Understanding Polymeric Materials, their Structure and Properties
Polymeric molecules are gigantic in comparison to the hydrocarbon molecules with which you are probably familiar. Therefore, they are often referred to as macromolecules. Within each molecule, the atoms are bound together by covalent bonds. For most polymers, these molecules are in the form of long and flexible chains in which a string of carbon atoms constitutes the backbone. Furthermore, these long molecules are composed of structural entities called mer units, which are repeated along the chain. A single mer is called a monomer, and the term polymer means many mer units. As an illustration, a mer unit and the zigzag backbone structure of polyethylene are shown schematically in Fig. 3.1.
Figure 3.1. Schematic representations of polyethylene. (a) The "mer" and chain structure of carbon and hydrogen atoms. (b) A perspective view of the molecule, showing the zigzag backbone structure.
By using different starting materials and processing techniques, we can produce polymers having different molecular structures. As illustrated in Fig. 3.2 these structures can be classified into four different categories: (i) linear, (ii) branched, (iii) crosslinked, and (iv) network. In linear polymers, the mers are joined together end to end in single chains (see Fig. 3.2a). The long chains are flexible and may be considered as a mass of spaghetti. Extensive van der Waals bonding between the chains exist in these polymers. Some of the common linear polymers are polyethylene, polyvinyl chloride, polystyrene, nylon and the fluorocarbons.
Figure 3.2. Schematic illustrations of (a) linear, (b) branched, (c) crosslinked, and (d) network (three-dimensional) molecular structures. The circle designate individual mer units.
Polymers may also have a molecular structure in which side-branch chains are connected to the main ones, as shown schematically in Fig. 3.2(b). These polymers are called branched polymers. The branches result from side reactions that occur during the synthesis of the polymer. The formation of side branches reduces the chain packing efficiency, resulting in a lowering of the polymer density.
In crosslinked polymers, adjacent linear chains are joined to one and another at various positions along their lengths as depicted in Fig. 3.2(c). Generally, crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains. Many of the rubber materials consist of polybutadiene crosslinked with S atoms.
Trifunctional mer units, having three active covalent bonds, form three-dimensional networks as shown in Fig. 3.2(d). Polymers consisting of trifunctional units are termed network polymers. Epoxies belong to this group.
The molecular structure of a polymer has significant effects on its mechanical and thermal properties. In general, as the strength of the connections between the chains increases, the thermal and mechanical stability of the material increases. These connections might be intermolecular bonds (van der Waals, dipolar, or H bonds) or covalent crosslinks. In this lab, we will test the mechanical properties of polymers with differing strengths.
While many polymeric materials are amorphous, some can also exist in a crystalline state. The atomic arrangements in polymers are more complex than in metals, since it involves the configuration of large (macro) molecules instead of individual atoms. We think of polymer crystallinity as the packing of molecular chains so that an ordered molecular array is produced. Crystal structures in these materials may also be specified in terms of unit cells. For example, Fig. 3.3 shows the unit cell for polyethylene and its relationship to the molecular chain structure; this unit cell has orthorhombic symmetry.
Figure 3.3. The arrangement of molecular chains in a unit cell of polyethylene. The larger circles represent C and the smaller circles represent H.
In most cases, polymers are only partially crystalline, having crystalline regions interspersed with amorphous material. The degree of crystallinity may range from completely amorphous to almost entirely crystalline (~95%). Furthermore, the density of a crystalline polymer will be greater than that of its amorphous counterpart, since the chains are more closely packed together in the crystalline polymer. For example, high density polyethylene (HDPE) is more crystalline and is more thermally stable than low density polyethylene.
Many bulk polymers that are crystallized from a melt form spherulites. Each spherulite may grow to be spherical in shape and consists of an aggregate of ribbon like chain folded crystallites (lamellae) approximately 10 nm thick that radiate from the center outward. This is schematically illustrated in Fig. 3.4. Tie-chain molecules that act as connecting links between adjacent lamellae pass through these amorphous regions. As the crystallization of a spherulitic structure nears completion, the surfaces of adjacent spherulites begin to impinge on one another, forming more or less planar boundaries. Thus, spherulites can be considered to be the polymer analog of grains in polycrystalline metals and ceramics. In this lab, we will crystallize high density polyethylene and observe the spherulitic microstructure.
Figure 3.4. Schematic representation of the detailed structure of a spherulite.
The response of a semicrystalline polymer to an applied tensile load can be understood by referring to Fig. 3.5. Two adjacent chain-folded lamellae and the interlamellar amorphous material, prior to deformation, are shown in Fig. 3.5(a). During the initial stage of deformation (Fig. 3.5b), the lamellae slide past each other as the tie chains within the amorphous regions become extended. Continued deformation occurs by the tilting of the lamellae so that the chain folds become aligned with the tensile axis (see Fig. 3.5c). On additional deformation, separation between crystalline segments occur as shown in Fig. 3.5(d). In the final stage, the blocks and tie chains are aligned along the direction of the tensile axis as depicted in Fig. 3.5(e), resulting in a highly oriented structure. As in the case of metals, the mechanical properties of semicrystalline polymers can be affected by manipulating their microstructures. For example, polymers can be strengthened by increasing the degree of crosslinking.
Figure 3.5 (a-c) Stages in the deformation of a semicrystalline polymer. (a) Two adjacent chain folded lamellae and interlamellar amorphous material before deformation. (b) Elongation of amorphous tie chains during the first stage of deformation. (c) Tilting of lamellar chain folds during the second stage.
Figure 3.5 (d,e) Stages in the deformation of a semicrystalline polymer. (d) Separation of the crystalline block segments during the third stage. (e) Orientation of the block segments and tie chains with the tensile axis in the final deformation stage.
The temperatures at which melting and/or the glass transition occur for a polymer are determined from a plot of specific volume versus temperature. Figure 3.6 shows such a plot, where curves A and C are for amorphous and crystalline polymers, respectively. For the crystalline material, there is a discontinuous change in specific volume at the melting temperature, Tm. The curve for the amorphous material is continuous, but upon cooling experiences a slight decrease in slope at the glass transition temperature, Tg. Below Tg, the material is considered to be amorphous; above Tg it is rubbery solid and then a viscous liquid. For a semicrystalline polymer (curve B), the behavior is intermediate between these extremes. Both melting and glass transition temperatures are observed; Tm and Tg are properties of the respective crystalline and amorphous phases.
Figure 3.6. Specific volume vs. temperature, upon cooling form the liquid melt, for totally amorphous (curve A) , semicrystalline (curve B), and crystalline (curve C) polymers.
I. Tensile properties of polyethylene and nylon.
In this experiment we will use standard tension testing techniques to obtain properties of three polymers, Kevlar, Nylon, and Lexan. The molecular structure of polyethylene was discussed earlier; the three polymers that we will test in the lab have a similar chain structure, but are built from different mers. Nylons are based on an amide linkage; this allows H-bonding to occur between the chains (see Fig. 3.7a). Kevlar is a highly crystalline polymer with rigid chains that resist bending. In Kevlar, H-bonding between the chains is also important. Lexan is an acrylic polymer built from the mers shown in Fig. 3.7c.
Figure 3.7. The building blocks found in (a) Nylon, (b) Kevlar, and (c) Lexan.
1) For each sample, make 2 marks at opposing ends of the gage section.
2) Measure the distance between the two marks, Lo. You will use this to obtain an elongation at the end of each experiment.
3) Measure the gage width and thickness for each sample. Are these dimensions consistent along the length of the gage?
4) Insert each tension sample into the Instron test machine and commence loading.
5) Following testing for each sample,
(i) Determine the final gage length, Lf. From this, calculate the total plastic strain (ep):
(ii) Determine the elastic modulus (in MPa).
(iii) Determine the stress at 1% strain (in MPa).
II. Polymer crystallization.
1) Cut a very thin sliver of high density polyethylene.
2) Turn the hot plate to "high" (at least 100 °C).
3) Place the glass slide on the hot plate and the polyethylene on the glass slide.
4) As the polyethylene softens, put a second glass slide over it and compress it slowly to spread the polyethylene into the thinnest possible layer.
5) In your notebook, describe the changes that you observe while the polyethylene is heating and cooling .
6) Examine the specimen with a light microscope, at 200X or higher magnification.
7) Make a sketch of what you see in your notebook and comment on any observed crystallinity.
8) Obtain a color micrograph using polarized light.
III. The temperature dependence of silicone rubber properties
In this experiment, we will study the elastic deformation of a crosslinked silicone rubber by measuring the rebound height of a ball as a function of temperature. The mer from which silicone rubber is built is illustrated in Fig. 3.8. The objective is to relate the rebound height of the ball to the temperature dependent elastic moduli and associated modes of deformation discussed in class.
Figure 3.8. The mer used to build silicone rubber.
1) Drill a 1 cm deep hole in a silicone rubber ball. The hole should be large enough to accommodate a type E thermocouple.
2) Immerse the ball in liquid N2 and soak for approximately 10 min.
3) Wearing protective gloves, insert the thermocouple into the hole in the ball. After it equilibrates, read the temperature.
4) As soon as possible after the temperature measurement, release the ball from the drop tube and measure the rebound height.
5) Repeat this procedure as the ball warms to room temperature, taking approximately 20 readings over the course of 30 min.
6) In your notebook, plot the rebound height as a function of temperature.
7) Identify the glass transition temperature.
8) Explain the observed trend in rebound height with temperature.