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.

The synthesis of bi-and tetra-functional tri-aryl ether epoxy resin isomers and their subsequent cure with 44 diamino diphenyl sulphone (DDS) is presented here. The effect of varying aromatic substitution and cross-link density on the structure, property, and processing relationships is explored for 1,3 bis (3-glycidyloxyphenoxy)benzene (133 BGOPB), 1,4 bis(4-glycidyloxyphenoxy) benzene (144 BGOPB), N,N,N,N-tetraglycidyl 1,3-bis (3-aminophenoxy) benzene (133 TGAPB), and N,N,N,N-tetraglycidyl 1,4-bis (4-aminophenoxy) benzene (144 TGAPB). Meta substitution to the aromatic ring reduces the rate of reaction, glass transition temperature, yield strain and crosslink density, coefficient of thermal expansion, and side reactions, while increasing strain softening, compressive modulus and strength, and methyl ethyl ketone ingress. Increasing crosslink density increases the glass transition temperature, promotes side reactions during cure, and increases compressive modulus, strength, and yield strain, while reducing coefficients of thermal expansion, methyl ethyl ketone ingress, and density. The results are discussed in terms of packing efficiency of the meta-substituted epoxy resins and the role of short range molecular mobility caused by the lack of an aromatic axis of rotation. K E Y W O R D Sisomerism, network properties, reaction kinetics, synthesis

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The synthesis of inherently ductile epoxy network as measured by their resistance to deformation however is less reported and is the subject of this study. Common strategies have been to incorporate structures that are inherently rigid, [8] such as liquid crystalline [9,10] or highly aromatic, [11,12] while others have sought to build a combination of both rigid and flexible features into a network. [13][14][15] Highly aromatic structures will generally have excellent mechanical and thermal properties and resistance to solvent ingress, but are also notoriously difficult to process. As a result, the following studies are a reflection of the variety of strategies attempting to break this paradigm. Karimata et al. (2016) [16] synthesized a tri-aryl di-epoxide monomer linked by flexible carbosilane segments. After curing with an aromatic amine, the glass transition temperature (T g ) of the network was still below room temperature, suggesting potential application as a reactive flexibilizer. Wan et al. (2016) [17] synthesized a bioderived eugenol epoxy resin, essentially a tri-aryl ether linked epoxy resin with a superior modulus, hardness, and char yield compared with a related epoxy amine cured network. It also had a higher creep displacement attributed to increased chain mobility in the glassy state. Yu et al. (2018) [18] combined a rigid carboxamide structure with flexible aliphatic ethers to synthesize an amine, which when cured with an epoxy resin reported improvements in tensile strength, failure strain, and impact strength without any commensurate deterioration in flexural strength. Varley et al. (2019) synthesized highly aromatic ether-linked epoxy monomers and amines, exploring the impact of substitution patterns on the properties and processability. [19] Improvements in yield and failure strain, referred to as distortional behavior, were attributed to an increase in energy dissipated through phenylene rotations available to para-substituted amine or epoxy resins. [20,21] In this work, two tri-aryl epoxy resins containing flexible methylene linkages instead of ether linkages and a rigid bi-aryl epoxy resin have been synthesized and compared with the commercially available diglycidyl ether of bis phenol F (BisF). A central hypothesis of this work is that flexibility or molecular mobility within a rigid network may impart enhanced distortional behavior of the epoxy amine network as measured by compression. The epoxy resins synthesized were bis[(glycidyloxy)phenyl)]-m-xylene (BGOPmX), bis[(glycidyloxy)phenyl)]-p-xylene (BGOPpX), and In this work, two tri-aryl and one bi-aryl epoxy resin, bis[(glycidyloxy)phenyl)]m-xylene (BGOPmX), bis[(glycidyloxy)phenyl)]-p-xylene (BGOPpX), and bis(glycidyloxy) biphenyl (BGOBP) are synthesized and cured with methylene dianiline and 4,4′-diamino diphenyl sulfone. Structure, property, and processing relationships are investigated and compared against diglycidyl ether of bis-phenol F epoxy resin to better understand the impact of rigid and flexible subunits within the network st...

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