Bimodal melting

Synthetic waxes consist of Fischer-Tropsch, polyethylene, and specialty waxes. Fischer-Tropsch waxes are produced from synthesis gas (CO and H2). They are often termed synthetic paraffin . Crystallinity is similar to paraffin, but with a higher and bimodal melting point (see Figs. 11 and 12). F-T waxes are used instead of paraffin where higher heat resistance is needed. 

It is the virtue of the NSE experiments that they confirmed the CLF mechanism quantitatively on a molecular level in space and time. On the other hand, the constraint release process still remains to be probed on a molecular scale and it is expected that future NSE experiments on bimodal melts will clarify these processes on a molecular scale also. 

Aydin et al. reported the influence of alkali treatment on the mechanical, thermal, and morphological properties of eco-composites made by short flax fiber reinforced PLA Scanning electron microscopy (SEM) revealed that the packed structure of the fibrils was deformed by the removal non-cellulosic materials. The mechanical tests indicated that the modulus of the untreated flax fiber/PLA composites was higher than that of PLA on the other hand, the modulus of alkali treated flax/PLA was lower than that of PLA. Thermal properties of the PLA in the treated flax fiber reinforced composites were also affected. Tg values of treated flax fiber reinforced composites were lowered by nearly 10°C for 10% NaOH treatment and 15°C for 30% NaOH treatment. A bimodal melting behavior was observed for treated fiber composites different than both of neat PLA and untreated fiber composites [60]. 

Neuutn scattering is often perftrmed as a function cf particle concentration to determine specific interactions between components. Blends cf metallocene synthesized polyolefins (32) and efi ds cf solvent on dendrima- size (33) have been studied using SANS. Interface widths between two polymer blend components in the melt have been studied by neuu-on reflectivity (34). Dynamic studies have been undertaken on bimodal melts (35) using neutron spin echo techniques. Some of these recent developments will be reported in ttie following chrqiters. 

Figure 7.6 Schematic diagram to show the origin of bimodal lamellar-size distribution giving rise to the bimodal melting (see Figure 7.5).

Figure 10.18 Heating thermograms showing different bimodal melting traces. Schematic curves.

However, there are also aspects of the crystallization process that are particular to statistical copolymers [16, 69, 71-76]. In a series of studies on the crystallization and melting behavior of ethylene-butene statistical copolymers, it was observed that after isothermal crystallization, while polyethylene homopolymer exhibited a single-peak melting endotherm, all the copolymers showed bimodal melting behavior [16,71]. Each melting peak of the copolymer endotherm was determined to represent a distinct crystal population because its shape... 

Besides its effects on morphology, comonomer sequence distribution also affects copolymer crystallization kinetics. In statistical copolymers, due to the broad distribution of crystaUizable sequence lengths, bimodal melting endotherms are typically observed. In block copolymers, the dynamics of crystallization have features characteristic of both homopolymer crystallization and microphase separation in amorphous block copolymers. In addition, the presence of order in the melt, even if the segregation strength is weak, hinders the development of the equihbrium spacing in the block copolymer solid-state structure. 

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

Figure 17 Correlation of failure time and melt flow rate MFR 190/5 of sharp notched bars under stress for unimodal homopolymer and copolymers, and bimodal copolymers. Source Ref. 130.

Strain-induced crystallization would presumably further improve the ultimate properties of a bimodal network. It would therefore obviously be of considerable importance to study the effect of chain length distribution on the ultimate properties of bimodal networks prepared from chains having melting points well above the very low value characteristic of PDMS. Studies of this type are being carried out on bimodal networks of polyethylene oxide) (55), poly(caprolactone) (55), and polyisobutylene (56). 

The silica carrier of a sulphuric acid catalyst, which has a relatively low surface area, serves as an inert support for the melt. It must be chemically resistant to the very corrosive pyrosulphate melt and the pore structure of the carrier should be designed for optimum melt distribution and minimum pore diffusion restriction. Diatomaceous earth or synthetic silica may be used as the silica raw material for carrier production. The diatomaceous earth, which is also referred to as diatomite or kieselguhr, is a siliceous, sedimentary rock consisting principally of the fossilised skeletal remains of the diatom, which is a unicellular aquatic plant related to the algae. The supports made from diatomaceous earth, which may be pretreated by calcination or flux-calcination, exhibit bimodal pore size distributions due to the microstructure of the skeletons, cf. Fig. 5. 

The addition of bilirubin or uremic toxin CMPF, which are strongly bound to HSA, alters the shape of the melting curve of serum albumin producing a bimodal curve, and causes a shift of the temperature maximum of the first peak to the right (Fig. 29.2) [7,8]. 

The PP exhibits a sharp peak at the maximum on CL intensity whereas the HDPE curve shows a broad bimodal behavior that has been thoroughly described elsewhere [127]. In the CL curves of the blend all these features were observed, which may be a strong indication of the existence of a two-phase system in the molten state [128], although based on peroxide treatment of PP/PE blend melts. It appears that PP oxidises first and the oxidation sites created during this process accelerate, to some extent, the oxidation of the PE phase. The overlap between the PP and PE traces in the blend can be interpreted as the interface of these two phases where the PE starts oxidising. In addition, the shape of the curves confirms that the oxidation mechanisms of the resins are different and that this difference remains during the oxidation of the blend in the molten state. 

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