7 research outputs found
Self-Diffusion and Constraint Release in Isotropic Entangled RodâCoil Block Copolymers
Understanding dynamic relaxation
mechanisms in self-diffusion and
constraint release processes of rodâcoil block copolymers is
important for many technological applications that employ neat melts
or concentrated solutions. Using a model system composed of polyÂ(alkoxyÂphenyleneÂvinylene)
rods and polyisoprene coils, reptation theories of entangled rodâcoil
block copolymers are investigated in the isotropic melt state. Self-diffusion
was measured by forced Rayleigh scattering using a red laser line
and a new blue photoswitchable dye that allow operation above the
bandgap of most semiconducting polymers. In contrast to previous tracer
studies where the diffusion of rodâcoils through a coil homopolymer
matrix is slowed relative to coil homopolymers because of a mismatch
in the curvature of the rod and coil entanglement tubes, slowed diffusion
is only present in self-diffusion measurements above a critical molecular
weight. An activated reptation mechanism with constraint release is
proposed as a modification to the description of entangled rodâcoil
block copolymer dynamics, where the slowing occurs when the time scale
of rod block reptation is faster than the reorganization of the surrounding
entanglement tube. This mechanism is supported by additional tracer
diffusion experiments on polyalanine-<i>b</i>-polyÂ(ethylene
oxide) diblocks in aqueous entangled polyÂ(ethylene oxide) matrix solutions
and KremerâGrest simulations where the matrix molecular weight
is varied. The slowing of tracer diffusion in rodâcoil block
copolymers relative to coil homopolymers is significantly weaker for
smaller matrix polymers, confirming the proposed constraint release
effects
Diffusion Mechanisms of Entangled RodâCoil Diblock Copolymers
Mechanisms of entangled rodâcoil
diblock copolymer diffusion
are investigated using tracer diffusion simulations and experiments
in a matrix of entangled coil homopolymers, demonstrating that the
diffusion mechanisms first identified in coilârodâcoil
triblocks are universal for various molecular architectures. Diffusion
measurements were performed using both KremerâGrest molecular
dynamics simulations and forced Rayleigh scattering experiments. In
the large rod regime, diffusivity decreases exponentially with increasing
coil size as predicted by an arm retraction mechanism. The ratio of
diblock to rod homopolymer diffusivity was approximately equal to
the ratio for triblocks squared, suggesting that the two coil blocks
of the coilârodâcoil triblock relax independently. In
the small rod regime, both experiments and simulation show that the
slowing of diffusion with increasing rod length is the same for rodâcoil
diblock and coilârodâcoil triblock copolymers. This
behavior occurs because both types of molecules reptate through tubes
with Gaussian statistics, so diffusional slowing results from entropic
barriers to rod reptation through curved sections of the entanglement
tube
Anomalous Self-Diffusion and Sticky Rouse Dynamics in Associative Protein Hydrogels
Natural and synthetic materials based
on associating polymers possess
diverse mechanical behavior, transport properties and responsiveness
to external stimuli. Although much is known about their dynamics on
the molecular and macroscopic level, knowledge of self-diffusive dynamics
of the network-forming constituents remains limited. Using forced
Rayleigh scattering, anomalous self-diffusion is observed in model
associating protein hydrogels originating from the interconversion
between species that diffuse in both the molecular and associated
state. The diffusion can be quantitatively modeled using a two-state
model for polymers in the gel, where diffusivity in the associated
state is critical to the super diffusive behavior. The dissociation
time from bulk rheology measurements was 2â3 orders of magnitude
smaller than the one measured by diffusion, because the former characterizes
submolecular dissociation dynamics, whereas the latter depicts single
protein molecules completely disengaging from the network. Rheological
data also show a sticky Rouse-like relaxation at long times due to
collective relaxation of large groups of proteins, suggesting mobility
of associated molecules. This study experimentally demonstrates a
hierarchy of relaxation processes in associating polymer networks,
and it is anticipated that the results can be generalized to other
associative systems to better understand the relationship of dynamics
among sticky bonds, single molecules, and the entire network
Experimental Measurement of CoilâRodâCoil Block Copolymer Tracer Diffusion through Entangled Coil Homopolymers
The diffusion of coilârodâcoil triblock
copolymers
in entangled coil homopolymers is experimentally measured and demonstrated
to be significantly slower than rod or coil homopolymers of the same
molecular weight. A model coilârodâcoil triblock was
prepared by expressing rodlike alanine-rich α-helical polypeptides
in <i>E. coli</i> and conjugating coil-like polyÂ(ethylene
oxide) (PEO) to both ends to form coilârodâcoil triblock
copolymers. Tracer diffusion through entangled PEO homopolymer solutions
was measured using forced Rayleigh scattering at various rod lengths
and coil molecular weights for the tracer, and various concentrations
for the coil homopolymer solutions. For rod lengths, <i>L</i>, that are close to the entanglementh length, <i>a</i>,
the ratio between the diffusivity of a triblock and the diffusivity
of a coil homopolymer of the same molecular weight decreases monotonically
and is only a function of <i>L</i>/<i>a</i>, in
quantitative agreement with previous simulation results. For large
rod lengths, diffusion follows an arm retraction scaling, which is
also consistent with previous theoretical predictions. These experimental
results support the key predictions of theory and simulation, suggesting
that the mismatch in curvature between rod and coil entanglement tubes
leads to the observed diffusional slowing
Diffusion of Entangled RodâCoil Block Copolymers
The diffusion of entangled rodâcoil block copolymers
is
investigated by molecular dynamics (MD) simulations, and theories
are introduced that describe the observed features and underlying
physics. The reptation of rodâcoil block copolymers is dominated
by the mismatch between the curvature of the rod and coil entanglement
tubes, which results in dramatically slower diffusion of rodâcoils
compared to the rod and coil homopolymers. For small rods, a local
curvature-dependent free energy penalty results in a rough energy
surface inside the entanglement tube, causing diffusivity to decrease
with rod length. For large rods, rotational hindrances on the rod
dominate, causing the coil block to relax by an arm retraction mechanism
and diffusivity to decrease exponentially with coil size
<i>In Situ</i> Investigations of Microstructure Formation in Interpenetrating Polymer Networks
Interpenetrating
polymer networks (IPNs) represent an effective
strategy for compatibilizing immiscible polymers to enhance the mechanical
properties of the final material. While it has been established that
the macroscopic properties are dependent on the microstructure, it
is unknown why various microstructures are formed in IPNs because
the microstructure is often trapped in a nonequilibrium state. To
explore this, we conducted a study to establish a relationship between
polymerization kinetics and microstructure formation in polydimethylsiloxane/poly(methyl
methacrylate) (PDMS/PMMA) IPNs. By manipulating the UV curing intensity,
we observed three distinct morphologies: isolated PMMA-rich spheres
within a PDMS matrix with a monomodal domain size distribution, spheres
with a bimodal size distribution, and a clustered domain microstructure.
To investigate the different phase separation mechanisms, we correlated in situ small-angle X-ray scattering (SAXS) to track microstructure
formation and Fourier transform infrared spectroscopy (FT-IR) to track
polymerization kinetics. Based on our findings, we propose that the
monomodal sphere microstructure formed via spinodal decomposition.
The positions of the domains are kinetically trapped in the PDMS network,
preventing macrophase separation. Similarly, the clustered domain
microstructure also arises from spinodal decomposition, but increased
mobility within the PDMS matrix enables domains to aggregate after
network percolation. In contrast, the bimodal spherical morphology
is attributed to a combination of nucleation and growth, and spinodal
decomposition. We postulate that these different mechanisms are dictated
by changes in the PMMA molecular weight during polymerization. Through
the examination of polymerization kinetics and microstructure formation,
we have proposed multiple mechanisms that explain the microstructure
formation in IPNs
Crossover Experiments Applied to Network Formation Reactions: Improved Strategies for Counting Elastically Inactive Molecular Defects in PEG Gels and Hyperbranched Polymers
Molecular
defects critically impact the properties of materials.
Here we introduce a paradigm called âisotopic labeling disassembly
spectrometryâ (ILDaS) that facilitates unprecedented precise
experimental correlations between elastically inactive network defects
(dangling chains and primary loops) and network formation kinetics
and precursor structure. ILDaS is inspired by classical crossover
experiments, which are often used to interrogate whether a reaction
mechanism proceeds via an inter- or intramolecular pathway. We show
that if networks are designed from labeled bifunctional monomers that
transfer their labels to multifunctional junctions upon network formation,
then the extent of junction labeling correlates directly with the
number of dangling chains and cyclic imperfections within the network.
We demonstrate two complementary ILDaS approaches that enable defect
measurements with short analysis times, low cost, and synthetic versatility
applicable to a broad range of network materials including polydisperse
polymer precursors. The results will spur new experimental and theoretical
investigations into the interplay between polymer network structure
and properties