Polymers Prior Knowledge

Polymers Prior Knowledge:  Intermolecular Forces  Molar mass  Molecular and Empirical Formulas  Stoichiometry Learning Objectives Students will be able to: Content:  Define what a polymer is.  Identify the relationship of small molecules (monomers) to polymers  Understand the concept of polymerization  Understand the relationship between polymers and bioplastics. Process  Determine the properties of polymers as related to its monomer properties.  Compare the relationship of a polymer’s properties to its structure.  Interpret data to describe the process of polymerization. Techniques  Use molecular models to represent polymer structure and polymer synthesis. Introduction to Polymers Polymers are substances containing many structural units joined by the same type of linkage. These substances often form into a chain-like structure. Polymers in the natural world have been around since the beginning of time. For example, DNA (deoxyribose nucleic acid) is a natural occurring polymer found in most organisms. Starch, cellulose, and rubber all possess polymeric properties. Polymers can be categorized as organic base (i.e., latex, proteins, polyethylene, etc.) or inorganic (i.e., graphite, silicone rubber (polysiloxanes, etc.) A common misconception is the term polymer is synonymous with the term plastics. While plastics are a category of polymers, polymers are a diverse group of compounds. Synthetic polymers (i.e. man-made) have been studied since 1832. These materials that have become commonplace in our daily lives. Polymers already have a range of applications that far exceeds that of any other class of material available to man. Current applications extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, elastomers, and structural plastics. Polymers are also used for most composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics. In 2013 alone, over 300 million pounds of plastics were produced worldwide. About half of all chemistry related jobs are currently centered on the topic of polymers. Today, the polymer industry has grown to be larger than the aluminum, copper and steel industries combined. This laboratory exercise will introduce you to the basic concepts of polymer structure and formation as well as relating polymer structure to its physical properties. You will also learn the relationship between synthetic polymers and bioplastics. 1st Question of the Day: What is a polymer? Background: The field of polymer science is vast, and its applications varied, that it is important to understand how polymers are made and used. There are over 60,000 different plastics vying for a place in the market. Polymers are amorphous-like solids. They are composed of long chains of small units, referred to as monomers; with the monomer linked together to form a large molecule. This large molecule is called a macromolecule. Macromolecules are giant, chain-like compounds that can contain up to a million atoms. A polymer’s molecular weight can range from 14,000 to 500,000. Examples of macromolecules are proteins, nucleic acid, carbohydrates, natural rubber, and synthetic polymers. The word polymer comes from the Greek polys, meaning many and meros means part or unit. These polymer units can be either small molecules or atoms that have been chemically linked. Polymer monomers can be of the same molecule, such as polyethylene, which is composed of ethylene molecules or the units can be combinations of two or more different units which are referred to as co-polymers. The type (i.e. same or copolymer units) and arrangement of the monomers within the macromolecule affect the physical and chemical properties of the polymer. Monomers can be arranged in several different ways: linear, branched, or cross-linked. Linear polymers are made up of one long continuous chain, without any excess appendages or attachments. Branched polymers have a chain structure that consists of one main chain of molecules with smaller molecular chains branching from it. A branched chain-structure tends to lower the degree of crystallinity and density of a polymer. Cross-linking in polymers occurs when primary valence bonds (i.e. covalent or ionic) are formed between separate polymer chain molecules. Figure 1: Illustration of linear, branched, and crosslinked/network polymers structures. (polymerinnovationblog.com/polymers) 2nd Question of the Day: How does a polymer’s chemical and physical structure affect its physical properties and the type of intermolecular forces present in the molecule? The chemical and physical characteristics of polymers are determined and influenced by the chemical composition of its monomer’s functional group. A functional group is a group of atoms (i.e. hydroxyl (-OH), carbonyl (C=O), etc.) and/or bonding type (i.e. single, double, or triple) which affect the chemical and physical characteristics of the molecule. Molecular structure is affected by the placement of functional groups within the polymer strand and are responsible for the type of intermolecular attractions within the polymer and between polymer strands. Examples on functional group placement: Synthetic rubber polymers of Polychloroprene & Polynorborene Polychloroprene (i.e. neoprene) and polynorbornene are synthetic rubber polymers whose chemical and physical properties are influenced by the polymer’s monomer functional groups. Neoprene’s functional group is a double bonded carbon atoms with an attached chlorine atom. Its unit molecular mass is 88.5 g/mol with an entangled molecular mass having a range from 5.0 x10 3 to 2.0 x 104 g/mol. Its density is 1.12 – 1.23 g/cm3. The double bonded carbon atoms somewhat restrict movement around the bond but remains flexible over a wide range of temperatures. Examples of neoprene products include O-rings, scuba suits, shock absorber seals, braking systems, and fan belts. Polymerization of chloroprene (2-chlor-1.3-butadiene) Neoprene Polynorbornene (i.e. Norsorex@) has shape memory. Polynorborene is a bridged cyclic hydrocarbon consisting of a cyclohexene ring with an ethylene bridge at carbon 1 and 4 on the ring. This ethylene bridge not only restricts movement of the molecule, but also its rigidity. Polynorbornene’s density is 1.10 – 1.17 g/cm 3 with a unit molecular mass of 94.2 g/mol and an entangled molecular mass of 3.0 x 10 6 g/mol. Polynorborene is used in ballistic containers, dampers in loudspeakers, and shoe soles. Polymerization for the Synthesis of norbornene The molecular structure of synthetic polymers and biological polymers (i.e. nucleic acids, proteins, starches, silk, natural rubber, etc.), are categorized as being primary, secondary, tertiary, and quaternary. Primary structure refers to the ordering of the atoms within the long chains, whereas secondary structure is about how these long molecular chains coil, entwine with one another. Tertiary structure is the 3-D arrangement of the molecular chain and quaternary structure is the 3-D arrangement of the molecular chains with either another smaller molecular or metal ion. Polymer strands are held together by intermolecular forces. The type and strength of these intermolecular attraction forces are dictated by the polymer’s chemical composition (I.e. functional group) and placement (i.e. primary structure). Intermolecular forces influence the polymer’s secondary & tertiary structure; they act as stabilizing forces. As noted earlier, polymer chain structure can vary. A straight chain polymer is the simplest form, followed by branched and then crosslinked polymer chains. Polymeric fluids, both natural (i.e. honey, tree sap) and synthetic (i.e. Silly Putty, slime) are examples of cross-linked polymeric substances. Polymeric fluids, as known as nonNewtonian fluids, do not behave like normal fluids and do not obey the usual laws of viscosity or flow of liquids. Figure 2. Illustration of branched polymer chains. Figure 3. Illustration of a cross-linked polymer. Polymeric fluids tend to expand rather than compress when placed under stress or pressure. For example, by slowly pulling on a piece of silly putty, the silly putty flows (i.e. elongates) forming a long thin strand. The act of pulling slowly is low stress. However, if you sharply tug (high stress) at a piece of silly putty it will snap and break apart. Both slime and silly putty are composed of linear polymer chains, that are cross-linked with sodium borate. The strength of crosslinks affects the Figure 4: An illustration of sodium borate crosslinked polymer elasticity of the polymeric fluid. If the cross-linked bonds in a polymer are permanent, the polymer is a classified as a thermosetting plastic. If the bonds are non-permanent, meaning the bond can be broken if enough energy or force is supplied, then the polymer is categorized as an elastomer. 3rd Question of the Day: How are polymer strands formed? The chemical reaction in which high molecular mass molecules are formed from monomers is known as polymerization (Figure 5). Below is the addition polymerization reaction for high density polyethylene (HPDE). There are two basic types of polymerization reactions: addition (i.e. chain-reaction) and condensation (i.e. stepreaction) The formation of HDPE is an addition type polymerization. Figure 5: Polymerization Addition reaction for polyethylene (https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/polymers.htm) An addition type reaction is usually a three-step process involving two chemical units (monomers) that will be linked together with the assistance of a catalyst. In nearly all cases, the monomers have at least one carbon-carbon double bond. A condensation reaction involves two different types of monomer units having end functional groups that react (i.e. link) with one another to form a chain. During the linking/polymerization process, a small molecule by-product (i.e. HCl, H 2O, CO2 etc.) is produced. Once the polymerization reaction is initiated the polymer can continue to lengthen until no further monomer with reactive functional end groups are available. Condensation reactions are slow chemical reactions. Both linear and branch chains type polymers with or without crosslinking can be formed via condensation reactions. Figure is the condensation reach associated with the preparation of nylon-66. The merger of adipic acid (functional group is the - COOH) and hexanediamine (functional group is –HNH) results in the formation of nylon66 and a molecule of water (H2O). Figure 6: Condensation polymerization for formation of Nylon-66(chem.libretexts.org) Polymer chains, whether formed via an addition or condensation reaction can be classified as homopolymers or copolymers. Homopolymers are polymers consisting of the same type of monomer such a HDPE. Copolymers have been formed with at least two different types of monomer units. The arrangement of the monomer units (i.e. primary structure) plays a role in the structure and the physical properties of the polymer. 4th Question of the Day: How does knowing the molecular structure of polymers help predict polymer behavior? Physical Properties of Polymers The properties that have made polymers essential to modern society are strength, resistance to temperature change, flexibility, and the ability to be molded into various shapes. The polymers that were synthesized in have different properties, due to differing interactions/bonds within each polymer. These molecular level interactions manifest in the macroscopic world as polymer properties, such as rigidity, flexibility, transparency, durability, and others. Below is a listing of common polymeric physical properties:       Flexibility: the ability to bend without breaking. Rigidity: the inability to bend or be forced out of shape. Elongation: the ability to stretch or extend without breaking or taring. Recoil/Bounce: the ability to return to an original shape if pressure is applied. Transparency: the ability for light pass through a sample without being scattered. Durability: the ability to withstand prolonged wear, pressure, or damage. An important physical characteristic of polymers is their glass transition temperature (Tg). Glass transition temperature (Tg) is the temperature at or above where the molecular structure exhibits macromolecular mobility. Polymeric properties such as rigidity, elongation, elasticity can differ greatly depending on the deviation from the glass transition temperature. For example, there is a direct relationship of a polymer’s rigidity to its Tg; and increase in rigidity is seen with an increase in the polymer’s Tg. 5th Question of the Day: What makes thermoplastic or thermosetting polymer recyclable? Most synthetic plastics are classified as either thermoplastics or thermosetting. The prefix "thermo" comes from the Greek word meaning heat. Thermoplastics are materials which become soft when heated and hard again when cooled. Materials with these characteristics are recyclable. They can be heated (melted down) and remade (remolded) into new shapes and products to buy. Reheating and cooling do not change the chemical properties of these materials. A good example of a thermoplastic material is candle wax. When a candle is lit, it melts and changes its shape, but it does not become a brand-new substance such as wood or metal. Thermosetting plastics, once they have been shaped, cannot be reheated and/or remolded. They maintain their shape when heated again. This is because the process used to mold these types of plastics initially causes a chemical change in the thermosetting plastic's original chemical make-up (i.e. it becomes a new and different substance). Thermosetting plastics are not as flexible as thermoplastics; they are harder and more rigid. A good example of a thermosetting plastic is Bakelite @ a synthetic polymeric material that has good thermal and electrical insulating properties. Bakelite@ is used in a variety of products such as billiard balls, telephones, hairdryers, and other electrical appliances. Thermosetting plastics can be recycled, but it is limited. These plastics are usually shredded or chipped and used as fillers. Elastomers are rubberlike polymers and depending on their synthesize process can be classified as thermosetting or thermoplastic materials. Elastomers are polymers who are grouped based on their physical properties and characteristics rather than by their chemical properties. Elastomers tend to exhibit more elastic behaviors (i.e. stretch, bounce, low strength) and have excellent impact resistance. They are not easily recyclable materials. Structurally they are long coiled chains with some crosslinking Figure 7: Structural representations of thermoplastic. Elastomer and thermoset plastics (adhesiveandglue.com) 6th Question of the Day: What is a Bioplastic? Currently, many plastics are derived from fossil fuels, which are non-renewable and are quickly being depleted. A greater concern is the harmful effects to humans and the environment caused by accumulation of plastics on both land and water. Though some plastics can be recycled, only 9.5% of the total plastic waste generated in 2014 was recovered for recycling. 3 Additionally, most plastics are non-degradable and will last hundreds, if not thousands, of years. Green Chemistry principles developed by Paul Anastas and John Warner focus on sustainability and provide a framework for scientists to use when designing new products or processes. Principle seven states “a raw material or feedstock should be renewable rather than depleting whenever technically and economically practical.” Principle ten states “not only do we want materials and products to come from renewable resources, but we would also like them to not persist in the environment”.4 Bioplastics are synthesized polymeric materials made from renewable and non-toxic components. Plant starches are polymeric substances that have been isolated from biomass sources such as corn, potato, and tapioca root, instead of oil (petroleum) The two main bioplastics produced nowadays are polylactide acid (PLA) and polyhydroxyalkanoate (PHA).5 In this laboratory exercise you will synthesize three bioplastic polymers and investigate their physical properties and characteristics. Polymer 1 & 2: Citric Acid and Glycerol or Starch: The combination of citric acid (a carboxylic acid) with either glycerol or starch (alcohols, -OH) involves the formation of an ester bond as illustrated in Figure 7 below. In this reaction the carboxylic acid groups (-COOH) of citric acid react with the alcohol groups of either the glycerol or starch to form the ester bond and eliminate water. Strong heating and longer reaction times are required to drive off the water. Figure 7: Generic chemical reaction for the formation of an ester. 1 Polymer 3: Starch and Glycerol: The glycerol and starch polymers are different from the citric acid polymers. First, vinegar is used to break down the amylopectin cross-linking and create linear chains of the amylose. Since there are no carboxylic acids in either the glycerol or amylose, you are not producing any ester bonds. Instead, you are creating a network of hydrogen bonds. The alcohol groups on the glycerol molecule can form hydrogen bonds with the alcohol groups associated with the starch (Figure 8). Longer reaction times and stronger heating are not required because water does not need to be driven off for the interactions to take place. Figure 8. Hydrogen bonding between glycerol and starch (amylose)1 References: 1. 2. 3. 4. 5. Modified from: Smith, J. Dyeing to Degrade… A Bioplastics Experiment, 2018, Laboratory Experiment, University of Minnesota Modified from: Umar, Y. Polymer Basics: classroom Activities Manipulating Paper Clips to Introduce the Structure and Properties of Polymers, J. Chem. Ed. 2014, 91, 1667-1670. U.S. Environmental Protection Agency, Wastes – Resource Conservation – Common Wastes & Materials Anastas, P. T.; Warner, J.C. Green Chemistry: Theory and Practice, Oxford University Press, 1998, p. 30. Washam, C. Plastics Go Green. Chemmatters 2010, April 10–12. GO TO ASSESSMENT/QUIZ TAB ON THE LABORATORY CLASS FOLIO WEBSITE TO COMPLETE QUIZ FOR THIS UNIT1