Enhancing toughness of poly (lactic acid)/Thermoplastic polyurethane blends via increasing interface compatibility by polyurethane elastomer prepolymer and its toughening mechanism
Introduction
Due to the environmental protection and energy crisis caused by the petroleum-based polymer, biodegradable polymers have attracted widespread attention in recent years [[1], [2], [3], [4], [5], [6], [7]]. Polylactic acid (PLA), as an aliphatic polyester derived from renewable resources with stiffness and high strength, excellent transparency, good biodegradability and biocompatibility, is widely used in biomedicine for surgical sutures, orthopedic fixation materials, drug delivery, tissue culture and drug packaging, etc. [[8], [9], [10], [11]] For most plastic products, sufficient safety must be ensured in case of falling and collision. Unfortunately, the poor impact resistance of PLA has restricted its broader application in the field of general plastics and engineering plastics [3,[12], [13], [14]]. Thus, lots of researchers tried to develop a tough PLA blend, for example, by facilitating PLA-based copolymers with complex architectures such as deblock copolymers, triblock copolymers, multiblock copolymers, star-shape block copolymers, comb-shape block copolymers and various PLA-grafted structures [[15], [16], [17], [18], [19], [20], [21], [22], [23]], or blending several flexible polymers, such as ethylene-co-vinyl acetate (EVA) [24], poly [(butylene succinate)-co-adipate] (PBSA) [25], poly (ether) urethane (PEU) elastomer [13], poly (caprolactone) (PCL) [26], natural rubber (NR) [27], and so on. However, in the absence of specific compatibilization methods, most polymer blends are incompatible and have obvious phase separation, which generally resulted in the brittleness and poor mechanical performances. Therefore, improving the compatibility of components in blends is very important approach for preparing high-performance polymer alloys. Sui et al. [28] using ethylene−methyl acrylate-glycidyl methacrylate (EGMA) terpolymer as interface compatibilizer to enhance the interface compatibility of PLA/olefin block copolymer (OBC) blends and obtained super-toughened PLA/OBC/EGMA ternary blends. Palai et al. [29] obtained PLA/thermoplastic starch (TPS) blends with excellent comprehensive properties via in-situ reactive compatibilization process in the presence of benzoyl peroxide (BPO) using a twin screw extruder. Zolali et al. [30] significantly improved the impact strength of PLA/linear low density polyethylene (LLDPE) blends via introducing EGMA as an interface compatibilizer. It is obvious that the PLA/flexible polymer blends could be toughened effectively by introducing appropriate interfacial compatibilizer.
Due to the flexibility and biocompatibility, thermoplastic polyurethane (TPU) is an ideal elastomer material and has been widely used in various fields [6,7,11,31]. The TPU elastomer consists of thermodynamically incompatible hard segments (diisocyanate and chain extender) and soft segments (high-molecular-weight polyether or polyester) [32,33]. For TPU elastomer, the hard segments provide high modulus, tear strength and hardness, and soft segments controls the toughness, respectively. According to published literatures, TPU elastomer would have a better compatibility with PLA due to its polyether-based or polyester-based soft segment [[34], [35], [36]]. However, most the reported PLA/TPU blends usually fail to achieve the theoretical high toughening effect, which may be related to the relatively easy formation of interface defects between components with large property differences. Therefore, in order for PLA/TPU blends to achieve the ultra-high toughness, it is very necessary to enhance the interface interaction between TPU elastomer and PLA resin, such as crosslinking, copolymerization or adding compatibilizers.
In our previous work [37], we found that the reactive isocyanate groups (−NCO) terminated polyurethane elastomer prepolymer (PUEP) was an good toughening agent for PLA to improving the impact toughness via dynamic vulcanization. The –NCO groups from PUEP and –OH groups from the PLA reacted effectively, so as to obtain the super-toughened PLA/polyurethane (PU) blend material with strong interface interaction. However, the direct use of a large amount of low viscosity PUEP as toughening component would lead to processing difficulties due to greatly reducing the processing efficiency. If only a small amount of PUEP is used as a compatibilizer for PLA and TPU components, the reactive groups in PUEP would reactive both PLA and PU, which could significantly enhance the interface interaction of PLA and PU and therefore to obtain the super-toughened PLA/TPU/PUEP blends theoretically. Compared with the PLA/TPU blends without PUEP, the processing efficiency will not be significantly affected. To the best of our knowledge, there is no literature for such similar toughening route. Thus, a super-toughened PLA/TPU/PUEP ternary blend is prepared by introducing the reactive –NCO group terminated PUEP as interfacial compatibilizer via reaction extrusion, and the morphologies, mechanical performances and thermal properties of the obtained PLA/TPU/PUEP ternary blends were investigated in detail. In addition, the dynamic impact fracture process was also analyzed by instrumented impact test in order to study the corresponding toughening mechanism of this system.
Section snippets
Raw materials
PLA (4032D) was obtained from Nature Works LLC (Minnetonka, MN). TPU (WHT1195, polyester type) was obtained from Yantai Wanhua Co., Ltd. (Yantai, China). PUEP (HC-5285, the content of reactive –NCO group: 4.5–4.8%, the melting temperature: 70 °C) was obtained from Shanghai Haksong Polymer Technology Co., Ltd. (Shanghai, China). Dibutyl tin dilaurate was obtained from Shao Yu Chemical Co., Ltd. (Guangzhou, China).
Sample preparation
The PLA/TPU/PUEP blends with various weight ratios (w/w/w, 70/30/0, 70/29.5/0.5,
Chemical reaction
Polyurethane reaction is an important organic synthesis reaction [39]. At the same time, isocyanate-terminated polyurethane prepolymer, such as isocyanate curing agent, isocyanate coating and isocyanate cross-linking agent, and so on, is also widely used in many application areas [37]. Unsaturated isocyanate groups (−NCO) exhibit good chemical reactivity. In the current system, the reactive –NCO groups from PUEP could efficiently react with the end –OH groups from PLA and formed urethane bonds
Conclusions
The PLA/TPU/PUEP blends with various PUEP loadings have been obtained and studied. The introduction of reactive compatibilizer PUEP could super toughened PLA/TPU/PUEP blends where the notched impact strength increases rapidly from 11.4 kJ/m2 to 81.3 kJ/m2. With the introduction of PUEP, there is only minor changes in relative crystallinity of PLA component observed. In addition, combining the notched impact results and the morphology of cryo-fracture surface and impacted fracture surface of the
CRediT authorship contribution statement
Hai-Chen Zhang: Data curation, Writing - original draft. Ben-hao Kang: Visualization, Investigation. Le-Shan Chen: Supervision, Writing - review & editing. Xiang Lu: Conceptualization, Writing - review & editing.
Acknowledgement
We acknowledge the Key Program of National Natural Science Foundation of China (Grant Nos. 51903092 and 21908031), the China Postdoctoral Science Foundation funded project (Grant No. 2019M652884), the Fundamental Research Funds for the Central Universities (Grant No. 2019MS059).
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These authors contributed equally to this work.