Computer simulation of crystallization kinetics and morphology in fiber-reinforced thermoplastic composites. I. Two-dimensional case

A body of experimental evidence suggests that reinforcing fibers influence both the crystallization kinetics and morphology of those composite materials that are based on crystallizable thermoplastics. The absence of an analytical model to predict the effect of fibers on crystallization has hindered data analysis. A new approach, using computer simulation of polymer crystallization, makes it possible to study the influence that reinforcing fibers have on the crystallization kinetics and morphology of semicrystalline polymers. Fibers depress the crystallization rate relative to an unreinforced polymer since they constrain spherulitic growth by an impingement mechanism. On the other hand, reinforcing fibers can also enhance crystallization rate by providing added surface nucleation sites. This work describes a two-dimensional simplification of the crystallization process that occurs in bulk materials. It is demonstrated that the relative bulk and fiber nucleation densities, in addition to the fiber fraction, fiber diameter, and spherulitic growth rate control the crystallization kinetics and also the spherulitic and transcrystalline morphologies that develop in reinforced thermoplastic composites. © 1993 John Wiley & Sons, Inc.     

Computer simulation of crystallization kinetics and morphology in fiber-reinforced thermoplastic composites. II. Three-dimensional case

A three-dimensional computer simulation has been used to predict crystallization kinetics and crystalline morphology in composite materials that are based on crystallizable thermoplastics. Reinforcing fibers in three-dimensional simulations show similar behavior to those in two-dimensional simulations; fibers suppress crystallization relative to an unreinforced polymer since they constrain spherulitic growth by an impingement mechanism, and also enhance crystallization by providing added surface nucleation sites. The effects of varying controlling parameters on crystallization kinetics and morphology are qualitatively the same as those observed in the two-dimensional case. The relative bulk and fiber nucleation denisities, in addition to the fiber volume fraction, fiber diameter, and spherulitic growth rate control the crystallization kinetics and crystalline morphology that develop in reinforced thermoplastic composites. It is more difficult to achieve the transcrystalline morphology in slices of three-dimensional composites than it is in two-dimensional composites because nuclei in 3-D systems are not constrained to positions in or near a 2-D plane. © 1993 John Wiley & Sons, Inc.     

Melt crystallization of polypropylene: Effect of contact with fiber substrates

The isothermal crystallization of isotactic polypropylene at different temperatures in the presence of fibrous substrates has been investigated. It is shown that preferential transcrystalline growth occurs at the fiber surface and that changes in nucleation density in the bulk material adjacent to the fibers also occur, the extent of which is dependent on temperature and fiber volume fraction. The effects are discussed in terms of the diffusion of heterogeneities in the bulk due to interaction and the adsorption on the fibers.     

碳纤维对聚丙烯结晶行为的影响

本文用偏光显微镜和示差扫描量热计(DSC)方法研究了碛纤维对聚丙烯结晶行为的影响。碳纤维表面对聚丙烯结晶过程具有明显的促进作用,纤维表面成核密度轻高,结晶生长成为横穿结晶,结晶特征温度随碳纤维加入而有不同程度的升高。结晶动力学表明:结晶生长本质仍是球晶机理,促进聚丙烯结晶的原因是碳纤维使结晶过程的表面自由能降低。     

Surface crystallization of polypropylene

Crystallization of compression-molded isotactic polypropylene and polyethylene is invariably spherulitic; generally, nucleation occurs randomly throughout the sample. In a special case where nucleation predominates at the surface, spherulitic growth centers become crowded and are forced to propagate unidirectionally into the bulk (transcrystallinity). Conditions for the formation of transcrystallinity have been investigated by optical and scanning electron microscopy. The occurrence of transcrystallinity is attributed to heterogeneous nucleation induced at the mold surface. To be effective, the mold surface must have a nucleating efficiency equal to or greater than that of adventitious nuclei present in the polymer. As the crystallization temperature approaches the melting point, the activity of mold surfaces is found to increase leading invariably to transcrystalline formation. The degree of activity of various mold surfaces correlates with the known activity of specific dispersed nucleating agents having similar chemical structures. Contrary to claims in the literature, the surface energy of the mold surface and temperature gradients across the melt surface do not play a primary role in transcrystalline formation of polypropylene.     

Transcrystallization of PTFE fiber/PP composites—III. Effect of fiber pulling on the crystallization kinetics

The effect of shear rates on the transcrystallization of polypropylene (PP) on the polytetrafluoroethylene (PTFE) fibers has been quantitatively investigated using a polarized optical microscope equipped with a hot stage and a tensile testing machine. The PTFE fibers were pulled at different rates, from 0.17 to 8.33 μm/s, to induce a range of shear rates, about 0.02 to 1.16 1/s, in the PP melt adjacent to the fiber. The induction time, nucleation rate, and saturated nucleation density at the fiber surface were determined at various crystallization temperatures. It was found that both the nucleation rate and the saturated nucleation density increase with increasing shear rates. However, the induction time is significantly reduced. Based on the theory of heterogeneous nucleation, the interfacial free energy difference functions Δσ;_TCL of PP on PTFE fibers at different levels of shear rates were determined and compared with that obtained from crystallization under quiescent conditions. Results showed that the magnitude of Δσ_TCL decreased to be about one-third of that for the quiescent crystallization, when a shear rate of 1.16 1/s was applied. The application of a shear stress to the supercooled PP melt by fiber pulling leads to enhance the development of transcrystallinity. Moreover, both the thickness and the crystal growth rate of transcrystalline layers were found to increase with the increasing rate of fiber pulling, especially at low crystallization temperatures where regime III prevails (see text). Surface morphology of PTFE fibers was revealed using a scanning electron microscope and an atomic force microscope. It is argued that the presence of fibrillar-type features at the fiber surface is the main factor responsible for the development of transcrystallinity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1361–1370, 1998     

Heterogeneous nucleation of crystallization of high polymers from the melt. III. Nucleation kinetics and interfacial energies

In the melt crystallization of isotactic polypropylene, poly(ethylene oxide) and poly(butene-1) in contact with substrates, the existence of a fixed number of nucleating sites on the substrate surfaces has been established. When these sites become active successively (the transient in the number of nuclei is long) during crystallization, pseudohomogeneous nucleation on the substrate occurs. Nucleation rates for poly(butene-1) and poly(ethylene oxide) on substrates and in bulk have been measured. These data can be used for comparing the nucleating ability of substrates. Estimates of the variation of bulk nucleation rates from one volume element to another as well as for repeated crystallization within a given volume element have been included. Finally, the temperature coefficients of heterogeneous nucleation rates have been combined with the temperature coefficient of spherulitic growth rate of poly(butene-1), to yield values of the interfacial energy parameters appearing in the theory of heterogeneous nucleation. The quantitative characterization of the nucleating ability of substrates by this method is an improvement over the mere use of nucleation densities or nucleation rates.     

橫晶与纤维界面的形态观察

在纤维表面的异相成核密度达到足够高的程度时,晶粒的生长由径向转变为沿垂直于纤维表面的方向生长,即横晶生长(transcrystalline growth)。在许多纤维增强的高聚物复合材料中都可以得到横晶结构。横晶的结构和球晶是完全相同的。横晶是球晶的一种特殊情况,即取向球晶。虽然横晶中片晶的生长方向是相同的,但是在横晶与纤维之间,即在纤维表面区域却是非取向的。利用刻蚀剂或离子对材料进行刻蚀处理,控制刻蚀程度,可以使纤维和取向的横晶之间的非取向区首先被刻蚀液或离子刻蚀破坏掉。而取向的横晶结构区域则被保留下来。用扫描电子显微镜就可以观察到这两种结构区。     

Crystallization kinetics of poly(glycolic acid-alt-6-aminohexanoic acid)

The crystallization behavior of a new regular poly(ester amide) constituted by glycolic acid and 6-aminohexanoic acid units under both isothermal and non-isothermal conditions is studied. Differential scanning calorimetry (DSC) is used to monitor bulk crystallization, and subsequently Avrami and Ozawa analyses are applied. A three-dimensional spherulitic growth from heterogeneous nuclei is deduced for isothermal crystallization, whereas higher exponents are obtained for non-isothermal crystallization when an Avrami equation is applied. However, modifications of the Ozawa methodology indicate a crystallization mechanism similar to that of the isothermal process.The maximum crystallization rate is deduced to take place at a temperature close to 91 °C by considering experimental data and theoretical equations with adjusted parameters. The equilibrium melting temperature is determined to be 168 °C by the characteristic Hoffman-Weeks plot. One crystallization regime is detected by using the Lauritzen-Hoffman kinetic theory for isothermal crystallization and also with an isoconversional method applied for non-isothermal crystallization. Activation energy of molecular transport and nucleation constant are close to 1500 cal/mol and 1.81 × 10⁵ K², respectively. Crystal morphology, nucleation, and spherulitic growth rates are also investigated with hot-stage optical microscopy (HSOM).     

Heterogeneous nucleation of crystallization of high polymers from the melt. I. Substrate-induced morphologies

Melt crystallization of isotactic polypropylene (iPP), poly(ethylene oxide), poly(butene-1), and polycaprolactone in contact with various substrates (mostly polymeric) has been studied by hot stage polarizing microscopy. Nucleating abilities of surfaces have been characterized qualitatively by examining the substrate-induced morphologies of the crystallizing polymer. These morphologies have been classified into three groups, depending on whether the substrate is very active (transcrystallinity), moderately active, or inactive as a nucleating agent. The morphologies observed are temperature-dependent, changing from transcrystalline to spherulitic upon increase of the crystallization temperature. At intermediate temperatures, mixed surface morphologies (transcrystalline plus spherulitic) are observed. The concentration of titanium and aluminum catalytic residues in isotactic polystyrene (iPS) samples can be reduced by two methods, i.e., (a) fractionating the polymer and (b) chelating Ti and Al with acetylacetone. The high nucleating ability of iPS samples in the crystallization of iPP has been shown to be due to the polymer (iPS) itself, and not to Ti and Al residues. Apart from iPS, other polymers (low energy surfaces) have also been found to induce transcrystallinity. From a survey of 43 substrate-crystallizing polymer pairs, conclusions have been drawn which are relevant to the following potential factors in heterogeneous nucleation processes: (a) chemical structure, (b) crystallographic unit cell type, (c) lattice parameters, (d) crystallinity of substrate, and (e) surface energy of substrate.     

Crystallization of isotactic polypropylene in a temperature gradient

The crystallization of isotactic polypropylene films was investigated in constant and in time-dependent temperature gradients. The temperature gradient influences a spherulitic pattern as well as an internal structure of spherulites. The gradient can accelerate conversion of the melt into spherulites although it has no effect on spherulitic nucleation. The acceleration of the local conversion results from a contribution of spherulites nucleated in colder parts of a sample. The observed effects intensify with the increase of the temperature gradient and they are also enhanced by a higher crystallization temperature. Received: 2 August 2000 Accepted: 21 February 2001     

Study of the carbon fiber–Poly(Ether–Ether–Ketone) (PEEK) interfaces, 3: influence and properties of interphases

The aim of this third part is to analyze the structure and properties of the interfacial region between carbon fibers and PEEK as a function of different thermal conditioning treatments. First, it is shown by means of optical microscopy that the interfacial zone is not different from the bulk matrix when standard cooling conditions are used. On the contrary, a transcrystalline interphase is formed near the carbon fiber surface in systems that have been subjected to isothermal treatments. By comparison with previous results concerning the mechanical properties of the fiber–matrix interface, it appears that the interfacial shear strength decreases in the presence of a transcrystalline interphase or when the crystallization rate of PEEK increases. Moreover, it seems that the “constraint state” of the amorphous phase of PEEK near the fiber surface could also play a role in the interfacial shear strength. Secondly, a method is proposed in order to estimate the elastic modulus of crystalline interphases. It seems that this modulus is strongly dependent on the crystallization rate of the polymer. Finally, the determination of the stress-free temperature, defined as the temperature at which a longitudinal compressive stress just appears on the carbon fiber during the processing of the composites, is performed by recording the acoustic events corresponding to the fragmentation process in single-fiber composites. The results confirm that the crystallization rate and the “constraint state” of the amorphous phase of the matrix play an important role in the mechanical behavior of carbon fiber–PEEK interfaces.     

Crystallization of poly(ethylene terephthalate) induced by inorganic compounds. I. Crystallization behavior from the glassy state in a low-temperature region

The kinetics of isothermal crystallization from the glassy state at low temperatures and the morphology of poly(ethylene terephthalate) (PET) filled with additives are reported. Talc, kaolin, silicon oxide, and titanium oxide have been used as fillers; they act as effective nucleating agents for PET. The overall rate of crystallization depends on the volume concentration, the size distribution, and the nucleating ability of the additives. An electron microscopic study reveals a transcrystalline morphology at the surface of the filler particles. The occurrence of transcrystallinity is attributed to extensive heterogeneous nucleation induced at the filler surface. From the shape of the crystallization isotherms, it can be concluded that the crystallization mechanism depends on the type of filler.     

Crystallization kinetics of poly(hexamethylene succinate)

Isothermal and nonisothermal crystallization kinetics of polyester 64 have been investigated by means of differential scanning calorimetry and optical microscopy. The Avrami analysis has been performed to obtain the kinetic parameters of primary crystallization. These indicate a three-dimensional spherulitic growth on heterogeneous nuclei for the isothermal crystallization, whereas an sporadic nucleation becomes dominant in the nonisothermal crystallization. The maximum crystallization rate of polyester 64 was deduced to take place at a temperature close to −3 °C. Polarizing light microscopy showed that spherulites with a negative birefringence are formed during isothermal crystallization, whereas transmission electron microscopy indicates that the b crystallographic axis is aligned parallel to the spherulitic radius.     

Estimation of the nucleation and crystal growth contributions to the overall crystallization energy barrier

Classical kinetic theories of polymer crystallization were applied to isothermal crystallization kinetics data obtained by polarized optical microscopy (PLOM) and differential scanning calorimetry (DSC). The fitted parameters that were proportional to the energy barriers obtained allow us to quantitatively estimate the nucleation and crystal growth contributions to the overall energy barrier associated to the crystallization process. It was shown that the spherulitic growth rate energy barrier found by fitting PLOM data is almost identical to that obtained by fitting the isothermal DSC crystallization data of previously self‐nucleated samples. Therefore, we demonstrated that by self‐nucleating the material at the ideal self‐nucleation (SN) temperature, the primary nucleation step can be entirely completed and the data obtained after subsequent isothermal crystallization by DSC contains only contributions from crystal growth or secondary nucleation. In this way, by employing SN followed by isothermal crystallization, we propose a simple method to obtain separate contributions of energy barriers for primary nucleation and for crystal growth, even in the case of polymers where PLOM data are very difficult to obtain (because they exhibit very small spherulites). Comparing the results obtained with poly(p‐dioxanone), poly(ε‐caprolactone), and a high 1,4 model hydrogenated polybutadiene, we have interpreted the differences in primary nucleation energy barriers as arising from differences in nuclei density. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1478–1487, 2008     

Transcrystallization of polypropylene at cellulose nanocrystal surfaces

There is a resurgence of interest in composite materials incorporating cellulose as fibrous reinforcement in semicrystalline melt-processed polymers. Potential natural cellulose sources range from flax and ramie fibres down to whiskers and nanocrystals isolated from bacteria. It has long been known that the crystallization of matrix polymers such as polypropylene may be preferentially nucleated by Cellulose I surfaces, leading to a “transcrystalline” layer around the fiber. In this note, a transcrystalline layer at the edge of films cast from cellulose nanocrystal suspensions is demonstrated, and preferential nucleation of polypropylene on nanocrystals deposited on a glass surface is also observed.     

Non‐isothermal crystallization kinetics and spherulitic morphology of nucleated poly(lactic acid): effect of dilithium cis‐4‐cyclohexene‐1,2‐dicarboxylate as a novel and efficient nucleating agent

The effect of dilithium cis‐4‐cyclohexene‐1,2‐dicarboxylate (CHDA‐Li) as a novel and efficient nucleating agent on the crystallization behaviors and spherulitic morphology of poly(lactic acid) (PLA) as well as non‐isothermal crystallization kinetics of the nucleated PLA was studied by means of differential scanning calorimetry and polarized light microscopy. The results show that CHDA‐Li serves as a good nucleating agent to accelerate the crystallization rate of PLA. The nucleation ability of CHDA‐Li is superior to octamethylenedicarboxylic dibenzoylhydrazide. With the incorporation of CHDA‐Li, the number of the spherulites increases, and the size decreases significantly. The non‐isothermal crystallization kinetics of the nucleated PLA can be well described by Jeziorny's and Mo's models. The activation energies (ΔE) of non‐isothermal crystallization were calculated by Kissinger's and Friedman's methods. The crystallization rate of PLA/0.5 wt% CHDA‐Li sample is faster than that of PLA/0.2 wt% CHDA‐Li sample, while the ΔE of the former is lower than that of the latter. Copyright © 2015 John Wiley & Sons, Ltd.     

Transient and athermal effects in the crystallization of polymers. I. Isothermal crystallization

The isothermal crystallization of poly(propylene) and poly(ethylene terephthalate) was investigated with differential scanning calorimetry and optical microscopy. It was found that the induction time depends on the cooling rate to a constant temperature. The isothermal crystallization of the investigated polymers is a complex process and cannot be adequately described by the simple Avrami equation with time‐independent parameters. The results indicate that crystallization is composed of several nucleation mechanisms. The homogeneous nucleation occurring from thermal fluctuations is preceded by the nucleation on not completely melted crystalline residues that can become stable by an athermal mechanism as well as nucleation on heterogeneities. The nucleation rate depends on time, with the maximum shortly after the start of crystallization attributed to nucleation on crystalline residues (possible athermal nucleation) and on heterogeneities. However, the spherulitic growth rate and the exponent n do not change with the time of crystallization. The time dependence of the crystallization rate corresponds to the changes in the nucleation rate with time. The steady‐state crystallization rate in thermal nucleation is lower than the rate determined in a classical way from the half‐time of crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1835–1849, 2002