Michael J. Birnkrant

Tuning Ion Conducting Pathways Using Holographic Polymerization

Polymer electrolyte membranes (PEMs) with high and controlled ionic conductivity are important for energy-related applications, such as solid-state batteries and fuel cells. Herein we disclose a new strategy to fabricate long-range ordered PEMs with tunable ion conducting pathways using a holographic polymerization (HP) method. By incorporating polymer electrolyte into the carefully selected HP system, electrolyte layers/channels with length scales of a few tens of nanometers to micrometers can be formed with controlled orientation and anisotropy; ionic conductivity anisotropy as high as 37 has been achieved.

Polymer crystallization/melting induced thermal switching in a series of holographically patterned Bragg reflectors

Holographic photopolymerization (H-P) is a simple, fast and attractive means to fabricate one-, two- and three-dimensional complex structures. Liquid crystals, nanoparticles and silicate nano-plates have been patterned into submicron periodical structures. In this article, we report fabrication of a one-dimensional reflection grating structure by patterning a semicrystalline polymer, polyethylene glycol (PEG), in Norland resin (thiol-ene based UV curable resin) matrix using the H-P technique. Sharp notches observed in the reflection grating of this Norland/PEG system indicate a finite Δ present in the system due to spatial segregation of the PEG and Norland resin. The notch position red shifts upon heating and the diffraction efficiency (ratio between diffraction and incident light intensity, DE) increases from ∼20% to 60% for the Norland 65/PEG 4600 grating. This dynamic behavior of the reflection grating is also fully reversible. The unique thermal switching behavior is attributed to the melting/formation of PEG crystals during heating/cooling. By employing different molecular weight PEGs which have different melting temperatures, a series of switching temperatures have been achieved. Since PEG can be easily coupled with a variety of functional groups, this research might shed light on fabricating multifunctional Bragg gratings using the H-P technique.

Permeable nanoconfinement of hierarchical block copolymer volume gratings

A hierarchical structure of poly(ethylene oxide)-b-poly(e-caprolactone) (PEO-b-PCL) block copolymer (BCP) confined between crosslinked resin was patterned into Bragg volume gratings using a holographic polymerization (HP) process. The BCP formed a lamellar structure confined between the layers of the grating created by HP. The periods of the volume grating and the BCP were controlled to be ∼200 nm and 20 nm, respectively. These two different length scale layers were aligned parallel to one another yielding a polymeric film which exhibits distinct diffraction behavior due to a periodic refractive index variation. This system exhibits complex thermo-optical behavior during heating and cooling cycles with reversible changes in both the diffraction wavelength and efficiency induced by BCP melting and crystallization in the confined region. Transmission electron microscopy studies show reversible diffusion of PEO-b-PCL into and out of the crosslinked resin, indicating that the nanoconfinement imposed by the resin is soft and permeable for the BCP. The morphological changes in nanoconfinement with temperature account for the complex thermo-optical behavior of the grating and the system provides an interesting platform to investigate soft nanoconfinement of BCP materials.

The structure of a polymer blend in a volume grating

Holographic polymerization (H-P) has been used to fabricate polymer-dispersed liquid crystals, block copolymers and pattern inert nanoparticles. In this article, one-dimensional grating structures of Norland resin and a polymer blend were achieved using the H-P technique. A reflection grating structure known as a Bragg reflector (BR) was fabricated. The hierarchical structure and morphology of the BR were studied using synchrotron X-ray, polarized light microscopy and transmission electron microscopy. The structure of the BR containing a polymer blend displayed lamellae structures formed with periodicity of 200 nm. Polycaprolactone and Poly(L-lactide) crystals were found to be confined in ~ 60 nm thick layers in the BR. The polymer chains tended to orient themselves parallel to the grating when the two polymers where blended together. The phase separation and structure of the polymer blend inside the H-P grating could be of great interest for multifunctional optical sensors or devices.