Rubber Compounding - Chemistry and Applications Part 6 pot

5
Thermoplastic Elastomers:
Fundamentals and Applications
Tonson Abraham and Colleen McMahan
Advanced Elastomer Systems, L.P., Akron, Ohio, U.S.A.
I. INTRODUCTION
In the fifteenth century, Christopher Columbus witnessed South Americans
playing a game centered around a bounceable ‘‘ solid’’ mass that was
produced from the exudate of a tree they called ‘‘weeping wood’’ (1). This
material was first scientifically described by C M. de la Condamine and
Francßois Fressneau of France following an expedition to South America in
1736 (2). The English chemist Joseph Priestley gave the name ‘‘ rubber’’ to the
material obtained by processing the sap from Hevea brasiliensis,atall
hardwood tree (angiosperm) originating in Brazil, when he found that it
could be used to rub out pencil marks (2). A rubber is a ‘‘solid’’ material that
can readily be deformed at room temperature and that upon release of the
deforming force will rapidly revert to its original dimensions.
Rubber products were plagued by the tendency to soften in the summer
and turn sticky when exposed to solvents. This problem associated with
natural rubber was overcome by Charles Goodyear in the 1840s by subjecting
the rubber to a vulcanization (after Vulcanus, the Roman god of fire) process.
Natural rubber was vulcanized by heating it with sulfur and ‘‘white lead’’
(lead monoxide) (2). In May 1920 the German chemist Hermann Staudinger
published a paper that demonstrated that natural rubber was composed of a
chain of isoprene units, that is, a polymer (from the Greek poly, many, and
mer, part) of isoprene (3). In vulcanization the rubber macromolecules are
chemically bonded to one another (‘‘ cross-linked’’ in a thermosetting process)
to form a three-dimensional network composing a giant molecule of infinite
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molecular weight. At present the word ‘‘rubber’’ is associated with macro-
molecules that exhibit glass transition below room temperature and have
‘‘long-chain,’’ ‘‘organic,’’ carbon-based backbones or ‘‘inorganic’’ back-
bones typified by polysiloxanes and polyphosphazenes.
‘‘Elastomer’’ is always used in reference to a cross-linked rubber that is
elastic (Greek elastikos,beatenout,extensible).Anelastomerishighly
extensible and reverts rapidly to its original shape after release of the
deforming force. Entropic forces best describe rubber elasticity (4). However,
it should be noted that under relatively much smaller deformation, plastic
materials and even metals can exhibit elasticity due to enthalpic factors (4).
Gases and liquids also exhibit elastic properties due to reversible volume
changes as a result of pressure and/or heat (4). Nevertheless, the term
‘‘elastomer’’ is always used in reference to rubber elasticity.
A plastic material is one that can be molded (Greek plastikos), and a
thermoplastic can be molded by the application of heat. A rubber compound
(a blend of rubber, process oil, filler, cross-linking chemicals, etc.) is thermo-
plastic and is ‘‘set’’ after several minutes in a hot mold, with loss of
thermoplasticity. A thermoplastic material can be molded in a matter of
seconds, and the molded part can be reprocessed. The viscous character of the
thermoplastic melt readily allows control of the appearance of the surface of
finished goods. In comparison, the effect of ‘‘ melt elasticity’’ of a rubber
compound on end product surface appearance is not as readily controlled.
The origin of the first thermoplastic material can be traced to Christian
Schonbein, a Swiss scientist who broke a beaker containing a mixture of nitric
and sulfuric acid and used his wife’s cotton apron to clean up the spillage!
Unfortunately for his wife, but fortunately for science, he left the washed
apron near a fireplace to dry. The cotton apron soon combusted without
leaving any residue! Schonbein realized that the cotton of the apron was
converted to ‘‘ gun cotton,’’ a nitro derivative of the naturally occurring poly-
mer cellulose (1). This learning may have been instrumental in the preparation

of the first plastic by the English chemist and inventor Alexander Parkes in
1862. First called Parkesine, it was later renamed Xylonite. This substance
was nitrocellulose softened by vegetable oils and a little camphor. During this
time, elephant tusks, which were used to make ivory billiard balls, among
other things, became scarce. In 1869, motivated by the need to find a suitable
substitute for ivory, John W. Hyatt in the United States recognized the vital
plasticizing effect of camphor on nitrocellulose and developed a product that
could be molded by heat. He named this product obtained from cellulose
‘‘Celluloid’’ (Greek oid, resembling). Though primarily regarded as a substi-
tute for ivory and tortoiseshell, Celluloid, despite its flammability, found
substantial early use in carriage and automobile windshields and motion
picture film (3).
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A. Definition of Thermoplastic Elastomer
A thermoplastic elastomer (TPE) is generally considered a bimicrophasic
material that exhibits rubber elasticity over a specified service temperature
range but at elevated temperature can be processed as a thermoplastic
(because of the thermoreversible physical cross-links present in the material).
It offers the processing advantages of a highly viscous melt behavior and a
short product cycle time in manufacturing due to rapid melt hardening on
cooling.
B. Classification of Commercially Available Thermoplastic
Elastomers
The TPE products of commerce listed in Table 1 are classified in Table 2 on
the basis of their polymer microstructure. Representative examples are
included for each polymer class. Segmented block copolymers, triblock
copolymers, and thermoplastic vulcanizates represent a significant portion
of the TPE family.

The fundamental aspects of structure–property relationships in ther-
moplastic polyurethanes (TPUs), styrenic block copolymers (SBCs) [with
emphasis on styrene/ethylene-1-butene/styrene (SEBS) copolymers and
SEBS compounds], and thermoplastic vulcanizates (TPVs) produced from
polypropylene and ethylene/propylene/diene monomer (EPDM) rubber were
selected for review in this chapter, as representative of the most commercially
significant and the closest in performance to thermoset elastomers.
Table 1 Thermoplastic Elastomer Products of Commerce
Product
First commercialized
(year, company)
Plasticized poly(vinyl chloride) 1935, B. F. Goodrich
Thermoplastic polyurethane 1943, Dynamit AG
PVC/NBR blends 1947, B. F. Goodrich
Styrenic block copolymers 1965, Shell
Thermoplastic polyolefin elastomers 1972, Uniroyal
Styrenic block copolymers (hydrogenated) 1972, Shell
Copolyester elastomers 1972, DuPont
Thermoplastic vulcanizates (PP/EPDM) 1981, Monsanto
Copolyamide elastomers 1982, Atochem
PP/NBR TPVs 1984, Monsanto
Chlorinated polyolefin/ethylene interpolymer rubber 1985, DuPont
UHMW PVC/NBR 1995, Teknor Apex
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Thermoplastic vulcanizates possess sufficient elastic recovery to chal-
lenge thermoset rubber in many applications, and insights into TPE elastic
recovery and processability are presented based upon the latest developments
in the field. The poor elastic recovery of TPEs at elevated temperature is a key

deficiency that has prevented these materials from completely replacing their
thermoset counterparts.
Thermoplastic elastomers owe their existence as products of commerce
to the fabrication economics and environmental advantage they offer over
thermoset rubber. TPEs, of course, are designed to flow under the action of
heat; hence their upper service temperature is limited in comparison to
thermoset rubber. Thus a major hurdle to overcome in the replacement of
thermoset rubber with TPEs is the improvement in elastic recovery, partic-
ularly at elevated temperature, especially compression set, because in many
applications elastomers are subjected to compression. The scope of this
chapter includes those TPEs that in our opinion come reasonably close in
properties to thermoset elastomers, as listed in Table 1. Not included, for
example, are plastomers that are ethylene/a-olefin copolymers generally
produced using metallocene catalysts (5).* These materials can be rubberlike
only at room temperature. They are thermoplastic owing to the thermorever-
sible cross-links provided by crystallization of the ethylene sequences in the
polymer but are deficient in elastomeric character above room temperature or
when under excessive strain. Thermoplastic elastomers based on melt-blended
polyolefins, ethylene/vinyl acetate copolymers, and ethylene/styrene co-
polymers are also omitted from the list (6,7). Although thermoplastic olefins
(TPOs) represent a commercially important class of materials, they are
included primarily as comparative points to their more elastomerically per-
forming counterparts, TPVs.
Plasticized poly(vinyl chloride) (PVC) is used as a flexible plastic and
not an elastomer but is included in Table 1 because it was the first commer-
Table 2 Thermoplastic Elastomer Classification
Segmented block
copolymers
Triblock
copolymers

Thermoplastic
vulcanizates
Polymer
blends
TPU SBC PP/EPDM PVC/NBR
COPE Hydrogenated SBC PP/NBR
COPA PP/IIR
*Note that Ziegler–Natta-based plastomers are also commercially available. For example, some
of Dow’s Flexomer products are based on ethylene/1-butene copolymers.
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cially produced thermoplastic elastomer. PVC, produced by free radical
polymerization, contains crystallizable syndiotactic segments, the crystalli-
zation of which is enhanced on mobilization of the polymer chain in the
presence of a plasticizer (8). However, imperfections in the crystalline phase
limit the upper service temperature of PVC.
II. THERMOPLASTIC ELASTOMERS: APPLICATIONS
OVERVIEW
Thermoplastic elastomers are found in thousands of applications, ranging
from commodity TPOs used in automotive bumper and facia applications,
through plastomers used as impact modifiers for plastics, and TPVs and SBCs
in sealing applications, to TPUs and copolyesters in numerous engineering
applications. TPEs replace EPDM rubber in many sealing applications, butyl
rubber where permeation resistance is required, and nitrile rubber for oil and
fuel resistance.
World demand for thermoplastic elastomers will grow at over 6% per
year through 2006, according to a recent study (9). The 1.6 million metric ton
TPE industry will remain concentrated in the United States, Western Europe,
and Japan, although underdeveloped markets such as Asia grow at a faster

rate.
The most important driver for TPE growth through thermoset rubber
replacement is cost savings. This is normally achieved through a combination
of material selection, part redesign, and fabrication economics. Recyclability
and weight reduction provide additional drivers in some markets. Colora-
bility is another important TPE attribute that increases design flexibility.
Further, use of TPEs allows introduction of designs, processes, and value-
added features not possible at any cost with thermoset rubber.
Almost all commercial TPEs have one feature in common: they are
microphase separated systems in which one phase is hard at room tempera-
ture while another phase is soft and elastomeric. The harder phase gives TPE
their strength and, when softened, their processability. The soft phase gives
TPEs their elasticity. Each phase has its own glass transition temperature, T
g
,
or crystal melting point, T
m
, and these in turn determine the temperatures at
which the TPEs exhibit their transition properties. Thus, the TPE service
temperature on the lower end is bounded by the T
g
of the elastomeric phase,
whereas the upper service temperature depends on the T
m
of the hard phase.
Note that the practical service range also depends on the softening point,
stress applied, and article design (10).
The ability of TPEs to repeatedly become fluid on heating and solidify
on cooling gives manufacturers the ability to produce rubberlike articles using
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the fast processing equipment designed for the plastics industry. Scrap can
usually be reground and recycled. Output of parts is generally increased and
labor requirements reduced compared to parts manufactured from thermoset
rubber. Thermoplastic elastomers can be fabricated by conventional thermo-
plastic methods including injection molding, blow molding, and extrusion.
Injection molding processes range from single- to multiple-cavity,
including up to 48 or more cavities per mold, hot runner mold technology
for runnerless part production, insert molding with other materials, and
coinjection molding of two materials sequentially or simultaneously. Tools
such as MoldflowR (11) allow fast development of tooling and process
conditions for many TPEs. Another significant advantage is that injection
molding of TPEs allows dimensional tolerances not achievable in thermoset
rubber. This allows snap fits and ‘‘living hinges’’ to be designed into the parts.
Flexible, nonblooming, flashless parts are easily produced on largely auto-
mated molding equipment. A compatible thermoplastic can give excellent
bond strength with two-shot injection molding. For noncompatible materials,
a physical lock or interference fit is used over a rigid substrate of metal, plastic,
or even glass (12).
Blow molding is practiced by injection blow molding, extrusion blow
molding, or press blow molding processes. Complex designs can be easily
manufactured by three-dimensional sequential blow molding with multiple
materials. Fabrication process equipment is available today that can blow
mold three-dimensional parts from combinations of thermoplastic and ther-
moplastic elastomer materials in up to seven layers by precise material de-
livery, robotic parison manipulation, and perfectly timed mold positioning,
all computer-controlled in a largely automated process (13).
Extrusion of thermoplastic elastomers includes single-extrusion, co-
extrusion, and triple-extrusion processes. Multiprofile dies for extrusions

from a single line provide important improvements in efficiency for simple
extrusions. Hard–soft combinations with other polymers, including polyole-
fins, polystyrene, and other TPEs, are commonly practiced. Recent develop-
ments include coextrusion of thermoset EPDM with TPVs (14,15). Special
extrusion processes have been developed to produce foamed profiles using
water as the blowing agent (16,17) and create low-friction surfaces with a
coextruded slipcoat, offering low-cost environmentally friendly alternatives
for specific applications. Robotic extrusion of TPVs, through a system
composed of a moving die, flexible heated hose, and 3D robot, has been used
to apply seals directly to automotive parts (18,19). Secondary processes such
as heat welding, thermoforming, coating, printing, and painting add signifi-
cant value at moderate cost in many applications.
Thermoplastic elastomers can offer the design engineer greater design
flexibility as well as part size and weight reduction. In the case of thermoset
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rubber replacement, the part is usually redesigned to leverage the physical
properties and processing characteristics of the TPE. The use of TPEs
frequently allows designers to reduce the amount of material per part and,
combined with the lower specific gravity of TPEs in comparison with thermo-
sets, significantly reduce the overall part weight compared with thermoset
rubber (20). An important advantage in redesign is the opportunity for parts
consolidation through combinations of thermoplastic elastomer and other
thermoplastic components.
Thermoplastic elastomer grades have been developed that bond to a
wide range of engineering thermoplastics, including polypropylene, polyeth-
ylene, polystyrene, polyamides, polyesters, acrylonitrile/butadiene/styrene
(ABS) rubber modified plastic, cured EPDM rubber, polycarbonates, and
copolyesters. The bond is typically formed through an autoadhesion (diffu-

sion) mechanism during thermoplastic processing (21,22). In many cases,
bond strengths at levels comparable to material strength can be achieved.
A. Thermoplastic Elastomers in Automotive Applications
The automotive industry has always been a major end-use market for TPEs
and accounts for about 60% of the total demand in North America. Tires
account for most of the thermoset elastomeric content in a vehicle. The rest is
spread over 600 or more elastomer applications from simple grommets to
complex constant-velocity joint boots and radial lip seals. Automotive
elastomeric parts serve in a wide range of operating environments. They also
provide numerous functions such as air, vacuum, and fluid seals; mechanical
shock absorption; flexible couplings; and soft-touch interior components. As
with any elastomer, TPEs have their limitations. They do not have the
combination of abrasion resistance, flexural strength, deformation resistance,
and high-temperature use that thermoset elastomers display; therefore, these
materials have found no significant use in pneumatic tires.
Key automotive trends have provided a demand for increasing use of
TPEs. The most important is the drive for cost reduction in every possible
component of the vehicle. Even though TPEs are more expensive as a raw
material than thermoset elastomers, the cost of the TPE finished part is
usually significantly lower than that of a functionally comparable thermoset
rubber part through redesign including lighter weight, shorter cycle time,
lower energy usage, lower scrap, and recyclability.
Another significant automotive trend is the increased level of govern-
ment regulations, which has forced the world’s automotive manufacturers to
put major emphasis on improving safety and increasing fuel efficiency,
recyclability, and the use of environmentally friendly materials. As Germany
led the world in reduction of nitrosamine-containing cure package compo-
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nents for thermoset rubber, the European Union leads with respect to
legislation requiring higher recyclable content and lower overall vehicle
emissions (23). Recyclability has provided a consistent driver in the Japanese
market. Thermoplastics and thermoplastic elastomers are key to reaching the
target (24). Vehicle manufacturers have taken a lead as well, including targets
for increase in recyclable content and elimination of PVC use in certain auto
interior skin applications. The relatively low price of PVC compounds,
however, makes replacement by olefinic systems difficult from a cost view-
point (25).
In addition, the automotive industry is trying to respond effectively to
an increased level of technical performance requirements. Higher perfor-
mance engines, operating at higher temperatures with lowered emissions,
coupled with improved aerodynamics due to decreased frontal and grille area,
contribute to increasing under-the-hood temperatures. Longer lived automo-
biles also require elastomers with improved ultraviolet resistance. Soft-touch,
color-matched interior parts, featuring low odor and low fogging, add to
esthetics and consumer-recognized value.
Engine compartment timing belt covers with a flexible segment of
rubber and a rigid segment of polypropylene have successfully employed
TPE. Fuel line covers from specially formulated flame-retardant grades, rack-
and-pinion boots taking advantage of the outstanding flex fatigue resistance
of TPVs, and clean air ducts featuring innovative convolute designs in
combination with polypropylene are just a few examples of automotive
applications that leverage the unique properties of TPVs. Thermoplastic
elastomers, especially thermoplastic vulcanizates, are moving quickly into
automotive weatherseal applications; this market provides significant growth
potential for TPEs in the future. TPEs are injection molded for glass
encapsulation and cutline seals. They are extruded for belt line and glass
run channel seals. Extruded seals can be coated with specially formulated low
friction TPEs and joined at the corners with specialty molding TPVs to

replace flocked thermoset EPDM seals with 100% recyclable parts.
B. Thermoplastic Elastomers in Industrial Applications
Thermoplastic vulcanizates are found in hundreds of industrial applications.
In most cases the drivers for TPE use are the same as in other industries, i.e.,
thermoset elastomer performance with the advantages of thermoplastic
economics. The building and construction industry takes advantage of TPE
performance to provide critical sealing in places such as architectural glazing
seals, bridge deck seals, pipe seals, and roofing. Industrial hose applications
form a growing segment of TPV applications, including fire hose, washdown
hoses, and specialty grades for handling potable water and food. Excellent
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TPV resistance to detergents, acids, and bases, combined with superior flex
life and weatherability compared to thermoset rubber, drive application in
thousands of small sealing parts such as gaskets and bushings in appliances
and mechanical devices worldwide. Specialty TPEs featuring low flame
retardancy, good abrasion resistance, dielectric strength, and wet electrical
performance are used in electrical applications, especially wire and cable
coverings, insulators, and flexible connectors (26). Conductive thermoplastic
elastomers incorporating carbon or metal powders are used for static
dissipative and conductive properties or in electromagnetic interference/radio
frequency interference (EMI/RFI) shielding (27).
Multilayer coated sheets are used in roofing, and their use is expanding
to innovative applications such as pillow tank liners.
C. Thermoplastic Elastomers in Consumer Applications
Thermoplastic vulcanizates are found in a variety of consumer products, most
recognizably those incorporating grips for soft but secure handling of power
tools, housewares, and toothbrushes. Good sealing properties and good
chemical resistance make them well suited for kitchen appliances (28).

Because many TPEs have consistent frictional characteristics over a range
of temperatures and in wet and dry conditions, they are well suited for use in
this growing market. The ability to adhere to a variety of substrates by two-
shot or overmolding allows processing ease with excellent adhesion. Trans-
parent and translucent products are readily available.
Many ballpoint pens now feature a soft grip made from a TPE.
Cosmetic containers, food containers, and water bottles incorporate TPEs
for soft-grip feel, color, and design innovation. The demand for thermoplastic
rubber soft grips is also growing in sports applications, such as tennis racket
or golf club grips. Other sports and leisure applications include toys, ski
equipment, and sports balls (e.g., soccer ball inner bladder) made from butyl
rubber–based TPVs. Consumer products emphasize good esthetic design as
well as functionality, and the ability of TPEs to be decorated is a real
advantage. Techniques such as permanent laser marking and the application
of hot stamping foils, heat transfer labels, or screen or tampo printing have
been used for marking various products, including multicolored flexible
labels. Logos can be integrally designed into products by using overmolding
of hard–soft combinations. Effects linked to other materials such as minerals
can be obtained through the use of innovative pigments; marble and granite
are the most commonly imitated materials (29). Newer application areas for
TPEs in consumer products include personal electronics and a growing range
of household and garden tools.
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III. SEGMENTED BLOCK COPOLYMER TPEs
The segmented block copolymer TPEs included in Table 1–3 contain sequen-
ces of ‘‘ hard’’ and ‘‘soft’’ blocks within the same polymer chain. Solubility
differences between the polymer segments and association and/or crystalliza-
tion of the hard blocks produce phase separation in the molten elastomer as it

cools. The hard blocks form the thermoreversible cross-links and reinforce-
ment (increasing stiffness) of the elastomeric soft phase. The rate of crystal-
lization or association of the hard blocks will impact product fabrication time.
Polymer microstructure and morphology is depicted in Figure 1. These TPEs
are produced by condensation or addition step growth polymerization and
have low molecular weight segments. Although this is desirable, segment mo-
lecular weight and molecular weight distribution cannot be readily controlled.
In a 40 Shore D copolyester (COPE) elastomer based upon poly(butylene
terephthalate) (PBT) hard blocks and poly(tetramethylene oxide/terephthal-
ate) (PTMO-T) soft blocks, the hard sequence length varies from 1 to 10 (30).
PBT molecular weight of sequence length 10 is 2200, whereas high molecular
weight PBT that is commercially available could easily have an M
n
of 50,000!
Thus, a sufficient number of hard blocks have to associate to produce a high
enough melting crystal phase to provide a reasonably high elastomer upper
service temperature. This necessitates increasing the hard-phase content of the
TPE, which results in a hard elastomer (‘‘filler’’ effect). Note that for a given
hard-phase content, the lower the number of hard domains (more hard
segments per domain), the greater the entropic penalty imposed on the
elastomeric phase and the less favored the phase-separated morphology.
Increased hard phase content also causes more hard segments to be
rejected into the amorphous elastomeric phase, thus raising the rubber glass
transition temperature (T
g
) and therefore also the TPE lower service temper-
ature. In the case of an increase in the number of hard domains, the soft-phase
T
g
is also elevated owing to the increased ‘‘cross-link density.’’ These

considerations allow the commercial viability of only hard COPEs. This is
a major deficiency in this class of TPEs as the softest product available has a
hardness of 35 Shore D. Also based on the above discussion, the more or less
continuous hard phase in commercially available COPEs where fibrillar
crystalline lamellae (due to short hard segments) are connected at the growth
faces by short tie molecules can readily be rationalized. The amorphous phase
is also continuous (31).
It is difficult to produce useful soft elastomeric products from segmented
block copolymers except in the case of thermoplastic polyurethanes (TPUs).
The strong association of hard blocks even at low hard block content allows
the preparation of soft elastomeric TPUs. TPUs with hardness as low as 70
Shore A are available commercially.
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Table 3 TPE Property Comparison
Manufacturer:
trade name TPE type
Hardness
(Shore A or D)
Compression set
(%, 22 hr, ASTM D 395B,
constant deflection) T
g
(jC, DSC) T
m
(jC, DSC peak)
Noveon: EstaneR
58134
TPU (ester) 45D 62 (70jC) À47 Multiple m.p. peaks,

highest at 218jC
Noveon: EstaneR
58137
TPU (ester) 70D 82 (70jC) À28 227
EMS-Chemie:
GrilonR
ELX2112
COPA 60D 85 (24 hr, 70jC) 30 (dry) 215
DuPont: HytrelR
7246 COPE 72D 80 (100jC),
5 (100jC ASTM D 395A,
constant load)
20 (DMA) 218
HytrelR 5526 COPE 55D 80 (100jC),
8 (100jC, constant load)
À25 (DMA) 203
HytrelR 4056 COPE 40D 89 (100jC),
12 (100jC, constant load)
À40 (DMA) 150
Zeon: VT355 NBR (30 wt% AN)
sulfur-cured
thermoset
76A 12 (100jC) À30 Amorphous
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IV. THERMOPLASTIC POLYURETHANES
Thermoplastic polyurethane (TPU) was the first thermoplastic product that
could truly be considered an elastomer (32). The bulk of commercially avail-
able TPUs are produced from hard segments based on 4,4V-diphenylmethane

diisocyanate (MDI) and 1,4-butanediol (BDO, a ‘‘ chain extender’’), with
either poly(tetramethylene oxide) (PTMO) glycol, or poly(1,4-tetramethylene
adipate) (PTMA) glycol or poly (q-caprolactone) (PCL) glycol as the soft
elastomeric segment (32). TPUs can be produced by a ‘‘ one-pot’’ method or in
Figure 1 Polymer microstructure and morphology of segmented block copolymers
(TPU, COPE, COPA). A, crystalline domain; B, junction area of crystalline lamella;
C, polymer hard segment that has not crystallized; D, polymer soft segment.
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a two-stage process. In the former, the diisocyanate, chain-extender diol, and
soft segment diol are mixed and heated to yield the final product, whereas in
the latter the soft-segment diol is first ‘‘ end-capped’’ by using an excess of
diisocyanate and the chain-extending short-chain diol is subsequently added
to form the hard segments and to attach them to the soft segments in an
alternating manner to yield a TPU of high molecular weight by addition step-
growth polymerization. A representation of a TPU molecule is presented in
Figure 2. A TPU’s M
w
can be as high as about 200,000, with M
n
about
100,000, although the individual hard and soft segments are of much lower
molecular weight. For example, poly(tetramethylene oxide) glycol of M
n
1000
or 2000 is used commercially for TPU production, thereby fixing the soft
block length. The longer the soft segment, the lower its hydroxyl end group
concentration, which would allow preferential step growth of the hard
segments by reaction of the short-chain diol with the diisocyanate. Hence,

the longer the soft segment, the longer the hard segment. Because the number
of soft segments will equal the number of hard segments, for a large number of
alternating segments,
Weight % SS
M
nss
¼
weight % HS
M
nhs
or
M
nhs
¼ weight % HS Â
M
nss
weight % SS
Figure 2 Polyether-based TPU.
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where soft segments and hard segments are abbreviated SS and HS, respec-
tively. For given soft segment molecular weight, the number-average molec-
ular weight of the hard segment is directly proportional to the hard segment
content and inversely proportional to the soft segment content (33).
M
wss
can
be obtained by measurements on the polyol, but obtaining the hard-segment
weight-average molecular weight is difficult. Bonart developed a theoretical

method to calculate
M
whs
(34). The average number of hard segments for a
TPU (MDI/BDO hard segments; polyoxypropylene end-capped with poly-
oxyethylene soft segments) with a 50 wt% hard phase has been calculated to
be six (35). Peebles mathematically modeled the soft and hard segment length
distribution in TPUs (36,37).
The infrared studies of Cooper demonstrated that the urethane NUHis
hydrogen-bonded to the oxygen atoms of the urethane moiety as well as to the
oxygen atoms of the polyether or polyester soft segments (38). This hydrogen
bonding and soft segment polarity can retard and lower the ultimate degree of
phase separation in TPUs. Poor phase separation is reflected in the increase in
T
g
of the mostly amorphous soft phase due to the presence of dissolved hard
segments. The hard microphase is formed by association of the relatively
short hard segments and by their crystallization into fibrillar microcrystals.
The poorer phase separation in polyester TPUs compared with polyether
TPUs is presumably due to the greater polarity of and stronger hydrogen
bonding (with the NUH of the hard segments) in the soft phase of the former
compared with the latter (39). A 1:2:1 (molar polyester:MDI:BDO) TPU
(polyester polyol M
n
= 1000) exhibited a single phase, but the corresponding
polyether-based TPU system was phase-separated (40). The degree of phase
mixing is also dependent upon soft segment content. For a polyether-based
TPU, complete phase mixing was observed at 80 wt% soft segment content
(41,42). Phase mixing is also dependent upon segment molecular weight, as
demonstrated in the case of TPUs containing low molecular weight poly-

caprolactone soft segments (43).
Phase separation in TPUs is driven by the solubility parameter differ-
ence between the polymer segments and by association and/or crystallization
of the hard segments and is limited by the geometry of the molecule and the
hydrogen bonding and polarity effects discussed. In addition, the kinetics of
TPU phase separation will also be influenced by the mobility (T
g
) of the
polymer segments.
A. TPU Morphology and Microstructure
The mechanical behavior (Young’s modulus, elastic recovery, elongation,
flexural modulus, heat sag, thermomechanical penetration probe behavior) of
TPUs suggests a transition from discrete to continuous hard microdomain
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morphology at hard segment content above about 45 wt% (33,41–46). The
small-angle X-ray studies of Abouzahr and Wilkes (42) and Cooper and
coworkers (43) and the small-angle X-ray and neutron scattering analysis of
Leung and Koberstein (41) suggested an interlocking hard domain morphol-
ogy at high hard segment content. Depending upon processing conditions and
hard phase type and content, crystalline TPU systems may exhibit a fringed
micellar texture of thickness equal to the hard segment length or clear-cut
connectivity of the crystalline hard phase. The hard domain diameter in a
TPU produced from a 1:6:5 polycaprolactone (M
n
= 2000)/MDI/BDO mole
ratio was estimated to be 400 A
˚
by transmission electron microscopy (TEM)

(46) (hence ‘‘hard microdomain’’), although for the typical TPU materials
mentioned in this review this number is expected to be about 100 A
˚
.
Using small-angle X-ray scattering (SAXS), Leung and Koberstein (41)
studied the hard segment microdomain thickness (which corresponds to the
length of the hard segments) in TPUs in which the hard segment content
varied from 30 to 80 wt%. The SAXS measurement provided an overall
characterization of the microdomain morphology averaged over crystalline
and noncrystalline structures. The hard microdomain thickness varied from
2 nm (corresponding to a hard segment length containing two MDI residues)
to 5.4 nm (hard segment length with four MDI residues) for the 60 wt% hard
segment content TPU, after which the thickness did not increase further with
increased TPU hard segment content. Because the hard segment length
increases with increased TPU hard segment content, chain folding via the
flexible BDO segments to accommodate longer hard sequences within the
crystal is thought to occur. Other possible explanations for this phenomenon
have been discounted. The extended chain crystal structure, irrespective of
TPU hard segment length, that has been demonstrated to occur by wide-angle
X-ray diffraction (WAXD) may well be characteristic of the TPU samples
studied that were treated (annealed, etc.) to maximize crystallinity so as to be
amenable to analysis by the WAXD method (41).
Spherulitic structure for high hard segment content (>40 wt%) TPUs
have been observed in samples crystallized in the laboratory (33,46,47). In one
case, because of the large spherulite diameter (several micrometers) and the
absence of a hard phase T
g
, the spherulites may have contained occluded soft
phase (33). Hard phase T
g

is rarely discernible even in high hard phase content
TPUs. A hard phase T
g
was observed in a melt-quenched TPU with 80 wt%
hard segment content (33). Owing to the tendency of the relatively short TPU
hard segments to associate or crystallize or to be miscible in the TPU soft
phase, amorphous hard segments may exist only as tie molecules connecting
microfibrillar crystalline segments. Low TPU amorphous hard phase content
would preclude T
g
detection. Moreover, hard phase T
g
observation would be
obscured by other transitions (discussed later). Spherulitic soft segment
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structure in a high PTMO soft segment content TPU has been observed (33).
Generally, TPU parts that are fabricated by commercial processing equip-
ment exhibit crystallinity but no spherulitic structure (48).
B. Thermal Characteristics of TPUs
Although the structure of TPUs changes constantly during differential
scanning calorimetry (DSC), DSC coupled with SAXS has proven to be a
powerful tool in uncovering TPU microstructure and thermal behavior, as in
the masterful research work of Koberstein and coworkers (35,41,49,50), who
studied polyether TPUs with MDI/BDO hard segments. Molten TPUs from a
homogeneous melt state were rapidly quenched to and held at various
annealing temperatures for specific time periods. Generally, three distinct
endotherms were observed by DSC of the annealed samples. The first
endotherm (T

I
) is dependent upon the annealing temperature, annealing
time, and TPU hard segment content. This endotherm is observed at 20–
40jC above the annealing temperature, which was varied from 30jC to 170jC,
depending upon TPU hard segment content. Higher hard segment content
TPUs gave higher T
I
values. The exact origin of T
I
is still unknown, but it is
linked to a short-range order dissociation endotherm in the hard microphase
and not in the interphase, because this transition is also observed in pure hard
segment materials as suggested by Cooper and coworkers (51,52). For a soft
TPU with a discrete hard phase and a total hard phase content of 30 wt%, the
T
g
of the soft phase kept increasing with increased annealing temperature up
to 170jC. Annealing above 170jC did not change the soft phase T
g
, indicating
that the microdomain structure is completely disordered above this temper-
ature (35). The T
g
increase of the soft phase was related to increased
solubilization of hard segments into the soft phase. Increasing annealing
temperature caused the solubilization of hard segments of high molecular
weight into the soft phase that already contained lower molecular weight hard
segments. It has also been suggested that ‘‘cross-linking’’ by soft segment–
hard segment hydrogen bonding is another factor that contributes to in-
creased soft phase T

g
in addition to the physical presence of TPU hard
segments in the soft phase (53). By studying the change in TPU heat capacity
at its glass transition temperature, it was concluded that below an annealing
temperature of 80jC hard segment solubilization into the soft phase occurs
and above 80jC, which is near the hard segment T
g
, soft segments that are
trapped in the hard microphase also enter the bulk soft phase in addition to
further hard segment dissolution into the soft phase.
The T
II
endotherm is also dependent upon annealing temperature, and
for the soft TPU under discussion the T
II
maximum is 175jC. This transition
was identified by Koberstein as the microphase separation transition (MST),
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where the partially ordered ‘‘noncrystalline’’ segments in the hard micro-
domain are mixed into the soft TPU phase. The TPU with 30 wt% hard
segment content did not exhibit a microcrystalline melting T
III
endotherm,
which is observed for higher hard segment content TPUs. The identification
of T
II
as the MST was further confirmed by simultaneous DSC/SAXS
measurements in a TPU with 50 wt% hard segment content (49). The TPU

interdomain spacing increased dramatically beginning at T
II
.ThisTPU
exhibited a higher T
III
endotherm corresponding to the melting of a micro-
crystalline hard phase within the ‘‘noncrystalline’’ ordered hard domain.
For the TPU with 50 wt% hard segment content, the T
I
endotherm
merged with the T
II
endotherm when annealing took place at 155jC.
Annealing above 155jC raised the T
II
endotherm and decreased its intensity
whereas the intensity of the T
III
microcrystalline peak melting endotherm
increased. T
III
was the only DSC peak endotherm observed at 210jC when
annealing was conducted at 175jC. At annealing temperatures of 175–190jC,
the T
III
endotherm diminished in magnitude and the T
II
endotherm reap-
peared. These findings are consistent with an expected decrease in crystallinity
at low undercoolings where crystallization is controlled by nucleation. Above

the MST, TPU crystallization occurs from a homogeneous mixed melt phase
(‘‘solution’’ crystallization). Crystallization occurs within the hard micro-
domains (‘‘bulk’’ crystallization) below the MST. For harder TPUs (70 wt%
hard segment content), melting endotherms corresponding to different crystal
structures have been observed, depending upon annealing conditions.
The thermogravimetric analysis (TGA) trace of the TPUs of the Hu and
Koberstein study (50) demonstrates initial weight loss around 300jC, which is
well above the annealing temperatures used to probe the TPU microstructure.
A small change in annealing temperature (from 190jCto195jC) exhibited a
dramatic increase in TPU M
n
and M
w
values [gel permeation chromatogra-
phy (GPC) measurements]. The increased MW is presumably the result of
‘‘trans urethanation’’ reactions that result from cleavage of the urethane bond
in a polymer segment back to the isocyanate and alcohol, and subsequent
allophanate formation by addition of the newly formed isocyanate to the
urethane NUH bond of another polymer chain, thus creating a branched
structure. Crystallization of the branched TPU molecules appears to be hind-
ered in comparison with their linear counterparts. Reduction in the heat of
fusion is observed for TPU samples where molecular weight was increased by
annealing at high temperature, due to ‘‘ trans-urethanation’’ reactions. It
should also be reiterated here that the sequence length of the hard segments
that are incorporated into the soft phase increases with increased annealing
temperature. For more on trans-urethanation reactions and TPU thermal
degradation mechanisms, the reader is referred to the work of Macosko and
coworkers (54).
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Copyright © 2004 by Taylor & Francis
According to Koberstein, all three TPU endotherms T
I
, T
II
,andT
III
are
accompanied by the mixing of hard and soft microphases. The Koberstein
schematic model for the morphological changes that occur during the DSC
scans of TPUs is presented in Figure 3. It should be noted that Koberstein’s
work is grounded on the pioneering TPU research work of Wilkes and
Cooper and coworkers, who had previously recognized the time- and
temperature-dependent morphological and mechanical properties of TPUs
(51,55–61). The increased mutual solubility of TPU hard and soft phases with
increasing temperature was recognized, as was the influence of hydrogen
bonding and soft phase T
g
on phase mixing and demixing over a broad temp-
erature range. Both phase mixing and demixing have been observed on TPU
mechanical deformation, depending upon sample thermal history, including
changes in phase continuity (59). TPU morphology is complex, and a small
change in the polymer segment type can result in diverse melting behavior.
For example, TPUs produced from MDI/BDO hard segments and poly(hexa-
methylene oxide) soft segments exhibited five melting endotherms that were
attributed to hard segment sequences containing one to five MDI-derived
units (62). There is continued interest in elucidating the origin of multiple
melting endotherms in TPUs (63).
It is now readily understood how TPU morphology is dependent upon
processing conditions and what thermally induced phase transitions can

occur that would be detrimental to product elastic recovery at elevated
temperature.
Based upon the information presented so far, it would appear that TPUs
that are designed for improved phase separation (decreased hard and soft
phase compatibility) should provide improved elastic recovery. However,
TPU mechanical properties are adversely affected when the desired micro-
structure is difficult to achieve due to incompatibility of the TPU building
blocks under the polymerization conditions, including incompatibility of the
reactants with the polymer produced. This is the case for TPUs (for improved
hydrolysis resistance) produced with polybutadiene diol or hydrogenated
polybutadiene diol (for improved heat and hydrolysis resistance) soft seg-
ments and MDI-based hard segments (64–68). Molecular heterogeneity in
chemical composition and average hard segment length is expected to be the
key factor contributing to the poor mechanical properties of these hydrocar-
bon soft segment TPUs compared with conventional TPUs, based on, for
example, MDI/BDO/PTMO (69–71). Hydrocarbon diols are being promoted
for nonelastomeric polyurethane applications, as in the preparation of
castable polyurethanes for moisture-resistant adhesives, coatings, and elec-
trical potting compounds (72).
Thermoplastic polyurethanes produced with 2,6-toluenediisocyanate
(2,6-TDI) hard segments with BDO as chain extender and PTMO as the soft
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Figure 3 Schematic model for the morphological changes that occur during DC
scans of polyurethane elastomer (a) below the microphase mixing transition temp-
erature, (b) between the microphase mixing temperature and the melting temperature,
and (c) above the melting temperature. The microcrystalline hard-segment domains
are indicated. (From Ref. 49.)
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phase undergo cleaner phase separation than the corresponding 2,4-TDI
based TPUs (53). The use of 2,6-TDI as the hard phase isocyanate may
provide TPUs with excellent elastic recovery, but difficulty in 2,6-TDI/2,4-
TDI isomer separation makes this approach commercially unfeasible. More-
over, the volatility of TDI over MDI makes the latter isocyanate preferable
because of toxicity considerations. However, TDI, the first isocyanate
developed for the thermoset polyurethane industry, is still used in North
America in the manufacture of thermoset polyurethane foam (73–75). TPUs
produced with aromatic diol chain extenders such as hydroquinone bis(2-
hydroxyethyl) ether in, for example, the conventional MDI/PTMO system
are emerging as elastomers with improved elastic recovery (76).
Aliphatic and aromatic diamines can be used as chain extenders to form
TPU ureas with high melting point hard segments, but these materials melt
with some decomposition and well above the processing temperature of TPUs
(32) and hence are not commercially feasible as thermoplastic elastomers with
improved elastic recovery.
However, owing to improved elastic recovery after high strain and a
higher use temperature due to the urea hard segments, solution-processed
aromatic polyurethaneureas are preferable to conventional melt-processed
aromatic polyurethanes in fiber applications (clothing, upholstery, and
carpet). Spandex is the generic trade name given by the Federal Trade
Commission to synthetic elastomeric fibers that contain at least 85% seg-
mented polyurethane. In comparison with natural rubber threads, Spandex
fibers are readily dyeable, lightweight materials with excellent abrasion
resistance, tensile strength, and tear strength. They have better resistance to
oxidation, sunlight, and dry cleaning fluids than natural rubber threads and
are also tolerant to bleach containing a low chlorine level. Although cured
natural rubber fibers have the advantage of low hysteresis and stretch

crystallinity, they are being replaced by Spandex, which can also be cured
during the fiber-forming process (77).
C. Aliphatic TPUs
Aliphatic TPUs are used in light-stable (nonyellowing) applications and can
have mechanical properties comparable to those of aromatic TPUs (78).
These materials are synthesized from hydrogenated MDI diisocyanate/BDO
or hexamethylenediamine diisocyanate/BDO hard segments and polyester
soft segments. (Polyether soft phase would reduce TPU UV resistance.)
Conventional MDI-based aromatic TPUs yellow on exposure to UV light
owing to the formation of quinone imides. The quinone imides are UV
absorbers that dissipate UV energy as heat and hence retard further TPU
degradation. On UV exposure, the aliphatic TPUs undergo a greater reduc-
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tion in mechanical properties than their aromatic counterparts but without
color change or loss of transparency. Hence, UV-stabilized aliphatic TPUs
are used in outdoor applications where the abrasion resistance of TPUs is
necessary. For example, some outdoor signs enclosed in transparent acrylic
are laminated with aliphatic TPUs. Aircraft canopies are fabricated with
high-impact-resistant layered structures produced from polycarbonate and a
‘‘flexibilizing’’ aliphatic TPU ‘‘glue.’’
As illustrated by the data in Table 3, the compression set of TPUs is
much poorer than that of thermoset rubber. Under compression at elevated
temperature, irreversible deformation in TPUs occurs by continued phase
separation and/or reorganization of the hard and soft segments over that
established after part manufacture.
Hydrogen bonding in the hard phase and in the interphase (the region
where the polymer composition changes from 100% hard segment to 100%
soft segment) between the hard and soft domains provides a ready mechanism

for chain slip because hydrogen bonds can reorganize readily by the partial
formation of ‘‘ new’’ hydrogen bonds as the ‘‘old’’ hydrogen bonds are
partially broken. Increasing the amount of the hard phase (to provide more
secure thermoreversible cross-links at the TPE upper service temperature)
increases compression set because the now higher modulus material is
subjected to much higher stress under compression compared to the
corresponding softer material (under constant deflection). Increased hard
phase volume fraction in TPUs also restricts polymer motion in the soft phase
(increased elastomer cross-link density), and there is an increased presence of
hard segments in the soft phase. These factors cause an increase in the soft
phase T
g
that raises the product’s lower use temperature. The hard TPU
product, of course, would have an advantage in constant load applications.
Thermoplastic polyurethanes may also contain thermoreversible allo-
phanate branch points resulting from the reaction of the urethane NUH bond
with excess diisocyanate. It is not feasible to design allophanate bonds into
a TPU, but these fortuitously present cross-links may contribute to improved
TPU elastic recovery. Nevertheless, elastic recovery in the various types of
TPUs does not approach that of thermoset rubber. In some cases the elastic
recovery of a soft product can be worse than that of a harder product because
of product design. For example, it may be necessary to produce a soft TPU
with a low rate of crystallization to achieve desirable processing character-
istics in film applications. This may be accomplished by the use of a low
molecular weight soft segment in which the TPU crystallization rate is
lowered owing to increased phase mixing. Continued phase separation in
the finished product is one factor that would raise set.
Amorphous materials exhibit a gradual decrease in viscosity with
increasing temperature beyond T
g

, compared with crystalline materials, in
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which viscosity drops sharply on melting due to the T
m
being much greater
than the T
g
. In crystalline hard phase TPUs the viscosity drop on crystal phase
melting may not be as precipitous as expected because of association among
the hard phase molecules that are still present just after melting because of
incompatibility with the soft phase. Even so, this viscosity drop in a crystalline
hard phase TPU may cause it to lack desirable processing characteristics, and
TPUs with a high amorphous hard segment content may be designed for an
improved processing window and for transparency. The excellent impact
properties, processability, and transparency of Dow’s Isoplast
TM
are credited
to the amorphous hard segment that makes up most of this TPU engineering
plastic. In the finished product, elastic recovery is controlled by both raw
material properties and part design.
V. ELASTOMERIC COPOLYESTERS AND COPOLYAMIDES
Elastomeric copolyesters (COPEs) (31) and elastomeric copolyamides
(COPAs) (79) are similar in structure to TPUs and suffer similar drawbacks
in rubber performance. The hydrogen bonding present in TPUs and COPAs is
absent from COPEs. Commercially available COPEs are based upon crys-
talline polybutyleneterephthalate (PBT) hard segments and poly(tetramethy-
lene oxide) (PTMO) soft segments. PBT monofilaments exhibit only a 1%
permanent set after 11% extension at room temperature, owing to a reversible

a-toh-crystal transition (80,81). This reversible crystal transition, which
would be beneficial in the elastic recovery of COPEs, has been observed in
PBT/PTMO COPEs with a high enough level of the PBT hard phase that the
amorphous phase is hard enough (due to the presence of PBT hard segments
in the amorphous phase) to bear the level of tensile stress necessary to cause
the reversible deformation behavior in the hard phase (82). Although it is
generally thought that segmented block copolymers have a homogeneous
amorphous phase consisting of hard and soft blocks, experimental evidence
indicates that a biphasic amorphous phase consisting of a PTMO phase and a
mixed PBT/PTMO phase can exist in certain COPEs (83,84). The lack of
hydrogen bonding in COPEs and the reversible crystal transformation
possible in the PBT hard phase are responsible for the modest improvement
in elastic recovery of these materials over TPUs and COPAs. However, at
elevated temperature, the motion of the soft segments cannot be adequately
restrained by the crystalline polymer chains, thus causing reorganization in
the hard phase that leads to irreversible deformation. COPEs cannot match
the elastic recovery of thermoset rubber (Table 3).
In addition to the disadvantage of poor elastic recovery at elevated
temperature that is characteristic of most TPEs, the segmented block copoly-
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mers suffer the additional disadvantage of the lack of commercially available
soft products due to inadequate physical properties as already discussed. In
the case of TPUs, the association of the hard segments is strong enough to
confer excellent physical properties to soft products (70 Shore A), but
difficulty in pelletization of the soft product during manufacture and pellet
agglomeration on storage have to be overcome.
Addition of plasticizer to hard segment block copolymers is not a viable
option for the production of soft products, because the plasticizer would

lower the melting point of the polar hard phase in addition to softening the
polar elastomeric phase, which, in any case, cannot hold a high level of added
plasticizer. Moreover, continued phase separation after processing can cause
the exudation of plasticizer from the molded product. Commercially available
segmented block copolymer TPEs are plasticizer-free.
Elastic recovery is an important property for elastomer performance.
Because of the price and performance requirements in diverse applications,
the hydrocarbon oil-resistant segmented block copolymers discussed are
successful products of commerce.
The most important end use of the polyurethane-elastomer, polyamide-
elastomer, and polyester-elastomer block copolymers has been in thermoset
rubber replacement. Their crystalline hard segments make them insoluble in
most liquids. Products feature exceptional toughness and resilience, creep and
flex fatigue resistance, impact resistance, and low-temperature flexibility. All
three types are generally used uncompounded, and the final parts can be
metallized or painted. Thus, they are often used as replacements for oil-
resistant rubbers such as neoprene because they have better tensile and tear
strength at temperatures up to about 100jC. Automotive applications include
flexible couplings, seal rings, gears, timing and drive belts, tire chains, and
brake hose. Special elastomeric paints have been developed that match the
appearance of automotive sheet metal; such parts have been used in car bodies
(31,32,79). Flexible membranes, tubing, hose, and wire and cable jackets are
included in the long list of applications.
VI. STYRENIC BLOCK COPOLYMERS
The advent of hydrogenated styrene/butadiene/styrene (SBS), i.e., styrene/
ethylene-1-butene/styrene (SEBS), triblock copolymer compounds repre-
sented an advance in the elastic performance of thermoplastic elastomers at
elevated temperature. SEBS is almost always compounded; one can achieve
processable soft compositions (0–30 Shore A) that are not possible in the case
of segmented block copolymers. The key features of SEBS will be described

before we discuss SEBS compounds. Phase separation in these triblock
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copolymers is more complete and occurs more readily than in the segmented
block copolymers. This is reflected in the T
g
of the rubber phase, which is
nearly unaffected by the polymer styrene content. The T
g
of the styrene phase
depends upon its molecular weight. More phase mixing with the rubber can be
expected with decreasing styrene molecular weight when the material is
heated to the T
g
of styrene (85). Both the polystyrene end block content
and polystyrene molecular weight in SEBS is designed to be lower than that of
the rubber midblock. For example, Kraton G1651(SEBS) of Kraton Poly-
mers has a plastic block of molecular weight 29,000 (33 wt%) and a rubber
block of molecular weight 116,000 (68 wt%) (86). The rubber block is
designed to have a 40 wt% butene content to limit crystallinity due to the
polyethylene segments (low crystallinity would increase the rubber’s oil-
holding capacity) and lower T
g
(low T
g
for improved low-temperature
performance) (87). Simplistically, SEBS has a ‘‘spaghetti and meatball’’
morphology, in which the styrenic microdomains (200–300 A
˚

) are dispersed
in a continuous rubber matrix (88). The polystyrene microdomain size reflects
the entropic penalty that would be imposed on the rubber in the case of larger
plastic domains. The higher molecular weight and narrower molecular weight
distribution of SEBS than those of the segmented block copolymers are
factors that favor improved phase separation in the former system in spite of
the smaller solubility parameter difference between the phases in SEBS versus
the segmented block copolymers (89,90). Molecular architecture also favors
better phase separation in SEBS than in the segmented block copolymers. The
polystyrene phase will flow above its T
g
(
f
95jC), and these microdomains
form the thermoreversible cross-links in the SEBS thermoplastic elastomer.
The styrenic cross-links, however, do not contribute much to the ‘‘ cross-link’’
density of the rubber phase that is dominated by the trapped entanglements
within it (91). This can readily be inferred by a comparison of the modulus
(initial slope of the stress–strain curve and also the plateau modulus) of SEBS
with other styrenic block copolymers such as styrene/butadiene/styrene (SBS)
and styrene/isoprene/styrene (SIS). The modulus in these systems is directly
related to the molecular weight between entanglements in the rubber phase
(88). The modulus of SEBS (lowest molecular weight between entanglements
and highest entanglement density) is greater than that of SBS, which in turn
has a higher modulus than SIS (highest molecular weight between entangle-
ments and lowest entanglement density).
Thus the function of the styrenic domains is to prevent disentanglement
of the rubber segments when these styrenic block copolymers (SBCs) are
subjected to load. For example, Kraton G1651 has a 33.3 wt% PS content and
a rubber molecular weight of 116,000 (M

n
g M
w
). Neglecting the interphase,
the total PS phase volume in 100 g of SEBS would be 31.71 cm
3
(PS density =
1.05 g/cm
3
). Assuming spherical 200 A
˚
diameter PS domains, the volume per
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Copyright © 2004 by Taylor & Francis
domain is 4.19
Â
10
À18
cm
3
, which translates to 7.57
Â
10
18
domains in the
SEBS sample. The number of PEB macromolecules is 35.29
Â
10
19

(68/
116,000 = 5.89
Â
10
À4
gÁmol = 5.86
Â
10
À4
Â
6.023
Â
10
23
macromolecules).
Assuming a molecular weight between entanglements for PEB of 1800, the
number of entanglements per chain is 64 (116,000/1800). If entanglements
occur only by the crossing of two different rubber chains, the total number of
entanglements in the rubber is 1129
Â
10
19
(35.29/2
Â
10
19
Â
64), which results
in 1490 entanglements in the rubber phase per PS domain. A representation of
SEBS polymer microstructure and morphology is presented in Figure 4. Note

that in SBS and SIS the rubber block has a high 1,4-copolymerized diene
content that maximizes phase separation (due to maximized incompatibility
between the plastic and rubber phases) for improved elastic properties but is
also detrimental to product processability. On the other hand, SEBS is
produced by the hydrogenation of high- ‘‘ vinyl’’ (low 1,4-copolymerized
diene) SBS for reasons already discussed. Hydrogenation of commercially
available SBS would yield a crystalline plastic instead of an elastomeric
polymer midblock.
The foregoing discussion is based upon the ‘‘spaghetti and meatball’’
SEBS morphology described earlier. In the case of lower molecular weight
SEBS, a higher modulus has been observed compared to those of the
corresponding higher molecular weight counterparts. This has been attribut-
ed to the presence of a larger interphase in the former case due to greater phase
mixing (92). If the TPE hard block content is high enough to form a
continuous phase, a higher modulus can be expected.
Upon increasing PS content, the discrete plastic phase morphology in
SEBS can change to a cocontinuous rubber and plastic phase, and further to a
discrete rubber phase in a plastic matrix. Also, the shape of the plastic phase
can change from spheroidal to cylindrical to plate-like with increasing SEBS
PS content. These regular shapes can be achieved only under carefully
controlled annealing or shearing conditions.
Compared with a corresponding low molecular weight polymer, high
molecular weight SEBS exhibits superior mechanical properties and can be
Figure 4 High rubber content SEBS triblock copolymer microstructure and mor-
phology.
4871-9_Rodgers_Ch05_R2_052704
MD: RODGERS, JOB: 03286, PAGE: 187
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