Semi-crystalline Polymer Composites Research Articles

Evaluation of Different ZX Tensile Coupon Designs in Additive Manufacturing of Amorphous and Semi-Crystalline Polymer Composites

The layer-by-layer deposition of molten polymer filament in fused deposition modeling (FDM) has evolved as a disruptive technology for building complex parts. This technology has drawbacks such as the anisotropic property of the printed parts resulting in lower strength for parts printed in the vertical Z direction compared with the other two planes. In this manuscript, we attempt to address these challenges as well as the lack of standardization in sample preparation and mechanical testing of the printed parts. The paper focuses on process parameters and design optimization of the ZX build orientation. Type I tensile bars in ZX orientation were printed as per the ASTM D638 standard using two (2B) and four (4B) tensile bar designs. The proposed design reduces material loss and post-processing to extract the test coupons. Printing a type I tensile bar in the ZX orientation is more challenging than type IV and type V due to the increased length of the specimen and changes in additional heat buildup during layer-by-layer deposition. Three different polymer composite systems were studied: fast-crystallizing nanofiller-based high-temperature nylon (HTN), slow-crystallizing nanofiller-based polycyclohexylene diethylene terephthalate glycol-modified (PCTG), and amorphous carbon fiber-filled polyetherimide (PEI-CF). For all the polymer composite systems, the 2B showed the highest strength properties due to the shorter layer time aiding the diffusion in the interlayers. Further, rheological studies and SEM imaging were carried out to understand the influence of the two designs on fracture mechanics and interlayer bonding, providing valuable insights for the field of additive manufacturing and material science.

Interlayer fusion bonding of semi-crystalline polymer composites in extrusion deposition additive manufacturing

This work focuses on the evolution of interlayer fracture toughness properties of fiber-reinforced, semi-crystalline polymers in the extrusion deposition additive manufacturing (EDAM) process. Further, this work bridges the gap between the additive process conditions (time-temperature history) and the effective layer-to-layer fracture properties developed within a printed component. This is the first step to predict delamination that can occur during printing, during cooling to room temperature after printing, and during service performance of an additively manufactured geometry. A phenomenological model is developed for fusion bonding of semi-crystalline polymer matrix composites by coupling the interdiffusion of polymer chains with the evolution of polymer crystallinity. While the interdiffusion is captured by reptation theory of polymer dynamics, the evolution of crystallinity is modeled by phenomenological crystallization kinetics and crystal melting dynamics. Further, a methodology is developed to determine the critical strain energy release rate, GICof the interlayer interface and experiments are conducted utilizing the double cantilever beam fracture test geometry. Predictions of GIC as a function of thermal history are compared with experiments.

Aggregation States of Poly(4-methylpentene-1) at a Solid Interface

A thin film of poly(4-methylpentene-1) (P4MP1) was prepared on a quartz substrate, which was a model system of an interface in filler-reinforced semicrystalline polymer composites. Grazing-incidence wide-angle X-ray diffraction measurements revealed that P4MP1 in the thin film after isothermal crystallization formed a Form I crystal polymorph composed of a tetragonal unit cell with a 72 helix, in which the chain axis was oriented along the direction parallel to the quartz interface. Combining sum-frequency generation vibrational spectroscopy with molecular dynamics simulation enabled us to gain access to the local conformation of P4MP1 chains at the quartz interface and the changes that occurred with isothermal crystallization. Finally, the way in which the initial chain orientation at the substrate interface impacted the crystalline structure in the thin film was discussed. A poly(4-methylpentene-1) (P4MP1) film was prepared on a quartz substrate, which was a model system of interfaces in a filler-reinforced semicrystalline polymer composite. Grazing-incidence wide-angle X-ray diffraction measurements revealed that P4MP1 in the film after the isothermal crystallization formed a Form I crystal polymorph, in which the chain axis was oriented along the direction parallel to the quartz interface. Combining sum-frequency generation vibrational spectroscopy with molecular dynamics simulation enabled us to study the local conformation of P4MP1 chains at the quartz interface and the changes that occurred with isothermal crystallization.

Damage Evaluation in Woven Glass Reinforced Polyamide 6.6/6 Composites Using Ultrasound Phase-Shift Analysis and X-ray Tomography

The paper proposes a new experimental methodology, based on ultrasonic measurements, that aims at evaluating the anisotropic damage in woven semi-crystalline polymer composites through new damage indicators. Due to their microstructure, woven composite materials are characterized by an anisotropic evolution of damage induced by different damage mechanisms occurring at the micro or mesoscopic scales. In this work, these damage modes in polyamide 6.6/6-woven glass fiber reinforced composites have been investigated qualitatively and quantitatively by X-ray micro-computed tomography (mCT) analysis on composite samples cut according to two orientations with respect to the mold flow direction. Composite samples are initially damaged at different levels during preliminary interrupted tensile tests. Ultrasonic investigations using C-scan imaging have been carried out without yielding significant results. Consequently, an ultrasonic method for stiffness constants estimation based on the bulk and guided wave velocity measurements is applied. Two damage indicators are then proposed. The first consists in calculating the Frobenius norm of the obtained stiffness matrix. The second is computed using the phase shift between two ultrasonic signals respectively measured on the tested samples and an undamaged reference sample. Both X-ray mCT and ultrasonic investigations show a higher damage evolution with respect to the applied stress for the samples oriented at $$45^{\circ }$$ from the warp direction compared to the samples in the $$0^{\circ }$$ configuration. The evolution of the second ultrasonic damage indicator exhibits a good correlation with the void volume fraction evolution estimated by mCT as well as with the damage calculated using the measured elastic modulus reduction. The merit of this research is of importance for the automotive industry.

Highly efficient load transfer in polypropylene/vapor grown carbon fiber nanocomposites

The alignment of fillers and the filler–matrix interfacial adhesion are crucial factors for improving the mechanical properties of nanocomposites. This study focuses on these effects in the melt-spun vapor grown carbon fibers and polypropylene (VGCFs/PP) composite fibers. By using different draw ratio, the alignment of VGCFs and the interfacial crystalline structure in the as-spun fibers can be easily controlled. Tensile testing showed that remarkable enhancements in tensile strength and modulus were achieved at high draw ratio. Topographic analysis of the fractured surfaces indicated that the enhancements were directly related to the strong interfacial adhesion and effective stress-transferring effect during the tensile test. Further analysis showed that the strong interfacial adhesion might be the result of stretching-induced transcrystalline structure. Our study sets a good example to realize a remarkable reinforcement in semicrystalline polymer composites by controlling the interfacial adhesion and filler alignment in a simple but efficient way.

Interfacial enhancement by shish–calabash crystal structure in polypropylene/inorganic whisker composites

Abstract The polymer matrix structure and the interface are strongly influenced by filler in semi-crystalline polymer composites because the fillers have the potential to nucleate the polymer crystallization. The structure of the nucleated crystalline polymer on filler is of particular interest and is a key to the interfacial enhancement. In this work, whiskers, with a large length/diameter ratio and with a diameter (0.2–2 μm) much larger than that of carbon nanotubes but much smaller than that of common fibers, were used to nucleate crystal morphology in polypropylene (PP)/whisker composites. The crystal morphology, interfacial adhesion and tensile properties of the composites were carefully investigated. A kind of peculiar shish–calabash crystallization morphology, with whisker serves as shish and PP spherulites serves as calabash, was observed for the first time in the thin film via PLM and in the injection molded bars by SEM. The formation mechanism of this shish–calabash structure was attributed to be that only a few nuclei could be induced on the whisker surface, which develop into large PP spherulites without hindrance, and finally stringed by the whisker, forming the shish–calabash structure. As a result, a significant improvement of interfacial interaction and tensile properties has been achieved.

Determination of Energy Release Rate in Carbon Fiber-Thermoplastic Composites and Comparison of Crack Propagation in Amorphous and Crystalline Interface By Single Fiber Pullout Test

The fracture toughness in the fiber-matrix composite by means of the single fiber pull-out test can be achievable, if it is possible to measure the compliance of partial debonded fiber, which necessitates a stable crack propagation and the determination of the corresponding crack length. A combination of high stiff test equipment and a simultaneous monitoring of the photo-elastic pattern of the embedded fibers with the help of polarization microscope enables to measure crack length and determination of Gc [Hampe, A and Marotzke, C. (1997). The Energy Release Rate of the Fiber/Polymer Matrix Interface: Measurement and Theoretical Analysis, J. Reinf. Plast. and Compo., 16: 341-352.]. This method is well-known for glass fiber-amorphous polymer composite. In this investigation, we tried to implement the same technique to C-fiber with amorphous and semi-crystalline polymer composites. The energy release rate (ERR) of the fiber-matrix interface was evaluated for the carbon fiber-reinforced in polycarbonate (PC) by interfacial crack propagation arising in the single fiber pullout test. Further, the crack propagation at the fiber-matrix interface with different matrix morphology was discussed with the use of photo-elastic pattern observed with the aid of polarization microscope.

Effects of thermal ageing treatment and antioxidants on the positive temperature coefficient characteristics of carbon black/polyethylene conductive composites

AbstractPolyethylene (PE)‐filled with carbon black (CB) is a prototypical composite that displays resistance switching. These materials can exhibit either a positive temperature coefficient (PTC) or negative temperature coefficient (NTC). The CB‐filled semicrystalline polymer composites ideally need antioxidants, which stabilize the composites against thermooxidative degradation, because they should be resistant to the severe conditions of high temperature. The characterization of PTC materials is affected by the crystallinity of the polymer, and the crystallinity of the polymer is changed with thermal ageing treatment. Thermal ageing of PTC samples was conducted in an oven in the range 50–140°C, in air. The composites, containing 0.5–3% (by weight) Irganox 1076 (Ciba‐Geigy), were irradiated under nitrogen at room temperature with different doses of gamma rays from a 60Co source. The resulting composites were analyzed by differential scanning calorimetry, gel fractionation, X‐ray diffraction, and dynamic mechanical analysis. The conductivity of the composites depended on the amounts of antioxidants and the duration of the thermal ageing treatment. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2316–2322, 2003