The ideal mix - polymeric blends and laminates to the fore

TWI Bulletin, November/December 2002

Sam Rostami has over 15 years experience in physics and physical chemistry of polymers, polymer interfaces and adhesion. He has worked at ICI's Advanced Materials and Acrylics R&D Departments. He is experienced in fundamental research, new product development and characterisation of multi-component materials. He is co-editor and author of a book on 'Multi-component polymeric systems', author of five chapters in technical books, over fifty technical papers and seven patents. He joined TWI in 2001.

Polymeric blends and laminates are used in auto manufacture to make your car journey to work more pleasant, comfortable and economical. Their use in electronics, medical and domestic appliances, architectural products and transport is ubiquitous these days. As Sam Rostami explains they have also been extensively researched for advanced technical applications such as polymeric light emitting devices, solar cells, polymeric lasers and electrically conductive polymers.

Polymeric blends and laminates are often used in applications where the technical requirements cannot be met by standard polymers. For example, polycarbonate-ABS blends are widely used in computers, electronics and mobile communication housings, ( Fig.1-3) since neither polycarbonate nor ABS can meet the techno-commercial demands of these applications. Multi-layered laminates are used in packaging for barrier resistance and in-mould surface decorative applications.

Due to the relatively short period of time required to develop polymeric blends and laminates, the industry has always considered this as an option to meet specific customer needs. As a result, the average annual growth rate of polymeric mixtures has been around six to seven per cent over the last three decades.

Unlike metallic alloys, it is difficult, but not impossible, to make true polymeric alloys miscible. However the majority of polymeric mixtures used are of multiphase structures or immiscible. In this paper, the key factors affecting immiscibility between two polymers are explored. Their effects on the properties of polymeric mixtures are highlighted.

Molecular interactions

• poly (methyl methacrylate), an electron rich polymer, mixes with electron deficient polymers such as poly (vinyl chloride), poly (vinylidine flouride), poly (vinyl phenol).
• Polar and non-polar polymers would not intimately mix.
• The miscibility between two non-polar polymers such as poly-propylene and polyethylene is greatly influenced by their chain conformations and the presence of minor chemical functional groups on the chains. ^[1]

The above analysis is only true as a first approximation. There are other factors, such as the change in free volume, isothermal compressibility and thermal expansion coefficients that affect the phase behaviours of polymeric mixtures. ^[2-4] More detailed considerations in the form of advanced thermodynamic models, that are currently being developed to explore the phase separation phenomena in polymer blends, are described in the literature. ^[5-6] The majority of existing models rely on at least one experimental data set to 'train' the models in order to simulate their phase behaviours. However, group contribution estimation ^[7] and polymer reference interaction site models ^[8-9] can potentially predict the phase behaviour of polymeric mixtures without the need for an experimental input.

In the absence of strong and favourable molecular interactions, the second derivative of change in free energy in respect of the composition, becomes negative causing phase separation immiscibility to occur. This is the case for the majority of polymeric mixtures that have been studied.

Immiscible blends

For most practical applications, the interfacial energy between the two components must be below 2mN/m.

As an example, a melt blended 30:70 wt% mixture of poly (ethersulphone) PES, and Nylon 66, with an estimated interfacial energy of around 2mN/m, was investigated. The mixture shows an almost uniform 2nm particle size of PES dispersed in Nylon 66, see Fig.4. The low interfacial energy between the two components is responsible for the fine phase structure of the blend, which in turn ensures enhanced and reproducible mechanical properties.

If the interfacial energy is too high, it can be reduced to around or below 2mN/m by addition of a third component, generally known as a compatibiliser. Like surfactants, the compatibilisers predominantly reside at the interfacial regions, reducing the interfacial energy and the phase sizes. Often they also enhance the mechanical toughness of the blends.

As an example, the interfacial energy between Nylon 66 and polypropylene, at 285°C, is measured to be about 15.5mN/m. ^[5] A 1:1 ratio of PP: Nylon 66, melt blended in a twin-screw extruder, produces a blend with a mean particle size of 500nm of PP dispersed Nylon 66. Addition of a maleinised polypropylene, MPP, as a compatibilising agent, reduces the PP domain size and enhances the fracture energy of the mixture, as shown in Table 1. As can be seen morphology, toughness and yield stress are the most affected properties observed in the presence of a compatibiliser.

Table 1: Mechanical properties of compatibilised and uncompatibilised blends of PP and Nylon 66

As mentioned earlier, the PES: Nylon 66 is fairly compatible as is evident from their low interfacial energy and uniform fine morphology. Since no copolymer of PES and Nylon can be synthesised, a co-compatibilising agent such Phenoxy is used to see if the particle size of PES in the blend can be further reduced and/or the toughness of the blend can be improved. The phenoxy resin is partially miscible with PES and can chemically react with Nylon 66. As shown in Table 2, the addition of phenoxy resin has no significant effect on the morphology or the toughness of the blend. Moreover, the morphology remains unchanged in the presence of any other compatibilisers used.

Table 2: Some properties of blend of PES: Nylon 66 in presence of the phenoxy resin.

Tables 1 and 2 suggest that blends with high interfacial energy and large phase size can mostly benefit from the addition of a suitable compatibilising agent, whereas blends with low interfacial energy and small particle size may not warrant any further interfacial modification. Experience in these blends and other similar systems show that phase morphologies with phase sizes below 2nm are generally robust enough for commercial applications and should not require compatibilisation.

Miscible blends

Homogeneous or miscible blends are mainly formed as a result of strong and favourable interactions, such as hydrogen bonding, between two different chains of high molecular weight polymers. The interactions are normally frozen-in below the glass transition temperature (or crystalline melting temperature when semi-crystalline components are involved) of the mixture and grow weaker as the temperature increases. The stronger the interaction, the higher the kinetic energy, in the form of heat, required to separate the two chains. At a certain high temperature the molecular interactions between the two chains, dissociate to cause phase separation of a type known as the lower critical solution temperature (LCST). An example of LCST for poly (ethersulphone) - poly (ethylene oxide) mixture is shown in Fig.5.

Transparency and glass transition temperature, Tg, are widely used to infer structural homogeneity or heterogeneity in polymeric materials. When the two polymers are miscible, their Tg lies between the Tgs of the parent polymers. Every homogenous composition behaves as a single component in its own right. For example poly (ether ketone), (PEK) forms miscible blends with poly (ether imide), (PEI) at all proportions. PEK is a semi-crystalline polymer with a Tg of 155°C and an experimental crystalline melting temperature, Tm, of 365°C whereas PEI is an amorphous polymer with a Tg of 220°C. The addition of PEI to PEK raises the Tg of PEK and enhances its high temperature capabilities whereas PEK introduces crystallinity into PEI and enhances PEI's environmental and chemical resistances. Furthermore, the miscibility in this case not only alters the Tg of the semi-crystalline polymer but also affects the crystallisation behaviour of the crystalline regions in PEK. The detailed aspects of the crystallisation of semi-crystalline polymer in miscible blends is discussed elsewhere. ^[10-11]

Table 3 shows the Tg and percentage crystallinity of various PEK:PEI blends. These results are obtained using re-heat runs in a differential scanning calorimeter on samples quenched from 400°C. A heating rate of 10°C/min was used for the re-heat process.

Table 3: Tg and the level of crystallinity in PEK: PEI miscible blends

As a further example, a 50:50 wt% blend of PEK with PEI has a Tg of 179°C and 19% crystallinity. This was then reinforced with 60wt% of continuous carbon fibre in a solution pre-preging operation. The pre-pregs were moulded into test specimens using a hot press. Results of mechanical testing the composite (with blend as its matrix) are compared with those of APC-2 in Table 4.

Table 4: Properties of continuous carbon fibre reinforced miscible blend

As shown in this table, good combinations of mechanical properties are obtained for the reinforced blend. Any other composition could similarly be treated depending on the property requirements and intended application.

Conclusions

In this article it is shown that morphology is a key factor in controlling the physical and mechanical properties of polymeric mixtures. The role of intra- and inter-molecular interactions between polymeric functional groups and resultant morphological developments has been discussed. Interfacial energy is a key macroscopic property that influences the morphology and property developments in polymeric mixtures and laminates. It is shown that low interfacial energy is a pre-requisite for robust morphological reproducibility.

When strong and favourable interactions, such as hydrogen bonding exist between two polymer chains it normally results in miscibility between the two components. As the hydrogen bonds become weaker at elevated temperatures, phase separation occurs.

Understanding the nature of molecular interactions between two polymers, as blends or laminates, opens up new opportunities to develop polymeric systems with unique properties to meet specific applications. The existing expertise in TWI can be used for consultation or new product developments in these areas.

References