Novel polymer/halloysite composites with high halloysite content and remarkable mechanical strength

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Abstract
Halloysite nanotubes (HNTs) are of interest for use in nanocomposites due to their unique cylindrical structure and resulting properties. Various polymer/HNT composites have been studied, but generally these composites have HNTs as a minor component. Here, we report novel polymer/HNT composites with high HNT content facilitated by strong hydrogen-bonding interactions between the polymer and HNTs. These composites with 50–75 wt% HNTs were prepared by in situ polymerization reactions of mixtures comprising HNTs, acrylic acid, triethylene glycol dimethacrylate, potassium persulfate, and water, followed by drying. The chemical structure of composites was verified by Fourier-transform infrared spectroscopy (FTIR). The high dispersity of HNTs in the poly(acrylic acid)-based matrix was demonstrated by scanning electron microscopy (SEM). Studies of mechanical properties illustrated greatly enhanced mechanical strength of the composites relative to the pure polymer matrix, with the highest flexural strength, microhardness, and ultimate tensile strength achieved for the composite with 66.7 wt% HNTs. Thus, the incorporation of high mass fractions of HNTs in a polymer matrix can offer potential benefits for applications requiring superior mechanical properties, in addition to other functions endowed by either HNTs or the polymer matrix. Differential scanning calorimetry (DSC) characterization did not show evidence of a glass transition in the polymer matrices of these composites, and thermogravimetric analysis (TGA) revealed increased thermal stability of the composites relative to the matrices. Swelling tests indicated that the swelling capacity is primarily determined by the amount of polymer present in the composite, and the presence of HNTs may facilitate mass transfer of water within the polymer matrix.

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