Development of in situ nanofibrillar poly (lactic acid)/poly (butylene terephthalate) composites: Non-isothermal crystallization and crystal morphology
Graphical abstract
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 TmA and TmB) 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 (Tprocess) between the melt temperature of the two polymers (TmA < Tprocessing < TmB). 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 form () was proposed for a PLLA sample crystallized below 120 °C [26]. The and forms are structurally similar, but 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 to 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 ( and ) 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.
Section snippets
Materials
PLA under the trade name of 2003D comprising ~4.5% of D-Lactide content and with a melt flow rate of 6 g/10 min (210 °C/2.16 kg) was purchased from Nature Works (U.S.A.). The values of its weight-average molecular weight (Mw), number-average molecular weight (Mn) and polydispersity index (PDI) were measured as 2.08 × 105, 1.35 × 105 g/mol and 1.54, respectively. PBT (Pocan B1300) with melting temperature of 225 °C and melt volume rate of 45 cm3/10 min (250 °C/2.16 kg) was kindly provided by
Solid state NMR spectra
CP-MAS 13C NMR spectra of neat PLA, neat PBT and NFC10 were derived in order to detect possible new chemical peaks and/or shifts which present special new groups or interactions generated by ester exchange reaction during the extrusion (Fig. 2B) [41]. Three peaks for PLA were observed which are corresponding to carbonyl, methine, and methyl groups at 170.07, 69.8 and 17.4 ppm, respectively [42], [43]. Carbon groups of PBT which are assigned as (a) to (e) on the structural schematic in Fig. 2A
Conclusion
Melt spinning of PLA/PBT blends containing 1, 3, 5 and 10 wt% of PBT resulted in the formation of nanofibrils of the minor phase in the PLA matrix at the optimized processing conditions. The microscopy results showed that the average diameter of the nanofibrils changed from 23 to 51.4 nm for the varying contents of the PBT and the aspect ratio of the PBT NFs was more than 400.
Hot stage polarized optical microscopy images demonstrated a pronounced nucleating effect of the PBT NFs regarding the
CRediT authorship contribution statement
Mahboobeh Shahnooshi: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Azizeh Javadi: Supervision, Project administration, Resources, Writing - review & editing. Hossein Nazockdast: Conceptualization, Resources. Volker Altstädt: Supervision, Project administration, Resources, Funding acquisition, 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.
Acknowledgement
We gratefully acknowledge the use of equipment and assistance offered in the Keylab “Small Scale Polymer Processing” of the Bavarian Polymer Institute at the University of Bayreuth. We appreciate the Northern Bavarian NMR Center for providing us solid state NMR results with the equipment in the University of Bayreuth. This research was funded by the German Research Foundation (DFG), grant number of AL474/33.
References (57)
- et al.
Poly (lactic acid) fiber: an overview
Prog. Polym. Sci.
(2007) - et al.
Microfibrillar reinforcement of polymer blends
Polymer
(1992) - et al.
Tuning viscoelastic and crystallization properties of polypropylene containing in-situ generated high aspect ratio polyethylene terephthalate fibrils
Polymer
(2015) - et al.
Tensile properties of poly (ethylene terephthalate) and polyethylene in-situ microfiber reinforced composite formed via slit die extrusion and hot-stretching
Mater. Lett.
(2002) - et al.
Nanofibril reinforced composites from polymer blends
Colloids Surf. A
(2008) - et al.
On transcrystallinity in semi-crystalline polymer composites
Compos. Sci. Technol.
(2005) - et al.
Nanotechnology: advantages and drawbacks in the field of construction and building materials
Constr. Build. Mater.
(2011) - et al.
Uneven distribution of nanoparticles in immiscible fluids: morphology development in polymer blends
Polymer
(2009) - et al.
Polylactide (PLA)-based nanocomposites
Prog. Polym. Sci.
(2013) - et al.
Poly (lactic acid) crystallization
Prog. Polym. Sci.
(2012)
Temperature dependent variations in the lamellar structure of poly (L-lactide)
Polymer
Miscibility and properties of poly (l-lactic acid)/poly (butylene terephthalate) blends
Eur. Polym. J.
Poly (lactic acid)-based in situ microfibrillar composites with enhanced crystallization kinetics, mechanical properties, rheological behavior, and foaming ability
Biomacromolecules
Nano-biocomposites: biodegradable polyester/nanoclay systems
Prog. Polym. Sci.
Effect of nucleation and plasticization on the crystallization of poly (lactic acid)
Polymer
PLA nanocomposites: effect of filler type on non-isothermal crystallization
Thermochim Acta
Nanoscale artifacts in RuO4-stained poly (styrene)
Polymer
Influence of different beta-nucleating agents on the morphology of isotactic polypropylene and their toughening effectiveness
Polymer
Enthalpy of melting of α′-and α-crystals of poly (l-lactic acid)
Eur. Polym. J.
Super-tough poly (lactic acid) materials: Reactive blending with ethylene copolymer
Polymer
Morphology, thermal behavior and mechanical properties of binary blends of compatible biosourced polymers: Polylactide/polyamide11
Polymer
Formation and morphology of cellulose acetate butyrate (CAB)/polyolefin and CAB/polyester in situ microfibrillar and lamellar hybrid blends
Eur. Polym. J.
Morphology, static and dynamic mechanical properties of in situ microfibrillar composites based on polypropylene/poly (ethylene terephthalate) blends
Compos. A Appl. Sci. Manuf.
Electrically conductive carbon black (CB) filled in situ microfibrillar poly (ethylene terephthalate)(PET)/polyethylene (PE) composite with a selective CB distribution
Polymer
Melting and recrystallization kinetics of poly (butylene terephthalate)
Polymer
Sustainable Polymers: Opportunities for the Next Decade
Poly (lactic acid): Synthesis, Structures, Properties, Processing, and Applications
Super toughened and high heat-resistant poly (lactic acid)(PLA)-based blends by enhancing interfacial bonding and PLA phase crystallization
Ind. Eng. Chem. Res.
Cited by (17)
In situ nanofibrillar fully-biobased poly (lactic acid)/poly (ethylene 2,5-furandicarboxylate) composites with promoted crystallization kinetics, mechanical properties, and heat resistance
2022, Polymer Degradation and StabilityCitation Excerpt :Intensive studies have demonstrated that the formation in situ nanofibrils could improve the mechanical properties of single polymer due to their large aspect ratio [26]. Furthermore, the fibers could work as nucleating agent and provide large amounts of nucleation site for crystals, which could significantly promote the crystallization ability of PLA [23,27,28]. For instance, the PLA's crystallization half-time (t1/2) (time required to achieve 50% of the final crystallinity) was about1412 s.
Entirely environment-friendly polylactide composites with outstanding heat resistance and superior mechanical performance fabricated by spunbond technology: Exploring the role of nanofibrillated stereocomplex polylactide crystals
2021, International Journal of Biological MacromoleculesCitation Excerpt :The SC-PLA70 simple blend showed a cold crystallization temperature, TCC, around 110 °C. TCC shifted to lower temperature, around 102 °C, after spinning at 190 °C and 230 °C, indicating that the crystallization kinetics is enhanced by spinning [79]. The SC-PLA70s simple blend also showed a TCC around 110 °C, whereas TCC was shifted to 102 °C for the SC-PLA70s spun at 190 °C, and almost no cold crystallization peak was noted for the SC-PLA70s spun at 230 °C.
3D fibrillated network of compatibilized linear low density polyethylene/polyamide with high melt strength and superior foamability
2021, PolymerCitation Excerpt :This technology has been exercised in various polymer/polymer composites in which the polymer matrix itself suffers from poor melt strength, and hence its use for foam applications is restricted. This approach showed significant efficacy to enhance the foaming performance of various composites such as PE/polypropylene (PP) [24], thermoplastic polyurethane/polytetrafluoroethylene (PTFE) [19], PP/PTFE [25,26], PP/polyethylene terephthalate [27,28], PP/polybutylene-terephthalate [29], polylactide/polyamide 6 (PA6) [30], polylactide/polybutylene-terephthalate [31–33], and polylactide/PTFE [34] through increasing their rheological performance and crystallinity. PA is among the most common candidates when blending PE with another component and is considered to achieve a unique combination of properties.
Rheological rationalization of in situ nanofibrillar structure development: Tailoring of nanohybrid shish-kebab superstructures of poly (lactic acid) crystalline phase
2020, PolymerCitation Excerpt :Interestingly NHSK and needle-like crystal morphology of PLLA was formed around melt-soluble self-assemblies molecules which act as strong nucleating agents [55]. As reported in our previous work, a unique structure of PBT nanofibrils with dimensions similar to those of well-dispersed MWCTs was generated in situ within a PLA matrix using a simple one-step melt spinning process [56]. The main objective of this work was to explore the role of viscoelastic behavior of blend components on the droplet deformation in shear flow and its influence in determining the morphology development in the elongational flow thereafter.
Highly toughened poly(lactic acid) (PLA) prepared through melt blending with ethylene-co-vinyl acetate (EVA) copolymer and simultaneous addition of hydrophilic silica nanoparticles and block copolymer compatibilizer
2020, Polymer TestingCitation Excerpt :The blending technique is the most economic, effective, and practical method. Various biodegradable or nonbiodegradable polymeric toughening agents, such as acrylonitrile–butadiene–styrene copolymer (ABS) [12], poly(caprolactone) (PCL) [4], polyamide11 (PA11) [13], natural rubber (NR) [14], poly (butylene terephthalate) (PBT) [15], thermoplastic polyurethane (TPU) [16], polyhydroxyalkanoate (PHA) [17], and ethylene-vinyl acetate (EVA) copolymer [18] have been blended with PLA. However, the relatively poor interfacial interaction between PLA and most of the common elastomers has resulted in rather low toughening efficiency.