and depended on the chemical composition of the solid surface and surface roughness. For the chemically homogeneous hydrophobic surface, WCA increased with increasing roughness, and, contrarily, WCA decreased for rougher, chemically homogeneous hydrophilic surfaces [53,54,55,56]. According to the results shown in Figure 4 and Figure 5 and Table 1, it can be seen that the surfaces of the tested PU and its composites were chemically inhomogeneous and had very different roughness and, therefore, the obtained WCA values varied.

4. Conclusions

The structure, morphology and hydrophobicity of the prepared PU network and its composites, based on unmodified and surface-modified TiO2 nanoparticles (0.5, 1 and 2 wt.%), were investigated in this study. The conducted analyses showed that the added TiO2 nanoparticles affected the cross-sectional and surface morphology, swelling behavior, and hydrophobicity of the prepared PU. The cross-sectional morphology of the PUCs prepared using lower contents of unmodified or modified TiO2 nanoparticles was less irregular and smoother, while, in PUCs with a higher content of TiO2 nanoparticles, certain micro-structures (clusters or agglomerates) of various sizes were created. AFM analysis confirmed the existence of a two-phase morphology. TEM analysis showed that the surface modification of the TiO2 nanoparticles with LG gave a much better, uniform and homogeneous dispersion of TiO2 nanoparticles inside the PU in comparison to the unmodified ones. According to the results obtained by the swelling measurements, the incorporation of unmodified and modified TiO2 nanoparticles led to a decrease in the crosslinking density of the PU network. Also, the obtained results showed that the kinetics of the swelling of the neat PU and prepared composites in THF, toluene and water could be described by a pseudo-second-order kinetic model. Based on the presented results, prepared PUCs may be considered to have potential usage as protective coatings.

Author Contributions

Conceptualization, I.S.S. and J.V.D.; methodology, I.S.S. and J.V.D.; validation, E.S.D., D.V.R., V.B.P., A.B. and C.M.; formal analysis, I.S.S. and J.V.D.; investigation, I.S.S., J.V.D., E.S.D., D.V.R., V.B.P. and A.B.; resources, I.S.S., J.V.D. and C.M.; data curation, I.S.S. and J.V.D.; writing—original draft preparation, I.S.S. and J.V.D.; writing—review and editing, E.S.D. and C.M.; visualization, I.S.S. and J.V.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science, Technological Development and Innovation of Republic of Serbia (contract nos.: 451-03-66/2024-03/200026 and 451-03-65/2024-03/200135) and the Science Fund of the Republic of Serbia, program PRISMA, grant no. 5354, Multifunctional visible-light-responsive inorganic-organic hybrids for efficient hydrogen production and disinfection—HYDIS.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 15 00231 g001
Figure 1. (a) Reactants and unmodified and modified TiO2 nanoparticles and (b) reaction pathway for the preparation of the neat PU and its composites.
Figure 1. (a) Reactants and unmodified and modified TiO2 nanoparticles and (b) reaction pathway for the preparation of the neat PU and its composites.
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Coatings 15 00231 g002
Figure 2. (a) FTIR spectra of TiO2 nanoparticles, neat PU and prepared PUCs with 0.5 and 2 wt.% of TiO2 nanoparticles and (b) FTIR spectra of TiO2-LG nanoparticles, neat PU and prepared PUCs with 0.5 and 2 wt.% of TiO2-LG nanoparticles.
Figure 2. (a) FTIR spectra of TiO2 nanoparticles, neat PU and prepared PUCs with 0.5 and 2 wt.% of TiO2 nanoparticles and (b) FTIR spectra of TiO2-LG nanoparticles, neat PU and prepared PUCs with 0.5 and 2 wt.% of TiO2-LG nanoparticles.
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Coatings 15 00231 g003
Figure 3. SEM micrographs taken of the cross-sectional surfaces of neat PU and its composites (magnification 10k×).
Figure 3. SEM micrographs taken of the cross-sectional surfaces of neat PU and its composites (magnification 10k×).
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Coatings 15 00231 g004
Figure 4. The 3D height AFM images of the surfaces of neat PU and PUCs. Scan size: 20 × 20 μm2.
Figure 4. The 3D height AFM images of the surfaces of neat PU and PUCs. Scan size: 20 × 20 μm2.
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Coatings 15 00231 g005
Figure 5. The 2D phase AFM images of the surfaces of neat PU and its composites. Scan size: 20 × 20 μm2.
Figure 5. The 2D phase AFM images of the surfaces of neat PU and its composites. Scan size: 20 × 20 μm2.
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Coatings 15 00231 g006
Figure 6. TEM micrographs of the selected PUCs (magnification: 60k×).
Figure 6. TEM micrographs of the selected PUCs (magnification: 60k×).
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Coatings 15 00231 g007
Figure 7. Swelling kinetic pseudo-second-order model for the neat PU and its composites in different solvents: (a) toluene and (b) THF.
Figure 7. Swelling kinetic pseudo-second-order model for the neat PU and its composites in different solvents: (a) toluene and (b) THF.
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Coatings 15 00231 g008
Figure 8. WCA values of the neat PU and its composites and photos of the water droplets placed on the surface of the films.
Figure 8. WCA values of the neat PU and its composites and photos of the water droplets placed on the surface of the films.
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Table 1. Surface roughness of neat PU and its composite films (scan sizes: 20 × 20 μm2).
Table 1. Surface roughness of neat PU and its composite films (scan sizes: 20 × 20 μm2).
SampleRa 1 (nm)Rq 2 (nm)
PU334408
PUC/Ti-0.5440545
PUC/Ti-1.0439533
PUC/Ti-2.0493589
PUC/LG-0.5367474
PUC/LG-1.0353431
PUC/LG-2.0446548
1 Ra—(mean roughness) the average value of the surface height deviations taken within a presented area. 2 Rq—(RMS—root mean square) the standard deviation of the height values within a presented area.
Table 2. Values of the equilibrium swelling degree, qe, determined in toluene and THF and the percentage of water absorption of the neat PU and its composites.
Table 2. Values of the equilibrium swelling degree, qe, determined in toluene and THF and the percentage of water absorption of the neat PU and its composites.
Sampleqe
Toluene		qe
THF		WA
(%)
PU0.5431.3884.052
PUC/Ti-0.50.5741.8179.840
PUC/Ti-1.00.6701.8635.208
PUC/Ti-2.00.5541.5113.380
PUC/LG-0.50.6331.7076.015
PUC/LG-1.00.7162.0334.856
PUC/LG-2.00.7211.5884.043
Table 3. Values of the rate constants (k2), theoretical equilibrium swelling degrees (qe2) and correlation coefficients (R2) of the pseudo-second-order kinetic model of the neat PU and its composites in different solvents.
Table 3. Values of the rate constants (k2), theoretical equilibrium swelling degrees (qe2) and correlation coefficients (R2) of the pseudo-second-order kinetic model of the neat PU and its composites in different solvents.
SampleTolueneTHFWater
k2
(g g−1min−1)		qe2R2k2
(g g−1min−1)		qe2R2k2
(g g−1min−1)		qe2R2
PU0.04390.5480.99940.12331.3891.00000.07230.0440.9979
PUC/Ti-0.50.03500.5830.99970.01891.8300.99980.04180.1050.9985
PUC/Ti-1.00.04380.6780.99990.76961.8641.00000.05660.0570.9989
PUC/Ti-2.00.07830.5570.99990.05311.5171.00000.03160.0420.9986
PUC/LG-0.50.04430.6410.99990.04461.7110.99990.07570.0650.9974
PUC/LG-1.00.06900.7190.99970.37272.0341.00000.03010.0560.9978
PUC/LG-2.00.05410.7280.99990.02631.5960.99980.03370.0490.9983
Table 4. Values of diffusion exponent (n) in water and diffusion coefficient (D) for the neat PU and its composites in different solvents.
Table 4. Values of diffusion exponent (n) in water and diffusion coefficient (D) for the neat PU and its composites in different solvents.
SamplenD × 108 (cm2 s−1)
TolueneTHFWater
PU0.388.3214.212.51
PUC/Ti-0.50.409.8325.924.08
PUC/Ti-1.00.5610.5629.983.44
PUC/Ti-2.01.148.6318.162.86
PUC/LG-0.51.008.2224.019.58
PUC/LG-1.01.4714.2344.108.61
PUC/LG-2.01.1715.2620.535.59
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MDPI and ACS Style

Stefanović, I.S.; Džunuzović, J.V.; Džunuzović, E.S.; Randjelović, D.V.; Pavlović, V.B.; Basagni, A.; Marega, C. Insight into the Morphology, Hydrophobicity and Swelling Behavior of TiO2-Reinforced Polyurethane. Coatings 2025, 15, 231. https://doi.org/10.3390/coatings15020231

AMA Style

Stefanović IS, Džunuzović JV, Džunuzović ES, Randjelović DV, Pavlović VB, Basagni A, Marega C. Insight into the Morphology, Hydrophobicity and Swelling Behavior of TiO2-Reinforced Polyurethane. Coatings. 2025; 15(2):231. https://doi.org/10.3390/coatings15020231

Chicago/Turabian Style

Stefanović, Ivan S., Jasna V. Džunuzović, Enis S. Džunuzović, Danijela V. Randjelović, Vladimir B. Pavlović, Andrea Basagni, and Carla Marega. 2025. "Insight into the Morphology, Hydrophobicity and Swelling Behavior of TiO2-Reinforced Polyurethane" Coatings 15, no. 2: 231. https://doi.org/10.3390/coatings15020231

APA Style

Stefanović, I. S., Džunuzović, J. V., Džunuzović, E. S., Randjelović, D. V., Pavlović, V. B., Basagni, A., & Marega, C. (2025). Insight into the Morphology, Hydrophobicity and Swelling Behavior of TiO2-Reinforced Polyurethane. Coatings, 15(2), 231. https://doi.org/10.3390/coatings15020231

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