Polymer surfaces differ from non-polymer surfaces in that the subunits that make up the surface are covalently bonded to one another. Non-polymer surfaces can be bound by ionic bonds, metallic bonds or intermolecular forces (IMFs). In a two component system, non-polymer surfaces form when a positive net amount of energy is required to break self-interactions and form non-self-interactions. Therefore, the energy of mixing (ΔG) is positive.  This amount of energy, as described by interfacial tension, varies for different combinations of materials. However, with polymer surfaces, the subunits are covalently bonded together and the bulk phase of the solid surface does not allow for surface tension to be measured directly. The intermolecular forces between the large polymer molecules are difficult to calculate and cannot be determined as easily as non-polymer surface molecular interactions. The covalently bonded subunits form a surface with differing properties as compared to non-polymer surfaces. Some examples of polymer surfaces include: polyvinyl chloride (PVC), nylon, polyethylene (PE), and polypropylene (PP). Polymer surfaces have been analyzed using a variety of techniques, including: scanning electron microscopy, scanning tunneling microscopy, and infrared spectroscopy.