The effect of molecular weight and hydrolysis degree of poly(vinyl alcohol)(PVA) on the thermal and mechanical properties of poly(lactic acid)/PVA blends

1Departmento de Ingeniería de Materiales, Universidad de Concepción, Concepción, Chile 2Unidad de Desarrollo Tecnológico, Universidad de Concepción, Coronel, Chile 3Departamento de Ingeniería Mecánica, Universidad de Concepción, Concepción, Chile 4Laboratorio de Materiales Avanzados, Departamento de Ingeniería Mecánica, Universidad de Chile, Santiago, Chile 5Centro de Investigación de Polímeros Avanzados, Concepción, Chile


Introduction
Polylactic acid (PLA) is a linear aliphatic polyester obtained from renewable sources such as starch and sugar. It is one of the most widely used bioplastics due to its optical, mechanical and barrier properties, and good processability by conventional transformation techniques of thermoplastics [1] . The worldwide production capacity of PLA in 2021 is estimated around 297.000 tons, 40% more than that produced in 2016 [2] . PLA is classified by the American food and drug administration as generally recognized as safe and hence it is very common in the food packaging industry. Its barrier, thermal and mechanical properties are in fact similar to those of synthetic polymers such as polystyrene [3][4][5][6] .
PLA can be biodegraded under compost conditions; it takes up to a year to degrade in real and simulated soil burial conditions. This slow degradation compared to other polymers such as for example poly (hydroxy alkanoates), is due to the fact that PLA must be hydrolyzed before microorganisms can use it as a source of nutrients [7,8] . Adding small quantities of compatible hydrophilic polymers such as PVA, which is biodegradable, hydrophilic and flexible [9] is a way to enhance the biodegradability of PLA. For example, it has been reported that polyvinyl alcohol (PVA) has the ability to accelerate the degradation of PLA, by increasing the hydrophilicity of the blend and breaking the crystallinity of PLA [10] . Furthermore, the hydroxyl groups in PVA readily form hydrogen bonds with the ester groups of PLA, which favors the compatibility of their blends. Yeh et al., (2008) [11] found that a compatible blend PLA: PVA (80: 20 wt. %: wt. %) prepared by melt blending method using PLA Mw = 37 kg/mol and PVA with HD = 97-98.5% and Mw = 75 kg/mol. Similar results were reported by Shuai et al., 2001 [12] for blends PLA/PVA 10 to 90 wt. % of PVA with polymerization degree of 2000 and HD=99%. They report the formation of interpolymer hydrogen bonds in the amorphous region of PLA/PVA blends (PVA content higher than 50 wt. %), which contributes to compatibility of these blends. However, PVA and PLA crystallized as isolated phases, for that tensile strength and elongation at break first declines for PVA content from 0 to 50 wt. % and then increases with increasing when the PVA content.
Several authors have reported the effect of PVA content with different HD and Mw on mechanical properties of PLA/PVA blends. For example, Lipsa et al. (2008) [13] prepared films by these blends using PVA with HD = 98% and Mw = 18 kg/mol. They found that the blends are partially compatible for PVA content from 70 to 90 wt. %, and slight reduction of mechanical properties, such as lower tensile strength were observed in blends. Tsuji and Muramatsu (2001) [14] found that the Young's modulus, tensile strength and percent elongation at break of the blends increase with increasing PVA content from 50 to 90 wt. % (HD=99.5%). Hoai N. et al. (2014) [15] prepared nanofibrillary structures from PLA/PVA blends, varying content of PVA from 0 to 100 wt. %. They found that increasing the amount of PVA results the thermal degradation of blends more stable.
However, a systematic study that show the influence of content, molecular weight and hydrolysis degree of PVA on compatibility, and therefore on the thermal and mechanical properties and crystallinity of PLA has not found. In this work, PLA/PVA blends prepared by the melt blending method was studied.

Materials
The commercial PLA grade 3251D, Mw of 55.4 kg/mol and isomer D lactic acid content of 1.2% [16] was purchased from Nature Works®, United States. Four different types of PVA with different hydrolysis degree and molecular weight were provided by Sigma Aldrich  . The main specifications of PVA are summarized in Table 1. The pure PLA processed under similar conditions was studied as reference material.

Processing
The PLA/PVA blends, ratios of 80/20, 90/10 and 97/3 wt. % for each type of PVA (Table 1), were prepared via melt blending in a Brabender mixer (Plastograph ® EC plus, Mixer 50EHT32, Germany) at 60 rpm for 8 min. The temperature was set at 190 °C. The PLA was previously dried at 40 °C overnight in a vacuum oven. After blending, the samples were pressure-molded at 30 bar and 10 min in order to obtain plates for XRD and impulse excitation technique (IET) tests. The temperature was set between 170 and 210 °C, depending on the type of PVA used in the preparation of the sample.

DSC
The thermal properties of PLA/PVA blends were evaluated using a differential scanning calorimetry (DSC) analyzer (Netzsch DSC 204 F1 Phoenix), Germany. DSC curves were scanned at 10 °C/min under nitrogen atmosphere (20 mL/min). The DSC scans were performed from room temperature to 250 °C. Then, the samples were cooled to 25 °C and a second heating scans were made up to 250 °C. Glass transition temperature (Tg), cold crystallization temperature (Tcc), cold crystallization enthalpy (ΔHcc), melting temperature (Tm), melting enthalpy (ΔHm), and degree of crystallinity (X C ) were determined from the second heating scans. X C of PLA in the blends was calculated by the Equation (1): where ΔH M is the melting enthalpy of the blends, W is the weight fraction of PLA and ΔH°M is the melting enthalpy of 100% crystalline PLA (93 J/g) [17,18] .

TGA
Thermal stability of pure PLA, PVA's and their blends was evaluated using a thermogravimetric analyzer Netzsch TGA 209 F3 (Tarsus, Selb, Germany). TGA scans were carried out at 10 °C/min under nitrogen atmosphere (20 ml/min), from 35 to 600 °C.
The following parameters were reported: (i) temperature at maximum decomposition rate for each step of decomposition (T max ), (ii) weight loss associated with T max (WL T max ), (iii) the onset temperature (T onset ) corresponding to each decomposition step and (iv) the temperature corresponding to 5% weight loss (T 5% ).

XRD
The XRD measurements were recorded on a Bruker diffractometer Endeavor model D4/MAX-B (United States) operated at 40 kV, 20 mA, at room temperature, using a CuKα source and λ= 1.5405 Å. The diffraction spectra were taken in the range 3°<2θ<45° at 0.02° steps and a scanning rate of 1°/min. The pure PLA and PLA/PVA blends were previously annealed for 20 min at 120 °C under vacuum conditions.

Tensile test
The mechanical properties of PLA/PVA blends were evaluated using tensile test performed on a Karg Industrietechnik machine (Germany) according to ASTM D638 standards. Samples type V were fabricated using a mini injection equipment type Haake Minijet II, and injection conditions were 350 bar of pressure and injection temperature of 180 °C. Prior testing, specimens were conditioned under 25°C and 50% relative humidity (R.H) for 7 days. The crosshead speed was set at 10 mm/min. The Young's modulus, strength and elongation percentage at break were obtained from the stress-strain curves. At least 5 individual measurements were carried out for each formulation and mean values and standard deviations were reported. Collected data were evaluated with a one-way analysis of variance at the 95% confidence level.
For comparison purpose, elastic modulus of PLA/PVA blends were evaluated by Impulse excitation technique (IET). The rectangular samples were suspended by soft and tiny tape sticks to simulate a "free-free" boundary condition and were excited by an impact hammer. The samples have an aspect ratio of a/b=1.5, where a, b are the plate's length and width dimensions, respectively. The vibration of the samples is captured by a microphone which was connected to a data acquisition system. A signal processing software computed the frequency content of the measured signals from which the experimental resonant frequencies were identified. Once the experimental resonant frequencies The effect of molecular weight and hydrolysis degree of poly(vinyl alcohol)(PVA) on the thermal and mechanical properties of poly(lactic acid)/PVA blends were identified, the elastic modulus was computed using the relationships presented in Equation (2) [19] : where ʋ = 0.4 is Poisson´s ratio, ρ is the density, E is the elastic modulus and h=0.85 and a=20 are plate's dimensions (thickness and length respectively, in millimetres). The variable ω corresponds to the second resonant frequency of the plate, which was easier to be excited and measured.

SEM
The morphology of cryogenic-fractured cross-sections of PLA/PVA blends was analyzed by scanning electron microscopy (JEOL JSM 6380 LV, Tokyo-Japan), operated at 20 and 5 kV. The samples were fractured under liquid nitrogen and sputtered with a gold coating of ca. 50 nm. The magnifications were 100x and 500x.

DSC Analysis
DSC thermograms of PLA, PVA's and PLA/PVA blends are shown in Figure 1. The thermograms for pure PLA and PLA/PVA blends display successive peaks corresponding to the glass transition, cold crystallization and melting. The Tg, Tcc and Tm are resumed in Table 2. The only one peak of Tg is observed for PLA/PVA blends, which is ranged from 42 to 55 °C, intermediate between the Tg values of pure PLA and PVA. This behavior is characteristic of compatible blends. Shuai et al., 2001 [12] found by 13 C solid-state nuclear magnetic resonance spectroscopy, the presence of hydrogen bonds between hydroxyl groups of PVA and carbonyl groups of PLA in the amorphous region of blends with  The blends and pure PLA exhibited two endothermic peaks of melting characteristic of the formation of two PLA crystal structures during its cold crystallization, known as α and β forms with melting temperatures Tm2 and Tm1 respectively. The melting temperature of the α-form is higher because of the better quality and higher size of its crystals [20,21] . The melting temperatures of the blends are also shifted toward lower temperatures, with increasing the PVA content irrespective of Mw and HD.
The Xc values for PLA in blends decrease, especially when 20 wt.% of PVA was added. Thus, the crystallization of PLA in presence of PVA is affected due to the formation of imperfect crystals in PLA by considering that the presence of the PVA can cause a dilution of the PLA nuclei, combined with the possible interference between the chains of PLA and PVA due to partial compatibility between them. Opposite results were obtained by Yeh et al., (2008) [11] who found that highest PLA crystallinity, appears in PLA/PVA blends containing 20 wt. % of PVA (MW: 75KDa, HD: 98%), attributed to the interactions between PLA and PVA that promote the crystallization. However, Tsuji and Muramatsu (2001) [14] report a zero Xc value of PLA for all PLA/PVA blends (ratio 90:10 to 50:50 wt. %) indicating that PLA is amorphous in the blend. On the other hand, Shuai et al., (2001) [12] reported a decrease in Xc values of PLA when PVA is added in contents more than 70 wt.%, result of some depression of the PLA crystallinity upon blending with a large amount of PVA.
Variations in values of Tcc, Tm, and Xc in PLA/PVA blends are usually attributed to interactions between the components. In general, the decrease in PLA crystallinity is related to the partially compatible nature of the blends [11,22] .

TGA Analysis
Thermal stability of pure PLA, PVA's and PLA/PVA blends was evaluated by TGA analysis. Table 3 summarizes the thermal parameters. The TGA curve for pure PLA showed one-step degradation with a weight loss of 99%, associated with the loss of ester groups by unzipping depolymerization [23,24] . T max for pure PLA was 364 °C. In contrast, pure PVA's displayed three degradation steps. The first one, between 31 and 188 °C and a weight loss below 5 wt. %, is associated with the loss of absorbed moisture. The second step, between 190 and 388 °C and a weight loss ranged from 65 to 77%, is related to the loss of low molecular weight substances, such as residual acetate groups, non-conjugated polyenes, acetic acid and H 2 O. The third step, between 351 and 426 °C, is associated to the breakdown of polymer backbone [25] . The main product of thermal degradation of PVA is water, which is formed by the elimination of hydroxyl side-groups [26] .
For the same Mw, PVA with higher HD (98%) showed the lowest values of T max-1 in the second step of degradation with values around 260 °C and weight loss associated with this T max-1 was 69-71% (see Table 3). Therefore, PVA with HD of 87-89%, present better thermal stability. Acetate groups present in PVA with lower HD, confer higher thermal decomposition temperatures to PVA, favoring its thermal stability [27] .
Two degradation steps were observed for PLA/PVA blends, (see Figure 2) suggesting that thermal decomposition of PVA and PLA happens as a combined process [25] . The first step of decomposition was between 188 and 386 °C with a weight loss of 75-98 wt.%. The second one was between 381 and 507 °C with a weight loss 0.2-2.2 wt. %. With increasing PVA content in the blends, the main degradation step is shifted significantly to lower temperatures. Water absorbed by PVA, could be favoring hydrolytic degradation of PLA. The effect of molecular weight and hydrolysis degree of poly(vinyl alcohol)(PVA) on the thermal and mechanical properties of poly(lactic acid)/PVA blends The Tmax-1 shifts to higher values for those blends with PVA's of higher Mw and higher HD. With increasing Mw of PVA, the entanglement along the chains of PLA and PVA is favored. These results are in agreement with some literature reports [28][29][30] for higher Mw of PVA, which favors intramolecular entanglement between PVA and PLA, primarily by virtue of esterification of PVA hydroxylic groups and PLA carboxylic groups, improving the thermal stability of PVA in the blend.

XRD analysis
The XRD diffraction patterns for pure PLA, PVA and selected PLA/PVA blends are shown in Figure 3. The results are shown only for two types of blends, considering that the diffraction patterns for all PLA/PVA blends and pure PLA were very similar.
The absence of characteristic PVA peaks in blends is attributed to the absence of PVA crystallization in the presence of PLA suggesting that PVA molecules was trapped in an amorphous state in the PLA phase and/or PLA molecules reduce the nucleus density of PVA crystallites [14] . In this context, Shuai et al. (2001) [12] reported the presence of two isolated crystalline phases (called co-crystalline phase) coexisting in PLA/PVA blends (ratio 1:1), that have the same crystal structure without inter-polymer interactions between them. Furthermore, with the addition of PVA in PLA, all of the blends show no shift in characteristic diffraction peaks of PLA, implying that, there is no significant effect of PVA on the crystal structure of PLA [11] .

Mechanical analysis
Tensile strength of pure PLA and PLA/PVA blends are shown in Figure 4. Compared with pure PLA, the tensile strength of blends containing 3 wt. % of PVA, is very similar (irrespective of Mw and HD). However, as the PVA content increases, tensile strength tends to lower values, especially for blends with 20 wt.% of PVA, with 98% of HD.
Influence of Mw is observed for blends with 10 wt.% of PVA. Thereby, blends containing PVA with higher Mw showed higher values in the tensile strength, irrespective of the HD.
Young's modulus results from tensile test analysis for pure PLA and PLA/PVA blends are presented in Figure 5. As compared with pure PLA, it is observed a decrease in Young´s modulus with increasing the content of PVA, particularly for blends with 20 wt.% of PVA (HD: 98%), indicating that presence of PVA affect the rigidity of PLA.
Influence of HD of PVA is clearer in Young´s modulus values obtained by IET analysis presented in Table 4. For higher HD, increasing content of PVA from 10 to 20 wt.%, decrease Young´s modulus values, and for the same Mw, samples containing 20 wt.% PVA with higher HD, present lower young´s modulus. PVA present flexible characteristic, in this way is expected a decrease of rigidity in PLA/PVA blends, suggesting that acetate groups present in PVA with lower HD influence the mobility of polymeric chains in the blends. In this context, according to DSC results, the lowering of crystallinity of PLA in presence of PVA can lead a decrease in mechanical properties of PLA/PVA blends.
Results reported for Shuai et al., 2001 andH. Tsuji, 2001 [12,14] , pointing out that the lowering of mechanical properties with increasing PVA content was attributed to the partial compatibility of the blends, due to the weak interfacial adhesion between the two phases in PLA/PVA blends. However, PVA with higher Mw conferring more resistance to PLA/PVA blends acts like a cross-linked component between molecules of PVA and PLA. Figure 6 shows cross-section SEM images of selected PLA/PVA blends. In general, is observed a porous morphology for all blends, related to the partially compatible nature of PLA/PVA blends.

SEM Analysis
However, the content of PVA had a remarkable influence on blends morphology, as evidenced in the characteristics of the observed pores. In this context, blends with higher content of PVA have higher density of pores, indicating a greater phase separation, unlike blends containing less   The effect of molecular weight and hydrolysis degree of poly(vinyl alcohol)(PVA) on the thermal and mechanical properties of poly(lactic acid)/PVA blends PVA (particularly with high Mw and independent of the HD), which have smaller pores, suggesting that the compatibility of PVA and PLA was improved on addition of low quantities of PVA with higher Mw. In this context, Zhang et al., (2012) [34] reported that increasing the PVA content (polymerization degree:1700, HD:88%) from 10 to 80 wt. % in PLA/PVA blends produced a less smooth surface. This was attributed to an excessive PVA content that led to strong and extensive intermolecular hydrogen bonding, which in turn resulted in PLA aggregation. Other reports showed development of pores with an average size of 5 µm for 50/50 PLA/PVA blends, suggesting the formation of two separate phases; a PLA-rich phase forming domains leading to porosity and a continuous PVA-rich phase [14] .

Conclusions
The partially compatible nature of PLA/PVA blends were evidenced from thermal and mechanical results. In this context, were observed one peak of Tg for PLA/PVA blends, intermediate between the Tg values of pure PLA and PVA, characteristic behavior of compatible blends. Additionally, for a higher content of PVA, decrease crystallinity of PLA, thus, PLA/PVA blends with 20 wt.% of PVA tend to reduce mechanical properties of PLA, particularly for those with 98% HD. However, the addition of low quantities of PVA (3 wt. %) do not reduce the tensile strength in PLA/PVA blends, because of better compatibility between PLA and PVA. PVA's with higher Mw favors the entanglement of PVA and PLA improving the thermal stability of PLA/PVA blends. With increasing PVA content in the blends, the main degradation step is shifted significantly to lower temperatures, however PVA with HD of 87-89%, present better thermal stability due to acetate groups conferring higher thermal decomposition temperatures to PVA, favoring the thermal stability of the PLA/PVA blends.

Acknowledgements
This work has been financed by projects of Comisión Nacional de Investigación Científica y Tecnológica CONICYT (Beca de Doctorado Nacional -Proyecto PAI 781411004), CONICYT-REGIONAL R08C1002 and Programa de Financiamiento Basal para Centros Científicos y Tecnológicos de Excelencia PFB-27. The authors thank to Carmen Pradenas for sample testing.