Question

What is the effect of time and temperatuure on the mechanical properties of polymers (in regards to polymer chains)?

Answer 1

EFFECT OF TEMPERATURE

When substances made of small molecules are heated, they simply melt and form a free-flowing liquid; however, with polymers the process is more complex. When polymers are cooled, they will often become very brittle. When heated, the polymers will first become flexible before melting. The individual molecules that make up polymeric materials are very large and have an extended chain-like shape that results in an entangled structure. This entanglement is beneficial in some respects. The relatively high levels of elongation that most polymers exhibit without breaking are due in large part to chain entanglement. However, this entanglement also restricts the freedom required at a molecular level to organize into crystals. Consequently, no polymer under normal processing conditions is fully crystalline, and some polymers do not crystallize to any significant degree.

This is due to their structure. In some polymers, there are areas of crystalline and amorphous regions. In the “glassy” state, the tangled polymer chains in the amorphous region become “frozen”, which prevents easy movement of the chains relative to each other. If the polymer is forced to change shape, it does so by breaking. When the glassy material is heated, the polymer chains will reach a temperature at which they can move relative to each other (the glass transition temperature T_g). On further heating of the glassy polymer, it becomes flexible and shows the more conventional properties expected of a plastic. Eventually, if the polymer is heated enough, the melting temperature T_m will be reached (the plastic becomes a viscous fluid).

a schematic representation of the modulus temperature behaviour which is common to linear amorphous polymers is presented. Here, the four major regions of viscoelastic behaviour are shown. These are the glassy region, characterized by a very high value of the modulus, the transition region from glassy to rubbery-like consistency, the rubbery plateau at much lower modulus values than those characteristics of glasses and finally, at high temperatures, a flow region. The behaviour of polymer molecules in each of these regions is very different. In the glassy domain the relatively low values of temperature result in limited extents of molecular motion Here, short range motions of side chains on the polymer backbone or perhaps limited backbone motions involving short segments of chains take place. As the temperature increases a situation is reached where substantial segments of polymer chains have enough energy to surmount local barriers which hinder molecular motion and these segments begin to move. In this range, motions on a scale of perhaps ten monomer units might occur. This is the transition zone. At higher temperatures still, even longer-range motion involving larger segments of chains become possible. However. due to the fact this long polymer molecules are entangled, whole scale translational motions of polymer chains are forbidden, This is the molecular interpretation of behaviour in the rubbery plateau region. At higher temperatures still. molecular motion is so extreme that even chain entanglements are no longer effective in restricting flow of one molecule past another and a liquid-like regime is realized

Time dependence

In order to understand creep and avoid product failure, there must be an appreciation of viscoelasticity. Viscoelasticity is the property of plastics in which they exhibit both viscous and elastic characteristics when placed under stress. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. On the other hand, elastic materials, like a steel rod, strain when stressed and quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent behaviour. Viscoelasticity is unique to thermoplastics because of the structure of the polymer molecules. The polymer chains are mobile and can slide past each other because they do not share chemical bonds with the other chains around them.

One of the key properties of plastics that results from viscoelasticity is creep. If a polymeric material is under constant stress, a continual change in strain will be observed. Simply stated, the stressed object will continue to deform over time. This change in dimension is known as creep. It’s important to understand the viscoelastic nature of plastic materials so that their behaviour in the intended application can be anticipated. One of the key properties of plastics that results from viscoelasticity is creep. If a polymeric material is under constant stress, a continual change in strain will be observed. Simply stated, the stressed object will continue to deform over time. This change in dimension is known as creep. When a material is placed under a constant stress, the response observed initially will be a function of the stiffness, or modulus, of the material. Modulus is expressed as the applied stress divided by the resulting strain. If stress is continuously applied to a thermoplastic, strain will continually increase. As a result, the calculated modulus at a point later in time will appear to have decreased. However, the stiffness of the material is not actually decreasing. Creep is the tendency of a solid material to deform permanently under the influence of constant stress: tensile, compressive, shear, or flexural. It occurs as a function of time through extended exposure to levels of stress that are below the yield strength of the material. Creep is the result of the inherent viscoelastic nature of polymers that causes time dependency.

Prolonged static stresses lead to a decay in apparent modulus that is associated with localized molecular reorganization of polymer chains. At stresses below the yield point, stress-relief molecular reorganization proceeds through disentanglement, as there is no opportunity for yielding. Stresses above the yield point result in plastic deformation, which is not fully recovered after removal of the stress. This macro response takes place through permanent molecular rearrangement. There are two main factors that affect polymer viscoelasticity, and accordingly, the creep behaviour of a plastic part: temperature and strain rate. As the temperature is increased, the polymer chains are further apart, there is more free volume and kinetic energy, and they can slide past one another to disentangle more easily. As the strain rate is increased, the polymer chains do not have enough time to undergo yielding, and disentanglement is favoured over yielding.