Melt crystallization INDEX

Floudas et al. [135] also studied the isothermal crystallization of PEO and PCL blocks within PS-b-PEO-h-PCL star triblock copolymers. In these systems the crystallization occurs from a homogeneous melt Avrami indexes higher than 1 are always observed since the crystallization drives structure formation and does not occur under confined conditions. A reduction in the equilibrium melting temperature in the star block copolymers was also observed. 

Figure 18.55 shows the predicted steady-state temperature and meridional flow streamlines for (a) no melt convection and stationary crystal, (b) no crystal rotation with buoyancy-induced melt flow, (c) modest rotation and mixed melt convection, and (d) high rotation rates with mixed melt convection [213], The emissivity of the crystal-vapor interface of the system was specified to be 0.3, while the melt-crystal interface emissivity was set to 0.9 for the GGG crystal of refractive index 1.8. Additional property values and geometric details are listed elsewhere [215]. The crucible diameter is 200 mm, the crystal diameter is 100 mm, and thermocapillary convection was not included in the analysis. 

Molar mass (inversely proportional to melt flow index, MFl) has a small influence. Generally in the chain folded crystal structure, the molar mass will not be important. It will mean that a particular molecule has more folds in the crystal. Melting is determined by the crystal thickness. Crystal thickness is determined by crystallization conditions and the presence of branches that sterically prevent crystallization of some segments. 

Isotactic PP has extremely good flow properties at a wide range of flow rates, and therefore good processing behavior. The melt flow index typically ranges from 0.5 to 50 g/10 min. Films, which can be produced by both blown and cast methods, can be oriented to provide improved optical characteristics and better strength. Because of the rapid crystallization of PP, blown films must be produced by either water quench or mandrel quench processes, unlike PE, which is cooled by air. 

Unmodified crystal polystyrene is relatively stable under oxidative conditions, so that for many applications the addition of an antioxidant is not required. Nevertheless, repeated processing may lead to oxidative damage of the material, leading to an increase of melt flow index and to embrittlement of the material. Stabilization is effected by the addition of octadecyl-3-(3,5-di-rerr-butyl-4-hydroxy-phenyl)-propionate at concentrations of up to 0.15%, if necessary in combination with phosphates or phosphonites to improve color. 

At a composition (in a/o) Ce2oAl35Si45 a ternary compound was observed, which tentatively was indexed hexagonal with c/a 1.17 H = 290 ( 10) kg/mm. The ternary phase was suspected to correspond to the compound CcjAljSij mentioned by Brauer and Haag (1952), who obtained this phase in an unsuccessful attempt to prepare CeSij by reaction of a CeAl melt with excess Si at 900°C in an AljOj crucible under a protective layer of NaCl. After 1 h at 900 0 and subsequent cooling to 600 0 in 2.5 h the obtained alloy was boiled in 2n-NaOH. The remainder was of metallic luster and well crystallized. Indexing was possible on the basis of a hexagonal cell a = 6.242, c = 7.304 (converted from kX units). 

Nadkami and Jog (1986), Nadkami et al. (1987), and Jog et al. (1993) investigated the crystallization in blends of PPS with three types of HOPE, having a different melt flow index. In contrast to the PPS/PET blends, PPS crystallizes now in a superheated HOPE melt environment. Erom the dynamic cooling experiments, it was found that the presence of the HDPE melt suppresses the crystallization of PPS. The crystal growth rate, G, of PPS was found to remain unchanged, but its nucleation density was reduced as the concentration of HDPE in the blend increased or when the melt viscosity of the HDPE phase decreased. As a consequence, the overall crystallization rate of PPS was found to be retarded. 

Keywords peroxide, molar weight distribution (MWD), rheology, crystallization, extrusion, melt flow index (MFI), controlled rheology (CR), peroxide-degradation, residence time distribution (RTD), halflifetime of peroxides, melt elasticity, die swell, viscosity curve, shear rate, elongational viscosity, melt fracture, heterophasic PR... 

A typical shear thinning behavior as well as a decrease in viscosity and an increase in the melt flow index while increasing the content of thermotropic liquid crystal copolyester was recorded. In addition, the oriented and ordered fibril structure of thermotropic liquid crystal copolyesters generated in the poly(ethylene 2,6-naphthalate) matrix is illustrated in Figure 6.2. 

The melt flow index, melting and crystallization temperatures, and heat of fusion of the samples are shown in Table 1. The difference in the melting point between the reference and the branched samples are caused by the presence of the branching and the reduced crystallinity. The molecular weights are shown in Table 2 based on a calibration curve determined using PMMA standards. 

As with the polysulphones, the deactivated aromatic nature of the polymer leads to a high degree of oxidative stability, with an indicated UL Temperature Index in excess of 250°C for PEEKK. The only other melt-processable polymers in the same league are poly(phenylene sulphides) and certain liquid crystal polyesters (see Chapter 25). 

When the first crystals just start to melt, record the temperature. When the last crystal just disappears, record the temperature. If both points appear to be the same, either the sample is extremely pure, or the temperature rise was too fast. If you record the temperature with the horizontal index line in the mirror matched to the lines etched on both sides of the periscope window and the top of the mercury thread at the same time, you ll be looking at the thermometer scale head on. This will give you the smallest error in reading the temperature (Fig. 38). 

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