Encapsulated Colorimetric Indicators used as Additives During the Polymerization of Smart Polyethylene

Abstract

A number of colorimetric pH indicators (phenolphthalein, bromocresol green, 4-nitrophenol, phenol red, and bromothymol blue) are encapsulated within silica particles via an acid-catalyzed sol–gel process. The fabricated indicator is added during ethylene polymerization with a metallocene (Cp₂ZrCl₂) catalyst. Introduction of the indicator into the silica matrix affects the surface area, probably by partially blocking some of the pores. The encapsulated pH indicator reduces the catalytic activity by up to 17%, whereas adding a solution of the indicator reduced the catalytic activity by up to 34%. The crystallinities of the samples containing the silica-encapsulated pH indicators are lower than those of the samples to which the pH indicator is added in solution form. The prepared encapsulated colorimetric pH indicators are analyzed by diffuse reflectance UV-vis spectroscopy. The measured color values are distributed according to the CIE Lab lightness psychometric color space. Addition of the indicator in solution form (free) to the ethylene polymerization reaction does not result in indicator activity in acidic or alkaline media. The pH indicators incorporated into the polyethylene show color changes when exposed to acidic and alkaline media. Samples to which the unencapsulated pH indicators (free) are added exhibit lower catalytic activities and higher crystallinities than the samples containing the encapsulated pH.