Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

No abstract

A sample of triblock copoly(oxyethylene/oxypropylene/oxyethylene) Synperonic F127 was purified. The micellisation and gelation properties of aqueous solutions of purified and unpurified copolymers were investigated by surface tension measurement, static and dynamic light scattering, differential scanning calorimetry and NMR spectroscopy. Generally, the results obtained for the two samples were similar: an exception was the surface tension. Endothermic standard enthalpies of micellisation were obtained over a wide concentration range, with corresponding endothermic standard enthalpies of gelation in the high concentration range. Considered on an equivalent basis, i.e. kJ mol-'(chains), gelation was found to be an almost athermal process compared to micellisation. Based on the presented evidence, particularly that from DSC, and considering other recent studies, it was concluded that the thermal gelation of F127 (i.e. gelation on raising the temperature) resulted essentially from the packing of spherical micelles. A small thermal event at the gelation point was ascribed to a disorder-order discontinuity.Block copolyethers composed of hydrophobic poly(oxypr0-pylene) (POP) and hydrophilic poly(oxyethy1ene) (POE) are non-ionic surface-active agents with a wide range of application. The most used commercial copolymers are triblocks : i.e. H[OCH,CH,],[OCH2CH(CH3)]n[OCH2CH2]mHor E,P,E, Variation of the ratio of oxyethylene to oxypropylene (rn : n) and the number of units in each block (i.e. the overall chain length, x = 2m + n) allows the properties of the block copolymers to be varied in a controlled way.The association properties of triblock copolymer F 127 in aqueous solution have attracted much research interest, not least because of the interesting thermally reversible gelation of its concentrated solutions. 'J According to the published grid,' F127 has the formula E1,$&1,6, where E represents an oxyethylene unit and P an oxypropylene unit. The micellisation of F127 in dilute aqueous solution has been studied p r e v i o~s l y ,~-~ as have the solubilisation properties of its micellar solutions.' The gelation properties of concentrated solutions of F127 have been studied more ~f t e n , ' ,~,~,~-'~ particularly with regard to application in controlled-drug-release systems. Detailed studies of micellisation and micellar solutions by Attwood et ~1 . ~7~ and by Wanka et a[.' reveal some divergence of quoted results: e.g. at 25 "C ref. 3, 4 ref. 5 critical micelle concentration aggregation number ca. 15 ca. 30 (c.m.c.)/g dm-1 0.04In part, these differences reflect a different choice of experimental methods and interpretation of results. A significant consideration, pointed out by Attwood et aI.,12 is the variability of the properties of different batches of F127 (and related copolymers) resulting from variation in their molecular composition. Apart from the natural width of the composition distribution, triblock copolyethers may contain diblock copolymers originating from the transfer reaction in the anionic polymerisat...