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Utilizing metal–organic frameworks (MOFs) as reinforcing fillers for polymer composites is a promising strategy because of the low density, high specific modulus, and tunable aspect ratio (AR). However, it has not been demonstrated for the MOF-reinforced polymer composite using MOFs with high AR and polymer-grafted surface, both of which are extremely important factors for efficient load transfer and favorable particle–matrix interaction. To this end, we designed an MOF–polymer composite system using high AR MOF PCN-222 as the mechanical reinforcer. Moreover, we developed a synthetic route to graft poly(methyl methacrylate) (PMMA) from the surface of PCN-222 through surface-initiated atomic transfer radical polymerization (SI-ATRP). The successful growth of PMMA on the surface of PCN-222 was confirmed via proton nuclear magnetic resonance and infrared spectroscopy. Through thermogravimetric analysis, the grafting density was found to be 0.18 chains/nm2. The grafted polymer molecular weight was controlled ranging from 50.3 to 158 kDa as suggested by size exclusion chromatography. Finally, we fabricated MOF–polymer composite films by the doctor-blading technique and measured the mechanical properties through the tension mode of dynamic mechanical analysis. We found that the mechanical properties of the composites were improved with increasing grafted PMMA molecular weight. The maximum reinforcement, a 114% increase in Young’s modulus at 0.5 wt % MOF loading in comparison to pristine PMMA films, was achieved when the grafted molecular weight was higher than the matrix molecular weight, which was in good agreement with previous literature. Moreover, our composite presents the highest reinforcement measured via Young’s modulus at low weight loading among MOF-reinforced polymer composites due to the high MOF AR and enhanced interface. Our approach offers great potential for lightweight mechanical reinforcement with high AR MOFs and a generalizable grafting-from strategy for porphyrin-based MOFs.
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Effect of Filler–Polymer Interface on Elastic Properties of Polymer Nanocomposites: A Molecular Dynamics Study
A coarse-grained model has been built to study the effect of the interfacial interaction between spherical filler particles and polymer on the mechanical properties of polymer nanocomposites. The polymer is modeled as bead-spring chains, and nano-fillers grafted with coupling agent are embedded into the polymer matrix. The potential parameters for polymer and filler are optimized to maximally match styrene-butadiene rubber reinforced with silica particles. The results indicated that, to play a noticeable role in mechanical reinforcement, a critical value exists for the grafting density of the filler–polymer coupling agent. After reaching the critical value, the increase of grafting density can substantially enhance mechanical properties. It is also observed that the increase of grafting density does not necessarily increase the amount of independent polymer chains connected to fillers. Instead, a significant amount of increased grafting sites serve to further strengthen already connected polymer and filler, indicating that mechanical reinforcement can occur through the locally strengthened confinement at the filler–polymer interface. These understandings based on microstructure visualization shed light on the development of new filler polymer interfaces with better mechanical properties.
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- Award ID(s):
- 1650460
- PAR ID:
- 10079322
- Date Published:
- Journal Name:
- Tire science & technology
- Volume:
- 45
- Issue:
- 3
- ISSN:
- 1945-5852
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Macromolecular architecture is a critical parameter when tuning polymer material properties. Although the implementation of non-linear polymers in different applications has grown over the years, polymer grafted surfaces such as nanoparticles have traditionally been composed of linear thermoplastic polymers, with a limited number of examples demonstrating a diversity in polymer architectures. In an effort to combine polymer architecturally dependent material properties with polymer grafted particles (PGPs), as opposed to conventional methods of tuning polymer grafting parameters such as the number of chains per surface area (i.e., polymer graft density), a series of bottlebrush grafted particles were synthesized using surface-initiated ring-opening metathesis polymerization (SI-ROMP). These bottlebrush PGPs are composed of glassy, semi-crystalline, and elastomeric polymer side chains with controlled backbone degrees of polymerization (Nbb) at relatively constant polymer graft density on the surface of silica particles with diameters equaling approximately 160 or 77 nm. Bottlebrush polymer chain conformations, evaluated by measuring the brush height of surface grafted polymer chains in solution and the melt, undergo drastic changes in macromolecular dimensions in different environments. In solution, brush heights increase linearly as a function of Nbb, consistent with fully stretched chains, which is confirmed using cryogenic transmission electron microscopy (Cryo-TEM). Meanwhile, brush heights are consistently at a minimum in the melt, indicative of chains collapsed on the particle surface. The conformational extremes for grafted bottlebrush polymers are unseen in any linear polymer chain systems, highlighting the effect of macromolecular architecture and surface grafting. Bottlebrush grafted particles are an exciting class of materials where diversifying polymer architectures will expand PGP material design rules that harness macromolecular architecture to dictate properties.more » « less
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Abstract Filler aggregation in polymer matrix nanocomposites leads to inhomogeneity in particle distribution and deterioration of mechanical properties. The use of polymer‐grafted nanoparticles (PGNPs) with polymers directly attached to the particle surfaces precludes aggregation of the filler. However, solids composed of PGNPs are mechanically weak unless the grafted chains are long enough to form entanglements between particles, and requiring long grafts limits the achievable filler density of the nanocomposite. In this work, long, entangled grafts are replaced with short reactive polymers that form covalent crosslinks between particles. Crosslinkable PGNPs, referred to as XNPs, can be easily processed from solution and subsequently cured to yield a highly filled yet mechanically robust composite. In this specific instance, silica nanoparticles are grafted with poly(glycidyl methacrylate), cast into films, and crosslinked with multifunctional amines at elevated temperatures. Indentation and scratch experiments show significant enhancement of hardness, modulus, and scratch resistance compared to non‐crosslinked PGNPs and to crosslinked polymer films without nanoparticle reinforcement. Loadings of up to 57 wt% are achieved while yielding uniform films that deform locally in a predominantly elastic manner. XNPs therefore potentially allow for the formulation of robust nanocomposites with a high level of functionality imparted by the selected filler particles.more » « less
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