| Abstract
| - Small-angle neutron scattering (SANS) has been used to measure the chain dimensions ofsyndiotactic polypropylene (s-PP) in the melt, using mixtures of 1H- and 2D-labeled molecules. This leadsto a segment length 7.6 Å normalized to a four-carbon repeat unit, which is in excellent agreement witha prediction based on a correlation of chain conformation and the plateau modulus obtained from rheology.The s-PP segment length is substantially higher than the values of 6.2 Å previously obtained for isotacticpolypropylene (i-PP) in the melt and 5.6 Å in a low-molecular-weight “polymeric solvent”. We have alsostudied its effect on the thermodynamics of polyolefin blends by investigating the miscibility of s-PPwith a range of atactic poly(ethylene/ethylethyelene) (PEEx) random copolymers, with x% ethylethylene,and comparing this with i-PP. The miscibility “window” of i-PP with PEEx ranges from x = 63−96%ethylethylene, while that of s-PP is shifted to x = 53−73%, in qualitative agreement with the conceptthat conformational symmetry matching favors miscibility. We have also performed a complementarymeasurement of the melt coil dimensions of s-PP, using a low-molecular-weight PEEx “polymeric solvent”similar to that previously employed for i-PP. A fit of the s-PP/PEE71 blend scattering yielded a segmentlength of 8.1 Å, supporting the previous finding that the coil dimensions of s-PP are substantially higherthan i-PP. These results emphasize the important influence of tacticity on the chain dimensions andthermodynamic properties in polyolefin systems.
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