Amorphous melting temperature

Melting temperature, °C Crystalline Amorphous Specific gravity Water absorption (24 h), % Dielectric strength, kV mm ... 

PVF displays several transitions below the melting temperature. The measured transition temperatures vary with the technique used for measurement. T (L) (lower) occurs at —15 to —20 " C and is ascribed to relaxation free from restraint by crystallites. T (U) (upper) is in the 40 to 50°C range and is associated with amorphous regions under restraint by crystallites (63). Another transition at —80° C has been ascribed to short-chain amorphous relaxation and one at 150°C associated with premelting intracrystalline relaxation. 

Eig. 15. Time—temperature transformation ia a thin-phase change layer during recording/reading/erasiug (3,105). C = Crystalline phase A = amorphous phase = melting temperature = glass-transition temperature RT = room temperature. 

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

The properties of PVDC (Table 3) ate usually modified by copolymerization. Copolymers of high VDC content have lower melting temperatures than PVDC. Copolymers containing mote than mol % acrylate or methacrylate ate amorphous. Substantially mote acrylonitrile (25%) or vinyl chloride (45%) is required to destroy crystallinity completely. 

The term glass has two meanings, ie, the material and a state of matter. The glassy or vitreous condition is where the atoms of the material have a random orientation. This amorphous or noncrystalline nature leads to physical properties typical of the product caHed glass, including unpredictable breaks, no sharp melting temperature, and no heat of fusion. 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by ... 

Fig. 7-17 Modulus behavior of amorphous and crystalline plastics showing Tg and melt temperatures.

Fig. 12. Free energy vs. temperature for FCC-cttrve /, for ECC-curve II and for the amorphous state (melt)-cur-ve III. Points 1 and 2 represent the melting temperatures of FCC and ECC, respectively, point 3 separates the ranges of the predominant existence of FCC and ECC (see text)...

Each crystallizable polymer exhibits a characteristic equilibrium melting temperature, at which the crystalline and amorphous states are in equilibrium. Above this temperature crystallites melt. Below this temperature a molten polymer begins to crystallize. 

Differential Thermal Analysis (DTA). One of the characteristics of a rubber useful in tire rubber compounds is that it is amorphous at room temperature but readily undergoes strain induced crystallization. For this reason, copolymers were prepared in order to appropriately adjust the crystalline melt temperature. 

The dynamic mechanical behavior indicates that the glass transition of the rubbery block is basically independent of the butadiene content. Moreover, the melting temperature of the semicrystalline HB block does not show any dependence on composition or architecture of the block copolymer. The above findings combined with the observation of the linear additivity of density and heat of fusion of the block copolymers as a function of composition support the fact that there is a good phase separation of the HI and HB amorphous phases in the solid state of these block copolymers. Future investigations will focus attention on characterizing the melt state of these systems to note if homogeneity exists above Tm. 

An X-ray diffraction study of PHA-10UND= showed that this PHA was amorphous however, the DSC thermogram of the polymer showed a small but clear endotherm at approximately 38 °C. The glass transition temperature of PHA-10UND= is approximately - 46 °C which is significantly lower than those of the poly(nHAMCL)s. Both the melting temperature and the glass transition tempe-... 

Most PHAs are partially crystalline polymers and therefore their thermal and mechanical properties are usually represented in terms of the glass-to-rubber transition temperature (Tg) of the amorphous phase and the melting temperature (Tm) of the crystalline phase of the material [55]. The melting temperature and glass transition temperature of several saturated and unsaturated PHAs have been summarized in Table 2. 

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