Effect of the simultaneous biaxial stretching on the ...

European Polymer Journal 68 (2015) 288–301

Contents lists available at ScienceDirect

European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Effect of the simultaneous biaxial stretching on the structural and mechanical properties of PLA, PBAT and their blends at rubbery state Racha Al-Itry a,b, Khalid Lamnawar a,c, Abderrahim Maazouz a,b,⇑, Noëlle Billon d, Christelle Combeaud d a

Université de Lyon, INSA de Lyon, CNRS, F-69361 Lyon, France UMR 5223, IMP, Ingénierie des matériaux polymères, INSA Lyon, F-69621 Villeurbanne, France UMR 5259, LaMCoS, Laboratoire de Mécanique des Contacts et des Structures, F69621 Villeurbanne, France d Mines ParisTech, PSL – Research University, CEMEF (CEntre de Mise En Forme des matériaux), CNRS UMR 7635, CS 10207 rue Claude Daunesse, 06904 Sophia Antipolis Cedex, France b c

a r t i c l e

i n f o

Article history: Received 22 March 2015 Received in revised form 1 May 2015 Accepted 4 May 2015 Available online 5 May 2015 Keywords: Biodegradable polymers Uniaxial stretching Simultaneous biaxial stretching Strain induced crystallization

a b s t r a c t This works deals with the study of the strain-induced structural changes of PLA, PBAT and their blends with/without multi-functional styrene-acrylic oligomers (Joncryl ADRÒ-4368). The films were prepared through a single-screw extrusion. Uni and bi-axially simultaneous stretching, at a temperature above the glass transition temperature, were performed. The experimental conditions were chosen so as not to generate significant thermal crystallization during tests duration. Stretching induced semi-crystalline films with a supposed in-plane mechanical isotropy. Their structural, morphological and thermo-mechanical properties were characterized using dynamic mechanical and thermal analysis (DMTA), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and transmission electron microscopy (TEM). The strain-induced crystals of PLA combined mesophase and a0 -form crystals. Besides, the stretched PBAT was found to be composed of mixed-crystal structure of BT and BA units in a PBT-like crystal form. The Joncryl addition promoted significant changes on the crystallization kinetics of PLA without affecting the crystallization kinetic of PBAT. In fact, Joncryl acted as a nucleating agent making thus faster the thermal crystallisation of PLA. In parallel, the strain induced crystallisation in PLA was accelerated and stresses increased. The same effect was still observed in the PLA_PBAT blends. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Driven by environmental concerns, packaging films made of bio-compostable and renewable raw materials are getting more and more attractive [1,2]. One of the most prominent polymers is the poly (lactic acid) (PLA). Unfortunately, some shortcomings like its poor ductility and poor melt properties regard to the petrochemical based-polymers (PET, PS, PP. . .) as well as its low crystallization kinetics have restricted its wide use in packaging applications. Taking into account the scientific challenges and the industrial needs, Al-Itry et al. [3,4] become interested in studying different modification routes ⇑ Corresponding author at: Université de Lyon, INSA de Lyon, CNRS, F-69361 Lyon, France. E-mail address: [email protected] (A. Maazouz). http://dx.doi.org/10.1016/j.eurpolymj.2015.05.001 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.

R. Al-Itry et al. / European Polymer Journal 68 (2015) 288–301

289

aiming at enlarging the processing window of PLA. They have demonstrated that the blending of PLA with the poly(butylene adipate-co-terephtalate) (PBAT) can be a suitable solution to upcoming its physical and mechanical drawbacks. Moreover, the incorporation of a chain extender/branching agent leads to an improvement of the melt strength of each polymer and to a better compatibilization for the blends [4]. Furthermore, the low kinetic of crystallization of PLA makes the crystallisation time incompatible with industrial processes such as extrusion, injection molding and foaming as well. In other words, as high crystallization is difficult to achieve during the processes [5,6], PLA can exhibit variable and not stable behavior and the gas (O2, N2, CO2 and CH4) barrier properties of PLA cannot be enhanced. The enhancement of the crystallization behavior and the increasing of crystallinity have been widely discussed in the literature. It has been reported that the high crystallinity can be achieved by isothermal annealing [7], polymer blending [8], addition of nucleating agents [9] and strain-induced crystallization [10,11]. The crystallinity development through the strain-induced crystallization (SIC) represents an important approach for manufacturing biodegradable PLA films with good properties for several practical applications such as blowing injection and thermoforming [6,12,13]. Crystal structure can be formed in the amorphous PLA films in uniaxial and biaxial stretching [10,11]. It is interesting to note that, four crystalline forms have been reported for PLA crystallizing under shear, namely: a (pseudo-orthorhombic, pseudo-hexagonal or orthorhombic), a0 , b (orthorhombic or trigonal), c and stereo-complex. The a form is usually formed during processing of the polymer via thermal treatment or from solutions. The b form is obtained via transformation of the a crystalline form during drawing of semicrystalline poly(lactide) to a large strain ratio above the glass transition (Tg). The ratio of the b to the a form increases with the temperature and the deformation. Kokturk et al. [11] studied the crystal structure development and orientation during uniaxial stretching of PLA from 65 to 85 °C that is above, but close to glass transition temperature. They found that the stretching led to a rapid orientation of an amorphous polymer followed by the appearance of a highly oriented crystalline a-phase. However, when the stretching temperature took place at or below Tg, crystalline phase did grow and preexisting crystalline unstructured due to chain extension in amorphous phase. In addition, Pluta and Galeski [14] revealed that both aging and orientation increased crystallinity. At a low stretch ratio and moderate deformation temperature (85 °C), molecular orientation in the crystalline regions was found to progress more slowly than chain orientation in the amorphous zones. In the same manner, it was revealed that the crystallinity and dimensional stability of bi-oriented PLA films are even more sensitive to the stretching and annealing temperatures [15]. In fact, the biaxial oriented PLA film with high crystallinity ensures dimensional stability at temperatures above 100 °C. The structural evolution during simultaneous and sequential rubbery state biaxial stretching has also been studied [13]. Compared to uniaxial stretching (which leads to gradual development of orientation particularly beyond the strain hardening and consequent oriented crystallization with well established three-dimensional order), simultaneous biaxial stretching always leads to films with in-plane isotropy and poor crystalline order. Otherwise, during sequential biaxial stretching oriented crystallization gradually developed. The application of transverse stretching destroys the crystalline structure oriented in ‘‘machine’’ direction during first stretching. It is well accepted that the ability to crystallize or orientate the polymers during these stretching is driven by chain architecture and resulting relaxation time of chains (related to the temperature and the molecular weight). Indeed, the strain induced crystallization can be promoted when the longest relaxation time exceeds the characteristic time for deformation [14,16]. For the P(BA-co-BT) or PBAT, the crystalline structure during uniaxial stretching have been reported by several research groups [17,18]. Solid state 13C NMR of the mobility of the aliphatic methylene did not allow discriminating whether the crystal structure was formed by pure BT unit or by both BT and BA units because of the presence of bulky and rigid benzene ring, as in the case of PBT since there is no difference in molecular mobility of soft methylene group in crystalline and amorphous regions [19]. Marchessault and co-workers [18] suggested a cocrystallization model in which the adipate units fit into the crystal lattice of PBT, which enable PBAT to form a well-developed PBT-like crystal structure despite of its randomness. However, the inclusion of soft BA unit into BT crystal lattice led to the significant lowering of the melting temperature compared with that of PBT.

Table 1 Composition of the different studied materials. Composition

Reference

100 wt% PLA 99.5 wt% PLA + 0.5 wt% Joncryl 99.3 wt% PLA + 0.7 wt% Joncryl 100 wt% PBAT 99.5 wt% PBAT + 0.5 wt% Joncryl 99.3 wt% PBAT + 0.7 wt% Joncryl 80 wt% PLA + 20 wt% PBAT 79.6 wt% PLA + 19.9 wt% PBAT + 0.5 wt% Joncryl 79.4 wt% PLA + 19.86 wt% PBAT + 0.7 wt% Joncryl 20 wt% PLA + 80 wt% PBAT 19.9 wt% PLA + 79.6 wt% PBAT + 0.5 wt% Joncryl 9.86 wt% PLA + 79.4 wt% PBAT + 0.7 wt% Joncryl

PLA PLA_0.5 PLA_0.7 PBAT PBAT_0.5 PBAT_0.7 80_20 80_20_0.5 80_20_0.7 20_80 20_80_0.5 20_80_0.7

290


(a)

(b) Head

Central zone

Fig. 1. Shaped specimen used for the uniaxial (a) and the biaxial (b) experiments.

The literature is very poor regarding the uniaxial and biaxial stretching properties of PLA blends. Chapleau et al. [10] showed that for PLA/TPS blends, at the lowest temperature of the process ability range (70 °C), the tensile modulus and tensile strength increased as a function of biaxial draw ratio and the elongation at break only increased significantly for the blends having low TPS contents. Despite this literature, few are known about strain induced crystallization of PLA_PBAT blends and induced properties. The key feature of our study is to shed further light on the relationship between mechanical and microstructural properties for films based on compatibilized and non-compatibilized PLA_PBAT blends. The microstructure of the materials (i.e. degree of crystallinity and conformation of molecular chain segments during drawing) will be studied by means of complementary methods including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and wide-angle X-ray scattering (WAXS). The information thus gathered made it possible to study the influence of stretching parameters on the strain-induced crystallization of all the blends. As a first step, individual polymers will be firstly studied. 2. Experimental 2.1. Materials The PLA Grade 4032D purchased from Natureworks with a D-isomer content of approximately 2%, with a weight- average molar weight of 100.000 g/mol (GPC analysis), a 60 °C glass transition temperature and a melting temperature of 170 °C (DSC analysis), was chosen. The PBAT copolymer was supplied by BASF, (Grade Ecoflex FBX 7011). The molar fraction of BT and BA units are 44% and 56%, respectively. It exhibits a weight- average molar weight of 40.000 g/mol (GPC analysis), a glass transition temperature and melting point of 30 °C and 110–120 °C (DSC analysis), respectively. Both polymers are supplied in pellets form. Commercial available Joncryl (BASF, Joncryl ADR-4368) has been used as chain extender molecules for polye sters. It is an epoxy functional oligomeric acrylic with the following physical characteristics: T g ¼ 54 C, EEW (epoxy equivalent weight) = 285 g/mol, Mw = 6800 g/mol, obtained in flake form. Table 1 summarizes the composition of the various samples used in our study. 2.2. Processing of the cast films The films were extruded in two steps. Firstly, the pure/modified PLA and PBAT polymers as well as their blends were mixed by means of a co-rotating twin screw extruder (Thermo Electron PolyLab System Rheocord RC400P, screw diameter of 16 mm). The profile temperature was set at 140, 200, 200, 180, 180 and 190 °C. The residence time was about 2 min. More details about their rheological, thermal, mechanical, morphological properties are presented in previous papers [3,4,21]. It should be pointed out that the incorporation of Joncryl makes the film processing more stable. Approximately 500 lm-thick cast-films were prepared with a 30 mm single-screw extruder and quenched with a 30 °C chill roll. The chilled roll was kept as close as possible to the die and two air fans were used as an air knife throughout the experiment. The die temperature was fixed at 190 °C and the residence time was set at less than one minute. 2.3. Film stretching The extrusion-cast films were stretched by a uniaxial and biaxial test machine at laboratory scale. Care was taken to cut the square test specimens from the central region of the extruded films, as showed in Fig. 1. The uniformity of each

291


Axial stress (MPa)

15

10

5

Machine Direction Transverse Direction

0 0.0

0.5

1.0

1.5

2.0

Hencky uniaxial strain Fig. 2. Example of stress–strain curves for PLA samples in both Machine (MD) and Transverse (TD) Directions.

specimen’s thickness was checked and validated. Uniaxial samples were extracted parallel to the extrusion direction (machine direction MD). Meanwhile, the biaxial samples were extracted so as to have one arm parallel to MD direction and a second arm perpendicular to that direction (transverse direction TD). The stretch abilities of the different films were investigated in uniaxial and simultaneous equi-biaxial mode, at constant arm displacement velocities: in both uniaxial and biaxial conditions, two, respectively four, tensile grips are moving simultaneously. Along one direction, force and displacement are measured and synchronized. In uniaxial conditions, supposing an homogeneous deformation along the specimen, the Hencky strain is defined by Eq. (1)

e ¼ ln

L ¼ LnðkUS Þ L0

ð1Þ

where L is the specimen length, L0 the initial length taken at 23 mm and kUS the uniaxial stretching ratio. We can deduce, supposing both the transverse isotropy and incompressibility hypothesis confirmed, the stress as follows:

F S

r¼ ¼

F e0 l0 ee

ð2Þ

where e0 is the initial thickness of samples, around 500 lm, l0 is the initial length and ee is the reciprocal of uniaxial strain exponential. In biaxial conditions, the stress is calculated on the specimen diagonal where the stress is supposed to be homogeneous during equilibrated stretching. Eq. (3) defines the biaxial stress.

pffiffiffi 2F F r¼ ¼ S e0 l0 ee

ð3Þ

where e is the uniaxial Hencky strain taken, in this study, as a comparison reference. In our case, the biaxial draw ratio kBS is then defined by Eq. (4)

kBS ¼ kMD kTD ¼

L L ¼ k2US L0 MD L0 TD

ð4Þ

where MD and TD are respectively the machine and the transverse directions. The uniaxial and biaxial stretching experiments, referred to as US and BS respectively, were conducted at 75 °C (Tg + 15 °C for PLA and Tg + 105 °C for PBAT). This temperature was chosen to be sure that both PLA and PBAT polymers were in their rubbery plateau. The specimens were heated by blowing heated air to the surface for 5 min to allow an homogeneous temperature in the whole sample thickness. The central 23 mm-central zone of the specimen and also the grips are then heated. The study of thermal crystallization kinetics at 75 °C confirms that no induced thermal crystallization could be developed during the stretching experiments since the duration of the test (preheating + stretching) was less than 10 min. The arm displacement velocity was thus chosen to be 0.1, 1 and 10 mm/s, which correspond to uniaxial strain rates of 0.004, 0.04 and 0.4 s1, respectively. For each condition, we carried out tensile tests until the rupture of the material. Finally, to differentiate the microstructural changes induced during the stretching from the ones that could develop during the cooling under stress, a quenching step to the ambient air temperature was used aiming to freeze the orientation and microstructure

292


Table 2 Thermal properties and the crystallinity evolution of unstretched and biaxially stretched samples. Sample

‘‘Tg’’ PLA (°C)

‘‘Tg’’ PBAT (°C)

Cold crystallization temperature ‘‘Tcc’’ (°C)

Cold crystallization enthalpy (J/g)

Melting temperature ‘‘Tm’’ (°C)

Melting enthalpy (J/g)

Calculated crystallinity (%)

Estimated crystallinity by ‘‘Hermans and Weidinger’’ method

PLA_Pellets PBAT_Pellets PLA_Cast film PLA_Stretched film PBAT_Cast film PBAT_Stretched film PLA_0.5_Cast film PLA_0.5_Stretched film PLA_0.7_Cast film PLA_0.7_Stretched film PBAT_0.5_Cast film PBAT_0.5_Stretched film PBAT_0.7_Cast film PBAT_0.7_Stretched film 80_20_0_Cast film 80_20_0_Stretched film 80_20_0.5_Cast film 80_20_0.5_Stretched film 80_20_0.7_Cast film 80_20_0.7_Stretched film 20_80_0_Cast film 20_80_0_Stretched film 20_80_0.5_Cast film 20_80_0.5_Stretched film 20_80_0.7_Cast film 20_80_0.7_Stretched film

62 – 60 69 – – 60 70

– 30 – – 29 29 – –

– – 107 – – – 105 –

– – 32 – – – 25 –

168 119 170 168 123 51–121 167 167

1.5 16 36 38.5 24 30 29 35.5

1.7 13.5 4 41 23 26 4.3 38

– – 1 30 3 11 1.07 35.5

60 69

– –

100 –

20 –

165 164

27 30

7.5 32

2 –

– –

29 28

– –

– –

120 51–117

23 27

20 24

5.7 7

– –

30 26

– –

– –

120 51–121

22 25

21 22

5.1 8

59 68.5

32 23

102 –

23 –

168.5 166.5

27 29

5.3 39

– –

59 70

27 22

101.5 –

16 –

166 165

19.5 28

4.7 38

– –

59 69

27 24

105 –

18 –

167 164

20 30

2.6 40

– –

60 70

28 24

103 –

2.7 –

168 168

12 22

12.5 24

– –

60 69

28 24

105 –

6 –

167 165

12 19

8 21

– –

60 67

29 23

107 –

1 –

166 165

7.5 17

8 19

– –

due to drawing. We should note that the stresses in both machine and transverse directions are similar, which is representative of the initial mechanical isotropy of the cast sheets, even for PBAT which is initially semi-crystalline, as shown in Fig. 2. 2.4. Strain induced microstructure 2.4.1. Qualitative method – Debye Scherrer method X-ray scattering allowed us to characterize both the crystalline phases and their in-plane orientation. Transmission Debye–Scherrer method and diffractometric 2h scans (XPERT-PRO) were used. Cu Ka radiation, with wavelength k = 1.54178 Å, was selected. The sample-to-detector distance (EO) was about 50 mm. This X-ray diffraction was conducted on both ‘‘Head’’ (clamping zone) and ‘‘central zone’’ (equi biaxial loading) of the specimens (see Fig. 1). Meanwhile, only the focus of experiments on the central part will be given. Indeed, this one underwent both thermal and mechanical treatment (stretching more precisely). The ‘‘Hermans and Weidinger’’ method (Eq. (5)) [20] was used to deduce the crystallinity ratio from X-ray scans;

Xc ¼

1 1 þ pq

Sa Sc

ð5Þ

where ‘‘ pq ’’ is a constant equal to 1.297, ‘‘Sa’’ the area of the amorphous halo, ‘‘Sc’’ the sum of the area of all crystalline reflections and ‘‘SSac ’’ the ratio of the area under the resolved Gaussian crystalline peaks to the amorphous area. 2.4.2. Dynamic scanning calorimetry (DSC) Thermal analysis were carried out using a TA instrument DSC Q10 from 80 °C to 200 °C range temperature with 10 °C/min heating rate under nitrogen atmosphere. The samples were crimped into Aluminum pans to a total weight of

293


35

Cast PLA film

30

Axial stress (MPa)

DT

Cast PBAT film

25

BS DM

PLA_BS

20 15 10

PBAT_BS

PBAT_US

DT

5

DM

PLA_US 0 0.0

0.5

1.0

US

1.5

Hencky uniaxial strain Fig. 3. Stress–strain curves for PLA and PBAT samples in uniaxial (US) and biaxial (BS) stretching mode and Debye–Scherrer patterns before (left) and after (right) stretching.

800

1800

(a)

(b)

1600 1400

600

Unstretched PLA

Intensity (Counts)

Intensity (Counts)

Central part _ Stretched PLA

Head _ Stretched PLA 400

200

Central part_Stretched PBAT

1200 1000

Head_Stretched PBAT 800 600

Unstretched PBAT

400 200

0

0 0

10

20

30

2θ (°)

40

50

60

0

10

20

30

40

50

60

2θ (°)

Fig. 4. Integrated intensity profiles of the 2D-patterns of unstretched PLA (a) and PBAT (b), central part and head of biaxially stretched PLA (a) and PBAT (b) samples.

5–10 mg. The crystalline weight fraction, Xc (%), was computed from the enthalpy of the melting endotherm according to the following equation (Eq. (6)):

X c ð%Þ ¼

DHm DHcc 100 DHm1

ð6Þ

where DHm is the melt enthalpy; DHcc is the cold crystallization enthalpy, DHm1 was taken as 93 J/g and 114 J/g for PLA and PBAT respectively. It represents the theoretical specific melt enthalpy of the perfect crystal.

2.5. Thermo-mechanical properties The thermo-mechanical properties were conducted through dynamic mechanical thermal analysis (DMTA) performed on a RDA II (TA instruments). The samples were cut from cast-films of 500 lm in a width of 10 mm and the length of 23 mm. Oscillating tensile-compression tests under a strain of 0.01% at a frequency of 1 Hz were performed during temperature sweeps from 80 °C up to 110 °C at a heating rate of 3 °C/min.

294


Table 3 The optimum strain hardening parameter and the interplanar spacing between the different crystalline planes for drawn neat and modified PLA and PBAT and their blends. Samples

PLA_0 PLA_0.5 PLA_0.7 80PLA_20PBAT_0 80PLA_20PBAT_0.5 80PLA_20PBAT_0.7

Uniaxial strainhardening parameter (SHP) or strain limit (Hencky) US

BS

1.1 1.1 0.85 1.1 1 1

0.9 0.85 0.75 0.9 0.8 0.8

dhkl_BS (Å)

Samples

2.5–2.9 1.5–1.8–2.4–2.6–2.8 1.5–1.9–2.3–2.5–2.7 2.9 1.5–1.8–3–2.85 1.4–1.85–2.4–2.85–2.2

PBAT_0 PBAT_0.5 PBAT_0.7 20PLA_80PBAT_0 20PLA_80PBAT_0.5 20PLA_80PBAT_0.7

Uniaxial strainhardening parameter (SHP) or strain limit (Hencky) US

BS

1.1 1.05 1.05 1.1 0.85 0.8

1.1 0.95 0.9 1.1 1.1 0.75

dhkl_BS (Å)

1.7–1.9–2–2.9 1.8–2–2.3–2.85 1.7–1.8–2.3–2.85 1.8–2.6–2.8–2–1.9 2–2.2–2.5–3 2–2.2–2.5–3

2.6. Morphological study To investigate the morphology of the blends, transmission electron microscopy (TEM) was carried out. The microtomed specimens were stained by ruthenium tetraoxide (RuO4) vapor for 30 min before observations.

3. Results and discussion 3.1. Biaxial stretching of neat PLA and PBAT polymers

1E10

E'_ PLA Cast film 3.5 E'_ PLA film_BS tan δ_PLA Cast film 3.0 tan δ_PLA film_BS

1E9

2.5 2.0 1.5

1E8

tan δ

Storage modulus E' (Pa)

Prior to the biaxial tests, a thermal analysis of static crystallization was performed to compare with strain induced crystallization. Results are depicted in Table 2. Upon heating, PLA pellets went through a glass transition at temperature of 62 °C followed by a small single endothermic peak ‘‘Tm’’ with a maximum at 168 °C. The crystallinity of the as-received polymer pellet is approximately 2%. No cold crystallization was observed for heating rate of 10 °C/min. Otherwise DSC thermogram of PLA extruded cast-film was very different from that of PLA pellets. The difference is highlighted by the appearance of an exothermic cold crystallization peak ‘‘Tcc’’ close to 110 °C followed by a more intense endothermic peak. The degradation of PLA chains, during process, can be evocated to explain this phenomenon although the decrease in the average molar weight from 100.000 g/mol (Pristine PLA) to 94.000 g/mol (Extruded PLA) remained low [3]. The by-products of the degradation (i.e. PLA oligomers and lactic acid) could then act as nucleating agent [3,4]. Some authors reported that the generated cold crystals at 110–120 °C have a-form [6]. The area under the cold crystallization peak (32 J/g) for the cast film is smaller than the melting enthalpy (36 J/g) indicating that the cast PLA film was slightly crystalline (4%) and it can further crystallize under stretching. PBAT polymer for its part is often known not to crystallize to a significant amount due to the random architecture of the copolymer. In our case, a crystallization ratio close to 23% is achieved. Therefore, the well-developed crystal structure in PBAT is a rare phenomenon though reported by some authors [22]. The determination of stretching conditions, specifically the deformation temperature, is essential. Generally, for polymer exhibiting cold crystallization as PLA, it is bounded by both glass transition and cold crystallisation temperatures. This range

1.0 0.5

1E7

0.0 40

60

80

100

Temperature (°C) Fig. 5. DMTA analysis for unstretched and biaxially stretched PLA films.

295


1.0

0.2

Heat Flow (W/g)

Stretched PLA_0.5_Central part 70.50°C

0.2 0.0

Stretched PLA_0.7_Central part

168.58°C

68.97°C

-0.2

-0.8 20

51.97°C

-28.78°C

121.53°C

Stretched PBAT_0.5_Central part

0.0

50.52°C

-0.1 -26.50°C

Stretched PBAT_0.7_Central part

117.18°C

167.73°C

-0.4 -0.6

Stretched PBAT_Central part

0.1

69.65°C

0.6 0.4

-29.19°C

Stretched PLA_Central part

Heat Flow (W/g)

0.8

-0.2

(a) 70

120

Exo Up

170

51.55°C

(b)

164.00°C

-0.3 -70

121.33°C

-20

30

80

130

Exo Up

Temperature (°C)

Universal V4.5A TA Instruments

Temperature (°C)

Universal V4.5A TA Instruments

Fig. 6. DSC thermograms of the stretched neat and modified PLA (a) and stretched neat and modified PBAT (b).

was estimated thanks to DMTA measurements between the a-relaxation temperature (peak of the loss factor tan d = 65 °C) and the onset cold crystallization temperature (85 °C). Ultimately, the drawing temperature was set at 75 °C, in the rubbery plateau of both PLA and PBAT polymers. Typical stress vs. strain curves at 75 °C up to rupture for PLA and PBAT are shown in Fig. 3. Some authors reported that PLA undergoes a plastic deformation above Tg [14]. In our case the shapes of the curves are quite close to elastomer behavior, which is usual in that range of temperature for amorphous (or slightly semi-crystalline) polymers. It would have been interesting to unload (before strain induced crystallization occurred) to analyze the possible existence of plastic behavior. PLA strain hardens above 0.9 Hencky strain in BS and 1.1 Hencky in US. This strain limit is deduced from the intersection point of tangent to the plateau region and upswing region in the stress–strain curve. Accounting for differences in loading path and our definition of bi axial strain one can conclude that strain hardening occurs in the same range regarding the thinning of the film. PLA that was slightly semi-crystalline, develop crystals which are uniaxially oriented in US conditions. In BS conditions, WAXS pattern is less clear though evidence for a certain level of periodic organization exists. BS type of pattern could be due to nematic-like order in the material which indicates that in this mode the three-dimensional order crystalline order is not established even well into the strain hardening range. The formation of the mesomorphic phase (or mesophase) was confirmed by the endothermic peak close to Tg, which is attributed to its melting as shown in Fig. 6 [27]. Some other authors reported that the deformation of an amorphous PLA films increases the tg0 t population. The notation tg0 t refers to a trans-planar ester C–O, gauche O–Ca, and trans Ca–C torsions [23]. It should be noted that a mesophase (formed by the orientation and periodic organization of amorphous chains) can exist in the earlier stages of crystallization, acting as precursor and disappearing to form an imperfect crystal which develops by becoming more and more ordered [24]. Indeed, a similar mechanical behavior was observed in PET stretched above the glass transition temperature [13,25,26]. The evolution of the diffraction peak at around 2h = 32° in Fig. 4 is subjected to a solid-state extrusion [14,16]. An important peak at 2h = 15.5 was observed following the biaxially stretching. It could correspond to (1 1 0) and (2 0 0) families of plans and related to the mesomorphic phase [6]. WAXS patterns (Fig. 3) and scans (Fig. 4) illustrated that PBAT, that was initially crystalline, remained crystalline even though diffraction seems to be weaker. This effect can be related to the thickness of the sample. This is clear in Fig. 4 on which scans clearly show discrete peaks for all samples. However, their orientation seems to be different as, in the chosen direction for the scan, only one peak remains for BS samples. Consequently, crystallinity ratio deduced for oriented samples appeared to be low (Table 2). In fact this only indicates that the method used here is highly dangerous when textured materials are considered. 0 1), 23.38 (1 0 0), 25.3 (1 1 1), 28.9° (1 0 4). The last peak is the For PBAT, the indexation could be 2h = 17.9 (0 1 0), 20.74 (1 most representative peak of b-forms of PBT [28]. Based on the analysis, the crystal structure of PBAT was characterized to be formed by mixed-crystallization of BT and BA units, where BA units were incorporated into the BT lattice. This mixed-crystal structure was found to undergo PBT-like reversible crystal modification. Table 3 summarizes the four interplanar spacing of PBAT determined according to Bragg’s law, which were evaluated to be 1.7, 1.9, 2 and 2.9 Å. It was demonstrated in the literature that the crystallized BT and BA units share a common crystal lattice, since soft BA unit was introduced into BT crystal lattice [18]. Main melting occurred at 123 °C and an additional endothermic peak at 51 °C having smaller endothermic enthalpy can be related to the formation of crystal lattice containing mainly BA units. Further verification will be given below. Unlike PLA, PBAT shrinks due to its elastomeric property during the transition from

296


0.05 PLA 0.7 PLA 0.5 PLA

Heat Flow (W/g)

0.04

0.03

0.02

PBAT 0.7 PBAT 0.5 PBAT

0.01 15

25

35

Exo Up

45

55

65 Universal V4.5A TA Instruments

Time (min)

Fig. 7. Isothermal crystallization of neat and modified PLA and PBAT polymers at 75 °C during 60 min.

35 Cast (PLA_0 .5) film

(a)

Cast (PLA_0.7) film

Axial stress (MPa)

30 25

PLA_BS

20 PLA_0.5_BS

15 PLA_0.7_BS

10 5

PLA_0.7_US PLA_US PLA_0.5_US

0 0.0

0.5

1.0

1.5

Hencky uniaxial strain 35

Cast (PBAT_0.5) film

(b)

Cast (PBAT_0.7) film

Axial stress (MPa)

30 25 20 PBAT_BS

15 PBAT_0.7_BS

10

PBAT_0.7_US

PBAT_0.5_BS

5 PBAT_0.5_US

0 0.0

PBAT_US

0.5

1.0

1.5

Hencky uniaxial strain Fig. 8. Stress–strain curves for the neat and modified PLA (a) and PBAT (b) films.

stretching to relaxation. This is related to the coexistence of soft (BA units) and hard segments (BT units), hence the need of ‘‘thermo-setting’’ at the end of the biaxial drawing experiments.

297


(0 0 10) (206) & (116) (203) & (113) (200) & (110)

Fig. 9. Debye–Scherrer pattern obtained with bi-axially stretched modified PLA film.

1E10

(a)



1E10

1E9

1E8

PLA_Cast film PLA_BS PLA_0.5_Cast film PLA_0.5_BS

1E7

20

40

60

(b) 1E9

1E8 PBAT_Cast film PBAT_BS PBAT_0.5_Cast film PBAT_0.5_BS

1E7 80

100

120

-80

-60

Temperature (°C)

-40

-20

0

20

40

60

80

100

Temperature (°C)

Fig. 10. DMTA analysis for cast and stretched neat and modified PLA (a) and PBAT (b).

The biaxial stretching was found to impact the microstructure and the thermo-mechanical behavior of the films, as shown in Fig. 5. In the case of the PLA, an increase of E0 between the drawn and the unstretched material from 20 to 120 °C is noted. This is due to the occurred crystallization under stretching. With respect to the transition peaks, the initial cat PLA exhibits sharp tan d peak at 65 °C whereas samples in which orientation-induced crystallization occurs exhibits weak broad peak around 79 °C. This increase suggests that a part of the amorphous phase could be constrained, i.e. extended. Secondly, the cold crystallization peak disappeared after stretching. This result is in good agreement with DSC scans presented in Fig. 6. The increase in E0 on the rubbery plateau together with the disappearance of crystallization above 100 °C is the sign that crystal appeared during tension and that crystallization was almost complete. 41% of crystals have been formed during the stretching (Cf. Table 2). In the PBAT case, the crystallinity also increased from 23% to 26% thanks to stretching. A slight increase of E0 was observed on DMTA scans under the whole range of temperature. The Ta values obtained by DMTA evolve from 24 °C to 26 °C, which is not really significant.

3.2. Biaxial stretching of long chain randomly branched PLA and PBAT polymers The chemical chain extension and branching mechanisms of both PLA and PBAT polymers using a chain extender is highly discussed by our team researchers. Some experiments have been conducted in previous works [3,4,21]. The authors showed that the structural modification highly increased the shear rheological properties (melt strength, zero-shear viscosity,

298


PLA_PBAT (20_80)

PLA_PBAT_Joncryl (20_80_0.7)

PLA_PBAT (80_20)

PLA_PBAT_Joncryl (80_20_0.7)

Fig. 11. TEM observations for the compatibilized and uncompatibilized PLA_PBAT blends.

35

45

(a)

25

80_20_0.7_BS

20 15

80_20_0.5_BS

10

(b)

40

Axial stress (MPa)

Axial stress (MPa)

30

PBAT_BS 80_20_BS

5

20_80_BS

35 30

20_80_0.5_BS

25 20 20_80_0.7_BS

15 10 5

PBAT_BS

PLA_BS 0

0.0

0.5

1.0

Hencky uniaxial strain

1.5

PLA_BS

0

0.0

0.5

1.0

1.5

Hencky uniaxial strain

Fig. 12. Stress–strain curves for the uncompatibilized and compatibilized PLA_PBAT (80_20) (a) and PLA_PBAT (20_80) blends (b).

elasticity, shear-thinning behavior) and contributed to very long relaxation process. A complex structure i.e. mixture of linear and randomly branched chains, was deduced according to Van–Gurp–Palmen plots, molar measurements and viscometric properties. Consequently, some interesting changes in the crystallization behavior, when going from neat to modified polymer, were observed (cf. Table 2). In the presence of Joncryl, the crystallization temperature is shifted to lower temperatures, suggesting a nucleating effect of the multifunctional epoxide. The effect of Joncryl on the isothermal crystallization at 75 °C was also investigated. As can be seen in Fig. 7, the thermal crystallization process is faster for the chain – extended/branched PLA with an amount of Joncryl close to 0.7%. As a matter of fact, the modified PLA with Joncryl strain hardens more easily then the unmodified counterpart (Fig. 8) whereas modified PBAT exhibit equivalent US and BS behavior as the unmodified one. The effect on PLA could be related to pre-nucleation of crystal during heating that can act as crosslinks, to an increase in stiffness (viscosity) [29] or simply an increase in crystallization kinetics.

299

R. Al-Itry et al. / European Polymer Journal 68 (2015) 288–301 Table 4 Debye–Scherrer patterns obtained for different blends bi-axially stretched. PLA_BS

80_20_BS

PBAT_BS

20_80_BS

PLA_0.5_BS

80_20_0.5_BS

PBAT_0.5_BS

20_80_0.5_BS

As for the neat PLA, a partially well-developed three-dimensionally ordered crystalline phase (a0 crystal form) was observed for the modified PLA in parallel with the mesomorphic form (200 & 110). Different crystalline peaks were also detected and may corresponded to (203 & 113), (206 & 116) and (0 0 10) as shown in Fig. 9 [30]. The interplanar spacing values between these plans were summarized in Table 3. Unlike PLA, the incorporation of Joncryl into PBAT matrix did not promote any structural changes. A similar crystalline structure as for neat PBAT was observed: neither the uniaxial nor the biaxial stretching lead to any observable texture signature. In addition, the glass transition is immediately followed by a distinct endotherm for the modified PLA films as already depicted in Fig. 6. All these observations were confirmed by other different studies [6,24]. None of the samples show double melting peaks but we can note, that the melt temperature becomes lower for modified stretched polymer as the amount of Joncryl is high. This comforts the fact that the crystalline structures built under stretching are not exactly the same if Joncryl is present or not: we had noticed lower interplanar spacing between the different crystalline planes by WAXS on modified PLA. The DMTA plots in Fig. 10 revealed, for the modified PLA, a reduced rubbery zone affected earlier by the cold crystallization. This observation was also confirmed by DSC results. The stretched specimen does not appear influenced by the presence of Joncryl. Nevertheless, as Joncryl is supposed to accelerate crystallization kinetics, it is not excluded that during tensile tests, a cold crystallization had appeared partially, especially when 0.7 wt% of Joncryl was added. This could explain the fact that the modified PLA does not behave mechanically exactly as the neat material: we noted that strain-hardening appeared sooner and that the material could reach higher rupture stresses. However, the addition of Joncryl into PBAT matrix (Fig. 10b) did not affect neither its thermo-mechanical properties nor its crystallization behavior. The organization of macromolecular PBAT chains to form crystals may be hindered by the incorporation of Joncryl. 3.3. Biaxial stretching properties of PLA_PBAT blends To study the effect of the composition on the uniaxial and biaxial deformation, blend films at different compositional ratios of PLA and PBAT with/without Joncryl were prepared. We have demonstrated that Joncryl acts as a compatibilizer for the blends [4]. The TEM observations related to the compatibilized and uncompatibilized PLA_PBAT films are shown in Fig. 11. A finer dispersion, in both cases, related to a decrease of the interfacial tension, of the dispersed phase into matrix was highlighted. Fig. 12 shows the stress vs. strain curves for PLA_PBAT blends drawn with a constant uniaxial extension rate of 1.2 Hencky. The drawing temperature (75 °C) remains above the glassy transition temperature for all the compositions. Conversely to the compatibilized blends, it was observed that the matrix governed the stretchability of the uncompatibilized blends in simultaneously biaxial stretching. The values of the strain limit are reported in Table 3.

300


The stretch ability of PLA is maintained when blended with PBAT. Under biaxial drawing, the stretch ability was found to be improved for high amounts of PBAT due to its elastomeric behavior, as plotted in Fig. 12b. As observed previously and regardless the compositional level of PLA, the incorporation of Joncryl caused an earlier strain-hardening phenomenon while increasing the maximum stress before breaking, especially with 0.7%wt of this reactive agent. This latter is due either to the physical entanglements by the crystallites that prevented chain relaxation during experiments or to some cold crystals formed during stretching. The resulted crystalline structure of the blends is almost governed by the pure matrix. More details are presented in Table 4. 4. Conclusion Basing on the thermal and thermomechanical characterizations of bi-axially stretched samples above Tg, several conclusions can be drawn: Slightly semi-crystalline PLA is able to be stretched above its Tg. It develops a certain level of organized crystalline texture. Same material initially crystalline is less convenient for stretching application, i.e. crystalline phase induces brittleness. Both PLA and PBAT are stretchable at 75 °C. Stretching of PBAT induces less modification, nevertheless. Joncryl added to compatibilize the blends may induce an increase in crystallization kinetics that may lower stretchability above Tg increasing strain hardening. This can be seen as a kind of physical network. Joncryl does not modify the mechanical behavior of PBAT polymer. The mechanical behavior of the compatibilized PLA_PBAT blends is between that of the neat polymers. The properties of each polymer are maintained during blending.

To conclude, it is possible to tailor the stretch ability and the microstructure evolution in the PLA_PBAT blends through compatibilization with Joncryl and/or by modifying their composition. Acknowledgments The authors gratefully acknowledge the editor and the reviewers for the meticulous assessment of this work. Many thanks also to the DGCIS, Ministry of Industry, for the financial support and to Mr. Gabriel MONGE from MINES ParisTech for the X-ray experiments. References [1] R.A. Gross, B. Kalra, Biodegradable polymers for the environment, Science 297 (2002) 803–807. [2] H. Schut, What’s ahead for green plastics?, Plast Technol. 54 (2008) 64–89. [3] R. Al-Itry, K. Lamnawar, A. Maazouz, Improvement of thermal stability, rheological and mechanical properties of PLA, PBAT and their blends by reactive extrusion with functionalized epoxy, Polym. Degrad. Stab. 97 (2012) 1898–1914. [4] R. Al-Itry, K. Lamnawar, A. Maazouz, Reactive extrusion of PLA, PBAT with a multi-functional epoxide: physico-chemical and rheological properties, Eur. Polym. J. 58 (2014) 90–102. [5] W. Zhai, Y. Ko, W. Zhu, A. Wong, C.B.A. Park, Study of the crystallization, melting, and foaming behaviors of polylactic acid in compressed CO2, Int. J. Mol. Sci. 10 (2009) 5381–5397. [6] G. Stoclet, R. Seguela, C. Vanmansart, C. Rochas, J.M. Lefebvre, WAXS study of the structural reorganization of semi-crystalline polylactide under tensile drawing, Polymer 53 (2012) 519–528. [7] H. Tsuji, H. Takai, S.K. Saha, Isothermal and non-isothermal crystallization behavior of poly(L-lactic acid): effect of stereocomplex as nucleating agent, Polymer 47 (2006) 3826–3837. [8] J.T. Yeh, C.J. Wu, C.H. Tsou, W.L. Chai, J.D. Chow, C.Y. Huang, K.N. Chen, C.S. Wu, Study on the crystallization, miscibility, morphology, properties of poly(lactic acid)/poly(e-caprolactone) blends, Polym. Plast. Technol. Eng. 48 (2009) 571–578. [9] A. Maazouz, K. Lamnawar, B. Mallet, Patent FR2941702. Composition polymère, procédé d’obtention de la composition et pièce obtenue à partir de la composition, 2010. [10] N. Chapleau, M.A. Huneault, H. Li, Biaxial orientation of polylactide/thermoplastic starch blends, Int. Polym. Process. 22 (2007) 402–409. [11] G. Kokturk, T.F. Serhatkulu, M. Cakmak, E. Piskin, Evolution of phase behavior and orientation in uniaxially deformed polylactic acid films, Polym. Eng. Sci. 42 (2002) 1619–1628. [12] S. Tabatabaei, A. Ajji, Crystal structure and orientation of uniaxially and biaxially oriented PLA and PP nanoclay composites films, J. Appl. Polym. 124 (2012) 4854–4863. [13] X. Ou, M. Cakmak, Comparative study on development of structural hierarchy in constrained annealed simultaneous and sequential biaxially stretched polylactic acid films, Polymer 51 (2010) 783–792. [14] M. Pluta, A. Galeski, Plastic deformation of amorphous poly (L/DL-lactide): structure evolution and physical properties, Biomacromolecules 8 (2007) 1836–1843. [15] C.C. Tsai, R.J. Wu, H.Y. Cheng, S.C. Li, Y.Y. Siao, D.C. Kong, G.W. Jang, Crystallinity and dimensional stability of biaxial oriented poly (lactic acid) films, Polym. Degrad. Stab. 95 (2010) 1292–1298. [16] S.C. Lee, J. Han, Y.G. Jeong, M. Kwon, Strain-induced enthalpy relaxation in poly (lactic acid), Macromolecules 43 (2010) 25–28. [17] K. Kuwabara, Z. Gan, T. Nakamura, H. Abe, Y. Doi, Crystalline/amorphous phase structure and molecular mobility of biodegradable poly(butylene adipate-co-butylene terephthalate) and related polyesters, Biomacromolecules 3 (2002) 390–396. [18] E. Cranston, J. Kawada, S. Raymond, F.G. Morin, R.H. Marchessault, Cocrystallization model for synthetic biodegradable poly (butylene adipate-cobutylene terephthalate), Biomacromolecules 4 (2003) 995–999. [19] M.A. Gomez, M.H. Cozine, A.E. Tonelli, High-resolution solid-state carbon-13 NMR study of the alpha and beta crystalline forms of poly(butylene terephthalate), Macromolecules 21 (1988) 388–392. [20] A. Weidinger, P.H. Hermans, On the determination of the crystalline fraction of isotactic polypropylene from X-ray diffraction, Macromol. Chem. Phys. 50 (1961) 98–115.


301

[21] Racha Al-Itry, Khalid Lamnawar, Abderrahim Maazouz, Understanding of the compatibilization effect of the multifunctionalized epoxy in PLA/PBAT biopolymer blends: linear and non-linear rheology, morphology and interfacial properties, Rheol. Acta 53 (7) (2014) 501–517. [22] H.J. Kang, S.S. Park, Characterization and biodegradability of poly (butylenes adipate-co-succinate)/poly (butylenes terephtalates) copolyester, J. Appl. Polym. Sci. 72 (1999) 593–608. [23] K. Aou, S. Kang, S.L. Hsu, Morphological study on thermal shrinkage and dimensional stability associated with oriented poly (lactic acid), Macromolecules 38 (2005) 7730–7735. [24] G. Stoclet, R. Seguela, J.M. Lefebvre, S. Elkoun, C. Vanmasart, Strain-induced molecular ordering in polylactide upon uniaxial stretching, Macromolecules 43 (2010) 1488–1498. [25] E. Gorlier, J.M. Haudin, N. Billon, Strain-induced crystallisation in bulk amorphous PET under uni-axial loading, Polymer 42 (2001) 9541–9549. [26] X. Zhang, K. Schneider, G. Liu, J. Chen, B. Karsten, D. Wang, S. Manfred, Structure variation of tensile-deformed amorphous poly(L-lactic acid): effects of deformation rate and strain, Polymer 52 (2011) 4141–4149. [27] G. Stoclet, R. Seguela, J.M. Lefebvre, C. Rochas, New insights on the strain-induced mesophase of poly (D,L-lactide): in situ WAXS and DSC study of the thermo-mechanical stability, Macromolecules 43 (2010) 7228–7237. [28] X.Q. Shi, H. Ito, T. Kikutani, Characterization on mixed-crystal structure and properties of poly (butylene adipate-co-terephthalate) biodegradable fibers, Polymer 46 (2005) 11442–11450. [29] G.H. Menary, C.W. Tan, E.M.A. Harkin-Jonces, C.G. Armstrong, P.J. Martin, Biaxial deformation and experimental study of PET at conditions applicable to stretch blow molding, Polym. Eng. Sci. 52 (2012) 671–688. [30] X. Ou, M. Cakmak, Influence of biaxial stretching mode on the crystalline texture in polylactic acid films, Polymer 49 (2008) 5344–5352.