Synthetic Polymer-Polymer Composites

Composite materials consisting of two or more components are likely to be more difficult to process than isotropic one-component materials. However, it is obvious that the success of a material is always dependent on its processability. Over the last decades, enormous progress has been made in this field, leading composites out of its niche as material for military purpose and aviation use, to commercial applications and possibly for daily life.

Polymer-polymer composites offer outstanding mechanical and structural advantages. They are extremely lightweight and able to beat conventional organic fiber reinforced composites, especially in terms of impact resistance and recyclability [1]. Nevertheless, the processing of thermoplastic all-polymer composites is limited due to the heat sensitivity of the polymer reinforcement.

High performance polymer matrix composites based on carbon fibers (CFs) have been used extensively as structural materials in aerospace engineering and orthopedic applications due to their low density, high strength and stiffness as well as good biocompatibility. These properties derived from CFs having outstanding strength and modulus, superior electrical and thermal conductivity [1]. Therefore, much effort has been devoted to the use of CFs for improving and optimizing the properties of resulting composites.

Carbon nanotubes (CNTs) are an allotropic form of carbon [1]. Single-walled carbon nanotubes (SWCNTs) are the simplest structures where the tube wall is formed by a single graphene sheet. The chiral vector c characterizes the orientation of the graphene sheet around the circumference. It is given by a linear combination of the graphene lattice vectors a1 and a2: c = n×a1 + m×a2, commonly abbreviated as (n,m).

Fullerenes, discovered in 1985 by Kroto et al. [1], have provided an exciting new insight into carbon nanostructures with geometric architectures made up of sp² carbon units. Carbon nanotubes (CNT), first reported in 1991 by Iijima [2], can be described as fullerenes that exist in cylindrical form. Due to their unprecedented physical and chemical properties, CNTs have generated huge activities in the fields of chemistry, physics, materials science, and electrical engineering.

The development of aromatic polyamides (aramids) had its beginning in the early 1960s in an industrial corporation (DuPont) and was a combination of fundamental science, engineering, and applications research from its very early stages. The broad range of properties of aramids and their structural variety are the main reason for their utility in diverse applications [1].

The demand for structural materials is steadily increasing. Especially for materials having: (i) a desiredcombination of advanced mechanical properties (tensile, shear and compressive properties, toughness, resistance to fatigue) and low specific weight and/or costs, (ii)extraordinary high strength and high stiffness, and (iii) new or significantly improved functional properties.

Currently, the field of synthetic polymer-polymer composites (PPCs) is characterized by very rapid progress since new ideas form the basis of new technologies and often require new materials to be developed. Thus, new materials such as nanofibers and nanofiber reinforced composites can create newer ideas and further applications, which in turn, can result in the development of more specialized materials. The PPC industry requires advanced matrices, advanced reinforcements, advanced additives, advanced technologies and wider application fields.

The use of polymers is becoming more and more common, in place of more traditional materials such as metals, ceramics, and wood. This is because polymers are simple to work with, easy to transport, friendly to automation, and low in cost. However, generally, polymers by themselves have inferior mechanical properties and to meet the demands of industry for strength and stiffness, they have to be reinforced.

It is well accepted that blending polyolens with general engineering plastics (GEPs), including poly(ethylene terephthalate) (PET), polycarbonate (PC), polyamide (PA), etc., is a major route to improve the mechanical properties of polymeric materials. During polymer processing operations, a large variety of shapes of the dispersed phase can be formed, e.g., spheres or ellipsoids, brils or plates, which strongly affect the final properties of polymer blend [1,2].

Conductive polymer composites (CPC) exhibit a series of unique features, such as comparatively low room temperature resistivity, percolation phenomenon, resistivity sensitivity to temperature, pressure and gas, and nonlinear voltage-current relationship [1–5]. They found wide industrial applications in the fields of antistatic materials, self-regulating heaters, over-temperature protection devices, and electromagnetic interference shielding.

An acceptable composite material for use in engineering applications should satisfy the following three basic requirements [1]: (i) to consist of at least two physically distinct and mechanically separable materials, which, depending on their properties and amounts used, are called matrix and reinforcing component; (ii) there must be a possibility for its preparation by admixing of the matrix and reinforcement components (sometimes preceded or accompanied by some special treatment so as to achieve optimum properties); and (iii) the final material is expected to possess several properties that are superior to those of the individual components, i.e., some synergistic effect should be present.

The mechanical properties of polymer-polymer composites are closely related to their structure on the nanometer and sub-nanometer scale. In general, this structure is changing under load. Thus, in situ structure monitoring methods are expected to become useful tools in the quest to elucidate the relations between structure and properties of the composites. In a further step, the recorded data can be used to identify mechanisms of structure evolution that, ultimately, can be exploited to manufacture polymer-polymer composites tailored to their field of application.

Polymer-polymer composites or self-reinforced polymeric materials (SRPM) are materials where both the reinforcement and matrix components have been made from all polymeric materials. This can include two polymers coming from the same family of polymers or simply the same polymer. Micro- and nanofibrillar single polymer composites (SPCs) are a subcategory of self-reinforced polymeric materials which are made by combining the properties of molecularly aligned and oriented forms with randomly disoriented forms of the same polymer material.

It is well known that both the actual modulus and strength of a polymer are far lower than their theoretical limits because of the random arrangement of the macromolecules and internal defects. The load-bearing ability is mostly offered by the secondary forces like van derWaals force and hydrogen bonding, which are much weaker than C-C and CH covalent bonding strengths.

Among the materials used most often, the strength of metal and ceramic is almost equal to the theoretical values. However, the strength of the products of polymers is far lower than the theoretical strength. Despite the fact that the theoretical strength and modulus of high-density polyethylene (HDPE) can reach 27 GPa and 300 GPa, respectively, almost equal to those of steel and carbon fiber, it is difficult to attain those high values, due to the fact that most polymer materials are semicrystalline and the chains in amorphous phase are randomly coiled.