Enthalpy relaxation in sucrose-maltodextrin-sodium citrate bioglass

Glass transition characteristics of sucrose (SC)-maltodextrin (MD)-sodium citrate (NaCit) bioglass system were investigated using differential scanning calorimetry. Samples were formulated with different SC:MD (7:3, 5:5 and 3:7 by mass) and NaCit/SC (0, 0.1 and 0.2 by mole) ratios, and were equilibrated to residual moisture contents of 0.27-0.35 %wb. Isothermal aging experiments were conducted with the degree of undercooling from 12 to 57 °C and aging times of 8, 20, 47 and 71 h. In general, the Kohlrausch-Williams-Watts (KWW) decay function fit well with the experimental enthalpy relaxation data. The enthalpy relaxation time (τKWW) and the time required for 50% completion of theoretical possible maximum enthalpy relaxation at constant temperature (tϕ(t)=0.5) increased with increasing MD content; lowering the aging temperature had similar effects on τKWW and tϕ(t)=0.5. In the system with high SC and NaCit concentrations (SC:MD = 7:3 and NaCit/SC = 0.2), a substantial increase in τKWW and tϕ(t)=0.5 were observed when aging temperature decreased slightly; these systems also exhibited the greatest apparent activation energy. The findings reveal that NaCit can enhance the stability of low-moisture bioglass by primarily interacting with SC and form large less-mobile clusters, which helps to improve glass transition temperature and restrict the matrix mobility.