Unexpected Elasticity in Assemblies of Glassy Supra‐Nanoparticle Clusters

Abstract

Granular materials, composed of densely packed particles, are known to possess unique mechanical properties that are highly dependent on the surface structure of the particles. A microscopic understanding of the structure‐property relationship in these systems remains unclear. Here, supra‐nanoparticle clusters (SNPCs) with precise structures are developed as model systems to elucidate the unexpected elastic behaviors. SNPCs are prepared by coordination‐driven assembly of polyhedral oligomeric silsesquioxane (POSS) with metal‐organic polyhedron (MOP). Due to the disparity in sizes, the POSS‐MOP assemblies, like their classic nanoparticles counterparts, ordering is suppressed, and the POSS‐MOP mixtures will vitrify or jam as a function of decreasing temperature. An unexpected elasticity is observed for the SNPC assemblies with a high modulus that is maintained at temperatures far beyond the glass transition temperature. From studies on the dynamics of the hierarchical structures of SNPCs and molecular dynamic simulation, the elasticity has its origins in the interpenetration of POSS‐ended arms. The physical molecular interpenetration and inter‐locking phenomenon favors the convenient solution or pressing processing of the novel cluster‐based elastomers.

Polymer Topology Reinforced Synergistic Interactions among Nanoscale Molecular Clusters for Impact Resistance with Facile Processability and Recoverability

“…The chain dynamics on the mechanical performance enhancement is analogous to the sticky nanofilled polymers with sufficiently high NP loadings [29] . Herein, the interpenetration of the OPOSS side chains and the high graft densities of OPOSS units lead to robust physical interlocking among the polymer chains, evidenced by the similar mechanical strength (≈10 MPa) to that of well‐defined granular systems of spherical assemblies of OPOSS (≈17 MPa) [12] . The OPOSS clusters are either structurally correlated via their covalent bonding to polynorbornene backbones or spatially correlated from the side chain interdigitation, which may result in the absence of rubbery plateau in rheological data of PolyPOSSs.…”

“…The relaxation time for each relaxation process is evaluated to be 3.51 s, 1.59×10 −3 s, and 1.28×10 −4 s, respectively, corresponding to the relaxations of polymer chains (PolyPOSS chains), chain segments (cooperative motion of multiple OPOSSs), and local structures (individual OPOSS). When compares to the star‐shaped OPOSS assemblies, [12] stronger temperature dependence of the time‐temperature superposition shifting factors ( α T ) is clearly detected for PolyPOSS 29 , arising from its more condensed and crowded interpenetrated microstructures (Figure 4 c).…”

“…Entanglements are not favorable for the rigid polymer chains, [16] while the van der Waals forces individually cannot contribute to the mechanical behaviors. Therefore, it most probably originates from the interlocking of polymer chains induced by the interpenetration of the OPOSS pendants (Figure S13) [12] . By performing in situ SAXS studies, highly orientated structures are observed for the stretched sample (Figure 2 g).…”

“…The afforded various relaxation dynamics may lead to unique viscoelastic behaviors [11] . Recently, star‐shaped supramolecular assemblies of MCs, [12] are reported to deliver unexpected elastic behaviors, arising from the physical interpenetration of the MC units, whereas the limited stretchability indicates the moderate inter‐MC attractions. More specifically, sphere‐like topology and soft linkers endow the terminal MCs with high spatial mobility, effective synergistic correlations are, thus, hard to achieve even if the intrinsic interpenetration structure can be formed.…”

“…Only short‐range ordering can be observed from the small angle and wide angle X‐ray scattering (SAXS and WAXS) studies (Figure 2 c and Figure S10). The polymer backbone‐to‐backbone, OPOSS‐to‐OPOSS, and OPOSS‐to‐backbone distances are calculated to be 3.0, 1.2, and 0.55 nm, respectively, based on the scattering maxima of the WAXS patterns [12] . The backbone‐to‐backbone distance is smaller than the cross‐section diameter of a single PolyPOSS 29 chain in solution (Figure 2 b), which implies the possible interpenetration among OPOSS pendants in the polymer melts.…”