polymeric materials characteristics applications mechanisms of polymerization structure–property...

POLYMERIC MATERIALS

• Characteristics

• Applications

• Mechanisms of Polymerization

• Structure–Property Relationships

• Classification of Polymers

• Glasses

• Processing of Polymers

• Mechanical Properties

• Polymer Types

POLYMERIC MATERIALS Important Characteristics of Polymers

The mechanical properties of polymers generally include low strength and high toughness. Their strength is often improved using reinforced composite structures.

Size: Single polymer molecules typically have molcular weights between 10,000 and 1,000,000 g/mol--that can be more than 2000 repeating units depending on the polymer structure! The mechanical properties of a polymer are significantly affected by the molecular weight, with better engineering properties at higher molecular weights.

Thermal Transitions: The softening point (glass transition temperature) and the melting point of a polymer will determine which applications it will be suitable for.

Crystallinity: Polymers can be crystalline or amorphous, but they usually have a combination of crystalline and amorphous structures (semi-crystalline).

Interchain Interactions: The polymer chains can be free to slide past one another (thermoplastic) or they can be connected to each other with crosslinks ( thermoset or elastomer). Thermoplastics can be reformed and recycled, while thermosets and elastomers are not reworkable.

Interchain Structure: The chemical structure of the chains also has a tremendous effect on the properties. Depending on the structure the polymer may be hydrophillic or hydrophobic (likes or hates water), stiff or flexible, crystalline or amorphous, reactive or unreactive.

http://www.mse.cornell.edu/courses/engri111/compo.htm

http://www.mse.cornell.edu/courses/engri111/elastome.htm

POLYMERIC MATERIALS

POLYMER composed primarily of C and H, they have low melting temperature, poor thermal and electrical conductors. Some are crystals, many are not, high plasticity, a few have good elasticity. Some are transparent, some are opaque. Low density structures.

Main applications

• Films, foams, paints, fibers, and structural materials.

• Microelectronics industry, fabrication of semiconductor devices.

• Plastics, Liquid crystals, Adhesives and glues

• Containers and Water-resistant coatings (latex)

• Moldable products (computer casings, telephone handsets)

• Clothing (vinyl , polyesters, nylon), Biomaterials (organic/inorganic)

• Low-friction materials (teflon), Synthetic oils and greases

• Gaskets and O-rings (rubber), Soaps and surfactants

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Table 1 Composition and Molecular Structures for some of

the Paraffin compounds CnH2n+2

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Industrially Important Polymers

Polymer Example Applications

Polyethylene (PE) Electrical wire insulation, flexible tubing, bottles

Polypropylene (PP) carpet fibers, liquid containers (cups, buckets, tanks), pipes

Polystyrene (PS) packaging foams, egg cartons, lighting panels, electrical components

Polyvinyl chloride (PVC) bottles, pipes, valves, electrical wire insulation, toys, raincoats, automobile roofs


Table 2 Repeat units and applications for selected addition thermoplastics

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Table 2 Repeat units and applications for selected addition thermoplastics (Continued)

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Table 3 Repeat units and applications for complex thermoplastics

Table 3 Repeat units and applications for complex thermoplastics (Continued)

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Table 4 Repeat units and applications for selected elastomers

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Table 4 Repeat units and applications for selected elastomers (continued)

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Table 5 Repeat units and applications for selected thermosets

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Mechanisms of PolymerizationPolymerization is the formation of chemical linkages between relatively small molecules or monomers to form very large molecules or polymers. These linkages are formed by either one or two of the following two types: addition or condensation.

Addition polymerization process is characterized by the simple combination of molecules without the generation of any by-products as a result of the combination. The original molecules do not decompose to form reaction debris. When units of single monomers are hooked together, the resulting product is a homopolymer, such as polyethylene, that is made from the ethylene monomer. When two or more polymers are used in the process, the product is a co-polymer.

Condensation polymerization involves the chemical reaction of two or more chemicals to form a new molecule. The chemical union of two molecules can be only achieved by the formation of a by-product molecule with atoms from the two molecules to create the link for the polymerization to continue. This chemical reaction produces a condensate or non-polymerizable byproduct, usually water. A catalyst is often required to start and maintain the reaction. It can also be used to control the reaction rate.

Addition polymerization - Process by which polymer chains are built up by adding monomers together without creating a byproduct.

Unsaturated bond - The double- or even triple-covalent bond joining two atoms together in an organic molecule.

Functionality - The number of sites on a monomer at which polymerization can occur.

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The addition reaction for producing polyethylene from ethylene molecules. The unsaturated double bond in the monomer is broken to produce active sites, which then attract additional repeat units to either end to produce a chain.

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Addition (Chain) Polymerization

– Initiation

– Propagation

– Termination

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Condensation polymerization - A polymerization mechanism in which a small molecule (e.g., water, methanol, etc.) is condensed out as a byproduct.

Condensation (Step) Polymerization

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The condensation reaction for polyethylene terephthalate (PET), a common polyester. The OCH3 group and a hydrogen atom are removed from the monomers, permitting the two monomers to join and producing methyl alcohol as a byproduct.

Condensation (Step) Polymerization

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Degree of polymerization - The average molecular weight of the polymer divided by the molecular weight of the monomer.

Example: Calculate the degree of polymerization if 6,6-nylon has a molecular weight of 120,000 g/mol.

SOLUTION: The molecular weights are 116 g/mol for hexamethylene diamine, 146 g/mol for adipic acid, and 18 g/mol for water. The repeat unit for 6,6-nylon is


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Example SOLUTION (Continued)

The molecular weight of the repeat unit is the sum of the molecular weights of the monomers, minus that of the two water molecules that are evolved:

Mrepeat unit = 116 + 146 – 2(18) = 226 g/mol

Degree of polymerization = 120,000/226 = 531

The degree of polymerization refers to the total number of repeat units in the chain. The chain contains 531 hexamethylene diamine and 531 adipic acid molecules.

POLYMERIC MATERIALS

Structure–Property Relationships in Thermoplastics

Branched polymer - Any polymer consisting of chains that consist of a main chain and secondary chains that branch off from the main chain.

Crystallinity is important in polymers since it affects mechanical and optical properties.

Tacticity - Describes the location in the polymer chain of atoms or atom groups in nonsymmetrical monomers.

Liquid-crystalline polymers - Exceptionally stiff polymer chains that act as rigid rods, even above their melting point.

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Polymeric structurePolymers are a group of materials characterized by chains of molecules made up of smaller units called monomers, a majority of which is joined artificially.

Most polymers are organic (carbon-based) materials that contain molecules composed of various combinations of hydrogen, oxygen, nitrogen and carbon. These four elements are among the most common found in organic polymers. Carbon forms the spine in the polymer chain, and the other constituents attach themselves to the carbon.

Crystalline polymers usually contain regions of well-packed chains separated by amorphous (liquid-like) regions. Pulling a fiber of a linear polymer causes the chains to line up, and can induce crystallization. Besides viscosity, there are other factors that influence the ability of a polymer to crystallize. One of them is the nature of the side groups on the polymer chains. With very bulky side groups, or side groups that vary in an irregular way, the chains have a hard time organizing into an ordered, crystalline solid. This effect is important, because crystalline polymers tend to be much stiffer, harder, and denser than amorphous polymers. A good example of this phenomenon is polypropylene, which can be made in either atactic, isotactic, or syndiotactic forms.

The atactic form doesn't pack well and therefore is normally amorphous, and is used in making garbage bags and other applications where flexible plastics are needed. The isotactic form is crystalline at ordinary temperatures (its melting point is 160oC), is translucent and much stiffer, and is used to make jars, tupperware, etc.

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Three possible arrangements of nonsymmetrical monomers: (a) isotactic, (b)syndiotactic, and (c)atactic.

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The relationship between the density and the temperature of the polymer shows the melting and glass temperatures. Note that Tg and Tm are not fixed; rather, they are ranges of temperatures.

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Both Tm and Tg increase with increasing chain stiffness.

Regularity – effects Tm only

Adapted from Fig. 15.18, Callister 7e.

Melting vs. Glass Transition Temp.

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Table 4 Melting and glass temperature ranges (0C) for selected

thermoplastics and elastomers


The folded chain ,model for crystallinity in polymers, shown in (a) two dimensions and (b) three dimensions.

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Photograph of spherulitic crystals in an amorphous matrix of nylon ( 200). (From R. Brick, A. Pense and R. Gordon, Structure and Properties of Engineering Materials, 4th Ed., McGraw-Hill, 1977.)

Crystal structure of several polymers

POLYMERIC MATERIALS Classification of Polymers

The terms Thermoplastic and thermoset refer to the properties of polymers. Polymers are separated into these two general classifications. In general, the properties of polymers depend on the additives used, materials added to increase polymer's strength; the amounts and properties of fillers used; coloring agents used; and plasticizers, which are added as internal lubricants.

- Thermoplastic polymersThermoplastic polymers are generally available in films, sheets, rods, tubing, and several molded or extruded shapes. Thermoplastic polymers often exhibit plastic, ductile properties. They can be formed at elevated temperatures, cooled, remelted, and reformed into different shapes without changing the properties of the polymer. The properties of thermoplastic polymers are determined by the bonding method between polymer chains; in thermoplastic materials these bonds are weak. Common thermoplastic polymers include acrylic, nylon, cellulose, polystyrene, fluorocarbons, and vinyl.

- Thermosetting polymersThermosetting polymers have strong primary bonds, often formed by condensation polymerization. Thermosetting polymers have strong primary bonds throughout, and their structure resembles one large molecule. Their properties are the result of chemical changes undergone during processing, under heat or through the application of a catalyst. Once hardened, thermosets can not be softened or reshaped, due to the loss of a part of the molecule during the curing process. Once cured, if further heat is applied to a thermosetting material, it will char, burn, or decompose.

• Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene

• Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin

Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)

Thermoplastics vs. Thermosets

Callister, Fig. 16.9

T

Molecular weight

Tg

Tmmobile liquid

viscous liquid

rubber

tough plastic

partially crystalline solid

crystalline solid

Polymer Additives

Improve mechanical properties, processability, durability, etc.

Fillers– Added to improve tensile strength & abrasion resistance,

toughness & decrease cost– ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc.

• Plasticizers – Added to reduce the glass transition

temperature Tg

– commonly added to PVC - otherwise it is brittle

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Polymer Additives

Stabilizers– Antioxidants, Antistatic Agent– UV protectants, Catalysts

• Lubricants – Added to allow easier processing – “slides” through dies easier – ex: Na stearate

• Colorants – Dyes or pigments

• Flame Retardants– Cl/F & B

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• Reinforcements

POLYMERIC MATERIALS Glasses

A glass is an inorganic nonmetallic material that does not have a crystalline structure. Such materials are said to be amorphous.

Glasses have historically been used for low technology applications such as soda bottles and window panes. However, glasses, like ceramics, have recently found new application in high technology fields, particularly the semiconductor microelectronics industry where silica is widely used as an insulator in transistors and the fiber optic cable industry where high purity silica glass has made advanced telecommunications possible.

Three of the most common uses for glasses: windows, liquid crystal displays, and optical fibers.

Examples of glasses range from the soda-lime silicate glass in soda bottles to the extremely high purity silica glass in optical fiber. As with ceramics, the list of industrially important glasses also continues to grow.

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Industrially Important Glasses

Glass Ex. Properties

Silica glassSiO2 Used for optical fibers when it is very pure

Soda-lime glass SiO2-Na2O-

CaO

standard glass used for bottles and windows due to its low cost and easy manufacturing

Borosilicate glass

SiO2-B2O3 thermal shock resistance (glassware) and low coefficient of thermal expansion

Lead glass SiO2-PbO high index of refraction

Polymer Crystallinity

Polymers rarely 100% crystalline Too difficult to get all those chains

aligned

• % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases.

Adapted from Fig. 14.11, Callister 6e.(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

crystalline region

amorphousregion

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Polymer Processing and Recycling

Forming Processes for Thermoplastics:• Extrusion• Blow Molding• Injection Molding• Thermoforming

Forming Processes for Thermosetting polymers:• Calendaring• Spinning• Compression Molding• Transfer Molding

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POLYMERIC MATERIALSExtrusion: The polymer is heated to the liquid state and forced through a die under pressure resulting in an endless product of constant cross section. Examples include: tubing, pipes, window frames, sheet, and insulated wire.

Film Blowing: Using the same method as extrusion the material coming out of the die is blown into a film. An example is plastic wrap.

Injection molding: Similar to extrusion, the polymer is heated to the liquid state, but it is prepared in metered amounts, and the melt is forced into a mold to create the part. It is not a continuous process. Many toys are made by injection molding.

Blow molding: The melted polymer is put into a mold, and then compressed air is used to spread the polymer into the mold. It is used to make many containers such as plastic soda containers and milk jugs.

Compression molding: Solid polymer is placed in a mold; the mold is heated and puts pressure on the polymer to form the part.

Reaction injection molding: Liquid monomers are placed in the mold avoiding the need to use temperature to melt the polymer or pressure to inject it. The monomers polymerize in the mold forming the part.

Processing Plastics - Molding

Injection molding– thermoplastic & some thermosets

Adapted from Fig. 15.24, Callister 7e. (Fig. 15.24 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, 1971. )

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Processing Plastics – Extrusion

Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.)

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Blown-Film Extrusion

Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclopædia Britannica, 1997.)

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Mechanical Propertiesbrittle polymer

plasticelastomer

FS of polymer

10% that of metals

Strains – deformations > 1000% possible (for metals, maximum strain ca. 10% or less)

elastic modulus – less than metal

Adapted from Fig. 15.1, Callister 7e.

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• Decreasing T... -- increases E -- increases TS -- decreases %EL

• Increasing strain rate... -- same effects as decreasing T.

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The stress-strain curve for 6,6-nylon, a typical thermoplastic polymer.

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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under

license.

Necks are not stable in amorphous polymers, because local alignment strengthens the necked region and reduces its rate of deformation.

Mechanical Properties of Thermoplastics

Compare the mechanical properties of LD polyethylene, HD polyethylene, polyvinyl chloride, polypropylene, and polystyrene, and explain their differences in terms of their structures.

Example 15.8 SOLUTION

Let us look at the maximum tensile strength and modulus of elasticity for each polymer.

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Example 15.8 SOLUTION (Continued)

We can conclude that:

1. Branching, which reduces the density and close packing of chains, reduces the mechanical properties of polyethylene.

2. Adding atoms or atom groups other than hydrogen to the chain increases strength and stiffness. The methyl group in polypropylene provides some improvement, the benzene ring of styrene provides higher properties, and the chlorine atom in polyvinyl chloride provides a large increase in properties.


The effect of temperature on the stress-rupture behavior of high-density polyethylene.

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Creep curves for acrylic (PMMA) (colored lines) and polypropylene (black lines) at 20°C and several applied stresses.


The effect of applied stress on the percent creep strain for three polymers

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Table 5 Deflection temperatures for selected polymers for a 264-Psi load


The stress-strain curve for an elastomer. Virtually all of the deformation is elastic; therefore, the modulus of elasticity varies as the strain changes.

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Table 6 properties of selected elastomerTable 7 properties of typical thermosetting polymer.

Polymer Types: Elastomers

Elastomers – rubber Crosslinked materials

– Natural rubber

– Synthetic rubber and thermoplastic elastomers

» SBR- styrene-butadiene rubber

styrene

– Silicone rubber

butadiene

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Polymer Types: Fibers

Fibers - length/diameter >100 Textiles are main use

– Must have high tensile strength

– Usually highly crystalline & highly polar

• Formed by spinning– ex: extrude polymer through a spinnerette

• Pt plate with 1000’s of holes for nylon

• ex: rayon – dissolved in solvent then pumped through die head to make fibers

– the fibers are drawn

– leads to highly aligned chains- fibrillar structure

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Polymer Types

Coatings – thin film on surface – i.e. paint, varnish– To protect item

– Improve appearance

– Electrical insulation

• Adhesives –Usually bonded by Secondary and Mechanical bonding

1.Chemically Reactive Adhesives

2.Evaporation or Diffusion Adhesives

3.Hot-Melt Adhesives

4.Pressure-Sensitive Adhesives

5.Conductive Adhesives• Films – blown film extrusion

• Foams – gas bubbles in plastic

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Advanced Polymers

Ultrahigh molecular weight polyethylene (UHMWPE)

– Molecular weight ca. 4 x 106 g/mol

– Excellent properties for variety of applications

» bullet-proof vest, golf ball covers, hip joints, etc.

UHMWPE

Adapted from chapter-opening photograph, Chapter 22, Callister 7e.

POLYMERIC MATERIALS

polymeric materials characteristics applications mechanisms of polymerization structure–property...

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