Kinetics of rapid structural changes during heat setting of preoriented PEEK/PEI blend films as followed by spectral birefringence technique

The influence of composition and preorientation on the development of structural hierarchy during heat setting of PEEK/PEI films was investigated using on-line birefringence, and off-line wide-angle and small-angle X-ray scattering, infrared dichroism and thermal analysis techniques. When the PEEK/PEI blends are drawn to deformation levels below the onset of strain hardening, the subsequent heat setting at high temperature starts with a large relaxation process followed by a fast crystallization and a long-term slow structural rearrangement stages. When the films are predrawn beyond a critical structural level (crystallinity and orientation), the initial relaxation stage disappears. This signifies that beyond a critical structural order a long-range physical network, where the nodes consist of crystallized domains and chain—self and cross-entanglements—are formed. This physical network allows the entropy driven shrinkage stresses to be maintained that results in the development of oriented crystalline phase. The addition of non-crystallizable PEI chains was found to retard the formation of this ‘network structure’ resulting in lower orientation levels.     

Irreversible structural changes during PET recycling by extrusion

Steric exclusion chromatography (SEC) and rheometry were used to study irreversible structural changes occurring during poly(ethylene terephthalate) (PET) extrusion. It appeared that polymer gelation could occur after three successive extrusions, showing the existence of a crosslinking process. This latter occurs only in the presence of oxygen at low oxygen pressures (typically P_O2<9×10³ Pa) as a result of coupling of alkyl radicals generated by the oxidation chain process. A kinetic model based on this assumption but taking also into account chemical reactions occurring in nitrogen, generates kinetic curves of melt (Newtonian) viscosity changes in good agreement with experimental ones.     

Nucleation of PET crystallization by metal hydroxides

PET formulations containing small amounts of nonalkali metal hydroxides were prepared by extrusion from the melt. The efficiency of the hydroxides as nucleating agents was estimated from the elevation of T_cc, the crystallization temperature upon cooling from the melt, normalized for the purpose of comparison to a single convenient M_w (or the corresponding intrinsic viscosity). The results show that metal hydroxides capable of releasing water within the processing temperature interval of PET are effective nucleating agents for the crystallization of the polymer. Hydroxides releasing water out of said interval do not nucleate PET. There are two proposed nucleation mechanisms: (a) a localized severe hydrolysis of PET by the water released in the immediate neighborhood of the hydroxide particles and (b) a localized supercooling of the PET in the vicinity of the particles by the released water. The hydrolyzed PET of sharply reduced molecular weight has a T_cc significantly higher than the T_cc of the unhydrolyzed majority of the polymer. Upon cooling from the melt, and with the possible localized supercooling, the PET in the immediate vicinity of the hydroxide particles crystallizes first, nucleating the rest of the polymer. At present we can not verify whether both mechanisms operate concomitantly and which one of them is dominant.     

纳米粒子改性PET的非等温结晶动力学

研究了有机纳米丁苯吡粉末橡胶、无机纳米粒子添加到聚对苯二甲酸乙二酯(PET)后对PET结晶行为的影响.并探讨了不同降温速率下3种体系降温过程中的结晶行为及基于Jeziomy理论和莫氏理论的结晶动力学.研究表明,3种纳米粒子对PET结晶动力学的影响是不同的,纳米粉末橡胶在较低结晶度时符合Jeziorny理论,而无机纳米粒子则对莫氏理论有较好的相关性.由莫氏理论计算出的参数表明,达到同样结晶度时有机化黏土促进PET结晶的效果比纳米CaCO3好.     

Kinetics and thermal crystallinity of recycled PET. I. Dynamic cooling crystallization studies on blends recycled with engineering PET

The recycled poly(ethylene terephthalate) (R-PET) from the recovery of a blow-molded bottle is studied with its crystalline behavior in terms of glass transition temperature (T_g), crystallization temperature (T_c), melting temperature (T_m), and its dynamic crystallization kinetics. These crystalline behaviors offer an explanation of the better mechanical properties of the R-PET. Thermal cycles of the processes of the R-PET and its blending specimens with engineering PET (E-PET) show the importance of the thermal treatment of the plastic PET in the improvements of mechanical strength and increased crystallinity. Tenfold and fivefold increases, respectively, of elongation and impact strength are observed in the specimen of R-/E-PET of 20/80 weight ratio blend, better than that of E-PET alone. A nearly 20-fold increase of the crystallization rate constant (K) at 190°C for the same R-/E-20/80 blend is observed. The Avrami exponent (“n”) is found to be variable with temperature. The changing crystallization mechanisms are mainly the result of the competition between the nucleating and growing of crystallites in response to the temperature-controlling factor at the melt state. © 1996 John Wiley & Sons, Inc.     

Mechanisms of structural organizational processes as revealed by real time mechano optical behavior of PET film during sequential biaxial stretching

The main focus of this research is to identify the structural mechanisms that are responsible for changes observed in various stages of coupled mechano-optical (stress and strain optical) relationships as they are influenced by the deformation mode, level and rate. This involves in situ real time studies of the true-stress-strain-birefringence of sequential biaxially stretched PET films using a highly instrumented biaxial stretching machine and ex situ structural studies. These ex-situ studies include wide angle X-ray diffraction, Raman spectroscopy and DSC thermal analysis to examine the orientation, conformation and crystallization behaviors as well as formation of long range connectivity through physical network. The relationship between the stress and birefringence exhibited three Regimes. Regime I, linear relationship with a stress optical constant of 5.8 GPa⁻¹. Regime II, non-linear relationship with the establishment of chain tautness and nematic like two-dimensional order. Regime III, where the chains reach their finite extensibility.     

双波长K系数分光光度法同时测定日落黄与柠檬黄

采用双波长K系数分光光度法同时测定饮料中的日落黄和柠檬黄,其中日落黄的回归方程为A1/A2=0.9083+2·604×10~(-2)c/A2,r_日=0.9998,线性范围为5～60μg/mL,柠檬黄的回归方程为A1/A2=0.9105+3.204×10~(-2)c/A2,r_柠=0.9999,线性范围为10～55μg/mL。对实际样品的测定表明,日落黄的加标回收率为99.72％,柠檬黄的加标回收率为94.76％,比双波长法更符合要求。     

Real time development of structure in partially molten state stretching of PP as detected by spectral birefringence technique

Our main focus in this study is to investigate the deformation behavior of PP in temperature range where the PP is partially molten using a newly developed uniaxial stretching system. This system allows the real time study of the structural reorganization processes as reflected in birefringence coupled with true stress and true strain at temperature and deformation rates used in industrial film process. This instrument revealed that birefringence-stress relationship is linear beyond an initial yielding point until the onset of strain hardening beyond which negative deviation from this linearity is observed. At lower strains the films were found to continue to change significantly during holding stage leading to increase in birefringence. These changes observed in holding stage decreases with decrease of stretching speed and temperature and with increase of total strain. Increased strain rates result in destruction of crystallites that involves block rotations of these regions leading to observation of lower birefringence at faster rates. The latter process generates large amount of stretched amorphous chains that gradually convert to crystalline state during holding.     

Estimation of heat evolution during viscoelastic crack propagation by liquid crystal film technique

Heat generated during dynamic crack propagation in viscoelastic solids was investigated by visible liquid crystal film technique to observe the thermal boundary front emanating from a running crack tip. The crack propagation velocity was also measured by the velocity gauge method. The heat so estimated is correlated with the crack propagation velocity.     

Properties and kinetics of thermally crystallized oriented poly(ethylene terephthalate) (PET). I: Kinetics of crystallization

The crystallization kinetics of previously oriented poly(ethylene terephthalate) (PET) sheets have been studied under unconstrained conditions, as functions of stretch ratio, mode of extension, and crystallization temperature. Isothermal and dynamic crystallization conditions have been utilized for the determination of half-times of crystallization and activation energies, respectively. Results indicate that rates of crystallization of pre-oriented PET increase substantially, with increasing stretch ratios. Half-times of crystallization, at a given isothermal temperature, were found to decrease from a few minutes to fractions of seconds. Similarly, the activation energies of thermal crystallization have been shown to decrease from 75 kcal/mol for unoriented PET, to less than 20 kcal/mol for highly oriented material. These changes are most apparent among samples with low to moderate levels of orientation. Highly oriented samples have proven to be less sensitive, yielding similar extents of thermal crystallization, regardless of specific initial extension ratios. These samples have also shown less sensitivity to temperature of crystallization, exhibiting only slightly higher levels of crystallinity as a result of exposure to increasing temperatures in the range from 100°C to 120°C.     

Study of structural evolution during controlled degradation of ultrathin polymer films

The structural aspects of polyacrylamide thin films annealed at degradation threshold temperature have been studied as a function of annealing time using in situ X-ray reflectivity technique in vacuum. We observe significant decrease of thickness and increase of density with annealing time for all the films. The dynamical behavior of the changes was modeled in terms of two distinct exponential decay functions, following our earlier observation of two different time scales for the chemical modification pathways, and was found to be in excellent agreement with the data. The diffusion coefficients of the polymer chains corresponding to the two modes are found to be different by an order of magnitude. It was found that the two dynamical modes correspond to the formation of two degradation products at two different rates. The larger time constants for both the modes in case of thickness reduction compare to the chemical changes was explained in terms of inter-chain entanglement and attachment of the polymer with the substrate.     

Microstructural evolution during fabrication of alumina via laser stereolithography technique

Application of additive manufacturing (AM) technology in production of ceramic parts is considered as a state-of-the-art technique which has been recently introduced to industry. In the current study the imperative microstructural characteristics of the alumina manufactured via laser stereolithography (SLA) has been investigated. The microstructural characteristics of the developed ceramic parts and components are still unknown and require detailed investigation. A combination of optical microscopy and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), image analysis, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and micro-computed tomography (micro-CT) scans was used to evaluate the microstructural features of the alumina samples after each step of the manufacturing process (i.e. printing, debinding and sintering). In addition, the apparent density of each sample was measured using water displacement method. Results indicated that the porosity of printed alumina samples was significantly reduced after sintering process. EDS analysis confirmed elimination of binder material after debinding and sintering processes. XRD analysis detected existence of α-Al₂O₃ in initial printed samples which was not changed during debinding and sintering processes. Due to detection of identical peaks for all samples, only one set of lattice parameters ($\mathrm{a}$ and $\mathrm{c}$) was calculated from XRD patterns of all samples which was close to the ones reported in literature for alumina. TEM micrographs and corresponding diffraction patterns confirmed polycrystalline structure from different layers of the samples. High resolution transmission electron microscopy (HRTEM) and diffraction patterns from single layers were used to calculate lattice parameters for each sample. A slight increase was noticed in unit cell and grain size after sintering process. The obtained results help for better understanding of the properties through microstructural characteristics of the 3D printed ceramic parts.     

Preparation and structural evolution of SiOC preceramic aerogel during high-temperature treatment

Silicon oxycarbide (SiOC) ceramic aerogels in mesopores range have been fabricated by pyrolyzing polycarbosilane aerogels in nitrogen (N₂) atmosphere. The reactants, poly(methylhydrosiloxane) and 2, 4, 6, 8-tetramethyl-2, 4, 6, 8-tetravinylcyclotetrasiloxane have been heated in the presence of hydrochloroplatinic acid. As-prepared SiOC preceramic aerogel has specific surface area of 299 m²/g at room temperature, and decomposes during pyrolysis. Structural evolution of the aerogels as a function of heat-treatment temperature has been investigated by Fourier transform infrared spectrophotometer, X-ray diffraction analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. Results indicate that tetrahedral Si–O–C network underwent four structural changes during thermal treatment from room temperature to 1600 °C.     

The structural evolution of thin multi-walled carbon nanotubes during isothermal annealing

Systematic isothermal annealing has been carried out on catalytic chemical vapor deposition-derived highly disordered multi-walled carbon nanotubes in order to understand the structural evolution of carbon nanotubes kinetically. After specified isothermal time the crystallographic parameters strongly depend on annealing temperature, and can be divided into three stages. Rapid structural enhancement in the range from 1800 to 2200 °C originates from the abrupt evolution of the residual catalytic compounds and is seen as partially aligned small fringes along the tube length. The lack of further structural ordering at temperatures above 2400 °C and the low activation energy (ca. 90 kJ/mol) are thought to be the result of the limited space available for further structural development considering the nano-sized multi-walled carbon nanotubes.     

Application of methoxymercuration-demercuration followed by mass spectrometry as a convenient microanalytical technique for double-bond location in insect-derived alkenes

The positions of double bonds in olefins can be readily determined by a sodium borohydride reduction of their methoxymercuration products followed by mass spectrometry. Fragmentation of the methoxy derivative in the mass spectrometer results in cleavage on either side of the methoxy group to give intense fragment ions which are characteristic of each isomer. This simple and convenient microanalytical technique was applied to several synthetic standards and insect derived olefins, including the alkenes from the cuticular lipids of the honeybeeApis mellifera L.Mention of trade names or companies is solely to identify materials used and does not imply endorsement by the USDA.     

Polymorph control of calcium carbonate by reactive crystallization using microbubble technique

In this study, we used the minute gas–liquid interfaces around CO₂/NH₃ microbubbles as new reaction fields where the crystal nucleation progresses and developed a crystallization technique to control the polymorphism of calcium carbonate (CaCO₃). In the regions around the gas–liquid interfaces of CO₂/NH₃ microbubbles, Ca²⁺ and CO₃²⁻ ion concentrations can be adjusted because of the characteristic of the electric charge on the bubble surface and the decrease in CO₂ concentration based on unit bubble caused by minimization of bubble size, and because of the pH difference between local pH at the gas–liquid interface and overall pH in the bulk liquid caused by mixing of NH₃ with CO₂; hence, the polymorph change of CaCO₃ is expected to occur. CaCO₃ was crystallized at 298 K by a semi-batch type reaction in which CO₂/NH₃/N₂ bubbles were continuously supplied to an aqueous Ca(NO₃)₂ solution using a self-supporting bubble generator. The solution pH during crystallization was maintained at a constant level of 6.9–12.0 by adding HNO₃ and NH₄OH solution. The average bubble size (d_bbl) was varied in the range of 40–1000 μm by controlling the N₂ flow rate, and the molar ratio of CO₂/NH₃ (αCO₂/NH₃${\alpha }_{\text{C}{\text{O}}_2\text{/N}{\text{H}}_3}$) was set at a specified value of 0.20–1.00 at a constant CO₂ flow rate. The following results were obtained by varying solution pH, d_bbl, and αCO₂/NH₃${\alpha }_{\text{C}{\text{O}}_2\text{/N}{\text{H}}_3}$: at a constant d_bbl of 40 μm and αCO₂/NH₃${\alpha }_{\text{C}{\text{O}}_2\text{/N}{\text{H}}_3}$ of 0.20, vaterite and calcite were major products at a solution pH lower than 9.0 and at a solution pH greater than 11.0, respectively, while aragonite was crystallized predominantly in the solution pH range of 9.7–10.5; at a constant solution pH of 9.7 the crystallization of aragonite was accelerated remarkably with a decrease in αCO₂/NH₃${\alpha }_{\text{C}{\text{O}}_2\text{/N}{\text{H}}_3}$ and d_bbl.     

PET/分子筛原位聚合复合材料的等温结晶性能

采用原位聚合的方法制备了聚对苯二甲酸乙二酯(PET),分子筛复合材料,并通过差示扫描量热仪研究了不同分子筛含量复合材料的等温结晶性能。结果表明：分子筛的加入有明显的异相成核效果,加快了结晶速率,增加了结晶度,减小了晶粒尺寸分布。在210℃的结晶温度下,当分子筛质量分数为4％时,半结晶时间为156s,结晶热焓为37．84J/g,结晶峰半峰宽为1．88s。     

Preparation of high performance PET fiber by solution spinning technique

High molecular weight poly(ethylene terephthalate) (PET, IV = 3.30 dL/g) was extruded in attempts to prepare high performance fiber using the solution spinning method. Solution preparation, fiber coagulation, and mechanical properties of resultant fibers were examined. An as-spun fiber exhibited high deformability when appropriate coagulation conditions were used. Tenacity and modulus of the resultant drawn fibers achieved 12.9 and 230 gpd, respectively, at draw ratio above 10. © 1995 John Wiley & Sons, Inc.     

Self-reinforcement of polypropylene by flow-induced crystallization during continuous extrusion

Self-reinforced polypropylene (PP) sheets have been prepared from melt flow-induced crystallization through a conical slit die fed by a conventional extruder. Their structure and properties, influenced by the die pressure ranging from 20 to 50 MPa and die outlet temperature, are studied by scanning electron microscopy observation, differential scanning calorimetry analyses, tensile strength, and light transmittance measurements. At a die outlet temperature of 162°C and a pressure above 30 MPa, conspicuous increases in the melting peak, tensile strength, and light transmittance (they can be used to characterize the self-reinforcement degree of sheet) are observed. The self-reinforcement degree, however, increases only slightly with increasing pressure as it exceeds 40 MPa. Raising the die outlet temperature from 162 to 172°C results in a further increase in the self-reinforcement degree (for example, a highest tensile strength of 288 MPa) while keeping the pressure at 40 MPa, so bulk PP materials with high properties can be produced from continuous melt extrusion under pressures lower than 40 MPa. Furthermore, the melt temperature plays an important role in determining the properties of self-reinforced polymeric materials. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2111–2118, 1998     

Kinetics of strain-induced crystallization during injection molding of short fiber composites of poly(ether ether ketone)

Crystallization kinetics of short glass and carbon fiber composites of poly(ether ether ketone) (PEEK) under melt-strain conditions have been obtained for the first time, using in-situ wide angle X-ray scattering, and have been correlated to a model based on the Avrami equation in order to enable minimization of the processing time for injection molding of these materials. It has been demonstrated that increased flow rate of the melt in the mold and, consequently, increased shear rate accelerates the crystallization process of PEEK composites, analogous to similar trends observed previously in PEEK resin. Short glass fiber composites of PEEK crystallize slower than the resin under identical processing conditions, while short carbon fiber composites crystallize faster than the resin, except at the highest mold temperatures and the lowest flow rates. A model based on the Avrami equation has been proposed to fit the kinetics data obtained experimentally. The Avrami coefficient has been calculated and Arrhenius plots have been used to predict the crystallization kinetics at temperatures lower than those at which experimental data have been obtained here. Fiber orientation, flexural elastic modulus, and flexural fracture toughness of the composites have also been evaluated.