PB-PS IPNs

While several experimental techniques provide Information relating to dual phase continuity, the two most important methods Involve scanning electron microscopy and dynamic mechanical spectroscopy [16,22-2A]. Donatelll, et al [1 ] performed the first mechanical study on PB/PS IPN s. Figure 5 [ 6] illustrates the fit provided by the Davies equation [22] and the Budlansky equation [25,26], both of these equations derived on the assumption of dual phase continuity. 

Table III shows the result of SANS analysis on fully polymerized PB/PS IPN s, seml-IPN s, and chemical blends by Fernandez et al. [ n.] The specific interfacial surface area was shown to increase with Increasing crosslink density, S decreasing in the order full-IPN s, semi-I IPN s, seml-II IPN s afid chemical blends, as expected from many earlier studies. Its value ranges from 20 to 200 m /gm, in the range of true colloids. This result is particularly important because interfacial surface area is closely related to toughness and impact strength.

In order to understand the domain formation process, an investigation of the Intermediate stages before formation of the final morphology is required. There are several different ways to prepare such intermediate materials [3,A2,A3], see Figure 9. The characteristic domain dimensions of PB/PS IPN s are compared in Figures 10 and 11 [3,12,A1]. 

Since this paper will be restricted to sequential IPN s based on cross-poly butadiene-inter-cross-polystyrene. PB/PS, it is valuable to examine the range of possible compositions, see Figure 2 ( ). The PB/PS IPN polymer pair models high-impact polystyrene, and in fact, many of the combinations made are actually more impact resistant than the commercial materials. In general, with the addition of crosslinks, especially in network I, the phase domains become smaller. The impact resistance of high-impact polystyrene, upper left, is about 80 J/ra. In the same experiment, the semi-I IPN, middle left is about 160 J/m, and the full IPN, lower left, is about 265 J/m (g). Since the commercial material had perhaps dozens of man-years of development, and the IPN composition was made simply for doctoral research with substantially no optimization, it was obvious that these materials warranted further study. 

In TEM studies by Fernandez, et al. (9) on thin-sliced materials, it was shown that early in the polymerization of the styrene in PB/PS IPN s the domains tended to be spherical, while later in the polymerization the domains tended to be ellipsoidal in nature. The latter were modeled as irregularly shaped cylinders, which resemble ellipsoidal structures on thin sectioning. In more recent experiments involving small-angle neutron scattering, SANS, it was concluded that the phase separation involved a mixture of nucleation and growth, and spinodal decomposition kinetics (10). 

Figure 4. SANS intensities of a PB/PS IPN, on samples made by adding limited amounts of styrene monomer, and polymerizing to completion. Number following the S represents the weight-fraction of polystyrene. (Reproduced from ret 10. Copyright 1988 American Chemical Society.)...

Figure 9. Splnodal decomposition kinetics for PB/PS IPN polymerization. Same data as in Figure 8. Dashed line is the best fit assuming splnodal decomposition throughout.

With both the PEA/P(S-co-MMA) and PB/PS IPN s, an important variable is the ratio of elastomer to plastic in the final material. When the plastic component predominates, a type of impact-resistant plastic results. In this manner the PB/PS IPN s are analogous to the impact-resistant graft copolymers. When the elastomer component predominates, a self-reinforced elastomer results, the behavior resembling that of the ABA-type block copolymers (thermoplastic elastomers) described in Section 4.4. When the overall compositions of both the PB/PS and the PEA/P(S-co-MMA) series are close to 50/50, the materials behave like leathers. 

Figure 8.1. Electron micrograph of a cis-PB/PS IPN containing 76 % PS. The dark portion is the PB phase stained with osmium tetroxide. Although the cell shape is irregular, the cell wall thickness appears nearly constant. (Curtius et al, 1972.)...

Figure 8.3. Electron micrograph of an 85/15 R-PB/PS IPN showing a two-phase, cellular structure. The symbol R denotes random (mixed cis and trans). This particular composition had an impact strength of over 5 ft-lb/in. of notch. (Curtius et al, 1972.)...

Figure 8.4. Electron micrograph of a PB/PS IPN containing 72% PS (Curtius et ai, 1972). This structure is much finer, and more suggestive of molecular interpenetration, than the structure shown in Figure 8.3. The major difference is the level of crosslinking, being greater by a factor of three in the more finely divided material.

Figure 8.12. Modulus-temperature behavior of cis-PB/PS IPN s (Curtius et al, 1972). Two transitions are observed for all IPN compositions. The IPN s with midrange compositions behave in a leathery manner at room temperature. The sharp rise at — 80°C for the pure ds-PB is due to crystallization. The fact that none of the IPN s shows PB crystallization indicates the existence of molecular mixing. (Modulus taken at 10 sec, using a Gehman torsional tester.)...

The ultimate utility of any material lies in its performance. Let us examine now the types of reinforcement obtained with IPN s. Stress-strain curves for random (R) PB/PS IPN s are shown in Figure 8.22. As is well known, random cis-trans mixture) polybutadiene homopolymer is very weak, breaking at a rather low elongation. Both ultimate elongation and stress to break are increased by addition of the polystyrene network the work required to break, as measured by the area under the curves, is vastly increased. Further, the shapes of the curves are affected by the presence... 

Figure 8.22. Stress-strain behavior of random PB/PS IPN s. Both ultimate elongation and stress to break are significantly increased. (Curtius et a/., 1972.)...

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