* why does the glass treanition for
Polymer
yield
no exothermic
endothermie Peak?
Answers
Answer:
Ok sure mate
Explanation:
Glass transition is typically detected by observing changes in dielectric (dielectric constant), mechanical (modulus, viscosity), and thermodynamic (enthalpy, free volume, heat capacity, thermal expansion coefficient) properties of amorphous materials (White and Cakebread, 1966; Wunderlich, 1981; Sperling, 1992; Roos, 2008). One individual amorphous material can have an indefinite number of solid, glassy states and corresponding relaxations can be observed at the solid–liquid change in state around the glass transition. Changes in material properties around the glass transition are time-dependent and may differ depending on frequency, temperature, and time-scale of experiments (Roos, 1995). Hence, glass transition is a time-dependent change of non-equilibrium states which can be characterized by the dramatic change in relaxation times around the transition (Slade and Levine, 1995).
The most common method for observing a glass transition is differential scanning calorimetry (DSC). DSC techniques can be used to detect the change in heat capacity around the glass transition (e.g., Wunderlich, 1981; Roos, 1995). The glass transition temperature, Tg, is a temperature value characteristic to the temperature range of the changes in thermodynamic material properties. As shown in Figure 8.1, glass transition is often associated with enthalpy changes resulting from the non-equilibrium characteristics of the glassy state and freezing of glass-forming molecules at various thermodynamic states in the vitrification process (Roos, 2008). DSC scans are commonly repeated to observe and confirm heat capacity changes apart from endothermal and exothermal changes around the glass transition (Roos, 1995, 2008). Glass-transition-associated changes in relaxation times can be derived from material responses to dielectric and mechanical perturbations. Relaxations can be observed by dynamic mechanical thermal analysis (DMA/DMTA) and mechanical spectroscopy which measure the effects of a sinusoidally varying stress on dynamic moduli (Roos, 2008). Changes in dielectric properties can be detected with dielectric thermal analysis (Laaksonen et al., 2002; Roos, 2008). Other important techniques in observing glass transition from changes in molecular mobility and diffusion are electron spin resonance spectroscopy (ESR) and nuclear magnetic resonance (NMR) spectroscopic methods. Fourier transfer infra-red (FTIR) and Raman spectroscopies may also be used to observe changes in molecular bonding occurring in amorphous systems over the glass transition (Söderholm et al., 1998).
Although novel thermal analytical and spectroscopic techniques allow detection of the glass transition, relaxations and molecular mobility associated with the transition, numerous empirical methods are commonly used to observe changes in flow resulting from the change in molecular mobility and the state of the material around the glass transition. Examples of empirical measurements of solid to liquid changes in foods are the observation of sticky points of powders (Lazar et al., 1956) and measurements of collapse in frozen (Bellows and King, 1971) and dehydrated systems (To and Flink, 1978a, 1978b, 1978c). The state diagrams with glass transitions and relaxations data have advanced understanding of the relationships of the various methods. The state diagrams also explain differences in various time-dependent characteristics of materials associated with empirical observations of material properties.
Explanation:
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