Hydrophobic Polymer Chain in Water That Undergoes a Coil-to-Globule Transition Near Room Temperature

Phase Transition Control of pNIPAAm-b-pMMA Thin Films via UV/Ozone Irradiation and Direct Immersion Annealing for Biomedical Applications

Poly(N-isopropylacrylamide) (pNIPAAm) and its copolymers with poly(methyl methacrylate) (pMMA), due to their lower critical solution temperature (LCST) within the physiological temperature range, have attracted attention in many studies as responsive surfaces for cell sheet growth. The behavior of pNIPAAm and its block copolymer with pMMA (pNIPAAm-b-pMMA) films is not fully understood. In this study, the effects of ultraviolet/ozone (UV/O3), interdiffusion, and direct immersion annealing (DIA) treatments on the responsiveness and adhesion of spin-coated thin films of pNIPAAm and pNIPAAm-b-pMMA were investigated. The water contact angle (WCA) measurements were used to record changes in hydrophilicity and relaxation time for the films immersed in deionized water, as well as hydrophilicity changes during cyclic measurements. Atomic force microscopy was utilized to track phase transitions in the films over three complete and continuous thermal cycles. The phase transition behavior of the films across the LCST range remains insufficiently understood, particularly in relation to the influence of different treatment types and the underlying chemophysical mechanisms that regulate the orientation of functional groups along the polymer chains. In this study, we address these gaps by examining how these factors, along with the globule-to-coil transition of pNIPAAm chains, impact the films' thermoresponsiveness and the time required for complete cell sheet detachment. This study evaluated the potential of utilizing straightforward and environmentally sustainable methods for producing cell culture dishes as alternatives to conventional, costly, and environmentally detrimental techniques. The findings confirmed the significant promise of these methods for the intended applications.

Antimicrobial Hydrogels Based on Cationic Curdlan Derivatives for Biomedical Applications

Hydrogels based on biocompatible polysaccharides with biological activity that can slowly release an active principle at the wound site represent promising alternatives to traditional wound dressing materials. In this respect, new hydrogels based on curdlan derivative with 2-hydroxypropyl dimethyl octyl ammonium groups (QCurd) and native curdlan (Curd) were obtained at room temperature by covalent cross-linking using a diepoxy cross-linking agent. The chemical structure of the QCurd/Curd hydrogels was investigated by Fourier transform infrared spectroscopy (FTIR) spectroscopy. Scanning electron microscopy (SEM) revealed well-defined regulated pores with an average diameter between 50 and 75 μm, and hydrophobic micro-domains of about 5 μm on the pore walls. The high swelling rate (21-24 gwater/ghydrogel) and low elastic modulus values (7-14 kPa) make them ideal for medical applications as wound dressings. To evaluate the possible use of the curdlan-based hydrogels as active dressings, the loading capacity and release kinetics of diclofenac, taken as a model drug, were studied under simulated physiological skin conditions. Several mathematical models have been applied to evaluate drug transport processes and to calculate the diffusion coefficients. The prepared QCurd/Curd hydrogels were found to have good antibacterial properties, showing a bacteriostatic effect after 48 h against S. aureus, MRSA, E. coli, and P. aeruginosa. The retarded drug delivery and antimicrobial properties of the new hydrogels support our hypothesis that they are candidates for the manufacture of wound dressings.

Sustainable 3D printing by reversible salting-out effects with aqueous salt solutions

Achieving a simple yet sustainable printing technique with minimal instruments and energy remains challenging. Here, a facile and sustainable 3D printing technique is developed by utilizing a reversible salting-out effect. The salting-out effect induced by aqueous salt solutions lowers the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) solutions to below 10 °C. It enables the spontaneous and instant formation of physical crosslinks within PNIPAM chains at room temperature, thus allowing the PNIPAM solution to solidify upon contact with a salt solution. The PNIPAM solutions are extrudable through needles and can immediately solidify by salt ions, preserving printed structures, without rheological modifiers, chemical crosslinkers, and additional post-processing steps/equipment. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, which offers great potential for developing water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators.

Aggregation and structural phase transitions of semiflexible polymer bundles: A braided circuit topology approach

We present a braided circuit topology framework for investigating topology and structural phase transitions in aggregates of semiflexible polymers. In the conventional approach to circuit topology, which specifically applies to single isolated folded linear chains, the number and arrangement of contacts within the circuitry of a folded chain give rise to increasingly complex fold topologies. Another avenue for achieving complexity is through the interaction and entanglement of two or more folded linear chains. The braided circuit topology approach describes the topology of such multiple-chain systems and offers topological measures such as writhe, complexity, braid length, and isotopy class. This extension of circuit topology to multichains reveals the interplay between collapse, aggregation, and entanglement. In this work, we show that circuit topological motif fractions are ideally suited order parameters to characterize structural phase transitions in entangled systems that can detect structural re-ordering other measures cannot.

Interactions between Water and a Hydrophobic Polymer

To investigate the mechanisms of interactions between a hydrophobic polymer and water, molecular dynamics calculations and Raman spectroscopic measurements of cis-1,4-polyisoprene (PI)-water systems were performed. The results show that PI in water undergoes a coil-globule transition at around 248 K. The transition is attributed to changes in the density and diffusivity of water. The volume expansion of the supercooled liquid water induces the coil structure of PI. The phase separation of PI from water with an increase in the self-diffusion coefficient of water molecules results in the globule structure of PI. The self-diffusion coefficient of free water with PI is larger than that of pure water because PI has an effect to decrease the hydrogen-bonding strength of water. The result suggests that the effects of the coexisting water are important factors governing the physical and chemical properties of hydrophobic polymers.

Interaction of Macromolecular Chain with Phospholipid Membranes in Solutions: A Dissipative Particle Dynamics Simulation Study

The interaction between macromolecular chains and phospholipid membranes in aqueous solution was investigated using dissipative particle dynamics simulations. Two cases were considered, one in which the macromolecular chains were pulled along parallel to the membrane surfaces and another in which they were pulled vertical to the membrane surfaces. Several parameters, including the radius of gyration, shape factor, particle number, and order parameter, were used to investigate the interaction mechanisms during the dynamics processes by adjusting the pulling force strength of the chains. In both cases, the results showed that the macromolecular chains undergo conformational transitions from a coiled to a rod-like structure. Furthermore, the simulations revealed that the membranes can be damaged and repaired during the dynamic processes. The role of the pulling forces and the adsorption interactions between the chains and membranes differed in the parallel and perpendicular pulling cases. These findings contribute to our understanding of the interaction mechanisms between macromolecules and membranes, and they may have potential applications in biology and medicine.

Osmotic second virial coefficients for hydrophobic interactions as a function of solute size

To gain quantitative insight into how the overall strength of the hydrophobic interaction varies with the molecular size, we calculate osmotic second virial coefficients B for hydrophobic spherical molecules of different diameters σ in water based on molecular simulation with corrections to the finite-size and finite-concentration effects. It is shown that B (<0) changes by two orders of magnitude greater as σ increases twofold and its solute-size dependence is best fit by a power law B ∝ σ^α with the exponent α ≃ 6, which contrasts with the cubic power law that the second virial coefficients of gases obey. It is also found that values of B for the solutes in a nonpolar solvent are positive but they obey the same power law as in water. A thermodynamic identity for B derived earlier [K. Koga, V. Holten, and B. Widom, J. Phys. Chem. B 119, 13391 (2015)] indicates that if B is asymptotically proportional to a power of σ, the exponent α must be equal to or greater than 6.

Solubility of Hydrophobes into Macrocyclic Hosts

The intrinsic nature of macrocyclic molecules to preferentially absorb a specific solute has been opening up supramolecular chemistry. Nevertheless, the determinant factor with molecular perspectives in promoting host-guest complexations remains inconclusive, due to the lack of rigorous thermodynamic examination on the guest solubility inside the host. Here, we quantify the solute-solvent energetic and entropic contributions between the end states and on the docking route during inclusion of noble gases in cucurbit[5]uril, cucurbit[6]uril, and α-cyclodextrin, using molecular dynamics simulations in combination with the potential distribution theorem. Results show that in all of the pairs examined both the solute-solvent energy and entropy favor the inclusion, while the former is rather dominant. The frequency of interior drying, which pertains to the entropic contribution, differs between the hosts and is controlled by the existence of lid water at portal and the flexibility of host framework. Moreover, the hosts exhibit various types of absorption manners, involving non-, single-, and double-free-energy barriers.

Less-Ordered Hydration Shell around Poly(N,N-diethylacrylamide) Is Insensitive to the Clouding Transition

Raman multivariate curve resolution (Raman-MCR) is applied to examine how the hydration shell around poly(N,N-diethylacrylamide) (PDEAM) changes upon heating, in comparison with poly(N-isopropylacrylamide) (PNIPAM), both of which undergo a clouding transition near room temperature. We report that PDEAM possesses a less-ordered and smaller hydration shell than PNIPAM. Furthermore, the PDEAM hydration-shell structure is insensitive to the occurrence of clouding, indicating the coil-globule transition and aggregation of multiple chains can be achieved without the hydration-shell structural transformation.

Thin Thermoresponsive Polymer Films for Cell Culture: Elucidating an Unexpected Thermal Phase Behavior by Atomic Force Microscopy

Application of poly-N-isopropylacrylamide (PNIPAM) and its more hydrophobic copolymers with N-tert-butylacrylamide (NtBA) as supports for cell sheets has been validated in numerous studies. The binary systems of these polymers with water are characterized by a lower critical solution temperature (LCST) in a physiologically favorable region. Upon lowering the temperature below the LCST, PNIPAM chains undergo a globule-to-coil transition, causing the film dissolution and cell sheet detachment. The character of the PNIPAM-water miscibility behavior is rather complex and not completely understood. Here, we applied atomic force microscopy to track the phase transition in thin films of linear thermoresponsive (co)polymers (PNIPAM and PNIPAM-co-NtBA) prepared by spin-coating. We studied the films' Young's modulus, roughness, and thickness in air and in distilled water in a full thermal cycle. In dry films, in the absence of water, all the measured parameters remained invariant. The swollen films in water above the LCST were softer by 2-3 orders of magnitude and about 10 times rougher than the corresponding dry films. Upon lowering the temperature to the LCST, the films passed through the phase transition observed as a drastic drop of Young's modulus (about an order of magnitude) and decrease in roughness in both polymers in a narrow temperature range. However, the films did not lose their integrity and demonstrated almost fully reversible changes in the mechanical properties and roughness. The thermal dependence of the films' thickness confirmed that they dissolved only partially and required an external force to induce the complete destruction. The reversible thermal behavior which is generally not expected from non-cross-linked polymers is a key finding, especially with respect to their practical application in cell culture. Both the thermodynamic and kinetic factors, as well as the confinement effect, may be responsible for this peculiar film robustness, which requires overcooling and the aid of an external force to destroy the film.

Study on the stability and cellular affinity of gelatin-polysaccharide composite films

The gelatin film has great potential in biomedical applications, especially in wound healing. The combination of gelatin films and stem cells could further accelerate the skin regeneration. Although polysaccharide modification can improve the mechanical property and biological activity of gelatin films, information about the stability and cellular affinity is still limited. This study investigated the influence of polysaccharides on the stability and cellular affinity of gelatin films. Two kinds of gelatin-polysaccharide composite films, including gelatin-hyaluronic acid (G-HA) and gelatin-chitosan (G-CS), were prepared in this study. It was found that G-HA composite film had better short-term and long-term stability compared with G-CS composite film. And G-HA composite film also had better biological safety than G-CS film. Moreover, the surface of G-HA composite film supported the adhesion and growth of human umbilical cord Wharton's jelly-derived mesenchymal stem cells (WJ MSCs) better than G-CS film surface. These data illustrated that G-HA composite film has better stability and cellular affinity compared with G-CS film, which could be considered a promising delivery system of stem cells for further in vivo studies. Therefore, this work would be very helpful to optimize the preparation of gelatin-polysaccharide composite films.

Reduction of water-mediated repulsion drives poly(N-vinylcaprolactam) collapse upon heating

Thermo-sensitive aqueous polymers undergo a coil-to-globule transition on heating, with drastic chemical and structural changes. We performed molecular dynamics simulations for PVCL in water to study the driving forces for the polymer's collapse.

The folding pathways and thermodynamics of semiflexible polymers

Inspired by the protein folding and DNA packing, we have systematically studied the thermodynamic and kinetic behaviors of single semiflexible homopolymers by Langevin dynamics simulations. In line with experiments, a rich variety of folding products, such as rod-like bundles, hairpins, toroids, and a mixture of them, are observed in the complete diagram of states. Moreover, knotted structures with a significant population are found in a certain range of bending stiffness in thermal equilibrium. As the solvent quality becomes poorer, the population of the intermediate occurring in the folding process increases, which leads to a severe chevron rollover for the folding arm. However, the population of the intermediates in the unfolding process is very low, insufficient to induce unfolding arm rollover. The total types of folding pathways from the coil state to the toroidal state for a semiflexible polymer chain remain unchanged by varying the solvent quality or temperature, whereas the kinetic partitioning into different folding events can be tuned significantly. In the process of knotting, three types of mechanisms, namely, plugging, slipknotting, and sliding, are discovered. Along the folding evolution, a semiflexible homopolymer chain can knot at any stage of folding upon leaving the extended coil state, and the probability to find a knot increases with chain compactness. In addition, we find rich types of knotted topologies during the folding of a semiflexible homopolymer chain. This study should be helpful in gaining insight into the general principles of biopolymer folding.

Cationic hydrophobicity promotes dissolution of cellulose in aqueous basic solution by freezing–thawing

Hydrophobic cations accumulate at the cellulose interface, favouring the physical dissolution of cellulose in aqueous quaternary ammonium hydroxides.

Hydration-Shell Transformation of Thermosensitive Aqueous Polymers

Although water plays a key role in the coil-globule transition of polymers and biomolecules, it is not clear whether a change in water structure drives or follows polymer collapse. Here, we address this question by using Raman multivariate curve resolution (Raman-MCR) spectroscopy to investigate the hydration shell structure around poly(N-isopropylacrylamide) (PNIPAM) and poly(propylene oxide) (PPO), both below and above the cloud point temperature at which the polymers collapse and form mesoscopic polymer-rich aggregates. We find that, upon clouding, the water surrounding long PNIPAM chains transforms to a less ordered and more weakly hydrogen bonded structure, while the water surrounding short PNIPAM and PPO chains remains similar above and below the cloud point. Furthermore, microfluidic temperature jump studies demonstrate that the onset of clouding precedes the hydration-shell structural transformation, and thus the observed water structural transformation is associated with ripening of aggregates composed of long-chain polymers, on a time scale that is long compared to the onset of clouding.