Halogen exchange of alkyl bromide (R–Br) with sodium iodide (NaI) was used for the transformation of R–Br to alkyl iodide (R–I) in situ in organocatalyzed living radical polymerization (LRP). R–Br ...was employed as a starting compound (precursor), and the R–I formed in situ was employed as an initiating dormant species for the polymerization. The efficiency of the transformation significantly depends on the R group of R–Br. By the rational selection of the R group and reaction temperature along with the use of tetrabutylammonium iodide (Bu4NI) as a catalyst, low-polydispersity (M w/M n = 1.1–1.4) polymers were obtained with high conversions (e.g., 70–90%) in reasonably short periods of time (typically 3–10 h) in the polymerizations of methyl methacrylate, butyl acrylate, styrene, acrylonitrile, and functional methacrylates. Well-defined diblock and triblock copolymers and a chain-end functional polymer were also obtained. R–Br is generally much more stable than R–I upon storage. Various R–Br are commercially available. The use of simple, stable, and inexpensive R–Br as precursors of the dormant species is an attractive feature of this system. The high monomer versatility and the accessibility to a wide range of polymer structural designs demonstrated in this work show the capability of this system for use in a range of applications.
Cellulose nanofibers (CNFs) hold great potential as sustainable reinforcement fillers with excellent mechanical, thermal, and chemical properties. However, in polyolefin nanocomposite materials, the ...rational control of dispersion and the improvement of interfacial strength remain challenging. Herein we propose the tuning of the interface between CNF and high-density polyethylene by the design of polymer dispersants on the basis of surface free energy and the glass transition temperature. The former is related to the wettability against the polymer matrix and is therefore critical to the dispersion of CNF whereas the latter is related to the interfacial strength between CNF and HDPE. As a result of this investigation, we discovered a suitable dispersant for CNFs, poly(dicyclopentenyloxyethyl methacrylate)-block-poly(2-hydroxyethyl methacrylate), which played a pivotal role in achieving both a uniform dispersion of CNF and greatly improved mechanical properties, including a 4-fold increase of the Young’s modulus over that of neat HDPE with 10 wt % CNF loading.
An effective approach for the dispersion of hydrophilic cellulose nanofiber (CNF) in hydrophobic high-density polyethylene (HDPE) is presented using adsorption of a diblock copolymer dispersant. The ...dispersant consists of both resin compatible poly(lauryl methacrylate) (PLMA) and cellulose interactive poly(2-hydroxyethyl methacrylate) blocks. The PLMA-adsorbed CNFs are characterized by FT-IR and contact angle measurement, revealing successful hydrophobization. X-ray CT imaging shows there are apparently less CNF aggregates in the nanocomposites if adding amount of the dispersant was enough. The good dispersion results in a high mechanical reinforcement, corresponding to 140% higher Young’s modulus and 84% higher tensile strength than the neat HDPE. This approach is broadly applicable and allows for easy manufacturing process for strong and lightweight CNF-reinforced nanocomposite materials.
In this article, we describe the synthesis and self-assembly of novel Janus bottlebrushes with cellulose as a polymeric backbone. The cellulosic Janus bottlebrushes, bearing poly(ε-caprolactone) ...(PCL) and polystyrene (PS) branches at the C-2,3 and C-6 positions of anhydroglucose units, respectively, were successfully prepared using a bulky
p
-methoxytrityl protecting group and by combining the “grafting-from” ring-opening polymerization of ε-caprolactone with the “grafting-to” click chemistry of PS. The degree of substitution of PCL at the C-2,3 positions was estimated to be 1.5, whereas values of 0.64–0.84 were estimated for the C-6 position of PSs with different molecular weights (number-average molecular weight = 4.0, 9.2, or 16 kg mol
−1
), depending on the length of the PS graft. The Janus bottlebrushes (PS/PCL = 50:50 v/v) self-assembled to form a lamellar microdomain structure comprising three layers (amorphous PCL, crystalline PCL, and amorphous PS layers), as characterized by small-angle X-ray scattering and transmission electron microscopy. The interlamellar distance was approximately 30 nm, depending on the length of the PS graft. The results illustrate that cellulosic polymeric backbones for Janus bottlebrushes can exhibit microscale morphologies, potentially leading to the development of polymers with new morphologies and functionalities.
Graphic abstract
The lubrication mechanism of concentrated polymer brushes (CPBs) exhibiting ultralow frictional property was investigated. The frictional force and hence the frictional coefficient μ between CPBs of ...polystyrene (PS) were measured as a function of shear velocity υ and degree of swelling. The degree of swelling was precisely controlled by varying the composition of solvent , which consisted of a mixture of toluene (good solvent for PS) and 2-propanol (nonsolvent for PS), from the brush highly stretched state (toluene rich) to glassy state (2-propanol rich). The μ data of the mixtures revealed two lubrication mechanisms, i.e., boundary and hydrodynamic lubrication. Boundary lubrication with μ values less dependent on shear velocity was observed both in ultralow (μ on the order of 10–4) and high frictional (μ on the order of 0.1) regimes. On the other hand, hydrodynamic lubrication was well described by the relation μ = β·υ α with α having an almost constant value of ca. 0.7. It was found that parameter β depended on the solvent composition and was scaled by the degree of swelling. It should be noted that the confronted polymer brushes interacted with each other even in this regime. Thus, CPBs in solvents may be employed as an efficient lubricating layer due to their unique features.
Concentrated polymer brushes (CPBs) synthesized by advanced surface-initiated controlled radical polymerization have received increasing attention in the tribological field because CPBs in good ...solvents exhibit superlow friction phenomena under macroscopic severe sliding conditions. To realize tribological applications of CPBs, an understanding of the wear mechanisms of CPBs is required to develop a method to enhance their durability. In this study, an operando tribo-Raman spectroscopic experiment, which is an effective approach for revealing the wear mechanisms of tribo-materials, was conducted on the frictional interface between a polymethyl methacrylate (PMMA)-CPB and a bearing steel ball immersed in an ionic liquid (IL), which is a good solvent for PMMA-CPBs. From the operando tribo-Raman results, the concentrations and molecular structures of the PMMA-CPBs and IL at the frictional interface were found to be constantly changing with the sliding cycles during operation. Moreover, their time-dependent changes strongly affected the wear process and superlow friction phenomena of the PMMA-CPB lubricated with the IL. Consequently, our results suggest that controlling the degree of swelling of CPBs at frictional interface is an effective approach for realizing a durable-superlow frictional tribo-system of CPBs.
•Tribological phenomena of CPB–steel tribo-systems in ionic liquids were investigated.•The Tribo-Raman method was conducted to reveal the tribological phenomena.•The conformation of CPB chains and ILs were changed during friction test.•The concentration of CPB chains and ILs decreased during friction test.•Their conformation is a key factor for achieving superlubricity of the system.
Concentrated Polymer Brushes (CPBs) have great potential to improve tribological properties under severe sliding conditions, even at the macroscopic scale, and thus are expected to apply to ...mechanical sliding components. However, the durability of CPBs has remained as a challenge for industrial applications. Here, we focused on a combination of CPBs and surface texturing technology to increase their durability. In this work, the effects of surface texturing, such as geometrical patterns on the durability of CPBs under severe sliding conditions, were investigated. The results demonstrate that applying a surface texture onto a substrate of CPBs can effectively improve the friction durability of CPBs because the polymers inside the textures, which have a locally milder contact condition, are prone to survive. By applying an appropriate surface texture, we achieved up to a 36% increase in durability.
•Concentrated Polymer Brushes (CPBs) have excellent frictional properties (lower than 0.03 in friction coefficient).•Applying a groove texture onto the substrate of CPBs increases their friction durability up to 36% compared to nontextured CPBs.
A novel class of living radical polymerization using amines as organic catalysts was developed. It is based on a new reversible activation mechanism, reversible complexation (RC). The polymer ...molecular weight and its distribution (M w/M n = 1.1–1.4) were well controlled in the polymerizations of methyl methacrylate (MMA), styrene, acrylonitrile, and some functional methacrylates with a fairly high conversion in hours in many cases. The catalysts include such common amines as triethylamine and tetramethylethylenediamine (TMEDA). Their low cost, good environmental safety, and ease of handling may be attractive for possible applications. Kinetic studies supported the RC mechanism. The activation rate constant for the MMA/TMEDA system was large enough to explain why the system provides low-polydispersity polymers from an early stage of polymerization.
The degree of chain-end functionality in a densely grafted, concentrated polymer brush is a critical parameter. In this study, p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) ...technique was applied to monitor the emergence and disappearance of substituted azide (N3) at the chain ends of polymer brushes, allowing the accurate determination of the azide density and the click reaction efficacy. The peak separation of the N3 absorption bands in the pMAIRS spectra showed that azide substitution occurred not only at the brush ends but also at the underlying initiators that did not react during surface-initiated atom transfer radical polymerization. Hence, the density of each type of azide group and the efficiencies of the azidation and click reaction could successfully be estimated. Furthermore, a chain-end-selective click reaction was demonstrated for the azide-functionalized polymethacrylate-type polymer brushes based on size exclusion effects and polarity differences. Thus, the pMAIRS method provides important quantitative information that can expand the applications of chain-end functionalized polymer brushes.
Friction continues to account for the bulk of energy losses in mechanical systems, with an estimated 23% of the world’s total energy consumption used to overcome friction. Concentrated polymer ...brushes (CPBs) have recently attracted significant scientific and industrial attention, given their ability to achieve superlubricity (i.e., coefficients of friction below 0.01); however, understanding the mechanistic interactions underlying their wear performance has been largely overlooked. Herein, we employ a custom-built optical test apparatus to investigate the inter-dependencies between CPBs and laser-produced surface texture (LST), assessing for the first time the friction, film thickness, and wear behavior in situ and simultaneously. Recent developments in picosecond laser etching allowed us to graft CPBs atop the finest laser-etched matrix of micron-sized dimples reported in literature to date. At low sliding speeds, combined CPB–LST reduces the coefficient of friction to 0.0006, while increasing the CPB durability by up to 34% through a lateral support mechanism offered by the textured micro-features. Furthermore, the imaging results shed light on CPB failure mechanisms. Both these mechanisms of lateral support and failure propagation impact the wear resistance of CPBs and are important in the development of CPBs for future applications (e.g., in low-speed bearings functioning under controlled abrasive wear conditions).
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