Multiple melting and partial miscibility of ethylene‐vinyl acetate copolymer/low density polyethylene blends

Multiple melting behaviors and partial miscibility of ethylene‐vinyl acetate (EVA) copolymer/low density polyethylene (LDPE) binary blend via isothermal crystallization are investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). Crystallization temperature T (°C) is designed as 30, 50, 70, 80°C with different crystallization times t (min) of 10, 30, 60, 300, 600 min. The increase of crystallization temperature and time can facilitate the growth in lateral crystal size, and also the shift of melting peak, which means the completion of defective secondary crystallization. For blends of various fractions, sequence distribution of ethylene segments results in complex multiple melting behaviors during isothermal crystallization process. Overlapping endothermic peaks and drops of equilibrium melting points of LDPE component extrapolated from Hoffman–Weeks plots clarify the existence of partial miscibility in crystalline region between EVA and LDPE. WAXD results show that variables have no perceptible influence on the predominant existence of orthorhombic crystalline phase structure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Melt crystallization of syndiotactic polypropylene in nanolayer confinement impacting structure

Layer multiplying coextrusion technique was used to fabricate films with hundreds of alternating layers of a crystallizable polymer, syndiotactic polypropylene (sPP), and an amorphous polymer, polycarbonate (PC). Atomic force microscopy and wide-angle X-ray scattering revealed the absence of any oriented crystal morphology of sPP in the extruded layered films. An approach of isothermal melt recrystallization of sPP nanolayers revealed the formation of oriented lamellae under the rigid confinement of hard glassy PC layers. X-ray scattering data showed that sPP crystallized as stacks of single crystal lamellae oriented parallel to the layers at high crystallization temperatures. As the crystallization temperature decreased, on-edge lamellar orientation was preferred. Formation of in-plane lamellae was attributed to heterogeneous bulk nucleation, while nucleation of on-edge lamellae was initiated at substrate interface. It was observed that as the layers thickness reduced, the orientations of both in-plane and on-edge lamellae became sharper.Detailed analysis of crystal orientations in 30 and 120 nm sPP layers was carried out. Melt recrystallization of 30 nm layers revealed formation of in-plane lamellae above 90 °C and mainly on-edge lamellae below 70 °C. At intermediate temperatures, formation of mixed crystals was reported. In 120 nm layers, crystallization temperature of 100 °C was required to form in-plane crystals, while on-edge lamellae were formed below 90 °C.We also investigated crystallization onset for on-edge and in-plane lamellar nucleation. Although, the two crystal fractions were significantly affected as a function of crystallization temperature, it was noticeable that both crystal habits were initiated at the same time. The results suggested that the relative growth rates of in-plane and on-edge crystal orientations was responsible for different fractions of the two crystal orientations at a given crystallization temperature.Oxygen transport properties of melt recrystallized sPP layers were measured. When the melt recrystallization temperature increased from 85 to 105 °C in 120 nm sPP layers, at least one order of magnitude enhancement in the barrier properties was observed. It was evident from the X-ray data that the amount of in-plane crystal fraction increased with increasing crystallization temperature. In-plane crystals acted as impermeable platelets to oxygen flux resulting in improved gas barrier properties. A similar effect was observed in 30 nm sPP layers over a temperature range of 60-105 °C. A correlation between in-plane crystal fraction and the oxygen permeability was obtained from X-ray and oxygen transport data analysis. It was shown that the permeability decreased exponentially with increasing in-plane crystal fraction.     

Confined crystallization of nanolayered poly(ethylene terephthalate) using X-ray diffraction methods

The development of crystalline lamellae in ultra-thin layers of poly(ethylene terephthalate) PET confined between polycarbonate (PC) layers in an alternating assembly is investigated as a function of layer thickness by means of X-ray diffraction methods. Isothermal crystallization from the glassy state is in-situ followed by means of small-angle X-ray diffraction. It is found that the reduced size of the PET layers influences the lamellar nanostructure and induces a preferential lamellar orientation. Two lamellar populations, flat-on and edge-on, are found to coexist in a wide range of crystallization temperatures (T_c = 117–150 °C) and within layer thicknesses down to 35 nm. Flat-on lamellae appear at a reduced crystallization rate with respect to bulk PET giving rise to crystals of similar dimensions separated by larger amorphous regions. In addition, a narrower distribution of lamellar orientations develops when the layer thickness is reduced or the crystallization temperature is raised. In case of edge-on lamellae, crystallization conditions also influence the development of lamellar orientation; however, the latter is little affected by the reduced size of the layers. Results suggest that flat-on lamellae arise as a consequence of spatial confinement and edge-on lamellae could be generated due to the interactions with the PC interface.     

In situ WAXD study of structure changes during uniaxial deformation of ethylene-based semicrystalline ethylene-propylene copolymer

In situ strain-induced structure changes during uniaxial deformation of an ethylene-propylene copolymer, containing 78 wt% (or 85 mol%) of ethylene moiety, were studied by synchrotron wide-angle X-ray diffraction (WAXD). The chosen sample could crystallize into orthorhombic, pseudo hexagonal or a mixed form, depending on the annealing conditions. Crystallization at high temperatures (e.g. 50 °C) favored the formation of orthorhombic form, while crystallization at low temperatures (e.g. 20 °C) favored the formation of pseudo hexagonal form. Under deformation, the transition from orthorhombic to pseudo hexagonal form was observed at relatively low strains (e.g. 0.12). At higher strains, WAXD data indicated the occurrence of direction-dependent crystal destruction at strains <0.25 and subsequent re-crystallization with extended-chain conformation at high strains (>1.0) all of the pseudo hexagonal form. The drastic changes in the crystalline structures (orthorhombic to pseudo hexagonal) and phase transitions (crystal destruction and re-crystallization) at modest strains can be attributed to the high mobility of the amorphous ethylene-propylene segments at room temperature.     

Hierarchical Growth of ZnO Particles by a Hydrothermal Route

The crystallization of ZnO microrods by hydrothermal treatment of a suspension formed from reaction of zinc acetate and sodium hydroxide has been examined using scanning and transmission electron microscopy. Polycrystalline hexagonal ZnO microrods first appeared after 0.5 h reaction time at 120°C. These early stage rods were composed of stacks of hexagonal layers, each ~50 nm in thickness containing closely aligned assemblies of nanocrystallites <20 nm in size. Further growth of the microrods involved columns of nanoparticles extending from the basal layers of the preformed hexagonal stacks. Re‐crystallization produced single‐crystal microrods, many of which existed as twin particles.     

Crystallization of polylactide films: An atomic force microscopy study of the effects of temperature and blending

Surface crystallinity on films of poly(l-lactide), poly(l/d-lactide) and their blends with poly(d-lactide) was studied. The isothermal spherulitic growth rate and its dependence on temperature were studied using tapping mode atomic force microscopy and ex situ isothermal crystallization. Using this technique, it is possible to extend spherulitic growth rate measurements to the region of significantly higher supercooling where nucleation concentration makes the use of in situ hot stage optical microscopy impossible. It was confirmed that while a poly(l/d-lactide) copolymer exhibits the typical “bell” shaped crystallization rate–temperature dependence, poly(l-lactide) exhibits a nonsymmetrical behavior having two crystallization rate maxima at 105 °C and 130 °C. As expected, the spherulitic growth rate of poly(l-lactide) was significantly higher than that of poly(l/d-lactide). The different types of crystalline formations exhibited at the surface of polylactide films are shown and discussed. The crystalline long spacing of poly(l-lactide) was also measured directly using tapping mode AFM and was found to be 19 nm at 165–170 °C. At low supercooling, several different scenarios of individual crystal formation were observed: purely flat-on stacks, purely edge-on stacks and scenarios where edge-on crystals flip to flat-on crystals and vice versa, where flat-on crystals yield edge-on sprouts. The preferred direction of growth of lamellae of both poly(l-lactide) and poly(d-lactide) was found to be counter-clockwise relative to the free surface.Finally, the crystallization kinetics of blends of poly(l-lactide) and poly(l/d-lactide) with poly(d-lactide) were studied. In such blends a triclinic stereocomplex crystalline structure forms between chains of opposite chirality and a pseudo-orthorhombic α-crystal structure forms between chains of like chirality. The presence of the stereocomplex crystals affects both the nucleation and the growth of the α-crystals. In fact depending on the stereocomplex content and the crystallization temperature the α-crystallization can either be enhanced or be inhibited. Interestingly it was found that the presence of the stereocomplex had a much stronger effect on the α-crystallization of poly(l/d-lactide) than on the α-crystallization of poly(l-lactide).     

Investigation of UV aging influences on the crystallization of ethylene-vinyl acetate copolymer via successive self-nucleation and annealing treatment

Influences of UV aging on the crystallization of ethylene-vinyl acetate copolymer (EVA) were researched via successive self-nucleation and annealing (SSA) treatment. During the aging process, the polar vinyl acetate (VAc) units in the amorphous region were the most vulnerable structure. FTIR results demonstrated that, VAc units were initially attacked by the UV radiation, which further resulted in chain scission of molecules. Degradation expanded from the amorphous region to the crystal region gradually. Chain scission reaction freed crystallizable ethylene sequences from inter-/intra-molecular tanglement and confinement of neighboring VAc units. Although the crystallinity decreased after aging, newly freed crystallizable sequences preferentially arranged into more densely packed lamellae, leaving less residual fraction to arrange into loosely packed crystal region. WAXD patterns showed that the predominant orthorhombic crystal phase did not vary during aging. Simultaneously, re-arrangement in crystallization also resulted in the growth in lateral crystal size of EVA.     

In-situ synchrotron x-ray scattering study on isothermal crystallization of ethylene-vinyl acetate copolymers containing a high weight fraction of carbon nanotubes and graphene nanosheets

The effect of two dimensional graphene nanosheets (2D GNSs) and one dimensional carbon nanotubes (1D CNTs) on the isothermal crystallization of an ethylene–vinyl acetate (EVA) copolymer at a high loading (5 wt.%) was studied by in-situ synchrotron wide-angle X-ray diffraction. The morphology observations indicated a homogeneous dispersion of GNSs and CNTs in EVA which formed a constrained environment (3D network or 2D layered structure) for ethylene–vinyl acetate EVA crystallization, respectively. The results of wide-angle X-ray diffraction showed a high weight fraction of nano-fillers slowed the crystallization rate of composites. The crystallization behavior of EVA was confined by 2D planar GNSs more seriously than 1D tubular CNTs owing to a more completed and closed layered GNS network.     

Study of morphology and crystal growth behaviour of nanoclay-containing biodegradable polymer blend thin films using atomic force microscopy

Thin films of unmodified and nanoclay-modified polylactide/poly(butylene succinate) (PLA/PBS) blends were prepared on a glass substrate with a spin coater. The morphology and crystal growth behaviours for the films, crystallized at different temperatures, were visualized with atomic force microscope (AFM). AFM images showed that the size of the dispersed PBS phase was reduced on the addition of 2 wt% clay to the PLA/PBS blend, and the size of the dispersed phase increases with the further addition of clay. Transmission electron microscopy studies indicated that this behaviour was due to the preferential location of silicates in the PBS phase than in the PLA phase. A similar effect of clay to the blend thin films on the dispersed phase and the crystalline morphology were observed when annealed at 60 °C and 120 °C. However, at 60 °C the addition of clay to the blend quenched the growth of edge-on lamellae. The crystalline morphologies at 120 °C were dominated by edge-on lamellae grown around the PBS phase to form spherulites. Morphologies of thin films crystallized at 120 °C from melt were dominated by the flat-on lamellae, while the ones crystallized at 70 °C from melt were dominated by the edge-on lamellae. The degree of clay silicate dispersion in the blend matrix was characterized by X-ray diffraction. These results show how the crystallization temperatures and the addition of the clay particles influence the morphology of the thin films.     

Polymorphism and side group location of ethylene copolymers characterized by FTIR and NMR spectroscopy

Fourier transform infrared spectroscopy (FTIR) was used to characterize the polymorphism, crystalline phase transformation, and side group location of ethylene copolymers (poly(ethylene-co-vinyl acetate), ab. EVA and poly(ethylene-co-vinyl alcohol), ab. EVOH). Sample films were prepared through a melt-pressing method, and submitted to cryogenic FTIR spectra measurement. Besides the occurrence of a normally orthorhombic crystalline phase (OCP), a monoclinic crystalline phase (MCP) was also detected for EVA copolymers with a relatively higher comonomer content (>14 wt%). The annealing temperatures greatly influence the transformation from MCP to OCP. As for EVOH copolymers, only thermodynamically stable OCP was found to exist in the crystalline region, and the hydroxyl groups were proved to enter the crystalline region of ethylene segments, while the side groups of EVA exist predominantly in the amorphous region.     

The observation of rapid surface growth during the crystallization of polyhydroxybutyrate

A two stage growth during isothermal cold crystallization of PHB has been observed in the temperature range 5-40 °C when a free surface is present. This growth has been investigated with optical and atomic force microscopy both in-situ and ex-situ. Initially, crystal growth is observed to be composed of lamellae oriented approximately flat-on relative to the free surface. At later stages of growth there can be a change to a distinctly different form of crystal growth that is composed of edge-on lamellae which grow at a substantially higher crystallization rate. This change in growth rate at constant temperature gives rise to curved interfaces between the slower growing flat-on growth and the faster growing edge-on growth. Several possible explanations for this change in growth rate are put forward.     

Structural and Kinetic Analysis of BaCl2 Nanocrystals in Fluorochlorozirconate Glass‐Ceramics

The presence of BaCl₂ nanocrystals and the crystallographic phase that they adopt controls the optical behavior of fluorochlorozirconate glass‐ceramics. We have used in situ X‐ray diffraction heating experiments and ex situ transmission electron microscopy to follow the BaCl₂ nanocrystal nucleation and growth processes as a function of heating rate and isothermal hold temperature. The BaCl₂ nanocrystals nucleate with the hexagonal crystal structure and grow as spherical particles to a size of ~10 to 20 nm. They then undergo a structural transformation to the orthorhombic phase and their shape changes to rounded disks, with diameters ranging from 150 to 250 nm, and thicknesses ranging from 80 to 120 nm. The change in size results from Ostwald ripening of the hexagonal BaCl₂ nanocrystals to form the orthorhombic BaCl₂ nanocrystals.     

Influence of processing condition and HA content on the crystallization behavior of HA‐filled EVA biocomposite

Hydroxyapatite/ethylene‐vinyl acetate (HA/EVA) composites with a HA content of 30 and 50 wt% were prepared by injection molding. The crystallization behaviors of EVA under different injection pressure, annealing temperature, and HA content were investigated. Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscope were used to evaluate the composites. The result of FTIR analysis infers the occurrence of hydrogen bonding between HA and EVA. XRD and DSC analyses show that the increasing injection pressure can accelerate the crystallization rate of EVA but it tends to decrease the crystallization degree slightly, which may be caused by the increase of EVA segmental activity and the loss of EVA crystallization order with the increase of pressure. The EVA crystallization degree can be improved by the annealing process. It is found that HA can induce more nucleation sites of EVA, but the crystallization degree of EVA decreases with the increase of HA content. The large content of HA acts to reduce the mobility of EVA crystallizable chain segments and inhibits the crystal growth of EVA. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Cellulose nanofibrils as templates for the design of poly(l-lactide)-nucleating surfaces

The abilities of surface-grafted cellulose nanofibrils for the nucleation of poly(l-lactide) (PLLA) were investigated. Cellulose nanofibrils with a width of ∼3 nm were obtained from wood cellulose via the oxidation using 2,2,6,6-tetramethylpiperidiniyl-1-oxyl as a catalyst and successive mechanical treatment. The cellulose nanofibril surfaces were selectively modified with amine-terminated poly(ethylene glycol) chains, via simple ionic bonds. The PEG-grafted cellulose nanofibril/PLLA composite films were prepared using a solvent casting method with chloroform. The isothermal and non-isothermal crystallization kinetics of the PLLA in the composites was studied using differential scanning calorimetry and polarized optical microscopy. The PEG chains were densely immobilized on the surface of the cellulose nanofibril templates, which had extraordinarily large specific surface areas. As a result, the surface-PEG layers effectively increased the rate of crystallization of the PLLA in the composites. Because of the increased degree of crystallinity after the isothermal crystallization, the composites showed better heat resistance than neat PLLA.     

Investigation of nonisothermal crystallization of hydroxyapatite/ethylene‐vinyl acetate (HA/EVA) composite

Hydroxyapatite/ethylene‐vinyl acetate (HA/EVA) composites were prepared by injection molding and characterized by X‐ray diffraction (XRD) and attenuated total multiple reflection infrared (ATR‐IR) spectroscopy. The nonisothermal crystallization behavior of HA/EVA composites at different cooling rates and with different HA content were examined by differential scanning calorimetry (DSC). The results exhibit the occurrence of interaction between HA and EVA, and the HA particles in EVA matrix act as effective nucleation agent. The addition of HA influences the mechanism of nucleation and growth of EVA crystallites. HA particles, as nucleus, are efficient to promote EVA crystallization at early stage but prevent EVA crystal growth in the late stage. The EVA crystallization in the composite is mainly through heterogeneous nucleation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Fabrication and characterization of HCl‐treated clinoptilolite filled ethylene vinyl acetate composite films

As received and HCl treated Clinoptilolite (C)‐ethylene vinyl acetate (EVA) composites were prepared via the melt‐mixing technique, and extruded through a single‐screw extruder to obtain composite strips with an average thickness of 0.5 mm. The films were then characterized for their morphological, structural, thermal, and mechanical properties. Optical micrographs show that at higher C loading, the particles form large agglomerates, resulting in the formation of voids on the surface of the films. With increasing zeolite loading, the films become brittle, resulting in reduced Young's modulus. Acid treatment of the C tends to affect the crystal structure of the zeolite, resulting in poor tensile properties of the HCl‐treated zeolite‐filled EVA films. Addition of the zeolite also increased the crystallinity of the structure, acting as a nucleating agent in the EVA crystallization. Modeling of the tensile yield data with Pukanszky model indicate that there is poor interfacial adhesion between the polymer matrix and the filler particles. Thermal characterization studies showed that addition of the zeolites retarded the onset degradation temperature of EVA. However, degradation temperatures including T_max and the final decomposed temperature were increased, suggesting improved thermal stability due to reduced inter‐chain mobility in the composite materials as a result of increased zeolite loading. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Selective area synthesis of aligned carbon nanofibers by laser-assisted catalytic chemical vapor deposition

Arrays of freestanding bamboo-type carbon nanofibers were grown on the surface of a porous alumina substrate by laser-assisted catalytic chemical vapor deposition. A continuous wave argon ion laser operated at a wavelength of 488 nm was used to thermally decompose pure ethylene over nickel catalysts. Two different catalyst preparation methods were used and are compared with respect to the synthesis of aligned nanofibers. First, a thin nickel film (50 nm) was evaporated on the substrate and was subsequently laser annealed into nanoparticles. This preparation produced non-aligned nanofiber films. Second, a 50 nm thick catalyst layer was electrochemically deposited within the pores of an alumina substrate. This preparation produced an array of vertically aligned nanofibers. A growth rate dependence on radial position within the irradiated area was observed. Average linear growth rates ranging from 554 nm/s to 25 μm/s are reported. The nanofibers were examined by scanning electron microscopy and Raman spectroscopy. Fiber texture and nanotexture were determined by lattice fringe analysis from high resolution transmission electron microscopy images. The alignment mechanism is also discussed.     

Micro-phase separation and crystallization behavior of amorphous oriented PLLA/PVAc blends during heat treatment under strain

Amorphous oriented poly(l-lactide) (PLLA)/poly(vinyl acetate) (PVAc) 50/50 films were prepared by uniaxial drawing of melt-mixed blends at 65 °C. The morphology development and crystal organization of the blends during heat treatment under strain were investigated using small angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Equatorial scattering maxima in the SAXS patterns for samples annealed at 75 °C were observed before the appearance of crystal reflections. Further annealing of the samples at higher temperature induced two further discrete meridian scattering maxima. The observations indicated that homogenous oriented PLLA/PVAc film undergoes micro-phase separation first, followed by crystallization of PLLA in the PLLA-rich phase. The micro-phase separated PVAc nanodomains are aligned parallel to the stretching direction, whereas the crystallized PLLA lamellae are oriented perpendicular to the stretching direction (crystal c-axis along the stretching direction). Micro-phase separation was not observed when films were annealed at 120 °C, at which temperature the high crystallization rate of PLLA overwhelmed the micro-phase separation process.     

Time resolved WAXS/SAXS observations of crystallisation in oriented melts of ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been drawn in the melt state at 140, 145 and 150 °C at extension rates ∼1 s⁻¹ while simultaneously recording two dimensional SAXS and WAXS with a time resolution of 0.1 s. The first observable crystallisation is mainly in the orthorhombic form at a level of about ∼1 wt%. At higher draw ratios additional crystallisation is in the hexagonal form up to ∼10 wt%. The crystallisation is accompanied by strong SAXS equatorial scatter with maxima at ∼25 nm period; in some cases meridional maxima are also visible at ∼120 nm. Substantial crystallisation occurs on subsequent cooling to 130 °C, accompanied by strong meridional maxima of narrow lateral width. The observed crystal forms are consistent with a temperature-strain phase diagram, favouring hexagonal at higher strains. There are indications that the thermodynamic orthorhombic to hexagonal transition T_tr is above 150 °C so that all the observable hexagonal structures are metastable. The initial orthorhombic crystals are associated with the high molecular weight tail and provide the strain hardening to enable the formation of subsequent hexagonal crystals. The equatorial SAXS lobes are interpreted in terms of lateral density fluctuations that are associated with an arrangement of columns of oriented chains comprising both orthorhombic and hexagonal structures. The columns are embryonic shish structures that on cooling nucleate kebab overgrowths.