Thermally induced conformational and structural disordering in polyethylene crystal studied by near-infrared spectroscopy

Thermally induced structural and conformational changes in polyethylene (PE) samples were explored by using near-infrared (NIR) spectroscopy. The differences in the temperature-dependent structural disordering process among six PE samples were depicted by monitoring the intensities of NIR bands characteristic of orthorhombic crystalline phase. The temperature dependency of bands in the NIR region that have been considered to be due to orthorhombic crystalline lattice was compared to that of a band at 1378 cm⁻¹ due to the methyl symmetric bending mode. The intensity decrease of the band in the mid-infrared (MIR) region seems to sensitively reflect the overall disordering of orthorhombic crystalline structure. As a result of this study, the intensity decrease of the bands in the NIR spectral region was found to proceed at lower temperature than that of the band at 1378 cm⁻¹. This finding suggests the status of orthorhombic crystalline structure probed by the intensity of the band at 1378 cm⁻¹ and that by the “crystalline” bands in the NIR spectral region may not be identical. The NIR spectra were further analyzed by two-dimensional (2D) correlation spectroscopy to provide the in-depth analysis of NIR bands. The 2D correlation spectroscopy has detected the presence of two NIR bands at 4342 and 4290 cm⁻¹ due to orthorhombic crystalline phase and those at 5840 and 5640 cm⁻¹ due to amorphous phase. The hetero-spectral 2D correlation analysis was carried out between the NIR spectral region of 4365-4240 cm⁻¹ and the well-established MIR spectral region for CH₂ wagging deformation region of 1390-1240 cm⁻¹, where bands due to nonplanar conformer are detected. This approach allowed us to determine NIR bands, which behave in a way similar to MIR bands originating from conformational defect sequences that exist in the orthorhombic crystalline lattice, the amorphous domain and the chain fold regions. As a result of the hetero-spectral 2D NIR-MIR correlation spectroscopic studies on the development of conformational defect sequence in three types of PE samples, it was concluded that the intensity of a band at 4265 cm⁻¹ changes in the same manner as the MIR bands at 1368, 1353 and 1308 cm⁻¹ assignable to gtg, gg and gtg′ (kink) conformations. This finding means that the state of conformational disorder in PE crystal can be studied by monitoring the intensity of the NIR band at 4265 cm⁻¹. The use of NIR spectroscopy makes it possible to directly probe the degree in the formation of conformational defect sequences in thick PE products typically produced in industry, which cannot be studied by MIR spectroscopy. This paper thus provides in-depth fundamental understandings on NIR spectra of PE as well as the results of our study regarding structural and conformational changes in PE crystals probed by NIR spectroscopy.     

Solvent-free slack wax de-oiling—Physical limits

In order to get more knowledge about the fundamentals of solvent-free slack wax de-oiling by melt crystallization, investigations on multicomponent paraffin waxes have been performed, which included X-ray powder diffraction and optical microscopy. The tested neutral oil and medium slack waxes consisted mainly of n- and iso-paraffins with different chain lengths. The results suggest that only n-paraffins formed orthorhombic crystals at room temperature with c-axes corresponding to the average chain length of the paraffin waxes. The crystals were surrounded by less crystalline, rather amorphous, solid phase containing mainly iso-paraffins. The typical solid–solid transition of crystal lattice in neutral oil slack waxes from orthorhombic to hexagonal system at temperatures below melting point increases the mobility of the low melting components. This is the key for the applicability of the melt crystallization process. Investigations on sweating suggest a strong influence of the n-paraffins content on mechanical stability of crystal structures and on the separation efficiency of sweating.     

Polymer nanofibers prepared by template melt extrusion

Combining the template method with the extrusion technology, polymer nanofibers have been prepared when molten polymer is forced through the pores of anodic aluminum membrane and cooled to complete the nanofiber formation process. The microstructures of nanofibers are determined by SEM, TEM, XRD, and DSC. The results suggest that the PE nanofibers consist of extended‐chain crystals and the transition from an orthorhombic to a hexagonal phase and the latter phase melting occurs at 159.8°C. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1018–1023, 2006     

A single crystal of polyethylene crystallized under high pressure

A single crystal of polyethylene about 2 μm thick has for the first time been obtained through high pressure crystallization and studied by X-ray diffraction and scanning electron microscopy. A band of a few μm thickness observed on the fractured surface of high pressure crystallized polyethylene is identified as a single crystal at least several tens of μm wide. Furthermore, a bulk composed of several bands stacked approximately parallel gives hexagonal symmetry in an X-ray oscillation photograph, the oscillation axis being parallel to the end surface of the bands. On the basis of these observations, it is concluded that a thick single crystal in the hexagonal phase splits into several bands on the transition from the hexagonal to the orthorhombic phase; 〈110〉 or 〈010〉 of each band in the orthorhombic phase corresponds to one of three possible a-axes in the hexagonal phase.     

Phase transition at a temperature immediately below the melting point of poly(vinylidene fluoride) from I: A proposition for the ferroelectric Curie point

A phase transition at a temperature immediately below the melting point of poly(vinylidene fluoride) form I has been found by means of differential scanning calorimetry (d.s.c.) and infra-red (i.r.) vibrational spectroscopy. An endothermic d.s.c. shoulder has been observed at a temperature about 10°C below the melting point, in the vicinity of which the i.r. crystalline trans bands decrease in intensity steeply and the crystalline gauche bands increase in intensity, indicating the conformational change from all-trans to T₃GT₃G type. These observations have been found to be detectable more clearly for samples subjected to the poling treatment under a d.c. high voltage. The transition shows the characteristic behaviour essentially identical to those observed for ferroelectric copolymers of vinylidene fluoride and trifluoroethylene, except for the irreversibility of the structural change, suggesting that the phase transformation revealed here may be a ferroelectric-to-paraelectric phase transition of polar form I crystal and the the Curie point may be about 172°C. It is consistent with Micheron's measurement of the temperature dependence of the dielectric constant. Other structural changes in the form I sample occurring in the temperature range from 20° to 170°C have also been discussed based on the i.r. spectral measurements.     

Crystal structure transformations and orientation in crosslinked elastic fibers of an ethylene‐octene copolymer in response to deformation and heat treatment

Crosslinked elastic fibers, made from a low density (0.875 g/cc) ethylene‐octene copolymer, were studied after constrained at 300% elongation and annealed at different temperatures (40–80°C) to simulate conditions encountered in yarn and textile processing. It is surprisingly found that the transition from pseudo hexagonal to orthorhombic structure is much faster under simultaneously constraining and annealing than that without strain. Almost a neat orthorhombic structure can be produced when the fiber is annealed at 60°C. Annealing above 60°C leads to mixed orthorhombic and pseudo‐hexagonal structures. The average melting point increases with an increase in the fraction of orthorhombic phase. It is also surprisingly noted that the simultaneously constraining and annealing of the fiber can produce highly oriented crystals, even annealed at 80°C (above the average melting point of 65°C). The unique effect of annealing under large strain can be attributed to the crosslinking of the fiber, which makes it possible for the fiber to have strong chain orientation (even in molten state) under large strain. The strong chain orientation in melt leads to a faster structural transition from pseudo hexagonal to more stable orthorhombic structure. The strong chain orientation is also very likely the reason why highly oriented crystal and amorphous phases are formed, including the case where the fiber is annealed above melting point. These findings could be leveraged for improving thermal and mechanical properties of the fabrics made with such fibers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3565–3573, 2013     

UHMWPE纤维的熔融行为和结晶结构的研究

用DSC法研究了超高相对分子质量聚乙烯(UHMWPE)纤维的熔融行为。随着拉伸倍数提高,纤维经松弛状态下热处理后,其熔融峰数目由1个变为2个;而经部分张力和张力状态下热处理后,其熔融峰数目均由1个变为3个。熔融峰出现的位置可分为4个区:131-138℃,141-143℃,150-152℃,155- 157℃。松弛与张力状态下热处理纤维的主熔融峰不同,前者主要是正交晶系的熔融,后者则分别对应于正交晶相向六方晶相的转变和六方晶相的熔融。经热处理的UHMWPE纤维的DSC图谱表明,代表正交晶相向六方晶相转化的熔融峰面积均有所增大,同时纤维的拉伸强度均有提高,提高幅度达31.1%。     

Influence of Nb5+ ions on phase transitions and polar disorder above TC in PbZrO3 studied by Raman spectroscopy

Micro-Raman light scattering experiments on PbZrO₃ (PZO) single crystal doped with Nb⁵⁺ have been investigated. Special attention was paid to the paraelectric (PE) phase in which nominally forbidden first-order Raman spectra were detected at temperatures far above the phase transition T_C. Complex Raman spectra were observed in the vicinity of three structural phase transitions. These results mainly from the coexistence of phases with different symmetries in a wide temperature range below T_C. The Raman measurements have been compared with dielectric and optical observations and proved that polar nanoregions in a centrosymmetric lattice appear well above T_C. It was shown that doping ABO₃ perovskites with heterovalent ions like Nb⁵⁺ unbalances charge neutrality of the lattice and strongly extends the temperature range of polar regions. The investigations performed point out that in the PE matrix the interaction between electrons and lattice vibrations, as recently suggested for pure PZO, plays an important role.     

Understanding the effects of nanocarbons on flexible polymer chain orientation and crystallization: Polyethylene/carbon nanochip hybrid fibrillar crystal growth

Polyethylene (PE)/carbon nanochip fibers (CNCF) fibrillar crystals were grown under shear flow. The fibrillar crystals ranged from 25 to 200 nm in diameter and bundled to form PE/CNCF macro‐fibers. The PE/CNCF fibers were further processed by hot‐drawing near the PE melting temperature. On the basis of small‐angle X‐ray analysis, it was found that during drawing the presence of CNCF led to continuous extended‐chain PE crystal growth. DSC melting peaks associated with transformation of the orthorhombic to a hexagonal lattice phase in the PE crystal structure (i.e., increased ordering and extension of the polymer chain along the fiber axis) was also observed for drawn hybrid PE/CNCF fibrillar crystals. The orientation factor for the extended‐chain (i.e., shish) crystals in the hybrid as compared to the control fibers improved from 0.88 to 0.93. This work shows the direct evidence that these nanocarbon fillers can promote extended‐chain polymer crystal growth under shear. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40763.     

Morphological effects of ballistic impact on fabrics of highly drawn polyethylene fibers

Changes in crystalline structure of high-tenacity ultra-high-molecular-weight polyethylene fabric brought on by ballistic impact from a small projectile were determined by X-ray diffractometry. A suitable X-ray diffraction method that averages out the fiber orientation effects in the diffraction pattern was used. The Spectra 1000? polyethylene fabrics were successfully characterized in terms of both the predominant orthorhombic and the minor monoclinic crystal content. Crystallinity values for the undamaged fabric are consistent from sample to sample and show an average orthorhombic fraction of 0.61 and an average monoclinic fraction of 0.04. Fabric damage by the projectile impact results in either an increase in monoclinic fraction, attributed to recrystallization at temperatures nearing the normal polyethylene melting point, or disappearance of monoclinic material as that temperature is exceeded. The latter predominates where ballistic penetration is complete. However, actual melting need not be involved: Transformation to the hexagonal (“rotator”) phase and the disappearance of the monoclinic phase could have occurred rather than true melting. © 1993 John Wiley & Sons, Inc.     

Phase Transition and Failure at High Temperature of Bismuth-Layered Piezoelectric Ceramics

High-temperature bismuth-layered piezoelectric ceramics (Ca _x Sr_{1− x} )Bi₄Ti₄O₁₅ have been prepared by the conventional solid reaction method. Our results reveal that there is a "phase transition induced by the composition" taking place in the solid solution at around x =0.4. During transition, the crystal structure changes from orthorhombic to pseudo-tetragonal and then back to orthorhombic. Although all (Ca _x Sr_{1− x} )Bi₄Ti₄O₁₅ (with any x value) undergo a second-order ferroelectric-to-ferroelectric phase transition at about 200°C below their Curie temperature, only the ceramic with x =0.4 exhibits a failure in piezoelectric properties during the transition. It is suggested that, on the basis of the X-ray photoelectron spectroscopy results, this material failure is induced by the high concentration of oxygen vacancies in the material during the phase transition.     

FIR-spectroscopy in the high pressure phase of polyethylene

Summary Despite the experimental problems of the high pressure and the high temperature necessary to reach the hexagonal phase of polyethylene a new absorption band was discovered and ascribed to a lattice vibration of this phase. This implies a well ordered lattice over considerable distances. From the frequency which lies between the frequencies of the B_1u– and _B2u– lattice vibrations of the orthorhombic crystal it is deduced that the hexagonal phase consists mainly of helical sequences and that the lattice vibrations do contribute little to the entropy difference orthorhombic hexagonal.     

Chain packing and phase transition of N-hexacosylated polyethyleneimine comb-like polymer: A combined investigation by synchrotron X-ray scattering and FTIR spectroscopy

In this paper, the chain packing and phase transition of comb-like polymer has been deeply analyzed with N-hexacosylated polyethyleneimine (PEI26C) as a template, fabricated through the reaction between n-hexacosyl bromide and PEI in a homogenous solution. The effect of long alkyl groups on the chain packing and phase transition of PEI26C was systematically investigated by synchrotron X-ray scattering and variable-temperature FTIR spectroscopy. PEI26C comb-like polymer exhibited an interesting structure-evolution process, and phase transformation from orthorhombic (β_O), monoclinic (β_M), to hexagonal (α_H) phase, and finally to amorphous state was demonstrated, indicating that large alkyl domains induced the complicated structures. Size-depended phase transition behavior provides an insight into the formation of metastable structure during the early stage of polymer crystallization.     

Morphological study of chain-extended growth in polyethylene: 2. Annealed bulk polymer

The morphology of initially spherulitic, chain-folded polyethylene has been observed after specimens had been annealed at temperatures up to and including the melting point at 5.3₅ kbar. The pattern of the various phenomena corresponds to a transition from orthorhombic to hexagonal structures prior to melting in accordance with other recent work. Certain errors in the literature are corrected and additional evidence is provided for the view that the molecular mechanism of annealing in polyethylene is little different from local melting followed by recrystallization provided the effect of constraints is recognized.     

Order-disorder transition in eicosylated polyethyleneimine comblike polymers

Order-disorder transition (ODT) behavior in eicosylated polyethyleneimine (PEI20C) comblike polymer obtained by grafting n-eicosyl group on polyethyleneimine backbone was systematically investigated by the combination of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) spectroscopy as well as solid-state high resolution nuclear magnetic resonance (NMR) spectroscopy. DSC investigations showed two obvious transitions, assigned to the transitions (1) from orthorhombic to hexagonal and (2) from hexagonal to amorphous phase, respectively. These transitions are induced by the variations of alkyl side chain conformation and packing structure with temperature changing, which consequently lead to the destruction of original phase equilibrium. The ODT behavior can also be confirmed by spectroscopic methods like WAXD, FTIR and NMR. The ordered structure and the transition behavior of the alkyl side chains confined by the PEI backbone are obviously different from those of pristine normal alkanes. The transition mechanism of ODT and the origin of the phase transition behavior in PEI20C comblike polymer were discussed in detail in this paper.     

Phase-Transformation-Induced Anti-Phase Boundary Domains in Barium Cerate Perovskite

Anti-phase boundary (APB) domains resulting from the orthorhombic ( o ₁, Ibmm (No. 74)) to orthorhombic ( o ₂, Pbnm (No. 62)) phase transformation at ∼290°C in pressureless-sintered barium cerate (BaCeO₃) ceramics have been investigated by transmission electron microscopy and large-angle convergent beam electron diffraction (LACBED). APB exhibiting symmetric π-fringe patterns in both bright-field and dark-field images, lying in {011) with the fault vector R =1/2〈111〉, were determined adopting the invisibility criteria of 2π g · R =0 or 2 n π. The curved boundaries due to symmetry change can be related to the low symmetry phase by a pure translation upon phase transition. The displacement vector, R , relating the domains is the translational symmetry element from the origin to the lattice point that was lost upon transition. The formation of such domains induced by the o ₁→ o ₂ phase transition is discussed in terms of change in the Bravais lattice and loss of lattice point upon the transition. The identification has also confirmed the existence of a higher symmetry orthorhombic o ₁ phase along the transition sequence of R 3 c → Ibmm → Pbnm . The increased spatial frequency of APB domains is attributed to A-site ordering due to vacancies created by BaO loss and/or disordering of the B-site substitutional defects generated extrinsically when doped with either a Y³⁺-acceptor or a Nb⁵⁺-donor.     

The phase diagram of poly(4-hydroxybenzoic acid) and poly(2,6-hydroxynaphthoic acid) and their copolymers from X-ray diffraction and thermal analysis

Homopolymers and copolymers of 4-hydroxybenzoic acid (HBA) and 2,6-hydroxynaphthoic acid (HNA) have been studied with differential scanning calorimetry and temperature-resolved wide angle X-ray diffraction. All polymers have more than one disordering transition between the glass transition (between 400 and 430 K) and decomposition (between 710 and 750 K). The first transition in PHBA at 616-633 K is from orthorhombic rigid crystals to a conformationally disordered pseudo-hexagonal phase (condis phase). The two higher transitions are first, a further disordering process to a hexagonal condis crystal, and then a change to an anisotropic melt (liquid crystal) at about 800 K, with increasing decomposition above 750 K. In PHNA, orthorhombic crystals change above 600 K to an orthorhombic condis crystal structure, which go to an anisotropic melt at 750 K, and subsequent decomposition. In addition, using empirical entropy rules that account for the changes during the transitions from the crystal to the disordered mobile phases, an effort is made to understand the disorder and mobility, and to arrive at a non-equilibrium phase diagram of the copolymer system. The existence of a single, but up to 200 K wide, glass transition and remaining high crystallinity of the copolyesters, indicate partial solubility of the repeating units in all phases. The new data are compared to and brought into agreement with the large number of prior measurements and often unclear interpretations.     

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.     

Phase Transitions of Nonionic Surfactant C18:1E10 in Mixed Media of Water with Ionic Liquids

Complicated temperature‐induced phase transitions were shown in two hexagonal liquid crystal mesophases constructed by nonionic surfactant C_18:1E₁₀ in the media of water with ionic liquids, 1‐butyl‐3‐methylimidazolium tetrofluoroborate (bmim‐BF₄) and 1‐butyl‐3‐methylimidazolium hexafluorophosphate (bmim‐PF₆), respectively. Detailed phase transition information was extracted from rheological, polarized optical microscopy, deuterium magnetic resonance spectroscopy and small‐angle X‐ray scattering measurements, with good agreement among these techniques. It is shown that the order‐to‐disorder phase transition occurred in a narrow range of temperatures for both the two hexagonal mesophases. In particular, the mesophases experienced a biphasic hexagonal‐lamellar coexisting before completely melting, which was not found in the corresponding aqueous hexagonal system of the same surfactant concentration. Of most importance, the two target mesophases exhibited some clearly different behaviors. The phase transition temperature is much higher (ca. 10 °C) in a (water + bmim‐BF₄) system than in a (water + bmim‐PF₆) system. After melting, a fast transition from elastic micelle to Newtonian fluid was observed in the (water + bmim‐BF₄) system, while the micelle related to the (water + bmim‐PF₆) system remained elastic over a relatively wide temperature region. The results obtained give a better insight of how the mixed solvents support mesophase formation and modulate the phase transition.     

The effect of temperature on the pulsed current processing behaviour and structural characteristics of porous ZSM-5 and zeolite Y monoliths

Hierachically porous monoliths of ZSM-5 and Y zeolites have been prepared by pulsed current processing (PCP). The densification behaviour and structural characteristics during rapid thermal treatment of the zeolites have been determined and related to the influence of the Si:Al ratio on the thermal stability of the zeolites. Monoliths of macroscopic shape can be prepared with an insignificant loss of surface area and micropore volume when the PCP-treatment was performed at temperatures below a critical maximum PCP temperature (T_PCP). Full-profile fittings of the powder X-ray diffraction patterns showed that the lattice strain of zeolite Y increases rapidly above the critical T_PCP while the ZSM-5 zeolites undergo a phase transition from orthorhombic to monoclinic. The use of a novel ceramic processing route for the production of the zeolite monoliths that do not significantly influence the structural characteristics and surface area of the starting materials has a potential to be of importance in catalysis and gas separation applications.