Melting at alkyl side chain comb polymer interfaces

Infrared visible sum frequency generation (SFG) spectroscopy has been used to study structure and melting transition temperatures of alkyl-side chain-acrylate comb polymers at air and solid interfaces. At the air interface, the SFG spectra show methyl bands and two transitions are observed: the first, near the bulk melting temperature, T(m), and the second 10-20 degrees C higher than T(m). The shorter the alkyl side chain, the larger the difference between the two transition temperatures. In contrast, methylene bands are observed at sapphire interface with a single transition near T(m) (C18).     

Anchoring of alkyl chain molecules on oxide surface using silicon alkoxide

Chemical states of the interfaces between octadecyl-triethoxy-silane (ODTS) molecules and sapphire surface were measured by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) using synchrotron soft X-rays. The nearly self-assembled monolayer of ODTS was formed on the sapphire surface. For XPS and NEXAFS measurements, it was elucidated that the chemical bond between silicon alkoxide in ODTS and the surface was formed, and the alkane chain of ODTS locates upper side on the surface. As a result, it was elucidated that the silicon alkoxide is a good anchor for the immobilization of organic molecules on oxides.     

Substrate and chain size dependence of near surface dynamics of glassy polymers

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) (i-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk Tg value, even on systems where the thin film Tg is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large (Mw independent) limiting thickness of approximately 180 nm where the free surface relaxation is not affected by the substrate. For thick films (h>200 nm) we find an unexpected linear Mw dependence of the near surface relaxation time.     

Determination of polymer chain conformation in amorphous polymers

We present the results of a careful examination of neutron scattering from polymer chains in bulk; we have arrived at the following conclusions: 1) Macromolecular chains (in bulk) have the same dimensions as in a θ solvent; 2) They obey the Debye formula q^?1 as low as 5 Å. In order to show this clearly, we compare the theoretical Debye curve and the experimental result for deuterated polystyrene (M_w = 21,000) in a matrix of ordinary polystyrene. The fit is remarkably good and leaves very little room for other interpretations.     

Some remarks on the chain segment dynamics in confined polymers

Recently, the chain dynamics of molten polymers in confined geometries has been investigated, using NMR. Some of these experiments have been performed on poydimethylsiloxane (PDMS) confined in planar nanolayers involving non-adsorbant solid surfaces. Here the data obtained on various PDMS systems are compared. The common property to these systems is that the local dynamics is anisotropic and the chain segments are undergoing uniaxial fluctuations around the normal n to the layers. Our data clearly show that the sign of the degree of orientational order S ( i.e. the direction of the fluctuations around n) and the broadening of the order distribution P( S) both result from the relative influence of the impenetrable interfaces and anchoring junctions on the segment behaviour.     

Periodic surface photoreliefs in glassy and hyperelastic polymers

The formation and degradation of periodic photoreliefs on the surface of polymer layers having significantly different glass-transition temperatures are investigated for various process activation temperatures. It is established that the main factor limiting the resolution of periodic relaxation photoreliefs at the surface of glassy polymer layers containing dimerizing anthracene derivatives is the presence of shear stresses. Their action is suppressed by the thermal decomposition of dimers, a process which gives rise to inverted reliefs of higher spatial frequencies. It is shown that the resolution can be enhanced by more than an order of magnitude by using a polymer matrix in the hyperelastic state. Zh. Tekh. Fiz. 69, 79–83 (August 1999)     

Some remarks on the chain segment dynamics in confined polymers

The European Physical Journal E - Recently, the chain dynamics of molten polymers in confined geometries has been investigated, using NMR. Some of these experiments have been performed on...     

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.     

Dependence of the clearing temperature on alkyl chain length in nematic homologous series

In homologous series of nematic compounds with relatively high clearing temperatures, these temperatures are known to decrease with increasing chain length. Recently, however, series with relatively low clearing temperatures have been synthetized in which these temperatures increase with increasing chain length. In both cases the clearing temperatures alternate within a series. A qualitative explanation is given for these facts. The discussion is based on an expression for the free energy in which both the attractive dispersion forces and the excluded volume affects are taken into account.     

An investigation of phase transformation and crystallinity in laser surface modified H13 steel

This paper presents a laser surface modification process of AISI H13 tool steel using 0.09, 0.2 and 0.4 mm size of laser spot with an aim to increase hardness properties. A Rofin DC-015 diffusion-cooled CO₂ slab laser was used to process AISI H13 tool steel samples. Samples of 10 mm diameter were sectioned to 100 mm length in order to process a predefined circumferential area. The parameters selected for examination were laser peak power, overlap percentage and pulse repetition frequency (PRF). X-ray diffraction analysis (XRD) was conducted to measure crystallinity of the laser-modified surface. X-ray diffraction patterns of the samples were recorded using a Bruker D8 XRD system with Cu?K _α (λ=1.5405 Å) radiation. The diffraction patterns were recorded in the 2θ range of 20 to 80°. The hardness properties were tested at 981 mN force. The laser-modified surface exhibited reduced crystallinity compared to the un-processed samples. The presence of martensitic phase was detected in the samples processed using 0.4 mm spot size. Though there was reduced crystallinity, a high hardness was measured in the laser-modified surface. Hardness was increased more than 2.5 times compared to the as-received samples. These findings reveal the phase source of the hardening mechanism and grain composition in the laser-modified surface.     

Effect of alkyl chain length in protic ionic liquids: an AIMD perspective

In this study we have explored, by means of ab initio molecular dynamics, a subset of three different protic ionic liquids (ILs). We present both structural and dynamical information of the liquid state of these compounds as revealed by accurate ab initio computations of the interactions. Our analysis figures out the presence of a strong hydrogen bond network in the bulk state, that is more stable in those ILs characterised by a longer alkyl side chain. Indeed it becomes more long-lasting passing from ethyl ammonium to butyl ammonium, owing to the hydrophobic effects stemming from alkyl chain contacts. Furthermore, the relative free energy landscape of the cation–anion interaction exhibits a progressively deeper well as the side chain of the cation gets longer. The hydrogen bond interaction, as already mentioned in previous works, leads to loss of degeneracy of the asymmetric stretching vibrations of the nitrate anions. The resulting frequency splitting between the two normal modes is about 90 cm^?1.     

Charge freezing in the zigzag chain PrBa2Cu4O8 cuprate

We report nuclear quadrupole resonance (NQR) studies on the chain Cu sites of PrBa2Cu4O8, a quasi-one-dimensional conductor with a nearly quarter-filled band. The nuclear spin-lattice relaxation rate 1/T1 shows a pronounced peak near 100 K caused by fluctuations of electric field gradient. Similar peak was observed for the spin-echo decay rate 1/T2, however, at a different temperature near 50 K. These results and broadening of the NQR spectrum at low temperatures indicate that slow charge fluctuations of either electronic or ionic origin freeze gradually at low temperatures.     

Infrared studies of chain folding in polymers. X. polycaprolactam

The 1210-, 1233-, and 1288—cm^?1 infrared bands in the α form of nylon 6 are shown to result from a unique conformation in a tightly folded chain with adjacent reentry. The assignment of the 1210- and 1288-cm^?1 bands to motions of a gauche nitrogen-methylene group in the fold and the 1233-cm^?1 band to motions of a gauche carboxyl-methylene group in the fold is supported by the infrared studies of the α form of nylon 6, the γ form of nylon 6, poly-D(-)β-methyl- ? -caproamide (PBCA), and the high- and low-temperature forms of the cyclic dimer of nylon 6. The γ- form of nylon 6 and PBCA do not exhibit a unique conformation in the fold. The 1198-cm^?1 band can be assigned to the trans conformation of the amide group which exists in the normal planar zigzag conformation in the α-form crystal. A mechanism for the α to γ iodine-induced transition in nylon 6 is proposed showing the role of the fold in maintaining a minimum of molecular disordering during the transformation.     

Infrared studies of chain folding in polymers. VII. Amylose

Studies of chain folding in amylose employing spectroscopic techniques have led to an assignment of the 1295-cm^?1 band to a unique fold conformation. Irregular folds, present in single crystals, are regularized to the unique fold conformation during annealing. Annealing of solution-cast films, which initially contain a small amount of regular folding, increases the amount of regular folds. Swelling of these annealed films with water vapor destroys this unique folded conformation.     

Effects of constrained chain conformations on polymer-solute interactions in semicrystalline polymers

The chemical potential of a solute in a solid polymer includes contributions from solute-polymer chain conformations, Flory-Huggins type interaction, and elastic energy of swelling. Presence of impermeable and rigid crystallites in such systems is expected to affect all these contributions. Theoretical calculations have been performed to check the direct effects of constrained chain conformations in the amorphous domains in semicrystalline polymers. Experimental results are used to determine Flory-Huggins coefficient and elastic modulus. From all these, the primary effects are shown to be on the entropic part of the Flory-Huggins coefficient and an increase in the elastic modulus by one or two order of magnitude. Finally, these results are used to calculate the rates of solvent-induced crystallization to show that these rates can drop to negligible values as the amount of crystals formed rises. Thus, the actual degree of crystallization can lie well below the Flory-Yoon limit.     

Effect of the processes of orientation of nonlinear optical fragments of a side chain in comb-shaped polymers of the methacrylic series on the second-harmonic generation in thin films

New comblike copolymers of methacrylic acid esters with an optically nonlinear chromophore—a derivative of the 4′-(4-nitrobenzylideneamino)phenol—in a side chain were obtained. Processes of forced orientation of nonlinear optical chromophores covalently bonded with the main chains in an electric field and the effects of the conditions of orientation on the nonlinear second-order optical activity of the polymer films were studied in some detail. The existence of a correlation between the chemical structure and the composition of the copolymers with the magnitude and the stability of the surface electron potential of the corresponding films was shown. It was established that a maximum extent of orientation of chromophores in the bulk of the polymer and, as a consequence, a high intensity of the second-harmonic signal were achieved when the thickness of the region of the uniform electric field maximally approached the thickness of the sample.     

Infrared studies of chain folding in polymers. v. polyhexamethylene adipamide

Systematic infrared studies of a series of polyhexamethylene adipamides (nylon 66) have resulted in the assignment of two bands to the regularly folded polymer chain. Utilizing this assignment and long-period measurements by small-angle X-ray diffraction, a method has been developed to compare the amount of regular chain folding in solution-crystallized samples precipitated at different temperatures. At high supercoolings in solution, only 60% of the chain folding which occurs has a regular structure. Annealing of the solution-crystallized samples not only produced a longer fold period but also regularized the folds. Melt-crystallized samples when annealed do not show much increase in fold period or regularization of the folds. Glass-crystallized samples show a rapid initial increase with time at a particular temperature in long period and regular folding. For the glass-crystallized samples crystallized at different temperatures for the same period of time, higher temperatures of crystallization produce longer fold periods and greater fractions of regular folds. The infrared fold bands disappear when samples consisting of regular folds are deformed but reappear after annealing.     

Orbital free ab initio simulation of surface freezing in a dilute Ga-Tl alloy

We have used the orbital-free ab initio molecular dynamics method to study the phenomenon of surface freezing in a liquid Ga-Tl alloy (nominal Tl concentration of 0.14). Several thermodynamic states are considered from a high temperature of 675 K to a low temperature of 350 K. At the higher temperature the Tl atoms segregate to the surface and form a liquid layer. Upon cooling the layer eventually crystallizes into a close-packed hexagonal solid layer. These results agree with experiments for a dilute liquid alloy of Tl in Ga, except for some overestimation of the surface freezing temperature. X-ray reflectivity data are also well reproduced. The wavelength of the oscillatory density profile coincides with that deduced from experiments and with that obtained in earlier computer simulation results. We also analyze the consequences of surface freezing on the structure of the liquid in contact with the solid layer, finding no influence whatsoever, as the structure basically coincides with the bulk liquid one.