 Polymer erosion

Polymer erosion is degradation that begins at a polymer surface and progresses inward. This is the specific IUPAC usage: it identifies where degradation proceeds, rather than serving as a synonym for every reduction in specimen mass. The recommendation discourages the expression “bulk erosion” in this terminology.^[ref]

A surface-controlled process can occur when a degrading reagent reacts faster than it penetrates the interior by diffusion. Conversely, reagent penetration before substantial reaction permits internal degradation without establishing the surface-inward mechanism. The relative time scales depend on the material, reagent and specimen geometry. A measured loss of dry mass alone cannot decide between these possibilities.^[ref]

An original ideal front model gives a clear quantitative prediction. Suppose a spherical specimen has constant density, initial radius $R_0$, no swelling and a sharp boundary receding at constant speed $v$. ThenThe polymer erosion front speed is constant, but the mass-loss rate decreases as the exposed area shrinks. For stipulated $R_0=1\mathrm{mm}$, $v=0.01\mathrm{mm}\mathrm{da}{\mathrm{y}}^{-1}$ and $t=10\mathrm{days}$, the retained mass fraction is $0.9^3=0.729$.

A flat sheet of initial thickness $h_0$, attacked on both large faces with negligible edge effects, instead gives $M(t)/M(0)=1-2vt/h_0$ before the fronts meet. At the same stipulated speed and time with $h_0=1\mathrm{mm}$, the fraction is 0.8. Geometry alone changes the mass curve; neither calculation is a measured degradation rate for a particular polymer.

Experimentally, combine timed mass measurements with evidence about depth and structure, such as cross-sectional imaging or measurements of chemical change at different depths. Retained mass, dissolved fragments and chain scission answer different questions. Swelling, fragment detachment and density changes can invalidate the ideal front model.

Polyglycolic-acid chain scission and mass loss supplies a useful comparison: a molecular chain can break many times before material leaves the specimen. Establish the spatial mechanism before calling that change erosion in the IUPAC sense. A good mass fit alone is insufficient proof of an inward-moving front.