Hydrophilic Polymers - American Chemical Society

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Downloaded by UNIV LAVAL on October 26, 2015 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch022

Fluorescence Studies of Cellulose Ethers Synthesis, Characterization, and Spectroscopic Properties of Labeled Polymers Françoise M. Winnik Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada

Examples are given of the use of pyrene-labeled polymers as models for the study of solution properties of hydrophobically modified polymers acting as gelling agents or associative thickeners in waterborne fluids. Labeled polymers were obtained from three commercial cellulose ethers: Tylose M-300 (hydroxyethylmethylcellulose), Methocel-50-F (hydroxypropylmethylcellulose), and Klucel-L (hydroxypropylcellulose). The polymers contain on average one to five pyrene groups per chain. Procedures for labeling, purification, and characterization of the polymers are described. Spectroscopic parameters such as pyrene excimer-to-monomer intensity ratios, absorption spectra, excitation spectra, and excited-state lifetimes are reported for solutions of the polymers in water and in an organic medium (dichloromethane—methanol). These parameters present evidence that in water pyrene groups form hydrophobic clusters that are destroyed at the temperature corresponding to the cloud point. These clusters form in samples that undergo heat-induced phase separation, such as pyrene-labeled hydroxypropylcellulose in water.

0065-2393/96/0248-0409$12.00/0 © 1996 American Chemical Society

In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


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HYDROPHILIC POLYMERS

CÎELLULOSE ETHERS H A V E A WIDE RANGE of industrial applications. They are used as additives in materials such as paints, inks, cosmetics, pharmaceuticals, foods, and ceramics (I ). They may function as thickeners, flow control agents, film-forming agents, or packaging materials for controlled drug release. The solution properties of cellulose ethers are dictated primarily by the chemical structure of the ether substituent and the degree of substitution of the cellulose and to a lesser extent by the molecular weight of the polymer. Methylcellulose, hydroxypropylcellulose, and hydroxyalkylmethylcellulose at modest concentrations are soluble in cold water. Upon being heated, their aqueous solutions undergo characteristic phase changes. Methylcellulose solutions form gels (2). Solutions of hydroxypropylcellulose that are clear at room temperature become opaque above a well-defined temperature, the cloud point (3). Both gelation and clouding are reversible. With methylcellulose, a negative hysteresis was observed between rising and decreasing transition temperatures (4). Hydroxyalkylmethylcelluloses in water exhibit various reversible thermally induced phase changes that depend on the chemical structures of the hydroxyalkyl groups and their level of incorporation along the polymer backbone. The properties of cellulose ethers are also influenced by the fabrication process. A typical procedure for the production of methylcellulose involves reacting methyl chloride with wood pulp pretreated with a caustic solution ( I ). In the manufacture of hydroxyalkyl methylcelluloses, an alkyl oxide such as ethylene oxide or propylene oxide is used in addition to methyl chloride to attach the hydroxyalkyl groups to the anhydroglucose units. The degree of substitution (i.e., the average number of substituted hydroxyl positions per glucopyranose unit) can be varied over a wide range of values, giving a large number of commercial grades. Moreover, hydroxyalkylation can take place at the terminal substituent hydroxyl position and lead to materials with molar substitution values (i.e., the average number of hydroxyalkyl substituents per glucose unit) larger than 3, the initial number of free hydroxyl groups on each anhydroglucose monomer. If methylation occurs after hydroxyalkylation, end capping of hydroxyalkyl substituents may occur as well. This heterogeneous chemical structure may be responsible for the unique properties of cellulose ethers. Industrial users require materials that meet strict specifications. These specifications are controlled by the molecular characteristics of the polymers, so it is important to analyze not only macroscopic properties but also interactions taking place on the molecular level. Analytical techniques frequently used for the characterization of cellulose ethers include C N M R (5) and E P R (6) spectroscopy, calorimetry (7), viscometry (8, 9), and interfacial measurements (JO). Fluorés1 3



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cence spectroscopy has been applied to a limited extent to the study of solution properties of cellulose ethers either by means of dyes added to the polymer solutions (JI, 22) or by fluorescent tags attached to the polymer itself (13, 14). The fluorescent-tag method yields direct information on the molecular properties of the polymers, but it is more difficult to carry out in practice. Two major problems are associated with the use of fluorescence-labeled water-soluble polymers: (1) Their synthesis and purification are often difficult to carry out, and (2) the fluorescent labels may act as hydrophobic modifiers and alter the properties of the starting material. This second drawback can be turned into an advantage if one sets as a goal the study of hydrophobically modified cellulose ethers, a class of polysaccharides gaining industrial importance (15). These polymers incorporate a small number of hydrophobic groups that are grafted along their backbones. Solutions of these polymers in water exhibit significantly enhanced viscosity compared to that of the original polymer as a result of the clustering of the hydrophobic pendant groups. Most intensely studied to date are the hydrophobic derivatives of hydroxyethylcellulose (II, 16-18), but other hydrophobically modified cellulose ethers will probably become available, given their low cost, lack of toxicity, and biodegradability. The synthesis and characterization of pyrene-labeled cellulose ethers prepared from hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, and hydroxypropylcellulose are discussed in this chapter. Pyrene was selected as the fluorescent tag because its spectroscopy is well documented (19). Particular attention is given to the preparation of materials and to purification procedures. Then the spectroscopic characterizations of the polymers in aqueous solution and in organic solvents are compared by using the absorption and excitation spectra, the pyrene excimer-to-monomer emission intensity ratios, and the lifetimes of the pyrene monomer and excimer as observable experimental parameters. Previously reported applications of fluorescence spectroscopy to the study of cellulose ethers are reviewed briefly to illustrate how the technique provides information on the molecular events associated with macroscopic phase changes.

Experimental Methods Materials. Hydroxyethylmethylcellulose (Tylose, grade M H 300, Werk Kalle-Albert) was purchased from Fluka; hydroxypropylcellulose (Klucel-L, Hercules) was purchasedfromAldrich Chemical Corp. Hydroxypropylmethylcellulose (Methocel-50-F) was a gift from Dow Chemicals. Pyrene-labeled hydroxypropylcellulose (HPC-Py/216) was prepared as described previously (13). Chemicals for the syntheses were purchased from Aldrich Chemical Corp. Water was deionized with a Millipore Milli-


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Q water purification system. Dimethylformamide (DMF) was dried by reflux over calcium hydride followed by distillation at reduced pressure. Spectroscopy-grade solvents were used for all spectroscopic measurements. Instrumentation. UV spectra were recorded with a Varian UV-VisNIR Cary-5 spectrophotometer. Steady-state fluorescence spectra were measured with a SPEX Fluorolog 212 fluorescence spectrometer equipped with a DM3000F data analysis system. Fluorescence lifetimes were determined with an LS-1 instrument from Photon Technology International equipped with a thyratron-gated N2 lamp and a proprietary analog stroboscopic boxcar detection system. Gel permeation chromatography (GPC) measurements were performed with a Shimadzu size exclusion chromatography system equipped with an SPD-6A UV-visible light detector and an RID-6A differential refractive index detector. Data analysis was performed with Shimadzu Chromatopac software provided by the manufacturer. Two Progel brand columns (G3000 and G5000, Toyo-Soda) were used. Separations were carried out at 34 °C. The eluent (flow rate of 0.6 mL min" ) was 0.1 M N a N 0 . 1

3

Fluorescence Measurements. Emission spectra (not corrected) were obtained with an excitation wavelength of343 nm. Excitation spectra were monitored in the ratio mode at 480 nm for the excimer and 390 nm for the monomer. The excimer-to-monomer emission ratio (/E//M) was calculated as the ratio of the emission intensity at 480 nm to the half-sum of the emission intensities at 376 and 396 nm. The concentration of the solutions for fluorescence analysis was 0.1 g L " . Under these conditions, the absorbance of the solutions at 343 nm was kept below 0.08 in all experiments. 1

Syntheses. 4-( 1-Pyrenyl)butyl Tosylate. To a solution of 4-(l-pyrenyl)butanol (20) (1.0 g, 3.65 mmol) in chloroform (10 mL, flushed through alumina) were added first pyridine (0.58 g, 7.3 mmol) and then p-toluenesulfonyl chloride (1.04 g, 5.48 mmol) in small portions over a period of 5 min. The mixture was stirred at 22 ° C in the dark under nitrogen for 1.5 h. The resulting mixture, diluted to 20 mL with diethyl ether, was extracted successively with 10% aqueous HC1 (twice), water (twice), and saturated brine (twice). The organic layer was dried over M g S 0 and evaporated to yield an oil. Crystallization from ethyl acetate—hexane (25 mL, 1/2 vol/vol) yielded 4-(l-pyrenyl)butyl tosylate (1.34 g, 86%) (mp, 91-92 °C; A (e, tetrahydrofuran), 343 nm (38,500), 327 nm (25,000)). 4

max

Pyrene-Labeled Tylose (Ty-Py/190). To a gel ofTylose MH-300 (2.0 g, dried by azeotropic distillation of toluene) in D M F (30 mL) kept under nitrogen was added a solution of 4-(l-pyrenyl)butyl to sylate (0.25 g, 0.58 mmol) in D M F . The mixture was stirred at room temperature for 2 h. A suspension of sodium hydride (250 mg, 60% dispersion in oil, washed twice with dry hexane) in D M F (1 mL) was added. The reaction mixture was kept in the dark at room temperature under nitrogen for 3 days. Excess base was destroyed by the addition of dilute acetic acid (5 mL, C H 3 C O 2 H - H 2 O , 1/2 vol/vol). A fluid solution formed immediately upon acidification. The fluid was stirred for 1 h at room temperature. The sol-


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Ethers

vent was removed by high-vacuum distillation (bath temperature, 50 °C). The resulting amber foamy material was dissolved in CH2CI2—MeOH (50 mL, 3/1 vol/vol). The polymer was isolated by precipitation of this solution into hexane (500 mL) and was purified by three successive precipitations from C H 2 C l 2 - M e O H into hexane. It was dried in vacuo at 30 °C. Redissolution of the polymer in water (100 mL) and freeze-drying yielded Ty-Py/ 190 (1.78 g) as a slightly tan solid: [Py] = 3 x 1 0 mol g " of polymer or ca. 1 pyrene per 190 anhydroglucose units as determined by UV absorption in C H C l - M e O H (3/1 vol/vol) using 4-(l-pyrenyl)butanol in the same solvent as was used with the reference material. - 5


2

1

2

Pyrene-Labeled Methocel (Me-Py/250). A solution of Methocel-50F (8.0 g, previously dried by azeotropic distillation of toluene) in D M F (50 mL) was prepared by stirring at room temperature under nitrogen for 24 h. This solution was then reacted with 4-(l-pyrenyl)butyl tosylate (1.0 g, 2.3 mmol) under the same conditions used with pyrene-labeled Tylose (described previously). An amber foamy material was recovered after evaporation of D M F . This material was dissolved in C H 2 C l - M e O H (50 mL, 3/1 vol/vol). The polymer was isolated by precipitation of this solution into hexane (500 mL) and was purified by three successive precipitations from C H 2 C l 2 - M e O H into hexane. It was dried in vacuo at 30 °C. Redissolution of the polymer in water (100 mL) and freeze-drying yielded MePy/250 (7.24 g) as a white solid: [Py] = 6.4 x 10" mol g" or ca. 1 pyrene per 250 anhydroglucose units. 2

6

1

Pyrene-Labeled Methocel (Me-Pyll00). The same procedure was employed starting with Methocel-50-F (5.0 g), 4-(l-pyrenyl)butyl tosylate (0.25 g, 0.58 mmol), and NaH (250 mg) in dry D M F (25 mL) to yield a labeled sample with [Py] = 4.16 mol g or ca. 1 pyrene per 100 anhydroglucose units. - 1

Results and Discussion Synthesis a n d Characterization of P y r e n e - L a b e l e d Polym e r s . The fluorescent labels were attached via ether linkages to three cellulose ethers: hydroxyethylmethylcellulose (Tylose M H 300), hydroxypropylmethylcellulose (Methocel-50-F), and hydroxypropylcellulose (Klucel-L). The molecular characteristics of the starting polymers are listed in Table I, and their structures, together with those of the corresponding labeled derivatives, are represented in Figure 1. The pyrene groups were attached to the polymers by reaction of 4-(l-pyrenyl)butyl tosylate with predried cellulose ether in alkaline D M F . Figure 2 illustrates the procedure for labeling Tylose. Because of the marked tendency of most cellulose ethers to form gels in D M F , it was necessary to invert the normal order of adding reagents in the Williamson ether synthesis. Instead of adding the tosylate to the preformed alkoxide, the tosylate was mixed first with a solution of dry


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Table I. Substitution Patterns and Molecular-Weight Ranges of Polymers Polymer Tylose

DS MH-300

Methocel-50-F

Klucel-L

1.5-1.6*

0.1-0.16*

130,(MX) (700)

1.6-1.8

0.1-0.2^

65,000-80,000

Û

2.9

Ref.

MW(DP)

MS

5.9*

c (100)

100,000

14 24

NOTE: DS, degree of substitution; MS, molar substitution; MW, weight-average molecular weight; DP, degree of polymerization (average number of glucose units per polymer chain). Methyl substituent was used. Hydroxyethyl substituent was used. Information was provided by the manufacturer. Hydroxypropyl substituent was used.

a

b c


d

cellulose ether in D M F . After several hours, when the mixture became homogeneous, sodium hydride was added to generate alkoxide groups on the polymer. Under these conditions, the syntheses could be carried out on a 1- to 10-g scale. Moreover, by changing the initial weight ratio of polymer to pyrene derivative, it was possible to vary the degree of incorporation of the chromophores. For example, two syntheses carried out with Methocel-50-F yielded samples with levels of pyrene incorporation of 4.1 x 10~" mol (Py) g ~ of polymer (Me-Py/100) and 2.1 x 10" mol (Py) g " of polymer (Me-Py/250). In the acronyms of the polymers, the digit refers to the average number of anhydroglucose units per pyrene group {see Experimental Methods). 5

5

1

1

Figure 1. Structures of the cellulose ethers studied.


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(a)

Figure 2. Synthetic scheme for the preparation of 4-(l-pyrenyl)butyl tosylate and pyrene-labeled Tylose (Ty-Py/190). THF is tetrahydrofuran. For each sample the amount of pyrene incorporation was determined from U V absorption data of polymer solutions in C H C l 2 - M e O H ; solutions of 4-(l-pyrenyl)butanol in the same solvent were used as standards (see Experimental Methods). From these data and from the reported degrees of polymerization of the respective cellulose ethers, it can be estimated that there are on average approximately four pyrenes per macromolecule in Ty-Py/190 and one pyrene or fewer in Me-Py/100 and Me-Py/250. 2


416


R E F R A C T I V E INDEX DETECTION


ι—ι—ι—ι—ι—ι—ι—

UV (λ=345ηπι) DETECTION

Figure 3. GPC elution profiles of pyrene-labeled Tylose (Ty-Py/190) monitored by refractive index detection and UV absorption detection (λ = 345 nm).

f Q

I 2

I 4

ι 6

ι ι ι 8 1 0 1 2

ELUTION T I M E ( m i n )

The purity of the pyrene-labeled polymers is an important aspect of the planned fluorescence experiments. The absence of low-molecu lar-weight chromophores has to be ascertained. This information was found by using UV-visible and refractive index detectors in tandem in the G P C analysis of the polymers. Thus it was established for each labeled sample that the pyrene groups were covalently attached to the polymers and that the chemical transformations did not alter the molecular weights and molecular-weight distributions of the poly mers. The technique is also useful for monitoring the purification pro cedure and for ascertaining that the reprecipitations are effective in removing pyrene-containing low-molecular-weight impurities. Hy droxypropylcellulose is soluble in several organic solvents as well as in water; thus in this case, it is possible to carry out the G P C analysis either in tetrahydrofuran, with calibration of the data against polysty rene reference samples, or in water. Tylose and Methocel-50-F do not exhibit appreciable solubility in organic media. In such cases, one is limited to chromatography in aqueous medium. The use of water as an eluent gave irreproducible results. However, acceptable and repro ducible G P C traces were obtained with aqueous sodium nitrate as the eluent, as shown in Figure 3 for the labeled Tylose (Ty-Py/190). Spectroscopic Properties of L a b e l e d Polymers. PyreneLabeled Tylose. The fluorescence spectra of Ty-Py/190 in water and in C H C l 2 - M e O H (3/1 vol/vol) are presented in Figure 3. Each 2



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Figure 4. Fluorescence spectra of pyrene-labeled Tylose (Ty-Py/190) in water (a) and in CH Cl -MeOH (3/1 vol/vol) (b). k = 343 nm; polymer concentration, 0.1 g L~ . 2

2

exc

l

spectrum shows two emissions: One is due to locally excited pyrenes (intensity J M , monomer emission) and has the (0,0) band located at 376 nm, and one is a broad emission centered at 480 nm that results from pyrene excimers (intensity J ) . For solutions of the same concentration, the relative contribution of the excimer to the total emission is stronger in the organic medium than in water. For samples in C H C l - M e O H , identical excitation spectra were obtained for emissions monitored at 396 nm (monomer) and 480 nm (excimer), and the maxima corresponded to those in the U V absorption spectrum (Figure 4a). Time-dependent fluorescence measurements in the nanosecond E

2

2


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Table II. Spectroscopic Properties of Pyrene-Labeled Cellulose Ethers


Polymer

Solvent

A W A337

IEIIM

1.32

0.21

m 6.4 (0.71) 97.3 (0.29) = 84.7 26.1 (0.10) 102.0 (0.90) = 94.2

a

Ty-Py/190

water

Ty-Py/190

CH Cl -MeOH (3/1 vol/vol)

2.10

0.40

Me-Py/100

water

1.32

0.10

Me-Py/100

CH Cl -MeOH (3/1 vol/vol) water water

2.18

0.01

1.60 2.16

0.01 0.10

MeOH

2.50

0.04

2

2

Me-Py/250 HPC-Py/216

c

HPC-Py/216

c

2

2

TM

(ns)

28 (0.17) 116 (0.83) = 112 19 (0.17) 114 (0.83) = 108

τ (nsjp

b

Ε

12.6 (0.34) 63.1 (0.66) = 58.4 7 (-0.98) 58 (1.0)

3.5 (0.64) 84 (0.36) = 78

Peak-to-valley ratios of the absorption spectra. See reference 19 for further information. Values in parentheses are prefactor values, T M representsfluorescencelifetime of the pyrene monomer emission, T E representsfluorescencelifetime of the pyrene excimer emission. Values arefromreference 13.

a

b

c

time domain were performed for samples in the organic medium. The excimer time-dependent profile showed a growing-in component and a decaying component (7 and 58 ns, respectively). The monomer emis sion showed a nonexponential decay that could be fit to a sum of two exponential terms with decay times of 6.4 and 97.3 ns (Table II). Taken together, the data indicate that the pyrene excimer in organic solutions of Ty-Py/190 is formed by a dynamic mechanism via encounter of an excited pyrene and a ground-state pyrene, as described by Birks (21 ). Time-dependent measurements were carried out next with solu tions of Ty-Py/190 in water. In this situation it was not possible to detect a rising component in the excimer profile within the nano second time domain accessible to the instrumentation. The excimer decay could be fit satisfactorily to a sum of two exponential terms with decay times of 12.6 and 63.1 ns. The monomer profile obeyed an exponential decay with a lifetime of 96 ns. Further steady-state measurements revealed that the excitation spectra monitored for the monomer and excimer emissions were different. The monomer spec trum was blue-shifted by about 3 nm relative to the excimer spectrum



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300

419

350

WAVELENGTH

(nm)

Figure 5. UV absorption spectra of pyrene-labeled Tylose (Ty-Py/190) in water (a) and in CH2CÎ2-MeOH (3/1 vol/vol) (b); polymer concentration, 0.3 g L " . 1

and relative to the U V absorption spectrum of Ty-Py/190 in water (Figure 5). Taken together, the experimental observations lead to the conclusion that the pyrene excimer emission originates from aggregates of pyrenes that exist prior to excitation, as is observed also in aqueous solutions of pyrene-labeled hydroxypropylcelluloses (23). In most conditions the formation of pyrene dimers or higher aggregates is precluded in solution. The situation in aqueous polymeric solutions is unique. It is postulated that the pyrene aggregates are stabilized by hydrophobic interactions. The nonpolar assemblies are surrounded by a cage of highly organized water molecules tightly bound through hydrogen bonding. This formation of dimers or higher aggregates has a positive entropy and a positive enthalpy. The entropie term is dominant, rendering the free energy of dimer formation favorable at room temperature. The spectroscopic information available does not allow one to distinguish between aggregates formed among pyrene groups attached to the same polymer or to several chains. It is expected that the latter situation predominates in view of the low level of pyrene incorporation along the macromolecules and of the restricted flexibility of the chains.



420


Figure 6. Fluorescence spectra of pyrene-labeled Methocel (Me-Py/100) in water; emission spectrum, \ - 343 nm; excitation spectra, X = 480 nm (excimer) and k = 396 nm (monomer); polymer concentration, 0.1 g L- . em

exc

exc

1

Pyrene-Labeled Methocel Derivatives. The fluorescence spectrum of Me-Py/100 in water exhibits a strong emission attributed to isolated excited pyrenes as well as a contribution from pyrene excimers (Figure 6). As with Ty-Py/190, the excitation spectra monitored for the excimer and the monomer emissions are different: The excitation spectrum monitored at 480 nm (excimer) is red-shifted by ca. 4 nm and exhibits significant line broadening compared to the excitation spectrum monitored for the monomer emission (Figure 6). Timeresolved measurements (Table II) and absorption spectra offer further support to the steady-state fluorescence data to establish that the excimer emission originates from preassociated pyrenes. Thus, as with Ty-Py/190, the chromophores form hydrophobic clusters in aqueous solutions of Me-Py/100. The pyrene excimer contribution to the total emission is significantly weaker than that of Ty-Py/190, even though the degree of pyrene incorporation on the Methocel-50-F sample is higher (4.2 x 10" mol g " in Me-Py/100 versus 3.0 x 10" mol g " in Ty-Py/190). The two cellulose ethers exhibit different macroscopic 5

1

5


1


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properties in water prior to labeling. It may be inferred from the preliminary spectroscopic data that corresponding differences are maintained in the labeled materials. Further work with the labeled Tylose and Methocel samples will focus on this issue. First, it is important to correlate the spectroscopic characteristics of the polymers to their macroscopic solution properties, such as viscosity and gelation. The effects of external stimuli, such as heat or the addition of surfactants, on pyrene photophysics will be monitored. Thus it should be possible to uncover detailed aspects of the molecular origin of macroscopic phenomena, such as the temperature-induced phase changes that take place in aqueous solutions of cellulose ethers. Key results for thermally induced changes in pyrene emission from aqueous solutions of pyrene-labeled hydroxypropylcellulose are reviewed next. A complete study of this system is reported elsewhere (22).

Application of Fluorescence Spectroscopy To Study Temperature-Induced Phase Changes in Aqueous Solutions of Cellulose Ethers. Hydroxypropylcellulose solutions in water exhibit a cloud point at 42 °C for polymer concentrations lower than 10 g L " (3). The cloud point of the labeled polymer HPC-Py/216 (Figure 1, Table I) occurs at a slightly lower temperature, 40 to 41 °C (22). This small but significant decrease is consistent with the usual effect of hydrophobic group substitution on the cloud point of water-soluble polymers (23). At 25 °C in water, HPC-Py/216 shows an emission due to pyrene monomers and excimers, with a ratio of excimer-to-monomer intensities (IE/IM) of 0.10. As the temperature of the solution is increased, the following events occur: First, the ratio of excimer-tomonomer intensity increases to a maximum at 35 °C. Above 35 °C the ratio decreases sharply to reach its limiting value of 0.01 at 50 °C. The midpoint of the transition is ca. 42 °C. When the sample temperature is decreased, a sharp increase in the ratio occurs, with the midpoint of the transition occurring at ca. 41 °C. At all temperatures lower than 50 °C, the I E / I M ratio measured upon cooling is slightly smaller than that measured upon heating. After a few hours at 25 °C, a cooled sample recovers the initial ratio value. Consecutive heating-cooling cycles performed with the same sample yield the same results. The ratio of excimer to monomer intensity is a useful parameter for monitoring solution phase changes. This ratio is obtained from simple steady-state fluorescence measurements, and it can serve as a tool to correlate, at least qualitatively, heat-induced spectroscopic changes to changes in macroscopic solution properties, such as turbidity. The sharp decrease in I E / I M at the phase transition reflects an increase in the absolute pyrene monomer intensity at the expense of 1


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the pyrene excimer fluorescence intensity. The increase in pyrene monomer intensity must be due to an increase either in the number of emitting species or in the fluorescence lifetime of the pyrene. No significant changes were observed in the decay times of either the pyrene monomers or the pyrene excimers as the samples were heated through their cloud points. Therefore the changes in relative fluorescence intensities indicate changes in the relative numbers of isolated and aggregated pyrenes. During the phase transition, the aggregated pyrenes separate. They become accommodated within hydrophobic cavities in the polymer-rich phase.

Conclusion Hydrophobic chromophores have been linked to cellulose ethers via chemical modifications akin to those employed in the preparation of hydrophobically m o d i f i e d cellulose ethers bearing alkyl or perfluoroalkyl groups. Spectroscopic measurements show that even at low solution concentrations, fluorescence-labeled cellulose ethers in water form interpolymeric aggregates via hydrophobic clusters of chromophores, a situation that parallels the behavior of commercial hydrophobically modified cellulose ethers. For pyrene-labeled cellulose ethers that exhibit a well-defined heat-induced phase transition in aqueous solutions, all spectral parameters point to the existence of organized assemblies of the pyrenes below the cloud point. Above the cloud point, the chromophores behave in patterns typical of those observed in isotropic solutions. Thus heat induces a complete reorganization of the interpolymeric association. Further experiments are required to answer questions related to temperature-triggered changes in the conformations of individual polymer chains associated with the macroscopic phase separation.

Acknowledgments I thank the Xerox Research Center of Canada, Mississauga, Ontario, Canada, where part of the work was carried out. G P C measurements were performed by D . Boils-Boissier of the Xerox Research Center of Canada.

References 1. Just, E. K.; Majewicz, T. G. Encyclopedia of Polymer Science and Engineering, 2nd ed.; John Wiley: New York, 1985; Vol. 3, pp 226-269. 2. Kato, T.; Yokoyama, M.; Takahashi, A. Colloid Polym. Sci. 1978,265,15. 3. Fortin, S.; Charlet, G. Macromolecules 1989, 22, 2286.



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4. Klug, E. D. J. Polym. Sci. C 1971, 36, 491. 5. Ibbett, R. N.; Philp, K.; Price, D. M. Polymer 1992,33,4087and references therein. 6. Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. In Cellulose: Structural and Functional Aspects; Kennedy, J. F.; Phillips, G. O.; Wil liams, P. Α., Eds.; Ellis Horwood: Chichester, United Kingdom, 1990; pp 323-327. 7. Robitaille, L.; Turcotte, N.; Fortin, S.; Charlet, G. Macromolecules 1991, 24, 2413. 8. Wirick, M.G.;Waldman, M. H. J. Appl. Polym. Sci. 1970,14,579. 9. Huikari, Α.; Karlsson, A. Acta Pharm. Fenn. 1989, 98, 231. 10. Sarkar, N. Polymer 1984, 25, 481. 11. Varelas, C. G.; Steiner, C. A. J. Polym. Sci., Polym. Phys. Ed. 1992, 30, 1233. 12. Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443. 13. Winnik, F. M.; Winnik, M. Α.; Tazuke, S.; Ober, C. K. Macromolecules 1987, 20, 38. 14. Winnik, F. M. Macromolecules 1987, 20, 2745. 15. Polymers in Aqueous Media: Performance Through Association; Glass, J. E., Ed.; Advances in Chemistry 223; American Chemical Society: Wash ington, DC, 1989. 16. Landoll, L. M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 443. 17. Hwang, F. S.; Hogen-Esch, T. E. Macromolecules 1993, 26, 3156. 18. Dualeh, A. J.; Steiner, C. A. Macromolecules 1991, 24, 112. 19. Winnik, F. M. Chem. Rev. 1993, 93, 587 and references therein. 20. Schnieders, C; Müllen, K.; Huber, W. Tetrahedron 1984, 40, 1701. 21. Birks, J. B. Photophysics of Aromatic Molecules; John Wiley: New York, 1970; Chapter 7. 22. Winnik, F. M . ; Tamai, N.; Yonezawa, J.; Nishimura, Y.; Yamazaki, I. J. Phys. Chem. 1992, 96, 1967. 23. Taylor, L. D.; Cerankowski, J. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 2551. 24. Sarkar, N. J. Appl. Polym. Sci. 1979, 24, 1073. RECEIVED for review November 18,1993. ACCEPTED revised manuscript September 27, 1994.


Hydrophilic Polymers - American Chemical Society

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