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End Blocks (end + block)

Distribution by Scientific Domains

Polymers and Materials Science	75%

Selected Abstracts

Innovation in material science: The chameleon block copolymer

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2006
Judit E. Puskas
This note to the editor is the first journal publication of the chameleon thermoplastic elastomer with entropy-driven phase separation. We have discovered that block copolymers with a high-molecular-weight, dendritic (arborescent) polyisobutylene core and poly(para -methylstyrene) end blocks can manifest themselves either as rubber or as plastic, depending on their environment. The behavior is thermally irreversible. This material represents a new concept in material science: the entropy-driven thermoplastic elastomer. [source]

Formation of Two Kinds of Hexagonally Arranged Structures in ABC Triblock Copolymer Thin Films Induced by a Strongly Selective Solvent Vapor

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 22 2009
Chunxia Luo
Abstract An order,order transition (OOT) in the sequence of a hexagonally arranged core,shell cylinder to a double-hexagonally arranged dot in polystyrene- block -poly(butadiene)- block -poly(2-vinylpyridine) (SBV) triblock copolymer thin films is reported to be induced upon exposure to a solvent vapor that is strongly selective for the two end blocks. These two kinds of hexagonally arranged structures could form when the film thickness is 44, 123, and 223,nm. When the film thickness is decreased to 13,nm, the ordered structure is absent. The sizes of the core,shell cylinder structures formed with the same annealing time in films of different thickness are compared to address the effects of film thickness on the phase structure. The mechanism is analyzed from the total surface area of the blocks and the effective interaction parameter in the solvent vapor. [source]

Biomacromolecular engineering: design, synthesis and characterization.

POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 9-10 2006
One-pot synthesis of block copolymers of arborescent polyisobutylene, polystyrene
Abstract Novel arborescent block copolymers comprising of an arborescent rubbery polyisobutylene (PIB) midsegment and glassy polystyrene (PSt) end blocks were prepared by sequential addition of monomers. Synthesis was conducted by the use of 4-(2-methoxyisopropyl) styrene as an inimer-type initiator in conjunction with titanium tetrachloride (TiCl4) in 60:40 (v/v) methylcyclohexane/methyl chloride solvent mixture. Isobutylene was polymerized for 1,2,hr and then prechilled styrene in the same solvent mixture was sequentially added with select additives to the reaction flask. The recovered block copolymers were purified and then characterized by 1H-NMR, size exclusion chromatograph (SEC), tensile test, atomic force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Samples with 16.4,32.8,wt% PSt and Mn,=,47,600,125,900,g/mol (Mw/Mn,=,2.02,4.45) displayed thermoplastic elastomeric properties with 5,9.2,MPa tensile strength and 490,920% elongation. The arborescent block copolymers showed surface morphologies ranging from spherical to cylindrical/lamellar nanometer-sized discreet PSt phases dispersed in a continuous PIB matrix, with a 10,nm PIB layer on the surface. Drug release profiles of paclitaxel from two arborescent blocks were found to be similar to that measured from Translute®. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Supramolecular assembly of collagen triblock peptides

BIOPOLYMERS, Issue 4 2003
Raquel Martin
Abstract The relationship between primary sequence and collagen triple-helix formation is relatively well characterized, while higher levels of structural assembly from these sequences is poorly understood. To address this gap, a new collagen-like triblock peptide design was used to study the relationship between amino acid sequence and supramolecular assembly. Four collagen-like peptides with the sequence (Glu)5(Gly,Xaa,Hyp,Gly,Pro,Hyp)6(Glu)5 and corresponding to Xaa = alanine, proline, serine, or valine, and an analogous peptide without the glutamic acid end blocks, were solubilized in water at high concentrations (20,150 mg/mL) and analyzed in optical polarizing microscopy and transmission electron microscopy. Some of the peptides self-assembled into supramolecular structures, the nature of which was determined by the core collagen-like sequence. The globular end blocks appeared necessary for these short triple-helix-forming peptides to spontaneously organize into supramolecular structures in solution and also provided enhanced thermal stability based on CD analysis. The results indicate a strong dependence of the peptide triblock assembly behavior on the identity of the guest residue Xaa; nematic order when Xaa was valine, no organization when Xaa was serine, and banded spherulites displaying a cholesteric-like twist when Xaa was proline or alanine. According to these results, the identity of the amino acid in position Xaa of the triplet Gly,Xaa,Yaa dramatically determined the type of supramolecular assembly formed by short triple helices based on collagen-triblock like sequences. Moreover, the structural organization observed for these collagen-triblock peptides was analogous to some assemblies observed for native collagen in vivo and in vitro. The amino acid sequence in the native collagen proteins may therefore be a direct determinant of the different supramolecular architectures found in connective tissues. © 2003 Wiley Periodicals, Inc. Biopolymers 70:435,444, 2003 [source]