Thermoset Network Research Articles

A hydrophilic coumarin-based polyester for ambient-temperature initiator-free 3D printing: Chemistry, rheology and interface formation

Initiator-free, photocurable, low-toxicity, viscoelastic polymer inks are attractive materials for creating low-modulus 3D-printed elastomeric biomedical constructs. In this work, we describe the synthesis, characterization, and 3D printing/crosslinking of a hydrophilic coumarin-PEG polyester (CPP). The viscoelastic CPP melt can be extruded at room temperature and deposited into layer-by-layer patterns. Upon UV irradiation, the coumarin pendant groups undergo [2 + 2] photocyclization, creating a thermoset network without the use of monomers, photoinitiators or propagating radicals. Voxel-voxel adhesion experiments between crosslinked and uncrosslinked material were performed to examine interfacial dynamics in a native, unperturbed state, and various attempts at 3D printing were made to assess the impact of printing conditions on the ultimate printed structure. Rheological analysis indicates a transition point where elastic behavior overtakes viscous behavior at increasing shear rate. It is hypothesized that this transition point corresponds with changes in interface formation from “sticky” to “bouncy” behavior in voxel-voxel adhesion and 3D printing processes. Tensile-mode dynamic mechanical analysis (DMA) and X-Ray microtomography (microCT) experiments reveal interfacial defect accumulation that results in deterioration of macroscopic structural and mechanical properties.

The effects of competitive primary and secondary amine reactivity on the structural evolution and properties of an epoxy thermoset resin during cure: A molecular dynamics study

Epoxy thermoset resins are widely used in industry due to their favorable thermomechanical properties. These properties depend strongly on the resin network topology formed during cure. In the past, there has been significant work to develop methodologies that simulate the cure process to predict the thermoset network topology and the properties of that network. However, very few of these simulations include anything more than a distance based criteria when determining which species in the simulation will bond. In this work, we show the importance of including additional bonding criteria based on the reaction kinetics for the system of interest. We simulate the cure of a model epoxy resin system, DGEBF and 44DDS, where the relative reaction rates, k2/k1, of epoxy-primary amine, and epoxy-secondary amine reactions are systematically varied to investigate the effect of competitive primary and secondary amine reactivity on the development of network structure and properties.

High enthalpy storage thermoset network with giant stress and energy output in rubbery state

Low output in stress and energy in rubbery state has been a bottleneck for wide-spread applications of thermoset shape memory polymers (SMPs). Traditionally, stress or energy storage in thermoset network is through entropy reduction by mechanical deformation or programming. We here report another mechanism for energy storage, which stores energy primarily through enthalpy increase by stretched bonds during programming. As compared to entropy-driven counterparts, which usually have a stable recovery stress from tenths to several MPa and energy output of several tenths MJ/m3, our rubbery network achieved a recovery stress of 17.0 MPa and energy output of 2.12 MJ/m3 in bulk form. The giant stress and energy release in the rubbery state will enhance applications of thermoset SMPs in engineering structures and devices.

Recyclable high-performance epoxy based on transesterification reaction

A repeatedly recyclable thermoset network with strength and stiffness similar to conventional epoxy achieved healing efficiency as high as 88.1%.

Rheological study on the cure kinetics of two-component addition cured silicone rubber

The cure kinetics for two-component silicone rubber formed by addition reaction was studied by the rheological method. The influence of reaction temperature (T) on the cure kinetics was explored in detail. It was observed that the data of gel time (tgel, i.e. the time when the reaction reaches the gel point) or a specific reaction time (tnc) (defined as the reaction time before which time the influence of confinement of network on the diffusion of reaction components can be neglected) versus T obey certain functional relationship, which was well explained by the cure kinetics model of thermoset network. The cure kinetics for the two-component silicone rubber can be well fitted by the Kamal-Sourour(autocatalyst) reaction model rather than Kissinger model. When the reaction time was before or equal to tnc, the reaction order obtained by the Kamal-Sourour reaction model was 2, which was consistent with the reaction order inferred from the two components chemical reaction when the diffusion of reaction components was not influenced by the formed cross-linked polymer network. When the reaction time was larger than tnc, such as to the end of reaction (te), the influence of confinement of network on the diffusion of reaction components cannot be neglected, and the reaction order obtained by the Kamal-Sourour reaction model was larger than 2. It was concluded that the confinement effect of network had a greater influence on the cure kinetics of the silicone rubber. The reaction rate constants (kr) under different temperatures were also determined by Kamal-Sourour reaction model. The activation energy (E) for the two-component silicone rubber was also calculated from the results of lntgel, lntnc, and lnkr versus 1/T, respectively. The three values of E were close, which indicated that above analyses were self-consistent.

Stress-sensing thermoset polymer networks via grafted cinnamoyl/cyclobutane mechanophore units in epoxy

A study of novel direct covalent grafting of photoactive mechanophore units into an epoxy matrix to create self-sensing thermoset network nanocomposites.

On the synthesis and structure of resorcinol-formaldehyde polymeric networks – Precursors to 3D-carbon macroassemblies

With the new impetus towards the development of hierarchical graphene and CNT macro-assemblies for application in fields such as advanced energy storage, catalysis and electronics; there is much renewed interest in organic carbon-based sol–gel processes as a synthetically convenient and versatile means of forming three dimensional, covalently bonded organic/inorganic networks. Such matrices can act as highly effective precursors, scaffolds or molecular ‘glues’ for the assembly of a wide variety of functional carbon macro-assemblies. However, despite the utility and broad use of organic sol–gel processes – such as the ubiquitous resorcinol-formaldehyde (RF) reaction, there are details of the reaction chemistries of these important sol–gel processes that remain poorly understood at present. It is therefore both timely and necessary to examine these reactions in more detail using modern analytical techniques in order to gain a more rigorous understanding of the mechanisms by which these organic networks form. The goal of such studies is to obtain improved and rational control over the organic network structure, in order to better direct and tailor the architecture of the final inorganic carbon matrix. In this study we have investigated in detail, the mechanism of the organic sol–gel network forming reaction of resorcinol and formaldehyde from a structural and kinetic standpoint, by using a combination of real-time high field solution state nuclear magnetic resonance (NMR), low field NMR relaxometry and differential scanning calorimetry (DSC). These investigations have allowed us to track the network formation processes in real-time, gain both detailed structural information on the mechanisms of the RF sol–gel process and a quantitative assessment of the kinetics of the global network formation process. It has been shown that the mechanism, by which the RF organic network forms, proceeds via an initial exothermic step correlated to the formation of a free aromatic aldehyde. The network growth reaction then proceeds in a statistical manner following a first order Arrhenius type kinetic relationship – characteristic of a typical thermoset network poly-condensation process. And despite the relative complexity and ill-defined nature of the formaldehyde staring material, the final network structure is to a large extent, governed by the substitution pattern of the resorcinol molecule.

Thermoset-cross-linked lignocellulose: a moldable plant biomass.

The present work demonstrates a high biomass content (i.e., up to 90% by weight) and moldable material by controlled covalent cross-linking of lignocellulosic particles by a thermoset through epoxide-hydroxyl reactions. As an example for lignocellulosic biomass, Eastern redcedar was employed. Using scanning fluorescence microscopy and vibrational spectroscopy, macroscopic to molecular scale interactions of the thermoset with the lignocellulose have been revealed. Impregnation of the polymer resin into the biomass cellular network by capillary action as well as applied pressure results in a self-organizing structure in the form of thermoset microrods in a matrix of lignocellulose. We also infer permeation of the thermoset into the cell walls from the reaction of epoxides with the hydroxyls of the lignin. Compression tests reveal, at 30% thermoset content, thermoset-cross-linked lignocellulose has superior mechanical properties over a commercial wood plastic composite while comparable stiffness and strength to bulk epoxy and wood, respectively. The failure mechanism is understood to be crack propagation along the particle-thermoset interface and/or interparticle thermoset network.

Kinetic Study of Curing Phenol-Formaldehyde/Tannin-Formaldehyde Composite Resins

This work presents a study on the uses of tannin-formadehyde derivative into phenolic resins. Eucalyptus tannins (T) were reacted with formaldehyde to form tannin-formaldehyde resin (TF). Then this derivative was used to prepare tannin-formaldehyde/phenol-formaldehyde resins (TFPF) at 20 and 40 %w/w. The kinetic values of thermal curing of Phenol-formaldehyde (PF), tannin-formaldehyde and tannin-formaldehyde/phenol-formaldehyde resins (TFPF) at 20 and 40 wt% from TF were studied by monitoring the weight changes which occurred in the samples weight during thermosetting process at four temperature (160°C, 180°C, 200°C and 220°C). The total evolved condensation products from curing reactions were about 32% - 36% per sample weight, and the rate of curing reaction constants was ranged between 0.163 %wt·min-1 at 160°C and 0.50 %wt·min-1 at 220°C. The path of TFPF curing and kinetic values indicated that these resins could be cured with the behavior and velocity comparable to that of PF. The activation energy of TFPF cross-linking was higher than that of PF. Increasing TF level to 20% and 40% into PF can reduce the amount of PF curing reactions density and weight loss percentage. The global kinetic properties showed that the TF participated in the thermoset network formation with acceptable activity and performance. The general results of this paper show that the TF is a suitable alternative material for partially replacement into PF resin.

Influence of stoichiometry on the glass transition and bond exchange reactions in epoxy thermoset polymers

Thermally malleable polymers which undergo covalent bond exchange reactions (BERs) have been shown to be able to rearrange their network topology at high temperatures without impairing the network integrity. At low temperatures, the BERs are so sluggish that the materials behave like traditional thermosetting polymers. In this paper, we demonstrated that the temperature dependent BER rate could be tuned by adjusting the stoichiometry of monomers. As the ratio of hard segments in the epoxy thermoset network is increased, the material's glass transition temperature (Tg) is increased, with a corresponding increase in the temperature required to achieve a given stress relaxation rate. The material stress relaxation behavior was studied from both a theoretical and experimental point of view. Based on the kinetics of BERs, we derived the detailed expression of stress relaxation time, which reveals an Arrhenius type dependency of material relaxation behavior on the applied temperature. Subsequently, from the experimental stress relaxation curves, we determined the energy barrier for the BERs in different networks. With the Tg being elevated from 30.3 °C to 63.0 °C, the BER energy barrier is linearly increased from 68.2 kJ mol−1 to 97.3 kJ mol−1. Such a correlation between these two thermomechanical behaviors provides an additional design parameter (beyond catalyst choice) which can aid in achieving highly tunable service conditions for practical engineering applications of thermally malleable thermosets.

Photoresponsive Polyesters for Tailorable Shape Memory Biomaterials

The synthesis of a library of poly(ester urethane)s (PEUs) containing pendant photoresponsive moieties afforded through the incorporation of one of two novel bifunctional monomers resulted in degradable materials with a range of tunable thermal and mechanical properties. Utilizing light irradiation, macroscopic temporary shapes were fixed by increasing the cross-link density of a thermoset network via photoinduced reversible [2 + 2] cycloaddition of cinnamamide or cinnamate pendant groups under UV light (λ = 302 nm). Further irradiation with UV light (λ = 254 nm) led to the cleaving of the temporary cross-links and recovery of the original shape. Examination of these materials under physiological conditions displayed tunable degradation with rates faster than PCL-based materials, and initial biocompatibility studies exhibited negligible cytotoxicity for HeLa cells based on results of ATP assay. The ability to tune thermal properties also allowed specific polymer compositions to boast transition temperatures within a range of applicable temperature for thermal shape memory.

Flexible aliphatic-bridged bisphenol-based polybenzoxazines

In this paper, we report the synthesis of a series of novel aliphatic-bridged bisphenol-based benzoxazine monomers comprising four to ten methylene unit spacers (BZ(n)BA). Cationic ring-opening polymerization of these monomers provides flexible polybenzoxazine thermosets with good film forming characteristics under solvent-free processing conditions. The effects of aliphatic bisphenol chain length on polymerization behavior, thermomechanical transitions, and mechanical properties of the polybenzoxazine thermosets are reported. Fourier transform-infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) show the ring-opening polymerization proceeds to high conversion with minimal dependence on spacer chain length. Thermomechanical properties of the pBZ(n)BA thermosets, such as rubbery storage modulus and glass transition temperature (Tg), show a strong dependence on the length of the aliphatic-bridged bisphenol linker where both properties decrease with increasing linker length. In particular, changing the length of the aliphatic-bridged bisphenol linker enables tailoring the Tg of the pBZ(n)BA series from 67 °C to 101 °C, as determined by dynamic mechanical analysis (DMA). Tensile properties of the pBZ(n)BA series exhibit similar trends with Young's modulus decreasing and elongation at break increasing with increasing aliphatic-bridged bisphenol linker length. The pBZ(n)BA materials all show a similar three mode degradation process by thermogravimetric analysis (TGA) consistent with other bisphenol based polybenzoxazines, and additionally exhibit a decrease in char yield with increasing aliphatic chain length owing to a decrease in aromatic content in the thermoset network.

Photo-oxidation of acrylic-urethane thermoset networks. Relating materials properties to changes of chemical structure

Acrylic-urethane thermoset photo-ageing was investigated by various techniques including IR spectroscopy, dynamic-mechanical thermal analysis (DMTA), and micro-hardness testing. These results were used to identify the sites at which the chain oxidation reaction is initiated and the main pathways through which the degradation reaction proceeds. The chemical modifications induced by photo-ageing were qualitatively and quantitatively correlated with the modifications observed in the architecture and mechanical properties of the thermoset network. The results of this work also allow for the development of a quantitative kinetic model based on the identified mechanisms and a multi-scale approach from the molecular to the macroscopic level, which highlights the effect of the changes of the chemical structure on the modification of the macromolecule arrangement and thus on the mechanical properties. Finally, the impact of stabilisers on material ageing was studied.

Advanced chemically induced phase separation in thermosets: Polybenzoxazines toughened with multifunctional thermoplastic main-chain benzoxazine prepolymers

Multifunctional thermoplastic main-chain benzoxazine prepolymers were synthesized and systematically varied in their structure in order to function as high-performance toughener additives. Their unique chemical composition allows multiple covalent crosslinking with many thermoset network systems including benzoxazines and epoxides in conjunction with a defined chemical induced phase separation (CIPS) upon curing. This was successfully shown using a benzoxazine-based thermoset resin matrix as an example. The corresponding morphologies were addressable in a predictable manner and brought into context with the obtained macroscopic mechanical and thermal properties. In this relationship the CIPS process was classified and compared with the literature in more general means for advanced morphology control by differentiating between covalently attached and so-called gradient domain structures. The prepolymers were characterized by 1H NMR, FT-IR, DSC and TGA. The thermoset morphologies were investigated by TEM and AFM. The fracture toughness ( K Ic) and the elastic modulus ( E) were measured by fracture and three point bending experiments. Thermal properties of the resulting films have been tested by DMA.

Blends of a new thermoplastic in a thermoset epoxy matrix

AbstractProperties of thermoplastic modified epoxy network have been studied. The particularity of this work is the use of new thermoplastic epoxies whose structure is close to the final matrix. Blends of thermoset epoxy (Diglycidyl Ether of Bisphenol A/4‐4′ methylenebis [3‐chloro 2,6‐diethylaniline]) with a thermoplastic content from 5 to 40%w have been realised. Initial miscibility in the thermoset precursors shows an UCST behaviour with a maximal value near 130°C for a thermoplastic content of 10%. Due to the presence of tertiary amine and pendant hydroxyl groups on the thermoplastic backbone, epoxy amine reactions are faster than for the neat system but the thermoplastic seems not to have reacted with the thermoset network. The final blends are transparent but toughening increase is rather low.

Modeling domain mixing in semi‐interpenetrating polymer networks composed of poly(vinyl chloride) and 5% to 15% of crosslinked thermosets

AbstractThe nature of phase mixing in semi‐interpenetrating polymer networks (SIPNs) of poly(vinyl chloride) (PVC)/thermoset blends was studied by using both the glass transition temperature third power composition equation and DMTA modeling. From 5% to 15% by weight of an oligomeric MDI isocyanate or a low viscosity epoxy were blended separately with PVC to make two series of SIPNs. The DMTA‐derived Tg data were modeled by the Tg third power composition equation to characterize “energy interaction” features of PVC/thermoset phase mixing. Fitting experimental Tg values gave estimations of the characteristic parameters, K1 and K2 of the Tg power equation. K1 and K2 were each positive for the PVC/epoxy (K1 = 1.1, K2 = 7.2) and the PVC/isocyanate (K1 = 29.9, K2 = 38) blends, showing that binary hetero‐interactions (enthalpic effects) and conformational redistributions (entropic effects) during the binary hetero‐interactions both contributed favorably to phase mixing. Negative K1 ‐ K2 values for both groups of blends indicate entropic contributions predominate. The thermoset dilution produced a lightly crosslinked thermoset network, which was locked into the amorphous PVC, forming a mixed thermoset/PVC SIPN domain. Conformational redistributions of PVC and thermoset segments continue to occur within the developing SIPN domain before phase separation can occur. The combined series‐parallel Takayanagi coupling model, which assumed that a PVC phase and a mixed PVC/thermoset SIPN phase coexisted, was employed to describe the viscoelastic behavior of PVC/thermoset blends. Reasonable fits between the experimental DMTA plots and modeling predictions were found. The predictions were not sensitive to the degree of series‐parallel coupling. The modeled DMTA plots, like the experimental results, exhibited only a single E″ peak in the glass transition temperature range for both the PVC/PAPI and PVC/epoxy systems. Thus, very small PVC/thermoset SIPN domains (< 20 nm diameter) that are dispersed in separate domains of a PVC‐rich phase (despersive phase mixing characteristics) provide a reasonable explanation of the blends' phase structures.

Effect of thermoplastic modifier variables on toughening a bismaleimide matrix resin for high-performance composite materials

A series of engineering thermoplastic toughness modifiers were used to modify the fracture toughness properties of a bismaleimide resin. The effects of thermoplastic loading, molecular weight and functionality were examined. Substantial improvements in K Ic stress intensity values were found when modifiers possessing reactive end-groups were used. Increases in modifier molecular weight and weight per cent loading produced steady increases in K Ic values and a limiting value at high concentrations and molecular weights. These observations were attributed to the thermoset network being the limiting material with respect to fracture toughness. By controlling molecular weight and adding reactive end-groups to the toughener, the fracture toughness properties of the commercial bismaleimide resin were successfully improved without sacrificing its desirable hot-melt processing characteristics. A unidirectional carbon-fibre prepreg was prepared and mode I and II fracture toughness tests were performed using double cantilever beam and end notch flexure specimens. Thermoplastic loadings of 15 and 20% of maleimide-terminated poly(ether sulphone) ( M n = 12 800 g mol −1) yielded composite G Ic values of 489 ± 25 and 734 ± 1 J m −2, respectively—a substantial improvement over the unmodified composite value of 359 ± 17 J m −2.

The curing mechanism and properties of bismaleimide–biscyanamide resin

AbstractThermoset networks consisting of N, N‐bismaleimidediphenylmethane (BMI) and bis [4‐(4‐cyanamide‐3‐methylphenyl)] methane (BCMM) were investigated with the aim of correlating macroscopic behavior and structure as network composition was varied. The glass transition temperatures of these cured resins obtained from the dynamic mechanical data and thermal expansion data had a distinct minimum at the 50/50 molar ratio of BMI/BCMM. Also, at this molar ratio, the decomposition temperature at a 5% weight loss in helium atmosphere for cured resins was the lowest. These results provided a rough idea of the thermoset network structure.

Relaxations in thermosets: 15. Curing kinetics and dielectric behaviour of butadiene-acrylonitrile-containing epoxide thermosets

The dielectric permittivity, ε′, and loss, ε″, during the crosslinking or curing of diglycidyl ether of bisphenol-A with diamino diphenyl methane and various amounts of an elastomer, amine-terminated butadiene acrylonitrile (ATBN), have been measured from their sol to gel to glass-formation regions. As the elastomer-containing thermoset cures, both ε′ and ε″ initially decrease and then increase with time. ε′ finally decreases to a value near 3, but ε″ goes through a sharp minimum followed by a peak and ultimately decreases to about 10 −3. The initial increase in both ε′ and ε″ has been interpreted as an onset of liquid-liquid phase separation, which becomes nearly complete before the monotonic decrease in ε′ begins as a result of network formation in the curing thermoset. The time for the onset of phase separation is relatively insensitive to the amount of elastomer in the thermoset, and indicates the time of cure at which the network structure reaches a molecular weight when the elastomer becomes immiscible in it. Both the conductivity and permittivity increase with the increase in the elastomer content, with the consequence that the onset of gelation, which is marked by an approach of conductivity to near zero, is masked by the dipolar relaxation process. Corresponding analysis in the electrical modulus formalism confirms these observations and shows further that the conductivity follows a Maxwell relaxation. The ε″ of the cured thermoset plotted against temperature shows broad peaks and shoulders to these peaks. These are a combination of the sub- T g relaxations in both the elastomer and the thermoset's network structure and the α-relaxation of the elastomer which appears at 240 K at 1 kHz. The permittivity and dielectric loss of the cured thermoset increase with an increase in the weight fraction of ATBN.

Effect of hygrothermal fatigue on dynamic mechanical properties of glass‐reinforced polyesters

AbstractSeveral nonreinforced and reinforced crosslinked polyester formulations were prepared and investigated. Solid glass microspheres, with and without coupling agent, were used as reinforcement. All samples were exposed to various cyclic hygrothermal fatigue patterns, differing from one another by the temperature and duration of a cycle. Dynamic mechanical measurements were used to evaluate the effect of hygrothermal fatigue on the mechanical properties of all formulations. Fatigue induced changes in the glass–matrix interphase led to the appearance of relaxation peaks (β′ and β″) in the temperature range between −50°C and −10°C. Electron microscopic evidence was obtained for the existence of nodular morphology on all fracture surfaces. The character of nodular morphology was described in terms of an inhomogeneous thermoset network.