Abstract
Low-temperature plasmas are a well-known technique for material synthesis and chemical reactions. They offer a unique combination of energetic electrons, radicals, and ions to trigger plasma chemistry efficiently. Here, we performed a modified plasma process to induce polymerization from L-lactic acid (LLA) vapors and liquid LLA oligomers. We also aimed to convert carbon dioxide (CO2) into fuels by plasma activation without using catalysts. The high-efficient, low energy waste and environmental-friendly of plasma processes are beneficial for biomedical and environmental applications. Firstly, novel biocompatible polymer films were derived from LLA with molecularly smooth surfaces by using a plasma deposition method significantly enhanced with higher monomer vapor densities. It was found that hydrocarbons and chain crosslinks increased relative to the oxygen-containing moieties as plasma power or reaction time increased, following a reaction scenario dominated by energy-mediated molecular scission pathways. With the hydroxyl groups being retained, the films of excellent mechanical strength were highly hydrophilic and manifested excellent biocompatibility applicable for a wide range of biomedical coatings. For the polymerization from liquid LLA oligomers, the polymerization was dominated by esterification processes activated by increased molecular vibrational energies imparted by electron bombardments from the plasma. The effects of oligomer molecular weight were studied and found useful for tailoring the properties of the final polymers. This work demonstrated the feasibility of converting common molecules to useful polymers under appropriate plasma conditions. Secondly, selected reactions between carbon dioxide and small hydrocarbon molecules (CnHm) of various bond structures and sizes (6≤n≤12) were investigated under plasma activation without catalysts. CO2 broke up into CO and O to form oxygenated functionalities in liquid and solid products. The liquid/solid ratio depended on plasma energy and molecular structures of the hydrocarbons. A high yield of liquids was obtained when enough hydrogen atoms were provided to saturate the active sites on CO2 and hydrocarbon fragments. Hence, the liquid yield from CO2 conversion with saturated hydrocarbons is higher than unsaturated hydrocarbons. For unsaturated hydrocarbons, the yield of solid products increases with increasing the number of C=C bonds. The bi-functional radicals produced through pi-bond dissociation can propagate into polymers easily. Such high solid yields from unsaturated hydrocarbons are able to decrease by adding water into the plasma system. In addition, the C=C bonds were found to have high activities with CO2 due to an effective recombination process of CO and O radicals with C=C bonds. This study clarifies the reaction routes for CO2 and hydrocarbon molecules under plasma activation and affords proper selections of molecules for optimal syntheses with CO2 without catalysts.