Fluorescence study of the dynamics of a star-shaped poly(ε-caprolactone)s in THF: A comparison with a star-shaped poly(L-lactide)s

Molecular dynamics of 2, 4- and 6-arm star-shaped poly(epsilon-caprolactone)s (PCLs) functionalized with pyrene at all chain ends followed by excimer formation were investigated in THF as a solvent. Dilute solutions (10(-8) to 10(-6) mol L-1) of these polymers revealed excimer emission due to the intramolecular cyclization, proceeding via chain end interaction. Time-resolved fluorescence measurements of star-shaped PCLs showed complex decay profiles of monomer and excimer fluorescence. An intensity rise component of decay profiles monitored in excimer region (500 nm) was observed for star-shaped polymers due to dynamic formation of excimers. Dipyrene-PCL (2-arm) also showed partial formation of static pyrene excimers as followed from measurement of excitation spectra at ambient temperature in THE. Excimer intensity of di- and four pyrene-telechelic PCLs was found to be comparable with that of di- and four pyrene-telechelic polylactide (PLA) polyesters having similar molar masses. 6-Arm PCL showed depressed excimer formation with regard to 6-arm PLA under the otherwise identical conditions contrary to the conclusion expected from the higher flexibility of the PCL chain. Eventually, molecular dynamic simulations showed that possible explanation can be related to different conformations of 6-arm PLA versus PCL stars. (C) 2009 Elsevier Ltd. All rights reserved.