Melting behavior

The observed melting temperature T ) of pure iPP, pure PB-1, and blends linearly increases with the crystallization temperature. The experimental data can be fitted by the Hoffman equation (45)  

As reported in Table 6.3, for all blend systems examined, the lower the extrapolated value, the higher the content of the second component in the blend. 

In Equation 6.6, 1 /y assumes values between 0 (I m = for all T ) and 1 (Tm = To). Therefore, the crystals are most stable at 1/y = 0 and inherently unstable at 1/y = 1 (46). For the PB-l/HOCP and the iPP/PB-1 blends, the 1/y values decrease slightly with increase in the second component content, indicating an increase in the crystal stability. However, the values of 1/y are very similar for the iPP/HOCP blends, that is, independent of composition. 

According to the Flory-Huggins theory, the equilibrium melting point depression can be related to the polymer-polymer interaction parameter, Xi2 by (46,47)  

The straight line calculated with the aforementioned parameters is plotted in Fig. 6.6, it shows an intercept on the ordinate axis of about 1.3 x 10 and a slope of —0.196 which corresponds to the xn of the mixture. 

The melting behavior of polypropylene has been studied by a number of authors [83,211-214]. Jaffe [211] and Samuels [83] have reported results for fibers. 

FIGURE 3.50 Tenacity based on denier at break versus elongation for drawn polypropylene yarns spun at various spinning speeds and at various spirmeret outputs. (From Hagler, G.E. unpublished research.) 

Jaffe melted and isothermally recrystallized fibers spun under different stress levels. The bulk crystallization kinetics was measured with both DSC and optical microscopy techniques. Table 3.30 lists the samples studied. The film samples were included to ensure the absence of spurious DSC effects due to fiber packing. The samples were heated from 50°C to the melt temperature at 80°C/min. The melt temperatures ranged from 170 to 230°C. The samples were held in the melt for a specified time and then cooled to the 130°C crystallization temperature at 40°C/min. 

Spin speed/ D. R. Unannealed Free Constrained Free Constrained Free Constrained 

For a crystalline polymer in a blend, the melting behavior is strictly influenced by the miscibihty, that is, by the blend composition and thermodynamics, as well as by kinetic factors controlhng the growth, structure, geometry, and perfection of the crystals. 

According to the kinetic theories, the melting point, Fm, of a polymer crystal with flnite size (and in absence of recrystalhzation phenomena consequent on heating or annealing processes) can be related to the crystallization temperature, r by the Hoffman-Weeks relation [30]  

A plot of the melting point difference (F°b-F ) versus the square of volume fraction of the noncrystal-lizable component, a, should be linear with an intercept at the origin (if there is no enfropic contribution to the melting point depression), and the value of Xab can be determined from the slope of the hnear plot, as shown in Rgure 10.6. 

A depression of the equilibrium melting temperature implies, for a miscible blend, a negative value of XabI in the absence of interactions between the polymer components (when iLB is zero), no depression of the melting equihbrium temperature is recorded. Values of the interaction parameters and density derived from Equation (10.20) are reported in Table 10.1 for various blend systems. 

In many polymer blends, a depression of the experimental melting temperature is observed, which can be ascribed to kinetic and/or morphological effects, different from the thermodynamic effects reported earlier. Kinetic effects may be derived from the growth process of the crystals at large undercoolings and in nonequiUbrium 

Multiple melting peaks (two to four peaks) have been observed on heating scans of SPS samples that present a combination of a and p crystals. However, multiple melting peaks can also be observed in samples that are known to contain only a single type of unit cell [73]. Apparently, the multiple peaks phenomenon cannot be entirely attributed to multiple types of unit cells. Variation in the lamellar morphology may also be responsible for the complex thermal behavior. 

The number of melting peaks decreases with increasing scan rates. By contrast, only one broad and irregularly shaped peak for cold-crystallized SPS is observed [73]. 

Two main mechanisms have been commonly debated and proposed to explain the phenomenon of multiple melting endotherms dual/multiple modifications and reorganization via melting/recrystallization/remelting [76]. 

In one case, each of the multiple melting peaks can be attributed to the various crystalline substructures, such as unit cells, lamellae, or spherulites, present in the polymer. In the alternative interpretation, multiple melting was attributed to the melting of thinner lameUae/crystals, recrystallization to thicker ones, and remelting of the thickened lamellae during DSC scanning. 

Woo et al. [90] suggest that a combination of these two mechanisms might be appropriate for providing a plausible explanation of the multiple melting behavior in SPS systems. 

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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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