PMMA, craze zone

Fig. 8. Measured craze zone at the crack tip and fit by the Dugdale model (—) for different polymers r + PES, o plasticized PMMA x PC , A plasticized PVC ...

Fig. It a and b. Variation with crack speed of the dimensions of the craze zone at the tip of a propagating crack in PMMA a maximum craze width 2Vj b craze length s...

In all papers mentioned above the characterization of the craze growth has been performed by measuring the craze length. By application of the optical interference method the essential information on the growth of craze width is awiilable. Figure 22 shows the interference fringe patterns and measured craze zone sizes at a stationary crack tip in PMMA loaded in a creep test at a constant K,-value at loading times of 150 s and 2 1(F s... 

Fig. 43. Optical interference photograph showing fatigue striations on the fracture surface and craze zone in a partially broken PMMA specimen...

Fig. 41. Sketch of cycKc loading together with interferent fringe patterns of crack opening and craze zone in PMMA at upper and lower load level (ro = 50 Hz R g 0.1 da/dN = 1.2 10 mm/ cycle)...

The effect of R-ratio (0.1 R 0.7) on the craze zone size of PMMA has also been investigated in the range of 0.4 g 1.0 MPa /m at 1 Hz no consistent or significant influence could be established this context it should be... 

Doll, W. and Weidmann, G.W. (1976) Interferential optical measurement of craze zone before fracture process in PMMA with different molecular weights. Colloid. Polym. Sci., 254, 205. 

Studies of PMMA-based ionomers also demonstrate the influence of thermal treatment on deformation modes (16). For Na salts of PMMA-based ionomers of 6 and 12 mol% that were cast from DMF, only crazes were observed on straining. However, after an additional heat treatment (48 h at 160°C), which also removes any DMF solvent that is present, shear deformation zones are induced. Hence, the ionic cluster phase, which was destroyed by the polar solvent, has been restored by the heat treatment. 

The combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

The fibrillar structure of the crazes is t5 ical of PS. Another glassy polymer is styrene-acrylonitrile (SAN) copolymer with a PS content of usually about 74%. In this material the dominant deformation structures are homogeneously deformed zones, but in many cases they coexist with the fibrillated crazes (2,29). Depending on the loading conditions (stress state, loading velocity, temperature), the deformation character can be shifted from one to another. Another typical glassy polymer is PMMA with a t5 ical appearance of homogeneously deformed crazes at room temperature (29). 

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