In situ polymerization of styrene–clay nanocomposites and their properties

Recent advances in materials technology have fostered the development of various preparation strategies and applications of new polymer–clay nanocomposites. Several synthesis pathways have resulted in new nanocomposites with properties that have been successfully incorporated into various application areas. Nanocomposites have attracted great interest as promising advanced materials due to their superior properties compared to conventional polymers [1, 2], such as density, high resistance to surface treatment, high modulus of elasticity, nonflammability and thermomechanical/optoelectronic/magnetic properties [3‐5]. Indeed, the use of a polymer matrix by adding a well-defined percentage of clay as reinforcement leads to the improvement in the physicochemical properties of the resulting nanocomposite and increase in the biodegradability of the polymer [6‐10]. All these improvements depend on several parameters such as clay distribution (dimensions, shape factor, exfoliation …) and polymer–clay interaction [7]. The improved properties of polymer–clay nanocomposites can be explained by the special properties of clays, such as swelling and ion exchange, but also by their structural nature [11]. Their two-dimensional structures in lamellar form and their electronegative colloidal charge make them the compounds of choice for various organic molecules, including cationic surfactant molecules. The resulting hybrid materials are used in various fields such as adsorption, barrier applications and supports for the development of nanocomposites [12]. Polystyrene (PS) belongs to the group of standard thermoplastics, which also include polyethylene, polypropylene and polyvinyl chloride. Due to its special properties, PS can be used in a very wide range of applications. It is a versatile polymer with the following main characteristics: transparency, simplicity of colouring and processing and low cost [13]. The objective of this work is to prepare new polystyrene–clay nanocomposites, and we are interested in the realization of polystyrene–clay nanocomposites reinforced by the organically modified maghnite. The technique used to incorporate the modified maghnite into the polymer is polymerization in situ by intercalation of polymers in interfoliar galleries. In situ polymerization is widely used in the preparation of polymer-based nanocomposites, which tends to favour the formation of exfoliated structures. Indeed, this mode is based on the dispersion of the monomer in the clay and then its polymerization allows a better dispersion of the clay in the polymer obtained. Initially, montmorillonite is organically modified by cation exchange with the cetyltrimethylammonium bromide molecule, in order to make it organophilic. This step promotes the insertion of the organic polymers into the interpolar galleries. The method of preparation of nanocomposites is a very important parameter that has a direct impact on the structural, thermal and mechanical properties of the materials obtained.

This simple analysis technique allows us to easily highlight the presence of surfactant molecules in the montmorillonite gallery, based on the comparison of the different absorption bands of functional groups present in the clay before and after its modification. The spectra of the organophilic maghnites (Mag–CTA) show the appearance of new peaks that indicate the insertion of alkylamines in the interleaving galleries of our clays. The analysis of these spectra shows the main absorption bands of the vibration modes of the different functional groups. An absorption band centred on 3620 cm⁻¹ is due to the valence vibrations of the OH groups linked to the octahedral cations Al(Al–OH–Al) [22] and the band at 3440 cm⁻¹ which is due to the OH–Fe⁺³ valence vibrations which widens the adsorption band. The band centred at 1642 cm⁻¹ is attributed to the H–O–H deformation vibrations of water molecules. The adsorption bands from 1000 to 500 cm⁻¹ are attributed to the valence and deformation vibrations of octahedral ions substituted for hydroxyl groups and the intense band observed at 1030 cm⁻¹ which corresponds to the valence vibrations of the Si–O bond in the plane [23]. The bands centred at 915, 865, 792, 624, 520 cm⁻¹ are attributed, respectively, to the deformation vibrations of the Al–OH–AL, Si–O–Al/Al–OH–Mg, cristobalite, Si–O–Mg and Mg–OH bonds [24]. Figure 1 shows the FTIR spectra of the welded maghnites; examination of these spectra shows: as in the maghnite spectrum, there are two bands characteristic of the valence vibrations of the OH groups of the octahedral layer (3626 cm⁻¹) and the OH–Fe⁺³ valence vibrations (3445 cm⁻¹): the characteristic band with H–O–H deformation vibrations of water molecules (1637 cm⁻¹) and the band corresponding to the valence vibrations of the Si–O bond (located between 1000 and 500 cm⁻¹ and centred around 1034 cm⁻¹). The spectrum of sodium montmorillonite also illustrates characteristic bands to the deformation vibrations related to substitutions in the octahedral layer (between 920 and 525 cm⁻¹). The spectra of organophilic maghnite (Mag–CTA) show the appearance of new peaks that indicate the insertion of alkylamines into the interfoliary galleries of our clays. These peaks are attributed to the valence and deformation vibrations of the different alkylamine groups. This band is slightly shifted (Fig. 1) and may be due to an interaction between the carbonyl function of the surfactant and the clay surface and more probably via the establishment of hydrogen bonds with hydroxyl groups located at the extreme surface of the clay. These vibration modes are CH₂ valence vibrations between 2950 and 2850 cm⁻¹, shear vibrations between 1470 and 1450 cm⁻¹ and swing vibrations between 600 and 650 cm⁻¹; this indicates that the organic group is present with maghnite [25]. All the FTIR spectra of the prepared organoclays confirm the incorporation of surfactants into the clay.

The different characteristic bands and their attributions are grouped in Table 1.

The FTIR spectra of the nanocomposites obtained by mass polymerization are almost identical and contain both the characteristic absorption bands of polystyrene and modified clay (Fig. 2). The different characteristic bands of PS, PS/Mag–Na⁺ and PS/Mag–CTA and their allocations are grouped in Table 1. The absorption bands characterizing the Mag–CTA and PS groups are present in the spectrum of PS nanocomposites such that the absorption band of the C–H range of the –CH₃ end groups appears between 2843 and 2949 cm⁻¹ and that of the CH₂ groups of PS and Mag–CTA appears between 1450 and 1482 cm⁻¹. The 3622 cm⁻¹ band characterizing the free OH groups of the octahedral layers of Mag–CTA disappears in the PSNC nanocomposite, showing that interactions between PS and Mag–CTA chains are possible. This confirms the incorporation of CTA clay in polystyrene matrices.

The basal spacings (interlayer distance) of the raw clay (Mag), clay treated with NaCl (Mag–Na⁺), organophilic montmorillonite (Mag–CTA⁺) and their nanocomposites with PS were obtained from the peak position of the XRD pattern. The interfoliar distance of Mag−Na⁺ increased from 12.27 to 33.53 Å for the prepared Mag-CTA, which corresponds to a bimodal profile, centred on two values of 19.27 Å and 33.53 Å (Fig. 3). The high relative peak intensity in the (001) plane is due to a narrower distribution of the interlamellar spacing when the CTA ions are incorporated into the clay sheets. Several (00l) basal reflections can be observed, corresponding to the crystallographic planes of the clay layer (110) and (020) [26] and its position being independent of the basal spacing.

Mass polymerization has shown that this mode of synthesis seems to favour the exfoliated morphology with the exception of PSNC15% nanocomposites, which will lead to an intercalated structure (presence of a peak at 2θ = 5.88°). The basal reflection (001) of the Mag-CTA has totally disappeared. Figure 4 shows the SAXS spectra of PSNC nanocomposites obtained by adding different percentages by weight of modified clay to styrene, while 1 and 10% nanocomposites give exfoliated morphologies (Table 2). The PSNC15% nanocomposite tends to have a morphology intercalated with a slight displacement of the basal distance of 2θ = 5.8° smaller compared to the organoclay used (2θ = 5.2°). This result indicates that the clay has kept its orderly state with a small amount of polystyrene between the layers of the clay. This effect has already been noted during the intercalation of polymerizable surfactants in clay and has been explained by a “pinning” effect of the clay layers following the reaction of growing polystyrene chains with several polymerizable surfactant molecules present on the surface of the clay, which induces a narrowing of the lamellar structure and thus a slight decrease in the basal space while maintaining the ordered structure of the clay [27‐29]. The 10% composite has an intermediate behaviour; in fact, its DRX diffractometer has a broad peak towards 2θ = 3.5°, which can be interpreted either by partial exfoliation or by a disorder in the tactoids of clay [30]. Note that for 10 and 15% nanocomposites, there is a small peak at 2θ = 2.1° which corresponds to the presence of small occasional tactoids. This behaviour is often observed when using long-chain alkyl surfactants as clay modifiers [31].

Transmission electron microscopy is the most widely used qualitative technique for the characterization of polymer–clay nanocomposite dispersions. Figure 5a, b, c of the PSNC% nanocomposites shows a degree of disorder in the morphology of the clay. The dark lines correspond to the silicate monolayers, and the clay platelets are separated from each other, which is in favour of an exfoliated structure. The silicate nanoplaquettes are oriented more or less parallel to each other but are separated enough from each other so that they do not interact. The TEM image of the PSNC 15% nanocomposite (d) shows an intercalated morphology, which corresponds to the results found by DRX with the presence of small tactoids. Indeed, in addition to the exfoliated nanoplaquettes, we note the presence of some aggregates.

Figure 6 shows the SEM images obtained for the nanocomposites PSNC1%, PSNC3%, PSNC5% PSNC7%. The micrographs of the PSNC nanocomposite show the presence of tortuous fissure propagation lines leading to a porous appearance, and comparison of the images shows that as the amount of organoclay dispersed in the PS matrix increases, the appearance of the nanocomposite becomes more irregular and porous. It has been described in the literature that when silicate polymers are well dispersed, many nonlinear cracks are formed and tend to develop until they interfere with each other [32]. Stresses at the ends of fracture lines interact with each other and hinder the growth of these stresses. Since the strength of the polymeric material is closely related to crack formation at the molecular level, the greater the number of these tortuous cracks during the failure process, the more energy will have to be absorbed to break the material.

The AFM characterization also gives the value of the RMS (root-mean-square) roughness and the average roughness Ra of the surface of each layer analysed. These two roughnesses are given by the relationships V.1 and V.2.

The RMS roughness is the average of the distance differences between the altitude z of each point and the average altitude of the line. The average roughness is given by:

It is the average of the differences between the attitudes z of each point on the line and the value of the average attitude with \(\left\langle Z \right\rangle = \frac{{ \mathop \sum \nolimits_{ij} Zij}}{N}\)

The AFM image processing software directly gives the value of the RMS and Ra roughness. Figure7 shows how the roughness varies as a function of Load rate by weight%. The roughness values indicate that the surfaces have slightly more hollow profiles and the decrease in roughness of the sample is related to the decrease in grain size, thus making the surface less rough; the variation in roughness follows the variation in load rate by weight. It can be seen that the PSNC5 nanocomposites have the lowest roughness values. This is due to the exfoliation of the PS/Mag-CTA nanocomposite. Comparison of the images shows that as the organic clay dispersed in the PS matrix increases, the appearance of the nanocomposite becomes more irregular and porous. Many nonlinear cracks form and tend to grow until they interfere with each other. This change in clay particle size was observed by SEM on samples PSNC1, PSNC3, PSNC5 and PSNC7.

Figure 8 shows a micrograph to the right of a morphological examination of 5 µm × 5 µm. The surface of a PSCN in real space has three dimensions to see the shape of the reliefs. The one on the left evokes the observation of this surface in 2D but with the calculation of depth Z of these reliefs, which could not be seen on the photographs of the SEM, on the micrograph of PSNC nanocomposites, the layers are smooth with a significant roughness; we can clearly see that the light areas are higher and the dark areas deeper; these two areas appear better on the 3D micrograph.

The thermal stability of nanocomposites has been studied by thermogravimetric analysis; these results show that the thermal stability of prepared nanocomposites is not only related to the clay content, but is also much more related to the condition of the clay in the polymeric matrix, on the surface between the polymeric matrix and the clay. The addition of only 1% modified clay improves the degradation temperatures T₂₀ and T₅₀ by 6 °C and 10 °C, respectively, compared to polystyrene. Above a concentration of 5%, there is little difference between the degradation profiles suggesting a threshold effect. The thermal stability of the PS/Mag–CTA nanocomposites obtained by incorporating 1, 5, 3% by weight of Mag–CTA-based organoclay with respect to styrene has been determined. The data obtained are grouped in Table 3 and Figure 9.

This result is in agreement with those reported in similar research studies where the authors generally observe an increase in the degradation temperature of nanocomposites due to good polymer–clay interactions or clay platelets slow the diffusion of degradation products. Zang et al. [33] described the synthesis of three quaternary polystyrene ammonium surfactants for the modification of montmorillonite. Clay modified by reaction with chloromethyl polystyrene gives the most thermally stable nanocomposites. Essawy et al. [34] observed that PS-clay nanocomposites modified with cetylpyridinium chloride were stable above 400 °C compared to PS and PS-clay nanocomposites modified by CTA. Uthirakumar et al. [35] showed an increase in the temperature at the beginning of degradation of nearly 35 °C by adding 1% by weight of clay. Chen et al. [36] observed the increase in this temperature by 11 °C by adding 5% weight of clay modified by CTA. Some authors have also studied the effect of the way PS nanocomposites are prepared on their thermal properties. Chigwada et al. [37] observed that the thermal stability of the PS was improved when the modified clay was added during mass polymerization compared to the melt mixing method. One possible interpretation is the formation of carbonaceous structures that limit diffusion phenomena. It also appears that optimal stabilization is obtained for charging rates ranging from 2.5 to 5% by mass. The results showed that the PS/maghnite–CTA nanocomposites had better thermal stability.

The gain in stability of modified PS–clay nanocomposites is due to the formation of a carbonized protective layer. The formation of this layer is stimulated by the fine dispersion of intercalated or exfoliated montmorillonite particles that act as an inorganic carrier [38]. It is important to note that the TGA trace recorded on the PSNC material indicates that the onset of PS decomposition is about 350 °C higher and the rate of polymer matrix degradation in these nanocomposites is significantly reduced. Indeed, the degradation temperature of the polymers is often improved by the incorporation of silicates in the exfoliated layers [39, 40], which increases the value of these polymers and allows them to be used at higher temperatures.

This work made it possible to prepare modified polystyrene–clay nanocomposites using different quantities of organoclays ranging from 1 to 15% using the in situ polymerization technique. The characterization of the organoclays by different analyses (IR, ATG and DRX, SEM, TEM, AFM) made it possible to quantify the quantity of surfactants incorporated in the clay and the spacing between the clay layers. X-ray diffraction analyses have shown that the majority of the nanocomposites obtained have an exfoliated morphology and the nanocomposites prepared with 15% by mass of Mag-CTA are of the intercalated type; however, the TEM and SEM images obtained are consistent with the DRX data. The mass polymerization technique has shown the positive effect of the introduction of a cetyltrimethylammonium bromide chain on the thermal stability of nanocomposites. The presence of organically modified maghnite in the polystyrene significantly improves the Young's modulus of the matrix and makes it more resistant. Therefore, a relatively flexible polystyrene-type matrix reinforced with such fillers becomes mechanically very strong, and the maghnite silicate layers, which have a very high specific surface area, bring considerable improvements in Young's modulus. The results obtained showed that Algerian clay can be used as a reinforcement for the development of polystyrene-based nanocomposites.