Polymer composites are a class of multiphase materials in which a polymer matrix is reinforced with either a fiber or filler phase. They are traditionally valued for their stiffness, high strength to weight ratio, and corrosion resistance. In emerging high-end applications, electronic, optical, thermal, chemical and barrier properties are additionally of interest, casting them now as multifunctional materials.
Polymer composite materials vary widely due to the many types of polymer matrices and reinforcement types that are possible, but may be most broadly classified by reinforcement type. Fiber reinforced plastics (FRPs), sometimes called "traditional composites", refers to materials in which the polymer matrices are reinforced with either continuous or chopped fibers. Fibers are most commonly glass, carbon, or aramid, but additional types such as natural fibers are now possible. FRPs are widely used in aerospace, automotive, marine, and construction applications, as well as specialty applications such as ballistic armor. Infrastructure applications are also becoming increasingly targeted. In these cases, the combination of excellent structural properties and light-weighting are paramount to their usage. The newer and still emerging area of polymer nanocomposites (PNCs) refers to materials in which the reinforcement phase has two or three dimensions that are less than 100 nm. These reinforcements are thus nanoparticles, nanosheets or nano-dimensional fibers such as carbon nanotubes or nanocellulose, amongst others. Nanocomposites differ from conventional composite materials by the much lower volume fraction, higher surface to volume ratio, and/or exceptionally high aspect ratio of the reinforcing phase, making the science of these materials in some respects different than the former.
While composite materials vary, their properties depend on essential features regardless of type and these become the objects of classification and investigation for the Materials Genome Initiative (MGI). In terms of thermomechanical properties, the first function of the reinforcement is to transfer loads from the weaker polymer phase to the more durable reinforcement. The material properties of the individual phases are therefore the entities of first order interest.
Beyond that, the properties of polymer composites materials are highly dependent on molecular scale interactions between the polymer matrix and filler. The first issue of fundamental importance is the interface where the matrix material comes into contact with the reinforcement. High-performance fibers give strength to composites via the interface between the matrix and the reinforcement surface. The stronger the interface, the greater the load transfer function. Interfacial bond strength, a measure of the strength of adhesion between the matrix and reinforcement, is a common design feature and critical property that must be engineered, controlled and measured. Structural properties such as interlaminar shear and flexural strength, as well as failure modes, are controlled by the interfacial bond strength. Thus, engineering the fiber surface chemistry to optimize the adhesion between fiber and matrix is of paramount importance. Optimized adhesion may also be achieved by means of chemical coupling agents which covalently bond the matrix and reinforcement.
The second issue of fundamental importance is the polymer/filler interphase. The presence of filler modifies the matrix properties, and the interphase is defined as the region in the vicinity of the filler where the matrix properties are modified from the bulk. The ability to tailor the polymer/filler interaction so as to optimize the impact of the interphase on macroscopic properties is another key to the intelligent design of composites. This is especially true for PNCs, where the surface to volume ratio is exceptionally high, making the interphase effectively a third component of the material.
In nanocomposites, another fundamental issue is filler dispersion. Mechanical properties are enhanced and optimized when dispersion is uniform and stable, and filler aggregates are minimized. At a molecular level, filler dispersion becomes an issue because strong van der Waal forces between the particles tend to produced high levels of aggregation in the absence of strategies to modulate these interactions. A common and widely adopted strategy for controlling interparticle interactions is to graft short molecules or polymer chains to the filler surface. However, this approach also affects the interphase, coupling the two problems. Finally, the polymer matrix itself also plays a role in dispersion (regardless of particle interaction properties) as dispersion is difficult to achieve in systems where the particle size is greater than polymer radius of gyration.
A final fundamental issue of fundamental importance is sorption of environmental components such as water, salts, or oxygen into the composite materials. While diffusion is slow in polymer matrices, it is finite, and environmental components indeed infiltrate the matrix usually finding home at polymer/reinforcement interface. Over time, this can lead to property degradation, loss of functionality, and promote failure. Measuring and controlling sorption is therefore an important component of composite research efforts.
MGI Related Composite Projects
This page describes NIST projects currently in place to understand the fundamental scientific issues related to polymer composite materials that are outlined above. Some of these are implemented in internal programs, others are taking place with external collaborators, most notably our partners at the Center for Hierarchical Materials Design (CHiMaD). CHiMaD collaborations are listed below. The goals is to increasingly coordinate these projects using MGI principles. The rising demand for multifunctionality makes the importance of progressively shifting composites design into the realm of the MGI even more imperative -- we expect that the combination of experimental, database, computational and machine learning tools will more and more provide a leveraged approach to the problem, decreasing the time and cost to develop new and better materials.