12 research outputs found
Tunable, Liquid Resistant Tip Enhanced Raman Spectroscopy Probes: Toward Label-Free Nano-Resolved Imaging of Biological Systems
Tip
enhanced Raman spectroscopy (TERS) has been established as
a powerful, noninvasive technique for chemical identification at the
nanoscale. However, difficulties, including the degradation of probes,
limit its use in liquid systems. Here TERS probes for studies in aqueous
environments have been demonstrated using titanium nitride coatings
with an alumina protective layer. The probes show enhancement in signal
intensity as high as 380% in liquid measurements, and the probe resonance
can be tuned by varying deposition conditions to optimize performance
for different laser sources and types of samples. This development
of inexpensively produced probes suited for studies in aqueous environments
enables its wider use for fields such as biology and biomedicine in
which aqueous environments are the norm
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
Manipulation of Polymer/Polymer Interface Width from Nonequilibrium Deposition
We demonstrate, using neutron reflectivity,
that the width of a
nonequilibrium interface between an organo-soluble aromatic polyimide
film and triacetate cellulose (TAC) support film created by spin-coating
or solution-casting can be broadened in a controllable way using a
âswelling agentâ in the deposition process. In a favorable
case, the adhesion, as measured by T-peel tests, can be increased
by a factor of 7 by adjustment of the solvent composition. The morphologies
of the TAC fractured surfaces after peeling tests measured by AFM
reveal that broadening of the interfacial width causes an interconnected
network in the interface, leading to a sharp increase in the interfacial
adhesion. Differences in the chemistry (solubility) of the materials
being deposited do make a difference in the effectiveness of this
strategy of using a âswelling agentâ. For one polyimide,
a 3-fold increase in adhesion can be obtained by optimizing the deposition
temperature, but this approach for improving adhesion is less effective
than that of adding âswelling agentâ. The formation
of robust interfaces of this type is important because of the critical
roles that multilayer films containing polymers with special properties
and tailored structures play in applications as diverse as computer
displays, photovoltaic devices, and polymeric electronics. The âswelling
agentâ strategy makes it possible to produce polymer multilayer
structures in a cost-effective way with roll-to-roll mass production
using direct solution coating
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
Synthesis and Isomeric Characterization of Well-Defined 8âShaped Polystyrene Using Anionic Polymerization, Silicon Chloride Linking Chemistry, and Metathesis Ring Closure
A methodology to efficiently synthesize
well-defined, 8-shaped
polystyrene using anionic polymerization, silicon chloride linking
chemistry, and metathesis ring closure has been developed, and the
8-shaped architecture was ascertained using the fragmentation pattern
of the corresponding Ag<sup>+</sup> adduct, acquired with tandem mass
spectrometry. The 4-arm star precursor, 4-<i>star</i>-α-4-pentenylÂpolystyrene,
was formed by linking α-4-pentenylÂpolyÂ(styryl)Âlithium
(PSLi) with 1,2-bisÂ(methylÂdichlorosilyl)Âethane and reacting
the excess PSLi with 1,2-epoxybutane to facilitate purification. Ring
closure of 4-<i>star</i>-α-4-pentenylÂpolystyrene
was carried out in dichloromethane under mild conditions using a Grubbs
metathesis catalyst, bisÂ(tricyclohexylÂphosphine)Âbenzylidine
rutheniumÂ(IV) chloride. Both the 4-arm star precursor and resulting
8-shaped polystyrene were characterized using SEC, NMR, and MALDI-ToF
mass spectrometry (MS). Tandem mass spectrometry (MS<sup>2</sup>)
was used for the first time to study the fragmentation pattern of
8-shaped polystyrene. The results confirmed the formation of the intra-silicon-linked,
8-shaped polystyrene isomer, but the observed spectra left open the
possibility that the inter-silicon-linked, 8-shaped polystyrene isomer
was also produced
Detection of Surface Enrichment Driven by Molecular Weight Disparity in Virtually Monodisperse Polymers
The preference for a shorter chain
component at a polymer blend
surface impacts surface properties key to application-specific performance.
While such segregation is known for blends containing low molecular
weight additives or systems with large polydispersity, it has not
been reported for anionically polymerized polymers that are viewed,
in practice, as monodisperse. Observations with surface layer matrix-assisted
laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS),
which distinguishes surface species without labeling and provides
the entire molecular weight distribution, demonstrate that entropically
driven surface enrichment of shorter chains occurs even in low polydispersity
materials. For 6 kDa polystyrene the number-average molecular weight
(<i>M</i><sub><i>n</i></sub>) at the surface is
ca. 300 Da (5%) lower than that in the bulk, and for 7 kDa polyÂ(methyl
methacryalate) the shift is ca. 500 Da. These observations are in
qualitative agreement with results from a mean-field theory that considers
a homopolymer melt with a molecular-weight distribution matched to
the experiments
Anomalous Confinement Slows Surface Fluctuations of Star Polymer Melt Films
The unusually large film thickness
at which confinement effects
manifest themselves in surface fluctuations of unentangled four-arm
star polymers has been defined using film thicknesses from 10<i>R</i><sub>g</sub> to 107<i>R</i><sub>g</sub>. For
15k four-arm star polystyrene (SPS), confinement appears at a thickness
between 112 nm (40<i>R</i><sub>g</sub>) and 72 nm (26<i>R</i><sub>g</sub>), which is remarkably larger than the thicknesses
at which confinement appears for unentangled 6k linear (<15 nm,
<7<i>R</i><sub>g</sub>) and 6k and 14k cyclic (24 and
22 nm, respectively) polystyrenes. Data for 15k star films can be
rationalized using a two-layer model with a 17 nm (6<i>R</i><sub>g</sub>) thick highly viscous layer at the substrate, which
is significantly thicker than the 1<i>R</i><sub>g</sub> thick
âirreversibly adsorbedâ layer. For a 29 nm (10<i>R</i><sub>g</sub>) thick film, more striking confinement occurs
due to the combined influence of both interfaces. These results underscore
the extraordinary role long-chain branching plays in dictating surface
fluctuations of thin films
Scaling Behavior and Segment Concentration Profile of Densely Grafted Polymer Brushes Swollen in Vapor
The
scaling of the thickness, <i>h</i><sub>s</sub>, of
a densely grafted polymer brush of chain length <i>N</i> and grafting density Ï swollen in vapor agrees quantitatively
with the scaling reported by Kuhl et al. for densely grafted brushes
swollen in liquid. Deep in the brush, next to the substrate, the shape
of the segment concentration profile is the same whether the brush
is swollen by liquid or by vapor. Differences in the segment concentration
profile are manifested primarily in the swollen brush interface with
the surrounding fluid. The interface of the polymer brush swollen
in vapor is much more abrupt than that of the same brush swollen in
liquid. This has implications for the compressibility of the swollen
brush surface and for fluctuations at that surface
Modifying Surface Fluctuations of Polymer Melt Films with Substrate Modification
Deposition
of a plasma polymerized film on a silicon substrate
substantially changes the fluctuations on the surface of a sufficiently
thin melt polystyrene (PS) film atop the substrate. Surface fluctuation
relaxation times measured with X-ray photon correlation spectroscopy
(XPCS) for ca. 4<i>R</i><sub><i>g</i></sub> thick
melt films of 131 kg/mol linear PS on hydrogen-passivated silicon
(HâSi) and on a plasma polymer modified silicon wafer can both
be described using a hydrodynamic continuum theory (HCT) that assumes
the film is characterized throughout its depth by the bulk viscosity.
However, when the film thickness is reduced to âŒ3<i>R<sub>g</sub></i>, confinement effects are evident. The surface fluctuations
are slower than predicted using the HCT, and the confinement effect
for the PS on HâSi is larger than that for the PS on the plasma
polymerized film. This deviation is due to a difference in the thicknesses
of the strongly adsorbed layers at the substrate which are impacted
by the substrate surface energy