Crystallinity in polymers

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2.7 Polymeric structures .1 Thermoplastics

2.7.4 Crystallinity in polymers

38 Modem Physical Metallurgy and Materials Engineering relatively unreactive novolac resin. (Control of the ini- tial phenol/formaldehyde ratio above unity ensures that a deficiency of formaldehyde will inhibit crosslinking during this first stage.) After drying, grinding and the addition of fillers and colourants, the partly-condensed resin is treated with a catalysed curing agent which acts as a source of formaldehyde. Network formation then proceeds during hot-moulding at a temperature of 200-300~ Each phenol group is said to be tri- functional because it can contribute three links to the three-dimensional random network (Figure 2.25c).

Another type of phenolic resin, the resole, is pro- duced by using an initial phenol/formaldehyde ratio of less than unity and a different catalyst. Because of the excess of formaldehyde groups, it is then possible to form the network structure by heating without the addition of a curing agent.

Atomic arrangements in materials 39

1

Specific volume

(a)

J J

J J

I

!

Temperature

(b)

l J

!

! |

I !

i I

! !

,[,

^{. :rm}

Temperature

!

!

!

!

!

A

Temperature

Figure 2.27 Specific volume versus temperature plots for (a) 100% amorphous polymer, (b) partially-crystalline polymer, (c) 100% crystalline polymer.

At this temperature, which is higher in value than Tg, the crystalline components of the structure break down completely on heating (Figure 2.27). For a given poly- mer, Tm increases with the degree of crystallinity; for example, its values for LDPE and HDPE are 110~

and 135~ respectively. Holding a polymer at a tem- perature between Tg and Tm (annealing) is sometimes used as a method for increasing existing crystallinity.

Turning now to the matter of structural complex- ity, crystallization is less likely as molecules become longer and more complex. Thus the presence of side-branching is a steric hindrance to crystallization, particularly if the molecules are atactic in character.

Iso- and syndio-tacticity are more readily accommo- dated; for example, isotactic polypropylene (iPP) is a material with useful engineering strength whereas atactic PP is a sticky gum. Scrutiny of the various repeat units shown earlier in Table 2.6 shows that they are usually asymmetrical and may be said to possess a 'head' and a 'tail'. Various combinations, or con- figurations, can result from addition polymerization, such as 'head to tail' (XYXYXYXY...) or 'head to head' (XYYXXYYXX...), etc. If the molecules have a similar and consistent configuration, crystallization is favoured. For instance, the previously-mentioned poly- mer, isotactic PP, has a 'head to tail' configuration throughout and can develop a high degree of crys- tallinity. Matching configurations favour crystallinity.

Crystallinity is therefore more likely in copolymers with regular block patterns of constituents than in ran- dom copolymers.

Summarizing, regular conformations and/or regular configurations favour crystallization. Each of these two characteristics is manipulated in a different way. Con- formations are changed by physical means (annealing, application of stress): changes in configuration require the breaking of bonds and are achieved by chemical means.

The fine structure of crystalline regions in commer- cial polymers and their relation to associated amor- phous regions have been the subject of much research.

An important advance was made in the 1950s when single crystals of polyethylene were produced for the first time. A typical method of preparation is to dissolve <0.01% PE in xylene at a temperature of 135-138~ and then cool slowly to 70-80~

The small PE crystals that precipitate are several microns across and only 10-20 nm thick. The thick- ness of these platey crystals (lamellae) is temperature- dependent. Diffraction studies by transmission electron microscope showed that, surprisingly, the axes of the chain molecules were approximately perpendicular to the two large faces of each lamella. In view of the smallness of the crystal thickness relative to the aver- age length of molecules, it was deduced that multiple chain-folding had occurred during crystallization from the mother liquor. In other words, a molecule could exhibit an extended and/or folded conformation. The chain-folding model has been disputed but is now generally accepted. The exact nature of the fold sur- face has also been the subject of much debate; typical models for folding are shown in Figure 2.28. The mea- sured density of these single crystals is less than the theoretical value; this feature indicates that irregular arrangements exist at fold surfaces and that the crystal itself contains defects. Most of the folds or loops are tight and each requires only two or three repeat units of molecular structure. 'Loose' loops of larger radius and chain ends (cilia) project from the surfaces of lamel- lae. These fundamental studies on single crystals had a far-reaching effect upon polymer physics, compara- ble in importance to that of single metal crystals upon metallurgical science.

In contrast to the relatively uncongested conditions that exist at the surface of an isolated crystal growing from a dilute solution, entanglement of chain molecules is more likely when a polymer

40 Modern Physical Metallurgy and Materials Engineering

(a) C-,. ' I~ (b)

r'~

Figure 2.28 Folded chain model for crystallinity in polymers shown in (a) two dimensions and (b) three dimensions (after Askeland, 1990, p. 534; by permission of Chapman and Hall, UK and PWS Publishers, USA).

Figure 2.29 Polarized light micrograph of two-dimensional spherulites grown in a thin film of polyethylene oxide (from Mills, 1986; by permission of Edward Arnold).

crystallizes from the molten state. Consequently, melt- grown crystallites are more complex in physical character. Microscopical examination of thin sections of certain crystallizable polymers (PE, PS or nylon) can reveal a visually-striking spherulitic state of crystalline aggregation. In Figure 2.29, three- dimensional spherulites have grown in a radial manner from a number of nucleating points scattered throughout the melt. Nucleation can occur if a few molecular segments chance to order locally at a point (homogeneous nucleation). However, it is more likely that nucleation is heterogeneous, being initiated by the presence of particles of foreign matter or deliberately- added nucleating agents. Radial growth continues until spherulites impinge upon each other. Spherulites are much larger than the isolated single crystals previously

described and range in diameter from microns to millimetres, depending upon conditions of growth.

Thus crystallization at a temperature just below Tm will proceed from relatively few nucleating points and will ultimately produce a coarse spherulitic structure.

However, this prolonged 'annealing', or very slow cooling from the molten state, can produce cracks between the spherulites (over-crystallization).

Internally, spherulites consist of lamellae. During crystallization, the lamellae grow radially from the nucleus. As with solution-grown single crystals, these lamellae develop by chain-folding and are about 10 nm thick. Space-filling lamellar branches also form. Fre- quently, a chain molecule extends within one lamella and then leaves to enter another. The resultant inter- lamellar ties or links have an important role during deformation, as will be discussed later. Inevitably, the outward growth of lamellae traps amorphous material.

Spherulites are about 7 0 - 8 0 % crystalline if the con- stituent molecules are simple. The layered mixture of strong lamellae and weaker amorphous material is rem- iniscent of pearlite in steel and, as such, is sometimes regarded as a self-assembled (in situ) composite.

The distinctive patterns seen when spherulitic aggre- gates are examined between crossed polars in a light microscope (Figure 2.29) provide evidence that the lamellae radiating from the nucleus twist in synchro- nism. For orthorhombic PE, the c-axes of lamellae lie parallel to the length of the extended chain molecules and are tangential to the spherulite (Figure 2.30). The a- or b-axes are radial in direction. Lamellae twist as they grow and the c-axes remain normal to the growth direction. The refractive index gradually changes for each lamella, causing incident plane-polarized light to become elliptically polarized in four quadrants of each spherulite and to form the characteristic 'Mal- tese cross' figure. The grain boundary structures of

n f ~ f t

Atomic arrangements in materials 41

Growth direction ^f

b

Figure 2.30 Schematic representation of a possible model for twisted lamellae in spherulitic polyethylene showing

chain-folds and intercrystalline links (from Young, 1991).

polycrystalline metals and alloys are reminiscent of spherulitic structures in polymers. Indeed, control of spherulite size in partially-crystallized polymers and of grain (crystal) size in fully-crystallized metals and alloys are well-known means of manipulating strength and deformability.

The mechanism of molecular movement in a poly- meric melt has presented a puzzling scientific problem.

In particular, it was not known exactly how a molecule is able to move among entangled molecules and to par- ticipate in the progressive chain-folding action that is the outstanding feature of crystallization. Clearly, it is completely different from atomic and ionic diffusion in metals and ceramics. This long-standing problem

/

Figure 2.31 Movement of a polymer molecule by reptation.

was convincingly resolved by de Gennes, l who pio- neered the idea of reptation, a powerful concept that serves to explain various viscous and elastic effects in polymers. He proposed that a long-chain molecule, acting as an individual, is able to creep lengthwise in snake-like movements through the entangled mass of molecules. It moves within a constraining 'reptation tube' (Figure 2.31) which occupies free space between molecules; the diameter of this convoluted 'tube' is the minimum distance between two entangling molecules.

The reptant motion enables a molecule to shift its cen- tre of mass along a 'tube' and to progress through a tangled polymeric structure. Reptation is more difficult for the longer molecules. At the surface of a growing crystalline lamella, a molecule can be 'reeled in from its tube' and become part of the chain-folding process.

Further reading

Barrett, C. S. and Massalski, T. B. (1966). Structure of Met- als, 3rd edn. McGraw-Hill, New York.

Brydson, J. A. (1989). Plastics Materials, 5th edn. Butter- worths, London.

Evans, R. C. (1966) An Introduction to Crystal Chemistry, 2nd edn. Cambridge University Press, Cambridge.

Huheey, J. E. (1978). Inorganic Chemistry: Principles of Structure and Reactivity, 2nd edn. Harper and Row, New York.

Hume-Rothery, W., Smallman, R. E. and Haworth, C. W.

(1988). The Structure of Metals and Alloys, revised 5th edn. Institute of Metals, London.

Kelly, A. and Groves, G. W. (1973). Crystallography and Crystal Defects. Longmans, Harlow.

Kingery, W. D., Bowen, H. K. and Uhlmann, D. R. (1976).

Introduction to Ceramics, 2nd edn. John Wiley and Sons, Chichester.

Mills, N. J. (1986). Plastics: Microstructure, Properties and Applications. Edward Arnold, London.

Morton, M. (ed.) (1987). Rubber Technology, 3rd edn, Van Nostrand Reinhold, New York.

Young, R. J. and LoveU P. A. (1991). Introduction to Poly- mers. 2nd edn, Chapman and Hall, London.

l Pierre-Gilles de Gennes, physicist, was awarded the Nobel Prize for Physics (1991) for his theoretical work on liquid crystals and macromolecular motion in polymers; de Gennes acknowledged the stimulating influence of the ideas of Professor S. F. Edwards, University of Cambridge.