Glass, transition measurement methods

The Tg determination, while not a thermodynamic criteria for misdbility, is an easy and effective method for determination of the phase behavior. The author, having conducted literally thousands of these measurements (primarily by dynamical mechanical methods), has found this to be a viable screening method that is highly reliable. In the borderline cases (xu 0), the experimental protocol is critical and with proper consideration of the experimental details, exceptions to these observations are rare. In the cases of specific interactions xn < 0), the literature is even clearer as glass transition measurements agree well with the results of other characterization methods, including those which meet the criteria of thermodynamic misdbility. 

Solution Polymers. Acryflc solution polymers are usually characterized by their composition, solids content, viscosity, molecular weight, glass-transition temperature, and solvent. The compositions of acryflc polymers are most readily determined by physicochemical methods such as spectroscopy, pyrolytic gas—liquid chromatography, and refractive index measurements (97,158). The solids content of acryflc polymers is determined by dilution followed by solvent evaporation to constant weight. Viscosities are most conveniently determined with a Brookfield viscometer, molecular weight by intrinsic viscosity (158), and glass-transition temperature by calorimetry. 

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. 

As-polymerized PVDC does not have a well-defined glass-transition temperature because of its high crystallinity. However, a sample can be melted at 210°C and quenched rapidly to an amorphous state at <—20°C. The amorphous polymer has a glass-transition temperature of — 17°C as shown by dilatometry (70). Glass-transition temperature values of —19 to — 11°C, depending on both method of measurement and sample preparation, have been determined. 

Two crystalline forms have been observed.One is formed by slow cooling from the melt and the other by slow heating of the amorphous polymer. The properties of the commercial products were therefore to some extent dependent on their heat history. Glass transition temperatures observed range from 7 to 32°C and depend on the time scale of the method of measurement. ... 

The method (27) can best be explained with reference to Figure 2. After stretching to 10, the force f is measured as a function of time. The strain is kept constant throughout the entire experiment. At a certain time, the sample is quenched to a temperature well below the glass-transition temperature, Tg, and cross-linked. Then the temperature is raised to the relaxation temperature, and the equilibrium force is determined. A direct comparison of the equilibrium force to the non-equilibrium stress-relaxation force can then be made. The experimental set-up is shown in Figure 4. 

In contrast to the mature instrumental techniques discussed above, a hitherto nonexistent class of techniques will require substantial development effort. The new instruments will be capable of measuring the thermal (e.g., glass transition temperatures for amorphous or semicrystalline polymers and melting temperatures for materials in the crystalline phase), chemical, and mechanical (e.g., viscoelastic) properties of nanoscale films in confined geometries, and their creation will require rethinking of conventional methods that are used for bulk measurements. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

The objectives of this review are to discuss the fundamental and more recently discovered properties of water alone and to critically examine the system properties and measurement methods used to measure the mobility of water and solids in foods—specifically water activity, nuclear magnetic resonance (NMR), and the glass transition. 

These polymerization methods were carried out to produce small quantities of polymers for DSC and GPC analysis. These analyses were used to measure the efficiency of the polymerization and the glass transition temperature of the resulting polymers, as a screen for further development of selected materials. It was deemed at the time of the tests that a high glass transition temperature was... 

The measurements of Young s modulus in dependence of the temperature (dynamic-mechanical measurements, see Sect. 2.3.5.2) and the differential thermal analysis (DTA or DSC) are the most frequently used methods for determination of the glass transition temperature. In Table 2.10 are listed and values for several amorphous and crystalline polymers. 

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