Bovine Serum Albumin Effect on Collapsing PNIPAM Chains in Aqueous Solutions: Spin Label and Spin Probe Study.

The influence of bovine serum albumin (BSA) on collapsing poly(N-isopropylacrylamide) (PNIPAM) chains was studied with turbidimetry and spin probe and spin label electron paramagnetic resonance spectroscopy. An increased ratio of collapsed chains in aqueous solutions in the narrow temperature region near the LCST appeared in the presence of 2.5-10 wt% BSA. The spin probe EPR data indicate that the inner cavities of the BSA dimers are probably responsive to the capture of small hydrophobic or amphiphilic molecules, such as TEMPO nitroxyl radical. The observed features of the structure and dynamics of inhomogeneities of aqueous PNIPAM-BSA solutions, including their mutual influence on the behavior of the polymer and protein below the LCST, should be considered when developing and investigating PNIPAM-based drug delivery systems.

Phosphazene Functionalized Silsesquioxane-Based Porous Polymer as Thermally Stable and Reusable Catalyst for Bulk Ring-Opening Polymerization of ε-Caprolactone.

The bulk ring-opening polymerization (ROP) of ε-caprolactone using phosphazene-containing porous polymeric material (HPCP) has been studied at high reaction temperatures (130-150 °C). HPCP in conjunction with benzyl alcohol as an initiator induced the living ROP of ε-caprolactone, affording polyesters with a controlled molecular weight up to 6000 g mol-1 and moderate polydispersity (Ð~1.5) under optimized conditions ([BnOH]/[CL] = 50; HPCP: 0.63 mM; 150 °C). Poly(ε-caprolactone)s with higher molecular weight (up to Mn = 14,000 g mol-1, Ð~1.9) were obtained at a lower temperature, at 130 °C. Due to its high thermal and chemical stability, HPCP can be reused for at least three consecutive cycles without a significant decrease in the catalyst efficiency. The tentative mechanism of the HPCP-catalyzed ROP of ε-caprolactone, the key stage of which consists of the activation of the initiator through the basic sites of the catalyst, was proposed.

Structure and Dynamics of Inhomogeneities in Aqueous Solutions of Graft Copolymers of N-Isopropylacrylamide with Lactide (P(NIPAM-graft-PLA)) by Spin Probe EPR Spectroscopy.

Coil-to-globule transition and dynamics of inhomogeneities in aqueous solutions of graft copolymers of NIPAM with different content of oligolactide groups were studied using spin probe continuous wave EPR spectroscopy. The technique of the suppressing of TEMPO as spin probe by spin exchange with Cu2+ ions was applied. This approach allowed us to detect individual EPR spectra of the probe in collapsed globules and estimate its magnetic and dynamic parameters reliably. The formation of inhomogeneities at temperatures lower than the volume phase transition temperature measured via transmission, and differential scanning calorimetry was fixed. An increase in oligolactide content in copolymers leads to the formation of looser globules, allowing for the exchange of the probe molecules between the globules and the external solution.

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.

Design of cross-linked polyisobutylene matrix for efficient encapsulation of quantum dots.

Photoluminescent quantum dots (QDs) are a prominent example of nanomaterials used in practical applications, especially in light-emitting and light-converting devices. Most of the current applications of QDs require formation of thin films or their incorporation in solid matrices. The choice of an appropriate host material capable of preventing QDs from degradation and developing a process of uniform incorporation of QDs in the matrix have become essential scientific and technological challenges. In this work, we developed a method of uniform incorporation of Cu-Zn-In-S (CZIS) QDs into a highly protective cross-linked polyisobutylene (PIB) matrix with high chemical resistance and low gas permeability. Our approach involves the synthesis of a methacrylate-terminated three-arm star-shaped PIB oligomeric precursor capable of quick formation of a robust 3D polymer network upon exposure to UV-light, as well as the design of a special ligand introducing short PIB chains onto the surface of the QDs, thus providing compatibility with the matrix. The obtained cross-linked QDs-in-polymer composites underwent a complex photostability test in air and under vacuum as well as a chemical stability test. These tests found that CZIS QDs in a cross-linked PIB matrix demonstrated excellent photo- and chemical stability when compared to identical QDs in widely used polyacrylate-based matrices. These results make the composites developed excellent materials for the fabrication of robust, stable and durable transparent light conversion layers.