Melting behaviors,isothermal crystallization kinetics,and morphology of poly(trimethylene terephthalate)/stainless steel fiber composites

The melting behavior and crystallization kinetics of poly(trimethylene terephthalate) (PTT)/stainless steel fiber (SSF) composites were investigated with differential scanning calorimetry. The morphology was studied with scanning electron microscopy and polarized optical microscopy. Differential scanning calorimetry analysis revealed that the crystallization temperature increased by 27°C with the addition of 1 vol % SSF to the matrix. The Avrami exponents, analyzed in isothermal crystallization kinetics, were determined to be 2–3 for both neat PTT and PTT/SSF composites. SSF, as a nucleating agent in the composites, greatly increased the crystallization rate. The activation energies of the composites were obviously lower than that of pure PTT, and this indicated much easier crystallization of the composites. All these samples exhibited banded spherulites, and the spherulite size gradually decreased with the SSF loading increasing. Subsequent melting behaviors revealed that all of these samples, especially of the composites, exhibited triple melting peaks at all crystallization temperatures studied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of crystallization behavior on morphological change in poly(trimethylene terephthalate) spherulites

In poly(trimethylene terephthalate) (PTT) spherulites during isothermal crystallization, the morphological changed from an axialite/or elliptical banded spherulite to banded spherulite and then non-banded spherulite with temperature decreasing were studied by following the lamellar growth behaviors. We report lamellar growth mechanism on varied crystallization temperature, which explicitly probes the link between microscopic structure and macroscopic morphology in the development of patterns. Fibrillation of the edge-on lamellae was observed on the surfaces of axialite and the convex bands of banded spherulite. Terrace-like lamellae were observed on the surface of the non-banded spherulite and the concave bands of banded-spherulite. In thin film crystallization, PTT banded spherulite exhibits a texture of alternate edge-on and flat-on lamellae, wavy-like surface and rhythmic growth. The deceleration of growth rate takes place in convex bands with a growth habit of fibrillation of the edge-on lamellae for emerging ridge surface. On the other hand, the acceleration of growth rate appears in concave bands with a growth habit of terrace-like lamellae for emerging valley surface. The alternating growth mechanism of the lamellae was considered to be related with the formation of spatiotemporal self-organization patterns far from equilibrium. In order to explain the rhythmic growth and periodic growth of the lamellae, we may conjecture that the emergence of PTT banded spherulite in thin film crystallization is associated with an oscillatory dynamics of the spherulite growth front driven by latent heat diffusion. We present some tentative ideas on the possibility of band-to-nonband (BNB) morphological transition, which might be analogous with the second order transition in non-equilibrium phase transition.     

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。     

PTT／纳米CaCO3复合材料流变行为及结晶性能研究

以毛细管流变仪研究了聚对苯二甲酸丙二酯(PTT)/纳米CaCO3复合材料的流变行为,讨论了复合材料的组成、剪切应力和剪切速率及温度对熔体流变行为、熔体黏度的影响,测定了不同配比的复合材料熔体的非牛顿指数 n。结果表明,PTT/纳米CaCO3复合材料熔体为假塑性流体,表观黏度随着剪切速率增加而降低。纳米CaCO3的加入量较少(1％)时,熔体黏度较纯PTT迅速下降;随着纳米CaCO3含量增加(2％-20％),熔体黏度随之上升,但都小于纯PTT的;直到含量为30％时,熔体黏度才超过纯PTT的。差示扫描量热仪测定复合材料的结晶和熔融性能发现,复合材料的熔体结晶温度Tpc和熔融温度Tm较纯PTT、都有所升高,说明纳米CaCO3的加入对PTT的结晶起到了异相成核作用。     

Morphological features and crystallization behavior of the conductive composites of poly(trimethylene terephthalate)/graphene nanosheets

Poly(trimethylene terephthalate) (PTT) composites filled with well‐dispersed graphene nanosheets (GNSs) were prepared through a coagulation method. The effects of increased GNS concentration on variations in the structure and properties of the PTT matrix, such as its electrical conductivity, crystallization kinetics, melting behavior, and crystal morphology, were investigated. Several analytical techniques were used, including electrical conductivity measurement, differential scanning calorimetry, Fourier transform infrared spectroscopy, wide‐angle X‐ray diffraction, polarized light microscopy, transmission electron microscopy (TEM), and thermo‐gravimetric analysis (TGA). Electrical conductivity increased from 1.8 × 10^?17 S/cm for neat PTT to 0.33 ± 0.23 S/cm for PTT/GNS composites with 2.97 vol % GNS content. Percolation scaling laws were applied, and then threshold concentration and exponent were determined. In the case wherein liquid nitrogen was used to quench the melt, a mesomorphic phase was formed despite the extremely short crystallization time after adding high GNS contents. PTT crystallization rate increased with the gradual addition of GNSs. The enhanced crystallization kinetics was attributed to the high nucleation ability of GNSs to induce epitaxially grown lamellae on their surfaces, as revealed by TEM. PTT nuclei were randomly developed on the GNS surface to form the lamellae. However, crystallinity reached its maximum value near the electrical percolation threshold because the PTT chain mobility was confined after the GNS–GNS network formed. The growth of PTT banded spherulites in the bulk was still observed for composites with high GNS content, and TGA results revealed that the GNS‐filled PTT composites had excellent thermal stability. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43419.     

Nucleation effect of clay on crystallization of polypropylene under carbon dioxide

We investigated the crystallization growth of isotactic polypropylene (PP)/clay phase‐separated microcomposites under carbon dioxide (CO₂) by in situ observation with a specially designed high‐pressure visualized cell. The number density of the spherulite was larger and the size of the spherulite filled in the whole space was smaller in the PP/clay microcomposites than those in the neat PP, indicating nucleation effect of clay on the crystallization of PP. The increase of the number density of the spherulite by addition of clay was much larger under CO₂ than that under air at ambient pressure, suggesting that the nucleation effect of clay is enhanced under CO₂. Microscopic observation and small angle X‐ray scattering measurement revealed that lamellae grow from the surface of the clay tactoids, and the lamellae and lamellar stacks arrange irregularly due to the growth of lamellae from the irregular shaped clay in different directions. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

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 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.     

Effects of crosslinked elastomer particles on heterogeneous nucleation of isotactic PP in dynamically vulcanized EPDM/PP and EOC/PP blends

The isothermal and non-isothermal crystallizations of PP in neat form and in the TPVs EPDM/PP and EOC/PP were investigated using differential scanning calorimetry (DSC). The crystallization of PP was systematically studied by fitting mathematical models, and was later confirmed by X-ray diffraction (XRD) and by scanning electron microscopy (SEM). The experiments revealed that crosslinked elastomer particles first accelerated the primary nucleation of the PP matrix, acting effectively as a nucleating agent that reduces the induction time while increasing the nucleation efficiency. In the secondary nucleation regime (growth of spherulites), the crosslinked elastomer particles enhanced crystal growth rate, reducing the nucleation energy contribution from PP chain folding. Moreover, the crosslinked elastomer particles increased the final thickness of PP lamellae from that of neat PP, and this was corroborated by the XRD results. On comparing the two types of elastomer, it was found that the EOC particles were more effective in heterogeneous cell nucleation than the EPDM particles. The morphological study by SEM revealed completely altered PP spherulite size and shape, as well as their altered distribution, affected by heterogeneous nucleation effects of the crosslinked elastomer particles.     

Isothermal crystallization kinetics of poly(phenylene sulfide)/TLCP composites

The isothermal crystallization kinetics, the morphology, and the melting behavior of melt‐processed composites of poly(phenylene sulfide) (PPS) with a thermotropic liquid crystalline copolyester, Vectra A950, (TLCP) were studied by differential scanning calorimetry (DSC) and optical microscopy. The crystallization behavior of PPS in PPS/TLCP composites is observed to be highly sensitive to T_c and immiscible TLCP content in the composites. The spherulite growth rate, the overall crystallization rate, and the activation energy of PPS in PPS/TLCP composites are markedly depressed by the presence of TLCP. The analysis of the Avrami kinetic parameters (n and k) indicates that blending of TLCP with PPS causes heterogeneous growth process and nucleation mechanisms. At low T_cs, the PPS crystallization rate is faster than that neat PPS with ≤30 wt% TLCP loading whereas at high T_cs it remains almost unchanged. The analysis of the melting behavior of these composites indicates that the stability of PPS crystals and their reorganization is influenced both by the T_cs and the composite compositions. The sizes and the number of spherulites change a great extent with composite composition with a drop of spherulite rapid growth rate, at constant T_c, with increasing content of TLCP in composites. The analysis based on the Lauritzen‐Hoffmann secondary nucleation theory, using present DSC data, indicates that present data predominantly follow a linear growth trend over a present range of T_cs and PPS crystallization in composites still occurs according to regime II kinetics, whereby multiple surface nuclei form on the substrate with multiple nucleation acts commencing before initially formed growth layer is completed. The fold surface free energy of PPS chains in composites is found higher than that of neat PPS, leading to an average higher work of chain folding and is ascribed to a general development of the PPS chain mobility in the composite melt. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Nonisothermal crystallization behavior and crystals morphology of poly(trimethylene terephthalate)/nano‐calcium carbonate composites

Nonisothermal crystallization behavior and crystal morphology of poly(trimethylene terephthalate) (PTT) composites filled with modified nano‐calcium carbonate (CaCO₃) had been investigated by using differential scanning calorimetry and polarized optical microscopy. The modified Avrami equation and Ozawa theory were used to investigate the nonisothermal crystallization, respectively. The particles of nano‐CaCO₃, acting as a nucleation agent in composites, accelerated the crystallization rate by decreasing the half‐time of crystallization or increasing the parameters of Z_c and K(T). Moreover, the nano‐composite with 2 wt% nano‐CaCO₃ exhibited the highest crystallization rate. The Avrami and the Ozawa exponents, n and m of the nano‐composites, were higher than those of neat PTT, suggesting more complicated interaction between molecular chains and the nanoparticles that cause the changes of the nucleation mode and the crystal growth dimension. The effective activation energy calculated from the Friedman formula was reduced as nano‐CaCO₃ content increased, suggesting that the nano‐CaCO₃ made the molecular chains of PTT easier to crystallize during the nonisothermal crystallization process. The optical micrographs showed that much smaller or less perfect crystals were formed in composites because of the presence of the nano‐CaCO₃ particles. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

聚对苯二甲酸丙二酯/蒙脱土复合材料的制备及性能研究

通过熔融共混法制备了聚对苯二甲酸丙二酯/蒙脱土(PTT/MMT)复合材料。采用DSC法对其非等温结晶性能进行了研究,结果表明MMT在复合材料中起到了异相成核作用,随着MMT含量的增加,PTT/MMT复合材料的结晶峰温θp向高温方向移动;采用偏光显微镜观察PTT及PTT/MMT复合材料在190℃等温结晶的情况下的球晶形态,PTT/MMT复合材料的球晶尺寸比PTT球晶尺寸大大减小;采用电子万能试验机、邵氏硬度计和D JF-20动态冲击分析仪对PTT及PTT/MMT复合材料的综合力学性能进行了测试,结果表明当MMT的质量分数为2%时的PTT/MMT复合材料的综合力学性能最佳。     

Crystallization and spherulitic growth kinetics of poly(trimethylene terephthalate)/polycarbonate blends

The macroscopic and microscopic melt‐crystallization kinetics of poly(trimethylene terphthalate) (PTT)/polycarbonate (PC) blends have been measured by differential scanning calorimetry (DSC), and optical microscopy (OM). The results are analyzed in terms of the Avrami equation and the Hoffman–Lauritzen crystallization theory (HL model). Blending with PC did not change the crystallization mechanism of PTT, but reduced the crystallization rate compared with that of neat PTT at the same crystallization temperature. The crystallization rate decreased with increasing crystallization temperature. The spherulitic morphology of PTT was influenced apparently by the crystallization temperature and by the addition of PC. X‐ray diffraction shows no change in the unit cell dimension of PTT was observed after blending. Through the HL theory, the classical regime II→III transition was detected for the neat PTT and the blends. The nucleation parameter (K_g), the fold‐surface free energy (σ_e), and the work of chain folding (q) were calculated. Blending with PC decreased all the aforementioned parameters compared with those of neat PTT. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Crystal morphology,melting behaviors and isothermal crystallization kinetics of SCF/PTT composites

Isothermal crystallization kinetics, subsequent melting behavior, and the crystal morphology of short carbon fiber and poly(trimethylene terephthalate) composites (SCF/PTT) were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The crystal morphology of the composites isothermally crystallized at T_c = 205°C is predominantly banded spherulites observed under polarizing micrographs, while the pattern of banded spherulites changed from ring to serration as the SCF content added into the PTT. Moreover, nonbanded spherulites formed at T_c = 180°C. The commonly used Avrami equation was used to fit the primary stage of the isothermal crystallization. The Avrami exponents n are evaluated to be 1.6–2.0 for the neat PTT and 2.7–3.0 for SCF/PTT composites, and the SCF acting as nucleation agents in composites accelerates the crystallization rate with decreasing the half‐time of crystallization and the sample with SCF component of 2% has the fastest crystallization rate. The crystallization activation energy calculated from the Arrhenius formula suggests that the adding SCF component improved the crystallization ability of the PTT matrix greatly, and the sample with of 2% SCF component has the most crystallization ability. Subsequent melting scans of the isothermally crystallized composites all exhibited triple melting endotherms, in which the more the component of SCF, the lower temperature of the melting peak, indicating the less perfect crystallites formed in those composites. Furthermore, the melting peaks of the same sample are shifted to higher temperature with increasing T_c, suggesting the more perfect crystallites formed at higher T_c. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

分散相含量对POE/PTT原位成纤增强复合材料性能的影响

采用微纳叠层共挤制备了乙烯-辛烯共聚物/聚对苯二甲酸丙二醇酯（POE/PTT）原位成纤复合材料,通过扫描电子显微镜分析了分散相PTT含量对其在基体中的形态及分布的影响;讨论了PTT含量对复合材料静态力学性能的影响,利用差示扫描量热仪分析了PTT对POE基体结晶性能的影响。结果表明,随着分散相含量的提高,PTT微纤的数量逐渐增加,降低了POE基体的结晶度,当POE/PTT质量比为85/15时,拉伸强度较纯POE提高了16.9 %。     

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.     

Miscible blends of poly(4‐vinyl phenol)/poly (trimethylene terephthalate)

A new miscible blend of all compositions comprising poly(4‐vinyl phenol) (PVPh) and poly(trimethylene terephthalate) (PTT) was discovered and reported. The blends exhibit a single composition‐dependent glass transition and homogeneous phase morphology, with no lower critical solution temperature (LCST) behavior upon heating to high temperatures. Interactions and spherulite growth kinetics in the blends were also investigated. The Flory–Huggins interaction parameter (χ₁₂) and interaction energy density (B) obtained from analysis of melting point depression are negative (χ₁₂ = ?0.74 and B = ?32.49 J cm^?3), proving that the PVPh/PTT blends are miscible over a wide temperature range from ambient up to high temperatures in the melt state. FTIR studies showed evidence of hydrogen‐bonding interactions between the two polymers. The miscibility of PVPh with PTT also resulted in a reduction in spherulite growth rate of PTT in the miscible blend. The Lauritzen–Hoffman model was used to analyze the spherulite growth kinetics, which showed a lower fold‐surface free energy (σ_e) of the blends than that of the neat PTT. The decrease in the fold‐surface free energy has been attributed to disruption of the PTT lamellae exerted by PVPh in an intimately interacted miscible state. Copyright © 2004 Society of Chemical Industry     

Effect of sepiolite on the crystallization behavior of biodegradable poly(lactic acid) as an efficient nucleating agent

To enhance the crystallization kinetics of poly(lactic acid) (PLA), fibrous sepiolite was explored for nucleating the crystallization of PLA. PLA/sepiolite nanocomposites were prepared via the melt‐extrusion method. The effect of sepiolite on the crystallization behavior, spherulite growth and crystal structure of PLA were investigated by means of differential scanning calorimetry (DSC), polarized optical microscope (POM), wide angle X‐ray diffraction (WAXD), Fourier transform infrared (FTIR), and scanning election microscope (SEM). On the basis of DSC and POM results, the overall crystallization kinetics of PLA/sepiolite nanocomposites were significantly enhanced leading to higher crystallinity and nucleation density, faster spherulite growth rate (G) and lower crystallization half‐time (t_1/2) compared with the neat PLA. Under non‐isothermal conditions, the PLA blend comprising 1.0 wt% of sepiolite still revealed two crystallization peaks upon cooling at a rate of 35°C/min. Above phenomena strongly suggested that sepiolite was an effective nucleating agent for PLA. FTIR and WAXD analyses confirmed that the crystal structure of PLA matrix was the most common α‐form. SEM micrographics illustrated the fine three‐dimensional spherulite structures with the lath‐shape lamellae regularly arranged in radial directions. POLYM. ENG. SCI., 55:1104–1112, 2015. © 2014 Society of Plastics Engineers     

Non‐Isothermal Crystallization Behavior of Poly(trimethylene terephthalate) Synthesized with Different Catalysts

Summary: Non‐isothermal crystallization behavior of PTT resins synthesized with different catalysts was studied by using differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The results showed that with the increase of the cooling rate, the crystallization temperature for poly(trimethylene terephthalate) (PTT) resin decreased, which indicated that the crystallization process was controlled by the nucleation. Catalyst had no effect on the crystallization development process, but had somewhat effect on the non‐isothermal crystallization mechanism. The average values of Avrami exponent, for PTT with different catalysts were between 3 and 4. It was assumed that the non‐isothermal crystallization mechanism for PTT with or without catalyst was the combination of homogenous and heterogeneous nucleation and spherulite growth, but it mainly depended on the latter. For sample 4, the non‐isothermal crystallization underwent secondary crystallization process when cooling rate was over 20 °C/min. At the same cooling rate, the crystallization temperature, the crystallization ability and the crystallization rate of PTT resins followed the sequence as: sample 2 > sample 1 ≈ sample 3 ≈ sample 4, which proved that catalysts could significantly prompt crystallization. The cooling rate had significant effect on the crystallization ability parameters of PTT, i.e., with the increase of cooling rate, the crystallization ability declined. Although catalyst could increase the crystallization ability of PTT, the effect was very limited because the effect of the molecular weight on the crystallization ability would be superior to the catalyst when the molecular weight of PTT was significantly high. The specific surface area of catalyst had also a great influence on the spherulitic morphology of PTT formed in the cooling process. The spherulite dimensions decrease with increasing the specific surface area of catalyst because of an increase in the nucleation rate, which produces more and smaller spherulites that can not grow larger before impinging on each other.

     

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.