Synthesis and morphology of TPEs.
Thermoplastic processing is potentially accomplished by one of several reversible physical or chemical crosslinking mechanisms. Namely:
* Block copolymer association to form glassy or crystalline domains;
* random copolymer segment association (glass or crystalline);
* specific site association, e.g. ionomers; and
* reversible chemical bonding.
Technologically important TPEs have been developed based on block copolymer, random copolymer and specific site association.
The elastic nature of the TPEs is accomplished by having rubbery chains connected by the above reversible physical crosslinks. The combination of physical crosslinks and entanglements among the chains results in a rubbery network somewhat like a chemically crosslinked rubber and the retractive force is entropic in nature. In other TPEs an elastic structure is the result of a cocontinuous phase morphology. In this case, one of the phases is semi-crystalline and deformation and recovery depends to some extent on this phase and is enthalpic in nature.
Synthesis of TPEs depends on several polymerization technologies. Anionic polymerization of styrene and 1,3 dienes is extensively employed for A-B-A block copolymers. This unique living polymerization is exploited to make various polymeric structures. Multi-block copolymers (A-B)n are usually produced with step-growth polymerizations with polyester, polyamide or urethane linkages. Random copolymers, which act as TPEs, are synthesized by a variety of polymerization mechanisms. Polyolefins of this type use coordination catalysis.
A two-step synthesis of TPEs that first uses polymerization followed by blending has been recently employed. One phase is chemically crosslinked and polymeric compatibilizers are employed to give highly elastic composites that are easily melt processable.
Anionic polymerization of block copolymers
Anionic polymerization is commonly used with styrene, butadiene and isoprene to form block copolymers (refs I and 2). An inert hydrocarbon solvent such as cyclohexane or toluene that is free of any materials that will react with the anion is employed in a batch reactor The preferred initiators are organolithiums. The polymerizations are conducted to make block copolymers either by sequential monomer addition, coupling of polymer chains or by multi-functional initiation.
Synthesis of thermoplastic polyurethanes
Thermoplastic polyurethanes (TPU) are synthesized by reacting a diisocyanate, a polyol and a diol extender to produce a copolymer having a multi-block structure ref 3). The most important diisocyanate for TPUs is MDI (4,4 diphenylmethane diisocyanate. Linear glycols are used as chain extenders such as 1,4, butanediol, hydroquinone bis (2-hydroxyethyl) ether and 1,6-hexanediol. MDI and these glycols form urethanes and have Tgs and crystalline melting points that result in crystallization at use temperature and can be processed without decomposition or crosslinking reactions.
The soft segments of TPUs depend on the polyol used (figure 1). The elastic, chemical and polarity of these segments controls the temperature dependent physical properties, weather and solvent resistance of the polymers.
Polyesters which are hydroxy terminated result from stepgrowth polymerization of adipic acid and an excess of a low molecular weight glycol (ethylene glycol, butanediol, hexanediol). Polyethers are made by ring opening polymerization of propylene oxide and/or ethylene oxide with the base catalyzed addition to a difunction initiator (propylene glycol or water). Cationic polymerization of tetrahydrofuran yields poly (oxytetramethylene) glycol.
Synthesis of the high MW TPU is either done in a single batch reaction or by a prepolymer method where the polyol and MDI are reacted first.
Synthesis of thermoplastic polyesters
Thermoplastic polyesters have hard segments based largely on crystalline tetramethylene terephthalate (4GT) repeat units and low Tg rubbery segments consisting of poly (tetramethylene oxide) glycol (PTMO) (ref. 4). The polyester TPEs are multi-block copolymers with (A-B) structure similar to a TPU.
Like the TPUs many structural and compositional variations are possible due to the richness of the condensation polymerization chemistry involved. These structure variations allow a wide variety of elastic materials to be synthesized. Copolymers are readily prepared by melt copolymerization of dimethyl terephthalate, the high molecular weight glycol and excess tetramethylene glycol. Esterification is aided by the presence of titanate catalysts and the excess methanol is removed by reduced pressure.
Synthesis of random copolymers
Semi-crystalline random copolymers exhibit the properties of TPEs. Examples are plasticized PVC and ethylene propylene copolymers (EPM). PVC has crystalline sequences due to chain-end directed stereochemistry while EPM has blocks of ethylene long enough to crystallize at use temperatures. These types of materials are synthesized by typical free radical dispersion polymerization (PVC) and Ziegler-Natta coordination solution or dispersion polymerization (EPM).
Synthesis of TPEs by blending (ref. 5)
Blends have long been an important aspect of rubber science and technology and have been important in thermosetting systems. Compounding with fillers and plasticizers has also been carried out extensively to provide required physical and processing properties in vulcanized rubber systems. Indeed, thermoplastic elastomers based on blending technology date from the 1940s using PVC and butadiene-acrylonitrile copolymers. The PVC/NBR blend systems achieve their rubber-like properties by lowering the Tg of one phase of the PVC into the rubber region while the crystalline segments in PVC provide thermally reversible physical crosslinks.
Thermoplastic elastomers based on blends of polyolefins are an important family of engineering materials. Their importance arises from a combination of rubbery properties along with their thermoplastic nature in contrast to thermoset elastomers. The development of polyolefin thermoplastic elastomer blends follows somewhat that of thermoplastic elastomers based on block copolymers such as styrene-butadiene-styrene triblock copolymer and multisegmented polyurethane thermoplastic elastomers which were instrumental in showing the utility of thermoplastic processing methods.
Polyolefins are based on coordination catalysts that do not easily lend themselves to block or multisegmented copolymer synthesis. However, polyolefins have many important attributes favorable to useful elastomeric systems. These considerations led to the development of polyolefin thermoplastic elastomer blends, and two types are now widely used: blends of ethylene-propylene rubber (EPM) with propylene (PP) and blends of EPDM and PP in which the rubber phase is highly crosslinked.
Polyolefin blends: In the simplest blends with polyolefins, such as high-molecular-weight EPM and PP (ref. 9), intensive mixing results in two continuous phases. By adjusting the viscosity ratios, both phases can be kept continuous over a considerable range of volume fractions in this blend (e.g., 80/20 to 20/80).
At the typical copolymer monomer ratios and molecular weights of EPM copolymers used in blends, the elastomers are insoluble in PP when held quiescently in the melt. This has been inferred from studies of the glass transition of EP atactic PP blends by DSC which do not have the single narrow change in heat capacity ([DELTA][C.sup.p]) found in soluble polymer systems. Scattering studies of deuterium-labeled EPM blends with isotactic PP indicate a two-phase melt. Shear could influence solubility during mixing of simple blends, but the nodular nature of the phases and phase size is more consistent with a shear dispersion mechanism than a spinodal decomposition or crystallization on cooling from a thermodynamically soluble system.
For high-molecular-weight polyisobutylene-based polymers, the solubility in the melt will be less than for EPM in PP, so it is likely that the morphology of these blends results from mixing a two-phase system.
For simple blends, the PP phase is continuous and exhibits elastic properties due to the open fiber-like microstructure. The EPM phase will be somewhat elastic if it is semicrystallizing or if it is highly branched, and if this phase is continuous, it will also provide some elastic response, particularly at short time scales.
Dynamically vulcanized blends: In blends where the rubber phase is crosslinked during the mixing process, the crosslinked phase must be discontinuous. In the absence of such discontinuities, shear deformation during flow would not be possible without fracture of the rubber phase. The viscosity would, of course, be unacceptably high because the shear-stress would have to be high enough to fracture the continuous phase during melt processing.
The break-up mechanism(s) that control particle size in dynamically cured systems have not been extensively investigated. Depending on the crosslinking rate of the EPDM phase, this could be the result of increased viscosity due to the long-chain branching prior to gel formation. In bicomponent blend systems, the most viscous phase tends to become the discontinuous phase during intensive mixing. Thus, as the crosslinking progresses during mixing, the viscosity increase of the major elastomer phase could either result in the inversion of phase continuity or go from a bicontinuous to a dispersed EPDM phase. Plasticizer oils complicate this picture considerably, since the solubility and viscosity of the components could be altered.
Polymers are synthesized with very controlled chemical structures to produce TPEs. The chemical structure variables (composition, molecular weight, block strengths and tacticity) and how these are inter- and intra-molecularly distributed along with processing/thermal history result in characteristic morphologies for TPEs. With this number of interrelated variables it is not surprising that many unique morphologies are obtained.
The elastic nature of these materials depends strongly on morphology. Moreover, the morphology of the melt also plays a large role in rheological properties and processability.
Styrene-diene-styrene block copolymers
The morphology of styrene-diene triblock thermoplastic elastomers is represented in figure 2 (ref. 6). At the proper concentration of polystyrene ( 20 wt%) the end-blocks form domains some 300-350 A in diameter. With the chain-ends fixed the diene segments form an elastic network consisting of entangled chains.
Different morphologies are obtained as the polystyrene content is increased. Starting from spherical domains at less than 20% polystyrene, they change to cylinders and then to lamellae as the content is increased to 50%. Beyond this point there is phase inversion with the polydiene becoming the continuous phase.
The styrene-diene ratio controls phase morphology. The molecular weights of the blocks must be high enough to yield incompatibility of the phases at use temperatures.
Block copolymers undergo an order-disorder transition (ref. 7). Depending on temperature, composition, block molecular weight, etc., the polymers can form a homogeneous, molecularly mixed melt where there is no microphase separation. In this disordered state the block segments are soluble in each other.
The order-disorder transition is thermodynamic in nature which is controlled by physical factors; energetics and entropy.
If processing is carried out above the Tg of both phases but below the order-disorder transition, the polymers have high viscosity, are very non-Newtonian and quite elastic in the melt. Viscosity and elasticity are lower in the disordered state.
Morphology of multi-segmented block copolymers
The morphology of multi-segmented TPU and polyester elastomers plays the major role in controlling final stress-strain and other physical properties. The morphology is a consequence of the initial chemical structure and the processing history of the materials. Because of the ability to synthesize many chemical structural variations and processing involves crystallization, flow and diffusion, the nature of the morphology has been a formidable scientific challenge.
As in the A-B-A block copolymers, phase separation occurs due to thermodynamic incompatibility of the hard and soft segment phases. Phase separation is more difficult as the number of blocks increases. Moreover, in the multi-segmented block copolymers the blocks are of random length.
While there are many differences in morphology within these types of copolymers the general picture is that of continuous phases of both hard and soft segments with some blocks (usually short) mixed between or dissolved in the contrasting phase.
Thermoplastic polyurethane elastomers: The hard segments in TPU are usually paracrystalline to crystalline depending on chemical structure and thermal history. Long annealing times produce higher levels of crystallinity. WAXS, SAXS and DSC analysis have been used extensively on these polymers to probe morphology. The degree of phase separation controls crystalline melting points and the Tg of the soft segments.
Thermoplastic polyester elastomers: The morphology of the block polyesters is characterized by a more or less continuous phase of crystalline lamellae extending through a continuous elastic phase. The lamellae are typically organized into spherulites with the proper thermal history. The lamellar morphology can be seen by transmission electron microscopy. TEMs show lamellae approximately 100 A thick by several thousand A long. As usual with crystallization phenomena, very slow cooling from the melt or solution casting yields the largest lamellae. Spherulite morphology is observed when samples are melt or solution cast.
WAX data indicate crystalline orientation during elongation.
Morphology of polvolefin thermoplastic elastomer blends Polvolefin blends: Blends of high molecular weight EPM and i-PP are produced over the entire composition range. At low concentration of EPM (5 to 30 wt%) the rubber is the dispersed phase. If high density polyethylene is included in these blends a shell core morphology is obtained, the copolymer being between the IPP and HDPE phases.
Kresge reviewed rubbery thermoplastic blends in 1978. The morphology of EPM/PP thermoplastic elastomers was shown to have continuous interpenetrating phases. The minor phase in this blend is PP (70/30 EPM/PP by weight). The phase size ranges from about 2[mu]m to less than 0.1[mu]m across the PP strands. Later work by Lohse showed that EPM and PP are also immiscible two-phase mixtures in the melt. Thus, the morphology of these blends is not dependent on the crystallization of the PP. Moreover, the PP is nonspherulitic in nature with a well-defined x-ray diffraction pattern of monoclinic PP.
Compatibilizers also are effective at altering the morphology of blends of EPM and i-PP. Graft copolymers of EPDM with i-PP side chains result in a large reduction in phase size. The size is also stabilized on holding the blend in the melt.
PP and several of the crystalline polymers were also found to be transformable into elastic materials by changing their crystal structure. With isotactic PP it is possible to obtain 97% recovery from 100% extension by using high stress during crystallization of fibers. The fibers were shown to have a non-rubber-like stress-temperature response and that the elastic nature was due to morphology. Electron microscopy showed close-packed lamellae with normals mainly parallel to the fiber axis which on extension tilt and split apart, creating voids. This nonentropic elastic nature of isotactic polypropylene may play a role in blend technology.
Isotactic PP and other crystalline poloyolefins with low glass-transition temperatures also have the unique characteristic of very high flexural fatigue life. Thin sections of a highmolecular-weight PP will withstand repeated extension without failure as used in designing self hinging in various molded articles.
Dynamically vulcanized blends: Crosslinking of an elastomer phase during mixing with a thermoplastic results in cured particles of rubber dispersed in a continuous matrix.
The morphology of dynamically cured blends of EPDM and PP were also explored by peroxide vulcanization of the rubber phase. In this case, the EPDM was shown to be a discontinuous dispersed phase with gelled particles ranging from less than 0.5 to 10 [mu]m.
Particle size in the dynamically vulcanized blends has a pronounced effect on mechanical properties as shown by Coran and Patel. As the particle size is decreased, ultimate elongation and tensile strength increased rapidly. In these experiments, the large particles were formed by press curing the elastomer and then grinding by tight roll-milling to various sized (72 to 7 [mu]m) and, therefore, were formed by a fracture mechanism rather than formation during crosslinking.
One of the major advantages of the dynamically cured blends over unvulcanized blends is that the morphology is fixed on crosslinking and is not altered by subsequent melt processing.
More complex morphologies are possible for thermoplastic elastomers produced by dynamic vulcanization. Puydak and Hazelton produced a dynamically cured blend containing two elastomer phases in a continuous semicrystalline polyolefin matrix. Both neoprene (CR) and IIR are separate dispersed phases. The CR phase consists of larger particles 1-2 [mu]m in size while the IIR phase is smaller. This apparently is caused by the differences in surface energy among the three phases.
A three-phase blend has also been produced where a crosslinked-elastomer phase is completely surrounded by an uncured phase in a continuous thermoplastic polymer matrix. This shell and core morphology must result from a minimization of the surface energy among the three phases along with the sharp viscosity increase due to crosslinking.
Coran and Patel showed that compatibilization could be used to control the morphology of NBR-polyolefin blends. This was accomplished by the in-situ formation of block copolymers by reacting amine-terminated NBR with maleicanhydride-modified PP. Only a small amount of block copolymer compatibilizer is necessary to produce small particle size and adequate adhesion between the PP and the NBR phases. The ability to control morphology in polyolefin thermoplastic elastomer blends allows tailoring of mechanical properties to meet a great variety of end-use applications. For example, simple blends EPM and PP have a crystalline polyolefin surface, and this results in a glossy appearance with a plastic-like feel. In contrast, dynamically vulcanized blends of the same materials typically have elastomeric surfaces with low gloss, a high coefficient-of-friction and a rubberlike feel.
[1.] M. Morton, Anionic polymerization, principles and practice, Academic Press, Inc., New York, 1983. [2.] S. Bywater, Encyclopedia of polymer science and engineering, John Wiley & Sons, Inc., 1990. [3.] W. Meckel W. Goyert, W. Wieder, Chpt. 2, Thermoplastic elastomers, N.R. Legge, G. Holden, H.E. Schroeder, Eds., Hanser Publishers, New York, 1987. [4.] R.K. Adams, G.K Hoeschele, Chpt. 8, ibid. [5.] E.N. Kresge, Rubber Chem. Tech., 64, 469 (1991). [6.] G. Holden, N.R. Legge, Chpt. 3, Thermoplastic elastomers, N.R. Legge, G. Holden, H.E. Schroeder, Eds., Hanser Publishers, New York, 1987. [7.] T Hashimoto, Chpt. 12, Section 3, ibid. [8.] A. Y. Coran, Chpt. 7, ibid. [9.] E.N. Kresge, D.J. Lohse, S. Datta, Makromol. Chem., Macromol Symp. 53,173 (1992).
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|Title Annotation:||thermoplastic elastomers|
|Date:||May 1, 1993|
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