The influence of chain-ends on the thermal and rheological properties of some 40/60 PES/PEES copolymers.
In recent years our group has been interested in the investigation of aromatic thermoplastic polymers containing ether, ketone, and sulfone groups (1-9). These polymers show usually good thermal and mechanical properties, and it has been reported in literature that a wide spectrum of characteristics can be obtained by balancing opportunely various groups (10). Moreover these polymers are of increasing technological importance; in particular, polyethersulfones (PESs) are used as adhesives for metal-to-metal bonds (11), as membranes in the separation of gases and solids from solutions (12) and as membranes for fuel cells (13). More recently the use of PESs as toughening agents for brittle thermosetting resin has been attempted (14).
As we are interested in synthesizing new polymers for these purposes, for which not only low viscosity is requested as well as fairly good thermal stability and glass transition temperatures, we thus considered PES/polyetherethersulfone (PES/PEES) copolymers, whose chains are more flexible than PES because of the presence of a larger number of ether links. We thus investigated in the past low molar mass CI- and [NH.sub.2]-ended PES/PEES copolymers, at various PES/PEES ratios, in order to compare their comprehensive thermal stabilities and rheological properties (8), (9). Both thermal and rheological parameters were widely dependent on copolymer composition in every case, but the trends observed for the two series of copolymers were different from each other. On the basis of the comprehensive picture of the results in Refs. 8 and 9, the 40/60 PES/PEES copolymer appeared to us as the most suitable for the present work, because its thermal and rheological parameters reasonably average among the needs of various uses. This choice was also in agreement with the literature report on the employment of 40/60 PES/PEES copolymer as toughening agent (15).
In this work, we thus studied some random differently (-CI, [-NH.sub.2], -OH, and [-COO.sup.-]) ended PES/PEES copolymers, having the same PES/PEES ratio (40:60) as well as the same number average molar mass ([M.sub.n] [approximately equal to] 9500 [g.mol.sup.-1]), and whose repeating units were the following
(where Ar is 1,4-substituted phenylene).
We determined here the glass transition temperature ([T.sub.g]) and the complex viscosity ([eta]*) of the studied copolymers, as well as the characteristic parameters of decomposition, initial decomposition temperature ([T.sub.i]) and apparent activation energy ([E.sub.a]) of degradation in both inert and oxidative environments, with the aim of investigating if and how much these parameters are affected by terminal groups.
The dependence of the thermal, mechanical, and rheological properties of polymers on the various functional groups, as well as on hydrogen bonds, present in the backbone is commonly accepted, and many articles on this topic are found in literature. By contrast the influence of end-groups on the physical and chemico-physical properties of polymers is usually small at the degrees of polymerization as used in practice. Only few papers treating the dependence of thermal and rheological parameters on the end chain groups have been reported (16-24), but, mostly, they concern the change of the reactivity of terminal groups or the increase of thermal stability due to chain end protection.
Commercial Aldrich 4,4'-dichlorodiphenylsulfone (DCDPS), hydroquinone (HQ), 4,4'-dihydroxydiphenylsulfone (DHDPS), tetramethylenesulfone (sulfolane), m-aminophenol, m-hydroxybenzoic acid, potassium carbonate and methanol were used for syntheses. DCDPS was crystallized twice from toluene; sulfolane was distilled under vacuum and stored under nitrogen until use, whereas the other products were used without any further purification.
The syntheses of Cl- and OH-ended copolymers were performed as follows: HQ and DHDPS (0.217 mol overall, with DHDPS/HQ ratio of 0.4/0.6) and DCDPS (0.226 mol) were dissolved in 200 ml of sulfolane. A little DCDPS excess was used to obtain Cl-ended copolymer whereas HQ in excess was used to obtain OH-ended copolymer. [K.sub.2.CO.sub.3] (about 0.224 mol) was thus added slowly to the solution. The obtained mixture was then heated, in flowing nitrogen and under stirring, at 180[degrees]C for 30 min and then at 200[degrees]C for 1 h and at 220[degrees]C for 3 h. The reaction mixture was then cooled to room temperature and poured into a large excess of cold methanol. The precipitated copolymers were washed several times, first in water and then in methanol, and successively dried under vacuum at 80[degrees]C overnight.
The amino-ended copolymer was prepared according to the method reported elsewhere (25), by nucleophilic reaction of the Cl-ended copolymer with m-aminophenol, in the presence of potassium carbonate, at 205[degrees]C. The precipitated amino-ended copolymer was washed several times with water and methanol and then dried under vacuum at 80[degrees]C overnight.
The [-COO.sup.-]-ended copolymer was prepared by nucleophilic reaction of the Cl-ended copolymer with m-hydroxybenzoic acid in sulfolane, in the presence of potassium carbonate, at 205[degrees]C. The precipitated carboxylate-ended copolymer was washed several times with anhydrous methanol and then dispersed in ethanol-KOH mixture for 24 h under stirring. The solid copolymer was then filtered and dried under vacuum at 80[degrees]C overnight. Samples were stored in a desiccator at room temperature until use.
The structure, the number average molar mass ([approximately equal to] 9500 [g.mol.sup.-1]) and the PES/PEES molar ratio (40/60) of all copolymers were checked and confirmed by (1).H NMR spectra according to the method below reported.
[sup.1.H] NMR Measurements
[sup.1.H] NMR spectra were performed at 50[degrees]C, by a Varian Unity Inova 500 MHz spectrometer, without any internal standard and by using deuterated dimethyl sulfoxide ([DMSO.-d.sub.6]) as solvent. Both the number average molar masses ([M.sub.n]) and the PES/PEES molar ratios of the various copolymers were determined according to a literature method (26). In particular, the number average molar masses were determined by the integration of chain end peaks, via the comparison with the peaks due to the aromatic protons in ortho position to the ether-oxygen groups in the backbone; the percentages of PES and PEES repeating units were determined through the evaluation of the peaks assigned to protons in ortho position in respect to sulfone and ether groups. The results confirmed that, for all investigated copolymers, the PES/PEES ratio was 40:60, whereas the number average molar mass was about 9500 [g.mol.sup.-1].
Differential Scanning Calorimetry Measurements
The glass transition temperatures ([T.sub.g]) were determined calorimetrically, by using a Mettler DSC 20 differential scanning calorimeter, coupled with a Mettler TC 10A processor as control and evaluation unit. Both heat flow and temperature calibrations of the apparatus were made according to the procedure suggested by the manufacturer and reported in the operating instructions of the equipment (27). Samples of about 6.0 x [10.sup.-3] g, held in sealed aluminum crucibles, a scanning rate of 10[degrees]C*[min.sup.-1], and a static air atmosphere were used for measurements.
TG and DTG Measurements
A Mettler TA 3000 thermobalance, coupled with the same Mettler TC 10 A processor used for calorimetric measurements, was employed for thermal degradations. The temperature calibration was made following the procedure reported in the user's manual of the equipment (27). based on the change of the magnetic properties of three metal samples (Isatherm, Nickel and Trafoperm) at their Curie points (142.5, 357.0, and 749.0[degrees]C, respectively). The temperature calibration was repeated every month.
Degradations were carried out in the scanning mode, under flowing nitrogen (0.02 [L.min.sup.-1]) and in a static air atmosphere, in the temperature range 35-650[degrees]C, at various selected heating rates ([PHI] = 2, 5, 7.5, 10, 12.5, 15, 17.5, and 20[degrees]C*[min.sup.-1]). Samples of 4-6 x [10.sup.-3] g, held in alumina open crucibles were used for degradations, and their weights as a function of temperature were stored in the list of data of the appropriate built-in program of processor. These experimental data were elaborated by the TC 10A processor at the end of each experiment and the corresponding TG and DTG curves were immediately printed. The same data were thus transferred to a PC, and then used to plot the percentage of undegraded polymer (1-D)% as a function of temperature, where D = ([W.sub.0] -W)/[W.sub.0], and [W.sub.0] and W were the weights of sample at the starting point and during scanning.
A rotational rheometer ARES by TA Instruments, equipped with parallel plates of 25 mm diameter, was used for rheological measurements. The polymer powder was pressed in a Carver press at room temperature to obtain an uniform cylinder (thickness about 2.5 mm), which was charged on the pre-heated lower plate of the rheometer, and the upper plate was then lowered to obtain a gap of 1.2 mm. Sample melted and the excess of molten polymer was removed with a clean wood spatula.
Frequency sweep tests were carried out at 270[degrees]C by using frequencies from 0.1 to 100 rad*[s.sup.-1]. The strain of experiments was fixed at 5% after some preliminary strain sweep tests confirming that this value is in the linear viscoelastic region.
RESULTS AND DISCUSSION
Some random degradation experiments, at various heating rates, were preliminarily carried out in both flowing nitrogen and a static air atmosphere, in the temperature range 35-800[degrees]C, and showed different behavior between the investigated environments.
Under flowing nitrogen, a single degradation process was observed in every case, followed by the formation of a stable residue. Several papers on the PES degradation are reported in the literature (28-32). The primary degradation process of PES was attributed by Montaudo et al. (33) to the thermal cleavage of two different bridged units, diphenylsulfone and diphenylether. The results of Ref. 33 were confirmed by the experiments under vacuum on Poly(etherketone)/poly(ethersulfone) copolymer (34) and copoly(arylene ether sulfone)s (35) and by the experiments in flowing nitrogen on chemically modified polysulfones (36). We thus explained the single degradation peak found with our 40/60 PES/PEES copolymers as due to the cleavage of the weaker diphenylsulfone and diphenylether groups. Moreover, the formation of a stable residue appears in agreement with cross-linking process, which also occurs, as reported in some literature reports (31), (32).
Also in a static air atmosphere, at various heating rates, a thermal process similar to that under nitrogen was observed, immediately followed, before its completion, by another one, very probably oxidative in nature, which gave rise to about complete mass loss. This finding is in agreement with some results obtained by us on PESs and PEEs (1), (2).
The initial decomposition temperatures and the apparent activation energies of degradation were thus thermog-ravimetrically determined in both studied environments, in the selected temperature range 35-650[degrees]C. The TG curves at 10[degrees]C*[min.sup.-1] are reported as an example in Figs. 1 and 2.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
The [T.sub.i] values at the various heating rates were obtained by the TG curves as the intersection between the starting mass line and the maximum gradient tangent to the TG curve. The [T.sub.i] values of each copolymer at various heating rates were different with each other, but the trend was the same independently on the used scanning rate. For the sake of simplicity, only the values at 10[degrees]C [min.sup.-1] were considered.
The apparent activation energies of degradation were obtained by the classical Kissinger method (37), which is based on the following equation
In([PHI]/[T.sub.m.sup.2]) = In(n[RAW.sub.m.sup.[n-1]]/[E.sub.a])-[E.sub.a]/[RT.sub.m] (1)
where [PHI] is the heating rate and [T.sub.m] is the temperature at the maximum rate of weight loss. We used as [T.sub.m] values the temperatures of DTG peaks. Several runs were carried out at various selected scanning rates and the average [T.sub.m] values were determined at every scanning rate (Table 1). Data in Table 1 were then smoothed according to Eq. 1, and the apparent activation energies of degradation in both studied environments were determined through the slopes of the linear equations obtained (Tables 2 and 3). The Kissinger straight lines of the Cl-ended copolymer are reported as an example in Fig. 3.
TABLE 1. Temperatures at maximum rate of weight loss ([T.sub.m]) for the degradation stage of various PES/PEES copolymers in static air atmosphere and in flowing nitrogen at the several used heating rates ([PHI]). Static air atmosphere Copolymer -OH [-NH.sub.2] -C1 [-COO.sup.-] [PHI]/ [T.sub.m]/K [T.sub.m]/K [T.sub.m]/K [T.sub.m]/K [degrees]C* [min.sup.-1] 2 812.1 803.2 803.6 793.5 5 829.1 822.6 818.7 816.0 7.5 837.6 830.9 833.2 828.1 10 839.6 835.2 839.2 831.7 12.5 845.6 840.0 843.3 839.8 15 848.1 841.8 844.4 845.8 17.5 849.9 844.3 848.0 847.7 20 852.1 845 853.1 848.3 Static air atmosphere Nitrogen flow Nitrogen flow Copolymer -OH [-NH.sub.2] -Cl [-COO.sup.-] [PHI]/ [T.sub.m]/K [T.sub.m]/K [T.sub.m]/K [T.sub.m]/K [degrees]C* [min.sup.-1] 2 800.6 805.7 812.28 810.1 5 810.0 822.6 826.06 827.2 7.5 818.1 830.9 834.16 833.1 10 821.1 833.7 843.63 842.5 12.5 823.1 837.6 845.28 848.6 15 827.9 841.4 848.24 850.4 17.5 831.9 843.2 851.96 854.6 20 834.1 844.7 852.69 855.6 TABLE 2. Regression coefficients and degradation apparent activation energies (E.sub.a]) by the Kissinger equation for various PES/PEES copolymers in static air atmosphere. Copolymer a (a) b x [10.sup.-3]/K (b) r (c) -OH 34.6 (+ or -1.6) 38.5 (+ or -1.4) 0.9962 [-NH.sup.2] 30.3 (+ or -2.0) 34.6 (+ or -1.7) 0.9929 -Cl 24.6 (+ or -1.8) 29.9 (+ or -1.5) 0.9925 -COO 19.7 (+ or -1.2) 25.7 (+ or -1.0) 0.9957 Copolymer E (a)/kJ*[mol.sup.-1] -OH 320 (+ or -12) [-NH.sup.2] 288 (+ or -14) -Cl 249 (+ or -12) -COO 214 (+ or -8) (a) a = In ([nRAW.sub.m.sup.[n-1]]/[E.sub.a]). (b) b = [E.sub.a]R. (c) Product moment correlation coefficient. TABLE 3. Regression coefficients and degradation apparent activation energies (E.sub.a]) by the Kissinger equation for various PES/PEES copolymers in static air atmosphere. Copolymer a (a) b x [10.sup.-3] /K (b) r (c) -OH 42.0 (+ or -3.1) 43.7 (+ or -2.6) 0.9896 [-NH.sub.2] 34.6 (+ or -1.7) 38.2 (+ or -1.5) 0.9957 -Cl 31.2 (+ or -2.1) 35.6 (+ or -1.8) 0.9926 -COO 26.8 (+ or -1.5) 32.0 (+ or -1.3) 0.9952 Copolymer E (a)/kJ*[mol.sup.-1] -OH 363 (+ or -22) [-NH.sub.2] 318 (+ or -12) -Cl 296 (+ or -15) -COO 266 (+ or -11) (a) a = In ([nRAW.sub.m.sup.[n-1]]/[E.sub.a]). (b) b = [E.sub.a]R. (c) Product moment correlation coefficient.
[FIGURE 3 OMITTED]
The glass transition temperatures were determined by DSC technique in a static air atmosphere, at the heating rate of 10 [degrees]C*[min.sup.-1]. The considered [T.sub.g] values were averaged from those of three runs, the maximum differences between the average and the experimental values being within [+or-] 0.5[degrees]C.
Finally. the complex viscosity ([eta]*) was determined in isothermal heating conditions (270[degrees]C), by dynamic oscillatory tests, at various selected frequencies ([omega]), in the 0.1-100 rad*[s.sup.-1]range. Experiments at [omega] < 0.1 rad*[s.sup.-1] were not carried out in order to limit the exposure time of samples at high temperature. The rheological behavior of the Cl-ended derivative is reported as an example in Fig. 4. The [eta]* values of all studies 40:60 PES/PEES copolymers decreased as a function of [omega] (Fig. 5); the values at [omega] = 10 rad*[s.sup.-1] were selected in order to observe the trend among various compounds.
[FIGURE 4 OMITTED]
The initial decomposition temperatures and the apparent activation energies of degradation of all the investigated copolymers in both inert and oxidative environments are reported in Table 4, together with the glass transition temperatures and the complex viscosities at 10 rad*[s.sup.-1].
TABLE 4. Glass transition temperatures ([T.sub.g]), initial decomposition temperatures ([T.sub.i]), apparent activation energies of degradation ([E.sub.a]) and complex viscosities ([eta]*) of various 40:60 PES/PESS copolymers in static air atmosphere and in flowing nitrogen. Static air atmosphere Copolymer [T.sub.g]/K [T.sub.i]/K E (a)/ [[eta]*]/Pa* (a) (a) kJ.[mol.sup.-1] s (b) -OH 471.0 805.1 320 (+ or -12) 1273 [-NH.sub.2] 462.8 794.2 288 (+ or -14) 757 -Cl 449.5 810.8 249 (+ or -12) 372 [-COO.sup.-] 436.0 798.9 214 (+ or -8) 158 Static air atmosphere Nitrogen flow Copolymer [T.sub.i]/K (a) E (a)/kJ.[mol.sup.-1] -OH 791.1 363 (+ or -22) [-NH.sub.2] 780.2 318 (+ or -12) -Cl 807.0 296 (+ or -15) [-COO.sup.-] 806.3 266 (+ or -11) (a) Determined at 10[degrees]C.[min.sub.-1]. (b) Determined at 10 rad*[s.sub.-1] in isothermal heating conditions (270[degrees]C).
The overall picture of the results of Table 4 clearly indicates that all the parameters determined are largely affected by copolymer chain ends.
Concerning the overall thermal stability, the characteristic parameters of degradation ([E sub.a] and [T.sub.i]) appear largely affected by the used atmosphere.
The [E.sub.a] values found in oxidative conditions are lower than those under nitrogen, but the same trend (-OH > [-NH.sub.2] > -C1 > [-COO.sup.-]) than in inert environment was observed. It suggests that, in a static air atmosphere, a different degradation mechanism, probably oxidative in nature, superimposes to the thermal degradation process, as also evidenced by the TG curves (Figs. 1 and 2).
This trend is different from that found for the initial decomposition temperatures. This finding is fairly in agreement with the results we obtained in the past, where lower [T.sub.i] values were found for polymers showing higher [E.sub.a] values (4), (8).
It is worth to note that the [T.sub.g], [eta]*, and [E.sub.a] values (in both studied environments) of the investigated compounds decrease according to the same order: -OH > [-NH.sub.2] > -Cl > [-COO.sup.-], thus suggesting that the same factor is the driving force of this behavior.
Transition temperatures are considered strongly "structure-sensitive," partly because of steric effects and partly because of intra- and inter-molecular interactions (38). In particular, the [T.sub.g] increase has been explained in literature with the increasing of chain rigidity (39) and the decreasing of free volume (40). As the [sup.1.H] NMR spectra gave evidence that the average molar mass and PES/PEES ratio of the studied copolymers were about the same, the intrinsic rigidity of the backbones must be considered about equivalent. The differences among the glass transition temperatures could then be explained by an additional chain rigidity contribution due to the terminal groups. The hydrogen bond appears, in our opinion, the driving factor of the differences observed. Both -OH and [-NH.sub.2] groups form strong intra- and inter-molecular hydrogen bonds, so giving rise to higher chain rigidity than the Cl-ended copolymer, in which hydrogen bonding is not present. The [T.sub.g] value of OH- derivative, which is higher than that of amino-derivative, can be explained with its stronger hydrogen bond (41), (42), whereas the lower [T.sub.g] value of [COO.sup.-] -ended copolymer appears in agreement with the repulsion interactions among negative carboxylate groups.
The formation of intermolecular hydrogen bonds, which slow down the movement of adjacent liquid layers, can also explain the higher complex viscosities found for OH- and [NH.sub.2]-ended copolymers, as well as their strong shear thinning behavior, in comparison with the Cl-ended, and more and more with the [COO.sup.-] -ended derivatives (Fig. 5).
[FIGURE 5 OMITTED]
The rheological behavior of the Cl-ended 40/60 copolymer shown in Fig. 4 was the same than that of the other copolymers here investigated, as well as of all other 40/60 PES/PEES copolymers at various PES/PEES ratios we studied in the past (8), (9). The presence of a shoulder in G' trace has been reported in the literature for polydisperse polymers (43), which is the case of our copolymers in principle. The high polydispersity of the investigated samples can also explain the apparent absence of zero shear viscosity. Moreover it is well noted in the literature (44) that, when polydisperse samples are analyzed, the steepness of viscosity curve is higher, thus appearing only at frequencies lower than those we used in our experiments.
The degradation [E.sub.a] values of the studied copolymers in both used atmospheres quite linearly increase as a function of both glass transition temperature and complex viscosity (Figs. 6 and 7, respectively). We found in the past similar behavior for some PES/PEES copolymers, where the content of sulfone groups in copolymer chains increased as a function of PES/PEES ratio (9). This behavior was explained with an increasing percentage of double bond in copolymer chain, so giving rise to a higher dissociation energy of links and, simultaneously, to a higher chain rigidity ([T.sub.g]) and a slower segmental relaxation movements of macromolecules ([eta]*). In the case of the 40:60 PES/PEES copolymers here investigated this interpretation is not possible because the backbone of the various studied compounds is substantially identical.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The results obtained here clearly indicate that the apparent degradation activation energy values are also affected by chain ends of various copolymers. This finding can be explained if we consider that, when a sample decomposes, the higher or lower mass loss rate depends on the packing of sample. In the case of our copolymers, the presence of more or less strong hydrogen bonds or repulsion forces due to various chain ends gives rise, in our opinion, to different levels of packing, and then to degradation [E.sub.a] values increasing as a function of attractive interaction forces in copolymer chains.
The initial decomposition temperatures under nitrogen were lower than those in oxidative ambient, apart from for the carboxylate-ended copolymer. The [[delta]T.sub.i] differences ([[delta]T.sub.i] = [T.sub.i air] - [T.sub.i N2]) were 14.4, 14.0, 3.8, and -7.4[degrees] C for OH-, [NH.sub.2.]- Cl-, and [COO.sup.-]-derivatives, respectively, thus showing the same trend observed for [T.sub.g], [eta]*, and [E.sub.a].
On the basis of the results in Table 4 the Cl-ended 40/60 PES/PEES copolymer, which shows low complex viscosity and high initial decomposition temperature in both inert and oxidative environments, coupled with sufficiently high glass transition temperature and degradation activation energy, appears the most suitable for the use in the form of fibre. By contrast, the [NH.sub.2]-ended copolymer appears more suitable for the use as toughening agent because of the higher complex viscosity.
Moreover, a more interesting finding of this work is, in our opinion, the influence of terminal groups on the parameters here investigated, in particular on glass transition temperature and complex viscosity. Very few papers have been reported in literature on the influence of the nature of the end-groups on glass transition temperature (16), (18). Only little [T.sub.g] differences were observed when terminal groups of some end-functionalized block copolymers of styrene and isoprene were substituted by zwitterionic groups (16). By contrast, large differences were found for some hyperbranched polyetherketones (19), but, in this case, the number of terminal groups was so large that their influence could be considered such as that of functional groups present in the backbone.
In the case of our 40/60 PES/PEES copolymers this influence appears surprisingly large if we consider the low number of chain end groups, thus suggesting the possibility of the modification of some properties of polymers (at least those having low molar mass) by changing opportunely terminal groups. This hypothesis would be verified and our group plans to go deeper into this question in the next future.
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Lorenzo Abate, Ignazio Blanco, Gianluca Cicala, Giuseppe Recca, Andrea Scamporrino
Dipartimento di Metodologie Fisiche e Chimiche per I'lngegneria, University of Catania, Viale A. Doria, 6 95125 Catania, Italy
Correspondence to: L. Abate; e-mail: email@example.com
Contract grant sponsor: The Italian M.I.U.R.
Published online in Wiley InterScience (www.interscience.wiley.com).
[C]2009 Society of Plastics Engineers
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|Author:||Abate, Lorenzo; Blanco, Ignazio; Cicala, Gianluca; Recca, Giuseppe; Scamporrino, Andrea|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2009|
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