Glass Transition dilatometric

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. 

Figure 4.4 shows a dilatometric or calorimetric experiment to show structural relaxation (physical aging) and glass transition hysteresis. The sample is cooled from T0 to T it is kept at Tj for a certain time and heated again to T0. During the cooling step, the material vitrifies at B, resulting in an abrupt decrease in both the expansion coefficient and the specific heat. 

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

Figure 35. Glass transition temperature 7., (determined by dilatometry) and relaxation time r at 131°C as a function of annealing time in air at 180°C for a film thickness of 63 nm. The dotted lines serve as a guide for the reader. Inset Dilatometric determination of the glass transition temperature. Upper. Normalized capacitance Cn0nn versus temperature at 106Hz (the solid lines represent linear dependencies, the dotted line marks the position of the glass transition temperature). Lower. The corresponding first and second numerical derivatives of Cnonn (in arbitrary units) as a function of temperature.

BRI Briatico-Vangosa, F. and Rink, M., Dilatometric behavior and glass transition in a styrene-acrylonitrile copolymer, J. Polym. Sci. Part B Polym. Phys., 43, 1904, 2005. 

The glass transition temperature was determined using the dilatometric method. Volume dependence on temperature was measured on a rising temperature scale with a heating rate of 2° K/min. The films were carefully degassed at room temperature before filling the dilatometer with mercury. 

There are various methods of the glass transition temperature evaluation based on temperature dependence of polymer physical properties in the interval of glass transition 1) specific volume of polymer at slow cooling (dilatometric method) 2) heat capacity (calorimetric method),3) refraction index (refractometric method) 4) mechanical properties 5) electrical properties (temperature dependence of electric conductivity) or maximum of dielectric loss 6) NMR ° 7) electronic paramagnetic resonance, etc. 

Glass Transition. The acoustic properties of polymers, when plotted over broad ranges of frequency and temperature, are usually dominated by the glass transition. Typical data are shown in Figures 1 (49), and 2 (50). The change in slope of the soimd speed at about —40°C is independent of frequency and is eqnal to the dilatometric value of Tg. This method of determining Tg has been applied to a number of polymers (51-53). The change in slope at —40°C occurs as a result of... 

All polymers have a glass transition temperature. These can be determined experimentally from dilatometric data (see Fig. 1-5). Here the inflection point... 

Following this empirical discovery there was naturally some speculation as to whether the WLF equation has a ftindamental interpretation. To proceed further we must consider the dilatometric glass transition, and its interpretation in terms of free volume. 

Dilatometric data agree well with modulus-temperature studies, especially if the heating rates and/or length of times between measurements are controlled. (Raising the temperature l°C/min roughly corresponds to a 10 s mechanical measurement.) Besides being a direct measure of Tg, dilatometric studies provide free volume information, of use in theoretical studies of the glass transition phenomenon (see Section 8.6.1). 

The glass transition temperature can be determined readily only by observing the temperature at which a significant change takes place in a specific electric, mechanical, or other physical property. Moreover, the observed temperature can vary significantly, depending on the specific property chosen for observation and on details of the experimental technique (for example, the rate of heating, or frequency). Therefore, the observed Tg should be considered to be only an estimate. Ifhe most reliable estimates are normally obtained from the loss peak observed in dynamic mechanical tests or from dilatometric data (ASTM D-20). 

Tensile strength and elongation properties of the vinyl acetate copolymer have been briefly examined/ " The glass transition temperature (Tg) was determined to be 100-104°C, by using a volume dilatometric procedure/ The amorphous, hydroscopic copolymer is soluble in alkali, water, alcohols, acetone, THF, and Paper chromatographic methods have been... 

This has led to the view that the WLF equation should be related at molecular level to the temperature J = Tg - 51.6, which we will call Ti, rather than to the dilatometric glass transition Tg. 

The glass transition temperature (Tg) of blends of fully amorphous polymers is frequently measured to assess miscibility. There are a number of experimental techniques calorimetric (DSC/DTA), dilatometric, dynamic mechanical and dielectric. The resolution of the Tg methods has been the object of some discussion and the proposed resolution limit for domains should range from 2 to 15 nm. The presence of two Tg s in a binary system indicates phase separation on a level greater than this minimum domain size. Figure 4.16 shows schematically the recorded glass transition temperature(s) as a function of composition for case a (complete miscibility), case b (partial miscibility) and case c (complete immiscibility). 

---