(522d) Dynamic Mechanical Strengthening of Polycarbonate

Polymers are viscoelastic materials and exhibit a wide range of mechanical deformation behavior. Polycarbonate (PC) has excellent impact strength against small fragments, thus is the choice of material for use in safety lenses as well as in spall layers for plastic/plastic or glass/plastic composites. However, PC not only has very poor resistance to abrasion but also is very notch sensitive. PC becomes prone to crack initiation and propagation, which occurs often in a target with thickness greater than about 5-6 mm, particularly under extreme service (i.e. low temperature) conditions.  The geometry dependence has been attributed to a change of stress state from the plane stress to plane strain, thus suppressing yielding and favoring craze initiation and growth and subsequently leading to a catastrophic failure. This characteristic notch sensitivity limits the useful parts thickness of PC components.  A common solution is to laminate thin PC sheets together to achieve thicker parts but leads to other issues such as delamination.  To improve the performance of polycarbonate under extreme service conditions, we have prepared various co-polycarbonates with varying compositions of bisphenol A (BPA), 4,4’-dihydroxydiphenol (DOD), and triptycene hydroquinone (THQ).  Inclusion of DOD and THQ in the polymer composition is expected to reduce the cross-sectional area of the polymer chains and enhance chain packing to improve crazing thresholds.  Initial thermal and mechanical testing has shown an enhancement in the glass transition temperature, yield stress, and flow stress of the copolymers over commercial PC when DOD and THQ are incorporated in the feed stock.