Glass transition temperature from expansion coefficient

Two resin systems based on this chemical concept are commercially available from Shell Chemical Company/Technochemie under the COMPIMIDE trademark COMPIMIDE 183 (34) [98723-11-2], for use in printed circuit boards, and COMPIMIDE 796 [106856-59-1], as a resin for low pressure autoclave mol ding (35). Typical properties of COMPIMIDE 183 glass fabric—PCB laminates are provided in Table 8. COMPIMIDE 183 offers a combination of advantageous properties, such as a high glass transition temperature, low expansion coefficient, and flame resistance without bromine compound additives. 

FIGURE 10 Glass transition temperature from change of thermal expansion coefficient in isobaric V-T diagram for random copolymer poly(ethylene-co-vinyl alcohol) with 44 mol% of ethylene, data after Funke et al. (2007). 

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

The volume coefficient of expansion of Teflon AF is linear with temperature and quite low. The coefficients are 280 ppm/°C and 300 ppm/°C for AF-1600 and AF-2400, respectively. Above the glass transition temperature these values increase sharply. Thermal conductivity is quite low, increasing from only 0.05W/mK at 40°C to 0.2 W/mK at 260°C. Many of these properties are believed to be related to the very low (1.7-1.8 g/ml) densities of these dioxole... 

While physicochemical and spectroscopic techniques elucidate valuable physical and structural information, thermal analysis techniques offer an additional approach to characterize NOM with respect to thermal stability, thermal transitions, and even interactions with solvents. Information such as thermal degradation temperature (or peak temperature), glass transition temperature, heat capacity, thermal expansion coefficient, and enthalpy can be readily obtained from thermal analysis these properties, when correlated with structural information, may serve to provide additional insights into NOM s environmental reactivity. 

The kink observed around 367 K corresponds to a change of the thermal expansion coefficient from a glassy to a liquid-like state and, by that, marks the position of the glass transition temperature. Usually, the 7g is calculated as a intersection point between two linear dependencies. Nevertheless, a more convenient method is the calculation of the first and second numerical derivatives of the experimental data (Fig. 15b,c). In this case, the Tg is defined as the minimum position in the second numerical derivative plot (Fig. 15c). Down to a thickness of 20 nm, no shifts of 7g as determined by capacitive scanning dilatometry were found (Fig. 16). 

When an amorphous polymer is gradually cooled from above the glass transition temperature Tg its volume decreases (see Fig. 13.32) according to its thermal expansion coefficient aj. In the region around the Tg the volume decrease will lag behind, starting at temperature Tel because the rate of reorganisation process becomes too small. The polymer starts to vitrify and a temperature Tel will be reached where the reorganisation completely stops and where the vitrification process is completed. Decrease of volume is only the result of normal volume contraction with expansion coefficient ag. The relationship between both thermal expansion coefficients is... 

Another explanation for an abnormal increase in Tgl in polymer blends has been proposed by Manabe, Murakami, and Takayanagi 125). They used a three-layered shell model, which accounts for interaction between the dispsersed and continuous phases of the blend. Abnormal increases in the glass transition of polystyrene in blends with various rubbers were explained by thermal stresses which arise from the difference in thermal expansion coefficients of the component polymers. However shifts in the glass transition temperatures of the SIN s do not appear to arise from differences in the expansion coefficients of the components because samples with the same overall composition and almost identical microstructures have significantly different glass transition temperatures. 

The glass transition temperature can be determined from microscopic (e.g., relaxation time of fluctuating dipoles) or macroscopic (e.g., thermal expansion coefficient) quantities. Do these different approaches necessarily yield coinciding results ... 

Fig. 1. Isobaric relationship between volume and temperature in the liquid, glassy, and crystalline states. Fm is the melting temperature, and Fga and Fgb are the glass transition temperatures corresponding to slow (a) and fast (b) cooling rates. The lower diagram shows the behavior of the thermal expansion coefficient corresponding to curve b. (From Debenedetti, 1996.)...

At the glass transition temperature (Tg), a thermoplastic material changes from a glassy state to a rubbery state. The properties of the material also change significantly. Tg values most often listed for polymers correspond to stiffening temperatures [3], The coefficient of thermal expansion usually doubles below Tg for these materials. Materials above the 7 , may be functional, but the performance may become unpredictable because most thermoplastic components are designed based on properties tested below 7 ,. 

The transition between the viscous state and the solid state is characterized by a temperature Tg called glass transition temperature. This temperature corresponds to a discontinuity of physical parameters and in particular of the thermal expansion coefficient. The discontinuity observed when T decreases seems to result from the freezing, for T < Tt, of rotation and of large motions of chain elements. 

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