Achieving highly crystalline rate and crystallinity in Poly(l-lactide) via in-situ melting reaction with diisocyanate and benzohydrazine to form nucleating agents

Highlights

• High crystallinity was achieved in PLLA via the in-situ melting reaction.

• The in-situ modified PLLA exhibited high crystallization rate.

• Heat resistance performance of PLLA was greatly improved by the modification.

• High-efficient nucleating agents were formed in PLLA by the in-situ reaction.

• The in-situ modified PLLA was more toughness than that of neat PLLA.

Abstract

The drawback of the application for poly(_l-lactide) (PLLA) is the low crystalline rate and crystallinity obtaining via normal processing methods. Modifying crystallization of PLLA has been found to be an efficient way to improve its mechanical and heat resistance properties. In this wok, 4, 4′-diphenylmethane diisocyanate (M) and benzohydrazine (P) were employed into PLLA melt to in-situ form nucleating agents. The in-situ melting reaction was confirmed by a nuclear magnetic resonance spectroscopy. The crystallization behavior and crystalline morphology were investigated by a differential scanning calorimetry, a polarized optical microscopy and a field emission scanning electron microscope. The crystalline rate of PLLA was abruptly enhanced by adding (M+P) and melting reaction with PLLA. The crystallization half-time of PLLA dramatically decreased from 42.0 to 1.1 min at 130 °C by the in-situ formation of nucleating agents. The crystallinity of PLLA increased from 10.3 to 42.1 by adding 0.25% (M+P) and melting reaction for 8 min. Furthermore, the size of PLLA crystals was dramatically reduced because of the nucleating effect. Accompanied with improvement on crystallinity, the Vicat softening temperature of PLLA shifted from 57.4 °C to 93.7 °C by the in-situ reaction with 6.00% (M+P), and indicating heat resistance enhancement.

Introduction

As a bio-based and bio-degraded polymer, poly(_l-lactide) (PLLA) enjoys much popularities in both industrial and academic fields [[1], [2], [3], [4], [5], [6]]. Up to now, PLLA is recognized as one of the most pragmatic substitutes for fossil-based polymers in these fuel-consumption society. However, the inherent brittleness and low heat resistance temperatures of PLLA seriously limited its large-scale applications [[7], [8], [9]]. The slow crystalline rate and low crystallinity has been found to be the main reasons for these disadvantages. Therefore, increasing the crystallinity of PLLA has become the prerequisite of possessing other superior performances [[10], [11], [12]]. Generally, the crystallinity of PLLA is below 10% by the conventional processing techniques without annealing, such as injection molding, blow molding, extrusion, and foaming [[13], [14], [15]].

The addition of nucleating agents could enhance the crystallinity and crystalline rate of PLLA [[16], [17], [18]]. Some organic molecules have been found to have efficiently nucleating effect for PLLA [[19], [20], [21]]. For examples, a low molecular weight hydrazide compound, tetramethylenedicarboxylic di (2-hydroxy-benzohydrazide) (TMBH) has been reported to serve as effective nuclei and the degree of enhancing the crystallization rate as well as the nucleation density of PLLA [22]. The crystallization half-time of PLLA could reduce to 6.13 min by adding tetramethylenedicarboxylic dibenzoylhydrazide as nucleating agents at 130 °C [23]. Furthermore, the aromatic sulfonate derivative was added into PLLA to increase crystallinity content to 42%. As a result, the heat distortion temperature also increased from 55 to 85 °C [24]. In addition, some inorganic particles also exhibited nucleating effect for PLLA crystallization [[25], [26], [27]]. For example, the crystallinity of PLLA could increase from 2 to 25% by the incorporation of talc [28]. The crystallization rate of PLLA also enhanced by the addition of fumed silica nanofillers [29]. Furthermore, it was also found that the crystallographic relationships between the filler and the polymer crystals was also very important to enhance the nucleating effect of inorganic fillers [[30], [31], [32], [33]]. No matter the organic molecules or the inorganic fillers could provide a lower energy barrier for nucleation process thus endow PLLA crystallization at a higher crystallization temperatures [34]. However, the compatibility between PLLA and nucleating agents and the uniform dispersion of nucleating agents in PLLA were still the key issues for the addition of nucleating agents [35].

In-situ fabricating a nucleation agent to acquire a high-crystallinity PLLA product stands in overwhelming superiority, which is not only easy-processing, high-efficient, but also time-saving. However, reports upon “in-situ tailing crystallization” of PLLA mostly flocked in some polymer-to-polymer blending, like PLLA mixing with Poly(_d-lactide) (PDLA) [36,37], polyoxymethylene (POM) [38], poly [(R)-3-hydroxybutyrate] (PHB) [39], poly(-caprolactone) (PCL) [40] and polyamide-6 (PA6) [41] blending system, in which suggested that addition of polymer-additive in PLLA matric could act as nucleating agents and meanwhile significantly accelerate the crystallization rate. However, the uniform dispersion and good compatibility for the polymer blends remained the disadvantages.

Therefore, the nucleating agents were grafted from the PLLA chains by in-situ melting reaction was probably to overcome the issues of dispersion and compatibility between nucleating agents and PLLA matrix. Fortunately, diisocyanate which can react with the end group of PLLA chains has been well used to modify the mechanical properties of PLLA [[42], [43], [44], [45], [46]]. In this work, a small amount of diisocyanate and benzohydrazine were in-situ reacted with PLLA chains in melt state during processing. The PLLA with high crystallinity, high heat distortion temperature, and good mechanical properties was achieved by the in-situ melting reaction among PLLA, diisocyanate and benzohydrazine.