1. Introduction Nowadays, acrylic materials are widely present in various application fields and many commercial resins are available on the market. If these acrylic resins are currently used to elaborate photocured ma- terials, they can also be mixed with (in)organic fillers to obtain composite materials for dental applications [1–4], paints [5, 6], ceramics [7–10]. The main short- coming of the photopolymerization process applied to composite formulations is the low depth of cure due to the limited light penetration resulting from scattering by fillers and filler-matrix interface. The- oretical and experimental results have shown that matching filler and organic matrix refractive indices leads to a decrease in the light scattering phenome- non [11–14]. Because of the acrylic matrix volume shrinkage consecutive to its photopolymerization, it is noteworthy that the refractive index of this matrix increases, whereas the refractive index of filler is supposed to be constant. If the refractive index of (meth)acrylic resins is often known, its evolution during their polymerization is never specified in the technical datasheets. However, these data are essen- tial for applications in need of well controlled opti- cal properties (holographic data-storage (HDS) tech- nologies [15], transparent glass cloth reinforced plastics [16], gradient-index (GRIN) optics [17], transparent fiber glass reinforced composites [18] etc), as well as for the elaboration of thick homoge- neous composite materials, where the prediction of the cure light transmission evolution and its influ- ence on conversion profiles and cure depth is fun- damental [19]. This work aims to characterize the refractive index evolution of some usual and commercial photocur- able (meth)acrylic resins. For that, refractive index, double bond conversion, mechanical transition tem- perature T α and material density were measured ver- sus irradiation time. The paper will be decomposed in two parts. In the first part, an aromatic dimethacry- late resin (Bis-GMA) was chosen to accurately de- scribe the experimental process used. In the second 966 Refractive index evolution of various commercial acrylic resins during photopolymerization F. Aloui, L. Lecamp * , P. Lebaudy, F. Burel Normandie Université, INSA de Rouen, CNRS UMR 6270, FR 3038, Avenue de l’Université, F-76801 Saint Etienne du Rouvray Cedex, France Received 12 April 2018; accepted in revised form 2 June 2018 Abstract. A set of commercial (meth)acrylic resins was photopolymerized under identical irradiation conditions and evolution of their refractive index was monitored as a function of double bond conversion. Initial refractive index values ranged from 1.4445 to 1.5454 and then linearly increased with conversion as long as the material was not in the glassy state. This increase was related to an increase of the material density arising during polymerization. Final refractive index values ranged from 1.4804 to 1.5632. The knowledge of the refractive index and of its evolution during the photocuring is indispensable, in par- ticularly to elaborate composite materials (polymer matrix + filler) with well controlled optical properties. Keywords: industrial applications, photopolymerization, acrylic resin, refractive index eXPRESS Polymer Letters Vol.12, No.11 (2018) 966–971 Available online at www.expresspolymlett.com https://doi.org/10.3144/expresspolymlett.2018.83 * Corresponding author, e-mail: laurence.lecamp@insa-rouen.fr © BME-PT