The transient behavior of a polymeric nematic liquid crystal interface in extensional flows is studied both experimentally and theoretically. Monolayers of the hairy rod polymer phthalocyaninatopolysiloxane subjected to two-dimensional, transient, extensional flows are modeled with the two-dimensional analogue of the rigid rod model. The scalar order parameter and the director orientation are compared with the experimental observables. Two parameters appear in the model : an average rotational diffusivity and the intensity of the nematic field. The average rotational diffusivity is determined by fitting relaxation experiments. The intensity of the nematic field, which is modeled with the Onsager potential, is determined by starting from the molecular parameters. A good quantitative agreement is obtained between experiments and theoretical predictions.