Gelation and cross-link inhomogeneity of phenolic resins studied by small- and wide-angle X-ray scattering and 1H-pulse NMR spectroscopy (original) (raw)
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Polymer, 2016
The cross-link inhomogeneity of phenolic resins at the initial stage of curing in a temperature range of 110e130 C was investigated through structural analysis of the network structure evolution mechanism using 1 H-pulse nuclear magnetic resonance spectroscopy and complementary small-and wide-angle Xray and neutron scattering methods. Two types of phenolic resins, PR06 and PR12, were prepared with stoichiometrically insufficient and sufficient amounts of cross-linker, respectively, via curing of a novolac-type phenolic resin oligomer with hexamethylenetetramine as the curing agent. Their network structures comprised three different structural domains because of the cross-link inhomogeneity: the high-cross-link-density domain (HXD), the low-cross-link-density domain (LXD), and the interface region between these domains. Percolation of the HXD occurred at the beginning of the curing. Intradomain reactions inside both HXD and LXD proceeded as the dominant reactions accompanying minor interdomain reactions between the HXDs, resulting in no significant change in the spatial location and size of the HXDs and LXDs. Intradomain reactions inside the LXD involved reactions with dangling chains, which would not affect the average mesh size of the domain significantly. These behaviors of the network structure evolution mechanism at the initial stage of the curing are a general feature of phenolic resins that does not depend on the amount of cross-linker. The difference between the amount of the cross-linker present in PR06 and PR12 was manifested as a difference in the degree of cross-linking in the percolated HXDs, i.e., the HXD of PR12 exhibited a tightly cross-linked, well-developed network structure since the beginning of the curing process; however, that of PR06 exhibited a loosely cross-linked network structure, with the degree of cross-linking increasing as the curing proceeded.
Polymers and Polymer Composites, 2003
The physicochemical characterization of the structures of the oligomers (n < 4) in resols has been carried out by fragmenting monomers in LC/UV/MS and LC/UV/MS/MS. Fragmentation mechanisms are related to the numbers and positions of substituents on the aromatic ring and to the types of oligomer junctions. It was more difficult to determine the structures of phenol hemiacetals and dimer hemiacetals because of the large number of position isomers. The resols were prepared with differing molar ratios R = Formaldehyde/Phenol and catalysts. They were cross-linked using two industrially recommended heat cycles. The progression of resin cross-linking was determined by solid state 13C NMR (CP/MAS). The residual percentage of monomers and oligomers at n < 4 was determined in leachates (water and methanol) and characterized by LC/UV/MS. The results for cross-linking advancement were correlated with the various synthesis parameters (ratio R, type of catalyst and heat cycle).
Investigating Chain Dynamics in Highly Crosslinked Polymers using Solid‐State 1 H NMR Spectroscopy
Journal of Polymer Science Part B, 2019
Solid state 1 H NMR line-shape analysis and (double quantum) DQ 1 H NMR experiments have been used to investigate the segmental and polymer chain dynamics as a function of temperature for a series of thermosetting epoxy resins produced using different diamine curing agents. In these thermosets, chemical crosslinks introduce topological constraints leading to residual stresses during curing. Materials containing a unique ferrocene-based diamine (FcDA) curing agent were evaluated to address the role of the ferrocene fluxional process on the atomic-level polymer dynamics. At temperatures above the glass transition temperature (T g), the DQ 1 H NMR experiments provided a measure of the relative effective crosslink and entanglement densities for these materials and revealed significant polymer chain dynamic heterogeneity in the FcDA-cured thermosets.
Investigating Chain Dynamics in Highly Crosslinked Polymers using Solid‐State 1H NMR Spectroscopy
Journal of Polymer Science Part B: Polymer Physics, 2019
ABSTRACTSolid state 1H NMR line‐shape analysis and (double quantum) DQ 1H NMR experiments have been used to investigate the segmental and polymer chain dynamics as a function of temperature for a series of thermosetting epoxy resins produced using different diamine curing agents. In these thermosets, chemical crosslinks introduce topological constraints leading to residual stresses during curing. Materials containing a unique ferrocene‐based diamine (FcDA) curing agent were evaluated to address the role of the ferrocene fluxional process on the atomic‐level polymer dynamics. At temperatures above the glass transition temperature (Tg), the DQ 1H NMR experiments provided a measure of the relative effective crosslink and entanglement densities for these materials and revealed significant polymer chain dynamic heterogeneity in the FcDA‐cured thermosets. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1143–1156
Solid-state NMR relaxometry study of phenolic resins
1 H wideline and 13 C CP/MAS spin-lattice (T 1H) relaxation solid-state NMR experiments have been performed in order to investigate the cure state of phenolic novolac resins. Due to the process of spin diffusion, a single averaged T 1H decay time was found which was sensitive to the cure state of the network structure. Measurements as a function of temperature revealed that mobile end-groups and branches determine the most efficient pathway of T 1H relaxation. Combined with FTIR measurements, it is shown that rearrangements of the hydrogen bond structure take place upon heating-cooling. There are strong indications that the process of disruption-reformation of the hydrogen bonds is not completely reversible.
Journal of Applied Polymer Science, 2011
In this study, we present the experimental results for the crosslinking process of a commercial polyester resin based on measurements of the spin lattice relaxation time T1 of protons, as function of the crosslinking time evolution. Multiexponential decomposition of the evolution of magnetization measured in inversion‐recovery experiments is performed. The population of “rigid” and “mobile” nuclear spin sites was estimated as function of time evolution. In analogy to the usual monomer conversion u, site conversion from “mobile” to “rigid” sites uM were also estimated as a function of time evolution and initial concentrations of the reagents. The multiexponential decomposition approach of T1 relaxation data allows one to follow crosslinking processes. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
Journal of Applied Polymer Science, 1999
Wide-scope mathematical relationships have been established between the 13 C-NMR of liquid polycondensation resins, such as urea-formaldehyde and phenolformaldehyde resins, and the strength of the network formed by the same resin when hardened under well-defined conditions, the thermomechanical analysis deflection, the number average molecular mass and the number of degrees of freedom of the average polymer segment between crosslinking nodes in the hardened resin network, the resin network glass transition temperature, its solid-phase 13 C-NMR proton-rotating frame spin-lattice relaxation time, and the homogeneous and heterogeneous polymer segment/polymer segment interfacial interaction energy calculated by molecular mechanics. These mathematical relationships allow the calculation of any of these parameters from any of the techniques listed, provided that all of the systems are used under well-defined conditions. Under different conditions, the values of the numerical coefficients involved change; and, whereas the equations are still valid, a different set of coefficients needs to be recalculated.
Evidence for a Linear NMR−Elasticity Interrelationship in Polymeric Gels
Macromolecules, 1997
A linear relationship between the elastic modulus of polymer networks and the transverse relaxation rate of protons attached to the unconstrained polymer is established by considering poly-(dimethylsiloxane) (PDMS) chains, cross-linked at random according to two different ways. Networks of type A were composed of long poly(vinylmethyl-dimethylsiloxane) copolymers. With randomly distributed vinyl functions (molar fraction of vinyl functions: 2 × 10-3 per monomeric unit), links were formed between vinyl and methyl groups, at 150°C; then, networks were quenched at 0°C, during the gelation process. Networks of type B were composed of vinyl-terminated PDMS chains and poly(methylhydrosiloxanedimethylsiloxane) copolymers, with additional hydride-terminated PDMS chains (molar fraction of hydride functions: 10-2 per monomeric unit). For these systems, measurements of the storage modulus or of NMR were performed, in situ, during the gelation. This study extends the NMR-swelling interrelationship to the property of elasticity.
Phase transition occurring in three different types of poly(N-isopropylacrylamide) gelssnormal cross-linked gel, comb-type grafted gel, and comb-type grafted gel with styrene-modified comb chainsshas been investigated by variable-temperature measurements of 1 H NMR spectra and spin-spin relaxation time. Three different gels exhibit distinct collapse behaviors in response to increasing temperature. For the normal gel, remarkable network shrinking occurs in a relatively narrow temperature range from 32 to 35°C. For the styrenemodified comb-type gel, overall chain shrinkage appears in a very broad temperature range from 22 to 35°C in which the styrene-modified comb chains shrink at lower temperatures (22-32°C) than the backbone networks (32-35°C). In the comb-type gel without styrene modification, however, the backbone networks shrink first (at 32-35°C) on heating, followed by collapsing of comb chains (at 35-36°C). During shrinkage of backbone networks the comb chains are expulsed from the main gel networks which is revealed by abnormal T 2 increase of comb chains. T 2 measurements also reveal that the styrene-modified comb-type gel in the equilibrium swelling state has more rigid network structure than both conventional gel and comb-type gel without styrene modification.
Developments in Solid-State NMR Spectroscopy of Polymer Systems
InTech eBooks, 2017
Solid-state nuclear magnetic resonance (NMR) has long emerged as a valuable technique for characterizing the molecular structure, conformation, and dynamics of polymer chains in various polymer systems. The principles of the solid-state 13 C NMR cross-polarization experiment are described along with corresponding relaxation measurements. The ensuing recent applications of these techniques to different polymer systems are reviewed, with selected examples that have appeared in the recent literature. All of these applications of solid-state NMR to polymers have one feature in common: the interpretation of spectroscopic observations as related to the structural features and physical properties of the polymer.