Sub-100 nm Cocontinuous Structures Fabricated in Immiscible

Jan 18, 2019 - College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University , Hangzhou 311121 , People's Republic of China...
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Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Sub-100 nm Cocontinuous Structures Fabricated in Immiscible Commodity Polymer Blend with Extremely Low Volume/Viscosity Ratio Fei Li,† Xuewen Zhao,† Hengti Wang,† Qin Chen,† Shuhua Wang,‡ Zhenhua Chen,‡ Xiaoyong Zhou,‡ Wenchun Fan,§ Yongjin Li,*,† and Jichun You*,†

ACS Appl. Polym. Mater. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/27/19. For personal use only.

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College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, People’s Republic of China ‡ Zhejiang Juhua Technology Center Co., Ltd, Quzhou 324004, People’s Republic of China § Zhejiang Juhua Research Institute of New Materials Co., Ltd. No. 258, Juxian Street, Hanghzou 311321, People’s Republic of China S Supporting Information *

ABSTRACT: Sub-100 nm cocontinuous structures were prepared successfully in poly(vinylidene fluoride)/poly(Llactic acid) (PVDF/PLLA) blend with extremely low volume/viscosity ratio by melting blending. The reactive compatibilizer with poly(methyl methacrylate) (PMMA) side chains and random epoxide groups plays an important role in the formation and the size decrease of these structures. On one hand, PMMA side chains exhibit excellent entanglement with PVDF; on the other hand, the epoxide groups can react with carboxyl group of PLLA. The resultant comb-like compatibilizer exhibits greater capacity to maintain the stress balance on two sides. The repulsion state of the binary brushes enlarges the interface curvature radius, dominating the formation of cocontinuous structures. KEYWORDS: cocontinuous structures, compatibilization, PVDF, PLLA, blend

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Luo et al. utilized premade block copolymer composed by polystyrene (PS) and poly(methyl methacrylate) (PMMA) as a compatibilizer in PS/PMMA blends.10,11 Compatibilization of polymer blend by copolymer can reduce the size of cocontinuous structures to submicron at most. 12 The fabrication of sub-100 nm cocontinuous structures from immiscible polymer blend with extremely low volume/viscosity ratio remains as one of the greatest challenges in this field. To obtain this structure, the precise localization of the compatibilizer at interface dominated by the balanced stress on two sides is necessary to reduce the interface tension and structure size. For this purpose, our attention has been paid to reactive blending, in which the compatibilizer forms in situ at the immiscible interface. It exhibits higher compatibilization efficiency relative to premade copolymer and therefore has been regarded as a promising solution for the reduction of phase-separated structure size.13 Meanwhile, it is the interface curvature radius that dominates the formation of cocontinuous structures.14 It would be helpful to adopt the comb-like

ocontinuous structures in nanometers have been paid much attention in the past decades.1,2 Phase separation in immiscible polymer blend can serve as a template for their fabrication.3,4 In this strategy, however, there are two open problems. For one thing, the cocontinuous composition/ temperature window is very narrow in phase diagram. The approximate volume/viscosity ratios of two components are required to develop cocontinuous structures, which can be expressed as eq 1.5,6

( ) ≈1 () ϕ1 η1

ϕ2 η2

(1)

Where ϕ, η, and the subscripts 1 and 2 represent volume, viscosity, phase 1, and phase 2, respectively. The fabrication of cocontinuous structures from melting blending with extremely low (or high) volume/viscosity ratio seems very difficult or just impossible because the phase with lower volume and higher viscosity tends to form island phase and vice versa; for another thing, the characteristic sizes of phase-separated structures in blend systems locate mainly at micrometers.7,8 Therefore, copolymers have been employed to compatibilize the blend systems and decrease the characteristic sizes.9 For instance, © XXXX American Chemical Society

Received: November 28, 2018 Accepted: January 18, 2019 Published: January 18, 2019 A

DOI: 10.1021/acsapm.8b00174 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Polymer Materials

Scheme 1. Sea−Island Structures in PVDF/PLLA (A) and Bigger (B) and Smaller (C) Cocontinuous Structure Forms with the Help of RCC and Shear Effect; Panels D and E Are the Zoomed-In Area in the Indicated Part

structures, thin slices of samples were microtomed in the blend specimen and examined by transmission electron microscopy (TEM) (Figure 1A and 1B). The TEM image of the sample

compatibilizers with enhanced ability to improve the interface curvature radius relative to the linear counterparts.15,16 In this work, therefore, the reactive comb-like compatibilizer (RCC) (RCC, Fourier-transform infrared (FTIR), and gel permeation chromatography (GPC) are shown in Figures S1 and S2) with long PMMA side chains and epoxide groups randomly distributed along the PMMA backbone was employed to compatibilize commodity blends system by taking poly(vinylidene fluoride)/poly(L-lactic acid) (PVDF/ PLLA) as an example (Scheme 1). On the basis of the excellent entanglement of PVDF/PMMA (resulted from their thermodynamical miscibility)17 and the reaction between the epoxide groups (in RCC) and the terminal carboxyl group (of PLLA), the obtained comb-like molecules with double brushes are expected to play the following roles in compatibilizing PVDF/PLLA blend system under well-controlled conditions. First, it would be an effective strategy to balance the stress on two sides along the interface, resulting in the precise localization of comb compatibilizers at the interface, the decreased interface tension and lower magnitude of characteristic size of phase-separated structures (shown in Scheme 1A−1C); second, the balanced stress on two sides and the existence of PMMA backbones lead to the organization of them along the interface (Scheme 1D), and finally, the poor miscibility between PVDF (PMMA) and PLLA contributes to the repulsion state of the binary brushes (Scheme 1E). Both the PMMA backbone along the interface and the repulsion effect of side/grafted chains are a benefit to the forced improvement of interface curvature radius. On the basis of the smaller structure size and higher curvature radius, the sub-100 nm cocontinuous structures can be prepared in PVDF/PLLA with extremely low volume/viscosity ratio. To the best of our knowledge, this is the first time to prepare cocontinuous structures in commodity polymer blend based on the combination of physical entanglement and chemical reaction. First, we calculated the volume/viscosity ratio (≈0.03) in PVDF/PLLA according to the rheology data shown in Figure S3 (based on the viscosity at lower frequency, 5900 Pa·s and 360 Pa·s for PVDF and PLLA respectively). It is very difficult to obtain cocontinuous structures in this system according to the criterion shown in eq 1. To validate the phase-separated

Figure 1. (A and B) TEM images of PVDF/PLLA/RC (30/70/20%); (C and D) SEM images of porous PVDF membranes upon removing PLLA by acid hydrolysis.

without RCC has also been provided as reference (Figure S4). The white and black phases are PLLA and PVDF, respectively, as the latter is easily stained by the RuO4. The structures in TEM images (Figure 1A and 1B) are similar to the cocontinuous structures reported in the literature.7,10,11 The characteristic size of this structure is less than 100 nm. To validate the composition distribution in this blend, our specimens were immersed in 20% nitric acid solution at 90 °C for various periods to remove both the grafted and free PLLA in blend. The acid hydrolysis efficiency, defined as the ratio of weight loss during this process and the initial weight of PLLA, is under the control of the immersing time (Figure S5). It is facile to remove PLLA completely upon the treatment for 3 days, which has been confirmed by the weight calculation and the disappearance of melting peak of PLLA in DSC curves (Figure S6). The complete removal of PLLA indicates that it is B

DOI: 10.1021/acsapm.8b00174 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Polymer Materials

Figure 2. (A) Photos of PVDF/PLLA with (right) and without (left) RCC; the mechanical properties (B) DSC thermograms of the first heating process of PVDF/PLLA blends. (C) (a) PVDF, (b) PLLA, (c) PVDF/PLLA/RCC (30/70/0%), (d) PVDF/PLLA/RCC (30/70/5%), (e) PVDF/ PLLA/RCC (30/70/20%); SEM images of (D) PVDF/PLLA/RCC (30/70/0%), (E) PVDF/PLLA/RCC (30/70/5%), and (F) PVDF/PLLA/ RCC (30/70/20%).

cooling process. The decreases of the melting temperatures (both component) and the crystallinity suggest that the blend exhibits better compatibility due to the existence of compatibilizer,21 which can be interpreted as the miscibility of the PMMA side chains in the compatibilizer with PVDF and the confined crystallization of PVDF resulted from the smaller domain size (Figure S10 and Table S3). The morphologies of PVDF/PLLA with various compatibilizer contents were investigated by SEM. To identify the composition distribution, chloroform was used as the selective solvent to etch PLLA on the fracture of our specimens. As illustrated in Figure S11, SEM images before and after etching indicate that the matrix and island phase are PLLA and PVDF, respectively. In Figure 2D, PVDF domains with the average size of 2.8 ± 0.9 μm disperse in PLLA matrix. The higher domain size and polydispersity are typical morphologies of blend system with poor compatibility.22,23 When compatibilizer is added in the blend system, our attention should be paid to following issues. First, the characteristic sizes of phase-separated structures decrease to 0.35 and 0.10 μm with the compatibilizer content of 5 and 20 wt %, respectively (Figure 2E, 2F, and Figure S12); second, this size becomes more and more uniform with increasing the compatibilizer content, corresponding to the smaller error bar in Figure S12, and finally, the PVDF islands tend to connect with each other, producing cocontinuous structures. The phase conversion from island−sea to cocontinuous structures is obvious in Figure 2F. Both the decrease of PVDF domain size and the continuity increase of the phase-separated structures agree well with the better mechanical and optical performances (Figure 2B and 2A) and the enhanced compatibility (Figure 2C and 2D−2F) between PVDF and PLLA. There are two different reported mechanisms for the formation of cocontinuous structures.24,25 In the first one, the investigators owe it to the coalescence of preformed particles.25,26 The second one assumes that cocontinuous structures start from long fibers or sheets, which are followed by their breakup into networks.24,27−29 To validate the

continuous because it is impossible to do this when PLLA acts as islands. Meanwhile, the porous PVDF membranes are selfsupporting (shown in Figure S7), indicating the continuous state of PVDF phase. According to the discussion above, the cocontinuous state can be confirmed. It serves as the base for not only the successful removal of one component (PLLA) and the consequent interpenetrated pores but also the selfsupporting porous target (PVDF) membranes.18,19 The scanning electron microscopy (SEM) images of porous PVDF membranes are displayed in Figure 1C and 1D, in which the cocontinuous structures, including interpenetrated nanopores and PVDF, are obvious. In the SEM images, the average pore size is less than 100 nm. Mercury injection test was used to determine the pore size and its distribution of the porous PVDF membranes (shown in Figure S8). It is found that the average pore size locates at 50 nm, ranging from 20 to 100 nm. Compared with the characteristic size of the cocontinuous structures, the pore size shows lower magnitude, which can be ascribed to the shrinkage during drying. Furthermore, the sub-100 nm cocontinuous structure exhibits good thermal stability upon annealing (shown in Figure S9). To assess the compatibilizer dependence of the compatibility between PVDF/PLLA, several methods were employed. In blend system, the optical and mechanical properties depend crucially on the phase-separated structure size.20 As shown in Figure 2A, the specimen of PVDF/PLLA/RCC = 30/70/20 exhibits much lower haze (Table S1) relative to the reference (PVDF/PLLA = 30/70). Figure 2B shows the tensile curves of PVDF/PLLA/compatibilizer blend, in which the yielding stresses are close in all specimens. The elongation at break of PVDF/PLLA is roughly 68.9%. This value increases to 334.6% upon adding 20 wt % compatibilizer (Table S2). The improved mechanical and optical properties should be attributed to the enhanced compatibility and resultant smaller phase-separated structures in PVDF/PLLA blend. In DSC results (Figure 2C), the addition of compatibilizer produces remarkable effect on the melting behaviors of PVDF and PLLA, suggesting the different crystallization behaviors during C

DOI: 10.1021/acsapm.8b00174 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

Figure 3. SEM images of PVDF/PLLA (A), PVDF/PLLA/RCC20% upon shear for 2 min@20 rpm (B), 2 min@20 rpm and 1 min@50 rpm (C), 2 min@20 rpm and 3 min@50 rpm (D), 2 min@20 rpm and 5 min@50 rpm (E), 2 min@20 rpm and 10 min@50 rpm (F). The red circle in panel B represents the breakup of long PVDF fibers, which is the reason for cocontinuous structures.

for compatibilizers to react with PLLA at the interface. This is the reason for more comb-like molecules and further decrease of characteristic size. The migration of compatibilizers to the interface and the reduced structure size upon shear effect strengthen each other. As a result of this synergism effect, more and more compatibilizers migrate to the interface, resulting in the breakup of long PVDF fibers and the further decrease of interface tension and characteristic size (Figure 3C−3E). At last, a considerable number of compatibilizers with double brushes locate at the interface, producing sub-100 nm structures (Figure 3F and Scheme 1C). To clarify the reason for the occurrence of cocontinuous structures in the case of extremely low volume/viscosity ratio, attention should be paid to the following issues. First, the reaction between PLLA and epoxide group leads to much higher viscosity of PLLA phase (shown in Figure S3) and the increase of volume/viscosity ratio of PVDF/PLLA, which is of benefit to the formation of cocontinuous structures. This result has good agreement with the conclusion from Paul and Barlow;30 second, a considerable number of compatibilizers were localized at the interface because of the balanced stress on two sides of the side PMMA and the grafted PLLA chains. The systematic investigation on their localization and some evidence of this (concerning simulation, TEM, and atomic force microscopy-IR) will be discussed in future publications, and third, the poor interaction between PVDF/PLLA results in the repulsion effect of the binary brushes along PMMA backbone (Scheme 1D). Finally, the PMMA backbone along the interface and the repulsion effect of the binary brushes improve the interface curvature radius remarkably (Scheme 1E). As a result, we can find long fibers in Figure 3 and obtain cocontinuous structures because the interface curvature radius dominates their formation. In these structures, the localization of comb-like molecules is determined by the balanced stress on two sides and the lowest free energy of the blend system. Therefore, the cocontinuous structures exhibit excellent thermal stability (Figure S9). On the contrary, the cocontinuous structures prepared by means of phase inversion between the dispersed phases and the matrix

structure formation mechanism in our system, we tracked the structure evolution as a function of mixing time. There are long PVDF fibers when the blend has been premixed at 20 rpm for 2 min (Figure 3B). The breakup of these fibers is obvious, which is highlighted by the red circle. As a result, the cocontinuous structures in Figure 3C were obtained. Upon further mixing, the cocontinuous state remains stable, while the characteristic size drops from ∼750 nm (Figure 3C) to sub100 nm (Figure 3F). Their size was measured based on the software of Nano Measurer, which is shown in Figure S13. The size evolution (Figure S14) indicates a rapid compatibilization process. SEM results shown in Figure 3 suggest that cocontinuous structures come not from the coalescence of preformed dispersed particles but from the breakup of long PVDF fibers due to the shear effect and Laplace pressure. The PVDF/PLLA blend exhibits poor miscibility, leading to the formation of sea−island structures with the size of several microns. PVDF acts as the island phase due to the lower volume fraction, higher viscosity (shown in Figure S3) and resultant extremely low volume/viscosity ratio (≈0.03). This scenario is consistent with Scheme 1A and the SEM images shown in Figure 2D and 3A. When the reactive compatibilizer is added to the blend system in the mixer, a fraction of it migrates to the immiscible interface at the very beginning. The reaction between the epoxide group in RCC and the terminal carboxyl group of PLLA takes place (IR and Torque data shown as Figures S15 and S16), resulting in the grafted molecules of PLLA-g-PMMA. The PMMA side chains exhibit good entanglement with PVDF because of the excellent miscibility between them.17 Subsequently, the comb-like compatibilizer with double brushes along PMMA backbone has formed in situ. The existence of the comb-like molecule can decrease the interface tension between PVDF and PLLA. This accounts for the smaller characteristic size and more interfaces (Scheme 1B), which makes it easier for compatibilizers to migrate to the “new” interface. At the same time, PVDF, PLLA, and reactive compatibilizer are forced to deform into long fibers upon the shear effect, providing more chance D

DOI: 10.1021/acsapm.8b00174 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

2017YFB0307704), and Zhejiang Provincial Key R&D Program (Grant 2018C01038).

are not stable because they are an intermediate state. It is noteworthy that the adopted blend exhibiting extremely low volume/viscosity ratio is composed by commercially available components with very large molecular weight. The fabrication of cocontinuous structure with the size of sub-100 nm in such systems remains as one of the greatest challenges. Our results can serve as an effective solution for the key issue of cocontinuous structures. In this work, PVDF/PLLA (30/70) was adopted as a model system to investigate the formation of sub-100 nm cocontinuous structures in blend system with extremely low volume/viscosity ratio. PVDF tends to act as islands due to the lower volume fraction and higher viscosity. When reactive comb compatibilizers are mixed in the blend, the epoxide groups can react with the terminal carboxyl group of PLLA, while the PMMA side chains exhibit excellent physical entanglement with PVDF, producing comb-like compatibilizers with double brushes which play important roles in the formation of sub-100 nm cocontinuous structures. On one hand, the comb-like compatibilizer with double brushes stabilizes its location at the interface because of the stress balance. This is the reason for the reduced interface tension, higher interface fraction, and lower magnitude of the characteristic size; on the other hand, the PMMA backbones along immiscible interface and the repulsion effect of binary brushes result in the enlarged interface curvature radius, accounting for the occurrence of cocontinuous structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.8b00174.

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REFERENCES

Information on materials and sample preparation; basic characterizations (including FTIR and GPC spectra) of RCC; viscosity of components; acid hydrolysis efficiency; DSC of the blend; photos; pore size measurement, distribution, and thermal stability of selfsupporting porous PVDF membranes; identification of matrix and island phase by solvent etching; the average size of phase-separated structures; torque evolution during melting blend; optical properties of PLLA, PVDF, and the blend with/without compatibilizers; mechanical properties of PVDF/PLLA blends; and thermal properties (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yongjin Li: 0000-0001-6666-1336 Jichun You: 0000-0003-2033-7711 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Zhejiang Natural Science foundation (Grant LD19E030001), National Natural Science Foundation of China (Grants 21674033 and 21104013), National Key R&D Program of China (Grant E

DOI: 10.1021/acsapm.8b00174 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsapm.8b00174 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX