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. The main objective of the current study was to elucidate the relationship between the mechanical properties and network structure of phenolic resins. However, the influence of the excess amounts of HMTA on the resulting cross-linked structure has not been well studied owing to challenges associated with analyzing the structures of insoluble and infusible cured phenolic resins. 2, 3 Experimentally, the preparation of phenolic resins with an excess amount of HMTA results in an improvement in the mechanical strength and modulus of the cured resins. To obtain highly cured phenolic resins, an excess amount of HMTA (greater than the stoichiometric ratio) is frequently used for industrial curing of NV. The excellent properties of cured phenolic resins are derived from their three-dimensional, cross-linked network structures, as well as their high crosslink densities. Several studies have been performed on NV using infrared (IR) 8 spectroscopy, nuclear magnetic resonance (NMR) 9, 10, 11 spectroscopy and differential scanning calorimetry. Therefore, understanding and controlling the chemical structure and curing behavior of these resins are important. 5, 6, 7 NV are further heat treated in the presence of HMTA to obtain cured phenolic resins. 2 For phenolic resins, the degree of the reaction ( О±) is defined as the fraction of reacted ortho- and para-positions adjacent to hydroxyl groups and is based on classical gelation theory. NV can be obtained via the polycondensation of phenol and formaldehyde in the presence of an acid catalyst, such as oxalic acid. 2, 3, 4ДЇor general industrial use, cured phenolic resins are prepared by curing a novolac-type phenolic resin oligomer (NV) with hexamethylenetetramine (HMTA) as the curing agent ( Figure 1). 1 These polymers have been used as indispensable thermosetting resins in the electronic, automotive, aerospace, housing and other industries due to their excellent mechanical and electrical properties, as well as their resistance to heat and solvent. Phenolic resins, which are well-known phenol formaldehyde resins or Bakelite, were invented by Baekeland in 1907 and employed as the first commercial synthetic polymer. These results suggest that structural differences governing the mechanical properties of phenolic resins are initiated in the pregel stage, followed by noticeable differences in the mechanical properties during the subsequent curing process. The relationship between M w and R h for NV in tetrahydrofuran indicates two different polymer-growth mechanisms irrespective of the excess amount of HMTA (that is, power-law and subsequent deviation corresponding to the chain extension and intermolecular reactions between larger molecules, respectively). A larger quantity of HMTA resulted in faster growth and larger R h and M w values. The dependence of the number of mixing times on the weight-average molecular weight ( M w) and hydrodynamic radius ( R h) of NV differed significantly with the HMTA amount. A two-roll mixing mill process was applied to control the curing reaction degree. The curing behavior of a novolac resin (NV) cured with hexamethylenetetramine (HMTA), as well as the influence of an excess amount of HMTA on the curing reaction, were investigated by dynamic light scattering and gel permeation chromatography.

Dynamic light scattering mathematics