Toughness heterogeneous polymers

Finally, our interest will be limited here exclusively to the phenomenon of crazing in heterogeneous polymers. Thus, apart from the considerations of improving toughness by manipulation of the processes that govern the craze flow stress and, thus, rendering the extrinsic flaws inoperable that result in craze fracture, we will not consider the mechanics of fracture of crazable polymers. A brief survey of this subject related to the crazing process can be found elsewhere... 

Pukanszky, B. and Maurer, RH.J (1995) Composition dependence of the fracture toughness of heterogenous polymer systems. Polymer, 36,1617. 

Raman spectroscopy is sensitive to polymer conformation. For example, a polymer blend of polybutadiene-polystyrene in which polybutadiene is used to increase toughness of the polystyrene can be examined by Raman microscopy to identify its heterogeneity. Polybutadiene has three isomer conformations (cis-1,4, trans-1,4 and syndiotactic-1,2). These three types of isomers can be identified from C=C stretching modes as shown in Figure 9.36. The Raman spectra of the copolymer indicate the difference in amounts of isomer types at the edge and the center of the polybutadiene-polystyrene sample. Relative amounts of these isomer types affect the mechanical properties of the copolymer. 

Distinct improvement of MC in toughness was accomplished by employing the block copolymers. The typical examples are the blends of PPTA-b-nylon 6, 66/nylon 6, 66 [18] and PPTA-b-butadiene rubber (BDR)/ABS resin [19]. In the latter case, only 2.5 wt % of PPTA in MC improved the energy-to-fracture by a factor of four in comparison with that of unmodified ABS resin, while the blend employing homopolymer of PPTA was brittle. Thus, the block copolymerization of PPTA with flexible matrix polymer blocks has advantages of the removal of defects associated with heterogeneous texture in MC and the increase in the degree of dispersion of PPTA block in MC. 

Many thermoplastics are heterogeneous (or heterophase) because they contain liquid or rubber dispersions that improve their physical properties with respect to those of the continuous brittle phase. Examples of this are the softening of PVC by the presence of phthalate droplets and the improved toughness of HIPS or the polymer of acrylonitrile-butadiene-styrene (ABS) by addition of PBD-based rubber particles. This chapter will focus on the (heterogeneous, bulk and free-radical) polymerizations leading to the production of HIPS and PVC. 

More recently, the same research group also reported that in hydro-calcite nanoparticle modified PE fibers [21] the incorporation of clay improved the thermal stability and induced heterogeneous nucleation of polyethylene crystals. Hydrocalcite exhibited good dispersion into the polymer matrix, and hence positively affected the mechanical properties in terms of both stiffness and strength. The toughness of the nanocomposite as spun fibers was also increased up to 30% with respect to neat polymer. 

Polymer alloys often exhibit microphase separation. The heterogeneous morphologies are determined not only by the composition of the system but by the processing conditions as well. The microstructure influences the properties of polymeric alloys [40]. For example, the addition of a second phase of dispersed rubbery particles into the polymer matrix results in a great enhancement of toughness [41]. 

One of the most important focus areas of research in the development of natural fiber-reinforced polymer composites is characterisation of the fiber-matrix interface, since the interface alone can have a significant impact on the mechanical performance of the resulting composite materials, in terms of the strength and toughness. The properties of all heterogeneous materials are determined by component properties, composition, structure and interfacial interactions [62]. There have been a variety of methods used to characterize interfacial properties in natural fiber-reinforced polymer composites, however, the exact mechanism of the interaction between the natural fiber and the polymeric matrix has not been clearly studied on a fundamental level and is presently the major drawback for widespread utilization of such materials. The extent of interfacial adhesion in natural fiber-reinforced polymer composites utilizing PLA as the polymer matrix has been the subject of several recent investigations, hence the focus in this section will be on PLA-based natural fiber composites. 

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