Polyethylene microstructure

Polyethylene microstructure is defined by its distributions of chain length (CLD) or molecular weight (MWD), chemical composition (CCD), comonomer sequence length (CSLD), and LCB. In addition, polypropylene microstructure is further characterized by its distribution of regio- and stereoregularity [53, 54]. 

The polymers compared all have similar crystal structures but are different from polyethylene, which excludes the possibility for also including the latter in this series. Also note that the isotactic structure of these molecules permits crystallinity in the first place. With less regular microstructure, crystallization would not occur at all. 

Functionalized polyethylene would be of great industrial importance, and if synthetic methods to control the microstructure of functionalized polymers using transition-metal-based catalysis are developed, it would significantly broaden the utility and range of properties of this class of polymers. Recent progress in the field of late transition metal chemistry, such as Brookliart s use of nickel-based diimine catalysts, has enabled the copolymerization of ethylene with functional a-olefins.29 However, these systems incorporate functionalized olefins randomly and with limited quantity (mol percent) into the polymer backbone. 

Starnes and Bovey (1) pioneered the method of I3C NMR analysis of reduced poly(vinyl chloride) (PVC) to study the microstructure of PVC. Tri-n-butyltin hydride ((n-Bu)3SnH) was found to completely dechlorinate PVC resulting in polyethylene (PE) whose microstructure (branching, end-groups, etc.) could be sensitively studied by 13C NMR. 

Table 7 shows synthesis processes and microstructures of available polyethylenes together with several of their properties. 

Table 7 Synthesis processes and microstructures of polyethylenes with characteristic properties... 

Favorable rheological properties are an essential requirement for the commercialization of polyolefins like polyethylene. The ease of processability of the polymer melt, obtained through modifications in the microstructural features, is as important as the end use mechanical properties of these polymers. Presence of long-chain as well as short-chain branching, LCB and SCB, respectively, more or less dictates the rheological behavior of most commercial... 

The crystallization kinetics of commercial polyolefins is to a large extent determined by the chain microstructure [58-60]. The kinetics and the regime [60] of the crystallization process determine not only the crystalline content, but also the structure of the interfaces of the polymer crystals (see also Chapter 7). This has a direct bearing on the mechanical properties like the modulus, toughness, and other end use properties of the polymer in fabricated items such as impact resistance and tear resistance. Such structure-property relationships are particularly important for polymers with high commercial importance in terms of the shear tonnage of polymer produced globally, like polyethylene and polyethylene-based copolymers. It is seen that in the case of LLDPE, which is... 

In very few cases solid-state NMR has been used to determine molecular weights for polyethylene [99]. In general solid-state NMR is not so suitable due to the long relaxation times of the end groups, which lead to long measurement times. The strength of NMR is in chemical structure characterization and also the possibility to determine the chain branching, tacticity or to obtain further details of the microstructure. 

The closed structure of the cyclophane ligand also induces dramatic changes in the polymer microstructure. Polyethylene from the cyclophane catalyst 1.53a is considerably more branched than with catalysts bearing acyclic ligands, which suggests... 

In principle, therefore, the microstructure of such a polymer could be described by a sequence of m and r diads. However, the limits of NMR resolution is such that the microstructure can only be determined over a limited section of the polymer, usually no longer than 5-7 monomer units. NMR spectra on comparatively insoluble polymers such as polyethylene and polypropylene are usually recorded in 1,2,4-trichlorobenzene or tetrachloroethane-d2 at elevated temperatures (110-150 °C). 

Polyethylene (PE) is the largest synthetic commodity polymer in terms of annual production and is widely used throughout the world in a variety of applications. Based on the density, PE is classified as LDPE at 0.910-0.930g/cm3, high-density polyethylene (HDPE) at 0.931-0.970g/cm3, and linear low-density polyethylene (LLDPE) based on the polymer chain microstructure. At present, processes that produce PE use the following raw materials ... 

The micro devices were fabricated from a polyethylene terephthalate (PET) substrate using 193 nm ArF excimer laser ablation [59], Microstructures produced in this way were thermally sealed by a lamination machine. 

---