The mechanical properties of thermo-responsive nanoribbons made of poly(N-isopropylacrylamide)-block-poly(ethylene terephthalate) (PNIPAM-b-PET) have been investigated. The nanoribbons are produced by grafting PNIPAM chains from the walls of nanopores of PET track-etched membranes, by dissolving the membranes. The swelling ratio and elastic modulus of the nanoribbons are evaluated as a function of polymer chain length and temperature, from atomic force microscopy (AFM) images and using the thermodynamic theory of swelling. Whereas the elastic properties of nanoribbons are similar to those of PNIPAM gels and brushes when the PNIPAM chain length is low, they increase by 2 orders of magnitude when the chain length becomes similar to the diameter of the nanopores used to synthesize them. This indicates that PNIPAM brushes synthesized in severe conditions of confinement are strongly cross-linked, either due to irreversible trapping of entanglements, or chemical cross-linking occurring in the dense reaction medium. This provides a way to modulate mechanical properties of soft nanostructures, by playing with the degree of confinement of the chains during synthesis.