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Introduction

Inorgancially filled polymer composites have been used as dental restorative materials with great success for more than half a century, since Bowen first patented resins based on Bis-GMA in the 1960s . The lack of mercury in composite restoratives is one significant advantage over traditional amalgam materials, minimizing damage to human health and the environment. Moreover, composites’ color similarity to dentin is also a remarkable aesthetic improvement . However, amalgam still remains a widely used restorative material in dental clinics, and more than 70 million amalgam restorations are placed every year in the United States . The primary reason that polymer composites have not substantially replaced amalgam is due to amalgam’s better longevity and sustainability . Another barrier to universal acceptance of polymer composites result from toxic interactions of unpolymerized leachates with metabolic systems in the body . Massive effort has been applied to solving these problems, with notable but limited success .

The clinical performance of a dental restorative material depends on many interacting factors, and those relationships have proven to be difficult to understand. One of the targets of dental materials development has been high mechanical strength, assuming that higher values lead to better long-term clinical outcomes. Dental polymer composites consist of three main components: (1) the resin , consisting of the monomer, photo-initiators and other additives; (2) the filler , containing inorganic reinforcing materials such as silica, quartz or glass; and (3) coupling agents , that produce covalent linkages between matrix and filler . The primary contributor to mechanical strength in the cured composite is the reinforcing filler, but it is clear that the coupling agents and polymer matrix are also important to mechanical performance . Coupling agents are typically long chains of covalently bonded atoms that terminate in a polymerizable group that is chemically compatible with the monomers in the matrix resin. Thus, these linkages are covalent, and it is reasonable to assume that the resin matrix fails long before these bonds would break from mechanical forces. Based on these reasons, improvement of the mechanical toughness of the matrix resin system is the most effective way to increase the restorative’s performance and improve clinical outcomes.

The polymer component must exhibit high resistance to deformation, since teeth are regularly under substantial mechanical stress, and flexural strength has traditionally been a key indicator used to rank materials for this application . As such, one goal of dental restorative polymer design has been to produce materials with high flexural strength, in the hope that such materials will perform well when combined into restorative materials. Modulus is one of the mechanical properties that demonstrate a material’s resistance against deformation under load. The flexural modulus (FM), in particular, measures this behavior under simple beam loading. Shown in Fig. 1 is the three-point bend test geometry, the most common test for polymer flexural modulus, as standardized in U.S. ASTM D790 . In this experiment, the sample beam is under compressive stress at the concave surface and tensile stress at the convex surface, and the flexural modulus is expressed as the ratio between the stress and deflection. While results can be obtained rapidly and reproducibly from this experiment, the synthesis, characterization, and formulation of materials for testing are time-consuming and expensive. Like other mechanical properties, values for the FM depend on a variety of factors including the polymer’s molecular structure, sample shape/dimensions, processing methods, testing conditions, and others.

Three-point bend test geometry.

There are a potentially enormous number of imaginable structural variations on just the commonly used methacrylate backbone, each producing different properties. Testing even a small set of candidates with no a priori method to select promising candidates is simply prohibitive. Specifically this paper will explore the relationship between the FM of the polymer matrix and its chemical structure. We here describe a quantum mechanically based quantitative structure–activity relationship model (QSAR) that predicts the flexural modulus of homopolymers based on information encoded in the molecular structures on small polymer segments. While not treating the composite that is ultimately deployed, this model directly addresses the factor most amenable to modification and measurement related to clinical composite performance, the flexural modulus.

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Method