Citation for published version (APA): de Moel, C. (2001). Comb-shaped supramolecules: a concept for functional polymeric materials. Groningen: s.n.

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1 University of Groningen Comb-shaped supramolecules de Moel, C IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2001 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): de Moel, C. (2001). Comb-shaped supramolecules: a concept for functional polymeric materials. Groningen: s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Chapter Two Self-organized supramolecular comb-shaped copolymers Abstract Supramolecular comb-shaped copolymers, based on amphiphiles, such as pentadecyl phenol (PDP), hydrogen-bonded to polymers, such as poly(4- vinylpyridine) (P4VP), can self-organize into microstructures at various length scales. The simplest example, P4VP(PDP), forms lamellae with a long period of 36 Å below 67 C, due to the repulsion between the non-polar alkyl tails and the rest of the polar material. The strong hydrogen bond prevents the system from macrophase separation. When this comb copolymer complex is covalently bonded to a homopolymer like polystyrene (PS), an additional length scale is introduced. The PS-block and the P4VP(PDP)-block microphase separate into lamellae, spheres or cylinders of one component in a matrix of the other, depending on the volume fraction of the blocks. The long period is typically in the order of Å. The P4VP(PDP)-block always self-organizes in lamellae, provided the temperature is low enough. Charges can be added to these supramolecules by ionically complexing P4VP with a strong acid like methane sulphonic acid (MSA), before hydrogen bonding it subsequently to PDP. The polysalt P4VP(MSA) is a proton conductor. The supramolecular comb-shaped P4VP(MSA)(PDP) shows reentrant phase behavior. Switching of the proton-conductivity of PS-block- P4VP(MSA)(PDP) takes place upon increasing the temperature, because the conducting domains undergo a series of dimensionality transitions: from onedimensional lamellae-within-lamellae to two-dimensional lamellae upon crossing the order-disorder transition temperature of P4VP(MSA)(PDP) to one-dimensional cylinders. Namely, above 130 C the imminent macrophase separation of P4VP(MSA)(PDP) sets in, due to the breakage of most of the hydrogen bonds between MSA and PDP. Simultaneously, PDP becomes soluble in PS, increasing the volume of the PS-domains, which results in an order-order transition of the large length scale structure from lamellae to cylinders.

3 8 2.1 Introduction Self-organization of diblock copolymers Self-organization is an example of how nanoscale structures can be formed when mutually repulsive groups are chemically connected to the same molecules. A typical example of self-organization is self-assembled monolayers or Langmuir- Blotgett films formed on surfaces based on amphiphilic molecules. 1,2 Other examples are the ordered structures formed by block copolymers, 3 consisting of large blocks of mutually repulsive polymer chains, which are covalently bonded. They self-organize into microphase separated structures, due to a competition between the repulsion and attraction. Two chemically different blocks repel, which is the reason for most polymer mixtures to phase separate. 4,5 The covalent bond prohibits phase separation at a macroscopic level. Simple A-block-B copolymers self-organize into various morphologies in the melt, i.e., without solvent, as illustrated in Fig ,6 At low volume fractions of block A, f A, the system is initially disordered, i.e., the small A-block dissolves in the B- block. Above f A = 0.12, small spheres of A are formed in a matrix of B. Upon further increasing the length of the A-block, the system undergoes a change from a spherical to a cylindrical morphology and finally to a lamellar microphase near f A = More complex morphologies, such as the bicontinuous gyroid phase, perforated lamellae and the (metastable) modulated lamellae, 7-11 are found in a narrow composition window between lamellae and cylinders for sufficiently small values of the Flory-Huggins parameter χ 1/Τ. This parameter describes the strength of the enthalpic interaction between A and B. 4 The research in the block copolymer field has evolved from simple diblock copolymers to triblock- and multiblock copolymers and to block copolymers of a more complex architecture such as star and comb copolymers. The transition between two microphase is called an order-order transition (OOT). This generally requires a change in the lengths of the two blocks (see Fig. 2.1). Upon heating, the system undergoes an order-disorder transition (ODT) to a disordered state. For diblock copolymers, this usually occurs at very high temperatures as both the repulsion between the two blocks and the block lengths are generally very large. For moderate molecular weights the ODT may be accessible, e.g. at 164 C for a

4 9 Figure 2.1. Schematic phase diagram and characteristic microphase separated morphologies of A- block-b diblock copolymers. 3 Here f A is the volume fraction of the A-block. PS(10k)-block-PI(10k) copolymer. 12 In the case of comb copolymers, the orderdisorder transition temperature (T ODT ) and the characteristic length scale are determined by the repeat unit, which consists of the side chain combined with the part of the polymer backbone between this and the next side chain. Because this length scale usually is an order of magnitude smaller than that of conventional diblock copolymers, so is the periodicity of the ordered structures. This is usually in the order of Å for linear block copolymers and Å for comb copolymer architectures. Moreover, since this length scale also determines the order-disorder transition, this will also frequently occur at moderate temperatures Supramolecular assembly In classical chemistry, molecular building blocks are connected by permanent, covalent bonds. In supramolecular chemistry, however, these bonds are replaced by molecularly matching physical bonds, such as hydrogen bonds to form liquid crystals, 13 polymer-like chains, 14,15 side chain-liquid crystalline polymers, 16,17 or comb copolymers Other types of physical bonds include ionic complexation, 22,23 steric and charge match in the case of crown ethers/metal cation complexes, 14,24-26 π π stackings, 27 and coordination complexation. 28,29 These specific interactions are typical examples of molecular recognition. Molecular recognition is characterized by the simultaneous stability of the supramolecule and selectivity in its formation. It allows one to build highly specific complexes called

5 10 supramolecules, which, in turn, are able to form a hierarchy of structures. It plays an important role in biological systems, for example in the formation of the doublestranded structure of DNA and for the binding of enzymes. 30, Hierarchical comb-shaped supramolecules Homopolymer/amphiphile complexes: P4VP(PDP) and P4VP(NDP) Comb copolymer-like molecules can be obtained from polyelectrolyte/surfactant or homopolymer/amphiphile complexes. In the first case, polyelectrolytes ionically bind oppositely charged surfactants In this way, self-organized lamellae with a long period of 35.2 Å as well as cylindrical phases with fcc undulations in the structures have been found. 32,37 Alternatively, the lone electron pair of the pyridine nitrogen of poly(4-vinylpyridine) can coordinate with zinc dodecyl benzene sulphonate, also yielding lamellae with a periodicity of 28 Å in the bulk. 28,29 This thesis focuses on a third type of supramolecular comb copolymers, namely that in which amphiphiles are hydrogen-bonded to poly(4-vinyl pyridine) ,46-51 A schematic of the lamellar microphase-separated structure is given in Fig.2.2. In order for a homopolymer and surfactants to form mesomorphic structures, the attraction between the two components, i.e., the hydrogen bonding, and the repulsion between the polar material, consisting of the homopolymer and the polar head group and the apolar tail of the surfactant, need to be carefully balanced. 18,20 If the hydrogen bonding is too weak, no complexation or only partial complexation takes place. The repulsion dominates and macrophase separation occurs. On the other hand, if the repulsion is not strong enough, a homogeneous mixture is formed of homopolymer and surfactants, or of homopolymer-surfactant complexes. Poly(4-vinylpyridine) and poly(2-vinylpyridine) are hydrogen bonding acceptors, that have been used to make miscible polymer blends with polymers containing hydrogen bond donors, such as poly(hydroxy metacrylate)s, 52 or to make supramolecular side-chain liquid crystalline polymers. 16,17,23,53,54 A study of Poly(4-vinylpyridine) (P4VP) with alkyl phenols, alkyl amines, alkyl alcohols and alkyl carboxylic acids surfactants shows that only in the case of alkyl phenols and

6 11 a) b) c) Figure 2.2. Schematics of the supramolecular comb copolymer P4VP(PDP) 1.0. Comb-shaped supramolecules are formed by connecting pentadecyl alkyl tails (PDP), using phenolic hydrogen bonds to the pyridine groups of P4VP (a and b). A simple hydrogen bond suffices to achieve a strong enough physical bond and more complicated recognition is not needed. (c) The complex, in turn, selforganizes to form a lamellar microphase with a long period Lp 36 Å. 46,49 carboxylic acids mesomorphic structures might be formed. 18 Alkyl phenols are slightly acidic (pka ca. 10) and therefore they act as hydrogen bond donors. 55 In the case of pentadecylphenol, the tail of 15 methyl units provides a strong enough repulsion with the polar heads and backbone. Combined with sufficiently strong hydrogen bond, the complex, denoted as P4VP(PDP)x, self-organizes into a lamellar microphase for x sufficiently large (0.4 < x < 1.5). 19 Here x is the ratio between the number of amphiphiles and the number of pyridine groups in P4VP. Thus x = 1.0 corresponds to the stoichiometric composition, where each pyridine group can be complexed with an amphiphile. Infra red spectroscopy for P4VP(NDP) 1.0, where nonadecyl phenol (NDP) is a slightly longer alkyl phenol, shows that at 25 C most, but not all, of the pyridine groups of P4VP are hydrogen-bonded to NDP, as is clear from the absorption band at 1003 cm -1 in Fig Upon increasing the temperature, the fraction of free pyridine groups gradually increases, as indicated by the increase in peak height at 993 cm -1 in Fig Already at 120 C, a large fraction of the pyridine groups are free. The fraction of non-bonded pyridine groups is larger for NDP than for PDP, which can be attributed to the longer repulsive alkyl tail. 20 Still, even for NDP at 180 C, there is a considerable amount of hydrogen-bonded pyridine present.

7 12 Figure 2.3. Infra red spectroscopy of P4VP(NDP) 1.0 at different temperatures, showing the characteristic absorption bands of the free and hydrogen-bonded pyridine at about 993 cm -1 and 1003 cm -1 respectively. 20 The phase behavior of P4VP(PDP) 1.0, as shown by SAXS, is presented in Fig At 30 ºC a sharp peak is present at q = 0.13 Å -1, indicating that the long period of what happens to be a lamellar structure is ca. 36 Å. 20 Above 67 ºC, the peak abruptly broadens and decreases in height; a transition from an ordered state to a disordered state occurs. The scattering in the disordered state is due to the "correlation hole" effect, 4,56,57 and shows that at elevated temperatures a large portion of the hydrogen bonds between PDP and P4VP is still present. 19,56,57 Thus, the transition at 67 ºC is not due to the breakage of the bonds, but because the repulsion between the polymer backbone complex and the amphiphile tails becomes sufficiently small compared to the thermal energy k B T. 58 Below room temperature, alkyl side chain crystallization occurs and the second order peak becomes visible as well. Above this temperature the second order peak is absent due to symmetry. Due to the better packing on crystallization, the long period decreases to 34 Å. 59 Accordingly, the SAXS intensity maximum in Fig. 2.4 shifts to slightly higher q-values. Upon varying the ratio of amphiphiles to pyridine groups in P4VP (x) from 0.4 to 1.0 the long period decreases as Lp ~ 1/x. 46 The stretching of the alkyl chains is more or less independent of x, whereas the thickness of the polymer layer decreases with x. 46 For 1.0 < x < 1.5, the excess of PDP chains are located in the PDP layers and the lamellar structure is maintained. For x > 1.5 macrophase separation was

8 13 Figure 2.4. Small Angle X-ray Scattering (SAXS) patterns of P4VP(PDP) 1.0, recorded upon heating, showing a microstructure at low temperatures and a disordered phase above 67 ºC. 19 found. 46,55 In case of stoichiometric amounts of P4VP and PDP, x = 1.0, where the hydrogen bonding is nearly complete, the T ODT is highest, indicating the most stable configuration. This case will be considered throughout this thesis. In the case of nonadecyl phenol (NDP), the longer alkyl tail (19 instead of 15 methyl units) accounts for the larger long period of 45 Å. This has been visualized using TEM. 20 The complex has a higher order-disorder transition temperature of 100 C. Crystallization occurs around 55 C, due to which the long period changes to 41 Å. 20, Hierarchical self-organized supramolecular polymeric structures: PSblock-P4VP(PDP) A hierarchy of self-organized supramolecular polymer structures can be achieved if different self-organizing schemes, each with their own characteristic lengths, are combined. Here, we focus on combining the comb copolymer complex P4VP(PDP) x, discussed in the previous paragraph, with a linear block of PS This renders materials, which have a structure-within-structure morphology: the PS- and P4VP(PDP)x-block microphase separate at large length scales, typically in the order of 100 to 1000 Å, into lamellar, cylindrical and spherical morphologies

9 14 Figure 2.5. Lamellar-within-lamellar morphology of PS-block-P4VP(PDP) 1.0 as shown by transmission electron microscopy. The dark regions correspond to P4VP stained with iodine. 50 like ordinary diblock copolymers. Within the P4VP(PDP) x layers, the comb copolymer complex self-organizes to form lamellae with a long period of Å for 0.4 x ,51 Fig. 2.5 displays the lamellar-within-lamellar structure, showing the small lamellae are oriented approximately perpendicular to the larger lamellae. 50 The SAXS data in Fig. 2.6 show the phase behavior of PS-block-P4VP(PDP) At room temperature, almost all pyridine groups are hydrogen-bonded as was demonstrated by infrared spectroscopy. 50 However, with increasing temperature the number of free pyridine groups increases; significant decoupling of the hydrogen bonds takes place. Furthermore, it was separately observed that around 130 C PDP becomes soluble in PS. 50 Consequently, the volume fraction of the PS containing domains might increase due to the diffusion of PDP. This, in turn, might induce an order-order transition from lamellar to cylindrical. However, in contrast to the systems to be discussed next, such a transition has not been observed so far. 60 Up to elevated temperatures, as high as 220 C, PDP continues to behave as a preferential solvent for P4VP. Still, the interface will be enriched with free PDP thus reducing the number of unfavorable PS-P4VP interactions. The χ-parameter for this polymer pair is quite large, values of up to 2.4 have been reported, 61 although the most reliable data indicate a value of Consequently, the interface will become more diffuse at elevated temperatures, which is characteristic for the so-called intermediate segregation regime. 56,60 This might also explain the reduction in the large length scale small angle scattering peak and the disappearance of the corresponding second order peak at high temperatures.

10 15 Figure 2.6. SAXS patterns of PS-block-P4VP(PDP) 1.0 recorded during heating at 2 ºC/min. The weight fraction of polystyrene f PS = Proton conductivity of comb-shaped supramolecules Proton conductive comb-shaped supramolecules Charges can be added to the materials discussed in the previous paragraph by adding simple sulphonic acids, such as methane sulphonic acid (MSA), to P4VP. 49,55 Thus, a proton conducting charge-transfer complex is formed. 63 Wellknown examples of other charge-transfer complexes are poly(2-vinyl pyridine) and poly(4-vinyl pyridine) complexed with iodine. 64,65 Other examples are found in Chapter 8. The polysalt P4VP(MSA) shows normal Arrhenius behavior, i.e., conductivity that decreases exponentially with inverse temperature (see Fig. 2.8). 49 PDP hydrogen bonds to the sulphonate group. Thus, when PDP is added to the polysalt, a comb architecture is again obtained. Below the order-disorder temperature, the resulting complex microphase separates to form lamellae with a long period of approx. 48 Å as indicated by SAXS (see Fig. 2.7). Above ca. 100 C, the system becomes homogeneous. 49 With increasing temperature, more hydrogen bonds break, therefore the correlation hole peak decreases and gradually shifts to a smaller angle. Near 175 C strong forward scattering was observed, indicative of macrophase separation. Optical microscopy

11 16 Figure 2.7. SAXS of P4VP(MSA)PDP 1.0, recorded during heating at 2 ºC/min, illustrating the phase behavior of P4VP(MSA)PDP showed the system to become miscible again around 195 C. 49 This re-entrant phase behavior is a well-known phenomenon in mixtures of hydrogen-bonded molecules. 66 Above 220 C, most of the PDP molecules separate again from the P4VP(MSA) polysalt. This is the usual lower critical solution temperature behavior of polymer systems, induced by a considerable difference in thermal expansion of the components. 67 The layer of alkyl tails acts as an insulator. Therefore, the order-disorder transition acts as a dimensionality change from a two-dimensional to a three-dimensional conductive system. This, however, is not reflected in the conductivity in Fig The conductivity of P4VP(MSA)PDP 1.0 shows a continuous increase up to 160 C, above which the conductivity decreases due to the imminent macrophase separation. The conductivity starts to increase again above 195 C Switchable supramolecular self-organized materials Similar to P4VP(PDP) x, a hierarchy of self-organized structures can be achieved, when P4VP(MSA)PDP x is covalently bonded to a PS-block. For the simplest case, at room temperature a lamellar-within-lamellar structure is present, consisting of alternating layers of PS and P4VP(MSA)(PDP), which, in turn, forms alternating layers of the polysalt and the polar head of PDP, and of the apolair tails of PDP. 49 Above ca. 100 C, the P4VP(MSA)PDP 1.0 domains are in the disordered state. Increasing the temperature further results in a decrease in the number of hydrogen

12 17 a) b) Figure 2.8. Electrical conductivity (s) during heating at 5 ºC/min, based on ac impendance measurements extrapolated to zero frequency. a) P4VP(MSA) and b) P4VP(MSA)PDP bonds, eventually leading to macrophase separation in the corresponding homopolymer complex. Here, PDP, which becomes miscible with PS above ca. 135 C, will diffuse into the PS layers. As a result, the volume fraction of the PSblock gradually increases and the system undergoes an order-order transition (OOT) from a lamellar to a cylindrical microphase, i.e., to P4VP(MSA)PDP cylinders within a PS/PDP matrix (see Fig. 2.9). Upon cooling down, this transition turned out to be reversible. 49,68 Therefore, in contrast to the PS-b-P4VP(PDP) systems, where PDP continued to be a selective solvent for P4VP up to temperatures as high as 220 C, in the case of PS-b-P4VP(MSA)(PDP), PDP becomes a non-solvent for P4VP(MSA) and a selective solvent for PS, instead. Fig. 2.9 shows the conductivity of PS-block-P4VP(MSA)PDP 1.0 as a function of temperature. Both the alkyl layers and the PS layers are insulators and thus conductivity is in principle only possible along the P4VP(MSA)-rich layers. A dimensionality change from one-dimensional slabs to two-dimensional layers occurs on crossing the order-disorder transition of the small length scale structure. It is accompanied by a strong increase in the conductivity around 100 C. For the cylindrical microstructure, which is formed at ca. 150 C, conductivity is limited to the direction of the long axis of the cylinders only. Thus, at the order-order transition at ca. 135 C, another dimensionality transition occurs from twodimensional conducting layers to one-dimensional conducting cylinders, due to which the conductivity decreases. 49 The conductivity in Fig. 2.9 was measured for a macroscopically isotropic sample, i.e., the lamellar and cylindrical domains have

13 18 Figure 2.9. Electrical conductivity (s) of PS-block-P4VP(MSA)PDP 1.0 recorded during heating at 5 C/min based on ac impedance measurements extrapolated to zero frequency. Similar data were observed upon cooling. 49 The cartoons show the dimensionality transitions occurring upon heating: from 1-dimensional slabs to 2-dimensional lamellae to 1-dimensional cylinders. The ODT at 100 C and the OOT at 150 C are distinctly present in the conductivity. a random orientation. The results, therefore, correspond to the conductivity being averaged over all directions. For a macroscopically oriented sample, the conductivity is expected to become anisotropic. This fact is one of the motivations to study macroscopic orientation by oscillatory shear flow of the complex microphase separated morphologies formed by these supramolecular copolymer complexes. 3,69

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