Measuring the Spring Constant of a Single Polymer Chain

We present a measurement of the mechanical properties of single polystyrene molecules of size $R\approx 50\mathrm{nm}$. Using a micromechanical technique, we show directly that the polymer chain behaves like a spring, and we measure the spring constant. We examine both cross-linked and non-cross-linked polystyrene. The former forms a stiffer spring, and the value of the spring constant is in agreement with theory, while in the latter case the measured value is larger than expected.