SANS study of salt induced micellization in PEO±PPO±PEO block copolymers V.K. Aswal a, * , P.S. Goyal b , J. Kohlbrecher a , P. Bahadur c a PaulScherrerInstitut,CH-5232VilligenPSI,Switzerland b IUC-DAEF,MumbaiCentre,BhabhaAtomicResearchCentre,Mumbai400085,India c DepartmentofChemistry,SouthGujaratUniversity,Surat395007,India Received 7 September 2001; in ®nal form 17 October 2001 Abstract Small-angle neutron scattering SANS) measurements have been carried out from the aqueous solutions of poly- ethylene oxide PEO)±polypropylene oxide PPO)±PEO block copolymers of pluronics F88 and P84 in presence of salt KCl. It is seen that the eect of addition of salt is similar to that of increasing temperature. The pluronics exist as unimers at low temperatures and their micellization takes place when the salt is added or the temperature is increased. The measurements for dierent salts of the lyotropic series on the above pluronic systems show that micellar structure is sensitive to the sizes of the hydrated ions of these salts. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Pluronics are polyethylene oxide±polypropylene oxide±polyethylene oxide PEO±PPO±PEO) tri- block copolymers and are widely used for various applications in the pharmaceutical, textile and detergent industries [1]. PPO block exhibits dra- matic temperature dependence in aqueous solu- tions. Below 20 °C, water is good solvent for PPO, whereas PPO at higher temperature aggregates. PEO, on the other hand, is dominantly hydrophilic with the temperature range 0±100 °C. Therefore, with blocks of PEO and PPO combined into a single pluronic molecule, one expects amphiphilic characteristics with interesting phenomenon shown by these systems [2±5]. Pluronics form micelles in aqueous solutions composed of a core of PPO blocks and an outer shell consisting of PEO end blocks and water. These micelles can be formed by either increasing temperature and/or copolymer concentration at a speci®c concentration critical micelle concentra- tion CMC)) or temperature critical micellization temperature CMT)). The micellar structures of some pluronics have been studied using SANS [6±9]. The temperature dependence studies show that PEO±PPO±PEO exists as unimers at low temperatures and as micelles at high temperatures. Thestructureofmicelledependsonthetemperature and the eect is dierent for dierent pluronics. The dehydration of water from PPO blocks with the increase in temperature results in trans- 7 December 2001 Chemical Physics Letters 349 2001) 458±462 www.elsevier.com/locate/cplett * Corresponding author. Fax: +41-56-310-3131. E-mailaddress: vinod.aswal@psi.ch V.K. Aswal). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0009-261401)01249-0