Two dimensional polymers comprised of a combination of stiff and compliant …

Description (excerpt)

CROSS REFERENCE TO RELATED APPLICATION(S) This application is a continuation-in-part (CIP) of U.S. patent application Ser. No. 16/744,293 filed Jan. 16, 2020, herein incorporated by reference in its entirety for all purposes. The '293 application is a divisional of U.S. patent application Ser. No. 15/434,391 filed Feb. 16, 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/296,892 filed Feb. 18, 2016, both of which are herein incorporated by reference in their entirety for all purposes. GOVERNMENT INTEREST The invention described herein may be manufactured, used, and licensed by or for the United States Government. BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates generally to polymer compositions, and in particular those having a two-dimensional (2D) repeating structure. II. Description of Related Art 2D materials offer unprecedented mechanical properties that can potentially be exploited for structural and barrier applications. Graphene is one example of a 2D material. It is a fullerene compound consisting of solely bonded carbon atoms. The extraordinary in-plane stiffness and intrinsic strength of graphene in its pristine state have made it a desirable candidate as a structural material. Graphene has the theoretical potential to enable ballistic barriers that have 10-100× less weight than barriers composed of Kevlar with the same ballistic limit, and has also demonstrated a specific kinetic energy of penetration an order of magnitude greater than steel and 2-3× greater than Kevlar, as measured by microscale ballistic experiments. However, because graphene is a network of very stiff sp 2 double bonds, it is highly resistant to fracture initiation but, once formed, a crack will propagate in a brittle manner. This brittle behavior may limit graphene's potential as a structural engineering material, as local failure due to a flaw or stress concentration is likely to trigger a sudden and catastrophic global failure. Other 2D polymers have been recently theorized. Graphyne and its related allotropes are composed strictly of extremely stiff carbon-carbon double and triple bonds, likely leading to brittle behavior. Graphane adds single hydrogen bonds to each carbon atom in graphene, resulting in a hexagonal network of sp 3 bonds. Studies have also examined carbon allotropes that are randomly hydrogen functionalized. Stiffness and strength in these graphene-like polymers have been incompletely reported, while fracture has not been directly studied in any of these systems. Compared to linear polymers, such as those used in high performance fibers and textiles such as Nylon (polyamide), Kevlar (para-aramid), and Dyneema (ultrahigh molecular weight polyethylene), 2D polymers provide inherent mechanical advantages. Primarily, fibers composed of linear polymers possess stiffness and strength only along the fiber direction. Combining fibers of multiple orientations can provide effectively isotropic performance, but the effective isotropic stiffness and strength are approximately 50% lower than those of the longitudinal fiber properties. Improved 2D compounds would be useful. SUMMARY OF THE PRESENT INVENTION A new family of novel 2D polymer compounds is described that maintains much of the stiffness of graphene, while being more resistant to fracture and more chemically interactive with other materials. This 2D covalent polymer network can be described as a well-ordered hybrid network of nodal carbon-containing cyclic unit (also referred to as a “node” or “nodal unit”) that produce 3 or more bonds in the same plane connected by short linear bridge units (also referred to as “bridge units”) that may contain one or more sp, sp 2 and/or sp 3 bonds but must maintain the overall planarity of the 2D polymer such that most bonds within the 2D polymer exist within a few bond lengths of a single plane, where a polymer is defined to be one or more repeat units potentially in combination with similar or different molecules. Herein, “hybrid” structures refer the combination of nodes and bridge units. The cyclic nodal units provide stiffness while the linear bridge units contribute compliance to the overarching 2D polymer, and the use of a wider set of atoms and bond chemistries compared to graphene provides opportunities for a wider range of properties and applications. According to embodimen