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Standard resin reactivity
From practical and theoretical points of view, bismaleimide resins chemistry has always proved to be difficult to investigate. Some reactions are observed with little doubt, such as the Alder-ene reaction, responsible for the chain-extension, and maleimide polymerization. Other addition reactions such as Wagner-Jauregg and Diels-Alder are strongly supported by studying model compounds reactivity. Using physico-chemistry investigation instead of chemical analysis will give more information about the nature of reactions involved in key steps such as gelation. This last point will be developed in the following part.
Rheology of bismaleimide resins
Isothermal experiment with combined sinusoidal oscillations containing several frequencies (5 Hz, 10 Hz and 20 Hz) were used to determine the gel time starting from 130C.
The strain amplitude ( 0) was fixed at 1%. At this strain value, the initial material still follows a linear stress-strain behaviour. Figure 24 displays examples of experimental tan() curves for two temperatures. During the first part of the experiment, the viscosity is very low, which results into inaccurate measurements. The shear moduli G00 and especially G0 cannot be determined precisely. For longer times, smooth tan() curves are obtained. Gelation occurs during the transition at intermediate times. If the crossover point of tan() curves can be easily identified at 130C, it becomes more difficult to locate at higher temperatures.
The BMI/DBA gelation process appears to be fast, with a sharp change from the sol to the gel state. Attempts to improve the measurement accuracy, by changing the shear strain amplitude, the set of frequencies, the plates diameter and even using another rheometer, were unsuccessful. Not being able to point out the tan() crossover, a fair approximation is still possible by considering the liquid to soft solid transition (red line at 150C on Figure 24). A numerical approach is to determine the onset of a monotonic increase of G0 curves. The assessment is that when a consistent signal is being recorded, the gel state has already been reached.
Time-Temperature-Transformation diagram
The use of TTT diagrams for thermosetting polymers was made popular by Gillham [68]. This kind of graphical representation gathers physical and chemical information with an overview of involved kinetics. Events, such as gelation or vitrification, are represented by curves on a time-temperature plot, defining a sort of state diagram. Data relative to the polymerization reaction can be added with isoconversional curves. Defining (t, T) the reaction conversion, depending on the curing temperature T and the time t, those curves are obtained by sets of coordinates (t, T) for which is equal to a chosen value (ranging from 0 to 1). TTT-diagrams need to used along horizontal lines corresponding to isothermal curing.
Experimentally, is obtained by DSC measurements during isothermal curing. The reaction enthalpy is determined by the integrated heat flow over the whole reaction range during a heating experiment. Here rH = 340J/g at 5C/min. (t, T) is then measured during an isothermal curing experiment at T using the partial reaction enthalpy rHp(t): (t, T) = rHp(t) rH at T.
Inhomogeneity in thermosetting polymers
Topological inhomogeneity of thermosets always leads to numerous questions. Dušek reviewed the subject with a critical approach of different kinds of thermosetting polymers, depicting a field where theoretical scenarii are hardly confirmed by, sometimes misleading, observation methods [73]. Characterisations involve electron microscopy, scattering methods (X-rays, neutrons), NMR, EPR… Inhomogeneities of cross-link density find their origin in the network building process but some early-stage inhomogeneities may be eliminated after curing. Consequently, studying the curing process along with the final material brings complementary information.
A conclusion arising from Dušek’s review is that topological inhomogeneities are not commonly found in epoxy-based thermosets, synthesized from polyaddition. Using specific conditions, such as stoechiometry imbalance or particular curing agent composition, can promote inhomogeneity, due to change in components compatibility or reaction paths. On the other hand, unsaturated polyester-based networks, building up by free radical polymerization, are prone to develop inhomogeneous structures. At the root of this difference, the polymerization mechanisms are found. For stepwise reactions, polymer products grows up together with a relatively narrow molecular mass distribution. By comparison, chain-wise reactions result in long chains dissolved in a high volume fraction of unreacted monomers. Radicals have a high reactivity, diffusion of reactive species being passed over, reactions proceed in a limited area. In appropriate conditions, this lead to highly cross-linked nodules, called microgels (Figure 32).
Table of contents :
Résumé étendu
Introduction
Chimie des bismaléimides
Modification par des thermoplastiques
Rupture des bismaléimides
Rupture des matrices modifiées
Rupture des composites
Conclusions
Introduction
Composite materials for aeronautics
Materials
Interesting properties
Limiting properties
Going further with composite materials
Bismaleimide resins
Preamble to the study
1 Bismaleimide resins
1.1 Development of bismaleimide resins
1.1.1 Evolution of the BMI-based thermoset
1.1.2 A whole history of chemical investigation
1.2 Experimental details
1.2.1 Selected materials
1.2.2 Experimental procedures
1.2.2.1 Mixing protocol
1.2.2.2 Spectroscopy
1.2.2.3 Calorimetry
1.2.2.4 Rheological analysis
1.2.2.5 Degradation
1.3 Thermal properties
1.3.1 Thermal degradation
1.3.2 Mechanical properties preservation
1.3.3 Thermomechanical behaviour
1.3.4 Conclusion
1.4 Reactivity
1.4.1 Phenol reactivity
1.4.2 Maleimide/allyl reactivity
1.4.2.1 Alder-ene reaction
1.4.2.2 Addition reactions
1.4.3 Standard resin reactivity
1.5 Kinetics elements
1.5.1 Gelation
1.5.1.1 Theoretical considerations
1.5.1.2 Rheology of bismaleimide resins
1.5.2 Time-Temperature-Transformation diagram
1.5.3 Vitrification
1.6 Network structure
1.6.1 Inhomogeneity in thermosetting polymers
1.6.2 Network architecture analysis
1.6.3 Simulated networks
1.7 Conclusions
2 Morphologies of bismaleimide thermoset and thermoplastics blends
2.1 Phase separation in thermosetting polymers
2.1.1 Flory-Huggins theory
2.1.2 Flory parameter
2.1.3 Reaction-induced phase separation
2.1.3.1 Reactive solvent
2.1.3.2 Pseudo-ternary thermoset
2.2 Literature on bismaleimide/thermoplastic blends
2.2.1 Polyethersulfones
2.2.2 Polyetherimides
2.2.3 Polyesters
2.2.4 Polyimides
2.2.5 Polyether ketones
2.2.6 Other polymers
2.2.7 Conclusion
2.3 Experimental details
2.3.1 Selected materials
2.3.1.1 Thermoset
2.3.1.2 Thermoplastics
2.3.2 Methods
2.3.2.1 Blend preparation
2.3.2.2 In situ temperature controlled microscopy
2.3.2.3 Cured samples preparation
2.3.2.4 Optical microscopy on cured samples
2.3.2.5 Electron Dispersive X-ray Spectroscopy
2.3.2.6 Dynamic Mechanical Analysis
2.3.2.7 Rheology
2.4 Phase separation in bismaleimide/solubilised thermoplastic blends
2.4.1 Phase separation behaviours
2.4.1.1 Polyetherimide
2.4.1.2 Polyethersulfone
2.4.2 Final morphologies
2.4.2.1 Influence of curing conditions
2.4.2.2 Comparison between thermoplastics
2.4.2.3 Deeper look on morphologies
2.4.3 Thermomechanical analysis
2.4.4 First conclusions on solubilised thermoplastics
2.5 Bismaleimide/thermoplastic particles blends
2.5.1 Soluble particles
2.5.1.1 Experimental considerations
2.5.1.2 Influence of the curing conditions
2.5.1.3 Conclusion on morphological control
2.5.2 Non-soluble particles
2.6 Conclusion
3 Fracture of bismaleimide resins
3.1 Polymer fracture
3.1.1 Fracture mechanics
3.1.1.1 Linear elastic fracture mechanics
3.1.1.2 Crack propagation
3.1.1.3 Fracture modes
3.1.1.4 Stress limitation at a crack-tip
3.1.2 Failure of thermosets
3.1.3 Crack propagation in glassy polymers
3.2 Experimental details
3.2.1 Materials
3.2.2 Methods
3.2.2.1 Mechanical characterisation
3.2.2.2 Compact tension
3.2.2.3 Double Cantilever Drilled Compression
3.2.2.4 Optical Imaging
3.2.2.5 Interferometry
3.2.2.6 Atomic force microscopy
3.2.2.7 SEM Fractography
3.3 Neat bismaleimide network fracture
3.3.1 Mechanical properties
3.3.1.1 Uniaxial tension
3.3.1.2 Uniaxial compression
3.3.1.3 Extrapolated microscopic behaviour
3.3.1.4 Molecular behaviour
3.3.2 Failure at the macroscopic scale
3.3.3 Crack propagation kinetics
3.3.3.1 Evaluation of the stress intensity factor
3.3.3.2 Considerations on the crack opening measurement
3.3.3.3 Velocity measurements and K(v) curves
3.4 Influence of the network architecture
3.4.1 Non-monotonic crack propagation
3.4.2 Network modification
3.5 Modelling toughness
3.5.1 Steady state toughness
3.5.2 Material behaviour at the crack-tip
3.5.3 Steady state crack propagation
3.6 Conclusion
4 Fracture in heterogeneous materials
4.1 Material toughness
4.1.1 Toughening of thermosetting polymers
4.1.1.1 Modifiers for thermosets
4.1.1.2 Toughening mechanisms
4.1.2 Toughness in composite laminates
4.2 Experimental details
4.2.1 Materials
4.2.1.1 Modified resins
4.2.1.2 Composites
4.2.2 Methods
4.2.2.1 Tension
4.2.2.2 Compact tension
4.2.2.3 Double Cantilever Drilled Compression
4.2.2.4 Composite delamination
4.2.2.5 Atomic force microscopy
4.2.2.6 Fractography
4.3 Fracture behaviour in matrices
4.3.1 Mechanical properties
4.3.2 Toughening
4.3.3 Crack path in modified matrices
4.3.3.1 Particles from phase separation
4.3.3.2 Initially insoluble particles
4.3.4 Conclusions
4.4 Composite materials properties
4.4.1 Selected formulations
4.4.2 Morphologies
4.4.2.1 Neat bismaleimide-based composite
4.4.2.2 Polyethersulfone-modified composite
4.4.2.3 Polyamide-imide-modified composite
4.4.2.4 PES/PAI-modified composite
4.4.2.5 Conclusions
4.4.3 Toughening
4.4.4 Crack path
4.4.4.1 Neat bismaleimide-based composite
4.4.4.2 Polyethersulfone-modified composite
4.4.4.3 Polyamide-imide-modified composite
4.4.4.4 PES/PAI-modified composite
4.4.4.5 Matrix-composite comparison
4.5 Conclusions
4.5.1 On crack propagation in modified resins
4.5.2 On composite processing and toughening
5 Suggestions for further work and general conclusion
5.1 Reactivity of bismaleimides
5.1.1 Chemical characterisations
5.1.2 Simulated network architecture
5.2 Fracture mechanics of bismaleimides
5.3 Failure of modified resins
5.4 Composites properties
5.4.1 Fracture of composites
5.4.2 Compression after impact
5.4.3 Other types of alteration
5.5 Conclusions
Abbreviations
Bibliography
Appendix A: NMR spectra
Appendix B: Modulus loss calculation
Appendix C: Epoxy TTT diagram
Appendix D: Simulation program
Appendix E: Solubility parameters
Appendix F: Thermoplastics TGA
Appendix G: Edge detection tools
