Morphology Molecular Structure Mechanical Property

Morphology is the study of the physical form or chemical structure of a material; that is, the physical molecular structure. As a result of morphology differences among polymers, great differences exist in mechanical and other properties as well as processing plastics.

Knowledge of molecular size and flexibility explains how individual molecules behave when completely isolated. However, such isolated molecules are encountered only in theoretical studies of dilute solutions. In practice, molecules always occur in a mass, and the behavior of each individual molecule is very gready affected by its intermolecular relationships to adjacent molecules in the mass. Three basic molecular properties affect processing performances, such as flow conditions, that in turn affect product performances, such as strength or dimensional stability. They are (1) mass or density, (2) molecular weight (MW), and (3) molecular weight distribution (MWD).

Densities

Absolute density (d) is the mass of any substance per unit volume of a material. It is usually expressed in grams per cubic centimeter (g/cm3) or pounds per cubic inch (lb/in3) (Table 1.5). Specific gravity (s.g.) is the ratio of the mass in air of a given volume compared to the mass of the same volume of water. Both d and s.g. are measured at room temperature [23°C (73.4°F)]. Since s.g. is a dimensionless quantity, it is convenient for comparing different materials. Like density, specific gravity is used extensively in determining product cost vs. average product thickness, product weight, quality control, and so on. It is frequently used as a means of setting plastic specifications and monitoring product consistency.

In crystalline plastics, density has a direct effect on properties such as stiffness and permeability to gases and liquids. Changes in density may also affcct other mechanical properties.

The term apparent density of a material is sometimes used. It is the weight in air of a unit volume of material including voids usually inherent in the material. Also used is the term bulk density that is commonly used for compounds or materials such as molding powders, pellets, or flakes. Bulk density is the ratio of the weight of the compound to its volume of a solid material including voids.

lable I. 5 Comparing densities of different polyethylene thermoplastics

Type

Density, g/cm3 (lb/ft3)

LDPE

0.910-0.925 (56.8-57.7)

MDPE

0.926-0.940 (57.8-58.7)

HDPE

0.941-0.959 (58.7-59.9)

HMWPE

0.960 & above (59.9 & above)

Molecular Weights

MW is the sum of the atomic weights of all the atoms in a molecule. Atomic weight is the relative mass of an atom of any element based on a scale in which a specific carbon atom (carbon-12) is assigned a mass value of 12. For polymers, it represents a measure of the molecular chain length. MW of plastics influences their properties. With increasing MW, polymer properties increase for abrasion resistance, brittleness, chemical resistance, elongation, hardness, melt viscosity, tensile strength, modulus, toughness, and yield strength. Decreases occur for adhesion, melt index, and solubility.

Adequate MW is a fundamental requirement to achieve desired properties of plastics. If the MW of incoming material varies, the fabricating and fabricated product performance can be altered. The greater the differences, the more dramatic the changes that occur during processing.

Molecular Weight Distributions

MWD is basically the amounts of component polymers that make up a polymer (Fig. 1.6). Component polymers, in contrast, arc a convenient term that recognizes the fact that all polymeric materials comprise a mixture of different polymers of differing molecular weights. The ratio of the weight average molecular weight to the number average molecular weight gives an indication of the MWD.

One method of comparing the processability with product performances of plastics is to use their MWD. A narrow MWD enhances the performance of plastic products. Wide MWD permits easier processing. Melt flow rates are dependent on the MWD. With MWD differences of incoming material the fabricated performances can be altered requiring resetting process controls. The more the difference, the more dramatic changes that can occur in the products.

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