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PLLA
Journal Pre-proof Graft Ratio: Quantitative Measurement and Direct Evidence for its Blending Sequence Dependence during Reactive Compatibilization in PVDF/PLLA
Fei Li, Yan Zhang, Xuewen Zhao, Qin Chen, Yongjin Li, Jichun You PII:
S0032-3861(19)30977-2
DOI:
https://doi.org/10.1016/j.polymer.2019.121970
Reference:
JPOL 121970
To appear in:
Polymer
Received Date:
18 September 2019
Accepted Date:
03 November 2019
Please cite this article as: Fei Li, Yan Zhang, Xuewen Zhao, Qin Chen, Yongjin Li, Jichun You, Graft Ratio: Quantitative Measurement and Direct Evidence for its Blending Sequence Dependence during Reactive Compatibilization in PVDF/PLLA, Polymer (2019), https://doi.org/10.1016/j.polymer. 2019.121970
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Journal Pre-proof
Graft Ratio: Quantitative Measurement and Direct Evidence for its Blending Sequence Dependence during Reactive Compatibilization in PVDF/PLLA Fei Li, Yan Zhang, Xuewen Zhao, Qin Chen, Yongjin Li, Jichun You* College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, People’s Republic of China ABSTRACT：The graft ratio during reactive compatibilization plays important roles in determining phase morphology and consequent performance. The quantitative measurement of it remains absent, which does hinder the basic understanding and applications of reactive compatibilization. In this work, co-continuous structures have been fabricated in PVDF/PLLA with reactive comb compatibilizers (RCC). Based on the co-continuous structures, acid hydrolysis has been employed to remove both reacted and free PLLA, producing acid hydrolysis ratio (AR). On the contrary, only free PLLA can be extracted by chloroform (its selective solvent), corresponding to the extraction ratio (ER). The difference between AR and ER represents the reacted weight percent of PLLA, which is a good parameter to describe the graft ratio (GR). According to the quantitative measurement of GR, the direct evidence for the blending sequence dependence of the extent of graft reaction has been provided. By optimizing the migration and localization of compatibilizers, 50nm co-continuous structures have been fabricated in (PLLA/RCC)/PVDF, in which GR exhibits the highest magnitude. This is the reason for the best mechanical and optical performances in the resultant specimen. Our results open a new avenue for understanding the reactive compatibilization in polymer blend. Keywords: Graft ratio; Reactive compatibilization; Co-continuous structures; *corresponding author, [email protected] (Prof. You)
1
Journal Pre-proof 1. Introduction Polymer blending is an effective strategy to develop new materials with desired properties
[1-2].
However, phase separation, poor interface adhesion and mechanical
performance suffer from the thermodynamical immiscibility in most polymer pairs (polymer A and B) [3-6]. Two methods have been employed to compatibilize them. On one hand, premade copolymer (e.g. A-b-B or A-g-B) has been designed and synthesized to improve the interface adhesion via enhanced entanglements on two sides
[7-9].
Generally speaking, the compatibilization efficiency of this method is low because of the phenomena of “pull-in” and “pull-out”; on the other hand, when the reactive compatibilizer (RC) is added in the blend system, the reaction between RC and one component (e.g. B) can produce copolymers locating at the immiscible interface [10-11]. This is so-called reactive compatibilization, corresponding to higher efficiency relative to premade copolymers [12-14]. In this process, both block and graft copolymers can be obtained [15]. The latter has been paid more attention since this kind of compatibilizer with brushes exhibits greater capacity to enhance the interface adhesion in polymer blends
[16-17].
During reactive compatibilization, the precise localization of graft
copolymer at interface, determined by balanced stress on two sides, plays the key role [18].
Obviously, it is dominated by the graft ratio (GR) which is defined as the reacted
weight percent of one component (says B here). In literatures, the graft extent of one component onto the compatibilizer has been assessed by means of several qualitative methods. For instance, Paul and his co-workers tracked the reaction during melt processing with the help of rheology [19]. To understand the reactive compatibilization in depth, the quantitative measurement of GR is urgently required but still challenging. The blending sequence in reactive compatibilization has been paid much attention in the past decades
[20-23].
The results in our previous work concerning poly(L-
lactide)/acrylonitrile-butadiene-styrene/reactive
compatibilizers
(PLLA/ABS/RC)
indicated that blending sequences produced different extents of reaction (between compatibilizers and PLLA) and entanglement (of ABS and PMMA) [24]. For one thing, the epoxy groups can be diluted by ABS if compatibilizers were blended with ABS firstly; for another thing, there were micelles of highly graft copolymer (PLLA-gPMMA) in PLLA phase when it was premixed with compatibilizers. Fu et al. investigated the blend of PLLA and olefin block copolymer (OBC) by taking ethyleneglycidyl methacrylate (EGMA) as the compatibilizer
[25].
Upon one-step or two-steps
blending, salami or quasi-salami structures, whose characteristic size depended 2
Journal Pre-proof crucially on the blending sequence, have been fabricated. It has been well accepted that blending sequence produces significant influence on the phase morphologies and consequent performance. By optimizing the compatibilizer migration and localization, the smaller domain size and the improved mechanical performance have been achieved [26-27].
However, the direct evidence for the blending sequence dependence of graft ratio
remains absent so far. The key for this issue is to measure the graft ratio quantitatively.
Scheme 1. The formation of co-continuous structures and definition of extraction ratio (ER), acid hydrolysis ratio (AR) and graft ratio (GR) in PVDF/PLLA/RCC.
For this purpose, a novel method has been developed by taking poly(vinylidene fluoride)/poly(L-lactic acid)/reactive comb compatibilizers (PVDF/PLLA/RCC) as an example in this work (Scheme 1). The reactive comb compatibilizer (RCC) contains poly(methyl methacrylate) (PMMA) backbones with randomly distributed PMMA side chains and epoxy groups [28-29]. The epoxy group can react with terminal carboxyl group in PLLA while PMMA entangles with PVDF due to the miscibility between them [30]. This model system has been adopted to measure GR quantitatively due to the following reasons. Firstly, it is facile to prepare co-continuous structures (Scheme 1A) by means of reactive compatibilization in PVDF/PLLA blend, which has been evaluated in our previous work
[16,18].
The co-continuous structures are the base for the successful
removal of PLLA, self-supporting PVDF porous membrane and the subsequent weight 3
Journal Pre-proof calculation; secondly, both solvent extraction and acid hydrolysis have been employed to remove PLLA, which can be regarded as an efficient way to distinguish reacted and free PLLA chains. On one hand, chloroform has been adopted to extract the unreacted (free) PLLA chains, producing self-supporting porous membrane. The extraction ratio (ER), defined as the ratio of the weight loss during extraction and the original weight of PLLA, can be measured (Scheme 1C); on the other hand, both free and grafted PLLA can be removed by immersing specimens in nitric acid solution. As a result, the acid hydrolysis ratio (AR=weight loss/weight of PLLA) can also be obtained (Scheme 1B); thirdly, PVDF exhibits excellent chemical stability, accounting for the self-supporting membranes during acid hydrolysis; finally, the difference between AR and ER represents the reacted weight percent of PLLA (graft ratio, GR, Scheme 1D). Based on the quantitative measurement of GR, we can provide the direct evidence for its blending sequence dependence during reactive compatibilization. 2. Experimental section 2.1 Materials and sample preparation PLLA (Mw=209000 g/mol, PDI = 2.00) and PVDF (Mw=89300 g/mol, PDI = 1.77) were purchased from Nature Works (USA, 3001D) and Kureha Chemistry (Japan, tradename, KF850), respectively. The synthesis and parameters of reactive comb compatibilizer (RCC, C-1-1-8 S-2400) have been described in reference
[16].
PLLA,
PVDF and RCC were dried in a vacuum oven overnight at 80 oC before processing. The weight ratio of PVDF, PLLA and RCC is 30/70/20. The samples were prepared by melt mixing at 190 oC at 20 rpm and 50 rpm in Haake Polylab QC apparatus (Thermo Fisher Scientific, Inc.) upon different blending sequence. After blending, the samples were hot pressed at 200 oC for 3 min (10MPa), followed by cold press at room temperature (10 MPa). 2.2 Characterizations The morphology of blends was characterized using TEM (Hitachi HT-7700) operating at an accelerating voltage of 100 kV. Before TEM measurement, the specimen was ultramicrocut to a thickness of 90 nm and stained by ruthenium tetroxide (RuO4) for 3 hours. Differential scanning calorimetry (DSC) measurements were carried out with DSC Q2000 (TA instrument) at a heating or cooling rate 10 oC/min. 4
Journal Pre-proof TGA measurement was performed with thermogravimetric analyzer (TGA, TA-Q2000) under N2. Rheological measurements were carried out in Anton Paar Instrument (MCR301) with parallel configuration (diameter 25 mm). The test cavity was at a temperature of 200 °C and the frequency from 0.01 to 500rad/s. Optical property was measured with WGT-S Light transmittance/haze meter at room temperature. DMA (Dynamic mechanical analysis) was performed on the DMA Q800 from TA Instrument in nitrogen atmosphere. The mercury-injection test (Micromeritics Poresizer 9320) was employed to measure the porosity of the porous PVDF membranes. SEM (Scanning electron microscopy) was performed via Hitachi S-4800 with the accelerating voltage of 3.0 kV. The specimens were fractured in liquid nitrogen and sputter-coated with gold before observation. 3. Results and discussion
Fig. 1. TEM images of (PVDF/RCC)/PLLA (A), PVDF/PLLA/RCC (B) and (PLLA/RCC)/PVDF (C, D). The scale bars are 200nm. E to G shows the SEM images of porous structures upon acid hydrolysis. H is the photo of self-supporting porous membrane.
First of all, it is necessary to confirm the co-continuous structure which is the base for the successful removal of PLLA, self-supporting porous PVDF membranes and the measurement of ER, AR and GR by weight calculation. In TEM images (Figure 1A1D), the black and white phases are PVDF and PLLA respectively, since the former is easily stained by the RuO4. In all TEM images, there are worm-like structures which are typical morphologies of co-continuous structures
[31-33].
The co-continuous
structures have also been validated by means of porous structures (Figure 1E-1G) 35],
[34-
in which the specimens were immersed in 20% nitric acid solution at 90oC for 90 5
Journal Pre-proof hours to remove both the reacted and free PLLA. For one thing, the weight loss during acid hydrolysis equals to the original weight of PLLA; for another thing, the resultant PVDF porous membranes are self-supporting (Figure 1H). This result makes it clear that the co-continuous structures have been prepared successfully. The formation of cocontinuous structures in this asymmetrical blend system can be ascribed to the balanced stress on two sides, the localization of RCC at the interface and the bigger interface curvature radius, which has been discussed in our previous work
[18].
Moreover, the
characteristic size of the co-continuous structure exhibits obvious dependence on the blending sequence. In (PVDF/RCC)/PLLA (PVDF premixed with RCC firstly, sic passim), the value locates at 130nm (Figure 1A). It decreases to 90nm in PVDF/PLLA/RCC
(one-step
mixing)
(Figure
1B).
In
the
specimen
of
(PLLA/RCC)/PVDF, the co-continuous structures exhibit the smallest size (~50nm, Figure 1C). The TEM image with higher magnification (Figure 1D) shows the excellent continuities of PVDF and PLLA. In the resultant porous PVDF membranes, the average pore size and surface area have been measured by means of mercury-injection test. It is obvious that the characteristic size of co-continuous structures (Figure 1A to 1D) agrees well with the pore size shown in Table 1. Table 1. The pore size information obtained from mercury injection test Samples
Average pore size (nm)
Surface Area (m2/g)
(PVDF/RCC)/PLLA
117.9
8.2
PVDF/PLLA/RCC
63.1
14.1
(PLLA/RCC)/PVDF
49.9
16.9
Table 2. The extraction ratio, acid hydrolysis ratio and graft ratio in specimens with different blending sequences. Samples Extraction Ratio Acid Hydrolysis Ratio Grafted Ratio (ER) (AR) (GR) (PVDF/RCC)/PLLA
66%
100%
34%
PVDF/PLLA/RCC
53%
100%
47%
(PLLA/RCC)/PVDF
42%
100%
58%
In the co-continuous structures discussed above, PLLA can be removed by two methods. On one hand, chloroform has been employed to extract the unreacted (i.e. free) PLLA. The extraction ratio (ER) is defined as the ratio between weight loss in this process and the original weight of PLLA. By means of weight calculation, we can get the values of them (Table 2). It is important to note that there is a tiny amount of RCC 6
Journal Pre-proof in the extraction solution, which has been confirmed by means of Fourier Transform Infrared Spectrometer (FTIR, data not shown here). The extraction ratios listed in Table 2 have been corrected according to the data of infrared spectrum. In the specimen of (PVDF/RCC)/PLLA, it exhibits the highest magnitude (66%), indicating the high fraction of unreacted PLLA. In PVDF/PLLA/RCC and (PLLA/RCC)/PVDF, the values decrease to 53% and 42% respectively; on the other hand, both grafted and free PLLA can be removed by immersing specimens in 20% nitric acid solution (90oC). The acid hydrolysis ratio (AR), defined as the weight loss over the original weight of PLLA, can be measured. As shown in Table 2, all specimens exhibit the similar value of 100%. This result suggests that almost all PLLA has been removed during acid hydrolysis. The difference between AR and ER represents the weight fraction of the grafted PLLA, which has been defined as graft ratio (GR) and listed in Table 2. With the help of cocontinuous structures, AR and ER, a novel method has been developed to measure the exact value of graft ratio (GR). The comparison of GR (Table 2) can provide the direct evidence for its blending sequence dependence, which has been described as follow. When PVDF has been premixed with RCC [(PVDF/RCC)/PLLA], only 34% PLLA reacts with RCC, leading to the higher ER and lower GR. In the case of (PLLA/RCC)/PVDF, 58% PLLA has been grafted on RCC based on the reaction between epoxy group in RCC and terminal carboxyl group of PLLA. In the specimen upon one-step mixing (PVDF/PLLA/RCC), the value of GR (47%) locates in between them.
Fig. 2. DTG curves of specimens upon extraction by chloroform. 7
Journal Pre-proof Table 3. Area ratio of the degradation peak at 374 and 472oC. (PVDF/RCC)/PLLA PVDF/PLLA/RCC (PLLA/RCC)/PVDF A /A 374
472
0.82
0.96
1.13
To validate the graft ratio as well as its blending sequence dependence, the thermal gravity analysis (TGA) has been performed in the specimens upon chloroform extraction. As shown in Figure 2, the main degradation peaks of PVDF, PLLA and RCC locate at 465, 348 and 378oC respectively. In the blend samples, the degradation peak at 472oC can be indexed to PVDF. The peak at 374oC and the shoulder at 348oC can be attributed to the degradation of the graft copolymer (PLLA-g-PMMA) resulted from the reaction between RCC and PLLA since there are only PVDF and the copolymer in the specimen (upon extraction, free PLLA has been removed). The areas of the peak and shoulder have been calculated to compare the graft ratio of PLLA by taking the area of PVDF peak as a reference. The area ratio (Table 3) between the copolymer and PVDF exhibits the lowest magnitude (0.82) in the specimen of (PVDF/RCC)/PLLA, indicating the lowest weight percent of grafted PLLA. This result has good agreement with the highest ER and lowest GR shown in Table 2. On the contrary, the lowest ER and highest GR in (PLLA/RCC)/PVDF correspond to the highest area ratio (1.13) shown in Figure 2 and Table 3. DMA measurements have been performed to assess the relaxation behaviors of PVDF and free/grafted PLLA (Figure 3A). From the reference of neat PVDF, PLLA and RCC, the relaxations at -34.5, 68.1 and 123.1oC can be ascribed to the glass transition of PVDF, PLLA and PMMA in RCC respectively
[36-37].
In Figure 3B, we
can find weak peaks of PVDF glass transition in the red and blue lines while there is no signal about it in (PVDF/RCC)/PLLA (black line). This result indicates that the glass transition of PVDF has been influenced significantly when it is premixed with RCC. The shoulder at 105oC (Figure 3D) can be associated with the relaxations within the PVDF crystalline phase. It exhibits the highest intensity in the black line, which can be interpreted as follow. In the specimen of (PVDF/RCC)/PLLA, the crystallization of PVDF has been depressed remarkably due to the excellent miscibility between PMMA and PVDF, leading to the increased crystal defects. This is the reason for the enhanced relaxation and stronger shoulder in Figure 3D [38]. Figure 3C illustrates the relaxation behaviors of PLLA. Our attention should be paid to the following issues. For one thing, the glass transition peaks of black, red and blue lines locate at 66.7, 66.0 and 65.5oC respectively. Compared with the value of neat PLLA (68.1oC), they move to the lower 8
Journal Pre-proof temperature direction, indicating the enhanced mobility of PLLA resulted from the better compatibility with PVDF; for another thing, the relaxation behaviors of grafted PLLA locating at PVDF/PLLA interface can be influenced remarkably by the graft reaction. As a result, the shoulders at ~53oC can be attributed to the relaxation of the grafted PLLA, which can act as a parameter to describe the graft ratio of PLLA. Obviously, the blue line of (PLLA/RCC)/PVDF corresponds to the strongest shoulder, suggesting the highest amount of grafted PLLA. This result agrees well with the highest magnitude of GR shown in Table 2. On the contrary, the line of (PVDF/RCC)/PLLA exhibits a weak shoulder, contributing to the lowest value of GR.
Fig. 3. Tan delta as a function of temperature of specimens upon different blending sequences. B, C and D show the zoom-in curves of the indicated parts.
As a result of the different compatibilization effect, the blends exhibit different mechanical and optical performances. Figure 4A shows the blending sequence dependence of unnotched impact strength. In the reference specimen of PVDF/PLLA without RCC, the impact strength is 18.2kJ/m2. Upon adding RCC, the value increases significantly. The smaller the characteristic size of co-continuous structure is (Figure 1), the higher impact strength becomes (Figure 4A). In (PLLA/RCC)/PVDF, it reaches the maximum of 32.9kJ/m2. To assess the optical performance, the transmittance and haze have been measured (Table 4). The former exhibits the similar values while the latter depends crucially on the blending sequence. In PVDF/PLLA without RCC, the haze is 91.9%. This value drops to 13.0%, 9.8% and 7.4% in the specimen of (PVDF/RCC)/PLLA, PVDF/PLLA/RCC and (PLLA/RCC)/PVDF, respectively. From 9
Journal Pre-proof the photos of three typical specimens shown in Figure 4B to 4D, the specimen of (PLLA/RCC)/PVDF corresponds to the best optical performance. It is noteworthy that 50nm co-continuous structures have been fabricated successfully by optimizing blending sequence (Figure 1C and 1D). The higher toughness and better optical performance in (PLLA/RCC)/PVDF can be attributed to the smaller characteristic size of co-continuous structures, which comes from the higher graft ratio and compatibilization efficiency (Table 2) [39-40].
Fig. 4. Impact strengths (A) and photos of specimens upon different blending sequences, (PVDF/RCC)/PLLA (B), PVDF/PLLA/RCC (C) and (PLLA/RCC)/PVDF (D). Table 4. Transmittance and haze of (PVDF/RCC)/PLLA, PVDF/PLLA/RCC and (PLLA/RCC)/PVDF. Samples
Transmittance %
Haze %
PLLA
92.4±0.1
3.0±0.2
PVDF
89.3±0.2
74.0±1.9
30/70 (without RCC)
83.9±0.9
91.9±0.1
(PVDF/RCC)/PLLA
84.1±0.1
13.0±0.5
PVDF/PLLA/RCC
85.0±0.3
9.8±0.5
(PLLA/RCC)/PVDF
90.3±0.2
7.4±0.1
There are both PMMA (backbone and side chains) and epoxy groups in RCC. They account for the entanglement (with PVDF) and graft reaction (between RCC and 10
Journal Pre-proof PLLA) respectively. When PVDF is premixed with RCC, the segments of PMMA chains diffuse to the PVDF phase because of the thermodynamic miscibility between them [30, 38]. RCC molecules tend to stay in PVDF, leading to its reduced contact with PLLA. The reaction between epoxy group in RCC and terminal carboxyl group of PLLA has been depressed. This is the reason for the lower graft ratio shown in Table 2. In the case of (PLLA/RCC)/PVDF, the terminal carboxyl group and epoxy group contact with each other sufficiently, producing the higher graft ratio. Upon adding PVDF, the excellent miscibility between PVDF and PMMA (in RCC) results in the migration of the graft copolymer to PVDF phase. The copolymer (PLLA-g-PMMA) prefers to localize at the immiscible interface (rather than entering PVDF phase) because of the poor interaction between grafted PLLA and PVDF
[41].
The precise
localization of compatibilizers at interface contributes to the formation of 50nm cocontinuous structures and the resultant improved mechanical and optical performances (Figure 4). When the blend is mixed simultaneously (PVDF/PLLA/RCC), there are RCC molecules in both PVDF and PLLA phases. The value of GR, impact strength and haze locate between two scenarios discussed above. The compatibilization efficiencies in specimens have been validated by rheology data (Figure 5). In Figure 5A, the frequency dependencies of storage modulus, three specimens exhibit similar viscoelasticity behaviors in high frequency region. In the case of low frequency, however, there is more remarkable deviation in (PLLA/RCC)/PVDF, indicating the better compatibility. This result has good agreement with the lower phase angle shown in Figure 5B. The more remarkable deviation in low frequency zone and smaller phase angle suggest the better compatibility in this specimen [42-44].
Fig.5. Linear rheology properties obtained from SAOS experiments: (A) storage modulus G′ and (B) van Grup-Palmen (VGP) plots of the (PVDF/RCC)/PLLA, PVDF/PLLA/RCC and (PLLA/RCC)/PVDF blends. 11
Journal Pre-proof 4. Conclusions In this work, a novel strategy has been developed to measure the graft ratio quantitatively during reactive compatibilization. In the co-continuous structures fabricated in PVDF/PLLA blend with RCC, extraction and acid hydrolysis have been employed to remove PLLA. The former and the latter correspond to extraction ratio (ER) and acid hydrolysis ratio (AR) respectively. The difference between AR and ER represents the weight percent of grafted PLLA, which can act as a good parameter to describe the graft ratio (GR). Based on quantitative measurement of GR, the blending sequence dependences of the extent of graft reaction, the characteristic size of cocontinuous structures and the mechanical and optical performances have been investigated. By optimizing the migration and localization of compatibilizers, 50nm cocontinuous structures, corresponding to the highest impact strength and lowest haze, have been fabricated successfully in the specimen of (PLLA/RCC)/PVDF. Our result provides not only a novel strategy for the quantitative measurement of graft ratio, but also the direct evidence for its blending sequence dependence.
Acknowledgements This work was financially supported by the Zhejiang Natural Science Foundation (LD19E030001), National Natural Science Foundation of China (21674033 and 2017YFB0307704), and Zhejiang Provincial Key R&D Program (2018C01038)
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Journal Pre-proof [24] X.J. Cao, W.Y. Dong, M.F. He, J.Q. Zhang, F.L. Ren, Y.J. Li, Effects of blending sequences and molecular structures of the compatibilizers on the morphology and properties of PLLA/ABS blends, RSC Adv. 9 (2019) 2189-2198. [25] M. Wu, K. Wang, Q. Zhang, Q. Fu, Manipulation of multiphase morphology in the reactive blending system OBC/PLA/EGMA, RSC Adv. 5 (2015) 96353–96359. [26] G. Pu, Y. Luo, Q. Lou, B. Li, Co-continuous polymeric nanostructures via simple melt mixing of PS/PMMA, Macromol. Rapid Commun. 30 (2009) 133−137. [27] H. Pernot, M. Baumert, F. Court, L. Leibler, Design and properties of co-continuous nanostructured polymers by reactive blending, Nat. Mater. 1 (2002) 54-58. [28] W.Y. Dong, H.T. Wang, M.F. He, F.L. Ren, T. Wu, Q. Zheng, Y.J. Li, Synthesis of reactive comb polymers and their applications as a highly efficient compatibilizer in immiscible polymer blends, Ind. Eng. Chem. Res. 54 (2015) 2081−2089. [29] B. Wei, D.P. Chen, H.T. Wang, J.C. You, L. Wang, Y.J. Li, M.G. Zhang, In-situ grafting of carboxylic acid terminated poly(methyl methacrylate) onto ethylene-glycidyl methacrylate copolymers： one-pot strategy to compatibilize immiscible poly(vinylidene fluoride)/ low density polyethylene blends, Polymer 160 (2018)162–169. [30] H.M. Chen, X.F. Wang, D. Liu, Y.P. Wang, J.H. Yang, Y. Wang, C.L. Zhang, Z.W. Zhou, Tuning the interaction of an immiscible poly(llactide)/poly(vinylidene fluoride) blend by adding poly(methyl methacrylate) via a competition mechanism and the resultant mechanical properties, RSC Adv. 4 (2014) 40569−40579. [31] D. Ren, Z. Tu, C. Yu, H. Shi, T. Jiang, Y. Yang, D. Shi, J. Yin, Y.W. Mai, R.K. Y. Li, Effect of dual reactive compatibilizers on the formation of co-continuous morphology of low density polyethylene/polyamide 6 blends with low polyamide 6 content, Ind. Eng. Chem. Res. 55 (2016) 4515−4525. [32] D. Yuan, Z. Chen, C. Xu, K. Chen, Y. Chen, Fully biobased shape memory material based on novel cocontinuous structure in poly(lactic acid)/natural rubber TPVs fabricated via peroxideinduced dynamic vulcanization and in situ interfacial compatibilization, ACS Sustainable Chem. Eng. 3 (2015) 2856-2865. [33] E. Cohen, L. Zonder, A. Ophir, S. Kenig, S. McCarthy, C. Barry, J. Mead, Hierarchical structures composed of confined carbon nanotubes in cocontinuous ternary polymer blends, Macromolecules 46 (2013) 1851-1859. [34] P. Poetschke, D.R. Paul, Formation of co-continuous structures in melt-mixed immiscible polymer blends, J. Macro. Sci. C Polym. Rev. C43 (2003) 87-141. [35] L. Zhu, Y. Wang, X. Yu, X. Shen, X. Xu, Controllable fabrication of graded and gradient porous polypropylene, J. Porous Mat. 22 (2015) 119-125. [36] J. Mijovic, J.W. Sy, T.K. Kwei, Reorientational dynamics of dipoles in poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends by dielectric spectroscopy, Macromolecules 30 (1997) 3042-3050. [37] E. Oikonomou, S. Tence-Girault, P. Gerard, S. Norvez, Swelling of semi-crystalline PVDF by a PMMA-based nanostructured diblock copolymer: morphology and mechanical properties, Polymer 76 (2015) 89-97. [38] T. Nishi, T.T. Wang, Melting point depression and kinetic effects of cooling on crystallization in poly(viny1idene fluoride)-poly (methyl methacrylate) mixtures, Macromolecules 8 (1975) 909−915. [39] W. Liu, Y.Y. Pang, M.H. Wu, K.P. Qin, W.G. Zheng, Dependence of the foaming window of a polystyrene/poly(methyl methacrylate) blend on structural evolution driven by phase separation, Polymer 166 (2019) 63-71. [40] D. Garcia-Garcia, J. Lepez-Martinez, R. Balart, E. Stromberg, R. Moriana, Reinforcing 14
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Journal Pre-proof Declaration of interest statement: The authors declare no conflict of interest.
Journal Pre-proof 1. Co-continuous structures with characteristic size below 100nm has been fabricated. 2. A novel method has been developed to measure the graft ratio quantitatively. 3. The direct evidence of the blending sequence dependence of graft ratio has been provided.
Fei Li, Yan Zhang, Xuewen Zhao, Qin Chen, Yongjin Li, Jichun You PII:
S0032-3861(19)30977-2
DOI:
https://doi.org/10.1016/j.polymer.2019.121970
Reference:
JPOL 121970
To appear in:
Polymer
Received Date:
18 September 2019
Accepted Date:
03 November 2019
Please cite this article as: Fei Li, Yan Zhang, Xuewen Zhao, Qin Chen, Yongjin Li, Jichun You, Graft Ratio: Quantitative Measurement and Direct Evidence for its Blending Sequence Dependence during Reactive Compatibilization in PVDF/PLLA, Polymer (2019), https://doi.org/10.1016/j.polymer. 2019.121970
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Journal Pre-proof
1
Journal Pre-proof
Graft Ratio: Quantitative Measurement and Direct Evidence for its Blending Sequence Dependence during Reactive Compatibilization in PVDF/PLLA Fei Li, Yan Zhang, Xuewen Zhao, Qin Chen, Yongjin Li, Jichun You* College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, People’s Republic of China ABSTRACT：The graft ratio during reactive compatibilization plays important roles in determining phase morphology and consequent performance. The quantitative measurement of it remains absent, which does hinder the basic understanding and applications of reactive compatibilization. In this work, co-continuous structures have been fabricated in PVDF/PLLA with reactive comb compatibilizers (RCC). Based on the co-continuous structures, acid hydrolysis has been employed to remove both reacted and free PLLA, producing acid hydrolysis ratio (AR). On the contrary, only free PLLA can be extracted by chloroform (its selective solvent), corresponding to the extraction ratio (ER). The difference between AR and ER represents the reacted weight percent of PLLA, which is a good parameter to describe the graft ratio (GR). According to the quantitative measurement of GR, the direct evidence for the blending sequence dependence of the extent of graft reaction has been provided. By optimizing the migration and localization of compatibilizers, 50nm co-continuous structures have been fabricated in (PLLA/RCC)/PVDF, in which GR exhibits the highest magnitude. This is the reason for the best mechanical and optical performances in the resultant specimen. Our results open a new avenue for understanding the reactive compatibilization in polymer blend. Keywords: Graft ratio; Reactive compatibilization; Co-continuous structures; *corresponding author, [email protected] (Prof. You)
1
Journal Pre-proof 1. Introduction Polymer blending is an effective strategy to develop new materials with desired properties
[1-2].
However, phase separation, poor interface adhesion and mechanical
performance suffer from the thermodynamical immiscibility in most polymer pairs (polymer A and B) [3-6]. Two methods have been employed to compatibilize them. On one hand, premade copolymer (e.g. A-b-B or A-g-B) has been designed and synthesized to improve the interface adhesion via enhanced entanglements on two sides
[7-9].
Generally speaking, the compatibilization efficiency of this method is low because of the phenomena of “pull-in” and “pull-out”; on the other hand, when the reactive compatibilizer (RC) is added in the blend system, the reaction between RC and one component (e.g. B) can produce copolymers locating at the immiscible interface [10-11]. This is so-called reactive compatibilization, corresponding to higher efficiency relative to premade copolymers [12-14]. In this process, both block and graft copolymers can be obtained [15]. The latter has been paid more attention since this kind of compatibilizer with brushes exhibits greater capacity to enhance the interface adhesion in polymer blends
[16-17].
During reactive compatibilization, the precise localization of graft
copolymer at interface, determined by balanced stress on two sides, plays the key role [18].
Obviously, it is dominated by the graft ratio (GR) which is defined as the reacted
weight percent of one component (says B here). In literatures, the graft extent of one component onto the compatibilizer has been assessed by means of several qualitative methods. For instance, Paul and his co-workers tracked the reaction during melt processing with the help of rheology [19]. To understand the reactive compatibilization in depth, the quantitative measurement of GR is urgently required but still challenging. The blending sequence in reactive compatibilization has been paid much attention in the past decades
[20-23].
The results in our previous work concerning poly(L-
lactide)/acrylonitrile-butadiene-styrene/reactive
compatibilizers
(PLLA/ABS/RC)
indicated that blending sequences produced different extents of reaction (between compatibilizers and PLLA) and entanglement (of ABS and PMMA) [24]. For one thing, the epoxy groups can be diluted by ABS if compatibilizers were blended with ABS firstly; for another thing, there were micelles of highly graft copolymer (PLLA-gPMMA) in PLLA phase when it was premixed with compatibilizers. Fu et al. investigated the blend of PLLA and olefin block copolymer (OBC) by taking ethyleneglycidyl methacrylate (EGMA) as the compatibilizer
[25].
Upon one-step or two-steps
blending, salami or quasi-salami structures, whose characteristic size depended 2
Journal Pre-proof crucially on the blending sequence, have been fabricated. It has been well accepted that blending sequence produces significant influence on the phase morphologies and consequent performance. By optimizing the compatibilizer migration and localization, the smaller domain size and the improved mechanical performance have been achieved [26-27].
However, the direct evidence for the blending sequence dependence of graft ratio
remains absent so far. The key for this issue is to measure the graft ratio quantitatively.
Scheme 1. The formation of co-continuous structures and definition of extraction ratio (ER), acid hydrolysis ratio (AR) and graft ratio (GR) in PVDF/PLLA/RCC.
For this purpose, a novel method has been developed by taking poly(vinylidene fluoride)/poly(L-lactic acid)/reactive comb compatibilizers (PVDF/PLLA/RCC) as an example in this work (Scheme 1). The reactive comb compatibilizer (RCC) contains poly(methyl methacrylate) (PMMA) backbones with randomly distributed PMMA side chains and epoxy groups [28-29]. The epoxy group can react with terminal carboxyl group in PLLA while PMMA entangles with PVDF due to the miscibility between them [30]. This model system has been adopted to measure GR quantitatively due to the following reasons. Firstly, it is facile to prepare co-continuous structures (Scheme 1A) by means of reactive compatibilization in PVDF/PLLA blend, which has been evaluated in our previous work
[16,18].
The co-continuous structures are the base for the successful
removal of PLLA, self-supporting PVDF porous membrane and the subsequent weight 3
Journal Pre-proof calculation; secondly, both solvent extraction and acid hydrolysis have been employed to remove PLLA, which can be regarded as an efficient way to distinguish reacted and free PLLA chains. On one hand, chloroform has been adopted to extract the unreacted (free) PLLA chains, producing self-supporting porous membrane. The extraction ratio (ER), defined as the ratio of the weight loss during extraction and the original weight of PLLA, can be measured (Scheme 1C); on the other hand, both free and grafted PLLA can be removed by immersing specimens in nitric acid solution. As a result, the acid hydrolysis ratio (AR=weight loss/weight of PLLA) can also be obtained (Scheme 1B); thirdly, PVDF exhibits excellent chemical stability, accounting for the self-supporting membranes during acid hydrolysis; finally, the difference between AR and ER represents the reacted weight percent of PLLA (graft ratio, GR, Scheme 1D). Based on the quantitative measurement of GR, we can provide the direct evidence for its blending sequence dependence during reactive compatibilization. 2. Experimental section 2.1 Materials and sample preparation PLLA (Mw=209000 g/mol, PDI = 2.00) and PVDF (Mw=89300 g/mol, PDI = 1.77) were purchased from Nature Works (USA, 3001D) and Kureha Chemistry (Japan, tradename, KF850), respectively. The synthesis and parameters of reactive comb compatibilizer (RCC, C-1-1-8 S-2400) have been described in reference
[16].
PLLA,
PVDF and RCC were dried in a vacuum oven overnight at 80 oC before processing. The weight ratio of PVDF, PLLA and RCC is 30/70/20. The samples were prepared by melt mixing at 190 oC at 20 rpm and 50 rpm in Haake Polylab QC apparatus (Thermo Fisher Scientific, Inc.) upon different blending sequence. After blending, the samples were hot pressed at 200 oC for 3 min (10MPa), followed by cold press at room temperature (10 MPa). 2.2 Characterizations The morphology of blends was characterized using TEM (Hitachi HT-7700) operating at an accelerating voltage of 100 kV. Before TEM measurement, the specimen was ultramicrocut to a thickness of 90 nm and stained by ruthenium tetroxide (RuO4) for 3 hours. Differential scanning calorimetry (DSC) measurements were carried out with DSC Q2000 (TA instrument) at a heating or cooling rate 10 oC/min. 4
Journal Pre-proof TGA measurement was performed with thermogravimetric analyzer (TGA, TA-Q2000) under N2. Rheological measurements were carried out in Anton Paar Instrument (MCR301) with parallel configuration (diameter 25 mm). The test cavity was at a temperature of 200 °C and the frequency from 0.01 to 500rad/s. Optical property was measured with WGT-S Light transmittance/haze meter at room temperature. DMA (Dynamic mechanical analysis) was performed on the DMA Q800 from TA Instrument in nitrogen atmosphere. The mercury-injection test (Micromeritics Poresizer 9320) was employed to measure the porosity of the porous PVDF membranes. SEM (Scanning electron microscopy) was performed via Hitachi S-4800 with the accelerating voltage of 3.0 kV. The specimens were fractured in liquid nitrogen and sputter-coated with gold before observation. 3. Results and discussion
Fig. 1. TEM images of (PVDF/RCC)/PLLA (A), PVDF/PLLA/RCC (B) and (PLLA/RCC)/PVDF (C, D). The scale bars are 200nm. E to G shows the SEM images of porous structures upon acid hydrolysis. H is the photo of self-supporting porous membrane.
First of all, it is necessary to confirm the co-continuous structure which is the base for the successful removal of PLLA, self-supporting porous PVDF membranes and the measurement of ER, AR and GR by weight calculation. In TEM images (Figure 1A1D), the black and white phases are PVDF and PLLA respectively, since the former is easily stained by the RuO4. In all TEM images, there are worm-like structures which are typical morphologies of co-continuous structures
[31-33].
The co-continuous
structures have also been validated by means of porous structures (Figure 1E-1G) 35],
[34-
in which the specimens were immersed in 20% nitric acid solution at 90oC for 90 5
Journal Pre-proof hours to remove both the reacted and free PLLA. For one thing, the weight loss during acid hydrolysis equals to the original weight of PLLA; for another thing, the resultant PVDF porous membranes are self-supporting (Figure 1H). This result makes it clear that the co-continuous structures have been prepared successfully. The formation of cocontinuous structures in this asymmetrical blend system can be ascribed to the balanced stress on two sides, the localization of RCC at the interface and the bigger interface curvature radius, which has been discussed in our previous work
[18].
Moreover, the
characteristic size of the co-continuous structure exhibits obvious dependence on the blending sequence. In (PVDF/RCC)/PLLA (PVDF premixed with RCC firstly, sic passim), the value locates at 130nm (Figure 1A). It decreases to 90nm in PVDF/PLLA/RCC
(one-step
mixing)
(Figure
1B).
In
the
specimen
of
(PLLA/RCC)/PVDF, the co-continuous structures exhibit the smallest size (~50nm, Figure 1C). The TEM image with higher magnification (Figure 1D) shows the excellent continuities of PVDF and PLLA. In the resultant porous PVDF membranes, the average pore size and surface area have been measured by means of mercury-injection test. It is obvious that the characteristic size of co-continuous structures (Figure 1A to 1D) agrees well with the pore size shown in Table 1. Table 1. The pore size information obtained from mercury injection test Samples
Average pore size (nm)
Surface Area (m2/g)
(PVDF/RCC)/PLLA
117.9
8.2
PVDF/PLLA/RCC
63.1
14.1
(PLLA/RCC)/PVDF
49.9
16.9
Table 2. The extraction ratio, acid hydrolysis ratio and graft ratio in specimens with different blending sequences. Samples Extraction Ratio Acid Hydrolysis Ratio Grafted Ratio (ER) (AR) (GR) (PVDF/RCC)/PLLA
66%
100%
34%
PVDF/PLLA/RCC
53%
100%
47%
(PLLA/RCC)/PVDF
42%
100%
58%
In the co-continuous structures discussed above, PLLA can be removed by two methods. On one hand, chloroform has been employed to extract the unreacted (i.e. free) PLLA. The extraction ratio (ER) is defined as the ratio between weight loss in this process and the original weight of PLLA. By means of weight calculation, we can get the values of them (Table 2). It is important to note that there is a tiny amount of RCC 6
Journal Pre-proof in the extraction solution, which has been confirmed by means of Fourier Transform Infrared Spectrometer (FTIR, data not shown here). The extraction ratios listed in Table 2 have been corrected according to the data of infrared spectrum. In the specimen of (PVDF/RCC)/PLLA, it exhibits the highest magnitude (66%), indicating the high fraction of unreacted PLLA. In PVDF/PLLA/RCC and (PLLA/RCC)/PVDF, the values decrease to 53% and 42% respectively; on the other hand, both grafted and free PLLA can be removed by immersing specimens in 20% nitric acid solution (90oC). The acid hydrolysis ratio (AR), defined as the weight loss over the original weight of PLLA, can be measured. As shown in Table 2, all specimens exhibit the similar value of 100%. This result suggests that almost all PLLA has been removed during acid hydrolysis. The difference between AR and ER represents the weight fraction of the grafted PLLA, which has been defined as graft ratio (GR) and listed in Table 2. With the help of cocontinuous structures, AR and ER, a novel method has been developed to measure the exact value of graft ratio (GR). The comparison of GR (Table 2) can provide the direct evidence for its blending sequence dependence, which has been described as follow. When PVDF has been premixed with RCC [(PVDF/RCC)/PLLA], only 34% PLLA reacts with RCC, leading to the higher ER and lower GR. In the case of (PLLA/RCC)/PVDF, 58% PLLA has been grafted on RCC based on the reaction between epoxy group in RCC and terminal carboxyl group of PLLA. In the specimen upon one-step mixing (PVDF/PLLA/RCC), the value of GR (47%) locates in between them.
Fig. 2. DTG curves of specimens upon extraction by chloroform. 7
Journal Pre-proof Table 3. Area ratio of the degradation peak at 374 and 472oC. (PVDF/RCC)/PLLA PVDF/PLLA/RCC (PLLA/RCC)/PVDF A /A 374
472
0.82
0.96
1.13
To validate the graft ratio as well as its blending sequence dependence, the thermal gravity analysis (TGA) has been performed in the specimens upon chloroform extraction. As shown in Figure 2, the main degradation peaks of PVDF, PLLA and RCC locate at 465, 348 and 378oC respectively. In the blend samples, the degradation peak at 472oC can be indexed to PVDF. The peak at 374oC and the shoulder at 348oC can be attributed to the degradation of the graft copolymer (PLLA-g-PMMA) resulted from the reaction between RCC and PLLA since there are only PVDF and the copolymer in the specimen (upon extraction, free PLLA has been removed). The areas of the peak and shoulder have been calculated to compare the graft ratio of PLLA by taking the area of PVDF peak as a reference. The area ratio (Table 3) between the copolymer and PVDF exhibits the lowest magnitude (0.82) in the specimen of (PVDF/RCC)/PLLA, indicating the lowest weight percent of grafted PLLA. This result has good agreement with the highest ER and lowest GR shown in Table 2. On the contrary, the lowest ER and highest GR in (PLLA/RCC)/PVDF correspond to the highest area ratio (1.13) shown in Figure 2 and Table 3. DMA measurements have been performed to assess the relaxation behaviors of PVDF and free/grafted PLLA (Figure 3A). From the reference of neat PVDF, PLLA and RCC, the relaxations at -34.5, 68.1 and 123.1oC can be ascribed to the glass transition of PVDF, PLLA and PMMA in RCC respectively
[36-37].
In Figure 3B, we
can find weak peaks of PVDF glass transition in the red and blue lines while there is no signal about it in (PVDF/RCC)/PLLA (black line). This result indicates that the glass transition of PVDF has been influenced significantly when it is premixed with RCC. The shoulder at 105oC (Figure 3D) can be associated with the relaxations within the PVDF crystalline phase. It exhibits the highest intensity in the black line, which can be interpreted as follow. In the specimen of (PVDF/RCC)/PLLA, the crystallization of PVDF has been depressed remarkably due to the excellent miscibility between PMMA and PVDF, leading to the increased crystal defects. This is the reason for the enhanced relaxation and stronger shoulder in Figure 3D [38]. Figure 3C illustrates the relaxation behaviors of PLLA. Our attention should be paid to the following issues. For one thing, the glass transition peaks of black, red and blue lines locate at 66.7, 66.0 and 65.5oC respectively. Compared with the value of neat PLLA (68.1oC), they move to the lower 8
Journal Pre-proof temperature direction, indicating the enhanced mobility of PLLA resulted from the better compatibility with PVDF; for another thing, the relaxation behaviors of grafted PLLA locating at PVDF/PLLA interface can be influenced remarkably by the graft reaction. As a result, the shoulders at ~53oC can be attributed to the relaxation of the grafted PLLA, which can act as a parameter to describe the graft ratio of PLLA. Obviously, the blue line of (PLLA/RCC)/PVDF corresponds to the strongest shoulder, suggesting the highest amount of grafted PLLA. This result agrees well with the highest magnitude of GR shown in Table 2. On the contrary, the line of (PVDF/RCC)/PLLA exhibits a weak shoulder, contributing to the lowest value of GR.
Fig. 3. Tan delta as a function of temperature of specimens upon different blending sequences. B, C and D show the zoom-in curves of the indicated parts.
As a result of the different compatibilization effect, the blends exhibit different mechanical and optical performances. Figure 4A shows the blending sequence dependence of unnotched impact strength. In the reference specimen of PVDF/PLLA without RCC, the impact strength is 18.2kJ/m2. Upon adding RCC, the value increases significantly. The smaller the characteristic size of co-continuous structure is (Figure 1), the higher impact strength becomes (Figure 4A). In (PLLA/RCC)/PVDF, it reaches the maximum of 32.9kJ/m2. To assess the optical performance, the transmittance and haze have been measured (Table 4). The former exhibits the similar values while the latter depends crucially on the blending sequence. In PVDF/PLLA without RCC, the haze is 91.9%. This value drops to 13.0%, 9.8% and 7.4% in the specimen of (PVDF/RCC)/PLLA, PVDF/PLLA/RCC and (PLLA/RCC)/PVDF, respectively. From 9
Journal Pre-proof the photos of three typical specimens shown in Figure 4B to 4D, the specimen of (PLLA/RCC)/PVDF corresponds to the best optical performance. It is noteworthy that 50nm co-continuous structures have been fabricated successfully by optimizing blending sequence (Figure 1C and 1D). The higher toughness and better optical performance in (PLLA/RCC)/PVDF can be attributed to the smaller characteristic size of co-continuous structures, which comes from the higher graft ratio and compatibilization efficiency (Table 2) [39-40].
Fig. 4. Impact strengths (A) and photos of specimens upon different blending sequences, (PVDF/RCC)/PLLA (B), PVDF/PLLA/RCC (C) and (PLLA/RCC)/PVDF (D). Table 4. Transmittance and haze of (PVDF/RCC)/PLLA, PVDF/PLLA/RCC and (PLLA/RCC)/PVDF. Samples
Transmittance %
Haze %
PLLA
92.4±0.1
3.0±0.2
PVDF
89.3±0.2
74.0±1.9
30/70 (without RCC)
83.9±0.9
91.9±0.1
(PVDF/RCC)/PLLA
84.1±0.1
13.0±0.5
PVDF/PLLA/RCC
85.0±0.3
9.8±0.5
(PLLA/RCC)/PVDF
90.3±0.2
7.4±0.1
There are both PMMA (backbone and side chains) and epoxy groups in RCC. They account for the entanglement (with PVDF) and graft reaction (between RCC and 10
Journal Pre-proof PLLA) respectively. When PVDF is premixed with RCC, the segments of PMMA chains diffuse to the PVDF phase because of the thermodynamic miscibility between them [30, 38]. RCC molecules tend to stay in PVDF, leading to its reduced contact with PLLA. The reaction between epoxy group in RCC and terminal carboxyl group of PLLA has been depressed. This is the reason for the lower graft ratio shown in Table 2. In the case of (PLLA/RCC)/PVDF, the terminal carboxyl group and epoxy group contact with each other sufficiently, producing the higher graft ratio. Upon adding PVDF, the excellent miscibility between PVDF and PMMA (in RCC) results in the migration of the graft copolymer to PVDF phase. The copolymer (PLLA-g-PMMA) prefers to localize at the immiscible interface (rather than entering PVDF phase) because of the poor interaction between grafted PLLA and PVDF
[41].
The precise
localization of compatibilizers at interface contributes to the formation of 50nm cocontinuous structures and the resultant improved mechanical and optical performances (Figure 4). When the blend is mixed simultaneously (PVDF/PLLA/RCC), there are RCC molecules in both PVDF and PLLA phases. The value of GR, impact strength and haze locate between two scenarios discussed above. The compatibilization efficiencies in specimens have been validated by rheology data (Figure 5). In Figure 5A, the frequency dependencies of storage modulus, three specimens exhibit similar viscoelasticity behaviors in high frequency region. In the case of low frequency, however, there is more remarkable deviation in (PLLA/RCC)/PVDF, indicating the better compatibility. This result has good agreement with the lower phase angle shown in Figure 5B. The more remarkable deviation in low frequency zone and smaller phase angle suggest the better compatibility in this specimen [42-44].
Fig.5. Linear rheology properties obtained from SAOS experiments: (A) storage modulus G′ and (B) van Grup-Palmen (VGP) plots of the (PVDF/RCC)/PLLA, PVDF/PLLA/RCC and (PLLA/RCC)/PVDF blends. 11
Journal Pre-proof 4. Conclusions In this work, a novel strategy has been developed to measure the graft ratio quantitatively during reactive compatibilization. In the co-continuous structures fabricated in PVDF/PLLA blend with RCC, extraction and acid hydrolysis have been employed to remove PLLA. The former and the latter correspond to extraction ratio (ER) and acid hydrolysis ratio (AR) respectively. The difference between AR and ER represents the weight percent of grafted PLLA, which can act as a good parameter to describe the graft ratio (GR). Based on quantitative measurement of GR, the blending sequence dependences of the extent of graft reaction, the characteristic size of cocontinuous structures and the mechanical and optical performances have been investigated. By optimizing the migration and localization of compatibilizers, 50nm cocontinuous structures, corresponding to the highest impact strength and lowest haze, have been fabricated successfully in the specimen of (PLLA/RCC)/PVDF. Our result provides not only a novel strategy for the quantitative measurement of graft ratio, but also the direct evidence for its blending sequence dependence.
Acknowledgements This work was financially supported by the Zhejiang Natural Science Foundation (LD19E030001), National Natural Science Foundation of China (21674033 and 2017YFB0307704), and Zhejiang Provincial Key R&D Program (2018C01038)
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Journal Pre-proof Declaration of interest statement: The authors declare no conflict of interest.
Journal Pre-proof 1. Co-continuous structures with characteristic size below 100nm has been fabricated. 2. A novel method has been developed to measure the graft ratio quantitatively. 3. The direct evidence of the blending sequence dependence of graft ratio has been provided.