Rheological properties of hydroxyl-terminated and end-capped aliphatic hyperbranched polyesters

The rheological behavior of two series of aliphatic hyperbranched (HB) polyesters, based on 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) and di-trimethylol propane (Di-TMP) as a tetrafunctional core, was studied. The effect of the size (pseudo-generation number, from second to eight) and structure on the melt rheological properties was investigated for a series of hydroxyl-terminated HB polyesters. In addition, the influence of the nature and degree of modification of the terminal OH groups in a series of fourth-generation polyesters end-capped with short and long alkyl chains and some aryl groups on the rheological properties was analyzed. The time–temperature superposition procedure was applied for the construction of master curves and for the analysis of the rheological properties of HB polyesters. The data obtained from WLF analysis of the HB polyesters showed that the values of the thermal coefficient of expansion of free volume α_f and the fractional free volume at the glass transition temperature, f_g, increase with increasing size of the HB polyesters. It was shown that the modified HB polyesters exhibited lower T_g and T_G′=G″ temperatures, above which viscous became dominant over elastic behavior. From an analysis of the master curves of the modified HB polyesters, it was observed that with increasing degree of modification, both storage and loss modules and complex dynamic viscosity and apparent energy for viscoelastic relaxation decrease, because of reduced intermolecular hydrogen interactions. They do not exhibit a plateau of rubbery behavior, which confirms that no entanglements are present and that the molar masses are below the critical molar mass. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41479.