Mechanisms of water vapour transport in polyimide thin films for applications in humidity sensing

Ravji, Shabir Hussein (2015) Mechanisms of water vapour transport in polyimide thin films for applications in humidity sensing. PhD thesis, University of Glasgow.

Abstract

Polyimides are ubiquitous in the electronics, space science, and research industries due to their thermal stability, ease of use, lifetime, and high dielectric strength. These properties, along with the propensity of polyimides to absorb water vapour, has led to both their use as a common sensing element in humidity sensors, and additional challenges when utilising polyimides in solid state electronic devices. Consequently, a substantial amount of literature has been produced regarding the transport properties of water vapour in polyimide films. This has been found to be a complex process dependent on the morphology and chemistry of the particular polyimide in question. Accordingly, as part of an industrial collaboration with Honeywell Inc. several tools and probing techniques were developed to map and quantify the transport properties and characteristics of such materials.
The material of focus is a particular form of polyimide which is used as a sensing element by
Honeywell in their capacitance based humidity sensors. The cure procedures and preparation have been varied to understand the relationship between water transport and processing procedure.
Importantly, methods and equipment have been developed to measure and characterise the subtle difference in water transport resulting from variations in the preparation procedure. in-situ techniques to characterise transport have included the use of capacitance measurements, Attenuated Total Refectance Infra -Red spectroscopy (ATR-FTIR), Quartz Crystal Microbalance (QCM), and Neutron Refectivity. Other techniques to characterise the polymer have included Transmission Electron Microscopy(TEM) and Atomic Force Microscopy(AFM).
The use of Attenuated Transmission Reflectance Infrared Spectroscopy (ATR-FTIR) has indicated a chemical interaction between the polar elements of polymer backbone and the water molecule.
Permporometry and an analysis of the TEM images indicate a transport length scale ∼ 0.2nm,
similar to the size of a water molecule.
Density profles ftted to Neutron Refectivity measurements reveal a dense skin layer on the surface of the polymer, the characteristics of which vary with the sample curing procedure. The neutron refectivity technique was then used in time of fight mode (ToF) to map the ingress of vapour into the polymer, pushing the time resolution further than has previously been achieved. However, the box-car averaging technique which was used to gain sufficient counts of neutrons was found to obscure the longer timescale transport effects. The QCM procedures outlined have provided
Honeywell with a cost effective method of raw material measurements of transport properties for a range of materials. QCM measurements indicate dual diffusion coefficients, skin, and bulk, when Fickian transport models are applied to the system. The transport timescales were found to be thickness independent in the range studied (60nm-1.1µm), consequently diffusion is not the rate limiting factor in this system.
Thus, the key factors such as time and length scales of this diffusion system have been characterised with the customisation of a host of techniques. The distribution of water vapour within the samples is shown to be uniform in the bulk layer of the sample, and the rate limiting step in the transport of water vapour is demonstrated to be constant on all polyimide thin films. It is also indicated that oxygen plasma etching can be used to reduce the hystersis effect in this form of polyimide with some indication that such treatment impacts on the chemical interactions between the polyimide films and water vapour.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Polyimide, water vapour, humidity, sensing, sensor, Honeywell, TEM, SEM, neutron reflectivity, QCM, quartz crystal microbalance, polymer, thin films
• Subjects: Q Science > QC Physics; T Technology > TS Manufactures
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: McGrouther, Dr. Damien
• Date of Award: 2015
• Depositing User: Shabir Ravji
• Unique ID: glathesis:2015-7034
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 18 Jan 2016 10:14
• Last Modified: 10 Feb 2016 15:04
• URI: https://theses.gla.ac.uk/id/eprint/7034