Research Note pubs.acs.org/IECR
Rheo-Raman: A Promising Technique for In Situ Monitoring of Polymerization Reactions in Solution Marie-Claire Chevrel,† Sandrine Hoppe,† Laurent Falk,† Brun Nadège,‡ David Chapron,‡ Patrice Bourson,‡ and Alain Durand*,†,§ †
CNRS, LRGP, UPR 3349, Nancy, F-54001, France Université de Lorraine, LMOPS, EA 4423, Metz, F-57070, France § Université de Lorraine, LCPM, UMR 7568, Nancy, F-54001, France ‡
ABSTRACT: The use of a laboratory-made rheo-Raman device for continuous in situ monitoring of free-radical polymerization of acrylic acid in aqueous solution is reported for the first time. Combining a mixer-type rheometer with a quartz outer cylinder to a Raman spectrometer, this original experimental device has allowed us to overcome three current challenges of polymerization reaction monitoring. First, monomer conversion, polymer formation, and viscosity of reaction medium were continuously monitored with well-defined conditions (composition, temperature, and flow), using Couette analogy. Second, direct correlation between macroscopic viscosity and medium composition was established (accounting for the continuous variation of molar mass distribution). Third, using a mixer-type rheometer provided scalable results, in terms of stirring conditions.
■
INTRODUCTION Free-radical polymerization in aqueous solution is an industrial production process. One of the main challenging problems in designing continuous or batch industrial reactors is to conciliate an efficient heat transfer (high exothermicity of reaction) with the significant increase of viscosity that takes place upon monomer conversion. In addition, industrial performances require the reaction to be carried out in concentrated media (often more than 10 wt %). There is a need for experimental data about reaction progress and viscosity variation, despite significant difficulties. Concentrated reaction media generally exhibit non-Newtonian rheological behavior at high monomer conversions. The molar mass distribution of synthesized macromolecules is continuously varying during the progress of a reaction. Finally, stirring conditions should be precisely defined and scalable between laboratory experiments and industrial conditions. Until now, only partial experimental data have been available. On the one hand, rheokinetics experiments have been reported providing viscosity variation during polymerization in aqueous media but without direct information about monomer conversion and in geometries that are not completely representative of industrial reactors.1−3 On the other hand, continuous monitoring of polymerization reaction has been designed, by continuous characterization of automatically withdrawn samples, providing numerous data about macromolecular structure as a function of monomer conversion in defined reaction conditions but without direct access to the macroscopic properties of the reaction medium itself.4−9 Over the last 20 years, there has been a great deal of work about the coupling of mechanical solicitation experiments with spectroscopic techniques such as nuclear magnetic resonance (NMR),10−12 small-anglwe X-ray scattering (SAXS),13 smallangle neutron scattering (SANS),14 Fourier transform infrared spectroscopy (FTIR),15,16 or Raman spectroscopy.17−20 IR and © 2012 American Chemical Society
Raman spectroscopy are among the most useful techniques, since they provide direct information about the formation or dissociation of covalent bonds and, thus, information about reaction progress. Nevertheless, until now, the reported rheooptical devices involve mechanical tests coupled to synchronized spectroscopic measurements. Although these devices allow multiscale investigation and provide precious data, they are restricted to samples exhibiting a solid-like behavior (polymers for instance). So far, two examples of rheo-optical devices adapted to soft matter sample have been reported, coupling a parallel plate rheometer to a NIR spectrometer.21,22 Nevertheless, to the best of our knowledge, these devices have been focused on in situ monitoring of curing reactions.23 Because of its fast response and its important technological advances over the last ten years, Raman spectroscopy seems to be a tool of choice for the noninvasive investigation of soft matter at the molecular scale (interactions, deformations, chemical reactions, etc.), which can be also adapted to industrial equipments.24,25 In addition, water molecules have weak Raman signals. Nevertheless, only a few papers reported the use of Raman spectroscopy to monitor acrylic acid polymerization in aqueous solution26 or in bulk.27 In that work, we designed an experimental device that allowed simultaneous rheological measurements and Raman spectra acquisition. This was done by coupling rheometric measurements in a defined geometry to Raman spectrometer. We applied this device to monitor the free-radical polymerization of acrylic acid in aqueous solution. Received: Revised: Accepted: Published: 16151
August 1, 2012 November 17, 2012 November 18, 2012 November 19, 2012 dx.doi.org/10.1021/ie302054k | Ind. Eng. Chem. Res. 2012, 51, 16151−16156
Industrial & Engineering Chemistry Research
Research Note
Figure 1. Scheme of the rheometer−Raman spectrometer coupled experiment.
Figure 2. Couette analogy and calibration curve of the mixer-type geometry (obtained with glycerol at 30 °C).
■
Calibration of the Rheometer. Calibration was required to obtain viscosity from torque measurement based on the Couette analogy.28 Briefly, the helical ribbon was assumed to behave as a virtual cylinder of radius Ri (see Figure 2). Ri was determined by an accurate calibration with a Newtonian fluid (glycerol), as well as r*, which represents a given radius where shear rate (γ̇, in s−1) is independent of the nature of the fluid (Ri < r* < Re) and directly proportional to the rotational speed (γ̇ = Kγ̇N). At r*, the viscosity28,29 is accurately defined by the ratio of stress (τ in Pa) to shear rate (eq 1):
EXPERIMENTAL SECTION Design of the Rheo-Raman Setup. The setup (Figure 1) consists of a small-scale reactor (14 mL) wherein polymerization reaction takes place in a rheometer cell including an in situ Raman spectrometer. The rheometer used for those experiments was manufactured by TA Instruments (Model AresG2). The laboratory-made geometry, similar to industrial reactors, consisted of a mobile cylindrical chamber in quartz and a fixed helical ribbon that was used as a mixing device. The rotational speed of the outer cylinder is controlled while measuring the torque on the helical ribbon. Temperature is controlled by a convective oven that is continuously flushed with nitrogen. Because of the small volume of the cylinder where the reaction takes place, and considering the efficient mixing and heating devices, no temperature or concentration gradients were considered in the discussions. Noncontact Raman spectroscopy measurements were made through the oven window and the quartz outer cylinder, thanks to the focalizing system of the probe adapted to the rheometer size. The Raman spectrometer (Model RXN1, Kaiser Optical Systems) had a noncontact fiber-optic probe that offered a spectral resolution of 1.5 cm−1. The laser excitation wavelength was 532 nm, and the laser power was 100 mW.
η(r *) =
τ(r *) Γ = γ(̇ r *) 2π (r *)2 Lγ(̇ r *)
(1)
The calibration was performed with glycerol, within a range of shear rate between 0.1 s−1 and 100 s−1, and at a controlled temperature of 30 °C. The agreement between the values obtained with parallel plate and mixer-type geometries was considered as satisfactory (Figure 2). Two parameters were calculated: the shear rate constant and the stress constant, which are respectively defined as follows (eq 2). Kγ̇ = 16152
γ(̇ r *) N
and
Kτ =
τ Γ
(2)
dx.doi.org/10.1021/ie302054k | Ind. Eng. Chem. Res. 2012, 51, 16151−16156
Industrial & Engineering Chemistry Research
Research Note
Figure 3. (Left) Evolution of raw Raman spectrum during polymerization reaction; an expanded view of the monitored peak is shown in the inset. Reaction progress is indicated by polymer concentration (in wt %). (Right) Calibration curve showing the peak area at 2935 cm−1 measured in situ (after treatment) versus polymer concentration (in wt %) determined by offline 1H NMR analysis.
Figure 4. (a) Simplified kinetic scheme assumed for acrylic acid polymerization (M = AA monomer and P = terminated poly(acrylic acid) chains). (b) Chemical bonds ((1) and (2)) monitored by Raman spectroscopy and used as indicators of chain growth through propagation reactions.
Calibration of the Raman Signal. Several parts of the Raman spectra (Figure 3) could be used to monitor polymerization, especially a peak at 1635 cm−1, corresponding to a vibration mode of the double bond of monomer (CC),27 which decreases with monomer conversion, but also a peak at 2935 cm−1, which characterizes an asymmetric vibration mode of CH bond30 and, thus, the formation of poly(acrylic acid). The second peak was chosen to monitor the reaction. The intensity of Raman spectra were normalized to the OH band of water at ∼3400 cm−1, which remained constant during the reaction. Raman spectra collection was performed during a period of 1 min every 2 min, and each Raman data point was centered on the midtime collection. This interval of time was sufficient considering reaction rates observed in that work and could be drastically reduced if working with more concentrated media. We confirmed the direct correlation between peak area and polymer concentration by analyses performed by 1H NMR on real samples (Figure 3). In addition, we checked that shear rate (between 0.1 and 100 s−1, data not shown) had no significant
effect on this peak area. On the contrary, temperature had a significant influence on peak area. Nevertheless, throughout the reaction process, the temperature of the reaction medium was maintained equal to 60 ± 1 °C, so that any variation of the peak area because of temperature fluctuation could be discarded. Polymerization Reactions in the Rheo-Raman Device. All chemicals were supplied by Aldrich and used without further purification. The required amounts of acrylic acid (AA) and sodium persulfate (NPS) were dissolved in water. Nitrogen was bubbled into the solution during 30 min, before the mixture was introduced into the rheometer. After closing the oven and setting the Raman spectrometer, rotation of the external quartz cylinder was started as well as heating under nitrogen flush. Raman spectrometer and rheometer signals were acquired from that time to the end of the experiment. Constant temperature was reached within 1 min. Once the reaction time elapsed, the oven was opened and a sample of reaction medium was extracted and added to a given amount of hydroquinone. This sample was used for complementary determination of monomer conversion by 1H NMR analysis, using a Bruker 300 16153
dx.doi.org/10.1021/ie302054k | Ind. Eng. Chem. Res. 2012, 51, 16151−16156
Industrial & Engineering Chemistry Research
Research Note
Figure 5. Evolution of viscosity (at 10 s−1) and concentration of polymer during the polymerization process. The initial concentration of monomer was varied between 5 wt % and 15 wt % while the initial concentration of the initiator was kept equal to 0.2 wt %. The abscissa corresponds to reaction time minus the duration of the induction period.
Figure 6. Viscosity variation as a function of polymer concentration during acrylic acid polymerization. Different initial monomer concentrations were used (as indicated on the graph), while the initial initiator concentration was kept equal to 0.2 wt %. The inset shows the final viscosity as a function of initial monomer concentration.
MHz spectrometer, after dissolving the dried extract into D2O. All reported experiments were performed at 60 °C and at 10 s−1. Initial concentrations studied vary from 5 wt % to 15 wt % for the monomer and from 0.04 wt % to 1 wt % for the initiator.
■
We focused our analysis of Raman spectra on the band at 2935 cm−1, characteristic of the asymmetric stretching of the CH bond ((1) in Figure 4b) and thus of poly(acrylic acid) formation.30 Similar results could be obtained with the band at 1635 cm−1, corrresponding to the stretching of the CC bond27 ((2) in Figure 4b). In the considered domain, no influence of initiator concentration on Raman spectra was observed, because its vibration modes were localized at much lower frequencies (strong bands at 1080 and 830 cm−1, data not shown). Experimental conditions allowed us to remain within the Newtonian domain. Reproducibility of experiments was verified and was of the order of experimental uncertainty, i.e., 1 mPa s at 10 s−1 in this geometry for viscosity measurements and negligible for Raman spectrometry, thanks to the high stability of the Raman signal over time. Intrinsic reproducibility of the latter also resulted from the use of peak ratios, which limited
RESULTS AND DISCUSSION
Continuous In Situ Monitoring of Acrylic Acid Polymerization in Aqueous Solution. Free-radical polymerization of acrylic acid in water was studied as a model reaction for which viscosity increases sharply within the course of polymerization and is closely linked to the final properties of the product. We assumed a usual kinetic scheme for acrylic acid polymerization, including initiation, propagation, and termination steps (Figure 4a). Other elementary reactions such as chain transfers to molecular reactants or to macromolecules were not considered in our study. 16154
dx.doi.org/10.1021/ie302054k | Ind. Eng. Chem. Res. 2012, 51, 16151−16156
Industrial & Engineering Chemistry Research
Research Note
Figure 7. Evolution of viscosity (at 10 s−1) and concentration of polymer during polymerization process. The initial concentration of monomer was 7.2 wt %, while the initial concentration of initiator varied between 0.04 wt % and 1.0 wt %. For the sake of clarity, curves have been displaced along the x-axis.
exponent of 2.8 (Figure 5, inset). Considering that the weight average molar mass is proportional to monomer concentration, this variation is consistent with the exponent 3.4 that was suggested previously as the order of magnitude for rheokinetics.34 Impact of Initiator Concentration. Similarly, various initial initiator concentrations were tested, at a given monomer concentration (see Figure 7). Decreasing the initiator concentration reduced the reaction rate but led to higher viscosities of the reaction medium for a given polymer concentration. This trend is consistent with the expected effect of the initiator on the molar distribution. The higher the initiator concentration, the faster the polymerization reaction and the shorter the formed macromolecules. For that series of experiments, initial monomer concentration being kept constant, the ionic strength of the aqueous medium was the same for all reactions. These results demonstrate that rheo-Raman device provides reliable in situ continuous monitoring of acrylic acid polymerization under defined flow conditions. These preliminary results aimed at establishing the feasibility and interest of such rheooptic monitoring of solution polymerization. A more systematic study of the effects of reactant concentrations (monomer and initiator), temperature, and stirring speed (i.e., shear rate) on polymerization kinetics and variation of rheological properties of reaction medium is ongoing.
the potential impact of viscosity variations on the spectra. Inductions periods were removed in all curves. First, we confirmed the correlation between peak area determined by in situ Raman measurements under flow and polymer concentration, based on offline analysis after sampling the final reaction medium at various initial monomer concentrations. These offline analyses were performed by 1H NMR in D2O. A linear relation was found between the area of Raman signal and polymer concentrations calculated using 1H NMR results (Figure 3). The reliability of polymer concentrations calculated from in-line Raman measurements under flow was demonstrated. Impact of Monomer Concentration. Using the rheoRaman device, viscosity variation (at a fixed shear rate of 10 s−1) as well as polymer formation (with the peak area at 2935 cm−1) were monitored at various initial monomer concentrations (see Figure 5). The higher the initial monomer concentration, the faster the polymer formation and the sharper the viscosity increase. For the most-concentrated reaction medium (initial acrylic acid concentration = 15 wt %), the viscosity increased over nearly four decades. Nevertheless, no gel effect could be detected either by reaction monitoring or by viscosity variation over that range of concentrations, as reported in the literature.31 Combining Raman and rheological data, a direct correlation between composition and viscosity of the reaction medium over the entire reaction progress was established (Figure 6). The rigorous discussion of viscosity evolution with monomer conversion must take into account at least three aspects: the effect of initial monomer concentration on the molar mass distribution of terminated macromolecules, the solution behavior of non-neutralized poly(acrylic acid) as a function of concentration, the effect of monomer and polymer concentration on the ionic strength of the aqueous medium (because of their self-dissociation). A complete discussion of these aspects would be out of the scope of this paper; thus, we will focus on the domain of polymer concentration, where viscosity is ∼50 times higher than the initial viscosity, which should correspond to the entangled regime.32,33 At a given polymer concentration of 4 wt %, the viscosity varied with initial monomer concentration, following a power law with an
■
CONCLUSION For the first time, rheo-Raman data were obtained and allowed a complete and complementary in situ monitoring of solution polymerization of acrylic acid in water. Data about polymer formation were compared to offline 1H NMR results, demonstrating the reliability of Raman spectrometry under flow. Furthermore, simultaneous rheological measurements were carried out using a mixer-type mobile, thus providing laboratory-scale data similar to industrial conditions. A morecomplete study of acrylic acid polymerization in aqueous solution is ongoing; in particular, the contribution of the rheoRaman technique to the study of gel effect will be investigated. The occurrence of a gel effect at sufficiently high monomer concentrations or polymer molar masses, as well as the 16155
dx.doi.org/10.1021/ie302054k | Ind. Eng. Chem. Res. 2012, 51, 16151−16156
Industrial & Engineering Chemistry Research
Research Note
(13) Somani, R. H.; Yang, L.; Hsiao, B. S.; Agarwal, P. K.; Fruitwala, H. A.; Tsou, A. H. Shear-induced precursor structures in isotactic polypropylene melt by in-situ rheo-SAXS and rheo-WAXD studies. Macromolecules 2002, 35, 9096−9104. (14) Porcar, L.; Eberle, A. P. R. Flow-SANS and rheo-SANS applied to soft matter. Curr. Opin. Colloid Interface Sci. 2012, 17, 33−43. (15) Hoffmann, U.; Pfeifer, F.; Okretic, S.; Völkl, N.; Zahedi, M.; Siesler, H. W. Rheo-optical Fourier transform infrared and Raman spectroscopy of polymers. Appl. Spectrosc. 1993, 47, 1531−1539. (16) Hofmann, G. R.; Sevegney, M. S.; Kannan, R. M. A rheo-optical FTIR spectrometer for investigating molecular orientation and viscoelastic behavior in polymers. Int. J. Polym. Anal. Charact. 2004, 9, 245−274. (17) Chai, C. K.; Dixon, N. M.; Gerrard, D. L.; Reed, W. RheoRaman studies of polyethylene melts. Polymer 1995, 36, 661−663. (18) Rodriguez-Cabello, J. C.; Merino, J. C.; Fernandez, M. R.; Pastor, J. M. Rheo-optical Fourier transform Raman study of the conformational changed in uniaxially stretched amorphous bulk poly(ethylene terephtalate). J. Raman Spectrosc. 1996, 27, 23−29. (19) Rodriguez-Cabello, J. C.; Merino, J. C.; Jawhari, T.; Pastor, J. M. Rheo-optical Raman study of chain deformation in uniaxially stretched bulk polyethylene. Polymer 1995, 36, 4233−4238. (20) Rodriguez-Cabello, J. C.; Merino, J. C.; Jawhari, T.; Pastor, J. M. Rheo-optical Raman study of chain deformation in uniaxially stretched bulk isotactic polypropylene. J. Raman Spectrosc. 1996, 27, 463−467. (21) Botella, A.; Dupuy, J.; Roche, A.-A.; Sautereau, H.; Verney, V. Photo-rheometry/NIR spectrometry: An in situ technique for monitoring conversion and viscoelastic properties during photopolymerization. Macromol. Rapid Commun. 2004, 25, 1155−1158. (22) Benali, S.; Bouchet, J.; Lachenal, G. Chemeorheology: a new design for simultaneous rheological and Fourier transform near infrared analysis. J. Near Infrared Spectrosc. 2004, 12, 5−13. (23) Verney, V.; Commereuc, S. Molecular evolution of polymers through photoageing: A new UV in situ viscoelastic technique. Macromol. Rapid Commun. 2005, 26, 868−873. (24) Mozharov, S.; Nordon, A.; Girkin, J. M.; Littlejohn, D. Noninvasive analysis in micro-reactors using Raman spectroscopy with a specially designed probe. Lab Chip 2010, 10, 2101−2107. (25) Fonseca, G. E.; Dubé, M. A.; Penlidis, A. A critical overview of sensors for monitoring polymerizations. Macromol. React. Eng. 2009, 3, 327−373. (26) Yu, J. A.; Liu, H. Z.; Chen, J. Y. FT-Raman spectroscopy for monitoring the polymerization of acrylic acid in aqueous solution. Chin. J. Polym. Sci. 1999, 17, 603−606. (27) Murli, C.; Song, Y. Pressure-induced polymerization of acrylic acid: A Raman spectroscopic study. J. Phys. Chem. B 2010, 114, 9744− 9750. (28) Aït-Kadi, A.; Marchal, P.; Choplin, L.; Chrissemant, A.-S.; Bousmina, M. Quantitative analysis of mixer-type rheometers using the Couette analogy. Can. J. Chem. Eng. 2002, 80, 1166−1174. (29) Choplin, L.; Marchal, P.; Baravian, C.; Langevin, D. Rhéologie et produits formulés complexes. Tech. Ing. 2010, (March 10), Ref J2145. (30) Walczak, W. J.; Hoagland, D. A.; Hsu, S. L. Spectroscopic evaluation of models for polyelectrolyte chain conformation in dilute solution. Macromolecules 1996, 29, 7514−7520. (31) Scott, R. A.; Peppas, N. A. Kinetic study of acrylic acid solution polymerization. AIChE J. 1997, 43, 135−144. (32) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. Scaling theory of polyelectrolyte solutions. Macromolecules 1995, 28, 1859−1871. (33) Harrington, J. C. Charge density effects on polyelectrolyte dynamic rheology. J. Appl. Polym. Sci. 2008, 107, 3310−3317. (34) Rosendale, D.; Biesenberger, J. A. Rheokinetic measurements of step- and chain-addition polymerizations. Adv. Chem. Ser. 1990, 227, 267−283.
consequences on kinetics of monomer consumption and viscosity variation will be described and discussed. Rheo-Raman appears to be a new and complementary method for the well-known continuous monitoring techniques.8,9 The obtained data are of primary interest for the design of batch or continuous polymerization reactors, especially concerning the effects of reaction conditions (initial monomer and initiator concentrations) and process parameters (temperature and stirring speed) on polymer characteristics and properties of reaction medium. Finally, this new coupled technique could be extended to other polymerization media such as organic solutions, bulk, or dispersions, as well as other polymerization mechanisms, such as step polymerizations. In addition, this device may be used to monitor the formation or dissociation of noncovalent bonds under shear in the case of soft matter samples. Such developments are ongoing.
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: + 33 (0)3 83 17 52 92. Fax: + 33 (0)3 83 37 99 77. Email:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We want to thank the financial support of EU (F3 Factory Project No. 228867) for the Ph.D. of M.-C.C. We are grateful to S. Ben Omrane for her experimental assistance in the rheological study.
■
REFERENCES
(1) Savart, T.; Dove, C.; Love, B. J. In situ dynamic rheological study of polyacrylamide during gelation coupled with mathematical models of viscosity advancement. Macromol. Mater. Eng. 2010, 295, 146−152. (2) Cioffi, M.; Hoffman, A. C.; Janssen, L. P. B. M. Rheokinetics and the influence of shear rate on the Trommsdorf (gel) effect during free radical polymerization. Polym. Eng. Sci. 2001, 41, 595−602. (3) Cioffi, M.; Ganzeveld, K. J.; Hoffman, A. C.; Janssen, L. P. B. M. Rheokinetics of linear polymerization. A literature review. Polym. Eng. Sci. 2002, 42, 2383−2392. (4) Alb, A. M.; Drenski, M. F.; Reed, W. F. Automatic continuous online monitoring of polymerization reactions (ACOMP). Polym. Int. 2008, 57, 390−396. (5) Alb, A. M.; Reed, W. F. Recent advances in automatic continuous online monitoring of polymerization reactions (ACOMP). Macromol. Symp. 2008, 271, 15−25. (6) Frauendorfer, E.; Wolf, A.; Hergeth, W.-D. Polymerization online monitoring. Chem. Eng. Technol. 2010, 33, 1767−1778. (7) Kulichikhin, S. G.; Malkin, A. Y.; Polushkina, O. M.; Kulichikhin, V. G. Rheokinetics of free-radical polymerization of acrylamide in an aqueous solution. Polym. Eng. Sci. 1997, 37, 1331−1338. (8) Alb, A. M.; Reed, W. F. Fundamental measurements in online polymerization reaction monitoring and control with a focus on ACOMP. Macromol. React. Eng. 2010, 4, 470−485. (9) Rosenfeld, C.; Serra, C.; O’Donohue, S.; Hadziioannou, G. Continuous online rapid size exclusion chromatography monitoring of polymerizationsCORSEMP. Macromol. React. Eng. 2007, 1, 547− 552. (10) Callaghan, P. T. Rheo-NMR and velocity imaging. Curr. Opin. Colloid Interface Sci. 2006, 11, 13−18. (11) Callaghan, P. T.; Gil, A. M. Rheo-NMR of semidilute polyacrylamide in water. Macromolecules 2000, 33, 4116−4124. (12) Nakatani, A. I.; Poliks, M. D.; Samulski, E. T. NMR investigation of chain deformation in sheared polymer fluids. Macromolecules 1990, 23, 2686−2692. 16156
dx.doi.org/10.1021/ie302054k | Ind. Eng. Chem. Res. 2012, 51, 16151−16156
