Degradation of Poly(ethylene oxide) in Aqueous Solutions by

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Degradation of Poly(ethylene oxide) in Aqueous Solutions by Ultrasonic Waves Michel Duval* and Elisa Gross Institut Charles Sadron (UPR 022) (CNRS−ULP), 23 Rue du Loess, BP 84047, 67034 Strasbourg Cedex 2, France ABSTRACT: The effects of ultrasonic waves on aqueous solutions of poly(ethylene oxide) (PEO) are reported. Static and dynamic light scattering measurements are performed on dilute irradiated solutions for various exposure times in order to characterize the polymer species that are present in the solutions at the end of the process. A progressive aggregation of the polymer chains is observed for the sample having the lower weight-average molecular weight (Mw = 6 kDa). These aggregates are stable but they are break by addition of sodium chloride (NaCl) in the aqueous solutions and the initial chains are recovered. The high molecular weight sample (Mw = 2 × 103 kDa) shows a different behavior under irradiation. The molecular weight of the processed polymer is lower than the initial molecular weight. However by addition of NaCl the molecular weight of the species decreases. This indicates that, in a first time, the large polymer chains are cut in smaller chains but furthermore they aggregate in stable clusters that are break by NaCl addition. The cluster formation is imputed to the instantaneous local increase of concentration of the hydrophobic −CH2−CH2− groups that belong to different PEO chains in the field of the ultrasonic waves.

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INTRODUCTION There has been a large amount of research interest in the field of the poly(ethylene oxide) (PEO) solutions. This interest is due to the fundamental importance and wide applications1−3 of this polymer. Two main problems have been observed and investigated on PEO/water solutions. The first problem concerns the tendency of PEO to aggregate in aqueous solution. However somewhat contradictory results are found in the literature about the ability of PEO to be perfectly soluble in water. Some results confirm that PEO is completely soluble in aqueous solutions4−8 but the formation of aggregates on the same system has been reported in many other studies.9−15 In a recent work we have shown that the cluster formation in PEO aqueous solutions depends on the history of the solutions16 and that, once the aggregates are formed, they can be break by addition of sodium chloride (NaCl) to the polymer solutions. The second specific effect observed in the aqueous PEO solutions is the ability of the chains to degraded easily under treatment such as mechanical stirring by a magnet17 or by submitting the solutions to a turbulent flow by passing for example these solutions through a syringe needle.18 For such treatments it is observed that low molecular weight samples (Mw = 6 kDa) lead to the formation of stable aggregates that are dissolved by NaCl addition while large molecular weight samples (Mw = 2 × 103 kDa) have a more complex behavior. In a first time we observe that, after treatment, the PEO chains have lower molecular weight than the initial chains. However, after addition of NaCl to the processed aqueous solutions this molecular weight decreases to lower value. The conclusion of this observation is that, in a first time, the PEO molecules are © XXXX American Chemical Society

cut in smaller chains but, in a second time, these smaller chains aggregate in clusters under the influence of the shear field to which they are submitted. In the current report we analyze the effects produced by ultrasonic waves on aqueous PEO solutions of low (Mw = 6 kDa) and high (Mw = 2 × 103 kDa) weightaverage molecular weight sample. It is with this aim in view that we have submitted aqueous dilute solutions to ultrasonic waves at several exposure times. The effects are analyzed by submitting directly the irradiated solutions to static and dynamic light scattering measurements in order to characterize the entities resulting from the processing by their molecular weight, their hydrodynamic radius and their concentration. The influence of ultrasonic waves in PEO solutions has already been studied.19−22 These studies show that the bonds in the PEO chains are ruptured due to the strong hydrodynamic shear force created by the ultrasonic waves. In the current study we analyze deeply these effects produced on a low molecular weight and a high molecular sample. In the light of the results we suggest a model for the general behavior of PEO chains in aqueous solution submitted to a shear field.

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EXPERIMENTAL SECTION

Materials and Preparation of the Solutions. All the solvents used in this study are spectroscopic purity grade products. Freshly deionized water is used to prepare aqueous PEO solutions. All the measurements are performed at 25 °C. Two PEO samples were used.

Received: April 11, 2013 Revised: May 27, 2013

A

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P(θ) is the particle scattering function and θ the scattering angle. In the case of vertically polarized incident light the optical constant K is given by

The low molecular weight sample PEO6 is a commercial sample from Fluka Co. The high molecular weight sample (PEO2000) is purchased from Aldrich Co. The characteristics of the samples are listed in Table 1.

K = 4π 2n0 2( dn/ dc)2 /λ 0 4Na

Table 1. Properties of PEO Samples sample

PEO6

PEO2000

Mwa (kDa) polydispersity indexa Mwb (kDa) A2b × 103 (cm3·mol·g−2) RGb (nm) RHb (nm)

7.4 1.02 6.0; 6.6; 6.5 2.0; 3.4; 3.8 3.0c 2.2; 2.5; 2.4

2130 2.60 1700; 1980; 2200 0.6; 0.7; 1.0 122; 125; 142 73; 81; 85

where n0 is the refractive index of the solvent, (dn/dc) is the refractive index increment of the polymer/solvent system, λ0 is the wavelength of the light source in vacuum, and Na is the Avogadro’s number. Dynamic Light Scattering (DLS). The analysis of the dynamic data is performed by fitting the experimentally measured g(2)(t), the normalized intensity autocorrelation function of the scattered light which is related to the electric field correlation function g(1)(t) by the Siegert relationship

a

Weight-average molecular weight (Mw) and polydispersity index measured by gel permeation chromatography (GPC) in tridistilled water/0.1N NaNO3 bWeight-average molecular weight (Mw), osmotic virial coefficient (A2), radius of gyration (RG) and hydrodynamic radius (RH) measured by SLS and DLS in methanol, deionized water, and deionized water/0.2N NaCl, respectively. cRadius of gyration (RG) measured by neutron scattering in deuterated water (D2O).

g(2)(t ) = 1 + β |g(1)(t )|2

(3)

where β is the coherence factor of the instrument. For polydisperse samples g(1)(t) can be written as the inverse Laplace transform of the relaxation time distribution function G(τ) g(1)(t ) =

It should be noted that the samples are characterized in three different solvents namely deionized water, (deionized water/0.2N NaCl) and methanol. This point is very important in the case when former history of the commercial sample is not known. One should be sure that the initial polymer does not contain aggregates16 from the beginning. It is verified by dynamic light scattering (DLS) that, before the irradiation, all the initial PEO solutions give a monomodal distribution of the relaxation time. The protocol for the preparation of the solutions is very strict. The polymer chains must be perfectly dissolved and one must avoid degradations of any sort either mechanical or chemical. All the solutions are prepared gravimetrically. At first they are maintained in an oven for 6 h at 55 °C. Then they are gently shaken in a dark room at 22 °C for 12 h. No magnetic stirrer is used to avoid cutting of the polymer chains. They are put again in the oven at 55 °C for 6 h and again agitated in the dark for 12 h. The solutions of lower concentration are prepared from stock solutions by dilution. The optical clarification of the solutions is carried out by filtering slowly without any pressure directly into the light scattering cells through 0.45 μm (nominal pore size) polytetrafluoroethylene (hydrophilic PTFE) filters from Millipore Co. The aqueous 0.2N NaCl/PEO solutions are prepared by directly adding the NaCl salt in weighted fraction in the irradiated solutions. Instruments. The irradiation of the solutions by ultrasound is performed using an Elmasonic S30/Elma GmbH&Co. KG setup. The frequency of the ultrasonic waves is 37 Khz and the input power 80 W. SLS and DLS measurements are run with an ALV/DLS/SLS-5020F experimental setup (ALV-Laser Vertriebsgesellshaft mbH, Langen, Germany) equipped with a vertically polarized light of a 22 mW He− Ne laser of 632.8 nm as an incident beam, a compact ALV/CGS-8 goniometer system and an ALV-5000 autocorrelator. The measurements are performed in the angular range 22° < θ < 145°.

∫ τG(τ) exp(−t /τ) d(ln τ)

(4)

where t is the lag-time. The relaxation time distribution function G(τ) is obtained by the CONTIN analysis24 of the correlation functions of the scattered light. The diffusion coefficient D is calculated from the second moment of the distribution as D = 1/τq2

(5)

where q is the scattering wave vector defined by q = (4π /λ 0)n0 sin(θ /2)

(6)

In the dilute regime D varies linearly with the polymer concentration according to D = D0(1 + kDC + ...)

(7)

D0 is the diffusion coefficient at infinite dilution and kD is the hydrodynamical virial coefficient. The Stokes−Einstein equation relates D0 to the hydrodynamic radius RH of the scattering particles

RH = kBT /6πηD0

(8)

kB is the Boltzmann’s constant, T is the absolute temperature, and η the viscosity of the solvent. Combined SLS and DLS. The correlation function of the light scattered by a mixture of monodisperse species consisting of aggregates of molecular weight Mag w and individual chains of the same chemical nature of molecular weight Miw shows two relaxation modes when the two scattering species have very different molecular weights. A combination of the SLS and DLS results provides information on the aggregation number as well as the weight fraction of the aggregates in the solution.25−27 The CONTIN analysis of the correlation functions of the scattered light gives two peaks and the area ratio A of these two peaks is given by

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THEORETICAL BACKGROUND FOR LIGHT SCATTERING MEASUREMENTS Static Light Scattering (SLS). The evaluation of the static light scattering measurements is carried out using the Zimm relationship23 KC /ΔR θ = 1/M w P(θ ) + 2A 2 C + 3A3C 2 + ...

(2)

(1)

−3

where C is the polymer concentration in g·cm , Mw is the weight-average molecular weight and A2 and A3 the second and third virial coefficients, respectively. K is the optical constant, and Rθ is the Rayleigh ratio of the measured excess of intensity of the light scattered by the solution over that of the solvent.

A = Ai /Aag = M w*iC i /M w*ag C ag

(9)

where Mw* and Mw* are the apparent weight-average molecular weight at finite concentration Ci and Cag of the well solvated molecules and aggregates, respectively. Extrapolated to zero i

B

ag

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inverse of the scattered light as a function of the scattering wave vector and the polymer concentration. The extrapolated value at zero angle and zero concentration leads to a molecular weight of 47 kDa much higher than the initial value before irradiation. Figure 2 shows the normalized intensity correlation function for the initial and the irradiated PEO6 aqueous solution after 24 h of exposure.

concentration and zero scattering wave vector this relation becomes →0 i i i ag ACq → 0 = x M w /(1 − x )MW

(10)

i

where x is the weight fraction of the nonaggregated species. The weight-average molecular weight Mapp w measured by SLS on the mixture is given by: MWapp = x iMWi + (1 − x i)MWag

(11)

Aq→0 C→0

Knowing the ratio measured by DLS we are able to determine the quantities xiMiw and (1 − xi)Mag w using eqs 10 and 11. Since the molecular weight Miw of the individual chains in the mixture is known provided the polymer is not degraded during the process we can calculate xi and hence Mag w.

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RESULTS AND DISCUSSION An aqueous solution (200 cm3) of the low molecular weight sample PEO6 is prepared by gravimetry at the concentration 2 × 10−2 g·cm−3. The distribution function of the relaxation time of the correlation function of the intensity of the light scattered by this solution is monomodal and the hydrodynamic radius associated with this relaxation time is 2.4 nm. Furthermore, starting from this concentrated solution we have prepared four dilutions (4C/5, 3C/5, 2C/5, and C/5). Classical integrated light scattering measurement has been made on these five solutions, and following the Zimm treatment, the extrapolated value at zero angle and zero concentration of the light scattered leads to a value of Mw = 7000 Da. These results (Mw = 7000 Da and RH = 2.4 nm) indicate that the PEO6 molecules in the aqueous solution at this mother concentration (2 × 10−2 g·cm−3) are well solvated. Four samples (30 cm3 each) of the mother solution are submitted successively to the ultrasonic waves at different exposure times (2, 4, 12, and 24 h respectively). At the end of the process four solutions (4C/5, 3C/5, 2C/5, and C/5 respectively) are prepared by adding freshly deionized water to each irradiated mother solution. Full SLS and DLS measurements are performed in the angular range 22° < θ < 145° on these solutions. Figure 1 is related to PEO6 aqueous solution irradiated for 24 h. It is the classical Zimm representation of the

Figure 2. Normalized intensity autocorrelation function for aqueous solution of PEO6 (×) and PEO6 after 24 h of irradiation (○). Polymer concentration C = 2 × 10−2 g·cm−3. Scattering angle θ = 22°.

The associated distribution function of the relaxation times as given by the CONTIN analysis is shown in Figure 3.

Figure 3. Distribution function of the relaxation times of the autocorrelation function of the intensity of the light scattered by aqueous solution of PEO6 (dash line) and PEO6 after 24 h of irradiation (dot line). Polymer concentration C = 2 × 10−2 g·cm−3. Scattering angle θ = 22°.

All the aqueous PEO6 solutions irradiated at the different exposure times investigated show this bimodal distribution. The two modes of relaxation are well separated and their respective amplitudes are significant. For example Figure 4 shows the variation of the relative amplitude of the fast mode A1 = Ai/(Ai + Aag) as a function of the square of the scattering wave vector

Figure 1. Variations of the inverse of the light scattered by PEO6 aqueous solutions as a function of the square of the scattering wave vector and the polymer concentration. The mother solution (C = 2 × 10−2g·cm−3) has been sonicated for 24 h. C

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−2

Figure 6. Variations of the hydrodynamic radius of the isolated molecules (RiH) and of the aggregates (Rag H ) of PEO6 aqueous solutions as a function of the polymer concentration. The mother solution (C = 2 × 10−2g·cm−3) has been sonicated for 24 h.

−3

Figure 4. Sonicated PEO6 aqueous solution (C = 2 × 10 g·cm ; exposure time, 24 h). Variations of the amplitude of the fast mode as a function of the square of the wave vector.

for the POE6 aqueous solution (C = 2 × 10−2 g·cm−3) irradiated for 24 h. The two relaxation modes observed by DLS in these solutions are diffusive modes as it appears in Figure 5 where the

Table 2 shows the variations of these two parameters and of the hydrodynamic radius of the species in the solution. Table 2. Variations of the Weight-Average Molecular Weight ag (Mag w ), the Weight Fraction (x ), and the Hydrodynamic ag Radius (RH ) of the Aggregates in the PEO6 Aqueous Solutions Submitted to Various Exposure Times to Ultrasonic Waves and after NaCl (0.2 N) Addition to the Irradiated Solutions (Miw, RiH) exposure time (h)

Mag w (kDa)

xag (%)

Rag H (nm)

Miw (kDa)

RiH (nm)

2 4 12 24

12 15 32 47

28 33 40 51

85 79 88 94

6.7 6.2 6.5 6.2

2.5 2.4 2.4 2.5

One can note that the hydrodynamic radius RiH associated with the fast relaxation time is constant with the exposure time and is equal to the RH value of the well-solvated PEO6 molecules. The non aggregated PEO6 molecules are not cut by the ultrasonic waves. This observation is coherent with the fact that, after addition of NaCl (0.2 N) to the irradiated solution, the value of the molecular weight of the solvated species measured by SLS is comparable to the value obtained on the initial solution before the process. Polik and Burchard9 have ascribed chain aggregation in aqueous solutions to hydrophobic interactions via the increase in the ordering of water molecules in the vicinity of PEO chains. The addition of salt breaks the structure of these solutions. We can note that the molecular weight and the weight fraction of the aggregates increase with the exposure time as indicated in Table 2 that means that the process of aggregation is continuous. However the hydrodynamic radius of these clusters seems to be constant but has a high value (≅85 nm) taking account of the low value of the molecular weight of the aggregates. The same observation has been made on PEO6 aqueous solutions that have been passed through the needle of a syringe.18 The same protocol as for the PEO6 is applied for the study of the PEO2000 aqueous solutions. These solutions of the high molecular weight sample have a different behavior. Indeed the correlation functions of the intensity of the light scattered by the solutions of PEO2000 are always monomodal as shown on

Figure 5. Sonicated PEO6 aqueous solution (C = 2 × 10−2 g·cm−3; exposure time: 24 h). Variations of the inverse of the relaxation time of the fast mode (τi) and of the slow mode (τag) as a function of the square of the wave vector.

linear variations of the inverse of the relaxation time of the fast mode (τi) and of the slow mode (τag) are shown as a function of the square of the scattering wave vector for the same solution (C = 2 × 10−2 g·cm−3; exposure time =24 h). Finally, Figure 6 shows the variation of the hydrodynamic radii of the two species (isolated molecules and aggregates) as a function of the polymer concentration. The extrapolation at zero concentration gives 2.5 nm for the isolated species equal to the measured value before irradiation and 94 nm for the aggregates. The combined SLS and DLS results allow us to calculate the weight fraction and the weight-average molecular weight of the aggregates (see eqs 9-11). These values are given in a first approximation taking account of the fact that the species are not strictly monodisperse and knowing that the values obtained by SLS are w-averaged and z-averaged for DLS measurements. D

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Figures 7 and 8 whatever is the exposure time to the ultrasonic waves.

Table 3. Variations of the Weight-Average Molecular Weight ag (Mag w ), and the Hydrodynamic Radius (RH ) of the Aggregates in the PEO2000 Aqueous Solutions Submitted to Various Exposure Times to Ultrasonic Waves and after NaCl (0.2 N) Addition to the Irradiated Solutions (Miw, RiH) exposure time (h)

Mag w (kDa)

Rag H (nm)

Miw (kDa)

RiH (nm)

2 4 12 24

1400 530 200 140

98 103 89 90

400 80 40 20

25.5 9.6 6.6 4.4

The same observation is made on the well-solvated chains after the break of the aggregates by NaCl addition. The values of the hydrodynamic radius of these well-solvated molecules measured by DLS are in agreement with the values obtained on low molecular weight PEO aqueous solutions. However, as for the case of the PEO6 solutions, the values of the hydrodynamic radii of the aggregated species are high as compared to their molecular weight. Differently of the aqueous solutions of the low molecular weight sample where the ultrasonic waves lead to the formation of stable aggregates in a one step process, the large chains of PEO2000 are first cut in small chains that afterward aggregate in clusters. We have noted that as well as for the processed solutions of PEO6 and PEO2000 the value of the hydrodynamic radius of the aggregates is high compared to the molecular weight. For example, in the case of the PEO2000, it clearly appears that the value of the hydrodynamic radius of the clusters that are created after 24 h of irradiation is of the same order of magnitude (90 nm) as the value of the initial PEO2000 sample (≅80 nm) while the value of the molecular weight of these clusters is much more lower (140 kDa) compare to the initial value (≅2000 kDa). This is a specificity of PEO clusters that has already been observed in other studies17,18 where the PEO chains have been submitted to other processes (stirring by a magnet or passing through the needle of a syringe). Neutron scattering measurements18 on these clusters have shown that they are loose structures of short chains. Finally we mention that PEO6 molecules solved in (water/0.2 N NaCl) and submitted to irradiation show no aggregation and PEO2000 molecules in the same solvent are cut but do not show any subsequent aggregation. These observations lead us to conclude that the cluster formation is due to the presence of hydrophobic interactions of the −CH2−CH2− groups of the PEO chains. The field of the ultrasonic waves creates possibly a local increase of the concentration of these hydrophobic groups associated with a stretching of the PEO molecules that favors permanent bonds between adjacent chains.

Figure 7. Normalized intensity autocorrelation function for aqueous solution of PEO2000 (×) and PEO2000 after 20 4 h of irradiation (○). Polymer concentration C = 4.5 × 10−4 g·cm−3. Scattering angle = 30°.

Figure 8. Distribution function of the relaxation time of the autocorrelation function of the intensity of the light scattered by aqueous solution of PEO2000 (dash line) and PEO2000 after 24 h of irradiation (dot line). Polymer concentration C = 4.5 × 10−4 g·cm−3. Scattering angle θ = 30°.

At first sight it seems that the correlation function is identical to the correlation function of the initial solution before irradiation. It is only after addition of NaCl to the processed aqueous solutions that it appears that the PEO molecules have been degraded. Indeed the molecular weight of the species in the (water/0.2N NaCl) solution measured by SLS is considerably low compared to the molecular weight of the initial PEO2000 molecule and of the aggregates. The results obtained by SLS and DLS measurements on the irradiated PEO2000 aqueous solutions are given in Table 3. It is shown that the molecular weight of the cut chains and of the aggregates decreases as the exposure time increases. Meanwhile the degree of association increases with the exposure time (between 3 and 4 for 2 h and 7 for 24 h) as observed for the low molecular weight sample.

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CONCLUSION The previous studies19−21 on the effects of the ultrasonic waves on the PEO dilute solutions have shown that the polymer chains are cut in smaller entities. This effect increases with the exposure time. The current study shows that in fact the situation is slightly more complex. For the small chains there is only aggregation of the molecules in the field of the ultrasonic waves. The degree of association and the weight fraction of aggregates increases with the time exposure. One can think that, for higher power of the ultrasonic waves or for higher exposure time the small chains could be degraded in smaller chains than in the initial state. For larger PEO chains a two step process is considered. In a first time the chains are break in E

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entities of lower molecular weight. Afterward they aggregate in clusters. The molecular weight of these clusters decreases when the exposure time increases. In the flow due to the high shear field created by the radiations, the linear PEO chains are stretched while there is an instantaneous local increase of the polymer concentration. The hydrophobic characteristic of the −CH2−CH2− groups that compose the PEO chains associated simultaneously with the local increase of concentration of these groups that belong to several neighboring chains leads to the formation of permanent loose clusters. In the current study we find similar effects as those observed when the chains are exposed to high shear field creates in a solution submitted to mechanical turbulence induced by a magnet17 or by crossing through the needle of a syringe.18 This phenomenon of break and aggregation seems to be a general behavior of the PEO chains in aqueous solutions under the effect of a shear field.

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AUTHOR INFORMATION

Corresponding Author

*(M.D.) Telephone: 33(0)388414113. Fax: 33(0)388414099. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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