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Telechelic ionomeric poly(butylene terephthalate): Synthesis, characterization and comparison with random ionomers
Reactive & Functional Polymers 70 (2010) 366–375
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Telechelic ionomeric poly(butylene terephthalate): Synthesis, characterization and comparison with random ionomers Corrado Berti a, Martino Colonna a,*, Enrico Binassi a, Maurizio Fiorini a, Sreepadaraj Karanam b, Daniel J. Brunelle c a b c
Dipartimento di Chimica Applicata e Scienza dei Materiali, Via Terracini 28, 40131 Bologna, Italy SABIC Innovative Plastics, Plasticslaan 1, 4600 AC Bergen op Zoom, The Netherlands General Electric Global Research, One Research Circle, Niskayuna, NY 12309, USA
a r t i c l e
i n f o
Article history: Received 3 November 2009 Received in revised form 23 February 2010 Accepted 27 February 2010 Available online 4 March 2010 Keywords: Poly(butylene terephthalate) Telechelic polymers Ionomers Polycondensation Crystallization
a b s t r a c t Telechelic poly(butylene terephthalate) (PBT) ionomers have been prepared by melt synthesis using a new polycondensation process that involves a pre-reaction of sulfobenzoic acid sodium salt with butanediol. No side reaction occurs and the incorporation of the ionic groups is quantitative. The addition of a buffer agent, such as Na3PO4, to the catalyst reduces the THF formation and improves the polycondensation rate. The comparison of the thermo-mechanical and physical properties between random and telechelic ionomers is also reported. The ionic groups act as chain-extension reversible electrostatic links for telechelic ionomers while act as cross-links in random ionomers. Therefore, random ionomers present a consistently higher melt viscosity compared to telechelic ionomers and to PBT and for this reason high molecular weight random ionomers cannot be obtained by melt polycondensation. Thermal and hydrolytic stabilities of telechelic ionomers are comparable with those of commercial PBT and consistently higher respect to those of random ionomers. The presence of ionic groups decreases the translation mobility of the polymer chains thus lowering the crystallization rate. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction It is reported in the literature [1–5] that the presence, even at very low concentrations, of ionic groups covalently bonded to the polymer backbone produces a consistent effect on the rheological and thermo-mechanical properties of polymers. The improvements in thermo-mechanical properties are generally ascribed to the formation of ionic aggregates that act as thermo-reversible cross-links [1] thus decreasing the translational mobility of the polymer backbone. Metal sulfonates are known [1] to strongly associate in the solid state as ionic aggregates that can be disrupted at higher temperatures. The thermo-reversibility of ionic aggregation provides the opportunity to solve several issues connected with covalently bonded high molecular weight polymers, such as poor melt processability, high melt viscosity and low thermal stability at high shear rate and temperature [1]. It has also been reported in the literature [2] that ionic interactions have consistent effects on the crystallization kinetics and on the morphology of ionomers. Moreover, the ionic groups can give rise to interactions with dyes [6] and with fillers, for example, organically modified clays [7–10], improving the adhesion of the polymer matrix with * Corresponding author. Fax: +39 0512090322. E-mail address: [email protected] (M. Colonna). 1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2010.02.011
the substrate and providing the thermodynamic driving force for the nanometric dispersion of the filler. The insertion of the ionic groups can be performed along the polymer chain (random ionomers) or only at the end of the chain (telechelic ionomer). Several differences can be expected between the two classes of ionomers. Most of the studies on polyester ionomers have been conducted on random polyester ionomers [11– 14]. On the contrary very few studies [2,15–18] concerning telechelic polyester ionomers have been conducted. The ionic aggregation in telechelic polymers occurs only at the end of the chain, giving rise to an electrostatic chain-extension while for random ionomers gellike or cross-linked aggregates are formed. For this reason, lower melt viscosities and therefore high molecular weight can be obtained for telechelic ionomers respect to random ionomers [7,8]. Indeed, only low molecular weight random poly(butylene terephthalate) (PBT) ionomers can be obtained by melt polycondensation due to the very high melt viscosity that occurs just after the beginning of the second stage of polymerization under dynamic vacuum [7]. This low Mw and the consequent brittleness are the main issues encountered [7] in developing PBT random ionomers for the preparation of nanocomposites with organo-clays. We have recently reported that telechelic sulfonated PBT ionomers present similar ionic interaction with the surface of the clay compared to random PBT ionomers and therefore give rise to similar improvement in
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the dispersion of the clay platelets and in thermo-mechanical properties of the nanocomposite [19]. Another drawback of random ionomers lies in their poor hydrolytic stability due to the activation effect of sulfonated groups towards hydrolysis [12]. Two methods are reported in the literature for the synthesis of terephthalate polyesters sulfonated telechelic ionomers: (a) Reactive blending with aliphatic esters of sulfobenzoic acid [18,19]. (b) Addition of sulfobenzoic acid at the beginning of the polymerization process [2,19]. In our previous works [18,19], poly(ethylene terephthalate) (PET) ionomers have been obtained by reactive blending of PET with the butyl ester of sulfonated benzoic acid. The main drawbacks of the use of benzoic acid derivatives are connected with their low solubility in the polymer melt and their high melting points. Therefore, high reaction temperatures and long residence times are required in order to obtain the ester exchange reaction of the sulfonated molecule with the polymer chain. Moreover, the reaction of the sulfobenzoate with the polymer gives rise to a chain scission and therefore, only low molecular PET ionomers have been obtained by this method. Kang et al. [2,20] report the synthesis of PET ionomers by melt polymerization, by adding the sodium salt of 3-sufobenzoic acid (SBANa) as end-capper at the beginning of the polymerization process. PET ionomers with ionic contents ranging from 1% to 5% have been synthesized by this method. These papers [2,20] reports a lower crystallization rate for telechelic ionomers respect to PET of similar molecular weight due to the higher melt viscosity caused by the ionic interactions. Two patents [21,22] claim the synthesis of PET sulfonated telechelic ionomers with the aim to increase the dyeability of the PET fibers and for blow molding applications. In the patents, the sulfobenzoic acid salt is used in combination with tetrafunctional carboxylic acids or alcohols as branching agents in order to increase the PET melt viscosity and the electrostatic interactions with the dyes. No paper reports up to now the synthesis and the study of the thermal and mechanical properties of PBT telechelic ionomers and, as will be presented in the following, the methods reported for telechelic PET synthesis cannot be successfully applied to PBT. We have patented [23] a method for preparing PBT telechelic ionomers by pre-reacting SBANa with 1,4-butanediol (BD) and then adding dimethyl terephthalate (DMT). In this paper we report a detailed study of telechelic ionomers synthesis, the scale-up of the polymerization process along with the chemical and physical characterization of the ionomers prepared. We also report a comparison with random PBT ionomers showing the improved thermo-mechanical and hydrolytic stability properties of telechelic ionomers respect to random ionomers. 2. Experimental 2.1. Materials 1,4-butanediol (BD), 3-sulfobenzoic acid sodium salt (SBANa), titanium tetrabutoxide (TBT), dimethyl 5-sulfoisophthalate sodium salt (DMSIP), dimethyl terephthalate (DMT) (all from Aldrich Chemicals) were high purity products and were not purified before use. 2.2. Sulfonated ionomeric telechelic PBT small scale synthesis A round bottom wide-neck glass reactor (250 ml capacity) was charged with BD (50.440 g; 0.560 mol), SBANa (2.686 g;
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0.0120 mol) and TBT (109 mg corresponding to 175 ppm as titanium with respect to the final polymer). The reactor was closed with a three-neck flat flange lid equipped with a mechanical stirrer and a torque meter. The system was then connected to a water-cooled condenser and immersed in a thermostatic oil-bath at 230 °C and the stirrer switched on at 100 rpm. After 1 h the reaction mixture became clear and the oilbath was cooled to 180 °C. DMT (77.624 g; 0.400 mol) was then carefully added and the temperature increased to 215 °C in 30 min and kept at this temperature for additional 90 min. The distillate recovered during this first stage in the condenser was collected and analyzed by 1H NMR in order to measure the amount of THF formed. The temperature was then increased to 245 °C, the lid was heated at a temperature of 120 °C with a heating band and the reactor connected to a liquid nitrogen cooled condenser. Dynamic vacuum was then applied in 20 min down to 0.2 mbar. After 100 min the very viscous pale yellow and transparent melt was discharged from the reactor. 2.3. Sulfonated ionomeric telechelic PBT micro-pilot plant synthesis The preparation of the telechelic ionomeric polyester has also been performed using a 1.8 L stainless steel batch reactor. In the first stage, 1.4-butanediol (458 g; 5.09 mol), 3-sulfobenzoic acid sodium salt (24.39 g; 0.11 mol) and TBT (1.05 g corresponding to 175 ppm as titanium with respect to the final polymer) were introduced into the reactor. The temperature of the reactor was increased to 220 °C and the stirrer switched on at 60 rpm. After 1 h the reactor was cooled to 180 °C and the dimethyl terephthalate (705 g; 3.63 mol) was then added (BD/DMT ratio 1.4:1) into the reactor and the temperature increased to 215 °C in 30 min. Volatile products (methanol and THF) were distilled off from the reactor, condensed in a water-cooled condenser and collected in a flask measuring the weight of the distillate in order to monitor the reaction kinetic. The distillate was analyzed by 1H NMR in order to measure the amount of THF formed. The temperature was then kept at 215 °C until 85% (corresponding to 245 ml) of the theoretical amount of methanol was distilled off. In the second stage the internal pressure was slowly reduced from atmospheric pressure down to 10 mbar in 10 min and below 1 mbar in another 10 min. At the same time the temperature of the reaction melt was increased to 245 °C and kept at this temperature until the end of the polymerization. The second stage was stopped when no further significant increase in strain gauge signal was detected. The preparation of telechelic ionomeric polyester using a neutralizing agent, was prepared according to the procedure reported above but with the addition of 0.012 mol of the neutralizing agent (0.34% by mol respect to the repeating unit of the final polymer) added at the beginning of the reaction. 2.4. Sulfonated ionomeric random PBT synthesis in glass reactor A round bottom wide-neck glass reactor (250 ml capacity) was charged with BD (50.440 g; 0.560 mol), DMT (75.3 g; 0.388 mol) (BD/DMT ratio 1.4:1), DMSIP (3.44 g; 11.6 mmol) and TBT (0.105 g corresponding to 175 ppm (ppm) as titanium with respect to the final polymer). The reactor was closed with a three-neck flat flange lid equipped with a mechanical stirrer and a torque meter. The system was then connected to a water-cooled condenser and immersed in a thermostatic oil-bath at 215 °C and the stirrer switched on at 100 rpm. The distillate recovered during this first stage in the condenser was collected and analyzed by 1H NMR in order to measure the amount of THF formed. The temperature was then increased to 245 °C, the lid was heated at a temperature of 120 °C with a heating band and the reactor connected to a liquid nitrogen cooled condenser. Dynamic vacuum was then applied in
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20 min down to 0.2 mbar. After 30 min the melt viscosity becomes to high to permit the stirring of the molten polymer and the very viscous pale yellow and transparent melt was discharged from the reactor. 2.5. Instrumental 1
H NMR and 13C NMR spectra were recorded with a Varian XL400 spectrometer (chemical shifts are downfield from TMS), using a CF3COOD/CDCl3 (1/4 v/v) mixture as solvent. The spectra have been recorded just after dissolution in order to avoid esterification reaction of end-groups with trifluoroacetic acid. The hydrolytic stability of polymer samples was determined on compression molding discs 25.0 mm in diameter and 1.5 mm thick. A set of discs was placed in an autoclave and samples removed with time. The temperature and pressure were 120 °C and 1.2 bar, respectively. The degree of hydrolytic degradation was monitored by measuring the carboxylic end-groups concentration of the polymer by potentiometric titration and by measuring the molecular weight drop by GPC. Carboxylic end-groups analysis: about 0.5 g of polymer was dissolved in 25 ml of a mixture of 75% o-cresol, 25% methylene chloride at reflux temperature. Once dissolved, the solution was allowed to cool to room temperature and then 50 ml of methylene chloride was added. The solution was titrated with a 0.01 N solution of tetrabutylammonium hydroxide. Gel permeation chromatography (GPC) analysis was performed using a mixture of chloroform/hexafluoroisopropanol (HFIP) (95/5 v/v) as eluent (elution rate of 0.3 ml min 1) on a HP 1100 Series apparatus equipped with a PL Gel 5 l Mini-Mixed-C column and a UV detector. Calibration was performed with polystyrene standards. Differential scanning calorimetry (DSC) analysis was performed using a Perkin Elmer DSC7. The instrument was calibrated with high purity standards (indium and cyclohexane). Dry nitrogen was used as purge gas. DSC heating and cooling rate 20 °C/min. All transitions have been measured after a heating scan to 250 °C and cooling down to room temperature in order to delete previous thermal history. The thermogravimetric analyses (TGA) were performed using a Perkin Elmer TGA7 apparatus in N2 (gas flow: 40 ml/min) at 10 °C/ min heating rate, from 25 °C to 800 °C. DMTA analyses were performed with a Rheometrics dynamic mechanic thermal analyzer DMTA 3E with a dual cantilever testing geometry. Test bars were injection molded at 275 °C using a Minimax Molder (Custom Scientific Instruments) equipped with a rectangular mold (30 8 1.6 mm3). The testing was done at a frequency of 3 Hz and temperature range was from 50 °C to 200 °C at a rate of 3 °C/min. 3. Results and discussion 3.1. Telechelic ionomers synthesis The method reported in the literature [2] for the synthesis of poly(ethylene terephthalate) (PET) telechelic ionomers cannot be applied to PBT. Indeed, if 3% by mol of the sodium salt of 3-sulfobenzoic acid (SBANa) is added to ethylene glycol (EG) and dimethyl terephthalate (DMT) in the presence of the catalyst, the heterogeneous mixture becomes clear after 15 min at 215 °C. This is not the case when butanediol (BD) is used since no homogenous melt was observed even after 2 h at 215 °C. If a full polymerization is conducted, the 1H NMR analysis of the polymer after dissolution in a trifluoroacetic acid/chloroform solution (1/4 v/v) and precipitation in methanol, shows that most of the sulfobenzoic acid has not reacted
and therefore is not linked to the PBT chain. One reason of this lower reactivity can be ascribed to the lower solubility of sulfobenzoic acid in the reaction medium caused by the lower polarity of BD respect to EG. Moreover, a smaller excess of BD (1.4 respect to DMT) compared to EG (2 respect to DMT) is normally used in order to reduce THF formation during the polymerization. Another reason can be that the transesterification of DMT with BD may be faster than the direct esterification of SBANa with BD and for this reason as the reaction proceeds, the DMT reacts with most of BD leading to the precipitation of the sodium salt. Therefore, in order to increase the polarity of the reaction medium, we first reacted butanediol and SBANa in the presence of TBT at 230 °C until the complete clear of the reaction mixture (1 h). A homogenous system was still observed after cooling at 180 °C and addition of DMT. After DMT addition the polymerization was conducted in a typical two stages polycondensation process. The polymerization scheme used for PBT telechelic ionomers is presented in Fig. 1. Also the methyl ester of sulfobenzoic acid can be used as ionic precursor. The use of the methyl ester should reduce the amount of THF formed at the beginning of the process and should not require the pre-reaction with BD. However, the methyl ester is not commercially available, therefore the process must be performed in two different reactors, one for the esterification with the excess methanol and one for the polycondensation. For this reason, we have decided to perform the process in one-pot prereacting SBANa with BD and then adding the DMT instead of making the methyl ester in a first stage and then run a standard polycondensation process. The polymer melt viscosity measured by the torque meter of the mechanical stirrer was slightly higher compared to that of standard PBT and it was possible to stir the polymer melt until the end of the polymerization process. The 1H NMR spectra of the polymers dissolved in a trifluoroacetic acid/chloroform solution (1/4 v/v) and precipitated in methanol (Fig. 2), showed the complete incorporation of the sulfobenzoic acid since the ratio of the integral of the signal at 8.55 ppm (ascribable to the proton a in Fig. 2) with the total aromatic signals does not change after the precipitation (SBANa is soluble in methanol). The signals have been attributed according to the literature [2]. The final polymer melt was transparent and clear and crystallized upon cooling after the extraction from the reactor providing a white material. A random sulfonated PBT with 3% (by mol) ionic content was also synthesized according to the literature [12] by adding dimethyl 5-sulfoisophthalate sodium salt (DMSIP) at the beginning of the polymerization. In this case, the second stage was stopped just after 30 min from the application of full vacuum since the melt viscosity was too high to permit the stirring and the removing from the reactor of the polymer melt. GPC analysis shows that only low molecular weight (Mw below 15,000) random ionomers with 3% of ionic groups can be obtained by melt polycondensation. This behavior confirms that, for random ionomers, the ionic interactions give rise to a gel-like or cross linked aggregation and therefore to a very high melt viscosity, while for the telechelics the ionic aggregation occurs only at the end of the chain, giving rise to an electrostatic chain-extension. The main results of the NMR and GPC characterization of PBT telechelic and random ionomers are reported in Table 1. The telechelic ionic content have been calculated by comparing the integral of the singlet at d = 8.55 ppm (corresponding to one aromatic proton of the sulfonated end-group) with that of the peak at 8.1 ppm ascribable to the four terephthalate protons. The telechelic ionic amounts reported in Table 1 have been measured on the crude final product. The ratio between the sulfonated end-groups and the total end-groups has been calculated by measuring sulfonated, methyl ester and OH end-groups by 1H NMR and COOH end-groups by titration.
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Fig. 1. PBT telechelic ionomer polymerization scheme.
The GPC results in Table 1 show a good reproducibility of the experimental procedure (compare Mw for runs 2 and 2 replicate). The telechelic ionomers are mainly terminated with sulfonated groups and the percentage of ionic end-groups increases with the amount of ionic end-capper added. The molecular weight after the precipitation presents a 5–10% increase due to the elimination of low molecular weight oligomers. The Mw as a function of ionic content is presented in Fig. 3. The molecular weight decreases, as expected, with the ionic content since the ionic groups acts as chain stoppers. With ionic content above 5% the molecular weight is low and therefore only brittle materials can be obtained. A scale-up of the process in a lab-scale (1.8 L) batch pilot plant has been conducted in order to measure the amount of THF formed in both stages, to follow the reaction kinetic and to prepare larger amounts for thermo-mechanical characterization. Polymers capped with benzoic acid (BA) have also been prepared for comparison. The main results are reported in Table 2. Some of the experiments have been conducted in duplicate showing a very good reproducibility of the results. The ionomeric content in Table 2 has been measured by 1H NMR after the dissolution–precipitation procedure reported above and it is possible to see that, again, the ionic groups are covalently bonded to the polyester chain. The use of the lab-scale pilot plant also permits the study of the first stage reaction kinetic (Fig. 4). The conversion has been evaluated by measuring the distilled volume and subtracting the amount of THF measured by NMR analysis. The curves in Fig. 4 show that the ionic groups have no inhibiting or catalytic effect on first stage reaction rate. A consistent increase in THF formation in both stages was observed. Both the THF amount and the carboxylic end-group content increase by increasing the ionic content. The presence of 5% of ionic groups gives rise to low molecular weight polymers (run 13). This low Mw can be ascribed to the fact that in the last part of the polymerization almost no OH end-groups are present (due to backbiting reaction and THF formation) and the molecular weight buildup is therefore stopped. For this reason, it is necessary to decrease the rate of side-reactions and drop down the amount of THF formed. On the basis of our previous works on PBT catalysis [24] we have tested phosphate salts in order to decrease the THF formation. The high THF formation can be due to the presence of SO3H
groups present as impurities in the sulfoaromatic carboxylic acid sodium salt source or generated during the polycondensation process. The amount of SO3H groups on the starting monomer was below 1% as measured by titration. The addition of Na3PO4 has a beneficial effect in reducing THF formation in both reaction stages and permits to obtain molecular weights comparable to those of polymers capped with the same molar amount of benzoic acid (run 15 and 16). As expected the reduction of THF formation gives rise to a lower COOH content. Using Na3PO4 during telechelic ionomers synthesis, THF and COOH end-groups contents are very similar compared to those produced during PBT synthesis with nonionic end-cappers. Other salts have been tested in order to assess their effect on the decrease in THF formation. Sodium phosphate, sodium dihydrogenphosphate, sodium carbonate and sodium acetate salts have been tested (Table 3). The results in Table 3 show that also sodium carbonate and sodium acetate salts give rise to reduced THF formation. For these runs, the amount of THF formed was 40–50% lower compared to the amount recovered if no neutralizing agent was used and similar to the amount observed for comparative example without ionic groups. On the contrary, NaH2PO4 has no significant effect on THF reduction. No significant further decrease in THF formation was observed increasing the buffer agent content. We have reported [24] that phosphonium salts not only reduce the THF formation but also have a catalytic effect when used in combination with titanium alcoxides. Indeed, the kinetic study of the polymerization in presence of Na3PO4 (Fig. 5) points out a consistent co-catalytic effect of this salt on titanium catalyst. The use of Na3PO4 permits to obtain the same DMT conversion (85%) in 55% shorter time respect to the use of titanium butoxide alone. The other salts (carbonates and acetates) do not give rise to improvements in reaction rate. According to our previous work [24] this co-catalytic effect can be ascribed to the formation of superacid adducts. 3.2. Thermal properties There are no significant differences in melting temperature and enthalpy of fusion (measured on second heating scan at 20 °C/min) for the telechelic polymers respect to control PBT (Table 4). On the
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Fig. 2. Aliphatic and aromatic regions of 1H NMR spectrum of PBT 3% sulfonated telechelic after dissolution and precipitation.
Table 1 Ionic groups content and Mw of telechelic and random ionomers. Run
3-SBANa added (mol.%)
Ionic content in crude polymer (mol.%)
Ionic content after precipitation (mol.%)
Sulfonated end-groups Total end-groups
Mw (GPC)
PBT control 1 2 2 replicate 3 4
– 1.5 3 3 5 3% DMSIP
– 1.4 2.8 2.9 4.8 3% random ionomer
– 1.4 2.8 2.8 4.8 2.9
– 0.75 0.85 0.78 0.90 –
113,500 56,630 45,800 44,300 29,650 12,300
contrary, according to the literature [25–27], random ionomers present a lower crystallinity. It is important to notice that the crystallization time (measured as peak time in isothermal conditions) in telechelic polymers increases with the ionic content even if the molecular weight follows an opposite trend. This means that the electrostatic interactions between the polymer chain ends give rise to
an increase in apparent melt viscosity that overcomes the decrease due to the lower molecular weight. The random ionomer presents a consistently slower crystallization rate even if its molecular weight is very low, confirming that the ionic groups along the polymer chains act as cross-link agents giving rise to a gel-like aggregate and therefore to a very high melt viscosity. This decreased chain mobility also explains the lower crystallinity
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Fig. 3. Mw as a function of sulfonated telechelic content.
Table 2 Overall results of micro-pilot plant polymerizations of telechelic ionomers and end-capped PBT with benzoic acid (BA).
a b c
Run
SBANa feda
Cocatalyst
Ionomeric contentb
Sulfonate end-groups Total end-groups
COOH end groups (meq/kg)
THF in first stagec
THF in second stagec
Mw (GPC)
PBT control 8 9 10 11 12 13 14 15 16
– 0.5 0.5 1 1 2 5 5 1% BA 5% BA
–
– 0.45 0.45 0.90 0.90 1.8 4.7 4.5 0.8 BA inserted 4.5 BA inserted
– 0.41 0.43 0.58 0.60 0.70 0.40 0.85 0.48 BA end-groups 0.84 BA end-groups
30 34 27 39 37 38 65 40 28 33
1.57 3.17 1.71 3.90 3.95 5.50 10.2 6.58 1.67 2.51
3.74 5.07 3.80 6.52 6.60 7.61 15.5 7.00 3.45 3.70
87,900 80,000 70,200 65,700 64,400 47,300 12,300 32,700 68,700 35,000
Na3PO4 – – – Na3PO4 – –
mol.% respect to DMT. mol.% respect to terephthalic units. mol.% respect to BD.
Fig. 4. First stage conversion in telechelic ionomers synthesis.
of random ionomers respect to standard PBT. Melt viscosity analyses and a more detailed DSC characterization (crystallization ki-
netic study) are in progress and will published in a following paper.
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Table 3 Effect of different buffer agents on THF reduction during telechelic ionomers synthesis.
a
Run
SBANa fed (mol.% respect to DMT)
Buffer agenta
THF in first stage (mol.% respect to BD)
PBT control 3% tel. control 17 18 19 20
–
–
1.6
3 3 3 3 3
– Na3PO4 Na2HPO4 Na2CO3 CH3COONa
4.5 2.1 4.0 2.1 1.9
0.34% by mol respect to the repeating unit of the final polymer.
3.3. Telechelic ionomers thermal stability The TGA analysis (Fig. 6) shows that the weight loss starts earlier for ionomers respect to standard PBT. Indeed, the onset values decrease by increasing the ionic content: the values are respectively 389 °C, 386 °C, 381 °C and 375 °C for standard PBT, PBT 1.5%, 3% and 5% telechelic. On the contrary, the residue at high temperature increases with the ionomeric content. There is also a correlation between thermal stability and carboxylic end-groups content. The TGA results in Fig. 7 show that telechelic ionomers are thermally more stable compared to random ionomers with the same ionic content. No significant effect of Na3PO4 has been found on thermal stability of telechelic polymers having similar amount of carboxylic end-groups.
3.4. Hydrolytic stability One of the main issues connected with the use of ionic groups in polyester chains is the loss in hydrolytic stability. For example, it is reported in the literature [12] that random ionomers are consistently less stable toward hydrolysis compared to standard PBT due to their higher water adsorption. The hydrolytic stability has been evaluated after degradation in a pressure vessel at 120 °C and 1.2 bar by following the Mw drop (Fig. 8) and by titration of COOH end-groups (Fig. 9). The comparison of random ionomers with telechelic ionomers have been performed only by COOH titration since the starting Mw of the random ionomer was consistently lower compared to those of telechelic ionomers. The results in Fig. 8 show that hydrolytic stability is not an issue for telechelic ionomers since they follow a similar degradation trend respect to PBT and that there is no consistent effect of the phosphate additive. On the contrary, COOH end-groups titration (Fig. 9) show that random ionomers are consistently less stable since the rate of COOH groups formation is higher compared to those of PBT and telechelic ionomers. The lower hydrolytic stability of random ionomers could be ascribable to their lower crystallinity (see Table 4) respect to telechelic ionomers that gives rise to a larger water uptake [12].
3.5. Dynamic-mechanical analysis of telechelic ionomers DMTA analysis (Fig. 10 and 11) shows a slight increase in glass transition temperature (Tg) and an increase in modulus above Tg by increasing the ionic content due to the formation of ionic interac-
Fig. 5. Co-catalytic effect of phosphates during first stage.
Table 4 Isothermal crystallization of sulfonated telechelic ionomers. Run
Ionic content (mol.%)
Mw
Tm ( °C)
DHm (J/g)
Cryst time at 200 °C (min)
Cryst time at 205 °C (min)
PBT control 1 PBT control 2 1.5% telechelic 3% telechelic 5% telechelic 3% Random
0 0 1.4 2.8 4.8 3
87,900 43,000 56,600 45,800 29,700 12,300
223.7 222.3 223.0 222.7 222.8 222.7
41.3 42.9 44.7 42.8 42.5 37.9
1.5 1.0 1.3 1.5 2.1 29.4
7.4 4.4 7.1 8.9 12.1 >120
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Fig. 6. TGA curves of telechelic ionomers.
Fig. 7. Comparison of TGA curves in N2 for PBT telechelic and random ionomers.
Fig. 8. Mw decrease during hydrolytic stability tests of telechelic ionomers.
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Fig. 9. Carboxylic end-groups increase during hydrolytic stability tests for telechelic and random ionomers.
tions between polymer chains. Multiple Tg were observed according to the results reported by Hara et al. [28] for sodium salts of ionomeric polystyrene. 4. Conclusions Telechelic PBT ionomers have been prepared by melt synthesis. No side reaction occurs and the incorporation of the ionic groups is quantitative. The ionic groups act as chain-extension reversible links for telechelic ionomers while act as cross-links for random ionomers. Indeed, random ionomers present a consistently higher melt viscosity compared to telechelic ionomers and to PBT that does not allow to reach high molecular weight polymers. A buffer agent such as Na3PO4 has to be added to the catalyst in order to reduce THF formation and improve the polycondensation rate. By this way, telechelic ionomers can be obtained with Mw, thermal and hydrolytic stability comparable to those of commercial PBT and consistently higher respect to random ionomers. Therefore, telechelic ionomers are very promising materials to be used in the preparation of nanocomposites and in the interaction with dyes. Fig. 10. Storage modulus of telechelic ionomers.
Acknowledgement This work was financed by General Electric Company. References
Fig. 11. Tan-delta of telechelic ionomers.
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Telechelic ionomeric poly(butylene terephthalate): Synthesis, characterization and comparison with random ionomers Corrado Berti a, Martino Colonna a,*, Enrico Binassi a, Maurizio Fiorini a, Sreepadaraj Karanam b, Daniel J. Brunelle c a b c
Dipartimento di Chimica Applicata e Scienza dei Materiali, Via Terracini 28, 40131 Bologna, Italy SABIC Innovative Plastics, Plasticslaan 1, 4600 AC Bergen op Zoom, The Netherlands General Electric Global Research, One Research Circle, Niskayuna, NY 12309, USA
a r t i c l e
i n f o
Article history: Received 3 November 2009 Received in revised form 23 February 2010 Accepted 27 February 2010 Available online 4 March 2010 Keywords: Poly(butylene terephthalate) Telechelic polymers Ionomers Polycondensation Crystallization
a b s t r a c t Telechelic poly(butylene terephthalate) (PBT) ionomers have been prepared by melt synthesis using a new polycondensation process that involves a pre-reaction of sulfobenzoic acid sodium salt with butanediol. No side reaction occurs and the incorporation of the ionic groups is quantitative. The addition of a buffer agent, such as Na3PO4, to the catalyst reduces the THF formation and improves the polycondensation rate. The comparison of the thermo-mechanical and physical properties between random and telechelic ionomers is also reported. The ionic groups act as chain-extension reversible electrostatic links for telechelic ionomers while act as cross-links in random ionomers. Therefore, random ionomers present a consistently higher melt viscosity compared to telechelic ionomers and to PBT and for this reason high molecular weight random ionomers cannot be obtained by melt polycondensation. Thermal and hydrolytic stabilities of telechelic ionomers are comparable with those of commercial PBT and consistently higher respect to those of random ionomers. The presence of ionic groups decreases the translation mobility of the polymer chains thus lowering the crystallization rate. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction It is reported in the literature [1–5] that the presence, even at very low concentrations, of ionic groups covalently bonded to the polymer backbone produces a consistent effect on the rheological and thermo-mechanical properties of polymers. The improvements in thermo-mechanical properties are generally ascribed to the formation of ionic aggregates that act as thermo-reversible cross-links [1] thus decreasing the translational mobility of the polymer backbone. Metal sulfonates are known [1] to strongly associate in the solid state as ionic aggregates that can be disrupted at higher temperatures. The thermo-reversibility of ionic aggregation provides the opportunity to solve several issues connected with covalently bonded high molecular weight polymers, such as poor melt processability, high melt viscosity and low thermal stability at high shear rate and temperature [1]. It has also been reported in the literature [2] that ionic interactions have consistent effects on the crystallization kinetics and on the morphology of ionomers. Moreover, the ionic groups can give rise to interactions with dyes [6] and with fillers, for example, organically modified clays [7–10], improving the adhesion of the polymer matrix with * Corresponding author. Fax: +39 0512090322. E-mail address: [email protected] (M. Colonna). 1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2010.02.011
the substrate and providing the thermodynamic driving force for the nanometric dispersion of the filler. The insertion of the ionic groups can be performed along the polymer chain (random ionomers) or only at the end of the chain (telechelic ionomer). Several differences can be expected between the two classes of ionomers. Most of the studies on polyester ionomers have been conducted on random polyester ionomers [11– 14]. On the contrary very few studies [2,15–18] concerning telechelic polyester ionomers have been conducted. The ionic aggregation in telechelic polymers occurs only at the end of the chain, giving rise to an electrostatic chain-extension while for random ionomers gellike or cross-linked aggregates are formed. For this reason, lower melt viscosities and therefore high molecular weight can be obtained for telechelic ionomers respect to random ionomers [7,8]. Indeed, only low molecular weight random poly(butylene terephthalate) (PBT) ionomers can be obtained by melt polycondensation due to the very high melt viscosity that occurs just after the beginning of the second stage of polymerization under dynamic vacuum [7]. This low Mw and the consequent brittleness are the main issues encountered [7] in developing PBT random ionomers for the preparation of nanocomposites with organo-clays. We have recently reported that telechelic sulfonated PBT ionomers present similar ionic interaction with the surface of the clay compared to random PBT ionomers and therefore give rise to similar improvement in
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the dispersion of the clay platelets and in thermo-mechanical properties of the nanocomposite [19]. Another drawback of random ionomers lies in their poor hydrolytic stability due to the activation effect of sulfonated groups towards hydrolysis [12]. Two methods are reported in the literature for the synthesis of terephthalate polyesters sulfonated telechelic ionomers: (a) Reactive blending with aliphatic esters of sulfobenzoic acid [18,19]. (b) Addition of sulfobenzoic acid at the beginning of the polymerization process [2,19]. In our previous works [18,19], poly(ethylene terephthalate) (PET) ionomers have been obtained by reactive blending of PET with the butyl ester of sulfonated benzoic acid. The main drawbacks of the use of benzoic acid derivatives are connected with their low solubility in the polymer melt and their high melting points. Therefore, high reaction temperatures and long residence times are required in order to obtain the ester exchange reaction of the sulfonated molecule with the polymer chain. Moreover, the reaction of the sulfobenzoate with the polymer gives rise to a chain scission and therefore, only low molecular PET ionomers have been obtained by this method. Kang et al. [2,20] report the synthesis of PET ionomers by melt polymerization, by adding the sodium salt of 3-sufobenzoic acid (SBANa) as end-capper at the beginning of the polymerization process. PET ionomers with ionic contents ranging from 1% to 5% have been synthesized by this method. These papers [2,20] reports a lower crystallization rate for telechelic ionomers respect to PET of similar molecular weight due to the higher melt viscosity caused by the ionic interactions. Two patents [21,22] claim the synthesis of PET sulfonated telechelic ionomers with the aim to increase the dyeability of the PET fibers and for blow molding applications. In the patents, the sulfobenzoic acid salt is used in combination with tetrafunctional carboxylic acids or alcohols as branching agents in order to increase the PET melt viscosity and the electrostatic interactions with the dyes. No paper reports up to now the synthesis and the study of the thermal and mechanical properties of PBT telechelic ionomers and, as will be presented in the following, the methods reported for telechelic PET synthesis cannot be successfully applied to PBT. We have patented [23] a method for preparing PBT telechelic ionomers by pre-reacting SBANa with 1,4-butanediol (BD) and then adding dimethyl terephthalate (DMT). In this paper we report a detailed study of telechelic ionomers synthesis, the scale-up of the polymerization process along with the chemical and physical characterization of the ionomers prepared. We also report a comparison with random PBT ionomers showing the improved thermo-mechanical and hydrolytic stability properties of telechelic ionomers respect to random ionomers. 2. Experimental 2.1. Materials 1,4-butanediol (BD), 3-sulfobenzoic acid sodium salt (SBANa), titanium tetrabutoxide (TBT), dimethyl 5-sulfoisophthalate sodium salt (DMSIP), dimethyl terephthalate (DMT) (all from Aldrich Chemicals) were high purity products and were not purified before use. 2.2. Sulfonated ionomeric telechelic PBT small scale synthesis A round bottom wide-neck glass reactor (250 ml capacity) was charged with BD (50.440 g; 0.560 mol), SBANa (2.686 g;
367
0.0120 mol) and TBT (109 mg corresponding to 175 ppm as titanium with respect to the final polymer). The reactor was closed with a three-neck flat flange lid equipped with a mechanical stirrer and a torque meter. The system was then connected to a water-cooled condenser and immersed in a thermostatic oil-bath at 230 °C and the stirrer switched on at 100 rpm. After 1 h the reaction mixture became clear and the oilbath was cooled to 180 °C. DMT (77.624 g; 0.400 mol) was then carefully added and the temperature increased to 215 °C in 30 min and kept at this temperature for additional 90 min. The distillate recovered during this first stage in the condenser was collected and analyzed by 1H NMR in order to measure the amount of THF formed. The temperature was then increased to 245 °C, the lid was heated at a temperature of 120 °C with a heating band and the reactor connected to a liquid nitrogen cooled condenser. Dynamic vacuum was then applied in 20 min down to 0.2 mbar. After 100 min the very viscous pale yellow and transparent melt was discharged from the reactor. 2.3. Sulfonated ionomeric telechelic PBT micro-pilot plant synthesis The preparation of the telechelic ionomeric polyester has also been performed using a 1.8 L stainless steel batch reactor. In the first stage, 1.4-butanediol (458 g; 5.09 mol), 3-sulfobenzoic acid sodium salt (24.39 g; 0.11 mol) and TBT (1.05 g corresponding to 175 ppm as titanium with respect to the final polymer) were introduced into the reactor. The temperature of the reactor was increased to 220 °C and the stirrer switched on at 60 rpm. After 1 h the reactor was cooled to 180 °C and the dimethyl terephthalate (705 g; 3.63 mol) was then added (BD/DMT ratio 1.4:1) into the reactor and the temperature increased to 215 °C in 30 min. Volatile products (methanol and THF) were distilled off from the reactor, condensed in a water-cooled condenser and collected in a flask measuring the weight of the distillate in order to monitor the reaction kinetic. The distillate was analyzed by 1H NMR in order to measure the amount of THF formed. The temperature was then kept at 215 °C until 85% (corresponding to 245 ml) of the theoretical amount of methanol was distilled off. In the second stage the internal pressure was slowly reduced from atmospheric pressure down to 10 mbar in 10 min and below 1 mbar in another 10 min. At the same time the temperature of the reaction melt was increased to 245 °C and kept at this temperature until the end of the polymerization. The second stage was stopped when no further significant increase in strain gauge signal was detected. The preparation of telechelic ionomeric polyester using a neutralizing agent, was prepared according to the procedure reported above but with the addition of 0.012 mol of the neutralizing agent (0.34% by mol respect to the repeating unit of the final polymer) added at the beginning of the reaction. 2.4. Sulfonated ionomeric random PBT synthesis in glass reactor A round bottom wide-neck glass reactor (250 ml capacity) was charged with BD (50.440 g; 0.560 mol), DMT (75.3 g; 0.388 mol) (BD/DMT ratio 1.4:1), DMSIP (3.44 g; 11.6 mmol) and TBT (0.105 g corresponding to 175 ppm (ppm) as titanium with respect to the final polymer). The reactor was closed with a three-neck flat flange lid equipped with a mechanical stirrer and a torque meter. The system was then connected to a water-cooled condenser and immersed in a thermostatic oil-bath at 215 °C and the stirrer switched on at 100 rpm. The distillate recovered during this first stage in the condenser was collected and analyzed by 1H NMR in order to measure the amount of THF formed. The temperature was then increased to 245 °C, the lid was heated at a temperature of 120 °C with a heating band and the reactor connected to a liquid nitrogen cooled condenser. Dynamic vacuum was then applied in
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20 min down to 0.2 mbar. After 30 min the melt viscosity becomes to high to permit the stirring of the molten polymer and the very viscous pale yellow and transparent melt was discharged from the reactor. 2.5. Instrumental 1
H NMR and 13C NMR spectra were recorded with a Varian XL400 spectrometer (chemical shifts are downfield from TMS), using a CF3COOD/CDCl3 (1/4 v/v) mixture as solvent. The spectra have been recorded just after dissolution in order to avoid esterification reaction of end-groups with trifluoroacetic acid. The hydrolytic stability of polymer samples was determined on compression molding discs 25.0 mm in diameter and 1.5 mm thick. A set of discs was placed in an autoclave and samples removed with time. The temperature and pressure were 120 °C and 1.2 bar, respectively. The degree of hydrolytic degradation was monitored by measuring the carboxylic end-groups concentration of the polymer by potentiometric titration and by measuring the molecular weight drop by GPC. Carboxylic end-groups analysis: about 0.5 g of polymer was dissolved in 25 ml of a mixture of 75% o-cresol, 25% methylene chloride at reflux temperature. Once dissolved, the solution was allowed to cool to room temperature and then 50 ml of methylene chloride was added. The solution was titrated with a 0.01 N solution of tetrabutylammonium hydroxide. Gel permeation chromatography (GPC) analysis was performed using a mixture of chloroform/hexafluoroisopropanol (HFIP) (95/5 v/v) as eluent (elution rate of 0.3 ml min 1) on a HP 1100 Series apparatus equipped with a PL Gel 5 l Mini-Mixed-C column and a UV detector. Calibration was performed with polystyrene standards. Differential scanning calorimetry (DSC) analysis was performed using a Perkin Elmer DSC7. The instrument was calibrated with high purity standards (indium and cyclohexane). Dry nitrogen was used as purge gas. DSC heating and cooling rate 20 °C/min. All transitions have been measured after a heating scan to 250 °C and cooling down to room temperature in order to delete previous thermal history. The thermogravimetric analyses (TGA) were performed using a Perkin Elmer TGA7 apparatus in N2 (gas flow: 40 ml/min) at 10 °C/ min heating rate, from 25 °C to 800 °C. DMTA analyses were performed with a Rheometrics dynamic mechanic thermal analyzer DMTA 3E with a dual cantilever testing geometry. Test bars were injection molded at 275 °C using a Minimax Molder (Custom Scientific Instruments) equipped with a rectangular mold (30 8 1.6 mm3). The testing was done at a frequency of 3 Hz and temperature range was from 50 °C to 200 °C at a rate of 3 °C/min. 3. Results and discussion 3.1. Telechelic ionomers synthesis The method reported in the literature [2] for the synthesis of poly(ethylene terephthalate) (PET) telechelic ionomers cannot be applied to PBT. Indeed, if 3% by mol of the sodium salt of 3-sulfobenzoic acid (SBANa) is added to ethylene glycol (EG) and dimethyl terephthalate (DMT) in the presence of the catalyst, the heterogeneous mixture becomes clear after 15 min at 215 °C. This is not the case when butanediol (BD) is used since no homogenous melt was observed even after 2 h at 215 °C. If a full polymerization is conducted, the 1H NMR analysis of the polymer after dissolution in a trifluoroacetic acid/chloroform solution (1/4 v/v) and precipitation in methanol, shows that most of the sulfobenzoic acid has not reacted
and therefore is not linked to the PBT chain. One reason of this lower reactivity can be ascribed to the lower solubility of sulfobenzoic acid in the reaction medium caused by the lower polarity of BD respect to EG. Moreover, a smaller excess of BD (1.4 respect to DMT) compared to EG (2 respect to DMT) is normally used in order to reduce THF formation during the polymerization. Another reason can be that the transesterification of DMT with BD may be faster than the direct esterification of SBANa with BD and for this reason as the reaction proceeds, the DMT reacts with most of BD leading to the precipitation of the sodium salt. Therefore, in order to increase the polarity of the reaction medium, we first reacted butanediol and SBANa in the presence of TBT at 230 °C until the complete clear of the reaction mixture (1 h). A homogenous system was still observed after cooling at 180 °C and addition of DMT. After DMT addition the polymerization was conducted in a typical two stages polycondensation process. The polymerization scheme used for PBT telechelic ionomers is presented in Fig. 1. Also the methyl ester of sulfobenzoic acid can be used as ionic precursor. The use of the methyl ester should reduce the amount of THF formed at the beginning of the process and should not require the pre-reaction with BD. However, the methyl ester is not commercially available, therefore the process must be performed in two different reactors, one for the esterification with the excess methanol and one for the polycondensation. For this reason, we have decided to perform the process in one-pot prereacting SBANa with BD and then adding the DMT instead of making the methyl ester in a first stage and then run a standard polycondensation process. The polymer melt viscosity measured by the torque meter of the mechanical stirrer was slightly higher compared to that of standard PBT and it was possible to stir the polymer melt until the end of the polymerization process. The 1H NMR spectra of the polymers dissolved in a trifluoroacetic acid/chloroform solution (1/4 v/v) and precipitated in methanol (Fig. 2), showed the complete incorporation of the sulfobenzoic acid since the ratio of the integral of the signal at 8.55 ppm (ascribable to the proton a in Fig. 2) with the total aromatic signals does not change after the precipitation (SBANa is soluble in methanol). The signals have been attributed according to the literature [2]. The final polymer melt was transparent and clear and crystallized upon cooling after the extraction from the reactor providing a white material. A random sulfonated PBT with 3% (by mol) ionic content was also synthesized according to the literature [12] by adding dimethyl 5-sulfoisophthalate sodium salt (DMSIP) at the beginning of the polymerization. In this case, the second stage was stopped just after 30 min from the application of full vacuum since the melt viscosity was too high to permit the stirring and the removing from the reactor of the polymer melt. GPC analysis shows that only low molecular weight (Mw below 15,000) random ionomers with 3% of ionic groups can be obtained by melt polycondensation. This behavior confirms that, for random ionomers, the ionic interactions give rise to a gel-like or cross linked aggregation and therefore to a very high melt viscosity, while for the telechelics the ionic aggregation occurs only at the end of the chain, giving rise to an electrostatic chain-extension. The main results of the NMR and GPC characterization of PBT telechelic and random ionomers are reported in Table 1. The telechelic ionic content have been calculated by comparing the integral of the singlet at d = 8.55 ppm (corresponding to one aromatic proton of the sulfonated end-group) with that of the peak at 8.1 ppm ascribable to the four terephthalate protons. The telechelic ionic amounts reported in Table 1 have been measured on the crude final product. The ratio between the sulfonated end-groups and the total end-groups has been calculated by measuring sulfonated, methyl ester and OH end-groups by 1H NMR and COOH end-groups by titration.
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Fig. 1. PBT telechelic ionomer polymerization scheme.
The GPC results in Table 1 show a good reproducibility of the experimental procedure (compare Mw for runs 2 and 2 replicate). The telechelic ionomers are mainly terminated with sulfonated groups and the percentage of ionic end-groups increases with the amount of ionic end-capper added. The molecular weight after the precipitation presents a 5–10% increase due to the elimination of low molecular weight oligomers. The Mw as a function of ionic content is presented in Fig. 3. The molecular weight decreases, as expected, with the ionic content since the ionic groups acts as chain stoppers. With ionic content above 5% the molecular weight is low and therefore only brittle materials can be obtained. A scale-up of the process in a lab-scale (1.8 L) batch pilot plant has been conducted in order to measure the amount of THF formed in both stages, to follow the reaction kinetic and to prepare larger amounts for thermo-mechanical characterization. Polymers capped with benzoic acid (BA) have also been prepared for comparison. The main results are reported in Table 2. Some of the experiments have been conducted in duplicate showing a very good reproducibility of the results. The ionomeric content in Table 2 has been measured by 1H NMR after the dissolution–precipitation procedure reported above and it is possible to see that, again, the ionic groups are covalently bonded to the polyester chain. The use of the lab-scale pilot plant also permits the study of the first stage reaction kinetic (Fig. 4). The conversion has been evaluated by measuring the distilled volume and subtracting the amount of THF measured by NMR analysis. The curves in Fig. 4 show that the ionic groups have no inhibiting or catalytic effect on first stage reaction rate. A consistent increase in THF formation in both stages was observed. Both the THF amount and the carboxylic end-group content increase by increasing the ionic content. The presence of 5% of ionic groups gives rise to low molecular weight polymers (run 13). This low Mw can be ascribed to the fact that in the last part of the polymerization almost no OH end-groups are present (due to backbiting reaction and THF formation) and the molecular weight buildup is therefore stopped. For this reason, it is necessary to decrease the rate of side-reactions and drop down the amount of THF formed. On the basis of our previous works on PBT catalysis [24] we have tested phosphate salts in order to decrease the THF formation. The high THF formation can be due to the presence of SO3H
groups present as impurities in the sulfoaromatic carboxylic acid sodium salt source or generated during the polycondensation process. The amount of SO3H groups on the starting monomer was below 1% as measured by titration. The addition of Na3PO4 has a beneficial effect in reducing THF formation in both reaction stages and permits to obtain molecular weights comparable to those of polymers capped with the same molar amount of benzoic acid (run 15 and 16). As expected the reduction of THF formation gives rise to a lower COOH content. Using Na3PO4 during telechelic ionomers synthesis, THF and COOH end-groups contents are very similar compared to those produced during PBT synthesis with nonionic end-cappers. Other salts have been tested in order to assess their effect on the decrease in THF formation. Sodium phosphate, sodium dihydrogenphosphate, sodium carbonate and sodium acetate salts have been tested (Table 3). The results in Table 3 show that also sodium carbonate and sodium acetate salts give rise to reduced THF formation. For these runs, the amount of THF formed was 40–50% lower compared to the amount recovered if no neutralizing agent was used and similar to the amount observed for comparative example without ionic groups. On the contrary, NaH2PO4 has no significant effect on THF reduction. No significant further decrease in THF formation was observed increasing the buffer agent content. We have reported [24] that phosphonium salts not only reduce the THF formation but also have a catalytic effect when used in combination with titanium alcoxides. Indeed, the kinetic study of the polymerization in presence of Na3PO4 (Fig. 5) points out a consistent co-catalytic effect of this salt on titanium catalyst. The use of Na3PO4 permits to obtain the same DMT conversion (85%) in 55% shorter time respect to the use of titanium butoxide alone. The other salts (carbonates and acetates) do not give rise to improvements in reaction rate. According to our previous work [24] this co-catalytic effect can be ascribed to the formation of superacid adducts. 3.2. Thermal properties There are no significant differences in melting temperature and enthalpy of fusion (measured on second heating scan at 20 °C/min) for the telechelic polymers respect to control PBT (Table 4). On the
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Fig. 2. Aliphatic and aromatic regions of 1H NMR spectrum of PBT 3% sulfonated telechelic after dissolution and precipitation.
Table 1 Ionic groups content and Mw of telechelic and random ionomers. Run
3-SBANa added (mol.%)
Ionic content in crude polymer (mol.%)
Ionic content after precipitation (mol.%)
Sulfonated end-groups Total end-groups
Mw (GPC)
PBT control 1 2 2 replicate 3 4
– 1.5 3 3 5 3% DMSIP
– 1.4 2.8 2.9 4.8 3% random ionomer
– 1.4 2.8 2.8 4.8 2.9
– 0.75 0.85 0.78 0.90 –
113,500 56,630 45,800 44,300 29,650 12,300
contrary, according to the literature [25–27], random ionomers present a lower crystallinity. It is important to notice that the crystallization time (measured as peak time in isothermal conditions) in telechelic polymers increases with the ionic content even if the molecular weight follows an opposite trend. This means that the electrostatic interactions between the polymer chain ends give rise to
an increase in apparent melt viscosity that overcomes the decrease due to the lower molecular weight. The random ionomer presents a consistently slower crystallization rate even if its molecular weight is very low, confirming that the ionic groups along the polymer chains act as cross-link agents giving rise to a gel-like aggregate and therefore to a very high melt viscosity. This decreased chain mobility also explains the lower crystallinity
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Fig. 3. Mw as a function of sulfonated telechelic content.
Table 2 Overall results of micro-pilot plant polymerizations of telechelic ionomers and end-capped PBT with benzoic acid (BA).
a b c
Run
SBANa feda
Cocatalyst
Ionomeric contentb
Sulfonate end-groups Total end-groups
COOH end groups (meq/kg)
THF in first stagec
THF in second stagec
Mw (GPC)
PBT control 8 9 10 11 12 13 14 15 16
– 0.5 0.5 1 1 2 5 5 1% BA 5% BA
–
– 0.45 0.45 0.90 0.90 1.8 4.7 4.5 0.8 BA inserted 4.5 BA inserted
– 0.41 0.43 0.58 0.60 0.70 0.40 0.85 0.48 BA end-groups 0.84 BA end-groups
30 34 27 39 37 38 65 40 28 33
1.57 3.17 1.71 3.90 3.95 5.50 10.2 6.58 1.67 2.51
3.74 5.07 3.80 6.52 6.60 7.61 15.5 7.00 3.45 3.70
87,900 80,000 70,200 65,700 64,400 47,300 12,300 32,700 68,700 35,000
Na3PO4 – – – Na3PO4 – –
mol.% respect to DMT. mol.% respect to terephthalic units. mol.% respect to BD.
Fig. 4. First stage conversion in telechelic ionomers synthesis.
of random ionomers respect to standard PBT. Melt viscosity analyses and a more detailed DSC characterization (crystallization ki-
netic study) are in progress and will published in a following paper.
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Table 3 Effect of different buffer agents on THF reduction during telechelic ionomers synthesis.
a
Run
SBANa fed (mol.% respect to DMT)
Buffer agenta
THF in first stage (mol.% respect to BD)
PBT control 3% tel. control 17 18 19 20
–
–
1.6
3 3 3 3 3
– Na3PO4 Na2HPO4 Na2CO3 CH3COONa
4.5 2.1 4.0 2.1 1.9
0.34% by mol respect to the repeating unit of the final polymer.
3.3. Telechelic ionomers thermal stability The TGA analysis (Fig. 6) shows that the weight loss starts earlier for ionomers respect to standard PBT. Indeed, the onset values decrease by increasing the ionic content: the values are respectively 389 °C, 386 °C, 381 °C and 375 °C for standard PBT, PBT 1.5%, 3% and 5% telechelic. On the contrary, the residue at high temperature increases with the ionomeric content. There is also a correlation between thermal stability and carboxylic end-groups content. The TGA results in Fig. 7 show that telechelic ionomers are thermally more stable compared to random ionomers with the same ionic content. No significant effect of Na3PO4 has been found on thermal stability of telechelic polymers having similar amount of carboxylic end-groups.
3.4. Hydrolytic stability One of the main issues connected with the use of ionic groups in polyester chains is the loss in hydrolytic stability. For example, it is reported in the literature [12] that random ionomers are consistently less stable toward hydrolysis compared to standard PBT due to their higher water adsorption. The hydrolytic stability has been evaluated after degradation in a pressure vessel at 120 °C and 1.2 bar by following the Mw drop (Fig. 8) and by titration of COOH end-groups (Fig. 9). The comparison of random ionomers with telechelic ionomers have been performed only by COOH titration since the starting Mw of the random ionomer was consistently lower compared to those of telechelic ionomers. The results in Fig. 8 show that hydrolytic stability is not an issue for telechelic ionomers since they follow a similar degradation trend respect to PBT and that there is no consistent effect of the phosphate additive. On the contrary, COOH end-groups titration (Fig. 9) show that random ionomers are consistently less stable since the rate of COOH groups formation is higher compared to those of PBT and telechelic ionomers. The lower hydrolytic stability of random ionomers could be ascribable to their lower crystallinity (see Table 4) respect to telechelic ionomers that gives rise to a larger water uptake [12].
3.5. Dynamic-mechanical analysis of telechelic ionomers DMTA analysis (Fig. 10 and 11) shows a slight increase in glass transition temperature (Tg) and an increase in modulus above Tg by increasing the ionic content due to the formation of ionic interac-
Fig. 5. Co-catalytic effect of phosphates during first stage.
Table 4 Isothermal crystallization of sulfonated telechelic ionomers. Run
Ionic content (mol.%)
Mw
Tm ( °C)
DHm (J/g)
Cryst time at 200 °C (min)
Cryst time at 205 °C (min)
PBT control 1 PBT control 2 1.5% telechelic 3% telechelic 5% telechelic 3% Random
0 0 1.4 2.8 4.8 3
87,900 43,000 56,600 45,800 29,700 12,300
223.7 222.3 223.0 222.7 222.8 222.7
41.3 42.9 44.7 42.8 42.5 37.9
1.5 1.0 1.3 1.5 2.1 29.4
7.4 4.4 7.1 8.9 12.1 >120
C. Berti et al. / Reactive & Functional Polymers 70 (2010) 366–375
Fig. 6. TGA curves of telechelic ionomers.
Fig. 7. Comparison of TGA curves in N2 for PBT telechelic and random ionomers.
Fig. 8. Mw decrease during hydrolytic stability tests of telechelic ionomers.
373
374
C. Berti et al. / Reactive & Functional Polymers 70 (2010) 366–375
Fig. 9. Carboxylic end-groups increase during hydrolytic stability tests for telechelic and random ionomers.
tions between polymer chains. Multiple Tg were observed according to the results reported by Hara et al. [28] for sodium salts of ionomeric polystyrene. 4. Conclusions Telechelic PBT ionomers have been prepared by melt synthesis. No side reaction occurs and the incorporation of the ionic groups is quantitative. The ionic groups act as chain-extension reversible links for telechelic ionomers while act as cross-links for random ionomers. Indeed, random ionomers present a consistently higher melt viscosity compared to telechelic ionomers and to PBT that does not allow to reach high molecular weight polymers. A buffer agent such as Na3PO4 has to be added to the catalyst in order to reduce THF formation and improve the polycondensation rate. By this way, telechelic ionomers can be obtained with Mw, thermal and hydrolytic stability comparable to those of commercial PBT and consistently higher respect to random ionomers. Therefore, telechelic ionomers are very promising materials to be used in the preparation of nanocomposites and in the interaction with dyes. Fig. 10. Storage modulus of telechelic ionomers.
Acknowledgement This work was financed by General Electric Company. References
Fig. 11. Tan-delta of telechelic ionomers.
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