Branched Polymers for Enhancing Polymer Gel Strength 

and Toughness (Final Report)

by Robert H. Lambeth, Randy A. Mrozek, Joseph L. Lenhart, 

Yelena R. Sliozberg, and Jan W. Andzelm

ARL-TR-6338 February 2013

Approved for public release; distribution unlimited. 
NOTICES

Disclaimers

The findings in this report are not to be construed as an official Department of the Army position 

unless so designated by other authorized documents.

Citation of manufacturer’s or trade names does not constitute an official endorsement or 

approval of the use thereof.

Destroy this report when it is no longer needed. Do not return it to the originator.
Army Research Laboratory

Aberdeen Proving Ground, MD 21005

ARL-TR-6338 February 2013

Branched Polymers for Enhancing Polymer Gel Strength 

and Toughness (Final Report)

Robert H. Lambeth, Randy A. Mrozek, Joseph L. Lenhart, 

Yelena R. Sliozberg, and Jan W. Andzelm

Weapons and Materials Research Directorate, ARL

Approved for public release; distribution unlimited. 
ii

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently 

valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

February 2013

2. REPORT TYPE

DRI

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Branched Polymers for Enhancing Polymer Gel Strength and Toughness (Final 

Report)

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Robert H. Lambeth, Randy A. Mrozek, Joseph L. Lenhart, Yelena R. Sliozberg, 

and Jan W. Andzelm

5d. PROJECT NUMBER

FY11-WMR-032

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Research Laboratory

ATTN: RDRL-WMM-G

Aberdeen Proving Ground, MD 21005

8. PERFORMING ORGANIZATION

 REPORT NUMBER

ARL-TR-6338

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The objective of this project is to develop robust and scalable synthetic methods to access polymer network and solvent 

macromolecules with complex chain architectures and highly branched molecular structures. The introduction of branching 

points in the polymer network and solvent will significantly enhance the strength and toughness of polymers gels through 

increased physical chain entanglements. In particular, we have focused on synthesizing and characterizing 

poly(dimethylsiloxane) (PDMS) based materials with well-defined chain lengths and branching points for use in polymer gels. 

This research has yielded four major accomplishments related to the proposal objectives including: (1) conducting successful 

synthesis of a “star shaped” PDMS solvent; (2) demonstrating greater than two times improvements in both strength and 

toughness of the polymer gel due to incorporation of the star shaped solvent; (3) identifying synthetic strategies for 

increasingly complicated chain structures, such as H, pom-pom, comb, and dumbbell shaped molecules; and (4) employing 

anionic polymerization to prepare additional “star shaped” PDMS solvents of varying molecular weight and arm number.

15. SUBJECT TERMS

branched polymers, polymer gels, anionic polymerization, Director’s Research Initiative (DRI)

16. SECURITY CLASSIFICATION OF:

17. LIMITATION

 OF 

 ABSTRACT

UU

18. NUMBER

 OF 

 PAGES

28

19a. NAME OF RESPONSIBLE PERSON

Robert H. Lambeth

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified

19b. TELEPHONE NUMBER (Include area code)

(410) 306-0281

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18
iii

Contents

List of Figures iv

Acknowledgments v

1. Objective 1

2. Approach 1

3. Results 5

3.1 Year 1 ..............................................................................................................................5

3.1.1 Star Polymer Synthesis........................................................................................5

3.1.2 Tensile Testing ....................................................................................................6

3.1.3 Tack Adhesion Testing........................................................................................7

3.1.4 Computational Modeling.....................................................................................8

3.2 Year 2 ..............................................................................................................................9

4. Conclusions 13

5. References 15

6. Transitions 17

List of Symbols, Abbreviations, and Acronyms 18

Distribution List 19
iv

List of Figures

Figure 1. Examples of branched additive architectures. .................................................................3

Figure 2. Synthetic scheme for PDMS star polymer via thiol-ene coupling chemistry (top). 

1H NMR analysis of vinyl-terminated PDMS showing complete disappearance of 

resonances associated with vinyl functionality after a 5-min reaction time (left). FTIR 

analysis of tetra-thiol showing disappearance of thiol peak after 5 min (right). .......................5

Figure 3. Synthetic scheme for platinum catalyzed hydrosilylation reaction (left). SEC 

analysis of the reaction showing shift of star polymer to higher molecular weight relative 

to linear peak with uniform peak shape and nearly 4-fold increase in molecular weight 

(right). ........................................................................................................................................6

Figure 4. Comparison of stress-strain tensile data for PDMS gels with either 50 vol.% of the 

star-branched additive or 50 vol.% of a small linear chain PDMS additive (1,100 g/mol). 

The strength (stress at break) increased twofold and the toughness (integral of the stressstrain curve) increased by threefold for the gel containing the star-branched additive 

compared to a linear small molecule solvent............................................................................7

Figure 5. Comparison of the stress-strain tack adhesion data for PDMS gels with either 

50 vol.% of the star-branched additive or 50 vol.% of a small molecule PDMS additive 

(1,100 g/mol). Similar to the tensile testing, the adhesive strength (maximum stress) and 

toughness/work of adhesion (integral of the stress-strain curve) are significantly 

improved for the gel containing the star-branched solvent when compared to a sample 

containing the small molecule additive......................................................................................8

Figure 6. Computational modeling of the impact of solvent size and architecture on the 

stress-strain behavior of a polymer network.............................................................................9

Figure 7. Synthetic scheme for preparation of four- and six-arm star polymers via 

chlorosilane coupling. ..............................................................................................................10

Figure 8. SEC analysis of chlorosilane coupling to form four- and six-arm star polymers. ........10

Figure 9. Organocatalytic ROP of D3 initiated by pyrene butanol and catalyzed TBD...............11

Figure 10. SEC analysis with light scattering and refractive index (left) and UV (right) of 

organocatalytic ROMP of D3. .................................................................................................11

Figure 11. Synthetic scheme for anionic polymerization of D3 (left). SEC analysis over time 

by light scattering and refractive index....................................................................................12

Figure 12. SEC analysis of star polymer synthesis via termination of anionic polymer of D3 

with multifunctional chlorosilane. ...........................................................................................13
v

Acknowledgments 

We would like to thank MyVan Baranoski and Sam Pederson for assistance with synthesis of the 

star polymers and preparation of gel formulations. We would also like to thank Dr. Yelena 

Sliozberg and Dr. Jan Andzelm for performing the computational modeling of the branched 

polymer additives.
vi

INTENTIONALLY LEFT BLANK.
1

1. Objective

The objective of this project is to develop robust and scalable synthetic methods to access 

polymer network and solvent macromolecules with complex chain architectures and highly 

branched molecular structures. The introduction of branching points in the polymer network and 

solvent will significantly enhance the strength and toughness of the polymers gels through

increased physical chain entanglements. These tougher polymer gels will enable a host of 

critical Army technologies including electronics packaging and shielding; soft robotics and 

morphing materials; stretchable and conformable electronics, smart textiles, sensors, and 

membranes; combat casualty care; prosthetics; bio-inspired materials; responsive coatings; and 

tissue simulants for ballistics testing. In particular, we have focused on synthesizing and 

characterizing poly(dimethylsiloxane) (PDMS) based materials with well-defined chain lengths 

and branching points for use in polymer gels.

2. Approach

Nature commonly employs complex and entangled architectures to control signaling, transport, 

and mechanical properties in biological systems. Interestingly, these soft biomaterials are 

amazingly tough and durable, despite the high water content (1). Synthetic polymer gels 

represent bio-inspired surrogates for these biotissues and are typically very soft elastomers 

composed of physically or chemically crosslinked polymer chains. Gels are loosely crosslinked 

and typically contain high solvent loadings to impart the softness. Many variables impact gel 

properties including the polymer/solvent chemistry, polymer/solvent chain architecture and size, 

solvent loading, and the incorporation of various fillers and additives. This tunability offers the 

potential to implement synthetic gels in a very wide range of Army applications including 

electronics packaging, soft robotics, textiles, sensors, membranes, and prosthetics; bio-inspired 

materials; and soft tissue surrogates for ballistics testing. While the potential applications are 

promising, synthetic approaches have not achieved the intricacy of biological materials. Critical 

challenges hinder broad utility of synthetic gels in defense-related applications including poor 

environmental stability; poor toughness, durability, and fatigue resistance; inability to function in 

extreme environments, lack of multifunctionality; and limited materials availability, 

processability, and reliability. These challenges must be addressed before widespread integration 

of gel-based materials can be realized in the defense community.

The presence of solvent in a crosslinked polymer network produces a polymer gel that behaves 

mechanically as a “soft solid” that is easily deformable yet still recovers from an applied strain 

due to the elastic nature of the crosslinked polymer chains (2). Polymer gels have received 
2

considerable attention for use in a wide range of applications including microvalves (3, 4), 

robotics (5), sensors (6, 7), electronic packaging (8), and biomedical technology (9, 10). A 

significant amount of this research effort has focused on water-based hydrogels. For example, 

hydrogels based on poly-(N-isopropylacrylamide) (PNIPAM) exhibit a volume shrinkage 

transition near 33 °C that has been exploited in the biological community for several applications 

including controlled release (11), cellular adhesion (12), and nucleic acid purification (13). 

However, widespread implementation of gel-based technology is often hindered by narrow 

operational temperature ranges, limited performance lifetimes, and poor toughness (14). 

To avoid the operational limitations of water-based hydrogels, the solvent can be composed of 

low volatility organic molecules. The incorporation of organic solvent coupled with other critical 

material requirements has led to the design of non-aqueous gels with very broad temperature 

performance (8, 15). However, the solvent molecules often have a significant impact on the 

small strain mechanical properties and high loadings of small molecule organic solvents almost 

exclusively results in a gel with very low toughness. Several approaches have been investigated 

to enhance the strength, toughness, and durability of polymer gels. Examples include slip-link 

gels, polymer hydrogel nanocomposites, and double-network gels. While these approaches 

enhanced the mechanical properties of the gel through different toughening mechanisms, each 

required complicated processing not amenable to scale up limiting their broad utility. 

Most research in polymer gels exploits a chemically or physically crosslinked polymer loaded 

with high fractions of a small molecule solvent (examples include physically crosslinked 

thermoplastic elastomer gels [16], chemically crosslinked polymer networks [15], hydrogels 

[17], and organogels [18]). Recent efforts from our laboratory demonstrated the importance of 

solvent molecular weight on the small strain rheological response and fracture behavior of the 

polymer gel. The impact of sol loading and molecular weight on the mechanical properties of a 

model end-linked PDMS gel that contains non-reactive PDMS soluble material was investigated. 

The networks were formed in the presence of non-reactive PDMS solvents with molecular 

weights from 1,250 to 423,000 g/mol, broadly spanning the molecular weight of entanglement 

for PDMS (MWENT ~ 29,000 g/mol) (19). The gels exhibited a strong frequency-dependent 

mechanical response when the solvent molecular weight > MWENT. In addition, scaling factors 

of shear storage modulus versus solvent loading displayed a distinct decrease from the 

theoretical value for networks formed in a theta solvent of 2.3 with increasing measurement 

frequency and solvent molecular weight. The frequency dependent shear storage modulus could 

be shifted by the ratio of solvent molecular weights to the 3.4 power to form a master curve at a 

particular solvent loading indicating that mobility of entangled solvent plays a critical role for the 

mechanical response. In addition, the incorporation of entangled solvent can increase the 

toughness of the PDMS gels. 

One approach that has not been considered is the introduction of branched polymer and solvent 

molecules into the network. Examples of branched structures include t-shaped, h-shaped, pompom, barbwire (flexible main chain with rigid side chains), star, comb-shaped, dumbbell-shaped, 
3

and a hybrid of multiple architectures illustrated in figure 1. The branched nature of the additive 

results in a range of diffusion timescales that are dependent on the molecular architecture (size 

and microstructure) when compared to linear polymers of similar molecular weight (20).

Incorporation of these branched molecules into a polymer network will result in a reduced 

mobility in the network relative to a linear molecule of the same molecular weight. As a result, 

when the deformation rate becomes faster than the characteristic mobility time of the additive in 

the network, the additive cannot migrate away from the deformation zone. Deformations faster 

than the characteristic mobility time of the branched additive will result in an increase in the 

modulus, strength, and toughness of the network enabling tenability of the strain-rate-dependent 

mechanical response.

Figure 1. Examples of branched additive architectures.

During deformation as the result of a fracture event, the branched additive will enhance the 

toughness of the material through several mechanisms: (1) the decreased mobility of the additive 

will act as additional chemical crosslinking requiring a larger number of covalent bonds to be 

broken to facilitate further fracture propagation; (2) the inability of the additive to migrate away 

from the crack tip will produce a larger zone of plastic deformation requiring additional energy 

to maintain crack propagation; and (3) the larger plastic deformation zone will produce greater 

energy dissipation to limit crack propagation. The increased energy required to initiate, 

propagate, and maintain crack growth due to these mechanisms will lead to enhanced fracture 

toughness of the polymer network.

The incorporation of branched additives will enhance the strength and toughness, and/or produce 

a strain-dependent response regardless of the modulus of the material ranging from conformal 

soft polymers and polymer gels or biomaterials (G’ ~ 100 Pa – 10 kPa) to structural resins 

(G’ ~ >10 MPa). In particular, this approach will address the prohibitively low fracture 

toughness of soft polymeric materials that has prevented their widespread implementation into a 

variety of practical applications (21). Examples of soft polymeric materials include low modulus 
4

network and elastomer (G’ < 1 MPa); solvent-swollen polymer gels, polymer networks, or gel 

containing unreacted polymer precursors; and many biomaterials. The enhanced toughness of 

soft materials will facilitate implementation into a wide variety of applications including 

robotics, prosthetics, damping, biotissue simulants, tissue regeneration, biomedical implants, 

coatings, and sensors. In addition, tailoring the strain-dependent mechanical response will allow 

for the material to remain soft and conformable at low strain rates but stiffen upon high 

deformation rates. This strain-dependent mechanical response can be utilized in wearable 

textiles, prosthetics, and coatings, and provide materials with enhanced blast, shock, and fracture 

protection. Incorporation of branched additives in rigid structural polymers will further enhance 

their fracture toughness leading to implementation into additional applications and the reduction 

of size and weight in current applications.

The enhancements from incorporating branched additives into a polymeric network are the result 

of the additive architecture, requiring no specific interactions, and can be used in any polymer 

network. This allows for a single additive to be used with a variety of network chemistries with 

similar solubilities. As a result, the branched additive production can be increased in scale to 

reduce the end cost of the product. While no specific interaction are required to obtain the 

aforementioned improvements, chemical functionalities that promote interaction of the additive 

with the network can provide further enhancements. Examples of chemical functionalities that 

promote interaction include hydrogen bond donors/acceptors, metal center ligands, and the 

production of covalent bonds. In addition, variations in chemical or mechanical properties could 

further improve toughness by acting similar to barbs on a fish hook, preventing disentanglement 

of the branches, to increase the zone of deformation and the extent of energy dissipation.

The primary challenge for implementation of this technology is the development of robust, 

simple, and scalable synthetic methods to access the branched polymer species with controlled 

branch architectures and molecular weight. Most approaches to branched polymers described in 

the literature require multiple synthetic steps and purifications, require very stringent reaction 

conditions, and are not amenable to scale up. As before, the model system consists of end-linked 

PDMS network swollen with non-reactive branched PDMS solvent molecules of varying 

architecture and molecular weight. In particular, we have focused on developing synthetic 

protocols for PDMS star polymers. Several different approaches were investigated accompanied 

by full molecular characterization. Upon development of synthetically viable approaches, the 

procedures were scaled up to produce enough material to formulate swollen PDMS network gels. 

The mechanical properties of the resulting gels were probed via stress-strain tensile 

measurements and tack adhesion testing. 
5

3. Results

3.1 Year 1 

3.1.1 Star Polymer Synthesis

The synthetic strategy chosen is based on coupling commercially available mono-functional 

(chain end functionalized) PDMS with a multi-functional core molecule to create a star-branched 

structure, as shown schematically in figure 1. Initially, thiol-ene linking chemistry was 

investigated to join the functional PDMS to a multi-reactive core. The reaction is highly 

versatile and quantitative, and can be conducted under a wide variety of conditions. The 

chemistry involves reaction between terminal thiols and terminal alkenes to form a thioether. In 

our case, an ene-terminated PDMS was reacted with a multi-functional thiol core in the presence 

of a photoactive radical initiator. The reaction products were characterized by Fourier transform 

infrared (FTIR) spectroscopy and 1H nuclear magnetic resonance (NMR) (figure 2). Each 

characterization method indicated complete attachment of the ene-functionalized polymer to the 

thiol core. 1H NMR spectroscopy indicated complete consumption of vinyl end groups in the 

end functionalized PDMS as evidenced by the complete disappearance of the resonances 

associated the vinyl group from 5.70–6.20 ppm. Reaction of the thiol group was monitored by 

FTIR, which confirmed complete consumption of the thiol group as well.

Figure 2. Synthetic scheme for PDMS star polymer via thiol-ene coupling chemistry (top). 1H NMR analysis of 

vinyl-terminated PDMS showing complete disappearance of resonances associated with vinyl 

functionality after a 5-min reaction time (left). FTIR analysis of tetra-thiol showing disappearance of 

thiol peak after 5 min (right).
6

After studying the reaction on gram scale, it was scaled up to 50-g batches and used to formulate 

the PDMS network gels. However, the gel did not cure in the presence of the PDMS star 

polymer. We reasoned the platinum catalyst used for curing the gel was being poisoned and 

rendered inactive either by impurities present from the reaction or unreacted thiol groups. The 

reaction products were subsequently rigorously purified by multiple precipitations and 

extractions and any potentially unreacted thiol groups capped by reaction with a large excess of 

ethyl vinyl ether. Despite these added steps, attempts to cure the PDMS were met with uniform 

failure. It is possible the thioether linkages formed from the thiol-ene reaction are coordinating 

and deactivating the catalyst as a number of platinum (II) thioether complexes are known in the 

literature.

Next, we turned our attention to another type of coupling chemistry to attach the mono end 

functionalized PDMS to a central core. The reaction investigated was a platinum catalyzed 

hydrosilylation between an ene-terminated PDMS linear polymer and a tetra-functional silane 

core molecule. This transformation is the same reaction used to cure the PDMS gels thus we 

were confidant the chemistry would be compatible for curing the gels. The reaction was 

characterized by size exclusion chromatography (SEC), which suggested nearly complete 

attachment of the mono vinyl terminated PDMS to the central core molecule (figure 3). The 

reaction was scaled up and a PDMS gel was prepared in the presence of the product. The gel 

appeared to be fully cured and the strength and toughness of the gels was evaluated relative to a 

linear PDMS solvent molecule. 

Figure 3. Synthetic scheme for platinum catalyzed hydrosilylation reaction (left). SEC analysis of the reaction 

showing shift of star polymer to higher molecular weight relative to linear peak with uniform peak 

shape and nearly 4-fold increase in molecular weight (right).

3.1.2 Tensile Testing

Tensile testing was performed using an MTS Synergie load frame at 10 in/min and a 500 N load 

cell on gel samples in a dogbone geometry. Figure 4 shows the stress-strain behavior of gel 

samples containing 50% of the star-branched additive and 50% of a small molecular weight 

linear chain (1,100 g/mol). The stress-strain tensile data exhibited a twofold enhancement of the 
7

strength (stress at break) and a threefold enhancement in toughness (integral of stress-strain 

curve) with the star-shaped additive relative to the small linear solvent.

Figure 4. Comparison of stress-strain tensile data for 

PDMS gels with either 50 vol.% of the star-branched 

additive or 50 vol.% of a small linear chain PDMS 

additive (1,100 g/mol). The strength (stress at break) 

increased twofold and the toughness (integral of the 

stress-strain curve) increased by threefold for the gel 

containing the star-branched additive compared to a 

linear small molecule solvent. 

3.1.3 Tack Adhesion Testing

Tack adhesion testing was performed on an Anton Paar rheometer using a tack probe with a 

circular cross section 8 mm in diameter. The tack probe was brought into contact with the gel 

sample under a force of 1.5 N for 120 s followed by drawing the tack probe away from the 

sample at a rate of 1 mm/s. The results were similar to the tensile data where the ultimate force 

was significantly larger and the work of adhesion (toughness) exhibited a threefold increase 

relative to the small linear additive (figure 5).
8

Figure 5. Comparison of the stress-strain tack adhesion 

data for PDMS gels with either 50 vol.% of the 

star-branched additive or 50 vol.% of a small molecule 

PDMS additive (1,100 g/mol). Similar to the tensile 

testing, the adhesive strength (maximum stress) and 

toughness/work of adhesion (integral of the stress-strain 

curve) are significantly improved for the gel containing 

the star-branched solvent when compared to a sample 

containing the small molecule additive.

3.1.4 Computational Modeling

As shown in the preceding examples, the mechanical properties of polymeric networks can be 

significantly modified through the incorporation of additives with topological constraints and/or 

entanglements. To identify the most promising of the substantial number of potential additives,

we are developing computational models. Direct atomistic modeling of the mechanical response 

of these systems is still unobtainable because of the slow dynamics and limited size of atomistic 

models. Coarse-graining models are being used to bridge this vast temporal and spatial scale. 

Conventional coarse-graining methods may use soft-repulsive interaction potentials between 

particles made of a large number of polymer monomers. Although the soft potential has 

substantial benefits allowing for long-time simulations, it also leads to unphysical crossing of the 

polymer chains making simulation of mechanical properties unrealistic. We addressed this issue 

by introducing a repulsive potential between bonds that prevents unphysical chain crossing. The 

models of polymer gel incorporating polymer network and a solvent composed of polymers 

exhibiting various shapes and lengths were built by applying recent U.S. Army Research 

Laboratory (ARL)-developed coarse-graining strategy. We have applied several coarse-grain 

techniques using both hard- and soft-repulsive potentials. In the latter care, a newly developed 

bond potential was also implemented. The polymer models were subjected to Molecular 

Dynamics simulations using the Large-Scale Atomic/Molecular Massively Parallel Simulator 

(LAMMPS) program and the stress-strain relations were calculated with varying strain-rates 

(figure 6). A fracture model of polymer networks, dependent on the network deformation, was 
9

introduced and applied to study toughness of polymeric gels. The computational modeling 

supports the observed improvements through the addition of branched and entangled solvent. In 

addition, the computational modeling suggests the most significant benefit from a “molecular 

barb-wire” architecture, where an entangled linear polymer has several short rigid side-chains 

resulting in a dramatic reduction in mobility.

Figure 6. Computational modeling of the impact of solvent size 

and architecture on the stress-strain behavior of a polymer 

network. 

3.2 Year 2 

To expand the scope of our study, we sought to prepare more branched polymers of various 

molecular weights and architecture. The hydrosilylation reaction used to prepare the four-arm 

star polymers described previously is only effective with low molecular weight precursors. As 

the molecular weight of the vinyl terminated polymer increases, the concentration of the reactive 

groups decrease, leading to lower conversions (21). To overcome this limitation, we explored 

the use of chlorosilane coupling to prepare the branched polymers. The chlorosilane bond is 

highly reactive towards nucleophiles and generally leads to quantitative conversion. In addition, 

there are a number of commercially available chlorosilane compounds from which a variety of 

polymer architectures can be derived.

Initially, we focused on preparing multi-arm star polymers using the chlorosilane coupling 

approach for comparison to the star polymers prepared via hydrosilylation. Commercially 

available mono-hydroxyl terminated PDMS (Mn ~ 10 kDa) was reacted with tetra- and hexyl-
10

functionalized chlorosilane compounds in the presence of triethylamine (figure 7). The reaction 

products were characterized by SEC (figure 8), which showed nearly complete coupling of the 

mono-hydroxyl terminated PDMS to the multi-functional chlorosilane. 

Figure 7. Synthetic scheme for preparation of four- and six-arm star polymers via chlorosilane coupling. 

Figure 8. SEC analysis of chlorosilane coupling to form four- and six-arm star polymers. 

Despite the success of using the chlorosilane coupling approach to prepare the star polymers, we 

were limited by the commercial availability of high molecular weight linear polymer precursors. 

The highest molecular weight available commercially is only 10 kDa. In order to expand the

scope of the study to include higher molecular weight polymers, it was necessary to prepare 

linear precursors ourselves. There are a variety of methods to synthesize PDMS. We initially 

choose to explore organocatalytic ring opening polymerization (ROP) of 

hexamethylcyclotrisiloxane (D3) (figure 9) (22). It was shown to be a robust method to produce 

polymers of predictable molecular weight and end group functionality from cyclic carbosiloxane 

monomers, while avoiding side reactions that accompany other methods such as anionic 

polymerization.
11

The polymerization was initiated by pyrene butanol which is highly ultraviolet (UV) active and 

provides an analytical handle to analyze the resulting polymers. Only chains containing the 

pyrene butanol would be observable by UV analysis and could potentially help differentiate 

between polymer species initiated by impurities such as water. The polymerization is catalyzed 

by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) which activates the initiator and subsequent 

propagating silanol through hydrogen bonding. We initially targeted a molecular weight of 

33 kDa and monitored the polymerization over time by SEC. A typical result is shown in 

figure 10. 

Figure 9. Organocatalytic ROP of D3 initiated by pyrene butanol and catalyzed TBD.

Figure 10. SEC analysis with light scattering and refractive index (left) and UV (right) of organocatalytic 

ROMP of D3.

In all of the polymerizations attempted, a significant shoulder was observed in all three detectors 

in the high molecular weight region of the SEC trace, especially at longer reaction times. Since a 

high molecular weight impurity appeared in the UV signal, it meant the UV active pyrene 

initiator was present in the impurity. This suggests the impurity is the result of backbiting, where 

the active propagating undergoes a silyl ether interchange reaction with another polymer chain 

causing scrambling. To test this, we performed a control experiment where a preformed monohydroxyl terminated PDMS was combined with the TBD catalyst (not shown). If 

transetherfication was occurring, it would be expected to observe a broadening of the molecular 

weight distribution with time. However, the molecular weight distribution remained constant for 

at least 24 h. This result was consistent with previous control experiments, which demonstrated 
12

silyl ether interchange did not occur when using the TBD catalyst (22). We performed multiple 

rigorous purifications to remove any impurities present in the starting materials. In addition, we 

experimented with reaction conditions varying the concentration of reactants, order of addition 

of reagents, and tried several other initiators but were unsuccessful in preparing a polymer with a 

monomodal molecular weight distribution.

Next, we turned our attention to preparing the polymers via anionic polymerization of D3

(figure 11). This method is commonly used to prepare well-defined PDMS chains (23, 24). 

Anionic polymerization requires very stringent reaction conditions excluding water and oxygen 

and is usually conducted using high vacuum techniques for which our lab currently does not the 

capability to do. In addition, at high conversions, the backbiting reaction described above occurs 

thus the polymerization must be terminated early (~85%). We performed the polymerizations in 

a mostly dry (<1.0 ppm H2O and O2) glove box atmosphere with a target Mn = 50 kDa at 100% 

conversion. The polymerization was monitored over time and analyzed by SEC (figure 11). The 

polymers produced had narrow molecular weight distributions (<1.1) and only a small shoulder 

in the high molecular weight region, possibly due initiation by trace amounts of H2O or from 

backbiting.

 

Figure 11. Synthetic scheme for anionic polymerization of D3 (left). SEC analysis over time by light scattering 

and refractive index.

With a satisfactory method in place to synthesize high molecular PDMS chains, we attempted to 

prepare a star polymer using anionic polymerization terminated by multifunctional chlorosilanes 

similar to the strategy outlined in figure 7. The linear chains had a target Mn = 25 kDa at 100% 

conversion. The polymerization was terminated with a tetrafunctional chlorosilane. A small 

aliquot was removed from the polymerization for analysis prior to termination. SEC analysis

(figure 12) showed the linear polymer had an Mn of 17.3 kDa (PDI = 1.08) and the star polymer 

had an Mn of 67.3 kDa (PDI= 1.14). 
13

Figure 12. SEC analysis of star polymer synthesis via termination of anionic 

polymer of D3 with multifunctional chlorosilane.

4. Conclusions

The design, synthesis, and characterization of branched polymer additives for enhancing polymer 

gel strength and toughness is reported. Several methods for synthesis of four-arm star PDMS 

based on coupling of vinyl-terminated PDMS to a multifunctional core were investigated. Thiolene coupling chemistry was shown to be an effective method for quantitative attachment of the 

polymer chains to the central core molecule. However, the thioether linkages that were formed 

appeared to be incompatible with the chemistry used to cure the gel. The platinum catalyzed 

hydrosilylation between a vinyl-terminated PDMS linear polymer and a tetra-functional silane 

core molecule was also investigated. Gel permeation chromatography (GPC) analysis of the 

reaction products suggested near complete attachment of the polymer chains to the central core 

molecule. The reaction was scaled up and a PDMS gel was prepared in the presence of the 

product. The gel appeared to be fully cured and the strength and toughness of the gels was

evaluated relative to a linear PDMS solvent molecule. Mechanical testing of the gels was 

performed via tensile testing and tack adhesion. The stress-strain tensile data exhibited over a 

two times enhancement of the strength (stress at break) and a threefold increase toughness 

(integral of stress-strain curve) when comparing the star-branched solvent with a linear solvent. 

Similar observations were also made when comparing the tack adhesion of the gels prepared in 

the presence of the different solvent molecules. Computation modeling of various branched 

polymer additives further confirmed the improvements observed and suggested additional 

branched architectures to investigate that were not previously conceptualized. 
14

Synthetic efforts were expanded to include a wider range of molecular weights and architectures 

for study. Chlorosilane coupling was determined to be an efficient method to prepare four- and 

six-arm star polymers from low molecular weight, commercially available mono-hydroxyl 

terminated polymers. Due to the limited availability of high molecular weight precursors, 

organocatalytic ROP was explored as method to prepare high molecular weight precursors. This 

method proved difficult to prepare polymers without significant high molecular weight 

impurities. Anionic polymerization proved to be a more robust technique to prepare welldefined PDMS and was coupled with chlorosilane coupling strategy to produce a well-defined 

star polymer. We are currently refining our synthetic techniques and procedures to further 

eliminate impurities in the polymer. Scale up of these methods will enable mechanical testing of 

wider array of branched polymer additives.
15

5. References

1. Chow, D.; Nunalee, M. L.; Lim, D. W.; Simnick, A. J.; Chilkoti, A. Materials Science and 

Engineering: R: Reports 2008, 62, 125.

2. Dusek K. Responsive Gels: Volume Phase Transitions, Advances in Polymer Science; 

Berlin: Springer, 1993

3. Beebe, D. J.; Moore, J. S. Nature 2000, 404, 588.

4. Liu, R. H.; Qing, Y.; Beebe, D. J. Journal of Microelectromechanical Systems 2002, 11, 45–

53.

5. Otake, M.; Kagami, Y.; Kuniyoshi, Y.; Inaba, M. A.; Inoue, H. A. IEEE Int. Conf. Robot 

Autom. 2002, 3, 3224–3229.

6. Hu, Z.; Chen, Y.; Wang, C.; Zheng, Y.; Li, Y. Nature 1998, 393, 149.

7. Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829.

8. Lenhart, J. L.; Cole, P. J.; Unal, B.; Hedden R. Appl. Phys. Lett. 2007, 91, 061929–061923.

9. Kwon, I. C.; Bae, Y. H.; Kim, S. W. Nature 1991, 354, 291–293.

10. Miyata, T.; Asami, N.; Uragami, T. Nature 1999, 399, 766–769.

11. Park, T. G. Biomaterials 1999, 20, 517–521.

12. Schmaljohann, D.; Oswald, J.; Jorgensen, B.; Nitschke, M.; Beyerlein, D.; Werner, C.

Biomacromolecules 2003, 4, 1733–1739.

13. Elaïssari, A.; Rodrigue, M.; Meunier, F.; Herve, C. J. Magn. Mater. 2001, 225, 127–133.

14. Calvert, P. Adv. Mater. 2009, 21, 743–756.

15. Mrozek, R. A.; Cole, P. J.; Cole, S. M.; Schroeder, J. L.; Schneider, D. A.; Hedden, R. C.;

Lenhart, J. L. J. Mater. Res. 2010, 25, 1105–1117.

16. Laurer, J. H.; Bukovnik, R.; Spontak, R. J. Macromolecules 1996, 29, 5760–5762.

17. Tanaka, T. Phys. Rev. Lett. 1978, 40, 820.

18. Twieg, R. J.; Russell, T. P.; Siemens, R.; Rabolt, J. F. Macromolecules 1985, 18, 1361–

1362.

19. Gent, A. N.; Liu, G. L.; Mazurek, M. Journal of Polymer Science, Part B: Polymer Physics

1994, 32, 271–279.
16

20. Fetters, L. J.; Kiss, A. D.; Pearson, D. S.; Quack, G. F.; Vitus, F. J. Macromolecules 1993, 

26, 647.

21. Hammouch, S. O.; Beinert, G. J.; Herz, J. E. Polymer 1996, 37, 3353.

22. Lohmeijer, B.G.G.; Dubois, G.; Leibfarth, F.; Pratt, R. C.; Nederberg, F.; Nelson, A.; 

Waymouth, R. M.; Wade, C.; Hedrick, J. L. Organic Letters 2006, 8, 4683. 

23. Zilliox, J. G.; Roovers, J.E.L.; Bywater, S. Macromolecules 1975, 8, 573.

24. Hammouch, S. O.; Beinert, G. J.; Zilliox, J. G.; Herz, J. E. Polymer 1995, 36, 421.
17

6. Transitions

The research is being continued under the Tissue Simulants program in the Materials for Soldier 

Protection technical area. A patent application (Mrozek, R. A.; Lenhart, J. L.; Lambeth, R. H.; 

Andzelm, J. W. Novel branched additives for polymer toughening, Patent Disclosure [S/N 

61/645,286]) was submitted and a peer-reviewed journal article (Polymer 2011, 52, 3422-3430) 

was also accepted.
18

List of Symbols, Abbreviations, and Acronyms

ARL U.S. Army Research Laboratory 

D3 hexamethylcyclotrisiloxane 

FTIR Fourier transform infrared 

GPC gel permeation chromatography 

LAMMPS Large-Scale Atomic/Molecular Massively Parallel Simulator 

NMR nuclear magnetic resonance 

PDMS poly(dimethylsiloxane)

PNIPAM poly-(N-isopropylacrylamide)

ROP ring opening polymerization

SEC size exclusion chromatography 

TBD triazabicyclo[4.4.0]dec-5-ene

UV ultraviolet
19

NO. OF

COPIES ORGANIZAATION

1 ADMNSTR

ELEC DEFNS TECHL INFO CTR

ATTN DTIC OCP

8725 JOHN J KINGMAN RD STE 0944

FT BELVOIR VA 22060-6218

2 PDFS US ARMY RSRCH LAB

ATTN RDRL WMM G J LENHART

ATTN RDRL WMM G R MROZEK

ATTN RDRL WMM G Y SLIOZBERG

ATTN RDRL WMM G J ANDZELM

BLDG 4600

ABERDEEN PROVING GROUND MD 21005

1 PDF US ARMY RSRCH LAB

ATTN RDRL WMM G R LAMBETH

ABERDEEN PROVING GROUND MD 21005-5066

2 PDFS US ARMY RSRCH LAB

ATTN IMAL HRA MAIL & RECORDS MGMT

ATTN RDRL CIO LL TECHL LIB

ADELPHI MD 20783-1197
20

INTENTIONALLY LEFT BLANK.