Self-nucleation Behavior Research Articles

The Influence of Thermal Parameters on the Self-Nucleation Behavior of Polyphenylene Sulfide (PPS) during Secondary Thermoforming.

During the secondary thermoforming of carbon fiber-reinforced polyphenylene sulfide (CF/PPS) composites, a vital material for the aerospace field, varied thermal parameters profoundly influence the crystallization behavior of the PPS matrix. Notably, PPS exhibits a distinctive self-nucleation (SN) behavior during repeated thermal cycles. This behavior not only affects its crystallization but also impacts the processing and mechanical properties of PPS and CF/PPS composites. In this article, the effects of various parameters on the SN and non-isothermal crystallization behavior of PPS during two thermal cycles were systematically investigated by differential scanning calorimetry. It was found that the SN behavior was not affected by the cooling rate in the second thermal cycle. Furthermore, the lamellar annealing resulting from the heating process in both thermal cycles affected the temperature range for forming the special SN domain, because of the refined lamellar structure, and expelled various defects. Finally, this study indicated that to control the strong melt memory effect in the first thermal cycle, both the heating rate and processing melt temperature need to be controlled simultaneously. This work reveals that through collaborative control of these parameters, the crystalline morphology, crystallization temperature and crystallization rate in two thermal cycles are controlled. Furthermore, it presents a new perspective for controlling the crystallization behavior of the thermoplastic composite matrix during the secondary thermoforming process.

How 4-(7-Octen-1-yl)-N,N-diphenylaniline co-units induce strong melt memory effects in isotactic polypropylene random copolymers

The random incorporation of comonomer units in isotactic polypropylene usually disrupts the chemical regularity of the macromolecular chain, thereby remarkably affecting the crystallization and melting processes. We study the influence of a novel comonomer 4-(7-Octen-1-yl)-N,N-diphenylaniline (ODPA), in the concentration range of 1.1–6.8 mol%, on the crystallization and self-nucleation behavior of isotactic polypropylene copolymers. Non-isothermal and isothermal differential scanning calorimetry experiments demonstrated that crystallization kinetics are significantly retarded upon comonomer incorporation as the ODPA co-units are excluded to the amorphous regions. Furthermore, ODPA comonomer incorporation leads to a sharp increase in nucleation density; thus, spherulitic superstructures become significantly smaller. Remarkably, the ODPA co-units induce a strong melt memory effect that can persist even above the equilibrium melting temperature for copolymers with 4.4–6.8 mol% ODPA. The width of Domain IIa (exclusive melt-memory domain), an indicator of the self-nucleation strength, starts to increase sharply once the content of ODPA exceeds 1.8 mol%. In contrast, the width of Domain IIb (self-seeding domain) remains constant. We attribute the self-nucleation effect in these ODPA copolymers to the aggregation of co-units in the interlamellar amorphous regions that hinder, upon melting, the motion of the chain sequences that were initially packed within the crystal lattice. In this way, to reach the isotropic melt state, the sample must be heated to much higher temperatures than its apparent or even equilibrium melting point, depending on the comonomer content. Consequently, nucleation upon cooling from the self-nucleated melt becomes easier and faster.

Peculiar self‐nucleation behavior of a polybutene‐1/ethylene random copolymer

The self-nucleation behavior of a polybutene-1/ethylene random copolymer, P(B1-ran-E), which undergoes a complex crystal-crystal transition behavior, has been studied in detail. Similar to PE random copolymers, this material shows a strong melt memory effect even above equilibrium melting point of PB-1 homopolymer. Different polymorphic forms can be obtained when P(B1-ran-E) is cooled from different self-nucleation domains. The trigonal form I′ could only be nucleated in the presence of remaining form I crystals via self-seeding, while the melt memory in Domain IIa could only act as self-nuclei for kinetically favored form II. Furthermore, observations from optical microscopy illustrated that melt memory is able to enhance nucleation density but it does not affect the spherulitic growth rate.

Suppression of the Self-Nucleation Effect of Semicrystalline Polymers by Confinement

The melt memory effect is well-known in polymer crystallization. It is caused by self-nuclei that persist above the melting temperature. The origin and physical nature of self-nuclei are still under debate. In this work, we studied the effect of confinement on the self-nucleation (SN) behavior of two typical semicrystalline polymers, poly(e-caprolactone) (PCL) and poly(butylene succinate) (PBS), using anodic aluminum oxide (AAO) templates. The density of AAO nanopores covers a range of 10¹¹–10¹³ cm–³. Narrowing of the SN region (Domain II) with a decrease of the AAO diameter was observed for both infiltrated PCL and PBS, indicating the suppression of the SN effect. When the diameter of AAO is below 60 nm, Domain II vanished. Further analysis indicated that Domain IIa (melt memory region) vanished first, followed by Domain IIb (self-seeding region). The results provide a method for estimating the self-nuclei density of different polymers at different temperatures.

The effect of self-nucleation on isothermal crystallization kinetics of poly(butylene succinate) (PBS) investigated by differential fast scanning calorimetry

Differential fast scanning calorimetry (DFSC) was employed on the study of self-nucleation behavior of poly(butylene succinate) (PBS). The ultra-fast cooling ability of DFSC allows investigating the effect of self-nucleation on the isothermal crystallization kinetics over a wide temperature range. Crystallization half-time, instead of crystallization peak temperature, was used to describe the self-nucleation behavior, and the self-nucleation domain for the samples crystallized at different temperatures was determined. Due to the competition between homogenous nucleation and self-nuclei, the effect of self-nucleation was less pronounced at high supercooling than that for the sample isothermally crystallized at higher temperature. An efficiency scale to judge the efficiency of nucleating agents from the crystallization half-time was also introduced in this work.

Effect of thermal history on nucleation and crystallization of poly(lactic acid)

In this paper, a successive heating and cooling protocol was designed to investigate the self-nucleation behavior of poly(lactic acid), PLA. The main objective of this investigation was to study the efficiency of the α and α′ crystalline modifications of PLA. This was carried out by comparing crystallization temperatures upon cooling after self-nucleation of samples previously crystallized at various isothermal temperatures ranging from 80 to 130 °C. During heating to the partial melting range, three different mechanisms were observed for crystallized samples. For samples crystallized below 100 °C, an exothermic peak was detected prior the main melting peak which is ascribed to the α′–α solid-state transition. For samples crystallized between 100 and 120 °C, a melt recrystallization mechanism was observed. Finally, for samples crystallized above 120 °C, only melting of the α phase was detected. Upon cooling after partial melting, it was found that samples comprising a mixture of α and α′ exhibited the highest crystallization temperature, the highest nuclei density, and the smallest spherulite size. Moreover, it was observed that samples that were isothermally crystallized between 100 and 120 °C, heated up to partial melting, and then cooled back to room temperature exhibited two peculiar crystallization peaks at 100 and 120° C. This phenomenon was ascribed to the formation of α and α′ crystalline phases as revealed by X-ray diffraction. In addition, by slightly changing the temperature within the self-nucleation temperature range, a change of the proportion of each peak was observed.

Investigation on the Self-nucleation Behavior of Controlled-rheology Polypropylene

Self-nucleation (SN) behavior is one of the most fascinating behaviors of semicrystalline polymers; it is of great scientific and practical importance. In this study, the crystallization and melting behavior of a series of controlled-rheology polypropylene (CRPP) with different molecular weights were studied by differential scanning calorimetry (DSC) scanning and successive self-nucleation and annealing (SSA) fractionation, and their self-nucleation behavior was studied according to the conventional SN measurement, in order to provide a new understanding on the relationship between the molecular weight and the SN behavior of polymers. The results of DSC scanning and SSA fractionation showed that the decrease of molecular weight led to higher molecular chains mobility, making the crystallization temperature higher and the crystalliation peak narrower; in addition, it significantly restrained the formation of thick lamellae. The results of SN measurements showed that both the upper and lower temperatures of the samples decreased as the molecular weight decreased, which was suggested to be closely related to the reduction of the formation of thick lamellae. The related mechanism is discussed, and it was found that the key parameter determining the occurrence of SN and annealing of CRPP was the ability to form thick lamellae with high melting point, which might be related to the ability for ordered structures to persist at high temperature.

Influence of melt structure on the crystallization behavior and polymorphic composition of polypropylene random copolymer

Polypropylene random copolymer (PPR) is one of important polypropylene types for the application fields. However, due to the random copolymer chain configuration, it is difficult to obtain high proportion of β-phase even under the influence of β-nucleating agent (β-NA). In this study, the melt structure (namely, the content of ordered structures in the melt) of β-nucleated ethylene-copolymerized PPR (β-PPR) was controlled by tuning the fusion temperature (Tf), and its impact on the crystallization and polymorphic behavior of β-PPR was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), polarized optical microscopy (PLM) and scanning electronic microscopy (SEM). The result revealed that compared with the β-nucleated iPP homo-polymer, it is more difficult for β-PPR to form β-crystals; interestingly, when Tf is in the temperature range of 162–173°C, the ordered structures survived in melt exhibit high β-nucleation efficiency under the influence of β-NA, resulting in significant increase of β-phase proportion and evident variation of crystalline morphology, which is called the Ordered Structure Effect (OSE). Moreover, through investigating the self-nucleation behavior and equilibrium melting temperature of pure PPR (non-nucleated PPR), the physical nature of the lower and upper limiting Tf temperatures for the occurrence of OSE in β-PPR was explored; the role of ethylene co-monomer in the occurrence of OSE was discussed.

Nucleation, crystallization, self-nucleation and thermal fractionation of cyclic and linear poly(ε-caprolactone)s

A series of low polydispersity cyclic PCL samples (C-PCLs), as well as their linear analogs (L-PCLs), were synthesized by click chemistry in a number average molecular weight (Mn) range of 2–22kg/mol. They were investigated by Polarized Light Optical Microscopy (PLOM) and Differential Scanning Calorimetry (DSC). The nucleation and overall crystallization kinetics were studied, as well as their self-nucleation behavior and SSA (Successive Self-nucleation and Annealing) thermal fractionation. Cyclic PCLs were found to nucleate and crystallize faster than linear PCLs due to: (a) faster diffusion of C-PCL chains and (b) larger supercoolings of C-PCLs at any given crystallization temperature, as compared to L-PCLs. A bell shape curve was obtained when the overall crystallization rate was examined as a function of Mn, this effect is probably due to a competition between nucleation and diffusion. It was found for the first time, that since cyclic molecules have lower entanglement densities, they can quickly recover their pseudo-equilibrium compact coil conformations upon melting and therefore exhibit much smaller crystalline memory effects than their linear counterparts of identical chain lengths. SSA revealed that C-PCLs are more sensitive to annealing than L-PCLs because their ring topology and limited lamellar chain folding facilitates crystal thickening.

Fractional Crystallization and Homogeneous Nucleation of Confined PEG Microdomains in PBS-PEG Multiblock Copolymers

Fractional crystallization, homogeneous nucleation of poly(ethylene glycol) (PEG) segment, and self-nucleation behavior of PEG segment within miscible double crystalline poly(butylene succinate)-poly(ethylene glycol) (PBSEG) multiblock copolymers with different composition and segment chain length were studied by differential scanning calorimetry (DSC). Surface morphology of PBSEG10K with different PEG content was investigated by atomic force microscope (AFM). Different from di- or triblock copolymers, the microstructure and confinement of PEG dispersed phase in PBS matrix phase highly depends on chain length and sequence as well as segment content. The transition point of the PEG segment content from heterogeneous to homogeneous nucleation mechanism decreased from 50 to 39 wt % with PEG segment chain length increasing from 1000 to 2000 g/mol. When PEG segment chain length increased further to 6000 and 10000 g/mol, homogeneous nucleation phenomenon took place at much lower PEG content and fractional crystallization was observed at 29 and 24 wt %, respectively. Homogeneous nucleation mechanism of PBSEG(1K-36), PBSEG(2K-26), PBSEG(6K-19), and PBSEG(10K-12) was evidenced by the large supercoolings needed for crystallization, as well as first-order crystallization kinetics obtained. Self-nucleation behaviors of PEG segment still rely on the composition of PBSEGs. In the case of heterogeneous nucleation crystallization, self-nucleation behaviors of PEG segment showed standard self-nucleation behavior with classical three self-nucleation domains. When the crystallizable chains were confined into isolated microdomains, however, self-nucleation domain (domain II) disappeared. The absence of III(A) was observed in PBSEG(2K-39), while PBSEG(6K-29) had both III(A) and III(SA). Furthermore, AFM morphology studies still indicated the confined degree of PEG segment by previous PBS crystals was profoundly influenced by segment fraction. The confinement of the PEG segment by previous PBS edge-on lamellae was observed in the sample which displays a homogeneous nucleation crystallization behavior.

Self-Nucleation and Kinetic Behavior of Nanocolloidal Sodalite Particles in Highly Caustic Liquors

Sodium aluminosilicate (SAS) crystallization from supersaturated caustic liquors is of high industrial significance with regards to intractable heat exchanger precipitation fouling issues in Bayer process alumina refining and high level nuclear waste processing. In the present work, isothermal (65 °C) homogeneous nucleation behavior of nanocolloidal SAS particles in optically clear solutions as a function of time was investigated by dynamic light scattering (DLS). At a high SiO2 relative supersaturation (σ) of 12, the solution was perennially metastable, reflecting a long nucleation induction time of 12 h. Upon ephemeral preheating at 100 °C, and/or a 25–40% increase in σ, the DLS analysis showed that rapid nucleation and moderate particle growth occurred in the optically clear solutions. Furthermore, evolution uni-, bi-, and trimodal particle size distributions in the range of 20–1800 nm with time was observed, accompanied by significant time-dependent distribution broadening effects. For growth mechanisms, both nanoparticle aggregation and surface integration of ionic growth units are revealed, manifesting in polydispersed, low mass density-contrast agglomerates in optically clear liquors. The SAS solid product observed after prolonged crystallization was high carbonate-sodalite crystals, comprising agglomerates of nanoparticles. The pivotal roles played by SiO2 supersaturation and temperature in the early stages of sodalite nucleation and growth are demonstrated by the results.

Crystallization of Isotactic Polypropylene under the Spatial Confinement Templated by Block Copolymer Microdomains

We investigate the crystallization behavior of isotactic polypropylene (iPP) under the influence of nanoscale confinement templated by the microphase-separated structure of an iPP-based diblock copolymer system, isotactic polypropylene-block-atactic polystyrene (iPP-b-aPS). Three types of iPP microdomains, i.e., lamellae, cylinder, and sphere, were generated by controlling the composition of the diblock. The effect of microdomain morphology on the nucleation mechanism, crystallization kinetics, self-nucleation behavior, the population of the helical sequence of iPP block in the melt state, and crystal orientation have been systematically studied. It was found that the crystallization rate of iPP was predominantly controlled by homogeneous nucleation when the crystallization process was largely confined within the individual cylindrical and spherical microdomains. Such a nucleation mechanism and the highly frustrated crystal growth in the isolated microdomains led to the absence of Domain II and atypical crystallization kinetics in Domain III in the self-nucleation study. The population of the longer helical sequence of iPP block revealed by infrared spectroscopy was found to be affected by temperature but not by the spatial confinement, chain stretching, and junction point constraint imposed by the microdomains. Finally, the orientation of α-form iPP crystals in the lamellae-forming iPP-b-aPS was identified over a broad range of crystallization temperatures (T(c)). Different from other crystalline-amorphous diblocks, a lamellar branching of α-form iPP was observed in the lamellar microdomains at T(c) lying between 15 and 80 °C, where the daughter lamellae developed from the perpendicularly orientated parent iPP crystals with a specific angle of 80° or 100°. Once the sample was crystallized at T(c) ≤ 10 °C, the iPP crystals became randomly oriented.

Nucleation and crystallization of H-shaped (PS)2PEG(PS)2 block copolymers

A series of H-shaped (PS)2PEG(PS)2 block copolymers with different PS chain lengths were prepared. The influence of different confinements active on the crystallization and self-nucleation (SN) behavior of the PEG blocks was investigated by differential scanning calorimetry (DSC). When the content of the crystalline block was high, a classical SN behavior was obtained. The block copolymer with PEG content of 49% (by weight) showed a classical SN behavior with a narrow self-nucleation domain and had bimodal crystallization exotherms. When the PEG dispersed as separated microdomains in the block copolymer, the self-nucleation domain disappeared and only annealing was observed.

Effect of sequence distribution on the isothermal crystallization kinetics and successive self-nucleation and annealing (SSA) behavior of poly(ε-caprolactone- co-ε-caprolactam) copolymers

Two types of miscible poly(ε-caprolactone- co-ε-caprolactam) copolymers were studied. In both cases catalyzed hydrolytic ring-opening polymerization was employed. For the first type, the comonomers were added simultaneously to obtain random copolymers. For the second type, the comonomers were added sequentially to obtain block copolymers. Successive self-nucleation and annealing (SSA) and isothermal crystallization studies were performed to both types of copolymers. The SSA results reflect the differences in molecular microstructure: block versus random copolymers. In a wide composition range only the polycaprolactam sequences were capable of crystallization in the random copolymers. Avrami indexes of approximately 3–4 were obtained corresponding to the spherulitic crystallization of these units within the copolymers. The block copolymer samples experienced a relatively small reduction of crystallization kinetics with composition, and this was attributed to the dilution effect caused by the miscible non-crystalline polycaprolactone units. On the other hand, for the random copolymers, the rate of crystallization strongly increased with polycaprolactam content while the energy barrier for secondary nucleation decreased exponentially. The comparison between miscible block and random copolymers provides a unique opportunity to distinguish the dilution effect of the polycaprolactone units (a moderate effect) on the isothermal crystallization and melting of the polyamide phase from the molecular microstructural effect in the random copolymers case (a dramatically strong effect), where the polycaprolactam sequences are interrupted statistically by polycaprolactone sequences.

Quantitative evaluation of fractionated and homogeneous nucleation of polydisperse distributions of water-dispersed maleic anhydride-grafted-polypropylene micro- and nano-sized droplets

Abstract The nucleation processes in waterborne Maleic Anhydride-grafted-Polypropylene micro- and nano-droplet suspensions have been studied. Compared to a previous report on this topic, an extended set of samples in combination with improved particle size distribution data of the samples have been used, which are both essential for the advancement of the analysis. Self-nucleation was utilized to ensure that the observed lowered fractionated crystallization (peak) temperatures – down to the extremely low value of 34 °C – are due to a lack of seeds in the droplets, which seeds for the polypropylene system used are normally active at the heterogeneous crystallization temperature of approximately 110 °C. An unusual self-nucleation behavior was observed in case of samples having a large amount of small droplets, requiring an extremely low self-nucleation temperature in order to suppress all crystallization at the lowest temperatures. Such behavior was observed for block copolymers but has not been reported so far for droplets dispersed in an immiscible matrix, polymeric or not. Another unusual behavior was observed for some self-nucleation temperatures for which apparently two different populations of self-nuclei are created that are suggestive of the α 1 and α 2 crystal structures of isotactic polypropylene. Next, two new methods are presented to quantify various crucial parameters of the nucleation process: one estimates the density of nucleants acting at different temperatures from the combination of dynamic DSC data and particle size distribution (PSD) data, and the other one focuses on the nature of the nucleation mechanism using both isothermal DSC data and PSD data, quantifying the nucleation rate at different temperatures. For the present MA-g-PP dispersions the latter method leads to the conclusion that the lowest crystallization temperatures reflect sporadic nucleation, probably by way of volume (homogeneous) nucleation. In the field of polymer crystallization, polymer dispersions are usually treated as being monodisperse, even though that is rarely the case. This simplification is inadequate for the present calculations, which is why polydispersity has been taken into account in order to quantify the density of nucleants and the kinetics of nucleation. Though in the present study DSC data are used for the calculations, the methods developed can be easily adapted to other techniques like time-resolved X-ray, rheometry and dilatometry.

Coincident or sequential crystallization of PCL and PEO blocks within polystyrene- b-poly(ethylene oxide)- b-poly(ε-caprolactone) linear triblock copolymers

In this work, the behavior of the crystallizable blocks within polystyrene- b-poly(ethylene oxide)- b-poly(ε-caprolactone) linear triblock copolymers is studied. The apparent crystallization and melting temperatures of PEO and PCL can be coincident as a result of their close values, their dependence on the molecular weight of the blocks and on the relative amount of each component within the copolymers. When the content of both PEO and PCL is low, typically 20% or less, both blocks crystallized coincidentally upon cooling from the melt at very large supercoolings ( T c=−40 °C), suggesting that homogeneous nucleation has taken place. A S 63EO 16C 20 copolymer (where subscripts denote the weight fraction of the components) exhibited a small angle X-ray scattering (SAXS) pattern at room temperature that suggests a cylindrical morphology for the minor components, where the crystallization of PEO and PCL blocks can take place subsequently near −40 °C in a hard confinement fashion within the vitreous PS matrix. We have employed differential scanning calorimetry (DSC) to study the self-nucleation process and the results indicate that the difficulty in generating enough self-seeds to nucleate every isolated microdomain causes the disappearance of Domain II (or exclusive self-nucleation Domain) and therefore a direct transition from complete melting ( Domain I) to self-nucleation and annealing ( Domain III) is observed. When the content of both PEO and PCL blocks is larger than 35%, a coincident crystallization can be observed as well as coincident melting. For a S 15EO 37C 48 triblock copolymer, PS is a minor phase and mixed spherulites composed of both PCL and PEO blocks were observed by Polarized Optical Microscope (POM) at appropriate supercoolings. In this case, the self-nucleation behavior is standard, and through its use the crystallization and melting process of both blocks can be separated. Wide angle X-ray scattering (WAXS) at different temperatures was used to corroborate the sequential melting of both semicrystalline blocks deduced by self-nucleation. Isothermal crystallizations were also followed by WAXS as a function of time and the results showed that depending on the crystallization temperature ( T c), only the PCL block crystallizes (at high T c), or the PCL block crystallizes first followed by the PEO at later times (at lower T c).

Crystallization in ABC Triblock Copolymers with Two Different Crystalline End Blocks: Influence of Confinement on Self‐Nucleation Behavior

AbstractThe influence of different confinements active during crystallization within polybutadiene‐block‐polyisoprene‐block‐poly(ethylene oxide) (PB‐b‐PI‐b‐PEO) and the corresponding hydrogenated polyethylene‐block‐poly(ethylene‐alt‐propylene)‐block‐poly(ethylene oxide) (PE‐b‐PEP‐b‐PEO) triblock copolymers on the self‐nucleation behavior of the crystallizable PEO and PE blocks is investigated by means of differential scanning calorimetry (DSC). In triblock copolymers with PEO contents ≤ 20 wt.‐% crystallization of PEO is confined within small isolated microdomains (spheres or cylinders), and PEO crystallization takes place exclusively at high supercoolings. Self‐nucleation experiments reveal an anomalous behavior in comparison to the classical self‐nucleation behavior found in semicrystalline homopolymers. In these systems, domain II (exclusive self‐nucleation domain) vanishes, and self‐nucleation can only take place at lower temperatures in domain IIISA, when annealing is already active. The self‐nucleation behavior of the PE blocks is significantly different compared with that of the PEO blocks. Regardless of the low PE content (10–25 wt.‐%) in the investigated PE‐b‐PEP‐b‐PEO triblock copolymers a classical self‐nucleation behavior is observed, i.e., all three self‐nucleation domains, usually present in crystallizable homopolymers, can be located. This is a direct result of the small segmental interaction parameter of the PEP and PE segments in the melt. As a consequence, crystallization of PE occurs without confinement from a homogeneous mixture of PE and PEP segments. Self‐nucleation regimes of a block copolymer showing confined crystallization by means of DSC.magnified imageSelf‐nucleation regimes of a block copolymer showing confined crystallization by means of DSC.

Thermal and self-nucleation behavior of molecular complexes formed by p-nitrophenol and the poly(ethylene oxide) end block within an ABC triblock copolymer

Thermal and self-nucleation behavior of molecular complexes formed by p-nitrophenol and the poly(ethylene oxide) end block within an ABC triblock copolymer

Homogeneous Nucleation and Fractionated Crystallization in Block Copolymers

The confinement of crystallizable blocks within AB or ABC microphase-separated block copolymers in the nanoscopic scale can be tailored by adequate choice of composition, molecular weight, and chemical structure. In this work we have examined the crystallization behavior of a series of AB and ABC block copolymers incorporating one or two of the following crystallizable blocks: polyethylene, poly(ε-caprolactone), and poly(ethylene oxide). The density of confined microdomain structures (MD) within block copolymers of specific compositions, in cases where the MD are dispersed as spheres, cylinders, or any other isolated morphology, is much higher than the number of heterogeneities available in each crystallizable block. Therefore, fractionated crystallization takes place with one or several crystallization steps at decreasing temperatures. In specific cases, the clear observation of exclusive crystallization from homogeneous nuclei was obtained. The results show that, regardless of the specific morphological features of the MD, it is their vast number as compared to the number of heterogeneities present in the system that determines the fractionated character of the crystallization or in extreme cases homogeneous nucleation. The self-nucleation behavior was also found to depend on the composition of the copolymers. When the crystallizable block is confined into spheres or cylinders and exhibits homogeneous nucleation, the self-nucleation domain disappears. This is a direct consequence of the extremely high density of microdomain structures that need to be self-seeded (on the order of 1015−1016/cm3). Therefore, to increase the density of self-nuclei, the self-nucleation temperature has to be decreased to values so low that extensive partial melting is achieved, and some of the unmelted crystal fragments can be annealed, in some cases even before self-nucleation takes place.

Crystallization of the polyethylene block in polystyrene-b-polyethylene-b-polycaprolactone triblock copolymers, 1. Self-nucleation behavior

The self-nucleation of branched polyethylene chains of different degrees of chain mobility was studied. The polyethylene block (PE block) within poly(styrene-b-ethylene-b-caprolactone) triblock copolymers (SEC) of varying compositions was studied. Differential scanning calorimetry was used to determine the self-nucleation domains as a function of the self-nucleation temperature (Ts). The self-nucleation behavior of PE chains within SEC block copolymers was found to be anomalous in comparison to the classical self-nucleation behavior exhibited by homopolymers. When the degree of chain constraint is high, as in the case where the SEC copolymer only contains 15% of PE, domain II (only self-nucleation domain) completely disappears and annealing can take place before self-nucleation occurs. This means that chain constraint complicates the self-nucleation process and this situation persists until, upon decreasing the self-nucleation temperature (Ts), annealing has generated crystals that are big enough to act as self-nuclei for the less restricted portions of the chain. If the PE content in the copolymer is very low (15%), two crystal populations can be distinguished. This may reflect the differences in diffusion of the PE chain segments close to the interfaces with the other two blocks and those segments that are close to the middle of the PE block. The influence of chain constraint on determining the difficulty of the chains to self-nucleate was further explored using a crosslinked low-density polyethylene (XLDPE). In this case, crosslinking junctions instead of covalent links with other blocks restrict chain mobility. Nevertheless, a similar difficulty in self-nucleation was found as in the case of the PE block within SEC triblock copolymers in contrast to neat LDPE, a polymer that exhibited the classical self-nucleation behavior with the usual three domains.