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Synthesis, characterization and thermal properties of oxo methacrylate-containing polymer/clay nanocomposites

In this study, synthesis and characterization of polymer/clay based nanocomposites was performed using 2-(4-methoxyphenyl amino)-2-oxoethyl methacrylate (MPAEMA) and organoclay. The amount of organoclay in nanocomposites synthesized by in situ polymerization method was determined as 3% and 5%. FTIR, XRD, and SEM were used as characterization techniques. It was determined from XRD and SEM results that the morphology of nanocomposites exfoliated. Then, the thermal properties of nanocomposites were investigated using the TGA/DTA/DTG simultaneous system. In thermal analysis, it was seen that the thermal stability increased when the clay amount increased. It is thought that this newly synthesized synthetic polymer/clay nanocomposites with natural clay content will serve different areas due to its environmentally friendly-biodegradable properties.


Originality
In this research, unlike polymer/organoclay composites made in the literature, MPAEMA polymer and C10A organoclay were used.

Findings
It was found that the composites synthesized by the in situ polymerization method were exfoliated. In addition, it has been observed from thermal analysis that thermal stability increases as the amount of clay increases.

Conclusion
It is thought that the newly synthesized synthetic polymer / clay nanocomposites with natural clay content will serve many different areas, due to their environmentally friendly and biodegradable nature.

Declaration of Ethical Standards
The author(s) of this article declare that the materials and methods used in this study do not require ethical committee permission and/or legal-special permission.

INTRODUCTION
Polymer/clay nanocomposites are structures formed by dispersing inorganic particles in a polymer matrix [1,2]. Thanks to the added inorganic materials, it is aimed to increase the strength of nanocomposites without increasing the hardness of the polymer matrix, and also to improve the thermal and mechanical properties and to reduce the gas permeability value, etc. [1][2][3][4][5][6][7]. Thanks to these advantages provided by nanocomposites, its use is becoming more and more common. On the other hand, with the increase of plastics and polymeric raw materials depending on the usage areas of traditional polymers, synthetic polymers in nature cause an important environmental problem since they cannot be biodegraded [8]. One of the novel and harmless ways of preventing plastics from damaging nature and the environment is to include plastics in the natural cycle, that is to make plastics degradable [8][9][10]. For this purpose, using naturally occurring clays with synthetic polymers and making polymer/clay composites are among the academic studies of the last 25 years. Polymers with hydrophobic properties are not compatible with hydrophilic clays. Therefore, to make it compatible, clays, ammonium salts, etc. are used. They are converted into organoclays by ion exchange reaction [10][11][12][13][14][15]. Clay layers can be intercalated type or exfoliated type throughout the polymer matrix. Intercalated structure occurs when high clay rates are used, or polymerization does not occur between clay layers if the clay layers do not separate much during polymerization. In the exfoliated structure, it usually occurs at low clay rates (~1%-15%) and by separating the clay layers as much as possible. In this case, the polymer/clay composite has been found to have the good thermal and mechanical properties. Achieving this situation is very important due to the strong electrostatic forces that hold the clay layers together [9][10][11][12][13][14][15].
In this study, C10A organoclay which was expanded with organic modifiers between Na+MMT clay species layers was used. Organoclay amount was used as low as 3% and 5%. The nanocomposites formed by the C10A organoclay and the MPAEMA (2-(4-methoxyphenyl amino)-2-oxoethyl methacrylate) monomer that we synthesized in our laboratory, were synthesized by using in situ polymerization and characterized. In addition, the morphology and thermal properties of nanocomposites were investigated.

Instrumental Measurements of Nanocomposite
The FTIR (Fourier Transform Infrared) spectra of all samples were performed with a PerkinElmer Spectrum Two (UATR) IR spectrometer in the range of 4000-450 cm-1. XRD (X-Ray Diffraction) patterns were obtained using a Bruker Axs D8 Advance diffractometer with a back monochromator and a Cu target and Kα (λ=1.5418 nm) in 2θ=10-45o (step of 0.01º, at room temperature). SEM (Scanning electron microscope) observation was recorded with a Zeiss Evo LS 10 at 25 kV. Thermal analyzes were obtained with a Hitachi 7000 TGA/DTA/DTG (Thermal Gravimetric Analysis/Differential Thermal Analysis/Differential Thermogravimetric Analysis) simultaneous system a heating rate of 10oC min-1, under nitrogen gas flow (0.2 L/min).

2.3.
Preparation of Poly(MPAEMA)/C10A Nanocomposites MPAEMA monomer was re-synthesized according to the literature ( Fig.1) [16,17]. Poly(MPAEMA)/organoclay nanocomposites were prepared with in situ method. 3% and 5% amount of C10A organonanoclay was dispersed in 1,4-dioxane and stirred by magnetic stirrer at 70 °C for 24 h. 1 M MPAEMA monomer was dissolved in 1,4dioxane at room temperature in another flask. BPO was added as a free radical initiator to the 3% and 5% organoclay added monomer mixture in separate bottles. In separate magnetic stirrer, polymer/organoclay composites were formed by mixing at 73 °C for 48 hours. The composites were precipitated in excess ethyl alcohol, removed from impurities, dried in the oven, and sifted through a 20micron sieve. The clay peaks are observed in the C10A organoclay as mentioned above. Also, includes dimethyl benzyl alkyl chain quaternary ammonium chloride structures used in the modification. The following are the peaks from the chemical modifier; aliphatic N-CH3 vibration at 2840 cm-1, aliphatic CH2 vibration at 1465 cm-1, symmetric and asymmetric C-H stretching vibration at 2920 cm-1 and aromatic C=C stretching vibration at 1644 cm-1 [10,18,19]. The most characteristic bands observed for poly(MPAEMA) units are seen in (cm-1) 3260 (N-H), 3444 (C Ar-H), ~2940 (aliphatic C-H), 1724 (C=O ester stretch), 1668 (C=O amide stretch), 1606 (C=C stretch on aromatic ring), 1246 and 1510 (symmetrical and asymmetrical C-O-C) [16,17]. In nanocomposites, all these peaks from the homopolymer are observed. On the other hand, some peaks characteristic of clay are also seen in composites. From these results, it can be said that the organomodified clay presents in polymer matrix as is reported by other articles [9][10][11][12][13][14][15].

3.2.
XRD Measurements X-ray diffraction is the preliminary technique to verify whether the layered structure has altered or not. In the literature, the characteristic crystalline peaks of the diffraction angle of C10A organoclay was 2θ = 5.4°, 20°, 22° (d=1.64, 0.44, 0.40 nm) [10,18,19]. Nanocomposites synthesized by the in situ polymerization method are obtained by polymerizing the monomer in these layers when the clay layers are spaced apart, so XRD peaks may appear amorphous. The distribution of the polymer between the layers of the clay causes that a clear XRD peak in the nanocomposites to be unreadable. The absence of component-specific peaks in nanocomposite materials can be explained by the fact that the polymer is intercalated between clay layers and clay layers become so irregular that they cannot give an XRD signal [10][11][12][13][14][15][17][18][19][20][21], clear XRD peak is not observed, and therefore it can be considered as an exfoliated structure. In addition, all peaks present in the XRD curve of the clays are not observed in the nanocomposites. The XRD patterns of the poly(MPAEMA)/organoclay nanocomposites are shown in Fig.3.a-c.

3.3.
SEM Measurements SEM micrographs were used for further characterization of nanocomposites. The homogeneous distribution of nanoparticles from SEM photographs is presented in Figure 4.a-b. As seen in the SEM micrographs, the clay was dispersed in the polymer matrix, and particle sizes are close to each other. This exfoliated structure was observed in the XRD results and confirmed with the help of SEM effects [10,15,17,[19][20][21]. Wavenumber (cm -1 ) % T (T r a n s m i t t a n c e)

3.4.
Thermal Measurements Thermal stabilities of polymer/clay nanocomposite were determined by TGA/DTA/DTG simultaneous method. For 3% and 5% clay additive nanocomposites at 10 °C/min heating rate, weight loss% decreased as the temperature increased. On the other hand, it is observed that the residue content increases with the increase of clay content in composites. Ash formation is very important for flame resistance and insulates the underlying polymer, thus it prevents flame feeding and air ingress [22,23]. These thermal results showed that the clay additive increased the thermal stability by increasing the activation energy level required for thermal decomposition. It was also observed that degradation occurred at two levels for both composites. Thermal curves of nanocomposites are given in Figure 5, and comparatively in Figure 6. Table 1 summarizes some of the thermal data of nanocomposites at different temperatures, such as weight loss% and residual ash. Similar approaches observed for polymer/clay nanocomposites have been reported in different studies [10,[13][14][15][17][18][19][20][21][22][23][24].

CONCLUSION
In this research, polymer/organoclay based nanocomposites synthesis, characterization, and thermal properties of poly(2-(4-methoxyphenyl amino)-2oxoethyl methacrylate) (MPAEMA) were investigated by in situ polymerization. FTIR, XRD, SEM, and TGA techniques were used at characterizations of nanomaterials. From XRD, SEM and thermal measurements, it was observed that the morphology of nanocomposites was exfoliated when the clay content in the polymer matrix was kept at 3% and 5%. It was observed that the thermal stability of nanomaterials increased as the clay rate increased from thermal analysis. Considering that environmentally friendly and biodegradable polymer/clay composites are used in various fields such as aviation, automobile, construction, packaging, petroleum, biomedical and wastewater treatment, it is hoped that this study will guide the literature and companies.