Calibration curve establishment and fractionation temperature selection of polyethylene for preparative temperature rising elution fractionation

A series of copolymers of ethylene with 1-hexene synthesized using a metallocene catalyst are selected and mixed. The blend is fractionated via preparative temperature rising elution fractionation(P-TREF). All fractions are characterized via high-temperature gel permeation chromatography(GPC), 13 C nuclear magnetic resonance spectroscopy(13C-NMR), and differential scanning calorimetry(DSC). The changes in the DSC melting peak temperatures of the fractions from P-TREF as a function of elution temperature are almost linear, thereby providing a reference through which the elution temperature of TREF experiments could be selected. Moreover, the standard calibration curve(ethylene/1-hexene) of P-TREF is established, which relates to the degree of short-chain branching of the fractions. The standard calibration curve of P-TREF is beneficial to study on the complicated branching structure of polyethylene. A convenient method for selecting the fractionation temperature for TREF experiments is elaborated. The polyethylene sample is fractionated via successive self-nucleation and annealing(SSA) thermal fractionation. A multiple-melting endotherm is obtained through the final DSC heating scan for the sample after SSA thermal fractionation. A series of fractionation temperatures are then selected through the relationship between the DSC melting peak temperature and TREF elution temperature.     

DSC investigation of an isothermal crystallized poly(ethylene terephthalate)

Two samples of poly(ethylene terephthalate), oriented and unoriented (annealed for several times and temperatures), were scanned by DSC. The density of the samples was also determined. A thermal process was observed in the region between the glass transition and the melting temperature. The position of this thermal process is related to the annealing temperature. The model of the process was proposed as a simultaneous partial melting and recrystallization. The shape of the heating curve depends on the rate of both processes.     

Annealing of Polypropylene/Polyethylene Blends Near to the Melting Points in TMDSC

Annealing experiments have been carried out just below the melting temperature of both polyethylene (LLDPE) and polypropylene (PP) and their blends. The total melting enthalpy measured after the annealing cycle was greater by 10-15% with respect to the value having been measured before it. During the annealing period the heat capacity decreases to a lower value within the first 2-3 min. Heat capacities of PP (either in pure form or in the blends) measured during the heating cycle following the annealing cycle have the same value as during the cooling section. The heat capacities of the LLDPE in the heating cycle following the annealing were those of the preceding heating cycle. The total heat flows in the cooling section following the annealing cycle were greater than those in another cooling cycle at the same temperatures indicating that the crystallisation takes place during the cooling rather than during the annealing periods. The presence of LLDPE decreases the crystallisation temperature of PP. The presence of SEBS in the blend results in a greater crystallisation temperature than that of pure PP. The crystallisation temperature of LLDPE increases with increasing levels of PP. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermally induced phase separation of polystyrene-poly(vinyl methyl ether) mixtures

Past differential scanning calorimetry and dielectric relaxation measurements have established that polystyrene (PS)-poly(vinyl methyl ether) (PVME) mixtures exhibit a degree of compatibility when cast from toluene, whereas they are incompatible when cast from chloroform or trichloroethylene. The present study reports that toluene-cast mixtures can be phase-separated by thermal treatment at temperatures exceeding 125°C. This is true for samples containing 20–80 wt-% PS. The temperature of phase separation varies with heating rate; isothermal heating times needed to cause phase separation increase rapidly as the temperature approaches 125°C. Reversibility of the phase separation process depends upon such factors as cooling rate, annealing time, treatment temperature, and thermal history. By annealing and/or slow cooling, all thermally phase-separated mixtures have been brought back to their original state of compatibility. That is, there is no evidence for true irreversiblity of phase separation in thermally treated samples. Quench-cooled samples remain phase-separated indefinitely at room temperature, but this is attributed to rapid cooling below the glass transition of the PS. Chloroform-cast and trichloroethylene-cast mixtures have not been brought to a compatible state by thermal treatment, even after lengthy annealing and slow cooling steps.     

Stereochemical heterogeneity of biodegradable poly(L‐lactide) homopolymer as revealed by temperature rising elution fractionation and successive self‐nucleation/annealing thermal fractionation

Commercial poly(L ‐lactide) is typically heterogeneous in chain structure due to the existence of a small amount of D ‐lactyl units that are produced by the racemization reactions during the synthesis. In this article, the stereochemical heterogeneity of two commercial poly(L ‐lactide) was investigated with temperature rising elution fractionation (TREF) and successive self‐nucleation/annealing (SSA) thermal fractionation. For both samples, three fractions were collected and characterized with rotatory power analysis and DSC. The fractions show distinct optical purity and DSC results, which reflect the structure differences among them directly. After SSA treatment, the observation of multiple endotherms for each physically separated fraction confirms the fractionated sample contains a heterogeneous intermolecular and intramolecular distribution of defects. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

L-丙交酯/ε-己内酯共聚物分子链结构非均匀性的研究

利用升温淋洗分级(TREF)和连续自成核退火热分级(SSA)等方法研究了L-丙交酯(LLA)和ε-己内酯(ε-CL)共聚物(PLC)分子链结构的非均匀性.合成了2种不同摩尔比的PLC共聚物,用TREF的方法,将同一个共聚物分级出3个级份.用核磁共振氢谱(1H-NMR)和凝胶渗透色谱(GPC)等方法对每个级份的分子结构进行了表征.结果表明,随着淋洗温度的升高,共聚物级份中的ε-CL单元的含量明显降低,而且级份的数均分子量增大,分子量分布指数降低.在SSA热分级研究中,发现PLC各级份间存在明显的分子链间和分子链内结构上的差异,这种链结构差异使得各个级份表现出不同的熔融行为.     

Absorption in the visible region of YSZ implanted with Ag ions

Ag ions were implanted into YSZ (yttrium-stabilized (cubic) zirconia) single crystals in two different energy regimes: kiloelectron volt and megaelectron volt. Optical absorption spectra were measured in the visible region at each stage in the annealing process of the sample. Depth profiles of Ag for the samples implanted at the energy of 20 keV were measured by X-ray photoelectron spectroscopy (XPS). For the samples implanted with Ag at the low energy of 20 keV, one large absorption peak appeared in the wavelength ranging from 470 to 536 nm, depending on the dose of Ag ions. As the sample was heated to 1000 degrees C, the intensity of the absorption peak decreased gradually, but a small, broad peak remains even at the temperature of 1000 degrees C. For the samples implanted with 2.8x10(16) Ag ion cm(-2) at the high energy of 3 MeV, one broad absorption peak was observed at around 470 nm. As the sample was heated sequentially to high temperatures, the peak gradually decreased and almost disappeared at 400 degrees C. When the sample was further heated to even higher temperatures, the absorption peak at 514 nm reappeared at 1000 degrees C and grew with heating time.     

Stability and phase transformations of the mesomorphic form of isotactic polypropylene in stereodefective polypropylene

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.     

Thermal history and melting behavior of poly(ethylene terephthalate)

Low molecular weight poly(ethylene terephthalate) samples were crystallized isothermally at 120–245°C from both the amorphous state and the melt. Isothermal annealing of these polymers at 215°C provided polymers which exhibited multiple melting peaks in thermal analysis, referred to as form I and form II, as assigned by Bell and Dumbleton. In these samples the peak temperature of the form II melting endotherm and the average crystallite size are dependent on the temperature of initial crystallization. This result requires a mechanism for retaining some structural feature during the conversion from morphological form I to form II. DSC thermograms obtained at varying heating rates on samples showing only form II endotherms support the assignment of superheating as the cause of the shift to higher peak temperatures with increasing heating rate.     

Discrimination of thermally treated low density polyethylenes using DSC and principal component analysis

The potential for differential scanning calorimetry (DSC) as a tool for the discrimination of forensic polymer specimens is investigated for a series of commercial low density polyethylene (LDPE) samples. Variation in the melting temperatures of ‘as received’ samples was found to be too small for its use in sample discrimination. The melting behaviour of thermally treated samples, quenched from the melt in liquid nitrogen followed by annealing at temperatures below the melting temperature, showed promise in discrimination potential. The application of principal component analysis to aid discrimination demonstrated the necessity in using a controlled thermal history to aid the discrimination process. The clustering of the LDPEs based on the factors selected demonstrated the potential of DSC for the discrimination of forensic LDPE samples.     

热历史对尼龙1212熔融行为的影响

利用DSC方法研究了不同热历史条件对尼龙1212熔融行为的影响.不同的热历史条件下,在DSC曲线上,观察到尼龙1212产生2个或3个熔融峰,依据聚合物结晶理论,对各峰的来源进行了分析.在160℃下不同温度退火120 min的尼龙1212样品DSC曲线上,低温结晶熔融峰主要由低温结晶形成的一些微晶体或者片晶熔融产生,其晶体完善程度较差,熔融峰值较低,峰面积较小;主熔融峰是由样品在淬火过程中形成的晶体和升温过程中低温结晶形成的晶体的熔融重结晶形成较为完善的晶体熔融所产生,熔融峰值较高,峰面积较大.在不同的升温速率条件下,熔融峰温度有所移动,表明不同升温速率条件下产生的熔融峰的结晶晶型是相同的.在不同结晶时间下结晶,延长结晶时间对较高完善程度晶体的生长有利.在不同温度下依次退火处理的样品,熔融产生两个附加峰,这两个附加峰的峰温都比它们相应的退火温度高,而峰高和峰面积随退火温度降低而减小.根据等温结晶结果,由Hoffman方法确定了尼龙1212的平衡熔融温度为202.8℃.     

Effect of the γ‐form crystal on the thermal fractionation of poly(propylene‐co‐ethylene)

The effect of the γ‐form crystal on the thermal fractionation of a commercial poly(propylene‐co‐ethylene) (PPE) has been studied by differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) techniques. Two thermal fractionation techniques, stepwise isothermal crystallization (SIC) and successive self‐nucleation and annealing (SSA), have been used to characterize the molecular heterogeneity of the PPE. The results indicate that the SSA technique possesses a stronger fractionation ability than that of the SIC technique. The heating scan of the SSA fractionated sample exhibits 12 endothermic peaks, whereas the scan of the SIC fractionated sample only shows eight melting peaks. The WAXD observations of the fractionated PPE samples prove that the content of the γ‐form crystals formed during the thermal treatment of the SIC technique is much higher than that of the SSA treatment. The former is 57.4%, whereas the later is 12.6%. The effect of theγ‐form crystals on thermal fractionation ability is discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4320–4325, 2004     

Evaluation of the physical stability and local crystallization of amorphous terfenadine using XRD-DSC and micro-TA

It is very difficult to follow rapid changes in polymorphic transformation and crystallization and to estimate the species recrystallized from the amorphous form. The aim of this study was to clarify the structural changes of amorphous terfenadine and to evaluate the polymorphs crystallized from amorphous samples using XRD-DSC and an atomic force microscope with a thermal probe (micro-TA). Amorphous samples were prepared by grinding or rapid cooling of the melt. The rapid structural transitions of samples were followed by the XRD-DSC system. On the DSC trace of the quenched terfenadine, two exotherms were observed, while only one exothermic peak was observed in the DSC scan of a ground sample. From the in situ data obtained by the XRD-DSC system, the stable form of terfenadine was recrystallized during heating of the ground amorphous sample, whereas the metastable form was recrystallized from the quenched amorphous sample and the crystallized polymorph changed to the stable form. Obtained data suggested that recrystallized species could be related to the homogeneity of samples. When the stored sample surface was scanned by atomic force microscopy (AFM), heterogeneous crystallization was observed. By using micro-TA, melting temperatures at various points were measured, and polymorph forms I and II were crystallized in each region. The percentages of the crystallized form I stored at 120 and 135 °C were 47 and 79%, respectively. This result suggested that increasing the storage temperature increased the crystallization of form I, the stable form, confirming the temperature dependency of the crystallized form. The crystallization behavior of amorphous drug was affected by the annealing temperature. Micro-TA would be useful for detecting the inhomogeneities in polymorphs crystallized from amorphous drug.     

Etude du comportement thermique de la poly(pivalolactone) par diffraction des rayons-x aux petits angles et par calorimetrie dynamique

Poly(α,α-dimethyl-β-propiolactone) (PPL), known as poly(pivalolactone), has been studied by differential scanning calorimetry (DSC) and small-angle X-ray diffraction (SAXR). DSC measurements indicate the presence of two melting endotherms. Peak 1 and Peak 2, the latter at lower temperatures. Peak 1 is relatively unaffected by the crystallization temperature and its relative intensity decreases with heating rate. Peak 2 is greatly influenced by the crystallization temperature of the sample and its relative intensity increases with heating rate. Peak 2 is associated with the true melting of the PPL samples and Peak 1 with a recrystallization process during the heating cycle. SAXR long periods increase with crystallization and annealing temperatures. Similar increases in density, in melting temperature, in lamella thickness, and in degree of crystallinity have been observed. These results lead to a thermodynamic melting temperature of 268 ± 3 for PPL, and to interfacial free energies of, respectively. 13 × 10^?7 J cm^?2 and (43 ± 4) × 10^?7 J cm^?2 for the lateral surface and the fold surface of the PPL crystal.     

Compatibility studies of polystyrene and poly(vinyl acetate) blends using electrostatic force microscopy

The effect of thermal treatment on the phase separation process of the components of a polymer blend was investigated using electrostatic force microscopy (EFM). EFM technique is an advance on conventional atomic force microscopy, which enables us to measure locally the dielectric properties of the samples under investigation providing compositional information. In this work, we studied the phase separation process of the polymer blend thin films made of polystyrene and poly(vinyl acetate) (PS/PVAc) (75/25 weight fraction). The samples were subjected to different thermal treatments. It was found that at low annealing temperature, PVAc forms many small islands within PS matrix. As the annealing temperature increases, the number of PVAc islands decreases with an increase in the size of the islands. These islands take spherical‐like shape when annealed at a temperature well above the glass transition temperatures of both the component polymers. Despite these morphological/topographical changes, EFM images evidence that there is no interdiffusion which was further confirmed by quantitatively measuring the value of the dielectric permittivity across the interphase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1332–1338, 2011     

Annealing of the β-modification of polypropylene

Expressive memory effect has been proven in the course of the annealing of the β-modification of polypropylene (β-PP). The character of the structural changes taking place in β-PP during annealing strongly depends on the thermal history. In dependence on the thermal history βα or ββ-recrystallization takes place during thermal treatment. In case the annealing starts from the crystallization temperature β-PP is recrystallized into a more perfect structure with higher melting point - similarly to thermodynamically stable modifications. The samples cooled below a certain temperature before subjected to annealing transform partly or fully into the α-modification. The βα-recrystallization tendency prevalent on the effect of recooling can be reduced or eliminated with the aid of stepwise annealing with decreasing temperature steps.     

Effect of thermal expansion on shape memory behavior of polyurethane and its nanocomposites

The effects of thermal expansion on shape memory performance of shape memory polyurethanes and their nanocomposites with organoclay, carbon nanofiber (CNF), silicon carbide (SiC), and carbon black (CB) were evaluated. The shape memory test cycle involved tensile deformation at above the trigger temperature to initiate shape memory function, cooling to room temperature to fix the shape, and shape recovery induced by heating to above the trigger temperature. Phenomenological models were used to interpret the experimental data on coefficient of thermal expansion (CTE). It was found that Kerner model showed good fit for composites of SiC and CB, and Halpin model gave better fit for composites of organoclay and CNF. It was observed that thermal expansion exerts negative effect on recovered strain, the extent of which depends on the magnitude of temperature gradient, CTE, and the level of tensile strain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1437–1449, 2008     

Calorimetric determination of head-to-head defect in poly(vinylidene fluoride)

Differential scanning calorimetric analyses were conducted on samples of poly(vinylidene fluoride) polymerized in an autoclave by tributylborane-oxygen by free-radical initiation at low temperature (-70-0°C). The peak melting points and the percent head-to-head defect in each polymer sample were determined by a reported calorimetric method. A commercial sample showed a melting temperature in the range 157–162°C and a percent head-to-head defect of 7.7%; whereas two experimental samples showed melting temperatures in the range 172–179°C with a percent head-to-head defect of 4.4 and 4.9%. The calorimetric procedure was modified by reducing annealing times to only 2 h, which saves time and, as shown in this study, avoids thermal polymer degradation.     

Relationship between transformation temperatures and alloying elements in Cu–Al–Ni shape memory alloys

Cu–Al–Ni shape memory alloys are good candidates for high temperature applications. We have investigated the effects of alloying elements on transformation temperatures, heat-capacity values, and structural properties of Cu–13.73Al–4.3Ni and Cu–13Al–4.3Ni (wt%) shape memory alloys. The evolution of the transformation temperatures was studied by differential scanning calorimetry with different heating/cooling rates. The heat-capacity measurements of the samples were made. It was found that the mass percentage of the alloying element has an important effect on the characteristic transformation temperatures and thermodynamic parameters. The structural changes of the samples were studied by X-ray diffraction measurements and optical microscope observations at room temperature. It is evaluated that the transformation parameters of CuAlNi shape memory alloy can be controlled by the change of the mass percentages of the alloying elements.     

Reversible Melting of Semi-Crystalline Polymers 2. Annealing Near to the Melting Point

Annealing experiments have been carried out at a few degrees below the melting point of different polyethylenes (LDPE, LLDPE, HDPE), of polypropylene (PP) and of Nylon-6. The heat capacities decrease during the annealing, within a 2-4 min time scale, to a lower value which corresponds to the extrapolated heat capacity values obtained for the cooling cycle when the polymer is cooled from the melt. Heat capacities in the heating cycle following the cooling cycle of PP, Nylon-6 and HDPE have the same value as during the cooling section. This is not the case for LDPE and LLDPE. Exothermic total heat flow in the cooling section following the annealing indicates that the crystallisation takes place during the cooling rather than during the annealing period. The total melting enthalpy measured before and after the annealing cycle is the same. The reversing heat flow shows an excellent fit to the change of the crystallinity measured by small angle scattering of synchrotron radiation during a heating cycle at temperatures below the melting peak. A coupled thermodynamic interaction of the crystalline and the amorphous phases is concluded from this study. This kind of interaction is possible at the lateral end of polymeric chains incorporated into the crystalline phase. This is an indication of the portion of tie molecules in the system, i.e. the portion of fringed micelle type of crystalline morphology with respect to that of folded chain lamellae. This revised version was published online in July 2006 with corrections to the Cover Date.