Nanooxide/Polymer Composites with Silica@PDMS and Ceria–Zirconia–Silica@PDMS: Textural, Morphological, and Hydrophilic/Hydrophobic Features
© The Author(s). 2017
Received: 23 December 2016
Accepted: 20 February 2017
Published: 27 February 2017
SiO2@PDMS and CeO2–ZrO2–SiO2@PDMS nanocomposites were prepared and studied using nitrogen adsorption–desorption, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), measurements of advancing and receding contact angles with water, and microcalorimetry. The pore size distributions indicate that the textural characteristics change after oxide modification by poly(dimethylsiloxane) (PDMS). Composites are characterized by mainly mesoporosity and macroporosity of aggregates of oxide nanoparticles or oxide@PDMS nanoparticles and their agglomerates. The FT-IR spectra show that PDMS molecules cover well the oxide surface, since the intensity of the band of free silanols at 3748 cm−1 decreases with increasing PDMS concentration and it is absent in the IR spectrum at C PDMS ≥ 20 wt% that occurs due to the hydrogen bonding of the PDMS molecules to the surface hydroxyls. SEM images reveal that the inter-particle voids are gradually filled and aggregates are re-arranged and increase from 20 to 200 nm in size with the increasing polymer concentration. The highest hydrophobicity (contact angle θ = 140° at C PDMS = 20–40 wt%) is obtained for the CeO2–ZrO2–SiO2@PDMS nanocomposites. The heat of composite immersion in water shows a tendency to decrease with increasing PDMS concentration.
In recent decades, hydrophobic hybrid metal or metalloid oxide (MO)–polymer composites are widely used in a variety of applications such as self-cleaning hydrophobic coatings [1–3], chemical separation of polar and nonpolar substances [4, 5], adsorption of organic contaminants, and removal of oil from the water surface [6, 7]. The main attention is paid to development of film coatings and membranes [1–5, 8–12], as well as to highly dispersed materials with developed surface area [13–16]. To prepare highly dispersed hydrophobic composites, silica [13–15], titania [16, 17], zinc oxide , magnetic nanoparticles (γ-Fe2O3) , and mixed oxides [20–22] are often used. Highly dispersed composites with an adsorbed polymer layer on a nanoparticle surface are referred to core–shell nanocomposites (NC)  having a number of peculiarities. In NC with MO/poly(dimethylsiloxane) (PDMS) at a low polymer content, a major fraction of the polymer is located at the interfaces with nanoparticles [24–26]. The interfacial polymer fraction [25, 27] is characterized by a modified structure [28, 29], slower dynamics [30–33], and increased thermal stability  in comparison to the bulk. The use of complex MO cannot only improve the performance characteristics of the NC (such as thermal stability, durability [20, 21]) but also significantly affect their structural characteristics [20, 34]. The structure of the polymer adsorption layer defines such NC surface properties as hydrophilicity–hydrophobicity [34–36], compatibility with organics, adsorption capability, and reactivity [37–39]. The helix shape of PDMS with six Si–O bonds in a cycle  restricts the number of segments which can directly interact with a solid surface to form the hydrogen bonds SiO–H···O(Si(CH3)2–)2. However, PDMS conformation can be changed in an adsorption layer depending on the PDMS content and MO structure (i.e., cores in the core–shell particles). A variation in the PDMS content can affect many properties of NC. Thus, such PDMS/zirconia/silica characteristics as the texture, hydrophobicity, and interfacial behavior strongly differ from those of PDMS/silica . Therefore, the effects of structure of complex MO, as well as the impact of the content and molecular weight of PDMS, on the structural and hydrophobic properties of core–shell NC are highly relevant for further development and enhancement of core–shell NC with improved characteristics and regulated properties.
Upon creation of NC with MO/PDMS, a particular interest should be paid to complex MO, in which each component can enhance certain properties of the whole composite. Since the presence of such oxides as CeO2 and ZrO2 in MO/PDMS increases the thermal stability [21, 34, 41], the NC based on CeO2–ZrO2–SiO2 could be a promising material. Preparation of similar complex nanooxides with a high specific surface was previously described in detail [42, 43].
Since the creation of the composites is primarily intended to obtain hydrophobic materials, the most important task is complete and correct characterization of their hydrophobic properties vs. structures. The hydrophobic properties of fine materials could be determined not only by measuring the contact angles [44, 45] but also by using a calorimetric technique to determine the heat of immersion in polar and nonpolar liquids [46, 47], and this method is optimal to evaluate the hydrophobicity of powder materials.
Thus, in the current study, the main attention is paid to the textural, morphological, and hydrophilic/hydrophobic properties of the polymer SiO2@PDMS and CeO2–ZrO2–SiO2@PDMS composites analyzed using SEM, adsorption, spectral, contact angle, and calorimetry methods.
Fumed silica (SiO2) (pilot plant of the Chuiko Institute of Surface Chemistry, Kalush, Ukraine) and CeO2–ZrO2/SiO2 were used as substrates for adsorption modification by poly(dimethylsiloxane). Silica-supported ceria–zirconia nanocomposites were prepared using a liquid-phase method and subjected to thermal treatment at 550 °C for 1 h. The content of grafted CeO2 was 3 and 10 wt%, and the ZrO2 content was constant at 10 wt% (CZS1 and CZS2, respectively). The synthesis and physicochemical characteristics of CeO2–ZrO2–SiO2 nanooxides were described in detail previously [42, 43].
Textural characteristics of initial oxides and oxide@PDMS nanocomposites
S BET (m2/g)
S micro (m2/g)
S meso (m2/g)
S macro (m2/g)
V micro (cm3/g)
V meso (cm3/g)
V macro (cm3/g)
V p (cm3/g)
R p,V (nm)
To analyze the textural characteristics of initial oxides and oxide@PDMS nanocomposites, low-temperature (77.4 K) nitrogen adsorption–desorption isotherms were recorded using an ASAP 2405N (Micromeritics Instrument Corp., USA) adsorption analyzer after outgassing the samples at 110 °C for 2 h in a vacuum chamber. The values of the specific surface area (S BET) were calculated according to the standard BET method . The total pore volume V p was evaluated from nitrogen adsorption at p/p 0 = 0.98–99 (p and p 0 denote the equilibrium and saturation pressures of nitrogen at 77.4 K, respectively). The nitrogen desorption data were used to compute the pore size distributions (PSD, differential f V~dV p/dR and f S~dS/dR) using a self-consistent regularization (SCR) procedure under nonnegativity condition (f V ≥ 0 at any pore radius R) at a fixed regularization parameter α = 0.01 with voids (V) between spherical nonporous nanoparticles packed in random aggregates (V/SCR model) . The differential PSD with respect to the pore volume f V~dV/dR, ∫f VdR~V p were re-calculated to the incremental PSD (IPSD) at ΦV(R i ) = (f V(R i+1) + f V(R i ))(R i+1 − R i )/2 at ∑ΦV(R i ) = V p. The f V and f S functions were also used to calculate contributions of micropores (V micro and S micro at 0.35 nm < R < 1 nm), mesopores (V meso and S meso at 1 nm < R < 25 nm), and macropores (V macro and S macro at 25 nm < R < 100 nm).
Fourier Transform Infrared Spectroscopy (FT-IR)
FT-IR spectra of powdered samples (ground with dry KBr at the mass ratio 1:9) were recorded over the 4000–400 cm−1 range using a ThermoNicolet FT-IR spectrometer with a diffuse reflectance mode. For quantitative analysis, some IR spectra were normalized using the intensity of the Si–O vibration overtone at 1865 cm−1 as an inner standard.
Scanning Electron Microscopy (SEM)
The particulate morphology was analyzed using field emission scanning electron microscopy employing a QuantaTM 3D FEG (FEI Company, USA) apparatus operating at the voltage of 30 kV.
Contact Angle Measurements
A Digidrop GBX Contact Angle Meter (France) equipped with a video camera and firm software was used in the contact angle (CA, θ) measurements. Using the tilting plate method, the contact angles were measured on the pressed (180 bars for 15 min) plates of samples inclined toward the optical axis and in that position liquid gathered on one side of the droplet and retracted on the other one. A 6-μl droplet  was set in a chamber in front of the Contact Angle Apparatus camera and then using a small table. The droplet was tilted at an appropriate angle. The whole process was filmed until it started to slide. The advancing contact angle (ACA) was measured just before the droplet sliding on the droplet front and the receding (RCA) on its rear. The values of CA were evaluated for both sides of the sample plate using the Win Drop++ program. To obtain the averaged CA values, the measurements were performed for 10 water droplets put on each sample. The measurements were conducted at 20 °C and humidity RH = 50%.
Apparent Surface Free Energy Determination
Microcalorimetric investigation of oxides and composites was carried out using a DAC1.1A (Chernogolovka, Russia) differential automatic calorimeter. Before the measurements of the heat of immersion of samples in polar (water) or nonpolar (n-decane) liquids, the samples were degassed at 130 °C and 0.01 Pa for 2 h and then used (100 ± 5 mg per 3 ml of wetting liquid) without contact with air. Since the total heat of immersion depends on the mass and surface area of a sample, the obtained results were normalized to both 1 g of a sample and 1 m2 of the surface area. To obtain the averaged Q values, the measurements were performed two to three times for each system and the average errors were ≤7%.
Results and Discussion
The specific surface area (Table 1, S BET) does not demonstrate a significant reduction after grafting of ceria–zirconia. All oxide samples (Fig. 2a) are mainly mesoporous (aggregates of SiO2 nanoparticles) and macroporous (aggregates of modified SiO2). In general, the average pore radii (Table 1, R p,V) are by almost twice larger in CZS1 and CZS2, as compared to unmodified silica. The textural characteristics of the materials change due to the adsorption of PDMS (Table 1 and Fig. 2b). The values of S BET of all composites decreased with the increasing polymer content by 74, 75, and 78% (in comparison to the initial oxides) for SiO2@P40, CZS1@P40, and CZS2@P40, respectively, after the PDMS adsorption in the amount of 40 wt% (Table 1). The polymer adsorption leads to suppression of the pore volume (V p) as well as V meso and V macro. After addition of PDMS, the average pore radii (Table 1, R p,V) decrease. In general, the addition of polymer can change the porosity characteristics because each long macromolecule can be able to bind several oxide nanoparticles and their aggregates in more compacted structures. This leads to the decrease in the volume of voids between particles .
Fourier Transform Infrared Spectroscopy (FT-IR)
Adsorption modification of initial silica and CeO2–ZrO2–SiO2 nanooxides was conducted with poly (dimethylsiloxane) due to the formation of hydrogen bonds ≡ MiO–H···O(Si(CH3)2–)2 (where M = Si, Ce, or Zr, i = 1 or 2) between the oxygen atoms in the PDMS polymer chain and hydrogen atoms of the surface hydroxyls (–OH) of the oxide particles.
The symmetric and asymmetric C−H stretching vibrations of the methyl groups of PDMS are observed at 2906 and 2963 cm−1 (Fig. 3), respectively, along with the deformation vibrations of the same groups at 1263 and 1406 cm−1 . The peak intensity depends monotonically on the amount of adsorbed polymer.
Scanning Electron Microscopy
Average diameter (d) of uncoated and coated nanoparticles and average thickness (h) of the PDMS layer
Hydrophilic/Hydrophobic Properties of Composites
Surface wettability is one of the research hotspots in surface science. Lately, hydrophobic surfaces, especially superhydrophobic surfaces, have attracted considerable attention because of their extensive applications in self-cleaning, biomaterials, droplet transportation, etc.
Evaluation of the hydrophobic properties of the composites was carried out by measurements of the contact angle of wetting with water. The apparent surface free energy (γ S) was determined using the ACA/RCA hysteresis approach .
Interactions of Composites with Polar (Water) and Nonpolar (Decane) Liquids
Conditions of interactions of water with composites in the microcalorimetry method differ from those measuring the CA by a lack of the gas phase, which affects the wetting. The Gibbs free energy of the system decreases upon immersion of a composite into a liquid. This occurs due to compensating high surface energy of a solid surface interacting with a liquid that is accompanied by the heat release .
According to the molecular theory of wetting , interfacial energy is considered as a measure of the balance of dispersive and polar intermolecular interactions, and the contributions of these interactions are additive. Depending on the nature of the solid surface and liquid interacting with it, the relationship between the polar and nonpolar interactions will determine the heat of immersion [59, 60].
The heat of immersion in water and decane liquids for the SiO2@PDMS and CeO2–ZrO2–SiO2@PDMS composites
C PDMS (wt%)
After modification of nanooxides with PDMS the surface hydrophilic active centers forming the hydrogen bonds with PDMS become less accessible or inaccessible for water molecules. For all composites SiO2@PDMS and CeO2–ZrO2–SiO2@PDMS, the heats of immersion in water significantly decrease with the increasing PDMS content. In general, the heat of immersion in water is determined by the total value of wetted composite surface, the structure of adsorbed PDMS surface layer and accessibility of oxide surface for interactions with the wetting liquid. Therefore, the decrease in the heat of immersion in water can be attributed to two factors: (i) reduction of specific surface of composites with the increasing PDMS content, and (ii) changes in the structure of the surface layer interacting with water.
Nanooxides (SiO2 and CeO2–ZrO2/SiO2) were used as substrates for adsorption modification by PDMS-400 in the amounts of 5, 10, 20, and 40 wt%. Effects of nanostructured oxides on the textural and morphological characteristics of polymer silica@PDMS and ceria–zirconia–silica@PDMS NC were studied employing structure (adsorption–desorption nitrogen isotherms, FT-IR) and morphology (SEM) techniques. Microcalorimetry and measurements of the contact angle of wetting with water were performed to investigate the hydrophobic properties of the materials.
It was found that the specific surface area of nanooxide@PDMS composites decreases with the increasing C PDMS, and this decrease is larger for the CZS@PDMS systems. In general, the polymer adsorption leads to decrease in the values of V p, V meso, and V macro as well as R p,V. The analysis of the FT-IR spectra shows that all surface OH groups of oxides are disturbed due to the interactions with PMDS and adsorbed water.
The SEM images showed that the composites retain in the disperse state at the concentration of 5–40 wt% PDMS, and they can be considered as core–shell nanocomposites due to the well-distributed PDMS molecules at the surfaces of all nanoparticles of oxides.
The nanocomposites displayed high values of the main parameters related to the hydrophobicity (ACA and RCA). The contact angles of water drops for the complex oxide with adsorbed PDMS are about 140° at C PDMS = 10–40 wt% while for the SiO2@PDMS composites it is of 120°. It was found that the surface free energies extremely decreased from 30.8 ± 3.2 mJ/m2 to 7.6 ± 1.3 mJ/m2 for SiO2@PDMS composites with the increasing amount of PDMS, while for ceria–zirconia–silica systems, this value is low (≤9.1 ± 1.3 mJ/m2) at any content of PDMS.
The heats of immersion in water significantly decrease with increasing PDMS content in the composites due to two factors: formation of the hydrogen bonds between the hydrophilic active sites of oxides and PDMS leading to the decrease in their accessibility to the interactions with the water molecules and reduction of the specific surface area of composite with the increasing PDMS concentration. However, at a high content of polymer (40 wt%), the interactions of composites with water can increase due to higher aggregation of PDMS molecules each other resulting to the adsorbed layer structure changes.
Thus, this study presents promising results for an easy and cost-effective alternative using such composites as protective coatings with appropriate hydrophobic properties.
The authors are grateful to the European Community, Seventh Framework Programme (FP7/2007–2013), Marie Curie International Research Staff Exchange Scheme (IRSES grant no 612484) for financial support of this work.
The research was partly carried out with the equipment purchased, thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-06-024/09 Center of Functional Nanomaterials).
IS carried out the synthesis and characterization of the nanocomposites by the FT-IR method. DS and ADM participated in the SEM and nitrogen adsorption–desorption studies. KT participated in the contact angle measurements for polymer composites as well as calculated the apparent surface free energy. OG carried out the microcalorimetric investigations of nanocomposites. OG and IS analyzed the data and drafted the manuscript. VMG, MVB, and ADM revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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