Interfacially located nanoparticles: Barren nanorods versus polymer grafted nanorods

https://doi.org/10.1016/j.compositesb.2020.108153Get rights and content

Highlights

  • Ternary Poly(l-lactide)/Poly(1,4-butylene succinate)/boehmite nanorods nanocomposites were prepared.

  • Both pristine boehmite nanorods and epoxide groups modified boehmite nanorods are thermodynamically located at the interface of PLLA/PBSU.

  • Barren nanorods deteriorate the mechanical properties of PLLA/PBSU blends, but surface grafted nanorods enhance the mechanical properties drastically.

  • Molecular entanglements of the grafted nanorods are the prerequisite for the compatibilization of immiscible polymer blends.

Abstract

Polymer blend nanocomposites with the nanoparticles exclusively located at the interface have attracted significant attention while the compatibilization functions of such nanoparticles in the blends are still controversial. Herein, the pristine boehmite nanorods (p-BNRs) and epoxide group modified boehmite nanorods (m-BNRs) have been synthesized and incorporated in the immiscible Poly(l-lactide)/Poly(1,4-butylene succinate) (PLLA/PBSU) blends. It is found that both p-BNRs and m-BNRs are thermodynamically located at the interface and the phase morphologies of the two nanocomposites are very similar at the same nanofiller loadings. However, the two nanocomposites exhibit different mechanical properties. The PLLA/PBSU blend nanocomposites with m-BNRs show increased elongation at break, enhanced notched impact strength and improved yield strength, as compared with the binary PLLA/PBSU blends. In contrast, p-BNRs lead to drastically deteriorating mechanical properties of the PLLA/PBSU blends even with all p-BNRs located at the interface. The investigation indicates that p-BNRs at the interface have barren surface and are lack of specific interactions with the polymers in the neighboring phases. In contrast, the epoxide groups on m-BNRs reacted with the end carboxylic acid groups of both PLLA and PBSU and the polymer chains were chemically bonded onto the surface of nanorods. The chemically bonded polymer chains on the nanorods at the interface can entangle with the molecular chains of the two polymers to enhance the interfacial adhesion. This paper demonstrates the significance of molecular chain entanglements of the compatibilizing nanoparticles and paves a new possibility to fabricate new materials with simultaneously enhanced strength and toughness.

Introduction

Polymer blend nanocomposites based on immiscible polymer blends and inorganic nanoparticles have attracted significant attentions because they can combine the advantages of polymer blends and the functionality of nanoparticles [[1], [2], [3], [4], [5]]. Moreover, the strategy to combine polymer blends and nanoparticles into one system provides a platform to tailor and design both the phase morphologies and the properties. The performance of polymer blend nanocomposites is greatly dependent on the phase morphologies of the multi-component/multi-phase systems [[6], [7], [8]]. Extensive investigations have been carried out on the location of nanoparticles in polymer blends and the effects of nanoparticles on phase morphologies of polymer blends as well over the past two decades [[9], [10], [11], [12], [13]].

Dependent on the interactions of specific nanoparticles with the two components of polymer blends, nanoparticles can be either located in one of the phases or distributed at the interface [[6], [7], [8], [9], [10], [11], [12], [13], [14]]. The thermodynamic equilibrium location of the nanoparticles in immiscible polymer blends can be predicted by the wetting parameter ω according to Young's equation [15]:ω=γfBγfAγAB

Inorganic nanoparticles will locate inside of B or A phase for the cases of ω<−1 or ω>1, respectively. The nanofillers distribute thermodynamically at the interface only when −1 < ω < 1. Based on the Young's equation, one can also design or tailor the phase structure of the blend nanocomposites with the nanoparticles distributed either in one of the phases or at the interface by the specific surface modification of the fillers [[16], [17], [18], [19]]. The blend nanocomposites with the interfacial distribution of nanoparticles attract the most attention because such a novel hierarchical structure is expected to exhibit additional functionality at very low nanofiller loadings, typically the so-called double percolation structure for the conductive polymer composites [20,21]. Numerous literatures reported the formation of the polymer nanocomposites with the nanoparticles located at the interface since the first report by Sumita [21].

On the other hand, the effects of nanoparticles on the phase morphologies of the immiscible polymer blends have also been well investigated [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. For blend nanocomposites with the nanofillers located in one of the two phases, the nanoparticles change the viscosity ratio of the two phases with less effect on the interfacial tension because nanoparticles usually increase the viscosity of the component [32,33]. Therefore, either the enlarged domain size or the decreased domain size has been observed depending on the distributions of nanoparticles in minor phase (domain) or major phase (matrix) [[31], [32], [33], [34], [35]]. Although many literatures concluded compatibilization effects of the nanoparticles for the immiscible polymer blends in the one phase distributed nanoparticle systems as evidenced by the decreased domains size [34,35], the nanoparticles are not the real compatibilizers because the compatibilizers should be at least located at the interface to bridge the neighboring phases and to decrease the interfacial tension. On the other hand, the interfacially located nanoparticles have been easily assigned as the effective compatibilizers for the immiscible polymer blends, especially for the system with decreased phase sizes as compared with the blends without nanoparticles [36]. However, the potential feasibility of nanoparticles compatibilizing is questionable and the strategy needs further exploration. Generally speaking, the effective compatibilizing of immiscible blends should enhance the mechanical properties, but most of the nanoparticles compatibilized immiscible polymer blends show even deteriorated mechanical properties, especially the toughness and ductility [[37], [38], [39], [40]]. We consider that the enhancing interfacial adhesion by molecular entanglements on the surface of nanoparticles will take the critical role for the mechanical enhancement for the real nanoparticle compatibilized system. Unfortunately, so far the importance of the molecular entanglements between the molecular chains on the nanoparticles and the molecules of components has not been confirmed. Especially, the comparison has not been made for the nanoparticle compatibilized polymer blends with and without chemical bonding polymer chains on the nanoparticles. The polymer chains chemically bonded on the nanoparticles may lead to the possible molecular entanglements of the grafted polymer chains with the individual components.

In this work, we prepared Poly(l-lactide)/Poly(1,4-butylene succinate) (PLLA/PBSU) blend nanocomposites with the incorporation of the pristine boehmite nanorods (p-BNRs) and the epoxide group grafted boehmite nanorods (m-BNRs) by melt blending. Interestingly, it is found that both p-BNRs and m-BNRs are mainly located at the interface between PLLA and PBSU and the two PLLA/PBSU blend nanocomposites have very similar morphologies at the same BNR loading. However, m-BNRs improve the mechanical properties of the blends and p-BNRs decrease the mechanical properties of the blends. This work presents the direct evidence, for the first time, of the prerequisite of the molecular entanglements for the nanoparticle compatibilized polymer blends. Moreover, the results pave a new possibility to fabricate materials with both high strength and toughness.

Section snippets

Materials

The chemical structures of the raw materials are shown in Scheme 1. Poly(l-lactide) (PLLA, 3001D, Mn = 1 × 105 g/mol, PDI = 1.45, density = 1.24 g/cm3) was purchased from NatureWorks, USA. Poly(1,4-butylene succinate), extended with 1,6-diisocyanatohexane (PBSU, Mn = 8.9 × 104 g/mol, PDI = 1.80, density = 1.3 g/cm3) was purchased from Sigma Aldrich. Aluminium isopropoxide, 3-glycidoxy-propyltrimethoxy-silane (GPS), chloroform, acetic acid, ethanol absolute, petroleum ether were all purchased

Morphology of the PLLA/PBSU blends with p-BNRs and m-BNRs

Fig. 1 shows the SEM images of PLLA/PBSU (70/30) blends without and with various amounts of p-BNRs and m-BNRs. The binary PLLA/PBSU (70/30) blend exhibits a typical two phase structure because of the immiscibility between PLLA and PBSU. The discrete droplets of the minor PBSU phase are dispersed in the PLLA matrix. The fact that PBSU domains show inhomogeneity with circular and oval form indicates poor compatibility between PLLA and PBSU. The average size of the dispersed PBSU phase in the

Discussion

It is interesting to find that both pristine boehmite nanorods and epoxide group modified boehmite nanorods are exclusively located at the interface of the PLLA/PBSU blends and the two types of BNRs show similar functions on the phase morphologies of the blends. However, the two types of interfacially located BNRs exhibit totally different functions on the physical properties of the nanocomposites. Obviously, the interfacial location of p-BNRs is also originated from the thermodynamically

Conclusion

The structures and properties of the immiscible PLLA/PBSU blend nanocomposites with pristine and expoxide modified boehmite nanorods have been investigated. The locations of the nanorods and the phase structures of the blends are almost same in the blend nanocomposites incorporated with the two types of BNRs at the same loadings. M-BNRs formed a double grafted structure through the reactive blending so that those grafted chains had the molecular chain entanglements with the both components. The

Supporting information

The TEM image of the pristine BNRs, SEM image of the solution blended nanocomposites, the calculation of the wetting parameter of the p-BNRs with PLLA and PBSU, and contact angle of the p and m-BNRs can be found in the Supporting information. This is available free of charge on the website.

CRediT authorship contribution statement

Xuan Li: Data curation, Writing - original draft. Zhiang Fu: Visualization, Investigation, Data curation. Xiaoying Gu: Visualization, Investigation, Data curation. Huanhuan Liu: Writing - review & editing. Hengti Wang: Writing - review & editing. Yongjin Li: Conceptualization, Methodology, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the Zhejiang Natural Science Foundation (LD19E030001) and National Natural Science Foundation of China (51890872).

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