Do Y. Yoon

Novel second‐order nonlinear optical polymers via chemical cross‐linking‐induced vitrification under electric field

We present a novel method of preparing highly efficient and stable second‐order nonlinear optical (NLO) polymers via chemical cross‐linking induced vitrification under electric field. In this method a soluble prepolymer is first prepared that contains cross‐linking sites attached to the NLO‐active groups. Upon preparing samples of desired thicknesses, the prepolymer is heated (precured) to enable some chemical cross‐linking and thus to increase the glass transition temperature (Tg) to an optimum for poling. The precured polymer is then heated above its Tg and subjected to a high electric field to obtain the desired alignment of NLO moieties. Subsequent chemical cross‐linking (curing) under electric field continues to advance the Tg and hence leads to in situ vitrification of the polymer that stabilizes the electric‐field‐induced orientation of the NLO moieties. Detailed results of thermal, linear optical, poling kinetics, and NLO properties are described for the polymer system prepared from tetrafunctiona...

An optimized united atom model for simulations of polymethylene melts

We present here an optimized united atom model that is able to reproduce properties of melts of n‐alkane chains of varying molecular weights. This model differs from previous models in that the Lennard‐Jones well depth for the terminal methyl group (0.2264 kcal/mol) differs from that of the methylene units (0.093 kcal/mol). The position of the minimum is at 4.5 A for both units. Properties of n‐C44H90 melts from this model are compared with experiments and those from an explicit atom model. Good agreement with experiment is obtained for static properties of the melt, specifically P–V–T behavior, chain conformations, and x‐ray scattering profiles. The large‐scale dynamics, as measured by self‐diffusion, are found to agree reasonably well with experimental results, being about 30% faster with our best united atom force field. Analysis of the end‐to‐end vector orientation autocorrelation function in terms of the Rouse model yields a monomer friction coefficient somewhat greater than that determined from the ...

Off‐lattice Monte Carlo simulations of polymer melts confined between two plates

In this paper, we present the results of Monte Carlo simulations of the static properties of polymer melts confined between hard walls. The simulations are conducted in the canonical ensemble with a method that is a combination of reptation and crankshaft motions. 1250 polymer chains each comprising of 100 connected beads are placed in a box which allows for the simulation of a typical polymer melt confined between two hard plates at a separation of 51 bead diameters. Noncovalently bonded beads are assumed to interact with an empirical 6‐12 Lennard‐Jones potential which has parameters chosen to simulate a polyethylene melt at 400 K. From the analysis of the simulation results we show the existence of two relevant length scales in the problem. Single‐chain statistics are perturbed by the wall, and this effect is screened out only after one proceeds to a distance comparable to twice the unperturbed radius of gyration of the polymer chain. However, many‐chain statistics, i.e., packing and orientation of chai...

Corona poling and real‐time second‐harmonic generation study of a novel covalently functionalized amorphous nonlinear optical polymer

Thin films for optical second‐harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p‐nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second‐order nonlinear coefficient, d33=31 pm/V, measured by the Maker‐fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen‐in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a...

Highly efficient and stable nonlinear optical polymers via chemical cross‐linking under electric field

Extending the novel method of Eich et al. [J. Appl. Phys. 66, 3241 (1989)] to prepare highly stable second‐order nonlinear optical (NLO) polymers via chemical cross‐linking under electric field, we have obtained a new polymer exhibiting large and stable second‐order optical nonlinearities after relaxation at 80 °C. This was achieved by forming a network polymer from two NLO‐active monomers, bifunctional N,N‐(diglycidyl)‐4‐nitroaniline and trifunctional N‐(2‐aminophenyl)‐4‐nitroaniline. Here, every NLO moiety is connected to the network by a single covalent bond. After full cure under corona poling at 120 °C, the sample exhibited at ambient conditions d33≂50 pm/V and d31≂16 pm/V at 1064 nm fundamental wavelength, as estimated from the Maker fringe data. Upon heating to 80 °C, the nonlinearities decreased somewhat initially, but leveled off and remained stable at 80 °C. This stable sample gave d33≂42 pm/V and d31≂14 pm/V at ambient conditions. Furthermore, the linear electro‐optical coefficient of this samp...

Electrical conduction in polyimide films

Electrical conduction phenomena in polyimide (Kapton) films were studied with particular attention devoted to the separation of interface and bulk phenomena. The measurements were carried out with a variety of methods in the temperature range of 50 to 270 °C at electrical fields of 104 to 6×105 V/cm and at time intervals of up to 2×104 s after voltage application. Biased, two‐side metallized samples yield, after sufficiently long voltage application, interface‐controlled steady‐state currents described by Schottky injection, modified by space‐charge layers in the vicinity of the electrodes. The effective work function for aluminum‐polyimide is estimated to be 1.7 eV in the temperature range between 100 and 270 °C. A distinct dependence of these currents on electrode material is observed. Bulk phenomena were studied on one‐side metallized samples subject to positive‐corona charge injection. At temperatures below 200 °C, significantly larger currents than those for biased, two‐side metallized samples were o...

Polyimides derived from nonaromatic monomers: synthesis, characterization and potential applications

A number of mixed aromatic/cycloaliphatic as well as fully nonaromatic polyimides have been prepared. Whereas all the poly(amic acids) derived from nonaromatic diamines involved salt-formation during the initial stages of the polymerization, the majority of these eventually formed homogeneous, highly viscous polymer solutions. Only in a few select cases involving all nonaromatic monomers traditional solution polymerization was unsuccessful. The polyimide derived from hexafluoroisopropylidene diphthalic anhydride (6FDA) and trans- 1,4-diaminocyclohexane (DACH) yielded films with tough mechanical properties, a glass transition temperature of ∼360°C, good solvent resistance, and a low dielectric constant of 2.6. Thermal stability of this polyimide as determined by thermal gravimetric analysis in both air and nitrogen was quite good, exhibiting a weight loss of only 0.07 wt%/h at 350°C under isothermal conditions in nitrogen. However, mechanical properties as a function of thermal aging in both air and nitrogen indicated a maximum use temperature of only 350°C under inert conditions and less than 300°C in the presence of oxygen.

Large anisotropy in optical properties of thin polyimide films of poly( p‐phenylene biphenyltetracarboximide)

Thin films ranging from 400 nm to 4 μm thickness of poly( p‐phenylene biphenyltetracarboximide) (BPDA‐PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by the optical waveguide spectroscopy. These polyimide films, most prominent for potential applications as the interlevel dielectrics in multilevel interconnect technologies owing to their low coefficients of thermal expansion and excellent thermal/mechanical properties, are found to exhibit an extraordinarily large anisotropy in the refractive index, with the measured in‐plane refractive index n∥≂1.852 and the out‐of‐plane value n⊥≂1.612 at 632.8 nm wavelength, nearly independent of the film thickness. This large optical anisotropy indicates a very strong preference of polymer chains to orient along the film surface, and suggests a considerably larger (by ca. 27%) dielectric constant in the film plane than that along the film thickness. Moreover, there is some evidence for the existence of a very ...

Theory of nematic systems of semiflexible polymers. I. High molecular weight limit

A statistical theory of nematic systems has been developed for semiflexible polymers on the basis of a new wormlike model of chain statistics. This wormlike chain model closely represents the short‐range statistics of real chains and allows the evaluation of the orientation‐dependent configurational partition function, which replaces the rotational partition function employed in the classical treatments of nematic systems of rodlike particles. This theory has been applied to polyethylene (PE) and polytetrafluoroethylene (PTFE) chains in the bulk, and leads to the conclusion that PE chains form a stable isotropic melt (with its first order isotropic‐nematic transition expected to occur around ∼75 K), whereas PTFE forms a stable nematic phase in the melt. Furthermore, the entropy changes accompanying the isotropic‐nematic transitions of semiflexible polymers are predicted to be quite small, and the predicted order parameters at the transitions are not very high, falling in the range 0.4–0.7.

Spreading characteristics of thin liquid films of perfluoropolyalkylethers on solid surfaces. Effects of chain-end functionality and humidity

Spreading characteristics of thin liquid films of perfluoropolyalkyl ethers (PFPE) on silica surfaces and thermally bonded PFPE surfaces have been measured by scanning microellipsometer as function of molecular weight, chain-end functionality, and humidity. The effects of molecular weight are much smaller than those of chain-end functionality. Changes in spreading profiles with time show that interactions of chain-end groups with solid surfaces have dramatic effects on the structure and mobility of thin polymer films on solid surfaces. Wetting autophobicity is clearly manifested by PFPEs with functional chain-end groups on silica surfaces.