Structural analysis of cured phenolic resins using complementary small-angle neutron and X-ray scattering and scanning electron microscopy

Abstract

The structure of cured phenolic resins prepared by compression molding of a deuterated phenolic resin oligomer and nondeuterated hexamethylenetetramine as a curing agent was investigated using complementary small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and scanning electron microscopy (SEM). Cured thermosetting resins have been considered to have an inherent inhomogeneity of the cross-links with sizes ranging from tens to hundreds of nanometers based on SEM observations of fracture surfaces. However, such spatial inhomogeneity has not been observed for the phenolic resins by either SANS or SAXS. The present observation with SANS and SAXS indicates that the phenolic resins have an inhomogeneity associated with internal fractal interfaces between voids and phenolic resins, with a fractal dimension equal to 2.5-2.6 in the range of 3-1600 nm. The presence of voids in phenolic resins with sizes ranging from tens to hundreds of nanometers is clearly confirmed by an evaluation of the difference in scattering length densities between the SANS and SAXS functions and by SEM observations of etched surfaces prepared by focused-ion beam milling. Therefore, it can be concluded that (i) cross-links are randomly distributed over the range and (ii) the spatial inhomogeneity of the cross-links in that range is very small and negligible in comparison with the inhomogeneity associated with the internal fractal interfaces in terms of the fluctuations of the neutron and X-ray scattering length densities.

Molecular dynamics simulations of phenolic resin: Construction of atomistic models

“…When resins reach this final stage, they are often insoluble and exhibit relatively brittle properties at lower temperatures (T<T g ). [69,70] In our computational models, we track these structural transitions explicitly by identifying the first and second largest cluster sizes in the system and also by determining whether the largest cluster is percolating. The gel point can be identified as the point where the second largest cluster size drops rapidly, indicating the merger of two larger clusters into a single percolating cluster [20].…”

Dynamic light scattering study of the curing mechanisms of novolac-type phenolic resins

“…We also carried out SAXS and SANS investigation on the structural inhomogeneity of insoluble phenolic networks near the gel point, and highly cured networks reacted with HMTA. [18][19][20] Our previous study of the gelation mechanism using SAXS demonstrated that the scattering function of phenolic resins in a tetrahydrofuran (THF) solution or fully THF-swollen states can be described by a combination of the Ornstein-Zernike and squared Lorentzian equations, which have correlation lengths of several nanometers. 19,20 The change in correlation length during the gelation process indicates the presence of two different gelation mechanisms that are dependent on the amount of cross-linker.…”

“…The existence of voids was elucidated through complementary SAXS and SANS analyses. 18 Herein we report the evolution of NV clusters during the curing process in the pregel regime and the influence of an excess amount of HMTA on this evolution using DLS and gel permeation chromatography (GPC). An open two-roll mixing mill process was applied to control α by changing the number of roll-mixing times (N).…”

“…Previously, we investigated the inhomogeneity of phenolic resins using dynamic light scattering (DLS), small-angle X-ray (SAXS) and neutron scattering (SANS) and molecular dynamics simulations. [15][16][17][18][19][20] These scattering techniques are powerful tools for elucidating the inhomogeneity of cross-linked materials, including soluble oligomers, insoluble gels and infusible cured resins. [21][22][23][24] However, studies of phenolic resins using scattering techniques have been limited to polymer blends 25 and composite materials 26 with phenolic resins except for our previous studies.…”

“…[21][22][23][24] However, studies of phenolic resins using scattering techniques have been limited to polymer blends 25 and composite materials 26 with phenolic resins except for our previous studies. [16][17][18][19][20] In our previous studies, conformational analysis was performed in the solution state, which was achieved by dissolving bulk reaction mixtures into good solvents, using DLS to gain insight into the phenol-formaldehyde polycondensation mechanism below the gel point. 16 Well below the gel point, phenolic resins exhibit self-similar structures with respect to their molecular weights, which results in a power-law relationship between the hydrodynamic radius (R h ) and the weightaverage molecular weight (M w ).…”