Chain Interpenetration Research Articles

Controllable Synthesis of Chain Center Dye-Labeled Star Polymers for Quantitative Examination of Interchain Conformation by Time-Resolved Fluorescence Resonance Energy Transfer.

Using a "core-first" approach with atom transfer radical polymerization, fluorescent center-functional star polymers of equivalent molecular weight but with varying numbers of arms (di-, tri-, and tetra-arm) were prepared. The sensitivity of fluorescence, combined with a dye-labeling technique introducing a fluorescent donor (carbazole) and an acceptor (anthracene) at the center of poly(methyl methacrylate) (PMMA) chains, enabled the application of time-resolved fluorescence resonance energy transfer to obtain quantitative insights into the conformation of the star polymer chains in the film state. When the results of star-branched polymers were compared with those of linear polymers of identical type and molecular weight, the impact of branching on polymer behavior was isolated for examination. Although the star topology does not alter the average intercoil distance, it affects the distance dispersity. Star polymers with higher arm numbers display decreased dispersity from distance due to reduced intermolecular aggregation at their geometric centers. This study presents the first spectroscopic evidence regarding the distribution of geometric centers in star polymers, offering a physical understanding of chain interpenetration and entanglement within star polymers.

Adhesion and Contact Aging of Acrylic Pressure-Sensitive Adhesives to Swollen Elastomers.

Fluid-infused (or swollen) elastomers are known for their antiadhesive properties. The presence of excess fluid at their surface is the main contributor to limiting contact formation and minimizing adhesion. Despite their potential, the mechanisms for adhesion and contact aging to fluid-infused elastomers are poorly understood beyond contact with a few materials (ice, biofilms, glass). This study reports on adhesion to a model fluid-infused elastomer, poly(dimethylsiloxane) (PDMS), swollen with silicone oil. The effects of oil saturation, contact time, and the opposing surface are investigated. Specifically, adhesion to two different adherents with comparable surface energies but drastically different mechanical properties is investigated: a glass surface and a soft viscoelastic acrylic pressure-sensitive adhesive film (PSA, modulus ∼25 kPa). Adhesion between the PSA and swollen PDMS [with 23% (w/w) silicone oil] retains up to 60% of its value compared to contact with unswollen (dry) PDMS. In contrast, adhesion to glass nearly vanishes in contact with the same swollen elastomer. Adhesion to the PSA also displays stronger contact aging than adhesion to glass. Contact aging with the PSA is comparable for dry and unsaturated PDMS. Moreover, load relaxation when the PSA is in contact with the PDMS does not correlate with contact aging for contact with the dry or unsaturated elastomer, suggesting that contact aging is likely caused by chain interpenetration and polymer reorganization within the contact region. Closer to full saturation of the PDMS with oil, adhesion to the PSA decreases significantly and shows a delay in the onset of contact aging that is weakly correlated to the poroelastic relaxation of the elastomer. Additional confocal imaging suggests that the presence of a layer of fluid trapped at the interface between the two solids could explain the delayed (and limited) contact aging to the oil-saturated PDMS.

Interfacial polymer architecture can control nanoparticle dispersion and rheological behavior of nanocomposites

Polymer nanocomposites (PNCs) have become essential components in many advanced materials applications, from solid state battery electrolytes to high-performance structural components. In PNCs, the large fraction of interfacial polymers determines their outstanding rheological performance; yet, strategies to control the structure and dynamics of the interfacial polymer has been very limited. Here, using poly(ethylene oxide)-silica model nanocomposite system, we propose a facile approach based on varying macromolecular architecture of interfacial polymers. We show that by changing the topology of the bound polymer from linear to star and hyperbranched, polymer-NP interaction as well as chain interpenetration in the interphases can be effectively altered without changing type or molecular weight of the polymer, or NP surface chemistry in attractive PNCs. Our results show that both dispersion and rheological behavior of PNCs depend highly on functionality and arm length of the polymers used at the interfaces. These results can be used to design ion-conductive molten polymer electrolytes with significantly improved mechanical properties suitable for solid-state battery applications.

Bio-Inspired Muco-Adhesive Polymers for Drug Delivery Applications.

Muco-adhesive drug delivery systems continue to be one of the most studied for controlled pharmacokinetics and pharmacodynamics. Briefly, muco-adhesive polymers, can be described as bio-polymers that adhere to the mucosal (mucus) surface layer, for an extended residency period of time at the site of application, by the help of interfacial forces resulting in improved drug delivery. When compared to traditional drug delivery systems, muco-adhesive carriers have the potential to enhance therapeutic performance and efficacy, locally and systematically, in oral, rectal, vaginal, amongst other routes. Yet, the achieving successful muco-adhesion in a novel polymeric drug delivery solution is a complex process involving key physico-chemico-mechanical parameters such as adsorption, wettability, polymer chain length, inter-penetration and cross-linking, to list a few. Hence, and in light of accruing progress, evidence and interest, during the last decade, this review aims to provide the reader with an overview of the theories, principles, properties, and underlying mechanisms of muco-adhesive polymers for pharmaceutics; from basics to design to characterization to optimization to evaluation to market. A special focus is devoted to recent advances incorporating bio-inspired polymers for designing controlled muco-adhesive drug delivery systems.

Optimization of Processing Conditions and Mechanical Properties for PEEK/PEI Multilayered Blends.

The goal of producing polyetheretherketone/polyetherimide (PEEK/PEI) blends is to combine the outstanding properties that both polymers present separately. Despite being miscible polymers, it is possible to achieve PEEK/PEI multilayered blends in which PEEK crystallinity is not significantly inhibited, as opposed to conventional extruding processes that lead to homogeneous mixtures with total polymer chain interpenetration. This study investigated a 50/50 (volume fraction) PEEK/PEI multilayered polymer blend in which manufacturing parameters were tailored to simultaneously achieve PEEK-PEI adhesion while keeping PEEK crystallinity in order to optimize the mechanical properties of this heterogeneous polymer blend. The interface adhesion was characterized with the use of three-point bending tests, which proved that a processing temperature below the melting point of PEEK produced weak PEEK-PEI interfaces. Results from differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and X-ray diffraction analysis (XRD) showed that under a 350 °C consolidation temperature, there is a partial mixing of PEEK and PEI layers in the interface that provides good adhesion. The thickness of the mixed homogeneous region at this temperature exhibits reduced sensitivity to processing time, which ensures that both polymers essentially remain separate phases. This also entails that multilayered blends with good mechanical properties can be reliably produced with short manufacturing cycles. The combination of mechanical performance and potential joining capability supports their use in a wide range of applications in the automotive, marine, and aerospace industries.

Investigating the Molecular Mechanism of Protein-Polymer Binding with Direct Saturation Compensated Nuclear Magnetic Resonance.

Herein, we describe a new technique, direct saturation compensated transfer (DISCO) NMR, to characterize protein-macromolecule interactions. DISCO enables the direct observation of intermolecular interactions and is used to investigate mucoadhesion, a type of polymer-protein interaction that is widely implemented in drug delivery but remains poorly understood. In a model system of bovine submaxillary mucin and poly(acrylic acid), DISCO identifies selective backbone interactions that facilitate mucoadhesion through chain interpenetration. DISCO demonstrated distinct patterns of molecular selectivity between mucoadhesive polymers when applied to hydroxypropyl cellulose and carboxymethyl cellulose and that functionalizing adhesive polymers with strongly interacting moieties may be detrimental to the overall adhesive interaction. Additionally, DISCO was used to estimate polymer-protein dissociation constants using individual proton signals as reporters. Overall, DISCO can be used as a label-free screening tool to generate polymer-specific binding fingerprints to map and quantify interactions between macromolecules.

Gold nanoparticles endowed with low-temperature colloidal stability by cyclic polyethylene glycol in ethanol

The colloidal stability of metal nanoparticles is tremendously dependent on the thermal behavior of polymer brushes. Neat polyethylene glycol (PEG) presents an unconventional upper critical solution temperature in ethanol, where phase segregation and crystallization coexist. This thermal behavior translated to a PEG brush has serious consequences on the colloidal stability in ethanol of gold nanoparticles (AuNPs) modified with PEG brushes upon cooling. We observed that AuNPs (13 nm diameter) stabilized with conventional linear PEG brushes (Mn = 6 and 11 kg mol-1) in ethanol suffer from reversible phase separation upon a temperature drop over the course of a few hours. However, the use of a polymer brush with cyclic topology as a stabilizer prevents sedimentation, ensuring the colloidal stability in ethanol at -25 °C for, at least, four months. We postulate that temperature-driven collapse of chain brushes promotes the interpenetration of linear chains, causing progressive AuNP sedimentation, a process that is unfavorable for cyclic polymer brushes whose topology prevents chain interpenetration. This study reinforces the notion about the importance of polymer topology on the colloidal stability of AuNPs.

Effects of Geometric Confinement on Caging and Dynamics of Polymer-Tethered Nanoparticle Suspensions

Well-dispersed polymer-tethered silica nanoparticles exhibit soft glassy rheology and caging behaviors due to chain interpenetration driven by entropic attraction and geometric confinement. In this...

On the transformation mechanism of polyethylene glycol- and citrate-coated silver nanocolloids under sunlight exposure

As the use of nanomaterials continues to increase, their release into the environment is also expected to increase. During the use phase of nano-enhanced products containing silver nanoparticles (Ag NPs) or after the release of Ag NPs in the environment, the latter are likely to be altered by the conditions they will encounter, such as sunlight irradiation. In this work, suspensions of citrate-stabilized and PEG-stabilized Ag NPs of around 20 nm in diameter were synthesized and aged for a week in a climatic chamber under controlled temperature (40 °C) and sunlight irradiation. The suspensions were analyzed by UV/visible spectroscopy, asymmetric flow field flow fractionation—inductively coupled plasma mass spectrometry (AF4-ICP-MS), dynamic light scattering (DLS), and transmission electron microscopy (TEM). We found that the electrostatic-stabilizing citrate promoted particle morphology changes with the emergence of a transient and minor secondary population of truncated 2D prisms (110–120 nm in size) but postponed the final sedimentation step. Conversely, the initial protecting role of the steric-stabilizing PEG was reduced by the photodegradation of the polymer chains and eventually favored inter-particle bridging resulting in aggregation and sedimentation. It is hypothesized that the bridging process proceeds via PEG chain interpenetration between two adjacent particles and reduction by the polymer photodegradation products of the Ag+ ions generated by oxidative dissolution.

Universality of the Entanglement Plateau Modulus of Comb and Bottlebrush Polymer Melts

A combination of scaling analysis and rheological experiments was used to study correlations between the entanglement plateau modulus and grafting density of graft polymers in a melt. Using the crowding parameter Φ, which describes overlap of side chains belonging to neighboring macromolecules, we identified two classes of graft polymers—combs and bottlebrushes—that demonstrate distinct conformational and rheological behaviors. In comb systems, both the backbones and sparsely grafted side chains are coiled that allow side chains of neighboring macromolecules to overlap (Φ < 1). In bottlebrush systems, steric repulsion between densely grafted side chains causes chain extension and inhibits side chain interpenetration (Φ ≥ 1). The ratio Ge,gr/Ge,lin ≅ φ3(1 + (Φ/0.7)3) of the plateau modulus of a graft polymer melt, Ge,gr, to that of a melt of linear chains, Ge,lin, is a universal function of the crowding parameter Φ ≅ φ–1nsc–1/2 and graft polymer composition φ = ng/(ng + nsc), where nsc and ng are the degre...

Structure and Entanglement Network of Model Polymer-Grafted Nanoparticle Monolayers

Ultrathin films containing polymer-grafted nanoparticles (PGNs) show promise for use in hybrid electronics and high energy density materials. In this work, we use a coarse-grained model to simulate a hexagonally packed monolayer of PGNs adsorbed on a smooth surface that is attractive to both nanoparticles and polymer chains. We find that decreasing graft density at the same graft length increases interpenetration of the polymer-grafted layers, as expected. We quantify both overall and interparticle entanglements (between polymers grafted to different PGNs). While the higher graft density particles have a higher overall number of entanglements per chain, the lower graft density particles have higher interparticle entanglements per chain due to their increased chain interpenetration. Finally, we apply uniaxial tensile deformation to the monolayers; the peak stress occurs at lower strain values for higher graft density particles, which is attributed to the relatively lower number of interparticle entanglemen...

Reattachment of crosslinked poly(ethylene oxide) via chain interpenetration and reentanglement induced by a simple wetting process

We demonstrate the reattachment of two chemically-crosslinked poly(ethylene oxide) (PEO) matrix samples prepared with the γ-ray irradiation method by a simple wetting process with a good solvent. The fractured polymer samples can be attached through chain diffusion, interpenetration into a crosslinked network, and intermixing/reentanglement. Reattached samples exhibit similar macro/microstructure and mechanical properties to those of pristine samples. This effective recovery process without any additional reactive chemicals or external stimuli highlights the importance of extending the use of simple, but fundamental, dynamic behaviors of polymer chains to obtain the desired properties as well as offering a unique and useful route toward effective healable materials.

Synthesis of Site-Specific Dye-Labeled Polymer via Atom Transfer Radical Polymerization (ATRP) for Quantitative Characterization of the Well-Defined Interchain Distance.

Novel difunctional initiators that incorporate Förster/fluorescence resonance energy transfer (FRET) pairs are generated to carry out atom transfer radical polymerization of styrene, methyl methacrylate, and n-butyl methacrylate monomers by an efficient manner. Based on the chemical structures of the initiators, the locations of the fluorophore moiety are dictated to be in the center of the chain with accurately quantified chain functionality (>90% labeling ratio). The site-specific integration of FRET dyes into separate polymer chain centers allows for characterization of the well-defined interchain distance quantitatively based on the response between these fluorescent probes. The reliability of this technique is verified in bulk state, which is in well agreement with the theoretical ones. This well-defined FRET system is expected to be a promising candidate to provide a distinct physical image at a microscopic level regarding scaling chain dimension, chain interpenetration, and polymer compatibility.

Origin of the High Carrier Mobilities of Nonperipheral Octahexyl Substituted Phthalocyanine

The carrier transport properties of nonperipheral octahexyl substituted phthalocyanine H2Pc(C6H13)8np in its crystal and columnar (Col) liquid crystal (LC) phase were investigated using density functional theory (DFT) calculations in combination with molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations. In the crystal phase, we found that the nonperipherally substituted chains of H2Pc(C6H13)8np, that interpenetrate adjacent phthalocyanines (Pcs), significantly alter the Pc-core stacking such that higher hole mobilities are observed for this system than for the nonsubstituted H2Pc. This chain interpenetration was found to be inherited by the column stacking in the Col phase and hindered the Pc-core in-plane rotations between adjacent Pcs. This rotational hindrance further caused a nonuniform distribution of the adjacent dimer Pc-core in-plane orientation configurations. The relatively high carrier mobility in the Col phase in this system can be rationalized by the nonuniform distribution of the dimer configurations that give relatively high electronic coupling between the adjacent dimers. Our results show the remarkable effects of nonperipheral substitutions on the carrier transport properties in both the crystal and Col LC phases.

Tethered poly(2-isopropyl-2-oxazoline) chains: temperature effects on layer structure and interactions probed by AFM experiments and modeling.

Thermoresponsive polymer layers on silica surfaces have been obtained by utilizing electrostatically driven adsorption of a cationic-nonionic diblock copolymer. The cationic block provides strong anchoring to the surface for the nonionic block of poly(2-isopropyl-2-oxazoline), referred to as PIPOZ. The PIPOZ chain interacts favorably with water at low temperatures, but above 46 °C aqueous solutions of PIPOZ phase separate as water becomes a poor solvent for the polymer. We explore how a change in solvent condition affects interactions between such adsorbed layers and report temperature effects on both normal forces and friction forces. To gain further insight, we utilize self-consistent lattice mean-field theory to follow how changes in temperature affect the polymer segment density distributions and to calculate surface force curves. We find that with worsening of the solvent condition an attraction develops between the adsorbed PIPOZ layers, and this observation is in good agreement with predictions of the mean-field theory. The modeling also demonstrates that the segment density profile and the degree of chain interpenetration under a given load between two PIPOZ-coated surfaces rise significantly with increasing temperature.

Relaxation Processes and Polymeric Chain Interpenetration in a PMMA-Cz/ MEH-PPV Blend Studied by Fluorescence Spectroscopy

Miscibility and energy transfer processes of a blend containing 0.1 % of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) in a matrix of poly(methyl-methacrylate) containing 1% of attached carbazolyl groups (PMMA-Cz) were studied by fluorescence spectroscopy and microscopy techniques. From SEM micrographs, a uniform surface is found. The images on the cryogenic fracture of the blend does not show domains, although from fluorescence optical microscopy domains of MEH-PPV of about 30 mm in the PMMA-Cz matrix were observed, indicating the immiscibility of the blend components. By steady-state and time-resolved fluorescence spectroscopies measured at room temperature and at 77 K, it was found that there is an important non-radiative energy transfer occurring by Forster mechanism and due to that, there must be an interpenetration degree between polymeric chains specially at room temperature, which is evidence for miscibility. This can be inferred also by temperature-dependent fluorescence spectroscopy, which informs about relaxation processes occurring in the blend and also reveals the occurrence of an interface effect that is characteristic of miscible components.

Formation of Multilayers by Star Polyelectrolytes: Effect of Number of Arms on Chain Interpenetration

We have investigated the influence of number of arms on chain interpenetration in the growth of star poly[2-(dimethylamino)ethyl methacrylate] (PDEM)/star poly(acrylic acid) (PAA) multilayers using a quartz crystal microbalance with dissipation (QCM-D). The oscillations in the changes of dissipation and frequency reflect the chain interpenetration and the variation of the mass of multilayer, respectively. The QCM-D results demonstrate that the growth of multilayers has two different mechanisms in terms of chain interpenetration. That is, the arm chains of star PDEM insert into a predeposited PAA layer to form a swollen multilayer, but the complex of star PAA with predeposited star PDEM is an "octopus-like" structure forming a dense multilayer. The transition between these two penetration modes is controlled by the number of arms in the star polyelectrolytes. As the number of arms of either PAA or PDEM increases, it becomes more difficult for star PDEM to penetrate into the PAA layer, but star PAA can more easily penetrate into the PDEM layer. According to atomic force microscopy and water contact angle measurements, all eight-bilayer multilayer surfaces have similar roughness values, and the surface wettability of the multilayers is dominated by the outermost PDEM layer.

Hierarchical supramolecular structuring and dynamical properties of water soluble polyethylene glycol–perylene self-assemblies

The structural and dynamical properties of dilute aqueous solutions of poly(ethylene glycol)-perylene diimides (PEG(n)-PDI) have been investigated by means of static and dynamic light scattering, TEM microscopy, and small-angle X-ray scattering experiments. The amphiphilic PEG(n)-PDI molecules first self-assemble into stable and compact primary stacks of a few units of planar PDI through hydrophobic and π-π interactions. These primary stacks subsequently arrange in large and globular aggregates of typically 100-250 nm via weak PEG chain interpenetration. Surprisingly, the scattered electric field autocorrelation function g((1))(q,t) measured by dynamic light scattering evolves over very long periods of times (several months) and up to a bimodal distribution. The fast relaxation mechanism is associated to the diffusion of free primary stacks, whereas the slower relaxation still indicates the presence of large self-assemblies. Kinetic experiments show that the large supramolecular aggregates slowly release the free primary stacks whose proportion increases with time. This dissociation depends on several parameters such as PEG side chain length, total concentration, and shaking.

Thermal welding of biological tissues derived from porcine aorta for manufacturing bioprosthetic cardiac valves

Sutures in cardiac valve bioprostheses have several disadvantages as they have to be manually processed and the suturing region is always a mechanically weak spot. Thermal welding of biological tissues has been evaluated as a means of replacing sutures by the direct application of heat to tissues. The mechanical strength of the welds increased up to 50°C and with lower degrees of humidity and longer times of welding. Chemical fixation was essential for the stability of the weld during re-hydration. The average mechanical strength of the welds (0.87MPa) was lower than the strength of sutures (1.36MPa) but some results showed strengths that were similar to sutures. Raman and electron micrographs showed that weld formation is primarily associated with chemical bonds between collagen fibers rather than chain flow and interpenetration.

Characteristic Shear Rate for Nonlinear Viscoelastic Behavior in a Polydisperse Polymer Solution

The characteristic shear rate for nonlinear viscoelastic behavior in a polydisperse polymer system was investigated by the study of the stress overshoot behavior of polybutadiene solutions. The frequency at the crossing point of the dynamic moduli () was determined by oscillation frequency sweeps. For the stress overshoot observed in the shear flow at a fixed rate, the dependency of the stress drop, , defined as the difference between the stress at the peak and the stress at the steady state, on the shear rate can be well described by a fitting function: , and a critical shear rate () can be obtained using this fitting function. The correlation between and for the solutions can be expressed as and the proportionality factor is dependent on the degree of chain interpenetration. When the shear rate was normalized by , the dependency of the stress drop on this normalized shear rate for the solutions with different concentrations can be superposed well. This observation indicates that can be viewed as a characteristic shear rate for nonlinear viscoelastic behavior in a polydisperse polymer solution.