End Of Block Research Articles

Experimental, numerical and analytical studies on a novel external prestressing technique for concrete structural components

This paper presents the details of a novel external prestressing technique for strengthening of concrete members. In the proposed technique, transfer of external force is in shear mode on the end block thus creating a complex stress distribution and the required transverse prestressing force is lesser compared to conventional techniques. Steel brackets are provided on either side of the end block for transferring external prestressing force and these are connected to the anchor blocks by expansion type anchor bolts. In order to validate the technique, an experimental investigation has been carried out on post-tensioned end blocks. Performance of the end blocks have been studied for design, cracking and ultimate loads. Slip and slope of steel bracket have been recorded at various stages during the experiment. Finite element analysis has been carried out by simulating the test conditions and the responses have been compared. From the analysis, it has been observed that the computed slope and slip of the steel bracket are in good agreement with the corresponding experimental observations. A simplified analytical model has been proposed to compute load-deformation of the loaded steel bracket with respect to the end block. Yield and ultimate loads have been arrived at based on force/moment equilibrium equations at critical sections. Deformation analysis has been carried out based on the assumption that the ratio of axial deformation to vertical deformation of anchor bolt would follow the same ratio at the corresponding forces such as yield and ultimate. It is observed that the computed forces, slip and slopes are in good agreement with the corresponding experimental observations.

Simulation study of aggregate morphologies formed by ABC linear triblock copolymers in a selective solvent through the self‐consistent field theory

AbstractThe effect of the hydrophobic properties of blocks B and C on the aggregate morphologies formed by ABC linear triblock copolymers in selective solvent was studied through the self‐consistent field theory. Five typical micelles, such as core‐shell‐corona, hamburger‐like, segmented‐wormlike, were obtained by changing the hydrophobic properties of blocks B and C. The simulation results indicate that the shape and size of micelle are basically controlled by the hydrophobic degree of the middle block B, whereas the type of micelle is mainly determined by the hydrophobic degree of the end block C. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 484–492, 2009

Hydrolytic Degradation Behavior of a Renewable Thermoplastic Elastomer

The hydrolytic degradation of polylactide-polymenthide-polylactide triblock copolymers (37 degrees C, pH 7.4) is compared to that of the component homopolymers. In addition to mass loss and water uptake measurements, size exclusion chromatography (SEC), 1H NMR spectroscopy, differential scanning calorimetry (DSC), and mechanical testing were used to monitor property changes during degradation. The rate of copolymer degradation was significantly influenced by the molecular weight of the polylactide end blocks. Mass loss of the polylactide homopolymer and the copolymer samples was observed once a decrease in the total molecular weight of the samples of 20% occurred. 1H NMR spectroscopy and DSC analysis of the copolymers during degradation revealed that the released oligomers contained mostly polylactide. After initial water uptake in which the mechanical properties were compromised to an extent, the Young's modulus and elongation at break of the biorenewable copolymers remained relatively unperturbed for up to 16 weeks, with significant retention of thermoplastic elastomeric properties for up to 21 weeks.

Star-Shaped Cationic Polymers by Atom Transfer Radical Polymerization from β-Cyclodextrin Cores for Nonviral Gene Delivery

Cationic polymers with low cytotoxicity and high transfection efficiency have attracted considerable attention as nonviral carriers for gene delivery. Herein, well-defined and star-shaped CDPD consisting of beta-CD cores and P(DMAEMA) arms, and CDPDPE consisting of CDPD and P(PEGEEMA) end blocks (where CD = cyclodextrin, P(DMAEMA) = poly(2-(dimethylamino)ethyl methacrylate), P(PEGEEMA) = poly(poly(ethylene glycol)ethyl ether methacrylate)) for gene delivery were prepared via atom transfer radical polymerization (ATRP) from the bromoisobutyryl-terminated beta-CD core. The CDPD and CDPDPE exhibit good ability to condense plasmid DNA (pDNA) into 100-200 nm size nanoparticles with positive zeta potentials of 25-40 mV at nitrogen/phosphate (N/P) ratios of 10 or higher. CDPD and CDPDPE exhibit much lower cytotoxicity and higher gene transfection efficiency than high molecular weight P(DMAEMA) homopolymers. A comparison of the transfection efficiencies between CDPD and P(DMAEMA) homopolymer indicates that the unique star-shaped architecture involving the CD core can enhance the gene transfection efficiency. In addition to reducing cytotoxicity, the introduction of a biocompatible P(PEGEEMA) end block to the P(DMAEMA) arms in CDPDPE can further enhance the gene transfection efficiency.

Physical gels of telechelic triblock copolymers with precisely defined junction multiplicity

We study transient networks formed by monodisperse telechelic polypeptides with collagen-like end blocks and a random-coil-like middle block. These artificial proteins are created using recombinant DNA techniques. Upon cooling, the end blocks associate reversibly into triple helices, leading to gels with a well-defined junction multiplicity of three. Both the storage modulus and the relaxation time of the gel increase very strongly as a function of concentration, and decrease with increasing temperature. All the experimental results are described quantitatively by an analytical model, based on classical gel theory, that requires no adjustable parameters, and accounts for the molecular structure of the gel, and the presence of loops and dangling ends.

Styrene−Butadiene Gradient Block Copolymers for Transparent Impact Polystyrene

The mechanical behavior of styrene/butadiene triblock copolymers and their blends with high molecular weight polystyrene (PS) homopolymer was studied. Symmetric triblock copolymers with two PS end blocks of equal length (SBS) were compared with asymmetric triblocks with end blocks of different lengths (S1BS2) and with asymmetric triblocks comprising a styrene/butadiene composition gradient in the middle block (S1GS2). The precise molecular architecture of these copolymers and its role on morphology and properties were described elsewhere (Macromolecules 2007, 40, 2432.). Here, immiscible blends of these copolymers with long PS chains are discussed. While solvent casting yields the expected phase separation, melt extrusion produces tough and transparent blends with metastable submicrometer scale dispersions of soft microdomains highly controlled by processing. The particular molecular architecture of gradient asymmetric triblocks significantly improves ductility of these nanostructured blends. The volume fraction of soft microdomains extracted from dynamic mechanical data is identified as a universal parameter that controls tensile behavior of the different blends studied.

Efficient Synthesis of Unimolecular Polymeric Janus Nanoparticles and Their Unique Self-Assembly Behavior in a Common Solvent

In this paper, we report a facile large-scale synthesis of the “smallest” (i.e., unimolecular) polymeric Janus nanoparticles from a polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO or SVEO) triblock copolymer by efficient intramolecular cross-linking of the middle P2VP block using 1,4-dibromobutane (DBB) in a common solvent, N,N-dimethylformamide (DMF), due to effective steric shielding of PS and PEO end blocks. Size-exclusion chromatography results indicated that intramolecular cross-linking of the middle P2VP block could take place when the polymer concentration was relatively high (20 mg/mL) and/or the DBB-to-2VP molar ratio was high. Dynamic and static light scattering experiments confirmed the unimolecular form for these polymeric Janus nanoparticles. After intramolecular cross-linking, DMF changed from a good to a slightly poor solvent, as evidenced by the negative apparent second Virial coefficient for the unimolecular Janus nanoparticles determined by SLS. As a result, concentration-dependent self-assembly in DMF was observed. At low concentrations (<2.0 mg/mL), the majority of the unimolecular polymeric Janus nanoparticles existed in the unimolecular form. When the concentration gradually increased, the unimolecular polymeric Janus nanoparticles started to aggregate into supermicelles (Rh = 50−100 nm), where PS formed the supercore and PEO formed the corona with cross-linked P2VP nanoparticles in between. The amphiphilic nature of these unimolecular Janus nanoparticles will enable us to study programmable and hierarchical self-assembly of asymmetrically modified polymeric nanoparticles in various solvents.

Synthesis and characterization of poly(lactide-b-siloxane-b-lactide) copolymers as magnetite nanoparticle dispersants

Poly(lactide-b-siloxane-b-lactide) triblock copolymers with pendent carboxylate groups have been synthesized to serve as dispersants for magnetite nanoparticles. Magnetic nanoparticles are of interest for potential biomedical applications including magnetic field-directed drug delivery and magnetic cell separations. For in-vivo applications, it is important that the magnetic particle be coated with biocompatible organic materials to afford dispersion characteristics or to further modify the surfaces of the complexes with biospecific moieties. Synthesis of the triblock copolymer dispersants comprises three reactions. First, difunctional, controlled molecular weight, telechelic, polymethylvinylsiloxane (PMVS) oligomers with 3-aminopropyl endgroups were prepared in ring-opening redistribution reactions. Secondly, these oligomers were utilized as macroinitiators for ring-opening l-lactide or d,l-lactide to provide triblock materials with PMVS central blocks and polylactide end blocks. Both the PMVS oligomers and the poly(lactide-b-siloxane-b-lactide) copolymers had the expected molecular weights and compositions. Thirdly, the vinyl groups on the polysiloxane center were functionalized with carboxylic acids by adding mercaptoacetic acid across the pendent double bonds. At neutral pH, the carboxylate-functional polysiloxane central block binds to the surfaces of magnetite nanoparticles, while the polylactides serve as tail blocks to provide dispersibility in polylactide solvents through interparticle steric repulsive forces. Analyses confirmed that the copolymers indeed adsorbed onto the surfaces of the magnetite nanoparticles, but fractionations of these materials suggested that the composition distribution of the components was somewhat inhomogeneous.

Block polyelectrolytes and colloidal stability

We simulate interactions between charged flat surfaces in the presence of block polymers, where the end blocks carry a charge opposite to the surfaces. Using a recently developed simulation technique, we allow full equilibrium, i.e. the chemical potential of the polyelectrolyte is retained as the separation is changed. In general, the block polyions will adsorb strongly enough to overcharge the surfaces. This results in a double layer repulsion at large separation, with a concomitant free energy barrier. At short separations, the surfaces are pulled together by bridging forces. We make some efforts to theoretically design the polymers to be efficient flocculants, i.e. minimize the free energy barrier and still allow for a long-ranged bridging attraction. Here, we also take into account the possibility of nonequilibrium circumstances, which may be relevant in practice. Our results suggest that short chains, with small charged end blocks and a (relatively speaking) long neutral mid block, are likely to promote rapid flocculation.

Lamellar-in-lamellar structure of binary linear multiblock copolymers

A theoretical description of the lamellar-in-lamellar self-assembly of binary A-b-(B-b-A)(m)-b-B-b-A multiblock copolymers in the strong segregation limit is presented. The essential difference between this binary multiblock system and the previously considered C-b-(B-b-A)(m)-b-B-b-C ternary multiblock copolymer system is discussed. Considering the situation with long end blocks, the free energy of the lamellar-in-lamellar self-assembled state is analyzed as a function of the number k of "thin" internal lamellar domains for different numbers m of repeating (B-b-A) units and different values of the Flory-Huggins chi(AB) interaction parameter. The theoretical predictions are in excellent agreement with the available experimental data.

Fibrillar Constructs from Multilevel Hierarchical Self‐Assembly of Discotic and Calamitic Supramolecular Motifs

AbstractWe here report on polymeric solid‐state self‐assembly leading to organization over six length scales, ranging from the molecular scale up to the macroscopic length scale. We combine several concepts, i.e., rod‐like helical and disc‐like liquid crystallinity, block copolymer self‐assembly, DNA‐like interactions to form an ionic polypeptide–nucleotide complex and packing frustration to construct mesoscale fibrils. Ionic complexation of anionic deoxyguanosine monophosphate (dGMP) and triblock coil–rod–coil copolypeptides is used with cationic end blocks and a helical rod‐like midblock. The guanines undergo Hoogsteen pairing to form supramolecular discs, they π‐stack into columns that self‐assemble into hexagonal arrays that are controlled by the end blocks. Packing frustration between the helical rods from the block copolymer midblock and the discotic motif limits the lateral growth of the assembly thus affording mesoscale fibrils, which in turn, form an open fibrillar network. The concepts suggest new rational methodologies to construct structures on multiple length scales in order to tune polymer properties.

Synthesis and Properties of New Thermoplastic Elastomers Containing Poly[4-(1-adamantyl)styrene] Hard Segments

The anionic polymerization of 4-(1-adamantyl)styrene (AdS) was carried out with s-BuLi in cyclohexane at 50 °C. The resulting poly(AdS) had predicted molecular weights, narrow molecular weight distributions, and high glass transition temperature (Tg) around 232 °C. A series of well-defined ABA type triblock copolymers, where B midblock stood for 1,4-polyisoprene and A end blocks for poly(AdS) (A), poly(AdS-ran-styrene) (AS), and polystyrene (S), were successfully synthesized by the sequential copolymerization with s-BuLi in cyclohexane. The 1,4-polyisoprene midblocks of triblock copolymers were completely converted into the saturated poly(ethylene-alt-propylene) (EP) blocks via hydrogenation with p-toluenesulfonhydrazide. Novel tailored thermoplastic elastomers, A−EP−A and AS−EP−AS, carrying bulky adamantyl groups in the hard end blocks were analyzed by dynamic mechanical analysis, stress relaxation test, tensile test, and transmission electron microscopy. These triblock copolymers were phase separated, and showed the high tensile strength (22−24 MPa), high elongation at break (590−660%), and high upper service temperature (175−210 °C). The heat resistant properties of A−EP−A and AS−EP−AS were drastically enhanced by substitution of the outer S segments in S−EP−S with either A or AS segments.

A new photoiniferter/RAFT agent for ambient temperature rapid and well‐controlled radical polymerization

AbstractPhenacyl morpholine‐4‐dithiocarbamate is synthesized and characterized. Its capability to act as both a photoiniferter and reversible addition fragmentation chain transfer agent for the polymerization of styrene is examined. Polymerization carried out in bulk under ultra violet irradiation at above 300 nm at room temperature shows controlled free radical polymerization characteristics up to 50% conversions and produces well‐defined polymers with molecular weights close to those predicted from theory and relatively narrow poyldispersities (Mw/Mn ∼ 1.30). End group determination and block copolymerization with methyl acrylate suggest that morpholino dithiocarbamate groups were attained at the end of the polymer. Photolysis and polymerization studies revealed that polymerization proceeds via both reversible termination and RAFT mechanisms. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3387–3395, 2008

New target materials for innovative applications on glass

In the new emerging markets of flat panel display, photovoltaic and optical coating applications, the introduction of cylindrical rotating magnetron technology can accommodate the needs for faster, better and cheaper coating processes. Recent developments of hardware (compact end blocks, etc.) and target materials for rotatable magnetron technology offer a total solution to the innovative thin film applications.

Impact of Solvent Quality on the Density Profiles of Looped Triblock Copolymer Brushes by Neutron Reflectivity Measurements

Preferential adsorption of poly(2-vinylpyridine)-deuterated polystyrene-poly(2-vinylpyridine) (PVP-dPS-PVP) triblock copolymers from toluene onto silicon leads to the formation of dPS loops tethered by the PVP end blocks. Using neutron reflectometry, we have determined the segment density profiles of these looped polymer brushes in toluene, a good solvent for the dPS block, and in cyclohexane at 20 C (poor solvent), 32 C, (near- solvent), and 50 C (marginal solvent). While the swelling behavior qualitatively agrees with that observed for singly grafted brushes, there are interesting differences in the local structural details: In a good solvent, the segment density profiles are composed of an inner parabolic region and a long, extended tail. In cyclohexane, the profiles are described by exponential decays. We ascribe these features to a novel polydispersity effect that arises due to tethering the PS loops by both ends. The results also show that the less dense layers undergo more significant changes in swollen height as solvent quality is changed and that the looped brushes of different molecular weight, asymmetry, and tethering density adhere to scaling relationships derived for lightly cross-linked polymer gels.

Morphology and phase diagram ofABClinear triblock copolymers: Parallel real-space self-consistent-field-theory simulation

A parallel algorithm designed for widely used distributed computer clusters is developed for the real-space self-consistent-field theory for polymers. We adopt an efficient data partition method to ensure high performance of this parallel algorithm on clusters, which enables us to explore unknown phase structures of complex block polymers with high accuracy and efficiency. As a benchmark test, the algorithm is applied to an ABC linear triblock copolymer, in which the volume fractions of the two end blocks of the polymer chain are equal (f_(A)=f_(C)) , to simplify the discussion. The three-dimensional microphases and phase diagram of this triblock copolymer are investigated in detail.

Shear Effects on the Loop Bridge Ratio of Middle Block Chains in Sphere-Forming ABA Triblock Copolymers Examined by Simple Elongation Measurements

Anisotropy of the bridge fraction of middle isoprene block chains in the directions parallel and perpendicular to the shear flow direction in sheared films of sphere-forming polystyrene-b-polyisoprene-b-polystyrene triblock copolymers are examined by breaking stress under simple elongation measurements at a fast draw rate, which reflect the bridge fraction. There was no practical difference between the data obtained in two directions at tested shear rate region (5-15 sec-1). The absolute values are close to the values for non-sheared samples. Thus, we conclude that when flow is applied to sphere-forming triblocks, only small portion of end blocks are pulled out to flow and there is no orientation dependent anisotropy of bridge fractions.

A study on the impact behavior of adhesively-bonded composite materials

In this paper, a unidirectional carbon-fiber composite is both experimentally and numerically investigated to study the nonlinear material behavior of impacted double cantilever beam (DCB) specimens. For the impact analysis, the load and the displacement applied from pin onto end block as well as the crack energy release rate are measured and compared with the finite element analysis results. The energy release rate is a critical measure of the resistance to crack propagation, which can be estimated by the force and displacement at the crack tip. It is found that the energy release rate measured from impact tests on the specimens is well predicted by the finite element model suggested in this study.

Amphiphilic Poly[(propylene glycol)‐block‐(2‐methyl‐2‐oxazoline)] Copolymers for Gene Transfer in Skeletal Muscle

Amphiphilic triblock copolymers such as poly(ethylene glycol-b-propylene glycol-b-ethylene glycol) PE6400 (PEG(13)-PPG(30)-PEG(13)) have been recently shown to promote gene transfer in muscle. Herein we investigated the effect of a chemical change of the PEG moiety on the transfection activity of these compounds. We synthesized new amphiphilic copolymers in which the PEG end blocks are replaced by more hydrophilic poly(2-methyl-2-oxazoline) (PMeOxz) chains of various lengths. The resulting triblock PMeOxz-PPG-PMeOxz compounds were characterized by NMR, SEC, TGA, and DSC techniques and assayed for in vivo muscle gene transfer. The results confirm both the block structure and the monomer unit composition (DP(PG)/DP(MeOxz)) of the new PPG(34)-PMeOxz(41) and PPG(34)-PMeOxz(21) triblock copolymers. Furthermore, in vivo experiments show that these copolymers are able to significantly increase DNA transfection efficiency, despite the fact that their chemical nature and hydrophilic character are different from the poloxamers. Overall, these results show that the capacity to enhance DNA transfection in skeletal muscle is not restricted to PEG-PPG-PEG arrangements.

Aggregate Morphologies of Amphiphilic Graft Copolymers in Dilute Solution Studied by Self-Consistent Field Theory

Microstructures assembled by amphiphilic graft copolymers in a selective solvent (poor for the backbone chain and good for graft chains or poor for graft chains and good for the backbone chain) were investigated on the basis of a real-space algorithm of self-consistent field theory in two-dimensions. Circle-like micelles, line-like micelles, large compound micelles, and vesicles are obtained by tailoring the architectural parameters and interaction parameter between the graft blocks and solvents. The aggregate morphology stability regions of graft copolymers as functions of the position of first graft point and the number of branches are constructed. It is found that the architectural parameters play a remarkable role in the complex microstructure formation. The interaction between the graft blocks and solvents is also shown to exert an effect on the morphology stability regions. The distributions of the free end and inner blocks of the backbone are found to be different in various aggregate structures. For the circle-like micelles assembled by graft copolymers with a hydrophobic backbone and vesicles assembled by graft copolymers with a hydrophilic backbone, the free end and inner blocks segregate and localize in different parts of the aggregates depending on their length. However, with respect to the large compound micelles and vesicles assembled by graft copolymers with a hydrophobic backbone, the free end and inner blocks uniformly mix in the clusters.