Surface Thermodynamic Properties of 3D/4D Printing Poly Lactic Acid by Inverse Gas Chromatography

1. Introduction

Since the first description of three-dimensional (3D) printing in the 1980s, many publications were devoted to the additive manufacturing, also known as 3D printing, which was extremely developed in many applications such as engineering, materials science, physics and astronomy, computer science, chemistry, mathematics, genetics, and molecular biology, varying from biomedical engineering, and precision manufacturing to the aerospace and military industries [1,2,3,4,5,6,7,8,9,10]. In 3D printing, the three-dimensional object is created from a digital model by adding material, typically in successive layers, on the contrary of traditional manufacturing technologies such as machining, grinding, and casting, where molten material is filled in a mold to create a product [1,2,3,4]. Seven classes were distinguished in 3D printing technologies: material extrusion, material jetting, binder jetting, powder bed fusion, directed energy deposition, sheet lamination, and vat photopolymerization [1].

The increasing evolution of the spatial technology of 3D printing led to the fourth-dimensional printing (4D printing) by considering the fourth dimension of time to modulate one or more properties of the 3D printed objects with the help of smart materials that can control the application of any external stimulus implying light, water, self-diagnostic, heating, pressure, and shape-changing effects [1,2,3,4,11,12,13,14,15,16].

One of the most used materials in 3D/4D printing is the poly (lactic acid) (PLA), because of its unique properties such as good appearance, higher transparency, less toxicity, and low thermal expansion that help reduce the internal stresses caused during cooling [17,18,19,20,21]. The PLA, a bio-derived thermoplastic polymer, is a 100% biodegradable polymer with high tensile strength and modulus and easily synthesized from lactic acid obtained from corn, sugarcane, and other biomass. It can be recycled up to 8 times and is compostable at the end of life [1,22].

The highest tensile and flexural strengths of PLA and the use of its composites with bio-derived reinforcements such as flax, hemp, jute, bamboo, and other natural fibres were widely researched for 3D printing to enhance mechanical properties, reduce material and production cost, and improve the sustainability of manufactured products [1,22,23,24,25,26,27,28,29,30,31,32,33,34].

PLA, a biodegradable aliphatic polyester, is produced from renewal resources has received much attention in the research of alternative biodegradable polymers [35,36,37]. This PLA polymer is the most popular polymer in the world and may be processed using standard machines, equipment and technologies for classic polymers [35,38,39]. PLA shows good biocompatibility and physical properties, such as high mechanical strength, thermoplasticity and fabricability [35].

The biodegradable PLA polymer is the most used material worldwide for 3D printing [1,22,23,35], and very solicited in 4D printing technology. PLA is an excellent bio-derived polymer is now used as a shape-memory polymer in 4D printing applications [28,29,30,31,32,33]. The future of 4D printing biocomposites involves multi-disciplinary research to combine design strategies, material properties, stimulus properties, and composite mechanics [1].

The surface properties, the Lewis acid-base parameters, and the dispersive and polar energies of PLA polymer are very important to be determined in many 3D and 4D applications involving the mechanical, adhesion and surface properties. Such properties were not correctly determined in literature.

In this paper, we were interested in determining the surface thermodynamic properties and the various variables of interactions between PLA polymer and other organic molecules. The technique used to study the polymer material was the inverse gas chromatography (IGC) technique at infinite dilution (ID). Our new models [40,41,42,43,44,45,46] were applied to quantify the dispersive and polar interaction energies to understand the behavior of PLA polymer and, therefore, to predict the various superficial thermodynamic properties of this 3D/4D printing material in interaction with organic molecules.

The London dispersive interaction [47] between the solvents and the solid materials was determined by applying the London equation and the notion of polarizabilities and ionization energies of the organic molecules and the polymeric material. This new methodology led to the separation between the dispersive and polar surface free energies of PLA and to the accurate determination of the Lewis enthalpic and entropic acid–base constants, the polar acid and base surface energies, and the glass transition of the PLA polymer.

2. Methods and Materials

In this paper, the inverse gas chromatography (IGC) at infinite dilution (ID) was used to determine the net retention time of organic solvents adsorbed on the solid material [48,49,50,51,52,53,54,55,56,57,58,59,60]. This results in the calculation of the net retention volume $Vn$ of the adsorbed probes, and therefore to the values of the free energy of adsorption $∆G_a^0$ of organic molecules adsorbed on PLA polymer given by the following equation:

$$∆G_a^0\left(T\right)=-RTlnVn+K\left(T\right)$$

(1)

Where $T$ is the absolute temperature, $R$ the perfect constant gas and $K\left(T\right)$ a constant depending on the temperature and the interaction solvents-PLA

$∆G_a^0\left(T\right)$ is expressed at any temperature by the summation of the London dispersive energy $∆G_a^d\left(T\right)$ and the polar energy $∆G_a^{sp}\left(T\right)$:

$$∆G_a^0\left(T\right)=∆G_a^d\left(T\right)+∆G_a^p\left(T\right)$$

(2)

Many methods and molecular models were used in literature [48,49,50,51,52,53,54,55,56,57,61] to separate the two dispersive and polar contributions of the free energy of adsorption. It was previously showed [40,41,42] that the best method that gave the accurate separation between $∆G_a^d\left(T\right)$ and $∆G_a^p\left(T\right)$ was that based on the London dispersion interaction energy given by equation (3):

$$∆G_a^d\left(T\right)=-\frac{{\alpha }_{0S} }{ H^6}\left[\frac{3\mathcal{N}}{{2\left(4\pi {\epsilon }_0\right)}^2}\left(\frac{{\epsilon }_S {\epsilon }_X}{\left({\epsilon }_S+{\epsilon }_X\right)}{\alpha }_{0X}\right)\right]$$

(3)

Where $\mathcal{N}$ is the Avogadro number, ${\epsilon }_0$ the dielectric constant of vacuum, ${\alpha }_{0S}$and ${\alpha }_{0X}$ the respective deformation polarizabilities of the solid material denoted by S and the organic molecule denoted by X, separated by a distance $H$, and ${\epsilon }_S$ and ${\epsilon }_X$ their corresponding ionization energies.

By combining equations (1) to (3), equation (4) was obtained:

$$RTlnVn=\frac{{\alpha }_{0S} }{ H^6}\left[\frac{3\mathcal{N}}{{2\left(4\pi {\epsilon }_0\right)}^2}\left(\frac{{\epsilon }_S {\epsilon }_X}{\left({\epsilon }_S+{\epsilon }_X\right)}{\alpha }_{0X}\right)\right]-∆G_a^{sp}\left(T\right)+K\left(T\right)$$

(4)

The chosen interaction parameter ${\mathcal{P}}_{SX}$ was given by equation (5):

$${\mathcal{P}}_{SX}=\frac{{\epsilon }_S {\epsilon }_X}{\left({\epsilon }_S+{\epsilon }_X\right)}{\alpha }_{0X}$$

(5)

For non-polar molecules such as n-alkanes, the representation of $RTlnVn$ of as a function of $\left[\frac{3\mathcal{N}}{{2\left(4\pi {\epsilon }_0\right)}^2}\left(\frac{{\epsilon }_S {\epsilon }_X}{\left({\epsilon }_S+{\epsilon }_X\right)}{\alpha }_{0X}\right)\right]$ of adsorbed molecules is given by equation (6):

$$RTlnVn\left(non-polar\right)=A\left[\frac{3\mathcal{N}}{{2\left(4\pi {\epsilon }_0\right)}^2}{\mathcal{P}}_{SX}\left(non-polar\right)\right]-K(T)$$

(6)

where $A$ is the slope of the non-polar straight line given by:

$$A=\frac{{\alpha }_{0S} }{ H^6}$$

(7)

For a polar molecule adsorbed on PLA polymer, the geometric point representing the polar probe will be located outside the straight line of n-alkanes and the distance between the polar point and this straight line will be equal to $∆G_a^p\left(polar\right)$ of the polar molecule, at chosen temperature.

$$∆G_a^p\left(T, polar\right)=RTlnVn \left(T, polar\right)-A\left[\frac{3\mathcal{N}}{{2\left(4\pi {\epsilon }_0\right)}^2}{\mathcal{P}}_{SX}\left(polar\right)\right]+K(T)$$

(8)

In the case of linear variations of $∆G_a^p\left(T\right)$ of polar probes as a function of the temperature, it is possible to deduce the specific enthalpy $\left(-∆H_a^p\right)$ and entropy $\left(-∆S_a^p\right)$ of polar probes adsorbed on PLA polymer by using the classic thermodynamic relation (9)):

$$∆G_a^p\left(T\right)=∆H_a^p-T∆S_a^p$$

(9)

The determination of $\left(-∆H_a^p\right)$ and $\left(-∆S_a^p\right)$ of adsorbed polar molecules leads to characterize the Lewis’s acid-base properties of PLA polymer by its enthalpic (K_A, K_D) and entropic (${\omega }_A$, ${\omega }_D$) acid-base constants using the following relations:

$$\left\{\begin{array}{l}\left(-∆H^p\right)=K_A\times D{N}^{\prime }+K_D\times A{N}^{\prime } \\ \left(-∆S_a^p\right)={\omega }_A\times {DN}^{\prime }+{\omega }_D\times {AN}^{\prime }\end{array}\right.$$

(10)

where $DN\prime$and $AN\prime$are, respectively, the corrected electron donor and acceptor numbers of the polar molecule [62,63].

Experimental results showed that the relations (10) were not always satisfied. In similar cases, other relations (11) were proposed in literature [44,46,64] taking into consideration the amphoteric coupling constants $K_{CC}$ and ${\omega }_{CC}$ of solid materials:

$$\left\{\begin{array}{c}\left(-∆H^p\right)=K_A\times D{N}^{\prime }+K_D\times A{N}^{\prime }-K_{CC}\times AN\prime \times DN\prime \\ \left(-∆S_a^p\right)={\omega }_A\times {DN}^{\prime }+{\omega }_D\times {AN}^{\prime }-{\omega }_{CC}\times AN\prime \times DN\prime \end{array}\right.$$

(11)

Relations (11) can be written as:

$$\left\{\begin{array}{c}{a}_i K_A+K_D-{b_i K}_{CC}={\left(c_H\right)}_i \\ a_i {\omega }_A+{\omega }_D-b_i {\omega }_{CC}={\left(c_S\right)}_i \end{array}\right.$$

(12)

Where $a_i$, $b_i$, ${\left(c_H\right)}_i$ and ${\left(c_S\right)}_i$, relative to the adsorbed polar molecule denoted by $i$, are known experimental values given by equations (13), whereas, $K_D$, $K_A$, $K_{CC}$, ${\omega }_A$, ${\omega }_D$ and ${\omega }_{CC}$ are the unknown quantities of the problem (12)

$$\left\{\begin{array}{c}{a}_i={\left(\frac{D{N}^{\prime }}{A{N}^{\prime }}\right)}_i\\ b_i={\left(D{N}^{\prime }\right)}_i\\ {\left(c_H\right)}_i={\left(\frac{\left(-∆H^p\right)}{A{N}^{\prime }}\right) }_i\\ {\left(c_S\right)}_i={\left(\frac{\left(-∆S^p\right)}{A{N}^{\prime }}\right)}_i\\ \end{array}\right.$$

(13)

The unique solution of the system (12) can be obtained if the number $n$ of polar solvents satisfies $n\ge 3$, by using the least squares method. The obtained solution $\left(K_D; K_A; K_{CC}\right)$ or $\left({\omega }_D; {\omega }_A; {\omega }_{CC}\right)$ thus minimizing the sum of the squares of the residuals.

3. Results

3.1. London Dispersive Component of Surface Energy of PLA

By using the same procedure developed in previous studies [40,41] and by varying the temperature of the chromatographic column containing the PLA particles, IGC technique allowed us to obtain the net retention times of the various solvents adsorbed on the PLA polymer. This led to the net retention volumes of injected probes and therefore the values of $RTlnVn$ of adsorbed organic molecules. The experimental results were given in Table S1 and S2 (Supporting Materials).

The London dispersive component ${\gamma }_s^d\left(T\right)$ of the surface energy of PLA polymer was obtained by applying the Hamieh thermal model [43,44,45,64,65]. The variation of ${\gamma }_s^d\left(T\right)$ of PLA polymer as a function of the temperature was plotted on Figure 1.

A non-linear evolution of ${\gamma }_s^d\left(T\right)$ was observed (Figure 1a) showing an important change in the thermodynamic properties of PLA when the temperature varies. However, three linear variations were distinguished (Figure 1b) characterized by a decrease of ${\gamma }_s^d\left(T\right)$ in the temperature interval [313.15K, 333.15] given by the equation (14):

$${\gamma }_s^d\left(T\right)=-0.384 T+165.87$$

(14)

Passing by a minimum of the London dispersive surface energy equal to 35.8 mJ/m² at $T=343.15K$, followed by an increase variation of ${\gamma }_s^d(T$ to reach a maximum equal to 56.4 mJ/m² at $T=363.15K$, with the following straight-line equation (15):

$${\gamma }_s^d\left(T\right)=1.030 T+317.68$$

(15)

And finally, a decrease characterized by equation (16):

$${\gamma }_s^d\left(T\right)=-1.272 T+518.31$$

(16)

This interesting result highlighted the possible glass transition temperature of PLA around $T_g=343.15K$ (70°C) explained by the change in the variations of ${\gamma }_s^d\left(T\right)$ of PLA before and after $T_g$, when the temperature increases.

3.2. Polar Surface free Energy of PLA polymer

All thermodynamic surface properties of PLA polymer were obtained by using our new method of the harmonic mean of the ionization energies and the deformation polarizability of particles. Table 1 and Table 2 respectively presented the values of deformation polarizability, harmonic mean of the ionization energies and the parameter ${\mathcal{P}}_{PLA-X}$ of the various organic molecules in interaction with the PLA polymer. The Handbook of Physics and Chemistry [66] were used to determine the parameters of the different solvents.

The obtained experimental results (Tables S1 and S2), the values in Table 1 and Table 2 and the equations 2, 4, and 8 allowed us to calculate the polar free surface energy ($-∆G_a^{sp}\left(T\right)$) of the polar solvents adsorbed on PLA polymer as a function of the temperature T (Table 3).

The results in Table 3 showed an amphoteric behavior of PLA with stronger basic character which is clearly shown by the high values of $-∆G_a^{sp}\left(T\right)$ of dichloromethane, the most acidic solvent among the five used polar molecules, and then traducing the important interaction energy with PLA. It was observed in Table 3 that the variations of $-∆G_a^{sp}\left(T\right)$ for all polar molecules, are not linear. The curves of $-∆G_a^{sp}\left(T\right)$ of polar solvents adsorbed on PLA in Figure 2 proved this non-linearity against the temperature with a maximum temperature around 343.15 K confirming the presence of the glass temperature of PLA which was previously shown with the variations of London dispersive surface energy ${\gamma }_s^d\left(T\right)$ as a function of the temperature.

3.3. Lewis’s Acid–Base Constants of PLA

The curves of ($-∆G_a^{sp}\left(T\right)$) of polar molecules drawn in Figure 2 showed four different temperature intervals in which the variations of ($-∆G_a^{sp}\left(T\right)$) are represented by straight line with an excellent linear regression coefficient equal to 0.9990. The results were given in Table 4 with the different equations.

The values of the polar enthalpy ($-∆H_a^p)$ and entropy ($-∆S_a^p)$ of the polar molecules adsorbed on PLA polymer were deduced from the equations of $∆G_a^{sp}\left(T\right)$ in Table 4. The values of these polar thermodynamic parameters were given in Table 5.

Table 5 allowed us to draw on Figure 3 the curves of ($-∆H_a^p\left(T\right)$) and entropy ($-∆S_a^p\left(\mathrm{T}\right)$) of polar molecules as a function of the temperature

It was observed that the curves of Figure 3 present positive values of polar enthalpy and entropy of polar solvents before and after the glass transition temperature highlighting the adsorption phenomenon, whereas, the desorption was observed during the transition process showing a repulsive interaction around the glass transition.

By using the empirical relation (10) and the results in Table 5 and Figure 3, the Lewis enthalpic and entropic acid–base constants $K_A$, $K_D, {\omega }_A$, and ${\omega }_D$of PLA polymer as a function of the temperature with their ratios were given in Table 6.

The results in Table 6 showed that the behavior of the PLA surface is 5.5 times more basic than acidic for a temperature less than 333.15K (Figure 4). The desorption of the polar solvents in the glass transition process led to a neutral surface of the polymer for $333.15K < T < 343.15K$ characterized by negative values of the Lewis acid-base constants $K_A$, $K_D, {\omega }_A$, and ${\omega }_D$of PLA. After the glass transition temperature, two zones were distinguished (Figure 4):

-

Stronger amphoteric character of PLA with highest values of the Lewis acid-base constants for $343.15K<T<353.15K$

-

Decreasing amphoteric behavior of PLA surface with lowest values of $K_A$, $K_D, {\omega }_A$, and ${\omega }_D$of the polymer for $T>353.15K$

Table 6 and Figure 4 clearly showed the stronger basic character of PLA varying with the temperature and decreasing amphoteric behavior for larger temperatures ($T>353.15K)$

Correction of the acid-base parameters of PLA

The validity of the equations (10) was not always satisfied as was shown by the values of the linear regression coefficients R2 given in Table 6. The correction reported by Hamieh et al. in other papers was applied in this work to give more accurate values of the acid-base constants of PLA polymer. Equations (11) to (13) were used with procedure developed in a recent paper [42]. The corrected results were presented in Table 7.

The comparison between the results in Table 6 and Table 7 showed that the error committed by neglecting the amphoteric constant reaches 25%, however, the tendency of the acid-base behavior of PLA remains the same with two used methods.

3.3. Dispersive and Polar Free Energy of PLA

This new method applied on the PLA polymer using the London dispersion interaction equation resulted in the net separation of the London dispersive free energy $∆G_a^d\left(T\right)$ and the polar free energy $∆G_a^p\left(T\right)$ of interaction between the PLA and the adsorbed organic molecules. By using equation (3), it was possible to experimentally determine the values of $∆G_a^d\left(T\right)$ of all molecules adsorbed on the PLA polymer from the following equation:

$$∆G_a^d\left(T\right)=A\left[\frac{3\mathcal{N}}{{2\left(4\pi {\epsilon }_0\right)}^2}{\mathcal{P}}_{SX}\right]$$

(17)

Where the values of the parameter $A$ was determined from the experimental results (Table 8).

It was observed that the variations of the parameter $A$ given in Table 7 passed through a minimum corresponding exactly to the glass transition temperature $T_g=343.15K$.

The values of $∆G_a^d\left(T\right)$ of the adsorbed organic molecules were determined (Tables S3 and S4). The obtained results led drawing the variations of $∆G_a^d\left(T\right)$ also showed a minimum at glass transition as was shown in Figure 5.

The experimental determination of the dispersive $∆G_a^d\left(T\right)$ and polar $∆G_a^p\left(T\right)$ energy of interaction allowed us to obtain the total free energy $∆G_a^0\left(T\right)$ of organic molecules (Table S5 and Figure S1) that also highlighted the presence of the glass transition

3.4. Average Separation Distance H

Table 8 and equation 7 allowed determining the average separation distance H between PLA surface and the organic molecules as a function of the temperature. The obtained results are given in Table 9. These results showed a slight variation of the separation distance when the temperature varies, but it respects the general tendency observed with the other thermodynamic variables showing a signature at the glass transition temperature reported for $H=8.30\mathrm{\AA }$ (Figure S2). The results in Table 9 and Figure S2 showed a small increase of the separation distance when the temperature increases until the glass transition $T<343.15K$, followed by a slight decrease of $H$ after this temperature. This result confirmed that obtained by the stronger acid-base constants of PLA obtained for $T>343.15K$.

3.4. Lewis Acid-Base Surface Energies of PLA

To determine the acid ${\gamma }_s^{+}$ and base ${\gamma }_s^{-}$ surface energy of PLA polymer, Van Oss’s relation was used [67]:

$$-∆G_a^p\left(X-Polar\right)=2\mathcal{N}{a}_X\left(\sqrt{{\gamma }_{lX}^{-}{\gamma }_s^{+}}+\sqrt{{\gamma }_{lX}^{+}{\gamma }_s^{-}}\right)$$

(18)

Where ${\gamma }_{lX}^{+}$ and ${\gamma }_{lX}^{-}$ are the respective acid and base surface energy of the polar molecule $X$ adsorbed on PLA surface with $a_X$ the surface area of the adsorbed solvent.

Using the experimental values relative to ethyl acetate (EA) and dichloromethane (CH₂Cl₂) respectively given by ${\gamma }_{EA}^{+}=0,{\gamma }_{EA}^{-}=19.2 mJ/m^2$ and ${\gamma }_{CH2Cl2}^{+}=5.2 mJ/m^2,{\gamma }_{CH2Cl2}^{-}=0$; it was possible to determine the values of ${\gamma }_s^{+}$ and ${\gamma }_s^{-}$ of PLA polymer by using equations (19):

$$\left\{\begin{array}{l}\begin{array}{c}{\gamma }_s^{+}=\frac{{\left[∆G_a^{sp}\left(T\right)\left(EA\right)\right]}^2}{{4\mathcal{N}}^2{\left[a\left(EA\right)\right]}^2{\gamma }_{EA}^{-}} \end{array} \\ {\gamma }_s^{-}=\frac{{\left[∆G_a^{sp}\left(T\right)\left(CH2Cl2\right)\right]}^2}{{4\mathcal{N}}^2{\left[a\left(CH2Cl2\right)\right]}^2{\gamma }_{CH2Cl2}^{+}} \end{array}\right.$$

(19)

Whereas, the acid-base (polar) surface energy ${\gamma }_s^{AB}$ of PLA was obtained from equation (20):

$${\gamma }_s^{AB}=2\sqrt{{\gamma }_s^{+}{\gamma }_s^{-}}$$

(20)

The results were reported in Table 10 and Figure S3. The variations of the acid and base surface energies versus the temperature showed a decrease of these surface energy parameters and then an increase reaching a maximum at the glass transition temperature, followed by a final decrease until $T=373.15K$. An important basic surface energy ${\gamma }_s^{-}$ of PLA was observed with smaller acidic surface energy.

The results in Table 10 with those relative to the dispersive surface energy of PLA previously obtained in this work, led to the determination of the Lifshitz – Van der Waals (LW) surface energy ${\gamma }_s^{LW}$ (Table 11) by using equation (21):

$${\gamma }_s^{LW}={{\gamma }_s^d+\gamma }_s^{AB}$$

(21)

3.5. Polar Component of the Surface Energy of Polar Molecules

By using the previous results and the equation (22) relating the polar free energy of polar molecules adsorbed on PLA to the polar components of the surface energy of the PLA polymer ${\gamma }_s^{\mathrm{p}}$ and the polar organic molecules ${\gamma }_l^{\mathrm{p}}$.

$$\left\{\begin{array}{c}-∆G_a^p\left(X\right)=2N{a}_X \sqrt{{\gamma }_s^{\mathrm{p}}{\gamma }_l^{\mathrm{p}}}\\ or {\gamma }_l^{\mathrm{p}}=\frac{{\left(-∆G_a^p\left(X\right)\right)}^2}{4{\mathcal{N}}^2{a_X}^2{\gamma }_s^{\mathrm{p}}} \end{array}\right.$$

(22)

${\gamma }_l^{\mathrm{p}}$ of polar molecules was directly obtained from equation (22). The results were given in Table 12

Table 12 showed that among all obtained values of the polar components of the surface energy of polar molecules, the stronger ${\gamma }_l^{\mathrm{p}}$ values were obtained with the dichloromethane, the highest acidic solvent used in this study, once again proving the highest Lewis basicity of PLA polymer.

4. Conclusion

Inverse gas chromatography (IGC) at infinite dilution was used to determine the surface thermodynamic properties of the biodegradable poly lactic acid considered as the most interesting material that can be used in 3D and printing applications. The new method used was that based on the London dispersion interaction equation. This equation took into account the polarizability and the harmonic mean of the ionization energies PLA polymer and adsorbed organic solvents. The London dispersive energy of PLA material was determined by using the Hamieh thermal model. The free dispersive and polar energies of adsorbed solvents were obtained by using the new parameter ${\mathcal{P}}_{SX}$ and the net retention volumes of adsorbed probes from chromatographic measurements. The variations of all thermodynamic parameters of interaction of organic molecules adsorbed on PLA highlighted four temperature intervals with linear equations in each interval of temperature. A glass transition temperature of PLA was located at $T_g=343.15K$. The presence of this transition phenomenon had an important effect on the non-linearity in the domain of temperature containing the glass transition temperature. This conducted to the strong variation of the enthalpic and entropic acid base constants of PLA as a function of the temperature. A stronger basic character of PLA surface was highlighted before and after the glass transition. A slight variation of the average separation distance between the PLA polymer and the solvents.

The determination of the various components ${\gamma }_s^{+}$, ${\gamma }_s^{-}$, and ${\gamma }_s^{AB}$ of acid–base surface energies of PLA allowed us to calculate the Lifshitz – Van der Waals surface energy ${\gamma }_s^{LW}$. A dominant basic surface character was shown with a highest value of ${\gamma }_s^{-}$of PLA. All these surface parameters confirmed the presence of $T_g=343.15K$ for the poly lactic acid.

The application of this new method allowed a net separation between the polar and dispersive free energy and also the determination of the polar components of the surface energy of polar solvents adsorbed on PLA polymer. These new findings are very useful and can be directly applied to an accurate determination of the dispersive and polar works of adhesion between the PLA surface and other materials.