Influence of spatial constraints in non-crystalline regions on the polymer dynamics in semi-crystalline polyethylene: a solid state NMR study

A broad variety of solid state NMR techniques were used to investigate the chain dynamics in several polyethylene (PE) samples, including ultrahigh molecular weight PEs (UHMW-PEs) and low molecular weight PEs (LMW-PEs). Via changing the processing history, i.e. melt/solution crystallization and drawing processes, these samples gain different morphologies, leading to different molecular dynamics. Due to the long chain nature, the molecular dynamics of polyethylene can be distinguished in local fluctuation and long range motion. With the help of NMR these different kinds of molecular dynamics can be monitored separately. In this work the local chain dynamics in non-crystalline regions of polyethylene samples was investigated via measuring 1H-13C heteronuclear dipolar coupling and 13C chemical shift anisotropy (CSA). By analyzing the motionally averaged 1H-13C heteronuclear dipolar coupling and 13C CSA, the information about the local anisotropy and geometry of motion was obtained. Taking advantage of the big difference of the 13C T1 relaxation time in crystalline and non-crystalline regions of PEs, the 1D 13C MAS exchange experiment was used to investigate the cooperative chain motion between these regions. The different chain organizations in non-crystalline regions were used to explain the relationship between the local fluctuation and the long range motion of the samples. In a simple manner the cooperative chain motion between crystalline and non-crystalline regions of PE results in the experimentally observed diffusive behavior of PE chain. The morphological influences on the diffusion motion have been discussed. The morphological factors include lamellar thickness, chain organization in non-crystalline regions and chain entanglements. Thermodynamics of the diffusion motion in melt and solution crystallized UHMW-PEs is discussed, revealing entropy-controlled features of the chain diffusion in PE. This thermodynamic consideration explains the counterintuitive relationship between the local fluctuation and the long range motion of the samples. Using the chain diffusion coefficient, the rates of jump motion in crystals of the melt crystallized PE have been calculated. A concept of "effective" jump motion has been proposed to explain the difference between the values derived from the chain diffusion coefficients and those in literatures. The observations of this thesis give a clear demonstration of the strong relationship between the sample morphology and chain dynamics. The sample morphologies governed by the processing history lead to different spatial constraints for the molecular chains, leading to different features of the local and long range chain dynamics. The knowledge of the morphological influence on the microscopic chain motion has many implications in our understanding of the alpha-relaxation process in PE and the related phenomena such as crystal thickening, drawability of PE, the easy creep of PE fiber, etc.

Identification of optically clear regions of ternary polymer blends using a novel rapid screening method

The application of a rapid screening method for the construction of ternary phase diagrams is described for the first time, providing detailed visualization of phase boundaries in solvent-mediated blends. Our new approach rapidly identifies ternary blend compositions that afford optically clear materials, useful for applications where transparent films are necessary. The use of 96-well plates and a scanning plate reader has enabled rapid optical characterization to be carried out by transmission spectrophotometry (450 nm), whilst the nature and extent of crystallinity was examined subsequently by wide angle X-ray scattering (WAXS). The moderating effect of cellulose acetate butyrate can be visualized as driving the position of the phase boundaries in poly(l-lactic acid)/polycaprolactone (PLLA/PCL) blends. More surprisingly, the boundaries are critically dependent on the molecular weight of the crystallizable PLLA and PCL, with higher molecular weight polymers leading to blends with reduced phase separation. On the other hand, the propensity to crystallize was more evident in shorter chains. WAXS provides a convenient way of characterizing the contribution of the individual blend components to the crystalline regions across the range of blend compositions. © 2013 Society of Chemical Industry.

Studies of the radiation chemistry and grafting of a fluoropolymer

The radiation chemistry and the grafting of a fluoropolymer, poly(tetrafluoroethylene-coperfluoropropyl vinyl ether) (PFA), was investigated with the aim of developing a highly stable grafted support for use in solid phase organic chemistry (SPOC). A radiation-induced grafting method was used whereby the PFA was exposed to ionizing radiation to form free radicals capable of initiating graft copolymerization of styrene. To fully investigate this process, both the radiation chemistry of PFA and the grafting of styrene to PFA were examined. Radiation alone was found to have a detrimental effect on PFA when irradiated at 303 K. This was evident from the loss in the mechanical properties due to chain scission reactions. This meant that when radiation was used for the grafting reactions, the total radiation dose needed to be kept as low as possible. The radicals produced when PFA was exposed to radiation were examined using electron spin resonance spectroscopy. Both main-chain (–CF2–C.F–CF2-) and end-chain (–CF2–C.F2) radicals were identified. The stability of the majority of the main-chain radicals when the polymer was heated above the glass transition temperature suggested that they were present mainly in the crystalline regions of the polymer, while the end-chain radicals were predominately located in the amorphous regions. The radical yield at 77 K was lower than the radical yield at 303 K suggesting that cage recombination at low temperatures inhibited free radicals from stabilizing. High-speed MAS 19F NMR was used to identify the non-volatile products after irradiation of PFA over a wide temperature range. The major products observed over the irradiation temperature 303 to 633 K included new saturated chain ends, short fluoromethyl side chains in both the amorphous and crystalline regions, and long branch points. The proportion of the radiolytic products shifted from mainly chain scission products at low irradiation temperatures to extensive branching at higher irradiation temperatures. Calculations of G values revealed that net crosslinking only occurred when PFA was irradiated in the melt. Minor products after irradiation at elevated temperatures included internal and terminal double bonds and CF3 groups adjacent to double bonds. The volatile products after irradiation at 303 K included tetrafluoromethane (CF4) and oxygen-containing species from loss of the perfluoropropyl ether side chains of PFA as identified by mass spectrometry and FTIR spectroscopy. The chemical changes induced by radiation exposure were accompanied by changes in the thermal properties of the polymer. Changes in the crystallinity and thermal stability of PFA after irradiation were examined using DSC and TGA techniques. The equilibrium melting temperature of untreated PFA was 599 K as determined using a method of extrapolation of the melting temperatures of imperfectly formed crystals. After low temperature irradiation, radiation- induced crystallization was prevalent due to scission of strained tie molecules, loss of perfluoropropyl ether side chains, and lowering of the molecular weight which promoted chain alignment and hence higher crystallinity. After irradiation at high temperatures, the presence of short and long branches hindered crystallization, lowering the overall crystallinity. The thermal stability of the PFA decreased with increasing radiation dose and temperature due to the introduction of defect groups. Styrene was graft copolymerized to PFA using -radiation as the initiation source with the aim of preparing a graft copolymer suitable as a support for SPOC. Various grafting conditions were studied, such as the total dose, dose rate, solvent effects and addition of nitroxides to create “living” graft chains. The effect of dose rate was examined when grafting styrene vapour to PFA using the simultaneous grafting method. The initial rate of grafting was found to be independent of the dose rate which implied that the reaction was diffusion controlled. When the styrene was dissolved in various solvents for the grafting reaction, the graft yield was strongly dependent of the type and concentration of the solvent used. The greatest graft yield was observed when the solvent swelled the grafted layers and the substrate. Microprobe Raman spectroscopy was used to map the penetration of the graft into the substrate. The grafted layer was found to contain both poly(styrene) (PS) and PFA and became thicker with increasing radiation dose and graft yield which showed that grafting began at the surface and progressively penetrated the substrate as the grafted layer was swollen. The molecular weight of the grafted PS was estimated by measuring the molecular weight of the non-covalently bonded homopolymer formed in the grafted layers using SEC. The molecular weight of the occluded homopolymer was an order of magnitude greater than the free homopolymer formed in the surrounding solution suggesting that the high viscosity in the grafted regions led to long PS grafts. When a nitroxide mediated free radical polymerization was used, grafting occurred within the substrate and not on the surface due to diffusion of styrene into the substrate at the high temperatures needed for the reaction to proceed. Loading tests were used to measure the capacity of the PS graft to be functionialized with aminomethyl groups then further derivatized. These loading tests showed that samples grafted in a solution of styrene and methanol had superior loading capacity over samples graft using other solvents due to the shallow penetration and hence better accessibility of the graft when methanol was used as a solvent.

High resolution imaging of boron carbide microstructures

Two samples of boron carbide have been examined using high resolution transmission electron microscopy (HRTEM). A hot pressed B13C2 sample shows a high density of variable width twins normal to (10-11). Subtle shifts or offsets of lattice fringes along the twin plane and normal to (10 5) were also observed. A B4C powder showed little evidence of stacking disorder in crystalline regions.

Laser-fused refractory oxides for optical coatings

Laser sintering was carried out using a high power continuous-wave CO2 laser to prepare pellets of zirconia (ZrO2), hafnia (HfO2) and yttria (Y2O3) mixed oxides as starting materials in the deposition of optical coatings. Hardened recrystallized pellets appeared to have been formed during laser treatment. X-ray diffraction analysis revealed a monoclinic-to-tetragonal phase transformation in the binary system while the ternary system was found to have a mixture of two crystalline phases. Cross-sectional scanning electron microscopy showed two isothermal crystalline regions in the ternary system. The optical inhomogeneity was low in the films deposited from the laser-fused pellets, but the absorption at a wavelength of 351 nm increased with increasing HfO2 content. The films deposited from laser-fused pellets were analysed by electron spectroscopy for chemical analysis and found to be stoichiometric and homogeneous.

Wet chemical formation of nanoparticles of binary perovskites through isothermal gel to crystallite conversion

Coarse BO2·xH2O (2 < x < 80) gels, free of anion contaminants react with A(OH)2 under refluxing conditions at 70�100°C giving rise to crystallites of single phased, nanometer size powders of ABO3 perovskites (A = Ba, Sr, Ca, Mg, Pb; B = Zr, Ti, Sn). Solid solutions of perovskites could be prepared from compositionally modified gels or mixtures of A(OH)2. Donor doped perovskites could also be prepared from the same method so that the products after processing are often semiconducting. Faster interfacial diffusion of A2+ ions into the gel generates the crystalline regions whose composition is controllable by the A/B ratio as well as the A(OH)2 concentration.

Hall coefficient and resistivity of an amorphous palladium-silicon alloy

The Hall coefficient and resistance in several specimens of an amorphous metallic alloy containing 80 at.% palladium and 20 at.% silicon have been investigated at temperatures between 4.2°K and room temperature. An ideal limiting behavior of these transport coefficients was analyzed on the basis of the nearly free electron model to yield a carrier density of 9 x 10²² cm.^-3, or about 1.7 electrons per palladium atom, and a mean free path of about 9Å which is almost constant with temperature. The deviations of the individual specimens from this ideal behavior, which were small but noticeable in the relative resistivity and much greater in the Hall coefficient, can be explained by invoking disk-shaped crystalline regions with low resistivity and a positive Hall coefficient. A detailed calculation shows how a volume fraction of such crystalline material too small to be noticed in X-ray diffraction could have a significant effect on the resistivity and a much greater effect on the Hall coefficient.

Miscibility of poly(ether ether ketone)/poly(ether diphenyl ether ketone) blends

Poly(ether ether ketone) and poly(ether diphenyl ether ketone) homopolymers are prepared by nucleophilic substitution routes. Miscibility of PEEK/PEDEK blends has been studied by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (d.s.c.). The results indicate that for PEEK/PEDEK blends, when the PEDEK content (weight fraction) is greater than 0.20 and less than 0.75, PEEK and PEDEK components form independent crystalline regions, i.e. they are immiscible; when the PEDEK content is in the range W-PEDEK less than or equal to 0.20 or greater than or equal to 0.75, a rich PEEK- or PEDEK-rich content crystallizes from a mixed melt and PEEK and PEDEK are miscible. Copyright (C) 1996 Elsevier Science Ltd.

EPITAXIAL CRYSTALLIZATION OF HDPE ON IPP IN HDPE/IPP BLENDS

Unique crystalline morphologies of solution-cast films of HDPE/iPP blends were investigated by means of transmission electron microscopy (TEM), electron diffraction, metal shadowing and specimen-tilt techniques. The unique morphologies come from an epitaxial crystallization of HDPE on iPP. The contact planes of the two kinds of crystals are (100) of HDPE and (010) of iPP, while the intercrossing angle between their chain axes is about 50-degrees. The HDPE existed with different crystalline morphologies in the two kinds of crystalline regions of iPP spherulites, i.e. cross-hatched and single-crystal-type structures. Based on structural analysis, two models of epitaxial growth of HDPE on iPP are proposed.

Gelatinization properties of native starches and their Naegeli dextrins

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Simultaneous observation of the magnetic and electric behavior in a correlated system near a metal-semiconductor transition: ESR in pellets of conducting polymers

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural and physicochemical characteristics of lintnerized native and sour cassava starches

The comprehension of the structure of starch granules is important for the understanding of its physicochemical properties. Native and sour cassava starches after being analyzed with respect to their pasting properties and baking expansion capacity, were treated with 2.2 N HCl at 38 degreesC for a maximum of nine days. The starch granules remaining after lintnerization were analyzed for amylose content and intrinsic viscosity, by X-ray diffraction, scanning electron microscopy and chromatographic analysis. The results indicated that the acid hydrolysis on all starches occurred in two steps. The first one, with high hydrolysis rate, was characterized by a quick degradation of the amorphous part of the granules whereas the second step, with lower hydrolysis rate, was characterized by a higher resistance of the organized areas of the granules to acid treatment. Most of the amylose chains were found in the amorphous areas of starch granules only a small percentage was involved in the crystalline regions. The microscopic and chromatographic analysis demonstrated that the acid hydrolysis was not able to disrupt the entire granular crystalline structure. Fermented starch showed amylose and/or amylopectin chain fractions resistant to pullulanase, probably due to structural alterations during fermentation.

The ferromagnetic behaviour of conducting polymers revisited

The magnetic properties of doped pellets of poly(3-methylthiophene) showing room temperature ferromagnetic behaviour have been discussed in a previous article. The magnetic behaviour was attributed to a weak ferromagnetic phase, due to the superexchange interaction of polarons via the dopant anions. The Dzialoshinsky-Morya interaction among canted spins was proposed to explain the ferromagnetism. In this article the main conclusions of that work concerning the magnetic behaviour are revised. The basic assumption now is that the magnetic moments are spin 1/2 polarons that can interact antiferromagnetically and/or ferromagnetically. In the small crystalline regions of the polymer, which are identified with the polymer portion that remains ferromagnetic at room temperature, the interaction gives rise to S = 0 and 1 bipolarons and the S = 1 triplet state is lower in energy. In the disordered region, disorder will prevent the complete S = 1 and 0 coupling and bands of polarons ferromagnetically and antiferromagnetically coupled will appear. Using this approach, all the magnetization data can be qualitatively explained, as well as the electron spin resonance data.

The structure of waxy corn starch: Effect of granule size

The granules of waxy corn starch were isolated and various samples were separated by size and classified according to their average diameter in: non-separated granules (N), granules with diameter < 15 μm (S) and granules with diameter ≥ 15 μm (L). The samples were hydrolyzed by bacterial α-amylase and fungal amyloglucosidase. The starch granules remaining after enzymatic hydrolysis were analysed by X-ray diffraction and optical and scanning electron microscopy. Sephadex G-50 gel permeation chromatography of the dissolved residues from the hydrolysis of the N and S samples was performed directly and after successive enzymatic digestion with pullulanase and β-amylase. The results showed that the percentage of hydrolysis increased with a decrease in diameter. No apparent differences in waxy corn starch when observed under light and scanning electronic microscope were observed, regardless of diameter and enzyme action, although both large and small granules showed extensive surface corrosion after enzymatic attack. X-ray analysis suggested a decrease in the quantity of crystalline areas in the smaller granules, which would explain the high percentage of hydrolysis evidenced by these granules. The elution patterns of the α-glucans of both starches (N and S) were similar and reveled the presence of two fractions which were not susceptible to a-amylase and amyloglucosidase attack suggesting that these fractions were involved in the waxy corn starch crystalline regions. Debranching with pullulanase followed by gel-permeation chromatography showed that the amylopectins from the starch granules studied contained three groups of unit chains instead of the two reported in the literature.

Evidence of weak ferromagnetism in doped plasticized polyaniline (PANI-DDoESSA)0.5 from electron spin resonance measurements

X-band electron spin resonance (ESR) measurements have been performed on a conducting free-standing film of polyaniline plasticized and protonated with di-n-dodecyl ester of sulfosuccinic acid (DDoESSA). The magnetic field was applied parallel and perpendicular to the plane of the film. At around 75 K a transition is observed from Pauli susceptibility to a localized state in which the spin 1/2 polarons behave as spin 1/2 dimers. A rough estimation of the intradimer and interdimer exchange constants is obtained. Below 5 K, ESR data reveal a weak ferromagnetism with the Dzyaloshinskii-Moriya vector mainly oriented in the plane of the film. The existence of a relatively well-defined n-fold axis along the chain direction in the crystalline regions confers a symmetry compatible with such analysis. © 2013 IOP Publishing Ltd.