Polymers thickness strain response

Many electrostrictive polymers have been developed in the last decade [1] and these newly developed electrostrictive polymers exhibit a high electric-field-induced strain, as shown in Figiure 16.5a, where the maximum thickness strain response of the polymer at different fields is given. A typical relationship between the strain response and the electric field observed in these polymers is shown in Figure 16.5b. All these electrostrictive polymers are polar polymers that contain polar units in the polymer chain. The electrostrictive strain response reflects the change in these polar units due to an electric field. 

A thermoplastic composite pipe produced by the tape winding process consists of two building blocks the mandrel (wall thickness t , internal radius r ) and the load-bearing composite tape (thickness f ) wound under a winding angle a with respect to the axial direction of the pipe and restraining the mandrel (Figure 2). The extruded mandrel is based on one constituent only, i.e. a viscoelastic polymer of the stress/strain response described by ct = f A, B, C, e). The three parameters are relatable to the initial stiffness and the coordinates of the yield point. The... 

For an isotropic polymer, the thickness ( 3) and traverse (xi) strain responses due to the Maxwell effect are ... 

Figure 16.5 (a) The maximum thickness strain at different electric fields for three newly developed electrostrictive polymers, (b) The relationship between the strain response and electric field for a typical electrostrictive polymer... 

The fracture process is investigated for two glassy polymers polymethyl methacrylate (PMMA) and polycarbonate (PC) which are generally thou t to show a brittle and a ductile response respectively and thus selected to illustrate the method. These materials consist of commercial sheets (from Goodfellow) of 10 mm thickness which ensures plane strain conditions for both materials. Caution about plane strain conditions concerns primarily PC which is prone to develop plasticity and a 10 mm thickness appears reasonable according to analysis of the influence of the thickness on its toughness found in [6, 7]. 

Piezoelectricity links the fields of electricity and acoustics. Piezoelectric materials are key components in acoustic transducers such as microphones, loudspeakers, transmitters, burglar alarms and submarine detectors. The Curie brothers [7] in 1880 first observed the phenomenon in quartz crystals. Langevin [8] in 1916 first reported the application of piezoelectrics to acoustics. He used piezoelectric quartz crystals in an ultrasonic sending and detection system - a forerunner to present day sonar systems. Subsequently, other materials with piezoelectric properties were discovered. These included the crystal Rochelle salt [9], the ceramics lead barium titanate/zirconate (pzt) and barium titanate [10] and the polymer poly(vinylidene fluoride) [11]. Other polymers such as nylon 11 [12], poly(vinyl chloride) [13] and poly (vinyl fluoride) [14] exhibit piezoelectric behavior, but to a much smaller extent. Strain constants characterize the piezoelectric response. These relate a vector quantity, the electrical field, to a tensor quantity, the mechanical stress (or strain). In this convention, the film orientation direction is denoted by 1, the width by 2 and the thickness by 3. Thus, the piezoelectric strain constant dl3 refers to a polymer film held in the orientation direction with the electrical field applied parallel to the thickness or 3 direction. The requirements for observing piezoelectricity in materials are a non-symmetric unit cell and a net dipole movement in the structure. There are 32-point groups, but only 30 of these have non-symmetric unit cells and are therefore capable of exhibiting piezoelectricity. Further, only 10 out of these twenty point groups exhibit both piezoelectricity and pyroelectricity. The piezoelectric strain constant, d, is related to the piezoelectric stress coefficient, g, by... 

The speed with which the actuators can be switched between their expanded and contracted states depends on the polymer film thickness, since actuation depends on mass transport. Like the strain, speed also depends on the electrolyte and on the polymer structure, and the stmcture depends not only on the type of polymer, but also on the film preparation conditions. Response times for the thin films that are used in microactuators are of the order of a second, which is sufficiently fast for most biomedical applications. 

Figure 20.1 Principle of operation ofdieiectric eiastomer actuators, (a) Functionai element of dielectric elastomer actuators. Polymer film compresses in thickness and expands in area when a voltage is applied across the film, (b) Typical thickness or planar strain in response to applied...

Effects of Spin Coating and Substrate Interaction. Effects of spin coating and interactive interfaces on shear mechanical properties of ultrathin unannealed PEP elastomer films were discussed. It was found that a stressed boundary layer is formed, 7-10 Rg thick. This imexpected far-field effect for a polymer has recently also been observed for annealed PEP films (1), The degree of disentanglement depends strongly on the distance towards the interactive interface. Strained film surfaces were found to determine the film stability, and thus, can be responsible for spontaneous autophobicity. 

During compression of polymeric foams, three characteristic stages of deformation are commonly observed. At low deformations, the polymer foam is in the linear elastic response regime, i.e., the stress increases linearly with deformation and the strain is recoverable. The second phase is characterized by continued deformation at relatively constant stress, known as the stress collapse plateau. And the final phase of deformation is densification where the foam begins to respond as a compacted solid. At this point the cellular structure within the material is collapsed, and further deformation requires compression of the solid foam material (Ouellet et al. 2006). As mentioned above, a specific technique is required to obtain stress-strain curves of ferroelectrets in thickness direction because the thickness in ferroelectrets is normally very thin, corresponding to very small defiections. Dansachmiiller et al. developed an experimental technique that allows obtaining the stress-strain curves in ferroelectret films (Dansachmiiller et al. 2005). This method may also be used to obtain the stress-stain curve for a polymer foam film without oriented macro-dipoles. The schematic of the experimental setup is shown in Fig. 4. 

In crystalline polymer systems the tough response, besides cavitation and crazing, is crystallographic in natme. Crystallographic slips ai-e the main plastic deformation mechanisms that require generation and motion of crystallographic dislocations. The concepts of generation of monolithic and half-loop dislocations plausibly explain the observed yield stress dependences on crystal thickness, temperatm-e and strain rate. 

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