Jörg Kressler

Influence of nanofillers on the deformation process in layered silicate/polyamide-12 nanocomposites

Abstract Polymer nanocomposites based on a synthetic layered silicate and polyamide-12 (PA-12) are prepared by injection molding to study their morphology, and the influence of nanofiller particles on local deformation processes. One of the most striking results from morphological studies by transmission electron microscopy is that although the layered silicates, locally stacked in the PA-12 matrix, are arranged on planes parallel to the injection molding direction, the fine lamellae are oriented with their planes perpendicular to the injection molding direction owing to nucleation at the interface between layered silicate and polymer matrix. The dispersion of layered silicates and the orientation of lamellae are reflected in the complexity of the deformation mechanisms, which in turn determine the ultimate macroscopic properties. From studies of in situ deformation under the high voltage electron microscope, it is concluded that the main deformation mechanism is microvoid formation inside the stacks of layered silicates. According to the orientation of these stacks the applied energy is dissipated by splitting, opening or sliding of separate bundles in the stacks during deformation. The nanofiller particles are load-bearing because surfaces in the microvoids are connected and hinder further growth of the microvoids, thus preventing catastrophic failure. As a consequence, the stiffness/strength/toughness balance has been synergistically improved. Finally, based on the present experimental results, a molecular network in polymer nanocomposites is proposed, that leads to the desired superfunctional characteristics.

Nanocomposites based on a synthetic layer silicate and polyamide-12

The swelling behavior of a synthetic three-layer silicate caused by 12-aminolauric acid is studied by wide angle X-ray scattering. The swelling process as a function of the 12-aminolauric acid concentration can be separated into two regimes; i) cation exchange of inorganic cations by protonated 12-aminolauric acid and ii) further diffusion of excess zwitterionic 12-aminolauric acid into the interlayer space of the silicate. After the cation exchange an interlayer distance of the silicate of about 1.7 nm can be achieved, which can be increased to more than 2 nm after additional diffusion of zwitterionic 12-aminolauric acid. Also the distribution of the interlayer distance as a function of the swelling conditions is measured from the full width at half maximum values of the d 001 spacing of the silicate. Improvement of the mechanical properties of polyamide-12 by adding the swollen layer silicate is only achieved when the silicate is present during the polymerization. This is closely related to the morphology development. It is shown by transmission electron microscopy and atomic force microscopy measurements that defoliation of the silicate layers leading to a nanocomposite occurs only when the layer silicate is present during the polymerization process. As little as 2 wt.% of swollen layer silicate with an interlayer distance of about 2.1 nm in the nanocomposites leads to an improvement of the tensile modulus of about 1000 MPa without decreasing the impact strength.

Poly(vinyl alcohol)/poly(acrylic acid) hydrogels: FT-IR spectroscopic characterization of crosslinking reaction and work at transition point

The crosslinking reaction of pure poly(acrylic acid) and its blend with poly(vinyl alcohol) was studied by FT-IR spectroscopy. It is demonstrated that also in blends the anhydride formation characteristic for pure poly(acrylic acid) is the predominant crosslinking reaction upon heating. But the ester formation between poly(vinyl alcohol) and poly(acrylic acid) is detectable due to the ester C=O vibrations and C—O—C vibrations, respectively. The degree of swelling and the Youngs modulus of the crosslinked blend in deionized water depend on the time and temperature of the heat treatment. In dependence of the pH-value of the swelling agent the blends swell or shrink. The working energy at the shrinking or swelling process induced by a change of the pH-value of differently treated blends was measured. The values are in a range of technical interests and comparable with other microactuators.

Alginate encapsulation of genetically engineered mammalian cells: Comparison of production devices, methods and microcapsule characteristics

PRIMARY OBJECTIVE Microencapsulation is a novel method for in vivo immunoprotection of genetically engineered mammalian cells. This study aimed at optimizing alginate/poly-l-lysine/alginate (APA) microencapsulation of mammalian cells in small size (< 300 microm) hollow core microcapsules and at evaluating some of the physical characteristics of APA microcapsules produced by different devices and with different alginate preparations. METHODS AND PROCEDURES Alginate preparations with higher or lower viscosity were used with three different methods: (i) vibrating nozzle, (ii) coaxial gas flow extrusion (AirJet) and (iii) laminar jet break-up (JetCutter). Parameters and device settings for the production of microcapsules with specific characteristics were defined for all three methods. Mechanical stability of APA microcapsules and cell viability of encapsulated cells were investigated in long-term culture and in an animal model. MAIN RESULTS Uniform spherical beads with a mean diameter < 300 microm could be produced by all three encapsulation methods. For the production of uniform microcapsules with a small diameter (< 300 microm) the vibrating nozzle technique required a relatively low viscosity of alginate (< 0.2 Pa/s), while the AirJet and JetCutter devices performed equally well with alginate of higher viscosity. In all cases, alginate with a lower molar mass was inferior to higher molar mass alginate in terms of mechanical stability of the microcapsules. Microcapsules with optimized mechanical properties were injected intravascularly in rats and shown to maintain their shape and the viability of encapsulated cells. CONCLUSIONS The described methods and devices are able to produce APA microcapsules of small size and uniform shape which are mechanically stable in culture and may maintain the viability of the enclosed cells over extended periods of time. These microcapsules seem to be suitable for further therapeutic studies in an animal model of human disease.

Morphology and mechanical properties of blends of isotactic or syndiotactic polypropylene with SEBS block copolymers

Blends of poly(styrene)-block-poly(ethene-co-but-1-ene)-block-poly(styrene) (SEBS) with isotactic polypropylene (PP) and syndiotactic PP, respectively, were investigated. The morphology was observed by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The cryofracture surfaces studied by SEM did not show any particles that were pulled out, so that a good compatibility between SEBS and different PPs could be assumed. The multiphase character of the blends could be well detected by TEM of RuO4 stained samples. TEM micrographs of two-layer specimens revealed that SEBS tends to diffuse into the PP phase under formation of micelles. The block copolymer shows a reorientation phenomenon of large domains at the interface before the diffusion into the PP phase occurs. The interfacial strength as a function of annealing time was measured by a peel test of two-layer specimens. Mechanical properties are studied and related to the blend morphology.

Influence of stereoirregularities on the formation of the γ-phase in isotactic polypropene

Medium to high molar mass isotactic polypropenes with different amounts of stereoirregularities were characterised with respect to their crystallisation behaviour and for comparison a random copolymer of ethene and propene with 5.8 wt% ethene is used. The influence of stereoregularity and crystallisation temperature on the γ-content of the crystallised samples is studied by means of wide angle X-ray scattering, atomic force microscopy and light microscopy. The paper deals also with the temperature rising elution fractionation of an i-PP with large amounts of stereoirregularities and the influence of a nucleation agent on the γ-content. It is shown that effects which render the chainfolding in lamellae more difficult, enhance the formation of the γ-modification. The necessity of chainfolding in isotactic polypropene is discussed in terms of a model that is based on the number of chains that emerge from the lamellae surfaces of the α- and the γ-modification, respectively.

Morphology and phase behaviour of blends of syndiotactic and isotactic polypropylene: 1. X-ray scattering, light microscopy, atomic force microscopy, and scanning electron microscopy

Abstract Blends of isotactic and syndiotactic polypropylene were studied by wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), light microscopy, scanning electron microscopy and atomic force microscopy. WAXS measurements show that both polymers crystallize in different unit cells already during precipitation from a common solvent. Both polymers have a very similar long period and lamella thickness after isothermal crystallization at 135°C as revealed by SAXS. From the crystallization morphology, it can be concluded that the crystallization of isotactic and syndiotactic polypropylene after isothermal annealing in the melt occurs always in large, macroscopic domains. Isotactic polypropylene crystallizes preferably in different spherulitic forms which can usually not be detected for syndiotactic polypropylene crystallizing preferably as needle-like entities. The crystalline morphology of the blends is very complex and depends strongly on the thermal history in the melt, the crystallization temperature and blend composition. It can clearly be seen that the blends undergo liquid-liquid phase separation in the melt which yields isotactic polypropylene in a matrix of syndiotactic polypropylene; syndiotactic polypropylene in a matrix of isotactic polypropylene or a co-continuous morphology for nearly symmetric blends.

Characterization of PLGA Nanospheres Stabilized with Amphiphilic Polymers: Hydrophobically Modified Hydroxyethyl Starch vs Pluronics

Some Pluronics, particularly F127, are known to stabilize nanospheres and prolong their circulation time in vivo. However, these copolymers of poly(ethylene glycol) (PEG) and poly(propylene glycol) are not biodegradable, and despite the long history, there is no approved commercial product using F127 for parenteral administration until now. Meanwhile, hydroxyethyl starch (HES) is a biodegradable polymer that is currently investigated as a substitute for PEG. In order to produce a fully biodegradable amphiphilic polymer, we esterified different molar masses of HES with lauric acid to get different molar substitutions. These polymers, as well as Pluronic F68 and F127, were used to stabilize poly(lactic-co-glycolic acid) (PLGA) nanospheres prepared by nanoprecipitation. For physicochemical characterization, the particle size, zeta potential, and the thickness of the adsorbed polymer layer were measured. The ability of the polymer coating to prevent the adsorption of human serum albumin (HSA) and fibrinogen (FBG) was evaluated. Finally, the phagocytosis of the stabilized nanospheres by a monocyte macrophage cell line (J774.2) was assessed. Results show that the PLGA nanospheres had an average particle size of 110-140 nm. The thickness of the adsorbed polymer layer increases with the increase in molar mass, and is generally higher for HES laurates than the studied Pluronics. Pluronic F68, F127 as well as the HES laurates with low molar substitution prevented the adsorption of HSA. HES laurates with low molar substitution and F127, but not F68, prevented the adsorption of FBG. The phagocytosis experiments showed that the HES laurates, particularly the one with the highest molar mass, could reduce the uptake of the nanospheres better than F68 and comparable to F127. Finally, these results warrant in vivo experiments to evaluate how the HES laurates can affect the pharmacokinetics and fate of the nanospheres.

Enzymatically Catalyzed HES conjugation Using Microbial Transglutaminase: Proof of Feasibility

Polymer-drug and polymer-protein conjugates are promising candidates for the delivery of therapeutic agents. PEGylation, using poly(ethylene glycol) for the conjugation, is now the gold standard in this field, and some PEGylated proteins have successfully reached the market. Hydroxyethyl starch (HES) is a water-soluble, biodegradable derivative of starch that is currently being investigated as a substitute for PEG. So far, only chemical methods have been suggested for HES conjugation; however, these may have detrimental effects on proteins. Here, we report an enzymatic method for HES conjugation using a recombinant microbial transglutaminase (rMTG). The latter catalyzes the acyl transfer between the gamma-carboxamide group of a glutaminyl residue (acyl donors) and a variety of primary amines (acyl acceptors), including the amino group of lysine. HES was modified with N-carbobenzyloxy glutaminyl glycine (Z-QG) and hexamethylene diamine (HMDA) to act as acyl donor and acyl acceptor, respectively. Using (1)H NMR, the degree of modification with Z-QG and HMDA was found to be 4.6 and 3.9 mol%, respectively. Using SDS-PAGE, it was possible to show that the modified HES successfully coupled to test compounds, proving that it is accepted as a substrate by rMTG. Finally, the process described in this study is a simple, mild approach to produce fully biodegradable polymer-drug and polymer-protein conjugates.

Drug-Induced Morphology Switch in Drug Delivery Systems Based on Poly(2-oxazoline)s

Defined aggregates of polymers such as polymeric micelles are of great importance in the development of pharmaceutical formulations. The amount of drug that can be formulated by a drug delivery system is an important issue, and most drug delivery systems suffer from their relatively low drug-loading capacity. However, as the loading capacities increase, i.e., promoted by good drug–polymer interactions, the drug may affect the morphology and stability of the micellar system. We investigated this effect in a prominent system with very high capacity for hydrophobic drugs and found extraordinary stability as well as a profound morphology change upon incorporation of paclitaxel into micelles of amphiphilic ABA poly(2-oxazoline) triblock copolymers. The hydrophilic blocks A comprised poly(2-methyl-2-oxazoline), while the middle blocks B were either just barely hydrophobic poly(2-n-butyl-2-oxazoline) or highly hydrophobic poly(2-n-nonyl-2-oxazoline). The aggregation behavior of both polymers and their formulations with varying paclitaxel contents were investigated by means of dynamic light scattering, atomic force microscopy, (cryogenic) transmission electron microscopy, and small-angle neutron scattering. While without drug, wormlike micelles were present, after incorporation of small amounts of drugs only spherical morphologies remained. Furthermore, the much more hydrophobic poly(2-n-nonyl-2-oxazoline)-containing triblock copolymer exhibited only half the capacity for paclitaxel than the poly(2-n-butyl-2-oxazoline)-containing copolymer along with a lower stability. In the latter, contents of paclitaxel of 8 wt % or higher resulted in a raspberry-like micellar core.