Zhixin Dong

Phase Behavior of Poly(sulfobetaine methacrylate)-Grafted Silica Nanoparticles and Their Stability in Protein Solutions

Biocompatible and zwitterionic poly(sulfobetaine methacrylate) (PSBMA) was grafted onto the surface of initiator-modified silica nanoparticles via surface-initiated atom transfer radical polymerization. The resultant samples were characterized via nuclear magnetic resonance, Fourier transform infrared spectroscopy, transmission electron microscopy, and thermogravimetric analysis. Their molecular weights and molecular weight distributions were determined via gel permeation chromatography after the removal of silica by etching. Moreover, the phase behavior of these polyzwitterionic-grafted silica nanoparticles in aqueous solutions and stability in protein/PBS solutions were systematically investigated. Dynamic light scattering and UV-visible spectroscopy results indicate that the silica-g-PSBMA nanoparticles exhibit an upper critical solution temperature (UCST) in aqueous solutions, which can be controlled by varying the PSBMA molecular weight, ionic strength, silica-g-PSBMA nanoparticle concentration, and solvent polarity. The UCSTs shift toward high temperatures with increasing PSBMA molecular weight and silica-g-PSBMA nanoparticle concentration. However, increasing the ionic strength and solvent polarity leads to a lowering of the UCSTs. The silica-g-PSBMA nanoparticles are stable for at least 72 h in both negative and positive protein/PBS solutions at 37 °C. The current study is crucial for the translation of polyzwitterionic solution behavior to surfaces to exploit their diverse properties in the development of new, smart, and responsive coatings.

Synthesis and Multi-Stimuli-Responsive Behavior of Poly(N,N-dimethylaminoethyl methacrylate) Spherical Brushes under Different Modes of Confinement in Solution

We report the synthesis and solution behavior of photo-, temperature-, pH-, and ion-responsive weak polyelectrolyte spherical brushes under different modes of confinement. The spherical brushes were prepared by copolymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA) and 7-(2-methacryloyloxyethoxy)-4-methylcoumarin anchored to silica nanoparticles via surface-initiated atom transfer radical polymerization. The photo-cross-linking and reversibility of the nanoparticle-attached coumarin entities are detected by UV-visible spectroscopy and dynamic light scattering (DLS). The cross-linking density of poly(DMAEMA) (i.e., PDMAEMA) brushes could be easily controlled by alternating irradiation at wavelengths of 365 and 254 nm. Moreover, solution behavior under different pH levels and ionic strengths is systematically investigated in the PDMAEMA brush-polyelectrolyte chains confined only by a hard core, the cross-linked PDMAEMA brush-polyelectrolyte chains confined by a hard core and cross-linking points, and the corresponding hollow nanocapsules after removal of silica by etching-polyelectrolyte chains confined only by cross-linking points. These three models represent the different modes of confinement. DLS results indicate that the volume phase transition temperatures of the three models shift to lower temperatures with the increase in pH. The highest temperature is afforded to phase transition for hollow nanocapsules in solution, followed by the cross-linked PDMAEMA brushes. The hydrodynamic radius of the polyelectrolyte brush systems obviously decreases with the increase in ionic strength of the solution when adjusted by NaCl.

Comparison of different methods for determining the imidization degree of polyimide fibers

In this study, polyimide fibers at different stages of imidization were characterized by TGA, DSC, and FTIR. The imidization degree (ID) calculated by TGA was based on the weight loss of each sample, which was caused by the imidization of residual amic acid groups. The results of TGA showed good regularity with the thermal treatment temperature of the PI fibers. For DSC, the ID was calculated based on the area of endothermal peak of each sample. Compared with TGA, DSC showed a relatively higher value because the endothermal peak was reduced by the exothermic re-formation of polyamic acid which may be partially degraded during thermal treatment. The IDs obtained by the FTIR spectra generally showed poorer regularities than those obtained by both TGA and DSC, especially for the results calculated using the 730 cm−1 band. Based on the 1350 cm−1 band, the obtained IDs showed better agreement with the TGA or DSC results. The results obtained by these three methods were compared and analyzed. The ID obtained by TGA showed much more reliability among these three methods.

AO-resistant properties of polyimide fibers containing phosphorous groups in main chains

A series of polyimide fibers containing phosphorus element derived from (3-aminophenyl) methyl phosphine oxide (DAMPO) diamine was exposed to an artificial atomic oxygen environment which simulated the space environment in low earth orbit (LEO). The mass loss, surface morphology, chemical composition, and mechanical properties of the fibers before and after atomic oxygen (AO) exposure were compared in detail with a blank sample. Results showed that the phosphor-containing fibers demonstrated lower mass change and less tensile strength reduction. SEM results showed that the fibers with phosphorous element had relatively dense surface after AO exposure. Meanwhile, XPS results indicated that a passivated phosphate layer, which could protect the following under-layer from attacking by AO, was formed on the surface of the fibers. These results indicated that the incorporation of diamine (DAMPO) into the main chains could protect the fibers for avoiding further erosion from AO exposure. Hence, the phosphor-containing PI fibers exhibits potential application in space fields.

Mechanical properties of polyimide/multi-walled carbon nanotube composite fibers

A series of polyimide (PI)/multi-walled carbon nanotube (MWCNT) composite fibers were prepared by copolymerizing a mixture of monomers and carboxylic-functionalized MWCNTs, followed by dry-jet wet spinning, thermal imidization, and hot-drawing process. The content of the carboxylic groups of MWCNTs significantly increased when treated with mixed acid, whereas their length decreased with treatment time. Both the carboxylic content and length of MWCNTs influenced the mechanical properties of the composite fibers. Fiber added with 0.1 wt% MWCNTs treated for 4 h exhibited the best mechanical properties, i.e., 1.4 GPa tensile strength and 14.30% elongation at break, which were 51% and 32% higher than those of pure PI fibers, respectively. These results indicated that a suitable MWCNT content strengthened and toughened the resultant PI composite fibers, simultaneously. Moreover, raising draw ratio resulted in the increase of tensile strength and tensile modulus of the composite fibers.

Low-dielectric polyimide nanofoams derived from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and 2,2′-bis(trifluoromethyl)benzidine

Low-dielectric polyimide (PI) nanofoams were prepared by introducing nanopores into the PI matrix containing fluorine groups. The nanopores were formed by thermolysis of the thermally labile content, namely, polyethylene glycol (PEG) oligomers, in air. The prepared PI nanofoams were characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and broadband dielectric spectroscopy. Results indicated that the PI nanofoams showed nanosized closed pores, excellent thermal stability, and a low dielectric constant of 2.12. The dielectric constant of the as-prepared nanofoams was stable within −150 °C to 150 °C. The thermal decomposition process of PEG in the PI matrix was designed and optimized to control the decomposition rate of PEG and the diffusion rate of the decomposition products of PEG. The dielectric constant of the nanofoams significantly decreased from 2.45 to 2.12 as the heating rate decreased from 5 °C min−1 to 1 °C min−1. The as-prepared PI nanofoams exhibited excellent properties and thus could be used in the microelectronics industry as a dielectric layer, multi-chip modules, or integrated circuit chips.

Synthesis and properties of novel polyimide fibers containing phosphorus groups in the side chain (DATPPO)

A series of polyamic acid copolymers (co-PAAs) containing phosphorous groups in the side chains were synthesized from [2,5-bis(4-aminophenoxy) phenyl] diphenylphosphine oxide (DATPPO) and 4,4′-oxydianiline (ODA) with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) through the polycondensation in N,N′-dimethyacetamide (DMAc). The co-PAA solutions were spun into fibers by a dry-jet wet spinning process followed by thermal imidization to obtain co-polyimide (co-PI) fibers. FTIR spectra and elemental analysis confirmed the chemical structure of PI fibers. SEM results indicated that the resulting PI fibers had a smooth and dense surface, a uniform and circle-shape diameter. The thermogravimetric measurements showed that with the increase of DATPPO content, the resulting PI fibers possessed high decomposition temperature and residual char yield, indicating that the PI fibers had good thermal stability. The corresponding limiting oxygen index (LOI) values from the experiment results showed that the co-PI fibers possessed good flame-retardant property. Furthermore, the mechanical properties of the co-PI fibers were investigated systematically. When the DATPPO content increased, the tensile strength and initial modulus of the co-PI fibers decreased. However, the mechanical properties were improved by increasing the draw ratio of the fibers. When the draw ratio was up to 2.5, the tensile strength and initial modulus of the co-PI fibers reached up to 0.64 and 10.02 GPa, respectively. The WAXD results showed that the order degree of amorphous matter increased with increased stretching. In addition, the SAXS results displayed that valuably drawing the fibers could eliminate the voids inside and lead to better mechanical property. WAXD revealed that the orientation of the amorphous polymer influenced the mechanical properties of the fibers.

Atomic oxygen resistance of polyimide fibers with phosphorus-containing side chains

A series of polyimide (PI) fibers were spun and exposed to atomic oxygen (AO). The PI fibers contained phosphorus in the macromolecular side chain, which was derived from DATPPO diamine. An AO exposure experiment was conducted in a ground-based AO-effect simulation facility. The changes in the surface morphologies and compositions of PI fibers before and after AO erosion were investigated by field emission scanning electron microscopy and X-ray photoelectron spectrometry (XPS). After AO exposure, the phosphorus-containing PI fibers exhibited a denser surface morphology compared with that of the pure PI fibers. XPS results indicated that phosphate species formed on the surfaces of phosphorus-containing PI fibers after AO exposure protected against further erosion. After AO erosion, the mass loss of phosphorus-containing PI fibers was lower than that of pure PI fibers. Moreover, at an AO fluence of 5.0 × 1020 atoms per cm2, the retention of tensile strength and Youngs modulus of phosphorus-containing PI fibers were 64.87% and 66.04%, respectively, which were higher than those of pure PI fibers. The results of the current study are crucial for understanding the relationship between polymer structure and AO-resistant properties of PI fibers to develop new materials with low-earth orbit applications.

Interaction between poly (ethylene oxide) and silica nanoparticles in dilute solutions

A mixed system that includes poly(ethylene oxide) (PEO) and silica (SiO2) nanoparticles is prepared using two mixing methods. The interaction between PEO and the SiO2 nanoparticles in the dilute basic solution is investigated using the dynamic light scattering (DLS) and isothermal titration calorimetry (ITC) techniques. The DLS results show qualitatively that SiO2 nanoparticles interact with both random coils and aggregates of PEO through hydrogen bonding, and PEO-SiO2 complexes are formed. The degree of disaggregation of aggregates of PEO is readily adjusted by changing the concentration of SiO2 nanoparticle suspensions. Moreover, the ITC results also certify quantitatively the interaction between PEO and SiO2 nanoparticle, and give the evidence of formation of PEO-SiO2 complex.

Adsorption-desorption of silica nanoparticles on poly(ethylene glycol) brushes grafted onto au substrate studied by quartz crystal microbalance

The adsorption-desorption of silica nanoparticles(NPs) on poly(ethylene glycol)(PEG) grafted onto gold(Au) substrate was studied by quartz crystal microbalance with dissipation monitoring(QCM-D) technique. The results of frequency and dissipation show that SiO2 NPs can be adsorbed strongly on PEG-SH brushes at pH of 9.6, and a new dense and rigid construction is formed. Adjusting the pH from 9.6 to 12.3 resulted in the desorption of silica NPs from the PEG brushes because of a significant weakening of the hydrogen bond between the silica NPs and PEG chains. In addition, the viscoelastic properties of the system during the adsorption-desorption process were also analyzed via the relationship between the normalized frequency(Δf/n) and mass. And the corresponding atomic force microscopy(AFM) images also exhibit morphological changes during the above process, consistent with the changes in viscoelasticity.