Search

Crystallization of polymorphic polymers can lead to different structures, starting from the same melt or solution. Comprehensive understanding of crystallization modalities in such systems is of primary technological and scientific relevance, because it can enable prediction â€˵ and possible control â€˵ of the polymorphic composition of a material and, in turn, of its properties. Several structuring pathways are possible. A given structure can develop either directly or through successive metastable states. The latter case obeys the so-called Ostwaldâ€⠪s rule of stages. Under particular thermodynamic and kinetic conditions, two or more polymorphs can nucleate and grow concomitantly. Moreover, cross-nucleation is observed when a faster growing polymorph nucleates on a previously existing polymorph. This chapter is focused on concomitant crystallization and cross-nucleation between polymer polymorphs, two topics that have seldom been considered in the past and lack critical review. First, the scattered pieces of information in the polymer literature are collected and discussed. Within this framework, we present two relevant examples taken from our own work, concerning concomitant crystallization of poly(pivalolactone) polymorphs and cross-nucleation in seeded crystallization of isotactic poly(1-butene). [ABSTRACT FROM AUTHOR]

High hydrostatic pressure pulses, up to 80 MPa, have been imposed for different time and at various temperatures on molten iPP during cooling. The absence of any effect on the crystallization behavior after pressure release and further cooling demonstrates either that no nucleation precursor is produced by this thermo-mechanical history or that the lifetime of pressure-induced precursors is extremely short, as if no crystallization occurs during pressurization. Even when crystals develop during the pressurized stage, when high pressure is released, subsequent crystallization takes place in the same temperature range in which the polymer crystallizes at atmospheric pressure. Structure and morphology of samples partially crystallized under pressure were investigated. WAXD show that, as expected, the fraction of γ-crystallinity firstly increases with increasing the time under pressure up to impinging of γ-spherulites and then levels off at a constant value. Morphological investigation performed on microtomed samples reveals that the γ-spherulites formed under high pressure exhibit positive birefringence and that their skeleton continues to grow during cooling when pressure is released. On heating, these spherulites melted first, but incompletely: the radial lamellar texture is still present up to relatively high temperature. These features seem to be consistent with the spherulitic architecture described by Lotz: infilling γ-lamellae grow by epitaxy on a skeleton of α-lamellae. Under the adopted experimental conditions, pre-imposed pressure pulses do not promote formation of extra nuclei, while they largely affect the crystal structure and the morphology of melt crystallized isotactic polypropylene. [ABSTRACT FROM AUTHOR]

The crystallization behavior of random isotactic butene-1/propylene copolymers has been investigated as a function of the propylene concentration and as a function of the crystallization temperature in a wide temperature range from 273 to 343 K, with emphasis placed on the detection of formation of Form I′ crystals. Form I′ crystals in random butene-1/propylene copolymers develop if the propylene content is larger than about 10 mol%. For a random butene-1/propylene copolymer with 11.4 mol% propylene it was found that Form I′ crystals only form below 310 K, with the relative amount increasing with decreasing crystallization temperature. A maximum Form I′ crystal fraction of about 1/3 of the total crystal fraction has been observed on crystallization at 273 K. Regardless the temperature of its formation, the thermal stability of metastable Form I′ crystals is distinctly lower than the thermal stability of unstable Form II mesophase and stable Form I crystals, if formed at identical temperature. [ABSTRACT FROM AUTHOR]

Abstract: The effect of counit type on the kinetics of mesophase formation has been investigated by means of chip-calorimetry in propene/α-olefin random copolymers, containing counits which show large differences in their co-crystallization behavior with propene, i.e. 1-butene and 1-hexene. Non-isothermal crystallization experiments indicated that the minimum cooling rate at which mesophase formation is observed is directly related to the kinetic of α-phase crystallization, which is lower for the copolymer with the bulkier 1-hexene counit. Isothermal structuring was probed in a wide temperature range, revealing that a double bell-shaped curve is required to describe the temperature dependence of crystallization times of the two polymorphs. The ordering kinetics of the mesophase is the fastest in i-PP homopolymer and decreases with increasing comonomer bulkiness, analogous to what happens for the monoclinic phase. The results are discussed by considering the effect of comonomer on the driving force for mesophase formation, also at the light of new WAXD and density evidences, which prove different extents of inclusion of 1-butene and 1-hexene in the ordered phases. [Copyright &y& Elsevier]

The solidification of the quiescent polyamide 6 (PA 6) melt has been analyzed as a function of the cooling rate in a wide range between 1.67 × 10 and close to 2 × 10 K s, by means of differential scanning calorimetry at a low cooling rate of up to about 1 K s, and by the recording of continuous cooling curves and time-resolved X-ray diffraction on cooling at a higher rate. The performed experiments allowed for the first time to establish the relationship between the cooling rate, the crystallization temperature, and the X-ray structure of PA 6. The exclusive formation of monoclinic α-crystals is only detected if the crystallization temperature is higher than about 430 K or if the cooling rate is slower than about 5 K s, respectively. The formation of α-crystals is increasingly replaced by the development of mesophase with increasing cooling rate, accompanied with a decrease of the temperature of crystallization/ordering. Finally, completely amorphous samples were obtained on cooling faster than about 10 K s. The continuous decrease of the temperature of crystallization with increasing cooling rate, regardless of the specific structure formed, precludes a primary effect of the nucleation mechanism on the α-crystal/mesophase polymorphism of PA 6. A preliminary discussion of the effect of molar mass of PA 6 on the cooling rate-dependent polymorphism is also included. [ABSTRACT FROM AUTHOR]

A particular kind of heterogeneous nucleation, i.e., cross-nucleation, is sometimes observed in polymorphic substances, when a new crystal structure nucleates on the surface of a crystal of a different modification. Here we show a unique and apparently incongruous nucleation behavior in polymorphic isotactic polypropylene (i-PP). The rate of cross-nucleation of the monoclinic α-phase on the trigonal β-phase crystals increases with increasing temperature, in the vicinity of the α-crystals melting point. This behavior is contrary to that of the heterogeneous nucleation kinetics of the same crystal on various solid substrates, and also to the previously reported cases of cross-nucleation rate of other polymorphic systems, both exhibiting the expected decrease with temperature in the same range of undercoolings. i-PP α-on-β cross-nucleation apparently eludes the nucleation theory. The results are explained as a manifestation of a kinetic competition between α-on-β cross-nucleation and growth of β-crystalline seeds, and finally reconciled with the current understanding of nucleation. These new findings indicate that further theoretical efforts are needed to include the cross-nucleation phenomenon in the framework of a comprehensive understanding of polymorphic crystallization. Incidentally, this study highlights the intrinsic limits of the, industrially desirable, promotion of β-phase formation in polypropylene. [ABSTRACT FROM AUTHOR]

Cross-nucleation between polymorphs occurs when a new crystal structure nucleates on the surface of a pre-existing crystal of a different modification. The understanding of the phenomenon is still mostly phenomenological and qualitative. Here, we report quantitative measurements of cross-nucleation rate of the α phase of polypivalolactone on its Γ phase, during melt crystallization over a large temperature range. The cross-nucleation kinetics is well modeled as heterogeneous secondary nucleation of one polymorph on another, in which the formation of a viable α-phase nucleus on the surface of Γ-crystals is the rate-limiting step. According to this analysis, cross-nucleation can occur because the interfacial energy between the two structures is small, and the nucleation free energy barrier is similar to that for secondary nucleation within the same polymorph in crystal growth. Some peculiar aspects of cross-nucleation with respect to conventional heterogeneous nucleation, arising from the continuous growth of the nucleating substrate, are also highlighted. [ABSTRACT FROM AUTHOR]

The self-nucleation behaviour of isotactic poly(1-butene) in the trigonal modification was investigated. Large and isolated Form I spherulites, successfully generated via a purposely designed thermal history protocol, were self-nucleated at different temperatures and subsequently crystallized at 90 °C. The morphology that developed was analysed using polarized optical microscopy, and the polymorphic content and distribution were monitored using synchrotron IR microspectroscopy. Images of the polymorphic distribution were generated using both univariate (band ratio) and multivariate (chemometric) analyses of the spectral data. From the morphological point of view, polarised optical microscopy showed that low seeding temperatures (＜130 °C) enabled preservation of the original large spherulite that served as a seed for the growth of Form II at its periphery through cross-nucleation. However, with increasing seeding temperature, more complex morphologies arise, through progressive fading of the original morphology. Firstly, re-crystallization results in a defective large spherulite, and then at temperatures above 132 °C, it results in numerous highly-nucleated Form II spherulites. The IR microspectroscopy images generated either by univariate (band ratio) or multivariate (hierarchical cluster analysis) mappings provided concise information on the spatial distribution of the two polymorphs in the re-crystallized specimens, with a resolution in the range of several microns. It was shown that the content of Form II in the region of the self-nucleated Form I spherulite increases with increasing seeding temperature, demonstrating that the morphological changes accompany the gradual erosion of the memory of the original crystalline structure. [ABSTRACT FROM AUTHOR]

The self-nucleation behaviour of isotactic poly(1-butene) in the trigonal modification was investigated. Large and isolated Form I spherulites, successfully generated via a purposely designed thermal history protocol, were self-nucleated at different temperatures and subsequently crystallized at 90 °C. The morphology that developed was analysed using polarized optical microscopy, and the polymorphic content and distribution were monitored using synchrotron IR microspectroscopy. Images of the polymorphic distribution were generated using both univariate (band ratio) and multivariate (chemometric) analyses of the spectral data. From the morphological point of view, polarised optical microscopy showed that low seeding temperatures (＜130 °C) enabled preservation of the original large spherulite that served as a seed for the growth of Form II at its periphery through cross-nucleation. However, with increasing seeding temperature, more complex morphologies arise, through progressive fading of the original morphology. Firstly, re-crystallization results in a defective large spherulite, and then at temperatures above 132 °C, it results in numerous highly-nucleated Form II spherulites. The IR microspectroscopy images generated either by univariate (band ratio) or multivariate (hierarchical cluster analysis) mappings provided concise information on the spatial distribution of the two polymorphs in the re-crystallized specimens, with a resolution in the range of several microns. It was shown that the content of Form II in the region of the self-nucleated Form I spherulite increases with increasing seeding temperature, demonstrating that the morphological changes accompany the gradual erosion of the memory of the original crystalline structure. [ABSTRACT FROM AUTHOR]

We studied experimentally the structure development during film blowing for three different low-density polyethylenes at four different processing conditions. For this, we combined blown film extrusion with in situ X-ray diffraction at a synchrotron radiation beam line. The measurements were performed at rheological similar conditions at the die exit, with each grade possessing different molecular properties in terms of molecular weight distribution and branching content. The development of crystallinity and molecular orientation was determined as function of the distance from the die. It is shown that this approach provides a valuable method to study structure evolution during film blowing and can be used to validate results from numerical models and help to improve and/or extend these models. [ABSTRACT FROM AUTHOR]

Abstract: Isotactic poly(1-butene) (i-PBu) is a polyolefin of industrial relevance which exhibits an interesting polymorphism. Upon cooling from the relaxed melt at atmospheric pressure, a tetragonal phase (Form II) develops. However, being metastable, this structure slowly evolves upon aging in the stable trigonal modification (Form I). Another trigonal modification, denoted Form I′, can also crystallize directly from the melt if proper conditions are met, e.g., high pressure, low tacticity, ultrathin samples etc. In this work, we aim to verify whether, by acting on the nucleation stage via a proper thermal history, the direct formation of the trigonal polymorph from the melt in a bulk sample is possible. Nucleation of i-PBu has been tailored by means of the self-nucleation technique, imposed on previously aged samples (Form I). DSC and temperature-resolved WAXS show that different crystallization pathways can be observed, depending on the residual concentration of Form I self-nuclei. With decreasing self-nucleation temperature we first encounter “cross-nucleation” of the tetragonal crystal on trigonal nuclei, followed by concomitant crystallization of Form II and Form I′ and eventually the sole formation of the trigonal modification (Form I′). [Copyright &y& Elsevier]

The crystallization behavior of a polyamide 6/organo-modified montmorillonite (PA 6/OMMT) nanocomposite has been investigated by scanning chip calorimetry and wide-angle X-ray scattering, with emphasis placed on the evaluation of the effect of supercooling/cooling rate on the crystal/mesophase polymorphism of the PA 6 matrix. Presence of OMMT has negligible effect on the kinetics of formation of α-crystals of PA 6 at low cooling rate while there is observed enhanced nucleation of γ-mesophase on fast cooling. Furthermore, addition of OMMT leads to a distinct increase of the cooling rate required to completely vitrify the amorphous state. The performed experiments demonstrate that the nucleating effect of OMMT in PA 6/OMMT nanocomposites is of particular importance at cooling conditions relevant in polymer processing. [ABSTRACT FROM AUTHOR]

Abstract: Cross-nucleation of crystalline phases, i.e., the nucleation of a polymorph on a previously existing one, has recently been recognized to play a role among the different crystallization pathways of polymorphic substances. While several examples exist for small organic molecules, cross-nucleation has mostly been overlooked in semi-crystalline polymers. One of the reasons resides in the self-nucleation phenomenon of one-polymorph on itself, i.e., a nucleating “memory” of pristine crystalline structure surviving the melting process. In this work, we provide a new tool useful for the studying of cross-nucleation in semi-crystalline polymers. The possibility of self-nucleation of one polymorph on itself can be easily identified and eliminated via a prior thermal history, thus providing the basis for a true and correct comparison of cross-nucleation experiments among the domains of small and macro-molecules. If the two classes of materials are compared together, it appears that cross-nucleation might be a more widespread phenomenon than what previously thought, and that the key for its understanding lies in the heterogeneous nucleation on a crystalline substrate without requiring epitaxial relations with the nucleating structure. A novel example of cross-nucleation in polymers, the one of isotactic poly(1-butene) Form II-on-Form I, is presented. [Copyright &y& Elsevier]

Abstract: The effect of variation the cooling rate in a wide range between 10−2 and 103 Ks−1 on solidification the relaxed melt of random isotactic copolymers of propylene with low amount of 1-hexene or 1-octene has been studied. Emphasis has been placed on the structure formation at rapid cooling and an evaluation of the conditions required to permit crystallization, mesophase formation, or suppression of any ordering. The presence of low amount of either 1-hexene or 1-octene co-units in the propylene chain decreases drastically the critical cooling rate required for suppression of crystallization from about 150–200Ks−1 in the homopolymer to about only 10Ks−1 in the copolymers; increasing the cooling rate beyond these limits allowed mesophase formation or even generation of fully amorphous samples. The study of the kinetics of formation of specific structures is completed by a complementary analysis of the X-ray structure, morphology and superstructure of the ordered phase. The hindrance of non-isothermal crystallization and mesophase formation of random copolymers of propylene with 1-hexene or 1-octene is compared with that in propylene–1-butene copolymers; addition of only 2–3mol% 1-hexene or 1-octene into the propylene chain leads to even larger hindrance of the ordering process than the addition of more than 10mol% 1-butene. [Copyright &y& Elsevier]

The solidification of random isotactic copolymers of propylene and 1-butene has been followed in real time by wide-angle X-ray scattering as a function of the rate of cooling the quiescent liquid. The experimental setup allowed simultaneous recording of cooling curves-sample temperature as a function of time-and X-ray patterns at high sampling rate of 20 Hz. This approach allowed establishing a correlation between cooling rate, temperature of crystallization/mesophase formation, and X-ray structure, which formerly has only been observed ex situ, after completion of structure formation during cooling and subsequent aging. It is quantitatively confirmed that addition of 1-butene co-units into the propylene chain allows mesophase formation on cooling the melt at distinctly lower rate than in case of the homopolymer. The experimental results are compiled into a continuous cooling transformation (CCT) diagram and compared with data obtained earlier on random copolymers of propylene with ethylene. [ABSTRACT FROM AUTHOR]

Abstract: We report results from time-resolved small/wide angle X-ray scattering of well-characterized isotactic polypropylene (iPP), in its pure form as well as loaded with nucleating additives of different shapes (isotropic, elongated, platelet-like). Isothermal crystallization of these systems has been studied both under quiescent conditions and after application of a shear pulse. A quantitative analysis is presented that couples the applied shear field and the specific additive to the nucleation of iPP and the subsequent kinetics of crystallization. The effect of flow is only relevant if its contribution to the nucleation density is at least of the same order of magnitude as the nucleation density under quiescent conditions. Anisotropic additives show strong interactions with the flow field leading to extra nucleating sites. [Copyright &y& Elsevier]