Article pubs.acs.org/JPCB
Tailor-Made Dispersion and Distribution of Stereocomplex Crystallites in Poly(L‑lactide)/Elastomer Blends toward Largely Enhanced Crystallization Rate and Impact Toughness Yuanlin Luo, Yilong Ju, Hongwei Bai,* Zhenwei Liu, Qin Zhang, and Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China ABSTRACT: Stereocomplex (SC) crystallites, formed between poly(L-lactide) (PLLA) and poly(D-lactide), exhibit great potential to substantially enhance crystallization rate of PLLA-based materials as an eco-friendly nucleating agent. However, the nucleation efficiency of the SC crystallites is still far below an expected level, mostly on account of their strong aggregation tendency in PLLA/ PDLA melts. Herein, taking PLLA/poly(ethylene-methyl acrylateglycidyl methacrylate) (E-MA-GMA) blends as an example, we report a unique and facile strategy to control the dispersion and distribution of SC crystallites within the PLLA matrix by using elastomeric E-MA-GMA as carrier for the incorporation of PDLA. To do this, PDLA was first blended with E-MA-GMA or chemically grafted onto the E-MA-GMA. During subsequent meltblending of PLLA and the E-MA-GMA/PDLA master batch, the PDLA chain clusters predispersed in the E-MA-GMA phase can gradually migrate into PLLA matrix and then collaborate with the matrix chains to form large amounts of tiny and well-dispersed SC crystallites. Compared with the SC-crystallite agglomerates formed by the direct melt-blending of PLLA and PDLA components, such tiny SC crystallites are much more effective in accelerating PLLA matrix crystallization. More interestingly, when PDLA chains are grafted onto the EMA-GMA, the formed SC crystallites tend to preferentially distribute at the blend interface and thus induce not only optimal nucleation efficiency but also superior impact toughness because these interfacelocalized SC crystallites can also serve as bridges to enhance interface adhesion. This work could open a new avenue in designing heat-resistant and supertough PLLA blends via controllable construction of SC crystallites.
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INTRODUCTION Over the last few decades, both increasing environmental problems and decreasing petroleum resources have driven tremendous efforts to develop biobased and biodegradable polymers capable of replacing traditional synthetic polymers.1,2 Among these emerging substitutions, poly(L-lactide) (PLLA) has been recognized as the most promising frontrunner with a huge application potential in numerous fields owing to its complete renewability from plant resources (e.g., corn), full biodegradability in solid, impressive transparency, high mechanical strength and modulus, good melt-processing, as well as relatively low production cost.3−5 Nevertheless, the large-scale practical application of PLLA as a competitive commodity thermoplastic has been severely hampered by some drawbacks, notably its inherent brittleness and poor heat resistance.5−7 PLLA crystallizes very slowly and thus it is hard to achieve articles with a high level of crystallinity under practical melt-processing conditions, such as high cooling rate and fast molding cycle.8−10 The heat distortion temperature (HDT) of amorphous PLLA articles is inferior to the highly crystalline ones because only the crystalline phase can confer sufficiently high resistance against heat-deformation above the low glass transition temperature (Tg, ca. 55−65 °C). So far, several effective strategies have been frequently used to toughen © 2017 American Chemical Society
PLLA, especially the melt blending with various elastomers including poly(ethylene-methyl acrylate-glycidyl methacrylate) (E-MA-GMA) random copolymer.6,7,11−17 However, despite significant progress being made in the design of supertough PLLA materials, all the toughening inevitably causes an unfavorable deterioration in the poor heat resistance of PLLA.6,7 Thus, there is a growing interest in simultaneously enhancing the fracture toughness and crystallization rate of PLLA so as to meet the essential requirements for various practical applications.7,14,18 Very interestingly, increasing PLLA matrix crystallinity has been evidenced to be a simple and versatile approach to enhance the heat resistance of supertough PLLA/elastomer blends, while maintaining or even further improving the blend toughness.14,19−21 Some inorganic/organic compounds and macromolecules can be utilized as highly efficient nucleating agents (NAs) to dramatically accelerate crystallization kinetic of PLLA,22−30 but this could bring the issues of incompatibility and unknown environmental harm. In recent years, stereocomplex (SC) crystallites formed between enantiomeric PLLA Received: April 27, 2017 Revised: June 4, 2017 Published: June 6, 2017 6271
DOI: 10.1021/acs.jpcb.7b03976 J. Phys. Chem. B 2017, 121, 6271−6279
Article
The Journal of Physical Chemistry B and poly(D-lactide) (PDLA) has aroused immense interest as a new promising type of compatible and eco-friendly NA.31−37 The SC crystallites can be survived in PLLA melt and provide heterogeneous nucleation surface for PLLA homocrystallization because their melting temperature is ca. 50 °C higher than that of the PLLA or PDLA homocrystallites.31−38 It has been reported that incorporating small amounts (1−3 wt%) of PDLA can enhance the nuclei density of PLLA greatly and thus accelerate its crystallization rate significantly through the in situ formation of SC crystallites in PLLA melt.31−33 The NAs with higher surface area and larger aspect ratio could impart PLLA matrix with much faster crystallization rate at low concentrations.39,40 Unfortunately, direct incorporation of PDLA into PLLA matrix by melt blending represents the simplest way toward PLLA/elastomer/PDLA blends, this method may lead to the formation of SC crystallite agglomerates since the rapid stereocomplexation (the stereocomplextion could occur immediately as soon as PDLA and PLLA pellets are melted41) makes PDLA chain clusters unable to be uniformly dispersed in the PLLA matrix melt before collaborating with the matrix chains (Figure 1 a1 and a2), thereby the nucleation efficiency of the formed SC crystallites on PLLA matrix crystallization is usually far below an expected level. Moreover, high-concentration SC crystallites could provide more nucleation surface for PLLA matrix but exhibit an evident confining effect on the matrix crystallization because they tend to form network structure with a strong ability to restrain the PLLA chain diffusion required for crystal growth.33 Therefore, exploring an elegant and powerful way to remarkably improve the dispersion of the SC crystallites is highly desirable for substantially enhancing the matrix crystallization rate and finally obtaining supertough PLLA/elastomer blends with good heat resistance. Recently, BiBi and co-workers have made an attempt to realize homogeneous dispersion of SC crystallites by incorporating presynthesized SC submicronparticles to the PLLA matrix.39 However, synthesizing these particles requires not only a tedious and complex synthetic process but also large amounts of toxic organic solvents, which significantly retards the progress of its industrial-scale applications. In light of the existing challenges, herein we develop a new and facile strategy to homogeneously disperse the SC crystallites in the PLLA matrix of PLLA/elastomer/PDLA blends by using elastomer as carrier for the incorporation of PDLA. To do this, PLLA/EMA-GMA/PDLA blends were taken as a model system and the blends were prepared by melt-blending the preprepared E-MAGMA/PDLA master batch with excessive PLLA. It was expected that the PDLA chain clusters encapsulated by the thermodynamically unfavorable E-MA-GMA phase could be gradually released into the more favorable PLLA matrix during the melt-blending process, thereby allowing for the in situ formation of many tiny and well-dispersed SC crystallites in the matrix (Figure 1 b1 and b2). Specially, when the PDLA chains are chemically grafted onto the backbone of E-MA-GMA copolymer, the formed SC crystallites could be selectively distributed at the blend interface (Figure 1 c1 and c2). The effect of the obtained SC crystallites with different dispersion and distribution states on the matrix crystallization behavior and blend toughness has been comparatively investigated. Moreover, the important positive roles of the homogeneous dispersion and selective distribution of these SC crystallites in enhancing the performance of PLLA/elastomer blends have been highlighted. The key advantage of the presented strategy
Figure 1. Schematic representations showing the formation process of SC crystallites from PLLA/elastomer/PDLA blend melts under different melt-blending procedures: (a1,a2) direct blending of PDLA and PLLA/elastomer components could cause the formation of SC crystallite agglomerates; (b1,b2) if PDLA are first dispersed in E-MAGMA (i.e., EG) phase, the PDLA chain clusters could migrate from EMA-GMA domains into PLLA matrix and then cocrystallize with the matrix chains into many tiny and well-dispersed SC crystallites; (c1,c2) if the PDLA is first chemically grafted onto E-MA-GMA copolymer (i.e., EG-g-PDLA), the PDLA chain segments could collaborate with PLLA matrix chains at the blend interface and thus cocrystallizing into the interface-localized SC crystallites.
is that the tailor-made SC crystallites can be controllably constructed using a simple and industrially meaningful meltblending technology, without any special processing equipment. To the best of our knowledge, there have been no similar investigations on the controllable construction of SC crystallites in PLLA/elastomer blends for achieving high-performance PLLA-based materials in the literature.
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EXPERIMENTAL SECTION Materials. PLLA (Mw = 1.7 × 105 g/mol, PDI = 1.7) was obtained from NatureWorks LLC, USA. PDLA (Mw = 1.2 × 105 g/mol, PDI = 1.6) was kindly provided by Zhejiang Hisun Biomaterial Co. Ltd., China. EMA-GMA random copolymer (E/MA/GMA = 68/24/8 (wt%), Mw = 2.4 × 105 g/mol, PDI = 3.5) was purchased from Arkema Inc., France. PDLA grafted EMA-GMA (EMA-GMA-g-PDLA) copolymer was synthesized by reactive blending of the EMA-GMA copolymer and PDLA under the catalysis of N,N-dimethylstearylamine, according to the protocol described in our previous work.14 All of the plastic 6272
DOI: 10.1021/acs.jpcb.7b03976 J. Phys. Chem. B 2017, 121, 6271−6279
Article
The Journal of Physical Chemistry B
Figure 2. XRD patterns of the melt-quenched blends with various PDLA concentrations: (a) PLLA/15EMA-GMA/PDLA, (b) PLLA/15EMA-GMA +PDLA, and (c) PLLA/15EG-g-PDLA.
pellets were vacuum-dried at 60 °C for at least 12 h before using. Sample Preparation. PLLA/EMA-GMA/PDLA and PLLA/EMA-GMA-g-PDLA blends with two different distribution states of SC crystallites (i.e., selectively localized in PLLA matrix and at the blend interface, respectively, as illustrated in Figure 1) were prepared by one-step melt blending of the blend components directly in a Rheomix 600 internal mixer (Haake Technik GmbH, Germany). The blending was performed at 190 °C for 6 min with a rotation speed of 60 rpm. The weight ratio of PLLA/EMA-GMA or PLLA/EMA-GMA-g-PDLA was fixed at 85/15, and the weight fraction of PDLA component in these blends was varied from 0.06 to 0.95 wt%. For convenience, the obtained blends were abbreviated as PLLA/ 15EG/xPDLA and PLLA/15EG-g-xPDLA, respectively. PLLA/ EMA-GMA (85/15) blend without PDLA (abbreviated as PLLA/15EG) and PLLA/PDLA blends without EMA-GMA were also prepared under the same conditions for comparison. Specially, in order to clarify the influence of SC crystallite dispersion on the blend performance, two-step blending procedure was used to prepare the PLLA/EMA-GMA/PDLA blends with greatly improved SC crystallite dispersion in the PLLA matrix, namely, blending of EMA-GMA/PDLA master batch with excessive PLLA (abbreviated as PLLA+15EG/ xPDLA). After melt blending, these as-prepared blends were vacuum-dried and subsequently injection-molded into standard specimens for impact test using a HAAKE MiniJet II injection molder (Thermo Scientific, Germany) at a barrel temperature of 200 °C and a mold temperature of 130 °C. To obtain the injection-molded blends with an amorphous PLLA matrix and a highly crystalline one, two isothermal annealing time of 0.5 and 5 min were applied to the blend melts in the hot mold cavity, respectively. Disk specimens (25 mm in diameter and 1.5 mm in thickness) for dynamic rheological analysis were processed by compression molding at 200 °C and 5 MPa.
Differential Scanning Calorimetry (DSC). Thermal analysis was carried out on a pyris-1 DSC (PerkinElmer Inc., USA) under a dry N2 atmosphere. For the analysis of isothermal crystallization kinetic, the specimen (∼5 mg) sealed in an aluminum pan was rapidly cooled to a desired crystallization temperature (ranging from 126 to 136 °C) at a rate of 100 °C/min after erasing any thermal history at 200 °C and then held at this temperature for complete crystallization of PLLA matrix. In order to evaluate the crystallinity of PLLA matrix (Xc,PLLA) in injection molded PLLA/elastomer blends, the melting behavior of the matrix was also analyzed using the PerkinElmer pyris-1 DSC by heating the blend specimen from 30 to 200 °C at a rate of 10 °C/min. The Xc,PLLA was calculated based on the following equation: Xc,PLLA =
ΔHm − ΔHcc wf ΔHmo
(1)
where ΔHm and ΔHcc are the measured enthalpies of melting and cold crystallization, respectively; ΔHom is the melting enthalpy of an infinitely large crystal (93.7 J/g38), and wf is the weight percent of the PLLA matrix in the blend. Polarized Optical Microscope (POM). Crystal morphology was observed on a DMLP POM (Leica Microsystems GmbH, Germany) equipped with a THMS 600 hot stage (Linkam Scientific Instruments Ltd., UK). Before the observation, the specimen (∼20 μm in thickness) sandwiched between two microscope coverslips was first melted at 200 °C for 5 min to erase any thermal history and then rapidly cooled to 140 °C at a cooling rate of 50 °C/min for isothermal crystallization. The morphological evolution of growing crystals in the crystallization process was monitored by taking POM micrographs with a Cannon digital camera. X-ray Diffraction (XRD). Crystal structure was characterized using an X’Pert pro MPD X-ray diffractometer (PANalytical B.V., Holland) equipped with a Cu Kα radiation source (λ = 0.154 nm, 40 kV, 40 mA). The XRD patterns were 6273
DOI: 10.1021/acs.jpcb.7b03976 J. Phys. Chem. B 2017, 121, 6271−6279
Article
The Journal of Physical Chemistry B
Figure 3. SEM micrographs showing the phase morphologies of the blends: (a) PLLA/15EMA-GMA/0.06PDLA, (b) PLLA/15EMA-GMA/ 0.25PDLA, (c) PLLA/15EMA-GMA/0.65PDLA, (d) PLLA+15EMA-GMA/0.06PDLA, (e) PLLA+15EMA-GMA/0.25PDLA, (f) PLLA+15EMAGMA/0.65PDLA, (g) PLLA/15EMA-g-0.06PDLA, (h) PLLA/15EMA-g-0.25PDLA, and (i) PLLA/15EMA-g-0.65PDLA; scale bar = 2 μm.
190 °C, which is a suitable temperature for the in situ formation of SC crystallites in the blend melts.42 The formed SC crystallites can be easily detected by XRD and the XRD patterns of these blends as a function of PDLA concentration are shown in Figure 2. Obviously, for the blends without PDLA, only a broad amorphous halo can be observed. However, with the incorporation of 0.25 wt% PDLA, two weak but still visible diffraction peaks appear at 2θ values of 12.0° and 20.9°, which are ascribed to the (110) and (300)/ (030) planes of SC crystallites, respectively.38 Moreover, with further increasing PDLA concentration up to 0.95 wt%, not only does the intensity of the two characteristic diffraction peaks become stronger but also another characteristic peak appears at 2θ value of 24.0°, which corresponds to the (220) plane of SC crystallites,38 indicating an enhanced content of SC crystallites. It is worth noting that the content of SC crystallites in the blends with low PDLA concentrations (
