Note: More generally, \(D\) is defined using vectorial flow as the proportionality constant, relating the flux per unit area, \(\boldsymbol{J_{n}}\) , of amount, \(n\), of entities \(\ce{B}\) to their concentration gradient, \(\boldsymbol{\nabla} c_{\ce{B}}\), i.e., \(J_{n} = -D \boldsymbol{\nabla} c_{\ce{B}}\). However, the more general definition is not needed in polymer science as experimental determinations of the translational diffusion coefficient all employ unidirectional flow.