Stretch Transition Research Articles

CO2 laser assisted vibrational overtone spectroscopy

We report the implementation of a new spectroscopic scheme for measuring vibrational overtone spectra of molecules in collisionless environments. This approach is based upon selective infrared multiphoton dissociation (IRMPD) of molecules that have undergone a light atom stretch vibrational overtone transition. Overtone excitation is ultimately monitored by laser induced fluorescence (LIF) detection of the dissociation products. Application of this technique to measure the 5←0 OH stretch transition methanol at 50 m Torr produces an excitation spectrum which is virtually identical to a photoacoustic spectrum recorded at 40 Torr, indicating that the former faithfully represents the vibrational overtone absorption. When applied to methanol cooled in a supersonic expansion, this method results in considerable simplification of the 5νOH vibrational overtone spectrum and reveals the presence of a strong Fermi resonance which divides the 5νOH oscillator strength equally between two vibrational levels. This new spectroscopic technique should permit the application of supersonic expansion and double resonance techniques to vibrational overtone spectroscopy of a wide variety of molecules.

Local modes of HOOH probed by optical-infrared double resonance

We have used an optical-infrared double resonance technique to probe the nature of the eigenstates prepared by 4νOH vibrational overtone excitation in hydrogen peroxide. A visible dye laser excites the 4←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=2 transition from the 4νOH level. Fixing the overtone excitation laser wavelength and scanning the wavelength of the infrared photon while monitoring the dissociation fragments by laser-induced fluorescence generates an infrared predissociation spectrum of the vibrationally excited molecule that contains information about vibrational state mixing at the 4νOH level. This spectrum indicates that the zeroth-order state that gives oscillator strength to the 4←0 OH stretch transition (i.e., the 4νOH bright state) is almost entirely comprised of a single vibrational eigenstate. Since the bright state is predominantly an OH stretch, the vibrational eigenstate prepared by 4νOH vibrational overtone excitation is well localized on the OH bond. This localization allows us to perform sequential local mode–local mode excitation of the two equivalent OH oscillators in HOOH.

Infrared spectroscopy of vibrationally excited HONO2: Shedding light on the dark states of intramolecular vibrational energy redistribution

Infrared predissociation spectroscopy of nitric acid subsequent to vibrational overtone excitation reveals vibrational state mixing of the highly excited levels and probes the character of the coupled dark states. A visible dye laser excites the 4←0 or 5←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=1 transition from the excited level. Scanning the optical parametric oscillator frequency while monitoring the predissociation products via laser-induced florescence produces an infrared spectrum of the excited molecules. Although the 4νOH vibrational overtone band consists of a single clean rotational contour that falls directly on a Birge–Sponer plot, the infrared transitions from this level indicate that the zeroth-order bright state is extensively mixed. On the average, the zeroth-order bright state is only a minor component of the eigenstates at this energy. The largest collective contribution is from zeroth-order states that have zero quanta of OH stretch.

Phase separation of polymer solutions in elongational flow

We have studied the phase separation behavior of polymer solutions in two-dimensional elongational flows that are sufficiently weak to avoid the coil–stretch transition. On the basis of a simple model that neglects hydrodynamic interactions, we find no shifts in the infinite molecular weight θ point, or coexistence curve. For finite chain lengths, there are flow-induced shifts of both quantities. These shifts are calculated to leading order in a loop expansion and are found to be order (Sζ)2N7/2, where S is the extension rate, ζ is the monomer friction coefficient, and N is the degree of polymerization. Our results are in qualitative agreement with experimental observations that find demixing promoted by flow.

The coil–stretch transition of polymers in external fields

We consider the exact statistical mechanical properties of a simple cubic lattice chain with one end anchored at the origin and with all other segments experiencing an external potential of one of two forms: −c‖x‖α (even Hamiltonian) or −c sgn(x)‖x‖α (odd Hamiltonian), for α an arbitrary exponent greater than zero and for c an arbitrary field strength. The problem is exactly soluble numerically for N not too large and for arbitrary α by transfer matrix techniques. In addition, the odd Hamiltonian with α=1 is especially simple to solve, yielding closed form expressions for a number of properties. Both Hamiltonians exhibit a first-order phase transition at c=0 in the limit of large N. The even Hamiltonian exhibits a coil (c<0) to stretch (c>0) transition. The odd Hamiltonian exhibits a left-stretched (c<0) to right-stretched (c>0) transition. For N sufficiently large and for α>1, the entire chain participates in the transition, becoming completely stretched for c only slightly greater than zero. When α<1, the transition is concentrated in one end of the chain. This transition is related to the coil–stretch transition of polymers in elongational flow. This model (unlike real polymers) does not exhibit hysteresis in the position of the transition, in agreement with the generally held belief that such hysteresis is due to hydrodynamic screening in the coil.

Elongational flow and rheology of monodisperse polymers in solution

We describe the utilization of idealized stagnation point extensional flows, produced by opposed jets, for birefringence visualization of induced molecular strain and flow resistance measurements. We identify rheological changes associated with the coilstretch transition which occurs beyond a critical strain-rate in elongational flow-fields. In dilute solutions of monodisperse atactic polystyrene, increases in extensional viscosity are observed as isolated molecules become stretched. The largest increases in extensional viscosity, however, are found only beyond a critical concentration and strain rate, and are associated with the stretching of transient networks of interacting molecules. These results parallel similar effects seen in porous media flow and capillary entrance experiments. We determine the molecular weight dependence of the critical concentration which scales as M −0.55 in agreement with pairwise interaction of coils, but is much lower than conventional values of the critical polymer concentration, c *. We believed that polydispersity may play an important role in the development of such transient networks, and in controlling the degradation behaviour during flow.

Dilute polystyrene solutions in extensional flows: Birefringence and flow modification

The coil—stretch transition in dilute polymer solutions has been studied using four- and two-roll mills to generate two-dimensional extensional flows. Flow birefringence measurements give the polymer conformation in the flow, while velocity gradient measurements using homodyne light scattering reveal the effects of the polymer on the flow field. When the polymers become sufficiently extended there is a distinct onset of measurable decreases in the velocity gradients. A necessary condition for this occur is a very high level of domain overlap between the extended molecules. If we estimate this by the volume concentration of circumscribed spheres, φ eff (i.e., the sphere diameter equals the extended length of the polymer), the onset of an effect of polymer on the flow does not occur until φ eff ≈ 4000–6000. Nevertheless, our measurements show that dilute polymer solutions do inhibit the development of large strain rates in regions of persistent extensional flow. The present flow birefringence measurements also extend previous studies to a wider range of flow types, thus providing new information that can be used in the evaluation of molecular models for polymer flow dynamics. In the present paper, we compare experimental data with predictions from the elastic dumbbell model with various combinations of a nonlinear spring, conformation-dependent anisotropic friction (including strain-inefficient rotation), and internal viscosity.

The behaviour of polyelectrolyte solutions in elongational flow; the determination of conformational relaxation times (with an Appendix of an anomalous adsorption effect)

The elongational flow method for determining conformational relaxation times of macromolecules (τ) in solution, based on the determination of the critical strain rate to induce the coil → stretch transition has been described previously for neutral polystyrene (PS) solution. In the present work it is applied to a polyelectrolyte, the Na + salt of polystyrene sulphonate (NaPSS), which is available in monodisperse form in several molecular weights. It was found that, in highly dilute solutions in the absence of added salt, τ is up to three orders of magnitude larger than for neutral PS indicating a more expanded and/or more permeable chain conformation. The existence of a critical strain rate plus the observed quadratic molecular-weight ( M) dependence suggest a free-draining coil at these low salt concentrations. On adding salt, a drastic and sharp decrease in τ, followed, thereafter, by a more gentle decrease on addition of further salt, is interpreted as a transition to a more collapsed non-free-draining coil behaviour, which is then retained. On raising and subsequently lowering of pH, τ increases and decreases in a reversible manner with a small hysteresis corresponding to the overall increase in salt concentration. Exchanging Na + or Ca ++ decreases τ drastically, from which a stiffer and more compact molecular conformation, assigned to intramolecular ionic bridging, is inferred. However, the same dependence on ionic strength of added salt is otherwise observed. For divalent salt forms, τ increases with polymer concentration until gelation occurs, indicating intermolecular ionic bridging. Many of the observed effects are in qualitative agreement with the accepted picture of polyelectrolyte behaviour, such as the pH-dependency of τ. However, some aspects provide a more detailed picture, for example, the quadratic-dependency of τ on M at low ionic strengths and the transition-like collapse of the coil to a non-free-draining state with increasing ionic strength. Other effects such as the competition between inter- and intramolecular linking with Ca ++ appear to be new. All these illustrate the power of the method at this initial stage. In addition, extraordinarily tenacious adhesion of the molecules to the glass walls of the channel into which they enter in an oriented state was observed and attributed to a previously unrealizable degree of multiple attachment of the chain when fully stretched out, resulting in drastic consequences for subsequent flow behaviour.

Coil-stretch transition of dilute flexible polymers under ultrahigh velocity gradients

Because the hydrodynamic interactions are reduced by stretching, a solute polymer coil should unwind abruptly when a certain critical value of the velocity gradient is reached. Depending on the details of the velocity field (``longitudinal'', or ``transverse'', or more complex gradients) this coil ↔ stretch transition may be continuous (``second order'') or discontinuous (``first order''). In the latter case hysteresis should often be observed; a qualitative discussion of the associated relaxation times is given. Simple shear flow is an exceptional case, with no sharp transition. Some expected effects of the C↔S transition on the mechanical behavior, on optical properties, and on chemical degradation in flow are briefly analyzed.