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.     

Role of the entangled amorphous network in tensile deformation of semicrystalline polymers

Being composed of crystalline lamellae and entangled amorphous polymeric chains in between, semicrystalline polymers always show a complicated deformation behavior under tensile deformation. In recent years, the process of tensile deformation was found to exhibit several regimes: intralamellar slipping of crystalline blocks occurs at small deformation whereas a stress-induced crystalline block disaggregation-recrystallization process occurs at a strain larger than the yield strain. The strain at this transition point is related to the interplay between the amorphous entanglement density and the stability of crystal blocks. We report experimental evidence from true stress-strain experiments that support this argument. It is emphasized that tie molecules, which connect adjacent lamellae, are of lesser importance with respect to the deformational behavior.     

CO2-induced Crystallization of Isotactic Polypropylene by Annealing Near Its Melting Temperature

CO₂-induced crystallization of isotactic polypropylene (iPP) by annealing had been studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). The iPP before annealed was in α-form and amorphous states. At lower temperatures by CO₂ isothermal treatments, iPP chains crystallized from the amorphous phase and only one crystal form, i.e., α-form, was observed. At higher temperatures by CO₂ isothermal treatments, both crystallization from the amorphous phase and thickening of existing crystal lamellae were observed. Moreover, light γ-form crystal appeared in the treated iPP. The crystalline lamellar thickness of iPP annealed at different CO₂ pressures had been determined. Using the Gibbs–Thomson plot method, the equilibrium melting temperature was found to be 187.6°C.     

Crystallization in Microdomains of a Block Copolymer Comprising Semicrystalline Block Observed by Simultaneous Measurement of SAXS and WAXS with H v‐SALS or DSC

Abstract

Semicrystalline block copolymers provide us with a fascinating model for studying the kinetics of crystallization. We performed the simultaneous measurement of small‐ (SAXS) and wide‐angle (WAXS) x‐ray scattering (SWAXS) with differential scanning calorimetry (DSC), or SWAXS with small‐angle light scattering (H _v‐SALS). The specimen used was polyethylene‐b‐poly(ethylene propylene) (PE‐b‐PEP) with the molecular weight of 44,200. The PE block has the melting point (T _m) at 108°C. We observed the time evolution of crystallization in the lamellar microdomains of PE‐b‐PEP after a temperature drop from 180°C (?T _m) to a variety of temperatures slightly below T _m. The exothermic signal was observed by DSC right after the temperature drop, while the four‐leaf‐clover pattern of H _v‐SALS and the SAXS peaks due to the lamellar microdomains were observed several minutes after the temperature equilibration. The WAXS peaks of (110) and (200) reflection were almost simultaneously detected with the H _v‐SALS and the SAXS peaks at crystallization temperature of 100°C. With the crystallization temperature closer to T _m, the WAXS crystalline signals showed up with longer time lag after the H _v‐SALS and the SAXS peaks began to appear. Interestingly, these phenomena are interpreted as that long‐range order of density fluctuation up to the order of micrometers was generated prior to the formation of crystals with partially ordered phase rather than the instantaneous crystalline nucleation.     

The role of the amorphous phase in the re-crystallization process of cold-crystallized poly(ethylene terephthalate)

The process of re-crystallization in poly(ethylene terephthalate) is studied by means of X-ray diffraction (SAXS and WAXS) and dynamical mechanical thermal analysis. Samples cold-crystallized for 9h at the temperatures T _c = 100 fcir#circ;C and T _c = 160 fcir#circ;C, i.e. in the middle of the relaxation region and close to its upper bound, respectively, are analyzed. During heating from room temperature, a structural rearrangement of the stacks is always found at T _r ≃ T _c + 20 fcir#circ;C. This process is characterized by a decrease of the linear crystallinity, irrespective of T_c; on the other hand, the WAXS crystallinity never increases with T below T_c+30fcir#circ;C. The lamellar thickness in the low-T_c sample decreases significantly after the structural transition, whereas in the high-T_c sample the lamellar thickness remains almost unchanged. In both, high- and low-T_c, the interlamellar thickness increases above T_r. Moreover, the high-T_c sample shows a lower rate of decrease of the mechanical performance with increasing T as the threshold T_r is crossed. This result is interpreted in terms of the formation of rigid amorphous domains where the chains are partially oriented. The presence of these domains would determine i) the stabilization of the crystalline lamellae from the thermodynamic point of view and ii) the increase of the elastic modulus of the amorphous interlamellar regions. This idea is discussed by resorting to a phase diagram. An estimation of the chemical-potential increase of the interlamellar amorphous regions, due to the enhancement of the structural constraints hindering segmental mobility, is offered. Finally, previous calculations developed within the framework of the Gaussian chain model (F.J. Baltá Calleja et al., Phys. Rev. B 75, 224201 (2007)) are used here to estimate the degree of chain orientation induced by the structural transition of the stacks.     

Infrared studies of drawn polyethylene part I. Changes in orientation and conformation of highly drawn linear polyethylene

Infrared spectroscopy is applied to discuss the orientation, the crystallinity, and the conformation of chain segments in the amorphous regions in drawn high-density polyethylene. The orientation of the crystals as well as the crystallinity are derived from the dichroism and the absorbance, respectively, of the band at 1894 cm-¹. The orientation and some aspects about the conformation of the chain segments in the amorphous regions can be obtained from the bands in the 1400-1300 cm-¹ region (gauche) and at 1078 cm-1 (gauche and trans). The dichroic studies show a high degree of orientation increasing with draw ratio λ for the chain segments in the crystals, but a low orientation reaching saturation at λ between 5 and 10 for those in the amorphous regions. The experiments indicate a change in crystallinity during the drawing process which depends on the thermal treatment of the undrawn sample. In the amorphous regions the number of CH₂ groups in gauche conformations decreases up to λ between 10 and 15 and remains nearly constant with further drawing. Since the sum of gauche and trans conformations remains unchanged, it can be deduced that the number of loops decreases and that of tie molecules increases with draw ratio.     

Chain orientation in natural rubber, Part II: 2H-NMR study

Stress-induced crystallisation (SIC) and stress-induced melting (SIM) in natural rubbers (NR), unfilled and filled with carbon black (CB) have been studied by ²H-NMR measurements. Various materials have been swollen with small amount (< 2%) of deuterated alkane chains. The orientation of the amorphous chains, then the local deformation of the amorphous chains during deformation cycles and during stress relaxation, permits to clarify the SIC and SIM processes during hardening and recovery. By mechanical, WAXS and NMR measurements one determines the same critical draw ratio for appearance λ_A and disappearance λ_E of the crystallites. It is demonstrated that the hysteresis observed by the different techniques (stress σ, crystallinity χ, NMR splitting Δν) are due to the supercooling effect ( λ_A > λ_E, at constant temperature). During hardening at constant strain rate it is found that the local draw ratio remains constant and equal to λ_A, whereas the crystallinity increases linearly with the macroscopic draw ratio λ. The hardening σ ∼ (λ - λ_A)² is then interpreted as a reinforcement effect due to the crystallites, which act as new crosslinks. This confirms the prediction of Flory. In filled rubber the same effects are observed, and the stress amplification factor is determined as a function of the CB content. It is found that the fillers act as nucleation centres for the NR crystallites. The reinforcement of such materials is due principally to this nucleation effect and to the presence of a super network formed by both the NR crystallites and the CB fillers.     

Biaxial Orientation Mechanism of Drawn Natural Rubber. I. Appearance of the Biaxial Orientation

Triple centrifuged natural rubber (NR) film was crystallized at room temperature by uniaxial drawing at draw ratio (λ) 6.0. Morphologies of the induced crystals were studied by wide angle X‐ray diffraction (WAXD). Using a special drawing apparatus for shifting the X‐ray beam positions, crystalline orientation was surveyed all over the drawn sample. WAXD results show the biaxial orientation (BO) with the c‐axis being parallel to the draw direction and the a‐axis parallel to the film surface. By means of the WAXD pattern with the incident beam perpendicular to the film, the drawn NR film was separated into three regions: (200+120) region indicating a low degree of BO, (200) region having a high degree of BO, and the intermediate region in between the above two regions. The degree of BO is enhanced from center to the clamping end depending on the increment of the sample width. At lower draw ratios (4.0<λ<6.0), BO was developed with increasing draw ratio. In the intermediate region, BO increases sharply with a small increment of the width. The degree of BO is lowered at the periphery of the film.     

ON FINE STRUCTURE OF NASCENT UHMWPE REACTOR POWDERS

Reactor powder of a number of commercial ultra-high molecular weight polyethylenes synthesized on heterogeneous Ziegler–Natta catalyst in a conventional slurry process have been investigated with the help of scanning electron microscopy (SEM), wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), and low-frequency Raman spectroscopy. The SEM study reveals a complicated structure of nascent particles consisting of a small (0.5–1.0 μm) spongy-like fibrillar spheres. It is suggested that the elementary morphological units are fibrils formed by the small defective crystallites with the dominating crystalline distortions caused by microstrains. The comparison of longitudinal crystallite sizes derived from WAXS data with the straight chain segment length calculated from Raman shift frequency and the length of extended segments computed from the true melting interval measured by DSC allows to conclude that there are a large number of extended tie chains bridging the neighboring crystallites, which is not in agreement with a lamellar model. The extended chains passing through the noncrystalline regions stabilize a structure and provides its thermal stability. A possible tilting of molecules in the crystals, the location of defects and the distribution of crystallite sizes are discussed.     

典型高分子材料的固体核磁共振研究

本论文通过固体核磁共振（NMR）谱及动力学参量的测量,并结合X-射线衍射技术和DSC测量等研究了两种典型高分子材料的相结构、链的运动以及相与相之间的关系.  乙烯-醋酸乙烯共聚物( EVA) 是最主要的乙烯共聚物之一. 研究发现,EVA的相组成非常复杂,共有5个不同的组分. 除了PE中所观察到的常规单斜晶相和刚性的正交晶相外,我们发现还存在第三个晶相分量－运动性较强的晶相（SOCP,可能是转动相）. 它不仅拥有自己的熔点,而且它的化学位移和分子运动性不同于刚性正交晶相（LOCP）. 另一方面,非晶相也由两种不同的分量组成：运动受限的各相异性的非晶界面相和高度可动的橡胶型的非晶相. 我们进一步详细研究了EVA中的晶区链动力学和非晶区的低温冻结行为. 实验发现,在正交晶相中,高分子链以180° flip-flop方式运动,同时伴随沿链方向的平移型跳跃运动,并引起正交晶相和非晶相之间的长程链扩散,通过NOE的测量证实了这种相间链扩散的存在,并进一步通过实验证实这种相间链扩散是一种受限扩散而不是自由扩散. 同时非晶相的两个组分具有不同的低温冻结行为：当温度低于-弛豫转变温度时,橡胶型的非晶相中的长程分子运动被冻结,但仍存在分子的局域运动;而界面非晶在低温时冻结成一种有序取向结构,并用质子自旋扩散实验证实该有序结构与正交晶相相邻近.  少量纳米级片层状粘土分散在聚合物中就可赋予材料许多优异的性能,我们用固体NMR技术对EVA/REC复合材料的结构和其中粘土的分散性质进行研究,发现上述复合材料中所形成的晶体类型不仅依赖于各组分的性质还依赖于所形成的复合材料的类型.  偏氟乙烯/三氟乙烯共聚物（P（VDF-TrFE））是最主要的铁电高聚物之一. 我们利用变温固体¹⁹F MAS NMR 谱及弛豫数据的测量详细研究了电子辐照对P(VDF-TrFE)共聚物的分子结构、构型、运动性以及相变等的影响. 发现,电子辐照不仅改变了分子链段的构型和运动性,同时也改变了局部分子化学结构. 电子辐照促使铁电相向顺电相(或者非晶相)转变,与此同时诱发了富含VDF和含-TrFE链段从全反式的构型到混合的反式-旁式构型的转变. 电子辐照加剧顺电区域中的分子运动而在高温熔融态中(>100 ℃）,分子的运动反而受限.     

Scanning force microscopy of the crystalline/amorphous interface of ultradrawn poly(ethylene)

We report on high-resolution experiments by Scanning Force Microscopy (SFM) indicating the tight folding of neighbouring molecular chains in Poly(Ethylene) (PE) crystals. Ultradrawn PE films were prepared in the stacked lamellar morphology exhibiting crystalline lamellae and amorphous parts. The [001] direction of the lamellae is aligned parallel to the substrate surface resulting in {hk0} planes perpendicular to the symmetry axis of the probing tip. This preparation technique allows the direct observation of the molecular arrangement in polymeric crystals as well as an investigation of the crystalline/amorphous interface by SFM: at lower magnification, crystalline and amorphous parts of the film can be distinguished clearly. High-resolution imaging on the crystalline lamellae reveals a PE pitch height of 0.26±0.02 nm while the interchain spacing measures 0.50±0.02 nm consistent with (100) lattice planes aligned parallel to the substrate surface. Finally, the molecular folding at the edges of the lamellae has been studied. Evidence is found for the adjacent reentry of individual molecules at the edge of (100) surfaces.     

Molecular mobility and fracture processes in ultimately drawn high-density polyethylene

The reasons for cessation of drawing and for the local fracture processes occurring in linear polyethylene near ultimate draw ratios, λ_max, have been studied by broad-line proton NMR and scanning electron microscopy. As λ_max was reached during drawing, the sample whitened and kink bands similar to those observed at deformation of low molecular solids were formed. The kink bands were clearly seen in the micrographs. As evidenced by NMR, the segmental mobility of chains in the amorphous regions is almost completely suppressed because of a sharp increase of the orientation stress near λ_max (though the temperature of drawing is close to the melting temperature). This phenomenon of mechanical vitrification is regarded as a fundamental reason for cessation of drawing and for transition to the “solid-state” fracture mechanism. Quantitative estimates are given for formation of submicrocracks arising at boundaries of kink bands.     

Applications of x-ray investigations of polymers under load. 1. Measurements of WAXS reflections shifts induced by elastic stretching of oriented polymer systems along their orientation axis

What information can be obtained by wide-angle x-ray scattering (WAXS) from elastic deformation of oriented polymer systems along their orientation axis? This article is a critical review and some generalization of answers for this question. Different cases are analyzed that occur in practice or that might occur in the future: (1) determination of Young's moduli E _c of crystalline lattices parallel to the polymer chains and establishment of correlations between E _c and chain conformation in crystalline lattices; (2) estimation of elastic properties of different fragments of polymer chains, especially of multiatomic complex fragments; (3) detection of conformational polymorphism in crystalline lattices; (4) determination of Poisson's coefficients of crystalline lattices; (5) estimation of potential ability of a given polymer for formation of systems with high moduli; (6) estimation of structural inhomogeneity of amorphous polymers; (7) study of interaction character of structural elements in oriented polymer blends; (8) investigations of systems with a combination of extended and folded-chain crystals; (9) study of influence of cross-linking on structure-mechanical behavior; (10) estimation of adhesive interaction of different components in polymer composites. Only Points 1 (see, e.g., Refs. 1–18) and 4 [19] have been considered in earlier literature by other authors, mainly for soft-chain polymers. Thus, similar results are considered here only briefly, and primary attention is focused on the peculiarities of investigations of rigid-chain polymers. As for the other points, they were considered earlier in our separate works [20–38] (or have not been considered at all) and up to now have not received wide acceptance. During the consideration here, both series and parallel models of interactions of different structural elements were used.     

Magnetic anisotropy,magnetostriction and intermediate range order in Co-P alloys

Coercivity, magnetostriction and saturation field as a measure of magnetic ansitropy energy have been investigated in Co_100-xP_x with 5 ? x ? 26. According to their magnetization curves and coercivities crystalline alloys (5 ? x ? 11) contain hcp-inclusions whereas amorphous material (12 ? x ? 26) is magnetically soft. The saturation magnetization is mainly determined by the nearest neighbour shell but magnetostriction is also correlated with the further environment of the cobalt atoms. The abrupt breakdown of the value |λ_s| of the saturation magnetostriction with increasing phosphorus content of the crystalline alloys points to a growing structural disorder. λ_s of the amorphous material depends on the saturation magnetization alone (λ_s ≈ σ₀²) which decreases with increasing phosphorus content. The small value of λ_s suggests that angular correlations do not exceed the second coordination shell.     

Quantitative structural characterization of the deformation and shrinkage of isotactic polypropylene fibers and films

A quantitative determination has been made of the structural elements which control deformation and shrinkage processes in isotactic polypropylene fibers and films. It is found that strain processes such as fabrication draw and shrinkage are controlled by the noncrystalline region of this highly crystalline polymer. Quantitative structure-property correlations are obtained for the polymer, which reveal the interactions between temperature, strain, and orientation. The thermal activation energy of the noncrystalline chains is also determined from these solid-state structure measurements.     

Structural characterization of Rh/pumice SMAD catalysts

The structure of Rh/pumice catalysts prepared by the SMAD (Solvated Metal Atoms Dispersion) technique at different metal loadings has been investigated by EXAFS (Extended X-ray Absorption Fine Structure Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), SAXS (Small-Angle X-ray Scattering), WAXS (Wide-Angle X-ray Scattering) and TEM (Transmission Electron Microscopy). According to EXAFS and XPS, a fraction of the Rh atoms is oxidised, but a noticeable part is also present as Rh ⁰. The Rh oxidation is attributed to the interaction of the Rh atoms with the hydroxyls of the support; after the formation of the oxide, the nucleation of metallic rhodium becomes possible. The WAXS data do not show evidence of rhodium fcc crystallites; the metal-bearing particles are probably amorphous and/or very small, as results from the SAXS and TEM data analysis. The disagreement between the latter two techniques, resulting in a small-angle determination of the average size of the particles that is about half that of TEM in the catalyst with the higher Rh loading, is acknowledged and discussed. Preliminary catalytic tests are described, demonstrating the suitability of using a low surface area support for the preparation of SMAD catalysts. Received 2 February 1999     

Effect of annealing on the structure and morphology of nylon 6 fibers

Changes in the structure of nylon 6 fibers annealed in dry and wet atmospheres were studied by small-and wide-angle x-ray diffraction. In the presence of water or saturated steam, fibers can be annealed to the same strucutral state at temperatures 70°C lower than in dry atmosphere. This is due to the enhanced mobility of the molecular segments in the amorphous region, a mechanism which is also known to lower the T_g by the same amount. Upon annealing under unconstrained conditions, lamellar spacing, crystallite size in the equatorial plane, crystalline as well as fiber density, and the chain-axis repeat increase with annealing-temperature; whereas crystalline orientation and the Van der Waals separation of the hydrogen-bonded sheets decrease. The monoclinic angle 8 remains constant at 66.7° (σ = 0.3°) and might depend on the starting fiber rather than on the treatment of the fiber. Most of these changes occur above a critical temperature of 170°C if dry, or 100°C if wet; rate of crystallization is also the highest under these conditions in nylon. The effect of these changes on such fiber properties as dyeing and the role of micro voids in dye diffusion and in dye uptake are discussed. Surface premelting and the accompanying changes in the surface structure of the lamellae, selective melting, and more importantly, the longitudinal motion of the nylon 6 chains and the resulting folding of interfibrillar extended amorphous chains are invoked to explain the shrinkage of the fiber, disorientation of the crystallites, increase in crystalline perfection, and the increase in lamellar spacing.     

Effect of Radiation from an Infrared Laser and <Emphasis Type="Italic">γ</Emphasis>-Rays from <Superscript>60</Superscript>Co on the Molecular–Topological Structure of Polyvinylidene Fluoride

We investigate the molecular–topological structure of polyvinylidene fluoride (PVDF) irradiated with γ-rays from ⁶⁰Co and IR radiation from a carbon dioxide laser by the thermomechanical spectroscopy method. The initial PVDF has a topological three-block network structure containing the low- and high-temperature amorphous blocks and crystalline fragments. Both types of irradiation can initiate interblock mass transfer of the macromolecular fragments from the amorphous to the crystalline form. As a result, unlike the predominantly amorphous structure of the native polymer, which is 7% crystalline, the weight fraction of the crystalline modification of the PVDF due to irradiation by an IR laser increases to 72%. Comparative analysis leads to the conclusion that the PVDF has a greater resistance to γ-irradiation than to IR laser irradiation. After IR laser irradiation, the pseudo-network structure of PVDF undergoes noticeable changes. The quantitative content of the crystalline fragments of macromolecules increases by almost an order of magnitude; the mobility of chains is reduced, and the rigidity of the chains is increased. However, the molecular flow of the polymer irradiated by the laser and γ-rays begins in the same temperature range (437 – 441 K) near where the native polymer is flowing (438 K).     

Creep in oriented thermoplastics

Sheets of oriented low-density polyethylene possessing transverse symmetry were prepared by simple tensile drawing at room temperature. The degree of anisotropy was varied by varying the draw ratio. Classic elasticity theory shows that five constants are necessary to characterize the deformation behavior of an elastic material possessing such symmetry. The time-dependent equivalents of these constants have been determined from the simultaneous measurement of longitudinal and lateral strain during tensile creep of specimens cut at 0, 45, and 90 deg to the draw direction. Special creep apparatus, able to handle small, flexible specimens, was developed for this study. The shear compliance obtained from the tensile creep studies was in good agreement with the value obtained from torsional creep measurements on a 0-deg specimen, for the entire range of draw ratios. The contraction measurements have been used to provide additional evidence for deformation mechanisms suggested by the longitudinal strain measurements and, when combined with tensile measurements, have allowed the volume changes occurring during tensile creep to be calculated.     

Strong polarization dependence and selenium lone-pairs in the photoemission spectra of Sb2Se3

The photoemission energy distribution curves (EDC's) of crystalline and amorphous Sb₂Se₃ were measured in the photon energy range hv=7 to 20 eV using polarized radiation from a synchroton storage ring. The EDC's show that the six electrons per Sb₂Se₃ molecule, attributed primarily to the selenium p-pairs, are clearly separated from the remaining part of the valence band of crystalline Sb₂Se₃. The optical transitions from these states occur with matrix elements strongly dependent on the orientation of the electrical vector of the polarized radiation as a result of crystal field effects. Model densities of states are constructed for both crystalline and amorphous Sb₂Se₃.