Crystal orientations and structures of poly(ethylene-ran-vinyl acetate) films coated onto silicon substrates

The crystal orientations and structures of poly(ethylene-ran-vinyl acetate) (EVA) films coated onto silicon substrates to a thickness of 5–200 nm were investigated by performing isothermal crystallization of the films. Bulk-like isotropic crystal orientations and orthorhombic crystalline phases were observed in films thicker than 50 nm. In the thinner films (<50 nm), the crystal orientations remained edge-on (with the chain axis parallel to the substrate), and two crystal structures, orthorhombic and hexagonal, were observed. The preference for an edge-on orientation was attributed to surface nucleation and repulsive interactions between the crystallizable ethylene components and the substrate. An orthorhombic structure developed at the surface from the primary crystallization during isothermal crystallization. A hexagonal structure formed near the interface as a result of secondary crystallization after cooling to room temperature. These results are discussed in terms of the favorable interactions between the polar vinyl acetate and the substrate, which affect the crystallization of the ethylene component in thin EVA films.     

Application of an extended Tool–Narayanaswamy–Moynihan model.Part 2. Frequency and cooling rate dependence of glass transition from temperature modulated DSC

In the first part of this paper a Tool–Narayanaswamy–Moynihan-model (TNM) extended by non-Arrhenius temperature dependence of the relaxation time was applied to describe results from temperature modulated DSC (TMDSC). The model is capable to describe the features of the heat capacities measured in TMDSC scan experiments in the glass transition region of polystyrene (PS). In this part the model is applied to bisphenol A-polycarbonate (PC). Both aspects of glass transition, vitrification as well as the dynamic glass transition are again well described by the model. The dynamic glass transition above T_g can be considered as a process in thermodynamic equilibrium. The non-linearity parameter (x) of the TNM model is not needed to describe complex heat capacity as long as the dynamic glass transition is well separated from vitrification. Under such conditions the relation between cooling rate (q₀), and the corresponding frequency (ω) can be found from the two independently observed glass transitions. Fictive temperature and the maximum of the imaginary part of complex heat capacity are used for comparison here. The measurement as well as the TNM-model confirm the relation derived from Donth's fluctuation approach to glass transition, ω=q₀/aδT, where a=5.5±0.1 (confirmed previously experimentally as 6±3) and δT is mean temperature fluctuation of the cooperatively rearranging regions (CRRs).     

Crystal lattice deformation and the mesophase in poly(ethylene terephthalate) uniaxially drawn by solid-state coextrusion

The development of crystalline and mesophase structure on drawing of poly(ethylene terephthalate) (PET) has been studied. The uniaxial drawing has been done by solid-state coextrusion from 50 to 90°C. The unit-cell parameters of stress-induced crystallites in extrudates have been determined as a function of extrusion draw ratio (EDR) up to 4.4 and at an extrusion temperature (ET) above T_g, at 70 and 90°C. The higher the EDR, the longer the c-axis chain direction, the shorter the a and b axes and the smaller the unit-cell volume. In comparison with the conventional lattice parameters obtained by Bunn and Fisher, the highest elongation of the c axis is near 10%. These features imply that the lattice of stress-induced crystallites is far from the closest packing. Coextrudates made below the T_g of PET differ markedly from those made above T_g. Wide-angle X-ray diffraction (WAXD) patterns of PET extruded at 50°C (below T_g) even at high draw ratio exhibit small and/or imperfect crystallites. They appear much as a mesophase. The distance between macromolecular chains ranges from 3.2 to 5.4 Å. The shortest value in the mesophase approaches the interplanar crystal distance of 3.4 Å for the (100) crystal face, on which the benzene rings lie. The crystalline peak separation of the PET, WAXD curve has been evaluated to obtain absolute crystallinities and the content of mesophase and amorphous phase.     

运用粒子图像测速仪研究双层桨搅拌槽内流体流动

The flow fields in a dual Rushton impeller stirred tank with diameter of 0.48 m （T） were measured by using Particle Image Velocimetry （PIV）. Three different size impellers were used in the experiments with diameters of D = 0.33T, 0.40T and 0.50T, respectively. The multi-block and 360° ensemble-averaged approaches were used to measure the radial and axial angle-resolved velocity distributions. Three typical flow patterns, named, merging flow, parallel flow and diverging flow, were obtained by changing the clearance of the bottom impeller above the tank base （C1） and the spacing between the two impellers （C2）. The results show that while C1 is equal to D, the parallel flow occurs as C2≥0.40T, C2≥0.38T and C2≥0.32T and the merging flow occurs as C2≤0.38T, C2≤0.36T and C2≤0.27T for the impellers with diameter of D=0.33T, 0.40T and 0.50T, respectively. When C2 is equal to D, the diverging flow occurs in the value of C1≤0.15T for all three impellers. The flow numbers of these impellers were calculated for the parallel flow. Trailing vortices generated by the lower impeller for the diverging flow were shown by the 10° angle-resolved velocity measurements. The peak value of turbulence kinetic energy （ k/V^2tip = 0.12-0.15 or above） appears along the center of the impeller discharging stream.     

Poly(oxyphosphazenes): A unifying look at structure and morphology

Variations in thermotropic behavior have been investigated for many crystalline polyphosphazenes. Experimental results obtained from DSC, solid-state MAS NMR, dilatometry, mechanical property (creep and dynamic) measurements, birefringence, X-ray (wide and small-angle), and electron diffraction have been made as a function of temperature in an attempt to elucidate the structure-morphology behavior in these polymers. Initially all polyphosphazenes described in this study crystallize from dilute or moderately concentrated solutions in the monoclinic form with relatively low crystallinity (50%). When heated, these specimens expand atT(1) and adopt a chain-extended 2D mesophase morphology in order to relieve congestion. Above this first-orderT(1) temperature a chain-extended –P=N– chain backbone morphology transforms into an isotropic phase at the melting temperatureT _m. When cooled, samples transform into the mesostate. However, crystallization from the mesophase belowT(1) takes place into a 3D orthorhombic state. These two modes of ordering/disordering are in accordance with (i) a hexatic chain packing [betweenT(1) andT _m] with variously mobile side groups that also exhibits smectic features when mesogenic side groups are present and (ii) a 3D orthorhombic structure of high crystallinity belowT(1). The initial monoclinic phase is accessible only via a solution crystallization route. Solid-state MAS NMR, crystallization kinetic measurements, and optical microscopy studies obtained from oriented and unoriented polymers have been especially germane in providing a unifying picture for mesophase formation and ordering in polyoxyphosphazenes. X-ray and electron diffraction evidence is complicated by critical overlapping peaks that are found under these same conditions. They demonstrate that (a) positional disorder exists along the chain direction and (b) the side chain above, as well as below, theT(1) transition is governed by the mobility, size, and type of side group in circumstances where phase transformations occur.     

Effects of crystallinity and molecular weight on crack behavior in crystalline poly(L‐lactic acid)

The crack behavior and spherulitic morphology in melt‐crystallized poly(L ‐lactic acid) (PLLA) were found to be molecular weight (MW) and crystallinity dependent, along with other key factors. With increasing MW in PLLA, the size of spherulites, band spacing of ring‐banded spherulites, and degree of crystallinity decreased, whereas cracks were increasingly less likely to occur. Multiple types of cracks, that is, circumferential and/or radial cracks, were massively present in low‐MW PLLA (PLLA‐11k), which had a high crystallinity. Upon cooling, in PLLA‐11k at most crystallization temperatures (T_c's), cracks formed, and the crack patterns were dependent on the lamellar morphology within the spherulites. Hexagonal, rather than circular, cracks occurred spontaneously during the cooling process of PLLA of a medium‐MW grade (PLLA‐120k) in PLLA film samples crystallized only at high T_c (135–138°C) and cooled to ambient temperature. However, no cracks of any types at all were present in PLLA films of high enough MWs (PLLA‐152k and PLLA‐258k) upon either slow air cooling or quench cooling when the samples were dipped into liquid nitrogen. Apparently, cooling‐induced contraction differences in different directions were invalid or not sufficient to address the complex cracking behavior in PLLA. In addition, for PLLA‐11k with a substantially high crystallinity, cracks were so prone to occur that even cover constraint imposed another factor in determining the crack and ring‐band patterns. More plausible mechanisms and correlations between the cracks, MW, crystallinity, spherulite size, and spherulite lamellar patterns of PLLA were analyzed in detail and proposed in this study. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

High birefringence of poly(trimethylene terephthalate) spherulite

Poly(trimethylene terephthalate) (PTT) spherulite shows interference color under polarized light microscope without a sensitive tint plate. The fact indicates that the retardation of PTT spherulite is high, while it was reported that the birefringence in PTT fiber is low. In this study, the reason why the high birefringence is observed in PTT spherulite was discussed. By small area X-ray diffraction measurement, it was confirmed that a-axis of unit cell of PTT crystal was parallel to the radial direction of the spherulite. Based on the result, we calculated the refractive indices of parallel to a-axis and the other orthogonal directions. It was clarified that the refractive index of a-axis is much lower than the others and the intrinsic birefringence for a-axis orientation is high. It is the reason why the PTT spherulite shows high and negative birefringence.     

Crystal structure of poly(ethylenimine)-hydrogen chloride complex

Linear poly(ethylenimine) was found to form a crystalline complex with hydrogen chloride when poly(ethylenimine) in the state of its anhydrate or hydrates was immersed in concentrated hydrochloric acid or exposed to HCl vapour. The crystal structure of the HCl complex was determined by X-ray diffraction. The crystals are metrically tetragonal with cell constants a = B = 5.06 Å and c (chain axis) = 7.57 Å and the unit cell includes one planar zigzag polymer chain (two monomeric units) and two HCl molecules (the molar ratio is 1:1). The space group of the crystals is, however, not of the tetragonal system: the structure determined can be expressed in terms of any of the following crystal systems and space groups: orthorhombic P222₁, orthorhombic Pcm2₁, orthorhombic Pc2m, and monoclinic P2₁/m (c unique). The Cl···N distance of 3.05 Å strongly indicates that every NH and HCl hydrogen atom participates in hydrogen bondings between nitrogen and chlorine atoms. The HCl complex has a fairly high melting temperature of 265°C, which is about 200°C higher than those of the anhydrate and hydrates.     

Stereocomplexation and morphology of enantiomeric poly(lactic acid)s with moderate‐molecular‐weight

The thermal behavior and spherulitic morphologies of poly(L ‐lactic acid) (PLLA)/poly(D ‐lactic acid) (PDLA) 1/1 blend with weight‐molecular‐weight of 10⁵ order, together with those of pure PLLA and PDLA, were investigated using differential scanning calorimetry and polarized optical microscopy. It was found that in the blend, stereocomplex crystallites could be formed exclusively or coexisted with homocrystallites depending on thermal history. Banded to nonbanded spherulitic morphological transition occurred for melt‐crystallized PLLA and PDLA, while the blend presented exclusively nonbanded spherulitic morphologies in the temperature range investigated. The spherulite growth of the blend occurred within a wider temperature range (≤180°C) compared with that of homopolymers (≤150°C), while the spherulite growth rates were comparable for both the blend and homopolymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Dieletric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg_1/3Nb_2/3O₃)_0.67(PbTiO₃)_0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ε'₁₁₁ (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.     

Comparison of the thermal behaviour of aqueous solutions and hydrogels of poly(N-methylol acrylamide)

The intrinsic viscosity ([η]) in water of a sample of linear poly(N-methylol acrylamide) (PMAM) has been measured over the range 278–348 K and the swelling ratio (Q) in water of a sample of chemically crosslinked PMAM has been determined within the interval 278–353 K. The plots of [η] and Q versus temperature (T) both exhibited a maximum. The viscometric behaviour is consistent with an analysis based on excluded volume theory, which indicates the existence of a critical solution temperature both below (263 K) and above (375 K) the maximum. The swelling data were analysed on the basis of the Flory-Rehner theory which yielded a predicted minimum in the dependence of the polymer-water interaction parameter (χ) with T, thus according with the observed maximum in Q versus T. Prolonged heating of linear PMAM and its xerogel yielded products, which were no longer soluble or swellable, respectively, in water. The induced crosslinking proposed as the cause has been confirmed by chemical and infra-red analysis.     

Effect of cooling rate and frequency on the calorimetric measurement of the glass transition

The glass transition of thermoplastics of different polydispersity and thermosets of different network structure has been studied by conventional differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The cooling rate dependence of the thermal glass transition temperature T_g measured by DSC, and the frequency dependence of the dynamic glass transition temperature T measured by TMDSC have been investigated. The relation between the cooling rate and the frequency necessary to achieve the same glass transition temperature has been quantified in terms of a logarithmic difference Δ=log₁₀[|q|]−log₁₀(ω), where |q| is the absolute value of the cooling rate in K s⁻¹ and ω is the angular frequency in rad s⁻¹ necessary to obtain T_g(q)=T(ω). The values of Δ obtained for various polymers at a modulation period of 120 s (frequency of 8.3 mHz) are between 0.14 and 0.81. These values agree reasonably well with the theoretical prediction [Hutchinson JM, Montserrat S. Thermochim Acta 2001;377:63 [6]] based on the model of Tool–Narayanaswamy–Moynihan with a distribution of relaxation times. The results are discussed and compared with those obtained by other authors in polymeric and other glass-forming systems.     

Crystallinity and lateral crystallite size of different UHMW PE materials

Nascent high-molecular-weight (UHMW) polyethylene (PE) samples of different origins show a rather high crystallinity of about 70–75% and contain both a major portion of orthorhombic extended chain crystallites and a minor portion of triclinic crystallites. The triclinic content is greater the higher the molecular weight of the sample and the higher the activity of the used catalyser. A melting / recooling treatment results in a reduction of crystallinity by about 15–25% and disappearance of the triclinic phase. Further, an irreversible conversion of nascent orthorhombic extended chain crystallites to orthorhombic folded chain crystallites of increased lateral dimensions and crystalline perfections takes place during the melting / recooling treatment. The results are compared to those obtained for lower-molecular-weight PE samples and for high-strength / modulus PE fibers of different origins.     

A near-infrared study of thermally induced structural changes in polyethylene crystal

Thermally induced structural changes of polyethylene (PE) have been studied by means of near-infrared (NIR) spectroscopy in the course of heating up to the melting temperature. NIR bands characteristic of the regular orthorhombic phase, the conformationally disordered hexagonal phase, and the amorphous phase have been successfully identified. It has been found that for the unoriented PE sample, the disordering process of orthorhombic lattice starts above room temperature and that it mostly occurs above 100 °C for the uniaxially oriented PE sample. In the latter case, the enhancement of crystallinity has clearly been detected just below T_m due to the reorganization of crystalline lattice. For the geometrically constrained ultradrawn PE sample, the phase transition from orthorhombic to hexagonal phase has been detected immediately below the melting point. The NIR bands characteristic of the hexagonal phase have been confirmed definitely. Usefulness of NIR spectroscopy has been demonstrated successfully in such a study of thermally induced phase transition behavior of PE samples with appreciable thickness, for which mid-IR spectroscopy is difficult to apply because of the intensity saturation of various key bands.     

The evolution of residual stresses in thermoplastic bonding to metals

The ability to create strong joints between thermoplastics and metals offers many advantages. Differential properties between the polymer and metal generate residual stresses during cooling. In our study, both amorphous thermoplastic thin films and semi-crystalline thermoplastic thin films are joined to metal strips and the curvature is measured during controlled cooling. A series of five designed experiments uses a fast cooling (30°C/s) and slow cooling (4.5–10°C/min) to create different residual stresses. Experimental evidence shows that the residual stresses begin to develop at 190°C for amorphous Poly Ether Imide (PEI, T_g = 210°C), but at 255°C for semi-crystalline Poly Ether Ether Ketone (PEEK, T_g = 143°C). A mechanics based curvature model, combined with the elasticity and viscoelasticity of thermoplastics, successfully predicts the residual stress development. An elastic behavior is exhibited during the fast cooling (30°C/s), whereas a viscoelastic behavior occurs during slow cooling (4.5°C/min).     

Spherulite Growth Rate in Polypropylene/Silica Nanoparticle Composites: Effect of Particle Morphology and Compatibilizer

Spherical and tube‐like (TL) silica nanoparticles were melt blended with an isotactic polypropylene (PP) matrix and its effect on the isothermal spherulite growth rate was analyzed by polarized optical microscope. The addition of low amount (≈1 wt.‐%) of either 15 nm spherical or TL particles raises the spherulite growth rate and the nucleation density of spherulites. Samples prepared with silica spheres of 80 nm otherwise do not show any change in the crystallization behavior. By adding a compatibilizer, both the nucleation density and the spherulite growth rate of the pure polymer are increased. Noteworthy, although the nanoparticles do not further increase the nucleation density of the PP/compatibilizer blend, independent of its form and size, they cause a decrease in its spherulite growth rate.

     

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     

Changes in optical texture of α-spherulites of isotactic polypropylene upon partial melting and recrystallization

Textural changes in -phase spherulites of isotactic polypropylene (iPP) in a sequence of thermal events were examined by means of polarized light microscopy (PLM). This sequence of thermal events involves isothermal crystallization (atT_c = 117 to 140 °C), followed by heating (at 5 °C/min) to nearly complete melting, and then recrystallization upon cooling (at -40°C/min) to T_c During isothermal crystallization, the a-spherulites were of mixed birefringenceat T_c = 117 to 127 °C or of negative birefringence at T_c = 140 °C; upon heating towards melting, the spherulitec birefringence consistently truned negative. More interestingly, after recrystallization during cooling back to Tc from nearcomplate melting, all spherulites exhibited positive birefringence. The recrystallization could also result in speckles of positive birefringence when T_c was high or upon slower cooling. The changes in optical texture are explained in terms of contributions from tangential (or, cross-hatched) subsidiary lamellae which (as compared to the radial dominant lamellae) are relatively low-melting but thicken and recrystallize more readily in the present temperature range.     

Banded spherulites of HDPE molded by gas-assisted and conventional injection molding

Crystal morphologies of high density polyethylene (HDPE) with low molecular weight obtained by gas-assisted injection molding (GAIM), conventional injection molding (CIM), and spontaneous cooling, respectively, were studied by scanning electronic microscopy (SEM). It is found that banded spherulites are generated in the inner zone of GAIM parts and the outer zone of CIM parts but are absent in quiescent parts. According to the results, the representative morphologies of crystal change with gradual increment of instantaneous flow field in crystallization from non-banded spherulite to banded spherulite and then to oriented lamellae. This morphological evolution indicates that banded spherulites could be induced by flow field with certain intensity, which is confined by both an upper critical value and a lower one.     

Effect of water on relaxations in the glassy and liquid states of poly(propylene oxide) of molecular weight 4000

The complex relative permittivity of poly(propylene oxide) (PPO) of molecular weight 4000 containing 1.23 wt% water has been measured in the temperature range 77 to 325 K and frequency range 12 Hz to 500 kHz, and the results are compared with the corresponding study of pure PPO-4000. On the addition of water, all the three processes, namely the β-process (at T < T_g) and the - and ′-processes (at T > T_g), are shifted to higher temperatures. The strength of the β-process remained unchanged but that of the and ′-processes increased. The halfwidths of the three processes remained unchanged on dilution with water. The decrease in the relaxation rate of the β-process is suggested to be due to hydrogen bonding of the ---CH(CH₃)---O---CH₂--- group with water molecules. Water antiplasticizes PPO-4000 and this is interpreted as due to the increased chain length when the chain ends become linked via hydrogen bonds. The static permittivity is increased by 30% on addition of 1.23 wt% water.