Surface reactivity of polyimide and its effect on adhesion to epoxy

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of properties such as low dielectric constant, high thermal stability, excellent mechanical properties and superior solvent resistance. Unfortunately, polyimides are well known to be difficult to bond to other materials, especially to epoxy resins. Many surface treatments have been developed to increase epoxy–polyimide adhesion. These treatments include exposure to ion beams, plasmas and chemical solutions. The goal of our research was to relate surface reactivity of epoxy and polyimide resins to the strength of epoxy–polyimide interfaces. The surface reactivity of four polyimides was studied and quantified using contact angle measurements, flow microcalorimetry (FMC), Fourier transform infrared (FT-IR) spectroscopy (using an attenuated total reflection (ATR) accessory) and X-ray photoelectron spectroscopy (XPS). Several ways of analyzing contact angles were tried and only a weak correlation between the polar component or the acid–base components of the surface free energy with the critical interfacial strain energy release rate (i.e., the interfacial fracture strength) was observed. FMC results suggest that the strength of epoxy–polyimide interfaces is related to the molecular interactions between the curing agent and polyimide. The molecular interactions between the curing agent and polyimide surfaces were found to be either greater than epoxy and polyimide interactions or more irreversible. Therefore, the curing agent (2,4-EMI) is thought to play a critical role in controlling adhesion strength.     

Evolution of the interfacial stress transfer ability between a glass fibre and a polypropylene matrix during polymer crystallization

A rigorous evaluation of the influence of a transcrystalline interphase on the adhesion between a fibre and a polymer matrix is difficult because it is generally not possible to consider this parameter without other factors. Indeed, in the case of a reinforcing material allowing spontaneous transcrystallization of a thermoplastic matrix, the inhibition of this phenomenon is, for instance, possible by modifying the surface topography or the physico-chemical nature of the fibre by appropriate coatings. In the same way, the fibres which do not intrinsically favour transcrystalline growth can behave as active substrates by applying a shear stress onto the fibre-matrix interface. In this case, however, processing of the sample for the classical micromechanical tests remains extremely difficult. In order to understand better the participation of a transcrystalline interphase in the interfacial adhesion in a polypropylene–glass fibre system, an experimental protocol has been developed which allows us to evaluate the evolution of the interfacial shear stress during the matrix crystallization, the latter being spherulitic or cylindritic. The results show that a transcrystalline interphase in a polypropylene–glass fibre composite does not significantly alter the adhesion between the two materials after total crystallization of the matrix. Nevertheless, it is shown that an important hooping of the fibre occurs during the development of a transcrystalline superstructure.     

Augmented Nonlinear PD Controller for a Redundantly Actuated Parallel Manipulator

In order to improve trajectory tracking accuracy for a redundantly actuated parallel manipulator, a so-called augmented nonlinear PD (ANPD) controller based on the conventional dynamic controller is proposed in this paper. The ANPD controller is designed by replacing the linear PD in the augmented PD controller with a nonlinear PD algorithm. The stability of the parallel manipulator system with the proposed ANPD controller is proven using the Lyapunov stability theorem, and the ANPD controller is further proven to guarantee asymptotic convergence to zero of both the tracking error and error rate. The superiority of the ANPD controller is verified through trajectory tracking experiments of an actual 2-d.o.f. redundantly actuated parallel manipulator.     

Design and validation of a complete haptic system for manipulative tasks

The present work deals with the design, implementation and assessment of a new haptic system specifically conceived for manipulative tasks in virtual environments. Such a system was designed by taking into account specific issues related to fine manipulation, such as multipoint haptics, coherence, transparency and physical representation. The haptic system described herein is integrated with a virtual environment engine for the simulation of multifinger manipulation. A preliminary evaluation of the system was conducted by comparing human performance in the manipulation of virtual objects with respect to real objects, according to the data available in the literature. The experiments confirm how the most relevant relationships among physiological and physical parameters involved in manipulation are also preserved during virtual manipulation. However, an in-depth analysis of the results shows that simulation parameters affect the level of force control during virtual manipulation and the quality of the perceived force feedback.