Effect of Thermal History and Microdomain Orientation on Deformation and Fracture Properties of Poly(cyclohexylethylene)−Polyethylene Triblock Copolymers Containing Cylindrical PE Domains

Abstract

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.

Self-assembled block copolymers: Bulk to thin film

“…360 This was explained by the SCMF theory that for a small difference in the surface energy between two blocks, the entropy penalty associated with the midblock looping in the surface wetting layer for the parallel orientation is larger than the enthalpic penalty arising between two blocks. 361 Once the microdomains are aligned vertically to the substrate, the microdomain size can be easily controlled by either using different molecular weights or adding homopolymers.…”

Novel Nanostructures from Self‐Assembly of Chiral Block Copolymers

“…In order to extend the number of varieties of self-assembled nanostructures of BCPs, various approaches have been tried, such as crystallizing semicrystalline BCPs, [35][36][37] selfassembling liquid crystalline BCPs, [38,39] ABC triblock terpolymers [14][15][16] and star copolymers, [40][41][42][43] as well as functionalizing constituent blocks. [44][45][46] Among these, a straightforward way to modify the performance of a BCP is through crystallization of the constituted blocks.…”

Chemical composition effects on the fracture of polystyrene‐block‐poly(methyl methacrylate)block copolymers

“…14 In particular, the mechanical behavior of BCPs with anisotropic ordered structures is further complicated 13,28 because it depends on the extent of long-range ordering and the direction of macroscopic deformation. Because of such morphological complexity, it is difficult to understand the fracture properties of BCPs in terms of their molecular factors.…”

“…Therefore, the fracture properties of BCPs depend not only on the molecular weight, 9 ordered structure, 10,11 and chain architecture 12 but also on the macroscopic alignment 10,13 and thermal aging. 14 Recent developments in high-performance liquid chromatography (HPLC) for BCPs have enabled the separation of BCPs by chemical composition differences. 15,16 For example, it has been shown that the chemical composition differences of HPLC-fractionated polystyrene-b-polyisoprenes (PS-b-PIs) are as high as 9 wt %.…”