Scanning densitometric and calorimetric studies of poly (Ethylene oxide) Poly(Propylene oxide) Poly(Ethylene oxide) Triblock copolymers (Poloxamers) in dilute aqueous-solution

The phase transition properties of poloxamers, in dilute aqueous solution (1 % w/v), detected by high-sensitivity differential scanning calorimetry (HSDSC) and differential scanning densitometry (DSD) at a scan rate of 30 K h-1 are reported. Poloxamers are ABA block copolymers of poly(oxyethylene) (A) and poly(oxypropylene) (B). The molecular weights of the poloxamers examined range from 2400 to 14000. The solutions are transparent over the temperature range of the observed phase transition. By HSDSC, the temperatures (T(m)) at which the differential excess heat capacity (C(p,m)) is a maximum range from 297 to 313 K and show good agreement with T(m) values obtained by DSD. The observed T(m) values decrease with increasing poly(oxypropylene) content. The calorimetric enthalpy (DELTAH(cal) of the phase transitions, which range from 133 to 337 kJ mol-1, increases with increasing poly(oxypropylene) content. The partial specific volume changes (DELTAvBAR)) of the phase transitions range from 5.6 x 10(-3) to 6.2 x 10(-2) cm3 g-1. The experimentally determined partial specific volumes (vBAR) for the poloxamers in dilute aqueous solution, at temperatures below the phase transition, agree reasonably well with those calculated for beta and gamma lattice structure forms (orthorhombic perpendicular or monoclinic lattice, respectively). At temperatures above the T(m), experimental vBAR values agree more closely with those calculated for the theoretical a form of hexagonally packed chains based on alkanols, characteristic of the more expanded form. The DSD data are discussed in terms of partial molal volume (V2(0)BAR) and expansibility (E2(0)BAR) at inifinte dilution in water. Both parameters show a dependence on temperature. This suggests that the poloxamers undergo a gradual change in molecular size as a function of temperature, possibly accompanied by a rapid conformational change around the observed T(m). Cooperativity values (DELTAH(vH)/DELTAH(cal)) of the phase transitions from HSDSC data are between 1.2 and 2.8; this together with the asymmetry of the transitions is indicative of an aggregation process. The relationships between DELTAV2(0)BAR and DELTAH(cal) per average monomer unit vs poly(oxypropylene) (POP)/poly(oxyethylene) (POE) ratio indicate that the observed phase transition is associated with the POP portion of the polymer, as both parameters approach zero as the POP/POE ratio approaches 0:1. The observed phase transitions are reversible and are unaffected by scan rate (10, 30 and 60 K h-1) using both techniques and hence show no kinetic limitations. The parameters derived from HSDSC and DSD data for the poloxamers are analyzed in terms of possible microscopic and macroscopic phenomena occurring during the phase transitions.