Arnold Chang-Mou Yang

Wear and friction in glassy polymers : Microscratch on blends of polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide)

The microscopic process of abrasive wear and friction in glassy polymers was studied by using a special microscratch technique. A miscible blend of polystyrene (PS) and poly(phenylene oxide) (PPO) was used. It was found that as the composition varies there seems to exist two wear regimes in the blends controlled by different breakdown mechanisms corresponding to the brittle—ductile transition. Detailed study of the contact loads and SEM micrographs indicate that abrasive wear in the glassy polymers is controlled by microcracking under the asperity contacts. The critical load τc for initiating microscopic cracks can be linked to the macroscopic wear via a statistical Weibull model where τc is taken to be the mean of a strength distribution function. On the other hand, the friction coefficient was found to be independent of the composition but to vary strongly with the contact load. It approaches zero at the extrapolated zero load, but increases rapidly and eventually levels off with contact load. This behavior can be understood by a simple frictional adhesion model in which the polymer deformation during a frictional contact is analyzed by considering the compressive plastic ploughing and shearing yielding around the asperity contact. The shear strength So of the polymer/asperity contacts was found to vary with the normal load. The vertical scratch hardness Hv, which characterizes the spontaneous indentation yielding on the polymer surface, was found to be independent of scratch length and depth, and indeed can be regarded as a material constant. Although both So and Hv can accurately describe the frictional behavior of the glassy polymers, they bear no correlation to abrasive wear in the same materials.

Filler-induced softening effect in thermally aged polydimethylsiloxane elastomers

Abstract An intriguing filler-induced softening effect was observed in filled polydimethylsiloxanes that aged at high temperatures. It was found that the Youngs modulus and hardness of the aged filled elastomers first decreased and then increased with filler fraction, remarkably different from those of the fresh samples which always increased monotonically. This softening effect cannot be explained by the original phase structure of rigid particles embedded in a soft matrix. However, due to the notable chain scissions and oxidative crosslinks detected in the aged neat resin, the softening was attributed to a heterogeneous oxidative hardening during the thermal ageing. It is proposed that filler particles behaved as secondary antioxidants to block the local reactions of free-radical crosslinks, effectively producing a soft elastomeric layer around each particle and thus this softening effect. This proposal is consistent with the fact that α-iron oxide, a strong free-radical scavenger, demonstrated the most pronounced softening effect. The proposal was further examined by comparing the mechanical strength at the filler/elastomer interfaces extracted from the wear rate data among the various fillers studied here. It was found that α-iron oxide/elastomer interfaces possess the lowest interfacial strength, in good agreement with the proposed mechanism.

Enhancing the Photoluminescence Emission of Conjugated MEH-PPV by Light Processing

We show here that treatment of thin films of conjugated polymers by illumination with light leads to an increase of the intensity of their photoluminescence by up to 42%. The corresponding enhancement of absorbance was much less pronounced. We explain this significant enhancement of photoluminescence by a planarization of the conjugated polymer chains induced by photoexcitations even below the glass transition temperature, possibly due to an increased conjugation length. Interestingly, the photoluminescence remains at the enhanced level for more than 71 h after treatment of the films by illumination with light, likely due to the fact that below the glass transition temperature no restoring force could return the conjugated chains into their initial conformational state.

Bioinspired hole-conducting polymers for application in organic light-emitting diodes

A novel uracil-functionalized poly(3-thiophene) as a hole-injecting/transporting layer in an organic light-emitting device is able to form physical crosslinkages resulting in high thermal stability, non-corrosion, excellent hole injection/transport and electron-blocking capabilities in the solid state, and it achieves up to 10 times higher performance than that of conventional poly(3-thiophene)s under similar experimental conditions.

Ductile-brittle transition induced by aging in poly(phenylene oxide) thin films

The mechanical toughness of poly(phenylene oxide) (PPO) thin films was found to degrade remarkably from physical aging and the fracture mode transformed from ductile shear into a brittle type that involved no stable deformation zone growth. In the beginning, the fresh PPO films deformed almost uniformly upon stretching up to more than 20% strain. However, when the film was isothermally aged, diffuse shear deformation zones grew in response to the external deformation. The deformation zones could sustain extremely large strain (>20% elongation) without cracking, but their breadth narrowed and boundaries sharpened as the aging time increased. As the aging time further increased, the diffuse zones were gradually replaced by the sharp straight shear zones that nucleated at a much lower strain and could break down during the course of deformation. These deformation zones had a generally smooth interior surface but sometimes showed no fibrillated microstructure and their growth followed a micro-necking mechanism. Finally, at the very long aging times, the PPO films cracked catastrophically at small strains around 1% elongation without the evidence of stable deformation zones. Increasing the aging temperature sped up this degradation process. A separate experiment using FTi.r. to examine the films cast from deuterated toluene indicated that the degradation was not due to the effect of solvent. The degradation, possibly due to the enthalpic relaxation during aging, however, requires further study to reveal its origin.

Massive enhancement of photoluminescence through nanofilm dewetting.

Due to the rather low efficiencies of conjugated polymers in solid films, their successful applications are scarce. However, recently several experiments indicated that a proper control of molecular conformations and stresses acting on the polymers may provide constructive ways to boost efficiency. Here, we report an amazingly large enhancement of photoluminescence as a consequence of strong shear forces acting on the polymer chains during nanofilm dewetting. Such sheared chains exhibited an emission probability many times higher than the nonsheared chains within a nondewetted film. This increase in emission probability was accompanied by the emergence of an additional blue-shifted emission peak, suggesting reductions in conjugation length induced by the dewetting-driven mass redistribution. Intriguingly, exciton quenching on narrow-band-gap substrates was also reduced, indicating suppression of vibronic interactions of excitons. Dewetting and related shearing processes resulting in enhanced photoluminescence efficiency are compatible with existing fabrication methods of polymer-based diodes and solar cells.

Characterization of vapor deposition polymerized polyimide thin films

Vapor deposition polymerized (VDP) polyimide (PI) thin films were prepared and characterized by using thermogravimetrical analysis (TGA), scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR), and bending-beam techniques. The film properties investigated were thermal stability, wet-etching characteristics, surface topology, imidization characteristics, internal stress upon curing and thermal cycling, and hygroscopic stress upon moisture diffusion. Markedly different characteristics are observed for the VDP-PI films when comparing with the conventional ones. They seem denser in film structure and have better mechanical properties, but are somewhat less stable in thermal resistance.

The solvent-induced cracking in glassy polymer coatings by atomic force microscopy

Abstract Polymer coatings for microelectronics packaging frequently undergo ductile-brittle transition when in contact with the specific solvents, which can cause the local deformation zones to grow from the stress concentrations and lead to cracking and debonding. Owing to the adhesion to the substrate, the surface opening of the deformation zones is very narrow, investigation of the microstructure cannot be made by optical microscopy or scanning electron microscopy. However, using the atomic force microscope, it was found that the depth of the zones increased with immersion time, indicating that they grew from the surface into the interior. The stress release induced by the deformation zones could be measured as a function of position. Furthermore, the zone depth increased linearly with the zone width. The zone growth and film-thickness effect, together with the cracking of the free-standing films in different solvents, were investigated.

Super-plastic behavior of the brittle polymer film in multilayer systems

Crazes are usually observed preceding brittle fracture of glassy polymers. They were believed to result from a necking process similar to that in fiber drawing. In this study, we further exploited the necking characteristic of crazing by sandwiching the craze-forming brittle polymer film between two ductile polymer films to examine the deformation behavior of the brittle polymer when necking is suppressed. We found that when necking was suppressed, the brittle polymer film demonstrated a super-plastic behavior in that the film could be stretched to a very large deformation without any strain localization or cracking, and this deformation was shown to be mostly plastic. The super-plastic behavior is remarkably dependent on the thickness of the outer ductile polymer layers. When the outer-layer thickness is less than a critical thickness, the brittle polymer film in combination with the sandwich structure demonstrated a different degree of strain localization with the critical strain increased with the thickness of the outer-layer. The microstructure of deformation zones in the multi-layer samples was investigated by atomic force microscopy (AFM). The effect of the interfacial strength at the polymer interfaces was also investigated by SIMS and discussed.

Young's moduli of material in polymer deformation zones by an AFM deflection technique

A new technique using contact mode atomic force microscopy is developed for probing the elastic properties of deformed material within the microscopic deformation zones in glassy polymers. By measuring the difference in local deflection to the contact force, the ratios of Youngs moduli of the matter inside the zone to that outside the zones were determined. Youngs modulus was found to decrease significantly after crazing or shear yielding, indicating that considerable strain softening was induced by large deformation of the polymer chains.