Crystallization and morphology of ultrathin films of homopolymers and polymer blends

Ultrathin films of thicknesses below 100 nm are now considered in different areas of applications. Their behavior in term of kinetics of crystallization is very different from that of bulk samples due to the film confinement in two-dimensions, and their morphologies are unique. In this review, recent advances in the crystallization of ultrathin films of homopolymers and miscible polymer blends will be described, with an emphasis on morphologies and, in the case of blends, on mixtures made of two crystalline polymers.     

Crystallization and melting of PEO:LiTFSI polymer electrolytes investigated simultaneously by impedance spectroscopy and polarizing microscopy

Simultaneous impedance measurements and optical observations of polymer electrolytes were conducted in an automated experimental setup that combined an impedance dielectric analyzer, a polarizing microscope with a heating stage and a digital camera. The polymer film was placed between glasses with ITO conductive layers, forming a transparent cell mounted in a custom designed holder that preserved argon atmosphere.Films of high molecular weight poly(ethylene oxide) with dissolved lithium bis(trifluoromethanesulfone) imide(LiTFSI) of two compositions: 50:1 and 6:1 (EO:Li molar ratio) were investigated in transparent cells above room temperature and in cells with gold electrodes in temperature range between −60 and 90 °C. Various heating and cooling runs enabled observation of the crystallization and melting.The results indicate that the decrease of conductivity observed in impedance spectra during crystallization is related to the closing of amorphous conductivity pathways by growing spherulites. In the dilute system, composition 50:1 EO:Li, amorphous areas were still visible in the film after the growth of spherulites ceased. In the film of composition 6:1, corresponding to the polymer-salt complex, densification of the structure and interfacial phenomena caused a large drop of conductivity at the late stage of crystallization. In the dense structure of crystallized P(EO)₆:LiN(CF₃SO₂)₂ film no amorphous areas were visible. Differences in the structure have a reflection in the relative change of conductivity caused by crystallization, which decreased six times for the 50:1 composition and 500 times for the 6:1 composition.     

Influence of the viscosity ratio in PC/SAN blends filled with MWCNTs on the morphological,electrical, and melt rheological properties

Co-continuous polycarbonate (PC)/poly(styrene-acrylonitrile) (SAN) = 60/40 wt.% blends were filled with 1 wt.% multi-walled carbon nanotubes (MWCNTs), which selectively localized within the PC component. To study the influence of the viscosity ratio, PCs with different viscosities were selected resulting in PC/SAN viscosity ratios (at 100 rad/s) between 1.2 and 4.5. With increasing viscosity ratio, smaller blend structures were observed. Furthermore, optical microscopy revealed that the filler dispersion was improved with decreasing PC viscosity. The highest electrical conductivity was achieved for the blend composite with the coarsest morphology, containing the low viscosity PC and having the lowest PC/SAN viscosity ratio. Transmission electron microscopy analysis indicated that for the composite prepared with high viscosity PC, not all of the incorporated MWCNTs were able to localize completely into the PC component. Instead, some MWCNTs were found to be stacked at the interface of the two polymers, indicating that the high PC melt viscosity had a restricting effect on the movement of the MWCNTs. Moreover, with electrical conductive atomic force microscopy, it was proven that small, spherical PC particles, even if filled with CNTs, do not take part in the conductive network of the blend composites. Rheological analyses showed a correlation with the morphological analysis and the electrical conductive behavior of the blend composites. In summary, a lower viscosity ratio between the blend components, in which upon addition due to thermodynamic reasons the CNTs localize (here PC), and the other component (here SAN) is favorable for high electrical conductivity values.     

Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s

The effects of incorporated amorphous poly(dl-lactide) (PDLLA) on the isothermal crystallization and spherulite growth of crystalline poly(l-lactide) (PLLA) and the structure of the PLLA/PDLLA blends were investigated in the crystallization temperature (T_c) range of 90-150 °C. The differential scanning calorimetry results indicated that PLLA and PDLLA were phase-separated during crystallization. The small-angle X-ray scattering results revealed that for T_c of 130 °C, the long period associated with the lamellae stacks and the mean lamellar thickness values of pure PLLA and PLLA/PDLLA blend films did not depend on the PDLLA content. This finding is indicative of the fact that the coexisting PDLLA should have been excluded from the PLLA lamellae and inter-lamella regions during crystallization. The decrease in the spherulite growth rate and the increase in the disorder of spherulite morphology with an increase in PDLLA content strongly suggest that the presence of a very small amount of PDLLA chains in PLLA-rich phase disturbed the diffusion of PLLA chains to the growth sites of crystallites and the lamella orientation. However, the wide-angle X-ray scattering analysis indicated that the crystalline form of PLLA remained unvaried in the presence of PDLLA.     

Improved mechanical properties of carbon nanotube/polymer composites through the use of carboxyl-epoxide functional group linkages

Single-walled and multi-walled carbon nanotubes (CNTs) were functionalized with carboxyl groups and dispersed in a polymer containing an epoxide group. We have then observed experimentally that mutual chemical reaction between the functional groups on the CNTs with the polymer epoxide group can enhance, two-fold, both the tensile strength and elastic modulus, E, of single walled CNT/polymer composites. A simple model was formulated to understand the variation of E with CNT volume fraction, considering agglomeration effects as well. An increase in the work of fracture, obtained from the experimental stress-strain curves, was seen at low nanotube filling fractions and is presumably due to crack bridging of the polymer matrix by CNTs. The influence of CNT length and geometry on mechanical properties, along with the influences of electrical and mechanical percolation thresholds was considered.     

Surface functionalities of multi-wall carbon nanotubes after UV/Ozone and TETA treatments

A carbon nanotube (CNT) surface was successfully modified using the UV/Ozone treatment and a triethylenetetramine (TETA) solution for use as the reinforcement for polymer matrix composites. These treatments along with ultra-sonication are aimed to disperse the CNTs uniformly in the resin matrix, as well as to provide the CNT surface with chemical functionalities for adhesion with epoxy resin. Fourier transform infra-red spectroscopy and X-ray photoelectron spectroscopy are performed to evaluate the changes in chemical structure and surface functional groups arising from the UV/O₃ and TETA treatments, confirming the efficiency of the processes. The practical implications of the surface functional groups for improving the interfacial adhesion in CNT-epoxy composites are discussed.     

The effect of high-pressure carbon dioxide treatment on the crystallization behavior and mechanical properties of poly(l-lactic acid)/poly(methyl methacrylate) blends

The purpose of this study is to investigate the effect of carbon dioxide (CO₂) on the crystallization behavior and the mechanical properties of PLLA/PMMA blends with various weight fraction of PMMA. PLLA/PMMA blends can be crystallized even at a low temperature of 0 °C under high-pressure CO₂. The films treated with high-pressure CO₂ at 0 °C have about three times larger strain at break than that of the amorphous and cold-crystallized film. The size of spherulites in the CO₂ treated film is considered to be smaller than the wavelength of the visible light because of a good transparency. The improvement of the strain at break is attributed to the reduction of the stress concentration during the deformation.     

Localization of carbon nanotubes at the interface in blends of polyamide and ethylene-acrylate copolymer

The present study demonstrates for the first time the possibility to jam unpurified and unfunctionalized multiwall carbon nanotubes (MWNTs) at the interface of an immiscible blend of polyamide (PA) and ethylene-acrylate (EA) copolymer. The confinement appears to be stable. The influence of the mixing strategy and of the polyamide type used has been examined. When the MWNTs are first dispersed in PA6, most of them migrate to the interface although some of them stay in the PA phase. When the MWNTs are first dispersed in PA12, they remain well dispersed in PA. When the MWNTs are first dispersed in the EA copolymer or when the three components are simultaneously mixed, a large part of the MWNTs migrate to the interface whatever the PA used. However, some of the MWNTs remain in the EA phase and when PA12 is used, part of the MWNTs penetrate inside the PA nodules. By a combination of TGA and separation techniques, we show that the first polymer to come in contact with the nanotubes during melt mixing is (at least partially) adsorbed irreversibly, by non-covalent adsorption. The resulting modification of interfacial thermodynamics explains the observed confinement.     

Crystallization of double crystalline block copolymer/crystalline homopolymer blends: 1. Crystalline morphology

The crystalline morphology formed in binary blends of poly(ε-caprolactone)- block-polyethylene (PCL-b-PE) copolymers and PCL homopolymers has been examined using synchrotron small-angle X-ray scattering (SR-SAXS) and differential scanning calorimetry (DSC) as a function of the homopolymer fraction in the blend. The PE block crystallized first on quenching from a lamellar microdomain structure to set a hard lamellar morphology (PE lamellar morphology) in the blend, followed by the crystallization of PCL chains (i.e., PCL homopolymers + PCL blocks). Two binary blends were studied by considering the miscible state of PCL homopolymers in the microdomain structure: when the PCL homopolymers were uniformly mixed with PCL blocks, they formed a mixed crystal. When the PCL homopolymers were localized between PCL blocks in the microdomain structure, DSC results suggested the possible formation of separate PCL crystals in the PE lamellar morphology. The effect of the advance crystallization of PE blocks on the subsequent crystallization of PCL chains was discussed as compared with the crystalline morphology formed in PCL-block-polybutadiene copolymer/PCL homopolymer blends, where the crystallization of PCL chains started directly from a microdomain structure without forming the hard lamellar morphology.     

Crystallization and orientation studies in polypropylene/single wall carbon nanotube composite

Crystallization behavior of melt-blended polypropylene (PP)/single wall carbon nanotube (SWNT) composites has been studied using optical microscopy and differential scanning calorimetry. Polypropylene containing 0.8 wt% SWNT exhibits faster crystallization rate as compared to pure polypropylene. PP/SWNT fibers have been spun using typical polypropylene melt spinning conditions. The PP crystallite orientation and the SWNT alignment in the fibers have been studied using X-ray diffraction and polarized Raman spectroscopy, respectively.     

聚乳酸（PLA）／己二酸-对苯二甲酸-丁二酯共聚物（PBAT）／乙酰化柠檬酸三丁酯（ATBC）共混物的结晶行为

采用熔融共混法制备聚乳酸（PLA）／己二酸-对苯二甲酸-丁二酯共聚物（PBAT）／乙酰化柠檬酸三丁酯（ATBC）共混物。用差示扫描量热仪（DSC）和偏光显微镜（POM）分析了乙酰化柠檬酸三丁酯（ATBC）对共混物PLA结晶行为的影响。结果表明当PLA／PBAT含量为80／20（质量比）时,随着ATBC量的增加,PLA的Tg、Tc和Tm降低,结晶度提高,球晶生长速率增加。     

Crystallization,melting behavior,and morphology of BPP/HDPE blend

The crystallization, melting behavior, and morphology of a low ethylene content block propylene–ethylene copolymer (BPP) and a high-density polyethylene (HDPE) blend were studied. It was found that the existence of ethylene–propylene rubber (EPR) in BPP has more influence on the crystallization of HDPE than on that of PP. This leads to the decreasing of the melting temperature of the HDPE component in the blends. It is suggested that the EPR component in BPP shifted to the HDPE component during the blending process. The crystallinity of the HDPE phase in the blends decreased with increasing BPP content. The morphology of these blends was studied by polarized light microscopy (PLM) and SEM. For a BPP-rich blend, it was observed that the HDPE phase formed particles dispersed in the PP matrix. The amorphous EPR chains may penetrate into HDPE particles to form a transition layer. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2469–2475, 1998     

Crystallization,melting, and rheology of reactive polyamide blends

The crystallization and melting characteristics of a series of polyamide blends based on PA 4,6 and PA 6I were investigated by calorimetric methods; preparation of the samples was conducted so as to control the extent of transamidation occurring in the melt before crystallization. Blend samples with minimal prior thermal history displayed a modest degree of melting point depression compared to the equilibrium melting temperature of PA 4,6 (T = 309.5°C). Application of the Nishi–Wang equation indicated a value of χ = ?0.25 for the blends. PA 4,6 and the blends followed Avrami crystallization kinetics with exponents in the range 2.0 to 2.5; no systematic variation of n with blend composition was observed. The influence of transamidation was investigated for samples exposed to varying melt temperatures and melt times with the extent of transreaction quantified using ¹³C‐NMR. Increasing extents of transreaction led to a decrease in both the rate of crystallization and the overall bulk crystallinity of the blends owing to a reduction in the length and number of crystallizable blocks present along the polymer chains. Capillary rheometry studies indicated a strong sensitivity to time in the melt for the PA 4,6 homopolymer, and the mechanism responsible for the observed decrease in apparent viscosity was also operative in the blend samples. As such, it was not possible to independently assess the influence of transreaction on the rheology of the blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1245–1252, 2004     

Crystallization kinetics and morphological behavior of reactively processed PBT/epoxy blends

Polybutylene terephthalate (PBT), a versatile engineering thermoplastic, has been processed using epoxy resin as a reactive solvent. Following processing of this blend, the epoxy was cured using a bi-functional amine curing agent, resulting in phase separation and phase inversion thus producing a different morphology. Change in crystallization kinetics of PBT in the presence of the epoxy monomer and cured epoxy resin has been studied using differential scanning calorimetry. Half time of crystallization (t_1/2) of PBT decreased in the presence of epoxy monomer while it remained constant in the presence of cured epoxy resin. The value of Avrami exponent varied between 1 and 2 in pure PBT as well as for uncured and cured blends, indicating mixed type of spherulitic growth. Morphology of the uncured and cured blends was studied using small angle light scattering (SALS) and polarizing microscopy for samples crystallized at different temperatures at all levels of the epoxy resin. Scattering pattern in H_v and V_v mode of SALS provided information about the type of spherulites as well as volume filling nature of the spherulites. In general, typical unusual type of spherulitic pattern for PBT, in which scattering lobes lie along the polar axis, changed to usual type of pattern for PBT/epoxy blends, in which scattering lobes lie at 45° to the polar axis.     

A change of phase morphology in poly(p-phenylene sulfide)/polyamide 66 blends induced by adding multi-walled carbon nanotubes

By adding a small amount of acid treated multi-walled carbon nanotubes (MWCNTs) into poly(p-phenylene sulfide)/polyamide 66 (60/40 w/w) blends, the morphology was found to change from sea-island to co-continuous structure. As the MWCNT content was increased, the morphology came back to sea-island but with increased domain size. It was very interesting to note that the MWCNTs were found to be selectively located in the PA66 phase, and their assembling determines the final morphology of PPS/PA66 blends. A dendritic contacted MWCNTs network was formed at low load, which leads to the formation of a co-continuous structure, and isolated MWCNT aggregates were observed at high load, which leads to the formation of sea-island morphology. Since the properties of multiphase polymeric materials are not only determined by the properties of the component polymers, but also by the morphology formed, our work indicates that the behavior of phase-separating polymer blends containing MWCNTs can be exploited to create a rich diversity of new structures and useful nanocomposites.     

Crystallization and coalescence of block copolymer micelles in semicrystalline block copolymer/amorphous homopolymer blends

Crystallization of two oxyethylene/oxybutylene block copolymers (E₇₆B₃₈ and E₁₅₅B₇₆) from micelles in block copolymer/amorphous homopolymer blends was studied by differential scanning calorimetry (DSC) and time-resolved small angle X-ray scattering (SAXS). Unlike the simultaneous crystallization and formation of superstructure in crystallization from an ordered structure, crystallization of block copolymer from micelles can be divided into two steps. The core of the micelles firstly crystallizes individually, with first-order crystallization kinetics and homogeneous nucleation mechanism. The SAXS revealed that crystallization-induced deformation occurs for the micelles, which strongly depends on microstructure of the block copolymers. For the shorter block copolymer E₇₆B₃₈, larger deformation induced by crystallization was observed, leading to coalescence of the micelles after crystallization, while for the longer block copolymer E₁₅₅B₇₆ the micelles show little deformation and the morphology of micelle is retained after crystallization.     

Electrical, mechanical, and glass transition behavior of polycarbonate-based nanocomposites with different multi-walled carbon nanotubes

Five commercially available multi-walled carbon nanotubes (MWNTs), with different characteristics, were melt mixed with polycarbonate (PC) in a twin-screw micro compounder to obtain nanocomposites containing 0.25-3.0 wt.% MWNT. The electrical properties of the composites were assessed using bulk electrical conductivity measurements, the mechanical properties of the composites were evaluated using tensile tests and dynamic mechanical analysis (DMA), and the thermal properties of the composites were investigated using differential scanning calorimetry (DSC). Electrical percolation thresholds (p_cs) were observed between 0.28 wt.% and 0.60 wt.%, which are comparable with other well-dispersed melt mixed materials. Based on measurements of diameter and length distributions of unprocessed tubes it was found that nanotubes with high aspect ratios exhibited lower p_cs, although one sample did show higher p_c than expected (based on aspect ratio) which was attributed to poorer dispersion achieved during mixing. The stress-strain behavior of the composites is only slightly altered with CNT addition; however, the strain at break is decreased even at low loadings. DMA tests suggest the formation of a combined polymer-CNT continuous network evidenced by measurable storage moduli at temperatures above the glass transition temperature (T_g), consistent with a mild reinforcement effect. The composites showed lower glass transition temperatures than that of pure PC. Lowering of the height of the tanδ peak from DMA and reductions in the heat capacity change at the glass transition from DSC indicate that MWNTs reduced the amount of polymer material that participates in the glass transition of the composites, consistent with immobilization of polymer at the nanotube interface.     

Dendritic crystallization in thin films of PEO/PMMA blends: A comparison to crystallization in small molecule liquids

Dendritic crystallization of poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) thin films is reported. The film thickness is kept constant while the PMMA molar mass and blend composition are varied. Some basic features of dendritic growth, such as the diffusion length and tip curvature are discussed. The diffusion coefficient is tuned by varying the molar mass of the non-crystallizable PMMA and the blend composition. The observed dendrite tip radius is on the order of 50 nm and the shape of the growth envelope varies from square to needle-like as the PMMA molar mass or PMMA content is increased. The sidebranch spacing increases with the distance from the dendrite trunk with a power-law relationship that is also dependent on the PMMA molar mass and PMMA content. This coarsening process is similar to that reported for other classes of materials. These similarities (the curved dendrite tip, power-law relationship of the sidebranches, and the sidebranch coarsening processes) indicate that the large scale crystallization morphologies of the polymeric materials we study are similar to those found in crystallization of small molecules and metals.     

Nonisothermal crystallization,melting behavior,and morphology of PP/EPPE blends

Nonisothermal crystallization, melting behavior, and morphology of polypropylene (PP)/Easy processing polyethylene (EPPE) blends were studied by differential scanning alorimetry (DSC) and scanning electron microscope (SEM). The results showed that PP and EPPE are miscible, and there is no obvious phase separation in microphotographs of the blends. The modified Avrami analysis, Ozawa equation, and also Mo Z.S. method were used to analyze the nonisothermal crystallization kinetics of the blends. Values of Avrami exponent indicated the crystallization nucleation of the blends is homogeneous, the growth of spherulites is tridimensional, and crystallization mechanism of PP is not affected much by EPPE. The crystallization activation energy was estimated by Kissinger method. The result obtained from modified Avrami analysis, Mo Z.S. method, and Kissinger methods were well agreed. The addition of minor EPPE phase favored to decrease the overall crystallization rate of PP, showing some dilution effect of EPPE on PP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010