Surface Glass Transition Research Articles

Free-volume distribution and glass transition of nano-scale polymeric films

Positron annihilation spectroscopy has been used to measure the ortho-positronium lifetime variation with respect to the temperature in an 80-nm polystyrene film on Si in different depths. The surface and interface glass transition temperatures were found to be significantly suppressed by 18 and 12 K at the depth of 5 and 70 nm from the surface, respectively. The observed T g-depth dependence is interpreted as a different degree of free-volume distributions at the surface and the interface with Si.

Investigation of the drastic change in the sputter rate of polymers at low ion fluence

The polymer sputter rate dependence on ion fluence and ion chemistry (Ar, N2, O2) at 1keV energy was investigated using a quartz crystal microbalance (QCM) which allowed to do real time etch rate measurements and to study kinetics of sputtering. The obtained sputter rates differed drastically from polymer to polymer showing, that the chemical structure of polymer is an important factor in the polymer etch yield. A decrease in the sputter rate was observed up to ion fluence of 5×1014–5×1015cm−2 (depending on the polymer type and ion chemistry) followed by the saturation in the rate at prolonged ion bombardment. Polymer removal was accompanied by the formation of degradation products, cross-linking or branching, modification of the surface chemical structure, which was studied in situ using XPS. The dependence of the surface glass transition temperature, Tgs on the ion fluence was studied using the method based on the embedding of metallic nanoparticles. The correlation between chemical yield data and ablation rate is discussed.

Measurement of surface glass transition temperature of amorphous cefditoren pivoxil granules by inverse gas chromatography

The aim of the present study is to increase the knowledge about the molecular mobility of cefditoren pivoxil (CDTR-PI) on the granules surface by studying the glass transition temperature (Tg) of the drug using inverse gas chromatography (IGC), and to clarify the inhomogeneity of the granule resulting in the physical stability of them. The Tg of the granulated amorphous CDTR-PI and the granulated CDTR-PI with polymers were measured by inverse gas chromatography (IGC) and differential scanning calorimetry (DSC). There was no difference among samples in the Tg measured by DSC. However, the Tg measured by IGC were different among samples; the value of CDTR-PI with hydroxypropylcellulose (HPC-L) was lower than the others, and showed a tendency similar to spray-dried samples. It was suggested that IGC measured the surface transition temperatures of granules that was impossible to measure by DSC. By comparison with the influence of the Tg on the surface and the whole granule on the crystallization rates of them, the surface Tg strongly affected the physicochemical stability of granules compared to those of the whole.

Investigation of the Surface Glass Transition Temperature by Embedding of Noble Metal Nanoclusters into Monodisperse Polystyrenes

Chain mobility in a near surface region at a polystyrene/vacuum interface was investigated by embedding of noble metal nanosized clusters. The embedding process was monitored in situ by X-ray photoelectron spectroscopy. The embedding of nanosized clusters needs long-range chain mobility of the polymer. Therefore, the embedding process is a probe for the glass transition in a near surface region. The clusters used in this study are formed by the dewetting of evaporated noble metals onto the polymer surface. First, methodical influences on the embedding process were investigated. An onset embedding temperature T* was defined. T* increases with a heating rate comparable to Tg(bulk) determined with a differential scanning calorimeter. Furthermore, T* increases with nominal metal coverage which results in increasing average cluster radius. The cluster size distribution was investigated by transmission electron microscopy. It is shown that T* is an upper limit for Tg in a near surface region with a depth of a few nanometers. With optimized probe conditions, embedding processes were performed on monodisperse polystyrene (Mw = 3−1000 kg/mol). The T* values fit quite well with the Fox−Flory relation, but with a saturation temperature of approximately 8 K below the bulk value. ΔT = Tg(bulk) − T* increases with molecular weight. This molecular weight dependence of T* will be discussed in terms of several models. The chain end segregation model can be ruled out. To investigate the kinetics of the embedding process, isothermal experiments were performed. From these experiments surface viscosities were derived, which are well below bulk values.

Study on Surface Glass Transition Behavior in Monodisperse Polystyrene Films

Surfaces in polymeric materials play an important role in many technological applications. Hence, systematical understanding of aggregation states and physical properties at the surfaces is of pivotal importance as the first benchmark so that highly functionalized polymeric materials based on surface peculiarity can be promisingly designed and constructed. Nevertheless, surface properties, especially rheological properties, had not been studied until early 90s because of technical difficulties. In 1993, we started to embark on this intriguing and challenging issue by mainly scanning force microscopy using monodisperse polystyrene (PS) solid films. Consequently, conclusions obtained thus far are consistently arrived that molecular motion at the PS surfaces is thermally activated in comparison with the corresponding internal bulk region. However, it is far from clear for the moment what the whole picture of such peculiar surface mobility is, although we believe that some parts of it have been successfully clarified to date. Here, experimental methods to examine surface rheological properties, which we have proposed, are mentioned at first, and then, essential points of what we have established are discussed.

Embedding of Gold Nanoclusters on Polystyrene Surfaces: Influence of the Surface Modification on the Glass Transition

The thermal behavior of surfaces of polystyrene (PS) films of 80−100 nm thickness decorated with gold nanoclusters with radii ranging from 9 to 18 Å has been investigated by in situ X-ray reflectivity measurements. Recently, the embedding of tiny clustersdriven by the large difference between the surface energies of the metal and the polymerhas been proposed to serve as a nanoprobe of the glass transition temperature at the polymer surface. In the present work for the first time a systematic study of the embedding process with a variable amount of metal has been carried out. Determining the density profile by X-ray reflectivity measurements, we monitored simultaneously the onset of the embedding at a temperature Te and the glass transition temperature of the whole film, which corresponds to the bulk value . We show that at sufficiently low coverages the gold clusters begin to embed at temperatures Te well below , which reveals the presence of a surface layer with an increased mobility of the polymer chains. Furthermore, it turned out that there is a strong effect due to the attractive polymer−metal interactions which again suppresses the kinetics at the surface and thus leads to an increase of Te with higher gold coverages. Therefore, the embedding temperature Te can be regarded as an upper limit for the glass transition temperature of the free polymer surface. In the present study we found for polystyrene with molecular weight Mw = 220 kg/mol (Mw = 3.7 kg/mol) a reduction for the surface glass transition of more than 37 K (22 K) with respect to its bulk value.

A direct comparison of surface and bulk chain-relaxation in polystyrene.

Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy was used to measure simultaneously the relaxation rates of polystyrene (PS) molecules at the free surface and in the bulk. The samples were uniaxially stretched and annealed at temperatures below the bulk glass transition temperature of PS. The surface and bulk chain relaxation was monitored by measuring the partial-electron and the fluorescence NEXAFS yields, respectively, both parallel and perpendicular to the stretching direction. The decay of the optical birefringence was also measured to provide an independent measure of the bulk relaxation. Relaxation of PS chains was found to occur faster on the surface relative to the bulk. The magnitude of the surface glass transition temperature suppression over the bulk was estimated based on the information on the temperature dependence of the rates.

Sum-Frequency Vibrational Spectroscopic Study of Surface Glass Transition of Poly(vinyl alcohol)

Sum-frequency vibrational spectroscopy was employed to study surface glass transition of poly(vinyl alcohol) by monitoring the relaxation of rubbing-induced alignment of surface chains with increase of temperature. The observed chain relaxation is two-dimensional, parallel to the surface. The surface transition temperature is 58 ± 2 °C, essentially the same as the bulk one.

Mechanisms of argon ion-beam surface modification of polystyrene

The surface characteristics of polymers are important factors determining their interfacial properties and their technological performance. Changes in physical and chemical properties of a polymer film may be induced by subjecting the material to a variety of surface modification techniques, one of which is ion-beam modification. In order to understand the underlying mechanisms X-ray photoelectron spectroscopy (XPS) was used to study the alterations of the polystyrene (PS) surface after Ar-ion treatment under well controlled conditions with low ion doses from 10 12 to 10 16 cm −2. The ion bombardment leads to surface functionalization, loss of aromaticity, and free radical formation. Induced surface cross-linking and the formation of polar groups raised the surface glass transition temperature of PS film.

Etching rate and structural modification of polymer films during low energy ion irradiation

Various polymers were sputtered with low energy Ar + ions of 1 keV in order to determine their etching rate. Hydrocarbons, oxygenated, fluorinated and nitrogen-containing glassy polymers with a broad range of the glass transition temperature ( T g) were chosen. The etching rate was measured using a profilometer, and X-ray photoelectron spectroscopy. At the same time the surface chemical modification, and the surface glass transition temperature were studied. Comparing the sputter rate to the various polymer properties a correlation among the T g, cross-link density, and sputter rate was found. In addition, the sputter rate as a function of the integral ion fluence proved to exhibit a sharp increase in the initial regime of very low fluence. The results are discussed in terms of the characteristics of the polymers.

Mobility Gradient in Surface Region of Monodisperse Polystyrene Films

Surface mobility in polystyrene (PS) films was studied using (PS/deuterated PS) bilayer films, which were prepared by attaching original two surfaces together. Time evolution of the bilayer interface at various temperatures was examined by dynamic secondary ion mass spectroscopy in conjunction with neutron reflectivity. When the bilayer was annealed at a temperature above bulk glass transition temperature, Tgb, the interfacial thickening was well expressed by the context of Fickian diffusion. On the other hand, in the case of an annealing temperature above surface glass transition temperature, Tgs, and below Tgb, the interface monotonically thickened with the time at first and then turned to be independent. This means that chains went across the “mobile” interface and then reached the “dead” bulk region in which the diffusivity should be frozen. Hence, it was claimed that chains could diffuse discernibly in the surface region even at a temperature lower than the Tgb. On the basis of temperature and molecu...

Adhesion behavior of blends of polybutadiene and tackifiers

The adhesion behavior of blends of polybutadiene (PB) and tackifiers was investigated. The peel strength was found to be significantly dependent on blend composition, and a maximum strength behavior was observed for all blend systems. The tack mechanisms were proposed to explain the observed adhesion behavior. The position of the glass transition temperature is related to where the highest tack appears for the miscible blend; for the partially miscible blend the dispersed phase plays an important role in determining the maximum adhesion, and the critical size of the dispersed phase and the Tg of the tackifier phase together determine what concentration of tackifier gives the maximum tack. For the immiscible blends, the maximum adhesion is controlled by both the surface glass transition temperature and the critical diameter of the domains of the tackifier phase.

Surface glass transition of polystyrene by X-ray reflectivity measurements

Surface glass transition of polystyrene by X-ray reflectivity measurements

Surface Glass Transition Temperature of Amorphous Polymers. A New Insight with SFM

The surface glass transition temperature, Tg, of several amorphous polystyrene (PS) samples each with a narrow molecular weight distribution covering the range of Mn from 3 900 to 1 340 000 has been measured by force−distance measurements using a scanning force microscope (SFM). Hysteresis in the loading−unloading cycles of the force−distance curves is observed above Tg and is ascribed to viscoelasticity at the polymer surface. The hysteresis behavior for different molecular weight samples above Tg is described in terms of the contact mechanics of the tip−surface interaction, by consideration of the relative roles of bulk viscoelasticity and peeling viscoelasticity. Data on the molecular mass dependence of thick films of the surface Tg are presented, along with the dependence of Tg on film thickness for thin films, and the annealing of films with a Langmuir−Blodgett overlayer. The results are related to existing theories describing the Tg depression at lower Mn as a result of the increased free volume in the system. The change in Tg with molecular weight of PS is steeper for the surface than the bulk Fox−Flory relationship. We show that polymer chain entanglement variation rather than the end group localization on the free surface is responsible for the observed surface Tg depression effect.

Measurements on the surface glass transition of PMMA from the decay of imprinted surface corrugation gratings: the influence of molecular weight

The near-surface mobility of poly-methylmethacrylate (PMMA) chains is probed by means of optical diffraction. Particularly, we monitor the surface tension-driven decay of shallow surface corrugation gratings. The probe depth of the technique is about 100 nm. We report on quasi-static experiments, in which the sample temperature was ramped slowly from room temperature to a temperature well above the glass temperature, T g. An estimate of the near-surface T g is derived from the temperature at which the decay sets in, T dec. For high molecular weight PMMA this decay temperature is much less than the bulk T g as determined by calorimetry on the same material. For low molecular weight PMMA, this effect is absent.

A Novel Method To Examine Surface Composition in Mixtures of Chemically Identical Two Polymers with Different Molecular Weights

The bulk glass transition temperature, Tgb, of miscible binary blends can be well expressed by the Gordon−Taylor equation. Extending this notion to surface, the surface composition in the blend would be obtained by measuring the surface glass transition temperature, Tgs, of each constituent as well as their blend. The most intriguing advantage of this technique is that labeling of a component is not necessary, unlike conventional spectroscopy. To confirm the validity of our technique, the surface composition in a mixture of polystyrene (PS) and deuterated PS (dPS) was evaluated and compared with the results by well-established surface characterization techniques, static secondary ion mass spectroscopy, and neutron reflectivity. They were in good accordance with one another within the experimental accuracy. Hence, it was claimed that the proposed method was powerful to study the surface concentration in miscible binary blends. Finally, the surface composition in blend films of two PSs with different molecular weights was experimentally and systematically elucidated. The surface enrichment of a smaller molecular weight component became more remarkable with increasing molecular weight disparity between the two components due to an entropic effect.

Elastomeric sizings for glass fibers and their role in fiber wetting and adhesion in resin transfer molded composites

Mold fill velocities of 0.067 cm3/s and 2.66 cm3/s were used to impregnate glass fiber preforms with different architectures and sizing types in two force-controlled resin transfer molding (RTM) fixtures. The fabrication of disk-shaped parts at high molding speed and high post-cure fill pressure was proven successful in reducing the amount of flow-induced defects for reinforcements with a random nonlayered structure. Investigations on the effect of fiber/matrix interface modification with controlled-thickness elastomeric films obtained by the admicellar polymerization technique were carried out to assess the structural integrity levels attained with these less expensive polymeric sizings. In particular, parts reinforced with fibers coated with a thin film of styrene-isoprene copolymer performed significantly better than the uncoated control samples in the tensile and flexural tests. For the same sizing type, the interlaminar shear strength was more than 30% higher than the desized composite and compared statistically to the adhesion level exhibited by commercially sized reinforcements. Greater data scatter and poorer adhesion performance was observed for those composites containing fibers with a thin polystyrene coat. We infer that beneficial effects of a nanometer-thick elastomer interlayer are more evident when extensive cooperative segmental motions take place, that is, when the surface glass transition temperature of the sizing is far below the room temperature. These results have implications for composite manufacture applications involving tailored interfaces with flexible sizings.

X-ray reflectivity study on the surface and bulk glass transition of polystyrene.

The surfaces of polystyrene (PS) films decorated with gold nanoclusters were investigated by x-ray reflectivity measurements. The thicknesses of the films are much larger than the radii of gyration of the different PS samples. By annealing the films above the glass transition temperature T(g) an embedding process of the clusters into the polymer is detected which is accompanied by a substantial increase in the cluster layer thickness due to Brownian motion. These processes start at a sufficiently low viscosity and may be regarded as a probe of the glass transition in the near surface region of the PS films. Simultaneously the thermal expansion of the entire film and hence its approximate bulk behavior were monitored. Two samples of different molecular weight do not show a significant difference between the surface and bulk T(g) values.

Surface Molecular Motion of Monodisperse α,ω-Diamino-Terminated and α,ω-Dicarboxy-Terminated Polystyrenes

Surface glass transition behaviors of monodisperse α,ω-diamino-terminated and α,ω-dicarboxy-terminated polystyrenes (α,ω-PS(NH2)2 and α,ω-PS(COOH)2) were studied by scanning force microscopy and were compared with the results of proton-terminated polystyrene (PS−H). All surface glass transition temperatures, Tgs, of PS−H, α,ω-PS(NH2)2, and α,ω-PS(COOH)2 were discernibly lower than each corresponding bulk glass transition temperature, Tgb. However, the magnitude of Tgs was strongly dependent on the chemical structure of chain end groups, because the surface concentration of chain ends varied with the surface free energy difference between the main chain part and the chain end portion, via the surface segregation or surface depletion of chain ends. This result makes it clear that chain end chemistry is one of determining factors on the magnitude of Tgs. On the basis of the time−temperature superposition principle applied to the scanning rate dependence of lateral force as a function of temperature, the appa...

Studies of polymer surfaces by sum frequency generation vibrational spectroscopy.

Recently, sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to study surfaces of polymer materials. This review summarizes the significant achievements in understanding surface molecular chemical structures of polymer materials obtained by SFG. It reviews in situ detection at the molecular level of surface structures of some common polymers in air, surface segregation of small end groups, polymer surface restructuring in water, and step-wise changed polymer blend surfaces. Studies of surface glass transition and surface structures modified by rubbing, plasma deposition, UV light irradiation, oxygen ion and radical irradiation, and wet etching are also discussed. SFG probing of polymer surfaces provides valuable insights into the relations between polymer surface structures and surface properties, which will assist in the design of polymer materials with desired surface properties.