Preparation, characterization and properties of PLA/TiO₂ nanocomposites based on a novel vane extruder

Abstract

Polylactide/TiO₂ nanocomposites with different contents of nanoscale TiO₂ were prepared by melt blending with a vane extruder in this work. SEM and TEM microphotographs indicate that the nanoparticles were well dispersed in the polymer matrix under the elongational flow field of the vane extruder. Crystallization and melting behavior were examined with DSC. The results show that TiO₂ nanoparticles have an inhibition for the cold crystallization process of PLA to some extent and the cold crystallization temperature of nanocomposites shifted to a maximum value at about 106 °C with low TiO₂ loadings (0.5 wt% or 1 wt%). Dynamic rheological measurements and TGA results show that the introduction of TiO₂ has significantly improved the thermal stability. The toughness, surface wettability and UV resistance of the composites were also investigated in detail.

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Introduction

Poly(lactic acid) (PLA) is an environmentally benign aliphatic bio-polyester which can be produced from renewable plant resources (mainly starch and sugar).^1,2 PLA has excellent comprehensive performance and it possesses reasonably good mechanical and optical properties, biocompatibility, processability and so on. PLA holds tremendous market potential for packaging materials, fibers, agricultural films, and biomaterials.^2–4

Despite the favorable properties, the poor thermal stability and the inherent brittleness of PLA severely limit its more widespread implementation, such as the engineering plastics and the electronics field. Several techniques including copolymerization and blending with other polymers have been developed to further improve the physical properties of PLA.^5–9 Besides, compounding with nanofillers,^5,10–14 such as silica, nanotubes, nanoclay, graphene and metal oxides nanoparticles (i.e. TiO₂, MgO and Fe₃O₄), is also an effective way to fabricate high performance composites. Among the metallic oxides, titanium dioxide (TiO₂), has been used most widely in paints, cosmetics, food packing materials, biomaterial as well as catalyst/photocatalyst in waste water treatment, since TiO₂ is inert, non-toxic, inexpensive, and environmentally-friendly with antimicrobial activity against a wide variety of microorganisms.^15–18

Biodegradable PLA nanocomposites with TiO₂ nanoparticles have gained much scientific attention due to the special surface characteristics of TiO₂. According to the literature, there exists plentiful dangling bonds on the surface of nano-TiO₂ which could interact with polymer molecules, thus resulting in improving the properties of nanocomposites.^16,17,19 Zhuang et al. have found that the PLA/TiO₂ nanocomposites, which prepared by in situ polymerization method, showed markedly improved thermal and mechanical properties with loading 3 wt% of TiO₂ nanoparticles.²⁰ Meng et al. reported that PLA can be also toughened by TiO₂ nanoparticles coated with polycaprolactone (PCL).²¹ Luo et al. have fabricated PLA/TiO₂ nanocomposites with improved mechanical properties by functionalization TiO₂.^22,23 Man and co-authors reported that the anti-UV performance of PLA was significantly improved by adding TiO₂ nanoparticles into PLA matrix resin.²⁴

Although many studies have demonstrated to improve the physical properties of PLA/TiO₂ nanocomposites with considering the influences of multifunctional modification or different loading content of TiO₂ nanoparticles, the effects of different flow field during processing on the degree of dispersion and the physical and mechanical behaviors have less investigation.

In the present study, PLA/TiO₂ nanocomposites have been prepared using vane extruder which the extruder is a novel polymer processing equipment dominated by elongational flow field. The vane extruder is beneficial for the dispersion of composites and especially suitable for the processing of thermo-sensitive polymer because of its short thermal–mechanical history and thus resulting in the reduction of degradation.^25–27 The plasticizing and conveying mechanism and the detailed structural design of vane extruder have been reported in detail in our previous work.^28,29 The purpose of this work is to detect the dispersion of nanoparticles in PLA matrix under elongational flow field and to determine the physical properties of its nanocomposites.

Experimental

Materials

A commercial PLA (trade name 4032D, 1.2–1.6% D-isomer lactide) was obtained from NatureWorks LLC, USA. The nanosized titanium dioxide (VK-TAKH570, Jing Rui New Material, China) was an anatase type with an average primary particle size of approximately 20 nm. PLA pellets and TiO₂ were dried to remove moisture in a vacuum before processing.

Preparation of PLA/TiO₂ nanocomposites by vane extruder

A novel polymer processing equipment known as the vane extruder was employed to compound PLA/TiO₂ nanocomposites in this research. The structure schematic diagram of the vane extruder is shown in Fig. 1. The vane extruder consists of a number of vane plasticizing and conveying units (VPCU).^25,28–30 The stator, vanes, baffles and the rotor of VPCU composes a closed chamber. Due to the stator has an eccentric distance to the rotor, the volume of the closed chamber is changed periodically when the rotor is rotating. A converging channel can be obtained in the circumferential direction, thus generating higher stress and dynamic elongational flow field. The detailed working principle and process of the vane extruder has been described detailedly in our previous works.^28,31,32 The temperature profile used for the extruder was 150, 165, 180 and 160 °C from hopper to die, respectively, and the rotation was maintained at 50 rpm.

Fig. 1 The structure schematic diagram of the vane extruder.

Before extrusion, PLA pellets and TiO₂ nanoparticles were dried in vacuum at 80 °C for more than 6 h. The mixture of PLA and TiO₂ was premixed in a high-blender at 400 rpm min⁻¹ for 5 min, and subsequently fed into the vane extruder for melt blending. The extrudant was cooled in air and subsequently granulated by a pelletizer.

The samples with 0, 0.5, 1.0, 2.0, 5.0, 10.0 and 15.0 wt% TiO₂ were prepared and marked as PLA, PLA-0.5, PLA-1, PLA-2, PLA-5, PLA-10 and PLA-15 nanocomposites. The pure PLA was also subjected to the mixing treatment so as to have the same thermal history as the composites.

Tensile test specimens were fabricated by injection molding. The barrel temperatures for the injection were set at 170, 180, 185 and 190 °C from hopper to the nozzle, respectively. Prior to injection molding, all pellets prepared by vane extruder were dried in vacuum oven at 50 °C for 12 h.

Characterization

SEM. The scanning electron microscopy (SEM) imaging was conducted to observe the morphology of the fracture surface of PLA/TiO₂ nanocomposites using a ZEISS-ATC scanning electron microscope using a 5 kV accelerating voltage. The specimens with 4 mm thickness were fractured to expose the internal structure after being immersed in liquid nitrogen for 30 min. Prior to the SEM investigation, the fractured surface were sputtered with gold to provide enhanced conductivity.

TEM. TEM observations were performed to determine the dispersion of nanoparticles in PLA matrix with a JEOL (JEM-100XII) field emission transmission electron microscopy at an acceleration voltage of 200 kV. The ultrathin sections with a thickness of ∼100 nm were prepared from the injection molded samples via using a Leica EMUC6/FC6 microtome.

Dynamic rheological measurements. Dynamic viscoelastic properties of pure PLA and its nanocomposites were analyzed by using a rheometer of Anton Paar Physica MCR 302. The measurements were performed in the linear viscoelastic region with dynamic oscillatory mode and 25 mm parallel cone-plate with gap setting of about 0.7 mm. All experiments were carried out under nitrogen atmosphere at 170 °C. Frequency scans were taken at low strain (5%) at the frequency range between 0.01 rad s⁻¹ and 100 rad s⁻¹.

DSC. The thermal properties of the composites were analyzed using a NETZSCH DSC204 F1 analyzer. The samples were first heated from room temperature to 200 °C at 10 °C min⁻¹ heating rate and held for 3 minutes to eliminate the thermal history, and then cooled at a rate of 10 °C min⁻¹ up to 30 °C and reheated from 30 °C to 200 °C at 10 °C min⁻¹. The glass transition temperature (T_g), cold crystallization temperature (T_cc), melting temperature (T_m) and melting enthalpy (ΔH_m) were determined from the second heating scan. The fractional crystallinity χ_c was calculated using the following equation:

χ_c (%) = (ΔH_c/ΔH_o) × (100/φ) (1)

where χ_c is the crystallinity of PLA in the composites, ΔH_c is the measure heat of crystallization, ΔH_o (93.6 J g⁻¹) is the heat of crystallization of a 100% crystalline PLA,² and φ is the weight fraction of PLA in the composites.

TGA. The thermal stability of the PLA and PLA/TiO₂ nanocomposites was determined by a thermogravimetric analyzer (NETZSCH TG209) at a heating rate of 10 °C min⁻¹ under air atmosphere, from room temperature to 500 °C. Samples for testing were approximately 7 mg.

DMA. The dynamic mechanical properties of neat PLA and PLA/TiO₂ nanocomposites were determined using a NETZSCH DMA 204C analyzer in three-point bending mode at a frequency of 1 Hz and a strain amplitude of 0.05%. Measurements of the storage modulus (E′), the loss modulus (E′′) and the dissipation factor (tan δ) were carried out in the temperature range of 0 to 100 °C at a heating rate of 3 °C min⁻¹. The test specimen dimensions were approximately 4.0 × 10.0 × 50.0 mm (thickness–width–length).

Tensile testing. Tensile tests of pure PLA and PLA/TiO₂ composites were performed on an INSTRON 5566 universal testing machine and conducted at room temperature using a cross-head rate of 10 mm min⁻¹. The injection-moulded specimens were used for tensile testing and the gauge length was 50 mm. The data reported were determined in an average of five determinations.

UV transmittance. UV transmittance spectra for the PLA and PLA/TiO₂ nanocomposites were obtained by a ultra-violet visible spectrophotometer (PerkinElmer Lambda 950, USA). The spectra were recorded in the 200–600 nm wavelength range at room temperature in air.

Wettability of nanocomposites. To examine the surface wettability, the contact angles of were pure PLA and its nanocomposites measured by a video-based contact angle measuring device OCA15 (Data Physics Instruments Company, Germany), and the average value of five measurements at different positions of the surfaces was taken as the contact angle.

Results and discussion

Morphology of TiO₂/PLA nanocomposites

It is well known that the dispersion of nanoparticles in the polymer matrix is the key factor influencing the physical properties of the nanocomposites. A homogeneous dispersion of TiO₂ nanoparticles, together with strong interfacial interactions between nanoparticles and the matrix, can effectively improve the thermal, rheological, and mechanical properties of the nanocomposites.

The fracture surface morphologies of PLA/TiO₂ nanocomposites were observed by scanning electron microscopy (SEM) as shown in Fig. 2 and the detail dispersion of the TiO₂ nanoparticles in the PLA matrix were determined by transmission electron microscopy (TEM) as shown in Fig. 3. It can be clearly seen that nanoparticles have been uniformly dispersed in the matrix when the TiO₂ content is less than or equal to 5 wt%. When the TiO₂ content was 10 wt%, some individual TiO₂ nanoparticles aggregated into clusters. This may be due to the strong interaction among the TiO₂ nanoparticles.¹⁷ However, the number of big clusters was not too much and the TiO₂ particles were evenly dispersed in the PLA matrix. It indicates that the continuous volume extensional flow generated in the vane extruder had good effects on the dispersion of nanoparticles in polymer matrix.

Fig. 2 SEM micrographs of fracture surfaces (a) 1 wt% TiO₂, (b) 2 wt% TiO₂, (c) 5 wt% TiO₂, (d) 10 wt% TiO₂; 5000×.

Fig. 3 TEM images of PLA/TiO₂ nanocomposites (a) 2 wt% TiO₂, (b) 5 wt% TiO₂, (c) 10 wt% TiO₂.

The possible dispersing mechanism for nanoparticles under the steady or elongational flow field is shown in Fig. 4. For the steady shear case, the velocity gradient is almost perpendicular to the flow direction. The nanoparticle aggregates were inclined to move in a shear layer and generated the rotation of itself (as shown in Fig. 4a). Thus the nanoparticle aggregates in steady shear flow were difficult to disperse as separated particles. However, according to recent published works, the melt drops of polymer are more efficiently broken under elongational flow than shear flow.^26,33–37 Logically, there is reason to believe that these nanoparticle aggregates will be easy to be separated if they withstand a strong stretched force during the polymer processing (as shown in Fig. 4b). In this work, the vane extruder has improved significantly the elongational flow during polymer processing and it is much larger than the shear flow in the vertical direction.^28,32 This is conducive to the uniform dispersion of TiO₂ nanoparticles in PLA matrix.

Fig. 4 The schematic diagram of dispersing process for the nanoparticles in different flow field (a) steady shear flow; (b) volume elongational flow.

Rheological measurements

The dynamic oscillatory shear measurements were performed on pure PLA and its nanocomposites to investigate the response of nanoparticles to the dynamic shearing. As shown in Fig. 5, the complex viscosities (η*) of the pure PLA and PLA/TiO₂ nanocomposites are plotted as a function of frequency (ω) in rad s⁻¹. The η* of the PLA and its nanocomposites melt show only a small frequency dependence, revealing a Newtonian plateau at low frequency. Fig. 5 also indicates the dependence of complex viscosities of nanocomposites on the TiO₂ content. Obviously, with low loading content of TiO₂ (0.5 wt% and 1 wt%), the nanocomposites melt has a higher complex viscosity and theirs values are twice higher than that of the pure PLA. However, the complex viscosity values are close to the neat matrix when the TiO₂ content is greater than 2 wt%.

Fig. 5 Complex viscosity curves of PLA and PLA/TiO₂ nanocomposites with different weight fractions of TiO₂ at 170 °C.

As shown in Fig. 6, the storage modulus G′ and loss modulus G′′ for the PLA/TiO₂ nanocomposites were increased as compared with the neat PLA. For the nanocomposites, the values of G′ increased with increasing TiO₂ content, which was more obvious in the lower frequency region. It is well known that the viscoelasticity of nanocomposites at low frequency (ω) range is sensitive to the structure evolution. It can be found in Fig. 6a that the second plateau appears in the logarithmic plots of the dynamic storage modulus versus frequency, G′ ∼ ω, in low frequency region for nanoparticles filled systems, confirming the occurrence of network structure of filler in nanocomposites. The second plateau starts to appear when the TiO₂ content is 0.5 wt% and begins to be more and more obvious with increasing nanoparticle content. The G′′ of the nanocomposites have a similar trend as pure PLA at all frequency range and the values of nanocomposites have a small increase as compared to the neat matrix.

Fig. 6 G′ (a) and G′′ (b) versus frequency for PLA and PLA/TiO₂ nanocomposites with different weight fractions of TiO₂ at 170 °C.

Effect of TiO₂ nanoparticles on the crystallization behavior

It is well known that nanoparticles could have considerable effect on the crystallization process of polymer, acting as nucleating. The thermal behaviors of pure PLA and its nanocomposites with TiO₂ were detected by the differential scanning calorimetry (DSC). The DSC cooling scans and the second heating scans of PLA/TiO₂ nanocomposites were performed after first melting runs.

The crystallization exotherms of the neat PLA and its nanocomposites are shown in Fig. 7a. At a constant cooling and heating rate of 10 °C min⁻¹, crystallization melting peak temperature (T_m) for all investigated samples are around 170 °C as listed in Table 1, showing considerable independence of the TiO₂ content. As shown in Table 1, comparing to the neat PLA, the degree of crystallinity (χ_c) for the nanocomposites has no significant difference when the TiO₂ nanoparticle content was increased to 15 wt%. These results might suggest that TiO₂ nanoparticles do not play active role in heterogeneous nucleation for the PLA matrix during the cooling process. Similar results could be found in other PLA nanocomposites.^19,22

Fig. 7 (a) DSC thermograms of the second heating for PLA and PLA/TiO₂ nanocomposites (b) the cold crystallization temperature of nanocomposites with different TiO₂ content.

Table 1 DSC results of PLA and nanocomposites with TiO₂

Samples	Tcc (°C)	Tm (°C)	χc (%)
Pure PLA	100.5	169.1	6.51
PLA-0.5	106.1	169.5	6.63
PLA-1	106.5	169.5	5.91
PLA-2	104.5	170.0	5.84
PLA-5	100.4	169.0	5.47
PLA-10	98.6	168.5	4.83
PLA-15	99.0	169.3	4.67

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As shown in Fig. 7a, each curve of the investigated samples appears a clear exothermic peak because of the cold crystallization process. It is interesting to note that the cold crystallization temperature (T_cc) seems to depend on the nanoparticle content. The relationship is shown in Fig. 7b by plotting T_cc versus TiO₂ content. Obviously, with the increase of TiO₂ content (up to 15 wt%), the cold crystallization peak of nanocomposites with low TiO₂ content (less than or equal to 2 wt%) is first shifted to a higher temperature and then it decreases to the characteristic cold crystallization temperature of PLA when the TiO₂ content is more than or equal to 5 wt%. The same examination for the nanocomposite with 0.2 wt% TiO₂ has also been performed under the same process and the cold crystallization temperature was about 104 °C. (The heating scan curve and detail results are not shown here.) Evidently, the cold crystallization temperature of nanocomposites reached a maximum value at about 106 °C when the nanoparticle content was 0.5 wt% or 1 wt%. This fact implies a stronger inhibition for cold crystallization process with appropriate nanoparticles content.

Although the exact cause of this phenomenon is not yet clear, but there are several aspects should be taken into account, such as the content and dispersion of nanoparticles, the initial crystallinity, intermolecular interactions between nanoparticles and the matrix, and so on. Obviously, due to the inactive heterogeneous nucleation of TiO₂ nanoparticles and the poor crystallization ability of the PLA resin itself, only a few crystalline was formed during the cooling process at a rate of 10 °C min⁻¹ and the nanocomposites have a low crystallinity degree about 5% which is close to the pure PLA sample. Therefore, the effect of the initial crystallinity on this change of T_cc can be ignored in the second heating process. Therefore, the dispersion and the intermolecular interactions should be two important factors. On one hand, TiO₂ nanoparticles could be easily dispersed into the macromolecular chains, which disrupted the regularity of the PLA chain structure and hindered the crystallization. On the other hand, according to the published researches, there were a lot of hydroxyl groups (Ti–OH) covering the surface region of TiO₂ nanoparticles which could form a strong interfacial bond Ti–O–C with carbonyl groups of PLA.^16,17,22,23 The strong interfacial bond would strengthen the molecular chain rigidity and decrease the mobility, thus resulting in a stronger inhibitory effect on the crystallization. Similar results or conclusions could be found in other reported researches.^13,14,38

Effect of TiO₂ nanoparticles on the thermal stability

PLA is a biodegradable polyester and it is prone to degradation induced by various factors, such as heat, moisture and UV radiation. Fig. 8a shows the thermogravimetric (TG) curves of pure PLA and PLA/TiO₂ nanocomposites at a heating rate of 10 °C min⁻¹ in an N₂ atmosphere. Fig. 8b shows their respective derivative thermograms (DTG) curves. For the neat PLA resin and its nanocomposites, the decomposition temperature at 5% and 10% weight loss (respectively recorded as T_0.05 and T_0.10), the maximum decomposition temperature (T_max) and the char formation at 500 °C are listed in Table 2. In this study, the temperature at 5% weight loss (T_0.05) was defined as the onset degradation temperature to evaluate the effect of TiO₂ nanoparticles on the thermal stability. It is obvious to note that the onset degradation temperature of nanocomposites shifted to a higher temperature with loading TiO₂ nanoparticles. The peaks in the DTG curves represented the temperatures at maximum degradation rates. As shown in Fig. 8b, the fastest degradation rate occurred at a higher temperature for PLA/TiO₂ nanocomposites than that for the neat PLA. Consequently, the thermal stability of PLA/TiO₂ was improved as compared with that of pure PLA resin. This is possible because that the TiO₂ particles acted as heat barrier in the early stages of thermal decomposition.^19,38

Fig. 8 (a) TGA profile of PLA and PLA/TiO₂ nanocomposites (b) derivative thermograms curves of TGA.

Table 2 TGA results of PLA and PLA/TiO₂ nanocomposites

Sample	T0.05 (°C)	T0.10 (°C)	Tmax (°C)	Char formation at 500 °C (wt%)
Pure PLA	338.2	347.9	372.6	1.38
PLA-0.5	343.5	352.0	376.6	1.78
PLA-5	347.3	354.6	374.8	7.15

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The influence of TiO₂ nanoparticles on the thermal stability for the nanocomposites can also be demonstrated by the dynamic rheological properties. It is well known that the pure PLA resin is prone to thermal oxidative degradation under long-time effect of heat and oxygen. At a constant strain of 3%, dynamic frequency sweep tests were performed from 100 to 0.01 rad s⁻¹ within the linear viscoelastic regime. For the pure PLA and PLA/TiO₂ nanocomposites, the running time of testing and the thermodynamic effect were consistent. As shown in Fig. 9, the complex viscosity of pure PLA was deviated from the Newtonian plateau at low frequency when the sweeping was in the range of 0.1 to 0.01 rad s⁻¹. However, the complex viscosity of PLA/TiO₂ did not deviated from the Newtonian plateau until finishing the sweeping from 100 to 0.01 rad s⁻¹. This indicates clearly that nano-TiO₂ contributed in promoting the thermal stability for the PLA resin.

Fig. 9 The influences of TiO₂ nanoparticles on the thermal stability of composites.

Dynamic mechanical properties

Fig. 10 illustrates the temperature dependence of storage modulus (E′) and tan δ of neat PLA and various PLA/TiO₂ nanocomposites. All the investigated samples show similar storage modulus behaviors with increasing temperature. It is seen that the storage modulus exhibits a long plateau in the temperature range 15 to 60 °C and then followed by a dramatic drop at about 50 to 60 °C. The variation of the damping factor tan δ as a function of temperature is shown in Fig. 10b. The maximum peak point corresponds to the glass transition temperature (T_g). It is clear to note that the T_g for PLA/TiO₂ nanocomposites with 1 wt% and 5 wt% is 64 °C and 63.8 °C, respectively, which are higher than that of neat PLA (62 °C). This phenomenon may be ascribed to the restricted chain mobility which caused by the TiO₂ nanoparticles.³⁹

Fig. 10 Temperature dependence of (a) storage modulus (E′) and (b) tan δ for pure PLA and various PLA/TiO₂ composites.

Tensile properties

Fig. 11 depicts typical stress–strain curves of pure PLA and nanocomposites with different TiO₂ loadings and the main tensile properties are listed in Table 3. Obviously, all investigated samples exhibit a clearly distinguished yield point as shown in Fig. 11. With increasing TiO₂ content (up to 2 wt%), the tensile strength of nanocomposites has a slightly shifting to a higher value as compared with pure PLA as listed in Table 3. When the TiO₂ content is more than or equal to 2 wt%, the nanocomposites show a lower tensile strength than neat PLA. Although there is no big variation in the tensile modulus and tensile strength for the nanocomposites, but the elongation at break show an obvious variation as increasing the content of nanoparticles. All samples except for 15 wt% TiO₂/PLA show a higher elongation at break which the detail is listed in Table 3. In particular, the elongations of nanocomposites with 1 wt% and 2 wt% TiO₂ are about 19.1% and 24%, respectively, which are 64.6% and 106.8% higher than the neat PLA. This indicates that an appropriate loadings of TiO₂, to some extent, can improve the toughness of the rigid PLA resin.²¹ A similar results were obtained in the g-TiO₂ (surface grafted TiO₂)/PLA nanocomposites by Wang group, which the strain at break was increased to 25.3% at 0.5 wt% content of g-TiO₂, and reached a maximum at approximately 37.6% at 1 wt% g-TiO₂, increasing 5 times larger than that of neat PLA.²²

Fig. 11 Stress–strain behavior of the pure PLA and PLA/TiO₂ nanocomposite with varying TiO₂ contents.

Table 3 Tensile properties of the nanocomposites

Sample	Tensile strength (MPa)	Tensile modulus (GPa)	Elongation at break (%)
Pure PLA	59.8 ± 0.8	2.3 ± 0.1	11.6 ± 0.2
PLA-0.5	61.7 ± 1.1	2.4 ± 0.3	12.9 ± 0.3
PLA-1	61.5 ± 1.0	2.4 ± 0.1	19.1 ± 0.3
PLA-2	60.7 ± 1.5	2.3 ± 0.5	24.0 ± 0.7
PLA-5	57.6 ± 0.5	2.4 ± 0.3	13.7 ± 0.5
PLA-10	55.9 ± 0.8	2.3 ± 0.2	13.7 ± 0.5
PLA-15	50.3 ± 0.7	2.8 ± 0.4	10.4 ± 0.1

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UV transmittance

Fig. 12 shows the UV transmittance spectra of the neat PLA and the nanocomposites with different TiO₂ loads. All films for investigating have the same thickness (0.07 cm). As shown in Fig. 12, the transmittance of pure PLA was approximately 87% and decreased as TiO₂ nanoparticles were added to the PLA matrix. With increasing the TiO₂ content, the transmittance of ultraviolet has greatly reduction. When the addition of TiO₂ was more than or equal to 2 wt%, the ultraviolet in the wavelength region from 250 to 350 nm was not allowed through and the transmittance was zero. Evidently, the TiO₂ particles have greatly improved the UV resistance of the nanocomposites. This improvement is probably ascribed to the excellent UV light screening effects of TiO₂ which is linked to the quantum size effect of nanoscale particles.^16,24,27 This would render a contribution in improving the aging resistance properties of composites. As shown in Fig. 12, the transmittance spectra for all samples in visible range (>400 nm) were also measured to determine the effect of TiO₂ on the transparency of nanocomposites. As an example, the transmittance of PLA, PLA-0.5, PLA-1, PLA-2, and PLA-5 in 600 nm which the value is in visible range are 93.1%, 89.5%, 86.7%, 71.6%, and 57.1%. The transmittance in other visible wavelength have the same trends. The results indicate that the transparency of nanocomposites trended to reduce with increasing the content of TiO₂ particles. However, the reduction of the transparency of nanocomposites films was not great when the content of TiO₂ nanoparticles was less than 2 wt%.

Fig. 12 UV transmittance spectra of the PLA/TiO₂ nanocomposites.

Wettability

The surface wettability of neat PLA and its nanocomposites were determined by water contact angle measurement. The contact angles of 0.5 wt% TiO₂/PLA, 2 wt% TiO₂/PLA and 5 wt% TiO₂/PLA nanocomposites are about 68.2°, 70.5° and 70.3°, respectively, which are slightly higher than that of pure PLA in about 66°. According to the literature, the surface wettability of nanocomposites is related to the physical–chemical properties of nanocomposites and the dispersion of nanoparticles.⁴⁰ The water drop will not easily penetrate into the pores on the rough surface of the polymer/particles system if the particles were uniformly distributed within the polymer matrix, resulting the increase of the contact angle.^41,42 TiO₂ nanoparticles were incorporated in PLA matrix and resulted in increasing the surface roughness of nanocomposites, then increased the contact angle slightly.

Conclusions

In this study, PLA/TiO₂ nanocomposites with different contents of nano-TiO₂ were prepared by vane extruder which it is a novel polymer processing equipment dominated by elongational flow field. The dispersion of nanoparticles and physical properties of PLA/TiO₂ composites were examined in detail. Results show that TiO₂ nanoparticles were well dispersed in the polymer matrix via elongational flow and these nanoparticles have an inhibition for cold crystallization process to some extent. Dynamic rheological measurements and TGA results show that the introduction of TiO₂ has improved the thermal stability of composites. The toughness and UV resistance of composites were also enhanced by adding TiO₂. This provides a method to develop functional PLA nanocomposites which it can be applied in food packaging and agriculture films applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China-Guangdong Joint Foundation Project (U1201242), 973 Program (2012CB025902), the National Research Foundation for the Doctoral Program of Higher Education of China (20120172130004), Program for the Fundamental Research Funds for the Central Universities (2014ZB0021), New Century Excellent Talents in University (NCET-11-0152) and the Pearl River Talent Fund for Young Sci-Tech Researchers of Guangzhou City (2011J2200058).

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