Tensile strength of oriented polymers below the glass transition temperature

A theory of tensile strength, based on the observation of cracks in specimens strained to breaking, is formulated. The treatment involves the assumption that a crack grows to a critical size by a nucleation process. When this critical size is exceeded the crack becomes unstable and propagates spontaneously to produce rupture. By comparing the predicted and measured strength, one can estimate the magnitude of the stress concentration factor in fibers. An interpretative analysis of experimental data obtained at various strain rates indicates that the resulting changes in tensile strength are due primarily to the changes in modulus.     

Physical aging and dye diffusion in polysulfone below the glass transition temperature

The effect of physical aging on the tracer diffusion coefficient D of camphorquinone in polysulfone is investigated. It is shown that if the sample is sufficiently annealed and physical aging is nearly complete, the temperature dependence of D will reflect the primary α-relaxation process of the host polymer. In the temperature range between T_g (=185°C) and 165°C, D is found to be a function of time, and the time dependence of D is given by D = At^?μ, with μ approximately equal to unity. © 1994 John Wiley & Sons, Inc.     

Kinetic analysis of isothermal cures performed below the limiting glass transition temperature

The paper addresses the problem of the application of isoconversional kinetic analysis to incomplete cures. Contrary to the expectations, a theoretical analysis suggests that correct estimates of the activation energy can be obtained if one uses the relative extents of cure instead of the absolute values. This conclusion is illustrated by simulated data for a hypothetical process and experimental data for epoxy‐novolac cure.     

Spatially heterogeneous dynamics in thermoset resins below the glass‐transition temperature

A photobleaching technique was used to measure the rotational dynamics of rubrene dispersed in thermoset resins. The matrices were polymerized from mixtures of two monomers with five different compositions. At temperatures below the glass‐transition temperature, probe rotational correlation times were shorter and showed a much weaker temperature dependence than those observed in glassy homopolymers. The probe correlation functions became increasingly nonexponential as the amount of the minor component in the matrix increased, presumably because a more heterogeneous set of environments resulted. Dynamics in the single‐component sample were quite homogeneous at room temperature. In contrast to homopolymer systems, a bimodal distribution of local relaxation times developed with the addition of the second component. At a given polymer composition, this bimodal distribution changed shape with temperature in a reversible manner. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2232–2239, 2000     

Photoinduced intramolecular charge separation in a polymer solid below the glass transition temperature

Photoinduced intramolecular charge separation (CS) in a polar polymer glass, cyanoethylated pullulan (CN-PUL), was studied below the glass transition temperature (Tg=395 K). A series of three carbazole (Cz: donor)-cyclohexane (S: spacer)-acceptor (A: acceptor) molecules (Cz-S-A) was used as intramolecular donor-acceptor dyads. The photoinduced CS rate was evaluated by the fluorescence decay measurement at temperatures from 100 to 400 K. The CS rate (kCS) increased above 200 K even far below Tg where micro-Brownian motions of the whole polymer chain are frozen. Below 200 K, on the other hand, kCS showed weak dependence on temperature. The temperature dependence of kCS is discussed in terms of the dielectric relaxation time of the polymer matrix. Consequently, CS below Tg was well explained by a thermally nonequilibrium electron transfer (ET) formula above 200 K and by a two-mode quantum-mechanical ET formula below 200 K. The increase in kCS above 200 K is mainly caused by a thermally activated low-frequency matrix mode originating from the side-chain relaxation of polar cyano groups. The weak temperature dependence of kCS can be explained by a nuclear-tunneling effect caused by a high-frequency matrix mode (variant Planck's over 2piomegH=250 cm-1) and an intramolecular vibrational mode (variant Planck's over 2piomegaQ=1300 cm-1). The high-frequency mode of the polymer matrix was attributed to a vibrational or librational motion of polar groups in the CN-PUL glassy solid.     

Infrared polarized spectra of single crystal of chloropentamethylbenzene above and below the temperature of transition

The i.r. spectra of polycrystalline sample and of single crystals of CPMB having ac and ab as predominant faces have been investigated above and below the temperature of transition to get information concerning the symmetrical modifications on the molecule or on the crystalline cell. Bands assigned to the stretching and bending vibrations CCH₃ groups become broader and less intense. Bands assigned to the rocking vibrations remain relatively unaffected. The general splitting of bands observed at low temperature is due to crystal field effect. Our results indicate a decrease of molecular symmetry through the phase transition I and III.     

Electron capture induced dissociation of nucleotide anions in water nanodroplets

We have studied the outcome of collisions between the hydrated nucleotide anion adenosine 5'-monophosphate (AMP) and sodium. Electron capture leads to hydrogen loss as well as water evaporation regardless of the initial number m of water molecules attached to the parent ion (m< or =16). The yield of dianions with microsecond lifetimes increases strongly with m, which is explained from dielectric screening of the two charges by the water nanodroplet. For comparison, collision induced dissociation results in water losses with no or very little damage of the AMP molecule itself.     

Amorphous fraction controlled mechanical and optical properties of polylactic acid below glass transition temperature

A negative relation between strength and crystallinity is observed in polylactic acid below glass transition temperature. Study indicates that entangled molecules in amorphous regions act as load bearing structures and are responsible for stress induced crazing. A three-phase model is proposed to explain how amorphous fraction changes with heat treatments and contributes to polymer modulus below glass transition temperature. Incorporation of carbon quantum dots into amorphous fraction dominated PLA creates a new type of luminescent composites that can be used for food labelling, tracking, packaging and production of origin.     

Probe rotation near and below the glass transition temperature: Relationship to viscoelasticity and physical aging

Reorientation times ť_c for two probes in several amorphous polymers near the glass transition temperature T_g are reported. T_g for these polymers ranges from 205 to 459 K. Probe reorientation was measured in the time window from 10⁻² to 10⁴ s with a recently developed photobleaching method. ť_c for a given probe at the T_gs of the different polymers varies more than three decades. Viscoelastic relaxation times characteristic of the Rouse modes of the matrix polymers are closely related to probe rotation times and thus also not constant at T_g. The characteristic length scale of motions responsible for the glass transition varies significantly for the three polymers studied. Preliminary physical aging results indicate that probe reorientation in polystyrene ages slightly faster than the volume.     

A simple model of entropy relaxation for explaining effective activation energy behavior below the glass transition temperature

Strong changes in relaxation rates observed at the glass transition region are frequently explained in terms of a physical singularity of the molecular motions. We show that the unexpected trends and values for activation energy and preexponential factor of the relaxation time tau, obtained at the glass transition from the analysis of the thermally stimulated current signal, result from the use of the Arrhenius law for treating the experimental data obtained in nonstationary experimental conditions. We then demonstrate that a simple model of structural relaxation based on a time dependent configurational entropy and Adam-Gibbs relaxation time is sufficient to explain the experimental behavior, without invoking a kinetic singularity at the glass transition region. The pronounced variation of the effective activation energy appears as a dynamic signature of entropy relaxation that governs the change of relaxation time in nonstationary conditions. A connection is demonstrated between the peak of apparent activation energy measured in nonequilibrium dielectric techniques, with the overshoot of the dynamic specific heat that is obtained in calorimetry techniques.     

A computer simulation of the iodine molecule dissociation reaction

The iodine dissociation reaction has been studied using a many-body classical trajectory technique. A qualitative analysis of the results shows that most of the molecules are both vibrationally and rotationally excited when they dissociate.     

A note on the dissociation energy of the excitonic molecule

Simple inequalities satisfied by the dissociation energy of the excitonic molecule are derived. They provide a useful check on the validity of calculated dissociation energies.     

Microdynamic characteristics of the proton of the water molecule over a broad temperature range

Physical Energy Institute, Obninsk. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 4, pp. 56–64, July–August, 1990.     

Unimolecular processes in CH2OH below the dissociation barrier: O-H stretch overtone excitation and dissociation

The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical, CH(2)OH, are reported in the region of the second and third overtones, which is above the thermochemical threshold to dissociation to H+CH(2)O (D(0)=9600 cm(-1)). The second overtone spectrum at 10 484 cm(-1) is obtained by double resonance IR-UV resonance enhanced multiphoton ionization (REMPI) spectroscopy via the 3p(z) electronic state. It is rotationally resolved with a linewidth of 0.4 cm(-1) and displays properties of local-mode vibration. No dissociation products are observed. The third overtone spectra of CH(2)OH and CD(2)OH are observed at approximately 13 600 cm(-1) by monitoring H-atom photofragments while scanning the excitation laser frequency. No double resonance REMPI spectrum is detected, and no D fragments are produced. The spectra of both isotope analogs can be simulated with a linewidth of 1.3 cm(-1), indicating dissociation via tunneling. By treating the tunneling as one dimensional and using the calculated imaginary frequency, the barrier to dissociation is estimated at about 15 200 cm(-1), in good agreement with theoretical estimations. The Birge-Sponer plot is linear for OH-stretch vibrations 1nu(1)-4nu(1), demonstrating behavior of a one-dimensional Morse oscillator. The anharmonicity parameter derived from the plot is similar to the values obtained for other small OH containing molecules. Isomerization to methoxy does not contribute to the predissociation signal and the mechanism appears to be direct O-H fission via tunneling. CH(2)OH presents a unique example in which the reaction coordinate is excited directly and leads to predissociation via tunneling while preserving the local-mode character of the stretch vibration.     

Response of water to electric fields at temperatures below the glass transition: a molecular dynamics analysis

The electric field dependence of the structure and dynamics of water at 77 K, i.e., below the glass transition temperature (136 K), is investigated using molecular dynamics simulations. Transitions are found at two critical field strengths, denoted E(1) and E(2). The transition around E(1)≈3.5 V/nm is characterized by the onset of significant structural disorder, a rapid increase in the orientational polarization, and a maximum in the dynamical fluctuations. At E(2)≈40 V/nm, the system crystallizes in discrete steps into a body-centered-cubic unit cell that minimizes the potential energy by simultaneous superpolarization of the water molecular dipoles and maximization of the intermolecular hydrogen bonds. The stepwise and discontinuous increase of the orientational polarization with the increasing electric field indicates that the dipole relaxation in the electric field is highly cooperative.     

New approach to the first-order phase transition of Lennard-Jones fluids

The multicanonical Monte Carlo method is applied to a bulk Lennard-Jones fluid system to investigate the liquid-solid phase transition. We take the example of a system of 108 argon particles. The multicanonical weight factor we determined turned out to be reliable for the energy range between -7.0 and -4.0 kJ/mol, which corresponds to the temperature range between 60 and 250 K. The expectation values of the thermodynamic quantities obtained from the multicanonical production run by the reweighting techniques exhibit the characteristics of first-order phase transitions between liquid and solid states around 150 K. The present study reveals that the multicanonical algorithm is particularly suitable for analyzing the transition state of the first-order phase transition in detail.