Chi Wu

Effects of surfactants on the phase transition of poly(N-isopropylacrylamide) in water

SYNOPSIS The effects of both anionic (sodium dodecyl sulfate, SDS) and cationic (dodecylpyridine bromide, DPB) surfactants on the phase transition of narrowly distributed poly(N-isopropylacrylamide) (PNIPAM) microgel particles were investigated by laser light scattering. The addition of SDS swells the particles and increases the phase transition temperature, while DPB has a much smaller effect. This difference cannot be due to an association between the surfactant hydrophobic tail and PNIPAM because DPB and SDS have an identical hydrophobic tail. The amide groups in PNIPAM are slightly protonized in deionized water (pH - 5.5). Our results contradict a previous prediction that oppositely charged surfactants will collapse a polyelectrolyte gel. After adding SDS, a two-step phase transition of the PNIPAM gel is observed. This suggests that SDS forms micelles inside the microgel with the help of the immobilized counter ions on the gel network. The SDS micelles are broken into individual SDS molecules in the first step of phase transition, while in the second step individual SDS molecules are gradually expelled. Surfactant effects on the microgel particles are compared with those of individual PNIPAM chains. 0 1996 John Wiley & Sons, Inc.

Nanopore Extrusion-Induced Transition from Spherical to Cylindrical Block Copolymer Micelles

Cylindrical micelles made of soluble and insoluble segments have found many interesting applications, for example, in templating the formation of inorganic nanocrystal arrays, controlled drug delivery, and bionanotechnology. Conventional methods for the preparation of cylindrical micelles are based on the judicious design of the molecular structures of the soluble and insoluble segments and the manipulation of the external parameters, such as the solvent, temperature, and guest ions/molecules. Here we report on the production of cylindrical micelles by extruding spherical micelles through nanoscale pores. The obtained cylindrical micelles and their transformation back into the spherical micelles are characterized using transmission electron microscopy imaging and dynamic laser light scattering. The extrusion process is continuous and simple and can be upgraded to large scales. We believe that our nanopore extrusion-based preparation method will provide enormous opportunities for the applications of block copolymer cylindrical micelles in nanotechnology and medicine.

A heterogeneous catalytic kinetics for enzymatic biodegradation of poly(ϵ-caprolactone) nanoparticles in aqueous solution

Abstract Water insoluble poly(ϵ-caprolactone) (PCL) was micronized into narrowly distributed stable nanoparticles. The biodegradation of such PCL nanoparticles in the presence of the enzyme, Lipase PS, was monitored by using laser light scattering because the scattering intensity is directly related to the particle concentration. The PCL and enzyme concentration dependence of the biodegradation rate supports a heterogeneous catalytic kinetics in which we have introduced an additional equilibrium between the inactive and active enzyme/substrate complexes. The initial rate equation derived on the basis of this mechanism was used to successfully explain the influence of surfactant, pH and temperature on the enzymatic biodegradation. Our results confirmed that both the adsorption and the enzymatic catalysis were important for the biodegradation of the PCL nanoparticles.

Viscometric investigation of intramolecular hydrogen bonding cohesional entanglement in extremely dilute aqueous solution of poly vinyl alcohol

An investigation of influence of cryogenic treatment on extremely dilute aqueous solution of poly(vinyl alcohol) (PVA) was performed by viscometry. The solution was frozen in liquid nitrogen or in a freezer at −25°C, then thawed at ambient temperature and concentrated by evacuation. The viscosity of the solution was measured using the dilute method. The experimental results indicated that the viscosity of the solution is related to N, the times of the freezing and thawing cycle, and the temperature for freezing. Undergoing a treatment of freezing and thawing, the viscosity of the solution is decreased, while it can be recovered the value of before the treatment as the solution had been heated at a high temperature. Thus, a conclusion may be obtained; that is, for an extremely dilute aqueous solution of PVA, which concentration is below Cgel, threshold concentration for gelation, an intramolecular hydrogen bonding cohesional entanglement can be formed by freezing as N 5, it not only formed an intramolecular hydrogen bonding but also produced an intermolecular hydrogen bonding. At the same time, the abnormal behavior of reduced viscosity of the solution in extremely dilute concentration region has been explained.

Light-scattering study of the coil-to-globule transition of linear poly(N-isopropylacrylamide) ionomers in water

The coil-to-globule transition of two poly(N-isopropylacrylamide) (PNI- PAM) ionomers with different ionic contents (0.8 and 4.5 mol %), but similar weight average molar masses, in deionized water was investigated by a combination of static and dynamic light scattering. In spite of the large difference in their ionic contents, both the ionomers have a nearly same lower critical solution temperature (LCST, C 32.57C). At temperatures higher than the LCST, the ionomer chains undergo a simultaneous intrachain coil-to-globule transition and interchain aggregation to form nanoparticles thermodynamically stable in water. The average size of the nanoparticles decreases respectively as the ionic content increases and the ionomer concentration decreases. The interchain aggregation can be completely suppressed in an extremely dilute ionomer solution ( oC 5 1 10 06 g/mL), so that the intrachain coil-to-globule transition leads to the collapse of the ionomer chains into individual single-chain nano- particles. Our results clearly indicate that there is a hysteresis in the colling process (the globule-to-coil transition). q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1501-1506, 1998

Laser Light-Scattering Studies of Poly(caprolactone-b- ethylene oxide-b-caprolactone) Nanoparticles and Their Enzymatic Biodegradation

Water-insoluble triblock poly(caprolactone-b-ethylene oxide-b-caprolac- tone) (PCL-PEO-PCL) was micronized into narrowly distributed nanoparticles stable in water. Using a combination of static and dynamic laser light scattering (LLS), we characterized the resultant nanoparticles and studied their biodegradation in the presence of enzyme lipase PS. The results revealed that the biodegradation rate was mainly dependent on the enzyme concentration. The scattering intensity decreased as the degradation proceeded, but there was no change in size of the remaining nanopar- ticles, indicating that the degradation of each particle was fast and the enzyme con- sumed the nanoparticles individually. We also found that different copolymer compo- sitions, i.e., different PCL-PEO molar ratios, led to different biodegradation rates. The pH and temperature dependence of the biodegradation rate were also studied. All results indicated that the biodegradation rate can be well controlled and the biodegra- dation essentially involves two processes: adsorption of lipase PS onto the nanopar- ticles, and enzymatic hydrolysis of the PCL blocks. The biomedical application of the enzymatic biodegradation of the copolymer nanoparticles is also envisioned.

Laser light scattering study of the degradation of poly(sebacic anhydride) nanoparticles

Poly(sebacic anhydride) (PSA) is biocompatible and degradable in basic media. We micronized this water-insoluble polymer into stable polymeric nanoparticles via a microphase inversion. Such PSA nanoparticles degraded much faster than bulk PSA. The influence of the surfactant, temperature, and pH on the degradation of the PSA nanoparticles was investigated by a combination of static and dynamic laser light scattering. Under each condition, the degradation rate was nearly constant up to a 75% weight loss; that is, the degradation was close to zero-order. The degradation rate increased with the pH and temperature. Biomedical applications of such PSA nanoparticles are suggested.

Formation and Structure of Pachyman Aggregates in Dimethyl Sulfoxide Containing Water

The aggregation of pachyman, b-(13 3)-D-glucan (Mw 5 1.68 3 10 5 ) from the Poria cocos mycelia, was investigated using static and dynamic laser light scatter- ing (LLS) in dimethyl sulfoxide (DMSO) containing about 15% water, which leads to large aggregates. Both the time dependence of hydrodynamic radius and the angle dependence of the scattering intensity were used to calculate the fractal dimension (df) of the aggregates. The aggregation rate and average size of aggregates increase dra- matically with increasing the polymer concentration from 1.7 3 10 24 g/mL to 8.6 3 10 24 g/mL, and with the decrease of the solvent quality, that is, water content from 13 to 15%. In the cases, the fractal dimensions change from 1.94 to 2.43 and from 1.92 to 2.54, respectively, suggesting that transforms of aggregation processes: a slow process called reaction-limited cluster aggregation (RLCA) to a fast process called diffusion-limited cluster aggregation (DLCA) in different polymer concentrations and water content. The fractal dimensions above 2 of the fast aggregation is larger than the 1.75 predicted for the ideal DLCA model, suggesting that the aggregation involves a restructuring process through the interchain hydrogen bonding interaction. There are no aggregates of pachyman in DMSO without water, but aggregates formed in the DMSO containing 15% water at 25°C as a compact structure.

Laser light scattering study of intrachain and interchain associations of poly(vinyl alcohol) in aqueous solution

Abstract Laser light scattering (LLS) was used to investigate the concentration dependence of intrachain and interchain associations of poly(vinyl alcohol) (PVA) in aqueous solution after different freezing-and-thawing cycles. Our results showed that if the PVA concentration (C) was much lower than its gelation concentration (Cgel), the average hydrodynamic size of the PVA chains became smaller after a cryogenic treatment, which was attributed to the change of the chain conformation from an extended coil to a compact globule because the static LLS results indicated that there was no change in the weight average molecular weight (Mw). This rules out a previously speculated degradation or scission of the PVA chains. When C was higher than Cgel, the apparent hydrodynamic size of the PVA chains was much larger and the scattering intensity became much stronger after the same cryogenic treatment, indicating the interchain association.

Investigation of the Solution Behavior of Organosoluble Aromatic Polyimides

Abstract Two high-molecular-weight organosoluble aromatic polyimides having the same diamine, 2,2′-(trifluoromethyl)-4,4′-diaminobiphenyl diamine (PFMB), but different dianhydrides, 2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride (HFBPDA) and 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), were prepared via one-step polycondensation reactions. Molecular weight fractionation of these two polymers was carried out and the polyimide fractions possess successively lower weight-average molecular weights. Intrinsic viscosity data from both sets of polyimide fractions gave the Mark-Houwink-Sakurada constants of these two polyimides, which reflect differing chain rigidity in tetrahydrofuran (THF) solutions. The MHS relations are [η] = 5.24 × 10−4 M0.65 and [η] = 3.77 × 10−5 M0.97 for 6FDA-PFMB and HFBPDA-PFMB, respectively. The physical meanings of these constants are discussed using the intrinsic viscosity data of these two polyimides in varying solvents. The persistence...