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  1. Polymers under nanoconfinement can exhibit large alterations in dynamics from their bulk values due to an interface effect. However, understanding the interface effect remains a challenge, especially in the ultrafine nanoconfinement region. In this work, we prepare new geometries with ultrafine nanoconfinement ∼10nm through controlled distributions of the crystalline phases and the amorphous phases of a model semi-crystalline polymer, i.e., the polylactic acid. The broadband dielectric spectroscopy measurements show that ultrafine nanoconfinement leads to a large elevation in the glass transition temperature and a strong increment in the polymer fragility index. Moreover, new relaxation time profile analyses demonstrate a spatial gradient that can be well described by either a single-exponential decay or a double-exponential decay functional form near the middle of the film with a collective interface effect. However, the dynamics at the 1–2 nm vicinity of the interface exhibit a power-law decay that is different from the single-exponential decay or double-exponential decay functional forms as predicted by theories. Thus, these results call for further investigations of the interface effect on polymer dynamics, especially for interfaces with perturbed chain packing.

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    Free, publicly-accessible full text available March 21, 2025
  2. Current polymer network design suffers from intrinsic trade-offs, where polymer networks with high modulus often turn out to be in short of stretchability or fracture toughness. Here, we show a novel polymer network design through polymer-nanoparticle alternating hybrids that enable integrating the non-polymeric colloid deformation into polymer network design. The new class of polymer network exhibits colloidal yielding at small deformation before conformational change at higher elongation ratios, enabling simultaneous achievement of high Young’s modulus of E≈10-50 MPa, high yield strength of σ_Y~ 3-5 MPa, large stretchability of λ~7-10, and high fracture energy density of Γ~30 MJ/m^3. These results demonstrate a successful strategy to decouple the molecular mechanics for yield from that for stretchability or toughness, leading to new polymer networks design. 
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    Free, publicly-accessible full text available March 1, 2025
  3. While tremendous progress has been made in the dynamic crosslinking of polypropylene (PP) for plastics upcycling, their efficacy in addressing low molecular weight (MW) PP waste remains untapped. In this work, we demonstrate a simple and scalable method to convert brittle low MW PP to vitrimer materials with enhanced thermal and mechanical properties, enabling their use in circular upcycling. Different from most previous work employing small molecule crosslinkers, we prepare PP vitrimers (PPv) using polymeric crosslinkers, containing polyethylene glycol segments (PEG), which lead to altered crystalline structures and network formation. Importantly, by increasing the MW of crosslinkers from 200 Da to 1000 Da, the PPv exhibit more than 50 times increase in their fracture energy with strong ductility, which can be attributed to combined effects of strengthened amorphous regions of semi-crystalline PP domains and the phase separation between soft PEG segments and PP matrix. Moreover, when blending the PPv with high MW PP (PPh), the PPh/PPv blends show comparable elastic modulus, yield strength, and stretchability to the PPh, in sharp contrast to the widely-known embrittlement of low MW PP/PPh blends. These results demonstrate the use of polymeric dynamic crosslinkers as an important strategy for upcycling low MW PP waste to value-added products. 
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