Superior Impact Toughness and Excellent Storage Modulus of Poly

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Superior Impact Toughness and Excellent Storage Modulus of Poly (lactic acid) Foams Reinforced by Shish-Kebab Nanoporous Structure Li-Hong Geng, Lengwan Li, Hao-Yang Mi, Bin-Yi Chen, Priyanka Sharma, Hongyang Ma, Benjamin S. Hsiao, Xiang-Fang Peng, and Tairong Kuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 11, 2017

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ACS Applied Materials & Interfaces

Superior Impact Toughness and Excellent Storage Modulus of Poly (lactic acid) Foams Reinforced by Shish-Kebab Nanoporous Structure

Lihong Geng1,2,3, Lengwan Li1, Haoyang Mi2, Binyi Chen2, Priyanka Sharma3, Hongyang Ma3, Benjamin S. Hsiao3, Xiangfang Peng1,2*, Tairong Kuang1,2*

1

National Engineer Research Center of Novel Equipment for Polymer Processing, The Key

Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Guangzhou, 510640, PR China 2

Department of Industrial Equipment and Control Engineering, School of Mechanical & Automotive

Engineering, South China University of Technology, Guangzhou, 510640, PR China 3

Department of Chemistry, Stony Brook University, Stony Brook, NY11794-3400, USA

*Corresponding Authors Xiangfang Peng, Email: [email protected]; Tairong Kuang, Email: [email protected] 1

ACS Paragon Plus Environment


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Abstract Poly(lactic acid) (PLA) foams, with the combination of shish-kebab and spherulite nanoporous structure in skin and core layer respectively, was prepared using a novel technique comprised of loop oscillating push-pull molding (LOPPM) and supercritical carbon dioxide low-temperature foaming process (Sc-CO2LTFP). The foams present superior impact toughness which is 6-fold higher than that of neat PLA, and no significant decrease was observed for the storage modulus. Moreover, Sc-CO2LTFP at soaking temperature ranging from 110°C to 150°C were performed to determine the evolution of pore morphology. The ultra-tough and super-moduli are unprecedented for PLA, and are in great need for broader applications.

Keywords: poly (lactic acid), shish-kebab, nanoscale pores, Sc-CO2LTFP (Supercritical carbon dioxide low temperature foaming process), soaking temperature.

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Poly(lactic acid) (PLA), the most widely applied biodegradable polymer derived from natural resources, possesses several exciting properties including renewability, high mechanical strength and relatively low cost.1 Hence, it has been considered as an excellent replacement of petroleum-based commodity polymers. Moreover, the biocompatible nature of PLA differentiated from other synthetic polymers made itself valuable in many areas of the medical field as drug delivery systems, artificial blood vessels and tissue engineering scaffolds.2 However, the inherent ability of crystallization in PLA, which mainly causes a reduction in crystallinity of PLA, leads to a decrease in strength, heat distortion temperature and gas barrier properties, and acts mainly as an obstacle for its applications in high performance products.3-4 Over the past decades, the most common approach to solve the crystallization issue of PLA, has been to incorporate nanoscale fillers into the polymer matrix. Here, the nanoscale fillers act as both reinforcements and heterogeneous nucleating agents to generally assist in enhancing the strength of the PLA matrix.5-8 Notably, the reinforcement of PLA using the incorporation of nanoscale fillers affects its biocompatibility and biodegradability properties, instigating major alterations in its innate behavior. It is known that dynamic molding techniques have been used to achieve self-reinforcement and high crystallization rate of polymer matrix such as polypropylene9 and polyethylene.10 In our previous studies, we have demonstrated a novel process: loop oscillating push–pull molding (LOPPM),11 by which a highly oriented and reinforced high impact polystyrene was successfully achieved due to more intensive flow field. In addition to the petroleum-based polymer, the self-reinforcement of PLA by means of a dynamic molding technique finds suitability and facileness due to not involving the use of other chemicals, and thereby guaranteeing its biocompatibility and 3



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biodegradability.12-14 A gradient two-fold increment in flexural strength was, for the first time, observed for PLA when it was processed through solid extrusion technology. Similarly, Li et al.

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prepared reinforced PLA using the oscillation shear injection molding technique and also observed a drastic increment in the strength of the PLA matrix due to the appearance of shish-kebabs and spherulite structures in the skin layer and the core of the PLA matrix, respectively. In fact, the dynamic molding technique has not been found effective in improving the toughness of the polymer, where the compression on the polymer matrix leads to a reduction of the space for molecular motion.10, 15 Therefore, it becomes more critical to deal with porous material, because porous materials are very tough thanks to a strong ability to absorb impact energy,16 which have found their advantages in applications as insulation, cushion, absorbents, especially weight-bearing structures such as furniture, transportation and packaging, having great demands on strength and toughness.17 Whereby, the supercritical carbon dioxide microcellular foaming (SC-CO2MF) technique has been widely applied in the manufacturing of porous materials,18 but it causes the reduction in tensile strength of porous material due to the development of large pores in polymer matrix.19 Recently, nanoscale porous polymers have also been developed using a supercritical carbon dioxide low temperature foaming process (SC-CO2LTFP), where the soaking temperature used is far below the melting temperature.20 Inspired by these processing techniques, this study aims to combine LOPPM and SC-CO2LTFP to prepare PLA foam with the combination of superior impact strength and high storage modulus properties. By combining these techniques, we have successfully designed tough nanoporous PLA foams with interlocked shish-kebab structure in the skin layer and spherulite morphology in the core layer. 4


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The standard type I dumbbell PLA samples were first prepared using self-designed loop oscillating push–pull molding (LOPPM), referred to as “LOPPM-PLA”, the schematic diagram and processing procedure is presented in Figure S1. For comparison, the regular PLA samples were also prepared using conventional injection molding (CIM), termed “CIM-PLA”. Samples prepared by LOPPM were foamed in a homemade batch foaming device. Differing from conventional SC-CO2 foaming processes, the LOPPM-PLA samples were soaked at a temperature far below melting temperature. The procedure of SC-CO2LTFP was shown in Figure S2 and the sample was named “LOPPM-PLA foam”. The novel processing technique, loop oscillating push–pull molding (LOPPM), deals with intensive shear force which causes changes in the arrangement of PLA crystal and was very effective in inducing shish-kebab structures.15, 21-22 SEM of the skin and core layers of etched LOPPM-PLA is presented in Figure 1(a) and Figure 1(d), respectively. SEM of the skin layer of etched LOPPM-PLA clearly depicts the presence of shish-kebab structure (Figure 1a), while SEM of the core layer of LOPPM-PLA indicates the existence of spherulites morphology (Figure 1d). The typical SAXS pattern with equatorial streaks and meridional lobes (inset right -Figure 1a) further confirmed the formation of shish-kebab structures in the skin layer of the PLA matrix. Moreover, the shish-kebabs arrange parallel to the ﬂow direction and are highly oriented, as indicated by the WAXD results (Figure 1b and 1c, Herman’s orientation factor is 0.31). The long period regarding lamellar spacing, calculated by the 1D-I(q) curve according to the Bragg equation, is 23.35 nm (Figure S4), while spherulites exist in a disordered state in the core layer (Figure 1e, and 1f, Herman’s orientation factor is 0.02). The interlocked shish-kebab structure in the PLA matrix has been found effective to improve the strength, 15 but it was not significant in the toughening of the sample. 5



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In this study, combined LOPPM and SC-CO2LTFP techniques have been applied to improve both impact strength and storage modulus of PLA. A schematic diagram of the combined technology is given in Figure 2a. It is explained that the intensive shear forces during the LOPPM process in the skin layer induced the orientation of long PLA chains and generated shish precursors along the flow direction, followed by growth of kebabs perpendicular to the shish. However, the shear force in the core was not found strong enough to induce or maintain the orientation of PLA chains, which resulted in the recovery of the chains into random coil. Therefore, spherulites form appeared in the core layer of LPPM-PLA. In the case of conventional SC-CO2 foaming techniques, the crystals start melting during the soaking step when the used soaking temperature is close to the melting temperature. Therefore, developed pores are large in size leading to low strength of PLA. However, for SC-CO2LTFP, the soaking temperature is far below melting temperature and hence SC-CO2 can only enter into amorphous regions without disturbing crystal regions. Thus, upon de-pressurizing, the crystal structure remains and pores start occupying the amorphous regions. SEM images of LOMMP-PLA foam (soaking temperature was 120 °C) in Figure 2b and Figure 2c indicate the presence of nanoscale pores in shish-kebab and spherulite forms in the respective skin and core layer of LOMMP-PLA foams, proving the feasibility of the SC-CO2LTFP viewpoint. Moreover, both crystallinity and degree of orientation have been improved for PLA foams (Figure S5 and S8). Most likely, the SC-CO2 treatment caused recrystallization of amorphous regions in PLA chains leading to more crystalline and ordered regions. And almost no degradation occurred during the PLA foams processing, indicated by similar weight loss temperature among PLA pellets, LOPPM-PLA and LOPPM-PLA foam in TGA analysis (Figure S6). Compared to CIM-PLA and LOPPM-PLA, LOPPM-PLA foam has exhibited outstanding 6


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toughness represented by impact strength as shown in Figure 3a. The impact strength of LOPPM-PLA measured was 3.2 KJ/m2, which was comparatively similar to the impact strength of CIM PLA of 3.8 KJ/m2. However, the impact strength observed for LOPPM-PLA foam was 20.5 KJ/m2, which is approximately six times more effective than CIM-PLA and LOPPM-PLA. Such a significant increment in the impact strength of PLA foams is attributed to the nanoscale pores, which cavitation relieves the triaxial stress state in front of the crack tip.23-24 The thermal mechanical properties of CIM-PLA, LOPPM-PLA and LOPPM-PLA foam are determined by DMA measurement using a damping parameter (tanδ) and storage modulus ( E ) which are depicted in Figure 3b and 3c. Compared to CIM-PLA, LOPPM-PLA foam showed a decrement in Tg corresponding to the peak of tanδ.25 LOPPM-PLA showed relatively high storage modulus E compared to CIM-PLA due to the presence of shish-kebab superstructure in LOPPM-PLA, which caused improvement in stiffness of PLA by suppressing deformation. Interestingly, even after the SC-CO2LTFP treatment, the E of the LOPPM-PLA foam was still found higher than CIM-PLA. Hence, the PLA foam with, combination of superior impact strength and excellent storage modulus properties, has been achieved by the combination of LOPPM and SC-CO2LTFP techniques. The melting temperatures (Tm) of LOPPM-PLA and LOPPM-PLA foam were measured using DSC, given in Figure S7. Compared to LOPPM-PLA, the Tm of LOPPM-PLA foam increased in both the skin and core layers, as recrystallization of PLA chains during soaking treatment contributed to formation of more ordered crystals. For SC-CO2LTFP, the soaking temperatures were maintained below Tm ranging from 110 °C to 150 °C. The evolution of pore morphologies of LOPPM-PLA foams soaked at different temperatures was presented in Figure 4. At the soaking temperature of 110 °C, the morphology of LOPPM-PLA 7



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foam indicates the presence of a non-foamed amorphous region. Whereby, the amorphous regions provided enough space to nucleate and further the growth of pores.26 However, the small pores only presented in the interlamellar regions having pore size less than 50 nm, which were found to be arranged along the flow direction. Therefore, it was worthwhile to further increase the soaking temperature to 120 °C. Interestingly, at the soaking temperature of 120 °C, most of the amorphous regions were occupied by pores. Also, shish-kebab and spherulite structured nanoscale pores were observed in the skin and core layer of PLA, where pores grew in the amorphous space between kebabs in the skin layer, while the pores in the core layer oriented along radial directions from center to the edge of spherulites. To further increase soaking temperature to 130 °C, the PLA crystal partially started melting, resulting in deformation and bending of crystal lamellae. The most probable reason is the collision of CO2 during depressurization. In this case the pore size obtained was large due to molten crystal which provided more sites for pores growth. Similarly, at the soaking temperature of 140 °C, most of the crystalline regions were melted because of the plasticization of SC-CO2. However, the pores in the skin layer were still oriented along the flow direction. The open pore structure in the core layer offers the potential application as a tissue engineer scaffold.27 At the soaking temperature of 150 °C, the PLA lost its melt strength, as a result, the pore wall was collapsed and broken. DSC thermograms of LOPPM-PLA foams in Figure S9 showed that both crystallinity and melting temperature increased with an increment of soaking temperature due to the recrystallization during SC-CO2 treatment, resulting in the significantly thickened and ordered lamellae.28 It was followed by a decrement with further increase in soaking temperature, because the lamellae was destroyed during depressurization due to the low strength at high temperature (140 °C and 150 °C). Compared to the skin layer, the broad melting peak in core layer is attributed to a broad 8


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distribution of lamellae thickness. In summary, PLA samples, which have selectively shish-kebab and spherulite forms in the respective skin and core layers, were prepared using a self-designed LOPPM device. Subsequently, SC-CO2 LTFP treatment assisted in producing the nanoscale pores in shish-kebab and spherulite structures. It was observed that the developed nanopores have occupied the amorphous regions in PLA chains because SC-CO2 could not dissolve the crystal region at a soaking temperature below melting temperature. Compared to CIM-PLA and LOPPM-PLA samples, the LOPPM-PLA foam exhibited superior impact strength while maintaining a high storage modulus. It is confirmed that the simultaneously superior impact strength and high storage modulus of PLA foams can be achieved by the combination of LOPPM and SC-CO2 LTFP techniques. The evolution of pore morphology was studied by using different soaking temperatures ranging from 110 °C to 150 °C. It is found that the optimum soaking temperature to obtain well defined shish-kebab and spherulite nanoscale pores in the skin and core-layer respectively is 120 °C. To further increase soaking temperature, the pore size gradually increased and the shish-kebab structured nanoscale pores transformed into microscale pores with thin pore walls due to the melting of crystal regions.

Acknowledgements The authors gratefully thank the National Natural Science Foundation of China (NO. 51573063), the Guangdong Natural Science Foundation (NO.S2013020013855), the Guangdong Science and Technology Planning Project (NO. 2014B010104004 and 2013B090600126), and National Basic Research Development Program 973 in China (NO. 2012CB025902). We also thank the Shanghai Synchrotron Radiation Facility (SSRF), Beamline BL16B for supporting X-ray measurements. 9



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Tairong Kuang would like to acknowledge the support of National Postdoctoral Program for Innovation Talents (NO. BX201700079). Lihong Geng would like to thank the Chinese Scholarship Council for their financial support. Supporting Information Materials, detailed methods and characterization, schematic diagram of loop oscillating push-pull molding device (Figure S1) and foaming process (Figure S2), SEM analyses of LOPPM-PLA and LOPPM-PLA foam samples (Figure S3), 1D-SAXS curves of the skin layer of LOPPM-PLA sample (Figure S4), 2D-WAXD pattern of LOPPM-PLA foams sample (Figure S5), TGA analysis and detailed data of neat PLA, LOPPM-PLA, and LOPPM-PLA foam (Figure S6 and Table S1), DSC analysis and crystallinity of LOPPM-PLA and LOPPM-PLA foams (Figure S7 and S8), and DSC thermograms curves and detailed data of LOPPM-PLA foams in the skin and core layer, respectively, at different soaking temperature (Figure S9 and Table S2).

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Figure 1. (a) SEM images of etched LOPPM-PLA (Inset right represents: 2D-SAXS pattern); (b) 2D-WAXD pattern; (c) azimuthal intensity distribution in skin layer; (d) SEM images of etched LOPPM-PLA; (e) 2D-WAXD pattern; (f) azimuthal intensity distribution in core layer.

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Figure 2. (a) Schematic diagram of combined LOPPM and Sc-CO2 LTFP; SEM images of LOPPM-PLA foam soaked at 120 °C in (b) skin layer and (c) core layer .

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Figure 3. (a) Impact strength of CIM-PLA, LOPPM-PLA and LOPPM-PLA foam; (b) the damping parameter (tanδ); (c) storage modulus (E' ) in DMA measurement.

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Figure 4. SEM images of the morphology of LOPPM-PLA foams soaked at: 110 °C (a: skin, b: core); 120 °C (c: skin, d: core); 130 °C (e: skin, f: core); 140 °C (g: skin, h: core); 150 °C (i: skin, j: core).

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Table of Contents (TOC) graphic

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Superior Impact Toughness and Excellent Storage Modulus of Poly

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