Synthesis and characterization of PMMA and organic modified montmorilonites nanocomposites via in situ polymerization assisted by sonication

Highlights

• Factorial design to study in situ polymerization of OMMT/PMMA under probe sonication

• Multiple techniques SAOS, XRD, TEM to assess clay dispersion in PMMA nanocomposite

• Intercalated/exfoliated OMMT in PMMA nanocomposites by in situ polymerization

• Flory-Huggins interaction parameter of organoclay, chloroform and methylmethacrylate

• Clay dispersion into PMMA nanocomposites confirmed by SAOS rheological measurements

Abstract

A variety of organo-modified montmorillonite (OMMT) and poly(methylmethacrylate) (PMMA) nanocomposites (CPN) were synthetized by in situ polymerization in a chloroform solution under probe sonication at clay loadings of 3 mass%. A factorial experimental design was used to study the effects of the synthesis variables: Flory-Huggins (F-H) interaction parameters between PMMA and clay organomodifier (three grades of OMMT) and sonication energy (26.0 kJ, 30.3 kJ and 34.6 kJ), aiming clay dispersion at nanoscale levels into the polymeric matrix and improvements in CPN properties. The distinct surfactant side groups of commercial OMMT were: hydroxyl (C30B), aryl (C10A) or alkyl (C25A), considering their similar interlayer space. Multiple techniques of analysis to evaluate the level of clay dispersion were used. Small-amplitude oscillatory shear rheology (SAOS) indicated a high level of clay dispersion for the PMMA/C25A CPN and a tendency to form a percolated network structure from the low shear thinning exponent n_ω of complex viscosity, either at low (n_ω = −0.97) or high (n_ω = −0.83) sonication energy. However, the rheological results showed a low level of clay dispersion for PMMA/C10A (n_ω = −0.70) and PMMA/C30B (n_ω = −0.40). The F-H interaction parameter was the statistically significant factor for the shear thinning exponent n_ω response. X-ray diffraction (XRD) of CPN exhibited no discernible reflections for PMMA/C25A-26 kJ and PMMA/C10A-30.3 kJ CPN, suggesting that a high level of clay dispersion was achieved. Transmission electronic microscopy (TEM) images of PMMA/C25A-36.4 kJ and PMMA/C30B-26 kJ CPN showed exfoliated/intercalated and intercalated morphologies, respectively. Thermogravimetry presented a significant increase in PMMA chain scission temperature of up to 82 °C for all CPN when compared to pristine PMMA. The UV–visible transmittance of PMMA/C25A-26 kJ CPN showed to be around 4.5% lower in comparison with pristine PMMA. The CPN refractive index showed a slight reduction, which could even candidate them for new materials in optical electronic devices. The improvements in the verified properties for PMMA/C25A CPN were attributed to the lower F-H interaction parameter of C25A alkyl surfactant with chloroform when considering the others OMMT side groups. The chemical affinity of the alkyl chain to the chloroform could contribute to the diffusion of the solvent/monomer solution into the interlayer space of the clay, prior to polymerization, and then the polymer chain growth to push the layers apart.

Introduction

Clay/polymer nanocomposites (CPN) have greatly called the attention of chemists of the academia and industry (Ruiz-Hitzky and Meerbeek, 2006). The efficiently dispersed clay in a polymer matrix can significantly improve mechanical properties and thermal stability (Alexandre and Dubois, 2000; Tsai et al., 2013; Barwood et al., 2014). These unique properties of polymer/clay nanocomposites from the interactions and extensive dispersion of clay in the polymer matrix can give rise to two distinct types of nanostructures: intercalated and exfoliated. Exfoliated organic modified montmorillonite (OMMT)/polymer nanocomposites are considered high performance materials because of the high aspect ratio of the exfoliated clay that offers a large superficial area of interaction with the polymer. However, obtaining the complete exfoliation of clays in polymers is still a challenging work despite the many methods that have been used, including in situ polymerization, melt blending and solution blending (Alexandre and Dubois, 2000; Ray and Okamoto, 2003). The in situ polymerization is an advantageous method to prepare exfoliated nanocomposites from melt intercalation. Namely, in polymerization, the process of incorporating the polymer into the clay gallery starts with the diffusion of the monomer, or even short propagating chains rather than the diffusion of long polymer chains, as in the case of melt intercalation (Cui et al., 2008; Elder, 2009).

Poly(methyl methacrylate) (PMMA) is a typical amorphous polymer that has the highest optical transmittance among thermoplastics, standing between commodity and engineering thermoplastics because of its relative low cost and high Young's modulus. Despite being a lightweight and shatter-resistant substitute to glass, PMMA is more susceptible to scratching (Slone, 2001). On the other hand, modified PMMA is sometimes able to achieve high scratch resistance and superior mechanical properties. Clay/PMMA nanocomposites can improve physical performance, increasing heat resistance and enhancing scratch resistance without significant reduction in optical clarity (Cui et al., 2008).

Price et al. (1992) and Price (1996) studied the polymerization of methyl methacrylate sonically initiated by an ultrasonic probe, showing that besides favoring the initiation step, the ultrasonic energy could increase the monomer conversion and polymerization rate. Ultrasound could form hot spots in a mixture of nanoparticles, MMA monomer, initiator and chloroform, where the temperature and pressure can instantaneously exceed 100 MPa and 5000 K (Price et al., 1992; Koshio et al., 2001). Organic molecules such as clay surfactant, MMA or PMMA could undergo decomposition at these hot spots, thus forming a new reactive species. So, the reaction mixture cavitation along the sonication process could assist the dispersion of nanoparticles and promote sonochemical reactions, allowing physicochemical interactions between polymer chains and organoclay. Chatel et al. (2016) reviewed the literature regarding the use of ultrasound-assisted methods to improve polymer intercalations into organoclay minerals and particles reduction and dispersion, to modify the textural properties of clays, and to favor the catalytic activity in selected reactions. They have emphasized the benefits provided by ultrasound irradiations in clay science, the current limitations of literature in report data on ultrasonic equipment and sonochemical parameters and the perspectives for this innovative clay science/sonochemistry combination.

Intercalated and semi-exfoliated clay in PMMA nanocomposites with several types and amounts of commercial OMMT clays were reported by several authors at different condition processes of probe sonication (Lee et al., 2002; Rajan et al., 2006; Ratinac et al., 2006, Ingram et al., 2008; Dhibar et al., 2009; Achilias et al., 2012). Lee et al. (2002) prepared PMMA nanocomposites using two kinds of OMMT, Cloisite 10A and 20A, through a two-stage sonication process. In the first-stage, a free radical polymerization of MMA with OMMT was carried out for 10 min of probe sonication; then, PMMA was melt and mixed with the first product for the same sonication period. Changes in phase structures towards an exfoliated clay structure were mainly found within Cloisite 20A. Nanocomposites prepared through a two-stage process revealed superior thermal stability and mechanical performance when compared to those that underwent a simple melt mixing process. Ingram et al. (2008) studied how OMMT with different organic modifiers (Cloisites 15A, 20A and 30B) and quantity of surface treatment (Cloisites 15A and 20A) affect the properties of in situ polymerized PMMA under probe sonication (8 min at 30% power). Glass transition temperatures (Tg) of nanocomposites were increased with only around 1 to 4% of clay, around 20 °C, for Cloisite 30B. The nanocomposite with Cloisite 30B had a slightly smaller d-value than the two other organoclays (XRD analysis), but no significant differences were found for those clays with different levels of surface modifier. All these synthetized nanocomposites presented mixed morphologies of intercalated and exfoliated species.

Transmission Electron Microscopy (TEM) and Wide Angle X-ray Diffraction (WAXRD or XRD) have been extensively used in the characterization of polymeric nanocomposite structures to identify the degree of intercalation, exfoliation, and dispersion of nanoparticles. TEM and XRD are effective but limited tools to scan small volumes of the sample. Morgan and Gilman (2003) and Zhao et al. (2005) recommended the use of multiple analysis techniques, usually XRD and TEM associated with another technique, to properly understand what type of nanocomposite has been made (Morgan and Gilman (2003); Zhao et al., 2005).

Zhao et al. (2005) combined the analytical methods XRD, TEM and Small Amplitude Oscillatory Shear (SAOS) to map a series of rheological responses of montmorillonite/polystyrene (PS) nanocomposites to various clay types, concentrations and degrees of dispersion. The objective of these authors was to develop a rheological technique to analyze the clay morphology in nanocomposites. In addition, they have reported the use of ultrasonic agitation in the synthesis from the rheological response of modified clay-polystyrene nanocomposites processed with high-energy sonication showed better clay dispersion than not processed with sonication, obtaining exfoliated and intercalated morphologies, respectively. Silva et al. (2016) used XRD, TEM and SAXS (Small Angle X-ray Scattering) to determine the nanostructure of OMMT/epoxy/PMMA nanocomposites prepared via in situ polymerization without sonication. These authors suggested an intercalated and disordered arrangements of organoclays inside the epoxy matrix.

The synthesis of polymer nanocomposites via in situ polymerization combined with ultrasonic energy is an ongoing and remarkable process to enhance nanoscale dispersion of organoclay into polymeric matrices. However, there is still a lack of systematic approaches in studies on synthesis variables of solution polymerization under probe sonication, as reviewed by Prado et al. (2014) for organoclays/PMMA nanocomposites. The amplitude or the energy of the ultrasonic probe was not stable during the processing variables used in several studies with different organoclays formulations. Besides, as a variety of commercial organoclays were reported in the literature on PMMA nanocomposites, it could be useful to evaluate their chemical affinity with the monomer and solvent. Previous studies (Galvan et al., 2012, Mazzucco et al., 2016) used the Flory-Huggins interaction parameter to investigate OMMT dispersion in acrylonitrile-butadiene-styrene copolymer (ABS) nanocomposites prepared by melt intercalation. Therefore, the interaction parameter of the classical Flory-Huggins lattice theory for solvent-polymer miscibility (Van Krevelen, 2009) is proposed here to study the interaction effects between organoclay-solvent and organoclay-monomer at clay dispersion degree into the polymer matrix. Three commercial OMMT clays and Cloisite grades 10A, 25A, and 30B were selected considering their similar interlayer space.

A systematic method was defined to study in situ solution polymerization of mehtylmethacrylate (MMA) with organoclays in chloroform considering its process and compounding factors. Further, a combination of XRD, TEM and SAOS analytical techniques was used to evaluate the level of clay dispersion of the nanocomposites synthetized in this study. The synthesis parameters were reviewed to set the boundaries for a factorial experimental design. The probe sonication energy was chosen as the process variable for nanoclay dispersion and polymerization. The kind of organic nanoclay phase was studied quantitatively instead of qualitatively; thus, the Flory-Huggins interaction parameter between organomodifier and MMA was estimated. Two factors with a center point experimental design was used at two levels (2²). The center point is the corresponding response to midway statistical analysis between the two levels of both factors. At this point running replicates provide an estimate of pure error and check the presence of curvature, testing whether the mathematical model between the response and the factors is linear or not. We aim to define the suitable organoclay surfactant through its interactions with the monomer and the solvent under sonication agitation, to identify the high level of clay dispersion into the PMMA synthetized by in situ polymerization in chloroform solution, and to improve on OMMT/PMMA nanocomposites.