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
- NSF-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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Abstract It is demonstrated that the addition of nanoparticles can decrease the viscosity of an entangled polymer matrix (plasticization). While there has been considerable effort in understanding the role of bare nanoparticles on viscosity, the corresponding study on grafted particles is nascent. Two nanometer radius zirconia fillers with a bimodal population of grafted PDMS is used. Two fillers are considered: one has grafted chains with a molecular weight, Mg, less than the entanglement M, Mg < Me(ZrO21k 10k), and the other filler has grafted chains M > Mc(the critical Me) (ZrO21k 36k). The ZrO21k 10k composites exhibit plasticization for large matrix molecular weight Mm. The ZrO21k 36k composites, in contrast, have Einstein‐like behavior. The plasticization effects are either due to: the small filler size relative to the tube diameter or an increase in chain mobility at the interface due to autophobic dewetting.
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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.
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Reinforcing fillers are necessary in rubber compounding to aid in enhancing the mechanical properties of the compound for various applications. Carbon black (CB) is currently the most common reinforcing filler used in tire compounding. Lignin, an amorphous polyphenolic material derived from plants and a by-product of the pulp and paper industry, is also an attractive material that can serve as a dispersant and as a reinforcing filler. This paper evaluates the interactions between styrene-butadiene rubber and reinforcing fillers with an electron-rich π- system, such as lignin and CB, in the presence of a graft copolymer (PB-g-PPFS) of PB and electron-deficient 2,3,4,5,6-pentafluorostyrene (PFS). The interactions are attributed to areneperfluoroarene interactions between the electron-deficient π-system of the polyperfluoroarene grafts and the electron-rich π-system of lignin and/or CB particles. The effects of improved fillerrubber interactions on mechanical properties and dynamic mechanical properties are analyzed. This paper will demonstrate the use of PB-g-PPFS as a coupling agent in rubber compounds to enhance the interaction between the filler, lignin and lignin-carbon black hybrid filler, and the rubber matrix to achieve a reduction in the hysteresis loss and enhanced filler dispersion.more » « less
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Abstract Poly(lactic acid) (PLA) is a commercially available bio‐based polymer that is a potential alternative to many commodity petrochemical‐based polymers. However, PLA's thermomechanical properties limit its use in many applications. Incorporating polymer‐grafted cellulose nanocrystals (CNCs) is one potential route to improving these mechanical properties. One key challenge in using these polymer‐grafted nanoparticles is to understand which variables associated with polymer grafting are most important for improving composite properties. In this work, poly(ethylene glycol)‐grafted CNCs are used to study the effects of polymer grafting density and molecular weight on the properties of PLA composites. All CNC nanofillers are found to reinforce PLA above the glass transition temperature, but non‐grafted CNCs and CNCs grafted with short PEG chains (<2 kg mol−1) are found to cause significant embrittlement, generally resulting in less than 3% elongation‐at‐break. By grafting higher molecular weight PEG (10 kg mol−1) onto the CNCs at a grafting density where the polymer chains are predicted to be in the semi‐dilute polymer brush conformation (~0.1 chains nm−2), embrittlement can be avoided.
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Accepted Manuscript1.0
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