PMMA, cavition

Figure 25 shows that the tensile behavior of PMMA and PS, stretched some 10-30 °C below Tg, is very similar to that of PC. Like PC both PMMA and PS exhibit extensive stress-whitening at high extension ratios, k", where the peak or inflection of the stress-strain curve is observed. In Table 3, the value of k" measured for PMMA is compared to the maximum extension ratio, k , of chains between entanglement points. The fact that k" = k provides further evidence that the cavitation in PMMA must be attributed to the same deformation mechanism as discussed in Section 4.2 for PC. The value of k" = kikj" = 6.8 measured for pre-oriented PS is higher than the theoretical value of k = 4.1 included in Table 3. This result is not... 

There is much evidence that the intrinsic crazing of PC is related to the existence of a general mode of cavitational plasticity in glassy polymers. Examples have been given for polymers which behave very similar to PC. In particular PMMA and pre-oriented PS exhibit an almost identical behavior when stretched at high stresses and strains in a temperature region close to Tg. 

Sternstein and Ongchin (28) considered that if cavitation occurs in crazes the criterion for crazing initiation should include the dilative stress component. They proposed the criterion to fit the experimental data for surface craze initiation in PMMA when the polymer is subjected to biaxial tension. The segmental mobility of the polymer will increase due to dilative stresses, thus provoking cavitation and the orientation of molecular segments along the maximum stress direction. 

Cavitation was also identified as an active mechanism in systems where a mbberlike phase (particles or interphase) is susceptible to implode under the effect of the hydrostatic stress induced by the applied tension. Fond (36) has recently revisited the critical conditions under which this form of damage becomes energetically favorable. In the case of core-shell rubber-toughened PMMA, he ascribed the extensive whitening under tension at room temperature to the profuse formation of voids in the mbber shell of the toughening particles. 

The cavitation mechanism within the modifier particles begins with microvoid formation and fibrillation in the butyl acrylate shell, as shown in higher magnification in Figure 10b. The fibrillated shells with the internal PMMA cores resemble spiders. The fibrils are connected by the PMMA cores, and the cavities in... 

Often, very small rubber particles or modifier particles are used to enhance the toughness, for instance, of PMMA or PP at lower temperatures. Figure 5.12 shows schematically one example with PBA core shell particles they consist of a hard core of PMMA (diameter about 180 nm) and a rubbery shell of poly(bu-tyl acrylate-co-styrene) (PBA) (approximately 40 nm thick). An outer PMMA shell increases compatibility between particles and matrix. The particles were preformed and possess spherical shapes with a narrow size distribution. Under load, the plastic deformation starts in the particles with cavitation and fibrillation of the rubbery shell. The second step is deformation in highly stressed zones between the particles in the form of crazes or homogeneous yielding. 

PBA shells are cavitated and fibrillated the PMMA cores (dark) are unchanged in the stress concentration fields the matrix strands are plastically deformed (homogeneously or with crazes) ... 

PBA shells cavitated and fibrillated, PMMA cores unchanged (dark), fibrillated crazes at particles (bright) ... 

PBA shells cavitated and fibril-lated (bright), PMMA cores gray, matrix dark, with crazes at particies (white) compare the contrast with the stained sampies in Figs, 5,83, 5,84 ... 

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