Thermal transitions melting temperature

Relatively few processible polyimides, particularly at a reasonable cost and iu rehable supply, are available commercially. Users of polyimides may have to produce iutractable polyimides by themselves in situ according to methods discussed earlier, or synthesize polyimides of unique compositions iu order to meet property requirements such as thermal and thermoxidative stabilities, mechanical and electrical properties, physical properties such as glass-transition temperature, crystalline melting temperature, density, solubility, optical properties, etc. It is, therefore, essential to thoroughly understand the stmcture—property relationships of polyimide systems, and excellent review articles are available (1—5,92). 

The Rheometric Scientific RDA II dynamic analy2er is designed for characteri2ation of polymer melts and soHds in the form of rectangular bars. It makes computer-controUed measurements of dynamic shear viscosity, elastic modulus, loss modulus, tan 5, and linear thermal expansion coefficient over a temperature range of ambient to 600°C (—150°C optional) at frequencies 10 -500 rad/s. It is particularly useful for the characteri2ation of materials that experience considerable changes in properties because of thermal transitions or chemical reactions. 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

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)... 

TDI isomers, 210 Tear strength tests, 242-243 TEDA. See Triethylene diamine (TEDA) Telechelic oligomers, 456, 457 copolymerization of, 453-454 Telechelics, from polybutadiene, 456-459 TEM technique, 163-164 Temperature, polyamide shear modulus and, 138. See also /3-transition temperature (7)>) Brill temperature Deblocking temperatures //-transition temperature (Ty) Glass transition temperature (7) ) Heat deflection temperature (HDT) Heat distortion temperature (HDT) High-temperature entries Low-temperature entries Melting temperature (Fm) Modulu s - temperature relationship Thermal entries Tensile strength, 3, 242 TEOS. See Tetraethoxysilane (TEOS)... 

A film of the branched chain 5-methylhexyl polymer absorbed at 315 nm and the position of this maximum was relatively insensitive to temperature. Likewise, no thermal transition was observed in the region where the side chain melting transition was seen for PDHS and its higher homologs. 

The heat flow into (endothermic) or out (exothermic) of a sample as a function of temperature and time is measured using the technique of DSC. In particular, it is used to study and determine the temperature of thermal transitions. For polymers, these include Tg, the glass transition temperature, Tc, the (exothermic) temperature of crystallisation for polymers that can crystallise, and Tm, the (endothermic) melting temperature. A DSC measurement requires only a small amount of sample 2-20 mg of a film, powder, fibre or liquid samples can be analysed in a DSC pan. 

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. 

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. 

X-ray diffraction studies are usually carried out at room temperature under ambient conditions. It is possible, however, to perform variable-temperature XPD, wherein powder patterns are obtained while the sample is heated or cooled. Such studies are invaluable for identifying thermally induced or subambient phase transitions. Variable-temperature XPD was used to study the solid state properties of lactose [20], Fawcett et al. have developed an instrument that permits simultaneous XPD and differential scanning calorimetry on the same sample [21], The instrument was used to characterize a compound that was capable of existing in two polymorphic forms, whose melting points were 146°C (form II) and 150°C (form I). Form II was heated, and x-ray powder patterns were obtained at room temperature, at 145°C (form II had just started to melt), and at 148°C (Fig. 2 one characteristic peak each of form I and form II are identified). The x-ray pattern obtained at 148°C revealed melting of form II but partial recrystallization of form I. When the sample was cooled to 110°C and reheated to 146°C, only crystalline form I was observed. Through these experiments, the authors established that melting of form II was accompanied by recrystallization of form I. 

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