Six types of spherulite morphologies with polymorphic crystals in poly(heptamethylene terephthalate)

Six types of spherulite morphologies packed with polymorphic crystals and their growth kinetics in melt-crystallized poly(heptamethylene terephthalate) (PHepT) were characterized using polarized-light optical microscopy (POM), Fourier transformed infrared microspectrometry (micro-FTIR), differential scanning calorimetry (DSC) and atomic-force microscopy (AFM). Two maximum melting temperatures (T_max), a higher 150 °C and a lower 110 °C, were used to melt the initially crystallized PHepT of either α- or β-crystal. The high T_max was enough to melt all nuclei, but the lower T_max was considered as near or slightly below the equilibrium melting temperatures of these two cells (if estimated by nonlinear methods). When crystallized at various T_c from these two T_max’s, PHepT can exhibit as many as six types of spherulites (Ring Type-I, -II, -III, Maltese-cross Type-1, -2, and -3) owing to different nucleations. Ring Type-I, Maltese-cross Type-1 and -3 spherulites are packed of the sole β-crystal, while Ring Type-II, -III and Maltese-cross Type-2 spherulites are attributed to the sole α-crystal. However, as the PHepT polymorphic cells are related to T_c, such correlations between the crystal cells and spherulite types (ring or ringless) cannot be ruled out to be a coincidence.     

Effects of molten poly(3-hydroxybutyrate) on crystalline morphology in stereocomplex of poly(L-lactic acid) with poly(D-lactic acid)

Effects of poly(3-hydroxybutyrate) (PHB) on crystalline morphology of stereocomplexing capacity of poly(L- and D-lactic acid) (PLLA and PDLA) were studied by differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), atomic-force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). When crystallized at high T_c (130 °C or above), morphology transition of stereocomplexed PLA (sc-PLA) occurs from original well-rounded Maltese-cross spherulites to dendritic form in blends of high PHB contents (50 wt.% or higher), where PHB acts as an amorphous species. Microscopy characterizations show that morphology of sc-PLA in PHB/sc-PLA blends crystallized at T_c = 170 °C no longer retain original complexed Maltese-cross well-rounded spherulites; instead, the spherulites are disintegrated and restructured into two types of dendrites: (1) edge-on feather-like dendrites (early growth) and (2) flat-on wedge-like crystal plates (later growth) by growing along different directions and exhibiting different optical brightness. The concentration and/or distribution of amorphous PHB at the crystal growth front, corresponding to variation of the slopes of spherulitic growth rates, is a factor resulting in alteration and restructuring of the sc-PLA spherulites in the blends. Despite of spherulite disintegration, WAXD result shows that these two PHB-induced dendrites still retain the original unit cells of complexes, and thus these two new dendrites are sc-PLA.     

Crystal morphology,melting behaviors and isothermal crystallization kinetics of SCF/PTT composites

Isothermal crystallization kinetics, subsequent melting behavior, and the crystal morphology of short carbon fiber and poly(trimethylene terephthalate) composites (SCF/PTT) were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The crystal morphology of the composites isothermally crystallized at T_c = 205°C is predominantly banded spherulites observed under polarizing micrographs, while the pattern of banded spherulites changed from ring to serration as the SCF content added into the PTT. Moreover, nonbanded spherulites formed at T_c = 180°C. The commonly used Avrami equation was used to fit the primary stage of the isothermal crystallization. The Avrami exponents n are evaluated to be 1.6–2.0 for the neat PTT and 2.7–3.0 for SCF/PTT composites, and the SCF acting as nucleation agents in composites accelerates the crystallization rate with decreasing the half‐time of crystallization and the sample with SCF component of 2% has the fastest crystallization rate. The crystallization activation energy calculated from the Arrhenius formula suggests that the adding SCF component improved the crystallization ability of the PTT matrix greatly, and the sample with of 2% SCF component has the most crystallization ability. Subsequent melting scans of the isothermally crystallized composites all exhibited triple melting endotherms, in which the more the component of SCF, the lower temperature of the melting peak, indicating the less perfect crystallites formed in those composites. Furthermore, the melting peaks of the same sample are shifted to higher temperature with increasing T_c, suggesting the more perfect crystallites formed at higher T_c. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Phase behavior,polymorphism and spherulite morphology in Poly(1,4-butylene adipate) interacting with two structurally similar acrylic polymers

Polymorphism and spherulite morphology of poly(1,4-butylene adipate) (PBA) influenced by two structurally similar acrylic polymers: poly(benzyl methacrylate) (PBzMA) and poly(phenyl methacrylate) (PPhMA) were investigated by differential scanning calorimeter (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). Characterization first proves that PBA is similarly miscible with PBzMA and PPhMA, respectively, but effects of these two polymers on polymorphism and spherulite morphology of PBA differ dramatically. The depression of PBA growth rate in blends retards the formation of kinetically stable β-form crystal and favors the formation of α-form crystal of PBA. In PBA/PBzMA blend, the spherulite morphology remains all ringless after addition of PBzMA at any T_c, regardless of polymorphism or not. By contrast, the spherulite morphology in PBA/PPhMA blend can change from ringless, ring-banded, to dendritic with respect to T_c. In general, the formation of ring-banded spherulites in either of blends has no relation with the polymorphic crystal cells. Effects of amorphous PBzMA or PPhMA polymers on the polymorphic and ring-banded patterns on PBA differ and were likely controlled by the interaction strength between the two constituents of the blends.     

Microscopy and microbeam X-ray analyses in poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with amorphous poly(vinyl acetate)

Double ring-banded spherulites of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV with 12 wt% 3HV) blending with 30 wt% amorphous poly(vinyl acetate) (PVAc) were examined using polarized light optical microscopy (POM), scanning electron microscopy (SEM), atomic-force microscopy (AFM) and micro-beam X-ray diffraction. A ring-banded spherulite of PHBV/PVAc 70/30 blend was linearly scanned across the bands in 5 μm steps by means of micro-beam X-ray diffraction. Solvent-etching and fracturing were utilized for probing the interior lamellar textures of the blend samples. Detail interior lamellar orientations in bulk film of PHBV three-dimensional ring-banded spherulites were revealed. SEM and micro-beam X-ray diffraction results suggest that the PHBV lamellar orientation gradually change along the radial growth direction with right-handed rotation sense. The blending effect in band pattern (width and regularity) of PHBV/PVAc blend was discussed.     

Nanostructure of atmospheric and high-pressure crystallized poly(ethylene-2,6-naphthalate). Part II: structure-microhardness correlations

The microhardness of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN), with a detailed characterized nanostructure has been investigated. PEN samples were crystallized from the glassy state at atmospheric pressure and from the melt at high pressure and were characterized using small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). Results show that the degree of crystallinity derived from WAXS, for both atmospheric and high-pressure crystallized PEN, is smaller than that obtained by density and calorimetry. For high-pressure crystallized samples, both, crystallinity and microhardness values are larger than those found for the material crystallized under atmospheric pressure. In the latter case, the hardness values depend on the volume fraction of lamellar stacks within spherulites X_L that depends on the crystallization temperature T_c. For T_c<200 °C, X_L is found to be less than 50%. Thus, for T_c<200 °C a linear relationship between H and T_c is observed provided a sufficiently long crystallization time is used. Results are discussed in terms of the rigid amorphous fraction that appears as a consequence of the molecular mobility restrictions due to the crystal stacks.     

Phase content and dielectrical properties of sintered BaSrTiO ceramics prepared by a high temperature hydrothermal technique

Ba_xSr_1−xTiO₃ ceramic powders with varying barium content were prepared by a high temperature hydrothermal technique and sintered at 1300 °C for 1–8 h. Their dielectrical, phase and structural properties were investigated. It was computed from the XRD spectra that the samples with a small amount of strontium, no more than 10% of the initial Ba:Sr share, had a single phase structure with x = 0.77–0.79 and Curie point T_c = 37–53 °C. Samples with a higher initial Sr ratio developed a two-phase structure and two Curie points. T_c(x) dependence showed that all the experimental data followed a linear trend and were close to the values obtained from the conventional solid state technique, while the dielectric constant was almost one order of magnitude smaller.     

Characterization of sepiolite as a support of silver catalyst in soot combustion

Turkish sepiolite–zirconium oxide mixtures were applied as a support for the silver catalyst in a soot combustion. Sepiolite–Zr–K–Ag–O catalyst was characterized by XRD, N₂ adsorption, SEM, TPR-H₂ and EGA-MS. The combustion of soot was studied with a thermobalance (TG-DTA). The modification resulted in a partial degradation of the sepiolite structure, however, the morphology was preserved. The adsorption of N₂ of the modified sepiolite is a characteristic for mesoporous materials with a wide distribution of pores. The specific surface area S_BET equals 83 m²/g and the pores volume is 0.23 cm³/g. The basic character of the surface centers of sepiolite is indicated by CO₂ desorption (TPD-MS) at 170 °C and at about 620 °C due to a surface carbonates decomposition. The thermodesorption of oxygen at 650–850 °C indicates the decomposition of AgO_x phases at the surface. The presence of AgO_x phases is also confirmed by TPR-H₂ spectrum (low temperature reduction peak at 130 and 180 °C). The high-temperature reduction at about 570 °C is probably related to Ag–O–M phases on the support.The soot combustion takes place at T₅₀ = 575 °C. Without silver (sepiolite–Zr–K–O) T₅₀ = 560 °C but sepiolite modified with silver (sepiolite–Zr–K–Ag–O) undergoes the same process at T₅₀ = 490 °C.     

Composition dependence of crystallized lamellar morphology formed in crystalline-crystalline diblock copolymers

We have investigated the crystallized morphology formed at each temperature T_c (20 °C ≤ T_c ≤ 45 °C) in double crystalline poly(?-caprolactone)-block-polyethylene (PCL-b-PE) copolymers as a function of composition (or volume fraction of PE blocks ?_PE) by employing small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) techniques. When PCL-b-PE with ?_PE ≤ 0.58 was quenched from a microphase-separated melt into T_c, the crystallization of PE blocks occurred first to yield an alternating structure consisting of thin PE crystals and amorphous PE + PCL layers (PE lamellar morphology) followed by the crystallization of PCL blocks, where we can expect a competition between the stability of the PE lamellar morphology (depending on ?_PE) and PCL crystallization (on T_c). Two different morphologies were formed in the system judging from a long period. That is, the PCL block crystallized within the existing PE lamellar morphology at lower T_c (<30 °C) to yield a double crystallized alternating structure while it crystallized by deforming or partially destroying the PE lamellar morphology at higher T_c (>35 °C) to result in a significant increase of the long period. However, the temperature at which the morphology changed was almost independent of ?_PE. For PCL-b-PE with ?_PE ≥ 0.73, on the other hand, the morphology after the crystallization of PE blocks was preserved at every T_c investigated.     

Magnetic properties of La0.7Sr0.3MnO3 under the pressure and the transport property under the magnetic field

Polycrystalline La_0.7Sr_0.3MnO₃ sample (LSMO) was synthesized by the solid phase reaction; it exhibits the paramagnetic-ferromagnetic transition at T_c = 362 K at the ambient pressure; it is paramagnetic metallic state above T_c and the ferromagnetic metallic state below T_c. It was observed that the pressure effect depends on the temperature range: (a) In the paramagnetic region, the magnetization M hardly changes with the pressure P, that is, ΔM≈0. There exist the antiferromagnetic (AFM) coupled ferromagnetic clusters in the paramagnetic region, and the pressure enhances the AFM coupling. (b) In the temperature range around T_c, the pressure increases M, that is, ΔM > 0, with the concomitant increase in T_c; the average pressure coefficient dT_c/dP is 5.40 K/GPa at P = .74 GPa, much smaller than 15.47 and 15.90 K/GPa for La_0.7Ca_0.3MnO₃ and La_0.9Ca_0.1MnO₃, respectively, due to the different distortion degree of MnO₆ octahedra in Ca and Sr doped manganites. (c) In the temperature region below T_c, the pressure reduces M, that is, ΔM < 0. M is determined by the competition between the Mn³⁺-O-Mn⁴⁺ double exchange and the interparticle dipolar interaction. The pressure enhances the interparticle dipolar interaction, leading to a significant decrease in magnetization. The resistivity of LSMO exhibits the metallic behavior in the temperature range of 5 K~370 K; it decreases as the applied magnetic field H increases from 0 to 7 T, that is, the magnetoresistance effect which is more significant around T_c. The fitting to the low-temperature resistivity shows that the applied magnetic field reduces the scattering from the grain boundary, electron, phonon, and magnon, especially reduces the electron-electron scattering.     

Effect of Pre-Melting Time on Crystallization of Poly(ethylene terephthalate)

In this study poly(ethylene terephthalate) (PET) was melted at 300°C, approximately 46°C above the crystalline melting point, T _m, for different times, i.e., Δt _m,=5, 8, and 10 min, and then quenched to different isothermal crystallization temperatures, T _c, ranging from 190°C to 230°C. The effect of pre-melting time, Δt _m, at 300°C on the degree of crystallinity and on crystalline morphology were investigated by differential scanning calorimetry (DSC) and polarized-light microscopy (PLM). After crystallization at low T _c, PLM data revealed the PET contained usual, positive, and unringed spherulites. After crystallization at high T _c PET contained unusual, ringed, and double-extinction spherulites. The experimental results reveal that increasing the pre-melting time Δt _m at 300°C causes an increment in T _c for usual–unusual, unringed–ringed, and positive–double-extinction transitions of the PET spherulites. The experimental results also show that PET with a pre-melting time Δt _m=8 min had higher crystallinity than those with pre-melting times Δt _m=5 and 10 min. These crystallization phenomena were attributed to the different numbers of residual unmelted PET crystallites as a result of the variation in pre-melting time, Δt _m, at 300°C.     

Crystallization controlled by layered silicates in nylon 6–clay nano-composite

To understand the effect of the montmorillonite (MMT) particles on the crystallization kinetics and crystalline morphology of nylon 6 upon nano-composite formation, we have characterized the crystallization behaviors by using light scattering, wide-angle X-ray diffraction (WAXD), transmission electron microscope (TEM) and rheological measurement. The correlation between the nucleating effect and the growth mechanism of the different polymorphism (γ-phase) of nylon 6 in the nano-composite (N6C3.7) was probed. N6C3.7 exhibited γ-phase crystal due to the nucleating effect of the dispersed MMT particles into the nylon 6 matrix throughout the whole T_c range (=150–215 °C). The lamellar growth of the γ-phase crystal took place on both sides of the dispersed MMT particles. In comparison between the temperature dependence of the characteristic relaxation time and the crystallization time, the lamellar growth of the γ-phase crystal has been discussed. The stable growth of the γ-phase was strongly disturbed at low T_c range (=160–190 °C) due to the lack of time for crystallization.     

Amorphous phase and crystalline morphology in blend of two polymorphic polyesters: Poly(hexamethylene terephthalate) and poly(heptamethylene terephthalate)

Crystalline/crystalline blends of two polymorphic aryl-polyesters, poly(hexamethylene terephthalate) (PHT) and poly(heptamethylene terephthalate) (PHepT), were prepared and the crystallization kinetics, polymorphism behavior, spherulite morphology, and miscibility in this blend system were probed using polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), temperature-resolved wide-angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS). The PHT/PHepT blends of all compositions were proven to be miscible in the melt state or quenched amorphous glassy phase. Miscibility in PHT/PHepT blend leads to the retardation in the crystallization rate of PHT; however, that of PHepT increases, being attributed to the nucleation effects of PHT crystals which are produced before the growth of PHepT crystals. In the miscible blend of polymorphic PHT with polymorphic PHepT, the polymorphism states of both PHT and PHepT in the blend are influenced by the other component. The fraction of the thermodynamically stable β-crystal of PHT in the blend increases with increasing PHepT content when melt-crystallized at 100 °C. In addition, when blended with PHT, the crystal stability of PHepT is altered and leads to that the originally polymorphic PHepT exhibits only the β-crystal when melt-crystallized at all T_c's. Apart from the noted polymorphism behavior, miscibility in the blend also shows great influence on the spherulite morphology of PHT crystallized at 100 °C, in which the dendritic morphology corresponding to the β-crystal of PHT changes to the ring-banded in the blend with higher than 50 wt% PHepT. In blends of PHT/PHepT one-step crystallized at 60 °C, PHepT is located in both PHT interlamellar and interfibrillar region analyzed using SAXS, which further manifests the miscibility between PHT and PHepT.     

Crystal orientation of poly(?-caprolactone) blocks confined in crystallized polyethylene lamellar morphology of poly(?-caprolactone)-block-polyethylene copolymers

The crystal orientation of poly(?-caprolactone) (PCL) blocks in PCL-block-polyethylene (PE) copolymers has been investigated using two-dimensional small-angle X-ray scattering (2D-SAXS) and 2D wide-angle X-ray diffraction (2D-WAXD) as a function of crystallization temperature T_c and thickness of PCL layers d_PCL. The PCL blocks were spatially confined in the solid lamellar morphology formed by the crystallization of PE blocks (PE lamellar morphology), an alternating structure of crystallized PE lamellae and amorphous PCL layers. This confinement is expected to be intermediate between hard confinement by glassy lamellar microdomains and soft confinement by rubbery ones, because the crystallized PE lamellae consist of hard PE crystals covered with amorphous (or soft) PE blocks. The 2D-SAXS results showed uniaxial orientation of the PE lamellar morphology after applying the rotational shear to the sample. Therefore, it was possible to investigate crystal orientation of PCL blocks within the oriented PE lamellar morphology. The 2D-WAXD results revealed that the c axis of PCL crystals (i.e., stem direction of PCL chains) was parallel to the lamellar surface normal irrespective of T_c when 16.5 nm ≥ d_PCL ≥ 10.7 nm. However, it changed significantly with changing T_c when d_PCL = 8.8 nm; the c axis was perpendicular to the lamellar surface normal at 45 °C ≥ T_c ≥ 25 °C while it was almost random at 20 °C ≥ T_c ≥ 0 °C. These results suggest that the PE lamellar morphology plays a similar role to glassy lamellar microdomains regarding spatial confinement against subsequent PCL crystallization.     

Isothermal and non-isothermal crystallization behavior of poly(l-lactic acid): Effects of stereocomplex as nucleating agent

The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (T_c)≥125 °C), the induction period for PLLA spherulite formation (t_i) (T_c≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤T_c≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (t_c(S), t_c(1/2), and t_c(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the t_c(S), t_c(1/2), and t_c(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long t_i and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3-3 wt%, the t_c(S), t_c(1/2), and t_c(E) were shorter than or similar to those at PDLA content of 0 wt% for the T_c range below 95 °C and above 125 °C, whereas this inclination was reversed for the T_c range of 100-120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected.     

Catalytic decomposition of alcohols over size-selected Pt nanoparticles supported on ZrO2: A study of activity, selectivity, and stability

This article discusses the performance of ZrO₂-supported size-selected Pt nanoparticles for the decomposition of methanol, ethanol, 2-propanol, and 2-butanol. The potential of each alcohol for the production of H₂ and other relevant products in the presence of a catalyst is studied in a packed-bed mass flow reactor operating at atmospheric pressure. All the alcohols studied show some decomposition activity below 200 °C which increased with increasing temperature. In all cases, high selectivity towards H₂ formation is observed. With the exception of methanol, all alcohol conversion reactions lead to catalyst deactivation at high temperatures (T > 250 °C for 2-propanol and 2-butanol, T > 325 °C for ethanol) due to carbon poisoning. However, long-term catalyst deactivation can be avoided by optimizing reaction conditions such as operating temperature.     

Simultaneous presence of positive and negative spherulites in syndiotactic polystyrene and its blends with atactic polystyrene

Crystal growth rates of syndiotactic polystyrene (sPS) and its blends with atactic polystyrene (aPS) at various temperatures (T_c) were measured using a polarized optical microscope (POM). In addition to the positively birefringent spherulites and axilites (P-spherulites and P-axilites) which are predominantly observed, small population of negatively birefringent spherulites (N-spherulites) is also detected in the neat sPS as well as in the sPS/aPS blends at a given T_c. Both P-spherulites and P-axilites possess a similar growth rate, whereas a smaller growth rate is found for N-spherulites at all T_c and samples investigated. Melting behavior of individual P- and N-spherulites was feasibly traced using hot-stage heating and a highly sensitive CCD through the decay of transmitted light intensity under cross-polars. Both P- and N-spherulites demonstrate exactly the same melting behavior under POM, which well corresponds to the differential scanning calorimetry measurements, suggesting no difference in lamellar thickness distribution or crystal perfection within P- and N-spherulites. Lamellar morphologies within spherulites were extensively investigated using transmission electron microscopy (TEM) as well as scanning electron microscopy (SEM). Results obtained from TEM and SEM show that the lamellar stacks within P-spherulites grow radially, whereas those within N-spherulites are packed relatively tangentially. The growth of P-spherulites is associated with the gradual increase of lamellae' lateral dimensions which follows the conventional theory of growth mechanism. However, the measured growth rate of N-spherulites is relevant to the gradual deposition of new lamellar nuclei adjacent to the fold surfaces of already-existing lamellar stacks. The difference in measured growth rate between P- and N-spherulites is attributed to the different energy barrier required to develop stable nuclei. Based on the exhaustive TEM and SEM observations, plausible origin of N-spherulites is provided and discussed as well.     

Isothermal crystallization behavior of metallocene‐catalyzed isotactic polypropylene

Isothermal crystallization behavior of isotactic polypropylene (iPP) synthesized using metallocene catalyst was investigated in this work. The isotacticity of the polypropylene was characterized by ¹³C‐NMR spectroscopy. It was found that the melting temperature (T_m) of the iPP is 123.51°C and the crystallization temperature (T_c) is 93°C. The iPP synthesized in this work did not show a general increase of T_m with an increase of crystallization temperature T_c, due to the short crystallization time of 20 min and low molecular weight (number average molecular weight = 6,300). The iPP showed a tendency of increasing heat of fusion (ΔH_f) with decreasing crystallization temperature. All the spherulites of iPP samples showed negative birefringence. For the iPP sample crystallized at the highest T_c (= 123°C, just below T_m), the spherulite showed a pronounced Maltese Cross and a continuous sheaf‐like texture aligning radially, which suggests that R‐lamellaes are dominant in this spherulite. The crystalline structure of the iPP was also investigated by X‐ray diffraction. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 231–237, 2005     

Shear-induced phase separation and crystallization in semidilute solution of ultrahigh molecular weight polyethylene: Phase diagram in the parameter space of temperature and shear rate

In the previous papers, we elucidated enhancement of concentration fluctuations, phase separation, and crystallization induced by steady state or step-up shear flow, as observed by shear small-angle light scattering, optical microscopy, and birefringence, for a semidilute solution of ultrahigh molecular weight polyethylene in paraffin as an athermal solvent. However the studies were done only at a given temperature of 124 °C, which is higher than the nominal melting temperature of the quiescent solution T_nm (115–119 °C). It is crucial to extend the studies over a wider temperature range in order to generalize shear-induced phase behavior of the solution. Thus in this work we constructed a kind of phase diagram in the parameter space of temperature (T) and shear rate (). The temperature range covered was higher than T_nm, so that the phase diagram is strictly concerned with shear-induced phase behavior (i.e., without shear the solution is homogeneous and in a single-phase state). The diagram identified Regimes I–III in the T– space as will be detailed in the text. In constructing the phase diagram we found the following new points also. (i) The critical shear rate _cx which defines the boundary between Regimes I and II was independent of T. (ii) Regime III identified previously through the dependence of the integrated scattered intensity only at a particular temperature T = 124 °C was further separated into two regimes of III_a and III_b below and above a critical temperature (147 °C), respectively, through the observation of the dependence as a function of T: In Regime III_a, the sheared solution developed the optically anisotropic fibrous structures, indicative of the shear-induced crystallization triggered by the shear-induced concentration fluctuations in Regime II; In Regime III_b, the solution is so stable that it did not show a trend of the shear-induced crystallization even at the highest shear rates accessible in this experiment, but it only showed the shear-induced phase separation. (iii) The critical shear rates _c,streak and _cz, which define respectively the boundary between Regimes II and III_a and that between Regimes II and III_b, are sensitive to temperature.     

Melting behavior of poly(l-lactic acid): Effects of crystallization temperature and time

Effects of crystallization temperature and time on the melting behavior of poly(l-lactic acid) were studied with differential scanning calorimetry (DSC). The isothermal crystallization was performed at various temperatures (T_cs), and DSC melting curves for the isothermally crystallized samples were obtained at 10 K min⁻¹. When T_c was lower than T_d (∼135 °C), the double melting peaks appeared. The melting behavior, especially T_c dependence of the melting temperature (T_m), discretely changed at T_b (=113 °C), in accordance with the discrete change of the crystallization behavior at T_b, which was previously reported. When T_c was higher than T_d, a single melting peak appeared. In addition, T_c dependence of dT_m/dT_c discretely changed at T_d. That is, the melting behavior, especially T_c dependence of T_m and dT_m/dT_c, are different in three temperature regions of T_c divided by T_b and T_d: Regions I (T_c ≤ T_b), II (T_b ≤ T_c ≤ T_d), and III (T_d ≤ T_c). The effect of crystallization time on the melting behavior, melting temperature and heat of fusion in each temperature region of T_c is also discussed.