Other Thermoplastic Elastomers

13 Other Thermoplastic Elastomers 13.1 Elastomeric Star Block Copolymers

and subsequently added divinylbenzene as the linking agent to the living charges.

Star-branched, radial, or multi-arm star copolymers are copolymers consisting of a core and arms held together by, or emanating from, the core (Fig. 13.1). Besides the chemical nature of the constituents, the properties of star copolymers mainly depend on two structural factors, namely, on the molecular weight of the arms and the number of arms. Certain properties of the star copolymers, such as intrinsic viscosity and shear sensitivity, depend only on the molecular weight of the arms, not on the total molecular weight of the polymer, and are independent of the number of arms above a certain number of arms [1, 2]. The core of a star copolymer is usually viewed as a volumeless and weightless point, since its volume and weight are negligible in comparison to the volume and weight of the entire star. A core can be a single linking agent, complex microgel, or a multifunctional initiator residue. The first report on the synthesis of nonlinear copolymers with controlled structures having extent of cross-linking without gelation was by Flory and coworkers [3]. The resulting copolymers having four or eight arms were prepared from e-caprolactam and tetra- or octacarboxylic acids as multifunctional reactants, respectively. Fetters and coworkers [4, 5] prepared star copolymers with polybutadiene arms using triand tetrachlorosilane as linking agents. Rempp and coworkers [6–10] synthesized polystyrene arms by living anionic polymerization techniques

13.1.1 General Methods for the Synthesis of Star Copolymers

Figure 13.1. Schematic drawing of a star (radial) blocks copolymer.

There are two major synthetic routes to star copolymers, namely, arm-first method and corefirst method. Besides these two major methods, several other methods have been reported in the literature. Arm-first method starts with the preparation of tailormade “prearms.” These prearms may be synthesized from a wide variety of monomers by living cationic [11–15], living anionic [4, 5, 16, 17], and by group transfer [18–24] polymerizations. Prearms can be linked by di- or multifunctional linking agents. Star copolymers with arms, consisting of polystyrene (PSt) [25, 26], polydiene (polyisoprene (PIP) and polybutadiene (PBD)) [27–29], poly(methyl methacrylate) (PMMA) [30, 31], poly(vinylpyridine) (PVP) [32], and polystyrene-block-polydiene (PSt-b-PIP and PSt-b-PBD) [33–36] have been synthesized by living anionic polymerization. In most cases divinylbenzene (DVB) has been used as the linking agent [32–37]. The advantage of the arm-first method with the use of difunctional polymers is the ease of producing fairly high degrees of branching (i.e., number of arms) and well-defined arms structures directly from precisely designed prearms. The major drawbacks of this method are the difficulty to form terminal arm functionalities on the arms, difficulty in achieving complete linking, ill-defined cores, and loss of control over the number of arms [38]. The preparation of star copolymers using multifunctional linking agents in cationic polymerization methods is described in [39–46]. In contrast to anionic systems, in cationic polymerization methods the living polymer chain ends are electrophilic; thus, the linking agents must be nucleophilic. A more detailed review of methods and monomers for both anionic and cationic systems used is in references [38] and [47]. In the core-first method, a multifunctional initiator containing a known number of initiating

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sites is prepared first. Then the star copolymer is formed by initiation from the initiating sites by living polymerization. The residue of the multifunctional initiator becomes the core. The Scheme 3 illustrates the process. This method has been used for the synthesis of star copolymers by anionic [48–50], cationic, group transfer, radical, and condensation polymerizations [51]. Multifunctional initiators were found to be more efficient in cationic polymerizations than in anionic polymerizations, for the preparation of stars [52]. The advantage of the core-first method is that the preparation of a star copolymer with a functional group at the polymeric arm end and the preparation of block star from the living chain end can be readily carried out. The maximum number of arms can be controlled when a well-defined core, that is, multifunctional initiator, is used. In addition, the star formation takes a shorter time than the arm-first method, since it is not diffusion controlled. The drawback of this method is its high dependence on initiating efficiency to obtain the target product with the correct number of arms and the loss of control over the precise molecular weight of the arms or the block composition [53]. Additional methods of preparation of star copolymers have been reported. Ito and coworkers synthesized star copolymers of poly(ethylene oxide) using homopolymerizable macromonomers [54]. 13.1.2 Physical Properties of Star Block Copolymers

Star block copolymers prepared by anionic methods have been shown to exhibit superior mechanical properties and lower melt viscosities than linear triblock copolymers with the same molecular weight [17]. The star copolymers with low-molecular weight arms exhibited low viscosities and were easily processed [17]. Dynamic viscosity studies performed on the above star block copolymers established that their melt viscosities were largely independent of the extent of branching [55–58]. Leblanc [58] observed that star block copolymers with four PSt-b-PBD arms exhibit lower activation energy of flow than that of linear triblock counterparts with comparable block composition. He assumed that an “aggregate flow” with low activation energy takes place. These results indicate that processing of star block thermoplastic elastomers (TPEs) should be easier than that of linear triblock copolymers.

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Table 13.1. Stress–Strain Behavior of Different Styrenic Block Copolymers Tensile Strength (MPa)

Elongation at Break (%)

S–I–S, linear

33.4

1,030

(SI)3-Si (3-arm star)

36.8

970

(SI)4-Si (4-arm star)

37.3

1010

(SI)6-DVB (6-arm star)

38.3

940

(SI)9-DVB (9-arm star)

42.2

1050

S–B–S, linear

29.4

1050

(SB)15-DVB (15-arm star)

41.2

730

Block Copolymer

Notes: Films cast from benzene/heptane 9/1 (v/v). S—polystyrene; I—polyisoprene; B—polybutadiene. Si—silicon tetrafluoride linking agent. DVB—divinyl benzene linking agent. Source: From reference [23].

Mechanical properties of TPEs are, like those of thermoset rubber materials, characterized by tensile tests. Star block copolymers prepared by anionic polymerization, that is, star PSt-b-PBD and PSt-bPIP, exhibit tensile properties, which are superior to those of corresponding linear triblock copolymers [17] (see Table 13.1). Polyisobutylene (PIB)-based TPEs, including linear triblock copolymers, are reported to exhibit excellent tensile properties [48]. High tensile strengths (up to 26 MPa) have been reported to form various PIB-based star block copolymers with arm numbers greater than three, prepared by living cationic polymerization techniques [13, 20, 59, 60]. Star block copolymers with PSt-b-PIP arms exhibit better adhesive properties than their linear counterparts [61].

13.2 TPEs Based on Interpenetrating Networks Generally, an interpenetrating polymer network (IPN) is defined as a combination of two polymers in network form, at least one of which is synthesized and/or cross-linked in the immediate presence of the other. A thermoplastic IPN contains physical cross-links in both polymers, rather than covalent cross-links [62]. To be of a practical utility, such a material has to behave as a thermoset at lower

13: OTHER THERMOPLASTIC ELASTOMERS temperatures and must be able to flow as a thermoplastic at higher temperatures. The physical cross-links that can be present in these networks can be introduced by block copolymers, partially crystalline or ionomeric structures. A thermoplastic IPN can be composed of networks of the same kind or of two completely different ones. Another important feature is dual phase continuity, sometimes referred to as “co-continuous phases.” The domains can be shaped as long cylinders, various interlocking structures, alternating lamellae, and so on [62]. In the IPNs containing covalent cross-links, the domain sizes are frequently governed by the crosslink density and are typically of the order of 0.05– 0.3 mm. Since many of the thermoplastic IPNs are prepared by blending (see below), the domains are almost always somewhat coarser [62]. 13.2.1 Synthesis of Thermoplastic IPNs

In general, thermoplastic IPNs are prepared by shearing or mixing either of the two polymers, or by polymerization of one or both of the polymers, or by the ionization of an ionomer component. At any rate, the final product has some kind of dual phase continuity [62]. An example is a thermoplastic IPN prepared from S–EB–S and polyamides, polyesters, or polycarbonates [63–65] with the addition of polypropylene. The S–EB–S is a triblock copolymer with hydrogenated polybutadiene center block and polystyrene end blocks and is a well-known commercial TPE. The other polymer added to the S–EB–S copolymer in reported studies is usually semi-crystalline. An example of such a blend is shown in Table 13.2. The final products exhibited dual phase continuity [66] and the authors claim that all three polymer phases (i.e., S–EB–S, the added semi-crystalline polymer and the polypropylene) are separate. The polypropylene makes up the interface between the other two polymers serving as a binder for them. The material is unusual by exhibiting a nearly constant modulus in the leathery range from the glass transition temperature of the center block (i.e., EB) near 60˚ C up to the melting temperature of the semi-crystalline component, which is typically 200˚ C or somewhat higher. This behavior was found advantageous by automotive design engineers and the material was used for the insulation of spark plug wires. Another possible combination is that of a triblock copolymer with an ionomer. The ionic groups

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Table 13.2. Example of a Composition for a Co-Continuous Interlocking Network Phases Component

Composition (parts by weight)

S–EB–Sa

100

Poly(butylene terephtalate)

100

Extending oil (paraffinic)

100

Polypropylene

10

HALS antioxidant

0.2

Non-staining antioxidant (e.g., LTDP)

0.5

Titanium dioxide

5

Note: Molecular weight distribution 25,000–100,000–25,000. Source: From reference [64]. a

in ionomers cluster together and form a physically bonded network [62]. The experiments performed to synthesize this type of copolymers employ two distinct methods [67, 68]: 

The chemically blended thermoplastic IPNs are prepared, for example, by dissolving S–EB–S in the mix of styrene monomer, methacrylic acid, and isoprene in the volume ratio 90/10/1. Then 0.4% of benzoin is added for subsequent photopolymerization.  The mechanically blended thermoplastic IPN is prepared in such a way that the styrenic monomer mix is photopolymerized separately and then melt blended with the S–EB–S triblock copolymer in an internal mixer. Both types of IPNs are neutralized by aqueous 10% solutions of either sodium hydroxide or cesium hydroxide in an internal mixer (as above). 13.2.2 Properties and Processing of Thermoplastic IPNs

The rheological and mechanical data are analyzed in reference [69]. It was concluded that on neutralization the triblock copolymer component, being the less viscous, assumes greater phase co-continuity. On the other hand, the mechanical data suggest that the phase inversion process remains somewhat incomplete, probably because of the relatively equal melt viscosities of the two polymers. Recent research in this area has concentrated on decross-linking and/or depolymerization [70] with the objective of creating thermoset polymeric

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materials that can flow at some later time by means of chemical reactions. Such materials are referred to as reworkable thermosets and show potential in electronics and biomaterials.

13.3 TPE Based on Polyacrylates One of the widely used and well-defined TPEs are triblock and radial-block copolymers of styrene and diene (butadiene, isoprene) based on the spontaneous and thermoreversible cross-linking in strong relation to the phase morphology [71]. However, the upper service temperature (UST) of a typical S–B–S block copolymer is limited to 60–70˚ C as a result of the partial miscibility of the styrene and butadiene blocks which decrease the glass transition temperature of polystyrene by about 20˚ C at Mn = 1.5  104 [72]. A substantial research work has been performed to alleviate this shortcoming by developing novel block copolymers. One approach is to prepare triblock copolymers containing at least one poly(alkyl methacrylate) constitutive block. Depending on the substitute, the Tg of this family of polymers can be extended over a large temperature range, for example, from 60˚ C for poly(2-ethylhexyl acrylate) up to 130˚ C for highly syndiotactic poly(methyl methacrylate), sPMMA. Thus, this sPMMA is an attractive substitute for PSt in the effort to increase the UST of a S–B–S block copolymer. Moreover, poly (alkyl acrylates) with low glass transition temperature have better heat and oxygen resistance than polydienes and are therefore a suitable replacement for them in the central block of the triblock copolymer [73]. 13.3.1 Synthesis of Triblock Copolymers Based on Methyl Methacrylate and Butadiene

Triblock copolymers with the combination of poly (methacrylate) and polybutadiene blocks (M–B–M) are prepared by sequential anionic polymerization but require several modifications to produce the desired rubbery cis-1,4 polybutadiene structure and a highly sPMMA block with a high Tg [74, 75]. These include the use of difunctional initiators and a seeding technique proposed in reference [76]. Detailed discussion of the synthesis is in reference [75]. 13.3.2 Properties of M–B–M Triblock Copolymers

The substitution of sPMMA for PSt in the traditional S–B–S TPEs results in the increase of

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UST by approximately 40˚ C without sacrificing the elastic properties of the final product. The ultimate mechanical properties of the two types of triblock copolymers of the same molecular weight (125,000), composition (64% of polybutadiene), and polybutadiene microstructure (45% of 1,2 units) are comparable. The values obtained from M–B–M triblock copolymers at 90˚ C (ultimate tensile strength = 14.3 MPa and elongation at break = 1,240%) are similar to those from S–B–S at 50˚ C (13 MPa and 1,200%, respectively) [77]. 13.3.3 Synthesis of Poly(MMA-tBA-MMA) Elastomers

Although there are several possible routes for the preparation of fully acrylic triblock copolymers the simplest method appears to be a two-stage process using difunctional anionic initiators. The most favorable situation is the initiation of MMA by living poly (t-BA) anions. Poly(MMA-b-tBA-b-MMA) can readily be converted into triblocks containing a central polyacrylate block of a low Tg. Transalcoholysis of poly(tert-butyl acrylate) PtBA by long-chain alcohols can be carried out selectively and quantitatively in the presence of PMMA by acid catalysis. Direct polymerization of this type of polymers is not possible because the anionic polymerization of primary and secondary alkyl acrylates cannot be controlled [78]. Using a similar technique, star-shaped copolymers of the type poly(MMA-b-alkylacrylate)nx or poly(St-b-alkylacrylate)nx have been synthesized by the “arm-first” method that consists of initiating the polymerization of a small amount of a bis-unsaturated monomer by the living diblock precursor (see Section 13.2.1.1) [79]. 13.3.4 Synthesis of Poly(MMA-b-Alkylacrylate -b-MMA)

The synthesis of fully acrylic TPEs by anionic polymerization has the major disadvantage of requiring the preliminary synthesis of a precursor triblock followed by its chemical modification into the desired polymer. Therefore, experiments were carried out to prepare them by sequential controlled radical polymerization [80, 81]. Triblocks containing 17–28 mol% of MMA, with central poly(n-BuA) block of Mn = 105,000 and Mw/Mn = 1.20 have been synthesized [82]. It was found that whenever radical process is used the PMMA outer block is less syndiotactic than the counterpart synthesized by anionic polymerization (57% vs 79%) [82].

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13.3.5 Mechanical Properties of Fully Acrylic Triblock and Branched Block Copolymers

A series of fully acrylic triblock and branched block copolymers were prepared by transalcoholysis of common poly(MMA-b-tBA-b-MMA) precursors by ethanol, n-propanol, n-butanol, and isooctyl alcohol. The resulting copolymers were then compared in stress–strain behavior to traditional S–I–S copolymer with comparable composition and molecular weights of the blocks. The welldefined and extended rubbery plateau typical for the S–I–S TPE was no longer evident in fully acrylic series [82]. The star-shaped copolymers behaved in the same way [79]. The ultimate mechanical properties (tensile strength and elongation at break) were found to be about half of those typical for S–B–S type TPE. A detailed discussion of polyacrylatebased TPEs and corresponding issues involving their synthesis and properties are in chapter 17 of Thermoplastic Elastomers, 3rd edition (Holden, G., Kricheldorf, H.R., and Quirk, R.P., Eds.), Hanser Publishers, Munich, 2004.

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