Bulk modification

In this surface modification method the surface structure is arranged through the bulk multi-component polymeric system. The relationship between the bulk and the surface structure of macromolecular system is considered in bulk modification study. It was mentioned previously that the surface composition is changed and modified by the external treatment (plasma, grafting,

etc.), whereas in the bulk modification method the surface composition is designed by the presence of the specific components resulting of their migration from the bulk of the polymer blend to the surface.

Blending of polymers is interesting way to improve the bulk properties. The polymer blending has been described in a lot of articles and books but the knowledge on the polymer surface through blending is much less considered. Recently some publications have discussed the several variables affecting the nature of the blend and polymer surface. Many articles also have described the effect of block copolymers on polymer surfaces with respect to their bulk properties (Li, Andruzzi et al. 2002).

It has long been known that low molecular weight additives blended in a polymer host might migrate to the surface of the host polymer under the shear field in melt processing equipment (Li, Andruzzi et al. 2002; Andruzzi, Hexemer et al. 2004). Such migration is a powerful mechanism to transport many desired materials with suitable functional groups to polymer surfaces during normal melt processing. The migration method is highly attractive because it utilizes physical processes to design and modify the chemical make-up of polymer surfaces and therefore does not require extra processing steps or new equipment. Furthermore, some specific surface properties could be imparted by flow-induced migration such as adhesion, printability, biocompatibility and lubricity. However, traditional surface modification methods have the drawbacks of requiring extra processing steps and providing little control over the resulting non-equilibrium surface structure and also no uniformity of modification. The problem is more pronounced when the shapes of the object is complex. It has been well known that in multi-component polymeric systems, the component with lower surface free energy preferentially migrates to the air/polymer interface in order to minimize the interfacial free energy.

Two objectives can be considered with respect to surface modification of polymers: increasing the surface energy and surface potential (e.g. adhesion promotion) or decreasing the surface energy and surface interaction with a given material. In general, we can say that all surface modification methods share a common goal to control the number of chemical functional groups⁵ at the polymer surfaces.

If decreasing of surface energy is the goal of the modification, some specific functional groups (low energy) are desired at the surface such as –CH2-, -CH3, –CF2- and -CF3 and the interaction is only London dispersive⁶. Such low energy functional groups would like migrating to the surface due to the fundamental thermodynamic theory. In order to get a high-energy surface, the specific high-energy functional groups or some nature reactive materials have to be delivered to the surface. For this goal of modification the incorporation of dipole-dipole interaction or hydrogen bonding can increase the thermodynamic work of adhesion.

Additionally, the control of the stability of the additives at the surface of polymer is one of the important steps within the modification process. Because surfaces are intended thermodynamically to lower their surface energy, therefore the high-energy surfaces created by the treatment methods are often unstable and susceptible to reorganization processes resulting in rapidly loss of surface functionality (Li, Andruzzi et al. 2002; Andruzzi, Hexemer et al. 2004).

5 Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

6 The London dispersion force is the weakest intermolecular force. The London dispersion force is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles.

It is desired to use of functional polymers as additives due to cost and kinetic factors that can affect on polymer surface by blending with nonfunctional polymer host. Surface-active block copolymers are perhaps the best-known examples of this class of additives.

In order to create a high-energy polymer surface, adding a small amount of polymer containing high-energy functional group cannot segregate to the surface. In fact, high-energy functional groups of additive molecules would not be located in the subsurface layer but would be dispersed throughout the polymer matrix. The solution for this problem is the use of a surface delivery vehicle, which can deliver high-energy functional groups to the surface. The simplest molecular design for a surface delivery vehicle is an end-functional block copolymer. The one side of this block copolymer comprises the same polymer as the substrate or a polymer that is miscible with the polymer substrate and the other block of the copolymer is a surface-active that causes the entire molecule to segregate preferentially to the surface. (Koberstein, Duch et al. 1998;

Anastasiadis, Retsos et al. 2003; Andruzzi, Hexemer et al. 2004). Figure 2-19 shows illustration of the mechanism by end functional copolymer schematically.

Figure 2-19: Schematic diagram of the self-assembly of a surface-active end-functional block copolymer at the surface of a polymeric substrate at an air–polymer interface and subsequent reorganization when it is exposed to water vapor.