The purpose of this study was to develop and evaluate a dynamic light scattering-based method for monitoring the polymerization reaction of a light activated dental composite.
Laser light back-scattered from thin disk-shaped composite samples was used to study the curing reaction kinetics. Samples were irradiated simultaneously on opposite surfaces with a 633 nm laser beam and a halogen curing lamp (320, 160, or 100 mW/cm 2 ). Dynamic laser speckle patterns were imaged onto a CCD camera at a rate of 32 frames/s for 2 min. The intensity decorrelation rate calculated from sequential speckle patterns was used to assess the rate of motion within the samples during the reaction.
Motion within the composite increased immediately upon the onset of light exposure for all trials. This was followed by a brief period characterized by a relatively constant high rate of motion. Finally the rate of motion decreased exponentially. The reaction acceleration, deceleration, and maximum rate were dependent upon the irradiance of the curing light source.
This method monitors reaction rate and the change in reaction rate at high temporal resolution without contact. Reaction kinetics was shown to begin immediately after light exposure suggesting limited opportunity for viscous flow and stress relief.
The curing kinetics of dental resin composites has been the subject of substantial research since the introduction of these materials over 40 years ago. One of the main issues driving continued research in this area is the clinically significant volumetric shrinkage that occurs in all such composites as they cure. The resulting stress that accumulates within the composite and the surrounding tooth structure can lead to a number of unwanted outcomes including pain, damage to the tooth, marginal failure between the restoration and tooth, as well as failure of the restoration itself . Many studies have focused on reducing this stress, both through the development of new composite materials as well as through the use of novel curing protocols . However, minimizing the shrinkage stress has proven difficult due to the many interrelated variables that play a role in the curing reaction , particularly when considering the necessary balance between reducing shrinkage stress while at the same time maintaining adequate mechanical and physical properties of the final restoration .
There have been numerous studies investigating how the polymerization kinetics affect shrinkage stress and the development of mechanical properties . The rate of polymerization of dimethacrylate monomers, which are commonly used in dental composite, has been shown to exhibit a rapid increase early in the reaction due to the auto-acceleration effect associated with free-radical termination becoming diffusion controlled. Soon thereafter, due to the increasing size and complexity of the polymer network, propagation also becomes diffusion controlled, causing a rapid decrease in polymerization rate, known as auto-deceleration . It has been shown that increasing rates of polymerization are associated with higher levels of shrinkage stress and the reaction rate also has an effect on final conversion of the composite . For the purpose of developing ways to reduce shrinkage stress, it is therefore important to have a means of monitoring polymerization kinetics, particularly early in the reaction when conversion rate is highest and changes rapidly with time. However there is currently no well-established technique, which can be used under clinically relevant curing and sample configurations, that is capable of measuring composite polymerization with the temporal resolution necessary to assess early reaction kinetics.
Degree of conversion (DC), the fraction of the initial monomer double bonds converted into polymer double bonds, is typically used to describe rate and extent of cure. A common means of measuring DC is by spectroscopy, specifically in the mid- or near-IR bands . With this method, IR spectra are typically obtained before and after completion of curing, and based on the change in magnitude of absorption peaks specific to unreacted monomer, the overall DC can be estimated. Time resolved conversion is occasionally measured with IR techniques , however due to the scanning time needed to obtain a single spectra and the fact that several spectra are usually averaged to reduce noise in the measurement, the sampling rate has generally been limited to less than 1 Hz. Coupled with the fact that, to obtain conversion rate information from DC, the data must be differentiated, thereby accentuating any noise, IR techniques are not well-suited for monitoring rapid changes in polymerization rate.
For studying time resolved conversion and conversion rate, thermal analysis methods, such as differential scanning calorimetry (DSC), have often been used . With this method the exothermic heat output during the reaction is continuously measured, and based on the known heat of reaction for polymerization, the rate of bond formation can be calculated and used to deduce conversion rate. DSC is generally capable of higher sampling rates than IR spectroscopy, however some machines suffer from a long response time, limiting the temporal resolution that can be achieved. Also DSC is greatly limited by stringent experimental conditions, based on the need for the sample to be placed in a thermally isolated chamber during the reaction, while also allowing for a port of entry for the curing light. In addition, sample size is typically limited to the milligram range, making it difficult to compare the results from this method to samples of clinically relevant dimension.
Another common method for monitoring the reaction kinetics has been through measurements of the rate of sample shrinkage, or strain rate. This can be accomplished through a number of experimental techniques including dilatometry and the bonded-disk method , among others. While several studies have shown final shrinkage and DC to be proportional others have suggested that when considering the time resolved reaction, there may be a lag between the development of conversion and shrinkage .
Dynamic light scattering (DLS) is a well-established optical technique used to study dynamic processes of liquids and solids . When a scattering medium is in motion, the light that it scatters will fluctuate with time. In DLS the intensity of this scattered light is measured and its temporal fluctuations are quantified to characterize the underlying motion. For the case in which each photon is scattered no more than once (the single scattering regime), the electric field at a point detector at a given time t can be described by
E ( t ) = ∑ m = 1 N a ( r m ( t ) ) e ( i q ⋅ r m ( t ) )