One route to increased aircraft performance is through the use of flexible, shape-changeable aerodynamics effectors. However, state of the art materials are not flexible or durable enough over the required broad temperature range. Mixed siloxanes were crosslinked by polyhedral oligomeric silsesquioxanes (POSS) producing novel materials that remained flexible and elastic from -55 to 94 C. POSS molecules were chemically modified to generate homogeneous distributions within the siloxane matrix. High resolution scanning electron microscope (HRSEM) images indicated homogenous POSS distribution up to 0.8 wt %. Above the solubility limit, POSS aggregates could be seen both macroscopically and via SEM (approx.60-120 nm). Tensile tests were performed to determine Young s modulus, tensile strength, and elongation at break over the range of temperatures associated with transonic aircraft use (-55 to 94 C; -65 to 200 F). The siloxane materials developed here maintained flexibility at -55 C, where previous candidate materials failed. At room temperature, films could be elongated up to 250 % before rupturing. At -55 and 94 C, however, films could be elongated up to 400 % and 125 %, respectively

Belcher, Marcus A.

Hinkley, Jeffrey A.

Kiri, Neha N.

Lillehei, Peter T.

NASA Technical Reports Server

NOVEL LOW-TEMPERATURE POSS-CONTAINING SILOXANE 
ELASTOMERS 
 
Marcus A. Belcher
1*
, Jeffrey A. Hinkley
2
, Neha N. Kiri
3
, Peter T. Lillehei
2
 
1
National Institute of Aerospace, Hampton, VA 23666-6147 
2
NASA Langley Research Center, Hampton, VA 23681-2199 
3
Dept of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854 
 
ABSTRACT 
One route to increased aircraft performance is through the use of flexible, shape-
changeable aerodynamics effectors.  However, state of the art materials are not flexible or 
durable enough over the required broad temperature range.  Mixed siloxanes were cross-
linked by polyhedral oligomeric silsesquioxanes (POSS) producing novel materials that 
remained flexible and elastic from -55 to 94 °C.  POSS molecules were chemically 
modified to generate homogeneous distributions within the siloxane matrix.  High 
resolution scanning electron microscope (HRSEM) images indicated homogenous POSS 
distribution up to 0.8 wt %.  Above the solubility limit, POSS aggregates could be seen 
both macroscopically and via SEM (~60-120 nm). Tensile tests were performed to 
determine Young’s modulus, tensile strength, and elongation at break over the range of 
temperatures associated with transonic aircraft use (-55 to 94 °C; -65 to 200 °F).  The 
siloxane materials developed here maintained flexibility at -55 °C, where previous 
candidate materials failed.  At room temperature, films could be elongated up to 250 % 
before rupturing.  At -55 and 94 °C, however, films could be elongated up to 400 % and 
125 %, respectively.   
 
This paper is work of the U. S. Government and is not subject to copyright protection in the U.S. 
* To whom correspondence should be addressed: tony.belcher@nianet.org, (757) 864-1083 
 
KEY WORDS: Polyhedral Oligomeric Silsesquioxane, Siloxane Elastomers, Molecular 
Reinforcement 
 
INTRODUCTION 
 
Three specific techniques for improving mechanical properties in elastomers 
include forming bimodal networks, using bulk fillers, and altering the cross-link 
chemistry with a reinforcing agent.  A bimodal network is comprised of polymer 
components with two different chain-length distributions.  A network composed of short 
and long chains is typically observed to have a high ultimate strength without 
compromising maximum elongation [1].  Previous research using bulk microscopic fillers 
as reinforcement, such as silica or carbon black, showed high ultimate strength and high 
elongation at break.  However, the highly viscous polymers generated by this technique 
are cumbersome to process [2].  While an abundance of literature exists on polymers 
https://ntrs.nasa.gov/search.jsp?R=20080020378 2019-08-30T04:30:27+00:00Z
containing polyhedral oligomeric silsesquioxanes (POSS) [3], POSS has primarily been 
used as a non-covalent polymer filler [4], pendant [5], or in-backbone modifier [6] where 
its use improved a variety of physical, thermal, and mechanical properties [7].   
In this work POSS molecules were incorporated as novel cross-linking agents 
(Figure 1) into flexible siloxane networks in an effort to enhance their tensile properties 
from -55 °C to 94 °C.  These nanoscale molecules were chosen because they have a well-
defined structure, silica-like core (e.g. Si8O12), and, in the case of Si8O12, eight organic 
groups at the corners of the cube (see inset of Figure 1).  Functional corner groups like 
vinyl, for example, enable POSS to be reacted with other molecules and thus they can be 
used to covalently reinforce a polymer network.  It was envisioned that covalent POSS 
incorporation would provide a greater degree of polymer matrix reinforcement compared 
to non-covalent incorporation and thus greater mechanical properties enhancement [8].   
 
EXPERIMENTAL SECTION 
 
Materials. Octavinyl-polyhedral oligomeric silsesquioxane (octavinyl-POSS) was used 
as received from Hybrid Plastics (Hattiesburg, MS).  Vinyl terminated (35-45% 
trifluoropropylmethylsiloxane)-(65-55% dimethylsiloxane) copolymer (FMV-4031; 
referred to as “trifluoro component”), trimethylsiloxane terminated (25-30% 
methylhydrosiloxane)-(75-70% dimethylsiloxane) copolymer (HMS-301; referred to as 
“hydromethyl component”), and diethylsilane (SID3415.0) were used as received from 
Gelest, Inc. (Tullytown, PA).  Platinum (0) 1,3-divinyl-1,1,3,3-tetramethyldisiloxane 
(0.05 M solution in vinyl terminated polydimethylsiloxane; also called Karstedt’s 
catalyst) was used as received from Aldrich (Milwaukee, WI).  [See Figure 2.]  
Methylene chloride was used as received from Fisher Scientific (Pittsburgh, PA).   
 
General Preparation Procedure. Octavinyl-POSS was chemically modified (see Figure 
3) to afford covalent incorporation into the siloxane network (see Figure 4) comprised of 
~30 % diethylsilane, ~60 % trifluoro component, ~8 % hydromethyl component and 
variable amounts of both octavinyl-POSS and Karstedt’s catalyst.  More specifically, 
octavinyl-POSS (from 0 to 1 wt %) was dissolved in a minimal amount of methylene 
chloride.  Diethylsilane and Karstedt’s catalyst (0.6 to 1.2 wt %) were added and the 
mixture was stirred by hand.  After ~30 s the trifluoro component was added with stirring 
until the reaction mixture was a uniform consistency followed by addition of the 
hydromethyl component and further stirring.  After all the components were stirred for 
~60 s, the reaction mixture was poured into non-stick perfluoroalkoxy (PFA) resin dishes 
(Savillex Corporation).  Individual bubbles in the wet film were popped using minimal 
pressure from a pressurized air canister.  Films were cured at room temperature overnight 
and were subsequently heated in a drying oven at 60 °C for a few hours.  After curing, 
films were peeled from the PFA dishes and die-cut into Type V specimens (ASTM 
Standard D 638-02a).     
 
Characterization.  Individual specimens were analyzed in several ways.  Scanning 
electron microscopy (SEM) images were collected on a Hitachi S-5200 Field Emission 
High Resolution Scanning Electron Microscope equipped with a through the lens (TTL) 
detector.  Differential scanning calorimetry (DSC) was completed on a DSC Q100 
Differential Scanning Calorimeter by TA Instruments (-80 to 300 °C at 20 °C/min). 
Thermal gravimetric analysis (TGA) was completed on a TGA Q500 Thermogravimetric 
Analyzer by TA Instruments under an atmosphere of air (25 to 100 °C at 20 °C/min, hold 
30 min, then 2.5 °C/min to 700 °C). Coefficient of thermal expansion (CTE) was 
determined per sample on a TMA Q400 Thermomechanical Analyzer by TA Instruments 
(2 °C/min from -60 to 250 °C).   
 
Tensile Testing.  Prior to mechanical testing, the thickness of each individual specimen 
was measured using either a digital caliper (Mitutoyo Corporation) or micrometer 
(Precision Micrometer, Testing Machines, Inc.), depending on overall thickness, which 
ranged from ~0.6 to ~1.0 mm.  Each sample was then installed on a test stand (Sintech) 
with a 1" gauge length between self-tightening roller grips (Instron) with a universal joint 
in the load train.  An SMT 1-11 lb load cell (Interface, Inc.) was used to determine load.   
Elongation was estimated using the crosshead displacement.  For each tensile test, the 
load and elongation data was collected using MTS Corporation TestWorks 4 software.  
For the tests conducted at non-ambient conditions, a Thermcraft, Inc. oven with a 
solenoid valve coupled to a liquid nitrogen Dewar was employed.  A second digital 
thermometer (Fluke 2170A) was used to verify the chamber temperature.  Once the 
temperature of the grips and sample reached equilibrium, typically after 10 min, each 
specimen was stretched at a rate of 7.5 in/min up to the moment of rupture, at which 
point the elongation at break, strain at break, and break load were recorded for later 
analysis.  
 
RESULTS AND DISCUSSION 
 
Direct incorporation of octavinyl-POSS into the proposed novel siloxane network 
was unsuccessful due to phase separation of the crosslinking agent and the unreacted 
matrix components.  To improve compatibility with the siloxane network, octavinyl-
POSS was chemically modified in-situ with diethylsilane (Figure 3).  The pre-treated 
reinforcing agent is likely comprised of a distribution of differing degrees of 
functionality, from zero to eight chemically modified corners.  HRSEM 
photomicrographs of some characteristic siloxane film samples are shown in Figure 5.  
The 0.3 wt % POSS film revealed no incongruities (Figures 5a and b), meaning no POSS 
aggregation, on either the surface (save for some apparent bubbles) or in the cross-
sectional portion (the rough textured portion at the right of Figure 5a).  Similarly, at 0.7 
wt % POSS, no aggregation on either the surface or at any specimen cracks is observed 
(Figures 5c and d).  However, at 1.2 wt % POSS, surface irregularities ~60-120 nm in 
size appear (Figures 5e and f), indicating that the matrix is over the POSS solubility limit, 
which is estimated to be 0.9 wt %.  Therefore, only materials near or below the solubility 
limit (≤ 1 wt % POSS) were used for further studies. 
Variation of POSS content from 0 to 1 wt % resulted in minimal change in the 
thermal properties of these elastomers.  DSC traces did not indicate distinct melting peaks 
(Tm) or glass transitions (Tg) for any sample.  Table 1 shows TGA and CTE data relative 
to POSS content.  TGA showed all samples to be thermally stable up to 100 °C followed 
by a gradual loss in mass to 350 ±3 °C (Td10, temperature by which 10 % of the material 
had been lost).  It was expected that CTE values for the reinforced films would decrease 
with increasing POSS content.  A 0 wt % POSS sample would have the largest CTE 
value, corresponding with greater elongation and apparent higher flexibility.  As POSS 
content increased, CTE values should decrease, reflecting an increase in crosslink density 
and thus higher rigidity.  However, preliminary data do not show the expected trend and 
overall the CTE values were higher than anticipated.   
 
Table 1. TGA and CTE data relative to POSS content 
 
POSS Content (wt %) Td10 (°C) CTE [m/(m•°C)] 
0 347 843 
0.2 348 934 
0.4 347 918 
0.6 348 626 
0.8 349 870 
1.0 353 666 
 
Figure 6, a plot of Young’s modulus versus POSS content, shows a direct 
relationship between polymer reinforcement and Young’s modulus values at -55 and 23 
ºC, yielding up to ~60 % improvement at room temperature and ~30 % improvement at 
the lower temperature.  At 94 ºC, however, the opposite trend is observed with up to a 
~60 % decrease in Young’s modulus.  Figure 7 shows a slight decrease in tensile strength 
with added polymer reinforcement at room temperature and a drastic decrease at low 
temperature.  At high temperature, however, a very subtle increase in tensile strength was 
observed.   
Dependence of elongation at break on POSS content is shown in Figure 8.  Of 
greatest significance for these results is that the POSS-reinforced elastomers remained 
flexible at -55 ºC.  That these samples exhibited greater elongation before failure than 
those tested at room temperature or 94 ºC by a factor of about 2 was quite interesting and 
might be explained by a delay in stretching of polymer network chains or slower chain 
motion due to the low temperature [9].  At all temperatures tested elongation was lower at 
higher POSS loadings, as could be expected.  
These novel elastomers exhibited a noticeable decrease in temperature 
dependence of the observed mechanical properties (Figures 6-8) for samples with 
intermediate POSS loadings (0.4 – 0.6 and in some cases 0.8 wt %).  These results 
suggest that incorporation of POSS into the siloxane network, at least at these loadings, 
yields thermal stabilization of mechanical properties while still providing desired changes 
in the behavior of the material (i.e., increased durability and flexibility over an expanded 
temperature range).  
 
CONCLUSIONS 
 
Octavinyl-POSS was employed as a covalent, nanoscale molecular reinforcement 
(i.e., not as a filler) to create novel siloxane systems.  POSS content was varied to allow 
for a systematic study.  Mechanical testing was done to determine the additive’s effect on 
Young’s modulus, tensile strength, and elongation at break of the novel reinforced 
polymers reported here. Data analysis indicated a positive correlation between POSS 
content and Young’s modulus, yielding up to 50 % improvement at room temperature.  
However, negative correlations were observed for both elongation at break (up to 50 % 
decrease) and tensile strength (up to 65 % decrease).  The structure/property relationships 
shown in Figures 6-8 may prove useful in considering future formulations, including 
work with films containing intermediate POSS content, particularly in the 0.3 to 0.7 wt % 
range.  Comparing collected mechanical property values with theoretical predictions 
concerning effective chain length and crosslinking density correlations with modulus and 
elongation are also of interest.  It is worth noting that the materials developed, tested, and 
reported here remained flexible at -55 ºC whereas previously considered and developed 
materials failed to meet this requirement, instead becoming brittle at -55 ºC.  It is also of 
interest that these materials exhibited greater elongation at -55 ºC than at room 
temperature.   
 
ACKNOWLEDGEMENTS 
 
The authors thank Drs. Chris Wohl and Sayata Ghose for helpful discussions. 
 
REFERENCES 
 
[1]  B.D. Viers and J.E. Mark, J. Macromol. Sci. Part A: Pure Appl. Chem. 44, 131 
(2007); B.D. Viers and J.E. Mark, J. Inorg. Organomet. Polym. 17, 283 (2007). 
 
[2]  C.C. Sun and J.E. Mark, Polymer 30, 104 (1989).   
 
[3]  S.H. Phillips, T.S. Haddad, and S.J. Tomczak, Curr. Opin. Solid State Mater. Sci. 8, 
21 (2004). 
 
[4]  E.T. Kopesky, T.S. Haddad, R.E. Cohen, and G.H. McKinley, Macromolecules 37, 
8992 (2004); E.T. Kopesky, T.S. Haddad, G.H. McKinley, and R.E. Cohen, Polymer 46, 
4743 (2005). 
 
[5]  G.Z. Li, H. Cho, L. Wang, H. Toghiani, and C.U. Pittman, Jr., J. Polym. Sci. Part A: 
Polym. Chem. 43, 355 (2005). 
 
[6]  M.E. Wright, D.A. Schorzman, F.J. Feher, and R.Z. Jin, Chem. Mater. 15, 264 
(2003); S. Wu, T. Hayakawa, R. Kikuchi, S.J. Grunzinger, M. Kakimoto, and H. Oikawa, 
Macromolecules 40, 5698 (2007). 
 
[7]  L. Zheng, R.J. Farris, and E.B. Coughlin, Macromolecules 34, 8034 (2001). 
 
[8]  G. Pan, J.E. Mark, and D.W. Schaefer, J. Polym. Sci. Part B: Polym. Phys. 41, 3314 
(2003) 
 
[9]  J.C. Halpin, J. Appl. Phys. 35, 3133 (1964). 
 
  LIST OF FIGURES 
 
Figure 1. Reinforcement of siloxane elastomers using POSS molecules 
 
Figure 2. Synthetic components used in the flexible network: a.) “trifluoro component”, 
b.) “hydromethyl component”, and c.) Karstedt’s catalyst 
 
Figure 3. Chemical modification of octavinyl-POSS (product shown with four 
diethylsilyl functionalities for illustrative purposes) 
 
Figure 4. Simplified network formation (0 % POSS shown) 
 
Figure 5. High Resolution Scanning Electron Microscope (HRSEM) photomicrographs of 
some characteristic siloxane film samples. 
 
Figure 6. Young’s modulus vs. POSS content 
 
Figure 7. Tensile strength vs. POSS content 
 
Figure 8. Elongation at break vs. POSS content 
 
 
 
Figure 1. Reinforcement of siloxane elastomers using POSS molecules   
 
 
 
O
Si
O
Si
O
SiSi
CF3
m n
O
Si
O
Si
O
Si
H
Si
p q
Si Si
O
Si
Si
O
PtSi
Si
O
Pt
c.
a.
b.
 
 
Figure 2. Synthetic components used in the flexible network: a.) “trifluoro component”, 
b.) “hydromethyl component”, and c.) Karstedt’s catalyst 
 
 
Si
O
Si
O
SiSi
O
O
Si
O
Si
O
Si
O
Si
O
O
O
O
O
Si
H
Si
H
Si
Si
H
H
Si
O
Si
O
SiSi
O
O
Si
O
Si
O
Si
O
Si
O
O
O
O
O
catalyst
Si
H H
 
 
Figure 3. Chemical modification of octavinyl-POSS (product shown with four 
diethylsilyl functionalities for illustrative purposes)  
 
 
 O
Si
O
Si
O
SiSi
CF3
m n
O
Si
O
Si
O
Si
H
Si
p q
O
Si
O
Si
O
SiSi
CF3
m n
Si
O
Si
O
SiSi
p x
Si
O
Si
O
Si
H
Si
p q-x
O
Si
O
x
Si
O
H
q-x
O
+
catalyst
 
 
Figure 4. Simplified network formation (0 % POSS shown) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
       
       (a) 0.3 wt % POSS film – surface           (b) a 0.3 wt % POSS film - depth 
         
             (c) a 0.7 wt % POSS film                          (d) a 0.7 wt % POSS film - magnified 
       
    (e) sample with 1.2 wt % POSS                   (f) sample with 1.2 wt % POSS - magnified 
 
Figure 5. High Resolution Scanning Electron Microscope (HRSEM) photomicrographs 
of some characteristic siloxane film samples 
 
 Figure 6. Young’s modulus vs. POSS content 
 
Figure 7. Tensile strength vs. POSS content 
Tensile Strength vs POSS Content
0
30
60
90
120
150
180
210
0 0.2 0.4 0.6 0.8 1 1.2
POSS Content (wt % )
T
e
n
s
il
e
 S
tr
e
n
g
th
 (
p
s
i)
-55 C
23 C
94 C
Young's Modulus vs POSS Content
25
30
35
40
45
50
55
60
65
70
75
0 0.2 0.4 0.6 0.8 1 1.2
POSS Content (wt % )
Y
o
u
n
g
's
 M
o
d
u
lu
s
 (
p
s
i)
-55 C
23 C
94 C
-55 C
23 C
94 C
Figure 8. Elongation at break vs. POSS content 
Elongation at Break vs POSS Content
0
50
100
150
200
250
300
350
400
0 0.2 0.4 0.6 0.8 1 1.2
POSS Content (wt % )
E
lo
n
g
a
ti
o
n
 a
t 
B
re
a
k
 (
%
)
-55 C
23 C
94 C


Novel Low-Temperature Poss-Containing Siloxane Elastomers

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