Controllability of Polymer Crystal Orientation Using Heterogeneous

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Controllability of Polymer Crystal Orientation Using Heterogeneous Nucleation of Deformed Polymer Loops Grafted on Two-Dimensional Nanofiller Yijing Nie, Zhouzhou Gu, Qiang Zhou, Ya Wei, Tongfan Hao, Yong Liu, Rongjuan Liu, and Zhiping Zhou J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b02861 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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The Journal of Physical Chemistry

Controllability of Polymer Crystal Orientation Using Heterogeneous Nucleation of Deformed Polymer Loops Grafted on Two-Dimensional Nanofiller Yijing Nie, Zhouzhou Gu, Qiang Zhou, Ya Wei, Tongfan Hao, Yong Liu, Rongjuan Liu, Zhiping Zhou* Institute of Polymer Materials, School of Materials Science and Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China

Abstract Nowadays, it is a research hotspot to realize the controllability of polymer crystal structure in polymer nanocomposites. However, polymer crystals induced by two-dimensional filler always exhibit random orientation, which somewhat limit the improvement of physical properties of polymer materials. In the current paper, dynamic Monte Carlo simulations were performed to explore the methods preparing crystals with uniform orientation. Heterogeneous nucleation of deformed polymer loops grafted on two-dimensional filler can induce the appearance of a special nanohybrid shish-kebab (NHSK) structure, in which the two-dimensional filler acts as “shish” and induces the formation of crystals with uniform orientation. The grafted deformed chains are first heterogeneously nucleated on filler surface, and then free chains participate in crystallization, resulting in the formation of the NHSK structure. The NHSK structure can only be formed in the systems with high interfacial interactions at high temperatures or moderate interfacial interactions at moderate temperatures or low interfacial interactions at low temperatures. The method proposed here can be used to achieve the controllability of polymer crystal orientation in experiments. *

Corresponding author. E-mail address: [email protected] (Z.P. Zhou).

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I. INTRODUCTION The presence of nanoparticles can induce dramatic changes of polymer crystallization behaviors. Firstly, ultimate crystallinity can be improved due to the addition of nanoparticles, such as carbon nanotubes (CNTs).1,2 Secondly, crystallization kinetics can be accelerated due to heterogeneous nucleation of polymer chains induced by nanofillers. Li et al. observed that induction periods for nucleation of poly (L-lactide) are significantly shortened due to the addition of CNTs or graphene.3-5 Thirdly, the inclusion of nanofillers can induce the changes of crystalline morphology of polymers. Li et al. observed the formation of a new crystalline structure, namely, nanohybrid shish-kebab (NHSK) structure in polymer solutions, in which CNTs act as the shish and induce the formation of polymer crystal lamellae with uniform orientation (kebabs) along the long axis of the fillers.6-8 Subsequently, Fu et al. found that the NHSK structure can even appear in sheared polymer melts.9-11 The appearance of the NHSK structure can cause the remarkable improvement of mechanical properties of polymer materials.9,10,12 In addition to the effective reinforcements for polymers, the NHSK structure also endows polymer materials with some other special potential functions. For instance, the NHSK structure can be used to functionalize nanoparticles. By incorporation of some functional groups into polymer chains, these groups can be directly introduced to the vicinity of nanoparticles without complex chemical surface modification.7,13,14 Additionally, due to the high specific surface area, these NHSK structures have potential application as catalyst supports.7 Vaia and Maguire mentioned in their review article that precise morphology control is paramout among many challenges as PNCs move beyond commodity plastic applications.15 For instance, polymer crystal orientation directly influences physical

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properties of polymer materials, and thus the control of crystal orientation should be conductive to the preparation of polymer materials with high-performance. In other words, the preparation of the crystals with uniform orientation (similar to the NHSK structure) in polymers filled with two-dimensional nanoparticles is very meaningful. However, it has been demonstrated that only the one-dimensional nanoparticles with small lateral size can induce the formation of the NHSK structure,6,12,16-18 while the crystals induced by the two-dimensional nanoparticles, such as graphene, exhibit random orientations.19-22 Nowadays, graphene becomes one of the most promising nanofillers.23 Apparently, the uncontrollable crystalline morphology somewhat limits the applications of graphene filled polymers in some industrial fields. Nowadays, applying shear flow field is the most commonly used method for the improvement of polymer crystal orientation. The oriented chains in sheared solutions or melts can induce the formation of oriented crystals or shish-kebabs.24 From another point of view, if oriented chains are fixed on the filler surface, oriented crystals may be obtained even without shear flow. In the current paper, using dynamic Monte Carlo (MC) simulations, we modified the two-dimensional nanofiller with two-end grafted oriented chains (loops), and then successfully obtained the crystal structure with uniform orientation in polymer solutions. Nowadays, molecular simulations are regarded as another powerful research tool that can be used to reveal microscopic mechanism of experimental results.16,25-28 Previously, we used dynamic MC simulations to investigate the nucleation details of NHSK structures in polymer solutions,16 the crystallization behaviors of grafted polymer chains26 and the synergistic effect of oriented polymer melts and nanotubes on polymer crystallization.29 In addition, by means of molecular simulations some new results may be found and then can be used to guide experimental works. For example, the

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simulation works on self-assembly of diblock copolymers under confinement not only have confirmed the experimental morphologies but also have predicted an even richer array of novel nanoscopic structures, which are then further observed in experiments.30 The current simulation findings not only provide a new method to control polymer crystal orientation, but also explain the microscopic mechanism of heterogeneous nucleation of oriented polymer loops on filler surface inducing the formation of the NHSK structures, which can provide theoretical guidance for the performance control of polymer materials.

II. SIMULATION DETAILS Firstly, we established a lattice box of 643 cubic cells, and then put 1012 free chains with extended conformation into it. One free chain contained 32 segments. In the box, one segment could only occupy one lattice site, and double occupation was not allowed. In addition, the coordination number of each segment was 26, and thus one bond formed by two consecutive segments had 13 possible orientations. Then, we placed a plane with the thickness of one lattice site, which was used to represent the two-dimensional nanofiller, at the bottom of the simulation box (the red sheet in Figure 1). Subsequently, 20 two-end grafted chains (polymer loops), two end segments of which were grafted on the filler surface, were put into the box in a certain arrangement, as shown in Figure 1a (the green cylinders). One grafted chain was formed by 12 segments, and the distance between its two ends was 8 lattice sites, as shown in Figure 1b. Thus, the degree of extension was 0.66 (the ratio of the distance between two ends to the length of the straight chain). Namely, the grafted chains were deformed along the X-axis direction. Five grafted chains were placed adjacently, and thus the 4 groups formed by 5 neighboring grafted chains could be considered as the

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bundles of deformed chains. The occupation density of polymers was about 0.124 to mimic a dilute polymer solution. The vacancy sites acted as the role of solvent. No interaction was introduced between polymer and solvent. In experiments, by reaction of functional groups at two ends of polymer chains with the functional groups at filler surface, polymer loops grafted on filler can be successfully prepared.31,32 In addition, using doubly anchored bifunctional RAFT agents, surface-bound polymer loops were successfully

synthesized

by

Rotzoll

and

Vana

according

to

reversible

addition-fragmentation chain transfer polymerization.33 It can be inferred that the

orientation of these grafted deformed chains may influence crystal orientation. During simulation, the free chains and the segments of the grafted chains except the two ends could move based on the micro-relaxation model. Segments could move from an occupied site to a neighboring vacancy site, or slip along the chain. In order to avoid the finite size effect, periodic boundary conditions were introduced along X- and Y-axis directions, but any attempts of chains passing through the filler should be rejected. For comparison, the system without grafted chains (the ungrafted system) was also established. In short, the grafted system is composed of the free chains and the grafted chains, while the ungrafted system only contains the free chains.

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Figure 1. (a) Snapshot of initially extended chains (yellow cylinders), cyclic grafted chains (green cylinders) and a sheet representing two-dimensional nanofiller (red sheet); (b) schematic diagram of one typical cyclic grafted chain (green spheres) that was put regularly into the box; (c) snapshot of polymer coils (blue cylinders and sky-blue cylinders) relaxed for 106 MC cycles under athermal conditions.

The conventional Metropolis sampling algorithm was used to judge whether segments can move during each micro-relaxation step. The change of the potential energy for each step is (1)

E = cEc + pE p + bEb + ∑ fi E f i

where Ec is the energy change caused by a non-collinear connection of two consecutive bonds along the chain, Ep is the energy change for two neighboring nonparallel packed bonds, reflecting driving force for polymer crystallization,34 Eb is the energy change due to the presence of a monomer-filler pair, representing the strength of polymer-filler interaction, Ef is the sliding-diffusion barrier for a bond sliding in the crystals,35 c is the net number of non-collinear connection pairs of consecutive bonds, p is the net number of nonparallel packed pairs of bonds, b is the net number of pairs of monomer unit and filler, and

∑f

i

is the sum of bonds with

i

parallel packing along the diffusion path of the sliding bonds. In the simulations, the value of Ep/Ec was fixed at 1 to ensure that the chains have a proper flexibility at crystallization temperatures, the value of Ef/Ec was set to 0.02 to endow the chains with a high sliding mobility in crystals, Eb/Ec was set to -0.4 or -0.5 (the minus means that attractive interaction exists between polymer and filler) to investigate the effect of polymer-filler interaction on the NHSK formation, and kT/Ec (k is the Boltzmann’s constant and T the temperature) representing the reduced system temperature was set

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to 2.6 or 2.8. The prepositioned chains were firstly relaxed for 106 MC cycles to obtain an equilibrium melt state (as seen in Figure 1c) under athermal condition (the athermal condition corresponds to an infinitely high temperature). Each segment has one move attempt on average in one MC cycle. Subsequently, the relaxed chains were quenched to a relatively low temperature to observe isothermal crystallization.

III. RESULTS AND DISCUSSION Firstly, crystallization behaviors of the grafted system with Eb/Ec = -0.4 at kT/Ec = 2.8 were investigated. Figures 2a and 2b comparatively show the crystalline morphologies for the ungrafted and grafted systems. It can be interestingly seen that the crystals formed in the ungrafted system exhibit random orientations (Figure 2a), while these in the grafted system have uniform orientation along the X-axis direction (Figure 2b). Apparently, the structure formed in the grafted system is different from the conventional NHSK structure and can be considered as a special NHSK structure, in which the two-dimensional filler acts as the “shish”, and the crystals with uniform orientation serve as the kebabs. The presence of the two-end grafted chains indeed contributes to the formation of the crystals with uniform orientation. To our knowledge, this special NHSK structure formed under quiescent state has only been observed in nylon 6/clay nanocomposites attributed to the presence of hydrogen bonds between polymer and filler.36 Thus, our simulation results provide a new approach to preparing the NHSK structure in the system filled with two-dimensional filler. Then, the microscopic mechanism of the formation of the NHSK structure is further investigated. Figure 2c depicts the evolutions of crystallinity for the free and

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grafted chains in the grafted system, respectively. Herein, the crystallinity was defined as the fraction of bonds that have more than five parallel neighboring ones. As shown in Figure 2c, the grafted chains exhibit shorter induction time for nucleation compared with the free chains, indicating that the grafted chains have stronger nucleation ability. The grafted chains are deformed along the X-axis direction, and thus the corresponding conformational entropy is reduced, resulting in the decrease of free energy barrier for nucleation.37,38 Meanwhile, the presence of the filler surface induces the decrease of the surface free energy of the nuclei, also resulting in the reduction of the nucleation energy barrier. Thus, during crystallization, the local bundles of the deformed grafted chains are firstly nucleated on the filler surface, as illustrated in Figure 3. Then, the free chains also participate in crystallization, finally leading to the formation of the NHSK structure, as demonstrated in Figure 3. This crystallization process is somewhat similar to the formation of shish-kebabs in sheared polymers.39,40

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Figure 2. (a) Snapshot of crystalline morphology for the ungrafted system. (b) Snapshot of crystalline morphology for the grafted system. The yellow cylinders denote the crystalline bonds of the free chains, and the green cylinders denote the crystalline bonds of the cyclic grafted chains. For clarity, only the crystalline bonds are shown. (c) Evolutions of crystallinity for the free and grafted chains, respectively, during crystallization.

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Figure 3. Snapshot of the evolutions of crystalline morphology during crystallization of the grafted system at different MC cycles. For clarity, only the crystalline bonds are shown.

There are several factors influencing the formation of the special NHSK structure, such as crystallization temperature, polymer-filler interaction and the number of grafted chains in a bundle. As a matter of fact, the formation of the special NHSK structure with uniform crystal orientation is governed by the heterogeneous nucleation of the two-end grafted chains. However, the filler surface can also act as the heterogeneous nucleation sites for the free chains in appropriate circumstances, resulting in random crystal orientation. In other words, the formation of this special NHSK structure is determined by the difference between the nucleation ability of the two different heterogeneous nucleation processes. Accordingly, if the grafted system is crystallized at a relatively lower temperature or the strength of polymer-filler interaction is relatively stronger, the filler surface can also induce crystallization of the free chains. As shown in Figure 4a, the crystals with other orientations appear, if the grafted system is crystallized at kT/Ec = 2.6 (a relatively lower temperature) without the changes of other simulation conditions. In addition, as shown in Figure 4b,

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if the value of Eb/Ec was changed to -0.5 (higher polymer-filler attractive interaction), it can be also seen that the crystals do not show uniform orientation. In order to systematically reveal the dependence of the formation of the NHSK structure on simulation temperature and interfacial interaction, more simulations on the crystallization of grafted systems with different interfacial interactions at different reduced temperatures were further carried out. Then, a phase diagram of polymer crystals as a function of reduced temperature and interfacial interaction (Eb/Ec) was obtained, as shown in Figure 5. In the systems with relatively high interfacial interactions, the NHSK structure could be only formed at relatively high temperatures. If the systems with relatively high interfacial interactions crystallize at relatively low temperatures, the filler can also induce the heterogeneous nucleation of the free chains, resulting in the formation of crystals with other orientations. With the decrease of the interfacial interactions, the temperature at which the crystals with uniform orientation can be formed also decreases. The decrease of the interfacial interactions will cause the increase of the free energy barrier for heterogeneous nucleation of free chains, and thus the temperature at which the NHSK structure can be formed also decreases. In addition, it can be seen that the NHSK structure cannot be formed in the system with no interfacial interaction. In this condition, the conformations of chain segments near the filler surface will be restricted by the filler, resulting in the decrease of conformational entropy. Thus, some segments will prefer to diffuse away from the interfacial regions to increase the conformational entropy, leading to the decrease of segment density in the interfacial regions. Then, the ability of the heterogeneous nucleation of polymer chains will be weakened, and the NHSK structure cannot be formed alone (some crystals with homogeneous nucleation can also be formed). On the other hand, some factors, which directly affect the nucleation ability of the grafted

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chains, can also influence the formation of the special NHSK structure. For instance, the number of the grafted chains in a deformed bundle affects the overall entropy reduction of the bundle, and then further influences the corresponding nucleation ability. If one bundle only contains one grafted chain, the NHSK structure cannot be obtained, as shown in Figure 4c.

Figure 4. (a) Snapshot of crystalline morphology for the grafted system with Eb/Ec = -0.4 at kT/Ec = 2.6. (b) Snapshot of crystalline morphology for the grafted system with Eb/Ec = -0.5 at kT/Ec = 2.8. (c) Snapshot of crystalline morphology for the grafted system, in which one bundle contains only one grafted chain.

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3.4 3.2 3.0

kT/Ec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.8 2.6 Crystals with uniform orientation Crystals with other orientations Crystals with homogeneous nucleation No crystallization

2.4 2.2 -2.0

-1.5

-1.0

-0.5

Eb/Ec

0.0

Figure 5. Phase diagram of polymer crystals as a function of reduced temperature and interfacial interaction.

Apparently, this nanohybrid shish-kebab structure can endow polymer materials with excellent mechanical properties, i.e. high tensile strength and high modulus. In addition, the method proposed here (grafting deformed polymer loops onto filler surface) could be used to control polymer crystal orientation. Namely, polymer crystal orientation is controlled by the orientation of the bundles of the deformed grafted polymer loops. Thus, in addition to the NHSK structure, the crystals with different designed orientations can also be obtained by changing the distribution and orientation of the bundles of the deformed grafted polymer loops. Furthermore, physical properties of semi-crystalline polymer nanocomposites could be regulated by precise tailoring of crystalline morphology, and thus the “engineered, designed and tailored” materials could be obtained.

IV. CONCLUSION In conclusion, using dynamic MC simulations, a special NHSK structure is obtained

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in polymer solutions containing two-dimensional filler grafted with oriented polymer loops. During crystallization, the deformed bundles of the polymer loops are firstly nucleated on the filler surface and then induce the crystallization of the free chains, resulting in the formation of the special NHSK structure. These findings can provide a new way to control crystal orientation and thus physical properties of polymer materials.

ACKNOWLEDGMENTS The financial supports from the National Natural Science Foundation of China (No. 21404050) are gratefully acknowledged. The National Basic Research Program of China (973 Program, No. 2012CB821500), the Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1402019A), the Postdoctoral Science Foundation of China (No. 2015M580394), and the Research Foundation of Jiangsu University (No. 14JDG059) are also appreciated.

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Zhou, Z. P. Relaxation and crystallization of oriented polymer melts with anisotropic filler networks. J. Phys. Chem. B 2017, 121, 1426–1437. (30) Shi, A. C.; Li, B. H. Self-assembly of diblock copolymers under confinement. Soft Matter 2013, 9, 1398–1413. (31) Ashcraft, E.; Ji, H.; Mays, J.; Dadmun, M. Grafting polymer loops onto functionalized nanotubes: monitoring grafting and loop formation. Macromol. Chem. Phys. 2011, 212, 465–477. (32) Haung, Z.; Ji, H.; Mays, J.; Dadmun, M. Polymer loop formation on a functionalized hard surface: quantitative insight by comparison of experimental and monte carlo simulation results. Langmuir 2010, 26, 202–209. (33) Rotzoll, R.; Vana, P. Synthesis of poly (methyl acrylate) loops grafted onto silica nanoparticles via reversible addition-fragmentation chain transfer polymerization. J. Polym. Sci. Part A: Polym. Chem. 2008, 46, 7656–7666. (34) Hu, W. B.; Frenkel, D. Polymer crystallization driven by anisotropic interactions. Adv. Polym. Sci. 2005, 191, 1–35. (35) Hu, W. B. Chain folding in polymer melt crystallization studied by dynamic Monte Carlo simulations. J. Chem. Phys. 2001, 115, 4395–4401. (36) Maiti, P.; Okamoto, M. Crystallization controlled by silicate surfaces in nylon 6-clay nanocomposites. Macromol. Mater. Eng., 2003, 288, 440–445. (37) Nie, Y. J.; Gao, H. H.; Yu, M. H.; Hu, Z. M.; Reiter, G.; Hu, W. B. Competition of crystal nucleation to fabricate the oriented semi-crystalline polymers. Polymer 2013, 54, 3402–3407. (38) Nie, Y. J.; Gao, H. H.; Hu, W. B. Variable trends of chain-folding in separate stages of strain-induced crystallization of bulk polymers. Polymer 2014, 55, 1267–1272.

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(39) Keller, A.; Machin, M. Oriented crystallization in polymers. J. Macromol. Sci. Phys. 1967, B1, 41–91. (40) Zhao, Y.; Hayasaka, K.; Matsuba, G.; Ito, H. In situ observations of flow-induced precursors during shear flow. Macromolecules 2013, 46, 172–178.

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TOC Graphic

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Figure 1. (a) Snapshot of initially extended chains (yellow cylinders), cyclic grafted chains (green cylinders) and a sheet representing two-dimensional nanofiller (red sheet); (b) schematic diagram of one typical cyclic grafted chain (green spheres) that was put regularly into the box; (c) snapshot of polymer coils (blue cylinders and sky-blue cylinders) relaxed for 106 MC cycles under athermal conditions. 254x190mm (96 x 96 DPI)



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Figure 2. (a) Snapshot of crystalline morphology for the ungrafted system. 254x190mm (96 x 96 DPI)


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Figure 2. (b) Snapshot of crystalline morphology for the grafted system. The yellow cylinders denote the crystalline bonds of the free chains, and the green cylinders denote the crystalline bonds of the cyclic grafted chains. For clarity, only the crystalline bonds are shown. 254x190mm (96 x 96 DPI)



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Figure 2. (c) Evolutions of crystallinity for the free and grafted chains, respectively, during crystallization. 254x190mm (96 x 96 DPI)


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Figure 3. Snapshot of the evolutions of crystalline morphology during crystallization of the grafted system at different MC cycles. 254x190mm (96 x 96 DPI)



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Figure 4. (a) Snapshot of crystalline morphology for the grafted system with Eb/Ec = -0.4 at kT/Ec = 2.6. (b) Snapshot of crystalline morphology for the grafted system with Eb/Ec = -0.5 at kT/Ec = 2.8. (c) Snapshot of crystalline morphology for the grafted system, in which one bundle contains only one grafted chain. 254x190mm (96 x 96 DPI)


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Figure 5. Phase diagram of polymer crystals as a function of reduced temperature and interfacial interaction. 254x190mm (96 x 96 DPI)


Controllability of Polymer Crystal Orientation Using Heterogeneous

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