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Jon Meegan, Mogon Patel, Anthony C. Swain et al.

Foamed variants of the above systems have been studied as the presence of a continuous

microstructure within the material allows for tailoring of the mechanical properties and

performance to suit specific end applications or working environments. We have employed 3

Dimensional X-Ray Computer Tomography (XRCT) to further develop the qualitative

understanding of foam microstructures within these materials.

The utilisation of fundamental ‘click’ type chemical reactions such as Diels Alder

cyclisation to introduce reversible cure functionality into crosslinking or polymer species has

been of significant interest to AWE in recent years. Materials containing such functionalities

can be designed to undergo a controlled and reversible crosslinking reaction on application of

an external stimulus which can be triggered when the material is no longer required to

perform its role. Thus, a material that reverts from a liquid to a solid and back to a liquid state

on application of heat would theoretically allow components to be separated without the

application of significant force.

Also highlighted within this chapter is an overview of AWEs efforts to develop and

formulate radiation and thermally stable polymeric materials through the incorporation of

icosahedral closo-dicarbaborane cages.

We also report a rationale for our recent studies into stress sensitive Poly

DimethylSiloxane (PDMS) elastomers and a generic materials qualification overview.



PDMS elastomers are typically reinforced with a filler phase to improve the physical

properties of the resulting composite material, this reinforcement is achieved through load

transfer between the polymer and the filler components [1]. Historically the most commonly

used fillers in PDMS systems have been particulate silica fillers due to their affordability,

high surface area and compatibility with the polymer matrix [2]. However, the incorporation

of these irregular shaped, hydroscopic, high surface area fillers is believed to lead to

undesirable and often complicated degradation mechanisms in both the filler [3] and the

polymer [4] phases as well as the exhibition of complex mechanical behaviours such as the

Mullins effect [5] Unwanted interfacial effects in filled systems such as an enhanced Mullins

effect can be inhibited or removed entirely from a material through the careful choice of filler

phase or use of a polymer component with a narrow (weight averaged) molecular weight

distribution; as the latter is both technically difficult and prohibitively expensive to achieve

on a production scale much of the work in the available literature concentrates on modifying

the filler component.

Due to the versatile nature and commercial viability of many existing PDMS

formulations there has been little published research in recent years into the effects of

incorporation of other filler types into PDMS elastomers. Much of the current research

indicates that the physical properties of other polymer networks, blends or melts are

dramatically improved and / or simplified through the incorporation of particulate nanofillers

[6,7,8,9]. Work within AWE has focused on the introduction of uniform nanotubular fillers or

Polyoctahedral OligomericSilSesquioxane (POSS) moieties into PDMS elastomers in an

effort to both modify and simplify the mechanical and ageing behaviours of these composite


Performance, Stability and Qualification ...


POSS [10,11] moieties represent an interesting class of filler material, formally they are

recognised as molecules but are often classed as nanofillers due to the dimensions of the

molecule and having a propensity to aggregate. The surface chemistry of the molecules is

readily tailored to suit a range of applications, including acting as a reactive filler [12,13] or a

mechanical filler via passivation of the surface. Incorporation of POSS fillers into polymeric

materials has been shown to improve properties such as mechanical strength and increased

glass transition or decomposition temperature [14] The molecular / nanoparticulate nature of

POSS fillers also allows for them to act as potential probes to further develop the molecular

nature of structure – property relationships which can then be exploited to optimise material

properties for specific applications.

Carbon nanotubes have been used as reinforcing agents within metals [15], ceramic

composites [16] and polymers [17], in all cases the presence of nanotubes in the bulk material

leads to improved physical properties such as mechanical strength or electric conductance

when compared to the non reinforced material. One of the drawbacks of these materials is that

until recently carbon nanotubes were difficult to produce and process in the quantities

required for component manufacture; of greater interest are graphite nanofibres (GN) as they

can be prepared in large quantities and display similar physical properties to carbon

nanotubes. Incorporation of GN into polystyrene foams has been shown to influence the

mechanical, heat transfer and thermal expansion coefficeint of the materials [18].

2.1. Preparation of Composites

It is universally recognised that in order to maximise the effect of incorporating a filler

into a polymer matrix, both components need to be homogenously distributed. To facilitate

this process there are three commonly used methods [16]:

Direct mixing –filler and polymer are directly mixed together prior to elastomer


Solution mixing –polymer and filler are dispersed in a common solvent which is

removed after mixing.

In situ polymersisation –the filler is mixed in with monomers which undergo

condensation events to form the polymer phase or the filler phase is simultaneously

formed during the evolution of the elastomeric network.

2.2. Structure Property Relationships of Selected Composites

Figure 1 shows that in the case of GN filled PDMS the composite material is stiffer and

exhibits a linear stress strain response, exhibiting more ideal mechanical behaviour than the

particulate silica filled sample. The degree of flow within the polymer is also reduced when

compared with the silica filled equivalent, this can been seen in the closer bunching of

compression / release cycles. The increase in stiffness and reduced flow properties are

empirically attributed to the combined reinforcing effect [19] of the nanofibres and their

random distribution throughout the material (Figure 2). Of greater interest is the reduction in


Jon Meegan, Mogon Patel, Anthony C. Swain et al.

non ideal behaviour and the resulting simplification of the bulk modulus when compared to

the particulate filled material; closer examination of the stress strain curves would suggest

that the inert surface of the GN and resulting decrease in polymer / filler interactions inhibits

the Mullins effect in the material.

The behaviour of the POSS filled materials is believed to be dependant on the dispersion

of POSS within the polymer matrix. Scanning Electron Microscopy (SEM) and Confocal

Raman (CFR) studies (Figure 2 and 3) indicated that the material comprised of polymer rich

and POSS rich areas and also a region which showed an anomalous Raman band, assigned to

a POSS / polymer composite rich zone.

Figure 1. SEM image of GN (top left), Molecular structure of POSS (top right) and Compressive Stress

Strain Plots for POSS filled9, Particulate filled and GN Filled PDMS (bottom) prepared using the direct

mixing protocol.

Performance, Stability and Qualification ...


Figure 2. SEM images of POSS filled (left) and GN filled (right) PDMS.

Figure 3. CFR study of POSS/PDMS composite (Octaphenyl POSS results shown) [21]. Optical

micrograph of POSS/PDMS surface (top left), CFR map of boxed region in micrograph (top right) and

Raman spectra of POSS (green) / PDMS (blue) and composite regions (red) in filled material.


Jon Meegan, Mogon Patel, Anthony C. Swain et al.

Examination of the stress strain traces for the POSS / PDMS composite demonstrates that

for equiaxial nanofillers, where there is no beneficial reinforcement arising form the aspect

ratio of the filler, the degree of non ideality and reinforcing behaviour can be qualitatively

accounted for by the surface chemistry of the filler phase. Silicic acid functionalities present

on dihydroxy POSS are known to readily incorporate into PDMS matrices [20], the

associated decrease in polymer chain mobility of the polymer component and effective fixed

location of the filler particles in this sample reduced the observed Mullins effect giving rise to

simplified bulk moduli (Figure 1). POSS fillers with non reactive surfaces (Octaphenyl

POSS) act as particulate fillers, offering improved mechanical reinforcement at the expense

of increased filler and polymer chain mobility caused by the inhomogenous distribution of

POSS through the polymer phase; this effect is evident in the increased non ideal character of

this trace.

2.3. Development of PDMS Foams

One approach used by AWE to control the mechanical properties of PDMS elastomers

has been to introduce a foam structure into the material. Foamed PDMS elastomers generally

play an important role in a number of specific stress absorbing applications. Examples of

Polystyrene and Polyurethane foams have tended to dominate the market and open literature

in recent years, generally exhibiting high stiffness to weight ratios and some unique

mechanical properties. The mechanical properties are strongly related to both the

microstructure of the foam and the properties of the polymer making up the cell walls [22]. In

general, the desirable material property requirements of foams may be summarised as shown


Uniform pore size/distribution.

Controlled or tuneable load-deflection properties.

Good batch to batch and sample reproducibility.

Low compression set.

Ability to control pore size leading to the development of scaled foam architectures.

The need to develop an overall understanding of how the microstructure in PDMS foams

influences mechanical performance is therefore of importance to AWE, one method reported

in the literature and used regularly by AWE is 3D X-Ray Computer Tomography (3DCT)

[23]. The technique offers insight into the behaviour of the foam structure (Figure 4) with and

without deformation and can be used, in conjunction with accurate baselining, to provide

porosity measurements.

Performance, Stability and Qualification ...


Figure 4. 3DCT model of a POSS filled foam.

If homogenous nano or molecular composite materials could be prepared many of the

beneficial effects discussed within this section such as quantitative structure - property

relationships or lower filler loadings to achieve a given mechanical strength may be realised.

Such materials could find uses within more specific applications and hostile environments

where reproducibility of mechanical behaviours, operation temperatures, stability and both

longer more accurate lifetime predictions are highly desirable. In order to achieve this goal

the authors believe that a significant amount of investigation must occur into the factors

affecting filler dispersion within polymer melts and ultimately elastomers.


Processes involving assembly and disassembly of complex components would be

facilitated by adhesives which could be switched between adhesive and non adhesive states as

required, one of the most elementary chemical reactions displaying this property is the

pericyclic reaction between an electron rich diene and an electron poor dienophile [24]

(Figure 5). Examples of this behaviour and its applicablility to functional materials are

numerous in the literature, a specific case is the observed reaction between discrete polymer

chains containing pendant furan (diene), or maleimide groups (dieneophile) groups [25] and

complimentary difunctional crosslinking moities, such as a bismaleimide or difuran

functionalised reagents to generate networks.

The reproducablity and efficiency of reactions displaying thermal hysteris has been

proven using cyclo addition chemistry [26] to develop a material which has found

applications within the electronics industry [27]

The Diels Alder reaction between furan and maleimide (Figure 5) is a thermally

reversible process, the reaction occurs at room temperature to form a kinetically stable adduct

which undergoes fragmentation at 90 °C, perturbing the equilibrium towards the entropically

favoured starting materials. The fragmentation reactions generate two isomeric products; an

endo product and an exo product.


Jon Meegan, Mogon Patel, Anthony C. Swain et al.

The temperature at which thermal hysterisis occurs can be tuned through subtle

modification of the diene and dienophile moieties. Making the the diene more or less

electron-rich and the dienophile more or less electron-poor has been shown to alter the

temperature at which the reaction proceeds and reverses [24].


E xo

















Figure 5. Diels-Alder reaction between a diene (furan) and a dienophile (maleimide).

3.1. Model Systems

Initial work conducted by AWE involved developing synthetic procedures to produce a

series of small molecule model PDMS based materials which incorporated furan and

maleimide units. Sufficient evidence was obtained to warrant extension of the study to

generate low molecular weight, linear, PDMS polymers.

Subsequently a series of difuran functionalised PDMS were generated and reacted with a

bismaleimide to yield low molecular weight model polymeric systems (Figure 6). This simple

approach allowed for the full characterisation of the core chemistry and facilitated the

development of PDMS materials containing substituted maleimide or furan functionalities.

Figure 6. Generic functionalized PDMS species used in the reversible Diels Alder reaction, n = 1 to 3.

Performance, Stability and Qualification ...


Figure 7. 1H VTNMR study, indicating the reversible nature of the Diels Alder reaction.

3.2. Novel Reversibly Curing Materials Using Diels-Alder Chemistry

Variable Temperature Nuclear Magnetic Resonance (VTNMR) studies of the Diels Alder

product in Figure 7 support the reversible nature of the reaction. Spectra obtained in the

reversible temperature region (60 °C - 90 °C) indicate that peaks assigned to the Diels Alder

linked product decrease in intensity while the peaks attributed to the momomeric products (δ

4.66, CH2 –difuran functionalised PDMS monomer, δ 6.88, CH -maleimide) are shown to

undergo a corresponding increase in intensity. Spectra obtained outside of this temperature

range do not show any increase or decrease of the monomer or product peaks. These

conclusions were verified by a Differential Scanning Calorimetry (DSC) investigation into

the material (not shown) which confirmed the presence of a broad endothermic event ranging

from 60 °C to 92 °C.

We believe that the reversible cure chemistry will give rise to a range of crosslinked

elastomeric or adhesive materials with designed functionality and operating regimes.

Physical characterisation of these potentially interesting systems will be of paramount

importance, as will the quantification of molecular constitution on the overall mechanical and

thermal stability of the materials in a range of operating environments.


Jon Meegan, Mogon Patel, Anthony C. Swain et al.



A previously reported study by AWE detailed the increase in stiffness of a commercial

silica filled PDMS formulation with increasing gamma dose3. The observed trend (Figure 8)

displayed non linear behaviour at low doses (red region) of gamma radiation which indicated

complex behaviour, believed to be caused by simultaneous degradation and radical mediated

crosslinking reactions of the polymer and filler phases at the polymer / filler interface [28].

At low gamma doses the stiffness of the polymer decreases, this is likely to be due to a

range of factors including chain scission and back biting to form cyclic materials. At high

gamma doses the PDMS polymer shows an increase in stiffness, caused by an increase in

crosslink density.

Figure 8. Polymer stiffness as a function of gamma dose.

The stability of polymers in a range of detrimental conditions including high temperature

or radioactivity, could be improved through the incorporation of ‘radical sinks’ within the

material., this approach has been shown to modify the glass transition temperature (Tg) and

decrease the rotational freedom of the polymer backbone; both factors cause a decrease in net

elasticity. Further investigation suggested that the physical and chemical properties of boron

containing siloxane polymers within such environments could be retained or improved,

relative to the non boron containing polymers [29] a generic structure for such a polymer is

shown in Figure 9.

Figure 9. Generic carboranylsiloxane material.

Performance, Stability and Qualification ...


4.1. Preparation of Poly(m-carboranyl-siloxane)

Poly(m-carborane-siloxane) elastomers are most commonly prepared by a ferric chloridecatalysed bulk condensation co-polymerization of dichloro- and dimethoxy-terminated

monomers with alkylchloro- or arylchloro-siloxanes. The nature of the substituent groups

within the siloxane blocks can be controlled through introduction of the appropriate silane

feedstock, phenyl and vinyl modified versions of poly(m-carborane-siloxane) have been

prepared in this manner [30].

4.2. Thermal and γ Stability of Poly(m-carboranyl-siloxane)

DSC investigations into the thermal behaviour of the materials showed that the Tg of

carboranyl modified elastomers synthesised by AWE were located within the range -30 °C to

-40 °C; this is significantly higher than that for standard PDMS [31]. The introduction of the

sterically bulky carborane unit into the siloxane backbone clearly elevates the glass-transition

temperature. However, although the carborane unit introduces conformational rigidity the

polymer chains retain sufficient flexibility and mobility which is indicated by a Tg < -30 °C.

Thermal Gravimetric Analysis (TGA) conducted in air showed that poly(m-carboranesiloxane) materials typically underwent 4 % cumulative mass loss up to 450 °C and that this

mass loss reached 7 % at 600 °C (Figure 10). In comparison, unmodified siloxanes are known

to undergo mass loss events of up to 50 % up to 450 °C caused by evolution of low molecular

weight siloxanes generated by chain scission events (as evidenced in Figure 8). The ca. 1 %

mass increase of the sample between 480 °C and 580 °C is attributed to the oxidative thermal

degradation of the carboranyl units and subsequent formation of a mixed boron oxide / boron

carbide surface coating. The rapid loss in mass above temperatures of 580 °C is due to the

thermal degradation of the boron containing species within the material.

Figure 10. TGA trace for carboranyl modified siloxane.


Jon Meegan, Mogon Patel, Anthony C. Swain et al.

Figure 11.

with a*.


B MAS NMR spectra of carbornyl modified siloxane. Spinning side bands are marked

The impact of heat and ionising radiation on the carborane cage is evident in the [11] B

Magic Angle Spinning NMR (MAS NMR) spectrum (Figure 11). At 580 °C the sacrifical

conversion of the carboranyl unit into amorphous boron oxide or carbide is indicated by the

presence of additional and enhanced shouldering of the signal which appears in the spectrum

obtained after heating at 480 °C. The broad line shape and chemical shifts of the signal in the

spectrum obtained at 580 °C and the resulting ceramic like residues observed in the sample

cell are typical of mixed boron oxides / carbides.

An 11B spectrum obtained for the irradiated sample closely resembles that of the pristine

material, indicating the remarkable γ stability of the carboranyl cage.

Our investigations have shown that the incorporation of carboranyl units into a

polysiloxane tends to strengthen the siloxane bond to thermal degradation. The typical high

temperature degradation process in siloxanes4 (depolymerisation or unzipping of the base

siloxane) is modified through increased inductive dπ - pπ contributions from the carboranyl



The ability of a material to provide diagnostic information relating to its condition or

local environment is of particular interest to AWE. Through external collaborations we have

sought to develop a range of fluorescent materials capable of providing qualitative

information regarding the stress / strain forces experienced by the material.

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