ABSTRACT:This article describes the use of hyperbranched polyester oligomers (HBPs) as modifiers for epoxy thermosets. The effect of HBP molar mass, end group, and loading on prepolymer viscosity, thermoset fracture toughness, T g , and high-temperature dynamic storage modulus (EЈ) were measured. The HBP molar mass was systematically increased from nominal values of ϳ 1750 g mol (Generation 2, or G2) up to ϳ 14,000 g mol (Generation 5, or G5), which corresponds from a low of two layers of monomer up to a maximum of five layers of monomer around the central core. Toughness increased only modestly with the molar mass of the HBP. At 7% loading in the epoxy thermoset, the G5 HBP increased toughness by ϳ 60% over the untoughened control. Toughness increased to 82% above the untoughened control at a loading of 19% G5 HBP, but the toughness decreased at 28% HBP loading. The T g and EЈ were influenced by the HBP modifier, but the effect was not systematic and may have been due to competing effects of HBP molar mass and end group. The effect of the architecture of the thermoplastic modifier was investigated by introducing a linear aliphatic polyester (ϳ 5400 g mol) with a repeat unit structure, which was similar to that of the HBP. At the molecular weight range investigated, neither the prepolymer viscosity nor the thermoset toughness of the HBP-epoxy was significantly different from that of the linear polyester in epoxy. Preliminary results are presented showing the effect of thermoplastic molecular weight and architecture on morphology.
This is the fourth article in a series describing efforts to produce tough, high-performance thermosets from very low viscosity prepolymers which are autoclave processable. Hydroxy-terminated hyperbranched polyester (HBP) with a systematically increased molar mass was used to toughen bismaleimide (BMI). HBP was dissolved in the allyl phenol component, B, of a two-part BMI, to yield homogeneous solutions. The BMI monomer, A, was dissolved in the solution of HBP in B to give homogeneous prepolymers. The fracture toughness (K Ic ) of neat resin plaques was measured by compact tension, while the T g and storage moduli (EЈ, at 55 and 200°C) were determined by DMA. At 9% loading, the K Ic of the BMI increased steadily with HBP molecular weight up to 138% over the control with G5 HBP (M n ϳ 14,000 g/mol); however, significant decreases in both the T g and EЈ resulted, indicating incomplete phase separation of the thermoplastic. A linear hydroxy-terminated polyester (M n ϳ 5400 g/mol) with a repeat unit structure which was similar to the HBP's was prepared and used as a control. The linear polyester (LPE) toughened the BMI nearly as effectively as did the HBP and caused a smaller decrease in the T g and EЈ. The viscosity of solutions of HBP and LPE in B were essentially the same at lower loadings in B, but at higher loadings, the HBP viscosity increased faster than did that of the LPE. The viscosity increase was end group-dependent. Preliminary morphological results are presented to show the effect of the thermoplastic architecture, loading, and end group on the cured thermoset.
ABSTRACT:Moderate increases (Ç 50-75%) in the toughness of bismaleimides (BMIs) were achieved with very low-molecular-weight ( Ç 1000 g/mol) imide thermoplastics at low levels of thermoplastic loading ( Ç 10-20%). The thermoplastic was introduced into the BMI using a simple, one-pot, reactive solvent approach. In this approach, the reactive diluent of a two-part BMI was used as the reaction solvent for the thermoplastic synthesis. The BMI monomer was then dissolved in the thermoplastic reaction solution to yield a low-viscosity homogenous prepolymer. The viscosity of the thermoplastic solution was Ç 6 Pa S at 55ЊC. The effect of thermoplastic loading and molecular weight on viscosity was determined by rheology, and the fracture toughness of neat resin plaques was determined by compact tension. Increasing the thermoplastic loading increased prepolymer viscosity without improving toughness, while increasing the thermoplastic molecular weight increased the toughness by only 25% more than the lowestmolecular-weight thermoplastic, yet increased viscosity fivefold. Fracture surfaces showed no obvious phase separation by scanning electron microscopy.
This is the third in a five-part series describing the preparation of tough, high-performance thermosets from low viscosity, autoclave-processable prepolymers. The first 2 articles described toughening of bismaleimides (BMI) and epoxy with linear imide thermoplastics of ϳ 1000 g/mol. Highly processable prepolymers were obtained, which resulted in increases in fracture toughness for BMI of ϳ 75-100%, while the fracture toughness of epoxy was increased by up to 220%. This article describes the preparation of a low-molecular-weight comb-shaped imide oligomer (ϳ 4100 g/mol) and the effect of the oligomer architecture and end-group on BMI and epoxy prepolymer viscosity and fracture toughness. When an unreactive comb-shaped oligomer was incorporated in a BMI prepolymer (10% thermoplastic loading in the thermoset), the fracture toughness increased by 67% over that of an untoughed control, while a reactive oligomer increased the fracture toughness by 150% over an untoughened control. At 55°C, the viscosity of the solution of the reactive comb-shaped imide in B was only 6.2 Pa ⅐ S. When the oligomer was dissolved in epoxy resin, the viscosity was less than 0.2 Pa ⅐ S at 90°C, and the fracture toughness increased by 110 and 133% (at ϳ 13% loading in the thermoset), relative to an untoughened control, depending on the reactivity of the end group. The T g and high-temperature modulus of BMI and epoxy remained approximately the same relative to the untoughened controls.
ABSTRACT:The fracture toughness of epoxy thermosets was increased by up to 220% using very low-molecular-weight (ϳ 1000 g/mol) imide thermoplastic. The objective was to produce a low-viscosity prepolymer that could be easily autoclave-processed to give a tough thermoset. Here, an homogenous epoxy prepolymer was prepared by first synthesizing very low-molecular-weight linear aromatic imide (ϳ 1000 g/mol) directly in a liquid allyl phenol reactive solvent, followed by dissolution of the epoxy (Epon® 825) and the cure agent (DDS) directly in the thermoplastic solution. The allyl phenol both cures into the epoxy network, through phenol functional groups, and accelerates the cure. The viscosity of the pure epoxy was 1.4 Pa ⅐ S at 30°C. The prepolymer formulations ranged from ϳ 5-33 Pa ⅐ S at 30°C, but all reduced to less than 1 Pa ⅐ S at 90°C. The onset of cure is well above 90°C so the prepolymer viscosity is within the range for autoclave processing. The cured resin plaques were not transparent, but phase-separated domains were not found by scanning electron microscopy, indicating that the domain size is below the detection limit of the instrument. The reactive solvent causes a decrease in both the T g and the high temperature modulus of the thermoset. Introduction of the thermoplastic results in partial recovery of the T g and modulus.
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