chemistry journal of moldova

Ion Dranca/Chem.J. Mold. 2008, 3 (1), 31-43cooling segment, the samples were heated at a rate whose absolute value was equal to the rate of preceding cooling. Theheat capacity was measured on ~27 mg sample by using a standard procedure that is precise to 1%. A sapphire sampleof 41.48 mg was used as the calibrant. The temperature program used for the heat capacity measurement involved 5min isothermal hold at 0C followed by heating at 10C min -1 to 90C and another 5 min isothermal hold at the finaltemperature.Anhydrous glucose (Dextrose) and maltitol were, respectively, purchased from Fisher and MP Biomedicals andused without purification. In order to produce amorphous (glassy) samples, ~15 mg of samples was placed in 40 lclosed Al pans and heated to ~10 o C above their respective melting points, 161 o C (glucose) and 149 o C (maltitol). Shortlyafter heating the samples were quenched into liquid nitrogen. The glass transition temperatures of amorphous sampleswere estimated as midpoint temperatures of the DSC glass transition steps measured at 10 o C min -1 .Sub-T gmeasurements, freshly quenched samples were quickly placed into the DSC cell that was maintained at-40 o C . From -40 o C the samples were heated to an annealing temperature, T a, and held at it for 30 min. After completionof the annealing segment, the samples were cooled down to -40 o C and immediately heated above T g. The heating rateswere 15, 20, 25, and 30 o C min -1 .All the aforementioned measurements we conducted by using a Mettler-Toledo heat flux DSC 822e in theatmosphere of nitrogen flow (80 ml min -1 ) at University of Alabama, USA. The temperature and heat flow calibrationwere performed by using an Indium standard.Results and DiscussionGlass transition or -relaxation in PS100 and nPS90. DSC data for both materials demonstrate (Fig. 1) typicalglass transition steps, the midpoint of which has been used as an estimate of the T gvalue. It is seen that in nPS90 theglass transition occurs at about 10 o C greater temperature than PS100.-2.0PS100nPS90Heat flow / mW-2.5-3.0Exo-40 60 80 100 120 140Fig. 2. DSC curves for the glass transition (-relaxation) in PS100 and nPS90 measured at 10 o C min -1The values of T ghave been determined at nine heating rates (two times at each heating rate). The heating ratedependence of Tg can be used to evaluate the effective activation energy of the glass transition from eq 1.T / o Cd ln E R(2)dT gln( / o C min -1 )3.53.02.52.0305 kJ mol -1PS100nPS90201 kJ mol -11.50.0026 0.0027T -1 /K -1Fig. 3. Evaluating activation energies (eq 2) for -relaxation in PS100 and nPS90 as proposed byMoynihan et al. [6, 47]34