The equilibrium melting of polymers

Equilibrium melting of polymer

Continuum Theory of Polymer Crystallization

We present a kinetic model of crystal growth of polymers of finite molecular weight. Experiments help to classify polymer crystallization broadly into two kinetic regimes. One is observed in melts or in high molar mass polymer solutions and is dominated by nucleation control with $G \sim \exp(1/T \Delta T)$, where $G$ is the growth rate and $\Delta T$ is the super-cooling. The other is observed in low molar mass solutions (as well as for small molecules) and is diffusion controlled with $G \sim \Delta T$, for small $\Delta T$. Our model unifies these two regimes in a single formalism. The model accounts for the accumulation of polymer chains near the growth front and invokes an entropic barrier theory to recover both limits of nucleation and diffusion control. The basic theory applies to both melts and solutions, and we numerically calculate the growth details of a single crystal in a dilute solution. The effects of molecular weight and concentration are also determined considering conventional polymer dynamics. Our theory shows that entropic considerations, in addition to the traditional energetic arguments, can capture general trends of a vast range of phenomenology. Unifying ideas on crystallization from small molecules and from flexible polymer chains emerge from our theory.Comment: 37 double-spaced pages including 8 figures, submitted to the Journal of Chemical Physic

Seeding of supercooled polyethylene with extended chain crystals

Seeding of supercooled polyethylene with extended chain crystal

Co-planar spin-polarized light emitting diode

Studies of spin manipulation in semiconductors has benefited from the possibility to grow these materials in high quality on top of optically active III-V systems. The induced electroluminescence in these layered semiconductor heterostructures has been used for a reliable spin detection. In semiconductors with strong spin-orbit interaction, the sensitivity of vertical devices may be insufficient, however, because of the sepration of the spin aligner part and the spin detection region by one or more heterointerfaces and becuse of the short spin coherence length. Here we demostrate that higly sensitive spin detection can be achieved using a lateral arrangement of the spin polarized and optically active regions. Using our co-planar spin-polarized light emitting diodes we detect electrical field induced spin generation in a semiconductor heterojunction two-dimensional hole gas. The polarization results from spin asymmetric recombination of injected electrons with strongly SO coupled two-dimensional holes. The possibility to detect magnetized Co particles deposited on the co-planar diode structure is also demonstrated.Comment: 8 pages, 3 figure

Measuring entanglement in condensed matter systems

We show how entanglement may be quantified in spin and cold atom many-body systems using standard experimental techniques only. The scheme requires no assumptions on the state in the laboratory and a lower bound to the entanglement can be read off directly from the scattering cross section of Neutrons deflected from solid state samples or the time-of-flight distribution of cold atoms in optical lattices, respectively. This removes a major obstacle which so far has prevented the direct and quantitative experimental study of genuine quantum correlations in many-body systems: The need for a full characterization of the state to quantify the entanglement contained in it. Instead, the scheme presented here relies solely on global measurements that are routinely performed and is versatile enough to accommodate systems and measurements different from the ones we exemplify in this work.Comment: 6 pages, 2 figure

Low-dimensional light-emitting transistor with tunable recombination zone

We present experimental and numerical studies of a light-emitting transistor comprising two quasi-lateral junctions between a two-dimensional electron and hole gas. These lithographically defined junctions are fabricated by etching of a modulation doped GaAs/AlGaAs heterostructure. In this device electrons and holes can be directed to the same area by drain and gate voltages, defining a recombination zone tunable in size and position. It could therefore provide an architecture for probing low-dimensional devices by analysing the emitted light of the recombination zone.Comment: 12 Pages, to be published in Journal of Modern Optic

Enhancement of laser cooling by the use of magnetic gradients

We present a laser cooling scheme for trapped ions and atoms using a combination of laser couplings and a magnetic gradient field. In a Schrieffer-Wolff transformed picture, this setup cancels the carrier and blue sideband terms completely resulting in an improved cooling behaviour compared to standard cooling schemes (e.g. sideband cooling) and allowing cooling to the vibrational ground state. A condition for optimal cooling rates is presented and the cooling behaviour for different Lamb-Dicke parameters and spontaneous decay rates is discussed. Cooling rates of one order of magnitude less than the trapping frequency are achieved using the new cooling method. Furthermore the scheme turns out to be robust under deviations from the optimal parameters and moreover provides good cooling rates also in the multi particle case.Comment: 14 pages, 8 figure

Time-dependent Reactions of the Crystallizing and Solidifying of Linear High Polymers

Polyethylene polymer crystal growth - effects of high pressure, high temperature, and formation time on thermodynamic stabilit