Polymer matrix composites drawbacks

Ultra-high modulus fibers such as aramid and carbon fibers have been currently utilized for composite material fabrication. Ultra-high modulus polyethylene (UHMPE) fiber is also applicable for composite fabrication because of the light weight in addition to its high modulus, vibration damping, and resistance to chemicals. However, this fiber has drawbacks such as poor interfacial adhesion with the polymer matrix of the composite because of highly hydrophobic nature of the fiber surface. 

The most desirable properties for electrically conductive polymeric materials are film-forming ability and thermal and electrical properties. These properties are conveniently attained by chemical modification of polymers such as polycation-7, 7,8, 8-tetracyanoqninodimethane (TCNQ) radical anion salt formation (1-3). However, a major drawback of such a system is the brittle nature of the films and their poor stability (4,5) resulting from the polymeric ionicity. In recent years, polymeric composites (6-8) comprising TCNQ salt dispersions in non-ionic polymer matrices have been found to have better properties. In addition, the range of conductivities desired can be controlled by adjusting the TCNQ salt concentration, and other physical properties can be modified by choosing an appropriate polymer matrix. Thus, the composite systems are expected to have important advantages for use in electronic devices. 

The major drawback of cellulose fibers in the present context resides in their highly polar and hydrophilic character, which make them both poorly compatible with commonly used non-polar matrices, such as polyolefins, and subject to loss of mechanical properties upon atmospheric moisture absorption. That is why they should be submitted to specific surface modifications in order to obtain an efficient hydrophobic barrier and to minimize their interfacial energy with the often nonpolar polymer matrix, and thus generate optimum adhesion. Further improvement of this interfacial strength, which is a basic requirement for the optimized mechanical performance of any composite, is attained by chain entanglement between the matrix macromolecules and the long chains appended to the fiber surface (brushes) or, better still, by the establishment of a continuity of covalent bonds at the interface between the two components of the composite. 

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. 

This technique produces an intimate mixture of cellulose and matrix polymer, which is preserved as the water is evaporated during matrix consolidation. High strength composite films have been produced. A major drawback with this approach is that it is only suitable for forming composite films, and that the consolidation of the polymer matrix requires volatilization and removal of the solvent phase, which may create defects (voids) in the final product and poses economic and environmental concerns. 

An important aspect in PBI/IL composite membranes is the IL lixiviation rate. When the IL were not effectively immobilized, a progressive release of the IL components during a long period of fuel cell operation took place, thus resulting in the decline of fuel cell performance [63]. In order to overcome this drawback, the srdfonation of polymer matrices led to a better dispersion of ionic domains and the retention of IL, along with a reduction of the proton conductivity loss, as shown by Ye et al. [68]. PolymerizatirMi techniques by introducing IL molecules in the polymer matrix were an actual alternative to overcome this drawback in polymer/IL composites. 

In polymer-clay nanocomposites, to truly reach the ultimate in property improvements requires full exfoliation. A fully exfoliate composite yields the maximum interfacial interaction between the nanoparticle and polymer matrix. In order to produce optimally exfoliated systems requires that direct methods be available to measure the level of exfoliation. The ideal analytical method should be rapid, nondestructive, applicable to many sample matrices, low cost, and should require minimal sample preparation. The only method that fits these criteria is wide-angle X-ray diffraction (WAXD). This method, however, has some major drawbacks that will be discussed in detail in this chapter. 

Natural or synthetic HA has been intensively nsed in pure ceramic scaffolds as well as in polymer-ceramic composite systems. In fact, dne to calcinm phosphate osteocon-ductive properties, HA, TCP and BCP can be nsed as a scaffold matrix for bone-tissue engineering. However, these ceramic phases do not possess osteoinductive ability and their biodegradability is relatively slow, particularly in the case of crystalline HA (see Section 15.4.1). To overcome these drawbacks, biodegradable polymers added with osteogenic potential cells are used to make new biocomposite materials. Some of the tissue-engineered CP-polymer nanocomposite scaffolds are briefly described in the following sections, showing that both natural and synthetic polymers can be used to this aim. 

Reinforcing fillers are often fibers or films that exhibit better mechanical and thermomechanical properties than the polymer matrix and develop strong molecular interactions with it. Upon mixing such fillers with polymers, materials called composites are obtained. Due the single orientation of fibers, the improvement in the mechanical properties is observed in one direction. To overcome this drawback, nonwoven fibers and fabrics can be utilized as bi-dimensional reinforcing fillers. 

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