Polymer Basics: Structure and Properties

Author: Dr Xiang Zhang

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Introduction

Although polymers have been the most widely used material in the pharmaceutical and medical devices industry for many years, they are still often the root cause of many problems, such as unexpected product failure or yield deterioration. This is usually down to the complexity of polymeric materials. Chemical and physical structure can change at any stage - during manufacturing, post treatment (e.g. during sterilization), in storage, transportation or in use. The resulting changes in structure, which can range from the nano and micro up to millimetre scales, consequently affect the performance of the product. What’s more, product failures are often due to several co-existing factors. It is important, therefore, to understand the factors that can affect a polymer’s structure and, hence, its properties.

This paper will introduce the basic concepts regarding the structure and properties of polymeric materials. It will be of particular interest to engineers, technologists, scientists, technical managers and QA/QC professionals; anyone who is involved in developing new products or finding root causes of failures.

How Polymers are Configured

Polyethylene (PE) has the simplest structure of all polymers. It is made from ethylene CH2=CH2 via a polymerisation process which opens its double bond and forms a structure with the following repeat unit:

Fig. 1

For a linear PE, its average molecular weight ranges from 200,000 g/mole to 500,000 g/mole. If stretched, the polymer chain has a diameter of approximately 0.5 nm and an average length of 304 nm to 760 nm. With such a high length to diameter ratio, what would this polymer chain look like?

Bear in mind that each of the thousands of -C-C units can rotate freely about the -C-C bond angle of 109.5° relative to the -C-C next to it, so thermal vibrations make it impossible to keep the structure in the stretched linear state, rather it will form a randomised coiled sphere-like structure. That is the reality of a single PE chain that changes its configuration instantly and randomly. With such a high length to diameter ratio, say from 608 to 1520 for the molecular weight 200,000 g/mole to 500,000 g/mole of PE, the long chain polymer should behave like a very soft rubber where highly entangled chains can be stretched out by force but will return to their favoured coiled and entangled state upon release (we will discuss why PE is not a rubber but a plastic polymer later).

If we replace one H from each repeat unit of PE with Cl, we create another polymer: poly-(vinyl chloride) (PVC).

Fig. 2

By changing one element, rotation of the -C-C- bonds in PVC becomes more difficult than in PE. This is because the covalent radius of Cl (= 0.099nm) is almost 3 times larger than that of H (=0.037 nm), meaning that it is too big to rotate easily without ‘bumping’ into its neighbour H.

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We can therefore see that if PE is termed ‘soft’ (i.e. easy to rotate the -C-C-), then PVC, in comparison, should be termed ‘stiff’ (difficult to rotate the -C-C-).

The structural change from PE to PVC has a huge impact on the properties of the two polymers and, hence, their applications. Pure PVC is a very hard plastic and very difficult to process without adding plasticisers. I will use the two polymers to both introduce other concepts of polymer science and to describe the complexity of mechanisms behind the variation in properties (and hence applications) of polymers.

Intra- and Inter-Polymer Characteristics

For the same molecular weight of PE ranging from 200,000 g/mole to 500,000 g/mole, one single molecular chain weighs from 3 x 10-19 g to 8 x 10-19 g respectively. This means that if we want 1 g PE, we need billions and billions of PE molecules. When such a large number of molecules are entangled together, how many different ways can the molecules be associated with one another? And what kind of physical state must they have to keep all the molecules within the 1 g of material?

The first question is impossible to answer because there are far too many possibilities at the molecular level, as we have just seen with polymer chain configuration. With regards to their physical state however, they do, collectively, have a ‘fixed’ physical state that provides ‘fixed’ properties which can be used for various applications.

Let’s now look at two concepts to explain this further:

Intra-polymer structure characteristics: Some polymer chains are ‘soft’ (such as PE, where the polymer chain is easily rotated) and some ‘stiff’ (like PVC, where it is more difficult to rotate the polymer chain). It is intra-polymer structure characteristics that decide if a long chain polymer is ‘soft’ or ‘stiff’ or something in between.

Inter-polymer forces: Some polymers have weak forces between their polymer chains, whereas some have strong. This inter-polymer force is decided by van der Waals forces. PE has a relatively weak van der Waals force while PVC has a stronger one.

These two physical factors are the features that can help us understand all of the varied properties of polymers. They are also the reason why polymers are extremely different to other materials such as metals and ceramics. For example, because of their relatively weak van der Waals forces, compared with those in metallic and ceramic bonding, polymers are easy to deform in most circumstances.

It is possible to chemically string chains of different polymers together (say forming block copolymers or through randomised copolymerisation), or physically melt different polymers in a blend to tailor the properties of the final product. The changed properties are also determined by the above two factors, i.e. intra and inter-polymer characteristics.

How can we determine the effect of these physical factors on polymer properties and on their applications? The glass transition temperature of a polymer can help us to determine these effects.

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Glass Transition Temperature Tg and its Physical Meaning

Tg is one important parameter that is unique to polymers. Before explaining the meaning of Tg, let’s use one example to explain the concept of the polymer chain segment. A polymer chain segment is not a simple repetition of units as shown in Figures 1 and 2. Nor is it the entire polymer chain, but rather a part of it. The length of one segment varies, depending on the stiffness of the polymer chain and inter-polymer forces, i.e. intra-and inter-polymer characteristics. So, what is a polymer chain segment?

When we play with a rope by holding one end by the handle and forcing the rope up and down, we will see its shape changing randomly, as shown in Figure 3.

Fig. 3

There appear to be several ‘segments’ along the rope, with the peak or valley positions of the segments not being fixed but rather varying with the rope movement. It is not difficult to imagine that the stiffer the rope, the longer the segments. Vice versa is also true. This phenomenon also applies to polymers although, with them, it is a far more complicated process than that with the rope.

Polymer chain movement appears in segments too, which can be treated in a statistical manner, i.e. a given polymer in a given environment has a certain statistical segment length. The larger the segment, the stiffer the polymer chain; with the reverse also being true. The physical meaning of Tg is directly relevant to the chain segment movement. Tg is a transitional temperature at which polymer segments start to move from the frozen state (with increasing temperature), or start to freeze (with decreasing temperature). This is shown in Figure 4.

Fig.4

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From a structural point of view, the softer the polymer chain, the smaller the segment and, hence, the lower the Tg. When we talk about plastics or rubbers we actually refer to Tg, and this is the physical meaning behind the parameter. Theoretically, when the application temperature is above Tg, the polymer behaves like a rubber and when below Tg, the polymer is a plastic (applicable to amorphous polymers only in this respect). For the two polymers discussed, PE has Tg around -80oC while PVC has a Tg around 68oC - what a difference!

PVC is undoubtedly a plastic as its Tg is above ambient temperature. However, PVC is often used with additives such as plasticisers. Plasticisers cannot change intra-polymer characteristics but can affect inter-polymer forces. Plasticisers, which have much smaller molecules than PVC, act as ‘lubricants’ and effectively reduce inter-polymer forces, leading to a reduction of Tg. When PVC compounds have a Tg below ambient temperature, they behave like a rubber.

(It is worth noting here that we have used PVC here to explain concepts, not to promote PVC. While it has been used the longest of all polymers, it has caused major environmental concerns. Actually there are many alternatives to PVC that have the same and even better properties but have no environmental hazards like PVC and its plasticisers.)

What about PE? Its Tg is very low, around -80oC. In theory, it should be a rubber, not a plastic, because its Tg is well below the ambient temperature. This issue is addressed in the next section.

Melting and Crystallisation Temperatures Tm and Tc

PE is a crystalline polymer. When people talk about crystalline polymers, they actually mean semi-crystalline polymers. This is because, unlike most inorganic crystalline materials, polymers cannot form 100% crystals. They always consist of, at least, two phases: amorphous and crystalline. PE is therefore a semi-crystalline polymer. Its Tg does not determine whether it is a rubbery material or a plastic, but rather its crystallinity does. For PE alone, its crystallinity ranges from 20% up to 80% depending both on its chemical structure, branched or linear, and on processing conditions. It is the crystals (hard phases) together with rubbery (soft phases) that make the PE polymer serve as a plastic with good toughness, and not a rubber.

Many people have a general knowledge of classical theories of crystals, melting and crystallisation. However, polymers are different from classical inorganic or organic (small molecule) crystals. Figure 5 is a schematic plot showing melting and crystallisation of a polymer. It is worth noting that both melting and crystallisation span a certain range of temperatures around the peak temperatures Tm and Tc. When people quote the melting temperature, Tm, or crystallisation temperature, Tc, they are talking about the peak temperatures. It is important to note the fact that a semi-crystalline polymer has a range of crystals that melt at different temperatures. Another factor to be borne in mind is that polymer re-crystallisation occurs on heating prior to melting, which adds even more complexity to polymers.

The issue of re-crystalllisation on heating is due to the uncompleted crystallisation process. Under most processing conditions, polymer chains do not have sufficient crystallisation time before they freeze, due to the fast cooling rate at manufacture. This is one reason that, when the polymer is heated, re-crystallisation will occur at temperatures prior to the melting temperature of the existing crystals.

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For this particular reason, it is not reliable to measure crystallinity using DSC (Differential Scanning Calorimetry), something which, unfortunately, is mostly used to measure this parameter. The measured crystallinity is not the same as that of the virgin polymer because new crystals will form on heating - thus creating the uncertainty of the crystallinity measured by the DSC method.

Fig.5

Polymer Molecular Weight and its Meanings

It is common practice to quote molecular weight as a parameter of the polymer concerned. There are two ways commonly used to present molecular weight. One is number average of molecular weight Mn, [Σ(Weights)/No of polymers] and the other is weight average Mw [Σ(Weight fractions*Weights)]. Why do we need two parameters? For ease of understanding we will assume that our polymeric material is a mixture of only two molecules as follows:

Case 1: One molecular weight is 2000 g/mole and the other 1000000 g/mole

Case 2: One molecular weight is 491000 g/mole and the other 511000 g/mole

Both cases have the same Mn of 501000 g/mole. However, they have very different Mw - Case 1: Mw = 998008 and Case 2: Mw = 500680. The two groups of polymers have many different properties, including polymer chain configuration, nano/micro phase distribution of amorphous and crystalline structure (if they are semi-crystalline polymers), viscoelastic property, rheology (processing flow behaviour), mechanical properties (performance of a product) and so on.

Figure 6 is a realistic molecular weight distribution, ranging from a few hundred up to hundreds of thousands and even millions gram/mole (like ultra-high molecular weight polyethylene) on a logarithmic scale. For a given processing stage, changing molecular weight and molecular weight distribution parameters can be highly problematic, depending on the degree of the variation.

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Fig.6

Mechanical Properties of Polymers

Figure 7 is a schematic plot of stress versus strain, where curve (1) is brittle fracture and curve (2) ductile fracture with large deformation. Two extreme cases are shown here to highlight the real performance of polymers. One is an example of a very brittle polymer and the other a very ductile one. There are, of course, many polymers which sit between the two extremes. Young’s modulus, E, for the polymer can be obtained from the slope of the initial linear elastic region of the stress/strain curve. Other mechanical properties are detailed as follows using curve (2):

A. The initial yield stress before the polymer starts to yield.

B. The minimum stress after initial yield, representing minimum forces required to draw the polymer along its stress direction (polymer chain orientation).

C. The strain hardening at which the gradient starts to increase due to the polymer chain being fully stretched along its stress direction (higher stress is needed for further deformation).

D. The failure stress at which the polymer has reached its maximum stress allowed.

The question is why, under normal application conditions, or well below the maximum forces designed and allowed, some devices made of polymer suddenly fail without warning. This is an issue which can be explained by polymer fracture mechanics.

Fig.7

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Fracture Mechanics of Polymers and its Applications

The concept of fracture toughness (KIC), in fracture mechanics, is defined by the following equation:

KIC = σc (πa)1/2 eq (1)

σc is the critical failure stress for a given defect size ‘a’. This means that the relationship between fracture stress and defect size is determined by the fracture toughness, which is a materials constant (for a given chemical structure and given nano/micro structure). Defects always exist, regardless of whether they are small (nano or micrometer scales) or large (up to mm scales). It is the largest defect that is a decisive factor that causes a polymer to fail or not.

Defect ‘a’ in equation (1) also represents a crack developed from a defect. Figure 8 plots the maximum defect (or crack) size as a function of critical stress σc applied for a given fracture toughness KIC, where A, B C stands for three polymers with increasing fracture toughness. For example, at an applied stress of say 50 MPa, a defect or crack greater than 32 µm will lead to failure of Polymer A that has a fracture toughness KIC of 0.5 MPa.m1/2 (the blue line in Figure 8). For the same stress, polymers B and C won’t fracture unless they have a defect or crack size greater than 125 µm for Polymer B and 290 µm for Polymer C (because they have a higher fracture toughness KIC of 1.0 and 1.5 MPa.m1/2 respectively - the red and green lines in Figure 8).

On the other hand, for a given defect or crack size of 30 µm, Polymers A, B and C won’t break until applied stress approaches stresses 52, 103 and 155 MPa respectively. This demonstrates the importance of increasing polymer fracture toughness to avoid product failure.

Fig.8

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Fundamental understanding of basic chemistry, physics and mechanics of polymers is necessary for scientists, engineers and technologists who are developing new products with polymers or who need to solve problems during manufacture. With a better understanding of the relationships between product performance and the required chemical and nano/micro structures of polymers, the identification of the root causes of polymer failure will be speeded up and future failures can be prevented.

Further reading on polymers:

1/2 Xiang Zhang, ‘Evaluation the Changing Characteristics of Polymers’, Med-Tech Innovation, November/December Issue, 2011

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About Ceram

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About the Author

Dr Xiang ZhangExpertise in: Medical DevicesPrincipal Consultant - Medical Devices

A materials scientist, Xiang undertook his PhD and postdoctoral research at Cranfield University where he studied micro-mechanics and micro-fracture mechanics of toughening plastics.

After spending a further four years on polymer research for industrial applications, Xiang was awarded an industrial fellowship at the University of Cambridge in 1995.

Xiang's industry experience was gained at Medisense, Abbott Laboratories, where, as Principal Scientist, his work covered almost all aspects of medical devices from R&D and manufacturing support to failure analysis and QC. Prior to joining Ceram, Xiang worked as Director of Cambridge NanoTech, in the field of nano conductive materials and diagnostic medical devices.

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