- Email: [email protected]
Polymer interfaces and the molecular basis of adhesion
673
Polymer interfaces and the molecular basis of adhesion Richard AL Jones Interfaces between immiscible polymers are typically rather narrow and weak, but they can be modified
in a variety of ways
to control both the interfacial strength and the morphology blends of immiscible polymers. Early work concentrated use of diblock copolymers, experimental
but recent attention,
and theoretical,
both
has been moving towards
the
use of molecules with different architectures.
Increasing
academic
the industrially
effort is being made to understand
of
on the
important but more complex process of reactive compatibilisation,
where inter-facial modifiers are formed in situ
by reactions at the interface.
Addresses Polymer and Colloid Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, UK; e-mail: [email protected] Current Opinion in Solid State & Materials Science 1997, 2~673-677 Electronic identifier: 1359-0286-002-00673 0 Current Chemistry Ltd ISSN 1359-0286
Introduction The nature of interfaces between immiscible polymers attracts study, both because such interfaces provide attractive model systems for illuminating fundamental general issues of the statistical mechanics of interfaces, and because it is hoped that an understanding of the microscopic structure of polymer interfaces will throw light on technologically important questions relating to the adhesion of polymers and the properties of multiphase polymer blends. In particular, substantial progress has been made in understanding the mechanisms by which interfacially active species, such as copolymers, modify the properties of interfaces, often leading to substantial improvements in adhesion. These studies are important both for providing guiding principles for the design of practically useful interfacial modifying agents, and for helping to elucidate the fundamental mechanisms of adhesion in polymer systems.
Interfaces
between
immiscible
polymers
A theory predicting the degree of interfacial mixing of immiscible polymers was provided by Helfand more than 25 years ago [l]; this theory predicts that the volume fraction profile through the interface has a hyperbolic tangent functional form, with a characteristic width WI related to the Flory-Huggins interaction parameter x and the statistical segment length a, as WI = .Za/d(6X). Reliable experimental measurements of such widths have only been available much more recently; the technique of neutron reflectivity provides the most accurate method. The experimental situation before 1995 was reviewed by Stamm and Schubert [Z].
A notable advance since then has been the measurement of the interfacial width between two of the most common commodity polymers, polystyrene and polyethylene [3”]. The relatively narrow value of the interfacial width obtained, 30 A, explains the well known mechanical incompatibility of polyethylene and polystyrene. However, this width is broader than that predicted by Helfand’s theory, a discrepancy noted in previous work on other polymers. A study of the interfacial width between poly(methy1 methacrylate) and thin films of polystyrene [4”] provides evidence that the origin of the discrepancy is that the mean-field profile predicted by the Helfand theory is broadened by a spectrum of thermally excited capillary waves. The experimental signature of such capillary waves is a logarithmic dependence of the apparent interfacial thickness on the dimensions of the system. An interesting counterpoint to this study is another experiment that found a dependence of the interfacial width on the film thickness, this time between two polymers not too far from their critical point [S”]. Here a square root degendence on system size was found, which again was ascribed to capillary waves, whose spectrum was cut off by an effective potential of short range arising from the truncation of the interfacial profile. Another way of testing the importance of capillary waves is through computer simulations, that is given the availability of enough computer power to simulate large enough systems. It is thus slightly puzzling that recent large Monte Carlo-simulations [6] show only rather small capillary wave broadening, although once again observed profiles are broader than the Helfand prediction. However, very recent molecular dynamics simulations have shown clear evidence of capillary waves (MD Lacasse, GS Grest, personal communication). If it does turn out that a correct accounting for capillary waves does render the Helfand theory quantitatively accurate, one useful consequence will be that the neutron reflectivity measurements of interfacial widths will provide one of the most practical and accurate methods of measuring the interaction parameter between highly immiscible polymers of high molecular weight. Theoretical developments have included the application of lattice fluid theory to interfaces [7], allowing a consistent treatment of compressibility effects, and a square gradient approach which allows the effects of finite chain length to be accounted for [8]. The effect of finite chain length on interfacial widths has been studied experimentally, the observed increase in width for decreasing molecular weight being in at least semi-quantitative agreement with theory [9]. On intuitive grounds it seems natural to assume that there is a direct relationship between the strength of an interface and its width, but up to now it has not been possible to
674
Polymers
make a very convincing test of this. Schnell et a/. (R Schnell, M Stamm, C Creton, personal communication) have been able to establish such a correlation by measuring both the width and toughness of interfaces between polystyrene and poly(paramethylstyrene) as a function of molecular weight and temperature. A strong correlation existed between the interfacial toughness and the interfacial width, with the toughness increasing markedly as the width increased from 9 nm to 12 nm, with no evidence for any further increase of toughness for interfacial widths larger than 1’2nm.
Modification
of immiscible polymer interfaces
The relatively small value of interfacial width between most immiscible polymers means that much effort has been devoted to finding ways to modify such interfaces to toughen them. A variety of strategies are employed to achieve this end, which is often referred to as compatibilisation; for a recent review see Ajji and Utracki [lo]. In all cases the aim is to arrange for some polymeric additive to be localised at the interface, with each such polymer chain crossing the interface at least once. This can be done by the addition of diblock copolymers, which act as polymeric surfactants, or the addition of copolymers with more complex architectures, such as triblock, comb, graft or random copolymers. Alternatively, systems can be designed in which the grafting reaction takes place in situ at the interface. Modification of interfaces in this way has three effects: reduction of the interfacial tension, sterically stabilising droplets against coalescence and increasing the toughness of the interface by promoting entanglements between the phases. Classically, any material which adsorbs at an interface must reduce the interfacial tension, and this may have beneficial effects on the morphology of a multiphase polymer. Directly measuring interfacial tensions between immiscible polymers is rather difficult, and can generally be done only for rather low molecular weight polymers, but for these substantial reductions are seen, the size of which depends on the degree of asymmetry of the diblock used [ 1 I]. Given these experimental difficulties theory has an especially important role to play in finding copolymer architectures which optimise the reduction in interfacial tension, as well as in predicting the conformation of the copolymers at the interface. In one such study self-consistent field methods were used to compare the reduction in interfacial tension predicted for random, alternating and diblock copolymers with star and comb copolymers [12”]. Another study concentrated on the effect of sequence distribution in linear homopolymers [ 131, and a third concentrated on determining the conformation of random copolymers at interfaces [ 141. Even simpler molecules than random copolymers can be interfacially active: in a blend of three homopolymers, at least two of which are immiscible, one component can usually be expected to segregate at the interface between the other two [l&16]. Monte Carlo methods were used to find the phase behaviour and interfacial structure of mixtures of
two immiscible homopolymers and a diblock, concentrating on situations where the interfacial amount of copolymer was rather small [17]. Another, more extensive, study using the same bond-fluctuation model was used examine both bulk and interfacial thermodynamics of polymer/polymer/ diblock blends [18]. This kind of work is extremely computationally expensive, but it is highly valuable in testing the more easily implemented self-consistent field methods. The effect of additives on the structure of the interface can be studied using the same techniques as are used for the homopolymer interface, in particular neutron reflection. This technique has been used to study block copolymers at rather weakly incompatible interfaces [19], while the effect of random copolymers on the interfacial width between polstyrene and poly(methy1 methacrylate) was studied [‘ZO”] in conjunction with a study of interfacial toughness. This revealed that optimum toughening was achieved in a situation when the interface between the polystyrene and the interfacial copolymer layer was broadened to the same degree as the interface between copolymer and poly(methy1 methacrylate). The microstructure of interfaces oversaturated with block copolymer typically display lamellar morphologies, which can be imaged with electron microscopy or by frictional force microscopy [Zl]. One striking effect of a reduction in interfacial tension will be a change in the morphology of a two-phase blend produced, for example, by melt-mixing immiscible components. The addition of diblock copolymers results in a reduction in phase particle size, which can be studied as a function of the molecular weight and architecture of the copolymers [Z’]. Estimates of the amount of copolymer required to saturate an interface can be made on geometrical considerations [23], in analogy to the methods used for small molecule surfactants. However, it has recently become apparent that substantial changes in morphology can occur even when the interface is far from saturated, and the reduction in surface tension is very small. Experiments have established that at least in one system the mechanism by which dispersion sizes is controlled is steric stabilisation, and not reduction in interfacial tension. Polystyrene and poly(methyl methacrylate) were melt blended in the presence and absence of a diblock copolymer and the morphology studied by light scattering and electron microscopy [24”,25”]; these studies established that the amounts of block copolymer required to modify the morphology were substantially less than that which would cause significant reduction in interfacial tension, suggesting that in this case the major role of the block copolymer is in inhibiting coalescence. This conclusion has received some theoretical support [26”].
The effect of copolymer additives on adhesion between immiscible polymers A more specific polymer effect occurs if the additive chains are sufficiently long such that each chain is well-
Polymer interfaces and the molecular basis of adhesion Jones
entangled on both sides of the interface. In this case the additive chains can act as mechanical connectors, greatly increasing the fracture toughness of the interface. Careful experiments, mainly on block copolymer reinforced interfaces, have contributed greatly to our understanding of this aspect [27,28]. Further experiments have studied the effects of architecture, comparing triblocks with corresponding diblocks [29]. Another study found that ABC triblocks with a rubbery polymer as the midblock B section did not reinforce an interface between two glassy homopolymers A and C [30]. Two studies have improved our understanding of the origin of the surprisingly efficient toughening effect of random copolymers; one, already mentioned [ZO”], directly correlated toughness measurements of random copolymer reinforced interfaces with neutron reflectivity measurements of interfacial width, and another [31’] used TEM to show that the copolymer effectively formed a separate phase at the interface. The importance of crazing, in which fibrils of polymer are drawn across the interface, as a dissipation mechanism in tough interfaces was underlined by a study of the effect of the molecular weight of the homopolymers on the toughness of a diblock reinforced interface [32’]. Purely theoretical work is still at rather an early stage. The problem can be considered in two parts; firstly the continuum mechanics of interfacial failure given some kind of constitutive model describing in a coarse-grained way the local motion of a reinforced interface, and secondly the derivation from molecular concepts of such a model. Continuum theories have been developed by Kogan et al. [33,34]; from a more molecular point of view Brownian dynamics and seem to offer a promising way of developing constitutive models and assessing optimum architectures for reinforcement [35”].
Reactive compatibilisation polymers
675
Theoretical studies of the kinetics of reactions at interfaces show that for long polymers the kinetics become very slow when the coverage achieves some low critical value [39”,40”,41]. This severely limits the coverage of graft copolymer that can be achieved, and thus means that long graft copolymers are unlikely to be formed in large enough quantities to significantly reduce the interfacial tension. This supports the conclusion mentioned above in the context of diblock copolymers, that the prevention of coalescence by steric stabilisation is at least as important a mechanism for the improved blend dispersions caused by compatibilisers, whether reactive or added block copolymers. There have now been some quantitative measurements of the effect of reactive compatibilisation on interfacial toughness. An example is a study of interfaces between poly(Z-vinyl pyridine) and a polystyrene which was lightly sulfonated, producing an acid functionality that reacts with the basic pyridine nitrogen. Increase of interfacial toughness by two orders of magnitude were observed [42’]. The same group studied both morphology and interfacial fracture energy for blends of amorphous polyamide with a polystyrene co-polymerised with vinyl oxazoline, a group which could react with the polyamide end-groups. The morphology was more dispersed, and the fracture energy showed (relatively small) increases [43]. In another study the interface between high-impact polystyrene and epoxy was modified by carboxylic-terminated polystyrene, which grafted to the epoxy network [44’]. A detailed picture of the relation between grafted chain length, grafting density and the fracture toughness could be built-up, allowing detailed conclusions about the mechanisms of toughening to be drawn in a similar way to the work by the same group on diblocks [28].
of immiscible
While most academic work on elucidating the mechanisms of reinforcement of interfaces has concentrated on the effect of added copolymers, industrial practise has been dominated by the use of in situ interfacial grafting reactions, either by functionalising one of the components of a two component blend, or by adding some third component that can form interfacially active graft copolymers at the interface. Both strategies are referred to as reactive compatibilization. Most studies of reactive compatibilisation over the years have concentrated on the effects on morphology and phase dispersion; some recently studied systems include blends of poly(butylene terephthalate) (PBT) and polypropylene with the addition of ethyleneco-glycidyl methacrylate copolymer, which forms interfacial grafts via reaction of the end-groups of the PBT with epoxy groups in the copolymer [36]. The same compatibiliser was used for blends of poly(ethylene-ran-acrylic acid) and polystyrene and poly(butylene terephthalate) and polstyrene [37]. Maleic anhydride functionalised styrene/ethylene-co-butylene/styrene triblock compatibilises blends of polyamide-6 and polycarbonate [38].
Finally, Boucher et al. [45”] have studied reactively compatibilised crystalline polymers. The system comprised a blend of crystalline polypropylene with terminally attached succinic acid groups and polyamide-6. The interfacial fracture energy varied as the square of the density of copolymers at the interface, as determined by X-ray photoelectron spectroscopy [45”]. Later the same group found that by annealing a similar system very close to the melting point even higher increases in fracture toughness were observed, associated with the formation of the Bphase of polypropylene near the interface [46].
Conclusions Theories of the microscopic structure of interfaces between immiscible polymers now seem to be in good shape. Such theories, combined with computer simulation, are leading to the possibility of designing optimal compatibilisers for modifying interfaces. Academic science has made a welcome move towards industrial practise, by turning attention to the important but complicated problems of reactive compatibilisation.
676
Polymers
References and recommended
reading
18.
Miiller M, Schick M: Bulk and interfacial thermodynamics of a symmetric, ternary homopolymer-copolymer mixture: a Monte Carlo study. J Chem Phys 1996, 105:8885-8901.
19.
Schnell R, Stamm M: The self-organisation of diblock copolymers at polymer blend interfaces. fhysica B 1997, 234:247-249.
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of special interest * of outstanding interest
1.
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2.
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immiscible
3. ..
Hermes HE, Higgins JS, Bucknall DG: Investigation of the melt interface between polyethylene and polystyrene using neutron reflectivity. Polymer 1997, 38:985-989. The interfacial width between two of the most important commodity polymers was measured by neutron reflectivity. The resulting small value explains the extreme incompatibility of these two materials. 4. ..
Sferrazza M, Xiao C, Jones RAL, Penfold J, Webster J: Evidence for capillary waves at immiscible polymer/polymer interfaces. Phys Rev Leti 1997,78:3693-3696. The interfacial width between two immiscible polymers, one present as a thin film, was measured by neutron reflectivity, and showed a logarithmic dependence on film thickness. This provides strong evidence that the origin of the discrepancy between experimental and theoretical values for the interfacial width is in thermally excited capillary waves.
Kulasekere R, Kaiser H, Ankner JF, Russell TP, Brown HR, Hawker CJ, Mayes AM: Homopolymer interfaces reinforced with random copolymers. Macromolecules 1996, 29:5493-5496. . MechanIcal aclheslon tests were combined with neutron retlecttvtty studies to provide a microscopic insight into the origin of the strengthening properties of random copolymers, and to rationalise the observed maximum in interfacial strength with copolymer composition. 20. ..
21.
22. .
Cigana P, Favis BD, Jerome R: Diblock copolymers as emulsifying agents in polymer blends: influence of molecular weight, architecture, and chemical composition. I Polym Sci 8 1996, 34:1691-1700. A systematic study of the effect of added diblock copolymers of different molecular weights and asymmetries and at different concentrations on the morphology of an immiscible polymer blend. 23.
5.
Kerle T, Klein J, Binder K: Evidence for size effects on interfacial widths in confined thin films. Phys Rev Left 1996, 77:1318-l 321. The interfacial width between weakly immiscible polymers is measured, and shown to vary with the square root of the film thickness, indicating an effective short range force acting on the interface. 6.
Binder K, Mijller M: Interfaces between coexisting phases of polymer mixtures: comparison between Monte Carlo simulations and theoretical predictions. Macromolecular Symposia 1997, 113:207-220.
7.
Ypma GJA, Cifra P, Nies E, Bergen ARDV: Interfacial behaviour of compressible polymer blends. Monte Carlo simulation and the lattice fluid theory. Macromolecules 1996, 29:1252-l 259.
8.
Ermoshkin AV, Semenov AN: Interfacial tension in binary polymer mixtures. Macromolecules 1996, 29:6294-6300.
9.
Schubert DW, Stamm M: Influence of chain length of the interface width of an incompatible blend. for Leff 1996, 35:419-424.
10.
Ajji A, Utracki LA: Interphase and compatibilisation blends. fo/yrn Eng Sci 1996, 36:1574-i 585.
11.
Jo WH, Nam KH, Cho JC: Effects of the molecular structure of styrene-isoprene block copolymers on the interfacial characteristics of polystyrene/polyisoprene blends. J folym Sci B 1996, 34:2169-2175.
of polymer
12. ..
Lyatskaya Y, Gersappe D, Gross NA, Balazs AC: Designing compatibilisers to reduce interfacial tension in polymer blends. J Phys Chem 1996, 100:1449-1458. Self-consistent field theory is used to evaluate the relative efficiencies of different copolymer architectures in reducing interfacial tension, illustrating how a first-principles approach to designing optimal compatibilisers might work. 13.
Dadmun M: Effect of copolymer architecture on the interfacial structure and miscibility of a ternary polymer blend containing a copolymer and two homopolymers. Macromolecules 1996, 29:3868-3874.
14.
Smith GD, Russell TP, Kulasekere R, Ankner JF, Kaiser H: A Monte Carlo simulation of asymmetric random copolymers at an immiscible interface. Macromolecules 1996, 29:4120-4124.
15.
Genzer J, Faldi A, Composto RJ: Mean-field theory of the interface between a homopolymer and a binary polymer mixture. J Chem Phys 1996, 105:10134-10144.
16.
He DQ, Kwak S, Nauman EB: On phase equilibria, interfacial tension and phase growth in ternary polymer blends. Macromol Theory Simu/l996,5:801-827.
1 7.
Werner A, Schmid F, Binder K, Muller M: Diblock copolymers homopolymer-homopolymer interface - a Monte-Carlo simulation. Macromolecules 1996. 29:8241-8248.
at a
Jandt KD, Dai C-A, Kramer EJ: Microstructure of block copolymer reinforced interfaces observed with frictional force microscopy. Adv Mater 1996, 8:660-682.
Lomellini P, Matos M, Favis BD: Interfacial modification of polymer blends - the emulsification curve: 2. Predicting the critical concentration of interfacial modifier from geometrical considerations. Polymer 1996, 375689-5694.
24. ..
Macosko CW, Guegan P, Khandpur AK: Compatibilisers for melt blending; pre-made block copolymers. Macromolecules 1996, 29:5590-5598. An experimental study of the role of diblock copolymers in modifying the way morphology evolves during the melt blending of poly(methyl methactylate) and polystyrene, focusing on the role of diblocks in modifying the coalescence process by imparting steric stabilisation to the dispersed phase droplets. Reference [25”] reports work with similar conclusions. 25. ..
Snedergaard K, Lyngaae-Jorgenson J: Coalescence in an interfacemodified polymer blend as studied by light scattering measurements. Polymer 1996, 37:509-517. The coalescence of particles in dispersions of poly(methyl methactylate) in polystyrene under shear flow was studied in the presence of diblock copolymers. As in [24”], the authors conclude that the major role of the diblocks is to inhibit coalescence by sterically stabilising the dispersed phase droplets, rather than in lowering the inter-facial tension. 26. ..
Milner ST, Xi H: How copolymers promote mixing of immiscible materials. J Rheol 1996,40:663-687. A theoretical rationale is developed for the view, expressed on the basis of experiments in references [24”] and [25”], that the major role of diblocks in modifying the morphology of immiscible blends is in inhibiting domain coalescence by steric stabilisation rather than lowering interfacial energy. 27.
Brown HR: The adhesion 1991, 21:463-489.
between
polymers. Annu Rev Mafer Sci
28.
Kramer, EJ, Norton, LJ, Dai, C-A., Sha, Y, Hui, C.-Y. : Strengthening polymer interfaces. Faraday Discuss 1994, 98:31-46.
29.
Dai C-A, Jandt KD, lyengar DR, Slack NL, Dai KH, Davidson WB, Kramer EJ: Strengthening polymer interfaces with triblock copolymers. Macromolecules 1996, 30:549-560.
30.
Brown HR, Krappe U, Stadler R: Effect of ABC triblock copolymers with an elastomeric midblock on the adhesion between immiscible polymers. Macromolecules 1996, 29:6582-6588.
31. .
Sikka M, Pellegrini NN, Schmitt EA, Winey KI: Modifying a polystyrene/poly(methyl methacrylate) interface with poly (styrene-co-methyl methacrylate) random copolymers. Macromolecules 1997, 30~445.455. Mechanical measurements of the reinforcing action of random copolymers are combined with transmission electron microscopy, which shows that the random copolymer forms a separate phase at the interface. 32. .
Dai C-A, Kramer EJ, Washiyama J, Hui C: Fracture toughness of polymer interface reinforced with diblock copolymer: effect of homopolymer molecular weight Macromolecules 1996, 29:75367543. Insight into the mechanisms of energy dissipation in copolymer reinforced interfaces is afforded by studying the dependence of interfacial strength on the entanglement properties of one of the homopolymers involved in the interface.
Polymer interfaces and the molecular basis of adhesion Jones
677
Kogan L, Hui C-Y, Ruina A: Theory of chain pull-out and stability of 1996, 29:4 101-4 106.
at an interface to form a diblock copolymer, concentrating on the regime when the reaction is controlled by diffusion of the entangled species.
Kogan L, Hui C-Y, Ruina A: Theory of chain pull-out and stability of weak interfaces. 1. Macromolecules 1996, 29:4090-4100.
41.
Pickett GT, Jasnow D, Balszs AC: Simulation of fracturing reinforced polymer blends. Phys Rev Lett 1996, 77:671-674. . . . . . .. . Browman dynamics slmulatlons were used to mvestlgate molecular mechanisms of energy dissipation in copolymer reinforced interfaces; the results are that a chain that crosses the interface many times is much more efficient at strengthening an interface than one that crosses just once, such as a diblock copolymer.
42. .
33
weak interfaces. 2. Macromolecules 34 35. ”
36.
Tsai CH, Chang FC: Polymer blends of PBT and PP compatibilised by ethylene-co-glycidyl methaclylate copolymers. J Appl Po/ym Sci 1996,61:321-332.
37.
Kim JK, Kim S, Park CE: Compatibllization mechanism of polymer blends with an in-situ compatibillzer. Polymer 1997, 36:2155-
38.
Horiuchi S, Matchariyaku, N, Yase K, Kitano T, Cho HK, Lee YM: Compatlbilising effect of a maleic anhydridge functionalised SEBS trlblock elastomer through a reaction induced phase
21 64.
formation In the blends of polyamide-6 and polycarbonate. 1. Morphology and interfacial situation. fo/ymer 1996,37:3065-3078.
Tan NCB, Pfeiffer DG, Briber RM: Reactive reinforcement of polystyrene/poly(2+inyl pyridine) interfaces. Macromolecules 1996, 29:4989-4975. The effect of reactive compatibilisation at an interface on the adhesive properties of an interface was investigated by direct mechanical tests. 43:
40. ke
Fredrickson GH: Diffusion controlled reactions at polymerpolymer interfaces. Phys Rev Lett 1996,76:3440-3443. authors calculate rate constants for the coupling of two long polymers
Tan NCB, Tai S-K, Bribe, RM: Morphology control and interfacial reinforcement in reactive polystyrene/amorphous polyamide blends. Polymer 1996,37:3509-3519.
44.
Sha Y. Hui CY. Kramer EJ, Hahn SF, Beralund CA: Fracture toughness and failure mechanisms of-epoxy/rubber-modified polystyrene (HIPS) interfaces reinforced by grafted chains. Ma&o~olecules 1996, 29~4728-4736. The adhesion at an interface between high-impact polystyrene and epoxy was measured as a function of the amount of grafting of carboxylicterminated polystyrene to the epoxy network, allowing a detailed picture of the relation between grafted chain length, grafting density and the fracture toughness to be built up.
.
45. O’Shaughnessy B, Sawhney U: Polymer reaction kinetics at interfaces. Phys Rev Leit 1996, 76~3444-3447. The authors calculate the expected reaction rate for the grafting of long polymers at polymer/polymer interfaces. They find that high degrees of grafting are practically unattainable, because the grafting reaction becomes exponentially slow when the grafted chains start to strongly overlap. A corollary of this is that in practise large decreases in interfacial tension are unlikely to be attainable in practise, and steric stabilisation is likely to be responsible for most of the modifications of morphology obtainable by reactive compatibilisation. 39. ”
Fredrickson GH, Milner ST: Time-dependent reactive coupling at polymer-polymer interfaces. Macromolecules 1996, 29:7386-7390.
u
Boucher E, Folkers JP, Hewet H, Uger L, Creton C: Effects of the formation of copolymer on the interfacial adhesion between semicrystalline polymers. Macromolecules 1996, 29~774.782.
A direct relationship was found between the amount of diblock copolymer formed in situ, by grafting at an interface between polypropylene and polyamide-8, and the fracture toughness of the interface. This relationship, quadratic in the areal density of copolymer, is the same as found previously for the toughness of interfaces between glassy polymers modified by added diblocks. 46.
Boucher E, Folkers JF’, Creton C, Hervet H, Lbger L: Enhanced adhesion between polypropylene and polyamide-6: role of interfacial nucleation of the beta-crystalline form of polypropylene. Macromolecules 1997,30:2102-2109.
Polymer interfaces and the molecular basis of adhesion Richard AL Jones Interfaces between immiscible polymers are typically rather narrow and weak, but they can be modified
in a variety of ways
to control both the interfacial strength and the morphology blends of immiscible polymers. Early work concentrated use of diblock copolymers, experimental
but recent attention,
and theoretical,
both
has been moving towards
the
use of molecules with different architectures.
Increasing
academic
the industrially
effort is being made to understand
of
on the
important but more complex process of reactive compatibilisation,
where inter-facial modifiers are formed in situ
by reactions at the interface.
Addresses Polymer and Colloid Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, UK; e-mail: [email protected] Current Opinion in Solid State & Materials Science 1997, 2~673-677 Electronic identifier: 1359-0286-002-00673 0 Current Chemistry Ltd ISSN 1359-0286
Introduction The nature of interfaces between immiscible polymers attracts study, both because such interfaces provide attractive model systems for illuminating fundamental general issues of the statistical mechanics of interfaces, and because it is hoped that an understanding of the microscopic structure of polymer interfaces will throw light on technologically important questions relating to the adhesion of polymers and the properties of multiphase polymer blends. In particular, substantial progress has been made in understanding the mechanisms by which interfacially active species, such as copolymers, modify the properties of interfaces, often leading to substantial improvements in adhesion. These studies are important both for providing guiding principles for the design of practically useful interfacial modifying agents, and for helping to elucidate the fundamental mechanisms of adhesion in polymer systems.
Interfaces
between
immiscible
polymers
A theory predicting the degree of interfacial mixing of immiscible polymers was provided by Helfand more than 25 years ago [l]; this theory predicts that the volume fraction profile through the interface has a hyperbolic tangent functional form, with a characteristic width WI related to the Flory-Huggins interaction parameter x and the statistical segment length a, as WI = .Za/d(6X). Reliable experimental measurements of such widths have only been available much more recently; the technique of neutron reflectivity provides the most accurate method. The experimental situation before 1995 was reviewed by Stamm and Schubert [Z].
A notable advance since then has been the measurement of the interfacial width between two of the most common commodity polymers, polystyrene and polyethylene [3”]. The relatively narrow value of the interfacial width obtained, 30 A, explains the well known mechanical incompatibility of polyethylene and polystyrene. However, this width is broader than that predicted by Helfand’s theory, a discrepancy noted in previous work on other polymers. A study of the interfacial width between poly(methy1 methacrylate) and thin films of polystyrene [4”] provides evidence that the origin of the discrepancy is that the mean-field profile predicted by the Helfand theory is broadened by a spectrum of thermally excited capillary waves. The experimental signature of such capillary waves is a logarithmic dependence of the apparent interfacial thickness on the dimensions of the system. An interesting counterpoint to this study is another experiment that found a dependence of the interfacial width on the film thickness, this time between two polymers not too far from their critical point [S”]. Here a square root degendence on system size was found, which again was ascribed to capillary waves, whose spectrum was cut off by an effective potential of short range arising from the truncation of the interfacial profile. Another way of testing the importance of capillary waves is through computer simulations, that is given the availability of enough computer power to simulate large enough systems. It is thus slightly puzzling that recent large Monte Carlo-simulations [6] show only rather small capillary wave broadening, although once again observed profiles are broader than the Helfand prediction. However, very recent molecular dynamics simulations have shown clear evidence of capillary waves (MD Lacasse, GS Grest, personal communication). If it does turn out that a correct accounting for capillary waves does render the Helfand theory quantitatively accurate, one useful consequence will be that the neutron reflectivity measurements of interfacial widths will provide one of the most practical and accurate methods of measuring the interaction parameter between highly immiscible polymers of high molecular weight. Theoretical developments have included the application of lattice fluid theory to interfaces [7], allowing a consistent treatment of compressibility effects, and a square gradient approach which allows the effects of finite chain length to be accounted for [8]. The effect of finite chain length on interfacial widths has been studied experimentally, the observed increase in width for decreasing molecular weight being in at least semi-quantitative agreement with theory [9]. On intuitive grounds it seems natural to assume that there is a direct relationship between the strength of an interface and its width, but up to now it has not been possible to
674
Polymers
make a very convincing test of this. Schnell et a/. (R Schnell, M Stamm, C Creton, personal communication) have been able to establish such a correlation by measuring both the width and toughness of interfaces between polystyrene and poly(paramethylstyrene) as a function of molecular weight and temperature. A strong correlation existed between the interfacial toughness and the interfacial width, with the toughness increasing markedly as the width increased from 9 nm to 12 nm, with no evidence for any further increase of toughness for interfacial widths larger than 1’2nm.
Modification
of immiscible polymer interfaces
The relatively small value of interfacial width between most immiscible polymers means that much effort has been devoted to finding ways to modify such interfaces to toughen them. A variety of strategies are employed to achieve this end, which is often referred to as compatibilisation; for a recent review see Ajji and Utracki [lo]. In all cases the aim is to arrange for some polymeric additive to be localised at the interface, with each such polymer chain crossing the interface at least once. This can be done by the addition of diblock copolymers, which act as polymeric surfactants, or the addition of copolymers with more complex architectures, such as triblock, comb, graft or random copolymers. Alternatively, systems can be designed in which the grafting reaction takes place in situ at the interface. Modification of interfaces in this way has three effects: reduction of the interfacial tension, sterically stabilising droplets against coalescence and increasing the toughness of the interface by promoting entanglements between the phases. Classically, any material which adsorbs at an interface must reduce the interfacial tension, and this may have beneficial effects on the morphology of a multiphase polymer. Directly measuring interfacial tensions between immiscible polymers is rather difficult, and can generally be done only for rather low molecular weight polymers, but for these substantial reductions are seen, the size of which depends on the degree of asymmetry of the diblock used [ 1 I]. Given these experimental difficulties theory has an especially important role to play in finding copolymer architectures which optimise the reduction in interfacial tension, as well as in predicting the conformation of the copolymers at the interface. In one such study self-consistent field methods were used to compare the reduction in interfacial tension predicted for random, alternating and diblock copolymers with star and comb copolymers [12”]. Another study concentrated on the effect of sequence distribution in linear homopolymers [ 131, and a third concentrated on determining the conformation of random copolymers at interfaces [ 141. Even simpler molecules than random copolymers can be interfacially active: in a blend of three homopolymers, at least two of which are immiscible, one component can usually be expected to segregate at the interface between the other two [l&16]. Monte Carlo methods were used to find the phase behaviour and interfacial structure of mixtures of
two immiscible homopolymers and a diblock, concentrating on situations where the interfacial amount of copolymer was rather small [17]. Another, more extensive, study using the same bond-fluctuation model was used examine both bulk and interfacial thermodynamics of polymer/polymer/ diblock blends [18]. This kind of work is extremely computationally expensive, but it is highly valuable in testing the more easily implemented self-consistent field methods. The effect of additives on the structure of the interface can be studied using the same techniques as are used for the homopolymer interface, in particular neutron reflection. This technique has been used to study block copolymers at rather weakly incompatible interfaces [19], while the effect of random copolymers on the interfacial width between polstyrene and poly(methy1 methacrylate) was studied [‘ZO”] in conjunction with a study of interfacial toughness. This revealed that optimum toughening was achieved in a situation when the interface between the polystyrene and the interfacial copolymer layer was broadened to the same degree as the interface between copolymer and poly(methy1 methacrylate). The microstructure of interfaces oversaturated with block copolymer typically display lamellar morphologies, which can be imaged with electron microscopy or by frictional force microscopy [Zl]. One striking effect of a reduction in interfacial tension will be a change in the morphology of a two-phase blend produced, for example, by melt-mixing immiscible components. The addition of diblock copolymers results in a reduction in phase particle size, which can be studied as a function of the molecular weight and architecture of the copolymers [Z’]. Estimates of the amount of copolymer required to saturate an interface can be made on geometrical considerations [23], in analogy to the methods used for small molecule surfactants. However, it has recently become apparent that substantial changes in morphology can occur even when the interface is far from saturated, and the reduction in surface tension is very small. Experiments have established that at least in one system the mechanism by which dispersion sizes is controlled is steric stabilisation, and not reduction in interfacial tension. Polystyrene and poly(methyl methacrylate) were melt blended in the presence and absence of a diblock copolymer and the morphology studied by light scattering and electron microscopy [24”,25”]; these studies established that the amounts of block copolymer required to modify the morphology were substantially less than that which would cause significant reduction in interfacial tension, suggesting that in this case the major role of the block copolymer is in inhibiting coalescence. This conclusion has received some theoretical support [26”].
The effect of copolymer additives on adhesion between immiscible polymers A more specific polymer effect occurs if the additive chains are sufficiently long such that each chain is well-
Polymer interfaces and the molecular basis of adhesion Jones
entangled on both sides of the interface. In this case the additive chains can act as mechanical connectors, greatly increasing the fracture toughness of the interface. Careful experiments, mainly on block copolymer reinforced interfaces, have contributed greatly to our understanding of this aspect [27,28]. Further experiments have studied the effects of architecture, comparing triblocks with corresponding diblocks [29]. Another study found that ABC triblocks with a rubbery polymer as the midblock B section did not reinforce an interface between two glassy homopolymers A and C [30]. Two studies have improved our understanding of the origin of the surprisingly efficient toughening effect of random copolymers; one, already mentioned [ZO”], directly correlated toughness measurements of random copolymer reinforced interfaces with neutron reflectivity measurements of interfacial width, and another [31’] used TEM to show that the copolymer effectively formed a separate phase at the interface. The importance of crazing, in which fibrils of polymer are drawn across the interface, as a dissipation mechanism in tough interfaces was underlined by a study of the effect of the molecular weight of the homopolymers on the toughness of a diblock reinforced interface [32’]. Purely theoretical work is still at rather an early stage. The problem can be considered in two parts; firstly the continuum mechanics of interfacial failure given some kind of constitutive model describing in a coarse-grained way the local motion of a reinforced interface, and secondly the derivation from molecular concepts of such a model. Continuum theories have been developed by Kogan et al. [33,34]; from a more molecular point of view Brownian dynamics and seem to offer a promising way of developing constitutive models and assessing optimum architectures for reinforcement [35”].
Reactive compatibilisation polymers
675
Theoretical studies of the kinetics of reactions at interfaces show that for long polymers the kinetics become very slow when the coverage achieves some low critical value [39”,40”,41]. This severely limits the coverage of graft copolymer that can be achieved, and thus means that long graft copolymers are unlikely to be formed in large enough quantities to significantly reduce the interfacial tension. This supports the conclusion mentioned above in the context of diblock copolymers, that the prevention of coalescence by steric stabilisation is at least as important a mechanism for the improved blend dispersions caused by compatibilisers, whether reactive or added block copolymers. There have now been some quantitative measurements of the effect of reactive compatibilisation on interfacial toughness. An example is a study of interfaces between poly(Z-vinyl pyridine) and a polystyrene which was lightly sulfonated, producing an acid functionality that reacts with the basic pyridine nitrogen. Increase of interfacial toughness by two orders of magnitude were observed [42’]. The same group studied both morphology and interfacial fracture energy for blends of amorphous polyamide with a polystyrene co-polymerised with vinyl oxazoline, a group which could react with the polyamide end-groups. The morphology was more dispersed, and the fracture energy showed (relatively small) increases [43]. In another study the interface between high-impact polystyrene and epoxy was modified by carboxylic-terminated polystyrene, which grafted to the epoxy network [44’]. A detailed picture of the relation between grafted chain length, grafting density and the fracture toughness could be built-up, allowing detailed conclusions about the mechanisms of toughening to be drawn in a similar way to the work by the same group on diblocks [28].
of immiscible
While most academic work on elucidating the mechanisms of reinforcement of interfaces has concentrated on the effect of added copolymers, industrial practise has been dominated by the use of in situ interfacial grafting reactions, either by functionalising one of the components of a two component blend, or by adding some third component that can form interfacially active graft copolymers at the interface. Both strategies are referred to as reactive compatibilization. Most studies of reactive compatibilisation over the years have concentrated on the effects on morphology and phase dispersion; some recently studied systems include blends of poly(butylene terephthalate) (PBT) and polypropylene with the addition of ethyleneco-glycidyl methacrylate copolymer, which forms interfacial grafts via reaction of the end-groups of the PBT with epoxy groups in the copolymer [36]. The same compatibiliser was used for blends of poly(ethylene-ran-acrylic acid) and polystyrene and poly(butylene terephthalate) and polstyrene [37]. Maleic anhydride functionalised styrene/ethylene-co-butylene/styrene triblock compatibilises blends of polyamide-6 and polycarbonate [38].
Finally, Boucher et al. [45”] have studied reactively compatibilised crystalline polymers. The system comprised a blend of crystalline polypropylene with terminally attached succinic acid groups and polyamide-6. The interfacial fracture energy varied as the square of the density of copolymers at the interface, as determined by X-ray photoelectron spectroscopy [45”]. Later the same group found that by annealing a similar system very close to the melting point even higher increases in fracture toughness were observed, associated with the formation of the Bphase of polypropylene near the interface [46].
Conclusions Theories of the microscopic structure of interfaces between immiscible polymers now seem to be in good shape. Such theories, combined with computer simulation, are leading to the possibility of designing optimal compatibilisers for modifying interfaces. Academic science has made a welcome move towards industrial practise, by turning attention to the important but complicated problems of reactive compatibilisation.
676
Polymers
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