13 research outputs found
Influence of the Chain Architecture and the Presence of End-Groups or Branching Units Chemically Different from Repeating Structural Units on the Critical Adsorption Point in Liquid Chromatography
The
critical adsorption point (CAP) of linear and star-shaped polymers
was investigated by normal phase and reversed phase liquid chromatography
(NPLC and RPLC) and computer simulation. Three sets of polystyrenes
(PS) differing in chain architecture and chemically distinct groups
were prepared: linear PS (<i>sec</i>-butyl and hydrogen
end group), 2-arm PS (linear, two <i>sec</i>-butyl end groups
and one silyl group in the middle of the chain) and 4-arm star-shaped
PS (four <i>sec</i>-butyl end groups and one silyl group
in the center of the star). It was found that the column temperature
at CAP, <i>T</i><sub>CAP</sub> (linear PS) = <i>T</i><sub>CAP</sub> (2-arm PS) > <i>T</i><sub>CAP</sub> (4-arm
PS) in both RPLC and NPLC which can be attributed to the variation
in chain architecture. However, the elution times at CAP of three
polymers are all different: In NPLC, <i>t</i><sub>E,CAP</sub> (linear) > <i>t</i><sub>E,CAP</sub> (2-arm PS) > <i>t</i><sub>E,CAP</sub> (4-arm PS) while in RPLC, <i>t</i><sub>E,CAP</sub> (4-arm PS) > <i>t</i><sub>E,CAP</sub> (2-arm
PS) > <i>t</i><sub>E,CAP</sub> (linear). The variation
of <i>t</i><sub>E,CAP</sub> can be explained by the contribution
of
the chemically distinct groups. The computer simulation results are
in good agreement with the chromatography experiments results and
support the interpretation of experimental data
Linear and Nonlinear Shear Rheology of a Marginally Entangled Ring Polymer
We present systematic, unique linear
and nonlinear shear rheology
data of an experimentally pure ring polystyrene and its linear precursor.
This polymer was synthesized anionically and characterized by interaction
chromatography and fractionation at the critical condition. Its weight-average
molar mass is 84 kg/mol; i.e., it is marginally entangled (entanglement
number <i>Z</i> â 5). Its linear viscoelastic response
appears to be better described by the Rouse model (accounting for
ring closure) rather than the lattice-animal-based model, suggesting
a transition from unentangled to entangled ring dynamics. The failure
of both models in the terminal region may reflect the remaining unlinked
linear contaminants and/or ringâring interpenetration. The
viscosity evolution at different shear rates was measured using a
homemade cone-partitioned plate fixture in order to avoid edge fracture
instabilities. Our findings suggest that rings are much less shear
thinning compared to their linear counterparts, whereas both obey
the CoxâMerz rule. The shear stress (or viscosity) overshoot
is much weaker for rings compared to linear chains, pointing to the
fact that their effective deformation is smaller. Finally, step strain
experiments indicate that the damping function data of ring polymers
clearly depart from the DoiâEdwards prediction for entangled
linear chains, exhibiting a weak thinning response. These findings
indicate that these marginally entangled rings behave like effectively
unentangled chains with finite extensibility and deform much less
in shear flow compared to linear polymers. They can serve as guideline
for further investigation of the nonlinear dynamics of ring polymers
and the development of constitutive equations
Determining the Origins of Impurities during AzideâAlkyne Click Cyclization of Polystyrene
Determining the Origins of Impurities during AzideâAlkyne
Click Cyclization of Polystyren
Model Branched Polymers: Synthesis and Characterization of Asymmetric H-Shaped Polybutadienes
A new type of model branched polymer, asymmetric H-shaped
polybutadienes,
consisting of central crossbars having various combinations of short
and long arms attached to the ends of the crossbars, was synthesized
using living anionic polymerization and chlorosilane linking chemistry.
The linking agent 4-(dichloromethylsilyl)Âdiphenylethylene provides
selective reactivity to attach short or long arms on one side or both
sides as desired. The samples were characterized thoroughly by size
exclusion chromatography with light scattering detection (SEC-LS)
and found to exhibit controlled molecular weights, as well as narrow
polydispersity indices (PDIs of 1.01â1.06). Temperature gradient
interaction chromatography, a method with far superior resolution
as compared to SEC, also shows that these materials are well-defined,
with minimal and identifiable impurities
Epitaxial Phase Transition between Double Gyroid and Cylinder Phase in Diblock Copolymer Thin Film
The epitaxial relationship in the
thermal phase transition between
double gyroid (DG) and hexagonally packed cylinder (HEX) phases in
polystyrene-<i>block</i>-polyisoprene thin films on Si wafer
was investigated using transmission electron microtomography and grazing
incidence small-angle X-ray scattering. Two different types of epitaxial
transitions were observed, and they appeared to be selectively favored
depending on the transition direction. One type of epitaxial relationship
prevails in the phase transition from DG to HEX upon heating in which
{121}<sub>DG</sub>, {111}<sub>DG</sub>, and {220}<sub>DG</sub> are
converted to {100}<sub>HEX</sub>, {110}<sub>HEX</sub>, and {001}<sub>HEX</sub>, respectively. The interphase planes are {220}<sub>DG</sub> and {001}<sub>HEX</sub>, and the cylinders meet the {220}<sub>DG</sub> plane perpendicularly (head-on, Type A) at the grain boundary between
DG and HEX. Although there are small dimensional mismatch and distortion
in the location of the cylinders in this epitaxial relationship, all
cylinders are formed along the topologically equivalent DG skeletal
path. On the other hand, in the transition from HEX to DG upon cooling,
another epitaxial relationship as well as the head-on type epitaxy
was observed, in which {100}<sub>HEX</sub>, {110}<sub>HEX</sub>, and
{001}<sub>HEX</sub> are converted to {121}<sub>DG</sub>, {220}<sub>DG</sub>, and {111}<sub>DG</sub>, respectively. The interphase planes
are {220}<sub>DG</sub> and {110}<sub>HEX</sub>, and the cylinders
meet the {220}<sub>DG</sub> plane in parallel (side-on, Type B) at
the grain boundary between HEX and DG. The domain spacing and the
symmetry of the two phases are matched near perfectly, but cylinders
are converted to two different DG skeletal paths. The Type B epitaxy
is hardly observed in the transition from DG to HEX
Figure-Eight-Shaped and Cage-Shaped Cyclic Polystyrenes
Nonlinear
polystyrenes (PS) with similar molecular weights but with different
molecular structures having star-, figure-8-, and cage-shaped architectures
were synthesized by combining atom transfer radical polymerization
(ATRP) and click chemistry. Figure-8- and cage-shaped PS were fractionated
by using a gradient normal phase liquid chromatography as confirmed
by SEC-LS, MALDIâTOF MS, <sup>1</sup>H NMR, and FT-IR spectrometry.
Their purities were estimated by using a two-dimensional liquid chromatography
(2D-LC). The glass transition temperatures of these topologically
different polymers were in the order of cage-, figure-8-, and star-shaped
polymers possibly due to the multiple links that constrain the overall
molecular diffusivity in the case of multicyclic polymers (figure-8,
and cage). Monte Carlo simulation on the glass transition behavior
of model system also agreed well with the experimental results
Comparison of Critical Adsorption Points of Ring Polymers with Linear Polymers
The
critical adsorption points (CAP) for ring and linear polymers
are determined and compared using Monte Carlo simulations and liquid
chromatography experiments. The CAP is defined as the coelution point
of ring or linear polymers with different molecular weights (MW).
Computational studies show that the temperature at the CAP, <i>T</i><sub>CAP</sub>, for rings is higher than <i>T</i><sub>CAP</sub> for linear polymers regardless of whether the chains
are modeled as random walks or self-avoiding walks. The difference
in the CAP can be attributed only to the architectural difference.
Experimentally, four pairs of linear and ring polystyrenes (PS) of
different MW were synthesized and purified. Care was taken to account
for the difference between the end-groups in linear polymers and the
linkage unit in ring polymers. Elution of these polymers using a C18
bonded silica stationary phase and a CH<sub>2</sub>Cl<sub>2</sub>/CH<sub>3</sub>CN mixed eluent were studied. The temperature at the coelution
point, <i>T</i><sub>CAP</sub>, and the coelution time at
the CAP, <i>t</i><sub>E,CAP</sub>, were determined for both
ring and linear polymers. Experimentally, it was found that <i>T</i><sub>CAP</sub> of linear PS is lower than <i>T</i><sub>CAP</sub> of cyclic PS and <i>t</i><sub>E,CAP</sub> of linear PS is shorter than <i>t</i><sub>E,CAP</sub> of
ring PS. Therefore, at the CAP of linear polymers, ring polymers elute
later in order of increasing MW while, at the CAP of ring polymers,
linear polymers elute earlier in order of decreasing MW. This is in
excellent agreement with the Monte Carlo computer simulation results.
We also found that the functionality effect can interfere in the LCCC
separation of ring polymers from their linear precursors
Thermodynamic and Kinetic Aspects of Coassembly of PEOâPMAA Block Copolymer and DPCl Surfactants into Ordered Nanoparticles in Aqueous Solutions Studied by ITC, NMR, and Time-Resolved SAXS Techniques
The electrostatic coassembly of a
block copolymer polyelectrolyte
polyÂ(ethylene oxide-<i>block</i>-polyÂ(methacrylic acid),
PEO<sub>705</sub>âPMAA<sub>476</sub>, and oppositely charged
surfactant, <i>N</i>-dodecylpyridinium chloride (DPCl),
has been investigated by a combination of isothermal titration calorimetry
(ITC), spin-echo NMR spectroscopy, and time-resolved SAXS measurements.
The study (i) confirms the conclusions drawn from our earlier study
[Macromolecules 2012, 45, 6474] by scattering and microscopy techniques (i.e., the ITC curves can
be interpreted using arguments consistent with conclusions of the
earlier study) and (ii) yields new insight into the thermodynamic
and kinetic behavior of the self-assembling system. The most important
finding obtained by stopped-flow time-resolved SAXS measurements concerns
the surprisingly high rate of processes of creation of structurally
ordered cores of self-assembled surfactantâpolyelectrolyte
nanoparticles (<50 ms)
Topologically Reversible Transformation of Tricyclic Polymer into Polyring Using Disulfide/Thiol Redox Chemistry
A polyring
capable of reversible growth and dissociation is synthesized
from a tricyclic polystyrene (PS) prepared by combining atom transfer
radical polymerization of a 4-arm star-shaped PS and azideâalkyne
click reactions. In the preparation of the tricyclic PS, a coupling
agent containing a disulfide linkage is used in the click cyclization
reaction. The reduction of the disulfide linkage in the tricyclic
PS results in an 8-shaped PS with thiol groups which on oxidation
leads to a high molecular weight polyring. The topology transformation
between the polymers occurs via reversible redox reaction of disulfide/thiol.
The high molecular weight of the polyring is realized due to the formation
of flexible SâS linkage between the 8-shaped PSs. Their structures
are confirmed by FT-IR, <sup>1</sup>H NMR, SEC, and MALDI-TOF MS analyses.
In addition, molecular weight control of the polyring according to
polymer concentration has been confirmed through SEC analysis
Correction to In Silico Molecular Design, Synthesis, Characterization, and Rheology of Dendritically Branched Polymers: Closing the Design Loop
Correction to In Silico
Molecular Design, Synthesis, Characterization, and Rheology of Dendritically
Branched Polymers:
Closing the Design Loo