Mechanical Properties of Polymers

1. Mechanical Properties of Polymers Polymers as materials need to be "strong" or "tough" or even "ductile". How can we define these Mechanical Properties? How do we measure how "strong" a polymer is? What is the difference between a "strong" polymer and a "tough" one? 1) STRENGTH is easiest to relate to, but there is more than one kind of strength. 1a) tensile strength. A polymer has good tensile strength if it is strong when one pulls on it in tension, stretching it (eg; fibres): The tensile strength is the highest load applied before it snaps in two

2. 1b) Then there is compression strength. A polymer sample has compressional strength if it is strong when one tries to squeeze it: The compression strength is the highest load applied before it fails. Similarly, 1c)shear strength can also be important, which describes the strength in resistance to bending until it snaps. A polymer sample has flexural strength if it is strong when one tries to bend it, like this:

3. 1d) torsional strength describes materials which resist twisting: Other types of strength can be very important practically, but much more difficult to describe quantitatively, such as 1e) flexural strength which describes the resistance to bending: and 1f)the vaguely defined impact strength, which one cares about if one’s material gets hit sharply and suddenly, as with a hammer.

4. To compare the Strength of Materials, and why our natural starting products (inorganics) are ‘low tech’ and unsuitable for ‘high tech’ : Inorganics are ‘strong’ materials, but ONLY in compression This means that when we carve them out of the ground, we are greatly limited in the types of forces we can expect them to withstand. Architecturally, stone is useful ONLY in certain geometries.

5. We chose one type of strength that we are most concerned about depending on the application we want the polymer for, and the rest of the description follows. Let’s use tensile strength as an example: To measure the tensile strength of a polymer sample, we take the sample and we try to stretch it with a machine. This machine simply clamps each end of the sample, then, when you turn it on it stretches the sample. While it is stretching the sample, it measures the amount of force (F) that it is exerting. When we know the force being exerted on the sample, we then divide that number by the cross-sectional area (A) of our sample. Stress (σ) = F/A The resulting change in shape (eg. Length L) is measured, and is called the Strain (ε) = ΔL/L0 in %. The max. stress is the tensile strength.

6. We chose one type of strength that we are most concerned about depending on the application we want the polymer for, and the rest of the description follows. Let’s use tensile strength as an example: (Text pages 482-494, and 295-300) L 0 Young’s Modulus E = slope = σ/ε Stress σ = F/A Force 1 Toughness = area under the curve (energy absorbed) Strain ε = ΔL / L0 in %

7. Stress – Strain Curve for a Tough Plastic Brittle plastic elastomer

8. In a similar fashion, measure the material’s response to Compression, Torsion, and Shear forces, we can obtain values for the: Bulk Modulus (M), Torsional Modulus (T) , and Shear Modulus (G) It is these stress/strain curves which allow us to characterize the mechanical properties of Materials most easily and effectively. A material which is strong, but not tough, is called brittle. Best are materials which are tough and strong. While it's good for materials in a lot of applications to have high moduli and resist deformation, it's usuallydesirable for a material to bend than to break,if this prevents the material from breaking. So when we design new polymers, or new composites (more later), we often sacrifice a little bit of strength in order to make the material tougher. Summary: Modulus absorbs FORCE, Toughness absorbs ENERGY

9. ‘Strength’ also depends strongly on the FREQUENCY of the action– quickly or slowly?, AND, the TEMPERATURE at which we do all this…

10. This all depends too on the FREQUENCY of the deformation event: log MPa Materials can respond over many orders of magnitude to how slowly or how quickly one applies the stress– here 8 decades of strains ( ε ) are observed for 12 different decades of frequency of the SAME stress ( σ ) applied on the sample. (flat spot usual)

11. This all depends too on the FREQUENCY of the deformation event: log MPa

12. And, most importantly, this depends strongly on TEMPERATURE : temperature

13. And, most importantly, this depends strongly on temperature : 8 7 6 5 4 3 2 temperature Amorphous polymers pass from a hard, brittle GLASS region, to a softer RUBBERY region at the Glass Transition Temperature, Tg. This Tg, the onset of long-range coordinated motion, is perhaps the single most important processing and mechanical parameter.

14. And, most importantly, this depends strongly on temperature : There is a MASTER CURVE for all amorphous polymers, and the Tg may be ABOVE room T (glassy material) or BELOW room T (rubbery) ( so just changing T changes the material– for good or for bad ! ) (usual range –40 C to 60 C… ship hulls, o-rings, CDs, cow patties… )

15. IMPORTANT PIECE of INSIGHT: THIS is why polymers are so interesting as materials: Unlike inorganics (nearly impossible) and metals (very difficult), the microstructure of polymers is relatively easy to vary by the chemist, or engineer, allowing GREAT control over the mechanical properties.

16. Biomimetic materials Another IMPORTANT PIECE of INSIGHT: Multicomponent materials, especially those developed by or inspired from Mother Nature, are engineered to provide some the highest strength and toughness values measured (covered in more detail later)

17. Polymer Tensile Strength Max Elongation Polystyrene (amorph) 50 Mpa 2 % PMMA (amorph) 65 Mpa 10 % PVC (amorph) 50 Mpa 30 % HDPE (semicrys) 30 Mpa 600 % PP (semicrys) 33 Mpa 400 % Polyamides (semicrys) 80 Mpa 200 % Polyamide (fibre) 800 Mpa 25 % PE (fibres, oriented) 3500 Mpa 5 %

18. SEMICRYSTALLINE POLYMERS SC Polymers contain many advantages in toughness, as careful ratios of amorphous/crystallinity can allow chains to slip, slide, and reorient instead of breaking, absorbing a lot of energy. Brittle polymers usually have no choice but to crack and fracture under high load. In addition to toughness, orientation of a polymer usually increases tensile strength too. Case 1) amorphous region RUBBERY: material will have lower modulus, but extension to break can be very large. Case 2) amorphous region GLASSY: material will have higher modulus, but extension to break will be much lower. The best approach for ultra high strength and toughness polymers is to combine materials as composites, blends, copolymers, often using fibres.

19. FAILURE in SEMICRYSTALLINE POLYMERS • Necking: (cold • drawing), pulls out • chains and orients • (viscoelastic flow) • strain of 100s of % • 2) Crazing: with more brittle • materials, fibrils pull out of • crack interfaces, and failure • occurs after 1 or 2% strain

20. FAILURE in AMORPHOUS, BRITTLE POLYMERS The problem with ‘pure’ substances (like most of our engineering materials) is that their overall strength is NOT the sum of all the bond strengths, but the ‘weakest link’ under stress: If we have a string of molecular bonds, the total force required to break it is equal to the strength of a single bond: The same is NOT true for multiple bonds however– they are not as strong as the sum of all of the individual bonds involved:

21. FAILURE in AMORPHOUS, BRITTLE POLYMERS The reason is that there is almost always a little twist, bend, or otherwise imperfect pull, which causes uneven stresses This means that the bonds one one side of the structure (here the top) experience more stress than on the other side (here the bottom), and they will not break at the same time, but one by one.

22. FAILURE in AMORPHOUS, BRITTLE POLYMERS So the material then tears one molecular bond at a time, unzipping bond after bond after bond… The total strength is therefore only as large as the number of bonds breaking at the same time, which is WAY fewer than the total number in the material. This is called crack propagation.

23. FAILURE in AMORPHOUS, BRITTLE POLYMERS Mother Nature has a clever plan to combat this however, based on stopping the crack from propagating by using as many interfaces as possible to spread out the forces over many bonds. In this way, even filling a material with holes makes it stronger in many ways (and not just lighter)

24. FAILURE in AMORPHOUS, BRITTLE POLYMERS What layering does is to use a small bit of soft material to break just BEFORE the crack arrives, thereby halting it right at the interface from spreading further. this gives layered natural materials superior strength over artificial ones, and the best that technology can do is to mimic Mother Nature (kevlar, carbon fibre, folded steel copy bone, tendon, spider silk…)

25. To gain some insight into how to cleverly design polymer materials, we can turn to Mother Nature for lessons in Materials Engineering An excellent example of a VERY strong material is Eggshell: It is composed mainly of brittle calcium carbonate (like limestone), but a little added protein turns the brittle solid into an excellent material = calcium carbonate = soft protein

26. The secret is a layer-upon layer structure. Tendon is also a material which is composed of layer-upon-layer of microstructure. It is mainly tropocollagen, which is a triple helix of three polypeptide chains wound around each other and self-assembled

27. The structure of bone is similar too,

28. The structure of bone is similar too, with very many very thin wrapped layers. There are also many holes (pores) interspersed in the material, which provides the best overall strength. The ‘bio-mimetic’ approach in polymer science is to introduce the interfaces with multicomponent materials

29. (kevlar, carbon fibre, folded steel copy bone, tendon, spider silk…) These all have a fine layered structure, and our aerospace materials are basically just a mimic of mother nature)

30. Kevlar’s great strength for example, can be thought of as being derived more from the layering of the fibres than from the molecular structure itself (though it is a great structure too)