Chi Ming Chan

Polymer surface modification by plasmas and photons

Abstract Polymers have been applied successfully in fields such as adhesion, biomaterials, protective coatings, friction and wear, composites, microelectronic devices, and thin-film technology. In general, special surface properties with regard to chemical composition, hydrophilicity, roughness, crystallinity, conductivity, lubricity, and cross-linking density are required for the success of these applications. Polymers very often do not possess the surface properties needed for these applications. However, they have excellent bulk physical and chemical properties, are inexpensive, and are easy to process. For these reasons, surface modification techniques which can transform these inexpensive materials into highly valuable finished products have become an important part of the plastics and many other industries. In recent years, many advances have been made in developing surface treatments to alter the chemical and physical properties of polymer surfaces without affecting bulk properties. Common surface modification techniques include treatments by flame, corona, plasmas, photons, electron beams, ion beams, X-rays, and γ-rays. Plasma treatment is probably the most versatile surface treatment technique. Different types of gases such as argon, oxygen, nitrogen, fluorine, carbon dioxide, and water can produce the unique surface properties required by various applications. For example, oxygen-plasma treatment can increase the surface energy of polymers, whereas fluorine-plasma treatment can decrease the surface energy and improve the chemical inertness. Cross-linking at a polymer surface can be introduced by an inert-gas plasma. Modification by plasma treatment is usually confined to the top several hundred angstroms and does not affect the bulk properties. The main disadvantage of this technique is that it requires a vacuum system, which increases the cost of operation. Thin polymer films with unique chemical and physical properties are produced by plasma polymerization. This technology is still in its infancy, and the plasma chemical process is not fully understood. The films are prepared by vapor phase deposition and can be formed on practically any substrate with good adhesion between the film and the substrate. These films, which are usually highly cross-linked and pinhole-free, have very good barrier properties. Such films find great potential in biomaterial applications and in the microelectronics industry. Very high-power microwave-driven mercury lamps are available, and they are used in UV-hardening of photoresist patterns for image stabilization at high temperatures. Other applications of UV irradiation include surface photo-oxidation, increase of hydrophilicity, and photocuring of paintings. Pulsed UV-lasers are used in surface modification in many areas. Pulsed UV-laser irradiation can produce submicron periodic linear and dot patterns on polymer surfaces without photomask. These interference patterns can be used to increase surface roughness of inert polymers for improved adhesion. These images can also be transferred to silicon surfaces by reactive ion etching. Pulsed laser beams can be applied to inert polymer surfaces for increased hydrophilicity and wettability. Polymer surfaces treated by pulsed UV-laser irradiation can be positively or negatively charged to enhance chemical reactivity and processability. Pulsed UV-laser exposures with high fluence give rise to photoablation with a clean wall profile. There are many other practical applications of laser photoablation, including via-hole fabrication, and diamond-film deposition. The present review discusses all these current applications, especially in the biomedical and microelectronics areas.

Effect of fiber pretreatment condition on the interfacial strength and mechanical properties of wood fiber/PP composites

The effect of fiber surface pretreatment on the interfacial strength and mechanical properties of wood fiber/polypropylene (WF/PP) composites are investigated. The results demonstrate that fiber surface conditions significantly influence the fiber–matrix interfacial bond, which, in turn, determines the mechanical properties of the composites. The WF/PP composite containing fibers pretreated with an acid–silane aqueous solution exhibits the highest tensile properties among the materials studied. This observation is a direct result of the strong interfacial bond caused by the acid/water condition used in the fiber pretreatment. Evidence from coupling chemistry, rheological and electron microscopic studies support the above conclusion. When SEBS-g-MA copolymer is used, a synergistic toughening effect between the wood fiber and the copolymer is observed. The V-notch Charpy impact strength of the WF/PP/SEBS-g-MA composite is substantially higher than that of the WF/PP composite. The synergistic toughening mechanisms are discussed with respect to the interfacial bond strength, fiber-matrix debonding, and matrix plastic deformation.

On deformation mechanisms of β-polypropylene 2. Changes of lamellar structure caused by tensile load

Several β-polypropylene (β-PP) samples were stretched to various strains at room temperature. The morphologies of the deformed specimens were studied by scanning electron microscopy and transmission electron microscopy. The deformation was highly inhomogeneous in the β-PP specimens. In the early stage of deformation, horizontal lamellae were stretched to separation and deformation bands were formed within a spherulite in some regions. The deformation bands coalesced as strain increased. Near the yield point, some deformation bands developed into crazes across the spherulite boundaries. Meanwhile, melting spots and shear bands were found in the yielded sample. However, the main cause of failure was cracking across the specimen.

Confirmation of the missing-row model with three-layer relaxations for the reconstructed Ir(110)-(1 × 2) surface

Abstract The reconstruction of Ir(110)-(1 × 2) has been re-analyzed by low-energy electron diffraction. In this study, the missing-row model with paired rows in the second layer and buckled rows in the third layer, as well as the Bonzel-Ferrer (sawtooth) model have been examined. In addition, two other models, which are obtained by putting the missing-row atoms back onto the surface in other sites, have also been considered. It is found that the missing-row model with paired rows in the second layer and buckled rows in the third layer gives the best R -factor among all the models considered in this study. This missing-row model with a three-layer reconstruction is thus proposed to solve the Ir(110)-(1 × 2) structure.

An R-factor analysis of several models of the reconstructed Ir(110)-(1 × 2) surface a☆

The structure of the reconstructed Ir(110)-(1 × 2) surface has been analyzed by low-energy electron diffraction. The reliability (R)-factor analysis, proposed by Zanazzi and Jona, has been applied to determine quantitatively the level of agreement between the experimental and calculated beam intensities for different models proposed for the (1 × 2) structure. The models tested were the following: (1) the missing row model, (2) the missing row model with a slight movement of the second layer, (3) the paired-rows model, and (4) the buckled surface model. Based on the results of the R-factor analysis, the missing row model with a topmost layer spacing of 1.16 ± 0.07 A, which corresponds to approximately 15% contraction of the bulk interlayer spacing of 1.36 A, is the preferred structure.

High-impact polystyrene/halloysite nanocomposites prepared by emulsion polymerization using sodium dodecyl sulfate as surfactant

High-impact polystyrene (PS) nanocomposites filled with individually dispersed halloysite nanotubes (HNTs) were prepared by emulsion polymerization of styrene in the presence of HNTs with sodium dodecyl sulfate (SDS) as the emulsifier. The SDS is a good dispersing agent for HNTs in aqueous solution. The emulsion polymerization resulted in the formation of polystyrene nanospheres separating individual HNTs. Transmission electron microscopy revealed that the HNTs were uniformly dispersed in the PS matrix. Differential scanning calorimetry, Fourier-transform infrared spectroscopy and thermogravimetry were used to characterize the PS/HNT nanocomposites. The impact strength of the PS/HNTs nanocomposites was 300% higher than that of the neat PS. This paper presents a simple yet feasible method for the preparation of high-impact PS/halloysite nanocomposites.

On deformation mechanisms of β-polypropylene 3. Lamella structures after necking and cold drawing

Abstract A β-PP sample was stretched to necking at room temperature and the morphologies of the deformed material over the necking region were examined with SEM and TEM. Before yielding, the strain was mainly accommodated between horizontal lamellae. Nevertheless, some vertical lamellae were stretched to break when the material approached yielding. At the yield point, many melt spots and deformation bands across vertical lamellae emerged. Some vertical lamellae became highly stretched and their thickness reduced significantly while the neighboring horizontal lamellae rotated towards the loading direction. At the upper shoulder of neck, the melt spots near the poles of spherulites enlarged and elongated in the stress direction, meanwhile, strain-induced crystals appeared in the melt spots. Furthermore, many vertical lamellae were broken into short fragments and some horizontal lamellae were sheared in the loading direction when the adjacent vertical lamellae were stretched heavily. In the tapered section of the neck, the original β-spherulites were shattered due to excessive crazing and deformation bands and some highly drawn material domains were formed. In turn, more shear bands were generated roughly along the loading direction. The horizontal lamella fragments rotated towards the loading direction and fed into the material flow continually. Finally, the lamella–spherulite structure was converted into an oriented fibril structure in the cold drawn material. Based on the observed results the deformation mechanisms of necking and cold drawing were discussed.

Positive and negative temperature coefficient effects of an alternating copolymer of tetrafluoroethylene-ethylene containing carbon black-filled HDPE particles

Abstract A conductive polymer composite was prepared by melt-mixing of an immiscible semicrystalline polymer blend of an alternating copolymer of tetrafluoroethylene–ethylene (ETFE), high density polyethylene (HDPE), and carbon black (CB). The optical microscopy and time-of-flight secondary mass spectrometry results indicated that the CB particles were selectively localized in the HDPE phase. In addition, it was found that the CB-filled HDPE particles formed a dispersed phase in the ETFE matrix. A double-positive temperature coefficient (PTC) effect was observed in the composite, caused by the large thermal expansion due to the consecutive melting of HDPE and ETFE crystallites. The negative temperature coefficient (NTC) that was observed in this system could not have been caused by the formation of flocculated structures because the size of the CB-filled HDPE particles is significantly large, so that their mobility is extremely limited even at high temperatures. This conclusion was confirmed by observing the morphology of the composite at temperature ranging from 25 to 250°C. These results suggest that new mechanisms need to be uncovered to explain the NTC effect of conductive polymer composites.

Chain extension of poly(butylene terephthalate) by reactive extrusion

The chain extension reaction in poly(butylene terephthalate) (PBT) melt was studied in detail. A high-reactivity diepoxy, diglycidyl tetrahydrophthalate, was used as a chain extender that can react with the hydroxyl and carboxyl end groups of PBT at a very fast reaction rate and a relatively high temperature. A Haake mixer 600 was used to record the torque during the chain extension reaction. The data show that this chain extension reaction could be completed within 2 to 3 min at temperatures above 250°C, and the reaction time decreased very fast with an increase in the temperature. Shear rate also had some effects on the reaction rate. The effect of the diepoxy chain extender on the flowability, thermal stability, and mechanical properties of PBT were investigated. The melt flow index (MFI) of the chain-extended PBT dramatically decreased as the diepoxy was added to PBT. In addition, the notched Izod impact strength and elongation-at-break of the chain-extended PBT also increased. The chain-extended PBT is more stable thermally. Compared with the conventional solid post-polycondensation method, this approach is simpler and cheaper to obtain high-molecular-weight PBT resins.

The effects of specific interactions on the surface structure and composition of miscible blends of poly(vinyl alcohol) and poly(N-vinyl-2-pyrrolidone)

Poly(vinyl alcohol) (PVAL)/poly(N-vinyl-2-pyrrolidone) (PVP) blends were studied by differential scanning calorimetry (d.s.c.), Fourier transform infrared spectroscopy ( FT i.r.), and X-ray photoelectron spectroscopy (XPS). A single glass transition temperature ( T g ) was observed for all PVAL/PVP blends, suggesting that PVAL/PVP blends are miscible within the compositions considered. The specific interaction (hydrogen bonding) between PVAL and PVP was investigated by d.s.c. and FT i.r. The surface chemical composition of the blends was studied by XPS. The XPS result showed that PVAL is enriched on the surface of the PVAL/PVP blends in spite of the formation of intermolecular hydrogen bonds between the C=O in PVP and the OH in PVAL. The intermolecular hydrogen bonds also induce a change of 0.3–0.4 eV in the Ols binding energy difference between the oxygen atoms in the carbonyl and hydroxyl groups. It can be concluded that the formation of the intermolecular hydrogen bonds is a major factor for the miscibility of the PVAL/PVP blends in the bulk and that the difference in the surface energy between PVAL and PVP is the dominant factor that controls the surface composition of the blends.