Toward Copolymers with Ideal Thermosensitivity: Solution Properties

Sep 19, 2012 - Toward Copolymers with Ideal Thermosensitivity: Solution. Properties of Linear, Well-Defined Polymers of N‑Isopropyl. Acrylamide and ...
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Toward Copolymers with Ideal Thermosensitivity: Solution Properties of Linear, Well-Defined Polymers of N‑Isopropyl Acrylamide and N,N‑Diethyl Acrylamide Felix A. Plamper,*,† Alexander A. Steinschulte,† Christian H. Hofmann,† Natascha Drude,† Olga Mergel,† Christian Herbert,‡ Michael Erberich,‡ Bjoern Schulte,‡ Roland Winter,§ and Walter Richtering*,† †

Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany DWI an der RWTH Aachen e.V. and Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Pauwelsstr. 8, 52056 Aachen, Germany § Physical Chemistry I−Biophysical Chemistry, TU Dortmund University, Otto-Hahn Str. 6, 44227 Dortmund, Germany ‡

S Supporting Information *

ABSTRACT: Statistical copolymers of N-isopropyl acrylamide (NIPAM) and N,N-diethyl acrylamide (DEAAM) show a pronounced synergistic depression in their cloud points, though both homopolymers phase separate at significantly higher temperatures close to 30 °C (e.g., Polymer 2009, 50, 519). While phase separation occurs at 20 °C for the statistical copolymers, the influence of the monomeric sequential arrangement along the backbone was not addressed so far. Thus, we report on the thermosensitive properties of a diblock copolymer PDEAAM-b-PNIPAM and compare it to the homopolymers, mixtures thereof, and to the statistical copolymer of the same molecular weight. These polymers were prepared by controlled radical polymerization, namely Reversible Addition− Fragmentation Chain Transfer (RAFT). Their solution behavior was mainly studied by infrared spectroscopy (IR) of the amide I′ band and by turbidimetry. IR spectroscopy sees a decreasing hydration with heating even below the cloud point for all polymers. This results finally in phase separation, which induces further spectral changes. Rather unexpectedly, the diblock copolymer shows phase separation at temperatures close to the homopolymers, well above the cloud points of the homopolymer mixtures. In turn, the transition temperature of the homopolymer mixture is reduced compared to its homopolymers, which indicates intermolecular attraction between both partners. This behavior can be explained by taking the block length dependencies of the respective cloud points into account and assuming a rather independent phase behavior of each short block (within the copolymer). Then, the increased inherent cloud point of each “half-length” block (compared to the homopolymers) has a stronger effect than the aggregating tendency inherited by the connectivity of the comonomer units. As a result, IR spectroscopy reveals almost ideal behavior of the diblock copolymer, which can be comprehended as an ideal mixture of the homopolymers, each one contributing to the overall signal by its concentration. Finally, 1H NMR suggests that intermediate aggregation (as seen by light scattering) is not induced by segregation of just one block, but rather by partial and weak complexation between the two components within the diblock copolymer.

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INTRODUCTION The class of waterborne, thermoresponsive polymers has gained great attention in macromolecular science over the past 20 years. Poly(N-isopropyl acrylamide) (PNIPAM) is a famous example, which shows an LCST-behavior in water, meaning phase separation upon heating and upon exceeding the cloud point (the LCST is the Lower Critical Solution Temperature of the binary phase diagram). Only slightly dependent on the molar mass and on the concentration, PNIPAM turns typically water-insoluble above 31 °C.1,2 Interestingly, poly(N,N-diethyl acrylamide) (PDEAAM) exhibits a very similar LCST at approximately 30 °C, though the structure has changed by substitution of the polar N−H bond with another alkyl group. Despite the similarities in their phase behavior, differences for both polymers are known. For example, the phase transition is © 2012 American Chemical Society

rather broad for PDEAAM, whereas PNIPAM shows a sharp transition (as seen by calorimetry).3 The molecular weight has a slightly stronger influence on the cloud points of PDEAAM as compared to those of PNIPAM.2,4,5 PNIPAM exhibits cononsolvency (insolubility in a mixture of good solvents),6 which is hardly detectable for PDEAAM.7 Further, statistical copolymers of NIPAM and DEAAM show a nonideal, synergistic suppression of the cloud points.8−11 This is in contrast to the ideal behavior of other acrylamide copolymers (like those of NIPAM and N-isopropyl methacrylamide).12 The nonideal effect is most pronounced for Received: July 30, 2012 Revised: September 5, 2012 Published: September 19, 2012 8021

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different compositions and segmental arrangements (Table 1). Also other polymers were synthesized, which helped to explain the peculiarities encountered in the set of polymers shown in Table 1 (also see Supporting Information).

copolymers with equimolar amounts of incorporated monomers, leading to cloud points close to 20 °C.10,13 This phenomenon was explained by preferential hydrogen bonding between the hydrogen-bond donor (PNIPAM, which acts as an acceptor as well) and the hydrogen-bond acceptor (PDEAAM). In the first stage, the synergistic cloud point depression seems to be an intramolecular phenomenon, since mixtures of PNIPAM and PDEAAM homopolymers were reported to show no reduction of the LCST.13 In contrast, a hypothetical, strictly alternating polymer of NIPAM and DEAAM would most probably exhibit the lowest LCST, since hydrogenbonding pairs will find each other at ease. Therefore, we are interested in the influence of the monomer pattern along the polymer on its cloud point. By changing the length of NIPAM/DEAAM sequences along the polymer backbone, one would gain mechanistic insight in the intra- and intermolecular hydrogen bonding and would be further able to fine-tune macroscopic properties. Since full control of the monomer sequence is not at hand (e.g., a strictly alternating copolymer is not accessible by current synthetic means), we start with the influence of a block-like arrangement on the cloud point.14 Hereby, we mainly focus on turbidity and especially on infrared spectroscopy (IR) of well-defined and narrowdispersed polymers. Experimentally, IR spectroscopy has proven to be a useful tool for the investigation of hydration and hydrogen bonding with temperature.8,10,11,13,15−19 Especially the pronounced amide I′ band between 1560 and 1680 cm−1 probes the environment of the amide carbonyl group (the “prime” indicates the measurement in D2O, which helps to omit the disturbing H2O band close to the amide I band). The amide I′ band comes apart the most other characteristic IRbands. It turned out that this band consists of at least two subbands, whose ratio depends on temperature. A subband near 1620 cm−1 is the dominant contribution below the phase separation for PNIPAM, whereas a peak at 1650 cm−1 develops above the LCST.20 The latter was assigned to carbonyl groups in a rather nonpolar environment and/or to carbonyl groups involved in intramolecular H-bonding. In some cases, an additional subband at 1600 cm−1 is identified, which is assigned to a carbonyl group linked to hydrogen-bonded water.11 Interestingly, the subband at 1600 cm−1 is even more pronounced for PDEAAM below its LCST-type transition.3 Compared to PNIPAM, the third peak develops at lower wavenumbers (at 1640 cm−1; the peak is related to the carbonyl group in a less polar environment). By help of these assignments, the phase transition was described on a microscopic level for the homopolymers and statistical copolymers. However, the literature is lacking an IR-investigation of homopolymer mixtures and of block copolymers, which will be addressed in this work. Therefore, all polymers used in this study were prepared by the RAFT method (Reversible Addition−Fragmentation Chain Transfer).21−26 By this, we kept the overall molecular weight almost constant. At the same time, we are able to neglect influences of the polymer end groups on the cloud points. Then, the effect of the monomer pattern can be studied, since we varied the monomer composition and the monomer sequence.

Table 1. Molecular Characterization of Samples Used samplea PNIPAM70 PDEAAM55 PDEAAM30-b-PNIPAM27 statistical PDEAAM30-stPNIPAM22 homopolymer mixture61f PNIPAM70/ PDEAAM55

Mn (SEC) [kg/mol]b

Mn (NMR) [kg/mol]c

ĐMd

NIPAM [mol %]e

7.4 5.0 7.1 4.2

8.2 7.3 7.1 7.2

1.29 1.17 1.19 1.15

100 0 47 42

6

7.7

-

42

a

Number average degree of polymerization Pn of each component as obtained from Mn(NMR). bApparent number average molecular weight determined by SEC in DMF using PMMA standards. c Determined by NMR end group analysis taking the mass of the RAFT agent into account. dApparent dispersities determined by SEC in DMF. eMolar NIPAM ratio determined by NMR. fValues recalculated as molar-weighted (in regard to monomeric units) average of homopolymers PDEAAM55 and PNIPAM70 (mixed in a 6/4 mass ratio).

The polymers were dissolved in D2O at 20 g/L (3 mmol/L) and were used for the experiments within 1 day. Care was taken to exclude hydrolysis of the polymers in the time scale of the experiment (disulfide formation,27,28 which would originate from a hydrolyzed RAFT dithioester moiety,29 could not be detected by comparing the SEC traces of fresh samples and of samples taken after 1 month in nondeaired water; see Supporting Information). In order to investigate their thermoresponsive properties, we started with turbidimetric measurements to learn about solubility changes. Likewise, the solutions were used for IR investigations in order to follow microscopic changes in hydration and hydrogen bonding. It is important to note that the turbidity measurements and the IR investigations were made at an average heating rate of 12 K/h. At this rate, several observations were made when comparing the turbidity behavior of the samples (Figure 1). The transitions for PDEAAM and PNIPAM are close, as reported in the literature.8,13 Also the pronounced drop in the cloud point of the statistical copolymer

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RESULTS AND DISCUSSION The preparation of a set of polymers is described in the Supporting Information. For the following discussion, we have chosen polymers with almost the same molecular weight, but

Figure 1. Summary of turbidity measurements (20 g/L in D2O; black: PNIPAM70; red: PDEAAM55; green: diblock PDEAAM30-b-PNIPAM27; purple: statistical copolymer PDEAAM30-st-PNIPAM22; blue: mixture PNIPAM70/PDEAAM55). 8022

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Figure 2. Compilation of temperature-dependent IR spectra (amide I′ band) of all samples (left-hand side; every ∼2K one spectrum from blue to red) and the decomposition of the data into subbands (right-hand side; black: subband at 1620 cm−1; red: subband at 1600 cm−1; blue: subband at 1640 − 1650 cm−1; lines originate a sigmoidal fitting); from top to bottom: (a) PNIPAM70, (b) PDEAAM55, (c) diblock PDEAAM30-b-PNIPAM27, (d) statistical copolymer PDEAAM30-st-PNIPAM22, (e) mixture PNIPAM70/PDEAAM55.

compared to the homopolymers is reproduced by our results.13 However, all cloud points are reduced compared to the linear

polymers made by free radical polymerization (cloud point of PNIPAM now at 29 °C instead of its reported LCST at ∼31 8023

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Figure 3. Comparison of arithmetic (molar-weighted) average of PNIPAM70 and PDEAAM55 subbands (hollow circles), diblock copolymer PDEAAM30-b-PNIPAM27 (hollow squares), statistical copolymer PDEAAM30-st-PNIPAM22 (faint triangles), homopolymer mixture PNIPAM70/ PDEAAM55 (faint asterisks) and homopolymers (thin lines); black: subband at 1620 cm−1 (right side); red: subband at 1600 cm−1 (left side); blue: subband at 1640−1650 cm−1 (right side).

Table 2. Comparison of Transition Temperatures FT-IR transition point [°C] PNIPAM70 PDEAAM55 block PDEAAM30-b-PNIPAM27 statistic PDEAAM30-st-PNIPAM22 mixture PNIPAM70/ PDEAAM55

FT-IR onset [°C]

turbidity

1640−1650 cm−1

1600 cm−1

1640−1650 cm−1

1600 cm−1

transition point [°C]

onset [°C]

32.3 30.1 34.7 19.0 26.1

− − − 18.5 27

29 27 29 15 23

25 22 24 15 23

28.8 26.3 28.6 16.1 23.9

27.9 25.5 27.2 15.3 22.6

copolymer) give a higher contribution to the 1640 cm−1 band than the parental homopolymers, indicating a less hydrophilic environment for the carbonyl function both at low and at high temperature. This can be understood in terms of additional hydrogen bonding and complexation between both components. In contrast, the diblock copolymer PDEAAM30-bPNIPAM27 rather appears as an ideal copolymer (linear dependence of the properties with composition), since the components of the amide I′ band can be rather well described by an arithmetic average of the homopolymer band contributions (see Figure 3 and Supporting Information). Thus we can state that both blocks in the diblock copolymer PDEAAM30-b-PNIPAM27 behave rather independently. Furthermore, the macroscopic properties (turbidity) and microscopic properties (IR) rather resemble those of the homopolymers. Interestingly, the statistical copolymer PDEAAM30-st-PNIPAM22 and the homopolymer mixture PNIPAM70/PDEAAM55 show nonideal behavior. Further seen in Figure 2, the PNIPAM70 sedimentation is also reflected in the infrared spectroscopy as a missing isosbestic point.30 In all cases, we see a decrease of the subband at 1600 cm−1 with increasing temperature even far below the phase transition. This subband is reported to reflect the hydrogen bonding toward water. Close to the macroscopic phase transition, this band even exhibits a change in slope, which is probably the beginning of a step-like transition (as seen for the statistical copolymer). The band close to 1620 cm−1 increases in all cases without showing distinct features. This subband is usually assigned to amide carbonyls in a mixed hydrophilic/hydrophobic surrounding. However, the subband close to 1640 cm−1 is most sensitive to the phase transition, showing a clear step-like increase. It is assigned to either interpolymer hydrogen bonding or generally to a polymer in a hydrophobic environment.3,10,17,20 Above the phase transition,

°C, PDEAAM55 at 26 °C instead of ∼30 °C, stat. copolymer at 16 °C instead of ∼20 °C).13 Molecular weight can only indirectly explain this decrease by taking the end groups of the polymer into account. Otherwise, the rather short polymers used in this study should have higher transition points than their high molecular weight analogues. Thus, the phenyl end group originating from the RAFT agent adds hydrophobicity to the system, which is not totally compensated by the hydrophilic group at the other chain end (see Supporting Information on structural details). Further, Figure 1 shows that a mixture of homopolymers has a lower LCST than their single constituents. Interestingly, the mixture PNIPAM70/PDEAAM55 does not sediment, while pure PNIPAM70 sediments during the experiment above its LCST (decrease in optical density). Even more unexpectedly, the diblock PDEAAM30-b-PNIPAM27 shows behavior similar to that of pure PNIPAM70. In order to gain a microscopic impression of the processes involved, we performed temperature dependent IR spectroscopy of the amide I′ band (refer to Supporting Information for spectrum comparison at low and high temperature and for information on the amide II′ band). By decomposition of the amide I′ band into subbands with the help of Gaussian fitting, we obtain a more quantitative picture and even follow the subbands with temperature (Figure 2 and Table S3 in the Supporting Information). The samples, containing both NIPAM and DEAAM repeating units, have intermediate contributions to the subbands compared to the homopolymers (Figure 3). Only the band at 1640 cm−1 differs for the homopolymer mixture PNIPAM70/PDEAAM55 and the statistical copolymer PDEAAM30-st-PNIPAM22 (for copolymers and mixtures, this peak stays at approximately 1640 cm−1, whereas the subband is situated at 1650 cm−1 for pure PNIPAM70). Here, these two samples (mixtures and statistical 8024

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is now in between the ones of the short homopolymers (see Supporting Information). These block length effects seem to be stronger than the intermolecular tendency to aggregate (as seen for the homopolymer mixture PNIPAM70/PDEAAM55). Thus, a reduction of the cloud points is expected when elongating the block lengths. At the same time the influence of the end groups is reduced, which would result in an increase of the transition. However, the opposite is true, since block copolymers of approximately double molecular weight turn turbid already close to the homopolymer mixture PNIPAM70/PDEAAM55 (see Supporting Information). This again indicates that PDEAAM30-b-PNIPAM27 is rather peculiar, since the rather oligomeric blocks (Pn ≈ 30) still render a strong dependence of the observed properties on the block length. Apparently, these severe effects are weakened and this regime with strong dependency is already left by approximately doubling the molecular weight of the blocks. Then, the homo- and copolymers with larger blocks behave quite similarly, showing only a weak dependency of the properties with molecular weight. A combined dynamic and static light scattering approach (see Supporting Information16) shows no internal transition of the unimolecularly dissolved PDEAAM30-b-PNIPAM27 below the cloud point (hydrodynamic radius stays at approximately 2 nm). Also NMR indicates that both blocks start dehydration almost simultaneously (Supporting Information). It means that segregated, star-shaped micelles (with homopolymer core) are rather unlikely for our system, though some minor complexation is suggested by the NMR results. Therefore, the case, where the PDEAAM block would form the core of the micelle, and the PNIPAM is displayed toward to the solution, can be excluded and therefore cannot explain the special properties of the block copolymer studied here.

more carbonyl groups are in a hydrophobic surrounding. The IR spectra nicely demonstrate that the microscopic change correlates with the macroscopically observed turbidity change. However, the cloud points (transition points determined by sigmoidal fitting) exhibit an especially good fit to the onsets of the band at 1640 cm−1 (except for the samples with high transition temperatures, where the IR onset temperature is less reliably determined; see Table 2). Thus, already small microscopic changes have a considerable macroscopic response. This can be best understood when regarding the constant decrease of the band at 1600 cm−1 and the constant increase of the band at 1620 cm−1 up to the phase transition. This means that the polymer becomes increasingly dehydrated already below the cloud point, while the number of carbonyls in a mixed environment rises. At a certain temperature, solvation is so much reduced that the polymer aggregates, which is demonstrated by the resulting turbidity. At that point, the hydration is even further reduced as seen by the kink in the curves for the subband at 1600 cm−1. At the same time, the released carbonyls predominantly assemble into hydrophobic domains, as shown by the rather step-like increase at 1640 cm−1. IR also reflects the behavior of the homopolymer mixture, except that the transition of the band at 1640 cm−1 is now a bit broader compared to the other samples. This might indicate that there is some increasing hydrophobic interaction between the two polymers at temperatures even below the cloud point. This could also explain the stabilization of PNIPAM70, which no longer sediments in the mixture PNIPAM70/PDEAAM55. Clearly, the mixing of homopolymers leads to a decrease of the cloud point. This drop, which is in the range of 3 K, was not reported before. Actually, it was stated that there is no decrease of cloud points by mixing homopolymers,13 which might be the case for lower concentrations. The molecules in homopolymer mixtures need to interact first intermolecularly to enable a cloud point depression, which is more pronounced for higher concentrations (see Supporting Information). Thus, the difference between the cloud points of the mixture and the homopolymers increases with increasing concentration. In contrast, the synergistic depression of the cloud point is first and foremost an intramolecular phenomenon for the statistical copolymers. Both IR spectroscopy and turbidity revealed that the diblock copolymer PDEAAM30-b-PNIPAM27 has a transition very similar to pure PNIPAM70. This is rather unexpected, since both the homopolymer mixture PNIPAM70/PDEAAM55 and the statistical copolymer PDEAAM30-st-PNIPAM22 show a decrease in their cloud points compared to the phase transition temperature of the pure homopolymers. Thus, we expected cloud points between the mixture and the statistical copolymer. However, while mixing homopolymers reduces the cloud point, connecting them in the form of a diblock copolymer has no effect. While the connectivity of the different monomers reduces the LCST (as seen for the statistical copolymer), the diblock does not show this behavior. One explanation for these unexpected results can be found in the reduced block length within the diblock PDEAAM30-bPNIPAM27. In the time scale of our experiment, the whole diblock does not act as a copolymer as in the statistical case, but the blocks behave rather independently. By shortening the block lengths, the cloud point increases due to block length effects (especially for PDEAAM).2,4 Similarly, the cloud point of mixtures of short homopolymers (PNIPAM39/PDEAAM30)

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CONCLUSION Copolymers, which usually do not show linear dependencies of properties with composition, can be made to ideally behave by changing the monomer sequence along the backbone. Under certain circumstances, a block-like arrangement of the comonomers is sufficient, as in the case of NIPAM and DEAAM. At the same time, two effects cancel each other. Here, the block length dependency on the respective cloud point competes with the effect of connectivity. Thus, the macroscopic and microscopic properties resemble more the averaged properties of the homopolymers. In contrast, the homopolymer mixtures and the random copolymers exhibit nonideal behavior leading to a synergistic depression of the cloud points.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, note on time scale and concentration effect of aggregation, amide II′ band, light scattering and NMR results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; richtering@rwth-aachen. de. Notes

The authors declare no competing financial interest. 8025

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(30) Carlon, H. R. Infrared Phys. 1981, 21 (2), 93−9.

ACKNOWLEDGMENTS We thank Toni Gossen (Institute of Inorganic Chemistry) for help in NMR. We are grateful to Martin Möller, Heikki Tenhu and Jun Okuda for sharing laboratory space. Further, we acknowledge the support of the German Research Foundation DFG (RI 560/20-1).

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