Graphene Oxide Nanosheet Induced Intrachain Conformational

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Letter pubs.acs.org/JPCL

Graphene Oxide Nanosheet Induced Intrachain Conformational Ordering in a Semicrystalline Polymer Jia-Zhuang Xu,† Yuan-Ying Liang,† Gan-Ji Zhong,† Hai-Long Li,† Chen Chen,‡ Liang-Bin Li,§ and Zhong-Ming Li*,† †

College of Polymer Science and Engineering and State Key Laboratory of Polymer Materials Engineering and ‡Analytical and Testing Center, Sichuan University, Chengdu 610065, China § National Synchrotron Radiation Laboratory and College of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China S Supporting Information *

ABSTRACT: The physical origin of graphene oxide nanosheet (GONS)driven polymer crystallization was studied from the perspective of intrachain conformational ordering. Time-resolved Fourier-transform infrared spectroscopy indicated that both conformational ordering and crystallization of isotactic polypropylene (iPP) were obviously accelerated by the presence of GONSs, indicating their efficient nucleation activity for iPP crystallization. Furthermore, the ordering of long helical segments occurred prior to the crystallization of iPP, as revealed by two-dimensional correlation infrared analysis. Compared to pure bulk system, the presence of GONSs was in favor of the formation of long ordering segments, especially at the early stage, accompanied by considerable enhancement of the crystallization kinetics. GONS-driven iPP crystallization was suggested to be attributed to this GONS-induced intrachain conformational ordering. SECTION: Macromolecules, Soft Matter

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anofillers have been well-established as effective nucleating agents for various semicrystalline polymers that can promote crystallization kinetics and trigger special crystalline morphology, thus acting on the macroscopic properties of polymer nanocomposites. For instance, the polymer crystalline coating grown on the surface of the nanofillers, such as a nanohybrid shish kebab structure, may effectively enhance interfacial stress transfer, directly optimizing mechanical properties of nanocomposites.1,2 Therefore, nanofiller-induced polymer crystallization is of profound importance for achieving high-performance nanocomposites. Generally, nanofiller-driven polymer crystallization is attributed to their high specific surface area, which reduces the nucleation barrier, thereby exerting positive effects on the crystallization of polymers.3 Nevertheless, this nonprocedural conclusion cannot give a convincing explanation to clarify the dynamic process of nanofiller-induced polymer crystallization. Assemble kinetics of polymer chains, especially their early connection and epitaxial form on the surface of nanofillers, are still at the initial stage of exploration. By now, different theoretical models have been proposed to depict the panorama of polymer crystallization (i.e., Lauritzen− Hoffman theory,4 orientation fluctuation,5 density fluctuation,6 and multistage growth model7). It is commonly agreed that the molecular chains need to pass the way of conformational order to provoking crystallization. The ordering of polymer chains consists of intrachain conformational ordering and interchain positional and orientational ordering. Apparently, following the © 2012 American Chemical Society

trajectory of intrachain conformational ordering is the first and foremost task to comprehend the complex crystallization process. For the bulk polymer, some work has been performed. The increase of the trans conformation with a long persistence length and large excluded volume was observed to induce orientation fluctuations before crystallization in a syndiotactic polystyrene melt.8 The conformation rearrangement of different functionalized groups in the backbone was detected to be not fully cooperative but sequential during melt crystallization of poly(3-hydroxybutyrate).9 When the persistence length of helical sequences of isotactic polypropylene (iPP) exceeded the critical length, the 31 helical conformation began to congregate, and then, crystallization was triggered.10 Recently, the occurrence of conformation ordering even at a temperature above the melting point was reported to be effectively induced by the shear field, which had been well proven to accelerate the overall kinetics and modify the final morphology of polymer crystallization.11 Furthermore, at the normal melting point of iPP, second-order exponential decay was anastomosed on the relaxation kinetic of helices, suggesting the existence of extremely stable helices or helical bundles.12 These helical bundles (or coupling of helices) were postulated to be the Received: January 14, 2012 Accepted: February 6, 2012 Published: February 6, 2012 530

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Figure 1. Time-resolved spectra in the range of 1330−820 cm−1 of PPG0 (a), PPG05 (b), and PPG10 (c) isothermally crystallizing at 145 °C.

helical structures of iPP, especially at the early stage of crystallization, which is suggested to be the reason for GONSdriven polymer crystallization. In order to confirm the nucleation ability of GONSs, differential scanning calorimetry (DSC) measurement is first used to measure the crystallization of iPP with and without GONSs. Figure S3 (Supporting Information) shows typical exotherms of PPG0, PPG05, and PPG10 at a cooling rate of 10 °C/min. In contrast to PPG0, the crystallization peak temperature for PPG05 and PPG10 increases by 8.9 and 9.8 °C, respectively. The evidence of GONS-induced iPP crystallization can be further confirmed from the isothermal crystallization where the half crystallization time (t1/2) is respectively reduced to 16 and 14.3% for PPG05 and PPG10 (Figure S4, Supporting Information). Compared to other nanofillers (i.e., carbon nanotubes, zinc oxide, montmorillonite, gold nanoparticles, etc.), GONSs exhibit comparable or superior nucleation activity for iPP crystallization.20−23 Therefore, it could be concluded that GONSs are indeed an effective nucleating agent for iPP, even at a very low concentration. Our major concern is the physical origin of the strong inducing efficiency of GONSs for iPP crystallization. Conformational ordering plays a pivotal role during this complex process,15 and tracing intrachain ordering is suggested to be the first choice for gaining deep insight of GONS-driven polymer crystallization. FTIR is highly sensitive to conformation changing and the packing density of molecular chains, thus being adopted to detect the crystallization process of the iPP/ GONS nanocomposites. Figure 1 shows the time-resolved FTIR spectra of PPG0, PPG05, and PPG10 isothermally crystallizing at 145 °C. Intensity changes and band shifts of the crystalline-sensitive bands apparently occur as the time increases, indicating that FTIR spectra are highly sensitive to structural changes during

precursors of nuclei. In contrast, the conformational ordering of nanocomposites received very limited attention. Due to different hindrance, clay platelets with different exfoliation extent caused distinct conformational changes and thereby crystallization kinetics of their poly(L-lactic acid) (PLLA) nanocomposites.13 In our previous reports, the template effect of carbon nanotubes and graphene nanosheets for PLLA chains to land was confirmed, which was explained in terms of surfaceinduced conformational order.14,15 However, due to the complex and strong interchain interaction of PLLA, little information on intrachain conformation adjustment could be extracted, making it difficult to comprehend skeletal ordering when loaded with nanofillers.16 In this work, we attempted to reveal the relationship between intrachain ordering and crystallization of polymer nanocomposites. iPP was chosen as the model polymer, most absorption bands of which in the mid-infrared region belong to the regularity bands and are related to the intrachain vibration within a single chain. The nanofillers were selected as graphene oxide nanosheets (GONSs), a derivative of graphene, synthesized by a modified Hummers method.17,18 Their huge two-dimensional (2D) flat structures provided a large enough streambed for molecular chains to connect, leading to considerable elevation of the overall crystallization rate for iPP.19 Solution coagulation was utilized to prepare iPP/GONS nanocomposites in order to obtain well-dispersed GONSs in the matrix. The nanocomposites with 0.05 and 0.1 wt % GONSs were noted as PPG05 and PPG10, respectively, while neat iPP was noted as PPG0. Time-resolved Fourier-transform infrared spectroscopy (FTIR) and 2D infrared correlation analysis were employed to tracing the intrachain conformation ordering and crystallization evolution of iPP/GONS nanocomposites, as well as their time sequences. Our results indicate that the presence of GONSs facilitates the formation of long 531

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Figure 2. Normalized intensity of the crystalline band at 1303 cm−1 (a) and the conformational ordering band at 998 cm−1 (b) as a function of time for PPG10, PPG05, and PPG0 isothermally crystallizing at 145 °C.

Figure 3. Synchronous and asynchronous correlation spectra of PPG0 (a, a′), PPG05 (b, b′), and PPG10 (c, c′) in the regions of 1350−1150 (ν1) and 1100−900 cm−1 (ν2) calculated from the time-resolved spectra obtained during crystallization at 145 °C.

reasons. Short helical structures already exist in iPP melt with the observation of a strong peak at 973 cm−1 (helical length with 3−4 monomers). These short helices must experience some transformation to construct longer helices. Thereby, propagation and incorporation were proposed, while no suitable method could detect the competition of these growth modes so far.11 Therefore, the fluctuation of the short helices cannot be satisfactorily elucidated. According to Doi−

crystallization. The IR bands at 940, 1220, 1303, 1167, 841, 998, 900, and 973 cm−1 are nominated as regularity bands and successively correspond to 31 helical structures with a degree of order from high to low.10 The gain in conformational ordering bands is directly attributed to the augmentation of the helical population. The regularity bands representing the long helix are used to evaluate the conformation evolution of iPP chains. The bands for short helices are not adopted for the following 532

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Edwards’s dynamics theory, the critical persistence length of iPP for isotropic-to-nematic transition is 11 monomers in the 31 helical conformation.10,24,25 The conformational band at 998 cm−1 corresponds to 10 monomer units, which is suggested to be more sensitive to the stable-to-unstable transition when crystallization is triggered. The 998 cm−1 band is reasonably chosen for statistical analysis of the conformation evolution during crystallization, and the 1303 cm−1 bond corresponding to the helical length with 13 monomers is taken as the crystalline signal.26 Following the changing process of those two bands may help to understand the crystallization process, especially unveiling how GONSs accelerate iPP crystallization at the early stages. Figure 2a illustrates the normalized intensity of the crystalline band (I1303) as a function of crystallization time for iPP and its GONS nanocomposites. The crystallization of iPP is strongly accelerated in the presence of GONSs, which is consistent with the results of DSC and previous synchrotron wide-angle X-ray diffraction.19 As the GONS content increases from 0.05 to 0.1 wt %, t1/2 of iPP/GONS nanocomposites reduces from 27.5 to 15.0 min. More nucleation sites provided by GONSs would further elevate the crystallization kinetics of iPP. Interestingly, the corresponding variations of their conformational ordering band follow the same changing tendency as that presented in Figure 2b. It implies that GONSs also speed conformational ordering of iPP associated with accelerating the crystallization. In situ FTIR results remind us that we might get a deeper insight of GONS-driven polymer crystallization via establishing relationship with GONS-induced conformation ordering. Moreover, to a certain extent, the dispute between different crystallization models is whether the conformation changing and crystallization occur in succession or not. 2D correlation spectroscopy is a very powerful tool for studying the structural changes during polymer crystallization.16 Derived from the analysis of the asynchronous spectra, the order of structure changes is emphasized under an external field (i.e., temperature, time). Figure 3 presents the synchronous Φ(ν1,ν2) and asynchronous Ψ(ν1,ν2) 2D correlation spectra in the ranges of 1300−1000 with 1100−900 cm−1 of iPP and its GONS nanocomposites isothermally crystallized at 145 °C. The intensity in Φ(ν1,ν2) represents the coincidental changes of spectral variations measured at ν 1 and ν 2 during crystallization of iPP, while the intensity in Ψ(ν1,ν2) reflects sequential or successive changes of specific structures. According to Noda’s rules,27 the signs of the cross-peak (ν1,ν2) are the same (both positive or negative) in the synchronous and asynchronous spectra if the band at ν1 varies prior to that at ν2, whereas the sequence is reversed when the signs are different. From Figure 3, a common phenomenon could be observed in all three systems, Φ(1303,998) > 0 and Ψ(1303,998) < 0, indicating that the intensity change of the 998 cm−1 band occurs before the band at 1303 cm−1. That is, the conformational ordering of long ordering structures acts as a vanguard proceeding the crystallization of iPP. Naturally, one could infer that the degree of conformational ordering ahead of crystallization relies on the amount of ordering structures and the development of crystals. Here, we propose a synchronous reference method to evaluate this process. Figure 4 shows semilogarithm curves of I998 with the development of I1303 for iPP and its nanocomposites. The K line is representative of a synchronous reference frame, that is, I998 = I1303. It can be seen that all curves referring to PPG0, PPG05, and PPG10 are incipiently over and subsequently

Figure 4. Normalized intensity of the 998 cm−1 band with the evolution of the normalized intensity of the 1303 cm−1 band for PPG0, PPG05, and PPG10 isothermally crystallizing at 145 °C. The inset magnifies the 0−0.15 region of I1303. The K line is representative of I998 = I1303.

coincide with the K line. At the initial stage of crystallization, the iPP melting is predominant, and the I998 positively diverges from I1303. It is suggested that conformational ordering does occur prior to crystal growth, which is consistent with our 2D correlation results. Then, the long helices continuously pack into a crystal lattice or lamellar layer, leading to the synchronization of conformational ordering and crystal growth. A slight deviation is observed for PPG0. This implies that the spontaneous formation of ordered segments in the bulk melt is finite, which is consistent with the report by Li et al. They observed that a large number of conformational ordering segments existed at the boundary of growing spherulites.26 When GONSs are introduced, salient increase of I998 occurs compared to the development of I1303 at the early period of crystallization. A great deal of long helical structures is easily formed in the presence of GONSs. These interesting results allow us to conceive a perspective of the physical origin for GONS-induced iPP crystallization at the early stage of crystallization, which is schematically portrayed in Scheme 1. Intrachain conformational ordering of iPP chains is prone to happen probably because of the interactions between GONSs and iPP chains, such as interaction between protruding methyl groups of iPP and the graphene layer of sp2-bonded carbon.28 Therefore, at the initial stage of crystallization, a large number of long helices are stocked up near the surface of GONSs, as presented in Scheme 1a. This may be attributed to the surface-induced conformational ordering or surface-induced polymer crystallization.15,29 The recent investigation by Li et al. also confirmed that the molecular chains of polyethylene (Caxis of the crystal) are parallel to the basal plane of the reduced graphene sheets.29 When the stable system is sluiced (nucleation), the coupled helices or crystal lamellae appear because only isolated single long helices are not sufficient to trigger crystallization,11 and then, the preexisting long helices can pack into the formed coupled helices rapidly (Scheme 1b). The crystallization rate is hereby significantly sped up compared to that of neat iPP. Essentially, there is a competition between the formation and consumption of long helices as crystal growth proceeds. This competition may be maintained until the completion of crystallization. Our results reveal that the presence of GONSs facilitates the formation of long helical 533

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Scheme 1. Schematic Diagrams of Intrachain Conformation Ordering iPP in the Proximity of GONSs: Pileup of Long Ordering Helices (a) and Early Stage of Crystallization (b)

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segments, leading to a considerable enhancement of the crystallization kinetics of iPP. In summary, the dynamic process for GONS-induced iPP crystallization was investigated at the molecular level. GONSs exhibit a strong nucleating ability in enhancing the crystallization temperature and crystallization rate of iPP. Following the evolution of regularity bands with different helical segments, conformational ordering was demonstrated to play a vanguard role in iPP crystallization, especially at the very early stage. When crystallization was triggered, the long ordered segments could quickly pack into a crystal lattice. The presence of GONSs facilitated the occurrence of the above process, leading to a large number of long helical structures accumulated at the initial period of crystallization. Thus, GONS-driven crystallization kinetics of iPP was considerably improved though the epitaxial mode is not yet clear.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of sample preparation, typical TEM of GONSs in iPP/GONS nanocomposites, DSC exothermal curves of PPG0, PPG05, and PPG10 at a cooling rate of 10 °C/min, heat flow curves, and relative crystallinity of PPG10, PPG05, and PPG0 during isothermally crystallization at 138 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the National Outstanding Youth Foundation of China (Grant No. 50925311) and the National Natural Science of China (Grant No. 51033004, 51121001).

EXPERIMENTAL METHODS

Time-Resolved FTIR Characterization. The time-resolved FTIR measurements were performed on a Nicolet 6700 FTIR spectrometer (Thermal Scientific, USA) equipped with a heated transmission cell (HT-32). The membrane of iPP/ GONS nanocomposites was sandwiched between two ZnSe plates so as to adopt the transmission mode over the wavenumber range of 650−4000 cm−1. The spectra were obtained by averaging 16 scans at a resolution of 2 cm−1 with a 1 min interval, which had already been subtracted from the background spectra. Each sample was kept at 180 °C for 5 min to erase any thermal history and cooled down to the Tc in less than 4 min. When the temperature arrived at Tc, the data collection started until the completion of the crystallization. Here, the intensity refers to the peak height of characteristic bands, and the base lines are corrected for each spectrum at the same standard. 2D IR Correlation Analysis. Time-resolved spectra in the range of 1370−750 cm−1 were first subjected to the baseline correction to minimize the effect of baseline instabilities. This data treatment was automated by the program written in Thermo Nicolet’s software package. Then, 2D correlation analysis of the selected spectra was carried out at a certain time interval in the object regions (1350−1150 and 1100−900 cm−1). In the 2D correlation maps, the red loops indicate positive correlation intensities, while the blue loops indicate negative correlation intensities.

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