Complexation between poly(acrylic acid) and poly(vinylpyrrolidone): Influence of the molecular weight of poly(acrylic acid) and small molecule salt on the complexation

EUROPEAN POLYMER JOURNAL

European Polymer Journal 41 (2005) 2406–2415

www.elsevier.com/locate/europolj

Complexation between poly(acrylic acid) and poly(vinylpyrrolidone): Influence of the molecular weight of poly(acrylic acid) and small molecule salt on the complexation Shuping Jin a

a,b

, Mingzhu Liu

a,*

, Shilan Chen a, Yong Chen

a

Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China b Department of Chemistry, Hexi University, Zhangye 734000, PR China

Received 10 December 2004; received in revised form 29 April 2005; accepted 4 May 2005 Available online 18 July 2005

Abstract Poly(acrylic acid) (PAA) with different molecular weight and poly(vinylpyrrolidone) (PVP) were prepared by free radical polymerization using 2,2 0 -azoisobutyronitrile (AIBN) as initiator in anhydrous methanol for PAA, and in distilled water for PVP. Then, the complexation between PAA and PVP in aqueous solution was studied by UV transmittance measurement and fluorescence probe technique. The result shows that (1) at low pH, the formation of complexation between PAA and PVP bases on the intermacromolecular hydrogen bond and the composition of the formed complex is around 3:2 (the unit molar ratio of PAA to PVP) at pH 2.60 over the range of pH investigated. (2) The cooperative interaction through the formation of hydrogen bond among active sites plays an important role in complex formation, and depends on the pH of solution, the required minimum chain length of poly(acrylic acid). (3) The hydrogen bond is not affected by small molecular salt, which only affects those carboxylic groups without forming hydrogen bond on the PAA chain.
2005 Elsevier Ltd. All rights reserved. Keywords: Poly(acrylic acid); Poly(vinylpyrrolidone); Complexation; Transmittance; Fluorescence probe; Cooperative interaction

1. Introduction It is well known that in an aqueous solution, polybases such as poly(oxyethylene) (PEO) or poly(vinylpyrrolidone) (PVP) can form polymer complexes with polyacids like poly(acrylic acid) (PAA) or poly(metha-

* Corresponding author. Tel.: +86 931 8912387; fax: +86 931 8912582. E-mail address: [email protected] (M.Z. Liu).

crylic acid) (PMAA). The stabilization of such complexes is due to the cooperative interaction through the formation of hydrogen bonds between acid (as hydrogen bond donors) and base (as hydrogen bond acceptors) groups on the polymer chain [1–7]. When poly(vinylpyrrolidone) and poly(acrylic acid) or poly(methacrylic acid) solution are mixed, the hydrophilic carbonyl groups of poly(vinylpyrrolidone) and the carboxylic groups of the polyacid are ‘‘locked’’ by a cooperative mechanism through hydrogen bond. It has been suggested that the polymer complex precipitates if the

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2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.05.006

J. Shuping et al. / European Polymer Journal 41 (2005) 2406–2415

concentration is high enough because of the hydrophobic influence of the vinyl backbones and the side methyl groups on the poly(methacrylic acid) chain [8,9]. Papisov et al. investigated the importance of hydrophobic interaction in polycomplex formation by calorimeter measurement [10]. They found that hydrophobic interaction is also significant in PAA/PVP complexes due to the carbon–carbon backbone inherent hydrophobicity. It is worth noting that the complexation between poly(vinylpyrrolidone) and poly(acrylic acid) or poly(methacrylic acid) involves two types of equilibrium intramolecular dissociation of the polyacid (Eq. (1)) and the formation of intermacromolecular hydrogen bond between undissociated carboxylic groups and carbonyl groups (Eq. (2)) [11]: H

COOH

ð1Þ

COO

COOH

O

N

COO

H

O

N

ð2Þ Therefore, there is a dramatic influence of the number of the undissociated and complexible acid sequence, this means that a minimum chain length of the interactive chains is required for the formation of a stable complex between PAA and PVP [1,2,12]: A polyacid, which forms a complex with a long chain of polybase, must have a degree of polymerization larger than a critical number corresponding to the minimum chain length. From a thermodynamic point of view, the formation of one noncovalent bond (hydrogen bond, electrostatic interaction) between two macromolecular species in solution is quite improbable, while the formation of a large number of such bonds leads to a thermodynamically stable complex [2,17,19]. A great number of studies have been devoted to the influence of the copolymer structure and small molecule salt on the complexation between a polybase (PVP, PEO, or PVME) and poly(acrylic acid) (PAA). Poly(acrylic acid) can be partially neutralized by addition of a strong-base solution (NaOH), and thus it can be considered as a copolymer bearing acrylate groups (COO) and undissociated carboxylic groups (COOH). The carboxylic groups can form hydrogen bonds, whereas acrylate groups cannot interact with polybase (inactive groups), this result shows that the complex reaction and the structure of the complex depends on the parameters such as degree of neutralization of poly(acrylic acid), nature of the polybase, concentration of the polymer solution and molecular weight of the polymers, and the presence of 0.2 M tetramethylammonium chloride favors the formation of the complex [13–17]. Kirsh studied the interactions between PVP with differ-

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ent number average molecular weight (Mn) and small molecules (iodine and 1-anilinonaphthaline-8-sulphonate) and polymers (polymethacrylic and polyacrylic acids) in aqueous solution. The ability of PVP to form complexes with other compounds depends on chain length. The chain length effects for PVP are accounted for an unlike dehydration and unlike ‘‘locked’’ link concentration near links for these macromolecules in water [18]. In our previous work [19–21], we reported that interpolymer complexation between PMAA and PVP or PAA and poly(N,N-diethylacrylamide) (PDEA) were both pH and unit molar ratio polyacid to polybase dependence, and a major conformational change occurred when polyacid was mixed with polybase in aqueous phase at a certain pH. To obtain further information about the influence of the molecular weight of polymer, unit molar ratio polyacid to polybase and the small molecular salt on the complexes formation, the present investigation was carried out by UV transmittance and fluorescence probe measurement. Our work demonstrates that the cooperative interaction through the formation of hydrogen bond plays an important role in the process of the complexes formation and there is an optimum value of molecular weight for PAA at which the cooperative effect is stronger.

2. Experimental 2.1. Materials Acrylic acid is A.P. grade and was distilled under reduced pressure prior to use. 2,2 0 -Azoisobutyronitrile (AIBN) is C.P. grade and was recrystallized from 95% ethanol prior to use. N-Vinylpyrrolidone (98%, Acros) was distilled under reduced pressure prior to use. Perylene (Pe P 99%, Fluka) was used directly without further purification. PAA was prepared by free radical polymerization using AIBN as an initiator in anhydrous methanol, and then was purified by multiple dissolutions (·3) in anhydrous methanol followed by precipitation into anhydrous diethyl ether. PVP was prepared by free radical polymerization using AIBN as the initiator in distilled water, and was precipitated into acetone, purified three times. Then, both PAA and PVP were dried under vacuum at room temperature for 48 h to constant weight. The viscosity average molecular weight (Mg) was estimated for PAA and PVP from intrinsic viscosity of the polymer in 2 M NaOH aqueous solution (for PVP is in distilled water) at a constant temperature of 25 ± 0.10 C, using the Mark–Houwink–Saknrada (MHS) equation [22,23] [g] = kMga (k = 42.2 · 103 mL g1, a = 0.64 for PAA, and k = 1.4 · 104 mL g1, a = 0.7 for PVP). Other reagents are A.P. grade and were distilled prior to use.

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2.2. Sample preparation Polymer solutions were prepared from their stock solutions. The concentrations of the stock solutions for PAA and PVP are all 0.1% (w/v), then were diluted to a certain value just before use, it is noted that the pH of the solution of PAA (4 · 102%) (w/v) was around 4.60, whereas that of PVP (4 · 102%) (w/v) was around 5.70. For the experiments involving dissolution of organic probe molecules into the water-soluble polymer solutions, the probe (Pe) was initially dissolved in anhydrous diethyl ether to obtain a stock solution of known concentration (1 · 103 mol L1). This solution was diluted to 1 · 105 mol L1 just before use. One milliliter of the probe solution (1 · 105 mol L1) was injected into a 10 mL volumetric flask. The ether was evaporated at room temperature. Subsequently, a polymer solution (4 · 103%) (w/v) of known pH was added to the flask. To ensure solubilization and equilibration, the polymer/ probe solution was sonicated for 20 min, and then left at room temperature for 12 h. For complexation measurements, all samples of different PAA-to-PVP ratios (r = nPAA/nPVP = 0.111; 0.250; 0.429; 0.667; 1.000; 1.500; 2.333; 4.000; 9.000) were prepared in a similar manner. The method is described by using the example of the preparation of a solution (4 · 103%) (w/v) containing PAA10.3 (the molecular weight is 10,000 times the numerical index used for the sample designation, for example, PAA10.3 means Mg of PAA is 10.3 · 104 g mol1) and PVP2.64 (r = 1, pH = 4.00). To make this solution, 9.00 mL of PAA solution (4 · 103%) (w/v) and 13.83 mL of PVP solution (4 · 103%) (w/v) were added to a 50 mL volumetric flask with shaking, and then left at room temperature for 24 h, its pH was adjusted to 4.00 using 0.1 M HCl and 0.1 M NaOH aqueous solutions. 2.3. Analytical methods The measurements for solution pH were carried out at 25 C with PHS-3B pH meter. Viscosity measurements were carried out in a water bath, thermostated at 25 ± 0.10 C, using ubbelohde viscometer. Transmittance measurements were performed with a shimadzu UV/VIS recording spectrophotometer UV240 equipped with a temperature controlled cell holder. The transmittance of a light beam at the analyzing wavelength of 500 nm was measured. Fluorescence measurements were conducted with a shimadzu spectrofluorophotometer RF-540 at 25 C. The advantage of fluorescence technique is that information about the behavior of the polymer at the molecular lever can be obtained. Perylene was used as a micropolarity molecular probe. Final concentration of perylene was 1.0 · 106 mol L1. An excitation wavelength of

350 nm was used. In fact, the fluorescence emission intensities of some vibronic fine structures in the perylene fluorescence spectrum show strong environmental dependence [24]. In particular, the ratio between the intensity of peak I (k = 413.0 nm) and peak III (k = 446.0 nm), IIII/II, increases when perylene passes from a polar environment (such as the aqueous medium) to a hydrophobic environment (such as the inner hydrophobic region of the micelles).

3. Results and discussion 3.1. Transmittance studies Since polymer–polymer complexation in solution is always accompanied by the contraction or collapse of the component polymer coils, which results in the decrease of the viscosity and transmittance, or even precipitation if the concentration is high enough [19,25]. Therefore, the complexation of the polymers in aqueous solution was determined by studying the transmittance of PAA/PVP complex system aqueous solution and PAA, PVP pure aqueous solution, respectively, over a wide pH range from 2.60 to 10.0. Fig. 1 shows the variation of the transmittance, T%, of the PAA10.3/ PVP2.64 aqueous solution versus the different pH at room temperature. Firstly, when pH is higher than 4.00, the transmittance of the mixed solution is almost pH independent, this is because when pH is higher than 4.00, carboxylic groups dissociation results in the complexible acid sequence is shorter than the required minimum chain length (Scheme 1). The larger number of acrylate groups on the PAA chain prevents the complexation between PAA and PVP. When 3.00 < pH < 4.00, the transmittance of the polymer mixtures presents a light decrease on pH, this is because the PAA chain contains complexible and uncomplexible acid sequence (Scheme 2), there are a few complex reactions occurring between polymers. Finally, when pH is close to 2.60, only a few acrylate groups per PAA chain are present (Scheme 3). The length of undissociated acid sequence is long enough, so the PAA chain is almost entirely complexation with the PVP chain through hydrogen bond between the carboxylic groups on the PAA chain and the carbonyl group on the PVP chain, the cooperative interaction occurs by formation of hydrogen bond. The conformation of complex is compact, so the transmittance is close to zero [1,2,12,16]. As Fig. 1(a) and (b) shows the minimum values in transmittance are higher than those shown in Fig. 1(c) and (d), this attributes to the difference of the concentration of the mixed solutions. It is easy to understand that lower concentration leads to a relatively higher transmittance.

J. Shuping et al. / European Polymer Journal 41 (2005) 2406–2415

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Fig. 1. Plots of transmittance (T%) against pH in PAA10.3/PVP2.64 mixed aqueous solution (a: CPAA/PVP = 0.01% (w/v); b: CPAA/PVP = 0.02% (w/v); c: CPAA/PVP = 0.04% (w/v); d: CPAA/PVP = 0.05% (w/v). T = 25 C).

uncomplexible

acrylate group

carboxylic group Scheme 1.

complexible

complexible

uncomplexible

uncomplexible

Scheme 2.

complexible

Scheme 3.

Fig. 2 shows the variation of the transmittance of polymers mixed aqueous solutions with different pH as a function of various unit molar ratios of PAA10.3 to PVP2.64 (r = nPAA/nPVP). At pH 4.00, 3.50 and 3.00, respectively, the transmittance of mixtures has only a

little change corresponding to r = 1, this means that there is a little complexation occurring, which is stabilized through hydrogen bond. On the contrary, at pH 2.60, initially, the transmittance of mixed aqueous solution gradually decreases with increasing unit molar of

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100

carbonyl group

T%

80

Scheme 5.

60 40 20 0 -1

0

1

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9

10

r=n PAA /n PVP Fig. 2. Plots of transmittance (T %) versus unit molar ratio (r = nPAA/nPVP) in mixed aqueous solutions of PAA10.3/ PVP2.64 at different pH (CPAA/PVP = 0.04% (w/v), T = 25 C. (h) pH = 2.60; (j) pH = 3.00; (s) pH = 3.50; (d) pH = 4.00).

PAA up to nPAA/nPVP = 1.5, and then it increases again, the minimum value is close to zero. Together with the results shown in Figs. 1 and 2, these facts suggest that (1) the complex is not formed in the high pH region, but at low pH; (2) at low pH, the composition of the formed complex of PAA/PVP is 3:2 (the unit molar ratio of PAA to PVP); (3) the ratio of undissociated carboxylic groups, which are concerned with the hydrogen bond, plays an important role in the complexation [4]. By comparison with the results of Ref. [4], the composition is not 1:1 but 3:2, this is because of the strong power of hydrogen bond acceptor of PVP [14]. A first complexation of a carbonyl group of PVP with a carboxylic group of PAA may induce the formation of a second hydrogen bond between two vicinal carboxylic groups (Scheme 4a) or each carbonyl group can be complexed, at the same time, with two carboxylic groups (Scheme 4b). A sample complex mechanism should be proposed such that this step of forming a hydrogen bond and the following steps are cooperative processes and correspond to propagation (Scheme 5) due to the inherent hydrophobic interaction of carbon–carbon backbone. Initially, certain interacting sites of PVP bind to the sites of another polymer (PAA) mostly by chance, this

O

O

C O H O

C

N

O H O

H

O H

O

C

C O

O

a

b Scheme 4.

N

binding process is noncooperative. The formation of the first binding perturbs the conformations of several units for the binding of the interacting macromolecules and consequently makes the cooperative binding favorable. The next binding takes place at a site adjacent to the first binding, and the stability constant of this step is greater than that of first step, because of the extra entropy involved in bringing together two polymer chains [26]. Our experiments demonstrate that this phenomenon always exists for all of complexes between PVP2.64 and PAA with different molecular weight, PAA3.3, PAA10.3, PAA14.4, PAA19.7, PAA22.8, PAA26.1, and PAA30.1. In order to obtain more information about the influence of the length of polymer chain on complexation between PAA and PVP, we investigated the variation of transmittance against the molecular weight of PAA at different pH, and the cooperative interaction between polymers have been illustrated in Fig. 3a and b. At pH 4.00, 3.50, 3.00, respectively, the minimum value of the transmittance of the complex between polyacid and polybase always correspond to the PAA which with molecular weight of 1.97 · 105 g mol1. This means that the ability of forming hydrogen bond between carboxylic groups and carbonyl groups begins to weaken when molecular weight of PAA (MPAA) is lower or higher than a certain value, which is 1.97 · 105 g mol1 at least over the range of MPAA investigations. This result may be easily understood, when MPAA is lower than 1.97 · 105 g mol1, namely the length of PAA chain is shorter, the cooperative interaction of forming hydrogen bond interpolymers has not occurred because the hydrophobic interactions are weak between nonpolar carbon–carbon backbone of two macromolecules (PVP and PAA), apart from the hydrogen bonds interaction [14,18,27]. When MPAA is higher than 1.97 · 105 g mol1, the cooperative interaction is also weak, this reason is that increasing of the length of chain leads to decreasing of the flexibility of the macromolecule chain owing to the restricted local motion of segment. Fig. 4 further illustrates this result. It is well known that poly(acrylic acid) belongs to weak polyelectrolyte, and its aqueous solution will be stabilized through the competitive of the electrostatic repulsion and electrostatic attraction between the polyion and counterions (hydrogen ions). But this equilibrium would be markedly destroyed by the presence of small molecule sodium chloride when concentration of

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MPAA

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Fig. 4. Plots of transmittance (T %) against unit molar ratio (r) in mixed aqueous solutions of PAA/ PVP2.64 containing different molecular weight of PAA at pH 3.00 (CPAA/ PVP = 0.04% (w/v), T = 25 C. (j) PAA6.7; (h) PAA71.6; (d) PAA51.3; (m) PAA31.7; (s) PAA19.7).

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Fig. 3. Plots of transmittance (T %) versus molecular weight of PAA in mixed aqueous solutions of PAA/ PVP2.64 at different pH (CPAA/PVP = 0.04% (w/v), T = 25 C. (a) r = nPAA/ nPVP = 1.0, (j) pH = 4.00; (d) pH = 3.50; (m) pH = 3.00; (b) r = nPAA/nPVP = 1.5, (h) pH = 4.00; (d) pH = 3.50; (h) pH = 3.00; (m) pH = 2.70).

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sodium chloride reaches a certain value, at this time, the polymer mixed aqueous solution should conduct a phase separation. This reason is that the addition of sodium chloride to polymer-mixed aqueous solution leads to the decrease of solubility due to the destruction of hydration sheath. On the other hand, the electricity equilibrium of the mixed aqueous solution would be destroyed, coupling–coupling attraction increases, meanwhile, electrostatic repulsion decreases. That results in the conformation of PAA changes from a relatively loosely coiled to a hypercoiled form. In the present study, the influence of sodium chloride on transmittance of polymer mixed aqueous solution containing different molecular weight of PAA as shown in Fig. 5, the concentration of sodium chloride (CNaCl) is 0.0500 mol L1 and 0.1120 mol L1, respectively. Comparing Fig. 5 with Fig. 3(a), it can be seen that (1) the transmittance of PAA19.7/PVP2.64 mixed aqueous

Fig. 5. Plots of transmittance (T%) against different molecular weight of PAA in mixed aqueous solutions of PAA/ PVP2.64 in the presence of NaCl at pH 5.00 (CPAA/PVP = 0.04% (w/v), r = nPAA/nPVP = 1.5, T = 25 C. (s) CNaCl = 0.0500 mol L1; (d) CNaCl = 0.1120 mol L1).

solution is always maximum in the presence of sodium chloride. This means that the cooperative interaction between PAA and PVP depends on the molecular weight of PAA as we have discussed above. (2) Sodium chloride does not destroy the hydrogen bonds between PAA and PVP, but the size or structure of complex may change with the increasing of the concentration of sodium chloride. Fluorescence probe studies (Fig. 8) would further illustrate this in the following. This interesting effect may be easily understood. At pH > 4.00, the PAA would be ionized to a significant extent if attraction of hydrogen ions to the polyion did not impede the process.

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Therefore, sodium chloride addition reducing the interaction of polyion with counterions leads to a great deal of additional ionization, this means the complex body is not stable and will dissociate. But in the present studies, pH 6 4.00, ionization is so slight that this effect is unimportant [6], so sodium chloride does not destroy the hydrogen bonds, but only affects on the exposure carboxylic groups. The concentration of sodium chloride is higher and the effect is stronger. Fig. 6 shows the variation of transmittance of PAA10.3/PVP2.64 mixed aqueous solution, PAA10.3 and PVP2.64 pure aqueous solution, respectively, versus the concentration of sodium chloride (CNaCl). We can see that the transmittance of PVP2.64 pure aqueous solution does not depend on the concentration of sodium chloride, but those of mixed aqueous solution and PAA10.3 pure aqueous solution dramatically drop within very narrow range of CNaCl, and when transmit-

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80 60 40 20

tance starts to decrease, the concentration of sodium chloride addition to PAA pure aqueous solution is around 18 times of that addition to polymers mixed aqueous solution. This is a further demonstration for the result (2) shown in Fig. 5. 3.2. Fluorescence probe studies It is to be expected that the tightly coiled conformation of PAA might favor solubilization of organic guests into its hydrophobic microdomains, therefore, fluorescence probe studies should be useful in investigating the effects of complexation on the conformational behavior of PAA. The fine structure of fluorescence of Py emission spectrum is highly sensitive to the changes in the polarity of its microenvironment [19,20,28,29]. The probe used in the present studies was Pe. The Pe was used because its chemical structure and properties of fluorescence are similar to Py. The large value of IIII/I indicate a more hydrophobic environment, this property has been widely used to monitor the conformational behavior of water-soluble polymers in aqueous phase [30]. The effect of complexation on the fine structures of the fluorescence emission spectra of Pe solubilized in PVP2.64, PAA19.7, and PAA19.7/PVP2.64 (r = nPAA/nPVP = 9) aqueous solution, respectively, at pH 4.00 and at a polymer concentration (CPAA/PVP) of 4.0 · 103% (w/v) and probe concentration (CPe) of 1 · 106 mol L1 is depicted in Fig. 7. Complexation

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Fig. 6. Plots of transmittance (T %) against concentration of sodium chloride addition to polymer solutions at pH 5.00 (a: T = 25 C. (j) PAA10.3 pure aqueous solution, CPAA = 0.04% (w/v); b: T = 25 C. (d) mixed aqueous solutions of PAA10.3/ PVP2.64, CPAA/PVP = 0.04% (w/v), r = nPAA/nPVP = 1.0; (m) PVP2.64 pure aqueous solution, CPVP = 0.04% (w/v)).

Fig. 7. Fluorescence emission spectra of perylene dispersed in polymers aqueous solution (1: PVP pure aqueous solution, CPVP = 4 · 103% (w/v); 2: PAA19.7/PVP2.64 mixed aqueous solution, CPAA/PVP = 4 · 103% (w/v), r = 9:1; 3: PAA pure aqueous solution, CPAA = 4 · 103% (w/v). pH = 4.00, T = 25 C).

J. Shuping et al. / European Polymer Journal 41 (2005) 2406–2415

between PAA and PVP was accompanied, as expected, by a conformational change as proved by the increase in the hydrophobicity of the environment of the probe. The result is because the IIII/II ratio of the probe increased from 0.78 (for PAA19.7 aqueous solution) to 0.91 (for PAA19.7/PVP2.64 aqueous solution) to 0.92 (for PVP2.64 aqueous solution). This reason is discussed in detail below. Fig. 8 shows the IIII/II ratio which varies against the molecular weight of PAA in the presence and absence of sodium chloride. From these we can see (1) in the absence of sodium chloride, the IIII/II ratio increases with increasing of MPAA and then reaches the maximum corresponding to MPAA = 1.97 · 105 g mol1, and then the ratio begins to decrease. This suggests that the complexation proceeds through ‘‘cooperative interaction’’ between two macromolecules required having ‘‘optimum chain length’’ for the formation of a complex [26,31]. On the contrast, the IIII/II ratio reaches its minimum corresponding to MPAA = 1.97 · 105 g mol1 in the presence of sodium chloride, this reason is that we have discussed above. (2) As shown in Fig. 8, the IIII/II ratio in the presence of sodium chloride is lower than that in the absence of sodium chloride, this is because the addition of small molecule salt leads to an increase of the polarity of the solvent, this means that perylene passes from a weak polar environment to a strong polar environment. But at MPAA = 3.3 · 104 g mol1, the IIII/II ratio in the presence of sodium chloride is higher than that in the absence of sodium chloride, this is because the polycomplex may prefer to adopt a relative hypercoil conformation in the presence of sodium chloride when the polymer chain is short enough.

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Poly(acrylic acid) contains hydrophilic sites (carboxylic groups that can stabilize the aqueous solution by forming hydrogen bonds with the neighbouring water molecules, as well as with other hydrogen donor substances) and hydrophobic sites (vinyl backbone that can destabilize the aqueous solution through their aggregation and the release of the surrounding water molecules). These two effects compete with each other, giving the polymer its special phase behavior. Fig. 9 shows the variation of the IIII/III ratio against different pH in PAA19.7 and PVP2.64 pure aqueous solution and PAA19.7/PVP2.64 mixed aqueous solution. For PAA19.7 aqueous solution, the moleculeÕs protective hydration sheath is destroyed because of dissociation of carboxylic groups when pH changes from 3.00 to 9.80, hydrophobic interaction is relatively stronger than hydrophilic interaction, the IIII/II ratio would increase with increasing in pH and is almost pH independence at higher pH. This result is in contrast to PMAA [19] because of the presence of methyl group attached to the carbon–carbon backbone for PMAA [4,31]. However, for PVP2.64 aqueous solution, the IIII/II ratio for Pe is almost pH independence, indicating that the PVP may adopt a relatively stable compact coil conformation within the wide range of pH studied. For the PAA19.7/ PVP2.64 mixed system, the IIII/II ratio for Pe is significantly higher than that of PAA19.7 system over the range of studied and is almost pH independence when pH > 4.00, this clearly indicates that the complex body of PAA19.7/PVP2.64 adopt relatively hypercoil conformation caused by the cooperative interaction processes in the formation of hydrogen bond as we have discussed above (Scheme 5).

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Fig. 8. Plots of the IIII/II ratio of Pe fluorescence emission intensity against different molecular weight of PAA in mixed aqueous solutions of PAA/ PVP2.64 (CPAA/PVP = 0.004% (w/v), r = nPAA/nPVP = 1.5, CPe = 1 · 106 mol L1, T = 25 C. (s) in the absence of sodium chloride; (d) in the presence of sodium chloride, CNaCl = 3 · 106 mol L1).

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Fig. 9. Plots of the IIII/II ratio of Pe fluorescence emission intensity against different pH in polymers aqueous solution (CPe = 1 · 106 mol L1, T = 25 C. (h) mixed aqueous solutions of PAA19.7/PVP2.64, CPAA/PVP = 0.004% (w/v), r = nPAA/nPVP = 1.5; (m) PVP2.64 pure aqueous solution, CPVP = 0.004% (w/v); (d) PAA19.7 pure aqueous solution, CPAA = 0.004% (w/v)).

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Acknowledgements

1.01

The authors are grateful to the Special Doctoral Program Funds of the Ministry of Education of China (20030730013).

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10

r=nPAA/nPVP

Fig. 10. Plots of the IIII/II ratio of Pe fluorescence emission intensity against unit molar ratio r (r = nPAA/nPVP) in mixed aqueous solutions of PAA19.7/PVP2.64 (CPAA/PVP = 0.004% (w/v), CPe = 1 · 106 mol L1, pH = 4.00, T = 25 C).

Fig. 10 presents the change of the IIII/II ratio for the complex system at different unit molar ratio (r = nPAA/ nPVP) at pH 4.00. It reveals that when r is lower than 1.0, the IIII/II ratio for Pe in the complex system increases with the changing of unit molar ratio and reaches its maximum, and then decreases with further increasing of the unit molar ratio. Obviously, this is due to the complexation between PAA and PVP. When r is lower than 1.0, the IIII/II ratio for Pe is smaller because there is little a complexation between PAA and PVP, but at r = 1.0, the complexation between polymers occurs which results in the collapse of the macromolecule chain, both the size and the number of hydrophobic microdomain are larger, so the IIII/II ratio is higher. When further addition of PAA to the mixed system, PAA is relatively excess quantity, IIII/II ratio would decrease. The result accords with that shown in Fig. 2 and Fig. 9.

4. Conclusions Transmittance and fluorescence probe studies show that the interpolymer complexation is strongly dependent on the molecular weight (chain length) of the polymer, pH and unit molar ratio (r = nPAA/nPVP). A major conformational change occurs when PAA is mixed with PVP in aqueous phase at pH < 4.00. The conformation change of PAA from a loose coiled to a hypercoiled form is evidenced by the decrease in transmittance and the increase in the hydrophobic microdomain size or number. Small molecule salt (sodium chloride) does not affect the hydrogen bonds between PAA and PVP but the exposure carboxylic groups, and results in the change of the size or structure of the complex body.

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