RETRACTED: Fractal Structure in Poly(3-Alkylthiophene) Derivatives

Retracted papers: One of the major features of conjugated polymer is the rigid molecular conformation which is absolutely different with that of flexible polymer. In this study, the non-conjugated aggregate structure of conjugated polymers was explored for the first time by using Small Angle X-ray Scattering (SAXS). It is found that there is a large-scale non-conjugated packing structure inside the Poly (3-alkylthiophene) (P3AT), which can be attributed to loose stacked structure of rigid conjugated main chain in P3AT. Thus, a large number of pore structures with the porosity of about 10-4 to 10-3 are formed, and such pore structures exhibit the characteristics of a mass fractal structure.

Introduction. Discovery and development of conductive polymers realized the huge transition of polymer materials from insulation to conduction, and also made it possible for organic electronic devices. [1][2][3] At present, a large number of π-π conjugates have been discovered and created. It greatly enriches not only the organic conjugated system but also the academic theory and industrial innovation. [4][5][6][7][8][9][10] Among them, polythiophene and its derivatives (poly (3-alkylthiophene) (P3AT)) are a typical type of conjugated polymeric materials that has attracted wide attention. In fact, polythiophene has intrinsic nature of insoluble due to strong π-π intermolecular interaction. Therefore, alkyl chains as side group are usually grafted on its conjugated ring to enhance its solvent solubility. When the side chain is linear and the number of carbon atoms in the side chain exceeds 4, P3AT materials can be dissolved in most common solvents, such as chloroform, tetrahydrofuran, and chlorobenzene. According to the number of carbon atoms on linear alkyl side chain, poly (3hexylthiophene) (P3HT), poly(3-octylthiophene) (P3OT) and poly(3-dodecylthiophene) (P3DDT) were synthesized.
Usually, the intermolecular interaction between main chains of P3AT is far greater than the Van der Waals forces along the side chain direction, so it is very easy to crystallize by π-π stacking between the main chains, thereby forming a one-dimensional order structure. However, due to the thermodynamic incompatibility between the alkyl side chains and the conjugated main chains, a nanoscale phase separation structure exists in the P3AT. When the number of carbon atoms in the linear alkyl side chain beyond to 8, both the main chain and side chain will crystallize, 11 meaning multiphases and aggregated structures inside P3AT are presented. Because the macromolecules with π-π conjugated structure exhibits a rigid conformational feature, it is very different from the traditional flexible molecular chain. From the perspective of molecular stacking mode, for such a rigid conformation, will it form a special molecular stacking which is different with a stable π-π conjugate structure? Unfortunately, this topic has reported rarely.
In this work, three P3AT materials, namely P3HT, P3OT, P3DDT were chosen to explore the nonconjugated stacking structure by using X-ray scattering technique. Beside the crystals formed by π-π stacking, there is a complex non-conjugated stacking structure. This stacking mode is consistent with the characteristics of mass fractal with the porosity of ranging from 10 -4 to 10 -3 .

Experimental
Materials. The P3HT, P3OT and P3DDT materials used in this study were purchased from Sigma-Aldrich Co., Ltd. The number average molecular weight was 54 kg/mol, 30 kg/mol, 60 kg/mol, respectively, and the regional regularities were greater than 98.5%. The solvent used in this C T E D experiment was chloroform, which was purchased J&K Scientific Ltd. Polyethylene (PE), polypropylene (PP), polylactic acid (PLA) and poly(methyl methacrylate (PMMA) were purchased from J&K Scientific Ltd.
Preparation. First, the P3HT, P3OT, P3DDT materials were dissolved respectively into chloroform with the concentration of 3 mg/mL at room temperature, then the solution were dropped onto Teflon film. The dried films were put into vacuum oven for 24 h to clear solvent completely. The thickness is about 300 um. For some hot-pressed samples, the hot-pressing temperatures for P3HT, P3OT, P3DDT, PE, PP, PLA and PMMA is 300 °C, 250 °C, 200 °C, 180 °C, 200 °C, 200 °C and 220 °C respectively, and the pressure is 5 MPa.
Characterization. In-situ SAXS measurements with transmission geometry were carried out on a Xeuss 2.0 SAXS/WAXS system (Xenocs SA, France). A Cu Kα X-ray source (GeniX3D Cu ULD) generated at 50 kV and 0.6 mA was utilized to produce X-ray radiation with a wavelength of 0.15418 nm. A semiconductor detector (Pilatus 300K, DECTRIS, Swiss) with a resolution of 487 × 619 pixels (pixel size = 172 × 172 μm 2 ) was used to collect the scattering. The image acquisition time was 1800 s. Linkam TST 350 hot stage temperature control device was used for temperature change experiment. In order to make the sample heat evenly, the thin film sample was wrapped in the middle of two aluminum foils for data processing. The one-dimensional (1D) intensity profiles were integrated from background corrected two-dimensional patterns using the Fit2D program. Q2000 Differential Scanning Calorimeter (DSC) was carried out with the heating rate of 10 °C/min. About 5 mg of the sample was loaded into an aluminum dish for testing, and a nitrogen (flow rate of 50 mL/min) protection program was used. (d) shows 1D I-Q curves of P3AT films at room temperature.

Results and Discussion
The chemical structures and 2D-SAXS patterns of P3HT, P3OT and P3DDT films prepared by the solution method are shown in Figure 1 respectively. Clearly, there is a very strong scattering signals around the scattering center. With the scattering vector (Q) becomes larger, the scattering intensity decrease. Different from the P3HT and P3OT, P3DDT film shows a clear isotropic ring at larger scattering Q, indicates P3DDT contains a large number of lamellar crystal structures. The maximum of Q peak corresponding to the average length of lamellar is defined as L=2π/Q. Figure 1d shows the 1D I-Q curves of P3HT, P3OT and P3DDT films at room temperature. It can be found that there is also an ordered structure signal in the P3OT material at relative smaller Q, indicating the L of P3OT is larger than that of P3DDT. Because the molecular structure of P3AT materials are very similar, so it is believed that the reason why no obvious long-period signal in P3HT is the long-period signal overlapped by the characteristic scattering signal of the small-angle region.  Interestingly, there is very strong scattering at small scattering angle, and the relative intensity is as high as 10 3 , indicating that these conjugated polymers contain a larger-scale stacked structure. Due to the relative intensity is dependent on many factors i.e. the equipment used, exposure time, sample thickness, background scattering, etc. Therefore, the relative intensity is usually used to calculate the geometric parameters such as the long period in the lamellar structure, the radius of rotation of the polymer in the mixed solution, etc. In order to further understand the larger-scale stacked structure of in these conjugate materials, absolute intensity 12 should be shown because it only related to the scatter-self. Figure 2 shows the absolute intensity of P3AT materials at different temperature. Obviously, temperature has a certain effect on the absolute intensity. With temperature increasing, the absolute intensity increasing.
Generally, the strong scattering phenomenon at small angle region is caused by the existence of a second phase that has a large difference in electron density from the material itself. The second phase includes inorganic nanoparticles, pores, and/or other constitutes. In the study, all P3AT materials are homogeneous materials, which can exclude the influence of second fillers' scattering, so it was thought that there may be some micro-porous-like structure with a size larger than the long period of the crystals in the P3AT material. The results show that there is a log(I(q)) ~log(q -3 ) relationship between the absolute intensity and the scattering vector in double Log coordinates. According to the Porod's Law (log(I(q)) ~log(q -D )), when D = 4, means a dense system, and there is a clear phase interface between the two phases; when 3 <D <4, it is still belongs to a dense system, but the twophase interface becomes blurred, which is a surface fractal situation, where D is the fractal dimension; when D ≤ 3, it is a loose system, and the interface is fuzzy, which is a mass fractal. Here, the slope is or slightly smaller than of 3, suggests a mass fractal structure in the P3AT materials, which also proves a micro-porous-like structure inside the P3AT materials. To further understand the relationship between scattering intensity variations and temperature, the in-situ SAXS, take P3DDT as an example, was collected as shown in Figure 3a. Since the thickness and transmittance of the sample cannot be measured in real-time during the in-situ measurement, the Advanced Materials, Processing and Testing Technology II relative intensity is shown. Clearly, with the temperature increasing, the scattering intensity increases. Furthermore, the scattering intensity at Q of 0.01 Å -1 was collected for tracking, and compared it with the thermodynamic results measured by DSC (Figure 3b). It can be seen that the scattering intensity variations is closely related to the main chain arrangement. After the crystals formed by alkyl side chains is melted (corresponding to the low-temperature melting peak of the DSC curve), the scattering intensity of the small-angle region only changes gently. But the scattering intensity is strongly dependent on the crystal structure formed by the conjugated main chain. After thermal destruction, the main chain is no longer limited to a three-dimensional ordered arrangement, but a random arrangement of random clusters. At this time, the mass fractal is further clear, and the SAXS scattering signal becomes stronger. So it can be concluded that the micro-porous-like structure in P3AT materials is formed by the stacking structure of the rigid conjugated main chain because the rigid main chain cannot be stacked densely in the entire space. Due to the solvent evaporation will leave holes which also have a huge impact on the scattered signal. Here, the samples prepared by melting and hot pressing were further used to be investigated, as shown in Figure 4. Obviously, melted and hot pressed samples also show a very strong scattering phenomenon in the small angle region, the absolute intensity and Q still show log(I(q)) ~log(q -D ) relationship in double log coordinates, indicating the existence of the mass fractal structure is none of business of processing method and state (crystalline or amorphous). Furthermore, polyethylene (PE), polypropylene (PP), polylactic acid (PLA), and polymethylmethacrylate (PMMA) were chosen as the comparison. Their chemical structure and absolute intensity were shown in Figure 4, despite a scattering increase at the small-angle region, the absolute intensity of all these flexible chains is just one order of magnitude, which is largely smaller than that of the conjugated polymer. Therefore, it is believed that there is no mass fractal structure inside the flexible polymer, and the conjugated polymer mass inside the fractal structure derived from a conjugated backbone rigidity random arrangement.
According to the classical scattering theory of two-phase systems: Where, Q is the invariant; V is the volume of X-ray passing through the sample; and ρ1 and ρ2 are the two-phase electron density, respectively. φ1 and φ2 are the volume fractions of the two phases, and φ1 + φ2 = 1, Ie is the scattering intensity of the single electron, which is about 7.95 × 10 -26 cm 2 . The calculated results are listed in Table 1

Conclusion
In this study, the non-conjugated aggregate state structure of conjugated polymers were explored for the first time using SAXS technology. P3HT, P3OT and P3DDT exhibit an obvious nonconjugated stacking structure. This non-conjugated stacking structure can be attributed to the loose packed structure by rigid conjugated molecular chains, thereby forming a large number of pore structures, and such pore structures exhibit the characteristics of mass fractal. According to further calculations based on classical scattering theory, the porosity is about 10 -4 ~ 10 -3 .

R E T R
Advanced Materials, Processing and Testing Technology II