Moisture Effects on the Physical Properties of Cross-linked Phenolic

3 days ago - Silane Based Redox Initiating Systems: Toward a Safer Amine-Free, Peroxide-Free, and Metal-Free Approach. Macromolecules. Wang, Garra ...
0 downloads 0 Views 2MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Moisture Effects on the Physical Properties of Cross-linked Phenolic Resins Weiwei Zhao,† Shaw Ling Hsu,*,† Sethumadhavan Ravichandran,‡ and Anne M. Bonner‡ †

Polymer Science and Engineering, University of Massachusetts (Amherst), Amherst, Massachusetts 01003, United States Saint-Gobain Research North America, 9 Goddard Road, Northborough, Massachusetts 01532, United States

Downloaded via IDAHO STATE UNIV on April 24, 2019 at 23:07:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Using a combination of characterization techniques, we have been able to establish two types of relaxation mechanisms when moisture is introduced into typically cross-linked phenolic resins. The inherent rigidity of this class of polymer originates from the extensive intra- and intermolecular hydrogen bonding. To raise structural stability at elevated temperatures, inter methylene bridges between the aromatic units (chemical cross-links) need to be formed usually with specific cross-linking agents such as hexamethylenetetramine. Given the rigidity of the polymer, segmental mobility decreases quickly as cross-linking reaction proceeds, thus limiting the degree of cross-linking to be well short of the theoretical values. This leaves a significant fraction of unreacted and/or partially reacted components in the phenolic network. In the presence of moisture, the disruption of hydrogen bonding provides one mechanism for cross-linked phenolic resins to relax. Our analysis showed that a second relaxation mechanism exists due to the specific affinity of water molecules to the unreacted functional groups. The investigation of relaxation behavior of this phenolic system will provide a fundamental basis for understanding segmental dynamics of a broad spectrum of other cross-linkable polymers. polymer network.5,7 The exact amount of the cross-linker used dictates the mechanical strength and heat resistance.8−10 HMTA is 100% crystalline and has a high melting temperature of 280 °C.7 Often a plasticizer is also added to this combination of polymer and cross-linking agent. Its principal role is to plasticize the phenolic resin, thus freeing the hydroxyl groups to solubilize HMTA for the cross-linking reaction to occur. This is deemed as a two-step reaction.11−13 In addition, because of limited segmental mobility once the cross-linking reaction initiates, the plasticizer also enhances the reaction because of the higher segmental dynamics introduced.11−13 It should be noted that even with the assistance of plasticizers, the decrease in the mobility of the molecules and their reactive groups as cross-linking occurs would result in the incomplete reaction, leaving a significant amount of unreacted HMTA or intermediates in the cross-linked structure.12 This heterogeneous structure (incomplete cross-linked phenomenon) is common to other cross-linkable polymer systems.14−16 Cross-linked polymers are often exposed to humid environments, where water molecules can easily permeate into the materials, even at ambient temperatures, thus modifying their physical properties. This study was carried out to explore the moisture induced changes in phenolic systems which are

1. INTRODUCTION Cross-linked polymers have been widely used as adhesives, coatings, and structural materials for composites because of their superior thermal and mechanical properties.1 This class of polymers consists of macromolecular chains cross-linked by covalent bonding or physical bonding or both. The former, covalent type of structure can yield materials with high modulus and thermal stability. The latter type of network structure, such as hydrogen bonding, serves to reinforce the intermolecular interaction and thereby strengthen the polymer networks.2−4 There are many thermosetting resins, such as phenolic, polyurethanes, and polyimides, involving the combination of covalent bonds and hydrogen bonds. Our specific interest in this study is focused on the phenolic systems. Phenolics clearly possess the hardness and durability that are pertinent to many high performance applications. From the fundamental respective, it is the chemistry and physics governing the structural formation that is of great importance to their properties. Phenolic resins have been described as largely associated complexes at ambient temperatures, held together by the network of hydrogen bonds involving the numerous hydroxyl groups present. However, the resin by itself does not exhibit sufficient thermal stability with segmental relaxation occurring at around 60 °C.5,6 A cross-linker such as hexamethylenetetramine (HMTA) is usually introduced to enhance the formation of a mechanically robust, chemically cross-linked © XXXX American Chemical Society

Received: February 25, 2019 Revised: April 6, 2019

A

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

described below, a typical exotherm representing the cross-linking reaction initiates at approximately 120 °C, varying by a few degrees depending on the type of plasticizer used.11−13 There is a second small exotherm at high temperature region. Previous studies have suggested that the first reaction is related to the rearrangement and formation of intermolecular methylene linkages and the second reaction has remained unexplained.11,26 As before, we have not taken the second reaction into account in all our analyses. The cross-linking reaction can be considered being completed by 160 °C. Once the sample reaches this elevated temperature, the heat flow measured using differential scanning calorimetry (DSC) diminishes to the original baseline. In addition, when the sample temperature reaches 160 °C, there is no exotherm observed in the second heating. Therefore, we deem these samples to be cross-linked. However, even under these conditions, these samples can only best be described as partially cross-linked products. In most commercial applications, the cross-linking process involves much longer reaction time (>48 h). 2.2. Characterization. 2.2.1. Infrared Spectroscopy. Nearinfrared spectra (NIR) were obtained with a PerkinElmer Frontier spectrometer in the transmission mode. The mixtures were placed between two KBr windows in our home-made sample cell. The sample temperature can be varied from ambient to 160 °C. The heating rate usually is kept at 5 °C/min, identical to the one used in our thermal experiments. All spectra were measured at 2 cm−1 resolution signal averaged 128 scans. For quantitative analysis, all spectra were normalized to the intensity of the first overtone of −CH2− stretching vibration at 5670 cm−1. There may be some slight error using this normalization technique as the −CH2− of the small amount of HMTA has a band at ∼5740 cm−1. Mid-infrared spectra were collected using a PerkinElmer Frontier spectrometer with an attenuated total reflectance accessory. All spectra were measured at 2 cm−1 resolution using 128 scans. All spectra were normalized to the intensity of benzene ring stretching vibration at 1595 cm−1. 2.2.2. Low-Field NMR. The spin-lattice relaxation time (T1) was measured with a Bruker mq20 low-field spectrometer operating at 20 MHz. The sample temperature was controlled with a BVT3000 heater working with a flow of heated air. The T1 value was measured using traditional inverse pulse sequence method (180−τ−90) with 30 data points and four scans for each data point. To characterize the variation of segmental dynamics during cross-linking process, in situ variable-temperature T1 measurements were performed from 50 to 140 °C. For the study of temperature and moisture effects on the relaxation of cross-linked PF resins, the samples were heated from room temperature to 160 °C at 5 °C/min. The T1 value of the cross-linked samples was measured at different temperatures. To evaluate the effects of H2O on the T1 relaxation, these cross-linked samples were exposed at room temperature to 52% RH and 95% RH for 24 hrs. The variation in RH was obtained using saturated water solution of Mg(NO3)2 and KNO3 for low and high humidity conditions, respectively.27 All samples were equilibrated for 10 min before the T1 measurement. The collected data points were fitted with one exponential T1 relaxation curve (eq 1) using software provided by Bruker mq20

hydrophilic. Understanding the effects of moisture is critical to modify their structures and to tailor the final properties for this class of materials, which is also expected to shed insights into other thermosetting resins. Moisture degrades polymer properties usually because the absorbed water acts as a plasticizer to enhance the segmental mobility and disrupt intermolecular interactions.14,17 There is also evidence showing that interactions between water molecules and specific functional groups, on the contrary, may cause antiplasticization.18,19 Intuitively and experimentally observed, absorption of water into phenolic resins, even in cross-linked samples, can diminish the mechanical strength.20 This is primarily attributed to the disruption of strong hydrogen bonding in the system. However, the effects of moisture seem to be larger and at a time scale longer than the absorption kinetics expected for such cross-linked systems. Previous studies have revealed the high solubility of HMTA in water.21,22 It is then hypothesized that the effects of moisture are more than just the relaxation caused by the disruption of hydrogen bonds. Considering the complexity of the cross-linking reaction, it is possible that the presence of intermediates and remaining functional groups inside the phenolic network, especially these nitrogencontaining intermediates,23 introduce an additional factor in the increase of free volume due to specific affinity to moisture. However, past studies have not yet incorporated the effects of these unreacted and/or partially reacted components and their interaction with moisture and correlated them to the physical properties of cross-linked phenolic resins. In the current study, we have used a combination of techniques, including thermal analysis, mid/near-infrared spectroscopy, and low-field NMR (LFNMR), to follow the cross-linking process and to assess the resultant structure and effects of moisture. Because of its hardness, it is not possible to directly measure physical properties associated with the crosslinked products. In our previous studies, it has been established that the spin-lattice relaxation profile of LFNMR signal is related to the final physical properties, such as cross-linking density and molecular motions.12,13,24 The relaxation or plasticization effects involve an almost unmeasurable amount of water and happen so quickly. These macroscopic properties can be measured with great sensitivity using a cantilever technique.25 This cantilever technique is used to measure the relaxation kinetics in the cross-linked structure when they are exposed to moisture and at different temperatures.

2. EXPERIMENTAL SECTION 2.1. Materials. Phenolic formaldehyde (PF) prepolymer with HMTA at three different levels 2, 8, and 14% by weight of resin has been provided by Saint-Gobain Corporation. On the basis of the 13C solution NMR data, this prepolymer has been determined to be random configuration in nature, that is, ortho−ortho/ortho−para/ para−para being 1:2:1. The ortho- and para-unsubstituted phenolic positions can be considered as reactive sites, and the number of unreacted ortho and para site per phenolic ring is 1.73. The numberaveraged chain length of the prepolymer is approximately eight phenolic repeat units as determined by 1H NMR analysis.26 Methyl benzoate (MB) was purchased from Alfa Aesar. To ensure uniformity, as before, the mixtures with plasticizers were mixed at liquid nitrogen temperature and ground for 2 mins.11−13 The prepared sample was kept at 30% RH, 15 °C before use. Because of the rigid nature of the two reactants in this study, it is virtually impossible to achieve a high degree of cross-linking under most experimental conditions unless a plasticizer is used. We have utilized a thermal profile by increasing the sample temperature at 5 °C/min from ambient temperature to 160 °C. As shown before and

Mz = M 0(1 − 2e−τ / T1)

(1)

where Mz and M0 are the nuclear spin magnetization measured as a function of time and the initial value, respectively. 2.2.3. Cantilever Deflection. To investigate the structure-related relaxation behavior of PF resins, a cantilever deflection method is applied (see Figure 1).25 In this study, the interfacial stress developed between the phenolic film and underlying glass plate is used to describe the changes in the structures obtained. Samples were prepared by applying PF resin powders (75 mg) onto cantilevers that were cut from glass slides. The cantilevers were degreased in acetone and ethanol, rinsed in deionized water, and dried under an N2 atmosphere prior to use. Samples were heated from room temperature to 160 °C at 5 °C/min and then cooled down to room temperature. Assuming a perfect adhesion between the phenolic film and substrate B

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Schematic diagram of the experimental set-up of cantilever beam deflection. The cell was custom-designed for well-controlled temperature and relative humidity. and that an evenly stress is applied to the cantilever, the magnitude of the stress (σ) can be calculated28 σ=

2ΔEh l2

(2) 29

where E, h, and l are the Young’s modulus (70 GPa), thickness (0.1 cm), and length of (6.1 cm) of the cantilever, respectively; Δ is the deflection which can be determined using the laser beam reflected from the free end.30 In contrast to the experiments carried out before, we have built a closed cell that allows variation in sample temperature and relative humidity. To validate our cantilever technique, the structure development for epoxy based on diglycidyl ether of bisphenol A (DGEBA) cured with Jeffamine D230 was studied. The epoxy resin was cured at 160 °C for 2 hrs. The ultimate stress obtained in our study is 12.3 MPa, which is in reasonable agreement with the value measured using other techniques.31−33 This validation experiment enables us to measure sample shrinkage developed for PF with three different HMTA contents. For the study of relaxation under humidity, the initially cross-linked samples were exposed to relative humidity value produced by bubbling a dry air stream through the saturated Mg(NO3)2 salt solution at room temperature. Relative humidity in the cell was determined from the humidity of the H2O at the outlet, which was measured directly using a digital hygrometer. 2.2.4. Thermogravimetric Analysis. The content of H2O absorbed was determined by the thermogravimetric analysis (TGA, TA instrument Q50). Samples of ∼5 mg were placed in the sample pan and heated from 30 to 300 °C at a rate of 10 °C/min. Nitrogen was used as a purge gas at a flow rate of 60 mL/min. 2.2.5. Differential Scanning Calorimetry. DSC (TA Instrument Q100) was performed to measure the curing energy and quantify the chain mobility of cross-linked PF resins. Specimens of 3−5 mg were loaded in aluminum pans and heated at 5 °C/min from −20 to 280 °C under an atmosphere of nitrogen.

hydrogen bonds are disrupted. The structural stability then rests with the amount chemical cross-links formed and is directly proportional to the amount of HMTA used. The T1 measured in the range between 50 and 140 °C can be used as a probe to estimate the contribution of two types of cross-links in the network structures, as summarized in the Figure 2b. Hydrogen bonds dominate in the network structures of crosslinked PF resins with low amount of cross-linking and are extremely important to maintain stability of the condensed state at low temperatures. A similar trend is also observed when segmental relaxation is measured using the cantilever technique as shown in Figure 3. In this case, the stress was converted from the deflection measured using eq 2. The initial stress in the as-prepared cross-

3. RESULTS AND DISCUSSION The expected decrease of T1 in the range between 50 and 140 °C shown in Figure 2a indicates an increase in segmental motion as the sample temperature increases. The rate of decrease in T1 as a function of temperature depends significantly on the amount of HMTA used in the crosslinking process. At low temperatures, the T1 values are similar for all three samples. However, at elevated temperatures, significant differences were observed for T1 values for the three samples. A higher T1 value is found for cross-linked PF resin with a higher HMTA content. The changes observed as a function of temperature are consistent with expected structures. At low temperatures, the structural stability is attributed to the both chemical and physical cross-links (hydrogen bonds) present. At elevated temperatures, these

Figure 3. Stress relaxation of 160 °C cross-linked PF resins as a function of temperature.

Figure 2. (a) T1 relaxation time of 160 °C cross-linked PF resins as a function of temperature. (b) Contribution of hydrogen bonding and covalent bonding as evaluated from the T1 value.

C

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

absorption of H2O.38,39 Therefore, a set of separate LFNMR experiments were also carried out to measure the spin-lattice relaxations or the molecular response of cross-linked PF resins exposed to different RHs as shown in Figure 5. In this case, the

linked PF resins is contractive resulting from volumetric shrinkage during the curing and as sample cools after curing.32,34,35 As temperature increases, the samples expand (cantilever bends downwards), thus compensating for the initial contractive stress (cantilever bends upwards). The temperature at which the beam lays flat, with no deflection (no net stress), is in the same range of temperature when chain segments gain mobility. Using DSC, the glass transition temperature has been measured to be 74, 122, and 146 °C for PF/2%HMTA/10%MB, PF/8%HMTA/10%MB, and PF/14% HMTA/10%MB mixtures, respectively. Both DSC and this cantilever technique exhibit virtually identical transition temperatures. As expected, the transition temperature is higher for more densely cross-linked samples. In addition, the net stress or deflection of the cantilever is higher for samples with higher cross-link density at all temperatures below Tg at which relaxation behavior changes. The relaxation of cross-linked PF resins exposed to moisture was investigated by the cantilever deflection technique. Figure 4 shows the variation in stress in the three cross-linked PF

Figure 5. T1 relaxation time of 160 °C cross-linked PF resins exposed to different RHs.

temporal resolution (∼couple minutes) is not as fast as the cantilever measurements. However, these spin-lattice relaxations provide a measure of segmental dynamics as moisture is absorbed into the sample. As expected, because of the plasticization effects by absorbed H2O, T1 is observed to decrease as a function of RH. In addition, the T1 rate of decrease as measured from the slope is found to increase as the amount of HMTA increases used in the sample. These data, in combination with the results in Figure 4, strongly indicate that the cross-linked PF resins with higher amount of cross-linker are more sensitive to relaxation due to H2O absorption as compared to samples with lower HMTA content. It could be expected that changes in the chemical nature and structure of the cross-linked PF resins, as a result of different amounts of HMTA used, primarily impact the unexpected relaxation behavior observed. To correlate the physical properties of cross-linked PF resins with their structures, the cross-linking reaction and the resultant cross-linked structures as a function of HMTA content are discussed in greater details below. The thermograms of the samples with different amounts of HMTA are shown in Figure 6. They all exhibit a similar pattern. During the sample heating, the hydrogenbonded structure is disrupted, causing the dissociation of prepolymer. This is correlated to the endotherm at ∼60 °C, which is superimposed on the glass transition of phenolic resin.5 As mentioned above, there are two exotherms in the

Figure 4. Stress relaxation of 160 °C cross-linked PF resins after exposure to 52% RH at room temperature. The solid line guides the eye. Digits in the figure show the absorption amount of H2O during 48 h humidity exposure, as determined by TGA.

resins exposed to 52% RH. When dried samples are exposed to moisture, the stress change can be extremely rapid as shown in the figure. With respect to the relative stress reduction as a function of time, it can be estimated from Figure 4 that the overall relaxation kinetics are faster for cross-linked PF resins with a higher HMTA content. This result is surprising because it was expected that higher chemical cross-linking would imply greater structural stability. Although several previous studies have established the effects of moisture on polymer performance, some in fact have dealt with phenolic resins, to our knowledge, few have measured the kinetics of the relaxation process. Our analysis is based on the initial slope of the relaxation curve. The best fit of these observations can only be accomplished most effectively using two exponential decays with two time constants. Previous studies have suggested one of the two relaxations is attributed to either branches or chain ends.36 We do not have independent data to verify this explanation. Nevertheless, the sample cured with a high HMTA content, or the most densely cross-linked samples, exhibit the fastest relaxation when first exposed to moisture. The changing cantilever deflection, a macroscopic response, is fast and sensitive to the moisture uptake. The timedependent relaxation can be attributed to the changing segmental dynamics.19,37 On the other hand, this change observed can also be expected by sample swelling due to the

Figure 6. DSC curves of PF/2%HMTA/10%MB, PF/8%HMTA/ 10%MB, and PF/14%HMTA/10%MB mixtures. The DSC curves were normalized by the weight of PF prepolymer with HMTA. D

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules range of 120−280 °C. The usual curing temperature employed in the commercial sector is in the range of 160−180 °C.7 Therefore, we have focused on the first exothermic peak, the dominant one. It corresponds to the production of intermediates and further reactions to produce methylene linkages for chain extension and cross-linking.5,11−13,23 From the total energy as associated with the first exothermic peak, it is evident that the PF resin with a higher HMTA content undergoes more cross-linking. A complete decomposition of a HMTA molecule should yield maximum functionalities of 12. Using the group contribution method, the reaction energy obtained from DSC results is analyzed to derive the average functionalities associated with HMTA that have reacted (f) during the crosslinking process,13 as shown in Figure 7. It is found that f

Figure 8. T1 relaxation time of PF resins during cross-linking process.

linkages, restricting the molecular motion as the threedimensional cross-linking proceeds.24,40,41 When the amount of HMTA increased from 2 to 14%, the T1 value measured at 140 °C is also found to increase, indicating more cross-linking for PF resins with a higher HMTA content.12,13,24 A higher cross-link density gives rise to more steric restrictions on the freedom of chain segments.41 This is consistent with thermal data. Between 120 and 140 °C, the increase in T1 diminishes or even decreases slightly with temperature. These data are consistent with our hypothesis that increasing the amount of HMTA enhances the reaction causing more chemical crosslinks to form therefore the higher T1 values. We have also assigned the slight decrease of the T1 values to be associated with the deceleration of the cross-linking reaction governed by the diffusion process.42−44 This reduction in curing kinetics is compensated by the fact that segmental dynamics continue to increase as a function of temperature, especially when a plasticizer (MB) is present.12,13 Because the data presented above suggest the possibility of incomplete reaction especially when HMTA content is high, near-IR spectroscopy is used to monitor the structural characteristics of these samples during the cross-linking process. Figure 9a shows the first overtone region of −OH stretching vibration as a function of temperature for the PF resin cross-linking containing 2% HMTA. The strong and sharp peak around 6950 cm−1 is assigned to the free −OH group, while the hydrogen-bonded −OH group is in the range between 6200 and 6800 cm−1.45 Upon increasing the temperature, the intensity of the free −OH peak increases with a corresponding decrease of hydrogen-bonded component due to the disruption of hydrogen bonding. On the basis of the corresponding DSC data, there is an obvious decrease in the free −OH intensity in the same temperature range when cross-linking reaction is initiated. In addition, a characteristic N−H stretching vibration at 6525 cm−1 appears at 140 °C.45 The presence of a nitrogen-containing functional group derived from the incomplete reaction of HMTA with prepolymer has been reported previously.23,46 The variation of normalized intensity of free −OH and N− H information as a function of temperature during crosslinking process is summarized in Figure 9b. The amount of the nitrogen-containing functional group increases with the amount of HMTA used in the cross-linking process. The drop-off in the intensity of free −OH can be explained by the formation of the benzoxazine ring, which has been identified as one of the major intermediates involved in the cross-linking process. The reduction is more significant in samples with a higher amount of HMTA.23 It can be imagined that the polymer−water affinity is then weakened because of the

Figure 7. Calculated reacted functionalities of HMTA, f, as a function of temperature.

increases with the temperature and is highly dependent on the HMTA content. The efficiency of the reaction or f value is much higher in the sample containing the least amount of HMTA. In the case of a high HMTA content, only a small fraction of all possible reaction sites of HMTA molecules has reacted. This is not surprising. The cross-linking reaction is least hindered by the cross-links formed in the sample containing 2% HMTA. A higher HMTA content, corresponding to a higher degree of cross-linking, causes a significant decrease in segmental mobility and thus diminishes the possible reaction with HMTA. The deceleration of the chemical reaction has been observed in our LFNMR data, as will be shown below. These kinetic data suggest that the crosslinking reaction in these phenolic resins is generally incomplete, even with the assistance of plasticizers. Although the reacted functionalities are surprisingly different for PF resins with different amounts of HMTA, the total amount of linkages as characterized by all characterization methods is consistent with the idea that a higher HMTA content leads to a higher degree of cross-links. LFNMR measurement has been shown to be a convenient and direct method to obtain information on the cross-linking process and to assess the degree of cross-linking.12,13,24 The T1 relaxation time associated with the segmental dynamics has been measured during cross-linking, as shown in Figure 8. T1 values for three PF resin samples exhibit a steady decrease as the sample temperature is increased until cross-linking reaction initiates in the proximity of 110 °C. Consistent with DSC data, the disruption of hydrogen bonding accompanying an increasing segmental motion results in the decreased T1 value.36,40 Following the drop, T1 then exhibits a subsequent increase. This is attributed to the formation of chemical E

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 10. Mid-IR spectra characterizing (a) original C−N stretching vibration of HMTA in the uncross-linked samples and (b) residual HMTA in 160 °C cross-linked samples.

in this case, the effects of moisture for these partially crosslinked samples are amplified because of the specific affinity of HMTA with water. The solubility of HMTA in water is exceptionally high as we have verified previously.26 The unreacted and/or partially reacted HMTA attracts a significant amount of moisture thus plasticizing the phenolic resins and perturbing their physical properties. Interestingly, the structural stability resulting from densely cross-linked networks is far out-weighed by the plasticization effect caused by the high water uptake. This explains the surprising relaxation behavior as shown in Figures 4 and 5. To assess the amount of H2O absorbed under different relative humidity, near-IR spectra and TGA were obtained. The results are shown in Figure 11. The TGA data are inserted directly in the figures. The infrared active band at around 5200 cm−1 is attributed to a combination of asymmetric stretching and in-plane deformation of H2O.45 As observed, the amount of H2O absorption increases with relative humidity. It is also noted that the H2O absorption amount increases with crosslinked samples with a higher HMTA content. The NIR spectra of the three samples at 95% RH are compared in Figure 11d. The intensity increase is consistent with more water in crosslinked samples with a higher HMTA content. Furthermore, these absorption bands are characterized by a broadness and asymmetry toward the low frequency side. The lower frequency component (5150 cm−1) is most prominent in the resin containing the least amount of HMTA. It is also quite prominent in resin with little unmeasurable amount of water. Therefore, we have assigned it to be associated with hydroxyl groups along the phenolic resin.48 On the other hand, variations of the intensity of this band for three different cross-linked samples are consistent with the results derived from Figure 9. It should be noted that there is a slight frequency shift in the water band (5205 to 5220 cm−1). This shift could be due to the overlapping components of two broad bands. The data in Figure 5 are then replotted against the amount of H2O absorbed to illustrate the relationship between the partially reacted structure and the perturbing effects of H2O (Figure 12). An overall reduction in the T1 value is seen with increasing H2O absorbed. Plasticization by absorbed H2O is the primary reason to induce structural relaxation.14,17,37 The rate of decrease of the relaxation time as a function of H2O

Figure 9. (a) Temperature-dependent NIR spectra of PF/2%HMTA/ 10%MB mixture during the cross-linking process. (b) Normalized intensity of free −OH and N−H information as a function of temperature during cross-linking process.

reduction of hydroxyl groups capable of interacting with water. The drop-off observed in Figure 9 might yield insights into new methodologies that may lower the water absorption of this important class of material. The infrared signature of HMTA is quite clear in mid-IR spectra, which is used to qualitatively measure the remaining HMTA content of these three types of samples. The C−N stretching vibration of HMTA was used for such an analysis.47 This band is not seriously obscured by other infrared features of the prepolymer and methyl benzoate and, thus, can provide a window for comparing different cross-linked samples. Figure 10a shows the original band of HMTA molecule in the uncross-linked samples. After cross-linking, the intensity of C− N stretching decreases significantly, indicating the consumption of HMTA because of cross-linking. As shown by the intensity of C−N stretching in Figure 10b, after removing the contributions from MB (1015 cm−1) and PF resin (1026 cm−1), the residual HMTA amount increases with the increasing initial HMTA content. This is consistent with near-IR results, showing an increasing amount of unreacted functional groups. However, as estimated by the intensity of C−N stretching before and after cross-linking, the amount of HMTA consumed during cross-linking increases with a higher initial HMTA content. This also indicates the higher amount of cross-linking, which is in good agreement with above analysis. The finding of the incomplete cross-linking reaction is not surprising. Other types of polymers such as epoxy or polyimides all exhibit such behavior because of restricted segmental motion as the cross-linking proceeds.14−16 However, F

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 11. NIR spectra of 160 °C cross-linked PF resins (a) PF/2%HMTA/10%MB, (b) PF/8%HMTA/10%MB, and (c) PF/14%HMTA/10% MB exposed to different RHs for 24 hrs. (d) Comparison of NIR spectra of three cross-linked samples at 95% RH. Digits in the figure show the absorption amount of H2O as determined by TGA.

molecules that interact with residual HMTA are less efficient in plasticization. However, because of the high H2O absorption caused by residual HMTA, the overall effects of H2O on relaxation are significant for cross-linked PF resins with a higher HMTA content, as indicated in Figures 4 and 5. These analyses demonstrate that the effects of water on this crosslinked phenolic resin are more complex than initially hypothesized. Not only is the amount of water absorbed important but the sites where the water molecules are located also affect the segmental relaxation greatly and the associated physical properties.

4. CONCLUSION This study was carried out to characterize the formation of network structures and their contribution to the physical properties in cross-linked phenolic resins. It has been found that the phenolic resins cross-linked with a high amount of HMTA exhibit high structural stability at elevated temperatures, however, fast relaxation when exposed to moisture. The kinetic data obtained from DSC and LFNMR indicate that the cross-linking reaction in these phenolic resins is generally incomplete, primarily due to the decrease in the mobility of the molecules and their reactive groups as cross-linking occurs. As demonstrated with infrared results, in the cross-linked systems, there exist nitrogen-containing functional groups with a high polar nature, arising from the incomplete decomposition of HMTA. The content of these components increases with the amount of HMTA used in the cross-linking process. H2O absorbed by these hydrophilic groups tend to form clusters in the network, which will be likely to disrupt the resin structure, thus increasing the free volume and segmental mobility. This type of H2O shows less plasticization efficiency than the absorbed H2O upon breaking of hydrogen bonds in the hydroxyl groups along the network. Interestingly, changes in the number of hydroxyl groups have been found as a result of the different treatments (temperature and HMTA amount).

Figure 12. T1 relaxation as a function of H2O absorption amount.

absorbed is the highest for cross-linked PF resins with the least amount of the HMTA content (slope is 249). In contrast, the relaxation as a function of the water content is lower in samples with a high amount of HMTA (slope is 133). As mentioned above, water can interact with partially cross-linked phenolic resins in two ways. The H2O can directly interact with the hydroxyl groups along the phenolic chain thus breaking the associated hydrogen bonds. This type of water seems to have significant plasticization efficiency. As there is a significant amount of hydrogen bonds with low degree of cross-linking, the effects of H2O on the relaxation response is strongest for PF/2%HMTA/10%MB sample. The absorbed H2O caused by residual HMTA would also contribute to sample relaxation. The increase in free volume can be due to the dissolution of the HMTA crystals introduced initially. Additional free volume can also be increased because of specific interactions between water molecules and molecular HMTA. This contribution to sample relaxation increases as the amount of unreacted HMTA remaining in cross-linked resins increases. The water clusters with a high dielectric constant will be likely to disrupt the resin structure, thus increasing the free volume and segmental mobility.49 Obviously, the water G

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

tetrafunctional/phenol novolac epoxy mixtures cured with a diamine. J. Appl. Polym. Sci. 2000, 77, 2305−2313. (16) Diener, C. E.; Susko, J. R. Etching of Partially Cured Polyimide. In Polyimides: Synthesis, Characterization, and Applications; Mittal, K. L., Ed.; Plenum Press: New York, 1984; Vol. 1, pp 353−364. (17) Choi, S.; Douglas, E. P. Complex hygrothermal effects on the glass transition of an epoxy-amine thermoset. ACS Appl. Mater. Interfaces 2010, 2, 934−941. (18) Cais, R. E.; Nozomi, M.; Kawai, M.; Miyake, A. Antiplasticization and abrasion resistance of polycarbonates in the charge-transport layer of an organic photoconductor. Macromolecules 1992, 25, 4588− 4596. (19) Zhou, J.; Lucas, J. P. Hygrothermal effects of epoxy resin. Part I: the nature of water in epoxy. Polymer 1999, 40, 5505−5512. (20) Gellert, E. P.; Turley, D. M. Seawater immersion ageing of glass-fibre reinforced polymer laminates for marine applications. Composites, Part A 1999, 30, 1259−1265. (21) Blanco, L. H.; Sanabria, N. R.; Dávila, M. T. Solubility of 1,3,5,7-tetra azatricyclo[3.3.1.13,7]decane (HMT) in water from 275.15K to 313.15K. Thermochim. Acta 2006, 450, 73−75. (22) Wang, L.; Dai, L.-y.; Lei, M.; Chen, Y. Solubility of Hexamethylenetetramine in a Pure Water, Methanol, Acetic Acid, and Ethanol + Water Mixture from (299.38 to 340.35) K. J. Chem. Eng. Data 2008, 53, 2907−2909. (23) Zhang, X.; Looney, M. G.; Solomon, D. H.; Whittaker, A. K. The chemistry of novolac resins: 3. 13C and 15N n.m.r. studies of curing with hexamethylenetetramine. Polymer 1997, 38, 5835−5848. (24) Patel, J. P.; Hsu, S. L. Development of low field NMR technique for analyzing segmental mobility of crosslinked polymers. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 639−643. (25) Stolov, A. A.; Xie, T.; Penelle, J.; Hsu, S. L. Simultaneous measurement of polymerization kinetics snd stress development in radiation-cured coatings: A new experimental approach and relationship between the degree of conversion and stress. Macromolecules 2000, 33, 6970−6976. (26) Patel, J. Analyse of Densely Crosslinked Phenolic Systems Using Low Field NMR; University of Massachusetts Amherst, 2017. (27) Wexler, A.; Hasegawa, S. Relative humidity-temperature relationships of some saturated salt solutions in the temperature range 0 to 50 oC. J. Res. Natl. Bur. Stand. (U. S.) 1954, 53, 19−26. (28) Flugge, W. Handbook of Engineering Mechanics; McGraw-Hill: New York, 1962. (29) Lee, K.-Y.; Case, E. D. Effects of adhesion on the effective Young’s modulus in glass slide/glue laminates. J. Mater. Sci. 1996, 31, 2253−2264. (30) Adhikari, R.; Kaundal, R.; Sarkar, A.; Rana, P.; Das, A. K. The cantilever beam magnetometer: A simple teaching tool for magnetic characterization. Am. J. Phys. 2012, 80, 225−231. (31) Nakamura, Y.; Tabata, H.; Suzuki, H.; Iko, K.; Okubo, M.; Matsumoto, T. Internal stress of epoxy resin modified with acrylic core-shell particles prepared by seeded emulsion polymerization. J. Appl. Polym. Sci. 1986, 32, 4865−4871. (32) Lange, J.; Toll, S.; Månson, J.-A. E.; Hult, A. Residual stress build-up in thermoset films cured above their ultimate glass transition temperature. Polymer 1995, 36, 3135−3141. (33) Wang, H.-B.; Yang, Y.-G.; Yu, H.-H.; Sun, W.-M.; Zhang, Y.-H.; Zhou, H.-W. Assessment of residual stresses during cure and cooling of epoxy resins. Polym. Eng. Sci. 1995, 35, 1895−1898. (34) Ochi, M.; Yamashita, K.; Shimbo, M. The mechanism for occurrence of internal stress during curing epoxide resins. J. Appl. Polym. Sci. 1991, 43, 2013−2019. (35) Ishida, H.; Low, H. Y. A study on the volumetric expansion of benzoxazine-based phenolic resin. Macromolecules 1997, 30, 1099− 1106. (36) Adriaensens, P.; Rego, R.; Carleer, R.; Ottenbourgs, B.; Gelan, J. Solid-state NMR relaxometry study of phenolic resins. Polym. Int. 2003, 52, 1647−1652. (37) Moy, P.; Karasz, F. E. Epoxy-water interactions. Polym. Eng. Sci. 1980, 20, 315−319.

This might provide a novel way to modify their structures and to tailor the final properties for this important class of materials.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 413-577-1411. ORCID

Shaw Ling Hsu: 0000-0003-1822-1584 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge Saint-Gobain for the financial support. We are also grateful for Walter J. Bryskiewicz’s (Noorgard Engineering) assistance to construct a set of unique sampling cells used in our infrared and cantilever experiments.

■

REFERENCES

(1) Polymer Networks: Principles of Their Formation, Structure, and Properties; Stepto, R. F. T., Ed.; Blackie Academic & Professional: London, 1998. (2) Whittaker, A. K. The structure of polymer networks. In Modern Magnetic Resonance; Webb, G. A., Ed.; Springer Netherlands: Dordrecht, 2006; pp 583−589. (3) Kim, H.-D.; Ishida, H. A study on hydrogen-bonded network structure of polybenzoxazines. J. Phys. Chem. A 2002, 106, 3271− 3280. (4) Monk, J. D.; Bucholz, E. W.; Boghozian, T.; Deshpande, S.; Schieber, J.; Bauschlicher, C. W., Jr; Lawson, J. W. Computational and experimental study of phenolic resins: Thermal−mechanical properties and the role of hydrogen bonding. Macromolecules 2015, 48, 7670−7680. (5) Katovic, Z.; Stefanic, M. Intermolecular hydrogen bonding in novolacs. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 179−185. (6) Jones, T. T. The melt-viscosity of some phenol formaldehyde novolak resins. J. Appl. Chem. 1952, 2, 134−149. (7) Knop, A.; Pilato, L. A. Phenolic Resins: Chemistry, Applications, Performance and Future Direction; Springer: Berlin, 1985. (8) Drumm, M. F.; Dodge, C. W. H.; Nielsen, L. E. Cross linking of a phenol-formaldehyde novolac-Determination by dynamic-mechanical measurements. Ind. Eng. Chem. 1956, 48, 76−81. (9) Wang, Y.; Wang, S.; Bian, C.; Zhong, Y.; Jing, X. Effect of chemical structure and cross-link density on the heat resistance of phenolic resin. Polym. Degrad. Stab. 2015, 111, 239−246. (10) Narayanan, K. S.; Hickory, G. E. Phenolic Resin Bonding Grinding Wheels. U.S. Patent 4,682,988, 1987. (11) Patel, J. P.; Xiang, Z. G.; Hsu, S. L.; Schoch, A. B.; Carleen, S. A.; Matsumoto, D. Path to achieving molecular dispersion in a dense reactive mixture. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1519− 1526. (12) Patel, J. P.; Zhao, C. X.; Deshmukh, S.; Zou, G. X.; Wamuo, O.; Hsu, S. L.; Schoch, A. B.; Carleen, S. A.; Matsumoto, D. An analysis of the role of reactive plasticizers in the crosslinking reactions of a rigid resin. Polymer 2016, 107, 12−18. (13) Patel, J. P.; Deshmukh, S.; Zhao, C.; Wamuo, O.; Hsu, S. L.; Schoch, A. B.; Carleen, S. A.; Matsumoto, D. An analysis of the role of nonreactive plasticizers in the crosslinking reactions of a rigid resin. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 206−213. (14) Nogueira, P.; Ramírez, C.; Torres, A.; Abad, M. J.; Cano, J.; López, J.; López-Bueno, I.; Barral, L. Effect of water sorption on the structure and mechanical properties of an epoxy resin system. J. Appl. Polym. Sci. 2001, 80, 71−80. (15) Barral, L.; Cano, J.; López, J.; López-Bueno, I.; Nogueira, P.; Abad, M. J.; Torres, A.; Ramírez, C. Mechanical behavior of H

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (38) Adamson, M. J. Thermal expansion and swelling of cured epoxy resin used in graphite/epoxy composite materials. J. Mater. Sci. 1980, 15, 1736−1745. (39) Abdelkader, A. F.; White, J. R. Influence of relative humidity on the development of internal stresses in epoxy resin based coatings. J. Mater. Sci. 2002, 37, 4769−4773. (40) Chu, P. P.; Wu, H.-D. Solid state NMR studies of hydrogen bonding network formation of novolac type phenolic resin and poly(ethylene oxide) blend. Polymer 2000, 41, 101−109. (41) Hale, A.; Macosko, C. W.; Bair, H. E. Glass transition temperature as a function of conversion in thermosetting polymers. Macromolecules 1991, 24, 2610−2621. (42) Ducruet, N.; Delmotte, L.; Schrodj, G.; Stankiewicz, F.; Desgardin, N.; Vallat, M.-F.; Haidar, B. Evaluation of hydroxyl terminated polybutadiene-isophorone diisocyanate gel formation during crosslinking process. J. Appl. Polym. Sci. 2013, 128, 436−443. (43) Riedl, B.; He, G. Curing kinetics of phenol formaldehyde resin and wood-resin interactions in the presence of wood substrates. Wood Sci. Technol. 2004, 38, 69−81. (44) Izumi, A.; Shudo, Y.; Nakao, T.; Shibayama, M. Cross-link inhomogeneity in phenolic resins at the initial stage of curing studied by 1 H-pulse NMR spectroscopy and complementary SAXS/WAXS and SANS/WANS with a solvent-swelling technique. Polymer 2016, 103, 152−162. (45) Workman, J., Jr; Weyer, L., Practical Guide to Interpretive NearInfrared Spectroscopy. CRC press: Boca Raton, 2007. (46) Hatfield, G. R.; Maciel, G. E. Solid-state NMR study of the hexamethylenetetramine curing of phenolic resins. Macromolecules 1987, 20, 608−615. (47) Tonogai, S.; Sakaguchi, Y.; Seto, S. Curing behavior of two-step phenolics by solvent extraction. J. Appl. Polym. Sci. 1978, 22, 3225− 3234. (48) Weyer, L.; Lo, S. C., Spectra-structure correlations in the nearinfrared. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons, Ltd.: Chichester, 2006; pp 1817−1837. (49) Vyavahare, O.; Ng, D.; Hsu, S. L. Analysis of structural rearrangements of poly(lactic acid) in the presence of water. J. Phys. Chem. B 2014, 118, 4185−4193.

I

DOI: 10.1021/acs.macromol.9b00385 Macromolecules XXXX, XXX, XXX−XXX