Chain Lamellae Research Articles

Folded Chain Lamellae of Dynamic Helical Poly(phenylacetylene) in the Hexagonal Columnar Phase

Folded Chain Lamellae of Dynamic Helical Poly(phenylacetylene) in the Hexagonal Columnar Phase

Extended Chain Lamella Formation Characteristics of Main-Chain Smectic Liquid Crystalline Copolyesters Comprising Different Length Units

A series of liquid crystal (LC) PB-8/12 copolyesters have been synthesized from 4,4′-biphenol with sebacic acid and tetradecanedioic acid, and their LC structures and morphologies have been examined. The copolyesters formed smectic I (SmI) LCs similarly as to the corresponding PB-8 and PB-12 homopolyesters; however, dissimilarity of the comonomer lengths decreased the smectic layer order while sustaining the hexagonal order in the lateral packing of the chains. The SmI LCs consisted of 100-nm-thick lamellae stacked along the polymer chain direction. The lamella thicknesses are more than two times greater than the thicknesses of the homopolyesters and comparable to or greater than the chain contour lengths, indicating the formation of extended chain lamellae.

Crystallization and Melting of Poly(ethylene oxide) in Blends and Diblock Copolymers with Poly(methyl acrylate)

Blends of poly(ethylene oxide) (PEO) and poly(methyl acrylate) (PMA) as well as the respective diblock copolymers PEO-b-PMA form a homogeneous melt and undergo crystallization of PEO upon cooling. Although an identical PEO (Mn = 5000 g/mol) is used in blends and diblock copolymers, crystallization and melting behavior at comparable PMA contents differs strongly as revealed by temperature-resolved small-angle X-ray scattering (TR-SAXS) and differential scanning calorimetry (DSC) measurements. After isothermal crystallization, PEO lamellae in the blends thicken during heating from once-folded to extended chain crystals prior to melting as revealed by TR-SAXS. Contrarily, in PEO-b-PMA a thickening to extended chain lamellae is impossible when the PMA block exceeds an Mn of about 3000 g/mol. This behavior is caused by a balance between the tendency of the crystallizable block to form extended chain crystals, the tendency of the noncrystallizable chains to adopt a maximum in conformational entropy, and the spa...

Shear-induced crystallization in isotactic polypropylene containing ultra-high molecular weight polyethylene oriented precursor domains

Melt blends of short ultra-high molecular weight polyethylene (UHMWPE) fibers and isotactic polypropylene (iPP) were subjected to shear at 145 °C, above the melting point of polyethylene (PE). Structural evolution and final morphology were examined by in situ synchrotron X-ray scattering/diffraction as well as ex situ microbeam X-ray diffraction and high resolution scanning electron microscopy, respectively. Results indicate that the presence of oriented UHMWPE molten domains significantly facilitated the crystallization of iPP and enhanced the initial ‘shish-kebab’ structure leading to the final cylindritic morphology. It is argued that shear flow aligns the fibrillar UHMWPE domains, where the interfacial frictions between PE and iPP effectively retards the relaxation of iPP chains, allowing the aligned iPP chains to create a shish-like structure. Nucleation on the iPP shish initiates the folded chain lamellae (kebabs), which grow perpendicularly to the iPP/PE interface.

Characterization of polyethylene crystallization from an oriented melt by molecular dynamics simulation.

Molecular dynamics is used to characterize the process of crystallization for a united atom model of polyethylene. An oriented melt is produced by uniaxial deformation under constant load, followed by quenching below the melting temperature at zero load. The development of crystallinity is monitored simultaneously using molecular-based order parameters for density, energy, and orientation. For crystallization temperatures ranging from 325 to 375 K, these simulations clearly show the hallmarks of crystal nucleation and growth. We can identify multiple nucleation events, lamellar growth up to the limit imposed by periodic boundaries of the simulation cell, and lamellar thickening. We observe a competition between the rate of nucleation, which results in multiple crystallites, the rate of chain extension, which results in thicker lamellae, and the rate of chain conformational relaxation, which is manifested in lower degrees of residual order in the noncrystalline portion of the simulation. The temperature dependence of lamellar thickness is in accord with experimental data. At the higher temperatures, tilted chain lamellae are observed to form with lamellar interfaces corresponding approximately to the [201] facet, indicative of the influence of interfacial energy.

TIME-RESOLVED SAXS/WAXS STUDY OF PHASE BEHAVIOR AND CRYSTALLIZATION IN POLYMER BLENDS

Real-time SAXS and WAXS patterns have been simultaneously obtained during isothermal melt-crystallization of blends of low-molecular-weight poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA). The analysis of results shows that the originally homogeneous, single-phase polymer blend separates into two phases. The PMMA molecules diffuse from the blend and form completely segregated regions while PEO starts to crystallize. The first and dominating effect at the beginning of crystallization is the formation of unstable lamellae of nonintegrally folded chains (NIF). The real-time crystallinity and density of the PEO crystalline phase in absolute units were obtained from the time-resolved SAXS/WAXS results. The structure development proceeds in two steps. A very fast evolution of PEO crystals from the melt starts to crystallize in disordered NIF lamellae with thick amorphous interlayers and with a lower density of crystalline phase. The steep growth of crystallinity and crystalline density mean quick thickening of crystalline part of lamellae and improvement of their crystalline structure. In the second step, the structure of the crystalline phase gradually improves and crystallinity grows very slowly. The recrystallization of NIF lamellae into extended chain lamellae (EC) and lamellae with once folded chains (1F) proceeds during both stages of crystallization.

Morphology and thermal properties of polyethylenes made by metallocene and Ziegler-Natta catalysts

Morphology and thermal behavior of high density polyethylene and ethylene-octene copolymers prepared with classical Ziegler-Natta (zHDPE) and metallocene catalysts (mHDPE and mLLDPE) are comparatively discussed. Wide angle X-ray diffraction (WAXD) data at room temperature showed that mHDPE crystals are more perfect with smaller unit cell dimensions and have higher crystalline density and larger crystallite sizes when compared to zHDPEs. In the case of copolymers, the unit cell dimensions did not change significatively when the number of branches was increased. Average interplanar chain distances in the amorphous phase increased with increasing octene content in the copolymer, showing a marked influence on the packing of the amorphous phase and confirming that the branches are concentrated in this phase. The existence of second phase in the ethylene-octene copolymers prepared with metallocenes was identified by calorimetry. The existence of this phase is time-dependent and was interpreted as the result of hexyl branches ordering in the amorphous phase. A morphological model based on Vile three-phase model, WAXD results and calculations involving empirical equations for fold and extended chain lamellae crystals was proposed to explain the time-dependent thermal behavior of these copolymers.

Extended chain lamellae self-assembled in liquid crystal phase of soluble polydiacetylenes through chain segregation

This review article highlights our efforts in the past years toward a fundamental understanding of the supramolecular structures of a soluble but rigid polydiacetylene. In our earlier works we presented evidence to indicate that this polymer formed a lyotropic phase in solutions. TEM, AFM and SEM observations on solidified thin films showed an extended chain lamellar structure which is surmised to be self-assembled in the lyotropic phase through chain segregation. Our results revealed that the supramolecular structures formed are closely associated with the structures formed in the lyotropic state. Due to three attributes of rigid macromolecules at the molecular level-high molecular weight, broad molecular weight distribution, and worm-like chain conformation, the supramolecular structures observed are also greatly different from those of conventional nematics or smectics.

Monte-Carlo simulations of polymer crystallization in dilute solution

Polymer crystallization in dilute solution is studied by three-dimensional Monte-Carlo simulations using the bond fluctuation model. We study monodisperse chains of moderate length, intended to model recent experiments on monodisperse alkanes with length of a few hundred carbon atoms, and we also investigate chain folding of very long polymers. For monodisperse flexible chains we observe both extended-chain and once-folded-chain crystals. The simulations illustrate the range of defects and irregularities which we expect to find in polymer crystals. The roughness of the top and bottom surfaces of the lamellae is measured. Chain ends can be seen as cilia emerging from the surfaces. Folds are found to occur with approximately equal frequency on top and bottom surfaces. Although most chain folds are aligned perpendicular to the growth direction, a significant number of chains folding parallel to the growth direction are found as defects. The simulation includes a chain stiffness parameter which has an important effect on chain folding kinetics. When chains are semi-flexible the crystals formed are extremely irregular with many defects including holes and blocks of extended chains within the folded chain lamellae. For very long chains we show that the lamellar thickness is determined by the folding kinetics. The thickness diverges as the temperature approaches the infinite chain melting point T∞. For T→T∞, the thickness is close to the theoretical minimum thickness, which indicates the dominant importance of the entropic barrier in crystallization.

Reversible Melting of Semi-Crystalline Polymers 2. Annealing Near to the Melting Point

Annealing experiments have been carried out at a few degrees below the melting point of different polyethylenes (LDPE, LLDPE, HDPE), of polypropylene (PP) and of Nylon-6. The heat capacities decrease during the annealing, within a 2-4 min time scale, to a lower value which corresponds to the extrapolated heat capacity values obtained for the cooling cycle when the polymer is cooled from the melt. Heat capacities in the heating cycle following the cooling cycle of PP, Nylon-6 and HDPE have the same value as during the cooling section. This is not the case for LDPE and LLDPE. Exothermic total heat flow in the cooling section following the annealing indicates that the crystallisation takes place during the cooling rather than during the annealing period. The total melting enthalpy measured before and after the annealing cycle is the same. The reversing heat flow shows an excellent fit to the change of the crystallinity measured by small angle scattering of synchrotron radiation during a heating cycle at temperatures below the melting peak. A coupled thermodynamic interaction of the crystalline and the amorphous phases is concluded from this study. This kind of interaction is possible at the lateral end of polymeric chains incorporated into the crystalline phase. This is an indication of the portion of tie molecules in the system, i.e. the portion of fringed micelle type of crystalline morphology with respect to that of folded chain lamellae.

Structure and Phase Transitions of Poly(diethylsiloxane)

Phase transitions and morphology of poly(diethylsiloxane) have been studied with help of differential scanning calorimetry and transmission electron microscopy on carbon−platinum replicas of freeze-fractured samples. The TEM experiments showed distinct lamellae in the case of the β and γ crystalline polymorphs, whose thicknesses correlated well with the calculated chain lengths, demonstrating an extended chain conformation for both polymorphs. A low heating rate dependence of the β → μ transition temperature in DSC and a linear relationship between isotropization temperature and reciprocal molecular weight indicated the existence of extended chain lamellae for the mesophase as well. The α crystalline polymorph displayed a greater softness than β- and γ-PDES, and no lamellar structures could be observed. DSC experiments on bimodal mixtures of PDES samples with strongly different molecular weights demonstrated molecular weight dependent phase separation in the case of β-PDES and the occurrence of mixed lame...

On the γ-Phase of Isotactic Polypropylene

The crystalline morphology of a relatively high molecular weight isotactic polypropylene (i-PP) (Mw = 18000 g/mol) with well-defined regio- and stereoirregularities is studied by means of atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and light microscopy. This i-PP crystallizes under certain crystallization conditions nearly exclusively in the γ-modification. The growth of the γ-phase is often influenced by the initial growth of the α-phase, but also the neat γ-phase is observable. The formation of the γ-phase can be related to stereo- and regioirregularities in the polymer chain caused by the polymerization mechanism using metallocene catalysts. WAXS measurements show that at large supercoolings the formation of α-phase i-PP is preferred, whereas at low supercoolings this sample crystallizes nearly exclusively in the γ-modification. Light microscopy and AFM reveal for the γ-phase i-PP morphologies different from the well-known spherulites formed by the α- or β-modification. However, it can be shown that the morphology development of γ-phase i-PP can be widely influenced by very small amounts of α-phase i-PP. A lamella thickness of about 4 nm and a long period of 12 nm of γ-phase i-PP isothermally crystallized at 100 °C are obtained from SAXS measurements. At very low supercoolings, supermolecular structures of the neat γ-modification are formed in competition with mixed (α and γ) morphologies. In thin films the neat γ-modification shows network structures, which are formed by triangular entities. Etched bulk samples reveal columns with a rectangular cross-section which are densely packed. On the molecular scale, the surface topography of the flat-on extended chain lamellae with periodic distances of rows of methyl groups of 3.6 Å is measured. This value is in excellent agreement with the distance of the methyl group rows on the crystallographic (010) plane according to the nonparallel chain packing model by Brückner and Meille.

Folded Chain Lamellar Crystal Self-Decoration of Extended Chain Crystals of a Random Terpolymer Main Chain Liquid Crystal Polymer

Abstract Surface melting of extended chain crystals of a flexible segment containing liquid crystal polymer followed by quenching is shown to result in the epitaxial crystallization thereon of folded chain lamellae, i.e. self decoration. It is proposed that the initial extended chain lamellae form by chain folding in the mesomorphic state followed by rapid chain extension by the “chain sliding mechanism”, the same mechanism that has been in the literature for the proposed similar crystallization of polyethylene under anabaric conditions.

Morphology of a thermotropic random terpolymer liquid crystal polymer crystallized in the bulk: compression mouldings, extrudates and fibres

The morphology of extruded pellets, compression and injection moulded articles, and melt spun fibres of a random liquid crystal terpolymer has been investigated. Etching and self-decoration techniques have been used, which reveal a lamellar morphology in bulk samples. The lamellae, ranging from 50 to 1800 Å in thickness depending on the preparation technique, are oriented perpendicular to the fibre or extrusion direction in melt spun fibres and extruded pellets. Extruded pellets are analogous to a composite material with 0.2–1.8 μm diameter ‘fibres’ of the liquid crystal being embedded in a less oriented matrix; greater alignment of the lamellae is observed in the ‘fibres’. The thickness of the lamellae observed in melt spun fibres ranged from 500 to 800 A. Thinner lamellae (50–130 Ǎ) are observed on the surface of compression moulded specimens with thicker (200–1800 Å), presumably extended chain lamellae in the interior. The thickness and degree of alignment, or order, of the lamellae is dependent on the cooling rate. Selected area electron diffraction confirms that the chain axes are perpendicular to the lamellae.

Folded chain crystallization of thermotropic, random terpolymer, main chain liquid crystal polymers

AbstractSingle crystals, suitable for electron microscopy and diffraction, have been grown from the quiescent and sheared “liquid crystal” state and solution for several random terpolymer polyesters having a nearly equimolar amount of a flexible segment dicarboxylate (4–7 CH2), oxybenzoate and dioxyphenyl. In all cases, in thin films and on free surfaces, folded chain lamellae are observed. High temperature electron diffraction has permitted characterization of the transition behavior and structure of the liquid crystal state. Permanganate etching permitted characterization of the interior of molded, extruded or spun samples; in these cases extended chain lamellae are observed. The results suggest chain folding in the nematic state at least in the thin films and on surfaces, with chain folding also possible in the bulk followed by chain extension by “sliding” diffusion.

Regular, adjacent reentry folding in single crystals of a liquid crystal polymer crystallized from the nematic state

AbstractFolded chain single crystals 35 Å thick have been grown from the liquid crystal state of an aromatic‐aliphatic azomethine ether polymer (AZMEP‐n) having a 10‐carbon flexible segment (n = 10). Electron diffraction has permitted refinement of the triclinic unit cell. The molecular axes lie at an ca. 65° angle to the lamella normal and fold every third chemical repeat distance. For AZMEP‐1 and ‐8 extended chain lamellae are formed; for AZMEP‐7 both folded and extended chain lamellae are found. The observations of folded chain lamallae are in agreement with prior suggestions from our laboratory of chain folding in the liquid crystalline state in thin films. © 1992 John Wiley & Sons, Inc.

Cross-linking of porous gel-spun ultra-high molecular weight polyethylene by means of electron beam irradiation

Porous gel-spun ultra-high molecular weight polyethylene (UHMWPE) fibres, consisting of large folded chain lamellae linked to each other by means of tie molecules, have been Irradiated at room temperature by means of high energy electrons.

Influence of draw ratio on morphological and structural changes in hot-drawing of UHMW polyethylene fibres as revealed by DSC

The melting behaviour of gel-spun, ultra-high molecular weight polyethylene fibres was investigated in an attempt to characterize their morphology after various stages of hot-drawing at 148 ‡C. In this drawing process a shish-kebab morphology is transformed into a smooth fibrillar structure. It was concluded that this transition initially proceeds by pulling elastically inactive loops, originally present in the folded chain lamellae of the shish-kebabs, taut between entanglements. Thereafter a considerable amount of entanglements is removed by pulling molecular chain ends through them, until ca. 2.5 entangelements per molecule remain in the ultimately drawn fibres. The fibrils in the fully drawn fibres were found to be composed of chain-extended orthorhombic crystallites with an average length of 70 nm, which are interrupted by defect regions (containing trapped entanglements and chain ends) of about 4 nm in length. If free shrinkage of the fibre was allowed, this structure had an equilibrium melting temperature of 140.5 ‡C. Upon constrained melting of the filaments, a solid-solid phase transition could be observed in the DSC thermograms (at a temperature of ca. 150 ‡C), associated with a transition of the chain-extended orthorhombic blocks in the fibrils into a hexagonal phase. The heat effect associated with this solid-solid transition in perfectly crystalline polyethylene (δH o−h ) was estimated to be 205 kJ/kg, whereas for the heat of fusion of the hexagonal phase (δH h−m ) a value of 81 kJ/kg was assessed.

Dislocation melting inn-paraffin homologs

A formula for the melting temperature of $n$-paraffin homologs as a function of polymer chain length is derived in a simple model of dislocation catastrophe. The salient features of the melting curve can be understood as a consequence of the gradual transition with increasing chain length from a melting process dominated by dislocations which bridge the paraffin chain lamellae, to one in which nonbridging loops play the principal role.

The role of inter‐ and intra‐links in the transformation of folded chain lamellae into microfibrils

AbstractIn this study we investigate the effect of crystallization rate on the extensibility (draw ratio) of spherulitic polyethylene film uniaxially deformed at room temperature. Draw ratios for deformed films increased with increasing crystallization temperature. Transmission electron microscopy of the microfibrillar structures revealed and interconnected network of fibrils. The degree of connectivity between microfibrils was found to decrease with increasing‐crystallization temperature. A model of the deformation process is developed in which inter‐ and intra‐link (extended chain fibers formed during crystallization) distribute the applied stress, initiate the micronecking process and influence the amount of chain unfolding during drawing.