Effect of Reaction Media on Grafting Hydrophobic Polymers from Cellulose Nanocrystals via Surface-Initiated Atom-Transfer Radical Polymerization

Hydrophobic polymer-grafted cellulose nanocrystals (CNCs) were produced via surface-initiated atom-transfer radical polymerization (SI-ATRP) in two different solvents to examine the role of reaction media on the extent of surface modification. Poly(butyl acrylate)-grafted CNCs were synthesized in either dimethylformamide (DMF) (D-PBA-g-CNCs) or toluene (T-PBA-g-CNCs) alongside a free polymer from a sacrificial initiator. The colloidal stability of unmodified CNCs, initiator-modified CNCs, and PBA-g-CNCs in water, DMF, and toluene was evaluated by optical transmittance. The enhanced colloidal stability of initiator-modified CNCs in DMF led to improved accessibility to initiator groups during polymer grafting; D-PBA-g-CNCs had 30 times more grafted chains than T-PBA-g-CNCs, determined by thermogravimetric and elemental analysis. D-PBA-g-CNCs dispersed well in toluene and were hydrophobic with a water contact angle of 124° (for polymer grafts > 13 kDa) compared to 25° for T-PBA-g-CNCs. The cellulose crystal structure was preserved, and individual nanoparticles were retained when grafting was carried out in either solvent. This work highlights that optimizing CNC colloidal stability prior to grafting is more crucial than solvent-polymer compatibility to obtain high graft densities and highly hydrophobic CNCs via SI-ATRP.

In situ monitoring of SI-ATRP throughout multiple reinitiations under flow by means of a quartz crystal microbalance

An investigation of the polymerisation of 2-hydroxyethyl methacrylate (HEMA) by means of surface-initiated atom transfer radical polymerisation (SI-ATRP) has been carried out in situ using a quartz crystal microbalance, with multiple reinitiations under continuous flow of the reaction mixture. The SI-ATRP kinetics of HEMA were studied continuously by means of changes in the frequency, varying conditions including temperature and solvent composition, as well as monomer and catalyst concentrations, showing the influence of key reaction parameters on SI-ATRP kinetics. Such experiments enabled the design of a polymerisation protocol that leads to a reasonably fast but well-controlled growth of poly(HEMA) brushes. Furthermore, only a minor change in growth rate was observed when the polymerisation was stopped and reinitiated multiple times (essential for block synthesis), demonstrating the living nature of the SI-ATRP reaction under such conditions. The clean switching of reaction mixtures in the flow-based QCM has been shown to be a powerful tool for real-time in situ studies of surface-initiated polymerisation reactions, and a promising approach for the precise fabrication of block-containing brush structures.

The polymerisation of 2-hydroxyethyl methacrylate (HEMA) by means of surface-initiated atom transfer radical polymerisation (SI-ATRP) has been studied in situ using a quartz crystal microbalance, with multiple reinitiations under continuous flow of the reaction mixture.

Surface Modification of Wood Flour via ARGET ATRP and Its Application as Filler in Thermoplastics

Wood flour is particularly suitable as a filler in thermoplastics because it is environmentally friendly, readily available, and offers a high strength-to-density ratio. To overcome the insufficient interfacial adhesion between hydrophilic wood and a hydrophobic matrix, a thermoplastic polymer was grafted from wood flour via surface-initiated activators regenerated by electron transfer-atom transfer radical polymerization (SI-ARGET ATRP). Wood particles were modified with an ATRP initiator and subsequently grafted with methyl acrylate for different polymerization times in the absence of a sacrificial initiator. The successful grafting of poly(methyl acrylate) (PMA) was demonstrated using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and water contact angle (WCA) measurements. To confirm the control over the polymerization, a cleavable ATRP initiator was immobilized on the particles, allowing the detachment of the grafted polymer under mild conditions. The grafted particles were incorporated into a PMA matrix using solvent casting and their influence on the mechanical properties (Young's modulus, yield strength, and toughness) of the composite was investigated. Tensile testing showed that the mechanical properties improved with increasing polymerization time and increasing ratio of incorporated grafted particles.

Structure and characterisation of hydroxyethylcellulose-silica nanoparticles

Functionalising nanoparticles with polymers has gained much interest in recent years, as it aids colloidal stability and manipulation of surface properties. Here, polymer-coated thiolated silica nanoparticles were synthesised by self-condensation of 3-mercaptopropyltrimethoxysilane in the presence of hydroxyethylcellulose. These nanoparticles were characterised by dynamic light scattering, small angle neutron scattering, Nanoparticle Tracking Analysis, Raman spectroscopy, FT-IR spectroscopy, thermogravimetric analysis, Ellman's assay, transmission electron microscopy and cryo-transmission electron microscopy. It was found that increasing the amount of hydroxyethylcellulose in the reaction mixture increased the nanoparticle size and reduced the number of thiol groups on their surface. Additionally, by utilising small angle neutron scattering and dynamic light scattering, it was demonstrated that higher concentrations of polymer in the reaction mixture (0.5-2% w/v) resulted in the formation of aggregates, whereby several silica nanoparticles are bridged together with macromolecules of hydroxyethylcellulose. A correlation was identified between the aggregate size and number of particles per aggregate based on size discrepancies observed between DLS and SANS measurements. This information makes it possible to control the size of aggregates during a simple one-pot synthesis; a prospect highly desirable in the design of potential drug delivery systems.

Facile fabrication of a “Catch and Release” cellulose acetate nanofiber interface: a platform for reversible glycoprotein capture and bacterial attachment

We disclose boronic acid ligand-functionalized electrospun cellulose acetate nanofiber mats that can be used as a platform for reversible glycoprotein capture and bacterial attachment.

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.

Direct Mapping of RAFT Controlled Macromolecular Growth on Surfaces via Single Molecule Force Spectroscopy

Single molecule force spectroscopy (SMFS) is employed to gain insight into reversible addition-fragmentation chain transfer (RAFT) polymerization processes with living characteristics on glass surfaces. Surface-initiated (SI)-RAFT was selected to grow poly(hydroxyethyl methacrylate) (PHEMA). After aminolysis of the RAFT chain termini, thiol moieties serve as anchoring points for the gold tip of an atomic force microscope. The results allow to directly monitor the macromolecular growth of the surface-initiated polymerization. The obtained SMFS-based molecular weight distribution data of the polymers present on the surface indicate that the RAFT chain extension proceeds linearly with time up to high conversions. The current study thus adds SMFS as a valuable tool for the investigation of SI-RAFT polymerizations.

Cellulose grafting by photoinduced controlled radical polymerisation

Cellulose surfaces, in the form of filter paper, have been grafted utilizing UV-induced surface-initiated controlled radical polymerization of acrylates.

Polycarbonate-Based Brush Polymers with Detachable Disulfide-Linked Side Chains

We have successfully designed and synthesized polycarbonate-based brush polymers with detachable, disulfide-linked side chains. A polycarbonate backbone with disulfide-linked, hydroxyl-terminated pendant side chains was first prepared. Poly(trimethylene carbonate) or poly(l-lactide) brushes were then grafted from the terminal hydroxyl groups using an acid- or base-catalyzed ring-opening polymerization. Inspired by how cells use glutathione to mediated reduction of disulfides in cytoplasmic proteins, we also demonstrate that the side chains are easily detached under mild reductive conditions (e.g., with 1,4-dithiothreitol). l-Lactide and trimethylene carbonate were selected as model building blocks for the polymer grafts because of their commercial availability and routine use in polymeric drug delivery systems.