Basic longitudinal texture and fracturing process in thermoset polymers

Abstract

The “basic longitudinal texture”, which is present everywhere on the fracture surfaces of glassy thermosets and is the finest texture observed on such surfaces, consists of low ridges and shallow grooves that are aligned parallel with the direction of crack propagation. The periodicity of the basic longitudinal texture, i.e., the average lateral separation between the ridges (or grooves), has been found to be characteristic of materials. This and other properties were measured for a series of rigid epoxy specimens made from diglycidyl ether of bisphenol-A and methylhexahydrophthalic anhydride. For the series of epoxies studied, the glass transition temperatures varied from 76 to 143 °C, the room temperature Young's modulus varied from 2.29 to 2.97 G Pa, the room temperature yield stress in compression varied from 99 to 128 M Pa, the room temperature Knoop hardness numbers varied from 133.5 to 163.5, the rubbery modulus at 200'C varied from 12.8 to 21.6 MPa, and the periodicity of the basic longitudinal texture varied from 205 to 368 nm. Only properties of the liquid state, namely glass transition temperature and the rubbery modulus, correlated well with periodicity of the basic longitudinal texture. This suggests that the basic longitudinal texture is the remnant left on the fracture surfaces of a liquid state that must have developed during fracture. This suggests in turn that liquefaction is an intrinsic part of the brittle fracture of polymer network glasses.