Thermal transitions flow transition temperature

T and are the glass-transition temperatures in K of the homopolymers and are the weight fractions of the comonomers (49). Because the glass-transition temperature is directly related to many other material properties, changes in T by copolymerization cause changes in other properties too. Polymer properties that depend on the glass-transition temperature include physical state, rate of thermal expansion, thermal properties, torsional modulus, refractive index, dissipation factor, brittle impact resistance, flow and heat distortion properties, and minimum film-forming temperature of polymer latex... 

The heat flow into (endothermic) or out (exothermic) of a sample as a function of temperature and time is measured using the technique of DSC. In particular, it is used to study and determine the temperature of thermal transitions. For polymers, these include Tg, the glass transition temperature, Tc, the (exothermic) temperature of crystallisation for polymers that can crystallise, and Tm, the (endothermic) melting temperature. A DSC measurement requires only a small amount of sample 2-20 mg of a film, powder, fibre or liquid samples can be analysed in a DSC pan. 

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

Aj3th is the jump of the linear thermal expansion coefficient at the glass transition temperature. As Fig. 1 lb shows, da is proportional to the rate (1 — a) of the flowing units, that is to the molecular arrangements, which can form some confer-... 

It would be inappropriate to refer to an object as having heat . Rather it would be stated that it is at a certain temperature or has a certain thermal energy. Heat is thermal energy in transit heat flows across a boundary. If two objects at different temperatures are placed in thermal contact, they will, with time, reach a third equal temperature as a result of heat flowing from the higher temperature object to the colder one. 

The reference material is chosen to be one that exhibits no thermally induced transitions within the temperature range of interest. Thus, transitions that occur in the sample during the applied temperature programme appear as peaks or troughs, depending on whether they are exothermic or endothermic, on the plot of differential heat flow versus temperature (the thermogram) that is the output of the instrument (Figure 22.1). Commonly used reference materials are listed by Hatakeyama and Quinn (1994). 

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