Polyethylene molecular fractionation

Wild and coworkers reported [19] the wide distribution of branching in two LLDPE resins of commercial importance at the time. Both samples had a density of 0.919 g/cc and similar MI values of 1.0 and 1.2 but were produced with two different low-pressure processes. As expected, polyethylene molecular fractions were found with a very heterogeneous branching distribution from about nil to 44 methyl groups/1000 carbons. The slight differences in the TREE data of the two similar LLDPE samples were attributed to the different process used to produce the sample. 

Schreiber and co-workers have noted very persistent history effects in linear polyethylenes (69). Fractions which have been crystallized from dilute solution required times of the order of hours in the melt state at 190° C in order to attain a constant die swell behavior upon subsequent extrusion. The viscosity on the other hand reached its ultimate value almost immediately. The authors concluded from this result that different types of molecular interactions were responsible for elastic and viscous response. However, other less specific explanations might also suffice, since apparent viscosity might be relatively intensitive to the presence of incompletely healed domain surfaces, while die swell, requiring a coordinated motion of the entire extrudate, might be affected by planes of weakness. It would... 

Crystallisation of most polymers is accompanied by the separation of different molecular species, a process referred to as molecular fractionation. Bank and Krimm [147] provided the first direct evidence of molecular fractionation in polyethylene. The first extensive study performed by Wunderlich and Mehta [148] indicated that, at each crystallisation temperature, there exists a critical molar mass (MCIjt) such that the molecules of molar mass greater than Mcrjt, are able to crystallise at this temperature, whereas... 

Most of the early studies concerned with molecular fractionation dealt with samples having a broad molar mass distribution. The crystallisation of binary mixtures of sharp fractions was studied to a lesser degree. The crystallisation of binary mixtures of linear polyethylene sharp fractions in the molar mass range from 1000 to 20,000 g mol1 depended upon the cooling rate, and two types of crystallisation were observed [155] ... 

W. Honig and M. R. Kula, Anal. Biochem., 72 502-512 (1976). Selectivity of Protein Precipitation with Polyethylene Glycol Fractions of Various Molecular Weights. 

PST/MAS experiments the intensity of the peak is governed by the NOE enhancement. In the CP/MAS spectrum, the intensity of peak 1 is more intense than that of peak 2, but in the PST/MAS spectrum (Fig. 9.15(b)) the intensity of peak 2 is relatively increased compared with the CP/MAS spectrum. This means that the molecular motion of the methylene carbons for peak 2 is faster than that for peaks 1 and 3. The CP/MAS spectral pattern of adsorbed polyethylene is very different from that of bulk polyethylene. The fractions of the mobile components for these polyethylene samples are different from each other. The fraction of the mobile component in adsorbed polyethylene is larger than that of bulk polyethylene. The chemical shift value for peak 1 is close to that for tran -zigzag methylene carbons and the difference in C chemical shift between peaks 1 and 3 is 4.4 ppm which corresponds to the 1 y-gauche effect value. From these results, it can be said that peaks 1 and 3 come from the trans and gauche parts in which the molecular motion is frozen on the NMR timescale. This is caused by adsorption on the surface of silica gel (this part is designated as the region A). 

The source and characterization of the linear polyethylene molecular weight fractions have already been described in previous publications from this laboratory (33). A Perkin-Elmer DSC-2 differential scanning calorimeter was used. The heating rates and sample sizes are indicated in the specific experiments. Special thermal history procedures that were adopted are described in the text. 

F.M. Mirabella, E.A. Ford, Characterization of linear low-density polyethylene cross-fractionation according to copolymer composition and molecular weight. J. Polym. Sci. B Polym. Phys. 25(4), 777-790 (1987)... 

We have shown that the inclusion of small amounts of DBS or selected derivatives leads to the formation of a well-dispersed system of nanofibrils. These are particularly effective in directing the crystallisation of polyethylene, polypropylene and poly(e-caprolactone) due to the high number density of the fibrils in the sample. It may well be possible to extend to other polymer systems. Such work is underway. The approach offers distinct advantages over the other approaches described in other chapters in this volume. It avoids the use of viscous high molecular fractions to produce row nuclei. By using a nanoparticle it may be possible to achieve other... 

As in the case of other material systems, the macroscopic properties of nanocomposites are driven by their micro-/nanoscopic structure. From an electrical insulation perspective, polyethylene (PE) and epoxy resins constitute two technologically important material systems, each of which embodies in very different ways, a great deal of structural complexity. In the case of PE, the constituent molecules are the result of the inherently statistical polymerisation process, which can ultimately result in the formation of a hierarchical morphology in which different molecular fractions become segregated to specific morphological locations. In an epoxy resin, the epoxy monomer chemistry, the hardener and the stoichiometry can all be varied, to affect the network structure that evolves. In the case of nanocomposites, another layer of structural hierarchy is then overlaid upon and interacts with the inherent characteristics of the host matrix. 

The above describes how molecular fractionation occurs in a melt-crystallized polymer, and the effect that such a process can have under appropriate conditions. However, polyethylene fractionation does not only give rise to the concentration of particular molecular components at particular locations within the bulk, but also, under certain circumstances, can give rise to a variety of different lamellar habits. 

Fig. 11.23 (a) Plot of crystallization half-time, ti/2, against percent M = 9000 for binary blends of linear polyethylene molecular weight fractions. Pure components M = 9000and370000.Crystallizationtemperature 130°C Ol29°C Dl27°C. (b) Plot of crystallization half-time, ti/2, against percent M = 26000 for binary blends of linear polyethylene. Pure components M = 26000 and 3.8 x 10 . Crystallization temperature 130°C o 128 °C. (From Ohno (37))... 

Growth rate studies of linear polyethylene from other solvents have also been re-ported.(51,53) The results obtained with decalin, a relatively good solvent, are very similar, at all concentrations, to those obtained with p-xylene.(51) In contrast, in a poorer solvent, n -octane, a maximum in the growth rate is observed at all concentrations studied, 0.001 % to 0.1 wt%.(48) However, the rate of decrease is much steeper in this case. Growth rates of linear polyethylene crystallizing from tetradeconol (0.05%), over the molecular weightrangeM = 1.4x 10" to 1.2x 10, are essentially constant at the lower crystallization temperatures. (5 3) However, as the temperature is raised the crystallization rate of the lowest molecular fractions decreases. 

It may be shown that M > M. The two are equal only for a monodisperse material, in which all molecules are the same sise. The ratio MI /MI is known as the polydispersity index and is a measure of the breadth of the molecular weight distribution. Values range from about 1.02 for carefully fractionated samples or certain polymers produced by anionic polymerization, to 20 or more for some commercial polyethylenes. 

Some by-product polyethylene waxes have been recently introduced. The feedstock for these materials are mixtures of low molecular weight polyethylene fractions and solvent, generaHy hexane, produced in making polyethylene plastic resin. The solvent is stripped from the mixture, and the residual material offered as polyethylene wax. The products generaHy have a wider molecular weight distribution than the polyethylene waxes synthesised directly, and are offered to markets able to tolerate that characteristic. Some of the by-product polyethylene waxes are distHled under vacuum to obtain a narrower molecular weight distribution. 

Low molecular weight fractions can be detected by smelling the inside of almost any freshly made polyethylene container. The amount varies with the specific polyethylene resin and the type of processing that have been used. 

A prehminary study of the use of larch AGs in aqueous two-phase systems [394] revealed that this polysaccharide provides a low-cost alternative to fractionated dextrans for use in aqueous two-phase, two-polymer systems with polyethylene glycol (PEG). The narrow molecular-weight distribution (Mw/Mn of 1-2) and low viscosity at high concentration of AG can be exploited for reproducible separations of proteins under a variety of conditions. The AG/PEG systems were used with success for batch extractive bioconversions of cornstarch to cyclodextrin and glucose. 

Figure 8. Fractionation of polyethylene owing to phase splitting in ethylene solution molecular weight distributions in equuibrium phases at 260°C and 900...

Figure 15 Morphological map of linear polyethylene fractions. Plot of molecular weight against crystallization temperature. The types of supermolecular structures are represented by symbols. Patterns a, b and c represent spherulitic structures with deteriorating order from a to c. Patterns g and d represent rods or sheet-like structures whose breadth is comparable to their length g or display a different aspect ratio d. Pattern h represents randomly oriented lamellae. Neither h nor g patterns have azimuthal dependence of the scattering. Reproduced with permission from Ref. [223]. Copyright 1981 American Chemical Society. (See Ref. [223] for full details.) Note the pattern a is actually located as o in the figure this was an error on the original.

Figure 16 Typical electron micrographs of quenched samples of indicated molecular weight fractions of linear polyethylene. Reproduced with permission from Ref. [231]. Copyright 1984, John Wiley 8t Sons, Inc.

Figure 21 Plot of thickness values as a function of molecular weight for linear polyethylene fractions quenched to —78°C. (A), crystallite thickness, Lc (O), interlamellar thickness, La ( ), interfacial thickness, Lb. Reprinted with permission from Ref. [277]. Copyright 1990 American Chemical Society.

---