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Effect of mixing poly(lactic acid) With glycidyl methacrylate grafted poly(ethylene octene) on optical and mechanical properties of the blown films.

INTRODUCTION

In recent years, biodegradable polymers such as poly(e-caprolactone) (PCL), poly(lactic acid) (PLA), polyhydroxyalkanoates (PHAs), and poly(butylene succinate) (PBS) have been extensively investigated due to their attractive biomedical and practical applications [1-5]. Among them, PLA is the most promising material for the production of environmentally friendly high-performance plastics. It is proven to be superior to the conventional petrochemical polymers in terms of the total energy consumption and C[O.sub.2] emission in the life cycle assessment [6].

PLA is accepted as an environmentally friendly material and can be used for manufacturing blown film and injected commodity products. PLA films for packaging applications are commonly made through a blown film process. The packaging applications include food packaging, trashcan liners, shrink films, stretch films and merchandise packaging [7]. In most applications, packaging films are required to possess high clarity and high strength. Unfortunately, while there is an enhancement effect on strength, the incorporation of fillers decreases the clarity of the film.

The inherent brittleness, low deformation at break, low-melt viscosity, and low-heat distortion temperature of PLA have restricted its applications [8, 9]. In order to overcome these drawbacks, the processability of film blowing and the mechanical properties of the PLA resin can be improved through copolymerization or blending with other polymers or plasticizers [10-31]. For example, PLA is blended with other more flexible and biodegradable polymers such as PCL, PBS, poly(3-hydroxybutyrate) (PHB), and poly(butylene adipate-co-terephthalate) (PBAT) [32-35]. However, these materials prepared by simple blending still suffer from poor impact resistance due to the phase separation and poor interfacial adhesion between the two immiscible components.

Usually, PLA can be considered toughening with rubber. Poly(ethylene octene) (POE) is a kind of flexible polymers which is gained through copolymerization of octene and ethylene. Compared with conventional polyolefin elastomers, such as ethylene-propylene-diene (EPDM) rubber and ethylenepropylene rubber (EPR), POE exhibits the advantage of thermoplastic processability [36]. However, simple blending of PLA with other components usually cannot produce satisfactory properties because of the unfavorable interaction between molecular segments of the components, which is responsible for their immiscibility or poor interfacial adhesion. To improve the miscibility of polymer blends and increase the interfacial adhesion between the matrix and disperse phases, reactive compatibilization is very often used to obtain desirable properties [37]. Because most polymer blends do not have the appropriate functional groups, functionalization of the components is very often required. Fortunately, the inherent chemical functionality of PLA makes it an attractive candidate for modification [38]. Glycidyl methacrylate (GMA) grafted polymers are often used as reactive compatibilizers in polyester blends [39-42]. It is usually believed that epoxy groups can react with carboxyl or hydroxyl groups of polyester. The end hydroxyl and/or carboxyl groups of PLA can react with epoxy groups via nucleophilic substitution under appropriate conditions. Many authors have used the flexibilizers with reactive functional groups for toughening PLA, such as poly(ethylene-co-glycidyl methacrylate) (EGMA) [43], glycidyl methacrylate-functionalized methyl methacrylatebutyl acrylate (ACR-g-GMA) [37], epoxy-functionalized grafted acrylonitrile-butadiene-styrene (ABS-g-GMA) [38], and glycidyl methacrylate-functionalized methyl methacrylate-butadiene copolymers (MB-tj-GMA) [44]. Lin and Shimizu [45] explore to use of a reactive styrene-acrylonitrile-glycidyl methacrylate (SAN-GMA) copolymers as a compatibilizer for PLA/ABS blends. Therefore, it is expected that GMA grafted polyethylene octene) (GPOE) would have a significant toughening effect on PLA.

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Plasticizers can be added to PLA to decrease its glasstransition temperature ([T.sub.g]), which results in a lower stress at yield and a higher elongation at break at room temperature [46, 47], with these conditions being necessary to improve the flexibility of films and sheets. In the present article, acetyl tributyl citrate (ATBC) is used as plasticizer for the preparation of PLA films. The content of the plasticizer is an important factor to obtain a desired blend with excellent mechanical properties. The plasticizer content of the film is most preferably in the range from 8% to 15% by weight [7]. The main purpose of the incorporation of plasticizer is to enhance the flexibility and the processability of the PLA blown film.

In this paper, the miscibility, rheological behavior, mechanical properties, phase morphology, and crystallization behavior of PLA/ATBC/GPOE blown films are investigated.

EXPERIMENTAL

Materials

Semicrystalline PLA Grade 2003D (weight-average molecular weight ([M.sub.w]) was 1.93 x [10.sup.5] g/mol with a polydispersity index of 1.69, specific gravity was 1.24 g/[cm.sup.3], melt flowing rate was 6.0 g/10min) was obtained from Natureworks[TM] in pellet form. The GMA grafted POE (GPOE, specific gravity was 0.86 g/[cm.sup.3], melt flowing rate was 6.2 g/10 min, grafted ratio was 1.5 wt%, chemical structure as shown in Scheme 1) contained 25% by weight of octene was kindly supplied by Ketong Plastic Co., Ltd. (Shenyang, China). ATBC was purchased from Jinling Petrochemical Limited Co., Sinopec Corp.

Preparation of the Blown Films

PLA and GPOE were dried at 60[degrees]C for 8 h in a vacuum oven. First, PLA and GPOE were blended by using a high-speed mixer (GH-100DY). Then the composites were prepared by melt mixing with ATBC using a co-rotating twin-screw extruder (SHJ-20, length/diameter ratio (L/D ratio) was 32:1, screw diameter was 20 mm, rotating speed was 100 rpm) at 175[degrees]C. In addition, ATBC was added through the lateral line feeder in the twin-screw extruder by measuring pump. The mixing compositions of the PLA/ATBC/GPOE blends were summarized in Table 1. Then, all the samples and neat PLA were compression molded into sheets with thicknesses of 1.0 mm at 175[degrees]C for rheological measurements, dynamic mechanical analysis and phase morphology tests. All the PLA/ATBC/GPOE films were blown using a 64.5 mm smooth-bore single-screw extruder having an aspect ratio of 25:1 and fitted with a 6-inchdiameter blown film die using external cooling air with a temperature of 15[degrees]C. Moreover, the extruder output was 30 kg/h, the blow up ratio (BUR) was 5.6, the frost line height (distance from die exit) was 36 cm and the winding speed was 10.0 m/ min. The barrel temperature profile in different zones ranged from 165 to 175[degrees]C and the screw speed was kept at 38 rpm. The thermal properties, crystallite morphology, mechanical properties, optical properties and morphology of the tear-fracture surfaces of the PLA/ATBC/GPOE films were studied.

Fourier Transform-Infrared (FT-IR) Spectroscopy

Fourier transform-infrared spectra were measured with a Nicolet 6700 spectrophotometer using transmission mode. Before FTIR measurements, the samples for the blends were extracted in hot xylene for 20 h and in chloroform for another 20 h.

Fourier transform infrared ray (FT-IR) spectroscopy (Nicolet 6700, USA) was used to investigate the intermolecular interaction between constituents. Spectra were obtained at 4 [cm.sup.-1] resolution and averages were obtained from 64 scans in the standard wave number range from 500 to 4000 cm All samples had been dried under vacuum at 50[degrees]C for 8 h before testing.

Rheological Properties

Rheological measurements of the blends were carried out on a Physica MCR 2000 rheometer (AR 2000ex). Frequency sweep for the neat PLA and PLA/ATBC/GPOE samples were carried out under nitrogen using 25-mm plate-plate geometry. The gap distance between the parallel plates was 0.9 mm for all tests. The sheet samples were about 1.0 mm in thickness. A strain sweep test was initially conducted to determine the linear viscoelastic region of the materials. The angular frequency range used during testing was 0.1 to 100 rad/s. The temperature was plotted at 175[degrees]C.

Thermal Properties

Crystallization behavior of the composites was studied by differential scanning calorimetry (DSC) (TA Instruments DSC Q20 USA) on the specimens sliced from blown film samples. The heating and cooling rates were 10[degrees]C/min with nitrogen purge, and the sample weights were between 5 and 8 mg. The samples were heated first from -50 up to 180[degrees]C at 10[degrees]C/min and held at 180[degrees]C for 5 min to eliminate their previous thermal history, then cooled at the same rate and finally heated again. As there were no crystals of GPOE and ATBC, only the contribution of the PLA was considered. The degree of crystallinity of the samples was evaluated from the heat evolved during crystallization by the following relationship:

[x.sub.c] = [DELTA][H.sub.f]/[w.sub.PLA] x [DELTA][H.sup.0.sub.f] x 100% (1)

where [x.sub.c] is the degree of crystallinity of the samples, [DELTA][H.sub.f] is the heat of fusion of the PLA in the blend, [DELTA][H.sup.0.sub.f] is the heat of fusion for 100% crystalline PLA (93 J/g) [48] and [w.sub.PLA] is the weight fraction of PLA in the blend.

Crystalline Morphology

The crystallite morphology of PLA/ATBC/GPOE blown film samples was observed with a Leica DMLP polarized microscope (POM) equipped with a Linkam TM600 hot stage and a computer-controlled charged-coupled-device camera. A small amount of sample with a thickness of approximately 0.05 mm was sandwiched between two microscope cover glasses and then placed on the hot stage. The samples were heated from room temperature to

180[degrees]C at a rate of 30[degrees]C/min, held there for 5 min to eliminate any thermal and mechanical history, and then quenched to 110[degrees]C at a rate of 30[degrees]C/min for isothermal crystallization and held for 60 min. The morphology changes were recorded during the crystallization process.

Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) was carried out with a [DMA/SDTA861.sup.e] apparatus (Mettler-Toledo, Switzerland) in the tensile mode, which provided the plots of the storage modules (E') and the mechanical loss (tan[delta]) against temperature. The samples were sized W X H X L = 20 X 4 X 1 [mm.sup.3]. All tests were conducted at a frequency of 1 Hz and a heating rate of 3[degrees]C/min as a function of temperature from -80 to 120[degrees]C.

Mechanical Properties

The uniaxial tensile tests were carried out at 23 [+ or -] 2[degrees]C on an Instron 1121 testing machine (Canton MA). Specimens (20 X 4 X 0.05 [mm.sup.3]) were cut from the previously blown films into a dumbbell shape in the machine direction (MD) and transverse direction (TD), respectively. The measurements were conducted at a cross-head speed of 20 mm/min at room temperature according to ASTM D638-2008. At least five runs for each sample were measured, and the results were averaged.

The right angle tearing strength were measured at 23 [+ or -] 2[degrees]C on an Instron 1121 testing machine. The measurements were conducted at a cross-head speed of 20 mm/min at room temperature according to QB/T 1130-91 in the machine direction and transverse direction, respectively. At least five runs for each sample were measured, and the results were averaged.

Morphology of Films

The tear-fracture surfaces of the neat PLA and PLA/ATBC/ GPOE blown films were characterized with a scanning electron microscope (model Japan JXA-840 ESEMFE). A layer of gold was sputter-coated uniformly over all the fractured surfaces before SEM observations.

Optical Properties

Transmittance haze and clarity of the neat PLA and PLA/ ATBC/GPOE blown films were measured in an Optical Hazemeter WGT-S system (Shanghai Precision and Scientific Instrument Co. Ltd, China). The thickness of the samples was 40 [micro]m. The values obtained were averages of at least six determinations.

Surface Property Measurements

Contact angle measurements were performed with a Kruss DSA100 (Germany) apparatus. A water droplet was dropped on the surface of a small sample cut from a dumbbell-shaped piece. The evolution of the droplet shape was recorded by a CCD video camera and was analyzed to determine the contact angle. A droplet of distilled water (3-5 [micro]l) was placed on the films surface. The contact angles were measured on both sides of the drop. Each reported contact angle was the mean value of at least 10 measurements taken at different position on the film.

Migration of ATBC

For migration measurement, all reagents were in chromatographic grade. ATBC used as a standard solution was prepared in dehydrated ethanol; 1.000 g of composite films were introduced into volumetric flasks (10 ml) and extracted at room temperature with dehydrated ethanol with shaking. The experiments of ATBC extraction from the solvent were determined at room temperature using a Gas Chromatography (GC) apparatus. The GC unit was a GC-14C (Shimadzu, Kyoto, Japan) gas chromatograph with a flame ionization detector (FID), equipped with a 30 m x 0.25 mm x 2.5 [micro]m column (DB-5, Agilent, USA). The column temperature program was the following; 100[degrees]C, held 3 min; rating 10[degrees]C/min to a temperature of 260[degrees]C (5 min hold). The injector and detector temperatures were 280[degrees]C and 300[degrees]C, respectively. Injection volume was 1 [micro]l and detection was performed at a split mode (30:1). The carrier gas was helium at 75 kPa (1 ml/min).

The migration rate of the samples was calculated using the formula [49]:

migration rate (%) = [W.sub.1]/[W.sub.2] X 100 (2)

where [W.sub.1] is the weight of the extracted ATBC tested by GC; [W.sub.2], initial weight of the test specimen.

Enzymatic Degradation of Blown Films

The enzymatic degradation of the composites films was carried out in phosphate buffer (pH 8) containing Pseudomonas mendocina at 30[degrees]C with shaking at 140 rpm. The composite films from the pressed sheets with thicknesses of 0.1 mm were chopped into square with gauge dimensions of 10 mm X 10 mm. Then all samples were placed in small glass bottles containing the buffer and P. mendocina. The samples were picked up after a fixed time interval, washed with distilled water, and dried to constant weight in a vacuum, and then the weights of the films were measured. For comparison, neat PLA was treated with the same procedure.

RESULTS AND DISCUSSION

Reaction Mechanism

The FT-IR spectra were illustrated in Fig. 1 to elucidate the structural changes induced by the reaction between carboxylic acid and epoxy during the melt-processing. FT-IR spectra of individual PLA and GPOE were also shown in Fig. 1. Before FT-IR test, the neat PLA, neat GPOE and PLA/ATBC/GPOE blends were extracted in xylene and chloroform to remove the unreacted GPOE, PLA respectively. For PLA/ATBC/GPOE blends, Fig. 1 showed that the absorption at 3504 [cm.sup.-1] due to the end carboxylic group of PLA disappeared and confirmed the chemical reaction between carboxylic acid of PLA end groups and epoxide groups of GPOE.

The strong absorption at 1757 [cm.sup.-1] was attributed to the stretch vibration of ester carbonyl group of the PLA backbone, while the vibration of ester carbonyl of GMA in GPOE was located at 1730 [cm.sup.-1]. With the increasing of GPOE content in the blends, the intensity of absorption peak at 1757 [cm.sup.-1] increased and absorption peak of ester carbonyl for GPOE disappeared. Compared with GPOE, the intensity of the epoxy characteristic peak at 711 [cm.sup.-1] decreased and suggested that most of the GPOE was reacted with PLA [50].

Rheological Characterization

Rheological behavior played an important role in polymer processing. As it was known, foaming and film blowing especially required a relative high melt strength. Dynamic rheological experiments were carried out for the PLA/ATBC/GPOE blends over the whole composition range. Figure 2a displayed the relationship between storage modulus and angular frequency of the PLA/ATBC/GPOE blends at 175[degrees]C. Because of the plasticization of ATBC, the storage modulus of 100/10 PLA/ATBC was remarkably lower than that of neat PLA. It indicated that ATBC was an effective plasticizer of PLA. As shown in Fig. 2a, the dynamic storage modulus G' of PLA/ATBC/GPOE blends increased with angular frequency and also increased with increase of GPOE content in the blends. At low frequencies, compared with neat PLA the storage modulus of the 100/10/20 PLA/ ATBC/GPOE blend increased by about an order of magnitude. The higher absolute values of dynamic modulus indicated the formation of interaction and entangled structures between the PLA chains and the chains of polyolefin elastomer [39, 51, 52].

Figure 2b displayed the relationship between complex viscosity [[eta].sup.*] and angular frequency of the neat PLA and PLA/ATBC/ GPOE blends at 175[degrees]C. With the addition of ATBC, the [[eta].sup.*] of the PLA/ATBC blend at low frequencies decreased than that of the neat PLA. The decreased melt viscosity of the blend could be related to an increased free volume due to the plasticization by ATBC. With increasing the angular frequency, all composites showed shear-thinning behavior, which was mainly ascribed to the disentanglement of molecular chain between PLA and GPOE at high shearing rate. At low frequencies the addition of GPOE resulted in a gradual increase in the viscosity of the blends. The [[eta].sup.*] of the PLA/ATBC/GPOE blends was much higher than that of the PLA/ATBC blend due to the interaction of PLA and GPOE. It took placed the entanglements between PLA chains and the chains of polyolefin elastomer. The melt viscosity of the PLA/ATBC/GPOE blends increased with an increase in GPOE content because more GPOE obstacles existed and more PLA chains had been anchored at low shear rates. This significant enhancement in the melt viscosity of PLA/ ATBC/GPOE blends leaded to an increase in melt strength, which was favorable for the processing of blown films. At high frequencies, the [[eta].sup.*] of the PLA/ATBC/GPOE blends was decreased due to the disentanglement of molecular chain between PLA and GPOE.

As soon as the composites percolated, the large scale polymer chain relaxation behaviors might be highly influenced by the percolated GPOE network. Figure 2c showed the Han plots of G'-G" [53] for neat PLA and its blends at 175[degrees]C, respectively. In addition, the reduced slope with the addition of GPOE indicated that the blends became more heterogeneous. It should be noted that the inflection point where the slope was changed shifts to a higher frequency with increasing GPOE content. This indicated that much energy was needed to change the degree of heterogeneity due to the increased physical association within the composites at a high GPOE loading. Meanwhile, as could be seen from Fig. 2c, PLA/TABC/GPOE samples showed a solid-like (G' > G") melt behavior over the entire angular frequency range. Furthermore, the elasticity of the samples gradually increased with increasing GPOE content, this effect was more significant for blown films, which can be ascribed to the formation of cross-linking between PLA and GPOE. This significant enhancement in the elastic behavior of PLA/ATBC/GPOE blends led to an increase in melt strength.

In Fig. 2d, the phase angle, [delta], was plotted versus the absolute value of the complex modulus [absolute value of [G.sup.*]] for the neat PLA and PLA/ ATBC/GPOE blends. As it was known, this plot was the famous van Gurp-Palmen plot, which could be used to detect the rheological percolation of the filled polymeric composites [54-56]. The low-frequency [delta] of the neat PLA and the PLA/ATBC blend were close to 90[degrees], which was indicative of a flow behavior presented by the viscoelastic fluid. However, the low-frequency [delta] decreased to 80[degrees] with increasing GPOE content to 20 wt%, indicating an enhanced elastic behavior. Thus, the enhanced elastic behavior and melt strength would be benefit for film blowing during the actual production process.

Thermal Properties

The DSC heating thermograms shown in Fig. 3a and b exhibited three main transitions successively: a glass transition, a cold crystallization exotherm and a melting endotherm. The measured values of the phase transition parameters were summarized in Table 2.

During the first heating run (Fig. 3a) the glass transition temperature of the PLA/ATBC film decreased about 10.8[degrees]C compared with neat PLA due to the plasticization of ATBC. Neat PLA displayed a broad cold crystallization exotherm at approximately 122.8[degrees]C. The incorporation of ATBC decreased the cold crystallization temperature by approximately 19.3[degrees]C. It indicated that ATBC could be acted as a plasticizer for PLA. The glass transition temperatures of the PLA/ATBC/GPOE films decreased slightly with increasing GPOE content in the blends. With increasing GPOE content from 5% to 20%, the initial cold crystallization temperature had been shifted to a lower temperature. It indicated that the reaction had produced compatibilization effect between PLA and GPOE. The addition of GPOE restricted the crystalline ability of PLA. Double melting peaks appeared at about 141.4 and 150.8[degrees]C in the PLA/ATBC and PLA/ATBC/GPOE films. This bimodal melting peak was induced by ATBC during the melting process when the less perfect crystals had enough time to melt and reorganize into crystals with higher structural perfection and then remelt at higher temperature [57, 58].

Figure 3b shown that the glass transition temperature of PLA/ATBC film decreased about 13.9[degrees]C compared with neat PLA due to the plasticization of ATBC during the second heating run. Neat PLA displayed a broad cold crystallization exotherm at approximately 123.3[degrees]C. The incorporation of ATBC decreased the cold crystallization temperature by approximately 14.7[degrees]C. This indicated that the ATBC acted as a plasticizer and promoted the crystallization of PLA. Double melting peaks appeared at about 141.8 and 151.1[degrees]C in the PLA/ATBC and PLA/ATBC/GPOE films. This bimodal melting peak was induced by ATBC during the melting process when the less perfect crystals had enough time to melt and reorganize into crystals with higher structural perfection and then remelt at higher temperature.

Compared with the PLA/ATBC film, the glass transition temperatures of the PLA/ATBC/GPOE films decreased with increasing content of GPOE in the blends. That indicated that the reaction between the two polymers had produced compatibilization effect and the PLA amorphous region had been influenced by GPOE. As it was shown in Scheme 2, after reactive compatibilization between PLA and GPOE, on one hand, with the addition of GPOE, the chains of entangled structures would decrease the space between polymer chains. Accordingly, the chain mobility of the polymer segments would decrease during heating trace, so the cold crystallization temperature decreased by approximately 5.6[degrees]C. On the other hand, the chains of entangled structures, result in more defective crystals, therefore, the Tm would shift to low temperatures. In addition, the reaction point worked as nucleation sites, entangled structures in amorphous area impeded the transportation of macromolecular chains between polymer chains, restricting the crystalline ability of PLA and forming more amorphous phase.

Crystalline Morphology

Figure 4 shows polarized microscope photographs of PLA/ ATBC/GPOE films. Neat PLA spherulites appear as an obvious Maltese cross pattern in Fig. 4a. For the 100/10/0 PLA/ATBC/ GPOE film, with addition of ATBC the spherulites enlarge, which indicates that the ATBC leads to a perfect crystal structure (Fig. 4b). For the 100/10/5, 100/10/10, 100/10/15, 100/10/20 PLA/ ATBC/GPOE films, GPOE can be used as a heterogeneous nucleating agent of PLA, which causes the nucleating density of the spherulites to increase and the size of the spherulites to decrease. (Fig. 4c-f). At the same time, the amorphous phase increased and crystallized phase decreased. The Maltese cross pattern phenomenon weakens and the spherulite size becomes smaller.

Dynamic Mechanical Analysis

Figure 5 showed the dependence of storage modulus (E') and tan[delta] on temperature for PLA/ATBC blends modified with different contents of GPOE. In Fig. 5a, the E' of neat PLA gradually decreased with increasing temperature from -60 to 50[degrees]C, then dropped rapidly because of the glass transition and finally reached a minimal value around 78[degrees]C. The influence of GPOE on the E' curve depended on the temperature region. In Fig. 5a, between -40[degrees]C and 45[degrees]C, the E' of the PLA/ATBC/GPOE samples were lower than that of neat PLA. This was attributed to the increasing content of GPOE in the PLA/ATBC/GPOE blends, indicating a decreasing rigidity of the blends. On further increasing the temperature, E' increased and achieved a maximal value around 105[degrees]C. This reflected an enhancement of the sample rigidity resulting from the cold crystallization process detected by the DSC.

In Fig. 5b, the tan[delta] peak around -27[degrees]C was assigned to the [T.sub.g] of the neat GPOE, neat PLA showed a single peak for [T.sub.g] (69.1[degrees]C) and the 100/10 PLA/ATBC blend showed a single high temperature peak for [T.sub.g] around 56.3[degrees]C, which suggested that PLA was miscible with ATBC. Moreover, ATBC could significantly decrease the glass transition temperature of PLA and enhance the flexibility of PLA/ATBC/GPOE blends. The full width-at-half-maximum (FWHM) was calculated by tan[delta]. The I parameters of width were listed in Table 3. The transitional width of PLA increased from 7.25 to 9.64[degrees]C in DSC, the FWHM of PLA increased from 8.54 to 9.17[degrees]C in DMA. It indicated that the plasticity of PLA was enhanced due to the addition of ATBC. Likewise, the Hildebrand solubility parameter was used to further discuss the PLA/ATBC system. The Hildebrand solubility parameter equation [59, 60] was as follows:

[delta] = ([E.sub.coh]/V).sup.1/2] (3)

where [delta] is solubility parameter, [E.sub.coh] is the cohesive energy density, and V is the per unit molar volume. From the Eq. 3, the solubility parameter of ATBC was 17.9. That the solubility parameter of PLA has been reported by Van and Palmen [54] was 17.6. The results also indicated that PLA was miscible with ATBC, it was consistent with the results of DMA.

Compared with neat PLA, with increasing GPOE content, the glass transition temperatures of the PLA/ATBC/GPOE blends had shifted to lower temperature in Fig. 5b. That suggested that the compatibility of PLA and GPOE was improved, it was consistent with the results of DSC. Similar phenomenon had also been observed from the PLA/methyl methacrylate-butadiene-styrene (MBS) blends and PLA/acrylate copolymer (ACR) blends [51, 61]. Surprisedly, the shoulder peaks of 100/ 10/5 and 100/10/10 PLA/ATBC/GPOE blends appeared at temperature about 55[degrees]C, it indicated that the compatibility of PLA and GPOE was relatively poor for the 100/10/5 and 100/10/10 PLA/ATBC/GPOE blends. With the content of GPOE increased to 15 wt% and 20 wt%, the shoulder peaks of the PLA/ATBC/ GPOE blends disappeared, it indicated that the compatibility between PLA and GPOE had been improved with the content of GPOE increased.

In addition, Fox equation was used to further discuss the compatibility of PLA and GPOE. The Fox equation [62] was as follows:

1/[T.sub.g] = [W.sub.1]/[T.sub.g1] + [W.sub.2]/[T.sub.g2] (4)

where [W.sub.i] is weight fraction of neat component i, [T.sub.g] is the blend and [T.sub.gi] is the glass temperature of neat component i. [T.sub.g] of the blends taken from the Table 2, was plotted in Fig. 6 as a function of the blend composition. It could be seen that the variation of [T.sub.g]s on the composition of PLA-rich blend Fitted basically to the Fox equation, and the result indicated that the GPOE was miscible with PLA at selective content.

Mechanical Properties

The tensile properties and tear strength of PLA/ATBC/GPOE films were given in Table 4. The addition of GPOE changed the tensile behavior of the PLA and PLA/ATBC films significantly, as was shown in Fig. 7. The tensile strength of neat PLA film was 52.2 MPa (MD) and 52.1 MPa (TD), and the elongation at break was only about 6.1% (MD) and 6.0% (TD). The film showed brittle fracture upon tensile load. In contrast, all of the PLA/ATBC/GPOE films showed clear stress-strain curve yielding behavior upon stretching in Fig. 8c-f. After yielding occurred the strain developed continuously while the stress remained almost constant and the samples finally broke with a high elongation compared with that of neat PLA. It was very interesting to find that the 100/10/20 PLA/ATBC/GPOE film had a very high elongation at break of 203.2% and 228.9% in the machine direction and the transverse direction, respectively, while the tensile strength remained as high as 20.3 MPa and 21.9 MPa.

Measuring the Young's modulus was the common method of determining the stiffness [63]. From Table 4, neat PLA film exhibited a modulus value of 2350 MPa and 2380 MPa in the machine direction and the transverse direction, respectively. It was evident that the addition of ATBC led to a decrease of the modulus value to 2287 and 2247 MPa, respectively. The addition of ATBC decreaseed the [T.sub.g] of PLA and improved its ductility and softness. The addition of GPOE could further decrease the modulus of PLA/ATBC/GPOE films. When the content of GPOE increased from 5% to 20%, the modulus decreased from 1962 MPa and 2011 MPa to 1392 MPa and 1259 MPa in the machine direction and the transverse direction, respectively. This indicated that GPOE improved the flexibility of PLA/ ATBC/GPOE blown films.

It was worth noting that the tear strength of the films has improved in contrast with the tensile properties in Table 4. The neat PLA specimen exhibited relatively low tear strength values of 69.2 and 74.7 KN/m in the machine direction and the transverse direction, respectively. In contrast, the PLA/ATBC/GPOE films revealed a substantial increase in tear strength values as their GPOE contents increased. For example, with GPOE content increasing from 0 to 20 wt%, the tear strength values of PLA/ATBC/GPOE specimens in the machine direction and the transverse direction increased from 79.6 and 81.1 to 141 and 147 KN/m, respectively. For the PLA/ATBC/GPOE blown films, the tear resistance was always higher along the machine direction relative to the transverse direction. This difference could be ascribed to the varying degrees of orientation of the lamellae within the plane of the film.

These results suggested that the relatively poor tear strengths of the PLA could be significantly improved after melt blending with GPOE in PLA resins. Thus the physical entanglement increased by the incorporation of GPOE was conducive to enhancing the intermolecular force of the blends. For the PLA/ATBC/GPOE blown films, the great improvement of tensile strain at break and tear strengths might be ascribed to the intrinsic high flexibility of GPOE and the increased miscibility of two components due to the reaction between carboxyl of PLA and epoxy of GPOE. GPOE was compatible with PLA, which typically improved the impact strength by absorbing the energy of the impact through deformation of the elastomer. It could be used to improve the tear strength of the blown film.

In addition, from the Table 4, the transverse direction (TD) had better mechanical properties than the machine direction (MD). As the blown ration was 5.6, the film oriented at the transverse direction. Thus, the PLA molecular chain was more easily to arrange in transverse direction. During the right angle tearing strength process, the tearing strength and the elongation at break of the film at the transverse direction was better than those at the longitudinal direction.

Surprisingly, the plasticizer and elastomer in the PLA/ATBC/ GPOE blown films has been found to yield a highly desired combination of high elongation at break, high tear strength and low Young's modulus required for most packaging applications. GPOE could decrease the modulus of the blown films. The PLA/ATBC/GPOE blown films had excellent flexibility and mechanical properties.

Morphology of Films

SEM micrographs of the tearing fracture surface along the machine direction of PLA/ATBC/GPOE blown films are given in Fig. 8. The fracture surface of 100/0/0 and 100/10/0 PLA/ ATBC/GPOE films (Fig. 8a and b) appeared relatively smooth, indicating that little plastic deformation has taken place during the tearing test. This is in good agreement with their lower elongation at break. From Fig. 8c to d, the tearing fracture surfaces of 100/10/5 and 100/10/10 PLA/ATBC/GPOE blown films are rough and long stretches of ligaments, which suggested that the tear specimen broke yieldingly. Some cavitations and a clear matrix deformation can be clearly identified for the PLA/ATBC/ GPOE films. These cavities are formed when the volumetric strain energy released by forming a void is greater than the surface energy needs to form a new surface plus the energy needs to stretch the surrounding rubber to make space for the void [64]. Especially, 100/10/15 and 100/10/20 PLA/ATBC/GPOE films display significantly ductile fracture on which a rumpled surface can be seen (Fig. 8e and f). The rumples lie parallel to the notch of highly drawn material. The occurrence of rumples can be ascribed to the considerable tearing ahead of the crack tip before unstable fracture sets in. The extensive deformation ahead of the crack tip gives rise to these structures. In particular, extensive plastic deformation of the PLA matrix can be clearly observed, which implied that shear yielding of the PLA matrix has taken place. This is a typical feature of a ductile fracture [51]. This result was in consistent with the tear strength data from Table 4.

Optical Properties

Transparency was an important property of plastic films. The refractive index of ATBC and GPOE chosen in this study was 1.44 and 1.48, respectively. It was very close to that of the PLA matrix, which had a value of 1.45. The visible light transmittance and haze value of the neat PLA and PLA/ATBC/GPOE films were given in Table 5. With increasing content of GPOE, the transmittance and the clarity decreased, and the haze value of all the samples increased. When the GPOE content was 20%, the transmittance was 86.4%, the haze value was 7.0% and the clarity was 92.2%. It could be concluded that the PLA/ATBC/GPOE blown films had good optical properties, which also indicated that the PLA/ATBC/GPOE blown films were miscible system.

To further study the effect of the GPOE particles on the transparency of PLA, the reflectivity (R) and the absorption coefficient ([alpha]) were calculated as follows [61]:

R = [(n - 1).sup.2]/[(n - 1).sup.2] (5)

[DELTA]n = [n.sub.1] - [n.sub.2] (6)

[alpha] = 2 [[pi].sup.3] [([DELTA]n/[n.sub.1]).sup.2] ([V.sup.2][v.sub.s][[lambda].sup.4]) (7)

n is refractive index; [n.sub.1] is the refractive index of the matrix; [n.sub.2] is the refractive index of the disperse phase; [lambda] is wavelength; V is the volume of the disperse phase; and [v.sub.s] is the volume ratio of the disperse phase and the matrix.

It could be seen that the reflectivity of the blends increased with the incorporation of GPOE, in Table 5. The results indicated that the loss of transparency was due to the low reflectivity (R < 4%). The absorption coefficient ([alpha]) of the blends increased with increasing GPOE content, it indicated that the scattering intensity of the blends improved while the transparency decreased. That was because the interface interaction between the crystalline phase and the amorphous phase enhanced with the content of GPOE increased, leading to induced large light scattering and the transparency of PLA/ ATBC/GPOE blown films reduced.

Surface Property Measurements

The contact angles (0) for water and diiodomethane on PLA films were shown in Fig. 9. As shown, the contact angle values of bipolar liquid water were range from 72.4 to 76.3[degrees], which increased with the addition of GPOE. From Fig. 9, it could be seen that the values of contact angle for diiodomethane decreased from 29.3 to 22.0[degrees]. It was known that PLA was a linear aliphatic thermoplastic polyester and also had excellent hydrophobic characteristic. GPOE was polyolefin elastomers, which gained through copolymerization of octene and ethylene, and it was also a hydrophobic compound. These results indicated that the addition of GPOE had improved the water resistance of PLA. However, the addition of ATBC would slightly decrease the contact angle for water on the PLA film. ATBC contained hydrophobic groups and led to slightly increased hydrophobicity of PLA films. Accordingly, the contact angle of the PLA/ATBC film decreased slightly.

Migration of ATBC

The migration rates of plasticizer was also an essential criterion to value plastic films [65]. It was found that ATBC was easily soluble in dehydrated ethanol through test. Dehydrated ethanol was used as the extraction medium in the evaluation of ATBC extraction. It has been used to evaluate the migration stability of polyester plasticizers in plasticized PLA/ATBC/ GPOE films.

The ATBC migration rates of PLA/ATBC/GPOE films containing different amount of GPOE were shown in Fig. 10. With the addition of GPOE, the migration rate of the PLA/ATBC/ GPOE films decreased from 2.67% to 2.17%. The addition of GPOE suppressed ATBC migration slightly in PLA/ATBC/ GPOE films as compared with the PLA/ATBC film. PLA was easily crystallized under 110[degrees]C at room temperature, leading part of amorphous phase to crystallized phase. The plasticizer dispersed in amorphous phase was squeezed from the PLA matrix, it would be easily migrated to the surface under the same temperature. However, with the addition of GPOE, the crystallization degree of PLA was decreased, part of crystallized phase transferred into amorphous phase, which allowed PLA molecular chains to move in a more free space. Accordingly, ATBC was more easily dispersed in the amorphous phase of PLA, lowering the content of plasticizer migrating to the surface of PLA matrix. Therefore, from ATBC extraction test, the addition of GPOE could reduce slightly the migration rate of plasticizer in the PLA/ATBC/GPOE films.

Enzymatic Degradation of Blown Films

PLA was the biodegradable materials. The biodegradability of PLA/ATBC/GPOE blown films was extremely important for the potential application of the materials in future. Here, enzymatic degradation tests were used to evaluate the biodegradability of the composites. P. mendocina which could produce lipase was used to degrade the PLA and its composites. Figure 11 showed the weight loss profiles of neat PLA and PLA/ATBC/ GPOE blown films as a function of time during the enzymatic degradation. The weight losses of the films increased with time for all samples. The rate of enzymatic degradation could be determined from the slope of the weight loss against time. At the first stage, the rates of enzymatic degradation for all the samples were slow, because there was an incubation period for the P. mendocina. It became faster after 60 h for neat PLA and PLA/ATBC/GPOE blown films. However, the rate of enzymatic degradation decreased with the addition of ATBC. The rate of enzymatic degradation also decreased with increasing GPOE content. Consequently, the enzymatic degradation of PLA was retarded in the presence of both ATBC and GPOE. Since the degree of crystallinity value of PLA component in the PLA/ ATBC blown film was increased comparing with the neat PLA blown film. The crystallization retarded enzymatic degradation. The retarded enzymatic degradation of PLA/ATBC/GPOE blown films might arise from that the GPOE elastomer diluted the PLA domains, thus disturbed the adsorption process of the enzyme molecules on the film surface or retarded hydrolytic scission of the PLA chains by the enzyme molecules in the presence of GPOE elastomer in PLA matrix, causing the disturbed degradation of PLA chains [66].

CONCLUSIONS

The PLA/ATBC/GPOE films have been prepared by a melt blending and extrusion blow film process. The effects of ATBC and GPOE on the rheological and mechanical properties of the samples were investigated in detail. From the rheology and dynamic mechanical analyses, it was found that ATBC can plasticize PLA and enhance the flexibility of PLA/ATBC/GPOE films. The elongation at break as well as the tear strength of PLA/ATBC/GPOE films was improved significantly compared with neat PLA. The tear strength of the 100/10/20 PLA/ATBC/ GPOE film was about 2.0 times that of neat PLA film. It was proved that the PLA/ATBC/GPOE blown films were high tearing resistance films. Moreover, from the SEM of tear fractured surfaces, while the content of GPOE increased up to 15 wt% and 20 wt%, the large plastic deformation of PLA/ATBC/GPOE films appeared. This phenomenon indicated that the tear strength of PLA/ATBC/GPOE films was improved. GPOE could be used effectively as a tear resistance modifier of PLA films. Lastly, the PLA/ATBC/GPOE blown films had better water-resistance than the neat PLA blown films. With adding ATBC and GPOE, the biodegradability of the PLA/ATBC/GPOE films decreased slightly. In conclusion, the PLA/ATBC/GPOE films would wide used in the field of polymer packaging materials for above mentioned merits.

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Yan Zhao, (1,2) Xianzhong Lang, (1,2) Hongwei Pan, (1,2) Yajun Wang, (1) Huili Yang, (2) Huiliang Zhang, (2) Huixuan Zhang, (1) Lisong Dong (2)

(1) Synthetic Resins and Special Fiber Engineering Research Center, Ministry of Education, Changchun University of Technology, Changchun 130012, China

(2) Key Laboratory of Polymer Ecomaterials, Chinese Academy of Sciences, Changchun Institute of Applied Chemistry, Changchun 130022, China

Correspondence to: H. Zhang; e-mail: hlzhang@ciac.jl.cn

Contract grant sponsor: Chinese Science Academy (Changchun Branch); contract grant number: 2014SYHZ0019; contract grant sponsor: The National High Technology Research and Development Program of China (863 Program); contract grant number: 2012AA062904; contract grant sponsor: The National Science Foundation of China; contract grant number: 51021003.

DOI10.1002/pen.24171

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. The mixing compositions of the PLA/ATBC/GPOE blends.

PLA/ATBC/GPOE (w/w/w)   PLA (g)   ATBC (g)   GPOE (g)

100/0/0                  2500        0          0
100/10/0                 2500       250         0
100/10/5                 2500       250        150
100/10/10                2500       250        250
100/10/15                2500       250        375
100/10/20                2500       250        500

TABLE 2. Crystallization properties of the neat PLA and
PLA/ATBC/GPOE films.

                               Second heating run

                                                    [DELTA]
PLA/ATBC/             [T.sub.g]      [T.sub.cc]    [H.sub.cc]
GPOE(w/w/w)          ([degrees]C)   ([degrees]C)     (J/g)

First heating run
  100/0/0                60.0          122.8          22.5
  100/10/0               49.2          103.5          27.9
  100/10/5               46.3          101.5          26.5
  100/10/10              45.6          100.9          23.6
  100/10/15              45.1          100.7          21.4
  100/10/20              45.0          100.5          21.3
Second heating run
  100/0/0                60.3          123.3          21.6
  100/10/0               46.4          108.6          26.7
  100/10/5               44.7          108.2          24.5
  100/10/10              43.5          105.9          23.9
  100/10/15              42.1          103.6          22.1
  100/10/20              40.8          102.4          21.1

                                    Second heating run

                                                  [DELTA]
PLA/ATBC/             [T.sub.m1]    [T.sub.m2]   [H.sub.f]  [X.sub.c]
GPOE(w/w/w)          ([degrees]C)  ([degrees]C)    (J/g)       (%)

First heating run
  100/0/0                 --          150.6        21.9       23.5
  100/10/0              144.0         150.8        27.7       32.8
  100/10/5              141.9         148.2        26.3       32.5
  100/10/10             141.6         147.8        23.4       30.2
  100/10/15             141.8         147.5        21.7       29.2
  100/10/20             141.4         147.1        20.8       29.1
Second heating run
  100/0/0                 --          150.0        21.4       23.0
  100/10/0              145.7         151.1        26.6       31.5
  100/10/5              143.5         149.0        25.4       31.4
  100/10/10             142.9         148.3        24.2       31.2
  100/10/15             141.9         147.9        21.7       29.1
  100/10/20             141.8         147.7        20.8       29.0

[DELTA][H.sup.0.sub.f] of PLA is 93 J/g.

TABLE 3. The glass transition parameters of PLA/ATBC blends.

                                 PLA/ATBC

PLA/ATBC (w/w)   Width of transition in DSC   FWHM in DMA

100/0                       7.25                 8.54
100/10                      9.64                 9.17

TABLE 4. Mechanical properties of PLA/ATBC/GPOE blend films.

PLA/ATBC/GPOE (w/w/w)       Tensile strength MD/TD (MPa)

100/0/0                 52.2 [+ or -] 1.1/52.1 [+ or -] 1.2
100/10/0                50.9 [+ or -] 1.7/49.0 [+ or -] 1.9
100/10/5                29.8 [+ or -] 1.4/34.5 [+ or -] 1.9
100/10/10               28.6 [+ or -] 1.8/28.6 [+ or -] 2.3
100/10/15               22.5 [+ or -] 1.9/25.0 [+ or -] 1.7
100/10/20               20.3 [+ or -] 1.0/21.9 [+ or -] 1.1

PLA/ATBC/GPOE (w/w/w)        Elongation at break MD/TD (%)

100/0/0                   6.1 [+ or -] 0.8/6.0 [+ or -] 0.7
100/10/0                 14.8 [+ or -] 1.3/18.2 [+ or -] 1.8
100/10/5                102.0 [+ or -] 11.5/100.6 [+ or -] 13.6
100/10/10               131.7 [+ or -] 17.2/158.6 [+ or -] 16.4
100/10/15               174.2 [+ or -] 13.4/185.9 [+ or -] 14.7
100/10/20               203.2 [+ or -] 16.1/228.9 [+ or -] 12.8

PLA/ATBC/GPOE (w/w/w)       Young's modulus MD/TD (MPa)

100/0/0                 2350 [+ or -] 164/2380 [+ or -] 156
100/10/0                2287 [+ or -] 113/2247 [+ or -] 142
100/10/5                1962 [+ or -] 124/2011 [+ or -] 146
100/10/10               1919 [+ or -] 143/1951 [+ or -] 121
100/10/15               1509 [+ or -] 172/1526 [+ or -] 189
100/10/20               1392 [+ or -] 117/1259 [+ or -] 134

PLA/ATBC/GPOE (w/w/w)        Tear strength MD/TD (KN/m)

100/0/0                 69.2 [+ or -] 1.7/74.7 [+ or -] 1.2
100/10/0                79.6 [+ or -] 2.4/81.1 [+ or -] 2.9
100/10/5                 106 [+ or -] 14.1/110 [+ or -] 10.2
100/10/10                116 [+ or -] 12.1/123 [+ or -] 11.4
100/10/15                128 [+ or -] 10.5/136 [+ or -] 12.7
100/10/20                141 [+ or -] 13.4/147 [+ or -] 11.2

TABLE 5. The optical properties of pure PLA and PLA/ATBC/GPOE blend
films.

PLA/ATBC/GPOE   Transmittancy              Clarity
(w/w/w)              (%)        Haze (%)     (%)

100/0/0             94.3          2.8       98.0
100/10/0            94.4          2.6       98.1
100/10/5            92.8          3.1       96.8
100/10/10           88.6          3.9       95.4
100/10/15           86.5          5.6       93.5
100/10/20           86.4          7.0       92.2

PLA/ATBC/GPOE   Reflectivity
(w/w/w)             (%)             [alpha]

100/0/0             3.37              --
100/10/0            3.36       1.76 x [10.sup.7]
100/10/5            3.38       2.38 x [10.sup.7]
100/10/10           3.39       3.53 x [10.sup.7]
100/10/15           3.40       8.65 x [10.sup.7]
100/10/20           3.42       1.53 x [10.sup.8]
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Author:Zhao, Yan; Lang, Xianzhong; Pan, Hongwei; Wang, Yajun; Yang, Huili; Zhang, Huiliang; Zhang, Huixuan;
Publication:Polymer Engineering and Science
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Date:Dec 1, 2015
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