Determination of Tg

FIGURE 6.1 Specific volume (v) versus temperature for poly(vinyl acetate) [1], 

Tg characterizes the amorphous phase. Since all polymers have at least some amorphous material (they cannot be 100% crystalline), they all have a Tg, but not all polymers have a crystalline melting point, they cannot have if they do not crystallize (and many polymers will degrade before they melt). 

Transitions in polymers are rapidly and conveniently studied using differential scanning calorimetry (DSC) [3]. Small samples ( 10 mg) of the polymer and an inert reference 

FIGURE 6.2 Specific volume-temperature relations for linear polyethylene (Marlex-50). O Specimen slowly cooled from melt to room temperature prior to fusion. Specimen crystallized at 130 °C for 40 days, then cooled to room temperature prior to fusion [2], 

A DSC is programmed to heat the sample at a constant rate. The higher the rate, the quicker the measurement, a practically desirable result. Unfortunately, because polymer chains cannot respond instantaneously to the changing temperature, the measurement is further from equilibrium. The dependence of the measured Tg or 7) on heating rate is at least partially responsible for the range of values observed in the literature. To approach the true equilibrium values, very low heating rates should be used or, better yet, several heating 

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Dynamic mechanical techniques for studying polymers are described in detail in Chapter 7. For the moment we will restrict ourselves to a simple outline of the method of DMTA as it is applied to the determination of Tg. 

The characterization of block or graft copolymers is generally much more difficult than that of random copolymers (see Sect. 23.2.7). Especieilly, DSC measurements are useful for the characterization of the different segments (determination of Tg). Also dynamic-mechanical measurements are used to distinguish statistical copolymers from those with block or graft structure. 

For optical detection, the product to be determined must have a reasonable absorption coefficient. Saturation ofthe pyrimidine C(5)-C(6) bond destroys the chromophore, and for the determination of Tg yields excreted into the urine, this product was reduced with HI into Thy prior to an HPLC analysis (Cathcart et al. 1984). 

Factors that affect the determination of 8-oxo-G by this technique have been discussed in some detail (Moller et al. 1998). The determination of Tg by this technique (Hegi et al. 1989) is one of its most sensitive assays (Weinfeld and Soderlind 1991), many orders of magnitude higher than the earlier determination by HPLC (Frenkel et al. 1981). A 32P-postlabeling assay for the cA lesion which blocks gene... 

In the case of failure of a pipe made of polymeric material, the determination of Tg and 7m by thermochemical methods to ascertain the effects of thermal effects, determination of molecular weight and molecular weight distribution of the polymer, morphology of the sample or structure such as crystallinity and amorphous nature help to establish the cause of failure. 

After isothermal cure, temperature scans are conducted in order to measure the Tg after cure and Tg . However, due to thermal degradation, postcures can lead to lower glass transition temperatures than those obtained after cure. Thus, the determination of Tg , for high T, systems is a difficult problem. One approach is to establish a relationship between Tg and theoretical crosslink density for systems of lower Tgoc and similar chemical structure, and extrapolate to the system with higher crosslink density, thereby obtaining an estimate of Tg, ... 

Roos (1995) noted that the decrease in viscosity above Tg is responsible for various changes, such as stickiness and collapse of dried foods, agglomeration, and crystallization of food components (e.g., lactose in dried milk). In addition, the crispness of various low moisture foods is lost above Tg. Determination of Tg values as a function of solids or water content and water activity can be used to establish state... 

Temperature dependence of microhardness in polymer glasses determination of Tg... 

The degree of plasticization of a glassy polymer may be expressed as the depression of the glass transition temperature, which can be directly measured by DSC determinations of Tg on polymer samples containing known concentrations of penetrant. As an example, Figure 5 shows Tg as a function of volume fraction toluene in PVC, measured by DSC on polymer film samples equilibrated with toluene at varied activity ("41. These data, and similar results for acetone and benzene, show... 

In fact, the data, as well as the theoretical equation of state for polymer fluids, exhibit gradual curvature, but this does not have a substantial effect on the determination of Tg. 

One of the oldest methods for the determination of Tg involves the temperature dependence of viscosity. Figure 5-5 is a plot of the variation of viscosity with temperature for polystyrene. Various authors have argued that... 

Furthermore, in addition to the bulk thermal properties of polymers and resists, determination of Tg of film interfaces and of ultrathin films has become an important issue in thin film imaging (bilayer, 157 nm, and EUV). Various techniques have been employed, which include ellipsometry [481,482], positron annihilation spectroscopy (PALS) [483], QCM [484], scanning viscoelasticity microscope (SVM) [485],x-ray reflectivity [486,487], and thermal probe [488]. 

Determination of Tg using a DSC is based on a similar concept, with the replacement of the change in volume by the change in specific heat which also occurs at Tg. The specific heat of the material will change rather abruptly as the solid converts to the liquid. The onset of the change in specific heat is commonly used as an indicator of the temperature range of transition, just as the onset of the change... 

The presence of low MW carbohydrates in supports may contribute, via their amorphous state (T < Tg), to possible modification/deterioration of quality and functionality of powders linked with temperature and RH, especially during the spray-drying process and storage crystallization, stickiness, caking (Vega and Roos, 2006). The determination of Tg will be important in predicting the onset of deteriorative reactions. And the choice of supports (proteins adsorption) may help to modify the powder surface (Jayasundera et al., 2009). 

The conclusion was reached that solvents as well as non-solvents of the polymer are suitable in determination of Tg by gas chromatography. 

Reproducibility Duplicate determinations of TgS on specimens of the same sample analyzed in different laboratories should not differ by more than 4 "C [1]. 

Since DSC measures heat capacity directly, rapidly, and accurately, it is an ideal technique for the determination of Tg. The calorimeter accepts polymers in any form (powder, pellet, or fibre), and only a few milligrams of sample are required. Samples are placed in a standard aluminium sample pan, crimped by a crimping press to ensure good thermal contact, placed in the sample holder, and scanned at an appropriate rate over the temperature range of interest. 

DSC has been applied to the determination of Tg in a variety of other polymers including polyimines [1], polyurethanes (PU) [2], Novolac resins [3], polyisoprene, polybutadiene, polychloroprene, nitrile rubber, ethylene-propylene-diene terpolymer, and butyl rubber [4], and bisphenol-A epoxy diacrylate-trimethylolpropane triacrylate [1516]. 

TMA has been applied to the determination of Tg in neoprene, styrene-butadiene, polyisoprene, polybutadiene, polychloroprene, nitrile, ethylene-propylene-diene, and butyl rubbers [4]. 

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