Effect of Halloysite Nanotube Loading on Structur

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Effect of Halloysite Nanotube Loading on Structure and Properties of Poly(l-lactic acid) / Poly-(butylene succinate)

Blend

Journal: Polymer Composites

Manuscript ID PC-15-1865

Wiley - Manuscript type: Research Article

Date Submitted by the Author: 27-Oct-2015

Complete List of Authors: Liu, Huan-Yu; South China University of Technology, National Engineering

Research center of Novel Equipment for Polymer Processing

Meng, Cong ; South China University of Technology, National Engineering Research center of Novel Equipment for Polymer Processing

Wu, Ming-Kun; South China University of Technology, National Engineering Research center of Novel Equipment for Polymer Processing

Chen, Rong-yuan ; South China University of Technology, National

Engineering Research center of Novel Equipment for Polymer Processing Lu, Xiang; South China University of Technology, National Engineering Research center of Novel Equipment for Polymer Processing

Yin, xiaochun; Key Laboratory of Polymer Processing Engineering of Ministry of Education, south china university of technology,

Qu, Jin-Ping; South China University of Technology, The National

Engineering Research Center of Novel Equipment for Polymer Processing

Keywords: PLA/PBS blend, Halloysite nanotube loading, Structure, Properties

Polymer Composites

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w Effect of Halloysite Nanotube Loading on Structure and Properties of Poly(l-lactic acid) / Poly-(butylene succinate)

Blend

Huan-yu Liu, Cong Meng, Ming-kun Wu , Rong-yuan Chen, Xiang Lu, Xiao-Chun Yin, Jin-ping Qu The Key Laboratory of Polymer Processing, Ministry of Education, National Engineering Research center of Novel Equipment for Polymer Processing, South China University of Technology,

GuangZhou 510640, China.

Please Corresponding to: Xiao-Chun Yin; e-mail: mxcyin@https://www.wendangku.net/doc/88136094.html, , Jin-ping Qu; e-mail: jpqu@https://www.wendangku.net/doc/88136094.html,,

ABSTRACT:

Poly(l-lactic acid) (PLA) / poly-(butylene succinate) (PBS) were prepared via melting compounding. The weight ratio of PLA to PBS was set at a constant value of 70:30, whereas the weight fraction of Halloysite nanotube (HNT) was varied from 0 to 12 wt. %. Tensile properties showed dependence on HNT content. Both tensile strength and elongation at break increased initially for the nanocomposites within 6 wt. %, and then monotonically declined with the introduction of more HNT. Overall, the optimum tensile property was found for the composite with 6 wt.% of HNT. SEM and TEM revealed that a small incorporation of HNT (≤6 wt.%) can well disperse in two polymer matrix while luxurious HNT would highly aggregated. As demonstrated under dynamic mechanical analysis (DMA), the introduction of HNT led to enhancement of storage modulus and decreased the difference of shift glass transition temperature between PLA and PBS. On the other hand, thermal analysis detected that HNT acted as a nucleating agent and enhanced the crystallinity of PLA and PBS. Additionally, rheological properties revealed that PLA/PBS/HNT composites showed unique shear-thinning behavior, and HNT was found to significant increase complex viscosity and storage modulus of the composites.

Keywords: PLA/PBS blend; Halloysite nanotube loading; Structure; Properties INTRODUCTION

Due to increasing environmental concerns, biodegradable polyesters are receiving considerable attention. In competition of biodegradable polyester, the polymers (PLA and PBS)

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w originated from renewable resource have been extensively investigated due to environmental protection [1]. In particular, PLA has attractive properties, such as good strength, good biocompatibility and barrier properties [2]. For these reasons PLA is expected to be an interesting candidate for biomaterials and construction materials in near future [3]. However, PLA’s poor flexibility, high price and low heat resistance are major obstacles for the possibility of commercial applications, particularly for packaging industries [4-6]. In this respect, blending PLA with other flexible and biodegradable polyesters such as polycaprolactone (PCL)[7] and poly(butylene succinate)PBS[8-10] could overcome the brittleness of PLA in recent studies. More often, other polymer blend eventually leads to deterioration in some properties since PLA is immiscible with PBS. Use of readily available and cheaper clays is a novel way of stabilizing two components. Through the introduction of clay, it is predictable that clays could not only stabilize the blend morphology, but also influence the properties of the polymer blends. Therefore, various attempts are targeted to enhance the morphology of polymer blends and hence improve the flexibility as well as the crystallization properties and thermal stability of PLA/PBS composites by the incorporation of organo-modified layered silicates (OMMT), carbon nanotubes (CNT). Ojijo [8] investigated the properties PLA/PBS blends by varying OMMT concentration and demonstrated the significance of the clay content and localization on the properties of the PLA/PBSA blends. Also, Yu et al [11]investigated other PLLA/PCL/OMMT blends. In that case, certain properties were enhanced but the improvement depended on the clay loading.

HNT have recently received consideration attention as a new type of nanofiller to enhancing the mechanical, thermal and other specific properties of different polymers. As demonstrated elsewhere, the composition of HNT is similar to kaolin. HNT had predominantly hollow tubular structure with nanoscale lumen, relatively high length-to-diameter ratio, and low hydroxyl group density on their surface. As an alternative one-dimensional nanoparticles for CNT and OMMS, HNT do not require exfoliation, making possible the production of stronger, lighter materials without complexity and processing cost associated with intercalation and exfoliation [12]. Additionally, HNT was rather cheap and ecofriendly. Given to these characteristics, HNT has been regarded as a promising competitors and alternatives to both CNT and OMMT. Murariu [13] reported that the addition of HNT can result in an increase of thermal and mechanical properties at a small loading. A larger loading of HNT (about 30%) can tough PLA in PLA/HNT composites

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w compounded by Liu et al [14]. However, to the best of our knowledge, we did not find work on the application of HNT within PLA/PBS or how this affects the properties of PLA/PBS composites in literatures. Further research in the effect of HNT on PLA/PBS properties will help in the development of several properties of PLA/PBS composites, which is significant both in theory and practice.

This work, hence, reports on the influence of HNT addition on mechanical properties, morphological, thermal properties and rheological behavior of PLA/PBS blends. The objective of the work was to try to optimize the HNT concentration in the blend so as to get the best properties from the polymers.

EXPERIMENTAL

Materials

The PLA used in the study is a commercial grad (PLA 4032D), supplied from Natural Works LLC (USA). It had a specific gravity 1.25?????, glass transition temperature about 60 °C, melting temperature about 163 °C. On the other hand, PBS, with the designation BIONOLLE #1903MD, was obtained from Showa High Polymer (Japan). According to the supplier, it had density=1.26 ????? and a melting temperature of ~115 °C. Halloysite nanotubes (HNT) were supplied by Aldrich

with

the

following

characteristics:

nanotubes

length=1~3

um,

pore

volume=1.26-1.34 ????. HNT are in a hydrated state with water in the interlayer spaces. Before processing, PLA and PBS were dried in vacuum at 80 °C for 8h, whereas HNT was dried for 10 h.

Preparation of Polymer Blend and Blend-Clay Composites

Dried pellets of PLA and PBS in the weight proportion of 70:30 were premixed. The proportion was determined since PLA was our target polymer, whose properties need to be improved and the ratio also resulted in a good balance of properties. Blend was mixed with different weights of HNT (1, 3, 6, 9, 12 wt. %) in a plastic bag, and then melting blended in a Brabender counter-rotating twin-screw extruder (Germany) with a screw diameter of 25 mm and a length/diameter ratio of 20:1. The pro?le temperatures were 160, 170, 195, 195, 195, 195, 175, and 175 °C, and the screw speed was 100 rpm. All extruded composites were immediately cooled in a water bath, and a pelletizer was used to cut the extruded blends into pellets. These pellets were dried at 80 °C for more than 4 h. After being oven-dried, part of the pellets were

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w compression-molded into sheets of 150 x 80 x 2 mm using a hydraulic press at 190 °C, then the samples for TEM, mechanical properties, dynamic mechanical properties and dynamic rheological test were prepared from the sheets. Another part of dried pellets from the fresh extruder at the exit of die were prepared for DSC and SEM specimens. It is noted that all the

composites samples were labelled: B0, B1, B3, B6, B9 and B12 for blends with 0, 1, 3, 6, 9, 12 wt. % contents respectively.

Transmission Electron Microscopy (TEM)

Transmission Electron Microscopy (TEM) observations was used to evaluate the nanoscale structure of different composites dispersion of nanoparticles in PLA matrix with a JEOL (JEM-100XII) eld emission transmission electron microscopy at an acceleration voltage of 200 kV. The ultrathin sections with a thickness of ~100 nm were prepared from circular sheet using a Leica EMUC6/FC6 microtome.

Scanning Electron Microscopy (SEM)

The morphology of the blends were investigated using a scanning electronic microscope (SEM, Quanta 200 FEI Co.). The rod-like specimens from extruded die were fractured at liquid nitrogen temperature. Before recording the morphological observations, the sample surfaces were sputter-coated with Au to prevent build-up of electrostatic charge during observations.

Test testing

Tensile tests to determine the yield strength, and elongation at breaking were carried out using an Instron universal machine (model 5566, USA), in accordance with GB/T 1040.2 (2006), under tension mode at a single-strain rate of 5 mm·min ?1 at room temperature.

Dynamic mechanical Analysis (DMA)

Dynamic mechanical properties of neat PLA/PBS and PLA/PBS/HNT blends were determined using a Netzsch DMA242c instrument in tensile mode at a ?xed frequency of 1 Hz and oscillation amplitude of 0.04 mm. Measurements of the storage modulus, the loss modulus and the dissipation factor were carried out in the temperature range from? 65 to + 150 °C with a heating rate of 3 °C/min. The test specimen dimensions were approximately 1×5×32 mm (thick-width-length).

Differential Scanning Calorimetric (DSC)

The differential scanning calorimetric (DSC) measurements were performed in a Netzsch DSC204 (Germany) in a nitrogen atmosphere. Composite samples of 6-10mg were subjected to

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the whole DSC protocol, in which all the samples were heated to 210 °C at a rate of 10 °C/min, held for 5min then cooled to temperature at 10 °C/min, followed by a second heating scan monitored between room temperature to 210 °C at a rate of 10 °C/min. The glass transition temperature (??), cold crystallization temperature (???), melting temperature (??) and melting enthalpy (Δ??) were determined from the second heating scan. The fractional crystallinity ?? was calculated using the following equation:

??(%)=??????

?

???

×??.%

×100 (1)

Where ?? is the crystallinity of polymer (PLA or PBS) in the composite, Δ?? is the enthalpy of cold crystallization, Δ?? is the heat of melting enthalpy, Δ?? is the theoretical heat of fusion for a fully crystalline PBS or PLA, and ??.% represents the matrix weight fraction in the case of PLA/PBS/HNT blends. From previous literatures [11][15], Δ?? for PBS and PLA has been assigned to be 200 J/kg and 93J/kg, respectively.

Dynamic rheological measurements

Dynamic viscoelastic properties of pure PLA and its nanocomposites were analyzed using a rheometer of Anton Paar Physica MCR 302. The measurements were performed in the linear viscoelastic region with dynamic oscillatory mode and 25 mm parallel cone-plate with gap setting of about 0.7 mm. All experiments were carried out under nitrogen atmosphere at 180 °C. Frequency scans were taken at low strain (1%) at the frequency range between 0.06 rad/s and 628 rad/s.

Result and discussion

Morphology

It is well known that the dispersion of nanoparticles in the polymer blend is the key factor influencing its physical properties. A well dispersion of HNT nanoparticles, associated with strong interfacial interactions between HTN and matrix, would significantly influence the mechanical, thermal, and rheological properties of PLA/PBS/HNT blends.

Hence, the dispersion state of HNT in the polymer was determined by TEM technique, which provided actual images of the HNT identifying physical properties of HNT in the polymer blend matrix. Figure 1 shows selected pictures of PLA/PBS composites produced by addition 3 wt.% (Fig. 1(a)) and 9 wt.% (Fig. 1(b)) respectively. It is obvious from the TEM images recorded at lower magnification that a good quality of the distribution of HNT was reached. On the introduction of 9 wt.% in PLA/PBS composite , some small individual HNT particles aggregated

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w into clusters. However, the degree of big clusters was not too serious. Remarkable aggregation of HNT particles were evidenced in the blends after the introduction of HNT was 12 wt.%,

On the other hand, the SEM images of the PLA/PBS blend was used to evaluate the phase structures of PLA/PBS/HNT blends. The prepared samples were fractured in liquid nitrogen for characterizing via SEM which was operated at 10kV accelerating voltage. Figure 2 displays morphology of the PLA/PBS blend with various HNT ratios at a fixed weight proportion of PBS and PLA. In polymer blend systems, interaction between two polymers or enthalpy can determine their miscibility, while the temperature, composition, other factors affect phase separation, particle size[16]. Distinctive phase separation in the two immiscible polymer blends was observed in pure PLA/PBS blend (Fig.2 (a)). This indicated poor interfacial adhesion between the two phases. In addition, Fig.2 (b-d) shows HNT bundles were embed and partially well dispersed in the PLA/PBS immiscible polymer blend, whereas there were same HNT bundles aggregated each other in Fig.2 (f). It has been reported that the high amount of HNT i.e. 12 wt. % resulted in aggregation of HNT nanotubes [13].

Tensile properties

Both the yield strength and elongation at break for PLA-PBS blend and PLA-PBS blend-HNT composites are shown in Figure 3. Tensile strength of the composites tended to increase with an increase of clay content, peaking at around 6 wt. %. Tensile strength was improved by almost 130% separately at 6 wt% HNT loading than that of PLA/PBS blend (31.48 MPa). However, on further increasing HNT loading, tensile strength of HNT composites gradual decreased to 33.64 MPa at 9 wt. % and 30.18 MPa at 12 wt. %. At the same time, regarding PLA/PBS/HNT composites, the elongation at break climbed sharply to 17.86% at 6 wt. %, with values nearly 2 times better than that of PLA/PBS system (9.78%), whereas a small reduction occurred in the nanocomposite with more HNT fillers, such as 9 wt. % (10.86%). A small incorporation could act as reinforcing and plastic fillers while high loadings resulted in weak tensile properties due to somewhat inevitable aggregation of the nanofiller(SEM and TEM results). Also, similar trends for tensile strength were observed in PLA/HNT systems [13]. Unlike other nanofillers that leaded to composites with improved strength in the detriment of elongation at break (eg. expanded graphite [17] and MWCNT [18]), such behavior was quite remarkably distinct. The differences of reinforcing were commonly ascribed to intrinsic stiffness of the nanotubes resulting from their tubular structure, their good dispersion, as well as to the good interfacial properties between filler and matrix [19-21].

Dynamic mechanical Analysis

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w Dynamic Mechanical Analysis (DMA) was used to evaluate the viscoelastic properties of PLA/PBS/HNT nanocomposites below and around the glass transition temperature. Figure 4 shows the temperature dependencies of storage modulus of pure PLA/PBS and nanocomposites obtained by the addition of 3-12wt% HNT, where (a) displays Temperature region from -80 °C to 80 °C, (b) displays the temperature region from -60 °C to 20 °C. All investigated samples for storage modulus presented similar behavior with increasing temperature. It is obvious that storage modulus decreased gradually before 40 °C, and then followed by a dramatic decline in the ranges of 40-60 °C. However, it is worthy noted that the storage modulus at low temperature for nanocomposites significantly increased with the increasing of HNT before 6 wt. % , but the reinforcing effect of HNT declined at 12 wt. %. It was clear that B6 at low temperature had the highest storage modulus compared to others, which can be ascribed to the reinforcing effect of the halloysite nanotubes that proved to be finely distributed and dispersed within the polymer matrix. However, a decline of storage modulus was observed in PLA/PBS with high loading of HNT (12 wt. %) due to aggregation of HNT (based on TEM images).

In addition, the variation of the damping factor (tanδ) as a function of temperature was considered as the glass transition temperature of material, as shown in Figure 5. ?? of PBS in PLA/PBS/HNT blend system increased consistently with the incorporation of HNT, whereas Tg of PLA increased firstly and then fell to about 65 °C( close to the ?? of B0). The temperature values of composites were displayed in Table 1, for instance, ?? for PBS at 6 wt. % HNT content was -6.7 °C (B6), which was 17.2 °C higher than that of pristine PLA/PBS composites. Here, nanotubular clays restricted the movement of PBS chains which was the reason behind the improvement of ?? with the addition of HNT in the blend system [22]. On the other hand, it is noted that, for the PBS component in B12 sample, ?? was lower than that of PBS in B3 and B6. This phenomenon may be ascribed to aggregation HNT nanoparticles, weakening resistance of PBS chain and hence lowered the glass temperature.

Moreover, the difference of shift glass transition temperature for PLA and PBS became smaller after the loading of HNT. For the B6 sample, the values of decreased by 11.7 °C. The difference ?Tg between PLA and PBS suggested that interactions between PLA and PBS could be improved by the addition of HNT.

Differential Scanning Calorimetric

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w Figure 6 shows DSC thermograms of PLA/PBS blends with different weight fraction of HNT during a second heating scan, and the corresponding data were collected in Table 2. Neat PLA/PBS composite for PLA showed a broad cold crystallization exothermic peak at about 103.6 °C. Here, PLA’ crystal was not perfect in cold crystallization because of the lower crystal ability of PLA chain, and the imperfect crystals formed in cold crystallization would reorganize into more orderly crystal structures [15]. The cold crystallization peak of PLA (???) shifted to a lower temperature after the addition of HNT. On introduction of 6 wt. % HNT into the blend, the ??? of sample B6 decreased to a lower temperature of nearly 97.2 °C, which was 6.4 °C lower than that of sample B0. This could be attributed to the nucleating effect of HNT in PLA cooling crystallization process. The heterogeneous nucleation could lead to the increase of crystallinity of PLA [23]. The crystallinities of the HNT/PLA/PBS nanocomposites were substantially higher than that of neat PLA/PBS blend (Table 2). On the other hand, the melting temperature of PLA changed slightly with the loading of HNT.

However, for the PBS component, the effect of HNT on PBS’ melting peak was not obvious. The melt enthalpy for the PBS component in PLA/PBS blends grew with the addition of HNT, though the increasing trend was not consistent. Further, the introduction of HNT tended to enhance the crystallinity of PBS, which implied that HNT may also work as a nucleating agent of PBS in PLA/PBS blend. Similar results of the heterogeneous nucleation of nanosized particles for PBS were reported by Wu [15] .

Rheological behaviors

As a parallel disc oscillatory shear test is a small amplitude deformation test, which does not change the material structure, it is often used to characterize the relation between the melt rheology properties of materials and the frequency or temperature. In this work, the parallel disc shear rheology tests were performed as a function of frequency ranging from 1 to 600 rad/s.

Complex viscosity as a function of angular frequency is represented in Figure 7. Normally, the presence of fillers in polymer melts not only increases their shear viscosity but also affects their shear rate dependency [24]. Non-Newtonian and shear-thinning characteristics in the range of the applied shear rates were observed. Further, such behavior in PLA/PBS/HNT composites seemed to be more dependent on the frequency than that in pristine PLA/PBS. Figure 8 also indicates the dependence of complex viscosities of composites on the HNT content under a series

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w of frequency, such as 0.1, 1, 10, 100 rad/s respectively. Complex viscosities for all the PLA/PBS blends under various frequency showed similar trends. More specially, at the same frequency, the complex viscosity of nanocomposites was higher than that of pristine PLA/PBS. Obviously, in PLA/PBS blend with 6 wt. % loading of HNT, complex viscosities showed the highest values when the frequency was 0.1, 1, 10, 100 rad/s, and the value at a frequency of 10 rad/s were nearly threefold higher than that of pure PLA/PBS. This was a conmen phenomenon in filler reinforcing system, where the interaction between HNT and polymer matrix hindered the movement of polymer chains.

Figure 9 and Figure 10 represents the storage modulus (G ?) and loss modulus (G”) for PLA/PBS/HNT nanocomposites. We can observed that storage modulus (G ?) and loss modulus (G”) for PLA/PBS/HNT nanocomposites were increased as compared with pristine PLA/PBS. For the nanocomposites, the values of G ? increased with increasing HNT content, which was particularly obvious in the lower frequency region. The storage modulus of the samples B6 was higher than other composition at low frequencies, whereas lower storage modulus for samples B12 was observed. The idea of view was presented here to interpret the rheology properties. It is noted that storage modulus reveals the elastic properties of material. As demonstrated in the TEM, we have speculated that HNT bundles were highly aggregated in sample B12 (detected in Figure 10) but HNT bundles in other nanocomposites (Figure 6) were more dispersed in a broad region. B12 with aggregated HNT had little effect on elastic properties, thus the storage modulus of B12 composition at a low angular frequency region was lower than others. The trends of G” for HNT filling composites were as similar as pure PLA was. Further, a subtle increase of G” occurred for PLA/PBS blending system after adding HNT.

CONCLUSIONS

Ternary blends PLA/PBS/HNT with various HNT content were prepared via melt blending in a novel vane extruder, resulting in composites with different properties. SEM and TEM demonstrated that a small incorporation of HNT can well disperse in two polymer matrix while luxurious HNT (>6 wt.%) would highly aggregated, which influenced the properties of the composites. The most important of our work was the achievement of PLA/PBS/HNT composite (6 wt.%) with optimal mechanical properties. Great enhancement (2 times) of the elongation at

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w break, improvement (1.3 times) of tensile strength over those of pure PLA/PBS blends were achieved in a composite with 6 wt.% HNT. DMA revealed that blending PLA/PBS with 6 wt.% HNT showed the highest storage modulus compared to others. Moreover, thermal analysis detected that HNT acted as a nucleating agent and enhanced the crystallinity of PLA and PBS. Also, from rheological test, shear-thinning behavior was observed in PLA/PBS/HNT composites. Complex viscosity was significant increased due to the incorporation of HNT.

Acknowledgement

The authors wish to acknowledge the National Natural Science Foundation of China-Guangdong Joint Fundation Project (U1201242),973 Program (2012CB025902), the National Research Foundation for the Doctoral Program of Higher Education of China

(20120172130004).

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w [24] H.B. Kim, J.S. Choi, C.H. Lee, S.T. Lim, M.S. Jhon and H.J. Choi. Eur. Polym. J., 41,

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FIG. 1. TEM images of PLA/PBS/HNT nanocomposites: (a) B3, (b) B6, (c) B9, (d) B12.

139x139mm (300 x 300 DPI)

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FIG. 2. SEM images of PLA/PBS/HNT blends with different loading of HNT: (a) B0, (b) B1, (c) B3, (d) B6, (e)

B9, (f) B12.

164x193mm (300 x 300 DPI)

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FIG. 3. Tensile properties of samples of PLA/PBS/HNT Blends (Samples B0, B1, B3, B6, B9 and B12, each

having 0, 1, 3, 6, 9, 12 wt. % HNT respectively).

141x99mm (300 x 300 DPI)

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FIG. 4. Storage modulus of PLA/PBS/HNT blends as a function of temperature (Samples B0, B1, B3, B6, B9

and B12, each having 0, 1, 3, 6, 9, 12 wt. % HNT respectively).

147x109mm (300 x 300 DPI)

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FIG. 5. Damping factor (tanδ) of PLA/PBS/HNT blends as a function of temperature: (a) Temperature region

from -80 to 80 °C, (b) Temperature region from -60 to 20 °C.

160x56mm (300 x 300 DPI)

Page 17 of 24Polymer Composites

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r P e e r R

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FIG. 6. DSC thermograms of PLA/PBS/HNT nanocomposites.

149x111mm (300 x 300 DPI)

Page 18 of 24

Polymer Composites

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F o r P e e r R

e v i e

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FIG. 7. Complex viscosity of PLA/PBS/HNT nanocomposites at 180 °C.

147x108mm (300 x 300 DPI)

Page 19 of 24Polymer Composites

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