Spherulitic Morphology Research Articles

Probabilistic numerical simulation for predicting spherulitic morphology from calorimetric crystallization conversion curves: An isothermal case

The present work introduces a novel method to estimate the morphology of semicrystalline polymers, namely the average spherulite size, size distribution, and nucleus density based on experimental crystallization conversion curves recorded by differential scanning calorimetry (DSC). A fast and accurate probabilistic numerical simulation method describing random nuclei formation and subsequent growth is introduced to handle the crystallization process of polymers. The developed method was used in a curve-fitting procedure with experimental crystallization curves to obtain structural parameters of neat and nucleated isotactic polypropylene. The simulated morphology was used with success to predict haze of injection-molded samples, proving the reliability and applicability of our new approach. The method presented here can be a useful technique to construct the spherulitic morphology based on crystallization conversion curves and predict properties dependent on the spherulite size, size distribution, and nucleus density, even when microscopic measurement cannot be carried out.

Tensile deformation mechanism of propylene-1-butene random copolymer: The role of initial crystalline morphology

Structural evolution of two propylene 1-butene random copolymer (iPPBu) films with different initial crystalline morphology during uniaxial deformation were investigated by synchrotron wide-angle and small-angle X-ray scattering (WAXS/SAXS), Fourier-transform infrared (FTIR) spectroscopy. The iPPBu with α crystal crystallized via mesophase exhibits a nodular morphology, while melt-crystallized iPPBu exhibits a characteristic spherulitic structure. The initial crystalline morphology has been found to exert influence on the structural evolution of the samples, including cavitation, orientation behavior of molecular chains in crystalline and amorphous phases, the occurrence of mesophase, and reduction in crystallinity, etc. Compared with the iPPBu with spherulitic morphology, no obvious cavitation was observed around the yield point in iPPBu with nodular morphology. Moreover, the variation rate of crystallinity with respect to draw ratio is smaller in the iPPBu with nodular morphology. A plausible reason for the phenomenon is that the iPPBu with nodular morphology has the lower lateral extension of crystallites. The onset draw ratio (λonset) of α-crystal-to-mesophase transition in sample with nodular morphology is larger than that in the sample with spherulitic morphology. The molecular chains tend to be aligned perpendicular to the stretching direction in iPPBu with spherulitic morphology around yield point. This is attributed to the bending and separation of the lamellae with the normal parallel to the tensile direction. However, this phenomenon is not observed in the iPPBu with nodular morphology. In the strain-hardening regime, the strain hardening modulus Gp of iPPBu with nodular morphology is higher than that of iPPBu with spherulitic morphology, which results from the more cross-linking points and more tie chains in highly oriented the amorphous phase of the former. A deformation mechanism is proposed to understand the role played by the initial morphology of iPPBu based on the crystal transition, variation in crystallinity, cavitation, and orientation.

Eggshell derived nano-hydroxyapatite incorporated carboxymethyl chitosan scaffold for dentine regeneration: A laboratory investigation.

To assess odontogenic differentiation abilities of porous biomineralizable composite scaffolds comprising eggshell derived nano-hydroxyapatite (HAnp) and carboxymethyl chitosan (CMC) on cultured human dental pulp stem cells (hDPSCs). Nano-hydroxyapatite was derived from eggshells using a simple combustion method and CMC was prepared from chitosan through a chemical route. Several compositions of HAnp-CMC (0:5, 5:0, 1:5, 2:5, 3:5, 4:5 and 1:1 w/w%) scaffolds were prepared by magnetic stirring and freeze-drying methods. HAnp-CMC scaffolds were characterized using high-resolution scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and X-ray diffraction methods. In vitro bioactivity was determined following the interaction in simulated body fluid for 21days. The optimized composite was then loaded onto hDPSCs to assess cell viability/proliferation, dentine sialophosphoprotein (DSPP) and vascular endothelial growth factor (VEGF) expressions using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay, real-time quantitative polymerase chain reaction and flow cytometry methods, respectively, following 7, 14 and 21days. For intergroup and intragroup comparisons, Kruskal-Wallis and Friedman tests were employed, respectively, followed by appropriate post hoc test (Dunn). Significant levels were set at *p<.05 and *p<.01. Synthesized hydroxyapatite (HAp) comprised crystals ranging from 20 to 50nm (HAnp) with spherulite morphology and calcium/phosphorus (Ca/P) molar ratio of 1.67. The ultrastructure of all the scaffolds revealed a highly interconnected porous microstructure, whilst the chemical characterization displayed specific functional groups of both HAnp and CMC. In vitro bioactivity assessment confirmed the biomineralization potential of all scaffolds with an apatite-like crystal formation on the surface. The 1:5 HAnp-CMC revealed a favourable pore size (60-180µm) that was suitable for cell seeding and was chosen for further experiments. Cell viability/proliferation rates of hDPSCs loaded 1:5 HAnp-CMC at 21st day was significantly greater than that at 7th day (p<.05). The mean relative quantification of DSPP expression by the scaffold was significantly higher (p<.05) on day 21 (3.16) than on day 7 (1.67). Mean fluorescence intensity of the VEGF expression at day 21 (32.5) was also significantly higher (p<.01) than at day 7 (12.54). hDPSCs on 1:5 HAnp-CMC scaffolds displayed increased cell viability/proliferation and enhanced DSPP as well as VEGF expressions. The 1:5 HAnp-CMC composite has the potential to serve as a promising scaffold for dentine regeneration.

Crystallization and Morphology of Triple Crystalline Polyethylene-b-poly(ethylene oxide)-b-poly(ε-caprolactone) PE-b-PEO-b-PCL Triblock Terpolymers.

The morphology and crystallization behavior of two triblock terpolymers of polymethylene, equivalent to polyethylene (PE), poly (ethylene oxide) (PEO), and poly (ε-caprolactone) (PCL) are studied: PE227.1-b-PEO4615.1-b-PCL3210.4 (T1) and PE379.5-b-PEO348.8-b-PCL297.6 (T2) (superscripts give number average molecular weights in kg/mol and subscripts composition in wt %). The three blocks are potentially crystallizable, and the triple crystalline nature of the samples is investigated. Polyhomologation (C1 polymerization), ring-opening polymerization, and catalyst-switch strategies were combined to synthesize the triblock terpolymers. In addition, the corresponding PE-b-PEO diblock copolymers and PE homopolymers were also analyzed. The crystallization sequence of the blocks was determined via three independent but complementary techniques: differential scanning calorimetry (DSC), in situ SAXS/WAXS (small angle X-ray scattering/wide angle X-ray scattering), and polarized light optical microscopy (PLOM). The two terpolymers (T1 and T2) are weakly phase segregated in the melt according to SAXS. DSC and WAXS results demonstrate that in both triblock terpolymers the crystallization process starts with the PE block, continues with the PCL block, and ends with the PEO block. Hence triple crystalline materials are obtained. The crystallization of the PCL and the PEO block is coincident (i.e., it overlaps); however, WAXS and PLOM experiments can identify both transitions. In addition, PLOM shows a spherulitic morphology for the PE homopolymer and the T1 precursor diblock copolymer, while the other systems appear as non-spherulitic or microspherulitic at the last stage of the crystallization process. The complicated crystallization of tricrystalline triblock terpolymers can only be fully grasped when DSC, WAXS, and PLOM experiments are combined. This knowledge is fundamental to tailor the properties of these complex but fascinating materials.

Improvement of high voltage direct current material properties upon tailoring the morphology of crosslinked polyethylenes

Crosslinked linear low-density polyethylene (XLPE) is the most common polymeric cable insulation material for high-voltage applications with a high number of operating hours in high voltage alternating current (HVAC) systems. High voltage direct current (HVDC) power transmission and polymeric cable systems play a major role in the future and raise, besides numerous systemic benefits, challenges in the design of material properties. The main issue is injection and trapping of space charges in insulation materials under DC-fields. The objective of this work is to increase knowledge of the interplay between microstructure and material performance of XLPE under DC by tailoring its morphology beyond the capabilities of “common crystallization kinetics” upon constrained crystallization at certain elongations. It was found that the tailored oriented morphology influences the energetic depth of traps and a significant reduction of space charge density occurs. Moreover, the optimized oriented morphology leads to a significant reduction of field enhancement for field strengths ELaplace ≥ 20 kV mm−1 compared to unoriented XLPEs with spherulitic morphology. It is shown that this way of morphology tailoring results in a considerable, material dependent reduction of field exposure by a factor of 4, which promises a significant improvement in the electrical life time of polymeric insulation material used.

A new archaeobotanical proxy for plant food processing: Archaeological starch spherulites at the submerged 23,000-year-old site of Ohalo II

Archaeological starch spherulites discovered at the submerged 23,000-year-old site of Ohalo II, Sea of Galilee, Israel, provide a new line of archaeobotanical evidence for plant food processing. Six-hundred and thirty-two (632) starch spherulites were recovered from four stone implements. The analysis of starch spherulites from reliable archaeological contexts is a breakthrough that will potentially allow the identification of a range of plant processing and wet cooking activities. Our work provides a baseline for starch spherulite extraction and identification protocols. They were confirmed as spherulites using Lugol's iodine confirmation and the optical properties of the polarisation cross when rotated on a microscope stage under cross-polarised light. Their identification as starch spherulites was based on their archaeological context, size range and optical (polarized light and SEM) characteristics (including partial iodine uptake). Following our promising results, we encourage researchers to describe archaeological starch spherulite morphology and elemental characteristics in as much detail as possible. Future experimental archaeology work may find that these details provide evidence for how starch spherulites were formed through plant processing and cooking techniques.

Miscibility and enzymatic degradability of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)-based polyester blends by PHB depolymerase and lipase

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) containing 5 and 17 mol% 3-hydroxyhexanoate has been blended with atactic poly(3-hydroxybutyrate) (PHB), polycaprolactone (PCL), and polybutylene succinate (PBS) at different ratios by the solvent-casting method. PHBH-5%/atactic PHB and PHBH-17%/atactic PHB showed a single glass-transition temperature (Tg), indicating miscibility in the amorphous state. Immiscibility was observed in the PHBH-5%/PCL and PHBH-17%/PCL blends based on the non-transitioning Tg of the PHBH component, presence of two distinct Tg values for some of the blend films, and individual crystallization manner. The thermal properties and spherulitic morphologies of the PHBH-5%/PBS blend films suggested partial miscibility. The enzymatic degradability of the three types of blends by Ralstonia pickettii T1 PHB depolymerase and Burkholderia cepacia lipase was investigated. Depending on the enzyme–substrate specificity, miscibility, and phase structure of the blend, the variation of the enzymatic degradation rate with the blend ratio was different among the three types of blends. The weight loss rate of PHBH-5%/atactic PHB was higher than that of PHBH-17%/atactic PHB when degraded by PHB depolymerase. The PCL component accelerated the weight loss rate of PHBH-17%/PCL but decreased that of PHBH-5%/PCL. Lipase barely eroded PHBH-5%/PBS, but the weight loss rate increased as the PCL content increased. From quantitative analysis of the water-soluble products liberated during degradation, the degradation manners of the blends by PHB depolymerase and lipase are discussed.

Microstructural evidence of the toughening mechanisms of polyurethane reinforced with halloysite nanotubes under high strain-rate tensile loading

In this study, we have investigated the relationship between the spherulitic morphology and the dynamic tensile response of polyurethane reinforced with Halloysite nanotubes (HNTs). The polyurethane prepolymer is partially silane end-capped and filled with only 0.8 wt.% of acid-treated Halloysite nanotubes. The resultant nanocomposite material presents a 35% higher spall strength compared to the neat polyurethane and 21% higher fracture toughness. We show evidence that the HNTs are not the toughening phase in the nanocomposite, but rather it is their influence on the resultant spherulitic structures which alters the polymer microstructure and leads to a tougher dynamic response. Microstructural characterization is performed via Scanning Electron Microscopy, Atomic Force Microscopy and Field Emission Scanning Electron Microscopy, and crystallinity examination via X-ray diffraction. The spherulitic structures present a brittle fracture character, while the interspherulitic regions are more ductile and show large deformation. The nanocomposite presents a finer and more rigid spherulitic structure, and a more energy dissipative fracture mechanism characterized by a rougher fracture surface with highly deformed interspherulitic regions.

Superior thermal stability and fast crystallization behavior of a novel, biodegradable \u03b1-methylated bacterial polyester

Given their ubiquity in modern society, the development of biodegradable and renewably sourced plastics is essential for the creation of an environmentally sustainable society. One of the drawbacks for currently available biodegradable plastics such as poly(l-lactic acid) (PLLA) and polyhydroxyalkanoates (PHAs) is that it is difficult to simultaneously achieve mechanical flexibility and certain crystallization behavior in these materials, which limits their use as replacements for established petroleum-based plastics such as isotactic polypropylene (iPP). Here, we report the synthesis and characterization of a new biodegradable plastic, poly(3-hydroxy-2-methylbutyrate) [P(3H2MB)], which is a member of the bacterial PHA family whose members include an α-methylated monomer unit. Biosynthesis of P(3H2MB) was achieved using recombinant Escherichiacoli expressing an engineered pathway. Biosynthesized P(3H2MB) exhibited the highest melting temperature (197 °C) among the biosynthesized PHAs and improved thermal resistance. It also exhibited improved crystallization behavior and mechanical flexibility nearly equal to those of iPP. The primary nucleation rate of P(3H2MB) was faster than that of P(3HB), and the spherulite morphology of P(3H2MB) was much finer than that of P(3HB). This crystal morphology may result in more rapid crystallization behavior, increased transparency, and enhanced mechanical properties. The superior physical properties of P(3H2MB) have the potential to open new avenues for the production of high-performance biodegradable plastics for replacing petroleum-based bulk commodity plastics.

Unseeded, spontaneous nucleation of spherulitic magnesium calcite

Most of the sedimentary carbonates deposited in the marine environments are composed of calcium carbonate minerals with varying amounts of incorporated Mg2+. However, understanding how interactions of impurities with carbonate and their incorporation affect sediments behavior remains a challenge. Here, a new insight is obtained by monitoring solution composition, morphology, and electrokinetic potential of carbonate particles formed in a spontaneous unseeded batch precipitation experiment using electrochemical and scanning electron microscopy methods. The solid composition and growth rate are extracted from changes in the bulk composition and fitted to chemical affinity rate law, revealing that the precipitation pathway consists of second-order dissolution and first-order precipitation. The molecular dynamics simulations show that the lattice strain induced by randomly substituting Ca2+ by Mg2+ stabilizes spherical nanoparticles and reduces their surface area and volume.Combining kinetics and thermodynamics insight, we conclude that variation in the carbonate bulk and interfacial energies, along with the solution supersaturation, lead to the dissolution-precipitation transformation pathway from Mg-rich to Mg-poor carbonate phase that preserves spherulitic morphology. Our findings are relevant for long-standing questions of how impurities influence diagenesis of carbonate sediments and spherulitic carbonate particles' origin.

An effective nucleating agent for isotactic polypropylene (iPP): Zinc bis- (nadic anhydride) double-decker silsesquioxanes

The zinc bicyclo [2.2.1]-heptane-2, 3-dicarboxylate-double-decker silsesquioxanes (Zn2(DDSQ-ND)) was firstly synthesized and the crystallization behavior of isotactic polypropylene (iPP) filled with different amount of Zn2(DDSQ-ND) (0–1 wt %) was investigated in detail. With the addition of Zn2(DDSQ-ND), the crystallization temperature enhanced significantly, indicating that it acted novel and effective nucleating agent in the iPP system. The morphology of spherulites, nucleation density, and the crystal structure of the samples were characterized using POM and WAXD. Besides, ultra-fast scanning calorimetry (FSC) was employed to study the effects of Zn2(DDSQ-ND) on the non-isothermal and isothermal crystallization behaviors of the sample iPP/Zn2(DDSQ-ND) under the high cooling rates and wide crystallization temperature ranges, as well as the cold crystallization and recrystallization behavior during the heating process. The data suggested that Zn2(DDSQ-ND) accelerated the formation of α-phase iPP and acted as a novel α-nucleating agent in the iPP system.

Space–charge electret properties of polypropylene films with transcrystalline or spherulitic structures—A comparison of functionalities at interfaces

Spherulite-related space–charge electret properties of polypropylene (PP) have been widely discussed in the past decades. In the present paper, a less-common crystalline structure in PP—transcrystalline PP—is studied regarding its electret behavior in comparison with the typical spherulitic morphology. Polarized light microscopy and differential scanning calorimetry were employed to characterize the crystallite types and crystallinities of transcrystalline and spherulitic PP. Their electret functionality is investigated by means of thermally stimulated discharge experiments, where the cross-over phenomenon is observed on transcrystalline PP films, whereas surface-potential saturation and undercharging on the surface occur on the spherulitic samples. Besides, an asymmetrical behavior of positive and negative surface-charge stabilities is found on PP with spherulites, the negatively charged spherulitic surfaces show a better charge stability. It is shown that PP electrets are very sensitive to changes in the microscopic crystalline structures and their interfaces as well as in the molecular conformations controlled through adjustments of the respective processing steps. In addition, surface and bulk nanocomposites of PP or low-density polyethylene with inorganic particles are included in the comparison. In view of recent developments in the areas of PP-based electret-fiber filters and cellular-foam ferroelectrets, the observed changes in the charge-storage properties may have particular relevance, as the required film, fiber, or foam processing might significantly modify crystalline morphologies and nano-scale interfaces in PP electrets. Limitations in the charge-storage capabilities of interface structures may also be of interest in the context of high-voltage electrical-insulation materials where reduced space–charge accumulation and slightly increased charge transport can be advantageous.

АСМ исследование морфологии эластомеров на основе полиуретанимидов с различными жесткими и гибкими фрагментами

The morphological characteristics of the surface of a number of films of elastomers based on polyurethanimides (PUI) were studied by atomic force microscopy (AFM). Significant differences in the surface morphology of PUI films were found depending on the composition: from granular morphology with different grain sizes or spongy morphology with nano - and submicron pores and cavities on the surface, to spherulitic morphology. It is shown that the roughness of the free surfaces formed during the films formation from the solution is significantly lower than the roughness of the surface which was in contact with the substrate. The influence of the chemical structure and size of the rigid diamine fragments in PUI macromolecules on the morphology of the film surface and the presence of different phases in the AFM torsion mode is established. The comparison of the deformation and strength characteristics showed high values of the elongation at break (more than 1400 %) in the PUI films with a developed porous structure of both surfaces. The results obtained show the need to control and take into account the morphological parameters of the surface layers of products made of highly elastic imide-containing materials for their use for technical and other purposes.

Hydrolytic and enzymatic degradation of biobased poly(4-hydroxybutyrate) films. Selective etching of spherulites

Hydrolytic degradation of poly(4-hydroxybutyrate) (P4HB) films has been studied considering media of different pH values (i.e., 3, 7 and 10) and temperatures (i.e., 37 and 55 °C). Enzymatic degradation has also been evaluated at physiological conditions using two different lipases: Pseudomonas cepacia and Rhizopus oryzae.Different bulk and surface erosion mechanisms with random chain scissions and successive removal of monomer units have been supported through weight loss measurements, molecular weight determinations by GPC and NMR spectroscopy and changes on thermal properties by DSC. Thermal annealing during exposure to different media and even degradation influenced on the melting temperature and crystallinity of samples, as well as on the lamellar geometrical parameters as evaluated by SAXS. Enzymatic degradation was ideal to selectively eliminate the amorphous regions and highlight the spherulitic morphology. Presence of ringed textures were therefore evident in bright field optical micrographs in addition to SEM images, namely observations under polarized light was not necessary to distinguish the presence of banded spherulites. Rhizopus oryzae was revealed to be the most suitable enzyme to crop out the P4HB spherulites that form part of the initial smooth surfaces of solvent casting films. After determining the appropriate activity and exposure time, the presence of rings constituted by cooperative C-shaped edge-on lamellae and flat-on lamellae was highlighted.

Hydroxyl Groups Induce Bioactivity in Silica/Chitosan Aerogels Designed for Bone Tissue Engineering. In Vitro Model for the Assessment of Osteoblasts Behavior.

Silica (SiO2)/chitosan (CS) composite aerogels are bioactive when they are submerged in simulated body fluid (SBF), causing the formation of bone-like hydroxyapatite (HAp) layer. Silica-based hybrid aerogels improve the elastic behavior, and the combined CS modifies the network entanglement as a crosslinking biopolymer. Tetraethoxysilane (TEOS)/CS is used as network precursors by employing a sol-gel method assisted with high power ultrasound (600 W). Upon gelation and aging, gels are dried in supercritical CO2 to obtain monoliths. Thermograms provide information about the condensation of the remaining hydroxyl groups (400–700 °C). This step permits the evaluation of the hydroxyl group’s content of 2 to 5 OH nm−2. The formed Si-OH groups act as the inductor of apatite crystal nucleation in SBF. The N2 physisorption isotherms show a hysteresis loop of type H3, characteristic to good interconnected porosity, which facilitates both the bioactivity and the adhesion of osteoblasts cells. After two weeks of immersion in SBF, a layer of HAp microcrystals develops on the surface with a stoichiometric Ca/P molar ratio of 1.67 with spherulite morphology and uniform sizes of 6 μm. This fact asserts the bioactive behavior of these hybrid aerogels. Osteoblasts are cultured on the selected samples and immunolabeled for cytoskeletal and focal adhesion expression related to scaffold nanostructure and composition. The initial osteoconductive response observes points to a great potential of tissue engineering for the designed composite aerogels.

Detailed thermal analysis of crystallization kinetics and band morphology for future blending of absorbable poly(p‐dioxanone) monofilament surgical suture with poly(ε‐caprolactone)

AbstractThe thermal effects on crystallization kinetics and the band morphology of absorbable poly(p‐dioxanone) (PDS) were studied in detail with TGA, AFM, DSC and polarized optical microscopy techniques. The polymer film was crystallized with isothermal and non‐isothermal conditions over a wide temperature range, cooling from the melt at 140 °C (beyond the equilibrium melting temperature but below the degradation temperature). With the isothermal crystallization process, a well‐defined spherulitic morphology of the PDS polymer, linearly growing in all directions with time, was observed. A microstructure analysis by polarized optical microscopy and AFM revealed that the crystallized PDS shows large spherulites with band morphology, the nucleation density decreases and the spacing between the band increases with crystallization temperature. The periodic band morphology of the spherulites is related to edge‐on and flat‐on lamella orientation. The crystallization temperature dependent spherulite growth rate displayed a bell‐shaped curve, showing a highest spherulite growth rate at 60 °C, explained by classical nucleation theory. These results allowed a good understanding of the thermal and morphological properties of the PDS polymer over a wide temperatures range in both isothermal and non‐isothermal conditions. Additionally, the crystallization and melting behaviors of the PDS polymer were studied and compared for the future application of blending with other compatible polymers, e.g. poly(ε‐caprolactone). © 2020 Society of Industrial Chemistry

Unusual Spherulitic Morphology of Poly(propylene fumarate)

Spherulites are the most common crystalline morphology and thus the visual expression of crystal structures for polymers. The diversified patterns have provided intuitive morphology probes for various crystallization behaviors, while the correlations between them are still needed to be enriched. In this work, the complicated spherulitic morphology of poly(propylene fumarate) (PPF), which is sensitive to crystallization temperature, is investigated. PPF melt, respectively, crystallizes into rough spherulites, regularly banded spherulites, and spherulites containing both two kinds of morphology at low, high, and mediate temperatures. By systematically assaying, it is clear that the growth axis along the radial direction changes from a-axis to b-axis as the crystallization temperature increases, which leads to the formation of unique crystallization-temperature-dependent spherulites. Based on detailed characterization of Fourier transform infrared spectroscopy, the packing state of the specific hydrogen bonds of “C=C-H⋯O=C-C=C” in PPF crystal lattices is determined, and furthermore, the mechanism for temperature-dependent selection of growth axes for PPF spherulites in melt is reasonably speculated.

Multiple β Forms of Saturated Monoacid Triacylglycerol Crystals.

We have investigated the polymorphism of triacylglycerol (TAG) crystals as they affect the qualities such as shelf life, mouth feel, and texture of chocolate and other products. Saturated monoacid TAGs, like trilaurin, are considered as models for TAG crystallization; however, there is still debate about the number of their polymorphs that exist. In this study, we characterized a set of novel polymorphs, β forms of saturated monoacid TAGs, which were obtained via different pathways depending on the crystallization history, by polarized light microscopy, X-ray diffraction, and differential scanning calorimetry. Saturated monoacid TAGs were crystallized as the unstable polymorphs, the α or β’ forms first, and then they were transformed into β forms by solid–solid transformations. The β form that had transformed from β’ changed its morphology by a polymorphic transformation, while the β form made from the α form kept its spherulite morphology. The β forms obtained showed different melting points. Additional heat treatment promoted further polymorphic transformation. Four novel β forms were found for each of the saturated monoacid TAGs, trilaurin, trimyristin, tripalmitin, and tristearin. They showed similar polymorphism with the same subcell packing.

Dynamics affected memory for crystallization behaviors of poly (d-lactic acid)

Dynamics of polymer chains not only is a fundamental bridge between the structures and properties of polymers but also plays a crucial role in the formation of polymer structures. The slow crystallization caused low crystallinity of Poly (lactic acid) (PLA) is the bottleneck for its applications. The chain dynamics affected memory for recrystallization morphology and behavior of poly (d-lactic acid) (PDLA) were observed by POM and characterized by DSC. The POM images showed that the PDLA could outline the morphology of original banded spherulites after recrystallization from melting of the original spherulites. The re-formed crystal inherited the crystalline location from the original crystalline morphology, forming a circle ring morphology that is following up the initial banded spherulites, even though the circle ring was constructed of smaller spherulites. DSC results showed various crystallization behaviors of PLA recrystallization with different thermal history. The influence of heating temperature, keeping time, and cooling rate on the recrystallization morphology were investigated and discussed. It was proposed that the adjustment and change of PDLA chain conformation is more difficult in various states between the crystalline and melt than that of other polymers, which relates to the strength of melt and entropy.

Rheological rationalization of in situ nanofibrillar structure development: Tailoring of nanohybrid shish-kebab superstructures of poly (lactic acid) crystalline phase

The key rheological parameters for in situ development of poly (butylene terephthalate) nanofibrils (PBT NFs) in poly (lactic acid) (PLA) matrix using a tuned melt spinning process were studied. Observation of ellipsoidal PBT domains oriented along a short die was attributed to a low interfacial tension between the blend constituents and fulfilled melt rheological criteria for fibrillation in a simple shear flow. The stability of the deformed droplets under the shear and elongational flows was studied by comparing the droplet breakup time and residence time in each flow field. It was demonstrated that the deformed droplets (ellipses) with an increased robust interface underwent coalescence through pinch-off in the subsequent elongational flow field, and transformed to the nanofibrils with an average diameter comparable to the gyration radius (Rg) of PLA chains. In the fabricated isotropic in situ nanofibrillar composites (NFCs), a random orientation of 2D nanohybrid shish-kebab (NHSK) superstructure was observed as a result of the orthogonal pattern of melt crystallized PLA nanodomains on the PBT NFs surface. The NHSK superstructure established in quiescent bulk through mechanism of geometric confinement (soft epitaxy) enabled us to tailor the crystalline morphology via annealing temperature. In a constant nanofibril content, increasing the isothermal temperature (Tiso) up to 114 °C led to an increase in the kebabs length and interlocking of the adjacent superstructures. A transition to spherulitic crystal morphology was observed due to further temperature increase (Tiso≥117 °C). The observed transition indicated that at higher Tiso, the fewer raw nuclei, which were initiated by soft epitaxy mechanism of PLA chain segments on the PBT NFs at the early stage of nucleation, could conduct self-nucleation and 3D lateral growth. The Avrami model was found to be applicable for interpreting variations of crystallite morphology, crystallization rate, and nucleation mechanisms with PBT NF content and isotherm temperature.