Characterization of the Polylactic acid stretched uniaxial and annealed by RamanSpectrometry and Differential scaning calorimetry
A. Bouamer1, 2 ,N.Benrekaa2, A. Younes1, M. Zergoug1 and A. Hamouda1
1Research Center in Industrial Technologies (CRTI), Algiers, Algeria
2 Laboratory of Physics of Materials, Faculty of Physics, Faculty of Physics, USTHB, Algiers, Algeria
Abstract
in this work, we have been interested in the characterization of the effect of heat treatment and mechanical treatment on the crystallinity of a polylactic acid (PLA) film by two techniques, DSC and Raman spectroscopy. the results obtained by the dsc for the stretched film shows the appearance of a broad peak of crystallization around 120 ° C, a rise in melting peak in a significant way, which shows that the uniaxial stretching has increased the crystallinity of the PLA, whereas for the annealed film appearance of a double milting peak. The results obtained by Raman spectroscopy show new peak appears at 922 cm−1 after stretching process, indicating crystallization process occur.
Keywords: Raman, DSC,PLA, crystallinity, Introduction:
The characterization of materials by the Raman technique is based on the measurement of the vibrational states of the molecules or crystals. The use of this technique for the characterization of polymers is of great interest, given the amount of information can be collected such as the chemical composition of the macromolecules of the polymer including the identification of basic unit, the chemical structure, the interactions intra- or intermolecular, chain conformation, crystallinity and orientation of macromolecules. This technique is non- destructive, fast and will require minimal preparation of the sample [1] and [2]. Poly (lactic acid), also known as polylactide (PLA), is an aliphatic polyester known for its biodegradability and its biocompatibility is derived mainly from agricultural biopolymer resources such as corn and sugarcane [3].The aim of our work is to highlight by Raman
spectroscopy the effect of uniaxial stretching and rubber annealing on the microstructure and crystallinity of polylactic acid (PLA).
Experimental:
Figure(1) shows the chemical composition of polylactic acid (PLA) with these different chemical bonds identified by Raman spectroscopy.
Figure(1) chemical composition of acid polylactic (PLA) Raman spectroscopy:
The Raman spectroscopy analyzes were made using a LabRAM HR Evolution micro- spectrometer (HORIBA). For recording spectra, we used an exciting wavelength (λ = 633 nm) that is provided by a helium-neon laser. All peaks were obtained using an x50 lens.
Differential scanning calorimetry (DSC):
Differential calorimetry (DSC) thermal analysis gives for each sample a thermogram reflecting the variation of heat flux as a function of temperature. The device used for this analysis is a microcalorimeter DSC-7, the system is initially calibrated in temperature and heat output using high-purity Indium and Zinc sample. The baseline is determined with the capsule without sample at the same heating rate as for the analyzed samples.all samples are heated from room temperature to 190°C.
Results and discussions:
Raman Analysis
The Raman spectra of the stretched,unstretched and annealed PLA films are shown in figure 2,thedifferent peaks characteristic of PLA are obtained. The Raman scattering bands 3002 cm -1, 2947 cm -1 and 2882 cm -1 were assigned to the stretching modes of the C-H bond.
Symmetric elongation vibration of CH3 and CH groups. The CH3 stretching modes appeared at 3002 cm -1 and 2947 cm -1. And the stretching mode CH was observed at 2882 cm -1 [4].
The symmetric and asymmetric deformation modes of the CH 3 group were observed at 1386
cm -1 and 1453 cm -1 respectively. The typical carbonyl stretching (C = O) was centered at 1769 cm-1 [5]. In addition, the 1091 cm -1, 873 cm -1 and 740 cm -1 bands can be assigned to the COC, C-COO, C = O and C = O stretching modes respectively of PLA [6, 7, 8 ,]. The drawing effect can be seen first of all on the 873 cm-1 band with the increase of the intensity of the latter during the drawing, and the appearance of two peaks, one to 925cm-1 attributed to the swing mode of the CH3 group and the deformation mode of C-C and the other to 712 cm-1 which shows the out-of-plane deformation of the C = O double bond [9] are peaks that exist in semi-crystalline.
Figure (2):Superposition of Raman Spectra of stretched and unstretched PLA films DSC analysis:
FIG. 3 shows the superposition of the dscthermograms of the different states of the PLA films; The film undergoes uniaxial stretching, another unstretched and finally annealed film at a temperature of 95 ° C for 4 hours. The influence of stretching uniaxial on the thermal properties shows the appearance of a broad peak of crystallization around 120 ° C, a rise in melting peak in a significant way, which shows that the uniaxial stretching has increased the crystallinity of the PLA, whereas for the annealed film appearance of a double milting peak.a slight shift of the glass transition temperature (Tg) towards the high temperatures might be
explain by effect of uniaxial stretching on the polymer restricts the movement of the chains and causes an increase in the glass transition temperature,this reflects the rearrangement of the chain segments.
Figure (3):Superposition DSC thermograms of PLA ,stretched PLA and annealed PLA
Conclusion:
The effect of uni-axial stretching and rubber annealed on the microstructure of PLA was studied by Raman spectroscopy, which allowed us to identify the chemical bonds of the characteristic bands, and to see the influence of uni-axial stretching and rubber anealed on the chemical bonds. Functional groups through the variation of their frequency and intensity and the appearance of new frequencies in the stretched and annealed polymer, this result is verified by scaning differential calorimetry (DSC)indicating the influence of mechanical and thermal treatment on the microstructure of PLA, change of the latter from amorphous state to semi-crystalline state.
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