Proceedings of SPIE - The International Society for Optical Engineering
The electromechanical properties of ultrathin films of polyglutamates with either a preferential ... more The electromechanical properties of ultrathin films of polyglutamates with either a preferential polar upright orientation of the helical chains or an orientation parallel to the substrate plane are compared. The electric field- induced change in film thickness as determined by an electromechanical interferometer is used to measure of the degree of polar order and the mechanical compliance. Following the approach by Whitesell and Chang monomolecular films of 15 nanometer thickness of the helical polypeptide poly((gamma) -benzyl-L-glutamate) (PBLG) were grown directly from a flat aluminum surface. The measured PBLG film polarization was found to be comparable to that of conventional ferroelectric materials. This result demonstrates that the polymerization starting from the surface forces the helical chains into a polar arrangement. A large mechanical plate modulus of the film of approximately 35 GPa as determined by electrostriction agreed with the theoretical prediction for a single...
The continuous development of new biomaterials for tissue engineering and the enhancement of tiss... more The continuous development of new biomaterials for tissue engineering and the enhancement of tissue ingrowth into existing scaffolds, using growth factors, create the necessity for developing adequate tools to assess tissue ingrowth rates into porous biomaterials. Current histomorphometric techniques evaluating rates of tissue ingrowth tend either to measure the overall tissue content in an entire sample or to depend on the user to indicate a front of tissue ingrowth. Neither method is particularly suitable for the assessment of tissue ingrowth rates, as these methods either lack the sensitivity required or are problematic when there is a tissue ingrowth gradient rather than an obvious tissue ingrowth front. This study describes a histomorphometric method that requires little observer input, is sensitive, and renders detailed information for the assessment of tissue ingrowth rates into porous biomaterials. This is achieved by examining a number of computer-defined concentric zones, which are based on the distance of a pixel from the scaffold edge. Each zone is automatically analyzed for tissue content, eliminating the need for user definition of a tissue ingrowth front and thus reducing errors and observer dependence. Tissue ingrowth rates in two biodegradable polyurethane scaffolds (Estane and polycaprolactone-polyurethane [PCLPU]) specifically designed for tissue engineering of the knee meniscus were assessed. Samples were subcutaneously implanted in rats with follow-up until 6 months. Especially at the earlier follow-up points, PCLPU scaffolds showed significantly higher tissue ingrowth rates than Estane scaffolds, making the PCLPU scaffold a promising candidate for further studies investigating meniscus tissue engineering.
Kogelnik's coupled-wave theory for thick phase gratings predicts that the diffraction efficiency ... more Kogelnik's coupled-wave theory for thick phase gratings predicts that the diffraction efficiency is given by ϰ sin 2 with ϭ G⌬ nd/, where G is a geometrical factor that depends on the polarization of the beams and the experimental geometry, ⌬ n is the dynamic range, d is the thickness of the grating, and is the wavelength of the light.
Nonlinear optical chromophores can be organized as orientationally correlated side groups of poly... more Nonlinear optical chromophores can be organized as orientationally correlated side groups of polymers with a rigid backbone. In such an organization, each chromophore contributes coherently to the second-order nonlinear response of the polymer structure.
The synthesis of amphiphilic diblock copolypeptides consisting of poly(a-L-glutamic acid) (PLGA) ... more The synthesis of amphiphilic diblock copolypeptides consisting of poly(a-L-glutamic acid) (PLGA) and poly(g-methyl-L-glutamate-ran-g-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) and their monolayer behavior at the air-water interface have been studied. PLGA-b-PMLGSLG was synthesized via a diblock copolymer precursor consisting of poly(g-tert-butyl-L-glutamate) (PtBuLG) and PMLGSLG blocks, with the tert-butyl group as a mild acid-labile protecting group for the carboxylic acid.
Polypyrrole has been chemically synthesized on thin film nanostructures obtained from comb-shaped... more Polypyrrole has been chemically synthesized on thin film nanostructures obtained from comb-shaped supramolecules of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) hydrogen bonded with pentadecylphenol (PDP). PDP was washed from thin films of cylindrical and lamellar self-assembled combcopolymer systems, which resulted in removal of the upper layers of microdomains, leaving single cylindrical and lamellar layers covering a substrate, with P4VP segregated at the bottom as well as at the free air interface. This P4VP was complexed with Cu 2þ ions, after which chemical oxidation polymerization of pyrrole resulted in a thin polypyrrole layer covering the nanostructured block copolymer. The use of a catalytic amount of bipyrrole greatly improved the quality of the obtained product. The conductivity was measured to be w0.7 S cm À1 .
In situ SAXS is used to investigate the shear-induced alignment of a supramolecular system consis... more In situ SAXS is used to investigate the shear-induced alignment of a supramolecular system consisting of a polyisoprene-b-poly(2-vinylpyridine) diblock copolymer and octyl gallate (OG), where OG is hydrogen-bonded to the vinylpyridine block of the PI-b-P2VP copolymer. Due to microphase separation, the PI-b-P2VP(OG) system self-assembles forming a domain structure of hexagonally ordered P2VP(OG) cylinders embedded in the PI matrix. The results of the alignment showed that the orientation mobility of the cylinders, lying within the (10) planes parallel to the shear plane, is strongly dependent on the size of the cylindrical domains. Subjected to large amplitude oscillatory shear for 15 min a freshly loaded sample with a small grain size structure exhibited much better alignment compared to that of a pre-aligned/squeezed sample with a considerably larger grain size where the orientation had been first partly destroyed by squeezing.
Polycaprolactone (PCL) polyester and segmented aliphatic polyester urethanes based on PCL soft se... more Polycaprolactone (PCL) polyester and segmented aliphatic polyester urethanes based on PCL soft segment have been thoroughly investigated as biodegradable scaffolds for tissue engineering. Although proven beneficial as long term implants, these materials degrade very slowly and are therefore not suitable in applications in which scaffold support is needed for a shorter time. A recently developed class of polyacylurethanes (PAUs) is expected to fulfill such requirements. Our aim was to assess in vitro the degradation of PAUs and evaluate their suitability as temporary scaffold materials to support soft tissue repair. With both a mass loss of 2.5-3.0% and a decrease in molar mass of approx. 35% over a period of 80 days, PAUs were shown to degrade via both bulk and surface erosion mechanisms. Fourier Transform Infra Red (FTIR) spectroscopy was successfully applied to study the extent of PAUs microphase separation during in vitro degradation. The
The number of internal double bonds in poly(vinyl chloride) (PVC) samples was studied as a functi... more The number of internal double bonds in poly(vinyl chloride) (PVC) samples was studied as a function of molecular weight at various monomer conversions. These defect structures were found to exhibit end-group-like characteristics: their concentration per chain was largely constant as a function of molecular weight. This tendency was independent of the degree of conversion. An intramolecular mechanism for formation of unsaturated structures and their location between carbons 5-6 were confirmed via 13 C NMR studies. High-level ab initio calculations showed that a 1-6 hydrogen transfer reaction was the most likely origin for these structures, though a second mechanism involving backbiting of the 1-2 Cl shifted head-to-head radical followed by -chlorine elimination and then transfer to monomer could also contribute at lower conversions. From the experimental analysis and theoretical calculations, it emerged that this backbiting reaction is stereoselective, with the isotactic conformation appearing to be more resistant. However, from the ab initio calculations and earlier results of other research groups it also seems likely that hydrogen abstraction from chloroallylic end groups and further propagation of such radical is a concurrent route to internal double bonds. The evidence collected in this paper point to hydrogen abstraction reactions, especially backbiting and abstraction from chloroallylic end groups, as reactions for which inhibition should have a beneficial effect on the thermal stability of PVC.
Synchrotron radiation was used to investigate the self-assembly in two comb-shaped supramolecules... more Synchrotron radiation was used to investigate the self-assembly in two comb-shaped supramolecules systems consisting of octyl gallate (OG), i.e., 1-octyl-3,4,5-trihydroxybenzoate, hydrogen bonded to the pyridine groups of polyisoprene-block-poly(vinylpyridine) diblock copolymers. In the case of the 1,2-polyisoprene-block-poly(4-vinylpyridine)(OG)x system, self-assembly was only observed for x g0.5, where x denotes the number of OG molecules per pyridine group. For x ) 0.5, 0.75, 1.0, and 1.2 the system self-assembled in the form of hexagonally ordered cylinders of P4VP(OG) throughout the entire temperature range of 25-200°C investigated. For the 1,4-polyisoprene-block-poly(2-vinylpyridine)(OG)x system, on the other hand, a considerably more complex phase behavior was found, including the formation of cubic, hexagonally ordered cylinders and lamellar morphologies. In this case several order-order transitions were observed as a function of temperature, including a lamellar to lamellar transition involving a collapse of the layer thickness. The absence of hydrogen bonding between the octyl gallate molecules and the pyridine groups at elevated temperatures is argued to be a key factor for many of the phenomena observed.
The monomer conversion dependence of the formation of the various types of defect structures in r... more The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both 1H and 13C NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects.
An enzymatic tandem reaction is described in which the enzymes phosphorylase and Deinococcus geot... more An enzymatic tandem reaction is described in which the enzymes phosphorylase and Deinococcus geothermalis glycogen branching enzyme (Dg GBE) catalyze the synthesis of branched polyglucans from glucose-1-phosphate (G-1-P). Phosphorylase consumes G-1-P and polymerizes linear amylose while Dg GBE introduces branching points on the a-(1 ! 6) positions by reshuffling short oligosaccharides. The resulting branched polyglucans have an unusually high degree of branching of 11%.
Cover: The cover picture shows the acyl-enzyme intermediate formed between 2-azetidinone and Cand... more Cover: The cover picture shows the acyl-enzyme intermediate formed between 2-azetidinone and Candida antarctica lipase B derived via docking with subsequent combined QM/MM geometry optimization. The enzymecatalyzed ring-opening polymerization of 2-azetidinone proceeds via this intermediate. It is shown that poly(balanine) can be prepared by enzyme catalysis and that the ring structure of the 2-azetidinone is essential for the formation of the acyl-enzyme intermediate.
A method is presented to prevent microbial adhesion to solid surfaces exploiting the unique prope... more A method is presented to prevent microbial adhesion to solid surfaces exploiting the unique properties of polymer brushes. Polyacrylamide (PAAm) brushes were grown from silicon wafers by atom transfer radical polymerization (ATRP) using a three-step reaction procedure consisting of immobilization of a coupling agent γ-aminopropyltriethoxysilane, anchoring of an ATRP initiator 4-(chloromethyl)benzoyl chloride, and controlled radical polymerization of acrylamide. The surfaces were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, ellipsometry, and contact-angle measurements. The calculated grafting density pointed to the presence of a dense and homogeneous polymer brush. Initial deposition rates, adhesion after 4 h, and detachment of two bacterial strains (Staphylococcus aureus ATCC 12600 and Streptococcus saliVarius GB 24/9) and one yeast strain (Candida albicans GB 1/2) to both PAAm-coated and untreated silicon surfaces were investigated in a parallel plate flow chamber. A high reduction (70-92%) in microbial adhesion to the surface-grafted PAAm brush was observed, as compared with untreated silicon surfaces. Application of the proposed grafting method to silicone rubbers may offer great potential to prevent biomaterials-centered infection of implants.
The monolayer behavior at the air-water interface of a chiral nonlinear optical dye was evaluated... more The monolayer behavior at the air-water interface of a chiral nonlinear optical dye was evaluated. The dye alone, an ester of palmitic acid and 4-nitro-4'-[(3R)-hydroxypyrrolidinol-1,l'-azobis[benzenel, did not form a stable monolayer at 23 *C at the airwater interface. However, mixtures of the dye with amylose acetate formed stable monolayers when the amount of the dye did not exceed 50 mol 9%. The surface structure of monolayers of the dye alone and of the mixtures was studied by transmission electron microscopy. This showed that molecular mixing did not occur. The mixtures are composed of domains of amylose acetate and two-dimensional crystallites of the dye molecules. The domains of the dye behave like H aggregates. It is argued that boundary lines between the crystallites of the dye and amylose acetate might play an important role of the stabilization process. As far as this study goes, the stabilization of the dye in mixed systems is only found for amylose esters as matrices.
The surface potentials and effective dipole moments of R-helical amphiphilic diblock copolypeptid... more The surface potentials and effective dipole moments of R-helical amphiphilic diblock copolypeptides during monolayer compression at the air-water interface are reported. Amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(R-L-glutamic acid) (PLGA) and poly(γ-methyl-L-glutamate-ran-γ-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) of various block lengths were studied during the double-brush formation process at the water surface. Upon monolayer spreading of PLGA-b-PMLGSLGs, surface potentials of hundreds of millivolts were recorded, attributed to the dipole moments of water molecules reorienting due to interactions with the monolayers. Upon compression, the effective dipole moments derived from the surface potentials of the PLGA-b-PMLGSLG monolayers decrease gradually, most likely as a result of the immersion of the hydrophilic block in water and cancellation of the interactions between the hydrophobic block and the underlying water molecules. The polypeptide macrodipole moment immersed in water was apparently effectively screened out. The remaining effective dipole moment of the monolayer contributes mainly to the hydrophobic block, and upon tilting away from the water surface toward the surface normal, it was found to increase with the hydrophobic block length, indicating the gradual formation of unidirectional aligned polypeptide molecules in the double-brush monolayer.
The azimuthal helix orientation of the rigid-rod amphiphilic diblock copolypeptides (PLGA-b-PMLGS... more The azimuthal helix orientation of the rigid-rod amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(R-L-glutamic acid) (PLGA) and poly(γ-methyl-L-glutamate-ran-γ-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) in Langmuir-Blodgett (LB) monolayers was investigated using polarized transmission Fourier transform infrared spectroscopy. The relative position of dipping with respect to the previous transfer position can be used to manipulate the azimuthal orientation of the helices parallel to or tilted by an angle of 45°with respect to the dipping direction in the transferred films. The study of the azimuthal order for the LB monolayers of PLGA-b-PMLGSLGs of various block lengths revealed that the observed effect arises mainly from the deformation of the PMLGSLG top brush layer, induced by the flow orientation around the transfer region. In those cases where the PMLGSLG block is tilted by a sufficiently large angle with respect to the surface normal, high azimuthal order parameters of 0.5-0.75 were obtained.
The effect of annealing on the structure and the helix orientation in Langmuir-Blodgett (LB) mono... more The effect of annealing on the structure and the helix orientation in Langmuir-Blodgett (LB) monolayers of diblock copolymers (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) with unidirectional helix orientation deposited on hydrophilic silicon substrates was characterized by means of small-angle X-ray reflectivity, transmission Fourier transform infrared spectroscopy, and atomic force microscopy. Upon annealing at 100 degrees C for 24 h, the alpha-helices became less tilted toward the substrate surface normal. Surface area shrinkage accompanied the change in tilt, indicated by an increase in both film thickness and electron density, resulting in more compact and uniform films. The enhancement of the helix orientation by thermal annealing was greater for the PMLGSLG block and for the diblock copolymers with the shorter block lengths. For these diblock copolymers, annealing resulted in postorientation of the PMLGSLG block helices almost perpendicular to the substrate surface. This effect originates from a considerable increase in intermolecular packing of the PLGA block caused by hydrogen bonds between the carboxylic groups upon annealing, as well as the high mobility of the PMLGSLG block helices for rearrangement favored by the melted side chain mantle at elevated temperatures.
Journal of Biomedical Materials Research Part A, 2010
Polyacrylamide (PAAm) brushes, covalently grafted from silicon wafer surfaces were examined for t... more Polyacrylamide (PAAm) brushes, covalently grafted from silicon wafer surfaces were examined for their ability to inhibit microbial adhesion after long-term exposure to PBS or reconstituted freeze-dried saliva for time intervals from 48 h up to 1 month at 37 C. Microbial adhesion after exposure was studied in a parallel plate flow chamber. Infrared spectra showed that PAAm brushes exhibit good chemical stability upon incubation in both PBS and reconstituted freeze-dried saliva up to 1 month. Reductions in microbial adhesion on PAAm brushes after exposure to PBS or reconstituted freezedried saliva varied from 63 to 93% depending on the microbial strain considered, even after 1 month of exposure of the brushes to reconstituted freeze-dried saliva. V C 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 94A: 997-1000, 2010.
Proceedings of SPIE - The International Society for Optical Engineering
The electromechanical properties of ultrathin films of polyglutamates with either a preferential ... more The electromechanical properties of ultrathin films of polyglutamates with either a preferential polar upright orientation of the helical chains or an orientation parallel to the substrate plane are compared. The electric field- induced change in film thickness as determined by an electromechanical interferometer is used to measure of the degree of polar order and the mechanical compliance. Following the approach by Whitesell and Chang monomolecular films of 15 nanometer thickness of the helical polypeptide poly((gamma) -benzyl-L-glutamate) (PBLG) were grown directly from a flat aluminum surface. The measured PBLG film polarization was found to be comparable to that of conventional ferroelectric materials. This result demonstrates that the polymerization starting from the surface forces the helical chains into a polar arrangement. A large mechanical plate modulus of the film of approximately 35 GPa as determined by electrostriction agreed with the theoretical prediction for a single...
The continuous development of new biomaterials for tissue engineering and the enhancement of tiss... more The continuous development of new biomaterials for tissue engineering and the enhancement of tissue ingrowth into existing scaffolds, using growth factors, create the necessity for developing adequate tools to assess tissue ingrowth rates into porous biomaterials. Current histomorphometric techniques evaluating rates of tissue ingrowth tend either to measure the overall tissue content in an entire sample or to depend on the user to indicate a front of tissue ingrowth. Neither method is particularly suitable for the assessment of tissue ingrowth rates, as these methods either lack the sensitivity required or are problematic when there is a tissue ingrowth gradient rather than an obvious tissue ingrowth front. This study describes a histomorphometric method that requires little observer input, is sensitive, and renders detailed information for the assessment of tissue ingrowth rates into porous biomaterials. This is achieved by examining a number of computer-defined concentric zones, which are based on the distance of a pixel from the scaffold edge. Each zone is automatically analyzed for tissue content, eliminating the need for user definition of a tissue ingrowth front and thus reducing errors and observer dependence. Tissue ingrowth rates in two biodegradable polyurethane scaffolds (Estane and polycaprolactone-polyurethane [PCLPU]) specifically designed for tissue engineering of the knee meniscus were assessed. Samples were subcutaneously implanted in rats with follow-up until 6 months. Especially at the earlier follow-up points, PCLPU scaffolds showed significantly higher tissue ingrowth rates than Estane scaffolds, making the PCLPU scaffold a promising candidate for further studies investigating meniscus tissue engineering.
Kogelnik's coupled-wave theory for thick phase gratings predicts that the diffraction efficiency ... more Kogelnik's coupled-wave theory for thick phase gratings predicts that the diffraction efficiency is given by ϰ sin 2 with ϭ G⌬ nd/, where G is a geometrical factor that depends on the polarization of the beams and the experimental geometry, ⌬ n is the dynamic range, d is the thickness of the grating, and is the wavelength of the light.
Nonlinear optical chromophores can be organized as orientationally correlated side groups of poly... more Nonlinear optical chromophores can be organized as orientationally correlated side groups of polymers with a rigid backbone. In such an organization, each chromophore contributes coherently to the second-order nonlinear response of the polymer structure.
The synthesis of amphiphilic diblock copolypeptides consisting of poly(a-L-glutamic acid) (PLGA) ... more The synthesis of amphiphilic diblock copolypeptides consisting of poly(a-L-glutamic acid) (PLGA) and poly(g-methyl-L-glutamate-ran-g-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) and their monolayer behavior at the air-water interface have been studied. PLGA-b-PMLGSLG was synthesized via a diblock copolymer precursor consisting of poly(g-tert-butyl-L-glutamate) (PtBuLG) and PMLGSLG blocks, with the tert-butyl group as a mild acid-labile protecting group for the carboxylic acid.
Polypyrrole has been chemically synthesized on thin film nanostructures obtained from comb-shaped... more Polypyrrole has been chemically synthesized on thin film nanostructures obtained from comb-shaped supramolecules of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) hydrogen bonded with pentadecylphenol (PDP). PDP was washed from thin films of cylindrical and lamellar self-assembled combcopolymer systems, which resulted in removal of the upper layers of microdomains, leaving single cylindrical and lamellar layers covering a substrate, with P4VP segregated at the bottom as well as at the free air interface. This P4VP was complexed with Cu 2þ ions, after which chemical oxidation polymerization of pyrrole resulted in a thin polypyrrole layer covering the nanostructured block copolymer. The use of a catalytic amount of bipyrrole greatly improved the quality of the obtained product. The conductivity was measured to be w0.7 S cm À1 .
In situ SAXS is used to investigate the shear-induced alignment of a supramolecular system consis... more In situ SAXS is used to investigate the shear-induced alignment of a supramolecular system consisting of a polyisoprene-b-poly(2-vinylpyridine) diblock copolymer and octyl gallate (OG), where OG is hydrogen-bonded to the vinylpyridine block of the PI-b-P2VP copolymer. Due to microphase separation, the PI-b-P2VP(OG) system self-assembles forming a domain structure of hexagonally ordered P2VP(OG) cylinders embedded in the PI matrix. The results of the alignment showed that the orientation mobility of the cylinders, lying within the (10) planes parallel to the shear plane, is strongly dependent on the size of the cylindrical domains. Subjected to large amplitude oscillatory shear for 15 min a freshly loaded sample with a small grain size structure exhibited much better alignment compared to that of a pre-aligned/squeezed sample with a considerably larger grain size where the orientation had been first partly destroyed by squeezing.
Polycaprolactone (PCL) polyester and segmented aliphatic polyester urethanes based on PCL soft se... more Polycaprolactone (PCL) polyester and segmented aliphatic polyester urethanes based on PCL soft segment have been thoroughly investigated as biodegradable scaffolds for tissue engineering. Although proven beneficial as long term implants, these materials degrade very slowly and are therefore not suitable in applications in which scaffold support is needed for a shorter time. A recently developed class of polyacylurethanes (PAUs) is expected to fulfill such requirements. Our aim was to assess in vitro the degradation of PAUs and evaluate their suitability as temporary scaffold materials to support soft tissue repair. With both a mass loss of 2.5-3.0% and a decrease in molar mass of approx. 35% over a period of 80 days, PAUs were shown to degrade via both bulk and surface erosion mechanisms. Fourier Transform Infra Red (FTIR) spectroscopy was successfully applied to study the extent of PAUs microphase separation during in vitro degradation. The
The number of internal double bonds in poly(vinyl chloride) (PVC) samples was studied as a functi... more The number of internal double bonds in poly(vinyl chloride) (PVC) samples was studied as a function of molecular weight at various monomer conversions. These defect structures were found to exhibit end-group-like characteristics: their concentration per chain was largely constant as a function of molecular weight. This tendency was independent of the degree of conversion. An intramolecular mechanism for formation of unsaturated structures and their location between carbons 5-6 were confirmed via 13 C NMR studies. High-level ab initio calculations showed that a 1-6 hydrogen transfer reaction was the most likely origin for these structures, though a second mechanism involving backbiting of the 1-2 Cl shifted head-to-head radical followed by -chlorine elimination and then transfer to monomer could also contribute at lower conversions. From the experimental analysis and theoretical calculations, it emerged that this backbiting reaction is stereoselective, with the isotactic conformation appearing to be more resistant. However, from the ab initio calculations and earlier results of other research groups it also seems likely that hydrogen abstraction from chloroallylic end groups and further propagation of such radical is a concurrent route to internal double bonds. The evidence collected in this paper point to hydrogen abstraction reactions, especially backbiting and abstraction from chloroallylic end groups, as reactions for which inhibition should have a beneficial effect on the thermal stability of PVC.
Synchrotron radiation was used to investigate the self-assembly in two comb-shaped supramolecules... more Synchrotron radiation was used to investigate the self-assembly in two comb-shaped supramolecules systems consisting of octyl gallate (OG), i.e., 1-octyl-3,4,5-trihydroxybenzoate, hydrogen bonded to the pyridine groups of polyisoprene-block-poly(vinylpyridine) diblock copolymers. In the case of the 1,2-polyisoprene-block-poly(4-vinylpyridine)(OG)x system, self-assembly was only observed for x g0.5, where x denotes the number of OG molecules per pyridine group. For x ) 0.5, 0.75, 1.0, and 1.2 the system self-assembled in the form of hexagonally ordered cylinders of P4VP(OG) throughout the entire temperature range of 25-200°C investigated. For the 1,4-polyisoprene-block-poly(2-vinylpyridine)(OG)x system, on the other hand, a considerably more complex phase behavior was found, including the formation of cubic, hexagonally ordered cylinders and lamellar morphologies. In this case several order-order transitions were observed as a function of temperature, including a lamellar to lamellar transition involving a collapse of the layer thickness. The absence of hydrogen bonding between the octyl gallate molecules and the pyridine groups at elevated temperatures is argued to be a key factor for many of the phenomena observed.
The monomer conversion dependence of the formation of the various types of defect structures in r... more The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both 1H and 13C NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects.
An enzymatic tandem reaction is described in which the enzymes phosphorylase and Deinococcus geot... more An enzymatic tandem reaction is described in which the enzymes phosphorylase and Deinococcus geothermalis glycogen branching enzyme (Dg GBE) catalyze the synthesis of branched polyglucans from glucose-1-phosphate (G-1-P). Phosphorylase consumes G-1-P and polymerizes linear amylose while Dg GBE introduces branching points on the a-(1 ! 6) positions by reshuffling short oligosaccharides. The resulting branched polyglucans have an unusually high degree of branching of 11%.
Cover: The cover picture shows the acyl-enzyme intermediate formed between 2-azetidinone and Cand... more Cover: The cover picture shows the acyl-enzyme intermediate formed between 2-azetidinone and Candida antarctica lipase B derived via docking with subsequent combined QM/MM geometry optimization. The enzymecatalyzed ring-opening polymerization of 2-azetidinone proceeds via this intermediate. It is shown that poly(balanine) can be prepared by enzyme catalysis and that the ring structure of the 2-azetidinone is essential for the formation of the acyl-enzyme intermediate.
A method is presented to prevent microbial adhesion to solid surfaces exploiting the unique prope... more A method is presented to prevent microbial adhesion to solid surfaces exploiting the unique properties of polymer brushes. Polyacrylamide (PAAm) brushes were grown from silicon wafers by atom transfer radical polymerization (ATRP) using a three-step reaction procedure consisting of immobilization of a coupling agent γ-aminopropyltriethoxysilane, anchoring of an ATRP initiator 4-(chloromethyl)benzoyl chloride, and controlled radical polymerization of acrylamide. The surfaces were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, ellipsometry, and contact-angle measurements. The calculated grafting density pointed to the presence of a dense and homogeneous polymer brush. Initial deposition rates, adhesion after 4 h, and detachment of two bacterial strains (Staphylococcus aureus ATCC 12600 and Streptococcus saliVarius GB 24/9) and one yeast strain (Candida albicans GB 1/2) to both PAAm-coated and untreated silicon surfaces were investigated in a parallel plate flow chamber. A high reduction (70-92%) in microbial adhesion to the surface-grafted PAAm brush was observed, as compared with untreated silicon surfaces. Application of the proposed grafting method to silicone rubbers may offer great potential to prevent biomaterials-centered infection of implants.
The monolayer behavior at the air-water interface of a chiral nonlinear optical dye was evaluated... more The monolayer behavior at the air-water interface of a chiral nonlinear optical dye was evaluated. The dye alone, an ester of palmitic acid and 4-nitro-4'-[(3R)-hydroxypyrrolidinol-1,l'-azobis[benzenel, did not form a stable monolayer at 23 *C at the airwater interface. However, mixtures of the dye with amylose acetate formed stable monolayers when the amount of the dye did not exceed 50 mol 9%. The surface structure of monolayers of the dye alone and of the mixtures was studied by transmission electron microscopy. This showed that molecular mixing did not occur. The mixtures are composed of domains of amylose acetate and two-dimensional crystallites of the dye molecules. The domains of the dye behave like H aggregates. It is argued that boundary lines between the crystallites of the dye and amylose acetate might play an important role of the stabilization process. As far as this study goes, the stabilization of the dye in mixed systems is only found for amylose esters as matrices.
The surface potentials and effective dipole moments of R-helical amphiphilic diblock copolypeptid... more The surface potentials and effective dipole moments of R-helical amphiphilic diblock copolypeptides during monolayer compression at the air-water interface are reported. Amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(R-L-glutamic acid) (PLGA) and poly(γ-methyl-L-glutamate-ran-γ-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) of various block lengths were studied during the double-brush formation process at the water surface. Upon monolayer spreading of PLGA-b-PMLGSLGs, surface potentials of hundreds of millivolts were recorded, attributed to the dipole moments of water molecules reorienting due to interactions with the monolayers. Upon compression, the effective dipole moments derived from the surface potentials of the PLGA-b-PMLGSLG monolayers decrease gradually, most likely as a result of the immersion of the hydrophilic block in water and cancellation of the interactions between the hydrophobic block and the underlying water molecules. The polypeptide macrodipole moment immersed in water was apparently effectively screened out. The remaining effective dipole moment of the monolayer contributes mainly to the hydrophobic block, and upon tilting away from the water surface toward the surface normal, it was found to increase with the hydrophobic block length, indicating the gradual formation of unidirectional aligned polypeptide molecules in the double-brush monolayer.
The azimuthal helix orientation of the rigid-rod amphiphilic diblock copolypeptides (PLGA-b-PMLGS... more The azimuthal helix orientation of the rigid-rod amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(R-L-glutamic acid) (PLGA) and poly(γ-methyl-L-glutamate-ran-γ-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) in Langmuir-Blodgett (LB) monolayers was investigated using polarized transmission Fourier transform infrared spectroscopy. The relative position of dipping with respect to the previous transfer position can be used to manipulate the azimuthal orientation of the helices parallel to or tilted by an angle of 45°with respect to the dipping direction in the transferred films. The study of the azimuthal order for the LB monolayers of PLGA-b-PMLGSLGs of various block lengths revealed that the observed effect arises mainly from the deformation of the PMLGSLG top brush layer, induced by the flow orientation around the transfer region. In those cases where the PMLGSLG block is tilted by a sufficiently large angle with respect to the surface normal, high azimuthal order parameters of 0.5-0.75 were obtained.
The effect of annealing on the structure and the helix orientation in Langmuir-Blodgett (LB) mono... more The effect of annealing on the structure and the helix orientation in Langmuir-Blodgett (LB) monolayers of diblock copolymers (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) with unidirectional helix orientation deposited on hydrophilic silicon substrates was characterized by means of small-angle X-ray reflectivity, transmission Fourier transform infrared spectroscopy, and atomic force microscopy. Upon annealing at 100 degrees C for 24 h, the alpha-helices became less tilted toward the substrate surface normal. Surface area shrinkage accompanied the change in tilt, indicated by an increase in both film thickness and electron density, resulting in more compact and uniform films. The enhancement of the helix orientation by thermal annealing was greater for the PMLGSLG block and for the diblock copolymers with the shorter block lengths. For these diblock copolymers, annealing resulted in postorientation of the PMLGSLG block helices almost perpendicular to the substrate surface. This effect originates from a considerable increase in intermolecular packing of the PLGA block caused by hydrogen bonds between the carboxylic groups upon annealing, as well as the high mobility of the PMLGSLG block helices for rearrangement favored by the melted side chain mantle at elevated temperatures.
Journal of Biomedical Materials Research Part A, 2010
Polyacrylamide (PAAm) brushes, covalently grafted from silicon wafer surfaces were examined for t... more Polyacrylamide (PAAm) brushes, covalently grafted from silicon wafer surfaces were examined for their ability to inhibit microbial adhesion after long-term exposure to PBS or reconstituted freeze-dried saliva for time intervals from 48 h up to 1 month at 37 C. Microbial adhesion after exposure was studied in a parallel plate flow chamber. Infrared spectra showed that PAAm brushes exhibit good chemical stability upon incubation in both PBS and reconstituted freeze-dried saliva up to 1 month. Reductions in microbial adhesion on PAAm brushes after exposure to PBS or reconstituted freezedried saliva varied from 63 to 93% depending on the microbial strain considered, even after 1 month of exposure of the brushes to reconstituted freeze-dried saliva. V C 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 94A: 997-1000, 2010.
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Papers by Arend Schouten