Development of in situ nanofibrillar poly (lactic acid)/poly (butylene terephthalate) composites: Non-isothermal crystallization and crystal morphology

Highlights

• Tuned melt spinning process of PLA/PBT blends succeeded to in situ nanofibrillar morphology.

• Very small size crystals of PLA oriented in PBT NFs direction.

• Significant improvement of crystallinity and crystallization rate of PLA in NFCs was obtained.

• Population of PLA crystal forms nucleated during cold and melt crystallization were discussed.

• Crystallization behavior of NFCs was compared with PLA/PBT blends.

Abstract

Tuned one-step melt spinning of poly (lactic acid) (PLA)/poly (butylene terephthalate) (PBT) blends containing 1–10 wt% of PBT, was carried out successfully to induce in situ nanofibrils of PBT with average diameter of 23–51.4 nm. Scanning electron microscopy (SEM) images showed the morphology of PBT nanofibrils (NFs) in the PLA matrix and it was found that the aspect ratio of PBT NFs was more than 400. The crystal morphology of the PLA melt spun fibers in the presence and absence of PBT NFs was analyzed using polarized optical microscopy. Dramatic changes in the crystal morphology of PLA were observed in the presence of PBT NFs. Scattered spherulites in the PLA fibers changed to very small crystals oriented in the PBT NFs direction in the PLA/PBT fibers. Non-isothermal crystallization studies of PLA in the in situ nanofibrillar composites (NFCs) prepared by compression molding demonstrated a significant improvement of crystallinity (27.5%) as well as crystallization rate. Enhanced crystallization rate provided comparative conditions to study the crystal forms of PLA and their relative populations nucleated during the cold and melt crystallizations. Moreover, the crystallization behavior of PLA/PBT blend with droplet-matrix morphology was compared with that of the NFC sample at the same PBT content to emphasize the tremendous effect of the morphological tailoring of blends.

Introduction

Environmental concerns and our increasing dependence on plastics have devoted many researchers’ efforts to discover new methods of synthesizing, modifying and processing biodegradable polymers from sustainable resources [1]. Poly (lactic acid) (PLA) as a biodegradable and biocompatible semi-crystalline polyester can be produced from oil derivatives and renewable resources such as corn and sugar [2]. Its clarity, mechanical, thermal and processing properties, compared to those of the traditional polymers such as polystyrene, polypropylene and polyethylene, have made scientists hopeful to come true the dream of an environmentally friendly alternative for oil derivative polymers [3]. However, so far, some of PLA’s properties such as slow crystallization kinetics, low melt strength, brittleness and higher cost have delayed the realization of this goal [4].

Melt blending of polymers, with specifically tailored morphology has been studied as a promising method to gain satisfactory improvements in the different demanding properties such as thermal, mechanical and electrical properties [5], [6], [7]. In situ microfibrillar composites (MFCs) have been stablished by Evstatiev et al. for producing the polymer composites consisting of binary and ternary blends (e.g. poly (ethylene terephthalate) (PET), polyamide-6 (PA6) and poly (butylene terephthalate) (PBT)) to improve their crystallization behavior and mechanical properties with a designed morphology [8]. In this technique, a well-distributed polymer blend is compiled from two polymers (the major phase A as a matrix and the minor phase B as a dispersed phase with the melt temperature of T_mA and T_mB) by using conventional melt blending equipment. The melt passes through a spinneret die and forms fibers by hot stretching. Meanwhile, the minor phase in the blend is oriented in the die direction and subsequently transforms to highly oriented and high aspect ratio microfibrils [9], [10]. To transfer the fibers to isotropic or oriented MFCs (e.g. laminates), the spun fibers which contain the microfibrils are chopped and then processed (e.g. compression molding) in a temperature window (T_process) between the melt temperature of the two polymers (T_mA < T_processing < T_mB). Three key requirements must be fulfilled in order to manufacture the MFCs which are: (i) the polymer components should have a sufficient drawability for the formation of reinforcing microfibrils; (ii) processing of the polymers must be possible at a single temperature without significant degradation of one of the constituents; and (iii) the melting temperature of the reinforcing polymer should be at least 40 °C higher than the melting temperature of the matrix, to guarantee constancy of the microfibrils during the matrix consolidation [11]. On the other hand, for the preparation of MFCs, the two components should not be miscible, but should be highly compatible. The interactions of the two polymers at their interface, the viscosity ratio and the processing conditions determine the deformability of the droplets [12]. MFCs of polypropylene (PP)/PET were investigated and PET microfibrils of 200 nm diameter has been achieved by Fakirov et al. [13]. Dramatic effects of the PET fibrillar morphology on the kinetics of crystallization, the crystallization morphology and mechanical properties of the PP composites are reported [13], [14], [15]. The substantial advantage of MFCs is that the reinforcing microfibrils are created in situ; therefore, the main problems in preparation of micro and nanocomposite, namely achieving the proper dispersion of the particles and nanotoxicity, can be eliminated using an environmental friendly and cost effective process [16], [17], [18].

The strong dependency of physical and chemical properties of PLA to its crystallinity, crystallization kinetics and morphology has motivated the researchers to carry out intense investigations in this field and to incorporate different kinds of additives in order to benefit from their features [19], [20], [21], [22], [23]. The crystallinity percentage of PLA is intrinsically a function of its two enantiomeric isomers ratio (L-lactide to D-lactide) [24]. Poly (L-lactide lactic acid) (PLLA) crystal forms are known as α, β and γ [25]. The disorder $\alpha$ form (${\alpha }^{\text{'}}$) was proposed for a PLLA sample crystallized below 120 °C [26]. The $\alpha$ and ${\alpha }^{\text{'}}$ forms are structurally similar, but ${\alpha }^{\text{'}}$ form is smaller and has looser chain packing. Zhang et al. explained that during heating runs of PLLA in differential scanning calorimetry (DSC) measurements, the small exothermal peak corresponds to the transition of ${\alpha }^{\text{'}}$to $\alpha$ which makes these two kinds of crystals distinguishable [26], [27]. The phase separation and mechanical properties of immiscible PLLA/PBT blends with droplet-matrix and co-continuous morphology were studied [28]. Compatibility of the two polymers was described by interactions between the functional groups of the two polyesters. A higher amount of crystallinity and improved crystallization rate were reported using PBT concentrations higher than 10 wt% for both morphologies [29]. Samthong et al. investigated the effect of size and shape (droplets or in situ microfibrils) of PBT phase on the crystallization kinetics of the PLA matrix [30]. The enhanced crystallization and crystallization rate for both morphologies were reported due to the nucleating effects of minor phase. However, no effect of PBT morphologies on the nucleation mechanism was observed. Similar observations for PLA/PA6 and PP/PET blends and MFCs were published, emphasizing a heterogeneous crystal nucleation effect of the interface [9], [31].

Different types of nanoparticles have been reported to be efficient to increase the crystallization rate and/or crystallinity of PLA as nucleating agents [32], [33], [34]. Comparing the different types of nanoparticles, MWCNTs are claimed to be the most efficient additives to boost the crystallization rate of PLA [35]. Although all of the particles provide a surface with a reduced energy barrier for nucleation, the MWCNTs are superior due to their high specific surface areas and large aspect ratios [36], [37].

The aim of this research is to implement a one-step tuned melt spinning process to develop in situ nanofibrils of PBT in the PLA matrix with diameter and aspect ratio comparable to those of MWCNTs. The work studies the crystal morphology and non-isothermal crystallization characteristics of the developed system. The variation of the PLA crystal morphology is investigated in the presence of PBT NFs using the polarized optical microscopy. Significant enhancement of crystallization degree and crystallization rate is used to make a quantitative comparison of the PLA crystal forms (${\alpha }^{\text{'}}$ and $\alpha$) population nucleated during the cold and melt crystallizations. The dramatic effect of nanofibrillar morphology of PBT on PLA crystallization is compared to the case when PLA matrix contains spherical PBT domains. This study could be a foundation to industrialize production of non-toxic polymeric nanotubes without sacrificing the biodegradability of the systems.