Tailoring toughness of injection molded bar of polypropylene random copolymer through processing melt temperature

In this study, a facile route to realize the superior toughness of injection molded polypropylene random copolymer (PPR) is reported. The toughness of PPR is increased about twofold when the processing melt temperature increases from 180 to 250 °C. Systematic and detailed structural characterizations have been carried out to establish the structure–property relationships by using polarized light microscopy, scanning electron microscopy, infrared microscopy and dynamic mechanical analysis. It is found that increasing the melt temperature is beneficial for the coalescence of rubbery domains and enhanced molecular mobility which are mainly responsible for the improvement in toughness. Other factors, such as molecular orientation, crystallinity and so on, seem to have little effect. The vital role of enhanced molecular mobility in improving toughness is further demonstrated by the annealing of injection molded samples at elevated temperature, i.e. 110 °C. Copyright © 2011 Society of Chemical Industry     

The use of light microscopy for investigations of epitaxial growth of polymers on polymeric substrates

Summary In order to study the oriented (epitaxial) crystallization of thermoplastic polymers on oriented polymer substrates, generally the transmission electron microscopy (TEM) is used. With this instrument, the crystallized material can easily be resolved and orientation relationships can be monitored by electron diffraction. Disadvantages are the time consuming sample preparations and difficulties in the in-situ observations of the crystallization events, because of the radiation sensitivity of the polymer crystals. It is demonstrated that these disadvantages of the TEM can be eleminated by the use of different methods of light optical contrasts under specific preparation conditions of the samples and that the optical microscopy being a supplementary method to the TEM for investigations of epitaxial crystallization.     

Morphology and structure of poly(p-phenylene terephthalamide) crystallized from dilute organic solution

The high-temperature crystallization of poly(p-phenylene terephthalamide) (PPTA) from dilute organic solutions was achieved through the introduction of a non-solvent, or precipitating agent, at the desired crystallization temperature. The morphology and crystal structure were examined for crystals produced from PPTA polymer with two different molecular weights (M_w = 46000 and 3430 g mol⁻¹), using transmission electron microscopy. For the high-molecular-weight polymer, ribbon-like crystals were produced, while the low-molecular-weight polymer yielded small needles or platelets. In both cases, electron diffraction showed that the Northolt allomorph was obtained. For the high-molecular-weight polymer, the molecular axes were parallel to the ribbon axes in a chain-extended type structure. A hypothesis for the orientation of the low-molecular-weight PPTA in the small platelets, is also given.     

Fracture behavior of oriented poly(ether-ether-ketone) (PEEK)

Poly(ether-ether-ketone) (PEEK) is a newly developed engineering thermoplastic with potentially vast application in advanced composites due to its exceptional performance. It is thus desired to understand the relationship between physical processing, microstructure and fracture in this semicrystalline polymer. Both oriented and unoriented PEEK were mechanically characterized using static test of three-point bend specimens. The molecular chain orientation was imposed using a rolltrusion technique. The effects of thickness, strain rate, Initial crack length ratio, and orientation on fracture toughness (K_c) are investigated. The crystallinity is also examined by density measurement. The degree of orientation is determined qualitatively by wide-angle X-ray scattering diffraction patterns and quantitatively by further measurement using an image analysis system. Fractographic analysis, using scanning electron microscopy, provides precise information about the mode of fracture, Results indicate that both the modulus and the fracture toughness are remarkedly increased in the direction of drawing (T-type) as opposed to the transverse direction (L-type).     

Structure image of single crystal of polyethylene

High-resolution electron macroscopic images of lamellar single crystal of polyethylene (PE) have been successfully obtained using high-resolution electron microscopy (HREM), although so far the feasibility of obtaining HREM images from such a radiation sensitive polymer is still drastically questioned. The HREM images with a clear two-dimensional periodic structure reported here were recorded in a transmission electron microscope operated at 200 kV. The images consisted of lattice fringes derived from the 〈001〉 zone, and the structure images of different lattice fringes were resolved. To our knowledge, this is the first time that such clear structure images of PE have been reported at a molecular level.     

Eutectic solidification and oriented growth in mixtures of polyethylene and 1,3,5-tribromobenzene

The melting and crystallization behaviour of mixtures of linear polyethylene and 1,3,5-tribromobenzene has been investigated by differential scanning calorimetry, optical and electron microscopy. The phase diagram reveals the existence of a pseudo-binary eutectic (polymer volume fraction of about 58%; T_m=116°C). The experimental observations are compared with the predictions of the Flory-Huggins theory of melting point depression. The polyethylene component of the eutectic assumes a habit in which lamellae are highly oriented with the chains in the [110] contact plane parallel to the tribromobenzene needle axes. This well defined orientation is most probably based on an epitaxial relation between polymer and substrate.     

Aggregation behavior of two-arm fullerene-containing poly(ethylene oxide)

A water-soluble two-arm fullerene-containing poly(ethylene oxide) (PEO) was synthesized through isocyanate-hydroxy condensation reaction with fullerenol as a molecular core and characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetry, and matrix-assisted laser desorption ionization time of flight mass spectrometry. The aggregation behavior of the resulting amphiphilic polymer in water, THF, and DMF, was studied by gel permeation chromatography, laser light scattering, and transmission electron microscopy. The polymer forms spherical aggregates with an aggregation number around 540-1020.     

On-off switching properties of one-dimensional photonic crystals consisting of azo-functionalized polymer liquid crystals having different methylene spacers and polyvinyl alcohol

In this paper, photoresponsive behavior of multi-bilayered films having precisely controlled layer thickness prepared by stacking an azo-functionalized polymer liquid crystal, PMAzXAc, and polyvinyl alcohol alternatively, PVA, is described. The multi-bilayered films were found to reflect a light of specific wavelength depending on the layer thickness and refractive index, and showed the reversible change in the reflection intensity by irradiation with visible and UV lights. The change in the reflection intensity was brought about by change in the molecular orientation of PMAzXAc between an out-of-plane orientation and a photo-induced isotropic state, and was strongly dependent on the number of methylene spacer of PMAzXAc linking the azobenzene side group with the acrylate polymer main chain. PMAz6Ac with hexa-methylene spacer showed the largest change in the reflection intensity, while smaller change in the reflection intensity was observed for PMAzXAc having shorter or longer methylene spacer than 6. The effect of the methylene spacers on the photochemical change in the molecular orientation of azobenzene chromophores in the multi-bilayered films will be discussed.     

Orientated CNT‐polypropylene nanocomposite films made from extruded monofilaments

In this study, a method for making an orientated polymer nanocomposite film was developed. Melt‐drawn nanocomposite monofilaments of isotactic polypropylene and acicular nanofillers, i.e. carbon nanotubes (CNT) or vapor grown carbon nanofibers (VGCF), were prepared and characterized from the aspect of polymer chain orientation, mechanical properties, and overall morphology. A marked improvement in mechanical properties was observed as a function of the addition of CNT, increasing draw down ratio (DDR) and annealing. Nanocomposite films were prepared from drawn monofilaments by hot‐pressing under low pressure in order to maintain the orientation of the monofilaments. Wide angle X‐ray diffraction showed a high degree of residual orientation in the films. Electron microscopy (high‐resolution scanning electron microscopy, transmission electron microscopy) unexpectedly revealed that the CNT‐matrix interface is amorphous. However, differential scanning calorimetry found no measurable influence of the CNT on the overall crystallinity as determined by the enthalpy of melting of the matrix. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

液晶聚合物纤维的牵引工艺与性能

通过单螺杆挤出机挤出,利用牵引机的不同牵引速度获得不同液晶聚合物纤维试样,采用万能试验机、电子显微镜扫描(SEM)分析其力学性能和内部取向.结果表明:随着牵引速度增大,其内部的取向逐渐规整,内部组成的微纤排列越紧凑,导致液晶聚合物纤维的弹性模量增大;同时,当牵引速度到达一定数值时,取向程度不再增大,液晶聚合物纤维的弹性模量呈下降趋势;在一定时间段内,经过温度为150℃的后处理,将使液晶聚合物纤维内部分子结构达到更完善的取向状态,力学性能有所提高.     

Effects of thermal treatment on the microstructure and thermal and mechanical properties of poly(lactic acid) fibers

The microstructure and thermal and mechanical properties of poly(lactic acid) (PLA) fibers after thermal treatment under both taut and free conditions at different temperatures were studied by differential scanning calorimetry, wide‐angle X‐ray diffraction, microscopy with polarized light, an acoustic method and tensile testing. In particular, the effects of thermal treatment conditions on crystallinity, molecular orientation, and mechanical properties, as well as their interrelationships, were investigated. Both the crystal orientation and crystallinity of the PLA fibers were higher after thermal treatment under taut conditions, even though the molecular orientation was not higher. Thermal treatment relaxed orientated segments, which resulted in increasing crystallinity; however, it did not indicate that the overall molecular alignment of the chains (molecular orientation) would necessarily be high. Tensile strength and elongation were increased with increasing treatment temperature up to 150°C, which corresponded to crystallinity and chain relaxation. The crystallinity detected by XRD, enthalpy measured by DSC, birefringence, and molecular orientation were compared to distinguish the effects of crystallinity, orientation of polymer chains, and crystals. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Preparation of polyacrylonitrile/graphene oxide by in situ polymerization

Highly oriented molecular structure is essential for high‐performance carbon fibers. The addition of a small amount of graphene sheets may enhance the degree of molecular orientation of precursor fibers during spinning and stabilization by limiting the disorientation of the chain segments. Graphene sheets merge into the carbon fiber structure during carbonization. The structure and properties of polyacrylonitrile containing graphene oxide (GO) prepared by in situ polymerization were investigated. With increasing GO loading, the molecular weight of the polymer decreased gradually from 69 000 g mol^?1 for the sample without GO to 60 600 g mol^?1 for the sample with 2.5 wt% loading of GO. Scanning electron microscopy and X‐ray diffraction results indicated that GO was dispersed in single layers in the polymer matrix. The degree of crystallization of the polymer with 0.5 wt% GO was increased by 8%. Moreover, differential scanning calorimetry and thermogravimetric analysis showed that an appropriate amount of GO, e.g. 0.5 wt%, made the carbon yield of the polymer increase by 5.0 wt%, because the GO in the composite improved the intermolecular crosslinking reaction. Copyright © 2012 Society of Chemical Industry     

Enhanced Molecular Alignment in Poly‐l‐Lactic Acid Nanotubes Induced via Melt‐Press Template‐Wetting

Molecular ordering in polymers can have a drastic effect on their properties and can be used to induce or enhance functionality. In the case of poly‐l ‐lactic acid (PLLA), which is a widely used polymer in biomedicine, sensors, and actuators, preferential orientation of chains can lead to significantly enhanced electromechanical properties. In this context, template‐wetting is a straightforward method of producing polymer nanostructures, which can lead to some degree of molecular order in the polymer. Template‐wetting of PLLA has not been fully explored, especially in terms of morphological and/or structural characterization. In this work, PLLA nanotubes are grown via a modification of the template‐wetting process, referred to here as melt‐press template‐wetting. The nanotubes are thoroughly characterized with wide‐angle X‐ray diffraction, isothermal differential scanning calorimetry, and polarized light optical microscopy. This characterization indicates that the polymer chains in these PLLA nanotubes are aligned parallel to the cylindrical axis of the nanotube, which may be beneficial in certain applications.     

Structural development of HDPE in injection molding

This study investigated some relevant structure/properties relationships in shear‐controlled orientation in injection molding (SCORIM) of high‐density polyethylene (HDPE). SCORIM was used to deliberately induce a strong anisotropic character in the HDPE microstructure. Three grades with different molecular weight characteristics were molded into tensile test bars, which were subsequently characterized in terms of the mechanical behavior by tensile tests and microhardness measurements. The structure developed upon processing was also characterized by polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). SCORIM allows the production of very stiff molded parts, exhibiting a very well‐defined laminated morphology. This morphology is associated with both an M‐shaped microhardness profile and a pronounced mechanical anisotropy. These characteristics are supported by an analogous variation in the crystallinity and a high level of molecular orientation, as indicated, respectively, by calorimetric measurements and X‐ray diffraction results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2079–2087, 2003     

Ultraschallabbau und Mikromorphologie amorpher Hochpolymerer

The microstructure of polydimethylsiloxane and polychloroprene, thermically treated polychloroprene and natural rubber was investigated by ultrasonic degradation and electron microscopy. The rate of ultrasonic depolymerisation in solution was determined by viscosity measurements. The limits of molecular weight were found between about 10000 and 30000. The irradiation times needed differed very much from the investigated polymers. The ultrasonic intensity has no influence on the limit of molecular weight. For thermically not treated polymers was found that following molecular weights are identical: 1. the limiting molecular weight after ultrasonic degradation, measured by viscosimetric method, 2. the molecular weight of tertiary structural units of undegraded and degraded polymer, determined by measuring electron micrographs. The ultrasonic degradation consequently has no influence on the size of tertiary units of a polymer. This result proves that pearlstring-molecules also exist in solution. The result suggests following mechanism of ultrasonic degradation: the molecule resembling a string of pearls is broken at the chain between the spherical units. For thermically treated and not treated polychloroprene the same size of tertiary units was found, but in the thermically treated product there was a big share with smaller structural units.     

In Situ polymerization in binary polymer/monomer mixtures: A novel route to polymeric composites

The formation of a polymer/polymer composite by solid-state polymerization of trioxane (TOX) crystals grown within binary trioxane/polycaprolactone or trioxane/poly(oxythylene) mixtures is reported. At present, such composites have been formed with trioxane-rich (hypoeutectic) mixtures. It is observed that in this composition range, much higher yields are obtained through thermal orientation of the TOX crystals which result in very highly ordered systems as revealed by optical and electron microscopy. These POM-rich composites were not, however, amenable to mechanical testing.     

Forced assembly by multilayer coextrusion to create oriented graphene reinforced polymer nanocomposites

A potential advantage of platelet-like nanofillers as nanocomposite reinforcements is the possibility of achieving two-dimensional stiffening through planar orientation of the platelets. The ability to achieve improved properties through in-plane orientation of the platelets is a challenge and, here, we present the first results of using forced assembly to orient graphene nanoplatelets in poly(methyl methacrylate)/polystyrene (PMMA/PS) and PMMA/PMMA multilayer films produced through multilayer coextrusion. The films exhibited a multilayer structure made of alternating layers of polymer and polymer containing graphene as evidenced by electron microscopy. Significant single layer reinforcement of 118% at a concentration of 2 wt % graphene was achieved—higher than previously reported reinforcement for randomly dispersed graphene. The large reinforcement is attributed to the planar orientation of the graphene in the individual polymer layers. Anisotropy of the stiffening was also observed and attributed to imperfect planar orientation of the graphene lateral to the extrusion flow.     

Functionalized graphene nanoplatelets as a barrier enhancing filler in organic photovoltaic encapsulant

This research work has concerned the development of polymer films, reinforced with graphene nanoplatelets (GNP) for use as encapsulating films for organic photovoltaic (OPV) cells. The aim of this work was to investigate the effects of concentrations and orientations of GNP on mechanical, optical, and barrier properties of polymer composite films. In this regard, the neat GNP was modified with Fe₃O₄ prior to mixing with acrylate-based monomers. The mixture was then cured by photo-polymerization with and without the application of magnetic fields. Changes in orientation of the functionalized GNP with the direction of applied magnetic fields were analyzed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. From the results, it was found that by inducing the orientation of functionalized GNP to the horizontal direction (with respect to the OPV cell), the great enhancement in tensile and barrier properties of the polymer composite films was achieved. This led to the longer performance of the OPV cell encapsulated with the nanocomposite film with 0.1 phr of the horizontally oriented GNP in comparison with the OPV cell encapsulated with the film reinforced with randomly oriented GNP at the same content.     

A comparative study on the thermally enhanced reorientation between random and diblock methacrylate copolymers with photo-cross-linkable mesogenic side groups by irradiating with linearly polarized ultraviolet light

The thermally enhanced photoinduced reorientation behavior of random and diblock copolymer films of methyl methacrylate and methacrylate with a photo-cross-linkable 4-(4-methoxycinnamoyloxy)biphenyl (MCB) side groups was investigated by irradiating with linearly polarized ultraviolet light and subsequently annealing. Random copolymers were synthesized by free radical copolymerization, while diblock ones were obtained by an atom transfer radical polymerization method with a PMMA macroinitiator. The photoinduced optical anisotropy was thermally amplified when the copolymer exhibited a liquid crystalline phase. The random copolymers with a high composition of MCB side groups reverted the orientation direction and inhibited molecular aggregation. For the diblock copolymers, the reorientation behavior was analogous to the methacrylate homopolymer with MCB side groups and transmission electron microscopy revealed a phase separation structure with molecular orientation of the MCB groups.     

Characterization of nanoclay orientation in polymer nanocomposite film by small-angle X-ray scattering

The orientation distribution of layer-shaped nanoclays (e.g. organoclays and pristine clays) dispersed in a polymer matrix is an important parameter to control the properties of polymer nanocomposites. In this study, we demonstrate that the use of multi-directional 2-D small-angle X-ray scattering (SAXS) can quantitatively describe the orientation distribution of organoclays (e.g. Cloisite C20A) in melt-pressed nanocomposite films, containing ethylene-vinyl acetate (EVA) copolymers as polymer matrices. Different weight fractions of organoclays were used to alter the orientation profile of nanocomposite films, in which the dispersion and morphology of organoclays were also characterized by complementary 2-D and 3-D transmission electron microscopy (TEM). All nanocomposites exhibited mixed intercalation/exfoliation clay morphology, where the intercalated structure possessed partial orientation parallel to the in-plane direction of the film. The higher content of the clay loading showed a higher clay orientation. A simple analytical scheme for SAXS data analysis to determine the orientation parameter (P₂) was demonstrated, the results of which are in agreement with the gas permeation properties of the nanocomposite films.