Orientation studies of poly(ethylene terephthalate)

This paper discusses the results of a detailed study of the relationships between molecular orientation, physical properties, and molecular weight of polyethylene terephthalate (PET), and their dependence on orientation variables. The molecular weight range of the samples used in this study included weight average molecular weights, M_w, between 29,000 and 65,000 which correspond to inherent viscosities, I.V., from 0.5 to 0.9. The orientation temperatures investigated were between 80 and 120°C. The extent of molecular ordering imparted by the orientation process was studied by birefringence, density, light scattering, and depolarized light intensity techniques. The results show that the degree of molecular orientation and the physical properties are strongly dependent on strain rate, extension ratio, molecular weight, and orientation temperature. The mechanical and transport properties, of PET are directly related to the degree of orientation as measured by birefringence. It is found that at a comparable level of orientation, the mechanical properties are also dependent on molecular weight, whereas the transport properties are independent of molecular weight. The degree of orientation varies according to the molecular weight of PET and stretch temperature. It is shown that for the same stretch ratio and stretch speed, the birefringence decreases with increasing stretch temperature. The light scattering results indicate that biaxial orientation of PET can lead to strain-induced crystallization. The amount and form of the crystalline structures are dependent on strain rate and orientation temperature.     

The onset of orientational crystallization in poly(ethylene terephthalate) during low temperature drawing

It is well known that crystallization can be induced in amorphous poly(ethylene terephthalate) (PET) by orientation below the isotropic crystallization temperature. The magnitude of the strain necessary for crystallization varies inversely with molecular weight because of relaxation. However, lower molecular weight PET might be expected to, crystallize at a lower extent of molecular orientation, since the crystallization rate also varies inversely with molecular weight. Chain conformations were measured during low temperature drawing of PET of various molecular weights. The molecular configuration associated with strain induced crystallization was found to be independent of chain length. The onset of orientational crystallization was associated with a particular conformation, and this critical trans/gauche ratio was equivalent for PETs of various molecular weights. The drawing behavior is thus in accord with theory concerning the transition of flexible chain polymes from isotropy to an ordered state. This result is congruent with previous studies suggesting the presence of extended chain crystallinity in amorphous PET after low temperature drawing.     

A device for study of polymer crystallization kinetics via real-time image analysis of small angle light scattering

A two-dimensional position-sensitive detector for real-time small-angle light scattering (RTSALS) of polymers is described. The device performs real-time image analysis of the evolving light-scattering pattern during crystallization. Such an approach provides a means of simultaneously measuring the rate of spherulite growth and the overall rate of crystallization, allowing calculation of nucleation density and/or rate. Although separation of nucleation effects from growth effects typically requires the application of two different experimental techniques, this device is unique in that it allows these two effects to be easily separated using only time-dependent light-scattering data. The device incorporates a CCD camera, a personal computer, and an imaging board as its major hardware components. Software designed specifically for this application performs real-time analysis of the light-scattering pattern at a rate of 1–3 images per second, thus allowing study of even moderately rapidly crystallizing polymers. (More recent software modifications have resulted in a speed enhancement to 5 images per s and further improvement is expected.) Intensities at various scattering and azimuthal angles are plotted at each time. In addition, the average spherulite radius is calculated and plotted. Application of the device is illustrated by measuring the spherulite growth rate of poly(ethylene terephthalate) as a function of temperature. © 1993 John Wiley & Sons, Inc.     

Crystallization kinetics of polyethylene terephthalate. I. Isothermal crystallization from the melt

The crystallization behavior of polyethylene terephthalate (PET) was investigated as a function of molecular weight, temperature of crystallization, and polycondensation catalyst system. A detailed analysis of the crystallization course has been made utilizing the Avrami expression. The crystallization rate constants and the Avrami exponents were calculated. The results show that the rate constant and the mechanism of crystallization are dependent on the molecular weight, temperature, and the polycondensation catalyst system. The catalyst system often exhibits a more significant influence than the molecular weight in controlling the rate of crystallization of PET.     

螺环二醇改性PET共聚酯的合成及性能

为了扩大聚对苯二甲酸乙二醇酯（PET）的应用范围,常通过引入第三单体来改变其性能。本研究以螺环二醇（SPG）为原料,采用酯交换法熔融缩聚合成了一系列不同单体含量的共聚酯。采用超高效聚合物色谱-多角度激光光散射仪（APC-MALLS）联用分析了共聚酯的分子量与分子量分布,利用核磁氢谱（¹H NMR）、傅里叶红外光谱（FTIR）研究了共聚酯的化学结构,同时采用差示扫描量热仪（DSC）、热重分析仪（TGA）和X射线衍射仪（XRD）研究了共聚酯的热性能以及结晶性能,测试了共聚酯的拉伸性能。结果表明,成功合成了符合纤维级聚酯数均分子量（M_n）和重均分子量（M_w）要求的共聚酯,分子量分布较窄,随着第三单体含量的增加,玻璃化转变温度从77℃提升至85℃,而熔融温度从255℃降低至222℃,热分解温度无明显变化,结晶性能下降。     

聚酯固相缩聚等温结晶特性的研究

聚酯(PET)固相缩聚(SSP)中切片的结晶性能及其演变影响固相缩聚反应,采用差示扫描量热仪(DSC)和热台偏光显微镜研究了固相缩聚反应前后PET切片的等温结晶特性。结果表明:PET切片在DSC中的等温结晶符合Avrami 方程,等温结晶温度升高,结晶速率常数K值减小,即结晶速率降低;热台偏光显微镜中不同等温结晶温度下形成了不同的球晶形态:黑十字消光图以及环形消光图;随着PET特性粘数(平均分子质量)增大,结晶速率常数K值减小,球晶生长速率减小,Avrami指数n值增大,形成更加复杂的消光图。对于固相缩聚前PET基础切片,球晶最大结晶速率在190℃左右。     

On the nature of multiple melting in poly(ethylene terephthalate) (PET) and its copolymers with cyclohexylene dimethylene terephthalate (PET/CT)

The multiple melting behavior of poly(ethylene terephthalate) (PET) homopolymers of different molecular weights and its cyclohexylene dimethylene (PET/CT) copolymers was studied by time-resolved simultaneous small-angle X-ray scattering/wide-angle X-ray scattering diffraction and differential scanning calorimetry techniques using a heating rate of 2 °C/min after isothermal crystallization at 200 °C for 30 min. The copolymer containing random incorporation of 1,4-cyclohexylene dimethylene terephthalate monomer cannot be cocrystallized with the ethylene terephthalate moiety. Isothermally crystallized samples were found to possess primary and secondary crystals. The statistical distribution of the primary crystals was found to be broad compared to that of the secondary crystals. During heating, the following mechanisms were assumed to explain the multiple melting behavior. The first endotherm is related to the non-reversing melting of very thin and defective secondary crystals formed during the late stages of crystallization. The second endotherm is associated with the melting of secondary crystals and partial melting of less stable primary crystals. The third endotherm is associated with the melting of the remaining stable primary crystals and the recrystallized crystals. Due to their large statistical distribution, the primary crystals melt in a broad temperature range, which includes both second and third melting endotherms. The amounts of secondary, primary and recrystallized crystals, being molten in each endotherm, are different in various PET samples, depending on variables such as isothermal crystallization temperature, time, molecular weight and co-monomer content.     

Strain-induced crystallization of poly(ethylene terephthalate)

The fabrication of poly(ethylene terephthalate), PET, into fibers, films, and containers usually involves molecular orientation caused by molecular strain, which may lead to stress- or strain-induced crystallization (SIC). The SIC of PET was studied by the methods of birefringence, density, thermal analysis, light scattering, and wide-angle X-ray. The development of crystallinity is discussed in relation to the rate of crystallization, the residual degree of orientation, and stress relaxation. The experimental procedure involves stretching samples at temperatures above the glass transition temperature, Tg, to a given extension ratio and at a specific strain rate of an Instron machine. At the end of stretching, the sample is annealed in the stretched state and at the stretching temperature for various periods of time, after which the sample is quickly quenched to room temperature for subsequent measurements. During stretching, the stress strain and the stress relaxation curves are recorded. The results indicate that the SIC of annealed, stretched PET can proceed in three different paths depending on the residual degree of orientation. At a low degree of residual orientation, as indicated by the birefringence value, annealing of stretched PET leads only to molecular relaxation, resulting in a decrease of birefringence. At intermediate orientation levels, annealing causes an initial decrease in birefringence followed by a gradual increase and finally a leveling off of birefringence after a fairly long period of time. At higher orientation levels, annealing causes a rapid increase in birefringence before leveling off. The interpretation of the above results is made using the measurements of light scattering, differential scanning calorimetry, and wide-angle X-ray. The rate of the SIC of PET is also discussed in terms of specific data analysis.     

Ultrasonic characterization of the crystallization behavior of poly(ethylene terephthalate)

On the basis of the previous observations that the ultrasonic signals are sensitive to the crystallization of polymers (Tatibouet and Piché, Polymer 1991, 32, 3147), we have expanded our efforts to study the detail relationship between the ultrasonic signals and crystallization process in this work. The nonisothermal and isothermal crystallization of virgin poly(ethylene terephthalate) (PET) and PET samples after degradation were studied by using a specially designed pressure‐volume‐temperature (PVT) device, with which an ultrasonic detector was combined. The results showed that the evolution of the ultrasonic signals not only can be used to probe the crystallization process but also can qualitatively characterize the crystallization rate, crystallinity, crystallite size, and amorphous. DSC measurement was used to verify such results. Ultrasonic signals could be as a complementary tool to polymer chain movement and well be applied to characterize the crystallization behavior. Furthermore, the ultrasonic measurement has the potential use to characterize crystallization of products in‐line during processing (i.e., injection molding, micromoulding). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Static and dynamic light scattering studies of the crystallization of poly(ethylene oxide) from dilute solutions

We have monitored in situ the self-seeded crystallization of poly(ethylene oxide) from dilute toluene solutions by dynamic light scattering. In supercooled dilute solution, the radii (R) of the crystals grow linearly with time. the rate constant obtained from the slope of the plot R versus time depends on the temperature and the molecular weight of the polymer, both of which determine the degree of the supercooling. The maximum crystal size obtained from solutions of fixed concentration also depends on the temperature and the polymer molecular weight. It appears that crystal growth is limited because of molecular weight fractionation. Static light scattering from suspensions of stable crystals provides information on the crystal morphology. A comparison of experimental and theoretical particle scattering functions suggests that the crystals form short cylinders and that the crystal growth from the seeds is primarily two-dimensional. Some comparisons to melt crystallization are possible. The preparation and use of tiny seed crystals is critical to the success of these studies.     

PET化学结构与热力学行为的关系

采用差热分析和红外光谱分析、粘均相对分子质量测定的方法,研究了PET的相对分子质量、端羟基和端羧基含量等化学结构与其玻璃化转变、冷结晶、热结晶行为、熔融行为的关系。结果表明:由于玻璃态中分子链段的局部有序性,PET呈现出双玻璃化转变和双冷结晶峰;随着相对分子质量减小,端羟基和端羧基含量相对增加,PET的玻璃化转变温度升高,冷结晶能力和热结晶能力增强,熔点升高,有利于形成结构完善的增强微纤。相对分子质量和端羧基、端羟基含量对热结晶行为影响的程度比对冷结晶行为的大得多,热结晶过程也由均相成核到既有异相成核,又有均相成核,到以异相成核为主。     

PET/ZnO纳米复合材料的制备及结晶性能

通过纳米ZnO存在下的对苯二甲酸／乙二醇(TPA／EG)酯化和缩聚反应制备聚对苯二甲酸乙二醇酯(PET)／ZnO纳米复合材料，研究了纳米ZnO用量及其分散方式对PET粘均摩尔质量、纳米ZnO在复合物中的分散及聚乙二醇(PEG)结晶性能的影响。发现纳米ZnO及分散改性剂(PEG)的加入，对合成PET的粘均摩尔质量均有一定影响；纳米ZnO在EG中直接分散再缩聚形成的复合物中，纳米ZnO团聚严重、分散性差，PET的结晶度和结晶速率降低；在纳米ZnO分散过程中加入PEG可以降低纳米ZnO在复合物中的团聚，提高分散性，PET的结晶度和结晶速率提高。     

聚对苯二甲酸乙二醇酯／蒙脱土复合物的制备与表征

分别采用蒙脱土与对苯二甲酸乙二醇酯(BHET)和与乙二醇(EG)混合的方法，通过原位插层聚合制备聚对苯二甲酸乙二醇酯／蒙脱土(PET／MMT)复合材料，研究了MMT含量对PET粘均相对分子质量、复合材料微观结构及热和结晶性能的影响。结果表明，随有机化MMT的加入量增加，PET的粘均相对分子质量降低；将有机化MMT分散在乙二醇中再聚合所得复合物中MMT分散更为均匀；MMT的加入使PET的玻璃化温度、冷结晶温度和维卡软化温度降低，BHET法的PET结晶度和热结晶温度增加，而EG法的PET结晶度和热结晶温度下降。     

The influence of fibers on the crystallization of poly(ethylene terephthalate) as related to processing of composites

The effect of fiber reinforcement on the isothermal crystallization of poly(ethylene terephthalate) (PET) was investigated using differential scanning calorimetry (DSC). Unidirectional fiber composites were prepared using glass and aramid fibers in PET. The rate of crystallization, as reflected by crystallization half-time, and the degree of crystallinity of PET are seen to depend on the type of reinforcing fiber as well as on crystallization temperature. Crystallization kinetics are also analyzed using an Avrami model for the volume of polymer crystallized as a function of time. The crystalline morphology of PET in fiber-reinforced systems was studied using polarized light microscopy. Results concerning nucleation densities and growth morphologies are used in explaining differences seen in crystallization kinetics in fiber-reinforced systems.     

Untersuchung der molmassenverteilung von harnstoff-formaldehyd-leimharzen durch GPC gekoppelt mit lichtstreuung

UF-resins have been synthesized with different degrees of condensation by different duration of the acidic condensation step and were characterized by gel permeation chromatography (GPC) on-line with a low angle laser light scattering (LALLS) photometer as molecular weight detector. The molar mass function lg M_w (V) and the weight average of molar mass have been calculated from the concentration signal and the scattering signal. The weight average molecular mass increases with longer condensation analogously to the turbidity temperature from several thousands up to more than 100000g/mol. Additionally it was found that UF-resins contain fractions with molar masses up to 500000g/mol which are not only aggregates caused by physical intermolecular forces.     

High molecular weight tail and long‐chain branching in low‐density polyethylenes

The composition of the high molecular weight tail in branched low‐density polyethylenes made by both tubular and autoclave reactors was studied in detail using size‐exclusion chromatography (SEC) coupled with a viscosity detector (VD) and a two‐angle light‐scattering detector (LSD). The detection of a second peak at very small elution volumes in the light‐scattering chromatogram but not in the refractive index chromatogram and viscosity chromatogram is due to high molecular weight species. It is also indicative of a change in the long‐chain branching distribution. It was found that the intrinsic viscosity contraction factor g′ scales with the radius of the gyration contraction factor, g, with the exponent, ε, having a value in the range 0.4–1.4. Furthermore, ε shows significant molar mass dependence. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2815–2822, 2001     

PET结晶速率的研究进展

阐述了聚对苯二甲酸乙二醇酯(PET)的结晶机理,综述了相对分子质量、残余助剂及末端基、物理老化和应力4个因素对PET结晶速率的影响,并着重介绍了提高PET结晶速率的方法,包括加成核剂、结晶促进剂和与其他聚合物共聚、共混。     

Crystallization of polyformals: 2. Influence of molecular weight and temperature on the morphology and growth rate in poly(1,3-dioxolane)

The morphology and growth rates of crystallized molecular weight fractions of poly(1,3-dioxolane) covering the range M_n = 8 800 to 120 000 have been studied by polarized light microscopy. Two different supermolecular structures, dependent on molecular weight and crystallization temperature have been found. Spherulites are formed after rapid crystallization and a more disordered morphology is formed at the lowest undercoolings but there is a temperature region where both forms are observed. The disordered form appears first and a consecutive spherulitic growth takes place. The crystallization kinetics were analysed over the temperature range 10°C to 36°C. At crystallization temperatures lower than 15°–18°C, the growth rate is linear and only spherulites are found. In the temperature range from 18°C to 36°C a well defined break is observed in the growth rate but the spherulitic growth rate is always higher than that of the irregular form. The growth rate temperature coefficient was studied and the usual plots are not linear in the whole range of crystallization temperatures. For the high crystallization temperature region, the slope is about twice as great as the low crystallization temperature slope. This is the region where regular spherulites are formed. The comparison between dilatometric and growth rate data has shown that the overall rate and growth rate temperature coefficients are the same.     

Effects of clay and POSS nanoparticles on the quiescent and shear‐induced crystallization behavior of high molecular weight poly(ethylene terephthalate)

The effects of organoclay and polyhedral oligomeric silsesquioxanes (POSS) nanoparticles on the crystallization behavior of high molecular weight poly(ethylene terephthalate) (HMWPET; inherent viscosity of 1.05) were investigated in terms of nanoparticle content and shear rate. Both nanoparticles played a role of nucleating agent for PET and increased the cold crystallization temperature by about 24°C. The half‐time of crystallization was also decreased with increasing the nanoparticle content. Clay proved to be more effective than POSS; a notable nucleating effect was observed at 0.3 wt% for clay and 2 wt% for POSS. Introducing 1 wt% of clay gave the highest crystallization rate among all PET nanocomposite samples examined. Isothermal crystallization of the nanocomposites under dynamic shear exhibited similar crystallization behavior. As in the DSC measurement, clay appeared to be more effective to promote the crystallization of PET under shear. The nucleating effects were more noticeable at higher shear rate. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Effects of water absorption on physical properties and degree of molecular orientation of poly (ethylene terephthalate)

Poly(ethylene terephthalate) (PET) is known to be a hygroscopic thermoplastic, which absorbs moisture from its environment at a rapid rate. The water absorption characteristics of PET as a function of relative humidity, exposure time, temperature, thickness, and molecular weight are reported here. Results indicate that absorbed moisture has significant influences on the physical properties of PET, leading to large decreases in the glass transition temperature, crystallization temperature, and degree of molecular orientation.