Determination of Unknown Polymer Properties

Introduction and Background

A multi-division corporation had a small plastic piece fail in one of its specialty machines. The maintenance department has been asked to replace the part. However, the original manufacturer, Kaufmann Ltd., has gone out of business, and specifications for the part are unavailable. The purpose of this experiment, performed by Ryan Nelson, Peter Glascor, Eric Flowr was to determine the identity of the unknown polymer by comparing its properties to properties of other known polymers. Most likely, the part is one of the five following polymers: polyethylene, polypropylene, polystyrene, polycarbonate, poly(vinyl chloride). These are all extremely common, and have widespread applications in industry.

Each of these polymers exhibits properties which allow it to be of unique technological importance. Theoretical values for the properties utilized in determining the identity of the unknown sample are listed in Table 1.

Density (g/cm3) Molecular Weight (g/mol) Melting Point (Ñ"C)

Polypropylene 0.905 42.08 170

Polyethylene 0.92 28.05 130-145

Polycarbonate 1.2 158 310

Polystyrene 1.05 104 240

Polyvinyl chloride 1.4 62.5 160-172

Table 1 - Properties for five known polymers (Callister 737-775)

The melting temperature of a polymer is the point at which it transforms from the solid form to a viscous liquid in which there is little molecular order (Callister 500). This temperature depends on the stiffness of the molecular chain, molecular weight, and the degree or branching (Callister, 502). Chain stiffness is controlled by the ease of rotation about the chemical bonds along the chain. Bulky or large side groups can impede rotation, thus raising the melting temperature (Callister, 502). At low molecular weights, polymers with longer chains have higher melting temperatures that those with shorter chains. This is because increased chain length leads to increased Van der Waals intermolecular forces. A polymer with a high degree of branching generally has a lower melting temperature than a similar molecular weight polymer with a low degree of branching. Branching induces defects into the crystalline structure of a polymer, thus lowering the melting temperature (Callister, 502).

Polypropylene is a widely used polymer, best known for its resistance to heat distortion, excellent electrical properties and fatigue strength. It is also chemically inert and relatively inexpensive, but has a poor resistance to UV light. It is most commonly used for sterilizeable bottles, packaging film, TV cabinets, and luggage (Callister, 505). Polyethylene, another polymer in table 1 is chemically resistant, electrically insulating, tough, and has a low coefficient of friction, yet has low strength and poor resistance to weathering. Polyethylene is most commonly used for flexible bottles, toys, tumblers, battery parts, ice trays, and film wrapping materials (Callister, 504). Polycarbonates exhibit such characteristics as dimensional stability, low water absorption, good impact resistance, high ductility, transparency. Polycarbonates can be found in safety helmets, lenses, light globes, and as a base for photographic film (Callister, 504). Polystyrene, another common polymer, possesses excellent electrical properties and optical clarity, good thermal and dimensional stability, and is relatively inexpensive. Thus, it serves most effectively in applications involving wall tile, battery cases, toys, indoor lighting panels, and appliance housings (Callister, 505). Poly(vinyl chloride) is best suited for low-cost general purpose materials. It is ordinarily rigid, but can be made flexible with certain additives called plasticizers. This polymer is best utilized in floor coverings, piping, electrical wire insulation, garden hoses, and phonographic records (Callister, 505).

Experimental Procedure

In order to determine the identity of the unknown polymer, several tests were performed with the five known polymers as well as the unknown polymer. These tests include: melting tests at 140Ñ"C and 220Ñ"C, density tests at 1 g/cm3 and 1.05 g/cm3, a flame test, and a solubility test in acetone. For the melting tests, approximately 15-20 polymer beads were placed in an aluminum weigh boat. The boat was labeled and placed in a Blue Heating Oven. The polymer samples were checked when the oven reached 140Ñ"C. Melted samples were recorded. They were checked again when the oven reached 220Ñ"C. Samples that melted were recorded. For the density tests, polymer samples were placed in beakers containing a solution with density of 1 g/cm3 and a solution with density of 1.05 g/cm3. The sample's behavior, floating or sinking, was recorded. If the