Silicon-Containing Polymers: The Science and Technology of Their Synthesis and Applications

769 Pages·2000·38.645 MB·English

by  R.G. Jones, W. Ando, Julian Chojnowski

CHAPTER 7
THERMAL PROPERTIES OF POLYSILOXANES
PETAR R. DVORNIC
Michigan Molecular Institute,
I9I 0 W,St . Andrews Road, Midland, Michigan 48640, USA
1. Introduction
Thermal properties are among the most characteristic and at the same time the most
technologically important properties of polysiloxanes. They embrace a unique
combination of pronounced elasticity at unusually low temperatures and high thermal
and thermo-oxidative stability at elevated temperatures. These properties are
characteristic of polysiloxanes because they directly originate from a specific interplay
of some of the most fundamental features inherent to the basic structural building blocks
that make up their repeat units, chain segments and entire macromolecules. They are
therefore found in more or less all members of this family of polymers. At the same time
they are also of outstanding technological importance because not only that they clearly
distinguish these unique polymers from their purely organic, -C-C- type counterparts,
but they often make polysiloxanes the materials of choice for many applications where
performance under extreme service conditions is required and where no other polymer
can successfully satisfy the purpose.
At the macroscopic level, low temperature elasticity of polysiloxanes is primarily
manifest in some of the lowest glass transition temperatures (Tg) known to polymer
science, low crystalline melting points (T,J, unusually fast crystallisation, specific liquid
crystalline (LC) behaviour and small viscosity-temperature coefficients. At high
temperatures, polysiloxanes are generally capable of retaining their useful properties
either for longer exposure to a given temperature or for the same period of time at higher
temperatures than most comparable organic polymers. For example, while glass
transition temperatures of many polysiloxanes, including polydimethylsiloxane (PDMS),
the parent polymer of the family and still the most commercially important 'inorganic'
polymer available, range from about -150 to -70 "C depending on the particular
substituent groups to the main -(Si-O)x- chain backbone (see Table l), their upper-limit
temperatures for the onset of irreversible degradation may reach to over 300-350 "C.
Compared to most organic -C-C- type polymers (the T,s of which generally do not
extend below -70 'C, while degradation temperatures can rarely exceed 150-200 "C),
this is at least a 150-200 'C wider temperature range of potential applicability. Hence,
polysiloxanes are suitable for use wherever a polymer is required to withstand an
unusually broad range of application temperatures or to provide desired properties at
unusually high or low temperatures. Because of this, many linear and cross-linked
185
R.G. Jones et al. (eds.),S ilicon-Containing Polymers, 185-212.
@ 2000 Kluwer Academic Publishers. Printed in the Netherlands.
186 P. R. DVORNIC
members of the family have found application as thermally stable fluids, lubricants,
elastomers, sealants or coatings, in the automotive, the aero-space and naval industries,
in household appliances, metallurgy, high temperature insulation, electronics, etc.
In this chapter, thermal properties of polysiloxanes are discussed from a structural
perspective since they are a direct consequence of intrinsic features of their main
building blocks at submolecular, molecular and supramolecular levels of organisation. In
fact, it is not exaggeration that polysiloxanes provide some of the prime examples of the
validity of the principles of structure-property relationships in polymer science. Hence,
although this discussion is focused primarily on the linear members of this family of
polymers, most of it applies equally well to their cyclic and cross-linked architectural
isomers, However, for specific information about the cyclic and cross-linked isomers,
the reader is directed to other chapters of this volume that are devoted to such structures.
2. Structural Characteristics of Polysiloxanes that Determine their Thermal
Behaviour
The main structural elements of polysiloxanes that directly or indirectly determine their
macroscopic behaviour, including their thermal properties, are: (1) the siloxane, Si-0,
bond as the main building block, (2) the exactly alternating arrangement of the silicon
and oxygen atoms, not just within the polysiloxane, -(Si-O)x-,c hain segments but within
entire molecules, and (3) the type and arrangement of organic substituents that are
attached to the main chain silicon atoms. Since the first two of these structural elements
are inherent to the chemical composition and macromolecular organisation of the whole
class of these polymers, the resulting properties are to some extent expressed by all
members of the polysiloxane family. Conversely, the variations that appear between
specific polysiloxanes result primarily from the type, relative content and arrangement of
the organic groups involved.
2.1. THE SILOXANE BOND
The siloxane, Si-0, bond is a relatively long, partially ionic linkage with partial double-
bond character [ 13. In linear, long chain polysiloxanes its length is typically 1.64 k 0.03
A, which is longer than most of the well known carbon-element bonds [l], but at the
same time, considerably shorter than would be expected from the addition of the atomic
radii of silicon (1.17 A) and oxygen (0.66 A). Hence, this is not a simple o bond but
instead, a more complex interatomic linkage. Its partial ionic character is a direct
consequence of the relatively large difference in electronegativities of the silicon and
oxygen atoms, which, according to Pauling, are 1.8 and 3.5, respectively [2]. These
result in an estimated 37-51 % ionic character of the unit, depending on which of the
proposed empirical equations is used for the calculation [2,3]. In addition to this, the
partial double bond character results from the partial overlap of the vacant, low energy
3d orbitals of silicon with the p orbitals of oxygen. The relatively large difference in the
size of the atoms facilitates the back donation of the oxygen lone electron pairs and a d,-
pn linkage is formed in addition to the normal o-connection between the two atoms [4].
THERMAL PROPERTIES OF POLYSILOXANES 187
These fundamental properties of the siloxane bond directly determine some of the
most characteristic properties of polysiloxanes. Thus, while the partial ionic and double
bond characters of the linkage govern its strength (i.e. the dissociation energy), and
hence its behaviour in chemical transformations and on exposure to heat, its length is the
main reason for the unique behaviour of polysiloxanes at low temperatures. It
contributes to exceptional inherent conformational flexibility of the -(Si-O)x- chains and
their segments and it also plays an important role in determining their specific
intermolecular interactions. In turn, as detailed elsewhere in this section, these
characteristics are directly responsible for the unprecedented elasticity of polysiloxanes
at low temperatures, their unique surface properties, their viscoelastic behaviour and,
together with the high Si-0 bond dissociation energy, their pronounced stability at high
temperatures.
2.2. CONFORMATIONAL FLEXIBILITY OF POLYSILOXANE CHAINS AND
SEGMENTS
Linear connectivity of the siloxane units into the strings of alternating silicon and
oxygen atoms, -(Si-O)x-, results in the formation of the most flexible chain of atoms
known to polymer science [5]. This exceptional flexibility originates from the following
structural features [6,7]: (I) the relatively long Si-0 main chain bonds and Si-C
substituent linkages, and (2) the alternation of the Si-0-Si and 0-Si-0 bond angles along
the backbone. Of these, the Si-C bonds are even longer than the Si-0 bonds and usually
range from about 1.87 to about 1.90 A [7,8], while the angle at oxygen is remarkably
'soft' and can vary from as low as 104" up to almost 180" [9,10] depending on the
specific structure.
The length of the Si-0 bond provides for increased spatial separation of the
neighbouring organic substituents in polysiloxanes, which, in turn results in significantly
reduced steric hindrance and the relief of molecular congestion that would otherwise
occur. The effect is particularly important for those side groups that are relatively large
and bulky such as methyl and phenyl. Thus, the rotation of methyl groups in PDMS,
which takes place via an 'umbrella-type' motion comprised of three different motional
processes (thermally activated rotational hopping, librational transitions and rotational
tunnelling) [I 1 J, was found to occur as low as -196 "C [l l-131. In addition, 2H NMR
investigations showed a p relaxation in poly(methylphenylsiloxane), PMPhS, at -123 "C
(i.e., almost 100 "C below the polymer's T,, see Table l), which was explained as
'phenyl ring 180" n flip' and some mobility ofthe phenyl units even at -233 "C [14].
The 'softness' of the Si-0-Si bond angle in -(Si-O)x- segments causes these units to
have a lower barrier to linearisation of about 1.3 kJ mol-' [ 151 so that these angles in
siloxane polymers usually range between 140" and 150" [6,10,16]. In contrast, the 0-
Si-0 bond angles are considerably less flexible and are usually found between about
102" and 112" [9,10] depending on the nature of the substituents on silicon atoms.
Hence, the 'softness' of the angle at oxygen allows for considerable bending of the
polysiloxane chain segments about these atoms, which, in turn, allows the -(Si-O)x-
backbone to assume shapes that would otherwise be highly improbable if not impossible.
It has been suggested that this 'softness' of the Si-0-Si bond angle is a consequence of
188 P. R. DVORNIC
delocalisation of the lone electron pair of oxygen into the covalent bonding region
between the Si and 0 atoms so that the sp3 hybridisation is altered to widen the angle at
the latter [lo]. Consequently, polysiloxane chain segments exhibit an ability to very
effectively relieve internal tension via these two modes of short-range motion (i.e.,
rotational and bending). Because of this, the -[Si(R)2-O-Si(R)2]- constellation exhibits a
relatively unrestricted rotation around its skeletal Si-0 bonds, for which the energy
barriers (AE fi-ee.rot,) are unusually low, at most only several hundred cal mole-’ [17]. For
this reason, even at very low temperatures, it is easy for the -(Si-O)x- chain segments to
change their spatial arrangement and assume various conformations, while the entire
molecules can readily transform their shapes and sizes, including those that are relatively
compact, depending on the particular type of surrounding medium and temperature to
which they are exposed [7-91. For example, various dilute solution measurements [ 18-
241 and theoretical calculations [25-271 carried out on PDMS have shown that the value
of its unperturbed mean-square end-to-end distance, <r2>o, is very close to that of an
idealised freely rotating chain. In fact, the ratio <r2>o/<r2>[ofr ee ,Ot is only about 1.4 [ 19-
23,251, which is considerably smaller than the corresponding typical values found for
most flexible organic macromolecules, which generally range from about 1.8 to greater
than 3.5 [25]. On the other hand, the characteristic ratio, <r2>,/n12 (where n is the
number of bonds of length I), of PDMS was determined to range from 5.7 and 6.2 in.
most solvents [26] to as high as 7.6 in a low cohesive energy and dielectric constant
mixture Of C8F18 and C2CI4F2[ 22]. Hence, while the former parameter clearly indicates a
relatively unrestricted free rotation about the main-chain bonds, and high flexibility of
the polysiloxane chains and segments approaching that of an ideal freely rotating chain,
the diversity of the characteristic ratio values in different solvents shows that the chains
are also quite sensitive to the influence of the medium, probably because of pronounced
polarity of the partially ionic Si-0 bonds [27].
Froin dilute solution measurements and theoretical calculations based on the
isomeric state theory, Flory, Crescenzi and Mark (FCM) [27], proposed that the
preferred, lowest energy conformation of PDMS chains is all-trans (Figure 1A) [27],
which is quite unusual when compared to most other macromolecules. In polysiloxanes,
however, this conformation is possible because of the relatively long Si-0 and Si-C
bonds, which allow the neighbouring pairs of methyl groups that are on the same side of
the plane of the extended backbone to appear at a distance which is very close to the
sum of their van der Waals radii. Because of the separation, the interactions between the
nearest substituents become attractive and stabilise the conformation. From this
preferred state, the flexibility of PDMS chain segments results from an increase in the
number of gauche states which occurs on stretching [8,27]. With an increase in the size
of pendant groups, however, steric effects between the neighbouring substituents gain in
importance and the all-trans state ceases to be preferable for poly(diethy1-) (PDES) and
poly(di-n-propylsiloxane) (PD-n-PrS) [28]. As a result, the chain dimensions increase
relative to those of PDMS, whileconversely they decrease with a decrease in the size of
the substituents, as in the case of polymethylhydrosiloxane (i.e. polymethylsiloxane)
(PMHS) [29].
THERMAL PROPERTIES OF POLYSILOXANES 189
>
C"3 /O
s:
\
CH3
(A) All-trans segment of PDMS chain
(B) Trans-sym segment of PDMS chain
Figure /. Proposed segmental configuration of polydimethylsiloxane (PDMS) chain in accordance with
references 27 and 3 1
190 P. R. DVORNIC
The major inaccuracy of the FCM model, however, is its prediction that the
differences in the alternating bond angles at silicon and oxygen atoms should cause the
all-trans conformation of PDMS to close upon itself after approximately eleven
repeating units. As a consequence, the synthesis would result in preferential formation of
cyclic undecamer rather than long, open-chain polymer. Since, in contrast with theory
this does not happen, an attempt was made to explain the difference in terms of an
inaccurately determined value of 143' for the Si-0-Si bond angle used in the
calculations, and by possible transformation from a quasi-ordered, stretched
conformation of very short PDMS chains (those having degrees of polymerisation < 12)
into a coiled conformation when the degree of polymerisation exceeds 12 [30]. More
recently, however, another model, obtained by calculations of intramolecular rotational
potential energy surfaces using a self-consistent field molecular orbital ab initio
approach, was proposed, which indicated a trans-syn conformation for the PDMS chains
(Figure 1B) [31]. It seems that this model is able to better explain why, in spite of
pronounced chain flexibility, polysiloxanes do not close upon themselves into
oligomeric macrocyclic ring structures early in the course of their formation,
Both experiment and theory have therefore shown that an unusually pronounced
flexibility of polysiloxane chain segments and entire molecules is a direct consequence
of their high freedom of motion, both in the rotational (around the Si-0 bonds) and the
bending (bond angles at oxygen atoms) senses. Together, these factors, which in turn
arise from the fundamental structural features of these polymers listed in the first
paragraph of this section, provide for low energy barriers for free rotation around the
main chain -(Si-O)x- skeleton. As a consequence, since these are structural features
shared by all the members of this unique family of polymers, the resulting chain
flexibility is a characteristic of polysiloxanes which dictates many of their macroscopic
properties and distinguishes them from all other ~r~acromolecules.
2.3. INTERMOLECULAR INTERACTIONS IN POLYSILOXANES
The other major reason for pronounced elasticity of polysiloxanes, particularly at low
ambient temperatures, is the relatively weak intermolecular interactions between their
segments and entire molecules. Various rheological investigations have shown that the
activation energy for viscous flow of these polymers, AE,,,, , is generally very small and
rarely above about 40 kJ mol", indicating that only small frictional forces are associated
with the translational flow of the molecules relative to one another. However, the
specific values steadily increase in the order of increasing size of the side groups from
only 14.2 kJ mo1-I for PDMS to 18 kJ mol-' for poly(methy1-n-propyl-siloxane)( PM-n-
PrS), 33 kJ mol-' for poly(inethyl-3,3,3-trifluoropropylsiloxane) (PMTFPS) and about
50 kJ mol-' for poly(methylphenylsiloxane) PMPhS [32,33]. At lower molecular
weights, AE,,,, values are molecular weight dependent so that typically, in the PDMS
series they increase from 9.08 to 10.25 to 11.17 to 11.63 kJ mol-' for degrees of
polymerisation of 1, 2, 3 and 4, respectively [34]. However, this dependence disappears
above a degree of polymerisation of about 10 [9] and for truly high molecular weight
polymers AE,,,, becomes constant [32,33].
THERMAL PROPERTIES OF POLYSILOXANES 191
When subject to shear stress, the flow patterns of polysiloxanes depend on the nature of
their organic substituents and the non-Newtonian flow behaviour becomes more
prominent with increase in their bulk [6]. Rheologically the most interesting is the
surprisingly pronounced Newtonian flow of PDMS even at unusually high molecular
weights and its characteristically small viscosity-temperature coefficient. It is believed
that these properties result from the regularly coiled 61 helical conformation of the
molecules, depicted in Figure 2, which is not only characteristic for the crystalline state
[35] but may also be largely retained at low temperatures in the melt [6,28]. If so, the
more or less intact helices would be expected to project their substituents outward, away
from the chain axis and towards the neighbouring chains, owing to repulsive interactions
between the substituents and the backbone. There would then be only a relatively low
resistance to flow because of both weak interactions between the methyl groups of the
I
Figure 2. Helical segment conformation of polydimethylsilooxane (PDMS) chain in crystalline state in
accordance with reference 35
192 P. R. DVORNIC
neighbouring segments and very little interpenetration of the chains. The latter should be
the consequence of effective shielding of the polar -(Si-O)x- main-chain backbones by
the pendant organic groups that are not only bulky enough to successfully accomplish
this function but, as indicated earlier, can also undergo rather unrestricted 'umbrella-
type' rotational motions around their Si-C bonds. For purely geometrical reasons these
motions should enable the substituents to occupy a larger space than their actual volume
and thereby to create a larger free volume between the neighbouring chain segments
andor molecules. Consequently, since these motions are pronounced well below the
crystallisation temperature [ 1 11, this will result in only small frictional forces associated
with the translational flow of these molecules, and hence in low E,,,, values and low
viscosities for a polymer ofgiven molecular weight.
With increasing temperature, the helical structure will revert to the more common
random coil conformation, which permits greater interactions and more entanglement
couplings between the neighbouring chain segments. At the macroscopic level, this
should reflect in an increase in viscosity that would compensate for its expected
decrease with increasing temperature. The two effects would counterbalance, resulting
in an overall retention of Newtonian-type flow behaviour [5,36] and very little change in
polymer viscosity, which, for PDMS fluids, typically decreases by an unusually small
factor of about two over the temperature range from 40" to 100 "C [9]. With increasing
molecular weight of the polymers, this type of rheological behaviour, which is
prominent for samples of intermediate degrees of polymerisation, is gradually replaced
by the non-Newtonian viscous flow. It has been shown that PDMS chains achieve their
entanglement length at a critical value of degree of polymerisation of about 465
corresponding to a molecular weight of about 34,500 (Figure 3) [5]. This corresponds to
about 930 main chain atoms, which is one of the largest values known for a critical
number of chain atoms for the onset of entanglement couplings. Thus, PDMS remains
Newtonian to considerably longer chain lengths than most other flexible
macromolecules, which has been interpreted as further evidence for pronounced
flexibility of its -(Si-O)x- main chain backbone [5]. As seen from Figure 3, below this
critical chain length, the slope of the plot of zero-shear viscosity versus chain length for
this polymer is close to unity. This is as expected for a typical Rousean-type fluid, whilst
above this value, it becomes equal to 3.5, which is characteristic of many flexible chain
macromolecules [37]. Particularly, it should be noted that the viscosity values for PDMS
at any given molecular weight are generally lower than those of other comparable linear
polymers. Together with its high temperature stability, it is this property that makes
PDMS an exceptionally useful fluid for many high temperature applications.
THERMAL PROPERTIES OF POLYSILOXANES
193
I I I I
1
7
t
I
4
3
a"
W
P
3M 2
1
0
-1
-2
Lc= 930
-3
Figure 3. A plot of zero-shear viscosity (qo) against weight average of chain atoms (Z,J for bulk PDMS
at 25 in accordance with reference 5.
OC
194 P. R. DVORNIC
In summary, it follows that structural features of polysiloxanes that are responsible
for their low intermolecular interactions include the following: (1) their regularly coiled,
helical chain structure with substituents pointing outwards and towards the neighbouring
chains which is retained even at low temperatures in the melt; (2) the low intensity
interactions between the neighbouring organic substituents; (3) effective shielding of the
backbone by these substituents, which undergo relatively free rotation around their Si-C
bonds; (4) a relatively large free volume between the neighbouring chain segments
resulting from this pronounced mobility of the substituents; (5) significantly reduced
possibility of interchain interactions, resulting in little interpenetration of the
neighbouring chains and a suppressed likelihood of entanglement couplings until
unusually high degrees of polymerisation. Altogether, these factors contribute to the
relatively weak secondary van der Waals attractive forces between the neighbouring
chain segments. At the macroscopic level these are manifest as the ease with which the
polymers undergo viscous flow at low shear, together with their surprising Newtonian
flow with very small viscosity-temperature coefficients up to unusually high molecular
weights.
3. Thermal Properties of Polysiloxanes at Low Temperatures
The most characteristic thermal properties of polysiloxanes at low temperatures are their
glass transition temperatures, melting temperatures and liquid crystalline (LC)
behaviours. Essentially, all of these are directly determined by polymer segmental chain
mobilities and in the case of this family of polymers, they are all governed by the
previously described inherent chain flexibility and relatively weak intra- and
intermolecular interactions.
3.1, GLASS TRANSITION TEMPERATURES
Ainongst the most important manifestations of the pronounced low temperature
flexibility of polysiloxane chains and segments, and certainly the most striking, are their
glass transition temperatures. As can be seen from Table I, they are some of the lowest
glass transition temperatures known to polymer science and that of polydiethylsiloxane
is the lowest. While these low T, values are determined by the conformational flexibility
of the -(Si-O)x- main chain and are therefore characteristic for more or less all members
of this polymer family, the variations between them are mainly due to the cohesive
energy between the neighbouring chain segments (i.e., to the differences in the free
volume between the chains), and thus to the type and nature of the organic substituents.
Factors that determine specific variations include (1) the relative bulkiness of the
substituents, (2) their polarities, and (3) the relative amount of the free volume that can
be generated by particular mode(s) of mobility.
Taking PDMS as the reference polymer for the polysiloxane family (T, = -123 "C), it
can be seen from Table 1 that in general, an increase in the size and bulk of the
substituents increases the T,, as illustrated by the poly(di-n-alky1siloxane)s and
particularly for polydiphenylsiloxane, PDPhS, (T, = 40 'C) and poly(di-p-tolylsiloxane)