Abstract
Reinforcing polymers with discontinuous fibers improves mechanical properties, such as strength and stiffness, and in some cases achieve isotropic properties, rendering them suitable for various engineering applications. Matrix materials are generally highly engineered thermosets (e.g. crosslinked epoxies), bonded to the fiber periphery by proprietary surface and sizing chemistries. Semicrystalline thermoplastic matrices are less utilized due to poor fiber-matrix bonding resulting in inefficient interfacial load-transfer in reinforced composites. However, flexibility with melt-processing or molding conditions can be leveraged to promote non-covalent interfacial bonding between matrix and fiber via crystallization of the matrix onto fiber surface. In the present study, we utilize a co-mingle chopped carbon and isotactic polypropylene fibers to form isotropic composites, tailoring interfacial immobilized matrix or interphase morphology to optimize performance through precise control of thermal processing/molding windows. Calorimetry and optical microscopy were employed to investigate the impact of carbon fiber at various volume fractions (10, 20, and 30 %) on isotactic polypropylene crystallization and mechanical performance. Variations in mechanical properties correspond to the structural evolution of the interfacial region and are correlated to underlying microstructural attributes using wide-angle X-ray scattering, thermal analysis, and low-field nuclear magnetic resonance spectroscopy. These results provide a practical framework for the manufacturing of thermoplastic matrix composites. The results presented provide a guide for the strategic optimization of interphase design, showcasing tailorable tensile strengths which outperform any isotactic polypropylene carbon fiber composites previously reported in literature.
| Original language | English |
|---|---|
| Article number | 112027 |
| Journal | Composites Part B: Engineering |
| Volume | 291 |
| DOIs | |
| State | Published - Feb 15 2025 |
Funding
To contextualize the tensile testing results we also performed interlaminar shear strength (ILSS) measurements according to established protocols in ASTM D5379. The ILSS format produces results that are sensitive to the fiber-matrix interactions and provide useful indications of which polymer morphology produces the most constructive interactions with the reinforcing phase. It should be noted that the V-Notch or Iosipescu test that measures ILSS may induce impure shear in thermoplastic materials attributing to the limitations of the test standard in materials that exhibit high amounts of plasticity [69,70]. However, the iPP-CF10 press-cooled sample displayed the highest overall ILSS value of 40 MPa, indicative of a well adhered polymer interphase. Higher CF volume fractions within press profile have lower ILSS values ranging between 28 and 30 MPa, but still higher than any other process condition. A universal result is that the iPP-CF10 composition possesses the highest ILSS values for each cooling profile, suggesting that CF concentrations above 10 vol% restrict chain mobility and inhibit optimal crystal interphase formation. The macroscale tensile properties (Fig. 6A) result from the combined effects from increasing rigid CF fraction and the quality of the iPP microstructural environment. SEMs of fracture surfaces support these findings (Fig. 7), with bound polymer regions and fiber breakage. Fig. 7A displays iPP-CF20 press-cooled samples with greater residual iPP matrix attached to the CF fiber comparatively to iPP-CF20 quenched samples (Fig. 7B). Quench profile composites exhibited a cleaner fiber during fracture, as highlighted indicating insufficient interfacial adhesion between the iPP and the CF. Additional SEM images can be found in supplemental information (Fig. S7). These results emphasize the importance of composition selection and precise control of processing conditions to maximize manufacturing throughput without compromising performance.DMA is a viable tool for studying the dynamic thermomechanical properties of composites and is particularly sensitive to interphase characteristics [74,75]. Fig. 9 presents the results of a temperature sweep performed at 1 Hz with composite samples varying CF volume fractions and cooling protocols [39]. The storage modulus (E\u2032) quantifies the elastic energy storage capacity of a material and is used to scale the static deformation mechanical strength [76]. At temperature below glass transition temperatures (Tg), the polymer chains are in a glassy state, forming the so-called glassy plateau, until Tg is reached and the magnitude of E\u2032 decreases due to the \u03B1-relaxation transition [77]. The drop in E\u2019 is, however, dependent on the crystallinity of the matrix as the crystal phase remains rigid until it reaches melting temperature. Regardless of CF content, the press-cooling condition, denoted iPP-CFXXp, displayed the highest values of the glassy plateau, indicating a higher amount of crystallinity [78]. Conversely, the quench processed samples possess the lowest E\u2032 across all CF contents, which correlates well with the mechanical testing results. Minimal contrast, especially in the higher-loaded CF composites (iPP-CF20 and iPP-CF30) between the varying process conditions suggest that the isothermal conditioning temperature did not produce distinctive microstructural differences, possibly due to increased thermal conductivity of loaded CF and consequential heat loss. Furthermore, the lack of strong variation in higher CF loading values are attributed to a narrowing of the crystallization kinetic window with increased available surface area, as supported by the crystallization kinetics data from DSC (Fig. 3D).Tan \u03B4, a measure of the magnitude of the composite's damping, is defined as the ratio of the loss modulus to the storage modulus (E\u201D/E\u2032). The tan \u03B4 peak indicates glass transition temperature (Tg), where thermally induced segmental motions become coordinated over a longer length scale and the polymer chains become rubbery. For all CF loading compositions, the tan \u03B4 peak appears to have only moderately different temperature values. However, these very low-strain DMA tan \u03B4 peak magnitudes in higher strength composites with 20 and 30 % CF loadings are slightly higher than the quenched samples (0.010 vs. 0.007 and 0.007 vs. 0.005, respectively). Given that the iPP crystallites at higher CF loading exhibit thinner, more disorganized lamellae, we expect higher damping behavior from the amorphous phase fractions (that are slightly higher for higher fiber mass fractions). At each fiber loading condition, we observe very high storage modulus in high strength (press-cooled) samples compared to the quenched samples. Essentially, these transcrystalline iPP phases (though contains imperfection and thinner lamellae) on fiber surface enhance fiber-matrix load transfer and increase composite storage modulus significantly. An important note is that within the low-strain regime used for these tests, the storage modulus trends do not necessarily exactly translate to static loading mechanical strength. At -20C composites with 10 % CF loading exhibit 40,363 MPa storage modulus for press-cooled sample and 15,830 MPa for quenched samples. Thus, at \u221220 \u00B0C for composites with 20 % CF loading exhibit 15,300 MPa storage modulus for press-cooled sample and 5670 MPa for quenched samples. Similarly, at \u221220 \u00B0C for composites with 30 % CF loading exhibit 12,645 MPa storage modulus for press-cooled sample and 6479 MPa for quenched samples. These trends (significantly higher E\u2032 for press-cooled samples compared to the quenched samples) are maintained even at higher temperature range (50\u2013140C). The other contrasting observation was the lower tan \u03B4 values for all press-cooled samples compared to the quenched samples at high temperature range (80\u2013140 \u00B0C). This signifies that the immobilized iPP crystalline phases on CF surface (though imperfectly crystallized) suppresses mobility of the amorphous iPP chains due to physiosorbed lamellae or crystalline interphase formation [79]. Retaining high stiffness or dynamic storage modulus for all temperature range and delivering lower damping behavior at high temperature range in press-cooled composites supports our hypothesis of trans-crystallization of the iPP, that occurs from the CF surface, contributes to improved adhesion at the fiber-resin interface and interphase formation. This interphase formation causes a significant increase in the strength of the composite material. For ideal composites possessing chemically linked phases, interphases transfer load and do not contribute significantly to tan \u03B4. Under these conditions, damping can reasonably be predicted based on phase fraction of the matrix. Physiosorbed semi-crystalline polymer matrices like iPP complicate this molecular landscape, but generally show lower damping properties for better interphase quality. To better understand these results, we performed low field NMR to interrogate the phase fractions of polymer dynamics within the composite.The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Amit K. Naskar reports financial support was provided by US Department of Energy. Co-author Prof. Uday Vaidya is one of the Editors-in-Chief of this Journal (Composites Part B: Engineering) If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This manuscript had been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or repro-duce the published form of this manuscript or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC05-00OR22725, was sponsored by the Vehicle Technologies Office (VTO) within the Office of Energy Efficiency and Renewable Energy (EERE). LTK and JTD acknowledge support from the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences, and Engineering Division [FWP#ERKCK60] for WAXS and spectroscopic characterization of nanofiber-matrix interface. This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC05-00OR22725, was sponsored by the Vehicle Technologies Office ( VTO ) within the Office of Energy Efficiency and Renewable Energy ( EERE ). LTK and JTD acknowledge support from the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences, and Engineering Division [FWP#ERKCK60] for WAXS and spectroscopic characterization of nanofiber-matrix interface.
Keywords
- Compression molding
- Dynamic mechanical analysis
- Fiber reinforced polypropylene composite
- Interfacial bonding
- Interfacial crystallization
- Phase morphology
