Search

We provide unambiguous experimental evidence that ring polymers are stronger depleting agents in colloidal suspensions than their linear counterparts. We use an intermediate volume fraction (ϕ_{c}=0.44) colloidal gel based on the classic poly(methyl methacrylate) (PMMA) hard spheres, in which the polystyrene depletant is either linear or ring of the same molar mass or the same size. We systematically increase the depletant concentration from zero (no attraction) to well above the gelation point and find that in the presence of rings, gels are formed at smaller concentrations and possess a larger storage modulus in comparison to those induced by the linear chains. Consequently, the yield stress is enhanced; however, the yield strain (gel deformability) remains concomitantly unaffected. Our experimental findings are in agreement with theoretical calculations based on effective interaction potentials. Hence, polymer architecture is a powerful entropic tool to tailor the strength of colloidal gels.

A multiblock [poly(ethylene oxide)-b-spiro-polystyrene] ([(PEO-b-spiro-PS)]) copolymer with a topologically novel architecture was synthesized using thiol-ene step-growth polymerization reaction. Spiro-PS with dimercapto groups as the hard segment was synthesized in three main steps: (a) preparation of tetra-arm PS by atom transfer radical polymerization and the conversion of the chain-end group to azide functionality, (b) alkyne-azide click coupling reaction to synthesize a tricyclic PS, and (c) tactical ring opening of the tricyclic PS through disulfide/thiol redox reaction. The PEO soft segment was obtained as chain-ends modified with norbornene groups. Finally, the hydrothiolation of the highly reactive norbornene chain-ends of polyethylene glycol with the dimercapto groups of spiro-PS produced the multiblock ([(PEOb-spiro-PS)]) copolymer in quantitative yield. The multiblock copolymer was characterized using size-exclusion chromatography, proton nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. [ABSTRACT FROM AUTHOR]

The critical adsorption point (CAP) of linear and star-shaped polymers was investigated by normal phase and reversed phase liquid chromatography (NPLC and RPLC) and computer simulation. Three sets of polystyrenes (PS) differing in chain architecture and chemically distinct groups were prepared: linear PS (sec-butyl and hydrogen end group), 2-arm PS (linear, two sec-butyl end groups and one silyl group in the middle of the chain) and 4-arm star-shaped PS (four sec-butyl end groups and one silyl group in the center of the star). It was found that the column temperature at CAP, TCAP (linear PS) = TCAP (2-arm PS) > TCAP (4-arm PS) in both RPLC and NPLC which can be attributed to the variation in chain architecture. However, the elution times at CAP of three polymers are all different: In NPLC, tE,CAP (linear) > tE,CAP (2-arm PS) > tE,CAP (4-arm PS) while in RPLC, tE,CAP (4-arm PS) > tE,CAP (2-arm PS) > tE,CAP (linear). The variation of tE,CAP can be explained by the contribution of the chemically distinct groups. The computer simulation results are in good agreement with the chromatography experiments results and support the interpretation of experimental data. [ABSTRACT FROM AUTHOR]

This document defines terms relating to the properties of individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers. In the section on polymer solutions and amorphous bulk polymers, general and thermodynamic terms, dilute solutions, phase behaviour, transport properties, scattering methods, and separation methods are considered. The recommendations are a revision and expansion of the IUPAC terminology published in 1989 dealing with individual macromolecules, macromolecular assemblies, and dilute polymer solutions. New terms covering the principal theoretical and experimental developments that have occurred over the intervening years have been introduced. Polyelectrolytes are not included. [ABSTRACT FROM AUTHOR]

Poly(lactic-co-glycolic acid) (PLGA) were grafted to both ends of Pluronic® F68 ((EO)75(PO)30(EO)75) triblock copolymer to produce poly {(lactic acid)m-co-(glycolic acid)n}-b-poly(ethylene oxide)75-b-poly(propylene oxide)30-b-poly(ethylene oxide)75-b-poly {(lactic acid)m-co-(glycolic acid)n} (PLGA-F68-PLGA) pentablock copolymers. Molecular weights of PLGA blocks were controlled and five kinds of pentablock copolymers with different PLGA block lengths were synthesized using in-situ ring-opening polymerization of D,L-lactide and glycolide with tin(II) 2-ethylhexanoate (Sn(Oct)2) catalyst. PLGA-F68-PLGA pentablock copolymers were characterized by ¹H- and 13C-NMR, GPC, and TGA. The numbers (2m, 2n) of repeating units for lactic acid and glycolic acid inside PLGA segments were obtained as (48,17), (90,23), (125,40), (180,59), and (246,64), with 7#185;H-NMR measurement. From NMR data, the resultant molecular weights were determined in the range of 12,700-29,700, which were similar to those obtained from GPC. Polydispersity index was increased in the range of 1.32-1.91 as the content of PLGA blocks increased. TG and DTG thermograms showed discrete degradation traces for PLGA and F68 blocks, which indicate the weight fractions of PLGA blocks in pentablock copolymers can be calculated by TG profile and it is possible to remove PLGA block selectively. Hydrodynamic radius and radius of gyration of pentablock copolymer micelle were obtained in the range of 46-68 nm and 31-49 nm, respectively, in very dilute (i.e. 0.005 wt %) aqueous solution of THF:H2O = 10:90 by volume at 25 °C. [ABSTRACT FROM AUTHOR]

Grazing-incidence X-ray scattering (GIXS) formulas for hexagonally perforated layer (HPL) structures with ABC and AB stacking sequences were derived, and used in the quantitative analysis of the two-dimensional GIXS patterns of polystyrene-b-polyisoprene (PS-b-Pt) diblock copolymer thin films supported on silicon substrates. This quantitative analysis provided detailed information (shape, size and size distribution, packing order, layer packing sequence, and orientation) about the HPL structure of the diblock copolymer films that cannot be easily obtained with conventional X-ray and neutron scattering techniques or with conventional microscopic methods. [ABSTRACT FROM AUTHOR]

The miscibility and the mechanism for thermal nanopore templating in films prepared fromspin-coating and subsequent drying of homogenous solutions of curable polymethylsilsesquioxanedielectric precursor and thermally labile, reactive triethoxysilyl-terminated four-armedpoly(ɛ-caprolactone) porogen were investigated in detail by in situ two-dimensionalgrazing incidence small-angle x-ray scattering analysis. The dielectric precursor andporogen components in the film were fully miscible. On heating, limited aggregationsof the porogen, however, took place in only a small temperature range of100–140 C as a result of phase separation induced by the competition of the curing andhybridization reactions of the dielectric precursor and porogen; higher porogenloading resulted in relatively large porogen aggregates and a greater sizedistribution. The developed porogen aggregates underwent thermal firing above300 C without further growth and movement, and ultimately left their individual footprints in thefilm as spherical nanopores. [ABSTRACT FROM AUTHOR]

Temperature programming has been used increasingly in liquid chromatography in recent years. In particular, temperature gradient elution has shown great potential in the analysis of complex polymers. In this study, the polymer retention behavior in temperature gradient interaction chromatography is investigated based on thermodynamic consideration of the retention factor. The polymer retention predicted by the model calculation is in good agreement with the experimental results, and the model allows devising a temperature program for designed retention behaviors such as a linear dependence of retention volume on log(molecular weight) of polymers. In addition, the migration behavior of polymeric solute along the separation column can be simulated, which shows strong molecular weight dependence. The migration behavior is also confirmed experimentally by employing different length columns or delayed injection. [ABSTRACT FROM AUTHOR]

Presents the characterization of poly(L-lactide)-block-poly-(ethylene oxide)-block-poly(L-lactide) triblock copolymers by liquid chromatography. Retention of liquid chromatography at the critical condition; Separation of mechanism of block copolymer.

The mechanism of thermal pore generation in organosilicate thin films loaded with a six-armed star-shaped poly(ε-caprolactone) porogen was quantitatively investigated by using in-situ grazing-incidence small-angle X-ray scattering and thermogravimetry. These analyses found that the blend components have a limited miscibility that depends on the composition; for porogen loadings up to only 20 wt%, molecularly miscible blend films were obtained. Even for the miscible blend films, heating the films produced a curing reaction of the precursor matrix component, leading to the phase separation of the porogen component. This phase separation was found to begin at 393 K for 10 wt% porogen loaded films and at 373 K for 20 wt% porogen loaded films, and to continue for temperatures up to 423 K. The porogen aggregates remained and were confined within the matrix film without any further growth or movement until complete thermal decomposition above 564 K. [ABSTRACT FROM AUTHOR]

Discusses the single-solvent liquid chromatographic critical condition for polyisoprene. Absence of the chromatographic invisibility of polystyrene-polyisoprene diblock copolymers under the critical condition; Effect of the length of the block copolymers on the chromatographic retention of the block.

The article offers information on a newly found epitaxial phase transition path between the hexagonally packed cylinder (HEX) and double gyroid (DG) structure, studied in block copolymers or in micelles. Thin film on a silicon wafer substrate was used, in which {121}dg planes orient parallel to the substrate plane after thermal annealing without applying any external field. A three dimensional transmission electron microphotography image of the transitional structure was obtained.

The small, well-characterized hammerhead ribozyme has served as a useful model for understanding the catalytic activity of RNA for the last two decades. A wealth of biochemical data was brought into focus by a series of X-ray crystallographic structures, and a full picture of this remarkable molecule is still evolving as new information is discovered. Because the exact location of metals in solution can be difficult to identify with certainty, particularly the biologically ubiquitous magnesium, the full scope of metal binding to RNA is not yet defined.

We show for the first time that entangled polymeric liquids containing long-chain branching can exhibit strain hardening upon startup shear. As the significant long-chain branching impedes chain disentanglement, Gaussian coils between entanglements can deform to reach the finite extensibility limit where the intrachain retraction force exceeds the value expected from the usual conformational entropy loss evaluated based on Gaussian chain statistics. The phenomenon is expected to lead to further theoretical understanding. [ABSTRACT FROM AUTHOR]

Temperature gradient interaction chromatography (TGIC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) were applied for the characterization of highly branched polystyrenes (PS) prepared by linking living polystyryl anions using 4-chlorodimethylsilylstyrene. Reversed-phase (RP)-TGIC showed an unexpectedly high resolution according to the number of branches despite significant overlap of the molecular weight as confirmed by MALDI-TOF MS. The enhancement of the resolution is ascribed to the contribution of the nonpolar groups in the branched PS: the dimethylsilyl groups in the branching unit as well as the sec-butyl initiator groups. As the number of branches increases, the number of nonpolar groups increases, which in turn increases the RP-TGIC retention synergistically with increasing molecular weight. In contrast, a poorer resolution was found in normal-phase-TGIC, in which the nonpolar groups reduce the retention. The resolution in RP-TGIC appears superior to that of liquid chromatography at the chromatographic critical condition (LCCC) of PS. It is seemingly due to the synergistic contribution of the incremental PS molecular weight to the functionality in the branched PS in RP-TGIC while only the functionality contributes to the separation in LCCC. This type of resolution enhancement could be utilized efficiently for the analysis of highly branched polymers such as dendrimers or hyperbranched polymers. [ABSTRACT FROM AUTHOR]