In situ formed copolymer

Vilchis, L., Rios, L., Guyot A., Guillot, J., Villalobos, M. A., In-situ formed copolymer of acrylic acid-styrene as stabilizer during suspension polymerization of styrene, DECHEMA Monogr. 134 (1998) 249. 

The in situ-formed copolymers locate at the interface to prevent coalescence of the dispersed particles. The brush chains (the B chains of the in situ-formed A-B copolymer see Fig. 8.35) on the dispersed particles overlap when neighboring particles approach each other. By the chain overlap, the conformational entropy decreases to generate a repulsive interaction between the particles, which is different from the electrostatic repulsion in low molecular weight systems (oil/water/soap). 

The emulsifying effect of the in situ-formed copolymers allows a fine dispersion to be achieved by reactive blending. The particle size during reactive blending of... 

The in situ-formed copolymer reduces the interfacial tension. In ternary systems of a major component (3) and two minor components (1 and 2), as schematically shown in Fig. 8.36, component 2 spreads over the component 1 particles when the spreading coefficient S, determined by a balance between the interfacial tensions r,/y, is positive. The 5 in a ternary system of EPR (ethylene-propylene rubber) (1)/PA(2)/PPS (poly(phenylene sulfide)) (3) defined by... 

During reactive blending, the in situ-formed copolymers are sometimes pulled out from the interface and dispersed as micelles (domains) in the matrix, as shown in Fig. 8.37. The micelles are typically 20 nm in diameter (Ibuki et al. 1999). The pullout does not occur at the static state i.e., it is not caused by the interfacial instability of the highly crowded copolymers themselves. The pull-out takes place mechanically under the shear fields (Charoensirisomboon et al. 1999). 

Fig. 8.38 In situ-formed copolymers are pulled out (YES) or not pulled out but stay at interface (NO), depending on the molecular architecture. Figures are number average molecular weight of component polymers...

The in situ formed copolymer compatibilizer is located preferentially at the interface where they it most needed, reducing the size of the dispersed phase. 

Keywords compatibilization, functionalization, reactive processing, blends, copolymer crosslinking, in-situ formed copolymers, low molecular weight compatibilizers, miscibility. 

Finally, we would like to emphasize that, in most applications, in-situ formed copolymers are utilized, which are formed by the reaction of appropriately functionalized homopolymer additives at the polymer-polymer interface. A review article [106] cites not a single case where a premade copolymer had been used in a real application. Therefore, the interfacial behavior in such systems should be investigated fundamentally in greater detail in order to probe the effects of the characteristics of the reactive species on the kinetics of interfacial partitioning and the subsequent reaction, as well as on the effect of the resultant (diblock or graft or comb) copolymer on the interfacial tension and, thus, on the morphology of the macrophase-separated polymer blend. 

Kim HY, Ryu DY, Jeong U, Kim DH, Kim JK (2005) The effect of chain architecture of in-situ formed copolymers on interfacial morphology of reactive polymCT blends. Macromol Rapid Commun 26 1428-1433... 

The evaluation of interfacial behavior of die in situ formed copolymer allows to indicate whether or not the copolymer stays at the interface as a frmction of time under quiescent or dynamic conditions. It is important to emphasize that the location of the copolymer at the interface is one of the important requirements for the interfacial adhesion between the blend phases. The in situ formed copolymer should not leave the interface upon further melt-processing. Polymer blends are very often subjected to different melt-processing operations for the fabrication of end-use products. 

The characterization of the interfacial chemical reactions and the reaction kinetics are very challenging topics in this area. In fact the quantitative analysis of the interfacial chemical reactions and reaction kinetics has still to be performed for most of the melt reactions despite their crucial importance for the understanding of the relationship between melt reactions, blend phase morphology and ultimate properties. The copolymer generated as a result of the interfacial reactions is difficult to separate and to characterize. Several investigations are still being made to identify and characterize the in situ formed copolymer. 

The use of reactive precursors of the compatibilizer offers a series of advantages. Indeed, the reactive polymers can be formed by easily implemented techniques, such as free radical copolymerization and melt grafting of reactive groups onto existing polymers. The compatibilizer is formed where it has to be localized, i.e. at the interface of the polyblend. Moreover, when the interface is saturated, the compatibilizer is no longer formed, so that the chance that the critical micelle concentration is exceeded is low compared to the use of pre-made compatibilizer, even though the in situ formed copolymer can be repelled from the interface after formation. Finally, the melt viscosity of the reactive precursors is lower than that of the parent pre-made compatibilizer, which is beneficial to the blend processing. 

The reactive compatibilization of a binary A7B immiscible polymer blend is usually ensured via the use of a chemical reaction during the melt-blending operation. The reaction leads to the formation of block or graft copolymers miscible, or at least sufficiently compatible, with both polymers A and B. Depending on its chemical composition and molecular architecture, the in situ formed copolymer is able to locate at the interface, improves the adhesion between the two phases, and constitutes a stabilizing barrier against coalescence (Fig. 22.8). 

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