B. E. Eichinger

Computer simulations of poly(ethylene oxide): force field, pvt diagram and cyclization behaviour

Parametrization of a force field capable of quantitatively describing the gas, liquid and crystal phases of alcohols, ethers and polyethers is described. Two applications are reported, the first employing atomistic simulations to study PVT (pressure, volume, temperature) and cohesive properties of oligomeric poly(ethylene oxide) (PEO) and related small-molecule liquids, and the second to study the extent of ring formation in polymerization of poly(ethylene glycols) (PEGs) and hexamethylene diisocyanate (HDI). The atomistic simulations, focusing extensively on liquids and amorphous poly(ethylene oxides), demonstrate the ability to predict densities with an accuracy of 1%–2% over extended ranges of at least 200K in temperature and 180MPa in pressure. Densities of related small-molecule liquids, dimethyl and diethyl ether and ethanol at or close to saturation pressure are also well reproduced to temperatures close to the critical temperature. Densities calculated for methoxy-terminated oligomers are used to predict the density of melt and amorphous high-molar-mass PEO with an accuracy of better than 1%. Similarly, solubility parameters have been calculated as a function of chain length for poly(ethylene glycol) oligomers and used effectively to obtain estimates of the solubility parameter of high-molar-mass material. Additionally, crystal structures can also be well predicted. For the polymerization studies the Monte Carlo network simulation method was modified to mimic diffusion of reactants during the polymerization. Application to the PEG/HDI ‘linear’ polymerization system, using chain configurations generated with the atomistic force field, reveals a major improvement in the ability of the method to predict the extent of ring formation without adjustable parameters for polymerization conditions ranging from the bulk to highly dilute reaction conditions. ©1997 SCI

Rational Enhancement of Second-Order Nonlinearity: Bis-(4-methoxyphenyl)hetero-aryl-amino Donor-Based Chromophores: Design, Synthesis, and Electrooptic Activity

Two new highly hyperpolarizable chromophores, based on N,N- bis-(4-methoxyphenyl) aryl-amino donors and phenyl-trifluoromethyl-tricyanofuran (CF3-Ph-TCF) acceptor linked together via pi-conjugation through 2,5-divinylenethienyl moieties as the bridge, have been designed and synthesized successfully for the first time. The aryl moieties on the donor side of the chromophore molecules were varied as to be thiophene and 1-n-hexylpyrrole. The linear and nonlinear optical (NLO) properties of all compounds were evaluated in addition to recording relevant thermal and electrochemical data. The properties of the two new molecules were comparatively studied. These results are critically analyzed along with two other compounds, reported earlier from our laboratories and our collaborators, that contain (i) aliphatic chain-bearing aniline and (ii) dianisylaniline as donors, keeping the bridge (2,5-divinylenethienyl-), and the acceptor (CF3-Ph-TCF), constant. Trends in theoretically (density functional theory, DFT) predicted, zero-frequency gas-phase hyperpolarizability [beta(0;0,0)] values are shown to be consistent with the trends in beta HRS(-2omega;omega,omega), as measured by Hyper-Rayleigh Scattering (HRS), when corrected to zero-frequency using the two-level model (TLM) approximation. Similarly, trends in poling efficiency data (r33/E(p)) and wavelength dispersion measured by reflection ellipsometry (using a Teng-Man apparatus) and attenuated total reflection (ATR) are found to fit the TLM and DFT predictions. A 3-fold enhancement in bulk nonlinearity (r33) is realized as the donor subunits are changed from alkylaniline to dianisylaminopyrrole donors. The results of these studies provide insight into the complicated effects on molecular hyperpolarizability of substituting heteroaromatic subunits into the donor group structures. These studies also demonstrate that, when frequency dependence and electric-field-induced ordering behavior are correctly accounted for, ab initio DFT generated beta(0;0,0) is effective as a predictor of changes in r33 behavior based on chromophore structure modification. Thus DFT can provide valuable insight into the electronic structure origin of complex optical phenomena in organic media.

Computer simulation of end‐linked elastomers. I. Trifunctional networks cured in the bulk

The end linking of difunctional poly(dimethylsiloxane) with trifunctional cross linkers in the bulk has been simulated with a computer. A random array of primary chains with a Gaussian end‐to‐end distribution is generated in an image container on the computer. By joining chain ends at nearby junctions, inter‐ and intramolecular reactions occur in all species, allowing formation of cyclic structures of any size. At high extents of reaction, the sol components extracted from the spanning forest thus constructed consist of a substantial amount of cycles such as cyclic dimers. The effect of intramolecular reaction increases the extent of reaction by several percent for molecular weights in the range of thousands. Structural statistics of various types of network imperfections and the cycle ranks of the networks are reported. The gels obtained at complete conversions contain loop defects, their populations increasing with lower molecular weights of the prepolymers. The elastic activities of nonstoichiometric s...

Computer simulation of end-linked elastomers. II. Bulk cured tetrafunctional networks

Sol–gel distributions at high conversions, simulated by computer with spatial constraints on the distribution of chain ends, are reported for end‐linked tetrafunctional systems. The composition of the sol is found to be uniquely dependent upon the functionality f of the cross linker: for f=4, it mainly consists of linear monomers and oligomers containing one saturated cross linker. The molecular weight distribution of the sol exhibits a shoulder at the trimer due to the relative abundance of the ‘‘bow‐tie’’ molecule. The populations of various types of dangling ends and loop defects in the gel are given and discussed. Our simulations reveal empirically that the single‐loop probability varies with the −3/8 power of the molecular weight of the prepolymers. The power‐law dependence of the loop formation on chain length of the prepolymers and the nonrandom nature of the cross‐linking reaction is discussed.

Determination of Crystallinity and Morphology of Fibrous and Bulk Poly(ethylene terephthalate) by Near-Infrared Diffuse Reflectance Spectroscopy

Diffuse reflectance spectroscopy in the near-infrared (NIR) region (1100-2400 nm) was used to characterize the morphology of fibrous and bulk Polyethylene terephthalate) (PET). The method of Partial Least-Squares (PLS) was used to correlate NIR spectra of PET yarns with percent crystallinity. Standard error of prediction values for percent crystallinity in PET yarns are in the range of 2–3.5% crystallinity, and depended on the specific NIR spectral region used for analysis. Principal Components Analysis (PCA) was used to study the differences in NIR spectra of bulk PET samples that were crystallized at different temperatures. It was observed that NIR spectroscopy can detect changes in both intermolecular and intramolecular interactions in PET. The observed effect of crystallization temperature on the morphology of bulk PET is very similar to effects observed in earlier IR and thermal analyses. In addition, detection of moisture in bulk PET is demonstrated.

Distribution functions for Gaussian molecules. II. Reduction of the Kirchhoff matrix for large molecules

The eigenvalues of the Kirchhoff matrix for a molecule consisting of m chains and μ multifunctional junctions can be calculated from a reduced matrix of dimension μ×μ. Many of the eigenvalues for complicated molecules can be obtained exactly, and the remainder are accessible by numerical methods. We find small eigenvalues for trifunctional networks which can be associated with very slowly relaxing oscillations in gels. In addition, networks have μ high frequency modes, which are located in unique regions of the spectrum.

On shape asymmetry of Gaussian molecules

A method of calculating average moments, to an arbitrary order m, of the principal components of the gyration tensor of a Gaussian molecule imbedded in any k‐dimensional space is presented. These average moments are then used in generalized shape parameters Am of degree m≤k, which measure the asymmetry of the shapes of Gaussian molecules. Simple formulas for A2, A3, and the average moments up to third order are given. Explicit expressions for A2 and A3 for linear and circular chains, regular stars with infinitely long arms, double rings of a large number of beads, and combs with many side chains are obtained, and the shape characteristics of these molecules are discussed. It is found that Gaussian molecules are, on the average, prolate rather than oblate even in an infinite dimensional space, with the exception of regular stars with densely radiated long arms which exhibit perfect symmetry. The problem of analytic characterizations of shape asymmetry of Gaussian molecules or non‐self‐avoiding random walks...

Reduced dimensionality in organic electro-optic materials: theory and defined order.

Identification of electronic intermolecular electrostatic interactions that can significantly enhance poling-induced order is important to the advancement of the field of organic electro-optics. Here, we demonstrate an example of such improvement achieved through exploitation of the interaction of coumarin pendant groups in chromophore-containing macromolecules. Acentric order enhancement is explained in terms of lattice-symmetry effects, where constraint of orientational degrees of freedom alters the relationship between centrosymmetric and acentric order. We demonstrate both experimentally and theoretically that lattice dimensionality can be defined using the relationship between centrosymmetric order and acentric order. Experimentally: Acentric order is determined by attenuated total reflection measurement of electro-optic activity coupled with hyper-Rayleigh scattering measurement of molecular first hyperpolarizability, and centrosymmetric order is determined by the variable angle polarization referenced absorption spectroscopy method. Theoretically: Order is determined from statistical mechanical models that predict the properties of soft condensed matter.

Near-Infrared Spectroscopic Analyses of Poly(Ether Urethane Urea) Block Copolymers. Part II: Phase Separation

Second-overtone carbonyl bands at 1918 and 2036 nm in the near-infrared (NIR) spectrum are used to monitor the phase separation of poly(ether urethane urea) (PEUU) block copolymers at 100°C. Exact positions of urea and urethane carbonyl bands are determined by NIR spectroscopic analyses of model compounds in bulk and in dilute solution. Results indicate that the carbonyl groups in the copolymers are affected by two different structural processes that occur with annealing. A faster process causes an increase in the number of free urethane groups, and a slower process causes a decrease in the number of free urea and urethane carbonyl groups. Although several structural mechanisms can be used to describe these trends, more information is necessary to identify exact structural changes that occur with annealing.

Computer simulation of the structure and elasticity of polyurethane networks: 1. Polyoxypropylene tetrols and hexamethylene diisocyanate

Abstract Model urethane networks prepared from polyoxypropylene tetrols and hexamethylene diisocyanate have been studied with the aid of the computer. Good agreement with experiment is found for gel points when the crosslinkers are treated as sticks. The simulation-theory moduli for the phantom model show good agreement with experiments for networks prepared with Mn = 1200 in the bulk and at various dilutions, or with Mn = 2100 in the bulk. Simulations underestimate moduli for networks prepared from Mn = 2100 at various dilutions by factors of less than 2.