Melt behavior

A useful formula to predict the (low shear) melt viscosity of PET from its IV is as follows  

If moisture is present before the polymer is melted, hydrolytic degradation will occur upon heating. Each water molecule will break a chain, so increasing the total number of polymer chains by one. The effect on the average molecular weight will be as follows  

An amorphous polymer in a state of molecular alignment is not a stable structure - it is metastable. It can transition either to a more perfectly ordered, ciystalline structure, or to a more disordered, nonoriented structure In either case, tire free energy of the system is reduced. Given enough time and/or thermal energy, an oriented amorphous polymer will transition in eitlier or botli of these dhections. 

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As determined from X-ray diffraction measurements, die unit cell of ciystalline PET is triclinic widi a repeat distance of 1.075 nm along die major axis [5, 6]. This coiTCsponds to 98 % of the theoretical extended leiigdi of die monomer repeat unit [6]. There is very little moleculai extensibility remaining in a PET ciystal, resulting not only in a high modulus but also a relatively short extension range over which die ciystal can be extended and still recover elastically. The density of die ciystalline stracture is 1.45 g/ml, or about 9% higher diaii die amoiphous sdiicture [3]. 

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To develop a more quantitative relationship between particle size and T j, suppose we consider the melting behavior of the cylindrical crystal sketched in Fig. 4.4. Of particular interest in this model is the role played by surface effects. The illustration is used to define a model and should not be taken too literally, especially with respect to the following points ... 

Sohd fats may show drastically different melting behavior. Animal fats such as tallow have fatty acids distributed almost randomly over all positions on the glycerol chain. These fats melt over a fairly broad temperature range. Conversely, cocoa has unsaturated fatty acids predominantly in the 2 position and saturated acids in the 1 and 3 positions. Cocoa butter is a brittle sohd at ambient temperature but melts rapidly just below body temperature. 

Glass-Transition Temperature and Melt Behavior. The T of BPA polycarbonate is around 150°C, which is unusually high compared... 

Thermoplasticity. High molecular weight poly(ethylene oxide) can be molded, extmded, or calendered by means of conventional thermoplastic processing equipment (13). Films of poly(ethylene oxide) can be produced by the blown-film extmsion process and, in addition to complete water solubiUty, have the typical physical properties shown in Table 3. Films of poly(ethylene oxide) tend to orient under stress, resulting in high strength in the draw direction. The physical properties, melting behavior, and crystallinity of drawn films have been studied by several researchers (14—17). 

The melting behavior of PE depends on the size, content, and distribution of SCBs as these molecular... 

These techniques help in providing the following information specific heat, enthalpy changes, heat of transformation, crystallinity, melting behavior, evaporation, sublimation, glass transition, thermal decomposition, depolymerization, thermal stability, content analysis, chemical reactions/polymerization linear expansion, coefficient, and Young s modulus, etc. 

This generalization is of great value. It is based upon exactly the type of experiment you have performed. We have confidence in the rule because this type of experiment has been conducted successfully on hundreds of thousands of substances. The melting behavior is one of the most commonly used methods of characterizing a substance. It leads us to wonder if every solid can be converted to a liquid if the temperature is raised sufficiently. Further, it leads us to wonder... 

Here are three statements concerning the melting behavior of poradichlorobenzene ... 

This same situation exists with plastics. To be successful with plastics requires experience with their melt behavior, melt-flow behavior during processing, and the process controls needed to ensure meeting the dimensions that can be achieved in a complete processing operation. Based on the plastic to be used and the equipment available for processing, certain combinations will make it possible to meet extremely tight tolerances. 

A major difference between extrusion and IM is that the extruder processes plastics at a lower pressure and operates continuously. Its pressure usually ranges from 1.4 to 10.4 MPa (200 to 1,500 psi) and could go to 34.5 or 69 MPa (5,000 or possibly 10,000 psi). In IM, pressures go from 14 to 210 MPa (2,000 to 30,000 psi). However, the most important difference is that the IM melt is not continuous it experiences repeatable abrupt changes when the melt is forced into a mold cavity. With these significant differences, it is actually easier to theorize about the extrusion melt behavior as many more controls are required in IM. 

In the molten state, a Newtonian behavior was observed, a consequence of lack of entanglements. The melt behavior is also dependent on die structure of die endgroups. 

Although each of these cyclic siloxane monomers can be polymerized separately to synthesize the respective homopolymers, in practice they are primarily used to modify and further improve some specific properties of polydimethylsiloxanes. The properties that can be changed or modified by the variations in the siloxane backbone include the low temperature flexibility (glass transition temperature, crystallization and melting behavior), thermal, oxidation, and radiation stability, solubility characteristics and chemical reactivity. Table 9 summarizes the effect of various substituents on the physical properties of resulting siloxane homopolymers. The... 

Philips R.A., McKenna J.M., and Cooper S.L., Glass transition and melting behavior of poly(ether-ester) multihlock copol3miers with poly(ethyleneterephthalate) hard segments, J. Polym. Sci. Part B, 32, 791, 1994. 

The [n]pericyclines 3-6 are all colorless, crystalline,light- and air-stable solids which do not exhibit any shock sensitivity. They exhibit sharp melting behavior without showing signs of decomposition. However, octamethyl[5]pericycline 30... 

Liu, H.B., Ascencio, J.A., Perez-Alvarez, M. and Yacaman, M.J. (2001) Melting behavior of nanometer sized gold isomers. Surface Science, 491, 88-98. 

C. L. Jackson, G. B. McKenna 1990, (The melting behavior of organic material confined in porous solids), J. Chem. Phys. 93, 9002. 

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