Promoting effect of methyne/methylene moiety of bisphenol E/F on phthalonitrile resin curing: Expanding the structural design route of phthalonitrile resin

Preparation of phthalonitrile resins containing siloxane linkages with improved processability

To improve the processability of biphenyl phthalonitrile resin, a flexible siloxane structure was introduced into the phthalonitrile monomer through molecular design, which was then blended with a biphenyl monomer to prepare phthalonitrile alloy resins. When the ratio of phthalonitrile monomer containing flexible siloxane to biphenyl phthalonitrile monomer was 1:1, the processing window widened from 58 to 110°C, as compared to that of biphenyl phthalonitrile. Due to the introduction of the biphenyl structure into the phthalonitrile alloy resins, the initial decomposition temperature of the silicon-containing phthalonitrile resin increased from 385 to 516°C. More importantly, the phthalonitrile alloy resin exhibited a high bending strength (66 MPa) and bending modulus (3762 MPa), indicating that it could be potentially applied as high temperature structural composite matrices. Furthermore, it provides a new strategy for processing phthalonitrile resins with a high melting point and narrow processing window.

Improvements in Temperature Uniformity in Carbon Fiber Composites during Microwave-Curing Processes via a Recently Developed Microwave Equipped with a Three-Dimensional Motion System

Curing processes for carbon-fiber-reinforced polymer composites via microwave heating are promising alternatives to conventional thermal curing because this technology results in nonhomogeneous temperature distributions, which hinder its further development in industries. This paper proposes a novel method for improving heating homogeneities by employing three-dimensional motion with respect to the prepreg laminate used in the microwave field by using a recently developed microwave system. The maximum temperature deviation on the surface of the laminate can be controlled within 8.7 °C during the entire curing process, and it produces an average heating rate of 1.42 °C/min. The FT-IR analyses indicate that microwave heating would slightly influence hydroxyl and methylene contents in the cured laminate. The DMA measurements demonstrate that the glass transition temperatures can be improved by applying proper microwave-curing processes. Optical microscopy and mechanical tests reveal that curing the prepreg laminate by using a multistep curing process that initially cures the laminate at the resin's lowest viscosity for 10 min followed by curing the laminate at a high temperature for a short period of time would be favorable for yielding a sample with low void contents and the desired mechanical properties. All these analyses are supposed to prove the feasibility of controlling the temperature difference during microwave-curing processes within a reasonable range and provide a cured laminate with improved properties compared with conventional thermally cured products.

A Pyridazine-Containing Phthalonitrile Resin for Heat-Resistant and Flame-Retardant Polymer Materials

In this study, a novel phthalonitrile monomer containing a pyridazine ring, 3,6-bis[3-(3,4-dicyanophenoxy)phenoxy]pyridazine (BCPD) with a low melting point (74 °C) and wide processing window (178 °C), was prepared by a nucleophilic substitution reaction. The molecular structure of the BCPD monomer was identified by Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). Poly(BCPD) resins were derived from the formulations by curing at 350 and 370 °C. The thermoset that was post-cured at 370 °C demonstrated outstanding high heat-resistant (glass transition temperature (Tg) > 400 °C, 5% weight loss temperature (T5%) = 501 °C, Yc at 900 °C > 74%) and was flame-retardant (limiting oxygen index (LOI) = 48)). Further, the poly(BCPD) resin simultaneously exhibited a superior storage modulus, which could reach up to 3.8 Gpa at room temperature. Excellent processability and heat resistance were found for phthalonitrile thermosets containing the pyridazine ring, indicating poly(BCPD) resin could be potentially applied as high-temperature structural composite matrices.

Synthesis of oligomeric phthalonitrile resins containing imide units and study of the methylene-cyano thermal synergistic polymerization effect

The meta- and para-catenated methylene-containing phthalonitrile oligomers were prepared from the reaction of an excess amount of 4,4′-(4,4′-isopropylidenediphenoxy) bis-(phthalic anhydride) (BPADA) with 4,4′-Methylenedianiline (MDA) in a N, N-dimethylformamide/ toluene solvent mixture, followed by end-capping agent with 4-nitrophthalonitrile or 4-phenoxyaniline in a two-step, one-pot reaction. Differential scanning calorimetry (DSC) showed that both PN oligomers exhibited low softening points. The self-catalyzed curing reactivity of the PN oligomers was confirmed by the isothermal rheological measurements. Fourier transform infrared spectroscopy (FTIR) and ultraviolet and visible spectrophotometry (UV–Vis) data of the pre-curing resins were employed to investigate the chemical structure of the pre-cured resins, suggesting that oligomers generated crosslinking sites, including triazine, isoindoline, and phthalocyanine. The results further confirmed the self-catalyzed curing reactivity of the oligomers. Thermal properties were investigated by dynamic mechanical analysis (DMA) and thermal gravimetric analysis (TGA), demonstrating good thermal properties of the cured resins. The glass transition temperatures (Tgs) of PIPN-1-325, PIPN-1-350, PIPN-1-375 were in the range of 285–345°C, the 5% weight loss temperature (T5%) was observed at 482°C. The PIPN-2-325, PIPN-2-350, PIPN-2-375 showed Tgs ranging from 293 to 370°C, and T5% of the resins were in the range of 481–501°C. Then the isothermal rheological results of model compound and PN oligomers implied that the curing process of PN oligomers was closely related to the methylene-cyano radical thermal synergistic polymerization (TSP) effect proposed in our previous research, and then a revised curing mechanism (radical TSP mechanism) was proposed.

Catalytic Polymerization of Phthalonitrile Resins by Carborane with Enhanced Thermal Oxidation Resistance: Experimental and Molecular Simulation

Biphenyl phthalonitrile (BPh) resins with good thermal and thermo-oxidative stability demonstrate great application potential in aerospace and national defense industries. However, BPh monomer has a high melting point, poor solubility, slow curing speed and high curing temperature. It is difficult to control the polymerization process to obtain the resins with high performance. Here, a BPh prepolymer (BPh-Q) was prepared by reacting 1,7-bis(hydroxymethyl)-m-carborane (QCB) with BPh monomers. The BPh-Q exhibited much better solubility, faster curing speed and lower curing temperature compared with pure BPh and BPh modified with bisphenol A (BPh-B, a common prepolymer of BPh). Thus, the polymerization process of BPh was greatly accelerated at a low temperature, resulting in a BPh resin with enhanced thermostability and oxidation resistance. The experimental and theoretical models revealed the promotion effect of B-H bond on the curing reaction of phthalonitrile via Markovnikov addition reaction due to the special steric structure of carborane. This study provided an efficient method to obtain low-temperature curing phthalonitrile resins with high thermal and thermo-oxidative resistance, which would be potentially useful for the preparation of high-performance cyanide resin-based composites.

Novel self-promoted phthalonitrile monomer with siloxane segments: synthesis, curing kinetics, and thermal properties

In the study, we synthesize a novel auto-catalytic phthalonitrile monomer containing siloxane segments and secondary amino groups. The phthalonitrile monomer has good processability. And the new polymer shows a higher Tg.