Yiu Ting Richard Lau | Hong Kong University of Science and Technology (original) (raw)

Papers by Yiu Ting Richard Lau

Surface and Interface Analysis, 2006

Films of a series of polymers with variable CF2 segments were studied by angle-dependent X-ray ph... more Films of a series of polymers with variable CF2 segments were studied by angle-dependent X-ray photoelectron spectroscopy (ADXPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The effects of the length of the CF2 segment on the surface composition of the polymers were investigated. ADXPS results indicated that the CF2 segment segregated on the surface of the polymers after thermal annealing at temperatures above their bulk glass transition temperatures when the number of CF2 units was increased to 8. Because of the higher surface sensitivity, ToF-SIMS results showed more CF2 segments at the topmost surface than shown by ADXPS. Copyright © 2006 John Wiley & Sons, Ltd.

Applied Surface Science, 2008

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), conta... more The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH2 and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.

Langmuir, 2009

Small-angle X-ray scattering revealed that a semirigid oligomer of bisphenol-A-co-ether-octane wi... more Small-angle X-ray scattering revealed that a semirigid oligomer of bisphenol-A-co-ether-octane with a monodisperse chain length is capable of forming ciliated-folded, once-folded, ciliated-extended and fully extended lamellar structures. Isothermal crystallization studies suggested a sequence of structures with increasing crystallization temperature, from a ciliated-folded to a once-folded form and then to a ciliated-extended form as the degree of supercooling is decreased. The crystal surface thus changed from octane cilia to bisphenol A segments and then back to octane cilia as the lamellar structure changed. The results of time-of-flight secondary ion mass spectrometry analyses strongly supported the fold structural models.

Surface and Interface Analysis, 2010

In this work, we examined the effects of the liquid crystalline ordering on the surfaces of therm... more In this work, we examined the effects of the liquid crystalline ordering on the surfaces of thermotropic fluorocarbon-hydrocarbon polymers (FmHn, with m = 8; n = 8, 10) by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with principal component analysis (PCA). XPS results showed that the liquid crystal surfaces of the polymers were enriched with hydrocarbon segments. PCA of ToF-SIMS data further showed that reversible structural changes between the amorphous and ordered states of the polymers occurred. As these polymers ordered at room temperature, their surfaces were transformed from the amorphous state with fluorocarbon-segment enrichment to the fully ordered liquid crystal states with hydrocarbon-segment enrichment. All these results suggested that the molecular orientation controlled the surface composition of the liquid crystalline polymers notwithstanding the surface energy. The axial alignment of the rigid fluorocarbon mesogens anchored the flexible hydrocarbon segments at the surface. Copyright © 2010 John Wiley & Sons, Ltd.

Macromolecular Rapid Communications, 2010

Back Cover: The schematic image presents an oligomer that contains an active dipole group and can... more Back Cover: The schematic image presents an oligomer that contains an active dipole group and can be aligned by a low‐energy electron beam. This novel finding offers a facile method to control molecular ordering and allows versatile patterning of ordered and disordered regions as imprinted by a mask. Further details can be found in the article by Y. T. R. Lau, L. T. Weng, K. M. Ng, M. Kempf, V. Altstädt, J. M. Schultz, C.‐M. Chan* on page 634.

Macromolecular Rapid Communications, 2010

We reveal that a beam of low‐energy electrons (18 eV) can directly trigger long‐range molecular o... more We reveal that a beam of low‐energy electrons (18 eV) can directly trigger long‐range molecular ordering of an amorphous, semi‐flexible oligomer in a few minutes without the prerequisite of pre‐orientation. A strong endothermic transition was detected with a micro‐thermal analyzer on the areas that had been exposed to the electron irradiation while the areas that were shielded from the irradiation by a protective mask remained amorphous as usual. This result suggests that long‐range molecular ordering only develops in the area of the oligomer film under electron irradiation. This is the first‐time effort to use electron irradiation to control the long‐range ordering of an amorphous organic thin film above the glass transition temperature.

Analytical Chemistry, 2010

In this article, we addressed the applicability of time-of-flight secondary ion mass spectrometry... more In this article, we addressed the applicability of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to examine the effects of molecular weight and of flexible-segment length on the polymer chain arrangement at the folding surfaces of the lamellae. Poly(bisphenol A−etheralkane) (Cn) contains both rigid aromatic and flexible aliphatic CH2 segments. The number of CH2 units per flexible segment, n, varies from 8 to 12. Principal component analysis (PCA) of TOF-SIMS data revealed the chemical and structural variations of the folding surfaces of these polymers and identified the ion peaks contributing to these variations. We highlighted the discriminating power of PCA to distinguish the structural conformations of the amorphous and flat-on lamellar surfaces of these polymers. PCA loadings analyses showed that relatively more flexible structures were deposited on the folding surfaces when the flexible-segment length increased from 8 to 10 CH2 units. The concentration of short loops at folding surfaces and the disorder of folding surfaces increased when the molecular weight increased. All these results led us to conclude that TOF-SIMS has great potential for probing the chemical composition of the folding surfaces of polymers.

Surface and Interface Analysis, 2010

The effect of film thickness on the structural conformation of the surfaces of the amorphous stat... more The effect of film thickness on the structural conformation of the surfaces of the amorphous state, edge-on lamellae and flat-on lamellae of a semiflexible polymer, poly(bisphenol-A-etheroctane), was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) with the aid of principal component analysis (PCA). PCA results empirically indicate that a structurally regular polymer surface was obtained with the formation of the flat-on lamellae from the amorphous state at a low degree of supercooling. A higher concentration of end group and cilium ion fragments, which are indicative of free chain ends, was observed on the edge-on lamellar surfaces than on the amorphous and the flat-on lamellar surfaces. This finding was attributed to the fact that the lateral surface of the edge-on lamellae contains many growth fronts, leaving behind a large number of uncrystallized chain remnants on the surfaces. Structural disorder was facilitated on both edge-on and flat-on lamellar surfaces as the film thickness decreased. Hence, this PCA study offers new insights into the nonequilibrium nature of polymer crystals and the mechanism of polymer crystallization in thin and ultrathin films. Copyright © 2010 John Wiley & Sons, Ltd.

Journal of Colloid and Interface Science, 2010

CaCO3 nanoparticles were coated with stearate through a salt-exchange procedure. Their coverage h... more CaCO3 nanoparticles were coated with stearate through a salt-exchange procedure. Their coverage had been successfully controlled by extraction in a Soxhlet apparatus, based on which a series of CaCO3 nanoparticles were obtained with different surface coverages. They were characterized with thermogravimetric analysis, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was found that free stearate, intercalated stearate and chemically-bonded alkyl chains could be extracted sequentially with the Soxhlet apparatus. Thus, the coverages of CaCO3 nanoparticles could be adjusted through carefully extracting the stearates from the CaCO3 nanoparticles with multi-layer coverage. Spectroscopic results revealed that the alkyl chains tended to adopt an extended-chain conformation in the monolayer coverage as well as the bi-layer coverage, but less ordered conformation in partially-coated coverage and random orientation at the outmost surface of the coated nanoparticles.

Surface and Interface Analysis, 2010

Poly(bisphenol-A-etheralkane), BA-Cn(n, which is the number of aliphatic CH2 units, = 8 and 10), ... more Poly(bisphenol-A-etheralkane), BA-Cn(n, which is the number of aliphatic CH2 units, = 8 and 10), composed of rigid and flexible segments, is a semicrystalline polymer. In 50-nm-thick films, the polymers tend to develop flat-on lamellae at the surfaces. Principal component analyses (PCAs) of the ion images were used to discriminate the conformation between the amorphous and flat-on lamellar surfaces of the polymers. The changes of the surface structure were directly detected by time-of-flight secondary ion mass spectrometry chemical imaging when flat-on lamellae were developed from the polymer melts. These structural changes were related to the lengths of the folded segments, which were strongly influenced by the crystallization temperature. The PCA loading analyses suggested that the length of the folded segments, which contained flexible and the rigid segments with an ether linkage, increased when the crystallization temperature decreased, suggesting the kinetic origin of polymer crystallization. Copyright © 2010 John Wiley & Sons, Ltd.

Surface and Interface Analysis, 2011

This report provides detailed experimental results of thermal and surface characterization on unt... more This report provides detailed experimental results of thermal and surface characterization on untreated and surface-treated halloysite nanotubes (HNTs) obtained from two geographic areas. Surface characterization techniques, including XPS and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used. ToF-SIMS surface analysis experiments were performed with both atomic and cluster ion beams. Higher ion yields and more high-mass ions were obtained with the cluster ion beams. Static ToF-SIMS spectra were analyzed with principal component analysis (PCA). Morphological diversities were observed in the samples although they mainly contained tubular structures. Thermogravimetric data indicated that aqueous hydrogen peroxide solution could remove inorganic salt impurities, such as alkali metal salts. The amount of grafting of benzalkonium chloride of HNT surface was determined by thermogravimetic analysis. PCA of ToF-SIMS spectra could distinguish the samples mined from different geographical locations as well as among surface-treated and untreated samples. Copyright © 2010 John Wiley & Sons, Ltd.

Biomacromolecules, 2012

In addition to its role in the regulation of sex-related processes, 17β-estradiol (E2) participat... more In addition to its role in the regulation of sex-related processes, 17β-estradiol (E2) participates in the prevention and treatment of cardiovascular diseases via nongenomic pathways mediated by estrogen receptors (ER-α) located in the cell membrane. To achieve specific nongenomic activity of E2, we linked E2 (4.4 mol %) to chitosan-phosphorylcholine (CH-PC) (20 mol % PC). Injections of ER-α solutions (5 to 100 nmol L −1) over rehydrated CH-PC-E2 thin films led to permanent adsorption of ER-α to the film surface, as detected by quartz crystal microbalance with dissipation (QCM-D). However, ER-α did not bind onto CH-PC-E2 films formed in situ and never dried. X-ray photoelectron spectroscopy (XPS) analysis of spin-cast CH-PC-E2 films revealed significant E2 enrichment of the topmost section of the film, attributed to the preferential migration of E2 toward the film/air interface upon drying. Mechanical analysis of CH-PC-E2 films in the frequency domain probed by QCM-D indicated that rehydrated films behave as an entangled network with junction points formed by self-assembly of hydrophobic E2 moieties and by ion pairing among PC groups, whereas films formed in situ are entangled polymer solutions with temporary junctions. The structural analysis presented offers useful guidelines for the study of amphiphilic biomacromolecules designed for therapeutic use as thin films.

Surface and Interface Analysis, 2013

The thermal properties of the fluorocarbon–hydrocarbon copolymer [‐(CF2)8‐(CH2)10‐]N were charact... more The thermal properties of the fluorocarbon–hydrocarbon copolymer [‐(CF2)8‐(CH2)10‐]N were characterized using polarized optical microscopy, differential scanning calorimetry and X‐ray diffraction. The results revealed the presence of a crystalline phase and a threaded nematic liquid crystal phase. The changes in the surface composition during annealing were studied in situ using time‐of‐flight secondary ion mass spectrometry. The spectra were interpreted using principal component analysis (PCA). The surface compositions at different thermal states can be distinguished by PCA. Moreover, a PCA loadings analysis revealed a continuous increase in the concentration of the hydrocarbon segments on the surface as the polymer gradually changed from the amorphous state to the liquid crystal state and finally the crystalline state. This suggests that the chemical composition of the surface in the amorphous state is controlled by the surface energy difference between the fluorocarbon and hydrocarbon segments, while the surface composition in the crystalline state is controlled by the structural ordering of the fluorocarbon segments. The results also showed that the surface re‐construction of this polymer was highly reversible. Copyright © 2012 John Wiley & Sons, Ltd.

The Journal of Physical Chemistry C, 2013

We report a simple way to obtain polymer-coated multiwalled boron nitride nanotubes (BNNTs) condu... more We report a simple way to obtain polymer-coated multiwalled boron nitride nanotubes (BNNTs) conducted under mild conditions compatible with fragile biopolymers. The approach converts aggregated pristine BNNTs into colloidally stable dispersions in water without requiring treatment at high temperature or in strongly oxidative conditions. The method relies on our experimental observation that glycine (NH2–CH2–COOH, Gly) interacts with BNNTs, in accordance with theoretical calculations. The role of glycine in this process is 2-fold: the Gly amine group binds to the B-sites of BNNTs, while the Gly carboxylic acid function provides ionic anchoring sites for interactions with polyelectrolytes. The formation of Gly-BNNTs proved to be essential, since they readily disperse in water as disentangled objects and spontaneously adsorb polycations, such as chitosan (CH), polyanions, such as hyaluronan (HA), and polyzwitterionic polymers, such as chitosan-phosphorylcholine (CH-PC). Treatment of aqueous dispersions of coarsely coated BNNTs with an immiscible solvent (hexane) resulted in the complete coverage of the BNNT surfaces via oil/water interfacial assembly. This work provides a rapid, mild, and scalable route to water-dispersible biofunctional BNNTs that may serve as drug delivery vehicles or scaffolds in tissue engineering.

Analytical Chemistry, 2013

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of polystyrene (PS) films suppo... more Time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of polystyrene (PS) films supported on silicon wafers were obtained at temperatures ranging from room temperature to 100 °C. Principal component analysis (PCA) of the TOF-SIMS data revealed a transition temperature (TT) at which the surface structure of PS was rearranged. The TT of a 120-nm thick PS (weight-average molecular weight of 3 000 g/mol) thin film was determined to be about 36 °C, which is approximately 30 °C lower than the bulk glass transition temperature (Tg) of that PS. Similar TTs were observed on PSs with different molecular weights. As the TT is strongly related to the Tg and dependent on the molecular weight, it is believed that the TT determined by TOF-SIMS is related to the surface glass transition temperature (TgS) measured by other techniques. This suggests that TOF-SIMS combined with PCA can be used to determine the TgS of polymer films. Furthermore, the detailed PCA analyses indicate that the phenyl groups of PS tended to move away from the surface at temperatures above TT. This conclusion was further confirmed by contact angle and XPS measurements.

Reviews in Analytical Chemistry, 2014

This article reviews the recent applications of time-of-flight secondary ion mass spectrometry in... more This article reviews the recent applications of time-of-flight secondary ion mass spectrometry in the determination of surface structures of semicrystralline and amorphous polymer films. Examples given include the determination of end-group distributions on semicrystalline polymer surfaces, lamellar orientation on thin-film surfaces, the structures of the folds of lamellae, the morphologies of polymer blends, the relationship between film stability and the concentration of the end groups, and the structures of thin polymer films.

Langmuir, 2014

The fractionation by length of multiwalled boron nitride nanotubes (BNNTs) was achieved by emulsi... more The fractionation by length of multiwalled boron nitride nanotubes (BNNTs) was achieved by emulsification and creaming of an oil/water/surfactant mixture. The length separation is based on the fact that nanoparticle-coated oil droplets polydisperse in size move toward the upper surface or the bottom of an emulsified mixture depending on the density of the droplets, such that droplets of different sizes are located at different heights. By sampling heightwise an unstable hexane/water/Tween 20/ BNNT (1−20 μm long) emulsion, we observed that the lengths of the BNNTs adsorbed on the droplets display a strong correlation with the droplets sizes, thus leading to selective separation of the BNNT lengths, as confirmed by dark-field optical imaging and dynamic light scattering. This method may potentially be extended to other high aspect ratio nanomaterials exhibiting emulsification properties similar to those of BNNTs.

Journal of Colloid and Interface Science, 2014

Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The s... more Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The spin-cast films of this polymer were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) at various temperatures ranging from room temperature to 120 °C. Principal component analysis (PCA) of the ToF-SIMS data revealed a transition temperature (T T) at which the surface structure of 5FPS was rearranged. A comparison between the results of the PCA of ToF-SIMS spectra obtained on 5FPS and polystyrene (PS) indicate that the pendant groups of 5FPS and PS moved in exactly opposite directions as the temperature increased. More pendant groups of 5FPS and PS migrated from the bulk to the surface and verse versa, respectively, as the temperature increased. These results clearly support the view that the abrupt changes in the normalized principal component 1 value was caused by the surface reorientation of the polymers and not by a change in the ion fragmentation mechanism at temperatures above the T T. Contact angle measurement, which is another extremely surface sensitive technique, was used to monitor the change in the surface tension as a function of temperature. A clear T T was determined by the contact angle measurements. The T T values determined by contact angle measurements and ToF-SIMS were very similar.

Journal of Colloid and Interface Science, 2017

The surface chain conformations of poly(methyl methacrylate) (PMMA) at different temperatures wer... more The surface chain conformations of poly(methyl methacrylate) (PMMA) at different temperatures were extensively studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Similar to our previous experimental studies on polystyrene (PS) and poly(2, 3, 4, 5, 6-pentafluorostyrene) (5FPS), a transition temperature () could be identified through the principal component analysis (PCA) of the ToF-SIMS spectra obtained from the PMMA samples annealed at different temperatures. Interestingly, our results show that the depended on molecular weight and was about 50–60˚C below the bulk glass transition temperature () and therefore could possibly be related to the surface glass transition temperature (). These results were confirmed by contact angle measurements. ToF-SIMS results showed higher peak intensities of several low-mass oxygen-containing positive ions, hydrocarbon positive ions and OCH3− negative ion at higher temperatures, which can be interpreted by a higher surface concentration of methoxy groups at the surface.

Surface and Interface Analysis, 2018

We studied the time-of-flight secondary ion mass spectrometry fragmentation mechanisms of polysty... more We studied the time-of-flight secondary ion mass spectrometry fragmentation mechanisms of polystyrenes-phenyl-fluorinated polystyrene (5FPS), phenyl-deuterated polystyrene (5DPS), and hydrogenated polystyrene (PS). From the positive ion spectra of 5FPS, we identified some characteristic molecular ion structures with isomeric geometries such as benzylic, benzocyclobutene, benzocyclopentene, cyclopentane, and tropylium systems. These structures were evaluated by the B3LYP-D/jun-cc-pVDZ computation method. The intensities of the C 7 H 2 F 5 + (m/z = 181), CyPent-C 9 H 3 F 4 + (m/z = 187), CyPent-C 9 H 4 F 5 + (m/z = 207), and CyPent-C 9 H 2 F 5 + (m/z = 205) ions were enhanced by resonance stabilization. The positive fluorinated ions from 5FPS tended to rearrange and produce fewer fluorine-containing molecular ions through the loss of F (m/z = 19), CF (m/z = 31), and CF 2 (m/z = 50) ion fragments. Consequently, the fluorine-containing polycyclic aromatic ions had much lower intensities than their hydrocarbon counterparts. We propose the fragmentation mechanisms for the formation of C 5 H 5 + , C 6 H 5 + , and C 7 H 7 + ion fragments, substantiated with detailed analyses of the negative ion spectra. These ions were created through elimination of a pentafluoro-phenyl anion (C 6 F 5 −) and H + , followed by a 1-electron-transfer process and then cyclization of the newly generated polyene with carbon-carbon bond formation. The pendant groups with elements of different electronegativities exerted strong influences on the intensities and fragmentation processes of their corresponding ions.

Surface and Interface Analysis, 2006

Films of a series of polymers with variable CF2 segments were studied by angle-dependent X-ray ph... more Films of a series of polymers with variable CF2 segments were studied by angle-dependent X-ray photoelectron spectroscopy (ADXPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The effects of the length of the CF2 segment on the surface composition of the polymers were investigated. ADXPS results indicated that the CF2 segment segregated on the surface of the polymers after thermal annealing at temperatures above their bulk glass transition temperatures when the number of CF2 units was increased to 8. Because of the higher surface sensitivity, ToF-SIMS results showed more CF2 segments at the topmost surface than shown by ADXPS. Copyright © 2006 John Wiley & Sons, Ltd.

Applied Surface Science, 2008

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), conta... more The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH2 and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.

Langmuir, 2009

Small-angle X-ray scattering revealed that a semirigid oligomer of bisphenol-A-co-ether-octane wi... more Small-angle X-ray scattering revealed that a semirigid oligomer of bisphenol-A-co-ether-octane with a monodisperse chain length is capable of forming ciliated-folded, once-folded, ciliated-extended and fully extended lamellar structures. Isothermal crystallization studies suggested a sequence of structures with increasing crystallization temperature, from a ciliated-folded to a once-folded form and then to a ciliated-extended form as the degree of supercooling is decreased. The crystal surface thus changed from octane cilia to bisphenol A segments and then back to octane cilia as the lamellar structure changed. The results of time-of-flight secondary ion mass spectrometry analyses strongly supported the fold structural models.

Surface and Interface Analysis, 2010

In this work, we examined the effects of the liquid crystalline ordering on the surfaces of therm... more In this work, we examined the effects of the liquid crystalline ordering on the surfaces of thermotropic fluorocarbon-hydrocarbon polymers (FmHn, with m = 8; n = 8, 10) by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with principal component analysis (PCA). XPS results showed that the liquid crystal surfaces of the polymers were enriched with hydrocarbon segments. PCA of ToF-SIMS data further showed that reversible structural changes between the amorphous and ordered states of the polymers occurred. As these polymers ordered at room temperature, their surfaces were transformed from the amorphous state with fluorocarbon-segment enrichment to the fully ordered liquid crystal states with hydrocarbon-segment enrichment. All these results suggested that the molecular orientation controlled the surface composition of the liquid crystalline polymers notwithstanding the surface energy. The axial alignment of the rigid fluorocarbon mesogens anchored the flexible hydrocarbon segments at the surface. Copyright © 2010 John Wiley & Sons, Ltd.

Macromolecular Rapid Communications, 2010

Back Cover: The schematic image presents an oligomer that contains an active dipole group and can... more Back Cover: The schematic image presents an oligomer that contains an active dipole group and can be aligned by a low‐energy electron beam. This novel finding offers a facile method to control molecular ordering and allows versatile patterning of ordered and disordered regions as imprinted by a mask. Further details can be found in the article by Y. T. R. Lau, L. T. Weng, K. M. Ng, M. Kempf, V. Altstädt, J. M. Schultz, C.‐M. Chan* on page 634.

Macromolecular Rapid Communications, 2010

We reveal that a beam of low‐energy electrons (18 eV) can directly trigger long‐range molecular o... more We reveal that a beam of low‐energy electrons (18 eV) can directly trigger long‐range molecular ordering of an amorphous, semi‐flexible oligomer in a few minutes without the prerequisite of pre‐orientation. A strong endothermic transition was detected with a micro‐thermal analyzer on the areas that had been exposed to the electron irradiation while the areas that were shielded from the irradiation by a protective mask remained amorphous as usual. This result suggests that long‐range molecular ordering only develops in the area of the oligomer film under electron irradiation. This is the first‐time effort to use electron irradiation to control the long‐range ordering of an amorphous organic thin film above the glass transition temperature.

Analytical Chemistry, 2010

In this article, we addressed the applicability of time-of-flight secondary ion mass spectrometry... more In this article, we addressed the applicability of time-of-flight secondary ion mass spectrometry (TOF-SIMS) to examine the effects of molecular weight and of flexible-segment length on the polymer chain arrangement at the folding surfaces of the lamellae. Poly(bisphenol A−etheralkane) (Cn) contains both rigid aromatic and flexible aliphatic CH2 segments. The number of CH2 units per flexible segment, n, varies from 8 to 12. Principal component analysis (PCA) of TOF-SIMS data revealed the chemical and structural variations of the folding surfaces of these polymers and identified the ion peaks contributing to these variations. We highlighted the discriminating power of PCA to distinguish the structural conformations of the amorphous and flat-on lamellar surfaces of these polymers. PCA loadings analyses showed that relatively more flexible structures were deposited on the folding surfaces when the flexible-segment length increased from 8 to 10 CH2 units. The concentration of short loops at folding surfaces and the disorder of folding surfaces increased when the molecular weight increased. All these results led us to conclude that TOF-SIMS has great potential for probing the chemical composition of the folding surfaces of polymers.

Surface and Interface Analysis, 2010

The effect of film thickness on the structural conformation of the surfaces of the amorphous stat... more The effect of film thickness on the structural conformation of the surfaces of the amorphous state, edge-on lamellae and flat-on lamellae of a semiflexible polymer, poly(bisphenol-A-etheroctane), was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) with the aid of principal component analysis (PCA). PCA results empirically indicate that a structurally regular polymer surface was obtained with the formation of the flat-on lamellae from the amorphous state at a low degree of supercooling. A higher concentration of end group and cilium ion fragments, which are indicative of free chain ends, was observed on the edge-on lamellar surfaces than on the amorphous and the flat-on lamellar surfaces. This finding was attributed to the fact that the lateral surface of the edge-on lamellae contains many growth fronts, leaving behind a large number of uncrystallized chain remnants on the surfaces. Structural disorder was facilitated on both edge-on and flat-on lamellar surfaces as the film thickness decreased. Hence, this PCA study offers new insights into the nonequilibrium nature of polymer crystals and the mechanism of polymer crystallization in thin and ultrathin films. Copyright © 2010 John Wiley & Sons, Ltd.

Journal of Colloid and Interface Science, 2010

CaCO3 nanoparticles were coated with stearate through a salt-exchange procedure. Their coverage h... more CaCO3 nanoparticles were coated with stearate through a salt-exchange procedure. Their coverage had been successfully controlled by extraction in a Soxhlet apparatus, based on which a series of CaCO3 nanoparticles were obtained with different surface coverages. They were characterized with thermogravimetric analysis, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was found that free stearate, intercalated stearate and chemically-bonded alkyl chains could be extracted sequentially with the Soxhlet apparatus. Thus, the coverages of CaCO3 nanoparticles could be adjusted through carefully extracting the stearates from the CaCO3 nanoparticles with multi-layer coverage. Spectroscopic results revealed that the alkyl chains tended to adopt an extended-chain conformation in the monolayer coverage as well as the bi-layer coverage, but less ordered conformation in partially-coated coverage and random orientation at the outmost surface of the coated nanoparticles.

Surface and Interface Analysis, 2010

Poly(bisphenol-A-etheralkane), BA-Cn(n, which is the number of aliphatic CH2 units, = 8 and 10), ... more Poly(bisphenol-A-etheralkane), BA-Cn(n, which is the number of aliphatic CH2 units, = 8 and 10), composed of rigid and flexible segments, is a semicrystalline polymer. In 50-nm-thick films, the polymers tend to develop flat-on lamellae at the surfaces. Principal component analyses (PCAs) of the ion images were used to discriminate the conformation between the amorphous and flat-on lamellar surfaces of the polymers. The changes of the surface structure were directly detected by time-of-flight secondary ion mass spectrometry chemical imaging when flat-on lamellae were developed from the polymer melts. These structural changes were related to the lengths of the folded segments, which were strongly influenced by the crystallization temperature. The PCA loading analyses suggested that the length of the folded segments, which contained flexible and the rigid segments with an ether linkage, increased when the crystallization temperature decreased, suggesting the kinetic origin of polymer crystallization. Copyright © 2010 John Wiley & Sons, Ltd.

Surface and Interface Analysis, 2011

This report provides detailed experimental results of thermal and surface characterization on unt... more This report provides detailed experimental results of thermal and surface characterization on untreated and surface-treated halloysite nanotubes (HNTs) obtained from two geographic areas. Surface characterization techniques, including XPS and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used. ToF-SIMS surface analysis experiments were performed with both atomic and cluster ion beams. Higher ion yields and more high-mass ions were obtained with the cluster ion beams. Static ToF-SIMS spectra were analyzed with principal component analysis (PCA). Morphological diversities were observed in the samples although they mainly contained tubular structures. Thermogravimetric data indicated that aqueous hydrogen peroxide solution could remove inorganic salt impurities, such as alkali metal salts. The amount of grafting of benzalkonium chloride of HNT surface was determined by thermogravimetic analysis. PCA of ToF-SIMS spectra could distinguish the samples mined from different geographical locations as well as among surface-treated and untreated samples. Copyright © 2010 John Wiley & Sons, Ltd.

Biomacromolecules, 2012

In addition to its role in the regulation of sex-related processes, 17β-estradiol (E2) participat... more In addition to its role in the regulation of sex-related processes, 17β-estradiol (E2) participates in the prevention and treatment of cardiovascular diseases via nongenomic pathways mediated by estrogen receptors (ER-α) located in the cell membrane. To achieve specific nongenomic activity of E2, we linked E2 (4.4 mol %) to chitosan-phosphorylcholine (CH-PC) (20 mol % PC). Injections of ER-α solutions (5 to 100 nmol L −1) over rehydrated CH-PC-E2 thin films led to permanent adsorption of ER-α to the film surface, as detected by quartz crystal microbalance with dissipation (QCM-D). However, ER-α did not bind onto CH-PC-E2 films formed in situ and never dried. X-ray photoelectron spectroscopy (XPS) analysis of spin-cast CH-PC-E2 films revealed significant E2 enrichment of the topmost section of the film, attributed to the preferential migration of E2 toward the film/air interface upon drying. Mechanical analysis of CH-PC-E2 films in the frequency domain probed by QCM-D indicated that rehydrated films behave as an entangled network with junction points formed by self-assembly of hydrophobic E2 moieties and by ion pairing among PC groups, whereas films formed in situ are entangled polymer solutions with temporary junctions. The structural analysis presented offers useful guidelines for the study of amphiphilic biomacromolecules designed for therapeutic use as thin films.

Surface and Interface Analysis, 2013

The thermal properties of the fluorocarbon–hydrocarbon copolymer [‐(CF2)8‐(CH2)10‐]N were charact... more The thermal properties of the fluorocarbon–hydrocarbon copolymer [‐(CF2)8‐(CH2)10‐]N were characterized using polarized optical microscopy, differential scanning calorimetry and X‐ray diffraction. The results revealed the presence of a crystalline phase and a threaded nematic liquid crystal phase. The changes in the surface composition during annealing were studied in situ using time‐of‐flight secondary ion mass spectrometry. The spectra were interpreted using principal component analysis (PCA). The surface compositions at different thermal states can be distinguished by PCA. Moreover, a PCA loadings analysis revealed a continuous increase in the concentration of the hydrocarbon segments on the surface as the polymer gradually changed from the amorphous state to the liquid crystal state and finally the crystalline state. This suggests that the chemical composition of the surface in the amorphous state is controlled by the surface energy difference between the fluorocarbon and hydrocarbon segments, while the surface composition in the crystalline state is controlled by the structural ordering of the fluorocarbon segments. The results also showed that the surface re‐construction of this polymer was highly reversible. Copyright © 2012 John Wiley & Sons, Ltd.

The Journal of Physical Chemistry C, 2013

We report a simple way to obtain polymer-coated multiwalled boron nitride nanotubes (BNNTs) condu... more We report a simple way to obtain polymer-coated multiwalled boron nitride nanotubes (BNNTs) conducted under mild conditions compatible with fragile biopolymers. The approach converts aggregated pristine BNNTs into colloidally stable dispersions in water without requiring treatment at high temperature or in strongly oxidative conditions. The method relies on our experimental observation that glycine (NH2–CH2–COOH, Gly) interacts with BNNTs, in accordance with theoretical calculations. The role of glycine in this process is 2-fold: the Gly amine group binds to the B-sites of BNNTs, while the Gly carboxylic acid function provides ionic anchoring sites for interactions with polyelectrolytes. The formation of Gly-BNNTs proved to be essential, since they readily disperse in water as disentangled objects and spontaneously adsorb polycations, such as chitosan (CH), polyanions, such as hyaluronan (HA), and polyzwitterionic polymers, such as chitosan-phosphorylcholine (CH-PC). Treatment of aqueous dispersions of coarsely coated BNNTs with an immiscible solvent (hexane) resulted in the complete coverage of the BNNT surfaces via oil/water interfacial assembly. This work provides a rapid, mild, and scalable route to water-dispersible biofunctional BNNTs that may serve as drug delivery vehicles or scaffolds in tissue engineering.

Analytical Chemistry, 2013

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of polystyrene (PS) films suppo... more Time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of polystyrene (PS) films supported on silicon wafers were obtained at temperatures ranging from room temperature to 100 °C. Principal component analysis (PCA) of the TOF-SIMS data revealed a transition temperature (TT) at which the surface structure of PS was rearranged. The TT of a 120-nm thick PS (weight-average molecular weight of 3 000 g/mol) thin film was determined to be about 36 °C, which is approximately 30 °C lower than the bulk glass transition temperature (Tg) of that PS. Similar TTs were observed on PSs with different molecular weights. As the TT is strongly related to the Tg and dependent on the molecular weight, it is believed that the TT determined by TOF-SIMS is related to the surface glass transition temperature (TgS) measured by other techniques. This suggests that TOF-SIMS combined with PCA can be used to determine the TgS of polymer films. Furthermore, the detailed PCA analyses indicate that the phenyl groups of PS tended to move away from the surface at temperatures above TT. This conclusion was further confirmed by contact angle and XPS measurements.

Reviews in Analytical Chemistry, 2014

This article reviews the recent applications of time-of-flight secondary ion mass spectrometry in... more This article reviews the recent applications of time-of-flight secondary ion mass spectrometry in the determination of surface structures of semicrystralline and amorphous polymer films. Examples given include the determination of end-group distributions on semicrystalline polymer surfaces, lamellar orientation on thin-film surfaces, the structures of the folds of lamellae, the morphologies of polymer blends, the relationship between film stability and the concentration of the end groups, and the structures of thin polymer films.

Langmuir, 2014

The fractionation by length of multiwalled boron nitride nanotubes (BNNTs) was achieved by emulsi... more The fractionation by length of multiwalled boron nitride nanotubes (BNNTs) was achieved by emulsification and creaming of an oil/water/surfactant mixture. The length separation is based on the fact that nanoparticle-coated oil droplets polydisperse in size move toward the upper surface or the bottom of an emulsified mixture depending on the density of the droplets, such that droplets of different sizes are located at different heights. By sampling heightwise an unstable hexane/water/Tween 20/ BNNT (1−20 μm long) emulsion, we observed that the lengths of the BNNTs adsorbed on the droplets display a strong correlation with the droplets sizes, thus leading to selective separation of the BNNT lengths, as confirmed by dark-field optical imaging and dynamic light scattering. This method may potentially be extended to other high aspect ratio nanomaterials exhibiting emulsification properties similar to those of BNNTs.

Journal of Colloid and Interface Science, 2014

Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The s... more Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The spin-cast films of this polymer were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) at various temperatures ranging from room temperature to 120 °C. Principal component analysis (PCA) of the ToF-SIMS data revealed a transition temperature (T T) at which the surface structure of 5FPS was rearranged. A comparison between the results of the PCA of ToF-SIMS spectra obtained on 5FPS and polystyrene (PS) indicate that the pendant groups of 5FPS and PS moved in exactly opposite directions as the temperature increased. More pendant groups of 5FPS and PS migrated from the bulk to the surface and verse versa, respectively, as the temperature increased. These results clearly support the view that the abrupt changes in the normalized principal component 1 value was caused by the surface reorientation of the polymers and not by a change in the ion fragmentation mechanism at temperatures above the T T. Contact angle measurement, which is another extremely surface sensitive technique, was used to monitor the change in the surface tension as a function of temperature. A clear T T was determined by the contact angle measurements. The T T values determined by contact angle measurements and ToF-SIMS were very similar.

Journal of Colloid and Interface Science, 2017

The surface chain conformations of poly(methyl methacrylate) (PMMA) at different temperatures wer... more The surface chain conformations of poly(methyl methacrylate) (PMMA) at different temperatures were extensively studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Similar to our previous experimental studies on polystyrene (PS) and poly(2, 3, 4, 5, 6-pentafluorostyrene) (5FPS), a transition temperature () could be identified through the principal component analysis (PCA) of the ToF-SIMS spectra obtained from the PMMA samples annealed at different temperatures. Interestingly, our results show that the depended on molecular weight and was about 50–60˚C below the bulk glass transition temperature () and therefore could possibly be related to the surface glass transition temperature (). These results were confirmed by contact angle measurements. ToF-SIMS results showed higher peak intensities of several low-mass oxygen-containing positive ions, hydrocarbon positive ions and OCH3− negative ion at higher temperatures, which can be interpreted by a higher surface concentration of methoxy groups at the surface.

Surface and Interface Analysis, 2018

We studied the time-of-flight secondary ion mass spectrometry fragmentation mechanisms of polysty... more We studied the time-of-flight secondary ion mass spectrometry fragmentation mechanisms of polystyrenes-phenyl-fluorinated polystyrene (5FPS), phenyl-deuterated polystyrene (5DPS), and hydrogenated polystyrene (PS). From the positive ion spectra of 5FPS, we identified some characteristic molecular ion structures with isomeric geometries such as benzylic, benzocyclobutene, benzocyclopentene, cyclopentane, and tropylium systems. These structures were evaluated by the B3LYP-D/jun-cc-pVDZ computation method. The intensities of the C 7 H 2 F 5 + (m/z = 181), CyPent-C 9 H 3 F 4 + (m/z = 187), CyPent-C 9 H 4 F 5 + (m/z = 207), and CyPent-C 9 H 2 F 5 + (m/z = 205) ions were enhanced by resonance stabilization. The positive fluorinated ions from 5FPS tended to rearrange and produce fewer fluorine-containing molecular ions through the loss of F (m/z = 19), CF (m/z = 31), and CF 2 (m/z = 50) ion fragments. Consequently, the fluorine-containing polycyclic aromatic ions had much lower intensities than their hydrocarbon counterparts. We propose the fragmentation mechanisms for the formation of C 5 H 5 + , C 6 H 5 + , and C 7 H 7 + ion fragments, substantiated with detailed analyses of the negative ion spectra. These ions were created through elimination of a pentafluoro-phenyl anion (C 6 F 5 −) and H + , followed by a 1-electron-transfer process and then cyclization of the newly generated polyene with carbon-carbon bond formation. The pendant groups with elements of different electronegativities exerted strong influences on the intensities and fragmentation processes of their corresponding ions.