Role of Thermal History and Entanglement Related Thickness Selection in Polymer Crystallization

Using molecular dynamics simulations and primitive path analysis, we show that hot entangled polymer melts can crystallize faster with higher crystallinities and larger crystalline stem lengths, as compared to cold melts under rapid quenching conditions or during cold-crystallization. This counterintuitive phenomenon similar to the so-called Mpemba effect observed for water can be explained by the temperature dependence of entanglements. Our results demonstrate the key role of the entanglement state for crystallization properties and provide a new approach to understand the role of thermal history and to the open question of thickness selection in polymer crystallization

Cononsolvency Effect: When the Hydrogen Bonding between a Polymer and a Cosolvent Matters

Despite the fact that the observation of cononsolvency was reported as early as four decades ago, its phase-transition mechanism is still under debate. In this work, we provided a comprehensive study of the phase behaviors of poly(N-isopropylacrylamide) (PNiPAAm) in sulfoxide or sulfone aqueous solutions. We observed a sharp collapse transition of PNiPAAm brushes in sulfoxide but not in sulfone aqueous solutions by equilibrium measurements of in situ spectroscopic ellipsometry. We found that the hydrogen-bond formation between sulfoxide oxygens and amide hydrogens of the polymer chains plays a critical role in regulating the cononsolvency of PNiPAAm. We have extended the concept of preferential adsorption by taking into account hydrophobic interactions between cosolvent molecules, which are adsorbed on the polymer by hydrogen bonds. This can explain the experimental observations of PNiPAAm brushes in these solvent mixtures and sheds light on understanding the phase behaviors of polymer solutions where hydrogen bonds and hydrophobic interaction play a critical role. Our results can also be of interest for the liquid–liquid phase separation in the living cell where the condensation of proteins bound to large biomacromolecules plays an essential role

Multicore Unimolecular Structure Formation in Single Dendritic–Linear Copolymers under Selective Solvent Conditions

The conformational and thermodynamic properties of single dendritic–linear copolymers are investigated by analytical models and computer simulations. Applying poor solvent conditions on the dendritic part, these molecules are known to form single unimolecular micelle-like structures. A mean-field model applying the Daoud–Cotton approach and a surface tension argument is presented and suggests the splitting of the unimolecular single-core structure into a multicore structure with increasing dendrimers generation and decreasing solvent selectivity. Monte Carlo simulations utilizing the bond fluctuation model with explicit solvent are performed which show the formation of multicore structures for trifunctional codendrimers of different generations and spacer lengths with linear chains attached to the terminal groups. These findings are aimed to understand the physics of spontaneous self-assembly of codendrimers in various well-defined macro-conformations under change of environmental conditions with potential applications such as drug delivery systems

Inclusion Free Energy of Nanoparticles in Polymer Brushes

Using molecular dynamics simulations, the forces acting on nanoparticles inside polymer brushes are computed. Vertical force profiles are obtained under variation of grafting density, nanoparticle (NP) size, solvent quality, and degree of polydispersity. The force profiles are integrated to obtain the inclusion free energies. If the NP size is fixed, this energy scales with the osmotic pressure, consistent with current theoretical models. These models also predict a scaling of the free energy proportional to the volume of the NP, which is verified in good solvent and at high densities. Otherwise, the power exponent remains lower, and surface tension as a possible cause for the observed deviation is discussed. Polydispersity is shown to reduce the inclusion free energy, while the power law scaling as a function of NP size remains unchanged. Finally, polymer brushes in Θ-solvent are shown to violate simple scaling predictions within the density regime covered in this work. Using a Flory–Huggins mean-field model, we demonstrate that the universal scaling regime is restricted to very low grafting densities below σ = 0.01, and the observed deviations are a result of higher order contributions to the virial expansion of the osmotic pressure

Thermal Tunneling of Homopolymers through Amphiphilic Membranes

We propose a theory to predict the passive translocation of flexible polymers through amphiphilic membranes. By using a generic model for the potential felt by a monomer across the membrane we calculate the free energy profile for homopolymers as a function of their hydrophobicity. Our model explains the translocation window and the translocation rates as a function of chain hydrophobicity in quantitative agreement with simulation results. The potential model leads to a new adsorption transition where chains switch from a one-sided bound adsorbed state into a bridging state through the membrane core by increasing the hydrophobicity beyond a critical value. We demonstrate that the hydrophobicity leading to the fastest translocation coincides with the solution for the critical point of adsorption in the limit of long chains

Nanoparticles of Various Degrees of Hydrophobicity Interacting with Lipid Membranes

Using coarse-grained molecular dynamics simulations, we study the passive translocation of nanoparticles with a size of about 1 nm and with tunable degrees of hydrophobicity through lipid bilayer membranes. We observe a window of translocation with a sharp maximum for nanoparticles having a hydrophobicity in between hydrophilic and hydrophobic. Passive translocation can be identified as diffusive motion of individual particles in a free energy landscape. By combining direct sampling with umbrella-sampling techniques we calculate the free energy landscape for nanoparticles covering a wide range of hydrophobicities. We show that the directly observed translocation rate of the nanoparticles can be mapped to the mean-escape-rate through the calculated free energy landscape, and the maximum of translocation can be related with the maximally flat free energy landscape. The limiting factor for the translocation rate of nanoparticles having an optimal hydrophobicity can be related with a trapping of the particles in the surface region of the membrane. Here, hydrophobic contacts can be formed but the free energy effort of insertion into the brush-like tail regions can still be avoided. The latter forms a remaining barrier of a few <i>k</i>_B<i>T</i> and can be spontaneously surmounted. We further investigate cooperative effects of a larger number of nanoparticles and their impact on the membrane properties such as solvent permeability, area per lipid, and the orientation order of the tails. By calculating the partition of nanoparticles at the phase boundary between water and oil, we map the microscopic parameter of nanoparticle hydrophobicity to an experimentally accessibly partition coefficient. Our studies reveal a generic mechanism for spherical nanoparticles to overcome biological membrane-barriers without the need of biologically activated processes

Nanoparticles of Various Degrees of Hydrophobicity Interacting with Lipid Membranes

Using coarse-grained molecular dynamics simulations, we study the passive translocation of nanoparticles with a size of about 1 nm and with tunable degrees of hydrophobicity through lipid bilayer membranes. We observe a window of translocation with a sharp maximum for nanoparticles having a hydrophobicity in between hydrophilic and hydrophobic. Passive translocation can be identified as diffusive motion of individual particles in a free energy landscape. By combining direct sampling with umbrella-sampling techniques we calculate the free energy landscape for nanoparticles covering a wide range of hydrophobicities. We show that the directly observed translocation rate of the nanoparticles can be mapped to the mean-escape-rate through the calculated free energy landscape, and the maximum of translocation can be related with the maximally flat free energy landscape. The limiting factor for the translocation rate of nanoparticles having an optimal hydrophobicity can be related with a trapping of the particles in the surface region of the membrane. Here, hydrophobic contacts can be formed but the free energy effort of insertion into the brush-like tail regions can still be avoided. The latter forms a remaining barrier of a few <i>k</i>_B<i>T</i> and can be spontaneously surmounted. We further investigate cooperative effects of a larger number of nanoparticles and their impact on the membrane properties such as solvent permeability, area per lipid, and the orientation order of the tails. By calculating the partition of nanoparticles at the phase boundary between water and oil, we map the microscopic parameter of nanoparticle hydrophobicity to an experimentally accessibly partition coefficient. Our studies reveal a generic mechanism for spherical nanoparticles to overcome biological membrane-barriers without the need of biologically activated processes

Nanoparticles of Various Degrees of Hydrophobicity Interacting with Lipid Membranes

Using coarse-grained molecular dynamics simulations, we study the passive translocation of nanoparticles with a size of about 1 nm and with tunable degrees of hydrophobicity through lipid bilayer membranes. We observe a window of translocation with a sharp maximum for nanoparticles having a hydrophobicity in between hydrophilic and hydrophobic. Passive translocation can be identified as diffusive motion of individual particles in a free energy landscape. By combining direct sampling with umbrella-sampling techniques we calculate the free energy landscape for nanoparticles covering a wide range of hydrophobicities. We show that the directly observed translocation rate of the nanoparticles can be mapped to the mean-escape-rate through the calculated free energy landscape, and the maximum of translocation can be related with the maximally flat free energy landscape. The limiting factor for the translocation rate of nanoparticles having an optimal hydrophobicity can be related with a trapping of the particles in the surface region of the membrane. Here, hydrophobic contacts can be formed but the free energy effort of insertion into the brush-like tail regions can still be avoided. The latter forms a remaining barrier of a few <i>k</i>_B<i>T</i> and can be spontaneously surmounted. We further investigate cooperative effects of a larger number of nanoparticles and their impact on the membrane properties such as solvent permeability, area per lipid, and the orientation order of the tails. By calculating the partition of nanoparticles at the phase boundary between water and oil, we map the microscopic parameter of nanoparticle hydrophobicity to an experimentally accessibly partition coefficient. Our studies reveal a generic mechanism for spherical nanoparticles to overcome biological membrane-barriers without the need of biologically activated processes

How do immobilised cell-adhesive Arg–Gly–Asp-containing peptides behave at the PAA brush surface?

<p>Bio-engineered surfaces that aim to induce normal cell behaviour <i>in vitro</i> need to ‘mimic’ the extracellular matrix in a way that allows cell adhesion. In this computational work, several model cell-binding peptides with a minimal cell-adhesive Arg–Gly–Asp sequence are investigated in the bulk as well as immobilised on a soft surface. For this reason, a combination of density functional theory and all-atom MD simulations is applied. The major goal of the modelling is to characterise the accessibility of the cell-recognition motif on the functionalised soft polymer surface. As a reference system, the behaviour of three peptide sequences is preliminarily studied in explicit water simulations. From the analysis of the MD trajectories, the solvent accessible surface area, the distribution of water molecules around peptide groups, the secondary structure and the thermodynamics of hydration are evaluated. Furthermore, each peptide is immobilised on the surface of a homopolymer poly(acrylic acid) brush. During MD simulations, all three peptides approach closely toward PAA brush, and their surface accessibility is characterised. Although the peptides are adsorbed onto the brush, they are not hidden by the polymer strands, with RGD unit accessible on the surface and available for guided cell adhesion.</p

Reversible Shape-Memory Effect in Cross-Linked Linear Poly(ε-caprolactone) under Stress and Stress-Free Conditions

The effect of cross-link density on the reversible shape-memory effect (SME) under constant load was systematically studied in cross-linked linear poly­(ε-caprolactone) (PCL). A remarkable reversible SME under stress-free conditions was observed in PCL with the highest achieved cross-link density. Thermal properties as well as morphology, size, and orientation of the nanocrystalline structure formed in covalent networks of PCL under load were compared with those in PCL crystallized under stress-free conditions. As shown, the oriented growth of crystals is the origin of both the reversible SME under and without load. Furthermore, a significant rise of crystallinity and crystal thickness was detected in PCL crystallized under constant load. The fitting curves of the temperature-dependent strain as well as the quantities of crystallinity, type of crystalline structure, size, and orientation of the crystals got by modeling the reversible SME in PCL under stress well correspond to their values obtained experimentally