A key means of creating an interface able to withstand all kinds of stresses developed during processing and device operation is to achieve substantial interdiffusion of the two polymers at the interface. There have been many studies of the structure of model polymer/polymer interfaces at thermodynamic equilibrium where detailed comparisons can be made with predictive theories. Also the connection between adhesion and interface width for these equilibrium interfaces has been investigated by others. Non-equilibrium interfaces are more complicated than their equilibrium analogs, but much more common, and therefore important, in practice. One example is the interface between direct solution cast polyimide (PI) films used as compensation layers and the films used for their support in liquid crystal displays (LCDs). We consider specifically PI copolymers that are soluble in common organic solvents and therefore suitable for solution casting on a substrate film, e.g. cellulose triacetate. To obtain high quality bilayers with excellent durability, sufficient interlayer adhesion strength is needed. This adhesion directly correlates with the structure of the interface between the polyimide layer and the cellulose triacetate substrate. It is important to understand how the non-equilibrium interface structure in such systems involving polymer layers of limited miscibility can be adjusted during the direct coating process to achieve better adhesion.
Polyimide/cellulose triacetate bilayer structures for interfacial width measurements are deposited by first spin coating the cellulose triacetate layer and then spin coating or solution casting the polyimide layer on top of the cellulose triacetate layer, while corresponding bilayers for adhesion measurements are deposited by sequential solution casting. Two fluorinated polyimides are studied: IN1 with 12.5 atomic wt% of fluorine and IN3 with 31.3 atomic wt% of fluorine content. Estimation of the segment-segment interaction parameter from solubility parameters suggests that IN3 is more compatible with TAC. The width of the nonequilibrium interface between PI and the substrate can be manipulated through an understanding of the competing processes of polymer plasticization, interdiffusion, miscibility enhancement and solvent evaporation that occur simultaneously in the direct solution casting process. Neutron reflectivity measurements reveal that the width of the nonequilibrium interface between IN3 and cellulose triacetate created by spin coating or solution casting can be increased in a controllable way using a "swelling agent", ethyl acetate, in the deposition process. This swelling agent promotes local mixing and interdiffusion at the interface over a time that is controlled by the evaporation of the mixed solvent. The adhesion, as measured by T-peel tests, can be increased by a factor of seven by adjustment of the amount of ethyl acetate in the IN3 coating solution. Differences in the chemistry (solubility and rigidity) of the material being deposited do make a difference in the effectiveness of this strategy using a “swelling agent”. No obvious change in interface width and adhesion between a solution cast PI film and cellulose triacetate observed when using the “swelling agent” ethyl acetate with IN1. For polyimide IN3, a threefold increase in adhesion is obtained by optimizing the deposition temperature, but this approach for improving adhesion is less effective than that of adding "swelling agent".
The morphologies of the cellulose triacetate fractured surfaces after peeling tests measured using atomic force microscope suggest that broadening of the interface is associated with formation of entanglements in the interface, leading to a sharp increase in the interfacial adhesion. The formation of stronger interfaces of this type is important for the critical roles that multilayer films containing polymers with special properties and tailored structures play in applications as diverse as computer displays, photovoltaic devices, and polymeric electronics. The elucidation here of how the "swelling agent" strategy modifies the interfacial structure and adhesion is an important contribution in understanding nonequilibrium interfaces that play such key roles in industrial applications, but have been the subject of little fundamental study.