Morphology of polypropylene/silica nano‐ and microcomposites

The aim of this study was to compare the effects of different silica grades on the structure and morphology of isotactic polypropylene (iPP)/silica composites to better understand their structure–property relationships. Isotactic polypropylene composites with 2, 4, 6, 8 vol % of added silica fillers differing in particle size (micro‐ vs. nanosilica) and surface modification (untreated vs. treated surface) were prepared by nonisothermal compression molding and characterized by different methods. The addition of all silica fillers grades to the iPP matrix significantly influenced the spherulitic morphology, while phase characteristics of the iPP matrix seemed to be unaffected. Surface modification of silica fillers exhibited stronger effects on spherulite size than size of silica particles. Nonpolar silica particles, more miscible or compatible with iPP chains than polar silica particles, enabled better spherulitic growth. The spherulite sizes tended to reach equal values at 8 vol % of added silicas showing that spherulite size became independent of filler concentration and surface modification above optimum filler concentration. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Crystallization of isotactic (D,L) poly(propylene oxide) in the presence of fine-particle silica. I: Radial growth rates of spherulites

The effect of added silica on the spherulite radial growth rates of isotactic poly(propylene oxide) (i-PPrO) has been investigated by optical photomicroscopy. Two different i-PPrO samples were used of different molecular weights and isotacticity, 87 and 100% as determined by ¹³C NMR. The addition of fine particle silica, an effective nucleating agent, depresses the spherulite growth rates throughout the entire temperature range, but the effect is more dramatic for the i-PPrO of lower isotacticity. For a given sample, the retardation increases as the quantity of filler increases. The growth rate-temperature behavior is analyzed in terms of the classical Hoffman-Lauritzen equation, modified to take into account the polymer-filler interaction.     

Spherulite Growth Rate in Polypropylene/Silica Nanoparticle Composites: Effect of Particle Morphology and Compatibilizer

Spherical and tube‐like (TL) silica nanoparticles were melt blended with an isotactic polypropylene (PP) matrix and its effect on the isothermal spherulite growth rate was analyzed by polarized optical microscope. The addition of low amount (≈1 wt.‐%) of either 15 nm spherical or TL particles raises the spherulite growth rate and the nucleation density of spherulites. Samples prepared with silica spheres of 80 nm otherwise do not show any change in the crystallization behavior. By adding a compatibilizer, both the nucleation density and the spherulite growth rate of the pure polymer are increased. Noteworthy, although the nanoparticles do not further increase the nucleation density of the PP/compatibilizer blend, independent of its form and size, they cause a decrease in its spherulite growth rate.

     

Redox-active silica nanoparticles : Part 2. Photochemical hydrosilylation on a hydride modified silica particle surface for the covalent immobilization of ferrocene

The surface of spherical nonporous monodisperse silica particles (diameter: 200 nm) is covalently modified with hydride by means of hydrosilanization. A direct silicon-carbon link between the hydride modified silica surface and 10-undecylenic acid is obtained by photochemical radical initiated hydrosilylation. A ferrocene unit is attached through an amide link to the carboxylic acid modified surface. Solid-state NMR and DRIFT spectroscopy indicate success of the reactions. The electrochemical oxidation and reduction of the ferrocene modified silica particles are observed after their adsorption on a platinum electrode. The cyclic voltammograms indicate, in addition to the charge transfer between ferrocene units on the particle surface, an interparticle charge transfer.     

Dispersion,crystallization behavior,and mechanical properties of poly(3-hydroxybutyrate) nanocomposites with various silica nanoparticles: effect of surface modifiers

Silica nanoparticles (NPs) with various surface properties were introduced in poly(3-hydroxybutyrate) (PHB) matrix and the their effect on the dispersion, crystallization behavior, and reinforcement in the nanocomposites was discussed in this article. Two kinds of commercial fumed silica NPs and two kinds of self-prepared sol-gel silica (bare and PEGylated) NPs were used. The cross-sectional SEM (scanning electron microscopy) images, provided the micrometer scale view (observation area: 12.6×8.2 μm²), showed that commercial fumed silica and PEG–silica NPs were aggregated and well-dispersed in PHB matrix, respectively. Similarly, Morisita’s analysis of TEM (transmission electron microscopy) images (observation area: 2.4×1.6 μm²) indicated that PEG-silica NPs were Poisson dispersed and commercial fumed silica NPs were serious aggregated in PHB matrix. However, SEM-EDX (energy dispersive X-ray analysis) Si-mapping micrographs, provided the millimeter scale view (observation area: 0.79×0.61 mm²), showed that four kinds of silica NPs were well-dispersed in PHB matrix. PLM (polarized light microscopy) images indicated that spherulite growth rate and morphology of PHB did not change obviously upon the addition of various silica NPs, except the PHB/PEG–silica system. PHB/PEG–silica showed a decreased spherulite growth rate, which was consistent with the DSC (differential scanning calorimetry) results, because the good miscibility between PHB and the grafted PEG chains on PEG–silica could decrease the polymer chain mobility during crystallization. The Young’s modulus and tensile strength of the PHB were enhanced by up to 34% and 63% by adding a small amount of PEG–silica. Fully well-dispersed PEG–silica NPs functioned as physical cross-linking centers for enhancing the mechanical properties of PHB but as retarding agents for reducing the crystallization rate.     

Spherulite growth in isotactic polypropylene-based blends: Energy and morphological considerations

The kinetics of isothermal crystallization of polymer blends in which the matrix is a crystallizable polymer is considered. It is shown that depending on the difference in interfacial energies the inclusions are rejected or engulfed by the growing spherulite. Other factors influencing rejection, engulfing, and/or deformation of dispersed particles of the second polymer are the viscosity of the melt, the spherulite growth rate, and the size of dispersed particles. If the difference in interfacial energies is positive, then rejection or engulfing requires additional work to be done by the crystallization front. This dissipation of energy decreases the spherulite growth rate. It is estimated that the rejection of the second component is the most important phenomenon in the crystallization of blends. The spherulite growth rate of isotactic polypropylene in blends with low-density polyethylene and several elastomers was studied as a function of crystallization temperature and concentration. The comparison of growth rate data with morphological changes occuring during crystallization of blends studied shows very good agreement with the theoretical predictions based on energetics considerations.     

The critical conditions for secondary nucleation of silica colloids in a batch Stöber growth process

In this work, we determined experimentally the critical total surface area of silica seeds in the solution to avoid the formation of new particles (i.e. secondary nucleation) in a batch Stöber growth process. This critical surface area corresponded to the balance between the generation rate of reaction intermediate from the hydrolysis of TEOS and its consumption rate via the growth of existing seeds. Further calculation showed that the secondary nucleation was probably related to the average distance between seed particles. When the distance between seeds exceeded a critical value for each growth condition, i.e. for each generation rate, one would then observe secondary nucleation. Otherwise, simple growth was observed. Higher generation rate corresponded to shorter distance.     

Non‐isothermal crystallization behavior of PLA/acetylated cellulose nanocrystal/silica nanocomposites

Poly(lactide) (PLA)/acetylated cellulose nanocrystals (ACN)/silica nanocomposites were prepared by solution casting. Surface modification of cellulose nanocrystal (CNC) was performed to prepare the ACN. The ACN and silica were expected to act as a mechanical reinforcement of PLA and a nucleation agent, respectively, to increase the crystallization rate. Introduction of acetyl groups on the surface of the cellulose nanocrystals was confirmed by Fourier transform infrared spectroscopy. A combined Avrami ? Ozawa analysis described the non‐isothermal crystallization effectively. The activation energy for the crystallization was calculated from the Kissinger and the Takhor equations. Spherulite growth behavior was observed by polarizing optical microscope and spherulite growth rate, the number of spherulite versus crystallization time have investigated. The development of PLA crystals and the thermal stability had a tendency to improve with increasing silica content. Increased tensile strength was observed due to the reinforcement effect of ACN and the morphology of the nanocomposites was investigated. © 2015 Society of Chemical Industry     

生物基尼龙56的等温结晶性能研究

采用差示扫描量热仪和带有热台的偏光显微镜对生物基尼龙56的等温结晶性能进行了研究。用Avrami方程对等温结晶过程及其动力学进行了分析,得出Avrami指数(n值)在2.30~3.37之间,推测其晶体生长方式为三维球状生长;采用Arrhenius方程计算了生物基尼龙56的等温结晶活化能(ΔE)为-99.04 kJ/mol。偏光显微镜研究证实了上述推测,同时发现球晶半径与结晶时间呈线性关系,求得了球晶的生长速率。     

Establishment of nucleation and growth model of silica nanostructured particles and comparison with experimental data

A mathematical model is developed for the calculation of the nucleation and growth process of silica nanostructured particles prepared by using the drop-by-drop method, and the calculation results of the proposed model is compared with the experimental value obtained from SAXS data. The model provides a non-ideal improvement in the supersaturation calculation and considers the impact of both mass transfer and surface reaction on the particle growth rate. The nucleation and growth rates are coupled depending on the change in monomer concentration over time, based on which the particle size and distribution are calculated. The growth curve of the silica particles from 3 nm to 20 nm and the change in particle number from 0 to over 10²⁰ are calculated, which are consistent with the experimental values, establishing the reliability of the model. The calculations of the growth rate reveal that mass transfer controls the growth of silica particles before 10 min and the surface reaction is the rate-determining step after 10 min. The changes in the model parameters obtained by fitting with the SAXS data under different reaction conditions indicate the sensitivity of the corresponding process to different conditions. Moreover, the relationship between the particle growth rate and monomer concentration change is analyzed using the proposed model.     

Morphology and spherulitic growth kinetics in poly(ethylene oxide)/poly(n‐butyl methacrylate) blends

Results of a study concerning the morphology and the spherulite growth rates of poly(ethylene oxide) (PEO) in binary blends with poly(n‐butyl methacrylate) (PnBMA) are reported. Microscopic observations show that blending causes the spherulite structure to become coarser and less birefringent and confirms that the spherulitic growth rates of PEO were reduced by the addition of PnBMA. X‐ray diffraction studies show no change in the unit cell dimensions and a decrease in the degree of crystallinity upon blending. Analysis of the spherulite radial growth rate data by using Lauritzen–Hoffman theory indicates that crystallization in the range of 300 to 330 K occurs solely within regime III. The calculated surface free energy of folding, σ_e, for pure PEO is 57 erg cm^?2 and decreases with increasing the content of PnBMA in the blend. Copyright © 2004 Society of Chemical Industry     

A DSC investigation of the crystallization kinetics of polyoxymethylene

The large spherulite size, heterogeneous nucleation and constant spherulite growth rate of POM suggests a straightforward two dimensional Avrami analysis, with two adjustable parameters, t_O(T) and K(T), to be appropriate to thin samples in the DSC. For T<147°C, spherulite densities inferred from results for the rate parameter K(T) and microscopical measurements of spherulite growth rates are consistent with observed spherulite sizes. The K(T) values may also be used to model non-isothermal crystallization in the same temperature range (dT/dt<10 K/minute). Whilst K(T) shows little dependence on molecular weight, samples containing 30 wt% TPU crystallize more slowly owing to a reduced nucleation density.     

Polymer crystallization from the metastable melt: The formation mechanism of spherulites

One of the most popular crystalline morphologies is a spherulite. An evidence is reported that the spherulite is crystallized through a dense packing state of small particles appearing in a droplet, which is caused by the primary phase separation of the melt in the metastable region of a phase diagram proposed by Olmsted et al. [Olmsted PD, Poon WCK, McLeish TCB, Terrill NJ, Ryan AJ. Phys Rev Lett 1998; 81: 373]. According to this phase diagram, the crystallization from the metastable state causes the nucleation and growth (N & G) of nematic domains, here named droplets, in the isotropic matrix. As a next step, the secondary phase separation of spinodal decomposition (SD) type into smectic and amorphous domains occurs inside the droplet where entanglements are excluded from the smectic to the amorphous domain; then such an SD structure turns into a densely packing structure of many small particles owing to surface tension. At this final stage of the induction period a long period peak of small-angle X-ray scattering (SAXS), so-called SAXS before WAXS, appears, which may be due to the average distance between these small particles. Furthermore, it is considered that crystalline lamellae are formed by radial and azimuthal fusion of these small particles inside the droplet, resulting in a spherulite. Such a type of crystallization occurs most commonly when flexible polymers are crystallized under the usual conditions. This tentative concept of spherulitic growth, which is completely different from a theory by Keith and Padden [Keith HD, Padden FJ. J Appl Phys 1963; 34: 2409], would give a new insight into problems of spherulites.     

Synthesis and crystallization behavior of acetal copolymer/silica nanocomposite by in situ cationic ring‐opening copolymerization of trioxane and 1,3‐dioxolane

The acetal copolymer/silica nanocomposite was prepared by in situ bulk cationic copolymerization of trioxane and 1,3‐dioxolane in the presence of nanosilica. The crystallization behavior of acetal copolymer/silica nanocomposite was studied by AFM, DSC, XRD, and CPOM, and the macromolecular structure of acetal copolymer/silica nanocomposite was characterized by FTIR and ¹H‐NMR. The ¹H‐NMR results showed that the macromolecular chain of acetal copolymer had more than two consecutive 1,3‐dioxolane units in an oxymethylene main chain, while that of acetal copolymer/silica nanocomposite had only one 1,3‐dioxolane unit in an oxymethylene main chain. There existed interaction between the macromolecular chains and nanoparticles (such as hydrogen bonds and coordination). On one hand, nanoparticles acted as nucleation center, which accelerated the crystallization rate but reduced the crystallinity. The spherulite sizes also decreased with addition of nanoparticles attributed to the nucleation effect. On the other hand, the presence of nanoparticles interrupted the spherical symmetry of the crystallite. In conclusion, the high surface energy and small scale of nanoparticles have a prominent impact on the polymerization mechanism and crystallization behavior of nanocomposite. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The effect of inorganic and organic nucleating agents on the electrical breakdown strength of polyethylene

Altering the morphology of polyethylene affects physical and electrical properties with reduced spherulite size correlating with higher electrical breakdown strength. Nucleating agents in polyethylene influence the final crystal morphology by increasing the number of spherulites and reducing spherulite size. Few studies are available that relate the nucleating activity to improved electrical breakdown strength. Although nanosilica is known to improve electrical breakdown strength of polyethylene in addition to serving as a nucleating agent, previous studies have not fully addressed the relationship between the improved breakdown strength and nucleating activity. In this article, direct current electrical breakdown strength and nucleation effects on morphology are assessed on a single set of controlled polyethylene compositions containing two types of surface treated nanosilica particles. The results are compared to composites with two types of organic nucleating agents 1,3:2,4‐bis(3,4‐dimethylbenzylidene) sorbitol or calcium 1,2‐cyclohexanedicarboxylate (CDA). CDA was the most effective organic nucleating agent and the hexamethyldisilizane treated nanosilica was the most effective inorganic nucleating agent in reducing spherulite sizes in low density polyethylene (LDPE). Reduced spherulite sizes in nucleated samples correlated with increased breakdown strength and lower conduction current compared to the neat LDPE. The LDPE sample with CDA also had the highest increase in crystallization temperature indicating stronger nucleating agent performance than the nanosilica and 1,3:2,4‐bis(3,4‐dimethylbenzylidene) sorbitol composite samples. The addition of these inorganic and organic nucleating agents all resulted in improvements in electrical breakdown strength. The results show that nucleation deserves more attention as a potential cause for improved breakdown strength observed with silica and organic nucleating agents. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46325.     

Effects of crystallization behavior on morphological change in poly(trimethylene terephthalate) spherulites

In poly(trimethylene terephthalate) (PTT) spherulites during isothermal crystallization, the morphological changed from an axialite/or elliptical banded spherulite to banded spherulite and then non-banded spherulite with temperature decreasing were studied by following the lamellar growth behaviors. We report lamellar growth mechanism on varied crystallization temperature, which explicitly probes the link between microscopic structure and macroscopic morphology in the development of patterns. Fibrillation of the edge-on lamellae was observed on the surfaces of axialite and the convex bands of banded spherulite. Terrace-like lamellae were observed on the surface of the non-banded spherulite and the concave bands of banded-spherulite. In thin film crystallization, PTT banded spherulite exhibits a texture of alternate edge-on and flat-on lamellae, wavy-like surface and rhythmic growth. The deceleration of growth rate takes place in convex bands with a growth habit of fibrillation of the edge-on lamellae for emerging ridge surface. On the other hand, the acceleration of growth rate appears in concave bands with a growth habit of terrace-like lamellae for emerging valley surface. The alternating growth mechanism of the lamellae was considered to be related with the formation of spatiotemporal self-organization patterns far from equilibrium. In order to explain the rhythmic growth and periodic growth of the lamellae, we may conjecture that the emergence of PTT banded spherulite in thin film crystallization is associated with an oscillatory dynamics of the spherulite growth front driven by latent heat diffusion. We present some tentative ideas on the possibility of band-to-nonband (BNB) morphological transition, which might be analogous with the second order transition in non-equilibrium phase transition.     

Isothermal crystallization and spherulite growth behavior of stereo multiblock poly(lactic acid)s: Effects of block length

Stereo multiblock poly(lactic acid)s (PLA)s and stereo diblock poly(lactic acid) (DB) with a wide variety of block length of 15.4–61.9 lactyl units are synthesized, and the effects of block length sequence on crystallization and spherulite growth behavior are investigated at different crystallization temperatures, in comparison with neat poly(L ‐lactide) (PLLA), poly(D ‐lactide) (PDLA), and PLLA/PDLA blend. Only stereocomplex crystallites as crystalline species are formed in the stereo multiblock PLAs and DB, irrespective of block length and crystallization temperature. The maximum crystallinities (33–61%), maximum radial growth rate of spherulites (0.7–56.7 μm min^?1), and equilibrium melting temperatures (182.0–216.5°C) increased with increasing block length but are less than those of PLLA/PDLA blend (67 %, 122.5 μm min^?1, and 246.0°C). The spherulite growth rates and overall crystallization rates of the stereo multiblock PLAs and DB increased with increasing block length and are lower than that of PLLA/PDLA blend. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

不同PEG含量对PLA/PEG共混物结晶动力学的影响

采用溶液共混法制备了一系列不同配比的聚乳酸(PLA)/聚乙二醇(PEG)共混物。通过偏光显微镜(POM)、扫描电镜(SEM)和差式扫描量热仪(DSC)研究了不同PEG含量的PLA/PEG共混物在不同结晶温度下,聚乳酸的晶体形貌、球晶生长速率及热力学性能。研究发现,PEG能够显著提高聚乳酸球晶的生长速率。当PEG含量为60%时,PLA/PEG共混物中聚乳酸球晶的生长速率最快,达到23.6μm/min,比纯聚乳酸的最快球晶生长速率(0.5μm/min)高47倍。但是,当PEG含量高于60%时,聚乳酸球晶的生长速率有所降低。同时,PLA/PEG共混物中聚乳酸球晶速率随结晶温度变化的取向,均向低温移动。另外,PLA/PEG共混物中聚乳酸球晶呈现环状花纹。DSC测试结果表明,随着PEG含量的增加,PLA/PEG共混物的玻璃化转变温度明显降低。     

聚丙烯/聚苯乙烯/膨润土共混体系的结晶性能和热性能研究

通过偏光显微镜(PLM)和光学解偏振仪对聚丙烯/聚苯乙烯/膨润土三元共混体系的结晶形态和等温结晶速率进行了研究。结果表明:共混体系所形成的球晶比纯聚丙烯(PP)所形成的球晶尺寸小,聚苯乙烯(PS)/膨润土(Garamite)复合粒子的加入导致PP的结晶成核和生长发生了改变,加快了PP的结晶速率。同时采用差示扫描量热仪(DSC)对该三元共混体系的热性能进行了研究,结果表明,随着膨润土含量的增加,共混体系熔融温度变高,结晶温度变高,结晶度下降。     

Nucleation and crystal growth of PEEK on carbon fiber

The necleation and crystal growth of poly(ether ether ketone) (PEEK) on carbon fiber during isothermal crystallization were studied using optical microscopy. It was shown that the nucleation rate on the fiber is affected by the crystallization and melting temperatures. Transcrystallinity of PEEK was found to appear in the range 280–315°C after PEEK was melted at rather high temperature. Its linear growth rates at various temperatures were found to be similar to those of the spherulite in the bulk. Also, its thickness depends on the crystallization and melting temperatures. Variation of the molecular weight of PEEK within a small range has no obvious influence on its nucleation rate on the fiber, but affects the transcrystalline growth rate. © 1993 John Wiley & Sons, Inc.