Unit 3 Polymers PDF

Polymers Unit 3. The Chemistry of Engineering Materials Learning Objectives Describe the properties and structure of polymers and know the common polymeric materials. Determine the average molecular weights of polymers and degree of polymerization. Cite the differences in behavior and molecu...

Polymers Unit 3. The Chemistry of Engineering Materials Learning Objectives Describe the properties and structure of polymers and know the common polymeric materials. Determine the average molecular weights of polymers and degree of polymerization. Cite the differences in behavior and molecular structure of thermoplastic and thermosetting polymers. Describe the sequencing arrangements along polymer chains and crystalline state in polymeric materials. Topic Outline 1. Properties and Structures of Polymers and Common Polymeric Materials 2. Molecular Weight and Degree of Polymerization 3. Thermoplastic and Thermosetting Polymers 4. Copolymers 5. Polymers Crystallinity  1 Properties and Structures of Polymers and Common Polymeric Materials Properties and Characterization of Polymers A polymer is a molecular compound that can be distinguished by a high molar mass, ranging into thousands and even millions of mass and they are made up of many repeating units. Properties and Characterization of Polymers Synthetic (man-made) polymers were first developed in the early 20th century, and these polymers remarkably transformed our world as different materials can be created with properties that are ideal for different applications. Natural polymers have been around since life itself began. Cellulose, starch, and other complex carbohydrates are examples of natural polymers. Natural rubber is a polymer obtained from rubber trees and even the code for life itself, DNA, is a natural polymer. Properties and Characterization of Polymers Monomers (mono meaning “one”; meros meaning “unit”) are the small molecules that are used for synthesizing polymers and each monomer is analogous to a link in a chain. Monomers, simple repeating units, and this type of composition markedly restricts the number of possible isomers. Polymers (poly means “many”) can be created from one monomer, of from a combination of two or more different monomers. Properties and Characterization of Polymers If a polymer is made up of only type of monomer (e.g. polyethylene), then it is known as homopolymer. Other homopolymer that are synthesized by the radical mechanism are TeflonTM, polytetrafuoroethylene and poly(vinyl chloride) (PVC). Properties and Characterization of Polymers Synthetic polymers can be made from many different starting materials which usually come from crude oil (raw material). Presently, crude oil is the starting material for many plastics, pharmaceuticals, fabrics, and other carbon-based products. Polymer Molecules The molecules in polymers are gigantic and because of their size they are often referred to as macromolecules. The backbone of each of a carbon-chain polymer is a string of carbon atoms and within each molecule, the atoms are bound together by covalent interatomic bonds. Polymer Molecules Many times each carbon atom singly bonds to two adjacent carbon atoms on either side which is represented as follows: Each of the two remaining valence electrons for every carbon atom may be involved in side bonding with atoms or radicals that are positioned adjacent to the chain. Of course, both chain and side double bonds are also possible. The Chemistry of Polymer Molecules The hydrocarbon ethylene (C2H4) is a gas at ambient temperature and pressure which has the following molecular structure: The Chemistry of Polymer Molecules Under appropriate conditions, ethylene gas reacted and it will transform to polyethylene (PE) which is a solid polymeric material. This process begins when an active center is formed by the reaction between an initiator or catalyst species (R·) and the ethylene monomer, as follows: The Chemistry of Polymer Molecules Next, the polymer chain forms by the sequential addition of monomer units to this actively growing chain molecule which is represented schematically as follows: The Chemistry of Polymer Molecules After the addition of many or alternatively as: ethylene monomer units, the final result is the polyethylene molecule (Figure 1). Representation of polyethylene chain structure is shown below: Here, the repeat units are enclosed in parentheses, and the subscript n indicates the number of times it repeats. The Chemistry of Polymer Molecules Figure 1. For polyethylene, (a) a schematic representation of repeat unit and chain structures, and (b) a perspective of the molecule, indicating the zigzag backbone structure (Callister & Rethwisch, 2014). The Chemistry of Polymer Molecules Other chemistry of polymer structure such as tetrafluoroethylene monomer to form polytetrafluoroethylene (PTFE) is shown below: Polytetrafluoroethylene (having the trade name Teflon) belongs to a family of polymers called the fluorocarbons. The Chemistry of Polymer Molecules The vinyl chloride monomer (CH2=CHCl) is a slight variant of that for ethylene, in which one of the four H atoms is replaced with a Cl atom. Its polymerization is represented as: and leads to poly(vinyl chloride) (PVC), another common polymer. The Chemistry of Polymer Molecules Some polymers may be represented using the following generalized form: where the R represents either an atom [i.e., H or Cl, for polyethylene or poly(vinyl chloride), respectively] or an organic group such as CH3, C2H5, and C6H5 (methyl, ethyl, and phenyl) (Figure 2). The Chemistry of Polymer Molecules Figure 2. Repeat unit and chain structures for (a) polytetrafluoroethylene, (b) poly(vinyl chloride), and (c) polypropylene (Callister & Rethwisch, 2014). Molecular Structures of Polymers Molecular weight and shape of a polymer is not the only basis of its physical characteristics, the difference in the structure of the molecular chains must also be considered. Table 1. Description and schematic representations of linear, branched, crosslinked, and network (three-dimensional) molecular structures. Circles designate individual repeat units. Molecular Structures of Polymers Molecular Structures of Polymers Molecular Structures of Polymers Note that polymers may have more than one distinctive structural type, for example, a linear polymer may have limited branching and crosslinking. Common Polymeric Material Presently, there are more than 60,000 synthetic polymers known, with this, six types of polymers (Table 2) account for roughly 75% of those used in both Europe and the United States. Table 2. Six Common Polymers (Symbols retrieved from: https://www.acmeplastics.com/content/your-guide-to-plastic-recycling-s ymbols/ ) Common Polymeric Material Common Polymeric Material Common Polymeric Material Common Polymeric Material  2 Molecular Weight and Degree of Polymerization Molecular Weight and Degree of Polymerization Polymers with very long chains has extremely large molecular weights but during polymerization process, not all polymer chains will grow to the same length and this results in a distribution of chain lengths or molecular weights. Usually, an average molecular weight is specified, which can be determined by the measurement of various physical properties such as viscosity and osmotic pressure. Molecular Weight and Degree of Polymerization There are several ways of defining average molecular weight. The number-average molecular weight Mn is obtained by dividing the chains into a series of size ranges and then determining the number fraction of chains within each size range (Figure 3). Molecular Weight and Degree of Polymerization The number- average molecular weight is expressed as: where Mi represents the mean (middle) molecular weight of size range i, and Xi is the fraction of the total number of chains within the corresponding size range. Molecular Weight and Degree of Polymerization A weight-average molecular weight Mw is based on the weight fraction of molecules within the various size ranges. It is calculated according to: where, again, Mi is the mean molecular weight within a size range, whereas Wi denotes the weight fraction of molecules within the same size interval. Degree of Polymerization Degree of Polymerization (DP) is an alternative way of expressing average chain size of a polymer. DP represents the average number of repeat units in a chain and it is related to the number-average molecular weight Mn by the equation: where m is the repeat unit molecular weight. Polymer Molecular Size Distribution Figure 3. Hypothetical polymer molecule size distributions on the basis of (a) number and (b) weight fractions of molecules Molecular Weight and Degree of Polymerization The length of polymer chains has affected many polymer properties. For example, as molecular weight (about 100,000 g/mol) of a polymer increases, its melting or softening temperature also increases. But for polymers with very short chains or having a molecular weights on the order of 100 g/mol, will usually exist as liquids at room temperature. Molecular Weight and Degree of Polymerization Those with molecular weights of approximately 1000 g/mol exists as waxy solids (e.g. paraffin wax) and soft resins. For polymers with molecular weights ranging between 10,000 and several million g/mol exist as solid, they are sometimes termed as high polymers. Therefore, the same polymer material can acquire various properties if it is produced with different molecular weight. Example 1. Assume that the molecular weight distributions shown in Figure 3 are for poly(vinyl chloride). For this material, compute (a) the number- average molecular weight, (b) the degree of polymerization, and (c) the weight-average molecular weight. Data to be used for Number/Weight-Average Molecular Weight Computations in Example 1.  3 Thermoplastic and Thermosetting Polymers Thermoplastic and Thermosetting Polymers Molecular structure has a great effect on how polymers react to mechanical forces at elevated temperatures. Indeed, one classification for these materials is according to behavior with rising temperature. Thermoplastics (or thermoplastic polymers) and thermosets (or thermosetting polymers) are the two subdivisions. Thermoplastic Polymers Thermoplastics soften upon heating and later liquefy, then it hardens when cooled. This process is reversible and can be repeated. Exposure of a molten thermoplastic polymer to a very high temperature results to an irreversible degradation. Examples of common thermoplastic polymers are polyethylenpe, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride). Thermosetting Polymers Thermosetting polymers are network polymers, they do not soften upon heating and they become permanently hard during their formation. Network polymers have covalent crosslinks between adjacent molecular chains. During heat treatment, the bonds fasten the chains together to resist the vibrational and rotational chain motions at high temperatures. Therefore, the materials do not soften when heated. Thermosetting Polymers Excessive heating temperatures will cause severance of these crosslink bonds and polymer degradation. As compared to thermoplastics, these thermoset polymers are generally harder and stronger and have better dimensional stability. Examples of these thermosets (crosslinked and network polymers) are vulcanized rubbers, epoxies, phenolics, and some polyester resins.  4 Copolymers Copolymers A copolymer is composed of two repeat units as represented in Table 3. It is possible that there are different sequencing arrangements along the polymer chains which depends on the polymerization process and the relative fractions of these repeat unit types. Synthetic rubbers are usually copolymers. Table 3. Schematic representations of random, alternating, block, and graft copolymers. The two different repeat unit types are designated by blue and red circles Copolymers  5 Polymer Crystallinity Polymer Crystallinity In crystalline state, the atomic arrangement in polymer materials are more complex as compared to metals and ceramics because in polymers it involves molecules instead of just atoms or ions. Polymer crystallinity is the packing of molecular chains to produce an ordered atomic array. Crystal structures may be specified in terms of unit cells, which are often quite complex. Figure 4 shows the example of a unit cell for polyethylene and its relationship to the molecular chain structure (unit has orthorhombic geometry). Polymer Crystallinity Figure 4. Arrangement of molecular chains in a unit cell for polyethylene Polymer Crystallinity Polymer molecules are often partially crystalline (or semicrystalline), having crystalline regions dispersed within the remaining amorphous material. An amorphous region is the result of any chain disorder or misalignment, a case that is quite common, because twisting, kinking, and coiling of the chains hinder the strict ordering of every segment of every chain. The extent of crystallinity may range from completely amorphous to almost entirely (up to about 95%) crystalline. If compared to metal specimens (almost always entirely crystalline) and many ceramics (either totally crystalline or totally noncrystalline) polymeric materials behave differently. Thank you!