Polyethylene melting range

The melt viscosity of a polymer at a given temperature is a measure of the rate at which chains can move relative to each other. This will be controlled by the ease of rotation about the backbone bonds, i.e. the chain flexibility, and on the degree of entanglement. Because of their low chain flexibility, polymers such as polytetrafluoroethylene, the aromatic polyimides, the aromatic polycarbonates and to a less extent poly(vinyl chloride) and poly(methyl methacrylate) are highly viscous in their melting range as compared with polyethylene and polystyrene. 

In the temperature range between 400 and 550 K, NSE spectra on the same undiluted polyethylene melt were recorded. These data were analyzed with respect to the entanglement distance. The result for the temperature-dependent entanglement distance d(T) is shown in Fig. 30. An increase in the tube diameter from about 38 to 44 A with rising temperature is found. 

Polyethylene is, depending on the molecular weight, a waxy or solid, crystalline substance. Following the above-mentioned procedure, a high molecular crystalline product with a melting range around 130 C is obtained. At room temperature it is insoluble in all solvents. At higher temperatures (100-150 °C) it can be dissolved in aliphatic and aromatic hydrocarbons.Viscosity measurements can be performed in xylene,tetralin or... 

The polypropylene so obtained has a high molecular weight and is crystalline.The proportion of isotactic polymer, determined by extracting with heptane for 10 h in a Soxh-let apparatus, is 98.5%. Isotactic polypropylene shows similar solubility behavior to polyethylene, but has a higher melting point (crystalline melting range 165-171 °C). 

The melting range of a semicrystalline polymer may be very broad. Branched (low-density) polyethylene is an extreme example of this behavior. Softening is first noticeable at about 75°C although the last traces of crystallinity do not disappear until about 115°C. Other polymers, like nylon-6,6, have much narrower melting ranges. 

Statistical copolymers of the types described in Chapter 8 tend to have broader glass transition regions than homopolymers. The two comonomers usually do not fit into a common crystal lattice and the melting points of copolymers will be lower and their melting ranges will be broader, if they crystallize at all. Branched and linear polyethylene provide a case in point since the branched polymer can be regarded as a copolymer of ethylene and higher 1-olefins. 

Table VI compares the key properties of these two types of thermotropic polymers category by category. The samples compared had the same melting ranges, but were very different in reduced viscosities and solubility characteristics. The data compared were those processed under the most favorable conditions. Interestingly enough, the as-spun fibers from the polyester-carbonate can be heat-treated more efficiently than those fibers (of same tenacity) spun from the polyester. Both of them gave fiber properties far superior to those of nylons and polyethylene terephthalate. These two classes of polymers also had comparative properties (such as tensile strength, tensile modulus, flex modulus, notched Izod impact strength) as plastics and their properties were far superior to most plastics without any reinforcement.

Within a BPE with a distribution of branches, there will be a distribution of lamella thickness. This will result in a broad melting range. BPE with a bimodal distribution of branches will have a bimodal distribution of lamella thickness and a corresponding melting temperature range. When two polyethylenes are blended, assuming they are miscible, they will cocrystallize only where they have common MSL. Some molecular segments in each BPE will crystallize independently of... 

Polyethylene and polypropylene are the polyolefins most commonly used as plastics. Polybutene-1 and poly-4-methylpentene-l are less common. Also important are certain copolymers of ethylene and also polyisobutylene, which is used for gaskets. The simplest method of identification of these materials is by infrared spectroscopy (see Section 8.2). However, some information can also be obtained from the melting range (see also Section 3.3.3) ... 

Even though CM and TM are used principally with TSs, these processes are sometimes used to mold conventional TPs for short runs with low-cost molds. They also are used with TPs that are difficult to melt, have too short a heat melt range for IM or other processes, and require high pressures. A typical example is ultrahigh molecular weight polyethylene (UHMWPE). 

The significance of melting range and melting point is illustrated for linear and branched polyethylene in Figure 2.10. Note that for both polymers above (115 C for BPE and 138 C for LPE), the volume increases linearly with T. As the temperature of the solid pofymer is... 

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