Slow regions percolate near glass transition.

A nanosecond scale in situ probe reveals that a bulk linear polymer undergoes a sharp phase transition as a function of the degree of conversion, as it nears the glass transition. The scaling behaviour is in the same universality class as percolation. The exponents gamma and beta are found to be 1.7 +/- 0.1 and 0.41 +/- 0.01 in agreement with the best percolation results in three dimensions.