10 research outputs found
Nonlinear ZieglerâNatta-Homopolyethylene with Enhanced Crystallinity: Physical and Macromolecular Characteristics
Polyolefin engineering and design are at the forefront
of a significant
number of research and development laboratories, helping to bring
about new and highly specific materials for tailored uses. Tailoring
the chain architecture of polyolefins improves their performance and
physical properties. Four unique polyethylene (PE) materials with
long-chain branches (LCBPE) are studied using advanced chromatographic
fractionation techniques alongside linear high-density PE (HDPE) and
typical commercial low-density PE (LDPE). The absence of short-chain
branching in the analyzed LCBPEs allows for a defined correlation
of long-chain branching (LCB) with specific physical properties. Possible
effects of side-chain crystallization on melt behavior and crystallinity
clearly show that the nonlinearity in architecture positively affects
crystallinity while simultaneously lowering melting temperature. The
separation of polyolefins according to the LCB content is demonstrated
for the first time by high-temperature interaction chromatography
and thermal analysis, in addition to size exclusion chromatography
coupled to differential viscometer and light scattering detectors.
This study is pioneering in applying solvent gradient interaction
chromatography and stationary-phase-assisted crystallization to the
separation of PE regarding long-chain branching
Alkoxide-Initiated Regioselective Coupling of Carbon Disulfide and Terminal Epoxides for the Synthesis of Strongly Alternating Copolymers
The synthesis of highly regioregular
and alternating polythiocarbonates
from carbon disulfide and terminal epoxides has been achieved. The
copolymerizations were performed under ambient and solvent-free conditions
in the presence of LiO<sup><i>t</i></sup>Bu (0.125â0.5
mol %) as initiator. At higher loadings the reaction pathway switched
in favor to the catalytic formation of cyclic dithiocarbonates. Under
optimized reaction conditions polymers with molecular weights up to
109 kg mol<sup>â1</sup> were isolated. The NMR spectroscopic
analysis of the polythiocarbonates revealed that 94% of backbone structure
is formed by strongly alternating head-to-head arrangement of epoxypropane
and 1,2-epoxybutane monomers, respectively, at a thiocarbonate group
âCHRâOCÂ(S)ÂOâCHRâ and tail-to-tail arrangement
at a trithiocarbonate group âCH<sub>2</sub>âSCÂ(S)ÂSâCH<sub>2</sub>â. Atactic polymers were obtained using racemic mixtures
of the epoxides, but an isotactic polymer was obtained when chiral
(<i>R</i>)-epoxyÂpropane was converted. A mechanism
is proposed which rationalizes the regio- and stereochemistry observed
for the alkoxide-initiated copolymerization of CS<sub>2</sub> and
terminal epoxides
Effect of Connectivity on the Structure and the LiquidâSolid Transition of Dense Suspensions of Soft Colloids
Aqueous solutions of multiarm flower-like
polyÂ(ethylene oxide)
(PEO) were formed and connected to various degrees by self-assembly.
The structure was rendered permanent by <i>in situ</i> UV-irradiation.
Dense suspensions of these single and connected soft colloids were
studied by static and dynamic light scattering and viscosity measurements.
The concentration dependence of the osmotic compressibility, the dynamic
correlation length, and the viscosity of single flowers was shown
to be close to that of equivalent PEO star-like polymers demonstrating
that the effect of forming loops on the interaction is small. It was
found that the osmotic compressibility and the dynamic correlation
length of dense suspensions are not influenced by the bridging. However,
when flower polymers are connected into clusters, motion in dense
suspensions needs to be collective over larger length scales. This
causes a much stronger increase of the viscosity for dense suspensions
of interpenetrated clusters compared to single-flower polymers
High Temperature Quadruple-Detector Size Exclusion Chromatography for Topological Characterization of Polyethylene
Modifying
material properties in simple macromolecules such as
polyethylene (PE) is achieved by different connection modes of ethylene
monomers resulting in a plurality of possible topologiesâfrom
highly linear to dendritic species. However, the challenge still lies
within the experimental identification of the topology and conformation
of the isolated macromolecules because of their low solubility, which
demands methods with specific solvents and high operating temperatures.
Additionally, a separation technique has to be coupled to different
detection methods to meet the specific demands of the respective characterization
goal. In this work, we report a quadruple-detector high temperature
size exclusion chromatography (HT-SEC) system which contains online
multiangle laser light scattering, dynamic light scattering, differential
viscometry, and differential refractometry detectors. Quadruple-detector
HT-SEC was successfully applied to explore the full range of physical
parameters of various PE samples with different branching topologies
ranging from highly linear macromolecules, polymers with moderate
level of branching, to highly branched PEs with hyperbranched structure.
This method is a useful tool not only to investigate molecular weight,
mass distribution, and size but also to enable access to important
factors which describe the conformation in dilute solution and branching
density
Sphere-Like ProteinâGlycopolymer Nanostructures Tailored by Polyassociation
Key parameters allow a reproducible
polyassociation between avidin
and biotinylated glycopolymers in order to fabricate defined supramolecular
nanostructures for future (bio)Âmedical and biotechnological applications.
Thus, the polymerization efficiency of biotinylated glycopolymers
in the fabrication of biohybrid structures (BHS) was investigated
with regard to the influence of (i) the degree of biotinylation of
the dendritic glycoarchitectures, (ii) two biotin linkers, (iii) the
dendritic scaffold (perfectly branched vs hyperbranched), and (iv)
the ligandâreceptor stoichiometry. The adjustment of all these
parameters opens the way to fabricate defined sizes of the final biohybrid
structures as a multifunctional platform ready for their use in different
applications. Various analytical techniques, including purification
of BHS, were used to gain fundamental insights into the structural
properties of the resulting proteinâglycopolymer BHS. Finally,
the elucidation of pivotal conformational properties of isolated BHS
with defined sizes by asymmetrical flow field flow fractionation study
revealed that they mainly possess spherical-/star-like properties.
From this study, the fundamental knowledge can be likely transferred
to other assemblies formed by molecular recognition processes (e.g.,
adamantane-β-cyclodextrin)
Coil-like Enzymatic Biohybrid Structures Fabricated by Rational Design: Controlling Size and Enzyme Activity over Sequential Nanoparticle Bioconjugation and Filtration Steps
Well-defined
enzymatic biohybrid structures (BHS) composed of avidin, biotinylated
polyÂ(propyleneimine) glycodendrimers, and biotinylated horseradish
peroxidase were fabricated by a sequential polyassociation reaction
to adopt directed enzyme prodrug therapy to proteinâglycopolymer
BHS for potential biomedical applications. To tailor and gain fundamental
insight into pivotal properties such as size and molar mass of these
BHS, the dependence on the fabrication sequence was probed and thoroughly
investigated by several complementary methods (e.g., UV/vis, DLS,
cryoTEM, AF4-LS). Subsequent purification by hollow fiber filtration
allowed us to obtain highly pure and well-defined BHS. Overall, by
rational design and control of preparation parameters, e.g., fabrication
sequence, ligandâreceptor stoichiometry, and degree of biotinylation,
well-defined BHS with stable and even strongly enhanced enzymatic
activities can be achieved. Open coil-like structures of BHS with
few branches are available by the sequential bioconjugation approach
between synthetic and biological macromolecules possessing similar
size dimensions
One-Pot Synthesis of All-Conjugated Block-Like BisthiopheneâNaphthalenediimide/Fluorene Copolymer
A copolymerization of electron-rich
and electron-deficient monomers
via the chain-growth catalyst-transfer polycondensation route is highly
challenging and has never been accomplished thus far, to the best
of our knowledge. Herein, we report a simple method to copolymerize
two monomers of a significantly different nature: anion-radical naphthalene
diimideâdithiophene-based and zinc-organic AB-type fluorenic
ones. We found that the copolymerization proceeds rapidly in the presence
of Pd catalyst having the bulky and electron-rich tri<i>-tert</i>-butylphosphine ligand. Despite the fact that the two monomers are
simultaneously added to the copolymerization (batch polymerization),
the polymerization leads to a gradient or even block-like copolymer
rather than to a random copolymer or to a mixture of homopolymers,
as evident from NMR, GPC, AFM, and fluorescence quenching experiments.
The block-like copolymer is formed because the fluorenic monomer polymerizes
much faster, yet because the resulting PF2/6 homopolymer is able to
initiate polymerization of the second monomer, presumably acting as
macroinitiator. Although the investigated copolymerization does not
involve a living propagation mechanism and the resulting product is
not a well-defined block copolymer, this result is an important step
toward a general protocol for preparation of all-conjugated donorâacceptor
block copolymers for optoelectronic applications
Supracolloidal Multivalent Interactions and Wrapping of Dendronized Glycopolymers on Native Cellulose Nanocrystals
Cellulose
nanocrystals (CNCs) are high aspect ratio colloidal rods
with nanoscale dimensions, attracting considerable interest recently
due to their high mechanical properties, chirality, sustainability,
and availability. In order to exploit them for advanced functions
in new materials, novel supracolloidal concepts are needed to manipulate
their self-assemblies. We report on exploring multivalent interactions
to CNC surface and show that dendronized polymers (DenPols) with maltose-based
sugar groups on the periphery of lysine dendrons and polyÂ(ethylene-<i>alt</i>-maleimide) polymer backbone interact with CNCs. The
interactions can be manipulated by the dendron generation suggesting
multivalent interactions. The complexation of the third generation
DenPol (G3) with CNCs allows aqueous colloidal stability and shows
wrapping around CNCs, as directly visualized by cryo high-resolution
transmission electron microscopy and electron tomography. More generally,
as the dimensions of G3 are in the colloidal range due to their âź6
nm lateral size and mesoscale length, the concept also suggests supracolloidal
multivalent interactions between other colloidal objects mediated
by sugar-functionalized dendrons giving rise to novel colloidal level
assemblies
Poly(ethylene oxide)â<i>b</i>âpoly(3-sulfopropyl methacrylate) Block Copolymers for Calcium Phosphate Mineralization and Biofilm Inhibition
PolyÂ(ethylene oxide) (PEO) has long
been used as an additive in
toothpaste, partly because it reduces biofilm formation on teeth.
It does not, however, reduce the formation of dental calculus or support
the remineralization of dental enamel or dentine. The present article
describes the synthesis of new block copolymers on the basis of PEO
and polyÂ(3-sulfopropyl methacrylate) blocks using atom transfer radical
polymerization. The polymers have very large molecular weights (over
10<sup>6</sup> g/mol) and are highly water-soluble. They delay the
precipitation of calcium phosphate from aqueous solution but, upon
precipitation, lead to relatively monodisperse hydroxyapatite (HAP)
spheres. Moreover, the polymers inhibit the bacterial colonization
of human enamel by <i>Streptococcus gordonii</i>, a pioneer
bacterium in oral biofilm formation, in vitro. The formation of well-defined
HAP spheres suggests that a polymer-induced liquid precursor phase
could be involved in the precipitation process. Moreover, the inhibition
of bacterial adhesion suggests that the polymers could be utilized
in caries prevention
Efficient Tin-Free Route to a DonorâAcceptor Semiconducting Copolymer with Variable Molecular Weights
For the fabrication of efficient
photovoltaic devices and thin-film
transistors, Ď-conjugated polymers with high molecular weight
are desirable as they frequently show superior charge transport, morphological,
and film-forming properties. Herein, we present an extremely fast
tin-free method to polymerize a naphthalene diimide-dithiophene-based
anion-radical monomer in the presence of Pd catalyst having bulky
and electron-rich tritert-butylphosphine ligands (Pd/P<sup><i>t</i></sup>Bu<sub>3</sub>). With this method, the corresponding
semiconducting polymer, PNDIT2 (also known as PÂ(NDI2OD-T2 or N2200)
with a molecular weight in excess of 1000 kg/mol can be obtained quickly
at room temperature and at rather low catalyst concentrations. In
general, molecular weights of resulting polymer can be regulated by
reaction conditions (e.g., catalyst concentration and reaction time).
Besides high molecular weight PNDIT2 (e.g., with <i>M</i><sub>N</sub> âź 350 kg/mol, <i>Ä</i><sub>M</sub> =2.9), PNDIT2 with moderate molecular weight (e.g., <i>M</i><sub>N</sub> âź 110 kg/mol, <i>Ä</i><sub>M</sub> = 2.3) and low molecular weight (e.g., <i>M</i><sub>W</sub> âź 12 kg/mol, <i>Ä</i><sub>M</sub> = 1.9),
can also be obtained. It was found that thus-prepared PNDIT2 exhibits
field-effect electron mobilities of up to âź0.31 cm<sup>2</sup>/(V s), similar to the Stille-derived N2200 control polymer (up to
âź0.33 cm<sup>2</sup>/(V s)). Preliminary studies demonstrated
that Pd/P<sup><i>t</i></sup>Bu<sub>3</sub> catalyst is remarkably
efficient in polymerizing of other anion-radical monomers, such as
isoindigo-, and diketopyrrolopyrrole-based ones, although conventional
Ni and Pd catalysts (e.g., NiÂ(dppp)ÂCl<sub>2</sub>, NiÂ(dppp)ÂCl<sub>2</sub>, PdÂ(PPh<sub>3</sub>)<sub>4</sub>) failed to polymerize these
monomers