Aging of Foamed Polysiloxanes Incorporating Nanoparticles - ACS

Chapter 22

Aging of Foamed Polysiloxanes Incorporating Nanoparticles J u l i a n J. M u r p h y , J o n M e e g a n , M o g o n Patel, P a u l R . M o r r e l l , Downloaded by CORNELL UNIV on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch022

A n t h o n y C . S w a i n , and J e n n y L. C u n n i n g h a m Atomic Weapons Establishment, Aldermaston, Reading RG7 4 P R , United Kingdom

Here we outline an approach that has been taken to develop Poly(dimethylsiloxane) ( P D M S ) systems, which have property changes that are easier to predict. The problems associated with the inclusion o f a filler phase that is required for P D M S systems to have many useful physical properties have been addressed by producing nano filled equivalents. It is shown that such systems offer easier control over the materials produced whilst also resulting in a simplification o f physical properties. The production o f foamed systems, which introduces an additional variable, is also discussed. The influence o f foam structure upon the measured properties o f a material is outlined and implications for sample production and the development o f predictive ageing models are explored.

N O T E : © C r o w n Copyright (2004) " T h i s Document is o f United K i n g d o m origin and contains proprietary information which is the property o f the Secretary o f State for Defence. It is furnished in confidence and may not be copied, used or disclosed in whole or part without prior written consent o f the Intellectual Property Rights G r o u p , I P R P L 1 , M i n i s t r y o f Defence, A b b e y W o o d Bristol B S 3 4 8 J H , England". Published 2009 American Chemical Society In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Downloaded by CORNELL UNIV on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch022

The development o f predictive ageing models for individual polymer systems is a very complex process. M a n y materials are formulations o f a number o f different components, any o f which can affect observed ageing trends. In filled systems there are additional effects since the properties o f the material are dependent upon interactions at the polymer/filler interface. These interactions can be highly complex and are often influenced by subtle chemical and morphological changes in the polymer phase. Understanding how the performance o f a complex component changes over time adds significantly to the level o f difficulty. In structured components such as foams the precise nature o f the cell structure has a significant influence on physical properties. The implications o f small differences in cell structure for accurate measurement o f physical properties are significant. V e r y small differences in manufacturing methodology or environmental conditions during production can tip a structure from fully open to partially closed cell. This causes a large change in the resulting compression properties since gas contained within closed cells has to be compressed during deformation. The properties are also time dependent since gas diffusion through the cell walls w i l l take place over extended timescales. Small changes in the size o f cells and/or the thickness o f cell walls w i l l also affect the properties o f a foamed component. Collapse o f the cell structure during deformation gives regions o f different stress strain response resulting in physical property measurements that are highly strain dependent. The following paper details the approach that has been taken to develop Poly(dimethylsiloxane) ( P D M S ) systems, which have property changes that are easier to predict. One o f the biggest problems with P D M S systems is associated with the inclusion o f a filler phase that is required for a system with useful physical properties (7) P D M S elastomers are typically reinforced with a filler phase to improve the physical properties o f the resulting composite material since load transfer between the soft polymer and hard filler components results in enhanced properties (2). The most commonly used fillers in P D M S systems are particulate silica fillers due to their affordability, high surface area and compatibility with the polymer matrix (5). However, the incorporation o f these irregular shaped, hydroscopic, high surface area fillers lead to complicated degradation trends (4,5) as well as the exhibition o f complex mechanical behaviors such as the M u l l i n s effect (6). Some o f the interfacial effects in filled systems can be inhibited through careful choice o f filler phase and there is a great deal o f work that concentrates on modifying the filler component (7, 8, 9). The introduction o f uniform nanotubular fillers (silica and carbon) or molecular equivalents (PolyoctahedralOligomericSilSesquioxane ( P O S S ) and Carboranes) into P D M S elastomers can be used to both modify and simplify the mechanical and ageing behaviors o f these composite materials, and work on such materials w i l l be discussed in detail.

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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A s P D M S elastomers are often used to produce foamed systems, understanding the influence that the foam structure has upon the measured properties o f a material is crucial to the development o f a confident ageing model. Moreover, understanding the variations in foam structures that are introduced by production processes is critical for the prediction o f long term performance for components. W o r k detailing the precise characterization o f foam structures using a number o f techniques is discussed. Variation introduced by production processes and the implications for performance prediction w i l l also be outlined. Strategies for producing idealized foam structures that overcoming such problems and the implication for performance prediction are detailed.

Experimental Details Poly(/w-carboranyl-siloxane) elastomers containing a mixture o f di-methyland methyl(phenyl)-silyl units were synthesised using the Ferric C h l o r i d e catalyzed condensation reaction between di-chloro-di-organosilane and 1,7bis(di-methyl(methoxy)silyl)-/w-carborane. S i l i c a N a n o tubes synthesised using insitu growth o f D L Tartaric acid nanocrystals and concurrent sol-gel hydrolysis and condensation o f silane. P O S S was mechanically dispersed in a fimctionalized polysiloxane gum using a speedimix (needs some details) and the m i x cured using tin catalyzed condensation chemistry. Silica and Carbon nano fiber composites were produced by m i x i n g the fiber into functionalized polysiloxane gum and the m i x cured using tin catalyzed condensation chemistry. Mechanical properties were assessed through Dynamic Mechanical Analysis ( D M A ) , Proton N M R relaxation ( T ) studies were used to assess segmental chain dynamics. X - r a y micro tamography was used to characterize foam structure. G a m m a irradiation was performed by exposure to a C o source. 2

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Results and Discussion Carborane Modified Materials The plots in Figure 1 and Figure 2 display changes in the T relaxation time for a series o f samples exposed to gamma radiation doses. The plots in Figure 1 were obtained using a conventionally silica filled P D M S material. The linear region o f the plot where 1/T increases after a dose o f approximately 0.1 M G y is caused by radiation induced crosslinking resulting in a stiffening o f the material. A t l o w doses, between 0 and 0.1 M G y , 1/T decreases, with dose. This is caused by radiation effects on the polymer filler interface, which reduce the extent o f 2

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258 interaction. The result is reduction in the effective crosslink density and a softening o f the system. The data for the plot in Figure 2 was obtained using a P D M S polymer modified with a carborane unit. A schematic o f the system is given in Figure 3. For this material there is no radiation effect on an interface and the plot is linear over the entire dose range. The development o f an ageing model that provided a predictive capability and high confidence is far easier in a system that displays a linear trend than on with a complex non linear relationship.


Polyhedral Oligomeric SilSesquioxane (POSS) Modified Materials A generic schematic o f P O S S in given in Figure 4, while Figure 5 displays compressive strain plots for a variety o f P D M S materials. The P O S S systems are functionalised with the different R groups indicated on the plot and are shown along with the virgin P D M S for comparison. The methyl and phenyl modified systems have a reinforcing effect on the resulting material whilst the hydroxyl functionalised system results in a softer material. This is caused by an overall reduction in crosslink density as the O H reacts during the curing process. The plots in Figure 6 show stress strain curves for two optimised P D M S systems formulated with 6% dihydroxyhexamethyl- and octaphenyl- modified P O S S . Although somewhat softer than the silica filled equivalent, also shown, plots indicate a far simpler behaviour with no obvious M u l l i n s effect and little

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Dose (MGy) Figure 1. Plot showing the changes in T for a series of silica filled PDMS samples exposed to a variety of gamma radiation doses. 2


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Dose (MGy) Figure 2. Plot showing the changes in T for a series of carborane modified PDMS samples exposed to a variety of gamma radiation doses. 2

Figure 3. Schematic of the carborane modified PDMS system used for the radiation degradation study.


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Aging of Foamed Polysiloxanes Incorporating Nanoparticles - ACS

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