Polymer Testing 22 (2003) 77–81 www.elsevier.com/locate/polytest Material Behaviour In situ evaluation of the polymerization kinetics and corresponding evolution of the mechanical properties of dental composites R.L. Ore ´fice a,* , J.A.C. Discacciati b , A.D. Neves b , H.S. Mansur a , W.C. Jansen b a Department of Metallurgical and Materials Engineering, UFMG—Federal University of Minas Gerais, Rua Espı ´rito Santo 35/206, Belo Horizonte, MG, 30160-030, Brazil b Dental School, LEPCom—Laboratory of Polymer and Composites Engineering, UFMG—Federal University of Minas Gerais, Rua Espı ´rito Santo 35/206, Belo Horizonte, MG, 30160-030, Brazil Received 27 March 2002; accepted 13 May 2002 Abstract Polymer composites have been used in dental applications for more than 25 years. Although their properties and behavior have been systematically improved, they still are not able to produce dental restorations chemically, dimen- sionally and mechanically stable for long periods of time. Low degrees of monomer conversion and poor processing control are some of the main features responsible for the materials instability. In this work, the monomer conversion of dental composites during visible light irradiation was in situ monitored by infrared spectroscopy. The relationship between degree of conversion and mechanical properties was obtained by evaluating the microhardness of composites with different degrees of conversion. The relationship obtained was then used to identify the evolution of the mechanical properties during photopolymerization.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Dental composites; Kinetics of polymerization; FTIR 1. Introduction Aesthetic concepts have been particularly important in guiding the development of dental materials in the last few years. Each year, more and more dentists and patients are looking for materials that can replace amal- gams and other metals while keeping the colors and glare as close as possible of the original teeth. These needs have then supported the use of polymer composites in dental applications, since this type of material can have its color easily altered by well-known additives. Dental materials based on polymer composites are also easily processed in different shapes by photopolymerizing * Corresponding author. Tel.: +55-31-238-1843; fax: +55- 31-238-1815. E-mail address: rorefice@demet.ufmg.br (R.L. Ore ´fice). 0142-9418/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII:S0142-9418(02)00052-1 dimethacrylate monomers to yield materials with high strength. These interesting features have motivated the spread of the use of polymer composites throughout the many applications related to the dentistry [1]. On the other hand, dental composites have not shown success in long-term applications yet due most of all to the lack of dimensional stability. During in situ poly- merization, conversion of monomers to polymers results in shrinkage that usually leads to the formation of tooth- material interfacial defects, represented by intercon- nected voids. These defects are powerful free paths for penetration of water and bacteria that cause infection and recurrent caries [2]. In order to reduce polymerization shrinkage, many approaches have been pursued and proposed. A series of new monomers that expands during polymerization have been proposed. Ring-opening, liquid crystalline, branched and dendritic monomers are examples of poly-