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Introduction

Dual-cured resin cements have been considered the material of choice to cement aesthetic indirect restorations . They can be photo-polymerized or a redox initiator system can initiate the polymerization . It is important for dual-cured resin cements to be formulated in such a way that they are capable of achieving a sufficient degree of cure with or without light-curing . This is desirable to ensure adequate polymerization of the cement in areas that are not readily accessible to light. Hence, their curing kinetics involves two distinct mechanisms. They present a physical curing, induced by means of a light source, and a chemical curing, conducive to its complete curing even in the deepest recesses where the light cannot reach . Despite their independent onset, the two ways of curing initiate a dynamics of free radical formation and monomer conversion which naturally overlap each other during the curing period. In general, the chemical-polymerizing mechanism for dual-cured resin-based materials alone is not only slower, but also less effective than when using light activation as a supplement to the final total conversion . Recently, the influence of curing protocol on the polymerization shrinkage kinetics of dual-cured resin cements (RelyX ARC, Bistite II, DuoLink, Panavia F, Variolink II and Choice) was evaluated and remarkable differences were reported on the light cure and chemical cure rates; light cure can occur about 320 times faster than chemical cure .

Dual-cured cement formulation includes base and catalyst pastes. As a general rule, it is known that these pastes must be mixed, applied and then photo-activated, and it can be expected that delaying or omitting the irradiation period may modify the polymeric structure and the extent of polymerization (EP). While the immediate photo-activation guarantees the initial stability necessary to withstand clinical tensions, the chemical curing will guarantee the scope of its maximum properties through time and where light cannot reach. Recent evidences suggest that the immediate photo-activation of some resin-based materials may compromise the final degree of conversion. Thus, the moment of light activation determines the way in which the structure networks will be formed and, as a consequence, determines the structural integrity of the materials. Insight into the effect of the moment of light activation on structural coherence, in particular in the early stages of hardening, may be of use in the clinical situation . It is expected that for a given resin cement, different curing protocols may result in different degrees of cure and polymeric network cross-link density . Resin cements that undergo different degrees of cure may present alterations in its mechanical properties . Therefore, the characteristics of handling such as work and setting times, if changed, may influence the clinical behavior of indirect restorations cemented and their mechanical properties .

Usually, the degree of conversion is determined from Fourier transform infrared spectroscopy (FTIR) or calorimetric measurements. However, it has been already shown that the method of evaluation with Stray-Field Magnetic Resonance Imaging (STRAFI-MRI) is an efficient tool to produce direct evidence of the molecular mobility , particularly in the kHz frequency range that is increasingly restricted as monomer transforms to polymer . Moreover, it is well known that mechanical properties depend strongly on molecular mobility in that frequency range, also the frequency of most of secondary mechanical relaxations. STRAFI-MRI already enabled mapping molecules from monomers until the formation of rigid polymers in order to determine the extent of polymerization of resinous components . Hence, it may be expected that, using STRAFI-MRI, a more complete evaluation of resin cements can be performed, which is based on molecular mobility analysis and is not confined to degree of conversion measurements, like FTIR. A key feature of the technique is the analysis of samples showing non-uniform polymerization. This effect is difficult or impossible to spatially resolve by FTIR or calorimetry due to experimental constraints, but can be directly evaluated by STRAFI-MRI. For instance, self- and light-curing kinetics can be better distinguished and separately analyzed using STRAFI-MRI . Therefore, the aim of this study was to probe molecular mobility and EP as a function of the curing protocol of some commercially available resin cements using ¹ H STRAFI MRI. It is hypothesized that molecular mobility and EP will vary as a function of the curing protocol and will be material dependent.

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Materials and methods