Enhanced Heat Deflection Resistance via Shear Flow-Induced

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Enhanced heat deflection resistance via shear flowinduced stereocomplex crystallization of polylactide systems Zheng-Chi Zhang, Zi-Hong Sang, Yan-Fei Huang, Jia-Feng Ru, Gan-Ji Zhong, Xu Ji, Ruyin Wang, and Zhong-Ming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02438 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

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ACS Sustainable Chemistry & Engineering

1

Enhanced heat deflection resistance

via

shear

flow-induced

2

stereocomplex crystallization of polylactide systems

3

Zheng-Chi Zhang,a Zi-Hong Sang,a Yan-Fei Huang,a Jia-Feng Ru,a Gan-Ji Zhong,a*

4

Xu Ji,b Ruyin Wang,c Zhong-Ming Lia*

5

a. College of Polymer Science and Engineering, State Key Laboratory of Polymer

6

Materials Engineering, Sichuan University, No.24 South Section 1, Yihuan Road,

7

Chengdu 610065, China

8

b. College of Chemical Engineering, Sichuan University, No.24 South Section 1,

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Yihuan Road, Chengdu 610065, China

10

c. Corbion Purac China, Unit 08-09, 30F, #6088 Humin Road, Minhang District,

11

Shanghai 201199, China

12

Email address of corresponding authours: *G.J.Z. [email protected]

13 14

ABSTRACT

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Stereocomplex crystal (SC) of polylactides (PLA) with melting point over 220 ºC

16

shows great potential to improve the heat deflection resistance of PLA. However, it is

17

still a challenge to fabricate PLA articles with high SC contents due to requirement of

18

high production efficiency and thus extremely large cooling rate. In the present work,

19

an upgraded injection molding, i.e. oscillation shear injection molding (OSIM), was

20

employed to impose intense shear flow on poly(L-lactide) (PLLA)/ poly(D-lactide)

21

(PDLA) samples. It is proved that even though large cooling rate existed, intense

22

shear flow provide by OSIM induced higher crystallinities of SC and well-defined 1

ACS Paragon Plus Environment


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lamellar structure in comparison with conventional injection-molded ones, which

2

subsequently resulted in high Vicat soften temperature (close to 200 ºC) and superb

3

heat deflection resistance in boiling water. To clarify the mechanism of shear-induced

4

SC formation, in-situ characterization with precisely controlled parameters was

5

performed by rheological measurements. More sensitive response of SC

6

crystallization kinetics to shear flow is observed compared to shear-induced

7

homocrystallites (HC), which is attributed to shear-induced stereoselective interaction

8

and the existence of transiently cross-linking network built through hydrogen bonds in

9

sheared PLLA/PDLA melts. These findings provide an effective method to prepare

10

PLA samples with promising heat deflection resistance without introducing any extra

11

component and scarifying its environmental friendliness.

12

KEY WORDS: injection-molded polylactide articles, stereocomplex crystal (SC),

13

flow-induced crystallization, highly heat deflection resistance.

14 15

INTRODUCTION

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As the most promising biodegradable thermoplastic and relatively low-cost

17

production from renewable resources, polylactides (PLA) possess considerable

18

mechanical properties and versatile processability.1,2 It has wide and potential

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applications in biomedical materials and other commodities for substitution of

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petroleum-based thermoplastics and has drawn a growing attention from both

21

academia and industry. However, due to the low crystallinity and low glass transition

22

temperature (Tg), PLA products usually exhibit a relatively low heat resistance in 2


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comparison with other commercial thermoplastics produced from petroleum. For

2

instance, the common PLA beverage cups with low crystallinity show obvious

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shrinkage and collapse after containing coffee at 100 ºC, vividly indicating

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unsatisfied heat resistance in the application area as daily necessities.3 Fortunately,

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lactic acid is a chiral molecule and PLA has two enantiomers, i.e. poly(L-lactide)

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(PLLA) and poly(D-lactide) (PDLA), which can pack into the unit cell

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simultaneously and yield stereocomplex crystal (SC).4,5 PLA SC with melting point

8

(Tm) over 220 ºC is the most promising method to improve the heat resistance of

9

PLA samples.6 This is because, in SCs, two complementary polymers usually have

10

specific interactions with each other, such as stereoselective van der Waals forces or

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intermolecular hydrogen bonds, thus resulting in denser chain packing than that in

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the homocrystallites (HC) of their parent enantiopure polymers.7-12 These

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interactions and denser unit structure grant PLA higher heat resistance6, better

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hydrolytic resistance13-14, and promising physical performance in biomedical

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applications such as drug delivery and tissue engineering15,16. Therefore, facilitating

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SC formation in PLA products has attracted great attentions in recent years.

17

Many previous results have revealed the formation of SC in PLLA/PDLA blends

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are affected by various factors, such as blend ratio17-22, optical purity21-23, thermal

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history24-27 and molecular weight17-19,28. Especially, when melt blending and

20

crystallization is employed to prepare PLLA/PDLA blends with Mw higher than 6000

21

g/mol, SC crystallization is significantly suppressed.20 Therefore, preparation of PLA

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samples with high SC content and high Mw through melt blending is a bottle-neck 3



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issue to obtain high heat-resistance and balanced performance. To solve this problem,

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many methods have been developed, including synthesis of macromolecules with

3

tailored topologies (stereoblock copolymer and star-shaped PLA)29-33, blending at a

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relatively low temperature between the melting point of SC and HC34, supercritical

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fluid treatment35 and the use of nucleating agents36. However, building PLA with

6

well-defined topology29-33 needs complicated and laborious synthetic process and

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organic solvents, which costs too much to be accepted by industry. Processing at low

8

temperature34 and introducing supercritical fluid35 only can provide powder with high

9

SC content but not real products. Note that, as reported by Xiong et al., favorable

10

formation of SC was achieved with the presence of nucleating agents such as N,

11

N’-dicyclohexylterephthalamide (TMB-5).36 However, the efficiency of nucleating

12

agents is usually very low in the case of large cooling rate during processing,

13

meanwhile, its introduction may cause some hygiene problems or unknown damage to

14

environment. When it comes to injection-molding, the widely used processing method

15

with high efficiency and large cooling rate, injection-molded PLLA/PDLA blends

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usually remain in amorphous state or exhibit very limited SC content due to the fast

17

cycle.37-39 In this case, as reported by Samuel et al., an obvious drop in storage

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modulus obtained by dynamic mechanical analysis (DMA) was observed lower than

19

60 oC in injection-molded PLLA/PDLA samples indicating poor heat resistance.37

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Thus, to the best of our knowledge, it is still challenging to obtain injection-molded

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PLA products with high SC content and reasonable heat deflection resistance without

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introducing any extra component. 4


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As well known, flow-induced crystallization (FIC) is broadly involved in

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industrial processing of semicrystalline polymer. Enhanced nucleation40 and unique

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crystalline superstructures41 can be expected in melt crystallization of polymers

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subjected to external flow field, which has attracted much attentions in past decades.

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Although some theoretical analysis42,43 and real processing (mainly spinning)26,44

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confirm that crystallization of SC can be accelerated with the presence of flow field,

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however, SC formation under high cooling rate and intense shear flow have not been

8

explored, which is the crucial characteristics of external field during injection molding

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over other processing. In our group, a type of injection molding method called

10

oscillation shear injection molding (OSIM) has been successfully upgraded, which is

11

able to impose intense shear flow (in an order of magnitude of 103-104 s-1) on polymer

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melts during solidification at the packing stage, aiming to tune crystallinities and

13

crystalline superstructure of PLA injection-molded parts. Consequently, PLLA parts

14

with enhanced crystallinities and numerous shish-kebabs are successfully prepared

15

accompanied by the increased mechanical performance.45 Analogously, by the

16

combination of shear flow and efficient nucleating agents (carbon nanotubes, CNT), a

17

remarkably high crystallinity in the injection-molded PLA part has been achieved

18

successfully through OSIM processing, which offers a new method to fabricate PLA

19

products with high crystallinity.46 Thus, OSIM is a desirable tool to investigate SC

20

formation under intense shear flow and high cooling rate, which will get more insight

21

into the formation of SC in real conditions of processing.

22

In the present work, we are curious about whether the intense shear flow provided 5



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by OSIM technique could induce SC formation in injection-molded PLA parts to

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improve their heat resistance. Injection-molded parts of equimolar PLLA/PDLA

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blends were fabricated via OSIM technique. Impressively, PLA parts with nearly

4

exclusive formation of SC and high total crystallinity were obtained with the help of

5

the intense shear flow in comparison with those amorphous samples prepared through

6

commercial injection-molding (CIM). Out of our expectation, superb heat resistance

7

was successfully achieved in injection molded PLA part. Based on these exciting

8

results, the mechanism of flow induced formation of SC was further clarified. Thus,

9

the crystallization of HC and SC was traced by rheometer after PLLA and

10

PLLA/PDLA melts subjected to shear flow with an identical Weissenberg number

11

under identical supercooling. Interestingly, SC’s response to shear flow was much

12

more sensitive than that of HC, indicating shear induced SC formation was not only

13

an illustrative example of shear induced crystallization but also involved other

14

procedure and mechanism to be settled. Our findings provide an effective method to

15

prepare PLA samples with good heat deflection resistance without introducing any

16

extra component and scarifying its environmental friendliness, which is meaningful to

17

enlarge the usages of PLA material.

18 19

EXPERIMENTAL SECTION

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Materials PLLA (trade mark L130, with Mw= 17.3 × 10 g/mol, Mn= 8.9 ×

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10 g/mol and PDI=1.95 according to GPC tests) and PDLA (trade mark D1010, with

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Mw=8.5 × 10 g/mol, Mn=4.7 × 10 g/mol and PDI=1.82 according to GPC tests), a 6


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semicrystalline grade PLA containing around 0.2% D-LA or L-LA, respectively, was

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kindly supplied by Corbion Company (Amsterdam, Netherland).

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Sample Preparation To avoid degradation due to hydrolysis and prevent the

4

formation of voids during processing, PLLA and PDLA pellets were dried at 80 °C

5

under vacuum overnight before extrusion. The PLLA/PDLA with equimolar ratio

6

were melt mixed by employing a twin-screw extruder. The screw speed of the

7

twin-screw extruder was kept constant at 200 rpm, and the processing temperature

8

profile was 190, 220, 230, 240, 240 and 200 ºC from hopper to die, respectively. The

9

blended pellets were injection-molded into dumbbell bars by utilizing the oscillation

10

shear injection molding (OSIM) technique and commercial injection-molded (CIM)

11

for comparison, the details of which will be described in the next section. PLA

12

samples prepared through OSIM and CIM were denoted as LD-OSIM and LD-CIM

13

for brief, respectively, where LD stands for PLLA/PDLA blends.

14

OSIM technique Different from CIM, the key feature of OSIM technique mainly

15

involves the modified injection mold and the packing stage of injection cycle, as

16

shown in Figure S1 in Supporting Information. During packing stage of OSIM

17

process, the PLA melt in cavity was subjected to a controlled shear flow continuously,

18

which was driven by two hydraulically actuated pistons those move reciprocally at the

19

same frequency of 0.3 Hz and the pressure of 12 MPa. The shear flow does not cease

20

until the gate solidified. The details of OSIM technique were described elsewhere

21

before.47-49

22

The temperature profile for OSIM was 220, 250, 250, 240, and 210 °C from hopper 7



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1

to nozzle, respectively. The temperature of hot runner was set at 260 ºC for all

2

samples. The mold temperature, packing time, and packing pressure during injection

3

molding were the same for all blends at 40 ºC, 270 s, and 60 MPa, respectively. The

4

CIM samples, just for comparison, were prepared with the same processing

5

parameters (only without the oscillation shear).

6

X-ray Diffraction (or Scattering) Measurements Two-dimensional (2D)

7

wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS)

8

measurements were conducted at the BL15U and BL16B (λ = 0.124 nm) in Shanghai

9

Synchrotron Radiation Facility (SSRF), with a Mar CCD (with a resolution of

10

1024×1024 pixels and pixel size=80 µm) as a detector. The distances between sample

11

and detector were 175 and 2240 mm for WAXD and SAXS, respectively.

12

The samples for X-ray measurements were obtained from the LD-OSIM and

13

LD-CIM parts as shown in Figure S2 in Supporting Information with a dimension of

14

6×6×1 mm3. Four positions from the surface to the core of injection-molded part, i.e.,

15

0, 1000, 2000 and 3000 µm away from surface, were scanned by X-ray beam,

16

respectively.

17

WAXD and SAXS Data Analysis The profiles of 1D-WAXD were gained on the

18

basis of the circularly integrated intensities of 2D-WAXD patterns. Then, by using the

19

deconvoluting-peak technique, the crystallinities of SC (Xc-sc) were calculated by the

20

following equation:

21

=

22

where A110, A300/030 and A220 are the area of (110), (300)/(030) and (220) reflection

/ ∑ ∑ !"

(1)

8


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1 2 3

peak of PLA SC, respectively.26,50 Similarly, the crystallinities of HC (Xc-sc) were given by /

# = ∑

∑

(2)

!"

4

where A110/200 and A203 are the area of (110) /(200) and (203) reflection peak of PLA

5

HC, respectively.

6

Linear SAXS profiles were obtained from circular integration of intensities from

7

2D-SAXS images, respectively. The intensity was plotted as a function of the

8

scattering vector, q, where |q|=4πsinθ/λ, with λ being the wavelength of the incident

9

beam (0.124nm) and 2θ being the scattering angle.

10

Vicat softening temperature (VST) tests The VST tests were carried out with

11

LD-OSIM and LD-CIM samples according to ISO-306. The heating rate during tests

12

was 12 ºC/6min and the area of indenter was 1 mm2 applying the load of 1 MPa.

13

Hot-water resistance tests Limited by the length of LD-CIM and OSIM samples

14

(80 mm), we could not conduct standard heat deflection temperature tests with these

15

samples. In this case, an analogous visible heat resistance test was designed and

16

employed to solve this problem. Firstly, PLA samples with two supports under a

17

loading of 100g were immersed in adequate water as heat-transfer medium. Then,

18

water was heated to boil at the rate of about 2 ºC/min and the temperature was

19

monitored by thermocouple. During heating, digital photos of PLA samples were

20

taken to detect their bending and deformation.

21

Rheological measurements of crystallization The crystallization of HC and SC

22

was monitored by a rotational rheometer (HAAK MARS, Thermo Scientific) with 9



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1

parallel plate geometry (20 mm diameter). The gap between two plates was 700 µm

2

during measurements. The shear rates ($% ) of pre-shear flow imposed on PLLA and

3

PLLA/PDLA melts were set as 150 and 1500 s-1, respectively, which makes sure that

4

the Weissenberg numbers (&' = $% × () , the related rheological tests and calculations

5

to determine () under Tc were also shown in Supporting Information) of PLLA

6

chains after shear keep a constant of 1.2 under different temperatures.51 The shear

7

temperature and isothermal crystallization temperature (Tc) of neat PLLA and

8

PLLA/PDLA blend were chosen at 166 and 215 ºC, respectively, i.e. 20 ºC below the

9

equilibrium melting point of HC (186 ºC) and SC (235 ºC). (The related experiments

10

and raw data to determine the equilibrium melting point of HC and SC are shown in

11

Supporting Information.) These experiments are named as identical supercooling &

12

Weissenberg-number experimental of shear induced crystallization (ISIW-SIC for

13

short) in this work.

14

The protocol of ISIW-SIC experiments is described below: (i) The samples

15

prepared through compression molding were heated to 5 ºC above the equilibrium

16

melting point of HC or SC and held at this temperature for 3 min in order to remove

17

any residual crystals before tests; (ii) the temperature was decreased to Tc and kept at

18

this value for a sufficient time to ensure the completion of isothermal crystallization;

19

(iii) a pre-shear at shear rate of 150 or 1500 s-1 was applied to neat PLLA or

20

PLLA/PDLA melts as soon as the temperature reached Tc (quiescent crystallization

21

processes without pre-shear were also performed for comparison); (iv) the time

22

evolution of the storage modulus (G’) was followed during the crystallization process 10


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1

by oscillatory tests that were conducted using an angular frequency of 1 Hz and a

2

strain of 1%.

3

Note that the relative crystallinity was estimated by applying a normalization of the

4

G′ data obtained in procedure (iv) as follows:

5

+,- =

6

4 where 234 and 2567 was the value of the starting storage modulus and ending

7

plateau storage modulus in the process of crystallization, respectively.

. / +0-.!/ .!/ . / 1

(3)

8 9

RESULTS

10 11

Figure 1. 2D-WAXD patterns at different positions of injection-molded PLA samples:

12

(a) LD-CIM and (b) LD-OSIM.

11



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1 2

Figure 2. 1D-WAXD profiles circularly integrated from 2D-WAXD patterns at

3

different positions of injection-molded PLA samples: (a) LD-CIM and (b) LD-OSIM.

4 5

Promoted SC formation in LD-OSIM samples To reveal the crystal forms in

6

injection-molded PLA samples, typically two dimensional WAXD (2D-WAXD)

7

patterns and corresponding one dimensional WAXD (1D-WAXD) profiles are shown

8

in Figure 1 and 2, respectively. In LD-CIM samples (Figure 1a and 2a), only very

9

weak diffraction rings and peaks of SC are noticed in the layer of 1000 µm away from

10

surface in addition with amorphous halo in other positions indicating that only

11

amorphous PLA parts can be obtained through CIM. For comparison, in LD-OSIM

12

samples, as shown in Figure 1b and 2b, only SC with the characteristic reflections

13

from lattice planes of (110), (300)/(030) and (220) is detected except the position

14

1000 µm away from surface where the diffraction rings of both SC and HC are

15

observed. This contrast between LD-OSIM and CIM samples clearly demonstrates

16

that the intense shear flow provided by OSIM is beneficial for the formation of SC

17

under large cooling rate in injection-molded parts. In addition, different from our

18

previous results45,46, SCs induced by shear flow are isotropic as confirmed by the 12


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1

isotropic diffraction rings shown in Figure 1b. The high melt temperature, leading to

2

fast relaxation of PLA chains, may be responsible for this phenomenon. The reason

3

why only isotropic SCs are induced in LD-OSIM samples will be further explained

4

with the help of rheology measurements in Discussion section.

5 6

Figure 3. Crystalline information (crystallinities of HC and SC) at different positions

7

in LD-CIM and OSIM sample.

8 9

To further understand the crystallinity of different samples as a function of

10

position, the estimated crystallinities of SC (Xc-sc) and HC (Xc-hc) in LD-OSIM and

11

CIM samples obtained by iterative peak fit procedure are shown in Figure 3. As a

12

whole, the so-called skin-core structure is observed in injection-molded PLA samples,

13

especially in LD-OSIM samples. The crystallinities in intermediate layers (from 1000

14

to 2000 µm away from surface) of LD-OSIM samples where the shear stress is

15

provided by OSIM are much higher than that in skin and core layers. The formation of

16

skin-core structure of OSIM samples will be further discussed in Discussion section. 13



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1

It is the most important that, in contrast with CIM samples, obviously increased

2

crystallinities, especially Xc-sc, are obtained in OSIM samples. As illustrated in Figure

3

3, LD-CIM samples seem to be nearly amorphous with very low content of SC and

4

the absence of HC. Xc-sc is in the range from 0.01 to 0.03 and Xc-hc are 0 in every layer.

5

The trace of SC in LD-CIM samples may be the results of shear-induced SC

6

formation because of the shear flow existing during mold filling stage of injection

7

molding cycles and appropriate thermal environment. In contrast, Xc-sc in intermediate

8

layer of LD-OSIM samples are much higher ranging from 0.24 to 0.35 accompanied

9

by the Xc-hc around 0.01-0.05. Although HC contents are also slightly promoted in

10

comparison with LD-CIM samples, crystal structure of LD-OSIM samples in

11

intermediate layer is SC dominant as reflected by the relative SC content (the

12

proportion of SC in total crystals, ca. 83% to 100%). A promoted, comparing with

13

LD-CIM samples, but relatively lower SC contents, ranging from 0.09 to 0.16, are

14

observed in skin (0 and 500 µm) and core (2500 and 3000 µm) layers of LD-OSIM

15

samples. In addition, no trace of HC is noticed in skin and core layers besides the

16

position 500 µm away from surface with Xc-hc of 0.03. From WAXD results, a rough

17

conclusion can be drawn that although low mold temperature (40 ºC) was employed

18

both in OSIM and CIM, SC formation at large cooling rate can be obviously induced

19

by the intense shear flow going with negligible effects on the formation of HC, which

20

leads to almost exclusive SC in LD-OSIM samples.

14


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1 2

Figure 4. (a) 2D-SAXS patterns and (b) corresponding 1D-SAXS curves of LD-CIM

3

and OSIM samples.

4 5

Well-defined lamellar structure in LD-OSIM samples The two-dimensional

6

SAXS (2D-SAXS) patterns and corresponding one-dimensional SAXS (1D-SAXS)

7

profiles of LD-OSIM and CIM samples are shown in Figure 4. No obvious scattering

8

signal is noticed in LD-CIM samples due to the absence of ordered lamellar structure

9

of crystals judging from the fuzzy scattering halo and flat 1D-SAXS curve. On the

10

contrary, a clear scattering pattern and sharper peak are observed in LD-OSIM

11

samples, indicating distinct electron density differences in these samples, i.e.

12

relatively well-defined and impeccable crystals are induced through OSIM process.

13

What’s more, in line with WAXD results, the scattering patterns of LD-OSIM samples

14

are also isotropic as shown in Figure 4a confirming once again that no oriented

15

nanostructure can be detected through SAXS tests. In short, well-defined lamellar

16

structure of crystals without preferential orientation can be induced by shear flow 15



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1

provided by OSIM. The long period (L) between the adjacent lamella is calculated

2

using the Bragg equation, 8 = 2:/; ∗ , here L is long period, and ;∗ represents the

3

peak position in the scattering curves. Apparently, in this work, the LD-OSIM

4

specimen containing SCs have a maxima around q=0.34 nm-1 in the scattering curves,

5

suggesting that the L is 18.5 nm. Due to the lack of ordered lamellar structure and

6

scattering maxima, L of LD-CIM samples can not be calculated. In our previous work,

7

HCs in neat PLLA sample with L of 20.8 nm were obtained through OSIM.45

8

Comparing with HCs, SCs show a reduction of ~2 nm in long period, which is

9

probably a result of more compact structure. And more remarkably promoted

10

nucleation sites of SC under shear flow should also be taken into account.

11

12 16


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1 2

Figure 5. (a) Vicat softening temperature of LD-CIM and OSIM samples; (b) digital

3

photos during visible heat deflection resistance test in hot water; (c) digital photo of

4

LD-CIM and OSIM sample after hot-water deflection resistance tests.

5 6

Heat deflection resistance of injection-molded PLA samples To evaluate the

7

heat deflection resistance of LD-CIM and OSIM samples and clarify the effect of SC

8

on the heat deflection resistance of PLA samples, the results of Vicat softening

9

temperature (VST) and hot-water heat resistance test are shown in Figure 5. The VST

10

of LD-CIM samples only reaches 61 ºC, around Tg of PLA, due to their amorphous

11

structure. By contrast, the VST of LD-OSIM samples with the formation of SC

12

induced by intense shear flow is 196 ºC, indicating the superior heat deflection

13

resistance. The hot-water heat resistance tests visibly show similar results, which is

14

close to the actual service conditions of PLA samples. As shown in Figure 5b, slight

15

bending of LD-CIM sample is noticed when water temperature is 58.0 ºC and

16

approaches Tg of PLA. With the temperature above Tg, i.e. 66.4 and 73.1 ºC, distinct 17



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1

blending is observed. When it comes to LD-OSIM samples, no bending or

2

deformation is detected even when water temperature reaches 96.5 ºC close to its

3

boiling point, showing a markedly promoted heat deflection resistance in comparison

4

with CIM ones. Furthermore, the digital photos of LD-CIM and OSIM sample after

5

hot-water heat resistance tests are shown in Figure 5c. No deformation or bending is

6

observed in LD-OSIM sample after maintained in boiling water for 10 min, however,

7

LD-CIM sample cannot bear the load any more when the temperature only reaches 80

8

ºC as illustrated by the distorted LD-CIM sample in Figure 5c. Thus, we are confident

9

to claim that LD-OSIM samples that can stand boiling water is adequate to serve as

10

daily necessities like containers and beverage cups.

11 12

DISCUSSION

13

As reported above, fully biodegradable PLA injection-molded parts with superior

14

heat deflection resistance can be obtained, without the addition of neither nucleating

15

agents nor other foreign components, which has no harm to their environmental

16

friendliness and wholesomeness. In comparison with amorphous LD-CIM samples,

17

the enhanced SC formation induced by intense shear flow provided by OSIM

18

processing should be responsible for this phenomenon. To the best of our knowledge,

19

this is first time that injection-molded PLA articles possess high SC content and super

20

heat deflection resistance with VST nearly 200 ºC. Although electrospinning was also

21

reported as an effective method to prepare SC nanofibers with high SC content,26,42

22

our method, i.e. a kind of modified injection-molding, is a more practical and 18


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1

widely-used way to fabricate three dimensional PLA products. Inspired by the

2

noticeable enhanced SC content due to the intense shear flow provided by OSIM, we

3

are interested to clarify the kinetics and mechanism of flow-induced SC formation.

4

Kinetics of shear-induced crystallization of PLLA/PDLA by identical

5

supercooling & Weissenberg-number experiment of shear induced crystallization

6

(ISIW-SIC) According to the theoretical analysis with the aid of in-situ

7

measurements, such as WAXD, SAXS and polarized optical microscopy (POM), the

8

mixing effect and promoted intermolecular nucleation were proposed to explain the

9

shear induced SC formation.42,43 However, solid conclusion still cannot be drawn due

10

to their defective experiments, where the un-blended PLA samples are omitted. Thus,

11

it is still under debate that the shear-induced SC formation either is an illustrative

12

example of shear induced crystallization or involves other un-settled procedure and

13

mechanism due to the lack of comparison with normal shear induced crystallization of

14

HC. To solve this problem, precisely controlled identical supercooling &

15

Weissenberg-number during shear induced crystallization (ISIW-SIC) experiments

16

were performed in both neat PLLA and PLLA/PDLA melts, where both supercooling

17

and Weissenberg number of neat PLLA and PLLA/PDLA melts are exactly identical.

18

From the aspect concerning flow-induced crystallization of polymers, the most

19

widely recognized mechanism is based on the idea of flow-induced entropic reduction

20

(∆>? ) of polymer melt due to orientation and stretch of chains.52-54 Through a simple

21

additive rule (∆F = ∆AB + D∆>? ), the effect of shear flow, i.e. ∆>? , is incorporated

22

into the classic nucleation theory, lowering nucleation barrier and promoting 19



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1

nucleation rate, where ∆F and ∆AB are the thermodynamic driving force for

2

nucleation under flow and quiescent conditions, respectively.55 In other words, driving

3

force of SC crystallization under shear flow can be divided into two parts, i.e. the

4

driving force dominated by supercooling (quiescent condition) and driving force

5

provided by shear flow. Thus, to clarify the mechanism of shear-induced SC

6

formation, the crystallization conditions for controlled neat PLLA and PLLA/PDLA

7

melts must be chosen carefully.

8

To ensure the same supercooling, the equilibrium melting points of both HC and

9

SC were determined through crystallization-melting experiments (details of these

10

experiments are shown in Supporting Information). The equilibrium melting point of

11

HC is 186 ºC and that of SC is 235 ºC. Thus, in ISIW-SIC experiments, the shear and

12

isothermal crystallization temperature was set at 166 ºC for neat PLLA melts and 215

13

ºC for PLLA/PDLA melts, which ensures the precisely identical supercooling (20 ºC)

14

for the crystallization of both HC and SC. In this case, the driving force provided by

15

supercooling (under quiescent condition) should be the same between neat PLLA and

16

PLLA/PDLA blends. Note, although higher melt temperature (260 ºC) was employed

17

during OSIM processing, the chosen crystallization temperature in ISIW-SIC

18

experiments is appropriate to track the crystallization of both HC and SC growth

19

during shear induced crystallization, higher crystallization temperature will lead to

20

slow crystallization rate and overlong experimental time, which may cause inevitable

21

degradation of PLA. On the other hand, the temperature in mold cavity decreases

22

rapidly during injection-molding processing because of the low mold temperature and 20


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1

the actual temperature for SC formation should be much lower than 260 ºC.

2

Besides the driving force provided by supercooling, the driving force resulted

3

from shear flow should also be taken into consideration. From a rheological viewpoint,

4

the effect of shear flow on the chain conformation can be assessed by defining two

5

characteristic Weissenberg numbers for molecular orientation and stretch: EF) =

6

() × $% and EFG = (G × $% with $% being the shear rate and () and (G being the

7

reptation time (disentanglement time) and the Rouse time (stretch relaxation time) of

8

polymer chains, respectively, as proposed by van Meerveld et al.,51,56 when

9

EF) < EFG < 1, the polymer chains are in their equilibrium state; thus, shear flow

10

has no effect on crystallization. Generally, polymer chains tend to be oriented for

11

EF) > 1 and EFG < 1, which is necessary for the enhancement of the number

12

density of activate nuclei. With the further increase of shear rate, EFG > 1 will be

13

reached where polymer chains tend to be stretched, which ensures sufficient stretch of

14

the polymer chains into a conformation ideal for the formation of oriented nuclei

15

(such as shish-kebab structure). In this work, only EF) > 1 is taken into

16

consideration because EFG > 1 is quite difficult to achieve at the relative high shear

17

and isothermal temperature (166 and 215 ºC), which is sufficient to promote the

18

nucleation and crystallization of both HC and SC.

19

Table 1. () of PLLA and PDLA chains under 166 and 215 ºC. 166 ºC () (s) PLLA 8 × 10J

215 ºC

1/() (s-1)

() (s)

1/() (s-1)

125

8 × 10

1250

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PDLA 1

-a

-a

3 × 10K

33333

a. () of PDLA under 166 ºC is not necessary for our experimentals.

2 3

To precisely control the EF) of PLA chains, the reptation time (() ) should be

4

determined under shear temperature through classical rheology tests and calculations.

5

The () of PLLA was confirmed as the reciprocal of the frequency where storage

6

and loss modulus have the same value under 166 ºC. And the () of PLLA and

7

PDLA under 215 ºC was calculated according to time-temperature superposition

8

principle. Details of these experiments and calculations are shown in Supporting

9

Information for brief. Finally, the () of PLLA chains under 166 and 215 ºC are

10

shown in Table 1 and the corresponding critical shear rate to ensure EF) > 1, i.e.

11

1/() , are also listed in this table. We have to mention that the () of PDLA chains

12

under 215 ºC is only 3 × 10K due to their low molecular weight and fast relaxation

13

at high temperature. Thus, to force PDLA chains orientation at 215 ºC is quite difficult,

14

which has been confirmed by the critical shear rate (over 30000 s-1) shown in Table 1.

15

Limited by the max shear rate (2000 s-1) can be provided by rheometer employed in

16

our work, EF) > 1 of PDLA chains is impossible to be reached at 215 ºC.

17

Based on aforementioned discussion, experiments and calculations, the controlled

18

EF) is chosen at 1.2 for both neat PLLA and PLLA/PDLA melts. Therefore, the

19

shear rates imposed on neat PLLA and PLLA/PDLA melts is 150 and 1500 s-1,

20

respectively. The shear rate for PLLA/PDLA melts (1500 s-1) is very close to that in

21

the mold cavity (~103 s−1) during OSIM processing. Meanwhile, it is still worth to 22


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1

mention that rheology tests can only reflect the general behavior of polymer chains

2

and average relaxation time in the melts regardless of molecular weight distribution.

3

Thus some PLLA chains with high molecular weight and long relaxation time could

4

be stretched even when EFG > 1 is not achieved calculated from average relaxation

5

time. In same way, high molecular weight PDLA chains might also be oriented under

6

215 ºC even the apparent relaxation of PDLA chains is fast and apparent EF) is

7

lower than 1.

8

Eventually, all experimental parameters of ISIW-SIC experiments are determined.

9

Thus, for neat PLLA melts, the shear and isothermal crystallization temperature and

10

shear rate of pre-shear are 166 ºC and 150 s-1, respectively. And those of PLLA/PDLA

11

melts are 215 ºC and 1500 s-1, respectively. Through the precisely controlled

12

experimental parameters, the crystallization driving force of HC and SC should be the

13

same. To be honest, it is quite difficult to simulate the shear induced crystallization of

14

SC during rheological measurements because OSIM features non-isothermal

15

crystallization, non-equilibrium and non-steady shear. Even though, we tried our best

16

to build a bridge between the real processing and ideal ISIW-SIC experiments and the

17

results of ISIW-SIC experiments should be helpful to understand the SC formation

18

and structure evolution in OSIM samples.

23



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Page 24 of 58

1 2

Figure 6. Relative crystallinity, Xc(t), as a function of time during ISIW-SIC

3

experimentals for (a) neat PLLA and (b) PLLA/PDLA blends.

4

Table 2. The kinetics information, i.e. t0 and t1/2, of sheared and quiescent melts. t0 (s)

t1/2 (s)

quiescent

sheared

quiescent sheared

Neat PLLA

17455

5666

20977

6730

PDLA/PDLA

4720

Not detected

7357

124

5 6

The results of ISIW-SIC experiments are shown below (Figure 6). As illustrated

7

in Figure 6a, for neat PLLA melts, the relative crystallinity ( +,-) vs. time curves

8

show a typical sigmoidal shape exhibiting a progression of +,- from about 0

9

before the starting of crystallization to a rapid increase and then to a plateau values at

10

the end stage of primary crystallization. The time at the intersection of the highest

11

slope of the +,--time curve with the horizontal line at 0 of +,- (indicated by the

12

blue dashed lines in Figure 6a) is often defined as the induction time, t0, for

13

crystallization.56 Analogously, the time at the intersection of the +,--time curve

24


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1

with the horizontal line at 0.5 of +,- (indicated by the blue dashed lines in Figure

2

6a) is often defined as the crystallization half-time, t1/2. As shown in Figure 6, the t0

3

and t1/2 of both HC (Figure 6a) and SC (Figure 6b) are obviously reduced after being

4

subjected to shear flow. Especially, the t0 of SC cannot be detected due to its fast

5

crystallization kinetics. According to the crystallization process shown in Figure 6, the

6

corresponding kinetics information are summarized in Table 2. The crystallization

7

process of neat PLLA under 166 ºC and quiescent condition lasts for a long time due

8

to the relative low super-cooling, exhibiting that t0 and t1/2 are 17455s and 20977s,

9

respectively. When neat PLLA is subjected to shear flow with EF) of 1.2, the

10

crystallization kinetics is obviously promoted with a reduced t0 of 5666s and t1/2 of

11

6730s. In PLLA/PDLA melts, t0 and t1/2 of SC crystallization are 4720s and 7357s,

12

respectively. After subjected to shear flow with EF) of 1.2, a dramatically enhanced

13

crystallization rate of SC is observed with undetectable t0 and tremendously reduced

14

t1/2 of only 124s.

15

Based on the ISIW-SIC results shown above, the first thing we want to claim is

16

that the intrinsic differences in crystallization kinetics between HC and SC are

17

observed under the same supercooling without shear flow. As observed in other

18

enantiopure

19

poly((R)-propylene

20

significantly enhanced crystallization and crystallinity have been reported due to

21

stereocomplexation.7,57 This phenomenon indicates that convincing conclusions

22

cannot be drawn as in many reported works through the promoted crystallization

polymers,

for

succinate)

instance and

poly((S)-propylene

enantiopure

succinate)

poly(limonene

25


and

carbonate),


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

kinetics of SC under shear flow in comparison with quiescent ones. In other words,

2

stereocomplexation must be taken into consideration during shear-induced SC

3

formation. We have to say that the lack of controlled samples and arbitrary choice of

4

experimental parameters (temperatures and shear rates) in published works should be

5

responsible for these unconvincing conclusions. Under this circumstances, only

6

performing a precisely controlled experiments like ISIW-SIC can we clarify the

7

mechanism of shear-induced SC formation.

8 9

Figure 7. The comparison of crystallization kinetics’ response to shear flow, i.e. the

10

ratio of t0 and t1/2 between sheared and quiescent melts, between HC in neat PLLA

11

and SC in PLLA/PDLA blends. The ratio of t0 between sheared and quiescent melts in

12

PLLA/PDLA blends is termed as null because the t0 is not detected in sheared

13

PLLA/PDLA blends.

14 15

Another important result should be paid attention to is the furious crystallization 26


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1

process of SC in sheared PLLA/PDLA melts in comparison with crystallization of HC

2

in sheared PLLA melts. Quantitively, the ratio of t0 and t1/2 between sheared and

3

quiescent melts calculated from the results in Table 2 are summarized in Figure 7. In

4

neat PLLA melts, the ratio of t0 is 3.08 and that of t1/2 is 3.13, indicating that the

5

crystallization rate of HC in sheared melts is about 3 time as fast as that in quiescent

6

melts. Based on the concept of ISIW-SIC experiments providing the same driving

7

force of crystallization, the ratio of t0 and t1/2 between sheared and quiescent

8

PLLA/PDLA melts should be around 3 as observed in neat PLLA melts. However, the

9

ratio of t0 is impossible to be calculated due to the undetectable t0 in of SC in sheared

10

PLLA/PDLA melts (an extremely large value outdistancing 3) and the ratio of t1/2 is

11

nearly 60 that is also far greater than 3, indicating that the crystallization kinetics’

12

response to shear flow in sheared PLLA/PDLA melts is more sensitive compared with

13

that of HC in neat PLLA melts. Consequently, different from the typical

14

shear-induced crystallization happening in neat PLLA melts, some new mechanism

15

must be involved during shear-induced SC formation in PLLA/PDLA melts.

16 17

Figure 8. Schematics of the crystallization process from sheared and quiescent 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

PLLA/PDLA melts.

2 3

Mechanism of shear-induced SC formation In the term of following parts of

4

Discussion, the mechanism of shear-induced SC formation is proposed and illustrated

5

in Figure 8.

6

Different from the crystallization of HC, the formation of SC is usually

7

accompanied by the stereoselective interaction (e.g., stereoselective van der Waals

8

forces or intermolecular hydrogen bonds) as reported previously.7-12 A red shift of

9

about 2 cm-1 was observed by Sarasua et al. for both the CH3 and CαH stretching

10

regions during SC crystallization, which increases by 4 cm-1 after cooling to room

11

temperature. These spectral changes suggested an arrangement for the hydrogen

12

bonds in the form of multicentric interactions.10 Furthermore, Zhang et al.

13

investigated the nature of the “peculiarly strong” interaction between PLLA and

14

PDLA during the isothermal melt crystallization of SC and confirmed that the

15

interaction between the PLLA/PDLA stereocomplex was ascribed to CH3···O=C

16

hydrogen bonding. And their results indicated that the CH3···O=C interaction was the

17

driving force for forming the racemic nucleation of the PLLA/PDLA stereocomplex.9

18

Pan et al. also reported the frequency shifts in stereocomplex crystallization and the

19

correlation field splittings at cryogenic condition suggesting the coexistence of weak

20

C-H···O=C hydrogen bonds and dipolar interactions between the PLLA and PDLA

21

chains packed in SC of PLA.11 Based on all these results, a clear conclusion can be

22

drawn that interchain interactions play an important role in the polymorphism of PLA, 28


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1

especially the SC formation. In other words, crystallization of SC can be accelerated

2

through the promoted intherchain interactions or stereoselective interactions. However,

3

in quiescent PLLA/PDLA melts, PLA chains are in coiled state and entangled with

4

each other as most polymers do. In this case, the stereoselective interactions between

5

PLLA and PDLA chains could be quite limited because these interactions only take

6

place on the surface of PLA coils where PLLA and PDLA can contact with each other.

7

Therefore, it is not wise to expect high SC content in quiescent PLLA/PDLA melts.

8

The situation will be different in sheared PLLA/PDLA melts. When subjected to shear

9

flow, both PLLA and PDLA chains tend to transform from random coil state to

10

orientation state (Route I in Figure 8). In this case, more carbonyl groups are exposed

11

in oriented PLA chains and better chances of interacting with each other for PLA

12

chains can be expected, which is beneficial for the formation of hydrogen bonds or

13

other stereoselective interactions between PLLA and PDLA chains. Therefore, more

14

SC nuclei are expected to form due to the promoted stereoselective interaction (e.g.,

15

stereoselective van der Waals forces or intermolecular hydrogen bonds) as shown in

16

Nuclei part (sheared melt) in Route I of Figure 8,7-12 which leads to high SC content

17

in the following crystallization process (Crystal part in Route I of Figure 8).

18

On the other hand, due to the stereoselective interaction, i.e. hydrogen bonds,

19

between PLLA and PDLA chains, a transiently cross-linking network might also be

20

established in PLLA/PDLA melts as illustrated in Melt part of Figure 8. As discussed

21

in recent researches, entangled network is responsible for the promoted crystallization

22

kinetics and oriented superstructure like shish-kebab.58-62 The results of Seki et al. 29



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1

indicated that the role of long chains in shear-induced oriented crystallization is

2

cooperative (rather than a single chain effect), enhanced by long chain-long chain

3

overlap.60 Based on detailed morphology observations, Zhang et al. proposed that

4

shear-induced crystallization involved a large number of entangled molecules and

5

oriented cylindrite core came from the stretched bundles of the entangled network

6

strands but not from the extended crystals of stretched single chains.59 Yang et al. also

7

suggested that adequate entanglement points which help the chains retain their

8

orientation during and after shear due to long relaxation time.62 In our work,

9

transiently cross-linking network built by PLLA and PDLA trough the formation of

10

hydrogen bonds (Melt part of Figure 8) may also contribute to chain orientation. To

11

some extent, the effect of shear is magnified by the presence of transiently

12

cross-linking networks due to their long relaxation time. Consequently, as illustrated

13

in Nuclei part in Route II of Figure 8, more SC nuclei are supposed to be induced,

14

which is also responsible for high SC content (Crystal part of Figure 8).

15

In addition, the effect of SC nuclei and crystals on shear-induced crystallization

16

should also be taken into consideration. Wei et al. found that the flow-induced

17

crystallization of PLLA was enhanced with the existence of SC due to the

18

combination of cross-linking and filler effect of SC.63 Similarly, Bai et al. also

19

reported that the crystallization process of PLLA in the blends was greatly accelerated

20

under shear conditions due to the existence of SC crystallites.64 Based on the

21

improved mechanical properties of PLLA/PDLA blends, the presence of a higher

22

density of intercrystalline connections through a mobile amorphous phase was 30


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1

proposed by López-Rodríguez et al..28 All these result indicate that a network can be

2

built in PLLA/PDLA melt with nuclei or primary crystals of SC acting as cross-link

3

point. This phenomenon can also happen in our work when SCs are efficiently

4

induced by shear as discussed above, which provides sufficient cross-link point and

5

results in the formation of network. The presence of network embedded in melts will

6

restrict the relaxation of PLA chains and amplify the effects of shear flow.

7

Consequently, extra nuclei can be formed and more SCs will be induced.

8

All the discussions above have clarified the mechanism of shear-induced SC

9

formation and can explain the results of ISIW-SIC experiments, which is helpful to

10

understand SC formation during OSIM processing. Owing to the clarified SC

11

formation mechanism with shear flow, the inner structure of OSIM samples is

12

illustrated below. During OSIM processing, melt is first injected into the mold cavity

13

after melting. Then the two pistons move out of phase during the packing stage

14

providing intense shear flow with a shear rate up to ~103 s−1 in the mold cavity during

15

solidification.65 Although the melts close to mold wall are subjected to intense shear

16

flow during filling stage, it is difficult to obtain high crystallinity in skin layer (0 and

17

500 µm away from surface) of OSIM samples due to the fast cooling. In intermediate

18

layer (from 1000 to 2000 µm away from surface), taking the melt temperature (260 ºC)

19

and intense shear rate into consideration, it seems that EF) higher than 1 that is

20

necessary for shear-induced nucleation is slightly difficult to achieve according to the

21

discussion shown above. Even though, PLA chains with high molecular weight and

22

transiently cross-linking networks built by hydrogen bonds have potential to be 31



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1

oriented or deformed due to their relatively long relaxation time. Meanwhile, shear

2

flow is continuous and melt temperature is decreasing during OSIM processing.

3

Therefore, more and more PLA chains tend to be oriented and built stereoselective

4

interaction with other chains, which subsequently results in extremely rapid SC

5

nucleation and crystallization. Consequently, SC contents in intermediated layer of

6

OSIM samples are obviously higher than those in skin and core layer. When it comes

7

to core layer (2500 and 3000 µm away from surface), the cooling rate is relative slow

8

due to the poor thermal conductivity of polymer, leading to partially relaxation of

9

oriented PLA chains, which is responsible for the relatively low SC contents in core

10

layer. Finally, the skin-core structure was obtained through OSIM processing as

11

probed by WAXD tests.

12

Another key factor that should be paid attention to is the temperature of melt (260

13

ºC), which has important influence on crystallization of both HC and SC. According

14

to the published results10,26, HCs cannot crystallize and the pre-existing HCs will

15

transform into SCs at the temperature higher the melting point of HC. In our work, to

16

ensure considerable processability, melt temperature was set at 260 ºC that is much

17

higher than HC’s melting point. With the decrease of temperature in mold cavity

18

during cooling stage, even HCs are induced by shear flow, they will recrystallize into

19

SCs at the temperature above HC′s melting point. In a world, the temperature window

20

that is benefit for SC formation is quite wide in our work, which plays an important

21

role in SC′s crystallization. However, the contributions of shear flow cannot be

22

neglected. On one hand, as shown in Figure 6 and 7 and Table 2, the SC′s response to 32


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1

shear flow is much more sensitive to that of HC when they are subjected to identical

2

supercooling or shear flow with the same Weissenberg number, indicating that the

3

shear-induced crystallization of SC may prevail over that of HC. Furthermore,

4

comparing with the CIM samples prepared under similarly thermal condition, OSIM

5

samples contain high SC content with the aid of shear flow. Therefore, only high melt

6

temperature shared by both CIM and OSIM is unable to lead to high SC content but to

7

amorphous samples due to the fast cooling rate during injection-molding, favorable

8

growth of SC is attributed to the synergy of high melt temperature and shear-induced

9

nuclei of SC.

10

Although the mechanism of shear-induced SC formation during OSIM processing

11

has been discussed above, another phenomenon that HC contents are quite limited in

12

OSIM samples shall be concerned. As shown in Figure 3, HCs are barely observed

13

with crystallinities less than 0.05. Apparently, the crystallization of HC is obviously

14

suppressed even with intense shear flow provided during OSIM processing that

15

should also induce HC formation in OSIM samples. There may be three reasons those

16

are responsible for the low HC content. Firstly, the melt temperature was set at 260 ºC

17

during OSIM processing, which is much higher than the melting point of HC. Thus,

18

no HC or nuclei of HC can be induced at this temperature. Secondly, when the melt

19

temperature cools down, abundant SCs have been induced in the melt with the aid of

20

shear flow, which results in the dramatical increase in viscosity of the melt. Thus,

21

shear force can not be conveyed to PLA chains due to the solid-like behavior of the

22

melt. In this case, shear-induced HC formation is difficult to happen. Thirdly, as 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

reported by Wei et al., crystallization of HC was obviously suppressed with the dense

2

SC network because the mobility of PLA chains was restricted.66 In our work, SC

3

contents are much higher than that in Wei’s work, which will also suppress the

4

crystallization of HC. Consequently, HC formation is quite limited in OSIM samples

5

even with shear flow.

6

Another fact should be paid attention to is the molecular weight difference

7

between PLLA (Mw=17.3 × 10 g/mol) and PDLA (Mw=8.5 × 10 g/mol) used in our

8

work. According to the results of Tsuji et al., when PDLA and PLLA with dissimilar

9

molecular weights were mixed, the lower molecular weight component might diffuse

10

to the higher molecular weight one, resulting in the nucleation and growth of SCs.18

11

Although PDLA with low molecular weight was employed in our work, Tsuji’s

12

viewpoint cannot be used to explain the efficient SC formation in OSIM samples,

13

because both the molecular weight of PLLA and PDLA is much higher than 6000

14

g/mol indicating that SC will be obviously suppressed in melting blended

15

PLLA/PDLA

16

formation in PLLA/PDLA blends with dissimilar molecular weights. Long relaxation

17

time is expected in PLLA due to its high molecular weight, which indicates that PLLA

18

chains are easier to be oriented with the presence of shear flow. On the other hand, the

19

mobility of PDLA chains with relatively low molecular weight are able to ensue SC

20

growth because low molecular weight is benefit for diffusion and conformation

21

adjustment. Thus, with the shear flow provided by OSIM, oriented PLLA chains with

22

high molecular weight can act as scaffold for SC formation and PDLA with relatively

20

. A conjecture is proposed here to explain the shear-induced SC

34


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1

low molecular weight can diffuse toward them to accomplish SC’s nucleation and

2

growth, which results in efficient SC formation in OSIM samples. In a word,

3

molecular difference between PLLA and PDLA makes contribution to shear-induced

4

SC formation in our work.

5

In addition, different from OSIM PLA samples in our reported works,45,46 SCs

6

induced by shear flow are isotropic but not oriented (see Figure 1a). Based on above

7

discussion and proposed mechanism. Firstly, when the shear temperature is 215 ºC

8

and shear rate is 1500 s-1, Weissenberg number (EF) ) of PLLA melts is only 1.2. In

9

this case, only accelerated nucleation but not the chain stretch and resulted oriented

10

crystals can be observed. For OSIM processing with higher melt temperature (260 ºC)

11

and shear rate up ~103 s−1, it is more difficult to induce oriented SC. Secondly, as

12

reported by Wei et al., the physical gel in PLLA/PDLA blends is formed at a PDLA

13

concentration of 2.0 wt% and the SC percolation concentration for the crystal

14

networks formation is 2.6 wt.%66, which is in agreement with Yang et al.67 and Bai et

15

al.64. As shown above, crystallization process of SC under shear flow is furious and

16

the physical gel and SC crystal network is quite easy to be achieved in mold cavity

17

during OSIM processing. Thus, a dramatically reduction in liquidity of PLLA/PDLA

18

melts may happen, which is also an adverse factor for the orientation of SC.

19 20

CONCLUSIONS

21

In this work, PLA articles with almost exclusive SC and superior heat deflection

22

resistance with enhanced VST near 200 ºC were successfully prepared through OSIM 35



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1

whose feature is providing continuously intense shear flow during packing stage.

2

More importantly, no obvious bending or deformation was observed in OSIM samples

3

after soaked in boiling water with a loading. These results provides an effective

4

method to prepare three dimensional PLA samples being adequate to serve as daily

5

necessities without introducing any extra component and scarifying its environmental

6

friendliness and wholesomeness, which is meaningful to enlarge the usages of PLA

7

material.

8

The mechanism of shear-induced SC formation was clarified through ISIW-SIC

9

experiments. When shear flow with the identical Weissenberg number was imposed,

10

the crystallization kinetics of HC and SC under the same supercooling was extremely

11

different. A rapid crystallization kinetics of SC was observed and crystallization

12

kinetics’ response to shear flow is more sensitive. The shear-induced stereoselective

13

interaction and the existence of transiently cross-linking network should be

14

responsible for the promoted crystallization kinetics. Although more detailed research

15

with the aid of in-situ WAXD and FTIR is still needed for further clarification, our

16

conclusion is in favor of the further understanding of SC formation under flow field.

17 18

ASSOCIATED CONTENT

19

Supporting Information

20

The Supporting Information is available free of charge on the ACS Publications

21

website at DOI: XXXX.

22

The schematic illustration of OSIM mold, diagrammatic drawing of X-ray 36


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1

measurements, the equilibrium melting point (Tm0) of HC and SC and the reptation

2

time (() ) of PLLA and PDLA, glass transition temperatures of injection-molded

3

samples and molecular weight of injection-molded samples are shown in Supporting

4

Information.

5 6

AUTHOR INFORMATION

7

Corresponding Authors

8

*G.J.Z. Telephone: +86 28 85400211. E-mail: [email protected]

9

*Z.M.L. Telephone: +86 28 85400211. E-mail: [email protected]

10

Notes

11

The authors declare no competing financial interest.

12 13

ACKOWNLEDGEMENTS

14

The authors gratefully thank the financial support from the National Natural Science

15

Foundation of China (51533004, 51473101, 51673135 and 51273131), the Innovation

16

Team Program of Science & Technology Department of Sichuan Province (Grant No.

17

2014TD0002), Doctoral Program of the Ministry of Education of China (Grant No.

18

20130181130012), and State Key Laboratory of Polymer Materials Engineering, PR

19

China (Grant No. sklpme 2014-3-08) and Purac Biochem B.V.. We also thank

20

Beamline BL16B and BL15U in Shanghai Synchrotron Radiation Facility (SSRF) for

21

supporting the X-ray measurement.

22 37



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branched

polylactide.


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Effects of high molecular weight component on crystallization of polyethylene under

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(62) Yang, H. R.; Lei, J.; Li, L. B.; Fu, Q.; Li, Z. M. Formation of Interlinked

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Shish-Kebabs in Injection-Molded Polyethylene under the Coexistence of Lightly

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Cross-Linked Chain Network and Oscillation Shear Flow. Macromolecules 2012, 45,

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G. Stereocomplex Crystallite-Assisted Shear-Induced Crystallization Kinetics at a

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High Temperature for Asymmetric Biodegradable PLLA/PDLA Blends. ACS

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Stereocomplex crystallite network in asymmetric PLLA/PDLA blends: Formation,

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structure, and confining effect on the crystallization rate of homocrystallites.

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Macromolecules 2014, 47, 1439-1448. 46


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(67) Yang, L.; Somani, R. H.; Sics, I.; Hsiao, B. S.; Kolb, R.; Fruitwala, H.; Ong, C.

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Shear-induced crystallization precursor studies in model polyethylene blends by in

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situ rheo-SAXS and rheo-WAXD. Macromolecules 2004, 37, 4845-4859.

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For Table of Contents Use Only

1 2

Enhanced heat deflection resistance

via

shear

flow-induced

3

stereocomplex crystallization of polylactide systems

4

Zheng-Chi Zhang,a Zi-Hong Sang,a Yan-Fei Huang.a Jia-Feng Ru,a Gan-Ji Zhong,a*

5

Xu Ji,b Ruyin Wang,c Zhong-Ming Lia*

6

a. College of Polymer Science and Engineering, State Key Laboratory of Polymer

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Materials Engineering, Sichuan University, No.24 South Section 1, Yihuan Road,

8

Chengdu 610065, China

9

b. College of Chemical Engineering, Sichuan University, No.24 South Section 1,

10

Yihuan Road, Chengdu 610065, China

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c. Corbion Purac China, Unit 08-09, 30F, #6088 Humin Road, Minhang District,

12

Shanghai 201199, China

13

Email address of corresponding authours: *G.J.Z. [email protected]

14 15

Heat resistance of injection-molded polylactide can be obviously improved with the

16

help of shear-induced stereocomplex crystals, which will enlarge the usages of

17

polylactide. 48


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Figure 1. 2D-WAXD patterns at different positions of injection-molded PLA samples: (a) LD-CIM and (b) LDOSIM. Figure 1 323x206mm (150 x 150 DPI)



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Figure 2. 1D-WAXD profiles circularly integrated from 2D-WAXD patterns at different positions of injectionmolded PLA samples: (a) LD-CIM and (b) LD-OSIM. Figure 2 442x206mm (150 x 150 DPI)


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Figure 3. Crystalline information (crystallinities of HC and SC) at different positions in LD-CIM and OSIM sample. Figure 3. 240x150mm (150 x 150 DPI)



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Figure 4. (a) 2D-SAXS patterns and (b) corresponding 1D-SAXS curves of LD-CIM and OSIM samples. Figure 4. 342x206mm (150 x 150 DPI)


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Figure 5. (a) Vicat softening temperature of LD-CIM and OSIM samples. Figure 5 272x181mm (150 x 150 DPI)



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Figure 5. (b) digital photos during visible heat deflection resistance test in hot water. Figure 5 338x116mm (150 x 150 DPI)


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Figure 5. (c) digital photo of LD-CIM and OSIM sample after hot-water deflection resistance tests. Figure 5 219x167mm (150 x 150 DPI)



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Figure 6. Relative crystallinity, Xc(t), as a function of time during ISIW-SIC experimentals for (a) neat PLLA and (b) PLLA/PDLA blends. Figure 6 466x214mm (150 x 150 DPI)


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Figure 7. The comparison of crystallization kinetics’ response to shear flow, i.e. the ratio of t0 and t1/2 between sheared and quiescent melts, between HC in neat PLLA and SC in PLLA/PDLA blends. The ratio of t0 between sheared and quiescent melts in PLLA/PDLA blends is termed as null because the t0 is not detected in sheared PLLA/PDLA blends.t1/2 between sheared and quiescent melts, between HC in neat PLLA and SC in PLLA/PDLA blends. Figure 7 270x194mm (150 x 150 DPI)



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Figure 8. Schematics of the crystallization process from sheared and quiescent PLLA/PDLA melts. Figure 8 856x383mm (150 x 150 DPI)


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Enhanced Heat Deflection Resistance via Shear Flow-Induced

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