Density degree of crystallinity

Polyethylene and polypropylene are semitransparent plastics made by polymerization. They are produced from ethylene and propylene in a variety of grades. Their mechanical properties are determined mainly by density (degree of crystallinity) and molecular weight, characterized by the Melt Index (MI). 

Diffusion and permeability are inversely related to the density, degree of crystallinity, orientation, filler concentration, and cross-link density of a polymeric film. Generally, the presence of smaller molecules, such as plasticizers, increases the rate of diffusion in polymers since they are more mobile and can create holes or vacancies within the polymer. The rate of diffusion or permeability is fairly independent of polymer chain length just as long as the polymer has a moderately high chain length. 

This table lists typical values of the dielectric constant (more properly called relative permittivity) of some important polymers. Values are given for frequencies of 1 kHz, 1 MHz, and 1 GHz in most cases the dielectric constant at frequencies below 1 kHz does not differ significantly from the value at 1 kHz. Since the dielectric constant of a polymeric material can vary with density, degree of crystallinity, and other details of a particular sample, the values given here should be regarded as only typical or average values. 

Specimen Conditionof Annealing Density Degree of Crystallinity ( %J ... 

Another key limitation of Z-N catalysts is the inability to incorporate very high levels of alpha-olefin comonomer such as 1-butene, 1-hexene, or 1-octene to make the density less than about 0.885 g/cm. The lowest density Z-N catalyzed VLDPE resin commercially available today (FLEXOMER from The Dow Chemical Company) has a target density of 0.885 g/cm (approximately 20 wt% crystallinity). Due to this, Z-N catalyzed VLDPE resins cannot be used in applications requiring very low modulus, low shore A hardness (density less than 0.885 g/cm ). Note that logarithm of modulus of polyethylene resins is related to density (degree of crystallinity) ... 

The polymerization rate is also controlled by the particle geometry, mass-transfer coefficient at the polymer surfece, and the diSusivity of the monomer through the layer of polymer coating the primary catalyst particles. The catalyst chemistry influences such polymer attributes as average molecular weight and distribution, bulk density, degree of crystallinity, particle size and morphology. 

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

Crystallization and Melting Point. The abihty of PVA to crystallize is the single most important physical property of PVA as it controls water solubiUty, water sensitivity, tensile strength, oxygen barrier properties, and thermoplastic properties. Thus, this feature has been and continues to be a focal point of academic and industrial research (9—50). The degree of crystallinity as measured by x-ray diffraction can be directly correlated to the density of the material or the swelling characteristic of the insoluble part (Fig. 2). 

From a brief consideration of the properties of the above three polymers it will be realised that there are substantial differences between the crystallisation of simple molecules such as water and copper sulphate and of polymers such as polyethylene. The lack of rigidity, for example, of polyethylene indicates a much lower degree of crystallinity than in the simple molecules. In spite of this the presence of crystalline regions in a polymer has large effects on such properties as density, stiffness and clarity. 

Draw ratio Density of the amorphous material da) (g/cm-" ) Amorphous orientation function fa) Crystallite length Oc) (nm) Long period (L) (nm) Degree of crystallinity (X=>) Substructure parameter (A) Axial elastic modulus ... 

Polymer Melting point range °C Density g/cm Degree of crystallinity % Stiffness modules psi X 10 ... 

High density polyethylene produced by a low-pressure low-temperature process involving Ziegler-Natta catalysts. This creates low levels of branching and hence a high degree of crystallinity. 

Fig. 9. a Density vs. birefringence (proportional to preorientation f ) for a polyethylene melt, b Degree of crystallinity vs. f vor polychloroprene... 

Stereodefects reduce the overall regularity of an isotactic polymer chain and hinder its ability to crystallize. As the concentration of defects increases, the degree of crystallinity falls, resulting in reduced density, reduced melting temperatures, lower heat distortion temperatures, reduced modulus, and reduced yield stress. 

In the case of a semicrystalline polymer, the two components are the crystalline and amorphous regions. If we know the densities of the crystalline and the amorphous regions, we can calculate a sample s degree of crystallinity from Eq. 7.5. 

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