Well‐Controlled Biodegradable Nanocomposite Foams: From Microcellular to Nanocellular

Via a batch process in an autoclave, the foam processing of neat polylactide (PLA) and two different types of PLA/layered silicate nanocomposites has been conducted using supercritical carbon dioxide as a foaming agent. The morphological correlation between the dispersed silicate particles with nanometer dimensions in the bulk and the obtained closed‐cell structure of the foam is discussed. This is the first report that deals with the possibility of preparing biodegradable nanocellular polymeric foams via nanocomposite technology.

SEM image of the freeze‐fracture surface of a PLA/layered silicate foam, exhibiting closed‐cell structure.     

New Insight into Biodegradation of Polylactide (PLA)/Clay Nanocomposites Using Molecular Ecological Techniques

Novel molecular ecological techniques were used to study changes in microbial community structure and population during degradation of polylactide (PLA)/organically modified layered silicates (OMLS) nanocomposites. Cloned gene sequences belonging to members of the phyla Actinobacteria and Ascomycota comprized the most dominant groups of microorganisms during biodegradation of PLA/OMLS nanocomposites. Due to their numerical abundance, members of these microbial groups are likely to play an important role during biodegradation process. This paper presents new insights into the biodegradability of PLA/OMLS nanocomposites and highlights the importance of using novel molecular ecological techniques for in situ identification of new microorganisms involved in biodegradation of polymeric materials.

     

Fabrication of Nanofillers into a Granular “Nanosupport” for Ziegler‐Natta Catalysts: Towards Scalable in situ Preparation of Polyolefin Nanocomposites

This communication reports a strategy for scale‐up of an in situ polymerization technique for polyolefin‐based nanocomposites preparation, taking layered silicate (clay) and multi‐walled carbon nanotubes (MWCNTs) as examples of nanofillers. The strategy is realized by transforming the nanofillers into granular “nanosupports” for Ziegler‐Natta catalysts. With a catalyst to polymer replication effect on particle morphology, the in situ prepared nanocomposites are of controlled granular particle morphology. With the polymer particle morphology controlled, the in situ polymerization technique becomes suitable for industrial olefin polymerization processes for mass production of polyolefin nanocomposites.

     

Exfoliated Polylactide/Clay Nanocomposites by In‐Situ Coordination–Insertion Polymerization

Poly(L ‐lactide)/layered aluminosilicate nanocomposites were synthesized in bulk by ring‐opening polymerization in the presence of two organo‐modified montmorillonites. When the organo‐modifier consisted of an ammonium cation bearing primary hydroxyl groups, polymerization was initiated by the alcohol functions after adequate activation. The growing polymer chains were directly “grafted” onto the clay surface through the hydroxyl‐functionalized ammonium cations yielding exfoliated nanocomposites with enhanced thermal stability.

TEM image of a fully exfoliated Cloisite®30B‐based nanocomposite, showing delamination of the silicate layers.     

Molecular Dynamics Simulation of the Fracture in Polymer‐Exfoliated Layered Silicate Nanocomposites

Summary: The MD technique was used to investigate the fracture behavior in fully exfoliated layered silicate (nanoplatelet)‐polymer nanocomposites. MD results reveal that the addition of the nanoplatelets can improve the fracture strength of polymers. The interactions between the surface of the nanoplatelets and the segments of the polymer, and the relaxation time of polymer chains have significant influences on the fracture strength of the polymer. For polymers with T_g below room temperature, such as polyurethane, or close to room temperature, such as nylon, the nanoplatelets are always working for the enhancement of the mechanical properties. However, for polymers with T_g above room temperature, such as epoxy and polystyrene, the addition of the nanoplatelets is not working well for toughening these polymers. If the nanoplatelets are to enhance the mechanical properties of these polymers, it is necessary to build up a stress relaxation interface between the polymer and the nanoplatelet in order to reduce the effect of the difference between the relaxation time of nanofillers and that of polymers.

Force per area versus distance curves as a function of the difference of the relaxation times of the nanoplatelets and polymer chains.     

New Polylactide/Layered Silicate Nanocomposite: Nanoscale Control Over Multiple Properties

The preparation, characterization, materials properties, crystallization behavior and biodegradability of a polylactide (PLA)/organically modified layered silicate nanocomposite is described. Wide‐angle X‐ray diffraction and high‐resolution TEM analyses confirmed that intercalated stacked and disordered/exfoliated mica layers coexist in the nanocomposite. The nanocomposite showed improved materials properties and crystallization behavior with a simultaneous improvement in biodegradability as compared to neat PLA.

Degree of biodegradation (i.e. CO₂ evolution) of neat PLA and PLACN4 under compost conditions.     

Organically Modified Layered Titanate: A New Nanofiller to Improve the Performance of Biodegradable Polylactide

Summary: The preparation and characterization of a new type of nanocomposite material that is based on biodegradable polylactide (PLA) and organically modified layered titanate, is reported. Layered titanate, modified with a N‐(cocoalkyl)‐N,N‐[bis(2‐hydroxyethyl)]‐N‐methylammonium cation, was used as a new nanofiller (OHTO) for the nanocomposite preparation. Wide‐angle X‐ray diffraction and transmission electron microscopy (TEM) analyses confirmed that titanate layers were intercalated and nicely distributed in the PLA matrix. The materials properties of neat PLA improved remarkably after nanocomposite preparation.

UV/vis transmission spectra of neat PLA and a nanocomposite.     

Nanocomposites of Size‐Tunable ZnO‐Nanoparticles and Amphiphilic Hyperbranched Polymers

Highly dispersed ZnO nanoparticles with variable particle sizes were successfully prepared within an amphiphilic hyperbranched polyetherpolyol matrix via decomposition of an organometallic precursor in the presence of air leading to stable nanocomposites. The high degree of stabilization during and after the synthesis by the polymer permits control over the nanoparticle size and therefore, due to the quantum‐size‐effect, the particle properties. Furthermore, these polymer‐inorganic nanocomposites can easily be dispersed in apolar solvents to yield highly transparent, stable solutions.

     

Effect of Drawing Induced Dispersion of Nano‐Silica on Performance Improvement of Poly(propylene)‐Based Nanocomposites

Summary: A new route that combines graft pre‐treatment and drawing techniques with melt mixing to prepare nanoparticle‐filled thermoplastic polymer composites is reported. Nano‐SiO₂ particles are first modified by graft polymerization and then the grafted nanoparticles are melt‐compounded with poly(propylene) (PP) to produce composite filaments via drawing. Finally, the filaments are injection molded into bulk materials. Because the proposed manufacturing method is able to induce separation of the nanoparticles and the formation of beta‐crystals in the PP matrix, the resultant PP‐based nanocomposites are much tougher than the unfilled polymers, as characterized by either static or dynamic tests, in addition to showing a simultaneous increase in strength and stiffness.

Force–time curves of PP and its nanocomposites recorded during notch impact tests.     

Polyhedral Oligomeric Silsesquioxane (POSS) Nanoscale Reinforcement of Thermosetting Resin from Benzoxazine and Bisoxazoline

Summary: The reaction between octaaminophenyl polyhedral oligomeric silsesquioxane (OAPS) and 2,2′‐(1,3‐phenylene)‐bis(4,5‐dihydro‐oxazoles) (PBO) over different temperature ranges was confirmed by FT‐IR spectroscopy. The OAPS was used to modify benzoxazine (BZ) in the presence of PBO. The novel polybenzoxazine (PBZ)‐PBO/OAPS hybrid nanocomposite was prepared by solvent methods. Dynamic mechanical analyses indicated that the nanocomposites exhibited much higher T_g values than the pristine PBZ and PBZ‐PBO resin, and the storage modulus of the nanocomposites was maintained at higher temperatures, although only a small amount of OAPS was incorporated into the systems. Dynamic thermogravimetric analysis showed that the thermal stability of the hybrid was also improved by the inclusion of OAPS.

DMA of PBZ (a), PBZ‐PBO (b), and PBZ‐PBO/OAPS nanocomposites (c–e).     

Shape Memory Effect of Bacterial Poly[(3‐hydroxybutyrate)‐co‐(3‐hydroxyvalerate)]

Summary: A bacterial poly[(3‐hydroxybutyrate)‐co‐(3‐hydroxyvalerate)] biosynthesized by Pseudomonas sp. HJ‐2 was found to be a shape memory polymer. Permanent shapes were set by annealing at room temperature the samples that had been pre‐treated above 95 °C in specified shapes. The temporary shapes were set by stretching and holding the elongated samples. Thermal shrinkage began at 45 °C and stopped at 75 °C to recover to their permanent shapes. Apparently, the orientation induced the formation of hard segments that were responsible for setting the temporary shapes. The shape memory effect of this polymer was explained based on the DSC and XRD results at different phases.

The recovery of a coil shape upon heating a strip of HJ‐2 PHB35V, demonstrating the polymers shape memory effect.     

Critical Conformational Change of Poly(oxypropylene)diamines in Layered Aluminosilicate Confinement

The intercalation of aluminosilicate clays proceeds via a critical conformation change. Ion exchange of the poly(oxypropylene)diamine salts allows widening of the clay layers from 12 Å to 20 Å and then a sharp increase to 58 Å. The data indicate a critical conformation change of the intercalated amines, thus providing a new way for manipulating nanomaterials and control of polymer conformations in layered silicate confinement.

Schematic illustration of POP‐amine self‐assembly in silicate confinements.     

Rod‐Like Silicate‐Epoxy Nanocomposites

Summary: Epoxy nanocomposites containing rod‐like silicate (attapulgite) were prepared using a simple organic modification to the nanorods. The modification led to effective interfacial adhesion between the ceramic nanorods and the epoxy resin and hence good load transfer. Scanning electron microscopy examination revealed a uniform dispersion of nanorods in the epoxy resin. Compared to the neat resin, nanocomposites with 7.47 vol.‐% nanorods exhibited an increase in the (rubbery state) storage modulus of 122.5%. In addition, the nanocomposites exhibited improved dimensional stability both above and below the T_g.

Storage modulus of the neat resin and nanocomposites.     

Cooperative Self‐Assembly of Nanoparticle Mixtures in Lamellar Diblock Copolymers: A Dissipative Particle Dynamics Study

Summary: Dissipative particle dynamics simulations are performed on the distribution of binary nanoparticle mixtures in lamellar diblock copolymers. The results show that the self‐assembly of nanoparticle mixtures in polymer matrix is a cooperative assembly that is affected by various factors, providing molecular‐level information for the rational design of new polymer nanocomposites with tailored properties.

The simulated polymer nanocomposite structure (the polymer matrix was omitted for clarity; P, gray; Q, black).     

Optical Probes for Monitoring Intercalation and Exfoliation in Melt‐Processed Polymer Nanocomposites

Summary: Laser‐induced fluorescence spectroscopy of the optical probe Nile Blue A in polymer clay nanocomposites is described. Concentration quenching of the fluorescence dominates the probe behavior until the clay platelets are physically separated by polymer intercalation. Further separation into an exfoliated structure results in an intense increase in probe fluorescence. Preliminary results indicate the ability to discriminate between intercalated and exfoliated structures in nanocomposites formed by melt processing.

Polyamide 6 nanocomposites: Purple, 1 minute processing (left). Red, 7 minute processing (right).     

Fabrication of Robust High‐Quality ORMOCER® Inverse Opals

Summary: The nanostructuring of ORMOCER® to form inverse opals is described. For this purpose a polymer opal is used as a template and infiltrated with liquid ORMOCER®. After photopolymerization of the resin the host opal is dissolved in tetrahydrofuran and an ORMOCER® inverse opal is obtained. It shows excellent periodicity (by SEM) and optical properties to reveal a high degree of face centered cubic order. This replication process leads to a nanostructured photonic crystal with the outstanding mechanical properties of ORMOCER® and high temperature stability up to 350 °C.

ORMOCER® inverse opal annealed at 200 °C for 10 h.     

Reactivity Study of Elementary Steps in the Polymerization Mechanism of Acrylfuranic Compounds by Frontier Molecular Orbital Theory

A reactivity study of the most important elementary steps (propagation, intermolecular degradative transfer, and re‐initiation) in free‐radical polymerization of acrylfuranic systems, furfuryl acrylate (FA), and furfuryl methacrylate (FM), using the frontier molecular orbital theory is described. A qualitative explanation of reactivity trends of these steps for both systems is given based on absolute values of the SOMO/HOMO gap. The small difference between values of k_p for FA and FM compared to that found for MA and MMA ( ) is justified semi‐quantitatively by applying a formulation for the change of energy in the transition state using second‐order perturbation theory.

     

Orienting the Demixion of a Diblock‐copolymer Using 193 nm Interferometric Lithography for the Controlled Deposition of Nanoparticles

DUV interferometric lithography and diblock copolymer self‐organization have successfully been combined to provide a simple and highly collective nanopatterning technique enabling the organization of nanoparticles over several orders of magnitude, from nanometre to millimetre. The nanostructural changes at the surface of the polymer film after thermal annealing have been monitored by AFM and the process parameters optimized for obtaining a long‐range organization of the lamellar domains. In particular, the impact of the annealing conditions and geometric parameters of the substrate patterns have been investigated. The nanopatterns resulting from the lamellar demixion of (PS‐b‐MMA) were used for a controlled deposition of nanoparticles. The affinity of the hydrophobic particles for the PS block was demonstrated, opening new doors towards the preparation of high‐density arrays of nanoparticles with potential applications in data storage.

     

Synthesis and Properties of Oligophenylene‐Layered Polymers

We report syntheses of phenylene‐, biphenylene‐, and terphenylene‐layered polymers with a xanthene scaffold by the modified Suzuki‐Miyaura coupling reaction. Their optical properties were studied in detail. The polymer end‐capped by nitrobenzene units, which act as fluorescence quenchers, exhibited the photo‐excited energy transfer from the layered oligophenylenes to the terminal units.

     

Crack Toughness Behaviour of Multiwalled Carbon Nanotube (MWNT)/Polycarbonate Nanocomposites

Summary: The morphology and fracture behaviour of polycarbonate (PC)/multiwalled carbon nanotube (MWNT) composites have been studied by AFM and post‐yield fracture mechanics. The essential work of fracture (EWF) method has been used to distinguish between two terms representing the resistance to crack initiation and crack propagation. A maximum in the non‐essential work of fracture was observed at 2 wt.‐% MWNT, demonstrating enhanced resistance to crack propagation compared to pure PC. At 4 wt.‐% MWNT, a tough‐to‐brittle transition has been observed. The time‐resolved in‐situ strain field analysis revealed that the onset of crack initiation was shifted to a shorter time for nanocomposites with 4 wt.‐% MWNT compared to that with 2 wt.‐%, and thus explained the existence of a tough‐to‐brittle transition in these nanocomposites.