Molecular dynamics simulation of amine groups formation during plasma processing of polystyrene surfaces

Miroslav Michlíček,Satoshi Hamaguchi,Lenka Zajíčková

doi:10.1088/1361-6595/abb2e8

Abstract

Plasma treatment and plasma polymerization processes aiming to form amine groups on polystyrene surfaces were studied in-silico with molecular dynamics simulations. The simulations were compared with two experiments, (i) plasma treatment in N2/H2 bipolar pulsed discharge and (ii) plasma polymerization in cyclopropylamine/Ar radio frequency (RF) capacitively coupled discharge. To model favorable conditions for the incorporation of primary amine groups, we assumed the plasma treatment as the flux of NH2 radicals and energetic NH3 ions, and the plasma polymerization as the flux of cyclopropylamine molecules and energetic argon ions. It is shown in both the simulation and the experiment that the polystyrene treatment by the bipolar pulsed N2/H2 plasmas with an applied voltage of about ±1 kV formed a nitrogen-rich layer of a thickness of only a few nm. The simulations also showed that, as the NH3 incident energy increases, the ratio of primary amines to the total number of N atoms on the surface decreases. It is because the energetic ion bombardment brakes up N–H bonds of primary amines, which are mostly brought to the surface by NH2 radical adsorption. Our previous experimental work on the CPA plasma polymerization showed that increased RF power invested in the plasma leads to the deposition of films with lower nitrogen content. The MD simulations showed an increase of the nitrogen content with the Ar energy and a limited impact of the energetic bombardment on the retention of primary amines. Thus, the results highlighted the importance of the gas-phase processes on the nitrogen incorporation and primary amines retention in the plasma polymers. However, the higher energy flux towards the growing film clearly decreases amount of hydrogen and increases the polymer cross-linking.

Highlights

Plasma modification of materials by amine groups is promising for applications in which cells interact with the surface as well as for immobilization of proteins and covalent bonding of drugs [1]
Other problems that can hinder the agreement between the simulation and experimental results are related to limitations inherent to molecular dynamics, (i) the discrepancy between the spatial and temporal scales accessible by the Molecular dynamics (MD) simulation and the experiment and (ii) the precision of potential functions, for slow processes and reactions
We have presented results of molecular dynamics simulations of the plasma treatment of polystyrene by the flux of NH2 radicals combined with NH3 energetic bombardment and the plasma polymerization of CPA under Ar energetic bombardment

Summary

Introduction

Plasma modification of materials by amine groups is promising for applications in which cells interact with the surface as well as for immobilization of proteins and covalent bonding of drugs [1]. The simplest technique relies on introducing amine groups on the polymer surface by plasma treatment in nitrogen (N2), nitrogen and hydrogen (N2/H2) or ammonia (NH3) discharges [2, 3]. Different approaches have been developed to maximize the concentration of the introduced primary amine groups. Plasmachemical mechanism of primary amine incorporation into hydrocarbons was studied for N2, N2/H2 and NH3, showing the importance of NH and NH2 radicals [2, 17]. The combined action of excited nitrogen N2(A) and ground state N radical was suggested as a plausible mechanism for the formation of labile nitrogen groups, which subsequently hydrolyze to a primary amine in the open air [17]

Results

Discussion

Conclusion