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Entanglement Molecular Weight (entanglement + molecular_weight)

Distribution by Scientific Domains

Polymers and Materials Science	100%

Selected Abstracts

Effect of Chain Straightening on Plateau Modulus and Entanglement Molecular Weight of Ni-diimine Poly(1-hexene)s

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 11 2006
Zhibin Ye
Abstract Summary: In this communication, we report the first rheological study on the chain-straightened Ni-diimine poly(1-hexene)s and investigate the unique effect of chain straightening on plateau modulus and entanglement molecular weight of this series of polymers. Two Ni-diimine poly(1-hexene) samples having different levels of chain straightening were prepared with a chain-walking Ni-diimine catalyst, (ArNC(An)C(An)NAr)NiBr2 (An,=,acenaphthene, Ar,=,2,6-(i -Pr)2C6H3) at two different temperatures. Rheological analyses show that the chain-straightened polymers exhibit significantly enhanced plateau modulus and reduced entanglement molecular weight compared to regular poly(1-hexene)s by metallocene catalysis. Such an effect becomes more pronounced with an increase in the level of chain straightening. Loss moduli G,(,) versus reduced angular frequency in a linear, natural logarithm plot for the three polymers at the reference temperature of 100,°C. [source]

Bimolecular radical termination: New perspectives and insights

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2008
Geoffrey Johnston-Hall
Abstract The reversible addition-fragmentation chain transfer-chain length dependent termination (RAFT-CLD-T) method has allowed us to answer a number of fundamental questions regarding the mechanism of diffusion-controlled bimolecular termination in free-radical polymerization (FRP). We carried out RAFT-mediated polymerizations of methyl acrylate (MA) in the presence of a star matrix to develop an understanding of the effect of polymer matrix architecture on the termination of linear polyMA radicals and compared this to polystyrene, polymethyl methacrylate, and polyvinyl acetate systems. It was found that the matrix architecture had little or no influence on termination in the dilute regime. However, due to the smaller hydrodynamic volumes of the stars in solution compared to linear polymer of the same molecular weight, the gel onset point occurred at greater conversions, and supported the postulate that chain overlap (or c*) is the main cause for the observed autoacceleration observed in FRP. Other theories based on "short,long" termination or free-volume should be disregarded. Additionally, since our systems are well below the entanglement molecular weight, entanglements should also be disregarded as the cause of the gel onset. The semidilute regime occurs over a small conversion range and is difficult to quantify. However, we obtain accurate dependencies for termination in the concentrated regime, and observed that the star polymers (through the tethering of the arms) provided constriction points in the matrix that significantly slow the diffusion of linear polymeric radicals. Although, this could at first sight be postulated to be due to reptation, the dependencies showed that reptation could be considered only at very high conversions (close to the glass transition regime). In general, we find from our data that the polymer matrix is much more mobile than what is expected if reptation were to dominate. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3155,3173, 2008 [source]

Effect of the molecular weight and architecture on the size and glass transition of arborescent polyisobutylenes

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 5 2006
Judit E. Puskas
Abstract This article discusses the characterization of arborescent (hyperbranched) polyisobutylenes (arb-PIBs) by size exclusion chromatography and differential scanning calorimetry, in comparison with linear PIB standards. The radius of gyration (,r,1/2 = Rz), measured from the angle dependence of light scattering of high-molecular-weight arb-PIBs, was significantly larger than the hydrodynamic radius (Rh) from size exclusion chromatography/viscometry, and the Rh values were significantly smaller than Rh of linear PIBs. The glass-transition temperature of arb-PIBs having a branch molecular weight higher than the critical entanglement molecular weight was dependent on both the total number-average molecular weight and BR up to BR , 15. A modified Fox,Flory equation is proposed to describe the effect of architecture on the thermal transition. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1770,1776, 2006 [source]