Filled elastomers: Mid- and Near-IR Spectroscopic Determination of Rubber Dimensions in Composite in Unstretched State and Under Uniaxial Extension Liliane Bokobza, Olivier Rapoport Laboratoire PCSM, E.S.P.C.I., 10 Rue Vauquelin, 75231 Paris Cedex, France Received 25 June 2001; accepted 17 April 2002 ABSTRACT: A Fourier transform IR investigation of rein- forced elastomers is presented. The work is essentially car- ried out in the near-IR range, that allows the analysis of thick samples. The dependence of the average rubber phase di- mensions on the filler loading is analyzed. The matrix chain deformation processes can be evaluated by looking at the strain dependence of the intensity of specific absorption bands of the polymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1204 –1208, 2003 Key words: filled elastomers; mid- and near-IR spectros- copy; rubber dimensions INTRODUCTION The addition of rigid particles into elastomers is well known as producing an improvement in their me- chanical properties. These properties are affected by a number of filler parameters such as the particle size, structure, distribution, and essentially the surface characteristics, which determine the interaction be- tween the filler and the polymer chains. The interfacial bond is considered to be the most important factor in rubber reinforcement. A poor polymer–filler interac- tion would result in dewetting (or cavitation) and vacuole formation upon a significant deformation, thus initiating cracks. One of the most characteristic features in filler rein- forcement is the obvious increase in the elastic mod- ulus. Several mechanisms have been invoked to ex- plain the enhancement in the modulus. The main con- tributions arise from the inclusion of rigid particles in the elastomeric medium and from filler–rubber inter- actions that lead to an increase in the effective degree of crosslinking. 1–13 In addition, strain amplification ef- fects caused by the inclusion of undeformable particles are expected to be important in filled systems. 14, 15 Various theoretical models, most of which lack ex- perimental proof, were developed to describe the ob- served moduli. All of them require knowledge of the distribution of the individual phases. The problem has been simplified to a two-phase model and the average behavior of the composite is defined in terms of a representative volume element. This work presents a direct evaluation of the aver- age dimensions of the rubber phase in the composite in the unstretched state and under an uniaxial defor- mation. The analysis of the deformed dimensions of the elastomeric matrix itself allows the investigation of the strain amplification concept. The experiments were carried out by using Fourier transform IR spec- troscopy (FTIR) performed in the mid- and near-IR (NIR) range. EXPERIMENTAL The samples employed, kindly supplied by Rhodia Silicones were silica-filled poly(dimethylsiloxane) (PDMS) networks. Pyrogenic silica was blended with PDMS chains containing a small amount of vinyl side chains (0.078% vinyl units) in order to carry out a peroxide cure. The amount of vinyl groups determine the molecular weight between crosslinks (M c ), which in this case was around 17,500 gmol -1 . The IR spectra were recorded with a Magna-IR 560 FTIR spectrometer equipped with a high-energy Ever- Glo source, an XT-KBr beamsplitter for a spectral range coverage from 11,000 to 375 cm -1 , and an MCT detector. The spectra were recorded with a resolution of 4 cm -1 and an accumulation of 32 scans. The macroscopic dimensions of the film were mea- sured with a micrometer comparator and averaged all along the specimen. Methods FTIR has been extensively used to evaluate the molec- ular structure and behavior of polymers. The specific- Correspondence to: Prof. L. Bokobza (Liliane.Bokobza@ espci.fr). Journal of Applied Polymer Science, Vol. 87, 1204 –1208 (2003) © 2002 Wiley Periodicals, Inc.