Morphology Of Lamellar Crystals Research Articles

In-situ assembly of Cu/CuxO composite with CNT/Bacterial cellulose matrix as a support for efficient CO2 electroreduction reaction to CO and C2H4

A CNT/Bacterial Cellulose (BC) nanocomposite with a large surface area and conductive properties was in-situ cultivated to retain the native 3D nanofibril network. The Cu-CuxO/CNT/BC nanocomposite electrode was in-situ fabricated via reduction with the CNT/BC composite as a support, comparing KBH4 and NaBH4 as a reducing agent to obtain efficient electrodes for CO2 reduction (ECR-CO2). Both electrodes exhibited higher performance than the traditional carbon cloth or pristine BC supported Cu-based catalysts for conversion of ECR-CO2 to CO and C2H4. The CNT/BC as a support provided not only more active sites for copper catalyst but also increased the charge transfer and selective performance of C2H4 product, due to the synergistic effect of CNT and copper. The smaller honeycomb and dendritic lamellar crystal morphology of the obtained Cu-CuxO/CNT/BCNaBH4 electrode possessed a large electrochemical specific surface area, providing more active sites for ECR-CO2. The double-layer capacitance and the current density reached 11.8 mF cm−2 and 33.3 mA cm−2, respectively. At a potential of −1.5 VRHE, the Faraday efficiency (FECO + FEC2H4) was 66%. Moreover, the Cu-CuxO/CNT/BC nanocomposite electrode was very stable, and withstood tolerance in ECR-CO2 for more than 20 h. This study possibly will provide a new design for catalyst support on CNT/BC substrate to regulate advanced catalysts/electrodes for different applications, especially to ECR-CO2 in terms of higher charge transport properties, lower contact resistance, higher current density, and more stable CO2 electroreduction performance.

Controlling Polymer Crystallization Kinetics by Sample History

AbstractIn this paper systematic changes in the isothermal crystallization kinetics (at Tc = 200 °C) of nonequilibrated isotactic polystyrene films, generated through spin coating are reported. Kinetics is tuned by annealing films at Ta = 250 °C, i.e., above their nominal melting temperature (TmDSC = 220 °C). An exponential decrease in growth rate and a significant decrease of crystal nucleation density by approximately three orders of magnitude are observed by increasing the time (ta) provided for equilibration at Ta. The characteristic exponential decay time is approximately four orders of magnitude longer than the equilibrium reptation time at Ta. Interestingly, with increasing ta, a change in the morphology of lamellar crystals varying from spherulites to hexagons of rather uniform height to finally hexagons exhibiting periodic modulations in height is observed. These observations reveal that understanding and tuning the crystallization kinetics of polymer films under quiescent conditions requires knowledge on how strongly chain conformations deviate from equilibrium.

Crystallization behavior of PEO in blends of poly(ethylene oxide)/poly(2‐vinyl pyridine)‐b‐(ethylene oxide) block copolymer

AbstractThe crystallization behavior of two molecular weight poly(ethylene oxide)s (PEO) and their blends with the block copolymer poly(2‐vinyl pyridine)‐b‐poly(ethylene oxide) (P2VP‐b‐PEO) was investigated by polarized optical microscopy, thermogravimetric analysis, differential scanning calorimetry, and atomic force microscopy (AFM). A sharp decreasing of the spherulite growth rate was observed with the increasing of the copolymer content in the blend. The addition of P2VP‐b‐PEO to PEO increases the degradation temperature becoming the thermal stability of the blend very similar to that of the block copolymer P2VP‐b‐PEO. Glass transition temperatures, Tg, for PEO/P2VP‐b‐PEO blends were intermediate between those of the pure components and the value increased as the content of PEO homopolymer decreased in the blend. AFM images showed spherulites with lamellar crystal morphology for the homopolymer PEO. Lamellar crystal morphology with sheaf‐like lamellar arrangement was observed for 80 wt% PEO(200M) and a lamellar crystal morphology with grain aggregation was observed for 50 and 20 wt% blends. The isothermal crystallization kinetics of PEO was progressively retarded as the copolymer content in the blend increased, since the copolymer hinders the molecular mobility in the miscible amorphous phase. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers

Effect of Liquid−Liquid Phase Separation on the Lamellar Crystal Morphology in PEH/PEB Blend

Optical microscopy (OM) and atomic force microscopy (AFM) were used to investigate the effect of liquid−liquid phase separation (LLPS) on the lamellar crystal morphology in a statistical copolymer blend of poly(ethylene-co-hexene) (PEH) and poly(ethylene-co-butene) (PEB). The quench depth and the time of liquid−liquid phase separation (LLPS) exhibit obvious influences on the lamellar crystal morphology. The values of lamellar long period were measured from the AFM micrographs. It is interesting to find that the lamellar long periods of the blend show minimum values with respect to the LLPS temperature, LLPS time, and the crystallization temperature. The lamellar insertion and lamellar thickening models and inclusion/exclusion of the amorphous component in the lamellar stacks are considered to cause this puzzling phenomenon.

Structural Data on Regular Poly(ester amide)s Derived from Even Diols, Glycine, and Terephthalic Acid

The crystalline structure of two sequential poly(ester amide)s derived from glycine, terephthalic acid, and 1,6-hexanediol or 1,12-dodecanediol has been investigated using X-ray and electron diffraction and real space electron microscopy. Crystal packing shows features of both polyamides and polyesters. The structures of the two studied poly(ester amide)s are characterized by the establishment of strong hydrogen-bond interactions along a single direction. The morphology of lamellar crystals has been investigated, their high aspect ratio being consistent with the existence of a preferential crystal growth direction. A deviation from an all-trans conformation has been detected for both polymers. Differences in crystal packing have been found depending on the number of aliphatic methylene groups. In this way, orthorhombic and triclinic unit cells have been deduced for the dodecanediol and the hexanediol derivatives, respectively. Their structures are based on a different arrangement of sheets constituted by hydrogen-bonded molecular chains.

Synthesis, Structure and Crystal Morphology of Nylon 2/16

The alternating copolyamide poly(glycine-co-16-aminohexadecanoic acid) (nylon 2/16) was synthesized by polycondensation of the “dimer” glycyl-16-aminohexadecanoic acid pentachlorophenyl ester hydrobromide in a N,N-dimethylformamide solution. Both X-ray diffraction and electron microscopy studies showed that this new 2/n copolyamide crystallized in a hexagonal lattice of a = 0.479 nm with a residue height of about 2.4 nm. The crystal structure was modeled for a 6-fold helix with a period of 14.4 nm that is linked to its six neighbors by a three-dimensional network of hydrogen bonds. The structure had the overall features of the so-called Form II typical of nylons 2/n and was able to reproduce satisfactorily the experimental diffraction data. 4.5 nm-thick lamellar single crystals accommodating only two nylon 2/16 chemical residues in height were obtained by crystallization from solution. Variations in the crystallization temperature produced changes in the lamellar crystal morphology consistent with the occurrence of preferential hydrogen bonding directions. Compared with all other members of the 2/n family studied already by us, no trace of crystal form with chains in fully extended conformation (Form I) was found for nylon 2/16.

Biodegradable poly(ethylene succinate) (PES). 1. Crystal growth kinetics and morphology.

Crystal growth rates of melt-crystallized poly(ethylene succinate) (PES) with a number-average molecular weight Mn of 21,100 and polydispersity of 1.86, respectively, were studied in an extensive temperature range. On the basis of secondary nucleation theory, only one transition between regime III and regime II was found at around 71 degrees C, but no transition of regime II-->I was detected by kinetic analysis. The ratio of nucleation constants KgIII in regime III to KgII in regime II was influenced by the value of activation energy U*, and tended to 2 as predicted by the theory when U* was set as 3688 cal/mol. The morphological changes at spherulitic level and lamellar level in regime II and regime III were examined by using an optical microscope (OM) and atomic force microscope (AFM). Although spherulitic morphologies were found in both regime II and regime III by optical microscopy, further investigations made by following the crystallization process using OM and by checking the lamellar morphology using AFM revealed morphological changes during the crystal growth process and different crystal morphologies in regime II and regime III. Meanwhile by using ultrathin film (approximately 100 nm in thickness), the morphology of lamellar crystals of lozenge-shape outline and composed of single crystals has been studied by AFM technique. The results indicate that there is indeed a transition from regime II to regime III at around 71 degrees C for the isothermal crystallization of PES with an increasing degree of undercooling.

A sheet structure in an alternate copolymer of 4-aminobutyric acid and α-isobutyl-l-glutamate

An alternating copolyamide derived from 4-aminobutyric acid and α-isobutyl-l-glutamate has been synthesized by the active ester method, and subsequently characterized. The structure and morphology of lamellar crystals have been investigated by using transmission electron microscopy. Additional data have been obtained from uniaxially oriented films by using X-ray diffraction. Experimental results agree with an orthorhombic unit cell of parameters a=4.88, b=14.0 and the chain axis c=10.5Å. A sheet structure with intermolecular hydrogen bonds was deduced taking also into account infrared dichroism data.

Structure and morphology of nylon 46 lamellar crystals: Electron microscopy and energy calculations

A detailed electron microscopy study of the structure and morphology of lamellar crystals of nylon 46 obtained by crystallization from solution has been carried out. Electron diffraction of crystals supported by X-ray diffraction of their sediments revealed that they consist of a twinned crystal lattice made of hydrogen-bonded sheets separated 0.376 nm and shifted along the a-axis (H-bond direction) with a shearing angle of 65°. The interchain distance within the sheets is 0.482 nm. These parameters are similar to those previously described for nylon 46 lamellar crystals grown at lower temperatures. A combined energy calculation and modeling simulation analysis of all possible arrangements for the crystal-packing of nylon 46 chains, in fully extended conformation, was performed. Molecular mechanics calculations showed very small energy differences between α (alternating intersheet shearing) and β (progressive intersheet shearing) structures with energy minima for successive sheets sheared at approximately 1/6 c and 1/3 c. A mixed lattice composed of a statistical array of α and β structures with such sheet displacements was found to be fully compatible with experimental data and most appropriate to describe nylon 46 lamellar crystals. Annealing of the crystals at temperatures closely below the Brill transition induced enrichment in β structure and increased chain-folding order. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 41–52, 2000

Structure of odd–even nylons derived from 2-methylpentamethylenediamine. Effect of the side methyl group

Structure and morphology of lamellar crystals of two polyamides derived from 2-methylpentamethylenediamine (MPMD) and dibasic acids with 4 or 8 methylenes have been studied by transmission electron microscopy, electron diffraction and X-ray diffraction. The crystallographic unit cell of MPMD-6 is triclinic with a=4.85, b=5.90, c (chain axis)=30.4 Å, α=115.40, β=99.9 and γ=61.1°, the chain axis direction is not coincident with the c ∗ reciprocal axis. These dimensions suggest a hydrogen-bonded sheet structure with a single hydrogen bond direction and a twist of 180° in the odd diamine unit. MPMD-10 shows two crystalline structures. The most abundant being a monoclinic unit cell with a=4.79, b=5.67, c (chain axis)=36.3 Å and γ=61.8°. The less abundant structure was only observed in oriented fibers and was similar to the MPMD-6 crystalline structure. Single crystals of both MPMD-6 and MPMD-10 polyamides grow preferentially along their single hydrogen bond direction.

Isothermal thickening and thinning processes in low molecular weight poly(ethylene oxide) fractions crystallized from the melt: 6. Configurational defects in molecules

Three two-arm poly(ethylene oxide) (PEO) fractions were prepared via a coupling reaction. These PEOs possess the same number average molecular weight of 2220gmol−1 for each arm and the coupling agents used are para-(1,4-), meta-(1,3-) and ortho-(1,2-) benzene dicarbonyl dichlorides. The configurations of these two-arm PEO molecules thus exhibit angles of 180, 120 and 60°, respectively, at the coupling agent. These coupling agents may substantially affect the overall molecular conformations (OMCs) in the crystals of the two-arm PEOs. Wide angle X-ray diffraction powder patterns indicate that these fractions have the same crystal structure regardless of the coupling agents (which act as defects) used, and therefore the defects must be excluded from the lamellar crystals. Differential scanning calorimetry results show that two populations of crystals exist when the samples are crystallized at relatively low undercoolings. This is particularly evident in the cases of 1,4- and 1,3-two-arm PEOs. Corresponding small angle X-ray scattering experiments show two different long periods in these fractions. Experimental results reveal the possibility that the crystals which correspond to the thinner long period possess an extended OMC in which one layer of defects exists between two neighboring lamellae. The crystals which exhibit the thicker long period possess a once-folded OMC, with two layers of defects lying between neighboring lamellae. During heating, the extended OMC crystals with the thinner long period melt at lower temperatures than the thicker, once-folded OMC crystals. Detailed studies on the efforts of these configurational defects on the lamellar crystal morphology and crystal stability are discussed in the main body of the article.

Crystallographic structures on the sequential copolymer of ε-caprolactam and pyrrolidinone (nylon 4/6)

The structure and morphology of lamellar crystals of a sequential copolymer of poly(pirrolidinone) with poly( ε-caprolactam) (nylon 4/6 copolymer) has been studied by transmission electron microscopy, electron diffraction and X-ray diffraction. The results obtained indicate that at room temperature single crystals of this copolymer have an α-form structure which is also the most stable one for both related homopolymers. The crystallographic unit cell is monoclinic with a=9.60 Å, b (chain axis)=14.80 Å, c=8.06 Å and β=67°. The effects of temperature on the lattice spacings show that the characteristic 200 and 002 equatorial spacings converge with increasing temperature but do not meet prior to the melting temperature. In conclusion, the copolymer and the related homopolymers behave similarly with respect to their room temperature most stable crystal structure and behavior on heating. The second characteristic crystalline phase ( γ-form) reported for even nylons could only be obtained by treating the polymer sample with iodine–potassium iodide solutions.

Incorporation of glycine residues in even–even polyamides. Part II: Nylons 6,10 and 12,10

Sequential copolymers of glycine and nylons 6,10 and 12,10 have been prepared by incorporating single glycine units at both ends of the diamine. These polymers have been obtained by interfacial polymerization and subsequently characterized by infrared and n.m.r. spectroscopies together with thermal analysis. The structure and morphology of lamellar crystals have also been investigated by X-ray diffraction and transmission electron microscopy, both imaging and diffraction. Additional data have been obtained from uniaxially oriented fibres. Results described in this paper complete previous studies on the incorporation of glycine units in nylons derived either from ω-amino acids or from diamines and diacids.

Incorporation of glycine residues in even–even nylons disrupts their characteristic all- trans conformation

A series of regular polymers derived from the inclusion of glycine in even–even nylons (2,4, 2,6, 4,4 and 4,6) has been prepared by the active ester method. The structure and morphology of lamellar crystals have also been investigated by using transmission electron microscopy, selected-area electron diffraction and X-ray diffraction. The results indicate that at room temperature the structure of these nylons is different from the typical forms ( α and β) of even–even polyamides. Observations described in this paper are complementary to previous work about the incorporation of glycine units in nylons derived from ω-amino acids and indicate that glycine residues always tend to adopt their characteristic polyglycine II conformation.

Crystal morphological investigation in thin films of poly(aryl ether ketone ketone) having a meta-linkage

The lamellar crystal morphology of thin films of poly(aryl ether ketone ketone) having a meta-linkage (PEKK(I)) is observed under transmission electron microscopy (TEM), utilizing samples with different thermal histories and preparation procedures. After isothermal crystallization from the melt, a double endothermic behaviour can be found through differential scanning calorimetry (d.s.c.) experiments. Partial melting of PEKK(I) can be obtained by heating the samples to a temperature between these two endotherms. By comparing the TEM morphological observations of the samples before and after partial melting, it can be shown that edge-on lamellar crystals, having different thermodynamic stabilities, form during isothermal crystallization. After partial melting, only the type of lamellar crystal exhibiting the higher thermodynamic stability remains. Small angle X-ray scattering (SAXS) results exhibit a change in the long period of the lamellar crystals before and after the partial melting process. For the solution-induced crystallization of PEKK(I), only a single endotherm exists in the d.s.c. observations. After annealing at high temperatures, however, a double endothermic behaviour appears. This also results in a decrease in the long period observed by SAXS. The double population of the lamellar crystals is difficult to observe in TEM, since the lamellar crystals in these thin film samples possess a random, rather than an edge-on, orientation. Detailed TEM imaging for observing these lamellar crystals is discussed.

The structure of nylon 12,5 is characterized by two hydrogen bond directions as are other polyamides derived from glutaric acid

The structure and morphology of lamellar crystals of nylon 12,5 have been investigated by transmission electron microscopy, electron diffraction and X-ray diffraction. The results obtained indicate that this nylon has a structure different from the conventional α, β and γ forms. Its features are similar to those previously reported for nylon 6,5, which has been shown to have its molecules packed in a network of hydrogen bonds with two spatial orientations (Navarro et al., Macromolecules, 28, 8742). Thus nylon 12,5 should be included in a new family of aliphatic polyamides. The structure derives from a particular conformation of the glutaric units and appears to be independent of the number of methylene groups present in the diamine residue. As a function of the method used in order to prepare the sample, a pseudohexagonal form can also be observed. It might be related to the γ form.

Synthesis and structural study of a new biodegradable copolymer of nylon-11 and l-alanine

The alternating copolyamide, poly( l-alanyl-11-aminoundecanoic acid), has been synthesized by the active ester method, and subsequently characterized. The structure and morphology of lamellar crystals have been investigated by using transmission electron microscopy, selected-area electron diffraction and X-ray diffraction. Additional data have been obtained from uniaxially oriented fibres. The deduced unit cell parameters ( a = 4.79, b = 10.35, c = 29.8Å, and α = β = γ = 90°) and the C222 1 space group symmetry indicate a layered structure which is related to that found in poly( l-alanine). A folded conformation is also deduced for the polymethylene segment. Results obtained from enzymatic degradation are also reported.

Model for the Solid-Liquid Phase Transitions (Melting) of Undiluted Random Semicrystalline Copolymers of Vinylidene Chloride with Methyl Acrylate Monomers

A comprehensive pseudoequilibrium melting model for pure random copolymers of vinylidene chloride (VDC) with methyl acrylate (MA) monomers is developed. The model incorporates a composition-dependent surface free energy contribution to the melting point depression of these copolymers. Total exclusion of noncrystallizable copolymer units from the crystal phase is assumed for this copolymer system, and this leads to a thin lamellar crystal morphology. A physical model for these crystals is shown, and a large lateral surface area relative to the volume of the crystal is suggested. The surface to volume ratio increases with increasing copolymer composition. The model developed in this work has an excellent fit to experimental melting point data