Concentration fluctuation and cooperative chain mobility of hyperbranched poly(ε-caprolactone)s investigated by photon correlation spectroscopy

Concentration fluctuation and cooperative chain mobility of the hyperbranched poly(ε-caprolactone)s (HPCLs) as well as that of the linear poly(ε-caprolactone) (LPCL) in their concentrated solutions in tetrahydrofuran (Cpolymer=1.0 g/mL) were systematically characterized using photon correlation spectroscopy (PCS). The results were interpreted in conjunction with the architectural characteristics of the HPCLs that are the different lengths of the linear segments and the different branching structures. The HPCLs were synthesized to incorporate the different lengths of the linear segments consisting of 5, 10, and 20 ε-caprolactone units on their backbone chains, and thereby referred to as HPCL-5, -10, and -20, respectively. The relative degrees of branching (DB) of the HPCLs, which is obtained by branching ratio values, were found to be in the order of HPCL-5>-10>-20. From the narrow distribution of relaxation times and q2-dependence of relaxation rates, it was verified that the relaxation processes concerned in this study were due to the concentration fluctuations caused by cooperative chain diffusion. The correlation times, τcʹs, and the corresponding apparent activation energies, Eaʹs, which provide a direct evaluation for the molecular mobility, were determined by non-linear curve fittings of measured correlation functions with Kohlrausch-Williams-Watts (KWW) equation and by the Arrhenius fits of temperature dependences of τcʹs, respectively. As a result, cooperative chain mobility of the HPCLs in the concentrated solution state was found to be higher than that of the LPCL, and was enhanced with the shorter linear segments and with the higher relative DBs of the HPCLs.