Polyimides

Fundamentals and Applications

Malay Ghosh

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9780824794668
• eBook ISBN:
  9781351423649
• Topic: Technology & Engineering
• Subtopic: Chemistry
• Index: Chemistry

Provides coverage on the full range of topics associated with polyimides, including structure, polymer fundamentals, and product areas. The text addresses both basic and applied aspects of the subject. It details the synthesis of polyimides, polyamideimides, and flourinated polyimides, explains the molecular design of photosensitive polyimides, and more.

14
Surface Characterization of Polyimides

Fabio Garbassi, Ernesto Occhiello and Marco Morra

Istituto Guido Donegani, Novara, Italy
Nobil Bio Ricerche, Villafranca d’Asti, Italy

I Introduction

A large number of techniques are available today for characterizing polymer surfaces [1]. With their use, unique information on the chemistry at the surface, surface morphology, surface energetics, and, to a smaller extent, structure, is now available [2].

The interest in such studies is strictly related to surface and interface properties such as adhesion and wettability. Characterization techniques are very valuable in assessing the results of surface modifications carried out with the aim to improve such properties. Adhesion aspects and surface modification techniques are the subjects of other chapters in this book; consequently, we will not deal with them and will focus only on the subject indicated our chapter’s title. It will be limited to a short description of the more relevant surface characterization methods and results obtained on commercial and laboratory-synthesized polyimides.

Surface characterization techniques will be discussed in five main groups: ion and electron spectroscopies; vibrational spectroscopies; other techniques (mainly bulk spectroscopies applied to thin films or to diffuse interface systems such as composites); surface energy measurements; and microscopic observations.

II Ion and Electron Spectroscopies

Ion and electron spectroscopies include several techniques that are widely used for the characterization of polymer surfaces. They are generally carried out in ultra-high-vacuum (UHV) systems. This is primarily due to instrumental requirements and also to limit surface contamination effects and the loss of information caused by the presence of residual atoms or molecules.

A Ion Spectroscopies

Ion spectroscopies imply the irradiation of the sample surface with an ion beam, followed by the collection and analysis of a specific response from the sample itself. The main ion techniques are ion scattering spectroscopy (ISS), sometimes named also LEIS (low-energy ion scattering), secondary ion mass spectroscopy (SIMS), and Rutherford backscattering (RBS).

1. Ion Scattering Spectroscopy

In ISS, the collision of an ion beam, generally constituted of noble gas ions, with the sample surface causes variations in the energy and direction of ions themselves, due to an elastic scattering process (billiard ball type collision). The energy of scattered ions depends on the angle of observation and on the ratio of the atomic masses present at the surface to the incident ion mass. By energy analysis, a series of peaks are obtained, each of them corresponding to a chemical element at the surface [3]. The highlight of ISS is its extreme surface sensitivity; in fact, the contribution to scattered ions from the bulk is very limited, and thus the information obtained is related mainly to the first atomic layer (and, to a smaller extent, to the second one). Quantitative information is difficult to obtain and requires the use of appropriate standards. In a limited number of cases, ISS results are useful to clarify the chemical state of surface elements. The application of ISS to polymeric materials has been quite limited up to now, primarily because of the competition with other surface spectroscopies.

2. Rutherford Backscattering Spectroscopy

If the energy of the ion beam is raised from the keV to MeV range, the experiment falls in the domain of RBS. Its theoretical basis and instrumental arrangement are similar to those of ISS. A beam of monoenergetic ions is produced and directed to strike the target. A fraction of these ions is elastically scattered, losing an amount of energy related to the mass of the atom being hit. The number of scattered particles depends on the number of scattering atoms. If one measures the energy and amount of scattered particles, the nature and quantity of atoms present on the target can be determined [4]. The primary beam should be in the 0.5-2.5 MeV range, with small energy spread (less than 0.1 %) and modest beam current. He⁺ ions are generally used, but H⁺ , N⁺ , 0⁺, and others have also been experimented with. They are produced by an accelerator that very often is an electrostatic one such as a Van de Graaf. As counting systems, solid state detectors combined with multichannel pulse-height analysers are preferred over magnetic or electrostatic spectrometers, because of their lower cost and higher count rate [4].

RBS is very suitably applied to surface problems in which impurities are concentrated in a thin layer or are buried within a few hundred nanometers from the surface of a given material. However, the atomic masses of impurities must be greater than that of the substrate. Some applications have been made on polyimides (e.g., studies of diffusion of iodine [5] and fluorine [6] into the polymer and interaction studies with metal layers [7, 8]). Iodine has been chosen to demonstrate that RBS can be used to measure concentration profiles of small molecules slowly diffusing into polymers [5]. Ni-polyimide and Cu-polyimide (DuPont Kapton H) profiles have been studied; after aging the persistence of a sharp interface in the former case is observed, while interdiffusion occurs in the latter case [7]. By heating at increasing temperatures after Cu deposition, peaks broadened and shifted down in energy, suggesting metal migration into the polyimide and formation of Cu islands [8].

Most of the published work deals with investigations after surface modification, carried out by various techniques such as cold plasma [9] and ion beam irradiation [10]. An unwanted side effect of RBS is the ion damage, which has been studied by Matienzo and Emmi [11]. Figure 1 reports the RBS spectrum of a pyromellitic dianhydride-oxydianiline (PMDA-ODA) film after plasma treatment with a 85% F₄/15% O₂ mixture. A signal due to fluorine is observed in the RBS spectrum of polyimide, in addition to those of C, N, and Ο atoms. The peak shape indicates that fluorine is localized as a thin film at the top of the polymer substrate [6].

Figure 1 RBS spectrum of a PMDA-ODA film treated in 85% CF₄/15% O₂ plasma for 30 min. (From Ref. 6.)

3. Secondary Ion Mass Spectroscopy

Among ion spectroscopies for polymer surfaces analysis, SIMS has gained increasing importance in recent years [2]. Secondary particles emitted from the surface by ion (or atom) bombardment are mass analyzed. SIMS can be performed in two modes: static or dynamic. In the first case (static SIMS or SSIMS), the primary ion intensity is very low, providing a high surface sensitivity (one or two atomic layers) and a very limited surface damage. In dynamic SIMS, progressive surface erosion is required in order to obtain a depth profile. As in conventional mass spectrometry, it is possible to obtain both positive-ion and negative-ion mass spectra. For polymers, the former generally gives information mainly inherent in the polymer backbone, while the latter is particularly useful in detecting groups containing electron-negative elements, such as oxygen and halogens. Details on the theory and experiments can be easily found in books dedicated to this topic [12, 13].

Positive ion spectra of several commercial polyimides have been published by van Ooij and co-workers [14], as a part of a study of interfaces between evaporated metal films and polyimides. The following materials have been analyzed: Kapton H, HN, and V from DuPont; Apical from Allied; Novax from Mitsubishi Chemical; Upilex R and S from Ube Industries; and Thermid MC-600 from National Starch. Kapton and Apical are PMDA-ODA based, and the others are not. Chemical structures of several polyimides are reported in Figure 2. Due to the extreme surface sensitivity of SIMS (which constitutes an advantage, but at the same time a problem) contaminants such as inorganic ions, solvents, unreacted poly(amic acid), residues from fabrication, or treatments were detected on the surface of samples. Particularly frequent and intense is the presence of alkali metal ions (Na⁺ and K⁺). For this reason, the published spectra are not alike, even if many fragments are common. The positive ion spectrum of a PMDA-ODA polyimide is shown in Figure 3 and is defined as clean by the authors [15]. Some peaks correspond to fragments containing oxygen (for instance, the peak at 55 D; see Table 1 for identification). Most of the peak...

Table of contents

• Cover Page
• Half title
• Title Page
• Copyright Page
• Preface
• Contents
• Contributors
• Introduction
• Synthesis
• Bulk Properties and Modifications
• Surface Characterization, Modification, and Adhesion Aspects
• Applications
• Index