Molecular design of polymers

Molecular design of polymers Anthony H. Willbourn Imperial Chemical Industries L td, Plastics Division, Welwyn Garden City, Hertfordshire, UK (Received 14 May 1976) This presentation is centred on what the chemist can achieve by control of molecular structure to achieve the 'simple properties' in linear, synthetic polymers which make them useful in a modern industrial environment. Transparency is a desirable property, readily accessible with amorphous polymers. Rather special structures are required to produce transparent crystalline polymers in the bulk state. The utility of many polymers is restricted by their susceptibility to chain scission, leading to a drop in molecular weight and hence loss of mechanical strength. A structural approach to defeating this effect is to make 'ladder' structures. An approach to the synthesis of novel ladder structures by bi-nuclear regulated copolymerization is suggested. Although copolymerization has been widely studied, little systematic work has been carried out on copolymer systems, which are more difficult to attain, to study the effect of copolymer composition on physical properties. The results are not always those expected, as illustrated by the acrylonitrile/styrene system. An example for further research of this type is proposed. The problems of correlating molecular structure with mechanical properties in any quantitative way are formidable, although much progress has been made on the phenomenological level in the understanding of the physical behaviour of polymers. Attention is drawn to a correlation which has been observed between modulus and certain structural parameters which may open the way to quantitative studies. Toughness is a more difficult property even to define, let alone correlate with structure. Nevertheless qualitative correlations may provide a useful starting point for future research.

INTRODUCTION Design is for a purpose. Design comes after the recognition of the need for what is to be achieved, and after a satisfactory answer to the question: why do it? It leads on to the question: how is it to be done? It is only in a mature field of science that these questions can be posed and answered at all, because they presuppose a wealth of knowledge about what it is possible to achieve and the ability either to do so, or to discover means of so doing by pursuing lines of research of known promise. Hence the fact that it is possible to talk on 'molecular design of polymers' is itself a tribute to the many able scientists who have, over the past 50 years or so, provided this wealth of background knowledge and who have discovered many remarkable ways of making polymers of all sorts of shapes and sizes. These different shapes and sizes result in an equal diversity of properties, and it is particularly in striving for understanding of some of these properties that Professor Geoffrey Gee has contributed so significantly. This discussion will be restricted to a consideration of the relationship between the molecular design of synthetic linear polymers and some of those simple properties which are relevant to their usefulness to man. This will be the unifying theme, which may not be scientifically very defensible but in practice it is among the most powerful motivators of scientific research. The 'simple properties' that are useful are not always simple in scientific terms and the future direction of much polymer research will be governed by efforts the better to * Presented at the Sixth Biennial Manchester Polymer Symposium, UMIST, Manchester, March 1976.

understand these properties and to improve on them. In reviewing what has been done, and what might be done in the future, emphasis will be placed on what can be achieved by the chemist by the control of the molecular architecture of polymers. There are other ways of changing properties, e.g. by adding plasticizers, stabilizers, 'reinforcing' agents, etc., which are often the only practicable means of producing polymeric compositions (plastics) with the processing characteristics and final properties to suit particular needs. However, on this occasion attention will be focused on what the chemist can do.

MOLECULAR ENGINEERING

Transparency Water-white transparency is on the face of it a very simple property. All that is required is that the material should neither absorb nor scatter radiation of wavelengths between 4000 and 7000 A. In principle the former should be no great problem with polymers based on carbon chemistry, since the electronic states of the ordinary simple carbon bonds in synthetic polymer do not give rise to significant absorption of this radiation. In practice however there are two major problem areas. The first problem is that most polymers when first made turn out to be coloured - usually dark brown, or at best yellow - and even if they start colourless they discolour on heating or on exposure to light. The pure scientist may dismiss this unfortunate result as due to an aberration-the use of impure reagents, the formation of ionic complex chromophores from metallic contamination, thermal or photochemical degradation of the polymer to form conjugated

POLYMER, 1976, Vol 17, November

965

Molecular design of polymers: A. H. Willbourn

b

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for 14 main chain carbons

31 helix

72 helix

Helical structures of (a) isotactic polypropylene and (b) isotactic poly(4-methylpentene-1). [Reproduced from Caunt, A.D. and Rose, J. B. 'Kirk-Othmer Encyclopaedia of Chemical Technology Supplement Volume', 2nd Edn, Wiley, New York, 1971, pp 7 7 3 - 8 0 7 by permission of John Wiley and Sons Inc., New York, Figure I

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diene sequences in the chain, and so on. He will be right, but nevertheless a vast amount of both industrial and academic research has been, and will be, aimed at eliminating colour in polymers, and in devizing stabilizers to prevent the formation of chrornophores (or to eliminate them by reacting with them). The problem arises not only from thermal decomposition during synthesis and processing, but from photochemical and thermal reactions during service. There is however one very helpful empirical rule to comfort the industrial chemist: the yellowness of a moulded thermoplastic polymer decreases with each increase in the scale and volume of commercial production and tends asymptotically to zero. This comes about for many reasons, the main one being that much effort is devoted to achieving it. Qualitatively the evidence for this 'law' may be seen in the history of PVC, polystyrene and polycarbonate among other commercial plastics. The second problem area is more fundamental and arises because, in general, those polymers having geometrically regular repeat units will crystallize to form crystalline domains larger than the wavelength of visible light. In polymer samples prepared by melting and forming (as most of them are), the individual lamellae are small, typically 100-300 A in thickness with lateral dimensions that can be up to several thousand A. But the lamellae ~,ggregate in twisted ribbons to form spherulitic structures of sizes from about a micron up to units easily visible in the microscope. Therefore crystalline polymers are opaque, or at best translucent, in the bulk state. In forming these polymers into films they can be made more transparent which is fortunate considering how important these materials are in such uses as photography. The rapid quenching which a thin film is subjected to produces some improvement in transparency since rapid quenching from the melt reduces the degree of crystallinity and retards aggregation into spherulites. More drastic modification is achieved by orientation which not only results in the absence of spherulites but can produce lamellar structures in which the orientation is correlated over distances large in relation to the wavelength of light, resulting in a marked decrease in intensity of scattered light ~. For example when

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POLYMER,

1 9 7 6 , Vol 17, November

polypropylene film is biaxially oriented, the spherulitic structure is completely disrupted and the final film has no visible structure and is highly transparent. However there are crystalline polymers which are transparent in the bulk state because of their molecular architecture, which is something that the chemist can control. An unusual method of achieving transparency is exemplified by poly(4-methylpentene-1). It is taken for granted that a polymer crystal has a greater density than the amorphous phase, a larger refractive index and is optically anisotropic. These things are not true for poly(4-methylpentene1)2. For instance the polymer chain in the crystalline unit cell forms a fairly open helix (less tightly coiled than the polypropylene helix, see Figure 1), and the packing of the large isobutyl side groups is such that the total assembly of bonds is vectorially quite well randomized. The result is that individual crystallites have low anisotropy and polymer spherulites exhibit unusually low birefringence compared, for instance, with polypropylene or polyethylene (Table 1). A second crucial and unusual factor is the fact that the specific volumes of the crystalline and amorphous regions are very close to each other; ill fact at room temperature the specific volume of the crystalline phase is greater (1.208 cm3/g) than that of the amorphous polymer (1.193 cm3/g). But the system is more complicated, otherwise the polymer would not crystallize at all. The amorphous polymer has a larger coefficient of thermal expansion than the crystalline phase, and has a greater specific volume above 60°C 3 (Figure 2). The difference is large at around 200°C which is the temperature region at which the polymer crystallizes on cooling slowly from the melt (m.p. = 245°C). Under these conditions large spherulites are formed, and in forming they produce voids and the optical transmission drops to about 20%. If however the samples are quenched, such crystallization as occurs is at a lower temperature; voiding is reduced, and transparency is good (~80% transmission). In practice quenching produces other problems and is not always practicable, and in fact the need to quench can be eliminated by some delicate molecular engineering. Copolymerization with a minor amount (~2%) of a linear 1-olefin helps, probably by lowering the temperature at which crystallization sets in. If in addition a very small amount (< 1%) of a high melting polyolefin is incorporated as a fine dispersion, to provide nuclei which persist in the molten polymer, then moulded parts can be made in which the average spherulite size is less than 5 pm, with optical transmissions around 97% (Table 2). There is another industrially important way chemically to affect the transparency of crystalline polymers if they can be made with ionizable groups. Examples are polyethylenes and polyoxymethylenes containing small molar percentages of carboxylic acid groups which, when neutralTable I

Spherulite birefringence of some polyolefins B irefringence,

Polymer

n r-

Polyethylene (low density) Polypropylene: Type I Type I I Type I I I Typical mouldings

-0.003

Poly(4-methylpentene- 1 )

nt

0.003 --0.002 --0.007 Varying from 0.003 to --0.002 0.0005

M o l e c u l a r design o f p o l y m e r s : A . H. W i l l b o u r n

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the ions form clusters a few tens of Angstroms in diameter. Other properties change also: ionomers are generally more transparent than the parent acid copolymers, depending on the type and concentration of the cation; they have greatly increased water absorption (which can be up to 25% by weight), the water presumably solvating the ionic clusters; ionomers are tougher than polyethylene, and are resistant to stress-cracking agents but they start to soften at quite low temperatures. In fact ionomers show some of the properties of crosslinked structures, but the 'crosslinks' are labile because ionomers are melt processable although they have much higher viscosities than the parent copoly-

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Figure 2 Specific volume-temperature curves for poly(4-methylpentene-1 ) samples: - - , quenched (more amorphous); -- -- -annealed (more crystalline). [Reproduced from Griffith, J. H. and Ranby, B. G. J. Polym. ScL 1960, 44, 369 by permissionof John Wiley and Sons Inc., New York (D]

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Electron micrograph of an acid copolymer [Reproduced from Longworth, R. 'Ionic Polymers', (Ed. L. Holliday), Applied

Figure 3 Table 2

Optical transmission of 1-olefin copolymers of 4methylpentene- 1 Optical transmission of mouldings (%)

Mean Comonomer (2 wt %) Nucleation

Science, London, 1975, Ch 2, pp 69-172 by permission of Applied Science Publishers Ltd, Barking ©]

spherulite

size (pm)

Quenched

Slow-cooled

>25 < 2

84 95

20 88

None

No Yes

No

>25

80

1-Hexene

Yes

< 2

98

38 93

1-Decene

No Yes

>25 < 2

90 98

47 97

ized with metallic cations, exhibit markedly increased transparency 4. These 'ionomers' (as they are known) are complex structures with many unusual features which can be illustrated by the Surlyn* range of plastics: these are ionomers based on ethylene copolymerized with methacrylic acid and (partly) 11eutralized with Na + or Zn ÷+ ions. The acid copolymers themselves with up to about 4 mol% of acid are structurally not very different from the parent low density polyethylenes, being translucent with typical spherulitic crystallinity (Figure 3). The introduction of cations however destroys the spherulitic structure (Figure 4), and the process is reversible. The 'ionomerized' structure is still crystalline (as shown by X-ray diffraction) and * Registered trademark, E. I. Du Pont de Nemours and Co. Inc.

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Electron micrograph of an acid copolymer after ionomerization. [Reproduced from Longworth R. "Ionic Polymers', (Ed. L. Holliday), Applied Science, London, 1975, Ch 2, pp. 69--172 by permission of Applied Science Publishers Ltd, Barking ©]

Figure 4

P O L Y M E R , 1976, Vol 17, November

967

Molecular design of pol/mers: A. H. Willbourn tant to penetration over the temperature range - 2 0 ° to +40°C, but at the same time the laminate must yield on impact to give controlled deceleration below the threshold value causing brain damage. On top of all that, when the windscreen is broken, the glass fragments should still adhere to the foil. To achieve these effects, the adhesion between foil and glass must be closely controlled, not too great, and not too small. There is only one foil material which has been found satisfactory on a commercial scale for this use. It is poly(vinyl butyral) (PVB), an amorphous polymer, compounded with a specific plasticizer, which was introduced some 30 years ago and which has of course been gradually improved over this period. A crucial requirement of PVB foil for this use is that its water content shall be carefully controlled at near 0.4%, because this governs not only the adhesion behaviour of laminates, but also the ability to make laminates and their integrity in service. PVB when sat arated takes up 3 to 5% of water. Various commercial ethylene copolymers and ionomers, which are partly crystalline, have some of the right sorts of properties for the windscreen application but none have the right balance. For example, ethylene/vinyl acetate copolymers may have the desired stiffness and toughness but have poor adhesion to glass and inadequate optical properties. Ionomers tend to be fairly transparent and give better adhesion to glass but are likely to be too stiff and to show too high a sensitivity to ambient relative humidity (r.h.). A much more suitable foil can be made by specific design of an ethylene/methacrylic acid terpolymer system. The level of acid copolymerized sets adhesion characteristics, while optical and mechanical characteristics can be optimized by controlling the content of a third monomer which can for instance be methyl methacrylate s. It turns out that a system with 4 mol% of each comonomer (Terpolymer X) possesses the right balance of properties: comparison of these materials is shown in Figure 5 and Table 3. An additional advantage of this terpolymer is that, unlike PVB, it has low water absorption and hence low sensitivity to ambient r.h. (Figure 6). One final thing is needed to achieve the highest transparency and minimum haze: the terpolymer foil should be cooled rapidly from the molten state during the process of making the windscreen. This quenching process renders the foil practically amorphous. Figure 7 shows what happened when a car fitted with a windscreen* made using Terpolymer X was involved in an (accidental) head-on collision, and illustrates the advantage of laminated screens. The driver's head impacted onto the

mers. Studies of other properties: dielectric relaxations, mechanical relaxations, thermal behaviour, X-ray diffraction etc. display a complex dependence of properties on composition and no very convincing correlation with molecular structure. There seems to be plenty of scope for further research to understand these effects and to seek novel combinations of properties in such systems. Seemingly simple uses often demand unusual property combinations. Consider the laminated winds.creen as used in motor cars. All that is apparently required to make such a windscreen is a transparent, colourless tough plastic foil, which can be sandwiched between two sheets of (curved) glass. Further study shows that the optical quality of the interlayer foilmust be of a very high order so that a glass laminate made with it has a minimum light transmission of 70%, negligible haze (i.e. wide-angle scattered light) and low optical distortion. The foil must also be tough over a wide temperature range so that laminates are highly resis-

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Variation of dynamic shear modulus G' with temperature. Method: torsion pendulum 0.3--3 cycles/sec. - . . . . , LDPE; . . . . . . ionomer B; , Terpolymer X; . . . . , plasticized

* Experimental windscreen made by Triplex Safety Glass Co. for BLMC 1800 car.

p o l y ( v i n y l butyral)

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Comparison of foil materials and glass laminates made from them Impact strength of laminate* (ft)

Methyl methacrylate (mol%)

at --20°C

at 25°C

Adhesion behaviour: glass retention after breakage

Transparency of laminate

Material

Cation

Methacrylic acid (mol%)

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-4

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Good Poor

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968

POLYMER,

1 9 7 6 , V o l 17, N o v e m b e r

Molecular design of polymers: A. H. Willbourn I.O

and fruitful field of research on the properties in their amorphous states of normally crystalline polymers, since this quenching technique can no doubt be applied to other crystalline polymers such as the aliphatic nylons, polyacetal and perhaps to polytetrafluoroethylene, polymers which have never been reported in the amorphous glassy state.

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Figure 6 Variation of equilibrium moisture content with relative humidity. Temperature 20 °-50°C. - - • Terpolymer X; . . . . plasticized poly(vinyl butyral)

screen, but he emerged without a scratch on his face (although he did break his arm!).

Glass transition temperatures of crystalline polymers No discussion of the temperature dependence of physical properties can be coherent without agreement about the nature of the glass transition temperature, Tg, and its relationship to features of molecular structure. This has been a fruitful field for research, and there is no need to emphasize its continuing importance. There is however one recent development worth drawing attention to because of its wider implications. For the simplest of polymers, viz. polyethylene, there is still controversy about its glass transition temperature: values of Tg are variously quoted as either around -120°C, or around -30°C 6. This disagreement leads to great confusion in the abundant literature on the properties of polyethylene and of related polymers (including the ionomers). Part of the problem undoubtedly arises because the 'glass transition temperature' can be defined in different ways which lead to different criteria for recognition and measurement of Tg. A definition favoured by the author (although it is admittedly incomplete) is 'that temperature at which the main polymer chain acquires large-scale mobility'. Following this definition there is a simple criterion for Tg for those crystalline polymers that can be quenched from the melt into the fully amorphous glassy state: Tg is that temperature at which crystallization occurs 7. Crystallization must involve fairly large scale chain movement, translational as well as rotational, especially as we now know that lamellae of folded chains are formed in the process. This criterion was of no value in the case of polyethylene because amorphous linear polyethylene had never been produced despite many efforts to do so. Quite recently however Hendra has succeeded in quenching a linear high density polyethylene to the fully amorphous state in which it is stable at -150°C. He has followed the onset of crystallization by observing infra-red spectra, Raman spectra and X-ray diffraction and has concluded that crystallization sets in at around -100°C which sets an upper litnit for Tg, using this criterion a'9. This work opens up a new

Permanence in physical behaviour is very desirable but most polymers are restricted in their utility by degradation, by light or by heat or by oxidation or by a combination of these. Typically such degradative reactions cause random chain scission, which results in a rapid drop in average molecular weight and this in turn results in a rapid fall in those properties dependent on average MW: impact strength, tensile strength and elongation at break. However there has been little work published on the quantitative aspects of this very important phenomenon. Adams has published data on MW changes of (unstabilized) polypropylene caused by thermal oxidation, photo-oxidation and by melt processing, and noted that samples were quite brittle by the time their MW were halved 1°. The effect is quite dramatic: polypropylene ofM n = 84 000 becomes brittle after 2 h exposure at 138°C if it is not stabilized. Similar data have been presented by Birley and Brackman on the photo-degradation of low density polyethylenell. Most commercially available thermoplastics are produced with molecular weights as low as is safe and reasonable to confer the required mechanical properties, in order to make fabrication by moulding, extrusion, etc. as easy as possible by keeping the melt viscosity low. With this in mind, and taking account of the published data, it seems that when degradation has proceeded to the extent of about one bond broken per molecule present (thus halving Mn) the mechanical strength of the polymer has been effectively destroyed. A means of defeating this effect by redesign of molecular structure is to form ladder polymers in which two parallel chains are connected together at regular intervals. An individual chain in a ladder polymer can undergo cleavage of any single bond without any effect on MW. If cleavage is a random process, then only after extensive degradation will two bonds in the same section of the ladder be cleaved

Figure 7 collision

Car fitted w i t h T e r p o l y m e r X windscreen after

head-on

P O L Y M E R , 1976, Vol 17, November

969

Molecular design of polymers: A. H. Willbourn X

X

Figure 8

A ladder structure can withstand many bond cleavages without reduction in overall length

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Taking the view that the Tg is below -100°C, then this amorphous branched polyethylene-type ladder polymer will have an effective Tg around -70°C and (if lightly crosslinked) will be elastomeric over a wide temperature range, say from -60°C up to its limit of stability which could be well over 100°C depending on the conditions, the life required and the effectiveness of stabilizing systems. Clearly some very advanced molecular engineering will be needed to synthesise a polyethylene ladder polymer. The obvious elegant way is to persuade, say 1,7-octadiene to copolymerize regularly, and coincidentally, between the same two growing polyethylene chains. Whilst this looks to be impossible to achieve in the way set out in Figure 11, published work on transition metal compounds which are catalysts for olefin polymerization 17perhaps provides the basis of a possible synthesis. These catalysts function by 'growing' the polymer chain from the metal atom, the monomer being first coordinated to the metal and then being inserted into the polymer chain. Thus zirconium tetrabenzyl is such a catalyst, and it retains its activity when bonded to a silica surface as a siloxy-tribenzyl zirconium. Under the right conditions structures of this

CH=CH 2

Figure 9 Diels--Alder condensation and polymerization of 2-vinylbutadiene and benzoquinone

and the chain itself be broken, with consequent drop in MW and in related physical properties (Figure 8). The first deliberate synthesis of a ladder polymer was by Bailey and Economy in 195412'13 of the polymer from 2-vinylbutadiene and benzoquinone by Diels-Alder condensation (Figure 9). A proportion of the product was of fairly high MW, softening at over 340°C and soluble only in powerful solvents such as hexafluoroisopropanol. Many such ladder polymers have since been made, often containing condensed heterocyclic rings and giving intractable infusible solids 14. However some polyester types with a significant aliphatic content have been synthesised, and degradation studies on these and other systems have demonstrated that these ladder structures are more resistant to degradation than the analogous single-strand polymers ls'~6. This is a tantalizing field because there are polymers it would be interesting ta examine but synthetic methods to make them have yet to be devized. One such is a polyethylene-type ladder polymer with the 'rungs' irregularly spaced at intervals with an average of around 4 per 100 chain atoms, i.e. at about the same frequency at the short branches in low density polyethylene (Figure 10). It is interesting to speculate what its properties would be. It is likely to be not very crystalline, the rungs not being regularly spaced, and also it is difficult to see such a ladder structure being incorporated in a chain-folded lamellar structure. A small amount of copolymerization with an a-olefin (say propylene or butene-1) to introduce some random short branches (R in Figure 10) might be expected to produce a completely amorphous polymer. Its thermal stability (and light stability) should be good, and although its melt viscosity would presumably be high for a given 'ladder length', it should be processable at high temperatures without significant degradation. What its mechanical properties would be depends on one's view of the Tg of polyethylene (hence the comments above on this matter).

970

POLYMER, 1976, Vol 17, November

(~R) -

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Figure 10

R)

A polyethylene-type ladder structure

Growth of polyethylene-type ladder structure by bi-copolvmerization with 1,7-octadiene (schematic)

Figure 11

Molecular design of polymers: A. H. Willbourn

Novel copolymer systems

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Bz

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Copolymerization is one of the most direct and widely used techniques of molecular engineering and the kinetics of copolymerization have rightly been the subject of extensive academic study. The principles of the free-radical copolymerization of vinyl monomers are to be found in standard texts, with tables of reactivity ratios, and Q - e diagrams. From these data it is apparent that there are many copolymers which are difficult to make. It is somewhat surprising that more of these systems have not been taken as projects in, say, chemical engineering, the objective being to make significant quantities of truly random, thermally stable copolymers using the special monomer composition control and feeding techniques now practicable with the advent of on-line process instrumentation and realtime computer control. It should be possible to make enough of these copolymers in laboratory scale apparatus to study their properties as a function of composition ~a. These properties sometimes turn out to be quite different from what was expected. Examples of 'difficult' copolymers which have become industrially important are the truly random copolymers of styrene and acrylonitrile (AN) containing 75 and higher mol% AN. This is a system in which~9:

Zr AN is monomer 1, Bz

~Bz

r 1 = 0.04

Styrene is monomer 2, r2 = 0.4

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I Figure 13

type can indeed be formed (Figure 12). In this state the bonds are tetrahedrally disposed, but it is probable that during polymerization the zirconium is 6 coordinated with an octagonal structure (Figure 13). Now the procedure would be to synthesise a binuclear catalyst, joining two Zr (Bz)3 moieties with a para-xylyl ligand to form (Bz)3Zr.CH2.C6H4.CH2.Zr(Bz)3 . This binuclear complex is then reacted onto a silica surface previously heated to about 400°C so as to leave the surface only sparsely covered with OH groups (so that paired structures like those in Figure 12 are formed). When ethylene is presented to the system, polyethylene chains will grow from each of the paired Zr nuclei, and the problem is: how to join these two growing chains together? In principle this might be done by copolymerizing with 1,4-dial lylbenzene if the two allyl groups could be placed simultaneously next to the two ~leighbouring Zr nuclei, To achieve this some 'template' mechanism must be provided to attract 1,4-diallylbenzene molecules onto the surface in between the paired Zr nuclei, so that their relative concentration is much greater there than at any other surface site. Such a mechanism could be the formation of a weak complex between diallylbenzene and the para-xylyl ligand by appropriate substitution of the respective aromatic nuclei by attracting groups X and Y. The complete system is shown in Figure 14. It would clearly be a formidable task to realize this system, but one might suggest an approach using X = F and Y = NR2 (R = alkyl), or vice versa. The effect of the substituent X on the stability of the xylyl ligand and on the polymerizing activity of the Zr nuclei may well be an important factor.

The copolymer composition plot is skew sigrnoidal, as shown in Figure 15. This reflects the fact that over much of the range of monomer compositions the copolymer (instantaneously) formed has a very different composition from the monomer mixture. Inevitably therefore if such a monomer mixture is fully polymerized, the copolymer formed will be structurally heterogeneous. This is illustrated in Figure 16 for a 25:75 molar mixture of styrene and AN. The properties of these heterogeneous copolymers are dreadful: thermal stability is poor and they are so brittle that the properties can hardly be measured. If however the monomer composition is controlled during reaction to produce a truly

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POLYMER, 1976, Vol 17, November

971

Molecular design o f polymers: A. H. Willbourn 1.0

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Figure 15 The relationship between monomer mixture composition and copolymer composition for acrylonitrile and styrene

and are used in the manufacture of bottles for carbonated beverages. Permeability is an important property which governs the use of many polymers in film form to protect foodstuffs, chemicals, and agricultural products. In its simplest terms, permeability is a function of both the solubility (S) of a gas in the polymer and of its rate of diffusion (D). Both S and D are dependent on polymer structure in ways that are qualitatively understood, but there is certainly scope for better understanding and there are structural effects which are still mysterious. One of these is the relatively low permeability of vinylidene polymers, shown by for instance polyisobutene compared with natural rubber and poly(vinylidene chloride) compared with poly(vinyl chloride) (Table 5). An interesting research project would be to synthesise a range of vinylidene chloridel/isobutene 2 copolymers, truly random composition. This is likely to be difficult since one can guess that for this system r 1 ~ 3, r 2 ~ 0. The structures, and hence the properties, of these copolymers might well show some interesting and unusual features. The methyl group, although slightly larger than the C1 atom a~ld not spherically symmetrical, is not very different in size from the C1 atom. The available evidence suggests that the chain configurations of the two polymers in their individual unit cells are somewhat similar. Polyisobutene (PIB) only crystallizes on stretching; however in the unit cell the conformation of the molecule is a 83 distorted helix with a repeat distance of 2.32 A per monomer unit2°; the corresTable 4 Styrene: acrylonitrile copolymers (20:80 mol ratio). Comparison of homogeneous and heterogeneous copolymers

50

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K

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123

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108

Table 5

Un-notched impact strength X-ray (kJ/m 2) Appearance observations 2.4

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24

Pale yellow, clear

Appreciable two-dimensional order Amorphous

Permeability of some polymers to oxygen and carbon

dioxide

Permeability coefficient at 25°C (cm 3 (STP)cm ) . . . . X 10 lo cm sec atm O

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50

Figure 16 The composition of the product resulting from the batch copolymerization of acrylonitrile (75 tool %) and styrene (25 mol %). [Reproduced from Hendy, B. N. 'Copolymers, Polyblends and Composites', Advances in Chemistry Series, No. 142, Am. Chem. Soc., Washington D.C., 1975, pp. 1 1 5 - 1 2 8 by permission of The American Chemical Society, Washington D.C. ©]

random copolymer having even as much as 80 mol% AN, then a strong, clear and thermally stable copolymer can be made 18. The contrast in properties is indicated in Table4. These particular copolymers, with high AN contents, are important because of their very low permeability to permanent gases and in particular to CO2 and oxygen (Table5). Such copolymers are classed industrially as 'barrier resins',

972

POLYMER,

1976, Vol 17, November

Oxygen (a) Thermoplastics: Polyethylene (low densityl Polypropytene ABS PVC (unplasticised) Nylon-6,6 (dry) Nitrile barrier resin (AN: styrene, 75:25) Oriented polyester film [(poly(ethylene terephthalate)] Saran film [poly(vinylidene chloride) copolymer] (b) Rubbers: Natural rubber Butyl rubber

Carbon dioxide

180 90 45 5 2 2

900 300 450 11 7 5

2

11

0.5

1500 100

4

11 000 400

Molecular design of polymers: A. H. Willbourn 250

20C

O

150

O

ioo

-20 u

50

-40

0

-60

-80

6

48

60

8o 18o

Vinylidcn¢ chloride (tool %)

Figure 17 Vinylidene chloride/isobutene copolyrners: dependence (hypothetical) of melting point (Tm) and of glasstransition temperature (Tg) on composition. A, Tin; B, Tg

ponding conformation for poly(vinylidene chloride) (PVdC) is a 21 helix with a repeat distance of 2.34 A per monomer unit 21. Both molecules are very overcrowded in any configuration, so much so that the main chain bond angles at the CH2 groups are opened up from the tetrahedral 109.5 ° to 128 ° for PIB and 123 ° for PVdC; the difference in bond angle at the CX2 group is rather greater, the angle being 110 ° for PIB and 120° for PVdC. These molecules are therefore rather similar in size and in preferred conformation, and in random copolymers of low isobutene content the comonomet units might be expected to cocrystallize. Some estimates can be made of the properties of such a cocrystallizing system. Although PIB does not crystallize unless stretched, from observations of the melting point of PIB at different elongations it appears that its notional melting point is around 0°C; the Tg of PIB is -70°C. For poly(vinylidene chloride) Tm ~ 210°C and Tg ~ -20°C. Hence one would expect a random, cocrystallizing, copolymer of 80:20 VdC/IB composition to have (Figure 17): Tm ~ 175°C: and Tg ~ -30°C. Quite apart from this being an interesting academic research project, these random copolymers should have technologically useful properties, especially if their lower melting points could be combined with thermal stability adequate to permit fabrication by melt processing. They should have good mechanical properties and be tough and extensible (the amorphous phase having such a low Tg), and might be expected to retain low permeability to CO 2 and oxygen.

MECHANICAL PROPERTIES Problem o f structure correlation It is evident that, however desirable may be properties such as transparency, resistance to degradation, low dielectric loss, low flammability and low permeability, these are of little practical value unless they can be combined with 'good mechanical properties' in a polymer which can be readily processed into desired shapes - mouldings, extrusions, sheet and film. It is impossible really to define 'good mechanical properties' because what is acceptable in practice is the result of a compromise between properties which

are essential for a particular application, adequate performance in other respects, and the overall cost involved. However, prescinding from specific properties and applications, it is reasonable to generalize to the extent of stating that for structural uses one is seeking for high strength, rigidity, toughness and a wide usable temperature range. Although the dependence of mechanical behaviour on a variety of parameters (time, stress, geometry, environment, temperature), has been the subject of detailed investigation and some progress has been made in an understanding in phenomenological terms, understanding at the level of molecular design is still qualitative, fragmentary and imperfect, and compares unfavourably with the state of knowledge about low molecular weight compounds, metals and even of elastomers. This point has recently been emphasized by Flory 22. He points the way to a rigorous mathematical approach to the calculation of physical properties using such basic data as energy barriers to bond rotation, intermolecular forces, the dimensions of chain links and of sub-units, the distribution of chain lengths, and the conformations of the long chain molecules. While in principle possible, this is a daunting task for the ordinary polymer scientist, if only because of the large amount of quantitative information needed for each polymer before a start can be made with the calculations, and also the difficulty of formulating the mathematical functions and programming the computations. A complementary approach is to consider the total system and to concentrate on those essential molecular features which determine a few basic properties. If one can establish some real correlations then these will point the way to profitable lines for future research, both practical and theoretical. Two examples of such correlations are proposed for consideration. Toughness and structure A tough material fails in ductile fashion, at large strains. Why is it that some glassy, amorphous (linear) polymers exhibit ductile failure, while others are brittle? Of course distinctions are not all clear cut, and are dependent on temperature, strain rate, etc. but the reality is quite recognizable in contrasting polystyrene with polycarbonate. There is no real correlation with modulus, glass transition temperature or secondary mechanical transitions. One clue is perhaps to be found in the shape of the molecule. In the 1930s and the 1940s when free-radical polymerization of vinyl compounds was the only readily available synthetic route to high molecular weight polymers, and most of these were amorphous being of the types - ( C H 2 - C H X ) - or -(CH2-CXY)-, much effort was put into seeking useful polymers with softening points above 110°C which was the top limit of Tg values for the polymers then in general use [polystyrene, poly(vinyl chloride), poly(methyl methacrylate)]. A favoured approach was to incorporate large side groups (X) in the vinyl (-CH2-CHX-)n chain units, but the effect of this was invariably to make brittle materials. Perhaps the ultimate example was polyvinylcarbazole, which has been used commercially because of its excellent dielectric properties and high softening point:

sC'~. CH

I

POLYMER, 1976, Vol 17, November

973

Molecular design of polymers: A. H. Willbourn Table 6

Effect of structure on impact behaviour of aromatic

sulphone polymers Impact

Structu re

behaviour

Me Tough

Me Ph Brittle

into these structures, then they become brittle also as was exemplified by Rose 23 for a series of aromatic sulphone polymers (Table 6). He also found that with the poly(.phenylene ethersulphone) series of polymers the all paralinked polymer was tough but the ortho-para and the metapara structures resulted in polymers which were brittle (Table 6). This is not inconsistent with the reasoning suggested, in that the ortho and meta linkages result in abrupt 'kinks' in the chains which could be envizaged as causing neighbouring chains to interlock. This approach to the understanding of 'toughness' is clearly much over-simplified but it does suggest that a systematic study of the failure behaviour of glassy amorphous polymers of widely different structures could be rewarding.

Ph Modulus and structure

- - - ~ S 0 2 ~ 0 - -

"--~S02~--0--

Tough

Brittle

Tough

Brittle

0w

~

S02~0

There is no satisfactory quantitative correlation between parameters dependent on molecular structure and elastic modulus, although in qualitative terms it is clear that modulus must be dependent on the energy of interaction between molecules and hence on parameters such as internal energy and coefficient of thermal expansion. Recognition of this has prompted a number of basic simple approaches to the problem. Combining a standard thermodynamic approach to the derivation of an equation of state of a simple molecular crystal with the Lennard-Jones model for the dependence of the energy of interaction between molecules on their distance of separatio~a, gives for the bulk modulus (B) of a face-centred cubic lattice at 0K24: B0=8.04

-

Brittle

In more recent years conditions for carrying out a variety of coupling and condensation reactions have been devised (and intermediates of the requisite purity have been synthesised) to produce a wide range of macromolecular structures. These routes have been exploited to make high softening amorphous polymers by synthesising more rigid chain structures in which large aromatic groups are incorporated in the main chain itself. If one compares these with polymers having bulky pendant groups, the striking difference is that the ring-in-the-chain polymers combine higher softening points (Tg) with increased toughness over vinyl polymers. They fail in ductile fashion, even at temperatures well below Tg, and therefore are able to absorb and dissipate mechanical energy: they are tough. It is in this context that the simple concept of shape becomes significant. Put in its crudest terms large pendant groups interlock. This inhibits relative movement of neighbouring chain segments and the dissipation of elastically stored energy and the redistribution of stresses over the system thereby, thus leading to sequential failure at local high stress concentrations and to brittle failure of the total system. This mechanism is widely accepted as relevant on the macroscopic scale. The ring-in-the-chain amorphous polymers which are tough have chain structures without protuberances. It is interesting to note that if large side groups are introduced

974 POLYMER, 1976, Vol 17, November

( E~~a p )

dyne/cm 2

(1)

where E0ap is the molar energy of vaporization at OK (erg/ mol) and vO is the mol ar volume (cm 3/mol). This equation holds well for neon, argon and nitrogen at cryogenic temperatures. Tobolsky24 suggested that equation (1) would be a fair approximation at higher temperatures, and might even be relevant to amorphous polymers in the glassy state. The t e r m (gvap/g) is the cohesive energy density (CED), and this is 83 cal/cm 3 at 25°C for polystyrene. With CED expressed in cal/cm 3 equation (I)becomes: B = 3.37 (CED) x 108 dyne/cm 3

(2)

which gives a modulus value for polystyrene of 2.8 x 1010 dyne/cm 2. The experimental value is 3.5 x 1010 dyne/cm 2, and at first sight this seems to be a very promising measure of agreement. A test of the general applicability (and utility) of equation (1) is to plot modulus vs. CED for a number of poly mers. Nielsen 2s collected such data but noted that there was no correlation. His data are plotted in Figure 18. Lack of interest in this approach is therefore understandable. However a broader consideration of the structural parameters which will determine modulus, combined with a similar approach to the glass transition temperature, enables one to demonstrate by an empirical procedure that there does indeed seem to be a relationship between modulus and cohesive energy density. Consider the simplest case of a linear, amorphous polymer of very high molecular weight (so that chain ends can be ignored) ~a the glassy state at temperature T < Tg. The indi-

Molecular design of polymers: A. H. Willbourn

NR PVAc PET PVC PC PS PMMA PES PSA PPO

200 Q

150

-~ I00 (9

50 /

8

° e

. . . .

s'.o

©

o

. . . . .

•

100

Young's modulus ot 4K (dyne/cmexI0-I°)

Figure 18 CryogenicYoung's modulus and cohesive energy density for various polymers. O, Amorphous polymers; O, crystalline polymers. - - , E = 3.37 (CED) X 108

vidual macromolecules will take up completely random configurations, such as they assume in infinitely dilute solution 2z. The force per unit area needed to produce uniaxial deformation at vanishingly small strain (i.e. the Young's modulus, E at zero strain) will in the ultimate be determined by three parameters: (1) A molecular flexibility parameter, F(T, t), which is a function of temperature, T, and of the time scale of the experiment, t; (2) a parameter which is a measure of interchain forces, assumed to be of the form of equation (2); (3) a parameter which is a measure of bond angle distortion, assumed to be negligible at temperatures significantly >OK. The glass transition temperature is clearly determined by two of the same parameters, namely molecular flexibility and interchain forces. At the glass transition temperature the Young's modulus of amorphous plastics drops from 3>1.0 GN/m 2 to ~0.01 GN/m z, the lower limit being set by the value for an ideal rubber-

E ~ 3pRT/M c

The E~0 values were taken from dynamic mechanical data measured in these laboratories between the years 1955 and 1965 except for the value for polysulphone (from Bisphenol A) which is a static modulus value and may therefore be slightly low relative to the other data. CED values were derived from solubility parameter values at 25°C calculated by the additive scheme proposed by Small26 and collated by van Krevelen and Hoftyzer27 who also give the specific heat data needed to make the derivations ~8, except for values for the two aromatic sulphone polymers which were estimated by Small:9. Tg values were taken as generally quoted in the literature, except that Tg for (amorphous) polyethylene was taken as -120°C. The line of slope 0.034 drawn in Figure 19 is simply the theoretical proportionality coefficient in equation (2), corrected for a change in units (1 GN/m 2 = 1010 dyne/cm 2) and assuming that the bulk modulus is (approximately) equal to the Young's modulus: E ~ 0.034 (CED) GN/m 2

(4)

In contrast to Figure 18, it is apparent that the data plotted in Figure 19 do indicate a significant correlation between 200

PAN O 150

(3)

which typically will be around 0.001 GN/m 2. So for amorphous linear pol~cmers at their glass transition temperatures E ~ 0.01 GN/m z. What this means is that, in a sense, Tg is a 'normalized' temperature and it suggests an approach to identifying the individual parameters which determine the modulus. Assume as an approximation that at temperatures just below Tg the parameter F (molecular flexibility) has the same dependence on temperature for all polymers, and hence that in this temperature region differences in modulus are determined by interchain forces (the contribution from the flexibility parameter F being constant). Pursuing this hypothesis, Figure 19 shows a plot of E~o versus 6~0 from data currently available on amorphous polymers, where: E~0 is the dynamic modulus in GN/m 2 at ~300 Hz measured at the temperature/'50; TS0 is the temperature 50°C below the Tg; 6~0 is the cohesive energy density at T50. The key to the data plotted is: PAN PEX

natural rubber, poly(vinyl acetate), poly(ethylene terephthalate) (amorphous), poly(vinyl chloride), polycarbonate (of Bisphenol A), polystyrene, poly(methyl methacrylate), poly(phenylene ethersulphone), polysulphone (of Bisphenol A), poly(dimethylphenylene oxide).

polyacrylonitrile, polyethylene (amorphous, crosslinked, see ref 7),

0 PET

NR

-6 u I0( PC O op S 0 PMMA

PES 0 PSA

0 PPO

O

I

I

I

2

I

[

3 4 E'(GN/m 2~

I

5

Figure 19 Relationbetween dynamic Young's modulus (E') and cohesive energy density (CED) for some amorphous polymers at (Tg -- 50)°C. E'/CED = 0.034

P O L Y M E R , 1976, Vol 17, November

975

Molecular design of polymers: A. H. Willbourn E~0 and CED which is not very different from equation (4). It is interesting to note that the Ts0 values range from - 1 7 0 ° to +175°C. While some relationship between the relaxed Young's modulus and short-range intermolecular forces (of which CED is a measure) is to be expected, the excellence of the particular correlation observed is surprising and is further discussed elsewhere a°. In the meantime it is clear that further work to explore the scope of these observations is called for, and it would be particularly interesting to obtain modulus data for the amorphous states of those polymers which are normally crystalline.

ACKNOWLEDGEMENTS

5 6 7 8 9 10 11 12 13 14 15 16 17

The author is indebted to his colleagues in the Research Department of the Plastics Division of ICI for helpful discussions and assistance in preparing this paper, and in particular to Mr I. T. Barrie, Dr D. J. Blundell, Dr J. D. Burnett, Dr A. D. Caunt, Mr P. R. Hendy, Mr A. Pajaczkowski, Dr J. E. Priddle, Dr J. B. Rose, Dr F. M. Willmouth and Dr D. G. M. Wood. The author would also like to thank Dr R. Longworth for permission to reproduce Figures from ref4.

21

REFERENCES

22 23 24

1 2 3 4

976

Stein, R. S. and Prud'homme, R. J. Polym. Sci. (Polym. Lett. Edn) 1971,9,595 Caunt, A. D. and Rose, J. B. 'Kirk-Othmer Encyclopaedia of Chemical Technology, Supplement Volume', 2nd Edn Wiley, New York, 1971, pp 773-807 Griffith, J. H. and R~nby, B. G. J. Polym. Sci. 1960, 44, 369 Longworth, R. 'Ionic Polymers' (Ed L. Holliday), Applied Science, London, 1975, Ch 2, pp 69-172

POLYMER, 1976, Vol 17, November

18 19 20

25 26 27 28 29 30

Priddle, J. E., Pajaczkowski, A. and Vincent, P. I. Br. Pat. 1 406 703 (1975) Boyer, R. F. Plast. Polym. 1973, 41, 15;Macromolecules 1973, 6,288 Winbourn, A.H. Trans. FaradaySoc. 1958,54,717 Hendra, P. J. J. Polym. Sci. (Polym. Lett. Edn) 1975, 13, 365 Hendra, P. J. unpublished results Adams,J. H. J. Polym. Sci. (A-l/, 1970, 8, 1077, 1269, 1279 Birley, A. W. and Brackman, D. S. Proc. Conf. Degradability Polym. Plast. Plast. Inst., November 1973 Bailey,W. J. and Economy, J. 126th Meet. Am. Chem. Soc. Div. Polym. Chem. New York, September 1954 Bailey,W. J., Economy, J. and Hermes, M. E. J Org. Chem. 1962, 27, 3295 Okada, M. and Marvel,C.S. Polym. Prepr. 1967,8,229 Bailey,W. J. and Feinberg, B. D. Polym. Prepr. 1967,8,165 Bailey,W. J. and Volpe, A. A. Polym. Prepr. 1967, 8,292 Ballard,D. G. H. 'Advances in Catalysis' (Eds D. D. Eley, H. Pines and P. B. Weisz), Academic Press, New York and London, 1973, pp 263-325 Hendy, B. N. 'Copolymers, Polyblends and Composites', Advances in Chemistry Series, No. 142, Am. Chem. Soc, Washington D.C., 1975, pp 115-128 Fordyce, R. G. and Chapin, E. C. J. Am. Chem. Soc. 1947, 69, 581 Tanaka, T., Chatani, Y. and Tadokoro, H. J. Polym. Sci. (Polym. Phys. Edn) 1974, 12, 515 Coiro, V. M., De Santis, P., Liquori, A. M. and Ripamonte, A. Ric. Sci. (2A) 1963, 3, 1043 Flory, P. J. Chem. Eng. News, 1974, 36 Rose, J. B. Polymer 1974, 15,464 Tobolsky, A. V. 'Properties and Structure of Polymers', Wiley, New York and London, 1960 Nielsen, L. E. 'Mechanical Properties of Polymers and Composites', Marcel Dekker, New York, 1974, pp 205-207 Small, P. A. J. Appl. Chem. 1953,3,71 van Krevelen, D. W. and Hoftyzer, P. J. 'Properties of Polymers: Correlations with Chemical Structure', Elsevier, 1972, Ch 8, p 140 van Krevelen, D. W. and Hoftyzer, P. J. ibid Ch 5, p 69 Small, P. A. personal communication WiUbourn,A. H. in press