Dynamics of Cellulose Whiskers in Agarose Gels. 1. Polarized Dynamic Light Scattering Clara I. D. Bica, †,§,|,⊥ Redouane Borsali, ‡,# Erik Geissler,* ,† and Cyrille Rochas †,& Laboratoire de Spectrome ´ trie Physique, CNRS UMR 5588, Joseph Fourier University Grenoble, B.P. 87, 38402 St. Martin d’He ` res, France; LCPO CNRS-ENSCPB (UMR 5629), Bordeaux 1 University, 16 Avenue Bey-Berland 33607 Pessac France; Instituto de Quı ´mica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc ¸ alves, 9500 CEP91501-970-Porto Alegre-RS, Brazil; and Cermav-CNRS, B.P. 53, 38041 Grenoble Cedex 9, France Received March 1, 2001; Revised Manuscript Received April 20, 2001 ABSTRACT: The dynamics of cotton cellulose microcrystals (“whiskers”) trapped in agarose hydrogels of varying concentrations was studied by polarized dynamic light scattering. The rigid agarose network strongly heterodynes the signal of the mobile whiskers, thereby enabling the measurement of their diffusion coefficient D and their fluctuating scattered intensity R θ. For low agarose gel concentrations, i.e., when the mesh size of the agarose network is larger than the length of the whiskers, the dynamics does not depend on the whisker content. The product of D and Rθ, which is inversely proportional to the friction coefficient, is independent of the gel concentration, indicating that the network does not hinder the whiskers. When the mesh size of the network is smaller than the length of the whiskers, the whisker content influences the dynamics and the product R θD becomes dependent on the gel concentration; i.e., frictional effects arise. Also in this condition domains with different whisker contents may develop in the gel. Introduction Cellulose microcrystals, called “whiskers”, have in- creasingly attracted attention due to their interesting physical properties related to their aspect ratio. By the addition of sulfate groups to the microcrystal surface, aqueous suspensions of cellulose whiskers may be stabilized via attraction/repulsion forces of electrical double layers 1 and yield chiral nematic phases at concentrations as low as 5% (w/w) for cotton microcrys- tals. 2 Cellulose whiskers are colloidal particles that have the form of rigid rods. In contrast to the dynamics of flexible coil polymers in solution, the dynamics of rigid rods is not yet completely understood. The dynamics of these whisker suspensions in water and in dextran solutions has been studied using dynamic light scatter- ing (DLS) and will be published in a forthcoming paper. 3 Agarose gels are widely used in the food and in the pharmaceutical industries. In recent years their impor- tance in gel chromatography has led to many studies into their structure and properties but a full under- standing is still lacking. 4 In previous studies 5,6 the dynamics of a flexible polymer (dextran), trapped in agarose gels of varying concentrations, was probed by dynamic light scattering. It was found that, for low molar mass dextran (M ) 70 000 g/mol), the diffusion coefficient decreases in the gel, but the intensity of the dynamically scattered light increases, with the result that the friction coefficient is the same whether it is in the free solution or in the gel. 5 The decrease in trans- lational diffusion coefficient is caused by a reduction of the osmotic pressure of the dextran confined in the agarose network. 6 For higher molar mass dextran (M ) 500 000 and 2 × 10 6 g/mol), the friction coefficient was found to be higher in the gel than in the free solution as far as the dilute regime of dextran is concerned. 6 This effect was attributed to entanglements between the free polymer and the network. To study the dynamics of rigid particles inside agarose hydrogels, in this work we used cellulose whiskers as a guest component in the host agarose matrix. Their diffusional behavior was investigated by polarized dy- namic light scattering (VV configuration, i.e., both incident and scattered light vertically polarized). Be- cause of their anisotropy, these whiskers strongly de- polarize light, and in a second part of this study using depolarized dynamic light scattering (VH configuration), results will be reported in a forthcoming paper. The whiskers originate from cotton and have an average length of 170 nm with an aspect ratio of about 10. 2 To investigate the role of the agarose network on the dynamics of trapped particles, the dynamics of whiskers was studied vs different agarose gel concentrations (Table 1). Theoretical Background At room-temperature agarose hydrogels are rigid, and the amplitude of their dynamics is therefore extremely small. Most of the light scattered by agarose is elastic. When this gel contains free particles, any quasi-elastic component in the scattered light may therefore be almost entirely attributed to the mobility of the free particles. The static component of the light scattered by the matrix is much stronger than the fluctuating one of the guest particles. As a consequence, the signal of the guest particles is strongly heterodyned. 5-8 For the whiskers trapped in the gel, the total average scattering intensity 〈I〉 is therefore * To whom correspondence should be addressed. E-mail: erik. geissler@ujf-grenoble.fr. † Joseph Fourier University Grenoble. ‡ Bordeaux 1 University. § Universidade Federal do Rio Grande do Sul. | Cermav-CNRS. ⊥ E-mail: claraism@iq.ufrgs.br. # E-mail: borsali@enscpb.u-bordeaux.fr. & cyrille.rochas@ujf-grenoble.fr. 5275 Macromolecules 2001, 34, 5275-5279 10.1021/ma010381g CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001