An undergraduate laboratory experiment on nucleation and growth via polarizing optical microscopy of semicrystalline polymers

Here, we describe a set of experiments that enables undergraduate students to perform and understand nucleation and growth utilizing the concept of birefringence. Semicrystalline polymers, e.g., polyethylene oxide, exhibit a Maltese cross extinction pattern when they are imaged under polarizing optical microscopy with crossed polars (i.e., two polarizers whose axes are oriented at right angle to each other). This behavior is due to the birefringence of the polymer's spherulite, which is its building block, analogous to the grains of a metal. In this instructional laboratory experiment, samples of polyethylene oxide were crystallized at three different transformation temperatures of 295, 273, and 77 K. Taking advantage of the birefringence of spherulites, the dependence of the microstructure with undercooling is shown. In particular, smaller spherulite sizes were obtained with larger undercoolings. It is also shown that the temperature dependence of the resulting polymer microstructure is in agreement with the analytical model for a nucleation and growth process. Finally, a live transformation of solidification of polyethylene oxide from the melt was recorded under crossed polars at room temperature via video microscopy. The time-dependent data that describe the advancement of the phase transformation are in excellent agreement with the Avrami model.