ONR Research Opportunities in Chemistry (1994)

Structural materials have experienced a revolutionary advance in recent years in response to customer demand for light weight, reduced maintenance, low cost, and other properties. Specific examples include high-strength steels for automobile weight reduction, ceramic compositions for low-dielectric-constant multilayer circuit boards, and organic based composites for aerospace and automobile weight reduction. Many of these materials are a result of improved understanding of chemical processes, and their integrity relies on the understanding of chemical mechanisms of degradation and their avoidance. In this section the panel touches on some specific issues that involve establishment of the chemical fundamentals required for rational design and for improving the properties of materials used in demanding applications. Delineation of the basic chemistry will enhance the probability that new materials can be adapted to perform advantageously in stringent naval applications.

A significant drawback to the use of organic matrix composites is their flammability. This problem has been attacked through addition of antimony oxide and halogenation, but flammability and smoke generation remain as problems in many applications, notably those with naval uses. Flammability can be reduced by inclusion of a glass reinforcement, and other inorganic additives are known to be beneficial as well. Opportunities still exist for substantially reducing flammability by admixture of inorganics (and possibly organics). Investigation of the fundamental chemistry of the burning process, including pyrolysis of the solid by flame radiation and gas-phase combustion reactions, by modern techniques could well suggest compositions that minimize hazards and give confidence for naval applications in which integrity is a prime concern. Smoke reduction and elimination of noxious constituents should also be targeted in such studies.

There has been unusual interest and activity in the area of surface chemistry in recent years, with applications ranging from the manufacture of integrated circuits to the chemistry of living organisms. The ability to prepare (e.g., by Langmuir-Blodgett techniques, chemical vapor deposition, and molecular beam epitaxy [MBE]) and manipulate surface coatings and films has advanced rapidly. New analytical tools (e.g., the scanning tunneling microscope, the near-field optical microscope, and the atomic force microscope) allow the elucidation of surface features at the molecular and atomic scale. The field of surface control and functionalization remains vigorous and promises further progress in fundamental chemical processes as well as applications that will be broadly useful.

The subject of adhesion has its origins in antiquity, and a rich body of materials and practical knowledge has been passed down. Physical and geometrical factors are important and an extensive literature exists, as does an abundant arsenal of practical compositions for adhesives. The basic molecular-level processes of adhesion are, however, not well described and understood. With the significant advances in analytical capability, encompassing computation and physical modeling as well as chemistry, there is an opportunity to achieve a new level of understanding of the phenomena of adhesion. The practical by-products of this improved scientific description should include new adhesive methods and materials and a

framework for predicting long-term performance of adhesive joints under the influence of various environmental factors.

The advances in adhesion chemistry and control of surface functional groups lead naturally to the design of interfaces between the matrix and the reinforcement in composites. Through the study of the chemistry of new generations of coupling agents and properties (e.g., tensile and impact strength), enhanced performance of structural composites may be achieved. Higher-molecular-weight coupling agents could provide affinity with both the matrix and the reinforcement and better matching of the mechanical properties of the two components. A better understanding of the chemistry of composite interfaces will enable rational design of high-performance materials and control of aging processes that limit applications. Owing to the interplay of chemical and mechanical factors, support of interdisciplinary programs that offer useful synergies should be given special priority.

There is a need for long-fiber composites that can be changed in shape (i.e., can be made formable) after the matrix is fully or partially cured. The introduction of thermoplastic matrices and possibly liquid crystalline matrices suggests a possible route to more formable composites and could broaden the range of applications of these materials. Fundamental studies of deformation and flow processes in such systems could lead to new classes of materials with useful structural properties.