Analysis of Multiple H-Bond Interactions in Self-Assembly between

Jan 30, 2008 - Strong evidence has been obtained for a type II H-bond, which is the specialty in PUc/P4VP assembly. The type I and type II H-bonding ...
1 downloads 0 Views 304KB Size
Download PDF
1926

J. Phys. Chem. B 2008, 112, 1926-1934

Analysis of Multiple H-Bond Interactions in Self-Assembly between Polyurethane with Pendent Carboxyl and Poly(4-vinylpyridine) Zhiyong Ren,*,† Senxiang Cheng,† Guobao Zhang,† Dezhu Ma,† and Xiaozhen Yang*,‡ 8224 Henan Key Laboratory of Fine Chemicals, Zhengzhou 450002, People’s Republic of China and 8225 State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry of Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: July 17, 2007; In Final Form: NoVember 5, 2007

As an extension study, FTIR and molecular simulation methods were combined in the present paper to analyze the H-bond interactions resulting from multiple donors and acceptors that have led to self-assembly based on segmented polyurethane with carboxyl (PUc) and poly(4-vinylpyridine) (P4VP) in our previous work. Of them, FTIR was used to analyze the H-bonding types and interactions as well as their changes before and after self-assembly; molecular mechanics (MM/COMPASS) was used to study the effect of possible conformations on the H-bonds involved and analyze the most probable H-bond patterns; quantum mechanics (QM/B3LYP) was used to help confirm the experimental FTIR band assignments and calculate the H-bond energy. It was found that two types of H-bonds exist, namely, COOH‚‚‚P4VP (type I) and (OCO)NH‚‚‚ P4VP (type II), based on OH and NH as the strong donors in the interaction between PUc and P4VP. Strong evidence has been obtained for a type II H-bond, which is the specialty in PUc/P4VP assembly. The type I and type II H-bonding energies are -11.293 and -7.150 kcal/mol, respectively. The forming probability of the type I H-bond accounts for 95.87%, while that of the type II H-bond is 4.13%, showing the primary driving force for the assembly based on PUc and P4VP is still the H-bond between COOH and P4VP, yet the H-bonds based on NH and pyridyl in P4VP cannot be ignored. 1. Introduction Research on H-bonds has been an active topic due to its importance in determining molecular conformation, molecular aggregation, and the function of a vast number of chemical systems ranging from inorganic to biological.1 It has been well accepted that H-bonding is also a driving force for selfassembly2-4 based on COOH-containing polymers and pyridyl polymer as well as for liquid crystal preparation5-6 based on pyridyl/acid-containing organic molecules or polymers. The COOH-containing polymers used for the assembly are usually carboxyl-terminated polystyrene (CPS),7-8 carboxyl-terminated polybutadiene,3 and carboxyl-terminated polyimide,2 whereas one of the pyridyl polymers is poly(4-vinylpyridine) (P4VP). Generally, carboxyl and pyridyl act as the H-bond donor and acceptor, respectively. The H-bond, based on OH as donor and pyridyl N as acceptor, is the sole strong H-bonding interaction in the conventional self-assembly. In the study on the H-bonds in COOH-containing polymer or on the H-bond between the carboxylic acid and pyridine, MacKnight et al.9 and Lee et al.10 show that carboxylic acid is strongly self-associated. The pyridyl-containing polymer is inherently weakly self-associated, while the strong intermolecular H-bonds can be formed between the carboxylic acid and pyridine groups.10 In order to study the H-bond interactions in polyurethane (PU) related self-assembly, we synthesized a type of segmented polyurethane with carboxyl (PUc), in which COOH is attached to its hard segment. The self-assembly based on PUc and P4VP * To whom correspondence should be addressed. E-mail: zyren23@ yahoo.com, [email protected]. † Henan Key Laboratory of Fine Chemicals. ‡ Institute of Chemistry of Chinese Academy of Sciences.

has been successfully made under selected conditions.11 Different from the conventional COOH-containing polymers that have been used for self-assembly, PUc is a segmented copolymer with alternating soft and hard segments. In addition, PUc itself has natively typical H-bond interactions based on urethane groups besides COOH. Therefore, it is interesting to know how multiple H-bonds function in such a self-assembly. H-bonds in PU have been widely studied since they closely relate to phase separation that is crucial to PU’s excellent properties.12-14 Previous studies on H-bonds in PU, however, mainly dealt with one H-bond donor (NH-based) system by FTIR12-18 and molecular simulations.19-23 As for multiple H-bonds based PUc, we performed preliminary studies on its H-bonds in a model PUc hard segment.24-25 Ambrozˇˇıcˇ et al. studied the liquid crystal PU (LCPU) based on multiple H-bonding donors and acceptors.26-28 Since in LCPU the COOH group is on the organic compounds while the pyridyl group is on the PU side chain, the urethane group only contributes to the binding of acid molecules to a minor extent28 besides the main H-bond between COOH and pyridyl N. PUc/P4VP selfassembly, however, is different from all the above-mentioned COOH- and pyridyl-containing polymers in either structure or type of H-bond donor-acceptor. Since the pyridyl N is not on the PU chain in PUc/P4VP assembly, it is completely possible for urethane NH to form the H-bond with the pyridyl besides the typical H-bond COOH‚‚‚pyridyl. This is expected to be of its specialty in stabilizing PUc/P4VP assembly. In the present work, FTIR and simulation methods have been combined to analyze the H-bond interactions leading to the selfassembly based on PUc and P4VP for the first time. The PUc used for the assembly is based on poly(tetrahydrofuran) (PTHF),

10.1021/jp075588b CCC: $40.75 © 2008 American Chemical Society Published on Web 01/30/2008

Analysis of Multiple H-Bond Interactions

J. Phys. Chem. B, Vol. 112, No. 7, 2008 1927

Figure 1. Reaction scheme of segmented PUc copolymer based on DMPA, TDI, and PTHF.

Figure 2. Comparative hydrodynamic radius (Rh) distributions of PUc/ P4VP assembly in mixed solvents (THF + CHCl3) at different time periods (total concentration is 9.64 × 10-4 g/mL)11.

2,2-hydroxymethyl butanoic acid (DMBA), and toluene diisocyanate (TDI) with the hard segment content of 45%. Two types of H-bonds, namely, OH‚‚‚P4VP (type I) and (OCO)NH‚‚‚P4VP (type II), have been found. Their H-bond energy and forming probability have been quantitatively studied. The objective of the present research is the continuation of our previous selfassembly work11 by further quantitatively analyzing the Hbonding interactions based on both carboxylic OH and especially urethane NH as donors in the assembly. 2. Experimental Section 2.1. Materials and Polymerization. Materials and polymerization as well as the preparation of PUc/P4VP self-assembly and their characterizations are available elsewhere.11 The hard segment concentration in PUc used for self-assembly is 45% (in weight), while the mole ratio among PTHF, TDI, and DMBA is 1:3:2. The concentration of -COOH groups in PUc is 5%. In preparation of PUc/P4VP self-assembly, P4VP/CHCl3 solution was added to PUc/THF solution with a total concentration [(PUc/P4VP) in (THF + CHCl3)] of 9.64 × 10-4 g/mL. The concentration of P4VP is 0.0156 mmol, while that of COOH in PUc is 0.0136 mmol, with P4VP being a bit excessive. The molar ratio of -COOH to pyridyl unit in the PUc/P4VP complex is about 1:1.15. Figures 1 and 2 were selected to show the scheme of the preparation of PUc and the laser light scattering result of self-assembly based on PUc and P4VP,11 which shows that the average hydrodynamic radius (Rh) is stable with time. 2.2. Instrumental Measurements. FTIR spectra were recorded on a Shimadzu FTIR-8700 spectrometer from 4000 to 400 cm-1, averaging 45 scans at a resolution of 2 cm-1 at room temperature. The PUc solution samples were directly coated onto a piece of KBr plate and measured after the acetone was completely removed, while precipitation (after being dried) from PUc/P4VP CHCI3 solution sample (with the same concentration ratio between PUc and P4VP in THF + CHCl3 solvents) was measured using KBr pellets.

2.3. Molecular Modeling. Combination of both quantum mechanics (QM) and molecular mechanics (MM) was used for modeling the H-bond interactions in order to better understand the H-bonding interactions in PUc/P4VP assembly. QM is suitable for obtaining H-bonding energy due to its accuracy of energy calculation; MM, on the other hand, has been approved to be an effective way of studying optimized H-bonding structure, especially when studying the effect of various conformers on the H-bonds.23,25 Model molecules that are used for calculating H-bonds are useful for explaining the H-bonds in PUc/P4VP assembly, although there would be differences between them and actual polymers. According to classical mechanics, the association energy between two single molecules includes all nonbonded molecular interactions such as the Lennard-Jones interaction, Coulombic interaction, and H-bond interaction. For the molecules with functional groups such as NH and CdO in PU and polyamide, however, the energies resulting from the first two are usually one magnitude smaller than that from H-bond; therefore, it has become a routine way to take the association energy, i.e., the difference between the sum energy of both individual molecules and the energy of the complex, ∆E, as the H-bonding energy.19-21,23,25 2.3.1. QM. Ab initio calculations of QM with the Gaussian 98 set of programs in the B3LYP method with a 6-31+G* basis set was used for studying the H-bonds as well as modeling the main FTIR spectra in PUc and its assembly with P4VP. The molecular models for PUc hard and soft segments as well as P4VP in modeling H-bonds OH‚‚‚P4VP, NH‚‚‚P4VP, and COOH dimer are 2-methyl-3-{[(methylamino)carbonyl]oxy}propanoic acid and pyridine, respectively; since it is more important for groups containing the H-bond donor and acceptor in simulating the IR spectra, two simpler models based on N-methyl methyl carbamate and dimethyl ether were used to simulate the infrared spectra related to NH‚‚‚OdC and NH‚‚‚ ether H-bonds (details for model molecules can be seen in section 3.2). 2.3.2. MM. All molecular models were built on a Silicon Graphics O2 workstation using the program Cerius 2, version 3.8, developed by Molecular Simulations Inc. (MSI). The COMPASS29 force field was used to optimize each model molecule. The high convergence option was adopted for the energy minimization of each model; the root-mean-square of force on each atom was controlled less than 0.001 kcal‚mol-1‚Å-1. For either individual molecule or their complex formed by the H-bond the structure of the model molecules was optimized to get the H-bond properties. For the H-bond complex, the two corresponding molecules were first located in a certain position where the H-bond donor atom, H, and the acceptor atom, O or N, are in line with a distance of about 3 Å before optimization. The energy of both the individual donor and acceptor molecule and the energy of the complex were then obtained.

1928 J. Phys. Chem. B, Vol. 112, No. 7, 2008

Ren et al.

Figure 3. Comparative FTIR spectra between PUc itself (before self-assembly) and PUc/P4VP complex (after self-assembly) in a range of 16003400 cm-1.

Figure 4. Comparative FTIR spectra between PUc itself (before self-assembly) and PUc/P4VP complex (after self-assembly) in a range of 9001500 cm-1.

2.4. Characterization. 2.4.1. FTIR. An obvious νN-H band at 3300 cm-1, amide II band at 1538 cm-1, and amide III band at 1225 cm-1 were found in the FTIR spectrum for PUc, indicating that the urethane group was formed. Meanwhile, there is a wide νCdO band centered at 1720 cm-1, showing the existence of different CdO from both urethane and carboxyl. In addition, the small band at 2606 cm-1 also suggests the existence of COOH. Moreover, the νC-O band at about 1100 cm-1 shows C-O-C group from PTHF. 2.4.2. NMR. 1H NMR (DMSO-d6, TMS): δ 12.76 (H from COOH), 9.50 and 8.86 (-NHCOO-), 7.47 and 7.06 (-ph-), 4.25 and 3.402 (-CH2O-, both from urethane and PTHF), 2.10 (CH3-ph), 1.59 (-CH2, attached to CH3 in DMBA), 0.86 (CH3 in DMBA). 13C NMR (DMSO-d , TMS): δ 174.78 (-COOH), 153.51 6 (-NHCOO-), 137.18, 130.16, 115.05 (-ph-), 69.68 and 63.86 (-CH2O-, both from urethane and PTHF). The above FTIR and NMR results show that the samples prepared are supposed to be the expected PUc with the structure of soft and hard segments (Figure 1). Of them, the molecular weights of the raw materials PTHF and P4VP are 1000 and 60 000, respectively, while Mn and Mw of the prepared PUc are 5400 and 13 000 with a molecular distribution of 2.4.11 3. Results and Discussion 3.1. FTIR Spectroscopic Analysis of H-Bonding Interactions in PUc/P4VP Self-Assembly. 3.1.1. General Analysis of H-Bonds Before and After Self-Assembly. Before discussing the

H-bonding interactions in the self-assembly, it is necessary to analyze the H-bond donors and acceptors in PUc itself and in the complex formed by PUc and P4VP after self-assembly. The PUc itself has two strong donors and three main strong acceptors. The two strong donors are NH from urethane and OH from COOH, while the three acceptors include urethane CdO and carboxyl CdO from hard segment as well as the ether group from soft segment. Theoretically the N and alkoxyl O in urethane can be also as acceptors. According to our simulation research on model PUc,25 however, these two acceptors can form the H-bond only in a special configuration. Therefore, they will not be considered here. Even so, the type of acceptors is one more than that of donors in PUc itself. Since carboxyl is on the hard segment, the amount of COOH and NH groups is expected to be the same in the hard segment, while the amount of ether group (one acceptor) is more than other acceptors (CdO from urethane and carboxyl) as each repeating unit of PTHF has one ether group in PTHF with a molecular weight of 1000. Namely, not only the type but also the amount of acceptor is more than donors in PUc. In P4VP itself, on the other hand, there is only one strong acceptor, namely, the pyridyl N from P4VP. When self-assembly is formed between PUc and P4VP, the type of acceptor will become four after adding the three above mentioned ones, while there are still only two strong H-bond donors (COOH and NH). In comparison, in most of the other self-assembly systems, there is only one type of H-bond, that is, the H-bond between carboxylic OH and pyridyl N.

Analysis of Multiple H-Bond Interactions

Figure 5. Comparative νNH bands between model soft segment (TDI/ PTHF) and model hard segment (TDI/DMBA) in a FTIR range of 2400-3600 cm-1.

Figures 3 and 4 show the comparative FTIR spectra of PUc itself (before self-assembly) and PUc/P4VP complex (after selfassembly was formed). It is seen that the IR spectra before and after assembly are quite different. The most obvious changes before and after self-assembly include the following: (1) one band at 3300 cm-1 becomes two bands at 3250 and 3313 cm-1, (2) the shoulder band at 2615 cm-1 becomes a peak band at 2483 cm-1, (3) one new band at 1943 cm-1 appears after selfassembly, (4) two peak bands at 1731 and 1713 cm-1 in PUc become one at 1716 cm-1, (5) one band at 1538 cm-1 in PUc becomes two bands at 1559 and 1542 cm-1, and (6) the band at 1109 shifts to 1113 cm-1. In order to help understand the spectral changes after assembly, we prepared the model soft segment TDI/PTHF and hard segment TDI/DMBA of PUc, whose comparative νNH bands are shown in Figure 5. It can be seen that the shoulder band at 2615 cm-1 in PUc (before self-assembly) on the left in Figure 3 is similar to that at 2619 cm-1 in Figure 5. Both should be the COOH dimer related OH band (sometimes called satellite bands10) of PUc, which is similar to the COOH-containing polymer (ethylene-methacrylic acid copolymers) studied by MacKnight et al.9 According to their research, COOH is easily formed in the way of dimer with the νOH band centered at about 3000 cm-1, in which there is a small but obvious band at about 2650 cm-1.9 This shoulder is characteristic of the stretching mode of the H-bonded hydroxyl group. Though the possible νOH band at about 3000 cm-1 in PUc is overlapped with CH2 and CH3 symmetric and asymmetric bands, the νOH band change can still be judged by making a comparison between the left and right parts of Figure 5. A remarkable difference can be found at about 3000 cm-1 for the model soft and hard segments with and without COOH group. In the model soft segment (left Figure 5) consisting of TDI and PTHF, the bands centered at 2900 cm-1 come only from the pure CH2 and CH3 symmetric and asymmetric bands, and there is no band between 3000 and 3200 cm-1, while there is a broad band between 2400 and 3100 cm-1 in the model hard segment (right Figure 5) consisting of TDI and BMBA. Obviously, the νOH band relating to COOH in the model hard segment has mixed with CH2 and CH3 symmetric and asymmetric bands. By carefully observing the band changes in the left and right parts of Figure 3, we can still see the band changes before and after self-assembly at around 3000 cm-1, although that region has similar different mixed bands. Therefore, the analyses of FTIR

J. Phys. Chem. B, Vol. 112, No. 7, 2008 1929 spectra on model hard and soft segments do give us the possibility for further analyzing FTIR spectra changes in PUc. The FTIR spectroscopic characteristic in PUc is that the two donors, NH and OH, are completely H-bonded, but there is obviously free νCdO. Such a result is reasonable as there are three strong H-bond acceptors while there are only two strong H-bond donors in PUc. If one donor forms a H-bond with one acceptor, there must be free acceptor left as well as an additional amount of ether group from the repeating unit of the soft segment. According to our experimental results we can preliminarily judge that the ether group in the soft segment has formed the H-bond with H-bond donors NH, which can be supported by the following three aspects. First, the νC-O-C (ether) is at 1110 cm-1, which is usually the H-bonded νC-O-C band. Second, DSC results30 also give additional proof from the Tg changes. Usually it is true that polyether soft segments in PU tend to phase separate to a higher degree than polyester soft segments. The way of judging it is to observe whether there is one Tg and the difference of Tg between pure soft segment and segmented PU. It is found that there is only one Tg in PUc based on PTHF as the soft segment, which was found at -3.6 °C,30 whereas the Tg of pure PTHF is -72 °C.31 The results of one Tg and a large ∆Tg between PTHF and the soft segments in PUc both show that there is good compatibility between the soft and hard segments in PUc after COOH has been introduced to PU. Third, we note that the νNH band in PUc is at 3300 cm-1, closer to the νNH band at 3296 cm-1 in the model soft segment while relatively further away from that (3313 cm-1) in the model hard segment. Since the νNH band in PUc can be considered as an average between the NH-soft segment and NH-hard segment interactions, this result possibly shows that the H-bond based on NH as a donor in PUc is expected to have formed more H-bonds with the ether group in the soft segment but fewer H-bonds with CdO in the hard segment. Therefore, it is not strange to have free CdO in PUc. Then where is the free CdO from? Since there is a band at 2615 cm-1 indicating the carboxylic dimer, the free νCdO may come from urethane CdO. Of course, we cannot rule out that the carboxylic OH may also form the H-bond with urethane CdO, leaving some carboxylic CdO free, while NH also forms the H-bond with carboxylic CdO, though still leaving urethane Cd O free in this way. That is not so important, however. The importance is that free CdO exists in PUc before making a complex with P4VP, which is helpful in studying the H-bond interaction change before and after self-assembly. It is more interesting that among other changes before and after assembly the free νCdO peak band disappears as the left part of Figure 3 shows. Since there are no free νOH and νNH bands and the type of strong H-bond acceptors becomes two more than that of H-bond donors after self-assembly, the disappearance of free νCdO suggests some other acceptor must become free. Of the four acceptors, two CdO groups coming from both carboxylic and urethane have been shown to be H-bonded, whereas the pyridyl N can also be taken as H-bonded according to the band at 1943 cm-1.10 In this way, only another acceptor, ether group, can be considered being possibly free. Since the νC-O-C band at 1109 cm-1 shifts to 1113 cm-1 after assembly, it is reasonable to think the H-bond NH‚‚‚ether may break after assembly, leading to free ether. Even if the ether group becomes free after assembly, however, there are still three types of acceptors versus two types of strong donors. The acceptor amount is still more than donor’s, and the concentration of P4VP added for self-assembly is more than

1930 J. Phys. Chem. B, Vol. 112, No. 7, 2008 that of COOH when the self-assembly between PUc and P4VP was prepared. In this case, νCdO should have remained free instead of being H-bonded. However, they are H-bonded, suggesting new interactions exist after the self-assembly is formed. Therefore, we can preliminarily judge from FTIR spectral changes that two types of H-bonds exist in PUc and P4VP interactions based on OH and NH as donors. The following sections will be used to make further analysis in detail. 3.1.2. H-Bond Based on COOH and P4VP (Type I H-Bond). A type I H-bond in the present paper means a H-bond interaction between COOH and P4VP, which can be indicated as COOH‚‚‚P4VP (type I H-bond). Strong evidence for a type I H-bond in PUc/P4VP self-assembly includes the newly appearing bands at 2483 and 1943 cm-1 as the right side of Figure 3 shows. They indicate a strong H-bond interaction between COOH and pyridyl N as Lee et al. indicated when they studied the complex between poly(ethylene-co-methacrylic acid) (EMAA) copolymer blends and poly(2-vinylpyridine) (P2VP).10 The new band at 2483 cm-1 after assembly can be possibly taken as the red shift of 2615 cm-1, suggesting OH from COOH has formed a H-bond with a stronger H-bond acceptor, which can only be the pyridyl N of P4VP. Namely, the strength of H-bond COOH ‚‚‚P4VP after assembly is obviously stronger than that of COOH dimer before assembly. That may be the reason why the COOH dimer would disassociate when P4VP was put together with PUc. The frequency difference between the band at 2530 cm-1 in Lee’s sample and the band at 2483 cm-1 in the PUc/P4VP complex obviously comes from different COOH-containing and pyridyl-containing polymers. The band at 2483 cm-1 suggests that the H-bond in the PUc/P4VP complex is stronger than that at 2530 cm-1 in the EMAA/P2VP complex. There is no doubt that the original H-bonding interactions would have readjustments after PUc was mixed with P4VP. Among all other bands changes, the change of the νCdO band may also relate to the type I H-bond. As discussed above, the appearance of the free νCdO band is reasonable in PUc but the question is why the free νCdO band disappears in PUc/ P4VP self-assembly where the amount of H-bond acceptors in PUc/P4VP self-assembly are even more than that in PUc itself when the H-bond donors remain unchanged. In other words, the free CdO should not disappear if only OH and NH are considered as H-bond donors and if one donor forms a H-bond with one acceptor. Instead, more free CdO groups should have been possibly left since there are no free νOH and νNH bands. When COOH forms the H-bond with P4VP, the original COOH dimer may dissociate and form a H-bond with P4VP. After that, the νOH band would certainly be still H-bonded but the CdO associated with OH from COOH would be free as in the assembly between EMAA and P2VP.10 Though we cannot rule out the possibility for these CdO groups to form the H-bond with NH or the possibility that two acceptors (e.g., both urethane and carboxylic CdO groups) form the H-bonds with one donor (e.g., OH or NH)), another possibility cannot be ruled out either. That is, these free CdO’s would form H-bonds with some additional acceptor, namely, the third donor. According to Sharma’s result,32 the H-bond interaction involves a strong H-bond OH‚‚‚pyridyl N and a weak H-bond CdO‚‚‚R-CH in P4VP. We think our FTIR result gives indirect proof for the weak H-bond associated with CdO. It is related to the H-bond patterns in the COOH‚‚‚P4VP interaction (see section 3.2). When CdO forms the H-bond with R-CH in P4VP, the νCdO frequency would also definitely shift to lower frequency, although such a shift is expected to be smaller than that when CdO forms strong H-bonds with OH or NH. In spite of that, it

Ren et al. is necessary to stress that the peak band at 1716 cm-1 is expected to represent the comprehensive results of the H-bonds based on CdO with different donors including not only R-CH but also OH and NH as the νCdO should also include urethane CdO. 3.1.3. H-Bond Based on Urethane and P4VP (Type II H-Bond). The type II H-bond in the present paper means the H-bond interaction between urethane and P4VP, which can be indicated as (OCO)NH‚‚‚P4VP (type II H-bond). As is seen from Figure 3, the νNH band at 3300 cm-1 before self-assembly has split into two bands at 3313 and 3250 cm-1 after selfassembly. The result suggests that the original H-bond-based NH as a donor turns into two distinct H-bonds of which one is weaker while another is stronger than the original H-bond according to the band wavenumbers. This is another proof for the original H-bond readjustments after introduction of a strong H-bond acceptor, the pyridyl N, into PUc, leading to formation of the self-assembly. Three acceptors can most possibly form the H-bonds with NH in the assembly. They are urethane Cd O, ether O in the soft segment, and pyridyl N in P4VP. Since the band at 3313 cm-1 in PUc/P4VP self-assembly has the same frequency as the band in model PUc hard segment (Figure 5), some of the NH in the self-assembly is expected to form the H-bond with CdO. This is consistent with the possible disassociation of the H-bond NH‚‚‚ether according to the νCO-C (ether) band shift from 1109 to 1113 cm-1 after selfassembly. In addition, since the H-bond NH‚‚‚ether-based νNH is lower in frequency (3296 cm-1) than the H-bond NH‚‚‚Od C-based νNH (3313 cm-1) (Figure 5), the above analysis is also reasonable. It is worth, however, paying special attention to another new band at 3250 cm-1. We note that there is no such band in PUc before assembly including in either the model hard or the model soft segment. The νNH band based on NH‚‚‚OdC in the model hard segment is at 3313 cm-1 (right Figure 5), while the νNH band resulting from NH‚‚‚ether is at 3296 cm-1(left Figure 5); both are at higher frequency than the new band at 3250 cm-1. It can then be concluded that the band at 3250 cm-1 is not due to the H-bond resulting from either the H-bond NH‚‚‚CdO (from urethane) or the H-bond NH‚‚‚O (ether) in PUc. The H-bond with the band at 3250 cm-1 is expected to be stronger than both NH‚‚‚OdC and NH‚‚‚O (ether) H-bonds. In addition, the band at 3250 cm-1 being from OH-related H-bonds can be eliminated. Since the νOH band from COOH is centered at about 3000 cm-1 and since the H-bond COOH‚‚‚P4VP after selfassembly is stronger than that of COOH dimer, the νOH band resulting from the H-bond COOH‚‚‚P4VP is expected to shift to lower wavenumbers (namely, lower than 3000 cm-1) after assembly, which is the reason why the new band at 3250 cm-1 is certainly not from the H-bond COOH‚‚‚P4VP. Therefore, the band at 3250 cm-1 can be preliminarily assigned to the νNH band resulting from the H-bond NH‚‚‚P4VP. Further proof can be found for the type II H-bond. We note from Figure 4 that the amide II and III NH bands at 1538 and 1225 cm-1 split, respectively, into two bands at 1559 and 1542 cm-1 as well as at 1248 cm-1 (one main band), respectively. The fact that both amide II and III shift to higher wavenumber (δ NH + νCN) shows the NH is in the stronger H-bond state after assembly, which is consistent with the νNH shifting from 3300 to 3250 cm-1. Therefore, the appearance of the band at 3250 cm-1 does suggest that the νNH-based band is stronger than that before assembly. 3.2. Confirmation of the Type II H-Bond by Simulation Method. In order to further confirm the type II H-bond, we

Analysis of Multiple H-Bond Interactions

J. Phys. Chem. B, Vol. 112, No. 7, 2008 1931

Figure 6. Models used for simulating infrared bands in PUc/P4VP assembly (among them, 2-methyl-3-{[(methylamino)carbonyl]oxy}propanoic acid stands for PUc, dimethyl ether for soft segment, pyridine for P4VP, and N-methyl methyl carbamate for general hard segment without COOH).

TABLE 1: FTIR and Simulated (QM/B3LYP) H-Bond Relating Bands

experimental FTIR band (cm-1) simulated IR band (cm-1) after correction

νOH-related bands in COOH dimer

νOH-related bands in OH‚‚‚P4VP complex

NH‚‚‚P4VP

NH‚‚‚O

NH‚‚‚OdC(NH)

2606 3201 3025

2483 2998 2836

3250 3399 3219

3296 3520 3296

3313 3538 3313

used QM to simulate the H-bonds relating infrared bands including the H-bonds based on COOH dimer, COOH‚‚‚P4VP, NH‚‚‚P4VP, NH‚‚‚O (ether), and NH‚‚‚OdC(NH). 2-Methyl3-{[(methylamino)carbonyl]oxy}propanoic acid (Figure 6) was used to be the model molecule representing the hard segment in which both urethane and carboxyl are in one molecule while dimethyl ether was used as the model soft segment and pyridine was to stand for P4VP. For the PU model hard segment without COOH, N-methyl methyl carbamate and dimethyl ether were used to model the H-bonds NH‚‚‚OdC and NH‚‚‚ether. Table 1compares band wavenumbers from both FTIR experiments and simulation, of which the QM result is helpful in assigning νOH and νNH bands. We will analyze the different H-bond interactions based on these bands. It can be seen from Table 1 that all the simulated νNH or νOH bands are of relatively higher wavenumbers than that in the actual IR spectra. One of the reasons for the difference in band position between simulated and experimental values is that the simulated spectra made by QM are based on two molecules while the actual spectra are a reflection of the condensed state of molecules. Though the simulated spectra are not from the structural environment of true molecules, it is helpful for making a relative comparison among the different H-bond interactions. It can be noted that the νOH from COOH‚‚‚P4VP is indeed of lower wavenumber than that of COOH dimer, as FTIR shows. Therefore, the newly appearing band at 3250 cm-1 cannot be from νOH relating band. Comparing the simulated three NHrelated bands with the experimental results in Table 1, we can see that the two simulated bands 3520 and 3538 cm-1 (relative to 3296 and 3313 cm-1 from FTIR) are also higher than 3438 cm-1 (relative to 3250 cm-1 from FTIR band). This result shows that the band at 3250 cm-1 is certainly not from νNH relating to NH‚‚‚OdC or NH‚‚‚ether either. Therefore, we can safely conclude that the band at 3250 cm-1 can only be from the H-bond NH‚‚‚P4VP. According to above analyses and the simulation in the following section, the two H-bond patterns can be possibly shown in Figure 7. Of them, type I has long been proved32 with a weak H-bond coexisting with a strong one while type II is the specialty in PUc/P4VP assembly. The type II H-bond also has a weak H-bond in the (OCO)NH/P4VP interaction according to the following simulated results. As there are possibly different conformers in urethane, two different H-bonds based on (OCO)NH and P4VP possibly exist, which are shown as conformers A and B of type II H-bond in right Figure 7. Though we confirmed the existence of a type II H-bond based on NH, in which conformer the type II H-bond adopts needs to further proof. The supposed two forms of type II H-bonds have the same strong H-bond donor NH but different weak H-bonds. The acceptor forming the weak H-bond with R-CH in conformer A

of the type II H-bond is CdO, whereas the acceptor forming the weak H-bond with R-CH in conformer B of the type II H-bond is alkoxyl O. The only way to distinguish it is to analyze the weak H-bond associated with these two forms of H-bonds. As the weak H-bond is not the main subject in the present paper, here we only suppose that these two forms of type II H-bonds both possibly exist. Further proof needs to occur in the future. 3.3. Forming Probability of the Two Types of H-Bonds. Since the type I and type II H-bonds both possibly contribute to the assembly, it is necessary to quantitatively know how much of each forming probability for the assembly will be, which can be done according to the Boltzmann equation. It is the first step to get the H-bond energy for the two types of H-bonds. Since the different conformers in each type of H-bond possibly exist, however, we cannot simply take one H-bond configuration from each of the two types of H-bonds to conduct such an analysis, for it is not considerate. Instead, the H-bond configuration with the highest probability in these two types of H-bonds should be considered in order to study the H-bonds reasonably. Namely, the main different conformers from these two types of H-bonds should be taken into account. Our previous work25 showed that various different conformers may exist if the relative position of NH and OH in the COOHcontaining urethane chain are considered. Among them, four conformers have the lowest energy (Figure 8) and similar high probability. Figure 9 shows the eight different H-bond complexes based on both OH and NH as donors in the COOH/ P4VP assembly. Therefore, all of the eight complexes should be selected in studying the forming probability of the H-bonds between PUc and P4VP. According to the simulation results, a pair of H-bonds exists for each complex in which one is a strong H-bond resulting from OH and pyridyl while the other is a weak H-bond formed by R-CH and CdO. This H-bond pattern is

Figure 7. Proposed two types of H-bonds existing in PUc/P4VP selfassembly (of which the second type of H-bond consists of different weak H-bonds based on conformers).

1932 J. Phys. Chem. B, Vol. 112, No. 7, 2008

Ren et al.

Figure 8. Four main conformers in PUc with lowest energies used as the basic conformers for further simulation.25

Figure 9. Eight possible H-bond complexes from four PUc conformers based on OH or NH as donor and pyridyl N as acceptor.

consistent with the X-ray result in studying the H-bonding interaction between COOH and P4VP.32 For calculating the probability of each type of H-bond it is necessary to make the following calculations for each of the eight H-bonded complexes according to the Boltzmann equation below

σ(i) ) e-∆Ec(i)/RT

(1)

∑i e-∆Ec(i)/RT

(2)

Z)

P(i) )

σ(i) Z

(3)

We first calculate the statistical weight factor σ(i) for each H-bond complex using eq 1, then calculate its partition function Z using eq 2, and finally obtain the probability P(i) for each H-bonded complex by eq 3. Here, ∆Ec(i) is the energy difference between the energy of each of the H-bond complexes and the one with the lowest energy (zero-point energy) in the different conformers. As Table 2 shows, the lowest energy is -66.792 kcal/mol, which is the one we selected for this zero-

TABLE 2: Probability of Each H-Bond Complex in PUc/P4VPa n

Etotal, kcal/mol

νσ(i) × 10-3

P (%)

OH/P4VP OH/P4VP OH/P4VP OH/P4VP NH/P4VP NH/P4VP NH/P4VP NH/P4VP

-66.482 -66.209 -66.628 -66.792 -63.811 -64.366 -65.062 -64.815

596.5 378.8 760.7 1000 7.0 17.6 56.0 37.1

20.90 13.27 26.66 35.04 0.25 0.62 1.96 1.30

a The total probability of H-bonds based on COOH‚‚‚P4VP is 95.87%, while the total probability of the H-bonds based on NH‚‚‚P4VP is 4.13%.

point energy. Since R is 1.987 E-03 kcal/mol and T is 300 K (at about room temperature), RT is simply taken as 0.6 in order to make calculation easier. The result calculated in this way is listed in Table 2. It shows that each H-bond complex has a different forming probability as they have a different H-bond energy value. Each of the H-bonds based on OH as a donor has a much higher probability than that of NH as a donor since the H-bond energy of the former is lower than that of the latter. The total probability of

Analysis of Multiple H-Bond Interactions

J. Phys. Chem. B, Vol. 112, No. 7, 2008 1933

Figure 10. Two types of H-bonds in PUc/P4VP self-assembly based on OH or NH as donor and pyridyl N as acceptor.

TABLE 3: Energy of H-Bonds Based on Different Donors and Acceptors in PUc/P4VP Interactions (QM/B3LYP) type II (NH‚‚‚P4VP) ν

type I (OH‚‚‚P4VP)

conformer A

conformer B

length (Å) angle (deg) ∆E (kcal/mol)

2.766 179.1 -11.293

3.054 172.3 -7.094 -7.150a

3.077 176.29 -7.205

a

An average for type II H-bond based on two conformers.

the type I H-bond accounts for 95.87%, while that of the total type II H-bond is 4.13% according to the calculation. It can then be concluded that the type II H-bond interaction plays a subsidiary role in stabilizing the PUc/P4VP assembly. However, considering the strong νNH band at 3250 cm-1 along with the large shifting of amide II and III bands after self-assembly, the type II H-bond, namely, the interaction between NH and P4VP, cannot be ignored. 3.4. H-Bond Energy of Type I and Type II H-Bonds in PUc/P4VP. Figure 10 shows the selected two types of H-bonds including two forms of type II H-bonds (resulting from urethane conformers) calculated by QM(B3LYP). According to MM results (Table 2), the H-bond energy in the same type of H-bond (type I or type II) are similar; therefore we select one of the H-bonding patterns to calculate the H-bond energy using QM. The H-bond energy for each of the complexes has been obtained by deducting the energy of the two single molecules. Table 3 lists the energy of these three forms of H-bonds as well as their properties. As seen in Figure 10 there is also a strong H-bond coexisting with a weak H-bond in each of the H-bond patterns according to the QM simulation. Obviously, the H-bond length based on a type I H-bond is shorter and the angle is closer to 180° than that in the type II H-bond. In addition, the weak H-bond length along with the type I H-bond is also shorter than that in the type II H-bond. The two forms of type II H-bonds have some little differences. The length of the strong H-bond based on conformer B is shorter and its angle closer to 180° than that based on conformer A, while the length of the weak H-bond in conformer A is shorter and the relative angle closer to 180° than that in conformer B. Since Table 3 shows two forms of type II H-bonds, the energy of the type II H-bond here has been taken as the average energy from two forms of type II H-bond. The above results suggest that the OH is a stronger H-bond donor than NH, and a weak H-bond has different roles depending upon the different strong H-bond along with it. Conclusions In summary, formation of H-bonds after assembly is expected to be a competitive process in a multiple H-bond donoracceptor system. The pyridyl N from P4VP, as a strong H-bond acceptor, forms two types of H-bonds, namely, a type I H-bond (P4VP‚‚‚COOH) and a type II H-bond (P4VP‚‚‚OCO(NH)). The

H-bonding energies of type I is -11.293 kcal/mol, while that of type II H-bond are -7.150 kcal/mol in the self-assembly. The forming probability of the type I H-bond accounts for 95.87%, showing that the primary driving force for the assembly based on multiple donors and acceptors is still the H-bond between COOH and P4VP, yet the type II H-bonds based on NH and pyridyl in P4VP cannot be ignored. The two types of H-bonds both contribute to the stability of the PUc assembly. Acknowledgment. This work has been supported by the National Natural Science Foundation of China (20474014, 20074041, 20274057, 20474073) and Henan Provincial Natural Science Foundation (0611020700). References and Notes (1) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. (2) Duan, H.; Chen, D.; Jiang, M.; Gan, W.; Li, S.; Wang, M.; Gong, J. J. Am. Chem. Soc. 2001, 123, 12097. (3) Wang, M.; Zhang, G.; Chen, D.; Jiang, M.; Liu, S. Macromolecules 2001, 34, 7172. (4) Jiang, M.; Eisenberg, A.; Liu, G. J.; Zhang, X. Macromolecular Self-Assembly; Academic Press: Beijing, 2006. (5) Kato, T.; Kihara, H.; Ujiie, S.; Uryu, T.; Frechet, J. M. J. Macromolecules 1996, 29, 8734. (6) Lin, H.-C.; Lin, Y.-S.; Lin, Y.-S.; Chen, Y.-T.; Chao, I.; Li, T.-W. Macromolecules 1998, 31, 7298. (7) Liu, S.; Zhang, G.; Jiang, M. Polymer 1999, 40, 5449. (8) Zhao, H.; Liu, S.; Jiang, M.; Yuan, X.; An, Y.; Liu, L. Polymer 2000, 41, 2705. (9) MacKnight, W. J.; McKenna, L. W.; Read, B. E.; Stein, R. S. J. Phys. Chem. 1968, 72, 1122. (10) Lee, J. Y.; Painter, P. C.; Coleman, M. M. Macromolecules 1988, 21, 954. (11) Ren, Z.; Guo, X.; Lu, Z.; Guo, X.; Ma, D. Polym. Bull. 2007, 59, 371. (12) Teo, L. S.; Chen, C. Y.; Kuo, J. F. Macromolecules 1997, 30, 1793. (13) Lee, H. S.; Wang, Y. K.; Hsu, S. L. Macromolecules 1987, 20, 2089. (14) Miller, C. E.; Edelman, P. G.; Ratner, B. D. Appl. Spectrosc. 1990, 44, 581. (15) Brunette, C. M.; Hsu, L. S.; MacKnight, W. J. Macromolecules 1982, 15, 71. (16) Coleman, M. M.; Lee, K. H.; Skrovanek, D. J.; Painter, P. C. Macromolecules 1986, 19, 2149. (17) Luo, N.; Wang, D.-N.; Ying, S.-K. Macromolecules 1997, 30, 4405. (18) Zhang, S.; Ren, Z.; He, S.; Zhu, Y.; Zhu, C. Spectrochim. Acta 2007, 66, 188. (19) Bandekar, J.; Okuzumi, Y. J. J. Mol. Struct. (Theochem) 1993, 281, 113. (20) Sun, H. Macromolecules 1993, 26, 5924. (21) Yilgor, E.; Yilgor, I.; Yurtsever, E. Polymer 2002, 43, 6551. (22) Furer, V. L. J. Mol. Struct. 2000, 520, 117. (23) Ren, Z.; Ma, D.; Yang, X. Polymer 2003, 44, 6419. (24) Ren, Z.; Wu, H.; Ma, J.; Ma, D. Chinese J. Polym. Sci. 2004, 22, 225. (25) Ren, Z.; Zeng, X.; Yang, X.; Ma, D.; Hsu, S. L. Polymer 2005, 46, 12337. (26) Ambrozˇˇıcˇ, G.; Zˇ igon, M. Macromol. Rapid Commun. 2000, 21, 53.

1934 J. Phys. Chem. B, Vol. 112, No. 7, 2008 (27) Ambrozˇˇıcˇ, G.; Mavri, J.; Zˇ igon, M. Macromol. Chem. Phys. 2002, 203, 439. (28) Ambrozˇˇıcˇ, G.; Zˇ igon, M. Polym. Int. 2005, 54, 606. (29) Sun, H. J. Phys. Chem. B 1998, 102, 7338.

Ren et al. (30) Ren, Z. Ph.D. dissertation, University of Science and Technology of China, 2003. (31) Al-Salah, H. A. Polym. Bull. 1991, 26, 169. (32) Sharma, C. V. K.; Zaworotko, M. J. Chem. Commun. 1996, 2655.