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Preparation and association behavior of diblock copolymer ionomers based on poly(styrene-b-ethylene-co-propylene)
European Polymer Journal 36 (2000) 61±68
Preparation and association behavior of diblock copolymer ionomers based on poly(styrene-b-ethylene-co-propylene) Guangzhao Zhang, Lu Liu, Hongmu Wang, Ming Jiang* Institute of Macromolecular Science and Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, People's Republic of China Received 25 September 1998; received in revised form 12 January 1999; accepted 13 January 1999
Abstract Two kinds of diblock ionomer were prepared by carboxylating and sulfonating the polystyrene (PS) block of a commercial product of poly (styrene-block-ethylene-co-propylene) (SEP). The PS block was partially carboxylated via a mild Friedel±Crafts acetylation and a subsequent haloform oxidation. The carboxylation only slightly changed the molecular weight distribution of the SEP at low carboxylation levels typical for ionomers. Acetyl sulfate was used as the sulfonating agent for sulfonation. Both carboxylated SEP and sulfonated SEP ionomers with either lithium or zinc as counterions exhibit increased decomposition temperature and higher Tg of the PS blocks compared with their base resin as a result of intermolecular association due to the interactions of ionic groups. The ionomers show apparently increased viscosity than SEP. The data indicate that the sulfonate groups have stronger ability of association than the corresponding carboxylate groups, while zinc ions stronger than lithium ions. A combination of static and dynamic light scattering directly provides semi-quantitative information of association of the diblock ionomers, e.g., for the ionomers with functionality of 5.3 mol%, the association number was found to range from 8 to 17 depending on the nature of the cationic and anionic groups incorporated. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Block ionomers composed of a hydrocarbon block and an ionomeric block, have attracted much attention for years [1±6], because they combine the characteristics of ionomers and block copolymers together, and, hence, they have many unique morphologies, and solid and solution properties [5,6]. Block ionomers are also potentially useful in compatibilizing two otherwise incompatible polymers in industry. The research program about block ionomers in this
* Corresponding author. Tel.: +86-21-65643919; fax: +8621-65640293. E-mail address: [email protected] (M. Jiang)
laboratory has been encouraged by our ®ndings of the miscibility-complexation transition [7±14] and the formation of stable surfactant-free ionomer nanoparticles in aqueous media [15±18]. In a systematic study on polymer blends with controllable speci®c interactions such as hydrogen bonding [7±14] and ionic interaction [19±21], Jiang et al. found that for some polymer pairs consisting of a hydroxyl-containing polystyrene and a polymer containing carbonyl groups, a miscibility-complexation transition can be achieved as hydrogen bonding is intensi®ed [7±14]. The complexation of metal sulfonated poly(styrene-block-ethylene-co-butyleneblock-styrene) (SSEBS) triblock ionomers with pyridine-unit-containing copolymers due to ionic interaction has also been reported [20,21]. Meanwhile, it was also found that sulfonated PS (SPS), SSEBS and
0014-3057/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 0 4 8 - 8
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G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
carboxylated SEBS (CSEBS) ionomers were able to form surfactant-free nanoparticles which are stable in water [15±18]. However, these unusual aggregation and complexation behavior of ionomers still remain open questions. Poly(styrene-block-ethylene-co-propylene) (SEP), prepared by hydrogenation of polystyrene-block-polyisoprene, has many interesting solution and solid-state properties itself [22±26], e.g., it can form micelles in selective organic solvents [22,23]. It can also be used for compatibilizing immiscible polymer blends, because the two kinds of blocks, i.e., polystyrene (PS) and poly (ethylene-co-propylene) (EP), can be entirely, or partially compatible with PS homopolymer and various polyole®ns, respectively. However, the non-polar nature of the two blocks precludes its use in high-performance blends containing polar components. The introduction of ionic groups to the block copolymers is expected to render its compatibility with some polar polymers. However, to our knowledge, there has been a little work published on the functionalization and properties of the diblock copolymer. Weiss et al. reported small angle X-ray scattering studies on microstructures of sulfonated SEP and its ionomers [27,28]. As a part of our research program investigating the complexation and aggregation of block ionomers and their role in compatibilization, this study reports preparation and thermal as well solution properties of both carboxylated and sulfonated diblock ionomers based on the commercial product SEP which possesses a well de®ned diblock structure and a narrow molecular weight distribution. 2. Experimental 2.1. Materials SEP is a commercial diblock copolymer of Shell Co. with a styrene content of 29.0 wt% and Mw 1:3 105 , Mw =Mn 1:10, as determined by size exclusion chromatograph (SEC). Before use, SEP was dried under vacuum at 708C for 72 h. Lithium methylate solution was prepared by reaction of lithium with methanol followed by mixing it with toluene. Other chemicals were puri®ed as described previously [16,29]. 2.2. Carboxylation Carboxylation of SEP included two steps, i.e., acetylation and oxidation. A modi®ed procedure for acetylation of SEBS was used here [29]. The mixture of nitrobenzene/aluminium trichloride (PhNO2/AlCl3) complex and acetyl chloride together with CS2 as diluent was added into SEP solution in CS2 (1 g dlÿ1) at 08C. The reaction mixture was stirred at 08C for 2 h
and re¯uxed for another 0.5 h. The reaction was then terminated by addition of hydrochloric acid and ice. The acetylated SEP (ASEP) was then recovered by precipitation in ethanol with a yield near 100%. The oxidation of acetyl groups to yield carboxyl groups was performed using 5% sodium hypochlorite aqueous solution and cetyltrimethylammonium bromide as the oxidizing agent and phase transfer catalyst, respectively. The procedure was described in detail in the previous publication [29]. The yield were about 80%. 2.3. Sulfonation Sulfonation of SEP was conducted in 1,2-dichloroethane (DCE) employing acetyl sulfate as the sulfonating agent. Acetyl sulfate was prepared by the reaction of concentrated sulfuric acid with 30 mol% excess of acetic anhydride in DCE. The sulfonation was typically carried out as follows: 10 g of SEP was dissolved in 100 ml of DCE at ambient temperature and the solution was allowed to cool to 08C when fresh acetyl sulfate was added dropwise within 0.5 h. The reaction proceeded for 2 h at 08C and another 2 h at 358C. Finally, 2-propanol was added to terminate the reaction. The sulfonation level was controlled by the amount of the acetyl sulfate. The sulfonated SEP (SSEP) was recovered by pouring the reaction mixture into distilled water and ¯ashing o the solvents by rotary evaporation. The product was then washed for 0.5 h in boiling water for six times and dried for 1 day followed by being dissolved in toluene/methanol mixture and precipitated into excess of ethanol. The products were then dried under vacuum at 508C for 72 h. 2.4. Neutralization The lithium salts of CSEP and SSEP were prepared by titration of their solution in tetrahydrofuran (THF) to a phenolphthalein end point with a solution of lithium methylate in toluene/methanol (90/10 v/v) mixture. For producing zinc salts of CSEP and SSEP, we used the procedure developed by JeroÂme et al. [30], which ensures complete neutralization. 30% excess of zinc acetate dihydrate in toluene/methanol mixture was added to CSEP and SSEP solution in THF and toluene/methanol, respectively. After re¯uxing for 3 h, an appropriate amount of cyclohexane which forms azeotrope with acetic acid was added. The solvents were then removed on a rotary evaporator. The ionomer was then dissolved in THF and the excessive zinc acetate was isolated by high-speed centrifugation. The clear solution was rotor-evaporated nearly to dryness followed by precipitation and thoroughly drying under vacuum. The nomenclature used for the SSEP and CSEP
G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
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Table 1 Characterization data of CSEP Run
Mole ratio of feeda (%)
Carboxylation levelb (mol%)
Mw =Mn
1 2 3 4 5 6 7
0 10.0 12.0 14.0 18.0 36.0 48.0
0 2.8 3.6 5.3 7.5 20.8 28.2
1.10 1.17 1.19 1.19 1.23 1.26 1.27
a b
Molar ratio of acetyl chloride to the monomeric units of PS block in SEP. Determined by titration.
ionomers are x-MCSEP and x-MSSEP, respectively, where x is the carboxylation or sulfonation level in molar percent of the PS block and M designates the counterion, either H, Li or Zn. 2.5. Characterization 1
H-NMR spectra were acquired on a Bruker AMX400 NMR spectrometer with tetramethylsilane as the internal standard. FTIR spectra of thin polymer ®lms on potassium bromide plates were obtained on a Nicolet Magna 550 spectrophotometer. SEC measurements were performed using a Waters Model 510 Pump and ERMA refractive index detector, and a set of Polymer Stands Service columns with THF as the mobile phase at a ¯ow rate of 1.0 ml minÿ1. Dierential Scanning Calorimetry (DSC) measurement was carried out at a heating rate of 108C minÿ1 with a Shimadzu DSC-50 under nitrogen atmosphere. Tg was de®ned as the temperature corresponding to the midpoint of heat capacity change. A NETZSCH TG 209 Thermische Analyse (TGA) was used to assess the stability of polymers at a heating rate of 108C minÿ1 under nitrogen. The carboxylation and sulfonation levels of CSEP and SSEP were determined by titration of their solutions in THF and toluene/methanol mixture, respectively to the phenolphthalein end point with a standard solution of sodium methylate in a toluene/methanol mixed solvent (90/10 v/v) under a nitrogen blanket. Reduced solution viscosity measurements were made with an Ubbelodhe viscometer at 30:020:18C. A modi®ed commercial LLS spectrometer (ALV/SP-150) equipped with a solid-state laser (ADLAS DPY425II, output power 1 400 mW at l 532 nm) as the light source and an ALV-5000 multi-t digital correlator were used to evaluate the average molar mass and hydrodynamic radius distribution (f Rh ) of the ionomer in dilute solution, respectively. The measurements were performed at 25:020:18C. The ionomer and SEP solutions were clari®ed using 0.5-mm and 0.1-mm Millipore ®lters, respectively. The
measured time correlation functions were analyzed by the cumulates and Laplace inversion program (CONTIN) provided with the correlator. The speci®c refractive index increment (dn=dC) of LiCSEP, ZnCSEP, LiSSEP and ZnSSEP in THF was determined to be 0.169, 0.152, 0.178 and 0.161, respectively. More details about the LLS measurements can be found elsewhere [31,32].
3. Results and discussion 3.1. Synthesis 3.1.1. Carboxylation Although the carboxylation procedure consisting of Friedel±Crafts acetylation with a mild catalyst and a subsequent oxidation by means of phase transfer catalysis has been successfully applied to SEBS [29] and PS [33] without degradation or crosslinking of the chains, some modi®cation of the procedure is found necessary when it is used to diblock copolymer SEP. Preliminary experiments revealed that the reaction mixture should be stirred at 08C at least for 2 h before re¯uxing and a low polymer concentration of 1.0 g dlÿ1 should be used. Otherwise, physical gelation would occur at high acetylation level even at room temperature, and a wide molecular weight distribution of ®nal polymer resulted. As shown in Table 1, even under these mild conditions, the molecular weight distribution broadened slightly in the case of high acetylation level. This is probably due to the presence of microgel of SEP in the reaction mixture during the acetylation, as CS2 is probably not a good solvent for the EP blocks of SEP. The HCSEP and ASEP had been characterized by FTIR, 1 H-NMR and SEC. For ASEP, a sharp absorption band at 1685 cmÿ1 in FTIR spectrum and an absorption at about d 2:56 ppm in 1 H-NMR spectrum can be attributed to the carbonyl and methyl of the acetyl group, respectively. The FTIR spectra of CSEP exhibit the bands characteristic of free and
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G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
Fig. 1. Degree of sulfonation of SEP vs. mole ratio of acetyl sulfate in feed to monomeric units of PS block.
hydrogen-bonded carbonyl groups at 1728 and 1685 cmÿ1, respectively. In the 1 H-NMR spectrum of CSEP, a small resonance peak arising from carboxyl groups at about d 11:01 ppm appears and the absorption at about d 2:56 ppm disappears. All the facts support that the acetyl groups were introduced in PS blocks of SEP and the groups were transformed completely into carboxyl groups consequently. Since the solvent quality is not the same for the two blocks of CSEP and the carboxyls associate each other in THF leading to the change of radius of gyration of the chain, we can not obtain the absolute molecular weight for CSEP by SEC with polystyrene standards for calibration. However, it is believed the complex factors can be canceled out mostly in measuring polydispersity index Mw =Mn which could give the information about the eect of functioalization on the polymer chains [33,34]. The Mw =Mn values of CSEP samples are listed in Table 1. It shows the carboxylation only slightly changed the molecular weight distribution of SEP, especially at low carboxylation levels typical for ionomer. 3.1.2. Sulfonation Sulfonation of SEP was based on the approach developed by Makowski et al. [35] for sulfonating PS and later for SEBS [36]. In our early experiments, we found that the solution would darken during sulfonation and samples with high sulfonation level would be insoluble in any solvents after 1 year standing. However, by using excessive acetic anhydride in the preparation of acetyl sulfonate solution and conducting the sulfonation below 358C, we obtained samples with good solubility even after a time as long as a year. The data in Fig. 1 show that the sulfonation level can be regulated over the range from 1.2 to 11.8 mol% by changing the mole ratio of acetyl sulfate to styrene units. The eciency of sulfonation shown in Fig. 1 is relatively low, which can be mainly attributed to the
Fig. 2. DSC curves of SEP and some SEP ionomers: (a) SEP; (b) 5.3HCSEP; (c) 5.3HSSEP; (d) 5.3LiCSEP; (e) 5.3LiSSEP; (f) 5.3ZnCSEP; and (g) 5.3ZnSSEP.
low reaction temperature. HSSEP had not been characterized by 1 H-NMR and SEC because of no appropriate solvent for the 1 H-NMR and SEC measurements. The FTIR spectra of HSSEP exhibit the bands at 1012 cmÿ1 due to the in-plane bending vibrations of a sulfonated phenyl ring and 1127 cmÿ1 due to the sulfonate anion attached to a phenyl ring, respectively. 3.1.3. Neutralization Because the basicity of zinc acetate is weak, it is dif®cult to completely neutralize the functionalized SEP, especially CSEP, whose carboxyls are weak acid groups, when a stoichiometric amount of zinc acetate is used. However, by using excessive zinc acetate dihydrate as described in the experimental part, the neutralization was performed completely as evidenced by the disappearance of peaks at 1685 and 1728 cmÿ1 in the FTIR spectra of LiCSEP and ZnCSEP.
Fig. 3. TGA themograms of SEP and SEP ionomers: (a) SEP; (b) 5.3LiCSEP; (c) 5.3CSEP; (d) 5.3ZnCSEP; (e) 5.3HSSEP; (f) 5.3LiSSEP; and (g) 5.3ZnSSEP.
G. Zhang et al. / European Polymer Journal 36 (2000) 61±68 Table 2 Thermal properties of typical CSEP and SSEP ionomers Polymer
SEP 5.3HCSEP 5.3LiCSEP 5.3ZnCSEP 5.3HSSEP 5.3LiSSEP 5.3ZnSSEP
Tg of PS block
92 95 100 104 103 103 105
Decomposition temperature Tonset (8C)
T1/2 (8C)
250 150 150 150 250 250 250
355 407 407 423 445 451 455
3.1.4. Association in bulk The thermal properties of the produced ionomers were investigated by DSC and TGA. Figs. 2 and 3 show the results for some CSEP and SSEP ionomers with the same functionality (5.3 mol%) together with SEP. Tg and decomposition temperature are summarized in Table 2. All the samples show two glass transitions associated with EP rubber phase and PS glass phase, respectively. The functionalization of PS block does not show any eect on the Tg of the rubber phase (ÿ598C) as a consequence of phase separation between the EP blocks and ion-containing PS blocks in the ionomers. However, all the ionomer samples show Tg of the PS block higher than that of SEP. It is generally accepted that the increase in Tg is the result of ion aggregation acting as physical crosslinks in polymer chains [37]. In other words, the association of the diblock ionomer chains due to the ionic crosslinks restricts the segment motion in the PS blocks leading to the increase in Tg of their PS blocks. It was reported that for some ionomers such as poly (styrene-co-sodium methacrylate) and sodium sulfonated polystyrene, two Tg's, respectively associated with unclustered and clustered regions were observed by dynamic mechanical analysis [37±39]. However, in the present case, for PS phase, the upper Tg associated with the clustered region is not observed. This is probably because DSC is not sensitive enough to detect the response of the clustered region. In addition, the low PS phase content (029 wt%) and ion content should also be responsible for the absence of the upper Tg. Fig. 3 shows typical TGA results of SEP and the ionomers. For convenience, we use Tonset and T1/2 to evaluate and compare their thermal stability, where Tonset is de®ned as the temperature at which the weight loss just becomes detectable and T1/2 as the temperature for 50% weight loss under the conditions described in the experimental part. All Tonset and T1/2 data for SEP and the ionomers with a functionality of 5.3 mol% are listed in Table 2. Fig. 3 and Table 2
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show that SEP starts its decomposition at 2508C and has a T1/2 at 3558C. The sulfonated samples in acid, lithium and zinc forms show a remarkable improvement in the thermal stability, as indicated by the little weight loss before the major decomposition and T1/2 of 445, 451, 4558C, much higher than that of base resin. Because the introduction of ionic groups cannot change the stability of the C±C bond along the backbone, the signi®cant increase in the decomposition temperature of the sulfonated ionomers cannot be directly attributed to the ionic association structure. However, the association of ionic groups may lead to higher viscosity at high temperature, which will retard the diusion of the degradation products out of the polymer causing a higher decomposition temperature in TGA curves. For the carboxylated samples, the situation is relatively complicated. Due to the thermal sensibility of the carboxylic groups associated with decarboxylation and dehydration, just like the cases reported for some ionomers such as carboxylated poly(2,6-dimethyl-1,4phenylene oxide) and carboxylated polysulfone ionomers [40,41], the carboxylated SEP samples show Tonset apparently lower than their base resin. However, the reactions of the carbonyl groups only causes a minor weight loss. The presence of the ionic groups still play a signi®cant role in retarding the major decomposition as evidenced by the about 50±708C increment in T1/2 compared with that of SEP. Obviously, this improvement in thermal stability in CSEP ionomers is less profound than in SSEP ionomers, which can also be attributed to the dierence in association ability as found in the viscosity measurements below. 3.1.5. Association in solution The introduction of ionic groups to SEP shows noticeable eects on its solubility behavior due to the tremendous dierence in polarity between the ionic groups and hydrocarbon main chain. Similar to what is observed with other ionomers [36], all the functionalized SEP samples studied are insoluble in toluene, which is a good solvent for SEP, but soluble in toluene mixture containing a small portion (010 wt%) of methanol. This is understandable as methanol may eectively solvate the ionic species and consequently weaken the ionic association to promote the dissolution of the ionomer chains [42]. THF as a solvent with a weak polarity was also found to be able to dissolve all the SEP ionomers except HSSEP, indicating that intermolecular association resulting from hydrogen bonding of sulfonic acid is stronger than the ionic association. Similar behavior has also been observed for SSEBS ionomers [36]. It should be noted that neither toluene/methanol mixed solvent nor THF can dissolve the SEP ionomers completely to produce solutions with concentration higher than about 1 g dlÿ1.
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G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
Fig. 4. Reduced viscosity vs. concentration for SEP and CSEP ionomers and in THF at 308C: (Q) SEP; (*) 2.8LiCSEP; (R) 2.8ZnCSEP; (T) 5.3LiCSEP; (W) 5.3ZnCSEP; (r) 7.5LiCSEP; (w) 7.5ZnCSEP.
All these facts indicate that the diblock ionomer chains form association in the weakly polar solvents. The reduced solution viscosities of CSEP and SSEP ionomers together with SEP in THF as a function of concentration are shown in Figs. 4 and 5, respectively. All the ionomer solutions exhibit a pronounced increase of viscosity compared with that of SEP as a result of the association of ionomer chains in the nonpolar environment due to the strong intermolecular ionic interaction between the solvated ionic pairs. Actually, in the ionomer solutions, associated chains instead of a single isolated chain serve as the basic ¯ow unit. Then, it is understandable that the higher level of carboxylation or sulfonation, the higher the viscosity of ionomer solutions. On the basis of the same argument, we can understand why the viscosity of a CSEP or SSEP zinc salt is higher than that of the corresponding lithium salt also as shown in Figs. 5 and 6, namely, divalent zinc ions can lead to a stronger association than the monovalent lithium ions for either
Fig. 5. Reduced viscosity vs. concentration for SEP and SSEP ionomers in THF at 308C: (Q) SEP; (*) 3.4LiSSEP; (R) 3.4ZnSSEP; (T) 5.3LiSSEP; (W) 5.3ZnSSEP; (r) 7.5LiSSEP; (w) 7.5ZnSSEP.
Fig. 6. Hydrodynamic radius distributions f Rh of SEP and SEP ionomers in THF. (W) SEP; (r) 5.3LiCSEP; (w) 5.3 ZnCSEP; (*) 5.3LiSSEP; (Q) 5.3ZnSSEP.
SSEP or CSEP ionomers. A comparison of Figs. 4 and 5 reveals that the viscosity of a SSEP ionomer is always higher than that of the corresponding CSEP ionomer with the same functionality and counterion. It indicates that the sulfonate groups can result in an stronger association than carboxyl groups. This result is in agreement with what we found for the eect of ionic groups on the thermal stability of the ionomers mentioned above. Another point here is that no crossover was found for both CSEP and SSEP ionomer solutions. It is well known that random ionomer solutions such as sulfonated polystyrene [42] in non-polar solvents have higher viscosity than that of the corresponding base resin over a broad concentration range but lower viscosities at very low concentrations. Therefore, the curves of the viscosity versus concentration of the random ionomers crossover that of the base resin. This low viscosity of the ionomers at very low concentrations has been attributed to the intramolecular association causing chain contraction overcoming the intermolecular association. However, for the block ionomers based on SEP, shown in Figs. 4 and 5, and those on SEBS reported by Weiss et al. [36], no crossover was found. In our opinion, the absence of crossover, does not exclude the occurrence of intramolecular association. Because the ionomeric PS block is only small (less than 30 wt%) portion of the block copolymer, the chain contraction of the PS block due to the intramolecular association is dicult and can only slightly aect the overall size of the copolymer chain as the ¯ow unit unless the ionomer has a high functionality or counterions leading to strong association. In fact, among a series of SEBS-based ionomers with various counterions, it was found that solutions of the cupric and nickel salts which display the strongest intermolecular association at high concentration show the crossover [43]. Laser light scattering can provide direct and semiquantitative information about the chain association.
G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
In this study, the association of both SSEP and CSEP ionomers in dilute solution were investigated by a combination of static and dynamic LLS. Fig. 6 shows f Rh of SEP and some SEP ionomers with the same functionality of 5.3 mol% in THF. The parent copolymer SEP, as expected, shows very narrow f Rh with average Rh being about 11 nm. Quite dierent from SEP, the ionomers show much broader distributions extending to 180 nm with average Rh about 20±30 nm. Furthermore, the static laser light reveals that the apparent average molar weights of 5.3LiCSEP, 5.3LiSSEP, 5.3ZnCSEP and 5.3ZnSSEP are 1:06 106 , 1:27 106 , 1:87 106 and 2:19 106 , respectively. Here, we de®ne association number as the average number of chains in a associated aggregate. It is known that when intramolecular association is predominant, ionomer chains exist as individual chain, so the association number should be about 1, whereas in the opposite case, ionomer chain exist as multichains due to intermolecular association, leading to an association number more than one. Thus, in comparison with the average weight of SEP (1:3 105 ), the apparent average association numbers of the above ionomer chains are found to be 8, 10, 14, 17, respectively. Obviously, both static and dynamic laser light scattering results indicate that the all ionomer chains associate in THF. The data also clearly reveal that the divalent zinc ions possess a stronger association than the monovalent lithium ions do, while sulfonate groups stronger than carboxylate groups. This result con®rms the conclusions drawn from the viscosity and thermal behavior studies mentioned above. It should be noted that the concentration of ionomer solutions in THF is as low as 10ÿ2 g dlÿ1, the LLS result reveals that the intermolecular association is still predominant over intramolecular association even at this low concentration. Similar results were also reported elsewhere [44,45]. This indicates that the absence of crossover observed in the viscosity measurements should result from the absent transition from inter- to intramolecular association at low concentration.
Acknowledgements We gratefully acknowledge the support of this research by the National Basic Research Project: `Macromolecular Condensed State Programme', the National Natural Science Foundation of China (Grant No. 29574154), and the Doctoral Programme Foundation of Institution High Education. We are also indebted to Prof. Chi Wu, The Chinese University of Hong Kong for his support in the LLS studies.
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Preparation and association behavior of diblock copolymer ionomers based on poly(styrene-b-ethylene-co-propylene) Guangzhao Zhang, Lu Liu, Hongmu Wang, Ming Jiang* Institute of Macromolecular Science and Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, People's Republic of China Received 25 September 1998; received in revised form 12 January 1999; accepted 13 January 1999
Abstract Two kinds of diblock ionomer were prepared by carboxylating and sulfonating the polystyrene (PS) block of a commercial product of poly (styrene-block-ethylene-co-propylene) (SEP). The PS block was partially carboxylated via a mild Friedel±Crafts acetylation and a subsequent haloform oxidation. The carboxylation only slightly changed the molecular weight distribution of the SEP at low carboxylation levels typical for ionomers. Acetyl sulfate was used as the sulfonating agent for sulfonation. Both carboxylated SEP and sulfonated SEP ionomers with either lithium or zinc as counterions exhibit increased decomposition temperature and higher Tg of the PS blocks compared with their base resin as a result of intermolecular association due to the interactions of ionic groups. The ionomers show apparently increased viscosity than SEP. The data indicate that the sulfonate groups have stronger ability of association than the corresponding carboxylate groups, while zinc ions stronger than lithium ions. A combination of static and dynamic light scattering directly provides semi-quantitative information of association of the diblock ionomers, e.g., for the ionomers with functionality of 5.3 mol%, the association number was found to range from 8 to 17 depending on the nature of the cationic and anionic groups incorporated. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Block ionomers composed of a hydrocarbon block and an ionomeric block, have attracted much attention for years [1±6], because they combine the characteristics of ionomers and block copolymers together, and, hence, they have many unique morphologies, and solid and solution properties [5,6]. Block ionomers are also potentially useful in compatibilizing two otherwise incompatible polymers in industry. The research program about block ionomers in this
* Corresponding author. Tel.: +86-21-65643919; fax: +8621-65640293. E-mail address: [email protected] (M. Jiang)
laboratory has been encouraged by our ®ndings of the miscibility-complexation transition [7±14] and the formation of stable surfactant-free ionomer nanoparticles in aqueous media [15±18]. In a systematic study on polymer blends with controllable speci®c interactions such as hydrogen bonding [7±14] and ionic interaction [19±21], Jiang et al. found that for some polymer pairs consisting of a hydroxyl-containing polystyrene and a polymer containing carbonyl groups, a miscibility-complexation transition can be achieved as hydrogen bonding is intensi®ed [7±14]. The complexation of metal sulfonated poly(styrene-block-ethylene-co-butyleneblock-styrene) (SSEBS) triblock ionomers with pyridine-unit-containing copolymers due to ionic interaction has also been reported [20,21]. Meanwhile, it was also found that sulfonated PS (SPS), SSEBS and
0014-3057/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 0 4 8 - 8
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carboxylated SEBS (CSEBS) ionomers were able to form surfactant-free nanoparticles which are stable in water [15±18]. However, these unusual aggregation and complexation behavior of ionomers still remain open questions. Poly(styrene-block-ethylene-co-propylene) (SEP), prepared by hydrogenation of polystyrene-block-polyisoprene, has many interesting solution and solid-state properties itself [22±26], e.g., it can form micelles in selective organic solvents [22,23]. It can also be used for compatibilizing immiscible polymer blends, because the two kinds of blocks, i.e., polystyrene (PS) and poly (ethylene-co-propylene) (EP), can be entirely, or partially compatible with PS homopolymer and various polyole®ns, respectively. However, the non-polar nature of the two blocks precludes its use in high-performance blends containing polar components. The introduction of ionic groups to the block copolymers is expected to render its compatibility with some polar polymers. However, to our knowledge, there has been a little work published on the functionalization and properties of the diblock copolymer. Weiss et al. reported small angle X-ray scattering studies on microstructures of sulfonated SEP and its ionomers [27,28]. As a part of our research program investigating the complexation and aggregation of block ionomers and their role in compatibilization, this study reports preparation and thermal as well solution properties of both carboxylated and sulfonated diblock ionomers based on the commercial product SEP which possesses a well de®ned diblock structure and a narrow molecular weight distribution. 2. Experimental 2.1. Materials SEP is a commercial diblock copolymer of Shell Co. with a styrene content of 29.0 wt% and Mw 1:3 105 , Mw =Mn 1:10, as determined by size exclusion chromatograph (SEC). Before use, SEP was dried under vacuum at 708C for 72 h. Lithium methylate solution was prepared by reaction of lithium with methanol followed by mixing it with toluene. Other chemicals were puri®ed as described previously [16,29]. 2.2. Carboxylation Carboxylation of SEP included two steps, i.e., acetylation and oxidation. A modi®ed procedure for acetylation of SEBS was used here [29]. The mixture of nitrobenzene/aluminium trichloride (PhNO2/AlCl3) complex and acetyl chloride together with CS2 as diluent was added into SEP solution in CS2 (1 g dlÿ1) at 08C. The reaction mixture was stirred at 08C for 2 h
and re¯uxed for another 0.5 h. The reaction was then terminated by addition of hydrochloric acid and ice. The acetylated SEP (ASEP) was then recovered by precipitation in ethanol with a yield near 100%. The oxidation of acetyl groups to yield carboxyl groups was performed using 5% sodium hypochlorite aqueous solution and cetyltrimethylammonium bromide as the oxidizing agent and phase transfer catalyst, respectively. The procedure was described in detail in the previous publication [29]. The yield were about 80%. 2.3. Sulfonation Sulfonation of SEP was conducted in 1,2-dichloroethane (DCE) employing acetyl sulfate as the sulfonating agent. Acetyl sulfate was prepared by the reaction of concentrated sulfuric acid with 30 mol% excess of acetic anhydride in DCE. The sulfonation was typically carried out as follows: 10 g of SEP was dissolved in 100 ml of DCE at ambient temperature and the solution was allowed to cool to 08C when fresh acetyl sulfate was added dropwise within 0.5 h. The reaction proceeded for 2 h at 08C and another 2 h at 358C. Finally, 2-propanol was added to terminate the reaction. The sulfonation level was controlled by the amount of the acetyl sulfate. The sulfonated SEP (SSEP) was recovered by pouring the reaction mixture into distilled water and ¯ashing o the solvents by rotary evaporation. The product was then washed for 0.5 h in boiling water for six times and dried for 1 day followed by being dissolved in toluene/methanol mixture and precipitated into excess of ethanol. The products were then dried under vacuum at 508C for 72 h. 2.4. Neutralization The lithium salts of CSEP and SSEP were prepared by titration of their solution in tetrahydrofuran (THF) to a phenolphthalein end point with a solution of lithium methylate in toluene/methanol (90/10 v/v) mixture. For producing zinc salts of CSEP and SSEP, we used the procedure developed by JeroÂme et al. [30], which ensures complete neutralization. 30% excess of zinc acetate dihydrate in toluene/methanol mixture was added to CSEP and SSEP solution in THF and toluene/methanol, respectively. After re¯uxing for 3 h, an appropriate amount of cyclohexane which forms azeotrope with acetic acid was added. The solvents were then removed on a rotary evaporator. The ionomer was then dissolved in THF and the excessive zinc acetate was isolated by high-speed centrifugation. The clear solution was rotor-evaporated nearly to dryness followed by precipitation and thoroughly drying under vacuum. The nomenclature used for the SSEP and CSEP
G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
63
Table 1 Characterization data of CSEP Run
Mole ratio of feeda (%)
Carboxylation levelb (mol%)
Mw =Mn
1 2 3 4 5 6 7
0 10.0 12.0 14.0 18.0 36.0 48.0
0 2.8 3.6 5.3 7.5 20.8 28.2
1.10 1.17 1.19 1.19 1.23 1.26 1.27
a b
Molar ratio of acetyl chloride to the monomeric units of PS block in SEP. Determined by titration.
ionomers are x-MCSEP and x-MSSEP, respectively, where x is the carboxylation or sulfonation level in molar percent of the PS block and M designates the counterion, either H, Li or Zn. 2.5. Characterization 1
H-NMR spectra were acquired on a Bruker AMX400 NMR spectrometer with tetramethylsilane as the internal standard. FTIR spectra of thin polymer ®lms on potassium bromide plates were obtained on a Nicolet Magna 550 spectrophotometer. SEC measurements were performed using a Waters Model 510 Pump and ERMA refractive index detector, and a set of Polymer Stands Service columns with THF as the mobile phase at a ¯ow rate of 1.0 ml minÿ1. Dierential Scanning Calorimetry (DSC) measurement was carried out at a heating rate of 108C minÿ1 with a Shimadzu DSC-50 under nitrogen atmosphere. Tg was de®ned as the temperature corresponding to the midpoint of heat capacity change. A NETZSCH TG 209 Thermische Analyse (TGA) was used to assess the stability of polymers at a heating rate of 108C minÿ1 under nitrogen. The carboxylation and sulfonation levels of CSEP and SSEP were determined by titration of their solutions in THF and toluene/methanol mixture, respectively to the phenolphthalein end point with a standard solution of sodium methylate in a toluene/methanol mixed solvent (90/10 v/v) under a nitrogen blanket. Reduced solution viscosity measurements were made with an Ubbelodhe viscometer at 30:020:18C. A modi®ed commercial LLS spectrometer (ALV/SP-150) equipped with a solid-state laser (ADLAS DPY425II, output power 1 400 mW at l 532 nm) as the light source and an ALV-5000 multi-t digital correlator were used to evaluate the average molar mass and hydrodynamic radius distribution (f Rh ) of the ionomer in dilute solution, respectively. The measurements were performed at 25:020:18C. The ionomer and SEP solutions were clari®ed using 0.5-mm and 0.1-mm Millipore ®lters, respectively. The
measured time correlation functions were analyzed by the cumulates and Laplace inversion program (CONTIN) provided with the correlator. The speci®c refractive index increment (dn=dC) of LiCSEP, ZnCSEP, LiSSEP and ZnSSEP in THF was determined to be 0.169, 0.152, 0.178 and 0.161, respectively. More details about the LLS measurements can be found elsewhere [31,32].
3. Results and discussion 3.1. Synthesis 3.1.1. Carboxylation Although the carboxylation procedure consisting of Friedel±Crafts acetylation with a mild catalyst and a subsequent oxidation by means of phase transfer catalysis has been successfully applied to SEBS [29] and PS [33] without degradation or crosslinking of the chains, some modi®cation of the procedure is found necessary when it is used to diblock copolymer SEP. Preliminary experiments revealed that the reaction mixture should be stirred at 08C at least for 2 h before re¯uxing and a low polymer concentration of 1.0 g dlÿ1 should be used. Otherwise, physical gelation would occur at high acetylation level even at room temperature, and a wide molecular weight distribution of ®nal polymer resulted. As shown in Table 1, even under these mild conditions, the molecular weight distribution broadened slightly in the case of high acetylation level. This is probably due to the presence of microgel of SEP in the reaction mixture during the acetylation, as CS2 is probably not a good solvent for the EP blocks of SEP. The HCSEP and ASEP had been characterized by FTIR, 1 H-NMR and SEC. For ASEP, a sharp absorption band at 1685 cmÿ1 in FTIR spectrum and an absorption at about d 2:56 ppm in 1 H-NMR spectrum can be attributed to the carbonyl and methyl of the acetyl group, respectively. The FTIR spectra of CSEP exhibit the bands characteristic of free and
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G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
Fig. 1. Degree of sulfonation of SEP vs. mole ratio of acetyl sulfate in feed to monomeric units of PS block.
hydrogen-bonded carbonyl groups at 1728 and 1685 cmÿ1, respectively. In the 1 H-NMR spectrum of CSEP, a small resonance peak arising from carboxyl groups at about d 11:01 ppm appears and the absorption at about d 2:56 ppm disappears. All the facts support that the acetyl groups were introduced in PS blocks of SEP and the groups were transformed completely into carboxyl groups consequently. Since the solvent quality is not the same for the two blocks of CSEP and the carboxyls associate each other in THF leading to the change of radius of gyration of the chain, we can not obtain the absolute molecular weight for CSEP by SEC with polystyrene standards for calibration. However, it is believed the complex factors can be canceled out mostly in measuring polydispersity index Mw =Mn which could give the information about the eect of functioalization on the polymer chains [33,34]. The Mw =Mn values of CSEP samples are listed in Table 1. It shows the carboxylation only slightly changed the molecular weight distribution of SEP, especially at low carboxylation levels typical for ionomer. 3.1.2. Sulfonation Sulfonation of SEP was based on the approach developed by Makowski et al. [35] for sulfonating PS and later for SEBS [36]. In our early experiments, we found that the solution would darken during sulfonation and samples with high sulfonation level would be insoluble in any solvents after 1 year standing. However, by using excessive acetic anhydride in the preparation of acetyl sulfonate solution and conducting the sulfonation below 358C, we obtained samples with good solubility even after a time as long as a year. The data in Fig. 1 show that the sulfonation level can be regulated over the range from 1.2 to 11.8 mol% by changing the mole ratio of acetyl sulfate to styrene units. The eciency of sulfonation shown in Fig. 1 is relatively low, which can be mainly attributed to the
Fig. 2. DSC curves of SEP and some SEP ionomers: (a) SEP; (b) 5.3HCSEP; (c) 5.3HSSEP; (d) 5.3LiCSEP; (e) 5.3LiSSEP; (f) 5.3ZnCSEP; and (g) 5.3ZnSSEP.
low reaction temperature. HSSEP had not been characterized by 1 H-NMR and SEC because of no appropriate solvent for the 1 H-NMR and SEC measurements. The FTIR spectra of HSSEP exhibit the bands at 1012 cmÿ1 due to the in-plane bending vibrations of a sulfonated phenyl ring and 1127 cmÿ1 due to the sulfonate anion attached to a phenyl ring, respectively. 3.1.3. Neutralization Because the basicity of zinc acetate is weak, it is dif®cult to completely neutralize the functionalized SEP, especially CSEP, whose carboxyls are weak acid groups, when a stoichiometric amount of zinc acetate is used. However, by using excessive zinc acetate dihydrate as described in the experimental part, the neutralization was performed completely as evidenced by the disappearance of peaks at 1685 and 1728 cmÿ1 in the FTIR spectra of LiCSEP and ZnCSEP.
Fig. 3. TGA themograms of SEP and SEP ionomers: (a) SEP; (b) 5.3LiCSEP; (c) 5.3CSEP; (d) 5.3ZnCSEP; (e) 5.3HSSEP; (f) 5.3LiSSEP; and (g) 5.3ZnSSEP.
G. Zhang et al. / European Polymer Journal 36 (2000) 61±68 Table 2 Thermal properties of typical CSEP and SSEP ionomers Polymer
SEP 5.3HCSEP 5.3LiCSEP 5.3ZnCSEP 5.3HSSEP 5.3LiSSEP 5.3ZnSSEP
Tg of PS block
92 95 100 104 103 103 105
Decomposition temperature Tonset (8C)
T1/2 (8C)
250 150 150 150 250 250 250
355 407 407 423 445 451 455
3.1.4. Association in bulk The thermal properties of the produced ionomers were investigated by DSC and TGA. Figs. 2 and 3 show the results for some CSEP and SSEP ionomers with the same functionality (5.3 mol%) together with SEP. Tg and decomposition temperature are summarized in Table 2. All the samples show two glass transitions associated with EP rubber phase and PS glass phase, respectively. The functionalization of PS block does not show any eect on the Tg of the rubber phase (ÿ598C) as a consequence of phase separation between the EP blocks and ion-containing PS blocks in the ionomers. However, all the ionomer samples show Tg of the PS block higher than that of SEP. It is generally accepted that the increase in Tg is the result of ion aggregation acting as physical crosslinks in polymer chains [37]. In other words, the association of the diblock ionomer chains due to the ionic crosslinks restricts the segment motion in the PS blocks leading to the increase in Tg of their PS blocks. It was reported that for some ionomers such as poly (styrene-co-sodium methacrylate) and sodium sulfonated polystyrene, two Tg's, respectively associated with unclustered and clustered regions were observed by dynamic mechanical analysis [37±39]. However, in the present case, for PS phase, the upper Tg associated with the clustered region is not observed. This is probably because DSC is not sensitive enough to detect the response of the clustered region. In addition, the low PS phase content (029 wt%) and ion content should also be responsible for the absence of the upper Tg. Fig. 3 shows typical TGA results of SEP and the ionomers. For convenience, we use Tonset and T1/2 to evaluate and compare their thermal stability, where Tonset is de®ned as the temperature at which the weight loss just becomes detectable and T1/2 as the temperature for 50% weight loss under the conditions described in the experimental part. All Tonset and T1/2 data for SEP and the ionomers with a functionality of 5.3 mol% are listed in Table 2. Fig. 3 and Table 2
65
show that SEP starts its decomposition at 2508C and has a T1/2 at 3558C. The sulfonated samples in acid, lithium and zinc forms show a remarkable improvement in the thermal stability, as indicated by the little weight loss before the major decomposition and T1/2 of 445, 451, 4558C, much higher than that of base resin. Because the introduction of ionic groups cannot change the stability of the C±C bond along the backbone, the signi®cant increase in the decomposition temperature of the sulfonated ionomers cannot be directly attributed to the ionic association structure. However, the association of ionic groups may lead to higher viscosity at high temperature, which will retard the diusion of the degradation products out of the polymer causing a higher decomposition temperature in TGA curves. For the carboxylated samples, the situation is relatively complicated. Due to the thermal sensibility of the carboxylic groups associated with decarboxylation and dehydration, just like the cases reported for some ionomers such as carboxylated poly(2,6-dimethyl-1,4phenylene oxide) and carboxylated polysulfone ionomers [40,41], the carboxylated SEP samples show Tonset apparently lower than their base resin. However, the reactions of the carbonyl groups only causes a minor weight loss. The presence of the ionic groups still play a signi®cant role in retarding the major decomposition as evidenced by the about 50±708C increment in T1/2 compared with that of SEP. Obviously, this improvement in thermal stability in CSEP ionomers is less profound than in SSEP ionomers, which can also be attributed to the dierence in association ability as found in the viscosity measurements below. 3.1.5. Association in solution The introduction of ionic groups to SEP shows noticeable eects on its solubility behavior due to the tremendous dierence in polarity between the ionic groups and hydrocarbon main chain. Similar to what is observed with other ionomers [36], all the functionalized SEP samples studied are insoluble in toluene, which is a good solvent for SEP, but soluble in toluene mixture containing a small portion (010 wt%) of methanol. This is understandable as methanol may eectively solvate the ionic species and consequently weaken the ionic association to promote the dissolution of the ionomer chains [42]. THF as a solvent with a weak polarity was also found to be able to dissolve all the SEP ionomers except HSSEP, indicating that intermolecular association resulting from hydrogen bonding of sulfonic acid is stronger than the ionic association. Similar behavior has also been observed for SSEBS ionomers [36]. It should be noted that neither toluene/methanol mixed solvent nor THF can dissolve the SEP ionomers completely to produce solutions with concentration higher than about 1 g dlÿ1.
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G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
Fig. 4. Reduced viscosity vs. concentration for SEP and CSEP ionomers and in THF at 308C: (Q) SEP; (*) 2.8LiCSEP; (R) 2.8ZnCSEP; (T) 5.3LiCSEP; (W) 5.3ZnCSEP; (r) 7.5LiCSEP; (w) 7.5ZnCSEP.
All these facts indicate that the diblock ionomer chains form association in the weakly polar solvents. The reduced solution viscosities of CSEP and SSEP ionomers together with SEP in THF as a function of concentration are shown in Figs. 4 and 5, respectively. All the ionomer solutions exhibit a pronounced increase of viscosity compared with that of SEP as a result of the association of ionomer chains in the nonpolar environment due to the strong intermolecular ionic interaction between the solvated ionic pairs. Actually, in the ionomer solutions, associated chains instead of a single isolated chain serve as the basic ¯ow unit. Then, it is understandable that the higher level of carboxylation or sulfonation, the higher the viscosity of ionomer solutions. On the basis of the same argument, we can understand why the viscosity of a CSEP or SSEP zinc salt is higher than that of the corresponding lithium salt also as shown in Figs. 5 and 6, namely, divalent zinc ions can lead to a stronger association than the monovalent lithium ions for either
Fig. 5. Reduced viscosity vs. concentration for SEP and SSEP ionomers in THF at 308C: (Q) SEP; (*) 3.4LiSSEP; (R) 3.4ZnSSEP; (T) 5.3LiSSEP; (W) 5.3ZnSSEP; (r) 7.5LiSSEP; (w) 7.5ZnSSEP.
Fig. 6. Hydrodynamic radius distributions f Rh of SEP and SEP ionomers in THF. (W) SEP; (r) 5.3LiCSEP; (w) 5.3 ZnCSEP; (*) 5.3LiSSEP; (Q) 5.3ZnSSEP.
SSEP or CSEP ionomers. A comparison of Figs. 4 and 5 reveals that the viscosity of a SSEP ionomer is always higher than that of the corresponding CSEP ionomer with the same functionality and counterion. It indicates that the sulfonate groups can result in an stronger association than carboxyl groups. This result is in agreement with what we found for the eect of ionic groups on the thermal stability of the ionomers mentioned above. Another point here is that no crossover was found for both CSEP and SSEP ionomer solutions. It is well known that random ionomer solutions such as sulfonated polystyrene [42] in non-polar solvents have higher viscosity than that of the corresponding base resin over a broad concentration range but lower viscosities at very low concentrations. Therefore, the curves of the viscosity versus concentration of the random ionomers crossover that of the base resin. This low viscosity of the ionomers at very low concentrations has been attributed to the intramolecular association causing chain contraction overcoming the intermolecular association. However, for the block ionomers based on SEP, shown in Figs. 4 and 5, and those on SEBS reported by Weiss et al. [36], no crossover was found. In our opinion, the absence of crossover, does not exclude the occurrence of intramolecular association. Because the ionomeric PS block is only small (less than 30 wt%) portion of the block copolymer, the chain contraction of the PS block due to the intramolecular association is dicult and can only slightly aect the overall size of the copolymer chain as the ¯ow unit unless the ionomer has a high functionality or counterions leading to strong association. In fact, among a series of SEBS-based ionomers with various counterions, it was found that solutions of the cupric and nickel salts which display the strongest intermolecular association at high concentration show the crossover [43]. Laser light scattering can provide direct and semiquantitative information about the chain association.
G. Zhang et al. / European Polymer Journal 36 (2000) 61±68
In this study, the association of both SSEP and CSEP ionomers in dilute solution were investigated by a combination of static and dynamic LLS. Fig. 6 shows f Rh of SEP and some SEP ionomers with the same functionality of 5.3 mol% in THF. The parent copolymer SEP, as expected, shows very narrow f Rh with average Rh being about 11 nm. Quite dierent from SEP, the ionomers show much broader distributions extending to 180 nm with average Rh about 20±30 nm. Furthermore, the static laser light reveals that the apparent average molar weights of 5.3LiCSEP, 5.3LiSSEP, 5.3ZnCSEP and 5.3ZnSSEP are 1:06 106 , 1:27 106 , 1:87 106 and 2:19 106 , respectively. Here, we de®ne association number as the average number of chains in a associated aggregate. It is known that when intramolecular association is predominant, ionomer chains exist as individual chain, so the association number should be about 1, whereas in the opposite case, ionomer chain exist as multichains due to intermolecular association, leading to an association number more than one. Thus, in comparison with the average weight of SEP (1:3 105 ), the apparent average association numbers of the above ionomer chains are found to be 8, 10, 14, 17, respectively. Obviously, both static and dynamic laser light scattering results indicate that the all ionomer chains associate in THF. The data also clearly reveal that the divalent zinc ions possess a stronger association than the monovalent lithium ions do, while sulfonate groups stronger than carboxylate groups. This result con®rms the conclusions drawn from the viscosity and thermal behavior studies mentioned above. It should be noted that the concentration of ionomer solutions in THF is as low as 10ÿ2 g dlÿ1, the LLS result reveals that the intermolecular association is still predominant over intramolecular association even at this low concentration. Similar results were also reported elsewhere [44,45]. This indicates that the absence of crossover observed in the viscosity measurements should result from the absent transition from inter- to intramolecular association at low concentration.
Acknowledgements We gratefully acknowledge the support of this research by the National Basic Research Project: `Macromolecular Condensed State Programme', the National Natural Science Foundation of China (Grant No. 29574154), and the Doctoral Programme Foundation of Institution High Education. We are also indebted to Prof. Chi Wu, The Chinese University of Hong Kong for his support in the LLS studies.
67
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