Preparation, Morphology and Properties of Poly(methyl acrylate-co-methyl methacrylate)/Montmorillonite Nanocomposite Films through Soap-free Emulsion Copolymerization
Date Issued
2011
Date
2011
Author(s)
Lin, Keng-Jen
Abstract
This research is mainly to investigate the exfoliation mechanism of montmorillonite (MMT) in the soapless emulsion polymerization. Based on the adsorption of anionic surfactant by MMT in aqueous, we can explain that the exfoliation behavior of MMT. Furthermore, the fabricated MMT/PMMA nanocomposite latex particles are too rigid to be cast into a film. To lower the glass transition temperature of nanocomposites, we have added methacrylate monomers to copolymerize with methylmethylacrylate in the presence of MMT during soap-free emulsion polymerization. Therefore, we reported the unique mechanical properties of the exfoliated MMT/P(MA-co-MMA) nanocomposite films
The study is divided into three parts; the first part of this research was to investigate the adsorption of sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) by MMT respectively in aqueous solution and their intercalation structures after removal of water. Based on the experimental observation, a justified exfoliation mechanism of MMT in the soapless emulsion polymerization under the presence of MMT was proposed. The second part of this research was to investigate the unique mechanical properties of exfoliated MMT/P(MA-co-MMA) nanocomposites. Finally, the third part of this research was to investigate the shear strength of fracture surface for MMT/P(MA-co-MMA) nanocomposite films utilized a unique test fixture.
In the first part of this research, the adsorption of SDS by MMT in aqueous solution was associated with the Donnan equilibrium for ions leading to the hydrophilic nature of SDS/MMT hybrid, whereas the adsorption of CTAB resulted from the Coulomb attraction or so-called cation exchange leading to the hydrophobic nature of CTAB/MMT. X-ray scattering (XRD) and transmission electron microscopic (TEM) investigations on the dried SDS/MMT hybrids revealed that the adsorbed SDS neither substantially increased the interlayer distance nor changed the rough spherical texture of pristine MMT. On the other hand, the adsorbed cetyltrimethyl ammonium cations increased the interlayer distance of MMT, resulting in a well integrated CTAB/MMT lamellar structure. Therefore, we proposed a justified exfoliation mechanism of MMT during soapless emulsion polymerization based on the above experimental results as that the polymerizing ionic radicals in water phase were considered as a major component to diffuse into the gallery of MMT. They have been observed to organize into disk-like micelles in the interlayer regions to exfoliate MMT. The exfoliation of MMT was almost completed before micellization stage was over. After exfoliation, the disk-like micelles became a polymerization loci for monomers. Because the disk-like micelles in numbers were substantially over the commonly formed spherical micelles in the typical soap-free emulsion polymerization, the conversion rate of MMA to MMT/PMMA nanocomposite latex was faster.
In the second part of this research, the exfoliated montmorillonite (MMT) nanoplatelets tended to re-stack with each other after casting the MMT/ poly(methylacrylate-co-methylmethacrylate) P(MA-co-MMA) latex solutions fabricated by soap-free emulsion polymerization into films as revealed by X-ray diffraction and transmission electron microscopy. As the content of MMT was increased from 0 to 20 wt %, the Tg measured by differential scanning calorimetry was slightly decreased from 19.2 to 17.2 0C, whereas that measured by dynamic mechanical analysis was increased from 22 to 32 0C, indicating that the local motion of polymer segments has been retarded by MMT nanoplatelets. Besides, the elongated elliptical voids appeared during stretching of 1 wt % MMT/P(MA-co-MMA) film to cracking also illustrated the pinning effect provided by the exfoliated MMT. As the content of MMT was increased more than 10 wt %, the mechanical behavior of MMT/P(MA-co-MMA) nanocomposite films was changed from ductile to brittle nature with significant increase of Young’s modulus and tensile strength owing to the restacking of exfoliated MMT nanoplatelets.
In the third part of this research, the shear strength of ductile polymer films was measured using a unique test fixture that is capable of controlling the normal to shear plane angle in a confined testing space. Two specimens were loaded in series in the test fixture such that as one fractured the other was in a state just prior to fracture and could be used for the investigation of shear bands by SEM. As the normal to shear plane angle was increased from 30 to 60°, the shear bands were more concentrated in the center region owing to the higher normal compressive stress. Interestingly, the fracture surface showed that the distance between two adjacent striations perpendicular to the fracture direction increased as the normal to shear plane angle was increased from 30 to 45° and the striations parallel to the fracture direction appeared. As the angle was further increased to 60°, small islands formed by cross-over between the parallel and perpendicular striations appeared in the fracture surface. The formation of islands was related to the flatness of the fracture surface because the shear fracture has been confined in the center of the specimens by high normal compressive stress. On the other hand, it was found that fracture of the films depended on both the shear and the normal stresses and could be described by Mohr-Coulomb criterion. The effects of MMT contents on the intrinsic shear strength (i.e., the shear strength at zero normal stress)
Subjects
Montmorillonite
ionic surfactant
soap-free emulsion polymerization
exfoliation
nanocomposite
grafting
poly(methyl methacrylate) poly(methylacrylate-co-methylmethacrylate)
shear strength
fracture surface
Type
thesis
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