Epoxy resin Thermomechanical properties

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

The majority of base materials for circuit boards are combinations of a copper foil with a laminate, where the laminate itself consists of a carrier material and a resin. Thus properties of the base material such as mechanical strength, dimensional stability, and processi-bility are determined primarily by the carrier material. On the other hand, the resin materials are responsible for the thermomechanical and electrical properties as well as for its resistance against chemicals and moisture. Frequently used carrier materials are based on glass and carbon fibers, papers, and polyamide, whereas the majority of the laminating resins are thermosets such as epoxies, phenolics, cyanates, bismaleimide triazine (BT) resins, maleimides, and various combinations of these [13]. 

The properties of a cured epoxy network depend primarily on two factors nature of the resin and nature of the curing agent. The functionality of an epoxy resin plays an important part in determining the thermomechanical properties. The properties of epoxy resins of various functionalities cured with DETDA are presented in Table 3.4. Multifunctional epoxies exhibit a higher T compared with the difunctional epoxy when cured with the same hardener. This is due to the increase in crosslink density as a result of an increase in epoxy functionality and the formation of a tighter network. This significantly reduces the free volume of the network, leading to an increase in the T. ... 

Liquid crystals exhibit a partially ordered state (anisotropic) which falls in-between the completely ordered solid state and completely disordered liquid state. It is sometimes referred to as the fourth state of matter . In recent years, interest in liquid crystalline thermosets (especially liquid crystalline epoxy) has increased tremendously [33-44]. If the liquid crystal epoxy is cured in the mesophase, the liquid crystalline superstructure is fixed permanently in the polymer network, even at higher temperature. Liquid crystal epoxies are prepared using a liquid crystal monomer [33-38] or by chemical modification of epoxy resin [43] which incorporates liquid crystal unit in the epoxy structure. Liquid crystalline epoxy resins with different types of mesogen such as benzaldehyde azine [33], binaphthyl ether [34, 35], phenyl ester [36, 37] and azomethine ethers [38, 39] have been reported. Depending on the chemical nature of the mesogen, the related epoxies display a wide range of thermomechanical properties. The resins can be cured chemically with an acid or amine [40, 41] or by photochemical curing in the presence of a photo-initiator [3]. Broer and co-workers [42] demonstrated the fabrication of uniaxially oriented nematic networks from a diepoxy monomer in the presence of a photo-initiator. 

The cured matrix of GMAEVC and epoxy resin contains hydrophilic functionality and easily absorbs water molecules. The absorbed water leads to dimensional variations in composites and also affects the mechanical properties of the composites. Water absorption tests were performed on the matrices prepared at different curing conditions and the thermomechanical performance of matrices was compared to the dry, original samples. 

From comparisons of the sorbed water effects on the polymer cured under different conditions, it can be concluded that the sorbed water in the polymer contributes to the changes of the thermomechanical properties differently depending on the crosslink structure. It is concluded that water absorption leads both to plasticization effects and chemical modification of the hydrophilic polymer matrix of GMAEVC and epoxy resin depending on the structure and crosslinks of the matrix. 

Block copolymers based on nitrile rubber and on epoxy and phenolic resins and on polystyrene (50-54) have been intensively studied in Russia The generated block copolymers were investigated by turbidimetric and IR methods. Thermomechanical experiments were also run on fractions. As may be seen from Fig 13, fractions which combine the properties of the polymers (Curves 2,3, and 4) were obtained together with fractions characteristic of the raw rubber (Curve 1) and of the resin (Curve 5). The copolymer is soluble in solvents which are typical for both components. Solubility studies on the products showed that for any given ratio of the original components, 15 to 20% of the resin combines with the rubber. The properties of the block copolymer, however, depend on the initial ratio of components nitrile rubber confers elasticity and the phenolic resin processability. 

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