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>_n ∼ 17 200 g/mol) and high (<i>M</i>_n ∼ 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

Role of Hydrogen Bonding on Nonlinear Mechano-Optical Behavior of l‑Phenylalanine-Based Poly(ester urea)s

The uniaxial mechano-optical behavior of a series of amorphous l-phenylalanine-based poly­(ester urea) (PEU) films was studied in the rubbery state. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during deformation. When the materials were subjected to deformation at temperatures near the glass transition temperature (<i>T</i>_g), the photoelastic behavior was manifested by a small increase in birefringence with a significant increase in true stress. At temperatures above <i>T</i>_g, PEUs with a shorter diol chain length exhibited a liquid–liquid (<i>T</i>_ll) transition (rubbery–viscous transition) at about 1.06<i>T</i>_g (K) under the tested strain rate of 0.017 s^–1 (stretching speed of 20 mm/min), above which the material transforms from a heterogeneous “liquid of fixed structure” to a “true liquid” state. The initial photoelastic behavior disappears with increasing temperature, as the initial slope of the stress optical curves becomes temperature independent. Fourier transform infrared spectroscopy (FTIR) was used to study the effect of hydrogen bonding on the physical properties of PEUs as a function of temperature. The average strength of hydrogen bonding diminishes with increasing temperature. For PEUs with the longest diol chain length, the area associated with N–H stretching region exhibits a linear temperature dependence. However, a three-stage temperature dependence was observed for PEUs with shorter diol chain length. The presence of hydrogen bonding enhances the “stiff” segmental correlations between adjacent chains in the PEU structure. As a result, the photoelastic constant decreases with increasing hydrogen bonding strength