Inorganic silica functionalized with PLLA chains via grafting methods to enhance the melt strength of PLLA/silica nanocomposites

Highlights

• Two kinds of PLLA-grafted silica nanoparticles with totally different topology are synthesized.

• Good dispersion of nanoparticle is obtained through grafting modification.

• The melt strength of PLA is increased by introducing graft-to silica nanoparticles.

• PLLA nanocomposite films are prepared by extrusion-film blowing.

Abstract

The low melt strength greatly limits the application of PLA as biodegradable package materials produced by film blowing method. Modified silica nanoparticles are introduced into PLA matrix to solve this problem in this study. To build Poly (l-lactide) nanocomposites successfully, two kinds of convenient and efficient methods are conducted to synthesize well-defined topological PLLA grafted SiO₂ nanoparticle. One is the ring-opening of l-lactide (Grafting from), and another is nucleophilic addition reaction (Grafting to). The structure, molecular weight of grafted PLLA chains, grafting density, and the thermal decomposition behavior of the nanoparticles prepared by different methods are characterized. By varying the contents of the initiator SiO₂ and the molecular weight of the reacted PLA chains, high density-low molecular weight PLLA grafted SiO₂ are obtained in “grafting from” while high molecular weight-low grafting density PLLA grafted SiO₂ are synthesized in “grafting to”. It is exactly in good agreement with the theoretic model. The spatial distribution of nanoparticles as well as the interaction force between nanoparticles and matrix is critical important to structuring bionanocomposites with desirable properties. So the two kinds of synthesized nanoparticles are introduced into PLA matrix in our contribution to evaluate these two factors, respectively. The TEM and SEM results both reveal the uniform dispersion of nanoparticles after modified. While the extension and shear rheology results show that the long grafted chains covalently connected on the surface of the silica via “grafting to” contribute more to enhance the melt strength of PLA. Meanwhile, stabilized PLA nanocomposites films with modified silica via “grafting to” method are successfully blown base on these researches. The research in this work constitutes a robust way to design melt-strengthen PLA/SiO₂ nanocomposites.

Introduction

Poly (lactic-acid) (PLA) holds a promise commercial application prospect to replace the traditional petroleum based plastics because it can be made from renewable agricultural products and is readily biodegradable [1]. It has been used as stretch-blown bottles [2], scaffold fabrication [3], tissue regeneration [4], and so on. If taking the environment problem into consideration, it is polymeric packaging materials that PLA mostly should be used as [5]. As we know, the biaxial extension of the film (film blowing) is the primary attractive method to prepare films, which increases the strength of the film in two directions and allows for precise control over the mechanical, shrink, and optical properties of the final products [6]. However, film-blowing is a complex three-dimensional deformation elongation flow process; the high melt strength of the material is the basic core for preparing blown films steadily and successfully. Thus, the most common plastic films produced by this method are linear low density polyethylene (LLDPE), branched low density polyethylene (LDPE) and linear high density polyethylene (HDPE). Unfortunately, the poor viscoelastic behavior, extremely low melt strength makes the PLA films production a great challenge in materials-processing science. To enlarge its processing window and prepare stabilized PLA films successfully, PLA needs to be melt-strengthened.

The melt strength is closely related to the entanglement and relaxation behavior of macromolecular chains, so the efforts on the improvement of melt strength of polymers are mainly proposed from the perspective of increasing chain entanglement or extending the melt relaxation times of PLLA chains. Increasing the molecular weight has been proved to be an effective method in polypropylene [7]. This is because the relaxation time λ₁ is sensitive to the molecular weight Mw, λ₁ ∝ Mw^3.0 has been established in the former study [8]. Branching is another effective method, as the long chain branched (LCB) has been proved to increase the entanglement in elongational flow [9], [10], especially the multiple branches per chain (comb-like) [11]. Although it has been known for a long time that high molecular weight PLA can be produced by the ring-opening polymerization of lactide [12], the Mw is still not high enough to increase the melt strength of commercial PLA; As for the synthesis of LCB-PLA, it is eventually hard to control the molecular weight and topology of the final products due to the randomness of branching [13]. Therefor, some other effective and well-controlled methods have been researched to increase the chain entanglement of PLA. For example, weaving a special cross-linking network in PLA matrix to enhance its melt strength has drawn someone's interest [14]; Some special additives has also been researched and used in PLA film blowing [15], [16]. In our opinion, introduction of nanoparticles into biopolymer to build special filler-matrix network is believed to be an effective way to improve the melt strength of PLA [17], [18], [19], [20], [21], as well as other properties. And the application fields of PLLA could be finally broadened [17], [22], [23].

To build useful nanofiller-matrix network in polymer science, the dispersion state of nanofillers and interaction between filler and matrix must be taken into deep consideration. However, because of the natural incompatibility between the hydrophilic nanoparticles and hydrophobic biopolymers such as Poly (l-lactide), the nanoparticles with high surface energy are easy to agglomerate in polymer matrix. Also the immiscibility will lead to the weak interaction force between the nanoparticles and matrix. To improve the distribution of nanoparticles and enhance the interaction force between the nanoparticles and matrix [24], one effective method is the surface pretreatment of nanoparticles. Among the surface modification of nanoparticles, covering the particles with a “brush” of grafted polymer having the same nature as the matrix via grafting chemistry is a significant route.

For the covalent functionalization of nanoparticles, two strategies have been well established, “grafting from (grafting side chains from the backbone)” and “grafting to (attachment of side chains to the backbone)” approaches [25], [26], [27]. Researches on “grafting from” are carried out by many institutions, and this method is widely used for the modification of particles. “Grafting from” polymerization reaction allows better control over the target molecular weight and number of the grafting molecular chains [28], especially when the side chains are grafted from the backbone or surface of particles via Atom Transfer Radical Polymerization (ATRP) [29], [30]. “Grafting to” method is also adopted by some researchers to modify the nanoparticles [31], [32]. Compared to the modified nanoparticles obtained by “grafting from” method, higher molecular weight of the side chains can be obtained via “grafting to”, even though the grafting density is lower. Typically, the reaction condition for this polymerization method (grafting to) is much harsher.

To obtain the positive effects of the nanoparticle on PLLA matrix, PLLA grafted nanoparticles have been prepared with different methods. The “grafting from” synthesis of PLLA-grafted nanoparticles by ring-opening polymerization of l-lactide with the surface active groups of particles as initiators [25], [33], [34], [35] or in situ melt polycondensation [36], [37] has been proposed by many authors. Corresponding, the “grafting to” method comprises amidation reactions or nucleophilic substitution [32]. In SS. Ray's group, a series of PLLA functionalized CNTs has been prepared by different methods [38], [39], while Young Gyu Jeongy and his coworkers found that the properties of PLA/MWCNT-g-PLA nanocomposites are strongly dependent on the length of the grafted PLA chain [25], [40]. If taking the use-cost and transparency of PLLA into consideration, SiO₂ or TiO₂ nanoparticles would be a more competitive candidate. So PLLA/SiO₂ or TiO₂ nanocomposites are also discussed by some researchers [41], [42], [43]. Among them, functionalization of TiO₂ particles with lactic acid through solution polycondensation or in situ melt polycondensation are prepared by Wang and his coworkers [36], [44]. PLLA grafted SiO₂ nanoparticles are also widely prepared by some research groups. The in-situ sol–gel method [43] or condensation reaction with l-lactic acid oligomer [42] by “grafting from” methods are also used in these researches. It is noteworthy that the nanoparticles have been modified with l-lactic acid and the performance of nanocomposites had been improved by incorporation of the modified nanoparticles prepared by different methods in these researches. Actually, the topology structure of the particles (mainly the molecular weight and grafting density) can be controlled by different methods from the previous studies. How to control the topologic structure of PLLA-grafted SiO₂ by different method and different reaction condition have not been considered by most of the published studies. So a study on controlling the topology structure of PLLA-grafted SiO₂ nanoparticles using “grafting from” and “grafting to” method are indicated in this study. Form the comparative study, two totally different kinds of modified silica are obtained, and then they are added to PLA matrix respectively to evaluate the final rheological properties of the nanocomposites.

In Jacques Jestin's research on PS-grafted SiO₂/PS nanocomposites, the mass ratio of grafted chain and free matrix chain (R) is a significant contributing factor which manages the particle dispersion in the matrix. When the ratio R is smaller than 0.24, large and compact aggregates are found in the PS matrix, however, individual nanoparticles dispersion state would be formed in the matrix when the R is bigger than 0.24 [27]. From the researches of Nicolas Jouault and Jacques Jestin [45], [46], [47], [48], we can find that the topology structure of the modified nanoparticles, including the molecular weight and density of the grafted molecular chains, would greatly influence the dispersion state of nanoparticles in matrix and the interface of filler and polymer, further influence the final properties of materials. So the modified nanofillers with different topology are introduced into PLA matrix to evaluate the contribution of the dispersion state and interaction force to the properties of nanocomposites. Controlling the properties of nanocomposites is still a great challenge both in industry and in academia. As Sanat K. Kumar has pointed out, understanding and relating the properties of polymer nanocomposite to the dispersion of nanoparticles and the interaction between polymer and nanoparticle are important challenges [49]. So the dispersion state and interfacial are characterized generally in polymer nanocomposites [50], [51]. Although the general literature on rheological properties of PLA nanocomposites is extensive, only a few references have considered the influence of the topology of nanofillers. Our interest in PLA/modified silica nanocomposites stems from the blown film processing of PLA, therefor, the morphology and rheological (the rheology behavior of nanocomposites is strongly influenced by the interfacial force) properties of PLLA nanocomposites are studied here.

Different from the previous reports, the SiO₂ nanoparticles are modified and introduced into the PLA matrix to improve the extension viscosity of PLA, and PLA/SiO₂ nanocomposites films are prepared firstly by us. Through the evaluation of the stretching properties of nanocomposites, the structure-property relationship of nanocomposites is researched. The objectives of this study involve: 1) A systematic study of the effects of reactant molar ratio on “grafting from” reaction as well as the molecular weight of reacted PLLA chains on “grafting to” polymerization; 2) two kinds of PLLA grafted silica with totally different topological structure are obtained by different methods; 3) The nanofillers are added into PLA matrix. The properties of PLA nanocomposites with two kinds of nanofillers are studied, and compared by morphology and extension rheology studies; 4) PLA nanocomposites films are successfully blown by us, through introducing long grafted PLLA chains onto the surface of Silica nanofiller.