The effect of cure rate on the mechanical properties of dental resins LaleG.Lovell a ,HuiLu a ,JeannineE.Elliott a ,JeffreyW.Stansbury b ,ChristopherN.Bowman a,b, * a Department of Chemical Engineering, University of Colorado, Boulder, CO 80309-0424, USA b Biomaterials Research Center, Department of Restorative Dentistry, University of Colorado Health Sciences Center, Denver, CO 80262, USA Received 29 August 2000; accepted 30 January 2001 Abstract Objective: Thisstudyinvestigatestheeffectofcurerateonthemechanicalpropertiesofacommondimethacrylatedentalresinformulation 75/25 wt% bis-GMA/TEGDMA). Methods: The polymerization rate and ®nal conversion of the exact specimens subsequently used for mechanical testing were monitored bynear-infrarednear-IR)spectroscopy.TheglasstransitiontemperatureT g )andmodulus,asafunctionoftemperature,weredeterminedby dynamic mechanical analysis DMA). Iniferter initiating systems were used to create partially cured networks that did not contain any trapped radicals. By the elimination of trapped radicals from the system, the formed networks can be characterized as a function of both temperature and double bond conversion without inducing additional thermal cure during testing. Results: CopolymerspecimenswerecuredwithUVandvisiblelightinitiatingsystems,UVlightintensitiesthatvariedbyoverfourorders ofmagnitude,andcuretemperaturesthatdifferedby608C.Eventhoughthepolymerizationratesfortheseresinswerevastlydifferent,similar T g and modulus were measured for specimens cured to the same ®nal double bond conversion. Signi®cance: This study shows that highly cross-linked dimethacrylate systems, such as bis-GMA/TEGDMA, exhibit similar network structure and properties as a function of double bond conversion, regardless of the method or rate of cure. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Dimethacrylate; Photopolymerization; Iniferter; Near-infrared spectroscopy; Dynamic mechanical analysis 1. Introduction Composite materials that contain dimethacrylate resins are commonly used as dental restorative materials. These materials are esthetically pleasing since a variety of added pigments allows for an excellent color match with adjacent enamel. In addition, polymer-based composites are perceived as `safe' alternatives to mercury-based amalgam ®llings. Furthermore, with the development of high- powered dental curing lamps, dimethacrylate resins can be quickly cured ,10 s) to form a strong, highly cross-linked polymermatrixthatactsasthestructuralsupportforcompo- site dental materials. However, the use of dental curing lampswithavarietyofspectralintensitieshascauseddebate about the effect of cure rate on the ®nal properties of the restoration. Some researchers have reported that resins cured with a high light intensity source exhibit higher contraction stresses and therefore, an increased potential for marginal gap formation [1,2]. Other researchers have found that degree of conversion, independent of cure method, has a critical effect on the ®nal mechanical proper- ties and wear rate of composite materials [3±6]. In recent work, the relationship between degree of conversion and shrinkage strain, for composites cured at different light intensities, was investigated. Over the conversion range studied 26±61%), shrinkage strain was shown to increase linearly with double bond conversion, regardless of the method of cure [7]. Therefore, to improve general understanding about the effects of cure rate on the ®nal properties of these materials, this work investigates the mechanical relaxation of dimetha- crylate resins as a function of both conversion and cure rate. Controllingthedegreeofconversionoftheseresinsystemsis dif®cult due to their rapid network formation and persistent trapped radical populations that may lead to a change in conversion as a function of storage time or temperature [8]. Forexample,testingofthespecimensatelevatedtemperatures i.e.,abovethecuretemperature)increasesthemobilityofthe radicals and additional polymerization ensues [9±11]. To avoid this result, most mechanical testing of dental resin systems has been done near the cure temperature. However, Dental Materials 17 2001) 504±511 dental materials www.elsevier.com/locate/dental 0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII:S0109-564101)00010-0 * Corresponding author. Tel.: 11-303-492-3247;fax: 11-303-492-4341. E-mail address: christopher.bowman@colorado.edu C.N. Bowman).