New Aspects of Viscosity Effects on the Photopolymerization Kinetics of the 2,2-Bis [4-(2-hydroxymethacryloxypropoxy)phenyl]propane/ Triethylene Glycol Dimethacrylate Monomer System Ewa Andrzejewska, Agnieszka Marcinkowska Institute of Chemical Technology and Engineering, Poznan University of Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland Received 7 March 2008; accepted 9 June 2008 DOI 10.1002/app.28831 Published online 26 August 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The photopolymerization kinetics and vis- cosity behavior of 11 2,2-bis[4-(2-hydroxymethacryloxypro- poxy)phenyl]propane/triethylene glycol dimethacrylate mixtures were investigated. The viscosity was studied at six temperatures (20–70  C), and the activation energies for the viscosity were determined. The excess logarithm vis- cosities were calculated and found to be negative over the whole composition and temperature ranges; they were fit- ted to the Redlish–Kister polynomial equation. The kinetic analysis of the photopolymerization was carried out at three polymerization temperatures (20, 40, and 60  C). The results proved the existence of the most reactive composi- tion (reaching the highest value of the maximum polymer- ization rate), but the ratio of the monomers in this composition, close to equimolar, showed a tendency to change with the polymerization temperature. The viscos- ities of the most reactive compositions lay in the range of about 0.1–1.2 Pa s, which was narrow in comparison with the range of viscosities of all the compositions used in the kinetic studies (from 3  10 3 to 1.5  10 3 Pa s). The acti- vation energies for the polymerization rates were calcu- lated and correlated with the viscosity changes. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2780–2786, 2008 Key words: activation energy; dental polymers; kinetics (polym.); photopolymerization; viscosity INTRODUCTION Dimethacrylate-based networks formed during pho- topolymerization are frequently used in high- performance applications such as dental restorative materials and information storage systems. Substan- tial effort has been devoted to understanding how varying the structure of methacrylate comonomers and the conditions of photopolymerization impacts the polymerization kinetics. 1,2 A system that has been intensively studied during the last several years is a mixture of two monomers: highly viscous 2,2-bis[4-(2-hydroxymethacryloxypro- poxy)phenyl]propane (bis-GMA) and low-viscosity triethylene glycol dimethacrylate (TEGDM). Their mixtures are often used as the organic phase of den- tal restorative materials. 3,4 Some kinetic studies have shown that both the addition of TEGDM to bis- GMA and the dilution of bis-GMA with TEGDM increase the maximum polymerization rate (R max p ) in comparison with the neat monomers. This leads to the appearance of a composition showing the maxi- mum reactivity (the highest value of R max p ). Although most often the composition of the most reactive resin is close to the equimolar mixture of the two mono- mers, 4–6 the number of investigated compositions with various monomer ratios is rather limited, and only in one case has it been as high as 9. 4 However, one can expect the composition of the most reactive mixture to be sensitive also to the ini- tiation rate (the type of initiating system used and the intensity of the incident light) as well as the polymerization temperature (mainly due to viscosity changes). In our study, we took into account the lat- ter factor and investigated the photopolymerization kinetics for 11 comonomer ratios at three polymer- ization temperatures. Special attention was paid to the viscosity behavior of the bis-GMA/TEGDM monomer mixtures at six temperatures. EXPERIMENTAL TEGDM was purchased from Aldrich (St. Louis, MO) and was purified by column chromatography before use. The removal of the inhibitor is important Journal of Applied Polymer Science, Vol. 110, 2780–2786 (2008) V V C 2008 Wiley Periodicals, Inc. Correspondence to: E. Andrzejewska (ewa.andrzejewska@ put.poznan.pl). Contract grant sponsor: Research Program of Poznan ´ University of Technology; contract grant number: 32-268/ 2008-DS.