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Phase Morphology, Rheological Behavior, and Mechanical Properties of Poly (lactic acid)/Poly (butylene succinate)/Hexamethylene Diisocyanate Reactive Blends Jintao Huang,1, 2,# Wei Zou,1, 3,# Yue Luo,1, 4 Qi-bao Wu,1, 5 Xiang Lu1, 6,* and Jinping Qu1,*

Abstract

Poly (lactic acid) (PLA) and poly (butylene succinate) (PBS) were melt-blended combined with hexamethylene diisocyanate (HDI). It was investigated that how the HDI content effected the blends phase morphology, the mechanical properties and rheological behavior. The increase in the complex viscosities of the blends indicated that compatibilization of the blend had occurred. It was shown that the size of the dispersed phase decreasing with the HDI content increases and appearing the fibrillation connection between the PLA and PBS phases by scanning electron micrographs, indicating that HDI plays a compatibilizing role. The crystallization rate of PLA was accelerated by PBS and chemical cross-link points as nucleating agents. Due to the good interface compatibility between PLA and PBS and the increased crystallinity of PLA and PBS, the impact toughness of the annealed PLA/PBS/HDI blend was significantly improved.

Keywords: PLA/PBS blends; Hexamethylene diisocyanate; In-situ reactive compatibilization; Phase morphology; Properties.

Received: 27 July 2020; Accepted: 7 January 2020.

Article type: Research article.

1. Introduction

Recently, biodegradable materials have received increasing

attention, mainly due to environmental issues related to

greenhouse gas production and the end of life of classic

petroleum-based polymers.[1-6] As one of the biodegradable

materials, the biodegradable polymer can be degraded by

microorganisms including algae, fungi and bacteria or

hydrolyzed in a buffer solution or seawater when exposed to a

biologically active environment. Among the biodegradable

polymers, polylactic acid (PLA) has received widespread

attention in various commercial applications because of its

high strength and stiffness, excellent transparency, and

biodegradability.[7-13] However, PLA has some serious

limitations such as low heat distortion temperature and

toughness. For improving the toughness of PLA, many

methods including copolymerization, plasticization and

blending with other polymers have been explored. In the

above method, blending with the ductile polymers proved to

be the most effective and convenient way to toughen PLA.[14-

23]

As a relatively environmentally friendly biodegradable

aliphatic polyester, polybutylene succinate (PBS) is

synthesized by the polycondensation reaction of succinic acid

and 1, 4-butanediol which are renewable and bio-based.[24] At

room temperature, PBS has good toughness and elongation at

break of more than 300%.[25-27] It is also a crystalline polymer,

whose melting point and glass transition temperature are about

114 oC and -35 oC, respectively. It is a simple method to

improve the toughness of PLA and tailor the properties of PBS

and PLA for different reasons by blending PLA and PBS,

because the properties of the two is complementary. Moreover,

the renewable and biodegradable properties of PLA/PBS

ES Energy & Environment DOI: https://dx.doi.org/10.30919/esee8c1017

1 Key Laboratory of Polymer Processing Engineering of the Ministry

of Education, National Engineering Research Center of Novel

Equipment for Polymer Processing, Guangdong Key Laboratory of

Technique and Equipment for Macromolecular Advanced

Manufacturing, South China University of Technology, Guangzhou,

510641, China. 2 Department of Polymeric Materials and Engineering, School of

Materials and Energy, Guangdong University of Technology,

Guangzhou, 510006, China. 3 School of Mechatronic Engineering, Jiangsu Normal University,

No.101, Shanghai Road, Xuzhou, Jiangsu, 221116, China. 4 School of Mechanical Engineering, Xiangtan University, Xiangtan,

411105, China. 5 School of Intelligent Manufacturing and Equipment, Shenzhen

Institute of Information Technology, Shenzhen, 518172, China. 6 School of Chemistry and Chemical Engineering, Huazhong

University of Science & Technology, Wuhan, 430074, China.

*Email: luxiang_1028@163.com (X. Lu); jpqu@scut.edu.cn (J. Qu) #These authors contributed equally to this work.

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blends are still retained.[28-31]

However, for all alloy systems, the key to determining

whether the two polymers are blended successfully is the

degree of miscibility of the two materials. Due to

incompatibility of polymer/polymer blends without

compatibilization, phase separation usually occurs, which may

result in probably decreased performance.[32-34] Reactive

processing is considered to be an effective method for

improving the miscibility of polymer/polymer blends.[35-37] The

use of pre-made or in-situ generated copolymers can

effectively modify morphology and increase the blends’

compatibility. Compared with specially-modified copolymers’

addition, which is usually expensive, in-situ reactive

compatibilization of polymer/polymer blends is technically

preferred and has been adopted by multiple authors. For

instance, Ojijo et al.[38] fabricated PLA/poly[(butylene

succinate)-co-adipate] (PBSA) blends with super toughness

by melt blending them with a multifunctional epoxy-

functional chain extender. Wang et al.[15] fabricated toughened

PLA/PBS blends by adding a small amount of dicumyl

peroxide (DCP) for in-situ compatibilization. Kumar et al.[39]

improved the interface between PLA and poly (butylene

adipate co-terephthalate) (PBAT) with glycidyl methacrylate

(GMA) as compatibilizers and toughened PLA/PBAT blends

were successfully prepared. With phthalic anhydride (PA) 2,

2-(1, 3-phenylene) and bis (2-oxazoline) (BOZ), Dong et al.[40]

fabricated PLA/PBAT blends by melt blending. Adding a

certain amount of PA or BOZ was proved an effective way to

greatly increase the elongation at break of the PLA/PBAT

blends while their tensile strength was still kept high.

As a very important and common isocyanate monomer,

Hexamethylene diisocyanate (HDI) has been widely applied

in the realm of high-performance polyurethane materials. The

-NCO groups with activity in HDI are easy to react with -OH

groups on polyester molecular chain. The reaction above can

give these blends outstanding properties including excellent

mechanical properties and thermal stability and outstanding

compatibility with other components.[24,41]

To our knowledge, no literature concerns PLA/PBS blends’

reactive compatibilization with HDI. In this study, using melt

blending techniques, PLA/PBS blends were prepared

compounding with different contents of HDI. It was

investigated that how the HDI content effected the blends

phase morphology, the mechanical properties and rheological

behavior.

2. Experimental section

2.1 Materials

The PLA (4032D) was supplied by NatureWorks company.

The melting temperature ™and glass transition temperature

(Tg) of the PLA were about170 oC and 60 oC, respectively. The

PBS (MI is 26g/10 min), whose brand name is Bionolle

1020MD, was obtained from Japan company named Showa

Highpolymer Co., Ltd. The HDI was purchased from Sigma-

Aldrich. Dibutyl tin dilaurate, which is the catalyst in this

study, was purchased from China company named Shao Yu

Chemical Co., Ltd.

2.1 Preparation of PLA/PBS Blends

Firstly, PBS and PLA were put into a vacuum oven to be dried

for 6 hours, with the temperature setting at 80 °C, then these

dry materials and HDI, were pre-mixed manually by rolling in

a plastic pull-seal bag in which the weight content of PLA,

PBS and HDI is 70%, 30% and ×% (× = 0, 0.2, 0.5, 1.0, 2.0),

respectively. Finally, PLA/PBS/HDI were melt blended by

vane extruder, a novel extruder, which was described in the

literatures.[42-44] The processing temperature and speed was

kept at 200 °C and 27 rpm, respectively. The extruded blends

were placed in the air to cool, and then made into pellets.

Finally, all the specimens were hot-pressed into 1mm and 4

mm sheets at 200 °C and 10 Mpa for 10 minutes.

2.2 Characterization

A scanning electronic microscope (SEM), production of the

HITACHI (model name:SE3400N), was used to image the

fracture surface of the blends. After immersing it in liquid

nitrogen for about 15 minutes, a cryo-fracture samples (4 mm

thick) were obtained. The specimen surface was sprayed with

gold to prevent static electricity from accumulating during the

observation.

A MCR302 rheometer, produced by Anton Paar, Austria,

was applied to study the rheological behavior of the blends

melt. When using the rheometer in this study, it was set at

dynamic oscillation mode, and the parallel-plate geometry is

25 mm diameter and 1mm gap. During the test, the frequency

sweep and amplitude was set in the range of 0.01–100 rad/s at

200 °C and 1%, respectively.

A Netzsch DMA242c instrument was used for dynamic

mechanical analysis (DMA) at 3 Hz frequency and 0.15 mm

oscillation amplitude. The heating rate and temperature range

were -60 °C to 100 °C and 3 °C/min, respectively.

A DSC instrument (Netzsch, model 204c, Germany), which

had liquid nitrogen cooling accessories, was used to perform

differential scanning calorimetry. The samples were scanned

twice. First, at a rate of 10 °C/min, the samples were heated

from room temperature to 210 °C, held there for 3 minutes,

then under nitrogen decreased to a 30 °C at a rate of 10 °C/min.

The second time, at a rate of 10 °C/min, the samples were

scanned from 30 °C to 210 °C.

Pure PBS and cross-linked PBS were analyzed by X-ray

diffraction (XRD) with Germany production named D8

ADVANCE instrument (Bruker). Scan were carried out with

Bragg angle between 5° and 40° at a scan rate of 2 degrees/min.

A tensile test was performed at room temperature in tensile

mode with a single strain rate of 20 mm/min by a USA Instron

machine (model 5566) according to ISO 527 to measure the

elongation at break and yield strength.

Two types of samples including the unannealed “as

prepared” and the annealed (annealed at 80 °C under vacuum

for 5 h) were analyzed. The average of less than five

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Fig. 1 Rheological behavior of the PLA/PBS blends with different HDI content: (a) complex viscosity, (b) storage modulus and (c)

loss modulus.

independent tests is the final results.

3. Results and discussion

3.1 Dynamic rheological properties

The dynamic oscillatory shear tests were carried out on neat

PLA/PBS and PLA/PBS/HDI blends to study the response of

the chemical reaction to dynamic shear in this system. It is

shown in Fig. 1a that relationship between the complex

viscosity (η*) of PLA/PBS blends with different HDI loading

at 200 oC and angular frequency (ω). It is obvious that the

complex viscosity (η*) of the PLA/PBS/HDI blends increases

monotonously with HDI content increase. When the content

of HDI is less than 0.5 wt%, the η* of the PLA/PBS/HDI

blends depends little on frequency, showing a plateau of

Newtonian at low shear rate (ω is between 0.01~0.1 rad s-1).

However, for PLA/PBS/HDI (70/30/2) blend, the η* shows an

obvious frequency dependence. This indicates that within the

range of applied shear rates, PLA/PBS/HDI (70/30/2) blends

exhibit non-Newtonian and shear thinning properties. Based

on the theory of entanglement, the existence of cross-linked

network structure or long-chain branch structure increased the

entanglement of PLA and PBS molecular chains, thus

affecting the polymer melt mobility.[45,46]

Compared with the neat PLA/PBS blend, the loss modulus

G’’ and storage modulus G’ for the PLA/PBS/HDI blends were

increased, as shown in Fig. 1b and Fig. 1c. For the

PLA/PBS/HDI blends, with the content of HDI increase, the

values of G’ increased, especially in the region of lower shear

rate. It is common sense that the PLA/PBS/HDI blends’

viscoelasticity is sensitive to structural evolution in the low

shear rate range. The relationship between the dynamic

storage modulus and frequency (G’~ω) in Fig. 1b, it can be

observed that the second plateau appears in the low shear rate

region for PLA/PBS/HDI (70/30/2) blend, which confirms the

network structure in the PLA/PBS/HDI system occur. As

shown in Fig. 1c, the G’’ of the blends have a similar trend in

all frequency range. The above results show that HDI is a

suitable chain extender and compatibilizer for PLA/PBS blend.

Fig. 2 SEM micrographs of fracture surfaces for PLA/PBS/HDI (70/30/x) blends: (a) x=0, (b) x=0.2, (c) x=0.5, (d) x=1.0, (e)

x=2.0.

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3.2 Phase morphology and compatibility of PLA/PBS

blends

SEM was used to characterize the phase structure of

PLA/PBS/HDI blends. Fig. 2 (a-e) shows the SEM images of

PLA/PBS blends with different HDI contents. It is obvious

that clearly two-phase island-sea morphology appears in the

neat PLA/PBS blend, in which the minor discrete phase (PBS)

like droplets are distributed in the matrix (PLA). For the neat

PLA/PBS blend, the average domain size is about 1.0-2.0 μm.

With increasing HDI loading (from 0.2 wt% to 0.5 wt%), it is

not obvious that the average domain size of PBS decreases.

For HDI loading from 1.0 wt% to 2.0 wt%, the average domain

size decreased to about 0.5 μm, which is approximately one

third of the neat PLA/PBS blend. The cryo-fractured surface

of the neat PLA/PBS blend is smooth, and the phase interface

is obvious, which indicates that PLA and PBS are

thermodynamically immiscible. However, with the

introduction of a very small amount of HDI, the reaction

products PLA-HDI-PBS bridge the PLA materials and the

PBS phase, and the phase interface between PBS and PLA

becomes more and more unclear, and phase morphology

becomes more relatively uniform especially when HDI

content is high.

DMA is a useful technique to track the movement of the

main chain and side groups, especially to study the different

relaxations (glass transition and secondary relaxation) of

polymer chains. Fig. 3a shows the relationship between graphs

of the dynamic loss (tanδ) and temperature for pure PLA/PBS

and PLA/PBS/HDI (70/30/0.5). There appeared two peaks at

about -25 oC and 65 oC in all the PLA/PBS/HDI blends, which

were related to the 𝑇𝑔 of PBS and PLA component,

respectively. Shown in Table 1, for the neat PLA/PBS blend,

the 𝑇𝑔 of PBS is -26.9 oC and 𝑇𝑔 of PLA is 66.6 oC. With the

introduction of 0.5 wt% HDI for PLA/PBS blend, the Tg of

PBS (𝑇𝑔(𝑃𝐵𝑆)was increased to -25.2 oC and 𝑇𝑔 of PLA (𝑇𝑔(𝑃𝐿𝐴))

dropped to 66.3 oC. Additionally, the value of ∆𝑇𝑔 (𝑇𝑔(𝑃𝐿𝐴) -

𝑇𝑔(𝑃𝐵𝑆) ) decreased from 93.5 to 91.5 oC with a 0.5 wt%

content of HDI. The above results showed that compatibility

of PLA and PBS was promoted by HDI, which is in agreement

with the SEM results.

Table 1. The glass transition temperature of PLA and PBS

component in the PLA/PBS/HDI blends.

HDI Content

(wt%) 𝑻𝒈(𝑷𝑩𝑺) (℃) 𝑻𝒈(𝑷𝑳𝑨) (℃)

𝑻𝒈(𝑷𝑳𝑨)-

𝑻𝒈(𝑷𝑩𝑺) (℃)

0 -26.9 66.6 93.5

0.5 -25.2 66.3 91.5

In this study, both PLA and PBS reacted with HDI leading

to a long-branched structure (including PLA-HDI-PLA, PBS-

HDI-PBS and PLA-HDI-PBS) through the process of reactive

extrusion. With the change of HDI content, the interfacial

tension and viscosity ratio of PBS and PLA changed. In

summary, the morphological changes in the PLA/PBS/HDI

blend are mainly due to two reasons: (1) compatibility

between PBS and PLA; (2) variety of the viscosity due to

interphase chain growth during processing.

Fig. 3 (a) tan δ versus temperature for PLA/PBS/HDI blends. (b) Storage modulus (E’) versus temperature for PLA/PBS/HDI blends.

(c) Melting curves of for PLA/PBS/HDI blends. (d) XRD curves for PLA/PBS/HDI blends.

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Table 2. Cold crystallization temperature (𝑇𝑐𝑐), melting temperature (𝑇𝑚), melting enthalpy (∆𝐻𝑚), cold crystallization enthalpy

(∆𝐻𝑐𝑐) and relative crystallinity (𝑋𝑐) for the PLA/PBS/HDI blends.

HDI Con-

tent (wt%)

PLA Component PBS Component

𝑇𝑐𝑐(oC) ∆𝐻𝑐𝑐(J/g) 𝑇𝑚(oC) ∆𝐻𝑚(J/g) 𝑋𝑐(%) 𝑇𝑚(oC) ∆𝐻𝑚(J/g) 𝑋𝑐(%)

0 98.2 31.1 168.1 36.5 8.3 112.0 3.2 5.4

0.2 102.1 11.7 168.3 23.5 18.1 112.8 12.8 21.4

0.5 101.5 10.0 167.8 22.7 19.5 112.2 10.9 18.2

1.0 102.6 8.5 168.7 22.1 20.9 111.5 7.3 12.1

2.0 103.8 3.3 167.8 20.9 17.0 108.3 2.8 4.7

3.3 Thermal properties and crystallization behavior

Fig. 3b shows the relationship between temperature and the

storage modulus (E’) of the neat PLA/PBS and PLA/PBS/HDI

(70/30/0.5) blends, which indicates E’ depend on the

temperature. From -60 °C to -40 °C, compared with the E’ of

neat PLA/PBS blend (about 3600 MPa), the E’ of

PLA/PBS/HDI (70/30/0.5) blend is increased to about 3850

MPa, the increments in E’ are about 7.0 % for 0.5 wt % HDI

contents. However, at 25 °C, all of the blends show the same

E’. Across the cross-linking points, the branched and cross-

linked structures in the PLA/PBS matrix facilitate load transfer.

Because of this, the modulus of the PLA/PBS/HDI blend was

somewhat enhanced.

Both PLA and PBS are semi-crystalline polymers, and their

molecular chain structure strongly determines their

crystallinity and melting behavior.[47] In order to prove the

effect of HDI on the PBS thermal performance, DSC was used

to test the melting difference and crystallinity of pure

PLA/PBS and PLA/PBS/HDI blends. Fig. 3c shows the

melting curves of the PLA/PBS/HDI blends with different

HDI content at a heating rate of 10 °C/min. The PLA relative

crystallinity ( 𝑋𝑐(𝑃𝐿𝐴) ) and PBS ( 𝑋𝑐(𝑃𝐵𝑆) ) component in all

samples of this study are calculated as follows:

𝑋𝑐(𝑃𝐵𝑆) =∆𝐻𝑚(𝑃𝐵𝑆)

∆𝐻𝑚(𝑃𝐵𝑆)𝑜 ×𝑤𝑓(𝑃𝐵𝑆)

(1)

𝑋𝑐(𝑃𝐿𝐴) =∆𝐻𝑚(𝑃𝐿𝐴)−∆𝐻𝑐𝑐(𝑃𝐿𝐴)

∆𝐻𝑚(𝑃𝐿𝐴)𝑜 ×𝑤𝑓(𝑃𝐿𝐴)

(2)

where ∆𝐻𝑚 is melting enthalpy measured from DSC, ∆𝐻𝑚𝑜 is

the original polymer crystal’s enthalpy (The PLA and PBS

crystal’s enthalpy is 200 J/g and 93 J/g, respectively.), 𝑤𝑓 is

the weight proportion of each component in the blends and

∆𝐻𝑐𝑐 is the cold crystallization measured enthalpies (form

DSC). Table 2 shows the DSC results. It indicates that the

𝑇𝑐𝑐(𝑃𝐿𝐴) and relative 𝑋𝑐(𝑃𝐿𝐴) of PLA increased with the HDI

loading, but the cold crystallization enthalpy ( ∆𝐻𝑐𝑐(𝑃𝐿𝐴) )

decreased. In the PLA/PBS/HDI blends, with the HDI content

increase, the relative crystallinity ( 𝑋𝑐(𝑃𝐵𝑆) ) of PBS first

increased and then decreased. As we all know, from the molten

state the crystallization of polymers consists of two stages:

first nucleation, homogeneous or heterogeneous; then crystal

growth.[48,49] In this research, the presence of PLA and

chemical cross-linking points inhibited the formation of

perfect crystals in the PBS polymer chain, which maybe the

main reason for the reduction of the reduction of 𝑋𝑐(𝑃𝐵𝑆) .

However, it is the presence of PBS and chemical cross-linking

points that promote more crystal nuclei to be formed by

heterogeneous nucleation, and therefore is the main reason for

the increase in 𝑋𝑐(𝑃𝐿𝐴).

The XRD patterns of the PLA/PBS/HDI blends with

different HDI contents are displayed in Fig. 3d. The

monoclinic crystal is the characteristic of the crystal unit cell

of PBS, and the four main peaks located at around 28.7°, 22.5°,

21.7° and 19.4° are attributed to the (111), (010), (021), and

(020) planes, respectively. But it is difficult for PLA to form

perfect crystals, and there are no obvious peaks for the PLA

component in the XRD curves. With increasing HDI loading,

the XRD pattern shows that the diffraction peaks of PBS

component keep similar, which indicates the crystal structure

of the PBS component in the PLA/PBS/HDI blend is almost

the same as the crystal structure of the PBS in the neat

PLA/PBS blend. However, there is a small peak located at

around 17° for the PLA when the HDI content is larger than

1.0 wt%. The results indicate that, for the PLA component,

heterogeneous nucleation is facilitated by the presence of PBS

and chemical cross-link points, forming more crystal nuclei

and perfect crystals. But for the PBS component, the effect of

chemical cross-link points and the presence of PLA mainly act

on the amorphous region, so the crystal structure of PBS does

not change.

3.4 Mechanical properties

The relationship between the content of HDI and the tensile

properties of PLA/PBS (70/30) blends, annealed and without

annealed, is shown in Fig. 4a and Fig. 4b. Five different tests

were performed to get the average value, where the standard

deviation was used as the error bar. With HDI content increase,

the tensile strength decreased slightly and the elongation at

break increased monotonically. For unannealed samples,

compared with neat PLA/PBS (47.4 MPa), 0.2 wt%, 1.0 wt%,

2.0 wt% HDI was added to the blend, the PLA/PBS/HDI

blends tensile strength was slightly reduced to 45.8 MPa, 44.1

MPa and 45.0 MPa, respectively. For the PLA/PBS/HDI

blends, when the HDI content is 2.0% by weight, the

elongation at break monotonically increases from 14.7% to

64.8%. For the annealed PLA/PBS/HDI samples, the tensile

strength is also reduced slightly with HDI content increase.

But the 70/30/0.2, 70/30/0.5, 70/30/1, 70/30/2 (PLA/PBS/HDI)

blends exhibited elongations at break of 177.1%, 264.2%,

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Fig. 4 Mechanical properties of PLA/PBS/HDI blends: (a, b) Tensile properties, (c, d) flexural properties and (e) Impact strength.

292.0% and 357.1%, respectively. These values were

approximately 7.6, 11.4, 12.6, and 15.4 times that of neat

PLA/PBS blend (23.2%). Fig. 4c and Fig. 4d show the flexural

properties of the PLA/PBS/HDI blends change with HDI

content change before and after annealing. With HDI content

increasing, for the PLA/PBS/HDI blends, the flexural modulus

and strength was reduced slightly.

The impact strength (samples with notch) of PLA/PBS/HDI

and PLA/PBS blends with various HDI contents is shown in

Fig. 4e. The five independent tests were conducted and the bar

graph shows the average value obtained. Since the impact

strength of the samples will be adversely affected by the

crystallinity, the effect of annealing was investigated by some

annealed samples (80 °C for 5 hours). As a flexible polyester,

PBS has a higher elongation at break but a lower notched

impact strength. Therefore, when the PLA/PBS was blended

with HDI, the impact resistance is only slightly increased.

Unexpectedly, the toughness improved significantly after

annealing. For annealed 70/30/1 and 70/30/2 (PLA/PBS/HDI)

blends, the PLA/PBS/HDI blends’ impact strength increased

from 2.16 kJ/m2 to 22.7 and 28.0 kJ/m2, respectively.

In summary, in our study, 2 wt% of HDI was regarded as

the optimal concentration because the blend achieved the

highest value in terms of elongation at break while its strength

and modulus decreased little.

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Fig. 5 SEM images impact-fractured surfaces of PLA/PBS/HDI (70/30/2) blend.

3.5 Toughening mechanism

For polymer blends, the toughening mechanism is due to the

matrix’s yield deformation. The improved impact resistance in

the blend is ascribed to the shear yield of the matrix, which is

caused by the microvoids degumming. In polymer blends, it is

considered widely that microvoids are the essential step to

shear yield the matrix and thus consume energy. Kim and

Michler [50,51] proposed a micromechanical deformation model

of the blend, and envisaged a process three stages: (1) stress

concentration in inclusions; (2) formation of voids and shear

bands; (3) induced shear yield. In the PLA/PBS blends, PBS

particles act as stress concentrators because both of them show

different impact resistance and PBS is tougher than PLA. For

neat blends, debonding can easily occur because the adhesion

between two phases is poor. Conversely, for compatible

blends, the fibrillated ligament at the interface will be

deformed through the debonding process, as shown in Fig. 5.

Premature interface failure happened because in neat blends,

the interfacial adhesion between PBS and PLA was poor,

which leaded to rapid diffusion of voids, and the occurrence

of matrix deformation was delayed by interfacial adhesion

which was moderately good in compatible blends. In

compatibilized blends, the above process consumes more

applied energy. Therefore, more toughening effects can be

obtained compared to neat blends.

It is well known that the polymers’ mechanical properties

and their blends are affected by crystal size and crystallinity

affect. The XRD diffractograms of blends, both HDI modified

blends and the unmodified blends, are shown in Fig. 3d. For

the PLA component, more crystal nuclei and perfect crystals

are formed by the heterogeneous nucleation promoted by the

presence of PBS and chemical cross-link points. In addition to

this, the first thermal DSC data is also provided because it is

main reason to determine the mechanical properties of the

blend that the crystalline state of the PBS and PLA

components in the original samples. Table 3 shows the DSC

results of the neat PLA/PBS (70/30) and PLA/PBS/HDI

(70/30/2) blends before and after annealing. It was observed

that the PLA and PBS samples’ crystallinity ( 𝑋𝑐 ) was

obviously improved after annealing compared with the

unannealed samples. Oyama et al.[52] proposed that the

increase of PLA matrix crystallinity is very important to

toughen the blends after studying the relationship between the

mechanical properties of PLA/poly (ethylene-co-glycidyl

methacrylate) (EGMA)and annealing of the blends. Therefore,

based on the above results, the annealed PLA/PBS/HDI blends’

impact toughness is significantly improved, which can be

ascribed to the good interface compatibilization between PBS

and PLA, and the increased crystallinity of PBS and PLA.

3.6 Chemical reaction of the PLA/PBS blends in the

presence of HDI

As an indispensable intermediate in the organic synthesis

process, isocyanate is widely applied in the preparation of

plastics, foams, elastomers and coatings. A variety of

polyurethane chemistries provide them with excellent

compatibility with other components and outstanding

mechanical and thermal properties.[53] In an isocyanate

monomer, unsaturated isocyanate groups (−NCO) have good

chemical reactivity, and urethane bonds can be formed by the

Table 3. Melting enthalpy (∆𝐻𝑚), cold crystallization enthalpy (∆𝐻𝑐𝑐) and relative crystallinity (𝑋𝑐) for the PLA/PBS/HDI blends.

Samples

(PLA/PBS/HDI) ∆𝑯𝒄𝒄(𝑷𝑳𝑨)(J/g) ∆𝑯𝒎(𝑷𝑳𝑨)(J/g) ∆𝑯𝒎(𝑷𝑩𝑺)(J/g) 𝑿𝒄(𝑷𝑳𝑨)(%) 𝑿𝒄(𝑷𝑩𝑺)(%)

Un-annealed 70/30/0 31.1 36.5 3.2 8.3 5.4

Annealed 70/30/0 - 32.3 20.2 49.6 33.7

Un-annealed 70/30/2 3.3 20.9 17.0 2.8 4.7

Annealed 70/30/2 - 32.1 20.9 49.3 34.8

ES Energy & Environment Research article

© Engineered Science Publisher LLC 2021 ES Energy. Environ., 2021, 12, 86-94 | 93

Scheme 1 Illustration of the Main Reactions in the PLA/PBS Blends in the Presence of HDI.

reaction between the -NCO groups of unsaturated isocyanates

and the -OH end groups of PLA and PBS. The reaction

products of PLA-HDI-PBS may also appear on the interface

through the combination of PLA and PBS molecular chain. As

a result, in this study both complex reaction products and some

side reactions can be obtained, but the later including

branching, crosslinking and degradation are not discussed.

Scheme 1 shows the typical chemical route used in this study.

4. Conclusions

After adding HDI to enhance in-situ compatibilization,

toughened PLA/PBS blends with biodegradation were

prepared by melt blending. Improved toughness was shown by

the well compatibilized blends, which depended on HDI

content. After the addition of HDI, the viscosity of the

PLA/PBS blend increased at low frequencies, indicating some

interaction between the two components. The PLA/PBS

(70/30) blend was a two-phase structure, which was shown by

SEM micrographs. Due to the addition of HDI, it was

observed that the domain size of the PBS particles was

reduced and the interface adhesion between the PBS and PLA

phases was better. From a two-phase island-sea type, the

morphology of the blends changed to a more uniform one. The

blends’ morphology improved due to the compatibilization

and the increased in-phase viscosity during processing.

Because of the deformation generated in the matrix, the impact

resistance and elongation at break of the blends improved,

which was caused by the debonding effect between the PBS

and PLA phases. In addition, the PLA crystallization rate was

accelerated by the PBS and chemical cross-link points via

acting as nucleating agents.

Acknowledgments

We acknowledge the National Natural Science Foundation of

China (Grant No. 51903092 and 52003111), the Key Program

of National Natural Science Foundation of China (Grant

No.51933004), the National Key Research and Development

Program of China (Grant No.2016YFB0302300), the Natural

Science Foundation of Guangdong Province

(2018A030313275), Guangdong Science and Technology

Plan Project (2019B020214003), Shenzhen Science and

Technology Plan Project (GJHZ20180929154602092).

Supporting information

Not applicable

Conflict of interest

The authors have no conflicts of interest to declare.

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