Editorial Feature

Polymers with Elasticity Gradients

A designer often needs to combine a soft and a hard material in a single item. For example, an engine mount usually has a rubber cushion attached to a metal base. This example also illustrates another common engineering problem - joining together soft and hard items. Mechanical stress concentrate at the boundary between hard and soft parts of a composite structure, which leads to failure along the boundary.

There have been attempts to avoid this boundary by mixing rubbers and plastics, and changing their proportions gradually along one direction of an item, so that it becomes soft at one end and hard at the other. However, such an article tends to creep under stress somewhere between the hard and the soft parts, which renders the whole component useless. Moreover, the position of this creeping transition zone is very sensitive to temperature, where a slight temperature change could render the whole item either plastic or rubbery.

Physical States of Polymers

Polymers can exist in three main physical states: the glassy state (with elasticity modulus, EM of 2000-3000 MPa), the rubbery state (EM = 0.1-10 MPa) and the viscous flow state (EM depends on the speed of deformation). Between the glassy and rubbery states there is a glass-to-rubber transition zone, which every known polymer (including the rubber-plastics mixtures mentioned above) inevitably passes through.

Glass Transition Zone

This transition zone is characterised by intermediate EM (between 3 MPa and 3000 MPa) and viscoelastic behaviour that causes pronounced creep, rendering all rubber-plastics mixtures unusable for practical purposes. The glass-to-rubber zone is sensitive to temperature, and in rubber-plastics mixtures ‘floats’ along the material if the temperature changes. Polymers with a variable degree of crosslinking also suffer from the same defect.

A New Physical State

Now, after seven years of intense R&D, a way has been found of modifying conventional materials such as polybutadienes and other rubbers, silicones etc, to control mechanical properties, including elastic modulus, the coefficient of thermal expansion and the refractive index. As a result it is possible to have a modified rubber, with EM between 1 MPa and 3000 MPa, while avoiding the glass to rubber transition zone with its viscoelastic behaviour, i.e. a new physical state for polymers has been created.

This new state offers a wide working temperature range from -70°C to 120°C. At higher temperatures the material goes into the rubbery state and behaves like conventional rubber up to 300°C (for silicones and fluorosilicones this figure might be much higher). The material has a low mechanical loss factor (0.01 to 0.05) and a low rate of stress relaxation and creep.

Methods have been developed for performing this modification on a number of commercially available polymers, using commercially available reagents. This allows the EM of a polymer to be varied gradually, from that of a hard plastics material to that of a rubber, within one piece of material without any joints (figure 1).

Figure 1. Gradient rigidity in a polymer sample.

Applications

Although the technology itself seems simple, it is based on huge experience and sophisticated calculations. An article with a predetermined EM gradient can be moulded, or objects such gaskets can be carved out of pre-fabricated sheets. Samples of rods, disks and flat sheets have been manufactured with EM gradient along each axis of an article.

The technology has a huge number of potential applications. Complicated items having parts with different EMs can be developed without using conventional methods of joining such as gluing, riveting, screwing, welding, etc. Apart from reducing production costs, jointless gradient materials are much more robust, as instead of being concentrated on a joint, the stress is distributed throughout the volume of the material.

The range of applications includes one-piece shoe soles, noiseless cogwheels and rollers, artificial limbs and various other applications where rubber parts need joining to metals or ceramics.

Gaskets

Gaskets are another application for this new modified polymer. Rubbers have a relatively low density of crosslinks, and they perform best when compressed hydrostatically. In the case of a gasket, the material is compressed in one direction only, which may contribute to its failure. Such compression could be brought closer to triaxial compression by reducing the thickness of the gasket. The minimum thickness is limited by the roughness of the surfaces. To this end gaskets having a thin compliant outer layer on both sides, which gradually becomes hard towards the middle have been manufactured and tested.

Lenses

The same technology could be used to produce transparent polymers with variable refractive index. The RI could be varied between 1.38 and 1.90, allowing, for example, the manufacture of lenses with uniform thickness.

Primary author: Andrei Askady and Andrei Astahov

Source: Materials World, Vol. 5 no. 10 p 587 October 1997.

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