Compatibilizing agents for polymer

Intense commercial and academic interest in block copolymers developed during the 1960s and continues today. These materials attract the attention of industry because of their potential for application as thermoplastic elastomers, tough plastics, compatibilizing agents for polymer blends, agents for surface and interface mo dification, polymer micelles, etc. Academic interest arises, primarily, from the use of these materials as model copolymer systems where effects of thermodynamic incompatibility of the two (or more) components on properties in bulk and solution can be probed. The synthesis, characterization, and properties of classical linear block copolymers (AB diblocks, ABA triblocks, and segmented (AB)n systems) have been well documented in a number of books and reviews [1-7] and will not be discussed herein except for the sake of comparison. 

Polymer blends and compatibilizing agents historically have been the subject of a wide variety of studies and an extensive body of literature on these materials exists. Without specific chenucal interactions between dissimilar polymers, most polymer mixtures tend to phase separate due to the unfavorable entropy of mixing between the polymer chains. Efforts to control or retard the phase separation process have led to the research and development of compatibilizing agents for polymer blends. For a variety of systems the dispersed phase particle size has been found to decrease with increasing copolymer concentration. Above a critical concentration of copolymer, the size of the dispersed phase remains constant. 

It has been found by Baird and others [74-77] that the presence of LCP may accelerate and presumably direct the crystallization of conventional polymers (PET, etc.). Porter [76] has shown that, by blending biphasic polymers such as the PET-poly HBA copolymers, miscibility may be achieved between the conventional phase of the biphasic polymer with another conventional polymer that component is miscible with, i.e., X7-G/PBT. The latter phenomena may offer direction in the search for useful compatibilizing agents for LCP/conven-tional polymer systems. 

Chain functionalized polymers or graft copolymers are of great technological importance. They are used as compatibilizing agents for immiscible polymer blends (8) and adhesive layers between polymer-polymer co-extruded surfaces (8). Currently, of all polymers sold, about 30% are in the form of compatibilized immiscible blends (9-12). Next we discuss a few examples of chain functionalization. 

The copolymer composition in miniemulsion copolymerization of vinyl acetate and butyl acrylate during the initial 70% conversion was found to be less rich in vinyl acetate monomer units [34]. Miniemulsion polymerization also allowed the synthesis of particles in which butyl acrylate and a PMMA macromonomer [83, 84] or styrene and a PMMA macromonomer [85] were copolymerized. The macromonomer acts as compatibilizing agent for the preparation of core/shell PBA/PMMA particles. The degree of phase separation between the two polymers in the composite particles is affected by the amount of macromonomer used in the seed latex preparation. 

A copolymer of the two immiscible polymers themselves would seem to be ideally suited to act as a compatibilizing agent for an immiscible blend. If the copolymer is at the interface of the two phases, then the segments of the copolymer dissolve in the respective bulk phases of the same identity. The copolymer acts as emulsifying agent for the blend resulting in reduced interfacial energy and improved interphase adhesion. 

Thielemans, W. and Wool, R.P. (2004) Butyrated kraft lignin as compatibilizing agent for natural fiber reinforced thermoset composites. Composites Part A Applied Science and Manufacturing, 35,327-338. Satheesh Kumar, M.N., Mohanty, A.K., Erickson, L. and Misra, M. (2009) Lignin and its applications with polymers. Journal of Biobased Materials and Bioenergy, 3, 1-24. 

Aminoacids, alkylammonium ions and silanes are commonly used compatibilizing agents for the effective dispersion of clay platelets in polymer. They contain both hydrophilic and hydrophobic groups, which make them compatible with clay and polymer. 

PVC/EVA is also produced by graffing techniques through vinyl chloride monomer (VCM) polymerization either in solution or suspension in the presence of EVA polymer. Such products arc described in Chapter 3. Such interpolymers find use in special purpose cable jackets, medical devices, and protective footwear. Another potential use is as a compatibilizing agent for otherwise incompatible PVC/LDPE blends. 

In addition, in blends which contain two-phase separated polymers, adhesion of the two phases is of primary importance for utilizing the blend. Additional research needs to be done to understand the features that provide compatibilization in high temperature polymer blends and the optimum structure of compatibilizing agents for such blends. An example of this has been presented in the case of blends which contain liquid crystal polymers, but the same point is true of all blends based on immiscible high temperature polymers. 

---