Characteristics and Applications of Polymers Chapter 15 and 17

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Characteristics and Applications of Polymers Chapter 15 and 17 by Mind Map: Characteristics and Applications of Polymers Chapter 15 and 17

1. Mechanical Behavior of Polymers

1.1. 15.2) Stress-Strain Behavior

1.1.1. -> Mechanical characteristics are highly sensitive to the rate of deformation, temperature and chemical nature of the environment. -> Three specific types of stress-strain behaviour: For plastic material the initial deformation is elastic and is followed by yielding and plastic deformation. Brittle polymers fracture while deforming elastically and finally elastomers has rubber-like elasticity. -> Tensile strength (TS) corresponds to the stress at which fracture occurs

1.2. 15.3) Macroscopic Deformation

1.2.1. -> While being tested on a stress-strain basis, at the upper yield point, a small neck forms within the gauge section. In this neck, molecule chains become orientated leading to localised strengthening. -> Consequently, there is a resistance to continued deformation at that point and elongation proceeds along the gauge length.

1.3. 15.4) Visco-Elastic Deformation -> For relatively small deformations, mechanical behaviour expected at low temperatures are elastic - within Hooke's law: σ = Eϵ. -> At the highest temperatures, viscous behaviour prevails. -> At intermediate temperatures, polymers tend to be rubbery exhibiting a combination of mechanical characteristics - known as visco-elasticity. -> Elastic deformation is instantaneous and upon release of a external stress, the deformation is totally recovered. -> By total contrast for viscous behaviour, deformation is delayed/dependent on time. -> For intermediate visco-elastic behaviour, the application of a stress results in an instantaneous elastic strain followed by a viscous, time dependent strain.

1.3.1. Visco-Elastic Relaxation Modulus -> Visco-elastic behaviour is dependent on time and temperature. -> Stress tend to decrease with time because of molecular relaxation processes happening in polymers. -> Relaxation modulus is defined as a time-dependent elastic modulus for visco-elastic polymers: Er(t) = σ(t)/ϵ0. -> The magnitude of Er is a function of temperature. -> At the lowest temperatures, in the glassy region, the material is brittle and rigid. -> As temperature is increased, Er drops abruptly by about 10^3 within a 20 C˚ range and is called the leathery/glassy transition region and within this region deformation is time dependent and not totally elastic. -> Within the rubbery plateau temperature region, polymers deform in a rubbery manner and both elastic and viscous flow are present.

1.3.2. Visco-Elastic Creep -> Visco-elastic creep: susceptibility to time-dependent deformation when the stress level is maintained constantly. -> Creep is represented by a time-dependent creep modulus Ec: Ec(t) = σ0/ϵ(t).

1.4. 15.5) Fracture of Polymers

1.4.1. -> The mode of fracture in thermosetting polymers is brittle. -> Covalent bonds in a network/cross-linked structure are severed during fracture. -> Factors favouring brittle fracture: Temperature reduction, strain rate increase, presence of a sharp notch, increase in specimen thickness and any change in the polymer structure that raises the glass transition temperature. -> Crazing is a phenomenon that frequently precedes fracture. -> A craze is different than a crack because a craze can support a load across its face. -> Craze growth before cracking absorbs fracture energy and effectively increases the fracture toughness of polymers. -> Crazes form at highly stressed regions associated with scratches, flaws and molecular inhomogeneties and propagate perpendicular to the applied tensile stress.

1.5. 15.6) Miscellaneous Mechanical Characteristics

1.5.1. Impact Strength ->Izod or Charpy tests are usually used to assess impact strength of polymers. -> Polymers have shown brittle or ductile fracture under impact loading conditions, dependent on temperature, specimen size, strain rate and mode of loading. -> Semi-crystalline and amorphous polymers are brittle at low temperatures and have low impact strengths. -> Impact strength gradually decreases at constant high temperatures as the material softens.

1.5.2. Fatigue -> Polymers can experience fatigue failure due to cyclic loading. -> Fatigue fractures occur at stress levels relatively lower than yield stresses. -> Fatigue Limit: stress level where the stress at which failure becomes independent of the number of loading cycles. -> Cycling polymers at high frequencies and large stresses can cause localised heating and may lead to failure due to softening.

1.5.3. Tear Strength and Hardness ->Tear Strength: energy required to tear apart a cut specimen of standard geometry. -> Hardness tests are conducted by penetration techniques similar to the tests used for metals. -> Rockwell, Durometer and Barcol tests are used.

2. Mechanisms of Deformation and for Strengthening of Polymers

2.1. 15.7) Deformation of Semi-Crystalline Polymers

2.1.1. Mechanism of Elastic Deformation ->Elastic deformation occurs at relatively low stress levels. -> Elastic deformation onset for semi-crystalline polymers leads to chain molecules forming in amorphous regions elongation in the direction of the applied tensile stress. ->Amorphous chains align and become elongated together with bending and stretching of the strong chain covalent bonds within the lamellar crystallites.

2.1.2. Mechanism of Plastic Deformation -> During transition from elastic to plastic deformation, adjacent chains in the lamellae slide past one another resulting in tilting lamellae and chains consequently become more aligned with the tensile axis. -> Crystalline block segments separate from lamellae and then become orientated in the direction of the tensile axis. This orientation process is known as drawing. -> Drawing is commonly used to improve mechanical properties of polymer fibres and films. -> During deformation, spherulites changes shape for moderate levels of elongation.

2.2. 15.8) Factors that influence the Mechanical Properties of Semi-Crystalline Polymers

2.2.1. Molecular Weight -> Molecular weight does not directly influence the magnitude of the tensile modulus. -> Tensile strength increases with increasing molecular weight. -> Tensile strength is defined as TS: TS = TS∞ - A/M ̅n

2.2.2. Degree of Crystallinity -> For specific polymers, the degree of crystallinity has a significant influence on the mechanical properties because it affects the extent of the inter-molecular secondary bonding. -> Secondary bonding is much less prevalent in amorphous regions due to chain misalignment and as a consequence the tensile modulus increases significantly with degree of crystallinity for semi-crystalline polymers. -> Increasing a polymers crystallinity generally makes it stronger and more brittle.

2.2.3. Predeformation by Drawing -> Drawing: permanently deform a polymer in tension (corresponding to the neck-extension process). -> Drawing is an important strengthening technique used in the production of fibres and films. -> During drawing molecular chains slip past one another and become more orientated. -> Degree of strengthening depends on how much the material is deformed. -> Materials drawn in uni-axial directions are stronger than those drawn in other directions.

2.2.4. Heat-Treating (Annealing) -> Can lead to increases in crystallinity and crystallite size and perfection and modifications of the spherulite structure. -> An increase in annealing temperature leads to the following: Higher tensile modulus, increased yield strength and a reduction in ductility. -> For some previously drawn polymers, the influence of annealing is in contrast of the previously mentioned because of the loss of chain orientation.

2.3. 15.9) Deformation of Elastomers

2.3.1. Vulcanization -> Vulcanization: crosslinking process in elastomers achieved by a non-reversible chemical reaction. -> In most vulcanization reactions, sulphur compounds are added to a heating elastomer and chains of sulphur atoms bond with adjacent polymer backbone chains, crosslinking with them. -> Unvulcanized rubber (few crosslinks) is soft and has poor resistance to abrasion. -> The magnitude of the modulus of elasticity is directly proportional to the density of the crosslinks.

3. Crystallization, Melting, and Glass-Transition Phenomena in Polymers

3.1. 15.10) Crystallization

3.1.1. -> The degree of crystallinity influences the mechanical and thermal properties of polymers. -> Upon cooling, nuclei form in which small regions of random molecules become ordered and align in chain-folded layers. -> Nuclei grow by the continued ordering and aligning of additional molecular chain segments; chain layers remain the same thickness but increase in lateral dimensions/spherulite radius at spherulites. -> Fractionally crystallized y is a function of time t (Avrami Equation): y = 1 - exp(-kt^n), k and n time independent constants. -> The extent of crystallization is measured by the volume change in a specimen. -> Crystallization rate is dependent on temperature and molecular weight.

3.2. 15.11) Melting

3.2.1. -> The transformation of a solid material with a ordered structure into a viscous liquid. -> The crystallization temperature affects the melting behavior. -> Impurities and imperfections in polymers also decrease the melting temperature. -> Annealing also raises the melting temperature.

3.3. 15.12) The Glass-Transition

3.3.1. -> Occurs in amorphous (glassy) and semi-crystalline polymers due to a reduction in motion of large segments of molecular chains with decreasing temperature. -> Abrupt changes in other physical properties are caused due to glass transition: stiffness, heat capacity and coefficient of thermal expansion.

3.4. 15.13) Melting and Glass Transition Temperatures

3.4.1. -> Tm (melting temperature) and Tg (glass transition temperature) influence the fabrication and processing procedures for polymers. -> These temperatures are determined in the same way it is done for ceramics: a plot of specific volume vs. temperature.

3.5. 15.14) Factors that influence Melting and Glass Transition Temperatures

3.5.1. Melting Temperatures ->During melting the ordered molecules states transforms to disordered molecular states. -> Double bonds in atomic groups in the polymer backbone lowers chain flexibility and causes a higher Tm. -> Molecular weight influences Tm as well. -> The degree of side branching of a polymer also affects the Tm.

3.5.2. Glass Transition Temperatures -> Tg depends on molecular characteristics that affect chain stiffness. -> Tg is increased by the following: Bulky side groups, Polar groups and double bonds and aromatic groups in the backbone. -> Tg lies between 0.5*Tm and 0.8*Tm for homopolymers.

4. Polymer Types

4.1. 15.15) Plastics

4.1.1. -> Polyethylene, polypropylene, poly vinyl chloride, fluorocarbons, epoxies and phenolics to name a few. -> Some are rigid while others are flexible, exhibiting elastic and plastic deformations. -> Plastics are either thermoplastic or thermosetting and may have a degree of crystallinity. -> See Table 15-3 (pg. 521) in textbook.

4.2. 15.16) Elastomers

4.2.1. -> Properties depend on the degree of vulcanization and on whether reinforcement is used or not. -> Most important man-made elastomer is SBR, used in vehicle tires; reinforced with carbon black. -> Silicone elastomers possess high flexibility at low temperatures (-90 degrees Celsius) and are still stable at high temperatures (250 degrees Celsius).

4.3. 15.17) Fibers

4.3.1. -> Fibers are able to be drawn into long filaments having a 50:1 length-to-radius ratio. -> Mostly used in textile industry, being woven/knitted into cloth/fabric. -> Fibers must have high tensile strength, high modulus of elasticity as well as abrasion resistance. -> Molecular weight should be relatively high to accommodate the drawing process. -> Exhibit chemical stability to withstand an extensive variety of environments.

4.4. 15.18) Miscellaneous Applications

4.4.1. Coatings -> Coatings are applied to material surfaces with the following functions in mind: Protect the material from the environment, to improve appearance and to provide electrical insulation is certain cases. -> Many coatings are organic: paint, varnish, enamel, lacquer and shellac. -> Latexes are common coatings. It's a stable suspension of small, insoluble polymer particles dispersed in water.

4.4.2. Adhesives -> Substance used to bond materials together. -> Mechanical Bonding: penetration of the adhesive into the surface pores and adherend. -> Chemical Bonding: involves inter-molecular forces between adhesive and adherend. -> Adhesive choice depends on the following: The materials being bonded, the required adhesive properties, maximum/minimum temperatures involved and processing conditions. -> Advantages of adhesives: lighter in weight, ability to join dissimilar materials, better fatigue resistance and lower manufacturing costs.

4.4.3. Films -> Films with a thickness of 0.025 - 0.125 mm are fabricated to be used as bags and other textile purposes. -> Materials used to produce films must have a low density, high degree of flexibility, high tensile and tear strengths, water resistant and low permeability to some gases.

4.4.4. Foams -> Plastic materials containing high volume percentage of small pores and gas bubbles. -> Commonly used in vehicles and furniture as cushions and in packaging and thermal insulation. -> Gas bubbles are generated throughout a fluid mass which remain solid after being cooled into a solid, giving a sponge-like structure.

4.5. 15.19) Advanced Polymeric Materials

4.5.1. Ultra-High-Molecular-Weight Polyethylene (UHMWPE) -> Linear polyethylene with an extremely high molecular weight (+- 4*10^6 g/mol). -> Characteristics: High impact resistance, good resistance to wear/abrasion, low friction coefficient, excellent low temperature properties, electrically insulating and excellent sound/energy absorption properties. -> Has a relatively low melting temperature - mechanical characteristics deteriorate quickly with increasing temperature. -> Used in bullet-proof vests, fishing line, golf-ball cores, bio-medical prostheses and blood filters to name a few.

4.5.2. Liquid Crystal Polymers -> Composed of extended, rod-shaped and rigid molecules. -> In liquid phase LCP molecules become aligned in highly ordered configurations and with transition into a solid, these configurations remain. -> Primary use is in the form of liquid crystal displays (LCDs) used in digital displays for example phones, digital watches and televisions. ->Some nematic types of LCPs are rigid solids at room temperature. -> Characteristics: good thermal stability, excellent stiffness/strength, chemically nonreactive to many acids. -> Processing/manufacturing characteristics: Conventional processing techniques available for thermoplastic materials can be used, low shrinkage during moulding, low melting viscosity.

4.5.3. Thermoplastic Elastomers (TPEs) -> Polymeric material that exhibits elastomeric behaviour at ambient conditions, but is thermoplastic. -> Best known use is a block co-polymer consisting of block segments of a hard and rigid thermoplastic. -> For common TPEs, hard, polymerised segments at chain ends with soft central regions consisting of polymerised butadiene/isoprene units (frequently termed styrenic block co-polymers). -> Tensile modulus is can be changed: increasing the amount of soft-component segments in the chains decreases the tensile modulus. -> Used in vehicle exteriors (bumpers, fascia), electrical insulation, connectors and gaskets in automobiles, shoe soles and sporting equipment.

5. Synthesis and Processing of Polymers (Chapter 17)

5.1. 17.12) Polymerization

5.1.1. Addition Polymerization -> Also called chain reaction polymerisation -> Process where monomer components are attached one by one in a chain to form a linear macro-molecule. ->Three stages: initiation, propagation and termination. -> Initiation: active centre capable of propagation is formed due to a reaction between a catalyst and the monomer component. -> Propagation: process that involves the linear growth of a polymer chain by the sequential addition of monomer units/components. -> Propagation ends either by two propagating chains forming one molecule of by two growing molecules reacting to form a 'dead chain'. This is known as termination. -> Molecular weight is controlled by the relative speeds of the three stages.

5.1.2. Condensation Polymerization -> Also called step reaction. -> The formation of polymers in step-based inter-molecular chemical reactions that can involve more than one monomer species. -> A low-molecular-weight by-product is usually eliminated (usually water being condensed). -> This step-wise process is repeated to form a linear molecule. ->Condensation reactions can include high functional monomers capable of forming cross-linked polymers.

5.2. 17.13) Polymer Additives

5.2.1. Fillers -> These are materials added to polymers to enhance tensile/compressive strengths, abrasion resistance, toughness and dimensional and thermal stability. -> Fillers include wood flour, silica flour and sand, glass, clay and even synthetic polymers. -> Fillers are usually cheap that replace some volume of a more expensive polymer.

5.2.2. Plasticizers -> The addition of plasticizers improves flexibility, ductility and toughness of polymers. -> Plasticizer molecules occupy spaces between large polymer chains, reducing secondary inter-molecular bonding, leading to a reduction in hardness/stiffness. -> Applications include films, tubing, raincoats and curtains.

5.2.3. Stabilizers -> Stabilisers counteract deteriorative processes. -> For example UV-absorbent material added to another material slows the deterioration of the material due to exposure to ultra violet radiation.

5.2.4. Colourants -> Includes dyes of pigments to impart a specific colour to a polymer. ->Dye molecules dissolve in a polymer while pigments are filler materials.

5.2.5. Flame Retardants -> The addition of flame retardants enhance the flammability resistance of flammable polymers. -> Function by interfering with the combustion process of by initiating a different combustion reaction that generates less heat, reducing the temperature.

5.3. 17.14) Forming Techniques for Plastics

5.3.1. Compression and Transfer Molding -> Male and female mould members are used for compression moulding with the correct amount of mixed polymer and additives placed between it. -> The mould pieces are heated and pressure is applied to melt the mixture. -> Preheating reduces moulding time and pressure and produces a more uniform finished piece. -> Transfer moulding is a variation of compression moulding where solid ingredients are melted in a heated transfer chamber. -> Process is used with thermosetting polymers with complex geometries.

5.3.2. Injection Molding -> Most used polymer fabrication method. -> The right amount of pelletised material is fed by a hopper into a cylinder by motion of a ram hydraulic press. -> The pellets are pressed into a spreader in a heating chamber to make better contact with the heater wall to melt and become a viscous liquid. -> Molten plastic is pushed through a nozzle by the hydraulic press into a mould and pressure is maintained until the piece has solidified. -> Cycle speed is fast due to the rapid solidification of the thermoplastic.

5.3.3. Extrusion -> The moulding of a viscous thermoplastic under pressure through an open-ended die by means of a mechanical screw/auger propelling pelletised material through a heater and finally a die. -> Technique is adapted to producing continuous lengths with a constant cross section (rods, tubes, sheets and filaments).

5.3.4. Blow Molding -> Similar to the fabrication process used to produce glass bottles. -> A parison (length of polymer tubing) is extruded and placed into a two-piece mould with the shape of the final piece. -> Air/steam is blown under pressure to form a hollow cavity due to the tube walls pressing against the mould contours.

5.3.5. Casting -> Casting is done similar to metal casting: melted material is poured into a mould and allowed to solidify. -> Solidification is done by cooling, but hardening is a consequence of polymerisation or curing process that is carried out at a higher temperature.

5.4. 17.15) Fabrication of Elastomers

5.4.1. -> Techniques used in the production of rubber parts are the same as those previously mentioned. -> Most rubbers are vulcanised and some are reinforced with carbon black.