Selective stacking of HT-poly(3-n-alkylthiophene-2,5-diyl)s in mixed systems

Stacking of polymer molecules containing a rigid main chain and flexible side chains has been the subject of recent interest.1 It has been established by many research groups that regioregular head-to-tail type poly(3-n-alkylthiophene-2,5-diyl), HT-P3RTh, forms a stacked structure in the solid.2-6 HT-P3RTh is considered to form the stacked structure in an end-to-end packing mode shown in Chart 1 and its XRD pattern exhibits a sharp diffraction peak corresponding to the space da (e.g., da = 16.1 Å for R = hexyl) in a low-angle region. End-to-end packing model of HT-P3RTh. One layer is depicted, and layers are considered to form the stacked structure by accumulation of the layers. Taking such a stacked structure renders unique optical and electrical properties to the polymer.2-4, 6, 7 The stacked solid is formed by drying a solution of HT-P3RTh or by adding a nonsolvent to the solution of HT-P3RTh. HT-P3RTh has good solubility in CHCl3. However, it is not soluble in methanol, and the precipitate of the polymer is obtained by adding an excess amount of methanol to the chloroform solution of HT-P3RTh. In spite of the presence of numerous papers on the stacking behavior of pure HT-P3RTh in the solid state, molecular behavior of HT-P3RTh in a mixed solid state has not been revealed. When a nonsolvent is added to a solution containing two kinds of HT-P3RThs, namely HT-P3R1Th and HT-P3R2Th with different R groups, there are several possibilities for the molecular behavior of HT-P3R1Th and HT-P3R2Th: HT-P3R1Th and HT-P3R2Th form the stacked structure separately. HT-P3R1Th forms the stacked structure only with HT-P3R1Th. HT-P3R1Th forms the stacked structure both with HT-P3R1Th and HT-P3R2Th. The end-to-end packing structure formed between HT-P3R1Th and HT-P3R2Th depicted in Chart 2 will also be formed in this case, in addition to the packing structures formed only among HT-P3R1Ths and among HT-P3R2Ths. Stacking of the layer shown in Chart 2 is also conceivable although it apparently requires specific arrangement of the layers during the accumulation of the layers. HT-P3R1Th and HT-P3R2Th interfere with the formation of the stacked crystalline structure of each other, and only amorphous solid is formed. Mixed aggregation. Herein, we report that HT-P3RTh with the long R groups has a strong tendency to stack with the same molecules and case 1 occurs in the mixed system. Polymer alloys and microphase separation of polymer blends have been the subject of polymer science.8 HT-P3BuTh (R = butyl), HT-P3HexTh (R = hexyl), and HT-P3DodTh (R = dodecyl) were purchased from Rieke Metals, Inc. (Lincoln, NE) and their chloroform-soluble parts were used in this study. The chloroform-soluble parts showed the following HT content (estimated from 1H NMR spectroscopy2-4, 6) and number-average molecular weight (Mn, estimated by GPC using chloroform as the eluent, polystyrene standards). HT content: 95.5, 96.7, and 95.7% for HT-P3BuTh, HT-P3HexTh, and HT-P3DodTh, respectively. Mn: 3500, 29000, and 23000 for HT-P3BuTh, HT-P3HexTh, and HT-P3DodTh, respectively. To a chloroform (3 mL) solution of a mixture of two kinds of the polymers (total amount = 40 mg) was added methanol (200 mL) to obtain a precipitate. Essentially all of the polymers used were considered to be recovered because the solution became almost colorless. The precipitate was dried under vacuum and submitted to the powder X-ray diffraction analysis using a Philips PW1000 X-ray diffractometer (Almelo, The Netherlands). UV-visible spectra were recorded on a Shimadzu 3100 PC spectrometer (Kyoto, Japan). Films of the polymers were prepared by casting chloroform solutions of the polymers on glass plates. Figure 1(a)–(e) exhibits XRD patterns of powders obtained by adding methanol to the CHCl3 solutions containing P3HexTh and P3DodTh in various ratios. On the other hand, the XRD patterns of powders recovered from the solutions containing HT-P3HexTh and HT-P3BuTh are shown in Figure 1(f)–(h). XRD patterns of powders obtained by adding methanol to the CHCl3 solutions containing the mixed HT-P3RThs. Figures 1(a), 1(e), and 1(h) depict XRD patterns of the powders obtained from the CHCl3 solutions of pure (a) HT-P3HexTh, (e) HT-P3DodTh, and (h) HT-P3BuTh, respectively. Molar ratios between the monomeric unit of the polymer are (b) HT-P3HexTh: HT-P3DodTh = 7 : 3; (c) HT-P3HexTh: HT-P3DodTh = 5 : 5; (d) HT-P3HexTh: HT-P3DodTh = 3 : 7; (f) HT-P3HexTh: HT-P3BuTh = 5 : 5; (g) HT-P3HexTh: HT-P3BuTh = 3 : 7. The d values of the peaks are given in these figures. These XRD patterns and UV-visible data of the polymers reveal the following behavior of the HT-P3RTh molecules in the mixed system. For the HT-P3HexTh and HT-P3DodTh system: As seen in Figure 1(a)–(e), the mixture of HT-P3HexTh and HT-P3DodTh gives the XRD peaks at the da values (cf. Chart 1) of HT-P3HexTh and HT-P3DodTh themselves. For example, Figure 1(b) shows two XRD peaks at d = 16.6 Å and 24.8 Å, which correspond to the da values of HT-P3HexTh [16.1 Å, Fig. 1(a)] and HT-P3DodTh [25.5 Å, Fig. 1(e)]. No XRD peak assigned to the mixed aggregation (cf. Chart 2) was observed. These results indicate that the HT-P3RTh molecule has a strong tendency to aggregate and form the stacked structure with the same HT-P3RTh molecules with the same R length. By changing the HT-P3HexTh: HT-P3DodTh ratio from 7 : 3 [Fig. 1(b)] to 5 : 5 [Fig. 1(c)], the XRD peak due to the stacked HT-P3DodTh becomes stronger at the expense of the XRD peak of HT-P3HexTh. This XRD peak of HT-P3HexTh becomes only a shoulder peak on further decrease in the proportion of HT-P3HexTh to 0.3 in the mixture [Fig. 1(d)]. HT-P3DodTh may have a stronger tendency to stack because of a stronger force of the side chain crystallization caused by the packing interaction9 between the longer alkyl chains. The XRD peak at d = about 3.8 Å is assigned to the distance between the stacked HT-P3RTh layers.2-6 The average interlayer distance estimated from the position of the XRD peak seems to somewhat increase with the increase in the fraction of HT-P3DodTh. The peak at d = about 3.8 Å seems to overlap with a broad diffraction at about 2θ = 20°, which may originate from the side-chain packing of the R group. Estimation of the crystallinity of polymers from their XRD peak patterns has been reported for various polymers.10 However, for the present system, the sharp two main peaks [e.g., Fig. 1(b)] appear in the low-angle region where the peak separation from the slope of intense X ray coming from the lower angle region was difficult, and determination of the crystallinity from their peak pattern was difficult. Estimation of the crystallinity from the pattern of the peak at d = about 3.8 Å was also difficult because of the above-mentioned reason (overlapping of the two peaks). However, the sharpness of the two XRD peaks in the low-angle region is comparable to those of original HT-P3HexTh and HT-P3DodTh. Furthermore, the UV-visible spectrum of the solid (vide infra) of the mixture of HT-P3HexTh and HT-P3DodTh indicates that most of the polymer molecules participate in the stacking in the mixed solid system. For the HT-P3HexTh and HT-P3BuTh system: In contrast to the case of the mixture of HT-P3HexTh and HT-P3DodTh, the recovered mixture of HT-P3HexTh and HT-P3BuTh gives only a sharp XRD peak of HT-P3HexTh in the low-angle region. Although pure HT-P3BuTh itself forms the stacked structure2-6 to exhibit the XRD peaks shown in Figure 1(h), its stacking tendency seems to be weaker. It may be ascribed to its having the shorter R groups, although one has to take account of its lower molecular weight (cf. Experimental). It may not be so easy for the HT-P3BuTh molecules to form the stacked structure in the mixed system. The above-shown XRD results that HT-P3RTh molecules with the long R side chains form the stacked structure even in the mixed system is consistent with the following observations in the UV-visible spectroscopy. Figure 2 shows UV-visible spectra of HT-P3RThs in chloroform solutions and in films. The UV-visible spectra in chloroform [Fig. 2(a)-1 through (a)-3] are considered to originate essentially from the single molecule of the polymer. Chloroform solutions containing the two kinds of HT-P3RThs also give UV-visible spectra originated only from each HT-P3RTh molecule, indicating that the two kinds of HT-P3RThs are dissolved homogeneously in the chloroform solution. UV-visible spectra of HT-P3RThs. Broken lines, spectra in CHCl3 solutions: (a)-1, HT-P3HexTh; (a)-2, HT-P3DodTh; (a)-3, HT-P3BuTh. Upper three solid lines, spectra of films cast on a glass plate: (b)-1, HT-P3HexTh; (b)-2, HT-P3DodTh; (b)-3, HT-P3BuTh. Lower two solid lines, spectra of films obtained by casting chloroform solutions of mixtures of two kinds of HT-P3RThs: (c)-1, mixture of HT-P3HexTh and HT-P3DodTh (1 : 1 monomer molar ratio); (c)-2, mixture of HT-P3HexTh and HT-P3BuTh (1 : 1 monomer molar ratio). In the film, the absorption peak is shifted to a longer wavelength, and this bathochromic shift is considered to be brought by the high coplanar structure of the polymer in the solid and/or π–π interaction between the stacked HT-P3RTh molecules.2-7 In contrast to regioregular HT-P3RTh, regiorandom P3RTh does not form the stacked structure and show only a minor bathochromic shift in its film.2-4, 6 As shown in Figure 2(b)-1 and (b)-2, the films of HT-P3HexTh and HT-P3DodTh exhibit the bathochromic shift typically caused by the stacking.2-7 Both films show a similar UV-visible spectrum. HT-P3BuTh also gives a similar bathochromic shift in the film, however, the degree of the shift is smaller [Fig. 2(b)-3], probably because of a weaker stacking interaction between the polymer molecules. The Figure 2(c)-1 exhibits the UV-visible spectrum of a film obtained by casting a chloroform solution containing a 1 : 1 mixture of HT-P3HexTh and HT-P3DodTh. The spectrum essentially agrees with the stacked structure of HT-P3HexTh and HT-P3DodTh, supporting that they form the stacked structure even in the mixed system. The strong tendency of HT-P3RTh toward the stacking was reported for a film obtained by casting a homogeneous chloroform solution containing poly(methyl methacrylate) and HT-P3HexTh in a 100 : 1 ratio.6 As shown in Figure 2(c)-2, the mixture of HT-P3HexTh and HT-P3BuTh also seems to give the stacked structure in the cast film. As described above, all of the obtained data disclose that (1) the HT-P3RTh molecule has a strong tendency to form the stacked structure with the HT-P3RTh molecules with the same R groups, and (2) the stacking force of the HT-P3RTh molecule tends to increase with increase in the length of the R group. Since HT-P3RTh gives stacked, aggregated colloidal particles in mixtures of chloroform and methanol,6 the selective stacking of HT-P3R1Th and HT-P3R2Th is considered to take place through the formation of similar colloidal particles. The obtained results will contribute to better understanding of the stacking of π-conjugated polymers. Investigations on the physical properties (e.g., optical properties and thermal properties) of the mixed system are under progress.