7 research outputs found
Real-Time Monitoring of Chemical and Topological Rearrangements in Solidifying Amphiphilic Polymer Co-Networks: Understanding Surface Demixing
Amphiphilic
polymer co-networks provide a unique route to integrating contrasting
attributes of otherwise immiscible components within a bicontinuous
percolating morphology and are anticipated to be valuable for applications
such as biocatalysis, sensing of metabolites, and dual dialysis membranes.
These co-networks are in essence chemically forced blends and have
been shown to selectively phase-separate at surfaces during film formation.
Here, we demonstrate that surface demixing at the air–film
interface in solidifying polymer co-networks is not a unidirectional
process; instead, a combination of kinetic and thermodynamic interactions
leads to dynamic molecular rearrangement during solidification. Time-resolved
gravimetry, low contact angles, and negative out-of-plane birefringence
provided strong experimental evidence of the transitory trapping of
thermodynamically unfavorable hydrophilic moieties at the air–film
interface due to fast asymmetric solvent depletion. We also find that
slow-drying hydrophobic elements progressively substitute hydrophilic
domains at the surface as the surface energy is minimized. These findings
are broadly applicable to common-solvent bicontinuous systems and
open the door for process-controlled performance improvements in diverse
applications. Similar observations could potentially be coupled with
controlled polymerization rates to maximize the intermingling of bicontinuous
phases at surfaces, thus generating true three-dimensional, bicontinuous,
and undisturbed percolation pathways throughout the material
Transport-Limited Adsorption of Plasma Proteins on Bimodal Amphiphilic Polymer Co-Networks: Real-Time Studies by Spectroscopic Ellipsometry
Traditional hydrogels
are commonly limited by poor mechanical properties
and low oxygen permeability. Bimodal amphiphilic co-networks (β-APCNs)
are a new class of materials that can overcome these limitations by
combining hydrophilic and hydrophobic polymer chains within a network
of co-continuous morphology. Applications that can benefit from these
improved properties include therapeutic contact lenses, enzymatic
catalysis supports, and immunoisolation membranes. The continuous
hydrophobic phase could potentially increase the adsorption of plasma
proteins in blood-contacting medical applications and compromise in
vivo material performance, so it is critical to understand the surface
characteristics of β-APCNs and adsorption of plasma proteins
on β-APCNs. From real-time spectroscopic visible (Vis) ellipsometry
measurements, plasma protein adsorption on β-APCNs is shown
to be transport-limited. The adsorption of proteins on the β-APCNs
is a multistep process with adsorption to the hydrophilic surface
initially, followed by diffusion into the material to the internal
hydrophilic/hydrophobic interfaces. Increasing the cross-linking of
the PDMS phase reduced the protein intake by limiting the transport
of large proteins. Moreover, the internalization of the proteins is
confirmed by the difference between the surface-adsorbed protein layer
determined from XPS and bulk thickness change from Vis ellipsometry,
which can differ up to 20-fold. Desorption kinetics depend on the
adsorption history with rapid desorption for slow adsorption rates
(i.e., slow-diffusing proteins within the network), whereas proteins
with fast adsorption kinetics do not readily desorb. This behavior
can be directly related to the ability of the protein to spread or
reorient, which affects the binding energy required to bind to the
internal hydrophobic interfaces
High Strength Bimodal Amphiphilic Conetworks for Immunoisolation Membranes: Synthesis, Characterization, and Properties
A strategy
for the synthesis of new cross-linkable bimodal amphiphilic grafts
(bAPGs) was developed. These grafts are of hydrophilic PDMAAm backbones
carrying low (<i>M</i><sub>n</sub> ∼ 17 200
g/mol) and high (<i>M</i><sub>n</sub> ∼ 117 000
g/mol) molecular weight hydrophobic PDMS branches, each branch carrying
a vinylsilyl end-group. The bAPGs were cross-linked by Karstedt catalyst
to bimodal amphiphilic conetworks (bAPCNs) by the use of polyhydrosiloxane-<i>co</i>-PDMS as the cross-linker. Membranes prepared from bAPCNs
exhibit mechanical properties surprisingly superior to earlier APCNs
prepared with APGs with monomodal low molecular weight branches. Membrane
bimodality controls surface morphology and topography by means of
elastic wrinkling instability during film formation. Semipermeable
bAPCN membranes with precisely controlled nanochannel dimensions were
prepared so as to allow rapid insulin diffusion and prevent passage
of IgG. bAPCN membranes were designed for immunoprotection of live
pancreatic islets and are thus key components for a bioartificial
pancreas
Three-Dimensional Printed Shape Memory Objects Based on an Olefin Ionomer of Zinc-Neutralized Poly(ethylene-<i>co</i>-methacrylic acid)
Three-dimensional
printing enables the net shape manufacturing of objects with minimal
material waste and low tooling costs, but the functionality is generally
limited by available materials, especially for extrusion-based printing,
such as fused deposition modeling (FDM). Here, we demonstrate shape
memory behavior of 3D printed objects with FDM using a commercially
available olefin ionomer, Surlyn 9520, which is zinc-neutralized polyÂ(ethylene-<i>co</i>-methacrylic acid). The initial fixity for 3D printed
and compression-molded samples was similar, but the initial recovery
was much lower for the 3D printed sample (<i>R</i> = 58%)
than that for the compression-molded sample (<i>R</i> =
83%). The poor recovery in the first cycle is attributed to polyethylene crystals formed
during programming that act to resist the permanent network recovery.
This effect is magnified in the 3D printed part due to the higher
strain (lower modulus in the 3D printed part) at a fixed programming
stress. The fixity and recovery in subsequent shape memory cycles
are greater for the 3D printed part than for the compression-molded
part. Moreover, the programmed strain can be systematically modulated
by inclusion of porosity in the printed part without adversely impacting
the fixity or recovery. These characteristics enable the direct formation
of complex shapes of thermoplastic shape memory polymers that can
be recovered in three dimensions with the appropriate trigger, such
as heat, through the use of FDM as a 3D printing technology
Three-Dimensional Printed Shape Memory Objects Based on an Olefin Ionomer of Zinc-Neutralized Poly(ethylene-<i>co</i>-methacrylic acid)
Three-dimensional
printing enables the net shape manufacturing of objects with minimal
material waste and low tooling costs, but the functionality is generally
limited by available materials, especially for extrusion-based printing,
such as fused deposition modeling (FDM). Here, we demonstrate shape
memory behavior of 3D printed objects with FDM using a commercially
available olefin ionomer, Surlyn 9520, which is zinc-neutralized polyÂ(ethylene-<i>co</i>-methacrylic acid). The initial fixity for 3D printed
and compression-molded samples was similar, but the initial recovery
was much lower for the 3D printed sample (<i>R</i> = 58%)
than that for the compression-molded sample (<i>R</i> =
83%). The poor recovery in the first cycle is attributed to polyethylene crystals formed
during programming that act to resist the permanent network recovery.
This effect is magnified in the 3D printed part due to the higher
strain (lower modulus in the 3D printed part) at a fixed programming
stress. The fixity and recovery in subsequent shape memory cycles
are greater for the 3D printed part than for the compression-molded
part. Moreover, the programmed strain can be systematically modulated
by inclusion of porosity in the printed part without adversely impacting
the fixity or recovery. These characteristics enable the direct formation
of complex shapes of thermoplastic shape memory polymers that can
be recovered in three dimensions with the appropriate trigger, such
as heat, through the use of FDM as a 3D printing technology
Rationally Designed Polyimides for High-Energy Density Capacitor Applications
Development
of new dielectric materials is of great importance
for a wide range of applications for modern electronics and electrical
power systems. The state-of-the-art polymer dielectric is a biaxially
oriented polypropylene (BOPP) film having a maximal energy density
of 5 J<b>/</b>cm<sup>3</sup> and a high breakdown field of 700
MV/m, but with a limited dielectric constant (∼2.2) and a reduced
breakdown strength above 85 °C. Great effort has been put into
exploring other materials to fulfill the demand of continuous miniaturization
and improved functionality. In this work, a series of polyimides were
investigated as potential polymer materials for this application.
Polyimide with high dielectric constants of up to 7.8 that exhibits
low dissipation factors (<1%) and high energy density around 15
J<b>/</b>cm<sup>3</sup>, which is 3 times that of BOPP, was
prepared. Our syntheses were guided by high-throughput density functional
theory calculations for rational design in terms of a high dielectric
constant and band gap. Correlations of experimental and theoretical
results through judicious variations of polyimide structures allowed
for a clear demonstration of the relationship between chemical functionalities
and dielectric properties
Poly(cannabinoid)s: Hemp-Derived Biocompatible Thermoplastic Polyesters with Inherent Antioxidant Properties
The
legalization of hemp cultivation in the United States has caused
the price of hemp-derived cannabinoids to decrease 10-fold within
2 years. Cannabidiol (CBD), one of many naturally occurring diols
found in hemp, can be purified in high yield for low cost, making
it an interesting candidate for polymer feedstock. In this study,
two polyesters were synthesized from the condensation of either CBD
or cannabigerol (CBG) with adipoyl chloride. Poly(CBD-Adipate) was
cast into free-standing films and subjected to thermal, mechanical,
and biological characterization. Poly(CBD-Adipate) films exhibited
a lack of cytotoxicity toward adipose-derived stem cells while displaying
an inherent antioxidant activity compared to poly(lactide) films.
Additionally, this material was found to be semi-crystalline and able
to be melt-processed into a plastic hemp leaf using a silicone baking
mold