Glass transition temperature analysis

Glass-transition temperatures are commonly determined by differential scanning calorimetry or dynamic mechanical analysis. Many reported values have been measured by dilatometric methods however, methods based on the torsional pendulum, strain gauge, and refractivity also give results which are ia good agreement. Vicat temperature and britde poiat yield only approximate transition temperature values but are useful because of the simplicity of measurement. The reported T values for a large number of polymers may be found ia References 5, 6, 12, and 13. 

Polycarbonate—polyester blends were introduced in 1980, and have steadily increased sales to a volume of about 70,000 t. This blend, which is used on exterior parts for the automotive industry, accounting for 85% of the volume, combines the toughness and impact strength of polycarbonate with the crystallinity and inherent solvent resistance of PBT, PET, and other polyesters. Although not quite miscible, polycarbonate and PBT form a fine-grained blend, which upon analysis shows the glass-transition temperature of the polycarbonate and the melting point of the polyester. 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

The dynamic mechanical properties of VDC—VC copolymers have been studied in detail. The incorporation of VC units in the polymer results in a drop in dynamic modulus because of the reduction in crystallinity. However, the glass-transition temperature is raised therefore, the softening effect observed at room temperature is accompanied by increased brittleness at lower temperatures. These copolymers are normally plasticized in order to avoid this. Small amounts of plasticizer (2—10 wt %) depress T significantly without loss of strength at room temperature. At higher levels of VC, the T of the copolymer is above room temperature and the modulus rises again. A minimum in modulus or maximum in softness is usually observed in copolymers in which T is above room temperature. A thermomechanical analysis of VDC—AN (acrylonitrile) and VDC—MMA (methyl methacrylate) copolymer systems shows a minimum in softening point at 79.4 and 68.1 mol % VDC, respectively (86). 

The thermal glass-transition temperatures of poly(vinyl acetal)s can be determined by dynamic mechanical analysis, differential scanning calorimetry, and nmr techniques (31). The thermal glass-transition temperature of poly(vinyl acetal) resins prepared from aliphatic aldehydes can be estimated from empirical relationships such as equation 1 where OH and OAc are the weight percent of vinyl alcohol and vinyl acetate units and C is the number of carbons in the chain derived from the aldehyde. The symbols with subscripts are the corresponding values for a standard (s) resin with known parameters (32). The formula accurately predicts that resin T increases as vinyl alcohol content increases, and decreases as vinyl acetate content and aldehyde carbon chain length increases. 

The advantage of the simulations compared to the experiments is that the correspondence between the tracer diffusion coefficient and the internal states of the chains can be investigated without additional assumptions. In order to perform a more complete analysis of the data one has to look at the quench-rate and chain-length dependence of the glass transition temperature for a given density [43]. A detailed discussion of these effects is far beyond the scope of this review. Here we just want to discuss a characteristic quantity which one can analyze in this context. 

The glass transition temperatures (Tg) of both modified and unmodified PSs were determined by DSC analysis, and thermomechanic analysis was controlled by TMK. The results are given in Table 8. It is seen from Table 8 that the highest glass transition temperature (410 K) was obtained with chlorohydrinated PS and that of the lowest (370 K) with olefinic PS. The lowest glass transition temperature in the alkenylated PS caused to elasticity properties on polybutadien and polyisopren fragments. 

The modified NBR samples were characterized by differential scanning calorimetry [11,78-80,98]. The glass-transition temperature (T ) decreased with the level of hydrogenation. In the case of HFNBR, Tg increased with an increase in the addition of aldehyde groups to the polymer chain. Thermogravimetric analysis of the modified polymers have also been carried out [15]. 

DSC helps in determining the glass-transition temperature, vulcanization, and oxidative stability. TG mainly is applied for the quantitative determination of major components of a polymer sample. TMA or DLTMA (dynamic load thermomechanical analysis) measures the elastic properties viz. modulus. 

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. 

Glass transition temperature (Tg), measured by means of dynamic mechanical analysis (DMA) of E-plastomers has been measured in binary blends of iPP and E-plastomer. These studies indicate some depression in the Tg in the binary, but incompatible, blends compared to the Tg of the corresponding neat E-plastomer. This is attributed to thermally induced internal stress resulting from differential volume contraction of the two phases during cooling from the melt. The temperature dependence of the specific volume of the blend components was determined by PVT measurement of temperatures between 30°C and 270°C and extrapolated to the elastomer Tg at —50°C. 

Another example involved a batch of isocyanate crosslinker which was too tacky. Upon comparing the HPGPC trace of this sample with that of a control as shown in Figure 9, it is seen that the major difference between these two samples was the level of free caprolactam. The high content of free caprolactam in sample CX-006 depressed the glass transition temperature (Tg) of the sample to such an extent that CX-006 became too tacky. This method of analysis has proved to be a reliable and useful technique for detecting low levels of free caprolactam in this type of oligomeric crosslinker. 

Both thermogravimetric analysis and differential scanning calorimetric studies were carried out on modified and unmodified PPO samples. Table IV presents the weight losses and the glass transition temperatures of the most representative polymers. 

Thermal Properties. The glass transition temperature (Tg) and the decomposition temperature (Td) were measured with a DuPont 910 Differential Scanning Calorimeter (DSC) calibrated with indium. The standard heating rate for all polymers was 10 °C/min. Thermogravimetric analysis (TGA) was performed on a DuPont 951 Thermogravimetric Analyzer at a heating rate of 20 °C/min. 

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