Jinxian Yang

Effect of urea on phase transition of poly(N-isopropylacrylamide) investigated by differential scanning calorimetry.

The effect of urea on the phase transition of PNIPAM was studied using differential scanning calorimetry (DSC). For a certain urea concentration, the enthalpy change of phase transition of poly(N-isopropylacrylamide) (PNIPAM) aqueous solution increases with the number of DSC cycles, presumably due to the displacement of water molecules bound to the amide groups of PNIPAM by urea molecules at the temperature higher than the lower critical solution temperature (LCST) of PNIPAM and causes the decrease in the absolute value of the exothermic heat related to the dehydration of hydrophilic groups and interactions of hydrophilic residues to around 0. Moreover, the enthalpy change decreases with the urea concentration during the heating process of the first DSC cycle, indicating the replacement of water molecules around the apolar isopropyl groups by urea molecules at the temperature lower than LCST, and the endothermic heat caused by the dehydration of apolar groups decreases. Furthermore, the urea molecules which replace the water molecules at high temperature can be replaced again by water molecules at the temperature lower than LCST, but this process needs several days to complete.

Scaling laws between the hydrodynamic parameters and molecular weight of linear poly(2-ethyl-2-oxazoline)

Poly(2-ethyl-2-oxazoline) (PEtOx), as an alternative polymer to poly(ethylene glycol), has potential applications in biomedical fields. The hydrodynamic parameters, such as the hydrodynamic radius and sedimentation coefficient, are important to understand its dynamics and properties, including its effect on the interactions between proteins and cells. In this study, we have investigated the hydrodynamic properties and thermodynamic parameters of a series of narrowly distributed PEtOx polymers with molecular weights ranging from 1.3 × 103 to 3.1 × 105 g mol−1, and the scaling laws between them by the use of a combination of analytical ultracentrifugation and laser light scattering. It is found that the sedimentation coefficient (s20,w) and hydrodynamic radius (Rh,0) at infinite dilution scale with molecular weight (Mw) as s20,w = Ks × Mαw = 0.0071 (S) × M0.462±0.019w and Rh,0 = KRh × Mβw = 0.0179 (nm) × M0.539±0.012w, respectively.

Effect of carbon chain length of monocarboxylic acids on cloud point temperature of poly(2-ethyl-2-oxazoline)

The temperature-induced phase transition of poly(2-ethyl-2-oxazoline) (PEtOx) aqueous solution under mixing with a series of small carboxylic acids has been studied by turbidity measurements and laser light scattering. It has been found that cloud point temperature (Tcp) of the PEtOx was changed to varying degrees depending upon the pH, ionic strength, molar ratio of acids to 2-ethyl-2-oxazoline unit, and carbon chain length of small carboxylic acids. Significant change in Tcp was observed in the case of hexanoic acid. At acidic pH, an increase in the molar ratio of hexanoic acid to the 2-ethyl-2-oxazoline unit gradually decreased the phase transition temperature of the polymer as compared to the Tcp of pure PEtOx. At original pH 6 (pH > pKa), Tcp shifts to higher value than that of pure PEtOx for lower molar ratios and decreased later on with increasing the molar ratio. The shift in the Tcp is described based on the differences in the driving force of phase transition, including hydrogen bonding between small carboxylic acids and PEtOx polymer and hydrophobic interaction.

Effect of Hydrophobic Chain Length on the Stability and Guest Exchange Behavior of Shell-Sheddable Micelles Formed by Disulfide-Linked Diblock Copolymers

Reduction-responsive micelles hold enormous promise for application as drug carriers due to the fast drug release triggered by reducing conditions and high anticancer activity. However, the effect of hydrophobic chain length on the stability and guest exchange of reduction-responsive micelles, especially for the micelles formed by diblock copolymers containing single disulfide group, is not fully understood. Here, shell-sheddable micelles formed by a series of disulfide-linked copolymer poly(ethylene glycol)-b-poly(ε-caprolactone) (PEG-SS-PCL) containing the same chain length of PEG but different chain lengths of hydrophobic block PCL were prepared and well characterized. The influence of the chain length of hydrophobic PCL block on the stability and guest exchange of PEG-SS-PCL micelles was studied by the use of both dynamic laser light scattering (DLS) and fluorescence resonance energy transfer (FRET). The results show that longer PCL chains lead to a slower aggregation rate and guest exchange of micelles in the aqueous solutions containing 10 mM dithiothreitol (DTT). The cell uptake of the shell-sheddable PEG-SS-PCL micelles in vitro shows that the amount of internalization of dyes loaded in PEG-SS-PCL micelles increases with the chain length of hydrophobic PCL block investigated by flow cytometric analysis and confocal fluorescence microscopy.

Degradable polyurethane with poly(2-ethyl-2-oxazoline) brushes for protein resistance

Linear poly(2-ethyl-2-oxazoline) (PEtOx) with two hydroxyl groups located at the ends of each chain [PEtOx(OH)2] was synthesized by cationic ring-opening polymerization (CROP) using 2,2-bis(hydroxymethyl)propionic acid as the end-capping agent. Further co-polycondensation of PEtOx and poly(e-caprolactone) (PCL) led to degradable polyurethane-graft-poly(2-ethyl-2-oxazoline) (PU-g-PEtOx) with PCL as the soft segment. The structures of PU-g-PEtOx with different graft densities of PEtOx as well as different chain lengths were confirmed by various characterization techniques. Moreover, protein resistance experiments were examined by a quartz crystal microbalance with dissipation (QCM-D). The results demonstrated that the adsorption of three model proteins decreased with the increase in chain length of PEtOx when PU-g-PEtOx possessed the same PEtOx graft density while the adsorption of proteins decreased with the increase in the graft density of PEtOx when the chain length was fixed. The adsorption of proteins on PU-g-PEtOx was reduced by 97% compared with that on unmodified PU. The degradation of PU-g-PEtOx was monitored by a combination of ellipsometry and QCM-D because of the biodegradation of PCL segments. Furthermore, marine field tests were also performed during the rich fouling season and our results showed that PU-g-PEtOx exhibited much better anti-biofouling performance than unmodified PU.

Poly(l-lactide-co-2-(2-methoxyethoxy)ethyl methacrylate): A biodegradable polymer with protein resistance

We have synthesized poly(l-lactide-co-2-(2-methoxyethoxy)ethyl methacrylate) (LA-co-MEO2MA) containing both degradable and protein resistant units via hybrid copolymerization with (1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phophoranylidenamino]-2Λ5,Λ5-catenadi(phosphazene) (t-BuP4) as the catalyst. Nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) show that LA-co-MEO2MA is a random copolymer. The studies of quartz crystal microbalance with dissipation (QCM-D) demonstrate that the copolymer enzymaticlly degrades much faster than poly(l-lactide) (PLA) homopolymer due to its lower crystallinity. We have also investigated the adsorption of bovine serum albumin (BSA), lysozyme or fibrinogen on a LA-co-MEO2MA surface in real time by use of QCM-D and surface plasmon resonance (SPR). Our studies reveal that the polymer is protein resistant depending on MEO2MA content. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay experiments demonstrate that the polymer has a low cytotoxicity.

The effect of surface poly(ethylene glycol) length on in vivo drug delivery behaviors of polymeric nanoparticles

Engineering nanoparticles of reasonable surface poly(ethylene glycol) (PEG) length is important for designing efficient drug delivery systems. Eliminating the disturbance by other nanoproperties, such as size, PEG density, etc., is crucial for systemically investigating the impact of surface PEG length on the biological behavior of nanoparticles. In the present study, nanoparticles with different surface PEG length but similar other nanoproperties were prepared by using poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL) copolymers of different molecular weights and incorporating different contents of PCL3500 homopolymer. The molecular weight of PEG block in PEG-PCL was between 3400 and 8000 Da, the sizes of nanoparticles were around 100 nm, the terminal PEG density was controlled at 0.4 PEG/nm2 (or the frontal PEG density was controlled at 0.16 PEG/nm2). Using these nanoproperties well-designed nanoparticles, we demonstrated PEG length-dependent changes in the biological behaviors of nanoparticles and exhibited nonmonotonic improvements as the PEG molecular weight increased from 3400 to 8000 Da. Moreover, under the experimental conditions, we found nanoparticles with a surface PEG length of 13.8 nm (MW = 5000 Da) significantly decreased the absorption with serum protein and interaction with macrophages, which led to prolonged blood circulation time, enhanced tumor accumulation and improved antitumor efficacy. The present study will help to establish a relatively precise relationship between surface PEG length and the in vivo behavior of nanoparticles.

Pulse detonation in a chamber with divergent nozzle

Detonation wave diffraction is a basic research topics of detonation dynamics and one of most important phenomena in pulse detonation engine with nozzle. Double exposure holographic interferometry, which can obtain more quantitative information compared with con- ventional visualization method such as schlieren photographic, was used to study the flow field after detonation wave. A numerical simulation based on the adaptive finite volume method with finite rate chemical reaction model was carried out to compare with the experiment result. It was found that the combination of numerical simulation with experiment can help us for better understanding of the mechanism of various phenomena accompanied with detonation diffraction process.

Visualization of a vortex interacting with an explosion initiated at center

In this paper, an explosion, which starts at the center of a vortex, is considered. A shock tube was used to conduct the experiment. A vortex is generated when the shock wave propagates around a wing-shaped model. The explosion is triggered at the time when the vortex arrives at the position where the explosion wire is located. Holographic interferometery was used to visualize the vortex-explosion interaction. Both experimental and numerical results showed that a vortex, after disturbed by an explosion, is strongly deformed and smeared out, which suggets that vortex might be suppressed or even destroyed with an explosion. More careful works need to be carried out in the near future in order to get more detailed and quantitative information for both academic and application interests.