We investigate the effects of polyethylene cylinder domain orientation and thermal history on the micromechanical deformation and fracture properties of poly(cyclohexylethylene)-poly(ethylene)poly(cyclohexylethylene) (CEC) triblock copolymer where the poly(cyclohexylethylene) (PCHE) blocks are unentangled. The properties are assessed using a "fragility" test in which copolymer films are strained in tension using a copper grid as a support. Optical microscopy was used to determine the statistics of deformation and fracture events while transmission electron microscopy (TEM) and scanning force microscopy (SFM) were used to investigate the morphological details of the plastically deformed regions. Films of CEC show much more ductility if the PE cylinders are oriented either randomly (spun-cast films) or parallel to the tensile direction than if the PE cylinders are perpendicular to the tensile direction. In the latter case craze breakdown and crack propagation can proceed within the unentangled PCHE-matrix domain without the highly entangled PE cylinders bridging such cracks. For the highest molecular weight CEC physical aging of the PCHE glassy domains is also important in causing embrittlement. Removing such physical aging by quenching from above the glass transition temperature T-g of PCHE causes a change in deformation mechanism from crazing in slowly cooled samples to a mixed case where crazing and shear deformation compete. Although the crystal morphology in the PE cylinders may also depend on the cooling rate during crystallization, it can be ruled out as the cause of the embrittlement since films physically aged below T-g of PCHE but above the melting temperature of PE show brittle behavior when quenched from the aging temperature.