Hybrid rigid/soft and biologic/synthetic materials: polymers grafted onto cellulose microcrystals.

Rigid nanoscale polymer rods were prepared by grafting preformed amine-terminated poly(styrene) and poly(tert-butyl acrylate) onto oxidized cellulose microcrystals. Low polydispersity polymers, grown using atom transfer radical polymerization, were characterized and purified prior to cellulose attachment. Oxidation of the cellulose microcrystal led to the formation of carboxylic acids on the surface of the microcrystals. Covalent attachment of the polymers onto the cellulose microcrystals was achieved via a carbodiimide-mediated amidation reaction. The length and diameter of the polymer-cellulose composites increased upon surface modification. Typically, polymer-cellulose composites are synthesized by a grafting-from method because it can be difficult to obtain sufficient graft density using a grafting-to preparation. However, the composites reported here comprised 60-64% grafted polymer by mass. This degree of grafting-to allowed the composite to form stable suspensions in organic solvents.

Bifunctional dendronized cellulose surfaces as biosensors.

Well-defined dendronized cellulose substrates displaying multiple representations of dual-functionality were constructed by taking advantage of the efficiency of the click reaction combined with traditional anhydride chemistry. First, activated cellulose surfaces were decorated with several generations of dendrons, and their peripheral reactive groups were subsequently reacted with a trifunctional orthogonal monomer. The generated substrate tool box was successfully explored by accurately tuning the surface function using a versatile orthogonal dual postfunctionalization approach. In general, the reactions were monitored by using a click-dye reagent or a quartz crystal microbalance (QCM) technique, and the resulting surfaces were well-characterized using XPS, FT-IR, and contact angle measurements. Utilizing this approach two different surfaces have been obtained; that is, triethylenglycol oligomers and amoxicillin molecules were efficiently introduced to the dendritic surface. As a second example, mannose-decorated hydroxyl functional surfaces illustrated their potential as biosensors by multivalent detection of lectin protein at concentration as low as 5 nM.

Biomimetic surface modification of honeycomb films via a "grafting from" approach.

Hydrophobic isoporous membranes were fabricated using the "breath figure" method from polystyrene stars synthesized via ATRP. The living polymer chain ends at the surface of the films were then used, without further modification, in a "grafting-from" approach to grow surface-linked polyglycidyl methacrylate chains under conditions that maintained the regular honeycomb structure. This versatile functional surface was then used as a platform to build a small library of surfaces using a variety of simple chemistries: (i) the acid hydrolysis of the epoxide to form bis-alcohol groups and (ii) utilizing the "click-like" epoxide-amine reaction to functionalize the surface with a model biomolecule-(biotinamido)pentylamine. The successful modifications were confirmed by a combination of spectroscopic and biological means. Changes in the growth characteristics of nonmotile Psychrobacter sp. strain, SW5, on the honeycomb films, provided further evidence confirming changes in the hydrophobicity of the surface upon grafting.

Bifunctional dendrimers: from robust synthesis and accelerated one-pot postfunctionalization strategy to potential applications.

A fourth wheel: Two sets of bifunctional AB(2)C dendrimers having internal acetylene/azides and external hydroxy groups were constructed utilizing benign synthetic protocols. An in situ postfunctionalization strategy was successfully carried out to illustrate the chemoselective nature of these dendrimers. The dendrimers were also transformed into dendritic nanoparticles or utilized as dendritic crosslinkers for the fabrication hydrogels.

Intelligent dual-responsive cellulose surfaces via surface-initiated ATRP.

Novel thermo-responsive cellulose (filter paper) surfaces of N-isopropylacrylamide (NIPAAm) and pH-responsive cellulose surfaces of 4-vinylpyridine (4VP) have been achieved via surface-initiated ATRP. Dual-responsive (pH and temperature) cellulose surfaces were also obtained through the synthesis of block-copolymer brushes of PNIPAAm and P4VP. With changes in pH and temperature, these "intelligent" surfaces showed a reversible response to both individual triggers, as indicated by the changes in wettability from highly hydrophilic to highly hydrophobic observed by water contact angle measurements. Adjusting the composition of the grafted block-copolymer brushes allowed for further tuning of the wettability of these "intelligent" cellulose surfaces.

A chemoselective approach for the accelerated synthesis of well-defined dendritic architectures.

A chemoselective and layered growth approach has been developed for the synthesis of dendrimers, combining Click chemistry with traditional esterification/etherification reactions, without the need for activation steps and with excellent overall yields.

Grafting liquid crystalline polymers from cellulose substrates using atom transfer radical polymerization.

Immobilizing liquid crystalline polymers on cellulose generates new possibilities of accomplishing addressable/responsive bio-based substrates. In this paper we report on our first steps to combine the electro-optic properties of liquid crystals with the versatility of paper as a displaying substrate. Electric current or magnetic fields can be used to manipulate the orientation of liquid crystals and thereby change the appearance and the properties of the material. Atom transfer radical polymerization (ATRP) can be used successfully to graft polymers from solid substrates in a controlled manner. In this study it is shown that the grafting of a liquid crystalline monomer, 11-(4'-cyanophenyl-4″-phenoxy)undecyl acrylate, onto cellulose by ATRP is possible, and that thicker films can be obtained by using PMA as a spacer in between the cellulose and the liquid crystalline block. The cellulose fibers become highly hydrophobic subsequent to grafting and the liquid crystalline polymer possesses mesophases accessible for further processing.

Superhydrophobic bio-fibre surfaces via tailored grafting architecture.

Superhydrophobic bio-fibre surfaces with a micro-nano-binary surface structure have been achieved via the surface-confined grafting of glycidyl methacrylate, using a branched "graft-on-graft" architecture, followed by post-functionalisation to obtain fluorinated brushes.

Stability and biocompatibility of a library of polyester dendrimers in comparison to polyamidoamine dendrimers.

Dendrimers can be designed for several biomedical applications due to their well-defined structure, functionality and dimensions. The present study focused on the in vitro biocompatibility evaluation of a library of aliphatic polyester dendrimers based on 2,2-bis(methylol)propionic acid (bis-MPA) with an overall diameter of 0.5-2 nm. In addition, dendrimers with two different chemical surfaces (neutral with hydroxyl end group and anionic with carboxylic end group) and dendrons corresponding to the structural fragments of the dendrimers were evaluated. Commercial polyamidoamine dendrimers (PAMAM) with cationic (amine) or neutral (hydroxyl) end group were also included for comparison. Cell viability studies were conducted in human cervical cancer (HeLa) and acute monocytic leukemia cells (THP.1) differentiated into macrophage-like cells as well as in primary human monocyte-derived macrophages. Excellent biocompatibility was observed for the entire hydroxyl functional bis-MPA dendrimer library, whereas the cationic, but not the neutral PAMAM exerted dose-dependent cytotoxicity in cell lines and primary macrophages. Studies to evaluate material stability as a function of pH, temperature, and time, demonstrated that the stability of the 4th generation hydroxyl functional bis-MPA dendrimer increased at acidic pH. Taken together, bis-MPA dendrimers are degradable and non-cytotoxic to human cell lines and primary cells.

Synthesis of polycaprolactone-grafted microfibrillated cellulose for use in novel bionanocomposites--influence of the graft length on the mechanical properties.

In the present work, microfibrillated cellulose (MFC) made from bleached sulfite softwood dissolving pulp was utilized to reinforce a poly(ε-caprolactone) (PCL) biopolymer matrix. To improve the dispersibility of the hydrophilic MFC in the nonpolar matrix and the interfacial adhesion in the composite material, we covalently grafted the MFC with PCL via ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). To be able to investigate the effect of the PCL graft length on the mechanical properties of the composite material, we performed ROP to different molecular weights of the grafts. Bionanocomposites containing 0, 3, and 10 wt % MFC were prepared via hot pressing using both unmodified and PCL grafted MFC (MFC-g-PCL) as reinforcement. PCL grafting resulted in improved dispersion of the MFC in a nonpolar solvent and in the PCL matrix. The mechanical testing of the biocomposites showed an improvement in the mechanical properties for the PCL grafted MFC in comparison to ungrafted MFC. It was also shown that there was an impact on the mechanical properties with respect to the PCL graft lengths, and the strongest biocomposites were obtained after reinforcement with MFC grafted with the longest PCL graft length.

Hard and flexible nanocomposite coatings using nanoclay-filled hyperbranched polymers.

The combination of hardness, scratch resistance, and flexibility is a highly desired feature in many coating applications. The aim of this study is to achieve this through the introduction of an unmodified nanoclay, montmorillonite (Na(+)MMT), in a polymer resin based on the hyperbranched polyester Boltorn H30. Smooth and transparent films were prepared from both the neat and the nanoparticle-filled hyperbranched resins. X-ray diffraction (XRD) and transmission electron microscopy (TEM) corroborated a mainly exfoliated structure in the nanocomposite films, which was also supported by results from dynamic mechanical analysis (DMA). Furthermore, DMA measurements showed a 9-16 degrees C increase in Tg and a higher storage modulus-above and below the T(g)-both indications of a more cross-linked network, for the clay-containing film. Thermogravimetric analysis (TGA) demonstrated the influence of the nanofiller on the thermal properties of the nanocomposites, where a shift upward of the decomposition temperature in oxygen atmosphere is attributed to the improved barrier properties of the nanoparticle-filled materials. Conventional coating characterization methods demonstrated an increase in the surface hardness, scratch resistance and flexibility, with the introduction of clay, and all coatings exhibited excellent chemical resistance and adhesion.

Poly(ethylene glycol)-based thiol-ene hydrogel coatings-curing chemistry, aqueous stability, and potential marine antifouling applications.

Photocured thiol-ene hydrogel coatings based on poly(ethylene glycol) (PEG) were investigated for marine antifouling purposes. By varying the PEG length, vinylic end-group, and thiol cross-linker, a library of hydrogel coatings with different structural composition was efficiently accomplished, with or without ester linkages. The thiol-methacrylate and thiol-allyl systems were evaluated with respect to curing, degradation, as well as antifouling properties. Methacrylate-based systems exhibited homopolymerization, whereas allyl-based systems reacted more selectively through thiol-ene couplings reaction. The ester-free hydrogels elucidated higher hydrolytic stability whereas longer PEG chains accelerated the degradation process. The antifouling properties were evaluated by protein adsorption with Bovine serum albumin (BSA) and bioassays with the marine bacteria, Cobetia marina, and the marine diatom, Amphora coffeaeformis; in all tests, longer PEG lengths improved the antifouling properties.

New methodologies in the construction of dendritic materials.

Dendritic polymers are highly branched polymer structures, with complex, secondary architectures and well-defined spatial location of functional groups. Due to their unique physical and chemical features, applications in areas such as targeted drug-delivery, macromolecular carriers, catalysis, sensors, light harvesting, surface engineering and biomimetic materials have been proposed. However, only a few dendritic materials have been exploited commercially due to time consuming syntheses and the generation of significant waste/presence of unreacted starting materials. This tutorial review describes traditional synthesis of dendritic materials as well as recent advances in synthetic strategies, for example the use of Click chemistry, as a tool to efficiently obtain complex, functional dendritic structures.

Adhesion dynamics for cellulose nanocomposites.

The efficiency of poly(ε-caprolactone) (PCL) as a matrix polymer for cellulose nanocomposites has been investigated at the macromolecular contact level using atomic force microscopy in a colloidal probe configuration. Model cellulose microspheres grafted with PCL were prepared via ring-opening polymerization. Force measurements between the functionalized particles revealed the adhesion to be highly dependent on the contact time because of a diffusion-controlled mechanism. Moreover, an increase of the temperature to 60 degrees C (close to T(m) for the PCL graft) greatly enhanced the adhesion at the polymer-polymer interface, demonstrating the importance of entanglements in the annealing of composite materials.

Superhydrophobic and self-cleaning bio-fiber surfaces via ATRP and subsequent postfunctionalization.

Superhydrophobic and self-cleaning cellulose surfaces have been obtained via surface-confined grafting of glycidyl methacrylate using atom transfer radical polymerization combined with postmodification reactions. Both linear and branched graft-on-graft architectures were used for the postmodification reactions to obtain highly hydrophobic bio-fiber surfaces by functionalization of the grafts with either poly(dimethylsiloxane), perfluorinated chains, or alkyl chains, respectively. Postfunctionalization using alkyl chains yielded results similar to those of surfaces modified by perfluorination, in terms of superhydrophobicity, self-cleaning properties, and the stability of these properties over time. In addition, highly oleophobic surfaces have been obtained when modification with perfluorinated chains was performed.

Self-assembly of poly(9,9'-dihexylfluorene) to form highly ordered isoporous films via blending.

Highly ordered hexagonal arrays of isoporous films prepared from poly(9,9'-dihexylfluorene) and polystyrene-grafted silica nanoparticles (Si-graft-PS) are presented. These close-packed arrays were formed in areas of many square millimeters. The pore size varied from 3.6 to 8.5 microm, depending on the concentration of Si-graft-PS and the processing conditions. Solid-state photoluminescence resulted in a significant red shift of up to 30 nm in these films compared to that in conventional processing techniques. These differences are attributed to enhanced aggregation of the polymers caused by polymer-solvent interactions. These highly ordered polymer films may find use in microelectronic and biological and/or chemical sensor applications.

Grafting of cellulose fibers with poly(epsilon-caprolactone) and poly(L-lactic acid) via ring-opening polymerization.

In this study, ring-opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL) and L-lactide (L-LA) has been performed from cellulose fibers. The hydroxyl groups on cellulose act as initiators in the polymerization, and the polymers are covalently bonded to the cellulose fiber. As an attempt to introduce more available hydroxyl groups on the surface, and thereby obtain higher grafting efficiency in the ROP of epsilon-CL and L-LA, unmodified paper was modified with xyloglucan-bis(methylol)-2-methylpropanamide (XG-bis-MPA) and 2,2-bis(methylol)propionic acid (bis-MPA), respectively. The grafted substrates were characterized via Fourier transform infrared spectroscopy (FTIR), contact angle measurement, atomic force microscopy, and enzymatic degradation. The results showed a successful grafting of poly(epsilon-caprolactone) (PCL) and poly(L-lactic acid) (PLLA) from the cellulose fiber surfaces. Furthermore, the results showed an improved grafting efficiency after activation of the cellulose surface with bis-MPA, and showed that the amount of grafted polymer could be controlled by the ratio of added free initiator to monomer.

Dendritic oxazoline ligands in enantioselective palladium-catalyzed allylic alkylations.

First to fourth generation dendritic substituents based on 2,2-bis(hydroxymethyl)propionic acid and (1R,2S,5R)-menthoxyacetic acid were attached to 2-(hydroxymethyl)pyridinooxazoline and bis[4-(hydroxymethyl)oxazoline] compounds. The new ligands obtained were assessed in palladium-catalyzed allylic alkylations. The first type of ligands exhibited enantioselectivity similar to that of a benzoyl ester derivative, whereas the latter type of ligands afforded products with higher selectivity than the analogous benzoyl ester. The activity of the dendritic catalysts decreased with increasing generation.

Immobilized oxazoline-containing ligands in asymmetric catalysis--a review.

Metal complexes of chiral oxazoline derivatives immobilized on soluble as well as insoluble supports serve as versatile asymmetric catalysts in a variety of applications. In a few cases recovery and reuse of the chiral ligands have been achieved.