Amphiphilic ABC Triblock Copolymers Tailored via RAFT

Article pubs.acs.org/Macromolecules

Amphiphilic ABC Triblock Copolymers Tailored via RAFT Polymerization as Textile Surface Modifiers with Dual-Action Properties Martin Messerschmidt,† Hartmut Komber,† Liane Haü ßler,† Christian Hanzelmann,† Manfred Stamm,†,§ Benedikt Raether,‡ Oswaldo da Costa e Silva,‡,∥ and Petra Uhlmann*,† †

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse, 6, D-01069 Dresden, Germany BASF SE, Carl-Bosch-Str. 38, D-67056 Ludwigshafen, Germany § Department of Chemistry, Technische Universität Dresden, 01069 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: We present the synthesis and characterization of two different kinds of triblock copolymers prepared via RAFT polymerization by only two synthetic steps applying a sequential monomer addition approach. Both triblock copolymers consist of a hydrophilic, a hydrophobic and an anchor block which is located in the middle or at the end of the polymer chains. In both triblock copolymer systems the hydrophobic blocks consist of randomly polymerized tert-butylstyrene (tBS) and n-hexyl acrylate (nHA) monomers in a ratio of approximately 2:1, respectively, whereas the hydrophilic blocks were accomplished by polymerization of N,N-dimethylacrylamide (DMA). As a result of the employed synthetic approach, every anchor block contains several N-acryloxysuccinimide (NAS) units besides tBS and nHA units in the case of the triblock copolymer with the central anchor block and DMA units in the case of the triblock copolymer with the terminal anchor block. The structures as well as the compositions of all synthesized polymers were verified by means of 1H, 13C NMR, and FT-IR analysis. Molar masses and molecular weight distributions were determined by GPC measurements. The thermogravimetric measurements showed a sufficient thermal stability for all prepared polymers up to 210 °C, while DSC investigations revealed that the glass transition temperatures of the hydrophobic blocks of both triblock copolymers are in the targeted range between 50 and 65 °C. The unique design of these triblock copolymer systems combined with the special properties of their blocks (polarity, Tg) predestinate these systems for an application as a durable dual-action texile finish rendered by virtue of a thermoresponsive block copolymer brush covalently attached on the texile fabric surface. The synthesis and the characterization of these triblock copolymer systems are emphasized in this paper.

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surface facilitating the cleaning process.9 However, as the higher hydrophilicity of the fabric surface is detrimental to the water and stain repellency, this approach represents only a compromise solution. As a result, a more sophisticated finishing system was developed. This system is based on a special copolymer that contains both nonpolar fluorochemical segments and hydrophilic moieties.10 In the dry state the fluorochemical segments are located directly at the top of the textile surface conferring hydrophobic as well as oleophobic surface characteristics and thus leading to an excellent water and soil repellency. During washing of the textile in aqueous media the situation is reversed. Since the copolymer segments are flexible and capable of restructuring, the hydrophilic segments are then concentrated at the interface between the textile and the water phase, giving rise to a more polar textile surface. Thus, nonpolar stain can be removed more easily

INTRODUCTION Textile fabrics have been finished with specific materials like paraffins, silicones, or fluorochemical compounds for many years in order to render them repellent to water and soil.1,2 However, these finishes are not able to prevent soiling in all cases, so specialized and professional laundering of soiled fabrics is still unavoidable. Generally, the removal of soil during laundering considerably depends on both the interaction of the soil with the textile and also the ability of the water to wet the textile surface. In particular, nonpolar oily soils and stains adhere much better onto a nonpolar hydrophobic textile surface during laundering than polar water molecules. As a consequence, the removal of this nonpolar soil by the “rollup” process in an aqueous detergent solution is inherently limited and very often unsatisfactory.3,4 To address this issue, so-called soil-release finishes were developed.5 As they contain also polar groups, they impart a permanent hydrophilicity on the fabric surface.6−8 Hence, nonpolar oily soil does not stick as strongly on the more nonpolar textile fabrics, and additionally the aqueous detergent solution is able to better wet the textile © 2013 American Chemical Society

Received: November 30, 2012 Revised: March 8, 2013 Published: March 21, 2013 2616

dx.doi.org/10.1021/ma302471q | Macromolecules 2013, 46, 2616−2627

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we anticipate that this requirement should be sufficiently met by a covalentor even betterby multiple covalent attachments of the polymer brush onto the textile fabric. Switchable block copolymer brushes with a multiple attachments to a substrate can be prepared via application of appropriate triblock copolymers.20,26−28 For our purpose, such copolymers should consist of a hydrophilic, a hydrophobic, and an anchor block containing several reactive anchoring groups. The anchor block can be located in the middle between the hydrophilic and hydrophobic block or at the end of the block copolymer chains as depicted in Figure 1.

during laundering by the roll-up mechanism. After washing and drying of the fabric, the nonpolar fluorochemical segments return to the top of the finish again. Because of this special behavior such a coating is also called a dual-action stain repellent and soil release finish. In order to generate oleophobic wetting characteristics the fluorinated segments contain linear perfluorinated alkyl chains with at least seven C atoms. Historically, perfluorinated alkyl chains based on perfluorooctanoic sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), or polyhedral oligomeric silsesquioxane (POSS)45 have been employed due to their excellent oleophobic wetting performance.1,11 However, these compounds and those with linear perfluorinated alkyl chains having more than eight C atoms were found to be bioaccumulative and biopersistent in nature and have also been detected in the human body.12 Recently published studies further indicate that these compounds have severe negative impacts on human health in different aspects.11,13,14 Thus, harmless, nontoxic, inexpensive, and environmentally benign replacements are highly desirable. Additionally, as many specific textile applications, such as sports apparel, do not necessarily need a strong oleophobicity, fluorocarbon-free finishing materials seem to be the alternatives of choice.15,46 On the basis of these considerations, we propose a new concept for finishes with dual-action characteristics utilizing special responsive block copolymer brush systems. Concept of Utilizing Amphiphilic Block Copolymer Brushes as Dual-Action Textile Finishes. Over the course of the past decades many different types of polymer brushes as well as preparation methods have been developed and were published in detail in the literature.16−19,44 However, only few polymer brushes have been used for the modification of textile fabrics. Exemplarily, Motornov et al. have employed mixed polymer brushes containing poly(2-vinylpyridine) and polystyrene chains to reversibly switch the wettability of a textile fabric from hydrophilic to hydrophobic and vice versa.20,21 Exposing of the brush into toluene leads to a switching of the brush into its hydrophobic state, while a treatment in ethanol or in an acidic aqueous environment of pH 3 gives rise to a switching into its hydrophilic state. The same author published also another amphiphilic polymer brush system employing the triblock copolymer poly(styrene-b-2-vinylpyridine-b-ethylene oxide).22 Here, in the first step, block copolymer brushes were prepared onto silica particles via the “grafing to” approach followed by application of these particles onto a polyamide textile fabric. A switching of the particle surface from the hydrophobic into the hydrophilic state was also achieved by exposing the fabric in an acidic aqueous medium of pH 3, whereas the reverse switching into the hydrophobic state was accomplished by either a thermal annealing of the fabric above 100 °C (the glass transition temperature of the polystyrene block) or alternatively by a treatment in toluene. Although the polymer brushes described above exhibit reversible switching characteristics, their stimuli parameters are incompatible with most commercial textiles and are also rather inappropriate for commonly used washing machines and laundry dryers in a typical household. Consequently, new block copolymer brush systems must be developed to enable a switching under conditions that are typically applied in a washing and drying procedure of a textile fabric. Moreover, the polymer brushes must possess excellent washing durability and wear resistance in order to ensure a sufficient longevity of the surface treatment. According to the literature23−25 describing other robust textile finishing systems,

Figure 1. Constitutional architecture of amphiphilic triblock copolymers having a central anchor block (a) or a terminal anchor block (b).

Polymer brushes prepared with such triblock copolymers are intended to conformationally arrange their chains in specific fashions. Scheme 1 illustrates the proposed morphologies of the Scheme 1. Conformational Rearrangement of the Polymer Chains of a Triblock Copolymer Brush with Central Anchoring Blocks (a) and Terminal Anchor Blocks (b) during Washing and Drying at Elevated Temperatures Rendering to a Reversibly Switching of the Textile Surface from a Hydrophobic to a Hydrophilic State and Vice Versa

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Figure 2. Chemical structures of the monomers and chain transfer agent (MCPDB) employed for the preparation of the triblock copolymers.

are solubilized and thus plasticized by an appropriate solvent. Since the hydrophobic blocks of the triblock copolymers are not water-soluble, only heating above the Tg imparts a sufficient flexibliliy to the polymer chains to switch the brush from a hydrophobic into a hydrophilic state and vice versa. Consequently, the Tg of the hydrophobic block is the crucial parameter, since it determines the temperature at which a switching of the brush is enabled. As the switching temperature should be significantly above room temperature, only a limited number of monomers can be used for the synthesis of this block. Particularly, polymers based on styrene and styrene derivatives have often glass transition temperatures around 100 °C or even higher, whereas the Tgs of most polyacrylates are quite low and often below room temperature.29 Hence, a random copolymerization with candidates of both monomer families in an appropriate ratio could directly provide a copolymer with a Tg in the desired range between 50 and 65 °C. Since both comonomers should be also quite nonpolar in order to generate a hydrophobic brush surface, monomers like 4-tert-butylstyrene (tBS) and n-hexyl acrylate (nHA) are ideal candidates for the synthesis of the hydrophobic blocks. For the attachment of the polymer brush onto the textile fabric surface, the anchor block needs a sufficient number of adequately reactive functional groups like epoxides or active ester groups. Monomers containing such reactive functional groups are glycidyl (meth)acrylate or N-acryloxysuccinimide (NAS). As the succinimide ester group is much less sensitive to moisture in comparison to the glycidyl group, the handling of NAS should be much easier and is thus the monomer of choice for the anchor block. A promising monomer for the hydrophilic block is dimethylacrylamide (DMA), since poly(dimethylacrylamide) is highly water-soluble and has a strong affinity to aqueous media. The preparation of the triblock copolymers requires polymerization techniques that are able to polymerize the monomers in a defined fashion, such as the controlled radical polymerization techniques atom transfer radical polymerization (ATRP), nitroxide-mediated radical polymerization (NMRP), and radical addition−fragmentation chain-transfer (RAFT) polymerization.30 Among these techniques, RAFT polymerization has emerged as a very promising method in terms of industrial applicability31 due to its ease of preparation, its tolerance toward many functional groups, and its ability to polymerize the most important monomer classes like styrenes, (meth)acrylates, acrylamides, vinyl acetates, etc., in a controlled fashion.32,33 Because all monomers selected for the preparation of the triblock copolymers herein belong to these monomer classes, RAFT was chosen as the preferred polymerization technique. Moreover, NAS and also DMA have already been successfully applied in RAFT polymerizations by different research groups employing also different chain transfer agents (CTAs).34−36 Among all possible and applicable CTAs for the RAFT polymerization of the favored monomers, S-

two different triblock copolymer brush systems based on the triblock copolymer with the central (Scheme 1a) and the terminal anchor block (Scheme 1b) together with the conformational rearrangement of their polymer chains within the laundering and drying procedure of the textile fabric. During the use of the textile fabric, the polymer brushes are in a dry state, where the nonpolar hydrophobic blocks are in a stretched conformation covering the collapsed hydrophilic blocks underneath them and thus conferring stain and waterrepellent wetting characteristics to the textile fabric. Likewise, the situation is reversed during laundering. Then, the brushes in both systems change their conformation in such a way that their hydrophilic blocks are at the top of the brushes imparting a polar surface. As a result, such an increase in the polarity of the textile surface leads directly to an enhanced soil release performance, particularly for nonpolar oily stains. However, a critical application requirement is that the rearrangement of the brushes from the hydrophobic into the hydrophilic state occurs only under the conditions of laundering in a washing machine and not during the use of the textile fabric, i.e., when it is getting wet in rain or when aqueous soil is deposited onto the fabric. This specific requirement can be directly implemented into the switching characteristics of the polymer brush via a precise adjustment of the glass transition temperature (Tg) of the hydrophobic block in an appropriate temperature range preferentially between 50 and 60 °C. Below the Tg no conformational rearrangement is feasible as the brush is frozen in a glassy state, even when it is exposed to an aqueous environment at ambient temperature. Only heating in an aqueous media above their Tg makes the hydrohobic chains flexible, so that a rearrangement of the brush morphology can take place. Therefore, both the laundering and the drying need to be performed at elevated temperatures, which is easily conducted in a washing machine and a laundry dryer. In contrast, the Tg of the hydrophilic blocks is considered to have not the same effect on the switching behavior of the polymer brush because the water of the aqueous detergent solution during laundering and also the absorbed water of the wet textile fabric at the beginning of the drying procedure function as a plasticizer for the hydrophilic polymer chains, so that their Tg should be significantly lowered beneath that of the hydrophobic block.

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RESULTS AND DISCUSSION Choice of Monomers and Polymerization Technique. The switching of a polymer brush requires polymer chain flexibility that is necessary for the rearrangement of the chain to a specific conformation. Normally, this prerequisite is fulfilled either when the temperature is above the glass transition temperature (Tg) of the polymer or when the polymer chains 2618


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Scheme 2. Synthetic Strategies for the Preparation of ABC Triblock Copolymers with (a) Central Anchor Block (Strategy I) and (b) Terminal Anchor Block (Strategy II)

methoxycarbonylphenylmethyl dithiobenzoate (MCPDB), first reported by Perrier et al., was chosen because of the its less demanding preparation procedure and its ability to polymerize the monomers styrene, methyl acrylate, DMA, and even methyl methacylate in a controlled fashion.37,38 Figure 2 depicts the chemical structures of the monomers and MCPDB. Synthetic Strategy. The approaches for the synthesis of the triblock copolymers with a central and a terminal anchor block, respectively, are depicted in Scheme 2. Strategy I comprises the preparation of the triblock copolymer with the central anchoring block. Here, in the first step 4-tert-butylstyrene (tBS) and n-hexyl acrylate (nHA) are randomly copolymerized building up the hydrophobic block. To this already running copolymerization N-acryloxysuccinimide (NAS) is added, leading to an anchor block consisting of all three components. By virtue of this sequential monomer addition approach already two blocks of the final triblock copolymer can be accomplished in only one synthetic step. The obtained and purified diblock copolymer (DBC1) is then used as a polymeric chain transfer agent in the RAFT polymerization of dimethylacrylamide (DMA), giving rise to the formation of the hydrophilic block and the final triblock copolymer TBC1. The synthetic strategy II describes the approach for the preparation of the triblock copolymer with the terminal anchor block which has some similarities with that of strategy I. Here, in the first step also tBS and nHA were randomly copolymerized, leading to the precursor polymer MacroCTA which constitutes the hydrophobic block of the final triblock copolymer TBC2. The purified MacroCTA is then used as a polymeric CTA in the following chain extension reaction using DMA as monomer which builds up the hydrophilic block. The

subsequent addition of NAS to the already running RAFT polymerization of DMA leads to the anchor block consisting of randomly copolymerized NAS and DMA repeating units. Hence, both strategies have in common that only two synthetic steps and purification procedures are required to obtain the desired triblock copolymers TBC1 and TBC2. RAFT Polymerizations. Synthesis of DBC1. Generally, both synthetic strategies commence with the preparation of the hydrophobic block which is based on a RAFT polymerization employing the chain transfer agent MCPDB.37 According to the synthetic protocols, the copolymerization of tBS and nHA was performed at 60 °C applying a ratio of 10:1 for MCPDB and AIBN, respectively. Additionally, anisole was added as solvent in order to ensure that the viscosity goes not increase too much with increasing monomer conversion, as this might adversely affect the control of the RAFT polymerization.33 The initial feed ratio of the copolymerizing monomers tBS and nHA was set to 2:1 since this resulted in hydrophobic polymer chains with a glass transition temperature in the range between 50 and 65 °C. A detailed discussion of the relevant aspects in terms of the thermal behavior of the polymers is presented below. As Motornov et al. used a triblock copolymer poly(styrene-b2-vinylpyridine-b-ethylene oxide) with a polystyrene block consisting of ca. 140 repeating units, we anticipated that the hydrophobic block in our triblock copolymer system with the central anchoring block should also have a similar number of repeating units.20,22 To this end, the initial feed ratio of the comonomers tBS and nHA and the MCPDB was set to 200:100:1, respectively, with a targeted monomer conversion of ca. 60%. The monomer conversion was set to only 60% because in RAFT polymerization higher conversions can result in an 2619


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Figure 3. 1H NMR spectrum of DBC1 (solvent: CDCl3).

acrylate (nBA). The reported copolymerization parameters of St and nBA are r1 = 0.20 and r2 = 0.76,47 so that the product r1r2 is 0.15, and thus between 0 and 1, indicating that crosspropagation is strongly favored over homopolymerization.30 Though, as the ratio of the comonomers shifted only slightly during the polymerization, it seems more likely that compared to St/nBA the tBS/nHA system has a significantly higher tendency to form statistical rather than alternating copolymers. However, such a finding could be also the result of the special case that we have coincidently set a comonomer feed ratio that is relatively close to the azeotropic composition of the system. To clarify this issue ultimately, the copolymerization parameters need to be determined. The addition of NAS to the reaction tube represents the starting point of the growing of the anchor block. After an additional polymerization period of 3.5 h the reaction was finally ceased by cooling in liquid nitrogen. Before the diblock copolymer DBC1 was purified by precipitation, a small sample of the crude reaction mixture was taken from the tube in order to determine the conversion of all three different monomers by 1 H NMR analysis analogously as described above (see Figure S4). For nHA the conversion increases during this time by 3 mol %, whereas in the same time 37 mol % of NAS were consumed, indicating that this monomer has a significantly higher reaction rate than nHA. The conversion of NAS was determined based on the comparison of the integration areas of the resonances of the vinyl protons at 6.16 ppm and the two methylene groups adjacent to the imide group within the ring appearing in the region between 3.0 and 2.5 ppm (see Figure S4). Interestingly, tBS was found to polymerize much faster after the addition of the NAS than before, clearly indicated by a significantly higher conversion of 6 mol % in this reaction step, compared to the 3 mol % of nHA. This phenomenon could be caused by the fact that an electron-deficient double bond like

additional broadening of the molecular weight distribution (MWD) due to an increasing viscosity, although the addition of an appropriate solvent like anisole might suppress this effect to some extent. After the polymerization proceeded for 64.5 h, and before the NAS was added to the reaction tube, a small amount of the crude reaction mixture was withdrawn from the tube in order to determine the monomer conversions of tBS and nHA by analysis of the 1H NMR spectrum (Figure S3). For the determination of the conversion of nHA the integrated signal intensities of the vinyl protons of the monomer appearing at 5.82 ppm and those of the methyl groups at 0.91 ppm originating from both the monomer as well as the polymer were used and revealed a conversion of 49 mol %. Because of intensive signal overlapping of tBS with other signals in the 1H NMR spectrum, the determination of the conversion of the tBS was accomplished by comparing the signal intensities of the vinyl protons of tBS at 5.73 ppm with that of the vinyl protons of nHA at 5.82 ppm. It turned out that the original feed ratio between tBS and nHA shifted from 2:1 at the beginning of the reaction to 1.8:1 after 64.5 h of polymerization. From this ratio the conversion of tBS was directly calculated to be 54 mol %. Consequently, both comonomers were consumed with relatively similar reaction rates, giving rise to the formation of a hydrophobic block with nearly randomly distributed repeating units along the polymer chains. This finding is rather favorable for our intent, since it facilitates the adjustment of the targeted amounts of the different comonomer units within the polymer chains by selecting the corresponding feed ratio of the comonomers in the batch. Unfortunately, a discussion of these results based on copolymerization parameters of tBS and nHA failed because they are not disclosed in the literature so far. Thus, we decided to assess their copolymerization behavior in free radical polymerization on the basis of data for structurally similar comonomers styrene (St) and n-butyl 2620


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that in the NAS monomer should build up a π-complex with the electron-rich double bond in the tBS monomer.30 As the NAS monomer is polymerizing very fast, also the complexed amount of the tBS monomers could be incorporated much faster into the growing polymer chain, resulting in an accelerated polymerization rate and a higher monomer conversion for tBS. The structure of the purified DBC1 was verified by means of 1H and 13C NMR analysis. Figure 3 depicts the 1H NMR spectrum of DBC1. Besides overlapping backbone and alkyl chain signals in the 1.0−2.5 ppm region, characteristic signals demonstrate the presence of all three different monomer units in the polymer chain of DBC1. Thus, the aromatic proton signals (7.3−6.2 ppm) and the intense tert-butyl group signal at 1.29 ppm represent the tBS units, whereas the ester methylene protons of nHA located in different microstructure sequences result in a signal group between 3.2 and 4.2 ppm. Also, the methyl group signal at 0.91 ppm is characteristic for nHA. Finally, the signal of the succinimide protons of the NAS units appear as broadened signal at 2.77 ppm. The assigned 13C NMR spectrum of DBC1 is presented in Figure S2. The molar mass and the polydispersity (PDI) of DBC1 were obtained by GPC measurements with THF as eluent (Figure 4). The monomodal, symmetrical main peak shows a small

shoulder does not substantially broaden the molecular weight distribution (MWD). Moreover, the low polydispersity (PDI = 1.13) suggests that the RAFT polymerization was conducted in a well-controlled fashion with a high chain transfer efficiency. As narrowly distributed polystyrene standards were used for the calibration of the GPC, the determined molar mass of DBC1 (Mn = 39 100 g/mol) represents only a relative molecular weight. From the initial CTA concentration [CTA]0 and the determined monomer conversions in the first (hydrophobic block) and second (anchor block) reaction step the number of monomer units Pn in DBC1 was calculated (Table 1). Based on this calculation, a more accurate molar mass of Mn,cal = 29 500 g/mol was determined which is more accurate compared to the GPC value. The hydrophobic block consists of 157 repeating units in total (108 tBS and 49 nHA; Mn,cal = 25 000 g/mol), indicating that the hydrophobic block should have a sufficient length for its final use in a switchable polymer brush system. Since the length of the anchor block has a severe impact on the switching performance of the final triblock copolymer brush, the chains of the anchor block need to have an adjusted specific length.48 On the one hand, they should not be too long in order to ensure that the distances between the hydrophilic and the hydrophobic polymer chains are small enough to form a polymer brush system. On the other hand, the anchor block should not be too small because a sufficient number of NAS repeating units are required to ensure a multiple covalent fixation of the final triblock copolymers onto the textile surface. Both premises should be sufficiently fulfilled by the 9 NAS anchoring units apart from 14 tBS and 3 nHA units within each anchoring block and its relatively low molecular weight of Mn,cal = 4200 g/mol. Synthesis of TBC1. The synthesis of the hydrophilic third block was performed using DBC1 as a polymeric chain transfer agent applying the same temperature (60 °C), amount of solvent (anisole), and ratio between MCPDB and AIBN (10:1) as for the preparation of DBC1. Only the ratio between the monomer DMA and the polymeric CTA was increased to ∼700 to 1 in order to obtain a relatively long hydrophilic block having more than 400 repeating units at a targeted conversion of ∼60%. Unlike the hydrophobic polymer chains, a significantly longer hydrophilic block was favored because Motornov et al. also used a triblock copolymer with a substantial longer hydrophilic block than the corresponding hydrophobic block (polystyrene).22 After the polymerization was allowed to run for 41 h the reaction was quenched by immersion into liquid

Figure 4. GPC elugram of DBC1 measured in THF as eluent.

shoulder toward higher molecular weight originating most likely from polymer chains which were formed by irreversible radical−radical coupling reactions that were not completely avoidable by the chosen reaction conditions. However, this

Table 1. Polymerization Conditions of DBC1 and TBC1 and Their Polymer Characteristics polymer sample

time [h]

conv [mol %]

repeating units Pn

Mn,cal [g/mol]

Mn,GPC [g/mol]

PDI

DBC1 (a) hydphob block (b) anchor block

67 (total) 64.5 +3.5

39 100e

1.13

41

tBS: 108b nHA: 49b tBS: 14b nHA: 3b NAS: 9b DMA: 430f

29 500d 25 000c 4 200c

TBC1 hydphil block

tBS: 54a nHA: 49a tBS: 7a nHA: 3a NAS: 37a DMA: 63g

72 200d 42 700c

60 000h

1.9

a Determined by 1H NMR analysis of the crude reaction mixture. bDetermined using eq 1: number of repeating units: Pn = ([monomer]0/[CTA]0) × conv(monomer). cDetermined by application of eq 2: Mn,cal(block) = Pn1 × M(monomer1) + Pn2 × M(monomer2) + .... dDetermined by application of eq 3: Mn,cal(block copolymer) = Mn,cal(block1) + Mn,cal(block2) + ... + M(CTA). eMeasured by GPC using THF as eluent and narrowdispersed polystyrene standards for calibration. fDetermined by analysis of the quantitative 13C NMR spectrum of TBC1 and by using the calculated number of tBS repeating units in DBC1. gDetermined by application of the calculated number of DMA repeating units on the basis of footnote f and application of eq 1. hMeasured by GPC using dimethylacetamide as eluent containing 3 g/L LiCl and 2 vol % water and narrow-dispersed poly(2vinylpyridine) standards for calibration.

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Figure 5. Quantitative 13C NMR spectrum of TBC1 (solvent: CDCl3).

solubility, which was a better solvent than THF. The molar mass of TBC1 was determined to Mn = 60 000 g/mol and deviates slightly from the calculated one (Mn,cal = 72 000 g/ mol) since linear poly(2-vinylpyridine) standards were applied for GPC calibration.

nitrogen. Purification of TBC1 by precipitation was not possible due to the amphiphilic character of the resulting triblock copolymer. Thus, the polymer was purified by removing most of the solvent and unreacted monomer (DMA) in vacuo followed by lyophilization from benzene. In contrast to DBC1 the conversion of DMA could not be determined by 1H NMR analysis of the crude reaction mixture and also not from the purified product (see Figure S5) due to multiple and intensive signal overlap. However, a quantitative 13 C NMR spectrum of the purified TBC1 (Figure 5) allowed estimation of the number of DMA units based on the number of tBS units which is already known from the analysis of DBC1. The total intensity of the overlapping signals C-11 (nHA) and C-25 (DMA) was corrected by the intensity of a nHA carbon obtained as averaged value of C-12, C-13, C-16, and C-17. The remaining intensity of C-25 was related to the averaged intensity of the three carbon atoms C-6 and C-5 of tBS resulting in a ratio of 3.54:1 of DMA and tBS units. As the number of tBS units is already known from the analysis of DBC1, the number of DMA repeating units could be calculated to Pn = 430 (3.54 × 122 tBS units) corresponding to a DMA conversion of 63 mol %. The chemical structure of TBC1 was further substantiated by characteristic vibrations of the different repeating units in the FT-IR spectrum (see Figure S6). Accordingly, the intensive bands of the carbonyl groups in the ester, amide, and imide moieties at 1735, 1643, 1782, and 1811 cm−1, respectively, verified the presence of the nHA, DMA, and NAS repeating units in the triblock copolymer, whereas the tBS units could be clearly identified by the aromatic C−H valence vibrations appearing in the region at 3100−3000 cm−1 and the deformation vibration of the 1,4-substituted aromatic ring at 829 cm−1. The molecular weight and the MWD of TBC1 was obtained from GPC analysis using dimethylacetamide (DMAc) as eluent, containing also 2 vol % water and 3 g/L LiCl for better

Figure 6. GPC curve of TBC1 measured in dimethylacetamide (DMAc) as eluent containing 2 vol % water and 3 g/L LiCl.

However, the GPC curve of TBC1 features a significant shoulder toward higher molecular weight, giving rise to a relatively broad MWD and a high PDI of 1.9. Two processes are considered to be responsible for this phenomenon. Basically, such a shoulder can be the direct result of irreversible radical−radical coupling reactions similarly as discussed for the GPC curve of DBC1. However, as the shoulder in the GPC curve of TBC1 is much more pronounced, it is very likely that also another process comes into play. This process could be a direct consequence of the fact that the polymeric chain transfer agent DBC1 contains indeed three different types of chain transfer agents depending on whether a tBS, a nHA, or a NAS repeating unit is adjacent to the dithioester group at the end of the polymer chains. According to the generally accepted 2622


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Table 2. Polymerization Conditions for MacroCTA and TBC2 and Their Characteristics polymer sample

time [h]

conv [mol %]

repeating units Pn

MacroCTA (hydphob block)

18

tBS: 21a nHA: 20a

tBS: 42b nHA: 21b

TBC2 hydphil block anchor block

43.5 1.5

DMA: 70f DMA: ≈2.5f NAS: 10e

DMA: 546b DMA: ≈14b NAS: ≈9b

Mn,cal [g/mol]

Mn,GPC [g/mol]

PDI

10 300c

12 400d

1.15

67 700h 54 600g ≈2 800g

49 000i

1.8

a Determined by 1H NMR analysis of the crude reaction mixture. bDetermined using eq 1. cDetermined by application of eqs 2 and 3. dMeasured by GPC using THF as eluent and narrow-dispersed polystyrene standards for calibration. eDetermined by analysis of the quantitative 13C NMR spectrum of TBC2 and by using of the calculated number of tBS repeating units in MacroCTA. fDetermined by analysis of the quantitative 13C NMR spectrum of TBC2, applying the calculated number of tBS repeating units in MacroCTA and the assumption that 70% of the overall DMA conversion is consumed before addition of the NAS monomer and 2.5% after the NAS addition. gDetermined by application of eq 2. hDetermined by application of eq 3. iMeasured by GPC using dimethylacetamide as eluent containing 3 g/L LiCl and 2 vol % water and narrow-dispersed poly(2vinylpyridine) standards for calibration.

that the RAFT polymerization proceeded in a very controlled fashion. The chemical structure of the purified copolymer MacroCTA was evidenced by means of 1H and 13C NMR spectroscopy (Figure S1). Synthesis of TBC2. For the preparation of the hydrophilic block, the ratio between the DMA and the MacroCTA was set to 700 to 1 in order to obtain a molecular weight of about 50 000 g/mol for the hydrophilic block at a targeted conversion of ∼70%. All other reaction parameters were kept the same as in the aforementioned RAFT polymerizations. After a period of 43.5 h a solution of NAS in anisole was added during the course of reaction, resulting in the formation of the terminal anchor block. The polymerization was allowed to run for another 1.5 h before the polymerization was ceased. Since TBC2 contains the same repeating units as TBC1, the 1 H and 13C NMR spectra of TBC2 are nearly identical to those of TBC1 except for varying signal intensities (see Figure S7). Again, intensive signal overlapping in the 1H NMR spectra requires the determination of the conversions of DMA and NAS from a quantitative 13C NMR spectrum of TBC2 analogously as described above for TBC1. Here, the comparison of the signal intensities of the carbonyl carbons of the NAS unit (C-20 and C-21) with that of the aromatic carbon C-5 of the tBS unit revealed a ratio NAS to tBS of 1:4.76. As the number of the tBS repeating units is known from the analysis of the MacroCTA, the number of the NAS repeating units could be calculated to 9, which is in a good agreement with the targeted value range of 7−10. By applying eq 1 (see Table 1), the conversion of the NAS monomer was calculated to 10 mol %. The number of the DMA repeating units and the overall conversion of DMA were determined as described for TBC1, revealing a ratio of 13.33:1 (DMA:tBS) and an overall conversion of 72.5% for both the hydrophilic and the anchor block. We assume that ∼70% of DMA was consumed before addition of the NAS monomer during the course of copolymerization and only ∼2.5% after the addition. According to eq 1, the hydrophilic block contains ∼546 DMA units, whereas ∼14 DMA units are located within the anchor block. Accordingly, the theoretical Mn,cal of the hydrophilic and the anchor block were determined to 54 600 and 2800 g/mol, respectively. The GPC elugram of TBC2 (Figure S9) shows an intense shoulder toward higher molecular weight similarly as in the case of TBC1. Likewise, this phenomenon can be attributed to the fact that MacroCTA consists of two different kinds of polymeric chain transfer agents depending on whether the

mechanism of the RAFT polymerization, the R group within a CTA [(SC(Z)R] has an enormous impact on the polymerization rate and kinetics.32 Therefore, it seems likely that the different types of CTAs within DBC1 and in particular the acrylate-based systems [SC(Ph)S-nHA-unit] and [SC(Ph)S-NAS-unit] show a significantly different efficiency in a RAFT polymerization than the styrene based one [SC(Ph)StBS-unit]. Hence, polymers with two different chain lengths could be formed, and thus a big shoulder should be discernible in the GPC curve. Table 1 summerizes relevant polymer characteristics of DBC1 and TBC1 as well as specific parameters of their synthesis. Synthesis of MacroCTA. As can be seen in Scheme 1, the switching mechanism of a triblock copolymer brush system with a terminal anchor block is quite different from that having a central anchor block. According to a schematic study made by Xu et al.,39 all blocks within such a triblock copolymer system have to be carefully adjusted in terms of their molar masses and their molar mass ratios as well. Only under these conditions the blocks are capable of fully switching from one state into the other. In addition, they further found that the topmost layer of such a block copolymer brush system needs to have a minimum thickness of 2−3 nm in order to ensure that the surface properties are exclusively governed by this top layer. Consequently, the hydrophobic blocks should be long enough to form such a hydrophobic layer, but on the other hand, the hydrophobic blocks should be not too long, since this would strongly hamper the switching from the hydrophobic into the hydrophilic state (see Scheme 1). We anticipate that a molar mass of ∼10 000 g/mol for the hydrophobic block and 60 000 g/mol for the hydrophilic block might be a proper choice for this new block copolymer brush system. The synthesis of the corresponding triblock copolymer starts with the preparation of the hydrophobic block. To this end, the RAFT polymerization with the comonomers tBS and nHA was performed in the same fashion than in the synthesis of the hydrophobic block in DBC1, except that the polymerization was ceased already after 18 h by immersion of the flask into liquid nitrogen. The monomer conversions of tBS (21%) and nHA (20%) were determined by 1H NMR analysis as described for DBC1. The calculated Mn,cal = 10 300 g/mol for the MacroCTA containing tBS and nHA in a 2:1 molar ratio is in good agreement with the targeted molar mass of ∼10 000 g/mol (Table 2). The molar mass of MacroCTA determined by GPC against polystyrene standards is 12 400 g/mol, only slightly higher than Mn,cal due to the calibration method. The low PDI (1.15) further indicates 2623


Macromolecules

Article

380 °C. All four polymers are stable up to at least 210 °C. Small weight losses below this temperature can be attributed to the evaporation of residual low molecular weight compounds like adsorbed water or solvent, etc. Then, the MacroCTA starts to decompose at 210 °C, which is clearly discernible in its DTG profile. The diblock copolymer DBC1 and the triblock copolymers TBC2 are slightly more stable and commence to degrade at 225 °C while TBC1 begins to decompose at ∼230 °C. Basically, the stability and the degradation mechanism depend significantly on the type of the thiocarbonylthio group at the end of the polymer chains (dithioester, trithiocarbonate, xanthate) as well as on the type of repeating units within the polymer chain and in particular of those that are adjacent and in relative close proximity to the CTA end group.40−43 Typically, RAFT synthesized polymers start to decompose by a thermolysis of the thiocarbonylthio moiety (dithioester) at the end of the polymer chain. Postma et al. have already shown40 that a RAFT synthesized polystyrene degrades in such a way that only the sulfur-containing end group is removed, while in contrast, an analogous RAFT synthesized poly(n-butyl acrylate) underwent a homolysis of the C−S bond leading to a poly(n-butyl acrylate) radical which decays by (consecutive) intramolecular backbiting and β-scission processes. Here, two or even more repeating units of the polymer chain can be cleaved so that the weight loss can be much higher than in the case of the polystyrene where only the thiocarbonlythio end group is affected.40 As MacroCTA, DBC1, TBC1, and TBC2 contain styrene, acrylate, and acrylamide based types of repeating units in their polymer chains that are adjacent and/ or in the proximity of the dithioester end group, it is obvious that all four polymers show also a different decomposition behavior with varying degradation rates. However, all polymers show a sufficient thermal stability concerning their end use and also in terms of the polymer brush preparation where they have to withstand a temperature of ∼150 °C. In Table 3 all relevant thermogravimetric data of the polymers are summerized. As the first and second degradation steps for each polymer overlap to some extent, the end temperatures of the first decomposition stage as well as the weight loss in the first step can only be evaluated with a limited accuracy. We assume that the end of the first degradation step and the beginning of the second decomposition step in which the polymer backbone chains start to degrade is located approximately at the intermediate minima of the DTG curves. These positions are marked with arrows in Figure 7b and were the basis for the evaluation of the mass losses in the first step. The polymers MacroCTA and DBC1 were completely degraded at Tf, leading to no detectable residue, while in case of the triblock copolymers TBC1 and TBC2 5 and 6 wt % were left over, respectively. Apparently, these small amounts of residues seem

dithioester group is adjacent to a tBS [SC(Ph)S-tBS-unit] or a nHA repeating unit [SC(Ph)S-nHA-unit]. As a consequence, the molecular weight distribution of TBC2 is quite broad indicated by a high PDI of 1.8. In Table 2, characteristic data of MacroCTA and TBC2 are summerized together with relevant parameters relating their preparation. Thermogravimetric Analysis (TGA). The thermal stability of the prepared polymers MacroCTA, DBC1, TBC1, and TBC2 was evaluated by TGA measurements. A close observation of the TGA curves presented in Figure 7a,b reveals

Figure 7. TGA thermograms and their corresponding derivative thermogravimetric (DTG) profiles of the polymers MacroCTA, DBC1, TBC1, and TBC2. (b) depicts the first decomposition stage in a magnified scale while (a) shows the overall curves. The approximate end temperatures of the first decomposition stage in (b) are marked with arrows for the DTG curves.

that the polymers degrade in a two-step process. The first decomposition step is depicted in Figure 7b in which the TGA curves are shown together with the derivative thermogravimetric (DTG) profiles in a magnified scale between 200 and

Table 3. TGA Characteristics of the Polymers MacroCTA, DBC1, TBC1, and TBC2 polymer sample

Tia [°C]

Tmax1b [°C]

Tf1c [%]

weight loss 1. stepd [%]

Tmax2e [°C]

Tff [°C]

residue at Tf [%]

MacroCTA DBC1 TBC1 TBC2

210 ∼225 ∼230 ∼225

249 311 319 289

279 334 335 318

∼5 ∼7 ∼3 ∼3

375 410 414 420

464 437 475 475

Amphiphilic ABC Triblock Copolymers Tailored via RAFT

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