What is the name of the mechanism of polymer degradation involving decreases in molecular weight and mechanical properties, but no change in external dimensions?

The mechanism of polymer degradation that involves decreases in molecular weight and mechanical properties without a change in external dimensions is known as bulk degradation. In this process, the polymer degrades throughout its entire volume, leading to a reduction in molecular weight and loss of mechanical integrity, but the overall shape and size of the material remain constant until the polymer is significantly weakened.

Bulk degradation typically occurs in hydrophilic polymers where the solvent can penetrate the bulk of the material. As the polymer breaks down on a molecular level, the degradation process is uniform throughout the entire mass of the material, thus maintaining its external dimensions while still experiencing mechanical changes. This type of degradation can be crucial in applications where a controlled release of the drug or a gradual loss of structural integrity is needed.

In contrast, surface degradation involves changes primarily at the surface of the material, which means that as the outer layer degrades, the dimensions can change over time. The other processes such as radiolysis or hydrolysis represent specific types of chemical degradation mechanisms that may or may not affect the molecular weight without being specifically tied to maintaining external dimensions. Understanding the differences among these mechanisms is vital in tissue engineering and materials science when designing polymers for specific applications.