Introduction
In recent years, Polylactide (PLA), as an aliphatic biodegradable material derived from renewable resources such as corn and plant straw, has attracted exuberant research interests due to its good biocompatibility and complete degradation into H2O and CO2[1]. PLA is also considered as a potential candidate for petrochemical polymers [2,3].
This sample in the material library is a polymer blend that bases on the PLA matrix. To identify the structure and properties of this sample, three techniques will be used for characterization. Infrared Spectroscopy is used to analyze the chemical composition and investigate the functional groups of the sample. Differential Scanning Calorimetric is used to identify the polymer type and the thermal properties of the sample. Besides, the qualitative information about crystallization behaviour can be obtained from the DSC curve. Finally, Scanning Electron Microscope was utilized to investigate the morphology of the surface of the sample, which can reveal the patterns on the PLA film.
Infrared Spectroscopy
Infrared (IR) Spectroscopy is the analysis of infrared light interacting with a molecule. The principle of IR is when infrared light passes through a sample, certain frequencies of the light are absorbed by the chemical bonds of the substance, leading to molecular vibrations. The frequencies and intensities of absorbed infrared light depend on the specific bond strengths and atoms of that molecule, and the absorption pattern (called a spectrum) is unique for each material. The condition of the material, such as the temperature, the state (solid, liquid, or gas), concentration, pressure, and other materials in a mixture also affects the spectrum. It is this spectrum that makes infrared spectroscopy a very powerful tool for both qualitative and quantitative analysis. [1,2]
For qualitative analysis, IR spectroscopy can be used to determine function groups in the sample and their interaction, then determine the composition. For quantitative analysis, IR spectroscopy can determine the percentage composition of a liquid sample mixture by the application of Beer's law with very high precision:
where A is absorbance; T is transmittance; ε is the molar absorptivity; l is the path length of the sample; c is the concentration of the compound in solution; C is the relative concentration and cn is the concentration of every compound. [1,2,3]
In the following case, ATR-FTIR （1wt% limit, 0.01cm-1 resolution） is used to investigate the fuction gruops and the specific intermolecular interaction of the PLA/PBS blends [3]
Table 1 The assignments of functional groups in PLA/PBS[3]
Fig.1 The spectra of PLA [5]
Table 1 shows the function groups in PLA/PBS blends. The IR spectra of PLA revealed characteristic absorption peaks of ester (1716.98 and 1119.50 cm-1 for -COO- and -CO-, respectively), -Ch3 (Stretching) group (2988.70) and -Ch3(2988.70). However, from IR spectra of PLA/PBS, wavelength of -Ch3 (Stretching) group is 2880, wavelength of -COO- is 868, -CO- is 1180. So, the wavelength of -COO- change a lot between PLA and PLA/PBS blends which shows a interaction between PLA and PBS.
Differential Scanning Calorimetric
Differential scanning calorimetric (DSC) was employed to identify the polymer type and character the degree of crystallinity. DSC can be divided into heat-flux DSC and Power-compensated DSC, they can measure the relationship between the heat (or power) difference input to the sample and the reference as a function of temperature.[8] The principle of DSC is the different amount of heat flows to the sample when it undergoes a phase transition under the increasing temperature compared with the reference. The x-axis of DSC curve represents the temperature (or time) and the y-axis represents the rate of heat flow, dH/dt, thus multiple thermodynamic parameters like Tg, Tm can be found in the curve. [9]
Thus, the polymer type of the material can be identified from the curve because the amorphous polymers have classical glass transition step while crystalline polymers only have a melting peak. The precision of Tg and Tm can be affected by the test conditions including heating rate, the mass of sample and atmosphere while DSC usually can give a result with the accuracy and precision are better than 1%. [10] Moreover, the melting enthalpy and crystallinity of material can be qualitatively calculated by the following formulas:
Where △Hm represents the melting enthalpy, K is the calorimetric constant, and A is the area of melting peak, Xc is the crystallinity, △Hm(0) represents the enthalpy required to melt pure samples.[11]
Fig 2 shows the DSC curve of pure PLA and PLA/PBS blends. It can be found that pure PLA curve exhibits glass transition, cold crystallization peak and melting peak, meaning that PLA is a semi-crystalline polymer and the crystallinity may be influenced by the thermal history. Besides, two melting peaks in one curve indicates the PLA is immiscible with secondary phase PBS. Tab 3 shows the detailed data obtained from Fig 2, the Tg of PLA is 59.3℃, the Tm of PLA and PBS are 166.2℃ and 116.2℃ respectively. The moderate melting temperature endows the polymer blend with good thermal processing ability. In addition, with the increasing content of PBS, the △Hm of PLA tends to increase while △Hc decreases, which implies that PBS facilitated the crystallization of PLA so it required more energy to destroy the PLA crystals.
Fig. 2 DSC heating curves of PLA and PLA/PBS with different blending ratio[12]
Table 2 DSC parameters of PLA and PLA/PBS blends[12]
Scanning Electron Microscope
The Scanning Electron Microscope (SEM) can be used to obtain magnified images that show the information about the shape and distribution of the phase.[13]
The electron emitted by the electron gun is accelerated along with the cylinder by the electrostatic field. Under the action of the electromagnetic lens and aperture in the cylinder, the electron is focused on an electron beam and incident into the sample. The scanning coil at the bottom of the tube controls the electron beam to scan on the surface of the sample to form a grating, whose strength changes with the structure and composition of the sample surface. The incoming beam electron interacts with the nucleus or the extranuclear electron of the sample and is scattered, which can be divided into elastic scattering and inelastic scattering. In the interaction, some electrons are affected by the nucleus（the attraction caused by opposite charges）, while the rest electrons gradually lose kinetic energy and are absorbed. In this process, the incident electron energy can excite various signals from the sample, such as secondary electron, backscatter electron, absorption electron, etc.[14]
The secondary electron is the extranuclear electron excited by the initial beam electron acting on the sample surface, with low energy and the best imaging resolution. Image contrast mainly depends on the appearance of the sample surface. The atoms in the convex part of the sample surface are easier to be excited than those in the concave part, and the number of secondary electrons escaping is more, so the image in the convex part is brighter. The secondary electronic signal is the main imaging signal of SEM.[15] In the SEM operation process, the sample has to be put in the gold steaming room to sputter an additional conductive thin gold layer on the surface of the sample to increase the sample’s conductivity.[16] Then the signal probe of SEM collects the corresponding signal, and after video amplification and processing, the electron beam current intensity in the CRT can be synchronously modulated to display the image of each point on the surface of the sample on the CRT display screen.[17]
This technique can offer qualitative information about the morphology of the sample structure. In the following figures, the resolutions of SEM can reach 1nm to 50 μm. The two resolutions 50 μm and 20 μm in this case correspond to the magnification of 500X and 2500X respectively. 50 μm’s resolution meets the requirement to observe the difference of these materials.[18]
Through the SEM picture (3), it is obvious that the PLA film’s surface is even and compacted. The photo (4) shows that the pure PLA film’s surface appeared smooth and dark. There are also some crack patterns formed in the pure PLA form, which could be explained by PLA’s natural brittleness so the cracks formed after the film dried.
Fig. 3 SEM micrographs of PLA(2500x)[19]
Fig. 4 SEM micrographs of PLA(500x)[20]
Reference
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[3]Wan Y, Chen W, Yang J, et al. Biodegradable poly (Llactide)- poly (ethylene glycol) multiblock copolymer: synthesis and evaluation of cell affinity. Biomaterials 2003; 24: 2195–2203.
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[7] Dr Runglawan Somsunan. Determination of the Interaction of Poly(lactic acid)/Poly(butylene succinate) Blends Containing Fatty Acids for Bioplastics Applications. Retrieved from https://asea-uninet.org/portfolio-item/determination-of-the-interaction-of-polylactic-acidpolybutylene-succinate-blends-containing-fatty-acids-for-bioplastics-applications/.4th,June,2020
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[19] Chetan Swaroop1, Mukul Shukla POLYLACTIC ACID/ MAGNESIUM OXIDE NANOCOMPOSITE FILMS
FOR FOOD PACKAGING APPLICATIONS 21st International Conference on Composite Materials, Xi’an, 20-25th August 2017
[20] Kurniawan Yuniarto Bruce Ari Welt Morphological, Thermal and Oxygen Barrier Properties Plasticized Film Polylactic Acid Journal of Applied Packaging Research Vol.9(2017) No.3