1) Present and explain the purposes of blending various types of polymers together.
Even the most versatile polymers have certain drawback such as difficulty to mold, or susceptible to plasticization by moisture, or low strength. Polymer blending is an alternative approach to obtaining new polymer materials with desirable properties based on commercially available polymers rather than to design and synthesize completely new polymers. In the blending of polymers two or more polymer components are added to create a single polymer matrix. The resulting polymer will exhibit the combined characteristics of each component. By controlling the fraction of each component the desired of the resulting polymer can be controlled. Usually the desired characteristic is to reach a balance between maximum temperatures, toughness, ease of fabrication, are resistance to flames or chemicals. Thus the main objectives of blending are to improve chemical and physical properties of the polymer material, and to recycle plastic scrap with the possibility of improving the mechanical properties.
2) Give definitions of miscible and immiscible blends with the emphasis on major differences. What equations and rules can you use to describe selected properties of truly miscible blends. Give an example of truly miscible blend and explain the advantages of using it for commercial applications.
In a Miscible blend there is a total interpenetration of molecular chains of both polymers components, as seen in figure 1a. In an immiscible blend the two polymers form separated domains (phases) with only slight interpenetration, as seen in figure 1b.
Figure 1 Miscible and immiscible blends
In a miscible blend the characteristic of the final polymer will exhibit properties somewhere in-between the unblended polymers, which depends on the fraction of each component. However, in an immiscible blend the characteristics of the final polymer depends on the spatial arrangement of the phases (morphology), and the nature of the interface between them. One unusual property of immiscible blends is that one made from two amorphous polymers has two glass transition temperatures. Since the two components are phase separated, they retain their separate glass transition temperatures. However a miscible blend are characterised by only one glass transition temperature.
A truly miscible blend is one that is defined by:
Where Φ1 and Φ1 are the fractional composition of each polymer component in the blend and Tg1 and Tg2 are their respective glass transition temperatures for each polymer in their pure state.
Excluding any synergistic effects, the “rule of mixtures” or additive behaviour (linear dependence vs. component fraction) guides the strength and toughness, and the modulus of elasticity.
An example of a truly miscible blend is PPO/PS (Polyhenylene oxide/ Polystyrene). Its properties are shown in table 1
Table 1 Properties of PPO/PS and components
PPO imparts high-temperature resistance and toughness; PS contributes to a lower cost. This blend is used in the automotive industry because of low cost and ease of processibility. PPO/PS is also good for arts requiring water absorption.
3) What is the synergistic effect and why does it occur in polymer blends? What is the microstructural mechanism responsible for the synergistic effect? (Describe it in your own words as YOU understand it after reading the section in the notes).
The synergistic effect occurs in polymer blends. It occurs when the combined effect of the blend doesn’t follow the “rule of mixtures” when summing the individual components of the blend. Typically the combined effect of the blend is greater than the sum of the individual components. Synergistic effect can only be described through experemention
For example the behaviour of modulus is generally described by the “rule of mixtures” however some treatments such as annealing may change this behaviour to a synergistic one. In this case the observed property will show a broad maximum. Figure 2 demonstrates this property, where annealing has caused a synergistic effect. Figure 3 further demonstrates the synergistic effect on toughness
Figure 2 the synergistic effect of composition and annealing conditions of PC/copolyester
Figure 3 Variation of yield strength with blend composition and annealing conditions
The micro-structural mechanism responsible for the synergistic effect is annealing, which is the result of the contraction of free volume or densification on mixing for highly miscible blends.
4) Describe methods that are used to improve the interfaces in immiscible blends. Give an example from the notes that they really work!
The interface between phases in an immiscible blend determines the physical characteristics of the polymer blend. The magnitude of the interfacial tension and adhesion affects the strength, because it governs the transfer of mechanical stresses between. Generally immiscible phases are not bonded very strongly. To improve the properties of an interface, a block or a graft co-polymer, is added to the blend. This coupling agent helps bond and lock the phases more strongly to each other. Figure 4 illustrates the coupling action of grafts and blocks.
Figure 4 Coupling agents between phases
Figure 5 shows the effect of adding the coupling agent, 5% Epcar, to PP/HDPE. Initially the blend has poor interfacial adhesion, which is characterize by a deep curve and a broad minimum. When the coupling agent is added the tensile strength is greatly enhance.
Figure 5 Effect if adding 5% EPCAR to PP/HDPE
5) Give 2 (two) examples of commercial immiscible blends and discuss what advantages they provide vis-à-vis their individual components.
An example of a commercial immiscible blend is PC/ABS (acrylonitrile-butadiene-styrene). The properties of the individual components are compared with the polymer blend in table 2 below
Table 2 Properties of PC/ABS and components
Clearly from table 2 the immiscible blend of PC/ABS is mechanically superior to either on of its components since it increases the toughness strength. Further more ABS is cheaper than PC, thus adding it to the blend reduces the overall cost.
Another example of a commercially blend is ABS/PVC
Table SEQ Table \* ARABIC 3 Properties of ABS/PVC and components
PVC is inherently flame retardant, and ABS has a very good processability and is resistant to moisture and chemicals. The blend of the two polymers creates a polymer which had good flame retarding characteristics and good processability.
6) Give major objectives for processing polymers by hot (warm) working. Which sketch in your notes illustrates very nicely the state of the microstructure that we want to obtain by hot working?
The major objective of processing polymers by hot (warm)-working is to obtain a highly aligned (oriented) microstructure. This reduces the force required to draw the polymer. The ideal microstructure that is desired from hot working is shown in figure 6 below.
Figure 6 highly aligned microstructure
Figure 7 highly aligned microstructure
7) What is the ‘alpha crystallization temperature’ and why is it so important for successful hot working of polymers?
The alpha crystallization temperature is the most effective temperature range for hot-working. It corresponds to a secondary transition of the polymer, at which crystal sub-units are capable of being moved within the larger crystal unit. Above this temperature lamellar slip can occur, and extended chain crystals are formed. It is difficult to orient polymers that do not have this transition temperature to any great extent, because their crystal segments cannot be readily rearranged into an aligned condition.
8) What is the ‘draw ratio’ and how does the elastic modulus of polymers depend on the draw ratio?
The draw (λ) ratio is the extent of drawing during simple tensile testing which simulates commercial drawing. It is defined as the ratio of the length of an element of the drawn material to its length before drawing.
The elastic modulus is a unique function of the draw ratio and as long as the material can be extended by the required amount. No matter what was done to draw the material, the appropriate modulus can be found. The relationship between modulus and draw ratio is illustrated in figure 8 for HDPE. The elastic modulus is also independent of molecular weight as shown in figure 9
Figure 8 Relationship between modulus and draw ratio for HDPE
Figure 9 Relationship between modulus and draw ratio for HDPE of different masses
Thus elastic modulus depends only on the draw ratio and is independent of the molecular weight. Thus the only factor controlling the modulus upon drawing is the draw ratio.
9) Describe in detail the dependence of the maximum draw ratio on the molecular weight, initial morphology, drawing temperature and strain rate.
The maximum draw ratio which can be achieved depends on factors such as molecular weight, initial morphology of the polymer and the drawing temperature.
The maximum draw ratio decrease as the molecular weight increases, irrespective of whether the undrawn material has been quenched or slow –cooled during solidification. This is shown in figure 10.
Figure 10 Draw ratio vs. molecular mass
The effect of initial morphology also effects the draw ratio. High draw ratios are attributed to the more lamellar structure, relatively small number of tie molecules, and segregation of low molecular weight material which can act as plasticizers. These morphological differences become more pronounced with decreasing molecular weigh. (a greater gap between curve a and b in figure 10)
The maximum draw ratio increases continuously with increasing draw temperatures. However the modulus of elasticity reaches a maximum, indicating the temperature at which most effective drawing is accomplished. This is shown in figure 11 below. The position ot the peak temperature is a function of ,molecular weight
Figure 11 Draw temperature vs. Modulus and draw ratio of HDPE
Increasing the strain rate increases the drawing because the time available for the heat to dissipate from the neck is reduced. However, at very huge strain rates, there is break down of the homogeneous deformation and the formation of voids cause the material to fracture.
10) What kind of change in properties would you expect from the warm rolling of HDPE and PP tapes? Describe in detail and give examples from the literature
Figures 12 to 15 describe various effects of warm rolling on HDPE and PP. Hot rolling results in a dramatic increase in elastic modulus for both HDPE and PP (figure 12 and 13). Chemical stability in oil increases dramatically with increasing Young’s modulus of rolled tape (figure 14 and 15).