Article pubs.acs.org/Macromolecules
Kinetics and Cure Mechanism in Aromatic Polybenzoxazines Modified Using Thermoplastic Oligomers and Telechelics Ian Hamerton,†,* Lisa T. McNamara,† Brendan J. Howlin,† Paul A. Smith,‡ Paul Cross,§ and Steven Ward§ †
Department of Chemistry and ‡Department of Mechanical Engineering Sciences, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, U.K. § Cytec, R414 Wilton Centre, Redcar, TS10 4RF, U.K. S Supporting Information *
ABSTRACT: A series of blends is prepared comprising 2,2bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane (BA-a) with variously 5, 10, or 20 wt % of a selected oligomer represented by poly(arylsulfone) (PSU) or poly(arylethersulfone) (PES). The oligomers, comprising either chloro-, hydroxyl- or benzoxazinyl- (Bz) terminal functionality, are of low molecular weight (3000−12000 g mol−1). The introduction of the oligomers is shown to initiate the polymerization of a bisbenzoxazine monomer where the terminal functionality of the oligomer is coreactive (e.g., hydroxyl or benzoxazine) without having a detrimental effect on the polymerization kinetics (similar values for the activation energy and orders of reaction are obtained). The introduction of the nonreactive chloro-terminated oligomer appears to favor the formation of an interpenetrating network (IPN) with a higher energy of activation. The thermal stability of the blends is generally increased compared with the polybenzoxazine homopolymer, regardless of the molecular weight or thermoplastic loading. Aside from the aforementioned PSUCl-containing IPN, the nature of the resulting network is slightly modified by the addition of the thermoplastic with similar or slightly elevated cross-link densities recorded (compared with the polybenzoxazine homopolymer). The heterogeneity of the network increases with a broadening of the tan δ response, suggesting an improvement in the toughness of the resulting blend.
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INTRODUCTION The potential for the use of oligomers based on engineering thermoplastics have been examined for some years as modifiers in aerospace composite matrices,1,2 due to their inherent toughness and the ability to blend them more easily than high molecular weight species without incurring the penalty of high melt viscosity. Poly(arylene ether sulfone)s are among the most widely reported high performance thermoplastics and were originally developed during the 1960s following independent research work by the 3 M Corporation,3 Union Carbide,4 and the Plastics Division of IC5 to develop thermally stable thermoplastics suitable for engineering applications, and their chemistry has recently been reviewed.6 The materials are highly aromatic polymers that comprise phenylene backbones bridged with heteroatoms (O, S) or groups (SO2, CH2, C(CH3)2, etc.), to offer thermal stability, good mechanical properties, creep resistance, and chemical resistance. These polymers have now reached a degree of maturity with many variants having been reported in both laboratory and commercial publications, and have been reviewed extensively.7 Commercial products (e.g., Udel, Radel, and Victrex) are now available in a variety of grades to satisfy different high performance applications and widely used. Poly(arylene ether sulfone)s display a wide range of glass transition temperatures (Tg) influenced to a large © 2014 American Chemical Society
degree by the chemical structure. Hence, polymers produced from dichlorodiphenylsulphone and simple bisphenols yield high Tg materials, typically in the range 180−230 °C with the magnitude being influenced by the bulk of the substituents on the central carbon atom. The prediction of thermal and mechanical properties in as yet unsynthesised polymers is beginning to be realized and we have demonstrated this in a variety of thermosetting polymers such as epoxy resins,8 cyanate esters,9 and polybenzoxazines,10 as well as engineering thermoplastics.11,12 In a previous publication,13 we reported the use of a quantitative structure property relationship (QSPR) to predict the Tg of a polymer of this type. In the present work, the preparation of low molecular weight oligomers (0.99) showed good linear fits to the DSC data.
between the benzoxazine and the thermoplastic can be observed and ultimately whether this influences the resin toughness (as has been found for epoxy blends containing similar polymers). It is hypothesized that the chloro-terminated thermoplastics are likely to inhibit the benzoxazine curing reaction compared to the other thermoplastics used in this work due to the presence of the thermoplastic diluting the benzoxazine system, so reducing the probability of a successful bridge forming reaction between open rings of the benzoxazine, and the absence of phenolic protons to encourage the ringopening reaction, thus potentially reducing the rate of reaction. In contrast, the hydroxyl-terminated thermoplastics may not only potentially improve toughness through hydrogen bonding interaction with the benzoxazine polymer network, but that the acidic protons of the hydroxyl terminated systems could also improve the reactivity of the benzoxazine curing reaction due to the protons initiating the ring-opening reaction.
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RESULTS AND DISCUSSION The dibenzoxazine monomer based on the reaction of bisphenol A and aniline 2,2-bis(3,4-dihydro-3-phenyl-2H-1,3benzoxazine)propane (BA-a, Figure 1) was chosen as the
Figure 1. Structures of the materials studied in this work (where X, Y are Cl, OH, or Bz).
baseline against which improvements might be measured. This is perhaps the most commonly reported and well-characterized benzoxazine monomer for which much data are available, making direct comparison easy to achieve. The polymerization of this monomer has been studied in great detail and a generally accepted mechanism proposed14 for the generation of phenoxytype and phenolic-type network structures. The selection of the oligomer loading was based on a mixture of pragmatism and practical considerations. Thus, all blends were prepared containing 10 wt % of the oligomers polyethersulphone (PES) or polysulphone (PSU) (Figure 1), the reasoning being that superior mechanical properties (and favorable morphologies) are often found at between 10 and 20 wt % incorporation of thermoplastics;15−17 while the formation of a stable blend containing 20 wt % PES was difficult to achieve and so a much lower loading (5 wt %) was explored instead. The blends were initially analyzed to examine their thermomechanical propertiesparticularly the influence of the nature and concentration of thermoplastic incorporated on the
ln
⎛ Q pAR ⎞ E β ⎟− a = ln⎜ 2 RTp Tp ⎝ Ea ⎠
(1)
where Qp = −[df(α)/d(α)]α=αp, Tmax is the maximum temperature of the exothermic peak, Ea = activation energy, R = gas constant, and β is the heating rate employed. The resulting activation energies are given in Table 2. It was assumed that the terminal functionalization would influence the manner in which the blend underwent polymerization (for a given molecular weight), although it is noted that the phenyl rings in the oligomer backbones are also potential sites for cross-linking as Chang et al. have identified in their studies of PBZs and polybenzimidazoles,26 albeit to a lesser degree than 1936
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Table 1. DSC Data for Cured Thermoplastic−Polybenzoxazine Blends Heated at 10 K/min under Nitrogen (50 cm3/min)a sample
[TP] (wt %)
To (°C)
Tmax (°C)
Tf (°C)
ΔHp (J g−1)
Tg (°C)
BA-a PESOH3000 PESOH6000 PESOH9000 PESOH3000 PESOH6000 PESOH9000 PSUOH3000 PSUOH6000 PSUOH10 000 PSUOH3000 PSUOH6000 PSUOH10 000 PSUBz3000 PSUBz6000 PSUBz12 000 PSUBz3000 PSUBz6000 PSUCl3000 PSUCl6000 PSUCl9000 PSUCl3000 PSUCl6000 PSUCl9000
− 5 5 5 10 10 10 10 10 10 20 20 20 10 10 10 20 20 10 10 10 20 20 20
192 204 204 205 200 195 203 199 203 210 195 194 200 199 203 192 195 194 210 208 213 207 201 203
240 243 243 244 240 242 243 239 239 244 237 243 246 239 239 236 237 243 247 246 247 248 245 246
291 285 282 290 288 293 291 289 287 285 289 287 288 289 287 281 289 287 286 290 291 289 285 291
309.9 262.0 274.1 284.6 274.6 280.0 291.6 259.7 271.4 258.7 246.3 223.2 231.1 259.7 271.4 248.2 246.3 223.2 259.0 261.4 266.3 233.8 235.0 249.3
157 149 139 151 150 161 161 142 149 133 151 147 159 142 149 148 151 147 127 140 162 161 148 159
a
Key: [TP] = concentration of thermoplastic oligomer in the blend, To = onset of polymerization exotherm, Tmax = temperature of exothermic peak maximum, Tf = final temperature of polymerization exotherm, ΔHp = enthalpy of polymerization exotherm, Tg = glass transition temperature (from DSC rescan).
(PSUBz3000) yields a similar activation energy (marginally reduced, along with a slightly lower value of m). The value of n is very similar across all the systems, the slightly higher value observed for PSUCl3000 might again be due to the presence of unreacted hydroxylated species. While the nature of the functionality of the oligomer might have little significant effect on the cross-link density, and thus the Tg, it might have a significant influence on the resulting morphology and molecular cohesion, affecting the moisture uptake and fracture toughness. Consequently, the next phase of the research concentrated on the use of spectroscopy to elucidate the nature of the network formation based on functional group chemistry. Cure Study of the Benzoxazine Blends Using FT Raman Spectroscopy. Although DSC (as previously described), torsional braid analysis (TBA)28 and rheokinetic analysis29 have been used to investigate kinetics and reactivity, these techniques are phenomenological in nature and chemical information features that might cause the differences in reactivity, must be inferred. Consequently, Raman spectroscopy was used to attain a better understanding of the chemical changes in the cure reaction of the benzoxazines blends, that relate to thermal events and physical processes such as gelation,30 by using multivariate data analysis it is possible to draw out changes in the spectra that may not easily be observed otherwise. We have already reported19 the power of this method to examine the polymerization mechanism and applied the technique directly. Thus, the blends were placed in Durham tubes and heated to the desired cure temperature within the Raman spectrometer while spectra are acquired (designated in situ). The resulting spectra were analyzed using principal components analysis (PCA) to uncover any patterns in the data and the key spectral bands are tentatively assigned from
Table 2. Average Kinetic Parameters for Differently Functionalized PSU Oligomers (Nominally 3000 g/mol) with BA-a over Conversion Range 20−40% sample
Ea (kJ/mol)a
ln Ab
mc
nd
BA-a PSUBz3000 PSUOH3000 PSUCl3000
81.37 80.42 85.86 93.22
9.31 10.96 10.98 10.95
2.49 1.99 2.87 2.97
1.39 1.34 1.21 1.79
a Ea = activation energy. bln A = Arrhenius pre-exponential (“collision”) factor. cm = reaction order for the early part of the reaction. dn = reaction order for the later part of the reaction.
the conventional mechanism. Thus, from a consideration of the terminal functional groups the ostensibly nonreactive chloroterminated oligomer (Figure 2a) would ultimately yield predominantly a semi-interpenetrating network;27 the hydroxyl-terminated oligomer (Figure 2b) would yield an interpenetrating network yielding both hydrogen bonded species and hydroxyl-mediated ring-opening; the full benzoxazine telechelic (Figure 2c) would be fully coreactive in a copolymeric network. The functionality of the oligomer has an influence on the activation of the polymerization reaction when compared with the BA-a monomer (Table 2). The chloro-terminated oligomer (PSUCl3000) displays a significantly higher activation energy indicating that the effect of the diluent is to inhibit reaction, although this is accompanied by a slight increase in reaction order (perhaps reflecting the presence of some hydroxylated oligomer). The hydroxylated oligomer (PSUOH3000) yields a slightly elevated activation energy suggesting that the presence of weakly acidic phenolic protons does not quite counteract the dilution effect of the PSU backbone. The addition of the benzoxazine telechelic 1937
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Figure 2. Schematic showing how (a) chloro-, (b) hydroxyl-, and (c) benzoxazine-terminated oligomers may interact with the benzoxazine network.
and spectra can be taken at fixed time intervals (thus the sample has a known thermal history); the free volume and network formation can develop as the scans are taken. However, the in situ heater could only attain a maximum of 180 °C as the fluorescence background produced by the thermal degradation at an impurity level caused the signal to be increasingly swamped between 1800 and 3400 cm−1 so that any data in this region could not be used quantitatively. Thus, no post-cure
literature sources31−33 along with data from ab initio calculations for infrared vibrations in phenolic species34 (Table 3). The heating regime was maintained (i.e., heated to 180 °C, held for 2 h, ramped to 200 °C and held for 2 h) and spectra were taken at set time intervals. Analyzing the reaction mechanism following heating in situ has both advantages and disadvantages. The sample is exposed to a constant temperature 1938
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(PCS), which explain a certain amount of information from the original data set, with PC1 containing the most information, and each subsequent PC containing less information, and also describing different variation so that later PCs not only contain less information, the information is also less relevant, e.g., noise. Plotting the PCs means sample and variable relationships can be observed, and interpretation of sample groupings to glean information, which in this case is the change with time to observe the cure reaction. The main results generated are: scores (which describe the properties of the samples related to the PCs and can also be plotted as a line plot to observe timed processes) and loadings (which describe the relationship between variables in relation to the PCs) and variance (which gives information about how much information is accounted for in each PC). Loading plots can be interpreted to deduce information about the mechanism due to the way the peaks relate to each other, for example, if peaks have a similar positive (or negative) loading, or intensity, they are positively correlated, but if they are of similar but opposite loading they have a negative correlation. So by examining at the value on the loading plot, relationships between peaks can be deduced. The loadings plot corresponds to the variables, i.e., the wave numbers. Although later stage rearrangements and cross-linking cannot be observed using the in situ technique, the early period of cure will show the most dramatic changes in the Raman spectra due to ring-opening and bridge forming reactions, compared with the later subtle molecular changes during the cross-linking stages. Although the entire data set is included initially (Figure 3a, left), the first eight samples of the in situ cure of the BA-a/ PSUBz3000 blend appear to undergo a different reaction from
Table 3. Assignments for Selected Key Bands Apparent in the PCA of the Raman Vibrational Spectra peak range (cm−1) 1615−1620 1584−1590 1575 1486−1489 1451−1455 1434 1308−1325 1285−1288 1205−1215 1192 1172−1174 1118 1104 1049−1052 1033 999 860 745−751
proposed assignment C−C stretch in benzene ring C−C stretch in benzene ring C−C stretch in benzene ring C−H bend in CH2 CH2 scissoring (ring) CH2 scissoring (ring) CH2 wag C−H bridge C−O−C or possibly C−N−C asymmetric stretch (ring closed) C−N−C asymmetric stretch (ring closed) C−H bending (ring) Wilson 18b (ring closed) C−N−C symmetric stretch (ring closed) C−O−C symmetric stretch of ring or during cure C−C stretching vibration of ring (substituted benzoxazine) C−C bending vibration of ring (mode 5 B2g) C−N−C symmetric stretch (ring closed benzoxazine) benzoxazine ring breathing
information is obtained and any rearrangements occurring at this stage are not observed. Statistical Analysis of Raman Spectral Data Using Principal Components Analysis (PCA). PCA is a multivariate analysis technique that can be used to find structure and patterns within large data sets. It works by taking information from the original variables and creating principal components
Figure 3. Scores and loadings plots for the blend containing BA-a and PSUBz3000 (a) all samples with PC1 (blue) and PC2 (red) overlaid, (b) with the first eight samples removed. 1939
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Figure 4. Scores and loadings plots for the blend containing BA-a and (a) PSUCl3000 (first three points removed), (b) PSUOH3000 (points 23−25 removed as outliers).
the remaining data set (N.B., the first 1−3 samples are usually removed the analysis as the sample undergoes melting). On removal of these spectra from the analysis (Figure 3b, left), the loadings plot remains unchanged which suggests that most of the changes occurring in the first eight samples can be described by PC2. The overlay of the PC1 and PC2 loadings plots (Figure 3a, right) shows that the spectral peak changes are the same, but the intensity and ratios both differ; these peak changes can be taken to be the most influential in the model. It is noteworthy that the curve in the scores plot (Figure 3b, left) shows that PC1 decreases as the cure progresses, contrary to the typical PCA output of a benzoxazine curing reaction.19Plots for PSUCl3000 and PSUOH3000 may be found in Figure 4. DMTA data are given in Table 4. Comparison of the fully cured spectra to those spectra taken prior to post cure (Figure 5) show that the differences between the spectra are very subtle with the PSUCl3000 appearing to be the most dissimilar in the group. The reductions in the 700− 820 cm−1 region (C−H out-of-plane bending vibrations) are less pronounced and there is a significant increase in the 990− 1050 cm−1 (C−H in-plane vibrations), 1170−1250 cm−1 (C− N−C stretch), and 1020−1070 cm−1 (C−O−C stretch), as opposed to the reductions in this band observed in the other species (BA-a, PSUOH3000, and PSUBz3000). There are also significant differences in the profile of the spectra in the 1260− 1300 cm−1 (C−O stretch) and 1400−1500 cm−1 (C−H deformations) regions. This is the strongest evidence for these blends containing the chlorinated oligomer undergoing a
different reaction mechanism, consistent with the formation of an IPN. The significant changes in the bands associated with the aromatic rings in the system, previously attributed to the change in substitution pattern following ring-opening might also support the aforementioned aromatic cross-linking mechanism,26 but further work would be required to substantiate this. Determination of Thermal Stability of Cured Polybenzoxazine−Thermoplastic Blends. The collected TGA data for the cured homopolymers35 and the cured blends (comprising both 5 and 10 wt % of each oligomer) are given in Table 5. The initial thermal stability (the temperatures at which mass losses of 5 and 10 wt % are recorded) and the char yield at 800 °C in flowing nitrogen are tabulated. In addition the TGA data for selected blends comprising oligomers of intermediate molecular weight are given in Figure 6, from which it is clear that the derivative degradation plots show a similar three-stage degradation mechanism for both the BA-a homopolymer and the blends, which is characteristic of polybenzoxazines in a nitrogen atmosphere.36,37 This is not unexpected given that the BA-a remains the major component in each blend. Considering Figure 6, which shows the highest molecular weights for each of the oligomers (at 10 wt % incorporation), the enhancement in thermal stability is most clearly seen. A smaller mass loss contributes to the second stage of the profile. Both the first and third steps in the derivative traces are shifted to higher temperatures relative to the BA-a homopolymer, particularly in the case of PSUCl9000. The 1940
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Table 4. DMTA Data for Cured Thermoplastic−Polybenzoxazine Blendsa cross-link density (10−3 mol dm−3)
E″max (°C)
a
average cross-link density (10−3 mol dm−3)
sample
[TP] (wt %)
BA-a PESOH3000 PESOH6000 PESOH9000 PESOH3000 PESOH6000 PESOH9000 PSUOH3000 PSUOH6000 PSUOH10 000 PSUBz3000 PSUBz6000 PSUCl3000 PSUCl6000 PSUCl9000 PSUOH3000 PSUOH6000 PSUOH10 000 PSUBz3000 PSUBz6000 PSUCl3000
− 5 5 5 10 10 10 10 10 10 10 10 10 10 10 20 20 20 20 20 20
165 181 182 178 179 182 183 179 175 178 174 171 179 174 180 174 162 169 164 166 120
PSUCl6000
20
PSUCl9000
20
127 170 147 178
− − − − 179 − − 177 175 163 172 171 177 173 180 167 167 − 159 − 129 157 127 171 −
− − − − − − − 176 − − − − − − − 166 − − − − 127 170 −
5.81 3.74 3.95 4.81 9.60 3.09 6.55 6.92 10.45 7.78 5.00 3.46 6.21 8.12 6.58 3.66 4.22 8.31 4.16 − 27.39
− − − − 9.67 − − 6.75 4.46 7.80 5.29 3.24 5.09 7.66 5.98 2.44 2.98 − 6.41 − 6.29
− − − − − − − 3.79 − − − − − − − 4.75 − − − − 4.65
5.81 3.74 − − 9.63 3.09 6.55 5.82 7.45 7.79 5.15 3.35 5.65 7.89 6.28 3.62 3.60 8.31 5.29 − 5.47
3.39
4.01
−
3.70
−
24.74
−
−
24.74
Key: [TP] = Concentration of oligomer; E″max = Temperature of maximum response in loss modulus.
Figure 5. Overlay of PC1 loadings plots for all blends containing BA-a. 1941
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hydrogen bonded results in a stable six-membered ring.40 This degradation pathway involves two routes: one producing aniline and the other a stable conjugated Schiff base. Primarily, cleavage of the C−N bond outside of the six-membered ring is more likely to occur as it is less energetically stable than that inside. The C−N bond is more susceptible to cleavage than the C−C bond of the Mannich base as it has a lower dissociation energy than the latter.41 Generally, the presence of thermoplastic increases the temperatures at which the 5 and 10 wt % losses are recorded (by some 20−40 K depending on the oligomer) compared with cured BA-a. The PSUCl oligomers confer the greatest improvement (increases of 32−44 K); the PSUBz oligomers the smallest (increases of 6−26 K). The increments in the temperature for 10 wt % loss and the char yields are more modest, but the PSUBz oligomers are still the best performing in this regard (Table 5). The formation of the char was studied by Hemvichian and Ishida,42 who used evolved gas analysis (EGA) to reveal a complex thermal degradation pathway involving polymer degradation and recombination of radical species to yield a range of primary decomposition products (substituted benzenes, anilines, phenols and Mannich bases), secondary products (biphenyl compounds, benzofurans, isoquinolines and phenathridines) and ultimately char. The TGA profile in Figure 6 reveals that the presence of the oligomers, which are predominantly aromatic, promotes the char-forming reactions at temperatures of 500 °C and above, leading to slightly elevated char yields of up to 10% in the case of PSUCl, compared with the BA-a homopolymer. Determination of Thermo-Mechanical Behavior of Cured Polybenzoxazine−Thermoplastic Blends. Owing to the potential problems associated a small unrepresentative sample in DSC, DMTA was also used in triplicate to examine the changes in the Tg. All of the blended systems are quite miscible, with only one peak visible in the tan δ trace, although it is fairly broad (PESOH9000 is shown as representative example in Figure 7). The shallow hump seen at around the 50−100 °C temperature range is typical of all of the DMTA data obtained in this study, and is thought to be a β transition. The glass transition from the maximum loss modulus and the calculated cross-link densities for the blends are given in Table 4. N.B., Entries highlighted in italics are likely to be anomalous. Where two distinctly different glass transition temperatures are
Table 5. TGA Data for Cured Thermoplastic− Polybenzoxazine Blendsa temperature (°C) at which mass loss (%) recorded sample
5% (°C)
BA-a PESOH6000 PSUOH6000
222 220 375 10 wt % thermoplastic
Yc800 (%)
10% (°C)
305 25 340 31 400 30 20 wt % thermoplastic
BA-a
5% (°C)
10% (°C)
Yc800 (%)
5% (°C)
10% (°C)
Yc800 (%)
PESOH3000 PESOH6000 PESOH9000 PSUOH3000 PSUOH6000 PSUOH10 000 PSUBz3000 PSUBz6000 PSUBz12 000 PSUCl3000 PSUCl6000 PSUCl9000
252 250 252 250 258 255 238 248 228 254 255 266
312 314 319 316 318 317 308 332 309 317 317 326
29 28 29 27 28 27 26 27 25 29 28 35
− − − 265 244 245 260 240 − 259 267 269
− − − 320 314 318 315 313 − 324 324 325
− − − 27 27 27 27 26 − 27 28 27
All samples were analyzed between 20 and 1000 °C at 10 K/min in and nitrogen (40 cm3/min). Key: [TP] = Concentration of oligomer. X% = temperature at which mass loss of X% recorded. Yc800 = temperature of char yield at 800 °C. a
Figure 6. Sample TGA thermograms for cured blends containing BA-a and 10 wt % of each of the different oligomer types (different molecular weights as indicated).
telechelic PSUBz12000 displays the smallest influence on the thermal stability, but nevertheless leads to modest improvements at these levels of incorporation. Low and Ishida38,39 identified aniline as a major degradation component in the initial stages of the thermal decomposition of PBA-a, which was postulated to arise from the cleavage of the Mannich base as evidenced by the presence of an infraredactive band at 1735 cm−1, consistent with a carbonyl group, that became negligible at 390 °C, arising from a secondary amide. In the same study it was proposed that the cured polybenzoxazines contain terminal Schiff base and secondary amides as defect structures. In contrast, the maximum derivative peak in the TGA data at 388 °C has also been assigned31 to the phenolic cleavage; the latter appears much more sensitive to heating rate. The degradation mechanism proposed by Low and Ishida31 where the nitrogen of the Mannich base is
Figure 7. DMTA data for blends of BA-a containing 10 wt % PESOH9000. 1942
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Figure 8. DMTA data for blends of BA-a containing (a) 10 wt % PSUCl3000, (b) 10 wt % PSUCl9000, (c) 20 wt % PSUCl3000, and (d) 20 wt % PSUCl9000.
containing PSUCl were very cloudy, especially compared with those containing PSUOH. This indicates that phase separation of the PSUCl has occurred, which appears to be caused by the functionality, as this is not observed in the other PSU systems (which in turn may be indicative of less interaction between the two polymers). SEM data for the various molecular weights of PSUCl are shown below (Figure 9), supporting the heterogeneity of the blend indicated by the DMTA data.
observed, the cross-link density is calculated using for the highest loss modulus maximum or, if similar, the higher temperature. The cross-link density (ν) for each PBA-a blend was calculated from the aforementioned DMTA data using eq 243 ν = Ge /φRTe
(2)
where ν is the cross-link density (mol dm−3), φ is a front factor taken as unity, Ge is ideally the storage modulus strictly from a sample at equilibrium, but is taken at Te, where Te = (Tg + 50 °C). This equation is technically most appropriate for lightly cross-linked materials in the rubbery state, but it is used in this instance as a comparison between similar samples. For comparison, Allen and Ishida36 obtained cross-link densities of (4−7) × 103 mol dm−3 for polybenzoxazines based on long chain aliphatic diamines. We have previously reported44 crosslink densities of similar magnitudes using this method for the BA-a homopolymer, but with the important finding that the network architecture (and cross-link density) appears to be dependent on the heating rate employed during cure. Other researchers have reported a drop in cross-link density or conversion when an epoxy is blended with a thermoplastic17,20,21 due to the thermoplastic chains interrupting the formation of the network. At 10% weight these systems generally give a cross-link density of a similar magnitude or larger than the BA-a homopolymer (Figure 8). However, to draw stronger conclusions on the effect of thermoplastic on cross-link density further repeats would need to be carried out, determining the standard deviation of the analysis; we will address this in future work. A shallow response can be observed at around 130−150 °C as a shoulder on the loss modulus of the blends containing 10 wt % PSUCl3000 and 10 wt % PSUCl9000 (Figure 8), which separates into two peaks in the system containing 20 wt %, indicating the formation of a two phase system (which was not observed in the DSC data). Furthermore, the plaques
Figure 9. SEM data for blends of BA-a containing (a) 10 wt % PSUCl9000 (×500 magnification), (b) 10 wt % PSUCl6000 (×500 magnification), (c) 10 wt % PSUCl9000 (×4000 magnification), (d) 20 wt % PSUCl6000 (×4000 magnification).
Samples containing PSU-Bz 3000 showed a very broad tan δ peak that is clearly made up of two overlapping peaks (Figure 10a), this was not seen as clearly in the PSU-Bz 6000 toughened system, although the presence of a very broad tan δ peak, is observed especially at 20% weight content (Figure 10b). Although it is not shown here, the storage moduli are all below the maximum achieved by the BA-a alone, suggesting 1943
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a similar magnitude or larger than the BA-a homopolymer ((4− 8) × 10−3 mol dm−3), the exception being PESOH6000 (3.09 × 10−3 mol dm−3) and PSUBz6000 (3.35 × 10−3 mol dm−3). The heterogeneity of the network increases with a broadening of the tan δ response, suggesting an improvement in the toughness of the resulting blend and these encouraging hints will be investigated in more depth in a subsequent paper.
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EXPERIMENTAL SECTION
Materials. The following chemicals were used in the synthetic steps and are listed with the purities and sources. 4,4′-Isopropylidenediphenol (97%), 4,4′-dichlorodiphenyl sulfone (98%), 4,4′sulfonylbisphenol (98%), 1-methyl-2-pyrrolidinone (A.C.S. reagent − >99%), paraformaldehyde (95%), sulfolane (99%), aniline (99%), and methanol were obtained from Sigma-Aldrich. Anhydrous potassium carbonate, acetic acid (Laboratory Reagent grade), toluene (LRG − low in sulfur), methanol, chloroform, and sodium hydroxide were all purchased from Fisher Scientific. N,N-Dimethylacetamide (DMAc) (99%) was obtained from ACROS Organic. Lithium bromide was obtained from Fluka Analytical. 4,4′- diaminodiphenyl sulfone was kindly supplied by Cytec Engineered Materials (Wilton, U.K.). 2,2Bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane (BA-a, Figure 1), 3,3′-thiodiphenol (TDP), and 3,3′-thiodipropionic acid (TDA) were all supplied by Huntsman Advanced Materials (Basel, Switzerland) and having been characterized using 1H NMR, Raman spectroscopy and elemental analysis, were generally used as received without further purification. In the interests of brevity the characterization data have been deposited as Supporting Information. The oligomers based on polyarylsulphone (PES) or polyarylethersulphone (PSU) (Figure 1) were prepared during a study of oligomeric engineering thermoplastics.45,46 The synthetic procedures (e.g., on a 20 g scale) used were all based on published methods,47−50 albeit modified versions in order to achieve lower molecular weight species. The preparation and characterization of the oligomers has previously been published27 and the salient features are deposited as Supporting Information. Instrumentation. Vibrational spectra were obtained using a Perkin-Elmer system 2000 FT-NIR-Raman spectrometer operating at 250−500 mW (Nd:YAG laser) and a Perkin-Elmer FTIR system 2000 spectrometer. Samples that were analyzed in situ during the cure process using Raman spectroscopy employed a heated cell that was ramped rapidly from 50 to 180 °C and held isothermally for 120 min; spectra being taken at intervals of 7.5 min. Chemometrics analysis using principal components analysis (PCA) was carried out on the spectral data using The Unscrambler X, v10.1 software (Camo, Oslo). Differential scanning calorimetry (DSC) was undertaken using a TA Instruments Q1000 running TA Q Series Advantage software on samples (4.0 ± 0.5 mg) in hermetically sealed aluminum pans. Experiments were conducted at a heating rate of 5, 8, 10, 12, and 15 K/min from room temperature to 300 °C (heat/cool/heat) under flowing nitrogen (50 cm3/min.). In order to gauge the reactivity of the monomer in the bulk, dynamic DSC analysis was performed on all of the systems. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 on milled, cured resin samples (5.5 ± 0.5 mg) in a platinum crucible from 20 to 1000 °C at 10 K/min in nitrogen (40 cm3/min). Dynamic mechanical thermal analysis (DMTA) (in single cantilever mode at a frequency of 1 Hz) was carried out on cured neat resin samples (2 mm ×10 mm ×17 mm) using a TA Instruments Q800 in static air. Blending and Cure of Polymer Samples for ThermoMechanical Analyses. Monomers were first degassed at 100−120 °C (2−3 h depending on the level of foaming) and cured in aluminum dishes (55 mm diameter, depth 10 mm) in a fan-assisted oven: heating at 2 K/min to 180 °C (2 h isothermal) + heating at 2 K/min to 200 °C (2 h isothermal) followed by a gradual cool (3 K/min) to room temperature. Cured samples were cut to the correct token size for analysis using a diamond saw. For the dynamic cure study, the BA-a
Figure 10. DMTA data for blends of BA-a containing (a) 10 wt % PSUBz3000 and (b) 20 wt % PSUBz6000.
that the stiffness is reduced by incorporating a thermoplastic into a benzoxazine network.
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CONCLUSIONS A series of blends is prepared comprising 2,2-bis(3,4-dihydro-3phenyl-2H-1,3-benzoxazine)propane (BA-a) with variously 5, 10, or 20 wt % of a selected oligomer represented by polyarylsulfone (PSU) or polyarylethersulfone (PES). The oligomers, comprising either chloro-, hydroxyl-, or benzoxazinyl- (Bz-) terminal functionality, are of low molecular weight (3000−12000 g mol−1). The addition of the oligomers generally does not significantly alter either the onset of the polymerization exotherm (around 192 °C for BA-a and between 194 and 210 °C depending on the backbone of the oligomer or the terminal functional group) or the final temperature of the exotherm (291 °C for BA-a and between 282 and 291 °C for the blends). The incorporation of thermoplastic always leads to a reduction in the polymerization enthalpy, but the values are not dissimilar for a given molecular weight. Compared with BA-a, the chloro-terminated oligomer (PSUCl3000) displays significantly higher activation energy for the polymerization reaction (93.2 kJ/mol), although this is accompanied by a slight increase in reaction order (m = 2.97, n = 1.79). The hydroxylated oligomer (PSUOH3000) yields slightly elevated activation energy (85.86 kJ/mol). Spectral analysis of the blend containing PSUCl3000 is consistent with the formation of an IPN. The addition of the oligomers leads to modest increases in thermal stability in the cured resins and at 10% weight these systems generally give a cross-link density of 1944
dx.doi.org/10.1021/ma500242w | Macromolecules 2014, 47, 1935−1945
Macromolecules
Article
monomer was first degassed at ca. 100 °C (1 h) and cured in aluminum dishes (55 mm diameter, depth 10 mm) and transferred to a fan-assisted oven set at 90 °C. The samples were allowed to equilibrate before being heated at 2, 8, or 15 K/min to 180 °C (2 h isothermal) + heating at 2 K/min to 200 °C (2 h isothermal) followed by a gradual cool (3 K/min) to room temperature. Cured samples were cut to the correct token size for analysis using a diamond saw. Producing flat, void-free plaques for DMTA provided a big challenge as the benzoxazines were difficult to degas and the addition of the initiators compounded the problem as they seemed to undergo degradation during cure to produce gas bubbles.
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ASSOCIATED CONTENT
S Supporting Information *
Spectroscopic data associated with the preparation and characterization of the monomer and the oligomers. This material is available free of charge via the Internet at http:// pubs.acs.org/
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AUTHOR INFORMATION
Corresponding Author
*(I.H.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Cytec (Wilton) and the Engineering and Physical Sciences Research Council (EPSRC) for funding (LTM) in the form of a studentship. At the University of Surrey, the authors are grateful to Miss Sin-Yi (Cindy) Yeung for conducting preliminary preparative work on the oligomers.
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