Viscosity Effects in the Photopolymerization of Two-Monomer Systems Agnieszka Marcinkowska, Ewa Andrzejewska Institute of Chemical Technology and Engineering, Poznan University of Technology, Poznan 60-965, Poland Received 15 May 2009; accepted 27 September 2009 DOI 10.1002/app.31491 Published online 1 December 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Photopolymerization kinetics and viscosity behavior of five different two-monomer systems forming hydrogen bonds and composed of mixtures of a high viscosity monomer (HVM) and a low viscosity monomer (LVM) at various molar ratios were investigated at six polymerization temperatures. The monomers used were mono- or dimethacrylates. Detailed viscosity measurements of the monomer mixtures showed significant negative deviations from the theoretical values (characterized by excess logarithm viscosities) indicating that interactions between the molecules of the same type (in individual monomers) are stronger than those between two molecules of different types (HVM and LVM). The photopolymeriza- tion kinetics were analyzed from the point of view of the appearance, viscosity and temperature behavior of the most reactive composition (MRC), the one showing the highest value of the maximum polymerization rate within a range of the HVM: LVM ratios. It was found that MRC appearance is determined mainly by the initial viscosity of the two- monomer system, whereas the functionality of the monomers (and network formation) is much less important (MRC is observed even in linear systems). The initial viscosity of all the monomer mixtures showing MRC lay in the range of 0.06–2 Pa s, which is narrow compared to the range of viscosities of the monomers (approximately 10 3 –10 3 Pa s). V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 116: 280–287, 2010 Key words: kinetics (polym.); photopolymerization; viscosity INTRODUCTION The photopolymerization is nowadays a widespread technique used for preparation of polymeric materials in many branches of industry. Photocurable composi- tions are based mainly on multifunctional methacry- lates and acrylates and are usually composed of highly viscous oligomers (responsible for mechanical properties of the product) and low viscosity mono- mers (LVM), working as reactive diluents. To the most popular oligomers belong 2,2-bis[4-(2-hydro- xymethacryloxypropoxy)phenyl]propane (bis-GMA) and 1,6-bis-(methacryloxy-2-ethoxycarbonylamino)- 2,4,4-trimethylhexane (UDMA), two basic resins used in dental restorative materials. The very high viscosity of bis-GMA is the main rea- son of its low polymerization rates and conversions; a dilution with a LVM, like triethylene glycol dimetha- crylate (TEGDM), increases the polymerization rate due to the increase in mobility of monomer molecules (causing reduction of diffusional limitations of propa- gation). However, too large content of the reactive dil- uent results in another decrease of the polymerization rate due to the increase in the mobility of the macro- radical chain ends and, in consequence, enhanced ter- mination rate. 1–4 This leads to the appearance of a composition showing a maximum reactivity (the high- est value of the maximum polymerization rate R p max ), 1,2 in which the ratio of the monomers changes with temperature. 1 Beside bis-GMA/TEGDM system, a similar behavior was also found for other systems, like UDMA/TEGDM, (ethoxylated bisphenol A dime- thacrylate)/TEGDM, bis-GMA/(phenyl glycidyl ether methacrylate) and bis-GMA/styrene systems. 2,5,6 In all these systems the high viscosity component was the crosslinking monomer whereas the reactive dilu- ent was a tetrafunctional or difunctional one (accord- ing to Flory’s definition difunctional monomers con- tain one double bond 7 ). The importance of the initial viscosity of the system as the main factor to obtain op- timum reactivity was indicated in some works 1,2 but also other explanations were proposed, like different reactivity of functional groups, change in pendant double bond fraction or change in initiator efficiency. 6 On the other hand, the existence of the most reac- tive composition (MRC) was also proved for mix- tures of two LVMs, one tetra- and one difunctional. In such a case the determining factor is the crosslink density. 8–11 If two monomers differ both with vis- cosity and functionality, changes in their ratio in the formulation cause changes both in the initial Correspondence to: E. Andrzejewska (ewa.andrzejewska@ put.poznan.pl). Contract grant sponsor: Poznan University of Technology; contract grant number: 32-061/2009-DS. Journal of Applied Polymer Science, Vol. 116, 280–287 (2010) V C 2009 Wiley Periodicals, Inc.