物理老化对非晶态PET结构、性能以及结晶的影响

采用密度、DSC、应力—应变测量、溶剂诱导结晶和声速测定等方法研究了物理老化对非晶态PET膜和纤维结构与性能的影响.研究结果表明,非晶态PET经物理老化后DSC曲线上的Tg增加,并出现吸热峰,热结晶变慢,溶剂诱导结晶和溶剂扩散速率变慢,初始模量和屈服应力增加,从物理老化引起PET中凝聚缠结增加这一结构变化的角度对上述实验现象进行了解释.     

PET 磁带带基的结构和性能

对各磁带厂使用的磁带用 PET 薄膜的力学性能和结构特征进行了分析和比较,发现有一些薄膜并不是纵向性能很好的加强膜。讨论了分子量、结晶程度和取向状态等和 PET 薄膜性能间的关系。提出可用折射方法得到的双折射位来表征 PET 薄膜的取向状态,并用于非加强膜的工艺试验和研究。     

物理松弛对硫化的天然橡胶化学应力松弛的影响

本文用在溶胀和干燥状态下制备的网络结构试样研究了物理松弛对化学应力松弛的影响。溶液状态下硫化的试样其应力松弛行为明显地不同于常规法的试样。观察两类试样链断裂数也有相同的结果。另一方面,用溶胀法测得的常规法交联的试样的链断裂数与用化学应力松弛测得的溶液状态下硫化的试样的结果非常一致。研究发现在平衡溶胀中,由链缠结引起的物理松弛只有很小或没有影响,而因链断裂形成的悬挂着的侧链则无影响。由溶胀法测定的网络链密度的相对变化也与由溶胶分数测定的一致。这些结果表明溶胀法可用于测量化学流变过程中的降解程度。考虑到物理松弛对化学应力松驰的影响,我们根据实验提出了相对应力损失与相对网络链密度的一种新的关系。     

PET初生纤维在存放和热处理时结构和性能的变化

用声速法、密度和双折射法研究了PET的POY和LOY丝在存放和热处理中的变化。结果表明,用声速法可以跟踪卷绕所得初生纤维在早期结构重排产生的变化。取决于PET初生纤维在热处理前的双折射、热处理温度和热处理时间,可以观察到分子取向的弛豫或结晶,初生纤维具有完全不同的性能。     

溶剂对PET纤维聚集态结构的影响

用密度法、DSC,偏光显微镜(PLM)和测定收缩率研究了1,2-二氯乙烷对不同纺速PET纤维聚集态结构的影响。结果表明,纺速不同,溶剂诱导结晶(SINC)及其动力学过程不同。高速纺纤维系取向晶态结构,对溶剂扩散有抑制作用。低、中速纺纤维基本为非晶态结构,SINC过程易于进行,但中速纺纤维取向非晶态含量高,溶胀度和收缩率较小。从而印证了关于不同纺速PET纤维结构特征的结论。     

聚对苯二甲酸乙二醇酯的拉伸应力波动行为研究

研究了不同相对分子质量聚对苯二甲酸乙二醇酯(PET)在不同拉伸速率下的应力波动行为。通过拉伸测试、差示扫描量热仪和扫描电子显微镜等方法对PET样条不同区域的应力波动行为、结晶度、宏观及微观形貌进行了分析。结果表明,PET应力波动行为对相对分子质量和拉伸速率具有依赖性;PET试样内部空洞不均匀现象只是应力波动行为的微观结果;拉伸诱导结晶过程中,明带暗带的结晶度大小不同是应力波动产生的原因。     

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。     

PET和PTT及PET/PTT复合纤维结构研究进展

概述了聚对苯二甲酸乙二醇酯(PET)、聚对苯二甲酸丙二醇酯(PTT)的结构异同点,以及高速纺丝工艺条件下纤维的超分子结构。PET和PTT纤维都只存在三斜晶系晶型,均属可高速纺拉伸诱导取向结晶类纤维。随着纺丝速度的增加,在纺丝速度为4000m/min左右时,PET和PTT纤维均出现取向诱导结晶现象,且晶体尺寸增大;双折射值则先增大后减小,但在相同的纺丝速度下,PTT初生纤维的双折射值要小于PET初生纤维的双折射值。综述了PET/PTT并列复合纤维的结构研究进展。单组分纤维和双组分纤维存在结构差异。指出应进一步研究PET/PTT并列复合纤维的结构和性能。     

成膜剂和剪切场对GF/PET结构与性能影响

采用不同的成膜剂处理玻纤(GF),改变制备试样过程中熔体流动场剪切速率来挤出制备了GF/聚对苯二甲酸乙二醇酯(PET)复合材料。通过测定PET及GF/PET复合材料试样流变性能和纵向拉伸强度,研究成膜剂和熔体剪切速率对GF/PET复合材料结构和性能的影响。结果表明,聚合物成膜剂与硅烷偶联剂并用处理GF,能提高GF/PET复合材料的力学性能,低相对分子质量PET成膜剂处理后的GF与PET的粘结性很好;在熔融成型GF/PET复合材料试样过程中,剪切速率影响GF在复合材料中取向,试样力学强度对应有最佳的熔体流动场剪切速率。     

PET卷绕丝预取向对后加工拉伸性能和纤维结构、性能的影响

应用ＷＡＸＤ、ＤＳＣ、声速法、密度法、双折射等，讨论了在提高卷绕丝纺速与喷丝头拉伸比后，纤维的预取向增大，对其在后加工中的拉伸性能及其结构、性能的影响。结果表明，预取向高的卷绕丝，在后加工一道拉伸过程中，除了非晶取向外，还产生了“应变结晶”（取向诱导结晶）。由此造成晶态、取向非晶态结构差异大，而导致强度、伸长不匀率上升，产生毛丝、缠辊，但可得到高强度纤维。调整拉伸浴槽温度、拉伸速度，可改变卷绕丝的拉伸性能。     

Uniaxial Orientation and Crystallization Behavior of Amorphous Poly(ethylene terephthalate) Fibers

The effects of drawing conditions on the orientation and crystallinity of poly(ethylene terephthalate) (PET) fibers were investigated by using optical birefringence, sonic velocity, and wide-angle X-ray diffraction measurements, respectively. The preferred condition for preparation of uniaxially oriented amorphous PET fibers was suggested. The crystallization behavior of oriented PET fibers under relaxed and fixed length conditions was investigated by using differential scanning calorimetry (DSC). The multi-overlapping peaks were observed in the non-isothermal DSC curves of oriented PET fibers under relaxed condition. The kinetics of non-isothermal crystallization of oriented PET fibers under relaxed condition was analyzed by using an equation which takes the multi-crystallization processes into account. The kinetic parameters of every process were obtained and the crystallization mechanism was discussed. The crystallization behavior under fixed length condition differs from that under relaxed condition.     

Crystallization in polymer melt spinning

The crystallization behavior of poly(ethylene) terephthalate (PET) melt spun into fiber monofilaments was examined using a laboratory set-up. The wind-up speeds ranged from free fall under gravity to 1500 m/min. The major additional variables that were manipulated included the mass flow rate and the filament temperature profile. The structure of the as-spun fibers was probed using tensile tests, differential scanning calorimetry, optical birefringence, and x-ray diffraction. It was found that while the filaments that had been spun nonisothermally were essentially amorphous, those that had been made under isothermal conditions at temperatures ranging from 180°C to 240°C were oriented and crystalline. In addition, the rate of oriented crystallization was much greater than that under quiescent conditions at the same temperature. This is perhaps the first published study which shows that highly crystalline (up to 40% crystallinity) PET fibers can be obtained at low spinning speeds merely by altering the fiber temperature profile while the material is still above the polymer glass transition temperature.     

Processing–structure–property relationships in poly(ethylene terephthalate) blown film

An investigation was undertaken to establish processing–structure–property relationships in poly(ethylene terephthalate) (PET) blown film. For the study, a commercial grade of PET was used to fabricate the film specimens by means of a tubular film blowing process. In this process, the stretch temperature was accurately controlled by an oven. The annealing treatment of the oriented specimens involved clamping the sample in an aluminum frame and then putting the clamped sample in an oven, controlled at a temperature between the glass transition temperature (70°C) and the melting point (255°C) of PET, for a specified annealing period. The structure of the blown film samples was characterized by density, bulk birefringence, flat plate wide-angle X-ray scattering, and pole figure analysis. The processing variables, namely, takeup ratio, blowup ratio, and stretch temperature were found to significantly affect the bulk birefringence and density of the oriented PET blown film samples. It was found that both the bulk birefringence and density of the specimens increased upon annealing at an elevated temperature. Both the crystalline and amorphous orientation functions were calculated from the data of bulk birefringence, density, and the pole figure analysis. Compared to the amorphous orientation functions, the crystalline orientation functions were found to be relatively insensitive to the processing variables. It was concluded that equibiaxially oriented PET films can be produced via a tubular film blowing process by judiciously controlling the processing and annealing conditions. It has also been observed that the tensile stress-at-break of equibiaxially oriented PET film increases with decreasing stretch temperature and increasing annealing temperature.     

Kinetics of non-isothermal cold crystallization of uniaxially oriented poly(ethylene terephthalate)

The non-isothermal equation was extended to describe non-isothermal cold crystallization kinetics of oriented polymers. The validity of the equation was examined by using a DSC crystallization curve of oriented poly(ethylene terephthalate) (PET) fibers with a constant heating rate. The double cold crystallization peaks appeared in the DSC curve. The relative degrees of crystallinity at different temperatures were analyzed by using the equation. The results show that the value of the Avrami exponent near to 1 at lower temperatures implies the bundle-like crystal growth geometry and the value of the Avrami exponent near to, 2, at higher temperatures implies the higher dimension crystal growth geometry. The first crystallization process crystallizes at faster rate than that of the isotropic sample, while the second process crystallizes at slower rate than that of isotropic sample. If a simple single process model was used, the value of the Avrami exponent, 0.77, was obtained. The result shows the simple single process model cannot describe the processes of crystallization of oriented PET fibers satisfactorily.     

Raman microprobe spectra of spin-oriented and drawn filaments of poly(ethylene terephthalate)

Polarized Raman spectra of single filaments of spin-oriented and drawn fibres of poly(ethylene terephthalate) were recorded. Various Raman bands and their intensities correlated with (1) conformation of the glycol linkage, (2) orientation of the chains, and (3) crystallinity. Because the degrees of crystallinity and orientation were known from X-ray diffraction, density and optical birefringence, it became possible to identify features of the Raman spectra which correlated with each of the effects noted above. The degree of orientation, as a function of take-up speed or draw ratio, was correlated with the intensity ratios of the various polarization components. The appearance and increases in intensity of bands due to trans glycol conformations and the disappearance of the gauche bands with increasing orientation could be followed readily. It appears that many of the Raman bands earlier assigned to crystallinity in PET actually represent the trans conformation of the glycol group as is also observed in i.r. spectra. This conclusion is based on the observation that in spinoriented amorphous materials the trans conformation bands increase in intensity with increasing birefringence. Also an amorphous highly oriented fibre shows these same transconformation bands with high intensity. On the other hand, the width of the carbonyl band, which is the classical indicator of the amorphous or crystalline character of PET, confirmed the amorphous nature of this sample. In conclusion, by monitoring appropriate features of the Raman spectra of spin-oriented and drawn fibres, we have been able to confirm that orientation can occur independently of crystallization in PET.     

The crystallization of oriented poly(ethylene terephthalate)

The rate of crystallization of oriented poly(ethylene terephthalate) has been measured at 100°, 120° and 150°C using carefully prepared amorphous fibre samples. The samples were held to length during crystallization so that shrinkage did not occur, and the course of crystallization was followed by measuring the changes in density and boiling water shrinkage of the samples. The results show that the rate of crystallization is strongly dependent on the degree of orientation. Nucleation and initial growth of crystallites occur in times of the order of milliseconds at 120°C in samples of birefringence 0.08 compared with times of several minutes in isotropic material. It was found that crystallization in oriented material cannot be described by the Avrami equation.     

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.     

Oxygen solubility and specific volume of rigid amorphous fraction in semicrystalline poly(ethylene terephthalate)

The existence of rigid amorphous fraction (RAF) in semicrystalline poly(ethylene terephthalate) (PET) is associated with the lamellar stack crystalline morphology of this polymer, the regions where several crystalline lamellas are separated by very thin (20-40 Å) amorphous layers. In contrast, regular or mobile amorphous fraction is associated with much thicker interstack regions. The oxygen transport properties of PET isothermally crystallized from the melt (melt-crystallization) or quenched to the glassy state and then isothermally crystallized by heating above T_g (cold-crystallization) were examined at 25 °C. Explanation of unexpectedly high solubility of crystalline PET was attributed to the formation of RAF, which in comparison with mobile amorphous phase is constrained and vitrifies at much higher than T_g temperature thus developing an additional excess-hole free volume upon cooling. Measurements of crystallinity and jump in the heat capacity at T_g were used to determine the amount of mobile and rigid amorphous fractions. Overall oxygen solubility was associated with the solubility of mobile and rigid amorphous fractions. The oxygen solubility of the RAF was determined and related to the specific volume of this fraction. The specific volume of the RAF showed a direct correlation with the crystallization temperature. It was shown that upon crystallization from either melt or glassy state, the constrained between crystalline lamellas PET chains consisting of the RAF, vitrify at the crystallization temperature and resemble the glassy behavior despite high temperature. When cooled to room temperature, the RAF preserves a memory about the melt state of polymer, which is uniquely defined by the crystallization temperature.     

Kinetics of structural evolution during crystallization of preoriented PET as followed by dual wavelength photometric birefringence technique

A two-color laser photometric measurement system was used to follow birefringence changes in annealing of prestretched PET films. This technique was shown to be capable of detecting large retardation (and thus birefringence) changes beyond one wavelength of the light. The use of green and red lasers in the photometric system allows the detection of the reversals in birefringence during the course of annealing. Unless critical orienialion/erystallinity levels are developed by stretching the films, heat setting the films at temperatures close to glass transition temper ature results in complete elimination of preferential orientation. At Intermediate stretch ratios, crystalline regions form and Ihe acl as anchor points establishing a network for the orienied amorphous chains. These Dims exhibit partial relaxation followed by rapid increase in birefringence due to the accelerating effect of orientation on crystallization. At high stretch ratios, where substantially oriented and strain crystallized structures are obtained, the initial relaxation stage disappears and birefringence continue to increase throughout the heat setting process even at temperatures very close to glass transition temperature. In these films, however, the total change of birefringence decreases as more of the chains are oriented and crystallized in the stretching stage, leaving a smaller fraction of polymer chains to rearrange during Ihe annealing stage. The kinetics of the structural change exhibit a complex behavior and the largest rates of structural changes were observed in films exhibiting intermidiate birefringence levels.     

Stress-induced crystallization of branched polyethylene terephthalate films

Polyethylene terephthalate (PET) prepolymer containing 0.0 to 0.3 mole percent pentaerythritol was polymerized to an inherent viscosity of 0.63-0.70 dl/gm by the fluidized bed technique. Rheological studies and gel permeation chromatography (GPC) examination showed the samples to be branched in character. Amorphous films were stretched at 82 and 93°C at elongation rates of 54, 161 and 267 percent-s^?1 using the T. M. Long machine. The extent of stress-induced crystallization was established by a density determination. The branched samples behaved very similarly to the linear PET material in crystallization and birefringence studies. Neither the percent crystallinity nor the birefringence appears to be a strong function of strain rate over the range 54-267 percent-s^?1. A cursory examination of the crystallization kinetics of the oriented samples suggests that extent of branching in our samples does not markedly reduce the crystallization rate at annealing temperatures of 180 and 220°C.