**1. Introduction**

Kapton HN films, which are well known to be made of polyimide polymer, are one of the most commonly used substrates for flexible electronics due to their excellent physical and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

electrical properties as well as exceptional thermal and chemical stability. However, such films feature an inert and highly hydrophobic surface. Hydrophilic (e.g. water-based) fluids (solutions, suspensions, inks, *etc*.) will ball up on such surfaces (due to "lotus effect") [1, 2]. However, for fabrication of an entire electronic device, both organic solvent- and water-based fluids are usually needed to deposit functional materials on the same substrate surface. As a result, surface modifying polyimide substrates to reduce their inherent surface hydrophobicity and/or inertness is usually needed to allow for the continuous and uniform deposition with both organic solvent- and water-based fluids.

**2. Characterization of Kapton HN films**

dibasic (CaHPO<sup>4</sup>

carbonate (CaCO<sup>3</sup>

and XRD analyses.

4.0 international license).

the CaHPO<sup>4</sup>

In this section, a number of characterizations were performed on as-received Kapton 500HN films (a gift from Dupont, Wilmington, DE, USA) particularly on their slip additive. The optical microscopy analysis of the films showed particles of varying sizes which were imbedded in Kapton HN polyimide matrix (**Figure 1a**). These particles have been shown to be the slip additive to the polyimide matrix of Kapton HN [14, 28]. As shown in **Figure 1a**, the majority of the slip additive particles exuded to the substrate surface, which is consistent with a previous observation [29]. The large hump (with 2θ ranging from ˜10° to ˜35°) and the sharp narrow peaks in the X-ray diffraction (XRD) pattern of the Kapton HN films (**Figure 1b**) indicated the presence of amorphous and crystalline components, respectively. Apparently the amorphous moiety was the polyimide polymer and crystalline moiety the slip additive. While calcium phosphate

peaks in the XRD pattern of the Kapton HN films matched very well with those of calcium

in the energy dispersive X-ray spectroscopy (EDX) pattern of the Kapton substrate (**Figure 1c**). To better characterize the slip additive in Kapton HN films, efforts were made to minimize the interference from the polyimide polymer matrix. Kapton HN films were fired at 800°C for 2 hours in air (this firing treatment has been shown to be efficient to pyrolyze the entire polyimide polymer moiety in Kapton HN [30]) to remove the polyimide polymer. The remaining inorganic components were characterized with scanning electron microscopy (SEM), EDX,

**Figure 1.** Kapton HN film characterization. (a) Optical microscopy analysis of a blank Kapton HN film. (b) XRD analysis

Locally enlarged XRD pattern to show the area with a 2θ of from 50° to 100°). (c) EDX analysis of the specimen shown in (a) (inset: Locally enlarged EDX pattern to show the calcium peak) [2] (licensed under creative commons attribution

(pattern ②, ICDD reference code 04–001-7249) (inset:

of the specimen shown in (a) (pattern ①) and reference CaCO<sup>3</sup>

) might be present in the additive as previously reported [14], the crystalline

Surface Modification of Polyimide Films for Inkjet-Printing of Flexible Electronic Devices

http://dx.doi.org/10.5772/intechopen.76450

3

) (ICDD reference code 04–001-7249) (**Figure 1b**) but did not match any of

peaks. Significant carbon and oxygen peaks and a small calcium peak showed up

Traditionally, polyimide substrates are surface modified with a number of methods including plasma [3, 4] and ion-beam [5, 6] etching, UV/ozone exposure [3, 7], acid [3, 8] and/or base [9, 10] treatments, and laser ablation [11, 12]. These methods, however, usually compromise the structural integrity and the properties (such as the cohesive strength, and the thermal and chemical stability) of the polyimide substrate, since they utilize relatively harsh conditions to oxidize and/or tear out part of the surface polyimide. Additionally, the wastes and by-products (such as acrolein which is extremely irritating, strong bases and acids which are corrosive, and benzene which is carcinogenic) generated from these harsh treatments can raise serious environmental and safety issues (especially when the treatments are performed indoors and/ or in large scales). For example, incubation with a sodium hydroxide solution has been one of the most common traditional methods to tune the surface properties of Kapton polyimide substrates [9, 10, 13], but such a treatment not only generates highly corrosive strong base waste but also tears out some surface polyimide resulting in pits in the Kapton surface [14]. The defects on structurally damaged Kapton films would result in not only poor deposition quality of the device components but also weakened mechanic strength of the resulting devices. The increasingly growing of flexible electronic devices (such as flexible displays [15], electronic paper [16, 17], photovoltaic cells [18, 19], sensors [1, 2, 20–23], LEDs [24], electronic textiles [25], RF tags [26], and electrochemical devices [27], *etc*.) is calling for mild and environmentally friendly surface modification approaches which can minimize the compromise to the structural integrity and the properties of Kapton polyimide substrates while efficiently tuning the surface properties of the substrates. To take full advantage of the properties of Kapton HN films, any surface modification to such films should avoid as much as possible compromising their structural integrity and properties. For extremely thin Kapton HN films, such as Kapton 30HN (thickness 7.5 μm), 50HN (thickness 12.7 μm), and 75HN (thickness 19.1 μm), it is critical to make sure that their surface modification is non- or minimally destructive. polyimide strength, air

Kapton HN films have a slip additive incorporated in the polyimide matrix to enhance their mechanical properties [13]. The nature of the additive, however, has been very scarcely reported in the literature. Williams *et al.* has mentioned, but without providing supporting data, that the additive in Kapton HN films was calcium phosphate dibasic (CaHPO<sup>4</sup> ) [14]. This chapter first describes the characterization of Kapton 500HN films particularly of their slip additive, then introduces two recently-developed mild and environmentally friendly wet chemical approaches for surface modification of Kapton HN films to allow for not only great printability of both water- and organic solvent-based inks but also strong adhesion between the inkjet-printed traces and the surface modified substrates. Unlike the aforementioned traditional Kapton surface modification methods which target, and oxidize and/or tear out part of, the surface polyimide matrix, the approaches described in this chapter target the electric charges on the slip additive particles.
