Influence of crystallization on microcellular foaming behavior of polyamide 6 in a supercritical CO2-assisted route

Supercritical CO₂ as a blowing agent has attracted increasing interest in the preparation of microcellular polyamide 6 (PA6) foams. In this work, we developed the supercritical CO₂-assisted method to prepare a series of different microcellular PA6 foams by controlling its crystallization properties in two steps and carefully investigated the corresponding crystallization properties of modified PA6 and foams using various techniques. Initially, a multifunctional epoxy-based chain extender (CE) was used to produce high-melt strength-modified PA6 with improved foaming ability; then, the resulting PA6 was foamed to prepare the microcellular foams of PA6 using supercritical CO₂ as a blowing agent in a batch foaming route. The CE effectively enhanced the melt strength of PA6, and CE usage was optimized to obtain a threshold of high branching without crosslinking. The number of crystals was also adjusted by the saturation temperature. Furthermore, these crystals that formed during the saturation process served as high-efficiency bubble nucleating agents and then limited the growth of bubbles at the same time. The microcellular foams of PA6 were successfully obtained with a cell size of 10.0 μm, and cell density of 2.0 × 10⁹ cells/cm³ at the saturation temperature of 225°C.     

Fabrication of polyamide 12T micro-cellular foams with ultra-high compressive strength via in situ polytetrafluoroethylene fibrillation using supercritical carbon dioxide foaming

The development of micro-cellular foams with ultra-high compressive strength and high volume expansion ratio (VER) is a challenging task. Herein, polyamide 12T (PA12T) micro-cellular foams with ultra-high compressive strength were fabricated via in situ polytetrafluoroethylene (PTFE) fibrillation using supercritical CO₂ foaming technology and a chain extender. The resulting branched structure showed considerably improved viscoelasticity and foaming performance, thus improving the cell morphology of the PA12T foam and exhibiting high VER. The PTFE fibrillation network induced melt strength enhancement, crystallization nucleation, and cell nucleation. The branched PA12T foam with 1.5 wt% PTFE exhibited the smallest cell diameter (15 μm) and highest cell density (3 × 10⁹ cells/cm³). The compressive strength of the foam (0.50 MPa under 5% strain) was 70% higher than that of pure PA12T. This research offers an effective method for producing high-VER PA12T foams with adjustable micro-cellular structures and excellent mechanical properties.     

Surlyn对PET及其共混体系流变与结晶性能的影响

通过熔融共混法制备了聚对苯二甲酸乙二醇酯 (PET)/聚酰胺6 (PA6)/Surlyn与PET/均苯四甲酸酐 (PMDA)/Surlyn共混材料,研究了Surlyn对共混体系的形貌以及共混体系中PET流变性能与结晶性能的影响。并使用扫描电子显微镜 观察了PET/PA6/Surlyn共混物的低温淬断断面,使用旋转流变仪与差示扫描量热仪研究共混物的流变性能与结晶性能。结果表明,Surlyn可以提高PET/PA6/Surlyn共混体系的熔体强度,促进共混体系中PET的结晶,使PET结晶温度提高10 ℃,过冷度与结晶半高宽显著下降,明显改善PET的流变性能与结晶性能,而对于PMDA扩链后的PET,Surlyn对其结晶性能的影响影响较小;Surlyn可以降低PA6分散相的粒径,提高PET与PA6两相之间的相容性。     

Preparation and characterization of biodegradable poly(ϵ‐caprolactone) self‐reinforced composites and their crystallization behavior

Self‐reinforced poly(?‐caprolactone) (PCL) composites were prepared by dispersing a homologous nucleating agent within the PCL matrix through melt mixing. Coalesced PCL, featuring more orderly chain arrangements, acted as the nucleating agent leading to improvement of crystallization for the melt PCL matrix. Non‐isothermal melt crystallization behavior, isothermal melt crystallization kinetics, spherulitic morphology and the crystal structure of neat PCL and the PCL self‐reinforced composites were studied in detail. The results indicated that both non‐isothermal and isothermal melt crystallization of PCL composites were enhanced significantly by the homologous nucleating agent, while the crystallization mechanism and crystal structures remained unchanged. The results of tensile mechanical tests showed that the Young's modulus of the composites was improved by up to 77% with the incorporation of 20 wt% nucleating agent. Biocompatibility tests demonstrated that the cells could adhere to and proliferate well on the surface of the self‐reinforced PCL composites. © 2017 Society of Chemical Industry     

聚(苯乙烯-丙烯酰胺)/蒙脱土核壳结构材料改性聚酰胺6的性能研究

用自制的以有机蒙脱土（OMMT）为核、聚苯乙烯为次外层、聚丙烯酰胺为外层的新型聚(苯乙烯-丙烯酰胺)/OMMT核壳结构,通过熔融共混法制备了含3 %（质量分数,下同）核壳结构的改性聚酰胺6（PA6）。研究了改性PA6的力学性能、加工流动性能、结晶性能和热稳定性能。结果表明,核壳结构改性PA6能明显提高材料的弯曲性能,其弯曲强度较纯PA6提高了1.44 %,弯曲模量提高了3.46 %;核壳结构对PA6的结晶有成核作用,材料的结晶温度提高了8.5 oC,结晶度提高了3.3 %;核壳结构改性PA6与纯PA6具有基本一致的热稳定行为;核壳结构还能有效提高PA6的加工流动性,其熔体流动速率是纯PA6的2.18倍。     

Influence of a synthetic ureido nucleating agent on crystallization behavior and mechanical properties of polyamide 6

A synthetic ureido mixture prepared from the reaction of 4,4′‐diphenylmethane disocynanate (MDI) and cyclohexylamine without using any harmful organic solvents, has been used as a nucleating agent (PNA) for polyamide 6 (PA6). The effect of PNA on the crystallization and mechanical properties of PA6 has been studied by means of differential scanning calorimetry (DSC), polarized optical microscopy (POM), tensile test, melt flow index (MFI), and X‐ray diffraction (XRD). The results show that PNA is an effective nucleation agent for PA6. PNA affects the nucleation mechanism of PA6, and substantially accelerates the crystallization rate of PA6 and gives rise to smaller crystal size. In comparison with PA6, the crystallization temperature (T_c) of PA6/PNA (100/0.5) increases 21.3°C and the degree of sub‐cooling (ΔT_c) decreases 23.7°C. Furthermore, because of the heterogeneous nucleation induced by PNA, the spherulites of PA6 become even and tiny based on POM observation. Polymorph transform has been obtained from XRD analysis. The virgin PA6 is free of γ‐phase crystals, presented as α‐phase crystals in this study, but γ‐phase crystal appears after the introduction of PNA. The mechanical and thermal properties of PA6 are obviously improved by the addition of PNA. POLYM. ENG. SCI., 55:2011–2017, 2015. © 2015 Society of Plastics Engineers     

无机成核剂改性聚酰胺6/碳纤维复合材料的结构与性能

以钛酸钾晶须（PTW）、高岭土（Kaolin）和滑石粉（Talc）为成核剂,制备了无机成核剂改性聚酰胺6/碳纤维（PA6/CF/NA）三元复合材料。通过分析复合材料的力学性能、动态热力学性能、微观形貌、结晶行为、晶体结构、热性能等对其结构和性能进行了系统的研究。结果表明,加入Talc可以大幅提高PA6/CF复合材料的冲击性能,添加2%（质量分数,下同）的Talc时,复合材料的冲击强度提高了44.5%;Talc在挤出过程中能够充分解离成片层并均匀地分散在PA6基体中,PA6/CF/Talc复合材料中存在大量纤维拔出后形成的孔洞,片层与基体黏结较好;与PTW和Kaolin相比,Talc突出的异相成核作用可以显著提高PA6/CF复合材料的结晶温度,并促进PA6形成更为完善的晶体结构。     

Crystallization and melting behavior of multi‐walled carbon nanotube‐reinforced nylon‐6 composites

The crystallization and melting behavior of neat nylon‐6 (PA6) and multi‐walled carbon nanotubes (MWNTs)/PA6 composites prepared by simple melt‐compounding was comparatively studied. Differential scanning calorimetry (DSC) results show two crystallization exotherms (T_{CC, 1} and T_{CC, 2}) for PA6/MWNTs composites instead of a single exotherm (T_{CC, 1}) for the neat matrix. The formation of the higher‐temperature exotherm T_{CC, 2} is closely related to the addition of MWNTs. X‐ray diffraction (XRD) results indicate that only the α‐phase crystalline structure is formed upon incorporating MWNTs into PA6 matrix, independently of the cooling rate and annealing conditions. These observations are significantly different from those for PA6 matrix, where the increase in cooling rate or decrease in annealing temperature results in the crystal transformation from α‐phase to γ‐phase. The crystallization behavior of PA6/MWNTs composites is also significantly different from those reported in PA6/nanoclay systems, probably due to the difference in nanofiller geometry between one‐dimensional MWNTs and two‐dimensional nanoclay platelets. The nucleation sites provided by carbon nanotubes seem to be favorable to the formation of thermodynamically stable α‐phase crystals of PA6. The dominant α‐phase crystals in PA6/MWNTs composites may play an important role in the remarkable enhancement of mechanical properties. Copyright © 2005 Society of Chemical Industry     

High performance polyamide 6 composites prepared by reactive extrusion

To improve the properties of polyamide 6 (PA6) composites, a series of modified PA6 composites was prepared by reaction extrusion. An amorphous PA6 was first obtained by the complexing reaction of Li⁺ in lithium chloride with amino groups, and then epoxy resins, nano‐SiO₂ as well as POE‐g‐MAH were in turn added into the PA6/LiCl system. The effect of different additives on the crystallization behavior and mechanical properties of PA6 composites was well‐studied by X‐ray diffraction (XRD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), and mechanical properties tests. The results demonstrated that PA6 was amorphous at 6 phr lithium chloride and a network structure was formed in PA6 matrix in the presence of epoxy resins, thus the mechanical properties of composites greatly were enhanced. However too many nano‐SiO₂ particles might impair the tensile strength of PA6 composites. Additionally, a PA6 composite with excellent properties was obtained in the presence of POE‐g‐MAH due to the crystal form change in PA6 matrix and the strong interaction between PA6 and POE‐g‐MAH. POLYM. COMPOS., 35:985–992, 2014. © 2013 Society of Plastics Engineers     

Effect of lignin particles as a nucleating agent on crystallization of poly(3‐hydroxybutyrate)

The effect of lignin fine powder, as a new kind of nucleating agent, on the crystallization process of poly(3‐hydroxybutyrate) (PHB) was studied. The kinetics of both isothermal and nonisothermal crystallization processes from the melt for both pure PHB and PHB/lignin blend was studied by means of differential scanning calorimetry. Lignin shortened the crystallization half‐time t_1/2 for isothermal crystallization. The activation energy ΔE for PHB/lignin and pure PHB in the isothermal crystallization process was ?237.40 and ?131.22 kJ/mol, respectively, clearly indicating that the crystallization of the PHB/lignin blend was more favorable than that of pure PHB from a thermodynamic perspective. At the same time, according to polarized optical microscopy, the rate of spherulitic growth from the melt increased with the addition of lignin, which is ascribed to the reduction of surface fold energy σ_e, that is, σ_e is 59.2 × 10^?3 and 41.6 × 10^?3 J m^?2 for pure PHB and PHB/lignin, respectively. Polarized optical microscopy also showed that the spherulites found in PHB with lignin were smaller in size and greater in number than those found in pure PHB. The wide‐angle X‐ray diffraction indicated that an addition of lignin caused no change in the crystal structure and degree of crystallinity. These results indicated that lignin is a good nucleating agent for the crystallization of PHB. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2466–2474, 2004     

Crystallization behavior of PA‐6 clay nanocomposite hybrid

Polyamide‐6 (PA‐6)/clay (modified montmorillonite) hybrid was synthesized by melt blending at high shear stress. ²⁷Al‐NMR of solid state shows that the clay is not modified after melt blending. Using wide‐line ¹H‐NMR and TEM, it is demonstrated that the nanocomposite exhibits mainly an exfoliated structure. It is shown that the modified montmorillonite induces the crystallization of PA‐6 predominantly in γ‐form. The presence of clay in PA‐6 increases the polymer crystallization temperature, and decreases its melting point. These phenomena show that a certain number of interactions develop near the reinforcing material, and that the latter plays a particular role of nucleating agent. However, the crystallization is not spherulitic and the assumption of macromolecular orientation in the vicinity of the clay is demonstrated by the observations carried out in DSC and AFM. These particular properties of orientation will have a particular importance on the mechanical behavior of the nanocomposite material. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2416–2423, 2002     

Enhancing crystallization rate and melt strength of PLLA with four‐arm PLLA grafted silica: The effect of molecular weight of the grafting PLLA chains

To improve the crystallization rate and melt strength of polylactide (PLLA), nano‐size amino silica grafted by four‐arm PLLA (4A‐PLLA) with different molecular weight was synthesized. ¹H nuclear magnetic resonance proved that 4A‐PLLA had been grafted onto the surface of SiO₂ successfully, and the grafting ratios and the degradation behaviors of the grafted SiO₂ nanoparticles (g‐SiO₂) were studied. When the grafted silica was introduced into PLLA matrix, the crystallization rate and melt strength of composites were found to be improved and the length of grafted chain played an important role. The extension rheology indicated that long grafted 4A‐PLLA on the surface of SiO₂ was more efficient in enhancing the elongational viscosity of PLLA, owing to the stronger interactions between the grafted chains and the matrix. The crystallization behavior of ungrafted silica filled composite was similar to that of neat PLA, while g‐SiO₂ played a role of nucleating agent. The crystallinities and the crystallization rates of the composites depended on the content of g‐SiO₂ and the grafted chain length of 4A‐PLLA, especially the latter. Longer grafted chain acted as nucleation site in the matrix and significantly improved the crystallization behaviors of PLLA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45675.     

Effect of different nucleating agent ratios on the crystallization and properties of MAS glass ceramics

The effects of different kinds of nucleating agents on crystallization, microstructure and performances of Magnesium Aluminosilicate (MgO-Al₂O₃-SiO₂, MAS) glass-ceramics which were fabricated by melting method in this study. Also, this paper systematically investigated the mechanism of glass stability, crystallization kinetics and element distribution of MAS glass-ceramics. Herein, we used three kinds of nucleating agents, which was TiO₂, ZrO₂ and composite nucleating agent (TiO₂/ZrO₂). The results showed after the doping of nucleating agent, the content of α-cordierite was increased, the stability and crystallization kinetics of glass was changed, the precipitated crystal phase was finer and more compact. Wherein, the sample with composite nucleating agents (TiO₂, ZrO₂) has the best performance due to the highest contents of α - cordierite, uniform distribution of elements without agglomeration in the crystal phase and the most compact structure, whose Vickers hardness and bending strength can reach 9.70 GPa and 312 MPa, respectively.     

PP/PA6复合材料结构与力学性能的研究

通过熔融共混法制备了聚丙烯/聚酰胺6(PP/PA6)复合材料。研究了PA6和不同增容剂(马来酸酐接枝PP和马来酸酐接枝聚烯烃弹性体)对PP性能的影响。通过力学性能测试、DSC和SEM对PP/PA6复合材料的结构和性能进行了系统研究。结果表明:加入8%PA6和5%增容剂时,复合材料的综合力学性能最佳;PA6对PP有异相成核作用;在增容剂的作用下,PA6均匀地分散于PP基体中,从而起到良好的改性作用。摘要:     

Study on the Crystallization Properties of Polypropylene/Montmorillonite Composites

The crystalline size of polypropylene (PP) filled with montmorillonite (MMT) was studied by X-ray diffraction (XRD). The isothermal crystallization behavior of polypropylene was studied by means of differential scanning calorimetry (DSC). The Avrami equation was used to describe the isothermal crystalline kinetics of PP/MMT composites. The result showed that the addition of MMT decreased the crystalline size L _hkl of the polymer. MMT was used as nucleating agent during isothermal crystallization process of polypropylene. The addition of montmorillonite decreased the crystallization time of the polypropylene and the melt point was raised. The value of Avrami exponent n was related with the crystallization temperature. The value of Avrami pre-index factor k of PP/MMT composite was decreased with increasing crystallization temperature. The value of half crystallization time t _1/2 of PP/MMT composite was less than that of PP at a given crystallization temperature, signifying that montmorillonite acted as nucleating agent, accelerated the overall crystallization process.     

Crystallization behavior of poly(vinylidene fluoride) composites containing zinc phenylphosphonate

Poly(vinylidene fluoride) (PVDF) composites with different ratios of zinc phenylphosphonate (ZPP) were prepared by mixing in a kneader. The crystallization behavior of the composites from the melt was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The POM observations directly showed that nucleation of the PVDF crystal was promoted by adding ZPP. The data obtained from DSC measurements also indicated that the crystallization temperature of the composites became higher than that of pure PVDF. These results suggested that ZPP acted as a good nucleating agent for PVDF. Nonisothermal crystallization behavior was analyzed by Avrami equation adopted by Jeziorny. The results of the analysis were found to be consistent with the POM observations for the composites. WAXD analysis suggested that the PVDF crystal structure was influenced by ZPP addition. These results proved that as a nucleating agent, ZPP increased the crystallization rate and temperature of PVDF. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Improving mechanical and thermal properties of short carbon fiber/polyamide 6 composites through a polydopamine/nano-silica interface layer

Carbon fiber (CF) reinforced matrix composites have been applied widely, however, the interfacial adhesion of composites is weak due to smooth and chemically inert of CF surface. To solve this problem, A polydopamine/nano-silica (PDA-SiO₂) interfacial layer on carbon fiber surface was constructed via polydopamine and nano- SiO₂ (CF-PDA-SiO₂) by a facile and effective method to reinforce polyamide 6 composites (CFs/PA6). The effects of PDA-SiO₂ interfacial layer on crystallization structure and behavior, thermal properties, and mechanical properties of CFs/PA6 composites were investigated. Furthermore, interfacial reinforcement mechanism of composites has been discussed. This interfacial layer greatly increased the number of active groups of CF surface and its wettability obviously. The tensile strength of CF-PDA-SiO₂/PA6 composites increased by 28.09%, 19.37%, and 26.22% compared to untreated-CF/PA6, CF-PDA/PA6, and CF-SiO₂/PA6 composites, respectively, which might be caused by the increased interfacial adhesion between CF and PA6 matrix. The thermal stability, crystallization temperature, crystallinity, and glass transition temperature (T_g) of CF-PDA-SiO₂/PA6 composites improved correspondingly, attributing to the heterogeneous nucleation of nano-SiO₂ in the crystalline area and hydrogen bonds with molecular chains of PA6 in the amorphous area. This work provides a novel strategy for the construction of interfaces suitable for advanced CF composites with different structures.     

熔融挤出温度对PA6/CaCl2复合材料结构与性能的影响

采用熔融挤出的方法,制备了PA6/CaCl2复合材料,研究了熔融挤出温度对PA6/CaCl2复合材料的结晶行为和性能的影响。实验结果表明：随熔融挤出温度的提高,CaCl2与PA6的络合反应程度越大,PA6/CaCl2复合材料的结晶度越小,结晶不完善程度越大,复合材料的粘度越大,熔体流动速率越小。随熔融挤出温度的提高,复合材料的拉伸强度先增大后减小,最后近似趋于定值,冲击强度、弯曲强度随熔融挤出温度的提高总体呈增加趋势。     

Effect of small amount of ultra high molecular weight component on the crystallization behaviors of bimodal high density polyethylene

In order to clarify the effect of high molecular weight component on the crystallization of bimodal high density polyethylene (HDPE), a commercial PE-100 pipe resin was blended with small loading of ultra high molecular weight polyethylene (UHMWPE). The isothermal crystallization kinetics and crystal morphology of HDPE/UHMWPE composites were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The presence of UHMWPE results in elevated initial crystallization temperature of HDPE and an accelerating effect on isothermal crystallization. Analysis of growth rate using Lauritzen-Hoffman model shows that the fold surface free energy (σ_e) of polymer chains in HDPE/UHMWPE composites was lower than that in neat HDPE. Morphological development during isothermal crystallization shows that UHMWPE can obviously promote the nucleation rate of HDPE. It should be reasonable to conclude that UHMWPE appeared as an effective nucleating agent in HDPE matrix. Rheological measurements were also performed and it is shown that HDPE/UHMWPE composites are easy to process and own higher melt viscosity at low shear rate. Combining with their faster solidification, gravity-induced sag in practical pipe production is expected to be effectively avoided.     

Crystallization behavior of polyamide 11/multiwalled carbon nanotube composites

Differential scanning calorimetry (DSC) was used to investigate the isothermal and nonisothermal crystallization kinetics of polyamide11 (PA11)/multiwalled carbon nanotube (MWNTs) composites. The Avrami equation was used for describing the isothermal crystallization behavior of neat PA11 and its nanocomposites. For nonisothermal studies, the Avrami model, the Ozawa model, and the method combining the Avrami and Ozawa theories were employed. It was found that the Avrami exponent n decreased with the addition of MWNTs during the isothermal crystallization, indicating that the MWNTs accelerated the crystallization process as nucleating agent. The kinetic analysis of nonisothermal crystallization process showed that the presence of carbon nanotubes hindered the mobility of polymer chain segments and dominated the nonisothermal crystallization process. The MWNTs played two competing roles on the crystallization of PA11 nanocomposites: on the one hand, the MWNTs serve as heterogeneous nucleating agent promoting the crystallization process of PA11; on the other hand, the MWNTs hinder the mobility of the polymer chains thus retarding the crystal growth process of PA11. The activation energies of PA11/MWNTs composites for the isothermal and nonisothermal crystallization are lower than neat PA11. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.