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Chi Wu

Publications and source records attributed to Chi Wu.

14 recordsLinked to original sources
PubMed2006

Observation of the first-order transition in ultrafiltration of flexible linear polymer chains.

Using a special double-layer membrane to avoid interaction among flow fields generated by different pores, we have, for the first time, observed the predicted discontinuous first-order transition in ultrafiltration of flexible linear polymer chains. Namely, the chain could pass through a pore much smaller than its unperturbed radius only when the flow rate is higher than a certain value. When only one chain and one pore are considered in theory, such a threshold is surprisingly independent of both the chain length and the pore size. Our results reveal that for a membrane with many pores and at a microscopic flow rate () lower than the threshold, the inevitable blocking of some pores by longer nonstretched coiled chains increases in those unblocked pores because the macroscopic flow rate () is a constant. Long chains have two populations, coiled and stretched, in a real ultrafiltration experiment when is lower than the threshold.

Computer Simulation↗
PubMed2006

Phase transition behavior of unimolecular micelles with thermoresponsive poly(N-isopropylacrylamide) coronas.

This paper describes the double phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush at the surface of a hydrophobic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was conducted by using a hyperbranched polyester (Boltorn H40) based macroRAFT agent. The resultant multiarm star block copolymer (H40-PNIPAM) exists as unimolecular micelles with hydrophobic H40 as the core, densely grafted PNIPAM brush as the shell. A combination of laser light scattering (LLS) and microdifferential scanning calorimetry (micro-DSC) studies of H40-PNIPAM in aqueous solution reveals double phase transitions of the PNIPAM corona, which is in contrast to the fact that free PNIPAM homopolymer in aqueous solution exhibits a lower critical solution temperature (LCST) at approximately 32 degrees C. The first phase transition takes place in the broad temperature range 20-30 degrees C, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of the PNIPAM brush close to the H40 core; the second phase transition occurs above 30 degrees C, which can be ascribed to the outer region of PNIPAM brush. Employing the RAFT chain extension technique, the inner and outer part of PNIPAM brush were then selectively labeled with pyrene derivatives, respectively; temperature-dependent excimer fluorescence measurements further support the conclusion that the inner part of PNIPAM brush collapses first at lower temperatures, followed by the collapse of the outer part at higher temperatures.

Journal Article↗
PubMed2006

First observation of two-stage collapsing kinetics of a single synthetic polymer chain.

Using a narrowly distributed poly(N-isoporpylacrylamide) (PNIPAM) chain with a degree of polymerization (N) of 3100, randomly labeled with pyrene, we have, for the first time, observed the two-stage kinetics of the coil-to-globule transition. Two characteristic relaxation times, tau(fast) for the crumpling of a random coil (approximately 12 ms) and tau(slow) for the collapsing of the crumpled chain to a compact globule (approximately 270 ms), were measured. To our knowledge, this is the first experimental evidence supporting the two-stage collapsing kinetics of single synthetic polymer chain previously proposed by de Gennes, Dawson, and Grosberg.

Journal Article↗
PubMed2005

Double hydrophilic block copolymer monolayer protected hybrid gold nanoparticles and their shell cross-linking.

This paper describes the syntheses of core/shell gold nanoparticles stabilized with a monolayer of double hydrophilic block copolymer and their stimuli responsiveness before and after shell cross-linking. The hybrid nanoparticles consist of gold core, cross-linkable poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner shell, and poly(ethylene oxide) (PEO) corona. First, diblock copolymer PEO-b-PDMA was prepared via the reversible addition-fragmentation chain transfer (RAFT) technique using a PEO-based macroRAFT agent. The dithioester end group of PEO-b-PDMA diblock copolymer was reduced to a thiol end group. The obtained PEO-b-PDMA-SH was then used to prepare diblock copolymer stabilized gold nanoparticles by the "grafting-to" approach. 1,2-Bis(2-iodoethoxy)ethane (BIEE) was utilized to selectively cross-link the PDMA residues in the inner shell. The stimuli responsiveness and colloidal stability of core/shell gold nanoparticles before and after shell cross-linking were characterized by laser light scattering (LLS), UV-vis transmittance, and transmission electron microscopy (TEM). At pH 9, the average hydrodynamic radius Rh of non-cross-linked hybrid gold nanoparticles starts to increase above 35 degrees C due to the lower critical solution temperature (LCST) phase behavior of the PDMA blocks in the inner shell. In contrast, Rh of the shell cross-linked gold nanoparticles were essentially independent of temperature. Core/shell gold nanoparticles before and after shell cross-linking exhibit reversible swelling on varying the solution pH. Compared to non-cross-linked core/shell gold nanoparticles, shell cross-linking of the hybrid gold nanoparticles leads to permanent core/shell nanostructures with much higher colloidal stability and physically isolates the gold core from the external environment.

Gold↗
PubMed2005

Double roles of stabilization and destabilization of initiator potassium persulfate in surfactant-free emulsion polymerization of styrene under microwave irradiation.

Emulsifier-free emulsion polymerization of styrene in a water/acetone mixture under microwave irradiation resulted in narrowly distributed stable polystyrene nanoparticles with an averaged hydrodynamic radius R(h) down to 35 nm when 50 wt % of acetone was added. For a given initiator potassium persulfate (KPS) concentration, R(h) was proportional to the monomer concentration in the range 1.2-7.0 wt %; while for a given monomer concentration, R(h) showed a minimum with an increasing initiator concentration. After both the roles of stabilization and destabilization of the initiator (KPS) were considered, we introduced a new parameter, the effective surface area stabilized per ionic group, and reformulated the Wu plot of R(h) proportional, variant W(monomer)/W(stabilizer) into R(h)/[k(1)(1 + k(2)W(KPS))] proportional, variant W(monomer)/W(initiator). It can qualitatively explain both the monomer and initiator concentration dependence of the particle size over a wide concentration range. The study of the salt effect showed that R(h) increased with the sodium sulfate concentration for fixed KPS and styrene concentrations.

Journal Article↗
PubMed2005

Modified structural model for predicting particle size in the microemulsion and emulsion polymerization of styrene under microwave irradiation.

In this study, the microemulsion and emulsion polymerization of styrene at 70 degrees C in the presence of sodium dodecyl sulfate (SDS, surfactant) and potassium persulfate (KPS, initiator) was conducted under microwave radiation. Laser light scattering was used to characterize the resultant polystyrene latex particles formed at different polymerization stages. The influence of the initial emulsion composition, that is, the SDS, KPS, and styrene concentrations, on the final particle size led us to a simple modified structural model in which we considered the stabilization effects of both the surfactant and the ionic end groups generated from the initiator. This model extended the application of the previous Wu plot from microemulsion polymerization to emulsion polymerization. Using this model, we were not only able to control the particle size but were also able to predict the monomer concentration dependence of the number of the resultant latex particles and the effect of diluting the reaction mixture on the resultant particle size.

Journal Article↗
PubMed2004

Investigation of extrinsic and intrinsic apoptosis pathways of new clerodane diterpenoids in human prostate cancer PC-3 cells.

In our continuing search to discover bioactive compounds from natural products, we isolated six new clerodane diterpenes, caseamembrins A to F, from Casearia membranacea and examined their antiproliferative activities in human hormone-resistant prostate cancer PC-3 cells. All of these compounds displayed effective antiproliferative activity using sulforhodamine B assays and induced cell apoptosis by a terminal deoxynucleotidyl transferase (TdT) dUTP nick-end labeling (TUNEL)-reaction technique. The data demonstrated that caseamembrin C was the most effective compound among these clerodane diterpenoids. Caseamembrin C induced down-regulation of Bcl-2 and Bcl-xL expression, while up-regulation of proapoptotic protein Mcl-1S (short chain), suggesting that these Bcl-2 family member proteins may play a role on arbitrating the apoptotic cell death. Caseamembrin C also induced the up-regulation of Fas ligand (FasL) expression, cleavage and activation of caspase-8 and caspase-9, Bid cleavage and activation of executor caspase-3. However, z-IETD-FMK (Z-Ile-Glu-Thr-Asp-fluoromethyl ketone, a selective caspase-8 inhibitor) almost completely inhibited caseamembrin C-induced Bid cleavage without any modification of caspase-9 activation, indicating that the extrinsic pathway of FasL/caspase-8/Bid cascade only played a minor role in the apoptotic signaling. Taken together, it is suggested that caseamembrin C-induced apoptosis is predominantly through the activation of intrinsic apoptosis pathways by causing the down-regulation of Bcl-2 and Bcl-xL expression, up-regulation of Mcl-1S protein and activation of caspase-9 and caspase-3.

Apoptosis↗
PubMed2004

Formation and degradation of poly(D,L-lactide) nanoparticles and their potential application as controllable releasing devices.

In the presence of surfactant, water-insoluble poly(D,L-lactide) (PLA) was dispersed into narrowly distributed nanoparticles stable in water via microphase inversion. The structure and degradation of such formed nanoparticles were investigated by a combination of static and dynamic laser light scattering. Our results revealed that the degradation rate increased with the temperature and pH so that the degradation could be regulated from minutes to days. Using anionic sodium dodecyl sulfate (SDS) as stabilizer resulted in a slower degradation than using cationic hexadecyltrimethylammonium bromide (HTAB). The phthalocyanine chromophores (PC) could be encapsulated inside these PLA nanoparticles. The degradation of individual PLA nanoparticles led to a controllable releasing of PC. The absorption and fluorescence studies revealed a correlation between the degradation and the releasing of PC. Our results showed that a higher PC/PLA ratio could lead to a faster degradation.

Biocompatible Materials↗
PubMed2004

Onset of tethered chain overcrowding.

We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature Tx increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density sigma; of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at sigma(*) approximately 3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and Tx. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks.

Journal Article↗
PubMed2004

Erosion induced controllable release of gliclazide encapsulated inside degradable polymeric particles.

The microphase inversion of water-insoluble poly(L-lactide), "poly[(L-lactide)-co-glycolide] and polylactide-block-poly(ethylene oxide)-block-polylactide from THF to water can result in narrowly distributed stable particles. Gliclazide, a commercial drug, can be encapsulated inside during the process. The formation and degradation of such particles was studied by laser light scattering. In comparison with the corrosion of a bulk material, the degradation of each particle is so fast that we only detect the decrease of the particle number, not the corrosion of individual particles. Therefore, the degradation is a "one-by-one" random process, just like the chemical reaction of molecules in solution. The disappearing rate of the particle number is nearly independent of time, ideal for the controlled release of drugs encapsulated inside. The amount of encapsulated gliclazide depends on the copolymer's hydrophobicity (composition), while the releasing rate mainly is, directly related to the disappearing rate of the particles. The correlation between the fluorescence intensity and the degradation was used to study the kinetics of gliclazide releasing. The releasing pattern is controllable with a proper choice of the drug loading, copolymer composition, pH and temperature.

Biodegradation, Environmental↗
PubMed2003

Structure of a collapsed polymer chain with stickers: a single- or multiflower?

Using an amphiphilic copolymer with evenly spaced hydrophobic styrene segments (stickers), poly(N-isopropylacrylamide-s-styrene), we recently confirmed a long-standing prediction that such a copolymer chain in a solvent selectively poor for the stickers could self-fold from a random coil to a single-flowerlike core-shell nanostructure. Moreover, we found that the self-folding involves the movement of the sticks to the center and the transition from the random coil to the collapsed globule passes through a proposed ordered coil state.

Journal Article↗
PubMed2003

Biodegradable, thermosensitive implant for approximating cylindrical structures: a preliminary study.

A new invention, i.e., a biodegradable, thermosensitive hybrid gel composite in the form of a thin sheet, was used as an implant inside the rat and guinea pig to test its tissue reactions, degradation, and function as a bridging agent. Tissue tractions in subcutaneous tissue, in muscle, around a peripheral nerve, and around an artery were mild. Degradation made rapid progress, starting on the third day and completed in 2-3 weeks. The hybrid gel, when wrapped around the cut ends of a peripheral nerve or an artery, functioned well as a bridging tube. In the case of the cut nerve, regenerating fascicles crept through the tube which protected them from fibrosis. In the case of the cut artery, patency of flow was maintained, and arterial wall healing was complete in 2-3 weeks. The hybrid gel composite contracted at body temperature, thus holding well to body tissues. Its biodegradable and inert nature offers the potential for future use as a tissue wrap and bridging agent.

Absorbable Implants↗
PubMedDate not supplied

Collapse and association of denatured lentinan in water/dimethlysulfoxide solutions.

Triple helical lentinan, beta-(1-->3)-D-glucan from Lentinus edodes, was denatured in dimethlysulfoxide (DMSO) into single random coils. The DMSO solutions of randomly coiled lentinan were diluted with pure water to different wH (the weight fraction of water in the mixed solvent), and their specific optical rotation [alpha]D, reduced viscosity (lnetar)/c, and hydrodynamic radius Rh were investigated as a function of wH and storage time t. With an increase of wH from 0.1 to 0.2, [alpha]D increased sharply, suggesting that transition of conformation of the macromolecules has occurred. When wH was lower than 0.1, (lnetar)/c of lentinan in water-diluted DMSO exhibited the almost same value as that in pure DMSO and changed hardly with increasing t. Interestingly, (lnetar)/c decreased to reach a minimum with a further increase of wH from 0.1 to 0.25 and then increased with a continuous increase of wH from 0.25 to 0.5. Both (lnetar)/c and Rh of the denatured lentinan in water-diluted DMSO with wH of approximately 0.25 both exhibited a minimum, indicating that collapsed coil chains have occurred. All of the experimental findings revealed that the behaviors of lentinan in water-diluted DMSO solution with wH < 0.1 were consistent with that in good solvent, DMSO. When wH = 0.25, the quality of the mixed solvents became worse, and the dominant intramacromolecular hydrogen-bond interaction enhanced, leading to minimum of viscosity and size of the chains as a result of the collapsed coils. When wH > 0.25, the quality of the mixture weakens further, and the intermolecular hydrogen-bond interaction enhanced and was dominant, leading to aggregation of the collapsed chains.

Dimethyl Sulfoxide↗