Article pubs.acs.org/IECR
Role of Nonbond Interactions in the Glass Transition of Novolac-Type Phenolic Resin: A Molecular Dynamics Study Cheng Bian,† Shujuan Wang,*,† Yuhong Liu,‡ Kehe Su,§ and Xinli Jing† †
Department of Applied Chemistry, School of Science and ‡Department of Chemical Engineering, School of Chemical Engineering and Technology, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an 710049, China § Key Laboratory of Space Applied Physics and Chemistry of the Ministry of Education, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710072, China S Supporting Information *
ABSTRACT: Nonbond interactions, such as hydrogen bonds and π−π/p-π interactions, have a decisive effect on the physical properties of phenolic-rich polymers, such as the novolac-type phenolic resins. To study the influence of nonbond interactions on the glass transition temperature (Tg) of novolac resins, both the molecular dynamics and experimental approaches were applied. The results show that, compared with the o-p′ type novolac, the o-o′ type novolac models have relatively lower gyration radiuses and higher Tg values because more intramolecular hydrogen bonds can be formed in the o-o′ type novolac models. As there are fewer hydrogen bonds formed in o-p′ type novolac, the π−π and p-π interactions, which decrease with the degree of polymerization and temperature, strongly influence the glass transition of o-p′ type novolac. These results provide a new perspective on the structural-property relationship of phenolic resin.
1. INTRODUCTION Phenolic resins (PRs) are one of the most important polymeric materials since the early part of the twentieth century. Due to their excellent mechanical and heat- and solvent-resistant properties, PRs are widely used in the fields of electronics, automotive, housing, and other industries.1 The excellent comprehensive properties of PRs largely depend on the crosslinked structure, which consists of a network of three functional phenols and two functional methylenes. There are two orthopositions and one para-position for each phenolic ring to link to the others; thus, the phenol pairs can be linked as o-o′, o-p′, and p-p′1 (Figure 1). The structure of PRs has been studied for
property relationship of PR. Novolac resins are one type of the typical intermediate products of PR, which are synthesized under an acidic catalyst and a molar excess of phenol to formaldehyde.6 The molecular weights of novolac resins are in the low thousands (200 to 2000 Da), typically consisting of 2− 20 phenolic units linked via methylene bridges.7 The physical properties of novolac resins largely depend on the flexibility of novolac chains and the nonbond interactions formed in and between novolac chains, such as hydrogen bond and π−π interactions. Given the similarity between the novolac resins and cured PRs, the investigation of the atomistic structure of novolac resins is expected to elucidate the structural-property relationship of the cured PR. A typical feature of novolac resins is the ability to form hydrogen bonds in novolac resins or novolac blends. Fahrenholtz et al.,8 Wu et al.,9 and Kuo et al.10 verified that the compatibility and glass transition temperatures (Tg values) of some novolac blends, such as with poly(methyl methacrylate) or poly(vinylpyridines), were significantly improved, because of the formation of hydrogen bonds network in the novolac blends. For this reason, novolac resins could be used not only as intermediate products of cured PR but also as a polar resin with good compatibility with other resins.11,12 The hydrogen bonds in novolac blends can easily be characterized by FTIR and NMR.7,9,10,13 However, the number and
Figure 1. Typical connection patterns between phenolic rings in PR.
several decades, and significant progress has been made.2−5 However, due to the insoluble and infusible properties of the cured 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. For example, the influence of the hydrogen bonds and π−π/p-π interaction on the chain conformation and glass transition of cross-linked PR is still unclear.3 For simplicity, novolac-type PRs (or novolac resins) are often chosen as a model compound to study the structural© XXXX American Chemical Society
Received: June 2, 2016 Revised: August 17, 2016 Accepted: August 22, 2016
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DOI: 10.1021/acs.iecr.6b02136 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
(RDFs), aRDG functions, mean square displacements (MSDs), and averaged radius of gyrations (Rgs) were calculated using molecular dynamics (MD) approach.
distribution of hydrogen bonds in novolac resins are difficult to be investigated using the above techniques, and it has also proved challenging to evaluate the influence of hydrogen bonds on the chain conformation and physical properties of novolac resins. Recent advances in molecular simulation technologies make it possible to study the structural details of novolac resins at the atomistic level. For example, Templeton et al.14 and Aoai et al.15 applied the Dreiding force field to search for the most stable conformations of single novolac chains. They found that more hydrogens bonds could be formed in o-o′ type novolac chains than in o-p′ type. Schürmannw et al.16 and Choi et al.17 speculated that the intra-hydrogen bonds strongly influence the Tg, viscosity, and solubility of novolac resins, especially for o-o′rich novolac model molecules. However, these simulations were based on energy minimization or simple molecular mechanics methods and isolated novolac model molecules. Further investigation was conducted by Pawloski et al.18 based on the bulk novolac models, and they concluded that the difference between the diffusivity of o-o′ and o-p′-rich novolac chains led to the discrepancies of Tg and viscosity. Considering the structure complexity caused by the molecular weight distribution and the variety of the connection patterns (o-o′, o-p′, and p-p′) between the phenolic pairs, more detailed studies are needed. The π−π or p-π (such as OH-π and CH-π) interactions are very likely to be formed in such phenolic-rich systems of novolac resins. π−π, OH-π, and CH-π interactions have already been confirmed in novolac-similar compounds, such as bisphenol F19 and calixarenes.20 Based on DFT calculations, Pillsbury et al. 21 found that one of the most stable conformations of the o-o′ type bisphenol F was the one containing a hydroxyl perpendicular to the next phenolic plane, in which OH-π interaction could be formed. Gruber et al.22 proposed that the CH-π interactions had considerable impact on the crystallization of p-p′ type bisphenol F. However, because of the composition complexity and amorphous nature, these methods are difficult to apply to characterize the π−π or p-π interactions in novolac resins. An alternative method is to analyze the number and distribution of the hydroxyl pairs and phenolic pairs using certain rules. For example, the typical rule for hydrogen bond formation in phenolic systems is “H···O length ≤2.0 Å, O−H···O or C−H···O bond angle ≥150°”, while the typical distance and angle for π−π interaction between the phenolic pairs can be within the range of 3.5 to 5.0 Å and 0 to 90°.23 However, because there are so many types of π−π or p-π interactions (e.g., sandwich, T-shaped or parallel displaced),24 it is still challenging to describe them solely by the centroid distances and planar angles. Instead, here we will use a visualization method based on the averaged reduced density gradient (aRDG) function,24,25 which can effectively display the π−π or p-π interactions, to describe the nonbond interactions in the novolac models. This study is aimed at illuminating the nonbond interactions (hydrogen bonds and π−π/p-π interactions) and their influences on the chain conformation and glass transition of novolac resins. To construct novolac models with realistic molecular weight (distribution), two types of commercial novolac resins (3490# and 2123 # ) were purified and characterized in detail. Novolac models with a different degree of polymerization (DP) and connection patterns, as well as models exhibiting the actual molecular weight distribution, were constructed. Their Tg values, radial distribution functions
2. EXPERIMENTAL AND SIMULATION DETAILS 2.1. Experiment Section. Sample Preparation. Industrial grade novolac resins (780 Da for 3490# and 824 Da for 2123#) were kindly supplied by Shandong Shengquan Chemical Co., Ltd. The resins were cooled and dried under vacuum at 60 °C overnight before purification by column chromatography.26 A resin solution prepared with n-hexane, charged to the silica gel column chromatographic purification, was adopted mainly to remove possible impurities (such as unreacted components and remaining catalyst). Structural Characterization. The content of elements C and H was measured by an elemental analyzer (Vario EL III, Elementar Instruments, Inc., Germany). An amount of 1−2 mg of novolac resin was burned at 1150 °C, the generated gases were separated, and the amount of each gas was determined. Fourier transform infrared spectroscopy (FTIR) was conducted with an infrared spectroscopy (TENSOR 27, Bruker) with the acquisition conditions of spectral width of 4000−400 cm−1, 32 accumulations, and 4 cm−1 resolution. A reference band of 1610 cm−1 was used, which corresponded to the stretching of aromatic ethylene bonds (CC) in the aromatic rings and appeared to be invariant. Proton and carbon-13 nuclear magnetic resonance (1H NMR and 13C NMR) spectra were acquired with a Bruker (Advance III) 400 MHz spectrometer with acetone-d6 as the solvent and tetramethylsilane as an internal standard. The novolac resins were analyzed by the matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) using a microTOF-Q II MALDI-TOF mass spectrometer (Bruker, Germany) and a linear model. The MALDI-TOF samples were prepared by adding the matrix [2,5-dihydroxybenzoic acid] into about 2 μL of the dilute oligomer solution in dimethylformamide dropped on the stainless-steel sample holder, and the solvent was evaporated on an electrical heater. The concentration of DHB in the solvent was approximately 10 mg/mL. The results of MALDI-TOF, NMR, and FTIR characterization were quite consistent with the literature27,28 (Figures S1, S2, and S3). Property Characterization. The densities of the bulk samples were measured by Archimedes’ technique with the water immersion method using a Standard Mettler Toledo Balance AG204 Densi-meter. Differential scanning calorimetry (DSC) analyses were performed on a NETZSCH DSC 200PC (NETZSCH Corporation, Germany) under a nitrogen atmosphere at a heating rate of 10 °C/min starting from room temperature and heating to 300 °C with an empty aluminum pan as the reference. Softening point tests of the resins were performed on a SYD-2806H fully automatic Softening Point Tester (Shanghai Changji Geological Instrument Co., Ltd., China). 2.2. Simulation Details. Model Construction. Boo-x and Bop-x. For simplicity, two types of all-o-o′ and all-o-p′ novolac models with different DP (termed as Boo-x and Bop-x, with x denoting DP, x = 2, 3, 4, 6, 8, 12, 16 and 20, see Figure 2 and Table S3) were constructed; the range of the DP was based on the MALDI-TOF results (Figure S1). The Amorphous Cell module in Materials Studio (v 6.0, Accelrys Inc.)29 was applied to construct the novolac models with chains randomly distributed in the cell. Considering the computational accuracy B
DOI: 10.1021/acs.iecr.6b02136 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
NVT-MD Simulation and RDF Calculation. For phenol and novolac models with DP = 2 to 6, 0.5 ns NVT-MD simulations at 300 and 540 K were conducted. The softening points of 3490# and 2123# are approximately 373 K, thus, the novolac models at 300 and 540 K could be considered to represent glass state and melting state, respectively. For the novolac models with DP = 2 to 20, the radial distribution function (RDF) of O−O pairs and phenol−phenol ring pairs, the mean square displacement (MSD) of all atoms and end phenolic rings, the averaged gyration radius (Rg) of novolac chains, and total number of hydrogen bonds were calculated. aRDG Analysis. To perform the aRDG analysis, one of the end phenolic rings was frozen in the last frame of the models, and a further 200 ps of NVT-MD simulations were conducted at both 300 and 540 K. Multiwfn software33 and molecular structure visualization software VMD 1.9.234 were adopted to visualize the averaged RDG iso-surface on the frozen phenolic ring. The aRDG (or s(r)) function for thermally fluctuating systems, for instance, trajectories generated from molecular dynamics simulations, was developed by Wu et al.25 based on Johnson et al. reduced density gradient (RDG, or s(r)),24 which derived from the electron density and its first derivative
Figure 2. Novolac models used in this study named as Boo-x (a) and Bop-x (b), respectively.
and expense, the model size was controlled at approximately 12000 atoms contained in each models. Boo-mix, Bop-mix, and PhOH. To compare the simulated results with the experimental values of novolac resins, two novolac models termed Boo-mix and Bop-mix were constructed according to the molecular weight distribution of 3490# and 2123# (Figures S1 and S4). A phenol model (termed PhOH) with realistic density (1.07 g·cm−3 at 300 K) was constructed and taken as a control model. Simulation Procedures. Geometric Optimization. For each model, 20 models were randomly generated by the Amorphous Cell and relaxed with precise geometry optimization; the configurations with relatively low energy were further annealed, relaxed, and used to sample the MD trajectories (Figure 3).
s(r) =
|∇ρ(r)| 1 2 1/3 2(3π ) ρ(r)4/3
(1)
Then the aRDG is described as s(r) =
|∇ρ(r)| 1 2 1/3 2(3π ) ρ(r)4/3
(2)
The promolecular density (ρpro = ∑iρatom ) obtained from i simple exponential atomic pieces (ρiatom) can predict lowdensity, low-reduced-gradient regions similar to density-functional results. To be exact, in molecular dynamics trajectories containing N snapshots Nsnapshots
Figure 3. Model construction, simulation, and analysis procedures.
⟨s(r)⟩ =
∑
si(r)/Nsnapshots
i=1
The Forcite geometry optimization with the Smart algorithm was employed to refine the geometries of the models until they satisfied the criteria of ‘energy
