In¯uence of UV radiation wavelength on conversion and temperature distribution pro®les within dimethacrylate thick material during photopolymerization L. Lecamp, P. Lebaudy * , B. Youssef, C. Bunel Laboratoire de Mate Âriaux Macromole Âculaires, Institut National des Sciences Applique ÂesdeRouen,UMRCNRS6522Polyme Áres, Biopolyme Áres, Membranes, Place E Blondel, BP 08, 76131 Mont Saint Aignan Ce Âdex, France Received 7 October 2000; received in revised form 17 April 2001; accepted 15 May 2001 Abstract UV-light initiation is now commonly used to induce polymerization of multifunctional monomers. The highly crosslinked networks obtained have a wide variety of applications. The thermal effects which take place during polymerization can be the cause of non-homo- geneity and defects in the ®nal material. These defects greatly alter the physical properties of the ®nal products, particularly the optical ones, which causes problems in the design of thick and optically perfect materials. To better control the homogeneity of photocured materials and to determine the in¯uence of different experimental parameters on them, conversion and temperature distribution pro®les within a material during photopolymerization were simulated numerically, using the general heat equation applied to one-dimensional system. To describe the true conditions of kinetic experiments, some necessary parameters were measured, like conversion, reaction rate, spectral irradiance of the Hg vapor lamp and dimethacrylate spectral absorbance. We focused our attention more particularly on the in¯uence of the irradiation wavelength. Indeed, the high values of the spectral absorbance coef®cient cause a great decrease in light intensity in the depth of the material and lead in turn to a sharp drop in conversion. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Photopolymerization; Conversion pro®le; Temperature pro®le 1. Introduction Photoinitiated polymerization of multifunctional mono- mers provides an easy and rapid method for producing highly crosslinked polymer networks. The rapid cure and excellent physical properties of these networks have led to a growing demand and new applications for these materials [1±4]. Unfortunately, in the case of thick material produc- tion, the thermal effects taking place during polymerization can be the cause of defects in the ®nal material [5,6]. These heterogeneities greatly alter the physical properties of the ®nal products, and particularly the optical ones, which causes problems in the design of thick and optically perfect materials. To ensure the effective use of these materials and to tailor them for a particular application, a good knowledge of conversion and temperature distribution pro®les within the material during photoinitiated polymerization is essential. However, the measurement of these parameters during the process is impossible because no suitable technique is avail- able to monitor their variations within a thick material. Moreover, it is well-known that polymerization continues after the end of irradiation [7±11]. This postpolymerization reaction is very fast during the ®rst seconds after the end of UV radiation and continues at a lower speed later in time. Thus,eventhoughitispossibletocutathickmaterialinthin slices after the end of UV radiation and to analyze them by FTIR spectroscopy, the obtained conversion values cannot be representative of the true ones at a time t and at a place x in the initial thick material. We can therefore conclude that no valid experimental measurements can be performed until optimum conversion is reached in all the material. Only the use of a suitable simulation may solve this problem. In this paper, our approach includes two steps. The ®rst one deals with the determination of some necessary para- meters for numerical analysis. First of all, the conversion and reaction rate versus temperature and UV light intensity were measured by photocalorimetry for thin materials. At the same time, the in¯uence of the radiation wavelength on kinetics was studied. Indeed, a previous study [12] has shown that the use of thin samples provides isothermal Polymer 42 2001) 8541±8547 0032-3861/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0032-386101)00353-6 www.elsevier.com/locate/polymer * Corresponding author. E-mail address: philippe.lebaudy@insa-rouen.fr P. Lebaudy).