Polymer Blends and Alloys

Mixing two or more polymers together to produce blends or alloys is a well-established strategy for achieving a specified portfolio of physical properties, without the need to synthesise specialised polymer systems. The subject is vast and has been the focus of much work, both theoretical and experimental. Much of the earlier work in this field was necessarily empirical and many of the blends produced were of academic rather than commercial interest.

This chapter appears amid an environment of intensive investigation into the thermodynamics and flow behavior of polymer blends themselves and an enhanced understanding of the kinematic behavior of mixing devices. As in other fields, the digital computer has multiplied the detail with which physicists can explain the observed development of blend morphology and engineers can elucidate on the deformation and stress effects of their mixer design. An argument could be made to let the controversy settle before going to print. On the other hand, a consensus of identifying cause and effect has developed which should be published.

Polomer blend (PB). A mixture of at least two polymers or copolymers. Miscible PB. PB homogeneous down to molecular level: ΔG _m ≅ΔH _m⩽0. Immiscible PB.ΔG _m≅ΔH _m> 0. Compatible PB (Utilitarian term). Homogeneous to the eye, commercially attractive PB. Polymer alloy (PA). Immiscible PB having a modified interface and/or morphology.

This chapter describes some methods which are used to characterise the microstructure of polymer blends. The main interests of those who are developing new polymer blends is to investigate and measure the degree of miscibility and the interaction of phases, as they can significantly influence the properties and behaviour of the compound. Phase size and size distribution can be revealed directly by microscopy but to obtain information on partial miscibility and interfacial interaction is more difficult and microscopy has to be supported by other techniques for the full characterisation of a polymer blend. All the techniques can provide some useful information in capable hands, but also frustration and wrong results for the less skilled. The preference for a particular technique is dependent on the polymer system, availability of instruments, experience and personal choice. For this reason this account is naturally biased and represents only one possible approach. There is an extensive literature on characterisation of polymer blends, indicating the importance of the subject (for review see e.g. [1-2]).

An attempt has been made in this chapter to provide a sound basis for understanding the mechanical behaviour of polymer blends and alloys. The presentation has not been over-simplified, for the author agrees with the point of view of Treloar [1] in the preface in the preface to the first edition of his book The Physics of Rubber Elasticity that ‘… if one is going to have a theory at all one may as well take some trouble to find the one which most nearly represents the known facts.’ It is relatively easy to produce equations which describe the behaviour of blends and alloys (and, indeed, of polymers in general) sufficiently well for the operation of, for example, extrusion machines or for the interpretation of standard test procedures on the solid material. However, it is not at all easy to explain their behaviour in a rational and scientific manner such that prediction of properties is possible and the understanding of materials scientists and engineers is increased. This the author has attempted to do in this short chapter but, for the serious materials scientist or technologist, the chapter should in any case be looked upon as a guide to further study, to the literature (references to some of which are given) and to the ever-increasing amount of new knowledge that is being gathered about these interesting and useful materials. Only in this way can progress be made to develop newer materials with novel properties.

Commodity plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinylchloride (PVC) make up a large of proportion of the total tonnage of plastic currently being used mainly for non-loadbearing applications (e.g. consumer products). However, with the ever-increasing use of plastics in the areas dominated by the use of metal or ceramics, e.g. in the automobile industry, new engineering plastics, both thermoplastics and thermoset resins, have been developed which provide the combinations of lightness and good balance of stiffness, and some also in toughness, over a wide range of temperature applications.

Liquid crystal polymers (LCPs) are used in many different commercial applications ranging from high modulus fibres (Kevlar) to microwave cook-ware (Xydar). Much of the driving force behind the development of LCPs had been the increasing desire to replace metal components with engineering polymers, for example in automotive and aerospace sectors. LCPs were proclaimed as a new class of engineering polymers, which would take a large share of this market. In the event they have achieved limited market penetration as engineering materials in their own right. Key properties turned out to be low thermal expansion coefficients leading to high quality mouldings, rather than exceptional modulus or strength. Suppliers have been withdrawing production from the area (e.g. ICI, Eastman Kodak). Over the same period commercial interest has grown in polymer blend technology. Thus it is not surprising that the focus of interest in LCPs has switched from their use as stand alone materials to their use in blends.

High performance fibre reinforced composites are now becoming important engineering materials in their own right and are penetrating many market sectors previously dominated by metals. One barrier to their more widespread use is the often protracted fabrication times associated with combining the fibre reinforcement and matrix to give predictable properties in the final artifact. The use of short fibre reinforced thermoplastics, which may be processed using extrusion and injection moulding technology, has brought about significant reductions in cycle times. However, difficulties still remain in terms of controlling the fibre length distribution and orientation in finished parts, and coping with the increased viscosity of the melt as a result of incorporation of the fibres.