Basic longitudinal texture and fracturing process in thermoset polymers

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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44718/1/10853_2005_Article_BF01197652.pd

Possible phase transformation toughening of thermoset polymers by poly(butylene terephthalate)

Mechanisms were explored by which particles of poly(butylene terephthalate) (PBT) are able to toughen a brittle epoxy. The epoxy studied was an aromatic amine-cured diglycidyl ether of bisphenol-A, which was toughened at about twice the rate with particles of poly(butylene terephthalate) as with particles of nylon 6, poly(vinylidene fluoride), or CTBN rubber. Many of the mechanisms of toughening are visible on the fracture surface of the PBT-epoxy blend, but a mechanism suggested to account for perhaps half of the increased toughness with PBT, phase transformation toughening, is not. The two types of experiment performed to detect phase transformation toughening were: (1) measurements of the rubber cavitation zone in PBT-CTBN rubber-epoxy ternary blends, which would detect an expansion of the PBT particles during fracture if it occurred, and (2) measurements of the fracture energy in PBT-epoxy blends in which the various mechanisms of toughening were selectively suppressed. Both types of experiment indicated the occurrence of phase transformation toughening in these PBT-epoxy blends.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44711/1/10853_2005_Article_BF01154110.pd

Particulate Fillers in Thermoplastics

The characteristics of particulate filled thermoplastics are determined by four factors: component properties, composition, structure and interfacial interactions. The most important filler characteristics are particle size, size distribution, specific surface area and particle shape, while the main matrix property is stiffness. Segregation, aggregation and the orientation of anisotropic particles determine structure. Interfacial interactions lead to the formation of a stiff interphase considerably influencing properties. Interactions are changed by surface modification, which must be always system specific and selected according to its goal. Under the effect of external load inhomogeneous stress distribution develops around heterogeneities, which initiate local micromechanical deformation processes determining the macroscopic properties of the composites