Interfacial compatibilization reaction

Figure 4.5 Interfacial compatibilization reaction catalyzed by Zn " present in the ionomer (reproduced with Elsevier permission from Ref. 79).

Generally, it is easier to obtain a fine domain size under conditions where the two melt viscosities are dose. Furthermore, the domain size reduction easily occurs when the interfacial tension (k) between each polymer particle is low, even if there exists a considerable melt viscosity difference. This means that both the addition of a compatibilizer and interfacial copolymerization reactions result in lower surface tension and, consequently, the domain size reduction is effectively accelerated. To develop and stabilize the optimum morphology of a polymer alloy, it is very important to utilize a particular interfacial reaction or a pre-designed compatibilizer to improve the surface tension or interaction between the domain and matrix polymeric components, as well as to select the optimum operational conditions for mixing. 

TPE (co)polymers from step-growth polymerization processes can be used as compatibilizers for melt blending of the respective polymers, e.g., polyesters with polyamides or polycarbonate. The interfacial chemical reactions between... 

We have previously used the term interfacial reaction to describe mixing between two reactive blend components. In reality, as we have seen in the Example 11.2, there is an interphase that is formed on the surface of the dispersed phase where molecules of both components can be found and react (66,67). If the nonfunctionalized blend components have high immiscibility, then the thickness, Si, of the interphase around the droplets, as well as the volume of the interphase, Vh will be small and, thus, the probability of the functional groups to react forming compatibilizing products will be low, giving rise to coarse and not very stable morphologies. Helfand (66) defines Si as... 

Apparently, with a very small interphase thickness the two end-cap groups are too few and not easily accessible to affect compatibilization. On the other hand, when four anhydride (An) groups are attached, randomly on each PDMS chain, then the blend of 20% PDMS/4-An and PA 6/di-amine have a very fine and stable morphology (ca 0.5 pm). Thus, the amount of interfacial reaction product, although diminished by small < / values of the unmodified polymer components, is promoted by the larger number and more accessible functional groups in either or both of the reactive components. Finally, Macosko and co-workers (62) have estimated that the minimum fraction of the interphase that has to be covered by reacted compatibilization products to achieve fine and stable morphologies is about 0.2. 

If chemical reaction can take place between the functional groups on the compatibilizer and the two phases, then this will result in high interfacial adhesion and the miscibility is important only insofar as the reacting groups need to approach one another in order to enable reaction. Often this functionalization is achieved in a separate reactive-processing step, such as the grafting of maleic anhydride to polyolefins. Scheme 1.48 (Moad, 1999). 

The formation of compatibilizer frequently occurs by interfacial reaction of the modified polymers. An example is the use of maleic anhydride grafted to a polyolefin elastomer (EPM/EPM-g-MA) as the interfacial additive for reactive blending with a polyamide, nylon-6 (Van Duin et al., 2001), in a twin-screw extruder. It was found that the consumption of MA occurred rapidly and different blend morphologies were produced. However, it was found that the nylon-6 degraded during processing, due to the reaction of anhydride with the... 

Reactive Compatibilization involves a heterogeneous reaction across a phase boundary. Such a reaction is limited by the interfacial volume available at this phase boundary. Most often, twin screw extruders (having screw diameter from 20 to >120 mm) are employed. The screws are designed using an appropriate sequence of screw elements and auxiliary conditions to promote generation of a large interfacial area for the desired chemical reaction to form copolymer. 

The chemical reaction at the interface during processing influences the morphology and thus the material properties. During the reaction, block or graft copolymers are formed. These copolymers are expected to reduce the interfacial tension coefficient, and to prevent coalescence of the dispersed particles. Furthermore, the chemical reaction influences the interfacial thickness. It was shown by ellipsometry that as a result of the reactive compatibilization, the interfacial thickness in the ternary system, PA/SMA/ SAN, increased up to Al = 50 nm [Yukioka and Inoue, 1994]. 

Reaction with vinyl acetate [81], hydroxypropylation [82], reactions with styrene [83], with ethylene glycol and other glycols giving rise to glucosides [63,84] or with acrylamide monomer [85,86] have also been described. Reactive extrusion is also used to decrease the melt viscosity and decrease the interfacial tension of TPS-based blends [76]. Ning et al. [74] studied the effect of adding citric acid on TPS and LLDPE, via a single-step reactive extrusion. The authors showed improvements in the compatibilization and the mechanical properties and shifts of polyethylene peaks observed by FTIR. 

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