Melting temperature, polymer crystal

Crystalline polyimide powders, 304 Crystalline transition temperature. See Melting temperature (Tm) Crystallization rate, for processing semicrystalline polymers, 44 CTE. See Coefficient of thermal expansion (CTE)... 

Temperature has a complex effect on crystallization rate. Initially, as the temperature falls below the equilibrium melting temperature, the crystallization rate increases because nucleation is favored. However, as the temperature continues to fall, the polymer s viscosity increases, which hampers crystallization. As a rule of thumb, a polymer crystallizes fastest at a temperature approximately mid-way between its glass transition temperature and its equilibrium melting temperature. 

Fig. 4.8.7. Typical DSC curves of a crystalline polymer melting temperature (T, ), crystallization temperature (7V), glass transition temperature (Tg), and cold crystallization temperature (T )...

Whether the polymer is totally amorphous or partially crystalline, the material will be glassy (hrittle) or ruhher-like (soft) depending on its temperature with respect to Tg. If an amorphous polymer is at a temperature helow Tg, it will be brittle and will show properties of a glassy material for example, it will fracture more easily. As the temperature of the sample increases and approaches Tg, it adopts a leathery behavior and its elastic modulus decreases. When the sample has reached several degrees above Tg, it shows a clear rubbery behavior and is easily deformable. If the temperature is increased even more, the polymer reaches liquid flow behavior. If the polymer is semicrystalline, it exhibits similar behavior, but when it reaches the melting temperature the crystals will break up, and the polymer will then reach the melted liquid state. This behavior is illustrated in Fig. 3.45 where the elastic modulus is plotted versus temperature. 

Much like polymers, organic chemicals have their own set of properties that can be used to provide quality control techniques. These properties can be very similar to those used in polymer systems and can include glass transition temperatures, melting (temperature, endialpy), crystallization (temperature, enthalpy), decomposition (thermal, oxidative), reaction (widi various other materials) and more. Thus, the techniques that could be used for quality control of starting materials, process intermediates and final products are similar to those used for polymers. 

Figure 3.14 Change in melting temperature with crystal (lamellar) thickness S. Kavesh, J.M Schultz, J. Polymer Sci. 1971, 9A, 285.

Figure 1. When a liquid polymer is cooled to a temperature below its melting temperature then crystallizing particles (spherulites) appear at random in the liquid and grow with time. Notice impinging (overlapping) particles and the gradual exclusion of liquid volume for the appearance of new nuclei. The release rate of latent heat of crystallization must be less than the rate of heat removal from the polymer for the growth to continue.

Okazaki I, Wunderlich, B (1997) Reversible Melting in Polymer Crystals Delected by Temperature Modulated Differential Scanning Calorimetry. Macromolecules 30 1758-1764. 

Why does the melting of polymer crystals exhibit a wide temperature range ... 

The melting temperatures and crystallization temperatures were measured by the differential scanning calorimetry (DSC) data obtained for the pristine P3HT and P3HT-MWNT nanocomposites and purified MWNTs (Table 12.1). The melting temperatures and crystallization temperatures of the composites also confirm that the composites prepared with low MWNT loading are more thermally stable. The probability of the P3HT interaction with the walls of more thermally stable CNTs is more via the main chain instead of side chains. The probabiUty of this interaction decreases as the MWNT content is further increased in the polymer matrix because now the probabiUty of the wall-wall interaction is more than that of the waU-chain interaction. 

Standard Method for Evaluation of Temperature Scale for Differential Thermal Analysis Standard Practice for Evaluating Temperature Scale for Thermogravimetry Standard Test Method for Transition Temperatures of Polymers by Thermal Analysis Standard Test Method for Melting Temperatures and Crystallization Temperatures by Thermal Analysis... 

For all polymers there is a maximum in the rate of crystallization versus quench temperature (Figure 2). As we decrease the quench temperature down from the melt temperature (Tm), crystallization rates continue to become faster until we reach a critical temperature where molecular mobility needed to achieve crystallization becomes the more important factor - and crystallization rate begins to decline. This temperature (Tk), is approximately 180 C for PET [8],[9] and can be related... 

Despite this inability to define a rmique melting temperature for a given sample, it is possible to define an equilibrium melting point, 7. This is the melting temperature for an infinitely thick crystal. In principle, this can be obtained from extrapolation of a plot of measured Tm values versus crystal thickness, 4. However, the latter is difficult to measure so it is more usual to plot melting temperature versus crystallization temperature. Observed melting temperatures for polymers are always... 

Figure 2.25 Schematic of melting temperature versus crystallization temperature for a polymer. The equilibrium melting temperature, 7, is obtained by extrapolation to the line 71 = X...

Temperature-modulated differential scanning calorimetry (T-MDSC) applies a thermal modulation in temperature to a conventional DSC mn and determines a dynamic heat capacity from the relationship between the modulation components of temperature and of heat flow. Primary application of this technique has been the measurement of specific heat capacity and the examination of the anomaly in a relaxation process such as alpha process related to the glass transition. An application to the first-order phase transitions of crystallisation and melting of polymer crystals has recently been suggested. The method and typical results are described. 13 refs. 

Polymer crystallization is also very sensitive to molecular orientation in the amorphous regions. Orientation affects the entropy and enthalpy of fusion, the nucleation rate, and so on, but the mathematics of the problem goes beyond the scope of the present textbook. Instead, we use Ziabicki s (1976) idea that any function of molecular orientation, X (/am), that is, melting temperature Tm, crystallization rate K, free energy AF, and so on, can be expanded as a series ... 

Other crystallization parameters have been determined for some of the polymers. The dependence of the melting temperature on the crystallization temperature for the orthorhombic form of POX (T = 323K) and both monoclinic (T = 348K) and orthorhombic (T = 329K) modifications of PDMOX has been determined (284). The enthalpy of fusion, Aff, for the same polymers has been determined by the polymer diluent method and by calorimetry at different levels of crystallinity (284). for POX was found to be 150.9 J/g (36.1 cal/g) for the dimethyl derivative, it ranged from 85.6 to 107.0 J/g (20.5—25.6 cal/g). Numerous crystal stmcture studies have been made (285—292). Isothermal crystallization rates of POX from the melt have been determined from 19 to —50 C (293,294). Similar studies have been made for PDMOX from 22 to 44°C (295,296). 

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