227 research outputs found
Consequences of Grafting Density on the Linear Viscoelastic Behavior of Graft Polymers
The linear viscoelastic behavior of poly(norbornene)-graft-poly(±-lactide) was investigated as a function of grafting density and overall molar mass. Eight sets of polymers with grafting densities ranging from 0 to 100% were synthesized by living ring-opening metathesis copolymerization. Within each set, the graft chain molar mass and spacing between grafts were fixed, while the total backbone length was varied. Dynamic master curves reveal that these polymers display Rouse and reptation dynamics with a sharp transition in the zero-shear viscosity data, demonstrating that grafting density strongly impacts the entanglement molar mass. The entanglement modulus (G_e) scales with inverse grafting density (n_g) as G_e ∼ n_g^(1.2) and G_e ∼ n_g^0 in accordance with scaling theory in the high and low grafting density limits, respectively. However, a sharp transition between these limiting behaviors occurs, which does not conform to existing theoretical models for graft polymers. A molecular interpretation based on thin flexible chains at low grafting density and thick semiflexible chains at high grafting density anticipates the sharp transition between the limiting dynamical regimes
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Polydispersity effects in poly(isoprene-b-styrene-b-ethylene oxide) triblock terpolymers
At time of publication all authors were at the University of Minnesota. Christopher Ellison is currently Asst. Professor at the University of Texas at Austin.Four hydroxyl-terminated poly (isoprene-b-styrene) diblock copolymers with comparable molecular
weights and compositions (equivalent volume fractions of polyisoprene and polystyrene) but
different polystyrene block polydispersity indices (Mw/Mn=1.06, 1.16, 1.31, 1.44) were synthesized
by anionic polymerization using either sec-butyllithium or the functional organolithium
3-triisopropylsilyloxy-1-propyllithium. Poly (ethylene oxide) (PEO) blocks were grown from the
end of each of these parent diblocks to yield four series of poly(isoprene-b-styrene-b-ethylene
oxide) (ISO) triblock terpolymers that were used to interrogate the effects of varying the
polydispersity of the middle bridged polystyrene block. In addition to the neat triblock samples, 13
multicomponent blends were prepared at four different compositions from the ISO materials
containing a polystyrene segment with Mw/Mn=1.06; these blends were used to probe the effects of
increasing the polydispersity of the terminal PEO block. The melt-phase behavior of all samples was
characterized using small-angle X-ray scattering and dynamic mechanical spectroscopy. Numerous
polydispersity-driven morphological transitions are reported, including transitions from lamellae to
core-shell gyroid, from core-shell gyroid to hexagonally packed cylinders, and from network
morphologies [either O70 (the orthorhombic Fddd network) or core-shell gyroid] to lamellae.
Domain periodicities and order-disorder transition temperatures also vary with block
polydispersities. Self-consistent field theory calculations were performed to supplement the
experimental investigations and help elucidate the molecular factors underlying the polydispersity
effects. The consequences of varying the polydispersity of the terminal PEO block are comparable
to the polydispersity effects previously reported in AB diblock copolymers. Namely, domain
periodicities increase with increasing polydispersity and domain interfaces tend to curve toward
polydisperse blocks. The changes in phase behavior that are associated with variations in the
polydispersity of the middle bridged polystyrene block, however, are not analogous to those
reported in AB diblock copolymers, as increases in this middle block polydispersity are not always
accompanied by (i) increased domain periodicities and (ii) a tendency for domain interfaces to curve
toward the polydisperse domain. These results highlight the utility of polydispersity as a tool to tune
the phase behavior of ABC block terpolymers.Chemical Engineerin
Chain Transfer in the Ring-Opening Metathesis Polymerization of Cyclooctadiene Using Discrete Metal Alkylidenes
The ring-opening metathesis polymerization of cyclooctadiene (COD) in the presence of a
difunctional chain transfer agent (CTA) was investigated. The effect of the CTA structure and choice of
metathesis catalyst on the overall efficiency of the polymerization reaction is discussed. A separation of
the oxygen functionality from the olefin in the CTA by two methylene units significantly enhances the
stability of the discrete, tungsten-based metathesis catalyst used for these polymerizations. Furthermore,
a more robust well-defined metathesis catalyst based on molybdenum was more effective than the
tungsten-based catalyst. The preparation oflow-molecular-weight hydroxytelechelic polybutadienes with
one type of regiochemistry, number-average functionalities that approach 2.0, and in high yield was
accomplished
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