= de Gennes confinement scaling
{c}
{title2=$X\sim L(b/D)^{2/3}$}
For a long self-avoiding <polymer> in a tube of diameter $D$, represent each confinement blob by $g$ statistical segments of length $b$, with $D\sim bg^{3/5}$. Excluded-volume repulsion orders the blobs along the tube, giving $X\sim(N/g)D=L(b/D)^{2/3}$. More generally, a segment excluded-volume parameter $v$ gives $X\sim L[v/(bD^2)]^{1/3}$. This requires many blobs and $D$ smaller than the free coil size; $b\ll D\ll L$ alone does not ensure that. An ideal chain without intersegment repulsion is not a one-dimensional string of nonoverlapping blobs.
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