Article pubs.acs.org/Macromolecules
Toughening Mechanisms in Aromatic Polybenzoxazines 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. ABSTRACT: 2,2-Bis(3,4-dihydro-3-phenyl-2H-1,3benzoxazine)propane (BA-a) is blended with oligomers of polyarylsulfone (PSU) and polyarylethersulfone (PES) of different low/intermediate molecular weights (3000−12 000 g mol−1) and terminal functionality (chloro-, hydroxyl- or benzoxazinyl(Bz)). Fracture toughness (KIC) is observed to increase from 0.8 MPa m0.5 for cured BA-a to 1 MPa m0.5 with the incorporation of 10 wt % PSU-Bz (12 000 g mol−1). Generally, greater improvements in KIC are observed for the PES oligomers compared with the PSU oligomers of equivalent molecular weight. The terminal functionality of the thermoplastic has a lesser effect on improving toughness than increasing the molecular weight or the nature of the polymer backbone. Surface analysis of the fractured surfaces show greater phase separation and crack pinning in the PES toughened system. Where crack pinning is less obvious, as in the case of hydroxyl-terminated PES (of 6000 g mol−1), this coincides with a drop in fracture toughness.
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INTRODUCTION Polymers and their composites are being applied increasingly in the aerospace industry in recent years;1 more recently, composites have been used in commercial airliners, most notably in the case of the Airbus A350 and Boeing 787, which comprise up to 50% composite by volume.2,3 Composites are now being applied in the primary structure of aircraft so their integrity is very important, developing methods to improve toughness of neat resins and trying to understand how the toughening agent works is the first step in developing robust composites. Polybenzoxazines show promising thermal and mechanical performance for these applications, but improvements in toughness are required before this can be a reality. The potential for the use of oligomers based on engineering thermoplastics have been examined for some years as modifiers in aerospace composite matrices,4,5 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,6 Union Carbide,7 and the Plastics Division of ICI8 to develop thermally stable thermoplastics suitable for engineering applications, their chemistry has recently been reviewed.9 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 © 2014 American Chemical Society
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.10 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 degree by the chemical structure. Hence, polymers produced from dichlorodiphenylsulfone 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,11 cyanate esters,12 and polybenzoxazines,13 as well as engineering thermoplastics.14,15 In a previous publication,16 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 ( 2.5(KQ /σy)2
where B, a, and W are as described in Figure 2a, KQ is the trial KIC value and σy is the yield stress taken from the maximum load of the uniaxial tensile test.27 K and G can then be calculated using the following equations:
Figure 2. Schematics showing (a) the dimensions and geometry of the CT testing specimen and (b) the direction of the load applied relative to the crack position. samples were mounted in an epoxy resin before polishing and then etching, though some samples were analyzed prior to etching, where X-rays could be used to discriminate between the two phases. The backscattered electron (BSE) analysis was also used in some cases to emphasize phase contrast as BSE can highlight areas of different molecular weight. A gold coating was used (2−5 nm thick) to minimize charging. An accelerating voltage of between 1 and 30 kV through a tungsten wire was used. 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. 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 Q800 in static air.
K = [PQ (BW )−1/2 ]f (x)
(4)
G = (1 − ν 2)K 2E−1
(5)
where E = Young’s modulus, B, W and a are as shown in Figure 2, x is a/W and PQ is the load a determined in Figure 3, where
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Figure 3. Schematic showing the method used to determine PQ.
RESULTS AND DISCUSSION In a previous publication it was reported25 that the selected thermoplastics presented herein could be blended with BA-a to yield a homogeneous system when up to 10 wt % was incorporated, while introducing 20 wt % thermoplastic led to cloudy sample with poor thermoplastic distribution. These findings informed the current research and consequently, in the present study the thermoplastic loading was maintained as 10 wt % in the blends with BA-a. To improve the toughness of a material an appreciation of how materials behave under damaged conditions is required. This behavior relates to linear elastic fracture mechanics (LEFM) theory, and in accordance with this theory, the condition for brittle failure can be expressed as KI = KIC (1)
AB is the linear fit of the load versus extension plot and AB′ has compliance 5% greater than AB. PQ is the point where AB′ and the experimental data cross over, but Pmax is used if the load versus extension plot falls within the are bounded by lines AB and AB′. The samples were tested at room temperature and KIC calculated using eq 4 and are presented in Table 1. Some samples were slightly outside the requirements of the calculation or problems with the testing method were encountered and these are highlighted in gray. The standard deviations are given both with these anomalous results included and excluded. Samples that did not fulfill the test requirements (i.e., failure occurred at a bubble) were not included. There might be several reasons for the variation in the data: the crack might develop in a flawed area, e.g. an area of high or low thermoplastic content (the dispersion of thermoplastic within the polybenzoxazine is not always even, especially in the case of PES25); microscopic air bubbles might exist in the crack region; the polymer network might be heterogeneous in this region (perhaps due to hot spots in the oven) as there is a general assumption made that polymerization occurs the same across the whole plaque. This method of toughness testing has its drawbacks for testing pure resin due to the difficulties in producing the cracks reproducibly, which can have a tendency to produce artificially high results, as the crack may not be fully formed (especially in the case of the brittle BA-a homopolymer), which might explain the standard deviations in the range of 0.03−1.15 that are often seen for KIC values determined in this way.28−34 An alternative method of presentation is to show
where KI is the mode I stress intensity factor and is dependent on the loading conditions and the size of the crack or defect in the material. It can be expressed as KI = Qσ(πa)1/2
(3)
(2)
where Q is a geometry correction factor, which depends on the geometry of the sample being tested and the crack or defect, σ is the applied stress, and a is the crack or defect size. KIC is the critical stress intensity factor expressed in the units (MPa m1/2) and represents the toughness of a material to resist a plane-strain fracture, and from this value GIC, which is the critical strain energy release rate, can be calculated which represents the amount of energy required to fracture a material. If the load−displacement curve of the specimen is almost linear until the sample breaks, the stress field near the crack tip is 1948
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Table 1. KIC Data Produced from CT Testing with Calculated Standard Deviation (SD) with Potential Outliers Highlighted in Gray mean (x) sample BA-a
PESOH3000
PESOH6000
PESOH9000
PSUOH3000
PSUOH6000
PSUOH10000 PSUCl3000
PSUCl6000
PSUCl9000
PSUBz3000
SD
[TP] (wt %)
KIC (MPa m1/2)
all data
minus highlighted
all data
minus highlighted
− − − 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
0.94 0.89 0.74 0.91 0.99 0.91 0.86 0.69 0.62 0.75 0.77 0.85 0.81 0.82 0.69 0.70 0.78 1.07 0.91 0.95 0.86 1.16 1.14 0.77 0.77 0.74 0.76 0.69 0.70 0.74 0.84 0.87 0.70 0.83 0.62 0.74 0.83 0.83 0.86 0.79 0.81 1.34 0.86 1.05 0.71 0.63 0.94 0.68 0.67 0.74 0.84 0.90 0.75 0.85 0.80 0.76 0.88 0.74
0.86
0.82
0.10
0.11
0.92
N/A
0.05
N/A
0.75
N/A
0.07
N/A
0.99
0.95
0.12
0.09
0.89
0.76
0.19
0.02
0.75
0.76
0.08
0.07
0.78
N/A
0.06
N/A
0.92
0.82
0.23
0.02
0.81
0.89
0.16
0.14
0.78
0.81
0.09
0.08
0.82
N/A
0.07
N/A
1949
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Table 1. continued mean (x) sample PSUBz6000
PSUBz12000
1/2
[TP] (wt %)
KIC (MPa m )
10 10 10 10 10 10 10 10 10 10 10 10
0.91 0.82 0.80 0.99 0.63 0.83 0.94 1.04 0.97 0.89 0.98 1.09
SD
all data
minus highlighted
all data
minus highlighted
0.82
0.86
0.13
0.09
0.98
1.00
0.07
0.06
10 wt % PSUBz12000, PESOH9000, and PESOH3000 (the blend containing PSUCl6000 also appears high, but this has not been included in this group due to the very large standard deviation). The data appear to show that (a) among the PSU samples fracture toughness increases with thermoplastic molecular weight; (b) the influence of functionality on fracture toughness falls into the trend Bz > Cl > OH. Owing to the large standard deviation in the results, apart from the aforementioned three systems, there is no striking difference in performance when the functional group of the thermoplastic is varied. The thermoplastic backbone appears to play a role in toughening as PES oligomers (apart from the 6000 sample) generally produce superior fracture toughness values to the corresponding PSU oligomers. Molecular weight is also significant as the blend containing PSUBz12000 displays the highest molecular weight of all the samples and out performs all of the samples. Effect of Oligomer Backbone on Fracture Toughness. The backbone structures of the two thermoplastics are illustrated in Figure 1. In general, it can be seen from Figure 4b that PES blends offer a higher degree of fracture toughness at the same molecular weight compared with the equivalent (in terms of end group and molecular weight) PSU blend. Previously reported data25 demonstrated that the PES blends underwent phase separation to form “particles” (i.e., globules of thermoplastic or varying size) of PES, whereas the PSU showed more of a phase transition between the two polymers. Mimura et al.36 observed a similar shift in phase separation from more to less well-defined regions of thermoplastic and this was associated with a decrease in the toughness of the system. During the blending operation, the PSU was miscible with the molten BA-a monomer, even at higher molecular weights, although PSUCl6000 and PSUCl9000 proved more challenging and required slower addition to the stirring benzoxazine. In contrast, the addition of PES tended to cause the blend to agglomerate around the stirrer paddle thus requiring more heat and agitation to effect dissolution. This may, in turn, account for the poor dispersion and sunken phases observed in the PES systems.25 The DMTA data recorded for these materials (Figure 5 and Table 3) confirm that one peak is observed in the tan δ response for blends containing up to 10 wt % PSU or PES (representative samples of PSUOH10000 are shown) and a single T g is observed, suggesting the formation of a homogeneous polymer network. Some blends (notably those containing PSUCl and the highest molecular weight PSUs) yielded slightly cloudy samples (implying a lack of homogeneity), but when the samples were
these data as bar graphs, these are also shown with the standard deviation represented as error bars (Figure 4). The numbers written within the bars in Figure 4a relate to the % of successful and usable samples produced (before samples have been excluded that did not comply with the ASTM), i.e. the amount of usable samples as a fraction of the number of plaques actually produced. As can be seen in Table 2, some results are produced from more samples than others, ideally at least five results would be required, more so to reduce the deviation. The reduced sample number in some cases is due to the difficulty not only of the test, but of producing usable plaques as sometimes the thermoplastic aggregated, which could be seen as cloudy areas in the cured plaque, or air bubbles formed as the degassing could only be taken up to a certain temperature (normally around 110 °C), due to the temperature of polymerization onset (To) (Table 2). As can be seen in Figure 4a, some samples such as PSUOH10000 had a very low “success rate” of around 4%, whereas some samples e.g. PSUBz3000 and PSUBz12000 were considerably higher at 50%, although this is still a relatively poor return (and expensive in terms of material consumption, although these broken samples were ultimately recycled for TGA measurements). Many of the problems arose from degassing the samples, particularly following the addition of the thermoplastic, which often made it difficult to remove all air and volatiles leading to porosity. The initiation of the crack with a razor blade was performed with a short, sharp tap with a mallet. However, if this force was applied too strongly then the sample was split in two; if too lightly then length of the crack fell outside the required magnitude of a/W = 0.5 ± 0.05. In this case, further taps were required to propagate the crack, which led to some crack tip blunting and a false value (this was particularly challenging in the untoughened BA-a), but as the amount of thermoplastic was added the increase in force required to initiate a crack was noticeable. The value of fracture toughness of ca. 0.8 MPa m1/2 recorded for BA-a in this study is broadly in line with the typical values of KIC for the untoughened cured BA-a (0.6−1.0 MPa m1/2) reported in the literature.35 However, owing to the low number of results achieved for the unmodified BA-a, it was more difficult to establish a reasonable range or average, whereas it was generally significantly easier to initiate the crack in those materials incorporating the oligomers and this is reflected in the % success rate recorded in Figure 4a. Examining the bar chart from Figure 4b, three samples stand out in terms of fracture toughness: the BA-a blends containing 1950
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Figure 4. Fracture data for all BA-a/TP blends with (a) all data included and with the % success of samples given within the bars and with (b) the highlighted data from Table 1 removed. The error bars represent the standard deviation for the data set. N.B., 35600 represents the BA-a homopolymer.
homogeneous blend between thermoplastics so that no phase separation can be observed using surface analysis37 while Hwang et al. found38 that increasing viscosity influences the phase separation behavior and morphology of the system. This phase separation is important in improving the fracture toughness, which has been reported extensively in the literature,17,39−41,43 but the loading plots produced from the tests, coupled with the SEM images, suggest the crack propagation largely undergoes a stable ductile failure. Some stick and slip behavior was also observed in the trace during
fractured and etched, the PES samples showed significant phase separation with particles or globules of PES acting to facilitate crack pinning. This in turn affects the formation of ridges and branching patterns in the polybenzoxazine matrix seen in Figure 6a thus suggesting the formation of a heterogeneous blend, but with a single Tg. The PSU did not show the same level of phase separation seen in the PES samples, as can be seen in Figure 6b, the branching pattern on the PSU surface does not stem from any visible thermoplastic particles, although the PSU phase might not be resolved at this magnification. Some researchers have observed the formation of a 1951
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the best results found for a bicontinuous or phase separated morphology. In the current study, PSU was functionalized with different end groups. The principle behind this was that the PSUCl would have less interaction with the benzoxazine polymer, although it is accepted 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 polybenzimidazoles51, albeit to a lesser degree than the conventional mechanism. On the other hand, PSUOH could form hydrogen bonds with the benzoxazine polymer and the PSUBz could coreact with the benzoxazine polymer during cure. Thus, these latter two polymers should improve the interfacial adhesion and thus toughness of the system compared to the chloro-terminated analogue. It has been shown25 that the spectral data acquired during the polymerization reactions of all three types of functionalized PSU/BA-a blends, suggest greater similarities exist between the mechanisms for PSUOH and PSUBz than for PSUCl (data for the latter are consistent with the formation of an interpenetrating network, IPN, although the data are not definitive). This is shown schematically in Figure 7: there is direct coreaction between the BA-a and the terminal groups of both PSUOH and PSUBz, although the kinetic parameters for the reactions are significantly different, with the polymerization of the BA-a/ PSUOH3000 having a higher activation energy (85.9 kJ/mol) than either the BA-a homopolymer (81.4 kJ/mol) or the BA-a/ PSUBz3000 (80.4 kJ/mol). In contrast, principal components analysis for the spectral data demonstrate that the PSUCl is involved in a different reaction mechanism and displays a much higher activation energy BA-a/PSUCl3000 (93.2 kJ/mol) reflecting the lower reactivity of the thermoplastic, wherein the terminal groups are not involved in reaction, but the crosslinking reaction is presumably due to the effect of the aromatic backbone. The SEM data presented in Figure 8 are representative of the fracture surfaces of the PSU blends. The crack pinning was observed at the beginning of the branches in the PES
Table 2. 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)
ΔH (J g−1)
Tg (°C)
BA-a PESOH3000 PESOH6000 PESOH9000 PSUOH3000 PSUOH6000 PSUOH10000 PSUBz3000 PSUBz6000 PSUBz12000 PSUCl3000 PSUCl6000 PSUCl9000
− 10 10 10 10 10 10 10 10 10 10 10 10
192 200 195 203 199 203 210 199 203 192 210 208 213
240 240 242 243 239 239 244 239 239 236 247 246 247
291 288 293 291 289 287 285 289 287 281 286 290 291
309.9 274.6 280.0 291.6 259.7 271.4 258.7 259.7 271.4 248.2 259.0 261.4 266.3
157 150 161 161 142 149 133 142 149 148 127 140 162
a Key: [TP] = concentration of thermoplastic in blend, To = onset of polymerization exotherm, Tmax = temperature of exothermic peak maximum, Tf = final temperature of polymerization exotherm, ΔHp = enthalpy of polymerization exotherm, and Tg = glass transition temperature (from DSC rescan).
final stages of the test and in the roughness of the fractured surface toward the end of the crack front. Effect of Functional Group on Fracture Toughness. It has been demonstrated in other thermoset polymers42−50 that by having a reactive or compatible end group on a thermoplastic can help to increases the toughness of the system due to greater interfacial adhesion, thus resulting in more energy being required to propagate the crack. McGrail et al.46 reported an increase in fracture toughness due to reduced cross-link density and phase separation of the PES with a reactive end group, but a slight reduction in flexural modulus. Girad-Reydet et al.47 saw similar effects with increased fracture toughness due to the presence of a reactive thermoplastic, with
Figure 5. Overlay of tan δ data from DMTA for BA-a blend containing PSUOH1000 at loadings of both 10 wt % (---) and 20 wt % (−). N.B., the development of a second phase is visible in the higher blend containing the higher loading. 1952
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Table 3. DMTA Data for Cured BA-a−Thermoplastic Polybenzoxazine Blendsa cross-link density (10−3 mol dm−3)
E″max (°C)
a
sample
[TP] (wt %)
BA-a PESOH3000 PESOH6000 PESOH9000 PSUOH3000 PSUOH6000 PSUOH10 000 PSUBz3000 PSUBz6000 PSUCl3000 PSUCl6000 PSUCl9000
− 10 10 10 10 10 10 10 10 10 10 10
average cross-link density (10−3 mol dm−3) 165 179 182 183 179 175 178 174 171 179 174 180
− 179 − − 177 175 163 172 171 177 173 180
− − − − 176 − − − − − − −
5.81 9.60 3.09 6.55 6.92 10.45 7.78 5.00 3.46 6.21 8.12 6.58
− 9.67 − − 6.75 4.46 7.80 5.29 3.24 5.09 7.66 5.98
− − − − 3.79 − − − − − − −
5.81 9.63 3.09 6.55 5.82 7.45 7.79 5.15 3.35 5.65 7.89 6.28
Key: [TP] = concentration of thermoplastic in blend; E″max = temperature of peak maximum in loss modulus data.
functionality of the polymer influencing toughness. However, the fracture surfaces of the chloro-terminated toughened systems appear much rougher at the branching areas than the OH systems (Figure 8) and surface roughness can be an indicator of toughness.52 However, when the PSU is functionalized with the benzoxazine end group, the toughness of the system is improved compared to other blends (bearing different functional end groups) of equivalent molecular weight. This is especially evident at higher molecular weights, due to covalent reaction between the thermoplastic and the benzoxazine. Following etching of the fracture surfaces of PSUBz12000, a larger area of higher PSU concentration can be observed (Figure 9) compared to the PSUCl9000 and PSUOH10000, as phase separation is more developed. This observation is likely to be aiding the increase in toughness recorded, as well as the increased interfacial adhesion achieved through covalent bonding between the two polymer networks. PSU samples, regardless of functionality, are more compatible with the BA-a benzoxazine than the PES oligomers at equivalent molecular weights, as the gross ‘sunken’ phase separation is not observed and they are physically easier to blend, although the PSUCl and PSUBz systems appear cloudy to the naked eye. Examining systems comprising higher molecular weight PSU oligomers, especially those functionalized with terminal chloro or benzoxazine species, a higher concentration of PSU can be observed at the bottom of the samples, following settling under gravity (Figure 9). Viscosity and compatibility of the polymers and cure temperature can strongly influence the phase separation behavior of the polymer blends, as Kim et al.38 reported for thermoset blends containing functionalized compared with unfunctionalized PES. Comparing the etched surfaces of the PSUOH10000, PSUCl9000, and PSUBz12000 in Figure 9, the phase separation of the three systems is clearly different, with little or no phase separation observed in PSUOH10000 at this resolution. PSU molecules would be more reactive with benzoxazine than PES due to the addition of the isopropylidene group, rather than the electron withdrawing effect of the sulfone alone. Effect of Oligomer Molecular Weight on Fracture Toughness. Molecular weight is known to have an influence on the ability of a linear polymer to toughen a thermoset polymer. Srinivasan and McGrath53 reported that the fracture toughness of a cyanate ester was improved with the introduction of increasing molecular weights of a hydroxyl-
Figure 6. SEM data (×500 magnification) for (a) PESOH9000 showing phase separation in fractured surface and (b) PSUCl6000 showing no phase separation at fracture surface. Arrow indicates direction of crack propagation.
toughened systems, but was not observable in the PSU system, which implies that the backbone structure of the has the greatest influence on which phase separation mechanism it undergoes (a not unreasonable premise given this feature dominates the lower concentration of functional terminal groups). The observation that similar toughness enhancement is recorded with the chloro-functionalized systems compared with the hydroxyl analogues runs counter to the concept of the 1953
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Figure 7. Schematic showing how (a) chloro-, (b) hydroxyl-, and (c) benzoxazine-terminated thermoplastics may interact with the benzoxazine network.
BA-a/PESOH3000 blend (0.92 MPa m1/2, based on 10 measurements, with an analysis success of 22%), although the sizes of some of the data sets in the present work must be noted with caution. Comparison of the fracture surfaces for PESOH6000 (Figure 10), PESOH3000 and PESOH9000 (Figure 8) reveals that branching is observed in the PESOH3000 and PESOH9000 but surprisingly not in the PESOH6000. Phase separation is also observed in the latter with a degree of crack pinning, inferred from the tails on the particles, but it is not as
terminated phenolphthalein-based poly(ether sulfone) and a poly(sulfone), but that significant improvements were not observed below a threshold molecular weight of between 15 0000 and 20 000 g mol−1. Unfortunately, although the molecular weights of the oligomers involved in the present work (
