Polystyrene brittle-ductile transition

It is tempting to relate the temperature at which the ductile-brittle transition takes place to either the glass transition or secondary transitions (Section 5.2.6) occurring within the polymer. In some polymers such as natural rubber or polystyrene Tb and Tg occur at approximately the same temperature. Many other polymers are ductile below the glass transition temperature (i.e. Tb < Tg). In this case it is sometimes possible to relate T to the occurrence of secondary low-temperature relaxations. However, more extensive investigations have shown that there is no general correlation between the brittle-ductile transition and molecular relaxations. This may not be too unexpected since these relaxations are detected at low strains whereas Tb is measured at high strains and depends upon factors such as the presence of notches which do not affect molecular relaxations. 

Guo, Z., Fang, Z., and Tong, L. 2007. Application of percolation model on the brittle to ductile transition for polystyrene and polyolefin elastomer blends. eXPRESS Polymer Letters 1 37-43. http //www. expresspolymlett.com/... 

Figure 10.7 shows that the tensile strength is improved as polystyrene is incorporated. Data for conventional melt-blended samples (Fayt et al., 1989) are provided for comparison. We note that the ductile-to-brittle transition for our system is shifted toward much higher polystyrene content. Fayt and others have shown that conventionally prepared polyethylene/ polystyrene blends are relatively poor materials (Barentsen and Heikens, 1973 Wycisk et al., 1990). Blends of most compositions are weaker than polystyrene or polyethylene homopolymers because of the poor interfacial adhesion between the two immiscible polymers. The electron micrographs and the mechanical data for the blends described here indicate that poly-... 

Qutubuddin and coworkers [43,44] were the first to report on the preparation of solid porous materials by polymerization of styrene in Winsor I, II, and III microemulsions stabilized by an anionic surfactant (SDS) and 2-pentanol or by nonionic surfactants. The porosity of materials obtained in the middle phase was greater than that obtained with either oil-continuous or water-continuous microemulsions. This is related to the structure of middle-phase microemulsions, which consist of oily and aqueous bicontinuous interconnected domains. A major difficulty encountered during the thermal polymerization was phase separation. A solid, opaque polymer was obtained in the middle with excess phases at the top (essentially 2-pentanol) and bottom (94% water). The nature of the surfactant had a profound effect on the mechanical properties of polymers. The polymers formed from nonionic microemulsions were ductile and nonconductive and exhibited a glass transition temperature lower than that of normal polystyrene. The polymers formed from anionic microemulsions were brittle and conductive and exhibited a higher Tj,. This was attributed to strong ionic interactions between polystyrene and SDS. 

The polymers used in commercial apphcations are solids at their use temperatures. The solid phase might be brittle or ductile, depending on the chemical composition and, to some extent, on the way in which the polymer has been processed. The chemical composition of the backbone of some polymers, such as polyethylene, is such that crystallization can occur other polymers, such as polystyrene, cannot form crystalline structures and sohdify only as amorphous sohds, or glasses. The glass transition occurs when the temperature is sufficiently low to prevent large-scale chain motion. Crystallization and glass transition temperatures are shown for... 

The nature of surfactant used in microemulsion has a pronounced effect on the above mentioned properties of polymer products. Thus, if the surfactant has any polar or electrostatic interaction with the polymer, this may lead to an improvement in its tensile properties. However, if the surfactant is not compatible with the polymerized matrix, it may simply act as a low-molecular additive (plasticizer) and reduce the mechanical strength of the material. For polystyrene, it has been observed that the polymer produced from a nonionic surfactant-based microemulsion is ductile while that obtained from anionic surfactant (e.g., SDS)-based microemulsion is brittle in nature and has much higher glass transition temperamre than the bulk-polymerized polymer. 

Polycarbonate is perhaps the most notoriously notch-sensitive of all thermoplastics, although nylons are also susceptible to ductile/brittle transitions in failure behaviour caused by notch sharpening. Other plastics such as acrylic, polystyrene and thermosets are always brittle - whatever the crack condition. 

---