Reinforced Polymers for Improved Strength

The mechanical strength of polymers can be improved by reinforcing the polymer matrix via the addition of micrometer scale filler materials (e.g. zirco-nia, alumina, silica) [19,20]. These micro-filled composite materials are stronger than the bulk un-reinforced polymers as a result of the high-strength reinforcement phase accommodating the applied loads. This improvement in mechanical performance is much more significant when the particles are chemically treated in order to functionalize them to ensure excellent bonding between the hard reinforcement and the polymer matrix [20]. The more effective this functiona-lization, the greater is the improvement in the composite strength. This is because better bonding improves the effectiveness of load transfer between the surrounding matrix and the high-strength reinforcement.

In the case of these reinforced composite however, it was noted that the improvement in mechanical characteristics do not monotonically increase as a function of increasing fraction of the filler material [19]. In fact it was demonstrated that there exists an optimal fraction beyond which mechanical performance such as fatigue life in fact declines in these polymer systems.

More recently, the advances in nanotechnology have led to the development of nano-filled polymers for achieving a step change in the mechanical performance of composite materials for dental restorations. The premise of these developments is that the unprecedented opportunities enabled by the novel mechanical phenomena operative at the nanometer length-scale can be exploited in suitably designed polymer matrices to create high-performance nanostructured systems. Figure 2.4 illustrates a nanostructured composite material reinforced by nano-scale silica particles.

Such nano-composites are commercially available under brand names such as Ceram X (Dentsply/Caulk, USA), Grandio (Voco, Germany), Tetric EvoCeram (Vivadent, Liechtenstein) and Filtek Supreme (3M ESPE, USA). It should however be noted that exploiting the mechanical consequences of nanostructured dental composites still requires the rigorous development of protocols for ensuring effective bonding between the matrix and the nano-particles as well as the generation of a fine dispersion of nano-particles throughout the matrix. This is because lack of insufficient bonding between the matrix and the nano-particle reinforcement and concomitantly, inefficient load transfer usually results in insignificant improvement of mechanical properties over conventional materials. Instead of the high-strength, high-elastic modulus nano-particle reinforcement accommodating the applied stresses, the poorly-bonded nano-composite would behave as if it were nano-porous. This would then result in little or no improvement over the non-reinforced bulk composite

Fig. 2.4 Nanostructured dental composite reinforced by nano-scale silica particles for superior mechanical performance (Courtesy: Elsevier [19])

material [20-22]. Furthermore, the nano-scale reinforcements are best exploited when they are uniformly dispersed in the softer matrix and there is negligible clumping or aggregation of the nano-particulates [21, 22]. Lack of aggregation is particularly important since, extended ensembles of the nano-particles would essentially behave as if they were micrometer-scale hard phases and offer little improvement over conventional polymer composites with micrometer sized reinforcements [23].

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