Two-dimensional polymers have numerous potential applications as surface coatings. In this work, we used the surface force apparatus to investigate the interfacial properties of monomeric and polymerized Langmuir-Blodgett films of the diacetylene 10,12-pentacosadiynoic acid (PCA). Using direct force and contact angle measurements, we investigated the impact of polymerization and film preparation conditions on the molecular surface properties of the layers. Humidity-dependent variations both in the water contact angles and in the directly measured interfacial energies of these layers were used to assess the film stability under different environmental conditions. Direct force measurements demonstrated directly that polymerization prevents molecular reorientations and consequent changes in the interfacial properties of the polymer films. Further, the polymerized diacetylene layers were stable to repeated subjection to large compressive loads over a course of several hours. A major limitation of the low-dimensional diacetylene polymers prepared from PCA is the difficulty of forming homogeneous, defect-free films over large areas. Our findings further demonstrate directly the impact of such heterogeneity on the interfacial properties of both monomeric and polymerized diacetylene films.