Lawrence B. Coleman

Future applications of ordered polymeric thin films

Abstract Polymeric ultrathin film systems need to be developed in the context of applications where their unique combinations of properties promise revolutionary improvements in performance or cost effectiveness. The applications examined include electron beam resists for microlithography and nanolithography, insulating films in semiconductor devices, non-linear optical elements and coatings for communications and computing, as well as highly permselective membranes in biotechnology. In this paper, we will review some of the most appealing suggestions and evaluate their current status. Improvements in film characterization and deposition based on Langmuir-Blodgett techniques are also suggested.

Far‐infrared reflection–absorption spectroscopy of thin polyethylene oxide films

We report the temperature dependent far‐infrared spectrum of ultra‐thin films of polyethylene oxide (PEO). Using the orientational specificity of infrared and far‐infrared reflection–absorption spectroscopy and in‐situ recrystallization, we find that during spin coating the PEO helices are initially in the plane of the film, but on crystallization reorient to be normal to the substrate. A splitting of the C–O torsional mode near 109 cm−1 is identified as arising from a distortion of the normal helical structure of PEO. Comparison with transmission spectra of cast films demonstrates the value of far‐infrared reflection–absorption spectroscopy (FIRRAS) in the study of crystalline polymers in the far infrared.

Sixteen years of Collaborative Learning through Active Sense-making in Physics (CLASP) at UC Davis

This paper describes our large reformed introductory physics course at UC Davis, which bioscience students have been taking since 1996. The central feature of this course is a focus on sense-making by the students during the 5 h per week discussion/labs in which the students take part in activities emphasizing peer-peer discussions, argumentation, and presentations of ideas. The course differs in many fundamental ways from traditionally taught introductory physics courses. After discussing the unique features of CLASP and its implementation at UC Davis, various student outcome measures are presented that show increased performance by students who took the CLASP course compared to students who took a traditionally taught introductory physics course. Measures we use include upper-division GPAs, MCAT scores, FCI gains, and MPEX-II scores.

Gas transfer in supported Langmuir-Blodgett films of polymeric lipids

Abstract Asymmetric membranes were fabricated by depositing Langmuir-Blodgett (LB) films of polymeric lipids on porous supports. Three polymeric lipids were used for deposition. Up to 50 Y-type layers were deposited on one side of the porous supports of polypropylene (Celgard) and polytetrafluoroethylene (GORE-TEX) membranes. A marked decrease in gas transfer with increasing number of LB layers was observed for the asymmetric membranes fabricated with the porous polypropylene. Gas transfer in the asymmetric membranes using porous polytetrafluoroethylene remained unchanged with the number of polymeric layers. The LB multilayers on polypropylene showed no evidence for pores or cracks as determined by scanning electron microscopy (at a magnification of 40 000x), but the LB multilayers on polytetrafluoroethylene revealed large cracks. The gas permeabilities of nitrogen, methane and carbon dioxide in the polypropylene-based asymmetric membranes were a function of the molecular weight of the gas for two of the three polymers which suggests that micropores are present in the LB films. Heat annealing decreased the gas permeabilities from 0% to 40%, but only one polymer showed a change in selectivity.

Selectivity of toxic and acid gases over methane and nitrogen in Langmuir-Blodgett films

Composite membranes for gas separation were fabricated by depositing polymeric Langmuir-Blodgett (LB) films on porous support membranes. By bridging over the pores of the porous support, the polymeric LB film acted as a selective barrier to gas diffusion. The gas selectivity of the LB films was demonstrated by measuring pure-component permeabilities as a function of temperature. LB composite membranes were selective for hydrogen sulfide (a toxic and acid gas) and carbon dioxide (an acid gas) over methane and nitrogen. Carbon dioxide selectivities were reduced by a slow reaction of H2S with the LB film. Grazing angle infrared reflection-absorption spectra (IRRAS) of separate films on aluminized glass slides showed irreversible disordering of the hydrocarbon side groups of the polymer at 35 °C. Disordering improved the selectivities of H2S and Co2 relative to CH4 and N2.

Effects of heating on the molecular orientation of polymeric lipids

Asymmetric membranes consisting of multilayers of polymeric lipids on a porous Teflon substrate were fabricated by the Langmuir-Blodgett technique. Fourier transform IR transmission spectroscopy was used to measure the level of molecular order in the n-alkyl side-chains of the polymeric lipids. The level of orientational order was monitored as a function of heat treatment temperature. We find that in the two polymer lipids studied, annealing below the bulk melting temperature leads to only a small decrease of order in the n-alkyl tails while removing a significant percentage of macroscopic defects. Heating to above the melting point results in an irreversible disordering of the alkyl chains.

CRYSTAL PERFECTION AND THE PHYSICAL PROPERTIES OF TTF‐TCNQ*

The temperature dependence and absolute value of the dc conductivity of tetrathiofulvalene tetracyanoquinodimethan (TTF-TCNQ) have been the most exhaustively studied properties of this compound. They have also been the most controversial, because of the wide range of results reported by the many research groups active in the field. This controversy sprung from two sources of experimental uncertainty. The first, which is the subject of this presentation, is the variable level of crystal perfection from sample to sample (and laboratory to laboratory). The second problem is the well-known difficulty inherent in measuring the conductivity of highly anisotropic materials. Discussed a t length in the literature,’-’the problem will not be discussed here.? The central question is, what is the intrinsic dc conductivity of TTF-TCNQ? Room-temperature conductivities have been measured between 100 and 1000 (Q-cm)-’, and the spread in the temperature dependence of the conductivity is demonstrated in FIGURES 1 and 2. We maintain that this varied behavior is due to widely different levels of crystal perfection (both chemical and structural) from sample to sample. Because the perfect crystal of TTF-TCNQ has yet to be produced, we must determine the intrinsic conductivity by first exploring the effects of imperfection and then using this information to extract the conductivity from the measured values. To accomplish this, a two-pronged attack was launched. First, an effort was made to produce higher quality samples, both chemically (this effort is reported on in the previous talk6) and physically by an investigation of the crystallization of TTF-TCNQ, which led to a model of the crystal growth and the production of larger and more perfect single crystals. Secondly, conductivity studies using samples of various levels of macroscopic crystal perfection and purity were made, to elucidate their effects on the conductivity.