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Introduction to Polymer Science and TechnologyMicrostructure
Î (n)CH2-CH-CH2-CI + (n)HO-< epichlorohydin ñí3 I ñí3 bisphenol-A ■ -OH (n)HCI +
d-o-' CH3 CH3 OH
I >-O-CH2-CH-CH2 bisphenol-A based epoxy prepolymer CH3 CH3 »-O-R
\ R = CH2 - CH - CH2 where ,R is the epoxy group Figure 4.16Formation of epoxy prepolymer from its monomers Î / \ - CH2- CH - CH2 + H2N - R - NH2 epoxy resin/prepolymer diamine CH2— CH — CH2 —- I I -CH,-CH-CH,-N OH
I OH N - CH2- CH - CH2 OH epoxy network polymer Figure 4.17Crosslinking of epoxy resin (low molecular weight) with an amine curing agent (note that the term epoxy resin is used to describe both the network polymer and the oligomeric prepolymers)
(a) rigid TS (e.g. PF) with low ductility strain(e) Figure 4.18a-e curves for crosslinked polymers: (a) high X-link density, (b) low X-link density Introduction to Polymer Science and Technology Microstructure 4.5 Copolymer arrangements Polymers can be tailored to produce unique/desirable combinations of properties by polymerising together two or more different types of monomers. The resultant polymer is called a copolymer to distinguish it from the homo-polymer that the individual monomers by themselves produce, e.g., acrylonitrile-butadiene-styrene copolymer, and polyacrylonitrile, polybutadiene and polystyrene homopolymers. While ABS is roughly twice as expensive as PS, it is more superior for its hardness, gloss, toughness, solvent resistance and electrical insulation properties. Note that copolymers should be distinguished from polymer blends. The arrangement of different monomers within the copolymer chain leads to the formation of different structures as delineated in Figure 4.19. The terms used to describe these different basic structures are self explanatory: in the random copolymer there is no detectable regularity to the sequence of different monomers within the polymer chain, in the alternating type the monomers are ordered regularly in an alternating sequence. In Block copolymer chains, the long segment (oligomers) of each type of monomer are joined together, and similarly in graft copolymers long segments of a monomer (string of Monomer  in Figure 4.19) is covalently attached as a side branch onto the main backbone chain consisting of the second type of monomer (Monomer A in the figure). Normally the random and alternating copolymers produce properties that are averages of the properties of homopolymers, whereas block and graft copolymers can exhibit a combination of properties that are unique to the individual homopolymers as in thermoplastic elastomers, covered in the next section. Introduction to Polymer Science and Technology Microstructure
Figure 4.19Illustration of different monomer arrangements in copolymers: (a) random, (b) alternating, (c) block, and (d) graft copolymers (source: Google images) 4.6 Domain structures Domain structures consisting of hard and soft segments are a feature of thermoplastic elastomers (TPE). Ordinary crosslinked (vulcanised) elastomers do not melt or dissolve and cannot, therefore, be processed using processing equipment suitable for thermoplastics, and also their waste cannot be reprocessed like thermoplastics. TPEs are attractive alternatives because they can be processed as a thermoplastic. Figure 4.20 shows an illustration of the microstructure for TPEs and also a product made from a TPE. The hard segments have T and Tm values well above room temperature and they perform the same function as crosslinks in thermosetting elastomers, but they are thermo-reversible physical crosslinks, making the material melt processable. The soft segments have T values well below ambient temperature and so exhibit good molecular flexibility at temperatures above its T . The soft segments impart rubbery characteristics to TPEs, as polybutadiene segments do in styrene-butadiene-styrene (SBS) block copolymer (Figure 4.20). Other examples of TPEs include thermoplastic polyurethane elastomer (TPU) in which the hard segments consist of urethane or urea groups (that include rigid aromatic rings of the isocyanate used), separated by soft blocs/segments of polyol. One particular TPU, a block copolymer of alternating soft (85 %) and hard segments, is used to produce Spandex fibre, which is popularly used to produce light-weight sports garments under such trade names as Lycra (Du Pont). Introduction to Polymer Science and Technology Microstructure
\ PBD-matrix (soft domain)
'Fashy' hot water bottles Figure 4.20Illustration of the structure of a TPE: (a) single molecule, (b) microstructure 4.7 Degree of molecular orientation Orientation causes alignment of the micro-structural units and polymer chains so it causes anisotropy in properties such that material becomes much stronger and stiffer along the orientation axis. Orientation is successfully exploited by industry - production of synthetic fibres depends on orientation. The packaging industry makes extensive use of uniaxial and biaxial orientation. Measuring orientation in polymers provides valuable information about micro-structure and therefore properties. It can be measured by birefringence, sonic modulus, X-ray diffraction, infrared dichroism, laser-Raman spectroscopy, etc. 4.7.1 Birefringence The measurement of optical anisotropy is a simple method of studying orientation in polymers. Birefringence is a measure of the total molecular orientation of a system (i.e., crystalline and amorphous components of the polymer). It is defined as the difference in the refractive index parallel, r^, and perpendicular, n±, to the stretch direction for a uniaxially oriented specimen. The refractive index is a measure of the velocity of light in the medium and is related to the polarisability of the chains. The birefringence for a uniaxial system, Ä, is therefore defined as A = nn - n± For a completely isotropic material Ä = 0. Anisotropy increases with the increased orientation in a material, and Ä increases too. To measure Ä, one method would be the direct measurement of the refractive indices, which is a tedious procedure. A more rapid method is the use of a compensator(e.g., Babinet compensator) to determine the phase difference between two perpendicular, plane-polarised wave motions emerging from the sample: since the sample is anisotropic, the velocity of the wave passing through the sample parallel to the stretch direction will be different than the one in the perpendicular direction. This velocity difference causes a phase difference in the emerging rays. Introduction to Polymer Science and Technology Microstructure Ä oc R (the retardation or phase difference as wave numbers). Birefringence is a suitable technique for transparent samples and requires a polarising microscope fitted with a compensator for measurement. 4.7.2 Sonic technique The orientation in polymers can be measured by propagating sound pulses through the material. This technique also gives an average orientation in the material. It is particularly suited for specimens of fibres/ribbons. The experimental set up consists of a sonic wave (pulse) transmitter and a wave detector; both placed on the sample a certain distance apart and a meter for measuring the time between the onset of the pulse and its detection after propagation. The sound velocity, v, is determined from the propagation distance and the time between the pulse and the signal detection. The sonic modulus,E = p (v2), where p is the density of the material. 4.7.3 X-ray method Wide-angle X-ray diffraction patterns of unoriented semi-crystalline polymers are characterised by a series of concentric rings. As the specimen is oriented, these rings break up into arcs and spots (Figure 4.21). From the intensity and the size of these arcs, the degree of orientation of the crystalline regions can be determined. Introduction to Polymer Science and Technology Microstructure
Figure 4.21 WAXS patterns for PP samples 4.7.4 Infra-red method The infra-red dichroism is used for the determination of crystalline and amorphous orientation by studying the appropriate absorption bands. The infra-red dichroic ratio, D = AN / A± where, AN and A± are the absorbances measured with radiation polarised parallel and perpendicular to the stretch direction. The orientation oc D. 4.8 Self-assessment questions 1. Polymers do not crystallise easily because: a) they are long chain molecules b) they contain covalent bonds c) the molecules are interconnected with H-bonding
2. Explain briefly the difference between crystalline and amorphous regions in a polymer. 3. What x-ray diffraction technique is used in determining crystallinity in polymers? 4. Describe crosslinking and how it affects properties in polymers. 5. What is the molecular difference between thermosetting and thermoplastic polymers? 6. Select the polymers that could show variation in tacticity: PP, PTFE, PS, and PE. 7. Indicate which of the following polymers could not exist as isotactic and syndiotactic stereo-isomers:
a) PP b) PMMA c) polyvinylidene chloride d) PTFE e) PS 8. Indicate if isotactic PP is a) thermoset b) polyolefin c) an amorphous polymer d) a semicrystalline TP Introduction to Polymer Science and Technology Microstructure 9. Select which of the following properties can be determined from a DSC scan: e) latent heat of fusion f) number average molecular weight g) tan 6 10. Indicate what is the effect of crosslinks in an elastomer a) reducing Youngs modulus b) increasing crystallinity c) decreasing degree of crosslinking d) enabling long-range elasticity 11. Indicate which of the following properties depend only on the chemical nature of its repeating units: a) crosslink density b) T M c) chain configuration d) chain conformation 12. Indicate if thermosetting polymers a) contain crystalline regions b) are more rigid than TPs c) consist of a 3D network of polymer chains d) exhibit T M 13. Indicate how neighbouring molecules are bonded in a TP: a) covalent bonds b) H-bonds or van der Waals forces c) crosslinks d) primary bonds 14. The number of tie-molecules between spherulites affects: a) the optical properties b) the impact properties c) density d) crystallinity 15. Indicate how to increase crystallinity in a vinyl polymer: a) change the tacticity from atactic to syndiotactic b) stretch it c) anneal it d) solidify from melt at a slow rate e) all of the above 16. Which of the following polymers is least likely to be optically transparent? a) isotactic polystyrene b) atactic polystyrene c) an ethylene/propylene block copolymer d) a styrene/butadiene random copolymer Introduction to Polymer Science and Technology Microstructure 17. HDPE cooled slowly from 160 °C to room temperature a) remains amorphous b) crystallises c) remains liquid d) crosslinks 18. A polypropylene sample is just buoyant in an alcohol of density p = 0.9 g cm"3. Calculate its mass fraction Answer: 39 %. 19. DCS traces of two PE samples produce melting enthalpies of 140 and 200 J/g for Specimens A and B, respectively. Introduction to Polymer Science and Technology Behaviour of polymers 5 Behaviour of polymers "Minds are like parachutes, they only function when they are open." James Dewar,1842-1923. After a very brief introduction to the degradation behaviour of polymeric materials, the chapter will concentrate on describing various basic concepts in association with the viscoelastic nature of polymers, so don't shut those parachutes yet! 5.1 Degradation of Polymers Polymers do not rust in the way metals do, but they also suffer degradation from environmental effects. The processes of degradation are different compared with metals: metallic corrosion is an electrochemicalreaction degradation of polymers is physiochemical(i.e., may involve physical and/or chemical processes). Polymers may deteriorate by swellingand dissolving- i.e., solute molecules enter and occupy positions between the polymer molecules. Note that plasticisation is achieved when this process is controlled. Polymers resist acids and alkaline solutions better than metals. Polymers are vulnerable to hydrocarbon liquids,the severity of which depends on the type of polymer. Polystyrene, for instance, with a benzene side group is sensitive to aromatic and chlorinated solvents and can be readily dissolved in these solvents. It is, however, resistant to water. Some polymers such as nylons and cellulosics are, on the other hand, susceptible to water. Bond rupturein polymer molecules (i.e., scission) may result from exposure to radiation or heat, and from chemical reaction. Not all radiation is deleterious: cross-linking may be achieved by irradiation, e.g., y-radiation is used to crosslink PE to increase its resistance to softening and flow at elevated temperatures. Degradation resulting from outdoor exposure is known as weatheringsuch that a combination of moisture, UV radiation, and heat causes deterioration by the process of oxidation.For example PVC can suffer degradation with the evolution of HC1 under long exposure to UV. An example can be seen in Figure 5.1: the part of a clear corrugated PVC roof that was shaded by a tree has not suffered any discolouration over time in comparison with the section exposed to direct sunshine.
Figure 5.1Specimens taken from a PVC clear corrugated roof: (a) represents the section of the roof exposed to sunshine, (b) represents the section of the roof shaded from the sun Introduction to Polymer Science and Technology Behaviour of polymers Oxygen and ozone also by themselves can cause the scission of the covalent bonds within the polymer molecules, particularly in rubber due to the presence of more vulnerable double covalent bonds along the backbone molecular chain. This phenomenon can be seen in ordinary balloons that are produced by dipping porcelain formers (covered with a coagulant to coagulate the latex) into natural-rubber latex. A suitable antioxidant is added to the latex and therefore the balloons are protected against degradation by oxidisation, but the protection is only effective when the balloons are not blown. The antioxidant reacts with the oxygen in the atmosphere and forms a protective layer. Once the balloons are blown up, however, the surface area increases and the protective layer breaks up. Consequently oxidisation occurs and causes chain scission, which manifests itself as a sticky mass of material. This can be seen in Figure 5.2, which shows a balloon that has not been blown up and also a few balloons that were blown up tied together and left in the blown up state for a few months after a children's birthday party, suffering degradation and becoming a gooey lump.
Figure 5.2Photos of rubber balloons: (a) a fresh balloon, (b) a few degraded balloons after being left blown up for a period of time Detailed information on weathering, aging, factors affecting aging, accelerated weathering outdoors and in devices, and guidance on selecting appropriate methods of testing is presented by Kockott (1999, p697). 5.2 Viscoelasticity Ordinary solids such as metals are immediateor instantaneous in responseto applied loads - i.e., they are elastic.In contrast polymers, from observation, are sluggish in response to applied loads - i.e., they are viscoelastic. Viscoelasticityis a material behaviour in which the relationship between stressand strainis time dependent.It should be possible to demonstrate some of these features using the items shown in the following demonstration-boxes. Demonstrations 5. lto 5.4 are aimed to highlight the sluggish recovery of viscoelastic materials/components, and Demonstration 5.5 should exhibit rate dependency in the behaviour of viscoelastic materials such as the "Smart putty". Smart putty can be obtained from Middlesex University, UK (www.mutr.co.uk). The human skin behaves in a viscoelastic manner - pinch the skin at the back of your hand and let go, the recovery is time dependent, particularly noticeable with the elderly. The extent of viscoelasticity depends on the temperatureof the material. A rigid plastic has near elastic behaviour at room temperature, but at T (more about this later) and beyond, the behaviour changes. Date: 2015-12-11; view: 937
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