Translated Abstract
Phenol formaldehyde resins, also known as phenolic resins (PRs), were the first commercial synthetic resins. Because of their high strength, low thermal conductivity and low-cost, PRs are widely applied in aerospace and military fields, used as the matrix resins in the heat shield materials of rockets, re-entry capsules and missiles. The current PRs, both the traditional and modified products, show good thermal resistance and ablative performance, but can hardly meet the growing demands from aerospace and defense industries. Thus, it is necessary to develop new PRs with improved thermal and mechanical properties. However, there still have many difficulties in developing such PRs. For examples, the hydroxyl and methylene groups, which are the main functional groups in the crosslinked network of PR, are relatively weaker to bear heat or more vulnerable to be attacked by the other free radicals, compared with the phenolic rings. Some modification methods based on the etherification or esterification of hydroxyl groups, or strengthening the methylene groups with ladderlike structures, could inadvertently cause some negative effects on the processability and mechanical properties. Meanwhile, due to the insoluble and infusible properties of the resin, as well as the complexity of the cross-linked structure, the structural-property relationship of the cured PR is challenging to study, and many unknowns still exist.
To furtherly improve the thermal and mechanical properties of PR, more knowledges about the structural-property relationship of PRs is badly needed. The rapid development of the modem molecular simulation technologies make it possible to study the structural-property relationship of PR from multi-level perspectives. In this study, with both the experimental and molecular simulation approaches, the following researches were carried out to study the structural details of some typical PRs.
1. Based on some experimental characterized data, a series of novolac-type PR (also known as novolac) models were constructed. The influences of hydrogen bonds and p-π/π-π interactions on the spatial distribution characteristics of hydroxyl and phenolic groups, as well as on the chain conformation and glass transition temperature (Tg), were studied in detail. The o-o＇ and o-p＇ type novolac models were represented for the high-ortho and random type novolac resins, respectively, which are two kinds of typical products of novolac resins. The hydrogen bond number in the o-o＇ type novolac models are significantly higher than that in the o-p＇ type models. This leads to more twisted conformation of the chains and higher Tg of the o-o＇ models, and helps explain the red shift of stretching vibration peak of OH in the infrared spectroscopy of high-ortho type novolac resins. The hindered rotation chain model fitted from the MD simulated parameters can well describe the relationship between the mean square radius of gyration and the degree of polymerization of the two novolac models, as well as their variation with temperature.
2. A series of crosslinked structural models for PR, with different O/P ratios and different cross-linking densities, were constructed through a model construction script programmed by myself. The spatial distribution of the hydroxyl and phenolic groups, as well as the influence of the hydrogen bonds and p-π/π-π interactions on the Tgs and mechanical properties of PR, were studied in detail. The restricted mobility of the segments in crosslinked network of PR will be the primary reason for the experimental phenomena that the with the increasing cross-link density, the Tg of the cured PRs was increased rapidly. A Venditti-Gillham matrix equation fitted from the MD simulation results can well describe the Tg ~ cross-link density relationship of the crosslinked PR models. There are more intermolecular hydrogen bonds and p-π/π-π interactions in the cross-linked PR models that build from the novolac models with lower O/P ratio, leads to higher Tg and Young＇s modulus, compared with that from novolac models with higher O/P ratio. The experimental phenomenon of volume shrinkage during the initial curing of PR can be attributed to the decrease of the fractional free volume; After that, the rebound of the model volume can be resulted from the densification of the crosslinked network.
3. Based on a high-throughput conformer screening from 1800 conformers, 180 conformers were selected for further study. The effects of the model size, the connection patterns between the phenolic rings, the types and number of the substituents (on the phenolic rings) and the oxygen-containing functional groups on the bond strength of C-C and C-O bonds in BPF-like models were investigated in detail. The influence of these structural factors on the thermal stability of PR was also discussed. Owing to the intra hydrogen bond formation in the o-o＇ type BPF, it is thermally stable than the o-p＇ and p-p＇ models. The types and number of the substituents on the phenolic ring have little effect on the bond strength of C-C and C-O bonds, but have significant influence on the radical reactivity of hydroxyl, phenolic and methylene groups. The tri-substituted phenolic rings have relatively lower radical reactivity. These results are helpful to explain some experimental phenomena like: The thermal stability of the high-ortho type novolac resins are always thermally stable than the random linear type novolac resins; The cured PRs with higher cross-linking density are generally more tolerant of heat.
4. To study the influence of the borate structure in the boric acid modified PR and phenylboronic acid modified PR on the thermal stability of the resins, the bond dissociation energies (BDEs) of the C-O, B-O and C-C bonds in and adjacent the borate structures were precisely calculated. The radical reactivity of the functional groups like borate structures, phenol rings, hydroxyl and methylene groups were also calculated. The introduction of the borate structures not only improves the bond dissociation energy of their adjacent C-C bonds, but also significantly decreases the radical reactivity of the adjacent methylene groups, which can directly and indirectly improve the thermal stability of PR. Compared with the borate structure, the phenyl borate structure is more beneficial to the thermal stability of PR as there is no HO· released during the initial pyrolysis of the later. These findings provide a reliable theoretical basis for understanding the relationship between the atomistic structure and thermal properties of boron-containing PRs, which will be conducive to guide the development of PRs with better thermal stability.
5. A series of graphene/PR (G/PR) and graphene oxide/PR (GO/PR) composite models were built to study the interface effects on the glass transition and stress-strain behavior of G/PR and GO/PR composites. The influence of the degree of polymerization and cross-linking density of the PR models, the types of oxygen-containing groups and oxidation degree of the GO surface on the interfacial strength were investigated in detail. It is revealed that due to the large amount of hydrogen bonds and p-π/π-π interactions formed between the G/PR and GO/PR interface, the composite models exhibit higher Tg and Young＇s modulus than that of fully crosslinked PR. The G/PR and GO/PR composites show higher modulus in the Z direction (parallel to the G or GO surface) than that in the X direction (perpendicular to the G or GO surface), which can be attributed to the higher modulus of the G or GO surface in the X direction. These results provide a reliable theoretical basis for the modification, preparation and processing of GO/PR and G/PR composites, or even the carbon fiber/PR composites.
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