2 research outputs found
Terminal and Internal Unsaturations in Poly(ethylene-<i>co</i>-1-octene)
Unsaturated structures in polyolefin
polymers are important in
many respects. In this work, new vinyl and vinylidene structures were
identified in polyÂ(ethylene-<i>co</i>-1-octene) copolymers.
The combination of careful sample selection and model compounds provided
clear evidence for the assignment of these structures. More importantly,
a new method was developed to differentiate and quantify for the first
time terminal and internal unsaturations in ethylene-<i>co</i>-1-octene copolymers. The method described here will be generally
applicable to many different polyolefins
Structure and Properties of Aqueous Methylcellulose Gels by Small-Angle Neutron Scattering
Cold, semidilute, aqueous solutions of methylcellulose
(MC) are
known to undergo thermoreversible gelation when warmed. This study
focuses on two MC materials with much different gelation performance
(gel temperature and hot gel modulus) even though they have similar
metrics of their coarse-grained chemical structure (degree-of-methylether
substitution and molecular weight distribution). Small-angle neutron
scattering (SANS) experiments were conducted to probe the structure
of the aqueous MC materials at pre- and postgel temperatures. One
material (MC1, higher gel temperature) exhibited a single <i>almost</i> temperature-insensitive gel characteristic length
scale (ζ<sub>c</sub> = 1090 ± 50 Å) at postgelation
temperatures. This length scale is thought to be the gel blob size
between network junctions. It also coincides with the length scale
between entanglement sites measured with rheology studies at pregel
temperatures. The other material (MC2, lower gel temperature) exhibited
two distinct length scales at all temperatures. The larger length
scale decreased as temperature increased. Its value (ζ<sub>c1</sub> = 1046 ± 19 Å) at the lowest pregel temperature was indistinguishable
from that measured for MC1, and reached a limiting value (ζ<sub>c1</sub> = 450 ± 19 Å) at high temperature. The smaller
length scale (ζ<sub>c2</sub> = 120 to 240 Å) increased
slightly as temperature increased, but remained on the order of the
chain persistence length (130 Ã…) measured at pregel temperatures.
The smaller blob size (ζ<sub>c1</sub>) of MC2 suggests a higher
bond energy or a stiffer connectivity between network junctions. Moreover,
the number density of these blobs, at the same reduced temperature
with respect to the gel temperature, is orders of magnitude higher
for the MC2 gels. Presumably, the smaller gel length scale and higher
number density lead to higher hot gel modulus for the low gel temperature
material