Crystallization/Melting Kinetics and Morphological Analysis of Polyphenylene Sulfide

Tailoring the Crystallinity of Ultrasonically Welded Interfaces in Glass Fiber-Reinforced Thermoplastic Composites

Ultrasonic welding (USW) is a fast and effective method for joining thermoplastic composites, offering excellent bonding strength that results in lightweight, durable structures, making it a cost-effective alternative to traditional joining techniques. The crystallinity at the weld interface impacts the mechanical properties and chemical resistance of the joint. The crystallization mechanisms at the bonded interface remain inadequately understood for the USW process, especially given its rapid cooling rates. This study investigates the use of polypropylene (PP) and multiwalled carbon nanotube (MWCNT)/PP films for ultrasonic welding of glass fiber (GF)/PP adherends, focusing on how process parameters influence the crystallinity degree, crystalline phases, crystallite size and spacing, lamellar structure and anisotropy, and molecular changes at the welded interface. Differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) were employed to gain a better understanding of crystalline structure at the interface. Four different sets of welding force and amplitude were tested: (1) 500 N, 38.1 μm, (2) 500 N, 54.0 μm, (3) 1500 N, 38.1 μm, and (4) 1500 N, 54.0 μm. The study demonstrated that despite fast cooling rates obtained during the process, higher welding force and amplitude significantly enhanced crystallinity, achieving 55% for welds with pure PP films and approximately 60% for MWCNT/PP films, compared to 35% and 41%, respectively, before welding. Notably, amplitude influenced the crystallinity at the welded interface more significantly compared to the force. SAXS experiments revealed that both pure PP and MWCNT/PP films exhibited isotropic structures prior to welding, but distinct anisotropy after welding. These findings suggest that strain-induced crystallization results from the welding process, with the degree of anisotropy correlating with the applied welding force and amplitude.

Additive Manufacturing of Poly(phenylene Sulfide) Aerogels via Simultaneous Material Extrusion and Thermally Induced Phase Separation

Additive manufacturing (AM) of aerogels increases the achievable geometric complexity, and affords fabrication of hierarchically porous structures. In this work, a custom heated material extrusion (MEX) device prints aerogels of poly(phenylene sulfide) (PPS), an engineering thermoplastic, via in situ thermally induced phase separation (TIPS). First, pre-prepared solid gel inks are dissolved at high temperatures in the heated extruder barrel to form a homogeneous polymer solution. Solutions are then extruded onto a room-temperature substrate, where printed roads maintain their bead shape and rapidly solidify via TIPS, thus enabling layer-wise MEX AM. Printed gels are converted to aerogels via postprocessing solvent exchange and freeze-drying. This work explores the effect of ink composition on printed aerogel morphology and thermomechanical properties. Scanning electron microscopy micrographs reveal complex hierarchical microstructures that are compositionally dependent. Printed aerogels demonstrate tailorable porosities (50.0-74.8%) and densities (0.345-0.684 g cm-3), which align well with cast aerogel analogs. Differential scanning calorimetry thermograms indicate printed aerogels are highly crystalline (≈43%), suggesting that printing does not inhibit the solidification process occurring during TIPS (polymer crystallization). Uniaxial compression testing reveals that compositionally dependent microstructure governs aerogel mechanical behavior, with compressive moduli ranging from 33.0 to 106.5 MPa.

The Influence of Thermal Parameters on the Self-Nucleation Behavior of Polyphenylene Sulfide (PPS) during Secondary Thermoforming

During the secondary thermoforming of carbon fiber-reinforced polyphenylene sulfide (CF/PPS) composites, a vital material for the aerospace field, varied thermal parameters profoundly influence the crystallization behavior of the PPS matrix. Notably, PPS exhibits a distinctive self-nucleation (SN) behavior during repeated thermal cycles. This behavior not only affects its crystallization but also impacts the processing and mechanical properties of PPS and CF/PPS composites. In this article, the effects of various parameters on the SN and non-isothermal crystallization behavior of PPS during two thermal cycles were systematically investigated by differential scanning calorimetry. It was found that the SN behavior was not affected by the cooling rate in the second thermal cycle. Furthermore, the lamellar annealing resulting from the heating process in both thermal cycles affected the temperature range for forming the special SN domain, because of the refined lamellar structure, and expelled various defects. Finally, this study indicated that to control the strong melt memory effect in the first thermal cycle, both the heating rate and processing melt temperature need to be controlled simultaneously. This work reveals that through collaborative control of these parameters, the crystalline morphology, crystallization temperature and crystallization rate in two thermal cycles are controlled. Furthermore, it presents a new perspective for controlling the crystallization behavior of the thermoplastic composite matrix during the secondary thermoforming process.