Subtle variations in the structure of crosslinked epoxy networks and the impact upon mechanical and thermal properties

Abstract

Presented here is an investigation of the structure-property relationships of crosslinked networks using three bi-functional glycidyl ether aromatic epoxy resins, two bi-aryl and one tri-aryl, cured with bi-and tri-aryl amines. Subtle changes to the monomer chemistry including changing aromatic substitution patterns from meta to para, methylene to isopropyl and isopropyl to ether were explored. Changing an epoxy resin backbone from methylene to isopropyl enhances backbone rigidity thus increasing glass transition temperature (T g ), yield strength, and strain despite reducing modulus. Changing meta-substitution to para increases T g and yield strain while leaving strength unaffected and reducing modulus. Changing isopropyl linkages to ether reduces modulus, strength, T g , and yield strain reflecting increased molecular flexibility. Using three instead of two aromatic rings increases the molecular weight between crosslinks thereby decreasing T g and yield strain while increasing modulus and strength. Despite the complexities of multiple systems for varying epoxy resins and amine hardeners, the effect upon network properties is explained in terms of short-and long-range molecular and segmental mobility, crosslink density, and equilibrium packing density.

Structural Variation and Chemical Performance—A Study of the Effects of Chemical Structure upon Epoxy Network Chemical Performance

“…Work by the current authors also shows the importance of regioisomerismnetworks from the three regioisomers of DGEBF (para–para (pp), para–ortho (po), ortho–ortho (oo)) showed considerable changes in properties and performance, where ortho-substitution was shown to reduce T g and to be detrimental to chemical performance . Recent work by Varley and co-workers also shows changes in properties due to regioisomerism and subtle structural differences. − In terms of T g , they show that para-substitution (relative to meta-) in epoxy resins increases network rigidity and T g , even where the changes are subtle.…”

“…The influence of subtler changes upon network properties is an area of increasing interest. − These subtler changes have also been shown to cause variation in properties/chemical performancefrom changes to the cure schedule , to changes in atmosphere of cure (processing) and cure method . Therefore, when attempting to draw comparisons, for changes between monomers, a standardized cure schedule should be used, to avoid variability due to processing.…”

Influence of POSS Type on the Space Environment Durability of Epoxy-POSS Nanocomposites

“…However, as the AM-POSS content was increased, T g values were similar to the T g of pristine epoxy for most of the compositions, and even slightly higher in the case of 50AM [51]. The reason for this is the increased presence of phenyl rings which increase the backbone rigidity of the network, increase the physical crosslinking density and T g , and decrease the chain mobility [63,64]. The lower chain mobility prevents uncured fragments from diffusing to the surface and outgassing from it [62].…”

Effect of the linking positions on the benzene ring on properties of poly(silane arylether arylacetylene)s

“…Although the chemical compositions of three isomeric resins are identical, the segmental motions in the crosslinking networks with the –Ph–O–Ph–O–Ph– unit (see Figure 5E) at elevated temperature can differently occur. The cured pmp ‐PSEA and pop ‐PSEA resins could undergo segment motions, resulting in higher β ‐transition temperature and stronger β ‐transition peak (tanδ) compared with cured ppp ‐PSEA resin 45–49 . For three cured resins, storage modulus drops at first and then increases corresponding to the movement of local segments and further crosslinking reactions of internal alkynyl groups, respectively.…”

“…The cured pmp-PSEA and pop-PSEA resins could undergo segment motions, resulting in higher β-transition temperature and stronger β-transition peak (tanδ) compared with cured ppp-PSEA resin. [45][46][47][48][49] For three cured resins, storage modulus drops at first and then increases corresponding to the movement of local segments and further crosslinking reactions of internal alkynyl groups, respectively.…”