4 research outputs found
Controlling Macroscopic Properties by Tailoring Nanoscopic Interfaces in Tapered Copolymers
Systematic
variation of comonomer concentration in individual polymer
chains is responsible for unique phase behaviors in different types
of copolymers. Controlled self-assembly of gradient copolymers into
desired morphologies is theoretically understood but practically challenging,
and identifying heterogeneous phase partitioning of individual comonomers
in those resulting morphological regions can be difficult. Building
on previous work where improved methods were used to elucidate heterogeneous
comonomer partitioning in styrene–butadiene gradient copolymers
[Clough Macromolecules 2014, 47, 2625], the arrangement of the styrene and butadiene monomers in only
a fraction of the total chain length is used here to significantly
perturb the overall morphology and physical properties of copolymers.
Importantly, the chemical composition of all copolymers was held nearly
fixed in this study. The resulting tapered and inverse tapered block
copolymers contain nanometer length scale interfaces that differ from
one another and differ dramatically from that observed in a control
block copolymer composed of chains with a discrete interface. Evidence
is presented that butadiene can reside in rigid environments, styrene
can reside in mobile environments, and their relative amounts can
be varied based on the gradient design. The connection between the
molecular design of the gradient, the resulting nanometer-length scale
interfacial structures, and mechanical properties is demonstrated
using a combination of variable temperature solid-state NMR, modulated
DSC (differential scanning calorimetry), AFM (atomic force microscopy),
and rheology experiments
Multiblock Inverse-Tapered Copolymers: Glass Transition Temperatures and Dynamic Heterogeneity as a Function of Chain Architecture
Systematic variation of the size
and number of inverse-tapered
blocks in styrene–butadiene copolymers results in a wide range
of accessible glass-transition temperatures (<i>T</i><sub>g</sub>), including <i>T</i><sub>g</sub>’s approaching
that predicted by the Fox equation. Composition-weighted average <i>T</i><sub>g</sub>’s are expected for miscible blends
or random copolymers, but such behavior has not previously been reported
for block copolymers made from immiscible styrene and butadiene segments.
In this work, 50:50 wt % multiblock copolymers with <i>M</i><sub>n</sub> = 120 000 kg/mol were synthesized using an inverse-tapered
block design for all blocks except the end blocks. The total composition
and molecular weight were held constant, but the type and number of
blocks were systematically varied in order to compare contributions
from the inverse-tapered chain interfaces to the overall glass transition
behavior. Discrete copolymers of similar block number and length were
investigated as controls to help separate contributions from the inverse-tapered
design and the molecular weight of individual blocks. Some copolymers
were intentionally designed such that individual block molecular weights
were between the entanglement molecular weight (<i>M</i><sub>e</sub>) of polystyrene (PS) and polybutadiene (PB). A range
of intermediate glass transitions was observed, but the inverse-tapered
copolymers that satisfied this latter condition were the only copolymers
that exhibited a <i>T</i><sub>g</sub> near a composition-weighted
average. Solid state NMR reveals dynamic heterogeneity among monomeric
components through chain-level identification of relatively large
amounts of rigid PB segments and mobile PS chain segments versus that
observed in discrete block analogues where essentially all PB segments
are mobile and all PS segments are rigid. NMR revealed subtle differences
in the temperature-dependent segmental chain dynamics of different
inverse-tapered blocks, which were not obvious from the calorimetric
studies but which presumably contribute to the longer length scale <i>T</i><sub>g</sub> behavior
Characterization of Kerogen and Source Rock Maturation Using Solid-State NMR Spectroscopy
Solid-state NMR methods common to
the analysis of polymers and
other rigid solids are utilized for the study of kerogen, bitumen,
and the organic content in source rocks. The use of straightforward
nondestructive techniques, primarily employing solid-state NMR, is
shown to provide useful information about both individual samples
and changes between samples that cover a range of thermal maturities
of type II kerogen. In addition to aromatic fraction and chemical
structure, one of the most striking changes to isolated kerogen with
maturity is the distribution of pore sizes, studied with both <sup>129</sup>Xe NMR and complementary nitrogen physisorption, that may
help to understand the process of bitumen generation. Ultimately,
direct in situ analysis of source rock samples that allow kerogen
and bitumen to be distinguished is desirable, as it would eliminate
the time and effort to isolate and prepare kerogen samples. By proper
consideration and removal of the background, we find that a clear <sup>13</sup>C NMR signal can be obtained from source rock with total
organic carbon weight as low as 2%. Simple <sup>1</sup>H NMR methods
are shown to quickly provide a qualitative measurement of the bitumen
in source rocks, while <sup>13</sup>C cross-polarization is found
to be an easy method to distinguish kerogen from bitumen
New Lithium Ion Clusters for Construction of Porous MOFs
Two novel types of lithium clusters,
Li<sub>4</sub> tetramer and
Li<sub>2</sub> dimer, have been created as the building blocks of
MOFs. The assembly of such unprecedented clusters with two types of
tricarboxylate ligands leads to two highly open frameworks, one of
which exhibits a very high CO<sub>2</sub> uptake capacity among Li-based
MOFs. The work reveals the interesting and unprecedented structural
chemistry of lithium ions