34 research outputs found
Computer Simulation of Concurrent Bulk- and Surface-Initiated Living Polymerization
We use Monte Carlo simulation implementing the bond fluctuation
model formalism in the canonical (NVT) ensemble to study living polymerization
initiated concurrently in bulk and on flat substrates. Our results
reveal that the molecular weights and molecular weight distributions
of both classes of polymers depend on the grafting density of the
surface-bound polymers (σ) and the fraction of polymers on the
surface (η) relative to that in bulk. In general, polymer grafts
on the surface possess lower molecular weight and higher polydispersity
index compared to their bulk counterparts. The difference between
the molecular weight of the two populations of polymers decreases
with decreasing σ and increasing η. Our work provides
evidence that the common practice of using the molecular weight of
bulk-initiated polymers in estimating the grafting density of polymeric
anchors on flat substrates is not generally valid
Swelling of Hydrophilic Polymer Brushes by Water and Alcohol Vapors
We
examine the effect of end-tethering, grafting density (σ),
chemistry of polymer side chain, and solvent type on the vapor swelling
of hydrophilic polymer brushes. Using a library of samples derived
by postpolymerization modification of a poly(2-(dimethylamino)ethyl
methacrylate) (PDMAEMA) brush, we determine the extent of vapor swelling
and solvent uptake at different vapor pressures of water, methanol,
and ethanol using spectroscopic ellipsometry. We compare the results
from neat PDMAEMA and PDMAEMA quaternized by methyl iodide with chemically
analogous samples prepared by spincasting bulk PDMAEMA. We find that
brush samples swell to greater extents than spuncast samples, indicating
a role for end-tethering in the vapor uptake process. Furthermore,
vapor swelling of polymer brushes depends strongly on both polymer
and solvent chemistry. We demonstrate that the extent to which σ
affects vapor sorption inside the brush depends on polymer side chain
chemistry, indicating an interdependence of the observed parameters
on each other. The implications of these findings for the use of polymer
brushes in technologies such as vapor sensing applications are discussed
Surface-Anchored Poly(<i>N</i>‑isopropylacrylamide) Orthogonal Gradient Networks
We present a versatile
synthetic route leading toward generating
surface-attached polyacrylamide gels, in which the cross-link density
varies continuously and gradually across the substrate in two orthogonal
directions. We employ free radical polymerization to synthesize random
copolymers comprising ∼5% of photoactive methacrylyloxybenzophenone
(MABP), ∼5% of thermally active styrene sulfonyl azide (SSAz),
and ∼90% of <i>N</i>-isopropylacrylamide (NIPAAm)
units. The presence of MABP and SSAz in the copolymer facilitates
control over the cross-link density of the gel in an orthogonal manner
using photoactivated and thermally activated cross-linking chemistries,
respectively. Spectroscopic ellipsometry is employed to determine
the degree of swelling of the gel in water and methanol as a function
of position on the substrate. Network swelling varies continuously
and gradually across the substrate and is high in regions of low gel
fractions and low in regions of high gel fractions
Effect of Protein-like Copolymers Composition on the Phase Separation Dynamics of a Polymer Blend: A Monte Carlo Simulation
We
use kinetic Monte Carlo simulation based on the bond fluctuation model
to investigate the dynamics of phase separation in immiscible 80/20
A/B binary polymer blends, comprising 80% and 20% of A and B components,
respectively, in the presence of ≈4.92% 30-mer protein-like
copolymer (PLC) made of C and D segments. The molecular interactions
are chosen such that there is an attraction between A and C and between
B and D segments and no interaction between like segments; all other
interaction energies have been chosen to be repulsive. The PLC migration
to and presence at the A/B interface effectively slow down the process
of phase separation in binary blends, thereby minimizing the unfavorable
A/B contacts and reducing the A/B interfacial tension. The ability
of PLCs to effectively retard the process of phase separation depends
sensitively on the PLC composition. PLCs with 0.3 ≤ <i>x</i><sub>C</sub> ≤ 0.5, where <i>x</i><sub>C</sub> is the mole fraction of C, are most effective in compatibilizing
the 80/20 A/B binary blend. The growth of phase-separated domains
follows a dynamical scaling law for both the binary and ternary blends
compatibilized by PLCs in the late stage of phase separation with
universal scaling functions that are nearly independent of PLC composition
Proteinlike Copolymers as Encapsulating Agents for Small-Molecule Solutes
We describe the utilization of proteinlike
copolymers (PLCs) as
encapsulating agents for small-molecule solutes. We perform Monte
Carlo simulations on systems containing PLCs and model solute molecules
in order to understand how PLCs assemble in solution and what system
conditions promote solute encapsulation. Specifically, we explore
how the chemical composition of the PLCs and the range and strength
of molecular interactions between hydrophobic segments on the PLC
and solute molecules affect the solute encapsulation efficiency. The
composition profiles of the hydrophobic and hydrophilic segments,
the solute, and implicit solvent (or voids) within the PLC globule
are evaluated to gain a complete understanding of the behavior in
the PLC/solute system. We find that a single-chain PLC encapsulates
solute successfully by collapsing the macromolecule to a well-defined
globular conformation when the hydrophobic/solute interaction is at
least as strong as the interaction strength among hydrophobic segments
and the interaction among solute molecules is at most as strong as
the hydrophobic/solute interaction strength. Our results can be used
by experimentalists as a framework for optimizing unimolecular PLC
solute encapsulation and can be extended potentially to applications
such as “drug” delivery via PLCs
Formation and Antifouling Properties of Amphiphilic Coatings on Polypropylene Fibers
We describe the formation of amphiphilic polymeric assemblies
via
a three-step functionalization process applied to polypropylene (PP)
nonwovens and to reference hydrophobic self-assembled <i>n</i>-octadecyltrichlorosilane (ODTS) monolayer surfaces. In the first
step, denatured proteins (lysozyme or fibrinogen) are adsorbed onto
the hydrophobic PP or the ODTS surfaces, followed by cross-linking
with glutaraldehyde in the presence of sodium borohydride (NaBH<sub>4</sub>). The hydroxyl and amine functional groups of the proteins
permit the attachment of initiator molecules, from which poly (2-hydroxyethyl
methacrylate) (PHEMA) polymer grafts are grown directly through “grafting
from” atom transfer radical polymerization. The terminal hydroxyls
of HEMA’s pendent groups are modified with fluorinating moieties
of different chain lengths, resulting in amphiphilic brushes. A palette
of analytical tools, including ellipsometry, contact angle goniometry,
Fourier transform infrared spectroscopy in the attenuated total reflection
mode, and X-ray photoelectron spectroscopy is employed to determine
the changes in physicochemical properties of the functionalized surfaces
after each modification step. Antifouling properties of the resultant
amphiphilic coatings on PP are analyzed by following the adsorption
of fluorescein isothiocyanate-labeled bovine serum albumin as a model
fouling protein. Our results suggest that amphiphilic coatings suppress
significantly adsorption of proteins as compared with PP fibers or
PP surfaces coated with PHEMA brushes. The type of fluorinated chain
grafted to PHEMA allows modulation of the surface composition of the
topmost layer of the amphiphilic coating and its antifouling capability
Formation and Antifouling Properties of Amphiphilic Coatings on Polypropylene Fibers
We describe the formation of amphiphilic polymeric assemblies
via
a three-step functionalization process applied to polypropylene (PP)
nonwovens and to reference hydrophobic self-assembled <i>n</i>-octadecyltrichlorosilane (ODTS) monolayer surfaces. In the first
step, denatured proteins (lysozyme or fibrinogen) are adsorbed onto
the hydrophobic PP or the ODTS surfaces, followed by cross-linking
with glutaraldehyde in the presence of sodium borohydride (NaBH<sub>4</sub>). The hydroxyl and amine functional groups of the proteins
permit the attachment of initiator molecules, from which poly (2-hydroxyethyl
methacrylate) (PHEMA) polymer grafts are grown directly through “grafting
from” atom transfer radical polymerization. The terminal hydroxyls
of HEMA’s pendent groups are modified with fluorinating moieties
of different chain lengths, resulting in amphiphilic brushes. A palette
of analytical tools, including ellipsometry, contact angle goniometry,
Fourier transform infrared spectroscopy in the attenuated total reflection
mode, and X-ray photoelectron spectroscopy is employed to determine
the changes in physicochemical properties of the functionalized surfaces
after each modification step. Antifouling properties of the resultant
amphiphilic coatings on PP are analyzed by following the adsorption
of fluorescein isothiocyanate-labeled bovine serum albumin as a model
fouling protein. Our results suggest that amphiphilic coatings suppress
significantly adsorption of proteins as compared with PP fibers or
PP surfaces coated with PHEMA brushes. The type of fluorinated chain
grafted to PHEMA allows modulation of the surface composition of the
topmost layer of the amphiphilic coating and its antifouling capability
Toward the Development of a Versatile Functionalized Silicone Coating
The development of a versatile silicone
copolymer coating prepared by the chemical coupling of trichlorosilane
(TCS) to the vinyl groups of poly(vinylmethylsiloxane) (PVMS) is reported.
The resultant PVMS-TCS copolymer can be deposited as a functional
organic layer on a hydrophobic poly(dimethylsiloxane) substrate and
its mechanical modulus can be regulated by varying the TCS coupling
ratio. In this paper, several case studies demonstrating the versatile
properties of these PVMS-TCS functional coatings on PDMS elastomer
substrates are presented. Numerous experimental probes, including
optical microscopy, Fourier-transform infrared spectroscopy, surface
contact angle, ellipsometry, and nanoindentation, are utilized to
interrogate the physical and chemical characteristics of these PVMS-TCS
coatings
Instability of Surface-Grafted Weak Polyacid Brushes on Flat Substrates
We study the stability of weak polyacid
brush (WPAB) gradients
in aqueous media covering a range in grafting density (σ) spanning
0.05–0.5 chains/nm<sup>2</sup> using two analogous surface-anchored
bromoisobutyrate-based initiators for atom transfer radical polymerization
(ATRP) bearing either an ester or amide linker. Variations in dry
thickness of ester-based WPABs as a function of time and pH are consistent
with WPAB degrafting via linker hydrolysis catalyzed by mechanical
tension in the grafted chains. Sources of tension considered include
high σ, as well as swelling and electrostatic repulsion associated
with increasing degree of deprotonation (α) of repeat units
in the WPAB. Normalized thickness of the WPAB decreases by a maximum
amount at intermediate σ between ∼0.05−0.15 chains/nm<sup>2</sup>, implying that contributions to tension by α are counterbalanced
by charge regulation in the WPAB at high σ. Amide-based WPABs
are more stable up to 264 h incubation, suggesting that commonly used
ester-bearing ATRP initiators are more susceptible to hydrolysis over
the time scales examined
Swelling of Polyelectrolyte and Polyzwitterion Brushes by Humid Vapors
Swelling
behavior of polyelectrolyte and polyzwitterion brushes
derived from poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) in
water vapor is investigated using a combination of neutron and X-ray
reflectivity and spectroscopic ellipsometry over a wide range of relative
humidity (RH) levels. The extent of swelling depends strongly on the
nature of the side-chain chemistry. For parent PDMAEMA, there is an
apparent enrichment of water vapor at the polymer/air interface. Despite
extensive swelling at high humidity level, no evidence of charge repulsion
is found in weak or strong polyelectrolyte brushes. Polyzwitterionic
brushes swell to a greater extent than the quaternized brushes studied.
However, for RH levels beyond 70%, the polyzwitterionic brushes take
up less water molecules, leading to a decline in water volume fraction
from the maximum of ∼0.30 down to ∼0.10. Using a gradient
in polymer chain grafting density (σ), we provide evidence that
this behavior stems from the formation of inter- and intramolecular
zwitterionic complexes