Flexible Aerogels from Hyperbranched Polyurethanes: Probing the Role of Molecular Rigidity with Poly(Urethane Acrylates) Versus Poly(Urethane Norbornenes)
Bibliographic record
Abstract
Flexible and foldable aerogels have commercial value for applications in thermal insulation. This study investigates the molecular connection of macroscopic flexibility using polymeric aerogels based on star-shaped polyurethane-acrylate versus urethane-norbornene monomers. The core of those monomers is based either on a rigid/aromatic, or a flexible/aliphatic triisocyanate. Terminal acrylates or norbornenes at the tips of the star branches were polymerized with free radical chemistry, or with ROMP, respectively. At the molecular level, aerogels were characterized with FTIR and solid-state 13 C NMR. The porous network was probed with N 2 -sorption and Hg-intrusion porosimetry, SEM and SAXS. The interparticle connectivity was assessed in a top-down fashion with thermal conductivity measurements and compression testing. All aerogels of this study consist of aggregates of nanoparticles, whose size depends on the aliphatic/aromatic content of the monomer, the rigidity/flexibility of the polymeric backbone, and generally varies with density. At higher densities (0.3–0.7 g cm –3 ), all materials were stiff, strong, and tough. Aerogels based on urethane-acrylates built around a rigid/aromatic core exhibited a rapid decrease of their elastic modulus with density (slopes of the log–log plots >5.0), and at about 0.14 g cm –3, they were foldable. Data support that molecular properties of the monomer affect macroscopic flexibility indirectly, not so through the particle size, but rather through the growth mechanism and consequently through the interparticle contact area. Thus, flexible aerogels of this study showed no indication for polymer accumulation onto the primary nanostructure (particle sizes via N 2 -sorption and SAXS were comparable), and their interparticle contact area was comparatively lower. Because for flexibility purposes, interparticle contact area is related to interparticle bonding, it is speculated that if the latter is controlled properly (e.g., through adjustment of the monomer functional group density) it might lead to superelasticity and shape-memory effects.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".