M7 Functional Classification: Thermoplastic, Thermostable and Elastomer

Topic

The fact that the molecular chains of this polymer—whose molecular weight, Kuhn segment configuration, and topological entanglements establish the underlying chain physics shared by three other models in this set—lack any chemical cross-linking between them enables the polymer's thermoplastic behavior: without chemical cross-link points to bind them permanently, the chains are free to slide past one another via reptation (a process described in detail by another model in this set), provided their molecular weight exceeds the corresponding critical value. Because this absence of chemical cross-linking persists—with no subsequent treatment chemically bonding the chains—the polymer's thermoplastic nature remains indefinitely reversible upon heating.

Conversely, if these same molecular chains are linked by dense chemical cross-linking—characterized by a high number of cross-link points per unit volume—the polymer exhibits thermoset behavior: this dense cross-linking creates a permanent network in which no chain segment can reptate freely beyond the cross-link points anchoring it to neighboring chains; consequently, no increase in temperature, however extreme, can break these chemical bonds or restore the polymer's ability to flow. A higher cross-link density—meaning more cross-link points per unit volume—renders the thermoset polymer even more incapable of flowing when heated. Thus, this model—which begins by addressing thermoplastic and thermoset states—also describes a third network architecture intermediate between the first two: when the chemical cross-linking is loose—with cross-link points spaced widely enough to leave long, free molecular chain segments between them—and the polymer is maintained well above its glass transition temperature, this loose cross-linking gives rise to an elastomeric state dominated by the entropic elasticity described in greater detail by another model in this set. These loose cross-link points prevent permanent flow (as would occur in the thermoplastic state) yet—thanks to the temperature being well above the glass transition—allow the long, free chain segments sufficient mobility to stretch and recover their conformational entropy, rather than undergoing the brittle fracture characteristic of the thermoset state. The fact that the same fundamental chain physics—molecular weight, Kuhn segment configuration, and topological entanglements—can produce three such distinct network architectures (thermoplastic, thermoset, and elastomer) depending on the absence, density, or looseness of the chemical cross-linking demonstrates that it is ultimately the cross-linking itself—rather than any change in the underlying chain physics—that determines which of these three states the polymer ultimately assumes.

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