TRADEOFFS IN OPTIMIZATION OF PROPERTY COMBINATIONS
The fundamental factors affecting various polymer properties make it difficult to obtain some combinations of properties simultaneously. For example, requiring a thermoset to have extremely high stiffness, strength, toughness, and heat resistance simultaneously tends to increase its brittleness, so that price-versus performance tradeoffs are often made.
There tends, therefore, to be a price premium on polymeric materials which combine exceptional characteristics that are difficult to attain simultaneously, as was discussed in a post titled PERFORMANCE VERSUS PRICE (created 08 November 2018).
The judicious selection of the monomer(s) which (co)polymerize into repeat unit(s) providing optimum property combinations is always the crucial first step towards optimum combinations of performance attributes.
In many instances, such as the case of a thermoset that we wish to have extremely high stiffness, strength, toughness and heat resistance, optimization of the molecular architecture is also very important, along with optimum repeat unit selection. One can consider the intrinsic toughness of a thermoset to arise from the superposition of (a) the intrinsic toughness of the underlying linear polymer, and (b) the effects of cross-linking. For example, polystyrene is the underlying linear polymer for styrene-divinylbenzene copolymers. So, the intrinsic toughness of a styrene-divinylbenzene copolymer is a function of (a) the intrinsic toughness of polystyrene, and (b) how this intrinsic toughness is modified as a result of cross-linking (and hence as a function of the amount of divinylbenzene used, as well as of the curing chemistry). Rubber-toughenable thermosets with high glass transition temperature (Tg) are more readily obtained if high Tg is attained by enhancing the chain stiffness than if it is attained by increasing the crosslink density. In other words, for highly effective rubber toughenability, it is better to obtain the high Tg by using monomers which produce stiff polymer chains with a high resulting Tg for the underlying linear polymer and then to increase this Tg further by a small amount via cross-linking, than it is to obtain the high Tg via dense cross-linking of an underlying linear polymer with a lower Tg. For this reason, thermosets prepared from “crosslinkable epoxy thermoplastic” resins are consistently tougher at room temperature than conventional epoxy thermosets of equal Tg.
The following image, which is reproduced from J. Bicerano, Prediction of Polymer Properties, third edition, Marcel Dekker, New York (2002), summarizes schematically some of the tradeoffs involved in cross-linking a polymer. It is seen that, as the cross-link density increases (as the average molecular weight Mc between cross-links decreases), the glass transition temperature (Tg) and the compressive yield stress (σcy) both increase, while the fracture energy RIc decreases, and that these trends accelerate when Mc becomes small.
Other examples of polymers where the molecular architecture (in addition monomer selection) is very important in determining the balance of properties as well as behavior during processing include many vinyl polymers, such as polypropylene, polystyrene, and poly(methyl methacrylate), whose Tg, crystalline percentage, and mechanical properties can be quite different depending on whether the polymer is isotactic, syndiotactic, or atactic. Hence each type of tacticity offers different tradeoffs and processing and performance characteristics that are optimal for different applications.
The effects of the distribution of the repeat units along polymer chains in copolymers are also examples where the molecular architecture is very important in determining the balance of properties as well as behavior during processing. For example, the processing and performance characteristics can be completely different depending on whether the same numbers of the same two repeat units are distributed in a statistically random manner, in an alternating manner, or in blocks of varying numbers and lengths.
The incorporation of any one or a combination of a broad range of formulation ingredients; such as plasticizers, rigid fillers, and/or elastomeric (rubbery) domains, is also often useful in optimizing property combinations and thus reducing the severity of the tradeoffs that would have been necessary in the absence of such additives.
Finally, optimizing the fabrication processing conditions, for example, to induce orientation and/or to take advantage of kinetic effects to “freeze in” an optimum morphology with different heterophasic morphology than would have developed if the polymer had been allowed to attain thermodynamic equilibrium, is another means for optimizing property combinations and thus reducing the severity of the needed tradeoffs.