Modification of the surface properties of a polyimide film during irradiation with polychromic light

Abstract

The behaviour of a polyimide film with the aromatic structure during the exposure to UV light with λ > 290 nm was studied. Significant changes in color surface and gloss surface were identified during irradiation. Sample became lighten and less glossy after exposure to the light.

These modifications were correlated with the structural changes in FTIR spectra. Based on changes in FTIR spectra recorded during irradiation, a mechanism for the photochemical degradation of polyimide film was proposed.

Introduction

Aromatic polyimides are a special class of high performance polymers well known for their outstanding thermal stability in air and good physico-mechanical properties at high temperature [1], [2], [3], [4], [5], [6], [7], [8]. The chain stiffness of the polyimides [9] gives high values of both glass transition and melting temperature. The low dielectric constant, the excellent processing characteristics, the chemical resistance and the smooth polished surface finish make pure and unfilled thermoset polyimide a desirable material for a high number of critical applications such as: thin film substrates, packaging applications, silicon and metal foil strain gauges, solar cell substrates. Polyimide coatings are often used to protect the optical fibres [10] and manufacturing of solar cells [11] or as photoresist [12].

The behaviour of polyimides during the prolonged exposure to sunlight or to UV light should be known in these cases. When considering polyimides for long-term applications, it is necessary to know how the polymers will behave during the intended service life. In recent years the polyimides are extensively used in vascular catheter applications in which walls, stiffness and strength are critical [13], [14]. In this case, exposure to light during the product storage may cause the decrease of polymer molecular weight leading to embrittlement and modification of some optical properties such as the color and the gloss [13]. The commercial polyimides Kapton are obtained from pyromellitic dianhydride and 4,4′ oxydianiline. Since the Kapton polyimides are insoluble and infusible, they are processed with difficulty. Perhaps the scarcity of data on the photodegradation of polyimides is due to the well-known relative stability of polyimides based on pyromellitic dianhydride and aromatic diamines [15], [16], [17], [18]. Because such polymers are only degraded upon prolonged exposure to a broad band UV source, the tendency is to assume that all aromatic-based polyimides are equally photostable regardless of structure [18].

However some articles on the photochemical degradation of polyimides obtained from hexafluorinated dianhydride irradiated with unfiltered UV light are published [15], [16], [17], [18], [19].

Photosensitive polyimides were synthesized by included of benzophenone moiety in the polymer chains [20]. The polyimides containing benzophenone are soluble and fusible polymers even in imidized form. Network structures are formed after the exposure of the photosensitive polyimides to the UV radiation [21], [22].

Unfortunately there are few data on their stability to UV radiation [23].

A new polyimide basis on 2,6-bis(3-aminophenoxy)benzonitrile and benzophenone tetracarboxylic dianhydride was synthesized for obtaining of composite materials with inorganic microparticles and for outdoor applications [6]. This new polymer is soluble and fusible and can be easily processed, being necessary only solvent elimination.

In this paper, the effect of UV light with λ > 300 nm on the photochemical stability of the polyimide was investigated. The structural changes in the polymer surface and variation of some surface properties (color and gloss) as a function of irradiation time were investigated. The study could help to find new possibilities for use of polymer as coating in outdoor applications such as the bundles of multi-fiber ruggedized with polyimide for use in aircraft, on wind turbine towers or satellite antennas.