Dynamics and Structure Formation of Confined Polymer Thin Films Supported on Solid Substrates.

Mujib Ur Rahman,Yonghao Xi,Dongjie Liu,Fei Chen,Haipeng Li,Jinjia Wei

doi:10.3390/polym13101621

Abstract

The stability/instability behavior of polystyrene (PS) films with tunable thickness ranging from higher as-cast to lower residual made on Si substrates with and without native oxide layer was studied in this paper. For further extraction of residual PS thin film (hresi) and to investigate the polymer–substrate interaction, Guiselin’s method was used by decomposing the polymer thin films in different solvents. The solvents for removing loosely adsorbed chains and extracting the strongly adsorbed irreversible chains were selected based on their relative desorption energy difference with polymer. The PS thin films rinsed in chloroform with higher polarity than that of toluene showed a higher decrease in the residual film thickness but exhibited earlier growth of holes and dewetting in the film. The un-annealed samples with a higher oxide film thickness showed a higher decrease in the PS residual film thickness. The effective viscosity of PS thin films spin-coated on H-Si substrates increased because of more resistance to flow dynamics due to the stronger polymer–substrate interaction as compared to that of Si-SiOx substrates. By decreasing the film thickness, the overall effective mobility of the film increased and led to the decrease in the effective viscosity, with matching results of the film morphology from atomic force microscopy (AFM). The polymer film maintained low viscosity until a certain period of time, whereupon further annealing occurred, and the formation of holes in the film grew, which ultimately dewetted the film. The residual film decrement, growth of holes in the film, and dewetting of the polymer-confined thin film showed dependence on the effective viscosity, the strength of solvent used, and various involved interactions on the surface of substrates.

Highlights

As new technologies are introduced to enable the manufacturing of ever-smaller devices, it has been shown that in the past two decades, the use of polymer nanometer films and structures underwent rapid growth, such as thin-film transistors, semi-conductor chips, biosensors, and organic photovoltaics [1]
The control of morphology and stability of polymer thin film on solid substrates is of paramount importance, especially for new emerging polymer-based nanotechnologies
Block copolymers, offering an attractive patterning technology because of the self-assembling into various morphologies, are considered to undergo extensive studies for wide applications, including biomolecules adsorption, nanolithography, nanoparticle synthesis, and high-density information storage media [2,3,4,5,6]

Summary

Introduction

As new technologies are introduced to enable the manufacturing of ever-smaller devices, it has been shown that in the past two decades, the use of polymer nanometer films and structures underwent rapid growth, such as thin-film transistors, semi-conductor chips, biosensors, and organic photovoltaics [1]. Chen et al [8] reported that unstable PS thin films form cylindrical holes on the solid substrate after a certain time of annealing, and the growth speed of the radius of the hole was related to polymer viscosity In this scenario, it becomes of interest to study how one can predict and improve the thermal stability of the thin films by controlling the dewetting phenomena. The outer layer at the polymer/air interface has higher mobility and enhanced chain dynamics, whose properties differ from that of the inner layer at the polymer/solid interface since in the inner layer the dynamics are reduced due to the presence of a strongly adsorbed polymer layer [41,42,43,44] By controlling this adsorption kinetics, the structural and dynamic properties, such as glass transition temperature, viscosity, segmental mobility, crystallization, wettability, and the thermal expansion coefficient of polymer thin films on solid substrates, could be tailored . The effective viscosity for two different substrates was calculated and discussed in order to facilitate the understanding of the chain dynamics and properties and control of the adsorption kinetics in polymer/substrate interfaces based on the AFM method

Materials

Spin Coating

Solvent Rinsing

Results and Discussion