Simulation of crystallization kinetics and morphology during nonisothermal crystallization in short fiber reinforced composites

Computer simulation for the nonisothermal crystallization of short fiber reinforced composites is presented. The pixel coloring technique is implemented to the study of crystal morphology evolution as well as the crystallization kinetics. A parametric study is used to explore the influences of thermal conditions and fibers on the crystallization in the reinforced system. We particularly focus on the roles of cooling rate, initial temperature, nucleation density on fibers, fiber content, fiber length, and fiber diameter. The results indicate that cooling rate is a significant factor to the crystallization kinetics as well as the morphology. The initial temperature only affects the crystallization kinetics and has minor impact on the morphology. The additional fibers have a dual effect on the crystallization. They depress the crystallization rate by hindering the spherulitic growth and accelerate the crystallization rate by providing nucleation sites. The constraining effect is mainly dependent on fiber content, whereas the enhancing effect is mainly determined by fiber surface and fiber nucleation density as well as surface nucleation mode. Present results are hoping to give more insight about the crystallization in short fiber reinforced composites and be more helpful to the industrial application. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

纤维增强PET复合材料非等温结晶行为的研究

用差示扫描量热法 (DSC)研究了 PET及增强 PET的非等温结晶动力学 ,结果表明 ,增强纤维的加入能促进 PET的结晶 ,且 Kevlar浆粕和碳纤维的效果较好。玻纤和碳纤维经等离子处理后 ,前者的成核效果下降而碳纤维的成核效果得以提高 ;在体系中加入成核剂能进一步促进 PET的结晶。另外还用偏光显微镜研究了体系的结晶形态 ,发现没有形成横穿晶 ,增强纤维的加入只是起了成核位的作用。     

Crystallization processes in poly(ethylene terephthalate) as modified by polymer additives and fiber reinforcement

The effect of fiber reinforcement on the crystallization kinetics of poly(ethylene terephthalate), or PET, was investigated using differential scanning calorimetry. The objective of the study was to determine how the effects of fiber reinforcement on PET crystallization are modified by the presence of polymer nucleating and plasticizing additives. Undirectional fiber composites were prepared using aramid and glass fibers in PET. The rate of crystallization of PET, as reflected by crystallization half-time, is seen to depend on reinforcing fiber type, crystallization temperature, and presence of nucleant or plasticizer. Howerver, degree of crystallinity of PET is largely unaffected by the presence of additives and reinforcing fibers. Crystallization kinetics are analyzed using the Avrami model for PET volume crystallized as a function of time. Avrami plots for PET and fiber-reinforced PET exhibit two linear regions, possibly corresponding to primary and secondary crystallization. The crystalline morphology of fiber-reinforced PET was also studied using polarized light microscopy. Results concerning nucleation density and growth morphology are used in explaining differences seen in crystallization kinetics in fiber-reinforced systems. © 1994 John Wiley & Sons, Inc.     

Crystallization kinetics of PPS composites and quantification of fiber nucleation density using a computer simulation

Differential scanning calorimetry and hot-stage optical microscopy were used to study the isothermal crystallization kinetics of unreinforced poly(phenylene sulfide) (PPS) and PPS reinforced with aramid, carbon, and glass fibers. The influence that fibers have on the crystallization kinetics of PPS was found to depend on the characteristics of the fiber as well as the type of PPS used. For one kind of PPS, fibers enhanced the crystallization rate, while for another type of PPS, reinforcing fibers had a moderate depressing effect on the polymer crystallization rate. To clarify these effects, we used a new method of quantifying the nucleation process in fiber-reinforced composites that employs a 3-D computer simulation of spherulitic crystallization. Using this method, the nucleation density in the bulk polymer, N_b, and the nucleation density on fiber surfaces, N_f, were calculated for PPS composites as a function of crystallization temperature. The calculated values of N_b and N_f were used to explain differences in the effectiveness of the fibers as well as differences in the nucleating characteristics of the two polymers. © 1995 John Wiley & Sons, Inc.     

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.     

玻璃纤维增强聚丙烯非等温结晶动力学研究

利用差示扫描量热仪结合Avrami方程研究了玻璃纤维（GF）增强聚丙烯（PP）复合体系的非等温结晶动力学,探讨了GF含量及长度对PP结晶行为的影响。通过偏光显微镜观察了复合材料的结晶形态。结果表明,引入GF改变了PP的结晶温度,对其结晶有成核作用,并随着GF含量的增加,PP的半结晶时间（t1/2）呈下降趋势,结晶速率逐渐上升;同时随着GF长度的增加,PP的结晶速率是先增大后减小。PP和PP/GF复合材料的Avrami指数均在2.99～3.40,说明PP的结晶成核机理和生长方式没有改变。     

Melt-electrospinning part I: processing parameters and geometric properties

The effects of various melt-electrospinning parameters on the morphology and fiber diameter of polypropylene of different tacticities were studied. The effect of the electric field strength at various melt flow indexes of polypropylene on fiber uniformity, morphology, and diameter was measured. It was shown that the molecular weight was the predominant factor in determining the fiber diameter of the collected fibers. Observations prove that the tacticity also influences the fiber diameter. Atactic polymers having molecular chains incapable of crystallization tend to produce larger diameter fibers than isotactic polymers capable of crystallization even at lower molecular weights. The polymer volume, at a given time, supplied to the electric field affected fiber diameter. Those systems supplying the smallest volume, at a given time, produced the smallest fiber diameter.     

Fabrication of long glass fiber reinforced polyacetal composites: Mechanical performance,microstructures, and isothermal crystallization kinetics

A thermoplastic pultrusion was carried out to prepare the long fiber reinforced thermoplastic (LFT) composites based on polyacetal (POM) matrix on the custom‐designed pultrusion equipment. The investigation on mechanical performance revealed that the POM‐based LFT composites achieved much higher tensile, flexural, and impact strength than the short glass fiber reinforced ones at the same fiber loadings. Such a promising reinforcement effect is attributed to the feature that the residual fiber length in the injection‐molded LFT products is greatly superior to that in short fiber reinforced ones. This takes full advantage of the strength of the reinforcing fiber itself. The scanning electronic microscopy demonstrated that the fiber fracture and fiber pull‐out concurred on the tensile and impact fracture surfaces, and the former preceded the latter. The isothermal crystallization kinetics of the POM‐based LFT composites was also intensively studied, and the results indicated that the crystallinity of POM domain was enhanced by the heterogeneous nucleation of glass fiber, but the crystallization rate was postponed due to the interspace restriction toward crystalline growth caused by long glass fiber. These kinetic parameters provided information on the processing conditions of POM‐based LFT composites for the injection and compression molding. POLYM. COMPOS., 36:1826–1839, 2015. © 2014 Society of Plastics Engineers     

Crystallization of poly(hydroxybutrate-co-hydroxyvalerate) in wood fiber-reinforced composites

Wood fiber-reinforced composites were prepared from poly(hydroxybutyrate) (PHB) and poly(hydroxybutyrate-co-hydroxyvalerate) (PHB/HV) copolymers containing 9 and 24% valerate. The effects of fibers on crystallization were investigated. Thermomechanical pulp, bleached Kraft fibers, and microcrystalline cellulose filler were used as the reinforcing phase. The crystallization of PHB/HV in composite materials was examined using Modulated Differential Scanning Calorimetry (MDSC) and hot-stage microscopy. Hot-stage microscopy showed that polymer crystallites are nucleated on the fiber surface and that the density of nuclei was greater in fiber-reinforced composites than in unfilled material. Dynamic crystallization experiments showed that bleached Kraft, thermomechanical pulp, and microcrystalline cellulose increased the crystallization rate of PHB and PHB/HV both from the glass and melt. However, ultimate crystallinity determined from the heat of crystallization was the same in unreinforced and reinforced materials. The kinetics of PHB/HV crystallization were examined using nonisothermal Avrami-type analysis. Unreinforced and Kraft-reinforced PHB were characterized and compared with unreinforced PHB/9%HV. The Avrami exponent of crystallization, related to nucleation mechanism and growth morphology, is 2.0 for unreinforced PHB, 2.8 for kraft-reinforced PHB, and 3.0 for unreinforced PHB/9%HV. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1785–1796, 1997     

碳纤维增强聚醚醚酮的非等温结晶行为与力学性能

利用聚酰亚胺(PI)作为碳纤维(CF)界面改性剂,制备了界面改性碳纤维增强聚醚醚酮(MCF/PEEK)复合材料。采用差示扫描量热仪(DSC)讨论了CF及其界面改性对PEEK非等温结晶行为的影响机制与作用规律,并基于莫志深法研究了MCF/PEEK的非等温结晶动力学;借助DSC和小角X射线散射仪(SAXS)表征不同降温速率下PEEK基体的结晶结构,采用万能试验机评价了MCF/PEEK的力学性能。结果发现:CF对PEEK的结晶有较为明显的异相成核促进作用,经过PI界面改性之后成核作用有所下降,但结晶行为仍较纯PEEK更容易发生,整体结晶速率更快;随冷却速率的增大,基体结晶度、片晶厚度与长周期均减小,MCF/PEEK的拉伸强度与模量也显著减小,层间断裂韧性提高。     

Thermal and crystallization behavior of cotton—Polypropylene commingled composite systems

Techniques like thermogravimetric analysis, differential scanning calorimetry, and polarized optical microscopy were used to study the thermal and crystallization behavior of cotton‐polypropylene (PP) commingled composite system. Thermal analysis was used to understand the structure‐property relationship and also to quantify the amount of moisture and volatiles, which causes the deterioration of the composite performance. Thermal stability of the composite was found be intermediate between that of PP and cotton fibers. Presence of treated reinforcements had increased the crystallinity of PP. Also, fibers act as heterogeneous nucleants and favor the early crystallization of PP in the composites. The crystallization and onset crystallization temperature values were increased by the presence of cotton fibers. The theories of heterogeneous nucleation and crystal growth kinetics were used to explain the growth of transcrystalline layer (TCL) of PP on cotton fibers. The interfacial free energy difference for nucleation of PP on fiber was found to be smaller compared with that in the bulk PP. This favors the formation and growth of TCL. The thickness of TCL and radius of the spherulites increase with the increase in the crystallization temperature. Fiber surface roughness and thermal stresses facilitate the growth of transcrystallinity on cotton fiber. POLYM. COMPOS., 31:1487–1494, 2010. © 2009 Society of Plastics Engineers     

The effects of a nucleating agent and of fibers on the crystallization of nylon 66 matrices

Crystallization from the melt of nylon 66 in the presence of carbon fiber, aramid fiber, or nucleating agent was studied using differential scanning calorimetry (DSC) and hot stage microscopy. The use of the nucleating agent resulted in an increase in crystallization rate and a decrease of induction time under both isothermal and nonisothermal conditions. The fibers were found to behave like a giant nucleating site producing a uniform transcrystalline layer having morphology and crystallization kinetics different from those of the bulk matrix. The influence of the cooling rate on the process of nonisothermal crystallization was analyzed, and the values of activation energy, calculated from the cooling rate—crystallization temperature relationship, appeared to be higher for the nucleated and for the reinforced nylon compared with that of the neat nylon 66. This implied that the presence of fibers or nucleating agent led to the development of a more ordered structure, which required a larger amount of energy for crystallization.     

Crystallization and chain adsorption of poly(etheretherketone) in discontinuous pitch-derived carbon fiber composites

The crystallization kinetics of poly(etherether ketone) (PEEK) in chopped mesophase pitch-derived carbon fiber/PEEK composites have been studied. Various processing techniques are used in order to obtain controlled fiber length in the composites. Scanning electron microscopy performed on properly etched long fiber composite samples reveals that the nucleating sites density is low at the carbon fiber surface: transcrystalline layers are rarely observed. This is confirmed by differential scanning calorimetry. However, samples processed by mixing carbon fibers and molten polymer in a high temperature mixer have a widely different behavior: the nucleation density and the crystallization rate increase, the glass transition of these samples is displaced towards higher temperatures, and the solubility is dramatically lowered. We ascribe these phenomena to the adsorption of the polymer chains on carbon particles created by attrition during the mixting.     

Crystallization kinetics by differential scanning calorimetry for PCL/starch and their reinforced sisal fiber composites

Calorimetric results obtained by differential scanning calorimetry (DSC) have been used to develop a kinetic model for the crystallization behavior of PCL/starch and their composites with sisal fibers. The model takes into account the effects of nucleation and crystal growth, and it is able to describe the isothermal and non‐isothermal conditions, especially for the low cooling rates. The effect of the sisal fiber has also been analyzed. The Avrami exponent was 2.0 for the crystallization of PCL/starch and sisal fiber reinforced composite. The activation energy of the crystallization process was 4.3 and 4.0 kJ/mol for PCL/starch and sisal composite, respectively. The induction time of the crystallization and the crystallization rate was not influenced by the presence of sisal fiber.     

Electrospun-fiber induced transcrystallization of isotactic polypropylene matrix

To reveal the influences of fiber diameter and characteristics on the surface-induced matrix crystallization, four different fibers, i.e. syndiotactic polystyrene (sPS), Nylon 6, and polyhydroxyamide (PHA) and poly(p-phenylene benzobisoxazole) (PBO) fibers, were used to study their nucleating abilities towards isotactic polypropylene (iPP) matrix. Among them, micron-sized PBO fibers were obtained from the supplier, whereas submicron-sized fibers of sPS, Nylon 6 and PHA were prepared by the solution electrospinning process developed in this laboratory. To resolve the observation difficulty due to the fast nucleation rate and crystal growth at high supercooling degrees (>60 °C), a high speed camera was mounted on the polarized optical microscope equipped with a hot stage to successfully snapshot the corresponding processes at various crystallization temperatures (T_c) in the range of 96-120 °C. For all the active fibers, only α-form iPP transcrystallites were observed at the fiber/matrix interface. Two crucial parameters were proposed for characterizing the fiber nucleating ability, i.e. the interfacial free energy difference (Δσ) based on the heterogeneous nucleation from a thermodynamic point of view, and the maximum temperature for transcrystalline layer observed (T_max) based on a kinetic consideration. Values of Δσ for different fibers were derived on the basis of the tertiary nucleation taking place in the selective “grooves” at the fiber surface. It was found that the nucleating rate of sPS fibers was scaled with the fiber diameter, and Δσ showed a negligible diameter dependence, but T_max slightly increased with increasing fiber diameter. For all the fibers investigated, an intimate relation between the Δσ and T_max was derived and used to compare their nucleating abilities.     

Transcrystalline morphology of nylon 6 on the surface of aramid fibers

In this paper, the isothermal crystallization of nylon 6 in the presence of Kevlar 129 fiber was investigated by polarized optical microscopy (POM). The formation of a transcrystalline domain was found to be mainly controlled by crystallization conditions, such as the temperature of the isothermal crystallization, residual time at melting temperature and the cooling rate of the melt. The nucleation rate of nylon 6 on the fibers was mainly affected by the crystallization temperature. The interfacial transcrystallinity of nylon 6 occurred on the surface of Kevlar 129 fiber in the temperature range 130–190 °C. The reason for the formation of interfacial transcrystalline morphology is discussed from the molecular level, based on the understanding of the packing mode of nylon 6 chains around fibers and the interaction between matrix and fibers. It was found that the lattice matching and hydrogen‐bonding between nylon 6 and poly(p‐phenylene terephthalamide) (PPTA) crystals play an important role in the epitaxial crystallization. Copyright © 2004 Society of Chemical Industry     

Kinetic crystallization of polypropylene in ternary composites based on fiber‐reinforced PP‐EPDM blends

The crystallization of isotactic polypropylene (iPP) in its blends with ethylene–propylene–diene terpolymer (EPDM), reinforced with different fibers, is described in this work. In particular, the effects of both the fibers and the EPDM on the crystallization kinetics and morphology of iPP are analyzed. The study was performed using differential scanning calorimetry (DSC) in dynamic and isothermal conditions and optical microscopy. It was found that all the fibers act as effective nucleant agents on iPP crystallization independently of the blend composition. The results obtained highlight the accelerating effect of the fibers and of the EPDM on the PP crystallization up to a certain EPDM percentage. The halftime of crystallization, τ_1/2, and the overall crystallization rate, K_n, increase in the presence of all the fibers analyzed, showed the aramidic ones the most effective. The isothermal crystallization kinetics of ternary composites based on PP–EPDM blend matrices reinforced with different types of fibers can be modeled using the Avrami equation. On the other hand, the kinetic curves obtained under nonisothermal conditions provide a further confirmation of the nucleating action of the fibers on the PP crystallization. Optical polarizing microscopy was also used to investigate the effect of EPDM on the spherulite growth and the transcrystallinity phenomenon on the surface of the fibers. The results of such analysis showed that the transcrystallinity phenomenon is hindered at high rubber percentages. As in the case of the rate of crystallization, the highest proportion of transcrystallinity was observed in the presence of the aramidic fibers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1063–1074, 2001     

Nano and submicrometric fibers of poly(D,L‐lactide) obtained by solution blow spinning: Process and solution variables

Nano and submicrometric fibers of poly(D ,L ‐lactide) (PDLLA or PLA) were spun from solutions using a solution blow spinning (SBS) apparatus. Fiber morphology and diameter were investigated by scanning electron microscopy as a function of polymer concentration, feed rate, and air pressure. A more systematic understanding of the SBS process parameters was obtained, and a quantitative relationship between these parameters and average fiber diameter was established by design of experiments and response surface methodology. It was observed that polymer concentration played an important role in fiber diameter, which ranges from 70 to 2000 nm, and its distribution. Lower polymer concentration tended to increase the formation of bead‐on‐string structures, whereas smooth fibers were formed at higher concentrations. Fiber diameter tended to increase with polymer concentration and decrease with feed rate. Based on these results, optimal conditions could be obtained for solution‐blow spun fibers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Surface Nucleation and Crystal Orientation in Lithium Silicate Glass Fibers

The effect of inorganic surface treatments on the orientation of lithium disilicate crystals formed in Li₂O·2.75SiO₂ glass fibers was studied. Glass fibers 0.5 mm and 9μ in diameter were subjected to various surface treatments at room temperature and then heated between 550O and 800°C. It was observed that metal salt solutions applied to the fiber surfaces at room temperature decreased the degree of orientation during heating only if the metal entered the glass and formed nucleation sites within the fibers. Orientation could also be decreased by removing lithium from the glass surfaces. Comparison of the crystallization characteristics of untreated and AgNO₃-treated fibers indicated that the crystallization behavior was controlled by either a growth process or a nucleation process, depending on whether the temperature was below or above 625°, respectively.     

Dry gel conversion synthesis of shape controlled MFI type zeolite materials

Acid treated silica fibers, coming either from asbestos cord waste or calcium aluminosilicate glass, were engaged as reactants to product MFI type zeolite fibers applying a dry gel conversion (DGC) type synthesis. The impact of amorphous silica fiber shape and synthesis conditions on morphology of the final product was investigated. Enhanced zeolite nucleation and restricted crystal growth are necessary to maintain the initial fiber morphology. Whereas a DGC type crystallization in hydroxide media results in the formation of zeolite fibers, the use of fluoride ions leads to the formation of large individual zeolite particles. Synthesis conditions have to be adapted to form zeolite particles with an average size smaller than the silica fiber diameter to allow a good conservation of the fiber morphology. Final zeolite fibers have textural properties comparable to a standard zeolite product and are promising for adsorption applications.