Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work

11th Young Faculty Meeting, 5th June 2018: SCNAT

Postprint (published version

Evaluating the use of calcium hydrogen phosphate dihydrate as a mineral-based fire retardant for application in melamine-urea-formaldehyde (MUF)-bonded wood-based composite materials

Calcium hydrogen phosphate dihydrate (DCPD) was evaluated for its potential as a mineral fire retardant (FR) for application in melamine-urea-formaldehyde (MUF)-bonded wood composites. The efficacy as FR was studied in melamine-urea-formaldehyde (MUF)-bonded three-layer particleboard as a function of addition quantities of 10-, 20- and 30 wt%. Resistance to fire and mechanical properties were determined by measuring the self-extinguishing time after flame exposure and internal bond strength, respectively. Combustion behavior was examined on samples with 20 wt% DCPD addition by performing cone calorimetry experiments. The efficacy of DCPD was evaluated by determining the heat release, total heat release rate, smoke production, and smoke production rate and compared to another promising mineral-based fire-retardant composition (FRC) based on hydroxyapatite (HA) with deliquescent salt and HA alone. The effect of FR on the curing behavior of MUF in relation to mechanical properties was determined through viscosity measurements of MUF with 10 wt% addition of FR. The results confirmed the fire-retardant characteristics of DCPD in wood composites, albeit at higher application rates when compared to the FRC, however with no negative impact on resin curing time or mechanical strength. Based on the demonstrated compatibility in MUF, DCPD is considered a promising mineral extender of other FRs for application in UF-based wood composites

Manipulation of cellulose nanocrystal surface sulfate groups toward biomimetic nanostructures in aqueous media

We report a facile aqueous procedure to create multivalent displays of sulfonated ligands on CNCs for future applications as viral inhibitors. CNCs were decorated with model compounds containing sulfonate groups via reactions of epoxides and isothiocyanates with amines under alkaline conditions. At first, surface sulfate groups of CNCs were hydrolytically cleaved by alkaline hydrolysis to increase the number of available surface hydroxyls. Success of desulfation was confirmed via dynamic light scattering (DLS), zeta potential measurements and thermogravimetric analysis (TGA). CNC surface hydroxyl groups were then activated with epichlorohydrin before subsequent reactions. As proof of concept toward aqueous pathways for functionalizing nanoparticles with sulfonated ligands, 3-chloro-2-hydroxy-1-propanesulfonic acid sodium salt hydrate (CPSA) and 4-sulfophenyl isothiocyanate sodium salt monohydrate (4-SPITC) were chosen as model compounds to react with homobifunctional 2,2'-(ethylenedioxy)bis(ethylamine) (EBEA) molecular spacer. The approaches presented are not only applicable to polysaccharide nanocrystals, but also other classes of polymeric and inorganic substrates presenting surface hydroxyl groups, as in the case of poly(2-hydroxyethyl methacrylate) (PHEMA), silica or glass. CNCs carrying sulfonated hgands were were characterized by ATR-FTIR and UV-vis spectroscopy. Surface chemical compositions of desired elements were determined via X-ray photoelectron spectroscopy (XPS). We anticipate that with these facile aqueous procedures as the proof of concept, a diverse library of target-specific functionalities can be conjugated to CNCs for applications in nanomedicine, especially related to viral inhibition. (C) 2015 Elsevier Ltd. All rights reserved

Cellulose Nanocrystals: Surface Modification, Applications and Opportunities at Interfaces

Cellulose nanocrystals (CNCs) are rod-like nano-scale particles that are widely available in nature and have recently gained great interest in both research and industry, due to their high strength, high crystallinity, high surface area, low density, biodegradability and low toxicity. CNCs can be easily extracted from natural cellulose sources and are broadly useful, for example in polymer reinforcement, paper manufacturing, and rheology modification. The high density of functional groups on the surface of CNCs allows various chemical surface modifications, which permit tuning the properties of CNCs over a wide range. This review gives a brief overview of surface chemical modification of CNCs, focusing especially on those often utilized for our own research, which focuses on some of the most prominent areas of interests of CNCs, notably polymer reinforcement, healable polymers, stimuli-responsive nanohybrids, Pickering emulsion stabilizers, viral inhibitors, and cholesteric liquid crystal assemblies

Patience is a virtue: self-assembly and physico-chemical properties of cellulose nanocrystal allomorphs

Cellulose nanocrystals (CNCs) are bio-based rod-like nanoparticles with a quickly expanding market. Despite the fact that a variety of production routes and starting cellulose sources are employed, all industrially produced CNCs consist of cellulose I (CNC-I), the native crystalline allomorph of cellulose. Here a comparative study of the physico-chemical properties and liquid crystalline behavior of CNCs produced from cellulose II (CNC-II) and typical CNC-I is reported. CNC-I and CNC-II are isolated by sulfuric acid hydrolysis of cotton and mercerized cotton, respectively. The two allomorphs display similar surface charge densities and ζ-potentials and both have a right-handed twist, but CNC-II have a slightly smaller average length and aspect ratio, and are less hygroscopic. Interestingly, the self-assembly behavior of CNC-I and CNC-II in water is different. Whilst CNC-I forms a chiral nematic phase, CNC-II initially phase separates into an upper isotropic and a lower nematic liquid crystalline phase, before a slow reorganization into a large-pitch chiral nematic texture occurs. This is potentially caused by a combination of factors, including the inferred faster rotational diffusion of CNC-II and the different crystal structures of CNC-I and CNC-II, which are responsible for the presence and absence of a giant dipole moment, respectively.status: publishe

Dynamic Light Scattering Plus Scanning Electron Microscopy: Usefulness and Limitations of a Simplified Estimation of Nanocellulose Dimensions

Measurements of nanocellulose size usually demand very high-resolution techniques and tedious image processing, mainly in what pertains to the length of nanofibers. Aiming to ease the process, this work assesses a relatively simple method to estimate the dimensions of nanocellulose particles with an aspect ratio greater than 1. Nanocellulose suspensions, both as nanofibers and as nanocrystals, are subjected to dynamic light scattering (DLS) and to field-emission scanning electron microscopy (FE-SEM). The former provides the hydrodynamic diameter, as long as the scatter angle and the consistency are adequate. Assays with different angles and concentrations compel us to recommend forward scattering (12.8°) and concentrations around 0.05–0.10 wt %. Then, FE-SEM with magnifications of ×5000–×20,000 generally suffices to obtain an acceptable approximation for the actual diameter, at least for bundles. Finally, length can be estimated by a simple geometric relationship. Regardless of whether they are collected from FE-SEM or DLS, size distributions are generally skewed to lower diameters. Width distributions from FE-SEM, in particular, are well fitted to log-normal functions. Overall, while this method is not valid for the thinnest fibrils or for single, small nanocrystals, it can be useful in lieu of very high-resolution techniques

One-Component Nanocomposites Based on Polymer-Grafted Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) are widely used as reinforcing fillers in polymers due to their exceptionally high stiffness and strength and because the biological species from which they are isolated represent renewable resources. However, aggregation of the CNCs, which is concomitant with limited reinforcement, is often difficult to avoid. One-component nanocomposites (OCNs) based on polymer-grafted nanoparticles can solve this problem because this approach affords, by design, materials in which no such aggregation is possible. At the same time, chain entanglements between the CNC-grafted polymer chains provide stress transfer among the particles. To demonstrate this, we investigated OCNs based on polymethacrylate-grafted CNCs. A previously unaccessed compositional space, that is, OCNs with a CNC content of 10 or 20 wt %, was explored. Cotton linter-based CNCs were modified via surface-photoinitiated free radical polymerization, which involved the functionalization of the CNC surfaces with benzophenone moieties as photoradical initiator species and the subsequent surface-photoinitiated polymerization of methyl or hexyl methacrylate under UV irradiation at 365 nm. The resulting particles readily dispersed in THF. Solvent-casting and compression-molding afforded films of homogeneous appearance, which display remarkable improvements in stiffness or toughness and strength in comparison to conventional two-component nanocomposites of unmodified CNCs and the respective polymers.Peer ReviewedPostprint (author's final draft

One-Component Nanocomposites Based on Polymer-Grafted Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) are widely used as reinforcing fillers in polymers due to their exceptionally high stiffness and strength and because the biological species from which they are isolated represent renewable resources. However, aggregation of the CNCs, which is concomitant with limited reinforcement, is often difficult to avoid. One-component nanocomposites (OCNs) based on polymer-grafted nanoparticles can solve this problem because this approach affords, by design, materials in which no such aggregation is possible. At the same time, chain entanglements between the CNC-grafted polymer chains provide stress transfer among the particles. To demonstrate this, we investigated OCNs based on polymethacrylate-grafted CNCs. A previously unaccessed compositional space, that is, OCNs with a CNC content of 10 or 20 wt %, was explored. Cotton linter-based CNCs were modified via surface-photoinitiated free radical polymerization, which involved the functionalization of the CNC surfaces with benzophenone moieties as photoradical initiator species and the subsequent surface-photoinitiated polymerization of methyl or hexyl methacrylate under UV irradiation at 365 nm. The resulting particles readily dispersed in THF. Solvent-casting and compression-molding afforded films of homogeneous appearance, which display remarkable improvements in stiffness or toughness and strength in comparison to conventional two-component nanocomposites of unmodified CNCs and the respective polymers