Tailoring terpenoid plasma polymer properties by controlling the substrate temperature during PECVD

Polymers derived from natural, minimally-processed materials have recently emerged as a more sustainable alternative to synthetic polymers, with promising applications in biocompatible and biodegradable devices. Plasma-enhanced deposition is well-suited to one-step, fast, and efficient synthesis of highly crosslinked inert polymers directly from natural resources, however, fabrication of biologically active polymers remains a challenge. Plasma processing parameters influence the properties such as surface energy, roughness, morphology, and chemical composition of deposited polymers and thus their final applications. This article reports on the important role of substrate temperature (T_S) in the chemical composition, wettability, refractive index, and crosslinking density of plasma polymers derived from terpenoids. Experiments are conducted as a function of deposition power P_d, and substrate temperature, T_S. T_S varied from 40 to 280 °C and is externally controlled. Atomic force microscopy analysis reveals the change in deposition mechanism attributed to shadowing effect at higher T_S and P_d. Increase in band gap (E_g) with high T_s deposition for terpenoid based plasma polymers is observed. Swelling behavior analyzed by in situ ellipsometry affirms the enhanced crosslink density with increasing deposition rate. Fourier transform infrared analysis exhibits the formation of additional chemical moieties with increasing T_S. Increase in deposition rate with increasing T_S at higher P_d supports the theory of direct incorporation of depositing particles as dominant mechanism of plasma polymerization in this study.