Abstract
Objectives
The change in optical characteristics through the bulk of curing photopolymers is not fully understood. Photopolymerization processes are accompanied by photoinitiator absorption, density changes and volumetric shrinkage, which alter optical properties and affects curing efficiency through depth.
Methods
This investigation demonstrates the use of a novel low coherence interferometry technique for simultaneous measurement of optical (refractive index) and physical (shrinkage) properties throughout curing of photoactive monomers containing various concentrations of bisphenol-A-diglycidyl ether dimethacrylate and triethylene glycol dimethacrylate.
Results
Reliability of the interferometry technique was compared with an Abbé refractometer and showed a significant linear regression relationship ( p < 0.001; adjusted R 2 > 0.99) for both uncured and cured resins. The extent and rate of refractive index change and magnitude of shrinkage strain was dependent upon monomer formulation.
Significance
The development of this interferometry technique provides a powerful non-invasive tool that will be useful for improving light transmission through photoactive resins and filled resin composites by precise control of optical properties through material bulk.
1
Introduction
The worldwide use of dental amalgam continues to decrease with several (Scandinavian) governments already prohibiting its application. The nearest chairside alternative in terms of mechanical and physical properties adequate for long-lasting, large stress-bearing restorations are resin-based composite (RBC) restorative materials polymerized by high intensity visible (blue) light. Unfortunately, there exist several shortcomings of RBCs in terms of technique sensitivity (partly a consequence of inadequate curing depths and multiple incremental placement steps) and dimensional change, which are partly a consequence of an incomplete understanding of the complex optical and physical material properties throughout cure.
Besides diffusion limited termination reactions (by free radicals trapped in the rapidly vitrifying matrix), inefficient light transmission and limited curing depths (generally less than 3 mm) are a result of surface reflection , photoinitiator and dye/pigment absorption , scattering by filler particles and interfacial filler/resin refraction . Although absorption characteristics of thin films are well characterised, the change in optical properties through the bulk of curing photopolymers (∼1 cm) are not fully understood due to issues associated with light attenuation with increasing sample depth and inherently non-uniform and complex curing rate profiles . As the resin matrix (typically, co-monomer mixtures of bisGMA and TEGDMA) cures, its optical properties change and refractive index rises due to a rapid increase in cross-link density and viscosity. As the refractive index of the resin approaches that of the filler, interfacial filler/resin scattering is reduced and light transmission is increased . The refractive index ( n ) of a medium at optical wavelengths is defined as the ratio of the phase velocity of light in a vacuum ( c ) to the phase velocity ( υ p ), in a given medium (Eq. (1) ).
n = c υ p
In most materials the velocity with which light propagates is dependent upon the optical wavelength λ . Hence, when broadband, rather than monochromatic, light illuminates a sample, the speed with which it propagates will have an associated distribution, the peak of which is known as the group velocity v g , with an associated group index n g .
n g = c υ g
As light passes through the interface of one medium to another, the change in velocity is also accompanied by an optical path diversion. The relationship between the phase velocity in two media ( V A and V B ) the angle of incidence ( A ) and refraction ( B ) and the refractive index of the two media ( n A and n B ) can be predicted by Snell’s law (Eq. (3) ).
V A V B = sin A sin B = n B n A
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