Molecular Engineering of Fracture Energy Dissipating Sacrificial Bonds Into Cellulose Nanocrystal Nanocomposites

Even though nanocomposites have provided a plethora of routes to increase stiffness and strength, achieving increased toughness with suppressed catastrophic crack growth has remained more challenging. Inspired by the concepts of mechanically excellent natural nanomaterials, one‐component nanocomposites were fabricated involving reinforcing colloidal nanorod cores with polymeric grafts containing supramolecular binding units. The concept is based on mechanically strong native cellulose nanocrystals (CNC) grafted with glassy polymethacrylate polymers, with side chains that contain 2‐ureido‐4[1H]‐pyrimidone (UPy) pendant groups. The interdigitation of the grafts and the ensuing UPy hydrogen bonds bind the nanocomposite network together. Under stress, UPy groups act as sacrificial bonds: simultaneously providing adhesion between the CNCs while allowing them to first orient and then gradually slide past each other, thus dissipating fracture energy. We propose that this architecture involving supramolecular binding units within side chains of polymer grafts attached to colloidal reinforcements opens generic approaches for tough nanocomposites.     

Latazienone, a new lathyrane-type diterpenoid from Euphorbia latazi Kunth

A new lathyrane-type diterpene 8alpha,15beta-diacetoxy-7beta-benzoyloxy-3beta-(2-methylpropanoyloxy)-4alphaH,9alphaH, 11alphaH-lathyra-5E,12E-dien-14-one (latazienone) has been isolated from the latex of Euphorbia latazi Kunth. The structure of the new diterpene was determined by a combination of ID- and 2D-NMR techniques.     

All-cellulose multilayers: long nanofibrils assembled with short nanocrystals

Self-organized multilayer films were formed by sequential addition of oppositely charged cellulose I nanoparticles. The all-cellulosic multilayers were prepared via adsorption of cationicially modified cellulose nanofibrils (cat NFC) and anionic short crystalline cellulose (CNC) at pH 4.5 and pH 8.3. The properties and build-up behavior of layer-by-layer-constructed films were studied with microgravimetry (QCM-D) and the direct surface forces in these systems were explored with colloidal probe microscopy to gain information about the fundamental interplay between cat NFC and anionic CNC. The importance of the first layer on the adsorption of the consecutive layers was demonstrated by comparing pure in situ adsorption in the QCM-D with multilayer films made by spin coating the first cationic NFC layer and then subsequently adsorbing the following layers in situ in the QCM-D chamber. Differences in adsorbed amount and viscoelastic behavior were observed between those two systems. In addition, a significant pH dependence of cat NFC charge was found for both direct surface interactions and layer properties. Moreover the underlying cellulose layer in multilayer film was established to influence the surface forces especially at lower pH, where the cat NFC chains extensions were facilitated and overall charge was affected by the cationic counterpart within the layers. This enhanced understanding the effect of charge and structure on the interaction between these renewable nanoparticles is valuable when designing novel materials based on nanocellulose.     

The behaviour of cationic NanoFibrillar Cellulose in aqueous media

This paper deals, with cationically modified NanoFibrillar Cellulose (cat NFC), obtained by reacting a dissolving pulp with 2,3-epoxypropyl trimethylammonium chloride (EPTMAC). The cat NFC was thoroughly characterized in terms of morphology and physical properties. The dimensions of individual cellulose nanofibrils were determined by atomic force microscopy (AFM) imaging in water and in air. Fibrils as thin as 0.8–1.2 nm were observed in water. The fibril diameter changed upon drying and the average size was further quantified by image analysis. The experiments showed the importance of characterizing nanocellulosic materials in situ before drying. The fibril size in air was confirmed by cryogenic transmission electron microscopy (cryo-TEM), and it was found to be 2.6–3.0 nm. Smooth ultrathin films of cationic NFC were prepared by spincoating on silica substrates. The effect of electrolyte concentration and pH on swelling of the cationic NFC film was studied using a quartz crystal microbalance with dissipation. The results showed that at pH = 8 the cat NFC film was insensitive to electrolyte changes while at pH = 4.5, the water content of the film decreased with increasing ionic strength. The electrophoretic mobility measurements showed a cationic zeta potential for the cat NFC that decreased at increasing pH, verifying the swelling behaviour.     

Enhanced hydrophobicity of rough polymer surfaces

Molecular dynamics simulations were used to study the effect of periodic roughness of PE and PVC polymer surfaces on the hydrophobicity. Pillars of different lateral dimensions and heights were derived from flat crystalline surfaces, and the results of nanoscale simulations on the structured surfaces were compared with theoretical predictions of the Wenzel and Cassie equations. Hydrophobicity increased on all rough surfaces, but the increase was greater on the structured PE surfaces because of the larger water contact angle on the flat PE surface than the corresponding PVC surface. Equally sized pillar structures on the two polymers resulted in different equilibrium wetting geometries. Composite contacts were observed on rough PE surfaces, and the contact angle increased with decreasing contact area between the solid and the liquid. Opposite results were obtained for rough PVC surfaces; the contact angle increased with the solid-liquid contact area, in agreement with Wenzel's equation. However, the composite contact was observed if the energies of the wetted and composite contacts were almost equal. Good agreement was obtained between the simulated contact angles and equilibrium droplet shapes and the theories but there were also some limitations of the nanoscale simulations.     

Functions of finite logarithmic order in the unit disc,Part I

The concept of logarithmic order in the unit disc forms a bridge between meromorphic functions of unbounded Nevanlinna characteristic and meromorphic functions of (usual) zero order of growth. A collection of fundamental results for meromorphic functions of finite logarithmic order is given. Some of these results are reminiscent from the finite order case. Part I of this paper culminates in solving the inverse problem related to the famous defect relation in the case of finite logarithmic order. Part II deals with the analytic case.     

Exponential polynomials as solutions of certain nonlinear difference equations

Recently, C.-C. Yang and I. Laine have investigated finite order entire solutions f of nonlinear differential-difference equations of the form fn + L(z, f ) = h, where n ≥ 2 is an integer. In particular, it is known that the equation f(z)2 + q(z)f (z + 1) = p(z), where p(z), q(z) are polynomials, has no transcendental entire solutions of finite order. Assuming that Q(z) is also a polynomial and c ∈ C, equations of the form f(z)n + q(z)e Q(z) f(z + c) = p(z) do posses finite order entire solutions. A classification of these solutions in terms of growth and zero distribution will be given. In particular, it is shown that any exponential polynomial solution must reduce to a rather specific form. This reasoning relies on an earlier paper due to N. Steinmetz.     

Facile aqueous synthesis and stabilization of nearly monodispersed gold nanospheres by poly(L‐proline)

A facile strategy is developed to synthesize Au nanoparticles (Au‐NPs) using water‐soluble poly(L ‐proline) (PLP). The synthesized NPs were characterized by TEM, FTIR and NMR spectroscopy, thermogravimetric analysis, and circular dichroism. It was found that PLP has a “dual” role as an efficient reductant of Au(III) and simultaneously as a stabilizing agent of Au‐NPs. The influence of PLP molecular weight, temperature, initial Au(III) concentration, and Au(III)/PLP molar ratio on the size and dispersity of Au‐NPs is examined. It was found that the unique extended secondary structure of PLP II resulted in the facile formation of highly crystalline Au‐NPs in water at a very low Au(III)/PLP molar ratio. These Au‐NPs have the smallest dimensions and size distributions among NPs synthesized so far by polymeric materials in aqueous media, and exhibit enduring colloidal stability. Therefore, by utilizing biocompatible and benign materials in water, we managed to obtain Au‐NPs, so as the final product is ready‐to‐use for biomedical applications. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013