Stress at yield examples MPa

Figure 4.51. (continued) (b) Neat PA stress at yield examples (MPa) (c) Neat PA modulus examples (GPa) (d) Neat PA HDTA (°C)... 

Polyamides and polyesteramides are more recent arrivals to the commercial biodegradable polymer field. Copolymers of either glycine or serine with e-aminocaproic acid are biodegradable. For example, biodegradable polyaspartic acid was synthesized (95% yield) at low cost [Koskan, 1992]. A copolymer of butylene-adipate and e-caprolactam was recently introduced by Bayer as BAK 1095. The material has T = 125°C, density of 1070 kg/m, tensile modulus of 180 MPa, maximum strain at break of 400%, tensile stress at break of 25 MPa, and it fully degrades in 300 days under the ASTM standard conditions. 

Another motivation for measurement of the microhardness of materials is the correlation of microhardness with other mechanical properties. For example, the microhardness value for a pyramid indenter producing plastic flow is approximately three times the yield stress, i.e. // 3T (Tabor, 1951). This is the basic relation between indentation microhardness and bulk properties. It is, however, only applicable to an ideally plastic solid showing no elastic strains. The correlation between H and Y is given in Fig. 1.1 for linear polyethylene (PE) and poly(ethylene terephthalate) (PET) samples with different morphologies. The lower hardness values of 30-45 MPa obtained for melt-crystallized PE materials fall below the /// T cu 3 value, which may be related to a lower stiff-compliant ratio for these lamellar structures (BaM Calleja, 1985b). PE annealed at ca 130 °C... 

Taking the two polymers discussed in the previous section as an example, PVC fracture toughness should be ATjc = 0.5—1 MPam and PE between 2 and 3 MPam. In fact, PVC is a brittle polymer, and its typical stress—strain curve is more or less like curve 1 in Fig. 1.7 with fractures stress (Black and Hastings, 1988) up to 75 MPa. On the other hand, PE is very ductile polymer with a stress—strain similar to curve 2 in the same Fig. 1.7. PE never reaches brittle failure but yield at various stresses between 10 and 30 MPa. 

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