Crystallinity first-order transitions

PTFE is known to undergo two crystalline first-order transitions at 19° and 30° and three viscoelastic relaxations have been observed. The temperature dependence of the relaxations (a,8,Y) as determined by dielectric and anelastic measurements is shown in the relaxation map of Figure 10 along with the temperatures of the crystalline phase transitions (vertical dotted lines) Also indicated in Figure 10 are the results of rotational diffusion rates obtained in this study. 

Figure 13.6 shows the influence of temperature on specific volume (reciprocal specific gravity). The exaet form of the eurve is somewhat dependent on the crystallinity and the rate of temperature change. A small transition is observed at about 19°C and a first order transition (melting) at about 327°C. Above this temperature the material does not exhibit true flow but is rubbery. A melt viseosity of 10 -10 poises has been measured at about 350°C. A slow rate of decomposition may be detected at the melting point and this increases with a further inerease in temperature. Processing temperatures, exeept possibly in the case of extrusion, are, however, rarely above 380°C. 

Phase transitions, whether first-order or second-order, are potent sources of instability of solid drugs and can usually be detected and studied by thermal methods of analysis (e.g., DSC, TGA, TMA, ODSC, DMA, DEA). In crystalline solids, typical first-order transitions are polymorphic or desolvation transitions. In amorphous solids, second-order transitions, such as glass transitions, are common. 

The LC nature of diols la-lg was demonstrated by DSC (Fig. 1). Two first order transitions were observed. The lower transition temperature appeared to be the crystalline-mesophase transition (T, ), and the higher transition temperature the mesophase-isotropic transition (T ). Other transitions were not evident in the DSC. The thermal data revealed an odd-even spacer effect for Tj . 

In contrast, diols 2a-2g and 3a-3g are not LC materials. Diols 2a-2g were shown to be crystalline by microscopy and by the existence of single first order transitions in the DSC. Diols 3a-3g appeared amorphous in the micrographs and had no first order transitions in the DSC. 

The first-order transition or melting point (Tm) is energywise larger than the Eg. Entirely crystalline polymers will have only a whereas a totally amorphous polymer will have only a Tg. Since most polymers are a combination of amorphous and crystalline regions, they have both a Tg and a T -... 

Both kinetic and thermodynamic approaches have been used to measure and explain the abrupt change in properties as a polymer changes from a glassy to a leathery state. These involve the coefficient of expansion, the compressibility, the index of refraction, and the specific heat values. In the thermodynamic approach used by Gibbs and DiMarzio, the process is considered to be related to conformational entropy changes with temperature and is related to a second-order transition. There is also an abrupt change from the solid crystalline to the liquid state at the first-order transition or melting point Tm. 

The T of crystalline polymers may be determined by observing the first-order transition (change in heat capacity value) by DTA or by DSC (ASTM-D3418). Some comparative information on thermal properties of polyolefins may be obtained from the melt index. To determine the melt index, the weight of extrudate or strand under a specified load and at a specified temperature is measured. Melt index values are inversely related to the melt viscosity. 

The most common applications of DSC are to the melting process which, in principle, contains information on both the quality (temperature) and the quantity (peak area) of crystallinity in a polymer [3]. The property changes at Tm are often far more dramatic than those at Tg, particularly if the polymer is highly crystalline. These changes are characteristic of a thermodynamic first-order transition and include a heat of fusion and discontinuous changes in heat capacity, volume or density, refractive index, birefringence, and transparency [3,8], All of these may be used to determine Tm [8],... 

We note here that gel is a coherent solid because its structure is characterized by a polymer network, and hence, the above theoretical considerations on crystalline alloys should be applicable to gels without essential alteration. It is expected that the curious features of the first-order transition of NIPA gels will be explained within the concept of the coherent phase equilibrium if the proper calculation of the coherent energy and the elastic energy of the gel network is made. This may be one of the most interesting unsolved problems related to the phase transitions of gels. 

Poly (diethyl siloxane) was suggested by Beatty et al. 1651 based on DSC, dielectric, NMR, and X-ray measurements to possess liquid crystalline type order between about 270 and 300 K. The macromolecule shows two large lower temperature first order transitions, one at about 200 K, the other at about 270 K166 ll,7). The transition of the possible mesophase to the isotropic liquid at 300 K is quite small and irre-producible, so that variable, partial crystallinity was proposed 165) [measured heat of transition about 150 J/mole1S8)], Very little can be said about this state which may even consist of residual crystals. It is of interest, however, to further analyze the high temperature crystal phase between 200 and 270 K. It is produced from the, most likely, fully ordered crystal with an estimated heat and entropy of transition of 5.62kJ/mol and 28J/(Kmol), respectively [calculated from calorimetric data 1S6)... 

Raman spectra for the sample were conducted in a compression-decompression cycle. In this experiment, the crystalline diffraction began to disappear above 7-8 GPa during compression, and pressure-induced amorphization was indicated by the Raman spectra above 13 GPa (Fig. 14). The resultant HDA Si exhibits the Raman spectrum that differs from the spectrum of normal -Si (LDA Si). Rather, the characteristics of the spectrum for HDA Si resemble those of the (3-tin crystal, which indicates that HDA Si has a (locally) analogous structure to the (3-tin structure. The synthesis of the HDA form of Si by Deb et al. [263] has a strong resemblance to that of water (ice) by Mishima et al. [149, 196]. Whereas compression induced amorphization that was almost completed at 13-15 GPa, decompression induced an HDA-LDA transition below 10 GPa, which is clearly shown in the Raman spectra (Fig. 14). This is the first direct observation of an amorphous-amorphous transition in Si. The spectrum at 0 GPa after the pressure release exhibits the characteristic bands of tetrahedrally coordinated -Si (LDA Si). Based on their experimental findings Deb et al. [263] discussed the possible existence of liquid-liquid transition in Si by invoking a bond-excitation model [258, 259]. They have predicted a first-order transition between high-density liquid (HDL) and low-density liquid... 

First order transition temperature in crystalline polymers. J. Appl. Phys. 34, 2442—2445 (1963). 

Secondary crystalline transitions (below Tm) occur if the material transforms from one type of crystal to another. These transitions are, like the melting point, thermodynamic first-order transitions. 

Crystallinity can be measured using the same thermal and dynamic mechanical methods described for measuring T however, the melting transition is much sharper than the Tg because it is a first-order transition compared with second-order for the T. 

In Raman scattering, the excitation light couples to changes in the polarizability and first order transitions are allowed in crystalline silicon. (See Lannin (1984) for a discussion of Raman scattering in amorphous silicon.) The scattering intensity, as a function of the phonon frequency, co, is approximately... 

PTFE resins exhibit a first-order transition at 19°C (66°F) due to a change of crystalline structure from triclinic to hexagonal unit cell (see Chapter 3, Section 3.2.1.3). A volume change of approximately 1% is associated with this transition (Figure 4.3). Another consequence is that the resin has a better powder flow below 19°C but responds more poorly to preform pressure. Billets prepared below this transition are weaker and tend to crack during sintering. For this reason, the resin should... 

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