Δ, and. where and indicate the change in enthalpy and entropy on φ ρ M are the

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1 1.1. Polymer Blends Polymer blends are physical mixtures of two or more polymers or copolymers. The blending of polymers provides a powerful route for obtaining materials with improved property/cost performances. A proper selection and combination of polymeric components in a precise ratio can result in blend with optimal properties for a specific application. The overall consumption of polymer blends ranges more than 20% among the engineering polymers owing to their wide spectrum of application in automotive, electrical, electronic, housing and medical fields. This in turn led to increased interest in the science and technology of polymer blends. The market of polymer blend based materials has increased continuously during the fast few decades and is expected to increase about 8-10% in the present decade [1]. An additional advantage of polymer blends is that a wide range of material properties is within reach by merely changing the blend composition [2-6]. Various methods are available for polymer blending which include mechanical as well as chemical methods [7]. The level of homogeneity obtained depends on the nature of the components to be blended and the blending technique employed. Different methods for blending are mechanical mixing, dissolution in co-solvent then film casting, freeze or spray drying, latex blending, fine powder mixing, use of monomer as solvent for the other component of the blend and then polymerising as in IPNs or HIPs manufacture etc. Mechanical mixing is preferred over the other methods owing to economic reasons.

2 2 Chapter Polymer Miscibility Polymer blends are either homogeneous or heterogeneous. The complete miscibility of polymers that can lead to homogeneous blends should require a negative free energy of mixing (ΔG m ). ΔG m = ΔH m TΔS m 0 (1.1) Δ = + 2B > 0 (1.2) 2 G m ρa ρ B RT 2 φi φ TP, AMA φbmb Δ H m Δ S m, and where and indicate the change in enthalpy and entropy on φ ρ M are the mixing respectively, T is the absolute temperature and i i i volume fraction, density and molecular weight of component i, respectively Due to the high molecular weight of polymers, the gain in entropy during mixing is negligible. Therefore a negative free energy of mixing requires negative heat of mixing which can only result from an exothermic mixing. This in turn suggests that a miscible polymer blend can result only if there exists specific interaction between the polymers. Usually, only van der Waals interactions occur during mixing, which explains why the number of miscible polymer blends are very less. Hence most of the polymers are immiscible between each other due to a positive ΔG m, forming heterogeneous blends. Miscible blends generally produce average properties between the two parent polymers. On the other hand, an immiscible blend can provide synergistic property improvements which exceed either of is components. The variation of free energy change with composition in binary polymer mixture can be schematically represented as in Figure 1.1.

3 Introduction 3 Change in Free energy 0 A C B A B C 0 1 Volume fraction Figure 1.1: Schematic representation for the variation of free energy change with composition in binary polymer mixtures [6] The curve A represents a case where ΔG m is positive and therefore the components are immiscible in the whole compositional range producing heterogeneous blends. Curve B represents a condition where ΔG m is negative and hence the polymer components form a completely miscible polymer pair in spite of a compositional variation leading to homogeneous blends. The system indicated by the curve C is partially miscible. In this case C, the mixture can develop an even lower free energy by splitting into two phases with compositions given by the two minima resulting in a miscibility gap or partial miscibility. Thus a second thermodynamic requirement (equation (1.2)) is applicable in the case of partially miscible systems according to which the second derivative of ΔG m with respect to the volume fraction of one of the components at constant temperature and pressure should be greater than zero to ensure stability against phase separation since in these systems the composition fluctuation may lead to phase separation.

4 4 Chapter 1 Based on the classical Flory-Huggins theory for polymer solutions, the free energy of mixing of two polymers A and B can be expressed as ρ AφAlnφA ρbφb lnφ B Δ Gm = Bφφ A B + RT + M A M (1.3) B where, φ, ρ and M represent the volume fraction, density and molar mass of the respective polymers, R is the universal gas constant, T is the absolute temperature. B is the binary interaction energy density and is related to the Flory-Huggins interaction parameter χ as BV χ ref 12 = (1.4) RT where V ref is the reference volume and B can be estimated from the solubility parameters ( δ ) of polymers as ( δ δ ) B = (1.5) A B 2 The Flory Huggins interaction parameter χ 12 is closely associated with the enthalpic interactions of the blend species. Extensive analysis of the Flory theory was carried out by Mc Master [8]. According to his findings compatibility between two polymers is favoured by (a) negative or very small positive values of χ 12 (b) low molecular weight by one or both polymers and (c) similar thermal expansion coefficient of the polymers Compatibilisation Since most of the polymer pairs fail to satisfy the thermodynamic criteria for miscibilility, the resulting blends will be heterogeneous in nature. This in turn will lead to blends with inferior properties. Owing to

5 Introduction 5 the lack of specific interactions between the polymer pairs, the interface formed will be very weak. In the case of polymer blends, the interface plays a decisive role in determining the blend properties. An interface is considered as a region having a finite distance neighbouring the dispersed phase. The properties of the interfacial region can differ from those of pure components. Schematic representation [9] of interface between immiscible polymers is given in Figure 1.2. Figure 1.2: (a) Interface between immiscible polymers and (b) interfacial density profile between immiscible polymers [9] It is clear from this figure that the interaction between two immiscible polymers is very weak resulting in a very thin interface [10]. For most polymer pairs, the interfacial width is very narrow of about 1-5nm, which implies that there is little penetration of polymer chains from one phase into another and vice versa resulting in very few entanglements across the interface [11]. The absence of strong interface between polymer pairs limits the stress transfer across the phase boundaries and requires only the breaking of

6 6 Chapter 1 van der Waals bonds during fracture. This implies that for most uncompatibilised polymer blends, interface are the most vulnerable location, which is most likely to fail well before the component polymers, on the application of an external stress. Another important parameter to be addressed in polymer blending is the phase morphology development during processing. It is well established that most of the ultimate properties (toughness, strength and crack resistance, optical, rheological and dielectrical properties) of polymer blends are strongly influenced by the type and fineness of phase structure. Therefore morphology control plays a major role in blending process. The morphology of a multi-component polymer system which refers to the shape, size and spatial distribution of the phases results from a complex interplay between viscosity (and elasticity) of the phases, interfacial properties, blend composition and processing conditions. Due to the high interfacial tension between the polymers, blending usually leads to a poor dispersion of one polymer phase in another. As the inclusions of the dispersed phase become larger, the interfacial contacts between the two phases will be small. Consequently, when the material is subjected to mechanical load, the two phases are unable to show their best features in a concerted manner in order to respond efficiently. Besides these factors, an immiscible polymer blend is thermodynamically unstable. A major problem caused by the instability of a phase separated blend is that its morphology is subjected to vary with conditions. Hence it requires that, for an immiscible blend to exhibit good properties, the interface should be modified and the phase morphology should be well controlled. This can be achieved by the incorporation of

7 Introduction 7 suitable agents called compatibilisers capable of modifying the interface of immiscible polymer blend, producing polymer alloy with desired properties. Compatibilisers are macromolecular species exhibiting interfacial activities in heterogeneous polymer blends. A suitably selected compatibiliser reduces the interfacial tension between the phases, enhances the interfacial adhesion and is also able to stabilise the developed phase morphology against coalescence. The term compatibility is more significant in a technological sense than thermodynamic sense. Thus, compatibilised blends indicate blends with satisfactory properties when compared to the immiscible system. In other words polymer blends with intentionally modified interfaces are called compatibilised blends. Let us discuss the various strategies for compatibilisation and the action of compatibilisers in detail Strategies for Compatibilisation in Polymer Blends The emulsification of polymer blends has been proposed as the most efficient tool for obtaining fine phase morphology and good mechanical properties which can be achieved by the addition of properly selected compatibilisers. The addition of a suitably selected compatibiliser to an immiscible binary blend (i) reduce the interfacial energy between the phases, (ii) permit finer dispersion during melt-mixing (iii) provide stability against gross segregation and (iv) result in improved interfacial adhesion [12]. Compatibilisation techniques can be broadly classified into two: 1) Physical compatibilisation and 2) Reactive compatibilisation Physical compatibilisation

8 8 Chapter 1 Physical compatibilisation/non-reactive compatibilisation involves the addition of pre-synthesised co-polymers into the blends. The copolymers should be selected in such a way that it could be able to locate at the interface between the components so as to reduce the interfacial tension and to promote adhesion between phases. Theoretical studies on physical compatibilisation by Dadmun and co-workers, states that the more times a copolymer molecule cross the interface, the more effective interfacial modifier it would be [13-16]. This leads to a thicker interface resulting from the presence of more joints sewing the two phases together through which stress can be transferred during failure. Therefore, the more the interfacial crossings by a copolymer molecule, the more will be the number of joints between the two phases that has to be broken to permit fracture. Early efforts on physical compatibilisation were concentrated on blends of diblock copolymers with homopolymers possessing repeat units identical or chemically similar to each segment of the copolymer. This type of compatibilised blend will be designated as an A/B/A-B system, where the A-B diblock is an entropic acting copolymer. Fayt et al. [17-21] reported extensively on this aspect. Recent studies using A-B type diblock copolymers are also cited in literature [22-27]. This approach has then been extended to A-C diblock copolymers, for filling the incompatibility gap between some A/B pairs, provided that C is miscible with B, so that the diblock is an enthalpic acting copolymer [28-38]. Literature presents reports on blends of two immiscible polymers (A and B) compatibilised with a C-D diblock copolymer, where A and B are miscible with C and D, respectively ( A/B/C-D system ) [39-41]. A diblock copolymer can cross a

9 Introduction 9 sharp interface only once where as with the increase in number of blocks increases the number of interface crossings. Compatibilisation using tri and penta block copolymers has been carried out by various researchers [42-50]. In addition to block copolymers, random [51-53] and graft copolymers [54-60] have also been extensively employed in physical compatibilisation of polymer blends. The random copolymer is also believed to cross the interface multiple times. Figure 1.3 represents the conformations of different types of pre-made copolymers at the interface in multiphase polymer systems. Figure 1.3: Conformations of different types of copolymers at the interface diblock copolymers (b) end grafted chains (c) triblock copolymers (d) multiply grafted chains (e) random copolymer [61] Based on their studies on compatibilisation of polyethylene (PE)/ polybutadiene (PB) system, Harrats et al. [37] reported that a copolymer to be effective as a compatibiliser should be designed in such a way that there should be an optimal interaction balance between its own sequences and the respective

10 10 Chapter 1 individual phases. Block copolymers were found to be effective in particle size reduction and improvement in mechanical properties of various blend systems. [25,30,34,38,42,44,57,59]. Very recently Dadmun et al. [49, 50] compared the compatibilisation efficiency of triblocks and multi blocks in PS/PMMA system. They found that in terms of the strength of interface the efficiency was in the following order pentablock > triblock > diblock > heptablock > random copolymers. According to them, the copolymer architecture and block length played crucial role in determining the compatibilisation efficiency. Hlavata et al. [45] observed that in PS/PP system compatibilised with SBS copolymer, location of the copolymer depends on the length of the blocks. Incorporation of poly (styrene-g-ethylene oxide) P(S-g-EO) copolymer in PS/ Poly (ethylene oxide) PEO, PS/PMMA and poly phenylene oxide) PPO/PMMA blends [60] revealed that even very short parts of the back bone of a graft copolymer can contribute to compatibilisation, especially when the backbone has a negative heat of mixing with one of the blend components. Oliva and co-workers [55] found that in PP/PS blends, compatibilisation efficiency of PP-g-PS improved by increasing either the length of the PP or PS sequences or the number of the side chains. Schulze et al. [56] reported that in the same system, graft with short PS side chains was more efficient than that with long PS chains at comparable compositions. Though physical compatibilisation is a classical approach to compatibilise immiscible polymer blends, the method has certain drawbacks. One of the major limitations is that compatibilisation of each immiscible polymer blend requires a specific block or graft copolymer which manifests the need for a specific synthetic procedure to obtain the

11 Introduction 11 copolymer. This in turn is time consuming and costly in addition to the fact that, in certain cases we still lack synthetic procedures for preparing corresponding block or graft copolymers. Another major limitation is regarding the amount of copolymers required for compatibilisation. Studies showed that the amount of the block or graft copolymers to be added is significantly higher than that required to saturate the interfaces. This is because for thermodynamic reasons, a portion of the added copolymer fails to reach the interface. During the extrusion process the residence time ~2-5min is too low for the copolymer to diffuse into the interfacial region which in turn lowers the compatibilisation efficiency. The addition of large amount of copolymers as compatibilisers is not recommended as they will form a separate phase contributing more towards immiscibility resulting in inferior properties of the blend. Moreover the control of molecular weight and the length of the copolymer segments which is a decisive factor in compatibilising efficiency is a difficult task. In spite of all these limitations, physical compatibilisation is considered as a convenient method to compatibilise immiscible polymer blends. The drawbacks of physical compatibilisation have triggered tremendous efforts to devise new alternatives which lead to the compatibilisation strategy by a reactive route. These days, reactive compatibilisation dominates the commercial blending practices in some way or another Reactive compatibilisation A look through the patent literature indicates that from the very beginning, blending involved both chemical and physical aspects. In the 1948 DuPont patent, PA-66 was first maleated, and then blended with

12 12 Chapter 1 PVAc. Starting in mid-1960, reactive extrusion and compounding (leading to controlled morphology, thus performance) began to be used for toughening and modification of engineering resins [62]. The technique has then been well established so that many commercial blends exist because of reactive processing. The basic concepts used in reactive blending involve the in-situ formation of a graft or a block copolymer via a suitable chemical reaction, and the ability of the in-situ formed copolymer to improve the compatibility between the immiscible polymer pairs. A schematic of the interfacial reaction of reactive chains demonstrating how the polymer chains on either side of the interface contain functional groups that can mutually react together, effectively forming a copolymer in-situ is presented in Figure 1.4. Figure 1.4: Schematic of the interfacial reaction of reactive chains [63] It should be taken into account that both the reactive and physical compatibilisation techniques have some similarities. The main similarities between the two strategies are; 1) the compatibilising agent is expected to locate at the interface between the phases, 2) compatibilisation in both the cases results in particle size reduction of the dispersed phase, enhanced interfacial adhesion between the phase, and a thermally stable phase morphology, 3) both the methods of blending result in compatibilised blends with attractive properties, 4) for industrial purposes, melt extrusion

13 Introduction 13 is the main compounding operation for both the physical and reactive blending. However reactive blending has certain advantages over physical blending which makes it the industrially preferred method. The copolymer responsible for compatibilisation is formed in-situ during blending which is expected to locate at the interface of the immiscible polymer blend. Hence micelle formation in one or the other phase is expected to be minor compared to when pre-made graft or blockcopolymers are used, which can easily self organise in the phase where they are the most energetically stable. Usually, the reactive molecules employed in reactive blending are prepared using less sophisticated synthetic routes whereas an additional step is required in physical blending for the synthesis and design of compatibilising agents. Thus reactive blending turns out to be cost effective and consequently, physical blending is mainly used when reactive blending is not technically feasible Chemical reactions in reactive compatibilisation Reactive compatibilisation strategy makes use of the common chemical reactions seen in low molecular weight (MW) materials. However, unlike in low MW materialas, the extent of reaction in polymers is controlled by various factors [64]. Major limitations involve the restricted diffusional mobility of the functional groups in the melt stage due to the high melt viscosity of polymers and the short reaction times. In reactive systems, whether small-molecule or macromolecular in nature, the two regimes that generally describe the rate limiting mechanism of the reaction are classified as diffusion- and reaction-controlled (DC and RC, respectively) regimes [65, 66]. de Gennes [67] reported that for bulk miscible macromolecular systems, the rate of chain coupling was

14 14 Chapter 1 proportional to the self diffusivity (chain mobility) of the functionalised polymers involved. Later, Durning et al. [68] and Fredrickson [69] proposed a similar relationship between chain coupling rates and selfdiffusivity for diffusion controlled reactions at the interfaces of immiscible polymers. For an effective compatibilisation process the following conditions are preferred: presence of sufficiently reactive functional groups in the polymers and selective as well as preferably irreversible mixing conditions to minimise mass transfer limitations. The types of chemical reactions that are frequently used in reactive blending can be grouped into imidisation, ring opening and amidation reactions, and interchange reactions between polycondensates. Other types of reactions that are less frequently used include esterification, urea and urethane links formation, ionic bonding, and concerted addition in which a comonomer such as maleic anhydride is copolymerised with a double bond of an unsaturated polymer. Some of the important reactions involved in reactive compatibilisation are summarised in the following Table 1.1 [70]. The common reactions involved in the formation of graft, block or random copolymers at the interface are discussed in the following section and the chemistry of reactions is represented schematically.

15 Introduction 15 Table 1.1: Reactions involved in reactive compatibilisation [70] Reaction type Reactive group Co-reactive group Remarks Amidation Carboxylic acid Amine Imidation Ahydride Amine Esterification Caboxyclic acid and anhydride Hydroxyl Urethane formation Isocyanate Hydroxyl Substitution Amine Hydroxyl, halide Ester interchange Ester Ester Transesterification Ester Hydroxyl/phenol Amide-ester exchange Amide Ester Aminolysis Ester Amine Amide interchange Amide Amide Acidolysis Ester Carboxylic acid Ring-opening reaction Ring-opening reaction Ring-opening reaction Epoxide Oxazoline Lactam Carboxylic acid, MAn, hydroxyl, amine Carboxylic acid, MAn, amine, halide Amine Addition/ Substitution Interchange reaction Ring-opening reaction Amine-anhydride (imidisation) reaction One of the most frequently used reaction in compatibilisation of polymer blends is imidisation reaction which is a very well known amineanhydride reaction in organic chemistry. The reaction is spontaneous and has certain advantages that no molecular catalyst is required. Besides that

16 16 Chapter 1 the reaction is feasible at high melt temperature in the absence of any solvent. The most vulnerable polymers for imidisation reaction are polyamides due the presence of amine end groups. Schematic representation of the interfacial chemical reaction with amino end groups of polyamide when maleic modified polymers are used as compatibilisers is shown in Figure 1.5. Literature survey reveals that the imidisation reaction has been extensively employed in compatibilisation of polymer blends [71-77]. Figure 1.5: Scheme of interface grafting by reaction between anhydride group and amine end group of polyamide [73] Amine-carboxylic acid (amidation) reaction Amidation is less efficient and less frequently used when compared to imidisation reaction. The efficiency of the reaction depends on the nature of the amine. Primary amine was found to be more reactive towards carboxylic acid groups attached to polymers than secondary or tertiary amine. This method has been used by several researchers for compatibilisation [78-82]. Figure 1.6 shows a schematic of amidation reaction.

17 Introduction 17 SAN PC O O C O SAN O O N H 2 C CH 2 N NH O O N O H 2 C CH 2 N N C PC O SAN- amine Polycarbonate PC Figure 1.6: Reaction of SAN-amine with PC to form SAN-g-PC [78] OH Amine-Epoxide reaction Amine-epoxide reaction has been successfully employed as a reactive compatibilisation in various systems [83-87]. A schematic of the reaction is given in Figure 1.7. O O C O CH 2 CH CH 2 NH 2 C O O CH 2 CH CH 2 OH NH Figure 1.7: Scheme of interface grafting by reaction between epoxide group and amine end group of polyamide [85] Carboxylic acid-epoxide reaction Reactive compatibilisation via acid-epoxide reaction has also been reported in literature and a schematic representation of the same is depicted in Figure 1.8 [88, 89]. O O O C O CH 2 CH CH 2 O HOOC C O CH 2 CH CH 2 OH O C Figure 1.8: Scheme of reaction between a carboxylic and an epoxide groups [88]

18 18 Chapter Ring opening reactions The reactive groups involved in this method include epoxide, oxazoline and lactam rings of which epoxide and oxazoline are the potential candidates. Epoxide can react with hydroxyl, carboxyl and amine groups, while oxazoline is able to react with carboxylic acid and phenol groups. Schematics of some ring opening reactions are shown in Figure 1.9(a-b). Several systems compatibilised by ring opening reactions are listed in literature [90-92]. O H R N 1 N R 2 NH 2 R 1 N O H R2 O R N 1 R 2 COOH R 1 N O R 2 O H O Figure 1.9(a): Scheme of reaction between oxazoline with amine and carboxyl end groups of polyamide [91] O R O HOOC PBT OH Epoxy PBT O a.) O R OH O PBT OH b.) R O PBT COOH O OH Figure 1.9(b): Scheme of reaction between epoxide and (a) with COOH (b) with -OH groups of PBT [92]

19 Introduction Transreactions Transreactions occur by chemical interchange of block segments of one polymer chain for corresponding segments of a second polymer chain. Table 1.2 shows possible interchange reactions during the melt blending of two polyesters, two PAs and polyester and PA. This compatibilisation strategy has been made use of by several researchers [93-96] in multiphase polymer systems Table 1.2: Interchange reactions between polycondensates [1] Alcoholysis P 1 -O-CO-P 2 +OH-P 3 P 1 -OH + P 3 -O-CO-P 2 Transesterification P 1 -O-CO-P 2 +P 3 CO-O-P 4 P 1 -O-CO-P 3 +P 2 -CO-O-P 4 Aminolysis P 1 -NH-CO-P 2 + H 2 N P 3 P 1 NH 2 +P 3 NH-CO-P 2 Transimidation P 1 -NH-CO-P 2 + P 3 CO-NH-P 4 P 1 -NH-CO-P 3 + P 2 CO-NHP 4 Acidolysis P 1 -O-CO-P 2 + HOOC-P 3 P 1 -O-CO-P 3 + HOOC-P 2 Ester-amide P 1 -NH-CO-P 2 + P 3 CO-O-P 4 P 1 -NH-CO-P 3 + interchange P 2 CO-O-P 4 Besides these frequently used reactions, compatibilisation can be effected by some other means which are discussed below Ionic Interaction Instead of covalent bonding ionic interactions enable compatibilising actions in certain polymer systems. Pyridine, imidazole and tertiary amine groups attached to polymers have been shown to form ionic bonding with sulfonic acid, carboxylic acid and ionomers and to promote compatibility of polymer blends [97, 98] Dynamic Vulcanisation.

20 20 Chapter 1 Dynamic vulcanistion is the process of vulcanising the elastomer during its intimate melt mixing with a non-vulcanising thermoplastic polymer. The introduction of crosslinks into one of the phases increases the viscosity of this phase leading to a change in the morphology of the blend [99,100]. The uncrosslinked phase is unaltered by the vulcanisation in the other phase, permitting easy processing of the blend. Improvements in properties include reduced permanent set, improved ultimate mechanical properties, fatigue resistance and high temperature utility, greater fluid resistance, melt strength and phase stability and more reliable thermoplastic fabricability. Several researchers have successfully employed this technique for the preparation of multiphase polymer systems with improved performance [ ] Addition of a Third Polymer (partially) Miscible with all Blend Phases This method involves the addition of polymer C (partially) miscible with the two constitutive polymers (A and B) of a two phase binary blend which is supposed to act as a common solvent thereby promoting complete or partial miscibility of the originally immiscible polymers. The choice of polymers A, B and C is such that the binary interaction parameter is negative for the A/C and B/C pairs and positive for the A/B pair. Tricomponent blends of PVC/CPE/EPDM has been investigated Lee and Chen [106] where CPE acts as the compatibiliser, because of its similarity in chemical structure with PVC and EPDM Addition of Reactive Fillers This compatibilisation strategy is similar to the one described in the above section using a third polymer. Shifrin et al. [107] have proposed that a two phase polyblend can be compatibilised by the addition of mineral

21 Introduction 21 filler. The selective localisation of the filler at the interface is a prerequisite, which is fulfilled (or not) depending on the balance of interaction between the filler and each constitutive polymer [108]. In the case of a comparable affinity of the filler for each blended polymer, the filler may be expected to accumulate in the interfacial region and provide the immiscible polyblend with an enhanced stability Addition of Low Molecular Weight Chemicals This strategy involves the addition of a (mixture of) low molecular weight chemicals such as peroxide and multifunctional chemicals during blending process. The actual compatibiliser, a branched, block or graft copolymer, is formed during a reactive blending process. Sato et al. [109, 110] reported the effect of addition of small amounts of maleic anhydride on the morphology, crystallinity and molecular structure of PA12/PE blends Solid-state Shear Pulverization This recently developed technique is done at temperatures below the melting transition or glass transition of semicrystalline and amorphous polymers respectively. Depending on the conditions employed chain scission may occur producing highly reactive free radicals at chain ends. The generated free radicals undergo combination at interfaces and in highly mixed regions may yield block copolymers [111] similar to those obtained in reactive compatibilisation during melt mixing of functionalised polymers Factors Affecting the Efficiency of Reactive Compatibilisation The phase morphology refinement and property enhancement in immiscible polymer pairs via reactive compatibilisation strongly depends on the kinetics and completeness of the interfacial chemical reaction with

22 22 Chapter 1 respect to blending time. Moreover, the location of the compatibiliser at the interface between the individual phases is a necessary requirement for an efficient compatibilisation. The important factors, which influence the efficiency of reactive compatibilisation include the reactive group content and end-group configuration, effect of miscibility of the reactive compatibiliser with one of the phases and the stability of the copolymer at the interface Reactive Group Content and End-group Configuration The limited yield of the interfacial reaction is the usual drawback of the reactive processing which requires the use of highly reactive groups and/or polymers of high content of mutually reactive functions. By increasing the amount of the reactive groups on a reactive polymer from one to multiple groups, the structure of the graft changes from a single to a multi-graft comp like structure. However it should be noted that the high functionality of component polymers or compatibilisers, which results in heavily grafted copolymers at the interface sometimes adversely affects the performance of the blends. Manson and co-workers [112] compared the compatibilising efficiency of hyperbranched polymer grafted polypropylene (PP-HBP) and Maleic anhydride grafted Poly propylene (PPMAH) in PP/PA6 blends. They reported that the interfacial adhesion between PP-HBP compatibilized bilayers was 10 times higher compared to maleic anhydride grafted PP (PP- MAH) compatibilized bilayers which was attributed to the higher diffusivity and functionality of PP-HBP giving more PP-PA6 copolymers at the interface [112] On the other hand Wong and Mai [113], on analysing PA6,6/PP blends find that an optimum functionality of SEBS-g-MA (0.74

23 Introduction 23 wt% MA) resulted in the better performance of the blend in terms of morphology and fracture toughness [113]. Triaca et al [114] reported that that when compatibilised with SMA25, the PA6/SAN blends became more brittle which was attributed to the formation of a highly branched comblike graft at the interface due to the high functionality of SMA25. Loyens and Groeninckx [115] analysed the morphology refinement of the PET/EPR blend with compatibilisers of different GMA content and established that the GMA-induced compatibilisation reaction (E-GMA8, E-GMA12 or EPR-g-GMA1.5) was much more effective than the reaction between MA (EPR-g-MA0.6) and the PET hydroxyl end groups. Huang et al. [116] reported that the in situ formation of styrene-glycidyl methacrylate-gpoly(ethylene-2,6naphthalate) (SG-g-PEN) copolymers with an optimal degree of grafting resulted in the best performance of PEN/PS blends compatibilised through SG copolymers Effect of miscibility of the reactive compatibiliser with one of the phases The localisation of the compatibiliser molecule at the interface is the prime and necessary requisite for an effective compatibilisation process. This demands that in addition to the reactivity of the compatibiliser molecule with one of the phases, it should be miscible with the other phase. A schematic representation showing the localisation of compatibiliser molecule depending on its miscibility is shown in Figure 1.10.

24 24 Chapter 1 Figure 1.10: Schematic representation of possible modes of location of graft copolymers in a reactively compatibilised blend depending on the level of its interaction with the dispersed phase [117] Figure 1.10(a) represents the case where the reactive compatibiliser that is fully miscible with the phase where it is initially incorporated. This type of interfacial arrangement can be expected to provide the most efficient reduction of the particle size through a decreased interfacial tension and increased interfacial stabilization. If the compatibiliser would show thermodynamic affinity for one of the phases, but is not miscible with it, an interlayer can be formed at the interface of the blend phases as shown in Figure 1.10(b). The coalescence suppression in this case will not be as effective in the case of fully miscible compatibiliser. At the same time, a very low affinity of the compatibiliser for one of the phases can cause its migration to the other phase as represented in Figure 1.10(c). However in the real case all the three cases co-exist in a reactively compatibilised system and can be represented as Figure 1.10(d).

25 Introduction 25 Harrats et al. [37] studied the miscibility of SMA copolymer with PMMA and their influence on PMMA/PA6 blend properties. The weight average particle diameter as a function of the extrusion time for the blend PA6/(PMMA/SMA)75(/20/5) with different MA contents for SMA is plotted in Figure Figure 1.11: Weight average particle diameter as a function of extrusion time for the blend PA6/(PMMA/SMA) 75(/20/5) with different MA contents of SMA [37] It can be noted from the figure that during the initial stages SMA-g- PA6 is formed at the interface, resulting a significant decrease in particle size. As the compounding process continues further, the SMA copolymer reacts more quantitatively to result in a heavily grafted copolymer at the interface and will be rejected from the interface. This tendency is more pronounced in the case of SMA28 and SMA33 where the extent of miscibility of SMA with PMMA, does not counterbalance the unfavourable compositional effect so that the copolymer richer in PA6 exhibits more

26 26 Chapter 1 interaction with PA6 matrix. It is the miscibility of reactive SMA compatibiliser with the PMMA phase that determines the phase stability in the blend at long extrusion times. This study suggests that the miscibility of the compatibiliser with one of the phases contributes significantly to the stability of copolymer at the interface. Takeda and Paul reported an increase in domain size at high acylonitrile (AN) contents in SMA2 compatibilised nylon6/san blends which was attributed to the immiscibilility SMA2 with SAN at high AN contents [118]. Dedecker in his studies on compatibilisation of PA6/PS blends found that PS-g-MA (0.17wt% MA) is more efficient when compared to SMA2 owing to its more stable location of the graft copolymer at the interface due to the better miscibility of the blend system PS/PS-g-MA [119] Stability of the copolymer at the interface As discussed in the earlier sections, for the better performance of compatibilised blends, the copolymer located at the interface should be stable during further processing, annealing etc so that they could provide a stable and uniform morphology to the compatibilised blends. Groeninckx and co-workers [ ] reported a series of studies regarding the interfacial stability of the copolymer. Dedecker and Groeninckx [120] reported that the morphology of the unccompatibilised PA6/PMMA blends were highly unstable up on annealing while the reactively compatibilised PA6/(PMMA/SMA) depicted an unchanged morphology under the same conditions. The influence of reactive compatibilisation on phase stability of blends in the melt can be understood from Table 1.3 [121].

27 Introduction 27 Table 1.3: Average dimensions of phases in annealed polymer melt blends [121] Blend system Blend ratio Mean phase dimensions (μm) 30min 60 min 90 min PS/Nylon6 60/ PS/ Nylon6/SAN 57/38/ PS/ Nylon6/SMA 57/38/ HDPE/ Nylon6 50/ HDPE/ Nylon6/ MA-g-PP 47.5/47.5/ SAN = styrene acrylonitrile copolymer, SMA = styrene-methacrylic acid copolymer, HDPE = high density polyethylene, MA-g-PP = maleic anhydride grafted polypropylene [121] The effect of the extrusion time on the phase morphology of the reactively compatibilised PA6/(PPO/SMA) 75/(20/5) blend has been reported by Dedecker and Groeninckx [122]. At very long extrusion times (30 min) a bimodal particle size distribution was found for the blend system with SMA8 as reactive compatibiliser. This has been accounted for by the fact that the graft copolymer already formed at the interface has left the interfacial region by the applied shear forces upon prolonged mixing. The shear forces must have overcome the thermodynamic compatibility between the SMA-g-PA6 copolymer and the respective homopolymer phases. The separated graft copolymer gave rise to the formation of micelles, which were detected by SEM as very small particles. As a result of this, the size of the dispersed phase increased dramatically. They found that even though the increase in the MA functionality of SMA resulted in large decrease of

28 28 Chapter 1 particle size, the functionality has adversely affected the stability of copolymers at longer extrusion time Molecular weight of the compatibiliser precursors Molecular characteristics of the compatibiliser precursors influence the blend viscosity which in turn contributes to the morphology development. According to Fortenly et. al. [123], the decrease in molecular weight and thus the viscosity of the matrix decreases the dispersive forces at the origin of particle breakup and the interface generation and makes the coalescence favourable. This suggests that the surface area of the interface is partly controlled by the reactive polymers. The molecular weight of the reacting polymers also shows a direct influence on the yield of compatibilisation reaction. At a constant weight fraction of the reacting polymers, the concentration of the reactive group attached as an end group of linear chains increases with decreasing molecular weight. The diffusion of the low molecular weight polymers to the interface will be faster again contributing to the interfacial reaction. Thus a decrease in the molecular weight of the reacting polymers favours the interfacial reaction Morphological Aspects of Reactive Polymer Blending The properties of polymer blends are strongly influenced by their morphology. Morphology development is the path of morphological change, which the material undergoes during its transformation from large to small domain size. From the point of view of performance polymer blends are expected to have two different types of morphology; blends with discrete phase structure (droplet or drop in matrix) and blends with bicontinuous phase structure (co-continuous). Other important types of

29 Introduction 29 morphologies include fibrillar, composite laminar, core shell, onion ring like, etc. A schematic representation of different types of morphologies of polymer blends is presented in Fig [124]. Examples of commercial blends with specific morphology are given in Table 1.4. Figure 1.12: A schematic representation of different types of morphologies of polymer blends [124] Table 1.4: Commercial examples of immiscible polymer blends [124] Product Manufacturer Components Matrix/disperse Morphology Property Dynamar Dyneon PE/PTFE dilute drops processability (adds-wall slip) MB Dow Corning PP/PDMS dilute drops lubricity Noryl GTX GE PA6/PPO/SB drops (60%) dimensional control (H 2 O uptake) Zytel ST Dupont PA66/EP double emulsion toughness Selar DuPont PE/PA66 lamellar diffusion barrier Vectra Hoechst PET/LCP fibres thermal expansion TSOP Mitsubishi/Toyota PP/PE/EP co-continuous electrical conductivity Stat-Rite Goodrich PP/PU co-continuous electrical conductivity

30 30 Chapter 1 The morphology of heterogeneous polymer blends in general depends on blend composition, interfacial tension between the component polymers, viscosity ratio and processing history [78,125,126]. Therefore it is important to study the relationship between processing conditions, blend composition and the final blend phase morphology. The knowledge of the mechanism of phase morphology development is not only important with respect to the blend properties but also for the design of intensive mixers with better dispersive mixing capabilities for reactive blending. Furthermore, information on the phase morphology development is essential for the understanding of the kinetics of generation of interfacial area during reactive blending and also with respect to the various aspects of the melt-mixing process General aspects of morphology development The morphology development during melt mixing of immiscible polymers is the resultant of two competing processes viz; deformationdisintegration (drop break-up) phenomena and coalescence. The balance of these competing processes determines the final particle size which results upon solidification of the blend. The pioneering studies on drop break up by Taylor [127, 128] on drop break up in Newtonian systems considers how the balance of applied shear forces and counteracting interfacial forces affects drop dimensions and stability. The results have been expressed in terms of the so-called capillary number, a dimensionless number expressing the relative importance of the applied viscous shearing forces and the counteracting interfacial forces.

31 Introduction 31 C a γη D m = (1.6) 2Γ where D is the diameter of the droplet, γ the shear rate, η m the matrix viscosity, and Γ the interfacial tension. According to Taylor s analysis, deformation of the droplet is enhanced by large shear rates, a high matrix viscosity, large droplet size and small interfacial tension. The deformation is retarded by large interfacial tension, high dispersed phase viscosity and small droplet size. Taylor reported that if a lower or equal viscosity droplet is placed in a high viscosity matrix, it is readily drawn out into a large filamentous ligament, which eventually breaks up. This was not the case for very high viscosity drops placed in a low viscosity matrix; these tend to retain their spherical shape i.e., when C a exceeds a critical value (C a crit ), the droplet will deform and subsequently break-up under the influence of the interfacial tension. If C a is small, the interfacial forces dominate and a steady drop shape is developed. Several studies on Newtonian system in both shear and extensional flow were in agreement with Taylor s prediction [129, 130]. However, it has been reported that drop break up was possible over a wide range of viscosity ratios in extensional flow for Newtonian systems [131, 132]. In the case of polymer blends, since the individual components exhibit large normal stresses in flow, the extension of Taylor s analysis to such systems has some limitations. For dilute Newtonian systems, the size of the smallest drop that can be broken could be calculated from Taylor's theory. For polymer systems, owing to the chances of coalescence, the equilibrium drop size is usually larger than predicted, and the deviation increases with

32 32 Chapter 1 concentration of the dispersed phase. This is due to the fact that most of the studies on polymer blends in literature regarding domain size, expressed in terms of capillary number and viscosity ratio (based on Taylor s analysis), do not account for the coalescence process. Elmendrop, [133] Fortelný and co-workers [ ] and Inoue and co-workers [138, 139] divided coalescence process into four stages; (i) approach of particles with and formation of the parallel film between the particles with thickness (ii) drainage of the matrix trapped between the particles until the film thickness reaches its critical value (iii) rupture of the film due to interfacial instability (iv) evolution of the dumbbell particle to a coalesced sphere particle (merging of particles). The coalescence process can be schematically represented as Figure Figure 1.13: Schematic diagram of the coarsening process [133] Coalescence occurs in shear as well as in quiescent systems. In the latter case the effect can be caused by molecular diffusion to regions of lower free energy by Brownian motion, dynamics of concentration fluctuation, etc. Diffusion is the mechanism responsible for coalescence known as Ostwald ripening. The process involves diffusion from smaller drops (high interfacial energy) to the larger ones. Shear flow enhances the process [140]. Flow-induced coalescence is accelerated by the same factors that favour drop break-up, e.g., higher shear rates, reduced dispersed-phase

33 Introduction 33 viscosity, etc. Therefore, theories that consider both the domain beak down and coalescence could explain the morphology development in polymer systems more precisely. Wu modified the Taylor s equation as follows [126] γηmd 2Γ = 4 (η d/η m ) ±0.84 (1.7) positive when η d /η m > 1; negative when η d /η m <1, η d is the dispersed phase viscosity. Wu s correlation suggests a minimum particle size when the viscosities of the two phases are closely matched. The validity of this equation has been testified by researchers [141,142] who have indicated some disagreement with respect to the dependence of the particle size on viscosity ratio and the correlation proposed by Wu. On the other hand Serpe et al. [143] found a good agreement of their experimental observations with Wu s results. Tokita [144] derived an expression for the particle size of the dispersed phase in polymer blends that incorporates composition as variable. According to this theory, at equilibrium, when coalescence and break down are balanced, the equilibrium particle size de is given by: { ( ) 2 } d 24Pσ πτ φ + 4PE πτ φ (1.8) e r 12 d r dk 12 d where τ 12 is the shear stress, σ the interfacial tension, E dk the bulk breaking energy, φ d the volume fraction of the dispersed phase and P r the probability that a collision to resulting coalescence. Therefore, it can be stated that the final morphology in a blend is the resultant of domain break-up and coalescence.

34 34 Chapter Morphology development during processing Phase morphology generation during melt blending is the evolution of the blend morphology from pellet-sized or powder-sized particles to submicron droplets which exist in the final blend. Morphology development during melt mixing is schematically represented in Figure Figure 1.14: Schematic representation of morphology development in compatibilised and uncompatibilised blends [145] During melting in batch mixers or screw extruders, high stresses are exerted on the softening pellets or powder which shears off the softening polymer into sheets. Scott demonstrated this mechanism by quenching samples from a batch mixer and dissolving the matrix polymer in a selected solvent [146,147]. Studies by Scott and Macosko et al. [145] also supported the same mechanism. They found that most of the reduction in dispersed phase size was demonstrated to occur in the very early stages of mixing in conjunction with the melting and softening process. At intermediate mixing times, the morphology consisted of a large number of small particles along

35 Introduction 35 with a small number of very large particles in the size distribution. The effect of subsequent mixing was mainly to reduce the size of the largest particles in the size distribution. Shih et al. [148] reported a phase inversion mechanism when the minor component melted or softened at a lower temperature than the major component. When two polymers are melt-blended in an extruder, the polymer with the lowest melting or flow temperature tends to form, in first instance, the matrix. After the second polymer has melted, phase inversion may occur depending on the viscosity ratio and blend composition. In order to understand the morphology development, a mathematical model has been proposed by Lindt and Ghosh [149] for the conversion of pellets into lamellar structures and striations during melting in a single screw extruder. Interestingly these findings were in agreement with those reported by Scott and Macosko [145, 146]. The investigations of Thomas and Groenincks [150] on the phase morphology development in thermoplastic/rubber blends of nylon 6 and ethylene propylene rubber (EPM) and PA6/PS blends also showed that the major break down of the particles occurs during the initial stages of mixing [151] Phase morphology development in reactive blending There are many similarities in morphology development of nonreactive and reactive blending. However the morphology development in reactive blending is greatly benefited from the interfacial chemical reaction. The morphology development in reactive blending is path dependent and is an outcome of the entire process starting from the onset of melting. It depends on the thermal flow and history. This is in contrast to uncompatibilised blends, where morphology at long mixing times is

36 36 Chapter 1 determined by a steady-state break up and coalescence equilibrium rather than the process path. Hence it is very important to understand and control the morphological and rheological evolution of the mixture. In Reactive blending the effects of chemical reaction at the interface is an important parameter which determines the phase morphology of the blend. The chemical reaction at the interface leads to a reduction in the average size of the dispersed phase due to the suppression of coalescence and the decrease of interfacial tension. an increase in the break-up of the largest particles in the size distribution of the dispersed phase. a substantial narrowing of the size distribution of the dispersed phase and a decrease of the mobility of the interface which slows down the coalescence rate Mechanism of Compatibilisation in Reactive Blending The incorporation of a reactive compatibiliser leads to a decreased dispersed phase size, enhances the break up process of larger particles and results in a narrow particle distribution. Experiments have consistently shown that even small amount of interfacial reaction can dramatically reduce coalescence under quiescent conditions [152]. The role of compatibilisers in the blending process is firstly to retard the formation of the interfacial (Rayleigh) disturbances on the generated threads of polymer as a result of a decreased interfacial tension [1]. The lower the interfacial tension, the longer the deformation tension exceeds the interfacial tension, the longer the stretching of the thread will proceed, the smaller the diameter of the resulting thread will become, and, consequently, the smaller the size

37 Introduction 37 of the generated droplets of polymer will be. Usually, an average particle size in the sub-micron range can be achieved. Once fine droplets are formed, the compatibilisers now prevent the coalescence between them by acting as steric barriers. Karim et al. [153] have reported that a copolymer at interface (in reactive blends) leads to a frustration in coalescence as shown in Figure Figure 1.15: Schematic illustration of droplet coalescence in immiscible polymer blends with and without a copolymer at interface [153] When the dispersed concentration increases in uncompatibilised polymer blends, the particle size increases, but for blends containing premade or insitu copolymers, the particle size remains fairly constant. This observation suggests that coalescence is suppressed by the addition of copolymers.

38 38 Chapter 1 Some researchers reported that the lowering of interfacial tension was responsible for the morphology stabilisation [72,154,155]. Two major mechanisms were proposed to describe the suppression of coalescence in presence of copolymer and is represented in Figure 1.16 (a-b). According to Sundararaj and Macosko [156] steric repulsive interaction between the copolymer layers in particle surfaces was thought to contribute to this coalescence suppression. Steric inhibition due to copolymers is schematically represented in Figure 1.16 (a). Lyu et al. [157] reported that the minimum coverage of copolymer required for coalescence suppression is independent of shear rate instead depends on the molecular weight of the copolymer which support the steric interaction mechanism suggested by Macosko. Milner and Xi [158] suggested that a local concentration gradient is caused by the flow between the approaching droplets. This inturn leads to the so called marangoni stress which retards the rate of film drainage between the droplets, thereby suppressing coalescence (Figure 1.16(b)). They predicted that the block copolymer concentration necessary for preventing coalescence was higher at higher shear rate. Even though there exists a conflict between two mechanisms, some researchers support the mechanism put forward by Milner and Xi [159]. The debate on the mechanism for coalescence still continues, but the stabilising effect of polymer blend morphology is an unequivocally established fact.

39 Introduction 39 Figure 1.16: Two mechanisms proposed for block copolymer suppression of coalescence (a) surface tension gradient (Marangoni force) and (b) steric repulsion [157] Besides thermodynamic interactions of the polymeric components, their rheological behaviour also determines to a great extent the morphology that results from the melt mixing process. In the case of reactively compatbilised systems, the strong chemical reactions affect the thermodynamic conditions and the interactions at the phase boundary layer producing large changes in the rheological behaviour and therefore changes in phase morphology as well. The interfacial reaction itself changes the rheology of the blend since it results in increasing molecular weights and in some cases long-chain branching or even cross-linking. Various reports on the

40 40 Chapter 1 rheology of reactively compatibilised blends are cited in literature [160,161]. Recently Chang and co-workers [161] have analysed the rheological behaviour of PEN/PS blends compatibilised with styrene-glycidyl methacrylate compounds. They reported that the increase in viscosity with compatibiliser content is due to the increase of molecular weight and interfacial friction under shear, which is attributed to the presence of in situ formed SG-g-PEN copolymers anchoring at the interface Theories of Compatibilisation Several researchers including Noolandi and Leibler proposed theoretical predictions regarding compatibilising action of copolymers. Some of the theories of compatibilisation are discussed below Noolandi s heory The model proposed by Noolandi [162, 163] was based on the assumption that part of the copolymer that does not localise at the interface will be randomly distributed in the bulk of the homopolymer phases as micelles. Localisation of the copolymer however results in a decrease in the entropy and ultimately limits the amount of copolymer at the interface. The efficiency of the copolymer is mainly influenced by a series of factors such as lowering the interaction energy between the immiscible homopolymers, the broadening of the interface between the homopolymers, a decrease in energy of interaction of the two blocks with each other and a large decrease in the interaction energy of the oriented blocks with the homopolymers. Various other factors such as mixing conditions, interaction of the compatibiliser with the dispersed phase, molecular weight and composition of the compatibiliser etc contribute towards the localization of the compatibiliser at the interface and thereby reducing the interfacial tension.

41 Introduction 41 The separation of the blocks and the consequent stretching of the blocks into corresponding homopolymers also cause a decrease of entropy. However, the main contribution to the interfacial tension reduction is the entropy loss of the copolymer that localises at the interface. The loss of conformational entropy of both the copolymer and homopolymer chains at the inter-face was shown to contribute very little to the interfacial tension reduction. An analytical expression for the interfacial tension reduction was derived by Noolandi and Hong by neglecting the loss of conformational entropy [164]. ( ) ΔΓ = dφc 12χ + 1Zc 1 Zc exp Zcχ 2 (1.9) where d is the width at half height of the copolymer profile reduced by the Kuhn statistical segment length, φc the bulk copolymer volume fraction of the copolymer in the system, Z c the degree of polymerisation of the copolymer and χ the Flory-Huggins interaction parameter between A and B segments Leibler s theory According to Leibler [165,166], emulsifying action of an A B copolymer at the interface in an immiscible blend of polymers A and B results in the reduction of interfacial tension [ΔΓ]. For long copolymer chains (the wet brush case i.e., copolymer overlaps strongly and stretches but still homopolymers can penetrate the brushes), the reduction of the interfacial tension coefficient should follow the relation [166]: ( kt a )( 34) ( a ) ( ZCAZ A ZCBZ B ) ΔΓ = Σ + (1.10)

42 42 Chapter 1 where Z CA and Z CB are the number of A and B units in the copolymer respectively, Z A and Z B the degree of polymerisation of A and B respectively, a the monomer s unit length, Σ the interfacial area per copolymer. Leibler suggested that at equilibrium, the droplet size distribution is controlled by rigidity and spontaneous curvature of radius of the interphase, both dependent on the copolymer s molecular constitution. For well-chosen compositions and molecular weights of the copolymer, low values of Δ γ are to be expected. This suggests a possible existence of thermodynamically controlled stable droplet phase, in which the minor phase homopolymer drops are protected by an interfacial film of the copolymer, interfacing the matrix polymer. In the case of dry brush limit [166] in which the homopolymer does not penetrate the brush formed by the copolymer, the reduction in interfacial tension is independent of homopolymer molecular weights. Hence the interfacial tension reduction (ΔΓ) can be expressed as. ΔΓ = μ Γ N 9 ( χ ) 12 (1.11) where Γ 0 is the interfacial tension of polymer blend without a compatibiliser and μ is the chemical potential which is given by the equation: + μ ln φ f χ N = + (1.12) where f is the volume fraction of the component in copolymer which is miscible to homopolymer forming the dispersed phase and

43 Introduction 43 φ φ + 0 = φm + φd exp χ A B { ( N N )} (1.13) where φ 0, φ m and φ d represent the volume fraction of the copolymer, matrix and dispersed phase, respectively, N A and are the number of segments of the component in the copolymer miscible to the homopolymer forming the dispersed phase and that miscible to homopolymer forming the matrix phase, respectively. Since the value of exp χ NA NB is negligible compared to m Paul and Newman s theory φ, φ + is expressed by φ0 φ m. N B { ( )} According to Paul and Newman [6] the interfacial area per unit volume occupied by each compatibiliser molecule is given by the expression: 3φ M Σ= (1.14) RNW where N is Avogadro number, M the number average MW of the compatibiliser, R the average radius of the dispersed phase, φ the volume fraction of the dispersed phase and W the weight of the compatibiliser required per unit volume of the blend. When φ and M are kept constant, Σ depends on the values of R and W. R decreases with increase in the weight fraction of the compatibiliser and Σ Favis theory may either decrease or not change or increase. Based on the geometrical considerations about the macromolecular size, Favis and co-workers [167] developed an expression to find the critical micelle concentration. In the case of dispersed spherical domains during melt

44 44 Chapter 1 mixing of immiscible polymer blends. According to the authors, the critical saturation concentration ( C cr ) value of the block copolymer is given as: C cr 2 π RM A 3 1 3kMA = k 2 3 = (1.15) 2 NR g 4πR ρwa 4RgRN ρwa where MA is the MW of block A, R g is the radius of gyration of A block, N is the Avogadro number, ρ is the density of the block copolymer, k is the number of block per cubic element, W A is the weight fraction of A phase in the copolymer and R is the radius of the dispersed phase Anastasiadis theory Based on the assumption that the composition gradient is small compared to the reciprocity of the intermolecular distances a generalised gradient theory of the interface was developed by Anastasiadis et al. [168]. The theory determines the difference in the density fluctuation per unit interfacial area between polymer mixture and a system in which the properties are homogenous. The theory predicts that, φβ ( ) 12 ν12 = κ g φ Δ dφ (1.16) φ α κ g φ φ g φ where g is the free energy density. (1.17) Utracki and Shi s theory Utracki and Shi [169] derived a semi-empirical relation between the interfacial tension coefficient ( γ 12 ) and compatibiliser concentration by

45 Introduction 45 assuming an analogy between addition of block copolymer to a polymer blend and titration of an emulsion with surfactant. According to the authors: {( CMC mean ) mean} γ = φγ + φ γ φ+ φ (1.18) 12 0 ( = ) ( ) where γ γ φ φ, φ φ + φ (1.19) CMC 12 c CMC mean Tang and Huang s theory CMC 0 2 Based on the fact that upon the addition of compatibiliser, interfacial tension γ decreases and on assumption that the decrease is directly proportional to the interfacial tension difference at a particular compatibiliser concentration C and CMC, then: dγ = dc K ( γ γ ) (1.20) s where K is the rate constant for the change in interfacial tension with concentration of the compatibiliser, γ the interfacial tension at a given compatibiliser concentration, C and γ s the interfacial tension at CMC. From the above expression, Tang and Huang [170] eventually derived the following equation: ( ) R R R R e KC s = 0 s (1.21) where R 0, R and R s are the average radius of dispersed particles without compatibiliser, at a given compatibiliser concentration and compatibiliser concentration at CMC respectively. A plot of ln( R R s ) versus C can be used to obtain K from the slope Reactive Compatibilisation of Polyamide/Polystyrene Based Blends

46 46 Chapter 1 Polyamides are an attractive class of polymers for engineering applications. Their inherent features include high strength, good abrasion resistance and chemical resistance. Polyamides are widely used for many industrial applications such as in bearings, fishing lines, ropes and also for the manufacture of electrical components. A particularly attractive feature of the nylons for reactive coupling to other polymer is their inherent chemical functionality through the amine or carboxyl end groups that may be presented, and potentially the amide linkage itself. Hence, there is no need to functionalise this component by further reaction or by physical addition of a functional component. However the major drawback of polyamides is their high moisture absorption and consequently, the dimensional instability. An efficient way to overcome the drawback of polyamides is blending with non-polar polymers so that the moisture absorption will be minimised. Literature presents several examples of blends of PA with different kinds of polymers including rubbers [150, ] and plastics [ ]. Blending with polystyrene [PS) is of course a good choice for reducing the moisture absorption of PA. However, the components being polar and non-polar, the resulting blends will be highly immiscible. To alleviate this problem reactive compatibilisation strategy has been employed by various researchers. Let us have a brief look into some of the works reported on PA/PS blends. Very recently, Omonov et al. [176] evaluated the morphology of ternary blends of PA/PP/PS. Reactive compatibilisation of the blends has been performed using two reactive precursors; maleic anhydride grafted polypropylene (PP-g-MA) and styrene maleic anhydride copolymer (SMA)

47 Introduction 47 for PA6/PP and PA6/PS pairs, respectively. They reported that the co continuous morphology exhibited by PA/PP/PS [40/30/30] composition turned out to dispersed phase morphology with PA dispersed in continuous PS phase when the blends were compatibilised. Li et al. [177] investigated the properties of PS/PA1212 blends compatibilised with SEBS-g-MA. Microscopic studies revealed that SEBS-g-MA was successful in morphology refinement of the system due to its reaction with PA. Differential scanning calorimetry data indicated that the strong interaction between SEBS-g-MA and PA 1212 in the blends retarded the crystallisation of PA Villarreal et al. [179] analysed the compatibilising effect of the ionomer, poly(styrene-co-sodium acrylate) (PSSAc), on immiscible blends of PS/PA6 by mechanical tests and scanning electron microscopy. The SEM micrographs of the fractured surface after tensile stress-strain tests of uncompatibilised and compatibilised PS/PA 6 blends are shown in Figure (a) (b) Figure 1.17: (a) PS/PA6 with 30% PA6 (b) PS/PSSAc/PA6 blends with 30% PA6 [179]