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。     

Crystallization behavior and mechanical properties of polypropylene/halloysite composites

In this work, halloysite nanotubes (HNTs), a new type of inexpensive filler, were used for the modification of polypropylene (PP). HNTs were first surface treated by methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium, then melt mixed with PP. Scanning electron microscope (SEM) was used to examine the dispersion of HNTs in PP matrix. Differential scanning calorimetry (DSC), polarized light microscope (PLM), dynamic melt rheometry and wide angle X-ray diffraction (WAXD) were employed to investigate the crystallization behavior of the prepared PP/HNT composites. The mechanical properties were evaluated by Instron and impact tests. SEM results revealed that HNTs could be well-dispersed in PP matrix and had a good interfacial interaction with PP, even up to a high content of 10 wt%. DSC data indicated that HNTs could serve as a nucleation agent, resulting in an enhancement of the overall crystallization rate and the non-isothermal crystallization temperature of PP. PLM showed a constant spherulite growth rate and a decreased spherulite size at given isothermal crystallization temperature, suggesting that nucleation and growth of a spherulite are two independent processes. The result obtained by dynamic melt rheometry indicated that HNTs mainly promoted nucleation and had not much influence on the growth of PP crystallization. Nevertheless, by fast cooling the samples, almost constant spherulite size can be obtained for both pure PP and PP/HNT composites due to the limited nucleation effect of HNTs on PP crystallization. WAXD showed that HNTs mainly facilitated α-crystal form of PP. Though a good dispersion of HNTs in PP matrix was observed, out of our expectation, not much enhancement on mechanical properties of PP/HNT composites had been achieved, and this could be mainly ascribed to the constant crystallinity and spherulite size of PP as well as the small length/diameter ratio of HNTs.     

Effects of processing on the microstructure,melting behavior,and equilibrium melting temperature of polypropylene

Polypropylene (PP) was extruded and injection-molded several times to mimic the effect of recycling procedures on PP. Differential scanning calorimetry (DSC) was used to follow crystallization rates under isothermal conditions in a temperature range of 120–150°C. Melting behavior and equilibrium melting temperatures were studied using the Hoffman-Weeks method of extrapolation. Optical microscopy combined with a hot stage was also used to follow the spherulite microstructure and crystal phase upon recycling of PP. Wide-angle X-ray spectroscopy identified the crystal phase at different isothermal crystallization temperatures. Twin melting peaks obtained for PP melting following isothermal crystallization were associated with crystal rearrangement during fusion. PP spherulite size and equilibrium melting temperatures were seen to increase with processing events, whereas reprocessing decreased nuclei density. © 1996 John Wiley & Sons, Inc.     

聚对苯二甲酸丙二醇酯的非等温结晶动力学研究

采用DSC研究了聚对苯二甲酸丙二醇酯(PTT)在不同等速降温速率下的结晶过程,并利用由等温Avrami方程推导得到的两种不同的等速降温非等温结晶方程,研究了其结晶动力学,得出PTT的成核方式为异相成核,同时利用拟和法计算了PTT的结晶能力,发现在同等条件下PTT的结晶能力大于PET。     

Crystallization of linear polyethylene from melt in isothermal compression

Crystallization and primary nucleation of linear polyethylene has been studied by means of a custom-made miniature pressure apparatus. It has been shown that during isothermal compression of linear polyethylene melt at a constant rate crystallization occurs. In the range of fastest conversion rates the crystallization assumes isobaric character. The level of pressure at which the crystallization occurs increases with the increase of the crystal-lization temperature and/or with the increase of the compression rate. The crystallization has a spherulitic character up to the highest pressure achieved in the apparatus (approx. 600 MPa). Surprisingly, there are no dependencies of average spherulite size, degree of crystallinity, and lamellae thickness on the pressure and the temperature of crystallization during melt compression, but there is a strong relation to the compression rate. Below 250 MPa and above 300 MPa the crystallization proceeds under pressure, ensuring a constant undercooling. The undercooling for the pressure above 300 MPa is approximately 10°C lower than that for the pressure below 250 MPa. For the pressure 250–300 MPa a change in a primary nucleation and spherulite crystallization has been observed that is connected with the transformation from orthorhombic to pseudohexagonal symmetry of crystals. No noticeable effect of molecular weight of linear polyethylene on crystallization during iso-thermal melt compression has been observed.     

Stochastic simulation for morphological development during the isothermal crystallization of semicrystalline polymers: A case study of syndiotactic polypropylene

A stochastic simulation scheme for predicting morphological development during nucleation and subsequent crystal growth based on predetermined crystallization kinetic data of a semicrystalline polymer under quiescent isothermal conditions is proposed. Based on previously obtained crystallization kinetic data for syndiotactic polypropylene (s‐PP) used as the input information, the simulation scheme was successful in predicting the morphological development of s‐PP during isothermal crystallization from the melt state. The predicted development of crystallinity during crystallization was reanalyzed with the Avrami macrokinetic model, and good agreement between the predicted and theoretical values for s‐PP was observed. On the basis of this simulation scheme, both the spherulite size and its distribution during the course of crystallization could also be predicted. Although the spherulitic growth rate influenced both the spherulite size and its distribution during the course of crystallization, it had no effect on the final spherulitic morphology or the resulting average spherulitic size. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Crystallization behavior of biodegradable poly(L‐lactic acid) filled with a powerful nucleating agent: N,N′‐bis(benzoyl) suberic acid dihydrazide

N,N′‐Bis(benzoyl) suberic acid dihydrazide (NA) as nucleating agent for poly(L ‐lactic acid) (PLLA) was synthesized from benzoyl hydrazine and suberoyl chloride, which was deprived from suberic acid via acylation. PLLA/NA samples were prepared by melt blending and a hot‐press forming process. The nonisothermal and isothermal crystallization, spherulite morphology, and melting behavior of PLLA/NA with different contents of NA were investigated with differential scanning calorimetry, depolarized‐light intensity measurement, scanning electron microscopy, polarized optical microscopy, and wide‐angle X‐ray diffraction. With the incorporation of NA, the crystallization peak became sharper and shifted to a higher temperature as the degree of supercooling decreased at a cooling rate of 1°C/min from the melt. Nonisothermal crystallization indicated that the presence of NA accelerated the overall PLLA crystallization. In isothermal crystallization from the melt, the presence of NA affected the isothermal crystalline behaviors of PLLA remarkably. The addition of NA led to a shorter crystallization time and a faster overall crystallization rate; this meant that there was a heterogeneous nucleation effect of NA on the crystallization of PLLA. With the addition of 0.8% NA, the crystallization half‐time of PLLA/NA decreased from 26.5 to 1.4 min at 115°C. The Avrami theory was used to describe the kinetics of isothermal crystallization of the PLLA/NA samples. Also, with the presence of NA, the spherulite number of PLLA increased, and the spherulite size decreased significantly. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

聚丙烯/纳米CaCO3复合材料的结晶行为研究

用超重力法制备的纳米CaCO3和PP熔融共混制备了PP/CaCO3复合材料,并对PP/CaCO3复合材料的结晶行为进行了详细研究。差示扫描量热分析表明,纳米CaCO3粒子的加入加快了PP的结晶速率,缩短了半结晶时间,130℃时含15份纳米CaCO3的PP/CaCO3复合材料的半结晶时间比纯PP的减少了8.92 min;结晶度有轻微下降,结晶温度为126.5 ℃时纯PP的结晶度为44.33 %,含15份纳米CaCO3的PP/CaCO3复合材料的结晶度为35.9 %。动力学研究数据表明,等温结晶过程符合Avrami方程,PP/CaCO3的n和k值都大于纯PP的;利用偏光显微镜观察了PP/CaCO3复合材料的结晶形貌及结晶生长过程,纳米CaCO3粒子的加入使球晶数量明显增多,意味着CaCO3起到了结晶成核剂的作用。     

SIMULATION OF CRYSTALLIZATION KINETICS AND MORPHOLOGICAL DEVELOPMENT DURING ISOTHERMAL CRYSTALLIZATION OF POLYMERS: EFFECT OF NUMBER OF NUCLEI AND GROWTH RATE

The effect of number of nuclei and growth rate on crystallization kinetics and detailed morphological development during isothermal crystallization of a polymer was investigated using a stochastic simulation. The results show that number of nuclei significantly affects both crystallization kinetics and polymer morphology. An increase in the number of nuclei hastens the crystallization process by speeding up the impingement phenomenon and increasing the levels of impingement. Growth rate has a stronger impact on crystallization kinetics, but it only helps speed up the impingement phenomenon without increasing the level of impingement. Although growth rate influences an average spherulite size and distribution of spherulite size during crystallization, it has no effect on final morphology. The quantitative understanding of morphological development obtained from this work will be a key element for constructing quantitative morphology-property relationships.     

生物基尼龙56的等温结晶性能研究

采用差示扫描量热仪和带有热台的偏光显微镜对生物基尼龙56的等温结晶性能进行了研究。用Avrami方程对等温结晶过程及其动力学进行了分析,得出Avrami指数(n值)在2.30~3.37之间,推测其晶体生长方式为三维球状生长;采用Arrhenius方程计算了生物基尼龙56的等温结晶活化能(ΔE)为-99.04 kJ/mol。偏光显微镜研究证实了上述推测,同时发现球晶半径与结晶时间呈线性关系,求得了球晶的生长速率。     

Spherulite growth in isotactic polypropylene-based blends: Energy and morphological considerations

The kinetics of isothermal crystallization of polymer blends in which the matrix is a crystallizable polymer is considered. It is shown that depending on the difference in interfacial energies the inclusions are rejected or engulfed by the growing spherulite. Other factors influencing rejection, engulfing, and/or deformation of dispersed particles of the second polymer are the viscosity of the melt, the spherulite growth rate, and the size of dispersed particles. If the difference in interfacial energies is positive, then rejection or engulfing requires additional work to be done by the crystallization front. This dissipation of energy decreases the spherulite growth rate. It is estimated that the rejection of the second component is the most important phenomenon in the crystallization of blends. The spherulite growth rate of isotactic polypropylene in blends with low-density polyethylene and several elastomers was studied as a function of crystallization temperature and concentration. The comparison of growth rate data with morphological changes occuring during crystallization of blends studied shows very good agreement with the theoretical predictions based on energetics considerations.     

Crystallization kinetics and morphology of poly(trimethylene terephthalate)

In this work, the isothermal crystallization kinetics of polytrimethylene terephthalate (PTT) was first investigated from two temperature limits of melt and glass states. For the isothermal melt crystallization, the values of Avrami exponent varied between 2 and 3 with changing crystallization temperature, indicating the mixed growth and nucleation mechanisms. Meanwhile, the cold crystallization with an Avrami exponent of 5 indicated a character of three-dimensional solid sheaf growth with athermal nucleation. Through the analysis of secondary nucleation theory, the classical regime I→II and regime II→III transitions occurred at the temperatures of 488 and 468 K, respectively. The average work of chain folding for nucleation was ca. 6.5 kcal mol⁻¹, and the maximum crystallization rate was found to be located at ca. 415 K. The crystallite morphologies of PTT from melt and cold crystallization exhibited typical negative spherulite and sheaf-like crystallite, respectively. Moreover, the regime I→II→III transition was accompanied by a morphological transition from axialite-like or elliptical-shaped structure to banded spherulite and then non-banded spherulite, indicating that the formation of banded spherulite is very sensitive to regime behavior of nucleation.     

Crystallization behavior and polymorphism of poly(1,4‐butylene adipate): Effect of anhydrous orotic acid as nucleating agent

The effect of anhydrous orotic acid (OA), as a biocompatible nucleating agent (NA), on the non‐isothermal and isothermal crystallization behaviors, polymorphic crystalline structure and phase transition of poly(1,4‐butylene adipate) (PBA) was investigated. It is found that the OA increased the crystallization temperature of the PBA in the non‐isothermal crystallization process and decreased the crystallization time of the PBA in the isothermal crystallization process. Meanwhile, the spherulite size decreased and spherulite density increased for the PBA. The OA favored the formation of the PBA α‐form crystal, compared to the neat PBA. In addition, upon incorporation of the OA, the β‐to‐α phase transition rate was enhanced significantly. Mechanisms for the preferential formation of the PBA α‐form crystal and the accelerated phase transition have also been proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42957.     

Spherulite nucleation in isotactic polypropylene based nanocomposites with montmorillonite under shear

The crystallization of nanocomposites of isotactic polypropylenes with organo modified montmorillonite compatibilized by maleic anhydride grafted polypropylene was studied by light microscopy in isothermal conditions in quiescent state and in shear. The isothermal and nonisothermal crystallization of the composites was also investigated by DSC method. Only weak nucleation activity of montmorillonite was observed during crystallization in static conditions. The clay nucleation activity was greatly enhanced in shear-induced crystallization and resulted in a drastic decrease of spherulite sizes. In nanocomposite films crystallized isothermally the intense nucleation of isotactic polypropylene spherulites was observed when the polymer was forced to flow to compensate the volume shrinkage due to crystallization. It was also found that the presence of a glass support enhances spherulite nucleation in nanocomposites which is caused possibly by shear due to a difference in thermal shrinkage of a polymer matrix and a glass support.     

Primary and secondary crystallization kinetic analysis of nylon 1212

The isothermal and non‐isothermal melt‐crystallization kinetics of nylon 1212 were investigated by differential scanning calorimetry. Primary and secondary crystallization behaviors were analysed based on different approaches. The results obtained suggested that primary crystallization under isothermal conditions involves three‐dimensional spherulite growth initiated by athermal nucleation, while under non‐isothermal conditions, the mechanism of primary crystallization is more complex. Secondary crystallization displays a lower‐dimensional crystal growth, both in the isothermal and non‐isothermal processes. The crystallite morphology of nylon 1212, isothermally crystallized at various temperatures, was observed by polarized optical microscopy. The activation energies of crystallization under isothermal and non‐isothermal conditions were also calculated based on different approaches. Copyright © 2004 Society of Chemical Industry     

Growth process of homogeneously and heterogeneously nucleated spherulites as observed by atomic force microscopy

A poly(bisphenol A octane ether) (BA-C8) was synthesized. The isothermal spherulitic growth process was studied in situ using atomic force microscopy (AFM) at room temperature. For spherulites formed by homogeneous nucleation, the growth process includes the birth of a primary nucleus, the development of a founding lamella and the growth of the founding lamella into a spherulite. An embryo below a critical size is unstable. A stable embryo grows into a founding lamella. There is only one founding lamella in each spherulite. All other lamellae originate from this founding lamella. Two eyes can be seen at the center of a spherulite. For spherulites formed through heterogeneous nucleation, many lamellae grow at the nucleus surface and propagate outward radially. The spherulites acquire spherical symmetry at the early stage of crystallization. No eyes are found for this kind of spherulites.     

Isothermal crystallization of polyethylene oxide/silver nanoplate composites

This research was accomplished to investigate the kinetics of isothermal crystallization of polyethylene oxide (PEO)/silver nanoplate composites. It was obtained that the spherulites increased in size and numbers with time for the composites with various particle loadings. Additionally, the spherulite growth rate of composites decreased with an increase in the crystallization temperature and increased with the addition of nanoplates. The spherulite growth rate was further analyzed by the theory developed by Lauritzen and Hoffman. The product of the lateral surface free energy (σ) and the end surface free energy (σ_e) decreased with an increase in the content of nanoplates. We proposed the possible crystallization mechanisms of these PEO/nanoplate composites according to the change of σ and σ_e with the presence of nanoplates. A controlled experiment showed a minor change in PEO crystallization with the presence of a surfactant C₁₆TAB. This implied that the unique size and shape of nanoplates plays a key role on hindering the primary nucleation of PEO and increasing the spherulite growth rate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

聚对苯二甲酸丙二醇酯的结晶性能研究

采用热台偏光显微镜和DSC差示扫描量热仪对PET、PTT和PBT的结晶性能进行了研究。实验得到的结果是:PBT具有极强的结晶能力。在相同的△T下,PTT的结晶诱导期和球晶出现的时间比PET短,球晶的生长速率也比PET快;同时,在相同的△T下,PTT的总结晶速率大于PET。PET和PTT在较高的温度下等温结晶时倾向于异相成核,而随着等温结晶温度的降低,开始倾向于均相成核。     

Study on the crystallization kinetics of PP/GF composites

The crystallization kinetics and morphology of glass fiber-reinforced polypropylene (PP/GF) were investigated in this work. Both isothermal and nonisothermal crystallization behaviors of 90PP/10GF, 80PP/20GF, and 70PP/30GF were examined with a DSC instrument. It was found that the addition of glass fiber would increase the crystallization rate of PP and increase the content of β spherulite, which was most likely formed at temperatures between 390 and 400 K. The morphology of spherulites of PP/GF composites were examined with SEM and a polarized microscope. All experimental observations conformed rather well with the theoretical approach, a dynamic crystallization model, proposed in our previous work. The size of α spherulites of PP would decrease at lower crystallization temperature, or at higher cooling rate, or by adding glass fiber in it.     

Crystallization and microstructure in quenched slabs of various molecular weight polypropylenes

The purpose of this study was to accumulate material data for use in a simulation of morphology and crystallization in quenched slabs of isotactic polypropylenes (i-PP's). The molecular weight effect on the crystallization kinetics of various i-PP's were studied using isothermal and nonisothermal experiments. An experimental setup was constructed to measure the spherulite growth rate of i-PP's using isothermal and nonisothermal melt crystallization experiments. The molecular weight effect on spherulite growth rate of i-PP's was determined. Molecular weight effect on nuclei concentrations as a function of temperature was determined from growth and crystallization rate constants. The morphology and crystallization in quenched slabs of i-PP's were determined from experiments and compared to computer simulations. The molecular weight effect on the degree of crystallinity and morphology was determined for i-PP during quenching.