Abstract
Recently, extensive research has been focused on the development of microcellular polymer foams because they offer multiple advantages relative to their solid analogs, such as substantial material savings and improved mechanical properties. Moreover, considerable effort has been devoted to polymer clay nanocomposites where addition of a small amount of clay can dramatically improve a wide variety of properties of the polymer matrix. In the present dissertation, the development of polymer clay nanocomposite and microcellular foams was studied. Understanding the parameters affecting the structure and properties of nanocomposite material is of fundamental importance in designing microcellular nanocomposite foams with desired properties. For this reason the modification of sodium montmorillonite (NaMMT) through the insertion of the amphiphilic hexadecylammonium cations into the clay’s interlayer spaces has been initially studied. Alkylammonium concentrations equivalent to 0.15 - 3.00 times ...
Recently, extensive research has been focused on the development of microcellular polymer foams because they offer multiple advantages relative to their solid analogs, such as substantial material savings and improved mechanical properties. Moreover, considerable effort has been devoted to polymer clay nanocomposites where addition of a small amount of clay can dramatically improve a wide variety of properties of the polymer matrix. In the present dissertation, the development of polymer clay nanocomposite and microcellular foams was studied. Understanding the parameters affecting the structure and properties of nanocomposite material is of fundamental importance in designing microcellular nanocomposite foams with desired properties. For this reason the modification of sodium montmorillonite (NaMMT) through the insertion of the amphiphilic hexadecylammonium cations into the clay’s interlayer spaces has been initially studied. Alkylammonium concentrations equivalent to 0.15 - 3.00 times the cation exchange capacity of the clay were used. The conformation of the surfactant cations in the confined space of the silicate galleries was investigated by XRD and SEM, while the organoclay’s thermal stability was examined by TGA. The clay’s surface properties induced by the ion-exchange process were followed by measurements of the mineral’s zeta potential as a function of pH and surfactant concentration, while the coagulation rates of organoclay suspensions in water and in chloroform were examined using the DLS technique. All the results are consistent with showing that the overall characteristics and thus, the behavior of the modified MMT particles strongly depends on the alkylammonium surfactant concentration used in the modification process. In the next stage of the thesis nanocomposite materials consisted of poly(L-lactic acid) and modified montmorillonite were prepared by the solution intercalation method and the role of the clay modification level on the structural, thermal and mechanical characteristics of the resultant hybrids was investigated. The results showed that proper separation of the clay layers due to the polymer insertion can be achieved, provided that the inorganic cations in the interlayer space are completely exchanged by the amphiphilic cations. Additionally, an ideal balance between thermal and mechanical properties can be obtained at surfactant quantity equivalent to 1.5 times the clay CEC. The role of nanostructure on the thermal and mechanical behavior of the hybrid materials was studied next. Nano- and microcomposites of PLLA with various loadings of natural and hexadecylamine-modified montmorillonite were prepared by the solvent casting method. The structure of the composites was investigated by XRD, TEM and AFM and their thermal behavior by TGA and DSC. Moreover, their mechanical properties were explored under tensile loading conditions. Nanocomposites were found to display increased thermal stability and improved tensile properties compared to conventional counterparts and pure polymer. SEM observation of the deformed surfaces affirmed a different deformation process mechanism between the two types of composites PLLA/organo-montmorillonite nanocomposites were also prepared using a micro-extruder and the effect of the preparation procedure on the composite’s structure and thermal properties was investigated. It was evident that clay dispersibility was higher at the hybrids prepared through a solution.
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