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Repeat-unit Polymer Tg,°C Tm,°CH I ^ (isotactic) CH3 CH3 Introduction to Polymer Science and Technology Behaviour of polymers H -CH2-C- PVC 87(81) 227(273) I Cl I chloride As shown in the above sections, the same factors tend to raise or lower both T and T , i.e., they are related and an g m' ' empirical rule exists for many polymers: T- —- = 0.5 to 0.8 when the temperature is in °K. ~ 0.5 for symmetrical polymers, e.g., PE. ~ 0.8 for unsymmetrical polymers, e.g., PS. Introduction to Polymer Science and Technology Behaviour of polymers 5.3.2 Secondary glass transitions Nearly all tough ductile polymers with high impact strength exhibit prominent secondary glass transitions, e.g., polycarbonates, nylons, PVC, polysulphones, poly(ethylene terephthalate)s and epoxy resins. Secondary glass transitions are mainly associated with the motions of side groups or the backbone chain motions over short segments, and therefore require less energy and occur at lower temperatures than the main (primary) glass transition. 5.3.3 Ways of controlling T Crosslinkingincreases T of a polymer by introducing restrictions on the molecular motions of the backbone chains. Plasticisationand copolymerisationalso affect T and Tm as illustrated in Figure 5.22 for a thermoplastic.
Figure 5.22Transition temperatures vs. % comonomer (—) or % plasticiser (—) content 5.4 Self-assessment questions 1. Consider the following mechanical models, which of these models is not appropriate for describing the delayed elastic response that is characteristic of creep behaviour?
2. Which of these models (in Question 1) is not appropriate for describing stress relaxation? 3. Which of these models (Question 1) would best give a description of viscoelastic behaviour in terms of a spectrum Introduction to Polymer Science and Technology Behaviour of polymers 4. The relationship below holds for the Maxwell model De ñ 1 dc — = —i Dt r\ E dt In a stress relaxation experiment which one of the following equations can be derived from this? ii. a = o0e(trt) iii. j = jo {1 - exp (-1/ T)}, where J is the compliance and ò = T|/E iv. a = Åå 5. Using the appropriate equation from Question 4, determine the relaxation time for a viscoelastic material that Answer: ò = 70.8 s. 6. The strain equation of a Kelvin model is e(t) = (ñò/Å2) (1-å(Ë)). Draw a sketch of a modified Kelvin model that has the strain equation of e(t) = ct/Ej + (ct/E2) (l-e(-rt)). 7. Immediately after applying the stress to the modified Kelvin model the strain is 0.001; after 1500 s the strain Answer: ò = 1638 s. 8. A strip of elastomer was stretched in tension and elongation was held constant. After 10 min the tensile stress • calculate the relaxation time (to the nearest whole number) (answer: ò = 78 min) • show that it takes 22 min for the stress to drop to 75% of its initial value. 9. If you hang a weight from a strip of rubber so that it stretches about 300%, then heat the rubber, which of the • stretches some more • contracts • maintains the same length 10. Consider the two transitions from the 'solid' to the liquid or rubbery state shown below on a plot of specific Introduction to Polymer Science and Technology Behaviour of polymers
a) the transition X is a T while transition Y is a crystalline melting point b) YistheT while X is the T g m c) X and Y are melting points, but X is the Tm of a semi-crystalline material and Y is the Tm of an almost 11. Which of the following statements are true? a) all polymers have a crystalline melting point b) all polymers have a glass transition c) the glass transition is a first order transition that occurs at a well defined temperature d) the crystalline melting point is not affected by the presence of solvent. Introduction to Polymer Science and Technology Behaviour of polymers
12. Consider the following polymers, which will have the lowest T ?
H I -ñóï CH3 (a) CH3 I -(CH2-C)-n CH3 (b) H I icu2-cyn (c) 13. Which of the polymers in Q. 12 is polar in nature? 14. Name polymer (c) in Q. 12. 15. Poly(n-butyl acrylate) (a) has a lower T than poly(methyl methacrylate) (b), because of: H ñóï COOC4H9 (a) CH3 I<ñè2-ñóï ÑÎÎÑÍç (b)
16.
a) weaker intermolecular attractions b) free volume effects due to the flexible side chain c) the stiffness of the side chain. The general chemical structure of aliphatic polyamides is given as -NH- (CH2)X-NHCO - (CH2)y-CO - Three specific nylons have values of (x = 4, ó = 6), (x = 6, ó = 6) and (x = 10, ó = 10); indicate which one of these nylons has the lowest and which the highest melting point. 17. A plot of DMTA damping term against temperature can be used to determine a temperature at which a) tensile strength becomes maximum b) the specific heat shows a minimum c) Youngs modulus undergoes a significant drop d) the crystalline phase melts. 18. The presence of aromatic groups in a polymer chain results in a) intermolecular attraction b) potential for crosslinking c) increase in T and T G m d) tensile strength becomes maximum. 19. Plot and compare schematically, specific volume vs. temperature curves for PS and PP. 20. Illustrate, with chemical formulae, the influence of the size of the side groups of a polymer molecule on T . 21. A strip of rubber is set in motion on a torsi onal pendulum. The amplitude of vibrations decay by 15% after each Answer: A = 0.163. Introduction to Polymer Science and Technology Behaviour of polymers 22. The amplitude of a torsional vibration decreases so that the amplitude on the 100th cycle is 13% of the amplitude Answer: A = 0.02. 23. Describe the efficiency, in general, of copolymerisation and plasticisation in lowering melting points and the 24. Briefly explain the shortcomings of a Maxwell mechanical model in describing the real behaviour of polymeric 25. Which of these polymer(s), PE, PS, PMMM, PC, would be best suited for use as ice cube trays? Why? 26. Plot specific volume against temperature, on the same graph, for two polyethylenes, one with 0.99 specific gravity 27. At room temperature, classify the following materials as elastomers, TP or TS polymers:
a) a lightly cross-linked copolymer with T = -45 °C b) a branched polypropylene of T = -8 °C c) epoxy resin matrix in advanced composites 28. Indicate true or false: a) Aliphatic molecular backbones, such as - CH2 - CH2 - in polymers increase T and lower Tm. b) Aromatic backbone chains (e.g., benzene rings in the chain) cause stiffness, thus, increase T and Tm. c) Presence of elements Î or Si or both in the backbone chain impart flexibility and, thus, lower both T M d) None of these are accurate. 29. How is the process of degradation in polymers described as? a) viscoelastic b) physiochemical c) electrochemical d) corrosion 30. Indicate true or false: a) Polystyrene has an aliphatic side group and thus its T = 100 °C b) Plasticised PVC has a Tg <25 °C c) Tg of an ordinary PVC is below 0 °C d) The chemical formula of polypropylene is ... - CH2 - CH2 - ... 31. A comparison of the primary and the secondary glass transitions in polymers indicate that the secondary glass a) occur at a lower temperature b) occur at a higher temperature c) improve impact performance in otherwise rigid plastics d) exhibit a higher intensity of energy dissipation Introduction to Polymer Science and Technology Mechanical properties 6 Mechanical properties "Satisfaction of ones curiosity is one of the greatest sources of happiness in life." Linus Pauling,1901-1994. Although a Nobel Laureate in Chemistry, Linus Pauling's sentiment applies to all subjects in education, particularly to an interdisciplinary subject such as materials, which combines chemistry, physics and engineering. With various chemical compositions, microstructures, processes and properties, there is so much material to be curious about. A healthy curiosity should generate a satisfactory outcome of how best to use materials so as to benefit from the wide range of properties they possess. 6.1 Introduction The properties of materials have been classified (Brown, 1996) as fundamental properties, apparent properties and functional properties. He distinguishes them using the example of strength. Fundamental strength of a material is that measured in such a way that the result becomes independent of test conditions. Apparent strength is that obtained by a method that has completely arbitrary conditions so that the data cannot be simply related to other conditions or specimen geometry. This classification applies to all kinds of properties (mechanical, electrical, chemical and thermal). It is important to be aware of the purpose of testing: in establishing design data, it is mostly fundamental properties that are needed, but most mechanical tests give apparent properties. In the absence of established and verified procedures for extrapolating results to other conditions, multipoint data have to be produced under conditions likely to influence the test result. Consequently, reliable tables of properties for designers are difficult and expensive to establish Introduction to Polymer Science and Technology Mechanical properties Standard test methods, giving apparent properties, are best suited for quality control, and only in relatively few cases are they ideal for design data. In recent years, the drive towards international standards has led to a close examination of long-established test methods, and it has been found that the reproducibility of many of the tests was poor. This in turn has not led to new tests but rather to the establishment of better standardisation of test procedures. There has also been a growing realisation of the need to calibrate test equipment with proper documentation of calibration procedures and results (James 1999, p8). There has been an increase in tests on actual products, which has resulted from a greater demand to prove product performance, and from specifications more often including such tests as part of the requirements. A test report should include information regarding the production/ preparation of the material being tested and its storage history, as well as the more obvious parameters such as test temperature and speed of test. While it is recognized that the result obtained depends on the conditions of the test, it is not always obvious that some of these conditions may have been established before the samples were received for testing. Sometimes the history of the samples is part of the test procedure, as in aged and unaged samples for example, but at other times it may not be at all clear that certain 'new' samples are already several months old, with their intervening history unknown. Degradative influences such as the action of ozone on rubber samples cannot be compensated for, but standard conditioning procedures are designed, as far as possible, to bring test pieces to an equilibrium state. The imposition of a standard thermal history before measuring the density of a crystalline polymer is a good example. Equally test piece geometry is important, and again if comparison is to be made, a standard and specified geometry should be adhered to. Rarely is it possible to convert from one geometry to another, since polymers are complex materials. The moulding conditions for the preparation of standard specimens, for example, by injection or compression moulding, should be taken into consideration. When prepared by injection moulding, the specimens will have molecular orientation and/ or fibre orientation (Akay & Barkley 1991) in reinforced mouldings with respect to the melt-flow direction. This means that many properties will be anisotropic. Tests should be designed and conducted in a manner that allows the application of statistical principles to test results. The accuracy of the quoted results depends not only on the accuracy of the original measurements but also on the validity of the data handling. The subject is comprehensively covered by Veith (1999) in Chapter 3 of Handbook of Polymer Testing. Engineering properties must always be accompanied by their appropriate units: the internationally recognised system of unit is the Systeme International d'Unites(SI, often referred to as "metric"), however, the unit system customarily used in a country no less than the U.S.A. is the imperial system. Mixing these units can result in very serious consequences, and has resulted in amazing clangers being dropped by such august establishments as NASA. The destruction, for example, of the Mars Climate Orbiter in 1999 was attributed to negligence over the units of engineering data. Press coverage on 2 October, 1999 included: "Simple maths mistake that destroyed £78m Mars mission" by R Price in Daily Mail, which read, "Converting imperial measurements into metric units is not exactly rocket science. But a failure in that most basic of calculations has been blamed for the disappearance of the Mars Climate Orbiter. An investigation has found that the engineers who built the spacecraft worked in imperial measures, while the Nasa scientists who launched it used the metric system. It is a mistake that has cost British scientists 11 years of painstaking work and the American space agency Nasa £78 million." Introduction to Polymer Science and Technology Mechanical properties The article further read, "The orbiter, which was carrying a batch of British experiments, was at first assumed to have smashed to pieces in the Martian atmosphere when radio contact was lost after it flew too close to the red planet last week. But Nasa experts now suspect it was bounced back into outer space. Yesterday it emerged that while one team of scientists was busily calculating how many miles the spacecraft needed to fly to reach Mars and how many pounds of thrust its engines must generate; the second was thinking in kilometres and measuring the thrust in the metric unit of newtons. It meant the orbiter was a crucial 60 miles off course at the end of its 416 million mile journey to the dark side of Mars. Confessing what went wrong, the agency's associate administrator Edward Weiler admitted yesterday: 'People sometimes make errors'. That meek explanation was met with disbelief by Patrick Moore, the astronomer ... ." The Independent (London, England) article by Charles Arthur read, "Converting imperial measurements into metric units isn't exactly rocket science. Maybe that's why the folks at the US space agency, Nasa, messed it up, and so lost their £78m Mars climate orbiter spacecraft late last month. The orbiter is believed to have burned up in the Martian atmosphere 37.5 miles above its surface after crossing 416 million miles of space apparently without a hitch. Why? Because the space-crafts builder, Lockheed Martin Astronautics, provided data for its controlling thrusters to Nasa in imperial units instead of Newtons, the scientific metric unit. But at Nasa's Jet Propulsion Lab (NJPL) the numbers were entered into a computer that assumed metric measurements." The Independent article poses the question, "Is there a difference? Yes. 1 pound-force, the imperial unit, equals 4.44 Newtons (sic), the metric unit. In interplanetary space, where the margins for error are huge, that made little difference. But closer to Mars, the orbiter was jockeying against the opposing forces of the solar wind, the buffeting of the atmosphere and the pull of the planets gravity. That made a big difference. Big enough to lead to the crafts destruction. Officials at Nasa and Lockheed were clearly ashamed at their failure. "In our previous Mars missions, we have always used metric," said Tom Gavin of NJPL adding: "It does not make us feel good that this has happened" Lockheed admitted that it should have submitted the data in metric units, although it was reviewing the contracts to see whether Nasa specified any units. ...". The properties of plastics at room temperature, in contrast to most metals, are time dependent. Superimposed on this are the effects of the level of stress, the temperature of the material, and its structure (molecular weight, molecular orientation, and crystallinity/density). In polypropylene (PP), for example, an increase in temperature from 20 °C to 60 °C may typically cause a 50% decrease in the allowable design stress. Also for each 0.001 g/cm3 change in density of PP, there is a corresponding 4% change in design stress (i.e., the allowable maximum stress that a machine part or member may be subjected to without failure). The material, moreover, will have enhanced strength in the direction of molecular alignment (that is, in the direction of flow in the mould) and less in the transverse direction (Design Council 1993, pl5). Because of the influence of so many additional factors on the behaviour of plastics, properties quoted as a single value will be applicable only for the conditions at which they are measured. The Design Council also points out that properties measured as single values following standard test procedures are useful primarily for quality control assessments. They are useless for design purposes, because to design a plastic component it is necessary to have complete information, at the relevant service temperature, on the time-dependent (viscoelastic) behaviour of the material over the whole range of stresses to be experienced by the component. There are many useful web-based sources that provide data on the properties of polymers. CAMPUS, ComputerAided Material Preselection by Uniform Standards, is one of these. It is internationally acknowledged database software for plastics Introduction to Polymer Science and Technology Mechanical properties materials, more than 40 plastics producers are participants of CAMPUS. The main feature of the CAMPUS database is to provide comparable data from participating suppliers. The acquisition and presentation of properties are based on the international standards of ISO 10350 for single-point data and ISO 11403-1, -2 for multi-point data. 6.2 Tensile properties The standard methods for conducting tests to measure tensile properties of plastics include various parts of ISO 527, BS 2782, or the harmonised method under the designation of BS EN ISO 527, and ASTM D638 or its metric version D638M. Alternative methods for testing rubber include ISO 37, BS 903, ASTM D412, and for films/sheeting ASTM D882. The tests can be conducted using a universal test machine (Figure 6.1 (a)) of appropriate load capacity in the tensile setting, with suitable grips and an extensometer to enable accurate extension measurements. Extensometers can be contact (Figure 6.1 (b)) or non-contact types (video and laser devices). For further information on the principles of non-contact extensometers see Bennett (1980).
Figure 6.1(a) a universal testing machine frame, (b) a tensile specimen with attached extensometer Test specimens are mainly flat (sometimes cylindrical) dumbbells or flat strips: dumbbell shape is aimed at achieving failure in the narrow-waist portion of the specimen and avoid premature failure at the grips. The shape and the dimensions of the specimens vary between standards, but often aim to avoid stress concentration and/or allow for circumstances where there is limited amount of material for testing. Important moulding conditions for the preparation of standard specimens by injection and compression moulding should be specified: when prepared by injection moulding, the dumbbell pieces will have molecular orientation along the length of the test piece and property values transverse to the orientation direction cannot be determined, unless specimens are cut from moulded plaques as described in ISO 294-5:2011. During a test, the machine measures load or force and displacement (in this case extension) and a dedicated PC can produce force-extension plots or, by pre-loading data on the specimen dimensions, stress-strain plots (see Figures 6.2). The stress (o) is related to force (F) and the specimen cross-sectional area (A) by î = F/A; and extension (AL) and specimen Introduction to Polymer Science and Technology Mechanical properties gauge length (L) to strain (e) by e = AL/L. Another parameter is the Poissons ratio, v, which indicates the change in the specimen cross-section as a result of axial strain and is expressed as a ratio of lateral strain to axial strain: v = - (Aw/w) / (AL/L) where, w is the specimen width. Note that when Aw is negative, i.e., a contraction, v becomes +ve. For the extreme cases in isotropic materials, v = -1, when the proportions of the specimen do not change (Aw/w = AL/L); and v= 0.5, when the specimen volume, V, remains constant, i.e., AV = 0. From AV = A(AL) = LAA + AAL = 0, by assuming a specimen with a square cross-section (A = w2), it can be shown that v = - (Aw/w) / (AL/L) = -0.5. Tensile properties of Youngs modulus (elastic modulus), the yield strength, the tensile strength (the maximum engineering stress, also termed the ultimate tensile strength (UTS)) and the elongation to failure can be extracted from the e-e curves. This is demonstrated in Figure 6.2, which shows a large deformation curve that includes elastic, viscoelastic and plastic deformation regions. In the elastic region, the strain is small and there is a linear relationship between stress and strain where Hookes law holds, and material can instantly revert back to its original form when unloaded. Youngs modulus, E = a/ s, is determined using the stress and strain data extracted from this elastic region. Beyond the elastic region and up to approximately the yield point, viscoelastic region, the material can recover to its original form over time. Introduction to Polymer Science and Technology Mechanical properties At yield point the material begins to deform plastically, i.e., the material will undergo permanent deformation and therefore, upon the release of load only the elastic portion of the strain will recover and plastic deformation will not recover. At yield point the material begins to neck, i.e., the specimen cross-sectional area undergoes a significant reduction, further elongation causes a fall in load and, therefore, in nominal stress (engineering stress) as shown in Figure 6.2. Note that the engineering stress is computed by dividing the load with the initial cross-sectional area of the specimen and it should be distinguished from the true stress, which is computed using the actual cross-section of the specimen at the time (at the instant) during the test. Accordingly, the true stress does not decrease at necking/yielding but either remains approximately constant or rises less steeply with increasing strain, depending on the extent of cold drawing. Cold drawing succeeds the yield point where material undergoes permanent deformation as a result of molecular slippage. Continuing extension of the waisted/narrow portion of the dumbbell specimen is achieved during drawing by causing the shoulders of the neck to travel along the specimen as it reduces from the initial cross-section to the drawn cross-section. At further elongations, the slope of the stress-strain curve increases again, due to "strain hardening'V'molecular orientation", and finally material ruptures. These features for a tensile tested dumbbell specimen are indicated in Figure 6.3.
cold drawing UTS strain (s), % Figure 6.2Typical stress-strain curve for a cold-drawing polymer
neck shoulder
Figure 6.3A broken half of a nylon dumbbell test piece Introduction to Polymer Science and Technology Mechanical properties In the absence of a clear yield point or where the departure from the linear elastic region cannot be easily identified, an offset yield strength or proof stress is determined. The proof strength is the stress at which the stress-strain curve deviates by a given strain (offset) from the tangent to the initial straight line portion of the curve). An offset is specified as a % of strain, and it is 0.2% for metals by ASTM E8, but for plastics a higher value of 2% is also sometimes used. Often stress-strain curves under tensile, flexure or compression may produce a spurious initial portion at the start of the curve, referred to as "toe region", and is caused by gripping and take-up of slack in the specimen, or alignment or seating of the specimen. This artefact can be compensated as illustrated in Figure 6.4 to obtain a correct zero point for strain in order to determine accurate properties.
Figure 6.4The toe compensation on a stress-strain curve: the toe region of AC is compensated for, by extending the linear/elastic portion of the curve, CD, and the intersection  becomes the corrected zero-strain point
Typical stress-strain curves for plastics with different levels of ductility are shown in Figure 6.5. Table 6.1 shows tensile properties (obtained under standard laboratory conditions of approximately 23 °C and 50 % relative humidity, and mostly in accordance with ASTM D638) for dry-as-moulded samples. Where there was access to sufficient information, the data in Table 6.1 is presented in the form of'the most quoted value', succeeded by minimum and maximum values in brackets.
Introduction to Polymer Science and Technology Mechanical properties Table 6.1Tensile properties of various polymers (sources: CAMPUS, Efunda, Ehrenstein (2001, p260), Fried (1995, p474), Callister (2007, Appendix B))
Introduction to Polymer Science and Technology Mechanical properties
Date: 2015-12-11; view: 2639
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