Ultimate Mechanical Behavior of IPNs

Both the modulus-temperature relationships presented in the preceding sections and the tensile data presented above are strikingly similar to those demonstrated for other rubber-plastic combinations, such as the thermoplastic elastomers (see Chapter 4 and the model system presented in Section 10.13) and the impact-resistant plastics (Chapter 3). The IPN s constitute another example of the simple requirement of needing only a hard or plastic phase sufficiently finely dispersed in an elastomer to yield significant reinforcement. Direct covalent chemical bonds between the phases are few in number in both the model system (Section 10.13) and present IPN materials. Also, as indicated in Chapter 10, finely divided carbon black and silicas greatly toughen elastomers, sometimes without the development of many covalent bonds between the polymer and the filler. 

Some of the plastic IPN s exhibit very high Charpy impact resistance impact strengths for these IPN s, as well as for ordinary polystyrene and a graft-type high-impact polystyrene (HiPS), are compared in Table 8.2. 

Material Sample thickness, in. Styrene content, % Impact resistance, ft-lb/in. 

The HiPS typically is about four times as tough as the unmodified plastic. The IPN s concerned exhibit values of impact strength about four times as high as the HiPS. The 15/85 composition of the random PB/PS is similar to the overall composition of the HiPS, but contains a few percent more rubber, which may contribute to the observed toughness. 

In contrast to the PB/PS compositions, the PEA/PS and PEA/PMMA IPN S exhibit only marginal increases in impact resistance values in comparison to the homopolymer PS and PMMA values of impact strength are typically below 1.0 ft-lb/in. of notch. This relatively poor performance may have two explanations (1) the domain sizes may be too small, and (2) the Tg of the PEA (-22°C) may be too high. According to the theory developed in Section 3.2, a 7 of about — 40 C of the rubber phase is required for impact resistance, suggesting that explanation 2 may be correct. 

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