高压下尼龙1010-单壁碳纳米管复合材料的结晶行为

 采用XKY-6×1200MN型六面顶压机,在不同温度、压力条件下处理30 min后制备了尼龙1010(PA1010)-单壁碳纳米管（SWCNT）复合材料的高压结晶样品,通过X射线衍射（XRD）、差热分析仪（DSC）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）,研究了高压处理样品的结晶行为、结构变化及形貌特征。结果表明：在1.0~2.5 GPa压力下,属于高压熔体结晶;在3.0和4.5 GPa压力下属于高压退火处理;高压结晶或高压退火均有助于聚合物片层晶体的增厚,并且高压熔体结晶的增厚效果优于高压退火处理。XRD结果表明,PA1010的三斜晶型在高压处理后保持不变,高压熔体结晶或高压退火都可以使(100)晶面和(010)晶面间距减小,即高压处理致使聚合物分子链紧密堆积。DSC结果表明：在高压熔体结晶过程中,升高压力和温度可以得到片层厚度较大的PA1010晶体;在2.0 GPa、350 ℃下获得的高压结晶样品的熔点和结晶度最高,分别达到208.5 ℃和64.6%。SEM和TEM结果表明：与常压结晶样品相比,高压结晶样品内部出现c轴厚度超过150 μm的大尺寸晶体;SWCNT与PA1010基体之间形成相互穿插的网络结构,刚性的SWCNT作为高压成核剂促进PA1010晶体生长和增厚。     

Exploring the melting of a semirigid-chain polymer with temperature-resolved small-angle <Emphasis Type="Italic">X</Emphasis>-ray scattering

The thermal behavior of semirigid semicrystalline polymers differs significantly from that of flexible-chain polymers. The origin of the differences is believed to lie in the higher energy expenditure associated with the formation of adjacent re-entry folds at the crystalline surface in the case of semirigid chains. The effect of constraints imposed by the interlamellar amorphous regions on the neighboring crystals was studied with temperature-resolved synchrotron radiation small-angle X-ray scattering (SAXS). The analysis of SAXS patterns with a generalized paracrystalline lamellar stack model indicates that melting of a semirigid-chain polymer is not a random process but that the crystals grown in the smallest amorphous gaps melt first. This suggests that the hitherto largely neglected geometrical confinement effects may play an important role in determining the thermodynamic stability of semirigid-chain polymer crystals.Received: 5 March 2004, Published online: 4 May 2004PACS: 61.41. + e Polymers, elastomers, and plastics - 64.70.Dv Solid-liquid transitions - 81.10.Aj Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation     

On the formation of lamellae during annealing of extended chain crystals of radiation-polymerized trioxane

The structure changes of radiation-polymerized trioxane taking place during annealing have been studied by means of electron microscopy, X-ray small- and wide-angle scattering, and differential thermal analysis. The original fibrillar crystals, supposedly consisting of extended chains, change into lamellar crystals due to annealing at temperatures between 150° and 190°C. Lamella formation can be connected with the appearance of a long period of about 200A which is not observed in the unannealed sample.

During annealing within the same temperature range the X-ray reflections due to the twin structure of the original polytrioxane disappear, whereas the orientation of the fraction with its c-axis parallel to the c-axis of the parent trioxane remains unaltered.

The melting point of the lamellar crystals obtained by annealing is 186°-187°C and, therefore, considerably higher than the melting point (175°C) of crystals grown during cooling of a polytrioxane melt. The equilibrium melting point of an undisturbed extended-chain poly-oxymethylene single crystal must be still higher and may even approach 200°C.

On the basis of the electron-microscopical observations and the X-ray results, it is supposed that the process of lamella formation takes place continuously by reorganization of the whole structure including the already grown lamellae. The rate of this process is the faster the higher the annealing temperature. Some possible mechanisms are discussed but the final reasons for the observed structure changes are not known.     

Size-Frustrated Thickening Growth of Extended-Chain Crystals in a Poly (Ethylene Terephthalate)/Bisphenol A Polycarbonate Blend Under High Pressure

High-pressure crystallized blend samples of poly (ethylene terephthalate)/bisphenol A polycarbonate (PET/BAPC) were investigated with scanning electron microscopy (SEM), confocal laser micro-Raman spectroscopy, and a standard general area detector diffraction system (GADDS) X-ray micro-diffraction system. The results showed that long time annealing at high pressure could result in extended-chain polymer single crystals with thicker lamellae. However, the large size of such crystals was also found to frustrate their thickening growth along the c-axis thickness, as demonstrated by the observation of a novel wrinkled crystal. The winkled crystals with different morphologies were thought to be formed as the compromise between the kinetic pathway and the thermodynamic driving force during the size-frustrated thickening growth of the large polymer crystals. Further observations revealed that environmental confinement was the main factor responsible for the creation of the wrinkled structures in polymer extended-chain crystals. The present study may promote understanding of the fundamental nature of the crystal thickening in these binary polyester blends.     

Structural and technical details of the Kirkwood-Buff integrals from the optimization of ionic force fields: focus on fluorides

Polymer crystals are metastable and exhibit morphological changes when being annealed. To observe morphological changes on molecular scales we started from small nanometer-sized crystals of highly folded long-chain polymers. Micron-sized stripes consisting of monolayers or stacks of several layers of flat-on oriented polyethylene nanocrystals were generated via evaporative dewetting from an aqueous dispersion. We followed the morphological changes in time and at progressively higher annealing temperatures by determining the topography and viscoelastic properties of such assemblies of nanocrystals using atomic force microscopy. Due to smallness and high surface-to-volume ratio of the nanocrystals, already at 75 °C, i.e. about 60 degrees below the nominal melting point, the lateral size of the crystal coarsened. Intriguingly, this occurred without a noticeable reduction in the number of folds per polymer chain. Starting at around 110 °C, chain folds were progressively removed leading to crystal thickening. At higher temperatures, but still below the melting point, prolonged annealing allowed for surface diffusion of molten polymers on the initially bare substrate, leading eventually to the disappearance of crystals. We compared these results to the behavior of the same nanocrystals annealed in an aqueous dispersion and to bulk samples.     

Lamellar changes during heat treatment in isotactic polystyrene crystals

In the present investigation, the melting and thickening processes in lamellar crystals of isotactic polystyrene have been studied by transmission electron microscopy. It is shown that under properly chosen experimental conditions for the polymer, one can continuously follow the physical changes involved during the thickening as well as melting of lamellar crystals on heat treatment. The study of crystals grown at different temperatures reveals that melting of a single lamella starts at various areas. A commonly observed feature is the preferential melting of elastically bent parts of a lamella. It is indicated that the occurrence of melting in the various parts is due to a structural variation along the surface of lamellae resulting in a hindrance of the lamellar thickening process. At particular temperatures, melting of lamellar crystals is followed by recrystallization. The occurrence of a solid-stage thickening process is the major process so far observed during slow heat treatments. Considerable change in surface structure of the crystals grown at different temperatures is clearly reflected during the heat treatment. The rates of heating have marked influence on the resulting morphology of the crystalline superstructures.     

Superheat of silicon crystals observed by live X-ray topography

In-situ observations of Si crystal growth and melting have been carried out by live X-ray diffraction topography. Superheated solid states beyond the melting point was observed for dislocation-free crystals with melting in their inside. Dislocations were found to impede superheat and to melt the crystal without an appreciable superheating. A slightly superheated state accompanying melting removes all dislocations including immobile ones by their climb motion. It is proposed that self-interstitials needed for the volume change by melting are supplied by climb of dislocations, in contrast to dislocation-free crystals creating the interstitials thermally. In real crystal growth, remelting occurs naturally by melt convection and acts to make the growing crystal dislocation-free.     

CHANGES IN MELTING BEHAVIOR AND MORPHOLOGY ON ANNEALING OF LINEAR AND BRANCHED POLYETHYLENE

Changes in morphology and melting behavior of various types of commercial polyethylenes as a result of annealing were studied using differential scanning calorimetry, transmission electron microscopy, and density measurements. The range of polyethylenes whose densities varied between 0.96 and 0.90 g/cm³ included linear polyethylene (LPE), high-density polyethylene, a 1-octene copolymer traditional linear low density polyethylene, low density polyethylene, and a 1-octene copolymer prepared by Dow's INSITE constrained geometry catalysts technology. Two sets of samples were initially prepared by fast cooling and slow cooling from the melt. Despite an initial lower crystalline content and crystal thickness for the fast cooled (FC) branched polyethylene samples, a higher melting temperature than for the slow cooled (SC) samples was found using a 10°C/min heating rate. In concordance with a recent work, melting–recrystallization processes are held responsible for the anomalous behavior. The annealing treatment consisted of heating the two sets of samples at 1°C/min from room temperature to a temperature located at the start of the endotherm. The thermal treatment stabilizes the crystals through an increase in their thickness, which prevents melting–recrystallization processes from taking place on subsequent heating. A lower melting temperature after annealing was observed for the FC branched polyethylene samples. No such behavior was found for the SC samples and even for the FC LPE sample.     

Crystallization Kinetics of Lamellar Crystals Confined in Polymer Thin Films

We used dynamic Monte Carlo simulations to investigate the crystallization kinetics of flat-on lamellar polymer crystals in variable thickness films. We found that the growth rates linearly reduced with decreasing film thickness for the films thinner than a transition thickness d_t , while they were constant for the films thicker than d_t . Moreover, the mean stem lengths (crystal thickness) we calculated decreased with film thickness in a similar way to the growth rates, and the intramolecular crystallinities we calculated confirmed the film thickness dependence of the crytsal thickness. Besides, the crystal melting rates in thin films were calculated and increased with decreasing film thickness. We proposed a new interpretation on the film thickness dependence of the crystal growth rate in thin films, suggesting that it is dominated by the crystal thickness in terms of the driving force term (l–l _min) expressed by Sadler, rather than the chain mobility based on experiments. The crystal thickness can determine whether a crystal grows or melts in a thin film at a fixed temperature, indicating the reversibility between the crystal growth and melting.     

Crystal growth and characterization of l-phenylalaninium trichloroacetate—A new organic nonlinear optical material

Amino acid based crystals exhibit excellent nonlinear and electro-optical properties. A new nonlinear optical single crystal l-phenylalaninium trichloroacetate (LPTCA) belonging to the amino acid group was grown by the slow evaporation solution growth method. The grown crystals have been subjected to powder X-ray diffraction studies to identify the crystalline nature. Single crystal X-ray diffraction study showed that LPTCA belongs to monoclinic crystal system. Fourier transform infrared study is used to confirm the presence of various functional groups in the grown crystal. Optical transparency of the grown crystals was investigated by UV-vis-NIR spectrum. The lower optical cutoff wavelength for this crystal is observed at 254 nm and energy band gap is 4.89 eV. Thermal properties of the LPTCA crystal were studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques, which indicate that the material does not decompose before melting. The melting point of grown crystal was observed to be 136 °C in a melting point apparatus. The existence of nonlinear optical (NLO) property of LPTCA was confirmed by second harmonic generation test using Nd:YAG laser fundamental wavelength of 1064 nm.     

Frequency and temperature amplitude dependence of complex heat capacity in the melting region of polymers

Amplitude and frequency dependence of reversible melting of polycaprolactone (PCL) and an ethylene octene copolymer (EOM) were studied using temperature-modulated differential scanning calorimetry (TMDSC) (2?10^?1 Hz <f < 0.05 Hz) and shear spectroscopy (dynamic mechanical analysis, DMA) (5?10^?4 Hz < f < 100 Hz). It was found that the excess heat capacity of PCL is constant for temperature amplitudes in the range 5 mK < A_T < 2 K. The excess heat capacity decreases with frequency of temperature perturbation and tends to zero at about 0.1 Hz and 100 Hz for PCL and EOM, respectively. The constant excess heat capacity and the frequency dependence support the idea that reversible melting is related to a relaxation process on the surface of the polymer crystals. The occurrence of such a relaxation process was shown by shear modulus measurements in the same frequency and temperature region. The relaxation process is, in the melting region, much slower than main relaxation (glass transition). At low temperatures, a crossover can be seen, indicating the independence of both processes because of spatial separation. The main relaxation is related to the melt, while the other is related to the crystal surface.     

Crystal melting processes of propylene carbonate and 1,3-propanediol investigated by the reed-vibration mechanical spectroscopy for liquids

The melting of crystals is one of the most common and general phase transition phenomena.However, the mechanism of crystal melting is not well understood, and more experimental measurements and explorations are still needed.The mechanical spectra of propylene carbonate and 1,3-propanediol during the crystal melting processes are measured by the reed vibration mechanical spectroscopy for liquids(RMS-L) for the first time.The experimental results show that as the temperature increases, the real part of the complex Young modulus first decreases slowly, and then quickly drops to zero;meanwhile, its imaginary part increases slowly at first, then goes up and drops quickly to zero, showing a peak of internal friction.Preliminary analyses indicate that both the real and imaginary parts can present some characteristics of the melting process, such as the transition from the disconnected liquid regions to the connected liquid regions, that from the connected crystal regions to the disconnected crystal regions, and so on.In addition, the results show that the melting rate per unit volume of crystalline phase versus temperature satisfies the Arrhenius relation at the initial stage of melting, and deviates from this relation as the temperature increases to a certain value.Therefore, the RMS-L will provide an effective supplement for the further study of melting.     

Morphology,Crystallization, and Melting of Single Crystals and Thin Films of Star‐branched Polyesters with Poly(ϵ‐caprolactone) Arms as Revealed by Atomic Force Microscopy

The morphology and thermal stability of different sectors in solution‐ and melt‐grown crystals of star‐branched polyesters with poly(?‐caprolactone) (PCL) arms, and of a reference linear PCL, have been studied by tapping‐mode atomic‐force microscopy (AFM). Real‐time monitoring of melt‐crystallization in thin films of star‐branched and linear PCL has been performed using hot‐stage AFM. A striated fold surface was observed in both solution‐ and melt‐grown crystals of both star‐branched and linear PCL. The presence of striations in the melt‐grown crystals proved that this structure was genuine and not due to the collapse of tent‐shaped crystals. The crystals of the star‐branched polymers had smoother fold surfaces, which can be explained by the presence of dendritic cores close to the fold surfaces. The single crystals of linear PCL grown from solution showed earlier melting in the {100} sectors than in the {110} sectors, whereas no such sectorial dependence of the melting was found in the solution‐grown crystals of the star‐branched polymers. The proximity of the dendritic cores to the fold surface yields at least one amorphous PCL repeating unit next to the dendritic core and more nonadjacent and less sharp chain folding than in linear PCL single crystals; this evidently erased the difference in thermal stability between the {110} and {100} sectors. Melt‐crystallization in thin polymer films at 53–55°C showed 4 times faster crystal growth along b than along a, and more irregular crystals with niches on the lateral faces in star‐branched PCL than in linear PCL. Crystal growth rate was strictly constant with time. Multilayer crystals with central screw dislocation (growing with or without reorientation of the b–axis) and twisting were observed in both classes of polymers.     

Coherent kinetic control over crystal orientation in macroscopic ensembles of polymer nanorods and nanotubes

We show that the crystal orientation in polymer nanotubes and nanorods inside porous templates is controlled by the kinetics of nucleation and growth under 2D confinement. Two clear limiting cases are identified: In separated nanostructures, any crystal orientation allowing the growth of lamellar crystals along the pores appears statistically. If a bulklike surface film connects the nanostructures, macroscopic arrays with uniform crystal orientation are obtained, in which the dominant growth direction of the crystals is aligned with the long axes of the pores of the template.     

金属片加热区熔法单晶生长及应用

在比较现有的各种溶体生长方法的基础上,针对氕物晶体生长的特点,提出了适合均匀生长非固液同成份氧化物晶体的金属片加热区溶法,对该方法的装置、优缺点以及国内外研究现状作了详细的介绍,报道并讨论了应用铂片加热区溶法生长化学计量比铌酸锂晶体的试验及结果。     

Growth Rates of Edge-on Lamellar Crystals Confined in Polymer Thin Films

The growth rates of edge-on lamellar polymer crystals in variable thickness films were investigated in terms of dynamic Monte Carlo (MC) simulations. The growth rates linearly decreased with decreasing film thickness for the thinner films and were nearly constant for the thicker films. The mean stem lengths (crystal thickness) were also constant in different thickness films. The crystal widths parallel to the film thickness increased more slowly with increasing film thickness in the thinner films than that in the thicker films, indicating they were restrained by the film thickness. We propose that the growth rate of edge-on lamellar crystals in thin films is dominanted by the crystal width in the thinner films and by the crystal thickness in the thicker films; the variation of the film thickness can change the three-dimensional shape of the crystal growth front, also affecting the growth rate of the edge-on lamellar crystal.     

The role of the amorphous fraction for the equilibrium shape of polymer single crystals

The equilibrium state of polymer single crystals is considered by explicitly taking into account the amorphous fraction formed by loops and tails of the chains using a statistical model introduced by Muthukumar (Philos. Trans. R. Soc. London, Ser. A 361, 539 (2003)). We show that under realistic conditions below the equilibrium melting temperature, tight loops and close re-entries are favored, and that the amorphous fraction can be mapped into an excess surface free energy. The model is extended to many-chain crystals where it is shown that the lamellar thickness increases with the number of chains in the crystal and extended-chain conformations are thermodynamically favored if the number of chains in the crystal is sufficiently large. The number of chains necessary to form an extended-chain crystal in thermodynamic equilibrium scales with the square of the degree of polymerization of the chains. We discuss the temperature behavior of the equilibrium crystal thickness in the under-cooled state.     

Polymer-mediated and ultrasound-assisted crystallization of ropivacaine: Crystal growth and morphology modulation

The objective of this research was to modify the crystal shape and size of poorly water-soluble drug ropivacaine, and to reveal the effects of polymeric additive and ultrasound on crystal nucleation and growth. Ropivacaine often grow as needle-like crystals extended along the a-axis and the shape was hardly controllable by altering solvent types and operating conditions for the crystallization process. We found that ropivacaine crystallized as block-like crystals when polyvinylpyrrolidone (PVP) was used. The control over crystal morphology by the additive was related to crystallization temperature, solute concentration, additive concentration, and molecular weight. SEM and AFM analyses were performed providing insights into crystal growth pattern and cavities on the surface induced by the polymeric additive. In ultrasound-assisted crystallization, the impacts of ultrasonic time, ultrasonic power, and additive concentration were investigated. The particles precipitated at extended ultrasonic time exhibited plate-like crystals with shorter aspect ratio. Combined use of polymeric additive and ultrasound led to rice-shaped crystals, which the average particle size was further decreased. The induction time measurement and single crystal growth experiments were carried out. The results suggested that PVP worked as strong nucleation and growth inhibitor. Molecular dynamics simulation was performed to explore the action mechanism of the polymer. The interaction energies between PVP and crystal faces were calculated, and mobility of the additive with different chain length in crystal-solution system was evaluated by mean square displacement. Based on the study, a possible mechanism for the morphological evolution of ropivacaine crystals assisted by PVP and ultrasound was proposed.     

掺杂钨酸铅晶体闪烁发光光谱研究

利用改进的晶体生长设备和工艺提高了PbWO₄闪烁晶体的光产额。通过对生长获得的PbWO₄、退火PbWO₄和BaF₂∶PbWO₄晶体的透过光谱,衰减时间和光产额等闪烁性质的研究,发现晶体退火和掺杂技术特别是阴离子掺杂技术能够显著提高晶体的闪烁发光性能。其中晶体掺杂全面提高了晶体的透过光谱强度,但是退火的影响较复杂。高温退火改善了PbWO₄晶体在360 nm以上波段的透过光谱的透过率,但是在320～360 nm波段其透过率反而降低。这些现象与晶体中缺陷在可见光波段产生的特征吸收有关。晶体的良好退火和掺杂提高了晶体的光产额,其中BaF₂∶PbWO₄掺杂晶体室温闪烁发光强度达到65 p.e.·(MeV)-1,接近PET的使用要求。这种提高与晶体F-离子掺杂引发晶体[WO₄]2-四面体基团畸变有关,F-离子进入该四面体产生了新的发光中心。     

Melting kinetics in polymers

In polymers, it is possible to obtain single chain forming single crystals. It is feasible to melt these crystals by simple consecutive detachment of chain segments from the crystalline substrate and its diffusion into the melt. However, complication in the melting process occurs when the chain in the process of detachment from the surface is shared between different crystals. Experimentally, a clear distinction in different melting processes is observed, by the differences in the activation energies required for the consecutive detachment of chain segments or of segments having topological constraints. The consecutive detachment of free chain segments starts at the melting temperature predicted from the Gibbs-Thomson equation, whereas higher temperature or time is required if the chain has to overcome the constraints.