Morphology, reorganization and stability of mesomorphic nanocrystals in isotactic polypropylene

The morphology and thermodynamic stability of crystals of isotactic polypropylene (iPP) were analyzed as a function of the path of crystallization by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Samples were melt-crystallized at different rates of cooling using a “controlled rapid cooling technique”, and subsequently annealed at elevated temperature. Mesomorphic equi-axed domains with a size less than 20 nm were obtained by fast cooling from the melt at a rate larger about 100 K s⁻¹. These domains stabilize on heating by growing in chain direction and cross-chain direction, to reach a maximum size of about 40-50 nm at a temperature of 433 K, with the quasi-globular shape preserved. Annealing at 433 K additionally triggers formation of different types of lamellae. It is suggested that these lamellae either develop by coalescence of nodules, or by recrystallization from the melt. The transition from the disordered mesomorphic structure, evident at ambient temperature after fast crystallization, to monoclinic structure on heating at about 340 K occurs at local scale within existing crystals, and cannot be linked to complete melting of mesomorphic domains and recrystallization of the melt. The temperature of melting of initial mesomorphic domains, after reorganization at elevated temperature, is identical to the temperature of melting of rather perfect lamellae, obtained by initial slow melt-crystallization, followed by annealing. The close-to-identical temperatures of melting of these crystals of largely different shapes are confirmed by model calculations, using the Gibbs-Thomson equation. Modeling of the melting temperature reveals that nodular crystals, stabilized by annealing at high temperature, exhibit a similar fold-surface as lamellar crystals.     

Direct analysis of annealing of nodular crystals in isotactic polypropylene by atomic force microscopy, and its correlation with calorimetric data

The process of isothermal annealing of nodular monoclinic crystals of isotactic polypropylene (iPP) was analyzed by atomic force microscopy (AFM) and temperature-modulated differential scanning calorimetry (TMDSC). Initially nodular and mesomorphic domains were obtained by controlled melt-crystallization at high cooling rate. Subsequent heating triggers transition from mesomorphic to monoclinic structure, and melting of unstable nodules. Annealing allows re-crystallization, which is recognized by enlargement of domains from initially about 20 nm to about 35 and 55 nm after annealing at 393 and 433 K, respectively. Furthermore, the re-crystallization process is connected with a slight change of the aspect ratio of crystals. The isothermal re-crystallization of the liquid is superimposed by aggregation of crystals, to yield blocky, and string-like objects. The direct analysis of structure on isothermal annealing by AFM is for the first time compared with the isothermal decrease of the apparent specific heat capacity, or change of enthalpy, monitored by TMDSC. The apparent specific heat capacity decreases during annealing with an identical non-linear time dependence as the directly observed growth of the crystal size. Analysis of the annealing processes at different temperatures yields proportionality between the increase of the crystal size and the reduction of the apparent specific heat capacity.     

Solid-state reorganization,melting and melt-recrystallization of conformationally disordered crystals (α′-phase) of poly (l-lactic acid)

Conformationally disordered α′-crystals of poly (l-lactic acid) were formed by crystallization of the melt at high supercooling at 95 °C. Analysis of their melting temperature as a function of the crystallinity revealed absence of crystal thickening during isothermal crystallization. Annealing of α′-crystals between the crystallization temperature of 95 °C and their zero-entropy production melting temperature of 150 °C leads to their stabilization, mainly by solid-state reorganization. Heating faster than 30 K s⁻¹ suppresses reorganization and permits superheating of the α′-phase. Consequently, isothermal melting followed by melt-recrystallization becomes accessible. Melting is completed within few hundreds of milliseconds, and melt-recrystallization is about two orders of magnitude faster than crystallization of the isotropic melt at identical temperature. The time required for melting decreases with superheating and increases with the lateral dimension of the lamellar crystals. Laterally extended lamellae require long time for melting of the outer crystal layers, which allows stabilization of the central crystal part. These crystal remnants then serve as seed for immediate recrystallization. In case of complete melting of smaller lamellae, melt-recrystallization is retarded but still distinctly faster than cold- and melt-crystallization, due to incomplete isotropization of the melt.     

Nanopatterned surfaces obtained with semicrystalline ABC triblock copolymers

The surface patterns resulting from fast crystallization of as-cast and annealed thin films (ca. 100 nm) of two polystyrene-b-polybutadiene-b-poly(ε-caprolactone) ABC triblock copolymers is investigated through atomic force microscopy (AFM). Two different substrates are used: silicon and mica. The behaviour is compared with the bulk morphology obtained through transmission electron microscopy (TEM). AFM images of the as-cast films revealed surfaces with lamellar patterns. Based on the observation of T-shaped grain boundaries between lamellae, and on a comparison of the microdomain dimensions obtained by TEM and AFM, the surface pattern is rationalized as being formed by amorphous and crystalline polycaprolactone (PCL), with the PCL/PB block copolymer interfaces located parallel to the susbstrate. The formation of islands and holes in annealed films with a lamellar ‘floor’, depending on the conmensurability between film thickness and long period, is also observed, indicating a parallel orientation of the block copolymer lamellae.     

Crystallization by post-treatment of reactive r.f.-magnetron-sputtered carbon nitride films

Carbon nitride (CN_x) films have been deposited on single crystal ZrO₂(100) substrates by reactive r.f. magnetron sputtering. The effect of thermal annealing at 900°C on the structural properties of the films has been studied by atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). AFM results show a fine spread of well-defined grains with an average size of 500 nm exhibited by the post-annealed films. TEM images confirmed that thermal annealing has transformed the predominantly amorphous as-deposited films into crystalline phases. The surface morphology observed is composed of carbon nitride nanofibres and p-C₃N₄ crystals in a less nitrogenated amorphous matrix. The size of the crystals ranges from about 0.05 to 1.5 μm. The smallest of the nanofibres appear to be approximately 20 nm in external diameter. Selected area electron diffraction indicates the fibre walls to be crystalline in nature. N 1s peaks in XPS spectra of the annealed films indicate the presence of two different bonding states, one attributed to nitrogen inserted into the graphitic ring structure, and the other attributed to nitrogen surrounded by three carbons in the NC network.     

Template-directed hydrothermal synthesis of dandelion-like hydroxyapatite in the presence of cetyltrimethylammonium bromide and polyethylene glycol

A template-directed synthetic method, using surfactant cetyltrimethylammonium bromide (CTAB) as a template and co-surfactant polyethylene glycol (PEG600) as a co-template under hydrothermal conditions, has been applied to obtain dandelion-like HAp. The morphology, size, crystalline phase, chemical composition, physical characteristics, and thermal behavior of the product were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), Fourier ransform infrared spectroscopy (FTIR), induced coupled plasma spectroscopy (ICP), BET (Brunauer, Emmett, and Teller) method, and simultaneous thermal analysis (STA). SEM and TEM observations showed in the presence of CTAB and a certain concentration of PEG600 (30%) HAp crystals have a uniform dandelion-like morphology with a diameter of about 80–150 nm and aspect ratio of about 20 for each tooth. Dandelion-like HAp crystals have a high surface area of 88 m² g⁻¹ showing potential applications. The template action of CTAB and co-template action of PEG600 are also discussed.     

Mesophase formation in poly(propylene-ran-1-butene) by rapid cooling

The effect of random insertion of low amount of 1-butene of less than about 11 mol% into the isotactic polypropylene chain on structure formation at non-isothermal crystallization at different rate of cooling was investigated by X-ray scattering, density measurements, and atomic force and polarizing optical microscopy. Emphasis is put on the evaluation of the condition of crystallization for replacement of lamellar crystals by mesomorphic nodules on increasing the cooling rate/supercooling. In the polypropylene homopolymer, mesophase formation occurs on cooling at rates larger about 150–200 K s⁻¹, while in case of poly(propylene-ran-1-butene) mesophase formation is observed on cooling at a lower rate of about 100 K s⁻¹. It is suggested that the lowering of the critical rate of cooling for mesophase formation in poly(propylene-ran-1-butene) is due to a reduction of the maximum rate of formation of monoclinic/orthorhombic crystals at low supercooling, compared to the homopolymer. The data of the present study allowed the establishment of a non-equilibrium phase diagram which shows ranges of existence of phases as a function of the cooling rate on solidification the quiescent liquid and the concentration on 1-butene co-units.     

Study on morphology and crystal growth of syndiotactic polystyrene solution-grown crystals by transmission electron and atomic force microscopy

The morphology of solution-grown crystals (SGCs) of syndiotactic polysthyrene (s-PS) has been investigated using transmission electron and atomic force microscopy (TEM and AFM). Well-defined s-PS SGCs were prepared from 0.005% (w/w) n-tetradecane/decahydronaphthalene solution 2:1 (v/v) by an isothermal crystallization method at 200 °C. Lattice constants for this crystal estimated by TEM with the diffraction mode are a=0.90 nm, b=2.88 nm. AFM images of the folded chains on the surface of s-PS SGC suggested their alignment parallel to the direction of the growth face of the truncated-lozenge shaped crystal. The angle between the growth faces obtained from s-PS SGCs was 130°. Since, the value of 130° correspond to the {230} angle, the primary growth face of s-PS SGC was assigned to the (230) plane.     

Controlled crystal nucleation in the melt-crystallization of poly(l-lactide) and poly(l-lactide)/poly(d-lactide) stereocomplex

Among the various inorganic nucleators examined, Talc and an aluminum complex of a phosphoric ester combined with hydrotalcite (NA) were found to be effective for the melt-crystallization of poly(l-lactide) (PLLA) and PLLA/poly(d-lactide) (PDLA) stereo mixture, respectively. NA (1.0 phr (per one hundred resin)) can exclusively nucleate the stereocomplex crystals, while Talc cannot suppress the homo crystallization of PLLA and PDLA in the stereo mixture. Double use of Talc and NA (in 1.0 phr each) is highly effective for enhancing the crystallization temperature of the stereo complex without forming the homo crystals. The stereocomplex crystals nucleated by NA show a significantly lower melting temperature (207 °C) than the single crystal of the stereocomplex (230 °C) in spite of recording a large heat of crystallization ΔH_c (54 J/g). Photomicrographic study suggests that the spherulites with a symmetric morphology are formed in the stereo mixture added with NA while the spherulites do not grow in size in the mixture added with Talc. The exclusive growth of the stereocomplex crystals by the melt-crystallization process will open a processing window for the PLLA/PDLA.     

Microscopic surface and bulk morphology of semicrystalline poly(dimethylsiloxane)–polyester copolymers

In this study, two series of semicrystalline poly(dimethylsiloxane) (PDMS)–polyester segmented copolymers with various PDMS contents were synthesized. One series was based on polybutylene adipate (PBA) as the polyester segment and the other was based on a polybutylene cyclohexanedicarboxylate ester (PBCH) segment. The copolymers were characterized using ¹H‐nuclear magnetic resonance, size exclusion chromatography, dynamic mechanical analyses, differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The microscopic surface morphology and the microscopic bulk morphology were investigated using atomic force microscopy (AFM) and transmission electron microscopy, respectively. The effects of the polyester type and the PDMS content on the crystallinity degree as well as the copolymer surface and bulk morphology at room temperature were investigated for each series. DSC and WAXD results showed the ability of the copolymers to crystallize, to various degrees, depending on the polyester type and the PDMS content. The results showed that the PDMS content had a greater influence on the crystallinity degree in the PDMS‐s‐PBCH (cycloaliphatic) copolymer series than in the PDMS‐s‐PBA (aliphatic) copolymer series. In the copolymers with a low PDMS content, the AFM images showed spherulitic crystal morphology and evidence of PDMS nanodomains in between the crystal lamellae of the ester phase on the copolymer surface. A heterogeneous distribution of the PDMS domains was also observed for these copolymers in the bulk morphology as a result of this segregation between the polyester lamellae. All the copolymers, in both series, showed microphase separation as a result of the incompatibility between the PDMS segment and the polyester segment. Three types of surfaces and bulk morphologies were observed: spherical microdomains of PDMS in a matrix of polyester, bicontinuous double‐diamond type morphology, and spherical microdomains of polyester in a matrix of PDMS as the PDMS content increases. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

In situ AFM study of near‐surface crystallization in PET and PEN

The surface crystallization behavior of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6‐naphthalate) (PEN) spin‐coated thin films was compared by means of atomic force microscopy (AFM) with an in situ heating stage. As the films were heated up stepwise, characteristic surface crystals appeared at a crystallization temperature (T_c) in the near‐surface region which is about 15 °C under the bulk T_c, and were replaced by bulk crystals when the temperature was increased to the bulk T_c. In the case of films whose thickness is less than 70 nm (PET) and 60 nm (PEN), significant increases in the bulk T_c were observed. Scanning force microscopy (SFM) force‐distance curve measurements showed that the glass transition temperature (T_g) of the near‐surface region of PET and PEN were 22.0 and 26.6 °C below their bulk T_g (obtained by DSC). After the onset of surface crystallization, edge‐on and flat‐on crystals appeared at the free surface of PET and PEN thin films, whose morphologies are very different to those of the bulk crystals. Although the same general behavior was observed for both polyesters, there are significant differences both the influence of the surface and substrate on the transition temperatures, and in morphology of the surface crystals. These phenomena are discussed in terms of the differences in the mobility of polymer chains near the surface. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44269.     

Effect of pressure on kaolinite nanomorphology under wet and dry conditions: Correlation with other kaolinite properties

The objectives of this investigation were to evaluate the change produced by isostatic pressure on nanomorphology of kaolins and to determine the effect of the presence or absence of water during these experiments. Nanomorphology was evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM), including roughness measurements by AFM height data. Nanomorphological changes were compared with variations in other kaolins properties as particle size distribution, fractal dimension, specific surface area and kaolinite structural order by X-ray diffraction (XRD).Nanomorphology changed by confined pressure, but the behaviour was different if the compaction was performed in the absence or presence of water. During pressure dry kaolinite crystals gradually lost their pseudohexagonal morphology, edges were rounded and books and vermicular stacks were fractured and distorted. On the contrary, kaolinite morphology did not show any change under wet conditions by SEM and TEM. AFM revealed less irregular edges of kaolinite crystals and a low amount of deformed crystals.In general, isostatic pressure in dry conditions increases roughness whereas in wet conditions roughness decreased (on average by 40 mass%). The nanomorphology changes of kaolinite were inversely correlated with the variation of the structural order as determined by XRD. The specific surface area decreased under pressure treatment especially in wet conditions.     

Influence of chemical structure on enzymatic degradation of single crystals of PCL-b-PEO amphiphilic block copolymer

Solution-grown lamellar single crystals of PCL homopolymer and amphiphilic block copolymers of PCL-b-PEO-b-PCL and PCL-b-PEO-FG (functional groups FG = NH₂, OCH₃) were prepared by self-seeding procedures. The crystal structure and morphology of these single crystals were mainly studied by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results indicated that the shorter PEO blocks as well as the functional groups bonded to the end of PEO block are in an amorphous state and located in the surface of single crystals. The enzymatic degradation of single crystals prepared from these homopolymer and amphiphilic block copolymers has been well-studied to demonstrate the effects of chemical structure on degradation behavior. The single crystals showed similar morphologies before enzymatic degradation but very different surface character after enzymatic degradation. Such differences resulted from the PEO block and functional end groups. The results of this work indicated the important role of chemical structure in determining biodegradation behavior.     

Electrochemical Deposition of 58SiO2‐33CaO‐9P2O5 Nanobioactive Glass Particles on Ti‐6Al‐4V Alloy for Biomedical Applications

The nanobioactive glass (58SiO₂‐33CaO‐9P₂O₅) powders were synthesized by simple sol–gel method. The prepared samples reveal amorphous nature, agglomerated spherical morphology with particle size of 100–150 nm. The specific surface area of nanobioactive glass (NBG) particle is 147 m²/g. The NBG samples were coated on titanium (Ti‐6Al‐4V) alloy through electrochemical deposition method. The particle size of the NBG‐coated surface was in the order of 200–300 nm, and it was confirmed by atomic force microscopy (AFM) analysis. In vitro and AFM studies reveal the existence of higher bioactivity and uniform coating of NBG on implants at 80 V for 1 h.     

Synthesis and characterization of poly(ethylene glycol)-b-poly(l-histidine)-b-poly(l-lactide) with pH-sensitivity

In this paper, amphiphilic biodegradable methoxy-poly(ethylene glycol)-poly(l-histidine)-poly(l-lactide) (mPEG–PH–PLLA) triblock copolymers with pH sensitivity were synthesized. The properties of mPEG–PH–PLLA triblock copolymers were investigated by GPC, ¹H NMR, DSC, TGA, XRD and polarized optical microscopy. The results showed that the thermal properties of the triblock copolymers varied with the chain length of PH blocks. The glass transition temperatures (Tg) of the triblock copolymers increased with increasing poly(l-histidine) chain length. The morphologies of PLLA crystals changed from spherulite to dendritic crystal. Moreover, the crystallization rate of triblock copolymers was faster than that of PLLA homopolymer. The pH sensitivity of the self-assembled mPEG–PH–PLLA nanoparticles was investigated. The mean diameter and morphology of the nanoparticles were characterized by DLS, AFM and TEM. The results showed that the mean diameter of mPEG₄₅–PH₃₀–PLLA₈₂ nanoparticles in pH = 5.0 was smaller than that in pH = 7.4.     

Crystallization behavior and morphology of poly(ethylene 2,6-naphthalate)

The crystallization behavior and morphology of poly(ethylene 2,6-naphthalte) (PEN) were investigated by means of differential scanning calorimetry (DSC), polarized optical microscopy (POM) and transmission electron microscopy (TEM). POM results revealed that PEN crystallized at 240 °C shows the coexistence of α and β-form spherulite morphology with different growth rates. In particular, when PEN crystallized at 250 °C, the morphology of spherulites showed a squeezed peanut shape. The Avrami exponents decreased from 3 to 2.8 above the crystallization temperature of 220 °C, indicating a decrease in growth dimension. Analysis from the secondary nucleation theory suggests that PEN crystallized at 240 °C has crystals with both regime I and regime II. In TEM observation, the ultra-thin PEN film crystallized at 200 °C showed the spherulitic texture with characteristic diffractions of α-form, while PEN crystallized at 240 °C generated an axialite structure with only β-form diffraction patterns. In addition, despite a long crystallization time of 24 h, amorphous regions were also observed in the same specimen. It was inferred that the initiation of PEN at 240 °C generates only β-form crystals from axialite structures.     

Synthesis of statistical PET/PEN random block copolymers and their crystallizability in the bulk and at the surface

A series of PET/PEN copolyesters were synthesized by molten transesterification. The degree of randomness and the sequence length of the copolymers were determined by ¹H NMR spectroscopy, and the changes in the bulk glass transition temperature (T_gB), bulk crystallization temperature (T_cB), and bulk melting temperature (T_mB) were observed by DSC. A clear relationship was obtained between the observed enthalpy of melting (ΔH_m) and degree of randomness (B), and T_mB was suppressed for the midcompositions. As with their homopolymer counterparts, there was significant depression in crystallization temperature at the surface (T_cS) of the random/block copolymers compared with the bulk, and so surface‐localized crystallization could be induced in spin‐coated thick films (thickness ranging from ca. 400 to 700 nm) by annealing at a temperature in which the surface region is mobile, but the bulk is not. The formation of these clear surface crystals allows the morphology to be directly imaged by AFM, and we observed that the crystallizability and the lamellar morphology of the surface crystals deviated from those of the original homopolymers, depending on the mixing ratio of PET/PEN and degree of randomness. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46515.     

Preparation of NaA Zeolite Membranes Using Poly(Ethyleneimine) as Buffer Layer,and Study of Their Permeation Behavior

Zeolite NaA membranes were prepared hydrothermally by secondary crystallization process at different temperatures (55°C–75°C) on porous α‐alumina‐based support tubes (inner side) precoated with poly(ethyleneimine) (PEI) buffer layer and NaA seed particles. The NaA seed crystals synthesized at 65°C/2 h in the size range 100–200 nm having BET surface area of 71.57 m²g^?1 were used for secondary crystallization of the membranes. The secondary crystallization at 65°C for (4 + 4) h (double‐stage) showed highly dense NaA grains in the microstructure of the membrane with a thickness of 5 μm. It rendered the permeance values of 50.6 × 10^?8, 2.47 × 10^?8, and 0.55 × 10^?8 molm^?2s^?1Pa^?1 for H₂, N₂, and CO₂, respectively, with their permselectivity of 20.48 (H₂/N₂), 92 (H₂/CO₂), and 4.49 (N₂/CO₂). A tentative mechanism was illustrated for the interaction of PEI with the support substrate and NaA seed crystals.     

Crystallization, and morphology of poly(trimethylene terephthalate)/poly(ethylene oxide terephthalate) segmented block copolymers

A new type of poly(ether-ester) based on poly(trimethylene terephthalate) as rigid segments and poly(ethylene oxide terephthalate) as soft segments was synthesized and its crystallization behavior and morphology were investigated. Differential Scanning Calorimetry revealed that the copolymer containing 57 wt% soft segments presented a low glass transition temperature (−46.4 °C) and a high melting temperature (201.8 °C), suggesting that it had the typical characteristic of thermoplastic elastomer. With increasing soft segment content from 35 to 57 wt%, the crystallization morphology transformed from banded spherulites to compact seaweed morphology at a certain film thickness, which was due to the change of surface tension and diffusivity caused by increasing the soft segment content. Moreover, with the decrease of film thickness from 15 to 2 μm, the crystallization morphology of the copolymer (57 wt% soft segment) changed from wheatear-like, compact seaweed to dendritic. Scanning Electron Microscopy revealed that some flower-like crystals presenting in the bulk, which had been surprisingly found in the poly(ether-ester) segmented block copolymers for the first time. Possible mechanism was discussed in the text.     

Effects of hard and soft components on the structure formation,crystallization behavior and mechanical properties of electrospun poly(l-lactic acid) nanofibers

The effects of multi-wall carbon nanotubes (MWCNTs) and poly(ethylene oxide) (PEO) on the structure formation, morphology, crystallization behavior and mechanical property of electrospun poly (l-lactic acid) (PLLA) nanofiber mats were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC) and mechanical test. If incorporate hard filler, MWCNTs into electrospun PLLA nanofiber, the crystallinity, chain orientation, and crystallization behaviors were almost not influenced by the MWCNTs content owing to the MWCNTs mainly acted as impeding the crystal growth and chain diffusion. If incorporate small content of soft and miscible component, PEO (10 wt%) into the electrospun PLLA and PLLA/MWCNTs nanofibers, the crystallinity and crystallization rate of PLLA in nanofibers were obviously enhanced. The synergistic effect of PEO and MWCNTs in PLLA nanofibers was observed during melt-crystallization behaviors of PLLA/MWCNTs fibers. Based on those results, we found that the chain mobility is an important factor to influence the structure formation and crystallization behaviors in the electrospun nanofibers. Our results indicated that the structure and properties of electrospun nanofibers could be optimized by compounding with hard inorganic filler and soft polymer components.