High Performance Polymers immiscible

This chapter discusses blends that are based on the use of high performance polymers. Both miscible and immiscible mixtures of such polymers are discussed and advantages that are provided by both types of blends are highlighted. It is pointed out that due primarily to the molecular conformation of high performance polymers the criteria for obtaining miscible mixtures of these type of polymers are different than for more flexible type polymers... 

This section of the chapter will focus on two applications of immiscible blends that contain high performance polymers. The first will be on the use of LCP s as processing aids and reinforcements of thermoplastics. There has been a great deal of both open literature and patents devoted to that topic. The second focus area will be on the use of polysulfone to enhance the fracture toughness of other thermoplastics. In that case, immiscibility is the desired phase structure for the observed effect. 

Polymer blends must provide a variety of performance parameters. Usually it is a set of performance criteria that determines if the material can be used or not. For specific application more weight can be given to one or another material property. The most important properties of polymer blends are mechanical. Two type of tests have been used the low rate of deformation — tensile, compressive or bending and the high speed impact. Immiscibility affects primarily the maximum elongation at break, and the yield stress. 

Most research on polymer blending has reported the successful application of the polymer-blending technique in the production of high-performance membrane with improved membrane properties. However, there are some major issues involved with polymer blending that need to be introduced. Blending of the inuniscible polymer pairs is one of the main problems. This contributes to three inherent problems (1) Poor dispersion of one polymer phase in the other (2) Weak interfacial adhesion between the two phases and (3) Instability of immiscible polymer blends (Baker et al. 2001).When all of these problems occur, the objective of polymer blending will not be achieved. 

In fact, this topic has evolved into a central area of polymer research during the last 40 years. One of the first ideas, that as a rule polymer blends are immiscible, needs to be reevaluated due to the increasing number of miscible or partially miscible polymer pairs reported in the literature (see, for example, Paul and Newman, 919) Despite this high level of activity, much of the work remains based on art and intuition rather than on science. Most of the work performed on high temperature polymer blends has involved the definition of miscible polymer pairs and their phase separation characteristics. Not much work has been done to predict miscible pairs. In fact, this is an open area of research in the entire area of polymer blends. 

Abstract The performance and subsequent properties of polymer blends are highly dependent on the blend s phase structure. For example, a miscible mixture of two polymers wiU have different features to an immiscible mixture of the same two polymers. Additionally, the manner in which a transformation from a miscible blend to an immiscible blend occurs will affect the ultimate properties. These features can be categorized under the topic of thermodynamics of polymer blends. This chapter discusses some features of the thermodynamics of blends that contain high temperature polymers, highlighting those which are most important in defining the blend phase structure. A comparison is made with other polymer blends, and important differences are noted. 

If one were to choose more reactive monomers, it would be possible to carry out polycondensations at considerably lower temperatures in solution. For example, consider the reaction of a diamine and a diacid to make a polyamide (nylon), a polymerization that requires relatively high temperatures (see Equation 9). A much faster reaction would occur between the diamine and a corresponding diacid chloride (see Equation 10). Both reactions would produce the same polymer, although the reaction conditions would be much different, and the byproduct HC1 from the acid chloride reaction would have to be carefully trapped. One technique for performing a polymerization such as that in Equation 10 is to dissolve the monomers in different, immiscible solvents, forcing the polymerization to occur only at the interface of the two solvents, a process called interfacial polymerization. Because of the high reactivity of an acid chloride, these reactions can be carried out at very low temperatures. This polymerization can be carried out rather dramatically in a beaker and is known as the nylon rope trick (see Section 4). 

Over the last decade, the poor economics of new polymer and copolymer production and the need for new materials whose performance/ cost ratios can be closely matched to specific applications have forced polymer researchers to seriously consider purely physical polymer blend systems. This approach has been comparatively slow to develop, however, because most physical blends of different high molecular weight polymers prove to be immiscible. That is, when mixed together, the blend components are likely to separate into phases containing predominantly their own kind. This characteristic, combined with the often low physical attraction forces across the immiscible phase boundaries, usually causes immiscible blend systems to have poorer mechanical properties than could be achieved by the copolymerization route. Despite this difficulty a number of physical blend systems have been commercialized, and some of these are discussed in a later section. Also, the level of technical activity in the physical blend area remains high, as indicated by the number of reviews published recently (1-10). 

It is usually difficult to isolate and characterize a copolymer from a melt-processed polymer blend. Model studies of copolymer formation between immiscible polymers have been performed either in solution (where there is unlimited interfacial volume for reaction) or using hot-pressed films of the polymers (where the interfacial volume for reaction is strictly controlled at a fixed phase interface). Model smdies using low molecular weight analogs of the reactive polymers are useful but their applicability to high molecular weight reacting systems is limited. 

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