Incompatible polyblends properties

A variety of other third polymers may be added to incompatible polyblends to improve compatibility. Most of them are random eopolymer structures with flexible or rubbery properties. Their compatibilizing action may be visualized in three ways ... 

VVThen two chemically different polymers are mixed, the usual result is a two-phase polyblend. This is true also when the compositional moities are part of the same polymer chain such as, for instance, in a block polymer. The criterion for the formation of a single phase is a negative free energy of mixing, but this condition is rarely realized because the small entropy of mixing is usually insufficient to overcome the positive enthalpy of mixing. The incompatibility of polymers in blends has important effects on their physical properties, which may be desirable or not, depending on the contemplated application. 

When the polymer components in a blend are less miscible, phase separation will form larger domains with weaker interfacial bonding between them. The interfaces will therefore fail under stress and properties of polyblends are thus likely to be poorer than for either of the polymers in the blend. U-shaped property curves (Figure 4.40c) thus provide a strong indication of immiscibility. In most cases they also signify practical incompatibility, and hence lack of practical utility. 

Whereas incompatible partners in polyblends usually result in heterogeneous morphologies (e.g., styrene-butadiene SB), and often in highly resilient plastics, compatible partners result in plastics with moderate property profiles in accordance with the mixing percentages (e.g., PC/PBT). 

In multiphase polyblends, a critical factor is the interface between the phases. If the two polymers reject each other and separate into phases, they are likely to reject each other at the interface as well. Such a weak interface will fail under stress, and most properties will suffer. Thus, most polymer blends are practically incompatible. Yef most snccessful commercial polyblends are multiphase systems. This means that there mnst he a mechanism to strengthen the interface. 

While such phase separation may benefit certain specialized properties such as melt fiuidity, easy-opening packaging, and lubricity, in most products the loss of strength, ductility, and impact strength is a serious handicap, so most polyblends are therefore labelled incompatible. ... 

Polyblending offers the possibility of combining the best properties of both polymers in the blend, particularly in a two-phase system [1-4]. Thus, when gross phase separation causes incompatibility, it is highly desirable to reduce the size and morphology of phase separation, and in particular to strengthen the interface... 

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