An upper-division instrumental chemistry laboratory has been developed in which students use atomic force microscopy (AFM) to collect force curves on polydimethylsiloxane (PDMS) elastomers. Force curves are used to quantify the stiffness of elastomers prepared with different base-to-curing agent ratios. Trends in observed spring constants of the elastomer samples are consistent with Young's elastic moduli measured in previous studies. This experiment has successfully exposed students to modern instrumental applications addressing surface chemistry in the context of examining cross-linked polymers. Ways to adapt the lab exercise to be used with diverse models of AFM are also discussed. (Contains 2 figures.)