Yiu-Ting R. Lau

Time-of-Flight-Secondary Ion Mass Spectrometry and Principal Component Analysis: Determination of Structures of Lamellar Surfaces

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 CH(2) segments. The number of CH(2) 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 CH(2) 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.

Preparation and characterization of coverage-controlled CaCO3 nanoparticles.

CaCO(3) 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 CaCO(3) 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 CaCO(3) nanoparticles could be adjusted through carefully extracting the stearates from the CaCO(3) 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.

Polymer Surface Structures Determined Using ToF-SIMS

Abstract 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.

Control of the Fold Surface Conformation of the Lamellae of an Oligomer

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.

Evidence of Enhanced Mobility at the Free Surface of Supported Polymer Films by in Situ Variable-Temperature Time-of-Flight-Secondary Ion Mass Spectrometry

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 (Tg(S)) measured by other techniques. This suggests that TOF-SIMS combined with PCA can be used to determine the Tg(S) 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.

Transition temperature of poly(methyl methacrylate) determined by time-of-flight secondary ion mass spectrometry and contact angle measurements

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 (TT) 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 TT depended on molecular weight and was about 50-60˚C below the bulk glass transition temperature (Tg) and therefore could possibly be related to the surface glass transition temperature (TgS). 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.

Detection of surface mobility of poly (2, 3, 4, 5, 6-pentafluorostyrene) films by in situ variable-temperature ToF-SIMS and contact angle measurements

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.

Induction of Molecular Organization of Oligomers by Low‐Energy Electrons

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.