Wettability and thermal analysis of hydrophobic poly(methyl methacrylate)/silica nanocomposites

Abstract

Hydrophobic poly(methyl methacrylate)/silica layers on the glass support activated with plasma were obtained. Wettability was investigated from the contact angle measurements and surface free energy estimation by means of using contact angle hysteresis approach. The thermal analysis of poly(methyl methacrylate)/silica films was conducted on a STA 449 Jupiter F1, Netzsch in temperature range of 30–950℃. It was found that the thermal stability of the poly(methyl methacrylate)/silica films decreases with increasing poly(methyl methacrylate) amount in the sample. Morphology of the obtained nanocomposites was investigated by means of scanning electron microscopy. The addition of silanized silica to the poly(methyl methacrylate) solution leads to obtaining the film with more hydrophobic properties, the contact angle value over 90°, and the apparent surface free energy lower than 30 mJ/m². The most hydrophobic film was obtained in the case of silica with Θ = 0.63 (Θ – surface fraction coverage with trimethylsilyl groups). Silica particles were well dispersed in the polymer solution which was confirmed by the scanning electron microscopy analysis.

Keywords

Hydrophobic layer contact angle surface free energy polymer silica

Introduction

In order to protect constructive materials from harmful atmospheric factors, for example glass used in solar panels or photovoltaic cells, it is necessary to cover them with special protective layers (Akamatsu et al., 2000; Kim et al., 2012). The lens surfaces with the reflective layer get dirty easily and have to be very often cleaned. The solution of such a problem seems to be a deposition of hydrophobic layer, so-called clean coat. Glass surface becomes smoother and more hydrophobic because water creates drops easier to remove and contamination embeds more difficult. Hydrophobic coats are widely used also on the concrete surfaces, polymers, or painted surfaces as protective layers for water and frost (Akamatsu et al., 2000). Unfortunately, the support coverage with a hydrophobic layer is very often problematic due to low adhesion. Because of that using plasma technique for surface modification is reasonable (Jung et al., 2011; Kang et al., 2010).

Using silica as a filler in polymers very often leads to mechanical properties improvement of organic–inorganic nanocomposites (Zhou et al., 2004). Yamano and Kozuka (2010) investigated thin poly(methyl methacrylate) (PMMA)/silica films on the glass. They have characterized the obtained films using scanning electron microscopy (SEM), wettability, and FT-IR spectroscopy. PMMA/silica films were optically transparent and exhibited slightly hydrophobic nature with the contact angle 54°. The addition of the PMMA resulted in photochromic changes in color and high chemical durability as well as mechanical hardness. Such films cannot be obtained by the conventional sol–gel method.

Tsai et al. (2011) obtained polyimide (PI)/silica layers on the glass surface. It was found that the addition of silica (from 10 to 40%) to PI increased hydrophobicity which ranged from 84 to 98° depending on the silica contents. On the other hand, transparency of such hybrid films is associated with improved compatibility between the polymer and silica. Hydrogen bonding between PI and silica leads to better dispersion of silica in the film. Pipatchanchai and Srikulkit (2007) investigated hydrophobicity modification of the cotton fabric surface with hexadecyltrimethoxysilane (HDTMS) and silica. The prepared coatings changed surface property from hydrophilic to hydrophobic. The confirmation of obtaining hydrophobic surface can be clearly seen in the contact angle values, which changed from 0° to even over 120° depending on the silica and HDTMS contents.

Experimental

Plasma treatment of glass surface

Air plasma treatment of glass surface was performed using a low-pressure plasma (system Pico, Diener Electronic, Germany). The round glass plates with a 20 mm diameter were placed in a vacuum chamber and the system was subjected to the pressure of 0.2 mbar with the gas flow 22 sccm (standard cubic centimeters per minute). The plates were treated with the plasma power 400 V for 30 s. To remove the gaseous products, the chamber was purged with air for 10 s and opened when the pressure inside was equal to the atmospheric pressure.

Modification of initial silica

The initial silica was modified by hexamethyldisilazane (HMDS, Merck Millipore) using the liquid phase method. Adsorption of HMDS on the silica surface was conducted from the hexane (chemically pure, Merck Millipore) at room temperature. After air drying, the samples were heated for 6 h at 130–150℃. The degree of surface modification by the trimethylsilyl (TMS) groups was controlled by varying concentration of HMDS in hexane. The degree of silica coverage was determined using IR spectroscopy (spectra 3750 and 2965 cm⁻¹) from the integral intensity of the corresponding to silica bands. The degrees of surface coverage with the TMS groups are shown in Table 1.

Table 1.

Properties of HMDS-modified silica.

	The amount of HMDS in the reaction mixture (mmol per 1 g SiO₂)	The degree of surface modification with TMS groups (Θ)
SiO₂ (initial)	–	0
P5	0.221	0.53
P6	0.268	0.63
P7	0.315	0.89
P8	0.689	1.00

HMDS: hexamethyldisilazane; TMS: trimethylsilyl

Deposition of polymer/silica layers on the glass surface

PMMA used in the experiments was from Organika S.A., Sarzyna, Poland, and chloroform (p.a.) was from Avantor Performance Materials, Poland. For the contact angle measurements water from Millipore Milli-Q System, USA, 18.2 MOhm/cm was used.

PMMA (0.2 g/100 cm³) (Chibowski et al., 2006) in chloroform was prepared and the determined amount of silanized silica with different surface fraction coverages with TMS groups (Θ) from 0.53 to 1.0 (origin of silica is given in Table 1) was dispersed (0.16 g) in the PMMA solution with a sonicator. Then, 0.5 ml of the PMMA solution with a filler was poured out in the middle of a circular glass plate and rotated in the spin coater (LOT, S.A., Germany) at 45 r/min for 15 s. The content of the fillers in the PMMA solution was 0.5 g/3 cm³ (Chibowski et al., 2006; Hołysz et al., 2013). Before contact angle measurements the plates were kept at room temperature for 24 h in a vacuum desiccator under 117 mbar pressure.

Contact angle measurements using the sessile drop method

For the contact angle measurements by the sessile drop method Digidrop GBX Contact Angle Meter, France video-camera system equipped with the computer software was used. The contact angles of probe liquid were measured at 20 ± 1℃ in a closed chamber. The advancing contact angles of water of 6 µl volume (Rymuszka et al., 2013) were measured after settling droplets on the glass surface. Then after sucking of one-third from the droplet into the syringe, the receding contact angle was measured.

Surface free energy calculation

Apparent surface free energy $(γ_{s})$ was evaluated from the contact angle hysteresis (CAH) using the model proposed by Chibowski and Perea-Carpio (2002), Chibowski et al. (2002), and Chibowski (2003, 2005). The CAH approach relates $(γ_{s})$ to the surface tension of the probe liquid $(γ_{l})$ and CAH, which is defined as the difference between the advancing $θ_{a}$ and receding $θ_{r}$ contact angles

γ_{s} = \frac{γ_{L} (1 + \cos θ a) 2}{2 + \cos θ_{a} + \cos θ_{r}}

(1)

where

γ_{s}

is the apparent surface free energy,

γ_{L}

is the water surface tension, and

θ_{a}

θ_{r}

are the advancing and receding contact angles of water.

SEM

Morphology observation was made using a high distributive field emission scanning electron-ion microscope (Quanta 3D FEG, FEI).

Thermal analysis

Thermal analysis was made on a STA 449 Jupiter F1, Netzsch, Germany under the following operational conditions: heating rate of 10℃/min, a dynamic atmosphere of synthetic air (50 ml/min), temperature range of 30–950℃, sample mass ∼ 16 mg, sensor thermocouple type S thermogravimetry–differential scanning calorimetry (TG–DSC). As a reference the empty Al₂O₃ crucible was used. The gaseous products emitted during decomposition of materials were analyzed by the quadrupole mass spectrometer QMS 403D Aeölos, Germany coupling online to the STA instrument by quartz capillary heated to 200℃. The mass spectra were recorded under electron impact ionization energy of 70 eV in the scan mode (scan bargraph) in the range from 10 to 100 amu.

Results and discussion

Nowadays glass is a very widely used type of support in everyday life and industry. From our previous investigation it can be concluded that the optimal air plasma treatment time of glass surface is 30 s (Terpiłowski and Rymuszka, 2016). Activation with plasma leads to improvement of the adhesion between the deposited layer and glass support. It was observed that modification leads to contact angle decrease about nine times and surface free energy increase (Terpiłowski et al., 2015). The calculated work of adhesion for the glass surface treated by air plasma amounts 145.60 ± 13.33 mJ/m² and is higher than for the unmodified surface (133.44 ± 12.84 mJ/m²). Therefore, it can be concluded that plasma treatment leads to adhesion improvement between the deposited layer and the glass surface.

Wettability and surface free energy

Figure 1 presents the water contact angle values measured for the PMMA/silica films deposited on the glass surface with a different surface coverage (Θ: 0.53–1.00). As can be seen the addition of silanized silica into the polymer solution leads to obtaining more hydrophobic properties of the obtained films. It can be observed that the highest value was attained with the addition of silica Θ = 0.63 and the advancing contact angle was 128.3° ± 7.4° and the receding contact angle 123.9° ± 7.3°. There is no characteristic dependence between the contact angle values and the degree of silica hydrophobization. On the other hand, the apparent surface free energy calculated for obtained films using CAH approach was much lower than its value for the glass covered only by PMMA which indicates that the prepared films are hydrophobic (Pipatchanchai and Srikulkit, 2007) (Figure 2).

Figure 1.

Advancing and receding contact angles of water measured on the PMMA/silica films deposited on the glass surface. PMMA: poly(methyl methacrylate).

Figure 2.

Apparent surface free energy calculated from CAH approach of PMMA/silica films deposited on the glass support. CAH: contact angle hysteresis; PMMA: poly(methyl methacrylate).

SEM analysis

The SEM micrographs of the films of PMMA and different content silica are shown in Figure 3. Angular structures of modified silica were observed in all samples. The modified silica particles are homogeneously dispersed in the PMMA/chloroform solution and the presence of big silica aggregations was not observed. Increasing of the hydrophobic group’s number on the silica surface can prevent the aggregation of the silica particles and leads to obtaining better surface properties of hydrophobic nanocomposites.

Figure 3.

SEM images of PMMA/silica films deposited on the glass support and modified silica (Θ = 0.86). (a) PMMA/silica (Θ = 0.53), (b) PMMA/silica (Θ = 0.63), (c) PMMA/silica (Θ = 0.86), (d) PMMA/silica (Θ = 1.00), and (e) silica Θ = 0.86. PMMA: poly(methyl methacrylate); SEM: scanning electron microscopy.

Thermal analysis

Thermal durability of PMMA coverage of silica powders was investigated by TG, derivative thermogravimetry (DTG), and DSC. In Figure 4 the TG, DTG, and DSC curves measured for the silica nanocomposites loaded with PMMA are presented. Analyzing these dependences in the range 30–900℃ indicates that thermo-oxidation of the all samples proceeds in two main stages. The first small weight loss (about 0.2%) at the temperatures below 100℃ corresponds to removal of physically adsorbed water from hydrophobic nanoparticles. The second exothermic peaks from 120 to 950℃ are due to the decomposition of organic molecules. According to the literature (Landau et al., 2002; Yariv, 2004) the oxidation process at 200–900℃ for organic compounds generally can be divided into a few main stages: defragmentation, oxidation, and burning carbonized residue. The TGA curves of silica nanocomposites reveal a major weight loss in the temperature range from 120 to 500℃. Calculations indicate that this degradation step results in weight loss of 4.11, 3.42, 3.23, and 3.46% for PMMA/silica (Θ = 0.53), PMMA/silica (Θ = 0.63), PMMA/silica (Θ = 0.86), and PMMA/silica (Θ = 1.00), respectively. This process depends on chemical composition and amounts of polymer, silica surface properties, and adsorbate–adsorbent interactions. The DTG dependences presented in Figure 4(b) for the samples show some differences. The degradation of PMMA/silica (Θ = 0.53) and PMMA/silica (Θ = 0.63) begins at 120℃ and seems to proceed in a one-reaction stage according to the DTG curves (wide peaks with the minimum at 310℃ for PMMA/silica (Θ = 0.53) and 340℃ for PMMA/silica (Θ = 0.63)). Similar changes were observed for thermal decomposition of the PMMA/palladium nanocomposites in air (Aymonier et al., 2003). However, appearance of two peaks on the DSC curve (Figure 4(c)) shows that two exothermic reactions take place in this region. The onset, maximum degradation, final temperatures (T_onset, T_max, T_end), and heat of the degradation step as well as the total mass loss are shown in Table 2. Regarding the complex character of PMMA interactions with silica, the processes of their thermal degradation are complicated and pass through many stages of degradation: unreacted modifiers, functionalities of silica aerogel nanoparticles, and different forms of polymer molecules (Fazli et al., 2015).

Figure 4.

TG (a), DTG (b), and DSC (c) curves for the silica nanocomposites: PMMA/silica (Θ = 0.53) (green line), PMMA/silica (Θ = 0.63) (red line), PMMA/silica (Θ = 0.86) (blue line), PMMA/silica (Θ = 1.00) (black line). DSC: differential scanning calorimetry; DTG: derivative thermogravimetry; PMMA: poly(methyl methacrylate); TG: thermogravimetry.

Table 2.

T_onset, T_max, T_end and the total mass loss of the thermal processes occurred for examined silica nanocomposites.

Additives	Degradation temperature (℃)				Total mass loss (%)
Additives	T_onset	T_max	T_end	ΔH, kJ/g	Total mass loss (%)
PMMA/silica (Θ = 0.53)	293.8	336.3	369	−0.011	5.21
	408.7	438.6	485.8	−0.098
PMMA/silica (Θ = 0.63)	310.5	435	466.7	−0.184	4.50
PMMA/silica (Θ = 0.86)	403.3	416.8	443.9	−0.312	4.65
PMMA/silica (Θ = 1.00)	341.9	404.5	443.2	−0.353	4.45

PMMA: poly(methyl methacrylate).

The biggest changes on the DTG curves took place for the samples modified in the presence of the highest TMS amount. In this case above 120℃ two peaks can be observed on the DTG curves. The first one with the minima at 257 and 272℃ (PMMA/silica (Θ = 0.86), PMMA/silica (Θ = 1.00)), respectively, is probably associated with the process of decomposition of weak head-to-head linkages and decomposition of PMMA chain ends (Ahmed et al., 2015; Hirat et al., 1985). A further increase of temperature (between 300 and 500℃) results in the exothermic reaction of random scission of the polymer chains and combustion of organic molecules (Akat et al., 2008; Fazli et al., 2015). With the increase in the concentration of modifier on the DSC curves, only one sharp exothermic peak appears. Due to the presence of larger amounts of PMMA, T_onset, T_max, T_final shifted toward lower temperature (Table 1).

For better illustration of the processes taking place during the thermal degradation of the examined samples, the measurements of the presence of gaseous products by mass spectrometry were performed. The intensity profiles of main decomposition products of PMMA (H₂O (18), CO₂ (44), the monomer after the loss of the entire side chain –COOCH₃ (41), the monomer after the loss of –OCH₃ (69), the entire monomer (100)) (Tatro et al., 2003) are presented in Figure 5. The analysis of M/Z = 41, 69, and 100 indicates that at low concentrations of PMMA (black curve) mainly defragmentation of polymer molecules is the dominant step in the temperature range 250–400℃. Due to strong interactions of polymer fragments with the silica surface, the process takes place at lower temperatures. In the case of gaseous profiles characteristic of CO₂ and H₂O, the appearance of distinct peaks in the temperature range 200–750℃ (with the maximum at about 300 and 450℃ for PMMA/silica (Θ = 0.53), and PMMA/silica (Θ = 0.63) and 400℃ for PMMA/silica (Θ = 0.86)) is the evidence of organic substance decomposition. It seems that with the increasing concentration of PMMA in the thermal degradation of the analyzed nanocomposites, combustion of polymer is the predominant process, as evidenced by the increase of negative value of decomposition enthalpy (Table 2).

Figure 5.

MS profiles of gaseous products characteristic of PMMA decomposition (m/z:18 (H₂O), 41 (the monomer after the loss of the entire side chain (–COOCH₃)), 44 (CO₂), 69 (the monomer after the loss of –OCH₃), 100 (the entire monomer) versus the temperature for the systems: PMMA/silica (Θ = 0.53) (black line), PMMA/silica (Θ = 0.63) (red line), PMMA/silica (Θ = 0.86) (blue line). PMMA: poly(methyl methacrylate).

Conclusions

Modification of glass surface by air plasma causes improvement of adhesion between the polymer/silica layers and the glass support which is related to the polar groups introduced on the glass surface. The contact angle values over 90° measured for hydrophobic PMMA/silica coatings and lower surface free energy calculated from the CAH approach allow to conclude that the modified silica addition leads to obtaining films with more hydrophobic properties than pure polymer coatings. From the SEM images it can be observed that silica particles are well dispersed in the PMMA matrix and aggregates are not formed.

These TG/MS data support the conclusion that the amount of monomer in the gaseous products decreases and thermal stability decreases with the increased amount of PMMA in the sample. With the increasing concentration of PMMA process in thermal degradation of the obtained composites, there are two predominant processes: defragmentation and combustion of the polymer. Moreover, the increase in enthalpy of the reaction suggests that polymer combustion is the predominant process in thermal degradation with the increasing concentration of PMMA.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research has been funded by the intentional grant of Ministry of Science and Higher Education to carry out research as well as do the related tasks, contributing to the development of junior researchers and doctoral students at the Faculty of Chemistry of the Maria Curie-Skłodowska University, project titled: The influence of plasma action on the adhesion of hydrophobic layers deposited on selected surfaces, the period of the project: 2015, Faculty of Chemistry, Maria Curie-Sklodowska University.

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