Effect of multistage sonication on dispersive mixing of polymer nanocomposites characterized via shear‐induced crystallization behavior

While nanoparticle dispersion is necessary to achieve optimal properties, it has long been recognized as a major technological hurdle for polymer nanocomposites. Here, a systematic study incorporated carbon nanotubes (CNTs) in polycaprolactone (PCL) using a multi‐stage sonication process, with Stage 1 sonication of CNT/solvent followed by Stage 2 sonication of the pre‐processed CNT/solvent with the dissolved polymer. Conventional dispersion characterization techniques were complemented with analysis of the shear‐induced crystallization (SIC) behavior of the semicrystalline nanocomposite, which was found to be particularly sensitive to the state of nanoparticle dispersion. While Stage 1 sonication was found to have a pronounced effect on the nanoparticle dispersion as characterized via SIC and thermal characterization, the impact of Stage 2 sonication on the level of nanoparticle dispersion was much smaller. Such results demonstrate the utility of characterization of the shear‐induced crystallization behavior as a means to analyze nanoparticle dispersion in semicrystalline polymer nanocomposites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44681.     

A novel kinetic approach to crystallization mechanisms in polymers

Kinetics of polymer crystallization is capable of modeling the polymer crystallization processes by revealing their mechanisms. Although complexities of polymer crystallization processes are familiar, a generalized approach for their kinetic modeling has not yet been suggested. This paper presents a concise review of the pre-existing kinetic approaches to polymer crystallization, including their pros and cons, putting a special emphasis on the necessity of approaches to polymer crystallization. A systematic and advanced approach comprising an innovative kinetic framework to model polymer crystallization processes is put forward using Hoffman-Lauritzen theory. Kinetic functions are developed for predicting the crystallization mechanisms of simple as well as complicated polymer crystallization processes. The suggested kinetic approach is experimentally justified by its effective applications to non-isothermal glass and melt crystallization of poly(ethylene 2,5-furandicarboxylate). An account of the soundness of the proposed kinetic approach and its prospective applications are also discussed.     

Effects of crystallization on dispersion of carbon nanofibers and electrical properties of polymer nanocomposites

Polymer nanocomposites filled with low volume fractions of carbon nanofibers (CNFs) were prepared by melt‐compounding. Three types of polymers with different crystallization behavior, i.e., weakly‐crystallized low density polyethylene (LDPE), strongly crystallized high density polyethylene (HDPE) and amorphous polystyrene (PS), were selected as matrices for the nanocomposites. The effects of polymer crystallization on the dispersion of CNFs were examined. Optical and electron microscopic examinations revealed that the dispersion of CNFs in the nanocomposite matrices was strongly depended on the crystallization behavior of polymer matrices. The CNFs were found to disperse uniformly in weakly crystallized LDPE and amorphous PS matrices, but agglomerated in HDPE due to its strong crystallization tendency. Such a distinct dispersion behavior of CNFs in polymers had a profound effect on the electrical properties of the nanocomposites investigated. The PS/CNF nanocomposites exhibited the lowest percolation threshold. The HDPE/CNF nanocomposites showed the largest percolation threshold due to the CNF agglomeration within the amorphous phase of HDPE. POLYM. ENG. SCI., 48:177–183, 2008. © 2007 Society of Plastics Engineers     

聚合物辅料对阿司匹林结晶动力学影响机制的非平衡热力学建模及预测

探究聚合物辅料对难溶性药物结晶的影响机制,是指导无定形固体分散体制剂设计和制备中辅料筛选的关键。研究了不同因素（温度、搅拌速率、聚合物浓度、聚合物分子量和聚合物种类等）对阿司匹林晶体生长动力学的影响。首先,采用基于三种不同晶体生长机制的化学势梯度模型结合UNIQUAC活度系数模型,描述和预测了阿司匹林在不同条件下的结晶动力学。进一步分析了不同因素对晶体生长速率常数k_t和结晶热力学推动力?\mu的影响以及对阿司匹林结晶动力学的影响机制。结果表明,阿司匹林晶体生长速率随着结晶温度、聚合物浓度的增加而降低,随搅拌速率的增加而升高;聚乙烯吡咯烷酮（PVP K25）和羟丙基甲基纤维素（HPMC E3）显著抑制了阿司匹林的晶体生长,在PVP K25和HPMC E3水溶液体系下阿司匹林的晶体生长属于二维成核机制,在纯水和PEGs水溶液体系下晶体生长属于粗糙生长机制。所采用的化学势梯度模型能很好预测不同温度和搅拌速率下阿司匹林的结晶动力学,可有效减少实验所需的人力、物力和财力。研究可为固体分散体制剂制备中聚合物的筛选提供理论研究基础。     

Effect of Clay Dispersion on the Nonisothermal and Isothermal Crystallization Behaviors of Polyethylene Composites

The presence of nanosized clay fillers can greatly influence the crystallization behaviors and crystalline morphologies of polymer composites. In this study, the modified montmorillonite polyethylene nanocomposites have been prepared by melt mixing, followed by injection molding. The morphology and microstructure of the composites were characterized by scanning electron microscopy and Fourier transform infrared spectrometry. The nanosized clay particles act as a nucleating agent in the solidification process of the matrix. The relative crystallinity ratio decreases with the decrement of polymer loading. Nonisothermal and isothermal crystallizations were conducted to characterize the crystallization kinetics of nanocomposites. The effects of clay dispersion on the crystallization of polymer were quantified by Avrami and Hoffman models. The addition of 1?wt% clay into polymer matrix has significantly increased the activation energy. The crystallization rate constants (G) have been determined which are higher for the composites than that for the polymer matrix.     

聚合物/纳米CaCO3复合材料研究进展

综述了表面处理对聚合物／纳米CaCO3复合材料力学性能的影响、纳米CaCO3在聚合物基体中的分散机理和对聚合物结晶行为的影响，并展望了聚合物／纳米CaCO3复合材料的发展方向和前景。     

Non-isothermal polymer crystallization kinetics and Avrami master curves

The kinetic crystallization model of Avrami is generally accepted as a starting point for the analysis of isothermal polymer crystallization. It is shown in this paper that, in the case of non-isothermal crystallization kinetics with constant heating or cooling rates, an apparent m -order reaction model is approximately equivalent to the nucleation and growth model of Avrami in the vicinity of the inflection points of the corresponding crystallization curves. Since the apparent m -order reaction model is defined for every real, positive apparent reaction-order m , a distinct Avrami index n , which is valid for the characterization of isothermal and non-isothermal crystallization experiments with constant heating or cooling rates, can always be related to any apparent reaction-order m . Therefore, two types of Avrami master curves, which are dependent merely on the Avrami index n and which describe the isothermal polymer crystallization thoroughly, can be obtained by performing non-isothermal experiments with constant heating or cooling rates.     

Adhesion of Polytetrafluoroethylene

Nucleation and crystallization of polytetrafluoroethylene in contact with gold produces a surface region of high mechanical strength in the polymer as evidenced from the strong adhesive joints prepared with a conventional epoxy adhesive. In addition, the wettability of the gold-nucleated polytetrafluoroethylene, after dissolution of the gold, is vastly improved. The critical surface tension of wetting increases from 18.5 dynes/cm to 40 dynes/cm, indicating an increase in the surface density of the gold-nucleated polymer. We conclude from this study, that the occurrence of the normal weak boundary layer on polytetrafluoroethylene is a consequence of the morphology of the surface region and is therefore influenced by the method of preparation.     

Influence of initial mixing methods on melt‐extruded single‐walled carbon nanotube–polypropylene nanocomposites

We report the first direct comparison of melt‐extruded polypropylene–single‐walled carbon nanotube (PP/SWNT) nanocomposites prepared by three different initial mixing methods. The standard deviation of the G‐band intensity obtained using Raman mapping was found to be the best measure of dispersion uniformity in the extruded composites, and dispersion uniformity was found to generally correlate with rheological and thermal properties. For all three initial mixing methods, both unmodified and sidewall‐functionalized purified SWNTs were evaluated. Surprisingly, in all cases, dodecylated SWNTs prepared using the reductive alkylation method were less uniformly dispersed in the final composite than the unmodified SWNTs. The simplest process, dry blending, resulted in poor nanotube dispersion and only polymer crystallization was significantly affected by the presence of the nanotubes. A slightly more complex rotary evaporation process resulted in significantly more uniform dispersion and significant changes in rheological properties, polymer crystallization, and thermal stability. The most elaborate process tested, hot coagulation, enabled the most uniform dispersion and the greatest change in properties but also resulted in some polymer degradation. POLYM. ENG. SCI., 50:1831–1842, 2010. © 2010 Society of Plastics Engineers     

Electrical conductivity and thermal stability of polypropylene containing well-dispersed multi-walled carbon nanotubes disentangled with exfoliated nanoplatelets

The morphology, crystallization behavior, electrical conductivity, and thermal stability of polypropylene (PP) modified with disentangled multi-walled carbon nanotubes (MWCNTs) is reported. Slightly oxidized MWCNT clusters were disentangled in solution by mild sonication in the presence of exfoliated α-zirconium phosphate nanoplatelets. The disentangled MWCNTs were isolated using acid-induced coagulation to precipitate the nanoplatelets, and were subsequently reacted with octadecylamine. The recovered functionalized MWCNTs (F-MWCNTs) are disentangled and easily dispersed in a commercial PP matrix, and serve as more efficient nucleating agents than the untreated MWCNTs. The PP/F-MWCNT composites exhibit an extremely low percolation-like transition in electrical conductivity, which is attributed to the preservation of a random dispersion of disentangled F-MWCNTs upon cooling from the melt. The thermal stability of PP in air is also substantially enhanced at loadings below the percolation threshold due to the tremendous interfacial area between the polymer chains and the free radical scavenging F-MWCNTs. The present approach provides an efficient and potentially scalable route for commercial production of conductive semi-crystalline thermoplastics. The method may be adapted to uniformly disperse MWCNTs in other polymer matrices by appropriate selection of surface functionality.     

A survey of the rheological properties,phase morphology,and crystallization behavior of PP-POSS materials with weak phase separation

Polypropylene (PP) compounds with varying amounts (0.4-8.2 vol%) of tailored allyl-isobutyl polyhedral oligomeric silsesquioxane (POSS) were prepared by melt-blending. The dependence of the crystallization behavior and crystalline structure of PP on the melt-state phase morphology of PP-POSS materials is addressed. PP-POSS systems were predicted to exhibit weak phase separation in both the molten and solid states based on Bagley plot using Hansen solubility parameters. Small-amplitude shear rheometry analysis suggested that the highest loaded (8.2 vol%) PP-POSS system behaves as a two-liquid emulsion, whereas the low-content POSS (0.4-4.0 vol%) systems deviate from this model, resembling a polymer nanocomposite. Based on these findings we hypothesized the formation of a heterogeneous phase morphology in the molten state comprised of nano-size “pseudo-solid” POSS clusters and micrometer-size “pseudo-liquid” POSS droplets dispersed within the PP matrix, depending on the POSS content. Upon cooling, POSS droplets comingle, forming cube-like micrometer-sized crystal domains. The nonisothermal crystallization of PP is enhanced by the presence of POSS clusters. Small-angle X-ray scattering analysis revealed the formation of thinner and more heterogeneous folded chain PP lamellae in the PP-POSS systems.     

Unsteady-state heat transfer in a crystallizing polymer

The processing of polymers typically includes steps involving heat transfer, such as quenching and crystallization. The mechanical properties of a polymer are thus affected by processing conditions, such as the amount of time spent by each portion of the polymer at each temperature in the processing sequence. This paper presents the results of a mathematical modeling study of the unsteady-state temperatures inside a crystallizing polymer during quenching. This model differs from usual methods in including temperature dependence of induction times and kinetic processes, and in not treating the polymer as a simple, small molecule system. The model permits the prediction of the final morphological distribution within the polymer. Use of the model will aid in tailoring processing conditions to produce a product with specified morphology. The method is applicable to a wide variety of problems in polymer processing, particularly for those polymers which exhibit crystallization.     

Morphology of nascent polyethylene prepared with the catalyst VOCl3/(C2H5)2AlCl

The catalyst VOCI₃/(C₂H₅)₂AICI was used to prepare polyethylene under various conditions of polymerization. The nascent polymer samples were examined by optical and electron microscopy, fuming nitric acid oxidation and differential scanning calorimetry. It was observed that the samples consisted of lamellar as well as fibrillar crystals. Examination of catalyst preparations by light-scattering photometry revealed the presence of a colloidal dispersion besides the soluble catalyst complex. It is proposed that the lamellar and fibrillar crystals have different origins. The lamellar crystals are formed by intramolecular crystallization of polymer chains generated from the soluble catalyst complex, whereas the fibrils are the product of intermolecular crystallization of polymer chains growing from adjacent active sites residing on the surface of the colloidal particles. The degree of chain extension in the fibrillar crystals is dependent on the conditions of polymerization.     

Structural characterization and thermal behaviour of wool keratin hydrolizates-polypropylene composites

Innovative polypropylene composites were prepared using as a biofiller, wool keratin hydrolizates obtained by a green process with superheated water in a microwave reactor. To promote the affinity between the hydrophobic polymer and the biofiller, maleic anhydride grafted polypropylene was used as a compatibilizer. The composites showed a homogeneous dispersion of the keratin particles in the polymer matrix. The thermal properties and the structure of the composites were investigated in dependence of keratin loading and crystallization conditions. The keratin particles had a heterogeneous nucleating action on polypropylene crystallization that increased the overall crystallization rate. The nucleation density increased as a function of the keratin amount in the composites. The crystallinity, the crystal dimension and the long period of the polypropylene were found to be dependent on the crystallization condition and the composite composition. In the crystallized composites, the keratin component, having dimension in the nano- and micro-scale length, was relegated to the intraspherulitic and/or interspherulitic polypropylene regions.     

Nucleation and crystallization of PET droplets dispersed in an amorphous PC matrix

The present work compares the nucleation and crystallization process of poly(ethylene terephthalate) (PET) in bulk and when it is finely dispersed in a polycarbonate (PC) matrix. Two types of 80/20 PC/PET immiscible blends were prepared by twin-screw extrusion at different screw rotation rates in order to produce fine dispersions of PET. The results indicate that the finer the dispersion, the greater the inhibition of the crystallization of the PET droplets. These results are explained by demonstrating (through self-nucleation experiments) that a fractionated crystallization process was developed in the dispersed PET, since the number of PET particles was much greater than the number of heterogeneities originally present in the bulk polymer. The dispersion of PET into droplets also affects its crystallization rate during isothermal crystallization at high temperatures and its reorganization capacity during heating. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1725–1735, 1998     

Heterogeneous nucleation and crystallization of polyethylene with gold: Dilatometry

Heterogeneous nucleation and crystallization data for polyethylene at a gold interface are reported. Isothermal crystallization is investigated by dilatometry. Avrami plots of the crystallization data suggest that the gold surface acts as a suitable nucleating agent resulting in heterogeneous nucleation and two-dimensional crystal growth at the polyethylene melt–gold interface. The deviation of measurements from Avrami's equation suggests the presence of intercrystalline links during the final growth stages of isothermal crystallization. A closely packed lamellar model is given for heterogeneous nucleation and two-dimensional crystallization which results in high density and rod-like crystals.     

Nanocomposites based on vermiculite clay and ternary blend of poly(L‐lactic acid), poly(methyl methacrylate), and poly(ethylene oxide)

Vermiculite (VMT) as clay was introduced into a ternary polymer blend composed of poly(L ‐lactic acid) (PLLA), poly(methyl methacrylate) (PMMA), and poly(ethylene oxide) (PEO), whose ternary miscibility was proven within low certain contents of PMMA and PEO in PLLA. VMT was incorporated to the ternary polymer blend as matrix after proper organic modification on the clay. The organically modified vermiculite (OVMT) shows good interaction and acceptable dispersion in the ternary polymer matrix without altering the crystal structures of PLLA/PEO constituents. The effect of OVMT addition was then analyzed in isothermal crystallization by using the Avrami kinetic analysis, and the addition of OVMT is evident in altering nucleation process of the polymer blend as well as the crystal perfection. The activation energy is much lowered by the addition of OVMT, as evident from the analysis; the overall crystallization kinetic rates are increased with the incorporation of OVMT. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

A mathematical model for diffusion with induced crystallization: 1

During the sorption of swelling agents in amorphous, but crystallizable, glassy polymers local crystallization takes place. Previous work has shown that the weight gain and overall crystallization kinetics depend on the polymer/penetrant interactions and the dimensions of the sample. In this paper we propose a mathematical model for the process which includes four important effects: time dependent polymer swelling, blocking of diffusion by crystallites, occlusion of penetrant by developing crystallites, and macrovoid formation. The model predicts four limiting regimes of behaviour according to the values of the dimensionless crystallization rate, Ω, and the dimensionless sample half thickness, λ_p. Analytical solutions are developed for each limiting case, with the corresponding criteria for their application to real systems. The model successfully predicts the variety of behaviours observed experimentally.     

填料对聚合物结晶行为的影响

综述了聚合物／填料体系的结晶行为,介绍了填料对聚合物结晶形态、尺寸的影响,阐述了聚合物／填料体系的结晶动力学过程,总结了填料对聚合物结晶速率、结晶温度、结晶度的影响,并描述了填充型聚合物的结晶结构模型。