17 research outputs found
Surface Layer Dynamics in Miscible Polymer Blends
In thin film A/B polymer/polymer
mixtures, the formation of a layer
at the free surface, with average composition that differs from the
bulk, due to the preferential segregation of the lower cohesive energy
density component, is well understood. While much is also understood
about this surface layer formation and growth to date, virtually nothing
is known about the surface dynamics of the chains in such mixtures.
Questions about the surface chain dynamics in relation to the bulk
have remained unanswered. With the use of X-ray photon correlation
spectroscopy (XPCS) we show that the dynamics of polyÂ(vinyl methyl
ether) (PVME) chains at the free surface of polystyrene (PS)/PVME
thin film mixtures can be orders of magnitude larger than the PVME
chains in the bulk. These dynamics manifest from differences between
the local compositions of the blend at the free surface and the bulk,
as well as film thickness constraints
Size-Dependent Particle Dynamics in Entangled Polymer Nanocomposites
Polymer-grafted
nanoparticles with diameter <i>d</i> homogeneously
dispersed in entangled polymer melts with varying random coil radius <i>R</i><sub>0</sub>, but fixed entanglement mesh size <i>a</i><sub>e</sub>, are used to study particle motions in entangled
polymers. We focus on materials in the transition region between the
continuum regime (<i>d</i> > <i>R</i><sub>0</sub>), where the classical StokesâEinstein (SâE) equation
is known to describe polymer drag on particles, and the noncontinuum
regime (<i>d</i> < <i>a</i><sub>e</sub>), in
which several recent studies report faster diffusion of particles
than expected from continuum SâE analysis, based on the bulk
polymer viscosity. Specifically, we consider dynamics of particles
with sizes <i>d</i> â„ <i>a</i><sub>e</sub> in entangled polymers with varying molecular weight <i>M</i><sub>w</sub> in order to investigate how the transition from noncontinuum
to continuum dynamics occur. We take advantage of favorable enthalpic
interactions between SiO<sub>2</sub> nanoparticles tethered with PEO
molecules and entangled PMMA host polymers to create model nanoparticleâpolymer
composites, in which spherical nanoparticles are uniformly dispersed
in entangled polymers. Investigation of the particle dynamics via
X-ray photon correlation spectroscopy measurements reveals a transition
from fast to slow particle motion as the PMMA molecular weight is
increased beyond the entanglement threshold, with a much weaker <i>M</i><sub>w</sub> dependence for <i>M</i><sub>w</sub> > <i>M</i><sub>e</sub> than expected from SâE
analysis
based on bulk viscosity of entangled PMMA melts. We rationalize these
observations using a simple force balance analysis around particles
and find that nanoparticle motion in entangled melts can be described
using a variant of the SâE analysis in which motion of particles
is assumed to only disturb subchain entangled host segments with sizes
comparable to the particle diameter
Nanorod Mobility within Entangled Wormlike Micelle Solutions
In the semidilute regime, wormlike
micelles form an isotropic entangled microstructure that is similar
to that of an entangled polymer solution with a characteristic, nanometer-scale
entanglement mesh size. We report a combined X-ray photon correlation
spectroscopy (XPCS) and rheology study to investigate the translational
dynamics of gold nanorods in semidilute solutions of entangled wormlike
micelles formed by the surfactant cetylpyridinium chloride (CPyCl)
and the counterion sodium salicylate (NaSal). The CPyCl concentration
is varied to tune the entanglement mesh size over a range that spans
from approximately equal to the nanorod diameter to larger than the
nanorod length. The NaSal concentration is varied along with the CPyCl
concentration so that the solutions have the maximum viscosity for
given CPyCl concentration. On short time scales the nanorods are localized
on a length scale matching that expected from the high-frequency elastic
modulus of the solutions as long as the mesh size is smaller than
the rod length. On longer time scales, the nanorods undergo free diffusion.
At the highest CPyCl concentrations, the nanorod diffusivity approaches
the value expected based on the macroscopic viscosity of the solutions,
but it increases with decreasing CPyCl concentration more rapidly
than expected from the macroscopic viscosity. A recent model by Cai
et al. [Cai, L.-H.; Panyukov, S.; Rubinstein, M. Macromolecules 2015, 48, 847â862] for nanoparticle
âhoppingâ diffusion in entangled polymer solutions accounts
quantitatively for this enhanced diffusivity
Polymer Film Surface Fluctuation Dynamics in the Limit of Very Dense Branching
The surface fluctuation dynamics
of melt films of densely branched
comb polystyrene of thickness greater than 55 nm and at temperatures
23â58 °C above the bulk <i>T</i><sub>g</sub> can be rationalized using the hydrodynamic continuum theory (HCT)
known to describe melts of unentangled linear and cyclic chains. Film
viscosities (η<sub>XPCS</sub>) inferred from fits of the HCT
to X-ray photon correlation spectroscopy (XPCS) data are the same
as those measured in bulk rheometry (η<sub>bulk</sub>) for three
combs. For the comb most like a star polymer and the comb closest
to showing bulk entanglement behavior, η<sub>XPCS</sub> >
η<sub>bulk</sub>. These discrepancies are much smaller than
those seen
for less densely branched polystyrenes. We conjecture that the smaller
magnitude of η<sub>XPCS</sub> â η<sub>bulk</sub> for the densely grafted combs is due to a lack of interpenetration
of the side chains when branching is most dense. Both <i>T</i><sub>g,bulk</sub> and the specific chain architecture play key roles
in determining the surface fluctuations
Dynamics of Surface Fluctuations on Macrocyclic Melts
A hydrodynamic continuum theory (HCT) of thermally stimulated
capillary
waves describing surface fluctuations of linear polystyrene melts
is found to describe surface fluctuations of sufficiently thick films
of unentangled cyclic polystyrene. However, for cyclic polystyrene
(CPS) films thinner than 10<i>R</i><sub>g</sub>, the surface
fluctuations are slower than expected from the HCT universal scaling,
revealing a confinement effect active over length scales much larger
than <i>R</i><sub>g</sub>. Surface fluctuations of CPS films
can be slower than those of films of linear polystyrene analogues,
due to differences between the glass transition temperatures, <i>T</i><sub>g</sub>, of the linear and cyclic chains. The temperature
dependences of the surface fluctuations match those of bulk viscosities,
suggesting that whole chain dynamics dictate the surface height fluctuation
dynamics at temperatures 25â60 °C above <i>T</i><sub>g</sub>. When normalized surface relaxation rates of thicker
films are plotted as a function of <i>T</i>/<i>T</i><sub>g</sub>, a universal temperature behavior is observed for linear
and cyclic chains
Microscopic Origins of the Nonlinear Behavior of Particle-Filled Rubber Probed with Dynamic Strain XPCS
The underlying microscopic response of filler networks
in reinforced
rubber to dynamic strain is not well understood due to the experimental
difficulty of directly measuring filler network behavior in samples
undergoing dynamic strain. This difficulty can be overcome with in
situ X-ray photon correlation spectroscopy (XPCS) measurements. The
contrast between the silica filler and the rubber matrix for X-ray
scattering allows us to isolate the filler network behavior from the
overall response of the rubber. This in situ XPCS technique probes
the microscopic breakdown and reforming of the filler network structure,
which are responsible for the nonlinear dependence of modulus on strain,
known in the rubber science community as the Payne effect. These microscopic
changes in the filler network structure have consequences for the
macroscopic material performance, especially for the fuel efficiency
of tire tread compounds. Here, we elucidate the behavior with in situ
dynamic strain XPCS experiments on industrially relevant, vulcanized
rubbers filled (13 vol %) with novel air-milled silica of ultrahigh-surface
area (UHSA) (250 m2/g). The addition of a silane coupling
agent to rubber containing this silica causes an unexpected and counterintuitive
increase in the Payne effect and decrease in energy dissipation. For
this rubber, we observe a nearly two-fold enhancement of the storage
modulus and virtually equivalent loss tangent compared to a rubber
containing a coupling agent and conventional silica. Interpretation
of our in situ XPCS results simultaneously with interpretation of
traditional dynamic mechanical analysis (DMA) strain sweep experiments
reveals that the debonding or yielding of bridged bound rubber layers
is key to understanding the behavior of rubber formulations containing
the silane coupling agent and high-surface area silica. These results
demonstrate that the combination of XPCS and DMA is a powerful method
for unraveling the microscale filler response to strain which dictates
the dynamic mechanical properties of reinforced soft matter composites.
With this combination of techniques, we have elucidated the great
promise of UHSA silica when used in concert with a silane coupling
agent in filled rubber. Such composites simultaneously exhibit large
moduli and low hysteresis under dynamic strain
Structure and Entanglement Factors on Dynamics of Polymer-Grafted Nanoparticles
Nanoparticles functionalized with
long polymer chains at low graft
density are interesting systems to study structureâdynamic
relationships in polymer nanocomposites since they are shown to aggregate
into strings in both solution and melts and also into spheres and
branched aggregates in the presence of free polymer chains. This work
investigates structure and entanglement effects in composites of polystyrene-grafted
iron oxide nanoparticles by measuring particle relaxations using X-ray
photon correlation spectroscopy. Particles within highly ordered strings
and aggregated systems experience a dynamically heterogeneous environment
displaying hyperdiffusive relaxation commonly observed in jammed soft
glassy systems. Furthermore, particle dynamics is diffusive for branched
aggregated structures which could be caused by less penetration of
long matrix chains into brushes. These results suggest that particle
motion is dictated by the strong interactions of chains grafted at
low density with the host matrix polymer
Structural Dynamics of Strongly Segregated Block Copolymer Electrolytes
Polymer
electrolytes are promising materials for high energy density
rechargeable batteries. However, they have low ion transport rates
and gradually lose electrode adhesion during cycling. These effects
are dependent on polymer structure and dynamics. This motivates an
investigation of diblock copolymer electrolyte dynamics. Structural
and stress relaxations have been measured with X-ray photon correlation
spectroscopy (XPCS) and rheology, respectively, as a function of salt
concentration and temperature. The polymer electrolyte studied in
this work is a mixture of polyÂ(styrene-<i>b</i>-ethylene
oxide), SEO, and lithium bistrifluoromethaneÂsulfonimide (LiTFSI).
Results from XPCS experiments showed hyperdiffusive motion for various
lithium salt concentrations and at varying temperatures, which is
indicative of soft glassy materials. This behavior is attributed to
cooperative dynamics. The decay time was a weak, nonmonotonic function
of salt concentration and decreased with increasing temperature, in
an Arrhenius fashion. In contrast, the shear modulus decreased with
increasing salt concentration and increasing temperature. The entanglement
relaxation from rheological measurements followed VogelâFulcherâTammann
behavior. The structural decay time was slower than the entanglement
relaxation time at temperatures above the glass transition temperature,
but they were approximately equal at <i>T</i><sub>g</sub> regardless of salt concentration. This may indicate a fundamental
connection between cooperative structural motion and polymer chain
motion in this material
Dynamics of Nanoparticles in Entangled Polymer Solutions
The mean square displacement
âš<i>r</i><sup>2</sup>â© of nanoparticle probes
dispersed in simple isotropic liquids
and in polymer solutions is interrogated using fluorescence correlation
spectroscopy and single-particle tracking (SPT) experiments. Probe
dynamics in different regimes of particle diameter (<i>d</i>), relative to characteristic polymer length scales, including the
correlation length (Ο), the entanglement mesh size (<i>a</i>), and the radius of gyration (<i>R</i><sub>g</sub>), are investigated. In simple fluids and for polymer solutions in
which <i>d</i> â« <i>R</i><sub>g</sub>,
long-time particle dynamics obey random-walk statistics âš<i>r</i><sup>2</sup>â©:<i>t</i>, with the bulk
zero-shear viscosity of the polymer solution determining the frictional
resistance to particle motion. In contrast, in polymer solutions with <i>d</i> < <i>R</i><sub>g</sub>, polymer molecules
in solution exert noncontinuum resistances to particle motion and
nanoparticle probes appear to interact hydrodynamically only with
a local fluid medium with effective drag comparable to that of a solution
of polymer chain segments with sizes similar to those of the nanoparticle
probes. Under these conditions, the nanoparticles exhibit orders of
magnitude faster dynamics than those expected from continuum predictions
based on the StokesâEinstein relation. SPT measurements further
show that when <i>d</i> > <i>a</i>, nanoparticle
dynamics transition from diffusive to subdiffusive on long timescales,
reminiscent of particle transport in a field with obstructions. This
last finding is in stark contrast to the nanoparticle dynamics observed
in entangled polymer melts, where X-ray photon correlation spectroscopy
measurements reveal faster but hyperdiffusive dynamics. We analyze
these results with the help of the hopping model for particle dynamics
in polymers proposed by Cai et al. and, on that basis, discuss the
physical origins of the local drag experienced by the nanoparticles
in entangled polymer solutions
Hyperdiffusive Dynamics in Newtonian Nanoparticle Fluids
Hyperdiffusive
relaxations in soft glassy materials are typically
associated with out-of-equilibrium states, and nonequilibrium physics
and aging are often invoked in explaining their origins. Here, we
report on hyperdiffusive motion in model soft materials comprised
of single-component polymer-tethered nanoparticles, which exhibit
a readily accessible Newtonian flow regime. In these materials, polymer-mediated
interactions lead to strong nanoparticle correlations, hyperdiffusive
relaxations, and unusual variations of properties with temperature.
We propose that hyperdiffusive relaxations in such materials can arise
naturally from nonequilibrium or non-Brownian volume fluctuations
forced by equilibrium thermal rearrangements of the particle pair
orientations corresponding to equilibrated shear modes