Polymers (1995)

This section summarizes polymer science and engineering research in progress at the NRL Chemistry Division and is based on the panel 's on-site interactions with NRL researchers. Polymer research and development at NRL is involved with the synthesis, design, characterization, and improvement of novel polymer systems of interest to the Navy. Further understanding, prediction, and control of the behavior of these polymer systems in particular hostile environments such as seawater, on the one hand, and high temperatures in various propulsion systems or during fires, on the other hand, have been of particular importance. Research on potential innovative analytical test methods, especially nondestructive ones, has also been of interest.

Much of the polymer-related research in progress at NRL can be placed in two categories: (1) materials—with specific properties and belonging to particular classes, and (2) characterization.

Materials with specific properties that have been studied include acoustical absorbing and vibration damping materials. Various conventional elastomers, polyurethanes, and interpenetrating network polymers have been studied for such acoustical applications. In addition, polymers are being used in the design of new piezoelectric composite materials containing a ceramic phase.

Polymeric materials that are being studied for use at high temperatures include phthalonitrile resins and composites using these resins. In addition, polymers made from high-carbon-content acetylenic monomers have been prepared and studied, especially with respect to pyrolysis, and inorganic-organic hybrid polymers have been prepared, especially for conversion to ceramics.

Flame-retardant polymers are especially important for naval applications, making this an important research area. For example, it has been shown that the phthalonitrile polymers have self-extinguishing properties on exposure to fire and, when used in composites, show superior flame resistance. These types of materials can be tested by the Navy Technology Center for Safety and Survivability, another part of the NRL Chemistry Division.

Another research area involves the study of different kinds of polymers that can be used as materials that have “low observable” properties with respect to radar. On the one hand, the high-frequency dielectric properties of such electronically conducting polymers as polyaniline and polypyrrole have been under investigation, and, on the other hand, a number of ultralow-dielectric-constant fluorinated polymers have been synthesized and studied.

Polymers used in coatings of many kinds have been synthesized and studied by many groups at NRL. The materials studied include various epoxies, polyurethanes, and fluorinated polyurethanes. Applications include nonskid coatings for horizontal surfaces on naval ships; water-shedding coatings on antennas; solar-heat-reflecting, anticorrosion, and antifouling coatings; and pipe, fuel storage, and sewage tank linings.

Ferroelectric liquid crystals with very fast switching times have been prepared for eventual use in various sensing and switching devices. Other areas of materials research include the synthesis of spin-labeled polymers for sensing composite interface properties, the structure and function of polymer-stabilized synthetic membranes, and polymers for various sensor applications.

In general, research on classes of materials is connected with that on materials with specific properties but includes somewhat more general research on composites, polyurethanes, epoxies, fluoropolymers, ferroelectric liquid crystals (especially those with fast switching times), polymer-polymer miscibility, double network elastomers, crystallization in polymers, polymeric Langmuir-Blodgett and other multilayer films, and polymer-stabilized synthetic membranes.

The application of composites to naval needs is the motif for research on joining techniques, hydrothermal effects and other damage and failure mechanisms, response to transient shock loading, and impact and underwater shock response. The research on ferroelectric liquid crystals includes their behavior as Langmuir-Blodgett multilayers.

Because of a general interest in polymer blends for various applications, some fundamental studies of such blends have been undertaken. For example, small-angle neutron scattering (SANS) and CP-MAS NMR studies showed that polyisoprene-polyvinylethylene (PIP-PVE) blends were miscible at molecular weights exceeding 10⁶ and had a negative Flory interaction parameter. Also, segmental dynamics have been studied in various polymer-polymer mixtures to gain a general understanding of the effects of different diluents. The morphology of a freeze-dried dilute polymer solution was also studied.

There are some ongoing general studies of surface modification of elastomers and studies on the synthesis and characterization of double-network elastomers. Research was done on the suppression of crystallization in blended natural rubber and neoprene. Other studies of crystallization, for example, on positron emission tomography (PET), have also been done.

A number of rather complex fluorinated monomers and the resulting polymers have been synthesized and studied because of the Navy's interest in low-permittivity materials.

The Navy's interest in coatings has led to the synthesis and study of various classes of polymers. For example, the curing reaction of epoxies with amidoamines has been studied, as has the polymerization of spirobislactones with epoxy resins.

Materials mentioned above are characterized and their properties measured by standard approaches for which NRL is well equipped, and these are discussed in this report. However, some novel characterization methods have been developed or extended at NRL.

The use of ¹²⁹Xe NMR to probe phase separation in polymer blends has been particularly useful for probing miscibility in those polymer blends in which the components have comparable glass transition temperatures, making it difficult, if not impossible, to study miscibility by using methods such as differential scanning calorimetry. There are plans to use this NMR method to measure domain size in phase-separated polymer blends.

Nondestructive characterization techniques such as NMR imaging of solid polymers are being studied. It has been possible to exploit methods that provide contrast discrimination based on molecular mobility. In addition, a method was recently demonstrated that provides an NMR image with contrast based on local polymer alignment in response to a strain field.

Research is in progress to use electron spin resonance (ESR) to characterize the interior surfaces in composites. Spin probes are to be deposited near the interface of a fiber/polymer matrix composite, and ESR is to be used to monitor the local orientation of the spin probes. The goal is to assess the spatial range of the “interphase” and to understand how the mechanical load is transferred from the fiber to the matrix.

Several groups at NRL have been working to better understand polymeric structure-property relationships with respect to particular uses of polymers. In addition, the composite

material characterization includes use of a structural response simulator.

It should also be noted that the NRL Center for Bio/Molecular Science and Engineering is involved in polymer research, e.g., in sensor design and fabrication, and in microlithography.

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