= Polymer erosion
\b[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.<Terminology for biorelated polymers and applications (IUPAC Recommendations 2012) by Michel Vert and others>[entry 70, erosion, printed p. 391; Gold Book entry 13906]{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.<Terminology for biorelated polymers and applications (IUPAC Recommendations 2012) by Michel Vert and others>[entry 70, notes on reagent diffusion and mechanism]{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 $v$. Then
$$
R(t)=R_0-vt,\qquad
\frac{M(t)}{M(0)}=\left(1-\frac{vt}{R_0}\right)^3,
\qquad 0\le t\le R_0/v.
$$
The <polymer erosion front speed> is constant, but the mass-loss rate decreases as the exposed area shrinks. For stipulated $R_0=1\,\mathrm{mm}$, $v=0.01\,\mathrm{mm\,day^{-1}}$ 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.
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