Phillip J. Cole

Adhesion Properties of Lightly Crosslinked Solvent-Swollen Polymer Gels

Polymer gels are crosslinked polymer networks, highly swollen with solvent. For practical gel applications adhesion to a wide range of substrates over a broad range of temperatures is desired. In this article the adhesive properties of two types of solvent-swollen elastomers were studied, utilizing a combination of tack, contact mechanics, and peel adhesion methods. The first gel was a crosslinked polybutadiene swollen with common polymer plasticizers, while the second was a commercially available silicone with high extractables content. Nominally, these solvent-swollen materials exhibit similar adhesive characteristics to nonsolvent swollen elastomers including: (1) an increase in tack adhesion energies with increasing pull-off rates and decreasing temperatures in the rubbery region, (2) qualitative correlation between the rheological loss tangent for the gel and the gel adhesion energy, (3) fibrillation and extension during adhesion testing for gels with a shear modulus value less than 105 Pa in the plateau region, and (4) a decrease in the adhesion energy with increasing crosslink density. However, the presence of solvent in the elastomer can lead to solvent exclusion effects that degrade tack adhesion and must be considered for gel design in practical applications.

Development of nonaqueous polymer gels that exhibit broad temperature performance

While significant work has focused on aqueous hydrogels for biotechnology applications, hydrogels suffer from a limited operating temperature range due to the moderate freezing point and high volatility of water. In this work, a nonaqueous, chemically cross-linked polybutadiene gel has been designed which exhibits stable properties over a temperature range of −60–70°C. A combination of rheology, neutron scattering, and tack adhesion testing was utilized to characterize the gel properties. The methodology employed to design the polybutadiene gel can be generalized to a variety of gel materials and applications.

Impact of precursor size on the chain structure and mechanical properties of solvent-swollen epoxy gels

The thermal and mechanical properties of solvent-swollen epoxy gels were investigated as a function of pre-cursor size in a model end-linked network. The epoxy networks were formed by cross-linking diepoxide and diamine precursors in the presence of a low volatility solvent, dibutylphthalate (DBP). Both precursors were end functionalized and contained a poly(propylene glycol) (PPG) spacer between the epoxy and amine functionality, respectively. The lengths of both precursors were controlled by varying the molecular weight of the PPG spacer between functional groups. The glass transition temperature for the gels as a function of solvent loading was well predicted by the Fox equation. The scaling factors of shear storage modulus versus solvent loading increased with increasing diamine precursor molecular weight to values much larger (3.91) than the theoretical value of 2.3, for entanglement dominated network formation in a theta solvent. In contrast, the scaling factor increased with decreasing epoxy precursor molecular weight to values near 4.5. The large, molecular weight dependent scaling factors are attributed to loop defect formation as the result of the amine cross-linker architecture, which consists of two difunctional reactive end groups separated by a spacer. Rather than equal spacing of all four reactive sites, the amine end groups contain two reactive hydrogens that increase the local concentration of reactive species, and facilitates loop formation. We anticipate that this work will aid in the development of non-aqueous gels and provide enhanced tailoring of the gel properties over a broad range of stiffness.

Radiation tolerant polymeric films through the incorporation of small molecule dopants in the polymer matrix

Radiation induced conductivity (RIC) in semicrystalline polyethylene terephthalate (PET) films can be reduced by incorporating small molecule electron traps into the polymer. The electron traps contained an aromatic core with strong electron withdrawing functionality pendant to the core and were incorporated into the PET film by immersing the polymer in a solution of dopant and solvent at elevated temperatures. The chemical functionality of the electron trapping molecule and the number of pendant functional groups had a strong impact on the equilibrium doping level and the most effective doping solvent. In addition, all of the electron traps exhibited effectiveness at reducing the RIC. The technique of incorporating small molecule dopants into the polymer matrix in order to reduce the RIC can potentially be exploited with other polymers films and coatings utilized in electronics devices such as encapsulants, conformal coatings, and polymeric underfills.

Multilayer co-extrusion technique for developing high energy density organic devices.

The purpose of this project is to develop multi-layered co-extrusion (MLCE) capabilities at Sandia National Laboratories to produce multifunctional polymeric structures. Multi-layered structures containing layers of alternating electrical, mechanical, optical, or structural properties can be applied to a variety of potential applications including energy storage, optics, sensors, mechanical, and barrier applications relevant to the internal and external community. To obtain the desired properties, fillers must be added to the polymer materials that are much smaller than the end layer thickness. We developed two filled polymer systems, one for conductive layers and one for dielectric layers and demonstrated the potential for using MLCE to manufacture capacitors. We also developed numerical models to help determine the material and processing parameters that impact processing and layer stability.