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 solut ion (1 % w/v), detected by high-sensitivity differential scanning calo rimetry (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 weigh ts of the poloxamers examined range from 2400 to 14000. The solutions are transparent over the temperature range of the observed phase trans ition. By HSDSC, the temperatures (T(m)) at which the differential exc ess heat capacity (C(p,m)) is a maximum range from 297 to 313 K and sh ow good agreement with T(m) values obtained by DSD. The observed T(m) values decrease with increasing poly(oxypropylene) content. The calori metric enthalpy (DELTAH(cal) of the phase transitions, which range fro m 133 to 337 kJ mol-1, increases with increasing poly(oxypropylene) co ntent. The partial specific volume changes (DELTAvBAR)) of the phase t ransitions range from 5.6 x 10(-3) to 6.2 x 10(-2) cm3 g-1. The experi mentally determined partial specific volumes (vBAR) for the poloxamers in dilute aqueous solution, at temperatures below the phase transitio n, agree reasonably well with those calculated for beta and gamma latt ice structure forms (orthorhombic perpendicular or monoclinic lattice, respectively). At temperatures above the T(m), experimental vBAR valu es 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 mo lal volume (V2(0)BAR) and expansibility (E2(0)BAR) at inifinte dilutio n in water. Both parameters show a dependence on temperature. This sug gests that the poloxamers undergo a gradual change in molecular size a s a function of temperature, possibly accompanied by a rapid conformat ional 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 indi cative of an aggregation process. The relationships between DELTAV2(0) BAR and DELTAH(cal) per average monomer unit vs poly(oxypropylene) (PO P)/poly(oxyethylene) (POE) ratio indicate that the observed phase tran sition is associated with the POP portion of the polymer, as both para meters 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 limi tations. The parameters derived from HSDSC and DSD data for the poloxa mers are analyzed in terms of possible microscopic and macroscopic phe nomena occurring during the phase transitions.