Negrican Sandalcı 030070084 1st Week

GLASS TRANSITION TEMPERATURE (NEW / BETTER)

Equilibrium T_g breadth 

The glass transition represents a change of a material from a disordered glassy solid with very limited mobility to a liquid melt with translational, rotational and conformational molecular motions. Calorimetrically, this transition is characterized by a step-change in heat capacity ΔC_p, but without an associated heat of transition.Maltodextrins typically consist of a mixture of small molecular weight oligosaccharides along with larger polysaccharides. Orford et al.showed that, for most binary mixtures of low molecular weight sugars, a linear relationship was found between T_g and the composition (mole fraction) of the mixture. Additionally, it is well-known that T_g is a function of molecular weight. Wunderlich described that compatible blends of homopolymers of plastics demonstrate a broadening of the glass transition region compared to the breadth of the transition of the pure homopolymers. Therefore, it is assumed here that a commercial maltodextrin, consisting of a distribution of molecular weights, would have a distribution of glass transition temperatures as well.

Furthermore, it has been well established that moisture can act as a plasticizer and lower the T_g of amorphous glasses. Often the “Couchman–Karasz” relationship is used to predict the T_g of a mixture of compatible ingredients as a function of weight fractions and pure-compound properties of its constituents:(1)In eq , W₁ and W₂ are the mass fractions of components 1 and 2, respectively (e.g., water and polymer). The terms T_g1 and T_g2 represent the respective glass transition temperatures of these pure components and ΔC_p1 and ΔC_p2 stand for the change in heat capacity at the transition.

Nonequilibrium T_g Breadth

As discussed by Gunning et al. and Bohn et al.,during moisture sorption processes in amorphous carbohydrate particles, variations in local moisture content may develop. Hence, spatial glass transition temperatures may differ from “mean” or “average” glass transition temperatures for the entire object. As Meyers et al. argue, thermal methods, such as DSC, suffer from the limitation that they provide sample-averaged responses without information on local variations within the sample. Nevertheless, in this study we explore the use of DSC to determine spatial glass transition temperature distributions in nonequilibrated amorphous carbohydrate particles. As will be explained below, the approach will not truly measure a localized T_g. Instead, the range of glass transitions (or T_g breadth) that exists within the sample is measured.

Moisture and T_g profiles will be predicted using a numerical mass transfer simulation that divides a cylindrical microcapsule into finite ‘shells’ (see Figure ). When such a hypothetical equilibrated particle has no spatial moisture distribution, the distribution of glass transition temperatures is mainly governed by the breadth of the distribution of the components that make up the glassy particle. Therefore, the mass or volume average T_g equals the localized T_g. However, a nonequilibrated particle with a moisture content and T_g distribution from center to surface will have regions with a T_g above and below the mass average T_g. Furthermore, spherical or cylindrical samples have more volume or mass located near the surface. Hence, the volume contribution of each “shell” is incorporated to predict the overall glass transition distribution of the whole particle.

( Thies, C. In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; John Wiley & Sons, Inc.: New York, 2005; pp 438– 463 )

Glass Transition Temperature (OLD)

The glass transition temperature (Tg) is a key parameter in thermosetting polymers, not only from the product performance point of view, but also from the processing point of view, since it may strongly affect the reaction kinetics. The glass transition temperature marks the boundry bbetween the glassy, rigid state of a polymer and the soft, flexible (or fluid) state of the polymer. Below the glass tranition temperature, the available energy is insufficient to allow the molecules coordinated mobility (although there may be some localized motion), so the material is rigid; above the glass transition, the molecules can flow past each other above the glass transition temperature - the polymer is a "melt". In the case of thermoset polymers above the glass transition temperature, the chemical crosslinks prevent the molecules from flowing, but there is enough mobility for molecules to cooperatively relax, and the polymer becomes flexible and "rubbery".

(Cheng S.Z.D., Handbook of thermal analysis and calorimetry: applications to polymers and plastics, 2002, pg.315,316)