Witold Brostow(*,a), Antonio M. Cunha(b), John Quintanilla(c) and Ricardo Simoes(a,b)

(a) Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science, University of North Texas, Denton, TX 76203-5310, USA; and Department of Physics, University of North Texas, Denton, TX 76203, USA; brostow@unt.edu
(b) Department of Polymer Engineering, University of Minho, 4800 Guimaraes, Portugal
(c) Department of Mathematics, University of North Texas, Denton, TX 76203, USA; johnq@unt.edu

ABSTRACT

In recent papers [1-2] we have used statistical mechanics to predict multiple phase formation in polymer liquid crystals (PLCs). Now we have performed molecular dynamics simulations of PLC copolymers as materials consisting of LC islands in flexible matrices. A method for creating such materials on a computer is described. Overall concentration of the LC units, island size and spatial distribution of the islands (random, in rows and evenly distributed throughout the material) were varied. Crack formation and propagation as a function of these parameters were investigated. The local concentration of LC units in each chain has been defined. We found that the probability of a crack initiation site goes symbatically with the local LC concentration. The first small crack is sometimes a part of the path through which the material breaks; however, several small cracks may evolve at first, but then some of these never evolve into larger cracks since crack arrest occurs. The results can be used for creation of real materials with improved mechanical properties.

Keywords: Computer Simulations, Fracture Analysis, Molecular Dynamics, Polymer Liquid Crystals, Structure-Property Relations