Abstract
<jats:p>Microparticle-based polymer films that combine high toughness with closed-loop recyclability have recently emerged as sustainable materials. Their mechanical response is thought to be governed by interfacial chain interpenetration, and hence by crosslinker density, but this picture rests on ensemble-averaged, static measurements that cannot show how individual particles and their interfaces deform during stretching. Here, using an atomic force microscope equipped with a uniaxial stretching stage, we tracked individual particles in the same field of view through loading and unloading cycles in films of 0.1, 3, and 5 mol% crosslinker. The net particle elongation showed no systematic dependence on crosslinker density (+0.62, +0.53, +0.65 in units of the interparticle spacing), whereas the width of the deformation distribution narrowed sharply from the 0.1 mol% film to the comparable 3 and 5 mol% films. This heterogeneity is inherited from the initial texture: the low-crosslinker film forms a fine-grained polycrystal with a six-fold higher grain-boundary fraction than the single-crystal-like high-crosslinker films, and its stretch distribution remains about twice as broad even when each particle is referenced to its own initial size. Its early-stage non-affine residual is also several-fold larger, and it sits in the mesoscopic displacement field rather than within the particle cages. Both the packing order and the distribution width recover on unloading, indicating an elastic rather than damage-driven origin. Crosslinker density thus acts as a formulation-level control on strain heterogeneity, the real-space counterpart of ductility and toughness.</jats:p>