Polymer yield strength

Third, numerical calculations based on the inequalities that we have proposed lead to qualitative predictions of the transfer of matter, and of general strength in adhering systems, that are in agreement with observations. The trends of these practical properties, with variation in fundamental properties such as polymer yield strength and free energy of adhesion, are in excellent agreement with observed results. 

Although polymer crystal structures are known, and some slip mechanisms (slip plane and slip direction) determined, these are less important than for metals. Firstly, the amorphous phase plays an important part in the mechanical properties. Secondly, polymer yield strengths are not determined by obstacles to dislocation movement. However, it is possible to fabricate highly anisotropic forms of semi-crystalline polymers, so crystal characterization and orientation are important. 

Class Polymer Yield Strength, (ASTM D638) (10 psi)... 

Let us now see whether materials really show this strength. The bar-chart (Fig. 9.2) shows values of Oy/E for materials. The heavy broken line at the top is drawn at the level it/E = 1/15. Glasses, and some ceramics, lie close to this line - they exhibit their ideal strength, and we could not expect them to be stronger than this. Most polymers, too, lie near the line - although they have low yield strengths, these are low because the moduli are low. 

In this chapter we show that k = Oy/2, and use k to relate the hardness to the yield strength of a solid. We then examine tensile instabilities which appear in the drawing of metals and polymers. 

Cellular materials can collapse by another mechanism. If the cell-wall material is plastic (as many polymers are) then the foam as a whole shows plastic behaviour. The stress-strain curve still looks like Fig. 25.9, but now the plateau is caused by plastic collapse. Plastic collapse occurs when the moment exerted on the cell walls exceeds its fully plastic moment, creating plastic hinges as shown in Fig. 25.12. Then the collapse stress (7 1 of the foam is related to the yield strength Gy of the wall by... 

At and near room temperature, metals have well-defined, almost constant, moduli and yield strengths (in contrast to polymers, which do not). And most metallic alloys have a ductility of 20% or better. Certain high-strength alloys (spring steel, for instance) and components made by powder methods, have less - as little as 2%. But even this is enough to ensure that an unnotched component yields before it fractures, and that fracture, when it occurs, is of a tough, ductile, type. But - partly because of their ductility - metals are prey to cyclic fatigue and, of all the classes of materials, they are the least resistant to corrosion and oxidation. 

On comparison of the yield strengths and elastic moduli of amorphous polymers well below their glass transition temperature it is observed that the differences between polymers are quite small. Yield strengths are of the order of 8000 Ibf/in (55 MPa) and tension modulus values are of the order of 500 000 Ibf/in (3450 MPa). In the molecular weight range in which these materials are used differences in molecular weight have little effect. 

In the case of commercial crystalline polymers wider differences are to be noted. Many polyethylenes have a yield strength below 20001bf/in (14 MPa) whilst the nylons may have a value of 12 000 Ibf/in (83 MPa). In these polymers the intermolecular attraction, the molecular weight and the type and amount of crystalline structure all influence the mechanical properties. 

The polymer is reported to have a heat deflection temperature of 198°C, and a tensile yield strength of 93.2 MPa, and to be flame retardant. 

Apart from that, the smaller the particle size, the stronger the structure formed by filler in the melt (that is, the yield strength of polymer ry is affected). Table 5 below demonstrates how the yield strength of molten polycarbonate composites depends on the size of CaC03 particles. 

When the load is high enough, a polymer yields and loses its resistance. The corresponding stress level is specific to the polymer and the actual temperature. Knowledge of the yield strength of a polymer is crucial in order to avoid the risk of failure in application. However, the polymer can fracture even at loads below... 

Yield strength as determined in tensile tests [53] at ambient temperature was plotted in Fig. 6.1 against M 1, the inverse molecular mass between crosslinks. All the samples of polymer A (the most crosslinked polymer) failed before the polymer started to yield. Therefore, load-extension-curves were extrapolated up to a hypothetical yield strain in this case. The extrapolated tensile is marked by brackets (Table 6.1). 

Fig. 6.1. Yield strengths of the five polymers are plotted against 1/MC that is the inverse molecular mass between crosslinks. The diamond represents polymer E. Test temperature 23 °C. a and b represent results of flexural tests on small samples (thickness 1.3 mm) a annealed, b quenched,...

Fig. 6.2. Yield strengths from tensile tests at 23 °C are plotted against the glass transition temperatures (T,max) of the five polymers [] result of extrapolated stress-strain-curve...

The flexural strength of the annealed polymers proved to be consistently about 30% higher than the strength of the quenched polymers as shown in Fig. 6.1. Tests were evaluated in accordance with ISO 178 [54]. As the samples yielded, they deformed plastically. Therefore, the assumptions of the simple beam theory were no longer justified and consequently the yield strength was overestimated. 

Fig. 6.3. Yield strengths from flexural tests are plotted against the densities of the polymers. The annealed samples were noticeably stronger than the quenched ones of similar density. Rigidity (Fig. 5.3.) was governed by the density of the polymer whereas yield strength seemed to depend mostly on molecular conformations...

At temperatures below the glass transition, the polymers are fairly rigid and the yield strength xy is high. vxy > kT is plausible in the case of solid polymers. Eq. (6.3) can be rearranged to read ... 

The yield strengths of the polymers A, B and E from flexural tests are plotted in Fig. 6.5 against the strain rate on a logarithmic scale. The crosshead speed was... 

Fig. 6.5. Yield strengths from flexural tests are plotted against strain rates at the surface of the samples. Tests were performed on polymers A, B, and E test temperature 23 °C. The slope of the three lines correspond to similar activation volumes v = 2 0.1 nm3...

The resolution of infra-red densitometry (IR-D) is on the other hand more in the region of some micrometers even with the use of IR-microscopes. The interface is also viewed from the side (Fig. 4d) and the density profile is obtained mostly between deuterated and protonated polymers. The strength of specific IR-bands is monitored during a scan across the interface to yield a concentration profile of species. While in the initial experiments on polyethylene diffusion the resolution was of the order of 60 pm [69] it has been improved e.g. in polystyrene diffusion experiments [70] to 10 pm by the application of a Fourier transform-IR-microscope. This technique is nicely suited to measure profiles on a micrometer scale as well as interdiffusion coefficients of polymers but it is far from reaching molecular resolution. 

Polymer Tensile strength (kg/cm2) Tensile modulus (kg/cm2) Elongation (%) yield break ... 

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