Roland Weidisch

Influence of interfacial structure on morphology and deformation behavior of SBS block copolymers

Linear styrene-block-butadiene-block-styrene (SBS) triblock copolymers having different interfacial structures were investigated. In spite of the nearly equivalent chemical composition (about 70 vol% of styrene), these copolymers show significantly different morphologies. It was shown that the origin of the modified morphology in asymmetric block copolymers is the intermixing of short polystyrene (PS) chains or chain segments into the polybutadiene (PB) phase. It has a consequence of an increase in the glass transition temperature of the soft phase (PB phase here) and a significant decrease of the whole relaxation time of the materials. The larger the interfacial volume, the more PS molecules can mix into the PB phase. Moreover, it seems that the extent of the stress transfer in heterogeneous polymeric systems is crucially influenced by the interface. The tapered interface in an SBS block copolymer, for example, permits a more effective stress transfer compared to the sharp interface resulting in a higher degree of orientation in the individual phases of the materials.

Micro-structured smart hydrogels with enhanced protein loading and release efficiency.

A series of temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels with highly porous microstructures were successfully prepared by using hydrophobic polydimethylsiloxane (PDMS) and sodium dodecyl sulfate as liquid template and stabilizer, respectively. These newly prepared hydrogels possess highly porous structures. In contrast to the conventional PNIPAAm hydrogel, the swelling ratios of the porous gels at room temperature were higher, and their response rates were significantly faster as the temperature was raised above the lower critical solution temperature. For example, the novel hydrogel prepared with 40% PDMS template lost over 95% water within 5 min, while the conventional PNIPAAm gel only lost approximately 14% water in the same time. The improved properties are achieved due to the presence of liquid PDMS templates in the reaction solutions, which lead to the formation of porous structures during the polymerization/crosslinking. Lysozyme and bovine serum albumin (BSA) as protein models were for the first time loaded into these micro-structured smart hydrogels through a physical absorption method. The experimental results show that the loading efficiency of BSA with a higher molecular weight is lower than that of lysozyme due to the size exclusion effect, and the loading efficiencies of both proteins in the porous hydrogel are much higher than those in the conventional PNIPAAm hydrogel. For example, the loading efficiency of BSA in porous hydrogel is 0.114, approximately 200% higher than that in conventional hydrogel (0.035). Both lysozyme and BSA were completely released from the porous hydrogel at 22 degrees C. Furthermore, the release kinetics of the proteins from the porous hydrogel could be modulated by tuning the environmental temperature. These newly prepared porous materials provide an avenue to increase the loading efficiency and to control the release patterns of macromolecular drugs from hydrogels, and show great promise for application in protein or gene delivery.

Crack toughness behavior of binary poly(styrene-butadiene) block copolymer blends

Fracture behavior of binary blends comprising styrene-butadiene block copolymers having star and triblock architectures was studied by instrumented Charpy impact test. The toughness of the ductile blends was characterized by the dynamic crack resistance concept (R curves). While the lamellar thermoplastic star block copolymer shows elastic behavior (small scale yielding and unstable crack growth), adding 20 wt% of a triblock copolymer (thermoplastic elastomer, TPE) leads to a strong increase in crack toughness. The stable crack propagation behavior of these blends was described by the crack resistance curve (R) concept of elastic-plastic fracture mechanics. This concept allows the determination of fracture mechanics parameters as resistance against stable crack initiation and propagation. Two brittle to tough transitions (BTT) are observed in the binary block copolymer blend: BTT1 at 20% TPE and BTT2 at about 60% TPE. The strong increase of toughness at 60 wt% TPE indicates a ‘tough/high-impact’ transition as a measure for the protection against stable crack initiation.The kinetics of stable crack propagation is discussed with respect to deformation mechanisms and crack-tip blunting behavior. The analysis of fracture surface by SEM revealed three different types of deformation mechanisms depending on the weight fraction of TPE: coalescence of microvoids (similar to semicrystalline polymers), shear flow (typical of many amorphous polymers like polycarbonate) and tearing (similar to elastomers). Our investigations on nanostructured binary block copolymer blends show new possibilities to tailor the toughness of polymer materials associated with complex morphology-toughness correlations. This may lead to new materials concepts for toughened nanostructured polymers, which still maintain excellent transparency.

Determination of the viscoelastic properties of hydrogels based on polyethylene glycol diacrylate (PEG-DA) and human articular cartilage

In this work, a systematic study of the viscoelastic properties of hydrogels based on polyethylene glycol diacrylate (PEG-DA) is presented. In addition to artificial PEG-DA-based hydrogels, natural hydrogels in the form of human articular cartilage were examined. Specimens were (unconfined) compression tested under static and dynamic load. Besides this, instrumented indentation tests with different indenter geometries (cylindrical, spherical) and load ranges (macro- and nano-indentation) were carried out and relaxation tests for the determination of moduli and relaxation time were performed. Tensile tests completed the list of measurement techniques. The measured initial moduli of the evaluated hydrogels range from 104?107 Pa. Spherical indentation was used in testing human articular cartilage in phosphate buffered saline (PBS). Cartilage samples were measured shortly after explantation, being stored at room temperature. The influence of freezing and shock-freezing was evaluated. It turned out that freezing has a massive impact on sample properties, especially on the stress relaxation time and the ratio of initial to equilibrium modulus.

CORRELATION BETWEEN MORPHOLOGY AND MECHANICAL PROPERTIES OF DIFFERENT STYRENE/BUTADIENE TRIBLOCK COPOLYMERS: A SCANNING FORCE MICROSCOPY STUDY*

The morphology of different styrene/butadiene (SB) block copolymers with triblock architectures was investigated using tapping mode scanning force microscopy (SFM). Comparative analysis of the morphology of the samples at the polymer/substrate interface of solution-cast films and in bulk was performed. It was found that, besides the total phase volume ratio, the interfacial structure between the incompatible chains determines the phase morphology and mechanical properties of the investigated block copolymers. The asymmetric SBS triblock copolymer (φps( 74 vol%) forms, as expected, a cylindrical morphology with hexagonally packed polybutadiene (PB) cylinders in the polystyrene (PS) matrix. Depending on the interfacial structure, block configuration, and the hard/soft phase ratio, other triblock copolymers (φps( 74 vol% and 65 vol%) show lamellae and randomly distributed PS cylinders in a random styrene/butadiene copolymer S/B matrix, respectively. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.

Mechanical Properties of Weakly Segregated Block Copolymers: 1. Synergism on Tensile Properties of Poly(styrene-b-n-butylmethacrylate) Diblock Copolymers

Mechanical properties of poly(styrene-b-n-butylmethacrylate) diblock copolymers, PS-b-PBMA, with different lengths of the polystyrene block were investigated. The copolymers display a composition range where the tensile strength of the block copolymers exceeds the values of the corresponding homopolymers. At a PS-content of 74 vol% not only the tensile strength but also the strain at break is higher than that of pure polystyrene. Furthermore, the absorbed energy shows a maximum at a polystyrene content of 29 vol%. This composition reveals a morphology of hexagonally packed PS-cylinders which turns out to be effective for the increase of toughness in PS-b-PBMA diblock copolymers. The improved mechnical properties of PS-b-PBMA diblock copolymers are discussed with respect to a possible correlation and synergism between phase behaviour, morphology, deformation mechanism, and interface formation.

Mechanical properties of weakly segregated block copolymers Part IV Influence of chain architecture and miscibility on tensile properties of block copolymers

Different types of weakly segregated block copolymers are investigated with respect to the influence of chain architecture and miscibility on tensile properties. Poly(styrene-b-butylmethacrylate) diblock copolymers (PS-b-PBMA) as well as poly(butylmethacrylate-b-polystyrene-b-butylmethacrylate) triblock copolymers (PBMA-b-PS-b-PBMA) show synergistic effects on tensile properties. The triblock copolymers show a higher tensile strength and stiffness compared to that of the diblock copolymers. In addition, the triblock copolymers exhibit a larger composition range for which the tensile strength exceeds that of the respective homopolymers. In order to investigate the influence of block miscibility on tensile properties, poly(methylmethacrylate-b-butylmethacrylate) diblock copolymers (PMMA-b-PBMA) are compared with PS-b-PBMA diblock copolymers.

Feature Article: Experimental Design and Molecular Modeling of Novel Graft Copolymers

The tensile properties of tetra-functional multigraft copolymers have been shown to have surprising high strain at break (~2100%), about double that of commercial thermoplastic elastomers such as Kraton! Currently, multigraft copolymers can be synthesized with a variety of branches (single, bi, tri-, tetra, and with different lengths) at each branch point and there can be a large number of branch points per molecule that are regularly, randomly, or heterogeneously spaced, each of which can have effects on mechanical properties. Unfortunately experimental synthesis and characterization of these novel polymer systems is quite time consuming. This is where molecular modeling and simulation can be critical for mapping out the fundamental mechanisms responsible for the observed behavior and to optimize/focus the experimental efforts. In this article we report details of our experimental synthetic and characterization effort along with some preliminary results from molecular dynamics, molecular mechanics, Monte...

Synthesis, dynamic‐mechanical and morphological behavior of styrene/butyl methacrylate diblock copolymers

In order to investigate the structure-property relationship of styrene/butyl methacrylate (PS/PBMA) diblock copolymers, two ranges of polymers with Mn ≈ 100 000 and Mn > 200 000, respectively, with polydispersity indexes Mw/Mn < 1.15 are synthesized with varying ratios of the block lengths. The molecular weights of these polymers are determined by size exclusion chromatography (SEC). Thermal behavior and morphology are characterized by dynamic-mechanical analysis (DMA) and transmission electron microscopy, respectively. Whereas for unsymmetrical diblock copolymers with an overall molecular weight of Mn ≈ 100 000 only one broadened glass transition due to their disordered state is found, nearly symmetrical diblock copolymers show a partial miscibility which can be ascribed to the proximity of the order-disorder transition and the annealing temperature of the samples. In the case of unsymmetrical high molecular weight diblock copolymers a partial miscibility is observed as well. The high molecular weight diblock copolymers with Mn > 200 000 at all compositions show microphase-separated structures in contrast to the diblock copolymers with an overall molecular weight of Mn ≈ 100 000. Compared with styrene/isoprene diblock copolymers a relatively wide range for lamellar structures is found.

Interfaces Between Incompatible Polymers

Many polymers are incompatible with each other and form phase segregated morphologies in binary blends and copolymers. Properties are largely determined by an interplay between morphologies, interfaces and properties of components. Neutron and X-Ray reflectometry are powerful techniques to determine the interface width between homopolymers, polymer blends, components of block copolymers and other organic materials. The techniques are described, several examples are discussed and compared with real space methods.