Objective: The purpose of this study was to evaluate a nano-filled dental composite, with varying cure irradiation-time, in terms of the spatial distribution of dynamic-mechanical properties determined at nanometre scale and the resultant distinction between filler, matrix and inter-phase regions.
Materials and methods: Specimen groups (n=5) of the composite Filtek Supreme XT were cured in 2mm deep molds for 5, 10, 20 and 40s, and stored for 24h in distilled water at 37 degrees C. Properties were measured at 2mm depth, on the lower specimen surfaces. Nano-dynamic-mechanical parameters (complex, storage and loss modulus, tandelta) were determined at an array of 65,000 locations in a 5microm x 5microm area. Micro-mechanical properties (hardness, modulus of elasticity, creep and elastic/plastic deformation) were also measured and additionally the real-time degree of cure, by ATR-FTIR, for 10min after photo-initiation and after storage.
Results: The spatial distribution of nano-dynamic-mechanical properties varied significantly enabling four distinguishable matrix, filler-cluster and inter-phase regions to be identified. Proceeding from matrix to filler-cluster locations, complex-moduli increased linearly and loss-factors decreased linearly, consistent with visco-elastic composite theory. Curing time strongly affected all measured properties at 2mm depth. The organic matrix was shown to be inhomogeneous for all curing times. By increasing cure-time, the proportion of less well polymerized area decreased from 37.7 to 1.1%, resulting in a more homogeneous organic matrix.
Significance: The experimentally observed graduated transition, in complex modulus and related dynamic-mechanical properties, across the matrix - inter-phases - filler-cluster regions is conducive to low internal stresses, in contrast to the abrupt modulus transitions anticipated or observed in many other particulate composite structures. The identification of these phase-regions provides a realistic basis for accurate nano- and micro-mechanical computational modelling.