Showing papers by "Alan J. Lesser published in 2011"

A Physical and Mechanical Study of Prestressed Competitive Double Network Thermoplastic Elastomers

Abstract: A new approach to prepare and characterize prestressed competitive double network elastomeric systems was investigated. A styrene−butadiene−styrene (SBS) triblock copolymer system containing physical cross-links was used to achieve a double network by additional chemical cross-linking using ultraviolet (UV) light. Properties measured from conventional monotonic tensile tests, stress relaxation, thermomechanical, hysteresis, and swelling analysis were investigated and related to their network structure. These double network elastomers show a transition between competitive and collaborative behavior in their mechanical properties at different strain regimes. These elastomers also show lower permanent set in both low and high strain regimes along with lower hysteresis. These networks exhibit lower modulus along with lower coefficient of thermal expansion, still showing lower swelling ratios, which results from a competition between the networks.

Shear band formation and mode II fracture of polymeric glasses

Abstract: Mode I and II fracture studies were performed from quasistatic to low velocity impact rates on polymethyl methacrylate (PMMA) and polycarbonate (PC). Mode II tests used an angled double-edge notched specimen loaded in compression. The shear banding response of PMMA is shown to be highly sensitive to rate, with diffuse shear bands forming at low rates and sharp distinct shear bands forming at high rates. As the rate increases, shear deformation becomes more localized to the point where Mode II fracture occurs. PC is much less rate dependent and stable shear band propagation is observed over the range of rates studied with lesser amounts of localization. A new theory is formulated relating orientation in a shear band to intrinsic material properties obtained from true-stress true-strain tests. In a qualitative sense the theory predicts the high rate sensitivity of PMMA. A kinematic limit for orientation within a shear band is also derived based on entanglement network parameters. Mode II fracture in PMMA is shown to occur at this kinematic limit. For the case of PC, the maximum impact rates were not high enough to reach the kinematic limit.

Reinforcing and Toughening of Polypropylene With Self-Assembled Low Molar Mass Additives

Abstract: This study presents a new approach to reinforce and toughen thermoplastic polymers by using a low molecular weight additive; calcium stearate (CaSt2). The method involves melt blending isotactic polypropylene (iPP) and CaSt2 at a process temperature where the CaSt2 reduces the process viscosity through its characteristic morphology. Once the mixture cools, CaSt2 coalesces and solidifies to develop reinforcing domains. Differential scanning calorimeter experiments demonstrate a significant reduction in the percent crystallinity of iPP with the addition of CaSt2. The changes in yield strength and modulus suggest a synergistic effect between the CaSt2 domains and iPP. Addition of 10% CaSt2 increases the fracture energy by almost four-fold with a simultaneous increase in the elastic modulus. The melt flow rate of the composite is dramatically increased with CaSt2 incorporation. Reduction in coefficient of thermal expansion was also observed. Simultaneous positive effects of CaSt2 filler is attributed to its rich mesomorphic structure and its interaction with the iPP matrix. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers