Characterization of the structure and the size distribution of branched polymers formed by co-polymerization of MMA and EGDMA

Free radical co-polymerization of methyl methacrylate (MMA) and ethyl glycol dimethyl methacrylate (EGDMA) in solution leads to the formation of polydisperse branched PMMA which grows in size until the system gels. The structure and the size distribution of the PMMA aggregates were characterized at infinite dilution using static and dynamic light scattering and size exclusion chromatography (SEC). The reaction extent was measured using SEC and Raman spectroscopy. The results show that the structure and size distribution of PMMA aggregates formed close to the gel point are compatible with those of percolating clusters. The structure factor of semi-dilute solutions of PMMA aggregates is the same as that of dilute solutions at distance scales much smaller than the correlation length of the concentration fluctuations (). However, the cut-off function of the pair correlation function at for semi-dilute solutions is more gradual than the cut-off function at for dilute solutions. Received 11 May 1998 and Received in final form 22 October 1998     

Distribution of molar mass and branching index of natural rubber from hevea brasiliensis trees of different age by size exclusion chromatography coupled with online viscometry

Natural rubber from hevea brasiliensis trees (Thailand, RRIM 600 clone) of different age (8, 20, and 35 years) were characterized by size exclusion chromatography (SEC) coupled with online viscometry with respect to their distribution of molar mass and branching index at a temperature of 70°C using cyclohexane as solvent. The natural rubber samples were purified by washing with an aqueous solution of sodium dodecylsulfate and subsequent saponification. With this procedure physical branching points caused by phospholipids, proteins and hydrophobic terminal units, mainly fatty acids, of the natural rubber (cis-1,4-polyisoprene) molecule, could be removed leading to completely soluble polymer samples. All samples investigated possess a very broad (10 to 50,000 kg/mol) and distinct bimodal molar mass distribution. With increasing age of the trees the peak area in the low molar mass region decreases favoring the peak area in the high molar mass region. By plotting the branching index as a function of the molar mass it could be shown that branching of the natural rubber increases with both, the molar mass and the age of the trees.     

ChemInform Abstract: Electronic Structure Calculations for the Ternary Intermetallic Compounds A2MX and AMX2 (A: Li; M: Rh,Pd, Ir,Pt; X: Al,Ga, In) Using Density‐Functional Theory.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Influence of crosslinking conditions on the phase behavior of a polyacrylate-based liquid-crystalline elastomer

The influence of crosslinking conditions (γ-radiation-induced and chemical crosslinking above and below the clearing point) on the phase behavior of a liquid-crystalline polyacrylate-based elastomer is shown and discussed. The network formation in samples γ-irradiated in the nematic phase results in an increase of the clearing point T_n-i which does not depend on the crosslinking density expressed as the ratio of weight-average molecular weight over number-average molecular weight between crosslinks M _w/M _c within a wide M _w/M _c range. The distorting action of the crosslinking agent at the same temperature leads to the opposite tendency in the change of T_n-i. Crosslinking in the isotropic phase shows a strong decrease in T_n-i.