Diffusion Effects During Cure

Recently, an approach was used based on dielectric measurements of the mobility-related properties, including ion conductivity and relaxation time, to estimate the diffusion coefficient [66,67]. The effect of diffusion on cure kinetics appeared to be insignificant in the pre-gel stage, but close to vitrification, the curing reaction became significantly slower. 

Note that although the results reported by Yu and von Meerwall [65] and by Deng and Martin [66,67] showed a dependency of diffusion coefficient on molecular weight, the diffusion of reactive groups toward each other concerned with epoxy cure reactions (step-growth polymerisation reactions) is only controlled by segmental diffusion irrespective of the size of molecules to which they are attached. Neither molecular weight nor symmetry or composition play any role within overall diffusion control. 

---

The formation and structure of interfaces prepared by deposition of metals onto fully cured polymers is strongly affected by the extent to which tnetal diffusion takes place during metallization. Very reactive metals, which appear to be practically immobile, form relatively sharp boundaries, and thin films in the monolayer range act as very effective diffosion barriers. In contrast, interfaces with noble metals can be much more spread-out, particularly when the metal is deposited at elevated temperatures and low rates. As mentioned earlier, pronounced diffusion and, at sufficiently high metal concentrations, formation of metal clusters inside the polymer may take place under these conditions. 

Acrylic esters and unsaturated polyesters are commercially cured with peroxides or peresters. The choice of per compound is determined on the basis of price, the achievable polymerization rate, and the side products formed. The polymerization rate is determined by the decomposition rate of the initiator, when mixed with the material to be cured, as well as on the free radical yield. In addition, attention should be paid to the fact that many per compounds decompose slowly during storage, thus reducing the polymerization activity per unit initiator mass. For this reason, crystalline per compounds are more stable because of the lower diffusion than amorphous or dissolved per compounds. Side products of initiator compounds can have an unfavorable effect on the long-term thermoset properties dibenzoyl peroxide, for example, forms acids dicumyl peroxide forms ketones. Acids can hydrolyze the ester bonds of polyester chains, causing scission, and ketones can... 

For the epoxy resins studied, the mobility factor based on heat capacity coincides very well with the diffusion factor, calculated from the nonreversing heat flow via chemical kinetics modelling, and describing the effects of diffusion control on the rate of conversion of the cure reaction. Although the two resins behave quite differently, this coincidence between the mobility factor and diffusion factor is valid for both systems. Therefore, the mobility factor can be used for a quantitative description of then-altered rate of conversion in the (partially) vitrified state for the decrease in rate during vitrification, the increase in rate during devitrification and the diffusion-controlled rate in the (partially) vitrified region in between both processes. 

---